Nitrogen fixation in solution

Tunable N2 Fixation Enabled by Ferroelectric Switching in Doped Graphene/In2Se3 Dual-Atom Catalysts

The electrochemical nitrogen reduction reaction (NRR) provides a sustainable alternative to ammonia synthesis. However, the development of catalysts with high activity and selectivity under ambient conditions remains a significant challenge. In this work, we propose a class of dual-atom catalysts (DACs), consisting of two metal atoms embedded in nitrogen-doped porous graphene (M2NPG) supported on a ferroelectric α-In2Se3 monolayer. Using density functional theory (DFT) calculations, we explore the effect of ferroelectric polarization switching on the structural stability, catalytic performance, and reaction mechanisms of these DACs. By computationally screening 27 metal atoms as active sites, we identify four promising candidates (V, Co, Ru, and Ta) with V2NPG@In2Se3 standing out due to its exceptional properties. The precise control of NRR pathways, along with tunable limiting potentials and selective product formation, can be achieved through the polarization switching of the α-In2Se3 monolayer. The combination of low limiting potential, abundant active sites, tunable catalytic behavior, and high selectivity against the hydrogen evolution reaction (HER) highlights the potential of V2NPG@In2Se3 as a promising alternative to traditional single-atom catalysts. This work demonstrates a versatile strategy for integrating DACs with ferroelectric materials, offering valuable insights into designing next-generation catalysts for NRR and beyond.

Dinitrogen Activation and Conversion by Actinide Complexes

The efficient activation and conversion of dinitrogen (N2) represent a significant challenge in sustainable chemistry, offering potential pathways for synthesizing valuable nitrogen-containing compounds while reducing the environmental impact of traditional nitrogen fixation processes. While transition metal catalysts have been extensively studied for this purpose, actinide complexes have been less explored but have recently emerged as promising candidates due to their unique electronic properties and reactivity. This Perspective systematically examines the recent advances in N2 activation and conversion mediated by actinide complexes, with a particular focus on their synthesis, mechanistic insights, and catalytic capabilities.

Beyond the triple bond: unlocking dinitrogen activation with tailored superbase phosphines

Activating atmospheric dinitrogen (N2), a molecule with a remarkably strong triple bond, remains a major challenge in chemistry. This theoretical study explores the potential of superbase phosphines, specifically those decorated with imidazolin-2-imine ((ImN)3P) and imidazolin-2-methylidene ((ImCH)3P) to facilitate N2 activation and subsequent hydrazine (H2NNH2) formation. Using density functional theory (DFT) at the M06L/6-311++G(d,p) level, we investigated the interactions between these phosphines and N2. Mono-phosphine-N2 complexes exhibit weak, noncovalent interactions (-0.6 to -7.1 kcal mol-1). Notably, two superbasic phosphines also form high-energy hypervalent complexes with N2, albeit at significantly higher energies. The superbasic nature and potential for the hypervalency of these phosphines lead to substantial N2 activation in bis-phosphine-N2 complexes, where N2 is "sandwiched" between two phosphine moieties through hypervalent P-N bonds. Among the phosphines studied, only (ImN)3P forms an exothermic sandwich complex with N2, stabilized by hydrogen bonding between the ImN substituents and the central N2 molecule. A two-step, exothermic hydrogen transfer pathway from (ImN)3P to N2 results in the formation of a bis-phosphine-diimine (HNNH) sandwich complex. Subsequent hydrogen transfer leads to the formation of a bis-phosphine-hydrazine (H2NNH2) complex, a process that, although endothermic, exhibits surmountable activation barriers. The relatively low energy requirements for this overall transformation suggest its potential feasibility under the optimized conditions. This theoretical exploration highlights the promise of superbase phosphines as a strategy for metal-free N2 activation, opening doors for the development of more efficient and sustainable nitrogen fixation and utilization methods.

The activating capture of N2 at the active site of Mo-nitrogenase

Dinitrogen is inherently inert. This report describes detailed density functional calculations (with a 485+ atom model) of mechanistic steps by which the enzyme nitrogenase activates unreactive N2 at the intact active site FeMo-co, to form a key intermediate with bound HNNH. This mechanism does not bind N2 first and then add H atoms, but rather captures N2 ('N2-ready') that diffuses in through the substrate channel and enters a strategic gallery of H atom donors in the reaction zone, between Fe2 and Fe6. This occurs at the E4 stage of the complete mechanism. Exploration of possible reactions of N2 in this space leads to the conclusion that the first reaction step is transfer of H on Fe7 to one end of N2-ready, soon followed by Fe-N bond formation, and then a second H transfer from bridging S2BH to the other N. Two H-N bonds and one or two N-Fe bonds are formed, in some cases with a single transition state. The variable positions and orientations of N2-ready lead to various reaction trajectories and products. The favourable products resulting from this capture, judged by the criteria of reaction energies, reaction barriers, and mechanistic competence for further hydrogenation reactions in the nitrogenase cycle, have Fe2-NH-NH bonding. The trajectory of one N2 capture reaction is described in detail, and calculations that separate the H atom component and the 'heavy atom' components of the classical activation energy are described, in the context of possible H atom tunneling in the activation of N2-ready. I present arguments for the activation of N2 by the pathway of concerted hydrogenation and binding of N2-ready, alternative to the commonly assumed pathway of binding N2 first, with subsequent hydrogenation. The active site of nitrogenase is well primed for the thermodynamic and kinetic advantages of N2 capture.

