Synthesis and characterization of silver(I) complexes of thioureas and thiocyanate: crystal structure of polymeric (1,3-diazinane-2-thione)thiocyanato silver(I)

2.1 Chemicals

AgNO₃ was a product of Panreac Química S.A. (Castellar del Vallès, Spain). KSCN was obtained from Merck (Darmstadt, Germany). N-Methylthiourea (Metu), N,N′-dimethylthiourea (Dmtu), and N,N,N′,N′-tetramethylthiourea (Tmtu) were obtained from Acros Organics (Pittsburgh, PA, USA). Imidazolidine-2-thione (Imt) and 1,3-diazinane-2-thione (Diaz) were synthesized according to the procedures described in the literature [16].

2.2 Preparation of (thione)–AgSCN complexes

The complexes were prepared by adding 1 or 2 mmolar solutions of Dmtu, Tmtu, and Diaz (for Tu, Metu, and Imt complexes only 2 mmolar) in methanol (Tu in water and Imt in acetonitrile) to 0.17 g AgNO₃ (1 mmol) in methanol followed by the addition of an aqueous solution of 0.10 g KSCN (1 mmol). In the case of Tu, Metu, and Tmtu, mixing of thiones to AgNO₃ resulted in colorless solutions, while for Dmtu, Diaz, and Imt white precipitates formed on mixing. The mixtures were stirred for 15 min and then one equivalent KSCN solution was added. The addition of KSCN to Tu, Metu, and Tmtu solutions resulted in white precipitates, while in the case of the 2:1 system of Dmtu, Diaz, or Imt:Ag(I), a colorless solution was obtained, which upon evaporation of some solvent yielded white precipitates. The white precipitates of the 1:1 mixture of Dmtu and Ag(I) did not dissolve on addition of KSCN and therefore the product was collected from the filtrate (the residue was discarded). The white precipitates in all cases were filtered, washed with methanol, and air dried. The product yield is about 40–50 %, except for the 1:1 system of Diaz:Ag(I), for which it is only 15 %. The elemental analysis and melting points of the complexes are given in Table 1.

2.3 Spectroscopic data

IR (KBr pellet, cm^–1): AgSCN, ν = 2142; [(Tu)Ag(SCN)], ν = 705, 2099, 3190, 3383 (Tu, ν = 732, 3156, 3365); [(Metu)Ag(SCN)], ν = 627, 2110, 3375, 3177 (Metu, ν = 634, 3163, 3245); [(Dmtu)Ag(SCN)], ν = 638, 2096, 3226 (Dmtu, ν = 641, 3203); [(Tmtu)(AgSCN)_1.5], 610, 2105 (Tmtu, ν = 622); [(Diaz)Ag(SCN)], ν = 520, 2100, 3250 (Diaz, ν = 510, 3200). – ¹H NMR (500 MHz, DMSO, 24 °C, TMS, ppm): [(Tu)Ag(SCN)], δ = 7.65, 8.10 (Tu, δ = 6.98, 7.25); [(Metu)Ag(SCN)], δ = 2.78, 2.79, 7.88, 8.42, 8.61 (Metu, δ = 2.68, 2.87, 6.95, 7.45, 7.65); [(Dmtu)Ag(SCN)], δ = 2.80, 3.04, 8.13, 8.43 (Dmtu, δ = 2.85, 7.38); [(Imt)₂Ag(SCN)], δ = 3.70, 8.75 (Imt, δ = 3.62, 7.98); [(Diaz)Ag(SCN)], δ = 1.78, 3.22, 8.66 (Diaz, δ = 1.75, 3.15, 7.81). – ¹³C NMR (125.65 MHz, DMSO, TMS, ppm): [(Tu)Ag(SCN)], δ = 178.7, 124.5 (Tu, δ = 183.81); [(Metu)Ag(SCN)], δ = 30.2, 31.5, 124.6, 175.4, 179.4 (Metu, δ = 29.9, 31.1, 181.1, 184.1); [(Dmtu)Ag(SCN)], δ = 29.6, 32.1, 124.3, 176.1 (Dmtu, δ = 30.7, 182.7); [(Imt)₂Ag(SCN)], δ = 44.7, 126.8, 179.5 (Imt, δ = 44.0, 183.4); [(Diaz)Ag(SCN)], δ = 18.2, 40.0, 124.5, 169.38 (Diaz, δ = 19.2, 39.8, 175.6).

2.4 IR and thermal measurements

The solid-state IR spectra of the ligands and their thiocyanato silver(I) complexes were recorded on a Perkin-Elmer FTIR 180 spectrophotometer using KBr pellets over the range 4000–400 cm^–1. Thermal analysis was carried out on a Mettler Tolledo TGA/SDTA 851e analyzer USA under argon atmosphere at a heating rate of 10 °C min^–1.

