Terminal Thiolate-Dominated H/D Exchanges and H2 Release: Diiron Thiol-Hydride.

Abstract

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)3S22-, 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.