Molecular versus crystal symmetry in tri-substituted triazine, benzene and isocyanurate derivatives

The crystal structures of triethyl-1,3,5-triazine- 2,4,6-tricarboxylate ( I), triethyl-1,3,5-benzenetricarboxylate ( II) and tris-2-hydroxyethyl isocyanurate ( III) have been determined from conventional laboratory X-ray powder diffraction data using the differential evolution structure solution technique. The determination of these structures presented an unexpectedly wide variation in levels of difficulty, with only the determination of ( III) being without complication. In the case of ( I) structure solution resulted in a Rietveld refinement profile that was not ideal, but was subsequently rationalized by single-crystal diffraction as resulting from disorder. Refinement of structure ( II) showed significant variation in side-chain conformation from the initial powder structure solution. Further investigation showed that the structure solution optimization had indeed been successful, and that preferred orientation had a dramatic effect on the structure-solution R-factor search surface. Despite the presence of identical side chains in ( I) and ( II), only the triazine-based system retains threefold molecular symmetry in the crystal structure. The lack of use of the heterocyclic N atom as a hydrogen-bond acceptor in this structure results in the formation of a similar non-centrosymmetric network to the benzene-based structure, but with overall three-dimensional centrosymmetry. The hydrogen-bonded layer structure of ( III) is similar to that of other isocyanurate-based structures of this type.