Iron Hydride Research Articles

Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies.

The reduction of CO2 to formic acid by transition metal hydrides is a potential pathway to access reactive C1 compounds. To date, no kinetic study has been reported for insertion of a bridging hydride in a weak-field ligated complex into CO2; such centers have relevance to metalloenzymes that catalyze this reaction. Herein, we report the kinetic study of the reaction of a tri(μ-hydride)triiron(II/II/II) cluster supported by a tris(β-diketimine) cyclophane (1) with CO2 monitored by 1H-NMR and temperature-controlled UV-vis spectroscopy. We found that 1 reacts with CO2 to traverse the reported monoformate (1-CO 2 ) and a diformate complex (1-2CO 2 ) at 298 K in toluene, and ultimately yields the triformate species (1-3CO 2 ) at elevated temperature. The second order rate constant, H/D kinetic isotope effect, ∆H ‡,and ∆S ‡for formation of 1-CO 2 were determined as 8.4(3)×10-4 M-1·s-1, 1.08(9), 11(1) kcal·mol-1, and -3(1)×10 cal·mol-1·K-1, respectively at 298 K. These parameters suggest that CO2 coordination to the iron centers does not coordinate prior to the rate controlling step whereas Fe-H bond cleavage does.

Synthesis and Catalytic Activity of Iron Hydride Ligated with Bidentate N-Heterocyclic Silylenes for Hydroboration of Carbonyl Compounds

We report the synthesis of a novel bidentate N-heterocyclic silylene (NHSi) ligand, N-(LSi:)-N-methyl-2-pyridinamine (1) (L = PhC(NtBu)2), and the first bischelate disilylene iron hydride, [(Si,N)(Si,C)Fe(H)(PMe3)] (2), and monosilylene iron hydride, [(Si,C)Fe(H)(PMe3)3] (2′), through Csp2–H activation of the NHSi ligand. Compounds 1 and 2 were fully characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. Density functional theory calculations indicated the multiple-bond character of the Fe–Si bonds and the π back-donation from Fe(II) to the Si(II) center. Moreover, the strong donor character of ligand 1 enables 2 to act as an efficient catalyst for the hydroboration reaction of carbonyl compounds at room temperature. Chemoselective hydroboration is attained under these conditions. This might be the first example of hydroboration of ketones and aldehydes catalyzed by a silylene hydrido iron complex. A catalytic mechanism was suggested and partially experimentally verified.

Solvent-Controlled CO2 Reduction by a Triphos–Iron Hydride Complex

The selective reduction of CO2 is of high interest toward future applications as a C1-building block. Therefore, metal complexes that allow for the formation of specific CO2 reduction products under distinct reaction conditions are necessary. A detailed understanding of the CO2 reduction pathways on a molecular level is, however, required to help in designing catalytic platforms for efficient CO2 conversion with specific product formation. Reported herein is a unique example of a solvent-controlled reduction of CO2 using a Triphos-based iron hydride complex. In THF, CO2 reduction selectively leads to CO formation, whereas experiments in acetonitrile exclusively afford formate, HCOO–. In order to explain the experimental findings, theoretical calculations of the reaction pathways were performed and further demonstrate the importance of the applied solvent for a selective reduction of CO2.

Outer Coordination Sphere Proton Relay Base and Proximity Effects on Hydrogen Oxidation with Iron Electrocatalysts

The intra- and intermolecular deprotonation reactions of [(CpC5F4N)Fe(PEtNR′PEt)H]+ (PEtNR′PEt = (Et2PCH2)2NR′) complexes are rate-limiting processes in the electrocatalytic oxidation of hydrogen and can be mediated by an amine base in the outer coordination sphere (OCS) by facilitating the matching of energy of deprotonation intermediates. A series of complexes with different N-substituents (R′ = Me, Et, CH2OMe, CH2N(Ph)Me, and (CH2)nNMe2; n = 1–4) was examined computationally using density functional theory to determine the influence of the identity and proximity of the OCS base on the intramolecular deprotonation. Complexes with an OCS dimethylamino substituent were the most suitably energy matched. The number of carbon atoms between the secondary and OCS nitrogen atoms dictated the strength of the hydrogen-bonding interaction and ring strain following deprotonation of the iron hydride. The previously reported [(CpC5F4N)Fe(PEtNR′PEt)H]+ (R′ = (CH2)3NMe2) electrocatalyst was predicted to have the most f...

Mechanism of atom economical conversion of alcohols and amines to amides using Fe(ii) pincer catalyst. An outer-sphere metal-ligand pathway or an inner-sphere elimination pathway?

