Non-innocence and mixed valency in tri- and tetranuclear ruthenium complexes of a heteroquinone bridging ligand

The redox-active ligand 5,7,12,14-tetraazapentacene-6,13-quinone = L forms structurally characterised compounds with three (1) or four (2) [Ru(acac)2] complex fragments in which each of the metals is N,O-chelated. The new tris- and tetrakis-bidentate chelate compounds exhibit ruthenium centres bridged at about 4 A by quinone O atoms which are then situated across the pentacene π system at about 6–8 A distance. Several electron transfer processes were observed by voltammetry (CV, DPV) and the intermediates identified by EPR and UV-Vis-NIR spectroelectrochemistry. TD-DFT calculations were applied to assign the proper oxidation states within the multistep redox systems {(μn-L)[Ru(acac)2]n}k, n = 3 or 4, revealing both metal and ligand based electron transfer.