Dendrimers metallodendrimers

Dendrimers can be constructed from chemical species other than purely organic monomers. For example, they can be built up from metal branching centres such as ruthenium or osmium with multidentate ligands. The resulting molecules are known as metallodendrimers. Such molecules can retain their structure by a variety of mechanisms, including complexation, hydrogen bonding and ionic interactions. 

Dendrimers are attractive nanosize model compounds because of their globular architecture and their highly functionalized surface. These hyperbranched compounds are synthesized in a repetitive reaction sequence of nearly quantitative reactions. The synthetic route can either be divergent, starting from the nucleus toward the surface, or convergent, where dendrons or wedges are covalently linked to a polyfunctional nucleus. The number of metallodendrimers is still limited.493-506... 

In a recent report [171] Newkome and He extended this concept and described the use of two ruthenium centers per appendage [—(Ru)—(x)—(Ru)—] towards construction of a four-directional dendrimer (e.g., 81, Fig. 36). A combination of convergent and divergent approaches, hence, allowed the stepwise construction of metallodendrimers via controlled metal complexation. 

Metallodendrimers can be constructed via binding of groups with suitable donor atoms (e.g., polydentate ligands) on either the periphery or the core of the dendrimer and the subsequent complexation/coordination of these ligands to an appropriate metal salt. Ultimately, this binding can involve the formation of a direct a bond linkage (i.e., a M-C bond). This chapter describes various... 

Beside these catalytically active metallophosphine dendrimers (see above), preliminary studies on the chemical properties of phoshorus-based dendrimers complexed to metals such as platinum, palladium and rhodium have been described by Majoral, Caminade and Chaudret [21], They showed that these macromolecules (see Scheme 13) could be useful for the (in situ) generation of metallodendrimer catalysts. 

The research group of Van Leeuwen reported the use of carbosilane de-ndrimers appended with peripherial diphenylphosphino end groups (i.e. 25, Scheme 26) [37]. After in situ complexation with allylpalladium chloride, the resultant metallodendrimer 25 was used as catalyst in the allylic alkylation of sodium diethyl malonate with allyl trifluoroacetate in a continuous flow reactor. Unlike in the batch reaction, in which a very high activity of the dendrimer catalyst and quantitative conversion of the substrate was observed, a rapid decrease in space time yield of the product was noted inside the membrane reactor. The authors concluded that this can most probably be ascribed to catalyst decomposition. The product flow (i.e. outside the membrane reactor)... 

The incorporation or complexation of transition metal fragments by dendrimers has led to a broad spectrum of metallodendrimers. Dendrimers are well-defined branched structures (Fig. 13.2). The dimensions of a dendrimer can easily be adjusted by changing its generation, which can be very practical for their application... 

At about the same time that these fundamental studies of molecular encapsulation were beginning to appear, other groups were exploring dendrimer topology broadly. Historically, at this point, there were few examples of metallodendrimers, so many creative example architectures were possible and were shown. Many different architectures and behaviors were being explored, so thematically these examples are diverse. 

Another interesting class of metallodendrimers are zwitterionic dendrimers in which the positive charge on the metal ion is balanced by carboxylate moieties present in the dendritic branches.58ab Dendrimers 20 and 21 (Fig. 6.14) that contain four [Ru(tpy)2]2+ complexes along the branches and eight carboxylate units either in the internal branching points or at the periphery are examples of this class of zwitterionic dendrimers. 

ECL investigations of dinuclear or polynuclear Ru(II) complexes have been recently performed with hope for developing more efficient electrochemiluminescent materials. Centrally or peripherally functionalized dendrimers with active RuL32 + chelate units can produce higher (up to four to five times) ECL intensities as compared to their monomeric RuL32 + precursors alone. It was also found that the ECL intensities of metallodendrimers become larger as the multiplicity of the involved Ru(II) units increases. Similar observations have been reported for binuclear Ru(II) complexes with weak interaction between both metallic centers.84-88 These results indicate that further studies in such direction may result in design of still more efficient ECL systems based on Ru(II) luminophores. 

The oligo- or multi-functional core unit also plays a role in determining the space occupied by a dendrimer. The core itself can exercise a function, as demonstrated by metallodendrimers (see Section 4.1.11), in which the metal ion core in a supramolecular or coordinatively constructed architecture coordinates with the surrounding branching units - and in this way can influence catalytic and photochemical processes. 

Balzani et al. prepared dendrimers with metal complexes serving both as core [36] and as branching unit The metallodendrimer in Fig. 2.10 is constructed solely from polypyridine ligands and transition metal ions. Such dendritic transition metal complexes can be synthesised both convergently and divergently and different transition metal ions (ruthenium/osmium) can be incorporated. This provides a means of influencing the luminescence properties of the den-drimer. Thus the energy transfer process proceeds from the inside outwards in... 

On account of their physical, photo-physical, or catalytic properties, metallodendrimers have become a widespread class of compounds. Combination of the characteristics of dendrimers with those of transition metals can, for example, produce light-harvesting effects (see Section 5.2) and energy-transfer gradients. 

Metallocenes have frequently been used as terminal moieties in dendrimer chemistry - as already demonstrated in previous sections. They are of interest primarily because of their potential application in catalysis [123]. An unusual metallodendrimer with peripheral ferrocene entities and optically active ferro-cenyldiphosphine ligands (josiphos ligands) was prepared by Togni et al. (Fig. 4.58) [124]. Adamantanetetracarboxlic acid was one of the core units employed. 

Dendrimers with metal complex moieties in their branches require the prior incorporation of specific coordination sites into the dendrimer scaffold. Newkome et al. used such a dendrimer with twelve alkyne units for spot-on introduction of l,2-dicarba-c oso-dodecaborane groups (Fig. 4.59, above right) [127]. Moreover, on-target coordination with dicobalt-octacarbonyl to form a metallodendrimer with twelve dicobalt-hexacarbonyl units was also accomplished. These units can serve as protective groups on the one hand [128], and as catalysts on the other [129]. 

Due to the magnitude of research that has been reported and the variety of dendrimers and metallodendrimers that have been developed for this purpose, throughout the chapter the reader will be directed to reviews and to the other chapters in this book for further information on selected topics. 

Although most of the dendrimers are synthesized by one of these two strategies, examples of metallodendrimer syntheses applying both methodologies have also been reported, which is done to improve their overall synthesis and/or purity (polydispersity) by minimizing the possible formation of defects in the dendritic framework. 

The design of metallodendrimers involves considering the position and repetition of the (catalytically active) metal site in the dendrimer framework, such as on the periphery (A) or at the core (B). Figure 2 shows schematic representations of different types of metallodendrimers. 

Core-functionalized metallodendrimers have the advantage of creating isolated sites due to the environment of the dendritic framework. In the case of core-functionalized dendrimers, the molecular weight per catalytic site (ligand/catalyst) is higher than for periphery-functionalized dendrimers, which therefore involves higher costs from a commercial point of view. The... 

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