In this present theoretical study, we investigated the reaction mechanism of atom-economical amide formation from alcohols and amines mediated by iron(ii) hydride complex (iPrPNP)Fe(H)(CO) (iPrPNP = N[CH2CH2(PiPr2)]2) using state-of-the-art density functional theory. Two scenarios of mechanistic pathways were considered, the inner-sphere and the outer-sphere pathways. In former case, the reaction of encounter complex of formaldehyde with amine is the rate-determining step with ΔG298 K = 33.75 kcal mol−1 while as in latter case dehydrogenation from trans-hydride is the rate-determining step having ΔG298 K = 21.34 kcal mol−1. Both the mechanistic scenarios operate through stepwise ionic pathways. The assessment of computational results demonstrate that inner-sphere pathway is energetically demanding and thus rendering outer-sphere pathway to be the most plausible mechanism of amide formation. Ligand modifications reveal that electron-withdrawing groups like CF3 near N of PNP ligand reduce the catalytic efficiency of the catalyst. Furthermore, changing the isopropyl moiety of phosphine scaffold with CH3 has a minimal impact on catalytic activity of the catalyst. Overall, our computational results provide new insights for the design and development of new Fe(ii) based pincer catalysts for atom economical amide formation from alcohols and amines.

Anion control of tautomeric equilibria: Fe-H vs. N-H influenced by NH···F hydrogen bonding.

Counterions can play an active role in chemical reactivity, modulating reaction pathways, energetics and selectivity. We investigated the tautomeric equilibrium resulting from protonation of Fe(PEtNMePEt)(CO)3 (PEtNMePEt = (Et2PCH2)2NMe) at Fe or N. Protonation of Fe(PEtNMePEt)(CO)3 by [(Et2O)2H]+[B(C6F5)4]- occurs at the metal to give the iron hydride [Fe(PEtNMePEt)(CO)3H]+[B(C6F5)4]-. In contrast, treatment with HBF4·OEt2 gives protonation at the iron and at the pendant amine. Both the FeH and NH tautomers were characterized by single crystal X-ray diffraction. Addition of excess BF4 - to the equilibrium mixture leads to the NH tautomer being exclusively observed, due to NH···F hydrogen bonding. A quantum chemical analysis of the bonding properties of these systems provided a quantification of hydrogen bonding of the NH to BF4 - and to OTf-. Treatment of Fe(PEtNMePEt)(CO)3 with excess HOTf gives a dicationic complex where both the iron and nitrogen are protonated. Isomerization of the dicationic complex was studied by NOESY NMR spectroscopy.

Catalytic Effect of Iron Hydrides on Dehydration of Primary Amides to Nitriles

Catalytic Effect of Iron Hydrides on Dehydration of Primary Amides to Nitriles

Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides.

Geometrically flexible 9-borabicyclo[3.3.1]nonyl units within the secondary coordination sphere enable isolation of high-spin Fe(ii)-dihydrides stabilized by boron-hydride interactions and a rare example of an isolable S = 3/2 reduction product. The borane-capped Fe(ii)-dihydride: (1) rapidly deprotonates E-H (E = N, O, P, S) bonds to afford borane-stabilized Fe adducts and (2) releases H2 upon exposure to π-acids. The Lewis acids provide an avenue for redox-leveling in analogy to the near constant operating potential for N2 reduction in nitrogenase.

Synthesis of Bis(phosphino)silyl Pincer-Supported Iron Hydrides for the Catalytic Hydrogenation of Alkenes

The synthesis and characterization of Fe pincer complexes supported by a bis(phosphino)silyl (PSiP) ligand are described. While four-coordinate species of the type (PSiP)FeX (X = halide) proved challenging to access, examples of five-coordinate (PSiP)Fe(II) and (PSiP)Fe(I) species were prepared and crystallographically characterized. In studying the reactivity of such (PSiP)Fe precursors, a variety of iron hydride species were observed and characterized, and interconversion among such complexes facilitated by the coordination of N2 was noted. The structures and spectroscopic features of several such diamagnetic Fe(II) hydrides were elucidated, including that of a unique and highly stable η2-(Si–H)Fe(II) dihydride complex. A surrogate for a low coordinate (PSiP)FeH species in the form of its bis(dinitrogen) adduct was found to be an effective precatalyst for the direct hydrogenation of alkenes, including various mono- and disubstituted aliphatic alkenes, as well as a trisubstituted example. Esters and ethe...

