Metal-Containing Dendritic Polymers

Fiona J. Stoddart and Thomas Welton*

Department of Chemistry, Imperial College of Science, Technology and Medicine, South

Kensington, London SW7 2AY, UK.

Abstract

Metal-containing dendrimers (metallodendrimers) have attracted a great deal of attention

recently and their study is becoming a growing field. Many workers have entered the field

and it is rapidly developing. In this review, the preparation, characterisation and applications

of metal-containing denrimers are discussed. The principal methodologies for the

preparation of dendrimers are first demonstrated and then the derivatisation of organic

dendrimers to form suitable potential ligands is presented. Finally the formation of

transition-metal complexes of the dendrimers is discussed. The manuscript is organised such

that the metallodendrimers are discussed by donor element in the dendrimer. As one might

expect, phoshine and nitrogen-donor complexes have dominated this initial phase of

synthesis. However, there are reports of metallodendrimers with a wide variety of donor

atoms. In the few years since the first metallodendrimers were prepared the field has moved

rapidly towards potential applications, and this has been noted.

1

Introduction

Over the last twenty years, a new class of polymers known as dendrimers has fascinated

many chemists. This review concentrates on those dendrimers that contain metals.

However, a brief introduction to dendrimers in general and the major approaches to their

syntheses is given. More detailed reviews on this subject have been given elsewhere.1

The term dendrimer is derived from the Greek word dendra meaning tree. These

highly branched macromolecules have compelling molecular structures that are reminiscent

of patterns often observed in nature and particularly those found in trees and in coral.

Dendrimers – also called arborols2 or cascade3 molecules – exhibit controlled patterns of

branching and ideally are monodisperse, i.e.; all the molecules should have exactly the same

molecular masses, constitutions and average dimensions. The larger dendrimers, which have

globular structures, carry many close-packed surface end groups and contain internal cavities.

The interest in dendritic polymers stems from the possibility that their architectures, which

differ from those of traditional linear step-growth polymers, offer exciting prospects of new

applications.4

Before 1940, branched molecular structures had been considered to be responsible for

the insoluble and intractable materials formed during polymerisations.Error: Reference

source not foundb These materials were largely ignored since it was invariably impossible to

isolate discrete molecular compounds and assign them definite structures.

In 1978 Vögtle and co-workers published a synthetic strategy which involved the

“cascade-like” synthesis of acyclic, branched polyamines.5 The synthesis, which is illustrated

in Scheme 1, began with an exhaustive Michael addition of the monoamine 1 to acrylonitrile,

leading to the annexation of two branches per amino group, thus affording the bisnitrile 2.

The nitrile groups were then reduced to amine functions, using cobalt(II)/sodium

borohydride to give the bisamine 3. Repetition of these two steps afforded the hexa-2

branched tetraamine 5, via the tetranitrile 4. Although this synthesis was not continued

beyond this point because of problems encountered in the reduction step, the principle that

repeated cycles of reactions could lead to controlled polymer growth had been demonstrated.

N

H2N NH2

N

N

NH2H2N

R

N

NC NC

N

N

CNCN

R

H2N

N

NH2

R

NC

N

CN

R

NH2

1

R

2

Co(III)/NaBH4

Co(III)/NaBH4

3

4 5

AcOH MeOH

AcOH MeOH

CN

CN

Scheme 1. “Cascade-like” synthesis of acyclic, branched polyamines

In 1981, Denkewalter et al.6 patented the synthesis of highly branched polylysine derivatives.

Each member of this series of compounds was monodisperse, consisting of branching units

of differing lengths. From 1985 onwards, two research groups, one headed by Tomalia7 and

the other by Newkome,Error: Reference source not found,8 simultaneously developed

families of dendrimers synthesised using this divergent method (see below). In 1990,

Fréchet and Hawker9 employed a different method, the convergent approach (see below), to

prepare poly(aryl ether) dendrimers.

3

Dendritic Structure

Figure 1 depicts the structure of a typical dendrimer. The following points must be

considered and, where appropriate, adapted when describing the structures of dendrimers:-

(i) There is a central point known as the initiator core: in the dendrimer shown in Figure

1, four branches emanate from a core and so the core multiplicity (Nc) is four.

(ii) Each branch contains further branching sites: in the example illustrated in Figure 1,

the degree of branching (Nb) is two.

(iii) Each new layer of branches that are constructed upon old branch points is called a

generation (G): generations are numbered at 0, 1, 2, 3 ... and so on.

(iv) The branch cell unit lengths (l) are determined by the choice of branched monomers.

l

G = 2

G = 1

Figure 1. Schematic representation of a dendrimer

4

The number of monomer units in a dendrimer increases exponentially as a function of the

generation. As the dendrimer grows in size, the end groups reside closer and closer to one

another. Eventually, this branch-growing process results in surface congestion, a feature that

prevents further growth from all branch points with the consequence that the dendrimer can

no longer be monodisperse. The highest generation at which the dendrimer is still potentially

monodisperse is described as its “starburst limit”.

5

Dendrimer Synthesis

Dendrimers are constructed in stages using repetitive synthetic strategies. Both the divergent

and convergent approaches to dendrimer synthesis have advantages and disadvantages.

The Divergent Approach

The synthetic approach to dendrimer formation, which has become known as the divergent

method, emerged during the period 1978-1987 with many of the seminal contributions

coming from Newkome Error: Reference source not found,Error: Reference source not found and Tomalia.Error:

Reference source not found The basic concept, which is that of starting at the core and

working outwards in a divergent fashion to create a highly branched structure, has

subsequently been developed and exploited by many research groups world-wide.10 An

illustration of the divergent approach to the synthesis of a dendrimer is shown in Scheme 2.

8 x

Coremolecule

First-generationdendrimer

Second-generationmolecule

4 x

Scheme 2. Schematic representation of the divergent synthesis of dendrimers

6

A multifunctional core molecule – in this case, one with four functional groups – is reacted

with four monomer molecules to give the first generation dendrimer. Repetitive addition of

similar building blocks – usually achieved by a protection-deprotection procedure – affords

successive generations. It is important to ensure that each set of reactions leading to these

new generations has been completed before the next cycle of reactions is commenced, if

defects in the dendritic structure are to be avoided.

Using the divergent approach, it is possible to prepare up to tenth generation

dendrimers with molecular weights of the order of 700,000 and with more than 3,000 end

groups per molecule.11 The advantage of the divergent method is that the production of

several grams of dendrimer is easily attainable since, with each subsequent generation, the

molar mass of the dendrimer is greatly increased.

This method is not without its drawbacks. As the dendrimer grows in size, the

number of end groups involved in the reaction increases and the likelihood of incomplete

growth steps leading to defects in the structure becomes greater. It is often difficult to detect

the precise extent of conversion from one generation to the next. As a consequence,

imperfect samples of dendrimers, which are virtually impossible to purify and characterise,

since they may differ only slightly from the desired monodisperse samples, are obtained.

