Nitrogen fixation and transformation with main group elements

Observing C-N bond formation in plasma: a case study of benzene and dinitrogen coupling via an arylnitrenium ion intermediate

Directly fixing dinitrogen into value-added organics is one of the core issues, and yet a long-standing challenge, in chemical synthesis. In earlier discrete studies, direct amination of benzene with N2 has been achieved via non-thermal plasma-liquid reaction. Nonetheless, the reaction mechanism thereof remains elusive and the amination product was non-selective primarily including aniline and diphenylamine. Herein, non-thermal plasma reaction in combination with on-line mass spectrometry was employed to probe the reaction pathway by on-line detection of the transient intermediate and the stable amination product. The long-lived atomic nitrogen ions N+(3P) as well as the arylnitrenium ions' intermediacy were shown to play a pivotal role in the amination process, and the product distribution was affected by an external hydrogen source and likely dependent on the competing hydrogen abstraction reaction and intersystem crossing of the initially generated triplet state arylnitrenium ions. The mechanistic investigation in this work has implications for plasma-based nitrogen conversion into organics, but also has broader relevance for understanding the C-N coupling by other means directly with N2.

Low oxidation state and hydrido group 2 complexes: synthesis and applications in the activation of gaseous substrates

Numerous industrial processes utilise gaseous chemical feedstocks to produce useful chemical products. Atmospheric and other small molecule gases, including anthropogenic waste products (e.g. carbon dioxide), can be viewed as sustainable building blocks to access value-added chemical commodities and materials. While transition metal complexes have been well documented in the reduction and transformation of these substrates, molecular complexes of the terrestrially abundant alkaline earth metals have also demonstrated promise with remarkable reactivity reported towards an array of industrially relevant gases over the past two decades. This review covers low oxidation state and hydrido group 2 complexes and their role in the reduction and transformation of a selection of important gaseous substrates towards value-added chemical products.

N2 cleavage by silylene and formation of H2Si(μ-N)2SiH2

Fixation and functionalisation of N2 by main-group elements has remained scarce. Herein, we report a fixation and cleavage of the N ≡ N triple bond achieved in a dinitrogen (N2) matrix by the reaction of hydrogen and laser-ablated silicon atoms. The four-membered heterocycle H2Si(μ-N)2SiH2, the H2SiNN(H2) and HNSiNH complexes are characterized by infrared spectroscopy in conjunction with quantum-chemical calculations. The synergistic interaction of the two SiH2 moieties with N2 results in the formation of final product H2Si(μ-N)2SiH2, and theoretical calculations reveal the donation of electron density of Si to π* antibonding orbitals and the removal of electron density from the π bonding orbitals of N2, leading to cleave the non-polar and strong NN triple bond.

A Novel Bubble-based Microreactor for Enhanced Mass Transfer Dynamics toward Efficient Electrocatalytic Nitrogen Reduction

Electrocatalytic nitrogen reduction reaction (eNRR) is a promising method for sustainable ammonia production. Although the majority of studies on the eNRR are devoted to developing efficient electrocatalysts, it is critical to study the influence of mass transfer because of the poor N2 transfer efficiency. Herein, a novel bubble-based microreactor (BBMR) is proposed that efficiently promotes the mass transfer behavior during the eNRR using microfluidic strategies. The BBMR possesses abundant triphasic interfaces and provides spatial confinement and accurate potential control, ensuring rapid mass transfer dynamics and improved eNRR performance, as confirmed by experimental and simulation studies. The ammonia yield of the reaction over Ag nanoparticles can be enhanced to 31.35 µg h-1 mgcat. -1, which is twice that of the H-cell. Excellent improvements are also achieved using Ru/C and Fe/g-CN catalysts, with 5.0 and 8.5 times increase in ammonia yield, respectively. This work further demonstrates the significant effect of mass transfer on the eNRR performance and provides an effective strategy for process enhancement through electrode design.

Unusual Electronic Structures of an Electron Transfer Series of [Cr(μ-η1 : η1 -N2 )Cr]0/1+/2

In dinitrogen (N2 ) fixation chemistry, bimetallic end-on bridging N2 complexes M(μ-η1  : η1 -N2 )M can split N2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)]2 (μ-N2 )}0/1+/2+ (1-3) are incapable to cleave N2 , in contrast to their Mo congeners. Remarkably, cross this series the N-N bond length of the N2 ligand and the N-N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand-mediated two-center spin transitions, distinct from discrete dinuclear spin crossovers. In-depth analyses using wave function based ab initio calculations reveal that the Cr-N2 -Cr bonding in complexes 1-3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1-3 is at least a key factor that contributes to their kinetic inertness toward N2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ-η1  : η1 -N2 )M complexes compared to their 4d and 5d analogs.

