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Metallodendrimers towards enzyme mimics and molecular electronics: new-generation catalysts, sensors and molecular batteries
MISE AU POINT / ACCOUNT
C. R. Acad. Sci. Paris, Chimie / Chemistry 4 (2001) 173–180 © 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1387160900012251/REV
Metallodendrimers towards enzyme mimics and molecular electronics: new-generation catalysts, sensors and molecular batteries Beatriz 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 la Libération, 33405 Talence cedex, France b Laboratoire de chimie structurale organique et biologique, EP CNRS n° 103, université Paris-6, 4, place Jussieu, 75252 Paris cedex 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 who rationalised the approach to molecular magnetism.
Abstract – Large supramolecular metallodendrimers can now be synthesised rapidly, reaching a high number of branches in only a few generations, and approaching the de Gennes steric limit. They are characterised by their MALDI TOF mass spectra, in particular by the molecular peak, and by 1H, 13C and 31P NMR. Following rational molecular engineering, they can be designed to achieve essential functions such as molecular batteries, catalysts and sensors for the recognition of various 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 moléculaire : batteries moléculaires, nouveaux catalyseurs et capteurs. La synthèse rapide de grands dendrimères a été réalisée grâce à l’activation du mésitylène par le greffon cationique à 12 électrons CpFe+, permettant de remplacer les neuf H benzyliques par neuf branches allyles, ce qui produit un cœur de dendrimère. Avec le p-éthoxytoluène, cette même activation conduit directement au dendron phénol trialkylé, greffable sur le cœur, c’est-à-dire à la construction divergente du 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 à température ambiante conduit aux dendrimères polyferrocéniques, ce qui permet une fabrication facile d’électrodes modifiées d’autant plus stables que le nombre de groupements ferrocényles dans le dendrimère est plus élevé (électrode de platine modifiée avec ce même métallodendrimère, figure 8). Les groupements polyferrocéniques constituent des batteries moléculaires, dans lesquelles tous les centres redox sont oxydés (à peu près) au même potentiel (voir le spectre Mössbauer du dendrimère bleu, comportant un nombre théorique de 243 ferriciniums, figure 6). Les métallodendrimères peuvent être habillés de clusters Ru3(CO)11 par réaction d’échange de ligands entre un carbonyle du cluster [Ru3(CO)12] et une terminaison phosphine de dendrimère phosphoré à 32 ou 64 branches. Cette réaction est catalysée par le complexe réservoir d’électron [FeICp(η6-C6Me6)] et procède suivant le mécanisme de transfert d’électron en chaîne, ce qui permet d’introduire 92 ou 184 Ru à la périphérie du dendrimère. Ces dendrimères-clusters devraient s’avérer d’excellents catalyseurs, 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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B. Alonso et al. / C. R. Acad. Sci. Paris, Chimie / Chemistry 4 (2001) 173–180 topologie évite les contraintes stériques périphériques auxquelles cette catalyse est sensible. Les constantes de vitesse obtenues 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’autant meilleure que la génération dendritique est plus élevée (3 < 9 < 18 branches amidoferrocènes). Les conditions sont à l’étude pour obtenir à la fois les fonctions de reconnaissance d’un oxoanion tel que le nitrate et de catalyse de sa réduction en ammoniac. L’addition de ces deux propriétés étant le propre des enzymes, il serait alors possible de mimer une 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 supramolecular chemistry to fill the gap between molecular ferro/ferrimagnetism [1, 2] and molecular-based ferromagnetic materials [3]. Indeed, supramolecular chemistry, initiated by Jean-Marie Lehn [4], is the science of weak bonding energies, which are involved in a large variety of systems, from simple ones such as the hydrogen bonding in water to sophisticated photochemical machineries [5] and the complicated biological processes in which these forces play a key role in recognition and catalysis [4–6]. Typically, these properties are also met in enzymes. Thus, enzyme models which can mimic these functions have been popular and could be useful as many biomimetic processes in bio-organic and bio-inorganic chemistry [7]. Dendrimers [8–19], the first precise synthetic macromolecules (theoretical polydispersity = 1.0) belong to the world of supramolecular chemistry. Moreover, their size matches that of biocomponents and the fractality of their surfaces resembles that of viruses, enzymes and proteins [20–22]. In the present mini-review article, we would like to underline their potential as a reaction medium, i.e. a molecular medium able to achieve several functions such as molecular batteries (electron-reservoirs), molecular recognition and catalysis in a single dendritic molecule with specified topological, chemical and physicochemical properties. Altogether, these properties would be such that the dendrimer could mimic an enzyme and be useful for the design of electronic and bio-electronic devices. Although these goals are still in front of us, we would like to show that this research line is realistic and we will indicate here our efforts in this direction. These include the synthetic aspects [23–25], investigation of the redox chemistry of metallodendrimers, studies of their use for the molecular recognition of various anions (oxo-anions, halides) [26–28] and catalysis of nitrate and nitrite reduction [29]. The most important physical properties are those involving the redox
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behavior of these metallodendrimers which can be viewed as molecular batteries given the large number of identical redox centres and their stability in two redox forms (i.e. their electron-reservoir properties) [30, 31].
