Infinite stacking of alternating polyfluoroaryl rings and bromide anions

Abstract

The crystal structure of 1-(4-bromo-2,3,5,6-tetrafluorophenyl)-3benzylimidazolium bromide comprises columns of parallel bromotetrafluorophenyl rings with an interplanar distance of 6.936(6) Å separated by bromide anions. The - stacking between arenes and polyfluoroarenes is an increasingly important interaction in crystal engineering. 1,2 It is expected that other electron-rich species, in particular anions, would also interact with the electron-poor polyfluoroarenes and that this interaction may also be important. A number of crystal structures do indeed possess anions positioned a short distance from the face of a polyfluoroarene (represented as ArF•••X-), suggesting an attractive interaction, 3 in particular where a polyfluoroaryl group is a component of a cation. 4-8 These observations are supported by theroretical calculations that indicate that interactions between anions and hexafluorobenzene are attractive. 9-11 For example, the energies of the optimised C6F6•••Xinteraction where the anion lies on the normal from the centroid were calculated (using the MP2 method and the 6-31++G** basis set with corrections for basis set superposition and zero-point errors) to be-76.1,-52.7 and-48.5 kJmol-1 for fluoride, chloride and bromide respectively; the anions poisitioned at 2.570, 3.148 and 3.201 Å respectively from

The structures of 1-(2-halo-6-fluorophenylmethyl)-1-methylimidazolium bromide salts

“…The cations of 1 and 3 adopt almost identical conformations ( Figure 7a) in which the plane of the imidazolium ring is perpendicular to the C(5)─C (6) bond. This conformation is calculated to be the most favoured ( Table 2).…”

“…The crystal structures of fluoroaryl-substituted imidazolium salts are a valuable source of information on a range of non-covalent interactions, in particular, π-π stacking [1][2][3][4][5] and anion-π interactions [2,3,[5][6][7][8], arising from the presence of fluorine atoms, along with charge-assisted hydrogen bonding [3,9]. These interactions have importance in crystal engineering and through the judicious choice of fluoroaryl substituents polar crystal structures can be realized [1][2][3][4][5].…”

“…These interactions have importance in crystal engineering and through the judicious choice of fluoroaryl substituents polar crystal structures can be realized [1][2][3][4][5]. Furthermore, the incorporation of other halogen atoms into the polyfluoroaryl group provides the opportunity for halogen bonding when the anion is a halide [5,6].…”

“…It is expected that non-covalent interactions can be turned on or off for imidazolium halide salts by variation of the substituent aryl group. As an example, although the structures of the related cations of 1-pentafluorophenylmethyl-3-methylimidazolium bromide (ROTMEU) [7] and 1-(2-bromophenylmethyl)-3-methylimidazolium bromide (EVACIO) [11] adopt similar conformations, because of the difference in the electronic nature of the phenyl ring the crystal structure of the former possesses an anion-π interaction [2,3,[5][6][7][8]12], whereas that of the latter does not (Figure 1). In order to further probe these structural features and non-covalent interactions we are investigating fluoroaryl-substituted imidazolium salts.…”

Anion−π Interactions with Fluoroarenes

“…116 The 1-(4-bromo-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromide 11 ( Figure 29) crystallized in the chiral space group P1 showing columnar arrangements of anion−π complexes. Each bromide is sandwiched between two 4-bromo-2,3,5,6-tetrafluorophenyl units.…”