XIONG Xin-Xing LI Di ZHOU Jun-Li XU Bao-Ming HUANG Qin WANG Long-Sheng（Institute of POM-based Materials， School of Chemistry and Chemical Engineering，Hubei University of Technology， Wuhan 430068， China）

Syntheses, Characterizations and Crystal Structures of (E)-2-Methyl-1,3-diphenylisothiouronium Iodide and 1,3-Dibenzyl-2-methylisothiouronium Iodide①

XIONG Xin-Xing LI Di ZHOU Jun-Li XU Bao-Ming HUANG Qin WANG Long-Sheng②
（Institute of POM-based Materials， School of Chemistry and Chemical Engineering，Hubei University of Technology， Wuhan 430068， China）

Two thiouronium iodide salts， （E）-2-methyl-1，3-diphenylisothiouronium iodide （1）and 1，3-dibenzyl-2-methylisothiouronium iodide （2）， were synthesized and characterized by single-crystal X-ray diffraction，1H NMR， IR， UV-vis and luminescence spectra. Single-crystal X-ray diffraction reveals that compound 1 crystallizes in monoclinic system， P21/n space group with a = 11.273（2）， b = 10.318（2）， c = 12.962（3） Å， β = 99.03（3）°， V = 1489.0（5） Å3and Z = 4；compound 2 crystallizes in monoclinic system， C2/c space group with a = 11.706（2）， b = 14.119（3），c = 20.930（4） Å， β = 103.84（3）°， V = 3358.8（12） Å3and Z = 8. The thiouroniums of both compounds are Y-shaped planar structures to form a cross-conjugated system. The averaged bond lengths of C-N and C-S indicate the presence of “Y” aromaticity. The researches on UV-vis and luminescence spectra of compounds 1 and 2 indicate that they can form inclusion complex with β -CD in the aqueous solution.

organosulfur, thiouronium, "Y" aromaticity, hydrogen bond, β-CD;

1 INTRODUCTION

Organosulfur compounds are widely distributed in natural world， which not only play an important role in the life process， but also have important potentials and applications in the fields of materials，medicine， catalysts and organic transformations［1， 2］. As one class of organosulfur compounds， thiouronium salts were one kind of important synthetic intermediates in the conversion of alkyl halide to the corresponding mercaptans or the conversion of thiourea to the corresponding guanidinium salts［3， 4］. On the other side， thiouronium salts bearing different organic groups can be used as chemical sensors to recognize different anions owing to their unique and adjustable cation structures［5， 6］. A thiouronium receptor containing naphthalene moiety was found to show different fluorescent responses depending on different anions［7］. Boerrigter et al. reported a series of thiourea resorcinarene cavitands which allow the transportation of halides anions through the liquid membrane［8］. Gunaratne et al. reported a series of chiral thiouronium salts，which can be used for chiral discrimination of oxoanions， e.g. mandelate［9］. Moreover， thiouronium salts can be utilized as stabilisers for vul-canized rubber［10］， and chelating agents for noble metals［11］. However， only a few structures of thiouronium salts have been reported up to now［12， 13］.

During our researches on carboximidamide， two new thiouronium iodide salts， （E）-2-methyl-1，3-diphenzyl-2-methylisothiouronium iodide （1） and 1，3-dibenzyl-2-methylisothiouronium iodide （2），were obtained. Herein， we report their syntheses，crystal structures and primary assembly researches with β-CD.

2 EXPERIMENTAL

2. 1 Materials and measurements

1，3-Diphenylthiourea and 1，3-dibenzylthiourea were prepared according to a literature procedure［14］and characterized by IR and melt point. β-CD was recrystallized three times using distilled water before use. All the other chemicals were of analytical reagent grade and used as received without further purification. The crystal structures were determined with a Bruker Smart APEX CCD diffractometer. Elemental analysis was performed by （Elementar An alysensysteme GmbH） Vario EL.1H NMR spectra were recorded in deuterated DMSO using Bruker Advance 400 NMR. IR spectra were recorded within the 400～4000 cm-1region on a Perkin Elmer Spectrum 100 FTIR Spectrometer. UV spectra were conducted on Shimadzu UV Probe 2700. Luminescence spectra were collected on a Cary Eclipse Spectrofluorometer.

