Hydrogen‐Abstraction/Acetylene‐Addition Exposed

Polycyclic aromatic hydrocarbons (PAHs) are omnipresent in the interstellar medium (ISM) and also in carbonaceous meteorites (CM) such as Murchison. However, the basic reaction routes leading to the formation of even the simplest PAH—naphthalene (C₁₀H₈)—via the hydrogen‐abstraction/acetylene‐addition (HACA) mechanism still remain ambiguous. Here, by revealing the uncharted fundamental chemistry of the styrenyl (C₈H₇) and the ortho‐vinylphenyl radicals (C₈H₇)—key transient species of the HACA mechanism—with acetylene (C₂H₂), we provide the first solid experimental evidence on the facile formation of naphthalene in a simulated combustion environment validating the previously postulated HACA mechanism for these two radicals. This study highlights, at the molecular level spanning combustion and astrochemistry, the importance of the HACA mechanism to the formation of the prototype PAH naphthalene.     

Selective Formation of Indene through the Reaction of Benzyl Radicals with Acetylene

The combustion of fossil fuels forms polycyclic aromatic hydrocarbons (PAHs) composed of five‐ and six‐ membered aromatic rings, such as indene (C₉H₈), which are carcinogenic, mutagenic, and deleterious to the environment. Indene, the simplest PAH with single five‐ and six‐membered rings, has been predicted theoretically to be formed through the reaction of benzyl radicals with acetylene. Benzyl radicals are found in significant concentrations in combustion flames, owing to their highly stable aromatic and resonantly stabilized free‐radical character. We provide compelling experimental evidence that indene is synthesized through the reaction of the benzyl radical (C₇H₇) with acetylene (C₂H₂) under combustion‐like conditions at 600 K. The mechanism involves an initial addition step followed by cyclization and aromatization through atomic hydrogen loss. This reaction was found to form the indene isomer exclusively, which, in conjunction with the high concentrations of benzyl and acetylene in combustion environments, indicates that this pathway is the predominant route to synthesize the prototypical five‐ and six‐membered PAH.     

Hydrogen Abstraction/Acetylene Addition Revealed

For almost half a century, polycyclic aromatic hydrocarbons (PAHs) have been proposed to play a key role in the astrochemical evolution of the interstellar medium (ISM) and in the chemistry of combustion systems. However, even the most fundamental reaction mechanism assumed to lead to the simplest PAH naphthalene—the hydrogen abstraction–acetylene addition (HACA) mechanism—has eluded experimental observation. Here, by probing the phenylacetylene (C₈H₆) intermediate together with naphthalene (C₁₀H₈) under combustion‐like conditions by photo‐ionization mass spectrometry, the very first direct experimental evidence for the validity of the HACA mechanism which so far had only been speculated theoretically is reported.     

Theoretical study on the HACA chemistry of naphthalenyl radicals and acetylene: The formation of C12H8, C14H8, and C14H10 species

The hydrogen-abstraction-C₂H₂-addition (HACA) chemistry of naphthalenyl radicals has been studied extensively, but there is a significant discrepancy in product distributions reported or predicted in literature regarding appearance of C₁₄H₈ and C₁₄H₁₀ species. Starting from ab initio calculations, a comprehensive theoretical model describing the HACA chemistry of both 1- and 2-naphthalenyl radicals is generated. Pressure-dependent kinetics are considered in the C₁₂H₉, C₁₄H₉, and C₁₄H₁₁ potential energy surfaces including formally direct well-skipping pathways. On the C₁₂H₉ PES, reaction pathways were found connecting two entry points: 1-naphthalenyl (1-C₁₀H₇) + acetylene (C₂H₂) and 2-C₁₀H₇ + C₂H₂. A significant amount of acenaphthylene is predicted to be formed from 2-C₁₀H₇ + C₂H₂, and the appearance of C₁₄H₈ isomers is predicted in the model simulation, consistent with high-temperature experimental results from Parker et al. At 1500 K, 1-C₁₀H₇ + C₂H₂ mostly generates acenaphthylene through a formally direct pathway, which predicted selectivity of 66% at 30 Torr and 56% at 300 Torr. The reaction of 2-C₁₀H₇ with C₂H₂ at 1500 K yields 2-ethynylnaphthalene as the most dominant product, followed by acenaphthylene mainly generated via isomerization of 2-C₁₀H₇ to 1-C₁₀H₇. Both the 1-C₁₀H₇ and 2-C₁₀H₇ reactions with C₂H₂ form some C₁₄H₈ products, but negligible phenanthrene and anthracene formation is predicted at 1500 K. A rate-of-production analysis reveals that C₁₄H₈ formation is strongly affected by the rates of H-abstraction from acenaphthylene, 1-ethynylnaphthalene, and 2-ethynylnaphthalene, so the kinetics of these reactions are accurately calculated at the high level G3(MP2,CC)//B3LYP/6-311G^** level of theory. At intermediate temperatures like 800 K, acenaphthylene + H are the leading bimolecular products of 1-C₁₀H₇ + C₂H₂, and 1-acenaphthenyl radical is the most abundant C₁₂H₉ isomer due to its stability. The predicted product distribution of 2-C₁₀H₇ + C₂H₂ at 800 K, in contrast to the results of Parker et al is predicted to consist primarily of species containing three fused benzene rings—for example, phenanthrene and anthracene—as the leading products, indicating HACA chemistry is valid from two to three ring polycyclic aromatic hydrocarbons under some conditions. Further experiments are needed for validation.     

