In search of conformationally chiral square-planar complexes: dichlorido­bis(2-methoxy­pyridine-κN)copper(II)

The title compound, [CuCl₂(C₆H₇NO)₂], was synthesized during a study of conformationally chiral square-planar coordination compounds. The coordination geometry deviates from the square-planar geometry generally adopted by copper(II) chloride complexes with pyridine ligands towards a tetra­hedral arrangement of ligands. The complex is conformationally chiral but crystallizes in a centrosymmetric space group with both enantiomers present in the unit cell.

S1. Comment

Mononuclear square-planar complexes may be chiral under certain circumstances, but this phenomenon has gained little attention. It was first pointed out by Werner (1893) that four-coordinate complexes of platinum were most likely to be square-planar rather than tetrahedral. An elegant evidence for this hypothesis appeared in 1935 (Mills & Quibell, 1935), the first optical resolution of a square-planar complex (with no chiral ligands). The complex was designed to display a mirror plane in the case of tetrahedral coordination geometry, and to be chiral in the case of square-planar geometry (Fig. 1). In 1999, the chirality of square-planar complexes was discussed and exemplified by four complexes of PdCl₂ and PtCl₂ with substituted pyridine ligands (Fig. 2), although none of the complexes were resolved (Biagini et al., 1999). Inspired by this work, we prepared a series of CuCl₂ complexes with different pyridine ligands in order to find a square-planar chiral compound that crystallizes in a Sohncke space group (Flack, 2003), which would make crystallization-induced asymmetric transformation (Jacques et al., 1984) possible.

Using 2-methoxypyridine as ligand, we obtained blue crystals of dichlorobis(2-methoxypyridine-N)copper(II), (I). Compared with the structures of CuCl₂–pyridine complexes found in the Cambridge Structural Database (CSD; Version 5.28 of November 2006; Allen 2002), the coordination geometry in (I) deviates considerably from ideal square-planar geometry (Fig. 3). The N—Cu—Cl angles are all between 91.09 (4) and 94.08 (4)°, but the N1—Cu1—N2 and Cl1—Cu1—Cl2 angles are 164.05 (5) and 142.640 (19)°, respectively (Table 1). The coordination geometry around atom Cu1 may be explained by the donor atom being in the ortho position. Weak Cu1···O1 and Cu1···O2 interactions might decrease the N1—Cu1—N2 angle and the two Cl atoms are forced out of the plane (Fig. 4). There are no other copper complexes of 2-methoxypyridine in the CSD for comparison, but there is one comparable example of a copper(II)–chlorido–pyridine complex with an O-donor atom in the ortho position, viz. catena-poly[[[dichlorocopper(II)]- µ-1,3,5-tris(2-pyridyloxymethyl)benzene] methanol solvate] (Bray et al., 2004). In this case, a similar effect was observed; the compound has a Cl—Cu—Cl angle of 131.983 (17)° and an N—Cu—N angle of 147.39 (5)°. The distances from the central Cu atom to the ortho O atoms are 2.6445 (11) and 2.8146 (11) Å. In trans-(dichloro)bis[4-(N-t-butyl-N-oxyamino)-2-(methoxymethyl)pyridine-N,O]copper(II), there is a one-C-atom bridge between the O atom and the pyridine ring, and 2-(methoxymethyl)pyridine acts as a bidentate ligand with Cu—O distances of 2.477 (3) and 2.549 (3) Å for the two independent molecules in the asymmetric unit (Zhu et al., 2005). Two metal complexes of 2-methoxypyridine are found in the CSD, viz. (dichloro)bis(2-methoxypyridine-N)cobalt(II) (Allan et al., 1981) and cis-(dichloro)(dimethylsulphoxide-S)(2-methoxypyridine-N)platinum(II) (Arvanitis et al., 2000). In both cases, 2-methoxypyridine is best described as a monodentate ligand with only weak metal–oxygen interactions. Since the two aromatic rings in (I) are not in the same plane, molecules of (I) are conformationally chiral, the C1—N1—N2—C7 torsion angle being -33.4 (2)° (cf. Fig. 2). Only one of the complexes of CuCl₂ with pyridine ligands found in the CSD is chiral in the sense shown in Fig. 2, namely cis-bis(1,8-naphthyridine)(dichloro)copper(II) (Enwall & Emerson, 1979). However, since (I) crystallizes in a centrosymmetric space group, both enantiomers are present in the unit cell and no spontaneous resolution occurs in (I).