Computational design of cooperatively acting molecular catalyst systems: carbene based tungsten- or molybdenum-catalysts with rhodium- or iridium-complexes for the ionic hydrogenation of N2 to NH3

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol-1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol-1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected.

Manipulating Reactivity of Ir(CH2)0-2+ Cations toward Dinitrogen at Room Temperature: A Unique Dependence on the Organic Ligand Structures

Nitrogen (N2) activation at room temperature has long been a great challenge. Therefore, the rational design of reactive species to adsorb N2, which is a prerequisite for cleavage of the strong N≡N triple bond in industrial and biological processes, is highly desirable and meaningful. Herein, the N2 adsorption process is controlled by regulating the types and numbers of organic ligands, and the organic ligands are produced through the reactions of Ir+ with methane and ethane. CH4 molecules dissociate on the Ir+ cations to form Ir(CH2)1,2+. The reaction of Ir+ with C2H6 can generate HIrC2H3+, which is different from the structure of Ir(CH2)2+ obtained from Ir+/CH4. The reactivity order of N2 adsorption is Ir(CH2)2+ > HIrC2H3+ ≫ HIrCH+ ≈ Ir+ (almost inert under similar reaction conditions), indicating that different organic ligand structures affect reactivity dramatically. The main reason for this interesting reactivity difference is that the lowest unoccupied molecular orbital (LUMO) level of Ir(CH2)2+ is much closer to the highest occupied molecular orbital (HOMO) level of N2 than those of the other three systems. This study provides new insights into the adsorption of N2 on metal-organic ligand species, in which the organic ligand dominates the reactivity, and it discovers new clues in designing effective transition metal carbine species for N2 activation.

Dinitrogen Activation and Functionalization Affording Chromium Diazenido and Hydrazido Complexes

ConspectusThe activation and functionalization of N2 to form nitrogen-element bonds have long posed challenges to industrial, biological, and synthetic chemists. The first transition-metal dinitrogen complex prepared by Allen and Senoff in 1965 provoked researchers to explore homogeneous N2 fixation. Despite intensive research in the last six decades, efficient and quantitative conversion of N2 to diazenido and hydrazido species remains problematic. Relative to a plethora of reactions to generate N2 complexes, their functionalization reactions are rather rare, and the yields are often unsatisfactory, emphasizing the need for systematic investigations of the reaction mechanisms.In this Account, we summarize our recent work on the synthesis, spectroscopic features, electronic structures, and reactivities of several Cr-N2 complexes. Initially, a series of dinuclear and trinuclear Cr(I)-N2 complexes bearing cyclopentadienyl-phosphine ligands were accessed. However, they cannot achieve N2 functionalization but undergo oxidative addition reactions with phenylsilane, azobenzene, and other unsaturated organic compounds at the low-valent Cr(I) centers rather than at the N2 unit. Further reduction of these Cr(I) complexes leads to the formation of more activated mononuclear Cr(0) bis-dinitrogen complexes. Remarkably, silylation of the cyclopentadienyl-phosphine Cr(0)-N2 complex with Me3SiCl afforded the first Cr hydrazido complex. This process follows the distal pathway to functionalize the Nβ atom twice, yielding an end-on η1-hydrazido complex, Cr(III)═N-N(SiMe3)2. In contrast, upon substitution of the phosphine ligand in the Cr(0)-N2 complex with a N-heterocyclic carbene (NHC) ligand, the corresponding reaction with Me3SiCl proceeds via the alternating pathway; the silylation occurs at both Nα and Nβ atoms and generates a side-on η2-hydrazido complex, Cr(III)(η2-Me3SiN-NSiMe3). Both silylation reactions are inevitably accompanied by the formation of Cr(III) hydrazido complexes and Cr(II) chlorides with a 2:1 ratio. These processes exhibit a peculiar '3-4-2-1' stoichiometry (i.e., treating 3 equiv of Cr(0)-N2 complexes with 4 equiv of Me3SiCl yields 2 equiv of Cr(III) disilyl-hydrazido complexes and 1 equiv of Cr(II) chloride). Upon replacing the monodentate phosphine and/or NHC ligand with a bisphosphine ligand, a monodinitrogen Cr(0) complex, instead of the bis-dinitrogen Cr(0) complexes, is obtained; consequently, the silylation reactions progress via the normal two-electron route, which passes through Cr(II)-N═N-R diazenido species as an intermediate and furnishes [Cr(IV)═N-NR2]+ hydrazido as the final products. More importantly, this type of Cr(0)-N2 complex can be not only silylated but also protonated and alkylated proficiently. All of the second-order reaction rates of the first and second transformations are determined along with the lifetimes of the intervening diazenido species. Based on these findings, we have successfully carried out nearly quantitative preparations of the Cr(IV) hydrazido species with unmixed or hybrid substituents.The studies of Cr-N2 systems provide effective approaches for the activation and functionalization of N2, deepening the understanding of N2 electrophilic attack. We hope that this Account will inspire more discoveries related to the transformation of gaseous N2 to high-value-added nitrogen-containing organic compounds.

