Covalent Capture of Nitrous Oxide by N-Heterocyclic Carbenes

Progress and challenges in nitrous oxide decomposition and valorization

Nitrous oxide (N2O) decomposition is increasingly acknowledged as a viable strategy for mitigating greenhouse gas emissions and addressing ozone depletion, aligning significantly with the UN's sustainable development goals (SDGs) and carbon neutrality objectives. To enhance efficiency in treatment and explore potential valorization, recent developments have introduced novel N2O reduction catalysts and pathways. Despite these advancements, a comprehensive and comparative review is absent. In this review, we undertake a thorough evaluation of N2O treatment technologies from a holistic perspective. First, we summarize and update the recent progress in thermal decomposition, direct catalytic decomposition (deN2O), and selective catalytic reduction of N2O. The scope extends to the catalytic activity of emerging catalysts, including nanostructured materials and single-atom catalysts. Furthermore, we present a detailed account of the mechanisms and applications of room-temperature techniques characterized by low energy consumption and sustainable merits, including photocatalytic and electrocatalytic N2O reduction. This article also underscores the extensive and effective utilization of N2O resources in chemical synthesis scenarios, providing potential avenues for future resource reuse. This review provides an accessible theoretical foundation and a panoramic vision for practical N2O emission controls.

Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Covalent Adsorption of N-Heterocyclic Carbenes on a Copper Oxide Surface

Tuning the properties of oxide surfaces through the adsorption of designed ligands is highly desirable for several applications, such as catalysis. N-Heterocyclic carbenes (NHCs) have been successfully employed as ligands for the modification of metallic surfaces. On the other hand, their potential as modifiers of ubiquitous oxide surfaces still needs to be developed. Here we show that a model NHC binds covalently to a copper oxide surface under UHV conditions. In particular, we report the first example of a covalent bond between NHCs and oxygen atoms from the oxide layer. This study demonstrates that NHC can also act as a strong anchor on oxide surfaces.

Frustrated Lewis Pair Stabilized Phosphoryl Nitride (NPO), a Monophosphorus Analogue of Nitrous Oxide (N2O)

Phosphoryl nitride (NPO) is a highly reactive intermediate, and its chemistry has only been explored under matrix isolation conditions so far. Here we report the synthesis of an anthracene (A) and phosphoryl azide based molecule (N₃P(O)A) that acts as a molecular synthon of NPO. Experimentally, N₃P(O)A dissociates thermally with a first-order kinetic half-life that is associated with an activation enthalpy of ΔH^⧧ = 27.5 ± 0.3 kcal mol^-1 and an activation entropy of ΔS^⧧ = 10.6 ± 0.3 cal mol^-1 K^-1 that are in good agreement with calculated DLPNO-CCSD(T)/cc-pVTZ//PBE0-D3(BJ)/cc-pVTZ energies. In solution N₃P(O)A undergoes Staudinger reactivity with tricyclohexylphosphine (PCy₃) and subsequent complexation with tris(pentafluorophenyl)borane (B(C₆F₅)₃, BCF) to form Cy₃P-NP(A)O-B(C₆F₅)₃. Anthracene is cleaved off photochemically to form the frustrated Lewis pair (FLP) stabilized NPO complex Cy₃P^⊕-N═P-O-B^⊖(C₆F₅)₃. An intrinsic bond orbital (IBO) analysis suggests that the adduct is zwitterionic, with a positive and negative charge localized on the complexing Cy₃P and BCF, respectively.

2-Pyridylselenenyl versus 2-Pyridyltellurenyl Halides: Symmetrical Chalcogen Bonding in the Solid State and Reactivity towards Nitriles

The synthesis of 2-pyridyltellurenyl bromide via Br2 oxidative cleavage of the Te–Te bond of dipyridylditelluride is reported. Single-crystal X-ray diffraction analysis of 2-pyridyltellurenyl bromide demonstrated that the Te atom of 2-pyridyltellurenyl bromide was involved in four different noncovalent contacts: Te⋯Te interactions, two Te⋯Br ChB, and one Te⋯N ChB contact forming 3D supramolecular symmetrical framework. In contrast to 2-pyridylselenenyl halides, the Te congener does not react with nitriles furnishing cyclization products. 2-Pyridylselenenyl chloride was demonstrated to easily form the corresponding adduct with benzonitrile. The cyclization product was studied by the single-crystal X-ray diffraction analysis, which revealed that in contrast to earlier studied cationic 1,2,4-selenadiazoles, here we observed that the adduct with benzonitrile formed supramolecular dimers via Se⋯Se interactions in the solid state, which were never observed before for 1,2,4-selenadiazoles.

