K₂SbB₃O₈: A Novel Boroantimonate with Isolated [B₃O₈]^7- Groups

1. INTRODUCTION

Organotin compounds have been widely used in industry, agriculture, medicine and other fields due to their variable structures and rich properties. Especially, their potential values^[1-6] in high-efficiency anti-cancer drug have been found, because many organotin compounds have extremely high broad-spectrum anti-cancer activities, which are much higher than cisplatin widely used as the current anti-cancer drugs. Therefore, the organotin compounds are expected as a type of anti-cancer drug in the future. However, many researches show the structure and performance of the organotin complexes are determined by the structures of alkyl connected to the tin atom and diverse ligands^[7-10]. Furthermore, the functional ligands can change the coordination modes of the tin atom, affect the biological activity of the organotin compound, and regulate the balance between its toxicity and biological activity. Some organic compounds containing many hetero-atoms including N, S are important bioactive ligands, which can be explained as follows: firstly, the hetero-atoms have strong coordination capabilities and flexible coordination mode to build different structural organic tin compounds; secondly, the ligands have special biological activities and the formation of organic tin can significantly enhance their biological activities. Because N, N-dialkyl-dithiocarbamonic acid is one of the above-mentioned ligands, many studies have focus on the organic tin compounds constructed by such important ligands in recent years^[11-15]. To investigate more systematical ligands, the tri(o-bromobenzyl)tin diethyldithiocarbamate (1) and tri(m-fluorobenzyl)tin pyrrolidine dithiocarbamate (2) have successfully been synthesized and characterized by elemental analysis, IR spectroscopy, NMR (¹H, ¹³C and ¹¹⁹Sn) and single-crystal X-ray diffraction in the work. The structures were optimized with ab initio calculations to study the stability of the compound, and the molecular orbital energy and composition characteristics of some frontier molecular orbital were also discussed. In addition, the thermal stabilities and anti-cancer activities in vitro of complexes were investigated.

2. EXPERIMENTAL

2.1 Reagents and instruments

The synthesis reactions were carried out in the microwave organic synthesis system (Micro-SYNTL-ab station for Microwave assisted, Italy). The IR spectra were recorded with a Japan Shimadzu FTIR-8700 infrared spectrometer in the 4000~400 cm^–1 region using KBr pellets. The elemental analysis was determined by PE-2400 (Ⅱ) elemental analyzer. Crystallographic data of the complexes were collected on a Bruker SMART APEX Ⅱ CCD diffractometer. The melting points were measured using an X4 digital microscopic melting point apparatus without correction. The nuclear magnetic data were determined with Avance Ⅲ HD 500MHz Fully Digital Superconductive Nuclear Magnetic Resonance Spectrometer (Switzerland, TMS for the inner standard). The thermogravimetric analyses were executed on a TG209F3 thermogravimetric analyzer. All reagents were analytically pure.

2.2 Synthesis of complexes

Complex 1  Tri(o-bromobenzyl) bromine tin (0.708 g, 1.0 mmol) and diethyldithiocarbamate sodium (0.225 g, 1.0 mmol) were dissolved in methanol (40.0 mL), and the solution was transferred into the Teflon microwave reaction tank which was then sealed and heated at 120 ℃ under microwave radiation for 2.0 h. After the solution was slowly cooled down to room temperature, filtered, and the partial solvent was removed by rotary evaporation and white solid was deposited. The solid recrystallizes with CH₂Cl₂-methanol, obtaining colorless transparent crystals of complex 1 with 0.526 g (yield 67.69%). Melting point: 72~73 ℃. Anal. Calcd. (%) for C₂₆H₂₈Br₃NS₂Sn: C, 40.19; H, 3.63; N, 1.80. Found (%): C, 41.03; H, 3.58; N, 1.84. IR (KBr, cm^–1): 3049, 2980, 2932 v(C–H), 1140 v_as(CS₂), 1014 v_as(CS₂), 561 v(Sn–C), 428ν(Sn–S). ¹H NMR(CDCl₃, 500 MHz), δ(ppm): 7.50~7.45 (m, 3H), 7.11~7.08 (m, 3H), 7.00~6.98 (m, 3H), 6.91~6.87 (m, 3H), 3.76 (q, J = 7.0 Hz, 4H), 2.86 (s, 6H), 1.24 (t, J = 7.0 Hz, 6H). ¹³C NMR (CDCl₃, 125 MHz), δ(ppm): 196.40, 141.38, 132.21, 130.36, 127.20, 125.77, 123.73, 50.11, 31.32, 12.11. ¹¹⁹Sn NMR (CDCl₃, 186 MHz), δ(ppm): –81.14.

