Conformational and configurational diversity of solvent inclusion complexes of some calix[4]pyrroles and their anion recognition properties

Sterically hindered meso-tetramethyl-meso-tetraarylcalix4]pyrroles 1-4 where aryl is p-fluorophenyl 1, p-chlorophenyl 2, and p-methylphenyl 3, 4 (configurational isomers) are synthesized and purified by the recrystallization technique. They are characterized by IR, ¹H and ¹³C NMR, and mass spectroscopy. Configurational isomers ααββ (3) and αααα (4) of meso-tetramethyl-meso-tetramethylphenylcalix 4]pyrroles are assigned by the ¹H NMR studies and confirmed by the X-ray diffraction analysis. The single crystal X-ray diffraction analysis reveals that the ethanol adduct of 1, the acetone adduct of 2 and 3 adopt the 1,2-conformation while the acetone-water adduct of 1 and the acetone adduct of 4 adopt partial cone and cone conformations respectively. The conformational diversity is due to non-covalent interactions among the encapsulated guest, pyrrolic NH protons, and meso- substituents. Anion binding studies (F^–, Cl^–, CH₃COO^–, HSO ₄ ^? ) are carried out through ¹H NMR titrations; the binding constants are evaluated using the EQNMR program, displaying that they are more selective towards fluoride rather than other anions with the 1:1 stoichiometry. The configuration of compounds drastically influences the ion-recognition processes.