Calix[6]arene-based N-3-donors - A versatile supramolecular system with tunable electronic and steric properties - Study on the formation of tetrahedral dicationic zinc complexes in a biomimetic environment

Novel tridentate N-ligands containing tertiary amines, pyrazoles, or benzim idazole groups were synthesized from tBu-calix[6]arene, Together with the p reviously described pyridine and imidazole-based ligands, they form a large family of biomimetic ligands (X6Me3N3) with different electronic and steri c properties. Their capacity at stabilizing a tetrahedral Zn dicationic cen ter in acetonitrile was investigated. Tertiary amines were too basic and st erically hindered, leading to precipitation of Zn(OH)(2). The resulting pro tonated ligand was, in one case, structurally characterized by X-ray analys is. Ligands with pyrazole, benzimidazole and imidazole donors, all formed a stable Zn complex under stoichiometric conditions in acetonitrile. An H-1 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 calixar ene functionalized by three pyridine groups, on the other hand, did not app ear to be a good ligand, which stands in contrast with its remarkable abili ty 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 barri er above 16 kcal/mol.