Atoms Cl1 and Cl2 both form two short contacts within van der Waals radii (geometric parameters and symmetry codes are given in Table 2). The H atoms are attached directly to the aromatic rings in all four cases. The short contacts involving atom Cl2 give rise to infinite racemic chains extended along the b axis, since atom H8 forms a short contact with atom Cl2ⁱⁱⁱ, and atom H10 forms a short contact with atom Cl2^iv. The Cl1···H3 contacts give rise to infinite chains extending along the c axis, and the Cl1···H4 contacts give rise to chains extending along the b axis. The C—H···Cl angles deviate considerably from 180°. Altogether, this results in a network structure (Fig. 5).

Related literature

For related literature, see: Allan et al. (1981); Allen (2002); Arvanitis et al. (2000); Biagini et al. (1999); Bray et al. (2004); Enwall & Emerson (1979); Flack (2003); Jacques et al. (1984); Mills & Quibell (1935); Werner (1893); Zhu et al. (2005).

S2. Experimental

Copper(II) chloride dihydrate (ca 10 mg) was dissolved in hot 2-methoxypyridine (ca 1 ml) and a clear dark-green solution was obtained. Small blue crystals were formed on slow cooling to ambient temperature.

S3. Refinement

All H atoms were included in calculated positions (C—H = 0.93 or 0.96 Å) and refined using a riding model with U_iso(H) values of 1.2 or 1.5 times U_eq(C).

Computing details

Data collection: CrystalClear(Rigaku, 2000); cell refinement: CrystalClear(Rigaku, 2000); data reduction: CrystalClear(Rigaku, 2000); program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. Mills & Quibell (1935)'s chiral square-planar complex (left). Tetrahedral coordination geometry around the central atom would give an achiral complex (right).
	Fig. 2. Two different possibilities of obtaining chiral square-planar complexes using two substituted pyridine ligands coordinated by a divalent central atom.
	Fig. 3. The molecular structure of (I), showing the crystallographic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. All H atoms have been omitted.
	Fig. 4. A spacefilling plot of (I), showing the Cu···O interactions. The interactions decrease the N1—Cu1—N2 angle and the two Cl atoms are forced out of the plane.
	Fig. 5. The packing of (I), viewed along the b axis. Four different Cl···H contacts result in a three-dimensional network structure. Molecules marked with an asterisk (*), number sign (#), ampersand (&), dollar sign ($) or section sign (?) are at the symmetry positions (x, -y + 1/2, z - 1/2), (x, -y + 1/2, z + 1/2), (-x + 1, y + 1/2, -z + 1/2), (-x + 1, -y + 1, -z + 1) and (-x + 1, y + 1/2, -z + 3/2), respectively.

dichloridobis(2-methoxypyridine-κN)copper(II)

Crystal data

[CuCl2(C6H7NO)2]	F(000) = 716
Mr = 352.69	Dx = 1.635 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P2ybc	Cell parameters from 2762 reflections
a = 10.5303 (19) Å	θ = 1.9–26.0°
b = 8.0531 (13) Å	µ = 1.89 mm^−1
c = 16.977 (3) Å	T = 100 K
β = 95.435 (7)°	Prism, blue
V = 1433.2 (4) Å^3	0.35 × 0.2 × 0.1 mm
Z = 4

Data collection

Rigaku R-AXIS IIC image-plate system diffractometer	2762 independent reflections
Radiation source: rotating-anode X-ray tube, Rigaku RU-H3R	2534 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.036
Detector resolution: 105 pixels mm^-1	θmax = 26.0°, θmin = 1.9°
ϕ scans	h = −12→12
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000)	k = −9→8
Tmin = 0.511, Tmax = 0.827	l = −20→20
9641 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)] = 0.023	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.060	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0333P)^2 + 0.3122P] where P = (Fo^2 + 2Fc^2)/3
2762 reflections	(Δ/σ)max = 0.002
172 parameters	Δρmax = 0.30 e Å^−3
0 restraints	Δρmin = −0.31 e Å^−3

