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Polycyclic aromatic hydrocarbons (PAHs) are widely recognized as catalysts for interstellar H2 formation. Extensive exploration into the catalytic potential of various PAHs has encompassed both theoretical investigations and experimental studies. In the present study, we focused on studying the reactivity of an imperfect aromatic molecule, indene (C9H8), and its hydrogenated counterpart, indane (C9H10), as potential catalysts for H2 formation within the interstellar medium. The reactions of these molecules with H atoms at 3.1 K were investigated experimentally using the para-H2 matrix isolation technique. Our experimental results demonstrate that both indene and indane are reactive toward H atoms. Indene can participate in H-atom-abstraction and H-atom-addition reactions, whereas indane primarily undergoes H-atom-abstraction reactions. The H-atom-abstraction reaction of indene results in the formation of the 1-indenyl radical (R1) (C9H7) and H2 molecule. Simultaneously, an H-atom-addition reaction forms the 1,2-dihydro-indene-3-yl radical (R2) (C9H9). Experiments also reveal that the H-atom-abstraction reaction of indane also produces the R2 radical. To the best of our knowledge, this study represents the first reporting of the infrared spectra of R1 and R2 radicals. The experimental results, combined with theoretical findings, suggest that indane and indene may play a role in the catalytic formation of interstellar H2. Furthermore, these results imply a quasi-equilibrium between the investigated molecules and the formed radicals via H-atom-addition and H-atom-abstraction reactions. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are widely recognized as catalysts for interstellar H2 formation. Extensive exploration into the catalytic potential of various PAHs has encompassed both theoretical investigations and experimental studies. In the present study, we focused on studying the reactivity of an imperfect aromatic molecule, indene (C9H8), and its hydrogenated counterpart, indane (C9H10), as potential catalysts for H2 formation within the interstellar medium. The reactions of these molecules with H atoms at 3.1 K were investigated experimentally using the para-H2 matrix isolation technique. Our experimental results demonstrate that both indene and indane are reactive toward H atoms. Indene can participate in H-atom-abstraction and H-atom-addition reactions, whereas indane primarily undergoes H-atom-abstraction reactions. The H-atom-abstraction reaction of indene results in the formation of the 1-indenyl radical (R1) (C9H7) and H2 molecule. Simultaneously, an H-atom-addition reaction forms the 1,2-dihydro-indene-3-yl radical (R2) (C9H9). Experiments also reveal that the H-atom-abstraction reaction of indane also produces the R2 radical. To the best of our knowledge, this study represents the first reporting of the infrared spectra of R1 and R2 radicals. The experimental results, combined with theoretical findings, suggest that indane and indene may play a role in the catalytic formation of interstellar H2. Furthermore, these results imply a quasi-equilibrium between the investigated molecules and the formed radicals via H-atom-addition and H-atom-abstraction reactions. [ABSTRACT FROM AUTHOR]

Realization of planar tetracoordinate arrangements of nitrogen atoms is challenging because their preference for localized bonding (caused by its high electronegativity) makes them typically tricoordinate. This is especially true for the more electronegative oxygen atoms. Herein, we computationally designed two clusters NBe4H4− and OBe4H4; they contain a planar tetracoordinate nitrogen (ptN) and planar tetracoordinate oxygen (ptO) atom, respectively. Remarkably, the former is a dynamically stable global minimum, while the latter is not. The bonding analysis proves that planar tetracoordination in NBe4H4− favors over tricoordination because of the presence of multicenter delocalized bonds. In contrast, the planar tricoordination dominates due to its weak delocalized bonding ability of oxygen in the OBe4H4 cluster. Moreover, the 6σ/2π double aromaticity due to multicenter delocalized bonds allows the NBe4H4− cluster to obtain additional stability. This cluster is a promising synthetic due its dynamic and thermodynamic stability. [ABSTRACT FROM AUTHOR]

Comprehensive Summary: Carbonylation reactions are a valuable synthetic method to construct carbonyl compounds and carbonylation reactions of aryl halides stand out as a highly significant tool for generating carbonyl substituted arenes. However, the important reactions have never been realized in aromatic metallacycles. Herein, we present the first carbonylation reactions of metallaaromatics, specifically alkoxycarbonylation and aminocarbonylation reactions of an osmapentalyne. During the carbonylation process, the electronic and steric properties of nucleophiles are regarded as critical factors. The alcohols with bulky substituents (isopropanol) require more reaction time and tert‐butyl alcohol is inert in the reaction. Comparatively, amines, being stronger nucleophiles, exhibit divergent behaviors. Bulky amines undergo aminocarbonylation, whereas small amines prefer direct nucleophilic additions. Control experiments revealed that the intermediate derived from coupling of metal carbyne with CO plays a significant role in the carbonylation reaction. According to these observations, a divergent pathway for the reaction is proposed. Furthermore, the photophysical properties of these carbonyl‐functionalized osmapentalene complexes are studied, and the maximum absorption peak of compound with a carboxylic group exhibits a significant red‐shift due to the smaller HOMO‐LUMO gap. These findings contribute to expanding the reactivity of metallaaromatics and offer new opportunities for the synthesis of carbonyl‐functionalized metallacycles. [ABSTRACT FROM AUTHOR]

The possible aromatic conjugation between 3D and 2D aromatic units is in the focus since the synthesis of benzocarborane. It has been showed that in the 3D aromatic icosahedral 1,2‐dicarba‐closo‐dodecaborane systems fused with 2D aromatic rings a global 3D/2D aromaticity does not exist. Despite this fact during the last years several studies proposed interactions between 2D and 3D moieties. Herein, while tuning the size and the effective charge of the (car)borane systems, we demonstrate that global aromatic character can be excluded in any investigated cases, and the detectable conjugative properties can be explained the effect of the well‐known negative hyperconjugation. [ABSTRACT FROM AUTHOR]

Doping of polycyclic conjugated hydrocarbons (PCHs) with sulfur atoms is becoming more and more important as a means of creating unique functional materials. Recently, thiophene‐containing multiple helicenes have garnered enormous attention due to their intriguing electronic and (chir)optical properties compared with carbohelicenes. However, the efficient synthesis of thiopyran‐containing multiple helicenes and the underlying sulfur doping mechanisms are rather unexplored. Herein, the synthesis and structural analysis of a thiopyran‐containing double [7]helicene 3 are reported. X‐ray crystallographic analysis reveals 3 and its dication with C2‐symmetric propeller‐shape structures and compact interactions in the solid state. 3 exhibits deep‐red to near‐infrared (NIR) fluorescence emission. Tunable aromaticity of the central benzene ring and thiopyran rings is found by chemical oxidation, which is further confirmed by nucleus‐independent chemical shift (NICS), anisotropy of the induced current density (ACID) and harmonic oscillator model of aromaticity (HOMA) analysis. Furthermore, the chiral and photosensitizing characters of 3 are investigated. The excellent deep‐red to NIR fluorescence, circularly polarized luminescence (CPL) and photosensitizing activities suggest that 3 can be used as an outstanding photosensitizer in photodynamic therapy (PDT) and bioimaging, especially paving the way for future CPL‐PDT and CPL‐bio‐probe applications. [ABSTRACT FROM AUTHOR]

