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(mu-S2C3H6)(CO)(6) and Fe-2(mu-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 Fe-2(mu-S2C3H6)(CO)(6) were studied with density functional theory (DFT) calculations. For Fe-2(mu-S2C3H6)(CO)(6) excited with 266 nm, long-lived signals (tau = 3.7 +/- 0.26 mu s) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (tau = 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(mu-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.