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8.1 Introduction

Dendrimers have attracted considerable attention since their discovery three decades ago [1–43]. Potential applications involve supramolecular properties [11] in the fields of nanomedicine [29], materials science [4–13] and catalysis [16, 30, 38–43]. Since the late 1980s, we have focused our attention on metallodendrimers [14, 44] with the aim to develop knowledge concerning redox properties that are useful for redox sensing and catalysis as well as for the design of molecular batteries. In this microreview article, we will illustrate the design of our recent series of metallodendrimers and some of their properties and applications.

8.2 The Complexes [FeCp(η6-arene)][PF6] as a Source of Dendritic Core, Dendrons and Dendrimers

In the complexes, [FeCp(η6-arene)][PF6], the arene ligand undergoes reactions resulting from “Umpolung” of the arene reactivity [45], i.e. the benzylic groups are easily deprotonated [46], the chloride arene substituent is easily substituted by nucleophiles such as alkoxyde [45] and the aryl ethers are heterolytically cleaved by t-BuOK in THF below room temperature in the presence of an inorganic salt such as KBr [47]. Moreover, these organometallic cations can be reduced to 19-electron FeI complexes that have a specific radical- and electron-transfer reactivity [48]. The removal of the arene ligand from the complex can easily proceed either via the 19-electron complexes or using visible photolysis of the 18-electron cations [49].

Using these properties, dendritic cores are quantitatively synthesized under ambient conditions from the mesitylene complex, whereas a simple tripodal dendron is prepared in a one-pot eight-step synthesis from the p-ethoxytoluene complex (Scheme 8.1). With a nona-allyl arene core and the “phenoltriallyl” brick, dendrimers containing 3n+2 terminal allyl tethers (n = generation number) could be constructed [15] using the 1 →3 C connectivity pioneered by Newkome with arborols [50] by a series of hydrosilylation-Williamson reactions. The hydrosilylation was carried out using chloromethyldimethylsilane [51] and Karsted catalyst at 40°C whereas the Williamson step was performed between the chloromethyl-terminated dendrimers and phenoltriallyl using a catalytic amount of NaI and K2CO3 in DMF at 80°C. Each step was checked by 1H, 13C and 29Si NMR and gave virtually pure dendrimers at the accuracy of NMR. MALDI TOF mass spectra show, however, that if the molecular peak largely predominates for the second generation 81-allyl dendrimer, the defects predominate in the spectrum of the 3rd generation 243-allyl dendrimer and the molecular peak is not even seen in that of the 4th generation 729-allyl dendrimer which shows massifs near the molecular peak (Scheme 8.2).

Scheme 8.1
scheme 1scheme 1

Mechanism of the one-pot eight-step synthesis of the phenoltriallyl dendron

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Scheme 8.2
scheme 2scheme 2

Construction of giant dendrimers starting from ferrocene with 3n+2 terminal tethers (n = generation number) until G9 (theoretical number of 311 terminal tethers). Each dendrimer along the construction was characterized by 1H, 13C and 29Si NMR (till G9), MALDI TOF mass spectrometry (till G4), SEC (PI = 1.00 to 1.02 till G5), TEM and AFM (till G9) showing the steady size increase

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The dendrimers were characterized by size exclusion chromatography until generation five showing a polydispersity between 1.00 and 1.02, atomic force microscopy showing the progression of the height of the monolayer from the first to the 9th generation and transmission electron microscopy of the polyiodo derivative of the last generation. Although the number of defects becomes larger and larger as the generation number increases, it may be estimated that the last generation reaches a number of terminal tethers of the order of 105. Beyond generation 5 (theoretical number: 37 = 2187 terminal tethers), it is compulsory that further dendritic construction reactions occur inside the dendrimers interior because the small termini must back fold toward the center in order to avoid the bulk at the periphery and fill the interior cavities. Thus the dendrimer construction becomes limited by the volume rather than by the surface. The reactions become slower and slower and the yields are lower as the generation number increases beyond generation 5.

