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

Two mononuclear Mn complexes [Mn(3,5-Fsalpn)(HO)][B(CH)]·2HO (1·2HO) and [Mn(3,5-Clsalpn)(OAc)(HO)]·HO (2·HO), where Hsalpn=1,3-bis(salicylidenamino)propane, have been prepared and characterized. The crystal structure of 1·HO 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 [Mn(3,5-X-salpn)(μ-O)] in which each X-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 HO disproportionation through a cycle involving Mn and Mn dimers, unlike related complexes with a more rigid and smaller chelate ring, which employ Mn/MnO monomers.