2. Synthetic tools and achievement of nanoscale dendritic syntheses The CpFe+ induced perallylation and perbenzylation of hexamethylbenzene has led to star-shaped cores (figure 1), which are excellent starting points to synthesise star-shaped nanostructures containing redox centres such as fullerenes (figure 2) [32, 33] or iron-sandwich moieties functioning as redox catalysts (figure 3) [29]. The advantage of the star topology is that no steric bulk interferes at the periphery where the redox centres are located, whereas the periphery of large dendrimers is marred with steric problems as first indicated by de Gennes [34] with Tomalia’s famous PAMAM dendrimers [35, 36]. The CpM+ induced perallylation and perbenzylation reactions have been extended to various polymethylaromatics (M = Fe [37] or Ru [38]) and to the pentamethyl-cyclopentadienyl ligand (M = Co [39, 40] or Rh [41]) providing chiral as well as non chiral dendritic cores. The most used system in our hand so far has been the CpFe+-induced nonaallylation of mesitylene in which all the nine benzylic hydrogens are substituted by nine allyl groups at 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 synthesise hexafunctional star cores using the CpFe+ activation.
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Figure 2. Synthesis of a star polystyrene polymer of low polydispersity (1.4) with C60 termini.
Figure 3. Star-shaped water-soluble organometallic redox catalyst for the cathodic reduction of NO2– and NO3– to NH3.
tion, which is quantitative and can be performed on large scales, illustrates the proton-reservoir properties of the [FeCp(arene)]+ complexes. It is due to both the robustness of the complexes and to the enhanced acidity of the benzylic hydrogen (by 15 pKa units in DMSO) upon complexation of the aromatic by FeCp+ [43, 44]. This system is not cata-
lytic, but represents a new kind of molecular activation by transition metals which is intermediate between stoichiometric and catalytic reactions, since nine identical deprotonation–allylation sequences (i.e. 18 reactions) are performed on the same metal without decomplexation. Regioselective hydroelementation (hydroboration, hydrosilylation or hydrozirconation) of the allyl groups of the core branches proceed smoothly and provide potential for further development of the dendritic construction. An example of such a strategy is shown in figure 4; it leads to a dendrimer with a theoretical number of 243 branches (figure 5) after only three generations. The molecular peak in the MALDI TOF mass spectrum of the second-generation dendrimer (81 branches) is larger than those of the side products, and the third-generation dendrimer, which was characterised by its 13C NMR spectrum, is presumably polydispersed. Higher-generation dendrimers have also been synthesised in our laboratory with such strategies and characterised by 13C NMR and transmission electron microscopy.
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Figure 5. Dendrimer with a theoretical number of 243 allyl branches (characterised by 1H and 13C NMR, see the 13C NMR spectrum in reference [23–25] obtained in only three generations according to the scheme in figure 4.
could be oxidised to blue polyferricinium dendrimers using NO+ (note that this method is a chemical equivalent of coulometry which can also be performed). The polyferricinium dendrimers, which have a huge molecular spin [3] are stable and could be characterised by Mössbauer spectroscopy (figure 6) and reduced back quantitatively to the polyferrocene dendrimers. The overall synthetic redox cycle proceeds without any decomposition even with the polydispersed 243-ferrocene dendrimers Figure 4. Synthetic strategy for the rapid construction of polyallyl dendron, dendritic cores and dendrimers using the CpFe+ activation.
