Control of Conformational Flexibility in Calix[6]Arenes: Synthesis and Characterisation of Triply Bridged Calix[6]Arene–-10,15-Dihydro-5 H -Tribenzo [ a,d,g ]Cyclononene Conjugates

Abstract

A series of triply bridged symmetrical tert-butylcalix[6]arene–-10,15-dihydro-5H-tribenzo[a,d,g]cyclononene conjugates have been synthesised in excellent yields. It has been observed that the synthesised multi-cavity molecular receptors retain the calix[6]arene and derivatised 10,15-dihydro-5H-tribenzo[a,d,g]cyclononene units in their cone conformations in solution. While the employed alkyl spacers confer flexibility to the conjugate units at room temperature, the synthesised receptors have been observed to exist in flattened cone conformations at low temperatures as determined by variable temperature NMR measurements. It has been observed that 6b shows a significant selectivity towards Ba²⁺ over other metal ions while compound 6c shows selectivity towards NH₄⁺ ions from amongst alkali, alkaline-earth and transition metal picrates. Alkali and transition metal ions are poorly extracted by the hosts 6a–c. The extraction abilities of host molecules 5a–c towards metal picrates were found to be much less than that of 6a–c.