Effect of ring size in macrocyclic dinuclear manganese(II) complexes upon their structure, properties and reactivity towards H2O2

Abstract

A dinuclear manganese(II) complex [Mn ₂ L ^4,4 (O ₂ CMe) ₂ ] has been prepared and its structure, properties and catalase-like function studied in comparison with the analogous [Mn ₂ L ^{m , n} (O ₂ CMe) ₂ ] [m,n = 2,3; 2,4; or 3,3; (L ^{m , n} ) ^2- denotes macrocycles containing two 2,6-bis(iminomethyl)-4-methylphenolate entities bridged through two lateral chains, (CH ₂ ) _m and (CH ₂ ) _n , at the imino nitrogens]. In the centrosymmetric [Mn ₂ L ^4,4 (O ₂ CMe) ₂ ] a pair of manganese(II) ions are bridged by two phenolic oxygens of (L ^4,4 ) ^2- in the equatorial plane and by two acetate groups at the axial sites. The configuration about each Mn is pseudo-octahedral. The Mn · · · Mn separation is 2.978(1) Å. Cryomagnetic studies (4.2–300 K) indicated a significantly strong antiferromagnetic interaction (J = -5.0 cm ^-1 based on H = -2JS ₁ S ₂ ). The complex is oxidized at +0.35 V (vs. saturated calomel electrode) at a platinum electrode to a Mn ₂ ^II,III complex. It catalyses the disproportionation of hydrogen peroxide in aqueous dimethylformamide. Based on ESR and visible spectroscopic studies, a catalytic mechanism involving the interconversion between Mn ^II Mn ^III (OH) and Mn ^II Mn ^IV (O) species is proposed.