Insights into the Structure of Halide-Rich Hydrochloric and Hydrobromic Acid: A Structural and Quantum-Chemical Investigation of the [H6X4O2]2− (X = Cl, Br) Anions

The previously unknown dianions [H6Cl4O2]2− and [H6Br4O2]2− were prepared as their bis(triphenyl-λ5-phosphanylidene)ammonium ([Ph3PNPPh3]+ ≡ [PNP]+) salts from [PNP]+ halides and aqueous hydrochloric or hydrobromic acid, respectively. The crystal structures of the two salts [PNP]2[H6Cl4O2] and [PNP]2[H6Br4O2] are reported. The unprecedented dianions [H6Cl4O2]2− and [H6Br4O2]2−, which represent a section of the structure of halide-rich hydrochloric or hydrobromic acid, are discussed. Quantum-chemical calculations support the findings. The crystal structure of [PNP][HBr2], a rare example for a crystallographically determined hydrogen dibromide structure, is presented as well.