ISOCYANIDE COMPLEXES OF RHENIUMXANDRI SCHOULTZ Submitted in partial fulfilment of the requirements...

ISOCYANIDE COMPLEXES OF

RHENIUM

X. SCHOULTZ

2013

ISOCYANIDE COMPLEXES OF

RHENIUM

by

XANDRI SCHOULTZ

Submitted in partial fulfilment of the requirements for the

degree of

Magister Scientiae

in the Faculty of Science at the

Nelson Mandela Metropolitan University

December 2013

Supervisor: Prof. T.I.A. Gerber

Table of Contents

i | P a g e Nelson Mandela Metropolitan University

Table of Contents

Contents i

Acknowledgements iii

Abstract iv

Crystallographic Data vi

Abbreviations vii

Chapter 1

Introduction

1.1 Rhenium Background 1

1.2 Aim and Motivation of Research 2

1.3 Rhenium in Nuclear Medicine 3

1.4 Low Valent Rhenium Chemistry 7

1.5 High Valent Rhenium Coordination Chemistry 14

1.6 References 18

Chapter 2

Experimental

2.1 Handling of Rhenium 23

2.2 Materials 23

2.3 Instrumentation 26

2.4 References 27

Table of Contents

ii | P a g e Nelson Mandela Metropolitan University

Chapter 3

Complexes of Aniline Derivatives Containing the ReCl3(CNR)

Core

3.1 Introduction 29

3.2 Experimental 32

3.3 Results and Discussion 35

3.4 References 59

Chapter 4

Isolation of Bis-isocyanide Complexes of Re(I)

4.1 Introduction 72

4.2 Experimental 73


4.4 References 85

Chapter 5

Reactivity of Re(V) with Isocyanides

5.1 Introduction 92

5.2 Experimental 93


5.4 References 102

Chapter 6

Conclusion and Future Work 109

Acknowledgements

iii | P a g e Nelson Mandela Metropolitan University

Acknowledgements

I would like to express my very great appreciation to Prof. T.I.A. Gerber for the

opportunity of working under his supervision. All his patient guidance, enthusiastic

encouragement and useful critiques are much appreciated and is truly motivating.

I would like to offer my special thanks to Dr. E. Hosten, Dr. R. Betz and Dr. B.J.A.M van

Brecht for their assistance with the crystallographic analysis.

I am particularly grateful to H. Schalekamp for the exceptional technical assistance he

has given me.

Technical assistance given by E. Bashman, P. Gaika, H. Marchant and V. Maqoko are

also greatly appreciated.

My special thanks are extended to all the rhenium chemistry students and other

colleagues who are so helpful and motivating.

I am grateful toward Nelson Mandela Metropolitan University, National Research

Foundation and NTemBi/NECSA for financial assistance.

I would also like to express my gratitude toward my family and friends for their moral

support and encouragement.

Finally, I would also like to thank God for giving me this amazing opportunity, and for

giving the strength and determination to succeed.

Abstract

iv | P a g e Nelson Mandela Metropolitan University

Abstract

This study investigates the synthesis of rhenium(III)-isocyanide complexes with

potentially bidentate ligands, as well as the reactivity of isocyanide ligands toward

rhenium(I) and (V). The crystal structures of all the complexes have been determined by

X-ray diffraction methods.

The coordination behaviour of trans-[ReIIICl3(t-BuNC)(PPh3)2] with aniline and its

derivatives were investigated. The isocyanide-containing rhenium(V) complexes

[ReCl3(t-BuNC)(L)(PPh3)] were isolated, with the ligands H2L (aniline, o-

phenylenediamine and anthranilic acid). In all these complexes the dianionic ligands L

are coordinated monodentately as the imide. However, with 2-aminophenol the

complexes [ReVCl2(t-BuNC)(L)(PPh3)2](ReO4) and [ReIIICl2(t-BuNC)(ibq)(PPh3)2] (ibq- =

2-iminobenzoquinonate) were identified as the products. [ReCl2(t-

BuNC)(L)(PPh3)2](ReO4) is the product of a disproportionation reaction from Re(III) to

Re(VII) and Re(V). All the above complexes show a distorted octahedral geometry

around the rhenium.

The products of the reaction of the Re(I) complex [Re(CO)5Cl] with isocyanides (tert-

butyl- and cyclohexylisocyanide) are reported. Rhenium(I) tricarbonyl complexes of the

form [Re(CO)3(CNR)2Cl] were isolated and they were characterized structurally and

spectroscopically. The tricarbonyls are coordinated in the typical facial-fashion, whereas

the isocyanides are coordinated cis to each other. The reaction of [Re(CO)3(t-BuNC)2Cl]

with H2O led to the formation of the rhenium(I) complex [Re(CO)3(t-BuNC)2(OH2)] in

which the aquo ligand can readily be substituted by a more complex ligand.

Abstract

v | P a g e Nelson Mandela Metropolitan University

The reaction of the rhenium(V) complexes cis-[ReO2I(PPh3)2] and mer-

[ReOCl3(SMe2)(OPPh3)] with isocyanides were studied. The seven-coordinate trigonal

prismatic, square faced monocapped rhenium(III) complex [ReI3(t-BuNC)3(PPh3)] was

surprisingly isolated upon reacting cis-[ReO2I(PPh3)2] with tert-butyl isocyanide. The

dimeric complex (μ-O)[ReOCl2(CNC6H11)2]2 was obtained from the reaction of mer-

[ReOCl3(SMe2)(OPPh3)] with cyclohexyl isocyanide.

Keywords: Rhenium, isocyanides, aniline, derivatives, dimeric

Crystallographic Data

vi | P a g e Nelson Mandela Metropolitan University

Crystallographic Data

Additional data and refinement details for all crystal structures analysed in this study are

stored on the compact disk included in this thesis (attached at the back).

These crystallographic data include the following:

All crystal data and details of the structure determinations;

Final coordinates and equivalent isotropic displacement parameters of the non-

hydrogen atoms;

Hydrogen atom positions and isotropic displacement parameters;

Anisotropic and isotropic displacement parameters;

Bond distances;

Bond angles;

Torsion angles;

Contact distances;

Hydrogen bonds.

Abbreviations

vii | P a g e Nelson Mandela Metropolitan University

Abbreviations

anal. calcd. elemental analysis calculated

CyNC cyclohexyl isocyanide

d doublet

DMF dimethylformamide

dppe diphosphine

H2aa anthranilic acid

H2an aniline

H2ap 2-aminophenol

H2pd o-phenylenediamine

ibq 2-iminobenzoquinonate

IR infrared

MeCN acetonitrile

m multiplet

NMR nuclear magnetic resonance

Ph phenyl

phen o-phenanthroline

pyr pyridine

s singlet

t triplet

t-BuNC tert-butyl isocyanide

Chapter 1 Introduction

1 | P a g e Nelson Mandela Metropolitan University

Chapter 1

Introduction

1.1 Rhenium Background

Rhenium has expansive coordination chemistry due to its characteristics of both early

and late transition metal complexes. It can exist in many oxidation states and binds to

various ligands systems to give stable complexes. Oxo- and imido-ligands are known to

form stable rhenium(V) cores, such as [ReO]3+, [ReO2]+, [ReN]2+ and [ReNR]3+, which

are important constituents of radio-labeled therapeutic agents [1]. Ligands such as

carbonyls with significant π-back bonding capabilities also bind to rhenium in various

oxidation states and is known to be part of the stable Re(I) (d6) core [2].

Due to its extensive coordination chemistry, and the fact that it has two radioactive

isotopes 188Re and 186Re, which are β—emitters, makes this unique metal applicable to

nuclear medicine therapy [3]. Successful employment in nuclear medicine requires

these rhenium compounds to be thermodynamically stable under both thermal and

hydrolytic conditions [4].

The 188Re isotope has up until now been the isotope which shows most promise in the

radiotherapeutic industry, due to its high beta energy. However, 186Re has proved to

also possess exceptional properties, as illustrated in Figure 1.1, giving it wide potential

radiopharmaceutical uses [3, 5]. The 188Re is relatively easy to produce from the

188W/188Re generator; however, it is limited to some extent in radiopharmaceutical uses

due to its relatively short half life [5, 6]. The 186Re radionuclide is generated by the

radioactive decay of 186W or via commercially available generators [5].



Figure 1.1 - Rhenium radionuclide physical properties

1.2 Aim and Motivation of Research

This project mainly deals with the reactivity of isocyanides towards Re(III) and the more

developed Re(I) and Re(V) oxidation states. The particular interest in isocyanide ligands

are due to their unique physical, chemical and ligation properties. The isocyanide

coordination chemistry towards technetium has been studied quite well and they are

known to form extremely stable complexes with technetium [7]. The technetium

compound used as a radiopharmaceutical in heart-imaging (Tc(MIBI)6+, where MIBI = 2-

methyoxy-2-methylpropyl-isocyanide) is a well known example, and serves as

motivation for this particular interest in isocyanide chemistry.

The chemistry of Re(I) and Re(V) has been investigated quite extensively, leading to the

discovery of the stable cores [Re(CO)3]+ and [ReO]3+ respectively. Limited research has

been done on the +III oxidation state of rhenium and no stable core is yet known. One

0

2

4

6

8

10

12

14

16

Halflife (hr) ( × 10) Max beta energy(MeV)

Range in tissue(mm)

Gamma energy(keV) (× 10)

Re-186

Re-188



of the main objectives in this study was to investigate the reactivity of rhenium(III)

compounds, with the aim of developing such a stable core.

Rhenium complexes containing multidentate ligands are potential models for

radiotherapeutic agents based on 186Re, especially if they can be coupled to a biological

relevant receptor [8]. Rhenium(III) complexes of the form trans-[ReCl3(CNR)(PPh3)2]

could be a potential precursor for the preparation of rhenium(III) complexes, where the

ligands consist of a multidentate ligand and isocyanide [8]. Considering the stability of

the rhenium-isocyanide bond, reaction of bidentate N-, O-, S-donor ligands should not

affect the rhenium-isocyanide bond, thus leading to a stable [ReIII(CNR)] core.

Re(I) complexes containing the fac-[Re(CO)3]+ core are known to be kinetically and

thermodynamically unreactive, and by adjusting the specific ligand, different

characteristics can be shaped, depending on the complexes of interest [9]. The

combination of the tricarbonyl moiety with isonitriles as a ligand system might give the

metal complex the required properties to be suitable as a therapeutic agent, specifically

in the heart due to its cationic nature. This study thus also deals with the preparation of

such complexes.

Lastly, the reactivity of Re(V), in particular [ReO2I(PPh3)2], with isocyanides were also

investigated. After seeing the interesting coordination of isonitriles with Re(I) and Re(III),

its reactivity with Re(V) was also intriguing.

1.3 Rhenium in Nuclear Medicine

Radiopharmaceuticals have shown to be very effective for diagnostic purposes in

nuclear medicine. The pattern of distribution of radiation in an organ system over a

certain period of time enables one to make an appropriate diagnostic evaluation.

Therapeutic radiopharmaceuticals have also shown very promising applications over



the last decade, with rhenium being one of the elements possessing radioactive

properties which shows most potential [10].

1.3.1 Rhenium vs. Technetium

Rhenium is a congener of technetium with very similar physical characteristics [11].

99mTc is the ideal radionuclide in diagnosis, but for therapeutic uses the requirements

are complex, since it not only depends on the nuclear, chemical and biological

properties, but also on the nature and localization of the pathological processes [6].

Although technetium and rhenium are so similar in terms of most properties, rhenium is

more easily oxidized and kinetically inert in comparison to technetium. This means that

in vivo oxidation to ReO4- occurs more readily than technetium - thus making it ideal for

therapeutic purposes [12].

The similarities between these two elements is highly advantageous, since extensive

research has been done on the technetium radio isotopes, and approaches to labeling

agents with 99mTc could theoretically be adapted for use with rhenium [6].

1.3.2 Rhenium Nuclides in Therapy

The use of antibody therapy for tumour destruction was based on the natural

mechanism of antibodies to perform different effector functions, such as lysis of tumour

cells by programmed cell death (apoptosis). These antibodies can produce cytotoxic

effects on the tumour cells in addition to the effects of radiation, thus making it highly

advantageous [10].

The ideal radiopharmaceutical would be one that is relatively easy to synthesize, would

have stability as well as selective biodistribution in vivo, after which it is able to clear the

body with minimal side-effects [13]. In radiotherapy, it is necessary to use radiation that

can penetrate the body to the location of the tumour. Thus it is dependent upon the

radionuclide’s radiation properties [14].



186Re has a half-life of 89.3 hours and upon decaying, a β-particle is emitted with a

maximum energy of 1.1 MeV, as well as a 135 keV gamma particle enabling imaging.

Due to its relatively long half-life, it enables efficient distribution in the body. This nuclide

is produced in a nuclear reactor easily obtainable world-wide, where the choice of

reactor depends on the specific activity requirements. The rhenium radionuclide 188Re

has a half-life of 16.9 hours and its β-particle has a much higher maximum energy of

2.12 MeV with a 135 keV gamma photon, thus also giving it imaging capabilities [15]. Its

half-life is much shorter than that of 186Re, giving it a higher specific activity, and

consequently lower doses are required. It can be disadvantageous in that it decays too

quickly, thus not reaching its final destination before emitting radiation [16]. It can be

produced either using a nuclear reactor, or in a generator – the W188/Re188-generator is

the most common method [17].

Based on the above, it is clear that receptor-targeted radiopharmaceuticals have great

potential in diagnostic imaging and therapy, especially the radioactive rhenium nuclides

due to their exceptional physical properties [18].

1.3.3 Technetium Nuclides for Imaging

The use of 99mTc complexes as imaging agents has been extremely successful in the

past two decades. There has been great success in many regions, including brain,

kidney, liver, heart and bone imaging [5]. The 99mTc complex of particular interest,

especially in this study, is [99mTc(MIBI)6]+ (Figure 1.2), where MIBI represents 2-

methoxy-2-methylpropyl-isocyanide [19]. This positively-charged complex is known to

accumulate in the heart via the Na/K-ATPase pump, with its lipophilicity being an

important physical property enabling this uptake [5, 20]. The overall negative potentials

in the cytosol and mitochondria allows the positively-charged complex to accumulate in

the heart [20]. The isocyanide ligands have great ligation properties and stabilizes the

low oxidation state well [20]. The methoxy group enhances the metabolization of the

untrapped complex by cleavage of the ether to an alcohol, and subsequent excretion by

the kidneys [20].



The 99mTc-MIBI analogue consisting of aryl bidentate isocyanides have also been

synthesized, showing similar myocardial uptake to Cardiolite (Figure 1.2) [21]. The

bidentate nature of the isocyanide ligands allows better charge distribution and also

increases the lipophilicity [21].

Figure 1.2 - Structure of CardioliteTM (A) and its bidentate analogue (B)

1.3.4 Clinical Applications of 186/188Re

Rhenium radio-isotopes have a broad spectrum of clinical applications due to its

advantageous properties. Both 186Re- and 188Re-hydroxyethylidenediphosphonate

(HEDP) are radiopharmaceuticals that have been used for the palliation of painful bone

metastases in patients [22]. The rhenium radio-isotopes was used to selectively

accumulate in the bone by adsorption to the surface of the hydroxyapatite mediated by

hydroxide bridges [23]. This is metabolically controlled to ensure that hypermetabolic

regions of the bone and joints will receive significant higher radiation than other skeletal

regions with normal metabolism [22]. 186Re-HEDP results in considerable pain relief and

mild toxicity at low doses. More than 50% of the administered activity is cleared through



the kidneys within 24 hours, thus making the radiation-exposure side-effects minimal.

The 188Re-HEDP have also been synthesized, as well as 188Re-dimercaptosuccinic acid

(DMSA) which is specific to bone metastases from prostate cancer [22, 24].

Due to the effectiveness of rhenium radioactive targeting in bone metastasis, its

applicability to osteosarcoma’s (relatively radio-resistant tumours) have also been

investigated. It was found that it significantly decreased radiation side-effects and

resulted in excellent long-term control [25].

Other common applications include 188Re-perrhenate and 188Re-mercaptoacetyl

triglycine in endovascular radiation therapy, 188Re-peptides in tumour therapy, 188Re-

particles in catheter administration, 188Re-antigranulocyte antibodies in marrow ablation

prior to stem cell rescue, and 186Re-sulphide in synovectomy [6]. Restenosis, which is a

process relatively common in cases of artherosclerotic coronary artery disease, could

possibly be prevented by the use of the β-emitting 188Re isotope [26].

These are just some main indications of successful applications of

radiopharmaceuticals labeled with rhenium radioactive isotopes; however its

applications are much broader. By using rhenium radiopharmaceuticals, it could not

only enhance the therapeutic effectiveness toward cancer cells, but also potentially treat

arthiritis and inhibit arterial restenosis [27].

1.4 Low Valent Rhenium Chemistry

Rhenium is known to exist in many oxidation states, from -I to +VII. The chemistry of the

+I, +V and +VII oxidation states have been investigated most extensively, with the

higher oxidation states being the most common [11]. This is readily seen from the stable

metal cores [ReO]3+, [ReO2]+, [ReN]2+, [ReNR]3+ and [Re(CO)3]+ [1]. The need to

stabilize the metal by pi-acceptors ligands increases as one goes to the lower oxidation

states of rhenium, and thus these classes of ligands will predominate the discussion of

low valent rhenium complexes.



