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

N-(2-pyridil)-salicylidene (1) and N-(2-pyridil)-2-oxo-1-naphthylidene-meth ylamine (2) were studied by elemental analysis, IR, H-1 NMR and UV-visible techniques and the structure of compound (2) was examined crystallographica lly. The UV-visible spectra of 2-hydroxy Schiff bases are investigated in d ifferent solvents, acidic and basic media. Compound (2) is in tautomeric eq uilibrium (phenol-imine, O-H ... N reversible arrow keto-amine, O ... H-N f orms) in polar and non-polar solvents. These tautomers are not observed in polar and non-polar solvents for (1) as also supported by H-1 NMR and UV-vi sible data. The keto-amine form of compound (2) was observed in basic solut ions of DMSO, ethanol, chloroform, benzene, cyclohexane and in acidic solut ions of chloroform and benzene, but not in acidic solutions of DMSO and eth anol. On the contrary, this form for compound (1) was not observed in the s ame solutions. The asymmetric unit of compound (2) contains two independent molecules of (C16H12N2O) which constitute a tautomeric pair. The observed differences in the related C=N (1.317(4) and 1.330(4) Angstrom) and C-O (1. 279(4) and 1.263(4) Angstrom) bond lengths in the two crystallographically independent molecules indicate that the phenol-imine and the keto-amine for ms coexist in the solid state. Intramolecular hydrogen bond lengths (O-H .. . N) are 2.586(4) Angstrom and 2.518(4) Angstrom for the two individual tau tomers. It crystallizes in the monoclinic space group P2(1)/n with a = 5.83 7(2), b = 17.476(2), c = 24.295(3) Angstrom, beta = 91.95(4)degrees, V = 24 76.9(6) Angstrom(3), Z = 4 and Dx = 1.3315 g cm(-3). (C) 2000 Elsevier Scie nce B.V. All rights reserved.