Iron Hydride Research Articles

Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand

A bulky tris(carbene)borate ligand allows several low-valent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron(II) complex [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) ([PhB(MesIm3)]− = phenyltris(1-mesitylimidazol-2-ylidene)borate) yields the low-spin (S = 1/2) iron(I) complex PhB(MesIm)3Fe(CO)2 (2), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K][PhB(MesIm)3Fe(CO)2] (3), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of 3 yields the weakly acidic iron hydride PhB(MesIm)3Fe(CO)2H (4), which can also be isolated by treating the κ3-coordinated alkylborohydrido complex PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5) with CO. The strong donor ability of the tris(carbene)borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy.

Design and Characterization of Phosphine Iron Hydrides: Toward Hydrogen-Producing Catalysts.

Diamagnetic iron chloro compounds [(P(Ph)2N(Ph)2)FeCp*Cl] [1Cl] and [(P(Cy)2N(Ph)2)FeCp*Cl] [2Cl] and the corresponding hydrido complexes [(P(Ph)2N(Ph)2)FeCp*H] [1H] and [(P(Cy)2N(Ph)2)FeCp*H] [2H] have been synthesized and characterized by NMR spectroscopy, electrochemical studies, electronic absorption, and (57)Fe Mössbauer spectroscopy (P(Ph)2N(Ph)2 = 1,3,5,7-tetraphenyl-1,5-diphospha-3,7-diazacyclooctane, P(Cy)2N(Ph)2 = 1,5-dicyclohexyl-3,7-diphenyl-1,5-diphospha-3,7-diazacyclooctane, Cp* = pentamethylcyclopentadienyl). Molecular structures of [2Cl], [1H], and [2H], derived from single-crystal X-ray diffraction, revealed that these compounds have a typical piano-stool geometry. The results show that the electronic properties of the hydrido complexes are strongly influenced by the substituents at the phosphorus donor atoms of the P(R)2N(Ph)2 ligand, whereas those of the chloro complexes are less affected. These results illustrate that the hydride is a strong-field ligand, as compared to chloride, and thus leads to a significant degree of covalent character of the iron hydride bonds. This is important in the context of possible catalytic intermediates of iron hydrido species, as proposed for the catalytic cycle of [FeFe] hydrogenases and other synthetic catalysts. Both hydrido compounds [1H] and [2H] show enhanced catalytic currents in cyclic voltammetry upon addition of the strong acid trifluoromethanesulfonimide [NHTf2] (pKa(MeCN) = 1.0). In contrast to the related complex [(P(tBu)N(Bn))2FeCp(C6F5)H], which was reported by Liu et al. (Nat. Chem. 2013, 5, 228-233) to be an electrocatalyst for hydrogen splitting, the here presented hydride complexes [1H] and [2H] show the tendency for electrocatalytic hydrogen production. Hence, the catalytic direction of this class of monoiron compounds can be reversed by specific ligand modifications.

ChemInform Abstract: Exploiting Metal‐Ligand Bifunctional Reactions in the Design of Iron Asymmetric Hydrogenation Catalysts

AbstractReview: 74 refs.

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(Me)py and PN(Ph)py. With ferrous halides, PN(Ph)py gave the complexes FeX2(PN(Ph)py) (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(H)py(R))2](2+) (R = H, Me). Dicarbonyls Fe(PN(R)py)(CO)2 were produced by treating PN(Me)py with Fe(benzylideneacetone)(CO)3 and reduction of FeX2(PN(Ph)py) 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(Ph)py system uniquely sustains two one-electron reductions. Treatment of Fe(PN(Ph)py)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(Ph)py.

Synthesis of Iron Hydrides by Selective C–F/C–H Bond Activation in Fluoroarylimines and Their Applications in Catalytic Reduction Reactions