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
C1	0.32200 (15)	0.1880 (2)	0.77564 (10)	0.0121 (3)
C2	0.36889 (16)	0.2044 (2)	0.85480 (10)	0.0157 (3)
H2	0.3650	0.1169	0.8902	0.019*
C3	0.42149 (15)	0.3560 (2)	0.87870 (10)	0.0174 (4)
H3	0.4523	0.3719	0.9313	0.021*
C4	0.42856 (15)	0.4848 (2)	0.82464 (10)	0.0171 (3)
H4	0.4642	0.5868	0.8401	0.021*
C5	0.38097 (15)	0.4559 (2)	0.74759 (10)	0.0156 (3)
H5	0.3859	0.5405	0.7107	0.019*
C6	0.26005 (17)	−0.0946 (2)	0.79050 (11)	0.0227 (4)
H6A	0.2185	−0.1827	0.7598	0.034*
H6B	0.2127	−0.0697	0.8346	0.034*
H6C	0.3449	−0.1286	0.8095	0.034*
C7	0.18376 (14)	0.0160 (2)	0.49928 (9)	0.0129 (3)
C8	0.13283 (16)	−0.0604 (2)	0.42955 (10)	0.0173 (3)
H8	0.1425	−0.1739	0.4220	0.021*
C9	0.06774 (16)	0.0365 (2)	0.37196 (10)	0.0193 (4)
H9	0.0333	−0.0113	0.3248	0.023*
C10	0.05399 (16)	0.2059 (2)	0.38470 (10)	0.0182 (4)
H10	0.0119	0.2733	0.3461	0.022*
C11	0.10439 (16)	0.2713 (2)	0.45610 (10)	0.0148 (3)
H11	0.0935	0.3839	0.4655	0.018*
C12	0.27145 (18)	−0.2374 (2)	0.55302 (12)	0.0217 (4)
H12A	0.3197	−0.2777	0.5999	0.033*
H12B	0.3176	−0.2584	0.5079	0.033*
H12C	0.1906	−0.2933	0.5463	0.033*
N1	0.32771 (13)	0.31104 (17)	0.72339 (8)	0.0127 (3)
N2	0.16870 (12)	0.17876 (17)	0.51277 (8)	0.0116 (3)
O1	0.26626 (11)	0.05092 (14)	0.74182 (7)	0.0179 (3)
O2	0.25091 (11)	−0.06107 (14)	0.56033 (7)	0.0170 (3)
Cl1	0.43000 (4)	0.34400 (5)	0.56227 (2)	0.01579 (10)
Cl2	0.06144 (4)	0.38706 (5)	0.64643 (2)	0.01815 (11)
Cu1	0.246952 (17)	0.27789 (2)	0.613548 (11)	0.00970 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0096 (7)	0.0141 (8)	0.0128 (8)	0.0021 (6)	0.0017 (6)	−0.0003 (6)
C2	0.0140 (8)	0.0217 (9)	0.0116 (8)	0.0040 (7)	0.0020 (6)	0.0053 (6)
C3	0.0144 (8)	0.0261 (9)	0.0113 (8)	0.0028 (7)	−0.0012 (6)	−0.0022 (7)
C4	0.0154 (8)	0.0183 (8)	0.0174 (8)	−0.0018 (7)	0.0002 (6)	−0.0037 (7)
C5	0.0171 (8)	0.0157 (8)	0.0138 (8)	0.0003 (7)	0.0006 (6)	0.0023 (6)
C6	0.0224 (9)	0.0164 (9)	0.0289 (10)	−0.0022 (7)	0.0003 (7)	0.0093 (7)
C7	0.0096 (7)	0.0149 (8)	0.0144 (8)	−0.0011 (6)	0.0014 (6)	0.0003 (6)
C8	0.0182 (8)	0.0146 (8)	0.0192 (9)	−0.0026 (7)	0.0033 (7)	−0.0049 (7)
C9	0.0181 (8)	0.0258 (9)	0.0136 (8)	−0.0042 (7)	−0.0012 (6)	−0.0061 (7)
C10	0.0168 (8)	0.0237 (9)	0.0132 (8)	−0.0009 (7)	−0.0026 (7)	0.0027 (7)
C11	0.0146 (8)	0.0157 (8)	0.0140 (8)	−0.0002 (6)	0.0005 (6)	−0.0001 (6)
C12	0.0210 (9)	0.0129 (8)	0.0307 (10)	0.0018 (7)	0.0001 (8)	0.0024 (7)
N1	0.0137 (7)	0.0152 (7)	0.0091 (6)	0.0006 (5)	0.0008 (5)	0.0009 (5)
N2	0.0111 (6)	0.0139 (6)	0.0096 (6)	−0.0014 (5)	0.0003 (5)	−0.0006 (5)
O1	0.0218 (6)	0.0143 (6)	0.0173 (6)	−0.0029 (5)	−0.0004 (5)	0.0026 (5)
O2	0.0197 (6)	0.0125 (6)	0.0178 (6)	0.0014 (5)	−0.0029 (5)	0.0013 (5)
Cl1	0.01553 (19)	0.0222 (2)	0.00976 (18)	−0.00539 (16)	0.00169 (14)	−0.00016 (15)
Cl2	0.01474 (19)	0.0207 (2)	0.0187 (2)	0.00378 (16)	0.00013 (15)	−0.00649 (16)
Cu1	0.01151 (12)	0.01024 (12)	0.00706 (12)	−0.00023 (7)	−0.00065 (7)	0.00018 (7)