1.4.1 Ligands for Stabilizing Low Valent Rhenium Complexes

Low oxidation state rhenium metal centers readily accept electron density from electron-

rich ligands, thereby stabilizing the resulting metal complex. Carbonyls is an example of

such a ligand and is known to be part of the stable Re(I) core [Re(CO)3]+ [2].

The Re(I) tricarbonyl core possess a d6 electronic low-spin configuration, making it both

thermodynamically and kinetically inert – more so in comparison to the higher valent

rhenium complexes, such as Re(V). It is synthesized as an aqua-complex, namely

[Re(CO)3(OH2)3]+ via perrhenate reduction. The aqua ligands are easily substituted by

other ligands – ideally tridentate ligands to fill the three vacant coordination sites [27].

Isocyanides are versatile ligands which are isoelectronic with carbon monoxide. The

physical, steric and electronic properties of metal isonitriles can be modified by

changing the substituent on the isocyanide ligand – a characteristic that carbonyls

unfortunately lack [28]. The HOMO of the C=N-R ligand donates electrons from the filled

π-orbital or lone pair orbital to an empty orbital on rhenium (4s/4p) via σ-donation. The

LUMO of the C=N-R ligand is empty and accepts electrons from the 5d orbital of

rhenium. This electron transfer is known as the synergic effect (Figure 1.3). If π-back

bonding is exceptionally strong, the normal linear isocyanide ligand would become bent

at the nitrogen atom (129°-144°), indicating that the metal-carbon bond has a bond

order of two [29].

The coordination chemistry of rhenium with isocyanides is not nearly as developed as

that of rhenium-carbonyl chemistry. Other common ligands that have been found to

efficiently stabilize the lower oxidation states of rhenium are phosphines and polypyridyl

ligands, which are also pi-acids [2].



Figure 1.3 - Coordination modes of isocyanides and carbonyl to metals

1.4.2 Rhenium(I) Coordination Chemistry

(a) General chemistry of the Re(I)-tricarbonyl core

The chemistry of rhenium(I) has been explored extensively since the discovery of the

rhenium-tricarbonyl core [30]. The fascination with this specific core structure is the

fixed three carbonyls such that three facial positions are unoccupied allowing

multidentate ligands to readily bind to the metal core [31]. As stated earlier, Re(I)-

tricarbonyl complexes are known to be kinetically and thermodynamically stable, and by

varying the ligand coordinated to the metal centre the desired complexes with specific

properties can be fashioned [9]. It was found that even monodentate ligands with low

thermodynamic stability can give stable metal complexes, if the electronic properties of

the ligand allow it [32].

The fac-[Re(CO)3]+ core has been reacted with a variety of multidentate ligands - one of

the earliest studies was with imidazoles and histamine/histidines, as depicted in Figure

1.4 [32]. The ligands that have shown to efficiently bind and stabilize this core contain

amine, aromatic N-heterocycles and carboxylate donors [31].



Figure 1.4: Early reactivity of fac-[Re(CO)3]+

Rhenium pentacarbonyl chloride [Re(CO)5Cl] has been used as a precursor for many

stable complexes, including the examples illustrated in Figure 1.5 [33]. It is clear from

the Scheme below that the pentacarbonyl rhenium complex can be used as starting

material for various compounds containing the stable tricarbonyl core.

Figure 1.5: Reactivity of Re(CO)5Cl [33]



(b) Multidentate coordination towards the Re(I) tricarbonyl core

There have been multiple complexes containing bidentate coordinating chelates since

the discovery of the tricarbonyl core. The donor atoms are typically N-, O-, S- or P-

donor ligands. Tridentate chelators containing these donor atoms have shown to

produce stable organometallic compounds, ideal for in vivo conditions [34].

Some of the latest work include complexes with terpyridine (Figure 1.6). The unique

feature of these complexes are that the carbonyls are coordinated in a meridional

fashion, which can be more advantageous than the usual bidentate diimine Re(I)

tricarbonyl complexes [35].

Figure 1.6: Reactivity of terpyridine [35]

The preparation of tridentate chelating systems for coordination with [Re(CO)3]+ usually

requires multiple steps for successful synthesis and its subsequent reactivity under in

vivo conditions can be problematic, and therefore ligands must be carefully considered.

Recent successful work includes the reaction of azide-functionalized compounds with

alkyne prochelators to give a triazole-containing tridentate ligand [34]. Reaction of these



tridentate chelators to the tricarbonyl core gives a variety of complexes, structurally

different, with unique physical properties, as depicted in Figure 1.7 below [34].

Figure 1.7: Tridentate ligand coordination to [Re(CO)3]+

1.4.3 Rhenium(III) Coordination Chemistry

The chemistry of rhenium in the +III oxidation state have not been studied very

extensively, although a number of stable complexes have been isolated. An interesting

example is its complexes with organohydrazines. These ligands are versatile and

reducing in nature, thus making them ideal for radiopharmaceutical use. They have

shown to prefer Re(III) metal centres, evident from the fact that whether the precursor

compound was in +V or +VII state, it reduced to the +III state yielding an inert, stable

complex as depicted in Figure 1.8 [37].

Figure 1.8: Reactivity of ReOCl3(PPh3)2 with organohydrazines



Interestingly, very few penta-coordinate Re(III) compounds have been isolated thus far.

An example is the unexpected reduction of oxorhenium((V) upon coordination with a

thiolate ligand (Figure 1.9) [38].

Figure 1.9: Reactivity of [ReOCl3(PPh3)2] with p-methoxybenzyl mercaptan

A similar unexpected reduction of the [ReOCl3(PPh3)2] complex was observed by its

reaction with a Schiff base derivative as illustrated below (Figure 1.10) [39].

Figure 1.10: Reactivity of ReOCl3(PPh3)2 with 2-(2-aminobenzylideneimino)phenol



1.5 High Valent Rhenium Coordination Chemistry

Extensive research has been done on the +V oxidation of rhenium. The chemistry of

rhenium is thus dominated by this particular oxidation state, due to its unique and stable

physical properties.

1.5.1 Re(V)-oxo chemistry

The +V oxidation state of rhenium is the most diverse and explored oxidation state, and

is known to undergo a variety of reactions, as illustrated below.

(a) Oxidation

A novel cationic rhenium hydrazido complex [ReCl2(NNMePh)2(PPh3)](BPh4) was

isolated by the reaction of trans-[ReOCl3(PPh3)2] with an excess of the unsymmetrically

disubstituted organohydrazine MePhNNH2 in boiling methanol, as illustrated by the

equation below [40]. Upon recrystallization, the highly unstable PF6- salt decomposes to

give a dicationic complex [ReOCl(NNMePh)(PPh3)2](PF6)2 [40].

trans-[ReOCl3(PPh3)2] + MePhNNH2 [ReOCl(NNMePh)(PPh3)2]2+ + 2Cl- + H2O

(b) Reduction

Rhenium(III) complexes can readily be obtained by the reduction of Re(V) by PPh3 in

the presence of suitable ligands such as isonitriles [41].

trans-[ReOCl3(PPh3)2] + CNC(CH3)3 + PPh3 [ReCl3(CNC(CH3)3)(PPh3)2] + OPPh3



(c) Disproportionation

The reaction of trans-[ReOCl3(PPh3)2] with 1,10-phenanthroline in ethanol gives the

disproportionation products [ReIIICl2(phen)(PPh3)2]+ and perrhenate [42]. Similar

reaction products are obtained when using biimidazole as opposed to the

phenanthronline ligand [42].

trans-[ReOCl3(PPh3)2] + phen [ReCl2(phen)(PPh3)2]+ + [ReO4]-

(d) Ligand Substitution

There are various factors that affect the way in which a ligand coordinates to a metal. It

depends on the structural characteristics, the temperature of the reaction mixture, the

ligand to metal mole ratio and the solvent, to name a few. It is not that common for a

solvent to influence the product, although it has been seen multiple times, as in the

example below with diphosphine [43].

[ReOCl3(OPPh3)(Me2S)] + dppe 𝑇𝐻𝐹→ [ReOCl3(dppe)] + OPPh3 + Me2S

[ReOCl3(OPPh3)(Me2S)] + dppe 𝐸𝑡ℎ𝑎𝑛𝑜𝑙→ [ReOCl2(OEt)(dppe)] + OPPh3 + Me2S + HCl

1.5.2 Re(V)-imido chemistry

The organoimido core M=N-R has proven to be of great use in the radiopharmaceutical

industry, since different organic substituents can be integrated into this stable entity

[44]. The bond between the nitrogen atom and the rhenium metal centre consists of one

sigma and two pi-bonds [43]. Its hybridization depends on whether the bonding mode is

linear or bent, as illustrated in Figure 1.11 below [43].



Figure 1.11: Organoimido core bonding modes

The linear structure (a) is more common and usually written as M=NR, although

technically speaking it is a triple bond [43]. The nitrogen in structure (a) is a sp-

hybridized nitrogen. The bent structure (b) is fairly uncommon and its nitrogen is sp2-

hybridized. An example of a complex with a bent structure is [Mo(NPh)2(S2CNEt2)2]

where the M-N-C angle is 139° [44].

There have been numerous synthetic methods reported for the preparation of imido

complexes. The synthesis of imido complexes directly from perrhenate is essential for

its feasibility to act as a radiopharmaceutical [44]. Another common method is the use of

aniline or its derivatives with trans-[ReOCl3(PPh3)2], or the reaction of metal chlorides

with primary amines, as depicted below [44, 45].

trans-[ReOCl3(PPh3)2] + H2NC6H5 [ReCl3(NC6H5)(PPh3)2]

[ReCl4(Cp)] + H2NC6H5 [ReCl2(NC6H5)(Cp)] (Cp = cyclopentadienyl)

1.5.3 Re(V) coordination chemistry

Most current studies on the coordination chemistry of rhenium focus on the +V oxidation

state. Numerous complexes in this state have been prepared with a broad class of

ligands typically containing N, S, P and O donor atoms. Re(V) complexes have also

been used multiple times for the preparation of rhenium compounds in other oxidation

states.



Schiff bases are a versatile class of ligands, and due to its potential multidenticity a

great deal of attention has been paid to these types of ligands [46]. Therefore, the Re(V)

coordination chemistry discussion will focus on Schiff base coordination.

N-(2-amino-3-methylphenyl)salicylideneimine gave a rather interesting complex upon

reaction with ammonium perrhenate, triphenylphosphine and hydrochloric acid in glacial

acetic acid [47]. The ligand acts as a tridentate N,N,O-ligand coordinating through a

doubly deprotonated nitrogen atom (imido), a neutral imino and a phenolate oxygen

[46]. The imido nitrogen and phenolate oxygen lie in the trans axial positions, as

illustrated below (Figure 1.12) [47].

Figure 1.12: Structure of Re(V)-Schiff base complex

Another unusual example of Schiff base coordination is illustrated below, where the

Schiff base undergoes structural changes upon coordination. In the complexes with the

Schiff base in Figure 1.13, structure a shows the product of the reaction of the Re(V)-

oxo precursor [ReOCl3(PPh3)2] with 2-(2-hydroxyphenyl)phenol [46]. In structure b the

product as obtained from the reaction of [ReOCl3(PPh3)2] with 2-

(salicylideneamino)benzenethiol. The resulting complex contained this ligand in two

different forms, thus given a 2+3 ligand system of O,N and S,N,O respectively. This is

very unusual, and quite intriguing [46].



Figure 1.13: Re(V) complexes with structurally modified Schiff base ligands

1.6 References

[1] K.C. Potgieter, Complexes of the ReO3+/Re(CO)3+ cores with multidentate N,O-

donor chelates, MSc Dissertation, Nelson Mandela Metropolitan University,

South Africa, 2009, 2.

[2] N.C. Yumata, Reactivity of Rhenium(III) and Rhenium(V) with Multidentate NN-

and NO-Donor Ligands, MSc Dissertation, Nelson Mandela Metropolitan

University, South Africa, 2010.

[3] U. Abram, R. Alberto, J. Braz. Chem. Soc., 2006, 17, 1486.

[4] V. Bertolasi, C. Bianchini, I. de le Rios, A. Marchi, L. Marvelli, M. Peruzzini, R.

Rossi, Inorg. Chim. Acta, 2002, 327, 140.



[5] G. Kemp, Mechanistic study of rhenium(I) carbonyl complexes as model

radiophamaceuticals, PhD Thesis, The University of Johannesburg, South Africa,

2008.

[6] D. Lukic, J. Vucina, Physics, Chemistry and Technology, 2002, 2, 236.

[7] R. Alberto, H. Braband, N.I. Gorshkov, V.V. Gurzhiy, S.V. Krivovichev, E.M.

Levitskaya, A.A. Lumpov, A.E. Miroslavov, G.V. Sidorenko, D.N. Suglobov, I.G.

Tananaev, J. Organomet. Chem., 2008, 693, 7.

[8] F.E. Hahn, M. Glaser, T. Lugger, D. Scheller, H. Spies, Inorg. Chim. Acta, 1995,

232, 235.

[9] R. Alberto, R. Schibli, R. Waibel, U. Abram, A.P. Schubiger, Coord. Chem. Rev.,

1999, 190, 901.

[10] S.W. Falen, R.J. Kowalsky, Radiopharmaceuticals in Nuclear Pharmacy and

Nuclear Medicine, 2nd edition, American Pharmacists Association, Washington

DC, 2004, 1; 235; 767-768.

[11] C.S. Cutler, H.M. Hennkens, S. Huclier-Markai, S.S. Jurisson, N. Sisay, Chem.

Rev., 2013, 113, 870.

[12] M.S. Kovacs. The coordination chemistry of rhenium, group 13 and lanthanide

metal complexes: towards new radiotherapeutic agents, PhD Dissertation, The

University of British Columbia, Canada, 2001, 9.

[13] J.B. Arterburn, D.M. Goreham, M.S. Holguin, K.V. Rao, M.V. Valenzuela,

Organomettallics, 2009, 19, 1789.



[14] J.C. Kotz, P.M. Treichel, J.R. Townsend, Chemistry and Chemical Reactivity, 7th

edition, Thomson Brooks/Cole, 2008, pp 1086, 1085.

[15] A.L. Beets, F.F. Knapp, J. Krop, W.Y. Lin, J. Pinkert, S.Y. Wang, Rhenium

Radioisotopes for Therapeutic Radiopharmaceutical Development. In:

International Seminar on Therapeutic Applications of Radiopharmaceuticals, pp

1-2. Proceedings of an International Seminar Held in Hyderabad, India 18-22

January 1999. IAEA-SR-209.

[16] J. Wilson, W. Walker, Principles and Techniques of Biochemistry and

Microbiology, 7th edition, Cambridge University Press, New York, 2010.

[17] T. Bandurski, M. Derejko, G. Romanowicz, J. Scheffler, Nucl. Med. Rev., 2003,

6, 55.

[18] J.B. Arterburn, M.C. Perry, K.V. Rao, Angew. Chem. Int. Ed., 2000. 39, 771.

[19] C. Jonsson, H. Jacobsson, Ann. Nucl. Med., 1996, 10, 281.

[20] S. Bouquillon, N, Chemin, A, Du Moulinet D’Hardemarke, D. Fagret, M. Vidal,

Appl. Radiot. Isot., 1996, 47, 479.

[21] B. Das, D. Elmaleh, A. Fischman, L. McIntosh, J. Pitman, T. Shoup, J. Nucl.

Med., 2006, 47, 524.

[22] M. Bangard, H.J. Biersack, C. Menzel, H. Palmedo, J. Risse, J.K. Rockstroh, K.

Schliefer, Radiology, 2001, 221, 257.

[23] P.J. Blower, S. Prakash, Perspective on Bioinorganic Chemistry, 1999, 4, 123.

[24] H.G. van der Poel, EAU-EBU update series, 2007, 5, 117.



[25] A.M. Cassoni, E.J. Sawyer, W. Waddington, British J. Radiol., 1999, 72, 1226.

[26] B. Hsieh, J. Hsieh, H. Huang, W. Lin, G. Ting, S. Tsai, S. Wang, Nucl. Med. Biol.,

1999, 26, 967.

[27] M. Andreou, M. Argyrou, M. Lyra, A. Valassi, Intern. Mol. Imag., 2013, 2013, 6.

[28] A.W. Chung, C. Ko, L.T. Lo, C. Ng, S. Yiu, Coord. Chem. Rev., 2012, 256, 1547.

[29] R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, 3rd

edition, Wiley-Interscience Publication, Canada, 2001, 87-88.

[30] U. Abram, R. Alberto, A. Egli, T.A. Kaden, R. Schibli, A.P. Schubiger, J. Am.

Chem. Soc., 1998, 120, 7987.

[31] J. Babich, S.R. Banerjee, M.K. Levadala, L. Wei, J. Zubieta, Inorg. Chem.

Comm., 2003, 6, 1099.

[32] U. Abram, R. Alberto, R. Schibli, A.P. Schubiger, R. Waibel, Coord. Chem. Rev.,

1999, 190, 908.

[33] A. Earnshaw, N.N. Greenwood, Chemistry of the Elements. Butterworth-

Heinemannm Oxford, 1997.

[34] C.A. Kluba, T.L. Mindt, Molecules, 2013, 18, 3207.

[35] D.R. Black., S.E. Hightower, Inorg. Chem. Comm., 2012, 24, 16.

[36] H. Berke, O. Blacque, T. Fox, C.M. Frech, Y. Jiang, Chem. Eur. J., 2010, 16,

2240.



[37] W.M. Davis, A. Davison, M. Hirsch-Kuchma, A.G. Jones, T. Nicholson, Inorg.

Chem., 1997, 36, 3237.

[38] P. Bouziotis, E. Chiotellis, M.S. Papadopoulos, I.C. Pirmettis, C.P. Raptopoulou,

A. Terzis, Inorg. Chim. Acta, 1990, 290, 249.

[39] T.I.A. Gerber, K.C. Potgieter, P. Mayer, Polyhedron, 2010, 29, 1424.

[40] M.M. Lynam, J.C. Vites, Coord. Chem. Rev., 1995, 142, 3.

[41] F.E. Hahn, L. Imhoff , T. Lugger, Inorg Chim. Acta, 1998, 269, 347.

[42] M. Jaworska, J. Klak, R. Kruszynski, B. Machura, J. Mrozinski, Polyhedron,

2006, 25, 2538.

[43] A.L. Beauchamp, O. Sigouin, Inorg. Chim. Acta, 2005, 358, 4490.

[44] M.T. Ahmet, B. Coutinho, J.R. Dilworth, J.R. Miller, S.J. Parrott, Y. Zheng,

Polyhedron, 1996, 15, 2041.

[45] R.A. Eikey, M.M. Abu-Omar, Coord. Chem. Rev., 2003, 243, 85.

[46] J.Y. Kim, Y.J. Jon, H.J. Ha, H.K. Chae, Bull. Korean. Chem. Soc., 2003, 24, 504.

[47] I. Booysen, T.I.A. Gerber, E. Hosten, P. Mayer, Bull. Chem. Soc. Ethiop., 2008,

22, 101.

Chapter 2 Experimental


Chapter 2

Experimental

2.1 Handling of Rhenium Rhenium occurs naturally as a mixture of two non-radioactive isotopes, namely the

stable 185Re and the unstable 187Re, with natural abundances of 37.40% and 62.60%

respectively. The non-radioactive isotopes were used in this study, thus no special

safety precautions were taken in handling the metal.

2.2 Materials

2.2.1 Chemicals

Ammonium perrhenate (NH4)[ReO4] was obtained commercially from Sigma-Aldrich and

required no further purification. All common laboratory chemicals were of reagent grade

and were used as received from suppliers. Commercially obtained chemicals are

tabulated below (Table 2.1), and were used as received.

Table 2.1: Chemicals obtained commercially.

Chemical Supplier

2-aminophenol >99%, Fluka

2-aminothiophenol 98%, Alfa Aesa

o-phenylenediamine BDH Chem Ltd

anthranilic acid 99.5%, Riedel de Haen



2.2.2 Precursor compounds

(a) trans-[ReOCl3(PPh3)2]

A solution of perrhenic acid was prepared by dissolving ammonium perrhenate (1.0 g,

3.7 mmol) in 3 mL concentrated hydrochloric acid. This solution was added to a

suspension of triphenylphosphine (5.0 g, 19.1 mmol) in 50 mL glacial acetic acid under

nitrogen. A green solid was removed by vacuum filtration and washed with glacial acetic

acid and diethyl ether [1]. Yield = 97%, m.p. 252 °C. Anal. Calcd. for C36H30OCl3P2Re

(%): (Mw = 833.16 g/mol) C, 51.90; H, 3.63. Found: C, 52.12; H, 3.69. IR (νmax/cm-1):

ν(Re=O) 998(s).

(b) trans-[ReCl3(MeCN)(PPh3)2]

A mixture of trans-[ReOCl3(PPh3)2] (2.5 g, 3.0 mmol), triphenylphosphine (2.5 g, 9.5

mmol), acetonitrile (17.5 mL) and toluene (20 mL) was stirred for 2 hours at reflux under

nitrogen. After an hour an orange solid formed, which upon cooling to room

temperature, was collected through vacuum filtration and washed with 50 mL ethanol

[2]. Yield = 87%, m.p. 290-296 °C. Anal. Calcd. for C38H33Cl3NP2Re (%): (Mw = 858

g/mol) C, 53.20, H, 3.86, N, 1.63. Found: C, 54.06; H, 3.61; N, 2.25. IR (νmax/cm-1):

ν(Re-N) 501(s).

(c) trans-[ReCl3(CNC(CH3)3)(PPh3)2]

The method obtained from the literature [3] was adapted as follows: trans-

[ReCl3(MeCN)(PPh3)2] (200 mg, 0.233 mmol) was dissolved in 10 mL dry benzene

under nitrogen. tert-Butylisocyanide (29 mg, 0.350 mmol) was added and the reaction

mixture was refluxed under nitrogen for 3 hours. The volume was reduced under

nitrogen and a yellow precipitate was collected. Yield = 58%, m.p. 189 – 192 °C. Anal.



Calcd. for C41H39Cl3NP2Re (%):(Mw = 899 g/mol) C, 54.70; H, 4.37; N, 1.56. Found: C,

53.87; H, 4.51; N, 2.30. IR (νmax/cm-1): ν(C≡N) 2144(s).

(d) trans-[ReOI2(OEt)(PPh3)2]

5.0 g of Triphenylphosphine was added to 1.0 g of (NH4)[ReO4] in 30 mL ethanol and 5

mL hydroiodic acid (56%), and the mixture was heated under reflux for 15 minutes.

Upon cooling to room temperature, the green precipitate was filtered, washed with

ethanol and diethyl ether, and dried under vacuum [1]. Yield = 82 %. m.p. 150-154°C.

Anal. Calcd. for C38H35I2O2P2Re (%): (Mw = 1142.8 g/mol) C, 44.50; H, 3.44. Found: C,

43.09; H, 3.44. IR (νmax/cm-1): ν(Re=O) 906(s).

(e) cis-[ReO2I(PPh3)2]

A mass of 1.0 g of trans-[ReOI2(OEt)(PPh3)2] was stirred in a solution of 50 mL acetone

and 2 mL distilled H2O at room temperature for an hour. A purple crystalline product

was isolated, washed with acetone and diethyl ether, and dried under vacuum [1]. Yield

= 75 %. m.p. 169-171°C. Anal. Calcd. for C36H30IO2P2Re (%): (Mw = 869.68 g/mol) C,

49.72; H, 3.48. Found: C, 49.19; H, 2.61. IR (νmax/cm-1): ν(Re=O) 907(s).

(f) mer-[ReOCl3(SMe2)(OPPh3)]

A solution of 1.0 g of trans-[ReOCl3(PPh3)2] and 0.1 mL of Me2SO in 80 mL benzene

was stirred under nitrogen for 2 days at room temperature. The light green precipitate

was filtered under vacuum and washed with diethyl ether. [4] Yield = 70 %. m.p. 222-

227 °C. Anal. Calcd. for C20H19Cl3O2PRe (%): (Mw = 652.01 g/mol) C, 32.86; H, 3.71;

S, 4.92. Found: C, 32.30; H, 3.34; S, 4.06. IR (νmax/cm-1): ν(Re=O) 989(s).



2.3 Instrumentation

2.3.1 Analytical Techniques

All infrared spectra were obtained using a Bruker Tensor 27 spectrophotometer.

All NMR spectra were collected at 300K using a 300 MHz Bruker AMX-300

spectrometer with chemical shifts relative to SiMe4.

A Perkin-Elmer Lamda 35 UV/Vis spectrophotometer was used for absorption spectra.

Melting points were determined using a Lasec Stuart SMP30 melting point apparatus.

A BASi Epsilon EC-2000 cyclic voltameter was used for cyclic voltammetry. The three

electrode system consists of a platium working electrode, a platinum wire auxiliary

electrode and a pseudo silver/silver chloride reference electrode. The supporting

electrolyte tetrabutylammoniumtoluene-4-sulfonate had a final concentration of 0.1 M

and the respective compounds analyzed had concentrations of 10-3 M. All the cyclic

voltammetric runs were done at a scan rate of 100 mV/s, scanning between 1500 mV to

-1500 mV and back to 1500 mV. Each sample was deoxygenated before each run by

bubbling nitrogen through the solution for 10-15 minutes.

All conductivity measurements were done on a Hanna HI2300 EC/TDS/NaCl meter. The

conductivity measurements obtained were compared with expected conductivity values

at a concentration of 10-3 M (Table 2.2) [1, 5].



Table 2.2: Conductivity ranges expected for electrolyte types in organic solvents.

Solvent Conductivity measurement in

1:1 electrolyte (μS)

Conductivity measurement in

1:2 electrolyte (μS)

Acetonitrile 120-160 220-300

Methanol 80-115 160-220

DMF 65-90 130-170

2.3.2 Crystallography

A Bruker Kappa Apex II diffractometer in the conventional ω-2θ scan mode and

monochromatic Mo-Kα radiation (λ = 0.71073 Å) was used for X-ray crystallographic

analysis.

2.3.3 Computational studies

Density functional theory (DFT) and time-dependent DFT were performed by Dr. P.

Ramasam from the Computational Chemistry Group, Department of Chemistry, Faculty

of Science, University of Mauritius, Réduit, Mauritius.

2.4 References

[1] K.C. Potgieter, Rhenium Complexes with Multidentate Benzazoles and Related

N,X-Donor (X = N, O, S) Ligands, PhD Thesis, Nelson Mandela Metropolitan

University, South Africa, 2012, 28.

[2] N.C. Yumata, Reactivity of Rhenium(III) and Rhenium(V) with Multidentate NN-

and NO-Donor Ligands, MSc Dissertation, Nelson Mandela Metropolitan




[3] F.E. Hahn, M. Glaser, T. Lugger, D. Scheller, H. Spies, Inorg. Chim. Acta, 1995,

232, 232.

[4] J.C. Bryan, R.E. Stenkamp, T.H. Tulip, J.M. Mayer, Inorg. Chem., 1987, 26,

2283.

[5] H. Fazilati, H. Golchoubian, Iranica Journal of Energy & Environment, 2012, 3,

266.

Chapter 3 ReCl3(CNR) Core


Chapter 3

Complexes of Aniline Derivatives

Containing the ReCl3(CNR) Core

3.1 Introduction

The potential relevance of the 186/188Re radionuclides in nuclear medicine makes the

coordination behaviour of rhenium of particular interest [1]. Although Re(I) complexes of

isocyanides have been studied extensively, not many research efforts focused on their

reactivity towards Re(III) and Re(V). Our initial approach was that the compound trans-

[ReCl3(CNR)(PPh3)2] could be a potential precursor for the synthesis of Re(III)-isonitrile

complexes [2]. In this study, the products of the reaction of trans-[ReCl3(t-

BuNC)(PPh3)2] with aniline and selected potential bidentate derivatives were

investigated. The retainment of the Re(CNR) bond in the +III oxidation state could lead

to a novel core for radiopharmaceutical purposes.

Aniline derivatives have unique chelating abilities, and have therefore attracted a lot of

research interest [3]. The coordination behaviour of rhenium(V) with aniline and its

derivates H2L (H2L = anthranilic acid, o-phenylenediamine, 2-aminophenol) (Figure 3.1)

has been previously studied and leads to complexes in various oxidation states and

results in quite unusual structural properties [4, 5, 6, 7]. These ligands have the ability to

coordinate in a monodentate or a bidentate fashion and are strong enough to bond to

rhenium at low concentration to give a single product in high yield [5, 6, 8].

This study centers on the reaction of trans-[ReCl3(t-BuNC)(PPh3)2] with aniline (H2an)

and its derivatives anthranilic acid (H2aa), o-phenylenediamine (H2pd) and 2-



aminophenol (H2ap) in an attempt to synthesize complexes of rhenium(III) that contain

an isocyanide ligand. The choice of starting complex was influenced by the reported

higher yields obtained by using [ReCl3(t-BuNC)(PPh3)2] rather than trans-

[ReCl3(MeCN)(PPh3)2] or by the reduction of trans-[ReOCl3(PPh3)2] with an excess of

triphenylphosphine [1].

Figure 3.1: Ligands used in this study

The reaction of trans-[ReCl3(t-BuNC)(PPh3)2] with the potentially bidentate ligands H2L

(H2L = aniline (1), o-phenylenediamine (2), anthranilic acid (3)) surprisingly led to the

isolation of rhenium(V) complexes [ReCl3(t-BuNC)(L)(PPh3)2], in which the ligands L are

coordinated as dianionic monodentate imides (Figure 3.2). Complex 2 was also

synthesized by the reaction of trans-[ReOCl3(PPh3)2] with H2pd in ethanol to form trans-

[ReCl3(pd)(PPh3)2], which was subsequently reacted with t-BuNC in toluene.

By using 2-aminophenol (H2ap) as possible ligand, the rhenium(V) complex salt trans-

[ReCl2(t-BuCN)(ap)(PPh3)2](ReO4) (4) was obtained, with ap also coordinated as an

imide after several recrystallization steps. We can’t offer any explanation for this

anomalous behavior of H2ap. However, the crystals obtained from the mother liquor of

the synthetic solution of 4 led to the formation of the complex trans-[ReCl2(t-

BuCN)(ibq)(PPh3)2] (5) (ibq = 2-iminobenzoquinonate), where ibq coordinates through a

monoanionic imino nitrogen. The oxidation of the ligand is shown in Figure 3.3.



Figure 3.2: Reaction pathway of trans-[ReCl3(CNC(CH3)3)(PPh3)2] with H2L

Figure 3.3: 2-Aminophenol oxidation and coordination modes for 5

Re

N

Cl

Ph3P

ClN

Y

Re

N

PPh3

Ph3P

Cl

Cl

Cl

N2,

NH2

X

(1) X = H; Y = Cl; Z = H

(2) X = NH2; Y = Cl; Z = NH2

(3) X = COOH; Y = Cl; Z = COOH

(4) X = OH; Y = PPh3; Z = OH

(5) X = OH; Y = PPh3; Z = (=O)

5

4

3

2

6

1

Z

(1)/(2) Acetontrile (3)/(4)/(5) Benzene



3.2 Experimental

3.2.1 Synthesis of trans-[ReCl3(t-BuNC)(PPh3)(an)] (1)

To a solution of trans-[ReCl3(t-BuNC)(PPh3)2] (75 mg, 0.08 mmol) in 10 mL acetonitrile

was added 15.2 μL of aniline (H2an, 0.17 mmol), and the mixture was heated under

reflux in a nitrogen atmosphere for 3 hours. Cooling to room temperature and removal

of the solvent under vacuum gave a green precipitate, which was filtered off.

Recrystallization from hot ethanol gave dark green crystals. Yield = 47%, m.p. >300°C.

Anal. Calcd. (%): C, 47.78; H, 4.01; N, 3.84. Found: C, 47.71; H, 3.33; N, 3.92. IR

(νmax/cm-1): ν(C≡N) 2187(m); ν(Re=C) 1184(m); ν(Re=N) 1088(m). 1H-NMR (295K, ppm,

CDCl3): 7.80 (d, 3H), 7.52 (d, 3H), 7.33-7.41 (m, 6H, PPh3); 7.19 (m, 9H, PPh3); 1.37 (s,

9H, 3xCH3). UV/Vis (CH3CN, λmax (ε, M-1cm-1)): 390 (1175), 680 (100). Conductivity

(CH3CN, μS): 4.

3.2.2 Synthesis of trans-[ReCl3(t-BuNC)(PPh3)(pd)] (2)

trans-[ReCl3(t-BuNC)(PPh3)2] (75 mg, 0.08 mmol) and o-phenylenediamine (H2pd, 18

mg, 0.17 mmol) were dissolved in 10 mL acetonitrile, and the mixture was heated under

reflux under nitrogen for 2 hours. No precipitate was obtained, but slow evaporation of

the mother liquor gave dark brown crystals suitable for X-ray diffraction after 3 days.

The complex crystallized as the solvate 2∙CH3CN. Yield = 54%, m.p. 125-127°C. Anal.

Calcd. (%): C, 46.80; H, 4.34; N, 7.07. Found: C, 46.56; H, 4.06; N, 6.62. IR (νmax/cm-1):

ν(C≡N) 2196(m); ν(Re=C) 1192(m); ν(Re=N) 1090(m). 1H-NMR (295K, ppm, CDCl3):

7.09 (t, 1H), 6.34 (d, 1H), 7.33-7.75 (m, 9H, PPh3); 7.19 (m, 6H, PPh3 ); 6.08 (d, 1H),

5.95 (d, 1H); 1.18 (s, 9H, 3xCH3). UV/Vis (MeOH, λmax (ε, M-1cm-1)): 346 (740), 480

(550), 669 (370). Conductivity (MeOH, μS): 8.

Complex 2 was also synthesized by dissolving trans-[ReOCl3(PPh3)2] (150 mg, 0.18

mmol) and H2pd (22 mg, 0.20 mmol) in 20 mL ethanol and heated under reflux for an



hour. A red precipitate [Re(pd)Cl3(PPh3)2] [9] was obtained of which 50 mg (0.05 mmol)

was reacted with tert-butyl isocyanide (12.3 μL, 0.11 mmol) and refluxed for another

hour. Evaporation of the mother liquor gave X-ray quality crystals after 5 days. The

complex did not crystallize with any solvent of crystallization. An X-ray analysis showed

the product to be trans-[ReCl3(t-BuNC)(PPh3)(pd)]. Yield = 33 %, m.p. 124-128 °C. IR

(νmax/cm-1): ν(C≡N) 2190(m); ν(Re=C) 1188(m); ν(Re=N) 1089(m).

