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

The asymmetric unit of the title N-heterocyclic carbene compound, C₂₂H₃₂N₄²⁺·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.

Key indicators

• Single-crystal X-ray study
• T = 100 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.036
• wR factor = 0.121
• Data-to-parameter ratio = 29.2

checkCIF/PLATON results

No syntax errors found



Alert level B
PLAT432_ALERT_2_B Short Inter X...Y Contact  F1     ..  C5      ..       2.86 Ang.

Alert level C
PLAT432_ALERT_2_C Short Inter X...Y Contact  F1     ..  C7      ..       2.88 Ang.
PLAT910_ALERT_3_C Missing # of FCF Reflections Below Th(Min) .....          5
PLAT911_ALERT_3_C Missing # FCF Refl Between THmin & STh/L=  0.600         40
PLAT913_ALERT_3_C Missing # of Very Strong Reflections in FCF ....         13
PLAT244_ALERT_4_C Low   'Solvent' Ueq as Compared to Neighbors of          P1
PLAT912_ALERT_4_C Missing # of FCF Reflections Above STh/L=  0.600         23

Alert level G
PLAT960_ALERT_3_G Number of Intensities with I .LT. - 2*sig(I) ..           4

   0 ALERT level A = In general: serious problem
   1 ALERT level B = Potentially serious problem
   6 ALERT level C = Check and explain
   1 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   4 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. 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).

Related literature

For background to N-heterocyclic carbenes, see: Arduengo et al. (1991); Papini et al. (2008). For applications of N-heterocyclic carbene derivatives, see: Meyer et al. (2009); Barnard et al. (2004); Lin & Vasam (2007). For a related structure, see: Washeel et al. (2010). For the stability of the temperature controller used for the data collection, see: Cosier & Glazer (1986).

S2. 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). KPF₆ (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.

S3. 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). A rotating group model was applied for the methyl groups. Short intermolecular F1···C5 and F1···C7 of 2.8636 (12) and 2.8798 (13) Å contacts are observed.

Computing details

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures

	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).
	Fig. 2. Crystal packing of the title compound, viewed down the c axis, showing the molecules linked into a 3-D network. Intermolecular hydrogen bonds are shown as dashed lines.

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

Crystal data

C22H32N4^2+·2PF6^−	F(000) = 660
Mr = 642.46	Dx = 1.548 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 8699 reflections
a = 8.9802 (5) Å	θ = 3.3–35.1°
b = 17.8421 (10) Å	µ = 0.26 mm^−1
c = 9.3637 (5) Å	T = 100 K
β = 113.233 (1)°	Block, colourless
V = 1378.64 (13) Å^3	0.37 × 0.25 × 0.20 mm
Z = 2

Data collection

Bruker APEX Duo CCD area detector diffractometer	5550 independent reflections
Radiation source: fine-focus sealed tube	4750 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.027
ϕ and ω scans	θmax = 34.0°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −12→14
Tmin = 0.910, Tmax = 0.950	k = −25→28
21938 measured reflections	l = −14→14

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.121	H atoms treated by a mixture of independent and constrained refinement
S = 1.10	w = 1/[σ^2(Fo^2) + (0.0669P)^2 + 0.3404P] where P = (Fo^2 + 2Fc^2)/3
5550 reflections	(Δ/σ)max < 0.001
190 parameters	Δρmax = 0.52 e Å^−3
0 restraints	Δρmin = −0.35 e Å^−3

