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Putting the Anion into the Cage – Fluoride Inclusion in the Smallest Trisimidazolium Macrotricycle
Author(s) -
Amendola Valeria,
Boiocchi Massimo,
Fabbrizzi Luigi,
Fusco Nadia
Publication year - 2011
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201100902
Subject(s) - chemistry , steric effects , deprotonation , fluoride , ion , crystallography , polyatomic ion , ionic bonding , cyclophane , stereochemistry , medicinal chemistry , crystal structure , inorganic chemistry , organic chemistry
The trisbenzimidazolium cyclophane receptor 2 3+ incorporates the F – anion in MeCN solution, which was inferred by spectrophotometric and 1 H NMR titration experiments, with an association constant log K > 7. On the basis of geometric considerations, it is assumed that F – lies in the middle of the triangle 3C , whose vertices are the carbon atoms of the three imidazolium C–H fragments, and profits from three C–H ··· F H‐bonds. No other anion is encapsulated by 2 3+ for reasons of size. The parent trisbenzimidazolium tripodal receptor 4 3+ does not exert size exclusion selectivity and forms 1:1 complexes not only with F – , but also with other mono‐ (Br – ) and polyatomic anions (NO 3 – ). X‐ray diffraction studies on [ 4··· Br] 2+ and [ 4··· NO 3 ] 2+ complexes indicated that, due to the steric restraints of the tripodal receptor, the anion is not positioned in the middle of the 3C triangle but stays below it, profiting from H‐bonds. Cage effects are observed in the higher thermodynamic stability of the [ 2··· F] 2+ complex with respect to [ 4··· F] 2+ and in its resistance to excess F – . In fact, on addition of excess fluoride, the tripodal [ 4··· F] 2+ complex decomposes with deprotonation of a C–H fragment and formation of the very stable HF 2 – complex.