2,4,5-Tri­chloro-1-iodo­benzene

The crystal structure of 2,4,5-tri­chloro­iodo­benzene, C₆H₂Cl₃I, a precursor of polychlorinated bi­phenyls (PCBs), is described. The mol­ecule is disordered in two distinct ways. Despite its lack of inversion point symmetry, it straddles an inversion centre in space group P2₁/n. An additional minor disorder component occurs as a result of a 180° rotation of some mol­ecules about [011].

Key indicators

• Single-crystal X-ray study
• T = 90 K
• Mean [sigma](C-C) = 0.006 Å
• R factor = 0.035
• wR factor = 0.078
• Data-to-parameter ratio = 16.7

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

PCBs are an important group of widespread environmental contaminants, known to cause a variety of toxic effects (Robertson & Hansen, 2001). During our attempts to synthesize pure PCB congeners, we obtained crystals of the title compound, (I), whose structure is reported herein.

2,4,5-trichloro-1-iodobenzene is one compound in a series of chlorinated derivatives of iodo- or bromobenzenes, which are `building blocks' to obtain polychlorinated biphenyls (PCBs). The 2,4,5-trichlorophenyl moiety is commonly found in PCB congeners in combination with a second phenyl moiety, and its three-dimensional structure is correlated with the toxicity of PCBs (Lehmler & Robertson, 2001). The availability of crystal structure data will therefore aid us in our understanding of toxicity of PCB congeners with a 2,4,5-substitution pattern.

S2. Experimental

The 2,4,5-trichloroiodobenzene crystals were obtained while attempting to synthesize 2,4,5-trichlorobiphenyl in the Suzuki reaction (Lehmler et al., 2001). Colorless needle-shaped crystals formed from solution in methanol.

S3. Refinement

The crystals are extensively disordered. The asymmetric unit contains nominally half a molecule, but it is disordered such that the Cl and I atoms are partially superimposed. Given the known chemical composition, a satisfactory model requires that the sum of the Cl atom occupancies should sum to 1.5 over the two halogen sites in the asymmetric unit, which was accomplished with the SUMP command in SHELXL97 (Sheldrick, 1997). This in turn allowed the I occupancy total over the two sites to be 0.5 while at the same time ensuring that the total occupancy of each site itself is unity. Bond distances for C—Cl and C—I are heavily correlated as a result of the disorder and it was not possible to independently refine both. The length of C—I bonds was optimized to be 1.15 times that of the corresponding C—Cl bonds. This treatment ensured that the C—Cl bonds were typical for this class of compound (e.g. Lehmler et al., 2001). The C—I bond length reported here should thus be regarded as a compromise to improve the least-squares fit and no particular claims are made for the accuracy of this C—I distance. The anisotropic displacement parameters (ADPs) of each individual pair of overlapping Cl/I were constrained to be equal while the ADPs between each pair were restrained to be similar.

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: SCALEPACK (Otwinowski & Minor, 1997); data reduction: DENZO-SMN (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in Siemens SHELXTL (Sheldrick, 1994); software used to prepare material for publication: SHELX97-2 (Sheldrick, 1997) and local procedures.

Figures

	Fig. 1. An ellipsoid plot of 2,4,5-trichloroiodobenzene with non-H atom displacement ellipsoids drawn at the 50% probability level.
	Fig. 2. The nature of disorder in 2,4,5-trichloroiodobenzene at 90 K.

(I)

Crystal data

C6H2Cl3I	F(000) = 284
Mr = 307.33	Dx = 2.471 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 3.9191 (8) Å	Cell parameters from 993 reflections
b = 10.894 (2) Å	θ = 1.0–27.5°
c = 9.852 (2) Å	µ = 4.76 mm^−1
β = 100.87 (3)°	T = 90 K
V = 413.07 (15) Å^3	Block, colourless
Z = 2	0.20 × 0.10 × 0.10 mm

