Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and Tandem Frustrated Lewis Pair Hydrogenation to Ammonia

Although reductive cleavage of dinitrogen (N₂) to nitride (N^3?) and hydrogenation with dihydrogen (H₂) to yield ammonia (NH₃) is accomplished in heterogeneous Haber–Bosch industrial processes on a vast scale, sequentially coupling these elementary reactions together with a single metal complex remains a major challenge for homogeneous molecular complexes. Herein, we report that the reaction of a chloro titanium triamidoamine complex with magnesium effects complete reductive cleavage of N₂ to give a dinitride dititanium dimagnesium ditriamidoamine complex. Tandem H₂ splitting by a phosphine–borane frustrated Lewis pair (FLP) shuttles H atoms to the N^3?, evolving NH₃. Isotope labelling experiments confirmed N₂ and H₂ fixation. Though not yet catalytic, these results give unprecedented insight into coupling N₂ and H₂ cleavage and N?H bond formation steps together, highlight the importance of heterobimetallic cooperativity in N₂ activation, and establish FLPs in NH₃ synthesis.     

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.     

New Developments in Nitrogen Fixation

The production of ammonia from atmospheric dinitrogen at room temperature and ambient pressure in analogy to nature is a long-term goal for coordination chemists. Novel reactions of N₂-containing transition metal complexes with H₂, the first side-on N₂-bridged structure of an actinide complex, and an interesting variation of synthetic N₂ fixation are the key points addressed in this contribution. The results are related to the known chemistry of N₂ complexes, and their significance is discussed with respect to enzymatic N₂ fixation.     

REVIEW: SOME CONSIDERATIONS ABOUT COORDINATION COMPOUNDS WITH END-ON DINITROGEN

Abstract

There have been several reviews on dinitrogen coordination compounds but no special attention has been paid to correlate the electron configuration of the metal ions with the main features of the ligands in order to establish an electron configuration-stability relationship. In this article we consider nearly 200 complexes with terminal dinitrogen to find common characteristics that lead to the synthesis of other stable dinitrogen compounds. This survey shows that for coordination number 6 there is a strong tendency for a d ⁶ configuration in the metals, with oxidation states between 1- and 2.

On the basis of quantum chemistry, dinitrogen as a ligand can be compared with the isoelectronic species CO, CN^?, NO⁺. The MO and orbital energy diagrams indicate that N₂ is not a good donor neither a good acceptor, but with the appropriate symmetry and in the presence of a good π-donor metal it forms an N₂← M π-bond strengthened by an N₂→ M σ-back-bonding.     

Single-Atom Catalysis Using Chromium Embedded in Divacant Graphene for Conversion of Dinitrogen to Ammonia

Reduction of dinitrogen to ammonia under ambient conditions is a long-standing challenge. The few metal-based catalysts proposed have conspicuous disadvantages such as high cost, high energy consumption, and being hazardous to the environment. Single-atom catalysis has emerged as a new frontier in heterogeneous catalysis and metal atoms atomically dispersed on supports receive more and more attention owing to rapid advances in synthetic methodologies and computational modeling. Herein, we propose metal atoms embedded in divacant graphene as a catalyst for N₂ fixation based on density functional calculations. We systematically investigate the potential of using transition metal like Cr, Mn, Fe, Mo and Ru as catalysts and our study reveals that Cr embedded in graphene exhibit good catalytic activity for N₂ fixation. The synergy between the metal atoms and graphene surface provides a stable support to the metal center that has a high spin density to promote adsorption of N₂ and activation of its N≡N triple bond. Our study deciphers the mechanism of conversion of N₂ to ammonia following two possible reaction pathways, distal and enzymatic routes, via sequential protonation and reduction of activated N₂. The study provides a rational framework for conversion of dinitrogen to ammonia using single atom catalyst.     

A Cornucopia of Iridium Nitrogen Compounds Produced from Laser-Ablated Iridium Atoms and Dinitrogen

The reaction of laser-ablated iridium atoms with dinitrogen molecules and nitrogen atoms yield several neutral and ionic iridium dinitrogen complexes such as Ir(N₂), Ir(N₂)⁺, Ir(N₂)₂, Ir(N₂)₂⁻, IrNNIr, as well as the nitrido complexes IrN, Ir(N)₂ and IrIrN. These reaction products were deposited in solid neon, argon and nitrogen matrices and characterized by their infrared spectra. Assignments of vibrational bands are supported by ab initio and first principle calculations as well as ^14/15N isotope substitution experiments. The structural and electronic properties of the new dinitrogen and nitrido iridium complexes are discussed. While the formation of the elusive dinitrido complex Ir(N)₂ was observed in a subsequent reaction of IrN with N atoms within the cryogenic solid matrices, the threefold coordinated iridium trinitride Ir(N)₃ could not be observed so far.     

