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.