English

• 0.   Introduction

  The chemistry of organotin carboxylates continues to be an active research area in recent years because of their versatile structures^[1] as well as significant biological activities^[2], especially anticancer activity^[3-4]. Recently, the synthesis of organotin carboxylates of multifunctional carboxylic acids with additional O, S or N donor groups is drawing more and more attention due to the functionalized carboxylic acids showing variable coordination modes as well as notable bioactivities^[5-10]. Among these functionalized carboxylic acids, pyrazolyl carboxylic acids have attracted a special interest owing to the important biological activity of pyrazole derivatives^[11] as well as the good coordinating ability of pyrazole^[12]. Many organotin carboxylates derived from pyrazolyl carboxylic acids have been synthesized and characterized, which demonstrated fascinating structural features and excellent biological activities^[13-21], such as antitumor^[21] and antifungal activities^[14]. Polycarboxylic acids have been extensively used as multifunctional ligands in designing novel and fascinating supramolecular structures with special properties^[22]. Many organotin polycarboxylates based on di-, tri-and tetracarboxylic acids have been reported in the literatures^[23-28], which display interesting supramolecular architectures. It is known that the structures and biochemical behaviors of organotin carboxylates significantly depend on the type of the substituents attached to the tin atom and also on the kind of carboxylate ligands^[3-4]. As an extension of our investigations on biologically active organotin derivatives^[10], we herein report the synthesis, structure and antitumor activity in vitro of organotin 1, 1'-methylenebis(1H-pyrazole-4-carboxylates).

  1.   Experimental

  IR spectra were obtained from a Tensor 27 spectrometer as KBr discs. NMR spectra were obtained with a Bruker 400 spectrometer, and the chemical shifts were reported with respect to reference standards (internal SiMe₄ for ¹H and ¹³C NMR spectra, external SnMe₄ for ¹¹⁹Sn NMR). Elemental analyses were carried out on an Elementar Vario EL analyzer. HR mass spectra were carried out on a Varian QFT-ESI spectrometer. Melting points were measured with an X-4 digital micro melting-point apparatus and were uncorrected. Ethyl 1H-pyrazole-4-carboxylate and organotin reagents are commercially available and used as received without further purification.

  1.1   Synthesis of CH₂(PzCO₂H)₂ (L)

  Powdery potassium hydroxide (85%, 3.29 g, 50 mmol) was added to the solution of ethyl 1H-pyrazole-4-carboxylate (7 g, 50 mmol) in CH₃CN (100 mL). The reaction mixture was stirred and heated to reflux, then the solution of CH₂Br₂ (4.35 g, 25 mmol) in CH₃CN (20 mL) was added dropwise. After the completion of addition, the reaction mixture was continuously stirred and refluxed for 6 h. The solvent was removed in vacuo to give a white solid, which was dissolved in the mixed solvent of CH₂Cl₂ (50 mL) and H₂O (50 mL). The organic phase was separated, washed with H₂O (2×30 mL) and dried over Na₂SO₄. Removing the solvent, the residual was recrystallized from hexane to afford white crystals of diethyl 1, 1'-methylenebis(1H-pyrazole-4-carboxylate). Yield:68% (5 g).

  Diethyl 1, 1'-methylenebis(1H-pyrazole-4-carboxy-late) (2.34 g, 8 mmol) was added to the solution of NaOH (3.20 g, 80 mmol) in H₂O (100 mL). The reaction mixture was stirred and heated at reflux for 4 h. After cooling to room temperature, the pH value was adjusted to 5 by dilute hydrochloric acid (3 mol·L^-1) to give white solids, which were filtered off, washed with H₂O and dried in air to afford 1, 1'-methylenebis(1H-pyrazole-4-carboxylic acid) (L). Yield:90% (1.69 g). ¹H NMR (DMSO-d₆):δ 6.52 (s, 2H), 7.88 (s, 2H), 8.57 (s, 2H), 12.58 (s, 2H). ¹³C NMR (DMSO-d₆):δ 65.0, 116.4, 135.1, 142.2, 163.9. IR (cm^-1):ν(OH) 3 110~ 2 574 (br), ν(C=O) 1 697. HRMS (ESI, m/z):235.047 0 (Calcd. for C₉H₇N₄O₄:235.047 3, [M-H]^-).

