Catalysis of H2/D2 Scrambling and Other H/D Exchange Processes by [Fe]-Hydrogenase Model Complexes

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Abstract
Protonation of the [Fe]-hydrogenase model complex (μ-pdt)[Fe(CO)2(PMe3)]2 (pdt = SCH2CH2CH2S) produces a species with a high field 1H NMR resonance, isolated as the stable {(μ-H)(μ-pdt)[Fe(CO)2(PMe3)]2}+[PF6]- salt. Structural characterization found little difference in the 2Fe2S butterfly cores, with Fe···Fe distances of 2.555(2) and 2.578(1) Å for the Fe−Fe bonded neutral species and the bridging hydride species, respectively (Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.; Darensbourg, M. Y. J. Am. Chem. Soc.2001, 123, 9710). Both are similar to the average Fe···Fe distance found in structures of three Fe-only hydrogenase active site 2Fe2S clusters: 2.6 Å. A series of similar complexes (μ-edt)-, (μ-o-xyldt)-, and (μ-SEt)2[Fe(CO)2(PMe3)]2 (edt = SCH2CH2S; o-xyldt = SCH2C6H4CH2S), (μ-pdt)[Fe(CO)2(PMe2Ph)]2, and their protonated derivatives likewise show uniformity in the Fe−Fe bond lengths of the neutral complexes and Fe···Fe distances in the cationic bridging hydrides. The positions of the PMe3 and PMe2Ph ligands are dictated by the orientation of the S−C bonds in the (μ-SRS) or (μ-SR)2 bridges and the subsequent steric hindrance of R. The FeII(μ-H)FeII complexes were compared for their ability to facilitate H/D exchange reactions, as have been used as assays of H2ase activity. In a reaction that is promoted by light but inhibited by CO, the {(μ-H)(μ-pdt)[Fe(CO)2(PMe3)]2}+ complex shows H/D exchange activity with D2, producing {(μ-D)(μ-pdt)[Fe(CO)2(PMe3)]2}+ in CH2Cl2 and in acetone, but not in CH3CN. In the presence of light, H/D scrambling between D2O and H2 is also promoted by the FeII(μ-H)FeII catalyst. The requirement of an open site suggests that the key step in the reactions involves D2 or H2 binding to FeII followed by deprotonation by the internal hydride base, or by external water. As indicated by similar catalytic efficiencies of members of the series, the nature of the bridging thiolates has little influence on the reactions. Comparison to [Fe]H2ase enzyme active site redox levels suggests that at least one FeII must be available for H2 uptake while a reduced or an electron-rich FeIFeI metal−metal bonded redox level is required for proton uptake.