Metronidazole (MET) reacts with HAuCl4·H2O to give metronidazolium tetrachloridoaurate(III), [H(MET)][AuCl4], in which the cation and anion are linked via an O—H⋯Cl hydrogen-bonding interaction.
Keywords: crystal structure, gold, metronidazole, flagyl, tetrachloridoaurate(III), O—H⋯Cl hydrogen bonding
Abstract
Metronidazole (MET) [systematic names: 1-(2-hydroxyethyl)-2-methyl-5-nitro-1H-imidazole and 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol] is a medication that is used to treat infections from a variety of anaerobic organisms. As with other imidazole derivatives, metronidazole is also susceptible to protonation. However, there are few reports of the structures of metronidazolium derivatives. In the title compound, (C6H10N3O3)[AuCl4] [systematic name: 1-(2-hydroxyethyl)-2-methyl-5-nitro-1H-imidazol-3-ium tetrachloridoaurate(III)], the asymmetric unit consists of a metronidazolium cation, [H(MET)]+, and a tetrachloridoaurate(III) anion, [AuCl4]−, in which the AuIII ion is in a slightly distorted square-planar coordination environment. In the cation, the nitro group is essentially coplanar with the imidazole ring, as indicated by an O N—C=C torsion angle of −0.2 (4)°, while the hydroxyethyl group is in a coiled conformation, with an O(H)—C—C—N torsion angle of 62.3 (3)°. In the crystal, the anion and cation are linked by an intermolecular O—H⋯Cl hydrogen bond. In addition, the N—H group of the metronidazolium ion serves as a hydrogen-bond donor to the O atom of the hydroxyethyl group of a symmetry-related molecule, leading to the formation of chains along [010].
Chemical context
Metronidazole (MET), marketed as flagyl, and also known by the systematic names 1-(2-hydroxyethyl)-2-methyl-5-nitro-1H-imidazole and 2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol, is a medication that has been used for the treatment of parasitic infections, such as trichomoniasis, amoebiasis and giardiasis, and is also effective against anaerobic bacteria (Freeman et al., 1997 ▸; Miljkovic et al., 2014 ▸; Soares et al., 2012 ▸; Samuelson, 1999 ▸; Lofmark et al., 2010 ▸; Contreras et al., 2009 ▸). Metronidazole possesses a variety of functional groups, and the two-coordinate nitrogen atom of the imidazole ring has been shown to be an effective ligand for a variety of metals (Contreras et al., 2009 ▸). This nitrogen atom is also susceptible to protonation, but there are few structures of metronidazolium derivatives reported in the literature (Yang, 2008 ▸; Wang et al., 2010 ▸). We describe herein the structure of metronidazolium tetrachloridoaurate(III), which is obtained by the addition of MET to HAuCl4.
Structural commentary
The asymmetric unit of [H(MET)][AuCl4] consists of a metronidazolium cation, [H(MET)]+, hydrogen-bonded to a square-planar tetrachloridoaurate(III) anion, [AuCl4]−, by an O—H⋯Cl hydrogen bond as illustrated in Fig. 1 ▸. The O3⋯Cl3 distance of 3.169 (2) Å is comparable to the values in other tetrachloridoaurate(III) derivatives that exhibit O—H⋯Cl hydrogen bonds. As an illustration, bis{2-[(2-hydroxyethyl)iminomethyl]phenolato}gold(III) tetrachloridoaurate(III) possesses an O—H⋯Cl hydrogen bond between a hydroxyethyl group and [AuCl4]−, with an O(H)⋯Cl distance of 3.365 Å (Nockemann et al., 2007 ▸). For further reference, the average O⋯Cl distance in compounds that have O—H⋯Cl interactions is 3.196 (3) Å (Steiner, 2002 ▸). The nitro group is almost coplanar with the imidazole ring, as indicated by an O1—N3—C2—C1 torsion angle of −0.2 (4)°, while the hydroxyethyl group exhibits an O3—C6—C5—N2 torsion angle of 62.3 (3)°, describing a coiled conformation.
