Solution and solid state interplay of isomeric 4′-(pyridyl)-3,2′:6′,3″-terpyridines with p-sulfonatocalix[4]arene

p-Sulfonatocalix[4]arene, 1, forms inclusion complexes with the three isomeric 4′-(pyridyl)-3,2′:6′,3″-terpyridines in acidic aqueous medium. Both 4′-(2-pyridyl)-, 3, and 4′-(3′-pyridyl)-3,2′:6′,3″-terpyridine, 4, form 1 : 1 host–guest complexes in the solid state, in which the terminal 4′-(pyridyl) ring resides in the cavity of the calixarene. In the case of 4′-(4-pyridyl)-3,2′:6′,3″-terpyridine, 5, a molecular capsule is formed in which two calixarenes encapsulate one terpyridine guest molecule, with the terminal pyridine ring being exo to the calixarene cavities. NMR spectroscopy indicates that association between p-sulfonatocalix[4]arene and each terpyridine guest occurs in solution at elevated temperature (333 K) as a prelude to crystallisation of the inclusion complexes.