Photochemistry of Cp′Mn(CO) ₂(NHC) (Cp′ = η ⁵-C ₅H ₄Me) species: Synthesis, time-resolved ir spectroscopy, and DFT calculations

UV irradiation of Cp′Mn(CO) ₃ (Cp′ = η ⁵-C ₅H ₄Me) in the presence of the free N-heterocyclic carbenes IEt ₂Me ₂, I ⁱPr ₂Me ₂, 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 δ-agostic (1-CO, 2-CO), to ε-agostic (3-CO), to φ-agostic (4-CO). The atoms-in-molecules approach is used to characterize these species, along with the γ-agostic interaction seen in the CpMn(CO)(PPh ₃) analogue (5-CO), and shows that these species are distinguished primarily by the magnitude of the electron density at the agostic ring critical point.