Therefore, if the divergent method is to be employed successfully, extremely efficient and

high-yielding reactions are required in order to ensure the production of dendrimers with low

polydispersities. This often poses a great synthetic challenge.

The Convergent Approach

Fréchet and Hawker first proposed an alternative approach to dendrimer syntheses, known as

the convergent method.Error: Reference source not found,12 Here, the reverse of the

divergent method is applied; the synthesis starts at what will eventually become the periphery 7

of the dendrimer and progresses inwards. Surface units are linked together increasingly with

more monomers until a wedge-shaped molecule is generated, carrying a reactive group at its

apex. The final step of the synthesis involves attaching the desired number of wedges to a

multifunctional core. This approach is illustrated in Scheme 3.

The attraction of the convergent method lies in the fact that only a small number of

molecules are involved in the reaction steps that form each successive generation. In

contrast, increasing numbers of molecules are involved in the reactions in the later stages of a

synthesis using the divergent approach. Large excesses of reagents and slight impurities can

also be avoided, without sacrificing high yields and, because of easier purification, reactions

no longer need to be as efficient, meaning that a much larger choice of reaction types are

available.

+

+

Two of I

II

Two of II Repeat

n times

Three wedges

+

WedgeDendrimer

I

Scheme 3. Schematic representation of the convergent synthesis of dendrimers

8

The main disadvantage of the convergent approach is that it is not accompanied, after

each reaction cycle, by the marked increase in molar mass, which is observed in the case of

the divergent method. The total number of steps involved in the construction of the

dendrimer using the convergent method is not actually reduced compared with that needed in

the divergent approach, yet significantly more starting material is required. Also the higher

generation dendritic wedges can experience severe steric problems when reactions to attach

their reactive apex groups to core molecules are attempted. Thus, the convergent approach

has been found to be less useful than the divergent one for the synthesis of dendrimers

approaching their starburst limit.

Metallodendrimers

During the last decade, those working with dendrimers have switched their focus from the

initial synthetic directions explored mainly by organic chemists to a more applied emphasis.

Thus, metallodendrimers are becoming of interest from a materials science perspective

because of their unique physical properties, leading to potential photophysical and catalytic

applications. Metallodendrimers show substantial structural diversity and their properties and

applications are wide-ranging. Metallodendrimers may be classified by where the metal

appears in the dendrimer, at the centre, as connectors, as branching units, or as peripheral

units of the dendrimer.13 However, here the metallodendrimers are classified by ligand type,

i.e., the particular ligand which complexes the metal centre to/within the dendrimer – thus

viewing them from the perspective of the inorganic chemist. In many of the following

illustrations, only one section of the dendrimer has been portrayed and a “W” within a

wedge-shaped motif represents other dendritic arms identical to the one which has been

drawn out in full.

9

Dendrimers as Counter ions

Perhaps the simplest way in which to include metals in a dendritic structure is to use the

dendrimer as a counterion for a well-defined metal or metal complex. The metal may bind to

a surface site on the dendrimer (exo-receptor) or to a site within the internal cavaties of the

dendrimer (endo-receptor).

Hydrolysis of ester-terminated PAMAM dendrimers with Group 1 metal (Na+, K+,

Cs+ and Rb+) hydroxides resulted in the formation of salts as white hygroscopic

powders.Error: Reference source not foundd Direct observation of these single dendrimer

molecules by Channel Tunelling Electron Microscopy has been achieved. Further studies

were conducted using carboxylate-terminated PAMAMs and their complexes with Fe3+, Gd3+,

Mn2+, Pr3+ and Y3+ ions.Error: Reference source not foundg

In another investigation,14 which sought to support the molecular mechanics

simulations with experimental evidence, the properties of the carboxylate salts of the half-

generation PAMAM dendrimers were likened to those of anionic micelles. The ability of

these anionic dendrimers to effect the kinetics of the electron-transfer quenching of

photoexcited Ru(phen)32+ has been examined.15 The emission decay of the metal-to-ligand

charge transfer (MLCT) excited state of the probe – Ru(phen)32+ bound to half-generation

PAMAMs was analysed in the presence and absence of the quencher – Co(phen)33+. The

studies showed that the probe lifetimes were enhanced when the complexes were bound to

dendrimers as compared with unbound complexes. It was concluded that the quenching of

dendrimer-bound Ru(phen)32+ by Co(phen)3

2+ occurs at the surface of the dendrimer. These

results indicate that the cationic Ru(phen)32+ binds strongly to the negative surface of the

dendrimer. This has been confirmed by another study study of Ru(phen)32+ labeled with a

nitroxide radical, via –NHC(O)OCH2- or –O(CH2)8O- units, as an EPR probe.16 More

recently, similar results using protonated amino-terminated PAMAMs and Ru(4,7-10

(SO3C6H5)2-phen)34- as the probe have been reported.17 Hence, these systems provide

examples of a dendrimer acting as an exo-receptor.

Phosphorus-Donor Metallodendrimers

Phosphorus-containing dendrimers, in which the core and subsequent branch points are

pentavalent phosphorus atoms and which possess peripheral aldehyde groups have been

prepared by Majoral et al.Error: Reference source not foundd The dendrimers – up to the

tenth generation – were functionalised with phosphino groups and then reacted with

AuCl(tetrahydrothiophene) to give dendrimers with AuCl moieties as the peripheral units.18

The authors note that the reactivity of all generations towards gold complexation is similar,

and therefore, independent of the size of dendrimer used. Most recently, Majoral et al.19

have reported the incorporation of gold into different generational layers of dendritic

molecules. Complexation occurs both at the sulfur-donor P=N-P=N-P=S fragments and the

terminal CH2PPh2 moieties. The dendritic fragment, shown in Figure 2, has been modified

at the generation 1 level to introduce ligands, which are able to coordinate gold. The

metallodendrimer has eighteen internal AuCl units – six at the P=N-P=N-P=S linkages and

twelve at the phosphino groups. The complexes formed can be characterised unambiguously

by 31P NMR spectroscopy. Studies are currently underway to extend this methodology to

incorporate a variety of different metals within the cascade structure of dendrimers.

11

O CH

N N

Me

CH2

CN

N

MeP

CH N

N

Me

P

S

P N PO

O

CN

NMe

PS

CH

N N

MeP

HC N N

Me

P

S

SO

O

O

O

CN

N

Me

NP

O

HCN

NP

Ph

Ph

O

CHN

NMeP

Ph

Ph

S

OArOAr

OArOAr

OArS

OAr

Me

S

POAr

OAr

NP

NP

N

P

Au-ClCl-Au

Cl-Au

H

H

H

WW

W

W W

1 a) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci., 1998,

9, 54. b) Newkome, G. R.; Moorefield, C. N.; Vögtle, F. Dendritic Molecules; VCH:

Weinheim, Germany, 1996 c) Tomalia, D. A. Sci. Am., 1995, 272(5), 62. d) Dvornic,

P. R.; Tomalia, D. A. Chem. Br., 1994 (Aug), 641. e) Tomalia, D. A. Adv. Mat.,

1994, 6, 529. f) Topics in Current Chemistry, 1998, 197, g) Tomalia, D. A.; Durst, H.

