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.
Batool, M., Martin, T. A., Algarra, A. G., George, M. W., Macgregor, S. A., Mahon, M. F., & Whittlesey, M. K. (2012). Photochemistry of Cp′Mn(CO)2(NHC) (Cp′ = η5-C5H4Me) species: synthesis, time-resolved IR spectroscopy, and DFT calculations. Organometallics, 31(14), 4971-4979. https://doi.org/10.1021/om300209a