Requirements for Functional Models of the Iron Hydrogenase Active Site: D2/H2O Exchange Activity in {MU-SMe)(MU-pdt)[Fe(CO)2(PMe3)]2+}[BF4-]

Hydrogen uptake in hydrogenase enzymes can be assayed by H/D exchange reactivity in H2/DO2 or H2/D2/H2O mixtures. Diiron(l) complexes that serve as structural models for the active site of iron hydrogenase are not active in such isotope scrambling but serve as precursors to FeʹʹFe" complexes that are functional models of [Fe]H2ase. Using the same experimental protocol as used previously for {(mu-H)(M-pdt)[Fe(CO)2(PMe3)]2+}, 1-H+ (Zhao et al. J. Am. Chem. Soc. 2001, 123, 9710), we now report the results of studies of {(mu-SMe)(mu-pdt)[Fe(CO)2(PMe3)]2+}, 1-SMe+, toward H/D exchange. The 1-SMe+ complex can take up H2 and catalyze the H/D exchange reaction in D2/H2O mixtures under photolytic, CO-loss conditions. Unlike 1-H+, it does not catalyze H2/D2 scrambling under anhydrous conditions. The molecular structure of 1-SMe+ involves an elongated Fe...Fe separation, 3.11 A, relative to 2.58 A in 1-H+. It is proposed that the strong SMe- bridging ligand results in catalytic activity localized on a single Fe11 center, a scenario that is also a prominent possibility for the enzyme active site. The single requirement is an open site on Fe" available for binding of D2 (or H2), followed by deprotonation by the external base H20 (or D20).