N2 Binding Research Articles

Cation assisted binding and cleavage of dinitrogen by uranium complexes.

The role of alkali promoters in N2 cleavage by metal complexes remains poorly understood despite its relevance to the industrial production of ammonia from N2. Here we report a series of alkali bound-oxo-bridged diuranium(iii) complexes that provide a unique example of decreasing N2 binding affinity with increasing cation size (from K to Cs). N2 binding was found to be irreversible in the presence of K. A N2 complex could be isolated in the solid state in the presence of the Rb cation and crystallographically characterized, but N2 binding was found to be reversible under vacuum. In the presence of the Cs cation N2 binding could not be detected at 1 atm. Electrochemical and Computational studies suggest that the decrease in N2 binding affinity is due to steric rather than electronic effects. We also find that weak N2 binding in ambient conditions does not prevent alkali assisted N2 cleavage to nitride from occurring. More importantly, we present the first example of cesium assisted N2 cleavage leading to the isolation of a N2 derived multimetallic U/Cs bis-nitride. The nitrides readily react with protons and CO to yield ammonia, cyanate and cyanide.

Cryo spectroscopy of N2 on cationic iron clusters.

Infrared photodissociation (IR-PD) spectra of iron cluster dinitrogen adsorbate complexes [Fen(N2)m]+ for n = 8-20 reveal slightly redshifted IR active bands in the region of 2200-2340 cm-1. These bands mostly relate to stretching vibrations of end-on coordinated N2 chromophores, a μ1,end end-on binding motif. Density Functional Theory (DFT) modeling and detailed analysis of n = 13 complexes are consistent with an icosahedral Fe13 + core structure. The first adsorbate shell closure at (n,m) = (13,12)-as recognized by the accompanying paper on the kinetics of N2 uptake by cationic iron clusters-comes with extensive IR-PD band broadening resulting from enhanced couplings among adjacent N2 adsorbates. DFT modeling predicts spin quenching by N2 adsorption as evidenced by the shift of the computed spin minima among possible spin states (spin valleys). The IR-PD spectrum of (17,1) surprisingly reveals an absence of any structure but efficient non-resonant fragmentation, which might indicate some weakly bound (roaming) N2 adsorbate. The multiple and broad bands of (17,m) for all other cases than (17,1) and (17,7) indicate a high degree of variation in N2 binding motifs and couplings. In contrast, the (17,7) spectrum of six sharp bands suggests pairwise equivalent N2 adsorbates. The IR-PD spectra of (18,m) reveal additional features in the 2120-2200 cm-1 region, which we associate with a μ1,side side-on motif. Some additional features in the (18,m) spectra at high N2 loads indicate a μ1,tilt tilted end-on adsorption motif.

Estimations of Fe–N2 Intrinsic Interaction Energies of Iron–Sulfur/Nitrogen–Carbon Sites: A Deeper Bonding Insight by EDA-NOCV Analysis of a Model Complex of the Nitrogenase Cofactor

The MoFe7S9C1– unit of the nitrogenase cofactor (FeMoco) attracts chemists and biochemists due to its unusual ability to bind aerial dinitrogen (N2) at ambient condition and catalytically convert it into ammonia (NH3). The mode of N2 binding and its reaction pathways are yet not clear. An important conclusion has been made based on the very recent synthesis and isolation of model Fe(I/0)-complexes with sulfur-donor ligands under the cleavage of one Fe–S bond followed by binding of N2 at the Fe(0) center. These complexes are structurally relevant to the nitrogenase cofactor (MoFe7S9C1–). Herein, we report the EDA-NOCV analyses and NICS calculations of the dinitrogen-bonded dianionic complex Fe0–N2 (1) (having a CAr ← Fe π-bond) and monoanionic complex FeI–N2 (2) (having a CAr–Fe σ-bond) to provide a deeper insight into the Fe–N2 interacting orbitals and corresponding pairwise interaction energies (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; NICS = nucleus-independent chemical shifts). The orbital interaction in the Fe–N2 bond is significantly larger than Coulombic interactions, with major pairwise contributions coming from d(Fe) orbitals to the empty π* orbitals of N2 (three Fe → N2). ΔEint values are in the range of −61 to −77 kcal mol–1. Very interestingly, NICS calculations have been carried out for the fragments before and after binding of the N2 molecule. The computed σ- and π-aromaticity values are attributed to the position of the Fe atoms, oxidation states of Fe centers, and Fe–C bond lengths of these two complexes.

