Effect of solvent and auxiliary ligand on catecholase activities of Mn(III) complexes: A comparative study

A mononuclear Mn(III) complex with composition [MnIIIL(Cl)] (1) has been utilized to prepare two new complexes, [MnIIIL(N3)(CH3OH)] (2) and [MnIIIL(μ1,3-NCS)]n (3), by substitution of the auxiliary ligand. The ligand utilized for preparation of 1 is a Schiff base (H2L), obtained by 2:1 condensation of 3-Chlorosalicylaldehyde and ethylene diamine in methanol. The chloride anion, present in the coordination sphere of the central metal ion in 1 has been replaced by two other auxiliary ligands, azide and thiocyanate, in 2 and 3, respectively. All of the three complexes behaved as bio-functional models of catechol oxidase toward oxidation of 3,5-di-tert butyl catechol (3,5-DTBC). A detailed kinetic investigations were carried out for 2 and 3 in methanol, acetonitrile and N,N′-Dimethyl formamide (DMF) separately and the corresponding turn-over numbers have been determined. The ESI-MS spectra of the substrate-catalyst mixtures in acetonitrile and methanol confirmed formation of catecholate bound Mn(III) intermediate in the mass spectral time scale. The progress of the catechol oxidation was monitored by 1H-NMR spectroscopy with the reaction mixtures containing complex-substrate ratio at 1:20 after stirring in acetonitrile as well as methanol at room temperature in open air. NMR spectral data confirmed completion of the process in 90 minutes and 45 minutes for 2 and 3, respectively. Notably, in absence of any of these complexes, aerobic oxidation of 3,5-DTBC to the corresponding quinone is negligible even after twenty four hours of mixing in the same solvent. This observation is a clear indicative of high catalytic efficiencies of the prepared complexes.