Iron Hydride Research Articles

Isocyanide Substituent Influences Reductive Elimination versus Migratory Insertion in Reaction with an [Fe2(μ-H)2]2+ Complex.

Iron hydrides are proposed reactive intermediates for N2 and CO conversion in industrial and biological processes. Here, we report a reactivity study of a low-coordinate di(μ-hydrido)diiron(II) complex, Fe2(μ-H)2L, where L2- is a bis(β-diketiminate) cyclophane, with isocyanides, which have electronic structures related to N2 and CO. The reaction outcome is influenced by the isocyanide substituent, with 2,6-xylyl isocyanide leading to H2 loss, to form a bis(μ-1,1-isocyanide)diiron(I) complex, whereas all of the other tested isocyanides insert into the Fe-H bond to give (μ-1,2-iminoformyl) complexes. Steric bulk of the isocyanide substituent determines the extent of insertion (i.e., into one or both Fe-H-Fe units) with tert-butyl isocyanide reacting to yield the mono-(μ-1,2-iminoformyl)diiron(II) complex, exclusively, and isopropyl- and methyl isocyanides affording the bis(μ-1,2-iminoformyl)diiron(II) products. Treatment of Fe2(μ-1,2-CHNtBu)(μ-H)L with 2,6-xylyl isocyanide (or XylNC) yields Fe2(μ-XylNC)2L and tert-butylaldimine as one of the organic products.

Equation of State, Structure, and Transport Properties of Iron Hydride Melts at Planetary Interior Conditions

AbstractIron hydrides are a potentially dominant component of the metallic cores of planets, primarily because of hydrogen's ubiquity in the universe and affinity for iron. Using ab initio molecular dynamics, we examine iron hydrides with 0.1, 0.33, 0.5, and 0.6 mol fraction hydrogen up to 100 GPa between 3,000 and 5,000 K to describe how hydrogen content affects the melt structure, hydrogen speciation, equation of state (EOS), atomic diffusivity, and melt viscosity. We find that the addition of hydrogen decreases the average Fe–Fe coordination number and lengthens Fe–Fe bonds, while Fe–H coordination number increases. The pair distribution function of hydrogen at low pressure indicates the presence of molecular hydrogen. By tracking chemical speciation, we show that the amount of molecular hydrogen increases and the number of iron in Hx≥1Fey≥0 clusters decreases as the hydrogen concentration increases. We parameterize a pressure, volume, temperature, and composition EOS and show that the molar volume and Grüneisen parameter of the melts decrease while the compressibility and thermal expansivity increase as a function of hydrogen concentration. We find that hydrogen acts as a lubricant in the melts as the iron and hydrogen become more diffusive and the melts become more inviscid as the hydrogen concentration increases. We estimate 2.7 wt% hydrogen in the Martian core and 0.49–1.1 wt% hydrogen in Earth's outer core based on comparisons to seismic models, with the assumption that the cores are pure liquid iron‐hydrogen alloy, and we compare the small exoplanet population with mass‐radius curves of iron hydride planets.

Superionic iron hydride shapes ultralow-velocity zones at Earth’s core–mantle boundary

Seismological studies have exposed numerous ultralow velocity zones (ULVZs) exhibiting extraordinary physical attributes at Earth's core-mantle boundary, yet their compositions and origins remain controversial. Water-iron reaction can generate unique phases under lowermost-mantle conditions and likely plays a crucial role in forming ULVZs. Through first-principles molecular dynamic simulations with machine learning techniques, we determine that iron hydride, the product of water-iron reaction, is stable as a superionic phase at the core-mantle boundary. This superionic iron hydride has much slower velocities and a higher density than the ambient mantle under lowermost-mantle conditions. Accumulation of iron hydride, created through either a chemical reaction between subducted water and iron or solidification of core material entrained in the lower mantle by convection, can explain the seismic observations of ULVZs particularly those associated with subduction. This work suggests that water may have a substantial role in creating seismic heterogeneities at the core-mantle boundary.

