Insight into the origin of catechol oxidase activity in a rare MnII/MnIII mixed valence ion pair complex: An account of comparative biomimetic catalytic study

Abstract

A mixed valence ion pair complex, [MnII(ntb)Cl][MnIII(Br4Cat)2(H2O)2]·4H2O (2) and a manganese(III) compound, (Et3NH)2[Mn(Br4Cat)2(H2O)2][Mn(Br4Cat)2(H2O)(pm)]·H2O (3), where ntb is tris(2-benzimidazolylmethyl)amine, Br4CatH2 is tetrabromocatechol and pm is pyrimidine, have been synthesized and the solid state structures were determined by X-ray crystallography. Compound 2 was prepared by the reaction of [Mn(ntb)Cl]Cl·2.5MeOH (1) and tetrabromocatechol in aerobic condition, and is a rare example of complex ion pair in the class of metal-dioxolene chemistry. Compound 2 exhibits strong catecholase activity in which both Mn(II) and Mn(III) centers are catalytically active as suggested by the reactivity study of compounds 1 (identical to the cationic part of 2) and 3 (comprising an anionic part similar as in 2). The mechanistic investigation discloses that Mn(III) center in complex 3 and in anionic part of complex 2 oxidises 3,5-DTBCH2 and itself reduces to Mn(II) center with concomitant production of 3,5-DTBQ and H2O2. Whereas both dioxygen and substrate bound Mn(II) center in cationic part of 2 and in compound 1 is the active species in the catalytic cycle in which complete reduction of molecular oxygen to H2O occurs. Although some mixed valence manganese(II/III) complexes are known to exhibit catechol oxidase, the present report only provides the role of individual metal ions in a mixed valence manganese(II/III) system to the mechanistic pathways of functioning catechol oxidase activity.