Time-Resolved Infrared Studies of a Trimethylphosphine Model Derivative of [FeFe]-Hydrogenase

Model compounds that structurally mimic the hydrogen-producing active site of [FeFe]-hydrogenases have been studied to explore potential ground-state electronic structure effects on reaction mechanisms compared to hexacarbonyl derivatives. The time-dependent behavior of Fe₂(μ-S₂C₃H₆)­(CO)₄(PMe)₂ (<b>A</b>) in room temperature <i>n</i>-heptane and acetonitrile solutions was examined using various ultrafast UV and visible excitation pulses with broadband IR-probe spectroscopy of the carbonyl (CO) stretching region. Ground- and excited-state electronic and CO-stretching mode vibrational properties of the possible isomers of <b>A</b> were also examined using density functional theory (DFT) computations. In <i>n</i>-heptane, 355 and 532 nm excitation resulted in short-lived (135 ± 74 ps) bands assigned to excited-state, CO-loss photoproducts. These bands decay away, forming new long-lived absorptions that are likely a mixture of isomers of both three-CO and four-CO ground-state isomers. These new bands grow in with a time scale of 214 ± 119 ps and persist for more than 100 ns. In acetonitrile, similar results are seen with a 532 nm pump, but the 355 nm data lack evidence of the longer-lived bands. In either solvent, the 266 nm pump data seem to also lack longer-lived bands, but the intensities are significantly lower in this data, making firm conclusions more difficult. We suggest that these wavelength-dependent excitation dynamics significantly alter potential mechanisms and efficiencies for light-driven catalysis

Time-Resolved Vibrational Spectroscopy of [FeFe]-Hydrogenase Model Compounds

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe–Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe₂(μ-S₂C₃H₆)­(CO)₆ and Fe₂(μ-S₂C₂H₄)­(CO)₆ is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe₂(μ-S₂C₃H₆)­(CO)₆ were studied with density functional theory (DFT) calculations. For Fe₂(μ-S₂C₃H₆)­(CO)₆ excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe–Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe₂(μ-S₂C₃H₆)­(CO)₆ with a semibridging CO is reminiscent of the geometry of the Fe₂S₂ subcluster of the active site observed in Fe–Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis