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

• Published in: Journal of the American Chemical Society 2012-11, Vol.134 (45), p.18843-18852
• Main Authors: Carroll, Maria E, Barton, Bryan E, Rauchfuss, Thomas B, Carroll, Patrick J
• Format: Article
• Language: English
• Subjects: Biocatalysis; Catalytic Domain; Ferrous Compounds - chemistry; Ferrous Compounds - metabolism; Hydrogenase - chemistry; Hydrogenase - metabolism; Iron-Sulfur Proteins - chemistry; Iron-Sulfur Proteins - metabolism; Models, Molecular; Molecular Conformation; Oxidation-Reduction; Protons
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja309216v

This report compares biomimetic hydrogen evolution reaction catalysts with and without the amine cofactor (adtNH): Fe2(adtNH)(CO)2(dppv)2 (1 NH ) and Fe2(pdt)(CO)2(dppv)2 (2) [(adtNH)2– = HN(CH2S)2 2–, pdt2– = 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 NH2 ](BF4)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 NH2 ]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·Et2O, 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.