Unusual conformational control of mobile mono- and diionizable calix[4]arene ligands by alkali metal cations† ‡

Conformations adopted in CDCl3 solution by alkali metal salts of mobile calix[4]arene ligands with one and two pendent proton-ionizable groups have been studied by NMR spectroscopy. For a series of the ligands with two N-(R-sulfonyl)carbamoylmethoxy substituents, there is no significant change in the conformational preferences of the calix[4]arene unit upon variation of the NSO2R substituents. Systematic changes of the preferred conformation(s) for the calix[4]arene moiety from cone to partial cone to 1,3-alternate are observed for all five of the ligands as the alkali metal cation is varied from Li+ to Na+ to K+ to Rb+ to Cs+. For ligands with one proton-ionizable group [carboxylic acid or N-(trifluoromethylsulfonyl)carboxamide] the conformational preferences of the calix[4]arene unit are also controlled by the identity of the complexed metal ion. The Li+ salts prefer the cone conformation, while for the Na+ and K+ salts more than two significantly populated conformations are evident. Remarkably, Cs+ and Rb+ salts prefer a partial cone conformation, which provides the possibility for the metal ion to have three π-interactions with the arene units of the calix[4]arene moiety and a coulombic interaction with the ionized group.