  1.2   Synthesis of CH₂(PzCO₂SnPh₃)₂ (1)

  Compound L (0.12 g, 0.5 mmol) and Ph₃SnOH (0.37 g, 1 mmol) were added to anhydrous benzene (30 mL), and the reaction mixture was stirred and heated at reflux for 6 h. Removing the solvent in vacuo, the residual was recrystallized from benzene/hexane to give colorless crystals of 1, which was dried in vacuo before the characterization work. Yield:82% (0.38 g), m.p. 89~91℃. ¹H NMR (CDCl₃):δ 6.22 (s, 2H), 7.42~7.47 (m, 18H), 7.73~7.76 (m, 12H), 7.95 (s, 2H), 8.15 (s, 2H). ¹³C NMR (CDCl₃):δ 65.7, 117.0, 129.0 (³J(¹³C-^119/117Sn)=63.7 Hz), 130.2, 133.9, 136.9 (²J(¹³C-^119/117Sn)=48.2 Hz), 138.1, 143.3, 168.3. ¹¹⁹Sn NMR (CDCl₃):δ -106.3. IR (cm^-1):ν_as(COO) 1 634, ν_s(COO) 1 378. Anal. Calcd. for C₄₅H₃₆N₄O₄Sn₂(%):C 57.85, H 3.88, N 6.00; Found(%):C 57.43, H 4.25, N 6.34.

  1.3   Synthesis of CH₂(PzCO₂Sn(C₆H₁₁)₃)₂ (2)

  This complex was obtained similarly using tricyclohexyltin hydroxide instead of Ph₃SnOH as described above for 1. Yield:93%, m.p. 164~166℃. ¹H NMR (CDCl₃):δ 1.32~1.41 (m, 18H), 1.68~1.75 (m, 30H), 1.90~2.02 (m, 18H), 6.27 (s, 2H), 7.95 (s, 2H), 8.10 (s, 2H). ¹³C NMR (CDCl₃):δ 26.9, 28.9 (³J(¹³C-^119/117Sn)=64.3 Hz), 31.1 (²J(¹³C-^119/117Sn)=13.9 Hz), 33.9 (¹J(¹³C-^119/117Sn)=337.5, 323.4 Hz), 65.6, 118.6, 133.2, 143.0, 167.0. ¹¹⁹Sn NMR (CDCl₃):δ 19.2. IR (cm^-1): ν_as(COO) 1 644, ν_s(COO) 1 372. Anal. Calcd. for C₄₅H₇₂N₄O₄Sn₂(%):C 55.69, H 7.48, N 5.77; Found(%):C 55.46, H 7.64, N 5.88.

  1.4   Synthesis of CH₂(PzCO₂SnEt₃)₂ (3)

  This complex was obtained similarly using (Et₃Sn)₂O instead of Ph₃SnOH as described above for 1, but in a 1:1 molar ratio. Yield:90%, m.p. 93~95℃. ¹H NMR (CDCl₃):δ 1.20~1.42 (m, 30H), 6.27 (s, 2H), 7.93 (s, 2H), 8.10 (s, 2H). ¹³C NMR (CDCl₃):δ 8.0 (¹J(¹³C-^119/117Sn)=365.8, 349.9 Hz), 9.9 (²J(¹³C-^119/117Sn)=25.1 Hz), 65.7, 118.3, 133.3, 143.9, 167.5. ¹¹⁹Sn NMR (CDCl₃):δ 114.0. IR (cm^-1):ν_as(COO) 1 595, ν_s(COO) 1 375. Anal. Calcd. for C₂₁H₃₆N₄O₄Sn₂(%):C 39.05, H 5.62, N 8.67; Found(%):C 39.31, H 5.36, N 8.46.

