Synthesis and characterisation of dimanganese(iii) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- or 5-methoxy) and their catalytic activity towards hydrogen peroxide disproportionation

Abstract

Structural, magnetic, electrochemical, EPR, 1H NMR, and H2O2 reactivity studies are reported for [MnIII2(X-salpentO)(μ-AcO)(μ-MeO)(MeOH)2]Br (1: X = 5-OMe and 2: X = 3-OMe; salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol). The X-ray diffraction analysis of 2 reveals that the manganese atoms possess an axially elongated-octahedral geometry. The elongation is perpendicular to the bridging plane resulting in a short Mn⋯Mn distance of 2.9278(6) A. The ligand lies in the meridional plane and the sixth co-ordination position of each manganese atom is occupied by a methanol molecule providing two substitution-labile sites in cis-position. Solution structural studies indicate that complexes 1 and 2 are stable in methanol and dmf, even in the presence of water, and the solid state structure is retained in solution. The two complexes show catalytic activity toward disproportionation of H2O2 in methanol and dmf at 25 °C. The initial rate of oxygen evolution in the presence of 1 or 2 is first order in [catalyst] and show saturation kinetics with [H2O2]. Complexes 1 and 2 dismutate H2O2 without a kinetic lag phase and are implicated as the active form of the reduced catalyst. Comparison to other dimanganese complexes of this family reveals that the methoxy phenyl-ring substituted derivatives are more effective at reducing H2O2 but are less efficient at binding the H2O2 substrate (higher KM) than the 5-bromo or the unsubstituted complexes.