Functional model for catecholase-like activity: A mechanistic approach with manganese (III) complexes of salen type Schiff base ligands

Abstract

Three new Mn (III) complexes [MnL¹(OOCH)(OH₂)] (1), [MnL²(OH₂)₂][Mn₂L₂²(NO₂)₃] (2) and [Mn₂L₂¹(NO₂)₂] (3) (where H₂L¹ = H₂Me₂Salen = 2,7-bis(2-hydroxyphenyl)-2,6-diazaocta-2,6-diene and H₂L² = H₂Salpn = 1,7-bis(2-hydroxyphenyl)-2,6-diazahepta-1,6-diene) have been synthesized. X-ray crystal structure analysis reveals that 1 is a mononuclear species whereas 2 contains a mononuclear cationic and a dinuclear nitrite bridged (μ-1κO:2κO′) anionic unit. Complex 3 is a phenoxido bridged dimer containing terminally coordinated nitrite. Complexes 1–3 show excellent catecholase-like activity with 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate. Kinetic measurements suggest that the rate of catechol oxidation follows saturation kinetics with respect to the substrate and first order kinetics with respect to the catalyst. Formation of bis(μ-oxo)dimanganese (III,III) as an intermediate during the course of reaction is identified from ESI-MS spectra. The characteristic six line EPR spectra of complex 2 in the presence of 3,5-DTBC supports the formation of manganese (II)-semiquinonate as an intermediate species during the catalytic oxidation of 3,5-DTBC.