Four Polyoxonibate-Based Inorganic–Organic Hybrids Assembly from

Article pubs.acs.org/crystal

Four Polyoxonibate-Based Inorganic−Organic Hybrids Assembly from Bicapped Heteropolyoxonibate with Effective Antitumor Activity Ying Zhang,† Jian-Qiang Shen,† Li-Hua Zheng,‡ Zhi-Ming Zhang,*,† Yu-Xin Li,‡ and En-Bo Wang*,† †

Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, 130024 Changchun, Jilin, P. R. China ‡ National Engineering Laboratory for Druggable Gene and Protein Screening, Northeast Normal University, 130024 Changchun, Jilin, P. R. China S Supporting Information *

ABSTRACT: Four novel heteropolyoxonibate-based inorganic−organic hybrids {Cu(en)2}6{GeNb12VIV2O42}·20H2O (1), {Cu(en)2}3K2Na4{GeNb12VIV2O42}·23H2O (2), {Cu(en)2}6{SiNb12VIV2O42}·18H2O (3), and {Cu(en)2}3K2Na4{SiNb12VIV2O42}·19H2O (4) (en = ethanediamine), composed of polyoxoanions [TNb12O40]16− (T = Si and Ge) and [Cu(en)2]2+ building blocks, were successfully synthesized under hydrothermal conditions by reaction of K7HNb6O19·13H2O, Cu(Ac)2·3H2O, Na2VO3, Na2SiO3, or GeO2 and en molecules. Polyoxoanion [TNb12VIV2O42]12− (T = Si and Ge) can be best described as a α-Keggin core [TNb12O40] with two [VO] units capping on its two “opened windows”. Compounds 1 and 3 are both composed of the bicapped heteropolyoxonibate core surrounded by a shell consisting of twelve [Cu(en)2]2+ groups, which represent a promising structural model toward core−shell nanostructures. Compounds 2 and 4 are also composed of a bicapped polyoxoanion [TNb12VIV2O42]12− (T = Si, Ge) decorated by three metal− organic fragments [Cu(en)2]2+, forming a trisupporting polyoxoanion {[Cu(en)2]3[TNb12O42VIV2]}6−. Antitumor, electrochemical study, and UV−vis spectra indicate that compounds 1−4 exhibit effective antitumor activity against SGC7901 cells and HepG2 cells and could keep the structural integrity in this process.

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[Ti12Nb6O44]10−, [Cr2(OH)4Nb10O30]8−, [V4Nb10O40(OH)2]12−, [H6V4Nb6O30]4−, [CuNb11O35H4]9−, and [Cu24(Nb7O22)8H23NaO8]16−.9−11 In comparison, heteropolyniobates (HPONs) are rarely explored and quite difficult to be obtained in high yield. In 2002, the first classic Keggin-type HPON {[Ti2O2][SiNb12O40]}12−, where the Keggin ions [SiNb12O40]16− are linked along the c direction by a [Ti2O2]4− dimer into an infinite one-dimensional (1D) chain, and the lacunary derivative [H2Si4Nb16O56]14−, in which a fusion of Nb16 cage are wrapped around four SiO4 tetrahedra, were successfully recognized.12 After that, the single crystals of Na16[SiNb12O40]·4H2O and Na16[GeNb12O40]·4H2O were also prepared and identified.13 Subsequently, several Keggin-type HPONs [TNb12O40]16−, [Ti2O2][TNb12O40]12−, [Nb2O2][TNb12O40]10− (T = SiIV and GeIV), and [TNb12O40(VO)2]9− (T = PV and VV) were reported,3b,14 while a trivacant α-Keggin ion [(PO2)3PNb9O34]15− decorated by PO2+ units, a Nb16 cage