Electrical resistivity of fcc phase iron hydrides at high pressures and temperatures

We measured the electrical resistivity of face-centered-cubic (fcc) structured iron hydrides at high pressures up to 65GPa and high temperatures in a laser-heated diamond anvil cell. The results indicate that the resistivity of stoichiometric fcc FeHx (x ∼ 1.0) is smaller than that of fcc Fe at the same pressure and temperature conditions. The same behavior was also observed in fcc FeNiHx (x ∼ 1.0). On the other hand, hydrogen-poor fcc FeHx (x<0.77) showed a resistivity comparable to that of the fcc phase of pure iron. Therefore, we conclude that the stoichiometric fcc Fe (–Ni) hydride is more conductive than Fe (–Ni) with the same crystal symmetry, and the impurity resistivity of hydrogen in Fe is vanishingly small. Even if hydrogen is the major light element in the Earth's core, it would have little influence on the electrical and thermal conductivity of Fe–Ni alloys, and hence the thermal evolution of the core.

Hydrogen Atom Transfer (HAT)-Triggered Iron-Catalyzed Intra- and Intermolecular Coupling of Alkenes with Hydrazones: Access to Complex Amines

A methodology for the coupling of alkenes with aldehyde- or ketone-derived Cbz-hydrazones to form a new C–C bond through a radical process is described. The sequence comprises an initial in situ generation of a putative iron hydride followed by a hydrogen atom transfer to an alkene, a coupling with a hydrazone, and a final reduction of the nitrogen-centered radical. Hydrogenation of the obtained hydrazines renders amines, including valuable tert-alkyl amines.

Mg2FeH6 synthesized from plain steel and magnesium hydride

Mg2FeH6 was successfully synthesized using magnesium hydride and plain hypoeutectoid steels (with different carbon contents) as precursors. Mixtures of magnesium hydride (MgH2) and steels were milled in a planetary ball mill under argon and subsequently sintered under hydrogen pressure at 450 °C. It was shown that steel can be successfully used as a reaction substrate to obtain results similar to those obtained with pure iron. The developed method shows that magnesium iron hydride can be industrially manufactured at high yields using steel scrap, which can lower the cost of the manufacturing process. However, the reaction mixture must be mechanically milled more intensively because steel has better mechanical properties than iron; if it is not milled well, it does not readily react with magnesium hydride due to long diffusion distance.

Chemoselective transfer hydrogenation of aldehydes in aqueous media catalyzed by a well-defined iron(II) hydride complex

An iron(II) hydride PNP pincer complex is applied as catalyst for the chemoselective transfer hydrogenation of aldehydes using an aqueous solution of sodium formate as hydrogen source. A variety of aromatic, heteroaromatic, and aliphatic aldehydes could be reduced to the corresponding alcohols in good to excellent yields with a catalyst loading of 1.0 mol% at 80 °C and 1 h reaction time. If present, C–C double bonds remained unaffected in course of the reaction, even when they are conjugated to the carbonyl group of the aldehyde. The catalyst’s lifetime and activity could be improved when the reactions were conducted in an ionic liquid-based micro emulsion.Graphical abstract

Crystal Structures and Properties of Iron Hydrides at High Pressure

Evolutionary algorithms and the particle swarm optimization method have been used to predict stable and metastable high hydrides of iron between 150-300 GPa that have not been discussed in previous studies. Cmca FeH5, Pmma FeH6 and P2/c FeH6 contain hydrogenic lattices that result from slight distortions of the previously predicted I4/mmm FeH5 and Cmmm FeH6 structures. Density functional theory calculations show that neither the I4/mmm nor the Cmca symmetry FeH5 phases are superconducting. A P1 symmetry FeH7 phase, which is found to be dynamically stable at 200 and 300 GPa, adds another member to the set of predicted nonmetallic transition metal hydrides under pressure. Two metastable phases of FeH$_8$ are found, and the preferred structure at 300 GPa contains a unique 1-dimensional hydrogenic lattice.

Electronic and magnetic properties of iron hydride under pressure: An experimental and computational study using x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Fe K edge

The application of a 3.5 GPa pressure on Fe in a ${\mathrm{H}}_{2}$ environment leads to the formation of iron hydride FeH. Using a combination of high pressure x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the Fe K edge, we have investigated the modification of electronic and magnetic properties induced (i) by the transition from bcc-Fe to dhcp (double hexagonal)-FeH under pressure and (ii) by the compression of FeH up to 28 GPa. XAS and XMCD spectra under pressure have been computed in bcc-Fe and dhcp-FeH within a monoelectronic framework. Our approach is based on a semirelativistic density-functional theory (DFT) calculation of the electron density in the presence of a core hole using plane waves and pseudopotentials. Our method has been successful to reproduce the experimental spectra and to interpret the magnetic and electronic structure of FeH. In addition, we have identified a transition around 28 GPa, which is a purely magnetic transition from a ferromagnetic state to a paramagnetic state.

Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane.

