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 cal ix[4]arene ligands with one and two pendent proton-ionizable groups have be en studied by NMR spectroscopy. For a series of the ligands with two N-(R-s ulfonyl)carbamoylmethoxy substituents, there is no significant change in th e conformational preferences of the calix[4]arene unit upon variation of th e NSO2R substituents. Systematic changes of the preferred conformation(s) f or 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 f rom Li+ to Na+ to K+ to Rb+ to Cs+. For ligands with one proton-ionizable g roup [carboxylic acid or N-(trifluoromethylsulfonyl)carboxamide] the confor mational preferences of the calix[4]arene unit are also controlled by the i dentity of the complexed metal ion. The Li+ salts prefer the cone conformat ion, while for the Na+ and K+ salts more than two significantly populated c onformations are evident. Remarkably, Cs+ and Rb+ salts prefer a partial co ne conformation, which provides the possibility for the metal ion to have t hree pi -interactions with the arene units of the calix[4]arene moiety and a coulombic interaction with the ionized group.