trans-Dibromidobis(3-methylpyridine-κN)copper(II)

The asymmetric unit of the title compound, [CuBr2(C6H7N)2], contains one half-molecule, the whole molecule being generated by inversion through a center located at the CuII atom. The geometry around the CuII atom is square planar. Semicoordinate Cu⋯Br bonds [3.269 (1) Å] and nonclassical C—H⋯Br hydrogen bonds connect the molecules, forming chains running parallel to the a axis. These chains are further linked via additional C—H⋯Br hydrogen bonds into a three-dimensional network.

The asymmetric unit of the title compound, [CuBr 2 (C 6 H 7 N) 2 ], contains one half-molecule, the whole molecule being generated by inversion through a center located at the Cu II atom. The geometry around the Cu II atom is square planar. Semicoordinate CuÁ Á ÁBr bonds [3.269 (1) Å ] and nonclassical C-HÁ Á ÁBr hydrogen bonds connect the molecules, forming chains running parallel to the a axis. These chains are further linked via additional C-HÁ Á ÁBr hydrogen bonds into a threedimensional network.

Comment
The molecular units ( Fig. 1) of the title compound are linked via Cu···Br semi-coordinate bonds to form a chain structure that runs parallel to the a-axis (Fig. 2). These chains are reinforced by C6-H6···Br1 and C2-H2···Br1 hydrogen bonding interactions. The data summarizing these interactions are shown in Table 1. These chains are interlinked using non-classical C5-H5···Br1 hydrogen bonding interactions to form the final three dimensional structure (Fig. 3).
The title compound was prepared to investigate chloro-methyl and bromo-methyl exchange rules in the crystal structures of Cu(3YP) 2 Br 2 complexes, where 3YP = 3-substituted pyridine and Y = Cl, Br and methyl (Awwadi et al., 2006 andAwwadi et al., 2011). These three compounds are isostructural in the solid state, hence, the halo-methyl exchange rule is not violated. Desiraju showed that the chloro-methyl exchange rule is obeyed if the final structure is stabilized by dispersive forces (Desiraju & Sarma, 1986). This indicates that the Cu···Br semi-coordinate bonds play the crucial role in determining the final structure of these compounds. The volume of the methyl group is ca 24 Å 3 which is in between the volume of chlorine (ca 19 Å 3 ) and bromine (ca 27 Å 3 ). In contrast, if directional forces are involved, the chloro-methyl exchange rule is violated.
Experimental 2 mmol of 3-methylpyridine were dissolved in 20 mL of acetonitrile. One mmol of CuBr 2 was dissolved in 20 mL of acetonitrile. The two solutions were mixed. The resulting solution was gently heated with stirring for 15 minutes. The solution was filtered and left to slowly evaporate at the room temperature. Green crystals with a needle habit were formed. One of these crystals was used for single-crystal X-ray data collection.

Refinement
The structure was solved by direct methods and refined by least squares method on F 2 using the SHELXTL program package. The structure was solved in the space group P2(1)/c (# 14) by analysis of systematic absences. All atoms were refined anisotropically. Hydrogen atoms were placed at the calculated positions using a riding model with C(aromatic)-H = 0.95 Å and Uiso(H) = 1.2Ueq(C), and with C(aliphatic)-H = 0.98 Å and Uiso(H) = 1.5Ueq(C).

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq