Search

Transformations of nitrogen-oxyanions (NOx- ) to ammonia impart pivotal roles in sustainable biogeochemical processes. While metal-mediated reductions of NOx- are relatively well known, this report illustrates proton-assisted transformations of NOx- anions in the presence of electron-rich aromatics such as 1,3,5-trimethoxybenzene (TMB-H, 1 a) leading to the formation of diaryl oxoammonium salt [(TMB)2 N+ =O][NO3- ] (2 a) via the intermediacy of nitrosonium cation (NO+ ). Detailed characterizations including UV/Vis, multinuclear NMR, FT-IR, HRMS, X-ray analyses on a set of closely related metastable diaryl oxoammonium [Ar2 N+ =O] species disclose unambiguous structural and spectroscopic signatures. Oxoammonium salt 2 a exhibits 2 e- oxidative reactivity in the presence of oxidizable substrates such as benzylamine, thiol, and ferrocene. Intriguingly, reaction of 2 a with water affords ammonia. Perhaps of broader significance, this work reveals a new metal-free route germane to the conversion of NOx to NH3 .

This paper presents new results that confirm our previous inference that the binuclear form of biologically active dinitrosyl iron complexes (B-DNICs) with thiol-containing ligands (glutathione or N-acetyl-L-cysteine) may act as a donor of nitrosonium cations, which are responsible for S-nitrosothiol formation during B-DNIC decomposition in acid solutions under both aerobic and anaerobic conditions. The presence of nitrosonium cations within B-DNICs is determined by the dispropoportionation reaction of free-radical nitric oxide (NO) molecules while binding to Fe2+ cations (two molecules per one ion) during B-DNIC synthesis. When thiolic ligands are oxidized in DNICs or inactivated by thiol-specific reagents, the nitrosonium cations released during decomposition of these DNICs at neutral pH values are hydrolyzed and transformed to nitrite anions. A similar transformation occurs when mononuclear DNICs (M-DNICs) with nonthiolic ligands are decomposed at neutral pH values. It has been found that S-nitrosothiol formation in the decomposition of B-DNICs with thiolic ligands at acidic pH values can be inhibited by the presence of a two to threefold excess of free thiol molecules (outside the B-DNIC) with regard to the B-DNIC level. This inhibition is due to the reduction of nitrosonium cations induced by free thiol molecules and catalyzed by iron ions. The NO molecules that result from the reduction are released from the DNICs. Thus, both forms of DNICs, M and B, that form in living systems can act not only as donors of NO, which is now recognized as one of the universal regulators of metabolic processes, but also as donors of nitrosonium cations, which initiate S-nitrosation of low- and high-molecular-weight (protein-bound) thiols.

The dynamic character of the active centers has made it difficult to unravel the reaction path for NH3-assisted selective catalytic reduction (SCR) of nitrogen oxides over Cu-CHA. Herein, we use density functional theory calculations to suggest a complete reaction mechanism for low-temperature NH3-SCR The reaction is found to proceed in a multisite fashion over ammonia-solvated Cu cations Cu(NH3)(2+) and Bronsted acid sites. The activation of oxygen and the formation of the key intermediates HONO and H2NNO occur on the Cu sites, whereas the Bronsted acid sites facilitate the decomposition of HONO and H2NNO to N-2 and H2O. The activation and reaction of NO is found to proceed via the formation of nitrosonium (NO+) or nitrite (NO2-) intermediates. These low-temperature mechanisms take the dynamic character of Cu sites into account where oxygen activation requires pairs of Cu(NH3)(2+) complexes, whereas HO-NO and H3N-NO coupling may occur on single complexes. The formation and separation of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured kinetic data and provides a solid basis for future improvements of the low-temperature NH3-SCR reaction.

In this article we minutely discuss the so-called “oxidative” mechanism of mononuclear form of dinitrosyl iron complexes (M-DNICs) formations proposed by the author. M-DNICs are proposed to be formed from their building material—neutral NO molecules, Fe2+ ions and anionic non-thiol (L−) and thiol (RS−) ligands based on the disproportionation reaction of NO molecules binding with divalent ion irons in pairs. Then a protonated form of nitroxyl anion (NO−) appearing in the reaction is released from this group and a neutral NO molecule is included instead. As a result, M-DNICs are produced. Their resonance structure is described as [(L−)2Fe2+(NO)(NO+)], in which nitrosyl ligands are represented by NO molecules and nitrosonium cations in equal proportions. Binding of hydroxyl ions with the latter causes conversion of these cations into nitrite anions at neutral pH values and therefore transformation of DNICs into the corresponding high-spin mononitrosyl iron complexes (MNICs) with the resonance structure described as [(L−)2Fe2+(NO)]. In case of replacing L− by thiol-containing ligands, which are characterized by high π-donor activity, electron density transferred from sulfur atoms to iron-dinitrosyl groups neutralizes the positive charge on nitrosonium cations, which prevents their hydrolysis, ensuring relatively a high stability of the corresponding M-DNICs with the resonance structure [(RS−)2Fe2+ (NO, NO+)]. Therefore, M-DNICs with thiol-containing ligands, as well as their binuclear analogs (B-DNICs, respective resonance structure [(RS−)2Fe2+2 (NO, NO+)2]), can serve donors of both NO and NO+. Experiments with solutions of B-DNICs with glutathione or N-acetyl-L-cysteine (B-DNIC-GSH or B-DNIC-NAC) showed that these complexes release both NO and NO+ in case of decomposition in the presence of acid or after oxidation of thiol-containing ligands in them. The level of released NO was measured via optical absorption intensity of NO in the gaseous phase, while the number of released nitrosonium cations was determined based on their inclusion in S-nitrosothiols or their conversion into nitrite anions. Biomedical research showed the ability of DNICs with thiol-containing ligands to be donors of NO and NO+ and produce various biological effects on living organisms. At the same time, NO molecules released from DNICs usually have a positive and regulatory effect on organisms, while nitrosonium cations have a negative and cytotoxic effect.

This work reports the first success of the nitroso-Povarov reaction, involving gold-catalyzed [4+2] annulations of nitrsoarenes with substituted cyclopentadienes. In this catalytic sequence, nitrosoarenes presumably attack gold-π-dienes by a 1,4-addition pathway, generating allylgold nitrosonium intermediates to complete an intramolecular cyclization. Acyclic dienes are also applicable substrates, and affording oxidative nitroso-Povarov products.

The synthesis of metallaisoxazole by the [3+2] cycloaddition reaction of metallacyclopropene with nitrosonium tetra-fluoroborate has been achieved under mild conditions. Nuclear magnetic resonance spectra, X-ray crystallographic analysis, and density functional theory calculations all suggest that the metallaisoxazole exhibits an aromatic character.

The nitrolysis of 3,7-dinitro-1,3,5,7-tetraazabicyclo[3.3.1]nonane (DPT) to prepare 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) in fuming HNO3 between −8 °C to −36 °C was tracked by 1H-NMR. It is found that DPT converted to 1-nitroso-3,5,7-trinitro-1,3,5,7-tetraazacyclooctane (MNX) immediately at the end of the feeding course; MNX was then gradually nitrolyzed to HMX. The latter reaction followed a pseudo-first order kinetics, and the selectivity of HMX was almost unaffected with the change in reaction temperature based on the kinetics study. The effects of water and N2O4 on this reaction were investigated and the results indicated that a small amount of water and N2O4 was beneficial to the generation of the intermediate product MNX. On the contrary, the presence of water and N2O4 hindered the conversion of MNX to HMX. Nitrosonium in the reaction system has two sources: one is from the nitrogen oxides such as N2O4 in fuming nitric acid; the other is from the redox reaction between nitric acid and formaldehyde, which was promoted by the H2O in fuming nitric acid. Based on the these results, the overall yield of the stepwise method for preparing HMX was up to 82.7% under the optimized reaction conditions.

The nitrite anion (NO2−) is an important source of nitric oxide (NO) or nitrosonium cations (NO+). In the present study, we have shown that [RuIII(edta)(NO+)] (edta4− = ethylendiaminetetraacetate) generated via decomposition of [RuIII(edta)(NO2)]2− at lower pH (3.5) could react with H2S to form a putative thionitrous acid (HSNO) bound Ru(edta) complex in aqueous solution. The reaction product [RuIII(edta)(NOSH)]− was characterized by IR, EPR and ESI-MS spectroscopy. Formation of N-coordinated [RuIII(edta)(NOSH)]− was further corroborated by DFT calculations.

Selective oxidative homo- and cross-coupling of electron-rich phenols and anilides was developed using nitrosonium tetrafluoroborate as a catalyst. Oxidative coupling of phenols revealed unusual selectivities, which translated into the unprecedented synthesis of inverse Pummerer-type ketones. Mechanistic studies suggest that oxidative coupling of phenols and anilides shares a common pathway via homolytical heteroatom-hydrogen bond cleavage. Nitrosonium salt catalysis was applied for cross-dehydrogenative coupling initiated by generation of heteroatom-centered radicals.

A catalytic, metal-free intramolecular rearrangement of benzyl phenyl ethers using nitrosonium salt as a catalyst is described. The optimized reaction conditions enabled a catalytic and metal-free Friedel–Crafts alkylation reaction with benzylic alcohols, producing water as the stoichiometric byproduct. A comprehensive scope (>50 examples) for both approaches and application in drug synthesis were demonstrated. Mechanistic studies suggest a Lewis acid-based mechanism for the metal-free Friedel–Crafts reaction.

The aim of this work is to introduce, model, and optimize a new non-acid-catalyzed system for a direct N $$=$$ N–C bond formation. By reacting naphthols or phenol with anilines in the presence of the sodium nitrite as nitrosonium ( $$\hbox {NO}^{+})$$ source and triethylammonium acetate (TEAA), a N $$=$$ N–C group can be formed in non-acid media. Modeling and optimization of the reaction conditions were investigated by response surface method. Sodium nitrite, TEAA, and water were chosen as variables, and reaction yield was also monitored. Analysis of variance indicates that a second-order polynomial model with F value of 35.7, a P value of 0.0001, and regression coefficient of 0.93 is able to predict the response. Based on the model, the optimum process conditions were introduced as 2.2 mmol sodium nitrite, 2.2 mL of TEAA, and 0.5 mL $$\hbox {H}_{2}\hbox {O}$$ at room temperature. A quadratic (second-order) polynomial model, by analysis of variance, was able to predict the response for a direct N=N–C group formation. Predicted response values were in good agreement with the experimental values. Electrochemistry studies were done to introduce new Michael acceptor moieties. Broad scope, high yields, short reaction time, and mild conditions are some advantages of the presented method.

Carboxyl and nitrite functionalized graphene quantum dots (CNGQDs) was used for the efficient synthesis of α-oximinoketones under free mineral acid conditions at room temperature. CNGQDs was prepared via o-nitrozation of carboxyl and hydroxyl graphene quantum dots (CHGQDs) and used as a nitrosonium source and also as an efficient acidic catalyst for the synthesis of α-oximinoketones. The structure of the catalyst was characterized by FT-IR, XRD, TGA and photoluminescence techniques. The structures of the synthesized products were confirmed by FT-IR, 1H-NMR and 13C-NMR spectroscopic techniques. Stability and recyclability of the prepared homogeneous catalyst was studied in details. Reaction times and yields of the products were compared with previous reported methods resulting high yields and also shorter reaction times.

Natural purified mordenite from Palmarito de Cauto (ZP) deposit, Cuba, was subjected to a hydrothermal ion exchange process in acid medium with Fe2+ or Fe3+ salts (Fe2+ZP and Fe3+ZP). The set of samples was characterized regarding their textural properties, morphology, and crystallinity, and tested in the NO reduction with CO/C3H6. Infrared spectroscopy coupled with NO as a probe molecule was used to give a qualitative description of the Fe species&rsquo, nature and distribution. The exchange process caused an increase in the iron loading of the samples and a redistribution, resulting in more dispersed Fe2+ and Fe3+ species. When contacted with the NO probe, Fe2+ZP showed the highest intensity of nitrosyl bands, assigned to NO adducts on isolated/highly dispersed Fe2+/Fe3+ extra-framework sites and FexOy clusters. This sample is also characterized by the highest NO sorption capacity and activity in NO reduction. Fe3+ZP showed a higher intensity of nitrosonium (NO+) species, without a correlation to NO storage and conversion, pointing to the reactivity of small FexOy aggregates in providing oxygen atoms for the NO to NO+ reaction. The same sites are proposed to be responsible for the higher production of CO2 observed on this sample, and thus to be detrimental to the activity in NO SCR.

Nitrosonium ions are versatile and mild oxidants, which were successfully employed in organic transformations. The applications cover different fields, such as the functionalization of carbon-carbon and carbon-heteroatom bonds, functionalization of unsaturated bonds and the oxidative coupling of arenes, catalyzed by nitrosonium ions. Due to the ability of nitrosonium ions to modify various types of bonds with different modes of action, a variety of applications has been established, which addresses current challenges in organic chemistry research. By considering additional points, such as safety of reagents, by-product formation, employed solvents and energy consumption, several aspects of green and sustainable chemistry can be addressed. Within this review, synthetic applications of nitrosonium ions under transition metal-free reaction conditions are summarized.

An efficient and a simple approach for the synthesis of azo dyes have been developed by the diazo coupling reactions of active aromatic compounds in the presence of in-situ nitrite functionalized graphene oxide polyvinyl alcohol GO-PVA composite as a highly efficient nitrosonium ion source. This methodology was objected to defeat the limitations of the earlier reported method such as use of acids, alkalis and toxic solvents, to reduce the stability of diazonium salts at room temperature, modest yields and long reaction time. Additionally, the attractive advantages of the process are that it incorporated mild conditions with excellent yield, simple product isolation process, fastest reaction time and recyclability of catalyst. The isolated products were characterized by FT-IR spectrum, UV-spectroscopy and 1H-NMR. The as-prepared composites were confirmed by UV-Visible, XRD, XPS, FT-IR spectrum, SEM and TGA analysis.

The formation of electron donor-acceptor complexes is studied with global and local charge transfer partitionings. The 1-parabola model is applied to the bromination reaction of alkenes and the correlations found between the global and local charge transferred with the transition energy of the charge transfer bands and the kinetic rate constants indicate that the nucleophilic attack of alkenes to bromine is the electronic process controlling the reactivity in the formation of the electron donor-acceptor complexes in this reaction. The 2-parabolas model is used in studying the nitrosation of aromatic compounds where colorful electron donor-acceptor complexes are formed. In this case, and like previous applications of the 2-parabolas model, the consistent usage of the model mandates the explicit consideration of reaction conditions in preparing the reactants to have a direction of electron transfer that is consistent with the chemical potential differences. For the nitrosation reaction this implies considering the nitrosonium cation as the charge acceptor. Both applications support that the charge transferred predicted from chemical reactivity models can be used as a scale to measure the nucleophilicity in reactivity trends. Graphical Abstract ᅟ.

The nitrosonium ion-catalyzed dehydrogenative coupling of heteroarenes under mild reaction conditions is reported. The developed method utilizes ambient molecular oxygen as a terminal oxidant, and only water is produced as byproduct. Dehydrogenative coupling of heteroarenes translated into the rapid discovery of novel hedgehog signaling pathway inhibitors, emphasizing the importance of the developed methodology.

