3,5-Bis(3-butylimidazolium-1-ylmethyl)toluene bis(hexafluorophosphate)

In the title compound [systematic name: 3,3′-Dibutyl-1,1′-(5-methyl-m-phenylenedimethylene)diimidazol-1-ium bis(hexafluoridophosphate)], C23H34N4 2+·2PF6 −, the imidazole rings are inclined at angles of 68.06 (7) and 75.05 (8)° with respect to the central benzene ring. In the crystal, molecules are linked into one-dimensional columns along [010] via weak intermolecular C—H⋯F hydrogen bonds. The crystal structure is further consolidated by weak C—H⋯π(arene) interactions. One of the n-butyl groups is disordered over two sites with refined occupancies of 0.694 (5) and 0.306 (5). In addition, four of the F atoms of one of the PF6 − cations are disordered over two sites with occupancies of 0.64 (3) and 0.36 (3).


Comment
Since the discovery of stable imidazoline-2-ylidenes, which were isolated and structurally characterized by Arduengo et al. (1991), the organometallic and coordination chemistry of N-heterocyclic carbene (NHC) ligands have been receiving great attention in recent years and much interest has been generated in the chemistry of the metal complexes of these ligands (Chen et al., 2002;Zhou et al., 2008). NHC carbene, a strong σ-donor and a weak π-acceptor, strongly interacts with different transitions metals in various oxidation states (Hahn & Jahnke, 2008;Danopoulos et al., 2007). Heterocyclic carbenes derived from imidazolium ions form complexes with many transition metals; heterocyclic carbene complexes of Pd, Ni, Pt, Rh, Ru, Ag and Au have been reported (Bourissou et al., 2000;McGuinness & Cavell, 2000;Garrison et al., 2001). Extensive catalytic studies on the application to organic synthesis have also been reported (Cavell & McGuinness, 2004;Liu et al., 2007).
Experimental N-butylimidazole (0.9 g, 7.2 mmol) was added to a stirred solution of 3,5-bis(bromomethyl)toluene (1.0 g, 3.6 mmol) in 20 ml of 1,4-dioxane. The mixture was refluxed at 373 K for 24 h. The sticky product was isolated by decantation and washed with fresh 1,4-dioxane (2 x 5 ml) and diethyl ether (2 x 3 ml). The resulting bromide salt was converted directly to its hexafluorophosphate salt by metathesis reaction using KPF 6 (1.3 g, 7.2 mmol) in 20 ml of methanol. The colourless precipitate formed was collected and washed with distilled water (2 x 5 ml), and then recrystallized from acetonitrile to give colourless crystals. Yield: 1.9 g (79%), m. p.: 369-371 K. Crystals suitable for X-ray diffraction studies were obtained by slow evaporation of the salt solution in acetonitrile at 281 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 > 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 )