The low‐melting compounds 1,4‐diethyl‐, 1,2‐diethyl‐ and ethylbenzene

Crystals of 1,4‐diethyl‐ and 1,2‐diethylbenzene, both C₁₀H₁₄, and ethylbenzene, C₈H₉, have been grown in situ. The molecules of 1,4‐diethyl‐ and 1,2‐diethylbenzene are located about a centre of inversion and across a twofold axis, respectively. In both molecules, the terminal methyl groups are located on opposite sides of the plane of the aromatic ring. In the crystal structures of all three compounds, molecules are linked together by (Ar)C—H...π and CH₂...π contacts. The methyl H atoms do not form close contacts with any of the aromatic π systems.     

Dynamics simulation and reaction pathway analysis of characteristics of soot particles in ethylene oxidation at high temperature

Soot particles characteristics were investigated numerically for high temperature oxidation of C₂H₄/O₂/N₂ (C/O ratio of 2.2) in a closed jet-stirred/plug-flow reactor (JSR/PFR) system. Based on the growth mechanism of polycyclic aromatic hydrocarbons (PAHs), two mechanisms were used to explore the formation pathways of soot precursors and soot. Numerical results were compared with the experimental and reference data. The simulation results show that the value predicted for small molecule intermediates within A1 gives a strong regularity, consistent trend with reference data. However, with the hydrogen-abstraction-carbon-addition (HACA) growth mechanism, the predicted value for beyond-A1 PAH macromolecules and soot volume fraction are smaller than the experimental data. The results also show that the predicted soot volume fraction is in good agreement with experimental data when a combination of the HACA and PAHs condensation (HACA + PAH-PAH) growth mechanisms is used. Analyses of the A1 sensitivity and reaction pathway elucidated that A1 are mainly formed from C₂H₃, C₂H₂, C₃H₃, C₆H₅OH, A1C₂H and A1-. The reaction 2C₃H₃ → A1 is the dominant route of benzene formation. The prediction results and an analysis of the A3 reaction pathway indicate that the growth process from benzene to larger aromatic hydrocarbons (beyond two-ring polycyclic aromatic hydrocarbons [PAHs]) goes by two pathways, i.e., HACA combined with the PAH-PAH radical recombination and addition reaction growth mechanisms.     

Luminescent Di‐ and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis,Characterization, and Application in OLEDs

