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

A dinuclear manganese(II) complex [Mn2L4,4(O2CMe)(2)] has been prepared and its structure, properties and catalase-like function studied in comparison with the analogous [Mn2Lm,n(O2CMe)(2)] [m,n=2,3; 2,4; or 3,3; (L-m,L-n)(2-) denotes macrocycles containing two 2,6-bis(iminomethyl)-4-methylphenolate entities bridged through two lateral chains, (CH2)(m) and (CH2)(n), at the imino nitrogens]. In the centrosymmetric [Mn2L4,4(O2CMe)(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) Angstrom. 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 (MnMnT)-Mn-II-T-III(OH) and (MnMnIV)-Mn-II(=O) species is proposed.