Highly Selective Fe-Catalyzed Nitrogen Fixation to Hydrazine Enabled by Sm(II) Reagents with Tailored Redox Potential and pKa

Controlling product selectivity in multiproton, multielectron reductions of unsaturated small molecules is of fundamental interest in catalysis. For the N2 reduction reaction (N2RR) in particular, parameters that dictate selectivity for either the 6H+/6e- product ammonia (NH3) or the 4H+/4e- product hydrazine (N2H4) are poorly understood. To probe this issue, we have developed conditions to invert the selectivity of a tris(phosphino)borane iron catalyst (Fe), with which NH3 is typically the major product of N2R, to instead favor N2H4 as the sole observed fixed-N product (>99:1). This dramatic shift is achieved by replacing moderate reductants and strong acids with a very strongly reducing but weakly acidic SmII-(2-pyrrolidone) core supported by a hexadentate dianionic macrocyclic ligand (SmII-PH) as the net hydrogen-atom donor. The activity and efficiency of the catalyst with this reagent remain high (up to 69 equiv of N2H4 per Fe and 67% fixed-N yield per H+). However, by generating N2H4 as the kinetic product, the overpotential of this Sm-driven reaction is 700 mV lower than that of the mildest reported set of NH3-selective conditions with Fe. Mechanistic data support assignment of iron hydrazido(2-) species FeNNH2 as selectivity-determining: we infer that protonation of FeNNH2 at Nβ, favored by strong acids, releases NH3, whereas one-electron reduction to FeNNH2-, favored by strong reductants such as SmII-PH, produces N2H4 via reactivity initiated at Nα. Spectroscopic data also implicate a role for SmIII-binding to anionic FeN2- (via an Fe-N2- -SmIII species) with respect to catalytic efficacy.

Advancing MXene Electrocatalysts for Energy Conversion Reactions: Surface, Stoichiometry, and Stability

MXenes, due to their tailorable chemistry and favourable physical properties, have great promise in electrocatalytic energy conversion reactions. To exploit fully their enormous potential, further advances specific to electrocatalysis revolving around their performance, stability, compositional discovery and synthesis are required. The most recent advances in these aspects are discussed in detail: surface functional and stoichiometric modifications which can improve performance, Pourbaix stability related to their electrocatalytic operating conditions, density functional theory and advances in machine learning for their discovery, and prospects in large scale synthesis and solution processing techniques to produce membrane electrode assemblies and integrated electrodes. This Review provides a perspective that is complemented by new density functional theory calculations which show how these recent advances in MXene material design are paving the way for effective electrocatalysts required for the transition to integrated renewable energy systems.

Recent progress of photocatalysts based on tungsten and related metals for nitrogen reduction to ammonia

Photocatalytic nitrogen reduction reaction (NRR) to ammonia holds a great promise for substituting the traditional energy-intensive Haber–Bosch process, which entails sunlight as an inexhaustible resource and water as a hydrogen source under mild conditions. Remarkable progress has been achieved regarding the activation and solar conversion of N₂ to NH₃ with the rapid development of emerging photocatalysts, but it still suffers from low efficiency. A comprehensive review on photocatalysts covering tungsten and related metals as well as their broad ranges of alloys and compounds is lacking. This article aims to summarize recent advances in this regard, focusing on the strategies to enhance the photocatalytic performance of tungsten and related metal semiconductors for the NRR. The fundamentals of solar-to-NH₃ photocatalysis, reaction pathways, and NH₃ quantification methods are presented, and the concomitant challenges are also revealed. Finally, we cast insights into the future development of sustainable NH₃ production, and highlight some potential directions for further research in this vibrant field.

