Dendrimers II Episode 5 pdf

This review summarizes our original organometallic route to stars, dendrimers, metallostars and metallodendrimers and the redox functions of these macromolecules in catalysis and anionic

This review summarizes our original organometallic route to stars, dendrimers, metallostars and metallodendrimers and the redox functions of these macromolecules in catalysis and anionic recognition The synthesis of metal-sandwich stars and dendritic cores was achieved using the CpM + induced polyallylation and polybenzylation of polymethylbenzenes (M = Fe or Ru) and pentamethylcyclopentadienyl ligands (M = Co or Rh) Subsequent functionalization of the polyallyl dendritic cores yielded polyols which are precursors of polyiodo, polymesylates, polynitriles, polyamines and polybenzaldehaldehyde cores The synthesis of dendrimers up to 144-nitrile and 243-allyl was subsequently achieved starting from mesitylene Functionaliza- tion of the polybenzyl dendritic cores was achieved by regiospecific Friedel-Crafts reactions (acetylation, chlorocarbonylation) in the para position Various metallodendrimers were syn-

thesized with amidoferrocene, amidocobaltocenium and FeCp*(h6-N-alkylaniline)+ termini in which the redox centers show a reversible behavior and are all independent as observed by cyclic voltammetry The 9-, 18- and 24-amidometallocene dendrimers were used for the recog- nition of the oxo anions H 2 PO 4 and HSO 4 by cyclic voltammetry, whereas a 24-iron-alkylaniline dendrimer was efficient to recognize Cl – and Br – anions by 1 H NMR with sharp dendritic effects Differences between the responses to the different anions were large and the largest effects were found for the 18-Fc dendrimer (dendritic effect) A water-soluble star-shaped hexa-iron redox catalyst was as efficient as the mononuclear species for the cathodic reduction of NO 3 and NO 2

in water In conclusion, metallostars are suitable for catalysis, and metallodendrimers present optimal topologies for molecular recognition These specific functions related to the topologies cannot be interchanged between the metallostars and the metallodendrimers with optimized efficiency in the present examples.

Keywords:Dendrimers, Supramolecular chemistry, Molecular recognition, Catalysis, molecular, Organometallic

Macro-1 Introduction 230

2 CpFe + Mediated Synthesis of Stars and Dendritic Cores 232

2.1 Syntheses of Hexa-Arm Stars Starting from Hexamethylbenzene 2322.2 Syntheses of Octafunctional Dendritic Cores Starting from Durene 2342.3 Syntheses of Nonafunctional Dendritic Cores Starting

from Mesitylene 2392.4 Syntheses of New Polyamine Dendrimers 2422.5 A Fast Organoiron Route to Large Dendrimers 242

The First Organometallic Dendrimers:

Design and Redox Functions

Didier Astruc1· Jean-Claude Blais2· Eric Cloutet1· Laurent Djakovitch1· Stéphane Rigaut1· Jaime Ruiz1· Valérie Sartor1· Christine Valério1

1 Groupe de Chimie Supramoléculaire des Métaux de Transition, LCOO, UMR CNRS No 5802, Université Bordeaux I, 33405 Talence Cédex, France

E-mail: d.astruc@lcoo.u-bordeaux.fr

2 Laboratoire de Chimie Structurale Organique et Biologique, EP CNRS No 103,

Université Paris VI, 4 Place Jussieu, 75252 Paris, France

Topics in Current Chemistry, Vol 210

© Springer-Verlag Berlin Heidelberg 2000

4 Redox Recognition of Inorganic Anions 248

5 Redox Catalysis by Metallostars 255

6 Conclusions 255

7 References 256

1

Introduction

Redox processes are essential in Nature and technology [1], and are intimately connected to supramolecular chemistry [2, 3] Thus, the redox properties of dendrimers, a now well-established field of supramolecular chemistry [4, 5], are likely to play an increasing role in the future Recent reviews on dendrimers are numerous [6–28], and we shall concentrate here on metallodendrimers in

which reversible redox centers have been attached in any way, allowing applica-tions to processes which involve the use of the redox funcapplica-tions Specifically, we

will compare the redox properties of metallostars and metallodendrimers with

respect to two functions: catalysis and molecular recognition.

