Di­chloro­bis(2-chloro-5-methyl­pyridine-κN)copper(II)

In the crystals of the title compound, [CuCl₂(C₆H₆ClN)₂], the Cu atom lies on an inversion centre and is four-coordinated by two pyridine N atoms and two Cl atoms in trans positions. The coordination geometry is square planar, with Cu—N and Cu—Cl distances of 1.986 (2) and 2.2536 (11) Å, respectively. The two pyridine rings are parallel, but twist from the CuN₂Cl₂ coordination plane by about 95° in the complex mol­ecule. There are three kinds of intermolecular C—H...Cl hydrogen bonds in the crystals. Two of these types generate two-dimensional molecular networks, viewed in the direction of the a axis, and the other connects adjacent molecular networks.

S1. Comment

2-Chloro-5-methylpyridine is a very important intermediate for the preparation of biological activity compounds, especially insecticides (Guenth, 1991) (e.g. imidacloprid; Diehr, 1990). 2-Chloro-5-methylpyridine is usually manufactured from 3-methylpyridine-N-oxide, but ?the product also contains 3-methylpyridine and its isomer 2-chloro-3-methylpyridine?? (Kaufmann et al., 1991). Because the properties of isomers are similar, it is difficult to separate them by ordinary methods, such as distillation. We have found that the crystalline complex of 2-chloro-5-methylpyridine can be formed easily when crude 2-chloro-5-methylpyridine is mixed with CuCl₂·2H₂O in absolute ethanol and hence can be isolated with over 99% purity. In order to look for specific structural features, we have performed an X-ray structural analysis of the title compound, (I).

The molecular structure of (I) is shown in Fig.1. The Cu atom lies on a crystallographic inversion center so that the angles N—Cu—N and Cl—Cu—Cl are 180°. The two pyridine rings are coplanar in the complex molecule because of crystallographic symmetry, but they twist from the CuN₂Cl₂ coordination plane with a torsion angle Cl2—Cu—N—C1 of 95.2 (2)°. The bond distances Cu—N and Cu—Cl [1.986 (2) and 2.2536 (11) Å, respectively (Table 1)] agree with the corresponding values for other Cu(II) complexes (Silva et al., 2001; Zavalij et al., 2002). The bond lengths and angles in the complex molecule are largely common.

A further analysis of the short intermolecular contacts shows that there are three kinds of C—H···Cl interactions (Table. 2). In the first type, the H···Cl distance is 2.76 Å, which is obviously shorter than the sum of the van der Waals radii of these two atoms (2.95 Å). The angle C—H···Cl is 170.5°, which is close to 180°. These parameters indicate a hydrogen bonding interaction (Aullon et al., 1998). In the other types of interaction, the H···Cl distances are very close to 2.95 Å. If the C—H···Cl distances and angles are normalized (Jeffrey & Lewis, 1978; Taylor & Kennard, 1983), the H···Cl distances are 2.82 and 2.87 Å, and the angles are 159.3 and 155.4°, respectively. These values suggest that the second and third C—H···Cl interactions can also be considered as hydrogen-bonding interactions. Among these three kinds of interactions, the first one is the strongest, the second is less strong and the third is weak.

A detailed analysis of the crystal packing shows that the hydrogen bonds involving the first and the third C—H···Cl interactions [i.e. C2 to Cl2(-x, y − 1/2, −z + 3/2) and C6 to Cl2(x, −y + 1/2, z − 1/2), respectively] generate two-dimensional networks when viewed in the direction of the α axis (Fig.2). The molecular networks are stacked one upon another. Because the centroid distances between neighbouring pyridine rings in adjacent molecular networks are 5.862 Å, any intermolecular forces between these rings should be very weak (Panda et al., 2001). The C5—H···Cl2(-x + 1, −y + 1, −z + 1) interactions (Fig.3) enforce the connection between adjacent molecular networks. Therefore, the C—H···Cl interactions are likely to be the major intermolecular forces, which cause complex molecules to be packed compactly and be isolated from the reactant mixture.

All three kinds of C—H···Cl interactions acting on the same pyridine ring in the complex molecule force the pyridine ring to rotate around the N—Cu bond, so that the pyridine ring twists from the CuN₂Cl₂ coordination plane with a Cl2—Cu—N—C1 torsion angle of 95.2 (2)° rather than 90°.

The crystal packing diagram also shows that although each Cl atom in a Cu—Cl interaction can form all three kinds of C—H···Cl connection, ??the interaction does not fully fit the concept of a hydrogen bond.?? Moreover, the H···Cl distances in the second and third C—H···Cl interactions, before normalizing, are 2.96 and 2.99 Å, respectively, which are a little longer than 2.95 Å. It would be better to describe these three kinds of C—H···Cl interactions as hydrogen bridges (Desiraju, 2002). The complex molecules form a supramolecular structure in the crystals via these hydrogen bridges.