AbstractThe reactions of Fe(PMe3)4 with different 2,6‐diflurophenylarylimines 1–5 were explored. Fluoroarylimines 1–3, the aryl rings of which are substituted with electron‐withdrawing groups, reacted with Fe(PMe3)4 to afford the C–H activation products 6–8. However, if the aryl rings of the fluoroarylimines were substituted with electron‐donating groups, the iron hydrides 9 and 10 were obtained from the reactions of the fluoroarylimines with Fe(PMe3)4 through C–F bond activation. In a further study, silanes, especially triethoxysilane, were found to benefit the reactions and improve the yields of the hydridoiron complexes. The three‐component reaction of Fe(PMe3)4, a fluoroarylimine, and a silane could also be utilized in reactions involving 2,6‐(CH3)2C6H3–C(=NH)–2,6‐F2C6H3 (13) and 2,6‐F2C6H3–C(=NH)–C6F5 (16) to synthesize iron hydrides (15 and 18). The hydridoiron complexes could be utilized as efficient catalysts in the hydrosilylation of aldehydes and ketones. Furthermore, cinnamaldehydes were selectively reduced to the corresponding cinnamyl alcohols in high yields. The mechanism of the catalytic reduction reaction was studied extensively through operando IR spectroscopy.

Synthesis and reactivity of iron(II) hydride complexes containing diphenylphosphine ligands

A series of iron(II) hydride complexes containing diphenylphosphine ligands is reported. In the presence of five equivalents of Ph2PH, the ionic complex trans-[(Ph2PH)4Fe(H)(NCMe)](Ph2P{BH3}2) (1) with the rather unusual Ph2P{BH3}2-anion is formed, whereas with four equivalents of Ph2PH, the fluxional cis-[(Ph2PH)4Fe(H)2] (2) can be isolated under otherwise identical conditions. Furthermore, our attempts to synthesize a mixed phosphine iron(II) hydride complex resulted in the isolation of [(dppe)(Ph2PH)Fe(H)(BH4)] (3, dppe = 1,2-bis(diphenylphosphino)ethane). The behavior of complexes 1-3 in solution was investigated by 1H NOESY NMR spectroscopy and compared with those of the corresponding ruthenium complex cis-[(Ph2PH)4Ru(H)2], revealing that no chemical exchange between the hydride ligands and the P–H-proton of the coordinated Ph2PH can be observed.

Exploiting metal-ligand bifunctional reactions in the design of iron asymmetric hydrogenation catalysts.

This is an Account of our development of iron-based catalysts for the asymmetric transfer hydrogenation (ATH) and asymmetric pressure hydrogenation (AH) of ketones and imines. These chemical processes provide enantiopure alcohols and amines for use in the pharmaceutical, agrochemical, fragrance, and other fine chemical industries. Fundamental principles of bifunctional reactivity obtained by studies of ruthenium catalysts by Noyori's group and our own with tetradentate ligands with tertiary phosphine and secondary amine donor groups were applied to improve the performance of these first iron(II) catalysts. In particular the correct positioning of a bifunctional H-Fe-NH unit in an iron hydride amine complex leads to exceptional catalyst activity because of the low energy barrier of dihydrogen transfer to the polar bond of the substrate. In addition the ligand structure with this NH group along with an asymmetric array of aryl groups orients the incoming substrate by hydrogen-bonding, and steric interactions provide the hydrogenated product in high enantioselectivity for several classes of substrates. Enantiomerically pure diamines or diphenylphosphino-amine compounds are used as the source of the asymmetry in the tetradentate ligands formed by the condensation of the amines with dialkyl- or diaryl-phosphinoaldehydes, a synthesis that is templated by Fe(II). The commercially available ortho-diphenylphosphinobenzaldehyde was used in the initial studies, but then diaryl-phosphinoacetaldehydes were found to produce much more effective ligands for iron(II). Once the mechanism of catalysis became clearer, the iron-templated synthesis of (S,S)-PAr2CH2CH2NHCHPhCHPhNH2 ligand precursors was developed to specifically introduce a secondary amine in the precatalyst structures. The reaction of a precatalyst with strong base yields a key iron-amido complex that reacts with isopropanol (in ATH) or dihydrogen (in AH) to generate an iron hydride with the Fe-H bond parallel to the secondary amine N-H. In the AH reactions, the correct acidity of the intermediate iron-dihydrogen complex and correct basicity of the amide are important factors for the heterolytic splitting of the dihydrogen to generate the H-Fe-N-H unit; the acidity of dihydrogen complexes including those found in hydrogenases can be estimated by a simple additive ligand acidity constant method. The placement of the hydridic-protonic Fe-H···HN interaction in the asymmetric catalyst structure influences the enantioinduction. The sense of enantioinduction is predictable from the structure of the H-Fe-N-H-containing catalyst interacting with the ketone in the same way as related H-Ru-N-H-containing catalysts. The modular construction of the catalysts permits large variations in order to produce alcohol or amine products with enantiomeric excess in the 90-100% range in several cases.

