Crystal structure of hexa-μ-chlorido-μ4-oxido-tetrakis{[1-(2-hydroxyethyl)-2-methyl-5-nitro-1H-imidazole-κN 3]copper(II)} containing short NO2⋯NO2 contacts

The title tetranuclear cluster contains a tetrahedral arrangement of copper(II) ions bonded to a central oxygen atom. The extended structure shows short O⋯N interactions between the nitro groups of adjacent clusters, which are oriented perpendicular to each other in a manner that has previously been described as an ONO2⋯π(N)NO2 interaction.


Chemical context
Metronidazole (C 6 H 9 N 3 O 3 ; MET) is a medication that was discovered to be effective against both bacteria and parasites more than 50 years ago (Samuelson, 1999). MET is currently incorporated in the World Health Organization (WHO) list of essential medicines, i.e. medications that are considered to be effective and safe to meet the most important needs in a health system (WHO, 2015). Despite the widespread use of MET as a drug, relatively little structural data concerning its interactions with metal ions exist, and there are few structurally characterized copper compounds of MET (Galvá n-Tejada et al., 2002;Barba-Behrens et al., 1991;Athar et al., 2005;Ratajczak-Sitarz et al., 1998;Bharti et al., 2002). Our recent work has sought to develop further metal-MET chemistry and we have reported structures containing Cu Quinlivan & Upmacis, 2016), as well as Ag  and Au (Quinlivan et al., 2015). Tetranuclear copper(II) compounds of the form [Cu 4 OX 6 L 4 ] are relatively well known, with the first example described in 1996 (Bertrand & Kelley, 1966). In this regard, although the structure of a [Cu 4 OX 6 L 4 ] structure, where L = imidazole, has been previously described (Atria et al., 1999), a counterpart containing L = MET has not been reported. Herein, we describe the structure of a tetranuclear Cu-MET complex [Cu 4 Cl 6 O(MET) 4 ] that is obtained by the reaction of anhydrous copper(I) chloride with MET in MeOH under aerobic conditions.

Structural commentary
The structure of the [Cu 4 Cl 6 O(MET) 4 ] complex is shown in Fig. 1. Four copper atoms are arranged around an oxygen atom in a tetrahedral fashion, with Cu-O distances ranging from 1.8960 (18) to 1.913 (2) Å . The Cu-O-Cu angles range from 108.36 (10) to 110.80 (9) , indicating a fairly uniform tetrahedron with little distortion. In fact, the degree of distortion from a tetrahedral arrangement can be readily quantified by the 4 four-coordinate geometry index that is reported and discussed elsewhere (Yang et al., 2007;, Brescia et al., 2018. Briefly, 4 is obtained from the expression, 4 = [360 À ( + )]/141, where and represent the two largest angles; a 4 value of 1.00 indicates an idealized tetrahedral geometry, whereas a value of 0.00 indicates an idealized square-planar geometry. In the title complex, = 110.80 (9) and = 109.55 (9) , such that 4 is 0.990, which indicates negligible deviation from a tetrahedral geometry for oxygen (Yang et al., 2007).
Each of the four copper atoms is linked to the other three copper atoms via three chloride bridges, with the Cu-Cl bridging distances varying from 2.3579 (10) to 2.4435 (9) Å (for Cu2-Cl6 and Cu1-Cl2, respectively). Each copper atom is also bound to a nitrogen atom of a MET ligand. The Cu-N lengths range from 1.949 (2) to 1.972 (3) Å (for Cu1-N11 and Cu4-N41, respectively). Thus, each copper atom sits within a trigonal-bipyramidal arrangement, with the oxygen and nitrogen atoms forming the axial coordination points, and the bridging chloride ligands occupying the equatorial plane. The trigonal-bipyramidal structure is somewhat distorted, as indicated by the fact that the O-Cu-N angles are less than 180 , ranging from 173.12 (10) to 176.91 (10) (for O1-Cu1-N11 and O1-Cu2-N21, respectively), and the Cl-Cu-Cl angles differ significantly from 120 , ranging from 109.97 (3) to 134.02 (3) (for Cl2-Cu2-Cl4 and Cl3-Cu1-Cl2, respectively). Furthermore, the O-Cu-Cl angles are all less than 90 , ranging from 83.33 (6) to 86.13 (6) (for O1-Cu1-Cl2 and O1-Cu-Cl1, respectively), indicating that the equatorial chloride ligands are displaced slightly more towards the axial oxygen atom in the center of the molecule, than towards the nitrogen-containing ligand in the opposite axial position.
The 5 geometry index is a general descriptor of five-coordinate molecules and provides a way to determine the extent of distortion of a molecule from trigonal bipyramidal to square pyramidal (Addison et al., 1984). The 5 geometry index is calculated by using the equation: 5 = ( À )/60, where À is the difference between the two largest angles (Addison et al., 1984;Palmer & Parkin, 2014). The values for 5 are calculated to be 0.65 (Cu1), 0.74 (Cu2), 0.84 (Cu3) and 0.73 (Cu4) for the five-coordinate copper centers, giving an average 5 value of 0.74. The 5 values obtained indicate that the copper-centered structures are closer to an idealized trigonalbipyramidal (1.00) than a square-pyramidal geometry (0.00).      Table 1, O11-H11A and O21-H21A probably form links to the disordered solvent molecules removed with SQUEEZE (see Experimental). The most interesting observation is the existence of short OÁ Á ÁN interactions between the N13/O12/O13 and N33/O32/O33 nitro groups of adjacent clusters that are oriented perpendicular to each other, as illustrated in Fig. 3 with O12Á Á ÁN33 = 2.775 (4) Å . This type of contact has previously been described as an O NO2 Á Á Á(N) NO2 interaction (Daszkiewicz, 2013); such contacts are typically shorter than 3 Å .

Synthesis and crystallization
Anhydrous copper(I) chloride (0.015 g, 0.00015 mol) was mixed with MET (0.05075 g, 0.00030 mol) in methanol (2 ml) in a glass vial, forming a dark olive-colored solution. After allowing the solution to evaporate for eight days, gold-colored plates, suitable for X-ray diffraction, were obtained.

Figure 3
Detail of the OÁ Á ÁN interaction between the nitro groups of adjacent clusters.

Hexa-µ-chlorido-µ 4 -oxido-tetrakis{[1-(2-hydroxyethyl)-2-methyl-5-nitro-1H-imidazole-κN 3 ]copper(II)}
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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (