Insights into Second-Sphere Effects on Redox Potentials, Spectroscopic Properties, and Superoxide Dismutase Activity of Manganese Complexes with Schiff-Base Ligands

Six Mn-Schiff base complexes, [Mn(X-salpn)]0/+ (salpn = 1,3-bis(sal-ic-ylidenamino)propane, X = H [1], 5-Cl [2], 2,5-F2 [3], 3,5-Cl2 [4], 5-NO2 [5], 3,5-(NO2)2 [6]), were synthesized and characterized in solution, and second-sphere effects on their electrochemical and spectroscopic properties were analyzed. The six complexes catalyze the dismutation of superoxide with catalytic rate constants in the range 0.65 to 1.54 × 106 M–1 s–1 obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutases assay, in aqueous medium of pH 7.8. In solution, these compounds possess two labile solvent molecules in the axial positions favoring coordination of the highly nucleophilic O2•– to the metal center. Even complex 5, [Mn(5-(NO2)salpn) (OAc) (H2O)], with an axial acetate in the solid state, behaves as a 1:1 electrolyte in methanolic solution. Electron paramagnetic resonance and UV–vis monitoring of the reaction of [Mn(X-salpn)]0/+ with KO2 demonstrates that in diluted solutions these compl...