Metallodendrimers towards enzyme mimics and molecular electronics: new-generation catalysts, sensors and molecular batteries

Metallodendrimers towards enzyme mimicsand molecular electronics: new-generationcatalysts, sensors and molecular batteriesBeatriz Alonsoa, Didier Astruca*‡, Jean-Claude Blaisb, Sylvain Nlatea, Stéphane Rigauta,Jaime Ruiza, Valérie Sartora, Christine Valérioa

a Laboratoire de chimie organique et organométallique, UMR CNRS n° 5802, université Bordeaux–1, 351, cours de laLibération, 33405 Talence cedex, Franceb Laboratoire de chimie structurale organique et biologique, EP CNRS n° 103, université Paris-6, 4, place Jussieu, 75252 Pariscedex 05, France

Received 31 August 2000; accepted 14 September 2000

Article dedicated to the memory of Olivier Kahn, a stimulating friend and an outstanding scientist whorationalised the approach to molecular magnetism.

Abstract – Large supramolecular metallodendrimers can now be synthesised rapidly, reaching a high number of branchesin only a few generations, and approaching the de Gennes steric limit. They are characterised by their MALDI TOF massspectra, in particular by the molecular peak, and by 1H, 13C and 31P NMR. Following rational molecular engineering, theycan be designed to achieve essential functions such as molecular batteries, catalysts and sensors for the recognition ofvarious anions. © 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS

metallodendrimers / supramolecular / batteries / catalysts / sensors

Version française abrégée – Les métallodendrimères vers le mimétisme des enzymes et l’électronique molécu-laire : batteries moléculaires, nouveaux catalyseurs et capteurs. La synthèse rapide de grands dendrimères a été réa-lisée grâce à l’activation du mésitylène par le greffon cationique à 12 électrons CpFe+, permettant de remplacer les neuf Hbenzyliques par neuf branches allyles, ce qui produit un cœur de dendrimère. Avec le p-éthoxytoluène, cette mêmeactivation conduit directement au dendron phénol trialkylé, greffable sur le cœur, c’est-à-dire à la construction divergentedu dendrimère par multiplication par trois du nombre de branches à chaque génération (figure 1 : 9 → 27 → 81 → 243).L’hydrosilylation de ces dendrimères polyallyles par le ferrocenyldimethylsilane en présence du catalyseur de Karsted àtempérature ambiante conduit aux dendrimères polyferrocéniques, ce qui permet une fabrication facile d’électrodesmodifiées d’autant plus stables que le nombre de groupements ferrocényles dans le dendrimère est plus élevé (électrodede platine modifiée avec ce même métallodendrimère, figure 8). Les groupements polyferrocéniques constituent desbatteries moléculaires, dans lesquelles tous les centres redox sont oxydés (à peu près) au même potentiel (voir le spectreMössbauer du dendrimère bleu, comportant un nombre théorique de 243 ferriciniums, figure 6). Les métallodendrimèrespeuvent être habillés de clusters Ru3(CO)11 par réaction d’échange de ligands entre un carbonyle du cluster [Ru3(CO)12] etune terminaison phosphine de dendrimère phosphoré à 32 ou 64 branches. Cette réaction est catalysée par le complexeréservoir d’électron [FeICp(η6-C6Me6)] et procède suivant le mécanisme de transfert d’électron en chaîne, ce qui permetd’introduire 92 ou 184 Ru à la périphérie du dendrimère. Ces dendrimères-clusters devraient s’avérer d’excellents cataly-seurs, du fait des propriétés catalytiques connues des clusters [Ru3(CO)11(phosphine)]. D’autres catalyseurs, de type redox,pour la réduction de nitrate et de nitrite en ammoniac dans l’eau peuvent être synthétisés en forme d’étoile. Ce type de

* Correspondence and reprints.E-mail address: [email protected] (D. Astruc).

‡ Membre de l’Institut universitaire de France.

