Arene-Perfluoroarene-Anion Stacking and Hydrogen Bonding Interactions in Imidazolium Salts for the Crystal Engineering of Polarity

Abstract

The crystal structure of 1-(2,3,5,6-tetrafluoropyridyl)-3-benzylimidazolium bromide possesses C6H5···C5F4N···Br– interactions that link the cations into chains, N­(C)­C–H···Br– interactions that link the chains into sheets, and N2C–H···Br– interactions that link the sheets to one another. As a consequence of these, it is polar (Pna21). Density functional theory calculations indicate that the strength of the interaction between a cation and a bromide anion lies in the order N2C–H···Br– > N­(C)­C–H···Br– > C6H5···C5F4N···Br–. Prevention of the N2C–H···Br– interaction by substitution of the hydrogen atom with a methyl group leads to dimers linked by two C6H5···C5F4N···Br– interactions. Prevention of the N­(C)­C–H···Br– interaction by substitution of the hydrogen with a methyl group permits chains of cations, but because the N2C–H···Br– interactions link the chains there are no strong interactions between the sheets. Chains of cations linked by Ar···C5F4N···Br– interactions also arise when the benzyl group is replaced by 3-phenylbenzyl and 2-naphthylmethyl groups. The former also contains N2C–H···Br– and N­(C)­C–H···Br– interactions and is centrosymmetric. The latter does not contain N­(C)­C–H···Br– interactions and is chiral and polar (P21). Exchanging the positions of the aryl and polyfluoroaryl groups results in a crystal structure with no π–π stacking between the aryl and polyfluoroaryl groups although N2C–H···Br– and N­(C)­C–H···Br– interactions persist.

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

“…The 2,6-disubstituted phenylmethylimidazolium bromide salts [MeNC 3 H 3 NCH 2 C 6 H 3 X-2-F-6] + .Br -[X = F (1), Cl (2), Br (3), I (4), CF 3 (5)] were 5 prepared by treatment of 1-methylimidazole with the appropriate benzyl bromide. The salts were characterized by mass spectrometry and 1 H, 13 C and 19 F NMR spectroscopy.…”

“…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]. 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.…”

Arene-polyfluoroarene π-π stacking between N-heterocyclic carbene ligands of pentamethylcyclopentadienyl group 9 metal complexes

“…Until recently its use in crystal engineering had been restricted to relatively simple molecules [2], but more recently more elaborate systems that allowed the crystal engineering of crystal structures with particular properties, such as polarity, have been devised [3]. In particular, aryl-and polyfluoroaryl-substituted imidazolium salts have proved to be fruitful [4][5][6][7]. For example, 1-(2,3,4,5,6-pentafluorophenylmethyl)-3-benzylimidazolium bromide [4] and 1-(4-chloro-2,3,5,6-tetrafluorophenyl)-3-benzylimidazolium bromide [6] crystallize in the polar space groups P1 and Cc respectively.…”

“…undergo undesired reactions in their syntheses [13], and the sole example of an imidazolium salt that is a precursor to 1-polyfluoroarylmethyl-3-arylimidazol-2-ylidene (III) does not possess ─ stacking [7]. Consequently a structural study of ( 5 -pentamethylcyclopentadienyl)rhodium and iridium complexes of 1-(2,3,4,5,6-pentafluorophenylmethyl)-3-benzylimidazol-2-ylidene and its analogue 1- undertaken.…”