NMR and X-ray studies of 2,6-bis(alkylimino)phenol Schiff bases

• Published in: Journal of molecular structure Vol. 844; pp. 94 - 101
• Main Authors: Schilf, Wojciech, Kamieński, Bohdan, Szady-Chełmieniecka, Anna, Grech, Eugeniusz, Makal, Anna, Woźniak, Krzysztof
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 12-11-2007
• Subjects: NMR studies; Schiff bases; X-ray analysis; Schiff bases; X-ray analysis; NMR studies
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2007.05.029

A series of model Schiff bases – 2,6-bis(alkylimino)phenol derivatives (Me, c-Pr, c-Bu, c-Pen and Ph) – has been studied by the solution and solid state (c-Pr and Ph derivatives) NMR methods. All compounds under investigations exhibit tautomeric equilibria in which proton is distributed between two imine sites. For the whole series, the strongest intramolecular H-bond is formed by the methyl derivative. The cycloaliphatic derivatives form weaker H-bonds. Their strength is proportional to the size of the aliphatic ring. The principal components of
15N chemical shift anisotropy (CSA) tensor have been measured in the solid state for the cyclopropyl derivative. The first two principal components of the CSA tensor change in the same way with respects to hydrogen bond formation as the isotropic nitrogen chemical shift values. A relationship between the CSA principal components and structure of tautomers present in the system has been discussed. In chloroform solutions we also measured the coalescence temperatures to estimate the reaction rate and activation energy.
For the c-Pr Schiff-base derivative, the X-ray structure has been established. This compound crystallizes in the monoclinic
P2
1 space group. There are two independent molecules in the crystal lattice. The geometry of the two moieties is very similar with the exception of some subtle changes in the intramolecular hydrogen bond and its vicinity. Although the molecules seem to be related by a glide plane, the symmetry is non-crystallographic and there is no indication of the existence of such a plane in the intensities of the reflections. We present details of the hydrogen bonds in both moieties.