QUT

THE SYNTHESIS AND

INVESTIGATION OF NOVEL

1]1-0RGANOMETALLIC PORPHYRINS

A thesis presented to

THE QUEENSLAND UNIVERSITY OF TECHNOLOGY

In fulfilment of the requirements for the degree of

Doctor of Philosophy

By

REGAN D. HARTNELL

Bachelor of Applied Science (Applied Chemistry)

Masters of Science

Synthesis and Molecular Recognition Program

School of Physical and Chemical Sciences

September, 2003

QUT

QUEENSLAND UNIVERSITY OF TECHNOLOGY

DOCTOR OF PHILOSOPHY THESIS EXAMINATION

CANDIDATE NAME:

FACULTY:

SCHOOL:

CENTRE:

PRINCIPAL SUPERVISOR:

ASSOCIATE SUPERVISORS:

THESIS TITLE:

Regan David Hartnell

Science

Physical and Chemical Sciences

Instrumental and Developmental Chemistry

Dr Dennis Arnold

Dr Godwin Ayoko Dr Steven Bottle

Synthesis and Investigations of Novel meso-n 1 Organometallic Porphyrins

Under the requirements of PhD regulations~ Section 16, It is hereby certified that the thesis of the above-named candidate has been examined. On advice from the Principal Supervisor and Head of School and the School Director Research, I recommend on behalf of the University that the thesis be accepted in fulfilment of the conditions for the award of the degree of Doctor of Philosophy.

ProfessorA4ahendr.an Acting Chair of Research Degrees Committee

... ~ .. ~ .. : ... ~.~ .. : .... ~.~.::.~ .................... . Date

1

2

ABSTRACT

Porphyrins and modified porphyrins play an integral part in many aspects of our day­

to-day lives. These include photosynthesis in green plants and oxygen transport and

storage within all living animals. The biomimicry of these and other natural systems,

has inspired much porphyrin research. The remarkable optical, electronic, and

coordinative properties of porphyrins have also seen them intimately involved with

the preparation of devices for advanced technologies. Organometallic porphyrins are

a relatively new area of chemical research that has attracted a great deal of interest.

This interest is due to the biologically important roles that some organometallic

porphyrins and modified porphyrins play in natural and chemical models of

coenzyme B12, cytochrome P450 and other haem-containing biomolecules. Recently,

meso-111-organometallic porphyrins have emerged as a rich area of chemical

research. This interest stems from their synthetic usefulness and the critical roles that

they play in bond forming reactions that involve porphyrins. Various stable entities

are known and have made an ideal starting point for further investigation of systems

that involve the combination of the remarkable optical- and electronic properties of a

porphyrin macrocycle with the various benefits (synthetic, chemical and physical) of

an organometallic fragment.

Attempted metathesis reactions of peripherally-metallated meso-11 1-

porphyrinylplatinum(II) complexes such as trans-[PtBr(NiDPP)(PPh3)2] (H2DPP =

5,15-diphenylporphyrin) with organolithium reagents fail due to competitive addition

of the organic fragment at the porphyrin ring carbon opposite to the metal

substituent. This reaction can be prevented by using 5,10,15-triarylporphyrins. These

triarylporphyrins are readily prepared using the method of Senge and co-workers by

addition of phenyllithium to the appropriate 5,15-diarylporphyrins, followed by

aqueous protolysis and oxidation. They are convenient, soluble building blocks for

selective substitutions and subsequent transformations at the remaining free meso

carbon. Oxidative addition of Pt(O) phosphine complexes generates the

organoplatinum porphyrins in high overall yields. The bromo ligand on the external

metal fragment undergoes exchange with alkynyl nucleophiles, including 5-

3

ethynylNiDPP, to form the first examples of meso-11 1-porphyrinylplatinum(II)

complexes with a second Pt-C bond. The range of porphyrinylplatinum(II)

bis(tertiary phosphine) complexes was extended to the triethylphosphine analogues,

by oxidative addition ofH2TrPPBr to Pt(PEt3)3, and the initially-formed cis adduct is

only slowly thermally transformed to trans-[PtBr(H2TrPP)(PEt3)2]

The oxidative addition of bis(dibenzylideneacetone)platinum(O) to meso-bromo-

5,15-diarylporphyrins in the presence ofPPh3 has also been shown to be an effective

way of synthesising 11 1-organoplatinum porphyrins in high yields. This methodology

has been extended to synthesise various palladio- and platinioporphyrins that utilise

bidentate nitrogen donor ligands [N,N,N',N'-tetramethylethylenediamine (tmeda) and

2,2'-bipyridyl (bpy)] in order to enforce a cis configuration at the metal centre.

The modification of peripherally-metallated meso-11 1-platiniometalloporphyrins, such

as trans-[PtBr(NiDAPP)(PPh3)2] [H2DAPP = 5-phenyl-10,20-bis(3' ,5'-di-t­

butylphenyl)porphyrin] leads to the corresponding platinum(II) nitrato and triflato

electrophiles in almost quantitative yields. Self-assembly reactions of these meso­

platinioporphyrin tectons with pyridine, 4,4' -bipyridine or various meso-4-

pyridylporphyrins in chloroform generate new multi-component organometallic

porphyrin arrays containing up to five porphyrin units. These new types of

supramolecular arrays are formed exclusively in high yields and are stable in solution

or in the solid state for extended periods.

A platinio-bis(phosphine) rrorganometallic porphyrin monomer of Zn(II)DPP was

prepared by the oxidative addition of a Pt(O) species that was formed in situ. A meso­

meso directly linked dimeric porphyrin was prepared from this monomer by Ag(I)

promoted oxidative coupling and planarised by oxidative double-ring closure to yield

a triply linked diporphyrin. Electrochemical measurements of this series indicate that

the organometallic fragment is a strong electron donor. The fusion and planarisation

continues this trend with substantial lowering of the first one-electron oxidation

potential and narrowing of the HOMO-LUMO gap. Preliminary investigations

indicate that the triply linked dimer is easily oxidised to the cation radical.

TABLE of CONTENTS

DECLARATION

ACKNOWLEDGMENTS

NOTE FOR READERS

CHAPTERl.

Introduction

1.1. Description ofResearch Problem Investigated

1.2 Overall Objectives ofthe Study

1.3 Specific Aims of the Study

1.4 Account of Research Progress Linking the Research Papers

CHAPTER2.

Literature Review

6

7

8

9

4

10

10

11

11

13

2.1 Introduction to Porphyrins 14

2.2. UV -visible Absorption Spectroscopy of Porphyrins 17

2.3 Introduction to Supramolecular Arrays ofPorphyrins 18

2.4 Porphyrin Assemblies Mimicking the Photosynthetic Reaction 20

Centre.

2.5 Strategies for the Preparation ofMulticomponent Arrays that 24

Contain Porphyrins

2.5.1 Supramolecular assemblies containing porphyrins 24

linked via covalent bonds

2.5.2 Supramolecular assemblies through axially coordinated 27

ligands

2.5.3 Coordination polymers containing porphyrin units

2.5.4 Molecular squares and cyclic arrays

2.6 Introduction to Organometallic Porphyrins

2.6.1 Porphyrin analogues

2.6.2 meso-111-0rganometallic porphyrins

2. 7 Conclusion

2.8 Project Aims

2.9 References

36

32

31

34

37

40

41

42

5

CHAPTER3. 54

Remarkable homology in the electronic spectra of the mixed-valence

cation and anion radicals of a conjugated bis(porphyrinyl)butadiyne

CHAPTER4~ 63

Peripherally-metallated porphyrins: mesa-11 1-porphyrinyl-platinum(II)

complexes of 5,15-diaryl- and 5,10,15-triarylporphyrins

INTRODUCTION 67

EXPERIMENTAL 68

General 68

Synthesis 69

Crystallography 76

RESULTS AND DISCUSSION 76

Studies of the addition of carbon nucleophiles to 76

porphyrinylplatinum(II) complexes

11 1-Platinioporphyrins with triethylphosphine ligands 84

CONCLUSION 86

Acknowledgements 87

REFERENCES 87

CHAPTERS. 90

Peripherally metallated porphyrins: the first examples of meso-11 1-

palladio(II) and -platinio(II) complexes with chelating diamine ligands

INTRODUCTION 93

RESULTS AND DISCUSSION 94

Syntheses of Palladio- and Platinioporphyrins. 94

NMR Spectra ofPalladio- and Platinioporphyrins. 99

Mass and UV-visible Absorption Spectra ofPalladio- and 101

Platinioporphyrins.

CONCLUSION 104

EXPERIMENTAL SECTION 105

General Remarks 105

Acknowledgments 110

REFERENCES 110

6

CHAPTER6. 113

Peripherally 11 1-Platinated Organometallic Porphyrins as Building

Blocks for Multiporphyrin Arrays

INTRODUCTION 116

RESULTS AND DISCUSSION 118

Syntheses ofPorphyrin Starting Materials. 119

Self-assembly with Pyridyl Moieties. 122

Self-assembly of Multi-porphyrin Arrays. 125

Electronic absorption spectra. 128

CONCLUSION 129

EXPERIMENTAL SECTION 130

General Procedures. 130

Acknowledgments. 135

REFERENCES 135

SUPPORTING INFORMATION 141

CHAPTER 7. 143

Bromobis(phosphine )platinum(II) as a cation-stabilising and solubilising

substituent on directly-linked and fused triply-linked diporphyrins

INTRODUCTION 146

RESULTS AND DISCUSSION 147

Electronic absorbance spectra 154

Electrochemical investigations 155

CONCLUSION 158

EXPERIMENTAL SECTION 159

General Procedures 159

Acknowledgements 162

REFERENCES 162

CHAPTER 8. 166

Discussion, Conclusions, and Future Work

8.1 Discussion and Conclusions

8.2 Future Work

167

174

7

DECLARATION

The work contained in this thesis has not been previously submitted for a degree or

diploma at any higher educational institution. To the best of my knowledge and

belief, the thesis contains no material previously published or written by another

person except where due reference is made.

R. D. Hartnell

September, 2003

ACKNOWLEDGMENTS

R.D.H wishes to thank:

His principal supervisor, Dr Dennis Arnold

Dr Steven Bottle and Dr John Bartley

Staff at the School of Physical and Chemical Sciences, QUT, for their

assistance

Some fellow chemistry post grads

Friends and family

8

9

NOTE FOR READERS Due to the required layout of this thesis, there may be some confusion with the

numbering scheme of compounds. Within the individual chapters and papers the

compounds are numbered consecutively as they are encountered in the text. If in a

latter chapter or discussion the compound is referred to again it will be numbered as

chapter number first followed by the compound number. For example, compound

3.12 is compound 12 from chapter 3. Any inconsistencies in formatting from

chapter to chapter are to be expected since each chapter is formatted to meet the

requirements of the journal that the research paper was submitted to. Some diagrams

within these papers have been moved to make it easier for the reader.

10

CHAPTERl

Introduction

11

1.1 INTRODUCTION

1.1. Description of Research Problem Investigated

Porphyrins and modified porphyrins are involved in many essential systems that

allow for the survival of life on this planet. These include photosynthesis, which

generates the oxygen that we breathe and the food that we eat as well as providing

the cells in all living animals with oxygen via haemoglobin in the bloodstream.

Organometallic porphyrins are a relatively new area of chemical research that has

attracted a great deal of interest. One of the key reasons for this attention is the

biologically important roles that some play in natural and chemical models of

coenzyme B12, cytochrome P450 and other haem-containing biomolecules. Recently,

mesa-11 1-organometallic porphyrins have emerged as a rich area of chemical research

due to their synthetic usefulness and the critical roles that they play in bond forming

reactions that involve porphyrins. Several stable entities are known and would make

an ideal starting point for further investigation of systems that involve the

combination of the remarkable optical- and electronic properties of a porphyrin

macrocycle with the various benefits (synthetic, chemical and physical) of an

organometallic fragment. This thesis is concerned with the results of investigations

along this line.

1.2 Overall Objectives ofthe Study

The overall objectives of this PhD project are the preparation and investigations of

novel mesa-111-organometallic porphyrins. This especially relates to YJ 1-palladio(II)­

and platinio(II)-porphyrin containing entities. This overall encompassing objective

umbrellas several more specific goals, which include:

• Preparation and investigation of new synthetically useful porphyrin

macrocycles.

• Preparation and investigation of various novel organometallic porphyrins.

• Incorporation of these organometallic porphyrins in supramolecular systems.

• Investigation of these supramolecular systems.

12

1.3 Specific Aims of the Study

• To investigate and optimise methodologies for the preparation of

synthetically useful, soluble and easily functionisable porphyrin macrocycles

in gram quantities.

• To prepare a range of novel porphyrins with a metal fragment attached to the

periphery of the porphyrin macrocycle in the meso positions.

• To investigate novel ways of preparing the aforementioned 11 1-

metalloporphyrins by means of a porphyrin nucleophile or by

transmetallation at the porphyrin's peripheral positions.

• To investigate the reactivity of subsidiary ligands bonded to a metal moiety

that is in turn directly bonded to the periphery of a porphyrin macrocycle as a

means of ligand substitution.

• To investigate the use of organometallic-porphyrins i.e. 1 11 -meso-

metalloporphyrins in order to prepare supramolecular systems with

predictable architectures.

• To delineate systematically the spectroscopic properties of all prepared

complexes by the means of:

• 1 H NMR spectroscopy

• 31P NMR spectroscopy

• Multi-dimensional (DQF-COSY, NOESY etc.) NMR spectroscopy

• To characterise all novel compounds (where suitable) by instrumental

methods e.g. elemental analysis, electronic absorption UV -visible and Near

IR and mass spectroscopies and single crystal X-ray crystallography.

• To study the redox behaviour of 11 1-metalloporphyrins by the means of

electrochemistry.

1.4 Account of Research Progress Linking the Research Papers

The relationship between the progression of research to the preparation of papers for

publication is explained in the flow diagram below. This diagram also shows how the

individual chapters in this thesis relate to the extension of ideas and research results.

Chapter 2

Literature Review * Introduction to porphyrins * Introduction to supramolecular assemblies of

porphyrins * Introduction to organometallic porphyrins

Chapters 3 and 4

Method development and optimisation for the synthesis of

soluble, easily functionisable porphyrin starting materials

Chapters 4, 5, 6 and 7

Preparation and methodology optimisation for the synthesis of

novel1/ -organometallic porphyrins

Chapters 4, 6 and 7

Preparation of supramolecular systems that contain 11 1

-

organometallic porphyrins

Chapters 4, 6 and 7

Characterisation and investigation of properties of supramolecular systems that

contain 11 1 -organometallic porphyrins

Chapter 3

Synthesis, properties, and investigation of dimeric

porphyrin systems

Chapters 4, 5, 6 and 7

Characterisation and investigation of properties of 111 -organometallic porphyrins

13

Figure 1.1. The flow of research progress and the relationship between research

papers and chapters in this thesis.

14

CHAPTER2

Literature Review

15

2. LITERATURE REVIEW

2.1 Introduction to Porphyrins

Porphyrins are deeply coloured red or purple, fluorescent, crystalline pigments of

natural or synthetic origin, having in common a substituted aromatic macrocyclic

ring consisting of four pyrrole type moieties, linked together by four bridging

methine groups. 1 The structure of the parent macrocycle and the lUP AC numbering

of the ring positions is shown below.

Figure 2.1 The structure of porphin and the TIJP AC numbering system

Positions 5,10,15,20 are usually referred to as the meso-positions and positions

2,3,7,8,12,13,17,18 are usually referred to as the f3-positions. The porphyrin ring

system is a planar, 18n electron aromatic system.2 Porphyrins are essentially flat

molecules, however circumstances can prevail when binding metal atoms, for

example, where the macrocycle becomes distinctly buckled with major distortions

from planarity. The central cavity of the macrocycle can complex a huge variety of

metal ions, with some ions, such as nickel(II), being slightly too small to fit into this

cavity. In this case the macrocycle distorts and twists to maximise its binding to

nickel.3

Modem scientific methods and instrumentation, including femtosecond lasers,

synchrotron radiation, supercomputers, and recombinant DNA techniques are

routinely exploited in order to unravel the mysteries of porphyrin chemistry and to

determine the modus operandi of these systems. In 1964, F. E Falk published a thin

266 page book on Porphyrins and Metalloporphyrins4 which represented the first

attempt to apply the principles of modem chemistry and physics to the structure and

function of porphyrin systems. This treatise on the subject soon became obsolete and

16

was replaced with an updated multiauthored volume. 5 This new volume further

described many of the unique systems, especially in nature that porphyrins are

intimately involved with. However this effort was soon overwhelmed by this field

that was rapidly spreading in every direction across the whole spectrum of science.

In the late 1970's, David Dolphin edited a multiauthored seven volume series6

comprising of many thousands of pages which covered in detail the current status

quo of the field of porphyrin chemistry and enjoyed success as the defmitive

porphyrin reference series for several decades. However, due to the burgeoning

interest in this field this text inevitably aged and has been replaced recently by a

twenty volume, 7000 page series, that combines the expertise of over 200 authors in

more than 120 chapters.7 These quantum leaps of interest in porphyrin chemistry

indicate the crucial role that it plays in our daily lives. For example, chlorophylls

help to fix carbon dioxide, create our food chain, and generate the oxygen that we

breathe. 8 Photosynthesis remnants are the source of the coal and oil we use as fuels,

and haemoglobin carries the oxygen in our bloodstream. Biologists and biochemists

are constantly uncovering new biological functions of porphyrins in reactions as

diverse as nitrogen assimilation, methane generation and a range of electron transfer

reactions, as well as oddities such as the porphyrin that controls the sex of the larvae

of the Mediterranean sea worm, Bonnelia viridis.8

One of the main thrusts of porphyrin research seeks to understand both the

fundamental principles of photosynthesis (the harvesting of solar energy and its

transduction into chemical species that drive the biochemistry of green plants, algae

and bacteria), and the catalytic functions of porphyrins in a wide range of

bioenergetic reactions. The aim of the work is to design synthetic systems that mimic

the efficiency and selectivity of biological reactions without biochemical by­

products. Conversely, simple synthetic models can offer valuable insights into

reactions that occur on picosecond timescales. 8

17

Metalloporphyrins play vital roles in many biological systems. 3 The main function of

porphyrins and porphyrin-like compounds in nature is to bind metal atoms, which

can then act as centres for significant biochemical events. Thus, protoporphyrin IX in

haem 1, complexes iron, which in haemoglobin and myoglobin reversibly binds

oxygen so that. it can be transported around the body (haemoglobin) or stored in

muscle tissue (myoglobin).

Photosynthesis captures most of the energy that drives most living things. This is

achieved by trapping energy contained in sunlight and using it to convert carbon

dioxide and water into energy-rich carbohydrates. In this process the water is

oxidised to oxygen. 3

Phytyl C02

2

In chlorophyll a 2, the coordinated metal is magnesium. Here, the function of the

macrocycle is to capture light in the near-ultraviolet (400 nm) and red (650-700 nm)

regions of the visible spectrum. The substituents around the macrocycle serve to fme­

tune light-absorption characteristics of the molecule. The metal also serves to

modulate the light-absorbing and energy-transfer characteristics of the chlorophyll,

while acting as a centre for binding water, which acts as an electron source.3

18

2.2. UV-visible Absorption Spectroscopy of Porphyrins

One ofthe more fascinating features of porphyrins is their striking colour, which is a

consequence of their remarkable UV -visible spectra. The characteristic electronic

absorption spectrum of a typical porphyrin such as nickel(II)octaethylporphyrin

(NiOEP) is shown in Figure 2.2.

nickel(ll)octaethylporphyrin (N iOEP)

3oo 556

wav-el~ngth/ nm

Figure 2.2 UV/visible spectrum ofNiOEP (recorded in CHCh)

700 760

From this it can be seen that the spectrum consists of two distinct regions. The first

region at around 400 nm (the Soret orB-Band) corresponds to a strong transition

from ground to the second excited state (So~ S2), and a weaker transition to the first

excited state (So ~ Sr) is seen at about 550 nm (the Q-band). Internal conversion

from S2 ~ St is rapid so fluorescence is only detected from St. The B- and the Q­

bands both arise from n - n* transitions and can be explained by considering four

orbitals (the "Gouterman four-orbital model")9-11

: two n orbitals (a1u and a2u) and a

degenerate pair of n* orbitals (egx and egy), as shown in Figure 2.3. The two highest

occupied n orbitals have similar energies, so there is strong configurational

interaction between a1u ~ eg and a2u ~ eg transitions. Constructive interference leads

to the more intense B-bands while the weaker Q-bands result from destructive

combinations. The a1u orbital has nodes at all four meso positions while the a2u

orbital has high coefficients at these sites. 12

19

B

egy(LUMO) egx(LUMO)

Q

a1u (HOMO) ~u(HOMO)

Figure 2.3 Schematic of the Gouterman four-orbital model.9-11

•13

Metalloporphyrin spectra are classified as either "regular" or "irregular" depending

on whether the metal that they contain has a closed or open shell of valence

electrons.3 Regular metalloporphyrins give normal spectra, i.e. a B-band in the near­

UV (390-425 nm depending on the macrocyclic substitution pattern), and two Q­

bands, a and J3. The a band occurs within the range 570-610 nm for complexes in

which the macrocycle is substituted in the J3-positions. For complexes in which the

macrocycle is substituted in the mesa-positions, the a band occurs between 590 and

630 nm. These positions depend on the metal that is complexed within the central

cavity of the porphyrin macrocycle and whether this coordinated metal has axial

ligands.

2.3 Introduction to Supramolecular Arrays ofPorphyrins

The crystal structure of the reaction centre (RC) from photosynthetic bacteria3•14

-19

confirmed what biologists already new about the spatial arrangement of the various

components of the RC, but it also inspired many research groups and thus initiated

the design and synthesis of numerous porphyrin-containing multi component systems

expected botft to display structural analogy with the RC and to fulfil some of its

photochemical and electron-transfer functions.

20

Figure 2.4 Schematic representation of the photosynthetic reaction centre isolated from

Rh d d . "d" 3,14-19 o opseu omonas vzn zs .

A fragment of the photosynthetic reaction centre, as isolated from

Rhodopseudomonas viridis is shown above, with the special pair bacteriochlorophyll

molecules labelled as such. The bacteriochlorophyll molecules in the special pair act

as the primary electron donors, while the adjacent bacteriochlorophyll (BCl)

facilitates electron transfer to the bacteriopheophytin (BPh) which acts as the primary

electron acceptor. The menaquinone moiety acts as an electron relay agent to

facilitate the fmal stage of electron transfer to the final electron acceptor, namely

ubiquinone, through a non-haem iron atom. There are two distinct branches of the

reaction centre, (M and L branches), however due to minor conformational changes

between the two sides, the L branch is the major pathway for electron transfer.

The main function of the RC is to generate an energy gradient using successive

electron transfer processes to span the cytoplasmic membrane. During

photosynthesis, extremely long -lived, charge-separated states ( t 112 > 1 s) are very

efficiently formed (quantum efficiency ;:::: 1 ), facilitated by the redox sites being

physically separated by a relatively large distance (ca. 70 A.). The series of many

small reaction steps that provide for long-range, efficient charge-separation are

21

sufficiently energy releasing that the unwanted, inherent reverse charge

recombination steps are effectively reduced. 20

A large range of molecular entities, mostly acceptor-substituted porphyrins have

been synthesized and used to determine rates of transfer as a function of separation

distance, mutual orientation, nature of insulating spacer group, energy gap, solvent

polarity, and the nature of the reactants.21-25 Synthetic porphyrin models with either

flexible or constrained geometries have proven useful in exploring how factors such

as distance, donor-acceptor orientation and energy, and solvent, mediate

photoinduced electron transfer reactions. For ease of synthesis and the stability of

model compounds, most artificial photosynthesis models are based on porphyrins or

metalloporphyrins, instead ofbacteriopheophytin, chlorophyll, or bacteriochlorophyll

found in the natural RC. 22•25

•26 In addition, in order to obtain a precise spatial distance

control, most artificial photosynthesis systems utilise coordination or covalent bonds,

instead of hydrogen bonds with proteins as found in natural systems. Replacing weak

hydrogen bonds or van der Waals forces with stronger chemical bonds enhances the

efficiency of electronic coupling between the donor and the acceptor.

The synthesis, characterisation and investigation of supramolecular entities that

contain porphyrin chromophores are not only of interest to research groups

concerned with the biomimicking of the photosynthetic RC, but such entities have

also enjoyed success in related areas of research such as non-linear optics,27-31

photoinduced picosecond molecular switches/2•33 optoelectronic gates,34

fluorescence quenching sensors,35 photonic wires,32•36 photodynamic reagents/741

catalysis, and molecular metals and conducting polymers. 13•42

•43

2.4 Porphyrin Assemblies Mimicking the Photosynthetic Reaction Centre.

For a well-designed short donor-acceptor dyad, D-A, the quantum yield of the light­

induced charge separation state (D•+_A._) can be assumed to be close to 1, although

quite often as a result of this very efficient charge separation the lifetime of this

separation is usually very short (less than a nanosecond) due to back charge

recombination processes. Factors such as direct covalent bonds between the donor

and acceptor moieties which increase the charge separation quantum yield of these

22

systems will also allow for rapid charge recombination to yield the initial ground

state configuration (D•+ -A •-----+ D-A) and hence the short lifetimes of the separated

state. A porphyrin containing synthetic system designed for the biomimicry of the

photosynthetic' reaction centre has been put forward by Gust, Moore and co­

workers 44-48 and it has been found that many features of this system mimic those

found in nature. These photosynthetic models 3 and 4 consisted of a covalently

linked arrangement of chromophores and electron donors and electron acceptors,

namely, porphyrins (P), carotenoid (C) and quinone (Q) modules.

p

3M=Zn

4M=Hz

p

0~0 ~ 0

0

Q Q

The strategy used in the design of this model is that multiple short-range electron

transfer steps are overall much more efficient than one long-range electron transfer.

Upon light excitation of the Zn porphyrin chromophore in 3 (C-ZnP*-H2P-Q-Q)

several small, fast and very efficient electron transfer steps occur in order to give rise

to the charge separated species (c•+_znP-H2P-Q-Q•-). The lifetime of charge

separation of 3 was found to be in the order of 50 f!S with a quantum yield of 83%

(similar to that found in natural systems). It was found that by demetallating the Zn

porphyrin of 3 to give rise to the entirely free base system 4, the lifetime of the

charge-separated species was somewhat increased to 340 f!S, however the overall

quantum yield was reduced to 15%.

Osuka and co-workers49-59 have also reported several synthetic models of the

photosynthetic RC. These models consist of several Zn porphyrins covalently linked

together and held in a restricted conformation with relatively low conformational

freedom.

5n=l

6n=2

7

23

Q

lm

The stacked porphyrin dimer (PP) mimics the special pair in the RC and the electron

acceptor unit is the quinone (Q) in the case of 5 and 6, and the pyromellitinide (Im)

fragment in 7. These dyads were examined by picosecond transient optical

spectroscopy, and the lifetime of the charge separated ion pair investigated. The

charge separation lifetimes of 5 and 6 were found to be 50 ps and 500 ps,

respectively. Two different charge separated species were detected for 7, namely

(PP-Pe+-Im•-) and (PPe+-P-Im•-) which had lifetimes of 50 ps and 2.5 J!S,

respectively. However, the quantum yield for charge separation in 5-7 was almost

100%. It is known that the more strongly coupled sub-units in 5 - 7 allow for higher

quantum yields, however this greater coupling also makes charge recombination

more facile and hence gives rise to the shorter lifetimes than the semiconjugated

amide linkages of 3 and 4. 22•44

-59

24

Imahori and co-workers60-64 have recently reviewed their work in nanostructured

artificial photosynthesis which utilises combinations of porphyrins and fullerenes as

monolayers on gold or indium-tin oxide (ITO) surfaces. An example of such

arrangements is shown below in Figure 2.5.

hv

CONH

Ar

Fe p

Figure 2.5 Photo-induced electron transfer and electron transfer at gold electrodes modified with

mixed self-assembled mono layers of ferrocene-porphyrin-C60 alkanethiol and boron dipyrrin thiol. 60-

64

It was found that these combinations of porphyrins and fullerenes are excellent

building blocks for artificial photosynthesis by examining the photo-induced

intramolecular electron transfer in the porphyrin-fullerene linked systems using time­

resolved spectroscopic methods covering from picosecond to second time regions.

One of the major benefits of utilising systems like these was found to be the

acceleration of photo-induced charge separation and the deceleration of charge

recombination. It was shown that these acceleration and deceleration effects arise

from the small reorganisation energies of fullerenes during charge separation as

opposed to more conventional acceptor moieties like quinones and diimides. The

electron transfer properties of the fullerenes were associated with the three­

dimensional delocalised n-systems with large size, spherical, rigid, strained shape,

and high symmetry. These and other porphyrin-fullerene triads and tetrads displayed

25

stepwise electron transfer relay, mimicking the primary charge separation in the

photosynthetic RC.

2.5 Strategies for the Preparation of Multicomponent Arrays That Contain

Porphyrins

There have been many different strategies employed to synthesise supramolecular

assemblies that contain porphyrin chromophores. A detailed explanation of these

numerous methodologies is beyond the scope of this brief literature review; however

an overview of some of the most popular methods and the type of assemblies

available by these methods will be discussed.

2.5.1 Supramolecular assemblies containing porphyrins linked via covalent

bonds.

Arnold and co-workers65-79 have synthesised systems like 8 which consists of a

simple bis(porphyrin) system linked via a butadiyne bridge which allows for

extensive electron transfer between macrocycles via the conjugated bridge.

8

More complicated systems like that of Mak80 may also be constructed by covalent

bonds as demonstrated by the metalloporphyrin dendrimer 9. It was found that this

dendrimer cooperatively bound to four molecules of 1,4-diazabicyclo[2.2.2]octane

(DABCO) with a folded arm arrangement. 80

26

9

Lindsey et al. have investigated many phenylacetylene linked multiporphyrin

systems (some shown in Figure 2.6) in order to investigate the fundamentals of

electronic communication between the constituents of covalently linked

multicomponent arrays. It is well known that understanding the basics of electronic

interactions is essential for the rational design of molecular devices for photonic,

electronic, or optoelectronic applications34,36

,81

-90

molecular wire

t l input output

linear gate

l redox-switched site

input output

T gate

l input output

redox-switched site

Figure 2.6 Molecular wire and two optoelectronic gates. The molecular wire contains a boron­

dipyrrin dye (input unit), three Zn porphyrins, and one free base porphyrin (output unit). Each

optoelectronic gate is comprised of a short molecular wire (boron-dipyrrin, Zn porphyrin, free base

porphyrin) and a redox-switched unit (Mg porphyrin). 32

27

Osuka and co-workers91-121 have prepared a large range of directly meso-meso singly

linked, meso-meso, ~-~ and meso- ~ doubly linked and meso-meso, ~-~, ~-~ triply

linked porphyrin dimers and oligoporphyrins up to a remarkable 128-mer. The fused

tape-like porphyrins show extremely red-shifted absorption bands that reflect the

extensively n-conjugated electronic system and a low excitation gap. The structure

and ultraviolet-visible-infrared absorption spectra of the dimer 10 to dodecamer 16

are shown below. The lowest energy electronic absorption bands become

increasingly intensified and red-shifted as the number of directly linked porphyrins

increases. The first one-electron oxidation potential also decreases steadily with an

increase in number of fully conjugated porphyrin macrocycles. The interesting

properties of these conjugated, rod-like molecules suggest their potential use as

molecular wires in nanoscale devices and their use as near-IR dyes, IR sensors and

dyes and materials for non-linear optics.