INTRODUCTION More keen attention of traditional molybdenum- and tungstenbased polyoxometalates (POMs) turns to polyoxoniobates (PONs), owing to their fascinating properties and multiple applications in virology, nuclear waste treatment, the basecatalyzed decomposition of biocontaminants, and photolysis of water to yield molecular hydrogen.1−4 However, in comparison with other traditional POMs, the development of PONs is still in its infancy, since the first Lindqvist-type hexaniobate anion [Nb6O19]8− was structurally described, which was restricted mainly by the narrow working pH range,5 as the well-known [Nb6O19]8− anion could only exist under alkaline conditions and change into Nb2O5 precipitate under acidic conditions.6 Given these difficulties, it is a great challenge for chemists to develop new approaches to extend the PON family. Even so, previous work is quite praiseworthy in isopolyoxoniobate (IPON) field, such as Nyman, Niu, Cronin, Hu, and Wang et al. have reported a series of IPON clusters: [Nb20O54]8−, [H9Nb24O72]15−, [HNb27O76]16−, [H10Nb31O92(CO3)]23−, [Nb32O96H28]4−,7,8 and transition-metal Ti-, Cr-, V-, and Cucontaining derivatives, including [TiNb9O28]7−, [Ti2Nb8O28]8−, © XXXX American Chemical Society

Received: August 12, 2013 Revised: October 8, 2013

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Table 1. Crystal Data and Structure Refinement for 1−4 formula M (g mol−1) λ (Å) T (K) a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (Mg m−3) μ (mm−1) F(000) θ range (deg) limiting indices

GOF on F2 data/res/para R1 [I > 2σ(I)]a wR2 (all data)b a

1

2

3

4

C24H135Cu6Ge N24Nb12O62V2 3423.19 0.71073 293(2) 20.709(4) 20.757(4) 14.872(3) 129.01(3) 4967.5(17) 2 2.289 3.159 3374 3.06−25.00 −24 ≤ h ≤ 24 −24 ≤ k ≤ 24 −16 ≤ l ≤ 17 1.048 4516/116/353 0.0720 0.2012

C12H94Cu3GeK2 N12Na4Nb12O65V2 3097.16 0.71073 293(2) 28.918(6) 27.275(5) 13.638(3) 93.88(3) 10732(4) 4 1.917 2.430 6036 2.99−27.46 −37 ≤ h ≤ 37 −35 ≤ k ≤ 35 −17 ≤ l ≤ 16 0.994 12276/66/553 0.0697 0.2126

C24H138Cu6N24 Nb12O63SiV2 3397.71 0.71073 293(2) 20.746(4) 20.789(4) 14.940(3) 129.11(3) 4999.7(17) 2 2.257 2.857 3360 3.05−27.48 −25 ≤ h ≤ 26 −26 ≤ k ≤ 26 −19 ≤ l ≤ 16 1.010 5846/100/369 0.0623 0.1887

C12H86Cu3K2N12 Na4Nb12O61SiV2 2980.60 0.71073 293(2) 28.942(6) 27.294(6) 13.568(3) 93.87(3) 10693(4) 4 1.851 2.169 5804 3.01−25.00 −34 ≤ h ≤ 33 −32 ≤ k ≤ 32 −14 ≤ l ≤ 16 1.053 9603/120/621 0.0824 0.2180

