Synthesis and characterization of gallium(III) dithiocarbamates as suitable nano-gallium(III) sulfide precursors

IR spectroscopic studies

For complexes (1) and (2), the ν_C-N (thioureide) bands were observed at 1478 and 1474 cm^-1, respectively. The ν_(C-S) stretching bands were observed at approximately 1000 cm^-1, supporting the bidentate coordination mode (Baskaran et al., 2008; Arul Prakasam et al., 2009; Marimuthu et al., 2012) of the dithiocarbamate to the metal center, and the ν_C-H bands were observed in the range of 2852–2929 cm^-1.

NMR spectroscopic studies

NMR spectra were recorded at room temperature with tetramethylsilane as an internal reference and CDCl₃ as the solvent. The chemical shifts reported in the Spectral Data Base for Organic Compounds (SDBS) for the parent amines are shown in brackets for comparison. The ¹H NMR of complex (1) showed an intense signal at 4.57(m) ppm corresponding to a single proton integration due to α-CH of the cyclohexyl ring, and CH₃ protons (α-CH₃) attached to the nitrogen appeared at 3.22(s) ppm with proton integration corresponding to three. The α-CH and α′-CH₃ protons were affected to a maximum extent on complexation. In the cyclohexyl ring, all equatorial protons were deshielded to a large extent compared to the axial protons. The ¹³C NMR spectrum of (1) showed a weak signal at 201.1(3) ppm due to the characteristic thioureide carbon (Van Gaal et al., 1979). In the case of (2), the ¹H NMR of α-CH proton of the cyclohexyl ring appeared at 4.56 ppm and α′-CH₂ protons at 3.73 ppm corresponding to two protons. The β′-CH₃ protons of the ethyl group appeared as a well-resolved triplet at 1.31–1.38 ppm. The proton signals in (2) appeared without much change as observed in (1) with slight deshielding. The ¹³C NMR of (2) showed the characteristic thioureide signal at 201.09 ppm. β′(H₃)C appeared at 13.99 ppm as a much shielded signal and the α′-CH₂ appeared highly deshielded at 44.15 ppm compared to complex (1). The ¹³C chemical shifts of the cyclic ring appeared similar in (1) and (2).

Thermogravimetric analysis

Thermograms of [Ga(chmdtc)₃] (1) and [Ga(chedtc)₃] (2) are shown in Figure 1A and B, respectively. Both complexes showed single-stage decompositions, and the thermogravimetric curves obtained for both complexes were almost similar. The percentages within the brackets refer to the moiety lost/residue left behind. The thermal decomposition of [Ga(chmdtc)₃] (1) started by losing the solvent toluene (0.5C₇H₈;% calcd.: 6.8, found: 6.5) in the first stage followed by the loss of chmdtc. The total mass loss corresponded to 78% and the final product of thermal decomposition was Ga₂S₃ (% calcd.: 17.5, found: 17.4). In the second stage, decomposition occurred from 280°C to 320°C due to the loss of the chmdtc ligand. Compound (2) started decomposing by losing toluene (% calcd.: 6.3, found: 5.8) up to 290°C. Considerable mass loss was observed from 290°C to 320°C. The final residue corresponded to Ga₂S₃ (% calcd.: 16.3, found: 16.1). The differential thermal analysis (DTA) curves of complex (1) showed exothermic signals at 290°C and 340°C. The exothermic signal indicates the decay of dithiocarbamates. Thermal analysis confirmed the proposed formulas of the compounds.

Figure 1:

Thermogravimetry-DTA: (A) [Ga(chmdtc)₃] and (B) [Ga(chedtc)₃].

XPS analysis

The survey scan of [Ga(chmdtc)₃] (1) and [Ga(chedtc)₃] (2) (Figure 2A and B) confirmed the presence of gallium, sulfur, nitrogen, and carbon in the samples. Gallium showed two signals corresponding to 2_p electrons at 1120.0(2_p3/2) and 1146.0(2_p1/2) eV in (1) and (2) (Figure 2C and D). The gallium atom showed the binding energy at 1116.7(2 _p3/2) eV (Ghosh et al., 2007; Budz et al., 2009). The experimentally observed binding energies were significantly higher than those reported for the elemental gallium, indicating the increased positive nature of the metal ion in the compounds. Elemental sulfur showed a binding energy corresponding to the removal of S2_p electron at 164.0 eV. In compounds (1) and (2), the signals were observed at 166.5 eV for both compounds (Figure 2E and F). The increase in binding energy is an indication of the loss of electron density from the sulfur atoms to the coordinated gallium. A very highly significant increase in binding energy was observed in the removal of N_1s electrons viz., 403.0 and 403.5 eV in (1) and (2), respectively, compared to the corresponding binding energy of the elemental N_1s electron (398.4 eV). The observation signifies the delocalization of the lone pair of electrons on the nitrogen leading to the thioureide from of the structure. The binding energy of C_1s electron in elemental carbon was 285.0 eV. In the present investigation, corresponding electrons were lost at 287.5 eV for both compounds. The similarity in binding energies indicated similarity in the electronic environments of carbon atoms in both compounds.

