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, (mu-H)Fe-2(pdt)(dppbz)(CO)(2)(SR) (1(mu-H)) [pdt(2-) = 1,3-(CH2)(3)S-2(2-), dppbz = 1,2-C6H4(PPh2)(2), RS- = 1,2-Cy2PC6H4S-)], was synthesized directly by photoassisted oxidative addition of 1,2-Cy2PC6H4SH to Fe-2(pdt)(dppbz)(CO)(4). The terminal thiolate in 1(mu-H) undergoes protonation, affording a thiol-hydride complex [1(mu-H)H](+). Placing an acidic SH site adjacent to the Fe-H-Fe site allows intramolecular thiol hydride coupling and releases H-2 from [1(mu-H)H](+). A diiron eta(2)-H-2 intermediate in the formation of H2 is proposed, and is evidenced by the H/D exchange reactions of [1(mu-H)H]+ with D-2, D2O, and CD3OD. Isotopic exchange in [1(mu-D)H](+) is driven by an equilibrium isotope effect with 2.1 kJ/mol difference in free energy that favors [1(mu-H)D](+). [1(mu y-H)H](+) catalyzes H/D scrambling between H-2 and D2O or CD3OD to produce HD. The reactions based on such a proton hydride model provide insights into the reversible heterolytic cleavage of H-2 by H(2)ases.