R1 = ∑||F0| − |FC||/∑|F0|. bwR2 = ∑[w(F02 − FC2)2]/∑[w(F02)2]1/2. stainless steel autoclave (23 mL), which was heated at 165 °C for 72 h and then cooled to room temperature at a rate of 10 °C/h. Purple block-like crystals of 1 were obtained in a yield of 72% (based on Nb). Elemental Anal. Calcd (found %) for 1: Cu, 11.14 (10.93); Nb, 32.57 (32.42); Ge, 2.12 (2.21); V, 2.98 (3.13). Synthesis of 2. Cu(Ac)2·3H2O (0.06 g, 0.3 mmol) and NaAc (0.04 g, 0.5 mmol) were added to 2 mL of distilled water with stirring. The resulting blue solution was added dropwise to a 4 mL aqueous solution containing K7HNb6O19·13H2O (0.137 g, 0.1 mmol), NaVO3 (0.024g, 0.2 mmol), and GeO2 (0.011 g, 0.1 mmol). Then, 2 mL of en was added dropwise to the mixture. Subsequently, the pH value of the mixture was adjusted to 12.5 using 2 M KOH solution and the mixture was transferred to a Teflon-lined stainless steel autoclave (23 mL). The Teflon-lined stainless steel autoclave was heated at 165 °C for 72 h and was then cooled to room temperature at a rate of 10 °C/h. Purple block-like crystals of 2 were obtained in a yield of 53% (based on Nb). Elemental Anal. Calcd (found %) for 2: K, 2.52 (2.39); Na, 2.97 (2.82); Cu, 6.16 (6.02); Nb, 36.00 (35.81); Ge, 2.35 (2.19); V, 3.29 (3.46). Synthesis of 3. Cu(Ac)2·3H2O (0.04 g, 0.2 mmol) was added to 2 mL of distilled water with stirring. The resulting blue solution was added dropwise to a 4 mL aqueous solution containing K7HNb6O19· 13H2O (0.137 g, 0.1 mmol), NaVO3 (0.024g, 0.2 mmol), and Na2SiO3·9H2O (0.028 g, 0.1 mmol). Then, 1 mL of en was added dropwise to the mixture. Subsequently, the pH value of the mixture was adjusted to 11.3 with a 4 M NaOH solution and the mixture was transferred to a Teflon-lined stainless steel autoclave (23 mL). The Teflon-lined stainless steel autoclave was heated at 165 °C for 72 h and then cooled to room temperature at a rate of 10 °C/h. Purple block-like crystals of 3 were obtained in a yield of 75% (based on Nb). Elemental Anal. Calcd (found %) for 3: Cu, 11.40 (11.07); Nb, 33.34 (32.61); Si, 0.84 (0.88); V, 3.05 (3.17). Synthesis of 4. Cu(Ac)2·3H2O (0.06 g, 0.3 mmol) and NaAc (0.04 g, 0.5 mmol) were added to 2 mL of distilled water with stirring. The resulting blue solution was added dropwise to a 4 mL aqueous solution containing K7HNb6O19·13H2O (0.137 g, 0.1 mmol), NaVO3 (0.024g, 0.2 mmol), and Na2SiO3·9H2O (0.028 g, 0.23 mmol). Then, 2 mL of en was added dropwise to the mixture. Subsequently, the pH value of the mixture was adjusted to 11.8 by using 2 M KOH solution. Then the mixture was transferred to a Teflon-lined stainless steel autoclave (23 mL). The Teflon-lined stainless steel autoclave was

capturing four GeO4 tetrahedra, and the C-type clusters [TNb18O54] (T = Si, Ga, or Al) were also recorded.15−17 Inspired by the prominent work, we considered that vanadium, possessing similar atomic radii and coordination numbers with niobium, may be a candidate for construction of Nb/V hybrid HPONs. Herein, we report the synthesis, structure, electrochemistry, and antitumor activity of four HPON-based inorganic−organic hybrids materials {Cu(en)2}6{GeNb12VIV2O42}·20H2O (1), {Cu(en)2}3K2Na4{GeNb12VIV2O42}·23H2O (2), {Cu(en)2}6{SiNb12VIV2O42}·18H2O (3), and {Cu(en) 2 } 3 K 2 Na 4 {SiNb 12 V I V 2 O 42 }·19H 2 O (4). The four compounds are all synthesized in high yields and contain a bicapped polyoxoanion [TNb12VIV2O40]12− (T = Si and Ge), which represent the first vanadium-capped TIV-centered HPON clusters. Interestingly, the bicapped polyoxoanion [TNb12VIV2O40]12− in 1 and 3 are surrounded by twelve Cu− organic groups, supplying a structural model for core−shell nanostructures.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All chemicals were commercially purchased and used without further purification. K7HNb6O19·13H2O was prepared according to the literature and identified by IR spectra.18 Elemental analysis for Nb, Cu, Na, K, Ge, Si, and V were determined by a Leaman inductively coupled plasma (ICP) spectrometer. IR spectra were recorded in the range of 400− 4000 cm−1 on an Alpha Centaurt FT-IR spectrophotometer, using KBr pellets. TG analyses were performed on a Perkin−Elmer TGA7 instrument in flowing N2, with a heating rate of 10 °C/min. UV−vis absorption spectra were recorded on a 756 CRT UV−vis spectrophotometer. Synthesis of 1. Cu(Ac)2·3H2O (0.04 g, 0.2 mmol) was added to 2 mL of distilled water with stirring. The resulting blue solution was added dropwise to a 4 mL aqueous solution containing K7HNb6O19· 13H2O (0.137 g, 0.1 mmol), NaVO3 (0.024g, 0.2 mmol), and GeO2 (0.011 g, 0.1 mmol). Then, 1 mL of en was added dropwise to the mixture and the pH value of the mixture was adjusted to 12.0 with 4 M NaOH solution. The mixture was transferred to a Teflon-lined B

