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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2012-07, Vol.116 (27), p.7261-7271
• Main Authors: Bingaman, Jamie L, Kohnhorst, Casey L, Van Meter, Glenn A, McElroy, Brent A, Rakowski, Elizabeth A, Caplins, Benjamin W, Gutowski, Tiffany A, Stromberg, Christopher J, Webster, Charles Edwin, Heilweil, Edwin J
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Bonding; Catalysis; Catalytic Domain; Cobalt; Coordination Complexes - chemical synthesis; Crystallography, X-Ray; Disulfides - chemistry; Electronics; Excitation; Hydrogen - chemistry; Hydrogenase; Hydrogenase - chemistry; Iron - chemistry; Iron-Sulfur Proteins - chemistry; Ligands; Models, Chemical; Quantum Theory; Spectroscopy; Spectrum Analysis; Time Factors; Vibration
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp2121774

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.