Formation, stability and catalase-like activity of mononuclear manganese(ii) and oxomanganese(iv) complexes in protic and aprotic solvents

Abstract

Synthetic compounds as biomimics of catalase enzymes may have potential biomedical applications as therapeutic agents for the detoxification of H₂O₂ and may give further information about the role of metal cofactors during the dismutation process. In this work, we present the synthesis and characterization of manganese(II) and oxomanganese(IV) complexes of a pentadentate ligand (N4Py* = N,N-bis(2-pyridylmethyl)-1,2-di(2-pyridyl)ethylamine), [Mn^II(N4Py*)(CH₃CN)](CF₃SO₃)₂ (1) and [Mn^IV(O)(N4Py*)](CF₃SO₃)₂ (2). Detailed pH-potentiometric titrations were also performed in order to gain insights into the complexation properties of the N4Py* ligand, and the equilibrium model used for fitting the pH-potentiometric titration data for the [Mn(N4Py*)]²⁺ complex was confirmed by ¹H-NMR relaxometry at 0.49 T field strength and 25 °C. Complex 1 has been shown to catalyze the dismutation of H₂O₂ into O₂ and H₂O in both aprotic (MeCN and DMF) and protic (TFE and buffered H₂O) solvents. The reactivity of 1 was higher in protic solvents, which was markedly influenced by the pH, and it revealed a Michaelis–Menten behaviour at pH 10.5 at 298 K. A mononuclear oxomanganese(IV) complex, 2, as a possible intermediate in the catalytic process, was also prepared, and its stability and reactivity towards H₂O₂ was also investigated in TFE and buffered H₂O/TFE solutions. The stoichiometric oxidation of H₂O₂ with 2 provided clear evidence (solvent isotope effect (SIE) of 6.2, pH dependence, saturation kinetics, etc.) of the rate-determining hydrogen atom transfer (HAT) mechanism between the H₂O₂ and oxomanganese(IV) species. The present results provide one of the first examples of a non-heme Mn^IVO complex that shows a catalase-like reaction in water.