Complexation of alkali-metal cations by conformationally rigid, stereoisomeric calix[4]arene crown ethers: A density functional theory study

The quantum mechanical study of the cone, partial-cone, and 1,3-alternate 1,3-diethoxy-p-tert-butylcalix[4]crown-5 and their interaction with the alkali-metal cations (Na+, K+, and Rb+) has been performed. Geometries, binding energies, and binding enthalpies are evaluated at the restricted hybrid Becke's three parameter exchange functional (B3LYP) level using standard 6-31G basis set and relativistic effective core potentials. The optimized geometric structures are used to perform natural bond orbital (NBO) analysis. The two main types of driving force metal-ligand and cation-π interactions are investigated. The results indicate that intermolecular electrostatic interactions are dominant and the electron-donating oxygen offer lone pair electrons to the contacting RY∗ (1-center Rydberg) or LP∗ (1-center valence antibond lone pair) orbitals of M+ (Na+, K+, and Rb+). What is more, the cation-π interactions between the metal ion and π-orbitals of the two rotated benzene rings play a certain role (especially in L3). Our calculations clearly show that solvation effects strongly influence cation selectivity. 1,3-diethoxy-p-tert-butylcalix[4]crown-5 isomers preferentially bind Na+, not K+ as found in aqueous environments. However, the calculated results indicate that K+ selectivity is recovered when even a few waters of hydration are considered.