Temperature, solvent, proton transfer and complexation effects on the conformational flexibility of the anionic host calix[6]arenesulfonate

Abstract

Calix[6]arenesulfonate is a flexible host that adopts the 1,2,3-alternate conformation in DMSO- d 6 solution at room temperature, as evidenced by 1H NMR spectroscopic data. This conformational preference is lost at higher temperatures or in the presence of acid. In aqueous media this calixarene exhibits a very simple proton NMR spectrum, reflecting the fast exchange between several conformations. Electrochemical data obtained with electroactive guests (ferrocenemethanol [FcOH], ferrocenylmethyldimethylammonium [FcN +Me 3], and cobaltocenium [Cob +]) indicate that the anionic calixarene host forms complexes with all of them. The stability of the corresponding complexes increases with the number of positive charges on the guest, but the uncharged forms of the guests are also bound. The affinity between the calixarene and the ferrocene-containing guests decreases substantially as the solution goes from neutral (pH 7.0) to acidic (pH 2.6). 1H NMR spectroscopic data in neutral aqueous solution suggests that the calixarene ( 1 8−) adopts the 1,2,3-alternate conformation as it forms very stable ternary complexes with either two Cob + or two FcN +Me 3 guests, respectively. The complex with FcOH is weaker. Overall, our data demonstrate that the conformational state of this anionic calixarene host is strongly affected by the nature of the solvent, pH, temperature, as well as by the presence of appropriate guests in the solution.