3.2.3 Synthesis of trans-[ReCl3(t-BuNC)(PPh3)(aa)] (3)

trans-[ReCl3(t-BuNC)(PPh3)2] (100 mg, 0.11mmol) and anthranilic acid (H2aa, 46 mg,

0.33 mmol) were dissolved in 20 mL benzene and heated under reflux under nitrogen

for 3 hours. A feather-like green precipitate was obtained, which was recrystallized from

dichloromethane:diethyl ether 1:2 v/v using the vapour diffusion crystallization method.

The green crystals isolated after 2 days were suitable for X-ray diffraction studies. Yield

= 39%, m.p. >300°C. Anal. Calcd. (%): C, 47.11; H, 3.78; N, 3.62. Found: C, 46.60; H,

4.26; N, 4.62. IR (νmax/cm-1): ν(C≡N) 2204(m); ν(CO2) 1663(m); ν(Re=C) 1192(m);

ν(Re=N) 1089(m). 1H-NMR (295K, ppm, d6-DMSO): 6.67 (d, 1H), 6.54 (t, 1H), 7.26 (t,

1H), 7.70 (t, 1H), 7.22-7.56 (m, 15H, PPh3), 7.85 (t, 1H), 1.39 (s, 9, 3xCH3). UV/Vis

(DMF, λmax (ε, M-1cm-1)): 384 (2030). Conductivity (DMF, μS): 27.

3.2.4 Synthesis of trans-[ReCl2(t-BuNC)(PPh3)2(ap)](ReO4) (4)

trans-[ReCl3(t-BuNC)(PPh3)2] (100 mg, 0.11 mmol) and 2-aminophenol (H2ap, 36 mg,

0.33 mmol) were dissolved in 20 mL benzene and heated to reflux under nitrogen for an

hour. The solution became red, forming a brown precipitate, which was filtered after

heating was stopped and cooled to room temperature. The material was recrystallized

from methanol to give yellow-brown crystals, suitable for X-ray diffraction. Yield = 30%,

m.p. 163-167°C. Anal. Calcd. (%): C, 45.5; H, 3.7; N, 2.3. Found: C, 47.0; H, 3.5; N,

2.1. IR (νmax/cm-1): ν(O-H) 3225(w), ν(C≡N) 2206(m); ν(Re=C) 1266(m); ν(Re=N)

1089(m), ν(ReO4-) 908(s). 1H-NMR (295K, ppm, CDCl3): 7.57-7.63 (m, 2H), 7.66 (m, 6H,

PPh3); 7.49 (t, 1H), 7.40 (t, 1H), 7.28 (m, 15H, PPh3); 7.19 (m, 9H, PPh3), 2.10 (s, 9H,



3xCH3). UV/Vis (DMF, λmax (ε, M-1cm-1)): 287 (1810), 408 (446), 499 (186).

Conductivity (DMF, μS): 69.

3.2.5 Synthesis of trans-[ReCl2(t-BuNC)(PPh3)2(ibq)] (5)

trans-[ReCl3(t-BuNC)(PPh3)2] (100 mg, 0.11 mmol) and H2ap (36 mg, 0.33 mmol) were

dissolved in 20 mL benzene and heated to reflux under nitrogen for an hour. The

solution became red, forming a brown precipitate which was filtered. The mother liquor

was evaporated slowly over a period of 3 days to give red-brown crystals, suitable for X-

ray diffraction. Yield = 59%, m.p. 189-183°C. Anal. Calcd. (%): C, 58.31; H, 4.67; N,

2.88. Found: C, 56.98; H, 5.19; N, 2.95. IR (νmax/cm-1): ν(C≡N) 2195(m); ν(Re=C)

1262(m); ν(Re=N) 1071(m), ν(C=O) 1606(m), ν(C=N) 1534(m). 1H-NMR (295K, ppm,

CDCl3): 7.58 (m, 2H), 7.67 (m, 6H, PPh3); 7.43 (d, 1H), 7.32 (t, 1H), 7.26 (m, 15H,

PPh3); 7.19 (m, 9H, PPh3), 2.11 (s, 9H, 3xCH3). UV/Vis (DMF, λmax (ε, M-1cm-1)): 326

(710), 437 (410), 501 (600), 549 (408). Conductivity (DMF, μS): 3.

3.2.6 X-ray crystallography

X-Ray diffraction studies of complexes 1 – 5 were performed at 200 K using a Bruker

Kappa Apex II diffractometer with graphite monochromated Mo Kα radiation (λ =

0.71073 Å). APEX-II was used for data collection and SAINT for cell refinement and

data reduction [10]. The structures were solved by direct methods using SHELXS-2013

[11] and refined by least-squares procedures using SHELXL-2013 [11] with SHELXLE

[12] as a graphical interface. Data were corrected for absorption effects using the

numerical method implemented in SADABS [10].

All non-hydrogen atoms were refined anisotropically. Carbon-bound H atoms were

placed in calculated positions (C-H 0.95 Å for aromatic carbon atoms and C-H 0.99 Å

for methylene groups) and were included in the refinement in the riding model

approximations, with Uiso(H) set to 1.2Ueq(C). The H atoms of the methyl groups were

allowed to rotate with a fixed angle around the C-C bond to best fit the experimental



electron density (HFIX 137 in SHELXL [BB]), with Uiso(H) set to 1.5Ueq(C). The H atom

of the hydroxyl groups were placed in calculated positions (HFIX 83 in SHELXL [11])

with Uiso(H) set to 1.5Ueq(O). Nitrogen-bound H atoms were located on a difference

Fourier map and refined riding the Uiso(H) set to 1.2Ueq(N).

In complexes 1, 3 and 4 reflections were omitted from the refinement since they were

obscured by the beam-stop. The crystal for 2 was twinned. A .hkl file suitable for twin

refinement was created using the TwinRotMat option in PLATON [13], and refined using

the HKLF 5 option in SHELXL [11] giving a final BASF value of 0.089. Structure 4

contains a solvent accessible void of 206 Å3; the SQUEEZE routine in PLATON [13]

was not used to remove any solvent electron densities. Structures 4 and 5 both have

water molecules where the hydrogen atoms could not be adequately located on the

difference Fourier map.

3.3 Results and Discussion

3.3.1 Synthesis and characterization of trans-[ReCl3(t-BuCN)(PPh3)(an)] (1),

trans-[ReCl3(t-BuCN)(PPh3)(pd)] (2) and trans-[ReCl3(t-BuCN)(PPh3)(aa)] (3)

Complexes 1 – 3 are structurally similar, the major difference being the free functional

group on the phenyl ligand. In complex 1 this is a hydrogen, in complex 2 a secondary

amine, and in complex 3 a carboxylic acid group. Due to their extreme similarities, the

three complexes will be discussed simultaneously.

For complexes 1 and 2, trans-[ReCl3(t-BuNC)(PPh3)2] was reacted with H2an and H2pd

respectively in refluxing acetonitrile with a molar ratio of 1:2 under nitrogen. Complex 3

was isolated by using the same starting material and reacting it with H2aa in a 1:3 molar

ratio in hot benzene under similar conditions as above. In all the above cases, the

ligand acts as a monodentate ligand, coordinating through a doubly deprotonated imido

nitrogen.



Complexes 1 – 3 are all non-electrolytes. Compound 1 appears as a green solid,

soluble in acetonitrile and dichloromethane, and sparingly soluble in ethanol. It is stable

in solution for weeks as a light green solution. Complex 2 is a brown solid, soluble in

methanol and acetonitrile, but insoluble in ethanol, acetone and dichloromethane.

Compound 3 is also a green solid, but upon dissolving it in an appropriate solvent, a

yellow/orange solution forms, which is stable for several weeks. This complex is soluble

in acetonitrile, dichloromethane and chloroform.

Compounds 1 – 3 have very similar structures, and their respective IR spectra are also

quite similar, as depicted in Figure 3.4 below. The three most important characteristic

bands for these complexes are the C≡N (of the isocyanide), Re=C and Re=N bands,

and they occur at similar frequencies respectively for all three complexes. The

coordinated isocyanide ligand is supported by the C≡N and Re=C absorption at about

2200 cm-1 and 1190 cm-1 respectively. The Re=N bond gave a strong stretching

frequency at approximately 1090 cm-1. The IR spectrum for 2 also shows two peaks at

3401 and 3307 cm-1 [ν(NH)] which signify the free uncoordinated amine group on the

aromatic ring. Complex 3 has a strong COO-H stretch, confirming the carboxylic acid

group to be free on the phenyl ring of the coordinated ligand aa2-.

The 1H NMR spectra for complex 1 is not very informative on the structure of the

complex, due to the fact that the signals of the phenyl protons overlap with that of the

protons on the triphenylphosphine ligand. The 1H NMR spectrum for 2 (Figure 3.5) show

very clear, distinct signals (as to be expected from a diamagnetic complex). The

triphenylphosphine ligand protons have rise to multiple signals in the range 7.19 – 7.75

ppm. The phenyl protons gave rise to two doublets at 6.34 and 6.08 ppm, which are

assigned to H2 and H5 respectively, and two triplets at 7.09 and 5.95 ppm due to H4 and

H3 respectively. The tert-butyl isocyanide protons show an intense signal at the highly

shielded range at about 1.2 ppm – the intensity ascribed to its symmetrical properties.



Figure 3.4: Infrared spectra of complexes 1–3

Although 1 and 2 are structurally very similar, their respective 1H-NMR spectra are

completely different. The an2- ligand in 1 is a mono-substituted phenyl ring in contrast to

pd2- in 2 being di-substituted. This greatly influences the observed chemical shifts, since

1 doesn’t contain the electron-withdrawing amine.

Complex 3 has a 1H-NMR spectrum very similar to that of 2, ascribed to the fact that the

two complexes are nearly identical – the major difference being the free functional

group on the phenyl ligand, as mentioned earlier. The free secondary amine and free

carboxylic acid on complexes 2 and 3 respectively have similar electronic properties,

explaining why the signals on the two spectra are similar. The proton-NMR spectrum

for 3 shows the presence of the triphenylphosphine ligand by the peaks in the range

7.19 – 7.66 ppm. The phenyl protons gave rise to two doublets at 6.67 (H2) and 7.70

(H5) ppm, and two triplets at 7.26 and 6.54 ppm due to H4 and H3 respectively. The t-

butyl isocyanide protons again resonate as a strong singlet at around 1.4 ppm.



Figure 3.5: 1H NMR spectrum of 2




The UV-Vis spectrum of the green complex 1 only reveals an extremely weak peak at

669 nm ascribed to weak metal-to-ligand charge-transfer. The spectrum of complex 2

shows three peaks. Firstly an intense band is seen at 346 nm, most probably due to the

combined ligand-to-metal charge-transfer transition of [pπ(Cl-) d*π(Re)] and [pπ(N2-)

d*π(Re)]. The less intense peak at about 480 nm is ascribed to being a ligand-to-

metal charge transitions [pπ(CNR) d*π(Re)]. There is also a very weak band at a low

energy (669 nm) due to [d*π(Re) pπ(CNR)] metal-to-ligand charge-transfer

transitions. Complex 3 only exhibits one very weak shoulder band at approximately 384

nm.

The cyclic voltammograms of complexes 1 – 3 show the current response when a

solution of 1 – 3 (0.001M) and 0.1M tetrabutylammonium perchlorate (TBAP) was

subjected to a cyclic excitation signal. The working electrode was a carefully polished

platinum electrode, and the reference electrode was a saturated Ag/AgCl electrode.

Figure 3.7: UV/Vis spectrum of 2



Consider the voltammogram for 1 (Figure 3.8) and its peak parameters as summarized

in Table 3.1. There is a redox couple observed at A and an irreversible cathodic peak at

B. The process at A is a one-electron quasi-reversible reduction, evident from the

cathodic/anodic current ratio.

Table 3.1: Peak parameters for the cyclic voltammogram of complexes 1 - 3.

1 2 3

Peak A B A B A B

Epc (mV) 1067 -1195 -595 - -825 -1437

Epa (mV) 1144 - - -995 804 -

E1/2 (mV) 1106 - - - -10.5 -

ic (μA) 6.376 14.49 14.1457 - 5.1606 2.7436

ia(μA) 7.111 - - 9.1092 9.4301 -

ia/ic 1.12 - - - 1.82 -

Figure 3.8: Cyclic voltammogram of 1

A

B





A

A B

B



The cyclic voltammogram for 2 (Figure 3.9) only show two irreversible processes – a

cathodic peak at A and an anodic peak at B. Complex 3 gave two processes upon

subjecting a solution of 3 to an excitation signal. Process A is a two-electron irreversible

process – the irreversibility indicated by the very high anodic to cathodic peak ratio.

Process B only shows an irreversible cathodic peak with a Epc of -1437 mV.

Single crystals for both 1 and 2 were obtained by the evaporation of the mother liquor.

Complex 2 crystallized as the solvate 2·CH3CN. Single crystals for 3 were obtained by

vapour diffusion of diethyl ether into a solution of 3 in dichloromethane. The three

compounds are nearly identical with the free substituent on the phenyl ligand being the

major structural difference. Therefore complexes 1 – 3 will be discussed together,

unless significant differences are applicable.

Perspective views of the asymmetric units of complexes 1 -3 are portrayed in Figures

3.11 – 3.13. The geometry is best described as slightly distorted octahedral around the

central rhenium(V) ion, which is evident from the non-linear bond angles for trans-

ligands Cl(1)-Re(1)-N(1) of 174.9(2)° for 1. Similar trans-angles are observed for 2 and

3 of 175.4(2)° and 170.2(3)° respectively. The basal planes are defined by the two

chlorides Cl(2) and Cl(3), the phosphorus atom P(1) and the carbon atom C(1) of the

isonitrile. The imido nitrogen N(1) and Cl(1) lie in the trans axial positions.

The Re(1)-N(11) bond distance of 1.728(2) Å in complex 2 clearly agrees with the

expected bond length for Re-imido double bonds [1.72(1) – 1.74(1)Å], thus confirming

the coordination mode being linear where the sp2-hybrized nitrogen acts as a 2e- donor

[2]. The analogous Re-N bond in 1 [1.715(2)] Å and 3 [1.728(2) Å] are also within the

expected range for Re(V)-imido bonds [2, 14].

The Re(1)-C(1) bond for 1 - 3 (about 2.03 Å) is significantly shorter than expected for a

Re-C single bond (2.13 Å) and somewhat longer than the distance expected for Re=C



double bond (1.91 Å), indicating that the Re(1)-C(1) bond has some double bond

character [15]. The Re-Cl and Re-P bond distances for 1 – 3 are as normally found [16].

Figure 3.11 – ORTEP diagram of 1, showing 40% ellipsoid probability

The tert-butyl isocyanide ligand in the complexes are essentially linear, as seen from

the C(1)-N(1)-C(2) angle of 176.8(3)° in 2, indicating the strong σ-donation, weaker π-

accepting nature of the ligand [17]. The isocyanide ligand in complex 3 deviates more

from linearity [169.9(5)°], which may be indicative of stronger pi-back bonding, although

not significantly. The C(1)-N(1) bond length of 2 [1.151(3) Å] is slightly longer than the

average C-N triple bond length, thus showing that is has double bond character [15].

The analogous bonds in complexes 1 and 3 show similar bond lengths. This



phenomenon is caused by π-back bonding from the metal, resulting in the Re-C bond

being strengthened and the unsaturated C-N bond being weakened [15].

Figure 3.12 – ORTEP diagram of 2∙CH3CN, showing 40% ellipsoid probability

Complex 2 was also synthesized by the reaction of trans-[ReOCl3(PPh3)2] with H2pd in

ethanol to form trans-[ReCl3(pd)(PPh3)2] [9], which was subsequently reacted with t-

BuNC in toluene. The reason behind using this method was to try and explain the

mechanism of the [ReCl3(CNR)]2+ formation. It was found that whether starting with the

Re(III)/Re(V) precursor, the same product will be isolated.



Figure 3.13: ORTEP diagram of 3, showing 40% ellipsoid probability

The crystal structures of 1 and 2 were superimposed to illustrate the extreme similarities

between these complexes (Figure 3.14). Except for the angles being bent slightly

differently, the only noticeable difference is the substituent on the phenol ring for 2.



Figure 3.14: Structure overlay of 1 and 2

The intramolecular hydrogen-bonds and interactions in 3 [C(10)-H(13)---O(2) and C(36)-

H(36)---O(1)] are quite strong, and correlates to the crystal packing in the unit cell for

complex 3 (Figure 3.15). As illustrated in Table 3.2, there are more hydrogen

interactions that occur in complex 2 in comparison to complex 1. The difference lies in

the crystal of 2 containing a solvent molecule which participates in hydrogen bonding,

as well as the free primary amine on the phenyl ring of the ligand which is absent in

complex 1. The two hydrogen atoms on the uncoordinated N(12) atom in 2 are involved

in hydrogen-bonds with the chlorides Cl(1) and Cl(3).



Table 3.2: Hydrogen-bonding and interactions for complexes 1 – 3.

