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?

Density functional theory has been used to study diiron dithiolates [HFe₂(xdt)­(PR₃)_<i>n</i>(CO)_5–<i>n</i>X] (<i>n</i> = 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 (<i>E</i>°_μ‑H > <i>E</i>°_t‑H). The nature and number of PR₃ unusually affect Δ<i>E</i>°_{t‑H−μ‑H}: 4PEt₃ models feature a μ-H with a milder <i>E</i>° than t-H, whereas the 4PMe₃ analogues behave oppositely. The correlation Δ<i>E</i>°_{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 <i>E</i>°_μ‑H > <i>E</i>°_t‑H in 4-PEt₃ species but not in 4PMe₃ analogues. Also determinant to observe <i>E</i>°_μ‑H > <i>E</i>°_t‑H is the presence of a Fe apical σ-donor because its replacement with a CO yields <i>E</i>°_μ‑H < <i>E</i>°_t‑H even in 4PEt₃ species. Variants with neutral NHC and PMe₃ in place of CH₃S^– still feature <i>E</i>°_μ‑H > <i>E</i>°_t‑H. Replacing pdt with (Hadt)⁺ lowers <i>E</i>° but yields <i>E</i>°_μ‑H < <i>E</i>°_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 Δ<i>E</i>°_{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