Synthesis, characterization and antioxidant activity of water soluble MnIII complexes of sulphonato-substituted Schiff base ligands

Two new MnIII complexes Na[Mn(5-SO3-salpnOH)(H2O)] ⋅ 5H2O (1) and Na[Mn(5-SO3-salpn)(MeOH)] ⋅ 4H2O (2) (5-SO3-salpnOH = 1,3-bis(5-sulphonatosalicylidenamino)propan-2-ol, 5-SO3-salpn = 1,3-bis(5-sulphonatosalicylidenamino)propane) have been prepared and characterized. Electrospray ionization-mass spectrometry, UV–visible and 1H NMR spectroscopic studies showed that the two complexes exist in solution as monoanions [Mn(5-SO3-salpn(OH))(solvent)2]−, with the ligand bound to MnIII through the two phenolato-O and two imino-N atoms located in the equatorial plane. The E1/2 of the MnIII/MnII couple (−47.11 (1) and −77.80 mV (2) vs. Ag/AgCl) allows these complexes to efficiently catalyze the dismutation of O2-, with catalytic rate constants 2.4 × 106 (1) and 3.6 × 106 (2) M−1 s−1, and IC50 values of 1.14 (1) and 0.77 (2) μM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH 7.8. The two complexes are also able to disproportionate up to 250 equivalents of H2O2 in aqueous solution of pH 8.0, with initial turnover rates of 178 (1) and 25.2 (2) mM H2O2 min−1 mM−1 catalyst−1. Their dual superoxide dismutase/catalase activity renders these compounds particularly attractive as catalytic antioxidants.

Graphical abstract1,3-Bis(5-sulphonatosalicylidenamino)propan-2-ol and 1,3-bis(5-sulphonatosalicylidenamino)propane afford mononuclear Mn complexes with the ligand bound to MnIII through the two phenolato-O and two imino-N   atoms located in the equatorial plane. These complexes are able to efficiently catalyze the dismutation of O2- and disproportionate up to 250 equivalents of H2O2 in aqueous solution of pH 8.0.Figure optionsDownload full-size imageDownload as PowerPoint slide