Synthesis and vibrational study of a 1:1 stoichiometric complex of hydronium tetrafluoroborate and 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6)

The macrocyclic ether 1,4,7,10,13,16-hexaoxaoctadecane (18-crown-6) has the remarkable property of being able to transfer the hydronium (H3O+) ion from an aqueous solution to an organic medium. This property has been used to synthetize a 1:1 stoichiometric complex of this ether with hydronium tetrafluoroborate. This complex is very stable (mp 145 °C) and dissolves readily, without decomposition, in solvents such as CHCl3. The presence of the H3O+ and BF4− ions in the complex has been confirmed by chemical analysis, and by 1H nmr, Raman, and ir spectroscopy. The vibrational spectra indicate that a conformational change of the crown ether takes place in the complex over the temperature range −50 °C to −100 °C. The symmetry of the crown ether, which appears to be relatively high at room temperature, is considerably reduced below the transition temperature.