Collect. Czech. Chem. Commun. 1999, 64, 1797-1806

Crystal and Molecular Structure of N-(3,5-Dichloro-2-hydroxybenzylidene)- and N-(2-Hydroxy-3-methoxybenzylidene)aniline Oxides. Delocalisation in the Spacer of the Intramolecular H-Bond and the Problem of Quasi-Aromaticity

Tadeusz M. Krygowskia,*, Beata Stępieńa, Romana Anulewicz-Ostrowskaa, Michał K. Cyrańskia,b, Sławomir J. Grabowskic, Zbigniew Rozwadowskid and Teresa Dziembowskad

a Department of Chemistry, University of Warsaw, L. Pasteura 1, 02-093 Warsaw, Poland
b Interdisciplinary Centre for Mathematical and Computational Modelling, Pawińskiego 5a, 02-106 Warsaw, Poland
c Institute of Chemistry, University in Białystok, Al. J. Pilsudskiego 11/4, Białystok, Poland
d Institute of Fundamental Chemistry, Technical University of Szczecin, Al. Piastów 42, 71-065 Szczecin, Poland


The hydrogen bond in the title compounds and their analogues with the spacer built of six heavy atoms (O,N,C,C,C,O) between the H-donor OH group and H-acceptor oxygen atom in the oxide group is much stronger than in the case of spacers with five heavy atoms. In spite of that, the delocalisation in the spacer does not depend on the strength of the H-bond, leading to the conclusion that quasi-aromaticity is not a proper term for these kinds of pseudorings. This is supported by analysis of the geometry of N-(3,5-dichloro-2-hydroxybenzylidene)- and N-(2-hydroxy-3-methoxybenzylidene)aniline oxides presented in this study and molecular geometries of five well solved structures retrieved from the CSD. Schleyer's index of aromaticity, NICS, is positive (i.e. indicating nonaromatic behaviour) for a model approximating the title H-bond system spacer indicating no ring current in the pseudoring whereas Li+-chelated system exhibits a slightly aromatic property (NICS < 0).

Keywords: H-bonds; Aromaticity; Schiff bases; Chelates; Crystal structure; X-Ray diffraction; Ab initio calculations.