Amnon Stanger

Energetic, electron density and magnetic criteria of aromaticity were applied to 15 cyclic two π and 9 six π electrons. There is no correlation between the aromaticity indices for the systems made of 3rd and 4th row elements.

Helili and Kerim report a study on the aromaticity of [n]Circulene ( n = 3–8), using topology‐based methods (TRE). Their assumptions regarding the geometries of the systems are that (a) the systems are planar and (b) all the six‐membered rings have C‐C bond lengths equal to that of benzene. This comment shows that these geometrical assumptions are too crude to allow correct estimations of the aromaticity.

Aromaticity is a fundamental concept in organic chemistry that has gradually been extended to many other areas in the chemical sciences, including organometallic chemistry. By replacing a CH unit in an aromatic hydrocarbon (e.g., benzene) with an organometallic fragment, novel metallaaromatic compounds with intriguing physicochemical properties can be synthesized. Yet, despite intense research efforts, the synthesis of their macrocyclic congeners remains challenging, mainly due to a lack of straightforward synthetic strategies. Here we report the synthesis of a carbene analog of the well-known PyBOX pincer ligand, termed CarBOX, that upon coordination to a low-valent metal center (e.g., Co or Rh) results in the formation of a bespoke macrocyclic metallaaromatic complex. 1H NMR analysis of the resulting cobalt complexes reveals a downfield chemical shift (Δδ ≈ 8.5 ppm) of a diagnostic methyl resonance, indicative of strong diatropic ring currents. The electronic properties of the resulting pincer complexes were further confirmed by density functional theory (DFT), including nucleus independent chemical shift (NICS), isomer stabilization energy (ISE), and the magnetically induced origin-independent electron current density, all of which support the metallaaromatic nature of the cobalt complexes. Overall, these findings establish a generalizable platform for accessing metallaaromaticity through coordination chemistry, expanding the design principles of aromatic systems to include through-metal π-delocalization opening up new avenues for electronic materials design and function.

When close to the molecular plane, the behavior of nucleus independent chemical shift (NICS) as a function of the distance from the molecular plane deviates from its behavior at larger distances. By using a dense grid of NICS‐probes (BQs) it is shown that, when close to the molecular plane, maximal (absolute) NICS values are obtained above the atoms. These maxima move towards the center as the grid is elevated until the (absolute) maximum NICS is obtained at the center and stay there when the grid is further elevated. It is shown that this behavior is a result of the current density, which is influenced by the electron density, according to the Biot‐Savart law, which, in turn, causes the induced magnetic field measured by the NICS. It is thus concluded that if magnetic aromaticity is studied, the NICS calculations should be carried out at a large enough distance so that only the π‐ring current affects the NICS. At distances ≥2 Å, NICS(r)_π,zz=A+B*C^r. Using non‐linear correlation for obtaining A, B and C and extrapolate to NICS(1)_π,zz and NICS(1.7)_π,zz is recommended as measures for aromaticity.

The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1‐dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch‐Buttenberg‐Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4‐tetramorpholino‐1‐buten‐3‐yne (4), which was used for the synthesis of four‐membered cyclic bent allene (CBA) transition‐metal complexes of the type [(CBA)ML_n] (5‐7; ML_n=AuCl, RhCl(COD), RhCl(CO)₂; CBA=1,3,4,4‐tetramorpholino‐1,2‐cyclobutadiene; COD=1,5‐cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4‐tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4‐tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C₄N₄ core as shown by X‐ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.

Qualitative aromaticity-based arguments are often used to explain singlet fission (SF) properties in polycyclic conjugated systems. It has been shown by Fowler and collaborators that magnetically induced ring currents are associated with transitions between occupied and unoccupied molecular orbitals (MOs). Since SF has to do with relative energies of electronic states, it was hypothesized that induced currents may indicate SF properties. The quantitative aromaticity of linear oligoacene and several doubly boron-doped anthracenes and phenanthrenes, in their closed-shell singlet, open-shell singlet (where applicable), and triplet electronic states, has been studied using nucleus-independent chemical shift (NICS)-XY scan methods. It is shown that quantitative magnetic aromaticity can be used to identify SF compounds, at least for initial screening. Thus, the induced current features that are indicative of singlet fission (SF) ability have been found to be global current and local current at each ring of the systems. This conclusion was verified for very different systems─a tetracyclic, nitrogen-containing quinone (6), diphenyl benzofurane (7), and cibalackrot (8), all reported to be SF systems. Finally, it is predicted that some isomers of the doubly boron-doped linear[4]phenylene should show SF properties.

The tropicity of the title system was studied using NICS(1.7)_π,zz-XY-scans and current density analysis, showing a global diatropic loop and local dia- and paratropic loops. This aromaticity picture is very different from the one proposed based on NICS(1) and HOMA ( Org. Lett. 2021, 23, 8794−8798). It is predicted that 1 should be suitable for singlet fission applications. It is concluded that local aromatic indices should not be used for the full analysis of multiring conjugated systems.

The Nucleus Independent Chemical Shift (NICS) method was invented in 1996 by the late Paul v. R. Schleyer as a computational tool for the assessment of aromaticity. In the intervening years, NICS has developed considerably. This Minireview describes the NICS‐based methods that are currently available and concludes with recommendations for choosing the appropriate NICS‐based methods to study a given problem.

The aromaticity of three non-planar, fully conjugated aza-nanographenes built around a pyrrolo[3,2-b]pyrrole core is assessed through application of two different computational procedures - GIMIC and NICS. We examine the calculated magnetically induced current densities (GIMIC) and nucleus independent chemical shifts (NICS). The structural differences between these three apparently similar molecules lead to significantly different aromatic properties. GIMIC analysis indicates that the peripheral diatropic ring current of 3.9 nA/T for the studied bowl-shaped diaza-nanographene is the strongest, followed by the double [6]helicene which lacks seven-membered rings, and is practically nonexistent for the double [5]helicene possessing seven-membered rings. The biggest difference however is that in the two not-fully-fused molecules the central pyrrole rings possess a significant diatropic current of about 4.1 nA/T, whereas there is no such current in the diaza-nanographene. Moreover, the antiaromaticity of the seven-membered rings is increasing while moving from double [5]helicene to diaza-nanographene (from -2.4 to -6.0 nA/T).The induced currents derived from NICS_π,zz-XY-scan analysis for all of the studied systems are in qualitative agreement with the GIMIC results. Subtle differences may originate from σ-electron currents in GIMIC or inaccuracy of NICS_π,zz values due to the non-planarity of the systems, but the general picture is similar.