Synthesis and reactivity of NHC-coordinated phosphinidene oxide

Here we report the synthesis of an N-heterocyclic carbene (NHC)-stabilised phosphinidene oxide by the controlled oxygenation of a phosphinidene under ambient conditions. This compound can be further oxygenated to a phosphinidene dioxide. The stoichiometric reduction of a phosphinidene oxide with KC₈ resembles the pinacol coupling reaction-the reduction of a carbonyl compound. We also looked at the stoichiometric oxidation of NHC-coordinated phosphinidene, phosphinidene oxide and phosphinidene dioxide with [NO][SbF₆].

Mechanistic Pathways for N2O Elimination from trans-R3Sn-O-N═N-O-SnR3 and for Reversible Binding of CO2 to R3Sn-O-SnR3 (R = Ph, Cy)

The rate and mechanism of the elimination of N₂O from trans-R₃Sn-O-N═N-O-SnR₃ (R = Ph (1^Ph) and R = Cy (1^Cy)) to form R₃Sn-O-SnR₃ (R = Ph (2^Ph) and R = Cy (2^Cy)) have been studied using both NMR and IR techniques to monitor the reactions in the temperature range of 39-79 °C in C₆D₆. Activation parameters for this reaction are ΔH^⧧ = 15.8 ± 2.0 kcal·mol^-1 and ΔS^⧧ = -28.5 ± 5 cal·mol^-1·K^-1 for 1^Ph and ΔH^⧧ = 22.7 ± 2.5 kcal·mol^-1 and ΔS^⧧ = -12.4 ± 6 cal·mol^-1·K^-1 for 1^Cy. Addition of O₂, CO₂, N₂O, or PPh₃ to sealed tube NMR experiments did not alter in a detectable way the rate or product distribution of the reactions. Computational DFT studies of elimination of hyponitrite from trans-Me₃Sn-O-N═N-O-SnMe₃ (1^Me) yield a mechanism involving initial migration of the R₃Sn group from O to N passing through a marginally stable intermediate product and subsequent N₂O elimination. Reactions of 1^Ph with protic acids HX are rapid and lead to formation of R₃SnX and trans-H₂N₂O₂. Reaction of 1^Ph with the metal radical •Cr(CO)₃C₅Me₅ at low concentrations results in rapid evolution of N₂O. At higher •Cr(CO)₃C₅Me₅ concentrations, evolution of CO₂ rather than N₂O is observed. Addition of 1 atm or less CO₂ to benzene or toluene solutions of 2^Ph and 2^Cy resulted in very rapid reaction to form the corresponding carbonates R₃Sn-O-C(═O)-O-SnR₃ (R = Ph (3^Ph) and R = Cy (3^Cy)) at room temperature. Evacuation results in fast loss of bound CO₂ and regeneration of 2^Ph and 2^Cy. Variable temperature data for formation of 3^Cy yield ΔH^o = -8.7 ± 0.6 kcal·mol^-1, ΔS^o = -17.1 ± 2.0 cal·mol^-1·K^-1, and ΔG^o_298K = -3.6 ± 1.2 kcal·mol^-1. DFT studies were performed and provide additional insight into the energetics and mechanisms for the reactions.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Side-on Coordination in Isostructural Nitrous Oxide and Carbon Dioxide Complexes of Nickel

A nickel complex incorporating an N₂ O ligand with a rare η² -N,N'-coordination mode was isolated and characterized by X-ray crystallography, as well as by IR and solid-state NMR spectroscopy augmented by ¹⁵ N-labeling experiments. The isoelectronic nickel CO₂ complex reported for comparison features a very similar solid-state structure. Computational studies revealed that η² -N₂ O binds to nickel slightly stronger than η² -CO₂ in this case, and comparably to or slightly stronger than η² -CO₂ to transition metals in general. Comparable transition-state energies for the formation of isomeric η² -N,N'- and η² -N,O-complexes, and a negligible activation barrier for the decomposition of the latter likely account for the limited stability of the N₂ O complex.