Nitrogen adsorption on Nb2C6H4+ cations: the important role of benzyne (ortho-C6H4)

N2 adsorption is a prerequisite for activation and transformation. Time-of-flight mass spectrometry experiments show that the Nb2C6H4+ cation, resulting from the gas-phase reaction of Nb2+ with C6H6, is more favorable for N2 adsorption than Nb+ and Nb2+ cations. Density functional theory calculations reveal the effect of the ortho-C6H4 ligand on N2 adsorption. In Nb2C6H4+, interactions between the Nb-4d and C-2p orbitals enable the Nb2+ cation to form coordination bonds with the ortho-C6H4 ligand. Although the ortho-C6H4 ligand in Nb2C6H4+ is not directly involved in the reaction, its presence increases the polarity of the cluster and brings the highest occupied molecular orbital (HOMO) closer to the lowest occupied molecular orbital (LUMO) of N2, thereby increasing the N2 adsorption energy, which effectively facilitates N2 adsorption and activation. This study provides fundamental insights into the mechanisms of N2 adsorption in "transition metal-organic ligand" systems.

The impact of Lewis acid variation on reactions with di-tert-butyl diazo diesters

Reactions of (tBuO2CN)2 with Lewis acids and FLPs have previously been shown to prompt the formation of diazene compounds. In this work, we show that the reaction of (tBuO2CN)2 with 9-BBN leads to a bicyclic heterocyclic product (tBuOCO(BBN)CN)21. In contrast, the reactions of (tBuO2CN)2 with BF3 or [Et3Si][B(C6F5)4] lead to the isolation of [tBuNHNH2tBu][BF4] 2 and [tBuN(H)NtBu][B(C6F5)4] 3, respectively. The mechanism for the formation of 2 is probed computationally, demonstrating that steric and electronic considerations of the Lewis acid impact the reaction pathway.

Fabrication of High Surface Area Fe/Fe3 O4 with Enhanced Performance for Electrocatalytic Nitrogen Reduction Reaction

The development of high-efficient and large-scale non-precious electrocatalysts to improve sluggish reaction kinetics plays a key role in enhancing electrocatalytic nitrogen reduction reaction (NRR) for ammonia production under mild condition. Herein, Fe3 O4 and Fe supported by porous carbon (denoted as Fe/Fe3 O4 /PC-800) composite with a high specific surface area of 1004.1 m2  g-1 was prepared via a simple template method. On one hand, the high surface area of Fe/Fe3 O4 /PC-800 provides a large area to enhance N2 adsorption and promote more protons and electrons to accelerate the reaction, thereby greatly improving the dynamics. On the other hand, mesoporous Fe/Fe3 O4 /PC-800 provides high electrochemically active surface area for promoting the occurrence of catalytic kinetics. As a result, Fe/Fe3 O4 /PC-800 exhibited significantly enhanced NRR performance with an ammonia yield of 31.15 μg h-1  mg-1 cat. and faraday efficiency of 22.26 % at -0.1 V vs. reversible hydrogen electrode (RHE). This study is expected to provide a new strategy for the synthesis of catalysts with large specific area and pave the way for the foundational research in NRR.

WOx nanoparticles coupled with nitrogen-doped porous carbon toward electrocatalytic N2 reduction

The electrocatalytic nitrogen reduction reaction (eNRR) is a sustainable and green alternative to the traditional Haber-Bosch process. However, the chemical inertness of nitrogen gas and the competitive hydrogen evolution reaction significantly limit the catalytic performance of eNRR. Although tungsten oxide-based eNRR catalysts could donate unpaired electrons to the antibonding orbitals of N2 and accept lone electron pairs from N2 to dissociate NN triple bonds, the low electrical conductivity and the influence of the variable valence of W still affect the catalytic activity. Herein, a high-performance eNRR catalyst WOx nanoparticle/nitrogen-doped porous carbon (WOx/NPC) was prepared by a one-step thermal pyrolysis method. The results reveal that WOx gradually changes from the dominant WO2 phase to the WO3 phase. WOx/NPC-700 °C with WO2 NPs anchored on the surfaces of NPC via W-N bonding could deliver a high NH3 yield of 46.8 μg h-1 mg-1 and a high faradaic efficiency (FE) of 10.2%. The edge W atomic site on WOx/NPC is demonstrated to be the active center which could activate a stable NN triple bond with an electron-donating ability. Benefiting from the covalent interaction between the WOx nanoparticles and NPC, WOx/NPC also shows high electrocatalytic stability.

