Syntheses, crystal structures and Hirshfeld surface analyses of four molecular salts of amitriptynol

The syntheses and crystal structures of four salts of amitriptynol (C₂₀H₂₅NO) with different carboxylic acids are described. The salts formed directly from solutions of amitriptyline (which first hydrolysed to amitriptynol) and the corresponding acid in acetonitrile to form amitriptynolium [systematic name: (3-{2-hydroxytricyclo[9.4.0.0^3,8]pentadeca-1(11),3,5,7,12,14-hexaen-2-yl}propyl)dimethylazanium] 4-methoxybenzoate monohydrate, C₂₀H₂₆NO⁺·C₈H₇O₃^-·H₂O, (I), amitriptynolium 3,4-dimethoxybenzoate trihydrate, C₂₀H₂₆NO⁺·C₉H₉O₄^-·3H₂O, (II), amitriptynolium 2-chlorobenzoate, C₂₀H₂₆NO⁺·C₇H₄ClO₂^-, (III), and amitriptynolium thiophene-2-carboxylate monohydrate, C₂₀H₂₆NO⁺·C₅H₃O₂S^-·H₂O, (IV). Compound (III) crystallizes with two cations, two anions and six water molecules in the asymmetric unit. The different conformations of the amitriptynolium cations are determined by the torsion angles in the dimethylamino-propyl chains and the –CH₂–CH₂- bridge between the benzene rings in the tricyclic ring system, and are complicated by disorder of the bridging unit in II and III. The packing in all four salts is dominated by N—H· · ·O and O—H· · ·O hydrogen bonds. Hirshfeld surface analyses show that the amitriptynolium cations make similar inter-species contacts, despite the distinctly different packing in each salt.