Mixed Valency vs Radical Bridge Formulation in Symmetrically and Asymmetrically Ligated Diruthenium Complexes

The asymmetrical dinuclear [{(trpy*)Ru}(2)(mu-adc-Salph)Cl](PF6) 1(PF6), trpy*=4,4',4"-tri-tert-butyl-2,6,2',6"-terpyridine, adc-Salph=1-benzoyl-2-salicyloylhydrazido(3-), and the related symmetrical dinuclear [{Cl(trpy*)Ru}(2)(mu,eta(2) : eta(2)-adc-Ph)](PF6) 2(PF6), adc-Ph=1,2-bis(benzoyl)hydrazido(2-), were synthesized and structurally characterized. Both paramagnetic compounds were compared with the previously reported symmetrical [{(trpy*)Ru}(2)(mu,eta(3) : eta(3)-adc-Sal)](PF6) 3(PF6) containing the bis-tridentate bridge 1,2-bis(salicyloyl)hydrazido(4-). Molecular structures and magnetic resonance features (H-1 NMR, EPR) indicate spin density distribution over the metal(s) and the bridging ligand. Reversible one-electron reduction and oxidation were possible in all instances yielding comproportionation constants K-c of about 10(9) for the paramagnetic intermediates 1(+)-3(+). Structural results, spin density distribution and UV-Vis-NIR spectroelectrochemistry were analyzed for 1(+) with the help of TD-DFT calculations for a model compound (tert-Bu -> Me). Intense absorptions around lambda(max)=1450-1650 nm for the cations were assigned to mixed metal/ligand transitions with significant inter-valence charge transfer (IVCT) character. For both the symmetrical and asymmetrical arrangements the cationic intermediates can be described as considerably mixed metal/ligand systems.