Chiral (Cyclopentadienone)iron Complexes for the Catalytic Asymmetric Hydrogenation of Ketones

AbstractThree chiral (cyclopentadienone)iron complexes derived from (R)‐BINOL (CK1–3) were synthesized and their structures unambiguously confirmed by X‐ray analysis (CK3). Under suitable conditions for the in situ conversion into the corresponding (hydroxycyclopentadienyl)iron hydrides (Me3NO, H2), the new chiral complexes were tested in the catalytic asymmetric hydrogenation of ketones, showing moderate to good enantioselectivity. In particular, the complex bearing methoxy substituents at the 3,3′‐positions of the binaphthyl moiety (CK2) proved remarkably more enantioselective than the unsubstituted one (CK1) and reached the highest level of enantioselectivity (up to 77 % ee) ever obtained with chiral (cyclopentadienone)iron complexes.

Synthesis of [POCOP]-pincer iron and cobalt complexes via Csp3–H activation and catalytic application of iron hydride in hydrosilylation reactions

Csp3–H bond activation in pincer ligand (Ph2PO(o-C6H2-(4,6-tBu2)))2CH2 (1) (POCH2OP) was achieved by Fe(PMe3)4 and CoMe(PMe3)4 to afford (POCHOP)Fe(H) (PMe3)2 (2) and (POCHOP)Co(PMe3)2 (4).

Thermal C-H borylation using a CO-free iron boryl complex.

New CO-free iron boryl complexes, CpFe(PR3)2(Bpin), are described. The CpFe(PEt3)2(Bpin) derivative is uniquely capable of UV-free arene borylation at 70 °C via a dissociative pathway. Catalytic C-H borylation does not proceed using either monometallic or heterobimetallic schemes, and this failure is rationalized through analysis of relative pKa values for the corresponding iron hydride species.

Hydricity of an Fe-H Species and Catalytic CO2 Hydrogenation.

Despite renewed interest in carbon dioxide (CO2) reduction chemistry, examples of homogeneous iron catalysts that hydrogenate CO2 are limited compared to their noble-metal counterparts. Knowledge of the thermodynamic properties of iron hydride complexes, including M-H hydricities (ΔGH(-)), could aid in the development of new iron-based catalysts. Here we present the experimentally determined hydricity of an iron hydride complex: (SiP(iPr)3)Fe(H2)(H), ΔGH(-) = 54.3 ± 0.9 kcal/mol [SiP(iPr)3 = [Si(o-C6H4PiPr2)3](-)]. We also explore the CO2 hydrogenation chemistry of a series of triphosphinoiron complexes, each with a distinct apical unit on the ligand chelate (Si(-), C(-), PhB(-), N, B). The silyliron (SiP(R)3)Fe (R = iPr and Ph) and boratoiron (PhBP(iPr)3)Fe (PhBP(iPr)3 = [PhB(CH2PiPr2)3](-)) systems, as well as the recently reported (CP(iPr)3)Fe (CP(iPr)3 = [C(o-C6H4PiPr2)3](-)), are also catalysts for CO2 hydrogenation in methanol and in the presence of triethylamine, generating methylformate and triethylammonium formate at up to 200 TON using (SiP(Ph)3)FeCl as the precatalyst. Under stoichiometric conditions, the iron hydride complexes of this series react with CO2 to give formate complexes. Finally, the proposed mechanism of the (SiP(iPr)3)-Fe system proceeds through a monohydride intermediate (SiP(iPr)3)Fe(H2)(H), in contrast to that of the known and highly active tetraphosphinoiron, (tetraphos)Fe (tetraphos = P(o-C6H4PPh2)3), CO2 hydrogenation catalyst.