Promotion of Nitrogen Fixation of Diverse Heterotrophs by Solid-Phase Humin

Although biological nitrogen fixation (BNF) proceeds under mild conditions compared to the energy-intensive Haber–Bosch process, the slow kinetics of BNF necessitate the promotion of BNF activity in its practical application. The BNF promotion using purified nitrogenases and using genetically modified microorganisms has been studied, but these enzymes are unstable and expensive; moreover, designing genetically modified microorganisms is also a difficult task. Alternatively, the BNF promotion in non-modified (wild-type) microorganisms (enriched consortia) with humin has been shown, which is a humic substance insoluble at any pH and functions as an extracellular electron mediator. However, the taxonomic distribution of the diazotrophs promoted by humin, the levels of BNF promotion, and the underlying mechanism in BNF promotion with humin remain unknown. In this study, we show that taxonomically diverse heterotrophic diazotrophs, harboring nifH clusters I, II, and III, promoted their BNF by accepting extracellular electrons from humin, based on the characterization of the individual responses of isolated diazotrophs to humin. The reduced humin increased the acetylene reduction activity of the diazotrophs by 194–916% compared to the level achieved by the organic carbon source, causing adenosine triphosphate (ATP) synthesis in the diazotroph cells without increase in the CO₂ production and direct electron donation to the MoFe protein of the nitrogenase in the cells without relying on the biological electron transfer system. These would result in BNF promotion in the wild-type diazotroph cells beyond their biochemical capacity. This significant promotion of BNF with humin would serve as a potential basis for sustainable technology for greener nitrogen fixation.

Reaction of Ta3 - Clusters with Molecular Nitrogen: A Mechanism Investigation

Because of the inertness of molecular nitrogen, its practicable activation under mild conditions is a fundamental challenge. Ta3 - is an exceptionally small cluster that reacts with N2 at room temperature, leading finally to Ta3N2 -; Ta3N2 - also could react with N2 at room temperature, leading finally to Ta3N4 -, a product of interest in its own right because of its potential as a nitrogen fixation medium. The mechanisms of the Ta3 -- and Ta3N2 --mediated activation of the N≡N triple bond have been investigated. Our extensive computations elucidate mechanisms for the ready reactions, leading to stepwise cleavage of the N≡N bond. Initial isomeric N2/Ta3 - complexes, N≡N elongation, undergo a N≡N split over generally low barriers in a highly exothermic process. The nitrogen-atom or molecular exchange reactions found in the Ta3N2 -/N2 system may be of paramount importance in both applied and fundamental studies.

Recent Advances in Application of Graphitic Carbon Nitride-Based Catalysts for Photocatalytic Nitrogen Fixation

Ammonia, the second most-produced chemical, is widely used in agricultural and industrial applications. However, traditional industrial ammonia production dominated by the Haber-Bosch process presents huge resource and environment issues due to the massive energy consumption and CO₂ emission. The newly emerged nitrogen fixation technology, photocatalytic N₂ reduction reaction (p-NRR), uses clean solar energy with zero-emission, holding great prospect to achieve sustainable ammonia synthesis. Although great efforts are made, the p-NRR catalysts still suffer from poor N₂ adsorption and activation, inferior light absorption, and fast recombination of photocarriers. Due to the tunable electronic structure of the metal-free polymeric graphitic carbon nitride (g-C₃ N₄ ), the above-mentioned issues can be significantly alleviated, making it the most promising p-NRR photocatalyst. This review summarizes the recent development of g-C₃ N₄ -based catalysts for p-NRR, including the working principle of p-NRR catalysts, the challenges of developing p-NRR catalysts, and corresponding solutions. Particularly, the roles of defect engineering and heterojunction construction on g-C₃ N₄ to the enhancement of photocatalytic performances are emphasized. In addition, computational studies are introduced to deepen the understanding of reaction pathways. At last, perspectives are provided on the development of p-NRR catalysts.

EDA-NOCV analysis of carbene-borylene bonded dinitrogen complexes for deeper bonding insight: A fair comparison with a metal-dinitrogen system

Binding of dinitrogen (N₂ ) to a transition metal center (M) and followed by its activation under milder conditions is no longer impossible; rather, it is routinely studied in laboratories by transition metal complexes. In contrast, binding of N₂ by main group elements has been a challenge for decades, until very recently, an exotic cAAC-borylene (cAAC = cyclic alkyl(amino) carbene) species showed similar binding affinity to kinetically inert and non-polar dinitrogen (N₂ ) gas under ambient conditions. Since then, N₂ binding by short lived borylene species has made a captivating news in different journals for its unusual features and future prospects. Herein, we carried out different types of DFT calculations, including EDA-NOCV analysis of the relevant cAAC-boron-dinitrogen complexes and their precursors, to shed light on the deeper insight of the bonding secret (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence). The hidden bonding aspects have been uncovered and are presented in details. Additionally, similar calculations have been carried out in comparison with a selected stable dinitrogen bridged-diiron(I) complex. Singlet cAAC ligand is known to be an exotic stable species which, combined with the BAr group, produces an intermediate singlet electron-deficient (cAAC)(BAr) species possessing a high lying HOMO suitable for overlapping with the high lying π*-orbital of N₂ via effective π-backdonation. The BN₂ interaction energy has been compared with that of the FeN₂ bond. Our thorough bonding analysis might answer the unasked questions of experimental chemists about how boron compounds could mimic the transition metal of dinitrogen binding and activation, uncovering hidden bonding aspects. Importantly, Pauling repulsion energy also plays a crucial role and decides the binding efficiency in terms of intrinsic interaction energy between the boron-center and the N₂ ligand.