D. in Top. Curr. Chem., Springer: Berlin, Germany, 1993, 165, 193. h) Tomalia, D.

A.; Naylor, A. M.; Goddard III, W. A. Angew. Chem., Int. Ed. Engl., 1990, 29, 138.

i) Archut, A.; Vögtle, F. Chem. Soc. Revs., 1998, 27, 233. j) Issberner, J.; Moors, R.;

Vögtle, F. Angew. Chem., Int. Ed. Engl., 1994, 33, 2413. k) Mekelburger, H.-B.;

Jaworek, W.; Vögtle, F. Angew. Chem., Int. Ed. Engl., 1992, 31, 1571. l) Fréchet, J.

M. J. Science, 1994, 263, 1710. m) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.;

Serroni, S.; Venturi, M. Coord. Chem. Rev., 1994, 137, 57. n) Newkome, G. R.;

Moorefield, C. N.; Baker, G. R. Aldrichim. Acta, 1992, 25(2), 31. o) Stinson, S. C.

Chem. Eng. News, 1997 (Sept. 22), 28. p) Dagani, R. Chem. Eng. News, 1993 (Feb.

1), 28.

2 Newkome, G. R.; Yao, Z.-Q.; Baker, G. R., Gupta, V. K. J. Org. Chem.,

1985, 50, 2003.

3 Engel, R. Polym. News, 1992, 17, 301.

4 a) Dagani, R. Chem. Eng. News, 1996, (June 3), 30. b) Service, R. F. Science,

1995, 267, 458. c) Tomalia, D. A.; Dvornic, P. R. Nature, 1994, 372, 617. d)

Grinthal, W. Chem. Eng., 1993 (Nov), 51.

5 Buhleier, E.; Wehner, W.; Vögtle, F. Synthesis, 1978, 155.12

Figure 2. Gold complexation within the cascade structure of a dendrimer

Majoral et al.20 have also prepared diphosphino-terminated dendrimers complexed to

rhodium, palladium, platinum and ruthenium. Addition of hydrazine to dendrimers –

generations 1-3 – with terminal aldehyde groups produced CH=NNH2 end groups and

subsequent reaction with Ph2PCH2OH (2 equivalents per NH2 group) led to the formation of

the desired diphosphine ligands. Reaction of these ligands with RuH2(PPh3)4 gave the

metallodendrimers shown in Scheme 4. The reactivity of these dendritic complexes was

found to be very limited, compared with the reactivity of the monomeric starting material

complex. However, the metallodendrimers reacted slowly with CO to give dihydrido

carbonyl derivatives, where the CO ligand is located trans to one of the hydrides. In order to

produce a more reactive ruthenium site, the diphosphino-terminated dendrimers were reacted

with RuH2(H2)2(PCy3)2 to give the isomeric dihydride dihydrogen derivatives shown in

Scheme 4. The isomer produced is dependent on the reaction conditions employed, but all

of the isomers reacted with CO to give one unique dihydrido carbonyl complex. Majoral et

al.Error: Reference source not found are currently investigating the extent of the chemical

reactivity displayed by these complexes and their application as catalysts for ketone

hydrogenation.

13

RuPCy3Ph2P

Ph2P HN

H

H H

RuHPh2P

Ph2P HN

PCy3

H H

RuHPh2P

Ph2P PCy3N

H

H H

RuPCy3Ph2P

Ph2P HN

H

CO

RuPPh3Ph2P

Ph2P HN

H

CO

RuPPh3Ph2P

Ph2P HN

H

PPh3PPh2

PPh2N

RuH2(H2)2(PCy3)2

RuH2(PPh3)4 CO

COCOCO

n n n

n n n

n

= Dendrimer G = 1, n = 6; G = 2, n = 12; G = 3, n = 24

17 Schwarz, P. F.; Turro, N. J.; Tomalia, D. A. J. Photochem. and Photobio. A: Chem., 1998, 112, 47.

6 Denkewalter, R. G.; Kolc,; Lukasavage, J. US Pat. 4 289 872 (1981)

7 a) Tomalia, D.A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin,

S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J., 1985, 17, 117. b) Tomalia, D. A.; Baker,

H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P.

Macromolecules, 1986, 19, 2466. c) Tomalia, D. A.; Hall, M.; Hedstrand, D. J. Am.

Chem. Soc., 1987, 109, 1601. d) Tomalia, D. A.; Berry, V.; Hall, M.; Hedstrand, D.

M. Macromolecules, 1987, 20, 1164.

9 Hawker, C.; Fréchet, J. M. J. J. Chem. Soc., Chem. Commun., 1990, 1010.

10 a) Jansen, J. F. G. A.; de Brabander-van den Berg, E. M. M.; Meijer, E. W.,

Science, 1994, 266, 1226. b) Roovers, J.; Toporowski, P. M.; Zhou, L.-L. Polym.

Preprints, 1992, 33, 573. c) Miedaner, A.; Curtis, C. J.; Barkley, R. M.; DuBois, D.

L. Inorg. Chem., 1994, 33, 5482. d) Launay, N.; Caminade, A.-M., Lahana, R.;

Majoral, J.-P. Angew. Chem., Int. Ed. Engl., 1994, 33, 1589.11 Hodge, P. Nature, 1993, 362, 18.

14

Scheme 4. Reactivity of diphosphine-terminated dendrimers

Schmidbaur et al.21 have used “spacers”, e.g., -(diphenylphosphino)propionic acid, to

functionalise third and fourth generation poly(propylene)imine dendrimers with terminal

diphenylphosphino groups. Subsequent addition of (dimethyl sulfide)gold chloride gave the

metallodendrimers as stable colourless solids (Scheme 5). Monomeric model compounds

were also synthesised from methylamine and ethylenediamine in order to ascertain suitable

coupling conditions for the synthesis of the dendritic N-alkylamides. Schmidbaur et al.Error:

Reference source not found envisage applications for these metallodendrimers in biochemical

diagnostics and imaging and as antiflammatory and antitumour drugs.

13 Gorman, G. Adv. Mater., 1998, 10, 295.

14 Gopidas, K.R.; Leheny, A. R.; Caminati, G.; Turro, N. J.; Tomalia D.

A.; J. Am. Chem. Soc., 1991, 113, 7335.

15 Turro, C.; Niu, S.; Bossmann, S. H.; Tomalia, D. A.; Turro N. J. J. Phys. Chem., 1995, 99, 5512.

16 Ottaviani, M. F.; Turro, C.; Turro, N. J.; Bossmann, S. H.; Tomalia, D. A. J. Phys. Chem., 1996, 100, 13667.

18 Slany, M.; Bardají, M.; Casanove, M-J.; Caminade, A-M.; Majoral, J-

P, Chaudret, B. J. Am. Chem. Soc., 1995, 117, 9764.

