[1,3-Bis(2,6-diisopropyl­phenyl)imidazol-2-yl­idene]chloridogold(I)

The mol­ecule of the title compound, [AuCl(C₂₇H₃₆N₂)], which belongs to a class of potentially catalytically active N-hetero­cyclic carbene complexes, has crystallographic C₂ symmetry and approximate C_2v symmetry. The structure is isostructural with the Cu^I and Ag^I analogues. A previous report of the structure of the title compound as its toluene solvate [Fructos et al. (2005). Angew. Chem. Int. Ed. 44, 5284-5288] has inaccurate geometry for the complex mol­ecule as a consequence of probable incorrect refinement in the space group Cc, instead of C2/c [Marsh (2009). Acta Cryst. B65, 782-783]. The Au-C bond length of 1.998 (4) Å in the title compound is more consistent with the mean distance of 1.979 (14) Å found in 52 other reported [AuCl(carbene)] complexes than with the shorter distance of 1.942 (3) Å given for the refinement in the space group Cc for the toluene solvate and the value of 1.939 Å obtained from the recalculation of that structure in C2/c.

S1. Comment

In recent years, the popularity of gold(I) and gold(III) complexes has developed exponentially due to their very broad catalytic activity (Fürstner & Davies, 2007; Hashmi, 2007; Jiménez-Nuñez & Echavarren, 2008; Li et al., 2008; Michelet et al., 2008). The use of N-heterocyclic carbenes (NHCs) as ancillary ligands in gold(I) complexes has received considerable attention as they provide unique properties due to the unprecedented σ-donation and significant steric bulk (Marion & Nolan, 2008; Nolan, 2011). In addition, gold(I) complexes based on N-heterocyclic carbenes have attracted increased attention in medicinal chemistry (Hindi et al., 2009) where they display antiarthritic (Gunatilleke & Barrios, 2006), antitumour (Barnard et al., 2004) and antimicrobial activity (Özdemir et al., 2010).

During our study of a gold-catalysed 1,2/1,2-bisacetoxy migration of 1,4-bispropargyl acetates to form (1Z,3Z)-2,3-diacetoxy-1,3-dienes (Huang et al., 2009), we synthesized and crystallized the title complex, (I).

The crystal structure of (I) as its toluene solvate was reported by Fructos et al. (2005) and discussed again by de Frémont, Scott, Stevens & Nolan (2005). The structure was reported in the space group Cc with Z' = 1. It was subsequently proposed by Marsh (2009) that the originally reported space group was probably incorrect and that the space group should be C2/c with Z' = 1/2, the complex molecule sitting on a C₂ axis and the solvent molecule disordered about a centre of inversion. Marsh noted from the originally deposited CIF [Cambridge Structural Database (CSD; Allen, 2002) refcode NATLAW, deposition No. 258274] that the original refinement was apparently unstable, as it had not converged, it required many geometric restraints and it had quite a wide range of C—C distances in the benzene rings (1.21–1.54 Å). This behaviour is consistent with attempting to refine a structure in a space group with lower symmetry than the true space group, especially when the molecule would have crystallographic symmetry but the chosen space group does not impose that symmetry on the molecule. The revised structure proposed by Marsh has more reasonable geometry, but that result was based on symmetrizing the geometry of the molecule refined in the original Cc structure. It was not refined against the original reflection data, so an accurate determination of the geometry of the title complex molecule was hitherto not available.

In view of the absence of a fully refined structure in the correct space group for the toluene solvate of (I), we determined the structure in its unsolvated form. A view of the molecular structure is shown in Fig. 1. The molecule of (I) has crystallographic C₂ symmetry and approximate C_2v symmetry, where the root mean-square fit of the non-H atoms to perfect C_2v symmetry is 0.11 Å. Consequently, the planes of the benzene and imidazolylidene rings are nearly perpendicular to each other, with a dihedral angle between them of 87.39 (15)°, and the isopropyl groups are oriented similarly on each side of the unique benzene ring. The C₂ axis passes through the Au, Cl and coordinated C atoms, so that atom Au1 necessarily has perfect linear coordination geometry. The Au1—C1 bond length of 1.998 (4) Å is more consistent with this distance in other [AuCl(NHC)] complexes [mean of 1.979 (14) Å over 52 entries in the CSD; Version 5.32 with November 2011 updates], compared with the value of 1.942 (3) Å given for the refinement in the space group Cc for the toluene solvate of (I) (de Frémont et al., 2005 [de Frémont, Scott, Stevens & Nolan, 2005 OR de Frémont, Scott, Stevens, Ramnial et al., 2005?]) and the value of 1.939 Å obtained from the recalculation of that structure in C2/c (Marsh, 2009). The Au1—Cl1 distance of 2.2718 (11) Å in (I) does not differ significantly from that obtained in the Cc refinement [2.2698 (11) Å].

