[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 C2h symmetry, and the bond lengths of the benzenoid/quinoid rings are averaged via resonance. One averaged 1H 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 (ΔES‑T = −6.23 kcal/mol). Its dication is anti­aromatic and open-shell, showing a smaller ΔES‑T value (−4.18 kcal/mol). Overall, [6]­CPPM behaves like an open-shell aromatic “super-benzene”.

本研究报道了一种[6]环对苯亚甲基（[6]cyclo-para-phenylmethine, [6]CPPM）大环化合物，其展现出类苯的电子性质。其均三甲苯基衍生物[6]CPPM-Mes以晶体形态分离得到。X射线衍射分析表明其具有C2h点群对称性，苯型/醌型环的键长因共振共轭效应趋于平均化。室温下可观测到主链质子的氢核磁共振峰呈现单一平均化峰形，但在198 K以下时，该峰分裂为一个屏蔽峰与一个去屏蔽峰，这与其全局芳香性特征相符，即分子外围存在占主导地位的30π共轭路径。该化合物表现出开壳层双自由基特性，单重态-三重态能隙适中（ΔES‑T = −6.23 kcal/mol）。其二价阳离子具有反芳香性且为开壳层结构，单重态-三重态能隙更小（ΔES‑T = −4.18 kcal/mol）。总体而言，[6]CPPM可被视作一种开壳层芳香性“超苯”。