The Formation of Surface Lithium–Iron Ternary Hydride and its Function on Catalytic Ammonia Synthesis at Low Temperatures

Lithium hydride (LiH) has a strong effect on iron leading to an approximately 3 orders of magnitude increase in catalytic ammonia synthesis. The existence of lithium–iron ternary hydride species at the surface/interface of the catalyst were identified and characterized for the first time by gas-phase optical spectroscopy coupled with mass spectrometry and quantum chemical calculations. The ternary hydride species may serve as centers that readily activate and hydrogenate dinitrogen, forming Fe-(NH₂)-Li and LiNH₂ moieties—possibly through a redox reaction of dinitrogen and hydridic hydrogen (LiH) that is mediated by iron—showing distinct differences from ammonia formation mediated by conventional iron or ruthenium-based catalysts. Hydrogen-associated activation and conversion of dinitrogen are discussed.     

Electrochemical N2 splitting at well-defined metal complexes

In this short review, we discuss recent examples of well-defined metal complexes capable to split dinitrogen electrochemically. Large progress has been made in the chemical dinitrogen splitting with molecular complexes during the last couple of years; however, electrochemical N₂ splitting remains scarce. Herein, three distinct examples, which were investigated in depth, are discussed. The iron complex 2⁺ converts N₂ to ammonia via an associative mechanism. With the rhenium pincer complex 3, N₂ is cleaved via a dissociative mechanism forming a very stable Re^V-nitride complex. The aluminium complex 6 also converts N₂ electrochemically to ammonia; however, the mechanism is distinctly different to that in 2⁺ or 3, as there is no evidence for a metal–N₂ interaction and likely the ligand acts as a hydride donor.     

Assessing the Brønsted Basicity of Diaminoboryl Anions: Reactivity toward Methylated Benzenes and Dihydrogen

Treatment of toluene or p‐xylene with diaminoboryllithium results in consecutive reactions, involving boryl‐anion‐mediated deprotonation at the benzylic position followed by nucleophilic substitution at the boron center, producing benzylborane species and LiH. Diaminoboryllithium also cleaves H₂ heterolytically affording diaminohydroborane and LiH, while the reaction of lithium diaminoboryl(bromo)cuprate with H₂ takes place accompanied by reduction of Cu^I to give diaminohydroborane, LiH, and Cu⁰.     

N2‐to‐NH3 Conversion by a triphos–Iron Catalyst and Enhanced Turnover under Photolysis

Bridging iron hydrides are proposed to form at the active site of MoFe‐nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N₂ via reductive elimination of H₂. This possibility inspires the investigation of well‐defined molecular iron hydrides as precursors for catalytic N₂‐to‐NH₃ conversion. Herein, we describe the synthesis and characterization of new P₂^P′PhFe(N₂)(H)_x systems that are active for catalytic N₂‐to‐NH₃ conversion. Most interestingly, we show that the yields of ammonia can be significantly increased if the catalysis is performed in the presence of mercury lamp irradiation. Evidence is provided to suggest that photo‐elimination of H₂ is one means by which the enhanced activity may arise.     

Photochemically Induced Reductive Elimination as a Route to a Zirconocene Complex with a Strongly Activated N2 Ligand

The zirconocene dinitrogen complex [{(η⁵‐C₅Me₄H)₂Zr}₂(μ₂,η²,η²‐N₂)] was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high‐yielding, alkali metal‐free route to strongly activated early‐metal N₂ complexes. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote C? H activation or undergo comproportion to formally Zr^III complexes. When N₂ is in excess arene displacement gives rise to strong dinitrogen activation.     

Iron–dinitrogen coordination chemistry: Dinitrogen activation and reactivity

Understanding the coordination of dinitrogen to iron is important for understanding biological nitrogen fixation as well as for designing synthetic systems that are capable of reducing N₂ to NH₃ under mild conditions. This review discusses recent advances in iron–dinitrogen coordination complexes and describes the factors that contribute to the degree of activation of the coordinated N₂. The reactivity of the N₂ ligand is also reviewed, with an emphasis on protonation reactions that yield ammonia and/or hydrazine. Coordination complexes containing N₂ reduction intermediates such as diazene (N₂H₂), hydrazido (N₂H₂^2?), hydrazine (N₂H₄), nitride (N^3?), imide (NH^2?), and amide (NH₂^?) are also discussed in the context of the mechanism of N₂ reduction to NH₃ mediated by iron coordination complexes.     