Complex 2  In accordance with the above method, the mixture was only replaced with tri(m-fluorobenzyl)chlorine tin(0.481 g, 1.0 mmol) and pyrrolidine dithiocarbamate sodium (0.205 g, 1.0 mmol), and colorless transparent crystals of 2 were obtained with 0.386 g (yield 65.20%). Melting point: 84~86 ℃. Anal. Calcd. (%) for C₂₆H₂₆F₃NS₂Sn: C, 52.72; H, 4.42; N, 2.36. Found (%): C, 52.58; H, 4.45; N, 2.40. IR (KBr, cm^–1): 3055, 2963, 2872 v(C–H), 1136 v_as(CS₂), 997 v_s (CS₂), 517 (Sn–C), 432 (Sn–S). ¹H NMR (CDCl₃, 500 MHz), δ (ppm): 7.16~7.12 (m, 3H), 6.75~6.72 (m, 3H), 6.60 (d, J = 7.5 Hz, 3H), 6.50 (d, J = 10 Hz, 3H), 3.72 (t, J = 6.5 Hz, 4H), 2.64 (s, 6H), 2.07 (t, J = 6.5 Hz, 4H). ¹³C NMR (CDCl₃, 125 MHz), δ (ppm): 192.16, 163.03 (J = 244.0 HZ), 143.25 (J = 7.8 HZ), 129.94 (J = 8.6 HZ), 123.52 (J = 2.5 HZ), 114.52 (J = 21.3 HZ), 111.18 (J = 21.0 HZ), 55.11, 26.84, 26.65. ¹¹⁹Sn NMR (CDCl₃, 186 MHz), δ (ppm): –90.40.

2.3 Crystal structure determination

The suitable samples with dimensions of 0.21mm × 0.20mm × 0.18mm (1) and 0.21mm × 0.20mm × 0.15mm (2) were chosen for the crystallographic study and then mounted at 296(2) K on a Bruker SMART APEX Ⅱ CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.071073 nm) with a φ-ω scan mode in the range of 2.43≤θ≤27.57° (1) and 2.93≤θ≤27.50° (2). A total of 17793 (6595 independent, R_int = 0.0285) (for 1) and 15660 (5796 independent, R_int = 0.0160) (for 2) reflections were measured, of which 5213 (for 1) and 5429 (for 2) were observed (Ι > 2σ(Ι)). All the data were corrected by Lp factors and empirical absorbance. The structures were solved by direct methods. All non-hydrogen atoms were determined in successive difference Fourier synthesis, and all hydrogen atoms were added according to theoretical models. All hydrogen and non-hydrogen atoms were refined by their isotropic and anisotropic thermal parameters through full-matrix least-squares techniques. All calculations were completed by the SHELXTL-97 program on the WINGX^[16]. The final R = 0.0434, wR = 0.1152 (1) and R = 0.0313, wR = 0.0934 (2); (Δρ)_max = 2195, (Δρ)_min = –1531 e/nm³ (1) and (Δρ)_max= 1260, (Δρ)_min = –591 e/nm³ (2). The selected bond lengths and bond angles for 1 and 2 are listed in Table 1.