Coordination and Activation of N2 at Low-Valent Magnesium using a Heterobimetallic Approach: Synthesis and Reactivity of a Masked Dimagnesium Diradical

The activation of dinitrogen (N2 ) by transition metals is central to the highly energy intensive, heterogeneous Haber-Bosch process. Considerable progress has been made towards more sustainable homogeneous activations of N2 with d- and f-block metals, though little success has been had with main group metals. Here we report that the reduction of a bulky magnesium(II) amide [(TCHP NON)Mg] (TCHP NON=4,5-bis(2,4,6-tricyclohexylanilido)-2,7-diethyl-9,9-dimethyl-xanthene) with 5 % w/w K/KI yields the magnesium-N2 complex [{K(TCHP NON)Mg}2 (μ-N2 )]. DFT calculations and experimental data show that the dinitrogen unit in the complex has been reduced to the N2 2- dianion, via a transient anionic magnesium(I) radical. The compound readily reductively activates CO, H2 and C2 H4 , in reactions in which it acts as a masked dimagnesium(I) diradical.

BN Analogue of Butadiyne: A Platform for Dinitrogen Release and Reduction

Exploration of the metallomimetic chemistry of main group elements is of the utmost importance from the perspective of both fundamental research and potential applications. Here, we report the synthesis, bonding analysis, and reactivities of an isolable diiminoborane, Mes*B≡N─N≡BMes* (Mes* = 2,4,6-tri-tert-butylphenyl) (1), a BN analogue of butadiyne. This species is characterized by a conjugated B≡N─N≡B moiety, a structural feature that enables the controlled release of N2 when it is exposed to organic nitriles. Furthermore, the N2 unit in 1 could be reduced to an ammonium salt via cleavage of the BN triple bond. Our work shows a rare example of an unsaturated BN system, serving as a platform for both the release and reduction of N2. This discovery opens new pathways and holds substantial influence on the future design of functional main group N2 species.

Concluding remarks: Sustainable nitrogen activation - are we there yet?

The activation of dinitrogen as a fundamental step in reactions to produce nitrogen compounds, including ammonia and nitrates, has a cornerstone role in chemistry. Bringing together research from disparate fields where this can be achieved sustainably, this Faraday Discussion seeks to build connections between approaches that can stimulate further advances. In this paper we set out to provide an overview of these different approaches and their commonalities. We explore experimental aspects including the positive role of increasing nitrogen pressure in some fields, as well as offering perspectives on when ¹⁵N₂ experiments might, and might not, be necessary. Deconstructing the nitrogen reduction reaction, we attempt to provide a common framework of energetic scales within which all of the different approaches and their components can be understood. On sustainability, we argue that although green ammonia produced from a green-H₂-fed Haber-Bosch process seems to fit the bill, there remain many real-world contexts in which other, sustainable, approaches to this vital reaction are urgently needed.

Reactivity of frustrated Lewis pairs with BOC protected diazocarboxylates: FLP capture of diazene

Reactions of (tBuO₂CN)₂ with FLPs are examined. B(C₆F₅)₃ interacts with the carbonyl oxygen atoms inducing loss of CH₂CMe₂; however, in the presence of basic donors, the protons are intercepted affording the salts [Hbase]₂ [((C₆F₅)₃BO₂CN)₂] (base = tBu₃P 1, NC₅H₂Ph₃2, HNC₅H₆Me₄3). In contrast, in the presence of (o-Tol)₃P, a proton transfers to the diazo-N atom affording (o-Tol)₃PN(CO₂tBu)NHB(C₆F₅)₃4. Further addition of B(C₆F₅)₃ to 4 prompts loss of olefin CH₂CMe₂ and CO₂ affording (o-Tol)₃PNHNHB(C₆F₅)₃5. The course of these reactions is examined by extensive DFT calculations. The nature of 5 is consistent with the FLP reduction of a diazene fragment.

P-Block Metal-Based Electrocatalysts for Nitrogen Reduction to Ammonia: A Minireview

Electrochemical nitrogen reduction reaction (NRR) to ammonia (NH₃ ) using renewable electricity provides a promising approach towards carbon neutral. What's more, it has been regarded as the most promising alternative to the traditional Haber-Bosch route in current context of developing sustainable technologies. The development of a class of highly efficient electrocatalysts with high selectivity and stability is the key to electrochemical NRR. Among them, P-block metal-based electrocatalysts have significant application potential in NRR for which possessing a strong interaction with the N 2p orbitals. Thus, it offers a good selectivity for NRR to NH₃ . The density of state (DOS) near the Fermi level is concentrated for the P-block metal-based catalysts, indicating the ability of P-block metal as active sites for N₂ adsorption and activation by donating p electrons. In this work, we systematically review the recent progress of P-block metal-based electrocatalysts for electrochemical NRR. The effect of P-block metal-based electrocatalysts on the NRR activity, selectivity and stability are discussed. Specifically, the catalyst design, the nature of the active sites of electrocatalysts and some strategies for boosting NRR performance, the reaction mechanism, and the impact of operating conditions are unveiled. Finally, some challenges and outlooks using P-block metal-based electrocatalysts are proposed.

