Intramolecular Hydrogen Bonding and Tautomerism in Schiff Bases: Synthesis, Spectroscopic and Theoretical Studies of N-(2,2 '-Methylenebis(4-methoxyphenyl)salicylidene and N-(2,2 '-Methylenebis(4-methoxyphenyl)-2-oxonaphthalidene-methylamine

Abstract

The new Schiff bases (4 and 5) were synthesized respectively from the reaction of 2,2'-methylenebis(4-aminomethoxybenzene) (3) with salicylaldehyde and 2-hydroxy-1-naphthaldehyde. The products have been characterized by elemental analysis, FTIR, UV-visible and NMR techniques. The tautomeric equilibria of 2-hydroxy Schiff bases were investigated in polar, non-polar, acidic and basic media by using UV-visible absorption spectra. Compound 4 is in, predominantly, phenol-imine (O-H center dot center dot center dot N) form, whereas compound (5) is in tautomeric equilibria (phenol-imine, O-H center dot center dot center dot N and keto-amine, O center dot center dot center dot H-N forms) in polar and non-polar solvents, as supported by H-1 NMR and UV-visible data. All of the NMR assignments were made using H-1, C-13 NMR and aided by 2D COSY, NOESY, HETCOR and HMBC heteronuclear correlation techniques. In addititon, theoretical calculations have been carried out PM6 methods in the MOPAC2009 and Marvinbeans-5_3_01-windows programs. The heat of formation (Delta H-f), enthalpy (Delta H), entropy (Delta S) Gibbs free energy (Delta G), conformations and tautomers of the synthesized compounds are estimated by using MOPAC2009 (PM6) program.