Ligand Rigidity Steers the Selectivity and Efficiency of the Photosubstitution Reaction of Strained Ruthenium Polypyridyl Complexes

While photosubstitution reactions in metal complexesare usuallythought of as dissociative processes poorly dependent on the environment,they are, in fact, very sensitive to solvent effects. Therefore, itis crucial to explicitly consider solvent molecules in theoreticalmodels of these reactions. Here, we experimentally and computationallyinvestigated the selectivity of the photosubstitution of diimine chelatesin a series of sterically strained ruthenium(II) polypyridyl complexesin water and acetonitrile. The complexes differ essentially by therigidity of the chelates, which strongly influenced the observed selectivityof the photosubstitution. As the ratio between the different photoproductswas also influenced by the solvent, we developed a full density functionaltheory modeling of the reaction mechanism that included explicit solventmolecules. Three reaction pathways leading to photodissociation wereidentified on the triplet hypersurface, each characterized by eitherone or two energy barriers. Photodissociation in water was promotedby a proton transfer in the triplet state, which was facilitated bythe dissociated pyridine ring acting as a pendent base. We show thatthe temperature variation of the photosubstitution quantum yield isan excellent tool to compare theory with experiments. An unusual phenomenonwas observed for one of the compounds in acetonitrile, for which anincrease in temperature led to a surprising decrease in the photosubstitutionreaction rate. We interpret this experimental observation based oncomplete mapping of the triplet hypersurface of this complex, revealingthermal deactivation to the singlet ground state through intersystemcrossing.