Cyclic nitrenium ions containing five‐membered and six‐membered rings are available, however, the seven‐membered cyclic nitrenium ions (azepinium ions) are rare. The chemistry of these species is related to their stability originating from the aromaticity due to 6π electrons. Very few theoretical and experimental studies have been conducted on the azepinium ions. Related clozapine and olanzapine cations (diazepinium ions) were observed during drug metabolism studies. In this work, quantum chemical analysis has been carried out to estimate the stability, aromaticity, and electrophilicity of several derivatives of azepinium ions. A few of the designed azepinium ions carry ΔES‐T values in the range of 50 kcal/mol favoring singlet state; π donating groups at the 2nd position increase the singlet‐triplet energy differences. Most of the substituents reduce the NICS(1) values compared to the parent system. Ring fusion with heterocyclic five‐membered rings generally increases the aromaticity and the stability of the azepinium ion ring systems. The electrophilicity parameters estimated in terms of HIA, FIA, and ω values indicate that it is possible to fine‐tune the chemical properties of azepinium ions with appropriate modulation. [ABSTRACT FROM AUTHOR]

The dearomatization of aromatic substrates via photocatalytic methodologies has gained escalating prominence to access molecular complexity. Although much focus has been put on non‐active arenes, naphthols and N‐based heteroaromatics, arylamines have so far remained in the background. Herein, we report a photocatalytic strategy for the aerobic, dearomative hydroxylation of naphthylamines under visible‐light irradiation. Our investigation highlights singlet oxygen generation as a viable mechanism for aromaticity disruption, mitigating selectivity issues deriving from the direct oxidation of targeted arylamines. Finally, we showcased the synthetic value of the obtained dearomatized products in a late‐stage diversification campaign. [ABSTRACT FROM AUTHOR]

Non‐benzenoid PAHs with pentagon/heptagon pairs have unique electronic structures and promising opto‐electronic properties. However, the construction of pentagon/heptagon pair units in these PAHs is challenging. Herein, we developed a Pd‐catalyzed (3+2) alkyne annulation reaction of heptagons, and constructed heptagon/pentagon pairs. The substrate scope is wide and the yield is good to moderate. A series of dibenzoauzlene derivatives were synthesized and their opto‐electronic properties were systematically studied by UV‐vis absorption and cyclic voltammetry. In addition, the dibenzoazulene derivative was further transformed to tropylium cation and tropyl radical, which showed bathochromic shifted absorption and switched aromaticity. [ABSTRACT FROM AUTHOR]

All‐benzenoid polycyclic aromatic hydrocarbons or macrocycles usually display localized aromaticity. On the other hand, incorporation of quinoidal units into the skeleton could lead to effective electron delocalization and global (anti)aromaticity. In this work, fully π‐conjugated macrocycle 1 and bismacrocycle 2 containing both para‐quinodimethane and triphenylamine units are efficiently synthesized mainly through intermolecular Friedel–Crafts alkylation reaction. They can be considered as a tetraazasuperbenzene and a hexaazasupernaphthalene, respectively, due to their similar geometry and electronic structures to the benzene and naphthalene. X‐ray crystallographic analyses reveal a largely planar geometry for both 1 and 2 and variable‐temperature NMR measurements disclose slow dynamic processes owing to restricted ring flipping of the phenyl rings. 1 and 2 can be easily oxidized into higher‐oxidation‐state species. NMR and theoretical calculations indicate that 12+ and 14+ show global anti‐aromaticity and aromaticity, respectively, with a dominant 32π and 30π conjugation pathway, while for the bismacrocycle 2, its dication 22+, tetracation 24+ and hexacation 26+ exhibit global aromaticity, antiaromaticity, and aromaticity with a 54π, 52π and 50π conjugation pathway along the outermost backbone, respectively. [ABSTRACT FROM AUTHOR]

Planar PV- or PIII-bridged bis(NHCs), which have only been employed in transition metal complex chemistry so far, were subjected to BCl3-containing solutions targeting the corresponding bis(NHC) BCl3 adducts. While the P(O)NEt2-bridged bis(NHC) showed the expected adduct formation, the PNEt2-bridged bis(NHC) reacted not only at the carbene moiety but also at the P-NEt2 functional group. The latter enabled access to the first 1,4-diphosphinine bis(NHC) main group adduct; its formation and properties were investigated by DFT calculations. Through the same route, a 1,4-diphosphinine bis(imidazolium) scaffold was generated and explored theoretically and experimentally. The new 1,4-diphosphinines are shown to possess high global aromaticity and a unique P-centred reactivity, allowing the formation of hitherto inaccessible [4 + 2]-cycloaddition products, thus suggesting potential new applications compared to previously known 1,4-diphosphinine derivatives. [ABSTRACT FROM AUTHOR]

The intrinsically disordered carboxy-terminal domain (CTD) of the largest subunit of RNA Polymerase II (RNAPII) consists of multiple tandem repeats of the consensus heptapeptide Y1-S2-P3-T4-S5-P6-S7. The CTD promotes liquid-liquid phase-separation (LLPS) of RNAPII in vivo. However, understanding the role of the conserved heptad residues in LLPS is hampered by the lack of direct biochemical characterization of the CTD. Here, we generated a systematic array of CTD variants to unravel the sequence-encoded molecular grammar underlying the LLPS of the human CTD. Using in vitro experiments and molecular dynamics simulations, we report that the aromaticity of tyrosine and cis-trans isomerization of prolines govern CTD phase-separation. The cis conformation of prolines and β-turns in the SPXX motif contribute to a more compact CTD ensemble, enhancing interactions among CTD residues. We further demonstrate that prolines and tyrosine in the CTD consensus sequence are required for phosphorylation by Cyclin-dependent kinase 7 (CDK7). Under phase-separation conditions, CDK7 associates with the surface of the CTD droplets, drastically accelerating phosphorylation and promoting the release of hyperphosphorylated CTD from the droplets. Our results highlight the importance of conformationally restricted local structures within spacer regions, separating uniformly spaced tyrosine stickers of the CTD heptads, which are required for CTD phase-separation. The authors explore sequence-encoded molecular grammar underlying the liquid liquid phase-separation of the human RNA polymerase II C-terminal domain and demonstrate that phase-separation appears to accelerate on phosphorylation by CDK7. [ABSTRACT FROM AUTHOR]

