Terminal Thiolate-Dominated H/D Exchanges and H 2 Release: Diiron Thiol-Hydride.

To determine the reaction pathways at a metal-ligand site in enzymes, we incorporated a terminal thiolate site into a diiron bridging hydride. Trithiolato diiron hydride, (μ-H)Fe2 (pdt)(dppbz)(CO)2 (SR) (1(μ-H)) [pdt2- = 1,3-(CH2 )3 S2 2- , dppbz = 1,2-C6 H4 (PPh2 )2 , RS- = 1,2-Cy2 PC6 H4 S- )], was synthesized directly by photoassisted oxidative addition of 1,2-Cy2 PC6 H4 SH to Fe2 (pdt)(dppbz)(CO)4 . The terminal thiolate in 1(μ-H) undergoes protonation, affording a thiol-hydride complex [1(μ-H)H]+ . Placing an acidic SH site adjacent to the Fe-H-Fe site allows intramolecular thiol-hydride coupling and releases H2 from [1(μ-H)H]+ . A diiron η2 -H2 intermediate in the formation of H2 is proposed, and is evidenced by the H/D exchange reactions of [1(μ-H)H]+ with D2 , D2 O, and CD3 OD. Isotopic exchange in [1(μ-D)H]+ is driven by an equilibrium isotope effect with 2.1 kJ/mol difference in free energy that favors [1(μ-H)D]+ . [1(μ-H)H]+ catalyzes H/D scrambling between H2 and D2 O or CD3 OD to produce HD. The reactions based on such a "proton-hydride" model provide insights into the reversible heterolytic cleavage of H2 by H2 ases.