3. Molecular batteries The 27-, 81- and 243-branch dendrimers above were hydrosilylated using ferrocenydimethylsilane and the Karsted catalyst. These hydrosilylation reactions were monitored by 1H and 13C NMR and were shown to proceed to completion giving the corresponding ferrocenylsilyldendrimers. Similar, but smaller ferrocene dendrimers could also be synthesised using a convergent strategy involving chromatographic purification of a nonaferrocenyldendron, which was attached to a benzene core yielding a dendrimer of excellent purity (from NMR and elemental analysis) with 54 ferrocenylsilyl termini. The yellow–orange polyferrocene dendrimers
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Figure 6. Zero-field Mössbauer spectrum of the 243-ferricinium dendrimer at 4 K showing the single line which corresponds to an almost zero quadrupole splitting. Isomer shift, IS = 0.57(1) mm·s–1 versus Fe. Γ = 100(4).
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larger, which allows fabricating modified electrodes in CH2Cl2 in which adsorption is most extensive [47, 48]. Thus, very stable modified electrodes were obtained with ferrocene dendrimers containing more than 27 ferrocene branches. Figure 8 shows the CV of the modified Pt electrode with the 243ferrocene dendrimer. Figure 7. Dendritic molecular batteries using both stable redox forms of the 243-ferrocene dendrimer.
(figure 7). Simple cyclic voltammetry (CV) also allows one to estimate the number of ferrocene centres in MeCN/DMF with 5 to 10 % errors up to the 81-ferrocene dendrimers using the Bard–Anson equation [45, 46]. With the 243-ferrocene dendrimer, no solvent can be found which would inhibit adsorption onto the platinum electrode. The result is that the numbers of ferrocene centres estimated are in excess if use of this equation is attempted. The adsorption onto the electrode is all the more marked as the ferrocene dendrimer is
4. Molecular recognition of small inorganic anions The area of anion recognition, pioneered by Lehn [4–6, 49, 50], is of particular importance for its biological implications, and various types of endoreceptor sensors are known [51–54]. On the other hand, dendrimers with redox sensors at the extremities of the branches could function as exoreceptors, especially if the surface covered with redox sensors is not too far from steric saturation. The principle is that the redox potential of the Fe(II/III) redox system of the ferrocene unit is not
Figure 8. Cyclic voltammogram of the 243-ferrocenyl dendrimer in CH2Cl2 solution containing 0.1 M [n-Bu4N][PF6]: (a) in solution (10–1 M) at a scan rate of 100 mV·s–1; (b) on a Pt anode modified with the 243-ferrocene dendrimer at various scan rates (inset: intensity as a function of scan rate).
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the same in the presence and absence of the substrate whose recognition is sought. The amidoferrocene fragment has the benefit of the acidic amide hydrogen atom and basic oxygen atom which can both form hydrogen bonds with an oxygen atom and a hydrogen atom of oxo-anions, as known from Beer’s work [51–54]. We have compared the 9-Fc and 18-Fc dendrimers with mono- and tripodal amidoferrocenes of closely related structure in order to investigate dendritic effects. Recognition studies have been carried out by cyclic voltammetry and 1H NMR. In each case, titration of the ferrocene dendrimers was effected by n-Bu4N+ salts of H2PO4–, HSO4–, Cl– and NO3–. By far, the most informative results were obtained by cyclic voltammetry by scanning the Fe(II/III) wave (figure 3). Before any titration, the CVs of the 9-Fc and 18-Fc dendrimers show a unique wave at 0.59 V/SCE in CH2Cl2, corresponding to the oxidation of the nine or 18 redox centres, which indicates that, as expected, the nine or 18 redox centres are approximately electrochemically equivalent, thus independent (when, for instance, two equivalent redox centres are not so far away from each other, two waves are observed at two distinct potentials, even if there is no electronic connection, because of the electrostatic effect). In the present situation, the redox centres are far from one another, thus the electrostatic effect is very weak and not detected. Upon addition of the anion, two situations can arise [55]. In the case of H2PO4, a new wave starts appearing at less positive potentials and correlatively, the intensity of the initial wave starts decreasing. When the equivalent of one anion per dendrimer branch has been added, the initial wave has disappeared, and upon addition of the anion, the intensity of the new wave does not increase any further. In the case of the other anions, no new wave appears, but the initial wave is progressively shifted to less positive potentials upon titration, until the equivalent of one anion has been added per dendrimer branch. It clearly appears that the shift ∆E0 of potentials observed after addition of one equivalent anion per dendrimer branch considerably increases in the series : 1-Fc → 3-Fc → 9-Fc → 18-Fc, which shows a dramatic dendritic effect, represented in figure 9 for the titration with the HSO4– anion. The magnitude of interaction with the anion increases as follows : −
−
−
H2 PO4 > HSO4 > Cl > NO3
−
In the amidoferrocene dendrimers, the amide H atom is located on the branch behind the ferrocene unit, which provides the surface bulk. Thus the anion must reach the inside of the microcavity
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formed by the amido-ferrocene units at the surface of the dendrimer. These conditions become optimal for redox sensing and recognition by the close ferrocene units at the 18-Fc generation, since the channels allowing the entry of the anions into the surface microcavity to reach the amide H atom are as narrow as possible. Note that with other metallodendrimers, the recognition of chloride and bromide is selective [56].
