Terminal Thiolate-Dominated H/D Exchanges and H 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)Fe(pdt)(dppbz)(CO)(SR) (1(μ-H)) [pdt = 1,3-(CH)S, dppbz = 1,2-CH(PPh), RS = 1,2-CyPCHS)], was synthesized directly by photoassisted oxidative addition of 1,2-CyPCHSH to Fe(pdt)(dppbz)(CO). 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 H from [1(μ-H)H]. A diiron η-H intermediate in the formation of H is proposed, and is evidenced by the H/D exchange reactions of [1(μ-H)H] with D, DO, and CDOD. 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 H and DO or CDOD to produce HD. The reactions based on such a "proton-hydride" model provide insights into the reversible heterolytic cleavage of H by Hases.