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

Abstract

Two structural isomers of 5-methoxysalicylaldimine Schiff-bases; N-( o-hydroxyphenyl)-5-methoxysalicylaldimine, [(Sal- o-OH-ph)H], and N-( m-hydroxyphenyl)-5-methoxysalicylaldimine, [(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 the 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)° 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.