5. Catalysis by metallostars and metallodendrimers Much work has already been carried out with dendritic catalysts [16], which represent a new generation of catalysts. Clusters such as [Ru3(CO)11(phosphine)] are very active in catalysis, but their introduction onto the termini of dendritic branches was a real challenge which could be fulfilled using the very precise electron-transfer-chain catalysis technique with the electron-reservoir complex [FeICp(η6-C6Me6)] as the catalyst (figure 2). The dendrimers containing respectively 32 and 64 Ru3 clusters could be readily identified and their purities checked from the unicity of the 31P NMR signal [57], a useful technique for phosphorouscontaining dendrimers [18]. Another example of the implication of nanoscopic catalysts is that of the electroreduction of nitrate and nitrite to ammonia, which is of environmental interest. This electroreduction can be catalysed in water by complexes of the [FeCp(arene)] family [58]. However, it is necessary to solubilise the redox catalysts in basic aqueous medium in order to be able to carry out kinetic studies of the reduction of the substrates by the reduced 19-electron form of the redox catalyst in aqueous solution. Kinetic studies were carried out by cyclic voltammetry in order to compare water-soluble mononuclear redox catalysts and hexanuclear systems in which the monomeric structure was branched to the extremities of nanoscopic hexaarm stars. The enhancement of the reduction wave of the catalysts observed on a mercury cathode upon addition of the substrate (NO3– or NO2–) leads to the measurement of the rate constant k according to the theory by Nicholson and Shain [59]. As indicated in the above section, suitable molecular engineering provided a hexanuclear catalyst, which was also stable, water soluble in basic aqueous medium (NaOH 0.1 N) and had the same redox potential as the monometallic compound. The comparison yielded data, which showed that the hexanuclear redox catalysts such as the one represented in figure 3 were as active as the mono-
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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 M in CH2Cl2 using cycling voltammetry (reference electrode: SCE; working electrode: Pt; sweep rate: 100 mV·s– 1).
nuclear catalyst of analogous driving force [29]. This enhancement was almost completely identical for the mono- and hexanuclear redox catalysts. For instance, the rate constant of reduction of nitrate by the 19-electron form of the catalyst is k = 3·103 mol–1·L·s–1, in agreement with literature data [58]. In conclusion, we have synthesised electronreservoir metallostars and metallodendrimers with a variable number of equivalent redox centres which can serve as molecular batteries, sensors for the recognition of small inorganic anions and catalysts. In particular, we have seen here that the ironsandwich metallodendrimers can recognise the nitrate ion and that related metallostars can be cata-
lysts for their reduction to ammonia. Although these stars and dendrimers resemble one another in that they bear iron-sandwich termini, precise definition was ultimately necessary in order to achieve each function. So far, however, none of the metallostar or metallodendrimer alone achieves all the three functions. We are presently working along this line in order to have metallodendrimers, which are able to achieve these functions within the same molecule. If this goal can be reached, we will have nanomolecules at hand, which will mimic the combination of properties specific to enzymes. Moreover, it should also be possible to use these nanomolecules for molecular-electronic devices for the achievement of unnatural functions.
