Abstract
1,3-Diisobutyl thiourea was synthesized and characterized by single crystal X-ray diffraction. It gives a monoclinic (α = γ = 90 and β ≠ 90) structure with the space group P21/c. The unit cell dimensions are a = 11.5131 (4) Å, b = 9.2355 (3) Å, c = 11.3093 (5) Å, α = 90°, β = 99.569° (2), γ = 90°, V = 1185.78 (8) Å3, and Z = 4. The crystal packing is stabilized by intermolecular (N–H⋯S) hydrogen bonding in the molecules. The optimized geometry and Mullikan's charges of the said molecule calculated with the help of DFT using B3LYP-6-311G model support the crystal structure.
1. Introduction
Thiourea derivatives are well known sulphur containing compounds acting as ligands in the field of coordination chemistry [1–3]. Such ligands are capable of affording complexes with metal ions; the resultant complexes have been reported to have imperative antimicrobial [4] and anticonvulsant [5] applications. Thiourea derivatives and their complexes have numerous catalytic applications in asymmetric organocatalysis [6–8], have been used as cocatalysts in Pauson-Khand reactions [1], Pd-catalyzed reactions [1, 9], Heck and Suzuki coupling reactions, and so forth [2, 9]. Thiourea derivatives got much more attention in the field of highly enantio- and diastereoselective catalysis [10] and have also been applied as cocatalysts in nanoparticles and a number of other types of reactions [11, 12]. The challenging task in the chemistry of thiourea is its syntheses and structural characterization. Several research groups are trying to make this versatile group of compounds easily accessible and have reported various methods [13–16].
In continuation to our previous work [17] the title compound was obtained unexpectedly. Surprisingly a comprehensive research survey does not show solid state structural data of title compound 1. The structure of the compound was determined by X-ray diffraction and its DFT optimized geometry was determined using B3LYP-6311G model of theory [18]. Theoretical data obtained for the compound compare well with the experimental data.
2. Experimental
All chemicals, that is, carbon disulphide and isobutyl amine, are commercially available and were used without further purification. Solvents were distilled prior to use.
2.1. Synthesis of 1,3-Di(isobutyl)thiourea
Solution of carbon disulphide (498.0 mmole; 50 mL) was prepared in petroleum ether (100 mL) and was cooled to 0°C in an ice bath, following the literature procedure [16, 17]. An excess amount of isobutylamine (996.0 mmole; 61.42 mL) was slowly and carefully added with constant stirring. The stirring was continued overnight. The solvent and all other volatiles were removed under reduced pressure and the oily compound was dissolved in ethyl acetate. After 2 days needle-like colorless crystals appeared in the solution; they were further allowed for few days to grow well. The same reaction was repeated by mixing equimolar amounts of CS2 and isobutylamine by adopting the same procedure but instead of the expected dithiocarbamate 2, product 1 was exclusively obtained (Scheme 1).
2.2. X-Ray Crystallography
A crystal of suitable dimensions was selected for X-ray structure analysis. The diffraction intensity data were collected on a Bruker kappa APEXII CCD diffractometer using graphite-monochromator Mo-Kα radiation (λ = 0.71073 Å) at ambient temperature. For data collection ω scan and multiscan absorption correction was applied. Final refinement on was carried out by full-matrix least-squares techniques. Structure solution and refinements were accomplished with SHELXL-97 [19] and publCIF [20].
2.3. DFT Calculations
The geometry of title compound (1) was obtained from X-ray crystallographic data. The molecular structure of (1) (C9H20N2S) in ground state is optimized by DFT method including correlation correction using B3LYP-6311G model of theory [18]. Mullikan’s charges were calculated by using the same model of theory; the data so-obtained are given in Table 5 and optimized structure of the compound is given in Figure 2. All calculations were performed by using Gauss-view molecule visualizer program and GAUSSIAN-03 program [18, 21, 22].
3. Results and Discussion
The reaction of primary or secondary amines with carbon disulphide gives dithiocarbamates; the reaction is straightforward and usually proceeds without the formation of side products. We treated isobutylamine with carbon disulphide in petroleum ether and instead of dithiocarbamate derivative 2, compound 1 was exclusively obtained as solid (Scheme 1). The reaction proceeded at ambient temperature without adding a catalyst and excellent yield was achieved. The reaction probably proceeds through formation of isothiocyanate intermediate which reacts further with amine and rearranges to thiourea [16].
