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.