The syn and anti isomers of the porphyrinogen-like precursor of calix[4]phyrin: isolation, X-ray structure, anion binding and fluoride-ion-mediated proton–deuterium exchange studies

Abstract

Porphyrinogen-like precursors of calix[4]phyrins are presumed to be unstable owing to their auto-oxidation. In contrast to this, the syn and the anti isomers of a calix[4]pyrrole molecule containing pyridine moieties at the meso positions were isolated and their structures were determined by single crystal X-ray diffraction studies. Both the isomers gave the same calix[4]phyrin molecule upon oxidation. The anion binding properties of both the isomers were studied in DMSO-d₆ by the EQNMR method, which showed that they have a preference of binding with the F⁻ ion over the other large sized halide and oxo anions. In addition, the F⁻ ion mediated H/D exchange process was monitored by the ¹⁹F NMR method. The solution state structures of the 1 : 1 F⁻ ion complexes containing deuterium atoms formed by a random but sequential substitution of NH protons by deuterium atoms were identified from their multiplicity patterns observed in the proton coupled ¹⁹F NMR spectrum, which are supported by the proton decoupled ¹⁹F NMR spectrum showing one singlet for each type of F⁻ ion complex in solution for both the syn and anti isomers, correlating with their solid state structures.