Single Crystal Studies. The molecular structure of compound 1 is given in Figure 1 with numbering scheme. It crystallizes in monoclinic crystal system (, ) with the space group P21/c; the crystal packing in unit cell is stabilized by intermolecular N–H⋯S bonding with an average distance of 2.819 Å, which is relatively stronger than the reported one 2.87 Å [23] and is closer to 2.837 Å [24]. The data pertinent to crystal structure determination are summarized in Table 1.
The geometry around nitrogen atom is distorted and cannot be predicted on the basis of hybridization. The distortion can easily be traced out by the involvement of lone pair of electrons on nitrogen in delocalization phenomenon with the π electrons of C=S moiety. The C5–N1 bond distance (1.328 Å) and C5–N2 (1.343 Å) are shorter than average C–N single bond distance (1.499 Å) and similarly that of C=S bond distance (1.698 Å) is longer than the average distance (1.599 Å) reported in literature [25]. These data support partial double bond characters between NC and flow of electron density from sulfur resulting in an elongation in S–C bond.
The elongation of C–S bond may also be explained on the basis of intermolecular hydrogen bonding (data reported in Table 2). The solid state crystal packing and intermolecular interactions are shown in Figure 2. The molecules of the title compound are organized and held together with the help of intermolecular N–H⋯S hydrogen bonding with an average distance 2.819 Å, which falls in the expected range, reported for analogous compounds, that is, 2.87 Å [23], 2.84 Å [26], and 2.837 Å [24]. The experimental and calculated data (for gaseous molecule), DFT (B3LYP/6-311G), are given in Table 3. The calculated and experimental data related to bond angles around N atom are approximately 120° which support the planarity and sp2 hybridization around nitrogen owing to delocalization of electrons in the molecule.
DFT-Studies and Vibrational Spectra Analysis. Experimental data correspond to solid phase while theoretical calculations belong to gaseous phase [22]. DFT calculations are usually performed on a single molecule in the unit cell [27]. DFT calculations for compound 1 were carried out by using the GAUSSIAN-03 program [18, 21, 22]. The geometry of the molecule was optimized by DFT/B3LYP with the 6-311G basis set [18]. The optimized geometry obtained through DFT was compared with crystal structure which supports the crystal structure. The crystallographic and optimized geometric bond lengths and bond angles of compound 1 are given in Tables 3 and 4, respectively.
Mullikan’s charges distribution on atoms of compound (1) is given in Table 5. It can be noticed from the table that negative charge density of nitrogen and terminal carbon appears in the range of −0.660 and −0.508, respectively, which is considerably larger than other atoms bearing negative charges, which shows that nitrogen is donor sites for traditional and terminal carbons are donor sites for nontraditional hydrogen bonding.
The experimental and calculated (B3LYP/6-311G) IR absorption frequency along with their respective intensities are given in Figure 3. The experimental bands are probably assigned comparatively with related molecules [28]. The presence of intermolecular interactions is responsible for the higher value of ν(C=S) stretching at 1227 cm−1. The absorption bands at 3346 and 3237 cm−1 are representing ν(N1–H) and ν(N2–H) stretching for title compound (1); their corresponding quantum chemical calculated ν(N–H) stretching appeared to be shifted to higher frequency. The bands at 2954–2868 cm−1 are assigned to ν(C–H) stretching similar to the calculated one. Strong IR bands in range 1600–1500 cm−1 represent ν(N–H) bending [29]. In addition strong band at 1227 cm−1 corresponds to ν(C=S) stretching. Lower value of ν(C=S) up to 750 cm−1 is also reported [29, 30] but intermolecular C=S⋯H–X interaction strongly affects the C=S stretching [30].
4. Conclusions
Reaction of CS2 and isobutylamine affords the corresponding thiourea instead of dithiocarbamate derivative. The compound could easily be purified with the help of crystallization. The crystal structure studies show the existence of intermolecular N–H⋯S type H-bonding. All structural parameters were also calculated (DFT) and were compared with the experimental data. Solid state and gaseous phase data (bonding and vibration energies) are in close agreement with each other.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work was supported by Higher Education Commission of Pakistan, under NRPU Project no. 20-1488/R&D/09-5432.
Supplementary Materials
Supplementary data include the crystallographic information (cif) file of compound 1, and the detailed list of FTIR data, both experimental and calculated).