28

Ar

Ar

Figure 2.7 Ultraviolet-visible-infrared spectra of porphyrins 10-16 recorded in CHCb.99

Covalently bound supramolecular structures are usually constructed in a step-wise

manner from the individual components forming new covalent bonds during each

addition.

2.5.2 Supramolecular assemblies through axially coordinated ligands.

There have been reports of the construction of numerous stable multi-porphyrin

arrays by supramolecular aggregation of individual porphyrin units that have been

pre-programmed with a recognition algorithm. Spontaneous self-assembly of these

units allows for the quick, often quantitative construction of larger arrays. There are

numerous examples of supramolecular porphyrinic arrays assembled through

29

coordination of phosphine groups, 122-126 coordination to centrally bound Ru(II)127

-144

or Zn(IIi9'81

'145

-167 or with other coordinating metal centres.168

-176

Haycock and co-workers146 have utilised the pre-designed Co(II) porphyrin 17 to

self-assemble in solution to the 12 porphyrin cyclic array 18.

cE N R

17 18

Structure 18 was identified after a series of Gel Permeation Chromatography (GPC)

experiments. These elegant yet simple experiments demonstrated that by the simple

and rational design of the unsymmetrical monomer, certain structures or

conformations may be preferentially formed. The macrocyclic architecture resembles

the bacterial light-harvesting complexes LHl and LH2 where close coupling of the

chromophores around the ring facilitates rapid and efficient energy transfer over long

distances.14-16

Metalloporphyrins can act as acceptor building blocks provided that the metal that is

coordinated within the central cavity of the porphyrin macrocycle has at least one

free additional coordination site that is available for axial coordination (see Figure

2.8). Oligomers, polymers (see below) and dendrimers of axially connected

metalloporphyrin tectons can be designed through the judicious selection of the

coordination geometry of the metal atoms (usually 5 or 6 coordinate) and the

geometry of the poly dentate bridging ligands.

The number of axial coordination sites that are available for self-assembly

coordination with donor atoms can be fine tuned by changing the nature and the

oxidation state of the metal atoms within the centre of the porphyrin rings. For

example five coordinate zinc atoms readily form adducts with N-donor heterocycles,

30

however these interactions are intrinsically weak and are fairly easily displaced

(association constant, ca. 103 M-1). On the other hand, stronger, more inert bonds are

formed between N-donor heterocycles and Rurr(CO)-, Osrr(CO)- or RhmX (where X

=a halide atom) porphyrins. It must be noted that although Run, Osrr and Rhm are all

six-coordinate metal centres, only one extra axial site may be exploited for

supramolecular coordination, because one is already occupied by the carbonyl ligand

or halide atom, and these are not easily displaced.127 However, Sn1v

metalloporphyrins prefer six-coordinate geometry and have a good affmity for

oxygen ligands.

Figure 2.8. Some schematic examples of metalloporphyrins as acceptor building blocks in the

construction of discrete supramolecular assemblies; the metal atom is five-coordinate in a-c, and six­

coordinate in d.127

Porphyrins may also act as donor building blocks, provided that they are furnished

with the appropriate ligating fragments at the periphery of the macrocycle that can

suitably coordinate to metal centres. Most examples reported in the literature take

advantage of porphyrin tectons elaborately decorated with covalently attached

hydroxy or N-donor ligands, usually at the meso positions.

The coordination chemistry of Rh(III) and Sn(IV) porphyrin building blocks was

utilised by Redman175'177 to build larger arrays, like 19. Generally in arrays like 19,

excitation of the rhodium porphyrin units leads to fast and efficient triplet-triplet

31

energy transfer to the pyridyl porphyrin. This process has been found to be

irreversible when the pyridylporphyrin is a free base but involves a reversible

equilibrium in the case of the Zn-metalated systems.

19

Chichak and co-workers130•134

•175

'177 utilised six metalloporphyrins to spontaneously

assemble into a octahedral array 20 through a metal-directed synthesis. 1H NMR

spectroscopy provided a convenient means for tracking the progress of the self­

assembly process and highlighted that the rates in which the octahedral complex

forms is influenced by the steric demands of the porphyrin building blocks. The

porphyrin building blocks were found to be particularly attractive supramolecular

building blocks, as they possess rich photo- and redox properties.

20

Compounds like 20 have been suggested to be possible models for solar energy

capture and transfer in naturally occurring photosystems, as well as to create artificial

photoactive molecular devices.

32

2.5.3 Coordination polymers containing porphyrin units

Polymeric porphyrins or related tetrapyrrole ring molecules have been formed in

several ways?2,1

70'178

-180 The porphyrin macrocycle can either be the pendent group

or part of the polymer main chain. For the former, attachment of the porphyrin can be

through the covalent bond (a), coordinative bond (b), or electrostatic interaction (c).

For the later, the polymer may be linear (d), zigzag (e), two dimensional (f), or

"shish-kebab" bridged type (g).

Porphyrins as pendent groups to the polymer chains

Porphyrins as part of the polymer chains or networks

* \

~ N N (f) N \ N

\:;M/ ~--}---· * *·-{----\:;M~~---}~-* N ,~N n ~ N , N ~ ~

* \ m *

Porphyrins as part ofligand-bridged coordination polymers

2.5.4 Molecular squares and cyclic arrays

By the appropriate choice of ligands and bridging groups, it is possible to build-up an

almost limitless number of pre-designed structures in both two (cyclic/squares) and

three (polyhedra) dimensions.23'24

'181

-187 These structures are of particular interest due

to their predictable architecture by the definitive choice of

33

components.127,140,158,168,169,176,183,186-195 As shown below in figures 2.9 and 2.1 0, by

careful selection of individual components an exotic range of products is available

after one self-assembly step.

', ""''1 ' ' . . A ' I ' ' ',Subuni ' .A

I

~ ' ...0... I

"V ' I ' ' Di!opl~, • I I I

ooo ' so· I 109.5" ' 120" ' 180°

Subunit', ' I • : ' . .

Figure 2.9 "Molecular Library" of cyclic molecular polygons created by the systematic combination

of ditopic building blocks with predetermined angles.185

'•,_ Ditopic •,, Subunit

'\•· .... ,

Tritopic •• •.. Subunit '·· ..•

A 180" 80-90"

Figure 2.10 "Molecular Library" for the formation of 3D-assemblies from ditopic and tritopic

subunits. 185

34

Drain and Lehn188 provide a representative example of how by choice of suitable

moieties, porphyrin macrocycles may be incorporated into molecular squares as

either comer groups 21 or as side groups 22.

21 22

Sanders and co-workers188'193'194 have developed a strategy of usmg template

molecules to form the platinum diyne-linked cyclic oligomers, like 23 . Depending

on the geometry of the template molecules, cyclic dimers, trimers or tetramers were

formed. The yields of these cyclic oligomers usually increase by a factor of two or

three in the presence of the template molecules. The complementary binding of the

octahedral aluminium tris[3-( 4-pyridyl)acetylacetonate] guest was detected by NMR

spectroscopy. Several chemical reactions including Diels-Alder and acyl transfer

reactions, have been demonstrated to be catalysed within the cavity of the cyclic

trimer.193,194,196-199

35

II

~ ,PEt3 .Pt

Et3P ~

23

2.6 Introduction to Organometallic Porphyrins

Much of the interest in the study of organometallic complexes that incorporate a

porphyrin macrocycle as a supporting ligand200-202 has been inspired by the detection

and/or postulation of porphyrin-containing organometallic species in the natural and

chemical models of coenzyme B12, cytochrome P450 and other haem-containing

biomolecules. In this regard several cobalt model systems for coenzyme B12 have

been synthesised and investigated. 203-207

The true description of an organometallic porphyrin is one that contains both a

porphyrin macrocycle and a metal-carbon bond. A classification system of

organometallic porphyrins (shown below) has been suggested in a recent

publication.202 This classification system covers most of the organometallic

porphyrins researched. 200-202

R

(a)

(d)

R==-==R

(b)

L'-Mln

(e)

Figure 2.11 Classical categorization of organometallic porphyrins.

36

R

(c)

X-M'Ln

MLn

Type (a) complexes contain one or two metal-carbon single or multiple bonds. This

class comprises the majority of the organometallic porphyrin complexes that contain

8-alkyl, 8-aryl, or carbene ligands. Both five- and six-coordination (with respect to

the metal atom) are possible. Type (b) complexes contain rc-ligands, including 112-

alkene, 112 -alkyne, 115 -cyclopentadienyl and related ligands. Type (c) complexes

involve bridging ).l-M,N groups in which the alkyl group bridges between the

coordinated metal and one porphyrin nitrogen atom. The M-N bond may or may not

be retained in these complexes. Type (d) complexes contain a metal-metal bond,

with the metal-carbon bonds associated with the metal not coordinated to the

porphyrin. The type (e) structure shows a number of possible arrangements for

coordination of a second metal fragment. It can be rc-coordinated to one of the

porphyrin pyrrole rings or to an aryl substituent on the periphery of the porphyrin.

The porphyrin macrocycle provides four donor nitrogen ligands in a square planar

arrangement with a hole size of approximately 2 A. Coordination numbers for a

metal coordinated to a porphyrin are usually 4, 5, or 6 although higher numbers are

known to exist. One of the major advantages that arises from utilising a porphyrin as

a supporting ligand is that the immediate coordination environment remains intact,

whilst a large number of changes can be made to the periphery of the ring in order to

tune the steric and electronic properties of the ligand. Another useful feature of

porphyrinic ligands is that the energies of the metal d orbital are often quite close to

37

that of the frontier molecular orbitals of the macrocycle, with the result that the redox

chemistry of the p~:>rphyrin and the coordinated metal are both changed. 200

2.6.1 Porphyrin analogues

Et Et

Me Et Et

Et Et 24 Oxybenziporphyrin 25 Carbaporphyrin

.. .,

26 Azuliporphryin 27

-N

g~ NH HC 7 NH

28 N-confused porphyrin 29 Doubly N-confused porphyrin

The synthesis and coordination chemistry of porphyrin analogues (24-29 for

example) have also been attracting considerable interest due to the possibility of

developing new porphyrin chemistry.Z08-234 Due to the presence of a carbon atom in

the direct coordination envelope of the internal cavity of the macrocycle, these

analogues often involve organometallic carbon-metal bonds after coordination of

various metals. Inverted or N-confused porphyrins are currently under investigation

in a number of research groups worldwide. N-confused porphyrins exhibit an

38

unusual tautomeric equilibirium, which affects their aromaticity, and undergo

several remarkable reactions including substitutions, internal fusion and ring

disruption. Especially interesting opportunities open up in the area of coordination

chemistry. These porphyrin analogues coordinate various transition and main-group

metals, often forming metal-carbon bonds or stabilizing unusual oxidation states.

Complexes of N-confused porphyrin are structurally diverse and often quite

reactive, which constitutes a rich field for chemical exploration.

2.6.2 meso-tJ1-0rganometallic porphyrins

Apart from the classes of organometallic porphyrins discussed above, there is

another group of '11 1-organometallic porphyrins that have been synthesised

recently.Z35-241 These '11 1-organometallic porphyrins have a metal atom directly

bonded to one or more of the meso carbons of the porphyrin macrocycle.

The publications of Arnold and coworkers236-239 have been the only reports of

isolated '11 1-organopalladio- and organoplatinioporphyrins. This particular version of

late transition metal chemistry leads to the compounds of type 30 that are involved in

reactions where meso-bromoporphyrins are coupled with simple terminal alkynes,242

alkynylstannanes or alkynylzincs243-246 and alkenylorganometallics.243

-246 It is

postulated in these examples that the key step in these coupling reactions, is the

oxidative-addition of the meso-carbon-to-bromine bond to a zerovalent palladium

species, usually a bis(phosphine) moiety (see Figure 2.12).

Ph

Ph

Ph

reductive-elimination

L'nMRX = ZnR2, CIZnR Bu3SnR

Ph

Ph

Ph \ /Ph

~ ligand exchange

39

Figure 2.12 Proposed Pd-catalysed cross-coupling scheme for a brominated porphyrin and a variety

of substrates. 245'246

The advance made by the Arnold group was to use a stoichiometric amount of the

Pd(O) precursor [Pd(PPh3)4 or the combination Pd2dba3 + PPh3] and the appropriate

5-bromo- or 5,15-dibromoporphyrin in toluene at 105°. This leads to the formation of

adducts such as 30 in near quantitative yields.

30

The Arnold porphyrin group236-239 is currently extending this new class of

compounds by systematically varying the central coordinated metal ion and types of

subsidiary ligands on the 11 1-transition metal, as well as developing bis(metallated)

derivatives from 5,10- and 5,15-dihalodiarylporphyrins.

40

31

Other reports of peripherally metallated porphyrins are rare. Therien and co­

workers235 have reported the first examples of a 11 1-meso-metalloporphyrin species in

which boronic esters were appended to a (porphyrinato )zinc(II) framework, like 31.

The archetypal members of this new class of macrocycle-derivatised porphyrins were

structurally characterised and their usefulness in carbon-carbon bond-forming

reactions was illustrated.

Ar Ar Ar Ar

Ar Ar

2TeCI4 -2 HCI - -Ar Ar Ar Ar Ar Ar

32 33 34 35

Sugiura et al. have utilised the meso-organometallic tellurium trichloride porphyrin

intermediate 33 in the preparation of a [2]porphyracene dimer (porphyrin + acene;

number [2] denotes the number of porphyrin nuclei). A mechanism was proposed for

the formation of 35 as shown above. The dimerisation was proposed to take place

through a cyclic intermediate 34, which undergoes a final detelluration reaction

similar to that of bis(aryl)tellurium dichloride to give aryl coupled products.

Tellurium trichloride analogues of 35 were also detected which could possibly

further couple to give entry into [ 4 ]porphyracene oligomeric systems.

41

HgCI

Me Me

36

Smith and co-workers247-250 utilised mercurated porphyrin 36 as a means of

functionalising zinc(II) deuteroporphyrin IX dimethyl ester. The bismercurated

deuteroporphyrin readily underwent Heck palladium-exchange reactions with various

substituted olefms. This also provided a means of fme tuning the optical spectra of

protoporphyrin IX derivatives, as well as attachment of a variety of potentially

interesting substituents to the porphyrin macrocycles. Electron-deficient acrylates

and styrenes were found to cause the Soret and satellite absorption bands to undergo

a red shift. Zinc and Grignard-like magnesium ring-metallated porphyrins were also

found by Therien and Deluca to be suitable for palladium-catalysed coupling with a

variety of aryl and vinyl halides.241

2. 7 Conclusion

As detailed throughout the previous sections, porphyrins play an integral part in our

daily lives and offer a fascinating avenue for research that encompasses not only the

field of chemistry but reaches across the many disciplines of science. Understanding

the dynamics and mechanisms of natural systems that contain porphyrins has not

only contributed to significant understanding of these natural examples but has also

inspired much interesting research. This work is not only concerned with the

biomimicry of natural systems but also with exploiting the remarkable properties of

"the pigments of life". Porphyrins exhibit a range of coordination, recognition,

donor-acceptor, optical and electronic properties that make them ideal fragments for

incorporation in advanced devices. Much of the research done in the area of

multicomponent arrays that incorporate porphyrin macrocycles has utilised covalent

linking of the individual components. This method has proven useful, and has

42

provided much knowledge about porphyrin-containing arrays. However, as the

complexity of the systems increases, yields are dramatically reduced and isolation

and purification methodologies are often fraught with difficulties. The self-assembly

of individual tectons that are pre-programmed with a recognition algorithm, offers an

attractive technique that is capable of assembling several components in a single

step, and often in quantitative yields. This methodology is used extensively in nature

and is attracting attention as a real life technique for the realisation and production of

useful nanoscale devices?51 Organometallic porphyrins play an essential role in

biological systems and offers a rich area of scientific research of compounds that

combine the properties of a porphyrin with those of organometallic chemistry.

2.8 Project Aims

As explained in section 1.1.2, the main objectives ofthis project are concerned with

extending the current knowledge base of t{organometallic porphyrins. The main

aims of this project are as follows:

• Preparation and investigation of new synthetically useful porphyrin

macrocycles.

• Preparation and investigation of various novel organometallic porphyrins.

• Incorporation of these organometallic porphyrins in supramolecular systems.

• Investigation of these supramolecular systems.

As part of this research it was desired to investigate new and familiar porphyrinic

systems in which to incorporate these organometallic fragments. By this systematic

study we wish to explore strategies that make it possible to exploit both the utility of

an organometallic fragment and the remarkable optical and electronic properties of

monomeric and multi-porphyrin systems. In this course of study we also hope to

expose these 11 1 -mesa-organometallicporphyrins to a large variety of reagents and

reaction conditions in order to fully investigate this new area of research. Results and

discussion along this line of research are presented in the following chapters.

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(242) Arnold, D. P.; Bott, R. C.; Eldridge, H.; Elms, F. M.; Smith, G.; Zojaji, M.

Aust. J. Chem. 1997, 50, 495.

(243) Lin, V. S.-Y.; Di Magno, S. G.; Therien, M. J. Science 1994, 264, 1105.

(244) Lin, V. S.-Y.; Therien, M. J. Chern. Eur. J1995, 1, 645.

(245) Di Magno, S. G.; Lin, V. S. Y.; Therien, M. J. J. Org. Chem. 1993, 58, 5983.

(246) Di Magno, S. G.; Lin, V. S. Y.; Therien, M. J. J. Am. Chem. Soc. 1993, 115,

2513.

(247) Morris, I. K.; Snow, K. M.; Smith, N. W.; Smith, K. M. J. Org. Chern. 1990,

55, 1231.

(248) Minnetian, 0. M.; Morris, I. K.; Snow, K. M.; Smith, K. M. J. Org. Chem.

1989, 54, 5567.

(249) Smith, K. M.; Langry, K. C.; Minnetian, 0. M. J. Org. Chem. 1984,49,4602.

(250) Smith, K. M.; Langry, K. C. J. Org. Chem. 1983,48,500.

(251) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; John Wiley & Sons:

West Sussex, 2000.

CHAPTER3

Remarkable homology in the electronic spectra of the

mixed-valence cation and anion radicals of a conjugated

bis(porphyrinyl)butadiyne,

55

Dennis P. Arnold, Regan D. Hartnell, Graham A. Heath, Leonora Newby, Richard

D. Webster,

Chemical Communications, 2002, 754-755

56

The research presented in this paper was a collaboration between QUT and ANU.

The original compounds that are presented in this paper were synthesised by me

(R.D.H), following methodologies and procedures that were developed and

optimised by myself. Characterisation of these compounds, especially NMR and UV­

visible spectroscopy was done by me. Mass spectra and microanalyses were obtained

commercially from independent providers. The spectroelectrochemical data were

gathered by D.P.A., G.A.H., and R.D.W., at the Research School of Chemistry,

Australian National University. Some repeat synthesis was carried out by L.N. under

my supervision.

Remarkable homology in the electronic spectra of the mixed-valence

cation and anion radicals of a conjugated bis(porphyrinyl)butadiynet

Dennis P. Amold,*a Regan D. Hartnell, a Graham A. Heath,b Leonora Newbya

and Richard D. Websterb

a School of Physical and Chemical Sciences, Queensland University of

Technology, G.P.O. Box 2434, Brisbane, Australia 4001. Fax: +61 7 3864

1804; Tel: +61 7 3864 2482; E-mail: d.arnold@qut.edu.au

b Research School of Chemistry, The Australian National University,

Canberra, Australia 0200. Fax: +61 2 6125 0750; Tel: +61 2 6125 4039; E­

mail: heath@rsc.anu.edu.au

57

The n-radical cation and anion of the dizinc complex of a

bis(triarylporphyrinyl)butadiyne, 1+ and r, respectively, display remarkable

spectral similarity in the near-IR region, in accord with the predictions of a

frontier orbital model for inter-porphyrin bonding across the conjugated

bridge.

58

Bis- and oligo(porphyrins) conjugated through alkyne-containing bridges linked

directly to the meso carbons display unique spectroscopic and redox properties,

making them promising· candidates for applications in non-linear optics and

molecular electronics. Their photophysical properties and electronic structures

have been investigated in detail by Anderson,1 Therien2 and others.3

Our work has focused on bis( octaethylporphyrin) systems of the form MOEP-C4-

MOEP.4 We have systematically varied the metal ion Min the parent systems4c,g and

explored the consequences of extending the conjugated bridge.4b,ef A distinctive

aspect is the in situ examination of UV to IR absorption spectra of the

electrogenerated mono- and dianions of these bis(porphyrins).4cf,g In both series,

remarkably intense near- to mid-IR bands exist, which we assigned to transitions

involving the newly-occupied rc-orbitals of the reduced forms. Instability and

apparent aggregation frustrated attempts to examine the corresponding cations. To

address this, we designed a new series of bis(porphyrins) in which the porphyrin

faces are encumbered by t-butyl groups on the lateral aryl substituents, and the

15,15'-meso carbons are protected by phenyl groups. The IR/near-IR spectra of the

cation and anion radicals derived from the dizinc complex 1, display a remarkable

homology (Fig. 1 ), confirming an important prediction of our earlier two-porphyrin

eight-orbital model for inter-porphyrin interactions.

h

Ar 1: M= Zn, Ar= 3,5-dt-t-butyl(flenyl

The conjugated bis(porphyrin) 1t was prepared by meso-bromination of

diaryl(phenyl)porphyrin free base, insertion of zinc(II), palladium/copper catalysed

coupling with trimethylsilylethyne, deprotection with fluoride, then oxidative

coupling of the ethynyl porphyrin using copper(II) acetate in pyridine?a,3a,4e The

diaryl(phenyl)porphyrin was prepared by treatment of 5,15-bis(3 ',5' -di-t­

butylphenyl)porphyrin with phenyllithium and aqueous oxidative work-up, according

to Senge and co-workers.5 There are only two previous examples of

bis(triarylporphyrinyl) butadiynes.6 The optical spectrum of 1 in CH2Ch or CHCh

resembles those of similar butadiyne-linked dizinc bis(porphyrins) of the 5,15-

59

diarylporphyrin type,2b except that the extinction coefficients for 1 are more than

double those of the diphenylporphyrin (DPP) complex?b Addition of the supporting

electrolyte, 0.5 M Bli4NPF6, causes red-shifts (400- 900 cm-1) and minor changes in

relative band intensities.

Binuclear 1§ displays similar voltammetric features to those reported by Therien

for the DPP analogue,2a and by ourselves for a range of OEP derivatives.4c/,g There

are two ring-centred one-electron oxidations separated by ca. 110 mV, representing

successive formation of the monocation and dication, and a corresponding pair of

reductions separated by ca. 100 mV.~ The voltammetric HOMO-LUMO gap M =

Fox- Fred of 1.85 V is typical ofbutadiyne-linked porphyrin dimers_2a,4cf,g

4 6 s 10 12xl03

wavenumber/cm"1

Fig. 1 Spectra of1+ (top) and 1· with inset

orbital schemes (see text).

The oxidation states implied by voltammetry were examined in situ from the UV

to the mid-IR (300 - 3200 nm) at 220 K in an optically transparent thin-layer

electrogenerative (OTTLE7) cell. Fig. 2 illustrates the progressive oxidation of 1 to

1+. Neutral 1 could be potentiostatically retrieved and recycled as desired. The

OTTLE progressions for 1 to 1· and for 1· to 12- were equally well-defmed and

reversible at this temperature.!! EPR spectra measured in situ under corresponding

conditions confirm the paramagnetism of 1 + and r. The g tensor splitting of r (g

2.003, 1.994) resembles that of (NiOEP-C4-NiOEPr. The influence of binuclearity

60

on the EPR signal is minor, unlike its profound effect on the optical spectra, because

monomeric porphyrin S = Ih paramagnets are already innately axial (whether the

splitting is resolved or not) and their symmetry may be reduced further by intrinsic

and environmental factors. 8

5 10 15 wavenumber/cm"

1 20

Fig. 2 Spectral progression during the one­

electron oxidation of 1.

In the IR/near-IR region, where 1 is transparent, 1 + has two intense bands denoted

VI(+) (4090 cm·I, c: 2: 40,000) and v2(+) (10710 cm-I, 74,000), as displayed in Fig. 1.

The strikingly similar spectrum ofr shows bands VI(-) (3635 cm·I, 32,000) and v2(-)

(10210 cm·I, 73,000). Monomeric porphyrin radical cations and anions also have

informative but much weaker near-IR bands which continue to attract close

attention. 4g'9

From our previous results on MOEP-C4-MOEP anions, we constructed a semi­

quantitative 'two-porphyrin eight-orbital' scheme based on the experimental band

energies and the symmetry properties of the monoporphyrin orbitals that either

interact strongly across the conjugated bridge (x-orbitals) or are orthogonal to this

axis (y-orbitals).4g The scheme is in accord with MO calculations on these

systems.2e,4d We designate that neutral 1 possesses eight frontier electrons (8e) and

we are seeking to characterise mixed-valence 1+ (7e) and r (9e), the former class

being unprecedented. The orbital occupancies and consequent transitions are shown

as insets in Fig. 1. The results obtained for the anions of 1 parallel the 8e/9ell Oe

progression gathered for our OEP series and reinforce the generic application of the

scheme to the reduced states of both triaryl- and octaethylporphyrin with respect to

electronic coupling and optical properties. The obvious homology between the

61

almost superimposable (within 500 cm-I) two-band IR/near-IR optical spectra for 1+

and r is quite ext~aordinary.

Qualitatively, it underscores the formal analogy between the transitions demanded

in 7e and 9e systems, namely the v2 ('Q-band') transition is available to both, and

the'basement' nln VI(+) promotion in the 7e cation is replaced by the corresponding

'upper-storey' n*ln* VI(-) low-energy promotion ofthe 9e anion. The typical Q-band

profile of a neutral porphyrin is maintained upon both oxidation and reduction (Figs.

1 and 2).

Quantitatively, the spectral match of Fig. 1 demonstrates that (i) the energy gaps

are broadly insensitive to successive changes in oxidation state; (ii) ring/ring

coupling through the conjugated bridge (probed by VI) is nearly identical in the 1t and

n* levels, as we had assumed elsewhere,4g and as borne out by calculations on both

NiP-C4-NiP (unsubstituted)4d and ZnDPP-C2-ZnDPP2

e cases. We note that electron­

pair correlation terms tend to cancel systematically in transitions that connect a

doubly-occupied level with a singly-occupied one, so that v2(-) converges on

'innocent' v2(+), while both VI(+) and VI(-) faithfully map the respective MO • IO spacrngs.

In summary, switching from ~-octaethyl to meso-triaryl substitution does not alter

the distinctive near-IR spectral signatures of the binuclear 9e and 10e anions and has

allowed us to access the corresponding dizinc monocation, the first representative of

the class of '7e' conjugated bis(porphyrin) pigments. The remarkable 7e/9e

congruency strongly reinforces the utility of a simple frontier-orbital model and the

discovery of new chromophores demonstrates the merit of pursuing such redox­

altered compounds.

D.P.A. thanks the Research School of Chemistry (A.N.U.) for a Visiting

Fellowship and Q.U.T. for an A.T.N. Small Grant.

Notes and references

t Electronic supporting information (ESI) available: Figs. S1 and S2: OTTLE-cell

progressions of 1 to r and r to 12-, respectively; Fig. S3: EPR spectral data. See

http:/ /www.rsc.org/suppdata/ cc/

~ Characterisation data for 1: OH (CDCh) 10.06, 9.18, 8.96, 8.90 (all d, ~-H), 8.22

(dd, o-H of 15,15'-Ph), 8.15 (d, o-H of Ar), 7.86 (t, p-H of Ar), 7.76 (m, m,p-H of

62

Ph), 1.60 (s, t-Bu); UVNIS (CH2Ch) Amax/nm (log s) 678 (4.79), 628 (4.54), 565

(4.33), 481 (5.28), 446 (5.35), 443sh (5.29), 424sh (5.15); FAB-MS maximum of

cluster 1699 (M'" requires 1698). Anal. Calcd. for C112H11oNsZn2·0.5CH2Ch C,

77.59; H, 6.43; N, 6.43. Found C, 77.68, H, 6.62, N, 6.33%.

§ Voltammetric data for 1: EfJ (a.c. voltammetry at 20 mvs-1 on Pt, 0.5 M

TBAPFJCH2Ch, 233 K, vs. Ag/AgCl in CH2Ch, Fe/Fe+ at +0.55 V): +0.83, + 0.72,-

1.13, -1.23 v.

~ Such closely-spaced redox potentials are fully compatible with strong coupling in

the intermediate hemi-bonded states as long as the ring/ring conjugation is

maintained in the respective two-electron-bonded dianion or dication. In such cases,

the resonance energy contributions systematically cancel from the free energy of

disproportionation.4g On spectroscopic evidence, we regard these mixed-valence

species as strongly-coupled (Class III) systems.

II The OTTLE-cell progressions confirm the redox equilibration of the spec1es

concerned, with discernible disproportionation of the odd-electron states.

1 (a) H. L. Anderson, Inorg. Chern., 1994, 33, 972; (b) H. L. Anderson, S. J. Martin

and D. D. C. Bradley, Angew. Chern., Int. Ed. Engl., 1994, 33, 655; (c) P. N.

Taylor, A. P. Wylie, J. Huuskonen and H. L. Anderson, Angew. Chern., Int. Ed.

Engl., 1998, 37, 986; (d) P. N. Taylor, J. Huuskonen, G. Rumbles, R. T. Aplin, E.

Williams and H. L. Anderson, Chern. Commun., 1998, 909; (e) H. L. Anderson,

Chern. Commun., 1999, 2323; (j) J. J. Piet, P. N. Taylor, H. L. Anderson, A.

Osuka and J. M. Warman, J. Am. Chern. Soc., 2000, 122, 1749.

2 (a) V. S.-Y. Lin, S. G. DiMagno and M. J. Therien, Science, 1994, 264, 1105; (b)

V. S.-Y. Lin and M. J. Therien, Chern. Eur. J., 1995, 1, 645; (c) P. J. Angiolillo,

V. S.-Y. Lin, J. M. Vanderkooi and M. J. Therien, J. Am. Chern. Soc., 1995, 117,

12514; (d) R. Kumble, S. Palese, V. S.-Y. Lin, M. J. Therien and R. M.

Hochstrasser, , J. Am. Chern. Soc., 1998, 120, 11489; (e) R. Shediac, M. H. B.

Gray, H. T. Uyeda, R. C. Johnson, J. T. Hupp, P. J. Angiolillo and M. J. Therien,

J. Am. Chern. Soc., 2000, 122, 7017.

3 (a) K.-i. Sugiura, Y. Fujimoto andY. Sakata, Chern. Commun., 2000, 1105; (b) H.

Higuchi, T. Ishikura, K. Mori, Y. Takayama, K. Yamamoto, K. Tani, K.

Miyabayashi and M. Miyake, Bull. Chern. Soc. Jpn., 2001, 74, 889; (c) K.

63

Susumu, H. Maruyama, H. Kobayashi and K. Tanaka, J. Mater. Chern., 2001, 11,

2262.

4 (a) D.P. Arnold, A. W. Johnson and M. Mahendran, J. Chern. Soc., Perkin Trans.

1, 1978, 366; (b) D.P. Arnold and L. J. Nitschinsk, Tetrahedron, 1992, 48, 8781;

(c) D. P. Arnold and G. A. Heath, J. Am. Chern. Soc., 1993, 115, 12197; (d) R.

