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]BF(4). One CO ligand in [1H]BF(4) is readily substituted by organophosphorus ligands to afford the substituted derivatives [HNiFe(pdt)(dppe)(PR(3))(CO)(2)]BF(4), where PR(3) = P(OPh)(3) ([2H]BF(4)); PPh(3) ([3H]BF(4)); PPh(2)Py ([4H]BF(4), 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)(PR(3)) center. In MeCN solution, 3, which can be prepared by deprotonation of [3H]BF(4) with NaOMe, is about 10(4) stronger base than is 1. X-ray crystallographic analysis of [3H]BF(4) revealed a highly unsymmetrical bridging hydride, the Fe-H bond being 0.40 Å shorter than the Ni-H distance. Complexes [2H]BF(4), [3H]BF(4), and [4H]BF(4) undergo reductions near -1.46 V vs Fc(0/+). For [2H]BF(4), 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 H(2) 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 H(2) evolution are estimated to be 430 mV, whereas the overpotential for the N-protonated pyridinium complex [4H(2)](2+) is estimated to be 260 mV. The mechanism of H(2) 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](+).

Figure 1

500 MHz ¹H NMR spectra (CD₂Cl₂ solution) of [1H]BF₄ (bottom, triplet, J_PH = 6 Hz) and of [3H]BF₄ (top, doublet of triplets, J_PH = 35 Hz).

Figure 2

FT-IR spectra in the ν_CO region for CH₂Cl₂ solutions of the nickel-iron hydride complexes described in this work (top to bottom): [1H]BF₄; [2H]BF₄; [3H]BF₄; [4H]BF₄. The ν_CO band for the Ni-R state occurs at 1936-1948 cm⁻¹, depending on the organism.

Figure 3

³¹P{¹H} NMR (161 MHz) spectra of CD₂Cl₂ solutions of [3H]BF₄ recorded at various temperatures. The signal at δ 68 is assigned to the Fe(PPh₃) center. The dynamic AB quartet at δ 65 is assigned to the Ni(dppe) center. Weak signals at δ 59 and 69 were verified to arise from trace impurities of Ni(pdt)(dppe) and [HNiFe(pdt)(dppe)(CO)₃]⁺, respectively.

Figure 4

Structure of [3H]BF₄. Selected distances (Å): Fe(1)-Ni(1), 2.6432(7); Fe(1)-H(1), 1.49(3); Ni(1)-H(1), 1.89(3). Selected Bond angles (°): S(2)-Fe(1)-S(1), 83.27(4); S(2)-Fe(1)-P(1), 93.58(4); C(1)-Fe(1)-C(2), 99.93(17).

Figure 5

FT-IR spectra of CH₂Cl₂ solutions of [3H]BF₄ (top) and 3 (bottom).

Figure 6

³¹P{¹H} NMR spectrum (161 MHz, CD₂Cl₂, ~20 °C) of 3. Signals at δ 77 and 45 are assigned to dppe, the signal at δ 55 is assigned to PPh₃.

Figure 7

Cyclic voltammograms of a 1.85 mM MeCN solution of [2H]⁺ (left) and a 1.68 mM 9:1 MeCN/CH₂Cl₂ solution (0.1 M [NBu₄]PF₆) of 2 (right) at various scan rates, denoted in mV/s.

Figure 8

Scan rate dependence of i_pc for the couples [2H]^+/0 and 2^0/+ in CH₂Cl₂ solution (~1.8 mM complex, 0.1 M [NBu₄]PF₆).

Figure 9

Cyclic voltammograms of [2H]BF₄ (left) and [3H]BF₄ (right) with increasing equiv of CF₃COOH (denoted on right). Overpotentials were estimated by the standard potential for hydrogen evolution from CF₃COOH in MeCN solution. Conditions: ~0.5 mM in CH₂Cl₂ (see experimental), 0.1M [NBu₄]PF₆, scan rate 0.1 V/s, glassy carbon working electrode (d = 3.0 mm); Ag wire pseudoreference with internal Fc standard at 0 V; Pt counter electrode.

Figure 10

Cyclic voltammograms of a CH₂Cl₂ solution (0.74 mM) of [4H]BF₄ with increasing equiv of CF₃CO₂H (denoted on right). Conditions: see Figure 9.

Figure 11

Influence of [acid]:catalyst ratio on catalytic current (i_c/i_p) for [1H]BF₄ (black circles), [2H]BF₄ (red squares), [3H]BF₄ (blue triangles), and [4H]BF₄ (green diamonds). Conditions: see Figure 9.

Scheme 1

Proposed Ni-centered dynamics for 1 (L = CO) and 2 (L = P(OPh)₃), 3 (L = PPh₃), and 4 (L = PPh₂py).

Scheme 2

Representation of the Fe- and Ni-centered dynamic processes proposed for [2H]BF₄. Variable temperature ¹³C NMR measurements indicate that pathway A has the lower barrier.

Scheme 3

Catalytic cycle proposed for hydrogen evolution by [HNiFe(pdt)(dppe)L₂(CO)]BF₄.