  1.5   Synthesis of CH₂(PzCO₂Sn(n-Bu)₃)₂ (4)

  This complex was obtained similarly using (n-Bu₃Sn)₂O instead of Ph₃SnOH as described above for 1, but in a 1:1 molar ratio. Hexane was used as the solvent in recrystallization. Yield:79%, m.p. 73~75℃. ¹H NMR (CDCl₃):δ 0.91 (t, J=7.3 Hz, 18H), 1.28~1.39 (m, 24H), 1.59~1.67 (m, 12H), 6.25 (s, 2H), 7.90 (s, 2H), 8.06 (s, 2H). ¹³C NMR (CDCl₃):δ 13.6, 16.5 (¹J(¹³C-^119/117Sn)=352.7, 341.6 Hz), 27.0 (³J(¹³C-^119/117Sn)=64.5 Hz), 27.8 (²J(¹³C-^119/117Sn)=20.5 Hz), 65.6, 118.5, 133.2, 142.9, 167.3. ¹¹⁹Sn NMR (CDCl₃):δ 115.4. IR (cm^-1):ν_as(COO) 1 593, ν_s(COO) 1 375. Anal. Calcd. for C₃₃H₆₀N₄O₄Sn₂(%):C 48.68, H 7.43, N 6.88; Found(%):C 48.45, H 7.33, N 7.04.

  1.6   Crystal structure determination

  Crystals of 1 and 3 suitable for X-ray analysis were obtained by slowly cooling their hot benzene/hexane solutions. All intensity data were collected with a SuperNova diffractometer using Mo Kα radiation (λ=0.071 073 nm). The structures were solved by direct methods and difference Fourier map using SHELXS of the SHELXTL package and refined with SHELXL^[29] by full-matrix least-squares on F². The SQUEEZE of the PLATON software^[30] was used in the refinement of the structures of 1 and 3. The highly disordered solvents (ca. 1.375 hexane molecules for 1 and 3 according to the number of removed electrons, respectively) in the voids were removed by the SQUEEZE process. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added geometrically and refined with riding model position parameters. A summary of the fundamental crystal data for these two complexes is listed in Table 1.

  表 1

  

  表 1  Crystallographic data and refinement parameters for complexes 1 and 3

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  Complex	1·1.375C6H14	3·1.375C6H14
  Formula	C53.25H55.25N4O4Sn2	C29.25H55.25N4O4Sn2
  Formula weight	1 052.64	764.40
  Crystal size/mm	0.50×0.10×0.05	0.80×0.20×0.20
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/c
  a/nm	0.917 48(2)	1.007 88(5)
  b/nm	1.847 59(4)	2.187 8(2)
  c/nm	2.888 60(6)	1.467 30(9)
  β/(°)	92.089(2)	93.472(5)
  T/K	120.0(1)	150.0(1)
  V/nm3	4.893 3(2)	3.229 5(4)
  Z	4	4
  Dc/(g·cm-3)	1.429	1.572
  θ range/(°)	3.030~25.010	2.933~25.009
  F(000)	2 139	1 563
  μ/mm-1	1.069	1.585
  Measured reflection	18 642	12 419
  Unique reflection (Rint)	8 598 (0.031 3)	5 681 (0.062 1)
  Observed reflection with[I ≥ 2σ(I)]	7 272	3 992
  Parameter	496	286
  GOF	1.047	1.137
  Residual R1, wR2	0.034 8, 0.075 4	0.088 0, 0.234 7

  CCDC:1949278, 1; 1949279, 3.