Elementary Steps of Iron Catalysis: Exploring the Links between Iron Alkyl and Iron Olefin Complexes for their Relevance in CH Activation and CC Bond Formation

The alkylation of complexes 2 and 7 with Grignard reagents containing β-hydrogen atoms is a process of considerable relevance for the understanding of C-H activation as well as C-C bond formation mediated by low-valent iron species. Specifically, reaction of 2 with EtMgBr under an ethylene atmosphere affords the bis-ethylene complex 1 which is an active precatalyst for prototype [2+2+2] cycloaddition reactions and a valuable probe for mechanistic studies. This aspect is illustrated by its conversion into the bis-alkyne complex 6 as an unprecedented representation of a cycloaddition catalyst loaded with two substrates molecules. On the other hand, alkylation of 2 with 1 equivalent of cyclohexylmagnesium bromide furnished the unique iron alkyl species 11 with a 14-electron count, which has no less than four β-H atoms but is nevertheless stable at low temperature against β-hydride elimination. In contrast, the exhaustive alkylation of 1 with cyclohexylmagnesium bromide triggers two consecutive C-H activation reactions mediated by a single iron center. The resulting complex has a diene dihydride character in solution (15), whereas its structure in the solid state is more consistent with an η(3) -allyl iron hydride rendition featuring an additional agostic interaction (14). Finally, the preparation of the cyclopentadienyl iron complex 25 illustrates how an iron-mediated C-H activation cascade can be coaxed to induce a stereoselective CC bond formation. The structures of all relevant new iron complexes in the solid state are presented.

Synthesis and Catalytic Property of Iron Pincer Complexes Generated by Csp3–H Activation

When the diphosphinito PCP ligand (Ph2P(C6H4))2CH2 (1) was treated with Fe(PMe3)4 and FeMe2(PMe3)4, the Csp3–H activation products [(Ph2P(C6H4))2CH]Fe(H)(PMe3)2 (2) and [(Ph2P(C6H4))(PhP(C6H4)2)CH]Fe(PMe3)2 (3) were obtained at room temperature. The generation of product 3 underwent one Csp3–H and one Csp2–H bond activation process. The new iron hydride complex 2 showed good activity in the catalytic hydrosilylation of aldehydes and ketones by using (EtO)3SiH as the hydrogen source under mild conditions. Complexes 2 and 3 were characterized by spectroscopic methods and X-ray diffraction analysis.

Synthesis of an Iron(II) Ethyl Complex Accompanied by Formation of an Unusual Dinitrogen‐Ligated Iron(I) Hydride

AbstractThe reaction of [(PNP)FeCl]·C7H8 [PNP– = bis(2‐(diisopropylphosphanyl)‐4‐methylphenyl)amido] with 1.0 equiv. of EtMgCl in Et2O under N2 produces the anticipated ethyl complex, [(PNP)FeEt] (1). However, the same reaction carried out using a slight excess of the Grignard, instead results in a mixture of species containing 1, but accompanied with formation of another iron‐containing species. This species was identified by single X‐ray diffraction studies, and IR spectroscopy to be the heterobimetallic complex, [(THF)2MgCl(μ‐(N)‐PNP)Fe(μ‐H)N2] (2), a rare example of a FeI hydride complex. The formation of compound 2 is proposed to result from single‐electron reduction of 1 by excess Grignard.

Hybrid functional calculations of potential hydrogen storage material: Complex dimagnesium iron hydride

By employing the state of art first principles approaches, comprehensive investigations of a very promising hydrogen storage material, Mg2FeH6 hydride, is presented. To expose its hydrogen storage capabilities, detailed structural, elastic, electronic, optical and dielectric aspects have been deeply analysed. The electronic band structure calculations demonstrate that Mg2FeH6 is semiconducting material. The obtained results of the optical bandgap (4.19eV) also indicate that it is a transparent material for ultraviolet light, thus demonstrating its potential for optoelectronics application. The calculated elastic properties reveal that Mg2FeH6 is highly stiff and stable hydride. Finally, the calculated hydrogen (H2) storage capacity (5.47 wt.%) within a reasonable formation energy of −78kJmol−1, at room temperature, can be easily achievable, thus making Mg2FeH6 as potential material for practical H2 storage applications.

Low temperature hydrogenation of iron nanoparticles on graphene

Hydrogenation of iron nanoparticles was performed both computationally and experimentally where previously chemically-bonded iron hydride is considered to be unachievable under ordinary conditions. Density functional theory (DFT) calculations predict that hydrogenated iron nanoparticles are stabilized on a single-layer graphene/Cu substrate. Experimentally, iron nanoparticles were deposited onto a graphene/Cu substrate by vacuum deposition. Hydrogenation was done at 1atm of hydrogen gas and under liquid nitrogen. Mass spectrometry peak confirmed the hydrogen release from hydrogenated iron nanoparticles while a scanning transmission electron microscopy is used in order to link a geometrical shape of iron hydride nanoparticles between experimental and theoretical treatments. The hydrogenated iron nanoparticles were successfully synthesized where hydrogenated iron nanoparticles are stable under ordinary conditions.

