Global Aromaticity and Antiaromaticity in Porphyrin Nanoring Anions

Abstract

Doping, through oxidation or reduction, is often used to modify the properties of π‐conjugated oligomers. In most cases, the resulting charge distribution is difficult to determine. If the oligomer is cyclic and doping establishes global aromaticity or antiaromaticity, then it is certain that the charge is fully delocalized over the entire perimeter of the ring. Herein we show that reduction of a six‐porphyrin nanoring using decamethylcobaltocene results in global aromaticity (in the 6− state; [90 π]) and antiaromaticity (in the 4− state; [88 π]), consistent with the Hückel rules. Aromaticity is assigned by NMR spectroscopy and density‐functional theory calculations.

Global aromaticity at the nanoscale

“…In these nanorings, each porphyrin contributes 10 electrons to the Hückel π-electron count, and each linking alkyne contributes 2 electrons, so a nanoring cation c- P N [b x e y ] Q + has an electron count of 10 N + 4 x + 2 y – Q (where N is the number of porphyrin units; x and y are the number of butadiyne and ethyne links respectively). Recently, we reported that the butadiyne-linked six-porphyrin nanoring c -P6[b 6 ] displays global aromaticity when oxidised 8 , reduced 17 or electronically excited 18 . According to Schleyer’s terminology 19 , all the nanorings discussed here are ‘trannulenes’ rather than ‘annulenes’, since the p-orbitals making up the aromatic π-system are oriented in the plane of the ring, whereas in annulenes the p-orbitals are perpendicular to the ring.…”

Aromatic Pathways in Porphyrinoids by Magnetically Induced Ring Currents

“…25,142,143 A large positive magnetic susceptibility was measured for the cation of a closed-shell porphyrin nanoring in its 4+ oxidation state and for the corresponding anion with a charge of -4. 143,144 Calculations of the magnetizabilities and ring-current strengths of strongly antiaromatic porphyrinoids showed that they are paramagnetic when the ring-current strength exceeds −25 nA/T. The large positive contribution to the magnetizability originates from the angular momentum operator, which is also responsible for the paratropic ring current.…”