Trends in the hydricities of iron, cobalt, and nickel complexes and the metal-hydride reactivities with CO2

Catalytic CO2 hydrogenation is considered one of the most efficient strategies for CO2 reduction, provided that we have efficient and selective catalysts. This paper explores a mass-spectrometric approach to rapidly compare the reactivities of 21 metal complexes (iron, cobalt, and nickel complexes with three bidentate N,N′-ligands and four bidentate P,P′-ligands) by energy-resolved MS/MS experiments. The experiments show relative hydricities of these complexes, singling out iron complexes with N,N′-ligands and cobalt complexes with P,P′-ligands as particularly reactive for hydride-donor reactions. Comparing these results with relative affinities of metal hydrides for CO2 led to the conclusion that iron hydrides should be particularly suited for CO2 hydrogenation reactions. The results, however, cannot account for the effects of the polar reaction environment in the condensed phase.

Low-Coordinate Iron Hydride Chemistry at an N,N,C-Heteroscorpionate Platform.

Locally high-spin iron hydrides are proposed to play a critical role as intermediates in iron-molybdenum cofactor (FeMoco)-catalyzed N2 fixation. Inspired by these biological systems, we report herein our initial investigations into low-coordinate iron hydride chemistry supported by our N,N,C-heteroscorpionate ligands. Those ligands with smaller steric profiles are unable to completely suppress the formation of a binuclear [Fe(μ2-H)]2 complex; however, the incorporation of more substantial steric bulk allows for the isolation of a rare example of a terminal, high-spin (S = 2) Fe2+ hydride. Fourier transform infrared spectroscopy suggests an unusually weak Fe-H bond in the latter molecule. Mössbauer spectroscopies, coupled with density functional theory calculations, highlights the substantial influence of the terminal hydride ligand on 57Fe isomer shift.

Synthetic and Mechanistic Studies into the Reductive Functionalization of Nitro Compounds Catalyzed by an Iron(salen) Complex.

We report on the use of a simple, bench-stable [Fe(salen)2]-μ-oxo precatalyst in the reduction of nitro compounds. The reaction proceeds at room temperature across a range of substrates, including nitro aromatics and aliphatics. By changing the reducing agent from pinacol borane (HBpin) to phenyl silane (H3SiPh), we can chemoselectively reduce nitro compounds while retaining carbonyl functionality. Our mechanistic studies, which include kinetics, electron paramagnetic resonance (EPR), mass spectrometry, and quantum chemistry, indicate the presence of a nitroso intermediate and the generation of an on-cycle iron hydride as a key catalytic intermediate. Based on this mechanistic insight, we were able to extend the chemistry to hydroamination and identified a simple substrate feature (alkene lowest unoccupied molecular orbital (LUMO) energy) that could be used to predict which alkenes would result in productive catalysis.

Directed Hydrogen Atom Transfer for Selective Reactions of Polyenols.

Directed hydrogen atom transfer to alkenes is described. The process is catalyzed by iron complexes and allows for the site-selective hydrofunctionalization of polyenols. Experimental data suggest that coordination of the hydroxy group to the iron hydride intermediate plays an important role in preferential engagement of the allylic alcohol motif and provides a new basis for selectivity in radical hydrofunctionalization events. As a proof of concept, β- and γ-amino alcohols are prepared from the corresponding polyenols in a selective manner.

Synthesis of Mg2FeH6 by High‐Temperature High‐Pressure Reactive Planetary Ball Milling

