Syntheses and structural characterizations of cobalt(II) complexes with N4-donor Schiff base ligands: Influence of methyl substitution on structural parameters and on phenoxazinone synthase activity

Abstract

The present report deals with the syntheses, crystal structures and phenoxazinone synthase mimicking activity of three new mononuclear complexes, [Co(L1)Cl(H2O)]Cl·H2O (1), [Co(L1)(NCS)2] (2) and [Co(L2)Cl2] (3), derived from the tetradentate ligands (L1=N,N′-bis(pyridin-2-ylmethylene)-2,2-dimethylpropane-1,3-diamine and L2=N,N′-bis(6-methylpyridin-2-ylmethylene)-2,2-dimethylpropane-1,3-diamine). Single crystal X-ray crystallography reveals that the geometry of the cobalt(II) center in these complexes is found to be distorted-octahedron in which the equatorial positions are occupied by tetradentate Schiff base ligands and remaining two apical positions are bonded to either solvent molecule or counter ion. The crystal packing of the complexes is mainly stabilized by the hydrogen bonding and π–π stacking interactions. All the complexes show moderate phenoxazinone synthase activity and their comparative catalytic activity has been described. The effect of methyl substitution in pyridine ring (in 3) on the structural factor and on the phenoxazinone synthase activity has been explored. The ESI-MS positive spectrum of a mixture of the representative complex (1 or 2) and o-aminophenol shows a peak corresponding to [Co(L1)(OAP)]+ (OAPH=o-aminophenol) that supports the complex substrate adduct formation in the catalytic cycle.