Lewis acid protection turns cyanide containing [FeFe]-hydrogenase mimics into proton reduction catalysts

Abstract

Sustainable sources of hydrogen are a vital component of the envisioned energy transition. Understanding and mimicking the [FeFe]-hydrogenase provides a route to achieving this goal. In this study we re-visit a molecular mimic of the hydrogenase, the propyl dithiolate bridged complex [Fe₂(μ-pdt)(CO)₄(CN)₂]^2-, in which the cyanide ligands are tuned via Lewis acid interactions. This system provides a rare example of a cyanide containing [FeFe]-hydrogenase mimic capable of catalytic proton reduction, as demonstrated by cyclic voltammetry. EPR, FTIR, UV-vis and X-ray absorption spectroscopy are employed to characterize the species produced by protonation, and reduction or oxidation of the complex. The results reveal that biologically relevant iron-oxidation states can be generated, potentially including short-lived mixed valent Fe(I)Fe(II) species. We propose that catalysis is initiated by protonation of the diiron complex and the resulting di-ferrous bridging hydride species can subsequently follow two different pathways to promote H₂ gas formation depending on the applied reduction potential.

Fig. 1. Panel A: The H-cluster, the active site of [FeFe]-hydrogenase, consisting of a [4Fe–4S] cluster fused with the dinuclear [2Fe]_H subsite (adapted from entry 6SG2 in the Protein Databank). The black dashed lines represent hydrogen bonding of the cyanide ligands of the [2Fe]_H subsite, to amino acids of the protein (P108 and A109; I204 and P203); and the yellow dashes denote linking of the [4Fe–4S]_H and [2Fe]_H sub-complexes by a bridging cystein thiol. Panel B: Cyanide substituted structural mimics of the [2Fe]_H subsite with different central groups in the bridging dithiolate ligand, [Fe₂(μ-adt)(CO)₄(CN)₂]²⁻ (1²⁻) and [Fe₂(μ-pdt)(CO)₄(CN)₂]²⁻ (2²⁻); [Fe₂(μ-pdt)(CO)₄(CN-BCF)₂]²⁻ (3²⁻) is formed upon BCF addition to 2²⁻; and [(μ-H)Fe₂(μ-pdt)(CO)₄(CN-BCF)₂]⁻ (4⁻) is a bridging hydride species formed by protonation of 3²⁻.

Fig. 2. FTIR spectra of studied complexes, 2²⁻ (black spectrum), its corresponding borane adduct 3²⁻ (red spectrum), and the protonated borane adduct 4⁻ (blue spectrum). Spectra were recorded on 0.5 mM solutions of the complexes in acetonitrile; 4⁻ was prepared by adding 4 eq. of HCl (2 mM) to a 0.5 mM solution of 3²⁻; and 3²⁻ was treated with AgNO₃ at room temperature to give 5 (green spectrum) IR band frequencies are summarised in Table 1 (corresponding EPR spectra are shown in Fig. S17†).

Fig. 3. The reduction of 4⁻ followed by FTIR (panel A) and EPR (panel B) spectroscopy. Panel A: FTIR spectra following the addition of CoCp* to complex 4⁻. 5 mM 3²⁻ (red spectrum); 5 mM 4⁻ (blue spectrum); 5 mM 4⁻ + 20 mM (4 eq.) CoCp* collected 3 min after mixing (orange spectrum), revealing a mixture of 3²⁻ and 4⁻; 5 mM 4⁻ + 20 mM (4 eq.) CoCp* collected 10 min after mixing (magenta spectrum), revealing complete conversion to 3²⁻. The contributions of a possible intermediate at the 3 min time-point is shown as a grey dashed line. Panel B: EPR spectrum following the addition of CoCp* to 4⁻ at −40 °C (green spectrum), showing a mixture of two paramagnetic species; simulated EPR spectrum following addition of CoCp* to 4⁻ (grey dashed line, for details see Fig. S20†) and spectrum recorded following incubation at room temperature for 5 min (magenta spectrum), yielding an EPR silent product.

Scheme 1. Schematic overview summarising the observed redox and protonation chemistry of 3²⁻. Note that 4²⁻ is proposed based on X-band EPR. The chemical reagents employed to trigger a specific reaction are shown, while an electrochemical redox process is indicated by “e⁻”.

Fig. 4. X-ray absorption spectroscopy data at the Fe K-edge of diiron complexes in MeCN solution. (A and B) X-ray absorption near edge structure (XANES) spectra of indicated complexes. (C) Fe K-edge energies (at 50% level) of the XANES spectra. (D) Fourier-transforms of the extended X-ray absorption fine structure (EXAFS) spectra in (E) of the complexes (black lines, experimental data; coloured lines, simulations with parameters in Table S4†). The annotations refer to the complexes shown in Fig. 1. The spectrum denoted 5 is the result of oxidizing 3²⁻ with AgNO₃ (5 is also observed by UV-vis and IR spectroscopy in Fig. S13 and S14†). 6 is Fe₂(μ-pdt)(CO)₆. The black dashed lines in (B and E) show the spectra of 4⁻ after reduction with CoCp to regain 3²⁻.

Fig. 5. Cyclic voltammograms showing oxidation features of complexes 3²⁻ and 4⁻ in acetonitrile. Complex 4⁻ is generated in situ via addition of HCl to a solution of 3²⁻. 5 mM analyte, 0.2 M TBAPF₆ (electrolyte), scan rate: 0.1 V s⁻¹, scan window: −1.0 to 0.2 V vs. Fc^+/0.

Fig. 6. Cyclic voltammograms showing the catalytic current response observed when adding HCl to 3²⁻ (5 mM) in acetonitrile. The first five titration points are shown; 0 mM HCl (black trace, also in inset); 5 mM HCl (red trace); 10 mM HCl (blue trace); 15 mM HCl (green trace); 20 mM HCl (purple trace). The full titration is shown in the ESI Fig. S22. 0.2 M TBAPF₆ (electrolyte), scan rate: 0.1 V s⁻¹, scan window: −1.25 to −2.01 V vs. Fc^+/0.