title: Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site.
abstract: A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni(I/II)(SCys)2 and Fe(II)(CN)2CO sites are represented with (RC5H4)Ni(I/II) and Fe(II)(diphos)(CO) modules, where diphos = 1,2-C2H4(PPh2)2(dppe) or cis-1,2-C2H2(PPh2)2(dppv). The two bridging thiolate ligands are represented by CH2(CH2S)2(2-) (pdt(2-)), Me2C(CH2S)2(2-) (Me2pdt(2-)), and (C6H5S)2(2-). The reaction of Fe(pdt)(CO)2(dppe) and [(C5H5)3Ni2]BF4 affords [(C5H5)Ni(pdt)Fe(dppe)(CO)]BF4 ([1a]BF4). Monocarbonyl [1a]BF4 features an S = 0 Ni(II)Fe(II) center with five-coordinated iron, as proposed for the Ni-SIa state of the enzyme. One-electron reduction of [1a](+) affords the S = 1/2 derivative [1a](0), which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni(I)Fe(II) compound. The Ni(I)Fe(II) assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni(II)Fe(I)-based models. Compound [1a](0) reacts with strong acids to liberate 0.5 equiv of H2 and regenerate [1a](+), indicating that H2 evolution is catalyzed by [1a](0). DFT calculations were used to investigate the pathway for H2 evolution and revealed that the mechanism can proceed through two isomers of [1a](0) that differ in the stereochemistry of the Fe(dppe)CO center. Calculations suggest that protonation of [1a](0) (both isomers) affords Ni(III)-H-Fe(II) intermediates, which represent mimics of the Ni-C state of the enzyme.
authors.affiliation: School of Chemical Sciences, University of Illinois , Urbana, Illinois 61801, United States.|International Center of Carbon-Neutral Energy Research, Kyushu University , 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.
authors.forename: Geoffrey M
authors.initials: GM
authors.lastname: Chambers
authors.affiliation: School of Chemical Sciences, University of Illinois , Urbana, Illinois 61801, United States.
authors.forename: Mioy T
authors.initials: MT
authors.lastname: Huynh
authors.affiliation: School of Chemical Sciences, University of Illinois , Urbana, Illinois 61801, United States.
authors.forename: Yulong
authors.initials: Y
authors.lastname: Li
authors.affiliation: School of Chemical Sciences, University of Illinois , Urbana, Illinois 61801, United States.
authors.forename: Sharon
authors.initials: S
authors.lastname: Hammes-Schiffer
authors.affiliation: School of Chemical Sciences, University of Illinois , Urbana, Illinois 61801, United States.|International Center of Carbon-Neutral Energy Research, Kyushu University , 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan.
authors.forename: Thomas B
authors.initials: TB
authors.lastname: Rauchfuss
authors.forename: Edward
authors.initials: E
authors.lastname: Reijerse
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lubitz
mesh_terms: D002384:Catalysis; D020134:Catalytic Domain; D004578:Electron Spin Resonance Spectroscopy; D006864:Hydrogenase / Q000737:chemistry*; D009532:Nickel / Q000737:chemistry*; D015204:Spectroscopy, Mossbauer; D013816:Thermodynamics
chemical_list: D009532:Nickel; C043296:nickel-iron hydrogenase; D006864:Hydrogenase