EDA-NOCV Calculation for Efficient N2 Binding to the Reduced Ni3S8 Complex: Estimation of Ni-N2 Intrinsic Interaction Energies.

The binding of the dinitrogen molecule to the metal center is the first and crucial step toward dinitrogen activation. Favorable interaction energies are desired by chemists and biochemists to study model complexes in the laboratory. An electrochemically reduced form of a previously isolated sulfur-bridged Ni3S8 complex is inferred to bind N2 at multiple Ni centers, and this bonded N2 undergoes reductive protonation to produce hydrazine (N2H4) as the product in the presence of a proton donor. Density functional theory (DFT) calculations and quantum theory of atoms in molecules (QTAIM) analysis have been carried out to shed light on the nature of N2 binding to an anionic trinuclear Ni3S8 complex. Additionally, energy decomposition analysis with the combination of natural orbital for chemical valence (EDA–NOCV) analysis has been performed to estimate the pairwise interaction energies between the Ni center and the N2 molecule under experimental conditions.

The activity evidence of Ti defect towards electrocatalytic N2 reduction

Electrochemical N2 reduction reaction (NRR) for NH3 synthesis, which usually needs highly-efficient electrocatalysts for N2 activation, is a carbon-neutral alternation compared to the traditional Haber–Bosch process. Although Ti-based compounds is widely used as electrocatalysts, what Ti defect affects NRR activity is still illusive. In this work, our systematic density functional calculations on Ti defect-decorated titanium oxide disclose that the unsaturated-Ti with the orbital splitting of defect electron states is the necessary feature for N2 binding and activation, which can be further enhanced by increasing the splitting degree. The bonding/antibonding orbital population and projected density of states indicate that the nature of N2 binding and activation on Ti-defect site is attributed to the elimination of the bonding orbital population in the conduction bands and the formation of * π back-bonding in the valence bands. For the whole NRR process, the synergy of Ti-defect and oxygen vacancy (VO) promotes N2 reduction, and the required maximum energy input scales quite well with the adsorption strength of *NNH. Finally, the formed volcano shape successfully predicts new candidate catalysts for ammonia synthesis, such as TiO2 combined VO with Ti interstitial or H atom. This work provides disclosure of the key elements on the rational design of Ti-based nanomaterial electrocatalysts for artificial N2 fixation.

Comparison of the accuracy of DFT methods for reactions with relevance to nitrogenase

We have studied the accuracy of 16 different density functional theory methods to reproduce experimental data for bond lengths, angles, vibrational frequencies, as well as enthalpies and entropies for the binding of N2, H2, CO and hydride ions to various transition-metal complexes (with Fe, Ni, Cr, Mo and W) with relation to nitrogenase. We show that generalized gradient approximation functionals give better structure-related parameters, whereas hybrid functionals often give better energies. However, the BLYP and B97D functionals seem to give reasonably accurate results for both types of properties. Geometries and entropies are converged with split-valence basis sets, but energies and vibrational frequencies (and therefore also thermal corrections) in general require a basis set of triple-zeta quality. Dispersion corrections are important to obtain accurate energies (contributing by up to 57 kJ mol−1), as well as structures.

Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12 X11 ]- (X=F, Cl, Br, I, CN).

Electrophilic anions of type [B12X11]− posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B12X11]− with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ‐donors and (ii) CO/N2 as σ‐donor‐π‐acceptors. Temperature‐dependent formation of [B12X11NG]− indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B12X11]− and steric effects. The binding of CO and N2 to [B12X11]− is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B12X11CO]− and [B12X11N2]− were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies νCO and νN2 , respectively. Observed shifts of νCO and νN2 are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π‐backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B12X11]− reactivety dependent on the substituent X and provides first systematic data on π‐backdonation from delocalized σ‐electron systems of closo‐borate anions.

Dinitrogen binding and activation at a molybdenum-iron-sulfur cluster.

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N2 to NH3. Although progress continues to be made in modelling the structural features of nitrogenase cofactors, no synthetic Fe-S cluster has been shown to form a well-defined coordination complex with N2. Here we report that embedding an [MoFe3S4] cluster in a protective ligand environment enables N2 binding at Fe. The bridging [MoFe3S4]2(μ-η1:η1-N2) complex thus prepared features a substantially weakened N-N bond despite the relatively high formal oxidation states of the metal centres. Substitution of one of the [MoFe3S4] cubanes with an electropositive Ti metalloradical induces additional charge transfer to the N2 ligand with generation of Fe-N multiple-bond character. Structural and spectroscopic analyses demonstrate that N2 activation is accompanied by shortened Fe-S distances and charge transfer from each Fe site, including those not directly bound to N2. These findings indicate that covalent interactions within the cluster play a critical role in N2 binding and activation.

