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