Transition Metal Complexes of Calix[4]arene: Theoretical Investigations into Small Guest Binding within the Host Cavity

The ability to selectively detect or store small molecules, such as gases, is of enormous commercial potential. Calixarenes have been studied extensively as host molecules; however, recent synthetic advances have seen the formation of new polymetallic calixarene clusters, which have not yet been explored for such purposes. We therefore present a theoretical study, using Density Functional Theory, to thoroughly investigate the binding preferences of calix[4]arene, with a variety of transition metal cations coordinated to the calixarene tetraphenolic pocket, toward a series of important small molecules, H2S, SO2, H2O, O2, H2, N2, N2O, CO2, NH3, and HCN. It was found that the inclusion of a metal atom at the lower-rim of the calixarene caused significant strengthening of binding energy with all of the small molecules in our study as compared to metal-free calixarene. The guests, SO2 and NH3, were found to bind strongest with H2 binding weakest. Our calculations predict that simply introducing metal coordination of any type to calix[4]arene will make the largest difference to the binding energies. Subsequently changing the type, oxidation state, or the spin state of the metal coordinated to the calixarene tetraphenolic pocket was found to have a lesser effect on these.