Synthesis of Diiron(I) Dithiolato Carbonyl Complexes

Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe₂(μ-SR)₂(CO)_<i>x</i> centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe₂(μ-SR)₂(CO)₆ involves the reaction of thiols with Fe₂(CO)₉ and Fe₃(CO)₁₂. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe₂(μ-S₂)­(CO)₆, Fe₂(μ-SH)₂(CO)₆, and Li₂Fe₂(μ-S)₂(CO)₆. This approach is especially useful for azadithiolates Fe₂[(μ-SCH₂)₂NR]­(CO)₆. Elaborate complexes arise via addition of the Fe<i>SH</i> group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe₂(μ-SR)₂(CO)₆ was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe­(II)­(SR)₂ + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe₃(CO)₁₂, S₈, and a variety of unsaturated substrates results in C–H activation, affording otherwise inaccessible derivatives. Thiones and related CS-containing reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C–H activation

Hydrogen Activation by Biomimetic [NiFe]-Hydrogenase Model Containing Protected Cyanide Cofactors

Described are experiments demonstrating incorporation of cyanide cofactors and hydride substrate into [NiFe]-hydrogenase (H₂ase) active site models. Complexes of the type (CO)₂(CN)₂Fe­(pdt)­Ni­(dxpe) (dxpe = dppe, <b>1</b>; dxpe = dcpe, <b>2</b>) bind the Lewis acid B­(C₆F₅)₃ (BAr^F₃) to give the adducts (CO)₂(CNBAr^F₃)₂­Fe­(pdt)­Ni­(dxpe), (<b>1</b>(BAr^F₃)₂, <b>2</b>(BAr^F₃)₂). Upon decarbonylation using amine oxides, these adducts react with H₂ to give hydrido derivatives [(CO)­(CNBAr^F₃)₂­Fe­(H)­(pdt)­Ni­(dxpe)]⁻ (dxpe = dppe, [H<b>3</b>(BAr^F₃)₂]⁻; dxpe = dcpe, [H<b>4</b>(BAr^F₃)₂]⁻). Crystallographic analysis shows that Et₄N­[H<b>3</b>(BAr^F₃)₂] generally resembles the active site of the enzyme in the reduced, hydride-containing states (Ni–C/R). The Fe–H···Ni center is unsymmetrical with <i>r</i>_Fe–H = 1.51(3) Å and <i>r</i>_Ni–H = 1.71(3) Å. Both crystallographic and ¹⁹F NMR analyses show that the CNBAr^F₃^– ligands occupy basal and apical sites. Unlike cationic Ni–Fe hydrides, [H<b>3</b>(BAr^F₃)₂]⁻ and [H<b>4</b>(BAr^F₃)₂]⁻ oxidize at mild potentials, near the Fc^+/0 couple. Electrochemical measurements indicate that in the presence of base, [H<b>3</b>(BAr^F₃)₂]⁻ catalyzes the oxidation of H₂. NMR evidence indicates dihydrogen bonding between these anionic hydrides and R₃NH⁺ salts, which is relevant to the mechanism of hydrogenogenesis. In the case of Et₄N­[H<b>3</b>(BAr^F₃)₂], strong acids such as HCl induce H₂ release to give the chloride Et₄N­[(CO)­(CNBAr^F₃)₂­Fe­(Cl)­(pdt)­Ni­(dppe)]

Connecting [NiFe]- and [FeFe]-Hydrogenases: Mixed-Valence Nickel–Iron Dithiolates with Rotated Structures

New mixed-valence iron–nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)­Ni­(pdt)­Fe­(CO)₃]­BF₄ ([<b>1</b>]­BF₄, where dppe = Ph₂PCH₂CH₂PPh₂ and pdt^2– = −SCH₂CH₂CH₂S−) with P-donor ligands (L) afforded the substituted derivatives [(dppe)­Ni­(pdt)­Fe­(CO)₂L]­BF₄ incorporating L = PHCy₂ ([<b>1a</b>]­BF₄), PPh­(NEt₂)₂ ([<b>1b</b>]­BF₄), P­(NMe₂)₃ ([<b>1c</b>]­BF₄), P­(<i>i</i>-Pr)₃ ([<b>1d</b>]­BF₄), and PCy₃ ([<b>1e</b>]­BF₄). The related precursor [(dcpe)­Ni­(pdt)­Fe­(CO)₃]­BF₄ ([<b>2</b>]­BF₄, where dcpe = Cy₂PCH₂CH₂PCy₂) gave the more electron-rich family of compounds [(dcpe)­Ni­(pdt)­Fe­(CO)₂L]­BF₄ for L = PPh₂(2-pyridyl) ([<b>2a</b>]­BF₄), PPh₃ ([<b>2b</b>]­BF₄), and PCy₃ ([<b>2c</b>]­BF₄). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [<b>1e</b>]­BF₄ and [<b>2c</b>]­BF₄ showed that they adopt “rotated” Fe^I centers, in which PCy₃ occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as Ni^IIFe^I (<i>S</i> = ¹/₂) systems according to electron paramagnetic resonance spectroscopy, although with attenuated ³¹P hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [<b>1e</b>]⁺ is mostly localized in a Fe^I-centered d­(<i>z</i>²) orbital, orthogonal to the Fe–P bond. The PCy₃ complexes, rare examples of species featuring “rotated” Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS₂Fe core of the [NiFe]-hydrogenase active site is reproduced, the “hybrid models” incorporate key features of the two major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)­Ni­(pdt)­Fe­(CO)₂L]^+/2+. The resulting unsaturated 32e^– dications represent the closest approach to modeling the highly electrophilic Ni–SI_a state. In the case of L = PPh₂ (2-pyridyl), chelation of this ligand accompanies the second oxidation

