Computational Modelling and Mechanistic Insight into Light-Driven CO Dissociation of Square-Planar Rhodium(I) Complexes

The activation step of Vaska-type Rh(I) complexes, such as the photocleavage of the Rh-CO bond, plays an important role in the subsequent C-H activation. To elucidate the details of the photochemistry of Vaska-type Rh(I) complexes, such as trans-Rh(PMe3)2(CO)(Cl), we here present a computationally derived picture as obtained at the density functional level of theory in combination with multireference wavefunction-based methods. We have identified that the photocleavage of CO proceeds via the metal-centered excited state (3MC, dRhz2 -> dRhx2-y2 ${{d\left({\rm R}{\rm h}\right)}_{{z}<^>{2}}\to {d\left({\rm R}{\rm h}\right)}_{{x}<^>{2}-{y}<^>{2}}}$), which is populated through intersystem crossing from the dipole-allowed excited state S1(dRhz2 -> pRhz ${{d\left({\rm R}{\rm h}\right)}_{{z}<^>{2}}\to {p\left({\rm R}{\rm h}\right)}_{z}}$-pi*CO ${{\pi }<^>{{\rm <^>{\ast}}}\left({\rm C}{\rm O}\right)}$). Moreover, the present study unraveled the reasons for the low C-H activation efficiency when using Rh featuring the bidentate ligand 1,2-bis(dimethylphosphino)ethane (dmpe), namely due to its unfavorable photochemical properties, i. e., the small driving force for light-induced CO loss and the fast deactivation of 3MC state back to the singlet ground state. In this study, we provide theoretical insight into mechanistic details underlying the light-induced CO dissociation process, for Rh complexes featuring PMe3 and dmpe ligands. C-H activation: Rh-phosphine complexes can activate C-H bonds in otherwise unreactive alkanes. Quantum chemical calculations provide mechanistic insights into the generation of the active species which is formed via the light-induced CO dissociation at Rh(I) complexes featuring trimethylphosphine and 1,2-bis(dimethylphosphino)ethane. The calculations align well with experimental photochemical studies on the Rh(I) complexes.image