CH/π hydrogen bonds in crystals

The nature and characteristics of the CH/pi interaction are discussed by comparison with other weak molecular forces such as the CH/O and OH/pi interaction. The CH/pi interaction is a kind of hydrogen bond operating between a soft acid CH and a soft base pi-system (double and triple bonds, C-6 and C-5 aromatic rings, heteroaromatics, convex surfaces of fullerenes and nanotubes). The consequences of CH/pi hydrogen bonds in supramolecular chemistry are reviewed on grounds of recent crystallographic findings and database analyses. The topics include intramolecular interactions, crystal packing (organic and organometallic compounds), host/guest complexes (cavity-type inclusion compounds of cyclodextrins and synthetic macrocyclic hosts such as calixarenes, catenanes, rotaxanes and pseudorotaxanes), lattice-inclusion type clathrates (including liquid crystals, porphyrin derivatives, cyclopentadienyl compounds and C-60 fullerenes), enantioselective clathrate formation, catalytic enantioface discriminating reactions and solid-state photoreaction. The implications of the CH/pi concept for crystal engineering and drug design are evident.