New NO donor ligands and complexes containing furfuryl or crown ether moiety: Syntheses, crystal structures and tautomerism in ortho-hydroxy substituted compounds as studied by UV–vis spectrophotometry

• New NO donor ligands and their nickel (II) and sodium complexes were prepared.
• The solid state structures of ligands and nickel (II) complex were verified by X-ray diffraction analysis.
• The tautomeric equilibria have been systematically studied.

NO donor ligands were prepared by the condensation of methoxy substituted salicylaldehyde with 5-methylfurfurylamine (1 and 2) and 4′-aminobenzo-15-crown 5 (3–5). New crown ether ligands of Schiff base type (3–5) containing recognition sites for alkali metal and transition guest cations. Ni(II) complexes (1a–5a) have been synthesized with bidentate NO donor Schiff base ligands (1–5) with Ni(CH3COO)2.·4H2O. Monotopic crystalline 1:1 (Na+:ligand) sodium complexes (3b–5b) of the crown ether ligands were also prepared. Schiff bases (1–5) and complexes (1a–5a, 3b–5b) were characterized by elemental analyses, FT-IR, 1H-, 13C-NMR and mass spectroscopies. The crystal structures of 1, 1a and 2 were verified by X-ray diffraction analysis. The tautomeric equilibria (phenol-imine, O–H⋯N and keto-amine, O⋯H–N forms) have been systematically studied by using UV–vis spectrophotometry for the o-hydroxy substituted compounds (1–5). The UV–visible spectra of these ligands (1–5) were recorded and commented in polar, non-polar, acidic and basic media.

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