Structural Criteria for the Rational Design of Selective Ligands:Extension of the MM3 Force Field to Aliphatic Ether Complexes of the Alkali and Alkaline Earth Cations

Abstract

Structural requirements for strain-free metal ion complexation by an aliphatic ether group are investigated through the use of both ab initio molecular orbital and molecular mechanics calculations. Hartree-Fock calculations on simple models, M-O(Me)[sub 2] and M-O(Me)(Et), reveal a preference for trigonal planar geometry when aliphatic ether oxygens are coordinated to alkali and alkaline earth cations. This preference is found to be strongest in small, high-valent cations and weakest in large, low-valent cations. Results from the Hartree-Fock calculations are used to extend the MM3 force field for calculation on aliphatic ether complexes with the alkali (Li to Cs) and alkaline earth (Mg to Ba) cations. The resulting molecular model (i) reproduces the experimental crystal structures of 51 different complexes of multidentate ethers with alkali and alkaline earth cations, (ii) explains experimental trends in the structure of five-membered chelate rings of aliphatic ethers, (iii) reveals a fundamental difference between the metal ion size selectivity of five-membered chelate rings of ethers versus that of amines, and (iv) rationalizes trends in the stability of four potassium complexes with the diasteriomers of dicyclohexyl-18-crown-6. 40 refs., 9 figs., 5 tabs.