A-H∙∙∙B A-H H∙∙∙B A∙∙∙B A-H∙∙∙B

1

C(36)-H(36)∙∙∙Cl(3) 0.9500 2.8100 3.652(2) 148

C(42)-H(42)∙∙∙Cl(3) 0.9500 2.7000 3.533(19) 146

C(46)-H(46)∙∙∙Cl(1) 0.9500 2.6400 3.249(2) 123

C(46)-H(46)∙∙∙Cl(3) 0.9500 2.7300 3.397(2) 128

2

N(12)-H(12A)∙∙∙Cl(3) 0.87(3) 2.68(3) 3.534(3) 168(3)

N(12)-H(12B)∙∙∙Cl(1) 0.88(2) 2.43(2) 3.300(3) 172(2)

C(3)-H(3A)∙∙∙Cl(3) 0.9800 2.8200 3.657(3) 144

C(5)-H(5C)∙∙∙N(2) 0.9800 2.5300 3.485(4) 166

C(36)-H(36)∙∙∙Cl(2) 0.9500 2.6300 3.481(3) 150

C(46)-H(46)∙∙∙Cl(2) 0.9500 2.7700 2.472(3) 132

3

O(2)-H(2)∙∙∙O(3) 0.8400 1.8400 2.651(6) 163

Cl(3)-H(13)∙∙∙O(2) 0.9500 2.3500 2.696(6) 101

C(26)-H(12B)∙∙∙Cl(3) 0.9500 2.6200 3.406(4) 131

C(36)-H(12B)∙∙∙O(1) 0.9500 2.4400 3.389(6) 172

C(43)-H(12B)∙∙∙Cl(3) 0.9500 2.7600 3.658(6) 157

Geometry optimization was done for all three complexes using the density functional

methods B3LYP and PBE1PBE, the latter being the preferred method for rhenium-

complexes and the former being an old, however very popular and efficient method. The

computational bond lengths and angles for 1 – 3 have quite a lot of similarities; however

there are also some cases in which the predicted structural properties are quite different

from that experimentally obtained. This can be ascribed to a number of reasons, one of

which can be due to the effect of the solvent present. The computational bond lengths

and angles are summarized in Tables 3.8 – 3.10.



Figure 3.15 - Packing diagram of the unit cell of complex 3

3.3.2 Synthesis and characterization of trans-[ReCl2(t-BuCN)(PPh3)2(ap)](ReO4)

(4)

Complex 4 was synthesized in relatively low yield by heating trans-[ReCl3(t-

BuNC)(PPh3)2] with H2ap in a 1:3 molar ratio in benzene in a nitrogen atmosphere, and

subsequent recrystallization using several solvents. One chloride ion ligand was

substituted by one ap2- unit, which coordinates in a monodentate fashion as an imide.

The reaction is a disproportionation reaction, giving two rhenium atoms in different

oxidation states, namely +V and +VII, in an intricate redox process.

Complex 4 is a 1:1 electrolyte in DMF. It is insoluble in most solvents, the exceptions

being DMF and chloroform. The yellow/orange solution is stable in solution for several

weeks.



The ligand in complex 4 is coordinated in a similar fashion to the aniline derivatives in

complexes 1 – 3; however the complex is a cation with perrhenate as the counter-ion.

This is confirmed by the strong stretching frequency in its IR spectrum at 908 cm-1

[v(Re=O)] (Figure 3.16). There is also a strong O-H stretch observed around 3600 cm-1,

as to be expected. The characteristic C≡N, Re=C and Re=N bands are observed at

2200 cm-1, 1190 cm-1 and 1090 cm-1 respectively for 4 – similar to that of complexes 1 -

3.

The proton NMR spectrum of 4 was difficult to assign due to the overlapping of the

phenyl protons with that of the triphenylphosphine ligands. The two triphenylphosphine

ligands gave multiplets at 7.66, 7.28 and 7.19 ppm, which integrate for 30 protons. The

other peaks in the aromatic region are assigned to the phenyl protons of the

coordinated ligand, and integrates for 4 protons. The t-butyl isocyanide protons appear

as a singlet integrating for 9 protons at 2.10 ppm.

The absorption spectrum for 4 contains three bands. The low energy, intense peak at

about 287 nm is most probably due to the ligand-to-metal transitions [pπ(Cl-) d*π(Re)]

and [pπ(N2-) d*π(Re)]. The band at 408 nm can be ascribed to the metal-to-ligand

transition from the rhenium metal to the pi-acceptor isocyanide ligand [d*π(Re)

pπ(CNR)]. The low intensity shoulder band at 499 nm can be due to d-d transitions.

The crystal structure of the complex salt 4 is shown in Figure 3.18. The cationic

complex adopts a distorted octahedral geometry around the rhenium(V) centre. The

metal coordinates to two phosphorus atoms, two chlorides, the isocyanide carbon atom

C(1) and the imido nitrogen atom N(2). The O(1)H group is left uncoordinated in the

complex.



Figure 3.16: Infrared spectrum of complexes 4


Complex 4 contains the ligand coordinated in a similar fashion to complexes 1 – 3, with

the unanticipated presence of the perrhenate counter ion. The recrystallization process

for 4 is much more complex and contains several more steps than that of 5, and it is



suspected that during this recrystallization, the perrhenate is somehow introduced. The

C-O bond length of 4 is 1.36(2) Å confirming its single bond character.

Selected bond lengths and angles are given in Table 3.11. Distortion from an ideal

metal-centered octahedron results in a non-linear N(2)-Re-Cl(2) axis of 175.2(4)°, due

to C(1)-Re-Cl(1) and P(1)-Re-P(2) angles of 178.6(4)° and 171.12(8)° respectively.

The rhenium atom is lifted out of the mean equatorial P2ClC plane by 0.160 Å towards

N(2), which is the result of the non-orthogonal angles N(2)-Re-P(1) = 93.74(6)°, N(2)-

Re-P(2) = 93.74(6)° and N(2)-Re-Cl(1) = 91.4(4)°. The N(2)-Re-C(1) angle is orthogonal

at 90.0(6)°.




The t-BuNC is coordinated in a linear fashion [Re-C(1)-N(1) = 176.6(1)°; C(1)-N(1)-C(2)

= 178.9(2)°]. The Re-C(1) and Re-N(2) bond lengths are 2.01(1) and 1.75(2) Å

respectively, with the Re-N(2)-C(41) bond angle equal to 178.6(1)°. The Re-P and Re-

Cl bond distances are within the expected ranges for these types of bonds [16]. The

packing diagram of 4 is given in Figure 3.19 which shows that each unit cell contains

four molecules of 4.

Figure 3.19: Packing diagram of 4.



3.3.3 Synthesis and characterization of trans-[ReCl2(t-BuCN)(PPh3)2(ibq)] (5)

H2ap was reacted with trans-[ReCl3(t-BuNC)(PPh3)2] in a 1:3 molar ratio in refluxing

benzene to form complex 5 in a relatively good yield. Atmospheric conditions were

carefully controlled by working in an inert nitrogen atmosphere. The crystals are not

stable in a solution of DMF, and go from an initial red to a yellow solution within a

couple of days. This compound was confirmed to be a non-electrolyte in DMF. Its

solubility is limited to solvents such as DMF and chloroform.

The IR spectrum of 5 is seen in Figure 3.20, and is overlaid with that of 4 in order to

characterize the spectrum more efficiently. The differences between the two complexes

can easily be identified in this manner. For instance, 5 has no ReVII=O absorption

frequency which is expected approximately at 900 cm-1. Complex 5 has an o-quinone-

imine coordinated to the metal centre, thus one should find a C=O stretching frequency.

The fact that this is not clearly seen can be ascribed to the delocalization of electrons

observed in the ligand (Figure 3.3). Complex 4 has an phenolic group, which explains

the strong signal at 3600 cm-1, which is absent in the spectrum of 5.

Complex 5 is paramagnetic, thus the observed chemical shifts on its proton NMR

spectrum appear over a wide range and the resolution is quite distorted. The protons on

the two triphenylphosphine moieties gave rise to three multiplets at 7.67, 7.26 and 7.19

ppm, integrating for 30 protons. The protons on the coordinated phenyl ligand gave

three signals which integrate for 4 protons at 7.58, 7.43 and 7.32 ppm.

Complex 5 shows two very clear distinct bands of relatively low intensity at 326 and 501

nm, ascribed to charge-transfer transitions [pπ(Cl-) d*π(Re)] and [pπ(N-) d*π(Re)].

There are also two weak shoulder bands observed at about 437 and 549 nm due to d-d

transitions. Their low intensity is a result of the transitions being Laporte forbidden.



Figure 3.20: IR spectra of 4 and 5






A



The cyclic voltammogram of complex 5 is shown in Figure 3.23. There is only one

process seen in the potential region scanned, namely process A. It is an irreversible

anodic peak with Eac = 711 mV and Ia = 4.5869 μA. One would expect to find a more

complex current response due to the intricate redox process which occurred upon

synthesizing complex 5. After a very weak response was obtained when using a

platinum working electrode, it was replaced by a glassy carbon electrode; however

similar results were obtained.

The asymmetric unit of complex 5 is given in Figure 3.24. The rhenium centre is

coordinated to two chlorides in cis positions, two phosphines in trans positions, and the

nitrogen N(1) at a site cis to the isonitrile carbon C(21). The oxygen O(1) is not

coordinated.

The coordination mode of the ap ligand in 5 is different to that in complex 4. The intra-

ligand bond distances of the mono-coordinated 2-aminophenol derivative assist in

determining the formal oxidation state of the rhenium. The O(1)-C(12) bond length of

1.266(6) Å implies that this bond is double compared to the similar bond of 1.36(2)Å in

4. The C(13)-C(14) [1.32(1) Å] and C(15)-C(16) [1.346(7) Å] bonds are double, with the

C(12)-C(13) [1.465(8) Å], C(14)-C(15) [1.412(8) Å], C(11)-C(16) [1.429(7) Å] and C(11)-

C(12) [1.450(7) Å] bonds all single. The N(1)-C(11) length of 1.334(5) Å implies that this

is also a double bond. These results indicate that the ligand is coordinated in the o-

quinone-imine form, as illustrated in Figure 3.25. This shows that the rhenium is in the

formal oxidation state of +III.

The Re-N(1) bond is a double bond [Re-N(1) = 1.748(3) Å], and the Re-N(1)-C(11)

angle of 172.6(3)° confirms the sp hybridization of N(1). The degree of distortion

between N(1) and the donor atoms cis to it differs remarkably from those in complex 4

[N(1)-Re-P(1) = 88.0(1)°, N(1)-Re-P(2) = 87.8(1)°, N91)-Re-C(21) = 94.1(2)°; N(1)-Re-

Cl(2) =104.6(1)°], with the result that the rhenium is lifted out of the mean P2CCl

equatorial plane by 0.098 Å towards N(1), compared to 0.160 Å in 4. The Re-C(21) and



C(21)-N(2) bond lengths are 2.016(5) and 1.162(6) Å respectively, and the Re-C(21)-

N(2) bond angle is linear [173.4(4)°].

It is clear from the crystal structure of 5 (Figure 3.24) that the phenolic oxygen did not

coordinate to the rhenium metal. There have been many cases in which a phenolic

oxygen coordinated to rhenium, but there is a relatively high energy barrier required for

the 1e- redox reaction which forms such a Re-O species [18]. In the case of 5 it was

more favorable to form the imido-species.




Figure 3.25: Coordination of the o-quinone-imine form of H2ap to Re(III)

Figure 3.26 illustrates the intramolecular hydrogen-bonding interactions.

Table 3.3: Hydrogen interactions for complex 5

A-H∙∙∙B A-H (Å) H∙∙∙B (Å) A∙∙∙B (Å) A-H∙∙∙B (°)

C(14)-H(14)∙∙∙Cl(2) 0.9500 2.6900 3.576(5) 156

C(15)-H(15)∙∙∙Cl(2) 0.9500 2.8000 3.640(5) 147

C(36)-H(36)∙∙∙Cl(1) 0.9500 2.5600 3.427(5) 152

C(42)-H(42)∙∙∙Cl(2) 0.9500 2.7000 3.513(6) 143

C(56)-H(56)∙∙∙O(1) 0.9500 2.4800 3.339(6) 150

C(62)-H(62)∙∙∙Cl(2) 0.9500 2.7400 3.510(5) 138

C(72)-H(72)∙∙∙Cl(1) 0.9500 2.6300 3.475(5) 148

C(82)-H(82)∙∙∙O(1) 0.9500 2.4600 3.331(6) 152

C(84)-H(84)∙∙∙Cl(1) 0.9500 2.8200 3.705(5) 155



Figure 3.26: Intramolecular hydrogen interactions separated by >5 bonds for 5

3.4 References

[1] R. Alberto, Eur. J. Nucl. Chem. Biol., 2003, 30, 1299; S.S. Jurisson, J.D. Lydon,

Chem. Rev., 1999, 99, 2205.

[2] F.E. Hahn, M. Glaser, T. Lugger, D. Scheller, H Spies, Inorg. Chim. Acta, 1995,

232, 238.

[3] K.C. Potgieter, PhD Thesis, Nelson Mandela Metropolitan Unviersity, 2012, 9.

[4] J.B. Arterburn, M.C. Perry, K.V. Rao, Angew. Chem. Int. Ed., 2000, 39, 771.



[5] J.B. Arterburn, D.M. Goreham, M.S. Holguin, K.V. Rao, M.V. Valenzuela,

Organomettallics, 2009, 19,1789.

[6] E.J. Sawyer, A.M. Cassoni, W. Waddington, J.B. Bomanji, T.W. Briggs, British J.

Radiol., 1999, 72, 1225.

[7] U. Abram, R. Alberto, G. Artus, A. Egli, W.A. Herrmann, T.A. Kaden, R. Schibli,

A. Schubiger, J. Organomet. Chem., 1995, 492, 217.

[8] U. Abram, R. Alberto, J. Braz. Chem. Soc., 2006, 17, 1486.

[9] I.N. Booysen, Rheniium(I) and (V) Complexes with Potentially Multidentate

Ligands containing the Amino Group, PhD Thesis, Nelson Mandela Metropolitan


[10] APEX2, SADABS and SAINT, Bruker AXS Inc., Madison, Wisconsin, USA, 2010.

[11] G.M. Sheldrick, A short history of SHELX, Acta Cryst., 2008, A64, 112.

[12] C.B. Hübschle, G.M. Sheldrick, B. Dittrich, ShelXle: a Qt graphical user interface

for SHELXL, J. Appl. Cryst., 2011, 44,1281.

[13] A. L. Spek, Acta Cryst., 2009, D65, 148.

[14] M.T. Ahmet, B. Coutinho, J.R. Dilworth, J.R. Miller, S.J. Parrott, Y. Zheng,

Polyhedron,1996, 15, 2042.

[15] M. Fatima, C. Guedes da Silva, R.A. Michelin, A.J.L. Pombeiro, Coord. Chem.

Rev., 2001, 218, 47.



[16] F. Refosco, F. Tisato, C. Bolzati, G. Bandoli, J. Chem. Soc., Dalton Trans., 1993,

605, and references therein.

[17] R.H. Crabtree, The Organometallic Chemistry of the Transition Metals, 3rd

edition, Wiley-Interscience Publication, Canada, 2001, 87.

[18] C.A. Lippert, K.I. Hardcastle, J.D. Soper, Inorg. Chem., 2011, 50, 9864, and

references therein.



Table 3.4 - Selected crystal and data collection details for complexes 1 - 3.

Complex 1 2 3

Formula C29H29Cl3N2PRe C29H29Cl3N2PRe.

C2H3N

C30H29Cl3N2O2PRe.

C4H10O

Formula weight 729.07 785.14 847.20

Crystal System Monoclinic Monoclinic Monoclinic

Space Group P21/c P21/c P21/c

a [Å] 13.5158(2) 9.1212(2) 10.7273(4)

b [Å] 13.9991(2) 16.7180(4) 17.4691(6)

c [Å] 15.6529(3) 21.0649(6) 10.8285(4)

α [°] 90 90 90

β [°] 96.058(1) 94.159(1) 115.866(1)

γ [°] 90 90 90

Volume [Å3] 2945.13(8) 3203.7(1) 1825.9(1)

Z 4 4 2

Dcalc [g/cm3] 1.644 1.628 1.541

Absorption

Coefficient (mm-1) 4.473 4.120 3.625

F(000) 1432 1552 844

Crystal size [mm] 0.13 × 0.17 ×

0.26 0.03 × 0.20 × 0.33 0.08 × 0.09 × 0.10

Theta Min-Max [°] 2.0 – 28.3 2.2 – 28.3 2.1 – 27.5

Index ranges

H -17 : 18 -12 : 12 -13 : 13

K -18 : 17 -22 : 22 -22 : 22

L -19 : 20 -3 : 28 -14 : 10

Goodness-of-fit 1.14 1.05 0.83

Observed data [I >

2.0 σ I] 6675 6976 7107

R 0.0154 0.0233 0.0161

wR2 0.0356 0.0455 0.0329



Table 3.5 - Selected crystal and data collection details for complexes 4 - 5.

Complex 4 5

Formula C47H44Cl2N2OP2Re, O4Re, O C47H43Cl2N2OP2Re

Formula weight 1238.10 970.88

Crystal System Orthorombic Monoclinic

Space Group Pmna P21/c

a [Å] 15.1070(5) 17.0475(5)

b [Å] 17.3430(6) 10.5682(2)

c [Å] 19.8490(7) 24.4531(6)

α [°] - 90

β [°] - 100.821(1)

γ [°] - 90

Volume [Å3] 5200.5(3) 4327.2(2)

Z 4 4

Dcalc [g/cm3] 1.581 1.490

Absorption Coefficient (mm-1) 4.8959 3.043

F(000) 2408 1944

Crystal size [mm] 0.03 × 0.02 × 0.10 0.05 × 0.06 × 0.30

Theta Min-Max [°] 1.7 – 28.4 1.7 – 28.3

Index ranges

h -20 : 19 -22 : 22

k -23 : 22 -10 : 14

l -21 : 26 -32 : 32

Goodness-of-fit 1.26 1.03

Observed data [I > 2.0 σ I] 5828 7903

R 0.0760 0.0355

wR2 0.1823 0.0930



Table 3.6 – Selected experimental bond distances [Å] for complexes 1 - 3.

