The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe 2 (xdt)(CO) 2 (dppv) 2 (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH 2 ) 2 NH, and oxadithiolate (odt), (SCH 2 ) 2 O; 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 CD 2 Cl 2 5.7-8.2) or oxonium (pK CD 2 Cl 2 −4.7-1.6) regulates the proton transfer. As 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 [HPMe 2 Ph]BF 4 (pK CD 2 Cl 2 = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe 2 (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.The [FeFe]-hydrogenases are among the very best catalysts known for the reduction of protons to dihydrogen, with turnover frequencies estimated to be ~6000 mol H 2 /mol enzyme per second operating at nearly Nerstian potentials. 1 The question about why the [FeFe]-hydrogenases are so efficient is topical, 2 and the answer is likely related to the incompletely characterized dithiolate cofactor that bridges the diiron subunit. In 2001, Nicolet et al. proposed that this dithiolate is the azadithiolate (adt, (SCH 2 ) 2 NH), wherein the amine functionality could relay protons to and from the apical site on the distal Fe center. 3 It is known that, unlike typical amine bases, transition metals can be slow to protonate. 4 The adt hypothesis is attractive because it potentially shows how to couple the kinetic facility of amine protonation with the redox abilities of iron hydrides. Indeed, DuBois has demonstrated that amine bases constrained within diphosphine ligands greatly accelerate both H 2 uptake and production for mononuclear iron and nickel phosphine complexes. 5 A recent DFT investigation suggests that the dithiolate cofactor is the oxadithiolate (odt, (SCH 2 ) 2 O), which also merits evaluation since protein crystallography cannot distinguish between C, N, and O. 6 rauchfuz@uiuc.edu. The recent discovery that diiron(I) dithiolates initially protonate to give terminal, not bridging, hydrides opens a new and potentially significant phase in elucidating the role of the dithiolate cofactor in the catalysis. 11 Terminal hydrid...

Using the thermally stable salts of [Fe 2 (SR) 2 (CO) 3 (PMe 3 )(dppv)]BAr F 4 , we found that the azadithiolates [Fe 2 (adtR)(CO) 3 (PMe 3 )(dppv)] + react with high pressures of H 2 to give the hydride [(μ-H)Fe 2 (adt)(CO) 3 (PMe 3 )(dppv)]BAr F 4 . The related oxadithiolate and propanedithiolate complexes are unreactive toward H 2 . Molecular hydrogen is proposed to undergo heterolysis assisted by the amine followed by isomerization of an initially formed terminal hydride. Use of H 2 and D 2 O gave the deuteride as well as the hydride, implicating protic intermediates.Hydrogenases are enzymes that catalyze the interconversion of dihydrogen with protons and reducing equivalents. 1 Understanding the reactivity of these enzymes via active site models remains topical, 2 especially since these catalysts rely on inexpensive first-row transition metals. 3 Significant progress has been made in [FeFe]-hydrogenases models, 4,5 but nearly all studies to date have focused on proton reduction. 6 The opposite reaction, hydrogen oxidation, has proven elusive. This lack of reactivity is surprising because [FeFe]-hydrogenases are exceptionally active towards H 2 oxidation. The translation of models to applications in fuel cells requires progress on hydrogen oxidation. Herein we report the activation of dihydrogen by a model for the [FeFe]-hydrogenase, as well as the associated advances that have facilitated this progress.Recently we reported [Fe 2 (pdt)(CO) 3 (PMe 3 )(dppv)]BF 4 ([1]BF 4 , pdt = S 2 C 3 H 6 ) ‡ , a paramagnetic (S = 1/2) spin-localized species that represents a useful structural model for the H ox state of the binuclear active site (dppv = cis-1,2-bis(diphenylphosphino)ethylene). 7,8 With a vacant coordination site on the Fe(I) center, H ox and its models are poised to activate H 2 . Although [1] + binds CO, it exhibits no discernable reactivity toward H 2 . The anticipated product of hydrogen activation, [Fe 2 (μ-H)(pdt)(CO) 3 (PMe 3 )(dppv)]BF 4 ([1H]BF 4 ), was prepared independently by protonation of the corresponding Fe(I)Fe(I) precursor; a terminal hydride is initially formed which rapidly isomerizes to bridging hydrides. 9 The thermal sensitivity of [1]BF 4 severely limits studies of its reactivity toward H 2 , but we found that the corresponding salt [1]BAr F 4 is stable in solution for days at room temperature (BAr F 4 = B(C 6 H 3 -3,5-(CF 3 ) 2 ) 4 − ). The sensitivity of electrophilic Fe carbonyls towards fluorinated counterions is precedented. 10

The report summarizes studies on the redox behavior of synthetic models for the [FeFe]hydrogenases, consisting of diiron dithiolato carbonyl complexes bearing the amine cofactor and its N-benzyl derivative. Of specific interest are the causes of the low reactivity of oxidized models toward H 2 , which contrasts with the high activity of these enzymes for H 2 oxidation.

Decades of biophysical study on the hydrogenase (H 2 ase) enzymes have yielded sufficient information to guide the synthesis of analogues of their active sites. Three families of enzymes serve as inspiration for this work: the [FeFe]-, [NiFe]-, and [Fe]-H 2 ases, all of which feature iron centers bound to both CO and thiolate. Artificial H 2 ases effect the oxidation of H 2 of H 2 and the reverse reaction, the reduction of protons. These reactions occur via the intermediacy of metal hydrides. The inclusion of amine bases within the catalysts is an important design feature that is emulated in related bioinspired catalysts. Continuing challenges are the low reactivity of H 2 towards biomimetic H 2 ases.

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