Highly selective complexation of metal ions by the self-tuning tetraazacalixpyridine macrocycles

By means of UV-vis and ¹H NMR titrations and X-ray crystallography, complexation of tetramethylazacalix4]pyridine L1 and tetramethylazacalix2]arene2]pyridine L2 with metal ions was studied. While no interaction was observed with alkali and alkaline earth metal ions, both ligands have been found to act as powerful and selective macrocyclic hosts to complex a number of transition and heavy metal ions. Due to the intrinsic nature of the bridging nitrogen atoms that can adopt different electronic configurations and form varied degrees of conjugations with their adjacent pyridine rings, tetramethylazacalix4]pyridine L1 regulated its conformation and cavity structure to best fit the guest metal ion species, yielding a 1:1 square planar L1-Mⁿ⁺ complex with binding constants log K_1:1 ranging from 2.7(1) to 8.2(8).