Bond 1 2 3

Re-N(1) 1.715(2) - 1.717(3)

Re-P(1) 2.4438(5) 2.4283(6) 2.447(1)

Re-C(1) 2.032(2) 2.024(4) -

Re-Cl(1) 2.4100(4) 2.4525(6) 2.4203(8)

Re-Cl(2) 2.4194(5) 2.4143(7) 2.419(1)

Re-Cl(3) 2.4026(5) 2.4150(7) 2.415(1)

C(1)-N(2) 1.157(3) - -

N(1)-C(11) 1.376(3) - 1.380(4)

Re-N(11) - 1.728(2) -

C(1)-N(1) - 1.151(3) -

N(11)-C(11) - 1.356(3) -

Re-C(2) - - 2.032(4)

C(2)-N(2) - - 1.146(6)

N(1)-C(11) - - 1.380(4)



Table 3.7 – Selected experimental bond angles [°] for complexes 1 - 3.

Angle 1 2 3

N(1)-Re-Cl(2) 96.55(5) - 97.9(1)

N(1)-Re-P(1) 93.23(5) - 91.3(1)

N(1)-Re-Cl(1) 170.22(5) - 95.3(1)

N(1)-Re-Cl(3) 97.36(6) - 172.6(1)

N(1)-Re-C(1) 90.85(8) - -

C(1)-N(2)-C(2) 177.1(2) - -

Re-C(1)-N(2) 177.0(2) - -

Cl(1)-Re-Cl(2) 87.30(2) - 87.97(4)

N(11)-Re-Cl(1) - 94.83(7) -

N(11)-Re-P(1) - 92.18(7) -

N(11)-Re-Cl(2) - 170.82(6) -

N(11)-Re-Cl(3) - 97.16(6) -

N(11)-Re-C(1) - 87.43(9) -

C(1)-N(1)-C(2) - 176.8(3) -

Re-C(1)-N(1) - 174.4(2) -

Cl(1)-Re-Cl(3) - 88.20(3) -

Re-N(11)-C(11) - 175.4(2) -

P(1)-Re-Cl(2) - 87.23(2) -

N(1)-Re-C(2) - - 93.9(1)

C(3)-N(2)-C(2) - - 169.9(5)

Re-C(2)-N(2) - - 174.1(4)



Table 3.8 – Computational bond distances and angles for 1.

Bond PBE1PBE B3LYP

Re-Cl(1) 2.457 2.484

Re-Cl(2) 2.475 2.520

Re-Cl(3) 2.441 2.479

Re-P(1) 2.479 2.529

Re-N(1) 1.709 1.719

Re-C(1) 2.014 2.030

P(1)-C(21) 1.829 1.842

P(1)-C(31) 1.832 1.845

P(1)-C(41) 1.831 1.841

N(1)-C(11) 1.368 1.373

N(2)-C(1) 1.165 1.167

Angle PBE1PBE B3LYP

Cl(1)-Re-Cl(2) 86.7 86.3

Cl(2)-Re-Cl(3) 86.5 86.7

Cl(2)-Re-P(1) 169.8 171.1

Cl(2)-Re-N(1) 95.8 94.4

Cl(2)-Re-C(1) 85.9 86.1

Cl(3)-Re-P(1) 86.9 87.4

Cl(1)-Re-Cl(3) 88.5 87.9

Cl(1)-Re-C(1) 79.8 80.2



Table 3.9 - Computational bond distances and angles for 2.

Bond PBE1PBE B3LYP

Re-Cl(1) 2.443 2.481

Re-Cl(2) 2.442 2.465

Re-Cl(3) 2.431 2.464

Re-P(1) 2.450 2.505

Re-N(11) 1.720 1.731

Re-C(1) 1.997 2.014

P(1)-C(21) 1.833 1.847

P(1)-C(31) 1.834 1.847

P(1)-C(41) 1.828 1.842

N(1)-C(1) 1.174 1.176

N(11)-C(11) 1.353 1.358

Angle PBE1PBE B3LYP

Cl(1)-Re-Cl(2) 88.2 88.1

Cl(2)-Re-Cl(3) 91.4 91.1

Cl(2)-Re-P(1) 82.3 83.6

Cl(2)-Re-N(11) 170.2 171.4

Cl(2)-Re-C(1) 88.2 79.3

Cl(3)-Re-P(1) 89.8 89.5

Cl(1)-Re-Cl(3) 86.5 86.9

Cl(1)-Re-C(1) 84.3 84.2



Table 3.10 – Computational bond distances and angles for 3.

Bond PBE1PBE B3LYP

Re-Cl(1) 2.425 2.457

Re-Cl(2) 2.427 2.468

Re-Cl(3) 2.445 2.464

Re-P(1) 2.464 2.519

Re-N(1) 1.714 1.726

Re-C(2) 2.010 2.025

P(1)-C(21) 1.835 1.848

P(1)-C(31) 1.828 1.839

P(1)-C(41) 1.835 1.847

N(1)-C(11) 1.366 1.372

N(2)-C(2) 1.167 1.171

Angle PBE1PBE B3LYP

Cl(1)-Re-Cl(2) 87.1 87.6

Cl(2)-Re-Cl(3) 87.5 87.6

Cl(2)-Re-P(1) 168.4 169.9

Cl(2)-Re-N(1) 94.6 93.2

Cl(3)-Re-P(1) 81.5 83.1

Cl(1)-Re-Cl(3) 91.2 91.0

Cl(1)-Re-N(1) 94.7 94.6



Table 3.11 – Selected experimental bond distances [Å] for complexes 4 - 5.

Bond 4 5

Re-N(2) 1.75(1) -

Re-P(1) 2.48(3) 2.472(1)

Re-C(1) 2.01(1) -

Re-P(2) 2.48(3) 2.471(1)

Re-N(1) - 1.748(1)

Re-Cl(1) 2.42(3) 2.478(1)

Re-Cl(2) 2.40(3) 2.403(1)

Re-C(21) - 2.016(5)

C(21)-N(2) - 1.162(6)

N(1)-C(11) - 1.334(5)

C(1)-N(1) 1.17(2) -

N(1)-C(2) 1.47(2) -



Table 3.12 - Experimental bond angles [°] for complexes 4 - 5.

4

N(1)-Re-Cl(1) 168.4(1)

N(1)-Re-P(1) 88.0(1)

N(1)-Re-P(2) 87.8(1)

Re-C(21)-N(2) 173.4(4)

N(1)-Re-Cl(2) 104.6(1)

N(1)-Re-C(21) 94.0(2)

C(21)-N(2)-C(22) 170.1(5)

Cl(1)-Re-Cl(2) 87.05(4)

5

N(2)-Re-Cl(2) 175.2(4)

N(2)-Re-P(1) 93.74(6)

P(1)-Re-P(2) 171.12(8)

C(1)-Re-N(2) 90.0(6)

C(1)-Re-Cl(1) 178.6(4)

C(1)-Re-P(1) 92.39(6)

C(1)-N(1)-C(2) 179(2)

C(1)-Re-Cl(2) 85.2(4)



Table 3.13 - Experimental intra-ligand bond distances [Å] for complexes 1 - 3.

1 2 3

C(11) – C(12) 1.390(3) 1.416(4) 1.412(5)

C(12) – C(13) 1.384(4) 1.404(4) 1.382(5)

C(13) – C(14) 1.370(4) 1.370(5) 1.380(7)

C(14) – C(15) 1.370(4) 1.401(4) 1.385(6)

C(15) – C(16) 1.385(4) 1.361(4) 1.380(5)

C(11) – C(16) 1.389(3) 1.416(4) 1.391(6)

C(12) – X - 1.344(4) 1.489(6)

X for (1) = H; (2) = N; (3) = C.

Table 3.14 - Experimental intra-ligand bond distances [Å] for complexes 4 - 5.

4 5

C(11) – C(12) 1.41(2) 1.450(7)

C(12) – C(13) 1.41(3) 1.465(8)

C(13) – C(14) 1.31(3) 1.32(1)

C(14) – C(15) 1.34(3) 1.412(8)

C(15) – C(16) 1.38(3) 1.346(7)

C(11) – C(16) 1.36(2) 1.429(7)

C(12) – O 1.36(2) 1.266(6)

Chapter 4 Bis-isocyanide Complexes


Chapter 4 Isolation of Bis-isocyanide

Complexes of Re(I)

4.1 Introduction

The radionuclides 186Re and 188Re, as well as 99mTc, are of great interest in

radiopharmacy, due to their highly favourable properties as therapeutic and diagnostic

agents respectively [1]. The coordination requirements of the relevant metal in a certain

oxidation state containing a specific ligand system are used as basis for radiolabeling

[2]. These requirements are usually characterized by chemically robust cores which are

used as foundation for the synthesis of radiopharmaceuticals [2].

The core of specific interest in this particular study is the [Re(CO)3]+ core. The Re(I)

tricarbonyl core is a small, kinetically inert moiety, which readily allows low molecular

weight biomolecules to bind without the loss of biological activity or specificity [2].

The reaction of [Re(CO)5Cl] with isocyanides have been investigated to some extent [3].

The isocyanide coordination chemistry of technetium has, however, been studied quite

well and they are known to form extremely stable complexes with technetium due to

their electronic properties [4]. The technetium compound used as a radiopharmaceutical

in heart-imaging [99mTc(MIBI)6]+ (MIBI = 2-methoxy-2-methylpropyl-isocyanide) is well

known and serves as motivation for the further study and expansion of Re(I)-isocyanide

coordination chemistry.



This chapter focuses on the reaction of [Re(CO)5Cl] with isocyanides and subsequent

reaction with aquo ligands to give a mixed ligand system with the tricarbonyl core.

These aquo ligands are very labile, and can readily be replaced by a desired ligand.

The reactions of the rhenium(I) complex [Re(CO)5Cl] with tert-butyl isocyanide (t-BuNC)

and cyclohexyl isocyanide (CyNC) led to the formation of [Re(CO)3(t-BuNC)2Cl] (1) and

[Re(CO)3(CyNC)2Cl] (2) respectively. The Re(I) precursor was first stirred in boiling

tetrahydrofuran for two days to remove two carbonyls, after which it was reacted with

the respective isocyanides at room temperature. Reaction of 1 with AgOTf (silver

trifluoromethanesulfonate) and subsequently with water, gave [Re(CO)3(t-

BuNC)2(OH2)](OTf) (3).

4.2 Experimental

4.2.1 Synthesis of fac-[Re(CO)3(t-BuNC)2Cl] (1)

A previously reported method [3] was adapted as follows: [Re(CO)5Cl] (424 mg, 1.2

mmol) was dissolved in 60 mL THF and refluxed under nitrogen for 12 hours. t-BuNC

(200 mg, 2.4 mmol) was added to the reaction mixture and it was stirred for an

additional 24 hours. After removal of the solvent under vacuum, a white precipitate

formed which was filtered and recrystallized from a dichloromethane-n-hexane (2:1 v/v)

mixture. The white crystals were washed with diethyl ether and dried. Yield = 20%, m.p.

160-163 °C. Anal. Calcd. (%): C, 33.08; H, 5.94; N, 3.84. Found: C, 33.47; H, 5.35; N,

4.63. IR (νmax/cm-1): ν(C≡O) 1890 (s), 1954 (s), 2030 (s) ; ν(C≡N) 2183 (s), 2207 (s).

1H-NMR (CDCl3) (ppm): 1.58 (s, 18H, C(CH3)3).

4.2.2 Synthesis of fac-[Re(CO)3(CyNC)2Cl] (2) (CyNC = Cyclohexyl isocyanide)

A mixture of [Re(CO)5Cl] (424 mg, 1.2 mmol) and CyNC (262 mg, 2.4 mmol) in 60 mL

THF was heated under reflux for 24 hours. The solvent was evaporated under vacuum

and formed a white precipitate, which was filtered off and recrystallized from a



dichloromethane-n-hexane (1:1 v/v) mixture. The white crystals were washed with

diethyl ether and were suitable for X-ray diffraction studies. Yield = 45%, m.p. 119-122

°C. Anal. Calcd. (%): C, 38.86; H, 4.19; N, 5.41. Found: C, 38.97; H, 4.23; N, 5.35. IR

(νmax/cm-1): ν(C≡O) 1892 (s), 1957 (s), 2030 (s) ; ν(C≡N) 2192 (s), 2210 (s). 1H-NMR

(CDCl3) (ppm): 1.49 (s, 2H, (CNCH-C5H10)), 1.40 (d, 8H, (m-H)), 1.73 (s, 8H, (o-H)),

1.86 (s, 4H, (p-H)). Conductivity (10-3 M, CH2Cl2): 1 μS.

4.2.3 Synthesis of fac-[Re(CO)3(t-BuNC)2(H2O)](OTf) (3)

(OTf = trifluoromethanesulfonate)

Complex 1 (27 mg, 0.058 mmol) was dissolved in 7 mL of dichloromethane and 0.017 g

AgOTf (0.066 mmol) was added. The reaction mixture was stirred for 30 minutes at

room temperature and the AgCl precipitate was filtered off. Four μL of distilled water

was then added to the filtrate, and stirred overnight. The solvent was partially removed

under vacuum, and the residue was filtered off, and recrystallized from ethanol to give

colourless crystals, suitable for X-ray diffraction studies. Yield = 57 %, m.p. 106-109 °C.

Anal. Calcd. (%): C, 28.22; H, 3.20; N, 4.41; S, 4.50. Found: C, 27.86; H, 3.34; N, 4.64;

S, 5.31. IR (νmax/cm-1): ν(C≡O) 1895 (s), 1931 (s), 2040 (s) ; ν(C≡N) 2199 (s), 2219 (s).

1H-NMR (DMSO) (ppm): 2.15 (s, 2H, H2O); 2.56 (s, 18H, C(CH3)3). Conductivity (10-

3M, MeCN): 100 μS.


4.3.1 Synthesis and characterization of fac-[Re(CO)3(t-BuNC)2Cl] (1) and fac-

[Re(CO)3(CyNC)2Cl] (2)

The neutral complexes fac-[Re(CO)3(t-BuNC)2Cl] (1) and fac-[Re(CO)3(CyNC)2Cl] (2)

were formed by the reaction of [Re(CO)5Cl] with tert-butyl isocyanide and cyclohexyl

isocyanide respectively. Two isocyanide ligands bind to the rhenium atom, substituting

two carbonyl groups in the process, as depicted below.



[Re(CO)5Cl] + 2CNR [Re(CO)3(CNR)2Cl] + 2CO

For both reactions no solid was isolated after refluxing for several hours. In both cases,

crystals of excellent quality were isolated upon removal of the solvent and

recrystallization of the residues in a suitable solvent. Both complexes 1 and 2 are

insoluble in acetonitrile, methanol and DMF, and soluble in ethanol as well as

dichloromethane.

Complex 1 and 2 contain three carbonyl groups centered around a central rhenium

atom, as well as two isonitrile ligands. The former is characterized by three intense

stretching frequencies in the IR spectrum (Figure 4.1), typical of fac-[Re(CO)3]+, at 1890,

1954 and 2030 cm-1. Nearly identical bands are observed for 2. The two isonitrile

ligands are also clearly distinguished from one another at frequencies of 2183 and 2207

cm-1 for 1, with similar bands seen for 2.

Figure 4.1: IR spectrum of 1 in the range 900 – 2400 cm-1




The simple 1H-NMR spectrum for 1 is quite useful in its characterization, since the only

protons present are those of the t-butyl isocyanide ligand which is highly symmetrical

and magnetically equivalent. These protons were assigned to the singlet at 1.58 ppm,

integrating for 18 protons. Complex 2 doesn’t contain these highly symmetrical

properties, and thus it was more difficult to assign their respective protons. The proton

next to the isonitrile functional group (CNCH-C5H10) is assigned to the singlet at 1.49

ppm, due to the shielding effects experienced by this particular proton. The peaks at

1.73, 1.40 and 1.86 were assigned to the ortho-, meta-, and para-hydrogens on the

cyclohexyl ring. The UV-Vis spectrum of both complexes 1 and 2 revealed no

observable transitions.

The cyclic voltammogram for 2 only shows one anodic peak at 1338 mV with an anodic

peak current of 19.8 μA. This is an irreversible process, since no corresponding

cathodic peak is observed, as can be seen from Figure 4.3. Complexes 1 and 2 are

structurally similar – the only difference being the type of isocyanide coordinated to the



rhenium. They are expected to behave similarly upon subjecting them to an excitation

signal, thus only the current response for 2 was analysed.


A perspective view of the asymmetric units of complexes 1 - 2 are portrayed in Figures

4.4 and 4.5 respectively. Complexes 1 and 2 are structurally similar, the major

difference being the type of isocyanide ligands bonded to the central rhenium atom.