The structure of the analogue of (I) in which the five-membered ring is saturated has also been reported by de Frémont et al. (2005) [de Frémont, Scott, Stevens & Nolan, 2005 OR de Frémont, Scott, Stevens, Ramnial et al., 2005?]. This structure is virtually isostructural with that of (I) and the unit-cell dimensions are very similar. The Au—C and Au—Cl distances in that structure [1.982 (2) and 2.2760 (8)Å, respectively] are also very similar to those in (I).

The structure of (I) is isostructural with the Cu^I and Ag^I analogues [CSD refcodes EVAHEO (Kaur et al., 2004) and NEHBIM (Yu et al., 2006), respectively]. The carbene N—C bonds have a similar length in all three structures, being 1.338 (3), 1.320 (7) and 1.3485 (17) Å for (I) and the Cu^I and Ag^I analogues, respectively. The metal—C bond lengths vary in the sequence Cu < Au < Ag [1.953 (8), 1.998 (4) and 2.077 (2) Å, respectively]. A similar trend is found for the corresponding metal—Cl bond lengths, viz. 2.089 (4), 2.2718 (11) and 2.3038 (7) Å, respectively. Although the Ag—C bond here is longer than the Au—C bond and seems counter-intuitive, considering that Au^I would be expected to have a larger ionic radius than Ag^I, it is not unusual. The mean Ag—C bond length from 51 entries for [AgCl(NHC)] complexes in the CSD is 2.079 (13) Å, while the mean Au—C distance, as mentioned above, is 1.98 (2) Å. Calculations on the [M(CN)₂]^- anion [M = Ag^I or Au^I] (Zaleski-Ejgierd et al., 2008; Wang et al., 2009) have found that the Au—C bond is shorter than the Ag—C bond, and this has been attributed to increased covalency in the Au—C bond on account of strong relativistic effects in Au (Wang et al., 2009).

The analogous Cu^I and Ag^I structures have also been determined as their dichloromethane solvates [CSD refcodes EVICER (Mankad et al., 2004) and HEBLUW (de Frémont, Scott, Stevens, Ramnial et al., 2005)]. While the M— Cl bond lengths [2.106 (2) and 2.3135 (18) Å for the Cu^I and Ag^I dichloromethane solvates, respectively] and the Ag—C distance [2.056 (7) Å] are similar to the corresponding distances in the unsolvated structures mentioned above, the Cu—C bond length of 1.881 (7) Å is 0.07 Å shorter in the dichloromethane solvate than in the unsolvated case. The mean Cu—C distance from 15 entries for [CuCl(NHC)] complexes in the CSD is 1.90 (4) Å, but this set has three structures, of which the unsolvated Cu^I structure (EVAHEO) is one, in which the Cu—C bond is up to 0.1 Å longer than for the other structures. Discarding these three outliers, the 12 remaining [CuCl(NHC)] structures have a mean Cu—C bond length of 1.88 (1) Å, which is very close to the corresponding bond length observed in the Cu^I dichloromethane solvate.

There are no significant intermolecular interactions in the structure of (I). The molecules stack in columns parallel to the [001] axis in which the C—Au—Cl vector points directly along the column axis and all molecules in the column are aligned unidirectional. Adjacent columns are antiparallel (Fig. 2).

Related literature

For related literature, see: Allen (2002); Barnard et al. (2004); Fürstner & Davies (2007); Frémont, Scott, Stevens & Nolan (2005); Frémont, Scott, Stevens, Ramnial, Lightbody, Macdonald, Clyburne, Abernethy & Nolan (2005); Fructos et al. (2005); Gunatilleke & Barrios (2006); Hashmi (2007); Hindi et al. (2009); Huang et al. (2009); Jiménez-Nuñez & Echavarren (2008); Kaur et al. (2004); Li et al. (2008); Mankad et al. (2004); Marion & Nolan (2008); Marsh (2009); Michelet et al. (2008); Nolan (2011); Wang et al. (2009); Yu et al. (2006); Zaleski-Ejgierd, Patzschke & Pyykkö (2008); Özdemir et al. (2010).

