New pharmaceutical salts containing pyridoxine

Two mixed crystals were obtained by crystallizing the active pharmaceutical ingredient pyridoxine [systematic name: 4,5-bis(hydroxymethyl)-2-methylpyridin-3-ol, PN] with (E)-3-(4-hydroxy-3-methoxyphenyl)prop-2-enoic acid (ferulic acid) and 4-hydroxy-3,5-dimethoxybenzoic acid (syringic acid). PN and the coformers crystallize in the form of pharmaceutical salts in a 1:1 stoichiometric ratio, namely 3-hydroxy-4,5-bis(hydroxymethyl)-2-methylpyridin-1-ium (E)-3-(4-hydroxy-3-methoxyphenyl)prop-2-enoate, C₈H₁₂NO₃⁺·C₉H₉O₅^-, and 3-hydroxy-4,5-bis(hydroxymethyl)-2-methylpyridin-1-ium 4-hydroxy-3,5-dimethoxybenzoate monohydrate, C₈H₁₂NO₃⁺·C₁₀H₁₁O₅^-·H₂O, the proton exchange between PN and the acidic partner being supported by the differences of the pK_a values of the two components and by the C-O bond lengths of the carboxylate groups. Besides complex hydrogen-bonding networks, π-π interactions between aromatic moieties have been found to be important for the packing architecture in both crystals. Hirshfeld surface analysis was used to explore the intermolecular interactions in detail and compare them with the interactions found in similar pyridoxine/carboxylic acid salts.