Table 1

Table 1.  Selected Bond Distances (nm) and Bond Angles (°) for 1 and 2

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Complex 1
Bond	Dist.		Bond	Dist.		Bond	Dist.
Sn(1)–C(8)	0.2162(5)		Sn(1)–C(15)	0.2194(5)		Sn(1)–S(2)	0.3103(1)
Sn(1)–C(1)	0.2168(5)		Sn(1)–S(1)	0.2459(1)
Angle	(°)		Angle	(°)		Angle	(°)
C(8)–Sn(1)–C(1)	118.32(19)		C(1)–Sn(1)–C(15)	107.1(2)		C(1)–Sn(1)–S(1)	116.02(16)
C(8)–Sn(1)–C(15)	111.2(2)		C(8)–Sn(1)–S(1)	107.41(15)		C(15)–Sn(1)–S(1)	94.17(17)
Complex 2
Bond	Dist.		Bond	Dist.		Bond	Dist.
Sn(1)–C(8)	0.2148(4)		Sn(1)–C(15)	0.2174(3)		Sn(1)–S(2)	0.3203(1)
Sn(1)–C(1)	0.2162(3)		Sn(1)–S(1)	0.2456(1)
Angle	(°)		Angle	(°)		Angle	(°)
C(8)–Sn(1)–C(1)	115.24(16)		C(1)–Sn(1)–C(15)	102.03(13)		C(1)–Sn(1)–S(1)	116.15(9)
C(8)–Sn(1)–C(15)	111.43(14)		C(8)–Sn(1)–S(1)	110.28(13)		C(15)–Sn(1)–S(1)	100.27(10)

2.4 Anti-tumor activity of the complexes

The Colo 205, HepG2, MCF7, Hela and H460 cells were taken from the U.S. Tissue Culture Library (ATCC) and cultured with RPMI1640 (GIBICO, Invitrogen) containing 10% bovine serum at 37 ℃ in an incubator with 5% (V/V) CO₂. The cell proliferation and growth inhibition were detected in the MTT method. The number of experimental cells was adjusted to the absorbance of 1.3~2.2 at 570 nm, and the compounds with 6 concentrations (0.1 nmol·L^–1~10.0 μmol·L^–1) were set to process cells for 72 h, the IC₅₀ value was determined used the GraphPad Prism5.0 software statistical analysis with at least 3 parallel and 3 repeat slabs per concentration.

3. RESULTS AND DISCUSSION

3.1 Spectral properties

In the IR spectra, the strong bands at 428 (1) and 432 cm^–1 (2) confirmed the formation of Sn–S. The asymmetric (ν_as(CS₂)) and symmetric (ν_s(CS₂)) stretching vibrations of C–S bonds appear 1140 (1), 1136 (2) and 1014 (1), 997 cm^–1 (2) with the Δν (ν_as(CS₂) – ν_s(CS₂)) values 126 (1) and 139 cm^–1 (2), respectively, indicating that dithiocarbamates in 1 and 2 coordinate with the tin atoms with non-homogenous bidentate-chelating modes, which are consistent with the structure of the single-crystal X-ray analyses.

In the ¹H NMR spectra, the multiple absorption peaks presented at 7.50~6.87 (1) and 7.16~6.50 (2), corresponding to the protons on the aromatic ring. The δ-proton peaks on the dithiocarbamatic group connected to the nitrogen atoms appeared at 3.76 (1) and 3.72 (2), respectively. Meanwhile, the absorption peaks of the methylene hydrogen connected to tin were at 2.86 (1) and 2.64 (2), respectively. The absorption peaks methyl hydrogen of 1 and two other methylene hydrogens of 2 belonged to 1.24 and 2.07, respectively. Furthermore, the ratio of the peak area of each proton was basically consistent with one of the number of protons in each group.

In the ¹³C NMR spectra, the absorption peaks of δ-S=C presented at 196.40 (1) and 192.12 (2), and the δ-C on the aromatic ring appeared at 141.38~123.73 (1) and 163.03~111.08 (2), respectively; δ-C of alkyl group was located at 50.11~12.11 (1) and 55.11~26.65 (2), respectively.

In the ¹¹⁹Sn NMR spectra, the absorption peaks presented at –81.14 (1) and –90.40 (2), respectively.

