Organic ammonium halides as analogues of main-group amide/imide complexes in the solid state: Extension of ring-laddering and ring-stacking principles

Abstract

The structural chemistry of primary, secondary and tertiary ammonium halides is reviewed with emphasis on application of ring-laddering and ring-stacking concepts in the organic solid state. Similarities between motifs observed in the crystal structures of the ammonium halides and in amide/imide complexes of main-group elements suggest a general analogy between R x NH 4− x +X − (X = Cl, Br, I) and R x N (3− x)− M (3− x)+ (M = principally Group 1/Group 2 cations and derivatives of Group 13 elements, e.g. [AlR] 2+, [AlR 2] +, [GaR] 2+, [GaR 2] +, etc.). The origin of the analogy lies in comparable directional preferences for association of the cations and anions in both the organic and main-group systems. Although similar motifs are observed amongst the organic and main-group structures, directly comparable ammonium and amide/imide moieties (i.e. R x NH 4− x + and R x N (3− x)− with identical R groups) rarely form comparable motifs. This is attributed primarily to different metric features: N +(−H)⋯X − distances in the organic sample are significantly longer than N⋯M distances in the main-group complexes. This affects the balance between electrostatic forces that promote further association of the cationic and anionic moieties and the (principally steric) interactions between the amine moieties that hinder further association. Additional factors that contribute to the observed differences include greater electrostatic energy for divalent RN 2−M 2+, directional bonding preferences for the M n+ cation and the influence of solvation in the main-group systems.