Different structural destinations: comparing reactions of [CuBr2(3-Brpy)(2)] crystals with HBr and HCl gas

Reaction of green crystalline solid trans-[CuBr2(3-Brpy)2] 1 (3-Brpy = 3-bromopyridine) with HBr (aq) vapour yields brown crystalline salt (3-BrpyH)2[CuBr4] 2 with quantitative conversion. Notably 2 adopts a different crystal structure to the three mutually isostructural compounds (3-XpyH)2[CuCl4] (X = Cl, Br) and (3-BrpyH)2[CuBr2Cl2] which result from reaction with HCl. Crystalline product 2 has been characterised by X-ray powder diffraction and its conversion back to 1 at 370–400 K has been followed in situ by synchrotron X-ray powder diffraction. Crystalline 1 and 2 are further notable for the presence of intermolecular C–BrBr–Cu halogen bonds and (only in the case of 2) N–HBr–Cu hydrogen bonds.