Catalytic Activation of N2O at a Low-Valent Bismuth Redox Platform

Herein we present the catalytic activation of N₂O at a Bi^I⇄Bi^III redox platform. The activation of such a kinetically inert molecule was achieved by the use of bismuthinidene catalysts, aided by HBpin as reducing agent. The protocol features remarkably mild conditions (25 °C, 1 bar N₂O), together with high turnover numbers (TON, up to 6700) and turnover frequencies (TOF). Analysis of the elementary steps enabled structural characterization of catalytically relevant intermediates after O-insertion, namely a rare arylbismuth oxo dimer and a unique monomeric arylbismuth hydroxide. This protocol represents a distinctive example of a main-group redox cycling for the catalytic activation of N₂O.

Activation of N2O and SO2 by the P-B Bond System. Reversible Binding of SO2 by the P-O-B Geminal Frustrated Lewis Pair

Herein, we present the first transformation of borylphosphine into borylphosphinite using nitrous oxide. Borylphosphine reacts with N2O via insertion of a single oxygen atom into the P-B bond and formation of a P-O-B bond system. Borylphosphine and borylphosphinite capture SO2 and activate it in an irreversible and reversible manner, respectively.

Small molecule activation by multimetallic uranium complexes supported by siloxide ligands

The synthesis and reactivity of uranium compounds supported by the tris-tert-butoxysiloxide ligand is surveyed. The multiple binding modes of the tert-butoxysiloxide ligand have proven very well suited to stabilize highly reactive homo- and heteropolymetallic complexes of uranium that have shown an unusual high reactivity towards small molecules such as CO2, CS2, chalcogens and azides. Moreover, these ligands have allowed the isolation of dinuclear nitride and oxide bridged complexes of uranium in various oxidation states. The ability of the tris-tert-butoxysiloxide ligands to trap alkali ions in these nitride or oxide complexes leads to unprecedented ligand based and metal based reduction and functionalization of N2, CO, CO2 and H2.

Synthesis of Imidazolidinone, Imidazolone, and Benzimidazolone Derivatives through Oxidation Using Copper and Air

A new synthetic method of urea derivatives using copper and air was developed. These mild conditions provided moderate to very good yields for 15 examples (53-93%), while low yields were obtained with sterically hindered substrates (3 examples). The reaction was found to go through an in situ generated copper- N-heterocyclic carbene, which was then oxidized into cyclic urea derivatives possessing alkyl, benzyl, aryl, hydroxy, Boc-protected, and tertiary amine groups.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Lewis Acid-Mediated One-Electron Reduction of Nitrous Oxide

The one-electron reduction of nitrous oxide (N₂ O) was achieved using strong Lewis acids E(C₆ F₅ )₃ (E=B or Al) in combination with metallocenes. In the case of B(C₆ F₅ )₃ , electron transfer to N₂ O required a powerful reducing agent such as Cp*₂ Co (Cp*=pentamethylcyclopentadienyl). In the presence of Al(C₆ F₅ )₃ , on the other hand, the reactions could be performed with weaker reducing agents such as Cp*₂ Fe or Cp₂ Fe (Cp=cyclopentadienyl). The Lewis acid-mediated electron transfer from the metallocene to N₂ O resulted in cleavage of the N-O bond, generating N₂ and the oxyl radical anion [OE(C₆ F₅ )₃ ]^⋅- . The latter is highly reactive and engages in C-H activation reactions. It was possible to trap the radical by addition of the Gomberg dimer, which acts as a source of the trityl radical.

Photochemistry of OPN: Formation of Cyclic PON and Reversible Combination with Carbon Monoxide

Cyclic PON, a first 16-electron triatomic ring consisting of two first-row elements, has been generated through 193 nm laser photolysis of OPN in cryogenic matrices. When the photolysis (365 nm) was performed in the presence of CO, OPN combines CO and furnishes an exotic acyclic molecule OPNCO. Upon the 193 nm laser irradiation, OPNCO dissociates mainly to OPN and CO with traces of PN and CO₂ . The identification of cyclic PON and OPNCO with IR spectroscopy is supported by ¹⁵ N-labeling experiments and quantum chemical calculations.

Trimethylsilyl-Induced N-O Bond Cleavage in Nitrous Oxide-Derived Aminodiazotates

The chemical activation of nitrous oxide (N₂O) typically results in O-atom transfer and the extrusion of N₂ gas. In contrast, reactions of N-trimethylsilyl (TMS)-substituted amides with N₂O give inorganic or organic azides, with concomitant formation of silanols or siloxanes. N-TMS-substituted amides are also able to induce N-O bond cleavage in N₂O-derived dialkylaminodiazotates, generating tetrazene salts. These results indicate the potential of silyl groups in devising transformations, in which N₂O acts as an N-atom donor.

