A New Mixed-Valence Mn(II)Mn(III) Compound With Catalase and Superoxide Dismutase Activities

Abstract

The synthesis, X-ray molecular structure, physico-chemical characterization and dual antioxidant activity (catalase and superoxide dismutase) of a new polymeric mixed valence Mn(III)Mn(II) complex, containing the ligand H₂BPClNOL (N-(2-hydroxybenzyl)-N-(2-pyridylmethyl)[(3-chloro)(2-hydroxy)] propylamine) is described. The monomeric unit is composed of a dinuclear Mn(II)Mn(III) moiety, [Mn(III)(μ-HBPClNOL)(μ-BPClNOL)Mn(II)(Cl)](ClO₄)·2H₂O, <b>1</b>, in which the Mn ions are connected by two different bridging groups provided by two molecules of the ligand H₂BPClNOL, a phenoxide and an alkoxide group. In the solid state, this mixed valence dinuclear unit is connected to its neighbors through chloro bridges. Magnetic measurements indicated the presence of ferromagnetic [<i>J</i> = +0.076(13) cm^-1] and antiferromagnetic [<i>J</i> = -5.224(13) cm^-1] interactions. The compound promotes O <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mrow></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mo>•</mml:mo> <mml:mo>-</mml:mo></mml:mrow> </mml:msubsup> </mml:math> dismutation in aqueous solution (IC₅₀ = 0.370 μmol dm^-3, <i>k</i> _cat = 3.6x10⁶ M^-1 s^-1). EPR studies revealed that a high-valent Mn(III)-O-Mn(IV) species is involved in the superoxide dismutation catalytic cycle. Complex <b>1</b> shows catalase activity only in the presence of a base, e.g., piperazine or triethylamine. Kinetic studies were carried out in the presence of piperazine and employing two different methods, resulting in <i>k</i> _cat values of 0.58 ± 0.03 s^-1 (detection of O₂ production employing a Clark electrode) and 2.59 ± 0.12 s^-1 (H₂O₂ consuption recorded via UV-Vis). EPR and ESI-(+)-MS studies indicate that piperazine induces the oxidation of <b>1</b>, resulting in the formation of the catalytically active Mn(III)-O-Mn(IV) species.