S2. Experimental

The title compound was prepared according to a previously reported procedure (de Frémont et al., 2005 [de Frémont, Scott, Stevens & Nolan, 2005 OR de Frémont, Scott, Stevens, Ramnial et al., 2005?]). Colourless single crystals suitable for a crystal structure analysis were grown in an NMR tube from a solution of (I) (10 mg) in CD₂Cl₂ (0.1 ml) and pentane (2 ml) that was cooled slowly to 273 K over a period of one week.

S3. Refinement

Methyl H atoms were constrained to ideal geometry, with C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C), but were allowed to rotate freely about the C—C bonds. All other H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C—H = 0.95 (aromatic) or 1.00 Å (methine) and with U_iso(H) = 1.2U_eq(C). One reflection, whose intensity was considered to be an extreme outlier, was omitted from the final refinement.

Computing details

Data collection: CrysAlis PRO (Agilent, 2011); cell refinement: CrysAlis PRO (Agilent, 2011); data reduction: CrysAlis PRO (Agilent, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2009).

Figures

	Fig. 1. A view of the molecule of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (i) -x + 3/2, -y + 1/2, z.]
	Fig. 2. The crystal packing of (I), projected down the b axis, showing the columns of aligned molecules progressing horizontally. All H atoms have been omitted for clarity.

[1,3-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene]chloridogold(I)

Crystal data

[AuCl(C27H36N2)]	F(000) = 1232
Mr = 621.01	Dx = 1.565 Mg m^−3
Orthorhombic, Pccn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 6205 reflections
a = 10.6655 (2) Å	θ = 2.2–30.2°
b = 12.6641 (2) Å	µ = 5.72 mm^−1
c = 19.5145 (4) Å	T = 160 K
V = 2635.82 (8) Å^3	Prism, colourless
Z = 4	0.18 × 0.15 × 0.12 mm

Data collection

Oxford SuperNova dual radiation diffractometer	3631 independent reflections
Radiation source: SuperNova (Mo) X-ray Source	2389 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.038
Detector resolution: 10.3801 pixels mm^-1	θmax = 30.3°, θmin = 2.5°
ω scans	h = −14→14
Absorption correction: multi-scan (CrysAlis PRO; Agilent, 2011)	k = −17→17
Tmin = 0.894, Tmax = 1.000	l = −25→26
17748 measured reflections

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)] = 0.024	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.050	H-atom parameters constrained
S = 1.01	w = 1/[σ^2(Fo^2) + (0.0145P)^2 + 1.527P] where P = (Fo^2 + 2Fc^2)/3
3630 reflections	(Δ/σ)max = 0.002
146 parameters	Δρmax = 0.57 e Å^−3
0 restraints	Δρmin = −1.03 e Å^−3

Special details

Experimental. Empirical absorption correction using spherical harmonics, as implemented in SCALE3 ABSPACK scaling algorithm of CrysAlis PRO. Solvent used: CD2Cl2/pentane Cooling Device: Oxford Instruments Cryojet XL Crystal mount: on a glass fibre Frames collected: 262 Seconds exposure per frame: 5.0 Degrees rotation per frame: 1.0 Crystal-detector distance (mm): 55.0 Client: Teresa De Haro Sample code: HAR-HO-04b (NV1110)

