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 [Mn^III₂(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) Å. 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 K_M) than the 5-bromo or the unsubstituted complexes.