Unsensitized Photochemical Hydrogen Production Catalyzed by Diiron Hydrides

The diiron hydride [(μ-H)­Fe₂(pdt)­(CO)₄(dppv)]⁺ ([H<b>2</b>]⁺, dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) is shown to be an effective photocatalyst for the H₂ evolution reaction (HER). These experiments establish the role of hydrides in photocatalysis by biomimetic diiron complexes. Trends in redox potentials suggests that other unsymmetrically substituted diiron hydrides are promising catalysts. Unlike previous catalysts for photo-HER, [H<b>2</b>]⁺ functions without sensitizers: irradiation of [H<b>2</b>]⁺ in the presence of triflic acid (HOTf) efficiently affords H₂. Instead of sacrificial electron donors, ferrocenes can be used as recyclable electron donors for the photocatalyzed HER, resulting in 4 turnovers

Characterization of a Borane σ Complex of a Diiron Dithiolate: Model for an Elusive Dihydrogen Adduct

The azadithiolate complex Fe₂[(SCH₂)₂NMe]­(CO)₆ reacts with borane to give an initial 1:1 adduct, which spontaneously decarbonylates to give Fe₂[(SCH₂)₂NMeBH₃]­(CO)₅. Featuring a Fe–H–B three-center, two-electron interaction, the pentacarbonyl complex is a structural model for H₂ complexes invoked in the [FeFe]-hydrogenases. The pentacarbonyl compound is a “σ complex”, where a B–H σ bond serves as a ligand for iron. The structure of this σ complex was characterized by variable-temperature NMR spectroscopy and X-ray crystallography. Complementary to the 1:1 borane adduct is the quaternary ammonium complex [Fe₂[(SCH₂)₂NMe₂]­(CO)₆]⁺, which was also characterized. It represents a kinetically robust analogue of the N-protonated amine cofactor, as indicated by its mild reduction potential

New Reactions of Terminal Hydrides on a Diiron Dithiolate

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe₂(SR)₂H₂ species is provided by the complex [(<i>term</i>-H)­(μ-H)­Fe₂(pdt)­(CO)­(dppv)₂] ([H<b>1</b>H]⁰). Spectroscopic and computational studies indicate that [H<b>1</b>H]⁰ contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)­Fe₂(pdt)­(CO)₂(dppv)₂]⁺ ([H<b>1</b>(CO)]⁺), which undergoes substitution to afford [(μ-H)­Fe₂(pdt)­(CO)­(NCMe)­(dppv)₂]⁺ ([H<b>1</b>(NCMe)]⁺). Upon treatment of [H<b>1</b>(NCMe)]⁺ with borohydride salts, the MeCN ligand is displaced to afford [H<b>1</b>H]⁰. DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s^–1 at −40 °C. The compound reacts with D₂ to afford [D<b>1</b>D]⁰, but not mixed isotopomers such as [H<b>1</b>D]⁰. The dihydride undergoes oxidation with Fc⁺ under CO to give [<b>1</b>(CO)]⁺ and H₂. Protonation in MeCN solution gives [H<b>1</b>(NCMe)]⁺ and H₂. Carbonylation converts [H<b>1</b>H]⁰ into [<b>1</b>(CO)]⁰

Excited State Properties of Diiron Dithiolate Hydrides: Implications in the Unsensitized Photocatalysis of H₂ Evolution

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to investigate how visible light photons can excite an asymmetrically substituted diiron hydride, [Fe₂(pdt)­(μ-H)­(CO)₄dppv]⁺ (<b>1</b>^<b>+</b>, dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂; pdt = 1,3-propanedithiolate), as well as the symmetric species [Fe₂(pdt)­(μ-H)­(CO)₄(PMe₃)₂]⁺ (<b>2</b>^<b>+</b>), which are the first photocatalysts of proton reduction operating without employing sensitizers (Wang, W.; Rauchfuss, T. B.; Bertini, L.; Zampella, G.; <i>J. Am. Chem. Soc.</i>, <b>2012</b>, <i>134</i>, 4525). Theoretical results illustrate that the peculiar reactivity associated to the excited states of <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> is compatible with three different scenarios: (i) it can arise from the movement of the hydride ligand from fully bridging to semibridging/terminal coordination, which is expected to be more reactive toward protons; (ii) reactivity could be related to cleavage of a Fe–S bond, which implies formation of a transient Fe penta-coordinate species that would trigger a facile turnstile hydride isomerization, if lifetime excitation is long enough; (iii) also in line with a Fe–S bond cleavage is the possibility that after excited state decay, a highly basic S center is protonated so that a species simultaneously containing S–H^δ+ and Fe–H^δ− moieties is formed and, once reduced by a suitable electron donor, it can readily afford H₂ plus an unprotonated form of the FeFe complex. This last possibility is consistent with ³¹P NMR and IR solution data. All the three possibilities are compatible with the capability of <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> to perform photocatalysis of hydrogen evolving reaction (HER) without sensitizer. Moreover, even though it turned out difficult to discriminate among the three scenarios, especially because of the lack of experimental excitation lifetimes, it is worth underscoring that all of the three pathways represent a novelty regarding diiron carbonyl photoreactivity, which is usually associated with CO loss. Results provide also a rationale to the experimental observations which showed that the simultaneous presence of donor ligands (dppv in the case of <b>1</b>^<b>+</b>) and a H ligand in the coordination environment of diiron complexes is a key factor to prevent CO photodissociation and catalyze HER. Finally, the comparison of photoexcitation behavior of <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> allows a sort of generalization about the functioning of such hydride species