Geometric parameters (Å, º)

C1—N1	1.335 (2)	C8—C9	1.381 (2)
C1—O1	1.353 (2)	C8—H8	0.9300
C1—C2	1.393 (2)	C9—C10	1.391 (3)
C2—C3	1.385 (2)	C9—H9	0.9300
C2—H2	0.9300	C10—C11	1.381 (2)
C3—C4	1.392 (2)	C10—H10	0.9300
C3—H3	0.9300	C11—N2	1.348 (2)
C4—C5	1.376 (2)	C11—H11	0.9300
C4—H4	0.9300	C12—O2	1.444 (2)
C5—N1	1.342 (2)	C12—H12A	0.9600
C5—H5	0.9300	C12—H12B	0.9600
C6—O1	1.439 (2)	C12—H12C	0.9600
C6—H6A	0.9600	N1—Cu1	1.9929 (14)
C6—H6B	0.9600	N2—Cu1	1.9941 (13)
C6—H6C	0.9600	Cl1—Cu1	2.2527 (5)
C7—N2	1.343 (2)	Cl2—Cu1	2.2599 (5)
C7—O2	1.349 (2)	Cu1—O1	2.835 (1)
C7—C8	1.395 (2)	Cu1—O2	2.877 (1)
N1—C1—O1	111.64 (14)	N2—C11—C10	122.77 (16)
N1—C1—C2	122.45 (15)	N2—C11—H11	118.6
O1—C1—C2	125.91 (15)	C10—C11—H11	118.6
C3—C2—C1	117.46 (16)	O2—C12—H12A	109.5
C3—C2—H2	121.3	O2—C12—H12B	109.5
C1—C2—H2	121.3	H12A—C12—H12B	109.5
C2—C3—C4	120.57 (15)	O2—C12—H12C	109.5
C2—C3—H3	119.7	H12A—C12—H12C	109.5
C4—C3—H3	119.7	H12B—C12—H12C	109.5
C5—C4—C3	117.63 (15)	C1—N1—C5	119.06 (14)
C5—C4—H4	121.2	C1—N1—Cu1	118.70 (11)
C3—C4—H4	121.2	C5—N1—Cu1	122.12 (11)
N1—C5—C4	122.82 (16)	C7—N2—C11	118.52 (14)
N1—C5—H5	118.6	C7—N2—Cu1	119.43 (11)
C4—C5—H5	118.6	C11—N2—Cu1	122.01 (11)
O1—C6—H6A	109.5	C1—O1—C6	117.56 (13)
O1—C6—H6B	109.5	C7—O2—C12	117.28 (14)
H6A—C6—H6B	109.5	N1—Cu1—N2	164.05 (5)
O1—C6—H6C	109.5	N1—Cu1—Cl1	91.43 (4)
H6A—C6—H6C	109.5	N2—Cu1—Cl1	93.57 (4)
H6B—C6—H6C	109.5	N1—Cu1—Cl2	91.09 (4)
N2—C7—O2	112.32 (14)	N2—Cu1—Cl2	94.08 (4)
N2—C7—C8	122.21 (15)	Cl1—Cu1—Cl2	142.640 (19)
O2—C7—C8	125.46 (15)	O2—Cu1—N2	50.79 (5)
C9—C8—C7	118.46 (16)	O2—Cu1—Cl1	93.70 (3)
C9—C8—H8	120.8	O2—Cu1—Cl2	119.03 (3)
C7—C8—H8	120.8	O2—Cu1—N1	113.79 (3)
C8—C9—C10	119.73 (16)	O2—Cu1—O1	68.13 (3)
C8—C9—H9	120.1	O1—Cu1—N2	113.24 (5)
C10—C9—H9	120.1	O1—Cu1—N1	51.28 (5)
C11—C10—C9	118.28 (16)	O1—Cu1—Cl2	93.55 (3)
C11—C10—H10	120.9	O1—Cu1—Cl1	116.58 (3)
C9—C10—H10	120.9

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C3^i—H3^i···Cl1	0.93	2.85	3.501 (2)	128
C4^ii—H4^ii···Cl1	0.93	2.82	3.705 (2)	160
C8^iii—H8^iii···Cl2	0.93	2.90	3.503 (2)	123
C10^iv—H10^iv···Cl2	0.93	2.85	3.519 (2)	130

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