A conformational study of the calixspherand and its complexes with alkalimetal cations

Abstract

The calixpherand 2 forms kinetically very stable complexes with alkalimetal cations. This molecule is not completely preorganized for binding of a cation, as is evidenced from the results of NOESY spectroscopy and X-ray diffraction measurements. Both in CDCl₃ solution and in the solid state the free ligand adopts a cone conformation, whereas the Na⁺ complex adopts a ‘flattened’ partial cone conformation. Molecular-mechanics calculations with different programs give rather biased results. Calculations with QUANTA(the all atom CHARMM-force field) correctly predict the conformation of the free ligand but not of the complexes, whereas with MACROMODEL(the united atom AMBER-force field) the experimentally observed conformation had the lowest energy only for the Na⁺ complex. The calculated geometries of the experimentally found conformations of the free ligand and the Na⁺ complex agree well with the X-ray structures, especially for the structures that were obtained with QUANTA. A comparison of the calculated structures of the Na⁺, K⁺ and Rb⁺ complexes showed that larger cations force the terphenyl bridge to bend away, thereby opening up the cage of the ligand and making the cation more accessible to solvent molecules. This might explain the considerably lower kinetic and thermodynamic stability of the Rb⁺ complex compared with those of the Na⁺ and K⁺ complexes.