The current study investigates the influence of several R substituents (e.g., Me, SiH3, F, Cl, Br, OH, NH2, etc.) on the aromaticity of borazine, also known as the "inorganic benzene". By performing hybrid DFT methods, blended with several computational techniques, e.g., Natural Bond Orbital (NBO), Quantum Theory of Atoms in Molecules (QTAIM), Gauge-Including Magnetically Induced Current (GIMIC), Nucleus-Independent Chemical Shift (NICS), and following a simultaneous evaluation of four different aromaticity indices (para-delocalization index (PDI), multi-centre bond order (MCBO), ring current strength (RCS), and NICS parameters), it is emphasized that the aromatic character of B-substituted (B3R3N3H3) and N-substituted (B3H3N3R3) borazine derivatives can be tailored by modulating the electronic effects of R groups. It is also highlighted that the position of R substituents on the ring structure is crucial in tuning the aromaticity. Systematic comparisons of calculated aromaticity index values (i.e., via regression analyses and correlation matrices) ensure that the reported trends in aromaticity variation are accurately described, while the influence of different R groups on electron delocalization and related aromaticity phenomena is quantitatively assessed based on NBO analyses. The most relevant interactions impacting the aromatic character of investigated systems are (i) the electron conjugations occurring between the p lone pair electrons (LP) on the F, Cl, Br, O or N atoms, of R groups, and the π*(B=N) orbitals on the borazine ring (i.e., LP(R)→π*(B=N) donations), and (ii) the steric-exchange (Pauli) interactions between the same LP and the π(B=N) bonds (i.e., LP(R)↔π(B=N) repulsions), while inductive/field effects influence the aromaticity of the investigated trisubstituted borazine systems to a much lesser extent. This work highlights that although the aromatic character of borazine can be enhanced by grafting electron-donor substituents (F, OH, NH2, O−, NH−) on the N atoms, the stabilization due to aromaticity has only a moderate impact on these systems. By replacing the H substituents on the B atoms with similar R groups, the aromatic character of borazine is decreased due to strong exocyclic LP(R)→π*(B=N) donations affecting the delocalization of π-electrons on the borazine ring. [ABSTRACT FROM AUTHOR]

Purine stretches, sequences of adenine (A) and guanine (G) in DNA, play critical roles in binding regulatory protein factors and influence gene expression by affecting DNA folding. This study investigates the relationship between purine stretches and cancer development, considering the aromaticity of purines, quantified by methods like Hückel's rule and NICS calculations, and the importance of the flanking sequence context. A pronounced avoidance of long purine stretches by typical cancer mutations was observed in public data on the intergenic regions of cancer patients, suggesting a role of intergenic sequences in chromatin reorganization and gene regulation. A statistically significant shortening of purine stretches in cancerous tumors (p value < 0.0001) was found. The insights into the aromatic nature of purines and their stacking energies explain the role of purine stretches in DNA structure, contributing to their role in cancer progression. This research lays the groundwork for understanding the nature of purine stretches, emphasizing their importance in gene regulation and chromatin restructuring, and offers potential avenues for novel cancer therapies and insights into cancer etiology. [ABSTRACT FROM AUTHOR]

Quantifying the relative influence of factors and processes controlling riverine ecosystem function is essential to predicting future conditions under global change. Dissolved organic matter (DOM) is a fundamental component of riverine ecosystems that fuels microbial food webs, influences nutrient and light availability, and represents a significant carbon flux globally. The heterogeneous nature of DOM molecular composition and its propensity for interaction (i.e., functional diversity) can characterize riverine ecosystem function across spatiotemporal scales. To investigate fundamental drivers of DOM diversity, we collected seasonal water samples from 42 nested locations within five watersheds spanning multiple watershed sizes (~ 5 to 30,000 km2) across the United States. Patterns in DOM molecular richness, aromaticity, relative abundance of N-containing formulas, and putative biochemical transformations derived from high-resolution mass spectrometry were assessed across gradients of explanatory variables associated with watershed characteristics (e.g., watershed area, water residence time, land cover). We found that putative biochemical transformations were more strongly related to explanatory variables across watersheds than common bulk DOM parameters and that watershed area, surface water residence time and derived Damköhler numbers representing DOM reactivity timescales were strong predictors of DOM diversity. The data also indicate that catchment-specific land cover factors can significantly influence DOM diversity in diverging directions. Overall, the results highlight the importance of considering water residence time and land cover when interpreting longitudinal patterns in DOM chemistry and the continued challenge of identifying generalizable drivers that are transferable across watershed and regional scales for application in Earth system models. This work also introduces a Findable Accessible Interoperable Reusable (FAIR) dataset (> 300 samples) to the community for future syntheses. [ABSTRACT FROM AUTHOR]

The problem of whether and how the captodative aromatic systems with the donor and acceptor substituents at the same carbon atom of the C=C bond can be more stable than the π‐conjugated push‐pull counterparts with the two substituents in the vicinal position is analyzed for cyclopentadienyl derivatives possessing an aromatic cyclopentadienyl ring. The analysis of electronic, magnetic, and structural criteria of aromaticity showed that among typical organic donors, such as amines NR3, ethers R2O, guanidine (NH2)2C=NH, siloxane O(SiH3)2, silatrane, the captodative isomers [cyclopentadienylium]−C(X+) = CH2 can be more stable than their push‐pull isomeric counterparts [cyclopentadienylium]−—CH=CH—X+. The largest energy difference in favor of the former was found to be ca. 13 kcal/mol for X = NH=C(NH2)2. [ABSTRACT FROM AUTHOR]

The free base form of doubly meso‐α linked N‐confused porphyrin (NCP) tape 3 was successfully synthesized via Ir(I) mediated intramolecular coupling. The following silver complexation afforded the Ag(III) complex of doubly meso‐α linked NCP tape 4. While 3 exhibited 38π aromatic characters, 4 exhibited not only 18π aromatic NCP‐type characteristics but also a decent antiaromatic contribution of 12π pyrrolo[2,3‐f]indole segment, as probed by NMR spectra, absorption spectra, and DFT calculations. [ABSTRACT FROM AUTHOR]

Sulfamethoxazole (SMX) is an antibiotic that is extensively used in veterinary medicine, and its occurrence in wastewater and surface water can reach up to 20 μg/L. SMX is categorized as a pollutant of emerging concern by the US EPA due to its persistence and effects on humans and the environment. In this study, photo-Fenton technology is proposed for the removal of SMX. Aqueous solutions of SMX (50.0 mg/L) are treated in a 150 W UV photoreactor, using [Fe2+]0 = 0.5 mg/L and varying [H2O2]0 = 0–3.0 mM. During the reaction, colour (AU) was assessed along with SMX (mg/L), turbidity (NTU), and TC (mg/L). SMX degrades to aromatic intermediates with chromophoric groups, exhibiting colour (yellow to brown) and turbidity. As these intermediates are mineralized into CO2 and H2O, the colour and turbidity of the water lose intensity. Using a molar ratio of 1 mol SMX:10 mol H2O2, the maximum degradation of aromatic species takes place (71% elimination), and colourless water with turbidity < 1 NTU is obtained. A kinetic modelling for aromaticity loss and colour formation as a function of the oxidant concentration has been proposed. The application of this model allows the estimation of oxidant amounts for an efficient removal of SMX under environmentally friendly conditions. [ABSTRACT FROM AUTHOR]