A challenge is the one-pot synthesis of dendrimers using such a strategy [52]. This was shown to be possible if chlorodimethylsilane [53] is used instead of the chloromethyldimethylsilane in the construction scheme. Indeed, the terminal Si-Cl bonds formed at the periphery of the dendrimer subsequent to hydrosilylation are much more reactive in the Williamson reaction with phenolates than the chloromethylsilyl termini, which permits the one-pot synthesis of up to the 243-allyl G3 dendrimer. The Si-phenolate link is less robust than the Si-CH2-phenolate link, but stable enough for extensive characterization. Such fragile dendrimers might be useful for applications requiring the decomposition of the dendrimer interior after using it as a template, for instance in materials chemistry (Scheme 8.3) [52].

Scheme 8.3
scheme 3scheme 3

One-pot synthesis of polyolefin dendrimers till G3 (35 = 243 terminal allyl groups) using the silane HSiMe2Cl

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8.3 Ferrocenyl Dendrimers

The first ferrocenyl dendrimers designed for function were synthesized by reaction of amine-terminated dendrimers with ferrocenoyl chloride, which yielded amidoferrocenyl dendrimers that were redox exo-receptors of oxo-anions [54]. It was subsequently found that silylferrocenylation of polyolefin dendrimers yielded polysilylferrocenyl dendrimers (Scheme 8.4).

Scheme 8.4
scheme 4scheme 4

Ferrocenyl-silylation of the “phenoltriallyl” dendron for the construction of ferrocenyl-terminated dendrimers

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Likewise, the silylferrocenylation of the “phenoltriallyl” brick yielded triferrocenyl dendrons that could be condensed onto a polyhalogeno core to form polyferrocenyl dendrimers (Scheme 8.5) [55].

Scheme 8.5
scheme 5scheme 5

Ferrocenylsilylation of the 81-allyl G2 dendrimer for the synthesis of ferrocenyl-terminated dendrimers

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With gold-nanoparticle-cored dendrimers, it was found that the silyl group was an excellent alternative to the amido group when it was attached to the ferrocenyl termini for the recognition of oxo-anions including ATP [56]. The factors involved in the redox recognition are the electrostatic attraction between the anion and the ferrocenium cation upon anodic oxidation and the supramolecular bonding between the amido group (hydrogen bonding) of the silyl group (Si hypervalence). The amidoferrocenyl or silylferrocenyl monomers do not show any effect, however. Therefore, the dendrimer topology is important for recognition of oxo-anions. The appropriate encapsulation of the anionic host between the dendritic tethers is a key factor that very much increases the interaction between the functional ferrocenyl termini and the guest (Scheme 8.6).

Scheme 8.6
scheme 6scheme 6

Gold-nanoparticle-cored dendrimer terminated by ferrocenylsilyl group that recognize and sense oxo-anions such as H2PO −4 and ATP

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8.4 Engineering the Dendrimer Family with Peripheral Ferrocenyltriazole Ligands: “Click” Dendrimers and Metallodendrimers for Oxo-Anion and Transition-Metal Cation Sensing

The 1,2,3-triazole is an ideal choice for the interaction with many substrates that have Brönsted or Lewis acid properties including transition metals and their complexes. Thus the encapsulation of such guests should prove feasible by introducing such triazole groups on the dendrimer tethers. The 1,2,3-triazole group is readily formed by “Click” chemistry recently reported by Sharpless to catalyze with CuI the regioselective Huisgens reaction between azido derivatives and terminal alkynes [57]. We used the dendrimer family that was constructed as indicated above and substituted the terminal halogeno group by azido upon reaction with NaN3. These azido-terminated dendrimers were engaged in reactions with ferrocenyl acetylene in order to locate the redox sensor directly on the triazole ring for adequate sensing of the interaction of guests with the triazole heterocycle by perturbation of the redox potential of the ferrocenyl system (Scheme 8.7).