In 1978, Vögtle published the first iteration of a reaction leading to the forma-tion of a tetraamine from a monoamine after two sequences consisting of

a Michael reaction followed by the reduction of the nitrile to the amine (Scheme 1) [29] In 1979, we independently reported the CpFe+mediated one-pot hexamethy-lation of the hexamethylbenzene ligand to hexaethylbenzene (Scheme 2) [30] This reaction comprises six deprotonation-alkylation sequences In this case, the iteration of the sequence was achieved without compulsory isolation of the inter-mediate products Although the reaction is not catalytic, the ligand is firmly held

on the metal center while the reaction sequences are repeated several times until the steric limit is reached This kind of reaction system represents a new type of process intermediate between stoichiometric reactions and catalysis It is made possible by the enhancement of the acidity of the benzylic protons in the cationic

complex The pKain dimethyl sulfoxide (DMSO) was indeed found to be about 14

units lower for the 18-electron complexes [MCp(h6-C6Me6)][PF6] (M = Fe, 1; Ru, 2)

(pKa= 29) than for the free arene (pKa= 43) [31–33] Thus, the organometallic

complex is a reservoir of protons in these reactions.

The use of this system with various polymethylbenzene ligands in the

com-plexes [MCp(h6-arene)][PF6] (M = Fe or Ru) and the pentamethylcyclopentadienyl

ligand in the complexes [M*Cp(h5-C5Me5)][PF6] (M = Co or Rh) led to a variety

of non-chiral and chiral dendritic cores starting from functionalizable halides such as benzyl bromide and benzyl bromide Subsequently, redox-active late-transition-metal sandwich units, ruthenium-polypyridine species and C60 frag-ments have been attached to the tethers of these stars and dendrimers We will first describe these syntheses, then address the redox properties and their uses

in molecular recognition and catalysis Other metallocene dendrimers, inparticular the polyferrocene dendrimers synthesized by the groups of Cuadrado,Jutzi and Togni, have appeared in the literature [34–39] Ru-polypyridine den-drimers were introduced in the seminal work of Balzani’s group [40–43],then by the groups of Newkome and Constable [44–47] Other redox-activedendrimers are those decorated with tetrathiafulvalene (TTF) units reported

by the groups of Bryce and Becher [48–51] and dendrimers centered onmetalloporphyrins [52, 53], metal-polypyridine units [54–57], metal clusters[58–60], ferrocene derivatives [61, 62], C60[63, 64] and naphthalene diimine [65, 66]

The First Organometallic Dendrimers: Design and Redox Functions 231

Scheme 1. The first iterative cascade synthesis of tetraamines reported by Vögtle [29]

+

-Scheme 2. One-pot hexamethylation of [MCp(C 6 Me 6 )][PF 6 ] (M = Fe or Ru) using excess

t-BuOK and methyl iodide in THF With Fe, the reaction occurs with a spontaneous smooth

reflux for 1 min (5 mmol-scale) upon addition by cannula of a THF solution of MeI to the other solid reactants with stirring With Ru, heating the reaction mixture for 1 d at 40 °C is needed

Syntheses of Hexa-Arm Stars Starting from Hexamethylbenzene

The reaction of the PF6 salt of 1 or 2 [67–69] with excess KOH (or t-BuOK) in

dimethyl ether (DME) and excess methyl iodide or benzyl bromide leads to

a one-pot hexa-substitution (Scheme 3, Fig 1) [30, 70] With allyl bromide (oriodide) in DME, either the hexa-allylation [71] or the dodeca-allylation [72]product is obtained, depending on the reaction time The prototypal hexafunc-tionalization is represented in Scheme 3 Both the hexa- and dodeca-reactionsare well controlled On the other hand, the reaction with excess benzyl bromide

or p-alkoxybenzyl bromide only gives the hexabenzylated [70, 73] or

hexa-alkoxybenzylated [74, 75] complex as the ultimate reaction product Similarly,

Fe

R

R R

+

-Scheme 3. One-pot hexafunctionalization of [FeCp(C 6 Me 6 )][PF 6 ] using various electrophiles Reaction temperatures vary between RT and 40 °C and reaction times are overnight or 1 d

