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

Abstract

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 (Bu(t)C4), the alkali metal salts of monoanionic Bu(t)C4 exist in monomeric or dimeric form in the solid state. Calix[6]arene (HC6) and p-tert-butylcalix[6]arene (Bu(t)C6) were treated with a 2:1 molar ratio of M(2)CO(3) (M = K, Rb, Cs) or a 1:1 molar ratio of MOC(CH(3))(3) (M = Li, Na) to give calix[6]arene monoanions, but calix[6]arenes react in a 1:1 molar ratio with M(2)CO(3) (M = K, Rb, Cs) to afford calix[6]arene dianions. Calix[8]arene (HC8) and p-tert-butylcalix[8]arene (Bu(t)()C8) 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-pi arene interactions were observed.

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

“…In the case of the dianions, OH signals are only found for the Bu t C5 Á Rb 2 and Bu t C5 Á Cs 2 salts (8.03 and 13.49 ppm, respectively) when the solvent used is CDCl 3 , but if we change the NMR solvent from CDCl 3 to DMSOd 6 , the OH signals for Bu t C5 Á Na 2 and Bu t C5 Á K 2 can be located at 15.38 and 15.15 ppm, respectively. This NMR behavior highlights the previously observed [in even numbered calix[n]arene salts (n = 4, 6, 8)] influence of the solvent polarity in the rate of conformational inversion of calixanions [20]. 13 C NMR spectra of all the mono and dianionic salts show four peaks for the aromatic carbons, one peak for the methylene groups and two peaks for tert-butyl groups.…”

“…As we previously reported, the reaction of calix [6]arene (Bu t C6) with 1 equivalent of M 2 CO 3 leads to the preparation of calix [6]arene dianions [20]. For Bu t C5, when the reaction ratio of the M 2 CO 3 (M = Na, K) salt was increased from 0.5 to 1-3 equivalents, the product obtained was surprisingly the monoanionic salt but in better yields.…”

“…In 2003 our group described the synthesis of a large series of mono-and dianionic calixarene alkali metal salts [20,21], observing that the size and nature of the alkali metal has a very important role in the conformation and the characteristics of the calixanions obtained. Solid state structures for monoanionic species (HC4 Á Li, HC4 Á Na, HC4 Á Rb, HC4 Á Cs) exhibit the cone conformation while the dianionic species [e.g.…”

“…Among the calix[n]arene anions reported in literature those for n = 4 are the most well represented [20,21,[29][30][31][32]. In 2003 our group described the synthesis of a large series of mono-and dianionic calixarene alkali metal salts [20,21], observing that the size and nature of the alkali metal has a very important role in the conformation and the characteristics of the calixanions obtained.…”

“…4]arenes have the ability to form complexes with alkali metal cations with stoichiometries higher than 1:1 [20][21][22]. The relatively small cavity of calix [4]arene can disfavor metal-metal interactions due to steric crowding in dimetallocalixarenes, for example by producing a 1,2-alternate conformation [7,23].…”