The influence of hydrogen bonding on the planar arrangement of melamine in crystal structures of its solvates, cocrystals and salts

Abstract

The hydrogen bonding patterns of melamine as well as mono- and diprotonated melamine have been analysed in five crystal structures of a solvate, a cocrystal and three organic and inorganic salts, namely, melamine DMSO solvate ([mel]·DMSO (1)), melamine theobromine cocrystal ([mel]·[TBR]₃ (2)), dimelaminium ethylenediaminetetraacetate ([mel-H]₂[EDTA-H₂]·2H₂O (3)), anhydrous dimelaminium sulfate ([mel-H]₂[SO₄] (4)), and anhydrous melaminium dinitrate ([mel-H₂][NO₃]₂ (5)). Melamine is a versatile molecular building block (tecton) in cocrystals, solvates and salts. Depending on the degree of protonation and/or other molecules or ions present in the structure, parallels could be drawn to determine whether melamine is arranged in a cross-linked manner, in undulating sheets or in the form of perfectly planar sheets in the structure. Graph set analysis was used to compare the geometry of hydrogen bond interactions of the new structures with those of other structures published in the literature. Solvent drop-assisted solid state reactions (kneading) were performed for “green” synthesis of the compounds. The organic salt 3 has high thermal stability as shown by variable-temperature X-ray powder diffraction, which is presumably related to its extensive hydrogen bond network. Rietveld refinements were carried out on laboratory powder diffraction data to confirm the structures of the compounds obtained from single-crystal data.