Intramolecular hydrogen bonding and tautomerism in Schiff bases. Structure of N-(2-pyridil)-2-oxo-1-naphthylidenemethylamine

Abstract

N-(2-pyridil)-salicylidene (1) and N-(2-pyridil)-2-oxo-1-naphthylidene-methylamine (2) were studied by elemental analysis, IR, 1H NMR and UV–visible techniques and the structure of compound (2) was examined crystallographically. The UV–visible spectra of 2-hydroxy Schiff bases are investigated in different solvents, acidic and basic media. Compound (2) is in tautomeric equilibrium (phenol–imine, 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 also supported by 1H NMR and UV–visible data. The keto–amine form of compound (2) was observed in basic solutions of DMSO, ethanol, chloroform, benzene, cyclohexane and in acidic solutions of chloroform and benzene, but not in acidic solutions of DMSO and ethanol. On the contrary, this form for compound (1) was not observed in the same solutions. The asymmetric unit of compound (2) contains two independent molecules of (C16H12N2O) which constitute a tautomeric pair. The observed differences in the related CN (1.317(4) and 1.330(4)Å) and C–O (1.279(4) and 1.263(4)Å) bond lengths in the two crystallographically independent molecules indicate that the phenol–imine and the keto–amine forms coexist in the solid state. Intramolecular hydrogen bond lengths (O–H⋯N) are 2.586(4)Å and 2.518(4)Å for the two individual tautomers. It crystallizes in the monoclinic space group P21/n with a=5.837(2), b=17.476(2), c=24.295(3)Å,β=91.95(4)°, V=2476.9(6)Å3,Z=4 and Dx=1.3315gcm−3.