1,3-Bis[(3-allylimidazol-3-ium-1-yl)methyl]benzene bis(hexafluoridophosphate)

In the title compound, C20H24N4 2+·2PF6 −, the ethene and 3-allylimidazolium moieties of the cation are disordered over two positions with refined site occupancies of 0.664 (19):0.336 (19) and 0.784 (7):0.216 (7), respectively, whereas four F atoms of one hexafluoridophosphate anion and all atoms in the other hexafluoridophosphate anion are disordered over two positions with refined site occupancies of 0.764 (5):0.2365) and 0.847 (9):0.153 (9), respectively. The benzene ring is inclined at angles of 78.2 (3), 81.3 (4) and 73.9 (12)° with the 1H-imidazol-3-ium ring and the major and minor components of the disordered 1H-imidazol-3-ium ring, respectively. In the crystal, the hexafluoridophosphate anions link the cations into two-dimensional networks parallel to (001) via intermolecular C—H⋯F hydrogen bonds. The crystal structure is further consolidated by π–π [centroid–centroid distance = 3.672 (3) Å] and C—H⋯π interactions.


Comment
Carbenes are compounds possessing a neutral divalent carbon atom with six electrons on its valence shell. N-heterocyclic carbenes (NHCs) are cyclic carbenes that are usually derived from the deprotonation of imidazolium salts. NHCs are strong σ-donating ligands (Yang & Nolan, 2001;Böhm et al., 2000) with negligible π -back bonding tendencies (Jafarpour & Nolan, 2001;Bourissou et al., 2000;Herrmann et al., 1996). The isolation of crystalline stable free NHC by Arduengo (Arduengo et al., 1991) provided information on their fundamental properties. Many researchers have since prepared free NHCs for use in other reactions (usually for metal complexation reactions). Danopoulos and co-worker (Danopoulos et al., 2002) isolated a stable phosphine-functionalised carbene. Various compounds that contain more than one carbene centres are known (Dias & Jin, 1994;Caballero et al. , 2001). Carbene have the ability to form complexes with many metals (Herrmann et al., 1997), of which transition metals are the most interesting due to their potentials for catalytic activity. All transition metals have been complexed with NHC and the applications of compounds came to focus in many areas including the medical and biological sciences (Thompson et al., 1999;Melaiye et al., 2005).

Experimental
A mixture of imidazole (9.0 g, 130 mmol) and sodium hydroxide (5.0 g, 120 mmol) in DMSO (30 ml) was heated to 90°C for 2 h. The mixture was cooled to room temperature and a solution of 1,3-bis(bromomethyl)benzene (15.0 g, 57 mmol) in DMSO (30 mL) was added, heated at 40 °C (1 h) and then poured into water (400 mL) followed by cooling in ice.
Further, a mixture of 1 (0.5 g, 2.1 mmol) and allyl bromide (0.5 g, 4.2 mmol) in acetonitrile (30 mL) was refluxed at 90 supplementary materials sup-2°C for 24 h. The solvent was removed under reduced pressure to give 2.2Br as a pale-brown oil which was then reacted with KPF 6 (0.22 g, 1.2 mmol) in 20 ml of methanol to yield the title compound. Recrystallisation from acetonitrile gives title compound as colourless solid. Yield: 0.4 g (60%). Crystals suitable for x-ray diffraction studies were obtained by slow evaporation of the salt solution in acetonitrile at room temperature.

Refinement
All H atoms were positioned geometrically and refined using a riding model with C-H = 0.93-0.97 Å and U iso (H) = 1.2 U eq (C). The highest residual electron density peak is located at 1.09 Å from P2B and the deepest hole is located at 0.76 Å from F10A. The ethene (C12/C13) and the 3-allylimidazolium (N3/N4/C15-C20) moieties of the cation are disordered

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.   (2)