Figure 1.
The asymmetric unit of the title compound, shown with 20% probability displacement ellipsoids. The O3—H3⋯Cl3 hydrogen bond is shown as an open bond.
Supramolecular features
In the crystal, the N—H group of the metronidazolium ion serves as a hydrogen-bond donor to the oxygen atom of the hydroxyethyl group of a symmetry-related molecule, forming a chain along [010] in which each O—H group is O—H⋯Cl hydrogen bonded to a [AuCl4]− ion (Table 1 ▸ and Fig. 2 ▸). The N⋯O distance of 2.729 (3) Å associated with the hydrogen bond is comparable to that observed for metronidazole [2.816 (2) Å] (Blaton et al., 1979 ▸; Galván-Tejada et al., 2002 ▸). However, an important difference between the hydrogen bonds in metronidazole and metronidazolium is that the alcohol O—H group is the hydrogen-bond donor for metronidazole (i.e. O—H⋯N), while the N—H group is the hydrogen-bond donor for metronidazolium (i.e. N—H⋯O).
Table 1. Hydrogen-bond geometry (, ).
| DHA | DH | HA | D A | DHA |
|---|---|---|---|---|
| N1H01O3i | 0.94(4) | 1.81(4) | 2.729(3) | 166(3) |
| O3H3Cl3 | 0.67(4) | 2.54(4) | 3.169(2) | 158(4) |
Symmetry code: (i)
.
Figure 2.

Part of the crystal structure showing a hydrogen-bonded chain (open bonds) along [010].
Database survey
Metronidazolium derivatives that feature other counter-ions, e.g. 3-carboxy-4-hydroxybenzenesulfonate and perchlorate have been reported (Yang, 2008 ▸; Wang et al., 2010 ▸), as have a variety of tetrachloridoaurate(III) complexes (Johnson & Steed, 1998 ▸; Pluzhnik-Gladyr et al., 2014 ▸; Fazaeli et al., 2010 ▸).
Synthesis and crystallization
Crystals of composition [H(MET)][AuCl4] were obtained by combining HAuCl4·H2O (0.12 mmol) with MET (0.20 mmol) in MeOH (2 ml), followed by evaporation of MeOH, and crystallization from Et2O.
Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. H atoms bonded to C atoms were refined with a riding model, with C—H = 0.95–0.99 Å and U iso(H) = 1.2U eq(C) or 1.5U eq(Cmethyl). H atoms bonded to N and O atoms were refined independently with isotropic displacement parameters.
Table 2. Experimental details.
| Crystal data | |
| Chemical formula | (C6H10N3O3)[AuCl4] |
| M r | 510.94 |
| Crystal system, space group | Monoclinic, P21/n |
| Temperature (K) | 130 |
| a, b, c () | 7.324(2), 11.972(4), 15.667(5) |
| () | 94.384(4) |
| V (3) | 1369.6(8) |
| Z | 4 |
| Radiation type | Mo K |
| (mm1) | 11.52 |
| Crystal size (mm) | 0.23 0.04 0.02 |
| Data collection | |
| Diffractometer | Bruker APEXII CCD |
| Absorption correction | Multi-scan (SADABS; Bruker, 2013 ▸) |
| T min, T max | 0.426, 0.746 |
| No. of measured, independent and observed [I > 2(I)] reflections | 22024, 4214, 3673 |
| R int | 0.041 |
| (sin /)max (1) | 0.718 |
| Refinement | |
| R[F 2 > 2(F 2)], wR(F 2), S | 0.021, 0.045, 1.16 |
| No. of reflections | 4214 |
| No. of parameters | 163 |
| H-atom treatment | H atoms treated by a mixture of independent and constrained refinement |
| max, min (e 3) | 1.30, 1.22 |
Supplementary Material
Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989015010798/lh5766sup1.cif
Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015010798/lh5766Isup3.hkl
CCDC reference: 1404845
Additional supporting information: crystallographic information; 3D view; checkCIF report
Acknowledgments
RKU would like to thank Pace University for research support. Gerard Parkin (Columbia University) is thanked for helpful discussions.