19 Larré, C.; Donnadieu, B.; Caminade, A-M.; Majoral, J-P. Chem. Eur. J.,

1998, 4, 2031.

20 a) Bardají, M.; Caminade, A-M.; Majoral, J-P.; Chaudret, B.

Organometallics, 1997, 16, 3489. b) Bardají, M.; Kustos, M.; Caminade, A-M.;

Majoral, J-P.; Chaudret, B. Organometallics, 1997, 16, 403.

15

NH2 N C

OPPh2 N C

OPPh2 AuClHOOC-C2H4-PPh2

EDC, NEt3

Me2SAuCl

n n n

= Dendrimer G = 3, n = 16; G = 4, n = 32

HH

Scheme 5. Synthesis of chlorogold(I)diphenylphosphino-terminated dendrimers

Second and third generation polypropylene(imine)22 dendrimers have also been surface

functionalised by Reetz et al.23 In this case, a double phosphinomethylation – similar to that

of Majoral et al.Error: Reference source not found described above – of each of the

peripheral primary amine functions was achieved. A variety of palladium, iridium, nickel

and rhodium complexes were reacted with the diphosphino-terminated dendrimers. The

palladium-containing dendrimers were employed as catalysts in the Heck reaction. A

significantly higher catalytic activity was observed for the dendritic catalysts compared with

the activities of the monomeric analogues. The authors attributed this enhancement of

activity to the thermal stability of the dendrimers which prevents the undesired formation of

elemental palladium from occurring – a major problem for the monomeric complexes. By

contrast, the rhodium-containing dendrimers display comparable catalytic activities in

hydroformylations to that of the monomeric complex.

16

Scheme 6. Synthesis of palladium-containing dendrimers

Palladium complexes of several small organophosphine dendrimers synthesised by

DuBois et al.Error: Reference source not foundc exhibit catalytic activity for the

electrochemical reduction of CO2 to CO. The synthesis of one example of these

metallodendrimers is shown in Scheme 6. Addition of diethyl vinylphosphonate to the

primary phosphine 6 gave the phosphonate 7 and subsequent reduction with lithium

aluminium hydride resulted in the phosphine 8. Repetition of these two steps afforded the

phosphine 9 which undergoes reaction with vinyldiphenylphosphine or

vinyldiethylphosphine to give dendrimers 10a and 10b, respectively. The reaction of 10a,b

with [Pd(MeCN)4][BF4]2 formed the metallodendrimers 11a,b containing five square planar

metal centres. These dendritic acetonitrile complexes catalyse electrochemical CO2 reduction

with rates and selectivities which are similar to analogous monomeric catalysts.17

P

PH2

PH2

P

P

PH2

PhPH2P

PH2

PH2PhP

P(OR)2

P(OR)2Ph

O

O

P(OR)2

O

PR2 [Pd(MeCN)4](BF4)2

PhPH2

P

P

P

P

PR2

PR2

R2PPR2

P

P

P

PR2

PR2

R2P R2P

Pd MeCN

PdNCMe

Pd

NCMe

Pd

NCMe

PdNCMe

Ph

P

P

P

PR2R2P

P

P

P

PR2

R2P

Ph

PR2

PR2

P

PR2

PR2

LAHLAH P(OR)2

O

6 7 8 9

10a,b 11a,bR = Ph, Et

18

Nitrogen-Donor Metallodendrimers

The majority of the metallodendrimers described in the literature belong to this category,

with many examples involving polypyridine-type ligands.

Balzani et al.24 have developed a synthetic procedure in which a complex is used as a

ligand and another is used as a metal (“complexes as ligands/complexes as metals”) to

prepare luminescent and redox-active metallodendrimers. Dendrimers that incorporate

specific “pieces of information” in their building blocks, such as the abilities to absorb and

emit visible light and to undergo reversible multielectron redox processes, have potential

applications in molecular electronics and photochemical molecular devices.

Both the divergent and convergent approaches to the synthesis of nitrogen-containing

metallodendrimers have been employed and are illustrated in Scheme 7. Using the divergent

method, the first step is the construction of the core, [Ru(2,3-dpp)3]2+ (12), which has three

chelate sites available and is, therefore, a “complex as ligand.” The building block, [Ru(2,3-

Medpp)2Cl2]2+ (13) has two labile chlorides – therefore representing a “complex as metal” –

and two bridging ligands, which have been protected to prevent further metal coordination.

The reaction of 12 with 13 gives the first generation metallodendrimer 14 and deprotection of

the six peripheral chelating sites yields compound 15. The second generation dendrimer 17,

which has ten ruthenium centres, is formed by the reaction of 15 with the capping unit,

[Ru(bpy)2Cl2] 16. In the convergent approach, the dendritic wedge 18 – “complex as metal”

is reacted with the core compound 12 to give the metallodendrimer 17.

19

Scheme 7. Convergent and divergent synthesis of the Balzani dendrimers

The absorption spectrum and redox patterns of the dendrimer resemble the sum of those

of its mononuclear component units and each of these units brings its own redox properties

into the macromolecular structure. By varying the metals and/or ligands of the building

20

N

N N

N

N

N

N

N

N

N

N

N

N

N N

N

NN

N

N

Ru

Ru

Ru

N

N

N

N

N

N

N

N

N

N N

N

N

NNN

N

N

NN

Ru

Ru Ru

N

N

N

N

N

N N

N

N

N

N

N

NNN

N

N

NN

N

Ru

Ru

Ru

Ru

N

N N

N

N

N

N

N

N

N

N

N

Ru

N

N

N

N

N

N

N

N

N

N N

NRu

N

N

N

N

N

N N

N

N

N

N

N

Ru

Ru

Me

Me

Me

Me

Me

Me

N

N N

N

N

N

N

N

N

N

N

N Ru

N

N N

N

N

N

N

N

N

N

N

N

Ru

N

N

N

N

N

N

N

N

N

N N

NRu

N

N

N

N

N

N N

N

N

N

N

N

Ru

Ru

N

N N

N

N

N

N

N

Ru

Me

Me

ClCl

Cl Cl

N

NN

N

Ru

Cl

Cl N

N

N

N

N

N

N

N

N

N N

N

NN

N

N

Ru

Ru

Ru

12

13

14 15

16

17

18

Deprotection

8+

20+

14+

2+

2+

4+

blocks employed, it is possible to design dendrimers with predetermined redox patterns.

Thus, synthetic control of the number of electrons exchanged at a certain given potential is

achieved and their application as multielectron-transfer catalysts is of potential interest.

Constable and co-workers25 have prepared a variety of metallodendrimers employing

terpyridine-based ligands using both divergent and convergent methodologies. Their latest

development, using convergent assembly, is shown in Scheme 8. A ruthenium complex 19

is reacted with an electrophile – bis(bromomethyl)benzene – to give complex 20 possessing

an electrophilic site remote from the metal centre. Reaction with a nucleophile – 4,4-

dihydroxy-2,2-bipyridine – yields the binuclear dendritic wedge 21 which has a bipyridine

ligand at its focal point. Thus, rapid coordination with either iron(II) or cobalt(II) leads to

the formation of the heptanuclear metallodendrimer 22.

