From molecular crystals to salt co-crystals of barbituric acid via the carbonate ion and an improvement of the solid state properties

Abstract

Co-crystals formed by neutral barbituric acid (HBA) molecules and alkaline barbiturate salts have been prepared by solvent-free mechanochemical treatment of solid HBA and alkaline carbonates or bicarbonates. During the reaction the carbonate salts release CO₂ and H₂O, affording the corresponding barbiturate salts which then co-crystallize with the neutral molecule. These “salt co-crystals” have the general formula M^x+(BA⁻)_x·HBA·nH₂O with (M = Na and K). The compounds can be obtained in a single step, by directly grinding HBA with M₂CO₃ (4 : 1 ratio) or MHCO₃ (2 : 1 ratio), or, alternatively, in two steps: the preparation of the BA salt followed by a further grinding process or crystallization of the resulting salts with a stoichiometric amount of neutral HBA. All compounds have been characterized by means of single-crystal and powder XRD, solid-state NMR (¹H MAS, ¹³C and ¹⁵N CPMAS), vibrational spectroscopies and thermal methods (DSC and TGA). A series of BA⁻ salts (either hydrated or anhydrous) of formula M^x+(BA⁻)_x·nH₂O (M = Na, K, Mg and Ca) have also been prepared and reported for comparison.