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heated at 165 °C for 72 h and was then cooled to room temperature at a rate of 10 °C/h. Purple block-like crystals of 4 were obtained in a yield of 65% (based on Nb). Elemental Anal. Calcd (found %) for 4: K, 2.58 (2.47); Na, 3.04 (2.95); Cu, 6.30 (6.26); Nb, 36.85 (37.21); Si, 0.93 (0.89); V, 3.37 (3.61). X−ray Crystallography. The crystallographic data of compounds 1−4 were performed on a Rigaku R-AXIS RAPID IP diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Suitable crystals were encapsulated into the glass capillary using silicone grease and transferred to the goniostat. The structures were solved by the direct method and refined by the full-matrix least-squares method on F2, using SHELXTL-97.19 During the refinement, all H atoms on water molecules were directly included in the molecular formula. Hydrogen atoms of organic ligands were fixed in the calculated positions. A summary of crystal data and structure refinement for compounds 1−4 is listed in Table 1. Crystallographic data has been deposited with the Cambridge Crystallography Data Centre (CCDC) as deposition numbers CCDC 931077, 931076, 931079, and 931078 for 1−4, respectively. The data can be obtained free of charge from the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. via fax (+44 1223 336033) or e-mail ([email protected]). Electrochemical Materials and Methods. Triply distilled water was used throughout the experiments. Solutions were deaerated by pure argon bubbling prior to the experiments. A CHI 660 electrochemical workstation was used for the electrochemical measurements. A conventional three-electrode system was used. The working electrode was a glassy carbon (GC) electrode, the reference electrode was the Hg/Hg2Cl2 electrode, and a platinum wire was used as a counter electrode. All potentials were measured and reported versus Hg/Hg2Cl2, and all experiments were carried out at room temperature (20−25 °C). Tumor Cells Culture and Treatment. Human gastric cancer SGC7901 cells and human liver carcinoma HepG2 cells were cultured in RPMI-1640 medium (Sigma), containing 10% serum with the concentration of 0.8−1 × 105 cells per milliliters. Two kinds of cells were placed into a 96-well plate (100 μL per well), respectively, which are cultured at 37 °C in humid air atmosphere, containing 5% CO2 for 24 h. Then, the used culture medium was separated out and the 1 mg/ mL drug solution (obtained by dissolving 20 mg of compounds 1−4 in 20 mL of 0.2 M KCl aqueous solution) was added into the 96-well plate. The drug concentration was diluted by 3% RPMI-1640 medium into 100, 10, 1, and 0.1 μg/mL, and the control group contains tantamount KCl with 3% RPMI-1640 medium. The system was further cultured for 48 h. Each experiment was performed three times. Then, 20 μL of MTT (2.5 mg/mL in PBS) was added in the culture system, which is further cultured at the same conditions for another 4 h. The used culture medium was separated out, and 100 μL of DMSO were added to each culture pore, which are oscillated for 10 min. The MTT−formazan product dissolved in DMSO was estimated by measuring absorbance at 570 nm with a micro plate reader. Then the inhibitory percentage of each compound at various concentrations was calculated, and the IC50 value was determined through SPSS.20

