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

The doubly deprotonated bridging ligand L 1 2- derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1',4'-benzoquinono[2',3'- d :5',6'- d ']diimidazole H 2 L 1 forms coordination compounds with two bis(2,2'-bipyridine)osmium(II) complex fragments in anti ([1](ClO 4 ) 2 ) and syn configurations ([2](ClO 4 ) 2 ) of {(μ-L 1 )[Os(bpy) 2 ] 2 }(ClO 4 ) 2 , as evident from crystal structure analyses. Exchange of the metal-coordinating 2-pyridyl functions in the bridge through non-coordinating 4-tolyl substituents (L 1 2- → L 2 2- ) leads to [3](ClO 4 ) 2 which involves chelation of the [Os(bpy) 2 ] 2+ groups through imidazole-N and carbonyl-O atoms of the central p -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 n + ]/[2 n + ] and [3 n + ] systems were found regarding variable but mostly metal centred oxidation, the two processes separated much more for [3 n + ]. 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.