Stranger, J. E. McGrady, D. P. Arnold, I. Lane and G. A. Heath, Inorg. Chern.,

1996, 35, 7791; (e) D.P. Arnold and D. A. James, J. Org. Chern., 1997, 62, 3460;

(j) D.P. Arnold, G. A. Heath and D. A. James, New J. Chern., 1998, 1377; (g) D.

P. Arnold, G. A. Heath and D. A. James, J. Porphyrins Phthalocyanines, 1999,3,

5.

5 M. 0. Senge and X. Feng,Perkin Trans. 1, 2000,3615.

6 (a) S. Smeets and W. Dehaen, Tetrahedron Lett., 1998, 39, 9841; (b) D. A.

Schultz, K. P. Gwaltney and H. Lee, J. Org. Chern., 1998, 63, 4034.

7 R. D. Webster, G. A. Heath and A.M. Bond, J. Chern. Soc., Dalton Trans., 2001,

3189.

8 (a) W. A. Kalsbeck, J. Seth and D. F. Bocian, Inorg. Chern., 1996, 35, 7935; (b) J.

Seth and D. F. Bocian, J. Am. Chern. Soc., 1994, 116, 143.

9 J. Mack and M. J. Stillman, J. Porphyrins Phthalocyanines, 2001, 5, 67.

10 (a) M. B. Hall, Polyhedron, 1987, 6, 679; (b) G. A. Heath, J. E. McGrady, R. G.

Raptis and A. C. Willis, Inorg. Chern., 1996, 35, 6838.

64

CHAPTER4

Peripherally-metallated porphyrins: meso-1'}1-porphyrinyl­

platinum(II) complexes of 5,15-diaryl- and 5,10,15-

triarylporphyrins

Regan D. Hartnell, Alison J. Edwards and Dennis P. Arnold

Journal of Porphyrins and Phthalocyanines, 2002,6,695-707.

65

The research presented in this paper is based on work done by the candidate (R.D.H.)

under the supervision of D.P.A. All of the original compounds that are presented in

this paper were synthesised by R.D.H., following methodologies and procedures that

were developed and optimised by the candidate. All of the spectral and optical

characterisation data of these compounds was collected and interpreted by myself.

Mass spectra and microanalyses were obtained commercially from independent

providers. Crystallographic data was collected by A.J.E. and was performed on

crystals that were prepared by myself The experimental and results sections were

written by myself.

66

Peripherally-metallated porphyrins: meso-111-

porphyri~yl-platinum(ll) complexes of 5, 15-diaryl- and

5,1 0, 15-triarylporphyrins

R~gan D. HartneiV Alison J. Edwardsb and Dennis P. Arnold*a

a Centre for Instrumental and Developmental Chemistry, Queensland University of

Technology, G.P.O. Box 2434, Brisbane, Australia 4001

b Research School of Chemistry, The Australian National University, Canberra,

Australia 0200.

Received 29 August 2002

Accepted 25 October 2002

ABSTRACT: Attempted metathesis reactions of peripherally-metallated

meso-'f] 1-porphyrinylplatinum(II) complexes such as trans­

[PtBr(NiDPP)(PPh3)2] (H2DPP = 5,15-diphenylporphyrin) with

organolithium reagents fail due to competitive addition at the porphyrin

ring carbon opposite to the metal substituent. This reaction can be

prevented by usmg 5,10,15-triarylporphyrins, e.g. 5,10,15-

triphenylporphyrin (H2 TrPP) and 5-phenyl-1 0,20-bis(3 ',5 '-di-t­

butylphenyl)porphyrin (H2DAPP) as substrates. These triarylporphyrins

are readily prepared using the method of Senge and co-workers by

addition of phenyllithium to the appropriate 5,15-diarylporphyrins,

followed by aqueous protolysis and oxidation. They are convenient,

soluble building blocks for selective substitutions and subsequent

transformations at the remaining free meso carbon. The sequence of

bromination, optional central metallation and oxidative addition of Pt(O)

tris(phosphine) complexes generates the organoplatinum porphyrins in

high overall yields. The bromo ligand on the Pt(II) centre can be

substituted by alkynyl nucleophiles, including 5-ethynylNiDPP, to form

the first examples of meso-11 1-porphyrinylplatinum(II) complexes with a

second Pt-C bond. The range of porphyrinylplatinum(II) bis(tertiary

phosphine) complexes was extended to the triethylphosphine analogues,

by oxidative addition ofH2TrPPBr to Pt(PEt3)3, and the initially-formed

cis adduct is only slowly thermally transformed to trans­

[PtBr(H2TrPP)(PEt3)2] 16. The molecular structures of NiDAPP 9b,

trans-[Pt(NiDPP)(C2NiDPP)(PPh3)2] 14 and 16 were determined by X­

ray crystallography.

KEYWORDS: porphyrin, organometallic, platinum, crystal structure,

phosphine

67

*Correspondence to: Dennis P. Arnold, phone: +61 7 3864 2482; fax: +61 7 3864

1804; email: d.amold@qut.edu.au

68

INTRODUCTION

For the entire history of porphyrin chemistry, scientists have studied the

coordination of metal ions in the central cavity of the macrocycle [1]. Until the recent

discovery and investigation of modified porphyrinoids such as inverted or "N­

confused" porphyrins [2] and azuliporphyrins [3], the coordination of metal ions in

or near the macrocycle plane has involved metal-nitrogen bonding. While there are

many examples of "organometallic" porphyrins, until recently almost all of these

have also involved the metal atom being placed at the centre of the ring, with various

types of metal-carbon bonds to out-of-plane organic fragments [4]. Reports of

externally-bonded porphyrin carbon-metal bonded species are still quite rare. They

include both cr- and n-bonded systems [5-8], and some examples are shown in Chart

1. We generally exclude from consideration species where the porphyrin is separated

from the metal by an intervening substituent (such as ferrocenylporphyrins), although

some interesting examples of such compounds with fused cyclopentadienyl rings

appended to porphyrin pyrrole rings were recently reported by Smith and co-workers

[9].

We entered this field as a result of our work on palladium-catalysed carbon-carbon

couplings involving porphyrins. Our work on the coupling of alkynyl porphyrins

with haloarenes and alkynes with bromoporphyrins [1 0] appeared almost

simultaneously with that of Therien on mesa-bromoporphyrin couplings [11]. The

first work on palladium-catalysed couplings involved mercurated porphyrins studied

by Smith's group [Sa]. More recently, many workers have employed the

haloporphyrin!alkyne coupling to prepare alkynylporphyrin monomers and oligomers

[12]. This is now the simplest route to alkynylporphyrins of the 5,15-diaryl type. The

initial step in the catalytic cycle of such a coupling is the oxidative addition of the

bromoarene to a 14- or 16-electron Pd(O) entity introduced as such or produced in

situ from a Pd(II) precursor [13]. Several years ago, we serendipitously isolated the

resulting mesa-11 1-organopalladium(II) porphyrin from one ofthese reactions [14].

Because such compounds had not previously been studied in their own right, we

embarked on a systematic study of this type of palladium compound, and have also

studied their more robust organoplatinum(II) analogues of the type shown on the

right of Chart 1 [14-16]. Apart from our reports, there appear to be no other examples

69

of compounds with direct transition metal to porphyrin M-C cr-bonds. The present

paper reports the results of our recent work on (i) substitution reactions at the Pt(II)

centre using simple carbon nucleophiles and (ii) extensions for the first time to

trialkylphosphines as supporting ligands. These studies produced several new types

of organometallic porphyrins and the molecular structures of two of these were

determined by X-ray crystallography. We found it to be advantageous on the grounds

of selectivity and solubility to use 5,10,15-triarylporphyrins as starting materials in

some parts of the work, and we also report here their synthesis and the molecular

structure of one example.

Ph Ar

e

Ph

Chart 1. Some examples of peripherally-metallated porphyrins with direct porphyrin-metal bonds.

EXPERIMENTAL

General

Syntheses involving zerovalent metal precursors were carried out in an atmosphere

of high-purity argon using conventional Schlenk techniques. Porphyrin starting

materials 5,15-diphenylporphyrin (H2DPP) 4a and 5,15-bis-(3,5-di-tert­

butylphenyl)porphyrin 4b [11,17] and trans-[PtBr(NiDPP)(PPh3)2] 1 [15] and the

zerovalent platinum precursors [18] were synthesised by literature methods.

Phenyllithium was freshly prepared from bromobenzene and lithium metal in ether.

All other reagents and ligands were used as received from Sigma-Aldrich. Toluene

was AR grade, stored over sodium wire, and degassed by heating and purging with

argon at 105 °C. Tetrahydrofuran was distilled from a dark blue solution of

potassiumlbenzophenone under an atmosphere of argon immediately before use.

Diethylamine was distilled under anhydrous conditions and stored over Molecular

Sieves. All other solvents were AR grade, and dichloromethane and chloroform were

70

stored over anhydrous sodium carbonate. Analytical TLC was performed using

Merck silica gel 60 F2s4 plates and column chromatography was performed using

Merck silica gel (230-400 mesh). NMR spectra were recorded on Bruker A vance 400

MHz or Varian Unity 300 MHz instruments in CDCh solutions, using CHCh as the

internal reference at 7.26 ppm for 1H spectra, and external 85% H3P04 as the

reference for proton-decoupled 31P spectra. UV - vis spectra were recorded on a

Cary 3 spectrometer in dichloromethane solutions. In some cases, both chloro and

bromo analogues are known to be present in the products, but extinction coefficients

are quoted using the molecular weight of the bromo species. Errors introduced by

this will be small since the molecular weights differ by only about 4%. Positive ion

FAB mass spectra were recorded on a VG-ZAB instrument at the Research School of

Chemistry, The Australian National University, or a Kratos Concept instrument at

the Central Science Laboratory, University of Tasmania. Samples were dissolved in

dichloromethane, and dispersed in a 4-nitrobenzyl alcohol matrix. In the data below,

masses given are for the strongest observed peak in the molecular ion cluster. In all

compounds this m/z value agreed with the predicted molecular mass, although in

most cases it represented a mixture of M and M+ 1. Elemental analyses were carried

out by the Microanalytical Service, The University of Queensland.

Synthesis

trans-Bromo [15-butyl-1 0,20-diphenylporphyrinatonickel(JI)-5-

yl]bis(triphenylphosphine)platinum(ll) 3. To a solution of trans­

[PtBr(NiDPP)(PPh3)2] 1 (20 mg, 0.015 mmol) in dry THF (20 cm3) under an

atmosphere of argon, BuLi (46 IlL of a 1.6 M solution in hexanes) was added

dropwise and stirred at room temperature for 1 hr. TLC analysis ( 40%

CHCh/hexane) showed the presence of some starting material and a more mobile

spot. The reaction was quenched by the addition of 50% aqueous THF and the

solvent was removed in vacuo. The residue was loaded onto a column and eluted

with 40% CHCh/hexane and the more mobile fraction was isolated. The solvent was

removed and the residue recrystallised from CHCh/hexane to give red crystals of 3

in a 28% yield. 1H-NMR: 8 0.88 (3H, t, J = 7.2 Hz, -(CH2)3-CH3), 1.57 (2H, m, -

(CH2)2-CHrCH3), 2.30 (2H, m, -CH2-CHrCH2-CH3), 4.48 (2H, br t, J = 7.2 Hz,

por-CHr(CH2)2-CH3), 6.50-6.70 (18H, m, PPh3), 7.20-7.30 (12H, m, PPh3), 7.60-

7.70 (6H, m, m,p-H on 10,20-phenyl), 7.80-7.90 (4H, m, o-H on 10,20-phenyl), 7.80,

71

8.56, 9.12, 9.35 (each 2H, d, J = 4.7 Hz, P-H); 31P-NMR: 8 23.20 (s, JPt-P 2942Hz),

FAB MS: 1373.3.

5,10,15-Triphenylporphyrin Sa and 5,15-bis(3 ',5 '-di-t-butylphenyl)-1 0-

phenylporphyrin Sb. As an example ofthis method, diphenylporphyrin 4a (100 mg,

0.22 mmol) was dissolved in dry THF (50 cm3) under an atmosphere of argon and

the solution cooled to 0 °C. PhLi (1.4 cm3 of 1.5 M solution in ether) was added

dropwise and the reaction vessel was removed from the cooling bath and allowed to

warm to room temperature and stirred for 30 min. To the reaction mixture 50%

aqueous THF (1 cm3) was added and stirring was continued for 10 min. DDQ (100

mg, 0.44 mmol as a DCM solution) was added and the solution was stirred for 15

min. The solvent was removed under vacuum and the residue dissolved in a

minimum of DCM. This solution was passed through a short column eluting with

DCM. The first major red fraction was collected and the solvent removed in vacuo

and the residue recrystallised from DCM/methanol to give dark purple crystals in

96% yield. 1H-NMR: 8 -2.85 (2H, br s, NH), 7.8-7.9 (9H, m, mJJ-H on 5,10,15-

phenyl), 8.3-8.4 (6H, m, o-H on 5,10,15-phenyl), 9.01, 9.03, 9.10, 9.33 (each 2H, d, J

= 4.7 Hz, P-H), 10.20 (1H, s, meso-H); UV- vis: Amax (a/103 M 1 cm-1) 411 (383),

508 (17.3), 542 (5.5), 583 (5.4), 637 (2.3) nm; FAB MS: 539.2. Anal. Calcd. for

C3sH26N4: C, 84.7; H, 4.9; N, 10.4. Found: C, 84.5; H, 4.9; N, 10.7.

A similar method was used for the synthesis of H2DAPP 5b, which was

recrystallised from DCM/methanol to give dark purple crystals in 84% yield. 1H

NMR: 8 -2.92 (2H, br s, NH), 1.57 (36H, s, tert-butyl-H on 5,15-aryl), 7.7-7.8 (3H,

m, m,p-H on 10-phenyl), 7.84 (1H, t, J = 1.8 Hz, 4-H on 5,15-aryl), 8.14(2H, d, J =

1.8 Hz, 2,5-H on 5,15-aryl), 8.2-8.3 (2H, m, o-H on 10-phenyl), 8.88, 8.98, 9.10,

9.36 (each 2H, d, J 4.7 Hz, P-H), 10.23 (lH, s, meso-H); UV- vis: Amax (a/103 M 1

cm-1) 413 (409), 510 (16.2), 545 (6.1), 584 (4.9), 638 (2.6) nm; FAB MS: 762.9.

Anal. Calcd. for Cs4HssN4·H20 : C, 83.0; H, 7.7; N, 7.2. Found: C, 82.5; H, 7.7; N,

7.0.

5-Bromo-1 0,15,20-triphenylporphyrin 6a and 5-bromo-1 0,20-bis(3 ',5 '-di-t­

butylphenyl)-15-phenylporphyrin 6b. Triphenylporphyrin 5a (100 mg, 0.19 mmol)

was dissolved in DCM (20 cm3) and the solution was cooled to 0 °C. N­

Bromosuccinimide (33 mg, 0.19 mmol) was added and the solution was stirred for 5

min, the progress of the reaction being checked by TLC (30% DCM!hexane ). It was

72

found that there was total consumption of the starting material and the presence of a

faster-moving spot. The solvent was removed in vacuo and the residue washed with

50% aqueous methanol and collected by filtration in order to remove the

succinimide. The solid was then recrystallised from DCM/methanol to give dark

brown/purple crystals of 6a in almost quantitative yield. 1H-NMR: o -2.77 (2H, br s,

NH), 7.7-7.9 (9H, m, m,p-H on 10,15,20-phenyl), 8.2-8.3 (6H, m, o-H on 10,15,20-

phenyl), 8.79, 8.80, 8.96, 9.70 (each 2H, d, J = 4.7 Hz, ~-H); UV- vis: Amax (s/103

M-1 cm-1) 419 (366), 517 (16.1), 552 (8.5), 594 (5.1), 651 (4.6) nm; FAB MS: 618.1.

Anal. Calcd. for C3sHzsBrN4: C, 73.9; H, 4.1; N, 9.1. Found: C, 73.4; H, 4.0; N, 8.8.

A similar method was used for the synthesis of H2DAPPBr 6b, which was

recrystallised from DCM/methanol to give dark purple crystals in 90% yield. 1H

NMR: o -2.65 (2H, br s, NH), 1.58 (36H, s, tert-butyl-H on 10,20-aryl), 7.75-7.85

(3H, m, m,p-H on 15-phenyl), 7.86 (1H, t, J = 1.8 Hz, 4-H on 10,20-aryl), 8.11 (2H,

d, J = 1.8 Hz, 2,5-H on 10,20-aryl), 8.2-8.25 (2H, m, o-H on 15-phenyl), 8.83, 8.89,

8.99, 9.72 (each 2H, d, J = 4.7 Hz, ~-H); UV- vis: Amax (s/103 M 1 cm-1) 422 (383),

519 (15.8), 555 (9.3), 596 (4.8), 652 (5.1) nm; FAB MS: 842.5. Anal. Calcd. for

Cs4Hs1BrN4: C, 77.0; H, 6.8; N, 6.7. Found: C, 76.4; H, 7.0; N, 6.4.

5-Bromo-1 0,15,20-triphenylporphyrinatonickel(II) 7 a and 5-bromo-1 0,20-

bis(3 ',5 '-di-t-butylphenyl)-15-phenylporphyrinatonickel(II) 7b. Bromoporphyrin 6a

(100 mg, 0.16 mmol) and Ni(acac)2 (222 mg, 0.8 mmol) were dissolved in toluene

(50 cm3) and refluxed. The progress of the reaction was monitored by TLC (30%

DCM!hexane) and was considered complete with the disappearance of the free base

porphyrin after 4 h. The solvent was removed in vacuo and the residue redissolved in

CHCh and passed through a short column, eluting with CHCh. The first red fraction

was collected and the solvent removed in vacuo and recrystallised from

DCM/methanol to give dark red crystals of7a in 95% yield. 1H-NMR: o 7.65-7.75

(9H, m, m,p-H on 10,15,20-phenyl), 7.95-8.05 (6H, m, o-H on 10,15,20-phenyl),

8.69, 8.71, 8.79, 9.52 (each 2H, d, J = 4.7 Hz, ~-H); UV- vis: Amax (s/103 M 1 cm-1)

417 (256), 530 (17.2) nm; FAB MS: 673.9. Anal. Calcd. for C3sH23BrN4Ni: C, 67.7;

H, 3.4; N, 8.3. Found: C, 67.4; H, 3.4; N, 8.3.

A similar method was used for the synthesis of NiDAPPBr 7b which was

recrystallised from DCM/methanol to give dark red crystals in 97% yield. 1H NMR:

o 1.49 (36H, s, tert-butyl-H on 10,20-aryl), 7.65-7.70 (3H, m, m,p-H on 15-phenyl),

73

7.74 (IH, t, J = 1.8 Hz, 4-H on 10,20-phenyl), 7.85 (2H, d, J = 1.8 Hz, 2,5-H on

10,20-aryl), 7.9-8.0 (2H, m, o-H on 15-phenyl), 8.70, 8.74, 8.84, 9.52 (each 2H, d, J

= 4.7 ,Hz, ~-H); UV- vis: Amax (c:/103 M 1 cm-1) 431 (139), 541 (9.5), 576 (3.3) nm;

FAB MS: 898.7. Anal. Calcd. for Cs4HssBrN4Ni·CH2Ch: C, 67.2; H, 5.8; N, 5.7.

Found: C, 66.9; H, 5.3; N, 5.3.

trans-Bromo[l 0,15,20-triphenylporphyrinatonickel(II)-5-

yl]bis(triphenylphosphine)-platinum(II) Sa and trans-bromo[l 0,20-bis(3 ',5 '-di-t­

butylphenyl)-15-phenylporphyrinato-nickel(II)-5-

yl]bis(triphenylphosphine)platinum(II) 8b. Toluene (50 cm3) was added to a

Schlenk flask and degassed at 105 °C under an atmosphere of argon.

Bromoporphyrin 7a (1 00 mg, 0.11 mmol) was added and stirred for 5 min.

Zerovalent Pt(PPh3)3 (130 mg, 13.2 mmol) was added and the solution stirred at 105

°C. TLC analysis (50% CHChlhexane) of the reaction mixture clearly showed

disappearance of the starting material after ca. 30 min and that the initially-formed

cis isomer was slowly being converted to the trans isomer. After ca. 6 hr the

isomerisation was considered complete and the reaction mixture was cooled to room

temperature and the solvent removed under high vacuum. The residue was purified

on a column eluting with 50% CHChlhexane and the major red fraction was

collected and the solvent removed in vacuo. The residue was recrystallised from

CHChlhexane to give Sa as dark red crystals in 84% yield. 1H-NMR: 8 6.60-6.75

(18H, m, PPh3), 7.25-7.40 (12H, m, PPh3), 7.60-7.70 (9H, m, m,p-H on 10,15,20-

phenyl), 7.85-7.95 (4H, m, o-H on 10,20-phenyl), 8.0-8.05 (2H, m, o-H on IS­

phenyl), 8.20, 8.53, 8.57, 9.48 (each 2H, d, J = 4.7 Hz, ~-H); 31P-NMR: 8 23.0 (s, JPt­

P 2936Hz), UV- vis: A-max (c/103 M 1 cm-1) 431 (235), 541 (14.9), 573 sh (4.5) nm;

FAB MS: 1393.3. Anal. Calcd. for C74Hs3BrN4NiP2Pt·O.SC6H14: C, 64.4; H, 4.2; N,

3.9. Found: C, 64.5; H, 4.3; N, 3.9.

In one case the chloro analogue of Sa was isolated by column chromatography by

elution with 30% CHChlhexane and its spectroscopic data follow. 1H-NMR: 8 6.60-

6.75 (18H, m, PPh3), 7.25-7.40 (12H, m, PPh3), 7.60-7.70 (9H, m, m,p-H on

10,15,20-phenyl), 7.80-7.90 (4H, m, o-H on 10,20-phenyl), 7.95-8.05 (2H, m, o-H on

15-phenyl), 8.16, 8.53, 8.56, 9.44 (each 2H, d, J = 4.7 Hz, ~-H); 31P-NMR: 8 24.2 (s,

JPt-P 2961 Hz), UV- vis: Amax (c:/103 M-1 cm-1) 430 (226), 541 (15.3), 573 sh (4.8)

74

nm; FAB MS: 1348.1. Anal. Calcd. for C74Hs3ClN4NiP2Pt·0.5C6Hf4: C, 66.4; H, 4.3;

N, 4.0. Found: C, 66.3; H, 4.4; N, 3.9.

A similar method was used for the synthesis of PtBr(NiDAPP)(PPh3)3 8b, which

was recrystallised from CHChihexane to give dark red crystals in 78% yield. 1H

NMR: 8 1.42 (36H, s, tert-butyl-H on 10,20-aryl), 6.60-6.75 (18H, m, PPh3), 7.20-

7.30 (12H, m, PPh3), 7.60-7.70 (3H, m, m,p-H on 15-phenyl), 7.60 (1H, t, J= 1.8 Hz,

4-H on 10,20-aryl), 7.72 (2H, d, J = 1.8 Hz, 2,5-H on 10,20-aryl), 7.95-8.05 (2H, m,

o-H on 15-phenyl), 8.28, 8.53, 8.60, 9.40 (each 2H, d, J = 4.7 Hz, ~-H); 31P-NMR: 8

23.0 (s, JPt-P 2941 Hz); UV- vis: Amax (E/103 M-1 cm-1) 432 (250), 542 (16.4), 576

(5.5) nm; FAB MS: 1618.3. Anal. Calcd. for C9oHssBrN4NiP2Pt: C, 66.8; H, 5.3; N,

3.5. Found: C, 66.9; H, 5.4; N, 3.3.

[5,15-Bis(3 ',5 '-di-t-butylphenyl)-1 0-phenylporphyrinatonickel(II) 9b. Free base

porphyrin Sb (50 mg, 0.065 mmol) and Ni(acac)2 (80 mg, 0.32 mmol) were dissolved

in toluene (20 cm3) and refluxed. The progress of the reaction was monitored by TLC

(30% DCM/hexane) and was considered complete with the total disappearance of the

free base porphyrin after 4 h. The solvent was removed in vacuo and the residue

redissolved in CHCh and passed through a short column, eluting with CHCh. The

first red fraction was collected and the solvent removed in vacuo and recrystallised

from CHCh/pentane to give dark red crystals in 98% yield. 1H NMR: 8 1.50 (36H, s,

tert-butyl-H on 5,15-aryl), 7.6-7.7 (3H, m, m,p-H on 10-phenyl), 7.76 (1H, t, J = 1.8

Hz, 4-H on 5,15-aryl), 7.92 (2H, d, J = 1.8 Hz, 2,5-H on 5,15-aryl), 8.0-8.1 (2H, m,

o-H on 10-phenyl), 8.79, 8.84, 8.96, 9.15 (each 2H, d, J = 4.7 Hz, ~-H), 9.85 (1H, s,

meso-H); UV- vis: Amax (s/103 M 1 cm-1) 409 (243), 521 (16.9), 549 (4.8) nm; FAB

MS: 818.3.

5-Butyl-1 0,15,20-triphenylporphyrinatonickel(II) 10 and 5,10,15,20-

tetraphenylporphyrinatonickel(II) 11. To a stirred solution of triphenylporphyrin

complex 8a (20 mg, 0.014 mmol) in dry THF (20 cm3) under an atmosphere of

argon, Bu'Li (45 I-lL of a 1.6 M solution in hexanes) was added dropwise. The

reaction mixture was stirred for 1 hr at room temperature. TLC analysis ( 40%

CHChlhexane) showed the consumption of most of the starting material and the

appearance of a more mobile spot. The reaction was quenched by the addition of

50% aqueous THF and the solvent was removed in vacuo. The residue was purified

on a column eluting with 40% CHChlhexane and the fastest-moving major red band

75

was collected. 1H NMR analysis showed the loss of the phosphine-containing moiety

and the spectrum ~greed with that in the literature [19d]. A similar experiment using

PhLi led to the isolation ofNiTPP 11, whose 1H NMR spectrum agreed with that of

an authentic sample.

trans-Phenylethynyl[1 0,20-diphenylporphyrinatonickel(II)-5-

yl]bis(triphenylphosphine)-platinum(ll) 12. To a degassed solution of trans­

[PtBr(NiDPP)(PPh3)2] 1 (20 mg, 0.015 mmol), Cui (ca. 2 mg) and Pd(PPh3)2Ch (ca.

2 mg) in dry THF (5 cm3) and Et2NH (15 cm3) under an atmosphere of argon,

phenylacetylene (15 mg, 0.15 mmol) was added. The solution was stirred at 60 °C

for 24 hr, after which TLC analysis ( 40% CHChlhexane) showed the disappearance

of starting material and the appearance of a less polar spot. The solvent was removed

in vacuo and the residue loaded on a column eluting with 40% CHChlhexane. The

major band was collected and the solvent removed and the residue recrystallised

from CHCh/pentane to give dark red needles of 12 in a 80% yield. 1H-NMR: o 6.25-

6.35 (2H, m, o-H on C2Ph), 6.60-6.70 (18H, m, PPh3), 6.90-6.95 (3H, m, mJJ-H on

C2Ph), 7.25-7.35 (12H, m, PPh3), 7.60-7.65 (6H, m, m,p-H on 10,20-phenyl), 7.85-

7.95 (4H, m, o-H on 10,20-phenyl), 8.11, 8.67, 8.94, 9.19 (each 2H, d, J = 4.7 Hz,~­

H), 9.54 (1H, s, meso-H); 31P-NMR: o 23.0 (s, JPt-P 2848Hz), UV- vis: Amax (s/103

M 1 cm-1) 424 (215), 532 (14.5), 563 sh (4.7) nm; FAB MS: 1338.0. Anal. Calcd. for

C16Hs4N4NiP2Pt: C, 68.2; H, 4.1; N, 4.2. Found: C, 68.0; H, 4.0; N, 4.0.

trans-{[10',20'-Diphenylporphyrinatonickel(JI)-5'-yl)ethynyl}[10,20-

diphenylporphyrinato-nickel(JI)-5-yl]bis(triphenylphosphine)platinum(II) 14. A

similar method to that above was used to prepare 14, namely trans­

[PtBr(NiDPP)(PPh3)2] 1 (20 mg, 0.015 mmol) was reacted with NiDPP-C2H 13 (9.2

mg, 0.015 mmol) in dry THF (10 cm3) and Et3N (10 cm3) for 48 hr to give 14 in 38%

yield. 1H-NMR: o 6.50-6.70 (18H, m, PPh3), 7.50-7.60 (12H, m, PPh3), 7.60-7.70

(12H, m, m,p-H on 10,10',20,20'-phenyl), 7.90-7.95 (8H, m, o-H on 10,10',20,20'­

phenyl), 8.20, 8.21, 8.40, 8.67, 8.69, 8.94, 8.95, 9.41 (each 2H, d, J = 4.7 Hz, ~-H),

9.55 (2H, s, meso-H of both porphyrins); 31P-NMR: o 22.85 (s, JPt-P 2850Hz); UV­

vis: Amax (s/103 M-1 cm-1) 442 (267), 540 (23.7), 579 (19.0) nm; FAB MS: 1778.6.

CHN analysis gave unusually high C and N results, but the crystal structure

confirmed the composition.

76

cis-Bromo(l 0,15,20-triphenylporphyrin-5-yl)bis(triethylphosphine)platinum(ll)

15. Toluene (20 cm3) was added to a Schlenk flask and degassed at 105 °C under an

atmosphere of argon. Bromoporphyrin 6a (20 mg, 0.032 mmol) was added and

stirred for 5 min. Zerovalent Pt(PEt3)3 (17 j..tL of a 0.18 M solution in toluene, 0.032

mmol) was added and the solution was stirred at 105 °C. TLC analysis (50%

CHChlhexane) of the reaction mixture after 5 min showed the disappearance of the

starting material and the presence of a much more polar spot. The mixture was

cooled to room temperature and the solvent removed under high vacuum. The

residue was purified by column chromatography eluting with 50% CHChlhexane

and the major purple/green fraction collected and the solvent removed in vacuo. The

residue was recrystallised from CHChlhexane to give 7 as dark purple crystals in a

67% yield. 1H-NMR: 8 -2.45 (2H, br s, NH), 0.70-0.90 (9H, m, PEt3 trans to Br),

1.05-1.25 (6H, m, PEt3 trans to Br), 1.40-1.60 (9H, m, PEt3 trans to porphyrin), 2.20-

2.40 (6H, m, PEt3 trans to porphyrin), 7.60-7.80 (9H, m, m,p-H on 10,15,20-phenyl),

8.00-8.10 (4H, m, o-H on 10,20-phenyl), 8.25-8.35 (2H, m, o-H on 15-phenyl), 8.70,

8.73, 8.79, 9.80 (each 2H, d, J = 4.7 Hz, f3-H); 31P-NMR: 8 1.7 (d, }p_p 18Hz, JPt-P

trans to Br 3972Hz), 7.3 (d, Jp_p 18Hz, JPt-P trans to porphyrin 1791Hz); UV- vis:

Amax (E/103 M-1 cm-1) 425 (226), 524 (13.9), 560 (11.8), 598 (5.1), 652 (8.0) nm; FAB

MS: 1049.1 (chloro analogue also present at 1005.2).

trans-Bromo(l 0,15,20-triphenylporphyrin-5-

yl)bis(triethylphosphine)platinum(ll) 16. A solution of the cis isomer 10 [10 mg,

0.012 mmol in xylene (5 cm3)] was degassed at 140 °C under an atmosphere of

argon. The mixture was stirred for 4 hr, cooled to room temperature and the solvent

removed under high vacuum. The residue was chromatographed on a short column

eluting with 30% ethyl acetate/hexane in order to remove a minor impurity of

unsubstituted porphyrin 4a, apparently formed by thermolysis of the Pt-C bond. The

major fraction was collected and the solvent removed in vacuo. The residue was

recrystallised from ethyl acetate/pentane to give the trans isomer 11 in 90% yield. 1H-NMR: 8 -2.30 (2H, br s, NH), 0.70-0.90 (18H, m, PEt3), 1.40-1.60 (12H, m,

PEt3), 7.60-7.80 (9H, m, m,p-H on 10,15,20-phenyl), 8.20-8.30 (6H, m, o-H on

10,15,20-phenyl), 8.70, 8.72, 8.79, 9.92 (each 2H, d, J = 4.7 Hz, f3-H); 31P-NMR: 8

10.04 (s, JP-Pt 2611 Hz); UV- vis: Amax (E/103 M-1 cm-1) 426 (207), 525 (10.0), 562

77

(8.4), 598 (4.2), 654 (5.6) run; FAB MS: 1049.1 (chloro analogue also present at

1005.2).

