Photochemistry of [CpMo(CO)₃]₂ (Cp = η⁵-C₅H₅) and [Cp*Fe(CO)₂]₂ (Cp* = η⁵-C₅Me₅) in Supercritical CO₂:  A Fast Time-Resolved Infrared Spectroscopic Study

Abstract

Fast (ns) time-resolved infrared spectroscopy has been used to follow the visible (532 nm) flash photolysis of trans-[CpMo(CO)3]2 (Cp = η5-C5H5) in supercritical CO2 (scCO2) (35 °C, 2100 psi). The primary photoproduct observed on this time scale is the CpMo(CO)3 radical, which dimerizes to form both trans- and gauche-[CpMo(CO)3]2. The dimerization of CpMo(CO)3 has been monitored as a function of pressure, and the estimated rate constant is slightly below the expected diffusion-controlled limit. The rate constant (2k2) decreases from 3.9 × 1010 to 9.9 × 109 M-1 s-1 as the pressure is increased from 79 to 213 bar. CpMo(CO)3 displays three ν(CO) bands in scCO2, compared to two ν(CO) bands in n-heptane solution and supercritical Xe (scXe), indicating that the radicals are interacting with CO2. We find that gauche-[CpMo(CO)3]2 decays (kobs = 3 (± 0.5) × 102 s-1) to the more stable trans isomer at similar rates in scCO2 and n-heptane solution. Visible photolysis (532 nm) of [Cp*Fe(CO)2]2 (Cp* = η5-C5Me5) in scCO2 generates Cp*Fe(CO)2 radicals which dimerize (2k2 = 9.7 (±0.3) M-1 s-1) to form both cis-[Cp*Fe(CO)2]2 and trans-[Cp*Fe(CO)2]2. We observed no spectroscopic evidence for the interaction of Cp*Fe(CO)2 with CO2, but the recombination of Cp*Fe(CO)2 in scCO2 is lower than the expected diffusion-controlled rate calculated using the Stokes−Einstein equation, which may indicate a radical−CO2 interaction. With increasing pressure, the rate constant for the dimerization of Cp*Fe(CO)2 radicals decreases from 3.0 × 1010 M-1 s-1 at 83 bar to 9.7 × 109 M-1 s-1 at 178 bar.