New benzo-15-crown-5 ethers featuring salicylic Schiff base substitutions -: Synthesis, complexes and structural study

Condensation reactions between 4'-formyl-5'-hydroxybenzo-15-crown-5 and 2-aminopyridine, 2-amino-6-methylpyridine, 2-amino-4-methylpyridine or 2-(aminomethyl)furan yielded the new laterally functionalized crown ethers 1-4. The crown compounds 1-3 form crystalline 1:1 (Na+:ligand) complexes 1a-3a with sodium perchlorate. Ligands and complexes have been characterized by elemental analyses, IR, UV-Vis, H-1-, C-13-NMR and mass spectra. The tautomeric equilibria (phenolimine, O-H...N and keto-amine, O...H-N forms) have been systematically studied by using UV-Vis absorption spectra. The spectra of the ligands 1-4 and complexes 1a-3a were recorded in polar, non-polar, acidic, and basic media. In solutions of polar solvents, tautomeric interconversion of the Schiff base into the keto-amine form has been observed. A crystal structure [monoclinic, space group P2(1)/c, a = 14.292(2), b = 9.449(6), c = 16.059(2) Angstrom, beta = 114.20(1)degrees, V = 1978.4(13) Angstrom(3), Z = 4 and D-x = 1.314 g cm(-3)] shows that compound 4 is in the form of phenol-imine in solid state. There is a strong intramolecular [O-H...N 1.78(6), O...N 2.581(7), O-H 0.89(6) Angstrom and N...H-O 148.4(5)degrees] hydrogen bond between the phenolic oxygen and imine nitrogen atoms. The C=N imine bond reveals a trans planar (1E) configuration. The molecules stack in columns parallel to the a/c plane of the unit cell.