Herein, a new approach for the synthesis of metal hydrides using simultaneous high‐temperature and high‐pressure reactive ball milling is demonstrated by preparing ternary magnesium iron hydride. The novelty and uniqueness of this technique are based on its integration of a specially designed and manufactured milling vial, allowing synthesis at controlled elevated temperatures (room temperature [RT]–400 °C) and pressures up to 100 bar. A Mg and Fe (2:1) mixture is used as a substrate for Mg2FeH6 synthesis. The effects of temperature on the synthesis kinetics and their outcomes are examined. An increase in the temperature accelerates the kinetics of hydrogen absorption for MgH2, while Mg2FeH6 formation is observed only above 250 °C. Increasing the reaction temperature not only causes magnesium particle refinement due to the hydrogenation and formation of magnesium hydrides but also leads to the agglomeration of iron caused by plastification due to a lack of strain hardening. The maximum conversion to Mg2FeH6 is observed for the sample premilled at RT. In this work, it is revealed that additional physical variables, e.g., pressure, temperature, time, and milling speed, during mechanochemical synthesis and material properties, need to be evaluated and considered to improve the reaction kinetics and yield of the synthesis.

Electride Formation of HCP-Iron at High Pressure: Unraveling the Origin of the Superionic State of Iron-Rich Compounds in Rocky Planets.

Electride possesses electrons localized at interstitial sites without attracting nuclei. It brings outstanding material properties not only originating from its own loosely bounded characteristics but also serving as a quasiatom, which even chemically interacts with other elemental ions. In elemental metals, electride transitions have been reported in alkali metals where valence electrons can easily gain enough kinetic energy to escape nuclei. However, there are few studies on transition metals. Especially iron, the key element of human technology and geophysics, has not been studied in respect of electride formation. In this study, it is demonstrated that electride formation drives the superionic state in iron hydride under high-pressure conditions of the earth's inner core. The electride stabilizes the iron lattice and provides a pathway for hydrogen diffusion by severing the direct interaction between the metal and the volatile element. The coupling between lattice stability and superionicity is triggered near 100GPa and enhanced at higher pressures. It is shown that the electride-driven superionicity can also be generalized for metal electrides and other rocky planetary cores by providing a fundamental interaction between the electride of the parent metal and doped light elements.

Machine learning-driven structure prediction for iron hydrides

Machine learning-driven structure prediction for iron hydrides

Synthesis and Superconducting Properties of Some Phases of Iron Polyhydrides at High Pressures

Experimental syntheses of two iron polyhydrides FeHx(I) and FeHx(II) have been carried out in diamond anvil cells by laser heating of metallic iron to temperatures of about 700 and 2000 K at pressures of 178 and 195 GPa, respectively. The initial sample is an iron plate enriched in the Fe-57 Mössbauer isotope placed in ammonia borane (BH3NH3). The electronic properties of FeHx compounds have been studied by measuring the electrical resistance R(T) at high pressures (180–216 GPa) in the temperature range of ~8–300 K. Based on the obtained R(T) data, two superconducting phases of FeHx compounds with the maximum critical transition temperatures Tc ≈ 25.0 and 27.7 K have been identified. It has been found that with increasing pressure, the temperature Tc in both hydrides increases linearly with the coefficients dTc/dP ~ 0.063 ± 0.001 K/GPa and 0.056 ± 0.003 K/GPa for the FeHx(I) and FeHx(II) phases, respectively. Superconductivity in iron hydrides revealed by the measured resistance R(T) has been confirmed by a number of additional methods.

Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraarylcyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations.

(Cyclopentadienone)iron carbonyl compounds are catalytically active in carbonyl/imine reductions, alcohol oxidations, and borrowing hydrogen reactions, but the effect of cyclopentadienone electronics on their activity is not well established. A series of (tetraarylcyclopentadienone)iron tricarbonyl compounds with varied electron densities on the cyclopentadienone were prepared, and their activities in transfer hydrogenations and dehydrogenations were explored. Additionally, mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, were undertaken to gain insights into catalyst resting states and turnover-limiting steps of these reactions. As the cyclopentadienone electron density increased, both the transfer hydrogenation and dehydrogenation rates increased. A catalytically relevant, trimethylamine-ligated iron compound was isolated and characterized and was observed in solution under both transfer hydrogenation and dehydrogenation conditions. Importantly, it was catalytically active in both reactions. Kinetic isotope effect data and initial rates in transfer hydrogenation reactions with 4'-substituted acetophenones provided evidence that hydrogen transfer from the catalyst to the carbonyl substrate occurred during the turnover-limiting step, and NMR spectroscopy supports the trimethylamine adduct as an off-cycle resting state and the (hydroxycyclopentadienyl)iron hydride as an on-cycle resting state. In transfer dehydrogenations of alcohols, the use of electronically modified benzylic alcohols provided evidence that the turnover-limiting step involves the transfer of hydrogen from the alcohol substrate to the catalyst. The trimethylamine-ligated compound was proposed as the primary catalyst resting state in dehydrogenations.

