DFT Dissection of the Reduction Step in H₂ Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?

Abstract

Density functional theory has been used to study diiron dithiolates [HFe2(xdt)(PR3)n(CO)5-nX] (n = 0, 2, 4; R = H, Me, Et; X = CH3S(-), PMe3, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H(+) + 2e(-) ↔ H2 reaction. Our study is focused on the reduction step following protonation of the Fe2(SR)2 core. Fe(H)s detected in solution are terminal (t-H) and bridging (μ-H) hydrides. Although unstable versus μ-Hs, synthetic t-Hs feature milder reduction potentials than μ-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to μ-H. Herein, we present another strategy: in place of preventing isomerization, μ-H could be made a stronger oxidant than t-H (E°μ-H > E°t-H). The nature and number of PR3 unusually affect ΔE°t-H-μ-H: 4PEt3 models feature a μ-H with a milder E° than t-H, whereas the 4PMe3 analogues behave oppositely. The correlation ΔE°t-H-μ-H ↔ stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt3 derivatives. One-electron reduction alleviates intramolecular repulsions only in μ-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe2 core. That implies that E°μ-H > E°t-H in 4-PEt3 species but not in 4PMe3 analogues. Also determinant to observe E°μ-H > E°t-H is the presence of a Fe apical σ-donor because its replacement with a CO yields E°μ-H < E°t-H even in 4PEt3 species. Variants with neutral NHC and PMe3 in place of CH3S(-) still feature E°μ-H > E°t-H. Replacing pdt with (Hadt)(+) lowers E° but yields E°μ-H < E°t-H, indicating that μ-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe2 core influences ΔE°t-H-μ-H, provided that (i) the R size of PR3 must be greater than that of Me and (ii) an electron donor must be bound to Fe apically.

Diiron Dithiolate Hydrides Complemented with Proton‐Responsive Phosphine–Amine Ligands

“…It is often observed that CO ligands preferentially occupy sites trans to hydride ligands in diferrous complexes of the type [(μ-H)Fe 2 (SR) 2 (CO) 6− x (PR 3 ) x ] +. [21] The N–H positions were refined crystallographically, revealing the presence of both amido and amino ligands. Thus, [H 1 H] 0 is described as HFe 2 (pdt)(CO) 2 (κ 2 -PNH 2 )(κ 2 -PNH).…”

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2(CO)4(κ2‐dmpe)(μ‐adtBn)] Related to the Active Site of [FeFe] Hydrogenases

“…DFT calculations have been performed with the pure functional BP86 and an all‐electron triple‐ζ basis set with polarization on all atoms (TZVP), as implemented in the TURBOMOLE 7.1.1 suite . This level of theory has provided good and consistent results in the reproduction of structural and reactivity features in bioinspired compounds and also very recently in other diiron catalysts for hydrogen evolution . In addition, the BP86/TZVP scheme was previously used to study the oxidative behavior of complex 1 in the absence and in the presence of CO .…”

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

“…These situations arise when bulky ligands are present, with reduction of the μ -H complex alleviating strain by allowing the metals to distance themselves from one another. 186 …”