Synthesis of Fe–H/Si–H and Fe–H/Ge–H Bifunctional Complexes and Their Catalytic Hydrogenation Reactions toward Nonpolar Unsaturated Organic Molecules

The iron hydride complexes bearing a Si–H or a Ge–H functional group in a ligand [2,5-SiPh3-3,4-butylene-(η5-C4COEEt2H)]Fe(CO)2H (E = Si (4), Ge (5)) and [2,5-SiPh3-3,4-butylene-(η5-C4COSiMe2OSiMe2H)]Fe(CO)2H (6), were synthesized in the reaction of the iron acetonitrile complex [2,5-SiPh3-3,4-butylene-(η4-C4CO)]Fe(CO)2(NCCH3) (3) with Et2EH2 or HSiMe2OSiMe2H. These complexes did not hydrogenate a polar unsaturated bond in either ketones or aldehydes but did hydrogenate a nonpolar unsaturated bond in alkynes and alkenes to give alkenes and alkanes selectively. These complexes also catalyzed a transfer hydrogenation reaction from isopropyl alcohol (IPA) to alkynes and alkenes.

Iron Complexes for the Electrocatalytic Oxidation of Hydrogen: Tuning Primary and Secondary Coordination Spheres

A series of iron hydride complexes featuring PRNR′PR (PRNR′PR = R2PCH2N(R′)CH2PR2 where R = Ph, R′ = Me; R = Et, R′ = Ph, Bn, Me, tBu) and cyclopentadienide (CpX = C5H4X where X = H, C5F4N) ligands has been synthesized; characterized by NMR spectroscopy, X-ray diffraction, and cyclic voltammetry; and examined by quantum chemistry calculations. Each compound was tested for the electrocatalytic oxidation of H2, and the most active complex, (CpC5F4N)Fe(PEtNMePEt)(H), exhibited a turnover frequency of 8.6 s–1 at 1 atm of H2 with an overpotential of 0.41 V, as measured at the potential at half of the catalytic current and using N-methylpyrrolidine as the exogenous base to remove protons. Control complexes that do not contain pendant amine groups were also prepared and characterized, but no catalysis was observed. The rate-limiting steps during catalysis are identified through combined experimental and computational studies as the intramolecular deprotonation of the FeIII hydride by the pendant amine and the subsequent deprotonation by an exogenous base.

Synthesis, spectroscopy, and hydrogen/deuterium exchange in high-spin iron(II) hydride complexes.

Very few hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin iron(II) hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin iron hydride complexes may be relevant to postulated nitrogenase mechanisms.

Theoretical Study of POCOP-Pincer Iridium(III)/Iron(II) Hydride Catalyzed Hydrosilylation of Carbonyl Compounds: Hydride Not Involved in the Iridium(III) System but Involved in the Iron(II) System

The catalytic hydrosilylation of carbonyl compounds by two POCOP-pincer transition-metal hydrides, (POCOP)Ir(H)(acetone)+ (1A-acetone) and (POCOP)Fe(H)(PMe3)2 (1B) (POCOP = 2,6-bis(dibutyl-/diisopropylphosphinito)phenyl), was theoretically investigated to determine the underlying reaction mechanism. Several plausible mechanisms were analyzed using density functional theory calculations. The 1A-acetone-catalyzed hydrosilylation of carbonyl compounds proceeds via the ionic hydrosilylation pathway, which is initiated by the nucleophilic attack of the η1-silane metal adduct by carbonyl substrate. This attack results in the heterolytic cleavage of the Si–H bond and the generation of a siloxy carbenium ion paired with a neutral iridium dihydride, [(POCOP)Ir(H)2][R3SiOCHR′]+, followed by transfer of hydride from the metal center to the siloxy carbenium ion to yield the silyl ether product. The activation energy of the turnover-limiting step was calculated as ∼15.2 kcal/mol. This value is energetically more favor...