Effect of Pyramidalization of the M₂(SR)₂ Center: The Case of (C₅H₅)₂Ni₂(SR)₂

The effects of simple thiolates vs chelating dithiolates on M–M bonding, redox potentials, and synthetic outcomes have been probed experimentally and computationally. Nickelocene (Cp₂Ni) has long been known to react with simple thiols to give diamagnetic Cp₂Ni₂(SR)₂ with planar Ni₂S₂ cores and long Ni- - -Ni distances. Ethane- and propanedithiol (edtH₂ and pdtH₂, respectively) instead give di-, tri-, and pentanickel complexes, with nonplanar Ni₂S₂ cores. The 36e Cp₂Ni₂(pdt) (<b>1</b>^<b>pdt</b>) adopts a symmetrical butterfly Ni₂S₂ structure. Variable-temperature NMR spectra indicate that <b>1</b>^<b>pdt</b> possesses a thermally accessible triplet state (Δ<i>G</i> = 2.65(5) kcal/mol) in equilibrium with a diamagnetic ground state. DFT calculations indicate that the singlet–triplet gap is highly sensitive to the nonplanarity of the Ni₂S₂ core. The calculations further reveal that only the high-spin form of <b>1</b>^<b>pdt</b> features Ni–Ni bonding, which is unprecedented. Cp₂Ni₃(pdt)₂ (<b>2</b>^<b>pdt</b>), which derives from <b>1</b>^<b>pdt</b>, crystallized as cis and trans isomers, both with a central Ni­(pdt)₂^2– unit that is S,S-chelated to two CpNi⁺ centers. Reaction of Cp₂Ni with 1,2-ethanedithiol (H₂edt) and 1,2-benzenedithiol (bdtH₂) exclusively gave the trinickel species <b>2</b>^<b>edt</b> and <b>2</b>^<b>bdt</b>, which are structurally analogous to <i>cis</i>-<b>2</b>^<b>pdt</b>. Solutions of <b>2</b>^<b>edt</b> are unstable, depositing crystals of Cp₂Ni₅(edt)₄ (<b>3</b>^<b>edt</b>). Cyclic voltammetric studies show that the Ni₂ species oxidize readily to give the mixed-valence cations [<b>1</b>^<b>pdt</b>]⁺ and [<b>1</b>^<b>edt</b>]⁺. Crystallographic and EPR analyses indicate that these cations are delocalized mixed-valence Ni­(II)–Ni­(III) species. Oxidation of Cp₂Ni₂(SEt)₂, which features a planar Ni₂S₂ core, afforded a mixed-valence cation, showing the pyramidal Ni₂S₂ core observed in [<b>1</b>^<b>pdt</b>]^0/+ and [<b>1</b>^<b>edt</b>]^0/+. Although not obtained from the Cp₂Ni/H₂edt reaction, the neutral complex <b>1</b>^<b>edt</b> was obtained by reduction of [<b>1</b>^<b>edt</b>]⁺. Variable-temperature NMR measurements and DFT calculations indicate that the triplet is further stabilized in this highly pyramidalized species

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

Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C–S Bond Homolysis in a Mixed Valence Diiron Dithiolate

The synthesis and redox properties are presented for the electron-rich bis­(monothiolate)­s Fe₂(SR)₂­(CO)₂­(dppv)₂ for R = Me ([<b>1</b>]⁰), Ph ([<b>2</b>]⁰), CH₂Ph ([<b>3</b>]⁰). Whereas related derivatives adopt <i>C</i>₂-symmetric Fe₂(CO)₂P₄ cores, [<b>1</b>]⁰–[<b>3</b>]⁰ have <i>C</i>_s symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [<b>1</b>]⁰–[<b>3</b>]⁰ undergo 1e^– oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe₂(SR)₂­(CO)₂­(dppv)₂]⁺. As established crystallographically, [<b>3</b>]⁺ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [<b>1</b>]⁺ and [<b>2</b>]⁺ and many other [Fe₂­(SR)₂L₆]⁺ derivatives, [<b>3</b>]⁺ undergoes C–S bond homolysis, affording the diferrous sulfido-thiolate [Fe₂­(SCH₂Ph)­(S)­(CO)₂­(dppv)₂]⁺ ([<b>4</b>]⁺). According to X-ray crystallography, the first coordination spheres of [<b>3</b>]⁺ and [<b>4</b>]⁺ are similar, but the Fe–sulfido bonds are short in [<b>4</b>]⁺. The conversion of [<b>3</b>]⁺ to [<b>4</b>]⁺ follows first-order kinetics, with <i>k</i> = 2.3 × 10^–6 s^–1 (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [<b>3</b>]⁺ to [<b>4</b>]⁺ is accelerated about 10×, the main organic product being TEMPO-CH₂Ph. DFT calculations predict that the homolysis of a C–S bond is exergonic for [Fe₂­(SCH₂Ph)₂­(CO)₂­(PR₃)₄]⁺ but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [<b>4</b>]⁺ is indicated by its double carbonylation to give [Fe₂­(SCH₂Ph)­(S)­(CO)₄­(dppv)₂]⁺ ([<b>5</b>]⁺), which adopts a bioctahedral structure

Preparation and Protonation of Fe₂(pdt)(CNR)₆, Electron-Rich Analogues of Fe₂(pdt)(CO)₆

