Two new dimanganese(II) complexes have been prepared and characterized as the first functional analogues of the manganese pseudocatalase enzyme of L. plantarum (Beyer, W. F.; Fridovich, I. Biochemistry 1986, 24, 6420). These have the formulas Mn2(L)Cl3 (1) and Mn2(L)(OH)Br2 (2) in which Cl1- and OH-, respectively, serve as one of two bridging ligands, the other coming from the alkoxide group of the binucleating ligand N,N,N′,N′-tetrakis(2-methylenebenz-imidazolyl)-1,3-diaminopropan-2- ol (HL). The solution structure of these complexes has been characterized by EPR spectroscopy at both 34 and 9 GHz. This reveals the presence of two equivalent high-spin Mn(II) ions electronically coupled by a weak electron spin exchange interaction. Analysis of the axial zero-field splitting (D = -0.072 cm-1) of this spin S = 5 complex in terms of the magnetic dipole interaction between the two Mn ions yields a lower limit to their separation of 3.2 Å. Cyclic voltammetry reveals that three separable oxidation processes occur for 2 at Ep = 0.60 V (A), 0.80 V (B), and 1.03 V (C), while 1 exhibits only two oxidations: a reversible one-electron process at 0.57 V (A) analogous to 2 and a second oxidation at 1.18 V corresponding to B + C. The hydroxide bridge in 2 thus appears to stabilize the Mn(III) oxidation state relative to Mn(II) in comparison with the chloride bridge in 1. The binuclear complexes 1 and 2 decompose H2O2 catalytically with an initial rate for 1 proportional to [H2O2]2[Mn2(L)Cl3] 1, while mononuclear Mn(II) is ineffective. The mechanism proceeds through the initial formation of the μ-oxo-containing MnIII intermediate, [Mn2 III(L)(O)]Cl2, which is reduced by a second H2O2 to release O2. A similar mechanism could be operating in the manganese pseudocatalase enzyme of L. plantarum, which is known to contain two Mn(III) per subunit and thus may have a binuclear Mn site.
|Number of pages||7|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1987|
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