  2.   Results and discussion

  2.1   Synthesis of complexes 1~4

  1, 1'-Methylenebis(1H-pyrazole-4-carboxylic acid) (L) was synthesized in the hydrolysis of its ethyl ester, which was obtained via the coupling reaction of ethyl 1H-pyrazole-4-carboxylate with CH₂Br₂ under basic condition. Reaction of L with organotin oxide or organotin hydroxide gave organotin 1, 1'-methylenebis(1H-pyrazole-4-carboxylates) (1~4) in good yields (Scheme 1). These complexes showed good soluble in chlorinated solvents. Complexes 1~4 have been chara-cterized by IR and NMR (¹H, ¹³C and ¹¹⁹Sn) spectra as well as elemental analyses. It should be pointed out that the vacuum-dried samples for above-mentioned characterization do not contain any solvent, though the single crystal samples contain partially crystallized solvents.

  Scheme 1

  

  Scheme 1.  Synthesis of complexes 1~ 4

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  In the IR spectra, the strong and broad band in L ascribed to the carboxyl group disappeared in compl-exes 1~4. Moreover, the carbonyl stretching frequencies in complexes 1~4 were significantly lower than that in L. These infrared absorption characteristics indicate the formation of the Sn-O bonds^[31]. In addition, the asymmetric stretching vibrations of the carboxylate group in 1 (1 634 cm^-1) and 2 (1 644 cm^-1) were significantly higher than those in 3 (1 595 cm^-1) and 4 (1 593 cm^-1), revealing that the carboxylate groups in these complexes possibly show different coordination modes^[31-32]. The larger values of Δν(ν_as(COO)-ν_s(COO)) for 1 (256 cm^-1) and 2 (272 cm^-1) compared with those of 3 (220 cm^-1) and 4 (218 cm^-1) imply the monodentate manner of the carboxylate group to the tin atom in 1 and 2 as well as the corresponding bidentate manner in 3 and 4^[31-32]. The ¹H NMR spectra of 1~4 exhibited the expected integral values and chemical shifts. The ¹³C NMR spectra of 2~4 clearly showed the ¹J(¹³C-^119/117Sn) coupling constant, which is consistent with the characteristic of 4-coordinated trialkyltin compounds (325~390 Hz)^[33]. The ¹¹⁹Sn NMR chemical shifts of 1~4 are also compared with those values reported in the corresponding 4-coordinated triphenyltin, tricyclohexy-ltin, triethyltin and tri(n-butyl)tin carboxylates^[1].

  2.2   Crystal structures of complexes 1 and 3

  The molecular structures of 1 and 3 have been confirmed by X-ray crystallography, and presented in Fig. 1 and 2, respectively. The selected bond distances and angles are listed in Table 2. Fig. 1 shows that the carboxylate group exhibits a monodentate coordination mode in 1 as above-mentioned by its IR spectrum, leading to each tin atom with a four-coordinated distorted tetrahedron geometry. The pyrazolyl nitrogen atoms do not coordinate to the tin atoms, possibly owing to the carboxylate groups on the pyrazolyl rings decreasing the donating ability of the pyrazolyl nitrogen atoms^[13]. The non-bond Sn…O distances (Sn1…O2 0.267 4(2) and Sn2…O4 0.284 1(2) nm) are considerably shorter than the sum of the van der Waals radii for the Sn and O atoms of 0.357 nm^[34], suggesting relatively strong non-bonded Sn…O interac-tions in 1. As shown in Fig. 2, the coordination mode of the carboxylate groups in 3 is significantly different from that in 1. The carboxylate groups act as a bridging bidentate ligand in 3, consistent with its IR spectrum. This complex forms a 2D-coordination polymer with 32-membered macrocyclic units through the carboxylate oxygen atoms (Fig. 3). Each tin atom adopts a five-coordinated distorted trigonal bipyramidal geometry with the electronegative oxygen atoms at the apical positions. The covalent Sn-O bond distances in bridging bidentate carboxylate groups, such as Sn2-O2 (0.216 2(9) nm), are shorter than the covalent-coordinated Sn-O bond distances, such as Sn1-O1 (0.245 0(8) nm). However, all these Sn-O bond distances fall in the typical range for triorganotin carboxylates^[1]. It is worthy of note that the polymeric structure of complex 3 in solid would break down in solution owing to the intermolecular weak Sn…O interactions, as indicated by its ¹³C and ¹¹⁹Sn NMR spectra.