Figure 2:

XPS: (A and B) survey scans of [Ga(chmdtc)₃] and [Ga(chedtc)₃], (C and D) 2_p3/2 electrons at 1120.0 eV and 2_p1/2 electrons at 1146.0 eV, and (E and F) S2_p electron at 166.5 eV in (1) and (2).

Structural analysis

Crystal data, data collection, and refinement parameters of complex (2) are given in Table 1. Selected bond parameters are given in Table 2. The ORTEP diagram of complex (2) is shown in Figure 3. In complex (2), the gallium center is coordinated by six sulfur atoms of three dithiocarbamate ligands. Four formula units are present in a unit cell. The compound is monomeric. Compound (2) crystallized with a toluene moiety, which was used for crystallization. A significant short contact of 2.855 Å was observed between H15B (cyclohexyl ring) and the S1 atom. Ga-S bonds and the associated C-S bonds showed asymmetry [Ga-S: 2.4164(12)–2.4481(12) Å and C-S: 1.714(4)–1.731(4) Å] as a requirement of packing. The mean Ga-S bond distance in the compound is 2.4362(13) Å. The complex shows distorted octahedral geometry due to its bite angle variations. The mean S-Ga-S and S-C-S angles are 73.64(2)° and 115.8(2)°, respectively. The thioureide bond shows a mean value of 1.332(4) Å and indicates a partial double-bonded nature of the C-N bond. The nature of the substituents associated with the dithiocarbamates does not alter the coordination environment around the gallium center to a large extent because of its relatively big ionic radius (76 nm; Andrews et al., 2000; Dutta et al., 2002; Goshal and Jain, 2007).

Figure 3:

ORTEP of [Ga(chedtc)₃] (2).

40% probability ellipsoids were used for the presentation.

Continuous shape measure (CShM)

CShM reckons the distance of a given structure from the desired ideal symmetry or from a reference shape (Zabrodsky et al., 1992; Keinan and Avnir, 2000; Alvarez et al., 2002; Ok et al., 2006). The descriptions of geometries such as ‘slightly distorted’ or ‘severely distorted’ relative to the reference polyhedron is qualitative in nature. The extent of distortion of a particular molecular structure from an ideal polyhedron can be quantified by symmetry measures. The CShM approach has been successful in such quantification in dealing with transition metal complexes and metal oxides. The ideal geometries associated with hexacoordinated atoms are ideal octahedron (iOh) or ideal trigonal prism (itp). In the present investigation, the coordination around gallium has been found to be distorted octahedron (dOh) in (2). In the present case, the CShM of GaS₆ core is 2.513. The observed CShM value clearly indicates the closeness of (2) to octahedral geometry.

Bond valence sum (BVS) calculation

BVS calculations proffer a reliable method of assigning formal oxidation states of an atom in a compound from the bond parameters. The method depends on the Rij value for an i-j bond in a compound that is principally ionic (Brown and Altermatt, 1985; Bresse and O’Keefe, 1991; Brown, 2002, 2009). In a compound, the oxidation state of a central atom i bonded to j matches the bond valence, S_ij, and the total valence of the central atom, which is its oxidation state, ∑S_ij=exp[(Ro-R_ij)/b], and Ro is used as reported (Brown and Altermatt, 1985) and R_ij is the experimentally determined bond distance. The constant b is assumed to be 0.37. For compound (2), BVS was calculated by summing up the bond valence contributions from six Ga-S interactions. The BVS value was found to be 2.987. In the present case, the BVS value clearly establishes the formal oxidation state of gallium as +3.0; therefore, a considerable ionic character to the predominantly covalent bonding in the compound is clear (Summers et al., 1994).

