Intervalence, electron transfer and redox properties of a triazolate-bridged ruthenium-polypyridine dinuclear complex

A new dinuclear complex of the type cis, cis [(bpy)(2)ClRu(mu-L-b)RuCl(bpy)(2)](n+) (bpy = 2,2'-bipyridine; L-b = benzotriazolate (bta); n = 1, 2, or 3) has been synthesized, isolated as a PF6- salt, and investigated in organic solutions by means of cyclic voltammetry and ultraviolet/visible/near-infrared spectroelectrochemistry. Particular emphasis has been given to the electron transfer (ET) properties of the mixed-valent species (n = 2), which displays a somewhat large metal-metal electronic coupling in the ground state with the complex featuring localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge-transfer (IVCT) band and electrochemical parameters. Analysis of the IVCT properties in the context of Hush's theory also supports a valence-trapped formulation. In spite of the class II categorization within the Robin-Day scheme, this system shows a remarkable intermetallic communication when compared with other analogues (e.g. Lb = pyrazine). Such aspect has also been stressed by comparison of the set of thermodynamic and mixed-valence parameters along with a series of L-bridged systems studied previously in aqueous solutions (in particular, [(edta)Ru(mu-bta)Ru(edta)](4-); edta = ethylenediamine-N,N,N',N'-tetraacetate), and the striking differences in their intervalence characteristics have been rationalized in terms of distinct types of electronic and structural effects. Despite the contrasting behavior, the same type of superexchange mechanism ('hole-transfer') seems to prevail in all these benzotriazolate-bridged mixed-valent species. In the 2,2'-bipyridine derivative, the synergistic charge-transfer effects are the most relevant factors on the great stabilization of the mixed-valence state. The combined pi-acceptor and sigma,pi-donor abilities of the ancillary (bpy) and bridging (bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (Ru-III-L-Ru-III) and fully reduced (Ru-II-L-Ru-II) isovalent species. From the IVCT band features, the rate of intramolecular thermal ET for the mixed-valent ion was estimated on the basis of the Hush and Marcus theories. (C) 2003 Elsevier Science Ltd. All rights reserved.