Intramolecular hydrogen bonding and tautomerism in Schiff bases. Part I. Structure of 1,8-di[N-2-oxyphenyl-salicylidene]-3,6-dioxaoctane

Abstract

1,8-Di[N-2-oxyphenyl-salicylidene]-3,6-dioxaoctane (1) and 1,8-di[N-2-oxyphenyl-2-oxo-1-naphthylidenemethylamino]-3,6-dioxaoctane (2) are obtained from the reaction of triethylene glycol bis(2-aminophenyl ether) with salicylaldehyde and 2-hydroxy-1-naphthaldehyde. Compounds (1) and (2) have been characterized by elemental analysis, IR, 1H-NMR and UV-visible techniques and the structure of compound (1) has been examined crystallographically. The UV-visible spectra of 2-hydroxy Schiff bases are studied in different solvents and acidic media. Compound (2) is in tautomeric equilibria (phenolimine, OH…N ⇌ keto-amine, O…HN forms) in polar and non-polar solvents. These tautomers are not observed in polar and non-polar solvents for (1) as supported by 1H-NMR and UV-visible data. The keto-amine form is observed in acidic solutions of chloroform and benzene, but it is not observed in solutions DMSO and EtOH for compounds (1) and (2). Compound (1) is in the form of phenol-imine in solid state and a crystallographically centrosymmetric multidentate Schiff base ligand, containing two imine nitrogen, two hydroxy and four etheric O atoms. There is a strong intramolecular [ OH⋯ N = 1.488(3) A ̊ ] hydrogen bond. The C  N imine bond and CNC bond angle are 1.270(3)Å and 123.5(2)°. It crystallizes in the monoclinic space group P2 1 n with a = 6.301(1), b = 13.624(1), c = 16.189(1) A ̊ , β = 100.28(1)°, V = 1367.4(3) A ̊ 3 , Z = 2 and D x = 1.313 g cm −3.