The on‐surface synthesis of an isomer of undecacene, bearing two four‐membered rings and two para‐quinodimethane moieties, starting from a tetramethyl‐substituted diepoxy precursor, is presented. The transformation implies a thermal double deoxygenation followed by a stepwise double dehydrogenation reaction on the Au(111) surface, locally induced by inelastic tunneling electrons. This results in the transformation of para‐dimethylbenzene moieties into non‐aromatic para‐quinodimethanes. The structures and electronic properties of the intermediate and final products are investigated at the single molecule level with high spatial resolution, using both scanning tunneling microscopy/spectroscopy and non‐contact atomic force microscopy. The experimental results are supported by density functional theory calculations. [ABSTRACT FROM AUTHOR]

Urban beaver and stormwater ponds provide hydrologic retention in the landscape while collecting dissolved organic matter (DOM)-rich runoff that can promote primary productivity. Our objective was to determine how the quantity, source, and bioavailability of DOM changed across urban stormwater and beaver pond systems, then compare the two pond types to each other. We measured dissolved organic carbon (DOC) and specific ultraviolet absorbance at 254 nm (SUVA254) from upstream, within, and downstream of seven ponds in Atlanta, GA, USA, biweekly from March to December 2021. Additionally, we completed 28-day laboratory microcosm incubations of pond in- and out-flow during summer and autumn of 2021. We found higher concentrations of DOC in the pond and outflows of stormwater ponds, whereas beaver ponds did not cause any change. Effects of pond type (beaver vs. stormwater) were greater than other controls on concentration, including flow and season. In contrast, SUVA254 showed a shift toward more aromatic carbon below both systems without a clear difference between pond types. Beaver and stormwater pond outflows had similar ranges of DOM bioavailability in summer, but during autumn bioavailability at both sites declined to near zero. Overall, we found that stormwater ponds and beaver ponds had similar impacts on aromaticity and bioavailability, however stormwater ponds increased the quantity of DOC while beaver ponds did not. This suggests that in addition to increasing hydrologic residence times in urbanized systems, urban beaver ponds may limit the export of bioavailable carbon and reduce microbial processing downstream. [ABSTRACT FROM AUTHOR]

Electrophilic activation of the aromatic cyclo‐P5 ligand in [Cp*Fe(η5‐P5)] is demonstrated to drastically enhance its reactivity towards weak nucleophiles. Unprecedented functionalized, contracted as well as complexly aggregated polyphosphorus compounds are accessed utilizing [Cp*Fe(η5‐P5Me)][OTf] (A), highlighting the great potential of this underexplored mode of reactivity. Addition of carbenes to A affords novel 1,2‐ or 1,1‐difunctionalized cyclo‐P5 complexes [Cp*Fe(η4‐P5(1‐L)(2‐Me)][OTf] (L=IDipp (1), EtCAAC (2), IiPr (3 b)) and [Cp*Fe(η4‐P5(1‐IiPr)(1‐Me)][OTf] (3 a). For the first time, the much smaller IMe4 leads to the contraction of the cyclo‐P5 ligand and formation of [Cp*Fe(η4‐P4(1‐IMe)(4‐Me)] (4). DFT calculations shed light on the delicate mechanism of this type of reaction, which is reinforced by the experimental identification of key intermediates. Even the comparably weak nucleophile IDippCH2 reacts with A to form [Cp*Fe(η4‐P5(1‐IDippCH2)(1/2‐Me)][OTf] (6 a/b), highlighting its explicitly more reactive nature. Moreover, exposure of A to IDippEH (E=N, P) leads to a unique aggregation reaction affording [{Cp*Fe}2{μ2,η4:3:1–P10Me2(IDippN)}][OTf] (8) and [{Cp*Fe}2{μ2,η4:1:1:1–P11Me2(IDipp)}][OTf] (9), respectively. [ABSTRACT FROM AUTHOR]

Carborazine (B2C2N2H6) and borazine (B3N3H6) are isoelectronic analogues of benzene (C6H6). The aromatic character of borazine have basically reached a consensus after a long period of controversy, but the related properties of carborazine are even rarely reported. In this work, we systematically investigated the geometric structure, charge distribution, frontier molecular orbital characteristics, bonding, electronic delocalization, magnetic shielding effect, and induced ring current of carborazine and borazine, and compared the studied characteristics with those of benzene to determine the aromatic character of the two analogues. The combination of multiple properties shows that although they are isoelectronic, carborazine is evidently aromatic, while borazine only exhibits rather weak aromaticity. The C atom acting as a connecting bridge between B and N atoms in carborazine reduces the electronegativity difference on the molecular backbone and enhances the electronic delocalization over the conjugated path, which is the essence of the distinct disparity of aromaticity between carborazine and borazine. [ABSTRACT FROM AUTHOR]

The ubiquitous chemistry of benzene led us to explore ways to stabilise analogous borozene, by capping them with appropriate groups. The mismatch in overlap of ring‐cap fragment molecular orbitals in [(HB)2B6H6]2− is overcome by replacing the two BH caps with higher congeners of boron. We calculated the relative energies of all the polyhedral structural candidates for [(HE)2B6H6]2− (E=Al−Tl) and found hexagonal bipyramid (HBP) to be more stable with Al−H caps. A global minimum search also gives HBP as the most stable structure for [Al2B6H8]2−. The capped B6H6 ring in [(HAl)2B6H6]2− has aromaticity comparable to that of benzene. [ABSTRACT FROM AUTHOR]

Molecules with a π-system that can be mapped onto a Möbius strip may display Möbius aromaticity. Such molecules are difficult to synthesize because they have a twisted structure. Recently, we combined chiral [6]helicene and fluorescent [7]cycloparaphenylene, and synthesized the first helicene para -phenylene ([6,7]HPP) carbon nanohoop. We have demonstrated that this design strategy ultimately provides a Möbius topology of the molecular π-electron system and, therefore, offers the potential to study Möbius aromaticity experimentally. In addition, the synthesized nanohoop exists as a mixture of conformers in solution. Some of the conformers possess a different orientability of their π-systems, i.e., they differ in their topology. As a result, the recorded circularly polarized luminescence of isolated enantiomers displays both left- and right-handedness of the emitted light, each emanating from a conformer with a different π-system topology. Therefore, [6,7]HPP provided the first experimental evidence of such topological bistability in carbon nanohoops. [ABSTRACT FROM AUTHOR]

