Diosmium compounds containing bis(imidazole)--quinone bridging ligands.

The doubly deprotonated bridging ligand L derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1',4'-benzoquinono[2',3'-:5',6'-']diimidazole HL forms coordination compounds with two bis(2,2'-bipyridine)osmium(II) complex fragments in ([1](ClO)) and configurations ([2](ClO)) of {(μ-L)[Os(bpy)]}(ClO), as evident from crystal structure analyses. Exchange of the metal-coordinating 2-pyridyl functions in the bridge through non-coordinating 4-tolyl substituents (L → L) leads to [3](ClO) which involves chelation of the [Os(bpy)] groups through imidazole-N and carbonyl-O atoms of the central -quinone function. In addition to identification, the compounds were subjected to electrochemical (CV, DPV) and spectroelectrochemical (UV-vis-NIR, EPR) analyses of electron transfer, the results being supported by results from TD-DFT calculations. Essential differences between [1]/[2] and [3] systems were found regarding variable but mostly metal centred oxidation, the two processes separated much more for [3]. The first reduction is bpy ([1], [2]) or quinone ligand centred ([3]). Electronic structures and electron transfer behaviour are thus highly sensitive to differences of configuration and coordination.