Synthetic Models for the Active Site of the [FeFe]-Hydrogenase: Catalytic Proton Reduction and the Structure of the Doubly Protonated Intermediate

School of Chemical Sciences, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
Journal of the American Chemical Society (Impact Factor: 12.11). 11/2012; 134(45). DOI: 10.1021/ja309216v
Source: PubMed


This report compares biomimetic hydrogen evolution reaction catalysts with and without the amine cofactor (adt(NH)): Fe(2)(adt(NH))(CO)(2)(dppv)(2) (1(NH)) and Fe(2)(pdt)(CO)(2)(dppv)(2) (2) [(adt(NH))(2-) = HN(CH(2)S)(2)(2-), pdt(2-) = 1,3-(CH(2))(3)S(2)(2-), and dppv = cis-C(2)H(2)(PPh(2))(2)]. These compounds are spectroscopically, structurally, and stereodynamically very similar but exhibit very different catalytic properties. Protonation of 1(NH) and 2 gives three isomeric hydrides each, beginning with the kinetically favored terminal hydride, which converts sequentially to sym and unsym isomers of the bridging hydrides. In the case of 1(NH), the corresponding ammonium hydrides are also observed. In the case of the terminal amine hydride [t-H1(NH)]BF(4), the ammonium/amine hydride equilibrium is sensitive to counteranions and solvent. The species [t-H1(NH(2))](BF(4))(2) represents the first example of a crystallographically characterized terminal hydride produced by protonation. The NH---HFe distance of 1.88(7) Å indicates dihydrogen-bonding. The bridging hydrides [μ-H1(NH)](+) and [μ-H2](+) reduce near -1.8 V, about 150 mV more negative than the reductions of the terminal hydride [t-H1(NH)](+) and [t-H2](+) at -1.65 V. Reductions of the amine hydrides [t-H1(NH)](+) and [t-H1(NH(2))](2+) are irreversible. For the pdt analogue, the [t-H2](+/0) couple is unaffected by weak acids (pK(a)(MeCN) = 15.3) but exhibits catalysis with HBF(4)·Et(2)O, albeit with a turnover frequency (TOF) around 4 s(-1) and an overpotential greater than 1 V. The voltammetry of [t-H1(NH)](+) is strongly affected by relatively weak acids and proceeds at 5000 s(-1) with an overpotential of 0.7 V. The ammonium hydride [t-H1(NH(2))](2+) is a faster catalyst, with an estimated TOF of 58 000 s(-1) and an overpotential of 0.5 V.

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    ABSTRACT: This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe2(xdt)(CO)2(PMe3)4, where xdt is edt (ethanedithiolate, 1), pdt (propanedithiolate, 2), and adt (2-aza-1,3-propanedithiolate, 3), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds 1-3 oxidize near -950 mV vs Fc+/0. Crystallographic analyses confirm that 1 and 2 adopt C2-symmetric structures (Fe-Fe = 2.616, 2.625 Å, respectively). Low temperature protonation of 1 afforded exclusively [μ-1H]+, establishing the nonintermediacy of the terminal hydride ([t-1H]+). At higher temperatures, protonation afforded mainly [t-1H]+. The temperature dependence of the ratio [t-1H]+/[μ-1H]+ indicates that the barriers for the two protonation pathways differ by ~4 kcal/mol. Low temperature 31P{1H} NMR measurements indicates that the protonation of 2 proceeds by an intermediate, proposed to be the S-protonated dithiolate [Fe2(Hpdt)(CO)2(PMe3)4]+ ([S-H2]+). This intermediate converts to [t-2H]+ and [μ-2H]+ by a first order (t1/2 ~ 2.5 h, 20 °C). Protonation of the 3 affords exclusively terminal hydrides, regardless of the acid or conditions to give [t-3H]+, which isomerizes to [t-3'H]+ wherein all PMe3 ligands are basal. DFT calculations support transient protonation at sulfur and the proposal that the S-protonated species (e.g., [S-H2]+) rearranges to the terminal hydride intramolecularly via a low energy pathway.
    Journal of the American Chemical Society 10/2012; 134(46). DOI:10.1021/ja3094394 · 12.11 Impact Factor
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