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.     

Room‐Temperature Functionalization of N2 to Borylamine at a Molybdenum Complex

Intermolecular, stepwise functionalization by BH bonds of a (triphosphine)Mo^IV–nitrido complex generated by N₂ splitting is reported. The imido–hydride and di‐hydride–amido Mo^IV complexes have been isolated and characterized. Addition of PinBH to the [Mo(H)₂(N(BPin)₂)]⁺ complex at room temperature results in the liberation of borylamines from the metal center.     

The Triple‐Bond Metathesis of Aryldiazonium Salts: A Prospect for Dinitrogen Cleavage

The {N₂} unit of aryldiazonium salts undergoes unusually facile triple‐bond metathesis on treatment with molybdenum or tungsten alkylidyne ate complexes endowed with triphenylsilanolate ligands. The reaction transforms the alkylidyne unit into a nitrile and the aryldiazonium entity into an imido ligand on the metal center, as unambiguously confirmed by X‐ray structure analysis of two representative examples. A tungsten nitride ate complex is shown to react analogously. Since the bonding situation of an aryldiazonium salt is similar to that of metal complexes with end‐on‐bound dinitrogen, in which {N₂}→M σ donation is dominant and electron back donation minimal, the metathesis described herein is thought to be a conceptually novel strategy toward dinitrogen cleavage devoid of any redox steps and, therefore, orthogonal to the established methods.     

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]^?.     

Dynamic 15N NMR studies of iron phosphine complexes containing coordinated dinitrogen

The ¹⁵N‐labelled iron dinitrogen complexes trans‐[FeH(N₂)(PP)₂]⁺[BPh₄]^? (PP = dppe, depe, dmpe) and cis‐[FeH(N₂)(PP₃)]⁺[BPh₄]^? were prepared in situ by exchange of unlabelled coordinated dinitrogen with ¹⁵N₂. ¹⁵N NMR chemical shifts and coupling constants are reported. The ¹⁵N spectra exhibit separate signals for the metal‐bound and terminal nitrogen atoms of the coordinated N₂. The ¹⁵N resonances display ¹⁵N, ¹⁵N coupling as well as ³¹P, ¹⁵N coupling and long‐range ¹⁵N, ¹H coupling when there is a metal‐bound hydrido ligand. Exchange between free and coordinated dinitrogen was monitored by magnetization transfer between ¹⁵N‐labelled sites using an inversion–transfer–recovery experiment. Exchange between the metal‐bound and terminal nitrogen atoms of coordinated N₂ was also monitored by magnetization transfer and this could proceed by N₂ dissociation or by an intramolecular process. Copyright © 2003 John Wiley & Sons, Ltd.     

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.     

Terminal end-on coordination of dinitrogen versus isoelectronic CO: A comparison using the charge displacement analysis

The metal dinitrogen bonding in a wide series of terminal end-on dinitrogen complexes is investigated with the charge displacement analysis based on natural orbitals of chemical valence (CD-NOCV). The effect of the σ donation and π backdonation on the N N bond are discussed and compared with the observations for a series of carbonyl complexes, published in 2016 by Tarantelli et al. The σ donation is relative invariant over the series of dinitrogen complexes and has no significant effect on the N N bond strength, whereas the π backdonation causes a considerable elongation of the N N bond. Some uncommon examples of weakly bound dinitrogen with blue-shifted stretching frequency compared to free N₂ (ν = 2330 cm⁻¹) are known. The dinitrogen bonding in these complexes is simulated with a point charge. Apparently, electrostatics account for the shortened N─N bond in these systems.     

Dinitrogen-Transition Metal Complexes: Synthesis,Properties, and Significance

Compared to the large number of CO complexes, N₂ complexes are still rare. In certain cases they may be formed from N₂ gas and metal compounds under physiological conditions, and are therefore frequently considered as possible intermediates in N₂ assimilation. However, despite numerous attempts it has not yet been possible to reduce the N₂ ligand of a fully characterized N₂ complex to NH₃. This negative evidence recently prompted the discoverers of the first dinitrogen complex, the [Ru(NH₃)₅N₂]^2⊕ ion isolated in 1965, to express doubt whether such complexes really do play a part in the enzymatic reduction of N₂. The latest findings nevertheless incline toward a somewhat more optimistic view. Thus, the mild partial reduction of the N₂ ligand in metal complexes with inert gas configuration, hitherto considered nonreducible, justifies the hope that a suitable system permitting the catalytic reduction of molecular nitrogen under normal conditions will one day be found. Even apart from any technical potential, the N₂ complexes constitute an interesting chapter of modern inorganic chemistry.     