Data collection

Nonius KappaCCD diffractometer	936 independent reflections
Radiation source: fine-focus sealed tube	884 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.035
Detector resolution: 18 pixels mm^-1	θmax = 27.5°, θmin = 2.8°
ω scans at fixed χ = 55°	h = −4→5
Absorption correction: multi-scan (SADABS; Sheldrick, 1997)	k = −13→14
Tmin = 0.570, Tmax = 0.621	l = −12→12
6306 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.035	H-atom parameters constrained
wR(F^2) = 0.078	w = 1/[σ^2(Fo^2))^2 + 1.7326P] where P = (Fo^2 + 2Fc^2)/3
S = 1.35	(Δ/σ)max < 0.001
936 reflections	Δρmax = 0.64 e Å^−3
56 parameters	Δρmin = −0.72 e Å^−3
5 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.011 (2)

Special details

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. 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	Occ. (<1)
Cl1	0.151 (2)	0.4432 (7)	0.2017 (8)	0.0208 (4)	0.6201 (14)
I1	0.1386 (9)	0.4246 (2)	0.1821 (3)	0.0208 (4)	0.3799 (14)
Cl2	−0.063 (3)	0.2275 (4)	0.4017 (10)	0.0314 (7)	0.8799 (14)
I2	−0.060 (6)	0.2056 (10)	0.403 (2)	0.0314 (7)	0.1201 (14)
C1	0.0622 (11)	0.4691 (4)	0.3699 (4)	0.0249 (9)
C2	−0.0299 (11)	0.3787 (4)	0.4563 (5)	0.0258 (9)
C3	−0.0924 (11)	0.4098 (4)	0.5855 (5)	0.0263 (9)
H3	−0.1562	0.3480	0.6441	0.032*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0303 (4)	0.0179 (10)	0.0161 (9)	0.0004 (6)	0.0089 (6)	−0.0089 (5)
I1	0.0303 (4)	0.0179 (10)	0.0161 (9)	0.0004 (6)	0.0089 (6)	−0.0089 (5)
Cl2	0.0475 (6)	0.0129 (19)	0.0321 (6)	−0.0048 (19)	0.0033 (4)	0.0012 (17)
I2	0.0475 (6)	0.0129 (19)	0.0321 (6)	−0.0048 (19)	0.0033 (4)	0.0012 (17)
C1	0.0204 (19)	0.031 (2)	0.021 (2)	0.0020 (17)	−0.0018 (15)	0.0022 (17)
C2	0.022 (2)	0.026 (2)	0.026 (2)	−0.0012 (16)	−0.0048 (16)	0.0023 (17)
C3	0.021 (2)	0.030 (2)	0.026 (2)	−0.0012 (17)	−0.0018 (16)	0.0066 (17)

Geometric parameters (Å, º)

Cl1—C1	1.778 (8)	C1—C2	1.393 (6)
I1—C1	1.989 (5)	C2—C3	1.383 (6)
Cl2—C2	1.729 (7)	C3—C1^i	1.389 (6)
I2—C2	1.955 (12)	C3—H3	0.9500
C1—C3^i	1.389 (6)
C3^i—C1—C2	119.7 (4)	C1—C2—Cl2	119.7 (5)
C3^i—C1—Cl1	115.2 (4)	C3—C2—I2	118.0 (7)
C2—C1—Cl1	125.1 (4)	C1—C2—I2	121.9 (7)
C3^i—C1—I1	120.5 (3)	C2—C3—C1^i	120.3 (4)
C2—C1—I1	119.8 (3)	C2—C3—H3	119.8
C3—C2—C1	120.0 (4)	C1^i—C3—H3	119.8
C3—C2—Cl2	120.3 (5)
C3^i—C1—C2—C3	0.3 (7)	C3^i—C1—C2—I2	−177.7 (8)
Cl1—C1—C2—C3	179.9 (5)	Cl1—C1—C2—I2	2.0 (10)
I1—C1—C2—C3	−178.3 (3)	I1—C1—C2—I2	3.8 (9)
C3^i—C1—C2—Cl2	−179.3 (5)	C1—C2—C3—C1^i	−0.3 (7)
Cl1—C1—C2—Cl2	0.4 (7)	Cl2—C2—C3—C1^i	179.3 (5)
I1—C1—C2—Cl2	2.2 (6)	I2—C2—C3—C1^i	177.8 (8)

Symmetry code: (i) −x, −y+1, −z+1.