  图 1

  

  图 1.  Molecular structure of 1 with 30% probability displacement ellipsoids

  H atoms are omitted for clarity

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  图 2

  

  图 2.  Asymmetrical structural unit of 3 with 30% probability displacement ellipsoids

  H atoms are omitted for clarity

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  图 3

  

  图 3.  Two dimensional supramolecular structure of 3

  H atoms are omitted for clarity; Symmetry codes: ⁱ 1-x, 0.5+y, 1.5-z; ⁱⁱ-x, 0.5+y, 1.5-z

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  表 2

  

  表 2  Selected bond distances (nm) and angles (°) for complexes 1 and 3

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  1
  Sn1-O1	0.207 6(2)	Sn1…O2	0.267 4(2)	Sn2-O3	0.205 2(2)
  Sn2…O4	0.284 1(2)	C1-O1	0.131 3(4)	C1-O2	0.122 8(4)
  C9-O3	0.131 9(4)	C9-O4	0.122 2(4)	C5-N2	0.143 8(4)
  C16-Sn1-O1	95.7(1)	C34-Sn2-O3	113.9(1)	C28-Sn2-C40	113.0(1)
  O1-C1-O2	121.4(3)	O3-C9-O4	121.5(3)	N2-C5-N3	111.8(3)
  3
  Sn1-O1	0.245 0(8)	Sn1-O4i	0.215 5(8)	Sn2-O2	0.216 2(9)
  Sn2-O3ii	0.250 9(8)	C1-O1	0.125 8(17)	C1-O2	0.127 4(14)
  C9-O3	0.122 8(15)	C9-O4	0.128 8(14)	C5-N2	0.148 0(17)
  O1-Sn1-O4i	173.2(3)	C10-Sn1-C12	118.9(7)	O1-C1-O2	121.7(13)
  O2-Sn2-O3ii	172.1(3)	C18-Sn2-C20	117.5(5)	O3-C9-O4	123.9(11)
  Sn1-O1-C1	154.9(8)	Sn2-O2-C1	123.4(9)	N2-C5-N3	109.5(9)
  Symmetry codes: i 1-x, 0.5+y, 1.5-z; ii -x, 0.5+y, 1.5-z.

  2.3   Cytostatic activity evaluation

  The cytotoxic activity of dried complexes 1~4 and the free acid (L) for MCF-7 and A549 cells in vitro was assayed by the MTT method^[35], and the data of IC₅₀ are summarized in Table 3. From these results, it is observed that the free acid has scarcely any activity against MCF-7 and A549 cells, but all complexes display higher activity to these two cells than cisplatin, especially complex 1, which is a promising candidate against these two cells.

  表 3

  

  表 3  IC₅₀ values of complexes 1~4 for MCF-7 and A549 cells μmol·L^-1

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  Compound	MCF-7	A549
  1	0.050	0.082
  2	1.05	1.20
  3	1.09	0.57
  4	1.03	0.61
  L	>100	>100
  Cisplatin	5.4	5

  3.   Conclusions

  In conclusion, four triorganotin 1, 1'-methylenebis(1H-pyrazole-4-carboxylates) have been synthesized and characterized. The crystal structural analyses of two of them reveal that the carboxylate groups show considerably different coordination modes in these complexes. The carboxylate group acts as a mono-dentate ligand in the triphenyltin complex, while a bridging bidentate coordinate mode is observed in the triethyltin complex. Moreover, the triphenyltin complex only shows a dinuclear structure, while the triethyltin complex forms a 2D supramolecular archite-cture with 32-membered macrocyclic units. All comp-lexes, especially the triphenyltin complex, display good cytotoxicities for MCF-7 and A549 cells in vitro.

