3,3′-Di-n-butyl-1,1′-(p-phenylenedimethylene)diimidazolium bis(hexafluorophosphate)

The asymmetric unit of the title N-heterocyclic carbene compound, C22H32N4 2+·2PF6 −, consists of one half of the N-heterocyclic carbene dication and one hexafluorophosphate anion. The dication lies across a crystallographic inversion center. The imidazole ring is twisted away from the central benzene ring, making a dihedral angle of 76.23 (6)°. The hexafluorophosphate anions link the cations into a three-dimensional network via intermolecular C—H⋯F hydrogen bonds. A weak C—H⋯π interaction further stabilizes the crystal structure.

The asymmetric unit of the title N-heterocyclic carbene compound, C 22 H 32 N 4 2+ Á2PF 6 À , consists of one half of the Nheterocyclic carbene dication and one hexafluorophosphate anion. The dication lies across a crystallographic inversion center. The imidazole ring is twisted away from the central benzene ring, making a dihedral angle of 76.23 (6) . The hexafluorophosphate anions link the cations into a threedimensional network via intermolecular C-HÁ Á ÁF hydrogen bonds. A weak C-HÁ Á Á interaction further stabilizes the crystal structure.
Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 3,3'-Di-n-butyl-1,1'-(p-phenylenedimethylene)diimidazolium bis(hexafluorophosphate) R. A. Haque, A. Washeel, S. F. Nasri, C. S. Yeap and H.-K. Fun Comment N-heterocyclic carbene (NHC) ligands have enjoyed wide applicability as ligands for transition and main group metals since the first crystalline free carbene were isolated in 1991 by Arduengo and co-workers (Arduengo et al., 1991). They display rich coordination chemistry and are able to form stable complexes with a large number of transition metals in both high and low oxidation states (Papini et al., 2008). The complexes are widely used in catalysis and are useful in medicinal science applications (Meyer et al., 2009;Barnard et al., 2004;Lin & Vasam, 2007). These compounds show unusually high thermal stability and nucleophilic behavior, in part due to the analogy of N-heterocyclic carbenes with strong Lewis-basic phosphines. NHCs are also cheap, non-toxic and easily prepared as an azolium salt precursor (Papini et al., 2008).
The asymmetric unit of the title compound consists of half of the N-heterocyclic carbene dication and one hexafluorophosphate anion (Fig. 1). The dication lies across a crystallograpic inversion center. The geometrical parameters are comparable to its related structure (Washeel et al., 2010). The imidazole ring (N1-C5-N2-C7-C6) is planar with a maximum deviation of 0.003 (1) Å for atom C6 and is twisted away from the central benzene ring making a dihedral angle of 76.23 (6)°. The hexafluorophosphate anions linked the molecules into a three-dimensional network via intermolecular C-H···F hydrogen bonds (Fig. 2, Table 1). Short intermolecular F1···C5 and F1···C7 of 2.8636 (12) and 2.8798 (13) Å contacts are observed. A weak C-H···π interaction further stabilizes the crystal structure (Table 1).

Experimental
To a solution of p-xylylene dichloride (1 g, 5.75 mmol) in 30 ml of 1,4-dioxane, 1-butylimidazole (1.42 g, 11.5 mmol) was added. The mixture was refluxed at 373 K for 24 h. The slurry product was isolated by decantation then washed with diethyl ether (2x3 ml). KPF 6 (2.1 g, 11.5 mmol) in 20 ml of distilled water was then added with stirring for 1 h and the suspension was left standing overnight. The white precipitate was filtered, washed with distilled water several times and recrystallized from acetonitrile. The yield was found to be 2.30 g (62.7 %), m.p.: 411-413 K. Crystals suitable for X-ray was obtained by slow evaporation of the salt solution in acetonitrile at 281 K.

Refinement
The H1A and H3A hydrogen atoms were located from a difference Fourier map and refined freely. The remaining H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93 or 0.97 Å and U iso (H) = 1.2 or 1.5 U eq (C  Fig. 1. The molecular structure of the title compound with 50% probability ellipsoids for non-H atoms. Atoms with suffix A are generated by the symmetry operation (1-x, -y, 1-z).

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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 > σ(F 2 ) is used only for calculating Rfactors(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. Hydrogen-bond geometry (Å, °)