were obtained by reaction of K7HNb6O19·13H2O, en, GeO2, or Na2SiO3 and Cu(Ac)2·3H2O (0.04 g) at the pH of 12.0 and 11.3, respectively. The in situ formation of the [Cu(en)2]2+ units surround the bicapped Keggin HPON clusters [TNb12VIV2O40]12− (T = Si and Ge), forming the inorganic− organic hybrids materials. In the synthesis, compounds 2 and 4 could not be isolated from this reaction system if sodium acetate was not added in this reaction system. Further, many parallel experiments reveal that compounds 1−4 could be obtained in the pH range of 11.3−12.5, reaching the highest yield at 12.0, 12.5, 11.3, and 11.8 for 1−4, respectively. If the pH value was out of this range, it is difficult to prepare the title compounds. Also, compounds 1−4 could not be synthesized by using Nb2O5 as the starting material to replace the K7HNb6O19· 13H2O precursors in equal mole of Nb atoms owing to the chemical inertness and insolubility of amorphous Nb2O5.15a,c Structure Description. Single-crystal X-ray diffraction analyses reveals that compound 1 crystallizes in the monoclinic space group C2/m and contains a typical Keggin-type structure [GeNb12O40] with two [VO] units capping on its two “opened windows” (Figure 1a). In polyoxoanion [GeNb12V2O42], the

Figure 1. (a) Structure of the bicapped polyoxoanion [GeNb12V2O42]12− in 1; (b and c) are the arrangement of the Cu(en)2 groups around the [GeNb12V2O42]12− core. Color codes: red spheres, O; red polyhedra, NbO6; yellow spheres and polyhedra V, green spheres and polyhedra, Ge; light blue spheres, Cu.

[GeNb12O40] exhibits the classical α-Keggin geometry of a tetrahedral arrangement of four triads, each composed of three edge-sharing NbO6 octahedra. The triply bridged oxygen of each triad bonded to the central GeO4 tetrahedron, the Ge−O distances are in the range of 1.73(2)−1.808(19) Å, and the O− Ge−O angles are in the range of 67.5(9)−180.0(13) Å. As shown in Figure 1a, two [VO] units capped upon the two opposite faces of the Keggin polyoxoanion [GeNb12O40]. The V center is in a square pyramidal coordination environment completed by four oxygen atoms from the HPON anion and a terminal oxygen atom. As a result, there are five types of oxygen atoms in the polyoxoanion [GeNb12V2O42], with two kinds of terminal oxygen atoms: Nb−Ot and V−Ot, the bridging μ2oxygen atoms: Nb−μ2−O−Nb, the bridging μ3−oxygen atoms: Nb−μ3−O−V, and central oxygen atoms: Ge−O. The bond lengths of Nb−Ot, Nb−μ−O and V−Ot in [GeNb12V2O42] are in the range of 1.711(11)−1.751(8) Å, 1.865(11)−2.44(2) Å, and 1.613(11)−1.951(8) Å, respectively. Interestingly, twelve [Cu(en)2]2+ fragments surround around a Keggin-type anion through static interactions and the hydrogen bond interactions

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RESULTS AND DISCUSSION Synthesis. In the last decades, the hydrothermal method has proven to be a powerful synthetic technique in making novel POM-based materials.21 In this paper, a series of interesting HPONs [Cu(en)2]n[TNb12O42VIV2] (T = Si and Ge) were obtained by reaction of K7HNb6O19·13H2O, Na2SiO3 or GeO2, Cu(Ac)2·3H2O, Na2VO3, and en molecules under the hydrothermal condition. It is worth noting that the heteroatoms were introduced into these four HPONs by using the inorganic raw materials Na2SiO3 or GeO2 instead of organic reagents tetraethylorthosilicate and tetraethoxygermane.17a,12,13 It might suggest useful information for the preparation of new HPONs. In the synthesis of compounds 1 and 3, the isolated bicapped Keggin HPON clusters [TNb12VIV2O40]12− (T = Si and Ge) C

