A coordination polymer of mercury(II) formed by triazole-based and chloride linkers

1 Introduction

Triazoles have been widely studied due to their broad spectrum of applications. One of the methods used to link a carbohydrate moiety with an additional function is via a triazole ring using the well-known “click chemistry” reaction [1]. Triazoles are bioisosteres of amide bonds [2]. Their units are present in the structure of compounds as receptors for anion recognition [3] and proton exchange membranes [4]. Triazole fungicides [5] are also used in wood preservatives, textiles, leather, adhesives, antifouling agents, and paints [5]. Properties such as high nitrogen content together with low sensitivity towards external forces like impact and friction [6, 7] make this compounds reliable as safe energetic material.

To extend the coordination chemistry of triazoles further, in this work, the preparation, characterization, and crystal structure of the triazole complex, [Hg₅(μ-DAMT)₂(μ-Cl)₆(μ³-Cl)₂Cl₂]_n, using 4,5-diamino-3-methyl-4H-1,2,4-triazol-1-ium chloride (DAMT·HCl, Scheme 1) is described.

Scheme 1:

Structure of the 4,5-diamino-3-methyl-4H-1,2,4-triazol-1-ium chloride (DAMT·HCl).

2 Results and discussion

Reaction between the ligand salt (DAMT·HCl) and an ethanolic solution of mercury(II) chloride under reflux conditions provides the title complex. The complex is air-stable and soluble in DMSO.

In the IR spectra of the ligand and the complex, the relatively strong absorption bands above 3100 cm⁻¹ are due to the symmetric and asymmetric stretching vibrations of the RNH₂ groups. In the IR spectrum of the complex, these bands are shifted to higher frequency (by 84 and 63 cm⁻¹, respectively) compared to the free ligand [8]. A band at about 1680 cm⁻¹ can be assigned to the vibration of the imine bond, ν(C=N)_ar, which is slightly shifted (5 cm⁻¹) to higher frequency upon complexation.

The ¹H NMR spectrum of the compound was recorded in [D₆]DMSO and is consistent with the presence of a deprotonated DAMT ligand. A singlet peak at the highest magnetic field was assigned to the hydrogen atoms of the methyl group of the ligand. With decreasing magnetic field, two singlet peaks belonging to the two amino groups are observed. Owing to the deprotonation of the ammonium group of the ligand and also to the decrease (or absence) of exchange between tautomeric forms of the ligand [8] (due to binding to mercury atom), the corresponding peak had a shift of 1.19 ppm to lower magnetic field as compared to the protonated ligand salt (7.57 ppm for the NH₃⁺ compound [8]).

X-ray structure determination of the complex revealed a 2D organic-inorganic hybrid coordination polymer (Figs. 1–3). In this structure, there are three mercury atoms with different coordination numbers in the range of 4–6. A search in the Cambridge Structural Database (CSD) [9] has shown that there are only two examples [10, 11] for this type of compounds, and this new complex is the third one. The Hg1 atom is coordinated by two nitrogen atoms of two ligands and four chloride ions with coordination number 6 and slightly distorted octahedral geometry (Fig. 2a). The Hg2 atom has coordination number four with a HgN(μ-Cl)₂(μ³-Cl) environment and seesaw geometry (Fig. 2b). There is only one previous example for HgNCl₃ with such a geometry [12]. The bond lengths of axial Hg2–Cl4 and Hg2–Cl5 have an average value of 2.326 Å and are shorter than the equatorial ones (3.054(4) and 2.52(2) Å, respectively, for Hg2–Cl1 and Hg2–N2 bond lengths) as in the CSD analogue. The axial (167.5°) and equatorial (77.26°) bond angles around the Hg2 atom are shorter than in its analogue (174.46 and 88.49°, respectively). In addition to these four dative interactions, the Hg2 cation interacts with Cl3 at a long distance (3.208(5) Å) to complete its pseudo-square planar geometry. These interactions also appeared as the only similar structure [12]. The Hg3 atom is coordinated by five chloride ions in a Hg(μ-Cl)₃(μ³-Cl)Cl coordinated sphere (Fig. 2c). The pentacoordinate geometry of Hg3 can be viewed either as a square pyramidal or as a trigonal bipyramidal structure [13]. To distinguish such structures, the formula of Addison et al. [14] was applied in which the angular structural parameter (τ) is represented as the index of trigonality. The parameter τ=(β−α)/60, where α and β are the two largest angles at the mercury atom with β≥α. An ideal square pyramid has β=180° and α=180° and, therefore, τ=0, but an ideal trigonal bipyramidal structure has β=180° and α=120° and, therefore, τ=1 [15]. The τ value is calculated to be 0.12 for Hg3, indicating an inclination to a square pyramidal geometry. Similarly to Hg2, this cation interacts with Cl4 (3.237(5) Å) to complete its pseudo-octahedral geometry.

Fig. 1:

View of the asymmetric unit of the complex.

Fig. 2:

Perspective view of the coordination environments of Hg²⁺.

Fig. 3:

Projection of the structure along the [101] direction.

DAMT acts as non-chelated N₂ donor forming a bridge between two mercury atoms. A mean plane through all atoms of DAMT showed this ligand to be planar (RMS deviation, 0.039 for N4 atom), but the coordinated mercury atoms (Hg1 and Hg2) do not lie on this plane (distances, 0.444 Å for Hg1 and 0.631 Å for Hg2).

In the crystal network of the complex (Fig. 4), intermolecular C–H···Cl and N–H···Cl hydrogen bonds appear between polymeric chains and extend the network into three dimensions. In addition to hydrogen bonds, the crystal network is stabilized by Hg···Cl interactions. In one chain, all DAMT ligands are coplanar (angle between mean planes through all atoms of ligands, 0°), but because of the large distance between their centers (9.346 Å), there are no π-π stacking interactions between rings.

Fig. 4:

View of hydrogen bonds found on the complex along c axis.

3 Conclusion

In this work, a new complex, [Hg₅(μ-DAMT)₂(μ-Cl)₆(μ³-Cl)₂Cl₂]_n, was synthesized and its spectral (IR, ¹H NMR) and structural properties were investigated. The crystal structure of the complex revealed a coordination polymer containing mercury atoms with three different coordination geometries. The Hg1 atom has a slightly distorted octahedral geometry and a HgN₂(μ-Cl)₂(μ³-Cl)₂ environment. The Hg2 atom shows a rare seesaw geometry, although this geometry, by participating in a Hg···Cl interaction, is inclined to be pseudo-square planar. The coordination sphere of Hg3 is Hg(μ-Cl)₃(μ³-Cl)Cl with a square-planar geometry. The geometry around the Hg3 atom is completed by Hg···Cl interaction to pseudo-octahedral. In the structure of complex, the DAMT ligand acts as N₂ donor and bridges between two mercury atoms. This ligand participates in hydrogen bonding as donor to extend the crystal network to 3D.

4 Experimental