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C. R. Acad. Sci. Paris, Chimie / Chemistry 4 (2001) 173–180 © 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservés S1387160900012251/REV
Metallodendrimers towards enzyme mimics and molecular electronics: new-generation catalysts, sensors and molecular batteries Beatriz 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 la Libération, 33405 Talence cedex, France b Laboratoire de chimie structurale organique et biologique, EP CNRS n° 103, université Paris-6, 4, place Jussieu, 75252 Paris cedex 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 who rationalised the approach to molecular magnetism.
Abstract – Large supramolecular metallodendrimers can now be synthesised rapidly, reaching a high number of branches in only a few generations, and approaching the de Gennes steric limit. They are characterised by their MALDI TOF mass spectra, in particular by the molecular peak, and by 1H, 13C and 31P NMR. Following rational molecular engineering, they can be designed to achieve essential functions such as molecular batteries, catalysts and sensors for the recognition of various 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 moléculaire : batteries moléculaires, nouveaux catalyseurs et capteurs. La synthèse rapide de grands dendrimères a été réalisée grâce à l’activation du mésitylène par le greffon cationique à 12 électrons CpFe+, permettant de remplacer les neuf H benzyliques par neuf branches allyles, ce qui produit un cœur de dendrimère. Avec le p-éthoxytoluène, cette même activation conduit directement au dendron phénol trialkylé, greffable sur le cœur, c’est-à-dire à la construction divergente du 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 à température ambiante conduit aux dendrimères polyferrocéniques, ce qui permet une fabrication facile d’électrodes modifiées d’autant plus stables que le nombre de groupements ferrocényles dans le dendrimère est plus élevé (électrode de platine modifiée avec ce même métallodendrimère, figure 8). Les groupements polyferrocéniques constituent des batteries moléculaires, dans lesquelles tous les centres redox sont oxydés (à peu près) au même potentiel (voir le spectre Mössbauer du dendrimère bleu, comportant un nombre théorique de 243 ferriciniums, figure 6). Les métallodendrimères peuvent être habillés de clusters Ru3(CO)11 par réaction d’échange de ligands entre un carbonyle du cluster [Ru3(CO)12] et une terminaison phosphine de dendrimère phosphoré à 32 ou 64 branches. Cette réaction est catalysée par le complexe réservoir d’électron [FeICp(η6-C6Me6)] et procède suivant le mécanisme de transfert d’électron en chaîne, ce qui permet d’introduire 92 ou 184 Ru à la périphérie du dendrimère. Ces dendrimères-clusters devraient s’avérer d’excellents catalyseurs, 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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B. Alonso et al. / C. R. Acad. Sci. Paris, Chimie / Chemistry 4 (2001) 173–180 topologie évite les contraintes stériques périphériques auxquelles cette catalyse est sensible. Les constantes de vitesse obtenues 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’autant meilleure que la génération dendritique est plus élevée (3 < 9 < 18 branches amidoferrocènes). Les conditions sont à l’étude pour obtenir à la fois les fonctions de reconnaissance d’un oxoanion tel que le nitrate et de catalyse de sa réduction en ammoniac. L’addition de ces deux propriétés étant le propre des enzymes, il serait alors possible de mimer une 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 supramolecular chemistry to fill the gap between molecular ferro/ferrimagnetism [1, 2] and molecular-based ferromagnetic materials [3]. Indeed, supramolecular chemistry, initiated by Jean-Marie Lehn [4], is the science of weak bonding energies, which are involved in a large variety of systems, from simple ones such as the hydrogen bonding in water to sophisticated photochemical machineries [5] and the complicated biological processes in which these forces play a key role in recognition and catalysis [4–6]. Typically, these properties are also met in enzymes. Thus, enzyme models which can mimic these functions have been popular and could be useful as many biomimetic processes in bio-organic and bio-inorganic chemistry [7]. Dendrimers [8–19], the first precise synthetic macromolecules (theoretical polydispersity = 1.0) belong to the world of supramolecular chemistry. Moreover, their size matches that of biocomponents and the fractality of their surfaces resembles that of viruses, enzymes and proteins [20–22]. In the present mini-review article, we would like to underline their potential as a reaction medium, i.e. a molecular medium able to achieve several functions such as molecular batteries (electron-reservoirs), molecular recognition and catalysis in a single dendritic molecule with specified topological, chemical and physicochemical properties. Altogether, these properties would be such that the dendrimer could mimic an enzyme and be useful for the design of electronic and bio-electronic devices. Although these goals are still in front of us, we would like to show that this research line is realistic and we will indicate here our efforts in this direction. These include the synthetic aspects [23–25], investigation of the redox chemistry of metallodendrimers, studies of their use for the molecular recognition of various anions (oxo-anions, halides) [26–28] and catalysis of nitrate and nitrite reduction [29]. The most important physical properties are those involving the redox
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behavior of these metallodendrimers which can be viewed as molecular batteries given the large number of identical redox centres and their stability in two redox forms (i.e. their electron-reservoir properties) [30, 31].
