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Time Resolved Infrared Spectroscopy: Kinetic Studies of Weakly Binding Ligands in an Iron–Iron Hydrogenase Model Compound

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journal contribution
posted on 20.02.2016, 18:15 by Sohail Muhammad, Salvador Moncho, Edward N. Brothers, Marcetta Y. Darensbourg, Donald J. Darensbourg, Ashfaq A. Bengali
Solution photochemistry of (μ-pdt)­[Fe­(CO)3]2 (pdt = μ2-S­(CH2)3S), a precursor model of the 2-Fe subsite of the H-cluster of the hydrogenase enzyme, has been studied using time-resolved infrared spectroscopy. Following the loss of CO, solvation of the Fe center by the weakly binding ligands cyclohexene, 3-hexyne, THF, and 2,3-dihydrofuran (DHF) occurred. Subsequent ligand substitution of these weakly bound ligands by pyridine or cyclooctene to afford a more stable complex was found to take place via a dissociative mechanism on a seconds time scale with activation parameters consistent with such a pathway. That is, the ΔS values were positive and the ΔH parameters closely agreed with bond dissociation enthalpies (BDEs) obtained from DFT calculations. For example, for cyclohexene replacement by pyridine, experimental ΔH and ΔS values were determined to be 19.7 ± 0.6 kcal/mol (versus a theoretical prediction of 19.8 kcal/mol) and 15 ± 2 eu, respectively. The ambidentate ligand 2,3-DHF was shown to initially bind to the iron center via its oxygen atom followed by an intramolecular rearrangement to the more stable η2-olefin bound species. DFT calculations revealed a transition state structure with the iron atom almost equidistant from the oxygen and one edge of the olefinic bond. The computed ΔH of 10.7 kcal/mol for this isomerization process was found to be in excellent agreement with the experimental value of 11.2 ± 0.3 kcal/mol.