DOI:
Authors:
Wen‐Yuan Hsieh
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 MnIV/V2=O complex as a model for the S4 state, [MnIII/IV2(2-OHsalpn) 2]+ (2-OHsalpn = N,N’-bis(salicylimino)-1,3-diaminopropan-2-ol) was oxidized at -78°C by meta-chloroperoxobenzoic acid (m-CPBA) to form [MnV/IV2=O(2-OHsalpn) 2]+. MnV=O reactivity was probed using a cyclohexene assay. Cyclohexene oxide was detected by GC-mass spectroscopy from the reaction of [MnIII/IV 2(2-OHsalpn)2]+ with m-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 MnIV/IV2 complex were detected. The generated MnV=O reacts with the product (m-chlorobenzoate) to form a radical then liberates CO2(g). Parallel mode EPR spectra also detected a multiline signal, which is identified as a MnIV 4 tetramer. The spectral features are very similar to the S1 state; however, the smaller spectral width indicates that the MnIV 4 model has a higher oxidation states than found in S 1. The proposed reaction mechanism suggests initial oxidation of Mn IV/IV2 to MnIV/V2=O, which can oxidize benzoate forming an organic radical species and a MnIV/IV 2 complex. Alternatively the MnIV/V2=O can rapidly comproportionate with a MnIII/IV2 species forming the MnIV4 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.