Crystallography

Crystal Structure Determinations. The data collection, extraction, solution and

refinement were all relatively routine although two of the crystals were very small,

one a needle of 0.06 x 0.06 mm cross-section and the other a plate only 20 ~-tm thick.

Crystal data are summarized in Table 1. Data were collected on a Nonius Kappa

CCD [20] diffractometer fitted with MoKa radiation using ~ and ro scans. Data were

integrated using the DENZO [21] package and a numerical absorption correction

was applied [22]. Structure solutions were by direct methods [23] and refmement

was by means of the CRYSTALS [24] program suite. For 9b a highly disordered

solvent region apparently occupied by pentane was subjected to the SQUEEZE

routine of the PLATON suite [25] and the contribution ofthis region to the data was

thereby eliminated. A solvent-accessible region was also evident in 14, but the low

electron density in this region was not suggestive of any significant solvent content.

There is some disorder of the phosphine ethyl groups in 16 and this was modelled in

terms of pairs of sites of fractional occupancy. Relatively high anisotropic

displacement ellipsoids are also observed for the 1 0,20-phenyl groups, although no

resolution of discrete disordered sites was apparent in a series of slant Fouriers

through the groups. The single site high ADP model was therefore preferred over a

disordered model.

CCDC deposition numbers: 191955-191957

RESULTS AND DISCUSSION

Studies of the addition of carbon nucleophiles to porphyrinylplatinum(II)

complexes

Our meso-11 1-organopalladium(II) and -platinum(II) porphyrins prepared by the

oxidative addition of the bromoporphyrin to Pd(O) and Pt(O) phosphine complexes

are the first reported examples of such compounds [14,15]. In the interests of

78

studying the chemistry of such species in more detail, and in particular their possible

use in preparing more elaborate structures based on the meso-metallo connection, we

attempted alkylation/arylation at the metal(II) centre. Successful exchange of

bromide at the metal would allow us to assess the stability of compounds possessing

both the porphyrin-metal and another carbon-metal bond. Such compounds constitute

a late intermediate along the carbon-carbon coupling pathway when palladium

complexes are used as catalysts. Given the predicted instability of such compounds

in the Pd series, these experiments were naturally carried out on the more robust

Pt(II) analogues, beginning with our well-characterised complex trans­

[PtBr(NiDPP)(PPh3)2] 1 (Scheme 1) [14,15].

Ph

Ph

1

Ph

Ph

Scheme 1. Attempted alkylations at platinum in 1.

Ph

PPh3 I lt-Br PPh3

Ph 3

Table 1. Crystal data for NiDAPP 9b, trans-[Pt(NiDPP)(C2NiDPP)(PPh3) 2] 14 and trans-[Pt(H2TrPP)(PEt3) 2] 16.

9b 14 16

a(A) 11.3026(2) 14.2070(2) 13.90680(10)

b (A) 11.3044(2) 16.5744(2) 14.78240(10)

c (A) 21.0858(3) 20.6061(3) 23.3665(2)

a (deg) 105.0783(6) 90.2470(5) 90

~ (deg) 97.7815(6) 101.7677(6) 102.6361(3)

y (deg) 103.5210(6) 113.9458(6) 90

Cell volume (N) 2473.40(7) 4320.73(10) 4687.24(6)

79

Crystal system triclinic triclinic monoclinic

Space group P-1 P-1 P21/n

R;nr 0.060 0.050 0.049

R 0.041 0.047 0.051

Rw 0.047 0.050 0.056

s 1.064 0.987 1.026

e limits 3-27.5° 3-25.4° 3-27S

No. unique/ No. 11356/4648 15830/9484 10756/5606

observed (obs. (I>3crl) (I>3crl) (I>3crl)

criterion)

Number refmed 525 1036 504

parameters

Crystal size (mm) 0.18 X 0.18 X 0.21 0.06 X 0.06 X 0.14 0.018 X 0.25 X 0.44

Initially, ligand exchange of the halogen on the metal by an alkyl moiety was

attempted with Grignard reagents. Unfortunately, a selective reaction was not found

with a range of such reagents, under various stoichiometric ratios and reaction

conditions. However when the reaction was repeated in THF with excess

alkyllithium reagents (PhLi, BuLi) as the source of the nucleophilic species, TLC

indicated that soon after the addition of RLi, a more mobile compound was present.

This fraction was isolated by column chromatography and recrystallised. However,

the 1H NMR spectrum of the product isolated from the BuLi reaction displayed a

broad, distorted triplet at 4.48 ppm which is characteristic of an alkyl chain directly

bonded to the porphyrin's meso position. This assignment was supported by the

disappearance of the meso-proton signal m the downfield region. It appeared

therefore that the nucleophile had added to the opposite meso position of the

porphyrin ring, rather than substituting the halide on Pt to give the desired 2.

Exposure to aerial oxygen during the work-up and chromatographic separation was

sufficient to oxidise the porphodimethene back to the tetra-substituted porphyrin 3.

The F AB mass spectrum also supported this conclusion with the parent ion of 3

clearly evident. This behaviour recalled the recent extensive work of the Senge group

80

on alkylation and arylation of porphyrins using alkyl- and aryllithiums [19]. It

seemed to be unlikely that we could escape this reaction, so with this in mind it was

decided to synthesise porphyrin precursors that have no unsubstituted meso-positions

available for attack by the alkyllithium reagent.

Ar

Ar

4a: Ar =phenyl

1. PhLi 2.Hz0 3.DDQ

THF/0°

4b: Ar = 3,5-di-t-butylphenyl

Ar

Ph

Ar

Ar

Ph

Ar

Ni(acac)z toluene reflux

Ar

Ph

Ar

Ar

NBS

Sa,b Ar 6a,b

\WI-Ni(acac)z reflux

Ar

PPh3 I Pt(PPh3)3 Pt-Br Ph

~Ph3 toluene 105°

Ar

Scheme 2. Preparations of triarylporphyrins and their derived platinioporphyrins.

Fig. 1. Molecular structure of NiDAPP 9b as determined by X-ray crystallography (hydrogen

atoms have been omitted for clarity). For one of the t-butyl substituents, only one of the disordered

sites is shown.

As shown in Scheme 2, triarylporphyrins Sa (Ar phenyl) and 5b (Ar = 3,5-di-t­

butylphenyl) were synthesised in high yields (>90%) from the respective 5,15-

diarylporphyrins 4a and 4b using the method of Senge [19]. After the initial addition

of an organolithium reagent to one meso position, quenching with aqueous THF

81

gives the porphodimethene which is re-oxidised to the respective triarylporphyrin by

the addition of dichlorodicyanoquinone (DDQ) to the reaction mixture. We use the

abbreviations H2TrPP (5,10,15-triphenylporphyrin, 5a) and H2DAPP

["diarylphenylporphyrin", 5,15-bis(3 ',5 '-di-t-butylphenyl)-1 0-phenylporphyrin, 5b]

from now on. The structures of both triarylporphyrins can easily be deduced by

inspection ofthe 1H NMR spectra. In the case ofH2TrPP the hydrogens of the phenyl

group on the 1 0-meso position understandably overlap with the signals arising from

the phenyl groups on the 5,15-meso positions. The reduced symmetry of the

molecule can be seen by the appearance of four doublets in the downfield region that

correspond to the four unique sets of P-hydrogens. Both triarylporphyrins were

brominated in almost quantitative yield using the standard procedure of N­

bromosuccinimide in dichloromethane at 0 °C [11,12b]. These bromo­

triarylporphyrins 6a and 6b offer an attractive entry into species elaborated at the

remaining meso carbon. The selective bromination at the only available meso­

position removes the need for the tedious chromatographic separation of the di-,

mono- and non-brominated porphyrins that is required if the halogenation is carried

out on the parent diarylporphyrins. Mono-halogenated derivatives of 5,15-

diphenylporphyrin have been synthesised in moderate yields (66%), however these

compounds tend to have limited solubilty in common organic solvents [11,12b]. In

our hands, the large-scale separation of the bromination products of the more soluble

bis(di-3,5-tert-butylphenyl)porphyrin is difficult and the use of these

triarylporphyrins has now overcome this problem. Both mono-brominated porphyrins

6a and 6b are considerably more soluble than the parent diaryl analogues. This is

especially the case with 6b, since it is even partially soluble in hexane and methanol,

in which the diaryl analogues have very limited solubility. Insertion of nickel into

both 6a and 6b to give NiTrPPBr 7a and NiDAPPBr 7b, respectively, was readily

carried out by standard metallation procedures, i.e. the action of Ni(II)

acetylacetonate in refluxing toluene over 4 hours.

The metallation of H2DAPP 5b was also undertaken, and the product NiDAPP 9b

was characterised crystallographically (Fig. 1 ). This is the first crystal structure of a

monomeric 5,10,15-triarylporphyrin, although two 5-alkyl DPP structures, namely 5-

isopropyl-1 0,20-diphenylporphyrin free base and 5-sec-butyl-1 0,20-

diphenylporphyrinatonickel(II) were reported by Senge and Feng [19b]. As shown in

Fig. 1, the porphyrin ring is mildly ruffled as is typical for most Ni(II) porphyrins.

82

The di-t-butylphenyl rings are almost orthogonal to the porphyrin mean plane, while

the unique phenyl ring lies at a more oblique angle.

We have also used 6b as an entry into conjugated bis(porphyrinyl) butadiynes

based on the DAPP unit, and their availability enabled us to advance our research

into the electrogenerated cations and anions from such bis(porphyrins) [26]. 5,10,15-

Triarylporphyrins have been prepared before by other routes [27], but the present

method is clean, high-yielding, and convenient. The only proviso is that the

phenyllithium should be in good condition, preferably freshly prepared, as degraded

solutions give poor results.

Bromometalloporphyrins 7a and 7b undergo the oxidative addition of zerovalent

Pt(PPh3)3 in hot, degassed toluene [14,15] to give the r{organometallic

platinioporphyrins 8a and 8b, respectively. The initial adduct is the cis isomer and its

thermal conversion to the trans isomer can be monitored by TLC. Isomerisation is

complete after approximately six hours. The identity of the fmal isomer is

unambiguously determined by 31P NMR, due to the presence of a sharp singlet at

approximately 23 ppm and a P-Pt coupling constant of about 2400Hz. It is known

that these 111-organometallic porphyrins undergo some halogen exchange in

chlorinated solvents [15]. In one case the chloro analogue of 8a was separated from

the bromo complex by column chromatography, and fully characterised.

With this new set of Pt(II) starting materials in hand, we returned to the alkylation

reactions (Scheme 3). When the 111-organometallic platinioporphyrin 8a was treated

with n-butyl- or phenyllithium in THF, the isolated products of the reaction were

simply the corresponding butyltriphenylporphyrin [19d] and tetraphenylporphyrin

nickel(II) complexes, 10 and 11, respectively. This loss of the platinum fragment was

evident in the 1H NMR by the lack of resonances near 7 ppm that arise from the

triphenylphosphine moieties, and the absence of any 31P NMR signal. The products

of these reactions can be rationalised by reductive elimination of the platinum

fragment to yield the new C-C bond. This mechanism is analogous to the fmal step

of several palladium-catalysed coupling reactions, such as the Stille and Suzuki

reactions [13]. It therefore appeared that preparation of the desired

alkyl(aryl)/porphyrinyl Pt(II) complexes would be frustrated by this reaction, so we

turned our attention to less basic organic ligands, namely alkynyls, which allowed us

to isolate the first examples of this class of compounds.

Ph

Ph

Sa Ph

PPh3 I lt-Br PPh3

"RLi THF/rt/1~ Ph

10:R=Bu 11: R=Ph

Ph

Ph

Scheme 3. Attempted alkylation at platinum in Sa.

PPh3 I lt-Br PPh3

1 Ph

Cui!Pd(PPh3)2Clz Et3N/THF/60°/48 h

Ph

H-=--0 Cui/Pd(PPh3hC12

Et2NH!fHF/60°/24 h

Ph

13

Ph

Ph

Scheme 4. Alkynylation at platinum in 1.

83

R

Ph

The alkynyl complexes 12 and 14 were obtained using the terminal alkynes and

the bromoplatinum NiDPP complex 1 as shown in Scheme 4. Phenylethynyl 12 was

prepared by the coupling of 1 with phenylacetylene in dry THF and diethylamine at

60 °C for 24 hours. TLC analysis showed the disappearance of starting material and

the appearance of a less polar spot. After isolation of the latter compound, its 1H

NMR spectrum clearly showed the presence of the extra phenyl moiety. The peaks.

corresponding to this phenyl group are somewhat upfield of the usual aromatic

84

region, presumably due to the shielding effects of the phenyl rings of the bulky

triphenylphosphine ligands. Given this favourable result, we next applied this

method to the 5-ethynylporphyrin 13, resulting in the isolation of novel

bis(porphyrin) 14. Compound 13 was obtained by reaction of the corresponding

trimethylsilylethynylporphyrin [12a] with fluoride ion. In the preparation of 14 the

usual 2° amine solvent diethylamine was replaced with triethylamine in order to

prevent consumption ofthe alkyne by the formation ofthe porphyrinylenamine [28].

The 1 H NMR spectrum of 14 clearly showed eight doublets in the down:field region

that correspond to the eight unique sets of ~-hydrogens distributed over the two

porphyrin rings. Not unexpectedly, the visible absorption of 14 showed a broad but

unsplit Soret band, indicating minimal ground state communication between the n­

systems of the two rings and also small through-space coupling.

Fig. 2. Molecular structure of trans-[Pt(NiDPPXC2NiDPP)(PPh3)z] 14 as determined by X-ray

crystallography (hydrogen atoms have been omitted for clarity).

We determined the molecular structure of this unusual complex by X-ray

crystallography (Fig. 2). It is the first example of a meso-11 1-platinioporphyrin with

two carbon-bound ligands and represents a new type of bis(porphyrin) with a unique

alkynylmetal spacer. As usual for Ni(II) porphyrins, both rings are slightly ruffled.

The coordination geometry at Pt is almost ideally square-planar, with no bond angle

deviating by more than 5° from 90°. The alkyne group is almost linear, the bond

angles· at the triply-bonded carbons being 176.2(6) (Pt-C42-C41) and 174.7(7)0

85

(C42-C41-C25). There is one other published alkynyl NiDPP structure, namely that

of 5-trimethylsilyl-1 0,20-diphenylporphyrinatonickel(II), and in that structure, the

alkyne unit is slightly more distorted from linearity, with angles of 169.7(5) and

172.6( 4)0 [12a]. The triphenylphosphine ligands, as usual in this type of

organometallic porphyrin, take up positions in which one phenyl group of each

ligand overlies the adjacent porphyrin ring in a nearly eclipsed relationship along the

P-Pt-P vector [15,16].

111- Platinioporphyrins with triethylphosphine ligands

The published structures of 111-platinioporphyrins with triphenylphosphine ligands

show the considerable steric demand of these ligands [14-16]. In some situations this

may be desirable as a way of protecting the faces of the porphyrin, but we are

naturally also interested in the preparation of analogues with different phosphine

groups that may leave these faces very open. As an entry point into this area the

smaller triethylphosphine ligand was chosen. This phosphine is also much more

electron-donating than triphenylphosphine, and Pt(PEt3)3 is much more reactive than

Pt(PPh3)3, possibly allowing easier synthesis of bis(platinio )porphyrins, whose

preparation has so far been somewhat difficult [16]. So bromo-triarylporphyrin free

base 6a was treated with Pt(PEt3)3 in hot, degassed toluene under an argon

atmosphere (Scheme 5). Immediately after the addition of the zerovalent Pt complex,

TLC showed the presence of a much more polar compound and almost complete

consumption of the bromo starting material. The high basicity conferred by the

electron-rich -PtBr(PEt3)2 moiety can be detected by TLC on silica gel, because the

product appears as an almost stationary bright green spot. This is due to the

protonation of the porphyrin nitrogens, and hence the addition of a base (e.g. Et3N) to

the TLC elution solvent is required in order to mobilise the spot (the colour also

changes to purple). Such behaviour was also observed for the PPh3 analogues [15].

The reaction was stopped at this point by the removal of the solvent under high

vacuum in order to prevent conversion to the trans isomer. The structure of the cis

isomer 15 was proved by 31P NMR, which revealed two sets of doublets for the non­

equivalent phosphines. These have a P-P coupling constant of 18 Hz, and P-Pt

coupling constants of 3972 Hz for the phosphine trans to the Br, and 1791 Hz for

that trans to the porphyrin. The inequivalence of the phosphine groups is also

86

reflected in the 1H NMR spectrum, which clearly shows two sets of ethyl signals in

the upfield region.

Ph

Ph Pt(PEt3)J

Br Ph toluene

105° 5min

Ph

PEt3 I xylene Pt-PEt3-Ph I 140° Br 2h

Ph

Scheme 5. Formation ofbis(triethylphosphine) complexes ofH2TrPP by oxidative addition.

Ph

Ph 16 (trans)

Fig. 3. Molecular structure of trans-[PtBr(H2TrPP)(PEt3) 2] 16 as determined by X-ray

crystallography (hydrogen atoms have been omitted for clarity). For each of the three disordered

carbons of the ethyl groups, only one position is shown.

By analogy with PPh3-cont~ining 11 1-organometallic platinioporphyrins,

isomerisation of the cis to the trans isomer was attempted by heating at 105 °C in

degassed toluene. Even after four hours of this treatment, isomerisation was

undetectable by 31P NMR. In fact it was found by 31P NMR that higher temperatures

(four hours in xylene at 140 °C) were required to complete the isomerisation. The 31P

NMR spectrum of trans isomer 16 showed a sharp singlet at 10.0 ppm with a Pt-P

coupling constant of 2611 Hz. This increased symmetry could also be seen in the 1 H

NMR with only a single set of ethyl resonances visible in the upfield region. So the

triethylphosphine ligand is advantageous if the cis isomer is the desired product,

87

offering improved selectivity in comparison to triphenylphosphine. For the latter

ligand, conversion to the trans adduct intervenes well before all the bromo starting

mater~al is consumed. These triethylphosphine complexes undergo halogen exchange

in chlorinated solvents more readily than the triphenylphosphine analogues, and

hence in the NMR spectra (both 31P and 1H) some peaks of the chloro complex are

discernible close to those of the bromo complex. Parent ions of both bromo and

chloro complexes are also visible in the F AB mass spectra, however no separation

was possible by TLC in a variety of solvent systems.

The molecular structure of 16 was determined by X-ray crystallography and is

shown in Fig. 3. As expected for a free base porphyrin without steric crowding on the

periphery, the macrocycle is nearly planar, with the P-Pt-P vector almost orthogonal

to that plane. The bond angles at the platinum atom are all 90 ± 2°. Although there is

some disorder in the triethylphosphine alkyl groups, the ligands adopt an eclipsed

relationship down the P-Pt-P direction as seen for the triphenylphosphine analogues.

However the flexibility of the ethyl groups allows the methyls nearest the porphyrin

to point directly away from that ring.

CONCLUSION

Triarylporphyrins of the 5,10,15 substitution pattern are formed in a clean and

selective manner by the method of Senge and co-workers [19]. Bromination is then

directed exclusively to the remaining meso position, enabling us to prepare the meso-

111-platinioporphyrins in a convenient manner. This series of compounds is

significantly more soluble in common solvents than the DPP analogues. The

alkylation of complexes of the type trans-[PtBr(porphyrin)(PPh3)2] with strongly

basic organometallic nucleophiles occurs at the opposite meso carbon, unless the

latter is already substituted. In that case, butyl- or phenyllithium attacks at platinum,

but reductive elimination occurs to extrude the Pt(O) moiety and form a new

porphyrin-carbon bond. However, less basic nucleophiles produced by the

combination phenyl- or porphyrinylethyne with catalytic Cui form stable

diorganoplatinum(II) bis(phosphine) complexes such as 12 and 14. Oxidative

addition of meso-bromoporphyrins to platinum(O) complexes of trialkylphosphines

was demonstrated for the first time, and the smaller bulk of triethylphosphine

compared with triphenylphosphine significantly increases the stability of the initial

88

cis adduct. Our present work involves the use of these novel organoplatinum(II)

porphyrins in the construction of supramolecular arrays and chirally-superstructured

porphyrins, as well as studies of the effects of the metallo substituents on

photophysical properti~s.

Acknowledgements.

D.P.A. thanks Dr Graham Heath and the Research School of Chemistry, The

Australian National University for a Visiting Fellowship and R.D.H. thanks the

Faculty of Science, Queensland University of Technology for a Post-Graduate

Scholarship.

REFERENCES

1. Sanders JKM, Bampos N, Clyde-Watson Z, Darling SL, Hawley JC, Kim H-J,

Mak CC and Webb SJ. in The Porphyrin Handbook, Kadish KM, Smith KM and

Guilard R. (Eds.) Academic Press: San Diego, 2000; Vol. 3, Ch. 15, 1-48.

2. See, for example, (a) Furuta H, Ogawa T, Uwatoko Y and Araki K. Inorg.

Chern. 1999; 38: 2676-2682. (b) P. J. Chmielewski and L. Latos-Grazynski,

Inorg. Chern., 1997, 36, 840-845. (c) Chmielewski PJ, Latos-Grazynski L and

Schmidt I. Inorg. Chern. 2000; 39: 5475-5482. (d) Araki K, Winnischofer H,

Toma HJ, Maeda H, Osuka A and Furuta H. Inorg. Chern. 2001; 40: 2020-2025.

(e) Schmidt I and Chmielewski PJ. Chern. Cornrnun. 2002; 92-93.

3. Graham SR, Ferrence GM and Lash TD. Chern. Cornrnun. 2002; 894-895.

4. (a) Brothers PJ. Adv. Organornet. Chern. 2001; 46: 223-321. (b) Guilard R,

Tabard A, van Caemelbecke E and Kadish KM. in The Porphyrin Handbook,

Kadish KM, Smith KM and Guilard R. (Eds.) Academic Press: San Diego, 2000;

Vol. 3, Ch. 21, 295-345.

5. (a) Smith KM and Langry, KC. J. Chern. Soc. Chern. Cornrnun. 1980; 217-218.

(b) Morris IK, Snow, KM, Smith, NW and Smith KM. J. Org. Chern. 1990; 55:

1231-1236.

6. (a) Dailey KK, Yap GPA, Rheingold ALand Rauchfuss TB. Angew. Chern. Int.

Ed. Engl. 1996; 35: 1833-1835. (b) Dailey KK and Rauchfuss TB. Polyhedron

1997; 16: 3129-3138.

89

7. Hyslop AG, Kellett MA, Iovine PM and Therien MJ. J. Am. Chern. Soc. 1998;

120: 12676-12677.

8. R:-uhlmann Land Giraudeau A. Eur. J. Inorg. Chern. 2001; 659-668.

9. Wang HJH, Jaquinod L, Nurco DJ, Vicente MGH and Smith KM. Chern.

Commun. 2001; 2646-2647.

10. Arnold DP and Nitschinsk LJ. Tetrahedron Lett. 1993; 34: 693-696.

11. DiMagno SG, Lin VS-Y and Therien MJ. J. Org. Chern. 1993; 58: 5983-5993.

12. See, for example, (a) Arnold DP, Bott RC, Eldridge H, Elms F, Smith G and

Zojaji M. Aust. J. Chern. 1997; 50: 495-503. (b) Shanmugathasan S, Johnson

CK, Edwards C, Matthews EK, Dolphin D and Boyle RW. J. Porphyrins

Phthalocyanines 2000; 4: 228-232. (c) Lin VS-Y, DiMagno SG and Therien MJ.

Science 1994; 264: 1105-1111. (d) Taylor PN, Wylie AP, Huuskonen J and

Anderson HL. Angew. Chern. Int. Ed. 1998; 37: 986-989. (e) Shultz DA,

Gwaltney KP and Lee H. J. Org. Chern. 1998; 63: 4034-4038. (f) Sugiura K,

Fujimoto Y and Sakata Y. Chern. Commun. 2000; 1105-1106.

13. Tsuji J. Palladium Reagents and Catalysts. Wiley: Chichester, U.K., 1995.

14. Arnold DP, Sakata Y, Sugiura K and Worthington EI. Chern. Commun. 1998;

2331-2332.

15. Arnold DP, Healy PC, Hodgson MJ and Williams ML. J. Organomet. Chern.

2000; 607: 41-50.

16. Hodgson MJ, Healy PC, Williams ML and Arnold DP. J. Chern. Soc., Dalton

Trans. in press.

17. Manka JS and Lawrence DS. Tetrahedron Lett. 1989; 30: 6989-6992.

18. (a) Ugo R, Cariati F and La Monica G. Inorg. Synth. 1990; 28: 123-126. (b)

Yoshida T, Matsuda T and Otsuka S. Inorg. Synth. 1990; 28: 119-121.

19. (a) Senge MO and Feng X. Tetrahedron Lett. 1999; 40: 4165-4168. (b) Senge

MO and Feng X. J. Chern. Soc., Perkin Trans. I 2000: 3615-3621. (c) Senge

MO, Kalisch WW and Bischoff!. Chern. Eur. J. 2000; 6: 2721-2738. (d) Feng X

and Senge MO. Tetrahedron 2000; 56: 587-590.

20. COLLECT software, Nonius BV 1997-2001.

21. Otwinowski Z and Minor W. in Methods in Enzymology Carter Jr CW and

Sweet RM. (Eds.) Academic Press: New York, 1997; Vol. 276,307-326.

22. Coppens P. in Crystallographic Computing, Ahmed FR, Hall SR and Huber CP.

(Eds.) Munksgaard: Copenhagen, 1970; 255-270.

90

23. (a) Altomare A, Cascarano G, Giacovazzo C, Guagliardi A, Burla MC, Polidori

G and Camalli M. J. Appl. Cryst. 1994; 27: 435. (b) Altomare A, Burla MC,

Camalli M, Cascarano GL, Giacovazzo C, Guagliardi A, Moliterni AGG,

Polidori G and Spagna R. J. Appl. Cryst. 1999; 32: 115-119.

24. Watkin DJ, Prout CK, Carruthers JR, Betteridge PW and Cooper RI.

CRYSTALS Issue 11, Chemical Crystallography Laboratory: Oxford; 2001.

25. VanderSluis P and Spek AL. Acta Cryst. 1990; A46: 194-201.

26. Arnold DP, Hartnell RD, Heath GA, Newby L and Webster RD. Chern.

Cornrnun. 2002; 754-755.

27. For examples, see (a) Shultz DA, Gwaltney KP and Lee H. J. Org. Chern. 1998;

63: 769-774. (b) Wojaczynski J, Latos-Grazynski L, Chmielewski PJ, Van

Calcar P and Balch AL. Inorg. Chern. 1999; 38: 3040-3050. (c) Yu L and

Lindsey JS. Tetrahedron 2001; 57: 9285-9298.

28. Arnold DP and James DA. J. Org. Chern. 1997; 62: 3460-3469.

91

CHAPTERS

Peripherally metallated porphyrins: the first examples of

meso-111-palladio(II) and -platinio(II) complexes with

chelating diamine ligands

Regan D. Hartnell and Dennis P. Arnold

European Journal of Inorganic Chemistry, 2003, in press.

92

The research presented in this paper is based on work done by R.D.H. under the

supervision ofD.P.A. All ofthe original compounds that are presented in this paper

were synthesised by myself, following methodologies and procedures that I

developed and optimised. All of the spectral and optical characterisation data of these

compounds was collected and interpreted by myself. Mass spectra and microanalyses

were obtained commercially from independent providers. The entire manuscript was

written by myself.

Peripherally metallated porphyrins: the first examples of meso-111 -palladio(II) and -platinio(II) complexes with chelating diamine ligands

Regan D. Hartnell and Dennis P. Arnold*

Synthesis and Molecular Recognition Program, School of Physical and Chemical

Sciences, Queensland University of Technology,

G.P.O. Box 2434, Brisbane, 4001, AUSTRALIA

Fax: +61 7 3864 1804

E-mail: d.amold@qut.edu.au

porphyrin I organometallic I palladium I platinum IN ligands

93

The oxidative addition of bis(dibenzylideneacetone)platinum(O) to meso-bromo-

5,15-diarylporphyrins in the presence ofPPh3 has been shown to be an effective way

of synthesising 11 1-organoplatinum porphyrins in high yields. This methodology has

been extended to synthesise various palladio- and platinioporphyrins that utilise

bidentate nitrogen donor ligands [N,N,N',N'-tetramethylethylenediamine (tmeda) and

2,2'-bipyridyl (bpy)] in order to enforce a cis configuration at the metal centre. The

products were characterised by multinuclear NMR and UV-visible spectroscopies as

well as fast atom bombardment and high-resolution electrospray ionisation mass

spectrometries.

94

Introduction

There has been considerable interest in the investigation of the synthesis and

properties of various types of organometallic porphyrins. Traditionally the

"organometallic" fragment of the molecule involves a metal-carbon bond between

the nietal bound in the central cavity of the porphyrin macrocycle and either an alkyl

or aryl moiety having either cr- or n-bonding character. [11 These investigations have

often been directed towards the catalytic activation of small molecules, usually in a

biomimetic sense. The recent discovery and investigation of porphyrinoid

macrocycles with modified cavities such as inverted or "N-confused" porphyrins[2]

and azuliporphyrins, [3] has somewhat increased the number of examples in this area,

however the centrally-coordinated metal is still intimately involved in the

organometallic character of the molecule. There are also several examples of

porphyrinoid compounds covalently bound to organometallic fragments such as

metallocenes,[4l or with one of the porphyrin pyrrole rings participating in a 115-

pyrrolyl-metal arrangement. [SJ

Ar

L I

Pd-X I

L

Ar

1

Besides the examples discussed above, there is also a small group of peripherally­

metallated 11 1-organometallic porphyrins. Examples in this area include mercurated

porphyrins,[6] meso-tellurium trichloride appended porphyrins,[7

] porphyrinyl

boronates[S] and the recent Grignard-like metalloporphyrin of Therien. [9] Our

publications have been the only reports of isolated 11 1-organopalladio- and

organoplatinioporphyrins[10-131 and these are the only examples with transition metals

directly bonded to porphyrin carbons. Compounds of type 1 (Chart 1) are involved in

95

the catalytic reactions for the coupling of mesa-haloporphyrins with simple terminal

alkynes, alkynylstannanes or alkynylzincs and alkenylorganometallics. [l4

-171 The key

initia~ step in these couplings is the oxidative addition of the mesa-carbon-to-halogen

bond to a zerovalent palladium species, usually a bis(phosphine) moiety. Several

years ago, we serendipitously isolated the resulting meso-11 1-organopalladium(II)

porphyrin from one of these reactions[121 and have recently embarked on a systematic

study of this type of palladium compound and their more robust organoplatinum(II)

analogues. One possible area of interest in meso-11 1-platinioporphyrins relates to the

combination of the cytotoxicity of the cis-platinum centre and the tumour selectivity

and photodynamic effect of the porphyrin. [IS-ZO] In this paper we report our synthesis

for the first time of meso-11 1-organometallic porphyrins of palladium(II) and

platinum(II) with chelating nitrogen ligands, namely N ,N ,N' ,N'­

tetramethylethylenediamine (tmeda) and 2,2'-bipyridyl (bpy).

