OCTAAZACRYPTAND COMPLEXATION OF THE FLUORIDE-ION

The fundamental properties of proton, fluoride, and chloride binding of the octaazacryptand L have been investigated by potentiometry and NMR spectroscopy. Successive protonation constants of L are 11.18(16), 9.43(08), 7.59(07), 5.78(02), and 4.39(33), with the last value corresponding to a two-proton step. H-1 NMR spectra at 250 MHz indicate that proton exchange is slow on the NMR time scale in the pH region where the two-proton step occurs. It is proposed that this results from disruption of an internal hydrogen bond network and heightened electrostatic repulsions present in tetraprotonated L. NMR data also suggest that protonation occurs predominantly at the secondary amines over the pH range studied. Triprotonated L binds fluoride significantly (log K = 3.6(4)) near neutral pH, and further protonation of L increases the fluoride complex stability (log K = 11.2(5) for the hexaprotonated fluoride complex). H-1 NMR data show that the rate of fluoride exchange decreases with solution pH. The magnitude of the aqueous fluoride binding constant, the substantial NMR shifts, and the published solid state structure suggest fluoride is bound inside the cavity of protonated L. MM2 calculations predict the strain energy of L increases by 10.2 kcal/mol when the expected nitrogen-halide hydrogen bond length increases from that of fluoride to that of chloride. Size selectivity of Lis proposed to explain the fact that fluoride is bound over 10(7) times stronger than chloride.