MODELING THE MULTINUCLEAR REDOX-ACTIVE MANGANESE ENZYMES - SYNTHESIS,STRUCTURE, AND PROPERTIES OF A BIS(DINUCLEAR MN(III)-MU-OXO-BIS(MU-ACETATO)) COMPLEX

The tetranuclear Mn(III) complex [Mn(4)(III)L(2)(mu-O)(2)(mu-OAC)(4)[( ClO4)(4) . 3CH(3)NO(2) (L = a,alpha'-bis(bis(2-pyridylmethyl)amino)-m- xylene), 2, has been synthesized, and its structural, spectroscopic, a nd magnetic properties have been investigated. The kinetics of the rea ction of its precursor Mn(II) complex (formulated as [Mn(2)L(OAc)(2)(C H3OH)](ClO4)(2), 1, with O-2 that leads ito the formation of 2 have be en studied. Complex 2 crystallizes on the orthorhombic crystal system, space group Pnma, with the cell dimensions a = 24.755(5) Angstrom, b = 28.854(4) Angstrom, c = 12.495(2) Angstrom, and Z = 4. The structure shows four Mn(III) ions in a dimer of two dinuclear centers. Each din uclear center has two Mn(III) ions triply bridged by two acetate group s and one oxo group with three nitrogen donors coordinated, forming a distorted octahedral coordination. The two dinuclear units are related to each other by a mirror plane. Within a dinuclear unit, the two Mn( III) ions show coordination differences in bond angles and bond distan ces arising from differences in the Jahn-Teller and lattice distortion s of the octahedral coordination. Temperature-dependent magnetic susce ptibility studies show very weak magnetic coupling (J = -0.4 cm(-1)) b etween Mn(III) ions. The stoichiometry of the O-2/complex 1 reaction t o form complex 2, the EPR, molar conductance, and magnetic susceptibil ity studies, and the structural data for complex 2 all support the for mulation of complex 2 as having four Mn(III) ions. IR spectral studies of samples of complex 2 prepared by the reaction of complex 1 with is otopically pure O-16(2) and O-18(2) show that the bridging oxo group i s derived from dioxygen. Kinetic studies show that the reaction of the precursor Mn(II) complex 1 with O-2 is first order in the concentrati ons of both the dinuclear Mn(II) complex and O-2. The reactivity of th e Mn(II) complex 1 with O-2 is in contrast to the lack of any reaction with O(?)2 under ordinary conditions of the Mn(II) complexes of dinuc leating ligands with two bis(2-pyridylmethyl)amine donor groups connec ted by straight-chain hydrocarbons (C(3) to C(5)). The rigidity of the m-xylene group connecting the two donor sets in the ligand L of compl ex 1 may be responsible for the contribution of the critical entropic factor in favor of the reaction, which may account for this difference in reactivities. The overall reaction of complex 1 with O-2 to form c omplex 2 is the reverse of the reaction at the water-oxidizing complex of photosystem II (PS II). The four Mn/dioxygen, four-electron redox reaction feature of the reaction complex 1 + O-2 --> complex 2 is rele vant to the chemistry of the water-oxidizing complex. Also the dinucle ar unit structure and the UV-visible absorption spectrum of 2 are feat ures with relevance to the active site of manganese catalase.