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topologie évite les contraintes stériques périphériques auxquelles cette catalyse est sensible. Les constantes de vitesseobtenues correspondent effectivement, sans diminution, à celles obtenues avec des monomères de même force motrice.Enfin, des dendrimères de structure métallocénique voisine servent de capteurs pour la reconnaissance d’oxo-anions(H2PO4

–, HSO4–, NO3

–) avec un effet dendritique très marqué, c’est-à-dire que la faculté de reconnaissance est d’autantmeilleure que la génération dendritique est plus élevée (3 < 9 < 18 branches amidoferrocènes). Les conditions sont àl’étude pour obtenir à la fois les fonctions de reconnaissance d’un oxoanion tel que le nitrate et de catalyse de saréduction en ammoniac. L’addition de ces deux propriétés étant le propre des enzymes, il serait alors possible de mimerune enzyme avec un métallodendrimère. © 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS

métallodendrimères / supramoléculaire / batteries / catalyseurs / capteurs

1. Introduction

Olivier Kahn used the concepts of supramolecu-lar chemistry to fill the gap between molecularferro/ferrimagnetism [1, 2] and molecular-based fer-romagnetic materials [3]. Indeed, supramolecularchemistry, initiated by Jean-Marie Lehn [4], is thescience of weak bonding energies, which areinvolved in a large variety of systems, from simpleones such as the hydrogen bonding in water tosophisticated photochemical machineries [5] andthe complicated biological processes in which theseforces play a key role in recognition and catalysis[4–6]. Typically, these properties are also met inenzymes. Thus, enzyme models which can mimicthese functions have been popular and could beuseful as many biomimetic processes in bio-organicand bio-inorganic chemistry [7]. Dendrimers [8–19],the first precise synthetic macromolecules (theoreti-cal polydispersity = 1.0) belong to the world ofsupramolecular chemistry. Moreover, their sizematches that of biocomponents and the fractality oftheir surfaces resembles that of viruses, enzymesand proteins [20–22]. In the present mini-reviewarticle, we would like to underline their potentialas a reaction medium, i.e. a molecular medium ableto achieve several functions such as molecular bat-teries (electron-reservoirs), molecular recognitionand catalysis in a single dendritic molecule withspecified topological, chemical and physico-chemical properties. Altogether, these propertieswould be such that the dendrimer could mimic anenzyme and be useful for the design of electronicand bio-electronic devices. Although these goalsare still in front of us, we would like to show thatthis research line is realistic and we will indicatehere our efforts in this direction. These include thesynthetic aspects [23–25], investigation of the redoxchemistry of metallodendrimers, studies of their usefor the molecular recognition of various anions(oxo-anions, halides) [26–28] and catalysis of nitrateand nitrite reduction [29]. The most importantphysical properties are those involving the redox

behavior of these metallodendrimers which can beviewed as molecular batteries given the large num-ber of identical redox centres and their stability intwo redox forms (i.e. their electron-reservoir prop-erties) [30, 31].

2. Synthetic tools and achievementof nanoscale dendritic syntheses

The CpFe+ induced perallylation and perbenzyla-tion of hexamethylbenzene has led to star-shapedcores (figure 1), which are excellent starting pointsto synthesise star-shaped nanostructures containingredox centres such as fullerenes (figure 2) [32, 33]or iron-sandwich moieties functioning as redoxcatalysts (figure 3) [29]. The advantage of the startopology is that no steric bulk interferes at theperiphery where the redox centres are located,whereas the periphery of large dendrimers ismarred with steric problems as first indicated by deGennes [34] with Tomalia’s famous PAMAM den-drimers [35, 36].

The CpM+ induced perallylation and perbenzyla-tion reactions have been extended to various poly-methylaromatics (M = Fe [37] or Ru [38]) and to thepentamethyl-cyclopentadienyl ligand (M = Co [39,40] or Rh [41]) providing chiral as well as nonchiral dendritic cores. The most used system in ourhand so far has been the CpFe+-induced nona-allylation of mesitylene in which all the nine ben-zylic hydrogens are substituted by nine allyl groupsat room temperature using KOH and allyl bromide(figures 4 and 5) [42]. This rather spectacular reac-

Figure 1. One-pot efficient and general method to synthesisehexafunctional star cores using the CpFe+ activation.

