Photodynamics of Asymmetric Di-Iron-Cyano Hydrogenases Examined by Time-Resolved Mid-Infrared Spectroscopy

Two anionic asymmetric Fe-Fe hydrogenase model compounds containing a single cyano (CN) and five carboxyl (CO) ligands, [Et₄N][Fe₂(micro-S₂C₃H₆)(CO)₅(CN)₁] and [Et₄N][Fe₂(micro-S₂C₂H₄)(CO)₅(CN)₁], dissolved in room-temperature acetonitrile, are examined. The molecular asymmetry affects the redox potentials of the central iron atoms, thus changing the photophysics and possible catalytic properties of the compounds. Femtosecond ultraviolet excitation with mid-infrared probe spectroscopy of the model compounds was employed to better understand the ultrafast dynamics of the enzyme-active site. Continuous ultraviolet lamp excitation with Fourier transform infrared (FTIR) spectroscopy was also used to explore stable product formation on the second timescale. For both model compounds, two timescales are observed; a 20-30 ps decay and the formation of a long-lived photoproduct. The picosecond decay is assigned to vibrational cooling and rotational dynamics, while the residual spectra remain for up to 300 ps, suggesting the formation of new photoproducts. Static FTIR spectroscopy yielded a different stable photoproduct than that observed on the ultrafast timescale. Density functional theory calculations simulated photoproducts for CO-loss and CN-loss isomers, and the resulting photoproduct spectra suggest that the picosecond transients arise from a complex mixture of isomerization after CO-loss, while dimerization and formation of a CN-containing Fe-CO-Fe bridged species are also considered.