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.