Di-μ-nitrito-κ4 O:O-bis[bis(1-ethyl-1H-imidazole-κN 3)(nitrito-κO)copper(II)]

In the structure of the title compound, [Cu2(NO2)4(C5H8N2)4], the asymmetric unit consists of two moieties containing one Cu ion, two nitrite ions and two 1-ethyl-1H-imidazole molecules associated via weak Cu—O interactions. Each CuII atom displays an elongted square-pyramidal CuN2O3 coordination geometry with a slight tetrahedral distortion in the basal plane. The dimeric units are linked into a three-dimensional network by C—H⋯O hydrogen bonds.


Comment
As part of our ongoing study of potential ferroelectric phase change materials we have determined the structures of several copper complexes and examined the changes in their dielectric constants with temperature. This is the usual method for detecting such behavior. (Fu et al., 2009;Ye et al., 2006;Zhang et al., 2008;Zhang et al., 2010). Unfortunately, the dielectric constant for (I) does not show any behavior indicating the onset of a ferroelectric phase change over the range 80 K to 298 K (m.p.219-229).
As shown in Fig. 1, the Cu ion adopts an elongated square pyramidal geometry with a slight tetrahedral distortion in the basal plane which is primarily associated with the coordination of the nitrite ions (O1-Cu1-O3 = 164.12 (11)°). This displaces O3 from the ideal coordination plane towards the centrosymmetrically-related copper atom (Cu1') resulting in an O3-Cu1' distance of 2.637 (2) Å. While this distance is considerably longer than the in-plane Cu1-O1 and Cu-O3 bond lengths of 2.025 (3) Å and 2.058 (5) Å, respectively, the direction of displacement of O3 and the orientations of the two nitrite ligands which place both O1 and O4 on the opposite side of the coordination plane from Cu1', suggests that there is a weak association of one Cu(NO 2 ) 2 (C 5 H 8 N 2 ) 2 unit with its centrosymmetrically-related counterpart. A similar weak association has been postulated to occur between two similar centrosymmetrically related Cu(NO 2 )(OC(CH 3 )CHC(CH 3 )N(CH 2 ) 2 NH 2 ) units (Cu-O = 2.014 (4) Å, Cu'-O = 2.634 (3) Å) (Costes, et al. 1995).
Slow evaporation of the solution following removal of the precipitated BaSO 4 yielded blue crystals after a few days.

Refinement
All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H = 0.93-0.96 Å, and with U iso (H) = 1.2 U iso (C) or 1.5 U iso (C) for ethy H atoms.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 Rfactors(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.