Photochemistry of Cp′Mn(CO)2(NHC) (Cp′ = η5-C5H4Me) Species: Synthesis, Time-Resolved IR Spectroscopy, and DFT Calculations

UV irradiation of Cp'Mn(CO)(3) (Cp' = eta(5)-C5H4Me) in the presence of the free N-heterocyclic carbenes IEt2Me2, (IPr2Me2)-Pr-i, IMes, and IPr affords the NHC dicarbonyl complexes Cp'Mn(CO)(2)(NHC) (1-4). Time-resolved infrared spectroscopy in alkane solution reveals that 1-4 photodissociate CO to generate Cp'Mn(CO)(NHC) (1-CO, 2-CO, 3-CO, 4-CO), which exhibit solvent-independent second-order rate constants (k(CO)) for reaction with CO. These observations are consistent with 1-CO to 4-CO being stabilized by intramolecular agostic interactions with the NHCs rather than intermolecular alkane coordination. Density functional theory calculations provide support for this hypothesis and locate a series of agostic structures varying from delta-agostic (1-CO, 2-CO), to epsilon-agostic (3-CO), to phi-agostic (4-CO). The atoms-in-molecules approach is used to characterize these species, along with the gamma-agostic interaction seen in the CpMn(CO)(PPh3) analogue (5-CO), and shows that these species are distinguished primarily by the magnitude of the electron density at the agostic ring critical point.