Scheme 8.7
scheme 7scheme 7

Synthesis of a “click” ferrocenyl dendrimer (G0)

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Ferrocenyl terminated dendrimers are known as very good sensors of oxo-anions with positive dendritic effects, i.e. the magnitude of the recognition effect increases together with generation number, because the dendrimer topology of higher generations involves narrower channels for a better interaction with the dendritic site on the tethers. Thus oxo-anions including ATP, a DNA fragment, are well recognized by the “Click” ferrocenyltriazolyl dendrimers. The additional electron density brought by the oxo-anions makes the ferrocenyl oxidation easier, i.e. at less positive oxidations potentials. On the other hand, the interaction with acetonitrile complexes of several transition metals (CuI, CuII, PdII, PtII) withdraws electron density from the ferrocenyltriazolyl system, the ferrocenyl oxidation is rendered more difficult, and its wave is found at more positive potentials (Scheme 8.8) [58].

Scheme 8.8
scheme 8scheme 8

Second-generation “click” ferrocenyl dendrimer (81 terminal ferrocenyltriazolyl groups) that recognizes both oxo-anions including ATP and transition-metal dications (CuI, CuII, PdII, PtII) with positive dendritic effect (i.e. recognition, characterized by the shift of potential of the ferrocenyl CV wave, works all the better as the dendrimer generation is higher)

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8.5 The Click Reaction as a Useful Iterative Method for Dendrimer Construction

In the preceding example, the “click” reaction was used for peripheral dendrimer functionalization. We then addressed the challenge of using the “click” reaction iteratively for divergent dendrimer construction. For this purpose, the “phenoltriallyl” brick used above was propargylated at the focal point before “click” reaction with an azido-terminated dendritic core as above. After the “click” reaction, the polyolefin dendrimer formed in which the number of terminal tethers has been multiplied by three is submitted to hydrosilylation with chloromethyldimethylsilane as in our classic dendrimer construction, then the terminal chloro groups are substituted by azido groups for further iteration of the “Click” reaction with the propargylated dendron (Scheme 8.9) [59].

Scheme 8.9
scheme 9scheme 9

Iterative construction of a G2 “click” dendrimer using a hydrosilylation-click-reaction sequence

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8.6 Dendrimers Containing Triazole Ligands and Ferrocenyl Termini as Useful Templates for Transition-Metal Ions and Transition-Metal Nanoparticles

The triazole ligands were introduced in these dendrimers in order to bind transition-metal cations before their reduction to metal (0) to form nanoparticles that are either stabilized inside the dendrimer or, if the dendrimer is too small, that are stabilized by the dendrimer without encapsulation. The ferrocenyl groups located at the dendrimer periphery just near the triazole rings allow titrating the metal cations that interact herewith. Palladium (II) was coordinated to the triazole ligands in the dendrimer interior using Pd(OAc)2, then reduced to Pd(0) using NaBH4 or methanol. The coordination of Pd(OAc)2 onto the triazole ligands was monitored by cyclic voltammetry, showing the appearance of a new wave corresponding to the ferrocenyl groups attached to Pd(II)-coordinated triazoles.

The outcome was a one-to-one stoichiometry that allowed designing a given number of Pd atoms in the Pd nanoparticles if the dendrimer is large enough for nanoparticle encapsulation. This aspect is very important for applications (Scheme 8.10) [59].

Scheme 8.10
scheme 10scheme 10

Coordination of the triazole ligand by Pd(OAc)2 monitored by ferrocenyl redox sensing followed by Pd(II) reduction to dendrimer-encapsulated Pd (0) nanoparticles used further in catalysis. The variety of nanoparticle sizes obtained with this strategy is crucial for catalyst optimization and mechanistic investigation

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8.7 Application in Catalysis of “Click” Dendrimers and Dendrimer-Stabilized Nanoparticles

Nanoparticles are attracting increasing attention as catalysts from both the homogeneous- and heterogeneous catalysis communities, because they are “ligandless” catalysts avoiding toxic phosphines, and they show remarkable activities and selectivities [60].

