Terminal Thiolate-Dominated H/D Exchanges and H₂ 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)) [pdt^2- = 1,3-(CH₂)₃S₂^2-, dppbz = 1,2-C₆H₄(PPh₂)₂, RS^- = 1,2-Cy₂PC₆H₄S^-)], was synthesized directly by photoassisted oxidative addition of 1,2-Cy₂PC₆H₄SH 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, D₂O, and CD₃OD. 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 D₂O or CD₃OD to produce HD. The reactions based on such a "proton-hydride" model provide insights into the reversible heterolytic cleavage of H₂ by H₂ases.