New bis‐ and tris(iminopyrrole)‐functionalized linear (1,2‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 2 ), 1,3‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 3 ), 1,4‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 4 ), 4,4′‐(HNC₄H₃‐C(H)?N)₂‐(C₆H₄‐C₆H₄) ( 5 ), 1,5‐(HNC₄H₃C‐(H)?N)₂‐C₁₀H₆ ( 6 ), 2,6‐(HNC₄H₃C‐(H)?N)₂‐C₁₀H₆ ( 7 ), 2,6‐(HNC₄H₃C‐(H)?N)₂‐C₁₄H₈ ( 8 )) and star‐shaped (1,3,5‐(HNC₄H₃‐C(H)?N‐1,4‐C₆H₄)₃‐C₆H₃ ( 9 )) π‐conjugated molecules were synthesized by the condensation reactions of 2‐formylpyrrole ( 1 ) with several aromatic di‐ and triamines. The corresponding linear diboron chelate complexes (Ph₂B[1,3‐bis(iminopyrrolyl)‐phenyl]BPh₂ ( 10 ), Ph₂B[1,4‐bis(iminopyrrolyl)‐phenyl]BPh₂ ( 11 ), Ph₂B[4,4′‐bis(iminopyrrolyl)‐biphenyl]BPh₂ ( 12 ), Ph₂B[1,5‐bis(iminopyrrolyl)‐naphthyl]BPh₂ ( 13 ), Ph₂B[2,6‐bis(iminopyrrolyl)‐naphthyl]BPh₂ ( 14 ), Ph₂B[2,6‐bis(iminopyrrolyl)‐anthracenyl]BPh₂ ( 15 )) and the star‐shaped triboron complex ([4′,4′′,4′′′‐tris(iminopyrrolyl)‐1,3,5‐triphenylbenzene](BPh₂)₃ ( 16 )) were obtained in moderate to good yields, by the treatment of 3 – 9 with B(C₆H₅)₃. The ligand precursors are non‐emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π‐conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time‐dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13 , in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples ( 11 , 12 , 14 , and 15 ), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π‐system, to afford green to yellow emissions. Organic light‐emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m^?2, for single layer devices, increasing to 4400 cd m^?2 when a hole‐transporting layer is used.     

A Free‐Radical Prompted Barrierless Gas‐Phase Synthesis of Pentacene

A representative, low‐temperature gas‐phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C₂₂H₁₄) along with its benzo[a]tetracene isomer (C₂₂H₁₄) is unraveled by probing the elementary reaction of the 2‐tetracenyl radical (C₁₈H₁₁^.) with vinylacetylene (C₄H₄). The pathway to pentacene—a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes—is facilitated by a barrierless, vinylacetylene mediated gas‐phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low‐temperature pathway can launch isomer‐selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free‐radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe.     

Cyclization Reactions of Aryl Propargyl Acetates with Tethered Epoxide Induced by Ruthenium Complex

Reactions of the ruthenium complex [Ru]Cl ([Ru]=Cp(PPh₃)₂Ru; Cp=η⁵‐C₅H₅) with several aryl propargyl acetates, each with an ortho ‐substituted chain of various length containing an epoxide on the aromatic ring and with or without methyl substitutents on the epoxide ring, bring about novel cyclizations. The cyclization reactions of HC≡CCH(OAc)(C₆H₄)CH₂(RC₂H₂O) (R=H, 6 a ; R=CH₃, 6 b , where RC₂H₂O is an epoxide ring) in MeOH give the vinylidene complexes 5 a – b , respectively, each with the Cβ integrated into a tetrahydro‐5H ‐benzo[7]annulen‐6‐ol ring. A C−C bond formation takes place between the propargyl acetate and the less substituted carbon of the epoxide ring. Further cyclizations of 5 a – b induced by HBF₄ give the corresponding vinylidene complexes 8 a – b each with a new 8‐oxabicyclo‐[3.2.1]octane ring by removal of a methanol molecule in high yield. For similar aryl propargyl acetates with a shorter epoxide chain, the cyclization gives a mixture of a vinylidene complex with a tetrahydronaphthalen‐1‐ol ring and a carbene complex with a tricyclic indeno‐furan ring. For the cyclization of 18 , with a longer epoxide chain, opening of the epoxide is required to afford the vicinal bromohydrin 22 , then tandem cyclization occurs in one pot. Products are characterized by spectroscopic methods as well as by XRD analysis.     