2. 2 Synthesis of compound 1

To a white slurry of 1，3-diphenylthiourea （1.14 g，5 mmol） in 10 mL alcohol， 0.315 mL CH3I was added and the mixture was stirred for 24 h at room temperature to form a yellow solution， which was refluxed for 10 h and cooled to room temperature. 30 mL ethyl ether was added to the solution and the mixture was stored at -20 ℃. Two days later， lots of colorless block crystals were obtained by filtration and washed with ethyl ether and dried in air. Weight 1.26 g， yield 68.6%. Elemental Anal. for C14H15N2IS: Calcd. （%）: C， 45.41； H， 4.08； N， 7.57. Found （%）: C， 45.70； H， 4.02； N， 7.63. IR（KBr pellet， cm-1）: 3420（w）， 3174（w）， 2981（w）， 1643（w），1425（m）， 1305（w）， 1184（w）， 939（s）， 920（m）， 906（s），845（s）， 974（w）， 687（s）.1H NMR（DMSO-d6， 300K）: 7.17～7.32 （m， 10H， -ArH）， 2.35（s， 3H， -SH3）.

2. 3 Synthesis of compound 2

To a white slurry of 1，3-dibenzylthiourea （1.28 g， 5 mmol） in 10 mL alcohol， 0.315 mL CH3I was added and stirred for 24 h at room temperature to form a yellow solution， then the mixture was refluxed for 10 h and cooled to room temperature. To the yellow solution， 30 mL ethyl ether was added and stored at -20 ℃. Two days later， a lot of colorless block crystals were obtained by filtration and washed with ethyl ether and dried in air. Weight 1.56 g， yield 78.4%. Elemental anal. for C16H19N2IS: Calcd. （%）: C， 48.25； H， 4.81； N， 7.03. Found （%）: C， 48.32； H， 4.75； N， 7.11. IR（KBr pellet， cm-1）: 3201（w）， 3087（w）， 2991（w）， 1481（m）， 1434（m），1391（m）， 938（s）， 908（shoulder）， 896（s）， 838（s），730（m）， 709（d）， 693（d）， 656（w）.1H NMR（DMSO-d6， 300K）: 9.80（s， 1H， -NH）， 9.37（s，1H， -NH）， 7.19～7.39 （m， 10H， -ArH）， 4.72（s， 2H，-CH2）， 4.61 （s， 2H， -CH2）， 2.71（s， 3H， -SCH3）.

2. 4 Structure determination

Single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a Bruker SMART APEX CCD diffractometer with graphite-monochromated MoKα radiation （λ = 0.071073 nm） at low temperature of 150（2） K. Both structures were solved by direct methods using the SHELXS program and refined by full-matrix least-squares methods with SHELXL［15-17］. Non-hydrogen atoms were refined with anisotropic thermal parameters on F2. Hydrogen atoms were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. Complex 1 is of monoclinic system， P21/n space group with a = 11.273（2），b = 10.318（2）， c = 12.962（3）Å， β = 99.03（3）º， V = 1489.0（5） Ǻ3， Z = 4， μ = 2.276 mm-1， reflections collected/unique data 8373/2632， crystal size 0.12mm × 0.10mm × 0.05mm， scan mode φ-ω，θ range for data collection 2.23～25.05o， Rint=0.0513， S = 1.019， Ra= 0.0413 and wRb= 0.0933. Complex 2 is of monoclinic system， C2/c space group with a = 11.706（2）， b = 14.119（3）， c = 20.930（4） Å， β = 103.84（3）°， V = 3358.8（12） Å3， Z = 8， μ = 2.024 mm-1， reflections collected/unique data 9467/2916， crystal size 0.21mm × 0.20mm × 0.18mm， scan mode φ-ω， θ range for data collection 2.00～24.93， Rint= 0.0243， S = 1.032， Ra= 0.0200 and wRb= 0.0507 （aR = Σ（||Fo| -|Fc||）/Σ|Fo|，bwR = ［Σw（|Fo|2- |Fc|2）2/Σw（Fo2）2］1/2）. Selected bond lengths and bond angles for complexes 1 and 2 are listed in Table 1. Hydrogen bond lengths （Ǻ） and bond angles （o） of both compounds are summarized in Table 2.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

