Synthesis, structure and catalase-like activity of dimanganese(iii) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

Abstract

The diMn^III complexes [Mn₂(5-Me-salpentO)(µ-MeO)(µ-AcO)(H₂O)Br] (1) and [Mn₂(3-Me-salpentO)(µ-MeO)(µ-AcO)(MeOH)₂]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(µ-alkoxo)(µ-acetato) triply-bridged diMn^III core with an Mn⋯Mn separation of 2.93–2.94 Å, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H₂O₂, with first-order dependence on the catalyst, and saturation kinetics on [H₂O₂], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H₂O₂ without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn^II species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn₂^III oxidation state during cycling. The correlation between log(k_cat) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H₂O₂ dismutation by these complexes converge at a catalytic cycle that involves the Mn₂^III and Mn₂^IV oxidation states.