Dinuclear manganese(II) complexes of unsymmetric phenol-based dinucleating ligands with amino and imino chelating arms: synthesis, structure and catalase-like activity

Abstract

Unsymmetric phenol-based dinucleating ligands with amino and imino chelating arms, 4-bromo-2-{[(2-dimethylaminoethyl)methylamino]methyl}-6-[2-(dialkylamino)ethyliminomethyl]phenolate(1–)[alkyl = methyl (L¹) or ethyl (L²)], formed dinuclear manganese(II) complexes [Mn₂L(RCO₂)₂(NCS)](L = L¹ or L², R = Me or Ph). The complex [Mn₂L²(MeCO₂)₂(NCS)] crystallizes in the monoclinic space group P2₁/n, a= 13.450(2), b= 18.743(3), c= 12.662(2)Å and β= 102.08(1)°. An X-ray diffraction analysis revealed that the dinuclear core structure is bridged by the phenolic oxygen of L² and by two acetate groups. The unsymmetric ligand provides different co-ordination geometries about the two manganese ions. The geometry of Mn at the imine site is distorted trigonal bipyramidal with the imine N and the two acetate O atoms in the basal plane and the phenolic O and the terminal N at the axial sites. The geometry of the Mn at the amine site is distorted octahedral involving also the nitrogen of the isothiocyanato group. Magnetic susceptibility measurements over the temperature range 4.2–300 K indicated weak antiferromagnetic interaction (J=–3 to –5 cm^–1 based on Ĥ=–2JS₁·S₂). Cyclic voltammetry for [Mn₂L¹(MeCO₂)₂(NCS)] in CH₂Cl₂ revealed three quasi-reversible redox couples (E_½= 0.63, 1.07 and 1.24 V vs. saturated calomel electrode) assignable to the stepwise oxidations Mn^IIMn^II→ Mn^IIMn^III→Mn^IIIMn^III→Mn^IIIMn^IV. All the complexes show a catalase-like activity, disproportionating H₂O₂ into O₂ and H₂O. The presence of oxomanganese(IV) intermediates is suggested based on visible and mass spectrometric investigations.