Detection of Aqueous Solvated Electrons Produced by Photoemission from Solids Using Transient Absorption Measurements

Solvated electrons in water have long been of interest to chemists. While readily produced using intense multiphoton excitation of water and/or irradiation with high-energy particles, the possible role of solvated electrons in electrochemical and photoelectrochemical reactions at electrodes has been controversial. Recent studies showed that excitation of electrons to the conduction band of diamond leads to barrier-free emission of electrons into water. While these electrons can be inferred from the reactions they induce, direct detection by transient absorption measurements provides more direct evidence. Here, we present studies demonstrating direct detection of solvated electrons produced at diamond electrode surfaces and the influence of electrochemical potential and solution-phase electron scavengers. We further present a more detailed analysis of experimental conditions needed to detect solvated electrons emitted from diamond and other solid materials through transient optical absorption measurements.

Bonding and stability of dinitrogen-bonded donor base-stabilized Si(0)/Ge(0) species [(cAACMe-Si/Ge)2(N2)]: EDA-NOCV analysis

Recently, dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAAC-borylene as (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino) carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = borylene]. It has attracted a lot of scientific attention from different research areas because of its future prospects as a potent species towards the metal free reduction of N₂ into ammonia (NH₃) under mild conditions. Two (cAAC)(B-Dur) units, each of which possesses six valence electrons around the B-centre, are shown to accept σ-donations from the N₂ ligand (B ← N₂). Two B-Dur further provide π-backdonations (B → N₂) to a central N₂ ligand to strengthen the B–N₂–B bond, providing maximum stability to the compound (cAAC)₂(B-Dur)₂(N₂) since the summation of each pair wise interaction accounted for the total stabilization energy of the molecule. (cAAC)(B-Dur) unit is isolobal to cAAC–E (E = Si, Ge) fragment. Herein, we report on the stability and bonding of cAAC–E bonded N₂-complex (cAAC–E)₂(N₂) (1–2; Si, Ge) by NBO, QTAIM and EDA-NOCV analyses (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; QTAIM = quantum theory of atoms in molecule). Our calculation suggested that syntheses of elusive (cAAC–E)₂(N₂) (1–2; Si, Ge) species may be possible with cAAC ligands having bulky substitutions adjacent to the C_cAAC atom by preventing the homo-dimerization of two (cAAC)(E) units which can lead to the formation of (cAAC–E)₂. The formation of E Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> E bond is thermodynamically more favourable (E = Si, Ge) over binding energy of N₂ inbetween two cAAC–E units.

Dinitrogen (N₂) binding and its activation have been achieved by non-metal compounds like intermediate cAACborylene with the general formula of (cAAC)₂(B-Dur)₂(N₂) [cAAC = cyclic alkyl(amino)carbene; Dur = aryl group, 2,3,5,6-tetramethylphenyl; B-Dur = aryl-borylene].

Synthesis, Crystal Structure, EPR, and DFT Studies of an Unusually Distorted Vanadium(II) Complex

We report a synthesis and structure of the most highly distorted four-coordinate d3 ion known to date that also serves as the second known example of a bis(biphenolato) transition metal...

Estimations of Fe0/−1–N2 interaction energies of iron(0)-dicarbene and its reduced analogue by EDA-NOCV analyses: crucial steps in dinitrogen activation under mild conditions

Metal complexes containing low valence iron atoms are often experimentally observed to bind with the dinitrogen (N₂) molecule. This phenomenon has attracted the attention of industrialists, chemists and bio-chemists since these N₂-bonded iron complexes can produce ammonia under suitable chemical or electrochemical conditions. The higher binding affinity of the Fe-atom towards N₂ is a bit ‘mysterious’ compared to that of the other first row transition metal atoms. Fine powders of α-Fe⁰ are even part of industrial ammonia production (Haber–Bosch process) which operates at high temperature and high pressure. Herein, we report the EDA-NOCV analyses of the previously reported dinitrogen-bonded neutral molecular complex (cAAC^R)₂Fe⁰–N₂ (1) and mono-anionic complex (cAAC^R)₂Fe⁻¹–N₂ (2) to give deeper insight of the Fe–N₂ interacting orbitals and corresponding pairwise intrinsic interaction energies (cAAC^R = cyclic alkyl(amino) carbene; R = Dipp or Me). The Fe⁰ atom of 1 prefers to accept electron densities from N₂via σ-donation while the comparatively electron rich Fe⁻¹ centre of 2 donates electron densities to N₂via π-backdonation. However, major stability due to the formation of an Fe–N₂ bond arises due to Fe → N₂ π-backdonation in both 1 and 2. The cAAC^R ligands act as a charge reservoir around the Fe centre. The electron densities drift away from cAAC ligands during the binding of N₂ molecules mostly via π-backdonation. EDA-NOCV analysis suggests that N₂ is a stronger π-acceptor rather than a σ-donor.

