Structural investigations of benzoyl fluoride and the benzoacyl cation of low-melting compounds and reactive intermediates.

Abstract

Acyl fluorides and acyl cations represent typical reactive intermediates in organic reactions, such as Friedel-Crafts acylation. However, the comparatively stable phenyl-substituted compounds have not been fully characterized yet, offering a promising backbone. Attempts to isolate the benzoacylium cation have only been carried out starting from the acyl chloride with weaker chloride-based Lewis acids. Therefore, only adducts of 1,4-stabilized acyl cations could be obtained. Due to the low melting point of benzoyl fluoride, together with its volitality and sensitivity toward hydrolysis, the structures of the acyl fluoride and its acylium cation have not been determined. Herein, we report the first crystal structure of benzoyl fluoride, C₇H₅FO or PhCOF (monoclinic P2₁/n, Z = 8) and the benzoacylium undecafluorodiarsenate, C₇H₅O⁺·As₂F₁₁^- or [PhCO]⁺[As₂F₁₁]^- (monoclinic P2₁/n, Z = 4). The compounds were characterized by low-temperature vibrational spectroscopy and single-crystal X-ray analysis, and are discussed together with quantum chemical calculations. In addition, their specific π-interactions were elucidated.