Nitrite functionalized star-like poly ionic (NFSPI) compound was synthesized and used as a highly efficient nitrosonium source and catalyst for the conversion of aniline derivatives to diazonium salts. Azo dyes were prepared via in situ azo-coupling reaction of these diazoniums with active aromatic compounds under solvent-free conditions in very short reaction time in excellent yields. NFSPI plays dual role as a three-dimensional nitrosonium source and catalyst because of its poly ionic characteristic. The isolated products were confirmed with FT-IR spectrum, 1H-NMR, 13C-NMR spectroscopy and CHNSO analysis. The structure of heterogeneous reagent and catalyst was confirmed by FT-IR spectrum, SEM images, EDX and CHNSO analysis. Yields and reaction times for the synthesis of a variety of products via this procedure were compared with reported values in literature.

We report a mechanistic DRIFTS in-situ study of NO2, NO + O2 and NO adsorption on a commercial Cu-CHA catalyst for NH3-SCR of NOx. Both pre-reduced and pre-oxidized catalyst samples were investigated with the aim of clarifying mechanistic aspects of the NO oxidation to NO2 as a preliminary step towards the study of the Standard SCR reaction mechanism at low temperatures. Nitrosonium cations (NO+, N formal oxidation state = +3) were identified as key surface intermediates in the process of NO (+2) oxidation to NO2 (+4) and nitrates (+5). While NO+ and nitrates were formed simultaneously upon catalyst exposure to NO2, nitrates evolved consecutively to NO+ when the catalyst was exposed to NO + O2, suggesting that nitrite-like species, and not NO2, are formed as the primary products of the NO oxidative activation over Cu-CHA. Upon catalyst exposure to NO only, i.e. in the absence of gaseous O2, NO+ and then nitrates were formed on a pre-oxidized sample but not on a pre-reduced one, which demonstrates the red-ox nature of the NO oxidation mechanism. The negative effect of H2O on NO+ and nitrates formation was also clearly established. Assuming Cu dimers as the active sites for NO oxidation to NO2, we propose a mechanism which reconciles all the experimental observations. In particular, we show that such a mechanism also explains the observed kinetic effects of H2O, O2 and NO2 on the NO oxidation activity of the investigated Cu zeolite catalyst.

The development of rechargeable Ca-ion batteries as an alternative to Li systems has been limited by the availability of suitable electrolyte salts. We present the synthesis of complexes of Ca(PF6)2 (a key potential Ca battery electrolyte salt) via the treatment of Ca metal with NOPF6, and explore their conversion to species containing PO2F2− under the reaction conditions.

A new ionic liquid, 1-methyl-3-(2-[2-(1-methyl-1H-imidazol-3-ium-3-yl)ethyloxy] ethyl)-1H-imidazol-3-ium dinitrite, was synthesized. This ionic liquid was used as a convenient nitrosonium source in diazotization of aryl amines into their corresponding diazonium salts, which were converted into aryl iodides and aryl azides using potassium iodide or sodium azide, respectively. Various aryl amines possessing electron-withdrawing groups or electron-donating groups were converted into the corresponding aryl iodides and aryl azides in excellent yields. Advantages of this methodology are the use of mild reaction conditions, short reaction times, and avoiding the use of toxic solvents.

Many studies have noted that mixtures of nitrogen oxides (or oxyanions; NOx = NO3–, NO2–, NO–, etc.) and halide (particularly bromine) with or without a transition metal salt (most frequently, copper and iron complexes) are fast and highly selective catalysts for aerobic sulfoxidation and decontamination of many oxidizable toxic compounds. However, the lack of extensive and rigorous kinetic information has kept unequivocal mechanistic inferences on these important catalyst systems to a minimum. Through a series of spectroscopic and kinetics studies on NOx/bromine systems in acetonitrile, the following clarifying technical points have been established: (1) bromine is the reactive species in these oxidations; (2) nitrosonium and hypobromite are not kinetically important, (3) tribromide (Br3–) functions as a stable reservoir for Br2; (4) salts of Br3– and nitrate are stable, storable decontamination catalyst precursors; and (5) most importantly, the multiple complex roles of water (required for catalysts yet...

Efficient transformation of alcohols to the corresponding aldehydes is an important process in organic synthesis and industry. In the absence of transition metal ion, an acid-activated NO x has been explored for the catalytic oxidation of benzyl alcohol. Under the optimal conditions, 95% yield of benzyl aldehyde could be achieved at room temperature with dioxygen as the terminal oxidant. The reaction mechanism was investigated through UV–visible spectrometry and cyclic voltammetry, and the key active species for oxidation have been assigned to the nitrosonium cation.

This work reviews the use of pyrrole, chloroform, methanol, as well as methoxy and nitrosonium groups generated ‘in situ’, as infrared and NMR probe molecules to characterize zeolites basicity. The main results reported in the bibliography about the correlation of the spectroscopic properties of the adsorbed molecule with the framework basicity, the host–guest interactions, and the limitations in the use of these molecules as probes for zeolite basicity are discussed. Special attention is paid to the results reported for the adsorption of pyrrole and halocarbons, most specially CHCl3 and CHClF2 over alkali-exchanged FAU-type zeolites using IR and NMR spectroscopies.

The specific behavior of HY in some steps that possibly occurred in the selective catalytic reduction of NO by hydrocarbon (HC-SCR) was investigated. Experimental results indicated that the activity of HY for NO oxidation to NO 2 was much lower than those of the pentasil zeolites (HZSM-5, HFER and HMOR). In addition, FTIR measurements showed that both nitrosonium ions (NO + ) and nitrate species were remarkably produced at 300 °C over the pentasil zeolites and they were highly active towards hydrocarbons at the temperature. On the contrary, no NO + species could be detected over HY and the nitrate species produced over the zeolite were almost inactive towards reduction at the same reaction condition. It is proposed that the low amount of strong Bronsted acids of HY accounts for the above inferior founctions of the zeolite required by HC-SCR, leading to the low NO reduction activity.