B-Heterocyclic Carbene Arising from Charge Shift: A Computational Verification

1-Borabicyclo[1.1.0]but-2(3)-ene (1BB) is a singlet biradical with two single electrons that can form an ionic resonance structure through a charge shift. The ionic resonance structure is a B-heterocyclic carbene (BHC), which can act as a carbene, Lewis base, or L- and Z-type ligand, to give adducts and complexes. Through a range of quantum methods, four types of stable compounds (A-D) derived from 1BB have been designed. These compounds retain the unique features of 1BB. As a consequence, the structures, stability, and Wiberg bond indices of the Lewis adducts of A-D with Lewis acids (BePh₂ , BH₃ , AlH₃ , AlCl₃ , C₅ BH₅ , and C₁₃ BH₉ ) and Cu^I , Ag^I , and Au^I complexes have been investigated. Results show that A-D can indeed react as carbenes. Interestingly, compounds A-D, as L-type ligands, can attach to BePh₂ , BH₃ , AlH₃ , AlCl₃ , C₅ BH₅ , C₁₃ BH₉ , and CuCl and form compounds with planar tetracoordinate carbon (ptC), whereas Z-type ligands A-D can bind to AgCl and AuCl to provide complexes with planar tetracoordinate boron (ptB). In addition, the binuclear complexes of ClX(1BB)CuCl (X=Ag, Au) have been studied and A-D behave as both L- and Z-type ligands, in which these complexes contain both ptC and ptB. Thus, a novel method for designing compounds with ptC and ptB is presented. These rationally designed compounds involve the elements of carbene, ptC, ptB, and L- and Z-type ligands, and are expected to be unique and useful in experimental chemistry once they are synthesized.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Palladium(II)-Stabilized Pyridine-2-Diazotates: Synthesis, Structural Characterization, and Cytotoxicity Studies

Well-defined diazotates are scarce. Here we report the synthesis of unprecedented homoleptic palladium(II) diazotate complexes. The palladium(II)-mediated nitrosylation of 2-aminopyridines with NaNO₂ results in the formation of metal-stabilized diazotates, which were found to be cytotoxic to human ovarian cancer cells.

Experimental observation and quantum chemical investigation of thallium(i) (Z)-methanediazotate: synthesis of a long sought and highly reactive species

Thallium(i) (Z)-methanediazotate has been synthesised through TlOPr induced cleavage of ¹⁵N-labelled N-methyl-N-nitrosourea, and confirmed by NMR analysis. CCSD(T)/CBS calculations reveal a kinetic product control.

Fixation of nitrous oxide by mesoionic and carbanionic N-heterocyclic carbenes

Double fixation of laughing gas (N₂O) can be achieved under mild conditions using mesoionic or ditopic carbanionic carbenes.

Computational Investigation on the Role of Disilene Substituents Toward N2O Activation

The effect of substituents in disilene mediated N₂O activation was studied at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between structural diversity and the corresponding reactivity of six disilenes (I_A-F^t) in the presence of four different substituents (-NMe₂, -Cl, -Me, -SiMe₃) is addressed in this investigation. We primarily propose two plausible mechanistic routes: Pathway I featuring disilene → silylene decomposition followed by N₂O coordination and Pathway II constituting the N₂O attack without Si-Si bond cleavage. Depending on the fashion of N₂O approach the latter route was further differentiated into Pathway IIa and Pathway IIb detailing the "end-on" and "side-on" attack to the disilene scaffold. Interestingly, the lone pair containing substituents (-NMe₂, -Cl,) facilitates disilene → silylene dissociation; on the contrary it reduces the electrophilicity at Si center in silylene, a feature manifested with higher activation barrier during N₂O attack. In the absence of any lone-pair influence from substituents (-Me, -SiMe₃), the decomposition of disilenes is considerably endothermic. Therefore, Pathway I appears to be the less preferred route for both types of substituents. In Pathway IIa, the N₂O moiety uniformly approaches via O-end to both the silicon centers in disilenes. However, the calculations reveal that Pathway IIa, although not operational for all disilenes, is unlikely to be a viable route due to the predominantly higher transition barrier (ca. 36 kcal/mol). The most feasible route in this current study accompanying moderately low activation barriers (∼19-26 kcal/mol) is Pathway IIb, which involves successive addition of two N₂O units proceeding via terminal N, O toward the Si centers and is applicable for all disilenes. The reactivity of substituted disilenes can be estimated in terms of the first activation barrier of N₂O attack. Surprisingly, in Pathway IIb, the initial activation barrier and hence the reactivity shows negligible correlation with Si-Si bond strength, indicating toward the versatility of the reaction route.

