Ultrafast Spectroscopy of [Mn(CO)] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding.

Manganese tricarbonyl complexes are promising catalysts for CO reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)(bpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (bpy, where R = Bu, H, CF, NO). Transient absorption spectroscopy measurements for Mn(CO)(bpy)Br coordination compounds with R = Bu, H, and CF in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)(bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.