Selective catalytic reduction of NO with acetylene (C2H2-SCR) over mordenite-based catalysts (HMOR, 0.5% Mo/HMOR and NaMOR) was investigated by in situ Fourier transform infrared spectroscopy. A possible mechanism was proposed to explain catalytic performance of the mordenite-based catalysts in the C2H2-SCR: Nitrosonium ions (NO+) and bidentate nitrate are reactive nitric species towards acetylene at 250 °C. Isocyanate species thus formed are then hydrolyzed to acid amide species that are crucial intermediate of the C2H2-SCR. Bridging nitrate species become reactive towards the reductant when reaction temperature increased to 300 °C. Molybdenum loading on HMOR zeolite considerably increased the population of bridging nitrate species and therefore enhanced the title reaction above 300 °C.

An efficient Michael addition of indoles to unsaturated enones, such as chalcones and β-nitrostyrenes, was achieved in the presence of a catalytic amount of nitrosonium tetrafluoroborate in ethyl ether.

We report the synthesis and encapsulation properties of long (up to 5 nm) molecular nanotubes 1-4, which are based on calix[4]arenes and can be filled with multiple nitrosonium (NO + ) ions upon reaction with NO 2 /N 2 O 4 gases. These are among the largest nanoscale molecular containers prepared to date and can entrap up to five guests. The structure and properties of tubular complexes 1·(NO + ) 2 -4·(NO + ) 5 were studied by UV/Vis, FTIR, and 1 H NMR spectroscopy in solution, and also by molecular modeling. Entrapment of NO + in 1·(NO + ) 2 -4·(NO + ) 5 is reversible, and addition of [18]crown-6 quickly recovers starting tubes 1-4. The FTIR and titration data revealed enhanced binding of NO+ in longer tubes, which may be due to cooperativity. The described nanotubes may serve as materials for storing and converting NO x and also offer a promise to further develop supramolecular chemistry of molecular containers. These findings also open wider perspectives towards applications of synthetic nanotubes as alternatives to carbon nanotubes.

The decomposition of S-nitrosothiols (RSNO) in solution under oxidative conditions is significantly faster than can be accounted for by homolysis of the S-N bond. Here we propose a cationic chain mechanism in which nitrosation of nitrosothiol produces a nitrosated cation that, in turn, reacts with a second nitrosothiol to produce nitrosated disulfide and the NO dimer. The nitrosated disulfide acts as a source of nitrosonium for nitrosothiol nitrosation, completing the catalytic cycle. The mechanism accounts for several unexplained facets of nitrosothiol chemistry in solution, including the observation that the decomposition of an RSNO is accelerated by O(2), mixtures of O(2) and NO, and other oxidants, that decomposition is inhibited by thiols and other antioxidants, that decomposition is dependent on sulfur substitution, and that decomposition often shows nonintegral kinetic orders.

The reactivity of a nitrosating agent (N2O3) on oxidations of alcohols and the methyl group of acetophenone were tested. Active electrophylic surface nitrosonium ions (NO+) was detected on H2SO4/SiO2 catalysts by Raman spectroscopy, suggesting a surface ionic mechanism of oxidation in agreement with the one proposed in acid aqueous solutions. Alcohols are selectively oxidized to ketones and aldehydes in high yield, useful for synthetic applications, at 25 °C in 1,2-dichlorethane and using commercial sulfonated styrene divinyl benzene resins (Amberlyst 15®) as catalysts. In addition, nitrous acid ester has been observed as intermediates according to an ionic mechanism by surface NO+. Under the same reaction conditions, acetophenone is selectively oxidized to benzoyl cyanide in high yield and selectivity. The comparison with oxidation carried out in aqueous solution of HNO2, where benzoyl formic acid was obtained, suggests that the differences in the final products are likely due to the specific stabilizing effect of each solvent. Moreover, the reactivity of the intermediates isolated in aqueous systems implies that α-nitroso-acetophenone is a probable reaction intermediate also in aprotic heterogeneous systems.

The use of simple calix[4]arenes for chemical conversion of NO2/N2O4 gases is demonstrated in solution and in the solid state. Upon reacting with these gases, calixarenes 1 encapsulate nitrosonium (NO+) cations within their cavities with the formation of stable calixarene-NO+ complexes 2. These complexes act as encapsulated nitrosating reagents; cavity effects control their reactivity and selectivity. Complexes 2 were effectively used for nitrosation of secondary amides 5, including chiral derivatives. Unique size-shape selectivity was observed, allowing for exclusive nitrosation of less crowded N-Me amides 5 a-e (up to 95 % yields). Bulkier N-Alk (Alk>Me) substrates 5 did not react due to the hindered approach to the encapsulated NO+ reagents. Robust, silica gel based calixarene material 3 was prepared, which reversibly traps NO2/N2O4 with the formation of NO+-storing silica gel 4. With material 4, similar size-shape selectivity was observed for nitrosation. The N-Me-N-nitroso derivatives 6 d,e were obtained with approximately 30 % yields, while bulkier amides were nitrosated with much lower yields (

The acid-promoted disproportionation of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) was studied in order to elucidate the recycling process of the catalytic oxidation of alcohol by nitroxides. Potentiometry, cyclic and hydrodynamic voltammetries with a rotating disk electrode (RDE) have been used to investigate the formation of a nitrosonium cation and a hydroxylamine by disproportionation of nitroxides in acetonitrile. The addition of p -toluenesulfonic acid caused the rest potential of the TEMPO/nitrosonium cation couple to shift in a more positive direction and decreased the peak current in the cyclic voltammograms. The subsequent addition of 2,6-lutidine as a Lewis base made the rest potential and the peak current recover. These phenomena corresponded to the recycling of the catalytic process by TEMPO in a basic aprotic solution.

