A DFT based study on complexation behavior of hexasulphonatocalix(6)arenes with Zr(IV) and Th(IV) metals

Calixarenes form a unique class of supramolecular compounds. Calixarenes and their derivatives have a distinct cavity which gives them promising recognition ability and because of this feature they are widely employed for the extraction of toxic, heavy and radioactive metals. Hexasulphonatedcalix(6)arenes (SCX) are one of the widely studied calixarenes. They are water soluble and this property makes them unique among other hydrophobic calixarene derivatives. Because of the presence of electron rich functionalities such as -SO3H groups (in addition to the -OH groups) they can act as a most promising host for various metal ions. In this paper, we report a systematic computational investigation carried out using quantum mechanics based density functional theory (DFT) to understanding the structures and stability of the ligand calixarene structure and the complexation behavior of ground state SCX with Zr and Th. Various models of SCX were optimized and the ground state conformation known as 123-alternate was obtained in agreement as proved experimentally elsewhere. IR calculations were carried out and compared with that of an experimental data from a lab synthesized sample. The calculated frequencies found to be in good agreement with that of the experimental values of both lab synthesized sample and also that reported in literature. A systematic IR spectral analysis was done on basis of the IR calculations carried out on various structural models with different possible cation locations. These models were optimized before the IR calculations. This study helped us to understand the possible modes of coordination of Zr and Th with different rims of the calixarene host structures and their stabilization effects. The most correlated stabilizations are due to weak interactions such as intramolecular hydrogen bondings etc. A comprehensive study using data such as energetics and structural parameters with number and nature of possible metal-oxygen bonds, charges on the metals etc., provided a deeper insight about the possible location of these metal cations in the ligand pouch and their stabilization effects. This was further substantiated by systematic comparison of the calculated IR values with the experimentally obtained values