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

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.