Aza- and oxadithiolates are probable proton relays in functional models for the [FeFe]-hydrogenases

The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe₂(xdt)(CO)₂(dppv)₂ (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH₂)₂NH, and oxadithiolate (odt), (SCH₂)₂O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK_a of the transiently formed ammonium (pK^CD2Cl2 5.7-8.2) or oxonium (pK^CD2Cl2 -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i_c/i_p vs [H⁺]. The adt derivative was capable of proton reduction from the weak acid [HPMe₂Ph]BF₄ (pK^CD2Cl2 = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe₂(xdt)(μ-H)(CO)₂(dppv)₂]⁺, where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these μ-H species can be deprotonated.