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

Abstract

The photochemistry of (η6-C6H6)M(CO)3 (M = Cr or Mo) is described. Photolysis with λexc. > 300 nm of (η6-C6H6)Cr(CO)3 in low-temperature matrixes containing CO produced the CO-loss product, while lower energy photolysis (λexc. > 400 nm) produced Cr(CO)6. Pulsed photolysis (λexc. = 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 (λexc. = 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.

Photo‐reduction of CO2 Using a Rhenium Complex Covalently Supported on a Graphene/TiO2 Composite

“…The equipment used for TRIR measurement has been described in detail previously. [17] The IR spectra were probed with IR pulses generated by difference-frequency generation (ca. 200 cm À1 wide).…”

“…This is attributed to the metalto-ligand charge transfer (MLCT)process in Re(PyBn)(CO) 3 Cl. [17] Owing to the light scattering of rGO, the TiO 2 -rGORe(PyBn)(CO) 3 Cl composite shows a broad absorption extending to the near-IR range. The emission spectra of the TiO 2 -rGO-Re(PyBn)(CO) 3 Cl composite exhibits significantly reduced emission compared with that of Re(PyBn)(CO) 3 Cl (Figure 2 d).…”

The photochemistry of transition metal complexes using density functional theory

“…However, the transition from the ground state to this MACT excited state has negligible oscillator strength (it is symmetry forbidden), thus the MCCT excited state is the lowest energy optically accessible excited state for [(η 6 -benzene)Cr(CO) 3 ] [23,24].…”

Photolysis of arene chromium tricarbonyl complexes in presence of amine–boranes: Observation of σ-borane complexes in solution

“…Formation of complex 3 is a two step process: loss of arene and CO from the starting (g 6 -arene)Cr(CO) 3 followed by a recombination of two CO ligands resulting in the formation of the Cr(CO) 5 species. The Cr(CO) 5 species then reacts with amine-borane to afford the Cr(CO) 5 (g 1 -HBH 2 ÁNMe 3 ) (3) [24]. Table 2 gives the ratio of 2 and 3 in terms of relative percentage of 2 calculated based on the integral ratio in the 1 H NMR spectra for all the three systems.…”