Preparation of Ga₂S₃ by nonconventional solvothermal decomposition of [Ga(chmdtc)₃]·0.5C₇H₈ (1)/[Ga(chedtc)₃]·0.5C₇H₈ (2)

A mixture of tris(cyclohexylmethyldithiocarbamato)gallium(III) or tris(cyclohexylethyldithiocarbamato)gallium(III) (1 g) as a clear solution in chloroform (100 mL) was heated with diethylenetriamine (2 mL) at 60°C for 45 min (Scheme 3). The solid black crystalline Ga₂S₃ obtained was separated from chloroform, washed with ether and chloroform in a drop of dodecylamine, and dried in air. The ease of formation of the nanosulfide from (1) was greater than (2).

Scheme 3:

Synthesis of nano-Ga₂S₃.

Powder XRD analysis

The powder XRD pattern of the nano-Ga₂S₃ corresponded to the hexagonal Ga₂S₃ nanocrystals with lattice constants of a=11.120 Å, b= 6.395 Å, and c=7.002 Å, consistent with the JCPDS 16-0500. The signals from (110), (-311), (020), (-112), (310), and (221) planes are characteristic signals of α-Ga₂S₃, which are clearly distinguishable in the wurtzite form of Ga₂S₃. Solvothermally prepared Ga₂S₃ showed sharp intense signals in its powder XRD, which indicates good crystalline nature of the material.

High-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) analysis

The HRTEM and selected area electron diffraction (SAED) of the nano-α-Ga₂S₃ are shown in Figure 4. The partly spherical nature of the particles was confirmed by the TEM micrographs, and the particles are in the range of 50 nm. SAED confirmed the crystalline nature of the nano-sized α-Ga₂S₃. The SAED pattern can be indexed to the (110) and (-311) planes, confirming the wurtzite phase of α-Ga₂S₃ nanoparticles. The HRTEM images of α-Ga₂S₃ display fringes with interlayer spacing in the order of 0.50±0.01 nm, equal to the lattice spacing of the (110) planes of hexagonal α-Ga₂S₃ nanocrystals. The corresponding EDX (Figure 5) indicates that the nanoparticles consisted of Ga and S in the ratio of 2:3.

Figure 4:

TEM-SAED patterns of nano-α-Ga₂S₃ (100 and 10 nm).

Figure 5:

EDX of α-Ga₂S₃.

Reagents and equipment

Gallium(III) chloride (Sigma-Aldrich, India; assay: 99.99%) and the cyclohexylmethyl amine and cyclohexylethyl amine (Sigma-Aldrich; assay: 98%) were used as supplied without further purification. IR spectra were recorded on an Avatar Nicolet Fourier transform IR spectrometer (range, 4000–400 cm^-1; USA) as KBr pellets of the compounds. Electronic spectra were recorded in CCl₄ on a Hitachi U-2001 double-beam spectrometer (Tokyo, Japan). Thermal analysis was carried out with NETZSCH STA 449F3 (Germany) instrument under nitrogen atmosphere with a heating rate of 20 K min^-1. ¹H (400 MHz; CDCl₃; Me₄Si) and ¹³C NMR (100.6 MHz; CDCl₃) spectra were recorded on a Bruker AMX-400 spectrometer (USA) at room temperature using CDCl₃ as a solvent. The powder diffraction data were collected in the 2θ range=2–80° using Bruker D8 X-ray diffractometer (USA) equipped with CuKα radiation at fixed current and potential. The scan speed and step sizes were 0.05° min^-1 and 0.00657, respectively. The scanning electron micrographs of the samples were recorded with JEOL JSM-5610Lv microscope (Tokyo, Japan). The HRTEM measurements were carried out on JEOL 2100 (field emission) with an accelerating voltage of 200 kV (Tokyo, Japan).

X-ray crystallography

Intensity data were collected at ambient temperature (295 K) on a Bruker APEX-II CCD diffractometer (USA) with graphite monochromated MoKα radiation (λ=0.71073 Å) and were corrected for absorptions with a multiscan technique (Walker and Stuart, 1983; Gluzinski, 1987). The structures were solved by direct methods using SIR97 (Altomare et al., 1999) and were refined by SHELX97 (Sheldrick, 2008). The nonhydrogen atoms were refined anisotropically and all hydrogen atoms were fixed geometrically. Molecular plots were obtained using the ORTEP-3 program (Farrugia, 1999).

Synthesis of the complexes

Supplementary material

CCDC 1025449 contains the supplementary crystallographic data for (2). The data can be obtained free of charge at http://www.ccdc.cam.ac.uk/con-ts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.