Metal complex dendrimers properties

Metal-acetylide complexes including metal-poly(yne) polymers often show unique properties [21-23]. Thus, metal-acetylide dendrimers are of interest because amplification of the functionality due to metal-acetylide units based on three-dimensional assembly with a regular dendritic structure is expected. 

When the only metal complex of a dendrimer is that constituting the core of the structure (Fig. la), the most interesting problem is whether and, if so, how much the electrochemical properties (potential value, kinetic reversibility) of the metal-based core are modified by the surrounding branches. 

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. 

Other first-generation dendrimers built around multichelating ligands are the trinuclear complexes 29 - 38 [50-52] and several hexanuclear complexes (for representative examples, see 39 and 40) [53]. In all of these compounds, the photophysical properties are equivalent to those of their building blocks. In the mixed-metal complexes 31 and 34 [50b] the Ru(II)- and Os(II)-based chromo-phores are only weakly-coupled and a dual luminescence is observed. 

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

V. Balzani, S. Campagna, G. Denti, A Juris, S. Serroni, M. Venturi, Designing Dendrimers Based on Transition-Metal Complexes. Light-harvesting Properties and Predetermined Redox Patterns , Acc Chem Res. 1998, 31, 26-34. 

Bimetallic metal particles are important materials because their characteristics, especially their catalytic properties, are often quite different from those of pure metal particles. Dendrimer-encapsulated bimetallic clusters can be synthesized by any of three methods (Fig. 17) (1) partial displacement of the dendrimer-encapsidated cluster, (2) simultaneous co-complexation of two different metal ions followed by reduction, or (3) sequential loading and reduction of two different metal ions. 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

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... 

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... 

Balzani V, Campagna S, Denti G, Juris A, Serroni S, Venturi M. Designing dendrimers based on transition-metal complexes. Light-harvesting properties and predetermined redox patterns. Acc Chem Res 1998 31 26-34. 

Very little is known about these dendrimer properties, although it has been noted that ester-terminated PAMAM dendrimers form deep blue complexes with CuS04 solutions, and that NH2-terminated homologues produce deep purple solutions [2, 79]. The well-known coordination properties of the amide bond, which lead to the formation of metal-ion complexes [163], should make this a very rich area for further investigation. 

Electrochemical characterization has been performed in acetonitrile solution on a great variety of dendrimers built with metal complexes as branching centers in the whole structure [16]. An interesting example is that represented by the decametallic dendrimers 30-32, where the metal centers are linked by bridging ligands made of two, closely connected bis-chelating units such as 2,3-dpp (2,3-dpp = 2,3-bis(2-pyridyl)pyrazine) [76, 78]. In the dendritic species, each mononuclear component brings its own redox properties, more or less affected by intercomponent inter-... 

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