Synthesis of Mn3O4 Nanozymes from Structurally Characterized Phenoxazinone Synthase Models Based on Manganese(iii) Schiff Base Complexes.

Three mononuclear and one hexanuclear manganese(iii) complexes, [Mn(L)(H2O)2]·Cl (1), [Mn(L)(H2O)2]·Br (2), [Mn(L)(H2O)2]·NO3 (3), and [Mn6(L)6(NCS)6] (4), have been synthesized using a Schiff-base ligand, namely (E)-2-((3-(2-hydroxyethylamino)propylimino)methyl)phenol (H2L), and structurally characterized by the usual physicochemical techniques such as UV-Vis, FT-IR, ESI-MS, EPR and single crystal XRD. The structure of complex 4 is unique among all four complexes as the sixth coordination position of manganese is fulfilled by the oxygen atom of a neighbouring unit by covalent interaction. The phenoxazinone synthase like activity of all four complexes has been thoroughly investigated using three different substrates, o-aminophenol (OAP), 2-amino-4-methylphenol (MAP) and 3-amino-4-hydroxybenzoic acid (CAP). All complexes were found to be active towards catalysis and complex 4 showed the highest activity. The EPR study reveals that the oxidative dimerization of the substrates occurred through metal centered redox participation rather than a radical formation pathway. The experimental observations have been supported by DFT calculations to put forward the most probable mechanistic pathways operating in the catalytic cycle. Moreover, Mn3O4 nanoparticles (NPs) having two different morphologies have been synthesized using complexes 1 and complex 4 simply by calcination, respectively, with the aim to prepare nanozymes. These two synthesized NPs were also able to show phenoxazinone synthase like activity and thus complexes 1 and 4 can be claimed as precursors of nanozymes.