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): Fe₂(adt ^NH)(CO)₂(dppv)₂ (1^NH) and Fe ₂(pdt)(CO)₂(dppv)₂ (2) [(adt^NH) ^2- = HN(CH₂S)₂^2-, pdt^2- = 1,3-(CH₂)₃S₂^2-, and dppv = cis-C₂H₂(PPh₂)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]BF₄, the ammonium/amine hydride equilibrium is sensitive to counteranions and solvent. The species [t-H1^NH ₂](BF₄)₂ represents the first example of a crystallographically characterized terminal hydride produced by protonation. The NH - -HFe distance of 1.88(7) - indicates dihydrogen-bonding. The bridging hydrides [-H1^NH]⁺ and [-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 HBF₄-Et ₂O, 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^NH₂]²⁺ is a faster catalyst, with an estimated TOF of 58 000 s^-1 and an overpotential of 0.5 V.