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

Abstract

The reaction of the complexes ctc-[Ru ^II L ₂ Cl ₂ ] (L = arylazopyridine, RC ₆ H ₄ N=NC ₅ H ₄ N, 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-SC ₆ H ₃ (R)N NC ₅ H ₄ N} ₂ ] where the o-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 ¹ H 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 ≈700 and ≈560 nm respectively and in the UV region intraligand (π-π, n-π) transitions. In acetonitrile solution the complexes exhibit one reversible ruthenium(II) 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-SC ₆ H ₃ (R)N NC ₅ H ₄ N} 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.