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

Abstract

The doubly deprotonated bridging ligand L₁²⁻ derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1′,4′-benzoquinono[2′,3′-d:5′,6′-d′]diimidazole H₂L₁ forms coordination compounds with two bis(2,2′-bipyridine)osmium(II) complex fragments in anti ([1](ClO₄)₂) and syn configurations ([2](ClO₄)₂) of {(μ-L₁)[Os(bpy)₂]₂}(ClO₄)₂, as evident from crystal structure analyses. Exchange of the metal-coordinating 2-pyridyl functions in the bridge through non-coordinating 4-tolyl substituents (L₁²⁻ → L₂²⁻) 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 [1ⁿ⁺]/[2ⁿ⁺] and [3ⁿ⁺] systems were found regarding variable but mostly metal centred oxidation, the two processes separated much more for [3ⁿ⁺]. 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.