Syntheses ofPalladio- and Platinioporphyrins.

As part of our ongoing investigation of 11 1-organometallic mesa-platinio- and

palladioporphyrins, we were very keen to synthesise platinioporphyrins with

chelating ligands, in order to enforce a cis arrangement of the platinum fragment.

This had been achieved for the palladioporphyrins by the use of various

diphosphines, for example 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-

bis( diphenylphosphino )propane ( dppp) and 1,1 '-bis( diphenylphosphino )ferrocene

( dppf). [l3] These 11 1-organometallic mesa-palladioporphyrins were readily

synthesised by the oxidative addition of a zero-valent palladium species to a 10-

bromo-5,15-diarylporphyrin. The zero-valent palladium species is usually prepared

in situ by the addition of the convenient palladium precursor, Pdz(dba)3 (dba =

dibenzylideneacetone, 1 ,5-diphenylpenta-1 ,4-dien-3-one) to the bidentate phosphine

in degassed toluene at 105 °C, to which the bromoporphyrin is later added. Under

these conditions the reaction is usually complete within 30 minutes. After removal of

the solvent and recrystallisation, the desired 11 1-organometallic mesa­

palladioporphyrin is obtained in high yields (>80% ).

Having developed this efficient methodology for our palladioporphyrin analogues,

we naturally sought to prepare the analogous platinioporphyrin species. After several

96

attempts it was soon apparent that the intervening zerovalent platinum species that is

formed in situ from Pt( dba )2 and a diphosphine ( dppe or dppp) is too stable to

undergo the oxidative addition step with the haloporphyrin. This appears to be

because the redistribution of ligands on Pt favours the apparently inert 18e species,

e.g. Pt( dppe )2. The analogous reaction was also attempted several times with less

stable, more sterically hindered zerovalent platinum species prepared from larger

bidentate diphosphines, namely 2,2'-bis( di-p-tolylphosphino )-1 ,1 '-binaphthyl (tol­

BINAP) and dppf. However these also failed to undergo the oxidative addition step

with a haloporphyrin to give the desired platinioporphyrin. In order to ensure that the

Pt( dba )2 that was being used throughout the investigations above was of high quality

and not the cause of our problematic reactions, we carried out a control reaction

between monodentate triphenylphosphine, Pt(dba)2 and a bromoporphyrin 2 (Table 1

shows the structures of compounds 2-14). This reaction proceeded smoothly, it being

evident by TLC that the Pt(dba)2 was consumed to form Pt(PPh3) 3 or Pt(dba)(PPh3)2

in situ and that 2 was being converted into a much more polar compound. Continual

TLC analysis showed that the initial more polar compound was slowly being

converted into a slightly more mobile product. This observation is in line with

others[10'

111 that the initially formed product of the oxidative addition step is the cis

isomer 3 which with continued heating slowly (over ca. 6 hours) isomerises to the

trans isomer 4. This method has the distinct advantage over the previous method[1o, 111 that it avoids the use of air- and moisture-sensitive Pt(PPh3) 3, which if not freshly

prepared may be of doubtful quality.

Ph

X y

Ph

Compound X y

2 H Br

3 4 5 6 7 8 9 10 11 12 13 14

H H H H Br Pd(tmeda)Br Ph H Ph Ph Ph Ph

cis-Pt(PPh3)2Br trans-Pt(PPh3)2Br Pd(tmeda)Br Pd(bipy)Br Br Pd(tmeda)Br I H H Pt(tmeda)l Pt(bipy)l trans-Pt(PPh3)2I

Table 1. Correspondence of compound numbers and structures

97

With the viability of the dba/ligand method proven, as explained above, a

different tactic was attempted in order to prepare the desired cis-orientated

organoplatinum porphyrins. It is well known that aromatic iodo groups undergo

palladium(O)-catalysed coupling reactions at a much greater rate than those of bromo

analogues.[211 This has also been shown to be true for 5-bromo-15-iodo-10,20-

diarylporphyrins, which are found to undergo palladium(O)-catalysed coupling

reactions at the iodo-substituted position preferentially over the bromo-substituted

position.P7'

221 With this in mind the oxidative addition reactions of the platinum(O)

diphosphine precursors were repeated with a range of iodoporphyrins, however

unfortunately these also failed to give the desired 111-organometallic porphyrins.

After these disappointing results with bidentate diphosphines, a new methodology

that utilises bidentate nitrogen donor ligands was developed in order to approach the

desired cis-orientated organoplatinum porphyrins. This reaction was initially

attempted with palladium, since it is known to undergo oxidative addition reactions

much more readily than platinum analogues. These reactions are well known from

the work of Canty and co-workers, who have used these N-ligands to make

interesting Pd(IV) complexes.[23l Thus, when Pd2(dba)3 was reacted with an excess of

tmeda and an equivalent of bromoporphyrin 2 in degassed toluene at 105 °C it was

soon evident that the starting material was being consumed and converted to a much

more polar compound. The reaction was completed after approximately one hour and

the solution was filtered to remove any palladium metal resulting from

decomposition of the Pd(O) precursors. After two recrystallisations from

CHCh/cyclohexane the product 5 was obtained in high yield as an air- and moisture-

98

stable dark purple solid. This procedure was similarly carried out with 2,2 '­

bipyridine as the nitrogen donor ligand to produce a high yield of the similar 111-

organometallic porphyrin 6. Unlike the porphyrinyl-palladium bidentate diphosphine

analogues, [12' l3] it was found that these compounds are relatively stable towards

silica and thus may be purified by column chromatography if required as long as

efforts are taken to ensure that any solvents used during the purification are

thoroughly de-acidified. The reaction was repeated with a dibromoporphyrin species

7, tmeda and Pd2(dba)3 in order to prepare the bis(palladated) species. TLC analysis

suggested that the desired double oxidative addition had occurred analogously,

however the product precipitated from the hot toluene solution. The precipitate was

collected but it was found to be extremely insoluble in all common organic solvents

and thus was not amenable to further purification. Other attempts at producing a

bis(palladated) porphyrin system using the more soluble 5,15-bis(3 ',5' -di-tert­

butylphenyl)porphyrin also gave insoluble products that were difficult to purify

thoroughly. FAB mass spectra of 8 (see below) indicated that the desired

bispalladium porphyrin was formed. Initial thoughts were that the bidentate tmeda

fragment could be flexible enough to act as a bridging group between two 11 1-

palladioporphyrin macrocycles, thus forming an oligomeric or polymeric species.

However, the same result was seen with the less flexible 2,2'-bipyridine ligand,

suggesting that they are not forming oligomers and that these examples of

bis(palladiated)porphyrin species with bidentate nitrogen ligands are inherently

insoluble.

Ph Pt(dba)2, PPh3

I - Ph toluene, 90°C

6h

Ph

9 Pt(dba)2, tmeda

~,90"C 24h

Ph

Ph

Ph

Ph

Ph

12 Scheme 1.

pp~ I rt-PP~- Ph I

Ph

Ph

pp~ I Pt-I I PPh3

99

Attempts at preparing the analogous platinioporphyrin systems with

bromoporphyrin 2 failed so the reactions were repeated with an iodoporphyrin. The

iodop~rphyrin chosen was 5-iodo-10,15,20-triphenylporphyrin (H2TrPP-I) 9. This

porphyrin was chosen due to its ease of synthesis in high yields, from the readily

available starting material diphenylporphyrin (H2DPP) 10. [ll, 241 The one free meso

position of triphenylporphyrin (H2 TrPP) 11 lends itself ideally to selective

iodination. [l7, 221 This method avoids the tedious chromatographic separation

procedures required to remove other iodoporphyrins that would be present if 10 itself

were iodinated by similar procedures. Thus, when the more reactive iodoporphyrin 9

was utilised, the desired platinioporphyrins with bidentate nitrogen ligands were

formed in good yields. As expected the formation of platinioporphyrins 12 and 13

was somewhat slower than the analogous palladioporphyrins 5 and 6. The latter were

formed in high yields within one hour, whilst 12 and 13 required overnight heating in

degassed toluene with an excess of the platinating agent, but were eventually formed

in good yields (ca. 80%). Platinioporphyrins 12 and 13 were purified by column

chromatography on a silica support in order to remove a trace of the starting

iodoporphyrin 9 and no degradation was seen during this procedure. It is important to

note that even though these oxidative addition reactions are carried out on the free

base porphyrins, no metallation of the central porphyrin cavity is seen in any of these

reactions, even after prolonged heating at 105 °C in toluene. This has been a general

observation in all our metallation reactions. Moreover, Pd-catalysed couplings on

free base porphyrins are readily carried out, as has been shown by several groups. [15'

16'

251 Indeed, we have found from qualitative reactivity comparisons of Pt insertions,

that the bromo free bases react faster than either Ni(II) or Zn(II) substrates. [261 For all

of these free base 11 1-organometallic porphyrins it was found that the addition of a

small amount of a base (1% triethylamine) to the mobile phase used during any

chromatographic procedures greatly improved the tractability of these species. If this

base were omitted, it was found that the porphyrin macrocycle tended to protonate

very readily on the column due to the strong electron donating properties of the 11 1-

organometallic fragment[10-l3] further enhanced by the electron-donating properties of

the bidentate nitrogen ligands.

With metalloporphyrins 12 and 13 in hand, we investigated the substitution of the

bidentate nitrogen donor ligands by chelating diphosphine ligands whose

coordination to the soft Pt(II) centre is expected to be favoured. Unfortunately it was

100

found that both 12 and 13 failed to undergo any ligand exchange reactions with a

variety of bis(phosphino) ligands ( dppe, dppp and dppf). It was surprising to fmd that

the ligand exchange did not occur at all even with excess phosphine ligand, elevated

temperatures, and prolonged reaction times. In all cases the bis(N-donor)-chelated

species was quantitatively recovered with no noticeable decomposition detectable by 1H NMR spectroscopy. It is however encouraging to observe that these species are

quite stable and unlikely to decompose under a variety of different solvents and

reaction conditions, so their use for the construction of multiporphyrin arrays with

Pt(II) connectors should be possible. The ligand exchange did occur with the simple

monophosphine, PPh3 to yield 14 in an approximate 50% yield, implying that the

problems may be steric in origin. However it was observed that H2 TrPP 11 was also

produced in an approximately equal amount. Clearly the organometallic

platinioporphyrin 14 is best prepared by using our original direct oxidative addition

of a zero valent Pt bis(phosphine) species (see Scheme 1 ), as discussed above. [IO-l3J

NMR Spectra ofPalladio- and Platinioporphyrins.

There are several aspects of interest in the NMR spectra of these YJ1-

organometallic porphyrins. The first observation is the facial asymmetry of the

porphyrin due to the favoured approximately orthogonal disposition ofthe N-Metal­

N plane of the organometallic fragment and the plane of the porphyrin macrocycle.

This arrangement gives rise to two different signals for the a-hydrogens of the phenyl

groups in the 5,15-positions of the porphyrin. The unequal faces of the porphyrin are

also clearly evident in the case of 5 where different non-equivalent m-hydrogens on

the 5,15-phenyl groups are also seen. This feature was less apparent in our previous

compounds with bidentate aryl diphosphines because of overlap of many aryl

signalsY2•

131 The porphyrin ~-hydrogens appear as four doublets ctJ = 4.7 Hz),

typical of substituted porphyrins of this symmetry and all peaks are shifted from

those encountered in the corresponding parent haloporphyrins. The biggest shifts are

understandably experienced by the peaks arising from the 3,7-~-hydrogens adjacent

to the organometallic fragment, which are shifted downfield by approximately 0.8

ppm. In the case of 5 and 6 this peak is even more downfield than the 20-meso­

proton which is observed as a sharp singlet at ca. 9.8 ppm. In those species

containing a 2,2'-bipyridyl ligand, namely 6 and 13, the peaks arising from the

101

bipyridine appear as a series of eight mutually coupled signals over a 4 ppm range

(see Figure 1). The signals from the protons that are above the porphyrin plane are

some~hat shielded by the macrocycle and this is demonstrated by large upfield shifts

of about 1 ppm from corresponding signals in the free ligand. Interestingly, the

signals arising from the 6' -proton of the bipyridine moiety in 6 and 13 are strongly

shifted downfield to 9. 7 and 10.4 ppm, respectively, demonstrating the effect of the

halogen as a neighbouring electronegative centre. In NOESY experiments on 6 and

13, a strong cross-peak is seen that represents the nOe between the 3,7-P-hydrogens

of the porphyrin and the 6"-proton of the bipyridine fragment. All peaks in these

spectra have been assigned by careful examination of the one-dimensional spectra

and a series of DQF-COSY and NOESY two-dimensional NMR experiments (see

Figure 2). Similar spectra are observed for 5 and 12, with signals for the tmeda

fragment appearing in the upfield region between 1.6 and 3.2 ppm. It is clear that the

metallo substituent is rotating slowly on the NMR timescale, residing mostly

orthogonal to the plane of the porphyrin, with the methyl groups being represented

by two singlets corresponding to those that are cis to the porphyrin and those that are

trans to the porphyrin. A dipolar coupling cross-peak from one of these methyl

singlets to the 3,7-p-hydrogens of the porphyrin reveals that the more upfield peak

around 1.6 ppm arises from the methyl groups that are cis to the porphyrin.

m,p-H

12,18-,SH

I CHCI3

I

13,17-,SH

3,7-,SH

\ 2,8-,SH I o-H

H3' H4"

Hs· \ • '!'

L~ U, I~·· Hs" H6"

10.8 9.8 8.8 ppm 7.8 6.8 5.8

Figure 1. 1H NMR spectrum of [Pti(H2TrPP)(bipy)] (13) in CDCh at 293K.

102

4'

6' 5'

Figure 2. Numbering scheme for bipy fragment and through-space correlations revealed by the NOESY spectrum of 13.

Mass and UV -visible Absorption Spectra of Palladio- and Platinioporphyrins.

All species gave mass spectra (ESI high-resolution for 5 and 6 and F AB for 12

and 13) that displayed either the molecular ions of the respective complexes or their

halide-exchanged analogues. For example 6 did not give any molecular ion signal

peaking at mlz 805.0744 for the bromo complex, rather the most abundant peak in

the spectrum corresponded extremely well with that for the iodo complex (calculated

mlz = 851.0622; found m/z 851.0722), indicating that iodide from the Nai internal

calibration standard had induced exchange. Also present in the spectra were the

analogous chloro complexes from CH2Ch used in the introduction of the samples

into the mass spectrometer. For platinioporphyrins 12 and 13, the FAB mass spectra

displayed the desired molecular ions for the appropriate parent compounds. In line

with our observations that the halogen is less labile on the platinioporphyrins

compared to the palladioporphyrins, 13 displayed only a very small (<10%) cluster

for the chloro complex, whilst 12 did not display any analogous peak at all. In all

spectra fragmentation occurred and appropiate masses were recognised. These

included fragments corresponding to the parent porphyrin macrocycle and fragments

after loss of the halide. On the other hand, the MALDI-TOF spectra of all

compounds displayed no parent molecular ions, but only clusters representing the

porphyrin macrocycle indicating that considerable fragmentation occurs during laser­

induced ionisation. Bis(palladated)porphyrin 8 gave a strong parent molecular ion at

m/z = 1065.2 (calculated mlz = 1065.1), as well as peaks corresponding to the loss of

either one or both of the organometallic fragments. Peaks for the chloro analogues

were also detected.

103

The UV-visible spectra of these organometallic porphyrins are quite typical for di­

and triphenylporphyrin, derivatives. The wavelengths of the principal visible

absorption bands for all complexes and the precursors are displayed in Table 2. Some

trends can be distinguished, even with this limited number of examples. All groups

other than H in the meso positions cause a red-shift, which is usual for these and

related porphyrinic systems. [151 It can be seen that the meso-11 1-organometallic

fragment exerts a similar effect on the electronic spectra to that of a simple halo

substituent. There is a red-shift of the Soret band of approximately 10-15 nm and

similar shifts for the Q bands when compared to the base porphyrin macrocycles,

after formation of the organometallic species. The nature of the nitrogen donor

ligands has very little effect on the electronic absorption spectrum for similar

metallated species, for example compare 5 and 6 or 12 and 13. When compared with

phosphine ligated species like 14 and those previously reported,P0-131 it can be seen

that they all have very similar electronic spectra, although the phosphine coordinated

species display slightly more red-shifted bands. The bis(palladated) species 8

continues the trend seen with mono(palladated) 5, the spectrum of 8 displaying an

even larger red-shift of all bands. This could be a sign of greater macrocycle

distortion in order to cope best with the electronic and steric demands of the two

organometallic fragments as well as the expected electron donating effect on the

energy ofthe HOMO.

Amax run Compound

So ret IV m n I

2 420 510 545 587 642 5 417 518 551 588 642 6 418 516 550 589 640 8 427 529 567 607 665 9 421 519 554 595 651 10 405 502 535 575 630 11 411 508 542 583 637 12 426 527 564 599 655 13 426 526 563 597 653 14 434 528 567 601 659

Table 2. Wavelengths for the principal UV-visible absorption bands for the meso-

metalloporphyrins and their precursors (in CH2Clz solutions).

104

Conclusion

The combination of Pt(dba)2 and monodentate phosphine ligands with various

haloporphyrins has been shown to be an effective way of synthesising 111-

organoplatinum porphyrins in high yields. An advantage of this method over

previous methods is that it avoids the use of unstable Pt(O) phosphine complexes.

This method has also been extended for the first time to synthesise various palladio­

and platinioporphyrins that utilise bidentate nitrogen donor ligands in order to

enforce a cis configuration of the metal centre. The availability of these stable cis

orientated organometallic porphyrins will allow for variation of architectures when

incorporated with other suitable tectons in self-assembled supramolecular systems.

Experimental

General Remarks: Syntheses involving zerovalent metal precursors were carried

out in an atmosphere of high-purity argon using conventional Schlenk techniques.

Porphyrin starting materials 10-bromo-5.15-diphenylporphyrin 2[171 and 5,10,15-

triphenylporphyrin 11[11] were prepared by literature procedures and Pt(dba)2 by the

method of Cherwinski and co-workers. [271 All other reagents and ligands were used

as received from Sigma-Aldrich. Toluene was AR grade, stored over sodium wire,

and degassed by heating and purging with argon at 105 °C. All other solvents were

AR grade, and dichloromethane and chloroform were stored over anhydrous sodium

carbonate. Analytical TLC was performed using Merck silica gel 60 F2s4 plates and

column chromatography was performed using Merck silica gel (230-400 mesh).

NMR spectra were recorded on Bruker Avance 400 MHz or Varian Unity 300 MHz

instruments in CDCh solutions, using CHCh as the internal reference at 7.26 ppm

for 1H spectra, and external 85% H3P04 as the reference for proton-decoupled 31P

spectra. UV - vis spectra were recorded on a Cary 3 spectrometer in

dichloromethane solutions. High resolution ESI mass spectra were recorded on a

Bruker BioApex 47e FTMS fitted with an Analytica Electrospray Source. The

samples were dissolved m dichloromethane and diluted with either

dichloromethane/methanol 1: 1 or methanol and solutions were introduced into the

105

source by direct infusion (syringe pump) at 60 J.!Lih, with a capillary voltage of 80 V.

The instrument was calibrated using internal Nal. Positive ion FAB mass spectra

were recorded on a Kratos Concept instrument at the Central Science Laboratory,

University of Tasmania. Samples were dissolved in dichloromethane, and dispersed

in a 4-nitrobenzyl alcohol matrix. In the data below, masses given are for the

strongest observed peak in the molecular ion cluster. In all compounds this mlz value

agreed with the predicted molecular mass, although in most cases it represented a

mixture of M and M+ 1 due to partial protonation of the free base porphyrin.

Elemental analyses were carried out by the Microanalytical Service, The University

of Queensland.

trans-[PtBr(H2DPP-)(PPh3)2] (4). Toluene (25 cm3) was added to a Schlenk flask

and degassed by bubbling argon through the solution at 90 °C. Bromoporphyrin 2

(20 mg, 0.037 mmol) was added and stirred for 5 min. Pt(dba)2 (29 mg, 0.044 mmol)

and triphenylphosphine (35 mg, 0.132 mmol) were added and the solution stirred at

105 °C. TLC analysis (50% CHCblhexane-1% Et3N) of the reaction mixture clearly

showed disappearance of the starting material after ca. 30 min and that the initially­

formed cis isomer was slowly being converted to the trans isomer. After ca. 6 h the

isomerisation was considered complete and the reaction mixture was cooled to room

temperature and the solvent removed in vacuo. The residue (now air stable) was

purified on a Si02 column eluting with 50% CHCI)/hexane-1% Et3N and the major

purple fraction was collected and the solvent removed in vacuo. The residue was

recrystallised from CHCblhexane to give 4 as dark purple crystals in 94% yield. The

spectroscopic data eH and 31P NMR) of this compound agreed well with those of a

genuine sample prepared previously using Pt(PPh3)3. [l3]

General Procedure for the Preparation of Compounds 5, 6 and 8. As an example

of this method, to a stirred solution ofbromoporphyrin 2 (25 mg, 0.046 mmol) in dry

toluene (15 ml) that had been degassed at 90 °C, tmeda (35 J.!L, 0.25 mmol) and

Pd2(dba)3 (43 mg, 0.046 mmol) were added sequentially. The reaction was

maintained at 90 °C under an argon atmosphere and monitored by TLC (50%

CHCI)/hexane-1% Et3N). When the reaction was considered complete by the total

disappearance of starting material, the solution (now air stable) was filtered through a

fine glass frit and the solvent removed in vacuo. The residue was recrystallised twice

106

from CHCb/ cyclohexane to give a dark purple solid that was dried thoroughly under

high vacuum.

[PdBr(H2DPP-)(tmeda)] '(5). The desired complex 5 (34 mg) was obtained in 93%

yield. 1H-NMR (400 MHz, CDCb, 25 °C): 8 -2.78 (br s, 2H, NH), 1.50 (s, 6H,

(CH3)2N), 2.37 (m, 2H, NCH2CH2N), 2.43 (m, 2H, NCH2CH2N), 2.70 (s, 6H,

(CH3)2N), 7.65-7.75 (m, 2H, one pair m-H on 10,20-phenyl), 7.75-7.85 (overlapping

m, 4H, one pair m-H on 10,20-phenyl and p-H on 10,20-phenyl), 8.05-8.10 (m, 2H,

o-H on 10,20-phenyl), 8.25-8.35 (m, 2H, o-H on 10,20-phenyl), 8.82, 8.90, 9.18,

10.04 (each d, 3JH,H = 4.7 Hz, 2H, ~-H), 9.92 (s, 1H, meso-H); UV- vis: Amax (e/103

M-1 cm-1) 417 (356), 518 (16.9), 551 (13.0), 588 (8.6), 642 (11.5) nm; High­

resolution ESI MS: [Mt accurate mass calculated for C3sH3sBrN6Pd(+1): 765.1375,

Found: 765.1385.

[PdBr(H2DPP-)(bpy)] (6). The desired complex 6 (36 mg) was obtained in 91%

yield. 1H-NMR (400 MHz, CDCh, 25 °C): 8 -2.93 (br s, 2H, NH), 5.95 (t, 3JH,H =

6.5 Hz, 1H, 5"-H), 6.02 (d, 3JH,H = 6.5 Hz, 1H, 6"-H), 7.05 (t, 3

JH,H = 6.5 Hz, 1H,

4"-H), 7.13 (d, 3 JH,H = 6.5 Hz, 1H, 3"-H), 7.32 (d, 3

JH,H 6.5 Hz, 1H, 3'-H), 7.48 (t, 3 JH,H = 6.5 Hz, 1H, 5' -H), 7.62 (t, 3

JH,H = 6.5 Hz, 1H, 4' -H), 7.65-7.75 (m, 6H, m,p-H

on 10,20-phenyl), 8.05-8.15 (m, 2H, o-H on 10,20-phenyl), 8.20-8.25 (m, 2H, o-H on

10,20-phenyl), 8.79, 8.92, 9.17, 10.25 (each d, 3 JH,H = 4.7 Hz, 2H, ~-H), 9.70 (d,

3 JH,H = 6.5 Hz, 1H, 6'-H), 9.91 (s, 1H, meso-H); UV- vis: Amax (e/103 M-1 cm-1

) 418

(492), 516 (15.2), 550 (9.9), 589 (6.3), 640 (7.0) nm; High-resolution ESI MS: I!Br

exchanged product [Mt accurate mass calculated for C42H3oiN6Pd( + 1 ): 851.0622,

Found: 851.0722.

Bis(palladio )porphyrin (8). This was prepared by a similar procedure to above, but

using dibromoporphyrin 7 and the appropriate amounts oftmeda and Pd2(dba)3. The

crude yield was quantitative, however the product is not sufficiently soluble for

further purification and 1H NMR analysis; UV- vis: Amax (rei. int.) 427 (37.8), 529

(3.3), 567 (5.6), 607 (1.0), 665 (4.9) nm; FAB MS: [Mt mass calculated for

C44Hs2Br2NsPd2(+ 1): 1065.2, Found: 1065.2.

H2TrPP-I (9). To a solution of triphenylporphyrin 13 (50 mg, 0.093 mmol)

dissolved in CHCh (25 em\ pyridine (250 J-LL), iodine (24 mg, 0.1 mmol) and

bis(trifluoroacetoxy)iodobenzene (40 mg, 0.1 mmol) were added. The reaction vessel

was protected from light and stirred at room temperature. Periodically, the progress

107

ofthe reaction was checked by TLC (30% CH2Chlhexane). After 36 hit was found

that there was total consumption of the starting material and the presence of a faster­

moving spot. The solvent was removed in vacuo and the residue purified by column

chromatography on Si02 eluting with 50% CH2Ch/ hexane. The major fraction was

collected and the solvent removed and the residue recrystallised from

CH2Ch/pentane. The desired haloporphyrin 9 was collected as bright purple crystals

in a 97% yield. 1H-NMR (400 MHz, CDCb, 25 °C): 8 -2.71 (br s, 2H, NH), 7.7-7.8

(m, 9H, m,p-H on 10,15,20-phenyl), 8.1-8.2 (m, 6H, o-H on 10,15,20-phenyl), 8.78,

8.80 (overlapping d), 8.87, 9.68 (each d, 3JH,H = 4.7 Hz, 2H, ~-H); UV- vis: Amax

(s/103 M-1 cm-1) 421 (433), 519 (19.7), 554 (11.6), 595 (5.5), 651 (4.9) nm; High­

resolution ESI MS: [M+Ht accurate mass calculated for C3sH26IN4(+1): 665.1202,

Found: 665.1212. Anal. Calcd. For C3sH25IN4: C, 68.68; H, 3.79; N, 8.43. Found: C,

68.54; H, 3.70; N, 8.27.

General Procedure for the Preparation of Compounds (12) and (13). As an

example of this method, to a stirred solution of iodoporphyrin 9 (25 mg, 0.038 mmol)

in dry toluene (15 ml) that had been degassed at 90 °C, tmeda (28 ~-tL, 0.19 mmol)

and Pt(dba)2 (125 mg, 0.19 mmol) were added sequentially. The reaction was

maintained at 90 °C under an argon atmosphere and monitored by TLC (50%

CHCblhexane-1% Et3N). When the reaction was considered complete after 24 h by

the total disappearance of starting material, the solution (now air stable) was filtered

through a fme glass frit and the solvent removed in vacuo. The residue was dissolved

in CHCh and loaded onto a Si02 column and eluted with CHCh-1% Et3N and the

major purple/green band collected. The solvent was removed in vacuo and the

residue recrystallised from CHCb/ cyclohexane to give a dark purple solid that was

dried thoroughly under high vacuum.

[Pti(H2TrPP-)(tmeda)] (12). The desired complex 12 (31 mg) was obtained in 83%

yield. 1H-NMR (400 MHz, CDCh, 25 °C): 8 -2.40 (br s, 2H, NH), 1.87 (s, 6H,

(CH3)2N), 3.27 (overlapping m, 4H, NCH2CH2N), 3.27 (s, 6H, (CH3)2N), 7.60-7.80

(m, 9H, m,p-H on 10,15,20-phenyl), 8.00-8.10 (m, 3H, o-H on 10,15,20-phenyl),

8.25-8.35 (m, 3H, o-H on 10,15,20-phenyl), 8.67, 8.72, 8.78, 10.12 (each d, 3JH,H =

4.7 Hz, 2H, ~-H); UV- vis: Amax (s/103 M 1 cm-1) 426 (395), 527 (8.9), 564 (10.5),

599 (2.8), 655 (8.1) nm; FAB MS: [Mt mass calculated for C44&1IN6Pt(+1): 976.2,

108

Found: 976.0. Anal. Calcd. for C44~1IN6Pt: C, 54.16, H, 4.23; N, 8.61. Found: C,

54.47; H, 4.13; N, 8.39%.

[Pti(H2TrPP-)(bpy)] (13). The desired complex 13 (30 mg) was obtained in 78%

yield. 1H-NMR (400 MHz, CDCIJ, 25 °C): 8-2.39 (br s, 2H, NH), 6.10 (t, 3JH,H =

6.2 Hz, 1H, 5"-H), 6.58 (d, 3JH,H = 6.2 Hz, 1H, 6"-H), 7.19 (d, 3JH,H = 6.2 Hz, 1H,

3"-H), 7.30 (t, 3JH,H = 6.2 Hz, 1H, 4"-H), 7.41 (d, 3

JH,H = 6.2 Hz, 1H, 3'-H), 7.60 (t, 3JH,H = 6.2 Hz, 1H, 5'-H), 7.60-7.75 (m, 9H, m,p-H on 10,15,20-phenyl), 7.80 (t,

3JH,H = 6.2 Hz, 1H, 4'-H), 8.10-8.25 (m, 6H, o-H on 10,15,20-phenyl), 8.70, 8.75,

8.80, 10.00 (each d, 3Ji1,H = 4.7 Hz, 2H, ~-H), 10.40 (d, 3JH,H = 6.2 Hz, 1H, 6'-H);

UV vis: Amax (c:/103 M-1 cm-1) 426 (379), 526 (13.0), 563 (14.1), 597 (5.9), 653

(8.6) nm; FAB MS: [Mt mass calculated for C4sH33IN~t(+1): 1016.2, Found:

1016.1; Anal. Calcd. for C4sH33IN6Pt: C, 56.76, H, 3.27; N, 8.27. Found: C, 56.53;

H, 4.38; N, 8.34%.

trans-[Pti(H2TrPP-)(PPh3)2] (14). Method A: To a refluxing, stirred solution of

complex 13 (10 mg, 0.01 mmol) in toluene, PPh3 (5.2 mg, 0.02 mmol) was added.

The solution was refluxed under an argon atmosphere for 24 h. The solvent was

removed in vacuo and the residue purified by column chromatography on Si02,

eluting with 50% CHCh/hexane-1% EhN to remove an impurity of 11. The solvent

was removed and the residue recrystallised from toluene/pentane to give the desired

complex 14 in a 52% yield.