Distance-Triggered Distinct Aryl Migrations on Azidodiboranes

Derived from structurally similar precursors, two different azidodiboranes went through distinct aryl migration reactions triggered by different boron-boron separation distances. Biphenylene based diborane with a shorter boron-boron distance underwent heterolateral aryl migration to form a seven-membered azadiborepin, while xanthrene based diborane with a longer boron-boron distance afforded a stable bis-azidoborane scaffold. The pyrolysis of such a bis-azidoborane led to eight-membered oxazadiborocine through homolateral aryl migration and subsequent [3+2] cycloaddition. Density functional theory (DFT) calculations unveiled that the boron-boron separation distances were the intrinsic factors for the distinct migrations.

Dinitrogen Activation and Addition to Unsaturated C-E (E=C, N, O, S) Bonds Mediated by Transition Metal Complexes

Dinitrogen (N₂ ) activation and functionalization is of fundamental interest and practical importance. This review focuses on N₂ activation and addition to unsaturated substrates, including carbon monoxide, carbon dioxide, heteroallenes, aldehydes, ketones, acid halides, nitriles, alkynes, and allenes, mediated by transition metal complexes, which afforded a variety of N-C bond formation products. Emphases are placed on the reaction modes and mechanisms. We hope that this work would stimulate further explorations in this challenging field.

Neutral Boryl Radicals in Mixed-Valent B(III) Br-B(II) Adducts

The general strategies to stabilize a boryl radical involve single electron delocalization by π-system and the steric hinderance from bulky groups. Herein, a new class of boryl radicals is reported, with intramolecular mixed-valent B^(III) Br-B^(II) adducts ligated by a cyclic (alkyl)(amino)carbene (CAAC). The radicals feature a large spin density on the boron center, which is ascertained by EPR spectroscopy and DFT calculations. Structural and computational analyses revealed that the stability of radical species was assisted by the CAAC ligand and a weak but significant B(III)Br-B(II) interaction, suggesting a cooperative avenue for stabilization of boryl radicals. Two-electron reduction of these new boryl radicals provides C-H insertion products via a borylene intermediate.

Single-Atom Low-Valent Alkaline-Earth-Metal Catalysts for Electrochemical Nitrogen Reduction with an Acceptance-Backdonation Mechanism

Single-atom catalysts (SACs) have drawn great attention in developing highly active and low-cost catalysts for electrocatalytic nitrogen reduction reaction (NRR) in ammonia synthesis, but the atomic metal centers are mainly limited to transition metals. Here, four stable alkaline-earth-metal (AEM)-based SACs are proposed by anchoring AEM on nitrogen-doped graphene nanoribbons, based on first-principles calculations. All SACs exhibit excellent NRR performance with competitive limiting potentials compared to stepped Ru (0001), and Ca-based SAC achieves optimal activity with a potential of -0.716 V. It is revealed that the low oxidation state of AEM is crucial for the activation of N₂ through an acceptance-backdonation mechanism. The antibonding 2π* orbital of N₂ can accept residual s electrons of low-valent AEM and backdonate electrons to the empty d orbitals of AEM, resulting in activation of N₂ molecules. In particular, the activation degree of N₂ and NRR activity is linearly associated with the charge states of AEMs. Our work reveals the underlying mechanism of AEMs for N₂ activation and reduction and presents the potential of AEM SACs as efficient electrochemical NRR catalysts.

The development of catalysts for electrochemical nitrogen reduction toward ammonia: theoretical and experimental advances

Ammonia (NH₃) is essential for the industrial production of fertilizers, pharmaceuticals, plastics, synthetic fibers, resins, and chemicals, and it is also a promising carbon-free energy carrier. The electrocatalytic nitrogen reduction reaction (eNRR) driven by renewable energy sources at ambient temperature and atmospheric pressure is an alternative approach to the Haber-Bosch process for NH₃ synthesis. However, the efficient electrocatalytic reduction of nitrogen (N₂) to NH₃ is challenging due to the lack of effective electrocatalysts. Tremendous effort has been made to develop high-performance electrocatalysts for the eNRR in the past few years. In this review, we summarize recent progress relating to electrocatalysts for the eNRR from both theoretical and experimental aspects. Remaining challenges and perspectives for promoting the eNRR to generate NH₃ are also discussed. This review hopes to guide the design and development of efficient electrocatalysts for the eNRR for NH₃ synthesis.

Reductive activation of N2 using a calcium/potassium bimetallic system supported by an extremely bulky diamide ligand

Potassium reduction of a bulky diamido-calcium complex under an N2 atmosphere afforded the first well-defined, hetero-bimetallic s-block metal complex of activated dinitrogen.