Spectroscopic properties are commonly used in the experimental evaluation of ground‐ and excited‐state aromaticity in expanded porphyrins. Herein, we investigate if the defining photophysical properties still hold for a diverse set of hexaphyrins with varying redox states, topologies, peripheral substitutions, and core‐modifications. By combining TD‐DFT calculations with several aromaticity descriptors and chemical compound space maps, the intricate interplay between structural planarity, aromaticity, and absorption spectra is elucidated. Our results emphasize that the general assumption that antiaromatic porphyrinoids exhibit significantly attenuated absorption bands as compared to aromatic counterparts does not hold even for the unsubstituted hexaphyrin macrocycles. To connect the spectroscopic properties to the hexaphyrins' aromaticity behaviour, we analyzed chemical compound space maps defined by the various aromaticity indices. The intensity of the Q‐band is not well described by the macrocyclic aromaticity. Instead, the degeneracy of the frontier molecular orbitals, the HOMO–LUMO gap, and the |ΔHOMO–ΔLUMO|2 values appear to be better indicators to identify hexaphyrins with enhanced light‐absorbing abilities in the near‐infrared region. Regions with highly planar hexaphyrin structures, both aromatic and antiaromatic, are characterized by an intense B‐band. Hence, we advise using a combination of global and local aromaticity descriptors rooted in different criteria to assess the aromaticity of expanded porphyrins instead of solely relying on the absorption spectra. [ABSTRACT FROM AUTHOR]

The design of boron‐based molecular rotors stems from boron‐carbon binary clusters containing multiple planar hypercoordinate carbons (phCs, such as C2B8). However, the design of boron‐coordinated phCs is challenging due to boron's tendency to occupy hypercoordinate centers more than carbon. Although this challenge has been addressed, the designed clusters of interest have not exhibited dynamic fluxionality similar to that of the initial C2B8. To address this issue, we report a σ/π doubly aromatic CB2H5+ cluster, the first global minimum containing a boron‐coordinated planar tetracoordinate carbon atom with dynamic fluxionality. Dynamics simulations show that two ligand H atoms exhibit alternate rotation, resulting in an intriguing dynamic fluxionality in this cluster. Electronic structure analysis reveals the flexible bonding positions of the ligand H atoms because they do not participate in π delocalized bonding nor bond to any other non‐carbon atom, highlighting this rotational fluxionality. Unprecedentedly, the fluxional process involves not only the usual conversion of the number of bonding atoms, but also the type of bonding (3c π bonds ↔4c σ bonds), which is an uncommon fluxional mechanism. The cluster represents an effort to apply phC species to molecular machines. [ABSTRACT FROM AUTHOR]

In the cross‐plane single‐molecule junctions, the correlation between molecular aromaticity and conductance remained puzzling. Cross‐plane break junction (XPBJ) provides new insight into understanding the role of aromaticity and conjugation to molecules on charge transport through the planar molecules. In this work, we investigated the modulation of cross‐plane charge transport in pyrene derivatives by hydrogenation and substituents based on the XPBJ method that differs from those used in‐plane transport. We measured the electrical conductance of the hydrogenated derivatives of the pyrenes and found that hydrogenation reduces conductance, and the fully hydrogenated molecule has the lowest conductance. Conductance of pyrene derivatives increased after substitution by both electron‐donating and electron‐withdrawing groups. By calculating, the trend in decreased conductance of hydrogenated pyrene was found to be consistent with the change in aromaticity. Electron‐withdrawing substituents reduce the aromaticity of the molecule and narrow the HOMO‐LUMO gap, while electron‐donating groups increase the aromaticity but also narrow the gap. Our work reveals the potential of fine‐tuning the structure of the pyrene molecule to control the cross‐plane charge transport through the single‐molecule junctions. [ABSTRACT FROM AUTHOR]

The formation of molecular-based functional materials is a key step towards the development of technologies at the nanoscale. Recently, it has been shown that after oxidation of closo-[B10H10]2− anions, an induced aggregation of two cluster units is achieved, retaining their parent B10 backbones as persistent building blocks. Such characterization provides an interesting scenario to further understand relevant factors leading to aggregation in a minimal structure involving two units. Here, we explore the interaction between closo-[B10H10]2− units in two isomers, namely, iso- and trans-[B20H18]2−, involving different intercluster contacts based on B–B and B–H interactions, respectively. Our results show that the inherent spherical aromatic characteristics of the parent closo-[B10H10]2− cluster are persistent in both iso- and trans-[B20H18]2− isomers as an interplay between the spherical aromatic properties from both B10 motifs, leading to an overlap of the shielding regions from shielding cone properties, ascribed as a dual spherical–spherical aromatic cluster. From 11B-NMR features, it came out that trans-[B20H18]2− involves larger differences in comparison to closo-[B10H10]2−, owing to the variation of the B10–B10 backbone provided by the intercluster B-interaction, thus resulting in a more effective aggregation connecting such building units, towards boron-based cluster materials. [ABSTRACT FROM AUTHOR]

Antiaromatic porphyrinoids have attracted significant attention owing to their unique electronic properties and potential applications. However, synthesis of antiaromatic contracted porphyrinoids is challenging owing to the inherent instability associated with smaller ring sizes. In this study, we report the synthesis and characterization of the first stable trioxa[16]triphyrin(2.1.1), a novel 16π antiaromatic contracted porphyrinoid. We utilized a core modification approach to stabilize the [16]triphyrin(2.1.1). X‐ray crystallographic analysis revealed a nearly planar structure. Electrochemical studies demonstrated reversible oxidation behavior and a small HOMO–LUMO gap, which was consistent with its antiaromatic nature. Chemical oxidation yielded an aromatic [14]triphyrin(2.1.1) dication, highlighting the antiaromaticity–aromaticity switching capability of this system. This synthesis involved the discovery of a key intermediate, dihydrotrioxatriphyrin(2.1.1), which underwent oxidative dehydrogenation to yield the target compound. Theoretical calculations suggested that dihydrotrioxatriphyrin(2.1.1) formed via a rare 16π electrocyclic reaction. The successful synthesis and characterization of this stable trioxa[16]triphyrin(2.1.1) underscores the potential of the core modification strategies for the rational design of novel antiaromatic systems with tunable properties. Moreover, the discovery of the rare 16π electrocyclic reaction advances the understanding of high‐order pericyclic processes and may inspire new synthetic strategies for complex macrocyclic compounds. [ABSTRACT FROM AUTHOR]

We present the visualization of the experimental valence electron‐density distribution (EDD) in the isolated 1,2‐digermacyclobutadiene ring system, revealing the unique 4π electron‐delocalization on the four‐membered Ge2C2 ring. A remarkably high Möbius 4π‐electron aromatic character in the Ge2C2 ring can be suggested from theoretical calculations, in sharp contrast to the significant antiaromaticity of the all‐carbon cyclobutadiene ring. [ABSTRACT FROM AUTHOR]