S2. Experimental

The crude 2-chloro-5-methylpyridine contained 2-chloro-5-methylpyridine (79.4%), 2-chloro-3-methylpyridine (13.6%) and 3-methylpyridine (5.2%) (determined by peak area % with GC—MS). Crude 2-chloro-5- methylpyridine (12.0 g) in absolute ethanol (30 ml) was mixed with CuCl₂·2H₂O (5.0 g, 0.029 mol, in 20 ml) in a round-bottom flask. The blue precipitate appeared immediately. More ethanol (50 ml) was added to the mixture, and it was refluxed for 15 min. The precipitate changed from blue to dark violet. After suction filtration, the precipitate was washed with absolute ethanol and dried to obtain a dark-violet crystalline precipitate (10.5 g; 73.6% recycle yield of 2-chloro-5-methyl pyridine). The product was recrystallized in absolute ethanol, and a single-crystal was obtained from the refined product mother solution.

A melting-point determination was performed on XRC1 melting-point apparatus (Science Instrument Company, Sichuan University). The crystal melted at 413 K (decomposed).

CHN analysis was obtained with an Eger 2000 elemental analyzer. Analysis; calculated for C₁₂H₁₂Cl₄CuN₂: C 37.02, H 3.08, N 7.19%; found: C 37.05, H 3.39, N 7.60%.

S3. Refinement

H atoms were added at calculated positions and refined using a riding model. H atoms were given isotropic displacement parameters equal to 1.2 (or 1.5 for methyl H atoms) times the equivalent isotropic displacement parameters of their parent atoms, and C—H distances were restrained to 0.95 Å for H atoms bonded to C2, C3 and C5, and 0.98 Å for methyl H.

Computing details

Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: DATARED, Enraf-Nonius; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX publication routines (Farrugia, 1999).

Figures

	Fig. 1. A view of a molecule of (I), showing the atomic numbering. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms.
	Fig. 2. The packing diagram of (I), viewed along the α axis. The C2—H2···Cl2(-x, y − 1/2, −z + 3/2) and C6—H6C···Cl2(x, −y + 1/2, z − 1/2) interactions are indicated by dashed lines.
	Fig. 3. The packing diagram of (I) viewed along the α axis. The C5—H5···Cl2(-x + 1, −y + 1, −z + 1) interactions are indicated by dashed lines.

(I)

Crystal data

[CuCl2(C6H6ClN)2]	F(000) = 390
Mr = 389.58	Dx = 1.676 Mg m^−3
Monoclinic, P21/c	Melting point: 140 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 5.862 (1) Å	Cell parameters from 25 reflections
b = 12.941 (4) Å	θ = 2.6–30.2°
c = 10.538 (4) Å	µ = 2.09 mm^−1
β = 105.12 (3)°	T = 293 K
V = 771.7 (4) Å^3	Prism, dark violet
Z = 2	0.35 × 0.3 × 0.2 mm

Data collection

CAD-4 diffractometer	Rint = 0.028
ω/2θ scans	θmax = 30.2°, θmin = 2.6°
Absorption correction: empirical (using intensity measurements) North, Phillips & Mathews, 1968	h = −8→8
Tmin = 0.528, Tmax = 0.680	k = −1→18
2585 measured reflections	l = 0→14
2284 independent reflections	3 standard reflections every 100 reflections
1352 reflections with I > 2σ(I)

Refinement

Refinement on F^2	H-atom parameters constrained
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0684P)^2 + 0.0962P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.037	(Δ/σ)max = 0.023
wR(F^2) = 0.116	Δρmax = 0.54 e Å^−3
S = 0.98	Δρmin = −0.64 e Å^−3
2284 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
90 parameters	Extinction coefficient: 0.053 (4)
0 restraints

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.

