Cobalt-mediated direct and selective aromatic thiolation in the complex [CoIII(o-SC6H4NNC 5H4N)2]ClO4. Synthesis, spectroscopic characterisation and electron-transfer properties

Abstract

The reaction of K[SC(S)OR] (R = Me, Et, Pr ⁿ , Pr ⁱ , Bu ⁿ , Bu ⁱ or CH ₂ Ph) with the complex [Co ^II L ₃ ][ClO ₄ ] ₂ ·H ₂ O 1 [L = phenyl(2-pyridyl)diazene, C ₆ H ₅ NNC ₅ H ₄ N] in boiling dimethylformamide resulted in [Co ^III L′ ₂ ]ClO ₄ 2 (L′ = o-SC ₆ H ₄ N NC ₅ H ₄ N). In complex 2 the o-carbon–hydrogen bond of the pendant phenyl ring of both the parent ligands L has been selectively and directly thiolated via carbon–sulfur bond cleavage of the dithiocarbonate. During the thiolation the metal ion is oxidised from the starting Co ^II in 1 to Co ^III in the final product 2. The reaction is highly sensitive to the nature of the solvent used, taking place only in those having high boiling points and relative permittivities. Its rate is dependent on the nature of the R group present in the thiolating agent, following the order Me ≈ Et > Pr ⁿ > Bu ⁿ > Pr ⁱ > Bu ⁱ benzyl. A meridional configuration (cis-trans-cis with respect to the sulfur, azo and pyridine nitrogens respectively) has been established by ¹ H and ¹³ C NMR spectroscopy. The complex exhibits reversible Co ^III ⇌ Co ^II reduction at -0.135 V and four ligand-based azo (NN) reductions at -0.51 (one electron) and at -1.175 V (simultaneous three-electron reduction) respectively versus saturated calomel electrode. The oxidation of the co-ordinated thiol group occurs at 0.90 V.