Protonation of nickel-iron hydrogenase models proceeds after isomerization at nickel

Abstract

Theory and experiment indicate that the protonation of reduced NiFe dithiolates proceeds via a previously undetected isomer with enhanced basicity. In particular, it is proposed that protonation of (OC)3Fe(pdt)Ni(dppe) (1; pdt(2-) = (-)S(CH2)3S(-); dppe = Ph2P(CH2)2PPh2) occurs at the Fe site of the two-electron mixed-valence Fe(0)Ni(II) species, not the Fe(I)-Ni(I) bond for the homovalence isomer of 1. The new pathway, which may have implications for protonation of other complexes and clusters, was uncovered through studies on the homologous series L(OC)2Fe(pdt)M(dppe), where M = Ni, Pd (2), and Pt (3) and L = CO, PCy3. Similar to 1, complexes 2 and 3 undergo both protonation and 1e(-) oxidation to afford well-characterized hydrides ([2H](+) and [3H](+)) and mixed-valence derivatives ([2](+) and [3](+)), respectively. Whereas the Pd site is tetrahedral in 2, the Pt site is square-planar in 3, indicating that this complex is best described as Fe(0)Pt(II). In view of the results on 2 and 3, the potential energy surface of 1 was reinvestigated with density functional theory. These calculations revealed the existence of an energetically accessible and more basic Fe(0)Ni(II) isomer with a square-planar Ni site.

Figure 1

Active site of [NiFe]-H₂ase, a bidirectional catalyst, participates in redox and acid–base chemistry (left). Similar behavior is observed for the prototypical model complex 1 (right).

Scheme 1

Scheme 2

Figure 2

ORTEP of 2 with ellipsoids drawn at the 50% probability level and H atoms omitted for clarity. Selected distances (Å) (mean DFT values in parentheses): Pd1–Fe1, 2.561 (2.56); Pd1–P1, 2.249; Pd1–P2, 2.271 (2.26); Pd1–S1, 2.573; Pd1–S2, 2.402 (2.49); Fe1–S1, 2.299; Fe1–S2, 2.275 (2.30); Fe1–C30, 1.795; Fe1–C31, 1.793; Fe1–C32, 1.798 (1.77). Selected angles (deg) (mean DFT values in parentheses): P1–Pd1–S1, 135.2; P1–Pd1–S2, 118.83; P2–Pd1–S1, 115.7; P2–Pd1–S2, 122.5 (135.1); S1–Fe1–C30, 162.2; S2–Fe1–C32, 163.1 (161.1).

Figure 3

Cyclic voltammograms of 2 (dotted trace) and [2′]BF₄ (solid trace) acquired in CH₂Cl₂ with 100 mM NBu₄PF₆. Potentials (V vs Fc^0/+) were swept at 100 mV s^–1.

Figure 4

X-band EPR spectra (CH₂Cl₂/PhMe, 110 K) of [2]BF₄ and [2′]BF₄.

Figure 5

ORTEP of one of two independent complexes in [2H]BF₄·THF·0.5Et₂O with ellipsoids drawn at the 50% probability level. The solvate molecules, BF₄^– anion and H atoms (except the H^– ligand) are omitted for clarity. Mean distances in the complexes (Å) (mean DFT values in parentheses): Pd2–Fe2, 2.882 (2.92); Pd2–H2, 2.173 (2.19); Pd2–P3, 2.253; Pd2–P4, 2.270 (2.30); Pd2–S3, 2.367; Pd2–S4, 2.363 (2.39); Fe2–S3, 2.330; Fe2–S4, 2.338 (2.34); Fe2–H2, 1.483 (1.54). Mean angles in the complexes (deg) (mean DFT values in parentheses): Pd2–H2–Fe2, 102.4 (101.2); P3–Pd2–S4, 171.5; P4–Pd2–S3, 177.7 (177.6); S3–Fe2–C35, 165.4; S4–Fe2–C33, 167.1 (165.8).

Figure 6

Energy-minimized structure of 3 obtained with DFT. Selected distances (Å): Fe–Pt, 2.87; Fe–C1, 1.75; Fe–C2, 1.75; Fe–C3, 1.76; Fe–S1, 2.34; Fe–S2, 2.37; Pt–S1, 2.37; Pt–S2, 2.38; Pt–P1, 2.28; Pt–P2, 2.28. Selected angles (deg): C1–Fe–S2, 136.8; C2–Fe–S1, 169.4; S1–Pt–P2, 176.1; S2–Pt–P1, 177.7.

Scheme 3

Scheme 4

Figure 7

Energy-minimized structures obtained with DFT for [1H]⁺ with the (pdt)Ni(dppe) moiety either square-planar (red) or distorted tetrahedral (blue). The optimized geometries are superimposed and presented in two views. Other (higher energy) isomers/tautomers proposed can be found in Figure S57.

Scheme 5

Scheme 6