Solid and solution structures of bulky tert-butyl substituted salicylaldimines

Abstract

A series of tert-butyl substituted salicylaldimines was synthesized and characterized by IR, 1H NMR, 13C NMR, UV–vis, X-ray diffraction and elemental analyses to investigate the effect of two bulky tert-butyl groups on the keto-enol tautomerization in solution and solid state. According to X-ray studies all salicylaldimines have an enol tautomeric form in solid state and due to strong O–H···N hydrogen bonding they form a nearly planar intramolecular six-membered ring. Based on NMR, IR and UV–vis studies, the enol form is also present in solutions. Computational studies carried out by the density function theory (DFT) method verifies that in each sterically hindered keto-enol pair the enol form is more stable, although the differences in energy vary from 5.5 to 10.1 kJ/mol (in terms of total energy obtained from Gaussian for the optimized moieties). For the tert-butyl substituted salicylaldimines, solvatochromism is not observed in polar hydrogen bonding solvents. As N-alkyl and N-alkylphenyl substituted salicylaldimines without tert-butyl groups have a solvatochromic band, the sterically bulky tert-butyl groups seem to have a key role in stabilizing the intramolecular hydrogen bonding.