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic FeII dimeric complex [Cp'FeI]2 (1) (Cp'=η5 -((1,2,4-tBu)3 C5 H2 )) was used as a precursor to a series of cyclopentadienyl FeII and FeIII mononuclear species. The complexes prepared were [Cp'Fe(η6 -Tol)][Cp'FeI2 ] (2) (Tol=C6 H5 Me), [Cp'Fe(η6 -Tol)][BArF4 ] (3) (BArF4 =[B(C6 H3 (m-CF3 )2 )4 ]- ), [N(nBu)4 ][Cp'FeI2 ] (4), Cp'FeI2 (5), and [Cp'Fe(MeCN)3 ][BArF4 ] (6). The electronic structure of the [Cp'FeI2 ]- anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1-6 were investigated as catalysts for the dehydrocoupling of Me2 NH⋅BH3 (I) in THF at 20 °C to yield the cyclodiborazane product [Me2 N-BH2 ]2 (IV). Complexes 1-4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1-4 proceeded via formation of the aminoborane Me2 N=BH2 (II) as the major intermediate, whereas for 6 the linear diborazane Me2 NH-BH2 -NMe2 -BH3 (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me3 N⋅BH3 were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ2 -H2 BH⋅NMe2 H][BArF4 ] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

Iron-Catalyzed Alkylation of Nitriles with Alcohols.

A general, efficient iron-catalyzed α-alkylation of nitriles with primary alcohols through a hydrogen-borrowing pathway has been developed, allowing a wide variety of alkylated nitriles to be readily accessible. Detailed mechanistic studies suggest that the reaction proceeds via an olefin intermediate with the turnover rate limited by the hydrogenation of the olefin with an iron hydride. Apart from participating in the alkylation, the nitrile is found to play an important role in promoting the formation of and stabilizing the active catalytic species.

The effects of ferromagnetism and interstitial hydrogen on the equation of states of hcp and dhcp FeHx: Implications for the Earth's inner core age

Hydrogen has been considered as an important candidate of light elements in the Earth's core. Because iron hydrides are unquenchable, hydrogen content is usually estimated from in situ X-ray diffraction measurements that assume the following linear relation: x = (V-FeHx - V-Fe)/Delta V-H, where x is the hydrogen content, Delta V-H is the volume expansion caused by unit concentration of hydrogen, and V-FeHx and V-Fe are volumes of FeHx and pure iron, respectively. To verify the linear relationship, we computed the equation of states of hexagonal iron with interstitial hydrogen by using the Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA). The results indicate a discontinuous volume change at the magnetic transition and almost no compositional (x) dependence in the ferromagnetic phase at 20 GPa, whereas the linearity is confirmed in the non-magnetic phase. In addition to their effect on the density-composition relationship in the Fe-FeHx system, which is important for estimating the hydrogen incorporation in planetary cores, the magnetism and interstitial hydrogen also affect the electrical resistivity of FeHx. The thermal conductivity can be calculated from the electrical resistivity by using the Wiedemann-Franz law, which is a critical parameter for modeling the thermal evolution of the Earth. Assuming an Fe1-ySiyHx ternary outer core model (0.0 <= x <= 0.7), we calculated the thermal conductivity and the age of the inner core. The resultant thermal conductivity is similar to 100 W/m/K and the maximum inner core age ranges from 0.49 to 0.86 Gyr.

High-Frequency Fe-H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT.

High-spin iron species with bridging hydrides have been detected in species trapped during nitrogenase catalysis, but there are few general methods of evaluating Fe-H bonds in high-spin multinuclear iron systems. An 57 Fe nuclear resonance vibrational spectroscopy (NRVS) study on an Fe(μ-H)2 Fe model complex reveals Fe-H stretching vibrations for bridging hydrides at frequencies greater than 1200 cm-1 . These isotope-sensitive vibrational bands are not evident in infrared (IR) spectra, showing the power of NRVS for identifying hydrides in this high-spin iron system. Complementary density functional theory (DFT) calculations elucidate the normal modes of the rhomboidal iron hydride core.

High‐Frequency Fe–H Vibrations in a Bridging Hydride Complex Characterized by NRVS and DFT

AbstractHigh‐spin iron species with bridging hydrides have been detected in species trapped during nitrogenase catalysis, but there are few general methods of evaluating Fe−H bonds in high‐spin multinuclear iron systems. An 57Fe nuclear resonance vibrational spectroscopy (NRVS) study on an Fe(μ‐H)2Fe model complex reveals Fe−H stretching vibrations for bridging hydrides at frequencies greater than 1200 cm−1. These isotope‐sensitive vibrational bands are not evident in infrared (IR) spectra, showing the power of NRVS for identifying hydrides in this high‐spin iron system. Complementary density functional theory (DFT) calculations elucidate the normal modes of the rhomboidal iron hydride core.