An Overlooked Supramolecular Synthon in Multicomponent Trimethylglycine Crystals: Moderate Hydrogen Bonding Between Carboxylate and H-N Groups of Guanidine Species

Three novel multicomponent crystals of trimethylglycine with 2-cyanoguanidine, guanidinium and aminoguanidinium chlorides are synthesized and structurally characterized. All three crystal packings are based on the supramolecular synthon formed by two N–H groups of the guanidine species and carboxylate group of trimethylglycine (graph set notation R22(8)). Its enthalpy is about 50 kJ•moL−1. The three-dimensional structure of crystals is stabilized by intermolecular interactions of various types. The energy of C–H∙∙∙X− interactions, where X = O, Cl, reaches 16 kJ•moL−1 due to the acidic nature of methyl hydrogens. The possible structure of the trimethylglycine–urea–2H2O complex is discussed. Its theoretical metric and spectroscopic parameters are in reasonable agreement with the available literature data on the deep eutectic solvent trimethylglycine–urea.