COORDINATION CHEMISTRY OF MANGANESE WITH 2,2'-BIPYRIDINE - SYNTHETIC ACCESS TO HIGH-VALENT POLYNUCLEAR OXOMANGANESE COMPLEXES OF BIOLOGICALRELEVANCE

Manganese is required in living systems to perform diverse redox trans formations, ranging from photosynthetic splitting of water to catalyti c disproportionation of hydrogen peroxide and dismutation of superoxid e radicals. Our knowledge about the function of Mn sites has been expa nding considerably over the past few years, thanks to extensive studie s using a variety of chemical and physical methods, but little is know n about their structural motifs. This has engendered numerous studies of low molecular weight Mn complexes that provide reasonable and likel y structural proposals. The coordination chemistry of Mn(III)/Mn(IV) c omplexes in aqueous media with the ligand 2,2'-bipyridine (bpy) is rev iewed with special emphasis on the redox and structural transformation s of the mu-oxo-bridged dimeric unit in different oxidation states. A variety of oxomanganese complexes in the di-, tri-, and tetranuclear f orms can be isolated by facile proton-regulated transformations, which are of particular relevance to the water oxidation center in green pl ant photosystem II (PSII). Thus, a proton induced two-electron oxidati ve charge storage on the Mn(III,III) dimer, [Mn2O(O2CCH3)2(H2O)2(bpy)2 ]2+, which leads to the formation of a Mn(IV,IV) complex, [Mn2O2(O2CCH 3)(H2O)2(bpy)2]3+, that retains two water molecules on the dimeric cen ter, provides an example of stabilized Mn-aqua and Mn-carboxylate bond s with respect to oxidation at the metal center. The formation of a tr imeric Mn(IV,IV,IV) complex, [Mn3O4(H2O)2(bpy)4]4+, from the hydrolysi s of Mn(IV,IV) complex accompanies self-liberation of protons that are essential for the stabilization of the increased nuclearity oxo clust er. The oxygen evolving center in PSII is believed to involve a tetran uclear Mn site of low symmetry with at least two di-mu-oxo units. The isolation and structural characterization of the bent-linear tetrameri c complex [Mn4O6(bpy)6]4+, which self-assembles in aqueous medium and consists of three di-mu-oxo Mn2(IV,IV) units in an open structure with a cis arrangement of the terminal Mn2O2 units with respect to the cen tral dimanganese cors, is reported. The cis arrangement of two bridgin g oxo atoms is discussed in connection with photosynthetic O-O bond fo rmation step.