supplementary crystallographic information
Crystal data
| (C6H10N3O3)[AuCl4] | F(000) = 952 |
| Mr = 510.94 | Dx = 2.478 Mg m−3 |
| Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
| a = 7.324 (2) Å | Cell parameters from 9874 reflections |
| b = 11.972 (4) Å | θ = 2.6–30.6° |
| c = 15.667 (5) Å | µ = 11.52 mm−1 |
| β = 94.384 (4)° | T = 130 K |
| V = 1369.6 (8) Å3 | Plate, yellow |
| Z = 4 | 0.23 × 0.04 × 0.02 mm |
Data collection
| Bruker APEXII CCD diffractometer | 3673 reflections with I > 2σ(I) |
| φ and ω scans | Rint = 0.041 |
| Absorption correction: multi-scan (SADABS; Bruker, 2013) | θmax = 30.7°, θmin = 2.1° |
| Tmin = 0.426, Tmax = 0.746 | h = −10→10 |
| 22024 measured reflections | k = −17→17 |
| 4214 independent reflections | l = −22→22 |
Refinement
| Refinement on F2 | 0 restraints |
| Least-squares matrix: full | Hydrogen site location: mixed |
| R[F2 > 2σ(F2)] = 0.021 | H atoms treated by a mixture of independent and constrained refinement |
| wR(F2) = 0.045 | w = 1/[σ2(Fo2) + (0.0129P)2] where P = (Fo2 + 2Fc2)/3 |
| S = 1.16 | (Δ/σ)max = 0.001 |
| 4214 reflections | Δρmax = 1.30 e Å−3 |
| 163 parameters | Δρmin = −1.22 e Å−3 |
Special details
| Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)
| x | y | z | Uiso*/Ueq | ||
| Au | 0.36629 (2) | 0.79983 (2) | 0.58625 (2) | 0.01868 (4) | |
| Cl1 | 0.26070 (11) | 0.84595 (7) | 0.45047 (5) | 0.03247 (17) | |
| Cl2 | 0.44092 (10) | 0.62541 (6) | 0.54111 (5) | 0.03012 (16) | |
| Cl3 | 0.46029 (10) | 0.75210 (6) | 0.72389 (5) | 0.02610 (15) | |
| Cl4 | 0.29498 (10) | 0.97418 (6) | 0.63205 (5) | 0.02831 (15) | |
| O1 | 0.8047 (3) | 0.7553 (2) | 0.41611 (14) | 0.0367 (5) | |
| O2 | 0.9242 (3) | 0.60835 (18) | 0.47815 (14) | 0.0331 (5) | |
| O3 | 0.7450 (3) | 0.55236 (18) | 0.73360 (15) | 0.0265 (5) | |
| H3 | 0.684 (5) | 0.591 (3) | 0.719 (2) | 0.042 (13)* | |
| N1 | 0.8446 (3) | 0.89061 (19) | 0.65413 (15) | 0.0175 (4) | |
| H01 | 0.810 (5) | 0.953 (3) | 0.685 (2) | 0.042 (10)* | |
| N2 | 0.9387 (3) | 0.72022 (17) | 0.63739 (14) | 0.0159 (4) | |
| N3 | 0.8670 (3) | 0.70319 (19) | 0.47846 (15) | 0.0223 (5) | |
| C1 | 0.8127 (4) | 0.8685 (2) | 0.56913 (17) | 0.0191 (5) | |
