Solid-state structures of some fluorescent macrocyclic complexes of alkali metal ions studied by single-crystal X-ray diffraction studies, vibrational spectroscopy and NMR spectroscopy: a review

The aim of this review is to compile important results of single-crystal X-ray diffraction method, vibrational and NMR studies conducted on alkali metal ion complexes of some small ring benzo-crown ethers namely benzo-15-crown-5, dibenzo-15-crown-5 and benzo-12-crown-4 to determine their structure, stoichiometry and conformation in the solid-state. This review focuses on solid-state architectural diversity of alkali metal ion complexes studied by single-crystal X-ray diffraction method. Investigations of these complexes are significant as the interactions of alkali metal ions with these crown ethers mimic those present in nature between univalent ions and several bioligands. The benzo group in the macrocycle allows to monitor the complexation event by fluorescence studies and extends the applications of these crown ethers to fluorescent ion sensors. The selected small ring benzo oxa-crown ethers have cavities that match the size of the alkali metal ions and give stable complexes with them. The review presents results related to variation in infrared and Raman spectra of the ligands with changes in their crystal structure and conformation on complexation with alkali salts. Single-crystal X-ray diffraction studies of these complexes help to confirm the chemical structure, stoichiometry, stable conformation and presence of associated solvent molecules in the solid state. As noncovalent interactions are involved in the formation of these complexes, they are capable of forming supramolecular assemblies for building smart functional materials.