Fig 1. X-ray crystal structures Ortep views of [FeCp{h6 -C 6 (CH 2 CH 2 -CH=CH 2 ) 6 }][PF 6] (left side

view) obtained by hexa-allylation of 1 and of [FeCp{h6 -C 6 (CH 2-pC6 H 4 OEt) 6 }][PF 6] (right top

view) obtained by hexaethoxybenzylation of 1

with ferrocenylalkyl iodide, the hexaferrocenylalkylation product [74] is

obtain-ed from 1, free of any more highly branchobtain-ed product This type of reaction can

only work with halides which are compatible with the presence of the base in

excess For instance, alkyl halides only react if the base is KOH, not t-BuOK,

since the latter leads to dehydrohalogenation [75] For this reason also, alkynylhalides cannot be used, but alkynyl substituents can be introduced from thehexaalkene derivative by bromination followed by dehydrohalogenation of thedodecabromo compound (Scheme 4) [76] The hexaalkene is also an excellent

The First Organometallic Dendrimers: Design and Redox Functions 233

Scheme 4. Synthesis (by reaction of the hexaalkene with Br 2 in CH 2 Cl 2 at RT followed by NaNH 2 in NH 3at –33 °C) and reactions of the hexaalkyne a Me2 NSnMe 3; b [Co2 (CO) 8 ], pen-

tane, RT; c nBuLi, THF, RT; d MeI, THF, RT; e MeSiCl, THF, RT; f CO, THF then aq HCl, RT

chain hexasilanes [71] and hydrometallations can also be achieved using[ZrCp2(H)(Cl)] [77] The hexazirconium compound obtained is an intermediatefor the synthesis of the hexaiodo derivative [77].

The most useful hydroelementation reaction, however, is hydroborationleading to the hexaborane which is oxidized to the hexol using H2O2under basicconditions [71] This chemistry can be carried out on the iron complex or, alter-natively, on the free hexaalkene, which may be liberated from the metal byphotolysis in CH2Cl2or MeCN using visible light [71, 78] Williamson couplingreactions between the hexol and 4-bromomethylpyridine or -polypyridine leads to hexapyridine and hexapolypyridine and their ruthenium complexes(Scheme 5) [79] The hexol is indeed the best source of the hexaiodo derivativeeither using HI in acetic acid or even better by trimethylsilylation using SiMe3Clfollowed by iodation using NaI [80] This hexaiodo star was condensed with

p-hydroxybenzaldehyde to give a hexabenzaldehyde star which could further

react with substrates bearing a primary amino group Indeed, this reactionyielded a water-soluble hexametallic redox catalyst which was active in theelectroreduction of nitrate and nitrate to ammonia on an mercury (Hg) cathode

in basic aqueous solution (vide infra) (Scheme 6).

Hexa-arm polystyrene polymers with Mn up to 90,000 g/mol with dispersities of 1.1 can be synthesized by regiospecific acetylation of the hexa-benzylated arene, followed by reduction to the hexa-secondary alcohol, chlori-nation with SOCl2 and living polymerization of styrene at –50 °C using SnCl4

poly-as the Lewis acid, n-Bu4NCl as the Cl source which quenches the living

carboca-tion, and 2,6-di-tert-butylpyridine as the base The hexa-arm polystyrene

polymer of Mn= 18,000 g/mol (30 repeat styrene units per branch) bearingsecondary chloro atoms at the termini of the branches can be transformed,using a 100-fold excess of Me3SiN3, to its hexaazido analogue which cleanlyreacts in refluxing PhCl with C60in one day to give a tetrahydofuran (THF)- and

CH2Cl2-soluble, hexa-C60star, characterized inter alia by 13C NMR, gravimetry, monomodal distribution in size-exclusion chromatography andcyclic voltammetry (Scheme 7) [81] Before closing this section, it is important

thermo-to note that various other symmetrically hexasubstituted benzene families areknown [82]

