On the Mechanism of Dioxygen Formation from a Di-μ-Oxo-Bridged Manganese Dinuclear Complex

Density functional theory (DFT) calculations, using the Becke−Perdew gradient-corrected functional with a triple-ζ-plus-polarization basis set, have been used to characterize the [(H2O)(H3N)3Mn(μ-O)2Mn(NH3)3(OH2)]q+ (q = 2−5) complexes. This structure has been proposed as a possible model for the oxygen-releasing site of the photosystem II (PSII) reaction center. We have performed full optimizations to locate stationary points in various spin states for each of the +2 to +5 charge states. Our calculations indicate that O2 release from the vacuum-phase +5 charge state complex is barrier inhibited, in contrast to the results of a recent DFT study. We report several new di-μ-oxo-bridged stationary points with spin multiplicities of S = 1/2, 3/2, and 5/2 and effective metal oxidation states of MnIVMnV for the +5 charge state. Finally, calculations employing the ‘conductorlike screening model‘ (COSMO), to address the inclusion of solvent effects, indicate that dissociative O2 release from the +5 charge state model complex is inhibited by a major barrier and is therefore apparently highly disfavored.