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atom is 50%, which is common in crystallography.11b In the unit cell, three [Cu(en)2]2+ fragments decorated the Keggin anion [GeNb12V2O42] via the terminal oxygen atoms of the HPON cluster forming a trisupporting {[GeNb12O42V2][Cu(en)2]3}6− polyoxoanion (Figure 3, panels a and b). The Cu−O bond lengths are 2.336(8) Å and 2.375(6) Å for Cu1−O24 and Cu2−O21, respectively. The Cu2 center possesses obvious distorted octahedral coordination geometry with four N atoms derived from two en ligands, one O atom originated from the [GeNb12V2O42] cluster and one solvent water molecule. Meanwhile, the Cu1 center exhibits a square−pyramid geometry with four N atoms from two en ligands and one O atom from the Keggin-type anion cluster. The bond distances of Cu−N and Cu−O are in the range of 2.004(9)−2.032(10) Å and 2.336(8)−2.375(6) Å, respectively. Interestingly, two K+ ions are jointed together via two μ2-Ow molecules to form a dimeric K2 cluster, which is further used as the linker to bridge the Keggin HPON anions into a 1D chain at the vertical direction against the V−V axis (Figure 3c). The K+ ions in 2 are in a nine-coordinated environment, and the K−O bond lengths are in the range of 2.793(9)−3.10(4) Å. Further, the 1D chains in 2 can be assembled into an infinitely extended threedimensional (3D) network through Na+ bridges. Structural analysis shows that both compounds 3 and 4 possess a similar bicapped [SiNb12V2O42] Keggin cluster, and they are isostructural with 1 and 2, respectively (Figure S2 of the Supporting Information). The Si−O band lengths of 3 and 4 are in the range of 1.633(12)−1.659(13) Å and 1.639(12)− 1.651(12) Å, respectively; the V−O band lengths of 3 and 4 fall in the range of 1.624(7)−1.943(8) Å and 1.598(11)−2.172(12) Å, respectively, and the Nb−O in these two compounds are in the range of 1.731(9)−2.538(9) Å and 1.754(9)−2.440(8) Å, respectively. Bond valence sum26 indicates that no protonation occurs on the surface of [TNb12VIV2O42] moieties, and all the Cu, V, and Nb centers in the four compounds are in the +2, +4, and +5 oxidation states. FT-IR Spectra and UV−vis Spectra. The IR spectra of 1− 4 display similar characteristic vibration patterns, resulting from the bicapped Keggin structure in the region of 400−1000 cm−1 (Figure S3 of the Supporting Information).18 The terminal M = Ot (M = V and Nb) vibrations appear at 920 and 861 cm−1 for 1, 916 and 854 cm−1 for 2, 986 and 925 cm−1 for 3, and 982 and 923 cm−1 for 4, respectively. The characteristic peaks at 678, 620, and 434 cm−1 for 1, 679, 619, and 433 cm−1 for 2, 687, 619, 523, and 475 cm−1 for 3, and 673, 521, and 472 cm−1 for 4 are assigned to the bridging M−Ob−M vibrations, respectively. Bands at 751, 743, 822, and 814 cm−1 are assigned to Ge (Si)−Oc vibration of 1−4.14b,17b In addition, the occurrence of characteristic vibrations between 1040−1490 cm−1 confirms the presence of en ligands.27 The UV−vis spectra of 1−4 were measured in 0.2 M KCl solution at room temperature. They display three absorption bands centered at about 239, 346, and 544 nm for 1, 244, 346, and 552 nm for 2, 241, 344, and 548 nm for 3, and 245, 341, and 524 nm for 4 (Figure 4). The higher energy absorption bands (239 nm for 1, 244 nm for 2, 241 nm for 3, and 245 nm for 4) are tentatively assigned to the O → M (M = Nb or V) charge-transfer transitions,28 whereas the lower energy band (346 nm for 1, 346 nm for 2, 344 nm for 3, and 341 nm for 4) is attributed to ligand-to-metal (N → CuII) charge-transfer.29 The absorption bands in visible region (544 nm for 1, 552 nm for 2, 548 nm for 3, and 524 nm for 4) results from the CuII d− d transitions. At the same time, the UV−vis spectra in solid

