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.
Electrochemical studies of [Fe (CO) (κ -dmpe)(μ-dithiolate)] (dithiolate=adt , pdt) and density functional theory (DFT) calculations reveal the striking influence of an amine functionality in the dithiolate bridge on their oxidative properties. [Fe (CO) (κ -dmpe)(μ-adt )] (1) undergoes two one-electron oxidation steps, with the first being partially reversible and the second irreversible. When the adt bridge is replaced with pdt, a shift of 60 mV towards more positive potentials is observed for the first oxidation whereas 290 mV separate the oxidation potentials of the two cations. Under CO, oxidation of azadithiolate compound 1 occurs according to an ECE process whereas an EC mechanism takes place for the propanedithiolate species 2. The dication species [1-CO] resulting from the two-electron oxidation of 1 has been spectroscopically and structurally characterized. The molecular details underlying the reactivity of oxidized species have been explored by DFT calculations. The differences in the behaviors of 1 and 2 are mainly due to the presence, or not, of favored interactions between the dithiolate bridge and the diiron site depending on the redox states, Fe Fe or Fe Fe , of the complexes.
Alzheimer's disease (AD) involves a number of factors including an anomalous interaction of copper with the amyloid peptide (Aβ), inducing oxidative stress with radical oxygen species (ROS) production through a three-step cycle in which O2 is gradually reduced to superoxide, oxygen peroxide and finally OH radicals.
DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe(CN){μ-CN(Me)}(μ-CO)(CO)(Cp)] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN and amino-carbyne ligands have been elucidated. After the first reduction, CN works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN that forms a Fe(CNH) moiety: i.e., the acidic source for the H generation. The hydride (formally 2e/H), necessary to the heterocoupling with H is thus provided by the μ-CN(Me) ligand and not by Fe centers, as occurs in typical LFeS derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN to CN(Me) is highlighted. The incompetence for the HER of the related species [Fe{μ-CN(Me)}(μ-CO)(CO)(Cp)] (2) has been investigated and attributed to the loss of proton responsiveness caused by CN replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe scaffold, could be exploited to develop more active and robust catalysts for the HER.
The complexes Fe2 (pdt)(CNR)6 (pdt2− = CH2(CH2S−)2) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C6H4-4-OMe (1), C6H4-4-Cl (2), Me (3). These complexes represent electron-rich analogues of the parent Fe2(pdt)(CO)6. Unlike most substituted derivatives of Fe2(pdt)(CO)6, these isocyanide complexes are sterically unencumbered and have the same idealized symmetry as the parent hexacarbonyl derivatives. Like the hexacarbonyls, the stereodynamics of 1–3 involve both turnstile rotation of the Fe(CNR)3 as well as the inversion of the chair conformation of the pdt ligand. Structural studies indicate that the basal isocyanide has nonlinear CNC bonds and short Fe–C distances, indicating that they engage in stronger Fe–C π-backbonding than the apical ligands. Cyclic voltammetry reveals that these new complexes are far more reducing than the hexacarbonyls, although the redox behavior is complex. Estimated reduction potentials are E1/2 ≈ −0.6 ([2]+/0), −0.7 ([1]+/0), and −1.25 ([3]+/0). According to DFT calculations, the rotated isomer of 3 is only 2.2 kcal/mol higher in energy than the crystallographically observed unrotated structure. The effects of rotated versus unrotated structure and of solvent coordination (THF, MeCN) on redox potentials were assessed computationally. These factors shift the redox couple by as much as 0.25 V, usually less. Compounds 1 and 2 protonate with strong acids to give the expected μ-hydrides [H1]+ and [H2]+. In contrast, 3 protonates with [HNEt3]BArF4 (pKaMeCN = 18.7) to give the aminocarbyne [Fe2(pdt)(CNMe)5(μ-CN(H)Me)]+ ([3H]+). According to NMR measurements and DFT calculations, this species adopts an unsymmetrical, rotated structure. DFT calculations further indicate that the previously described carbyne complex [Fe2(SMe)2(CO)3(PMe3)2(CCF3)]+ also adopts a rotated structure with a bridging carbyne ligand. Complex [3H]+ reversibly adds MeNC to give [Fe2(pdt)(CNR)6(μ-CN(H)Me)]+ ([3H(CNMe)]+). Near room temperature, [3H]+ isomerizes to the hydride [(μ-H)Fe2(pdt)(CNMe)6]+ ([H3]+) via a first-order pathway.
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