Bis(μ-nitrito-κ2 O:O)bis[bis(1-methyl-1H-imidazole-κN 3)(nitrito-κO)copper(II)]

In the binuclear title compound, [Cu2(NO2)4(C4H6N2)4], centrosymmetrically related complex molecules are linked via weak Cu—O interactions, forming dimeric units. The CuII atom displays an elongated square-pyramidal CuN2O3 coordination geometry with a slight tetrahedral distortion of the basal plane [maximum deviation = 0.249 (2) Å]. The dihedral angle formed by the imidazole rings is 26.20 (10)°.

In the binuclear title compound, [Cu 2 (NO 2 ) 4 (C 4 H 6 N 2 ) 4 ], centrosymmetrically related complex molecules are linked via weak Cu-O interactions, forming dimeric units. The Cu II atom displays an elongated square-pyramidal CuN 2 O 3 coordination geometry with a slight tetrahedral distortion of the basal plane [maximum deviation = 0.249 (2) Å ]. The dihedral angle formed by the imidazole rings is 26.20 (10) .

Related literature
The structure of the title compound was determined as part of our ongoing study of potential ferroelectric phase change materials. For general background to ferroelectric compounds with metal-organic framework structures, see: Fu et al. (2009);Ye et al. (2006); Zhang et al. (2008Zhang et al. ( , 2010. For a related structure, see: Costes et al. (1995).

Run-Qiang Zhu 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, which is the usual method for detecting such behaviour (Fu et al., 2009;Ye et al., 2006;Zhang et al., 2008;Zhang et al., 2010).
Unfortunately, the dielectric constant of the title compound indicates the onset of a ferroelectric phase change over the range 80-298 K.
As show in Fig. 1, the copper ion adopts an elongated square pyramidal geometry with a slight tetrahedral distortion in the basal plane (maximum deviation 0.249 (2) Å for atom N4) which is primarily associated with the coordination of the nitrite ions (O2-Cu1-O3 = 167.15 (8)°). As observed in a related compound (Costes et al., 1995), this geometry displaces atom O2 from the ideal coordination plane towards the centrosymmetrically related Cu1 i copper atom [symmetry code: (i) 1-x, 1-y, -z] resulting in an O2-Cu1 distance of 2.578 (5) Å. While this distance is considerably longer than those in basal plane (Cu1-O2 and Cu1-O3 are 2.0221 (19) and 2.0085 (19) Å, respectively), the direction of the displacement of atom O2 and the orientations of the two nitrite ligands which place both atoms O1 and O3 on the opposite side of the coordination plane, suggest that there is a weak association of one complex molecule with its centrosymmetrically related. The dihedral angle formed by the trans-arranged imidazole rings is 26.20 (10)°. The crystal packing ( Fig. 2) is governed only by van der Waals interactions.
Slow evaporation of the solution following removal of the precipitated BaSO 4 yielded blue crystals after a few days. M.

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 methy H atoms.

Bis(µ-nitrito-κ 2 O:O)bis[bis(1-methyl-1H-imidazole-κN 3 )(nitrito-κO)copper(II)]
Crystal data [Cu 2 (NO 2 ) 4 (C 4 H 6 N 2 ) 4 ] M r = 639.55 Triclinic, P1 Hall symbol: -P 1 a = 7.8281 (16)  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.