Unravelling the Photodissociative Mechanism of CO-Release in Re(I) Tricarbonyl Complex Using Density Functional Theory

The mechanism of the CO ligand dissociation of [Re(CO)3(Pyta)(TPP))]+ complex in aqueous medium has been investigated theoretically, as the dominant process of the photochemical ligand substitution (PLS) reaction of [Re(CO)3(Pyta)(TPP))]+, by using density functional theory (DFT). The PLS reactivity can be determined by the topology of the lowest triplet potential energy surface because the photoexcited complex is able to decay into lowest triplet state via intersystem crossing and internal conversion with sufficiently low energy barriers. The lowest triplet state has a character of metal-to-ligand-charge-transfer (3MLCT) around the Franck-Condon region, and it shifts to metal-centered (3MC) state as the axial Re-CO bond is bend and elongated. The equatorial CO ligand is not dissociated in the course of PLS reaction and only axial CO ligand is selectively dissociated. This study reveals that the 3MC connecting the 3MLCT is not dissociative in nature, rather, the dissociative 3MC is reached via multiple 3MCs. We also predict that the photoproduct [Re(CO)2(CH3CN)(Pyta)(TPP))]+ is formed via the formation of a singlet-adduct. The insights provided in this study shed light on, not-all 3MCs are dissociative in nature, and a step forward towards rationally-designing novel PhotoCORMs.