Department of Chemistry, Indian Institute of Science Education and Research (IISER), Pune-411008, Maharashtra (India).
Angew Chem Int Ed Engl. 2014 Oct 6;53(41):10984-7. doi: 10.1002/anie.201406893. Epub 2014 Aug 26.
Ever since the discovery of the trityl radical, isolation of a stable and neutral organic radical has been a synthetic challenge. A (4n+1)π open-shell configuration is one such possible neutral radical but an unusual state between aromatic (4n+2)π and antiaromatic (4n)π electronic circuits. The synthesis and characterization of an air- and water-stable neutral 25π pentathiophene macrocyclic radical is now described. It undergoes reversible one-electron oxidation to a 24π antiaromatic cation and reduction to a 26π aromatic anion, thus confirming its amphoteric behavior. Structural determination by single-crystal X-ray diffraction studies revealed a planar configuration for the neutral radical, antiaromatic cation, and aromatic anion. In the solution state, the cation shows the highest upfield chemical shift ever observed for a 4nπ system, while the anion adhered to aromatic nature. Computational studies revealed the delocalized nature of the unpaired electron as confirmed by EPR spectroscopy.
自从三价自由基被发现以来，分离稳定的中性有机自由基一直是一个合成上的挑战。（4n+1）π 开壳层构型就是这样一种可能的中性自由基，但它处于芳香（4n+2）π 和反芳香（4n）π 电子电路之间的一种不寻常状态。现在已经描述了一种空气和水稳定的中性 25π 五噻吩大环自由基的合成和表征。它可以可逆地被氧化为 24π 反芳香阳离子，被还原为 26π 芳香阴离子，从而证实了其两性行为。通过单晶 X 射线衍射研究确定了中性自由基、反芳香阳离子和芳香阴离子的平面构型。在溶液状态下，阳离子表现出迄今为止在 4nπ 体系中观察到的最高场化学位移，而阴离子则保持芳香性质。计算研究表明，未配对电子具有离域性质，这一点通过电子顺磁共振（EPR）光谱得到了证实。
