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 center dot center dot center dot X- and N-H center dot center dot center dot X- interactions link the cations and anions together into ladder structures; the C-Br center dot center dot center dot X- synthons form the rails while the N-H center dot center dot center dot X- synthons form the rungs. In (25DBP)I, chains formed from the C-Br center dot center dot center dot X- synthons are cross-linked into a two-dimensional network by the N-H center dot center dot center dot X- synthons. The competition between N-H center dot center dot center dot X- and C-Br center dot center dot center dot X- synthons indicates that the C-Br center dot center dot center dot X- synthons are of a comparable importance with strong N-H center dot center dot center dot X- hydrogen bonds. In the 35DBP(+) salts, C-Br center dot center dot center dot Br-C synthons compete with the C-Br center dot center dot center dot X- synthons, so the Br center dot center dot center dot 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 center dot center dot center dot X- and N-H center dot center dot center dot X- synthons alternate on the rails and C-Br center dot center dot center dot X- synthons form the rungs in the 35DBP(+) salts. The analysis of the experimental values of C-Br center dot center dot center dot X- (avg. = 169.7 degrees) and N-H center dot center dot center dot X- (avg. = 161.7 degrees) angles indicates that the C-Br center dot center dot center dot 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). (C) 2008 Elsevier B.V. All rights reserved.