Method B: Toluene (50 cm3) was added to a Schlenk flask and degassed by bubbling

argon through the solution at 90 °C. Iodoporphyrin 9 (50 mg, 0.075 mmol) was

added and stirred for 5 min. Pt(dba)2 (59 mg, 0.090 mmol) and triphenylphosphine

(47 mg, 0.18 mmol) were added and the solution stirred at 90 °C. TLC analysis (50%

CHCh/hexane-1% Et3N) of the reaction mixture clearly showed disappearance of the

starting material after ca. 30 min and that the initially-formed cis isomer was slowly

being converted to the trans isomer. After ca. 6 h the isomerisation was considered

complete and the reaction mixture was cooled to room temperature and the solvent

removed in vacuo. The residue (now air stable) was purified by column

chromatography eluting with 50% CHCh/hexane-1% Et3N and the major purple

fraction was collected and the solvent removed in vacuo. The residue was

recrystallised from CHCh/pentane to give 90 mg of 14 as dark purple crystals in

87% yield. Some I/Cl exchange was found to occur when left in chlorinated solvents

109

for extended periods. 1H-NMR (400 MHz, CDCh, 25 °C): 8 -3.13 (br s, 2H, NH),

6.40-6.60 (m, 18H, PPh3), 7.20-7.30 (m, 12H, PPh3), 7.60-7.70 (m, 9H, rn,p-H on

10,15,20-phenyl), 8.05-8.15 (m, 4H, o-H on 10,20-phenyl), 8.20-8.25 (m, 2H, o-H on

15-phenyl), 8.26, 8.63, 8.65 (overlapping d), 9.65 (each d, 3JH,H = 4.7 Hz, 2H, ~-H);

31P-NMR: 8 24.0 (s, 1JPt-P 2976Hz), UV- vis: Amax (e/103 M-1 cm-1) 434 (299), 528

(11.0), 567 (13.8), 601 (7.2), 659 (12.4) nm; High-resolution ESI MS: [Mt accurate

mass calculated for C74HsslN4PzPt(+ 1): 1385.2690, Found: 1385.2690

Acknowledgments

R.D.H. thanks the Faculty of Science, Queensland University of Technology for a

Post-Graduate Scholarship.

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1974, 1405-1409.

113

CHAPTER6

Peripherally 111- Platina ted Organometallic Porphyrins as

Building Blocks for Multiporphyrin Arrays

Regan D. Hartnell and Dennis P. Arnold

Submitted to Organometallics, 2003.

114

The research presented in this paper is based on work done by myself (R.D.H.) under

the supervision of D.P.A. All of the original compounds that are presented in this

paper were synthesised by myself, following methodologies and procedures that

were developed and optimised by me. All of the spectral and optical characterisation

data of these compounds was collected and interpreted by myself. Mass spectra and

microanalyses were obtained commercially from independent providers. The entire

manuscript was written by myself.

Peripherally 111-Platinated Organometallic Porphyrins as

Building Blocks for Multiporphyrin Arrays

Regan D. Hartnell and Dennis P. Arnold*

Synthesis and Molecular Recognition Program, School of Physical and Chemical

Sciences, Queensland University ofTechnology, G.P.O. Box 2434, Brisbane,

Australia 4001

115

The modification of peripherally-metallated meso-'11 1-platiniometalloporphyrins, such

as trans-[PtBr(NiDAPP)(PPh3)2] [H2DAPP = 5-phenyl-1 0,20-bis(3 ',5 '-di+

butylphenyl)porphyrin] leads to the analogous platinum(II) nitrato and triflato

electrophiles in almost quantitative yields. Self-assembly reactions of these meso­

platinioporphyrin tectons with pyridine, 4,4 '-bipyridine or various meso-4-

pyridylporphyrins in chloroform generate new multi-component organometallic

porphyrin arrays containing up to five porphyrin units. These new types of

supramolecular arrays are formed exclusively in high yields and are stable in solution

or in the solid state for extended periods. They were characterised by multinuclear

NMR and UV -visible spectroscopies as well as high-resolution electrospray

ionisation mass spectrometry.

116

Introduction

The rich photochenlical and redox properties of porphyrins have made them

attractive targets for incorporation into oligomeric and polymeric assemblies that

may exhibit useful photonic or electronic properties. Thus there has been much

interest recently in the design, synthesis and properties of discrete ordered arrays of

porphyrins and metalloporphyrins and the work has spanned many disciplines of

science.1-3 Generally, multi-porphyrin arrays have been considered as biomimetic

models of several natural systems or as materials for the transport of energy,

electrons and ions and as potential catalytic species. Thus these arrays are useful

whenever a specific arrangement of porphyrins and/or metalloporphyrins with low

conformational freedom is desired for the most efficient operation. A large range of

molecular entities has been synthesized and studied and these have provided much

valuable knowledge ofhow electron transfer is affected by factors such as separation

distance, mutual orientation, nature of insulating spacer group, energy gap, solvent

polarity, and the nature of the participating donors/acceptors.1-3 In order to increase

control over solution and solid state structural characteristics, most supramolecular

multi-porphyrin arrays take advantage of coordination or covalent bonds, instead of

hydrogen bonds with proteins as found in natural porphyrin-containing systems.

Substitution of weaker hydrogen bonds or van der Waals forces with stronger

chemical bonds has been successfully utilised to ensure the efficient electronic

coupling in either ground or excited states between the individual components of the

supramolecular system.1-3

The self-assembly of complementary components vm non-covalent

interactions such as metal-ligand coordination offers a popular synthetic

methodology for the controlled synthesis of large supramolecular entities. These

arrays usually possess well-defmed and predictable architectures. By this approach

and by choosing complementary components, elaborate multi-component arrays may

be constructed in a single step from a stoichiometric combination of the individual

tectons.4 Porphyrins represent ideal building blocks for the synthesis of oligomeric

and polymeric arrays via the self-assembly technique. They are fairly rigid and prefer

planarity, are easily modified and functionalised, and are able to coordinate a large

number of metals and pseudo-metals within their central cavity. There are numerous

examples of supramolecular porphyrinic arrays assembled through coordination of

117

phosphine groups,5 coordination to centrally bound Ru(II)6'7 or Zn(II)8 and

coordination to external metal centres like Pd(II) or Pt(II)9-15 or other metals.16

'17 The

vast ~ajority of the systems prepared so far have utilised a 'linker' (often a pyridyl

moiety) between the porphyrin macrocycle and the coordinating metal centre. Due to

steric constraints between the porphyrin's ~-hydrogens and the pyridyl ring, the

aromatic group will often lie perpendicular to the plane of the porphyrin ring. This

conformation reduces the amount of ground state electronic communication within

the molecule, which is often desirable to control the properties. However, it may also

be useful to have linkers that modify the redox properties of the porphyrin(s) by

more direct interactions. One way of combining redox control and array construction

is to have the metal fragment directly 11 1-bonded to the porphyrin macrocycle.

Several types of organometallic porphyrins are known. The vast majority of

examples in this area usually incorporate the organometallic bond between the

centrally bound metal in the porphyrin and an organic fragment. 18 The recent

discovery and investigation of modified porphyrin macrocycles such as inverted or

"N-confused" porphyrins19 or azuliporphyrins,20 have somewhat increased the

number of examples in this area, however the centrally coordinated metal still

participates in the organometallic character of the molecule. There are also limited

examples of porphyrins with an externally bound organometallic fragment, which

includes examples of both cr- and n-bonded organometallic moieties?1

Ar

Ar

1

L I Pt-X I

L

Apart from the examples of organometallic porphyrins discussed above, there

is another group of 11 1-organometallic porphyrins that has been synthesised recently.

Our publications have been the only reports of isolated 111-organopalladio- and -

organoplatinioporphyrins?2-24 This particular version of late transition metal

chemistry leads to the compounds of type 1 whose palladium(II) analogues are

118

involved in the catalytic couplings of meso-bromoporphyrins with simple terminal

alkynes, alkynylstannanes or alkynylzincs and alkenylorganometallics.25 It has been

assumed in these examples that the key step in the coupling reactions is the oxidative

addition of the meso-carbon-to-bromine bond to a zerovalent palladium species,

usually a bis(phosphine) moiety. Several years ago, we serendipitously isolated the

resulting meso-r{organopalladium(II) porphyrin from one of these reactions?4

Because such compounds had not previously been studied in their own right, we

embarked on a systematic study of this type of palladium compound, and have also

studied their more robust organoplatinum(II) analogues. Apart from our reports,

there appear to be no other examples of isolated compounds with direct transition

metal to porphyrin M-C cr-bonds. The present paper reports the results of our recent

work on (i) substitution reactions at the Pt(II) by various nucleophiles and (ii)

coordination for the first time of these Pt(II) centres to various pyridyl moieties.

Investigation of this pyridine coordination has allowed us to prepare and characterise

several novel self-assembled multiporphyrin arrays using the direct porphyrin­

platinum bond as a new construction principle.

Results and Discussion

Ar

X y

Ar Ar = 3,5-Bu1

2C6H3

Compound M X y

2 Ni Ph Br

3 H2 H H

4 Ni Ph trans-Pt(PPh3)2Br

5 Ni Ph trans-Pt(PPh3)2Cl

119

Ar Ar

Pt(dbab PPh3 ~Ph3 AgN03 ~Ph3 2 Ph Pt-Br Ph Pt-0-NO

I I 2 ~oluene, 105°C PPh3 acetone PPh3

6h (95%) (85%)

Ar Ar

Ar = 3,5-But2C6H3 4 ~Tf 6

benzene, DCM 5h

(100%) Ar

PPh3 0 F I II-A Ph Pt-0-S I II PPh

3 0 F F

Ar

7

Scheme 1

Syntheses of Porphyrin Starting Materials. The base porphyrin

chromophore chosen for this investigation was the readily synthesised 2 and its

transformations are shown in Scheme 1. This haloporphyrin is easily prepared from

the diarylporphyrin (Ar = 3 ',5 '-di-t-butylphenyl) via a simple phenylation,

bromination, metallation scheme in multi-gram quantities and in high yields (ca.

75% over three steps from 3).23'26 One of the major advantages stemming from the

use of this building block is that it is very soluble in many common organic solvents

(CHCh, CH2Ch, THF, DMSO, acetone, ether, toluene, benzene; partly soluble in

cyclohexane ). The inherent solubility of this macrocycle avoids the problems that

have plagued other multi-porphyrin arrays with 3'7'10'12'13'15'17 and without 2'27,28

coordinating metal centres. The 111 -organometallic porphyrin 4 has been previously

prepared by the oxidative addition of the zerovalent platinum complex, Pt(PPh3)3, to

the appropriate bromoporphyrin 2 in degassed toluene at 105 °C.23 It has since been

found that a more attractive way of preparing such platinioporphyrins is to use

Pt(dba)229 (dba = dibenzylideneacetone) in combination with triphenylphosphine to

generate the desired zerovalent platinum complex in situ. The very reactive Pt(PPh3)2

or Pt(dba)(PPh3)2 then undergoes oxidative addition with the haloporphyrin. The

initial product of the oxidative addition is the cis isomer, which with continued

heating slowly isomerises (over ca. 6 hours) to furnish the desired trans isomer in a

120

high yield. This method has the distinct advantage over the previous method that it

avoids the use of air-sensitive Pt(PPh3)3 which if not freshly prepared may be of

unknown or dubious quality. The platinum functionalised porphyrin 4 is an air- and

moisture-stable entity in line with other 111-organometallic platinio- and

palladioporphyrins. However, some Br/Cl exchange is known to occur in chlorinated

solvents. 22-24

Bromide abstraction with silver nitrate in acetone/CH2Ch proceeded

smoothly to give platinum nitrate 6 in 95% yield. The reaction was easily followed

by TLC, and the product was isolated by filtration to remove the silver bromide by­

product, and the residue was washed with water to remove excess silver nitrate. The

platinionitrate 6 was also found to be very soluble in common organic solvents and

air- and moisture-stable. Moreover the compound is stable in solution for several

weeks, with no detectable N03/Cl exchange (as measured by 1H NMR) when

dissolved in chlorinated solvents, and stable for over one year in the solid state. This

complex was fully characterised, as outlined in the Experimental Section, by 1H and 31P NMR, UV-visible spectroscopy, high-resolution ESI mass spectrometry and

elemental analysis. Understandably the proton NMR of 6 is similar to its parent

compound, however small (ca. 0.2 ppm) upfield shifts are seen for the porphyrin B­

protons nearest the organometallic fragment (i.e. 2,3,7,8 B-H). The 31P NMR of 6

shows no change of the chemical shift of the equivalent triphenylphosphine groups

when compared to 4 indicating similar polarity of the Pt-Br and Pt-ON02 bonds.

The high-resolution ESI mass spectrum of 6 (calibrated against an internal standard

ofNai clusters) shows a peak at m/z = 1622.5124, which corresponds to the sodium

adduct of [Mt (calculated 1622.5034). Similarly another cluster is seen at m/z =

1537.5221 corresponding to the organometallic fragment after the loss of nitrate, [M

- N03t.

It was also of interest to prepare the triflato platinioporphyrin due to the more

electrophilic nature of the Pt centre. Initially, it was found that treatment of 4 with

silver triflate in dichloromethane led to the rapid decomposition of the

organometallic species and formation of a green product. Decomposition of the

organometallic species was also encountered by Stang et al. 30 in studying their

anthracene "molecular clip" system. This observation is in line with the recent work

of Osuka and co-workers in their preparation of meso-meso linked porphyrin arrays

121

by Ag(l) oxidation of the porphyrin macrocycle.28'31 From this it can be seen that the

higher redox potential of Ag+ in dichloromethane (+0.65 V vs. SCE)32 favours

oxidation of the porphyrin macrocycle, rather than inducing metathesis. This

problem was overcome by carrying out the bromide abstraction reaction of 4 in

benzene, rather than dichloromethane, where the redox potential of the system is

apparently low enough to avoid porphyrin oxidation and hence achieve the anion

exchange reaction. The simple work-up of filtering offthe silver bromide by-product

and washing the porphyrin residue with water furnished the desired triflate

exchanged product 7 in an almost quantitative yield. Inspection of the 1H and 31P

NMR spectra of 7 showed that unless the sample was rigorously dried under high

vacuum it preferred to exist as the cationic platinum complex with a water molecule

as the coordinated ligand. This gave rise to broadened peaks in both the 1H and 31P

NMR spectra. The coordinated water peak can be seen at approximately 5.5 ppm as a

slightly broadened singlet. The identity of this peak was confirmed by the addition of

1-2 !lL of D20 to the NMR sample, after which the singlet at 5.5 ppm further

broadened, diminished in size, and moved upfield to about 4.7 ppm. Even in the most

thoroughly dried sample the aqua species can be seen in the 1H NMR spectrum in

approximately a 1:3 ratio with the triflato product. The 31P NMR spectrum of 7

demonstrates the effect of the bonded triflate on the equivalent neighbouring

triphenylphosphine groups in 7. The phosphine resonance occurs at 18.2 ppm

[flanked by 195Pt satellites, 1Je1P_195Pt) 2958Hz], considerably upfield of that of the

analogous resonance in the parent compound 4 that occurs at 23.0 ppm. The high­

resolution ESI mass spectrum supports the assigned structure and mainly consists of

the parent ion of7 at mlz = 1686.4776 (calculated mlz = 1686.4776) and [M- OTtt

at m/z 1537.5234.

pyridine

Ph

6or7

4,4'-Bipy

Ph

Ar

Ar

Ar

Ar

I PPh3

'D ft~N~ !/ 8a Y=PF6•

8b Y=OTf PPh3

y-

9a Y=PF6•

9b Y=OTf·

Scheme 2

122

Ar

Ar

Self-assembly with Pyridyl Moieties. With the 11 1-organometallic

platinioporphyrin tectons 6 and 7 in hand, the coordination of pyridines was

investigated (Scheme 2). This was initially attempted with the parent

platinioporphyrin bromide 4 in CHCh and pyridine. It was found that even after

stirring at elevated temperatures with an excess of pyridine the desired cationic

platinum complex was not formed. The reaction was repeated with the

nitratoplatinum complex 6 with a stoichiometric amount of pyridine. It was found

that elevated temperatures were also required for this reaction. Under these

conditions, it was seen by both 1H and 31P NMR that the desired cationic platinum

complex with coordinated pyridine was contaminated with the N03/Cl exchanged

product 5. In order to avoid the use of chlorinated solvents, the reaction was repeated

in acetone at 50 °C with a stoichiometric amount of pyridine. The reaction progress

was monitored by 1 H NMR and the conversion progressed smoothly and was

considered complete after approximately 5 hours. The product Sa was obtained as the

hexafluorophosphate salt by the addition of an excess of KPF6. This procedure was

repeated with half an equivalent of 4,4' -bipyridine to furnish the bipyridine linked

dimer 9a, which too was formed almost quantitatively. It was also found that with

the more electrophilic platinum triflate tecton 7, the reactions with both pyridine (to

form 8b) and 4,4'-bipyridine (to form 9b) proceeded smoothly and were complete

within three hours at room temperature (as monitored by 1H NMR). The products did

Ph

123

not require treatment with KPF 6, as they were quite stable as the triflate salts. This

allowed the use of chlorinated solvents for these simple self-assembly reactions as

the reaction proceeded, relatively quickly (ca. 3 hours) at room temperature, thus

avoiding complications from the production of the triflate/chloro exchanged product

5.

NMR spectroscopy eH and 31P) provides a convenient means of

characterising these products. The resonances of the pyridine and 4,4 '-bipyridine

moieties are all shifted from the usual positions of the free ligands indicating that the

desired coordination to the metal centre has occurred. The spectrum of 8b displays

resonances from the a-protons ofthe pyridine ring (H0-py) as a broad doublet at 8.54

ppm (J = 5.1 Hz) indicating that they are equivalent and have shifted upfield. Other

signals arising from the pyridine moiety are found as multiplets at 6.95 and 7.36 ppm

corresponding to them- (Hm-PY) and p-pyridyl (Hp-py) protons, respectively. These

two resonances too are shifted upfield from corresponding resonances in the free

ligand supporting the conclusion of coordination to platinum. Usually the resonances

arising from the pyridyl moiety are shifted somewhat downfield after coordination to

a cationic metal centre, 10'15'30'33 however these system are usually lacking coordinated

phosphine groups or utilise less sterically demanding and magnetically isotropic

trialkylphosphines. This effect was demonstrated by Stang and co-workers33 by the

synthesis and comparison of self-assembled phosphine-coordinated Pt and Pd

cationic molecular squares with either Et3P or dppp [1 ,3-

bis( diphenylphosphino )propane] ligands. It has been proposed in this example and

others12-14'33 that the significant shielding effects of the bulky phenylphosphine

groups offset the effect of the cationic metal centre to give an overall moderate

upfield shift of the pyridyl resonances. These assignments are supported by

significant cross-peaks in DQF-COSY NMR experiments. Inspection of the 1H NMR

spectrum of bipyridine-linked dimer 9b suggests a structure with high symmetry.

The bridging 4,4'-bipyridine group is seen as a pair of coupled resonances (J= 6.2

Hz) at 8.55 and 7.36 ppm indicative of the bipyridyl o- and m-protons, respectively.

The symmetry of this 4,4'-bipyridine group demonstrates that the proposed dinuclear

array 9b exists as the exclusive coordinated species in solution. These 4,4'-bipyridine

resonances are also significantly shifted upfield by 0.3- 0.4 ppm from the respective

signals of the free ligand, supporting the assigned structure. The resonances arising

from the porphyrin ~-hydrogens nearest the organometallic fragment (2,3,7,8-~-Hs)

124

are understandably affected most by the presence of a cationic platinum moiety.

These signals are shifted downfield from that of triflate 7 by approximately 0.3-0.4

ppm. The triphenylphosphine signals (6.8-7.0 ppm) also experience a slight upfield

shift after coordination of a pyridyl moiety. The symmetry of compounds 8b and 9b

is also reflected in the corresponding 31P NMR spectra, which consist of a single

resonance at approximately 19 ppm that is flanked by 195Pt satellites with a

corresponding 195Pt-31P coupling constant of about 2900Hz. This coupling constant

is slightly diminished from that of non-cationic 111-organometallic platinioporphyrins

by about 50-100 Hz, reflecting the different cis influences of pyridine and

bromide. 22-24

The ESI high-resolution mass spectra of the cationic complexes 8b and 9b

gave excellent agreement with the predicted isotopic patterns. The mass spectrum of

8b displayed a strong peak at m/z = 1616.5786 corresponding to the cationic

fragment, [M- OTt]+ (calculated m/z = 1616.5681 ). The bipyridine bridged dimer 9b

also displayed a significant cluster at m/z = 1616.0803, which corresponds well with

the calculated pattern for the dication after loss of both the triflate anions, namely [M

- 20Tt]2+ (calculated m/z = 1616.0603). This cluster displayed a line separation of

half a mass unit in the isotopic pattern, demonstrating that it is indeed the dication

that is being detected.

7

Ar = 3,5-Bu12C6H3

tol = p-tolyl

CDCI3 / 5 h, 88%

":{}· tol 10

CDCI3 / 5 h 89%

CDCI3 / 24 h 78%

Ph

12

Ar

Ph

Ar

Ar

Ar

Ph

Scheme3

11

to I

to I 13

Ph

Ph

15

to I

to I

125

to I

OTf-

Ar

Ar

Ph

40Tf-

Self-assembly of Multi-porphyrin Arrays. With a basic methodology for

the coordination of pyridyl moieties to 11 1-organometallic meso-platinioporphyrins

established, we wished to extend this to incorporate coordination to a range of

pyridyl-porphyrins (see Scheme 3). When triflate 7 was reacted with a single

equivalent of 5-(4-pyridyl)-10,15,20-tri(p-tolyl)porphyrin 10 in CDCb at room

temperature, formation of a new species 11 was observed by 1H NMR. Likewise,

bis(pyridyl)porphyrin 12 and tetra(pyridyl)porphyrin 14 when reacted with

respective stoichiometric amounts of 7 (CDCb, room temperature, 5-24 hours) gave

rise to multiporphyrin arrays 13 and 15, respectively. From the 31P and 1H NMR

Ph

20Tf-

126

spectra of 11, 13 and 15 it was clearly evident that the reactions were quite selective

with only one new species being produced in each case. The 31P NMR spectra of

these multiporphyrin arrays supported the assigned structures and showed a single

resonance at atound 20 ppm flanked by 195Pt satellites with a 1JPt-P coupling constant

of approximately 2900Hz. Peak shifts seen in the 1H NMR spectra of 11, 13 and 15

indicate moderate changes to the pyridine-appended porphyrins after coordination to

the 111-platinioporphyrins. These changes include upfield shifts of approximately 0.2

ppm of the a-pyridyl protons closest to the coordinating metal centre of 11, 13 and

15. This suggests moderate shielding from nearby triphenylphosphine groups. On the

other hand, the m-pyridyl protons, those closest to the porphyrin macrocycle,

underwent substantial downfield shifts (ca. 0.3 ppm) compared to the parent

porphyrin due to coordination ofthe pyridyl group to a metal. These shifts ofpyridyl

o-, m-H are in line with those previously reported for self-assembled arrays and

squares that contain coordinating metal centres in conjunction with porphyrin

macrocycles.I0,12,15,17

Figure 1. Through-space correlations revealed by the NOESY spectrum of 15.

The 1H NMR spectrum of 15 displays broadened peaks for all of the

resonances probably indicative of restricted rotation. Despite the uninformative

nature of the lD spectrum, the connectivity of the newly formed array has been

demonstrated by 2D NMR experiments (DQF-COSY and NOESY). These spectra

allowed complete assignment of all peaks even where overlapped by other

resonances (see Supporting Information). For example, strong nOe's were seen

between the pyridyl protons of the central free base porphyrin macrocycle and the

protons of the triphenylphosphine groups on the metallated porphyrin indicating the

127

existence of the desired array 15 in solution (Figure 1 ). These systematic changes in 1H and 31P NMR spectra and observed nOe's between the individual porphyrin

macr~cycles indicate that the desired conjugates are indeed exclusively formed and

that they are not just stoichiometric combinations of the individual tectons. In the

pentanuclear array 15 the P-Hs of the central free base pyridyl porphyrin are seen as

a single resonance at 8.60 ppm indicative of a highly symmetrical structure. The

inner free base protons for all complexes 11, 13 and 15 are seen as single broad

resonances at approximately -3 ppm indicating that these arrays comprised a single

structure in solution and the free base porphyrin is intact.

1858

1859

1860 ...... -=:t -=:t 00 00 V) -=:t '.0 MVl VlVl o..q ~ o..q ~ 0. 0 0 ...... V) '.0 '.0 '.0 00 00 00 00 ...-I __. '1""""'1 ...-1

I I I I

1861

1862 miz

1863 miz

Figure 2. Experimental high-resolution ESI (bottom) and predicted (top) mass

spectral patterns for the ion [M- 20Tf ]2+ for trinuclear array 13.

128

Arrays 11 and 13 displayed high-resolution mass spectra that agreed

extremely well with those predicted for the assigned structures (Figure 2). Both of

these mass spectra show strong parent ion peaks, which correspond to the mass of the

array after the loss of the triflate counterions. For example, 13 displayed a strong

peak at m/z = 1860.6544 [M- 20Tf]2+, the cluster abundances corresponding well

with the calculated isotopic pattern and the most intense ion matching at m/z =

1860.6576. This isotopic pattern of13 displayed the desired half mass unit separation

between neighbouring peaks, indicating that it is indeed the assigned dication that is

being detected. Unfortunately no peaks that were assignable to the molecular ion of

15 were detected by ESI, FAB, or MALDI-TOF mass spectrometric techniques. All

mass spectra of 15 displayed the singly-charged fragment assignable to the individual

porphyrin macrocycle with the platinum phosphine fragment attached at mlz =

1537.2 (calculated m/z = 1537) indicating that considerable fragmentation of the

molecule occurs under all these conditions. Elemental analyses returned carbon

percentages that are about 1% low, which fits a solvate containing one molecule of

chloroform from the recrystallization solvent. The NMR spectra are strong evidence

for the formulation of the species as 15. Unfortunately, we have been unable to grow

crystals suitable for X-ray analysis for any of these arrays.

Electronic absorption spectra. The electronic absorption spectra of the 11 1-

organometallic platinioporphyrins display some changes after coordination with the

various pyridyl moieties. The Soret band of the Ni(II) porphyrin (ca. 430 nm) is

slightly red-shifted by approximately 1-5 nm after coordination to the pyridine

fragment. Larger shifts are seen from the complexes that involve coordination to

free-base pyridyl porphyrins i.e. complexes 11, 13 and 15. There is a decrease in the

extinction coefficient per porphyrin [or per Ni(II) porphyrin] for these arrays. This

suggests that there is some through-space electronic coupling between the porphyrin

chromophores. However, the platinum fragment acts more as a structural component

than a conjugative linking group. These observations are in line with those of other

groups. 11-15 The coordinated free base pyridyl porphyrins show moderate changes in

their electronic absorption spectra after coordination to platinum centres. The bands

stemming from the free base porphyrins that are visible and not overlapped by those

of the Ni(II) 111-organometallic platinioporphyrins show larger 8-12 nm red shifts.

The largest of these shifts is seen in the Q band (IV) at highest energy for complexes

129

11 and 13. The spectrum of 13 is shown in Figure 3. However, in pentanuclear array

15 band (IV) was obscured by those of the Ni(II) porphyrins.

400 450 500 550

wavelengtblnm

Figure 3. Absorption spectrum of 13 in CH2Ch.

Conclusion

600 650 700

This report demonstrates that 11 1-organometallic platinioporphyrins may be

incorporated into multiporphyrin arrays and that predetermined structural

characteristics of the individual tectons may be used to control the architecture of the

newly self-assembled multicomponent systems. These meso- organometallic

porphyrins form stable conjugates with various pyridine ligands and pyridyl­

substituted porphyrins that are very stable in solution for extended periods and in the

solid state indefinitely. These conjugates self-assemble selectively and in high yields

when the individual components are reacted in stoichiometrically equivalent ratios.

The newly formed arrays are highly soluble in common organic solvents, a property

that stems from the use of trisubstituted parent monomers, and this avoids solubility

problems that have plagued other similar assemblies. High-resolution mass spectral

data and single and multi-dimensional NMR experiments have unambiguously

established their structures in solution. This fundamental design approach will allow

for the possible incorporation of a large number of versatile components tailored to

the specific requirements of the assembly. Various attractive components such as

different metalloporphyrins, and various electron donor or acceptor fragments may

be included as either substituents on a 11 1-organometallic platinioporphyrin or as the

pyridine-substituted moiety which is directly coordinated to an organometallic

porphyrin. The ability to tailor the surface properties by the use of other substituents

130

on the aryl rings and alternative phosphine or other ligands on the Pt(II) cationic

centre is an advantage. This methodology should make it possible to attach

individual porphyrin macrocycles to surfaces via the appropriately substituted

pyridylligand'in combination with an organometallic porphyrin. For example, a 111-

organometallic platinioporphyrin coordinated to an isonicotinic fragment will allow

for attachment to a nanoporous, nanocrystalline Ti02 film. Preparation of these and

other supramolecular assemblies that utilise bis( meso-11 1-organometallic )porphyrins

and studies of their electrochemical and photophysical properties are in progress.

Experimental Section

General Procedures. Syntheses involving zerovalent metal precursors were

carried out in an atmosphere of high-purity argon using conventional Schlenk

techniques. Porphyrin starting materials 5, 15-bis-(3 ',5 '-di-tert-butylphenyl)porphyrin

33\ 5-bromo-1 0,20-bis(3' ,5' -di-t-butylphenyl)-15-phenylporphyrin 223

, 5-( 4-

pyridyl)-10,15,20-tri-p-tolylporphyrin 10 and 5,15-bis(4-pyridyl)-10,20-di-p­

tolylporphyrin 1215 and platinum precursor, Pt(dba)2 29 were synthesised by literature

methods. All other reagents and ligands were used as received from Sigma-Aldrich.

Toluene was AR grade, stored over sodium wire, and degassed by heating and

purging with argon at 90 °C. All other solvents were AR grade, and dichloromethane

and chloroform were stored over anhydrous sodium carbonate. Analytical TLC was

performed using Merck silica gel 60 F2s4 plates and column chromatography was

performed using Merck silica gel (230-400 mesh). NMR spectra were recorded on

Bruker A vance 400 MHz or Varian Unity 300 MHz instruments in CDCh solutions,

using CHCb as the internal reference at 7.26 ppm for 1H spectra, and external 85%

H3P04 as the reference for proton-decoupled 31P spectra. UV -visible spectra were

recorded on a Cary 3 spectrometer in dichloromethane solutions. High-resolution ESI

mass spectra were recorded on a Bruker BioApex 47e FTMS fitted with an Analytica

Electrospray Source. The samples were dissolved in dichloromethane and diluted

with either dichloromethane/methanol 1: 1 or methanol and solutions were introduced

into the source by direct infusion (syringe pump) at 60 ~-tLih, with a capillary voltage

of 80 V. The instrument was calibrated using internal Nai. Positive ion FAB mass

spectra were recorded on a Kratos Concept instrument at the Central Science

Laboratory, University of Tasmania. Samples were dissolved in dichloromethane,

131

and dispersed in a 4-nitrobenzyl alcohol matrix. In the data below, masses given are

for the strongest observed peak in the molecular ion cluster. The Microanalytical

Servi~e, The University of Queensland, provided the elemental analyses.

trans-[PtBr(NiDAPP)(PPh3)2] (4). Toluene (250 cm3) was added to a

Schlenk flask and degassed by bubbling argon through the solution at 90 °C.

