Photoinduced β-Hydrogen Elimination and Radical Formation with CpW(CO)₃(CH₂CH₃):  Ultrafast IR and DFT Studies

The initial steps of β-hydrogen elimination from the complex CpW(CO)3(CH2CH3) were studied using ultrafast infrared and step-scan FTIR spectroscopy combined with density functional theory calculations. The reaction was initiated by a pulse of UV light at 266 nm and followed with infrared light on the femto- to microsecond timescale, allowing for the identification and kinetic characterization of transients. In the process of β-H elimination, the primary photoproduct was found to be an α-H stabilized complex and the dynamics of the elusive species cis-CpW(CO)3(H)(CH2CH2) were resolved, allowing for a full kinetic and thermodynamic analysis of the reaction. In a side reaction, CpW(CO)3• is formed via ethyl ligand dissociation; the measured formation rate indicates that the complex forms from a vibrationally excited parent complex. Density functional theory calculations of all relevant transient intermediates and transition states involved in β-H elimination of CpW(CO)3(CH2CH3) are presented. These calculations show two principle mechanisms of β-H elimination, which are discussed in the context of the experimental data.