Complexes 1 – 2 are typical tricarbonyl rhenium complexes, as indicated by the average

Re-CO bond length of 1.950 Å, which corresponds to the range normally observed

[1.900(2)-1.928(2) Å] for similar complexes [5]. A chloro ligand and a carbonyl ligand

occupy the axial sites, with two isocyanides and two carbonyls in the equatorial plane.

The three carbonyls are coordinated in a facial orientation to the rhenium(I) central atom

– characteristic of the chemically robust fac-[Re(CO)3]+ core [5].



Figure 4.4: ORTEP diagram of 1, showing 40% ellipsoid probability. Hydrogen atoms

have been omitted for clarity

The Re-CNR bonds all have some double bond character, as can be seen from their

average bond lengths of 2.096(5) Å (Tables 4.6 and 4.7), which are intermediate to that

normally found for typical rhenium(I)-carbon single and double bonds [6]. There is a

slight distortion from an ideal octahedral geometry, evident from the trans angles. This

distortion is not significant, due to the absence of multidentate ligands imposing further

constraints to the geometry. The Cl-Re-C(3) bond angle [89.9(2)° in 1] is extremely

close to orthogonality, which is not surprising at all. The tert-butyl isocyanides are

linear, deviating slightly from linearity, as seen from the C(1)-N(1)-C(11) [173.0(5)°] and

C(2)-N(2)-C(21) [177.(5)°] for complex 1. The cyclohexyl isocyanide ligands in complex

2 are more linear, with an average C-N-C angle of 176.3(2)°.

The central rhenium atom is not lifted out of the mean equatorial plane of the complexes

to any significant extent. This is probably due to the absence of multidentate ligands, as

mentioned above.



Figure 4.5: ORTEP diagram of 2, showing 40% ellipsoid probability. Hydrogen atoms

have been omitted for clarity

Figure 4.6: Unit cell of 2 showing short van der Waals intramolecular contact distances



The unit cell of 2 contains two complexes and is shown in Figure 4.6, which illustrates

intramolecular contact distances larger than three bonds. Only one was found between

N(1) and H(15B) at a distance of 2.72 Å. This contact distance is relatively short,

indicating that there are only van der Waals forces present in this particular complex [5].

4.3.2 Synthesis and characterization of fac-[Re(CO)3(t-BuNC)2(H2O)](OTf) (3)

The reaction of a mixture of [Re(CO)3(t-BuNC)2Cl] and AgOTf gave the intermediate

product [Re(CO)3(t-BuNC)2(OTf)], as well as AgCl. The AgCl was filtered off and

[Re(CO)3(t-BuNC)2(OTf)] was reacted with water to give fac-[Re(CO)3(t-

BuNC)2(H2O)](OTf) (3). The axial chloro ligand is substituted for an aquo ligand, as

illustrated below.

[Re(CO)3(t-BuNC)2Cl] + AgOTf [Re(CO)3(t-BuNC)2(OTf)] + AgCl

[Re(CO)3(t-BuNC)2(OTf)] + H2O [Re(CO)3(t-BuNC)2(OH2)]+ + OTf-

Recrystallization of the precipitate obtained from the mother liquor in ethanol gave white

crystals suitable for X-ray diffraction studies. Complex 3 is soluble in acetonitrile and

methanol. Its conductivity measurement confirms it being a 1:1 electrolyte in acetonitrle

(100 μS).

The IR spectrum of 3 is characterized by three intense stretching frequencies at 1895,

1931 and 2040 cm-1, which is the result of the v(C≡O) of its three carbonyl groups

(Figure 4.7). The isocyanide C≡N stretching frequencies are also observed at 2199 and

2219 cm-1. The presence of the aquo ligand is evident from the broad peak at 3000-

3400 cm-1, ascribed to the O-H stretching frequency bands.



Figure 4.7: IR spectrum for 3 in the range 1500 – 3500 cm-1

The proton NMR spectrum for 3 quite useful in characterization, due to the symmetrical

properties of the tert-butyl isocyanide ligand. The protons on the latter ligand are

assigned to the singlet resonating at 2.56 ppm, which integrates for 18 protons. The

protons on the aquo ligand resonate at 2.15 ppm.

Table 4.1: Peak parameters for the cyclic voltammogram of 3.

Peak A B

Epc (mV) -376 -1315

Epa (mV) - 1211

E1/2 (mV) - -52

ic (μA) 0.949 8.896

ia(μA) - 3.937

ia/ic - 0.45




The cyclic voltammogram of complex 3 shows the current response when a solution of

3 (0.001M) was subjected to a cyclic excitation signal. The working electrode was a

carefully polished glassy carbon electrode. From the summarized cyclic voltammetric

parameters in Table 4.1 and the cyclic voltammogram in Figure 4.8, it is clear that there

are two processes observed for 3. Process A is an irreversible process, whereas B is a

quasi-reversible two-electron redox couple.

A perspective view of the assymmetric unit of 3 is shown in Figure 4.9. An aquo ligand

and a carbonyl ligand occupy the axial sites, with two isocyanides and two carbonyls in

the equatorial plane. Overall, the structure is only slightly distorted. The trans axial

angle O(6)-Re-C(4) is extremely close to orthogonality [178.1(1)°]. The fact that there is

not a great deal of distortion from an ideal octahedral geometry is most probably due to

the relatively small, monodentate ligands present. Complex 3 is structurally similar to 1,

the axial substituent being the major difference. Complex 1 has a chloro ligand in the

axial site, whereas complex 3 has a labile aquo ligand, which can readily be substituted

with a ligand of interest. The structural similarities are clearly depicted in Figure 4.10.

A

B



Figure 4.9: ORTEP diagram of 3, showing 40% ellipsoid probability. The hydrogen

atoms have been omitted for clarity

Figure 4.10: Structure overlay of complexes 1 and 3



Table 4.8 shows the experimental bond lengths and angles for 3. Complex 3 also

contains the fac-[Re(CO)3]+ core, with an average Re-CO bond length of 1.943 Å, which

is concordant with expected values [1.900(2)-1.928(2) Å] [5]. The rhenium-carbonyl

bond [1.897(3) Å] trans to the aquo ligand is somewhat shorter than the corresponding

rhenium-carbonyl bond trans to the chloride in complex 1 [1.915(5) Å]. The rhenium-

isocyanide bond shows double bond character, as seen from the average value of

2.095(3) Å, identical to that of complex 1. The Re-O bond [2.22(2) Å] of H2O is typical of

rhenium-aquo bond distances and is significantly longer than other typical Re-O bonds

such as phenolic oxygens [1.84-1.97Å], which indicates that it is more weakly bound

and neutral, and thus easily replaceable [7, 8].

The hydrogens on the aquo ligand in 3 connects to the anion OTf- via two hydrogen-

bonds (Figure 4.11). These intramolecular hydrogen-bonds O(6)-H(6A)∙∙∙O(71) [1.87(2)

Å] and O(6)-H(6B)∙∙∙O(73) [1.81(2) Å] are relatively strong, and agree with other [O-

H∙∙∙O] hydrogen-bonds found in the literature [9].

Figure 4.11: Intra- and intermolecular hydrogen-bonding for 3



Table 4.2: Hydrogen-bond parameters for complex 3.

A-H∙∙∙B A-H H∙∙∙B A∙∙∙B A-H∙∙∙B

O(6)-H(6A)∙∙∙O(71) 0.84(2) 1.87(2) 2.687(4) 166

O(6)-H(6B)∙∙∙O(73) 0.83(2) 1.81(2) 2.642(4) 177

4.4 References

[1] U. Abram, R. Alberto, R. Schibli, A.P. Schubiger, R. Waibel, Coord. Chem. Rev.,

1999, 190-192, 903.

[2] J. Babich, S.R. Banerjee, M.K. Levadala, L. Wei, J. Zubieta, Inorg. Chem.

Commun., 2003, 6, 1099.

[3] K.K. Cheung, A. Mayer, L. Yang, J. Organomet. Chem., 1999, 585, 26.

[4] R. Alberto, H. Braband, N.I. Gorshkov, V.V. Gurzhiy, S.V. Krivovichev, E.M.

Levitskaya, A.A. Lumpov, A.E. Miroslavov, G.V. Sidorenko, D.N. Suglobov, I.G.

Tananaev, J. Organomet. Chem., 2008, 693, 7.

[5] I. Booysen, T.I.A. Gerber, E. Hosten, P. Mayer, J. Iran. Chem. Soc., 2008, 5, 93.

[6] M. Fatima, C. Guedes da Silva, R.A. Michelin, A.J.L. Pombeiro, Coord. Chem.

Rev., 2001, 218, 47.

[7] R. Mahfouz, E. Al-Frag, R.H. Siddiqui, W.Z. Al-kiali, O. Karama, Arab. J. Chem.,

2011, 4, 119.

[8] T.N. Mtshali, W. Purcell, H.G. Visser, S.S. Basson, Transition Met. Chem. 2008,

33, 711.

[9] W. Noh, G.S. Girolami, Dalton Trans., 2007, 674.



Table 4.3 - Selected crystal and data collection details for 1.

Complex 1

Formula C13H18ClN2O3Re

Formula weight 471.95

Crystal System Orthorhombic

Space Group P212121

a [Å] 5.9620(2)

b [Å] 9.8999(3)

c [Å] 28.8810(9)

Volume [Å3] 1704.65(9)

Z 4

Dcalc [g/cm3] 1.839

Absorption Coefficient (mm-1) 7.292

F(000) 904

Crystal size [mm] 0.06 × 0.06 × 0.56

Theta Min-Max [°] 2.2 – 28.4

Index ranges

h -7 : 7

k -13 : 10

l -38 : 38

Goodness-of-fit 1.17

Observed data [I > 2.0 σ I] 4141

R 0.0205

wR2 0.0418




Complex 2

Formula C17H22ClN2O3Re


Crystal System Triclinic

Space Group P-1

a [Å] 6.0170(2)

b [Å] 10.4477(3)

c [Å] 15.4208(5)

α [°] 98.801(1)

β [°] 99.740(1)

γ [°] 95.696(1)

Volume [Å3] 936.36(5)

Z 2

Dcalc [g/cm3] 1.859


F(000) 508

Crystal size [mm] 0.06 × 0.06 × 0.43

Theta Min-Max [°] 2.0 – 28.3

Index ranges

h -8 : 7

k -13 : 13

l -20 : 20



R 0.0147

wR2 0.0309




Complex 3

Formula C13H20N2O4Re. CF3O3S


Crystal System Triclinic

Space Group P-1

a [Å] 9.1575(4)

b [Å] 111.5182(5)

c [Å] 12.2140(6)

α [°] 67.073(2)

β [°] 70.216(2)

γ [°] 85.534(2)

Volume [Å3] 1114.33(9)

Z 2

Dcalc [g/cm3] 1.799


F(000) 584

Crystal size [mm] 0.13 × 0.66 × 0.72

Theta Min-Max [°] 1.9 – 28.3

Index ranges

h -12 : 12

k -15 : 15

l -16 : 16



R 0.0196

wR2 0.0458



Table 4.6 – Experimental bond distances and angles for 1.

Bond Bond length [Å]

Re-Cl(1) 2.494(1)

Re-C(1) 2.096(5)

Re-C(2) 2.095(5)

Re-C(3) 1.972(5)

Re-C(4) 1.950(6)

Re-C(5) 1,915(5)

C(1)-N(1) 1.144(7)

C(2)-N(2) 1.150(6)

C(3)-O(3) 1.132(7)

Angle Bond angle [°]

Cl(1)-Re-C(5) 177.5(2)

Cl(1)-Re-C(4) 92.3(2)

Cl(1)-Re-C(3) 89.9(2)

Re-C(1)-N(1) 173.0(5)

Re-C(2)-N(2) 173.8(4)

C(1)-N(1)-C(11) 173.0(5)

C(2)-N(2)-C(21) 177.(5)

C(3)-Re-C(5) 92.0(2)





Re-Cl(1) 2.4969(5)

Re-C(1) 2.099(2)

Re-C(2) 2.093(2)

Re-C(3) 1.956(2)

Re-C(4) 1.917(2)

Re-C(5) 1.957(2)

C(1)-N(1) 1.147(3)

C(2)-N(2) 1.143(3)

C(3)-O(3) 1.138(3)


Cl(1)-Re-C(4) 176.95(7)

Cl(1)-Re-C(3) 92.56(7)

Cl(1)-Re-C(5) 88.77(6)

Re-C(1)-N(1) 177.2(2)

Re-C(2)-N(2) 177.0(2)

C(1)-N(1)-C(11) 177.5(2)

C(2)-N(2)-C(21) 175.0(2)

C(4)-Re-C(5) 91.36(9)





Re-O(6) 2.220(2)

Re-C(11) 2.100(3)

Re-C(21) 2.090(3)

Re-C(3) 1.961(3)

Re-C(4) 1.897(3)

Re-C(5) 1.971(3)

C(11)-N(1) 1.140(4)

C(21)-N(2) 1.144(4)

C(3)-O(3) 1.134(5)


O(6)-Re-C(4) 178.1(1)

O(6)-Re-C(3) 92.2(1)

O(6)-Re-C(5) 92.9(1)

Re-C(11)-N(1) 176.6(3)

Re-C(21)-N(2) 173.3(3)

C(11)-N(1)-C(12) 178.7(3)

C(21)-N(2)-C(22) 173.4(3)

C(4)-Re-C(5) 88.8(1)

Chapter 5 Re(V) Reactivity


Chapter 5 Reactivity of Re(V) with Isocyanides

5.1 Introduction

The previous chapters deal with the chemistry of rhenium(I) and (III) and their reactivity

towards isocyanides. This chapter focuses on the reactivity of rhenium(V), since this

particular oxidation state is of great significance in radiopharmaceuticals due to its easy

accessibility from perrhenate. The reactivity of [ReVOCl3(PPh3)2] with isocyanides have

been investigated and led to the isolation of the rhenium(III) complex

[Re(CNR)Cl3(PPh3)2] [1]. However the reactivity of [ReVO2I(PPh3)2] with isocyanides has

not been studied.

The fac-[Re(CO)3]+ core has been proven to be effective in radiolabelling due to the

kinetic and thermodynamic stability of the d6 configuration. Isocyanides possess similar

electronic properties to that of carbonyls, and can therefore potentially also display the

necessary properties for application in radiopharmaceutical use. They have great

ligation properties and have been used in diagnostics, as seen from technetium

complexes of the form [Tc(CNR)6]+ [2].

This study describes the reactivity of [ReVO2I(PPh3)2] and [ReVOCl3(OPPh3)(SMe2)] with

the ligands tert-butyl isocyanide and cyclohexyl isocyanide respectively. The reaction of

tert-butyl isocyanide with [ReO2I(PPh3)2] surprisingly gave the seven-coordinated

rhenium(III) complex cis-[Re(t-BuNC)3I3(PPh3)] (1) - a triisocyanide complex containing

three bulky iodo-ligands. From the reaction of cyclohexyl isocyanide with

[ReOCl3(OPPh3)(SMe2)], the rhenium(V) complex (μ-O)[ReOCl2(CNC6H11)2]2 (2) was



isolated. A similar reaction has been previously reported and gave a different complex,

as depicted in Figure 5.1, but no crystal structure was reported to confirm the result [3].

Figure 5.1: Previously reported reaction of [ReVOCl3(OPPh3)(SMe2)] with cyclohexyl

isocyanide

5.2 Experimental

5.2.1 Synthesis of cis-[Re(t-BuNC)3I3(PPh3)] (1)

27.8 μL (0.25 mmol) of t-BuNC was added to a stirred suspension of 0.097 g (0.11

mmol) of [ReO2I(PPh3)2] in 20 mL benzene. The reaction mixture was heated under

reflux for an hour. After cooling to room temperature, the solvent was removed under

vacuum to produce a yellow residue. Recrystallization from ethanol gave yellow

crystals. Yield = 21 %, m.p. 194-196 °C. IR (νmax/cm-1): ν(C=N) 2121 (s), 2158 (s). 1H-

NMR (CDCl3) (ppm): 7.45 (m, 15H, PPh3), 2.16 (s, 27H, C(CH3)3). UV/Vis (CH2Cl2, λmax

(ε, M-1cm-1)): 544 (270).

5.2.2 Synthesis of (μ-O)[ReOCl2(CNC6H11)2]2 (2)

A suspension of 0.21 g (0.32 mmol) of [ReVOCl3(OPPh3)(SMe2)] in 10 mL

dichloromethane was mixed with 0.17 g of cyclohexyl isocyanide in a nitrogen

atmosphere and stirred for 2 hours at room temperature. The solution instantly changed

to a bright blue colour. The solvent was evaporated under vacuum and a blue



precipitate was filtered off. Recrystallization from acetonitrile produced blue crystals

suitable for X-ray diffraction studies. Yield = 52 %, m.p. 171-172 °C. IR (νmax/cm-1):

ν(C=N) 2224 (s), 2238 (s); ν(Re=O) 894 (s); ν(Re-O-Re) 720. 1H-NMR (CDCl3) (ppm):

2.18 (m, 8H, p-H); 2.00 (m, 16H, m-H); 1.52 (m, 16H, o-H); 1.28 (s, 4H, CNCH(C5H10).

UV/Vis (CH2Cl2, λmax (ε, M-1cm-1)): 583 (335), 664 (280).


