Generation of manganese-(III) versus -(IV) complexes with a conjugated ONS donor set: controlling effect of ligand substituents†

Abstract

Manganese(IV) complexes, [MnL ₂ ] [H ₂ L = MeC(OH)CHCMeNN C(SH)SR (R = Me 1a or CH ₂ Ph 1b)] and manganese(III) complexes, [Mn(O ₂ CMe)L] 1c or [Mn(acac)L] 1d [acac = acetylacetonate; H ₂ L = PhCH(OH)CPhNNC(SH)SCH ₂ Ph] have been synthesized and characterized. The Schiff-base ligands which are derived from an aliphatic carbonyl function, favour the facile oxidation of manganese-(II) to -(IV) under ambient conditions. The structure determination of 1a showed that the molecule is octahedral with the two equivalent tridentate ligands spanned meridionally. The EPR spectrum of 1a with a strong but structured signal at g ≈ 4.0 and a weak one at g ≈ 2.0 implies a large zero-field splitting, but the spectral profile differed from an ideal axial form. All the complexes exhibited reversible or quasi-reversible Mn ^IV –Mn ^III redox couples in their cyclic voltammograms at potentials commensurate with the nature of the substituents in the appropriate ligands. A reasonable basis is suggested by which one may predict whether a particular ligand will stabilize manganese-(II), -(III) or -(IV) in an aerobic medium.