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tion, which is quantitative and can be performedon large scales, illustrates the proton-reservoirproperties of the [FeCp(arene)]+ complexes. It isdue to both the robustness of the complexes and tothe enhanced acidity of the benzylic hydrogen (by15 pKa units in DMSO) upon complexation of thearomatic by FeCp+ [43, 44]. This system is not cata-

lytic, but represents a new kind of molecular acti-vation by transition metals which is intermediatebetween stoichiometric and catalytic reactions,since nine identical deprotonation–allylationsequences (i.e. 18 reactions) are performed on thesame metal without decomplexation. Regioselectivehydroelementation (hydroboration, hydrosilylationor hydrozirconation) of the allyl groups of the corebranches proceed smoothly and provide potentialfor further development of the dendritic construc-tion. An example of such a strategy is shown infigure 4; it leads to a dendrimer with a theoreticalnumber of 243 branches (figure 5) after only threegenerations. The molecular peak in the MALDI TOFmass spectrum of the second-generation dendrimer(81 branches) is larger than those of the side prod-ucts, and the third-generation dendrimer, whichwas characterised by its 13C NMR spectrum, is pre-sumably polydispersed. Higher-generation dendrim-ers have also been synthesised in our laboratorywith such strategies and characterised by 13C NMRand transmission electron microscopy.

Figure 2. Synthesis of a star polystyrene poly-mer of low polydispersity (1.4) with C60 termini.

Figure 3. Star-shaped water-soluble organometallic redox cata-lyst for the cathodic reduction of NO2

– and NO3– to NH3.

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3. Molecular batteries

The 27-, 81- and 243-branch dendrimers abovewere hydrosilylated using ferrocenydimethylsilaneand the Karsted catalyst. These hydrosilylation reac-tions were monitored by 1H and 13C NMR andwere shown to proceed to completion giving thecorresponding ferrocenylsilyldendrimers. Similar,but smaller ferrocene dendrimers could also besynthesised using a convergent strategy involvingchromatographic purification of a nonaferrocenyl-dendron, which was attached to a benzene coreyielding a dendrimer of excellent purity (from NMRand elemental analysis) with 54 ferrocenylsilyl ter-mini. The yellow–orange polyferrocene dendrimers

could be oxidised to blue polyferricinium dendrim-ers using NO+ (note that this method is a chemicalequivalent of coulometry which can also be per-formed). The polyferricinium dendrimers, whichhave a huge molecular spin [3] are stable and couldbe characterised by Mössbauer spectroscopy (fig-ure 6) and reduced back quantitatively to the poly-ferrocene dendrimers. The overall synthetic redoxcycle proceeds without any decomposition evenwith the polydispersed 243-ferrocene dendrimers

Figure 6. Zero-field Mössbauer spectrum of the 243-ferriciniumdendrimer at 4 K showing the single line which corresponds toan almost zero quadrupole splitting. Isomer shift,IS = 0.57(1) mm·s–1 versus Fe. Γ = 100(4).

Figure 4. Synthetic strategy for the rapid construction of polyal-lyl dendron, dendritic cores and dendrimers using the CpFe+

activation.

Figure 5. Dendrimer with a theoretical number of 243 allylbranches (characterised by 1H and 13C NMR, see the 13C NMRspectrum in reference [23–25] obtained in only three generationsaccording to the scheme in figure 4.