Single-crystal X-ray structural determination reveals that compound 1 crystallizes in monoclinic system， P21/n space group. As shown in Fig. 1，there are one anion of iodide and one cation of 1-（methylthio）-N，N΄-diphenylmethanediamium in the asymmetric unit. In the cation of 1-（methylthio）-N，N΄-diphenylmethanediamium， two phenyl ligands adopt a nonclockwise configuration， namely，one phenyl ring does not stand on the clockwise position of 2-methylisothiouronium （Scheme 1）. C（1） is one sp2hybridized carbon atom. C（1）， N（1），N（2） and S（1） are sited on the same plane with the mean deviation of 0.0011 Å. The C（1）-N（1）（1.340（7） Å） and C（1）-N（2） （1.309（7） Å） bonds are longer than the C=N double bond （1.260 Å）， but shorter than the C-N single bond （1.460 Å）. And the bond length of C（1）-S（1） （1.750（6） Å） is longer than that of the C=S double bond （1.60 Å）， but shorter than the value for C-S single bond （1.80 Å）. As for compound 2， it crystallizes in monoclinic system， C2/c space group. There are one anion of iodide and one cation of 1-（methylthio）-N，N΄-dibenzylmethanediamium in the asymmetric unit （Fig. 2）. In the cation of compound 2， two benzyl ligands in 2-methylisothiouronium adopt the clockwise configuration， namely， the three organic ligands stand in the clockwise direction of2-methylisothiouronium （Scheme 1）， which are different from that in compound 1. This is because two more flexible benzyl groups are bound to the thiouronium cation in compound 2， which can adopt a more stable clockwise configuration. C（1） is one sp2hybridized carbon atom similar to that in compound 1， and C（1）， N（1）， N（2） and S（1） are located on the same plane with the mean deviation of 0.0416 Å. The bond lengths of C（1）-N（1） （1.318Å） and C（1）-N（2） （1.324 Å） are longer than that of the C=N double bond （1.260 Å）， but shorter than that of the C-N single bond （1.460 Å）. The C（1）-S（1） bond in 1.756 Å is longer than the C=S double bond in 1.60 Å， but shorter than the C-S single bond （1.80 Å）.

Scheme 1. Nonclockwise configuration of compound 1 and clockwise configuration of compound 2

Fig. 1. ORTEP diagrams of (E)-2-methyl-1,3-diphenylisothiouronium iodide (1) (top) and 1,3-dibenzyl-2-methylisothiouronium iodide (2) (bottom) with thermal ellipsoids at 50% level

Fig. 2. Ball-stick viewing of hydrogen bonds and π··π interaction of compound 1 which are indicated by dashed lines(Dumb atom: ring centre; Symmetry codes: A: x + 1/2, –y + 1/2, z + 1/2; B: 1/2 + x, 1/2 – y, –1/2 + z, C: 2 – x, –y, 2 – z)