Neutral Aminyl Radicals Derived from Azoimidazolium Dyes

The synthesis and characterization of a new class of neutral aminyl radicals is reported. Monoradicals were obtained by reduction of azoimidazolium dyes with potassium. Structural, spectroscopic, and computational data suggest that the spin density is centered on one of the nitrogen atoms of the former azo group. The reduction of a dimeric dye with an octamethylbiphenylene bridge between the azo groups resulted in the formation of a biradical with largely independent unpaired electrons. Both the monoradicals and the biradical were found to display high stability in solution as well as in the solid state.

Theoretical study of the mechanism for the sequential N–O and N–N bond cleavage within N2O adducts of N-heterocyclic carbenes by a vanadium(iii) complex

The addition of N-heterocyclic carbene (NHC) increases the activity of N₂O towards cleavage of both the N–O and N–N bonds.

Activation of C–F bonds in fluoroarenes by N-heterocyclic carbenes as an effective route to synthesize abnormal NHC complexes

An unexpected consecutive C–F activation of fluoroarenes by NHCs is presented, offering a versatile strategy to access aNHC metal complexes.

Nitrogen-rich hypergolic ionic salts based on (2-methyltetrazol-5-yl)diazotates

Novel hypergolic energetic salts containing the (2-methyltetrazol-5-yl)diazotate anion were synthesized and exhibit attractive short ignition delay times with white fuming HNO₃.

Applications of N-heterocyclic imines in main group chemistry

A survey of the most recent progress in the applications of N-heterocyclic imines in main group compounds is given.

N-Heterocyclic carbene phosphaketene adducts as precursors to carbene–phosphinidene adducts and a rearranged π-system

Sodium phosphaethynolate, Na(OCP), is demonstrated to be successful for the synthesis of NHC–phosphinidene adducts and a linear π-conjugated molecule.

Synthesis of a "Masked" Terminal Nickel(II) Sulfide by Reductive Deprotection and its Reaction with Nitrous Oxide

The addition of 1 equiv of KSCPh3 to [L(R)NiCl] (L(R) = {(2,6-iPr2C6H3)NC(R)}2CH; R = Me, tBu) in C6H6 results in the formation of [L(R)Ni(SCPh3)] (1: R = Me; 2: R = tBu) in good yields. Subsequent reduction of 1 and 2 with 2 equiv of KC8 in cold (-25 °C) Et2O in the presence of 2 equiv of 18-crown-6 results in the formation of "masked" terminal Ni(II) sulfides, [K(18-crown-6)][L(R)Ni(S)] (3: R = Me; 4: R = tBu), also in good yields. An X-ray crystallographic analysis of these complexes suggests that they feature partial multiple-bond character in their Ni-S linkages. Addition of N2O to a toluene solution of 4 provides [K(18-crown-6)][L(tBu)Ni(SN=NO)], which features the first example of a thiohyponitrite (κ(2)-[SN=NO](2-)) ligand.

Facile reversibility by design: tuning small molecule capture and activation by single component frustrated Lewis pairs

A series of single component FLPs has been investigated for small molecule capture, with the finding that through tuning of both the thermodynamics of binding/activation and the degree of preorganization (i.e., ΔS(⧧)) reversibility can be brought about at (or close to) room temperature. Thus, the dimethylxanthene system {(C6H4)2(O)CMe2}(PMes2)(B(C6F5)2): (i) heterolytically cleaves dihydrogen to give an equilibrium mixture of FLP and H2 activation product in solution at room temperature and (ii) reversibly captures nitrous oxide (uptake at room temperature, 1 atm; release at 323 K).

Synthetic chemistry with nitrous oxide

Nitrous oxide (N₂O, ‘laughing gas’) is a very inert molecule. Still, it can be used as a reagent in synthetic organic and inorganic chemistry, serving as O-atom donor, as N-atom donor, or as a oxidant in metal-catalyzed reactions.