Solvent and structural effects on catalase-like function of binuclear manganese(II) compounds with μ-phenoxide bridge

The structures and ESR spectral properties of binuclear manganese(II) compounds which functionally mimic the bacterial dimanganese catalase enzyme have been developed. These compounds are formulated as [(L¹)Mn₂X₂]Y(X=Cl, Br, CH₃COO, C₆H₅COO, (C₆H₅)₂HCCOO; Y=ClO₄, PF₆), [(L²)Mn₂X₃] (X=Cl, Br), and [(L²)Mn₂(benzoate)₂ (NCS)], where H(L¹)=2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methyl-phenol and H(L²)=2,6-bis[N-(2-dimethylamino)ethyl]iminomethyl]-4-methyl-phenol. An X-ray diffraction structure determination of [(L¹)Mn₂Cl₂(CH₃OH) ₂]PF₆ confirmed the anticipated N₆O septadentate coordination of the (L¹)-ligand, binuclear manganese(II) complex with μ-phenoxide, and two halogenide ions and two methanol molecules. In the crystal of [(L²)Mn₂Cl₃]CH₃CN, there exists tetranuclear manganese(II) complex having center of symmetry. The terminal and central manganese(II) ions are bridged by phenoxide oxygen and a chloride ion (Mn⋯Mn = 3.440(1) Å), and two central atoms are bridged by two chloride ions with Mn⋯Mn distance of 3.804(1) Å. In acetonitrile solution, catalase-like function of the manganese(II) compounds with H(L¹)-ligand are found to be highly dependent the anions coordinating the Mn(II) ion; the activity of the X=Cl complex is much higher than those of the acetate and benzoate. On the other hand, the activity of [(L²)Mn₂Cl₃] is slightly lower than that of the [(L²)Mn₂(benzoate)₂(NCS)], where two manganese ions are bridged by benzoate anion. The Mn(II/III) complex of H(L¹), [(L¹)Mn₂(CH₃COO)₂]²⁺ exhibits much higher activity than that of the corresponding Mn(II/III) complex, [(L¹)Mn₂(CH₃COO)₂]⁺. In dimethylformamide(dmf) solutions, an induction period (at least 30 minutes) is necessary before vigorous evolution of oxygen gas occurs for both the Mn(II) complexes with (L¹) and (L²), however, some binuclear Mn₂(II/III) and Mn₂(III/III) complexes exhibited high catalase-like function in dmf. The latter fact may give a reasonable explanation for the fact that the manganese cluster in S₂-state of the Photosystem II shows a high catalase-like function instead of the negligible action in the S₁-state. Based on these facts, it is concluded that the most important factor to control the catalase-like function of binuclear Mn(II) complex should be the oxidizable nature of the Mn(II) ion to a higher oxidation state, and the unusual behavior in dmf solution was accounted for on the assumption that dmf molecule may contribute to the activation of peroxide ion through interaction with an Mn(II)-peroxide adduct, leading to oxidation of the Mn(II) ion.