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A chemical bonding of several metallabenzenes and metallabenzynes was studied via an adaptive natural density partitioning (AdNDP) algorithm and the induced magnetic field analysis. A unique chemical bonding pattern was discovered where the M=C (M: Os, Re) double bond coexists with the delocalized 6c-2e π-bonding elements responsible for aromatic properties of the investigated complexes. In opposition to the previous description where 8 delocalized π-electrons were reported in metallabenzenes and metallabenzynes, we showed that only six delocalized π-electrons are present in those molecules. Thus, there is no deviation from Hückel’s aromaticity rule for metallabenzynes/metallabenzenes complexes. Based on the discovered bonding pattern, we propose two thermodynamically stable novel molecules that possess not only π-delocalization but also retain six σ-delocalized electrons, rendering them as doubly aromatic species. As a result, our investigation gives a new direction for the search for carbon-metal doubly aromatic molecules.

Electrophilic aromatic substitution (EAS) reactions are widely regarded as characteristic reactions of aromatic species, but no comparable reaction has been reported for molecules with Craig-Möbius aromaticity. Here, we demonstrate successful EAS reactions of Craig-Möbius aromatics, osmapentalenes, and fused osmapentalenes. The highly reactive nature of osmapentalene makes it susceptible to electrophilic attack by halogens, thus osmapentalene, osmafuran-fused osmapentalene, and osmabenzene-fused osmapentalene can undergo typical EAS reactions. In addition, the selective formation of a series of halogen substituted metalla-aromatics via EAS reactions has revealed an unprecedented approach to otherwise elusive compounds such as the unsaturated cyclic chlorirenium ions. Density functional theory calculations were conducted to study the electronic effect on the regioselectivity of the EAS reactions.

In this work the relationship between the formal number of π‐electrons, d‐orbital conjugation topology, π‐electron delocalization and aromaticity in d‐block metallacycles is investigated in the context of recent findings concerning the correlation of π‐HOMO topology and the magnetic aromaticity indices in these species. It is demonstrated that for π‐electron rich d‐metallacycles the direct link between aromaticity, the number of π‐electrons and the frontier π‐orbital topology does not strictly hold and for such systems it is very difficult to unambiguously associate their aromaticity with the “4n+2” (Hückel) and “4n” (Möbius) rules. It is also shown that the recently proposed electron density of delocalized bonds (EDDB) method can successfully be used not only to quantify and visualize aromaticity in such difficult cases, but also – in contrast to magnetic aromaticity descriptors – to provide a great deal of information on the real role of d‐orbitals in metallacycles without the ambiguity of bookkeeping of electrons in the π‐subsystem of the molecular ring. Interestingly, some of the metallacycles studied cannot be classified exclusively as Hückel or Möbius because they have a hybrid Hückel‐Möbius or even quasi‐aromatic nature. To d or not to d? Aromaticity in the d‐block metallacycles is sometimes hardly assessable by means of topological, structural and magnetic criteria. In such difficult cases, the proposed orbital‐decomposition method based on the electron density of delocalized bonds (EDDB) provides a great deal of information on the real role of the transition metal d‐orbitals in aromatic stabilization.

Using a scanning tunneling microscope-based break-junction technique, we assessed the conductance of single-molecule junctions formed with three distinct molecular wires, each featuring a cyclic five-membered ring: furan, cyclopentanone or cyclopentadiene. We found that the conductance of these three wires correlates with their aromaticity; the non-aromatic cyclopentadienone derivative exhibits the highest conductance, while the most aromatic furan in this series displays the lowest conductance. Additionally, the lower HOMO-LUMO gap of anti-aromatic compounds may also contribute to this effect. This discovery aids in understanding the influence of molecular structure on conductive properties and provides crucial insights for the design and optimization of molecular electronic devices.

In 1931, Erich Hückel published a landmark paper — the seed of the now famous 4 n + 2 rule for aromaticity in annulenes that bears his name. Electron counting has since been extended to other classes of compounds, resulting in a multitude of rules aiming to capture the concept of aromaticity and its impact in chemistry.

Antiaromaticity is extended from aromaticity as a complement to describe the unsaturated cyclic molecules with antiaromatic destabilization. To prepare antiaromatic species is a particularly challenging goal in synthetic chemistry because of the thermodynamic instability of such molecules. Among that, both Hückel and Möbius antiaromatic species have been reported, whereas the Craig one has not been realized to date. Here, we report the first example of planar Craig antiaromatic species. Eight Craig antiaromatic compounds were synthesized by deprotonation-induced reduction process and were fully characterized as follows. Single-crystal X-ray crystallography showed that these complexes have planar structures composed of fused five-membered rings with clearly alternating carbon–carbon bond lengths. In addition, proton NMR (¹H NMR) spectroscopy in these structures showed distinctive upfield shifts of the proton peaks to the range of antiaromatic peripheral hydrogens. Experimental spectroscopy observations, along with density-functional theory (DFT) calculations, provided evidence for the Craig antiaromaticity of these complexes. Further study experimentally and theoretically revealed that the strong exothermicity of the acid-base neutralization process was the driving force for this challenging transformation forming Craig antiaromatic species. Our findings complete a full cycle of aromatic chemistry, opening an avenue for the development of new class of antiaromatic systems.

