Isolating Benzenium Ion Salts

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• 作者：Christopher A. Reed ; Kee-Chan Kim ; Evgenii S. Stoyanov ; Daniel Stasko ; Fook S. Tham ; Leonard J. Mueller ; and Peter D. W. Boyd
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：February 19, 2003
• 年：2003
• 卷：125
• 期：7
• 页码：1796 - 1804
• 全文大小：213K
• 年卷期：v.125,no.7(February 19, 2003)
• ISSN：1520-5126

文摘

When partnered with carborane anions, arenium ions are remarkably stable. Previouslyinvestigated only at subambient temperatures in highly superacidic media, protonated benzene is readilyisolated as a crystalline salt, thermally stable to >150 C. Salts of the type [H(arene)][carborane] havebeen prepared by protonating benzene, toluene, m-xylene, mesitylene, and hexamethylbenzene with thecarborane superacid H(CB₁₁HR₅X₆) (R = H, Me; X = Cl, Br). They have been characterized by elementalanalysis, X-ray crystallography, NMR and IR methods. Solid-state ¹³C NMR spectra are similar to thoseobserved earlier in solution, indicating that lattice interactions are comparable to solution solvation effects.The acidic proton(s) of the arenium cations interact weakly with the halide substituents of the anion via ionpairing. This is reflected in the dependence of the C-H stretching frequency on the basicity of the carboraneanion. Bond lengths in the arenium ions are consistent with predominant cyclohexadienyl cation character,but charge distribution within the cation is less well represented by this resonance form. Structural andvibrational comparison to theory is made for the benzenium ion (C₆H₇⁺) with density functional theory atB3LYP/6-31G* and B3P86/6-311+G(d,p) levels. The stability of these salts elevates arenium ions fromthe status of transients (Wheland intermediates) to reagents. They have been used to bracket the solution-phase basicity of C₆₀ between that of mesitylene and xylene.