Crystal and molecular structure of N-(3,5-dichloro-2-hydroxybenzylidene)- and N-(2-hydroxy-3-methoxybenzylidene)aniline oxides. Delocalisation in thespacer of the intramolecular H-bond and the problem of quasi-aromaticity

The hydrogen bond in the title compounds and their analogues with the space r 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 t he spacer does not depend on the strength of the H-bond, leading to the con clusion that quasi-aromaticity is not a proper term for these kinds of pseu dorings. This is supported by analysis of the geometry of N-(3,5-dichloro- 2-hydroxy-benzylidene)- and N-(2-hydroxy-3-methoxybenzylidene)anilin oxides presented in this study and molecular geometries of five well solved struc tures retrieved from the CSD. Schleyer's index of aromaticity, NICS, is pos itive (i.e. indicating nonaromatic behaviour) for a model approximating the title H-bond system spacer indicating no ring current in the pseudoring wh ereas Li+-chelated system exhibits a slightly aromatic property (NICS < 0).