Mixed Valency in Dinuclear Ruthenium Complexes Bridged by Tetrazine- and Azo-Cyanamide Ligands

The novel [Ru(II),Ru(II)] dinuclear complex [{Ru(bpy)(ttpy)}2(µ-tdpc)][PF6]2, where ttpy: 4-(tert-butylphenyl)-2,2′:6′,2″-terpyridine, bpy: 2,2′-bipyridine and tdpc2- is 3,6-bis(phenylcyanamido)-1,2,4,5-tetrazine, was synthesized and characterized by elemental analysis, 1H-NMR, IR and UV-Vis spectroscopies. Analysis of the cyclic voltammetry data combined with spectroelectrochemistry results indicates the [Ru(II),Ru(III)] complex is a weakly coupled Class II system. These results were compared against the properties of the strongly coupled Class II complex, [{Ru(bpy)(trpy)}2(µ-adpc)]3+, where trpy: 2,2′:6′,6″-terpyridine and adpc2-: 4,4′-azodi(phenylcyanamide). DFT calculations of the free ligands and mixed-valence complexes suggest that metal-metal coupling primarily occurs via the hole-transfer mechanism. Two dinuclear complexes, [{Ru(bpy)2}2(µ-adpc)2] and [{Ru(bpy)2(dmso)}2(µ-adpc)][PF6]2 where dmso: dimethylsulfoxide, were synthesized and characterized by elemental analysis, 1H-NMR and UV-Vis spectroscopies. Spectroelectrochemistry studies showed delocalized metal-ligand mixed-valence Class II description for [{Ru(bpy)2}2(µ-adpc)2]+. The [{Ru(bpy)2(dmso)}2(µ-adpc)][PF6]2 complex exhibits photo-induced Ru-S to Ru-O and thermal Ru-O to Ru-S linkage isomerism in DMSO solution. DFT calculations suggested the non-innocent behavior of phenylcyanamide ligand in both complexes.