catena-Poly[[(2,2'-biimidazole-[kappa]²N,N')chloro­copper(II)]-[mu]-chloro]

In the polymeric title compound, [CuCl₂(C₆H₆N₄)]_n, each Cu^II ion is five-coordinated by four basal atoms (two N atoms from a 2,2'-biimidazole mol­ecule and two Cl^- anions) and one axial Cl^- anion, in a distorted square-pyramidal coordination geometry. Cl^- anions bridge the {Cu(C₆H₆N₄)Cl} units into one-dimensional linear chains, which are reinforced by [pi]-[pi] inter­actions. Adjacent linear chains are linked by N-H...Cl hydrogen bonds, resulting in a grid layer. The hydrogen-bonding pattern can be described in graph-set notation as C(9)R(9)R(14). This study extends our knowledge of the multifunctional properties of the 2,2'-biimidazole ligand and of the coordination stereochemistry of copper(II).

S1. Comment

2,2'-Biimidazole is a multifunctional ligand that can be coordinated to a transition metal in non-deprotonated (neutral, H₂biim), mono-deprotonated (monoanion, Hbiim⁻) and di-deprotonated (dianion, biim²⁻) forms (Cancela et al., 2001). The metal-binding ability of its three forms has been well documented since the same 3d metal complexes with either neutral or deprotonated forms were first reported by Holmes et al. (1961). In the neutral state, H₂biim possesses a double property, namely it can bind metals as either a bidentate chelate, seen, for example, with Cu^II (Liu & Su, 1996), V^IV (Sang et al., 2002), and Co^II and Ni^II (Atencio et al., 2004), or as a bridging ligand between two metal centres (Kirchner & Krebs, 1987), and can act as a donor in hydrogen-bonding interactions. Coordinated H₂biim usually forms hydrogen bonds with counteranions and solvent molecules (Ye et al., 1999). On account of the bridging and counteranion properties of the Cl⁻ anion, as well as the flexibility of the coordination stereochemistry of the Cu^II cation, we are interested in copper complexes with the Cl⁻ anion and H₂biim. In addition, copper(II) chloride yields adducts with H₂biim in which the bi-heterocyclic ligand binds in the typical chelating mode, and one such complex, [Cu(H₂biim)₂]Cl₂, has been reported previously (Atencio et al., 2005). The present study uses a similar method for synthesizing a new adduct with copper(II) chloride and H₂biim by adjusting the molar ratio of copper(II) chloride and H₂biim, which yielded the title compound, [Cu(C₆H₆N₄)Cl₂]_n, (I).

In (I), each Cu^II ion is five-coordinated with a distorted square-pyramidal geometry (Fig. 1 and Table 1). The basal plane is formed by atoms N1 and N4 from one H₂biim ligand in chelating mode, along with anions Cl1 and Cl2, with an r.m.s. deviation of 0.018 Å. The apical site is occupied by a Cl⁻ anion, Cl2ⁱ [symmetry code: (i) x − 1, y, z], with a Cu—Cl bond distance of 2.892 (2) Å, which is longer than the average equatorial Cu—Cl bond distance of 2.271 (2) Å. The Cu—N bond distances and the chelating mode of the H₂biim ligand are similar to those observed in [CuCl(C₆H₆N₄)(H₂O)][Cu(C₄H₅NO₄Cl]·H₂O (Gao et al., 2005). Atom Cu1 is located 0.1623 (8) Å out of the basal plane towards Cl2ⁱ.

Five-coordinate copper(II) complexes have geometries ranging from trigonal–bipyramidal to square-pyramidal. Energetically, the limiting trigonal–bipyramidal and square-pyramidal forms are often almost equally favourable, with a low activation barrier to interconversion. In the present instance, the observed geometry is very nearly square-pyramidal, as determined by the observed distortion value τ (van Albada et al., 1999; Addison et al., 1984) of 0.05, which is very near the ideal value of τ = 0. For perfect trigonal–bipyramidal geometry, τ = 1.

The bond distances and angles of the complexed H₂biim ligand are unexceptional and compare well with those of the free ligand (Cromer et al., 1987). Of note is the dihedral angle between the two imidazole rings. The free ligand is found to crystallize in a trans conformation, with a dihedral angle of 4.6°, but in (I), the H₂biim ligand takes on a cis conformation, with a dihedral angle of 2.3 (4)°. This angle is smaller than that observed in [Ag(NO₃)(C₆H₆N₄)]_n [23.2 (2)°; Hester et al., 1997], in which H₂biim functions as a bridging ligand to link the Ag atoms, forming a right-hand helical chain. In (I), the Cl2ⁱ anion connects adjacent [Cu(C₆H₆N₄)Cl₂] units to produce a linear chain along the crystallographic a axis, with a Cu1···Cu1ⁱ separation of 3.8671 (12) Å (Fig 1). In the chain, the heterocyclic rings are either parallel or nearly parallel to each other.