Correction for ‘Synthesis of Ca(PF6)2, formed via nitrosonium oxidation of calcium’ by Evan N. Keyzer et al., Chem. Commun., 2017, 53, 4573–4576.

Intermolecular electron transfer from various arene donors (ArH) to the nitrosonium acceptor (NO+) proceeds via a series of unusual [1∶1] precursor complexes. Quantitative analysis of the accompanying pair of charge-transfer absorption bands (hνH and hνL) reveals the presence of strong electronic interactions between the donor/acceptor moieties in [ArH,NO+] complexes with the coupling elements lying in the range: Hab = 1.4 ± 0.5 eV. In the context of Sutin's development of Marcus–Hush theory, these dyads represent a Robin–Day Class III system for the 2-step transformation of the reactant diabatic state {ArH + NO+} to the final diabatic state {ArH+˙ + NO˙}, in which the unusually high values of Hab characterize a series of intermolecular (precursor) complexes that lie in a single (potential-energy) minimum. As such, the thermal electron transfer between the arene donor and the nitrosonium acceptor occurs first via the redistribution of a pair of electrons in the formation of the [ArH, NO]+ complex with Xmin = 1.0, and then followed by its homolytic dissociation to the {ArH+˙ and NO˙} products. This conclusion is experimentally confirmed by X-ray crystallographic and IR analyses of the precursor complexes that identify: (i) the strongly perturbed donor/acceptor moieties structurally akin to arene cation radicals (ArH+˙) and the reduced nitric oxide (NO˙), and (ii) the degree of charge transfer (Z) that is complete (100%). Optical electron transfer via the direct photoactivation of the precursor complex to the identical Franck–Condon state {ArH+˙, NO˙}* occurs independent of whether the high-energy (hνH) or the low-energy (hνL) absorption band is irradiated.

An octacationic homo[2]catenane comprised of two mechanically interlocked cyclobis(paraquat-p-phenylene) rings has been obtained from the oxidation of the septacationic monoradical with nitrosonium hexafluoroantimonate. The nanoconfinement of normally repulsive bipyridinium units results in the enforced π-overlap of eight positively charged pyridinium rings in a volume of

NO+ nitrosonium species have been evidenced by in situ FTIR spectroscopy, on H–ZSM-5 and H–Cu–ZSM-5 zeolites under NO+O2 flow (near reaction conditions) at 623 K. Propane introduction in the reaction stream leads to NO+ disappearing and to acrylonitrile detection among reaction products on H–ZSM-5. In the presence of copper, acrylonitrile adsorbed on Cu+ is rapidly decomposed into ethylene and cyanide Cu+CN in these conditions. In a further step, cyanide isomerises into isocyanide Cu+NC. A reaction pathway involving H+ and Cu+ zeolitic sites is proposed for isocyanide species formation from reactants. The transformation of isocyanide species into N2 (already published) is recalled in order to define a complete mechanism for NOx selective catalytic reduction.

Spontaneous formation of colored (1:1) complexes of various aromatic donors (ArH) with the nitrosonium acceptor (NO+) is accompanied by the appearance of two new (charge-transfer) absorption bands in the UV-vis spectrum. IR spectral and X-ray crystallographic analyses of the [ArH,NO+] complexes reveal their inner-sphere character by the ArH/NO+ separation that is substantially less than the van der Waals contact and by the significant enlargement of the aromatic chromophore. The reversible interchange between such an inner-sphere complex [ArH,NO+] and the redox product (ArH+.+ NO.) is quantitatively assessed for the first time to establish it as the critical intermediate in the overall electron-transfer process. Theoretical formulation of the NO+ binding to ArH is examined by LCAO-MO methodology sufficient to allow the unambiguous assignment of the pair of diagnostic (UV-vis) spectral bands. The MO treatment also provides quantitative insight into the high degree of charge-transfer extant in these inner-sphere complexes as a function of the HOMO-LUMO gap for the donor/acceptor pair. The relative stabilization of [ArH,NO+] is traced directly to the variation in the electronic coupling element H(AB), which is found to be substantially larger than the reorganization energy (lambda/2). In Sutin's development of Marcus-Hush theory, this inequality characterizes a completely delocalized Class III complex (which occupies a single potential well) according to the Robin-Day classification. The mechanistic relevance of such an unusual (precursor) complex to the inner-sphere mechanism for organic electron transfer is discussed.

