Supramolecular chemistry of phosphonic acid calixarenes

[Truncated abstract] Calix[n]arenes are an important class of bowl shaped macrocycles which are widely used for host-guest chemistry, stabilisation of nanoparticles, sensors and as scaffolds for building complex materials. They are prepared by the condensation reaction between a phenol and an aldehyde, and can be readily modified at the lower and upper rims with moieties including alkyl and phosphonic acid groups respectively. Their bowl shape allows for the confinement of space through the formation of molecular capsules and higher order self-assembled arrays. Hirshfeld surface analysis is a recently developed technique which allows for the fast and detailed understanding of the nature of interactions within the solid state, and is applicable to molecules of any size, including phenols and calixarenes. Phenols are one of the starting reagents in the synthesis of calixarenes, and a better understanding of their interactions in the solid state is important in further understanding the behaviour of calixarenes. To this end, the effect of para substitution on the interactions of phenols was investigated. Three different functionalised phenols were crystallised and their solid state structures elucidated. The three substituents included a tert-butyl group, a benzyl group and a polar nitro group, with the two former moieties resulting in a very similar crystal structure, both containing a remarkable six molecules in the asymmetric unit. Despite their apparently similar packing, Hirshfeld surface analysis established that the interactions within each structure are distinctly different, with the nature of the substituent predetermining the type of interactions. As expected, the nitro groups dominate the crystal packing and result in essentially columnar arrays of the phenol, with a significantly different proportion of interactions relative to the other two phenols.