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Au1	0.7500	0.2500	0.273821 (7)	0.02254 (5)
Cl1	0.7500	0.2500	0.15741 (6)	0.0445 (3)
N1	0.7199 (2)	0.16942 (16)	0.41739 (12)	0.0238 (5)
C1	0.7500	0.2500	0.37621 (19)	0.0223 (8)
C2	0.7313 (3)	0.1993 (2)	0.48525 (15)	0.0296 (7)
H2	0.7159	0.1565	0.5243	0.035*
C3	0.6762 (3)	0.0675 (2)	0.39390 (15)	0.0232 (6)
C4	0.5473 (3)	0.0534 (2)	0.38604 (16)	0.0305 (7)
C5	0.5073 (3)	−0.0439 (2)	0.36134 (18)	0.0388 (8)
H5	0.4201	−0.0570	0.3561	0.047*
C6	0.5920 (3)	−0.1215 (2)	0.34441 (18)	0.0428 (9)
H6	0.5629	−0.1868	0.3264	0.051*
C7	0.7186 (3)	−0.1055 (2)	0.35334 (19)	0.0418 (9)
H7	0.7756	−0.1603	0.3418	0.050*
C8	0.7647 (3)	−0.0100 (2)	0.37913 (16)	0.0298 (7)
C9	0.4543 (3)	0.1411 (2)	0.40055 (18)	0.0368 (8)
H9	0.4991	0.1968	0.4274	0.044*
C10	0.4095 (3)	0.1911 (3)	0.3341 (2)	0.0551 (11)
H10A	0.4821	0.2104	0.3059	0.083*
H10B	0.3605	0.2546	0.3445	0.083*
H10C	0.3570	0.1407	0.3090	0.083*
C11	0.3441 (3)	0.1033 (3)	0.4435 (2)	0.0606 (12)
H11A	0.2964	0.0503	0.4178	0.091*
H11B	0.2896	0.1633	0.4544	0.091*
H11C	0.3753	0.0718	0.4861	0.091*
C12	0.9042 (3)	0.0040 (2)	0.38976 (18)	0.0346 (8)
H12	0.9169	0.0714	0.4155	0.042*
C13	0.9729 (3)	0.0139 (3)	0.3216 (2)	0.0565 (10)
H13A	0.9667	−0.0529	0.2964	0.085*
H13B	1.0613	0.0304	0.3300	0.085*
H13C	0.9349	0.0706	0.2944	0.085*
C14	0.9607 (3)	−0.0848 (3)	0.43245 (19)	0.0471 (9)
H14A	0.9166	−0.0894	0.4764	0.071*
H14B	1.0497	−0.0702	0.4406	0.071*
H14C	0.9520	−0.1518	0.4078	0.071*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Au1	0.02696 (8)	0.02273 (7)	0.01793 (8)	0.00231 (7)	0.000	0.000
Cl1	0.0571 (7)	0.0552 (7)	0.0212 (5)	0.0190 (7)	0.000	0.000
N1	0.0356 (14)	0.0160 (10)	0.0197 (13)	0.0014 (9)	0.0004 (10)	−0.0016 (10)
C1	0.0263 (18)	0.0188 (15)	0.0217 (19)	0.0011 (19)	0.000	0.000
C2	0.049 (2)	0.0249 (14)	0.0152 (13)	0.0059 (14)	0.0040 (15)	0.0054 (12)
C3	0.0343 (16)	0.0154 (12)	0.0199 (15)	−0.0013 (11)	−0.0002 (13)	0.0022 (13)
C4	0.0347 (17)	0.0255 (14)	0.0312 (17)	0.0018 (13)	−0.0059 (15)	0.0073 (15)
C5	0.0371 (19)	0.0306 (16)	0.049 (2)	−0.0073 (14)	−0.0144 (17)	0.0112 (16)
C6	0.054 (2)	0.0218 (14)	0.053 (2)	−0.0037 (15)	−0.0150 (19)	0.0007 (17)
C7	0.050 (2)	0.0219 (14)	0.053 (2)	0.0060 (14)	−0.0096 (18)	−0.0060 (16)
C8	0.0365 (18)	0.0218 (12)	0.0310 (16)	0.0021 (13)	−0.0008 (15)	−0.0019 (13)
C9	0.0293 (16)	0.0306 (16)	0.050 (2)	0.0021 (13)	0.0007 (16)	0.0035 (17)
C10	0.0357 (19)	0.058 (2)	0.071 (3)	0.0189 (18)	0.011 (2)	0.034 (2)
C11	0.054 (2)	0.059 (2)	0.069 (3)	0.018 (2)	0.021 (2)	0.028 (2)
C12	0.0321 (17)	0.0295 (15)	0.042 (2)	0.0055 (13)	0.0006 (16)	−0.0082 (16)
C13	0.046 (2)	0.066 (3)	0.058 (3)	0.0104 (19)	0.007 (2)	0.005 (2)
C14	0.046 (2)	0.0438 (19)	0.052 (2)	0.0088 (16)	−0.0147 (19)	−0.0088 (19)

Geometric parameters (Å, º)