| H1A | 0.7604 | 0.9173 | 0.5261 | 0.023* | |
| C2 | 0.8708 (3) | 0.7626 (2) | 0.55853 (17) | 0.0158 (5) | |
| C3 | 0.9200 (4) | 0.8016 (2) | 0.69521 (17) | 0.0170 (5) | |
| C4 | 0.9783 (4) | 0.7982 (3) | 0.78720 (19) | 0.0272 (6) | |
| H4A | 0.9318 | 0.8643 | 0.8154 | 0.041* | |
| H4B | 1.1123 | 0.7972 | 0.7949 | 0.041* | |
| H4C | 0.9295 | 0.7307 | 0.8126 | 0.041* | |
| C5 | 1.0107 (4) | 0.6070 (2) | 0.65912 (18) | 0.0198 (5) | |
| H5A | 1.0863 | 0.6102 | 0.7142 | 0.024* | |
| H5B | 1.0904 | 0.5822 | 0.6145 | 0.024* | |
| C6 | 0.8579 (4) | 0.5230 (2) | 0.66604 (18) | 0.0227 (6) | |
| H6A | 0.7818 | 0.5199 | 0.6111 | 0.027* | |
| H6B | 0.9111 | 0.4479 | 0.6773 | 0.027* |
Atomic displacement parameters (Å2)
| U11 | U22 | U33 | U12 | U13 | U23 | |
| Au | 0.01490 (5) | 0.01746 (6) | 0.02409 (6) | −0.00147 (4) | 0.00416 (4) | −0.00253 (4) |
| Cl1 | 0.0438 (4) | 0.0304 (4) | 0.0234 (4) | 0.0042 (3) | 0.0037 (3) | −0.0009 (3) |
| Cl2 | 0.0286 (4) | 0.0222 (3) | 0.0400 (4) | 0.0024 (3) | 0.0057 (3) | −0.0092 (3) |
| Cl3 | 0.0255 (3) | 0.0251 (3) | 0.0272 (4) | 0.0036 (3) | −0.0008 (3) | −0.0012 (3) |
| Cl4 | 0.0360 (4) | 0.0193 (3) | 0.0292 (4) | 0.0033 (3) | 0.0002 (3) | −0.0038 (3) |
| O1 | 0.0516 (15) | 0.0375 (13) | 0.0193 (11) | 0.0047 (11) | −0.0090 (10) | −0.0008 (10) |
| O2 | 0.0468 (14) | 0.0221 (11) | 0.0309 (12) | 0.0056 (10) | 0.0058 (10) | −0.0077 (9) |
| O3 | 0.0250 (11) | 0.0233 (11) | 0.0327 (13) | 0.0053 (9) | 0.0109 (9) | 0.0075 (9) |
| N1 | 0.0172 (10) | 0.0152 (10) | 0.0203 (11) | −0.0003 (9) | 0.0023 (9) | −0.0003 (9) |
| N2 | 0.0146 (10) | 0.0157 (10) | 0.0175 (11) | 0.0000 (8) | 0.0021 (8) | 0.0014 (8) |
| N3 | 0.0253 (12) | 0.0233 (12) | 0.0181 (12) | −0.0032 (10) | 0.0010 (9) | −0.0019 (9) |
| C1 | 0.0217 (13) | 0.0198 (13) | 0.0157 (12) | 0.0003 (10) | −0.0001 (10) | 0.0012 (10) |
| C2 | 0.0184 (12) | 0.0152 (11) | 0.0138 (12) | −0.0023 (10) | 0.0008 (9) | −0.0006 (9) |
| C3 | 0.0146 (12) | 0.0188 (12) | 0.0180 (13) | −0.0012 (10) | 0.0038 (10) | 0.0002 (10) |
| C4 | 0.0292 (16) | 0.0355 (17) | 0.0167 (14) | 0.0031 (13) | 0.0011 (12) | −0.0009 (12) |
| C5 | 0.0181 (12) | 0.0170 (12) | 0.0245 (14) | 0.0051 (10) | 0.0026 (10) | 0.0064 (10) |
