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

Abstract

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

Investigation of the Ultrafast Dynamics Occurring during Unsensitized Photocatalytic H2 Evolution by an [FeFe]-Hydrogenase Subsite Analogue

“…21,22,24,31 The faster timescale was widely attributed to electronic relaxation or cooling of vibrationally hot species 22, 32 while the longer timescale was attributed to the geminate recombination of species that have undergone CO photolysis upon photoexcitation. [20][21][22][23][24][25] This model is broadly consistent with the data observed here.…”

“…These included species arising from CO-loss, which would be expected to be present based on previous studies of the photochemistry of hydrogenase sub-site analogues and is further indicated by the similar dynamics for the TRIR peak recovery reported above to previous work on similar systems. [20][21][22][23][24][25] Ground state carbonyl ligand dissociation energies were calculated to be 40, 47 and 57 kcal/mol for the CO ap , CO eq and CO dppv ligands respectively, in reasonably good agreement with the TD-DFT results reported previously (45, 51 and 58 kcal/mol, respectively). 14 It is noteworthy that the available photon energies at 335, 490 and 572 nm are 85, 58 and 50 kcal/mol, respectively, suggesting that, even at the longest wavelength studied, CO dissociation is by no means prohibited, though it may be approaching the limit at which it is possible for the CO dppv ligand at longer wavelengths.…”

“…[15][16][17][18][19] Studies of the photochemistry of hydrogenase-based biomimetic compounds are limited and tend toward excitation at wavelengths shorter than 400 nm but, in all of these cases, carbonyl ligand photolysis has been a predominant outcome and it has also been a feature of longer wavelength studies. [20][21][22][23][24][25] It is therefore imperative that experimental verification of predictions suggesting that CO photolysis is expected only to be a minor product of 1 excitation is obtained. In order to understand the intimate processes underpinning the photochemistry of this system, we report timeresolved infrared (TRIR) spectroscopy of 1 following photoexcitation at three different pump wavelengths pump = 335, 490 and 572 nm, resonant with three absorption bands in the UV/Vis spectrum (Fig.…”

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

“…With 355 nm excitation, an additional photoproduct consisting of electronic excitation accompanied by a longer Fe-Fe bond is observed. Using 532 nm excitation, both pathways are still observed, but the electronic excitation is significantly stronger [43]. Similar dynamics are seen in 1b but with significantly more spectral overlap between the two photoproducts.…”

“…Ultrafast time-resolved, UV-visible pump, IR probe spectroscopy (TRIR) was previously performed by our group on 1a , 1b , and 2a by exciting the sample at multiple wavelengths [43, 44]. These studies utilized 532 nm, 355 nm, and 266 nm excitation wavelengths which correspond to absorptions in the UV-visible spectra of these models.…”