Exclusive selectivity of multidentate ligands independent on the oxidation state of cobalt: influence of steric hindrance on dioxygen binding and phenoxazinone synthase activity

Abstract

The present report describes the syntheses, characterizations, crystal structures and study of the phenoxazinone synthase activity of two peroxo-bridged dicobalt(III) complexes, [Co₂(L¹)₂(μ-O₂)](ClO₄)₄·2CH₃CN (1) and [Co₂(L²)₂(μ-O₂)](ClO₄)₄ (2), and three mononuclear cobalt(II) complexes, [Co(L³)(CH₃CN)](ClO₄)₂ (3), [Co(L⁴)(H₂O)](ClO₄)₂ (4) and [Co(L⁵)(H₂O)](ClO₄)₂ (5), derived from the pentadentate ligands L¹–L⁵, which are the 1 : 2 condensation products of triamines and 2-acetylpyridine or 2-pyridinecarboxaldehyde (6-methyl-2-pyridinecarboxaldehyde). X-ray crystallography reveals exclusive selectivity of the acyclic Schiff dibasic form of the ligands over the heterocyclic analogues, and this selectivity is found to be insensitive to the oxidation state of cobalt. Other first row transition metals have been characterized in either form of the ligands in their complexes but it is specific for cobalt established in the present study. The pronounced effect of the methyl substitutions is observed from their crystal structures; substitution at imine-C does not have any significant influence on the peroxo-bridging but substitution at sixth position of pyridyl ring prevents the formation of peroxo-bridging, and both the steric and electronic factors play vital roles on such chemical diversity. All the complexes show the phenoxazinone synthase mimicking activity and the comparative catalytic activity has been explored. Although electrochemical behaviors of all the complexes are very similar, their relative catalytic activity mimicking the function of phenoxazinone synthase arises from the electronic and steric factors of the methyl substitution.