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

Evidence for the Formation of a Mo–H Intermediate in the Catalytic Cycle of Formate Dehydrogenase

DFT/BP86/TZVP and DFT/B3LYP/TZVP have been used to investigate systematically the reaction pathways associated with the H-transfer step, which is the rate-determining step of the reaction HCOO^– ⇄ CO₂ + H⁺ + 2e^–, as catalyzed by metalloenzyme formate dehydrogenase (FDH). Actually, the energetics associated with the transfer from formate to all H (proton or hydride) acceptors that are present within the FDH active site have been sampled. This study points to a viable intimate mechanism in which the metal center mediates H transfer from formate to the final acceptor, i.e. a selenocysteine residue. The Mo-based reaction pathway, consisting of a β-H elimination to metal with concerted decarboxylation, turned out to be favored over previously proposed routes in which proton transfer occurs directly from HCOO^– to selenocysteine. The proposed reaction pathway is reminiscent of the key step of metal-based catalysis of the water–gas shift reaction

Contrasting Protonation Behavior of Diphosphido vs Dithiolato Diiron(I) Carbonyl Complexes

This paper reports on the protonation of phosphine-substituted diiron diphosphido carbonyls, analogues of diiron dithiolato centers at the active sites of hydrogenase enzymes. Reaction of the diphosphines (CH₂)_<i>n</i>(PPhH)₂ (<i>n</i> = 2 (edpH₂) and <i>n</i> = 3 (pdpH₂)) with Fe₃(CO)₁₂ gave excellent yields of Fe₂(edp)­(CO)₆ (<b>1</b>) and Fe₂(pdp)­(CO)₆ (<b>2</b>). Substitution of Fe₂(edp)­(CO)₆ with PMe₃ afforded Fe₂(edp)­(CO)₂(PMe₃)₄ (<b>3</b>; ν_CO 1855 and 1836 cm^–1). Crystallographic analysis showed that <b>3</b> adopts an idealized <i>C</i>₂ symmetry, with pairs of phosphine ligands occupying apical–basal sites on each Fe center. Relative to that in the dithiolato complex, the Fe–Fe bond (2.7786(8) Å) is elongated by 0.15 Å. Treatment of <b>3</b> with H­(OEt₂)₂BAr^F₄ (Ar^F = C₆H₃-3,5-(CF₃)₂) gave exclusively the <i>C</i>₂-symmetric μ-hydride complex [HFe₂(edp)­(CO)₂(PMe₃)₄]⁺. This result contrasts with the behavior of the analogous ethanedi<i>thiolate</i> Fe₂(edt)­(CO)₂(PMe₃)₄ (edt = 1,2-C₂H₄S₂), protonation of which gives both the bridging <i>and terminal</i> hydride complexes. This difference points to the participation of the sulfur centers in the formation of terminal hydrides. The absence of terminal hydride intermediates was also revealed in the protonation of the diphosphine diphosphido complexes Fe₂(pdp)­(CO)₄(dppv) (<b>4</b>; dppv = <i>cis</i>-1,2-C₂H₂(PPh₂)₂) and Fe₂(edp)­(CO)₄(dppbz) (<b>5</b>; dppbz = 1,2-C₆H₄(PPh₂)₂). Protonation of these diphosphine complexes afforded μ-hydrido cations with apical–basal diphosphine ligands, which convert to the isomer where the diphosphine is dibasal. In contrast, protonation of the dithiolato complex Fe₂(pdt)­(CO)₄(dppv) gave terminal hydrides, which isomerize to μ-hydrides. In a competition experiment, <b>4</b> was shown to protonate faster than Fe₂(pdt)­(CO)₄(dppv)

