Manganese complexes as synthetic models for manganese peroxidase and the oxygen evolving complex.

Abstract

Manganese complexes with all-oxygen ligation were studied in relation to important biological processes: lignin degradation, photosynthesis and manganese transport. The $\alpha$-hydroxy acids provide three new structures that are useful for the study of the oxygen evolving complex (OEC) core and the mechanism of manganese peroxidase. Both bidentate (carboxylate/alkoxide) and monodentate (carboxylate) coordination is available through these ligands. The high oxidation state requires coordination of three alkoxide oxygens. NA$\sb2$ (Mn(HIB)$\sb3$).2MEOH is the first trigonally-compressed monomeric Mn(IV) complex with redox innocent-ligands. Two (Mn(HIB)$\sb3$) $\sp{2-}$ complexes are linked together by a sodium ion with face-sharing octahedra; this provides the shortest known Mn-Na distance of 2.97 A. Although the epr spectrum of this complex is closer to that of low field epr signal in the S$\sb2$ state of the OEC than previously made monomers, its spectrum is not symmetric and speaks against a monomeric Mn(IV) as the origin of the signal. Lactic acid formed the first tetragonally compressed manganese(III) dimer, TMA$\sb3$ (Mn$\sb2$(Lc)$\sb4$(HLc)).H$\sb2$O, due to bridging by the alkoxide oxygens. Na$\sb2$ (Mn(HEB)$\sb2$(HHEB)) has a chain structure with a carboxylate group bridging each two adjacent Mn(III) ions. Mn(III) complexes with malonic acid were structurally and magnetically characterized. NH$\sb4$Mn(malonate)$\sb{2}.$CH$\sb3$OH, a linear chain, and NaMn(malonate)$\sb{2}$.CH$\sb3$OH, a three-dimensional solid, where characterized. Linear chain manganese malonate complexes nicely fit a theoretical model for magnetic behavior which considers little interaction between adjacent Mn(III) ions along the chain. However, the three-dimensional solid deviates considerably. Mn complexes of $\alpha$-hydroxy acids and malonic acid oxidize vanillylacetone to pyruvaldehyde and vanillin; MN(II) is formed. A mechanism is proposed to interpret this result. Mn(III) oxidation of the substrate leads to an organic radical that undergoes rearrangement, hydrolysis and further oxidation by Mn(III) ions to form the products. Manganese(III) complexes of dihydroxamic acids are stable and behaves as monomers. They can be oxidized to the Mn(IV) analogs which are unstable and have high reduction potential. Manganese dihydroxamic acid complexes have properties similar to those of iron analogs which function as iron transport agents. The probable use of these acids as manganese transport agents is discussed.