2.2

Syntheses of Octafunctional Dendritic Cores Starting from Durene

In compound 1, the CpFe+induced perfunctionalization reaction is limited bythe bulk of the six alkyl substituents around the benzene ring Thus, the usualtrend is that only one hydrogen per methyl substituent can be replaced by thebranch introduced using the halide (the only exception being the prolongedreaction with allyl bromide which can be pushed to double substitution,Scheme 8) However, depending on the bulk around the methyl groups, the sub-stitution pattern varies Fortunately, reactions can always be made specific for

the formation of a single product In [FeCp(h6-durene)][PF] (3) each methyl

The First Organometallic Dendrimers: Design and Redox Functions 235

Scheme 5. Hydroelementation reactions of the hexaalkene derivative

The First Organometallic Dendrimers: Design and Redox Functions 237

Scheme 7. Synthesis of a star-shaped hexa-C 60 polymer derivative by CpFe + induced benzylation of C 6 Me 6 followed by regiospecific acetylation, reduction to the hexol with NaBH 4 , chlorination in the benzylic positions using SOCl 2 , living polymerization by reaction with SnCl 4 and styrene, formation of the hexaazido by reaction with NaN 3 , and reaction of the hexaazido with C 60

hexa-group has only one methyl neighbor, so that double branching proceeds easilyand selectively by reaction with excess methyl iodide, allyl bromide or benzylbromide (Scheme 9) [72] Regiospecific hydroboration of the octaallyl productfollowed by oxidation by H2O2/OH–gives the octol [72] whereas regiospecificchlorocarbonylation of the octabenzyl product selectively provides the octa-chlorocarbonyl derivatives in which chlorocarbonylation only occurs in thepara position [83] This compound is an excellent starting point for the synthe-sis of octaamide derivatives by reaction with amines This allows the branching

of ferrocene and tripodal units such as Newkome’s amino tripod (Scheme 10)which leads to a 24-nitrile dendrimer of generation 0 whose matrix-assistedlaser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum isshown in Fig 2 [83]

Scheme 8. Synthesis of the bulky dodecaallyl derivative and self-assembly of the two enantiomers with opposite directionality

R R

Kt-BuO or KOH (excess) RBr or RI (excess)

+ +

Syntheses of Nonafunctional Dendritic Cores Starting from Mesitylene

In [FeCp(mesitylene)][PF6] (4) [83], each methyl group is free of a methyl

neigh-bor on the adjacent arene positions This is indeed the optimal situation for mum substitution, i.e replacement of the nine H-atoms of the three methyl

maxi-groups by nine branches In the initial reaction of 4 with t-BuOK and MeI in THF,

the tris-tert-butyl benzene complex was obtained [70] However, the introduction

of nine bulkier groups by reactions of other alkyl and benzyl halides failed,

sub-stitution being incomplete [85] As for 1, this limit has an exception with allyl

The First Organometallic Dendrimers: Design and Redox Functions 239

C

C

C C

C

C

HN O O H

H O O HN NH

C O O O

C O O O

C O O C

NC

NC CN CN NC

CN CN

CN NC CN CN NC NC NC

C C

O Cl O Cl

O Cl O Cl O

Cl O Cl

O Cl

Cl O

HN O O H H O O HN NH O O H N

NH O

Fe

Fe

Fe Fe

Fe Fe

bromide; in this case, the reaction leads to nona-allylation in high yield (Fig 3)

[85] As for the hexa-allylation of 1 and 2, the facile nona-allylation of 4 leads to

subsequent synthetic developments, in particular via the quantitative tion followed by oxidation to the nonol.At this point, it is already possible to intro-duce metallocene redox centers [84], but molecular engineering is necessary inorder to match the required structure with the desired function (Scheme 11)

hydrobora-The First Organometallic Dendrimers: Design and Redox Functions 241

Fig 3.X-ray crystal structure of C 6 H 3 [C(CH 2 CH=CH 2 ) 3 ] 3 obtained by CpFe + induced allylation of mesitylene Ortep view along the plane of the benzene ring

nona-Scheme 11. CpFe + induced nona-allylation of mesitylene and regiospecific functionalization

of the nonaallyl derivative

The hexol and nonol can be easily transformed, via the nonaiodo and the anitrile, to hexa- and nonaamines in which the branches have the same length[85] However, we forecast that the branches of such hexa- and nonaamineswould be too short to provide soluble hexa- and nonametallocenes Thus, weused the Michael-type condensation of acrylonitrile with the nonol to increasethe length of the branches by three carbons and one nitrogen atom This reac-tion has been used by Newkome to synthesize a trinitrile tripod from a trioltripod [86] The nonanitrile was obtained in high yield, but its reduction to thenonaamine was a delicate task The Raney nickel catalyzed hydrogenation wasnot as efficient in our hands as announced in the recent literature [87], as only

non-up to 90% hydrogenation could be obtained after repeated attempts using thesame mixture

