On the Protonation of Fluoro Cryptands and the Possibility of CF…HN+ Hydrogen Bonds

Abstract

AbstractThe 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 1H‐, 13C‐ and 19F‐NMR spectroscopy, X‐ray crystal structure analysis and IR spectroscopy, with a view to detecting possible CF…HN hydrogen bonding. From the crystal structures of mono‐ and diprotonated 23‐fluoro‐4,7,20‐trioxa‐1,10‐diazatricyclo[8,7,5,112,16]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‐oxygen distances and consequently to O·HN hydrogen bonds. A related type of interaction involving fluorine appears possible since short N(H)…F distances (281.2–286.6 pm) with NHF angles between 130–140° are observed. A Cambridge Structural Database search was performed and 27 structures with short CF…HN contacts were found. On the other hand, IR spectra do not give any clear evidence in favor of CF…HN hydrogen bonding since the N–H vibrations in FN2O3…H+ and FN2O3…2H+ as well as in FN2O4…2H+ do not experience longwave shifts relative to the N–H vibrations of the reference systems HN2O3·H+, HN2O3…2H+ and HN2O4…2H+.HN2O3 and HN2O4 are almost identical to FN2O3 and HN2O4, respectively, the only difference being that the single fluorine atom of the fluoro cryptands is replaced by hydrogen. NMR spectroscopic evidence concerning CF…HN interactions is ambiguous: (i) A spin‐coupling between 1H and 19F (NH…F) is observed. (ii) The 1JCF value is reduced by up to 15 Hz upon protonation of the fluoro cryptand. (iii) An NMR competition experiment between FN2O3 and HN2O3 yields no evidence for an increased basicity of the fluoro cryptand. Finally, it can be stated that evidence in favor of CF…HN hydrogen bonds is inconclusive; should such an interaction exist it will certainly be very weak.