The icosahedral Au135+ core is a recurrent building block in ligand‐protected gold clusters involving an 8‐cluster electron 1S21P6 electronic shell. Such a prototypical structure enables a spherical aromatic behavior as given by long‐range magnetic shielding. Recently, the Au20(tBu3P)8 cluster featuring a contrasting cuboctahedral core with formally neutral gold atoms appears as a novel core architecture with the potential to be considered as another potential building block towards functional nanostructures. Here, we explore the ligand‐core interaction and spherical aromatic characteristics of Au20(tBu3P)8, in order to provide a direct connection to classical icosahedral spherical aromatic compounds, now involving a cuboctahedral core structure. Such characteristics suggest rationalization of their robustness in terms of certain electron counts, enabling a shielding cone property in ligand‐protected metallic clusters, which favors bridging organic and inorganic planar/spherical aromatic species towards the unification of the aromaticity concept and designing guidelines for further achievements. [ABSTRACT FROM AUTHOR]

The functionalization of small aromatic heterocycles is a straight and efficient method to access large polycyclic conjugated molecules. However, due to its unique reactivity, the derivatization of acridone usually gives the product in the angular shape and is difficult to obtain the linear ones. Here we report the direct functionalization of acridone to generate its linearly fused derivatives containing cyclobutadienoid via the palladium‐catalyzed arene‐(oxa)norbornene annulation followed by the dehydrated aromatization. The experimental characterizations demonstrate that the antiaromatic cyclobutadienoid ring can extend the molecular conjugation. Theoretical calculations reveal the weakened aromaticity of neighboring benzenoid rings of cyclobutadienoid with no influence on the electronic structure of central pyridone. The molecular stability is enhanced in comparison to that of linear acridone derivatives containing only six‐membered rings. [ABSTRACT FROM AUTHOR]

Supramolecular complexes of carbon nanohoops with fullerenes play a key role for the design of novel nanomaterials with technological applications. Herein we investigate with density functional theory (DFT) methods the capability of neutral and dioxidized cycloparaphenylenes (CPPs) to encapsulate all‐boron fullerene B40. Our results show that [9]CPP and [10]CPP are feasible host candidates to encapsulate B40 displaying comparable complexation energies with the all‐carbon analog [10]CPP⊃C60. Upon dioxidation the host‐guest interactions are not affected, whereas the positive charge is delocalized on the CPPs leading to global aromatic character of the hosts. Consequently, the dicationic complexes [n]CPP2+⊃B40 and [10]CPP2+⊃C60 display augmented global shielding cones that strongly shield the guests, as manifested by large upfield shifts in 11B‐NMR and 13C‐NMR signals. Hence, CPP complexes with carbon fullerenes can be extended borospherene B40 host‐guest complexes, as well as to doubly oxidized species stabilized by global host aromaticity, expanding our understanding of carbon nanohoop complexes to boron‐based fullerenes. [ABSTRACT FROM AUTHOR]

Phthalazine can behave as a diene in Diels–Alder (DA) cycloadditions, typically at the pyridazine ring, however, its application is somewhat limited because these reactions usually require harsh conditions or sophisticated catalysts. As an unconventional example, phthalazine was reported to undergo cycloaddition with the [PCO]− anion without any catalyst. In this computational study, we scrutinise the mechanism of the DA reactions between phthalazine and the so far known [ECX]− (E: P, As; X: O, S, Se) anions as dienophiles. In principle, the attack of an [ECX]− anion may occur at two different sites of phthalazine, either at the benzene or the pyridazine ring, and both of these possible reaction channels were juxtaposed on the basis of energetic aspects. In all of the investigated cases, the analysis of the energy profiles reveals a clear regioselectivity that favours the attack at the pyridazine ring. As a result, so far unprecedented 2‐pnictanaphth‐3‐olate analogues seem achievable as final products. Comparing the characteristics of these pathways allowed us to clarify the source of this regioselectivity: The pyridazine ring of phthalazine exhibits lower aromaticity than the benzene subring; therefore, in the DA step, the former ring shows a higher affinity toward a dienophile than the latter, leading to lower activation barriers. To further map the electronic and structural features of the cycloaddition steps, the local interactions evolving in the transition states were analysed and compared using global and local descriptors. In most aspects, the characteristics of both pathways were found to be rather similar, in contrast to the markedly differing activation barriers on the two routes. [ABSTRACT FROM AUTHOR]

The spatial magnetic properties (through‐space NMR shieldings—TSNMRSs—actually the ring current effect in 1H NMR spectroscopy) of the recently synthesized infinitene (the helically twisted [12]circulene) have been calculated using the GIAO perturbation method employing the nucleus‐independent chemical shift (NICS) concept and visualized as iso‐chemical‐shielding surfaces (ICSS) of various size and direction. Both 1H and 13C chemical shifts of infinitene and the aromaticity of this esthetically very appealing molecule have been studied subject to the ring current effect thus obtained. This spatial magnetic response property of TSNMRSs dominates the different magnitude of 1H and 13C chemical shifts, especially in the cross‐over section of infinitene, which is unequivocally classified as an aromatic molecule based on the deshielding belt of its ring current effect. Differences in aromaticity of infinitene compared with isolated benzene can also be qualified and quantified on the magnetic criterion. [ABSTRACT FROM AUTHOR]

Structures, 1H/13C chemical shifts, and the ring current effects (spatial magnetic properties: through‐space NMR shieldings [TSNMRSs]) of various π‐conjugated macrocyclic hydrocarbons and the corresponding charged analogues have been calculated at the B3LYP/6‐311G(d,p) theory level using the GIAO perturbation method and employing the nucleus‐independent chemical shift (NICS) characterization. The spatial magnetic properties (TSNMRS) are visualized as iso‐chemical shielding surfaces (ICSSs) of various size and direction and together with especially the δ(1H)/ppm chemical shifts employed to unequivocally qualify and quantify local 6π‐aromaticity of individual benzenoid building blocks and the global ([4n + 2], n > 1) aromaticity of the macrocyclic ring. [ABSTRACT FROM AUTHOR]

In the present study, conformations and aromaticity of seventeen [13], [23] and [33] double-stranded cyclophanes composed of three benzene moieties were investigated at B3LYP level of density functional theory. The stability of these conformers is different depending on the type of cycloalkane-like cycles formed by (CH2) n bridges and the peripheral benzene rings, and also the curvature of the latter. In [13] cyclophane, this leads to only one conformer containing puckered cyclobutane-like cycle, at an angle of about 37◦. In comparison, in cyclophanes [23] and [33], the corresponding cycles are cyclohexane-like and cyclooctane-like, respectively. While conformer 2a having TB-like cycles is the most stable member of the [23] family, of eight ground-state conformers obtained for [33], 3a with two Chair-Chair-like and one Twist-Chair-like cycles is the most stable one, and 3h with three Twist-Boat-like cycles being 6.5 kcal higher in energy than the former is the least stable member of series. In addition to the conformational analysis, the aromaticity of the series has been assessed using different indexes, including magnetic criteria nucleus-independent chemical shifts (NICS-based) at the cage and at the ring centers, HNMR chemical shift, Magnetic Susceptibility, HOMA and relative energy (RE). [ABSTRACT FROM AUTHOR]

