Dimerization, redox properties and antioxidant activity of two manganese(III) complexes of difluoro- and dichloro-substituted Schiff-base ligands

Two mononuclear MnIII complexes [Mn(3,5-F2salpn)(H2O)2][B(C6H5)4]·2H2O (1·2H2O) and [Mn(3,5-Cl2salpn)(OAc)(H2O)]·H2O (2·H2O), where H2salpn=1,3-bis(salicylidenamino)propane, have been prepared and characterized. The crystal structure of 1·H2O shows that this complex forms μ-aqua dimers with a short Mn⋯Mn distance of 4.93Å. Under anaerobic conditions, the two complexes are stable in solution and possess trans-diaxial symmetry with the tetradentate Schiff base ligand symmetrically arranged in the equatorial plane. When left in air, these complexes slowly dimerize to yield high-valent [MnIV2(3,5-X2-salpn)2(μ-O)2] in which each X2-salpn ligand wraps the two Mn ions. This process is favored in basic medium where the deprotonation of the bound water molecule is concomitant with air oxidation. The two complexes catalyze the dismutation of superoxide (superoxide dismutase (SOD) activity) and peroxide (catalase (CAT) activity) in basic medium. The phenyl-ring substituents play an important role on the CAT reaction but have little effect on SOD activity. Kinetics and spectroscopic results indicate that 1 and 2 catalyze H2O2 disproportionation through a cycle involving MnIII2 and MnIV2 dimers, unlike related complexes with a more rigid and smaller chelate ring, which employ MnIII/MnVO monomers.