Primary hydroxyl groups were oxidized regioselectively to carboxylic acid using organic nitrosonium salts generated on supported silver catalysts, which promote disproportionation of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) in aqueous solution. The oxidation reactions were performed at pH 9.5 in a batch reactor at room temperture using heterogeneous silver catalysts and peroxides as primary co-oxidants; e.g., 99 mol% selectivity to methyl-α-D-glucopyrasiduronic acid was obtained at 90% conversion of the pyranoside using a silver carbonate–celite catalyst . The efficiency of the system was increased by adding carbonates to the silver catalyst. This result is explained by the increase of the electron charge deficiency on silver in the presence of carbonate, which accelerates the nucleophilic attack of hydroxyls and/or TEMPO. In the case of the Ag–Al2O3 catalyst, this result was proved by temperature-programmed desorption measurements using ammonia. With primary/secondary polyols, the selectivity for the primary hydroxyl groups is high. In addition, primary hydroxyl groups, as in the case of pyranosides, were oxidized more selectively than those of the furanosides. The observed regioselectivity is due to the sterical hindrance caused by the four methyl groups in TEMPO.

The Wheland intermediate in electrophilic aromatic nitrosation, viz. the nitrosoarenium σ-complex, is directly observed by transient absorption spectroscopy. Femtosecond time-resolved laser experiments based on charge-transfer photoexcitation of electron donor/acceptor (EDA) complexes of nitrosonium cation with various arenes reveal the ultrafast formation of nitrosobenzenium to occur in less than 10 ps via the radical/radical coupling of arene cation radicals and nitric oxide. The lifetimes of the σ-complexes in dichloromethane solution are strongly temperature dependentvarying from nanoseconds (T = 298 K) to microseconds (T = 195 K). Steady-state photolysis of arene/NO+ complexes in n-BuCl glasses at T = 77 K leads to nitrosoarenium σ-complexes which persist for several hours. Based on a reaction scheme that includes an ultrafast equilibrium between the [ArH+•,NO•] radical pair and the nitrosoarenium σ-complex, energy diagrams are constructed which establish the highly endergonic reaction profile of ele...

Reactions of N2O4, formed by condensation of NO2 gas, with adsorbed water films as models for ice surfaces were studied on a Au(111) substrate at low temperatures under ultrahigh vacuum (UHV) conditions. Two thermal reaction paths were found, primarily by using infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) techniques. One path evolved gas phase HONO (nitrous acid) and HNO3 (nitric acid) below 150 K, independent of both the crystallinity of the ice film and the exposure of NO2. In contrast, another reaction pathway depended strongly on these variables and was conveniently monitored by the formation of oxygen adatoms on the Au(111) substrate surface. The latter reactions only occurred following multilayer adsorption of NO2 (present as N2O4) and only if the adsorption was on amorphous ice clusters and not crystalline ice. The extent of these reactions was proportional to the concentration of ‘free-OH’ groups on the ice film, indicating that water molecules with only two hydrogen bonds were required to initiate these reactions by strongly hydrating N2O4. Following adsorption of N2O4 multilayers on amorphous ice, we propose that isomerization of O2N–NO2 (D2h–N2O4) to ONO–NO2 (nitrite–N2O4) occurs upon heating to 130–185 K. Further heating to 200–260 K leads to formation of NO+NO−3 (nitrosonium nitrate). This compound is stable on the Au(111) surface up to at least 275 K. Decomposition of NO+NO−3 on the Au(111) surface at 275–400 K evolves gas-phase NO2, and possibly NO (nitric oxide), and also produces oxygen adatoms that recombine and thermally desorb as O2 at about 520 K. These investigations provide new data on reactions of nitrogen oxides and condensed phases of water, which are of general importance for several technologies and improved understanding of atmospheric chemistry. In addition, new details are revealed concerning a novel method for producing oxygen adatoms on Au(111) surfaces under UHV conditions.

Nitrosonium ions, NO+, are formed upon reaction between NO, NO2, and the Bronsted acid sites of H–MOR. It is shown that the NO+ ions are highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can be hydrolysed to yield adsorbed ammonium ions, which are known to be efficient in reducing nitrogen oxides to nitrogen. The dehydration of propenal oxime appears to be the rate determining step in nitrile formation.

Molecular association of various aromatic hydrocarbons (D, including sterically hindered donors) with a representative group of diverse acceptors (A = quinone, trinitrobenzene, tetracyanoethylene, tropylium, tetranitromethane, and nitrosonium) is visually apparent in solution by the spontaneous appearance of distinctive colors. Spectral (UV−vis) analyses of the colored solutions reveal their charge-transfer origin (λCT), and they provide quantitative information of the intermolecular association in the form of the KDA and eCT values for the formation and visualization, respectively, of different [D,A] complexes. Importantly, such measurements establish charge-transfer absorption to be a sensitive analytical tool for evaluating the steric inhibition of donor−acceptor association. For example, the steric differences among various hindered aromatic donors in their association with quinone are readily dramatized in their distinctive charge-transfer (color) absorptions and verified by X-ray crystallography of ...

The transient electron donor-acceptor (EDA) complexes of various olefins (dimethylbutene, etc.) with the nitrosonium acceptor (NO+) show diagnostic charge-transfer absorption bands in dichloromethane solutions at low temperatures. Since the same charge-transfer absorption bands are observed when olefins are exposed to dinitrogen tetraoxide, they are ascribed to analogous EDA complexes [olefin, NO+] NO3 − that are derived from the ionic disproportionation of nitrogen dioxide.

