Ruthenium-mediated selective activation of a C–H bond. Direct aromatic thiolation in the complexes [Ru^II{o-SC₆H₃(R)N=NC ₅H₄N}₂] (R = H, Me or Cl)

Abstract

The reaction of the complexes ctc-[Ru^IIL₂Cl₂] (L = arylazopyridine, RC₆H₄N=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 KS₂COR′ (R′ = Me, Et, Prⁿ, Buⁿ or CH₂Ph) in boiling dimethylformamide afforded [Ru^II{o-SC₆H₃(R)N=NC ₅H₄N}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 ¹H 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-SC₆H₃(R)N=NC ₅H₄N}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 g₁ = 2.359, g₂ = 2.300 and g₃ = 1.952.

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