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

Magnetic shielding studies demonstrate that successive hydrogenation of NiII 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 NiII 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.