The stable Fe–N₂ bond of stable complex should have a sufficiently high interaction energy.

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.

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 MoFe₇S₉C^1- 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 (MoFe₇S₉C^1-). Herein, we report the EDA-NOCV analyses and NICS calculations of the dinitrogen-bonded dianionic complex Fe⁰-N₂ (1) (having a C_Ar ← Fe π-bond) and monoanionic complex Fe^I-N₂ (2) (having a C_Ar-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). ΔE _int 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.

Semiconductor photocatalysts and mechanisms of carbon dioxide reduction and nitrogen fixation under UV and visible light

The review summarizes the current knowledge about heterogeneous semiconductor photocatalysts that are active towards photocatalytic reduction of carbon dioxide and molecular nitrogen under visible and near-UV light. The main classes of these photocatalysts and characteristic features of their application in the target processes are considered. Primary attention is given to photocatalysts based on titanium dioxide, which have high activity and stability in the carbon dioxide reduction. For the first time, the photofixation of nitrogen under irradiation in the presence of various semiconductor materials is considered in detail.

The bibliography includes 264 references.

Emerging two-dimensional nanomaterials for electrochemical nitrogen reduction

Ammonia (NH₃) is essential to serve as the biological building blocks for maintaining organism function, and as the indispensable nitrogenous fertilizers for increasing the yield of nutritious crops. The current Haber-Bosch process for industrial NH₃ production is highly energy- and capital-intensive. In light of this, the electroreduction of nitrogen (N₂) into valuable NH₃, as an alternative, offers a sustainable pathway for the Haber-Bosch transition, because it utilizes renewable electricity and operates under ambient conditions. Identifying highly efficient electrocatalysts remains the priority in the electrochemical nitrogen reduction reaction (NRR), marking superior selectivity, activity, and stability. Two-dimensional (2D) nanomaterials with sufficient exposed active sites, high specific surface area, good conductivity, rich surface defects, and easily tunable electronic properties hold great promise for the adsorption and activation of nitrogen towards sustainable NRR. Therefore, this Review focuses on the fundamental principles and the key metrics being pursued in NRR. Based on the fundamental understanding, the recent efforts devoted to engineering protocols for constructing 2D electrocatalysts towards NRR are presented. Then, the state-of-the-art 2D electrocatalysts for N₂ reduction to NH₃ are summarized, aiming at providing a comprehensive overview of the structure-performance relationships of 2D electrocatalysts towards NRR. Finally, we propose the challenges and future outlook in this prospective area.

Recent Advances and Perspective on Electrochemical Ammonia Synthesis under Ambient Conditions

Ammonia is an essential chemical for agriculture and industry. To date, NH₃ is mainly supplied by the traditional Haber-Bosch process, which is operated under high-temperature and high-pressure in a centralized way. To achieve ammonia production in an environmentally benign way, electrochemical NH₃ synthesis under ambient conditions has become the frontier of energy and chemical conversion schemes, as it can be powered by renewable energy and operates in a decentralized way. The recent progress on developing different strategies for NH₃ production, including 1) classic NH₃ synthesis pathways over nanomaterials; 2) the Mars-van Krevelen (MvK) mechanism over metal nitrides (MN_x ); 3) reducing the nitrate into NH₃ over Cu-based nanomaterial; and 4) metal-N₂ battery release of NH₃ from Li_x M. Moreover, the most recent advances in engineering strategies for developing highly active materials and the design of the reaction systems for NH₃ synthesis are covered.

Comprehensive Understanding of the Thriving Ambient Electrochemical Nitrogen Reduction Reaction

The electrochemical method of combining N₂ and H₂ O to produce ammonia (i.e., the electrochemical nitrogen reduction reaction [E-NRR]) continues to draw attention as it is both environmentally friendly and well suited for a progressively distributed farm economy. Despite the multitude of recent works on the E-NRR, further progress in this field faces a bottleneck. On the one hand, despite the extensive exploration and trial-and-error evaluation of E-NRR catalysts, no study has stood out to become the stage protagonist. On the other hand, the current level of ammonia production (microgram-scale) is an almost insurmountable obstacle for its qualitative and quantitative determination, hindering the discrimination between true activity and contamination. Herein i) the popular theory and mechanism of the NRR are introduced; ii) a comprehensive summary of the recent progress in the field of the E-NRR and related catalysts is provided; iii) the operational procedures of the E-NRR are addressed, including the acquisition of key metrics, the challenges faced, and the most suitable solutions; iv) the guiding principles and standardized recommendations for the E-NRR are emphasized and future research directions and prospects are provided.

