Metallodendrimers-metal centers

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 ruthenium-based metallodendrimer Go-8 was applied as catalyst in a ring-closure metathesis reaction. The activity per metal center of the dendritic catalysts was found to be comparable to that of the corresponding mononuclear catalyst. Unfortunately, the metathesis reaction conditions were not compatible with the nanofiltration membrane set-up used, since a black precipitate was formed in the vessel containing the catalyst. It was found that the conversion to diethyl-3-cyclopentene dicarboxylate product stopped... 

Nonetheless, the transposition of homogeneous catalytic reactions from unsupported to dendrimer-supported catalysts is still not straightforward. Various dendritic effects , positive and negative ones, on the activity, selectivity, stability and solubility of metallodendrimer catalysts have been observed in this respect. In our own research we have found that a high concentration of metal centers at periphery-functionalized metallodendrimers may translate into a decrease in the catalytic performance due to undesirable side-reactions between the catalytic sites at the dendrimer surface (Fig. 4 and Scheme 4). In contrast, when the exact same catalyst is located at the focal point of a dendron, this matter is avoided by isolating the active site, thereby providing a more stable albeit less active catalyst (Scheme 13). 

Metal centers can typically be included in the core or as the core of a dendrimer via metal-ligand coordination bonds. Complexes (M(L) ) of metals (M) containing ligands (L) that can readily be replaced are generally employed for synthesizing these metallodendrimers. Due to its remarkable ability to form stable coordination compounds, Ru(II) metal ion is an excellent candidate for such a build-up process. One of the first examples of Ru(II)-based metallodendrimers was prepared by reacting... 

Different types of metallodendrimers with metal compounds at each repeating unit throughout the dendrimer have been reviewed [195,197]. Because of their high concentration of metal centers, they can be considered as nanoparticle equivalents. One example is the platinum acetylide dendrimer 72 obtained by a stoichiometric transmetallation to form acetylides [204]. This metallodendrimer has a precedent in earlier syntheses of linear Pt-acetylide coordination polymers. Several papers describe the synthesis of various Ru pyridyl complexes as repeating units for photochemical investigations and non-... 

Metal centers may be introduced into metallodendrimers and metallostars at a number of conceptually different sites. A convenient classification has been introduced by Balzani and co-workers and is used here in a slightly modified form. Conceptually, the simplest site for metal ion incorporation is at the center of the complex. In principle, a mononuclear complex with dendron-functionalized ligands could be regarded as a metallodendrimer, although this chapter concentrates upon multinuclear systems. 

We will now briefly discuss the synthetic strategies that may be used for the preparation of metallostars and metallodendrimers. Clearly, different strategies will be appropriate to different sites of metal ion incorporation and the structural role of the metal center(s). 

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. 

Some metallodendrimers with one or more stereogenic centers have been prepared without control of the chirality. Vogtle and Balzani [74] have tried several strategies to prepare dendrimers in which a ruthenium cation is the core of the final compound. In these compounds, the only centre of chirality is that of the metal, but as it was not controlled racemic mixtures were obtained. Controlling the stereochemistry of the starting complex would have allowed the authors to prepare a optically pure metallodendrimer. Denti, Campagna, Balzani, and their co-workers have studied polymetallic dendrimers based on bipyridine and 2,3- 7s -(2-pyridyl)pyrazine (2,3-... 

For the formation of metallodendrimers of precise nature, a second favorable position in the overall structure for complexation can obviously be at the periphery. Excellent examples of such systems have been reported that include a silicon dendrimer decorated with 243 ferrocenyl units at the periphery with stable redox activity [58]. Catalytic activity of dendrimers with metals located at the periphery has also proven to be of great interest as it has been recently reviewed by several authors [59,60]. Placing photoactive centers at this specific location can nonetheless be more intricate in this case, as demonstrated by the limited number of reported examples. 

Metallodendrimers are an interesting class of molecules in the area of dendrimer chemistry. They combine dendritic structures with the specific activity of metal complex centers. Metal coordination has facilitated the synthesis of a number of dendritic, supramolecular structures. Metals have been incorporated in all of the topologically different parts of dendrimers in the repeat or branching unit, in the molecular core and in the peripheral units. Because this field of metallodendrimers has been reviewed recently [195-197], only a few examples are given below. Other supramolecular organizations such as catenanes and rotaxanes have been mentioned previously in this chapter. 

An interesting variant on metal polypyridyl-type metallodendrimers is the material 8.34, which was prepared by a convergent synthetic approach (Scheme 8.2) [70]. A further example is the dendrimer 8.35, which possesses 18 ruthenium centers [71]. 

Metallodendrimers can be classified according to the metal ion s position within the dendritic structure, such as (i) the structure s core (ii) connectors between branching centers (iii) branching centers (iv) terminal gronps or (v) metal ions attach using a supramolecular interaction to a preformed covalent dendrimer the latter will be considered as a host-guest interaction (Figure 5). 

Figure 5 Different positions that metal ions can play in metallodendrimers, such as the (a) core, (b) connectors, (c) branching centers, (d) termini, or (e) structural auxiliaries.

The first example of metallodendrimers containing metal ions as part of the connectors (Figure 5b) was synthesized by Newkome et which presented a dendrimer containing 12 pseudooctahedral Ru(II) centers via connectivity. An extension of this family was the synthesis of a tetrahedral metallodendrimer, wherein each arm, two ruthenium ions were linearly connected by a connectivity. ... 

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