Crystal structure of 2-chloro-1,3-(2,6-diisopropylphenyl)-4,5-dihydro-1H-imidazol-3-ium tetrakis(3,5-trifluoromethylphenyl)borate

The salt compound presented is an example of a 2-chloro imidazolidinium structure where the formerly carbene carbon has a trigonal–planar geometry.


Chemical context
The use of main group elements as a way to stabilize singlet carbenes was first investigated in-depth by Bertrand & Reed (1994), leading to the discovery of the first phosphino silyl carbenes (Igau et al., 1988) followed by other novel singlet carbenes (Lavallo et al., 2005;Frey et al., 2007;Aldeco-Perez et al., 2009). However, the report of the first 'bottleable' crystalline N-heterocyclic carbene (NHC) (Arduengo et al., 1991) initiated a new paradigm in synthetic chemistry (Bourissou et al., 2000). In particular, NHCs are favoured due to their stability and ease of synthesis. The ability of these stable carbenes to activate small molecules and to help stabilize highly reactive intermediates makes this an increasingly desirable area of research. The crystal structure of the compound under investigation incorporates a popular fivemembered saturated NHC (known as SIPr) coordinated with a Cl atom attached at the formally carbene atom as a borate salt.

Structural commentary
The molecular structure of the title salt compound is shown in Fig. 1. The formerly carbene carbon has a distorted trigonalplanar geometry and is flanked by the two sterically bulky Ndiisopropylphenyl groups of the heterocycle. The imidazolidinium ring is in a half-chair conformation having approximate C 2 symmetry. The dihedral angle between the mean planes of the benzene rings is 36.7 (1) . The isopropyl groups containing C12 and C27 are essentially bisected by the plane of the benzene ring to which they are attached, subtending dihedral angles of 116.0 (2) (C16/C21/C25/C27) and 112.4 (2) (C4/C9/C10/C12), relative to the ipso carbon atoms C4 and C16 while the isopropyl groups containing C15 and C23 deviate significantly from this bisected geometry with dihedral angles of 26.1 (2) (C4/C5/C13/C15) and 46.7 (2) (C16/C17/C22/C23) relative to the ipso carbon atoms C4 and C16. The C1-Cl1 bond length of 1.681 (2) Å is slightly less than the average value of 1.73 Å for a Csp 2 Á Á ÁCl bond length.

Figure 2
Part of the crystal structure with weak C-HÁ Á ÁF hydrogen bonds shown as dashed lines.

Database survey
A search of the Cambridge Structural Database (CSD; Groom et al., 2016) revealed two hits for structures which are imidazolidinium salts with N-methyl groups in place of the Ndiisopropylphenyl groups of the title compound. One of the structures contains a tetrachloronickel counter-anion and the other is that of a chloride [XAMQAE (Kremzow et al., 2005) and SISVUN (Bö ttcher et al., 2014)]. The CSD also contains two structures of unsaturated five-membered NHC compounds that contain C-Cl bonds in the C2 position [NUXPOL (Arduengo et al., 1997) and XOMMER (Kuhn et al., 2002)].

Synthesis and crystallization
In a glovebox, prior to the synthesis of the title compound, SIPrCO 2 (Zhou et al., 2008) was reacted with SOCl 2 in an attempt to synthesize SIPrCOCl 2 . The exact composition of the product was unconfirmed; however, the decision was made to take a portion of this product and move forward to test its chemistry. This product is the primary reagent for the synthesis of the title salt. In a vial equipped with a magnetic stirring bar was placed the resulting product from the SIPrCO 2 /SOCl 2 reaction (0.0478 g, 9.745 Â 10 À2 mmol), NaBARF (0.0863 g, 9.738 Â 10 À2 mmol) and 5 mL of dichloromethane. The mixture was left to stir overnight (18 h) after which the insoluble solids were removed by filtering the solution into a pre-weighed vial. This was done using a glass pipette containing a small layer of diatomaceous earth. Volatiles were removed in vacuo, leaving behind a pale-yellow-coloured solid (0.0596 g, 4.623 Â 10 À2 mmol). The purity of the sample was confirmed using 1 H NMR spectroscopy in deuterated chloroform (CDCl 3 ). The recrystallization was carried out by evaporation of CDCl 3 , followed by cooling in the freezer overnight, to afford colourless needle-shaped crystals. 1 H NMR (300 MHz, 298 K, C 6 D 6 ): 1.26 (d, CH(CH 3 ) 2 , 12H), 1.33 (d, CH(CH 3 ) 2 , 12H), 3.84 (sept., CH(CH 3 ) 2 , 4H), 4.52 (s, CH 2 , 4H), 7.34 (d, m-Ar-H, 4H), 7.50 (s, p-Ar-H, 4H), 7.56 (t, p-Ar-H, 2H), 7.68 ppm (t, m-Ar-H, 8H). 19 F NMR (282.5 MHz, 298 K, C 6 D 6 ): À63.1 ppm (s). 11 B NMR (96.3 MHz, 298 K, C 6 D 6 ): À6.18 ppm (s). Trifluorotoluene was used as an external reference for the 19 F NMR spectrum and boron trifluoride diethyl etherate was used as the external reference for the 11 B NMR spectrum.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were included at geometrically idealized positions and were included in a riding-motion approximation. For the methyl groups, the dihedral angle of the idealized tetrahedral CH 3 fragment was allowed to refine.
Prior to final refinement, there was significant disorder associated with one of the CF 3 groups attached to each of C34 and C58. After trying to assess whether the groups had two components of a disorder, it became clear that each of these CF 3 groups actually had four components of disorder that needed to be resolved. In order to do this, the SUMP command was applied to all of the fluorine atoms involved. This involved grouping the four components into PART 1, PART 2, PART 3, and PART 4, respectively, and assigning a free variable to each of the individual parts, where the weighted sum of the free variables was set to equal 1.0 (C58: 0.5: 0.3: 0.1: 0.1 and C34: 0.4: 0.3: 0.2: 0.1). Following refinement using the SUMP command, the EADP command was applied, which allowed for all of the anisotropic parameters of the fluorine ellipsoids to be similar in size. Lastly, the SADI command was applied to each of the affected C-F bonds in the disordered CF 3 groups in order to have similar bond lengths for each of the disordered F atoms (i.e. the bond lengths were approximately equal for C58-F16A-D, C58-F17A-D, etc). The combination of these commands allowed for complete refinement of the CF 3 disorder. + ÁC 32 H 12 BF 24 1473  Crystal structure of 2-chloro-1,3-(2,6-diisopropylphenyl)-4,5-dihydro-1Himidazol-3-ium tetrakis(3,5-trifluoromethylphenyl)borate Darcie L. Stack and Jason D. Masuda

Computing details
Data collection: APEX2 (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT (Bruker, 2008); program(s) used to solve structure: XS in SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and PLATON (Spek, 2009); software used to prepare material for publication: publCIF (Westrip, 2010).  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.