Structural Motifs in Secondary Ammonium Halides: Ring-Stacking and Ring-Laddering in the Organic Solid State

✍ Scribed by Andrew D. Bond

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 515 KB
• Volume: 10
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200305700

✦ Synopsis

Abstract

The ring‐stacking and ring‐laddering concepts of structural inorganic chemistry may be applied to rationalize motifs observed for secondary ammonium halides R~2~NH~2~X (X = Cl, Br) in the organic solid state. General examination of the directional preferences of N^+^⋅⋅⋅X^−^ contacts in 166 crystal structures confirms that the shortest contacts (3.0–3.2 and 3.2–3.4 Å, X = Cl, Br) are N^+^H⋅⋅⋅X^−^ hydrogen bonds lying approximately along the directions of the N^+^H bond vectors. The next shortest N^+^⋅⋅⋅X^−^ contacts display two preferred directions of approach: i) contacts in the distance range 3.2–3.5 (X = Cl) and 3.2–3.9 Å (X = Br) lie close to the HN^+^H plane, along the direction of the bisector of the HN^+^H angle; ii) contacts in the distance range 4.0–4.2 (X = Cl) and 4.0–4.4 Å (X = Br) lie close to the HN^+^H plane, along the direction of an axis extending to the rear of one of the N^+^H bonds. Both directions of approach lead frequently to association of R~2~NH~2~^+^X^−^ ion pairs into laddered motifs. Stacking association is also observed, giving rise in one case to discrete cubanes and in several other cases to extended stacked‐cube arrangements. In each case, the distribution of N^+^⋅⋅⋅X^−^ contacts reflects a balance between the directional properties of the N^+^H⋅⋅⋅X^−^ hydrogen bonds and (primarily steric) interactions between the R groups of the organic moieties. The ladder and stack motifs of the organic ammonium halides are in many cases directly comparable to those in alkali metal amides, [R~2~NM]~n~, and information derived from the extensive organic sample provides insight into the motifs adopted by the inorganic complexes.