Catalysis of H(2)/D(2) scrambling and other H/D exchange processes by [Fe]-hydrogenase model complexes

Protonation of the [Fe]-hydrogenase model complex (mu-pdt)[Fe(CO)(2)(PMe(3))](2) (pdt = SCH(2)CH(2)CH(2)S) produces a species with a high field (1)H NMR resonance, isolated as the stable [(mu-H)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+)[PF(6)](-) salt. Structural characterization found little difference in the 2Fe2S butterfly cores, with Fe.Fe distances of 2.555(2) and 2.578(1) A 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 A. A series of similar complexes (mu-edt)-, (mu-o-xyldt)-, and (mu-SEt)(2)[Fe(CO)(2)(PMe(3))](2) (edt = SCH(2)CH(2)S; o-xyldt = SCH(2)C(6)H(4)CH(2)S), (mu-pdt)[Fe(CO)(2)(PMe(2)Ph)](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 PMe(3) and PMe(2)Ph 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(II)(mu-H)Fe(II) 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)(PMe(3))](2)](+) complex shows H/D exchange activity with D(2), producing [(mu-D)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+) in CH(2)Cl(2) and in acetone, but not in CH(3)CN. In the presence of light, H/D scrambling between D(2)O and H(2) is also promoted by the Fe(II)(mu-H)Fe(II) catalyst. The requirement of an open site suggests that the key step in the reactions involves D(2) or H(2) binding to Fe(II) 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]H(2)ase enzyme active site redox levels suggests that at least one Fe(II) must be available for H(2) uptake while a reduced or an electron-rich Fe(I)Fe(I) metal-metal bonded redox level is required for proton uptake.