Vögtle and Balzani26 have employed a similar strategy to that described above in the

synthesis of ruthenium complexes of dendritic bipyridine ligands. The ligands have been

prepared by the attachment of branches at the 4 and 4 positions of bipyridine using a

procedure reported by Newkome et al.Error: Reference source not found The dendritic

bipyridines – generations 1-3 – were reacted with Ru(III) chloride to produce a

metallodendrimer where the metal is only present in the core. These complexes exhibit

similar absorption and emission properties to those of an unsubstituted Ru(II) bipyridine

complex. However, a longer excited-state lifetime was observed for the higher generation

dendrimers because of the shielding effect of the dendritic branches on the metal core, which

limits the quenching effect of molecular oxygen.

21

Scheme 8. Convergent synthesis of a Constable dendrimer

22

N

NN

N

NN

O

O

N

N

NN

N

NN

O

O

N

Ru

Ru

NN

N

NN

N

O

Br

RuN

NN

NN

N

OH

Ru

Br

Br OH

N

OH

N

N

NN

N

NN

O

O

N

N

NN

N

NN

O

O

N

Ru

Ru

NN

N

NN

N

O

O

N

NN

NN

N

N

O

O

N

Ru

Ru

NN

NN

N

N

O

O

N

NN

N

NN

N

O

O

N

Ru

Ru

M

19 20 21

22

M = Fe, Co

Recently, more attention has been focused on the ability to incorporate predetermined

subunits into the dendritic structure, thus possessing the synthetic control necessary to create

series of “dendritic assemblies.” With this aim in view, Newkome et al.27 investigated the

connection of two different sized dendritic fragments to a ruthenium centre, thus forming a

bis-dendrimer. A recent publication by Newkome et al.28 describes the connection of two

different dendritic fragments to two separate dendritic core molecules to give the snowflake-

like metallodendrimers shown in Figure 3. This shift in focus from the synthesis of

traditional dendrimers, where the repeat/branching units are identical, to the preparation of

macromolecular dendritic assemblies is becoming more apparent in recent research.

Chemists are interested in tailor-made dendritic molecules that allow a greater control of the

properties exhibited by the macromolecular array.

A convergent synthesis of organoplatinum dendrimers developed by Puddephatt et

al.29 is illustrated in Scheme 9. An oxidative addition reaction of the bromomethyl groups

on bipyridine 24 to two square planar dimethylplatinum centres on 23 gave the dendritic unit

25. The diimine group at the centre of the molecule was then complexed to a different

dimethylplatinum centre 26 to yield the trinuclear complex 27. These two steps were

repeated to give the dendritic wedge 28 and subsequent reaction with the core compound 29

– 1,2,4,5-tetrakis(bromomethyl)benzene – yielded metallodendrimer 30 containing 28

platinum centres.

Dendrimers that possess a metal porphyrin unit as the core have the potential to

mimic the biological functions of haemoproteins and act as sterically hindered oxidation

catalysts. Dendrimers (generations 1-3) with a zinc-porphyrin core and Newkome-typeError:

Reference source not found polyether amide branches synthesised by Diederich et al.30 can be

viewed as encapsulated redox-active centres. The influence of the close-packed dendritic

branches on the redox properties of the central zinc porphyrin unit was studied by cyclic 23

voltametry. A decrease in the first reduction potential of the zinc-porphyrins with increasing

dendrimer generation was observed. Thus, the dendritic fragments serve to shield the

porphyrin centre and hinder the addition of electrons to it. More recently, Diederich et al.31

have modified the peripheral groups on the dendrimer to prepare water-soluble dendritic

iron-porphyrins and similar electrochemical behaviour was observed.

Fréchet et al.32 have also investigated the effects of the dendrimer generation on the

properties of a porphyrin core. They discovered that although higher generation dendrimers

can bury the core site and hinder electron-transfer to it, small molecules such as

benzylviologen are able to penetrate the dendritic shell. Photophysical studies revealed that

the dendritic shell does not interfere with the ability of benzylviologen to quench the

fluorescence of the metalloporphyrin. This result suggests that dendrimers with

metalloporphyrin cores could be employed as catalysts. Modification at the periphery of the

dendrimer, or incorporation of rate enhancing ligands into the dendritic structure, would

allow their fine-tuning for specific catalytic applications.

These, apparently contradictory, results can be explained by the methods used to

study the electron transfer (cyclic voltametry) and excited state quenching (fluorescence

spectroscopy). The former is an interfacial experiment where the metal centre is required to

come into close contact with a large solid electrode. The dendritic structure around the metal

porphyrin prevents this close contact and hence inhibits electron transfer. The latter is a

solution experiment where the flexibility of the dendritic arms allows the probe molecule to

approach the metal centre.

Suslick et al.33 have studied the role of dendritic porphyrins as regioselective catalysts

in the epoxidation of olefins. The metallodendrimer shown in Figure 4 and its first

generation equivalent were prepared using a convergent synthesis. The peripheral tert-butyl

groups served to increase steric hinderance and enhance solubility. Epoxidation of 24

nonconjugated dienes and mixtures of linear and cyclo-alkenes were carried out using

iodosylbenzene as the oxygen donor. Using 1:1 alkene mixtures, the dendrimer-

metalloporphyrins showed greater selectivity for epoxidation of 1-alkenes over cyclooctene.

This selectivity was higher for the second generation metallodendrimer than for the first

generation one. The reason for this increase in selectivity shown by the dendritic catalyst

was attributed to the steric influence of the bulky second generation dendrimer which led to

preferential penetration of the linear alkenes.

The synthesis and properties of dendrimer porphyrins have also been reported by

Aida et al.34 Most recently, they have used negatively and positively charged dendrimer-

metalloporphyrins to construct electrostatic assemblies.35 Studies on these systems showed

that the spatial arrangement of these two communicating functionalities could be controlled

with nanometric precision. Thus, their potential application in nanomaterials science can be

anticipated.

25

O

OO

OO

OO

O

OO

O O

OO

O

OO

OO

O

O

O

OO

O

O

O

ONN

N NMnCl WW

W

Figure 4. A dendrimer-metalloporphyrin

Porphyrin-functionalised dendrimers have also been investigated for their

potential biological/therapeutic uses. The use of antibodies, modified with radioisotopes or

cytotoxic drugs in cancer imaging and therapy, is of great interest on account of the inherent

specificity of the antibody-antigen interaction. However, these modifications often diminish

or eliminate the biological activity of the macromolecule, therefore destroying its targeting

potential. In order to prevent these problems, intermediate linker molecules, which can be

highly modified with a drug, but which will only modify a single site on the surface of the

antibody, would be of great advantage. Roberts et al.36 have used PAMAM dendrimers to

covalently couple synthetic copper-chelated porphyrins to antibody molecules. The

antibody-dendrimer-porphyrin conjugate is illustrated in Figure 5.

