Influence of the geometry around the manganese ion on the peroxidase and catalase activities of Mn(III)–Schiff base complexes

Abstract

The peroxidase and catalase activities of eighteen manganese–Schiff base complexes have been studied. A correlation between the structure of the complexes and their catalytic activity is discussed on the basis of the variety of systems studied. Complexes 1– 18 have the general formulae [MnL n(D) 2](X)(H 2O/CH 3OH) m, where L n = L 1–L 13; D = H 2O, CH 3OH or Cl; m = 0–2.5 and X = NO 3 −, Cl −, ClO 4 −, CH 3COO −, C 2H 5COO − or C 5H 11COO −. The dianionic tetradentate Schiff base ligands H 2L n are the result of the condensation of different substituted (OMe-, OEt-, Br-, Cl-) hydroxybenzaldehyde with diverse diamines (1,2-diaminoethane for H 2L 1–H 2L 2; 1,2-diamino-2-methylethane for H 2L 3–H 2L 4; 1,2-diamino-2,2-dimethylethane for H 2L 5; 1,2-diphenylenediamine for H 2L 6–H 2L 7; 1,3-diaminopropane for H 2L 8–H 2L 11; 1,3-diamino-2,2-dimethylpropane for H 2L 12–H 2L 13). The new Mn(III) complexes [MnL 1(H 2O)Cl](H 2O) 2.5 ( 2), [MnL 2(H 2O) 2](NO 3)(H 2O) ( 4), [MnL 6(H 2O) 2][MnL 6(CH 3OH)(H 2O)](NO 3) 2(CH 3OH) ( 8), [MnL 6(H 2O)(OAc)](H 2O) ( 9) and [MnL 7(H 2O) 2](NO 3)(CH 3OH) 2 ( 12) were isolated and characterised by elemental analysis, magnetic susceptibility and conductivity measurements, redox studies, ESI spectrometry and UV, IR, paramagnetic 1H NMR, and EPR spectroscopies. X-ray crystallographic studies of these complexes and of the ligand H 2L 6 are also reported. The crystal structures of the rest of the complexes have been previously published and herein we have only revised their study by those techniques still not reported (EPR and 1H NMR for some of these compounds) and which help to establish their structures in solution. Complexes 1– 12 behave as more efficient mimics of peroxidase or catalase in contrast with 13– 18. The analysis between the catalytic activity and the structure of the compounds emphasises the significance of the existence of a vacant or a labile position in the coordination sphere of the catalyst.