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq
P1	0.24993 (3)	0.644667 (14)	0.41525 (3)	0.01504 (7)
F1	0.17538 (13)	0.58756 (4)	0.27368 (9)	0.0377 (2)
F2	0.32152 (11)	0.70282 (5)	0.55641 (8)	0.03461 (19)
F3	0.40070 (10)	0.59059 (5)	0.49878 (11)	0.0363 (2)
F4	0.15300 (10)	0.60287 (5)	0.50374 (10)	0.03387 (18)
F5	0.34534 (10)	0.68616 (5)	0.32536 (10)	0.03212 (18)
F6	0.09980 (9)	0.70032 (5)	0.33174 (9)	0.02912 (16)
N1	−0.09308 (9)	−0.12292 (5)	0.43634 (9)	0.01428 (14)
N2	0.12122 (9)	−0.11365 (5)	0.38494 (10)	0.01434 (14)
C1	0.42820 (12)	−0.01546 (6)	0.34135 (11)	0.01608 (16)
C2	0.39498 (11)	−0.06136 (5)	0.44620 (11)	0.01435 (15)
C3	0.46724 (12)	−0.04565 (6)	0.60501 (11)	0.01585 (16)
C4	0.28430 (11)	−0.12854 (6)	0.38858 (12)	0.01716 (17)
H4A	0.2753	−0.1415	0.2849	0.021*
H4B	0.3314	−0.1710	0.4560	0.021*
C5	0.05423 (11)	−0.15095 (5)	0.46797 (11)	0.01380 (15)
H5A	0.1022	−0.1900	0.5365	0.017*
C6	0.01313 (13)	−0.05969 (6)	0.29775 (13)	0.01997 (18)
H6A	0.0295	−0.0256	0.2301	0.024*
C7	−0.12157 (12)	−0.06587 (6)	0.32955 (13)	0.01986 (18)
H7A	−0.2152	−0.0371	0.2873	0.024*
C8	−0.20778 (12)	−0.14960 (6)	0.50215 (12)	0.01732 (17)
H8A	−0.2538	−0.1069	0.5344	0.021*
H8B	−0.1501	−0.1799	0.5934	0.021*
C9	−0.34402 (11)	−0.19587 (6)	0.38501 (12)	0.01788 (17)
H9A	−0.4209	−0.2086	0.4304	0.021*
H9B	−0.4004	−0.1652	0.2940	0.021*
C10	−0.28848 (13)	−0.26785 (6)	0.33359 (13)	0.02143 (19)
H10A	−0.3792	−0.2892	0.2475	0.026*
H10B	−0.2049	−0.2557	0.2961	0.026*
C11	−0.22253 (16)	−0.32643 (7)	0.46144 (18)	0.0314 (3)
H11B	−0.1973	−0.3714	0.4194	0.047*
H11C	−0.3025	−0.3372	0.5028	0.047*
H11D	−0.1262	−0.3076	0.5428	0.047*
H1A	0.372 (2)	−0.0256 (10)	0.227 (2)	0.026 (4)*
H3A	0.447 (2)	−0.0786 (10)	0.671 (2)	0.025 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
P1	0.01633 (12)	0.01343 (12)	0.01512 (11)	0.00062 (7)	0.00596 (9)	0.00052 (7)
F1	0.0666 (6)	0.0170 (3)	0.0251 (4)	−0.0065 (3)	0.0134 (4)	−0.0070 (3)
F2	0.0451 (5)	0.0276 (4)	0.0184 (3)	−0.0044 (3)	−0.0011 (3)	−0.0044 (3)
F3	0.0324 (4)	0.0346 (4)	0.0439 (5)	0.0182 (3)	0.0173 (3)	0.0191 (4)
F4	0.0300 (4)	0.0430 (5)	0.0339 (4)	−0.0048 (3)	0.0182 (3)	0.0085 (3)
F5	0.0322 (4)	0.0337 (4)	0.0369 (4)	−0.0006 (3)	0.0204 (3)	0.0123 (3)
F6	0.0231 (3)	0.0298 (4)	0.0278 (3)	0.0112 (3)	0.0029 (3)	0.0008 (3)
N1	0.0115 (3)	0.0145 (3)	0.0169 (3)	0.0003 (2)	0.0056 (3)	0.0004 (3)
N2	0.0127 (3)	0.0128 (3)	0.0185 (3)	−0.0015 (2)	0.0071 (3)	−0.0015 (3)
C1	0.0163 (4)	0.0168 (4)	0.0167 (4)	−0.0029 (3)	0.0081 (3)	−0.0029 (3)
C2	0.0125 (3)	0.0133 (4)	0.0192 (4)	−0.0020 (3)	0.0083 (3)	−0.0030 (3)
C3	0.0168 (4)	0.0158 (4)	0.0178 (4)	−0.0029 (3)	0.0099 (3)	−0.0010 (3)
C4	0.0147 (4)	0.0144 (4)	0.0258 (4)	−0.0035 (3)	0.0116 (3)	−0.0058 (3)
C5	0.0118 (3)	0.0133 (4)	0.0164 (4)	−0.0002 (3)	0.0056 (3)	−0.0003 (3)
C6	0.0180 (4)	0.0168 (4)	0.0248 (4)	−0.0006 (3)	0.0082 (3)	0.0057 (3)
C7	0.0156 (4)	0.0163 (4)	0.0265 (5)	0.0023 (3)	0.0070 (3)	0.0057 (3)
C8	0.0142 (4)	0.0214 (4)	0.0186 (4)	−0.0014 (3)	0.0089 (3)	−0.0021 (3)
C9	0.0124 (4)	0.0197 (4)	0.0218 (4)	−0.0007 (3)	0.0071 (3)	−0.0008 (3)
C10	0.0196 (4)	0.0212 (5)	0.0263 (5)	−0.0028 (3)	0.0121 (4)	−0.0044 (4)
C11	0.0290 (6)	0.0223 (5)	0.0447 (7)	0.0055 (4)	0.0166 (5)	0.0043 (5)

