Isolating Benzenium Ion Salts

Abstract

When partnered with carborane anions, arenium ions are remarkably stable. Previously investigated only at subambient temperatures in highly superacidic media, protonated benzene is readily isolated as a crystalline salt, thermally stable to >150 °C. Salts of the type [H(arene)][carborane] have been prepared by protonating benzene, toluene, m-xylene, mesitylene, and hexamethylbenzene with the carborane superacid H(CB11HR5X6) (R ) H, Me; X ) Cl, Br). They have been characterized by elemental analysis, X-ray crystallography, NMR and IR methods. Solid-state 13 C NMR spectra are similar to those observed 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 ion pairing. This is reflected in the dependence of the C-H stretching frequency on the basicity of the carborane anion. 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 and vibrational comparison to theory is made for the benzenium ion (C 6H7 + ) with density functional theory at B3LYP/6-31G* and B3P86/6-311+G(d,p) levels. The stability of these salts elevates arenium ions from the status of transients (Wheland intermediates) to reagents. They have been used to bracket the solutionphase basicity of C60 between that of mesitylene and xylene.

[FHF]−—The Strongest Hydrogen Bond under the Influence of External Interactions

“…It is interesting that the decomposition described here of hydrogen bifluoride is typical for some moieties commonly known as super-acids [57,58]. For example HBF 4 , HSbF 6 , HPF 6 or HB(C 6 F 5 ) 4 acids do not exist because of such decomposition, for the HB(C 6 F 5 ) 4 moiety the cleavage of a B-C bond is observed, similarly HBF 4 should be treated as a HF .…”

The Low Basicity of Phosphabenzenes: First Examples of Protonation, Alkylation, and Silylation Reactions

“…Solvents were dried over Na and distilled prior to use. In a typical protonation reaction, freshly prepared Et 3 Si(carborane) [12] (148 mg, 0.185 mmol) was combined with triphosphabenzene 4 (55.4 mg, 0.185 mmol) in a 5 mL vial with a stir bar. Benzene (approximately 1 mL) was added to give a pale yellow solution.…”

“…[11] Ion pairing of the cation with the carborane anion through the acidic proton (H + ···Br 3.13 ) is observed, a feature common to carborane salts of highly acidic cations, such as arenium ions [12] and tert-butyl cation. [13] The major structural effect of protonation on the phosphabenzene is a shortening of the P À C bonds adjacent to the site of protonation, indicative of developing P V character at the phosphininium center.…”

Can the hexamethylhydrazinium dication [Me3N–NMe3]2+ be prepared?

“…‡ The [HMe 2 N-NMe 2 H] 2+ cation 6 2+ is ion-paired with the carborane anions via NH · · · Cl H-bonding. This is a feature common to carborane salts of highly acidic cations such as arenium ions 11 and alkyl carbocations (CH · · · Cl = 2.71-2.85 Å ). 9 The shortest H · · · Cl contact in 6 2+ (CHB 11 Cl 11…”