Bromoporphyrin 2 (300 mg, 0.33 mmol) was added and stirred for 5 min. Pt(dba)2

(328 mg, 0.50 mmol) and triphenylphosphine (262 mg, 1.0 mmol) were added and

the solution stirred at 90 °C. TLC analysis (50% CHChlhexane) of the reaction

mixture clearly showed disappearance of the starting material after ca. 30 min and

that the initially-formed cis isomer was slowly being converted to the trans isomer.

After ca. 6 hr the isomerisation was considered complete and the reaction mixture

was cooled to room temperature and the solvent removed in vacuo. The residue (now

air stable) was purified by column chromatography eluting with 50% CHChlhexane

and the major red fraction was collected and the solvent removed in vacuo. The

residue was recrystallised from CHChlhexane to give 4 (460 mg, 85%) as dark red

crystals. The spectroscopic data ctH and 31P NMR) of this compound agreed well

with that of a sample prepared previously using Pt(PPh3)3.23

trans-[Pt(N03)(NiDAPP)(PPh3)2] (6). To a solution of platinioporphyrin 4

(200 mg, 0.12 mmol) dissolved in CH2Ch (10 cm3) and acetone (50 em\ silver

nitrate (210 mg, 1.2 mmol) was added. The solution was protected from light and

stirred at room temperature. Progress of the reaction was monitored by TLC (100%

CH2Ch) and conversion of the starting material to a slightly more polar red

compound was clearly evident. After 3 h the reaction was considered complete and

the solution was filtered through a fine glass frit in order to remove the white

suspension of silver bromide and any undissolved silver nitrate. The solvent was

removed in vacuo and the residue thoroughly washed with excess water. After

recrystallisation from CH2Cl;Jpentane and drying under high vacuum the product 6

(188 mg, 95%) was collected as bright red crystals. 1H NMR (CDCh, 400 MHz): 8

1.51 (s, 36H, tert-butyl-H on 10,20-aryl), 6.70-6.90 (m, J8H, PPh3), 6.90-7.00 (m,

12H, PPh3), 7.60-7.70 (m, 3H, mJJ-H on 15-phenyl), 7.67 (t, 2H, J= 1.8 Hz, 4'-H on

10,20-aryl), 7.71 (d, 4H, J= 1.8 Hz, 2',5',-H on 10,20-aryl), 7.95-8.05 (m, 2H, o-H

on 15-phenyl), 8.10, 8.54, 8.57, 9.47 (each d, 2H, J = 4.7 Hz, !3-H); 31P-NMR

(CDCh, 121.4 MHz): 8 22.1 (s, 1J Pt-P 3064Hz); UV-vis: Amax (c/103 M-1 cm-1) 429

132

(227), 537 (16.3), 57I sh (4.4) run; High-resolution ESI MS: [M +Nat, accurate

mass calculated for C9oHssNsNi03P2PtNa(+ I): I622.5034, Found: I622.5I24. Anal.

Calcd. for C9oHs5N5Ni03P2Pt: C, 67.54, H, 5.35 ; N, 4.3S. Found: C, 67.60; H, 5.4I;

N,4.30%.

trans-[Pt(OTf)(NiDAPP)(PPh3) 2] (6). Silver trifluoromethanesulfonate (15S

mg, 0.6 mmol) was added to a solution ofplatinum bromide 4 (200 mg, O.I2 mmol)

in benzene (50 cm3). The reaction vessel was protected from light and the solution

stirred at room temperature. After 5 h the halide abstraction was considered complete

by TLC analysis (CHCh), which showed the total disappearance of the starting

material and the presence of an immobile red spot. The white suspension of silver

bromide was removed by filtration through a fme glass frit and the solvent removed

in vacuo. The residue was washed thoroughly with water and dried in vacuo. After

recrystallisation from CH2Ch/pentane and drying under high vacuum the product 7

was collected as a red solid (20S mg) in a quantitative yield. 1H NMR (CDCh, 400

MHz): 8 1.47 (s, 36H, tert-butyl-H on I0,20-aryl), 6.70-6.SO (m, ISH, PPh3), 7.I0-

7.20 (m, I2H, PPh3), 7.60-7.70 (m, 3H, m,p-H on IS-phenyl), 7.6S (t, 2H, J= l.S Hz,

4'-H on I0,20-aryl), 7.70 (d, 4H, J= l.S Hz, 2',5',-H on I0,20-aryl), 7.95-S.OS (m,

2H, o-H on IS-phenyl), S.I6, S.54, S.56, 9.36 (each d, 2H, J = 4.7 Hz, 13-H); 31P­

NMR (CDCh, I21.4 MHz): 8 IS.2 (bs, 1J Pt-P 295S Hz); UV- vis: Amax (s/I03 M 1

cm-1) 429 (2I5), 537 (I7.4), 56S (5.6) run; High-resolution ESI MS: [Mt, accurate

mass calculated for C91HssF3N4Ni03P2PtS(+ I): I6S6.4776, Found: I6S6.4204.

In solution, this was found to be in equilibrium with the cationic aquated

analogue, namely trans-[Pt(NiDAPP)(H20)(PPh3)2](0Tf), in an approximate 3:I

ratio and its 1H NMR spectroscopic data are as follows. 1H NMR (CDCh, 400 MHz):

8 1.47 (s, 36H, tert-butyl-H on I0,20-aryl), 5.57 (s, 2H, coordinated H20) 6.S0-6.90

(m, ISH, PPh3), 7.00-7.IO (m, I2H, PPh3), 7.60-7.70 (m, 3H, m,p-H on IS-phenyl),

7.6S (t, 2H, J = l.S Hz, 4'-H on I0,20-aryl), 7.70 (d, 4H, J = l.S Hz, 2',5' ,-H on

I0,20-aryl), 7.95-S.OS (m, 2H, o-H on IS-phenyl), S.I7, S.52, S.54, 9.52 (each d, 2H,

J= 4.7 Hz, 13-H).

General Procedure for the Preparation of Compounds (8b), (9b), (11),

(13) and (15). To a solution of 7 (20.0 mg, O.OI2 mmol) in CDCh (3 cm3) a

stoichiometric amount of the pyridyl-containing ligand was added as a CDCh

solution. The vessel was sealed and stirred at room temperature. Afterwards an

133

aliquot was removed via syringe and transferred to an NMR tube for monitoring of

the reaction progress by 1H and 31P NMR. Once the reaction was considered

comp~ete (3-24 h) the NMR sample was re-combined with the bulk reaction sample

and the solvent removed in vacuo. The residue was recrystallised to yield the

appropriate cationic platinum complex as a red solid. Where required the product

was recrystallised again as noted and dried thoroughly under high vacuum at 80 °C

in order to obtain a sample for elemental analysis.

trans-[Pt(NiDAPP)(py)(PPh3)2](0Tt), (8b). Recrystallised from

CH2Ch/pentane to give 8b (21 mg) in 97% yield. 1H NMR (CDCh, 400 MHz): 8

1.47 (s, 36H, tert-butyl-H on 10,20-aryl), 6.70-6.90 (m, 18H, PPh3), 6.95 [m, 2H, m­

H on pyridine (signal overlapped by PPh3)] 6.90-7.00 (m, 12H, PPh3), 7.36 (m, 1H,

p-H on pyridine), 7.60-7.70 (m, 3H, m,p-H on 15-phenyl), 7.71 (t, 2H, J= 1.8 Hz, 4'­

H on 10,20-aryl), 7.75 (d, 4H, J= 1.8 Hz, 2',5',-H on 10,20-aryl), 7.95-8.05 (m, 2H,

o-H on 15-phenyl), 8.54 (d, 2H, J= 5.1 Hz, o-H on pyridine), 8.49, 8.58, 8.62, 9.70

(each d, 2H, J 4.7 Hz, P-H); 31P-NMR (CDCh, 121.4 MHz): 8 19.2 (s, 1J Pt-P 2906

Hz); UV-vis: Amax (s/103 M-1 cm-1) 429 (197), 538 (15.6), 574 (3.8) nm; High­

resolution ESI MS: [M - OTf t, accurate mass calculated for C9sH9oNsNiP2Pt( + 1 ):

1616.5681, Found: 1616.5786, Anal. Calcd. for C96H9oF3NsNi03P2PtS: C, 65.27; H,

5.14; N, 3.96. Found: C, 65.00; H, 4.88; N, 4.02%.

{Bis-[trans-Pt(NiD APP)(PPh3)2] ( 4,4' -bipyridine) }(OTt)2 (9b).

Recrystallised from CH2Ch/pentane, to give 9b (19 mg) in 93% yield. 1H NMR

(CDCh, 400 MHz): 8 1.48 (s, 72H, tert-butyl-H on 10,20-aryl), 6.70-6.90 (m, 36H,

PPh3), 6.90-7.00 (m, 24H, PPh3), 7.36 (d, 4H, J= 6.2 Hz, m-H on 4,4'-bipyridine),

7.60-7.70 (m, 6H, m,p-H on 15-phenyl), 7.74 (t, 4H, J= 1.8 Hz, 4'-H on 10,20-aryl),

7.78 (d, 8H, J 1.8 Hz, 2',5',-H on 10,20-aryl), 7.95-8.05 (m, 4H, o-H on 15-

phenyl), 8.55 (d, 4H, J= 6.2 Hz, o-H on 4,4'-bipyridine), 8.40, 8.58, 8.63, 9.67 (each

d, 4H, J= 4.7 Hz, P-H); 31P-NMR (CDCh, 121.4 MHz): 8 18.9 (s, 1JPt-P 2894Hz);

UV-vis: Amax (s/103 M 1 cm-1) 430 (387), 538 (35.4), 578 sh (9.4) nm; High­

resolution ESI MS: [M - 20Tf f+, accurate mass calculated for

C19oH17sN10NhP4Pt2(+2): 1616.0603, Found: 1616.0803. Anal. Calcd. for

C192H17sF6N10Nh06P4PhS2: C, 65.31; H, 5.08; N, 3.97. Found: C, 65.04; H, 4.86; N,

3.93%.

134

trans-[Pt(NiDAPP)(H2PyTTP)(PPh3)2](0Tf) (11). Recrystallised from

CHCh/pentane to yield 11 (26 mg) in 88% yield. 1H NMR (CDCh, 400 MHz): 8-

2.80 (bs, 2H, NH), 1.47 (s; 36H, tert-butyl-H on 10,20-aryl), 2.70 (s, 3H, 4'-CH3 on

15-tolyl), 2.80 (s, 6H, 4'-CH3 on 10,20-tolyl), 6.80-7.0 (m, 18H, PPh3), 7.15-7.30 (m,

12H, PPh3), 7.57 (m, 6H, m-H on 10,15,20-tolyl), 7.66 (m, 3H, m,p-H on 15-phenyl),

7.75 (t, 2H, J = 1.8 Hz, 4'-H on 10,20-aryl), 7.79 (d, 4H, J = 1.8 Hz, 2',5',-H on

10,20-aryl), 7.84 (m, 2H, m-H on 5-pyridyl), 8.00-8.05 (m, 2H, o-H on 15-phenyl),

8.10 (m, 6H, o-H on 10,15,20-tolyl), 8.18, 8.60, 8.62, 8.65 (each overlapping d, 2H, J

= 4.7 Hz, J3-H on free base porphyrin), 9.27 (m, 2H, o-H on 5-pyridyl), 8.42, 8.62,

8.64, 9.90 (each d, 2H, J = 4.7 Hz, J3-H on Ni porphyrin); 31P-NMR (CDCh, 121.4

MHz): 8 19.5 (s, 1J Pt-P 2907Hz); UV-vis: Amax (c/103 M 1 cm-1) 430 (367), 453 sh

(111), 522 sh (22.4), 536 (25.9), 587 (7.1), 647 (3.5) nm; High-resolution ESI MS:

[M- OTft, accurate mass calculated for C13<#12oN9NiP2Pt(+1): 2195.8170, Found:

2195.8270.

{Bis-[trans-Pt(NiDAPP)(PPh3)2](H2DPyDTP)}(OTf)2 (13). Recrystallised

from CHCh/pentane to yield 13 (22 mg) in 89% yield. 1H NMR (CDCh, 400 MHz):

8 -3.10 (bs, 2H, NH), 1.50 (s, 72H, tert-butyl-H on 10,20-aryl), 2.72 (s, 6H, 4'-CH3

on 10,20-tolyl), 6.80-7.0 (m, 36H, PPh3), 7.10-7.20 (m, 24H, PPh3), 7.64 (d, 4H, J=

8.1 Hz, m-H on 10,20-tolyl), 7.67 (m, 6H, m,p-H on 15-phenyl), 7.73 (t, 4H, J= 1.8

Hz, 4'-H on 10,20-aryl), 7.78 (d, 8H, J= 1.8 Hz, 2',5',-H on 10,20-aryl), 7.85 (m,

4H, m-H on 5,15-pyridyl), 8.00-8.05 (m, 2H, o-H on 15-phenyl), 8.10 (d, 4H, J= 8.1

Hz, o-H on 10,20-tolyl), 8.16, 8.88 (each d, 4H, J 4.7 Hz, J3-H on free base

porphyrin); 9.20 (d, 4H, o-H on 5,15-pyridyl), 8.42, 8.61, 8.63, 9.90 (each d, 2H,

4.7 Hz, J3-H on Ni porphyrin); 31P-NMR (CDCIJ, 121.4 MHz): 8 19.5 (s, 1J Pt-P 2895

Hz); UV-vis: Amax (c/103 M-1 cm-1) 433 (601), 519 sh (34.4), 357 (43.3), 586 (13.5),

648 (7.3) nm; High-resolution ESI MS: [M- 20Tf ]2+, accurate mass calculated for

C2o1H2o2N14NhP4Pt2(+2): 1860.6576, Found: 1860.6544.

{Tetrakis-[trans-Pt(NiD APP)(PPh3)z] (H2 TPy P) }( 0Tf)4 (15).

Recrystallised twice from CHCh/cydohexane to yield 15 (18 mg) in 78% yield. 1H

NMR (CDCh, 400 MHz): 8 -3.2 (bs, 2H, NH), 1.50 (s, 144H, tert-butyl-H on 10,20-

aryl), 6.75-7.10 (m, 48H, PPh3), 7.10-7.30 (m, 72H, PPh3), 7.60-7.70 (m, 12H, m,p-H

on 15-phenyl), 7.78 (bs, 8H, 4'-H on 10,20-aryl), 7.82 (bs, 16H, 2',5',-H on 10,20-

aryl), 7.98 (bd, 8H,J= 5.2Hz, m-H on 5,10,15,20-pyridyl), 8.00-8.10 (m, 8H, o-H on

135

15-phenyl), 8.60 (bs, 8H, J3-H on free base porphyrin (overlapped by J3-H on Ni

porphyrin)), 9.48 (bd, 8H, J = 5.2Hz, o-H on 5,10,15,20-pyridyl), 8.46, 8.61, 8.63

(overlapping), 9.92 (each d, 8H, J= 4.7 Hz, J3-H); 31P-NMR (CDCh, 121.4 MHz): 8

20.3 (s, 1JPt-P 2954Hz); UV-vis: Amax (s/103 M 1 cm-1) 434 (768), 538 (80.1), 587 sh

(20.1), 646 (4.4) nm; FAB MS: [Pt(NiDAPP)(PPh3)2t fragment 1537.2, mass

calculated for most intense peak of cluster C9oH85N4NiP2Pt(+): 1537.5. Anal. Calcd.

for C4o4H36()f12N24Ni4012PsP4S4·CHCh: C, 64.95, H, 4.97; N, 4.49;. Found: C,

64.87; H, 4.85; N, 4.50%.

Acknowledgments. R.D.H. thanks the Faculty of Science, Queensland University of

Technology for a Post-Graduate Scholarship.

Supporting Information Available: 1D, NOESY and COSY 1H NMR spectra of

pentanuclear array 15. This material is available free of charge via the Internet at

http:/ /pubs.acs.org.

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8.0

B.

9.5

lG.O

10.5

SUPPORTING INFORMATION

lD proton Nl\.ffi of 15 recorded in CDCI3

Supp. Info 1. 1D 1H NMR spectrum of 15 recorded in CDCb.

:i H2-P):

II Ni-beta

DQF-COSY of 15 recorded in CDCI3

H2-Py

~ o-Plt W

f! Ni-beta

f Ni-beta & H2-beta

s.o ..

PPb3.Jll"/ ~r

(t .In,Il-Ph

--~-

PPh3

.., " J ,v

Supp. Info 2. DQF-COSY NMR spectrum of 15 recorded in CDCh

141

142

7.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5~~~~~~~~~~~~~~~~~~~~~~· 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 ppm

Supp. Info 3. NOESY NMR spectrum of 15 recorded in CDCh

143

CHAPTER 7

Bromobis(phosphine )platinum(II) as a cation-stabilising

and solubilising substituent on directly-linked and fused

triply-linked diporphyrins

Regan D. Hartnell and Dennis P. Arnold

Manuscript prepared for submission to Organic Letters, 2003.

144

The research presented in this paper is based on work done by myself (R.D .H.) under

the supervision of D.P.A. All of the original compounds that are presented in this

paper were synthesised by myself, following methodologies and procedures that

were develope<l and optimised by me. All of the spectral and optical characterisation

data of these compounds was collected and interpreted by myself. Electrochemical

measurements were preformed by myself. Mass spectra and microanalyses were

obtained commercially from independent providers. The entire manuscript was

written by myself.

145

Bromobis(phosph ine )platin um(ll) as a cation-

stabilising and solubilising substituent on directly­

linked and fused triply-linked diporphyrins

Regan D. Hartnell and Dennis P. Arnold*

Synthesis and Molecular Recognition Program, School of Physical and Chemical Sciences, Queensland University ofTechnology, G.P.O. Box 2434, Brisbane, Australia 4001

ABSTRACT: A platinio-bis(phosphine) 111-organometallic porphyrin monomer of

Zn(TI)DPP (DPP = dianion of 5,15-diphenylporphyrin) was prepared by the

oxidative-addition of a Pt(O) species. A meso-meso directly linked dimeric

porphyrin was prepared from this monomer by Ag(I) promoted oxidative coupling

and planarised to a triply linked diporphyrin. Electrochemical measurements of

this series indicate that the organometallic fragment is a strong electron donor.

Oxidative double-ring closure continues this trend with substantial lowering of the

first one-electron oxidation potential and narrowing of the HOMO-LUMO gap.

Preliminary investigations indicate that the triply linked dimer is easily oxidised to

the cation radical.

*Correspondence to: Dennis P. Amold, phone: +61 7 3864 2482; fax: +61 7 3864 1804; email:

d.arnold@qut.edu.au

146

INTRODUCTION

There has been considerable interest in the investigation of the synthesis and

properties of various types of organometallic porphyrins. These organometallic

porphyrins have been investigated as model compounds for cytochrome P450 and

coenzyme B12 and to provide potential insight into several events of biological

importance. Organometallic porphyrins have also been used for activation of small

molecules. Traditionally the "organometallic" nature of the molecule arises from a

metal-carbon bond between the metal bound in the central cavity of the porphyrin

macrocycle and either an alkyl or aryl moiety of either o- or n-bonding character.1

Synthesis and metal coordination chemistry of porphyrinoid macrocycles with

modified cavities such as inverted or "N-confused" porphyrins2 and azuli- or

carbaporphyrins,3 have attracted considerable interest lately, and have extended the

realm of traditional metalloporphyrin chemistry. Metallated species of these

porphyrin analogues may have a metal-carbon bond; however the centrally

coordinated metal still participates in the organometallic character of the molecule.

There are also several examples of porphyrinoid compounds covalently bound to

organometallic fragments such as metallocenes4 or with one of the porphyrin pyrrolic

rings participating in a 115 -pyrrolyl- n-metal arrangement. 5

Ph

Ph 1

Apart from the examples of organometallic porphyrins discussed above, there is

another group of 11 1-organometallic porphyrins that have been investigated recently

where the metal fragment is directly bonded to the periphery of the macrocycle.

Examples in this area include the bis(mercurated) porphyrins of Smith and co­

workers,6 meso-tellurium trichloride appended porphyrins,7 porphyry! boronates8 and

the recent Grignard-like metalloporphyrin of Therien.9 Our publications have been

147

the only reports of isolated 11 1-organopalladio- and organoplatinioporphyrins. 10-13

This type of transition metal chemistry gives rise to compounds like 1 that are

believ;ed to be involved in palladium catalysed coupling reactions of meso­

haloporphyrins with various substrates.14'15 It is presumed in these examples, that the

key step in these couplings is the oxidative addition of the meso-carbon-to-bromine

bond to a zerovalent palladium species, usually a bis(phosphine) moiety. Several

years ago, we isolated the resulting meso-11 1-organopalladium(II) porphyrin from one

of these reactions. 12 Because such compounds had not previously been studied in

their own right, we are in the process of systematically studying these types of

palladium compounds and their more robust organoplatinum(II) analogues.

As part of this on-going research we wish to investigate new and familiar porphyrinic

systems in which to incorporate these organometallic fragments. By this systematic

study we wish to explore strategies that make it possible to exploit the utility of both

an organometallic fragment and the remarkable optical and electronic properties of

monomeric and multi-porphyrin systems. In this course of study we also hope to

expose these 11 1-meso-platinioporphyrins to a large variety of reagents and reaction

conditions in order to investigate this new area of research fully. Following this line

of study we have prepared several new porphyrin dimers furnished with

organometallic fragments and investigated their chemical, optical and

electrochemical properties.

RESULTS AND DISCUSSION

Ph

Ph

2

Br toluene I go•

6 hr 94%

Scheme 1

Ph Ph

~Ph3 Zn(OAc)2 ~Ph3 Pt-Br Pt-Br PPh CHCI3 / reflux I

3 1 h PPh3

97%

Ph Ph

3 4

Organometallic 11 1-meso-platinio- and palladioporphyrins are readily prepared by the

oxidative-addition of a zerovalent metal precursor to a haloporphyrin. Several

different examples have been published10-13 and encompass a variety of

148

haloporphyrins and ligands. It has been found that the easiest way to prepare those

that utilise monophosphines (e.g. PPh3 or PEt3) is by generating the desired

zerovalent precursor in situ so that it may then immediately react with the desired

haloporphyrin. For, example ifPt(dba)2 (dba = dibenzylideneacetone) is reacted with

triphenylphosphine and 2 in degassed toluene at 90 °C for 6 hours, excellent yields

(>90%) of 3 are achieved. It is postulated that the newly formed zero valent Pt species

[Pt(PPh3)3 or Pt(PPh3)2(dba)] undergoes an oxidative-addition reaction with the

haloporphyrin to form the organometallic mesa-platinioporphyrin. The haloporphyrin

is usually consumed within 1 hour, depending on the nature of the monophosphine

(PPh3 or PEt3), the haloporphyrin (whether Br or I) and the metal coordinated within

the central cavity of the porphyrin macrocycle. It is known10-13 that the initial product

formed after the oxidative-reaction is the cis isomer, which with continued heating

isomerises over a period of approximately 6 hours to give the more stable trans

isomer. These organometallic porphyrins are very air and moisture stable and stable

in solution for several weeks and indefmitely in the solid state. However, as expected

they are quite sensitive to strong acids, which may cleave the metal-carbon cr-bond,

and some halogen exchange may occur when chlorinated solvents are used,

especially in the case of r{mesa-palladioporphyrins. Organometallic r{mesa­

platinioporphyrins may be purified by column chromatography on a silica gel with

no degradation detectable, as long as efforts are taken to ensure that all solvents used

in the chromatographical purification are thoroughly de-acidified. However, the 111-

mesa-palladium analogues do undergo significant decomposition on silica gel to the

analogous parent porphyrin macrocycle. The addition of a base (1% v/v), such as

triethylamine to the eluant during column chromatography and TLC, greatly

improves the chromatographic behaviour of these compounds, especially those that

involve free base porphyrins. The addition of a base avoids any unwanted

protonation of the porphyrin core, which is greatly enhanced by the presence of the

strongly electron donating mesa-organometallic fragment. It should be noted that

although free base porphyrins are used for the oxidative-addition step no platinum

coordination to the central cavity of the porphyrin occurs. Metals may be coordinated

within the central cavity of the porphyrin using usual literature methods, with no

degradation of the organometallic fragment. For example, 4 may be prepared by

refluxing the free base organometallic porphyrin 3 in chloroform with a saturated

solution of zinc(II) acetate in methanol. This metal insertion reaction is easily

149

followed by TLC and UV -visible spectroscopy and is completed within 1 hour. 10 The

metallated porphyrin may be purified from any excess metal salts by simply washing

the product with water, or by filtering the reaction mixture through a short plug of

Si02. Recrystallisation of the residue and collection of the product gives an almost

quantitative yield (97%) of zinc(II) 4.

The Ag(I) promoted dimerisation of 4 was initially investigated using the procedure

of Osuka and co-workers.16-18 This utilises half an equivalent of the one-electron

oxidant (AgPF6) in order to maximise the populations of neutral and radical cationic

porphyrin species. It has been postulated that dimerisation occurs via a mechanism

that involves the nucleophilic attack of the porphyrin radical cation by a neutral

porphyrin. In our case the reaction was very slow under these conditions and the

reaction did not go to completion, even after prolonged reaction times. This is

possibly due to the electron donating effects of the organometallic fragment

stabilising the porphyrinic radical cation. However, it was found that when a five­

fold excess of the oxidant was used the dimerisation proceeded smoothly and the

starting material was consumed within one hour.

Ph Ph 1. AgPF6

2.LiBr 3.Zn(OAc),

4 92%

Ph Ph

5 M =Zn, X= Br 6 M = H2, X= Br 7 M = Zn, X= PF6

PI· X I PPh3 toluene

reflux 2h 68%

Scheme 2

Ph Ph

8

~Ph3 ~t-Br

PPh3

CHCI3 / 1 min ! 1. 2M HCI in MeOH 100% 2. TEA

Ph Ph

Ph Ph 9

~Ph3 ~t-Br

PPh3

A 1 H NMR spectrum of the reaction products showed that there were several

products present. The absence of any porphyrin meso-protons indicated that the

dimerisation did indeed go to completion and the other compounds were various

other meso-meso linked porphyrin dimers as well as the desired dimer 5. The

presence of an NH resonance around -3 ppm indicated that one of the contaminants

150

was the free base analogue 6. We were unable to easily separate the free base and

metallated analogues. However, the TLC did show the presence of a very polar spot,

which is postulated to be either the mono or his halo/PF 6 exchanged product 7, due to

bromide abstraction by excess Ag(I). Instead of separating the desired dimer from

various by-products it was found that the easiest method of purification was simply

to change the by-products into the desired dimer. This was easily carried out by

refluxing the reaction mixture with excess LiBr to revert 7 to the bromoplatinum­

appended porphyrin. Then all free base species were metallated by the addition of

excess zinc acetate as a saturated methanolic solution to the refluxing reaction

mixture. 1 H NMR analysis showed that indeed our postulations were correct and

within one hour only the desired meso-meso linked dimer 5 was present. The reaction

mixture was filtered through a short Si02 plug to remove the excess salts. After

recrystallisation 5 was collected as dark olive green crystals in 92% yield. There are

several significant changes in the 1H NMR spectrum, upon dimerisation of 4. Firstly,

the 1H NMR spectrum of 5 displayed one set of porphyrin resonances indicating that

as expected the two halves of the dimer are equivalent. The absence of any porphyrin

meso-proton resonances and the relative simplicity of the spectrum indicated that

dimerisation had occurred exclusively at the mesa-positions and that no P-coupling

reactions had complicated the preparation of 5. As expected the P-hydrogens nearest

the direct meso-meso link undergo the biggest changes as compared to those of

monomeric 4.10 In the NMR spectrum of 5 the signals of the inner P-hydrogens,

nearest to the meso-meso link, are shifted upfield by ~8 = 0.72 and 0.92 ppm relative

to those of 4, reflecting the shielding effect of the second porphyrin ring. The

resonances of the outer P-hydrogens are only slightly shifted after dimerisation.

Similar changes upon dimerisation have been reported by Osuka and co-workersY

The resonances from the chelated phosphine groups are shifted downfield by

approximately 0.2 ppm in 5 relative to those of 4. The 31P NMR spectra of 5 displays

a singlet resonance at 23.3 ppm for the four equivalent triphenylphosphine groups

and is flanked by 195Pt satellites, with a JPt-P coupling constant of 3003 Hz, a typical

value for trans phosphine groups. The oxidative double-ring closure and

planarisation of 5 was carried out using the methodology of Osuka.19 Thus, the

substrate 5 was refluxed with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)

and scandium triflate [Sc(OT6)] in toluene for one hour. During this procedure the

reaction mixture changed from an olive green to a bright blue/green indicating that a

151

significant reaction had occurred. After purification by column chromatography and

recrystallisation, the desired meso-meso,~-~,~-~-triply linked dimer 8 was obtained in

68% ~ield. The 1H NMR spectrum of8 is relatively simple due to the high symmetry

of the molecule; however it is very indicative that the desired planarisation has

occurred. The outer ~-protons of 8 are seen as a pair of broad doublets (J = 4.4 Hz) at

8.72 and 7.26 ppm. These resonances have been shifted significantly up field by .6.8

= 1.22 and 1.24 ppm, respectively, relative to 5, which demonstrates the large effect

that planarisation and the reduced ring current has on the porphyrin macrocycle.18 In

similar diporphyrins,18 the outer ~-protons occur near 7.70 ppm. In our case, the ~­

protons adjacent to the organometallic fragment are seen at 8.72 ppm, considerably

shifted downfield demonstrating the influence of the organometallic group. The inner

~-proton, nearest to the naphthalene bridging group, is seen as a broadened singlet at

7.19 ppm. The resonances arising from the phosphine groups are shifted slightly,

compared to 5, especially the m,p-proton resonance, which has moved downfield by

0.3 ppm. Complete assignment of all proton resonances was facilitated by several

two-dimensional NMR experiments (DQF-COSY and a proton NOESY). The 31P

NMR spectrum of 8 shows a singlet at 22.7 ppm with a P-Pt coupling constant of

3016Hz.

152

9.0 R8 3.6 R4 8.2 B.O ?.8 7.6 7.4 7.2 7.0 6.!! 6.6 6.4 6

Figure 1. Down:field region of the 1H NMR spectrum of 8 recorded in CDCh before (top) and after

(bottom) the addition of20 (.IL of80% hydrazine.

It was found on several occasions that if the NMR sample was left as a solution

overnight the signal of the porphyrin dimer 8 degraded and no resonances from the

P-H or phenyl groups could be detected. The only resonances detectable were those

of the phosphine groups, which were seen as broad humps. Recrystallisation of the

sample and preparation of a new NMR solution restored the spectrum to that

previously seen. Similar behaviour is known for zinc porphyrins that suffer from

aggregation problems; however these spectra are usually improved by the addition of

pyridine to act as an axial ligand, which effectively reduces aggregation artefacts.

The addition of pyridine-d5 to the NMR sample of 8 did not show any improvement

of the degraded spectrum. Contemplation of these facts has led us to believe that the

porphyrin dimer is being partially oxidised to the radical cation, thus producing a

paramagnetic species and leading to the degradation of the NMR signal. This fact

was further investigated by addition of hydrazine to the NMR sample in order to

reduce the cation radical back to the neutral species. This reduction immediately

restored the signal to full strength, as previously seen in the freshly prepared sample

(see Figure 1). Removal of the hydrazine by high vacuum and preparation of a new

153

NMR sample revealed that the sample initially started in the neutral state and slowly

oxidised to the cation radical. Addition of hydrazine to this sample fully reduced the

cation radical and restored full NMR sensitivity. Similar deterioration in NMR signal

was seen in the 31P NMR, which too was fully restored after the addition of

hydrazine. In order to rule out aggregation phenomena as the cause of the

problematic NMR spectroscopy, a small sample of 8 was fully demetallated with

methanolic 2M HCl, to produce free base 9. Addition of acid to 8, immediately

demetallated and protonated the porphyrin dimer to give a bright purple solution.