| C6 | 0.0254 (14) | 0.0164 (13) | 0.0271 (15) | 0.0029 (11) | 0.0072 (12) | 0.0046 (11) |
Geometric parameters (Å, º)
| Au—Cl1 | 2.2752 (10) | N2—C5 | 1.485 (3) |
| Au—Cl4 | 2.2807 (9) | N3—C2 | 1.441 (3) |
| Au—Cl2 | 2.2844 (9) | C1—C2 | 1.351 (4) |
| Au—Cl3 | 2.2855 (10) | C1—H1A | 0.9500 |
| O1—N3 | 1.218 (3) | C3—C4 | 1.472 (4) |
| O2—N3 | 1.210 (3) | C4—H4A | 0.9800 |
| O3—C6 | 1.436 (3) | C4—H4B | 0.9800 |
| O3—H3 | 0.67 (4) | C4—H4C | 0.9800 |
| N1—C3 | 1.341 (3) | C5—C6 | 1.515 (4) |
| N1—C1 | 1.360 (3) | C5—H5A | 0.9900 |
| N1—H01 | 0.94 (4) | C5—H5B | 0.9900 |
| N2—C3 | 1.345 (3) | C6—H6A | 0.9900 |
| N2—C2 | 1.392 (3) | C6—H6B | 0.9900 |
| Cl1—Au—Cl4 | 90.14 (3) | N1—C3—N2 | 108.2 (2) |
| Cl1—Au—Cl2 | 90.27 (3) | N1—C3—C4 | 124.7 (2) |
| Cl4—Au—Cl2 | 179.36 (3) | N2—C3—C4 | 127.0 (2) |
| Cl1—Au—Cl3 | 177.66 (3) | C3—C4—H4A | 109.5 |
| Cl4—Au—Cl3 | 89.52 (3) | C3—C4—H4B | 109.5 |
| Cl2—Au—Cl3 | 90.09 (3) | H4A—C4—H4B | 109.5 |
| C6—O3—H3 | 108 (3) | C3—C4—H4C | 109.5 |
| C3—N1—C1 | 110.4 (2) | H4A—C4—H4C | 109.5 |
| C3—N1—H01 | 120 (2) | H4B—C4—H4C | 109.5 |
| C1—N1—H01 | 129 (2) | N2—C5—C6 | 111.8 (2) |
| C3—N2—C2 | 106.6 (2) | N2—C5—H5A | 109.3 |
| C3—N2—C5 | 124.1 (2) | C6—C5—H5A | 109.3 |
| C2—N2—C5 | 129.3 (2) | N2—C5—H5B | 109.3 |
| O2—N3—O1 | 125.9 (3) | C6—C5—H5B | 109.3 |
| O2—N3—C2 | 118.9 (2) | H5A—C5—H5B | 107.9 |
| O1—N3—C2 | 115.2 (2) | O3—C6—C5 | 111.1 (2) |
| C2—C1—N1 | 105.7 (2) | O3—C6—H6A | 109.4 |
| C2—C1—H1A | 127.1 | C5—C6—H6A | 109.4 |
| N1—C1—H1A | 127.1 | O3—C6—H6B | 109.4 |
| C1—C2—N2 | 109.1 (2) | C5—C6—H6B | 109.4 |
| C1—C2—N3 | 125.8 (2) | H6A—C6—H6B | 108.0 |
| N2—C2—N3 | 125.1 (2) |
Hydrogen-bond geometry (Å, º)
| D—H···A | D—H | H···A | D···A | D—H···A |
| N1—H01···O3i | 0.94 (4) | 1.81 (4) | 2.729 (3) | 166 (3) |
| O3—H3···Cl3 | 0.67 (4) | 2.54 (4) | 3.169 (2) | 158 (4) |
Symmetry code: (i) −x+3/2, y+1/2, −z+3/2.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989015010798/lh5766sup1.cif
Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989015010798/lh5766Isup3.hkl
CCDC reference: 1404845
Additional supporting information: crystallographic information; 3D view; checkCIF report

