Tautomerism in novel oxocorrologens

A novel corrole-type macrocycle, oxocorrologen (2), substituted with hemiquinone groups, has been synthesized. It was found to undergo multiple tautomerism of its exchangeable protons between electronegative atom sites at the macrocyclic core (nitrogen atoms) and periphery (phenol oxygen atoms). Alkylation at one macrocyclic nitrogen atom with a 4-nitrobenzyl group gave 3, which can exist in only two tautomeric forms depending on the solvent. Tautomerism has been studied by means of (1)H NMR spectroscopy in a variety of solvents and solvent mixtures. Tautomer structure assignments have been supported by DFT calculations of the relative energies of the tautomers. X-ray crystallography of the N-nitrobenzyl derivative has revealed that intramolecular hydrogen bonding may be responsible for stabilizing the observed tautomers. The solvent dependence of the tautomerism of 2 and 3 confers solvatochromism. Electrochemical measurements on 2 and 3 in their respective quinone forms have revealed irreversible processes, but indicate that they are both electron-deficient with a small HOMO-LUMO gap and first reduction potentials close to those of fullerene electron acceptors.