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

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).