Salts of 4-aminobutyric acid and 6-aminohexanoic acid behaving as molecular Velcro

Abstract

The crystal structures of eight novel carboxyalkylammonium salts, (⁺H₃N(CH₂)nCOOH)X⁻, are reported, with n = 4 and X = Cl, Br and I in structures 1, 2 and 3, respectively, and n = 6 and X = Cl, Br·0.5H₂O, Cl·0.5H₂O, NO₃ and ClO₄ in structures 4, 5, 6, 7 and 8. The members of this family of compounds were found to display significant structural diversity, and a careful analysis of the structures employing the principles of crystal engineering was done to explain the observed trends and differences, specifically also the interdigitation or non-interdigitation of alkyl chains. It was found that a primary hydrogen bonding network formed between the ammonium groups and halide or oxo-anions, which plays a major structure-directing role. The structures may be likened to molecular Velcro, in which secondary hydrogen bonding interactions involving the carboxylic acid groups act as “hooks” to link primary networks.