2. Synthetic tools and achievement of nanoscale dendritic syntheses The CpFe+ induced perallylation and perbenzylation of hexamethylbenzene has led to star-shaped cores (figure 1), which are excellent starting points to synthesise star-shaped nanostructures containing redox centres such as fullerenes (figure 2) [32, 33] or iron-sandwich moieties functioning as redox catalysts (figure 3) [29]. The advantage of the star topology is that no steric bulk interferes at the periphery where the redox centres are located, whereas the periphery of large dendrimers is marred with steric problems as first indicated by de Gennes [34] with Tomalia’s famous PAMAM dendrimers [35, 36]. The CpM+ induced perallylation and perbenzylation reactions have been extended to various polymethylaromatics (M = Fe [37] or Ru [38]) and to the pentamethyl-cyclopentadienyl ligand (M = Co [39, 40] or Rh [41]) providing chiral as well as non chiral dendritic cores. The most used system in our hand so far has been the CpFe+-induced nonaallylation of mesitylene in which all the nine benzylic hydrogens are substituted by nine allyl groups at 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 synthesise hexafunctional star cores using the CpFe+ activation.
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Figure 2. Synthesis of a star polystyrene polymer of low polydispersity (1.4) with C60 termini.
Figure 3. Star-shaped water-soluble organometallic redox catalyst for the cathodic reduction of NO2– and NO3– to NH3.
tion, which is quantitative and can be performed on large scales, illustrates the proton-reservoir properties of the [FeCp(arene)]+ complexes. It is due to both the robustness of the complexes and to the enhanced acidity of the benzylic hydrogen (by 15 pKa units in DMSO) upon complexation of the aromatic by FeCp+ [43, 44]. This system is not cata-
lytic, but represents a new kind of molecular activation by transition metals which is intermediate between stoichiometric and catalytic reactions, since nine identical deprotonation–allylation sequences (i.e. 18 reactions) are performed on the same metal without decomplexation. Regioselective hydroelementation (hydroboration, hydrosilylation or hydrozirconation) of the allyl groups of the core branches proceed smoothly and provide potential for further development of the dendritic construction. An example of such a strategy is shown in figure 4; it leads to a dendrimer with a theoretical number of 243 branches (figure 5) after only three generations. The molecular peak in the MALDI TOF mass spectrum of the second-generation dendrimer (81 branches) is larger than those of the side products, and the third-generation dendrimer, which was characterised by its 13C NMR spectrum, is presumably polydispersed. Higher-generation dendrimers have also been synthesised in our laboratory with such strategies and characterised by 13C NMR and transmission electron microscopy.
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Figure 5. Dendrimer with a theoretical number of 243 allyl branches (characterised by 1H and 13C NMR, see the 13C NMR spectrum in reference [23–25] obtained in only three generations according to the scheme in figure 4.
could be oxidised to blue polyferricinium dendrimers using NO+ (note that this method is a chemical equivalent of coulometry which can also be performed). The polyferricinium dendrimers, which have a huge molecular spin [3] are stable and could be characterised by Mössbauer spectroscopy (figure 6) and reduced back quantitatively to the polyferrocene dendrimers. The overall synthetic redox cycle proceeds without any decomposition even with the polydispersed 243-ferrocene dendrimers Figure 4. Synthetic strategy for the rapid construction of polyallyl dendron, dendritic cores and dendrimers using the CpFe+ activation.