Role of Catalyst in Controlling N2 Reduction Selectivity: A Unified View of Nitrogenase and Solid Electrodes

The Haber–Bosch process conventionally reduces N2 to ammonia at 200 bar and 500 °C. Under ambient conditions, i.e., room temperature and ambient pressure, N2 can be converted into ammonia by the nitrogenase molecule and lithium-containing solid electrodes in nonaqueous media. In this work, we explore the catalyst space for the N2 reduction reaction under ambient conditions. We describe N2 reduction on the basis of the *N2 binding energy versus the *H binding energy; we find that under standard conditions, no catalyst can bind and reduce *N2 without producing H2. We show why a selective catalyst for N2 reduction will also likely be selective for CO2 reduction, but N2 reduction is intrinsically more challenging than CO2 reduction. Only by modulating the reaction pathway, like nitrogenase, or by tuning chemical potentials, like the Haber–Bosch and the Li-mediated process, N2 can be reduced.

Chromism of chemical compounds

Chromism, generally a reversible color change of substances, finds vital applications in the fields of switching, catalysis, molecule binding, drug delivery and energy storage applications. Chromism is caused by the change of electronic, conformational, crystal or physical structure of materials due to heat, light, solvent/vapour, applied electric potential, etc. There have been observed different types of chromism such as thermochromism, photochromism, solvetochromism, electrochromism and many others from different compounds. Here in we have discussed different types of chromisms but in brief as inherited by various types of materials. Reversible multicolor chromism by metal halide formamidinium perovskite attests solvetochromism and thermochromism due to the hydrogen bonding. Tetrathiazolylthiophene compound shows mechanochromism because of the structural deformation. Chain length of ligand influences the chromism in copper complex. Gold cluster complex, boron complex, Bi2WO6 also display specific chromic performance. Phosphomolybdate – malachite green dye moiety (PMMG) displays hydrochromism because of bridged water molecules. N2 (or O2) binding to PMMG has recently been shown as an extension of hydrochromism phenomonon. Thus, this account on chromism from various materials is expected to attract the attention of researchers.

Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System.

Bimetallic nitrogen (N2) splitting to form metal nitrides is an attractive method for N2 fixation. Although a growing number of pincer-supported systems can bind and split N2, the precise relationship between the ligand properties and N2 binding/splitting remains elusive. Here we report the first example of an N2-bridged rhenium(III) complex, [(trans-P2tBuPyrr)ReCl2]2(μ-η1:η1-N2) (P2tBuPyrr = [2,5-(CH2PtBu2)2C4H2N]-). In this case, N2 binding occurs at a higher oxidation level than that in other reported pincer analogues. Analysis of the electronic structure through computational studies shows that the weakly π-donor pincer ligand stabilizes an open-shell electronic configuration that leads to enhanced binding of N2 in the bridged complex. Utilizing SQUID magnetometry, we demonstrate a singlet ground state for this Re-N-N-Re complex, and we offer tentative explanations for antiferromagnetic coupling of the two local S = 1 sites. Reduction and subsequent heating of the rhenium(III)-dinitrogen complex leads to chloride loss and cleavage of the N-N bond with isolation of the terminal rhenium(V) nitride complex (P2tBuPyrr)ReNCl.

Critical evaluation of a crystal structure of nitrogenase with bound N2 ligands

Recently, a 1.83 Å crystallographic structure of nitrogenase was suggested to show N2-derived ligands at three sites in the catalytic FeMo cluster, replacing the three mu_{2} bridging sulfide ligands (two in one subunit and the third in the other subunit) (Kang et al. in Science 368: 1381–1385, 2020). Naturally, such a structure is sensational, having strong bearings on the reaction mechanism of the enzyme. Therefore, it is highly important to ensure that the interpretation of the structure is correct. Here, we use standard crystallographic refinement and quantum refinement to evaluate the structure. We show that the original crystallographic raw data are strongly anisotropic, with a much lower resolution in certain directions than others. This, together with the questionable use of anisotropic B factors, give atoms an elongated shape, which may look like diatomic atoms. In terms of standard electron-density maps and real-space Z scores, a resting-state structure with no dissociated sulfide ligands fits the raw data better than the interpretation suggested by the crystallographers. The anomalous electron density at 7100 eV is weaker for the putative N2 ligands, but not lower than for several of the {mu }_{3} bridging sulfide ions and not lower than what can be expected from a statistical analysis of the densities. Therefore, we find no convincing evidence for any N2 binding to the FeMo cluster. Instead, a standard resting state without any dissociated ligands seems to be the most likely interpretation of the structure. Likewise, we find no support that the homocitrate ligand should show monodentate binding.Graphic abstract

Structures and reaction dynamics of N2 and H2 binding at FeMo-co, the active site of nitrogenase.

