Hydrogen bonding, pi-pi stacking and cation-pi interactions are all noncovalent bonding interactions which are well-accepted in the field of supramolecular chemistry. However, in past five years a new type of supramolecular interaction, namely the anion-pi, and more generally the lone pair (lp)-pi interaction, has become recognised by chemists. 

This type of interaction involves an association between a neutral electron-rich molecule and an electron-poor pi ring.

Though lp-pi interactions appear to be of great importance for the stabilisation of biological macromoleucles, such as Z-DNA, remarkably they have hardly been studied to date.

 

However, in their CrystEngComm Highlight, Patrick Gamez, Jan Reedijk and Tiddo Mooibroek, from Leiden University, The Netherlands, show that lp-pi interactions are much more common than expected.

They investigated over 260,000 crystal structures deposited in the CSD and this study revealed that this type of supramolecular interaction is more prevalent in solid state-structures than pi-pi or cation-pi interactions. 

'This study clearly demonstrates that the lp-pi interaction should be considered as an important supramolecular interaction in crystal engineering as are the well-accepted hydrogen bonding, pi-pi stacking and cation-pi interactions,' say the authors.