Synthesis, Structures, and Conformational Characteristics of Calixarene Monoanions and Dianions

The synthesis, complete characterization, and solid state structural and solution conformation
determination of calix[n]arenes (n = 4, 6, 8) is reported. A complete series of X-ray structures of the alkali
metal salts of calix[4]arene (HC4) illustrate the great influence of the alkali metal ion on the solid state
structure of calixanions (e.g., the Li salt of monoanionic HC4 is a monomer; the Na salt of monoanionic
HC4 forms a dimer; and the K, Rb, and Cs salts exist in polymeric forms). Solution NMR spectra of alkali
metal salts of monoanionic calix[4]arenes indicate that they have the cone conformation in solution. Variable-temperature NMR spectra of salts HC4·M (M = Li, Na, K, Rb, Cs) show that they possess similar
coalescence temperatures, all higher than that of HC4. Due to steric hindrance from tert-butyl groups in
the para position of p-tert-butylcalix[4]arene (ButC4), the alkali metal salts of monoanionic ButC4 exist in
monomeric or dimeric form in the solid state. Calix[6]arene (HC6) and p-tert-butylcalix[6]arene (ButC6)
were treated with a 2:1 molar ratio of M2CO3 (M = K, Rb, Cs) or a 1:1 molar ratio of MOC(CH3)3 (M = Li,
Na) to give calix[6]arene monoanions, but calix[6]arenes react in a 1:1 molar ratio with M2CO3 (M = K, Rb,
Cs) to afford calix[6]arene dianions. Calix[8]arene (HC8) and p-tert-butylcalix[8]arene (ButC8) have similar
reactivity. The alkali metal salts of monoanionic calix[6]arenes are more conformationally flexible than the
alkali metal salts of dianionic calix[6]arenes, which has been shown by their solution NMR spectra. X-ray
crystal structures of HC6·Li and HC6·Cs indicate that the size of the alkali metal has some influence on
the conformation of calixanions; for example, HC6·Li has a cone-like conformation, and HC6·Cs has a
1,2,3-alternate conformation. The calix[6]arene dianions show roughly the same structural architecture,
and the salts tend to form polymeric chains. For most calixarene salts cation−π arene interactions were
observed.