The above cis conformation and parallel disposition of H₂biim preserve favourable π–π interactions between the two sets of parallel symmetry-related rings of the H₂biim moieties [at (x, y, z) and (x − 1, y, z) or (x + 1, y, z)]. Ring centroids are separated by 3.867 (2) Å in both cases, and the perpendicular separations between the two sets of planes are 3.377 (3) and 3.351 (3) Å. These π–π interactions reinforce the linear chains. Adjacent linear chains are linked by N2—H2A···Cl1ⁱⁱ and N3—H3A···Cl2ⁱⁱ [symmetry code: (ii) x − 1, y, z − 1] hydrogen bonds along the [101] direction, resulting in a grid layer parallel to (010) (Fig. 2 and Table 2). The hydrogen-bonding pattern, as shown in Fig. 2, can be described in graph-set notation (Etter, 1990; Grell et al., 2000) as C₂²(9), R₂²(9), R₂²(14).

S2. Experimental

CuCl₂·2H₂O (1 mmol, 0.17 g) and 2,2'-biimidazole (1 mmol, 0.14 g) were suspended in water (30 ml). To the resulting mixture, concentrated aqueous HCl was added until the suspension became clear. The resulting solution was filtered and allowed to evaporate slowly at room temperature. After three weeks, blue crystals of (I) appeared.

S3. Refinement

All H atoms were positioned geometrically and allowed to ride on their parent atoms at distances of C—H = 0.93 Å or N—H = 0.86 Å, and with U_iso(H) = 1.2U_eq(parent atom).

Computing details

Data collection: SMART (Bruker, 2002); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2002); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view along [010], showing the one-dimensional chain of (I) which propagates along the a axis. Atom numbering and displacement ellipsoids at the 50% probability level are also shown; H atoms have been omitted to improve clarity. [Symmetry code: (i) x − 1, y, z.]
	Fig. 2. A view along [010] of the two-dimensional layer in (I), formed by N—H···Cl hydrogen-bond interactions, shown as dashed lines. [Symmetry code: (ii) x − 1, y, z − 1.]

catena-Poly[[(2,2'-biimidazole-κ²N,N')chlorocopper(II)]-µ-chloro]

Crystal data

[CuCl2(C6H6N4)]	F(000) = 266
Mr = 268.59	Dx = 2.032 Mg m^−3
Monoclinic, P21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2yb	Cell parameters from 1408 reflections
a = 3.8671 (4) Å	θ = 2.7–24.2°
b = 14.8937 (17) Å	µ = 3.05 mm^−1
c = 7.6363 (9) Å	T = 298 K
β = 93.742 (2)°	Needle, blue
V = 438.88 (9) Å^3	0.34 × 0.06 × 0.05 mm
Z = 2

Data collection

Bruker APEX area-detector diffractometer	1590 independent reflections
Radiation source: fine-focus sealed tube	1522 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.038
ϕ and ω scans	θmax = 25.2°, θmin = 2.7°
Absorption correction: multi-scan (SADABS; Bruker, 2002)	h = −4→4
Tmin = 0.424, Tmax = 0.863	k = −17→17
4178 measured reflections	l = −9→9