Neutralisation of this solution with excess triethylamine caused an immediate

change, and the solution reverted to an intense green/blue colour. The NMR of free

base 9 was similar to that of metallated 8, indicating that cleavage of the Pt

fragments had not occurred and only demetallation of the macrocycle had resulted.

As for 8, the NMR spectrum of 9 dramatically improved after the addition of

hydrazine. This suggests that a small population of both 8 and 9, prefer to exist as the

radical cation in solution and this is easily converted to the neutral species by

treatment with hydrazine. The electrospray ionisation (ESI) mass spectrum (see

Figure 2) of 8 displayed a strong cluster at m/z = 2643.26, which corresponded very

well with the predicted isotope pattern (calculated for [M+Ht

C136H92Br2NsP4Pt2Zn2, m/z = 2643.27) for the assigned structure of8.

2637 2642 r-~r~,~

NNN ,....j~~ ~~~ 1.0\0\,0 NNN

II I

2640

2647

2650 mlz

Figure 2. Experimental (top) and calculated isotopic distribution pattern (bottom) of m + for 8

Electronic absorbance spectra

300 400 500 600 700 600 900 1000 1100 1200 1300

Wavelength (nm)

154

Figure 3. The electronic absorption spectra of monomeric 4 (-), meso-meso linked

5 (-) and triply linked 8 (-).

155

The electronic absorption spectra of monomeric 4 and dimeric 5 and 8 are compared

in Figure 3. From this it can be seen that the monomeric 4 displays a normal

electronic spectrum for a zinc metallated porphyrin. The Soret band can be seen at

430 nm with the weaker Q-band absorbances at 555 and 599 nm. Several interesting

changes are seen after dimerisation through a meso-meso link. The most dramatic of

these is the splitting of the Soret band in 5, which can be seen by the manifestation of

two peaks at 437 and 470 nm of similar intensity. Understandably there is a reduction

in the extinction coefficient of these two peaks, as compared to monomeric 4. The

presence of split Soret bands indicates that the planes of the two porphyrins are

essentially perpendicular to each other, possibly in a D2 twisted conformation.18'20

This has been interpreted as strong through-space electronic interaction between the

two porphyrin chromophores, however they are not conjugated. 18 The Q-bands of 5

are also red-shifted by 20 nm compared with 4. After oxidative double-ring closure

the absorption spectrum of 8 is heavily perturbed and indicative of a totally

conjugated bisporphyrin system.18'20 This displays a large splitting of the bands (I

and II) formerly attributable to the Soret bands in 4 and 5 and the lowest energy

band, (III), extends well into the near infra-red region beyond 1100 nm.

Qualitatively, it is understood that the forced planarisation of 8 removes the

coincidental degeneracy of the egx and egy orbitals. Bands I and II have been assigned

to the Soret-like transitions along the two axes of the molecule (short and long,

respectively). The third band arises from formerly forbidden Q-band-like transitions

in monomeric D4 symmetrical porphyrins. 18

The absorbance spectra of Zn(II) 8 and free base 9 did not show any substantial

changes after the addition of hydrazine. This indicates that there is only a small

population of the radical cation in solution that is not detectable by either UV-visible

or near infra-red spectroscopies. In the case of 8, there was a slight red-shift of all

peaks after the addition of hydrazine, which we interpreted as a manifestation of the

axial coordination of hydrazine to the zinc(II) centres.

Electrochemical investigations

Compound

ZnDPP

4

5

8

-1.2

-1.4

-0.76

0.85

0.75

0.70a 0.81a

-1.0 0.20

a Split 1st oxidation potential- one electron per porphyrin macrocycle.

1.3

1.1

1.3

0.62

156

Table 1. Electrochemical data for precursor (ZnDPP), monomeric 4 and dimeric 5

and 8 (in B114NPF6-CH2Ch vs. Ag/AgCl, 293 K)

The electrochemical data for precursor ZnDPP (DPP = dianion of 5,15-

diphenylporphyrin) , monomeric 4 and dimeric 5 and 8 are collected in Table 1.

Initially we wished to investigate the effect of the organometallic fragment on the

redox properties of the porphyrin ring. Measurements were carried out by cyclic

voltammetry on ZnDPP as a reference compound, and platinum complexes 4, 5, and

8. Experiments were carried out in CH2Ch containing 0.5 M tetrabutylammonium

hexafluorophosphate (B114NPF 6) as the supporting electrolyte, with a standard three

electrode cell under an atmosphere of nitrogen. Electrochemical measurements of

ZnDPP were carried out on a saturated solution, due to poor solubility of ZnDPP in

the electrolyte rich solvent which gave rise to relatively high background current.

However, interestingly for Pt complexes 4, 5, and 8 the presence of an electrolyte

rich solvent seemed to improve the solubility of the substrate, so the measurements

were performed on solutions with a porphyrin concentration of approximately 1 x 1 o-3 M. Unfortunately, in our hands, accurate measurements of both 1st and 2nd reductive

electrochemical waves could not be made on all samples and these measurements are

planned for future investigation. However, the full cyclic voltammogram of 8

including 1st and 2nd reduction and oxidation potentials was measured and revealed

some interesting electrochemical properties of this compound. The Pt-containing

species showed good reversible waves, except for 5 which underwent some

degradation at high positive potential during the second oxidation. The first oxidation

potentials of ZnDPP and 4, 0.85 V and 0.75 V (Fe/Fe+ at + 0.55 V) respectively,

clearly demonstrated the effect of the platinio-fragment. This effect was seen as a

cathodic shift of approximately 100 mV, which supports the observation that the

157

mesa-bonded -Pt(PPh3) 2Br fragment has a strong electron donating effect on the

porphyrin macrocycle. However, this cathodic shift brought about by the platinum

moiety is somewhat smaller than that reported for similar free base and Ni(II) mesa­

platinioporphyrins 10 that underwent a larger cathodic shift of approximately 300 m V.

A second fully reversible oxidation of 4 was seen at 1.41 V (!:illox = 0.39 V). The 1st

reduction half-wave potential of 4 is seen at -1.18 V and is fully reversible, which

translates to a HOMO-LUMO gap of approximately 1.92 V. After dimerisation,

meso-meso linked 5 displayed split 1st redox potentials, which have been observed

for other meso-meso linked dimers18 and have been interpreted as the removal of one

electron from each of the orthogonal porphyrin macrocycles?1 Using Osuka's18

definition, the split 1st and 2nd oxidation potentials for 5 occur at 0.70 V and 0.81 V

(!:ill = 113 mV), respectively, which compare well with that of the meso-meso

analogue lacking the organometallic fragments, which displays a difference in split

first oxidation potentials of 130 mV. Unfortunately, direct comparison of actual half­

wave potentials are not possible due to different solvents and electrolyte

concentrations used for the electrochemical measurements of our compounds and

those in the literature, however similar trends are seen.18 The 130 m V difference

between the two split 1st oxidation potentials of 5 indicates that there is little

electronic conjugation between the two porphyrin rings. Comparison of the 1st

oxidation potentials between 4 and 5, 0.75 V and 0.70 V respectively, confirms that

there is only slight elevation of the HOMO after dimerisation. The 1st reduction

potential of5 occurs at -1.39 V, and gives a HOMO-LUMO gap of2.09 V which is

comparable to that of 4 and also demonstrates very little electronic conjugation

between the individual porphyrin macrocycles.

158

-3

-2

-1

3

4

- .5 -1 -0.5 0 0.5 1.5

5L-------~------~----~--~~----~-------L------~ Potential (V)

Figure 4. Cyclic (100 mV s-1) voltammogram of triply linked 8 in CH2Ch-Bll4NPF6

at 293 K.

Planarisation of 5 via oxidative double-ring closure results in several interesting

electrochemical consequences. The first of these is the dramatic lowering of the 1st

oxidation potential (E1 ox 0.20 V) by approximately 500 m V when compared to 4 and

5. The 2nd fully reversible oxidation occurs at 0.62 V which translates to a ~x value

of0.42 V for 8. The 1st and 2nd reduction potentials were seen at -0.76 V and -1.02 V,

respectively, (Mred = 0.26 V) and were found to be reversible. From these half-wave

potentials it can be seen that planarisation to form 8 results in substantial raising of

the HOMO and lowering of the LUMO, to give a HOMO-LUMO gap of 0.964 V,

less than half of that for dimeric .5. This HOMO-LUMO gap is similar of that

reported by Anderson et al.20 for their quinoidal triply linked porphyrin dimer, which

has a HOMO-LUMO gap of 1.00 V.

CONCLUSION

In summary, it has been shown that monomeric meso-r{organometallic porphyrins

are facilely dimerised by Ag(I) oxidative coupling to give the meso-meso linked

dimer in high yields. Oxidative double-ring closure is easily carried out and provides

easy access to triply linked porphyrin dimers furnished with organometallic

159

fragments. Electrochemical measurements demonstrated that the organometallic

fragment is a good electron donor. This trend was further exacerbated after fusion

and ~lanarisation to generate a very easily oxidisable triply-linked dimer, with a

reduced HOMO-LUMO gap. Detailed spectroelectrochemical experiments to further

deduce and characterise the oxidised and reduced forms of this dimer are underway.

Methods for incorporating these advanced organometallic porphyrins into

supramolecular arrangements and for attachment to surfaces and electrodes are

planned for the future.

Experimental Section

General Procedures. Syntheses involving zerovalent metal precursors were carried

out in an atmosphere of high-purity argon using conventional Schlenk techniques.

Porphyrin starting material, 5,15-diphenylporphyrin15'22 and Pt(dba)l3 were prepared

by literature methods. All other reagents and ligands were used as received from

Sigma-Aldrich. Toluene was AR grade, stored over sodium wire, and degassed by

heating and purging with argon at 105 °C. All other solvents were AR grade, and

dichloromethane and chloroform were stored over anhydrous sodium carbonate.

Analytical TLC was performed using Merck silica gel 60 F2s4 plates and column

chromatography was performed using Merck silica gel (230-400 mesh). NMR

spectra were recorded on Bruker Avance 400 MHz or Varian Unity 300 MHz

instruments in CDCh solutions, using CHCb as the internal reference at 7.26 ppm

for 1H spectra, and external 85% H3P04 as the reference for proton-decoupled 31P

spectra. UV -vis spectra were recorded on a Cary 3 spectrometer in dichloromethane

solutions. NIR measurements were carried out on a Nicolet Nexus FT-IR equipped

with a quartz halogen lamp, quartz beam splitter and an InGaAs detector operating at

room temperature. ESI mass spectra were recorded on a Bruker BioApex 47e FTMS

fitted with an Analytica Electrospray Source. The samples were dissolved in

dichloromethane and diluted with either dichloromethane/methanol 1: 1 or methanol

and solutions were introduced into the source by direct infusion (syringe pump) at 60

)..lLih, with a capillary voltage of 80 V. The instrument was calibrated using internal

Nal clusters. Elemental analyses were carried out by the Microanalytical Service,

The University of Queensland. For the electrochemical measurements, a standard

three-electrode configuration was used, with a Pt wire working electrode immersed

in the solution and a Pt button counter electrode. The reference electrode comprised a

160

silver wire coated with silver chloride separated from the working solution by two

fritted jackets. The internal chamber containing the wire was filled with 0.05 M

Bl4NC1-0.45 M Bli4NPF6, and the external chamber contacting the working solution

was filled with 0.5 M Bti4PF6, all in CH2Ch. The working solution was pre-purged

and subsequently protected with very high purity N2. The voltammetry was carried

out in CH2Ch (freshly distilled from CaH2 under argon) containing 0.5 M Bli4NPF6

using EG&G Princeton Applied Research Model 272A Potentiostat linked to a PC

interface and controlled by PowerSuite software. Electrode potentials are reported

using ferrocence as an internal standard (E0 (ox) = +0.55 V).

trans-Bromo[l 0,20-diphenylporphyrinato-5-yl]bis(triphenylphosphine)

platinum(II) 3.

Toluene (200 cm3) was added to a Schlenk flask and degassed by bubbling argon

through the solution at 90 °C under an atmosphere of argon. Bromoporphyrin 2 (200

mg, 0.37 mmol) was added and stirred for 5 min. Pt(dba)2 (290 mg, 0.44 mmol) and

triphenylphosphine (346 mg, 1.32 mmol) were added and the solution stirred at 90

°C. TLC analysis (50% CHCh/hexane-1% Et3N) of the reaction mixture clearly

showed disappearance of the starting material after ca. 30 min and that the initially­

formed cis isomer was slowly being converted to the trans isomer. After ca. 6 h the

isomerisation was considered complete and the reaction mixture was cooled to room

temperature and the solvent removed under in vacuo. The residue (now air stable)

was purified on a Si02 column eluting with 50% CHCh/hexane-1% Et3N and the

major purple fraction was collected and the solvent removed in vacuo. The residue

was recrystallised from CHCh/pentane to give 431 mg of 3 as dark purple crystals in

94% yield. The spectroscopic data (1H and 31P NMR) of this compound agreed well

with that of a genuine sample prepared previously using Pt(PPh3)3. 13

trans-Bromo[l 0,20-diphenylporphyrinatozinc(II)-5-yl]bis(triphenylphosphine)

platinum(II) 4.

This was prepared in a similar manner to that outlined previously.1° Free base 3 (200

mg, 0.16 mmol) was dissolved in CHCh (100 cm3) and Zn(OAc)2 (200 mg, 1.1

mmol) added as a methanol solution. The solution was refluxed for 1 h and the

solution cooled to room temperature. This solution was concentrated to

approximately 50 cm3 and added to the top of a short Si02 chromatography column.

161

The column was eluted with CHCh and the major purple/green band collected. The

solvent was removed in vacuo and the residue recrystallised from CHCh/pentane to

give ~08 mg of 4 as metallic purple crystals in 97% yield. The spectroscopic data (1H

and 31P NMR) ofthis compound agreed well with that of a genuine sample prepared

previously. 10

meso-meso-linked platinated dimer 5.

To a solution of monomer 4 (100 mg, 0.075 mmol) in CHCh (50 cm3), AgPF6 (95

mg, 0.375 mmol) was added as an acetonitrile solution. The reaction vessel was

protected from light and stirred for 1 h. To the reaction mixture LiBr (100 mg, 1.15

mmol) was added and the solution refluxed for 30 min. A methanolic solution of

Zn(OAc)2 (100 mg, 0.55 mmol) was added and the solution further refluxed for 1 h.

The reaction mixture was cooled to room temperature and the solvent removed in

vacuo. The dark green residue was purified by column chromatography (66% CHCh/

hexane) and the major band collected. The solution was evaporated to dryness and

the residue recrystallised from CHCh/pentane to give 91 mg of dimer 5 in 92% yield.

1H-NMR: o 6.40-6.60 (m, 36H, PPh3), 7.30-7.40 (m, 12H, PPh3), 7.55-7.65 (m, 12 H,

m,p-H on 10,20-phenyl), 8.05-8.15 (m, 8H, o-H on 10,20-phenyl), 7.98, 8.47, 8.50,

9.30 (each d, 4H, J = 4.7 Hz, 13-H); 31P-NMR: o 23.5 (s, JPt-P 3003 Hz); UV- vis:

Amax (s/103 M-1 cm-1) 437 (210), 470 (221), 571 (42.0), 610 (17.0) nm; Anal. Calcd.

for C136H96Br2NsP4Pt2Zn2: C 61.71, H 3.66, N 4.23, Found: C 61.68, H 3.52, N 4.16.

fl-fl,Jl-fl,meso-meso-linked platinated dimer 8.

Directly meso-meso-linked dimer 5 (50 mg, 0.019 mmol)was dissolved in dry

toluene (25 cm3) under an argon atmosphere. Sc(OTf)3 (47 mg, 0.095 mmol) and

DDQ (22 mg, 0.095 mmol) were added to the reaction solution which was refluxed

for 1 h. The mixture was cooled to room temperature and the solvent removed in

vacuo. The residue was dissolved in a minimum of ethyl acetate and loaded onto a

short Si02 and eluted with ethyl acetate. The blue/green fraction was collected, the

solvent removed and the residue recrystallised from CH2Ch I cyclohexane. The

product was collected by filtration as a very dark blue/green solid (34 mg, 68%). 1H-

NMR: o 6.80-6.90 (m, 36H, PPh3), 7.19 (s, 4H, 2,2',8,8'-13-H), 7.26 (d, 4 H, J= 4.4

Hz, 12,12',18,18'- 13-H), 7.30-7.50 (m, 24H, PPh3) 7.50-7.60 (m, 12 H, m,p-H on

162

10,20-phenyl), 7.70-7.80 (m, 8H, o-H on 10,20-phenyl), 8.72 (d, 4 H, J = 4.4 Hz,

13,13',17,17'- P-H); 31P-NMR: 8 22.7 (s, JPt-P 3016Hz); UV- vis: Amax (c/103 M-1

cm-1) 426 (59.7), 489 (40.5), 608 (88.5), 670 sh (25.2), 936 sh (11.9), 1066 (33.4)

nm; ESI MS: [M+Ht, mass calculated for C136H93Br2NsP4PhZn2(+1): 2643.27,

Found: 2643.26; Anal. Calcd. for C136H92Br2NsP4Pt2Zn2.2CH2Ch : C 58.93, H 3.44,

N 3.98, Found: C 58.79, H 3.28, N 3.91.

Free base p-p,p-p,meso-meso-linked platinated dimer 9.

To a stirred solution ofmetallated 8 (3 mg, 0.0014 mmol) in CHCh, HCl (500 J..LL of

a 2M solution in methanol) was added. The solution immediately turned purple and

was neutralised with Et3N (1 em\ after which it changed back to a deep blue/green

colour. The CHCh layer was washed twice with water and dried over anhydrous

MgS04. The solvent was removed in vacuo and dried thoroughly under high vacuum

at 50 °C for 2 h. Free base 9 was formed quantitatively and pure as considered by 1H

and 31P NMR. The signal arising from the free base NH protons could not be

definitively identified, and is possibly obscured by the broad H20 signal near 1.5

ppm, which is where this NH occurs, as judged by similar compounds. 18 1H-NMR: 8

6.80-6.90 (m, 36H, PPh3), 7.0 (s, 4H, 2,2',8,8'-P-H), 7.19 (d, 4 H, J = 4.0 Hz,

12,12',18,18'- P-H), 7.40-7.50 (m, 24H, PPh3) 7.50-7.60 (m, 12 H, m,p-H on 10,20-

phenyl), 7.60-7.70 (m, 8H, o-H on 10,20-phenyl), 8.74 (d, 4 H, J = 4.0 Hz,

13,13',17,17'- P-H); 31P-NMR: 8 23.3 (s, JPt-P 2996Hz);. UV- vis: Amax (rel. int.)

422 (4.9), 488 (3.5), 604 (6.0), 934 sh (1.0), 1055 (2.1) nm.

Acknowledgements.

R.D.H. thanks the Faculty of Science, Queensland University of Technology for a

Post-Graduate Scholarship.

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166

Chapter 8

Discussion, Conclusions, and Future Work

167

8. Discussion, Conclusions, and Future Work

8.1 Discussion and Conclusions

Platinio- and palladio-111-organometallic porphyrins have been shown to be an

interesting field of chemical research that requires further investigation. Many of the

systems and methodologies presented in this thesis are just the starting point for

further work that is required in order to appreciate fully the possible potential of the

area. Porphyrins and related macrocycles have been shown to take part in critical

biological events and are intimately involved in current research that is underway for

the preparation of realistic systems or devices for future technologies. This research

is not only of interest to chemists but reaches across the many disciplines of science.

Organometallic porphyrins offer an attractive approach for combining the properties

of a porphyrin core with those of an organometallic fragment. The organometallic

fragment may be useful by itself or utilised as a linking group to arrange numerous

macrocycles in diverse, well-defined arrays with predictable architectures. This

ideology allows for the rational design of structures that are able to incorporate

several attractive tectons in one self-assembly step.

Triarylporphyrins of the 5,10,15 substitution pattern 4.6a,b and 4.7a,b are formed in

a clean and selective manner by the methods discussed in chapter 3 and 4.

Bromination of these porphryins is then directed exclusively to the remaining meso

position, which provides easy access in high yields to highly soluble porphyrin

starting materials. These halogenated porphyrins are not just of use to our research

group but are utilised across the entire field of synthetic porphyrin chemistry. This

methodology may be easily scaled up to gram-quantities.

These triarylporphyrins (Chapter 3) were simply functionalised with a terminal

alkyne moiety, which after deprotection were easily coupled to give the appropriate

butadiyne-linked dimer 3.1. Then-radical cation and anions of the dizinc complex of

this bis(triarylporphyrinyl)butadiyne, 3.1+ and 3.r, respectively, displayed

remarkable spectral similarity in the near-IR region. These observations were in

accord with the predictions of a frontier orbital model for inter-porphyrin bonding

across the conjugated bridge. The oxidised species had not been previously detected

168

spectroscopically, for any alkyne-linked porphyrin dimers due to molecular

aggregation of the charged intermediates. The incorporation of the bulky aryl

substituents allowed for these species to be spectroelectrochemically detected for the

first time.

Easy' access to the triarylporphyrin starting materials enabled the preparation of

meso-11 1-platinioporphyrins in a convenient manner. This series of compounds 4.8a,b

are significantly more soluble in common solvents than the DPP analogues. The

alkylation of complexes of the type trans-[PtBr(porphyrin)(PPh3) 2] 4.8a,b with

strongly basic organometallic nucleophiles occurs at the opposite meso carbon 4.3,

unless the latter is already substituted. In that case, butyl- or phenyllithium attacks at

platinum, but reductive elimination occurs to extrude the Pt(O) moiety and form a

new porphyrin-carbon bond. However, less basic nucleophiles produced by the

combination phenyl- or porphyrinylethyne with catalytic Cui form stable

diorganoplatinum(II) bis(phosphine) complexes such as 4.12 and 4.14. Oxidative

addition of meso-bromoporphyrins to platinum(O) complexes of trialkylphosphines

was demonstrated for the first time, and the smaller bulk of triethylphosphine

compared with triphenylphosphine significantly increases the stability of the initial

cis adduct. This methodology enables a choice (cis or trans) of the desired orientaion

of the phosphine ligand around the Pt(II) centre.

It was shown in chapter 5 and the associated publication that the combination of

Pt( dba )2 and monodentate phosphine ligands with various haloporphyrins is an

effective way of synthesising 111-organoplatinum porphyrins in high yields. An

advantage of this method over previous methods is that it avoids the use of air- and

moisture-sensitive Pt(O) phosphine complexes. This methodology was extended for

the first time to synthesise various palladio- and platinioporphyrins that utilise

bidentate nitrogen donor ligands in order to enforce a cis configuration of the metal

centre. This offers a new possibility for the preparation of anti-cancer drugs

comprising the cyto-toxic effects of a cis coordinated platinum fragment with the

photodynamic effect of a porphyrin macrocycle, for example compound 5.12. This

combination should prove useful as intercalators which bind to the DNA through the

square-planar metal centre through a combination of electrostatic and n-n

interactions. The presence of the planar aromatic macrocycle may also disrupt the

169

stacking of n-stacked nucleobases on the major groove side. Extensive studies on

porphyrin macrocycles for photodynamic therapy have shown that they selectively

accun;mlate in tumour cells, where the photodynamic effect of porhryins may also

lead to increased effect on tumours. These different strategies of cancer activity may

combine well to produce novel candidates with anti-cancer properties. The

availability of these stable cis orientated organometallic porphyrins will also allow

for further variation of configurations and architectures when incorporated with other

suitable tectons in self-assembled supramolecular systems.

In chapter 6, it was shown that ligand exchange of the halide for weakly basic

ligands is easily and efficiently achieved under mild conditions. Several ligand­

exchanged examples were prepared, nitrato 6.6 and triflato 6. 7 for example, and

allowed for the synthesis of several self-assembled entities. These examples,

especially 6. 7, were incorporated into multi porphyrin arrays and predetermined

structural characteristics of the individual tectons were used to control the

architecture of the newly self-assembled multicomponent systems. The mesa­

organometallic porphyrins formed stable conjugates with various pyridine ligands

and pyridyl-substituted porphyrins. The newly self-assembled arrays were found to

be very stable in solution for extended periods and in the solid state indefinitely.

These conjugates self-assemble selectively and in high yields when the individual

components are reacted in stoichiometrically equivalent ratios. The newly formed

arrays are highly soluble in common organic solvents, a property that stems from the

use of trisubstituted parent monomers, and this avoids solubility problems that have

plagued other similar assemblies. High-resolution mass spectral data and single and

multi-dimensional NMR experiments have unambiguously established their

structures in solution.

Porphyrin dimers appended with organometallic fragments were synthesised and

discussed in chapter 7. In summary, it was been shown that monomeric mesa-T]1-

organometallic porphyrin 7.4 is readily dimerised by Ag(I) oxidative coupling to

give the meso-meso linked dimer in high yields. Oxidative double-ring closure was

easily carried out and provided access to triply linked porphyrin dimers 7.8 and 7.9

furnished with organometallic fragments. Electrochemical measurements on the

monomer demonstrated that the platinio-organometallic fragment is a good electron

170

donor. This trend was further reinforced after fusion and planarisation to generate a

very easily oxidisable triply-linked dimer, with a reduced HOMO-LUMO gap. NMR

experiments demonstrated that the fused dimers 7.8 and 7.9 are partially oxidised in

solution to the n-cation radical species. The charged species were easily reduced with

aqueous hydrazine to the neutral species, which was accompanied by a dramatic

improvement in the appearance of the 1 H NMR signals of these compounds.

8.2 Future Work

Research never ends! Any half-rate researcher, student or academic can always think

of a couple more compounds to make, areas to investigate or experiments to try; ''just

quickly to see where it leads". Unfortunately, most of these little explorations

unearth more questions than they answer. However, more research is needed in this

almost virginal area of 11 1-meso-organometallic porphyrins. I am the first PhD

student to complete a thesis in this area, although I am sure more will follow. Some

basic foundations have been laid upon which many avenues may be built in order to

investigate fully and possibly realise useful real-world applications of this interesting

area of porphyrin research.

Methodologies for the incorporation of a wider range of metal fragments need to be

deduced. The oxidative addition methodology ideally lends itself to the incorporation

of Pd(II), Pt(II) and possibly Ni(II) fragments, however, the preparation of other

metallated systems would greatly extend this area of research.

Ph

Ph

organolithium reagent Rli

THF

R

H

Ph

Ph

R

H

Ph

Ph

! [0]

Ph

Ph

Figure 8.1 Preparation of organometallic porphyrins by organolithium reagents.

171

Some of the negative results that were originally part of this research project were

associated with this area. Detailed experiments were attempted with nucleophilic

porphyry! anions that were prepared by the action of organolithium reagents, (see

above) however the intermediate anion is thought to be too weakly nucleophilic to

add effectively to a metal fragment. The recent report of Grignard porphyrin

analogues may allow for the rapid preparation of a large number of various

metallated derivatives. This basic methodology is outlined below.

Ph

Ph

"activated Rieke Mg" Br

THF/ overnight

Ph

MgBr

Ph

For example M'Ln = CO(PPh3)zRh, Sn(n-Buh, AuPEt3, PPh2 etc.

Ph

Ph

Figure 8.2 Preparation of organometallic porphyrins from a Grignard intermediate.

The preparation of different organometallic porphyrins may extend the number of

examples in this area, as well as the possible applications of these types of

compounds.

172

Preparation of water-soluble porphyrins furnished with a cis square-planar Pt­

fragment with bidentate N-donor ligands, is required to evaluate the anti-cancer

properties of such a molecule. Detailed analysis with and without irradiation will

enable the cytotoxicity and ph ototoxicity of such a system to be investigated.

The 'Utilisation of r{meso-organometallic porphyrins for the preparation of

multiporphyrin arrays has been shown to be an effective way of arranging desirable

fragments in a controlled fashion with very predictable geometry. This fundamental

design approach will allow for the possible incorporation of a large number of

versatile components tailored to the specific requirements of the assembly. Various

attractive components such as different metalloporphyrins, and various electron

donor or acceptor fragments may be included as either substituents on a 11 1-

organometallic platinioporphyrin or as the pyridine-substituted moiety which is

directly coordinated to an organometallic porphyrin. The ability to tailor the surface

properties by the use of other substituents on the aryl rings and alternative phosphine

or other ligands on the Pt(II) cationic centre is an advantage. This methodology

should make it possible to attach individual porphyrin macrocycles to surfaces via

the appropriately substituted pyridyl ligand in combination with an organometallic

porphyrin.

Conducting glass (Sn02-F)

\

1111

lO,um

Nonporous-nanocrystalline Ti02

Figure 8.3 Utilisation of an organometallic porphyrin for attachment to a

nanoporous-nanocrystalline Ti02 surface.

173

For example, as shown above, a 11 1-organometallic platinioporphyrin coordinated to

an is~nicotinic fragment will allow for attachment to a nanoporous, nanocrystalline

TiOz film. Examples like this may be useful for the preparation of dye-sensitised

solar cells.

The specific design of porphyrinic arrays that incorporate long alkyl chains on the

porphyrin macrocycle, will allow for good stability on suitable surfaces, such as

highly ordered pyrolytic graphite (HOPG). Stable arrays that are attached to HOPG

will allow for scanning tunnelling microscopic (STM) imaging of these arrays at the

molecular level. This will further increase the knowledge base of both porphyrinic

arrays and STM imaging of these types of systems.

Preparation of heteronuclear arrays like that shown in Figure 8.4, will make it

possible to carry out detailed experiments in order to deduce the nature of electron

transfer within the arrays. The incorporation of differently metallated porphyrins may

facilitate tailored input/output functionality for advanced technologies. Detailed

spectroelectrochemical experiments to further characterise the oxidised and reduced

forms of these arrays may also be very informative.

~Ph3 ()-C" Pt-OTf I PPh3

j )Ph3

Pt-OTf I PPh3

Figure 8.4 Strategy for the simple preparation ofheterometallated porphyrin arrays.

174

The spectroelectrochemical studies of molecules like 7.8 and 7.9 are required in

order to clarify the exact nature of oxidised and reduced species. The ability of such

a porphyrin dimer to be so easily oxidised is an interesting fact in itself and requires

further attention. The coupling of such a triply-linked porphyrin dimer furnished with

organometallic fragments with a simple electron acceptor (as shown in Figure 8.5)

should produce a stable charge separated state.

L-Pt

·""'I L

Pt-L 1/ L

Figure 8.5 Donor-acceptor arrangement of a porphyrin dimer and a naphthalene

his( dicarboximide ).

Many of the possible uses of porphyrin-containing systems are dependant on their

ground and excited state photophysical properties. Further investigation in this area

is required for those that utilise 11 1-organometallic porphyrins, as little is known.

Whether energy transfer does occur and the exact nature and mechanisms of this

energy transfer needs to be systematically determined in order to fully characterise

already prepared and other potential supramolecular systems. Time-resolved

spectroscopic methods may be used in order to shed light on these details.

In summary, 11 1-organometallic porphyrins offer a rich area of chemical research.

This research project has initiated some preliminary investigations that exploit the

properties and characteristics of organometallic porphyrins in various ways.

Research never ends and neither will the investigation of remarkable compounds like

porphyrins and related macrocycles.