Molybdenum Complexes Supported by Mixed NHC/Phosphine Ligands: Activation of N2 and Reaction With P(OMe)3 to the First Meta‐Phosphite Complex

Molybdenum(0) dinitrogen complexes, supported by the mixed NHC/phosphine pincer ligand PCP, exhibit an extreme activation of the N₂ ligand due to a very π‐electron‐rich metal center. The low thermal stability of these compounds can be increased using phosphites instead of phosphines as coligands. Through an amalgam reduction of [MoCl₃(PCP)] in the presence of trimethyl phosphite and N₂ the highly activated and room‐temperature stable dinitrogen complex [Mo(N₂)(PCP)(P(OMe)₃)₂] is obtained. As a second product, the first transition metal complex containing the meta‐phosphite ligand P(O)(OMe) originates from this reaction.     

Counterion Dependence of Dinitrogen Activation and Functionalization by a Diniobium Hydride Anion

We report the synthesis of anionic diniobium hydride complexes with a series of alkali metal cations (Li⁺, Na⁺, and K⁺) and the counterion dependence of their reactivity with N₂. Exposure of these complexes to N₂ initially produces the corresponding side‐on end‐on N₂ complexes, the fate of which depends on the nature of countercations. The lithium derivative undergoes stepwise migratory insertion of the hydride ligands onto the aryloxide units, yielding the end‐on bridging N₂ complex. For the potassium derivative, the N?N bond cleavage takes place along with H₂ elimination to form the nitride complex. Treatment of the side‐on end‐on N₂ complex with Me₃SiCl results in silylation of the terminal N atom and subsequent N?N bond cleavage along with H₂ elimination, giving the nitride‐imide‐bridged diniobium complex.     

Beryllium Atom Mediated Dinitrogen Activation via Coupling with Carbon Monoxide

The reactions of laser‐ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end‐on bonded NNBeCO and side‐on bonded (η²‐N₂)BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end‐on bonded complex has a triplet ground state while the side‐on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge‐ and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s⁰2p² excited configuration and that the metal‐ligand bonds can be described in terms of N₂→Be←CO σ donation and concomitant N₂←Be→CO π backdonation. The results demonstrate that the activation of N₂ with the N?N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.     

Metal‐Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N?N bond energy. Photochemical strategies that drive N₂ fixation are scarcely developed. Here, the synthesis of a dinuclear N₂‐bridged complex is presented upon reduction of a rhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2 H⁺/2 e^? transfer of the pincer ligand. A three‐step cycle is demonstrated for N₂ to nitrile fixation that relies on electrochemical reduction, photochemical N₂‐splitting and thermal nitrogen transfer.     

Conversion of ammonia to dinitrogen in wastewater by Nitrosomonas europaea

Because Nitrosomonas europaea contains ammonia-oxidizing enzyme, nitrite reductase, and nitrous oxide reductase, the conversion of ammonia to dinitrogen was tried with different reaction conditions. In aerobic reaction conditions, ammonium was converted to nitrite (NO ₂ ⁻ ), while under oxygen-limiting or oxygen-free conditions, NO ₂ ⁻ -N formed from ammonia oxidation by N. europaea was reduced to N₂O and dinitrogen with 22% conversion. During denitrification, optimal pH for the production of N₂O and dinitrogen was found to be 7.0–8.0. Dinitrogen was not produced in acidic pH<7.0. A low partial oxygen pressure as well as oxygen-free conditions are favorable for high production of dinitrogen.     

Facile conversion of ammonia to a nitride in a rhenium system that cleaves dinitrogen

Rhenium complexes with aliphatic PNP pincer ligands have been shown to be capable of reductive N₂ splitting to nitride complexes. However, the conversion of the resulting nitride to ammonia has not been observed. Here, the thermodynamics and mechanism of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, conversion of ammonia to the nitride complex. Depending on the conditions, treatment of a rhenium(iii) precursor with ammonia gives either a bis(amine) complex [(PNP)Re(NH₂)₂Cl]⁺, or results in dehydrohalogenation to the rhenium(iii) amido complex, (PNP)Re(NH₂)Cl. The N–H hydrogen atoms in this amido complex can be abstracted by PCET reagents which implies that they are quite weak. Calorimetric measurements show that the average bond dissociation enthalpy of the two amido N–H bonds is 57 kcal mol⁻¹, while DFT computations indicate a substantially weaker N–H bond of the putative rhenium(iv)-imide intermediate (BDE = 38 kcal mol⁻¹). Our analysis demonstrates that addition of the first H atom to the nitride complex is a thermochemical bottleneck for NH₃ generation.

Rhenium–PNP complexes split N₂ to nitrides, but the nitrides do not give ammonia. Here, the thermodynamics of the hypothetical N–H bond forming steps are evaluated through the reverse reaction, showing that the first H addition is the bottleneck.