Density functional theory study of calix[4]arene-nazacrown-5, calix[4]arene-N-phenyl-azacrown-5, and their complexes with alkali-metal cations: Na+, K+, and Rb+

Theoretical studies of 1, 3-alternate-25, 27-bis(1-methoxyethyl)calix[4] arene-azacrown-5 (L₁), 1, 3-alternate-25, 27-bis(1-methoxyethyl) calix[4]arene-N-phenyl-azacrown-5 (L₂), and the corresponding complexes M⁺/L of L₁ and L₂ with the alkali-metal cations: Na⁺, K⁺, and Rb⁺ have been performed using density functional theory (DFT) at B3LYP/6-31Gß level. The optimized geometric structures obtained from DFT calculations 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 RYß (1-center Rydberg) or LPß (1-center valence antibond lone pair) orbitals of M1 (Na⁺, K⁺, and Rb⁺). What's more, the cation-π interactions between the metal ion and π-orbitals of the two rotated benzene rings play a minor role. For all the structures, the most pronounced changes in geometric parameters upon interaction are observed in the calix[4]arene molecule. In addition, an extra pendant phenyl group attached to nitrogen can promote metal complexation by 3D encapsulation greatly. In addition, the enthalpies of complexation reaction and hydrated cation exchange reaction had been studied by the calculated thermodynamic data. The calculated results of hydrated cation exchange reaction are in a good agreement with the experimental data for the complexes.