Intermolecular C—H...O and C—H...X interactions in substituted spiroacenaphthylene structures

In the three spiroacenaphthylene structures 5′′‐[(E)‐2,3‐dichlorobenzylidene]‐7′‐(2,3‐dichlorophenyl)‐1′′‐methyldispiro[acenaphthylene‐1,5′‐pyrrolo[1,2‐c][1,3]thiazole‐6′,3′′‐piperidine]‐2,4′′‐dione, C₃₅H₂₆Cl₄N₂O₂S, (I), 5′′‐[(E)‐4‐fluorobenzylidene]‐7′‐(4‐fluorophenyl)‐1′′‐methyldispiro[acenaphthylene‐1,5′‐pyrrolo[1,2‐c][1,3]thiazole‐6′,3′′‐piperidine]‐2,4′′‐dione, C₃₅H₂₈F₂N₂O₂S, (II), and 5′′‐[(E)‐4‐bromobenzylidene]‐7′‐(4‐bromophenyl)‐1′′‐methyldispiro[acenaphthylene‐1,5′‐pyrrolo[1,2‐c][1,3]thiazole‐6′,3′′‐piperidine]‐2,4′′‐dione, C₃₅H₂₈Br₂N₂O₂S, (III), the substituted aryl groups are 2,3‐dichloro‐, 4‐fluoro‐ and 4‐bromophenyl, respectively. The six‐membered piperidine ring in all three structures adopts a half‐chair conformation, the thiazolidine ring adopts a slightly twisted envelope and the pyrrolidine ring an envelope conformation; in each case, the C atom linking the rings is the flap atom. In all three structures, weak intramolecular C—H...O interactions are present. The crystal packing is stabilized through a number of intermolecular C—H...O and C—H...X interactions, where X = Cl in (I) and F or S in (II), and C—H...O interactions are observed predominantly in (III). In all three structures, molecules are linked through centrosymmetric ring motifs, further tailored through a relay of C—H...X [Cl in (I), Br in (II) and O in (III)] interactions.     

Bergenin monohydrate from the rhizomae of Astilbe chinensis

The title compound, 4‐methoxy‐2‐[(1S,2R,3S,4S,5R)‐3,4,5,6‐tetrahydro‐3,4,5‐tri­hydroxy‐6‐(hydroxy­methyl)‐2H‐­pyran‐2‐yl]‐α‐resorcylic acid δ‐lactone monohydrate, C₁₄H₁₆O₉·H₂O, is a C‐glucoside of 4‐O‐methylgallic acid which has antiasthmatic, antitussive, anti‐inflammatory, antifungal, anti‐HIV and antihepatotoxic activity. The mol­ecule is composed of three six‐membered rings: an aromatic ring, a glucopyran­ose ring and an annellated δ‐lactone ring. The glucopyran­ose ring exhibits only small deviations from an ideal chair conformation. The annellated δ‐lactone ring possesses the expected half‐chair conformation. There is one intra‐ and six intermolecular hydrogen bonds which form an extensive hydrogen‐bonding network within the crystal.     

A novel chelatofore functionalized spiropyran of the 2‐oxaindane series

A novel chelatofore functionalized spiropyran of the 2‐oxaindane series, namely 8‐formyl‐7‐hydroxy‐3′,3′‐dimethylspiro[2H‐chromene‐2,1′(3′H)‐2‐benzofuran], C₁₉H₁₆O₄, is reported. In the crystalline state, dimers are formed as a result of the π–π stacking of aromatic groups of the 2H‐chromene part of the molecule and C—H...O interactions. The C_spiro—O bond length in the pyran ring is 1.4558 (10) Å, which is longer than or equal to the bond length in thermo‐ and photochromic 2‐oxaindane spiropyrans synthesized previously, except for the 7,8‐benzo/6‐NO₂ derivative, in which this bond length is 1.465 (2) Å.     

C—H...π interactions in cocrystals of bis(trimethylsilyl)acetylene and diphenylacetylene with benzene

We present here the crystal structures of two acetylene derivatives cocrystallized with benzene, namely bis(trimethylsilyl)acetylene benzene solvate, C₈H₁₈Si₂·C₆H₆, (I), and diphenylacetylene benzene solvate, C₁₄H₁₀·C₆H₆, (II). In (I), both molecules belong to the symmetry point group C_2h and are located about special positions with site symmetry 2/m. In (II), both molecules show crystallographic inversion symmetry. In both structures, there are C—H...π contacts between aromatic H atoms and the π‐electrons of the triple bond. In addition to these, in (II) there are C—H...π contacts between aromatic H atoms and the π‐electron cloud of the benzene molecules.     

Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

The metalloradical activation of ortho‐benzallylaryl N‐tosyl hydrazones with [Co(TPP)] (TPP=tetraphenylporphyrin) as the catalyst enabled the controlled exploitation of the single‐electron reactivity of the redox non‐innocent carbene intermediate. This method offers a novel route to prepare eight‐membered rings, using base metal catalysis to construct a series of unique dibenzocyclooctenes through selective C_carbene?C_aryl cyclization. The desired eight‐membered‐ring products were obtained in good to excellent yields. A large variety of aromatic substituents are tolerated. The proposed reaction mechanism involves intramolecular hydrogen atom transfer (HAT) to Co^III–carbene radical intermediates followed by dissociation of an ortho‐quinodimethane that undergoes 8π cyclization. The mechanism is supported by DFT calculations, and the presence of radical‐type intermediates was confirmed by trapping experiments.     

Photochemische Reaktionen von Cyclopentadienylbis(ethen)rhodium mit Benzolderivaten

Photochemical Reactions of Cyclopentadienylbis(ethene)rhodium with Benzene Derivatives During UV irradiation of [CpRh(C₂H₄)₂] ( 1 ) (Cp = η⁵‐C₅H₅) in hexane in the presence of hexamethylbenzene the di‐ and trinuclear arene bridged complexes [(CpRh)₂(μ‐η³ : η³‐C₆Me₆)] ( 3 ) and [(CpRh)₃(μ₃‐η² : η² : η²‐C₆Me₆)] ( 4 ) are formed besides known [CpRh(η⁴‐C₆Me₆)] ( 2 ). It was shown by a separate experiment that 3 besides small amounts of 4 is formed by attack of photochemically from 1 arising CpRh fragments at the free double bond of the η⁴‐bonded benzene ring in 2 . Irradiation of 1 in the presence of diphenyl (C₁₂H₁₀) affords the compounds [(CpRh)₂(μ‐η³ : η³‐C₁₂H₁₀)] ( 5 ) and [(CpRh)₃(μ₃‐η² : η² : η²‐C₁₂H₁₀)] ( 6 ) as analogues of 3 and 4 , in the presence of triptycene (C₂₀H₁₄) only [(CpRh)₂(μ‐η³ : η³‐C₂₀H₁₄)] ( 7 ) is obtained; the bridging in 5 , 6 , and 7 always occurs via the same six‐membered ring of the corresponding ligand system. During the photochemical reaction of 1 in the presence of styrene (C₈H₈) substitution of the ethene ligands by the vinyl groups with formation of [CpRh(C₂H₄)(η²‐C₈H₈)] ( 8 ) and known [CpRh(η²‐C₈H₈)₂] ( 9 ) is observed exclusively. The new complexes were characterized analytically and spectroscopically, in the case of 3 also by X‐ray structure analysis.     

[1a(1aα,5β,9aα)]‐1,1a,4,5,7,8,9,9a‐Octahydro‐3‐hydroxy‐1,1,2,5‐tetramethyl‐7‐methylene‐6H‐cyclopropa[3,4]cyclo­hept[1,2‐e]inden‐6‐one

Interest in the title structure, C₂₀H₂₄O₂, lies in the novel cis ring junction between the three‐ and seven‐membered rings. This stereochemical arrangement causes the methylene moiety and the cycloheptane ring to be twisted out of the plane of the aromatic ring. The cyclopropane ring is also twisted out of the plane of the aromatic system. The molecules are linked by an O—H?O hydrogen bond [O?O 2.741 (3) Å].     

The effects of σC—Se–π hyperconjugation in 1‐methoxy‐ and 1‐nitro‐4‐[(phenylselanyl)methyl]benzene

The structures of 1‐methoxy‐4‐[(phenylselanyl)methyl]benzene, C₁₄H₁₄OSe, (1), and 1‐nitro‐4‐[(phenylselanyl)methyl]benzene, C₁₃H₁₁NO₂Se, (2), were determined at 130 K. The two structures, which differ in that (1) contains an electron‐rich aromatic ring and (2) contains an electron‐deficient aromatic ring, both adopt conformations which allow for σ_C—Se–π hyperconjugation. However, although there are significant differences in the ⁷⁷Se chemical shifts for these two compounds, they do not display significantly different H₂C—Se or H₂C—C_ar bond lengths, suggesting that the effects of σ_C—Se–π hyperconjugation in (1) and (2) are not strong enough to be manifested in measurable differences in the structural parameters.     