Non-Dissociative Activation of Chemisorbed Dinitrogen on One or Two Vanadium Atoms Supported by a Mo6 S8 Cluster

Adsorption of N₂ on Mo₆ S₈ ^q _V_x clusters (x=0, 1, 2; q=0, ±1) were systematically studied by density functional theory calculations with dispersion corrections. It was found that the N₂ can be chemisorbed and undergo non-dissociative activation on single or double metal atoms. The adsorption and activation are influenced by metal types (V or Mo), N₂ coordination modes and charge states of the clusters. Particularly, anionic Mo₆ S₈ ^- _V₂ clusters have remarkable ability to fix and activate N₂ . In Mo₆ S₈ ^- _V₂ , two V atoms prefer to adsorb on two adjacent S-Mo-S hollow sites, leading to the formation of a supported V…V unit. The N₂ is bridged side-on coordinated with these two V atoms with high adsorption energy and significant charge transfer. The bond order, bond length and vibration frequency of the adsorbed N₂ are close to those of a N-N single bond.

Ammonia Formation Catalyzed by a Dinitrogen-Bridged Dirhenium Complex Bearing PNP-Pincer Ligands under Mild Reaction Conditions*

A series of rhenium complexes bearing a pyridine-based PNP-type pincer ligand are synthesized from rhenium phosphine complexes as precursors. A dinitrogen-bridged dirhenium complex bearing the PNP-type pincer ligands catalytically converts dinitrogen into ammonia during the reaction with KC₈ as a reductant and [HPCy₃ ]BAr^F ₄ (Cy=cyclohexyl, Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) as a proton source at -78 °C to afford 8.4 equiv of ammonia based on the rhenium atom of the catalyst. The rhenium-dinitrogen complex also catalyzes silylation of dinitrogen in the reaction with KC₈ as a reductant and Me₃ SiCl as a silylating reagent under ambient reaction conditions to afford 11.7 equiv of tris(trimethylsilyl)amine based on the rhenium atom of the catalyst. These results demonstrate the first successful example of catalytic nitrogen fixation under mild reaction conditions using rhenium-dinitrogen complexes as catalysts.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Dinitrogen (N2 ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

Comparing Molecular Mechanisms in Solar NH3 Production and Relations with CO2 Reduction

Molecular mechanisms for N2 fixation (solar NH3) and CO2 conversion to C2+ products in enzymatic conversion (nitrogenase), electrocatalysis, metal complexes and plasma catalysis are analyzed and compared. It is evidenced that differently from what is present in thermal and plasma catalysis, the electrocatalytic path requires not only the direct coordination and hydrogenation of undissociated N2 molecules, but it is necessary to realize features present in the nitrogenase mechanism. There is the need for (i) a multi-electron and -proton simultaneous transfer, not as sequential steps, (ii) forming bridging metal hydride species, (iii) generating intermediates stabilized by bridging multiple metal atoms and (iv) the capability of the same sites to be effective both in N2 fixation and in COx reduction to C2+ products. Only iron oxide/hydroxide stabilized at defective sites of nanocarbons was found to have these features. This comparison of the molecular mechanisms in solar NH3 production and CO2 reduction is proposed to be a source of inspiration to develop the next generation electrocatalysts to address the challenging transition to future sustainable energy and chemistry beyond fossil fuels.

Exploration and Investigation of Periodic Elements for Electrocatalytic Nitrogen Reduction

High demand for green ecosystems has urged the human community to reconsider and revamp the traditional way of synthesis of several compounds. Ammonia (NH₃ ) is one such compound whose applications have been extended from fertilizers to explosives and is still being synthesized using the high energy inhaling Haber-Bosch process. Carbon free electrocatalytic nitrogen reduction reaction (NRR) is considered as a potential replacement for the Haber-Bosch method. However, it has few limitations such as low N₂ adsorption, selectivity (competitive HER reactions), low yield rate etc. Since it is at the early stage, tremendous efforts have been devoted in understanding the reaction mechanism and screening of the electrocatalysts and electrolytes. In this review, the electrocatalysts are classified based on the periodic table with heat maps of Faraday efficiency and yield rate of NH₃ in NRR and their electrocatalytic properties toward NRR are discussed. Also, the activity of each element is discussed and short tables and concise graphs are provided to enable the researchers to understand recent progress on each element. At the end, a perspective is provided on countering the current challenges in NRR. This review may act as handbook for basic NRR understandings, recent progress in NRR, and the design and development of advanced electrocatalysts and systems.

