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′ = η5-C5H4Me) in the presence of the free N-heterocyclic carbenes IEt2Me2, IiPr2Me2, 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 (kCO) 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 -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)(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.