Synthesis and cation binding properties of bis-bridged calix[6]arenes

Abstract

Calixarenes have been studied and utilized for their molecular recognition properties. A total of four bis-bridged calix[6]arenes have been synthesized and their metal binding properties and structural conformations were studied in solid, solution, and gas phase. Methods of testing include: NMR titrations, liquid-liquid extractions, x-ray crystallography, and electrospray ionization-MS. A dialkylated calix[6]arene was used as the principle starting material which predisposed the four remaining phenolic hydroxyl groups for placement of two bis-electrophiles. The bis-electrophiles were triethylene glycol di-p-tosylate, α,α-di-bromomethyl-m-xylene, and 2,6-bis(bromomethyl) pyridine. All of the compounds tested, with the exception of the bis-xylenyl, formed 1:1 complexes. The bis-xylenyl calix[6]arene is a poor cation binder that is conformationally mobile at room temperature. In contrast the bis-pyridinyl calix[6]arene was a non-selective strong binder of all alkali metal ions. Both bis-bridged aromatic compounds were in a 1,2,3-alternate conformation. Also synthesized were bis-crown-4-calix[6]arene in the cone and 1,2,3-alternate conformation. The calixcrown conformers were selective for the larger alkali metals, cesium and rubidium, with the Cs+/Na+ selectivity of the cone being 1,400 and that of the 1,2,3-alternate being 144, in solution. Similar results were observed in gas phase competition studies where both conformers exhibited exclusive Cs+ selectivity over Rb+ and a selectivity trend for consecutive larger cations in competition mixtures of Na+/K+, and K+/Rb+.