3. Molecular batteries The 27-, 81- and 243-branch dendrimers above were hydrosilylated using ferrocenydimethylsilane and the Karsted catalyst. These hydrosilylation reactions were monitored by 1H and 13C NMR and were shown to proceed to completion giving the corresponding ferrocenylsilyldendrimers. Similar, but smaller ferrocene dendrimers could also be synthesised using a convergent strategy involving chromatographic purification of a nonaferrocenyldendron, which was attached to a benzene core yielding a dendrimer of excellent purity (from NMR and elemental analysis) with 54 ferrocenylsilyl termini. The yellow–orange polyferrocene dendrimers
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Figure 6. Zero-field Mössbauer spectrum of the 243-ferricinium dendrimer at 4 K showing the single line which corresponds to an almost zero quadrupole splitting. Isomer shift, IS = 0.57(1) mm·s–1 versus Fe. Γ = 100(4).
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larger, which allows fabricating modified electrodes in CH2Cl2 in which adsorption is most extensive [47, 48]. Thus, very stable modified electrodes were obtained with ferrocene dendrimers containing more than 27 ferrocene branches. Figure 8 shows the CV of the modified Pt electrode with the 243ferrocene dendrimer. Figure 7. Dendritic molecular batteries using both stable redox forms of the 243-ferrocene dendrimer.
(figure 7). Simple cyclic voltammetry (CV) also allows one to estimate the number of ferrocene centres in MeCN/DMF with 5 to 10 % errors up to the 81-ferrocene dendrimers using the Bard–Anson equation [45, 46]. With the 243-ferrocene dendrimer, no solvent can be found which would inhibit adsorption onto the platinum electrode. The result is that the numbers of ferrocene centres estimated are in excess if use of this equation is attempted. The adsorption onto the electrode is all the more marked as the ferrocene dendrimer is
4. Molecular recognition of small inorganic anions The area of anion recognition, pioneered by Lehn [4–6, 49, 50], is of particular importance for its biological implications, and various types of endoreceptor sensors are known [51–54]. On the other hand, dendrimers with redox sensors at the extremities of the branches could function as exoreceptors, especially if the surface covered with redox sensors is not too far from steric saturation. The principle is that the redox potential of the Fe(II/III) redox system of the ferrocene unit is not
Figure 8. Cyclic voltammogram of the 243-ferrocenyl dendrimer in CH2Cl2 solution containing 0.1 M [n-Bu4N][PF6]: (a) in solution (10–1 M) at a scan rate of 100 mV·s–1; (b) on a Pt anode modified with the 243-ferrocene dendrimer at various scan rates (inset: intensity as a function of scan rate).
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the same in the presence and absence of the substrate whose recognition is sought. The amidoferrocene fragment has the benefit of the acidic amide hydrogen atom and basic oxygen atom which can both form hydrogen bonds with an oxygen atom and a hydrogen atom of oxo-anions, as known from Beer’s work [51–54]. We have compared the 9-Fc and 18-Fc dendrimers with mono- and tripodal amidoferrocenes of closely related structure in order to investigate dendritic effects. Recognition studies have been carried out by cyclic voltammetry and 1H NMR. In each case, titration of the ferrocene dendrimers was effected by n-Bu4N+ salts of H2PO4–, HSO4–, Cl– and NO3–. By far, the most informative results were obtained by cyclic voltammetry by scanning the Fe(II/III) wave (figure 3). Before any titration, the CVs of the 9-Fc and 18-Fc dendrimers show a unique wave at 0.59 V/SCE in CH2Cl2, corresponding to the oxidation of the nine or 18 redox centres, which indicates that, as expected, the nine or 18 redox centres are approximately electrochemically equivalent, thus independent (when, for instance, two equivalent redox centres are not so far away from each other, two waves are observed at two distinct potentials, even if there is no electronic connection, because of the electrostatic effect). In the present situation, the redox centres are far from one another, thus the electrostatic effect is very weak and not detected. Upon addition of the anion, two situations can arise [55]. In the case of H2PO4, a new wave starts appearing at less positive potentials and correlatively, the intensity of the initial wave starts decreasing. When the equivalent of one anion per dendrimer branch has been added, the initial wave has disappeared, and upon addition of the anion, the intensity of the new wave does not increase any further. In the case of the other anions, no new wave appears, but the initial wave is progressively shifted to less positive potentials upon titration, until the equivalent of one anion has been added per dendrimer branch. It clearly appears that the shift ∆E0 of potentials observed after addition of one equivalent anion per dendrimer branch considerably increases in the series : 1-Fc → 3-Fc → 9-Fc → 18-Fc, which shows a dramatic dendritic effect, represented in figure 9 for the titration with the HSO4– anion. The magnitude of interaction with the anion increases as follows : −
−
−
H2 PO4 > HSO4 > Cl > NO3
−
In the amidoferrocene dendrimers, the amide H atom is located on the branch behind the ferrocene unit, which provides the surface bulk. Thus the anion must reach the inside of the microcavity
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formed by the amido-ferrocene units at the surface of the dendrimer. These conditions become optimal for redox sensing and recognition by the close ferrocene units at the 18-Fc generation, since the channels allowing the entry of the anions into the surface microcavity to reach the amide H atom are as narrow as possible. Note that with other metallodendrimers, the recognition of chloride and bromide is selective [56].
