Efficient Synthesis and Crystal Structure of the HCO⁺ Cation

In this work, we present an efficient way to synthesize the elusive formylium ion, HCO⁺. Its high reactivity toward even weak nucleophiles necessitates the use of an unconventional solvent like the refrigerant 1,1,1,2-tetrafluoroethane. The stabilization and isolation of the smallest possible acyl cation at low temperatures allowed us to thoroughly investigate its [Sb₂F₁₁]⁻ salt via vibrational spectroscopy and single-crystal X-ray diffraction. The experimental data, together with quantum chemical calculations at the M06-2X/aug-cc-pVTZ-level of theory, help us to understand the stabilization of the elusive cation in the solid state.