Synthesis and crystal structures of bis(imidazolidine-2-thione-κS)bis(thiocyanato-κS)mercury(II) and bis(cyanido)bis(μ₂-imidazolidine-2-thione-κS)mercury(II).Hg(CN)₂

2.1 Materials

Mercury(II) chloride (HgCl₂) was obtained from Merck Chemical Company (Germany). Imidazolidine-2-thione (Imt) was synthesized according to the procedure described in the literature [33]. Hg(CN)₂ was prepared by reacting HgCl₂ with KCN in a 1:2 mole ratio in water-methanol medium.

2.2 Preparation of the complexes

For preparation of 1, 0.195 g (2 mmol) of KSCN in 10 mL water was added to 0.27 g (1.0 mmol) of mercury(II) chloride in 10 mL methanol. On mixing, a clear solution was obtained, which was stirred for 15 min. Then 2 equivalents of imidazolidine-2-thione (0.205 g, 2.0 mmol) in 15 mL methanol were added. After stirring for 15 min, the colorless solution was filtered and the filtrate was kept at room temperature for crystallization. A white crystalline product was obtained, which was washed with methanol and dried. Yield=75%, melting point=153–155°C.

For the preparation of 2, to 0.25 g (1.0 mmol) of mercury(II) cyanide in 10 mL methanol were added 2 equivalents of imidazolidine-2-thione (0.204 g, 2.0 mmol) in 15 mL methanol. (Caution: Potassium cyanide is extremely dangerous and must be handled with care.) On mixing, a clear solution was obtained, which was stirred for 30 min. The clear solution was kept at room temperature for crystallization. An off white crystalline product was obtained, which was washed with methanol and dried. Yield=60%, melting point=199–201°C.

2.3 Spectroscopic data

• IR: [Hg(Imt)₂(SCN)₂] (1), ν=1191, 1476, 1527, 2083, 3183 cm⁻¹; [Hg(Imt)₂(CN)₂]·Hg(CN)₂ (2), ν=1195, 1479, 1504, 1520, 2171, 3283 cm⁻¹ (Imt, ν=1195, 1456, 1497, 1515, 2882, 3236 cm⁻¹); Hg(SCN)₂, ν=2144 cm⁻¹; Hg(CN)₂, ν=2191 cm⁻¹.

• ¹H NMR (500 MHz, DMSO, 297 K, TMS, ppm): [Hg(Imt)₂(SCN)₂] (1), δ=3.63, 8.59; [Hg(Imt)₂(CN)₂].Hg(CN)₂ (2), δ=3.58, 8.27 (Imt, δ=3.62, 7.98). – ¹³C NMR (125.65 MHz, DMSO, ppm): [Hg(Imt)₂(SCN)₂], δ=44.39, 128.63, 179.51; [Hg(Imt)₂(CN)₂], δ=44.73, 144.72, 181.79 (Imt, δ=44.0, 183.4); Hg(SCN)₂, δ=115.88; Hg(CN)₂, δ=144.75 ppm.

2.4 IR and NMR measurements

The IR spectra were recorded on a Nicolet iS5 FTIR spectrophotometer using Diamond ATR accessories over the range 4000–500 cm⁻¹ with the resolution of 2 cm⁻¹. The ¹H and ¹³C{¹H} NMR spectra in [D₆]DMSO were obtained on a Jeol JNM-LA 500 NMR spectrometer operating at frequencies of 500.00 MHz and 125.65 MHz, respectively, at 297 K. The ¹³C chemical shifts were measured relative to TMS.

2.5 X-ray structure determinations

Single-crystal data collections were performed at 296 K on a Bruker Kappa APEXII CCD diffractometer equipped with a four-circle goniometer and using graphite mono-chromated MoKα radiation. The refinement and all further calculations were carried out using Shelxl-2014 [34]. Platon was used for molecular graphics [35]. Crystal data and details of the data collection are summarized in Table 1.

Fig. 1:

The molecular structure of 1 in the crystal with displacement ellipsoids drawn at the 50% probability level. Symmetry code: (i) 1−x, y, 0.5−z].

CCDC 1471633 (1) and 1471634 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 IR and NMR spectroscopy

The IR spectral data of free Imt and the mercury(II) complexes are given in the experimental section. The spectra are characterized by mainly four bands, ν(C=S), ν(N-H), ν(C-N), and ν(C≡N). They are observed around 1200, 3200, 1500, and 2100 cm⁻¹, respectively. The ν(C=S) band expected around 500 cm⁻¹ could not be observed because of the instrumental detection limit. The C=S stretching is shifted to lower frequencies upon Hg-S bonding, while the C-N band was shifted towards higher frequency. The ν(C≡N) stretching vibrations observed at 2083 cm⁻¹ and 2171 cm⁻¹ for 1 and 2, respectively, are close to those reported for other similar complexes [7], [15], [32].

