Mechanisms of Mn(V)-oxo to Mn(IV)-oxyl Conversion: From Closed-Cubane Photosystem II to Mn(V) Catalysts and the Role of the Entering Ligands.

The low activation barrier for O-O coupling in the closed-cubane Oxygen-Evolving Centre (OEC) of Photosystem II (PSII) requires water coordination with the Mn4 'dangler' ion in the Mn(V)-oxo fragment. This coordination transforms the Mn(V)-oxo into a more reactive Mn4(IV)-oxyl species, enhancing O-O coupling. This study explains the mechanism behind this and indicates that in the most stable form of the OEC, the Mn4 fragment adopts a trigonal bipyramidal geometry but needs to transition to a square pyramidal form to be activated for O-O coupling. This transition stabilizes the Mn4 dxy orbital, enabling electron transfer from the oxo ligand to the dxy orbital, converting the oxo ligand into an oxyl species. The role of the water is to coordinate with the square pyramidal structure, reducing the energy gap between the oxo and oxyl forms, thereby lowering the activation energy for O-O coupling. This mechanism applies not only to the OEC system but also to other Mn(V)-based catalysts. For other catalysts, ligands like OH- stabilize the Mn(IV)-oxyl species better than water, improving catalyst activation for reactions like C-H bond activation. This study is the first to explain the Mn(V)-oxo to Mn(IV)-oxyl conversion, providing new foundation for Mn-based catalyst design.

Elucidating the catalytic mechanisms of O2 generation by [Mn2(µ-O)2(terpy)2(OH2)2]3+ using DFT calculations: a focus on ClO- as oxidant.

The experimentally reported Mn(IV)Mn(III) complex [Mn2(µ-O)2(terpy)2(OH2)2]3+ has been observed catalyzing O2 generation with oxidants like ClO- and HSO5-. Previous mechanistic studies primarily focused on O2 generation with HSO5-, concluding that Mn(IV)Mn(III) acts as a catalyst, generating a Mn(IV)Mn(IV)-oxyl species as a key intermediate responsible for O-O bond formation. This computational study employs DFT calculations to investigate whether the catalytic generation of O2 using ClO- follows the same mechanism previously identified with HSO5- as the oxidant, or if it proceeds through an alternate pathway. To this end, we explored multiple pathways using ClO- as the oxidant. Interestingly, our findings confirm that in the case of ClO- as the oxidant, similar to what was observed with HSO5-, the Mn(IV)Mn(IV)-oxyl species indeed plays a crucial role in driving the catalytic evolution of O2 with the potential formation of the binuclear complexes Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH during the reaction. These complexes are reactive in producing O2, with activation free energies of 15.9 and 14.3 kcal mol-1, respectively. However, our calculations revealed that the Mn(IV)Mn(IV)-oxyl complex is significantly more reactive in producing O2 than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, with a lower free energy barrier of 8.1 kcal mol-1. Consequently, even though Mn(IV)Mn(IV)-oxyl is predicted to be present in much lower concentrations than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, it emerges as the species acting as the active catalyst for catalytic O2 generation. This study enhances our knowledge of high oxidation state (+3 and +4) manganese chemistry, highlighting its key role in catalysis and paving the way for more efficient Mn-based catalysts with broad applications.