Department of Chemistry, Johns Hopkins University, 3400 North Charles St., Baltimore, MD 21218, USA.
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 2, 8093, Zürich, Switzerland.
Angew Chem Int Ed Engl. 2018 Feb 12;57(7):1924-1927. doi: 10.1002/anie.201712021. Epub 2018 Jan 24.
We report the first spectroscopic evidence for a [C-F-C] fluoronium ion in solution. Extensive NMR studies ( F, H, C) characterize a symmetric cage-like species in which fluorine exhibits substantial covalent bonding to each of the two carbon atoms involved in the three-center interaction. Experimental NMR data comport well with calculated values to lend credence to the structural assignment. As the culminating experiment, a Saunders isotopic perturbation test confirmed the symmetric structure. Congruent with the trend in other types of onium ions, the calculated charge at fluorine moves in a more positive (less negative) direction from the neutral. It is this important trend that explains in part the extraordinary historical difficulty in making theoretical predictions of fluoronium ions come true in solution, and why it takes fluorine captured in a cage to produce, finally, a stable ion and complete the historical arc of the organic halonium ion story.
我们首次报道了在溶液中存在 [C-F-C] 氟鎓离子的光谱证据。广泛的 NMR 研究（ F、 H、 C）表征了一种对称的笼状物种，其中氟原子与参与三中心相互作用的两个碳原子中的每一个都表现出显著的共价键合。实验 NMR 数据与计算值非常吻合，为结构分配提供了可信度。作为最后的实验，桑德斯同位素扰动测试证实了对称结构。与其他类型的鎓离子的趋势一致，氟原子上的计算电荷朝着更正（更少负）的方向移动。正是这一重要趋势部分解释了在溶液中实现氟鎓离子的理论预测如此困难的历史原因，也解释了为什么只有在笼中捕获的氟才能最终产生稳定的离子，完成有机卤鎓离子故事的历史弧线。