2.5 ¹H and ¹³C NMR measurements

The ¹H NMR spectra of the complexes in [D₆]DMSO were obtained on a Jeol JNM-LA 500 NMR spectrometer operating at a frequency of 500.00 MHz at 297 K using 0.10 m solution. The ¹³C NMR spectra were obtained at a frequency of 125.65 MHz with ¹H broadband decoupling at 298 K. The spectral conditions were 32 K data points, 0.967 s acquisition time, 1.00 s pulse delay, and 45° pulse angle. The ¹H and ¹³C chemical shifts were measured relative to TMS.

2.6 X-ray structure determination

Single crystal data collection was performed at 296 K on a Bruker Kappa APEXII CCD diffractometer equipped with a four-circle goniometer and using graphite monochromatized MoK_α radiation. Crystal data and details of the data collection are summarized in Table 2. The investigated crystal was found to be twinned. The twin matrix (1 0 1.743, 0 1̅ 0, 0 0 1̅) was found using the program Rotax by Parsons and Gould [22] and subsequent refinement was done with an HKLF 5 type of file with Shelxl-97 [23]. The H atoms were positioned geometrically (C–H = 0.97 Å, N–H = 0.86 Å) and refined as riding with U_iso(H) = 1.2 × U_equ of the atoms to which they are attached. For molecular graphics the program Platon [24] was used.

CCDC 1030273 contains 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 studies

The selected IR frequencies of thioamides and their silver thiocyanate complexes are given in the Experimental section. In the IR spectra of thiones, the characteristic bands are expected in three frequency regions: ν(C=S) appears around 500 or 600 cm^–1, ν(C–N) bands at about 1500 cm^–1, and ν(N–H) near 3200 cm^–1. A low-frequency shift in the ν(C=S) band (except in 1) and a high-frequency shift in the ν(N–H) bands in the complexes compared to the free ligands indicate the existence of thione forms in the solid state. A sharp band around 2100 cm^–1 for SCN^– stretch was observed for all the complexes indicating its binding with silver(I).

The ¹H and ¹³C chemical shifts of the complexes in [D₆]DMSO are summarized in the Experimental section. In the ¹H NMR spectra of the complexes, the N–H signals of the thioamides were shifted upon coordination downfield from their positions in free ligands. A slight downfield shift was also observed for other protons, which is related to an increase in π electron density in the C–N bond upon coordination. The appearance of the N–H signal shows that the ligands are coordinated to silver(I) via the thione group. The N–H protons of Metu are nonequivalent. The N–H and CH₃ (2.85 ppm) protons of Dmtu are equivalent, but after coordination they become nonequivalent (δ CH₃ = 2.80, 3.04 ppm). A similar observation was made in the ¹H spectrum of [(Dmtu)AgCN] [12].

In ¹³C NMR, the >C=S resonance of the ligands in the complexes is shifted upfield by about 4–7 ppm as compared to the resonances of free ligands in accordance with the data observed for other complexes of copper(I) [25], silver(I) [12–15, 18, 26, 27], and gold(I) [16, 17, 28, 29] with thiones. A shift of this magnitude is diagnostic for S-bonded thiones, ascribed to back-bonding from the metal d orbitals to the antibonding π orbitals of sulfur in the >C=S bond, which will not only reduce the >C=S bond order but also shield the carbon atom of >C=S group resulting in a high field shift [12–18]. As the shift difference of the >C=S resonance may be related to the strength of the metal-sulfur bond [12–15], the data show that the Dmtu complex should be the most stable among the examples. A small deshielding effect is observed in other carbon atoms, which is due to an increase in π character of the C–N bond. It should be noted that Metu gives two signals for both >C=S and N–CH₃ carbons showing that the compound exists in two isomeric forms in solution form that could be due to a rotation barrier in the C–N bond. For [(Dmtu)AgSCN], two signals are observed for N–CH₃ groups suggesting that the methyl groups become nonequivalent upon coordination similar to our observation for AgCN complexes [12]. The presence of a resonance at about 125 ppm in ¹³C NMR indicates the coordination of SCN^– to silver(I). The CN resonance in the corresponding AgCN complexes was observed around 143 ppm [12, 13].

