Diosmium compounds containing bis(imidazote)-p-quinone bridging ligands

The doubly deprotonated bridging ligand L-1(2-) derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1',4'-benzoqui-nono(2',3'-d:5',6'-didiimidazole H2L1 forms coordination compounds with two bis(2,2'-bipyridine) osmium(o) complex fragments in anti ([1](ClO4)(2)) and syn configurations ([2](ClO4)(2)) of {(mu-L-1)[Os(bpy)(2)](2)) (ClO4)(2), as evident from crystal structure analyses. Exchange of the metal-coordinating 2-pyridyl func- tions in the bridge through non-coordinating 4-tolyl substituents (L-1(2)-> L-2(2-)) leads to [3](ClO4)(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.