Geometric parameters (Å, º)

P1—F3	1.5939 (8)	C4—H4A	0.9700
P1—F1	1.5948 (8)	C4—H4B	0.9700
P1—F5	1.5999 (8)	C5—H5A	0.9300
P1—F2	1.6020 (8)	C6—C7	1.3595 (15)
P1—F4	1.6029 (8)	C6—H6A	0.9300
P1—F6	1.6073 (7)	C7—H7A	0.9300
N1—C5	1.3337 (12)	C8—C9	1.5241 (14)
N1—C7	1.3782 (13)	C8—H8A	0.9700
N1—C8	1.4725 (12)	C8—H8B	0.9700
N2—C5	1.3335 (12)	C9—C10	1.5229 (15)
N2—C6	1.3822 (13)	C9—H9A	0.9700
N2—C4	1.4755 (12)	C9—H9B	0.9700
C1—C3^i	1.3966 (13)	C10—C11	1.5222 (17)
C1—C2	1.3981 (13)	C10—H10A	0.9700
C1—H1A	1.005 (17)	C10—H10B	0.9700
C2—C3	1.3964 (13)	C11—H11B	0.9600
C2—C4	1.5141 (13)	C11—H11C	0.9600
C3—C1^i	1.3966 (13)	C11—H11D	0.9600
C3—H3A	0.925 (17)
F3—P1—F1	91.03 (5)	H4A—C4—H4B	107.9
F3—P1—F5	90.61 (5)	N2—C5—N1	108.63 (8)
F1—P1—F5	89.70 (5)	N2—C5—H5A	125.7
F3—P1—F2	90.02 (5)	N1—C5—H5A	125.7
F1—P1—F2	178.91 (5)	C7—C6—N2	107.00 (9)
F5—P1—F2	90.60 (5)	C7—C6—H6A	126.5
F3—P1—F4	89.66 (5)	N2—C6—H6A	126.5
F1—P1—F4	89.82 (5)	C6—C7—N1	106.99 (9)
F5—P1—F4	179.45 (5)	C6—C7—H7A	126.5
F2—P1—F4	89.87 (5)	N1—C7—H7A	126.5
F3—P1—F6	179.09 (5)	N1—C8—C9	111.68 (8)
F1—P1—F6	89.64 (5)	N1—C8—H8A	109.3
F5—P1—F6	88.78 (4)	C9—C8—H8A	109.3
F2—P1—F6	89.31 (4)	N1—C8—H8B	109.3
F4—P1—F6	90.95 (5)	C9—C8—H8B	109.3
C5—N1—C7	108.79 (8)	H8A—C8—H8B	107.9
C5—N1—C8	125.60 (8)	C10—C9—C8	114.48 (8)
C7—N1—C8	125.60 (8)	C10—C9—H9A	108.6
C5—N2—C6	108.59 (8)	C8—C9—H9A	108.6
C5—N2—C4	124.66 (8)	C10—C9—H9B	108.6
C6—N2—C4	126.75 (8)	C8—C9—H9B	108.6
C3^i—C1—C2	120.28 (9)	H9A—C9—H9B	107.6
C3^i—C1—H1A	120.5 (10)	C11—C10—C9	113.87 (9)
C2—C1—H1A	119.2 (10)	C11—C10—H10A	108.8
