Efficient Synthesis of Calix[6]tmpa: A New Calix[6]azacryptand with Unique Conformational and Host–Guest Properties

Abstract

A new C(3v)-symmetrical calix[6]azacryptand, that is, calix[6]tmpa (11), was synthesized by efficient [1+1] macrocyclization reactions. Remarkably, both linear and convergent synthetic strategies that were applied lead to equally good overall yields. Calix[6]tmpa behaves as a single proton sponge and appeared reluctant to undergo polyprotonation, unlike classical tris(2-pyridylmethyl)amine (tmpa) derivatives. It also acts as a good host for ammonium ions. Interestingly, it strongly binds a sodium ion and a neutral guest molecule, such as a urea, an amide, or an alcohol, in a cooperative way. A (1)H NMR study indicated that the ligand, as well as its complexes, adopt a major flattened cone conformation that is the opposite of that observed with the previously reported calix[6]cryptands. Characterization of the monoprotonated derivative 11H(+) by X-ray diffraction also revealed the presence of a 1,3-alternate conformation, which is the first example of its kind in the calix[6]arene family. This conformer is probably also present in solution as a minor species. The important covalent constraint induced by the polyaromatic tmpa cap on the calixarene skeleton, and conversely from the calix core onto the tmpa moiety, is the likely basis for the unique conformational and chemical properties of this host.