Bipyridine dendritic ligands

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... 

Dendritic building blocks having the nature of complex ligands can coordinate with a central metal unit [29]. Such self-assembly opens up direct access to metallodendrimers (see also Section 4.1.7). It should, however, not go unmentioned in this context that the strong Ru2+ bond to the bipyridine nitrogen -although non-covalent - is not regarded as supramolecular by some chemists because of its lack of reversibility. That is why we consider it from a coordination-chemical viewpoint [30]. 

Figure 5.7 compares the luminescence lifetimes of various homo- and hetero-leptic [Ru(bipy)3]2+ complexes with acyclic and macrocyclic bipyridines together with the dendritic species just considered [4], Here too it is apparent that the stability towards photooxidation is again greater for the more sterically shielded homoleptic complexes than for the less strongly shielded heteroleptic species, which each contain another unsubstituted bipyridine ligand. However, the variant with dendritic protective shield proves to be the champion (dendritic effect). 

Newkome and coworkers also examined a dendritic system in which bipyridine molecules, incorporated into the branches, were subsequently complexed to yield dendrimers containing ruthenium tris-bipyridyl moieties dangling from the dendritic arms. In the electrochemistry of these compounds, the metal centers, surrounded by a dendritic network, exhibited irreversible kinetic behavior, as is commonly found in such systems. On the other hand, there are three separately identifiable ligand-based reductions, each of the... 

Coordination of Ag(I) with the bipyridine ligand (n = 1) that possesses a dendritic aliphatic side chain at C5 (X group in Figure 6b) gave different self-assembled products as a function of the nature and size of the counterion. The... 

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