The change in local, semi-local and peripheral aromaticity of coronene upon gradual replacement of its peripheral CC bonds with BN bonds, starting from the C3h symmetrical 3-BN derivative, was followed by calculating HOMA, PDI, FLU, EDDB and NICS-xy scans. They were calculated using B3LYP-D3(BJ), M06-2X and ωB97XD functionals, whose results agreed very well. It was found that introduction of BN units instead of CC units can, more or less, isolate aromatic carbocyclic subunits and increase peripheral electron delocalization. These subunits dominate the frontier orbital shapes and energy gap, thus enabling their modulation by applying BN/CC isosterism. [ABSTRACT FROM AUTHOR]

Non‐Kekulé quinoidal azaacences m‐A (1 a,b) were synthesized and compared to their para‐ and ortho‐quinodimethane analogues. m‐A display high diradical characters (1 b: y0 = 0.88) due to their meta‐quinodimethane (m‐QDM) topology. Electron paramagnetic, nuclear magnetic resonance spectroscopies and supraquantum interference device measurements in combination with quantum‐chemical calculations revealed singlet ground states for m‐A with singlet‐triplet gaps ΔEST (0.13–0.25 kcal mol−1) and thermally populated triplet states. These non‐Kekulé structures are over all void of zwitterionic character and possess record high two‐photon absorption cross sections over a broad spectral range in the near‐infrared. [ABSTRACT FROM AUTHOR]

The synthesis of diradical organic compounds has garnered significant attention due to their thermally accessible spin inversion and optoelectronic properties. Yet, preparing such stable structures with high open‐shell behavior remains challenging. Herein, we report the synthesis and properties of four π‐extended, fused fluorene derivatives with high diradical character, taking advantage of a molecular design where the closed‐shell does not include any Clar sextet, comparatively to a maximum of 5 in the corresponding open‐shell state. This led to an unusual open‐shell triplet ground state with an outstanding singlet‐triplet energy difference (ΔEST) of ca. 19 kcal/mol, one of the highest values reported to date for an all‐carbon conjugated scaffold. Incorporation of dithiafulvene units at each end of the molecule (at the five‐membered rings) furnishes extended tetrathiafulvalenes (TTFs) undergoing reversible oxidations to the radical cation and diradical dication. The various pro‐aromatic structures presented herein show highly localized spin density and a limited conjugation due to the confined π‐electrons in the aromatic cycles, as supported by 1H NMR, UV/Visible, EPR spectroscopy and DFT calculations. [ABSTRACT FROM AUTHOR]

Nominally, meso-hydroxyoxochlorins, like known 5-hydroxy-7-oxo-octaethylchlorin (9), its nickel complex [5-hydroxy-7-oxo-octaethylchlorinato]nickel(II) (9Ni), or the novel 5-hydroxy-7,17-dioxo-octaethylbacteriochlorin (10), incorporate an acetylacetonate (acac)-moiety in the enol form in their chromophore structures. X-Ray diffraction studies of the compounds show the presence of a strong H-bond between the enol and flanking β-ketone. Like acac, the functionality can be deprotonated. However, unlike regular acac-like moieties, we did not find any indication that this functionality is competent in chelating any of the 3d or 4d transition metal ions tested. Evidently, the conjugation that contributes to the stability of acac as a ligand cannot be expressed in the meso-hydroxyoxochlorins since it would perturb the aromaticity of the porphyrinic chromophores; in other words, the metal binding energies do not offset the loss in aromaticity. The halochromic properties of the molecules provide some more insight into the location of the protonation/deprotonation sites. The interpretation of the findings is supported by computations. [ABSTRACT FROM AUTHOR]

Exploration of expanded porphyrins with unprecedented reactivities has remained important. Here [22]pentaphyrins(2.0.1.1.0) were synthesized as a constitutional isomer of sapphyrin by acid‐catalyzed cyclization of 1,14‐dibromo‐5,10‐diaryltripyrrin with 1,2‐di(pyrro‐2‐ly)ethenes. These pentaphyrins display roughly planar structures and varying aromaticities depending upon the vinylene structures. The 19,20‐ditolyl pentaphyrin gave an N‐fused product and an unprecedented pyrrole‐rearranged product, depending upon the oxidation conditions. Remarkably, upon the metalation with CuCl, the N‐fused product and the pyrrole‐rearranged product afforded an inner β‐β coupled face‐to‐face CuII complex dimer and an outer β‐β coupled lateral CuII complex dimer, respectively, in fairly good yields. Further, [22]pentaphyrin(2.0.1.1.0) fused with a NiII porphyrin was effectively dimerized upon oxidation with MnO2 to give a 16–16' directly linked dl‐dimer. [ABSTRACT FROM AUTHOR]

Aromaticity and antiaromaticity are foundational principes in organic chemistry, regularly invoked to explain stability, structure, and magnetic and electronic properties. There are ongoing challenges in assigning molecules as aromatic or antiaromatic using optical spectroscopy. Here we report spectroelectrochemical and computational analyses of porphyrin (18π neutral, aromatic) and norcorrole (16π neutral, antiaromatic), and their oxidized (16π porphyrin dication) and reduced (norcorrole 18π dianion) forms. Our results show that while the visible spectra are characteristic of (anti)aromaticity consistent with Hückel's rules, the IR spectra are much less informative, owing to the relative rigidity of norcorrole. The results have implications for the assignment of (anti)aromaticity in both ground‐state and time‐resolved spectra. [ABSTRACT FROM AUTHOR]

Among the known planar pentacoordinate atoms, chlorine is missing due to its large radius and high electronegativity. Herein, we report the first star-like superhalogen anion D5h Cl©Li5Cl5− (1), which contains a planar pentacoordinate chlorine (ppCl) at the center. Computer structural searches and high-level calculations reveal that 1 is a true global minimum (GM) on the potential energy surfaces. Molecular dynamics simulations indicate it is kinetically stable against isomerization or decomposition. Although detailed chemical bonding analyses reveal one delocalized 6c-2e σ bond over the Cl©Li5 central unit and five delocalized 3c-2e σ bonds along the periphery, while aromaticity has very little beneficial effect on stability, instead, ionic interaction dominates the stability of the system. More encouragingly, with the large HOMO–LUMO energy gap of 7.66 eV and vertical detachment energy of 7.87 eV, the highly chemically inert 1 can be viewed as a typical superhalogen anion and is possible to be synthesized and characterized in future experiments. [ABSTRACT FROM AUTHOR]

