Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44780 Bochum (Germany).
Angew Chem Int Ed Engl. 2015 Feb 23;54(9):2656-60. doi: 10.1002/anie.201411234. Epub 2015 Feb 5.
The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five-membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible-light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.
芴阳离子是教科书中 4π 反芳香阳离子的一个典型例子。然而，关于稠合苯环如何补偿其核心中环的 4π 反芳香五元环的不稳定性，已经发表了相互矛盾的结果。尽管以前在超强酸性介质中合成这种阳离子的尝试仅得到了未定义的聚合物材料，但我们在此报告，通过二氮杂芴的光解，可以在低于 30 K 的温度下在非晶态冰中生成并分离这种阳离子。在这些条件下，芴亚基被水质子化生成芴阳离子，其可以通过光谱法进行表征。其在可见光范围内的吸收与以前通过超快吸收光谱获得的吸收相匹配，并且可以记录其 IR 光谱。非晶冰中的 IR 带非常好地与 DFT 和 DFT/MM 计算的预测相匹配，表明阳离子和周围水分子之间没有强烈的相互作用。