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(figure 7). Simple cyclic voltammetry (CV) alsoallows one to estimate the number of ferrocenecentres in MeCN/DMF with 5 to 10 % errors up tothe 81-ferrocene dendrimers using the Bard–Ansonequation [45, 46]. With the 243-ferrocene den-drimer, no solvent can be found which wouldinhibit adsorption onto the platinum electrode. Theresult is that the numbers of ferrocene centres esti-mated are in excess if use of this equation isattempted. The adsorption onto the electrode is allthe more marked as the ferrocene dendrimer is

larger, which allows fabricating modified electrodesin CH2Cl2 in which adsorption is most extensive[47, 48]. Thus, very stable modified electrodes wereobtained with ferrocene dendrimers containingmore than 27 ferrocene branches. Figure 8 showsthe CV of the modified Pt electrode with the 243-ferrocene dendrimer.

4. Molecular recognition of smallinorganic anions

The area of anion recognition, pioneered byLehn [4–6, 49, 50], is of particular importance for itsbiological implications, and various types of endo-receptor sensors are known [51–54]. On the otherhand, dendrimers with redox sensors at theextremities of the branches could function as exo-receptors, especially if the surface covered withredox sensors is not too far from steric saturation.The principle is that the redox potential of theFe(II/III) redox system of the ferrocene unit is not

Figure 7. Dendritic molecular batteries using both stable redoxforms of the 243-ferrocene dendrimer.

Figure 8. Cyclic voltammogramof the 243-ferrocenyl dendrimerin CH2Cl2 solution containing0.1 M [n-Bu4N][PF6]: (a) in solu-tion (10–1 M) at a scan rate of100 mV·s–1; (b) on a Pt anodemodified with the 243-ferrocenedendrimer at various scan rates(inset: intensity as a function ofscan rate).

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the same in the presence and absence of the sub-strate whose recognition is sought. The amidofer-rocene fragment has the benefit of the acidic amidehydrogen atom and basic oxygen atom which canboth form hydrogen bonds with an oxygen atomand a hydrogen atom of oxo-anions, as knownfrom Beer’s work [51–54]. We have compared the9-Fc and 18-Fc dendrimers with mono- and tripodalamidoferrocenes of closely related structure inorder to investigate dendritic effects. Recognitionstudies have been carried out by cyclic voltammetryand 1H NMR. In each case, titration of the fer-rocene dendrimers was effected by n-Bu4N

+ salts ofH2PO4

–, HSO4–, Cl– and NO3

–. By far, the mostinformative results were obtained by cyclic voltam-metry by scanning the Fe(II/III) wave (figure 3).Before any titration, the CVs of the 9-Fc and 18-Fcdendrimers show a unique wave at 0.59 V/SCE inCH2Cl2, corresponding to the oxidation of the nineor 18 redox centres, which indicates that, asexpected, the nine or 18 redox centres are approxi-mately electrochemically equivalent, thus indepen-dent (when, for instance, two equivalent redoxcentres are not so far away from each other, twowaves are observed at two distinct potentials, evenif there is no electronic connection, because of theelectrostatic effect). In the present situation, theredox centres are far from one another, thus theelectrostatic effect is very weak and not detected.Upon addition of the anion, two situations canarise [55]. In the case of H2PO4, a new wave startsappearing at less positive potentials and correla-tively, the intensity of the initial wave startsdecreasing. When the equivalent of one anion perdendrimer branch has been added, the initial wavehas disappeared, and upon addition of the anion,the intensity of the new wave does not increaseany further. In the case of the other anions, nonew wave appears, but the initial wave is progres-sively shifted to less positive potentials upontitration, until the equivalent of one anion hasbeen added per dendrimer branch. It clearlyappears that the shift ∆E0 of potentials observedafter addition of one equivalent anion per den-drimer branch considerably increases in the series :1-Fc → 3-Fc → 9-Fc → 18-Fc, which shows a dra-matic dendritic effect, represented in figure 9 forthe titration with the HSO4

– anion. The magnitudeof interaction with the anion increases as follows :

H2 PO4− > HSO4

− > Cl− > NO3−

In the amidoferrocene dendrimers, the amide Hatom is located on the branch behind the ferroceneunit, which provides the surface bulk. Thus theanion must reach the inside of the microcavity

formed by the amido-ferrocene units at the surfaceof the dendrimer. These conditions become optimalfor redox sensing and recognition by the close fer-rocene units at the 18-Fc generation, since thechannels allowing the entry of the anions into thesurface microcavity to reach the amide H atom areas narrow as possible. Note that with other metallo-dendrimers, the recognition of chloride and bro-mide is selective [56].