We turned our attention to the efficient reduction using the BH3/Me2S reagent[88] Reduction was quantitative and free of retro-Michael reaction using thisreagent at 20 °C, as shown by the mass spectrum of the nonaamine from whichboron edducts had been removed by methanolysis in refluxing methanol UsingVögtle’ seminal iteration (Scheme 1) [29, 35], consisting of the Michael reaction

of acrylonitrile with a diamine to form a tetranitrile, we performed the reaction

of the nonaamine with acrylonitrile which gave a 18-nitrile This dendriticstrategy was pursued until the 72-amine and 144-nitrile [90, 91] The 13C NMRspectra showed the absence of significant amounts of products resulting from sidereactions, and elemental analyses of the polynitriles were correct (Scheme 12)

2.5

A Fast Organoiron Route to Large Dendrimers

Polybranching using CpFe+activation of benzylic protons was extended to tional aromatics in order to open the route to dendrons For instance, starting

func-from [FeCp(h6-p-MeC6H4OEt)][PF6], reaction with excess allyl bromide and

KOH leads to the tripodal dendron p-OHC6H4C(CH2–CH=CH2)3in a one-potreaction consisting of eight steps which must proceed in the right order: threedeprotonation-allylation sequences, cleavage of the O–Et bond and decomplexa-tion (Scheme 13) This dendron can be branched onto a nonaiodo-, or better,onto a nonamesylate core (obtained from the nonol core) to give the 27-allyldendrimer which, in turn, can be transformed into the 27-alcohol dendrimerwhose MALDI-TOF mass spectrum is shown in Fig 4 Iteration of this processleads to the 81-allyl dendrimer whose molecular peak is still the major peak

in the MALDI-TOF mass spectrum and to the 243-allyl dendrimers whose

13C NMR spectrum shows complete substitution of the mesylate groups [78]

Scheme 12 a Synthesis of new polynitrile and polyamine dendrimers starting from the

nonaallyl compound (Scheme 11) and using the seminal strategy of Vögtle shown in Scheme 1;

b 144-CN, the ultimate dendrimer of Scheme 12a

The First Organometallic Dendrimers: Design and Redox Functions 243

78%

70%

43% after chromatog. 88% 50% after chromatog.

O O

O

NH 2

NH 2

NH 2 O

O O

NH 2

NH 2

NH 2

O O O

H 2 N

H 2 N

H 2 N

O O

O

CN

CN

CN O

O O

CN NC NC

O O O NC NC NC

O

N

N

N O

O O

N N N

O O O N

CN CN CN CN CN NC NC NC NC CN CN CN

O O

O

N

N

N O

O

O

N N

N

O

O

O N

CH 2 =CH-CN H2O, 80 C

80%

60% after chromtog.

84%

BH 3 :Me 2 S THF

64%

KOH, dioxane

18-CN 18-NH 2

O

O O N

N N

N

N N N N N N N N N N

O O

O

N N

N

N N N N N

N N

N N

N N

N N N N

N N

O O

O N

N N

N N N N

N N

N N N N N

N N N N

N

N

N

N N N N N N N N N N N

N

N N

N N N N N N

N

N N N

N N N

N NN N N N N N N

N N N N N N N N N N N N N N N N N N N N N N N

N C N C

C C C C

C C C

C CC CC

C C C C C C C C C C C

C CC CC

C C C C

C C C C C C C

CCCC

C C C C C C C C C C

C

CCCC C C C C C C

C C

C N

5 8

16

14 13 18

6 7 4

C

N

15

N N N N N N N N N

N N N

N N N N N N N N N N N N N N N N N N N N N N N N N