Unveiling a unique outer-sphere pathway in manganese-catalyzed acceptorless dehydrogenation reaction

First-row transition metals are widely used in acceptorless dehydrogenative coupling reactions, making the detailed investigation of their catalytic mechanisms crucial for the rational design of efficient catalysts. In this study, density functional theory (DFT) was employed to explore the mechanism of an acceptorless dehydrogenation reaction catalyzed by a bifunctional complex based on a high-spin Mn(II) center. The computational results reveal that the Mn(II) catalyst follows a novel dehydrogenation pathway, where the external base KOH first coordinates to the metal center, and dehydrogenation proceeds via an outer-sphere mechanism. The hydride transfer step is identified as the rate-determining step, with an energy barrier of 26.3 kcal mol^-1, consistent with experimental results. To further investigate the selectivity of the mechanism, energy decomposition analysis (EDA) and extended transition state-natural orbitals for chemical valence (ETS-NOCV) analysis were conducted on key transition states. The results show that the small steric hindrance and strong orbital interactions of the external base KOH are the key factors contributing to the selectivity of this mechanism. These findings not only deepen our understanding of the reaction mechanism but also provide valuable theoretical insights for the design and optimization of future acceptorless dehydrogenation catalysts.