5.3.1 Synthesis and characterization of cis-[Re(t-BuNC)3I3(PPh3)] (1)

The seven-coordinated Re(III) complex cis-[Re(t-BuCN)3I3(PPh3)] was isolated by the

reaction of [ReO2I(PPh3)2] with tert-butyl isocyanide in a 1:2 molar ratio in benzene. The

two cis-oxo ligands were substituted, giving three isocyanides and three iodides cis

relative to one another. This reaction thus involves the reduction of Re(V) to Re(III).

Recrystallization of 1 from ethanol gave yellow crystals which were suitable for X-ray

diffraction studies. It is insoluble in acetonitrile, methanol and ethanol, and soluble in

chlorinated solvents to give yellow solutions. The complex decomposes after a few days

in solution.

The infrared spectrum of 1 (Figure 5.2) is characterized by the intense C≡N stretching

frequencies of the three isocyanide ligands bonded to the central rhenium atom. One

intense band appears at 2121 cm-1, with a shoulder at 2158 cm-1. It is possible that the

stronger band obscured the peaks for the other two isocyanide ligands. A similar effect

is normally observed for the three C≡O ligands of the rhenium(I) tricarbonyl core.

The 1H-NMR spectrum for 1 is not very useful in its characterization, since the protons

on the tert-butyl isocyanide ligands are all highly symmetrical in nature, and thus they

are observed as one intense 27 proton singlet. The aromatic region contains a multiplet



integrating for 15 protons in the range 7.25 – 7.90 ppm. In the UV-Vis spectrum,

obtained in dichloromethane solution, there is only one weak peak at 544 nm. Ligand-

to-metal and metal-to-ligand charge transfer transitions are normally very intense, so

this peak of relatively low intensity is ascribed to d-d transitions.


The asymmetric unit of 1 is shown in Figure 5.3, showing the seven-coordinate Re(III)

complex. The geometry is best described as trigonal prismatic, square face

monocapped, with angles I(1)-Re-I(1i) = 86.70(2)°, P(1)-Re-C(1) = 75.88(8)°, I(1)-Re-

P(1) = 127.57(1) and C(1)-Re-C(1i) = 144.2(2)°. There is an isocyanide slightly trans to

each iodo-ligand, with an average angle of 156.55(8)°, instead of 180° for an ideal

octahedral arrangement. The isocyanides are all linear, as seen from the Re-C(1)-N(1)

angle of 178.1(2)°, while the C(1)-N(1)-C(2) angle deviates more from linearity by an

angle of 167.8(3)°. The Re-C bond lengths [2.013 Å] are as expected for rhenium-

isocyanides, as are the average isonitrile C≡N bond of 1.151(4) Å illustrating triple bond

character [4].



Figure 5.3: ORTEP diagram of 1, showing 40% ellipsoid probability. The hydrogen

atoms have been omitted for clarity

Figure 5.4 illustrates the very complex packing of the crystals of 1 in its unit cell. There

are six molecules of 1 per unit cell; however, no significant hydrogen-bonding is

observed in this complex.

Complexes of seven-coordinate rhenium(III) are scarce in the literature. The oxidative

addition of Br2 to [Re(CO)L4Br] (L = CNMe, p-CNC6H4Me) produced the complex

[ReL4Br3], a rhenium(III) complex containing four isocyanides. In this complex the metal

is seven-coordinated, with three isocyanides and three bromides in a fac-octahedral

arrangement, with the fourth isocyanide coordinated on the face formed by the three

isocyanides in a capped-octahedral geometry. The average C-Re-Br angle is 158°,

compared to the C-Re-I angles of average 156.55(8)° in complex 1 [5].



The reaction of two equivalents of tert-butyl isocyanide with [ReCl3(PMePh2)3] in toluene

led to the isolation of the seven-coordinae [ReCl3(t-BuNC)2(PMePh2)2]. However, with a

large excess of t-BuNC the complex salt [ReCl2(t-BuNC)3(PMePh2)2]Cl was isolated [6].

Figure 5.4: Packing diagram for 1



5.2.2. Synthesis and characterization of (μ-O)[ReOCl2(CNC6H11)2]2 (2)

Compound 2 was prepared by stirring [ReVOCl3(OPPh3)(SMe2)] and cyclohexyl

isocyanide in dichloromethane at room temperature for two hours. A similar reaction has

been reported previously, but the solvent used was tetrahydrofuran and the product

isolated was speculated to be [ReOCl3(CNC6H11)2], but was not confirmed since no

crystal structure was reported [3]. At first the reaction was done in tetrahydrofuran;

however, the [ReVOCl3(OPPh3)(SMe2)] did not react efficiently and no blue powder was

isolated as stated in literature [3]. The solvent was changed in an attempt to synthesize

[ReOCl3(CNC6H11)2], but upon recrystallization from acetonitrile complex 2 was

surprisingly isolated.

Complex 2 is stable in air for several days, and it is insoluble in alcohols, acetonitrile

and chloroform, but soluble in dichloromethane, DMF and DMSO.

The IR spectrum for 2 (Figure 5.5) shows two C≡N stretching frequencies at 2238 and

2224 cm-1. Only one Re=O band is seen at 893 cm-1, typical of rhenium-oxo bonds.

There is a strong absorption at 720 cm-1, which is ascribed to the bridging oxo species

[v(Re-O-Re)].

The 1H-NMR spectrum for 2 is not very informative on the structure due to the

symmetrical nature of the complex. The multiplets in the range 1.2 – 2.2 ppm are

assigned to the cyclohexyl protons, which integrate for 44 protons of the four ligands.

The electronic spectrum of the blue solution of 2 shows two bands at high energy range.

The more intense band at 583 nm in the electronic spectrum of 2 can be ascribed to the

ligand-to-metal charge transfer transition [pπ(Cl-) d*π(Re)]. The less intense shoulder

at 664 nm is assigned to a (dxy)2 (dxy)1 (dπ*)1

transition.

The cyclic voltammogram for 2 (Figure 5.7) shows both oxidative and reductive waves.

The negative sweep potentials show a one-electron reversible oxidation at peak A, the



reversibility indicated by the anodic to cathodic peak ratio which is extremely close to 1

(Table 5.1). The reductions at B and C are all irreversible, and as well as the oxidation

at peak D.

Figure 5.5: IR spectrum for 2

Figure 5.6: UV-Vis spectrum for 2



Table 5.1: Peak parameters for the cyclic voltammogram of complex 2.

Peak A B C D

Epc (mV) -802 -1281 -1497 -

Epa (mV) -721 - - 202

E1/2 (mV) -761 - - -

ic (μA) 2.5879 4.0589 2.4629 -

ia(μA) 2.5635 - - 0.9461

ia/ic 0.99 - - -

Figure 5.7: Cyclic voltammogram for complex 2

A perspective view of the crystal structure of 2 is shown in Figure 5.8. The dinuclear

molecule is symmetrical around the Re(1)-O(1)-Re(1i) centre, with the bridging O(1)

lying on a crystallographic inversion centre. The bond lengths between rhenium and the

bridging oxo are exactly equal at 1.9119(3) Å and it has a Re(1)-O(1)-Re(1i) angle of

exactly 180.0°. This Re-O-Re angle is precisely linear, which is not so common when

considering other complexes of this form [7]. This Re=O distance [1.702(3) Å] as well

as the Re-O bond lengths lie within the expected range for similar complexes containing

A

B

C

D



the O=Re-O-Re=O moiety [8-11]. This O=Re-O-Re=O moiety is commonly found in

rhenium(V) chemistry and bond distances and angles are consistent with those normally

found [7]. [Re2O3(S2CN(C2H5)2)4] is an example of a similar dinuclear Re(V) species, in

which the bridging species is essentially linear [12]. The bond distances also correlate

well with the mononuclear complex [ReOCl3(CN(CH3)3)(PPh3)] [7].

Each rhenium is centered in a distorted octahedral geometry. The basal plane is defined

by the two chloride and two isocyanide ligands, with the terminal and bridging oxo

ligands coordinated in the trans axial positions. The distortion is due to the non-linear

angles O(1)-Re-O(2) = 168.45(9)°, Cl(2)-Re-C(2) = 171.78(1)° and Cl(1)-Re-C(1) =

175.5(1)°. The angles Cl(1)-Re-Cl(2) = 88.67(3)°, Cl(1)-Re-O(1) = 90.37(3)° and Cl(1)-

Re-C(2) = 90.11(9)° are very close to orthogonality.

The isocyanides are all linear, evident from the angles Re-C(1)-N(1) and Re-C(2)-N(2)

of 174.7(3)° and 177.7(3)° respectively. Similarly, the intra-ligand angles C(1)-N(1)-

C(11) and C(2)-N(2)-C(21) of 174.3(4)° and 175.8(3)° respectively are also linear. The

bond lengths Re-C(1) [2.090(4) Å] and Re-C(2) [2.079(3) Å] are as expected for

rhenium-isocyanides [4]. The average Re-Cl bond distance of 2.39 Å is as normally

found for similar Re(V) complexes [3, 7, 13].

The hydrogen interactions for complex 2 are summarized in Table 5.2. It is evident from

this table that there are strong hydrogen interactions between C(25)-H(25b)∙∙∙O(2) and

C(26)-H(26a)∙∙∙O(2).

Table 5.2: Hydrogen interactions for complex 2

A-H∙∙∙B A-H (Å) H∙∙∙B (Å) A∙∙∙B (Å) A-H∙∙∙B (°)

C(25)-H(25b)∙∙∙O(2) 0.9900 2.5600 3.253(5) 127

C(26)-H(26a)∙∙∙O(2) 0.9900 2.5700 3.151(5) 118



Figure 5.8: ORTEP diagram of complex 2. Hydrogen atoms were omitted for clarity

Although dimeric complexes of rhenium(V) containing the Re2O34+ core are quite

common, most examples contain tetradentate N2O2-donor ligands. For example, with

the tetradentate ligands (1,2-ethylenebis(salicylideneimine) [H2sal2en] the complex

[Re2O3(sal2en)2] was isolated from its reaction with [ReOCl3(PPh3)2]. It was determined

that these type of μ-oxo dimers are usually formed if the reactions are carried out in air

and wet solvents [13].

5.4 References

[1] H. Spies, M. Glaser, F.E. Hahn, T. Lügger, D. Scheller, Inorg. Chim. Acta, 1995,

232, 235.



[2] F.E. Hahn, Angew. Chem. Int. Ed., 1993, 32, 650.

[3] J.C. Bryan, R.E. Stenkamp, T.H. Tulip, J.M. Mayer, Inorg. Chem., 1987, 26,

2283.

[4] P. Schaffer, J.F. Britten, A. Davison, A.G. Jones, J.F. Valliant, J. Organomet.

Chem., 2003, 680, 323.

[5] P.M. Treichel, J.P. Williams, W.A. Freeman, J.I. Gelder, J. Orgmet. Chem., 1979,

170, 247.

[6] S. Warner, L.K. Cheatham, T.H. Tulip, I.D. Williams, S.J. Lippard, Inorg. Chem.,

1991, 30, 1221.

[7] F.E. Hahn, T. Lügger, Inorg. Chim. Acta, 1998, 269, 347.

[8] B. Machura, R. Kruszynski, J. Kusz, Polyhedron, 2007, 26, 3054.

[9] B. Machura, Coord. Chem. Rev., 2005, 249, 591.

[10] N.H. Huy, U. Abram, Inorg. Chem., 2007, 46, 5310.

[11] S.R. Fletcher, A.C. Skapski, J. Chem. Soc., Dalton Trans., 1972, 10, 1073.

[12] R. Shandles, E.O. Schlemper, R.K. Murmann, Inorg. Chem., 1971, 10, 2785.

[13] F. Refosco, F. Tisato, C. Bolzati, G. Bandoli, J. Chem. Soc., Dalton Trans., 1993,

605, and references therein.

[14] T.I.A. Gerber, D. Luzipo, P. Mayer, J. Coord. Chem., 2005, 58, 1505.




Complex 1

Formula C33H42I3N3PRe


Crystal System Trigonal

Space Group R-3

a [Å] 16.8432(3)

b [Å] 16.8431(3)

c [Å] 23.5562(3)

α [°] 90

β [°] 90

γ [°] 120

Volume [Å3] 5787.4(3)

Z 6

Dcalc [g/cm3] 1.857


F(000) 3060

Crystal size [mm] 0.24 × 0.28 × 0.34

Theta Min-Max [°] 2.2, 28.3

Index ranges

h -22 : 22

k -22 : 22

l -31 : 29



R 0.0180

wR2 0.0522




Complex 2

Formula C28H44Cl4N4O3Re2


Crystal System Monoclinic

Space Group P21/n

a [Å] 12.1765(6)

b [Å] 9.8302(5)

c [Å] 14.3784(7)

α [°] 90

β [°] 99.695(2)

γ [°] 90

Volume [Å3] 1696.5(2)

Z 2

Dcalc [g/cm3] 1.956


F(000) 964

Crystal size [mm] 0.05 × 0.46 × 0.52

Theta Min-Max [°] 2.5, 28.3

Index ranges

h -15 : 16

k -13 : 13

l -19 : 19



R 0.0221

wR2 0.0643





Re-I(1) 2.8402(5)

Re-C(1) 2.013(3)

Re-P(1) 2.391(1)

N(1)-C(1) 1.151(4)

N(1)-C(2) 1.459(4)


I(1)-Re-P(1) 127.57(1)

I(1)-Re-I(1i) 86.70(2)

I(1)-Re-C(1i) 156.55(8)

P(1)-Re-C(1) 75.88(8)

C(1)-Re-C(1i) 144.2(2)

I(1i)-Re-C(1ii) 156.55(8)

C(1ii)-Re-C(1) 156.55(8)

Re-C(1)-N(1) 178.1(2)

C(1)-N(1)-C(2) 167.8(3)





Re-Cl(1) 2.392(1)

Re-Cl(2) 2.3953(9)

Re-O(1) 1.9119(3)

Re-O(3) 1.702(3)

Re-C(1) 2.090(4)

Re-C(2) 2.079(3)

N(1)-C(1) 1.137(5)

N(1)-C(11) 1.455(5)

N(2)-C(2) 1.142(4)

N(2)-C(21) 1.446(4)


Cl(1)-Re-Cl(2) 88.67(3)

Cl(1)-Re-O(1) 90.37(3)

Cl(1)-Re-O(2) 97.00(9)

Cl(1)-Re-C(1) 175.5(1)

Cl(1)-Re-C(2) 90.11(9)

Cl(2)-Re-C(2) 171.8(1)

Re(1)-O(1)-Re(1a) 180.00

O(1)-Re-O(2) 168.45(9)

C(1)-N(1)-C(11) 174.3(4)

C(2)-N(2)-C(21) 175.8(3)

Re-C(1)-N(1) 174.7(3)

Re-C(2)-N(2) 177.7(3)

Chapter 6 Conclusion


Chapter 6 Conclusion and Future Work

This study deals with the preparation of rhenium complexes in different oxidation states

containing isocyanide ligands. All of the complexes were analysed spectroscopically

and were structurally characterized.

The complexes synthesized in Chapter 3 contain potentially bidentate derivatives of

aniline and can be extended to include multifunctional ligands such as 3-nitrobenzene-

1,2-diamine (A) and 2-[(4-methylphenyl)sulfanyl]aniline (B), as depicted in Figure 6.1. It

has also been proposed that [Re(t-BuNC)Cl3(PPh3)2] can be reacted with tripodal

tetradentate ligands, in which the rhenium-isocyanide bond is retained [1].

Figure 6.1: Structure of multifunctional aniline derivates

Chapter 4 dealt with the preparation of rhenium-tricarbonyl complexes containing

isocyanide ligands. These complexes can be reacted with different aromatic,

multidentate ligands to stabilize the complexes. This is especially true for the positively

Chapter 6 Conclusion


charged complex [Re(CO)3(t-BuNC)2(OH2)]+ which contains the labile aquo ligand,

which can be substituted very easily.

Different rhenium(V) precursors were reacted with isocyanides in Chapter 5 to give

large, complex molecules. An unusual seven-coordinate trigonal prismatic, square face

monocapped rhenium complex was isolated containing three isocyanides coordinated in

a cis-fashion. The class of isocyanides can be varied to see whether it reacts similarly.

Reactivity of more bulky isocyanides would be particularly interesting, since the complex

already contains three bulky iodides.

In vivo cytotoxicity screening can be performed on the complexes synthesized against a

variety of cell lines. A particularly interesting complex to screen would be [Re(CO)3(t-

BuNC)2(OH2)]+ due to its cationic nature. Cationic complexes are known to be readily

taken up by the heart, thus its in vivo behavior is something to explore in the future.

Multidentate isocyanide ligands have more favorable properties compared to the

monodentate analogues and have been successful in diagnostic applications with

technetium [2, 3]. A logical extension to this study would be the synthesis of Re(I) and

(V) complexes containing bidentate isocyanides.

References

[1] H. Spies, M. Glaser, F.E. Hahn, T. Lügger, D. Scheller, Inorg. Chim. Acta, 1995,

232, 235.

[2] F.E. Hahn, M. Tamm, L. Imhof, T. Lügger, J. Organom. Chem., 1996, 526, 149.

[3] R.J. Angelici, M.H. Quick, G.A. Kraus, D.T. Plummer, Inorg. Chem., 1982, 21,

2178.

ISOCYANIDE COMPLEXES OF RHENIUMXANDRI SCHOULTZ Submitted in partial fulfilment of the requirements...

Documents

Transcript of ISOCYANIDE COMPLEXES OF RHENIUMXANDRI SCHOULTZ Submitted in partial fulfilment of the requirements...