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

	x	y	z	Uiso*/Ueq
Cu	0	0.5	0.5	0.03760 (17)
Cl2	0.30697 (13)	0.55558 (6)	0.66244 (8)	0.0532 (2)
Cl1	−0.19362 (18)	0.36842 (10)	0.69258 (10)	0.0771 (3)
N	0.0948 (4)	0.35446 (18)	0.5451 (2)	0.0401 (5)
C1	−0.0024 (5)	0.3008 (3)	0.6239 (3)	0.0501 (7)
C5	0.2460 (5)	0.3056 (2)	0.4889 (3)	0.0444 (6)
H5	0.3186	0.3434	0.4354	0.053*
C4	0.2983 (6)	0.2019 (2)	0.5072 (3)	0.0548 (8)
C2	0.0416 (7)	0.1966 (3)	0.6489 (3)	0.0669 (10)
H2	−0.0292	0.1606	0.7049	0.08*
C3	0.1919 (7)	0.1485 (3)	0.5888 (4)	0.0668 (10)
H3	0.223	0.0784	0.6033	0.08*
C6	0.4637 (7)	0.1515 (3)	0.4385 (5)	0.0821 (13)
H6A	0.5806	0.1125	0.5009	0.123*
H6B	0.5402	0.2036	0.3995	0.123*
H6C	0.3761	0.1061	0.3712	0.123*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu	0.0379 (3)	0.0379 (3)	0.0384 (3)	0.0041 (2)	0.01251 (18)	−0.0019 (2)
Cl2	0.0491 (4)	0.0521 (4)	0.0529 (4)	0.0019 (3)	0.0033 (3)	−0.0072 (3)
Cl1	0.0654 (5)	0.1123 (9)	0.0649 (5)	−0.0084 (5)	0.0371 (5)	−0.0050 (5)
N	0.0399 (11)	0.0398 (12)	0.0403 (11)	−0.0009 (10)	0.0099 (10)	0.0009 (10)
C1	0.0479 (16)	0.0586 (19)	0.0414 (15)	−0.0052 (14)	0.0075 (12)	0.0062 (14)
C5	0.0441 (14)	0.0411 (15)	0.0475 (15)	0.0037 (12)	0.0113 (12)	−0.0024 (12)
C4	0.0504 (17)	0.0439 (16)	0.0604 (19)	0.0062 (14)	−0.0026 (15)	−0.0062 (14)
C2	0.074 (2)	0.064 (2)	0.055 (2)	−0.0200 (19)	0.0025 (18)	0.0207 (17)
C3	0.072 (2)	0.0463 (18)	0.071 (2)	0.0009 (17)	−0.002 (2)	0.0107 (18)
C6	0.066 (2)	0.068 (3)	0.104 (3)	0.023 (2)	0.008 (2)	−0.030 (2)

Geometric parameters (Å, º)

Cu—N	1.986 (2)	C5—H5	0.93
Cu—N^i	1.986 (2)	C4—C3	1.373 (5)
Cu—Cl2^i	2.2536 (11)	C4—C6	1.503 (5)
Cu—Cl2	2.2536 (11)	C2—C3	1.363 (6)
Cl1—C1	1.723 (4)	C2—H2	0.93
N—C1	1.321 (4)	C3—H3	0.93
N—C5	1.345 (4)	C6—H6A	0.96
C1—C2	1.385 (5)	C6—H6B	0.96
C5—C4	1.378 (4)	C6—H6C	0.96
N—Cu—N^i	180.0000	C3—C4—C5	117.2 (3)
N—Cu—Cl2^i	89.84 (7)	C3—C4—C6	122.6 (3)
N^i—Cu—Cl2^i	90.16 (7)	C5—C4—C6	120.2 (3)
N—Cu—Cl2	90.16 (7)	C3—C2—C1	117.9 (3)
N^i—Cu—Cl2	89.84 (7)	C3—C2—H2	121.1
Cl2^i—Cu—Cl2	180	C1—C2—H2	121.1
C1—N—C5	118.1 (3)	C2—C3—C4	121.0 (3)
C1—N—Cu	120.6 (2)	C2—C3—H3	119.5
C5—N—Cu	121.09 (19)	C4—C3—H3	119.5
N—C1—C2	122.8 (3)	C4—C6—H6A	109.5
N—C1—Cl1	115.6 (2)	C4—C6—H6B	109.5
C2—C1—Cl1	121.6 (3)	H6A—C6—H6B	109.5
N—C5—C4	123.0 (3)	C4—C6—H6C	109.5
N—C5—H5	118.5	H6A—C6—H6C	109.5
C4—C5—H5	118.5	H6B—C6—H6C	109.5
Cl2^i—Cu—N—C1	−84.8 (2)	Cu—N—C5—C4	−173.3 (2)
Cl2—Cu—N—C1	95.2 (2)	N—C5—C4—C3	−1.2 (5)
Cl2^i—Cu—N—C5	90.3 (2)	N—C5—C4—C6	178.3 (3)
Cl2—Cu—N—C5	−89.7 (2)	N—C1—C2—C3	0.0 (5)
C5—N—C1—C2	−1.2 (4)	Cl1—C1—C2—C3	178.9 (3)
Cu—N—C1—C2	174.0 (2)	C1—C2—C3—C4	0.7 (5)
C5—N—C1—Cl1	179.9 (2)	C5—C4—C3—C2	−0.1 (5)
Cu—N—C1—Cl1	−4.9 (3)	C6—C4—C3—C2	−179.6 (4)
C1—N—C5—C4	1.9 (4)

Symmetry code: (i) −x, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···Cl2^ii	0.93	2.76	3.685 (4)	171
C5—H5···Cl2^iii	0.93	2.96	3.850 (3)	160
C6—H6C···Cl2^iv	0.96	2.99	3.885 (4)	156

Symmetry codes: (ii) −x, y−1/2, −z+3/2; (iii) −x+1, −y+1, −z+1; (iv) x, −y+1/2, z−1/2.