Synthesis, crystal structure, quantum chemical calculations, electrochemistry and electro-catalytical properties as cytochrome P-450 model of tetradentate Mn(III)-Schiff base complex

Abstract

The tetradentate Schiff base ligand has been obtained from condensation with mixing ethylenediamine and 2 mmoles of 5-methoxy-2-hydroxybenzaldehyde in absolute ethanol H₂L. To the ethanolic solution was added manganese(II)acetate tetrahydrated and lithium chloride (LiCl) to obtain the tetradentate manganese(III) Schiff base complex [Mn(III)(Cl)L]. The prepared compounds have been characterized by several spectroscopic techniques such as elemental analyses, FT-IR, UV–vis., ¹H NMR and HRMS. In this paper, the X-ray diffraction (XRD) and the computational studies (DFT) of the ligand (H₂L) with its manganese(III)-Schiff base complex [Mn(III)(Cl)L] are described and confirmed the given molecular structures. The crystallographic studies have been utilized toelucidate the kinetics, selectivity and stereochemistry of the transferred oxygen atomsto the substrate molecules when the considered complex is used as catalyst according thecytochrome P450 model. In addition, the density functional theory (DFT) calculation with B3LYP/6-31G(d,p) level is performed to obtain the optimized geometries and electronic properties of the prepared compounds. The global reactivity parameters have also been calculated using the energies of frontier molecular orbitals suggesting that the ligand H₂L is more stable than its Mn(III) complex. This may be due to the presence of hydrogen bonds in the ligand and the weaker energies of coordination bonds in the complex. The electrochemical behaviour of Mn(III)(Cl)L has been studied by cyclic voltammetry in acetonitrile solutions at room temperature. The resulting cyclic voltammogram shows Mn(III)/Mn(II) couple at E_1/2= -0.62V with glassy carbon (GC) electrode. This redox couple is involved in the electrocatalytic cycle where the manganese(III) cation is successively mono-electronated until the formation of superoxo intermediates and then the oxo species, respectively. These oxo forms, generated in situ, transfer their oxygen atoms to the substrate giving the oxidized product. So, the chemical and electrochemical reactions, implicated in this electrocatalytical process, obey to the biomimetic oxidation reactions as those of monooxygenase enzymes (Cytochrome P450).