into a structural model for core−shell nanostructures (Figure 1, panels b and c); well-known, core−shell nanostructures are one of the hottest topics in current materials science and chemistry as a result of the ease of their structure/property tuning and their great potential applications in adsorption, separation, catalysis and photonics et al.22 In this field, POMs, a class of nanosized clusters with controllable architecture and large negative charges, have been widely used as a core or template to construct the novel materials.23,24 So, the synthesis of the single crystal of 1 and 3 might supply a straightforward way to get a reasonable model for core−shell nanostructures.25 Further, the core−shell nanostructures were further connected by the Hbonding interactions into a three-dimensional supramolecular framework (Figure 2 and Figure S1 of the Supporting Information).

Figure 2. The 3D supermolecular structure of 1, viewed along different directions.

As shown in Figure 3, compound 2 is also composed of the bicapped Keggin anion [GeNb12V2O42] modified by three

Figure 3. (a) Ball and stick and (b) polyhedral representation of the polyoxoanion [GeNb12V2O42]12− in 2. (c) Mixed polyhedral and balland-stick representation of the 1D chain structure in 2.

metal−organic [Cu(en)2]2+ units via the Nb−O−Cu bonds. The anion structure in 2 is similar to that of 1, which was described as a α-Keggin core [GeNb12O40] with [VO] capping units. The GeO4 tetrahedron located in the center of cluster with the Ge−O bond lengths of 1.733(8), 1.734(5), 1.734(5), and 1.742(8) Å, and all the Nb atoms exhibit the NbO6 octahedral configuration with the bond lengths of Nb−O in the range of 1.758(6)−2.392(6) Å. The two units of [VO] occupy four empty “window” face-to-face due to the existence of positional disorder; the site occupancy of each vanadium D

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the redox peaks of Nb and V were observed but also the peaks of Cu were clearly observed in the potential range from −1.0 to +1.0 V (Figure 5). As shown in Figure 5, the redox processes I−I′ with E1/2 = −0.588 V, −0.610 V, −0.645 V, and −0.616 V(vs Hg/Hg2Cl2), respectively, for compounds 1−4, correspond to the redox of the V centers in the polyoxoanions.30 The oxidation peaks appeared at −0.313, −0.326, −0.302, and −0.308 V, and their reduction counterparts located at −0.576, −0.576, −0.612, and −0.604 V (II−II′, vs Hg/Hg2Cl2), respectively, for compounds 1−4, attributed to the redox processes of Nb centers in these HPON anions.11a For compound 1, two reduction peaks appear at −0.0100 and −0.210 V, and their oxidation counterpart is located at +0.0380 V. The characteristic shape usually encountered for the redox processes of the Cu2+ in the POM frameworks and the pattern features the two-step reduction process of Cu2+ to Cu0 through Cu+.17b,31 Similarly, in the CV curves of 2−4, the oxidation waves located at +0.0700 V for 2, +0.131 V for 3, and +0.0946 V for 4 and their counterparts of two reduction peaks located at −0.301 and −0.003 V for 2, −0.359 and −0.0115 V for 3, and −0.369 and −0.0115 V for 4 were also detected. The aqueous solution stabilities of PONs 1−4 have been investigated by UV−vis spectrum and CV in 0.2 M KCl solution (Figure S7 of the Supporting Information). The UV−vis spectra of the solutions were monitored in every 24 h and completely detected five times. These solutions can be well-stored, and no distinct changes were observed in their UV−vis spectra. Further, the CV behaviors of the above solutions were checked simultaneously and no obvious voltammetric characteristics have ever changed in the CV curves at 100 mV s−1. These