26

Gansow et al.37 have attached the nitrogen-donor macrocycle 1,4,7,10-

tetraazacyclododecanetetraacetic acid to PAMAM dendrimers and then formed antibody

conjugates. The chelator-dendrimer-antibody constructs were easily labeled with 90Y, 111In

and 212Bi, suggesting that these types of complexes could be used in radiotherapy and

imaging. In a more detailed study, dendritic magnetic resonance imaging contrast agents,

consisting of Gd(III) complexes of the chelator 2-(4-isothiocyanatobenzyl)-6-methyl-

diethylenetriaminepentaacetic acid anchored to amino-terminated PAMAMs, were developed

by Tomalia et al.38 These complexes enhanced magnetic resonance images and were found

to be more effective contrast agents than other commercially available macromolecule-

chelate complexes, such as those formed using albumin, polylysine and dextran.

NH

SO3–

NHN

O3S

N

SO3–

C

NO2

CH2O

CH

O

HN

NH2H2N

H2N

NHCH2

Antibody

Dendrimer

Porphyrin

NH2

–

Figure 5. Representation of an antibody-dendrimer-porphyrin conjugate

27

Nickel-containing dendritic catalysts have been designed and tested for their catalytic

activity in the Kharasch addition by van Koten et al.39 The catalysts – a second generation

example of which is shown in Figure 6 – are carbosilane dendrimers, which have been

surface-functionalised with nickel complexes. Although the catalytic activities of the

dendrimers were found to be lower than that of the monomeric analogue, the catalysts could

be easily precipitated from solution and therefore recycled. More recently, van Koten et al.40

have reported the preparation of platinum-containing dendrimers which possess the same

surface functionalities as those of the dendrimer shown in Figure 6, yet contain aryl-ester

branching units instead of carbosilane backbones. The complexes reversibly bind SO2 both

in the solid state and in solution and can therefore be used as molecular sensors for this toxic

gas. Desorption of SO2 is achieved using mild conditions, thus regenerating the “detector”

compounds.

Si Si

SiO

ONH

O

Ni

Me2N

NMe2

Br

SiO

O NH

O

Ni

NMe2

NMe2

Br

Si O

OHN

MeMe

Me

Me

O

Ni

NMe2

NMe2

Br

Me

Me

WW

W

28

Figure 6. Second generation silane dendrimer functionalised with nickel complexes

Chow and Mak41 have prepared dendritic bis(oxazoline) copper complexes (generations 0-3)

which catalyse the Diels-Alder reaction. The catalytic centre is at the focal point of a

dendritic wedge constructed of Fréchet-typeError: Reference source not found polyaryl ether

dendrons. The catalysis of the Diels-Alder reaction between cyclopentadiene and crotonyl

imide (Scheme 10) was studied. Lower rates of reaction were observed using generation 3

dendrimer catalysts than when generations 0-2 were used. This observation is thought to be a

consequence of the morphological change shown by the dendrimer as the generation

increases. The catalytic core is essentially open to the surroundings at lower generations but

is partially buried in the interior of the dendritic branches at generation 3. These and other

studies discussed above indicate that a variety of types of dendritic catalyst can be

synthesised but further work on the dendritic structure is needed before superior catalysts can

be prepared.

N

O

O

O

N O

O

O

N

O

N

O

Cu(OTf)2

WW

Scheme 10. Diels-Alder reaction catalysed by bis(oxazoline)copper(II) dendrimers

Organometallic Dendrimers

29

Astruc et al.42 reported the synthesis of ferrocenyl star polymers using the Fe(-C5H5)+

induced perfuctionalisation of polymethylaromatics. Very recently,43 they have reacted

amino-terminated dendrimers with ferrocenylcarbonylchlorides to give amidoferrocenes such

as the example illustrated in Figure 7. A dendritic effect in molecular recognition has been

demonstrated using these metallodendrimers. The binding of several inorganic anions to the

ferrocenyl units was investigated by examining the shift in the position of the cyclic

voltammetric wave. The apparent association constants were found to increase in the order

NO3- < Cl- < HSO4

- < H2PO4-. In addition, the magnitude of the interaction of higher

generation dendrimers with the anions was greater than that of the lower generation

analogues.

O

N

NH

NH

O N

NH

CO

OC

NH

CO

CO

O

N

HN

NH

CO

C

Fe

Fe

Fe

Fe

Fe

Fe

O

W

W

Figure 7. An amidoferrocene dendrimer30

Organosilicon dendrimers with ferrocenyl peripheral units have been prepared by Cuadrado

et al.44 and their electrochemical properties studied. Cyclic voltammograms of the first and

second generation dendrimers show a unique wave corresponding to the oxidation of all of

the redox centres. Therefore, the ferrocenyl moieties are electrochemically equivalent non-

interacting redox centres. Cuadrado et al.45 have also surface-functionalised poly(propylene

imine) dendrimers with ferrocenyl units. In order to prepare inclusion complexes where the

dendritic terminal groups act as the guests, the binding of cyclodextrin – a well known

molecular host – to the ferrocene moieties in these dendrimers was studied. The authors

discovered that although the aqueous solubility of the dendrimers was enhanced by the

presence of cyclodextrin, it decreased with increasing dendrimer generation. In addition to

this, they also observed two different voltammetric waves for the highest generation

dendrimer, indicating that complexed and uncomplexed ferrocene units were present and

thus, complete complexation of all surface moieties was not possible. Both these

observations were attributed to the steric congestion present at the surface of the larger

dendrimer, which limits the number of ferrocene residues that can be included by the bulky

cyclodextrin hosts.

Pugin et al.46 have attached chiral diphosphine ferrocenyl complexes to dendritic

ligands in order to examine their catalytic activity in hydrogenation reactions. The rhodium-

catalysed asymmetric hydrogenation of dimethyl itaconate was studied using different sizes

of dendritic complexes. The enantioselectivities shown by the dendritic catalysts were found

to be slightly lower than that of the corresponding mononuclear catalyst. Pugin et al.Error:

Reference source not found are currently investigating the use of larger dendrimers in other

catalytic reactions in order to ascertain the influence of the dendrimer backbone on the

selectivities of reactions. 31

O

NH

NHO

O

O

O

OO

OO

O

NH

OO

O

OOONH

O

O

OOO

O

Fe

Figure 8. Asymmetric redox-active dendrimer with ferrocene subunit

Another recent report by Kaifer et al.47 describes the synthesis of asymmetric redox-

active dendrimers. A single ferrocene unit is appended to a dendritic branch of variable size

to form compounds such as the one illustrated in Figure 8. The electrochemical behaviour

of these dendrimers is similar to that described by Diederich and co-workersError: Reference

source not found for their porphyrin-based systems. Again, the redox-active centre is

partially shielded by the higher generation dendritic branches, thus hindering electron

transfer to the ferrocene unit.

