The trimorphic structure of N,N'-bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine

The new Schiff base ligand, N,N'-bis(5-methylsalicylidene)-4-methyl-1,3-phenylenediamine (H2L), has been prepared by condensation of 5-methylsalicylaldehyde with 4-methyl-1,3-phenylenediamine. It was found that this ligand crystallizes in the monoclinic space group P21/c, with Z' = 1. Further studies showed that - as a result of the one-pot metal-promoted reactions - three different polymorphic forms of H2L were obtained, with different numbers (1, 3 or 4) of symmetry-independent molecules depending on the lanthanide metal ion present in the reaction media: La (Z' = 3), Nd (Z' = 4), Sm, Tb, Ho and Yb (all Z' = 1); the last form is identical with that obtained by crystallization of the ligand itself. The geometrical features of all eight independent molecules of H2L are very similar. The relatively strong intramolecular O-H...N hydrogen bonds stabilize almost co-planar conformations of terminal rings and C-C=N-C linkers, at the same time strong hydrogen-bond donors and acceptors involved in these interactions cannot take part in the determination of supramolecular architecture. Therefore, weak intermolecular interactions are important and this can be regarded as one of the reasons for packing conflicts that lead to the presence of polymorphic forms and multiple molecules. The pseudosymmetries are observed in both forms with Z' > 1 and the degree of pseudosymmetry is described by the values of appropriate combinations of the coordinates. In order to elucidate the differences in crystal structures the Hirshfeld surface method was applied. It shows that there are only small differences in the surface shape and in the fingerprint plots; however, the volumes of voids in three structures are significantly different.