[6]Cyclo-para-phenylmethine: An Analog of Benzene Showing Global Aromaticity and Open-Shell Diradical Character

We report a [6]cyclo- para -phenylmethine ( [6]CPPM ) macrocycle that shows benzene-like electronic properties. Its mesityl derivative, [6]CPPM-Mes, was isolated in crystalline form. X-ray analysis reveals a C 2 h symmetry, and the bond lengths of the benzenoid/quinoid rings are averaged via resonance. One averaged 1 H NMR peak for the protons on the backbone was observed at room temperature, but it was split into one shielded and one deshielded resonance below 198 K, consistent with its globally aromatic character with a dominant 30π conjugation pathway along the periphery. It exhibits open-shell diradical character with a moderate singlet–triplet energy gap (Δ E S-T = −6.23 kcal/mol). Its dication is antiaromatic and open-shell, showing a smaller Δ E S-T value (−4.18 kcal/mol). Overall, [6]CPPM behaves like an open-shell aromatic “super-benzene”.