32

27 Newkome, G. R.; Güther, R.; Moorefield, C. N.; Cardullo, F.; Echegoyen, L.;

Pérez-Cordero, E.; Luftmann, H. Angew. Chem. Int. Ed. Engl., 1995, 34, 2023.

28 Newkome, G. R.; He, E.; Godínez, L. A. Macromolecules, 1998, 31, 4382.

29 a) Achar, S,; Puddephatt, R. J. Angew. Chem. Int. Ed. Engl., 1994, 33, 847. b)

Achar, S.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun., 1994, 1895. c) Achar,

S.; Vittal, J. J.; Puddephatt, R. J. Organometallics, 1996, 15, 43.

31 Dandliker, P. J.; Diederich, F.; Gisselbrecht, J-P.; Louati, A.; Gross, M.

Angew. Chem. Int. Ed. Engl., 1995, 34, 2725.

32 Pollak, K. W.; Leon, J. W.; Fréchet, J. M. J.; Maskus, M.; Abruña, H. D.

Chem. Mater., 1998, 10, 30.

33 Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc.,

1996, 118, 5708.

34 a) Jiang, D-L.; Aida, T.; Chem. Commun., 1996, 1523. b) Sadamoto, R.;

Tomioka, N.; Aida, T. J. Am. Chem. Soc., 1996, 118, 3978. c) Yashima, E.;

Okamoto, Y. Macromolecules, 1996, 29, 5236.

33

OO

Pd

SPh

SPh

Pd

PhS

SPh

N

NH

O O

Pd

SPh

SPh

Pd

SPh

PhS

O

ClCl

OO

Pd

SPh

SPh

Pd

PhS

SPh

Cl Cl

CN

NNH

O

O

Pd

PhS

SPh

Pd SPhPhS

O

Cl

Cl

N

NHO

O

Pd SPhPhS

Pd

PhS

PhS

O

Cl

Cl

CN

AgBF431 32

33

Scheme 11. Convergent synthesis of palladium-containing dendritic wedge

Reinhoudt et al.48, 49 have utilised palladium-containing building blocks to construct

metallodendrimers using both convergent and divergent approaches. Dendritic growth was

achieved by the substitution of an N-donor ligand for a chloride in a two-step process.

Scheme 11 shows the convergent synthesis of a second generation dendritic wedge. Firstly,

the palladium complex 31 was activated by removing the chloride ion using Ag[BF4].

35 Tomioka, N.; Takasu, D.; Takahashi, T.; Aida, T. Angew. Chem. Int. Ed.,

1998, 37, 1531

36 Roberts, J. C.; Adams, Y. E.; Tomalia, D. A.; Mercer-Smith, J. A.; Lavallee, D. K. Bioconjugate Chem., 1990, 1, No. 5, 305.

37 Wu, C.; Brechbiel, M. W.; Kozak, R. W.; Gansow, O. A. Bioorganic & Medicinal Chemistry Lett., 1994, 4, 449.

38 Wiener, E. C.; Brechbiel, M. W.; Brothers, H.; Magin, R. L.; Gansow, O. A.; Tomalia, D. A.; Lauterbur, P. C. Mag. Res. Med., 1994, 31, 1.

39 a) Knappen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van Leeuwen, P.

W. N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature, 1994, 372, 659. b)

Gossage, R. A.; van de Kuil, L.; van Koten, G. Acc. Chem. Res., 1998, 31, 423.

40 Albrecht, M.; Gossage, R. A.; Spek, A. L.; van Koten, G. Chem. Commun.,

1998, 1003.

41 Mak, C. C.; Chow, H-F. Macromolecules, 1997, 30, 1228.

42 a) Moulines, F.; Gloaguen, B.; Astruc, D. Angew. Chem. Int. Ed. Engl., 1992,

31, 458. b) Moulines, F.; Djakovitch, L.; Boese, R.; Gloaguen, B.; Thiel, W.; Fillaut,

J-L.; Delville, M-H.; Astruc, D. Angew. Chem. Int. Ed. Engl., 1993, 32, 1075. c)

Cloutet, E.; Fillaut, J-L.; Gnanou, Y.; Astruc, D. J. Chem. Soc., Chem. Commun.,

1994, 2433. d) Fillaut, J-L.; Linares J.; Astruc, D. Angew. Chem. Int. Ed. Engl., 1994,

33, 2460.

34

Subsequent addition of two equivalents of pyridine-based building block 32 gave a second

generation dendritic wedge 33 with a cyano group at its focal point. Repetition of these two

steps afforded a third generation wedge. In the final stage of the synthesis, the dendritic

wedges were coupled to a trifunctional palladium-containing core molecule. In one report,

the dendrimers were synthesised using a combination of both coordinative and hydrogen

bonds.50

Another type of organopalladium dendrimer has been prepared by van Koten et al.51

via the insertion of palladium into peripheral carbon-iodine bonds of carbosilane dendrimers.

The organopalladium moieties were attached to the periphery of the dendrimer exclusively

via palladium-carbon bonds. Reactions of these complexes with transmetalation reagents

LiMe and SnMe4 were attempted but were unsuccessful.

43 a) Valério, C.; Fillaut, J-L.; Ruiz, J.; Guittard, J.; Blais, J-C.; Astruc, D. J.

Am. Chem. Soc., 1997, 119, 2588. b) Guittard, J.; Blais, J-C.; Astruc, D.; Valério, C.;

Alonso, E.; Ruiz, J.; Fillaut, J-L. Pure & Appl. Chem., 1998, 70, 809.

44 Alonso, B.; Cuadrado, I.; Morán, M.; Losada, J. J. Chem. Soc., Chem.

Commun., 1994, 2575.

45 a) Castro, R.; Cuadrado, I.; Alonso, I.; Casado, C. M.; Morán, M.; Kaifer, A.

E. J. Am. Chem. Soc., 1997, 119, 5760. b) Takada, K.; Díaz, D. J.; Abruna, H. D.;

Cuadrado, I.; Casado, C.; Alonso, B.; Morán, M.; Losada, J. J. Am. Chem. Soc., 1997,

119, 10763.

46 Köllner, C.; Pugin, B.; Togni, A. J. Am. Chem. Soc., 1998, 120, 10274.

47 Cardona, C. M.; Kaifer, A. E. J. Am. Chem. Soc., 1998, 120, 4023.

35

Pt

PBu3

PBu3

Bu3PPBu3

Ph

Pt

Ph

PBu3

Bu3PPt

PtBu3P

PBu3

PBu3

PBu3

Ph Pt

Ph

Bu3P

PBu3Pt

PtPBu3

Bu3P

Ph

PBu3

PBu3

Pt

Ph

PBu3

Bu3PPt

Figure 9. Organoplatinum dendrimer

Stang et al.52 and Takahashi et al.53 have also employed metal-carbon bonds to

construct their organoplatinum dendrimers. In these examples, the metals are present in

every generational layer of the dendrimer. Stang et al.Error: Reference source not found

used a stepwise divergent approach to synthesise first and second generation

metallodendrimers with a backbone of -bonded tri- and tetra-ethynylbenzene units. A

similar strategy devised by Takahashi et al.Error: Reference source not found used

triethynyl-trimethylbenzene as a building block to form metallodendrimers such as the one

illustrated in Figure 9.