Polycyclic aromatic hydrocarbons (PAHs) play a central role in materials science due to their extended π-conjugated systems, with their stability and reactivity depending critically on their aromatic character. In this work, we systematically investigated the aromaticity and stability of a broad range of linear (acenes, phenacenes, biphenylenes, and cyclobuta-acenes) and belt-like (cyclacenes, cyclophenacenes, and cyclobiphenylenes) PAHs containing five to twelve benzene rings. A diverse set of aromaticity descriptors was employed, including geometric (HOMA), electronic (MCI, FLU) and magnetic (NICS) descriptors, plus the recently developed Q2 indices, based on the components of the distributed multipole analysis (DMA) electric quadrupole tensor. These data were complemented by stability analyses using singlet–triplet energy splitting (ΔES–T) and fractional occupation number-weighted densities (NFOD) values. Our results indicate that acenes and phenacenes follow a comparable aromatic trend, with inner rings possessing lower aromaticity and the edge rings showing a more pronounced aromatic character. A subtle difference is observed in the position of the most aromatic ring, which lies slightly closer to the interior in acenes. Phenacenes, however, exhibit greater overall stability, attributed to their armchair edges. For biphenylenes and cyclobuta-acenes, the antiaromatic cyclobutadiene moiety perturbs the aromaticity only in its direct neighborhood and preserves the aromaticity in the remaining chains. In belt-like systems, cyclacenes exhibit strong radical character and low stability, consistent with longstanding synthetic challenges, whereas cyclophenacenes display enhanced aromaticity and stability with extending size. Cyclobiphenylenes combine localized antiaromatic centers with preserved benzene-like aromaticity in rings distant from the cyclobutadiene unit.

π-Aromaticity is an important driving force in directing the synthesis of aromatic compounds; in contrast, reactions induced by σ-aromaticity are uncommon. Here we report a strategy based on π- and σ-aromaticity relays to realize the structurally defined ring contraction of metallacyclobutadiene to metallacyclopropene. This reaction involves the release of the π-antiaromaticity of metallacyclobutadiene to afford a π-aromatic intermediate, followed by ring reclosure to generate σ-aromatic metallacyclopropene. The ring opening–reclosing mechanism and versatile switching of the aromaticity of the metallacyclic species are supported by experimental results and theoretical calculations. This work demonstrates the importance of aromaticity relay with the successive decrease of energy in reactions and will stimulate efforts in exploiting the vital role of aromaticity in synthetic chemistry.Reactions induced by σ-aromaticity are uncommon compared with those induced by π-aromaticity. Now, a π- and σ-aromaticity relay strategy is developed to realize the ring contraction of metallacyclobutadiene to metallacyclopropene. This reaction involves the release of π-antiaromaticity to afford a π-aromatic intermediate, followed by ring reclosure to form σ-aromatic metallacyclopropene.

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.

Synthetic carbon allotropes such as graphene 1 , carbon nanotubes 2 and fullerenes 3 have revolutionized materials science and led to new technologies. Many hypothetical carbon allotropes have been discussed 4 , but few have been studied experimentally. Recently, unconventional synthetic strategies such as dynamic covalent chemistry 5 and on-surface synthesis 6 have been used to create new forms of carbon, including γ-graphyne 7 , fullerene polymers 8 , biphenylene networks 9 and cyclocarbons 10 , 11 . Cyclo[ N ]carbons are molecular rings consisting of N carbon atoms 12 , 13 ; the three that have been reported to date ( N  = 10, 14 and 18) 10 , 11 are doubly aromatic, which prompts the question: is it possible to prepare doubly anti-aromatic versions? Here we report the synthesis and characterization of an anti-aromatic carbon allotrope, cyclo[16]carbon, by using tip-induced on-surface chemistry 6 . In addition to structural information from atomic force microscopy, we probed its electronic structure by recording orbital density maps 14 with scanning tunnelling microscopy. The observation of bond-length alternation in cyclo[16]carbon confirms its double anti-aromaticity, in concordance with theory. The simple structure of C 16 renders it an interesting model system for studying the limits of aromaticity, and its high reactivity makes it a promising precursor to novel carbon allotropes 15 . The carbon allotrope cyclo[16]carbon has been synthesized using on-surface chemistry and characterized with scanning tunnelling microscopy and atomic force microscopy, revealing orbital densities and bond-length alternation, which show it to be doubly anti-aromatic.