On tautomerism of two 5-methoxysalicylaldimine structural isomers in the solid state:: Structural study of N-(o-hydroxyphenyl)-5-methoxysalicylaldimine and N-(m-hydroxyphenyl)-5-methoxysalicylaldimine

Two structural isomers of 5-methoxysalicylaldimine Schiff-bases; N-(o-hydroxyphenyl)-5-methoxysalicylaldimine, [(Sal-o-OH-ph)H], and N-(m-hydroxyphenyl)-5-methoxysalicylaidimine, [(Sal-m-OH-ph)H], have been prepared and characterized in the solid state by means of the single crystal X-ray diffractometry at 298 and 200 K, and also by IR spectroscopy. Two structures, which are different only in the position of the N-substituent phenyl ring hydroxy group, are characterized by different tautomeric forms, i.e. ortho- and meta-hydroxy derivatives exist as the N-H...O and O-H...N tautomers, respectively. The tautomeric forms are influenced by life different intermolecular hydrogen bonding patterns of the molecules in the crystalline state. The stabilization of the non-favourable NH form of (Sal-o-OH-ph)H is ascribed to stronger intermolecular O-H...O hydrogen bonds in (Sal-o-OH-ph)H than in (Sal-m-OH-ph)H. Both compounds form discrete centrosymmetrical dimers leading to 14- and 16-membered rings for (Sal-o-OH-ph)H and (Sal-m-OH-ph)H, respectively. The molecules are non-planar and twisted around C-N single bond by 13.9(2) and 10.5(2)degrees in (Sal-o-OH-ph)H and (Sal-m-OH-ph)H, respectively. The crystal structure analyses at 200 K show that there is no change in the type of the tautomeric form in comparison to 298 K data. (C) 2002 Elsevier Science B.V. All rights reserved.