Three molecular salts, benzamidinium 3-methoxybenzoate, C7H9N2(+)·C8H7O3(-), (I), benzamidinium 4-methoxybenzoate, C7H9N2(+)·C8H7O3(-), (II), and benzamidinium 3,4,5-trimethoxybenzoate monohydrate, C7H9N2(+)·C10H11O5(-)·H2O, (III), were formed from the proton-transfer reactions of 3-methoxy, 4-methoxy- and 3,4,5-trimethoxybenzoic acids with benzamidine (benzenecarboximidamide, benzam). Monoclinic salts (I) and (II) have a 1:1 ratio of cation to anion. In monoclinic salt (III), two cation-anion pairs and two water molecules constitute the asymmetric unit. In all three molecular salts, the amidinium fragments and the carboxylate groups are completely delocalized, and the delocalization favours the aggregation of the molecular components into nonplanar dimers with an R(2)(2)(8) graph-set motif by N(+)-H···O(-) (±) charge-assisted hydrogen bonding (CAHB). Of the three molecular salts, (I) and (II) show similar conformations of the anionic components and exhibit bidimensional isostructurality, which consists of alternating R(2)(2)(8) and R(6)(4)(16) rings resulting in a corrugated sheet propagated parallel to the crystallographic ab plane. In molecular salt (III), the R(2)(2)(8) synthon is retained but the supramolecular structure is different, due to the presence of three bulky methoxy substituents and a water molecule. The structures reported here further demonstrate the robustness of R(2)(2)(8) hydrogen-bonded synthons having the benzamidinium cation as a building block, whereas N(+)-H···O(-) hydrogen bonds external to the salt bridge contribute to the overall structure organization.
Keywords: benzamidinium salts; charge-assisted hydrogen bonds; crystal structure; proton-transfer adducts.