Synthesis, structures and full characterization of p-tert-butylcalix[5]arene mono-, di-, tri- and pentaanionic ligand precursors

The synthesis, complete characterization, and solid state conformation of a new series of p-tert-butylcalix[5]arene (ButC5) mono-, di-, tri- and pentaanions are reported. X-ray structures of the alkali metal salts illustrate the strong influence of the alkali metal ion on the structure of the calixanion. The strength of the alkali metal base and its reaction stoichiometry play an important role in the conformation and level of deprotonation of the resulting anion. Reaction of ButC5 in a 2:1 molar ratio with alkali metal bases M2CO3 (M = Rb, Cs), or in a 1:1 ratio with M2CO3 (M = Na, K), MOH (M = Na, K, Rb, Cs) or MH (M = Li, Na) produces ButC5 monoanions, but ButC5 reacts in a 1:1 molar ratio with M2CO3 (M = Rb, Cs) or a 1:2 molar ratio with MOC(CH3)3 (M = Na, K) to afford ButC5 dianions. Due to the steric bulkiness of the But group no polymeric structures are observed. Alkali metal salts of trianionic ButC5 were obtained in high yields from reactions of ButC5 with MOC(CH3)3 (M = Li, Na, K), BunLi, LiH and LiOH in a 1:3 molar ratio. Pentaanionic ButC5 salts were obtained by the reaction with MOC(CH3)3 (M = Li, Na, K) or BunLi in a 5:1 ratio. X-ray crystal structures of ButC5 · Na and ButC5 · Cs indicate that the size of the alkali metal influences the level of cation-π arene interactions and therefore the conformation of the ButC5 unit; for example, ButC5 · Na has a cone conformation while ButC5 · Cs shows a flattened cone conformation. Cation-π arene interactions are observed in most of the calixarene salts.