3.2 Crystal structural description

As shown in Fig. 1 and Table 1, the two complexes are mononuclear structures, and the center Sn ion is five-coordinated [SnC₃S₂] by three carbon atoms of methylene and two S atoms of ligand, displaying a distorted trigonal bipyramid. The bond distances and bond angles of Sn–C are different due to the spatial action of ligands and bromobenzyl(fluorobenzyl). The distances of Sn(1)–S(1) with 0.24590 (1) and 0.24558 nm (2) suggest that Sn(1) and S(1) are easy to bond. However, the distances between Sn(1) and S(2) are 0.31029 (1) and 0.32030 nm (2), respectively, which is much smaller than the sum of the Van der waals radius (0.4 nm) of two atoms, indicating that Sn(1) and S(2) can bond but the strength of the bond is relatively weak. Therefore, the thionocarboxylic of ligand in the complexes coordinated with the Sn atoms in non-homogenous bidentate-chelating modes to construct the five-coordinated geometry, which is confirmed in the IR spectra.

Figure 1

Figure 1.  Molecular structures of 1 (a) and 2 (b) (thermal ellipsoids at 15% probability level)

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As shown in Fig. 2 and Table 2, one-dimensional infinite chain structure is formed through weak interactions of C(24ⁱ)–H(24ⁱ)···S(1) and C(4ⁱⁱ)– H(4ⁱⁱ)···π from two adjacent molecules in the crystal of complex 1, and the Cg represents the centroids of benzene rings composed of C(16), C(17), C(18), C(19), C(20) and C(21).

Figure 2

Figure 2.  One-dimensional infinite chain of complex 1 constructed by weak interaction

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Table 2

Table 2.  Hydrogen Bond Data of Compounds 1 and 2

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Complex	D–H···A	d(D–H)/nm	d(H···A)/nm	d(DCA)/nm	∠D–H···A/°
1	C(24i)–H(24i)···S(1)	0.0960	0.2975	0.3849	152
	C(4ii)–H(4ii)···Cg	0.0930	0.2868	0.3781	168
2	C(7)–H(7)···F(1i)	0.0929	0.2540	0.3277	137
	C(25ii)–H(25ii)···F(2)	0.0970	0.2261	0.3169	156
	C(12)–H(12)···F(1iii)	0.0930	0.2594	0.3468	157
Symmetry codes: 1 i 1–x, 1–y, 1–z; ii 1–x, 1–y, –z; 2 i 1+x, y, z; ii 1–x, 1–y, 1–z; iii –x, –y, 1–z

As shown in Fig. 3, there are also abundant hydrogen bonds in the crystals of complex 2. The hydrogen bond data are listed in Table 2, one-dimensional infinite band structure is formed between two adjacent molecules by weak effects of C(25ⁱⁱ)–H(25ⁱⁱ)···F(2) and C(12)–H(12)···F(1ⁱⁱⁱ). The two adjacent bands are further expanded into two-dimensional network structure by C(7)–H(7)···F(1ⁱ).

Figure 3

Figure 3.  Two-dimensional network of compound 2 constructed by hydrogen bond

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3.3 Quantum chemical analysis

According to the atomic coordinates of crystal structure, the molecular orbital energy and the composition of frontier molecular orbital have been elucidated theoretically with Gaussian 03W program using B3lyp/lanl2dz level.

To explore the electron structure and bonding characteristics of the complexes, the molecular orbitals of the compounds were analyzed, and the contributions of the atomic orbital coefficients in the combinations were expressed by the sum of squares and normalized. The atoms of complexes were divided into six parts. Complex 1: (a) Sn atom; (b) C and S atoms of thiocarboxylic acid of ligand L; (c) C of alkyl and N of ligand L1; (d) methylene C atom of benzyl; (e) C and Br atoms of bromophenyl M; (f) H atoms. Complex 2: (a) Sn atom; (b) C and S atoms of thiocarboxylic acid of ligand L; (c) C and N atoms of pyrrole ring L1; (d) C atom of benzylide methyl; (e) C and F atoms of fluorophenyl M; (f) H atoms. The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were calculated and the results are shown in Table 3 and Fig. 4.