Cyclo[n]carbon (Cn) is one member of the all-carbon allotrope family with potential applications in next-generation electronic devices. By employing first-principles quantum transport calculations, we have investigated the electronic transport properties of single-molecule junctions of Cn, with n = 14, 16, 18, and 20, connected to two bulk gold electrodes, uncovering notable distinctions arising from the varying aromaticities. For the doubly aromatic C14 and C18 molecules, slightly deformed complexes at the singlet state arise after bonding with one Au atom at each side; in contrast, the reduced energy gaps between the highest occupied and the lowest unoccupied molecular orbitals due to the orbital reordering observed in the doubly anti-aromatic C16 and C20 molecules lead to heavily deformed asymmetric complexes at the triplet state. Consequently, spin-unpolarized transmission functions are obtained for the Au-C14/18-Au junctions, while spin-polarized transmission appears in the Au-C16/20-Au junctions. Furthermore, the asymmetric in-plane π-type hybrid molecular orbitals of the Au-C16/20-Au junctions contribute to two broad but low transmission peaks far away from the Fermi level (Ef), while the out-of-plane π-type hybrid molecular orbitals dominate two sharp transmission peaks that are adjacent to Ef, thus resulting in much lower transmission coefficients at Ef compared to those of the Au-C14/18-Au junctions. Our findings are helpful for the design and application of future cyclo[n]carbon-based molecular electronic devices. [ABSTRACT FROM AUTHOR]

Kekulene, a cycloarene composed of 12 fused benzene rings in a circular arrangement, exhibits a highly planar and robust structure. Kekulene has been the subject of investigation into its aromaticity and electronic structure, particularly in relation to the cyclic benzenoid. We have successfully synthesized novel bowl‐shaped kekulene analogues with five‐membered rings incorporated into the kekulene structure. The results of DFT calculations and VT‐NMR spectra indicate that inversion of their concave‐convex structures occurs at room temperature. The NICS and AICD plots predict that the Clar's type resonance structure is found in a manner analogous to the pristine kekulene, albeit with the interruption of the π‐conjugation on the sp3 carbons at the five‐membered rings. Despite the presence of the Clar's resonance structure, the Diels‐Alder reaction proceeded smoothly with a dienophile, in contrast to the behavior of planar kekulene derivatives. This study will lead to the creation of novel bowl‐shaped compounds and development of reactivity in aromatic compounds. [ABSTRACT FROM AUTHOR]

Non‐benzenoid diradicaloids possessing high‐spin ground states are attractive synthetic targets given their high potential for spintronics and quantum computing. Nevertheless, the synthesis of such compounds remains highly challenging due to their inherent instability. In this work, we present the synthetic attempt towards creating a non‐benzenoid diradicaloid (1) with a triplet ground state by fusing two cyclohepta[def]fluorene units onto a benzene ring. Our synthetic approach involves both oxidation and reduction pathways. In the oxidation path, we obtained the partially dehydrogenated products 1+H and 1+2H containing an indeno[1,2‐b]fluorene core from the tetrahydro precursor (2). However, further dehydrogenation to afford the target molecule (1) did not proceed. On the other hand, with the reduction pathway, a novel tetraketone precursor (9) with two pairs of pentagons and heptagons was successfully synthesized. The subsequent nucleophilic attack however was proved to be difficult probably due to the unselective nucleophilic addition on the zigzag nanographene ketones. Furthermore, UV‐vis absorption, cyclic voltammetry, and theoretical calculations were conducted to explore the optical, electrochemical properties, and aromaticity of all the obtained molecules (1+H, 1+2H and 9). Although the desired target 1 is not achieved, our work provides insight into designing novel high‐spin non‐benzenoid NGs based on nonalternant cyclohepta[def]fluorene system. [ABSTRACT FROM AUTHOR]

Aromaticity is a fundamental concept in chemistry that explains the stability and reactivity of many compounds by identifying atoms within a molecule that form an aromatic ring. Reliable aromaticity indices focus on electron delocalization and depend on atomic partitions, which give rise to the concept of an atom‐in‐the‐molecule (AIM). Real‐space atomic partitions present two important drawbacks: a high computational cost and numerical errors, limiting some aromaticity measures to medium‐sized molecules with rings up to 12 atoms. This restriction hinders the study of large conjugated systems like porphyrins and nanorings. On the other hand, traditional Hilbert‐space schemes are free of the latter limitations but can be unreliable for the large basis sets required in modern computational chemistry. This paper explores AIMs based on three robust Hilbert‐space partitions – meta‐Löwdin, Natural Atomic Orbitals (NAO), and Intrinsic Atomic Orbitals (IAO) – which combine the advantages of real‐space partitions without their disadvantages. These partitions can effectively replace real‐space AIMs for evaluating the aromatic character. For the first time, we report multicenter index (MCI) and Iring values for large rings and introduce ESIpy, an open‐source Python code for aromaticity analysis in large conjugated rings. [ABSTRACT FROM AUTHOR]

An intriguing question in the general problem of aromaticity is whether captodative aromatic systems with the donor and acceptor substituents at the same carbon of the CC bond can be more stable than the π‐conjugated push‐pull counterparts? The analysis of electronic, magnetic, and structural criteria of aromaticity showed that for conventional organic substituents XO, TfN, (NC)2C, (NO2)2C, Tf2C, the push‐pull tropylidene derivatives [tropylium]+CHCHX− are expectedly more stable than their captodative isomers [tropylium]+C(X−)CH2, with the lowest ΔE for the most strong acceptor Tf2C. A different behavior is observed for XMHlg3 (MB, Al; HlgF, Cl). They are not only structurally and magnetically most aromatic in both series but show the inverse stability of the push‐pull and captodative isomers, the latter being more stable by up to 10 kcal/mol (in gas).The difference between the MHlg3 groups and conventional organic groups is that in the latter the electron density is transferred to the π‐system of the substituent, while the former can accept it only to the σ*(CM) orbital. Thus, when the electron donor and acceptor effects are separated between the σ and π systems, captodative isomers can be more stable than their push‐pull isomers with more extended conjugation. [ABSTRACT FROM AUTHOR]

A tricyclic annulated 1,4,2-diazaborole is readily accessed via reaction of a bidentate pyridyl-carbene ligand with MesBBr2 followed by reduction. Dearomatization of the flanking rings is shown to increase reactivity of this heterocycle in the form of a B-centred alkylation with MeI. Its reaction with hydrido-, fluoro-, and chloro-boranes reveal an unprecedented ring expansion reaction to form a diverse family of B2C2N2 heterocycles, reduction of which allows facile access to the first examples of Cs symmetric 1,4,2,5-diazadiborinines. DFT calculations have shed light on the electronic structures of the reduced species and provide insight into mechanistic aspects of the observed ring-expansion. [ABSTRACT FROM AUTHOR]