Visible-Light Activation of Photocatalytic for Reduction of Nitrogen to Ammonia by Introducing Impurity Defect Levels into Nanocrystalline Diamond

Nitrogen impurity has been introduced in diamond film to produce a nitrogen vacancy center (NV center) toward the solvated electron-initiated reduction of N₂ to NH₃ in liquids, giving rise to extend the wavelength region beyond the diamond’s band. Scanning electron microscopy and X-ray diffraction demonstrate the formation of the nanocrystalline nitrogen-doped diamond with an average diameter of ten nanometers. Raman spectroscopy and PhotoLuminescence (PL) spectrum show characteristics of the NV⁰ and NV⁻ charge states. Measurements of photocatalytic activity using supraband (λ < 225 nm) gap and sub-band gap (λ > 225 nm) excitation show the nitrogen-doped diamond significantly enhanced the ability to reduce N₂ to NH₃ compared to the polycrystalline diamond and single crystal diamond (SCD). Our results suggest an important process of internal photoemission, in which electrons are excited from negative charge states into conduction band edges, presenting remarkable photoinitiated electrons under ultraviolet and visible light. Other factors, including transitions between defect levels and processes of reaction, are also discussed. This approach can be especially advantageous to such as N₂ and CO₂ that bind only weakly to most surfaces and high energy conditions.

Electrocatalytic Reduction of Nitrogen to Hydrazine Using a Trinuclear Nickel Complex

Activation and reduction of N₂ have been a major challenge to chemists and the focus since now has mostly been on the synthesis of NH₃. Alternatively, reduction of N₂ to hydrazine is desirable because hydrazine is an excellent energy vector that can release the stored energy very conveniently without the need for catalysts. To date, only one molecular catalyst has been reported to be able to reduce N₂ to hydrazine chemically. A trinuclear T-shaped nickel thiolate molecular complex has been designed to activate dinitrogen. The electrochemically generated all Ni(I) state of this molecule can reduce N₂ in the presence of PhOH as a proton donor. Hydrazine is detected as the only nitrogen-containing product of the reaction, along with gaseous H₂. The complex reported here is selective for the 4e^-/4H⁺ reduction of nitrogen to hydrazine with a minor overpotential of ∼300 mV.

2D Electrocatalysts for Converting Earth-Abundant Simple Molecules into Value-Added Commodity Chemicals: Recent Progress and Perspectives

The electrocatalytic conversion of earth-abundant simple molecules into value-added commodity chemicals can transform current chemical production regimes with enormous socioeconomic and environmental benefits. For these applications, 2D electrocatalysts have emerged as a new class of high-performance electrocatalyst with massive forward-looking potential. Recent advances in 2D electrocatalysts are reviewed for emerging applications that utilize naturally existing H₂ O, N₂ , O₂ , Cl^- (seawater) and CH₄ (natural gas) as reactants for nitrogen reduction (N₂ → NH₃ ), two-electron oxygen reduction (O₂ → H₂ O₂ ), chlorine evolution (Cl^- → Cl₂ ), and methane partial oxidation (CH₄ → CH₃ OH) reactions to generate NH₃ , H₂ O₂ , Cl₂ , and CH₃ OH. The unique 2D features and effective approaches that take advantage of such features to create high-performance 2D electrocatalysts are articulated with emphasis. To benefit the readers and expedite future progress, the challenges facing the future development of 2D electrocatalysts for each of the above reactions and the related perspectives are provided.

Catalytic N2-to-NH3 (or -N2H4) Conversion by Well-Defined Molecular Coordination Complexes

Nitrogen fixation, the six-electron/six-proton reduction of N₂, to give NH₃, is one of the most challenging and important chemical transformations. Notwithstanding the barriers associated with this reaction, significant progress has been made in developing molecular complexes that reduce N₂ into its bioavailable form, NH₃. This progress is driven by the dual aims of better understanding biological nitrogenases and improving upon industrial nitrogen fixation. In this review, we highlight both mechanistic understanding of nitrogen fixation that has been developed, as well as advances in yields, efficiencies, and rates that make molecular alternatives to nitrogen fixation increasingly appealing. We begin with a historical discussion of N₂ functionalization chemistry that traverses a timeline of events leading up to the discovery of the first bona fide molecular catalyst system and follow with a comprehensive overview of d-block compounds that have been targeted as catalysts up to and including 2019. We end with a summary of lessons learned from this significant research effort and last offer a discussion of key remaining challenges in the field.

Relating N-H Bond Strengths to the Overpotential for Catalytic Nitrogen Fixation

Nitrogen (N₂) fixation to produce bio-available ammonia (NH₃) is essential to all life but is a challenging transformation to catalyse owing to the chemical inertness of N₂. Transition metals can, however, bind N₂ and activate it for functionalization. Significant opportunities remain in developing robust and efficient transition metal catalysts for the N₂ reduction reaction (N₂RR). One opportunity to target in catalyst design concerns the stabilization of transition metal diazenido species (M-NNH) that result from the first N₂ functionalization step. Well-characterized M-NNH species remain very rare, likely a consequence of their low N-H bond dissociation free energies (BDFEs). In this essay, we discuss the relationship between the BDFE_N-H of a given M-NNH species to the observed overpotential and selectivity for N₂RR catalysis with that catalyst system. We note that developing strategies to either increase the N-H BDFEs of M-NNH species, or to avoid M-NNH intermediates altogether, are potential routes to improved N₂RR efficiency.