The unshared pair electrons of two nitrogen atoms and sulfur atoms in both compounds are conjugated with the coplanar empty p orbital of the central sp2hybridized carbon atom to form a closed cross-conjugated system. Those cross-conjugated molecules， ions or radicals demonstrate some unusual chemical and physical properties. For example， guanidine is the most strong organic base （pKa 13.6， and pKa of OH-is 13 for comparison）and guanidinium cation is even stable in the boiling water［18］. To explain the extraordinary stability of guanidine and its derivatives， Pauling had estimated the resonance energy of guanidine （～47 kal/mol）and guanidium （53～55 kal/mol） using bond valence theory， which is even bigger than that of benzene （30～35 cal/mol）［19］. Peter Gund proposed the concept of “Y-Declocalization” and “Y-Aromaticity” to explain the exceptional stability of cross-conjugated guanidine and trimethylenemethane based on the HMO studies［20］. Gernot Frenking proved that the “Y-shaped” trimethylenemethane dianion is more stable than the linear forms because of a more favorable charge distribution［21］. Joel M. Karty discovered that the “Y-shaped”trimethylenemethane dication is more stable than the linear butadienyl dication owing to its higher resonance energy， which further proves the presence of Y-Aromaticity［22］. Besides the high delocalization energy of cross-conjugated system，the three bonds connected to the central atoms in the cross-conjugated molecules have the averaged bond lengths， for example， the bond lengths of three C-N bonds in the guanidium cation are 1.322， 1.323，and 1.326 Å， respectively［23］； and bond lengths of three C-O bonds in CO32-are 1.279 Å［24］. The averaged bond lengths of C-N and C-S in compounds 1 and 2 indicate a strong π electron delocalization in the cations of 1，3-dibenzyl-2-methylisothiouronium and （E）-2-methyl-1，3-diphenzyl-2-methylisothiouronium， which possibly suggests the presence of "Y" aromaticity in both compounds and needs more theoretical studies to prove it in the future.

Hydrogen bonds and π··π interaction play an important role in the packing of both compounds. The distances between I（1A） （A: x + 1/2， -y + 1/2， z + 1/2） and H（1A）， I（1） and H（2A） in compound 1 are 2.730（4） and 2.670（4） Å， respectively. And the distances between I（1（A）） （A: -x + 1/2， y - 1/2， -z + 3/2） and H（2A）， I（1） and H（1A） in compound 2 are 2.730（2） and 2.950（2） Å， respectively. They are all less than the sum of van de Waal radii of iodine and hydrogen atoms （3.30 Å）， indicating the presence of hydrogen bonds between I and N. In compound 1， the cation of 1-（methylthio）-N，N΄-diphenylmethanediamium and the anion of iodideare connected into a wavelike infinite chain via two hydrogen bonds of N（1）-H（1A）··I（1A） （2.730 Å，162.1°） and N（2）-H（2A）··I（1） （2.670 Å， 173.1°）， as shown in Fig. 2. The ring centre separation of Cg（1）and Cg（2） in the 1-（methylthio）-N，N΄-diphenylme-thanediamium is 3.819 Å， indicating the presence of intramolecular π··π interaction，which renders the thiouronium to adopt a nonclockwise configuration； the distance between （H15A（B）） and the ring centre of Cg（1） is 3.029 Å，suggesting the existence of intermolecular π··π interaction of C（15（B））- H（15A（B））··π （B: 1/2 + x，1/2 - y， -1/2 + z）， which further stabilizes the 1D infinite chain. The ring centre separation of Cg（1）and Cg（2（C）） （C: 2 - x， - y， 2 - z） is 3.850 Å， indicating the presence of intermolecular π··π interactions which connect these chains into a 2D layer.

