Primary Photochemical Dynamics of a Triply-Bonded Metal Carbonyl Dimer Probed Via Ultrafast Infrared Spectroscopy

The primary photochemical dynamics of [Cp*Cr(CO)2]2 have been studied using picosecond time-resolved infrared (TRIR) spectroscopy. Upon visible or UV photoexcitation, the primary photochemical pathway is formation of a transient rear- rangement isomer with a weakened Cr≡Cr bond and two terminal carbonyl ligands rearranged to a bridging conformation. This species reverts to the parent dimer on the time scale of 378 ± 15 ps, and Density Functional Theory calculations suggest that this transient species is characterized by a triplet spin state and a trans conformation of the two terminal CO ligands. Photolysis in neat THF solution is unable to trap the transient intermediate via solvent-coordination. The excited state transient rearrangement isomer appears to adopt a distorted structure in THF, relative to cyclohexane, evidenced by the observation of an additional bridging-CO stretching band in THF solution. The lifetime of the transient in THF is just slightly shorter at 344 ± 17 ps. The CO-loss product of 1 has been characterized previously and adopts an asymmetric arrangement of the three bridging CO ligands. In neat THF solution, the CO-loss complex is not observed to react with THF on the picosecond timescale, although a previous study on longer timescales observed formation of a THF adduct of the CO-loss complex in dilute alkane/THF solutions. Though the molybdenum congener, [Cp*Mo(CO)2]2, is unstable in solution, decaying on the timescale of a few hours in cyclohexane, TRIR experiments demonstrate that no bridged photoproducts (transient or long-lived) are formed from the Mo complex in cyclohexane solution.