The chemical reactions occurring at the Fe7MoS9C(homocitrate) cluster, FeMo-co, the active site of the enzyme nitrogenase (N2 → NH3), are enigmatic. Experimental information collected over a long period reveals aspects of the roles of N2 and H2, each with more than one type of reactivity. This paper reports investigations of the binding of H2 and N2 at intact FeMo-co, using density functional simulations of a large 486 atom relevant portion of the protein, resulting in 27 new structures containing H2 and/or N2 bound at the exo and endo coordination sites of the participating Fe atoms, Fe2 and Fe6. Binding energies and transition states for association/dissociation are determined, and trajectories for the approach, binding and separation of H2/N2 are described, including diffusion of these small molecules through proximal protein. Influences of surrounding amino acids are identified. FeMo-co deforms geometrically when binding H2 or N2, and a procedure for calculating the energy cost involved, the adaptation energy, is introduced here. Adaptation energies, which range from 7 to 36 kcal mol-1 for the reported structures, are influenced by the protonation state of the His195 side chain. Seven N2 structures and three H2 structures have negative binding free energies, which include the estimated entropy penalties for binding of N2, H2 from proximal protein. These favoured structures have N2 bound end-on at exo-Fe2, exo-Fe6 and endo-Fe2 positions of FeMo-co, and H2 bound at the endo-Fe2 position. Various postulated structures with N2 bridging Fe2 and Fe6 revert to end-on-N2 at endo positions. The structures are also assessed via the calculated potential energy barriers for association and dissociation. Barriers to the binding of H2 range from 1 to 20 kcal mol-1 and barriers to dissociation of H2 range from 3 to 18 kcal mol-1. Barriers to the binding of N2, in either side-on or end-on mode, range from 2 to 18 kcal mol-1, while dissociation of bound N2 encounters barriers of 3 to 8 kcal mol-1 for side-on bonding and 7 to 18 kcal mol-1 for end-on bonding. These results allow formulation of mechanisms for the H2/N2 exchange reaction, and three feasible mechanisms for associative exchange and three for dissociative exchange are identified. Consistent electronic structures and potential energy surfaces are maintained throughout. Changes in the spin populations of Fe2 and Fe6 connected with cluster deformation and with metal-ligand bond formation are identified.

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters.

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.

A Mononuclear and High-Spin Tetrahedral TiII Complex.

A high-spin, mononuclear TiII complex, [(TptBu,Me)TiCl] [TptBu,Me– = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(TptBu,Me)TiCl2] with KC8. Complex [(TptBu,Me)TiCl] has a 3A2 ground state (assuming C3v symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined TiII complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(TptBu,Me)TiCl(THF)], which is impervious to N2 binding. However, in the absence of THF, the TiII complex captures N2 to produce the diamagnetic complex [(TptBu,Me)TiCl]2(η1,η1;μ2-N2), with a linear Ti=N=N=Ti topology, established by single-crystal X-ray diffraction. The N2 complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two TiIII centers covalently bridged by an N22– unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(TptBu,Me)TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate TiII complex, namely, [(TptBu,Me)TiCl(CNAd)]. The reducing power of the coordinatively unsaturated TiII-containing [(ΤptBu,Me)TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear TiIV complexes of the type [(TptBu,Me)Ti=E(Cl)] (with E2– = NSiMe3, N2CPh2, O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N3SiMe3, N2CPh2, N2O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

Defect and Interface Engineering on Two‐Dimensional Nanosheets for the Photocatalytic Nitrogen Reduction Reaction

AbstractReducing dinitrogen (N2) to ammonia (NH3) under mild conditions is a very significant nitrogen cyclic process, which plays a vital role in agricultural, biological and industrial fields. The Haber‐Bosch process as a current mainstream way of N2 fixation has particularly high energy consumption, occupying about 1 % of the world energy production. In contrast, the photocatalytic N2 reduction reaction provides a green route almost without energy consumption and environmental pollution. However, there are many challenges that needs to be solved urgently, such as, for example, low quantum yield, inefficient N2 adsorption and activation and an unclear N2 fixation mechanism. Nowadays, tactics for improving the catalytic performance mainly focus on producing more active sites via defect or interface engineering. Generally, two‐dimensional materials with defect or interface engineering can not only accelerate photon‐exciton interactions, but also enhance sufficient N2 binding, activation and hydrogenation. In this Minireview, we will first summarize the principles of photocatalytic N2 fixation, and then discuss progress in the development two‐dimensional (2D) materials with defect and interface engineering for photocatalytic N2 fixation. Finally, we describe ammonia detection methods and recent important developments and challenges in this field.

