Dimanganese complexes of a septadentate ligand. Functional analogues of the manganese pseudocatalase

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 E_p = 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 Mn^III 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.