Calixarene-Based lead receptors: an NMR, DFT and X-Ray synergetic approach

The conformational changes of the homooxa adamantyl ketones 1 and 2, as well as of the calix[4]arene analogue derivative 3, all in the cone conformation, upon Pb²⁺ complexation were investigated by ¹H, ¹³C, ²⁰⁷Pb NMR and proton spin-lattice relaxation times (T ₁) experiments. The X-ray crystal structures of ketones 1 and 3 were determined. The inclusion of an acetonitrile molecule in the hydrophobic cavity of the ligands was observed in the solid state. In solution, inclusion was observed only in the case of Pb²⁺ ⊂ ketone 3 complex. DFT calculations were also performed to complement the NMR conformational analysis and to bring further insights to the cation complexation. The data confirmed the formation of 1:1 complexes between Pb²⁺ and the ligands, and that the cation is located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. In general, the ligand conformations became closer to a regular cone upon complexation, with the binding models found for the three ketones through the NMR studies corroborated by the DFT calculations.