CO as a substrate and inhibitor of H+ reduction for the Mo-, V-, and Fe-nitrogenase isozymes

Three known nitrogenase isozymes, Mo-, V-, and Fe-, catalyze biological reduction of dinitrogen (N2) to ammonia (NH3). All three utilize the same reductive elimination mechanism: an intermediate with two metal-bound hydrides reductively-eliminates hydrogen gas (H2) in a reaction coupled to binding and activation of N2. Nonetheless, the three isozymes show dramatically different relative rates of H2 formation and N2 reduction, revealing important differences in reactivity with substrates. Carbon monoxide (CO) has been characterized as both an inhibitor and substrate for Mo- and V‑nitrogenases, but not for the Fe‑nitrogenase. Here, we present a comparative study of the reactivity of the three isozymes with CO, examining CO both as a substrate and as an inhibitor of proton (H+) reduction under steady-state conditions. For Mo‑nitrogenase, there is neither detectable reduction of CO nor inhibition of H+ reduction. Fe- and V‑nitrogenase show CO reduction and inhibition of H+ reduction that depends on the CO partial pressure. For V‑nitrogenase, ethylene (C2H4) is the major reduction product with a maximum specific activity of ~7.5 nmol C2H4/nmol VFe protein/min at 1 atm CO. The major product of CO reduction for Fe‑nitrogenase is methane (CH4) with a maximum specific activity of ~4.8 nmol CH4/nmol FeFe protein/min at 0.05 atm CO. The rate of CH4 production by Fe‑nitrogenase progressively increases to a maximum at 0.05 atm CO and then declines by ~90% with increasing CO partial pressure up to 1 atm. CO does not inhibit proton reduction in Mo‑nitrogenase but shows 16% inhibition for V‑nitrogenase and 35% for Fe‑nitrogenase.

Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase.

Binding of N2 by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e- and 4 H+ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P6ArC)Fe2(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e- transfer which bear some resemblance to a pair of En and En+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe2(μ-H)}2 state of FeMoco (E4(4H)), the data and analysis presented herein suggest that the hydride ligands in E4(4H) bridge isovalent (most probably FeIII) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P6ArC)Fe2(μ-H)]+1 and [(P6ArC)Fe2(μ-H)]-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e- reduction), a conclusion further supported by X-ray absorption spectroscopy, 57Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

One-pot, room-temperature conversion of dinitrogen to ammonium chloride at a main-group element.

The industrial reduction of dinitrogen (N2) to ammonia is an energy-intensive process that consumes a considerable proportion of the global energy supply. As a consequence, species that can bind N2 and cleave its strong N-N bond under mild conditions have been sought for decades. Until recently, the only species known to support N2 fixation and functionalization were based on a handful of metals of the s and d blocks of the periodic table. Here we present one-pot binding, cleavage and reduction of N2 to ammonium by a main-group species. The reaction-a complex multiple reduction-protonation sequence-proceeds at room temperature in a single synthetic step through the use of solid-phase reductant and acid reagents. A simple acid quench of the mixture then provides ammonium, the protonated form of ammonia present in fertilizer. The elementary reaction steps in the process are elucidated, including the crucial N-N bond cleavage process, and all of the intermediates of the reaction are isolated.

Considering Electrocatalytic Ammonia Synthesis via Bimetallic Dinitrogen Cleavage

Despite advances in the development of molecular catalysts capable of reducing dinitrogen to ammonia using proton donors and chemical reductants, few molecular electrocatalysts have been discovered. This Perspective considers the prospects of electrocatalyst development based on a mechanism featuring the cleavage of N2 into metal nitride complexes. By understanding the factors that control the reactivity of individual steps along the electrochemical N2 cleavage path, opportunities for new advances are identified. Ligand design principles for facile electrochemical N2 binding, formation of bridging N2 complexes, thermal or photochemical N2 cleavage, and conversion of a nitride ligand into ammonia are described, featuring recent advances and the authors’ collaborative work on rhenium complexes.