Protonated oxalyl chloride and the ClCO⁺ cation

The reactions of oxalyl chloride were investigated in the binary superacidic systems HF/SbF₅ and DF/SbF₅. O-Monoprotonated oxalyl chloride was isolated and represents the first example of a protonated acyl chloride. Diprotonated oxalyl chloride is only stable in solution. Salts of the ClCO⁺ cation were synthesized from the reactions of oxalyl chloride or COClF with SbF₅ in 1,1,1,2-tetrafluoroethane (R-134a, CF₃CFH₂). The colourless salts were characterized by low-temperature vibrational spectroscopy, NMR spectroscopy and single-crystal X-ray diffraction. (1,2-Dichloro-2-oxoethylidene)oxidanium hexafluoridoantimonate(V), [C₂O(OH)Cl₂][SbF₆], crystallizes in the monoclinic space group P2₁ and carbonyl chloride hexadecafluoridotriarsenate(V) [ClCO][Sb₃F₁₆], in the trigonal space group P3₁, with two and three formula units per unit cell, respectively. Monoprotonated oxalyl chloride and the ClCO⁺ cation both display very short C-Cl bonds with a strong double-bond character.