The role of charge assisted arylhalogen–halide ion interactions in the structures of the dibromopyridinium halide salts

The role that the linear arylbromine–halide ion synthons, in conjunction with traditional hydrogen bonding synthons, play in the development in the crystal structures of the 2,5-dibromopyridinium halide salts, (25DBP)X, and the 3,5-dibromopyridinium halide salts, (35DBP)X (X = Cl, Br, I), are analyzed. In the Cl− and Br− salts of the 25DBP+ cation, the extremely short, nearly linear C–Br···X− and N–H···X− interactions link the cations and anions together into ladder structures; the C–Br···X− synthons form the rails while the N–H···X− synthons form the rungs. In (25DBP)I, chains formed from the C–Br···X− synthons are cross-linked into a two-dimensional network by the N–H···X− synthons. The competition between N–H···X− and C–Br···X− synthons indicates that the C–Br···X− synthons are of a comparable importance with strong N–H···X− hydrogen bonds. In the 35DBP+ salts, C–Br···Br–C synthons compete with the C–Br···X− synthons, so the Br···X− distances increase, but still significantly shorter than, the sum of the van der Waals radii. In the chloride and monoclinic form of the bromide salt, chains are again formed. However, in contrast, C–Br···X− and N–H···X− synthons alternate on the rails and C–Br···X− synthons form the rungs in the 35DBP+ salts. The analysis of the experimental values of C–Br···X− (avg. = 169.7°) and N–H···X− (avg. = 161.7°) angles indicates that the C–Br···X− angles are closer to a linear arrangement in comparison to the corresponding N–H···X− angles. This conclusion is authenticated through theoretical calculations of the optimized structures of the model n-bromopyridinum halids salts: (nBP)X (X = Cl, Br; n = 2 and 3).