Models of the Ni-L and Ni-SI_a States of the [NiFe]-Hydrogenase Active Site

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^IIFe^II center with five-coordinated iron, as proposed for the Ni-SI_a 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^IFe^II compound. The Ni^IFe^II assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni^IIFe^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.