5. Catalysis by metallostars and metallodendrimers Much work has already been carried out with dendritic catalysts [16], which represent a new generation of catalysts. Clusters such as [Ru3(CO)11(phosphine)] are very active in catalysis, but their introduction onto the termini of dendritic branches was a real challenge which could be fulfilled using the very precise electron-transfer-chain catalysis technique with the electron-reservoir complex [FeICp(η6-C6Me6)] as the catalyst (figure 2). The dendrimers containing respectively 32 and 64 Ru3 clusters could be readily identified and their purities checked from the unicity of the 31P NMR signal [57], a useful technique for phosphorouscontaining dendrimers [18]. Another example of the implication of nanoscopic catalysts is that of the electroreduction of nitrate and nitrite to ammonia, which is of environmental interest. This electroreduction can be catalysed in water by complexes of the [FeCp(arene)] family [58]. However, it is necessary to solubilise the redox catalysts in basic aqueous medium in order to be able to carry out kinetic studies of the reduction of the substrates by the reduced 19-electron form of the redox catalyst in aqueous solution. Kinetic studies were carried out by cyclic voltammetry in order to compare water-soluble mononuclear redox catalysts and hexanuclear systems in which the monomeric structure was branched to the extremities of nanoscopic hexaarm stars. The enhancement of the reduction wave of the catalysts observed on a mercury cathode upon addition of the substrate (NO3– or NO2–) leads to the measurement of the rate constant k according to the theory by Nicholson and Shain [59]. As indicated in the above section, suitable molecular engineering provided a hexanuclear catalyst, which was also stable, water soluble in basic aqueous medium (NaOH 0.1 N) and had the same redox potential as the monometallic compound. The comparison yielded data, which showed that the hexanuclear redox catalysts such as the one represented in figure 3 were as active as the mono-
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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 M in CH2Cl2 using cycling voltammetry (reference electrode: SCE; working electrode: Pt; sweep rate: 100 mV·s– 1).
nuclear catalyst of analogous driving force [29]. This enhancement was almost completely identical for the mono- and hexanuclear redox catalysts. For instance, the rate constant of reduction of nitrate by the 19-electron form of the catalyst is k = 3·103 mol–1·L·s–1, in agreement with literature data [58]. In conclusion, we have synthesised electronreservoir metallostars and metallodendrimers with a variable number of equivalent redox centres which can serve as molecular batteries, sensors for the recognition of small inorganic anions and catalysts. In particular, we have seen here that the ironsandwich metallodendrimers can recognise the nitrate ion and that related metallostars can be cata-
lysts for their reduction to ammonia. Although these stars and dendrimers resemble one another in that they bear iron-sandwich termini, precise definition was ultimately necessary in order to achieve each function. So far, however, none of the metallostar or metallodendrimer alone achieves all the three functions. We are presently working along this line in order to have metallodendrimers, which are able to achieve these functions within the same molecule. If this goal can be reached, we will have nanomolecules at hand, which will mimic the combination of properties specific to enzymes. Moreover, it should also be possible to use these nanomolecules for molecular-electronic devices for the achievement of unnatural functions.
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