Crystal structure of bis[1-(2-hydroxyethyl)-2-methyl-5-nitro-1H-imidazole-κN 3]silver(I) tetrafluoridoborate methanol monosolvate

1-(2-Hydroxyethyl)-2-methyl-5-nitro-1H-imidazole (metronidazole, MET) reacts with AgBF4 to give [Ag(MET)2]BF4, in which the Ag atom is coordinated by two MET ligands with a trans arrangement.

1-(2-Hydroxyethyl)-2-methyl-5-nitro-1H-imidazole (metronidazole, MET) is a medication that is used to treat infections by a variety of anaerobic organisms, but there are relatively few reports of the structures of metal compounds that exhibit coordination of metronidazole. We have demonstrated that MET reacts with AgBF 4 to give [Ag(MET) 2 ]BF 4 ÁCH 3 OH, in which the Ag I cation is coordinated by two MET ligands with a trans arrangement. The structure of [Ag(MET) 2 ]BF 4 exhibits some interesting differences from its nitrate counterpart, [Ag(MET) 2 ]NO 3 [Fun et al. (2008). Acta Cryst. E64, m668]. For instance, although the two MET ligands of both [Ag(MET) 2 ]BF 4 and [Ag(MET) 2 ]NO 3 are almost coplanar, the former compound has an anti-like geometry with a molecular inversion center, but the latter has a syn-like arrangement. In the crystal, the BF 4 À anion is linked by an O-HÁ Á ÁF hydrogen bond to the methanol solvent molecule, which is, in turn, linked to the cation by an O-HÁ Á ÁO hydrogen bond; the components of the structure are linked by O-HÁ Á ÁO hydrogen bonds, forming chains along [001]. One of the MET ligands and the BF 4 À anion are disordered over two sets of sites with ratios of refined occupancies 0.501 (17)

Chemical context
1-(2-Hydroxyethyl)-2-methyl-5-nitro-1H-imidazole, also known as metronidazole (MET) or Flagyl, is a medication used particularly for treatment of parasitic infections, such as trichomoniasis, amoebiasis and giardiasis, but is also effective against anaerobic bacteria (Freeman et al., 1997;Miljkovic et al., 2014;Soares et al., 2012;Samuelson, 1999;Lofmark et al., 2010). There are relatively few reports of the structures of metal compounds that exhibit coordination of MET. For example, with respect to silver, only the nitrate compound, [Ag(MET) 2 ]NO 3 , has been structurally characterized by X-ray diffraction (Fun et al., 2008). Herein, we describe the structure of the tetrafluoridoborate derivative, [Ag(MET) 2 ]-BF 4 , which is obtained by addition of MET to AgBF 4 in methanol (see Scheme).

Structural commentary
Crystals of composition [Ag(MET) 2 ]BF 4 ÁMeOH were obtained from a solution in methanol. The asymmetric unit consists of a silver cation, [Ag(MET) 2 ] + , a tetrafluoridoborate anion, BF 4 À , and a solvent methanol molecule. The silver atom of [Ag(MET) 2 ] + is coordinated by two MET ligands in a trans manner by their N 3 nitrogen atoms, as illustrated in Fig. 1. ISSN 2056-9890 One of the MET ligands exhibits disorder resulting from rotation about the Ag-N bond [the dihedral angle between the planes of the disordered 5-membered rings is 11.0 (9) ]. The Ag-N bond lengths [Ag-N11 = 2.082 (15), Ag-N11A = 2.163 (16) and Ag-N21 = 2.1193 (15) Å ] are comparable to those values in the nitrate derivative, [2.1489 (11) and 2.1475 (11) Å ; Fun et al., 2008). There are, however, some interesting differences between the two compounds.
First, while the two MET ligands of both [Ag(MET) 2 ]BF 4 and [Ag(MET) 2 ]NO 3 are almost coplanar, the former compound has an anti-like geometry, and the latter has a synlike arrangement. Thus, the C13-N11Á Á ÁN21-C23 torsion angle for [Ag(MET) 2 ]BF 4 is 160.8 (9) [148.6 (11) for the minor component of disorder], while the value for [Ag(MET) 2 ]NO 3 is 24.10 (Fun et al., 2008). These differences are illustrated in Fig. 2 Fun et al., 2008). It is possible that this could be attributed to the tetrafluoridoborate ligand being considered a non-coordinating ion relative to nitrate, and this is reflected by the fact that [Ag(MET) 2 ]NO 3 exhibits AgÁ Á ÁO contacts of 2.63 and 2.67 Å , which are comparable to distances in other silver nitrate compounds (Wu et al., 2012).

Supramolecular features
The hydroxyethyl group of one of the MET ligands [O21-H] serves as a donor in an intermolecular hydrogen-bonding interaction with the other hydroxyethyl group [O11-H] of an adjacent molecule. In turn, the latter hydroxyethyl group serves as a hydrogen-bond donor to a methanol molecule, which also hydrogen bonds to a tetrafluoridoborate anion. In  The molecular structure of the cation of the title compound, with displacement ellipsoids drawn at the 30% probability level. The disorder is not shown.

Synthesis and crystallization
Crystals of composition [Ag(MET) 2 ]BF 4 ÁMeOH were obtained by combining AgBF 4 with MET in a 1:2 molar ratio in methanol and allowing the solution to evaporate slowly at room temperature.