Unique halogen–π association detected in single crystals of C–N atropisomeric <i>N</i>-(2-halophenyl)quinolin-2-one derivatives and the thione analogue

In single crystals of C-N atropisomeric <i>N</i>-(2-halophenyl)quinolin-2-one and the thione analogue, a unique association based on a halogen-π interaction was detected. In racemic and optically pure <i>N</i>-(2-bromo- or 2-chlorophenyl)quinolin-2-ones, homochiral layered polymers, which consist of (<i>P</i>)- or (<i>M</i>)-atropisomers, were formed through intermolecular halogen-π association. The halogen-π association in the racemates is due to a halogen bond (C-X···π) between a σ-hole on the halogen atom and a π-electron on the quinolinone benzene ring, while that in optically pure forms is caused by an n-π* interaction between a lone electron pair on the halogen atom and a π* orbital of the quinolinone. In contrast to the formation of the homochiral layered polymer in quinolinones, in racemic <i>N</i>-(2-bromophenyl)quinoline-2-thione, heterochiral layered polymers, in which (<i>P</i>)- and (<i>M</i>)-atropisomers were alternately connected, were formed through an n-π* interaction between a lone electron pair on the bromine atom and a π* orbital of the quinoline-2-thione.