5. Catalysis by metallostarsand metallodendrimers

Much work has already been carried out withdendritic catalysts [16], which represent a new gen-eration of catalysts. Clusters such as[Ru3(CO)11(phosphine)] are very active in catalysis,but their introduction onto the termini of dendriticbranches was a real challenge which could be ful-filled using the very precise electron-transfer-chaincatalysis technique with the electron-reservoir com-plex [FeICp(η6-C6Me6)] as the catalyst (figure 2).The dendrimers containing respectively 32 and 64Ru3 clusters could be readily identified and theirpurities checked from the unicity of the 31P NMRsignal [57], a useful technique for phosphorous-containing dendrimers [18].

Another example of the implication of nano-scopic catalysts is that of the electroreduction ofnitrate and nitrite to ammonia, which is of environ-mental interest. This electroreduction can be cataly-sed in water by complexes of the [FeCp(arene)]family [58]. However, it is necessary to solubilisethe redox catalysts in basic aqueous medium inorder to be able to carry out kinetic studies of thereduction of the substrates by the reduced19-electron form of the redox catalyst in aqueoussolution. Kinetic studies were carried out by cyclicvoltammetry in order to compare water-solublemononuclear redox catalysts and hexanuclear sys-tems in which the monomeric structure wasbranched to the extremities of nanoscopic hexa-arm stars. The enhancement of the reduction waveof the catalysts observed on a mercury cathodeupon addition of the substrate (NO3

– or NO2–)

leads to the measurement of the rate constant kaccording to the theory by Nicholson and Shain[59]. As indicated in the above section, suitablemolecular engineering provided a hexanuclear cata-lyst, which was also stable, water soluble in basicaqueous medium (NaOH 0.1 N) and had the sameredox potential as the monometallic compound.The comparison yielded data, which showed thatthe hexanuclear redox catalysts such as the onerepresented in figure 3 were as active as the mono-

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nuclear catalyst of analogous driving force [29]. Thisenhancement was almost completely identical forthe mono- and hexanuclear redox catalysts. Forinstance, the rate constant of reduction of nitrateby the 19-electron form of the catalyst isk = 3·103 mol–1·L·s–1, in agreement with literaturedata [58].

In conclusion, we have synthesised electron-reservoir metallostars and metallodendrimers with avariable number of equivalent redox centres whichcan serve as molecular batteries, sensors for therecognition of small inorganic anions and catalysts.In particular, we have seen here that the iron-sandwich metallodendrimers can recognise thenitrate ion and that related metallostars can be cata-

lysts for their reduction to ammonia. Althoughthese stars and dendrimers resemble one another inthat they bear iron-sandwich termini, precise defini-tion was ultimately necessary in order to achieveeach function. So far, however, none of the metal-lostar or metallodendrimer alone achieves all thethree functions. We are presently working alongthis line in order to have metallodendrimers, whichare able to achieve these functions within the samemolecule. If this goal can be reached, we will havenanomolecules at hand, which will mimic the com-bination of properties specific to enzymes. More-over, it should also be possible to use these nano-molecules for molecular-electronic devices for theachievement of unnatural functions.

Figure 9. Titration of 1-Fc (1-Fc= [FeCp(g5-C5H4CONHCH2CH2

OPh]), 3-Fc, 9-Fc and 18-Fc(10– 3 M) by [n-Bu4N][BF4] 0.1 Min CH2Cl2 using cycling voltam-metry (reference electrode: SCE;working electrode: Pt; sweeprate: 100 mV·s– 1).

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Metallodendrimers towards enzyme mimics and molecular electronics: new-generation catalysts, sensors and molecular batteries