Catalysis of H2/D2 scrambling and other H/D exchange processes by [Fe]-hydrogenase model complexes

Protonation of the [Fe]-hydrogenase model complex (mu-pdt)[Fe(CO)(2)(PMe3)](2) (pdt = SCH2CH2CH2S) produces a species with a high field H-1 NMR resonance, isolated as the stable {(mu-H)(mu-pdt)[Fe(CO)(2)(PMe3)](2)} [PF6](-) salt. Structural characterization found little difference in the 2Fe2S butterfly cores, with (FeFe)-Fe-... distances of 2.555(2) and 2.578(1) Angstrom 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 (FeFe)-Fe-... distance found in structures of three Fe-only hydrogenase active site 2Fe2S clusters: 2.6 Angstrom. A series of similar complexes (mu-edt)-, (mu-o-xyldt)-, and (mu-SEt)(2)[Fe(CO)(2)(PMe3)](2) (edt = SCH2-CH2S; o-xyldt = SCH2C6H4CH2S), (mu-pdt)[Fe(CO)(2)(PMe2Ph)](2), and their protonated derivatives likewise show uniformity in the Fe-Fe bond lengths of the neutral complexes and (FeFe)-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 (mu-SRS) or (mu-SR)(2) bridges and the subsequent steric hindrance of R. The Fe-\\(mu-H)Fe-\\ complexes were compared for their ability to facilitate H/D exchange reactions, as have been used as assays of H(2)ase activity. In a reaction that is promoted by light but inhibited by CO, the {(mu-H)(mu-pdt)[Fe(CO)(2)(PMe3)](2)}(+) complex shows H/D exchange activity with D-2, producing {(mu-D)(mu-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 H-2 is also promoted by the Fe-\\(mu-H)Fe-\\ catalyst. The requirement of an open site suggests that the key step in the reactions involves D-2 or H-2 binding to Fell 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 Fell must be available for H2 uptake while a reduced or an electron-rich (FeFe\)-Fe-\ metal-metal bonded redox level is required for proton uptake.