Influence of anions and solvents on distinct coordination chemistry of cobalt and effect of coordination spheres on the biomimetic oxidation of o-aminophenols

Abstract

The present work reports the synthesis and structural characterizations of five new cobalt complexes (1–5) resulting from a N3O donor ligand, a Schiff base condensation product of N,N-dimethyldipropylenetriamine and 3-ethoxysalicylaldehyde, and their catalytic activity for the aerobic oxidation of various substrates, namely o-aminophenols and catechol. X-ray structural studies reveal that the Schiff base ligand can bind the metal centre either in the tetradentate fashion using all the available donor sites of the monoanionic deprotonated form (in 2 and 3) or in the tridentate fashion using the zwitterionic form of the Schiff base ligand with pendent quaternary amine nitrogen (in 1 and 4). Additionally, the monoanionic deprotonated ligand can bind the metal centre in the tridentate fashion (in 4 and 5) where the pendent tertiary amine nitrogen is involved in the intramolecular hydrogen bonding. All these adaptabilities associated with this triamine make it appealing candidate for exploration of the coordination chemistry with diverse structures. All complexes except compound 1 are efficient 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 the governing factor for higher catalytic activity in 2 and 3. Requisite of the hydrogen bond acceptor centre as well as proton abstraction centre at the second coordination sphere behind the facile oxidation of the substrate were also explored (reactivity of 1versus4 and 5). Moreover, the broader catalytic ability of these complexes was examined with substituted aminophenol and catechol as substrates, and the results were assembled and analysed. Furthermore, emphasis was given to get insight into the mechanistic pathway of functioning phenoxazinone synthase activity, well supported by the mass spectral study.