Cooperative Metal–Ligand Reactivity and Catalysis in Low-Spin Ferrous Alkoxides

This report describes examples of combined Fe- and O-centered reactivity of Fe­(P₂O₂)­(CO)₂ (<b>1</b>), where P₂O₂ is the diphosphinoglycolate (Ph₂PC₆H₄CHO)₂^2–. This 18e low-spin ferrous dialkoxide undergoes substitution of CO to give the labile monosubstituted derivatives Fe­(P₂O₂)­(CO)­(L) (L = PMe₃, pyridine, MeCN). Treatment of Fe­(P₂O₂)­(CO)₂ with Brønsted acids results in stepwise O-protonation, affording rare examples of low-spin Fe­(II) complexes containing alcohol ligands. Substitution reactions with amides (RC­(O)­NH₂) proceeds with binding of the carbonyl and formation of an intramolecular hydrogen bond between N<i>H</i> and the neighboring alkoxo ligand. This two-site binding was confirmed with crystallographic characterization of the thiourea-substituted derivative. Fe­(P₂O₂)­(CO)₂ reacts with Ph₂SiH₂ to give the O-silylated hydrido complex, which is inactive for hydrosilylation. The monocarbonyl derivatives Fe­(P₂O₂)­(CO)­(L) (L = NCMe, PMe₃, acetamide) are precursors to catalysts for the hydrosilylation of benzaldehyde, acetophenone, and styrene

Insights into the Hydrolytic Polymerization of Trimethoxymethylsilane. Crystal Structure of (MeO)₂MeSiONa

The commercially practiced conversion of trimethoxymethylsilane (MTM) to [OSi­(OMe)­Me)]_<i>n</i> polymers and resins is assumed to proceed via the silanol (MeO)₂MeSiOH. Access to this crucial silanol is gained via the synthesis of (MeO)₂MeSiONa, the first methoxysilanoate to be crystallographically characterized. Mild protonation of this silanoate gives (MeO)₂MeSiOH, which is shown to condense with (MeO)₂MeSiOH but not with MTM. Condensation generates reactive disiloxanols but does not produce symmetric disiloxanes. Parallel results were obtained with (MeO)₂PhSiOH

Phosphine-Iminopyridines as Platforms for Catalytic Hydrofunctionalization of Alkenes

A series of phosphine-diimine ligands were synthesized by the condensation of 2-(diphenylphosphino)­aniline (PNH₂) with a variety of formyl and ketopyridines. Condensation of PNH₂ with acetyl- and benzoylpyridine yielded the Ph₂P­(C₆H₄)­NC­(R)­(C₅H₄N), respectively abbreviated PN^Mepy and PN^Phpy. With ferrous halides, PN^Phpy gave the complexes FeX₂(PN^Phpy) (X = Cl, Br). Condensation of pyridine carboxaldehyde and its 6-methyl derivatives with PNH₂ was achieved using a ferrous template, affording low-spin complexes [Fe­(PN^Hpy^R)₂]²⁺ (R = H, Me). Dicarbonyls Fe­(PN^Rpy)­(CO)₂ were produced by treating PN^Mepy with Fe­(benzylideneacetone)­(CO)₃ and reduction of FeX₂(PN^Phpy) with NaBEt₃H under a CO atmosphere. Cyclic voltammetric studies show that the [FeL₃(CO)₂]^0/– and [FeL₃(CO)₂]^+/0 couples are similar for a range of tridentate ligands, but the PN^Phpy system uniquely sustains two one-electron reductions. Treatment of Fe­(PN^Phpy)­X₂ with NaBEt₃H gave active catalysts for the hydroboration of 1-octene with pinacolborane. Similarly, these catalysts proved active for the addition of diphenylsilane, but not HSiMe­(OSiMe₃)₂, to 1-octene and vinylsilanes. Evidence is presented that catalysis occurs via iron hydride complexes of intact PN^Phpy

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

Borane-Protected Cyanides as Surrogates of H‑Bonded Cyanides in [FeFe]-Hydrogenase Active Site Models

