Metal complexes, dendritic

We have prepared a variety of metalladendrimers from the reaction of phosphorus-containing dendrimers with various transition metal complexes. Dendritic complexes incorporating metals such as gold [28a, 57], iron [57,58,59], tungsten [58,59], rhodium [59, 60], platinum [60], palladium [61], ruthenium [62], or zirconium [63] were prepared. 

A number of attempts to produce tire refractory metals, such as titanium and zirconium, by molten chloride electrolysis have not met widr success with two exceptions. The electrolysis of caesium salts such as Cs2ZrCl6 and CsTaCle, and of the fluorides Na2ZrF6 and NaTaFg have produced satisfactoty products on the laboratory scale (Flengas and Pint, 1969) but other systems have produced merely metallic dusts aird dendritic deposits. These observations suggest tlrat, as in tire case of metal deposition from aqueous electrolytes, e.g. Ag from Ag(CN)/ instead of from AgNOj, tire formation of stable metal complexes in tire liquid electrolyte is the key to success. 

Dendritic Sphere Based on Aryl-Metal Complexes. 44... 

Dendritic Sphere Based on Alkynyl-Metal Complexes. 47... 

Complexation of gold ions, [Au(I)], with peripheral phosphine groups of a P-based dendrimer was reported by Majoral et al. [185]. Transmission electron microscopy (TEM) was used to analyze the large aggregates formed by the dendritic gold complexes and a direct correlation was observed between the size of the particles and the dendrimer generation number. In a recent report [186], Majoral et al. further demonstrated that up to 48 diphosphino groups could be anchored to the surface of dendrimers and various dendritic metal-complexes... 

Dendrimers built around a metal complex as a core. These compounds can be considered metal complexes of ligands carrying dendritic substituents (Fig. 1 a). The most commonly used metal complex cores are porphyrin complexes, polypyridine complexes, and ferrocene-type compounds. 

Dendrimers containing metal complexes in the branches. In these compounds (Fig. lc), metal complexes may play the role of connectors along the branches of a dendritic structure as in the case of (tpy)Ru(tpy)2+ (tpy=2,2 6, 2"-ter-pyridine) [6], or may be attached to specific sites as in the case of cobalt carbonyls [7]. 

For space reasons, we will deal mainly with the electrochemical behavior of large dendritic compounds. Therefore, the electrochemical properties of a number of borderline compounds [14] between metal complexes and dendrimers have not been included in this review. 

In 1977, Parshall and co-workers published their work on the separation of various homogeneous catalysts from reaction mixtures.[46] Homemade polyimide membranes, formed from a solution of polyamic acid were used. After reaction the mixture was subjected to reverse osmosis. Depending on the metal complex and the applied pressure, the permeate contained 4-40% of the original amount of metal. This publication was the beginning of research on unmodified or non-dendritic catalysts separated by commercial and homemade membranes. 

Tulchinsky and Miller (32) patented dendritic macromolecules, their metal complexes, and their use in catalytic processes such as hydroformylation. Dendrimers containing organophosphites, organophosphonites, and/or organophosphinites were... 

The properties induced by the dendritic framework depend on the location of the functional groups within the structure. Periphery-functionalized dendrimers offer high accessibility of the metal complex, which allows reaction rates that are... 

The interest in highly branched polynuclear metal complexes, and more generally in dendritic species, is related not so much to their size, but rather to the presence of different components. An ordered array of different components can in fact generate valuable properties, such as the presence of cavities having different size, surfaces with specific functions, gradients for photoinduced directional energy and electron transfer, and sites for multielectron transfer catalysis. Studies along these directions are underway in our laboratories. 

Pyridine-based ligands which have been used for dendrimers are 2,2-bipyridine (bpy) 17,2,3-bis(2-pyridyl)pyrazine (2,3-dpp) 18 and its monomethylated salt 19, and 2,2 6, 2"-terpyridine 20. Their transition metal complexes possessing dendritic structures were first reported in the collaborative work of Denti, Campagne, and Balzani whose divergent synthetic strategy has led to systems containing 22 ruthenium centers. - The core unit is [Ru(2,3-dpp)3] 21 which contains three... 

Transfer hydrogenation of ketones using metal complexes with a chiral water-soluble [97,98] and a dendritic ligand [99] was investigated for use in recycling catalysts. The reaction with immobilized catalysts has also been reported [100]. 

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. 

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... 

Fig. 2.11 Lanthanide ion as core unit of a dendritic metal complex...

E.g. dendritically substituted photoactive metal complexes Review V. Balzani, P. Ceroni, A. Juris, M. Venturi, S. Campagna, F. Puntoriero, S. Serroni, Coord. Chem. Rev. 2001, 219-221, 545-572. 

Synthesis of dendritic structures with a metal complex as core can proceed in two different ways ... 

Covalent coupling A pre-existing metal complex is coupled covalently with den-drons and is thus surrounded by a dendritic shell. Appropriate examples are the redox-active metallodendrimers of Kaifer et al. [117] or the dendritic porphyrin/ metal complexes prepared and characterised by Diederich et al. [118] and Aida et al. [119]. 

A dendrimer with metal complexes both in the core and in the branches was described by Balzani et al. The luminescent, heteroleptic (having different ligands), dendritic polypyridine-ruthenium or polypyridine-osmium complex can be prepared both divergently and convergently [130] (cf. Section 2.5.2). 

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