Norcorrole: Aromaticity and Antiaromaticity in Contest

Abstract

Magnetic shielding studies demonstrate that nickel norcorrole (NiNc) and norcorrole (H2Nc) provide unusual examples of stable molecules with high antagonistic levels of antiaromaticity and aromaticity: Both incorporate an antiaromatic "core", a 14-membered cyclic conjugated subsystem with 16 π electrons, surrounded by an aromatic "halo" in the form of a ring of either 14 atoms and 14 π electrons with a new type of homoconjugation (NiNc), or 18 atoms with 18 π electrons (H2Nc).

Visualisation of Chemical Shielding Tensors (VIST) to Elucidate Aromaticity and Antiaromaticity**

“…(3), but this is not related to aromatic ring currents. An enhancement of NICS values close to the bonds is also visible for NICS(1), in agreement with recent results of 2D and 3D scans [25,57] . This enhancement is lost in the case of the NICS(2) tensor in Figure 2.…”

“…This reduction in deshielding for the 5‐membered ring, which was also seen in Ref. 26, has been explained in the sense that antiaromatic currents are only present on the inner part of the ring and that norcorrole possesses an aromatic pathway on the perimeter [25] …”

“…NICS values can be visualised after computing them on a grid around the molecule of interest [45][46][47][48] and several recent applications present 1D scans, 2D contour plots or even 3D isosurfaces of the isotropic NICS values, e.g. to visualise local aromatic and antiaromatic parts of larger molecules, 6,22,35,49 to represent Clar sextets in condensed hydrocarbons, 50 or to study interactions in excimers. 51 These visualisations are almost exclusively based on isotropic NICS values.…”

Magnetic shielding paints an accurate and easy-to-visualize portrait of aromaticity

“…5 For a further example, see ref. 46 where s iso (r) isosurfaces are used in order to study bonding and aromaticity in another nonplanar molecule, norcorrole.…”

A Volumetric Analysis of the 1H NMR Chemical Shielding in Supramolecular Systems

“…The aromaticity of protons in conjugated systems was investigated for many decades, and various aspects of this topic were most recently covered in references [25][26][27]. The B3LYP-PCM-based methodology that is described in the preceding section was applied to two compounds, which are important for aromaticity/antiaromaticity studies of porphyrin analogues [13] (see the surveys [28,29]). These molecules are stable complexes of Ni II with substituted norcorroles, namely, an antiaromatic Ni II 6,15-dimesitylnorcorrole and aromatic Ni II 6,16-dimesityl-10-oxanorcorrole.…”