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

Abstract

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.