Terminal vs Bridging Hydrides of Diiron Dithiolates: Protonation of Fe₂(dithiolate)(CO)₂(PMe₃)₄

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe₂(xdt)­(CO)₂(PMe₃)₄, where xdt is edt (ethanedithiolate, <b>1</b>), pdt (propanedithiolate, <b>2</b>), and adt (2-aza-1,3-propanedithiolate, <b>3</b>), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds <b>1</b>–<b>3</b> oxidize near −950 mV vs Fc^+/0. Crystallographic analyses confirm that <b>1</b> and <b>2</b> adopt <i>C</i>₂-symmetric structures (Fe–Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of <b>1</b> afforded exclusively [μ-H<b>1</b>]⁺, establishing the <i>non</i>-intermediacy of the terminal hydride ([<i>t</i>-H<b>1</b>]⁺). At higher temperatures, protonation afforded mainly [<i>t</i>-H<b>1</b>]⁺. The temperature dependence of the ratio [<i>t</i>-H<b>1</b>]⁺/[μ-H<b>1</b>]⁺ indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature ³¹P­{¹H} NMR measurements indicate that the protonation of <b>2</b> proceeds by an intermediate, proposed to be the <i>S</i>-protonated dithiolate [Fe₂(Hpdt)­(CO)₂(PMe₃)₄]⁺ ([<i>S</i>-H<b>2</b>]⁺). This intermediate converts to [<i>t</i>-H<b>2</b>]⁺ and [μ-H<b>2</b>]⁺ by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the <i>S</i>-protonated species (e.g., [<i>S</i>-H<b>2</b>]⁺) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of <b>3</b> affords exclusively terminal hydrides, regardless of the acid or conditions, to give [<i>t</i>-H<b>3</b>]⁺, which isomerizes to [<i>t</i>-H<b>3′</b>]⁺, wherein all PMe₃ ligands are basal

Terminal vs Bridging Hydrides of Diiron Dithiolates: Protonation of Fe₂(dithiolate)(CO)₂(PMe₃)₄

This investigation examines the protonation of diiron dithiolates, exploiting the new family of exceptionally electron-rich complexes Fe₂(xdt)­(CO)₂(PMe₃)₄, where xdt is edt (ethanedithiolate, <b>1</b>), pdt (propanedithiolate, <b>2</b>), and adt (2-aza-1,3-propanedithiolate, <b>3</b>), prepared by the photochemical substitution of the corresponding hexacarbonyls. Compounds <b>1</b>–<b>3</b> oxidize near −950 mV vs Fc^+/0. Crystallographic analyses confirm that <b>1</b> and <b>2</b> adopt <i>C</i>₂-symmetric structures (Fe–Fe = 2.616 and 2.625 Å, respectively). Low-temperature protonation of <b>1</b> afforded exclusively [μ-H<b>1</b>]⁺, establishing the <i>non</i>-intermediacy of the terminal hydride ([<i>t</i>-H<b>1</b>]⁺). At higher temperatures, protonation afforded mainly [<i>t</i>-H<b>1</b>]⁺. The temperature dependence of the ratio [<i>t</i>-H<b>1</b>]⁺/[μ-H<b>1</b>]⁺ indicates that the barriers for the two protonation pathways differ by ∼4 kcal/mol. Low-temperature ³¹P­{¹H} NMR measurements indicate that the protonation of <b>2</b> proceeds by an intermediate, proposed to be the <i>S</i>-protonated dithiolate [Fe₂(Hpdt)­(CO)₂(PMe₃)₄]⁺ ([<i>S</i>-H<b>2</b>]⁺). This intermediate converts to [<i>t</i>-H<b>2</b>]⁺ and [μ-H<b>2</b>]⁺ by first-order and second-order processes, respectively. DFT calculations support transient protonation at sulfur and the proposal that the <i>S</i>-protonated species (e.g., [<i>S</i>-H<b>2</b>]⁺) rearranges to the terminal hydride intramolecularly via a low-energy pathway. Protonation of <b>3</b> affords exclusively terminal hydrides, regardless of the acid or conditions, to give [<i>t</i>-H<b>3</b>]⁺, which isomerizes to [<i>t</i>-H<b>3′</b>]⁺, wherein all PMe₃ ligands are basal