Relative Abilities of Fluorine and Chlorine to Stabilize Carbenium Ions: Crystal Structures of Two Fluoro-Substituted Carbocations and of As₂F₁₁^-

Abstract

The first crystal structures of fluoro-substituted carbocations and of the As2F11- anion are reported. The experimental geometries of the carbenium ions in [(CH3)2CF]+AsF6-, [(m-CF3C6H4)(C6H5)CF]+AsF6-, and [(m-CF3C6H4)(C6H5)CF]+As2F11- and their comparison with that of the [(o-ClC6H4)(C6H5)CCl]+ cation show that, in accord with previous theoretical calculations, chlorine stabilizes carbenium ions more efficiently than fluorine. The 13C and 19F NMR spectra of the [(m-CF3C6H4)(C6H5)CF]+ cation were recorded and analyzed with the help of RHF/6-31G(d,p) calculations using the GIAO method. In each of the three fluoro-substituted carbocation crystal structures studied, the carbenium centers are further stabilized by forming two fluorine bridges with the anions, resulting in distorted trigonal bipyramidal environments around the carbenium centers. The [F5As−F−AsF5]- anion in [(m-CF3C6H4)(C6H5)CF]+As2F11- possesses a symmetric fluorine bridge with an As−F−As angle of 156.5(13)° and staggered AsF4 groups.