Ruthenium-mediated selective activation of a C-H bond. Direct aromatic thiolation in the complexes [Ru-II{o-SC6H3(R)N=NC5H4N}(2)] (R=H, Me or Cl)

The reaction of the complexes ctc-[Ru(II)L(2)Cl(2)] (L = arylazopyridine, RC(6)H(4)N=NC5H4N, where R = H, m-Me, p-Me or p-Cl; ctc = cis-trans-cis with respect to chlorides, pyridine and azo nitrogens respectively) with KS(2)COR' (R' = Me, Et, Pr-n, Bu(n) or CH(2)Ph) in boiling dimethylformamide afforded [Ru-II{o-SC6H3(R)N=NC5H4N}(2)] where the a-carbon atom of the pendant phenyl ring of both ligands L has been selectively and directly thiolated. The newly formed tridentate thiolated ligands are bound in a meridional fashion. When one methyl group is present at the meta position of the pendant phenyl ring of L the reaction resulted in two isomeric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group. The molecular geometry of the complexes in solution has been determined by H-1 NMR spectroscopy. This revealed the presence of an intimate mixture of the two isomers in solution in a 2:1 ratio. In the visible region the complexes exhibit two metal-to-ligand charge-transfer transitions at approximate to 700 and approximate to 560 nm respectively and in the UV region intraligand (pi-pi*, n-pi*) transitions. In acetonitrile solution the complexes exhibit one reversible ruthenium(n) reversible arrow ruthenium(III) oxidation couple near 0.4 V and an irreversible oxidative response near 1 V due to oxidation of the co-ordinated thiol group. Reduction of the co-ordinated azo groups occurs at ca -0.8 and -1.4 V respectively. Coulometric oxidation of the complexes [Ru-II{o-SC6H3(R)N=NC5H4N}(2)] at 0.6 V versus the saturated calomel electrode in dichloromethane produced unstable ruthenium(III) congeners. When R = p-Me, the presence of trivalent ruthenium in the oxidised solution was evidenced by a rhombic EPR spectrum having g(1) = 2.359, g(2) = 2.300 and g(3) = 1.952.