Metal complex dendrimers synthesis

Polynuclear complexes based on octahedral building blocks may be structurally not well defined because of stereogenic problems [8]. However, clever synthetic strategies have recently been devised to obtain chirally pure species [61-65]. Synthesis and, of course, photophysical and photochemical studies of stereochemically pure metal-based dendrimers are still in their infancy. 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

Over the years, molecular design has allowed the synthesis of functional dendrimers possessing metal and semiconductor nanoparticles, metal complexes and macrocycles, dyes and biologically important carrier molecules in various parts of the dentritic structures [31-39], Such dendrimers incorporating functional units are considered as supramolecular species [16, 21], Functional dendrimers have been proved to be very useful for specific applications [40, 41], Some examples of functional dendrimers are shown in Fig.5. 

Brunner s concept (dendrizyme) of attaching dendrihc chiral wedges to a catalytically achve achiral metal complex represents the first example of asymmetric catalysis using a core-funchonalized dendrimer catalyst [21]. In view of the extremely poor asymmetric induction effected by the chiral dendritic shucture, the bulk of the attenhon has been focused on the immobilizahon of the well-established chiral hgands and/or their metal complexes into an achiral dendrimer core. The important early examples included TADDOL-centered chiral dendrimers, which were reported by Seebach et al. in 1999 [28]. In this section, we ahempt to summarize the recently reported chiral core-funchonalized dendrimers with special emphasis on their applications in asymmetric synthesis. 

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. 

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. 

The next step in the synthesis of supported metal catalysts via dendrimers is the immobilization of dendrimer-metal nanocomposites onto a solid support. An array of techniques exists for achieving this task. Wet impregnation and sol-gel incorporation of dendrimer-metal nanocomposites may lead to strongly adhered metal particles. Other techniques, such as functionalization of the support to facilitate dendrimer growth or adhesion, provide a route for deposition of empty dendrimers that can subsequently undergo complexation with metal precursors to form dendrimer-metal complexes and eventually zerovalent nanoparticles. Whereas the complexation and reduction phases of catalyst synthesis via dendrimers can be fairly complicated, most methods of dendrimer deposition are rather straightforward. 

Polypyridyl transition metal complexes, especially those of ruthenium(II), have been extensively apphed in light harvesting and information storage, because they exhibit a wide range of photophysical and electrochemical properties. Storrier et al. have reported the synthesis and characterization of PAMAM dendrimers functionalized with tris(bipyridyl) ruthenium(II) (dend- -[Ru(bpy)3] +) or bis (terpyridyl) ruthe-nium(II) (dend-n-[Ru(tpy)2] ) complexes (GO, 1, 2, 3, and 4 with 4, 8, 16, 32,... 

There are numerous examples of dendrimers and star polymers that contain metal coordination complexes based on pyridine ligands. There is also interest in the incorporation of more than one type of metal complex into these materials. Constable and coworkers have reported the synthesis of heptametallic complexes containing six peripheral ruthenium coordination complexes and a central iron or cobalt complex. Scheme 21 illustrates the reaction of 89 with either iron or cobalt complexes to produce the heptametallic dendrimer 90. A heptametallic star-shaped complex with a CpFe -coordinated arene as the core and ruthenium Aexa-pyridine complexes at the periphery has also been reported by Astruc. ... 

Storrier GD, Takada K, Abruna HD (1999) Synthesis, characterization, electrochemistry, and EQCM studies of polyamidoamine dendrimers surface-functionalized with polypyridyl metal complexes. Langmuir 15 872-884... 

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