Unusually Slow Photodissociation of CO from (η⁶-C₆H₆)Cr(CO)₃ (M = Cr or Mo): A Time-Resolved Infrared, Matrix Isolation, and DFT Investigation

The photochemistry of (η⁶-C₆H₆)M(CO)₃ (M = Cr or Mo) is described. Photolysis with λ_exc. > 300 nm of (η⁶-C₆H₆)Cr(CO)₃ in low-temperature matrixes containing CO produced the CO-loss product, while lower energy photolysis (λ_exc. > 400 nm) produced Cr(CO)₆. Pulsed photolysis (λ_exc. = 400 nm) of (η⁶-C₆H₆)Cr(CO)₃ in n-heptane solution at room temperature produced an excited-state species (1966 and 1888 cm⁻¹) that decays over 150 ps to (η⁶-C₆H₆)Cr(CO)₂(n-heptane) (70%) and (η⁶-C₆H₆)Cr(CO)₃ (30%). Pulsed photolysis (λ_exc. = 266 nm) of (η⁶-C₆H₆)Cr(CO)₃ in n-heptane produced bands assigned to (η⁶-C₆H₆)Cr(CO)₂(n-heptane) (1930 and 1870 cm⁻¹) within 1 ps. These bands increase with a rate identical to the rate of decay of the excited-state species and the rate of recovery of (η⁶-C₆H₆)Cr(CO)₃. Photolysis of (η⁶-C₆H₆)Mo(CO)₃ at 400 nm produced an excited-state species (1996 and 1898 cm⁻¹) and traces of (η⁶-C₆H₆)Mo(CO)₂(n-heptane) within 1 ps. For the chromium system CO-loss can occur following excitation at both 400 and 266 nm via an avoided crossing of a MACT (metal-to-arene charge transfer) and MCCT/LF (metal-to-carbonyl charge transfer/ligand field) states. This leads to an unusually slow CO-loss following excitation with 400 nm light. Rapid CO-loss is observed following 266 nm excitation because of direct population of the MCCT/LF state. The quantum yield for CO-loss in the chromium system decreases with increasing excitation energy because of the competing population of a high-energy unreactive MACT state. For the molydenum system CO-loss is a minor process for 400 nm excitation, and an unreactive MACT state is evident from the TRIR spectra. A higher quantum yield for CO-loss is observed following 266 nm excitation through both direct population of the MCCT/LF state and production of a vibrationally excited reactive MACT state. This results in the quantum yield for CO-loss increasing with increasing excitation energy.