A dinuclear manganese(II) complex with the {Mn2(μ-O2CCH3)3}+ core:: Synthesis, structure, characterization, electroinduced transformation, and catalase-like activity

Reactions of Mn-parallel to(PF6)(2) and Mn-parallel to(O2CCH3)(2).4H(2)O with the tridentate facially capping ligand NN-bis(2-pyridylmethyl)ethylamine (bpea) in ethanol solutions afforded the mononuclear [Mn-parallel to(bpea)](PF6)(2) (1) and the new binuclear [Mn-2(parallel to,parallel to)(mu-O2CCH3)(3)(bpea)(2)](PF6) (2) manganese(II) compounds, respectively. Both 1 and 2 were characterized by X-ray crystallographic studies. Complex 1 crystallizes in the monoclinic system, space group P2(1)/n, with a = 11.9288(7) Angstrom, b = 22.5424(13) Angstrom, c =13.0773(7) Angstrom, alpha = 90degrees, beta = 100.5780(10degrees), gamma = 90degrees, and Z = 4. Crystals of complex 2 are orthorhombic, space group C222(1), with a = 12.5686(16) Angstrom, b = 14.4059(16) Angstrom, c = 22.515(3) Angstrom, alpha = 90degrees, beta = 90degrees, gamma = 90degrees, and Z = 4 The three acetates bridge the two Mn(II) centers in a mu(1,3) syn-syn mode, with a Mn-Mn separation of 3.915 Angstrom. A detailed study of the electrochemical behavior of 1 and 2 in CH3CN medium has been made. Successive controlled potential oxidations at 0.6 and 0.9 V vs Ag/Ag+ for a 10 mM solution of 2 allowed the selective and nearly quantitative formation of [Mn-2(III)(mu-O)(mu-O2CCH3)(2)(bpea)(2)](2+) (3) and [Mn-2(IV)(mu-O)(2)(mu-O2CCH3)(bpea)(2)](3+) (4), respectively. These results have shown that each substitution of an acetate group by an oxo group is induced by a two-electron oxidation of the corresponding dimanganese complexes. Similar transformations have been obtained if 2 is formed in situ either by direct mixing of Mn2+ cations, bpea ligand, and CH3COO- anions with a 1:13 stoichiometry or by mixing of 1 and CH3COO- with a 1:1.5 stoichiometry. Associated electrochemical back-transformations were investigated. 2, 3, and the dimanganese [(MnMnIV)-Mn-III(mu-O)(2)(mu-O2CCH3)(bpea)(2)](2+) analogue (5) were also studied for their ability to disproportionate hydrogen peroxide. 2 is far more active compared to 3 and 5. The EPR monitoring of the catalase-like activity has shown that the same species are present in the reaction mixture abeit in slightly different proportions. 2 operates probably along a mechanism different from that of 3 and 5, and the formation of 3 competes with the disproportionation reaction catalyzed by 2. Indeed a solution of 2 exhibits the same activity as 3 for the disproportionation reaction of a second batch of H2O2 indicating that 3 is formed in the course of the reaction.