Au1—C1	1.998 (4)	C9—C10	1.521 (5)
Au1—Cl1	2.2718 (11)	C9—C11	1.521 (4)
N1—C1	1.338 (3)	C9—H9	1.0000
N1—C2	1.383 (3)	C10—H10A	0.9800
N1—C3	1.446 (3)	C10—H10B	0.9800
C2—C2^i	1.344 (6)	C10—H10C	0.9800
C2—H2	0.9500	C11—H11A	0.9800
C3—C8	1.392 (4)	C11—H11B	0.9800
C3—C4	1.395 (4)	C11—H11C	0.9800
C4—C5	1.391 (4)	C12—C14	1.523 (4)
C4—C9	1.515 (4)	C12—C13	1.525 (5)
C5—C6	1.375 (4)	C12—H12	1.0000
C5—H5	0.9500	C13—H13A	0.9800
C6—C7	1.376 (4)	C13—H13B	0.9800
C6—H6	0.9500	C13—H13C	0.9800
C7—C8	1.399 (4)	C14—H14A	0.9800
C7—H7	0.9500	C14—H14B	0.9800
C8—C12	1.512 (4)	C14—H14C	0.9800
C1—Au1—Cl1	180	C11—C9—H9	107.6
C1—N1—C2	110.2 (2)	C9—C10—H10A	109.5
C1—N1—C3	124.6 (2)	C9—C10—H10B	109.5
C2—N1—C3	125.2 (2)	H10A—C10—H10B	109.5
N1—C1—N1^i	106.2 (3)	C9—C10—H10C	109.5
N1—C1—Au1	126.92 (16)	H10A—C10—H10C	109.5
C2^i—C2—N1	106.71 (15)	H10B—C10—H10C	109.5
C2^i—C2—H2	126.6	C9—C11—H11A	109.5
N1—C2—H2	126.6	C9—C11—H11B	109.5
C8—C3—C4	123.7 (3)	H11A—C11—H11B	109.5
C8—C3—N1	118.5 (2)	C9—C11—H11C	109.5
C4—C3—N1	117.8 (2)	H11A—C11—H11C	109.5
C5—C4—C3	117.0 (3)	H11B—C11—H11C	109.5
C5—C4—C9	120.9 (3)	C8—C12—C14	112.2 (3)
C3—C4—C9	122.0 (3)	C8—C12—C13	111.3 (3)
C6—C5—C4	121.0 (3)	C14—C12—C13	110.4 (3)
C6—C5—H5	119.5	C8—C12—H12	107.6
C4—C5—H5	119.5	C14—C12—H12	107.6
C5—C6—C7	120.6 (3)	C13—C12—H12	107.6
C5—C6—H6	119.7	C12—C13—H13A	109.5
C7—C6—H6	119.7	C12—C13—H13B	109.5
C6—C7—C8	121.2 (3)	H13A—C13—H13B	109.5
C6—C7—H7	119.4	C12—C13—H13C	109.5
C8—C7—H7	119.4	H13A—C13—H13C	109.5
C3—C8—C7	116.5 (3)	H13B—C13—H13C	109.5
C3—C8—C12	123.7 (3)	C12—C14—H14A	109.5
C7—C8—C12	119.8 (3)	C12—C14—H14B	109.5
C4—C9—C10	110.6 (3)	H14A—C14—H14B	109.5
C4—C9—C11	112.2 (3)	C12—C14—H14C	109.5
C10—C9—C11	111.0 (3)	H14A—C14—H14C	109.5
C4—C9—H9	107.6	H14B—C14—H14C	109.5
C10—C9—H9	107.6
C2—N1—C1—N1^i	−0.14 (15)	C4—C5—C6—C7	1.8 (5)
C3—N1—C1—N1^i	177.3 (3)	C5—C6—C7—C8	−0.6 (6)
C2—N1—C1—Au1	179.86 (15)	C4—C3—C8—C7	1.7 (5)
C3—N1—C1—Au1	−2.7 (3)	N1—C3—C8—C7	−176.9 (3)
C1—N1—C2—C2^i	0.4 (4)	C4—C3—C8—C12	−178.0 (3)
C3—N1—C2—C2^i	−177.0 (3)	N1—C3—C8—C12	3.4 (4)
C1—N1—C3—C8	88.4 (3)	C6—C7—C8—C3	−1.1 (5)
C2—N1—C3—C8	−94.5 (3)	C6—C7—C8—C12	178.7 (3)
C1—N1—C3—C4	−90.3 (3)	C5—C4—C9—C10	−75.4 (4)
C2—N1—C3—C4	86.7 (4)	C3—C4—C9—C10	101.9 (4)
C8—C3—C4—C5	−0.7 (5)	C5—C4—C9—C11	49.2 (4)
N1—C3—C4—C5	178.0 (3)	C3—C4—C9—C11	−133.6 (3)
C8—C3—C4—C9	−178.0 (3)	C3—C8—C12—C14	127.2 (3)
N1—C3—C4—C9	0.7 (4)	C7—C8—C12—C14	−52.5 (4)
C3—C4—C5—C6	−1.1 (5)	C3—C8—C12—C13	−108.6 (3)
C9—C4—C5—C6	176.2 (3)	C7—C8—C12—C13	71.7 (4)

Symmetry code: (i) −x+3/2, −y+1/2, z.