The Effect of Hydrogenation on the Contest between Aromaticity and Antiaromaticity in Norcorrole

Off-nucleus isotropic shielding isosurfaces computed using DFT show that a key feature of Ni^II norcorrole (NiNc), its antiaromatic “core”, a 14-membered cyclic conjugated subsystem with 16 π electrons that is formally preserved in all hydrogenated norcorroles, unexpectedly becomes less and less antiaromatic in the series H₂NiNc−H₈NiNc, which is predicted to end in the completely nonaromatic H₈NiNc.

Abstract

Magnetic shielding studies demonstrate that successive hydrogenation of Ni^II norcorrole (NiNc), a stable molecule combining aromatic and antiaromatic features, first weakens and then eliminates the central antiaromatic region, even though the NiNc antiaromatic “core”, a 14-membered conjugated cycle with 16 π electrons, is formally preserved throughout the H₂NiNc−H₈NiNc series. The differences between aromatic and non-aromatic isotropic shielding distributions and nucleus-independent chemical shift (NICS) values in these hydrogenated porphyrin analogues are highlighted by comparing the results for the members of the H₂NiNc−H₈NiNc series to those for the aromatic Ni^II porphyrin complex. The results strongly support the unexpected and counterintuitive conclusion that H₈NiNc will be nonaromatic, without even a trace of antiaromaticity. Based on these findings, H₈NiNc is predicted to be the most stable member of the H₂NiNc−H₈NiNc series.

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