C3—C2—C1	119.47 (8)	C9—C10—H10A	108.8
C3—C2—C4	120.16 (9)	C11—C10—H10B	108.8
C1—C2—C4	120.36 (8)	C9—C10—H10B	108.8
C2—C3—C1^i	120.25 (9)	H10A—C10—H10B	107.7
C2—C3—H3A	117.1 (11)	C10—C11—H11B	109.5
C1^i—C3—H3A	122.5 (11)	C10—C11—H11C	109.5
N2—C4—C2	111.82 (8)	H11B—C11—H11C	109.5
N2—C4—H4A	109.3	C10—C11—H11D	109.5
C2—C4—H4A	109.3	H11B—C11—H11D	109.5
N2—C4—H4B	109.3	H11C—C11—H11D	109.5
C2—C4—H4B	109.3
C3^i—C1—C2—C3	−0.02 (16)	C8—N1—C5—N2	178.85 (8)
C3^i—C1—C2—C4	−178.77 (9)	C5—N2—C6—C7	−0.48 (12)
C1—C2—C3—C1^i	0.02 (16)	C4—N2—C6—C7	179.81 (9)
C4—C2—C3—C1^i	178.77 (9)	N2—C6—C7—N1	0.50 (12)
C5—N2—C4—C2	−118.10 (10)	C5—N1—C7—C6	−0.35 (12)
C6—N2—C4—C2	61.57 (13)	C8—N1—C7—C6	−179.15 (9)
C3—C2—C4—N2	78.63 (11)	C5—N1—C8—C9	−104.24 (11)
C1—C2—C4—N2	−102.63 (10)	C7—N1—C8—C9	74.36 (12)
C6—N2—C5—N1	0.26 (11)	N1—C8—C9—C10	63.05 (11)
C4—N2—C5—N1	179.98 (8)	C8—C9—C10—C11	67.80 (12)
C7—N1—C5—N2	0.05 (11)

Symmetry code: (i) −x+1, −y, −z+1.

Hydrogen-bond geometry (Å, º)

Table 1. Hydrogen bond geometry (Å, °). Cg1 is centroids of benzene ring C1-C2-C3-C1A-C2A-C3A.

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1A···F3^ii	1.004 (17)	2.532 (18)	3.3945 (14)	143.8 (15)
C4—H4A···F4^ii	0.97	2.52	3.3516 (14)	144
C4—H4B···F2^iii	0.97	2.45	3.3497 (14)	153
C7—H7A···F1^iv	0.93	2.36	2.8798 (13)	115
C8—H8B···F6^v	0.97	2.49	3.3537 (13)	148
C8—H8A···Cg1^vi	0.97	2.84	3.7376 (12)	154
C8—H8A···Cg1^vii	0.97	2.84	3.7376 (12)	154

Symmetry codes: (ii) x, −y+1/2, z−1/2; (iii) x, y−1, z; (iv) −x, y−1/2, −z+1/2; (v) x, −y+1/2, z+1/2; (vi) x−1, y, z; (vii) −x, −y, −z+1.