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

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posted on 24.04.2003, 00:00 by Tracy A. Hanna, Lihua Liu, Alfredo M. Angeles-Boza, Xiaodi Kou, C. David Gutsche, Krzysztof Ejsmont, William H. Watson, Lev N. Zakharov, Christopher D. Incarvito, Arnold L. Rheingold
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.