Complexation and transport of alkali, alkaline earth, transition and heavy metal cations by p-tert-butyldihomooxacalix[4]arene tetra(diethyl)amide

Abstract

The binding properties of the tetra(diethyl)amide (2) derived from p-tert-butyldihomooxacalix[4]arene, in the cone conformation, towards alkali, alkaline earth, transition (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) and heavy (Ag⁺, Cd²⁺, Hg²⁺ and Pb²⁺) metal cations have been established by extraction studies of metal picrates from an aqueous solution into dichloromethane, transport experiments with the same salts through a dichloromethane membrane and stability constant measurements in methanol. Results concerning the calorimetric study of Na⁺ and K⁺ complexes in methanol are presented. The affinity of 2 for some cations (Na⁺, K⁺, Ba²⁺, Ag⁺ and Zn²⁺) has been investigated by ¹H NMR spectrometry, as well. The results are compared to those obtained with the analogous calix[4]arene tetraamide derivative 3. Amide 2 displays a preference for the alkali cations (mainly Na⁺ and K⁺) in contrast to amide 3 that prefers the alkaline earth cations. Ag⁺ and Cd²⁺ soft Lewis acids are also strongly bound by both amides. ¹H NMR titrations confirm the formation of 1∶1 complexes between 2 and all cations studied, also suggesting that they should be located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. 2 shows transport rates that do not follow the same trends as the stability constants or extraction percentages; it can therefore be characterized as a selective receptor.