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

Abstract

The structure of the manganese cluster in photosystem II has not yet been elucidated. It is well established that some of the Mn ions are separated by 2.7 A distances. The same is true for a high valent form of a dinuclear manganese catalase. The most common structural motif in Mn model compounds that achieves this short separation is an edge-shared (MnO) $\sb2$ unit. However, neither water oxidation nor catalase activity have been previously demonstrated for complexes of this type. Before it is possible to assess proposed catalytic roles for the bridging oxides in such structures, the pathways of formation, decomposition and reactivity must be better understood. Formation of (Mn(IV)(SALPN)(O)) $\sb2$, 1, was examined using peroxidic oxidants and well defined Mn(III) precursors. Hydrogen peroxide reacts with Mn(III)(SALPN)(AcAc), (Mn(III)(SALPN)(OMe)) $\sb2$ or Mn(III)$\sb2$(SALPN)$\sb3$ to give 1 in quantitative yields with direct incorporation of both peroxidic oxygens into the bis-$\mu$-oxo unit. Isotopic labeling studies show that t-ButylOOH also reacts but follows a different pathway. The hydrogen peroxide reaction is suggested to proceed via a (Mn(III)(SALPN)) $\sb2$(O$\sb2\sp{-2}$) dimer while a Mn(V)O is implicated in the ROOH oxidation. Efficient catalase activity was demonstrated for 1 in contrast to other bis-$\mu$-oxo dimers. Isotopic labeling studies show bridge exchange which incorporates both oxygens from one hydrogen peroxide molecule into the catalyst and oxygen formation without O-O bond cleavage. Reversible protonation studies suggest increased basicity in the oxide bridges of 1 compared to other bis-$\mu$-oxo dimers. An empirical correlation is established between Mn-Mn distance and Mn-O-Mn bridging angle which is useful for interpreting metal-metal distances obtained from EXAFS of manganoenzymes. Also, a new Mn(III/IV) model dimer is reported which shows temperature dependent interconversion between a g = 2 multiline EPR signal and a high field feature interpreted as arising from an S = 3/2 spin level.