QUANTITATIVE STRUCTURE STABILITY RELATIONSHIP FOR POTASSIUM-ION COMPLEXATION BY CROWN-ETHERS - A MOLECULAR MECHANICS AND AB-INITIO STUDY

Molecular mechanics calculations with the MM2 program were used to examine the structures and strain energies 11 hexadentate crown ether ligands and their potassium complexes. With the exception of K-O stretching parameters, all force field parameters for interactions involving the potassium ion were obtained by fitting to ab initio potential energy surfaces for selected distortions in K-O(Me)2 and K-O(Me)(Et). The K-O stretching parameters were optimized relative to crystallographic data. Comparison of the molecular mechanics results to experimental log K values that were available for all 11 crown ethers revealed the complex stability to be strongly correlated with the difference in strain energy between the uncomplexed ligand and its potassium complex. The results establish that both the K-O length preference and bonding directionality at the ether oxygen donor atom are important factors in the determination of complex stability. It is concluded that the failure to consider the orientation of the ether C-O-C moiety, relative to the metal ion, is a serious flaw in the size-match selectivity theory.