A final example of the use of metal-carbon -bonds in the construction of

organotransition metal dendrimer synthesis has been reported by Liao and Moss.54 They 36

have prepared dendrimers with the functional group (CpM(CO)2CH2CH2CH2-) – where M =

Fe, Ru – located exclusively at the periphery. Using the convergent approach, the metal

complexes were attached to a Fréchet-typeError: Reference source not found poly(aryl ether)

dendritic wedge. The largest dendrimer synthesised – generation 4 – contained 48 ruthenium

atoms and has an estimated diameter of about 5 nm.

Sulfur-Donor Dendrimers

Majoral et al.Error: Reference source not found have prepared gold chloride containing

dendrimers with the gold atom coordinated to sulfur in the internal cavaties of the dendrimer

and to phosphorus at the surface (see Figure 2). The palladacycle dendrimers of Reinhouldt

21 a) Lange, P.; Schier, A.; Schmidbaur, H. Inorg. Chim. Acta, 1995, 235, 263.

b) Lange, P.; Schier, A.; Schmidbaur, H. Inorg. Chem., 1996, 35, 637.

24 a) Serroni, S.; Denti, G.; Campagna, S.; Juris, A.; Ciano, M.; Balzani,

V. Angew. Chem. Int. Ed. Engl., 1992, 31, 1493. b) Campagna, S.; Denti, G.;

Serroni, S.; Ciano, M.; Juris, A.; Balzani, V. Inorg. Chem., 1992, 31, 2982. c) Juris,

A.; Balzani, V.; Campagna, S.; Denti, G.; Serroni, S.; Frei, G.; Güdel, H. U. Inorg.

Chem., 1994, 33, 1491. d) Campagna, S.; Denti, G.; Serroni, S.; Juris, A.; Venturi,

M.; Ricevuto, V.; Balzani, V. Chem. Eur. J., 1995, 1, 211. e) Denti, G.; Campagna,

S.; Serroni, S.; Ciano, M.; Balzani, V. J. Am. Chem. Soc., 1992, 114, 2944. f)

Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem.

Res., 1998, 31, 26.

25 a) Newkome, G. R.; Cardullo, F.; Constable, E. C.; Moorefield, C. N.; Cargill

Thompson, A. N. W. J. Chem. Soc., Chem. Commun., 1993, 925. b) Constable, E. C.;

Haverson, P. Chem. Commun., 1996, 33. c) Constable, E. C.; Haverson. P.;

Oberholzer, M. Chem. Commun., 1996, 1821. d) Armspach, D.; Cattalini, M.;

Constable, E. C.; Housecroft, C. E.; Phillips, D. Chem. Commun., 1996, 1823. e)

Constable, E. C.; Haverson, P. Inorg. Chim. Acta, 1996, 252, 9. f) Constable, E. C. 37

et. al.Error: Reference source not found,Error: Reference source not found,Error: Reference source not found also use

sulfur donors in S-C-S double pincer ligands.

Welton et al. have prepared PAMAM dendrimers with a terminal secondary amine

rather than the normal primary amine.55 They have then used these to prepare sodium

dithiocarbamate dendrimers that they then coordinated to a variety of ruthenium complexes

(Scheme 12).

Chem. Commun., 1997, 1073

26 Issberner, J.; Vögtle, F.; De Cola, L.; Balzani, V. Chem. Eur. J., 1997, 3, 706.

48 Huck, W. T. S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Angew. Chem. Int.

Ed. Engl., 1996, 35, 1213.

49 Huck, W. T. S.; Prins, L. J.; Fokkens, R. H.; Nibbering, N. M. M.; van

Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc., 1998, 120, 6240.

50 Huck, W. T. S.; Hulst, R.; Timmerman, P.; van Veggel, F. C. J. M.;

Reinhoudt, D. N. Angew. Chem. Int. Ed. Engl., 1997, 36, 1006.

51 Hoare, J. L.; Lorenz, K.; Hovestad, N. J.; Smeets, W. J. J.; Spek, A. L.;

Canty, A. J.; Frey, H.; van Koten, G. Organometallics, 1997, 16, 4167.

52 Leininger, S.; Stang, P. J.; Organometallics, 1998, 17, 3981.

53 Ohshiro, N.; Takei, F.; Onitsuka, K.; Takahashi, S. J Organomet. Chem.,

1998, 569, 195.

54 a) Liao, Y-H.; Moss, J. R. J. Chem. Soc., Chem. Commun., 1993, 1774. b)

Liao, Y-H.; Moss, J. R. Organometallics, 1995, 14, 2130.

38

Conclusions

These aesthetically pleasing macromolecules known as dendrimers are attracting an

increasingly large amount of attention from chemists and biochemists in all research areas.

Over 600 papers in this field were published during 1998 alone. Metallodendrimers are

beginning to show real promise in catalysis, with catalytic activities per metal centre being

equivalent to those of monomeric analogues. The possibility of using dendrimers to prevent

catalyst losses from reaction mixtures could soon lead to commercialisation of these

materials.

Future progress will undoubtedly include the formation of tailor-made dendritic

assemblies where predetermined properties can be introduced into specific sites in the

dendritic structure. The synthetic control and fine-tuning of the dendrimer needed to produce

molecules with specific properties should lead to the proposed applications of these

molecules becoming a reality.

39

References

8 a) Newkome, G. R.; Baker, G. R.; Saunders, M. J.; Russo, P.S.; Gupta, V. K.;

Yao, Z.; Miller, J. E.; Bouillion, K. J. Chem. Soc., Chem. Commun., 1986, 752. b)

Newkome, G. R., Yao, Z.; Baker, G. R.; Gupta, V. K.; Russo, P.S.; Saunders, M. J. J.

Am. Chem. Soc., 1986, 108, 849.

12 Wooley, K. L.; Hawker, C. J.; Fréchet, J. M. J. J. Chem. Soc. Perkin Trans.,

1991, 1059

22 a) Wörner, C.; Mülhaupt, R. Angew. Chem. Int. Ed. Engl., 1993, 32, 1306. b)

Brabander-van den Berg, E. M. M.; Meijer, E. W. Angew. Chem. Int. Ed. Engl.,

1993, 32, 1308.

23 Reetz, M. T.; Lohmer, G.; Schwickardi, R. Angew. Chem. Int. Ed Engl., 1997,

36, 1526.

30 Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati, A.;

Sanford, E. M. Angew. Chem. Int. Ed. Engl., 1994, 33, 1739.

55 McCubbin, Q. J.; Stoddart, F. J.; Welton, T.; White, A. J. P.; and Williams, D. J. Inorg. Chem., 1998, 37, 3753.

40