Calix[6]arene-BasedN3-Donors − A Versatile Supramolecular System with Tunable Electronic and Steric Properties − Study on the Formation of Tetrahedral Dicationic Zinc Complexes in a Biomimetic Environment

Abstract

Novel tridentate N-ligands containing tertiary amines, pyrazoles, or benzimidazole groups were synthesized from tBu-calix[6]arene. Together with the previously described pyridine and imidazole-based ligands, they form a large family of biomimetic ligands (X6Me3N3) with different electronic and steric properties. Their capacity at stabilizing a tetrahedral Zn dicationic center in acetonitrile was investigated. Tertiary amines were too basic and sterically hindered, leading to precipitation of Zn(OH)2. The resulting protonated ligand was, in one case, structurally characterized by X-ray analysis. Ligands with pyrazole, benzimidazole and imidazole donors, all formed a stable Zn complex under stoichiometric conditions in acetonitrile. An 1H NMR spectroscopic study together with X-ray crystallography showed that the metal ion is coordinated to the three nitrogen arms with MeCN as a fourth ligand included in the calixarene conic pocket. These complexes provide new but rare examples of stable dicationic tetrahedral Zn species. The calixarene functionalized by three pyridine groups, on the other hand, did not appear to be a good ligand, which stands in contrast with its remarkable ability at stabilizing copper(I). Finally, these funnel complexes are chiral due to their helical shape. In solution, both enantiomers are in equilibrium. However, sterically hindered N-donors increased the enantiomerization barrier above 16 kcal/mol.