Metal complex dendrimers approach

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. 

The surface morphologies of PAMAM dendrimers have been studied extensively by Turro and co-workers [16-23]. As shown in Scheme 4, one approach was to study the adsorption of organic dye molecules and metal complexes on the dendrimer surface by UY-Vis and fluorescence spectroscopy another approach took advantages of electron transfer processes between two adsorbed species on a single dendrimer surface or between the adsorbed species on a dendrimer surface and other species in aqueous solution. 

Figure 7. Divergent synthetic strategy to obtain polynuclear metal complexes of dendrimer shape. - ° Each deprotected compound of the dive ent synthetic approach can be used as a core in convergent synthetic processes. Some of these routes starting from a tetranuclear core are illustrated in Figure 8.

As we have seen in Section IIl.A, the complexes-as-metals/complexes-as-ligands synthetic approach can be extended to obtain even laiger dendrimers. Most... 

There are basically two routes available for the preparation of dendrimers with a metal complex as core unit. One of these approaches starts from a preconstructed metal complex, whose ligand framework is covalently substituted with dendritic groups. Aida [31], Diederich [32], and Kaifer [33] were among those to use this strategy. 

Recently, we extended the approach to dendrimers immobilized on silica. For this purpose, the first generation (and also larger generations) DAB den-drimer was functionalized with urea propyl-(SiOR)3 groups that via an easy process were grafted on silica. The catalyst was noncovalently anchored by simply adding a solution of a metal complex with ancillary ligands utilized with the typical the complementary motif (Fig. 7) [29]. 

Dendrimers can be prepared using either a divergent or convergent approach. The former is more suited to the synthesis of supported metal complexes and is the most frequently used (Scheme 1). For higher-generation dendrimers however, incomplete conversion can be a problem as can purification of the materials. This can be partially overcome by building the dendrimer on an insoluble polymer support. In this case, it is possible to use an excess of reagents to drive the reaction to completion and also separation and purification is facilitated. 

There is a plethora of examples representing dendrimers with metal-binding sites on the periphery. Polycarboxylates were the simplest, early examples [179], although dendrimers possessing more complicated ligand complexing centers have invariably been built via a convergent approach. 

Molecular catalysts, often in the form of metal ions complexed to a suitable ligand, can also be attached to dendrimer surfaces [3,9,10,93,94,96,148,149]. Such materials are generally structurally better defined than catalysts bounded to linear polymers, but like random-polymer catalysts they can be easily separated from reaction products. Note, however, that this approach results in a synthetic dead-end as far as further manipulation of the terminal groups is concerned, and thus some of the advantages of using dendrimers, such as solubility modulation, are lost. 

Recently the divergent iterative approach, welt known in oiganic synthesis, has been successfully extended to prepare polynuclear dendrimer-type complexes containing up to 22 metal centers using 2,3-dpp as bridging ligand. The two... 

The divergent synthetic approach is schematically summarized in Figure 7. Reaction of the mononuclear complex-ligand tRu(2,3-dpp)j] with the complex-metal [Ru(2,3-Medpp)2Cl2] in a 1 3 molecular ratio leads to a tetranuclear complex (first dendrimer generation) which in its peiipheiy contains six protected chelating sites. Demethylation of this species restores its ligand ability and yields... 

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