Ring-Laddering and Ring-Stacking: Unifying Concepts in the Structural Chemistry of Organic Ammonium Halides

Abstract

The ring-laddering and ring-stacking concepts of structural inorganic chemistry, having been shown previously to be useful for rationalizing the crystal structures of organic secondary ammonium halides, are shown also to be applicable to describe the structural chemistry of tertiary and primary ammonium halides. General examination of the directional preferences of N+···X- contacts in the crystal structures of 196 tertiary ammonium halides (R3NH+X-) and 59 primary ammonium halides (RNH3+X-) confirms that the shortest contacts in each structure are N+−H···X- hydrogen bonds that are close to linear. The next shortest N+···X- contacts display preferred directions of approach toward the centers of the faces of the pseudo-tetrahedral R3NH+ and RNH3+ moieties. Association of R3NH+X- ion pairs according to these preferences leads frequently to the formation of dimers, ladders, and 2D nets in crystal structures of tertiary ammonium halides. Similar association of R3NH+X- ion pairs leads frequently to the formation of ladders, 2D nets, cubanes, and extended stacks in crystal structures of primary ammonium halides. The observed structural motifs, in particular the distribution of N+(−H)···X- distances, are influenced by interactions between the R groups of the organic moieties, both within and between motifs.