Iron-Catalyzed Parahydrogen Induced Polarization.

Parahydrogen induced polarization (PHIP) can address the low sensitivity problem intrinsic to nuclear magnetic resonance spectroscopy. Using a catalyst capable of reacting with parahydrogen and substrate in either a hydrogenative or nonhydrogenative manner can result in signal enhancement of the substrate. This work describes the development of a rare example of an iron catalyst capable of reacting with parahydrogen to hyperpolarize olefins. Complexes of the form (MesCCC)Fe(H)(L)(N2) (L = Py (Py = pyridine), PMe3, PPh3) were synthesized from the reaction of the parent complexes (MesCCC)FeMes(L) (Mes = mesityl) with H2. The isolated low-spin iron(II) hydride compounds were characterized via multinuclear NMR spectroscopy, infrared spectroscopy, and single crystal X-ray diffraction. (MesCCC)Fe(H)(Py)(N2) is competent in the hydrogenation of olefins and demonstrated high activity toward the hydrogenation of monosubstituted terminal olefins. Reactions with p-H2 resulted in the first PHIP effect mediated by iron which requires diamagnetism throughout the reaction sequence. This work represents the development of a new PHIP catalyst featuring iron, unlocking potential to develop more PHIP catalysts based on first-row transition metals.

The stability of FeHx and hydrogen transport at Earth’s core mantle boundary

Iron hydride in Earth’s interior can be formed by the reaction between hydrous minerals (water) and iron. Studying iron hydride improves our understanding of hydrogen transportation in Earth’s interior. Our high-pressure experiments found that face-centered cubic (fcc) FeHx (x ≤ 1) is stable up to 165 GPa, and our ab initio molecular dynamics simulations predicted that fcc FeHx transforms to a superionic state under lower mantle conditions. In the superionic state, H-ions in fcc FeH become highly diffusive-like fluids with a high diffusion coefficient of ∼3.7 × 10−4 cm2 s−1, which is comparable to that in the liquid Fe-H phase. The densities and melting temperatures of fcc FeHx were systematically calculated. Similar to superionic ice, the extra entropy of diffusive H-ions increases the melting temperature of fcc FeH. The wide stability field of fcc FeH enables hydrogen transport into the outer core to create a potential hydrogen reservoir in Earth’s interior, leaving oxygen-rich patches (ORP) above the core mantle boundary (CMB).

Four-Step Synthesis of (-)-4-epi-Presilphiperfolan-8α-ol by Intramolecular Iron Hydride Atom Transfer-Mediated Ketone-Alkene Coupling and Studies to Access trans-Hydrindanols with a Botryane Scaffold.

From an (R)-(+)-pulegone-derived building block that incorporates the stereo-defined tertiary carbon bearing a methyl group, as found in the targeted sesquiterpenoid, a four-step synthesis of (-)-4-epi-presilphiperfolan-8-α-ol was achieved. The key processes involved are a ring-closing metathesis leading to a bridged alkene-tethered ketone and its subsequent FeIII -mediated metal-hydride atom transfer (MHAT) transannular cyclization. This synthetic method, implying an irreversible addition of a carbon-centered radical upon a ketone by means of a hydrogen atom transfer upon the alkoxy radical intermediate, was also applied in the synthesis of trans-fused hydrindanols structurally related to botrydial compounds.

Methyl Effects on the Stereochemistry and Reactivity of PPP-Ligated Iron Hydride Complexes.