The ¹H and ¹³C NMR chemical shifts of the complexes in [d₆] DMSO are summarized in the Experimental part. In ¹H NMR spectra of the complexes, the N-H signal of Imt shifted downfield by 0.3–0.6 ppm from its position in the free ligand. Deshielding of the N-H proton is related to an increase of the π electron density in the C-N bond upon complexation [7], [15], [32], [33]. The greater ¹H shift in the thiocyanato complex may be due to strong intramolecular hydrogen bonding of these protons with the nitrogen atoms of the SCN groups (vide infra). In complexes 1 and 2, the C-2 resonance of Imt appeared upfield by 1.6 and 3.9 ppm, respectively, compared to the uncomplexed ligand as observed for other mercury(II) complexes of thiones [7], [15], [16], [17], [18], [19], [20], [32]. The upfield shift is attributed to a lowering of the C=S bond order upon coordination and a shift of N→C electron density producing a partial double bond character in the C-N bond [10], [26], [27]. As the difference in shielding at the C-2 resonance is associated with the strength of the Hg-S bond [15], the greater shift in the Hg(SCN)₂ complex than in [Hg(Imt)₂(CN)₂].Hg(CN)₂ reflects its higher stability. The large downfield shift (12.75 ppm) of the C (thiocyanate) in 1, relative to the chemical shift of this C atom in free mercuric thiocyanate, is in agreement with the strong binding of thiocyanate to the mercury atom in 1. No significant change was observed for the ¹³CN resonance of Hg(CN)₂ in 2 after complexing with Imt as reported previously [15]. The appearance of this resonance as a singlet suggests that there is a rapid exchange of the Imt ligands between the two Hg(CN)₂ components of 2 in DMSO solution.

3.2 Crystal and molecular structures

A view of the molecular structure of complex 1, [Hg(Imt)₂(SCN)₂], is shown in Fig. 1, and selected geometrical parameters are listed in Table 2. The Hg atom in 1 is coordinated to two S atoms of Imt ligands and two thiocyanate anions through S adopting a distorted tetrahedral geometry. The average S-Hg-S bond angle around the Hg atom is 108.58°. The S-Hg-S (Imt) bond angle [148.65(6)°] is much larger than the S-Hg-S (SCN) bond angle [104.96(4)°], and this difference can be attributed to the larger size of Imt. The involvement of thiocyanate (nitrogen) in hydrogen bonding with N-H may lead to the contraction of the bond angle. The Hg-S (Imt) distances are shorter than the sum of covalent radii of S and tetrahedral Hg (1.04 and 1.48 Å, respectively), indicating that the thione ligand forms a strong covalent bond to Hg. It is also shorter than the Hg-S (SCN) distance, indicating a stronger binding of Imt to mercury(II). These values are in agreement with the literature [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [36].

In the molecular packing of complex 1, the symmetry-related molecules are connected by intermolecular N-H···N hydrogen bonds, involving the N-H atoms of Imt and the N atoms of the SCN⁻ anions (Table 3). This gives rise to the formation of an infinite three-dimensional framework, as shown in Fig. 2.

Fig. 2:

Packing diagram of 1 showing hydrogen bond interactions.

The molecular structure of compound 2 along with the atomic numbering scheme is depicted in Fig. 3, and selected geometrical parameters are given in Table 2. Compound 2 is composed of a complex unit, [Hg(CN)₂(Imt)₂], and an independent Hg(CN)₂ molecule. In the complex molecule, the Hg atom is coordinated to two cyanide ions and four imidazolidine-2-thione (Imt) ligands through bridging S atoms adopting an irregular octahedral geometry. The cis bond angles around the Hg atom are in the range of 84.63(3)–95.37(3)°, while the trans angles are 180°. The Hg-S distances are larger than those reported for the related compounds [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. The longer Hg-S distances indicate that Imt ligands are weakly bound to Hg(II) [23]. However, the Hg-CN bond distances are in agreement with the reported values [5], [6], [7], [8], [15], [23]. The Hg-C≡N unit of complex Hg(CN)₂ is perfectly linear, while the non-coordinated Hg(CN)₂ shows some deviation from linearity [bond angle=174.7(3)°]. The title compound represents the first example of a mercury cyanide complex that exhibits an octahedral geometry.

Fig. 3:

Molecular structure of 2 in the crystal with displacement ellipsoids drawn at the 30% probability level. Symmetry codes: (i) 1−x, −y, 1−z; (ii) 0.5−x, −y, 0.5+z; (iii) 0.5+x, y, 0.5−z; (iv) −0.5+x, y, 0.5−z; (v) 0.5−x, −y, −0.5+z.

In the molecular packing of the complex (Fig. 4), the symmetry-related molecules are connected by intramolecular N-H…S and intermolecular N-H-N (CN) hydrogen bonds, involving the Imt NH atoms and the N atom of the CN⁻ anions (Table 3). This gives rise to the formation of a two-dimensional network.

Fig. 4:

Projection of 2 along the crystallographic c axis showing hydrogen bond interactions.

The present report shows that the interaction of imidazolidine-2-thione (Imt) with mercury thiocyanate and cyanide results in complexes with a distorted tetrahedral and octahedral geometry, respectively. In complexes 1 and 2, the Imt ligand is coordinating through its sulfur atom in monodentate terminal and bridging modes, respectively.