Nanoparticles can be stabilized by an extremely large variety of supports from organic to inorganic [61]. Polymers have been among the most popular supports for nanoparticle catalysts, [62] thus dendrimers also stabilize them, and dendrimer stabilization can proceed either by encapsulation [63] or, if the dendrimer is too small, by peripheral stabilization of the nanoparticle surrounded by a number of dendrimers [64]. Thus commercial polyamidoamine and polypropylene imine have been extensively used to stabilize nanoparticle catalysts [65].

Click-dendrimer-stabilized nanoparticles are a new family of dendrimer-stabilized nanoparticles that is particularly suitable for catalytic studies [59, 66]. Different Pd nanoparticles were synthesized from the dendrimers of generations 0 (9 tethers) to 2 (81 tethers). Transmission electron microscopy shows that generations G1 and G2 form dendrimer-encapsulated nanoparticles whose sizes correspond to Pd nanoparticles that contain the same number of Pd0 atoms as that of PdII ions initially coordinated to the triazoles inside the dendrimer, whereas G0 is too small to encapsulate the nanoparticle formed. In this case, the nanoparticle is surrounded by a number of dendrimers that provide stabilization (Scheme 8.11).

Scheme 8.11
scheme 11scheme 11

Pd nanoparticle surrounded and stabilized by several small G0 dendrimers

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The collection of different nanoparticles having different designed sizes is crucial to the study of the mechanisms in nanoparticle catalysis. These “click” dendrimer- stabilized nanoparticles are efficient catalysts for selective olefin hydrogenation under ambient conditions, and the turnover frequencies, turnover numbers and yields depend on the nanoparticle size. The smallest nanoparticles (from G1) are the most active ones, in agreement with a classic hydrogenation mechanism entirely proceeding at the nanoparticle surface [66]. On the other hand, the turnover numbers, turnover frequencies and yields are independent on the type of nanoparticle stabilization and sizes of the nanoparticles for the Suzuki cross coupling reaction between chlorobenzene or bromobenzene and PhB(OH)2. Moreover, the TON increases when the amount of nanoparticle catalyst is decreased or when the solution is diluted. The efficiency reaches 54% yield using 1 ppm Pd nanoparticles, i.e. the amount of nanoparticle catalyst is homeopathic. On the other hand, with high loading of catalyst, the yield is not quantitative, reaching only 70% at 1% Pd atom catalyst. These phenomena are taken into account by a leaching mechanism whereby one or two Pd atoms escape from the nanoparticle surface subsequent to the oxidative addition of the aryl halide onto the nanoparticle surface, then become extremely active in solution until it is quenched by the mother nanoparticle [66]. A similar mechanism had been proposed earlier by de Vries for the Heck reaction at high temperature (150–170°C) [67–69].

8.8 Conclusion and Outlook

The synthesis of high-generation dendrimers starting from organoiron activation provided suitable nanomaterials for molecular electronics, catalysis and sensing. In molecular electronics, the property of fast electron transfer (electrochemical reversibility) with metallocenyl-terminated dendrimers and the single wave of multi-ferrocenyl dendrimers in cyclic voltammetry leads to useful electrocatalytic and sensing properties. For sensing, the compared performances of the functional groups attached to the peripheral groups as exo-receptors offered flexibility of substrates using specific termini. Using the most recent “click” dendrimers with which the recognition can be achieved for both oxo-anions and transition-metal cations, redox recognition was very useful to determine the number of PdII ions coordinated into the dendrimer on the triazole ligands. The precise sizes of Pd nanoparticles designed in this way led to delineation of mechanistic experiments and catalyst optimization that significantly contribute to the knowledge and performances of Pd nanoparticle catalysis. This approach of dendrimer catalysis is complementary to the one introducing inorganic or organometallic catalysts at the core or periphery of dendrimers that was more classic and involved leaching and limited possibilities of catalyst recovery [70]. Studies are ongoing along this line to use suitable dendrimers for efficient “Green” catalysis [71].