Multiple-Timescale Photoreactivity of a Model Compound Related to the Active Site of [FeFe]-Hydrogenase

Abstract

Ultraviolet (UV) photolysis of (mu-S(CH 2) 3S)Fe 2(CO) 6 ( 1), a model compound of the Fe-hydrogenase enzyme system, has been carried out. When ultrafast UV-pump infrared (IR)-probe spectroscopy, steady-state Fourier transform IR spectroscopic methods, and density functional theory simulations are employed, it has been determined that irradiation of 1 in an alkane solution at 350 nm leads to the formation of two isomers of the 16-electron complex (mu-S(CH 2) 3S)Fe 2(CO) 5 within 50 ps with evidence of a weakly associated solvent adduct complex. 1 is subsequently recovered on timescales covering several minutes. These studies constitute the first attempt to study the photochemistry and reactivity of these enzyme active site models in solution following carbonyl ligand photolysis.

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

“…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.…”

“…Such biexponential kinetics have been observed previously in similar studies of hydrogenase model compounds. 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.…”

“…16,17,19,[22][23][24][25]27 Indeed, all the CO ligand vibrational mode frequencies were predicted by DFT calculations to redshift by an average of 6 cm -1 relative to those of 1 when simulations were performed on a MeCN molecule coordinating via its nitrogen atom lone pair as a solvent adduct to the vacant site on the Fe after CO loss (see SI, Fig. S5).…”

Transient 2D-IR spectroscopy of inorganic excited states

“…57 TR-IR spectroscopy of (µ-propanedithiolate)Fe 2 (CO) 6 (see Fig 5 for structure) following <350 nm excitation demonstrated that CO photodissociation was the main photochemical pathway observed, though the photoproduct spectra produced were found to be complex and assignment to a specific photoproduct or to a mixture was not possible, with density functional theory simulations suggesting more than one species could be present. 59 T-2D-IR spectroscopy was employed to unravel the spectroscopy subsequent to photodissociation. Once again, the experiments were performed in the pseudo 3 rd order limit in order to reveal the 2D-IR spectrum of the photoproduct species.…”