Molecular Recognition of Alkylamines: Conformational and Binding Properties of Calix[6]arene-based Ester Ligands

Abstract

A series of alkyl 1(5),3(5),5(5),7(5),9(5),11(5)-hexa-tert-butyl-1,3,5,7,9,11-hexa[1,3]benzenacyclododecaphane esters have been prepared that selectively bind to alkylammonium cations. Ester ligands showed extraction efficiency toward alkylamines in the sequence of ammonium < Me similar or equal to Pt > Pr > Pr-i > Bu > Bu(i) similar or equal to Bu(5) > tert-butylammonium guests. Single and competitive transport experiments of butylamines also confirmed the binding properties obtained with extraction experiments and the transport selectivity for Bu/Bu(t) pair exceeds 25. Conformational properties, such as T-c and Delta G(c)(double dagger) , of ester ligands were investigated by means of temperature-dependent H-1 NMR spectroscopy. Conformational freedom was generally found to decrease as the size of substituent of ester side chain increased. H-1 NMR complexation studies of ethyl ester 2 with caesium tetraphenylborate and butylammonium picrate guests suggest that the conformational reorganization into a cone conformation has provoked complex formation.