In compound 2， the distance of C（12）-H（12A）··Cg（2） is 3.124 Å， indicating the presence of intramolecular π··π interaction. These cations of compound 2 are connected into a 1D infinite chain via intermolecular π··π interaction of C（15（B））-H15（A（B））··Cg（2） （3.329 Å） （B: 1/2 + x， 1/2 + y， z）. These chains are further connected into a 2D network via two hydrogen bonds of N（1）-H（1A）··I（1） （2.950 Å， 132.6°） and N（2）-H（2A）··I（1（A）） （2.730 Å， 147.4°） （A: 1/2-x， -1/2 + y， 3/2 - z ） （Fig. 3）

Fig. 3. Ball-stick viewing of hydrogen bonds and π··π interaction of compound 2 which are indicated by dashed lines(Dumb atom: ring centre； Symmetry codes: A: 1/2 – x, –1/2 + y, 3/2 – z; B: 1/2 + x, 1/2 + y, z + 1/2)

3. 2 UV-vis and luminescence spectra

In the aqueous solution of compound 1 with concentration of 1.0 × 10-5mol·L-1， compound 1 demonstrates a maximum absorption band ca. 251 nm. The absorption intensity of compound 1 shows slight decrease in the presence of β-CD， and the maximum absorption band shifted bathochromically to 253 nm （Fig. 4）. Compound 1 has an intense emission with the maximum peak of 273 nm and a weak emission band around 300 nm upon the excitation of 250 nm. With increasing the concentration of β-CD， the peak intensity of 273 nm has no obvious change， but the peak intensity of the second emission band around 306 nm demonstrates an obvious enhancement. The third emission band around 360 nm appeared and became strong when increasing the concentration of β-CD.

In the aqueous solution with concentration of 2.5 × 10-5mol·L-1， compound 2 demonstrates a maximum absorption band ca. 218 nm. The intensity of this band shows a slight decrease in the presence of β-CD （Fig. 5）. Compound 2 has a weak emission with the maximum peak ca. 280 nm upon the excitation of 220 nm. The emission intensity of compound 2 increased greatly with the increase of β-CD concentration， and the maximum emission peak of 2 shifted to 300 nm. The primary UV and luminescent study of compounds 1 and 2 indicatethat they could be included into β-CD to form inclusion compounds.

Fig. 4. UV-vis (top) and luminescence spectra (bottom) of compound 1 in distilled water: I (compound 1, 1 × 10-5mol·L-1); II (compound 1, 1 × 10-5mol·L-1; β-CD 1 × 10-5mol·L-1); III (compound 1, 2.4 × 10-3mol·L-1; β-CD 4.81 × 10-3mol·L-1)

Fig. 5. UV-vis (top) and luminescence spectra (bottom) of compound 2 in distilled water: I(Compound 2, 2.5 × 10-5mol·L-1); II (Compound 2, 2.5 × 10-5mol·L-1; β-CD 8 × 10-3mol·L-1); III (Compound 2, 2.5 × 10-5mol·L-1; β-CD 1.6 × 10-2mol·L-1)

4 CONCLUSION

We had synthesized two thiouronium iodide salts via the nucleophilic substitution reaction of corresponding sulfocarbamide and methyl iodide. The cation of （E）-2-methyl-1，3-diphenylisothiouronium adopts a nonclockwsie configuration owing to the intermolecular π··π interaction， and the cation of 1，3-dibenzyl-2-methylisothiouronium iodide adopts a clockwise configuration. Both thiouroniums have the typical feature of “Y” aromaticity considering the averaged bond lengths of C-N and C-S. The luminescence intensity of compounds 1 and 2 becomes strong with increasing the concentration of β-CD， so they can form inclusion compounds with β-CD.

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Received 17 November 2014； accepted 6 March 2015 （CCDC 1048232 for 1 and 1048233 for 2）

① The authors thank the financial support of the National Natural Science Foundation of China （No. 21101062， 21271068， 21203059），Educational Commission of Hubei Province （No. Q21021402）， Chutian Scholar Fund of Hubei Province （No. 4032401），and National Training Programs of Innovation and Entrepreneurship for Undergraduates （No. 201210500012）

② Corresponding author. E-mail: wangls@mail.hbut.edu.cn

10.14102/j.cnki.0254-5861.2011-0577