Influence of the first and second coordination spheres on the diverse phenoxazinone synthase activity of cobalt complexes derived from a tetradentate Schiff base ligand

Abstract

This paper describes syntheses and structural characterization of four new cobalt(III) compounds (1–4) derived from a N₃O donor Schiff base ligand, a condensation product of N,N-dimethyldipropylenetriamine and o-vanillin, and their catalytic activity relating to the function of phenoxazinone synthase. X-ray crystallography reveals that the Schiff base can coordinate the metal centre either in a tetradentate fashion through the monoanionic deprotonated form using all four donor sites (in 1 and 2) or in a tridentate fashion using the zwitterionic form of the Schiff base ligand, leaving the quaternary amine nitrogen free from coordination (in 3). The monoanionic deprotonated ligand can also bind the metal centre in a tridentate fashion where the pendent tertiary amine nitrogen is engaged in intramolecular hydrogen bonding in 4. Moreover, the triamine part of the Schiff base ligand can bind a metal centre both facially and meridionally. Therefore, all these versatilities associated with this triamine make it appealing for the development of coordination chemistry with diverse structures. All complexes are active functional models for phenoxazinone synthase, and as expected the availability of labile sites at the first coordination sphere for the substrate, o-aminophenol, binding is responsible for higher catalytic activity in 1 and 2. The importance of a proton abstraction site at the second coordination sphere behind the facile oxidation of the substrate is also explored (reactivity of 3vs.4). The remarkable finding from the mass spectral study discloses several important intermediates, and thereby provides significant information relating to the mechanistic pathway of the functioning phenoxazinone synthase activity of the synthetic models.