Mechanistic Insights into Photocatalyzed C-H Borylation by a Rhodium Complex: MS-CASPT2 and DFT Study

The use of visible light to catalyze organic reactions has sparked significant interests, particularly in C-H borylation. A notable advancement is the visible-light-induced C-H borylation mediated by Rh complex, i.e., [(NHC)Rh(cod)]. Nevertheless, the fundamental role of light remains poorly understood. To establish the underlying structure-activity relationship, systematic theoretical investigations using multistate complete active space second-order perturbation theory (MS-CASPT2), density functional theory (DFT), and rate constant calculations are performed. For the initial photophysical process, the metal-to-ligand charge transfer (MLCT) state is first reached in the Franck-Condon region, which is followed by a cascade of nonradiative processes that populate the lowest triplet state with a distorted coordination. Two-step isomerization of 1,5-cod ligand affords a Rh complex coordinated with 1,3-cod. Subsequent thermal reactions, instead of photocatalytic reactions as experiments proposed, dominate the C-H borylation, accompanied by a catalytic cycle involving Rh(I) → Rh(III) → Rh(I) redox changes. Moreover, substrate modulation reveals the pivotal role of Rh-N coordination in facilitating the following C-H activation. This work not only elucidates the underlying reaction mechanism but also offers valuable insights for the improvement of current catalytic systems.