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)] = 0.053	H-atom parameters constrained
wR(F^2) = 0.103	w = 1/[σ^2(Fo^2) + (0.0324P)^2 + 0.9884P] where P = (Fo^2 + 2Fc^2)/3
S = 1.19	(Δ/σ)max < 0.001
1590 reflections	Δρmax = 0.70 e Å^−3
118 parameters	Δρmin = −0.76 e Å^−3
1 restraint	Absolute structure: Flack (1983), with 761 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.07 (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
Cu1	0.3637 (2)	0.86139 (5)	0.63901 (10)	0.0233 (2)
Cl1	0.3741 (5)	0.75984 (13)	0.8595 (2)	0.0297 (5)
Cl2	0.7743 (5)	0.95448 (14)	0.7671 (2)	0.0300 (5)
N1	0.0701 (15)	0.7804 (4)	0.4796 (8)	0.0204 (13)
N2	−0.1181 (16)	0.7476 (4)	0.2112 (9)	0.0285 (15)
H2A	−0.1604	0.7514	0.0995	0.034*
N3	0.2332 (18)	0.9406 (4)	0.1335 (8)	0.0317 (16)
H3A	0.1502	0.9267	0.0299	0.038*
N4	0.3698 (16)	0.9325 (4)	0.4157 (8)	0.0246 (14)
C1	−0.0925 (18)	0.6984 (5)	0.4822 (10)	0.0256 (18)
H1	−0.1154	0.6628	0.5808	0.031*
C2	−0.214 (2)	0.6785 (5)	0.3162 (11)	0.033 (2)
H2	−0.3380	0.6275	0.2804	0.040*
C3	0.0498 (18)	0.8069 (5)	0.3117 (9)	0.0188 (17)
C4	0.2134 (18)	0.8932 (5)	0.2766 (9)	0.0218 (17)
C5	0.499 (2)	1.0099 (5)	0.3543 (10)	0.0242 (17)
H5	0.6276	1.0520	0.4205	0.029*
C6	0.410 (2)	1.0157 (6)	0.1810 (11)	0.034 (2)
H6	0.4606	1.0630	0.1076	0.041*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cu1	0.0243 (4)	0.0286 (5)	0.0165 (4)	−0.0008 (5)	−0.0027 (3)	−0.0001 (5)
Cl1	0.0319 (10)	0.0354 (11)	0.0210 (10)	0.0008 (9)	−0.0032 (8)	0.0051 (8)
Cl2	0.0275 (10)	0.0375 (12)	0.0241 (10)	−0.0049 (9)	−0.0047 (8)	−0.0030 (9)
N1	0.020 (3)	0.026 (4)	0.014 (3)	0.000 (3)	−0.004 (2)	−0.002 (3)
N2	0.030 (4)	0.036 (4)	0.019 (3)	0.000 (3)	−0.005 (3)	−0.002 (3)
N3	0.049 (4)	0.029 (4)	0.016 (3)	0.005 (3)	−0.001 (3)	0.004 (3)
N4	0.030 (4)	0.024 (3)	0.019 (3)	0.002 (3)	−0.004 (3)	−0.002 (3)
C1	0.020 (4)	0.035 (5)	0.023 (4)	0.004 (3)	0.007 (3)	−0.005 (3)
C2	0.033 (5)	0.023 (4)	0.045 (5)	−0.008 (4)	0.011 (4)	−0.014 (4)
C3	0.020 (4)	0.030 (4)	0.006 (3)	0.008 (3)	−0.001 (3)	−0.008 (3)
C4	0.014 (4)	0.031 (4)	0.020 (4)	0.009 (3)	0.005 (3)	0.007 (3)
C5	0.025 (4)	0.020 (4)	0.029 (4)	0.000 (3)	0.005 (3)	−0.009 (3)
C6	0.047 (5)	0.030 (5)	0.027 (5)	0.005 (4)	0.008 (4)	0.001 (4)

Geometric parameters (Å, º)

Cu1—N4	2.009 (6)	N3—C6	1.347 (10)
Cu1—N1	2.011 (6)	N3—H3A	0.8600
Cu1—Cl1	2.262 (2)	N4—C4	1.324 (9)
Cu1—Cl2	2.280 (2)	N4—C5	1.352 (10)
Cu1—Cl2^i	2.892 (2)	C1—C2	1.355 (11)
Cu1—Cu1^i	3.8671 (12)	C1—H1	0.9300
N1—C3	1.339 (9)	C2—H2	0.9300
N1—C1	1.374 (10)	C3—C4	1.464 (10)
N2—C3	1.314 (10)	C5—C6	1.348 (12)
N2—C2	1.370 (10)	C5—H5	0.9300
N2—H2A	0.8600	C6—H6	0.9300
N3—C4	1.308 (9)
N4—Cu1—N1	80.8 (2)	C4—N4—Cu1	114.7 (5)
N4—Cu1—Cl1	169.71 (19)	C5—N4—Cu1	140.3 (5)
N1—Cu1—Cl1	91.79 (18)	C2—C1—N1	108.2 (7)
N4—Cu1—Cl2	89.83 (19)	C2—C1—H1	125.9
N1—Cu1—Cl2	166.96 (18)	N1—C1—H1	125.9
Cl1—Cu1—Cl2	96.27 (7)	C1—C2—N2	107.0 (7)
N4—Cu1—Cl2^i	95.03 (19)	C1—C2—H2	126.5
N1—Cu1—Cl2^i	93.80 (17)	N2—C2—H2	126.5
Cl1—Cu1—Cl2^i	92.58 (7)	N2—C3—N1	110.9 (7)
Cl2—Cu1—Cl2^i	96.07 (7)	N2—C3—C4	133.3 (7)
C3—N1—C1	106.1 (6)	N1—C3—C4	115.8 (7)
C3—N1—Cu1	113.4 (5)	N3—C4—N4	112.5 (7)
C1—N1—Cu1	139.9 (5)	N3—C4—C3	132.6 (7)
C3—N2—C2	107.8 (7)	N4—C4—C3	114.9 (6)
C3—N2—H2A	126.1	C6—C5—N4	108.6 (7)
C2—N2—H2A	126.1	C6—C5—H5	125.7
C4—N3—C6	106.2 (7)	N4—C5—H5	125.7
C4—N3—H3A	126.9	N3—C6—C5	107.7 (7)
C6—N3—H3A	126.9	N3—C6—H6	126.2
C4—N4—C5	105.0 (6)	C5—C6—H6	126.2

Symmetry code: (i) x−1, y, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N3—H3A···Cl2^ii	0.86	2.44	3.221 (7)	152
N2—H2A···Cl1^ii	0.86	2.49	3.227 (7)	145

Symmetry code: (ii) x−1, y, z−1.