Crystal structure of di-μ-chlorido-bis[chloridobis(1,2-dimethyl-5-nitro-1H-imidazole-κN 3)copper(II)] acetonitrile disolvate

1,2-Dimethyl-5-nitroimidazole (dimetridazole, dimet) reacts with copper(II) chloride to give dinuclear [Cu(dimet)2(μ-Cl)Cl]2, in which each copper moiety is coordinated to two dimet ligands in a trans arrangement.


Chemical context
1,2-Dimethyl-5-nitroimidazole, also known as dimetridazole (dimet), is structurally related to metronidazole [2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol, MET]. Thus, both compounds contain a 2-methyl-5-nitroimidazole core and are only differentiated according to whether one of the nitrogen atoms possesses a methyl substituent (as in dimet) or a hydroxyethyl substituent (as in MET), as illustrated in Fig. 1. Both MET and dimet are used to treat microbial infections, ISSN 2056-9890 Figure 1 A comparison of the structures of (a) metronidazole (MET) and (b) dimetridazole (dimet). but dimet has specifically been used in animals for the treatment of, for instance, bovine trichomoniasis (McLoughlin, 1968), giardiasis in birds (Panigrahy et al., 1978) and swine dysentery (Messier et al., 1990). In order to control outbreaks of infection, a previous common practice was to incorporate dimet as a feed additive given, for example, to poultry and pigs (Buizer & Severijnen, 1975). However, concerns about the mutagenic properties displayed by this class of drug (Voogd et al., 1974), and the fact that trace amounts can be detected in certain animal products intended for human consumption (Arias et al., 2016), have led to a discontinuation of this practice (EC bans use of dimetridazole in food animals, 1995). Reports of structures of metal compounds involving the coordination by dimetridazole are scarce. Herein, we describe the structure of the copper compound [Cu(-Cl)Cl(dimet) 2 ] 2 , which is obtained by the reaction of dimet with CuCl 2 ÁH 2 O (see Scheme).

Structural commentary
Crystals of composition [Cu(-Cl)Cl(dimet) 2 ] 2 were obtained by addition of dimet to CuCl 2 Á2H 2 O in chloroform, followed by recrystallization of the blue precipitate from acetonitrile. The molecular structure, as illustrated in Fig. 2, shows a centrosymmetric chlorido-bridged dimer. The coordination geometry around each copper atom is a slightly distorted trigonal-bipyramidal with two axial dimet ligands, and three chlorine ligands in the equatorial plane, two of which bridge to the adjacent copper. This structure is analogous to a previously reported copper(II) dimer containing MET, instead of dimet, [Cu(MET) 2 (-Cl)Cl] 2 ( Barba-Behrens et al., 1991), and a comparison of the two structures is shown in Fig. 3 Quinlivan et al., 2015).
Examination of the structure of the [Cu(-Cl)Cl(dimet) 2 ] 2 complex demonstrates that, interestingly, the chlorine atoms bridge in an asymmetric manner, with Cu-Cl bridge bond lengths of 2.3811 (3) and 2.6024 (3) Å , both of which are longer than the terminal Cu-Cl bond length of 2.2822 (3) Å . Of note, these features are also observed for the MET analog, The molecular structure of [Cu(-Cl)Cl(dimet) 2 ] 2 , with displacement ellipsoids depicted at the 30% probability level. H atoms associated with methyl groups are not shown [symmetry code ('): Àx, Ày + 1, Àz + 1].

Figure 3
A comparison of the structures of the dinuclear Cu complexes which are derived from (a) metronidazole (MET) and (b) dimetridazole (dimet).

Supramolecular features
The crystal structure displays a number of weak intermolecular interactions between hydrogen atoms of CH groups and the more electronegative atoms on adjacent molecules, such as the oxygen atoms in the nitro groups of the dimet ligand and also the terminal and bridging chlorine atoms (see Table 1 and Fig. 4). In this regard, one of the oxygen atoms of the nitro group participates in intermolecular hydrogenbonding interactions with CH 3 and CH groups of an adjacent molecule. For reference, intermolecular and intramolecular C-HÁ Á ÁO hydrogen bonds involving an O atom from a nitro group (or other O-containing groups) have been reported (Desiraju, 1991;Sharma & Desiraju, 1994;Forlani, 2009). As an illustration, intermolecular C-HÁ Á ÁO interactions (involving C-H motifs from an NMe 2 substituent and the O atoms of a nitro group) are reported at 2.71 (3) Å , with CÁ Á ÁO distances of 3.658 (4) and 3.725 (4) Å (Sharma & Desiraju, 1994). The results of our structure analysis are also comparable to the average values that have been reported for hydrogen-bonding interactions of (N,C)Csp 2 -H (2.48 and 3.47 Å ) and Csp 3 -CH 3 (2.63 and 3.61 Å ) groups with a water O atom (Steiner, 2002). For comparison, intramolecular N-HÁ Á ÁO interactions to an O atom of a nitro substituent form shorter contacts, e.g. 1.927 (15) Å for N-(2-nitrophenyl)benzamide (Saeed & Simpson, 2009) and 2.11 Å for 2-iodo-N-(2-nitrophenyl)benzamide (Wardell et al., 2005), which is in accord with the reports that C-HÁ Á ÁO bonds are weaker than N-HÁ Á ÁO bonds (Desiraju, 1991).
The bridging chlorine atoms also form weak intermolecular interactions with CH 3 and CH groups of an adjacent molecule. In addition, the terminal chlorine atom participates in a hydrogen-bonding interaction with a CH 3 group of an adjacent molecule.
While C-HÁ Á ÁO interactions are widely accepted (Desiraju, 1991), C-HÁ Á ÁCl interactions are considered more controversial, but a survey of the literature reveals that they also represent a common phenomenon (Aakerö y et al., 1999). For example, hydrogen-bonding interactions of sp 2 (N,C)C-H with Cl À are reported at 2.64 Å (Kovacs & Varga, 2006). However, when Cl is bonded to a metal, the average C-HÁ Á ÁCl-M hydrogen-bonding distance is 2.974 Å (Thallapally & Nangia, 2001). Fig. 4 illustrates some of these intermolecular interactions. An important difference between this structure and the MET analog is that the dimet compound lacks the hydroxyethyl group, which is involved in classical intermolecular hydrogenbonding interactions for the MET derivative (Barba-Behrens et al., 1991).

Synthesis and crystallization
CuCl 2 ÁH 2 O (3 mg, 0.018 mmol) was added to a solution of dimet (6 mg, 0.043 mmol) in chloroform (0.7 mL), resulting in the precipitation of a blue solid over the course of 1 h at room temperature. The blue solid was isolated by decantation and crystals of [Cu(-Cl)Cl(dimet) 2 ] 2 , suitable for X-ray diffraction, were obtained by slow evaporation from a solution in acetonitrile.

Di-µ-chlorido-bis[chloridobis(1,2-dimethyl-5-nitro-1H-imidazole-κN 3 )copper(II)] acetonitrile disolvate
Crystal data 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.