Dioxygen Sensitivity of [Fe]‐Hydrogenase in the Presence of Reducing Substrates

Mono‐iron hydrogenase ([Fe]‐hydrogenase) reversibly catalyzes the transfer of a hydride ion from H₂ to methenyltetrahydromethanopterin (methenyl‐H₄MPT⁺) to form methylene‐H₄MPT. Its iron guanylylpyridinol (FeGP) cofactor plays a key role in H₂ activation. Evidence is presented for O₂ sensitivity of [Fe]‐hydrogenase under turnover conditions in the presence of reducing substrates, methylene‐H₄MPT or methenyl‐H₄MPT⁺/H₂. Only then, H₂O₂ is generated, which decomposes the FeGP cofactor; as demonstrated by spectroscopic analyses and the crystal structure of the deactivated enzyme. O₂ reduction to H₂O₂ requires a reductant, which can be a catalytic intermediate transiently formed during the [Fe]‐hydrogenase reaction. The most probable candidate is an iron hydride species; its presence has already been predicted by theoretical studies of the catalytic reaction. The findings support predictions because the same type of reduction reaction is described for ruthenium hydride complexes that hydrogenate polar compounds.     

Enzymatic and catalytic reduction of dinitrogen to ammonia: Density functional theory characterization of alternative molybdenum active sites

We used density functional calculations to model dinitrogen reduction by a FeMo cofactor containing a central nitrogen atom and by a Mo‐based catalyst. Plausible intermediates, reaction pathways, and relative energetics in the enzymatic and catalytic reduction of N₂ to ammonia at a single Mo center are explored. Calculations indicate that the binding of N₂ to the Mo atom and the subsequent multiple proton–electron transfer to dinitrogen and its protonated species involved in the conversion of N₂ are feasible energetically. In the reduction of N₂ the Mo atom experiences a cycled oxidation state from Mo(IV) to Mo(VI) by nitrogenase and from Mo(III) to Mo(VI) by the molybdenum catalyst, respectively, tuning the gradual reduction of N₂. Such a wide range of oxidation states exhibited by the Mo center is crucial for the gradual reduction process via successive proton–electron transfer. Present results suggest that the Mo atom in the N‐centered FeMo cofactor is a likely alternative active site for dinitrogen binding and reduction under mild conditions once there is an empty site available at the Mo site. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Synthesis and Diverse Transformations of a Dinitrogen Dititanium Hydride Complex Bearing Rigid Acridane‐Based PNP‐Pincer Ligands

Studies on N₂ activation and transformation by transition metal hydride complexes are of particular interest and importance. The synthesis and diverse transformations of a dinitrogen dititanium hydride complex bearing the rigid acridane‐based ^acriPNP‐pincer ligands {[(^acriPNP)Ti]₂(μ₂‐η¹:η²‐N₂)(μ₂‐H)₂} are presented. This complex enabled N₂ cleavage and hydrogenation even without additional H₂ or other reducing agents. Furthermore, diverse transformations of the N₂ unit with a variety of organometallic compounds such as ZnMe₂, MgMe₂, AlMe₃, B(C₆F₅)₃, PinBH, and PhSiH₃ have been well established at the rigid ^acriPNP‐ligated dititanium framework, such as reversible bonding‐mode change between the end‐on and side‐on/end‐on fashions, diborylative N=N bond cleavage, the formal insertion of two dimethylaluminum species into the N=N bond, and the formal insertion of two silylene units into the N=N bond. This work has revealed many unprecedented aspects of dinitrogen reaction chemistry.     

Hydrogenated Bismuth Molybdate Nanoframe for Efficient Sunlight‐Driven Nitrogen Fixation from Air

Sunlight‐driven dinitrogen fixation can lead to a novel concept for the production of ammonia under mild conditions. However, the efficient artificial photosynthesis of ammonia from ordinary air (instead of high pure N₂) has never been implemented. Here, we report for the first time the intrinsic catalytic activity of Bi₂MoO₆ catalyst for direct ammonia synthesis under light irradiation. The edge‐exposed coordinatively unsaturated Mo atoms in an Mo?O coordination polyhedron can act as activation centers to achieve the chemisorption, activation, and photoreduction of dinitrogen efficiently. Using that insight as a starting point, through rational structure and defect engineering, the optimized Bi₂MoO₆ sunlight‐driven nitrogen fixation system, which simultaneously possesses robust nitrogen activation ability, excellent light‐harvesting performance, and efficient charge transmission was successfully constructed. As a surprising achievement, this photocatalytic system demonstrated for the first time ultra‐efficient (1.3 mmol g^?1 h^?1) and stable sunlight‐driven nitrogen fixation from air in the absence of any organic scavengers.     

Revealing Capacity Degradation of Ge Anodes in Lithium-Ion Batteries Triggered by Interfacial LiH

The Germanium (Ge), as a fast-charging and high specific capacity (1568 mAh g⁻¹) alloy anode, is greatly hampered in practical application by poor cyclability. To date, the understanding of cycling performance degradation remains elusive. This study illustrates that, contrary to conventional beliefs, most of the Ge material in failed anodes still retains good integrity and does not undergo severe pulverization. It is revealed that capacity degradation is clearly correlated to the interfacial evolution of lithium hydride (LiH). Tetralithium germanium hydride (Li₄Ge₂H), as a new species derived from LiH, is identified as the culprit of Ge anode degradation, which is the dominant crystalized component in an ever-growing and ever-insulating interphase. The significantly increased thickness of the solid electrolyte interface (SEI) is accompanied by the accumulation of insulating Li₄Ge₂H upon cycling, which severely retards the charge transport process and ultimately triggers the anode failure. We believe that the comprehensive understanding of the failure mechanism presented in this study is of great significance to promoting the design and development of alloy anode for the next generation of lithium-ion batteries.     

Dinitrogen Activation Upon Reduction of a Triiron(II) Complex

Reaction of a trinuclear iron(II) complex, Fe₃Br₃ L ( 1 ), with KC₈ under N₂ leads to dinitrogen activation products ( 2 ) from which Fe₃(NH)₃ L ( 2‐1 ; L is a cyclophane bridged by three β‐diketiminate arms) was characterized by X‐ray crystallography. ¹H NMR spectra of the protonolysis product of 2 synthesized under ¹⁴N₂ and ¹⁵N₂ confirm atmospheric N₂ reduction, and ammonia is detected by the indophenol assay (yield ～30 %). IR and Mössbauer spectroscopy, and elemental analysis on 2 and 2‐1 as well as the tri(amido)triiron(II) 3 and tri(methoxo)triiron 4 congeners support our assignment of the reduction product as containing protonated N‐atom bridges.     

Mononuclear Iridium Dinitrogen Complexes Bonded to Zeolite HY

The adsorption of N₂ on structurally well‐defined dealuminated HY zeolite‐supported iridium diethylene complexes was investigated. Iridium dinitrogen complexes formed when the sample was exposed to N₂ in H₂ at 298 K, as shown by infrared spectra recorded with isotopically labeled N₂. Four supported species formed in various flowing gases: Ir(N₂), Ir(N₂)(N₂), Ir(C₂H₅)(N₂), and Ir(H)(N₂). Their interconversions are summarized in a reaction network, showing, for example, that, in the presence of N₂, Ir(N₂) was the predominant dinitrogen species at temperatures of 273–373 K. Ir(CO)(N₂) formed transiently in flowing CO, and in the presence of H₂, rather stable iridium hydride complexes formed. Four structural models of each iridium complex bonded at the acidic sites of the zeolite were employed in a computational investigation, showing that the calculated vibrational frequencies agree well with experiment when full calculations are done at the level of density functional theory, independent of the size of the model of the zeolite.     

Hydrogen storage properties of Li-Mg-N-H systems with different ratios of LiH/Mg(NH2)2

In this work, the hydrogen desorption and structural properties of the Li-Mg-N-H systems with different LiH/Mg(NH2)2 ratios are systemically investigated. The results indicate that the system with the LiH/Mg(NH2)2 ratio of 6/3 transforms into Li2NH and MgNH, and then, the mixture forms an unknown phase by a solid-solid reaction, which presumably is the ternary imide Li2Mg(NH)2; the system with the LiH/Mg(NH2)2 ratio of 8/3 transforms into 4Li2NH and Mg3N2 after releasing H2 at T < 400 degrees C; the system with the LiH/Mg(NH2)2 ratio of 12/3 transforms into 4Li3N and Mg3N2 after releasing H2 at T > 400 degrees C, where the LiMgN phase is formed by the reaction between Li3N and Mg3N2. The characteristics of the phase transformations and the thermal gas desorption behaviors in these Li-Mg-N-H systems could be reasonably explained by the ammonia mediated reaction model, irrespective of the difference in the LiH/Mg(NH2)2 ratios.     