The complexes Fe₂(pdt)­(CNR)₆ (pdt^2– = CH₂(CH₂S^–)₂) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C₆H₄-4-OMe (<b>1</b>), C₆H₄-4-Cl (<b>2</b>), Me (<b>3</b>). 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 <b>1</b>–<b>3</b> 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 <i>E</i>_1/2 ≈ −0.6 ([<b>2</b>]^+/0), −0.7 ([<b>1</b>]^+/0), and −1.25 ([<b>3</b>]^+/0). According to DFT calculations, the rotated isomer of <b>3</b> 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 <b>1</b> and <b>2</b> protonate with strong acids to give the expected μ-hydrides [H<b>1</b>]⁺ and [H<b>2</b>]⁺. In contrast, <b>3</b> protonates with [HNEt₃]­BAr^F₄ (p<i>K</i>_a^MeCN = 18.7) to give the aminocarbyne [Fe₂(pdt)­(CNMe)₅­(μ-CN­(H)­Me)]⁺ ([<b>3</b>H]⁺). 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 [<b>3</b>H]⁺ reversibly adds MeNC to give [Fe₂(pdt)­(CNR)₆(μ-CN­(H)­Me)]⁺ ([<b>3</b>H­(CNMe)]⁺). Near room temperature, [<b>3</b>H]⁺ isomerizes to the hydride [(μ-H)­Fe₂­(pdt)­(CNMe)₆]⁺ ([H<b>3</b>]⁺) via a first-order pathway

Preparation and Protonation of Fe₂(pdt)(CNR)₆, Electron-Rich Analogues of Fe₂(pdt)(CO)₆

The complexes Fe₂(pdt)­(CNR)₆ (pdt^2– = CH₂(CH₂S^–)₂) were prepared by thermal substitution of the hexacarbonyl complex with the isocyanides RNC for R = C₆H₄-4-OMe (<b>1</b>), C₆H₄-4-Cl (<b>2</b>), Me (<b>3</b>). 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 <b>1</b>–<b>3</b> 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 <i>E</i>_1/2 ≈ −0.6 ([<b>2</b>]^+/0), −0.7 ([<b>1</b>]^+/0), and −1.25 ([<b>3</b>]^+/0). According to DFT calculations, the rotated isomer of <b>3</b> 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 <b>1</b> and <b>2</b> protonate with strong acids to give the expected μ-hydrides [H<b>1</b>]⁺ and [H<b>2</b>]⁺. In contrast, <b>3</b> protonates with [HNEt₃]­BAr^F₄ (p<i>K</i>_a^MeCN = 18.7) to give the aminocarbyne [Fe₂(pdt)­(CNMe)₅­(μ-CN­(H)­Me)]⁺ ([<b>3</b>H]⁺). 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 [<b>3</b>H]⁺ reversibly adds MeNC to give [Fe₂(pdt)­(CNR)₆(μ-CN­(H)­Me)]⁺ ([<b>3</b>H­(CNMe)]⁺). Near room temperature, [<b>3</b>H]⁺ isomerizes to the hydride [(μ-H)­Fe₂­(pdt)­(CNMe)₆]⁺ ([H<b>3</b>]⁺) via a first-order pathway

Mechanistic Insight into Electrocatalytic H₂ Production by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe₂(CN)­{μ-CN­(Me)₂}­(μ-CO)­(CO)­(Cp)₂] (<b>1</b>) 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 H₂ generation. The hydride (formally 2e/H⁺), necessary to the heterocoupling with H⁺ is thus provided by the μ-CN­(Me)₂ ligand and not by Fe centers, as occurs in typical L₆Fe₂S₂ 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)₂ is highlighted. The incompetence for the HER of the related species [Fe₂{μ-CN­(Me)₂}­(μ-CO)­(CO)₂(Cp)₂]⁺ (<b>2</b>^<b>+</b>) 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 Fe₂ 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 Fe₂ scaffold, could be exploited to develop more active and robust catalysts for the HER

Imine-Centered Reactions in Imino-Phosphine Complexes of Iron Carbonyls

Fundamental reactions of imino-phosphine ligands were elucidated through studies on Ph₂PC₆H₄CHNC₆H₄-4-Cl (PCHNAr^Cl) complexes of iron(0), iron­(I), and iron­(II). The reaction of PCHNAr^Cl with Fe­(bda)­(CO)₃ gives Fe­(PCHNAr^Cl)­(CO)₃ (<b>1</b>), featuring an η²-imine. DNMR studies, its optical properties, and DFT calculations suggest that <b>1</b> racemizes on the NMR time scale via an achiral N-bonded imine intermediate. The <i>N</i>-imine isomer is more stable in Fe­(PCHNAr^OMe)­(CO)₃ (<b>1</b>^<b>OMe</b>), which crystallized despite being the minor isomer in solution. Protonation of <b>1</b> by HBF₄·Et₂O gave the iminium complex [<b>1</b>H]­BF₄. The related diphosphine complex Fe­(PCHNAr^Cl)­(PMe₃)­(CO)₂ (<b>2</b>), which features an η²-imine, was shown to also undergo N protonation. Oxidation of <b>1</b> and <b>2</b> with FcBF₄ gave the Fe­(I) compounds [<b>1</b>]­BF₄ and [<b>2</b>]­BF₄. The oxidation-induced change in hapticity of the imine from η² in [<b>1</b>]⁰ to κ¹ in [<b>1</b>]⁺ was verified crystallographically. Substitution of a CO ligand in <b>1</b> with PCHNAr^Cl gave Fe­[P₂(NAr^Cl)₂]­(CO)₂ (<b>3</b>), which contains the tetradentate diamidodiphosphine ligand. This C–C coupling is reversed by chemical oxidation of <b>3</b> with FcOTf. The oxidized product of [Fe­(PCHNAr^Cl)₂(CO)₂]²⁺ ([<b>4</b>]²⁺) was prepared independently by the reaction of [<b>1</b>]⁺, PCHNAr^Cl, and Fc⁺. The C–C scission is proposed to proceed concomitantly with the reduction of Fe­(II) via an intermediate related to [<b>2</b>]⁺