Redox Ambi-Valence of Phenylcyanamide Ligands in Ruthenium Complexes

Four dinuclear ruthenium (II)-terpyridine-bipyridine complexes of bridging substituted (dimethyl-, unsubstituted dichloro- and tetrachloro-) 1,4-dicyanamidebenzene dianion ligands have been synthesized. The singly oxidized complexes were characterized by vis-NIR, IR and EPR spectroscopies which confirm a valence configuration with the oxidized bridging ligand. Six dinuclear ruthenium (II)-terpyridine-bipyridine complexes of bridging substituted (tetramethyl-, dimethyl-, unsubstituted, dichloro-, tetrachloro-) azodi(phenylcyanamide) ligands have been synthesized. The EPR spectroscopy, supported by IR and vis-NIR spectroelectrochemistry of singly oxidized complexes showed a transition in non-innocent behaviour of the bridging ligand and established the ligand-centred radical descriptions for tetramethyl-/dimethyl-, a delocalized metal-ligand mixed-valence description for unsubstituted, and a localized metal-centred mixed-valence descriptions for dichloro-/tetrachloro- azodi(phenylcyanamide) complexes. Six mononuclear ruthenium (II)-(dmso-S)-bipyridine complexes of substituted (pentachloro-, tetrachloro-, trichloro-, dichloro-, monochloro- and unsubstituted) phenylcyanamide monoanion ligands were synthesized and characterized. The complexes exhibited photo-induced Ru-S to Ru-O and thermal Ru-O to Ru-S linkage isomerism in solutions and solid polymer films. The quantum yields (0.43-0.06) of photo-induced Ru-S to Ru-O linkage isomerism are shown to decrease with increasing donor properties of the phenylcyanamide ligand. The thermal Ru-O to Ru-S back reaction (k = 4.52 – 2.61 x 10-3 s-1) showed a modest decrease with decrease in the donor properties of the phenylcyanamide ligand. The scan rate dependent cyclic voltammograms demonstrated redox-induced Ru-S to Ru-O and Ru-O to Ru-S linkage isomerism and provided estimates of ground state isomerism rates. DFT calculations established the non-innocent behaviour of phenylcyanamide ligands. Five mononuclear neutral ruthenium (III)-terpyridine-chloride complexes of substituted (pentachloro-, tetrachloro-, trichloro-, dichloro-, monochloro-) phenylcyanamide monoanion ligands were synthesized. The paramagnetic NMR spectroscopy of complexes in dimethylsulfoxide provided estimates of pi-carbon spin density distributions of phenylcyanamide ligands (0.7-3.0 %) and suggested a Ru(III)-centred spin in disagreement with the gas-phase DFT spin density calculations which gave mostly ligand-centred spin. It is suggested that specific solvent-solute interactions stabilize the Ru(III) oxidation state.