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 Fe ₂(μ-S ₂C ₃H ₆)(CO) ₆ and Fe ₂(μ-S ₂C ₂H ₄)(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 Fe ₂(μ-S ₂C ₃H ₆)(CO) ₆ were studied with density functional theory (DFT) calculations. For Fe ₂(μ-S ₂C ₃H ₆)(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 Fe ₂(μ-S ₂C ₃H ₆)(CO) ₆ with a semibridging CO is reminiscent of the geometry of the Fe ₂S ₂ 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.