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On the Protonation of Fluoro Cryptands and the Possibility of CF…HN + Hydrogen Bonds
Author(s) -
Plenio Herbert,
Diodone Ralph
Publication year - 1997
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19971300514
Subject(s) - chemistry , protonation , cryptand , hydrogen bond , tautomer , crystallography , crystal structure , infrared spectroscopy , fluorine , stereochemistry , molecule , organic chemistry , ion
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 1 H‐, 13 C‐ and 19 F‐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[8,7,5,1 12,16 ]tricosa‐12,14,16(23)‐triene ( FN 2 O 3 ·H + and FN 2 O 3 ·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 FN 2 O 3 … H + and FN 2 O 3 …2H + as well as in FN 2 O 4 …2H + do not experience longwave shifts relative to the N–H vibrations of the reference systems HN 2 O 3 ·H + , HN 2 O 3 …2H + and HN 2 O 4 …2H + . HN 2 O 3 and HN 2 O 4 are almost identical to FN 2 O 3 and HN 2 O 4 , 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 1 H and 19 F (NH…F) is observed. (ii) The 1 J CF value is reduced by up to 15 Hz upon protonation of the fluoro cryptand. (iii) An NMR competition experiment between FN 2 O 3 and HN 2 O 3 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.