3.2 Thermal studies

The isolated tetramethylthiourea complexes (expected to be [(Tmtu)₁Ag(SCN)] and [(Tmtu)₂Ag(SCN)]) were insoluble in DMSO; therefore, their NMR could not be recorded and they were characterized by thermal analysis. Thermal degradation of the complexes was monitored up to 740 °C. The thermal behavior of the expected [(Tmtu)₂Ag(SCN)] is illustrated in Fig. 1. The decomposition starts at 206 °C and is completed at about 740 °C. The endothermic transition at 206.4 °C is associated with the melting point of the complex. The decomposition of [(Tmtu)₁Ag(SCN)] also started at 206 °C associated with an endothermic transition and followed a similar pattern as observed for [(Tmtu)₂Ag(SCN)]. The calculated weight losses for the release of one or two Tmtu ligands from [(Tmtu)₁Ag(SCN)] and [(Tmtu)₂Ag(SCN)] are 44.3 % and 61.4 %, respectively. However, the thermogram (Fig. 1) shows that the weight loss due to removal of Tmtu ligands is about 35 % at 320 °C. This decomposition pattern suggests that the most probable formula of the complexes is [(Tmtu)(AgSCN)_1.5], for which the percentage value is 34.7 % for the loss of Tmtu. The remaining weight of ∼ 65 % beyond 320 °C corresponds to AgSCN, which slowly releases thiocyanate. Previously, we reported that the analogous cyanido complex exhibits a polymeric structure with the composition [(Tmtu)(AgCN)₂]_n [11].

Fig. 1:

Thermogravimetric and DSC (as well as differential DSC) curves of [(Tmtu)(AgSCN)_1.5]. The peak at 206.4 °C indicates the melting point of the complex.

Thermal degradation of intended [(Dmtu)₂Ag(SCN)] shows a weight loss of about 33 %, but the calculated value for the loss of Dmtu is 52.0. This value is consistent with the formula [(Dmtu)Ag(SCN)] (calculated 35.2 %). The expected Dmtu complexes, [(Dmtu)₁Ag(SCN)] and [(Dmtu)₂Ag(SCN)], exhibit similar decomposition behavior showing that the composition of the resulting compound is the same. The decomposition of the complex starts at 118 °C and is complete in one step at 478 °C. The exothermic transition represents the melting point of the complex as shown in Table 1. The weight loss of 40 % at 478 °C corresponds to the removal of only one Dmtu ligand from the complex (calcd wt loss = 38.5 %). AgSCN remains stable until 740 °C. Thus the proposed formula of both complexes is [(Dmtu)₁Ag(SCN)], although in the preparation, the molar ratio of Dmtu to AgSCN was different (1:1 and 2:1).

3.3 Crystal structure description of 1

The molecular structure of 1 together with the atomic labeling scheme is shown in Fig. 2. selected bond lengths and angles are presented in Table 3. The complex exists in the form of a polymer consisting of Ag(μ₂-Diaz)(μ₂-SCN) units. The silver atom is coordinated to two μ₂-sulfur atoms of Diaz and to two μ₂ thiocyanate sulfur atoms in a distorted tetrahedral geometry. The Ag(μ₂-Diaz)(μ₂-SCN) units are arranged in the form of a four-membered ring and are repeated to form a double stranded polymeric chain (Fig. 2). The S–Ag–S(Diaz) angles are greater than the S–Ag–S(SCN) angles. The Ag–S and other distances are comparable to the values reported for other Ag(I)–thione complexes [7, 11, 18–21, 30]. The longer Ag–S distance for SCN suggests its comparatively weaker binding compared to the Diaz ligand. The angles around the sulfur atom lie in the range expected for a tetrahedral environment except for Ag–S–Ag, which is 80.79(5)° (Table 3). The SCN moiety is linear with a bond angle of 178.6(8)°. Although the composition of the compound is similar as in [Ag(Tu)SCN], the two structures are significantly different [21].

Fig. 2:

The polymeric chain of [(Diaz)Ag(SCN)] (1) drawn parallel to the crystallographic a axis.

The silver atoms in the chain are connected to each other by weak argentophilic interactions. The Ag1···Ag2 distance is 3.3595(3) Å, which is somewhat shorter than the sum of the van der Waals radii of two silver atoms (3.44 Å) [31–33]. The corresponding distances in other reported complexes are [Ag(Tmtu)(AgCN)₂], 3.6965 Å [11], [Ag(Dmtu)₂Ag(CN)₂], 3.12 Å [7], and [Ag(Dmtu)₂]ClO₄, 3.21 Å [34]. The argentophilic interactions in crystals of 1 result in a stabilization of the chain structure.

In the crystals, hydrogen-bonding interactions occur, involving each of the two N–H groups and the thiocyanate sulfur or nitrogen atoms. Details are given in Table 4.

The present report shows that thioamides and thiocyanate react with AgNO₃ to form complexes of the type [(thioamide)_n(AgSCN)_n] in which the ligands coordinate in the thione form in solution as well as in the solid state. The crystal structure of one example shows that it exists in the form of a double stranded polymer, in which Diaz binds to silver(I) through bridging sulfur atom exhibiting a tetrahedral coordination.