Triarylborane Lewis acids bind [Fe₂(pdt)­(CO)₄(CN)₂]^2– [<b>1</b>]^2– (pdt^2– = 1,3-propanedithiolate) and [Fe₂(adt)­(CO)₄(CN)₂]^2– [<b>3</b>]^2– (adt^2– = 1,3-azadithiolate, HN­(CH₂S^–)₂) to give the 2:1 adducts [Fe₂(xdt)­(CO)₄(CNBAr₃)₂]^2–. Attempts to prepare the 1:1 adducts [<b>1</b>(BAr₃)]^2– (Ar = Ph, C₆F₅) were unsuccessful, but related 1:1 adducts were obtained using the bulky borane B­(C₆F₄-<i>o</i>-C₆F₅)₃ (BAr^F*₃). By virtue of the N-protection by the borane, salts of [Fe₂(pdt)­(CO)₄(CNBAr₃)₂]^2– sustain protonation to give hydrides that are stable (in contrast to [H<b>1</b>]⁻). The hydrides [H<b>1</b>(BAr₃)₂]⁻ are 2.5–5 p<i>K</i>_a units more acidic than the parent [H<b>1</b>]⁻. The adducts [<b>1</b>(BAr₃)₂]^2– oxidize quasi-reversibly around −0.3 V versus Fc^0/+ in contrast to ca. −0.8 V observed for the [<b>1</b>]^2–/– couple. A simplified synthesis of [<b>1</b>]^2–, [<b>3</b>]^2–, and [Fe₂(pdt)­(CO)₅(CN)]⁻ ([<b>2</b>]⁻) was developed, entailing reaction of the diiron hexacarbonyl complexes with KCN in MeCN

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

<i>C</i>₂‑Symmetric Iron(II) Diphosphine–Dialkoxide Dicarbonyl and Related Complexes

Reaction of Fe­(bda)­(CO)₃ (bda = benzylideneacetone) and Ph₂P-2-C₆H₄CHO (PCHO) affords the bis­phosphine bisalkoxide complex Fe­[(Ph₂PC₆H₄)₂C₂H₂O₂]­(CO)₂ (<b>1</b>) arising from the head-to-head coupling of two formyl groups concomitant with oxidation of Fe(0) to Fe­(II). Crystallographic studies show that <b>1</b> features <i>cis</i> alkoxide ligands that are <i>trans</i> to CO; the two phosphine groups are mutually <i>trans</i> with a P–Fe–P angle of 167.44(4)°. The pathway leading to <b>1</b> was examined, starting with the adduct Fe­(PCHO)­(CO)₄ (<b>2</b>), which was obtained by addition of PCHO to Fe₂(CO)₉. Compound <b>2</b> decarbonylates to give tricarbonyl Fe­(κ¹,η²-PCHO)­(CO)₃ (<b>3</b>), which features a π-bonded aldehyde. Photolysis of <b>2</b> gives a mixture of <b>3</b> and isomeric hydride HFe­(κ²-PCO)­(CO)₃. Complex <b>3</b> reacts with an additional equivalent of PCHO to afford <b>1</b>, whereas treatment with PPh₃ afforded the substituted product Fe­(κ¹,η²-PCHO)­(PPh₃)­(CO)₂ (<b>4</b>). In <b>4</b>, the phosphine ligands are <i>trans</i> and the aldehyde is π-bonded. The geometry around Fe is pseudo trigonal bipyramidal. To gain insights into the mechanism and scope of the C–C coupling reaction, complexes were prepared with the imine Ph₂PC₆H₄CHNC₆H₄Cl (abbreviated as PCHNAr), derived by condensation of 4-chloroaniline and PCHO. PCHNAr reacts with Fe₂(CO)₉ and with Fe­(bda)­(CO)₃ to afford the tetra- and tricarbonyl compounds Fe­(PCHNAr)­(CO)₄ (<b>5</b>) and Fe­(PCHNAr)­(CO)₃ (<b>6</b>), respectively. Treatment of <b>6</b> with PCHO gave the unsymmetrical C–C coupling complex Fe­[(Ph₂PC₆H₄)₂CH­(O)­CH­(NAr)]­(CO)₂ (<b>7</b>). Compound <b>7</b> was also prepared by the reaction of <b>3</b> and PCHNAr. The solid-state structure of <b>7</b>, as established by X-ray crystallography, is similar to that of <b>1</b> but with an amido group in place of one alkoxide. The deuterium-labeled phosphine aldehyde PCDO was prepared by the reaction of <i>ortho</i>-lithiated phosphine Ph₂PC₆H₄-2-Li with DMF-<i>d</i>₇. Reaction of <b>6</b> with PCDO gave <b>7</b>-<i>d</i>₁ with no scrambling of the deuterium label. Attempted oxidation of <b>1</b> with FcBF₄ (Fc⁺ = ferrocenium) gave the adduct Fe­[(Ph₂PC₆H₄)₂C₂H₂O₂(BF₃)₂]­(CO)₂ (<b>8</b>). The structures of <b>1</b> and <b>8</b> are almost identical. Compound <b>8</b> was independently synthesized by treating <b>1</b> with BF₃OEt₂ via the intermediacy of the 1:1 adduct, which was detected spectroscopically. Qualitative tests showed that <b>1</b> also reversibly protonates with HOSO₂CF₃ and binds TiCl₄