Photochemical dynamics of a trimethyl-phosphine derivatized [FeFe]-hydrogenase model compound

Abstract

Though there have been many studies on photosensitizers coupled to model complexes of the [FeFe]-hydrogenases, few have looked at how the models react upon exposure to light. To extract photoreaction information, ultrafast time-resolved UV/visible pump, IR probe spectroscopy was performed on Fe2(μ-S2C2H4)(CO)4(PMe3)2 (2b) dissolved in heptane and acetonitrile and the photochemical dynamics were determined. Excitation with 532 and 355 nm light produces bleaches and new absorptions that decay to half their original intensity with time constants of 300 ± 120 ps and 380 ± 210 ps in heptane and acetonitrile, respectively. These features persist to the microsecond timescale. The dynamics of 2b are assigned to formation of an initial set of photoproducts, which were a mixture of excited-state tricarbonyl isomers. These isomers decay into another set of long-lived photoproducts in which approximately half the excited-state tricarbonyl isomers recombine with CO to form another complex mixture of tricarbonyl and tetracarbonyl isomers.

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

“…We will show that these models exhibit a fast (20–30 ps) decay followed by a long-lived photoproduct offset. As was found for symmetric models containing two CN ligands, − these monosubstituted anions did not exhibit an initial (ca. 150 ps) decay, as previously observed in all CO or mixed CO/PMe 3 compounds, implying that the symmetry and inclusion of CN ligands alters the catalytic capabilities of these models. ,,− Several density function theory (DFT)-simulated spectra for CO-loss isomers also suggest the possibility of forming a bridging Fe–CO–Fe species, as found in the native structure.…”

“…IR spectra for isomeric and transient species were calculated using Gaussian 16W. , Gas-phase calculations were performed using the BP86 functional − with the Triple Zeta Valence plus Polarization (TZVP) basis set, , which has performed well for related model compound studies. − Whereas absolute frequency agreement between DFT results and experiments is generally found to be poor, relative frequency shifts and amplitudes adequately agree to make favorable product and intermediate structural assignments. − The calculated IR “stick” spectra were convolved with a 15 cm –1 fwhm Lorentzian function to mimic features broadened by the room-temperature MeCN solvent. Calculations were performed for each isomer of 1 and 2 and for all possible CO- and CN-loss photoproduct fragments that could result from UV excitation.…”

“…However, on their own, the photodynamics of the [FeFe]-hydrogenase active site (and similar model compounds), especially on the ultrafast timescale, are not well understood. Symmetric model compounds containing all CO, mixed CO and CN, and mixed CO and PMe 3 ligands have been studied by us using ultraviolet (UV) or visible excitation with ultrafast infrared (IR) probe spectroscopy. − However, asymmetric model compounds containing only one CN ligand, for example, have been synthesized but not yet investigated using time-resolved IR (TRIR) spectroscopy. These species are the focus of this study.…”

Ultrafast Photodynamics of Cyano-Functionalized [FeFe] Hydrogenase Model Compounds

“…18 The energy differences for isolated gas-phase molecules, as calculated using DFT, are shown in Figure 4 (these energy differences are similar to those seen in the model compounds with two trimethylphosphine ligands, rather than two cyano ligands). 40, 69 Assignments for the CO and CN vibrational stretches can be found in the supplementary information Table S1)…”

Photochemistry and photoinhibition of the H-cluster of FeFe hydrogenases

“…When Fe 2 (CO) 4 (L) 2 (bridging ligand) models are considered, ligand isomerization is observed, which is due to the concomitant CO dissociation and Fe-S bond breaking during the non-radiative decay of the first excited state to the ground state 149,150 . The latter result is reminiscent of the photo-inhibition of the enzyme, where photochemical channels are populated when the enzyme is irradiated in the UV range.…”