Switchable silver-ion complexation by triazolated calix[4]semitubes

Triazolated calix[4]semitubes comprising several binding sites were studied for complexation of Ag⁺ to get insight into the ability of the multitopic semi-tubular environment to host cation(s) in a structure-specific/switchable manner. For this purpose, a series of triazolated calix[4]semitubes having two or three 1,3-alternate calix[4]arene cores and crown-5-ether loops in the structures were prepared using the recently developed stepwise synthesis approach. Crown-5-ether loops were used as model receptor units which could be filled with K⁺ to charge positively either a specific 'end' or both 'ends' of the semi-tubular assemblies and to affect the complexation abilities of the internal binding sites of triazolated calix[4]semitubes. Comparative analysis of complexation-induced broadening in the ¹H NMR spectra of three isomeric tris(calixarenes) having different mutual arrangements of the internal binding sites upon addition of Ag⁺ suggested intramolecular rather than intermolecular migrations of the bound cation between two bistriazole units. The study on Ag⁺/K⁺ complexation by the crowned biscalixarene semitubes revealed strong dependence of the ditopic complexation mode on the mutual arrangement of the triazole and crownether sites within the molecule, which managed either the heterodinuclear Ag⁺/K⁺ binding, or switching between two single-nuclear complexes. In crowned triscalixarene semitubular systems, the same structure/complexation mode correlation was observed, but in this case, binding of K⁺ by an appropriately arranged crownether loop did not destroy the initial silver complex, but stopped the above migrations of Ag⁺ between the internal sites of heteromultitopic ligands, thus indicating the applicability of the triscalixarene semitubular core for the design of multipositioned molecular switches.