(n‐Nitrophenyl)acetonitrile,with n = 2, 3 and 4

The molecular structures of the three title nitro‐substituted phenyl­aceto­nitriles, C₈H₆N₂O₂, at 123 K show that the mol­ecules are linked together very differently. In the 2‐ and 4‐nitro compounds, there are both O?H and N_cyano?H interactions, whereas the crystal lattice of the 3‐nitro compound is essentially built up by O?H interactions. The O atoms seem to prefer the aromatic H atoms, while the cyano N atoms prefer the methyl­ene H atoms. The phenyl–nitro torsion angles are ?19.83 (13), ?5.69 (12) and ?2.88 (12)°, while the phenyl–cyano­methyl torsion angles are ?62.27 (12), ?147.99 (9) and ?16.75 (14)° in the 2‐, 3‐ and 4‐NO₂‐substituted compounds, respectively.     

[η6‐1‐Chloro‐2‐(pyrrolidin‐1‐yl)benzene](η5‐cyclopentadienyl)iron(II) hexafluoridophosphate and (η5‐cyclopentadienyl){2‐[η6‐2‐(pyrrolidin‐1‐yl)phenyl]phenol}iron(II) hexafluoridophosphate

In the complex salt [η⁶‐1‐chloro‐2‐(pyrrolidin‐1‐yl)benzene](η⁵‐cyclopentadienyl)iron(II) hexafluoridophosphate, [Fe(C₅H₅)(C₁₀H₁₂ClN)]PF₆, (I), the complexed cyclopentadienyl and benzene rings are almost parallel, with a dihedral angle between their planes of 2.3 (3)°. In a related complex salt, (η⁵‐cyclopentadienyl){2‐[η⁶‐2‐(pyrrolidin‐1‐yl)phenyl]phenol}iron(II) hexafluoridophosphate, [Fe(C₅H₅)(C₁₆H₁₇NO)]PF₆, (II), the analogous angle is 5.4 (1)°. In both complexes, the aromatic C atom bound to the pyrrolidine N atom is located out of the plane defined by the remaining five ring C atoms. The dihedral angles between the plane of these five ring atoms and a plane defined by the N‐bound aromatic C atom and two neighboring C atoms are 9.7 (8) and 5.6 (2)° for (I) and (II), respectively.     

The 1:1 proton‐transfer compounds of 4‐(phenyldiazenyl)aniline (aniline yellow) with 3‐nitrophthalic, 4‐nitrophthalic and 5‐nitroisophthalic acids

The structures of the anhydrous 1:1 proton‐transfer compounds of the dye precursor aniline yellow [4‐(phenyldiazenyl)aniline], namely isomeric 4‐(phenyldiazenyl)anilinium 2‐carboxy‐6‐nitrobenzoate, C₁₂H₁₂N₃⁺·C₈H₄NO₆⁻, (I), and 4‐(phenyldiazenyl)anilinium 2‐carboxy‐4‐nitrobenzoate, C₁₂H₁₂N₃⁺·C₈H₄NO₆⁻, (II), and 4‐(phenyldiazenyl)anilinium 3‐carboxy‐5‐nitrobenzoate monohydrate, C₁₂H₁₂N₃⁺·C₈H₄NO₆⁻·H₂O, (III), have been determined at 130 K. In (I) the cation has longitudinal rotational disorder. All three compounds have substructures comprising backbones formed through strong head‐to‐tail carboxyl–carboxylate hydrogen‐bond interactions [graph set C(7) in (I) and (II), and C(8) in (III)]. Two‐dimensional sheet structures are formed in all three compounds by the incorporation of the 4‐(phenyldiazenyl)anilinium cations into the substructures, including, in the cases of (I) and (II), infinite H—N—H to carboxylate O—C—O group interactions [graph set C(6)], and in the case of (III), bridging through the water molecule of solvation. The peripheral alternating aromatic ring residues of both cations and anions give only weakly π‐interactive step features which lie between the sheets.