ON THE PROTONATION OF FLUORO CRYPTANDS AND THE POSSIBILITY OF CF-CENTER-DOT-CENTER-DOT-CENTER-DOT-HN-BONDS( HYDROGEN)

The protonation of several fluoro cryptands synthesized from 1,3-bis ( bromomethyl)-2-fluorobenzene and diaza-macrocycles (diaza-12-crown-4, diaza-15-crown-5, diaza-18-crown-6 and 2,3-benzodiaza-15-crown-5) has been investigated by H-1-, C-13- and F-19-NMR spectroscopy, X-ray crys tal structure analysis and IR spectroscopy, with a view to detecting p ossible CF ... HN hydrogen bonding. From the crystal structures of mon o- and diprotonated ricyclo[8.7.5.1(12,16)]tricosa-12,14,16(23)-triene (FN2O3 . H+ and FN2O3 . 2H(+)) it is apparent that protonation leads to a shortening of the non-bonded nitrogen-fluorine and nitrogen-oxyge n distances and consequently to O ... HN hydrogen bonds. A related typ e of interaction involving fluorine appears possible since short N(H). .. F distances (281.2-286.6 pm) with NHF angles between 130-140 degree s are observed. A Cambridge Structural Database search was performed a nd 27 structures with short CF ... HN contacts were found. On the othe r hand, IR spectra do not give any clear evidence in favor of CF ... H N hydrogen bonding since the N-H vibrations in FN2O3 . H+ and FN2O3 . 2H(+) as well as in FN2O4 . 2H(+) do not experience longwave shifts re lative to the N-H vibrations of the reference systems HN2O3 . H+, HN2O 3 . 2H(+) and HN2O4 . 2H(+). HN2O3 and HN2O4 are almost identical to F N2O3 and HN2O4, respectively, the only difference being that the singl e fluorine atom of the fluoro cryptands is replaced by hydrogen. NMR s pectroscopic evidence concerning CF ... HN interactions is ambiguous: (i) A spin-coupling between H-1 and F-19 (NH ... F) is observed. (ii) The (1)J(CF) value is reduced by up to 15 Hz upon protonation of the f luoro cryptand. (iii) An NMR competition experiment between FN2O3 and HN2O3 yields no evidence for an increased basicity of the fluoro crypt and. Finally, it can be stated that evidence in favor of CF ... HN hyd rogen bonds is inconclusive; should such an interaction exist it will certainly be very weak.