  
  
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• 

  Scheme 1  Synthesis of complexes 1~ 4

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  图 1  Molecular structure of 1 with 30% probability displacement ellipsoids

  H atoms are omitted for clarity

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  图 2  Asymmetrical structural unit of 3 with 30% probability displacement ellipsoids

  H atoms are omitted for clarity

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  图 3  Two dimensional supramolecular structure of 3

  H atoms are omitted for clarity; Symmetry codes: ⁱ 1-x, 0.5+y, 1.5-z; ⁱⁱ-x, 0.5+y, 1.5-z

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  表 1  Crystallographic data and refinement parameters for complexes 1 and 3

  Complex	1·1.375C6H14	3·1.375C6H14
  Formula	C53.25H55.25N4O4Sn2	C29.25H55.25N4O4Sn2
  Formula weight	1 052.64	764.40
  Crystal size/mm	0.50×0.10×0.05	0.80×0.20×0.20
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/c
  a/nm	0.917 48(2)	1.007 88(5)
  b/nm	1.847 59(4)	2.187 8(2)
  c/nm	2.888 60(6)	1.467 30(9)
  β/(°)	92.089(2)	93.472(5)
  T/K	120.0(1)	150.0(1)
  V/nm3	4.893 3(2)	3.229 5(4)
  Z	4	4
  Dc/(g·cm-3)	1.429	1.572
  θ range/(°)	3.030~25.010	2.933~25.009
  F(000)	2 139	1 563
  μ/mm-1	1.069	1.585
  Measured reflection	18 642	12 419
  Unique reflection (Rint)	8 598 (0.031 3)	5 681 (0.062 1)
  Observed reflection with[I ≥ 2σ(I)]	7 272	3 992
  Parameter	496	286
  GOF	1.047	1.137
  Residual R1, wR2	0.034 8, 0.075 4	0.088 0, 0.234 7

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  表 2  Selected bond distances (nm) and angles (°) for complexes 1 and 3

  1
  Sn1-O1	0.207 6(2)	Sn1…O2	0.267 4(2)	Sn2-O3	0.205 2(2)
  Sn2…O4	0.284 1(2)	C1-O1	0.131 3(4)	C1-O2	0.122 8(4)
  C9-O3	0.131 9(4)	C9-O4	0.122 2(4)	C5-N2	0.143 8(4)
  C16-Sn1-O1	95.7(1)	C34-Sn2-O3	113.9(1)	C28-Sn2-C40	113.0(1)
  O1-C1-O2	121.4(3)	O3-C9-O4	121.5(3)	N2-C5-N3	111.8(3)
  3
  Sn1-O1	0.245 0(8)	Sn1-O4i	0.215 5(8)	Sn2-O2	0.216 2(9)
  Sn2-O3ii	0.250 9(8)	C1-O1	0.125 8(17)	C1-O2	0.127 4(14)
  C9-O3	0.122 8(15)	C9-O4	0.128 8(14)	C5-N2	0.148 0(17)
  O1-Sn1-O4i	173.2(3)	C10-Sn1-C12	118.9(7)	O1-C1-O2	121.7(13)
  O2-Sn2-O3ii	172.1(3)	C18-Sn2-C20	117.5(5)	O3-C9-O4	123.9(11)
  Sn1-O1-C1	154.9(8)	Sn2-O2-C1	123.4(9)	N2-C5-N3	109.5(9)
  Symmetry codes: i 1-x, 0.5+y, 1.5-z; ii -x, 0.5+y, 1.5-z.

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  表 3  IC₅₀ values of complexes 1~4 for MCF-7 and A549 cells μmol·L^-1

  Compound	MCF-7	A549
  1	0.050	0.082
  2	1.05	1.20
  3	1.09	0.57
  4	1.03	0.61
  L	>100	>100
  Cisplatin	5.4	5

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