Photochemistry of [CpMo(CO)(3)](2) (Cp = eta(5)-C5H5) and [Cp*Fe(CO)(2)](2) (Cp* = eta(5)-C5Me5) in supercritical CO2: A fast time-resolved infrared spectroscopic study

Fast (ns) time-resolved infrared spectroscopy has been used to follow the v isible (532 nm) flash photolysis of trans-[CpMo(CO)(3)](2) (Cp = eta (5)-C5 H5) in supercritical CO2 (scCO(2)) (35 degreesC, 2100 psi). The primary pho toproduct observed on this time scale is the CpMo(CO)(3) radical, which dim erizes 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 estimate d rate constant is slightly below the expected diffusion-controlled limit. The rate constant (2k(2)) decreases from 3.9 x 10(10) to 9.9 x 10(9) M-1 s( -1) as the pressure is increased from 79 to 213 bar. CpMo(CO)(3) displays t hree v(CO) bands in scCO(2), compared to two v(CO) bands in n-heptane solut ion and supercritical Xe (scXe), indicating that the radicals are interacti ng with CO2. We find that gauche-[CpMo(CO)(3)](2) decays (k(abs) = 3 (+/- 0 .5) x 10(2) s(-1)) to the more stable trans isomer at similar rates in scCO (2) and n-heptane solution. Visible photolysis (532 nm) of [Cp*Fe(CO)(2)](2 ) (Cp* = eta (5)-C5Me5) in scCO(2) generates Cp*Fe(CO)(2) radicals which di merize (2k(2) = 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 scCO(2) is lower than the expected diffusion-controlled rate calculated using the Stokes-Einstein equation, which may indicate a radical-CO2 inter action. With increasing pressure, the rate constant for the dimerization of Cp*Fe(CO)(2) radicals decreases from 3.0 x 10(10) M-1 s(-1) at 83 bar to 9 .7 x 10(9) M-1 s(-1) at 178 bar.