The reduction of nitric oxide by propene in the presence of oxygen over platinum-group metals supported on TiO2, ZnO, ZrO2, and Al2O3 has been investigated by combined diffuse reflectance FT-IR spectroscopy and catalytic activity studies under flow reaction conditions at 523–673 K and atmospheric pressure. The catalytic activity for the selective reduction of nitric oxide and the intensity of the IR bands due to reaction species depended strongly on the nature of the support, type of supported metal, reaction time and temperature. The main surface species detectable by IR were adsorbed hydrocarbons (2900–3080 cm−1), isocyanate (2180, and 2232–2254 cm−1), cyanide (2125 cm−1), nitrosonium (1901 cm−1), CO2 (2343–2357 cm−1), CO (2058 cm−1) and carbonate (1300–1650 cm−1) species. In the case of rhodium containing catalysts, when supported on Al2O3, they exhibited both the highest concentration of surface species and the highest activity for nitric oxide reduction and selectivity to nitrogen. The catalytic activity and the IR intensities of the nitrosonium and isocyanate bands increased with reaction temperature, reached their maximum between 570 and 620 K, and then decreased at higher temperatures. The IR band intensities due to nitrogen containing surface species were found to be strongly correlated to the activity for nitric oxide conversion and only slightly related to the selectivity to dinitrogen.

A Two series of oligothiophenes 2(nT) (n=4,5), annelated with bicyclo[2.2.2]octene (BCO) units at both ends, and quaterthiophenes 3 a-c, annelated with various numbers of BCO units at different positions, were newly synthesized to investigate the driving forces of π-dimerization and the structure-property relationships of the π-dimers of oligothiophene radical cations. Their radical-cation salts were prepared through chemical one-electron oxidation by using nitrosonium hexafluoroantimonate. From variable-temperature electron spin resonance and electronic absorption measurements, the π-dimerization capability was found to vary among the members of the 2(nT)(+)(·)SbF6(-) series and 3(+)(·)SbF6(-) series of compounds. To examine these results, density functional theory (DFT) calculations at the M06-2X/6-31G(d) level were conducted for the π-dimers. This level of theory was found to successfully reproduce the previously reported X-ray structure of (2(3T))2(2+) having a bent π-dimer structure with cis-cis conformations. The absorption bands obtained by time-dependent DFT calculations for the π-dimers were in reasonable agreement with the experimental spectra. The attractive and repulsive forces for the π-dimerization were divided into four factors: 1) SOMO-SOMO interactions, 2) van der Waals forces, 3) solvation, and 4) Coulomb repulsion, and the effects of each factor on the structural differences and chain-length dependence are discussed in detail.

An efficient catalytic method is described for the preparative conversion of hydroquinones to quinones with dioxygen under mild conditions. The use of the gaseous nitrogen oxide (NO x ) catalyst allows a simple workup procedure for the isolation of quinones in essentially quantitative yields by merely removing the low-boiling solvent dichloromethane in vacuo. The mechanism of the catalytic autoxidation of hydroquinones is ascribed to the critical role of nitrosonium (NO + ) in the one-electron oxidation of hydroquinone, followed by the reoxidation of the reduced nitric oxide (NO) with dioxygen. A extensive series of complex interchanges among various NO x species in nitrogen-(V),-(IV), -(III), and -(II) oxidation states, coupled with stepwise oxidation of hydroquinone via a successive series of one-electron/proton transfers, form the critical components of the catalytic cycle

The substitution reaction of methane with the nitrosonium cation, a model electrophile, was investigated computationally at the Hartree-Fock and correlated MP2, MP4SDTQ, and CISD levels of theory, using standard basis sets (6-31G(d), 6-31G(dp), and 6-31+G(dp) for geometry optimizations and TZ2P for energy single points on the most critical structures). The energetically favored reaction course leads to N-protonated nitrosomethane. H[sub 3]/CHNO[sup 4] (6). The initial complex of CH[sub 4] and NO[sup +] in C[sub s] symmetry is bound by -3.7 kcal mol[sup 1] MP4SDTQ/6-31+G(dp)/MP2/6-31+G(dp) + ZPVE/MP2/6-31+G(dp). In the critical step, electrophile NO[sup +] attacks carbon directly, rather than a C-H bond, to yield a pentacoordinate intermediate (3) with a hydrogen unit attached to a H[sub 2]CNO[sup +] cation moiety [[Delta]H[sub O](CISD+Q/TZ2P/MP2/6-31G(dp)+ZPVE//MP 2/6-31G(d p)) = 57.3 kcal mol[sup [minus]1]]. This unusual mode of attack, proceeding through a transition structure which also has three-center two-electron (3c-2e) CHH bonding, can be visualized in two ways. During the reaction, tetrachedral methane distorts to lower symmetry (C[sub s]) and binding between the electrophile and the developing lone pair occurs. The energy required for the methane distortion is partly recovered from the new bonding interaction to the electrophile. An alternative pathway involving the insertion of NO[sup +]more » into a CH bond is less favorable by 14.4 kcal mol[sup [minus]1]. 27 refs., 9 figs., 3 tabs.« less

The novel epoxidation of diadamantylidene 1 and related hindered olefins with NO2 proceeds by way of the cation radical 1+˙ generated during the electron-transfer chain (ETC) catalysis initiated by nitrosonium.