Table 3

Table 3.  Some Calculated Frontier Molecular Orbitals Composition of Complexes 1 and 2 (%)

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MO	E	Sn	L	L1	C	M	H
1
114HO	–0.20726	1.00395	57.12734	5.61426	15.88773	18.94138	1.41220
115LU	–0.02589	27.09916	25.23990	9.67591	6.06792	29.14020	2.45320
2
116HO	–0.21273	1.43774	52.49131	6.17040	14.54449	24.39840	0.95026
117LU	–0.03498	35.03867	17.54267	3.55513	6.56192	36.11115	0.95026

Figure 4

Figure 4.  Schematic diagram of frontier MO for complexes 1 (a) and 2 (b)

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It can be seen that the two complexes have similar bonding characteristics: first, the frontier MO for complexes shows the most contribution to molecular orbital is thionocarboxylic groups with 57.12% (1) and 52.49% (2). Moreover, the methylene C atom of benzyl has larger contribution to molecular orbital with 15.89% (1) and 14.54% (2), resulting in stable Sn–C and Sn–S bonds, especially the ligand and the tin atom have better binding; Second, the bromophenyl (fluorophenyl) has a greater contribution to HOMO and LUMO with 18.94% (24.40%) for 1 and 29.14% (36.11%) for 2, respectively, suggesting the bromophenyl (fluorophenyl) has a better conjugate delocalization and stability; Third, comparing various atomic orbital components of HOMO and LUMO, we can find that when electrons are excited from HOMO to LUMO orbitals, the electrons mainly transfer on the thionocarboxylic groups and the methylene C atom of benzyl, while the Sn atoms work as the main recipient of the electrons. Meanwhile, the bromophenyl (fluorophenyl) also received part of the electrons.

3.4 Thermal stability

The thermogravimetric analysis was carried out in air atmosphere from 40 to 700 ℃ at a heating rate of 20 ℃·min^–1 and gas flowing velocity of 20 mL·min^–1. As shown in Fig. 5, complex 1 had no loss before 220 ℃, then the weight loss from 210 to 350 ℃ was rapider than that at 350~540 ℃, and finally the loss stopped until 540 ℃. The total loss was 79.75%, with the final residue to be SnO₂ (obsd. 20.25%, calcd. 19.39%). Complex 2 had no loss until 200 ℃. The weight loss in the range of 200~310 ℃ was rapider than that at 310~550 ℃, and stopped until 550 ℃. The total loss was 74.21%, getting the final residue as SnO₂ (obsd. 25.79%, calcd. 25.44%).

Figure 5

Figure 5.  Thermogravimetric analysis curves of complexes 1 and 2

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3.5 Anti-tumor activity of the complexes

The in vitro growth inhibition activities of the complexes against tumor cells including human colon cancer (Colo205), liver cancer cells (HepG2), breast cancer (MCF-7), cervical cancer cells (Hela) and lung cancer cells (H460) were tested with a cisplatin control, as shown in Table 4. The results suggest that the two complexes have stronger inhibitory activities on human cancer cells than the clinical cisplatin. Therefore, the two complexes can be used as candidate drugs to broad-spectrum anti-cancer.

Table 4

Table 4.  IC₅₀ of Complexes and Cis-platinum on Tumor Cells in Vitro (μmol·L^–1)

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	Colo205	HepG2	MCF-7	Hela	H460
1	0.23	0.47	0.24	0.17	0.38
2	0.09	0.13	0.18	0.17	0.41
Cis-platinum	58.06	65.32	88.17	3.64	1.51

4. CONCLUSION

In summary, the two new organotin complexes, tri(o-bromobenzyl)tin diethyldithiocarbamate and tri(m-fluorobenzyl)tin pyrrolidine dithiocarbamate, have been successfully synthesized with methanol as the solvent in the microwave solvothermal. The in vitro anti-tumor activities of complexes show stronger inhibitory ability on human cancer cells (Colo205, HepG2, MCF-7, Hela and H460) than the clinical cisplatin. The two complexes can be used as candidate drugs to broad-spectrum anti-cancer.