Iron dihydride complexes are key intermediates in many iron-catalyzed reactions. Previous efforts to study molecules of this type have led to the discovery of a remarkably stable cis-FeH2 complex, which is supported by bis[2-(diisopropylphosphino)phenyl]phosphine (iPrPPHP) along with CO. In this work, the hydrogen on the central phosphorus has been replaced with a methyl group, and the corresponding iron carbonyl dichloride, hydrido chloride, and dihydride complexes have been synthesized. The addition of the methyl group favors the anti configuration for the Me-P-Fe-H moiety and the trans geometry for the H-Fe-CO motif, which is distinctively different from the iPrPPHP system. Furthermore, it increases the thermal stability of the dihydride complex, cis-(iPrPPMeP)Fe(CO)H2 (iPrPPMeP = bis[2-(diisopropylphosphino)phenyl]methylphosphine). The variations in stereochemistry and compound stability contribute greatly to the differences between the two PPP systems in reactions with PhCHO, CS2, and HCO2H.

Formation of distinct iron hydrides via mechanistic divergence in directed C-H Bond activation of aryl ketones, esters and amides.

Directing groups play an important role in controlling the selectivity of C-H bond activation. Here we demonstrate that for iron, the nature of the directing group (e.g., ketone, ester, or amide) influences the C-H activation process. In this study the C-H bond activation step either occurs with or without the assistance of the directing group resulting in distinct cis- and trans-isomers of the corresponding iron hydride.

Iron Hydride in the Earth's Inner Core and Its Geophysical Implications

AbstractHydrogen is potentially a key light element in the Earth's core. Determining the stability of iron hydride is essential for Earth's core mineralogy applications. We investigated the thermal stabilities of a range of Fe‐H binaries at core P‐T conditions. It is concluded that face‐centered cubic phase FeH is stable in the Earth's inner core. The high mobility of hydrogen in the Fe lattice suggests that hydrogen is transferred to a superionic state under the inner core conditions, where the superionic state transfer temperature of H in Fe fcc lattice is ∼500 K higher than that in hcp Fe system. The H concentration in the inner core is estimated to be ∼0.92 wt% to explain its density deficit, this value was further constrained to ∼0.21 wt% by matching the density jump at the inner‐core boundary. H alongside other light elements are required to account for the geophysical observations of the Earth's inner core.

New Insight into CO2 Reduction to Formate by In Situ Hydrogen Produced from Hydrothermal Reactions with Iron.

To reveal the nature of CO2 reduction to formate with high efficiency by in situ hydrogen produced from hydrothermal reactions with iron, DFT calculations were used. A reaction pathway was proposed in which the formate was produced through the key intermediate species, namely iron hydride, produced in situ in the process of hydrogen gas production. In the in situ hydrogenation of CO2, the charge of H in the iron hydride was -0.135, and the Fe-H bond distance was approximately 1.537 Å. A C-H bond was formed as a transition state during the attack of Hδ- on Cδ+. Finally, a HCOO species was formed. The distance of the C-H bond was 1.107 Å. The calculated free energy barrier was 16.43 kcal/mol. This study may provide new insight into CO2 reduction to formate in hydrothermal reactions with metal.

Formal Glycosylation of Quinones with exo-Glycals Enabled by Iron-Mediated Oxidative Radical-Polar Crossover.

The intermolecular C-O coupling reaction of 1,4-quinones with exo-glycals under iron hydride hydrogen atom transfer (HAT) conditions is described. This method provides a direct and regioselective access to a wide range of phenolic O-ketosides related to biologically relevant natural products in diastereomeric ratios up to >98:2 in the furanose and pyranose series. No trace of the corresponding C-glycosylated products that might have resulted from the radical alkylation of 1,4-quinones was observed. The results of mechanistic experiments suggest that the key C-O bond-forming event proceeds through an oxidative radical-polar crossover process involving a single-electron transfer between the HAT-generated glycosyl radical and the electron-acceptor quinone.