Eight Schiff bases derived from various salicylaldehydes: phenol-imine and keto-amine forms, conformational disorder, and supramolecular assembly in one and two dimensions.

Abstract

Structures are reported for eight Schiff bases derived from various salicylaldehydes: five are newly synthesized and re-investigations are reported for three previously reported structures, leading, in each case, to some revision of previous conclusions. In (E)-N-(3,4-dimethylisoxazol-5-yl)-4-[(2-hydroxybenzylidene)amino]benzenesulfonamide, C18H17N3O4S, (I), and (E)-4-[(5-bromo-2-hydroxy-3-methoxybenzylidene)amino]-N-(3,4-dimethylisoxazol-5-yl)benzenesulfonamide. C19H18BrN3O5S, (II), the isoxazole rings adopt different orientations relative to the rest of the molecules, despite the additional substituents in (II) being in the aryl ring remote from the isoxazole unit. The molecules of both (E)-4-bromo-2-[(2-hydroxyphenylimino)methyl]-6-methoxyphenol, C14H12BrNO3, (III), and (E)-4-bromo-2-methoxy-6-[(2-methoxyphenylimino)methyl]phenol, C15H14BrNO3, (IV), are both effectively planar; while (III) adopts the phenol-imine constitution, (IV) adopts the keto-amine constitution. (E)-2-Methoxy-6-[(2-methoxyphenylimino)methyl]phenol, C15H15NO3, (V), which was determined previously using powder X-ray data assuming the phenol-imine constitution, has now been refined from single-crystal X-ray data, confirming the phenol-imine constitution. In (E)-3-benzoyl-2-[(5-fluoro-2-hydroxybenzylidene)amino]-4,5,6,7-tetrahydrobenzo[b]thiophene, C22H18FNO2S, (VI), the fused carbocyclic ring exhibits conformational disorder; both disorder components, having populations of 0.705 (4) and 0.295 (4), adopt half-chair conformations. The isostructural (E)-3-benzoyl-2-[(2-hydroxybenzylidene)amino)]-4,5,6,7-tetrahydrobenzo[b]thiophene, C22H19NO2S, (VII), which was originally reported as having a fully ordered structure [Kaur et al. (2014). Acta Cryst. E70, o476-o477], has been rerefined using the original data set and found to exhibit the same type of disorder as found in (VI), with disordered populations having occupancies of 0.851 (3) and 0.149 (3). The triclinic polymorph of (E)-[(2-hydroxyphenylimino)methyl]phenol, C13H11NO2, (VIII), which crystallizes with Z' = 2 in the space group P-1, has been described variously as occurring as the keto-amine tautomer [Maciejewska et al. (1999). J. Phys. Org. Chem. 12, 875-880] and as the phenol-imine tautomer [Tunç et al. (2009). J. Chem. Crystallogr. 39, 672-676]. Rerefinement of this structure using one of the original data sets shows that both of the independent molecules exist in the keto-amine form. In the structures of compounds (I), (VI), (VII) and (VIII), hydrogen bonds generate simple chains, while a chain of rings is formed in (V). Sheets are formed by hydrogen bonds in both (II) and (III), while in (IV), the sheet structure is built from aromatic π-π stacking interactions.