Synthesis and reactivity of the high-valent monomeric and dimeric manganese complexes.

Abstract

Photosynthetic water oxidation to form dioxygen occurs using a manganese cluster in the oxygen evolving complex (OEC). The higher oxidation states (S₃ and S₄) of the Mn cluster are still unclear. To examine the viability of a Mn<super>IV/V</super>₂=O complex as a model for the S₄ state, [Mn<super>III/IV</super>₂(2-OHsalpn)₂]<super>+</super> (2-OHsalpn = N,N<super>'</super>-bis(salicylimino)-1,3-diaminopropan-2-ol) was oxidized at -78&deg;C by <italic>meta</italic>-chloroperoxobenzoic acid (<italic>m</italic>-CPBA) to form [Mn<super>V/IV</super>₂=O(2-OHsalpn)₂]<super>+</super>. MnV=O reactivity was probed using a cyclohexene assay. Cyclohexene oxide was detected by GC-mass spectroscopy from the reaction of [Mn<super>III/IV </super>₂(2-OHsalpn)₂]<super>+</super> with <italic> m</italic>-CPBA in the presence of cyclohexene. The reaction proceeded for 20 turnovers. CO₂(g) was observed from the same reaction in the absence of cyclohexene. The kinetics of the reaction in the absence of cyclohexene was followed by stopped-flow UV-Visible, freeze-quench EPR and XANES spectroscopies. Rapid formation (30 ms after the reaction was initiated) of a radical species and a Mn<super>IV/IV</super>₂ complex were detected. The generated Mn<super>V</super>=O reacts with the product (<italic>m</italic>-chlorobenzoate) to form a radical then liberates CO₂(g). Parallel mode EPR spectra also detected a multiline signal, which is identified as a Mn<super>IV</super>₄ tetramer. The spectral features are very similar to the S₁ state; however, the smaller spectral width indicates that the Mn<super>IV </super>₄ model has a higher oxidation states than found in S₁. The proposed reaction mechanism suggests initial oxidation of Mn<super> IV/IV</super>₂ to Mn<super>IV/V</super>₂=O, which can oxidize benzoate forming an organic radical species and a Mn<super>IV/IV</super>₂ complex. Alternatively the Mn<super>IV/V</super>₂=O can rapidly comproportionate with a Mn<super>III/IV</super>₂ species forming the Mn<super>IV</super>₄ tetramer. Despite previous literature reports that treat the manganyl group solely as a two-electron, oxo transfer moiety, this work demonstrates for the first time that the manganyl group can competitively undergo single electron reduction reactions.