Solid and solution structures of bulky tert-butyl substituted salicylaldimines

A series of tert-butyl substituted salicylaldimines was synthesized and characterized by IR, H-1 NMR, C-13 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 center dot center dot center dot 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. (C) 2011 Elsevier B.V. All rights reserved.