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, ¹H-, ¹³C-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 P2₁/c, a = 14.292(2), b = 9.449(6), c = 16.059(2) Å, β= 114.20(1)°, V = 1978.4(13) ų, 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) Å and N⋯H-O 148.4(5)°] 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.