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(OH ₂)]^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 Mn^IVMn^V 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.