Hydride-Containing Models for the Active Site of the Nickel−Iron Hydrogenases

Abstract

The [NiFe]-hydrogenase model complex NiFe(pdt)(dppe)(CO)₃ (1) (pdt = 1,3-propanedithiolate) has been efficiently synthesized and found to be robust. This neutral complex sustains protonation to give the first nickel−iron hydride [1H]BF₄. One CO ligand in [1H]BF₄ is readily substituted by organophosphorus ligands to afford the substituted derivatives [HNiFe(pdt)(dppe)(PR₃)(CO)₂]BF₄, where PR₃ = P(OPh)₃ ([2H]BF₄); PPh₃ ([3H]BF₄); PPh₂Py ([4H]BF₄, where Py = 2-pyridyl). Variable temperature NMR measurements show that the neutral and protonated derivatives are dynamic on the NMR time scale, which partially symmetrizes the phosphine complex. The proposed stereodynamics involve twisting of the Ni(dppe) center, not rotation at the Fe(CO)₂(PR₃) center. In MeCN solution, 3, which can be prepared by deprotonation of [3H]BF₄ with NaOMe, is about 10⁴ stronger base than is 1. X-ray crystallographic analysis of [3H]BF₄ revealed a highly unsymmetrical bridging hydride, the Fe−H bond being 0.40 Å shorter than the Ni−H distance. Complexes [2H]BF₄, [3H]BF₄, and [4H]BF₄ undergo reductions near −1.46 V vs Fc^0/+. For [2H]BF₄, this reduction process is reversible, and we assign it as a one-electron process. In the presence of trifluoroacetic acid, proton reduction catalysis coincides with this reductive event. The dependence of i_c/i_p on the concentration of the acid indicates that H₂ evolution entails protonation of a reduced hydride. For [2H]⁺, [3H]⁺, and [4H]⁺, the acid-independent rate constants are 50−75 s⁻¹. For [2H]⁺ and [3H]⁺, the overpotentials for H₂ evolution are estimated to be 430 mV, whereas the overpotential for the N-protonated pyridinium complex [4H₂]²⁺ is estimated to be 260 mV. The mechanism of H₂ evolution is proposed to follow an ECEC sequence, where E and C correspond to one-electron reductions and protonations, respectively. On the basis of their values for its pK_a and redox potentials, the room temperature values of ΔG_H• and ΔG_H- are estimated as respectively as 57 and 79 kcal/mol for [1H]⁺.