Selective Dinitrogen Conversion to Ammonia Using Water and Visible Light through Plasmon‐induced Charge Separation

The generation of ammonia from atmospheric nitrogen and water using sunlight is a preferable approach to obtaining ammonia as an energy carrier and potentially represents a new paradigm for achieving a low‐carbon and sustainable‐energy society. Herein, we report the selective conversion of dinitrogen into ammonia through plasmon‐induced charge separation by using a strontium titanate (SrTiO₃) photoelectrode loaded with gold nanoparticles (Au‐NPs) and a zirconium/zirconium oxide (Zr/ZrO_x) thin film. We observed the simultaneous stoichiometric production of ammonia and oxygen from nitrogen and water under visible‐light irradiation.     

Low‐Temperature N2 Binding to Two‐Coordinate L2Fe0 Enables Reductive Trapping of L2FeN2− and NH3 Generation

The two‐coordinate [(CAAC)₂Fe] complex [CAAC=cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T2 complex, [(CAAC)₂Fe(N₂)], was trapped by one‐electron reduction to its corresponding anion [(CAAC)₂FeN₂]^? at low temperature. This complex was structurally characterized and features an activated dinitrogen unit which can be silylated at the β‐nitrogen atom. The redox‐linked complexes [(CAAC)₂Fe^I][BAr^F₄], [(CAAC)₂Fe⁰], and [(CAAC)₂Fe^?IN₂]^? were all found to be active for the reduction of dinitrogen to ammonia upon treatment with KC₈ and HBAr^F₄?2 Et₂O at ?95 °C [up to (3.4±1.0) equivalents of ammonia per Fe center]. The N₂ reduction activity is highly temperature dependent, with significant N₂ reduction to NH₃ only occurring below ?78 °C. This reactivity profile tracks with the low temperatures needed for N₂ binding and an otherwise unavailable electron‐transfer step to generate reactive [(CAAC)₂FeN₂]^?.     

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 η³‐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.     

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 η³‐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.     

Formation of Gas‐Phase Formate in Thermal Reactions of Carbon Dioxide with Diatomic Iron Hydride Anions

The hydrogenation of carbon dioxide involves the activation of the thermodynamically very stable molecule CO₂ and formation of a C−H bond. Herein, we report that HCO₂⁻ and CO can be formed in the thermal reaction of CO₂ with a diatomic metal hydride species, FeH⁻. The FeH⁻ anions were produced by laser ablation, and the reaction with CO₂ was analyzed by mass spectrometry and quantum‐chemical calculations. Gas‐phase HCO₂⁻ was observed directly as a product, and its formation was predicted to proceed by facile hydride transfer. The mechanism of CO₂ hydrogenation in this gas‐phase study parallels similar behavior of a condensed‐phase iron catalyst.     

Conversion of Dinitrogen into Acetonitrile under Ambient Conditions

About 20 % of the ammonia production is used as the chemical feedstock for nitrogen‐containing chemicals. However, while synthetic nitrogen fixation at ambient conditions has had some groundbreaking contributions in recent years, progress for the direct conversion of N₂ into organic products remains limited and catalytic reactions are unknown. Herein, the rhenium‐mediated synthesis of acetonitrile using dinitrogen and ethyl triflate is presented. A synthetic cycle in three reaction steps with high individual isolated yields and recovery of the rhenium pincer starting complex is shown. The cycle comprises alkylation of a nitride that arises from N₂ splitting and subsequent imido ligand centered oxidation to nitrile via a 1‐azavinylidene (ketimido) intermediate. Different synthetic strategies for intra‐ and intermolecular imido ligand oxidation and associated metal reduction were evaluated that rely on simple proton, electron, and hydrogen‐atom transfer steps.