Figure 4. The UV−vis spectra of compounds 1−4 in aqueous solution.

state were also detected and provided in Figure S4 of the Supporting Information. Electrochemistry. 0.2 M KCl solution was used for the electrochemical measurement for compounds 1−4 and the K7HNb6O19 precursor. As shown in Figure S6 of the Supporting Information, the redox peaks of K7HNb6O19 were observed with the E1/2 = (Epa + Epc)/2 = (− 0.248 − 0.443)/2 = −0.346 V (vs Hg/Hg2Cl2) at 100 mVs−1. For compounds 1− 4, the cyclic voltammetric (CV) behaviors are similar; not only

Figure 5. (a−d) CV curves of compounds 1−4 with the concentration of 1.0 × 10−4 M in a pH = 0.2 M KCl solution at the scan rate of 100 mV s−1. A glassy carbon (GC) electrode was used as a working electrode, a platinum wire served as the counter electrode, and a Hg/Hg2Cl2 electrode served as the reference electrode. E

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results can confirm that PONs 1−4 are structurally stable in the 0.2 M KCl solution. Antitumor Activity. Taking into account that the POMs significantly exhibit biological activities,1a,4c,32 the ability of 1−4 for inhibiting tumor cell growth were assessed by inhibition rate and the IC50 values (50% inhibitory concentration) were calculated by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5diphenyl tetrazolium bromide] assay in vitro screening (Figure 6 and Figure S8 and Table S1 of the Supporting Information).

Article

ASSOCIATED CONTENT

S Supporting Information *

Additional structural figures and details of characterization data; crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Natural Science Foundation of China (Grant 21101022).

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Figure 6. The antitumor activities of compounds 1−4 against the human gastric cancer SGC7901 cells and human liver carcinoma HepG2 cells line.

It turns out that compounds 1−4 have a similar inhibitory effect on HepG2 cells (human liver carcinoma tumor cells) and SGC7901 cells (human gastric cancer cells) and have a certain amount of activity relationship based on the dose of the title compounds. The inhibition rates are 90.25%, 83.05%, 83.4%, and 83.4% against HepG2 cells, 82.8%, 73.3%, 83.8%, and 78.85% against SGC7901 cells, respectively, with the dose of 1−4 100 μg/mL (Figure S8 of the Supporting Information), indicating the effective antitumor activity of 1−4. IC50 value is another evaluation criterion for the ability of cell apoptosis induced by drug. Figure 5 shows that 1−4 could kill the HepG2 and SGC7901 cells, and the inhibition ability of 4 against HepG2 and SGC7901 cells is more superior than 1, 2, and 3 (IC50: 9.93 ± 1.83 μg/mL against HepG2 cells and 9.53 ± 2.53 μg/mL against SGC7901 cells (Table S1 of the Supporting Information). At the same time, the control group containing tantamount [Cu(en)2]2+ is operated under the same experimental conditions. The average inhibition rate of tantamount metal−organic fragment are 22.17 ± 1.41 (%) against HepG2 cells and 20.80 ± 0.21 (%) against SGC7901 cells. Therefore, it can be concluded that these clusters play a major role for the antitumor activity of 1−4 against SGC7901 and HepG2 cells.

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CONCLUSION

In summary, four vanadium-capped HPON compounds have been successfully synthesized. Polyoxoanion [TNb12VIV2O40]12− (T = Si and Ge) represents the first vanadium-capped TIV-centered HPON cluster. 1 and 3 are reasonable core−shell nanostructure models, while 2 and 4 are typical supporting frames, enriching the structural diversity of HPON chemistry. Furthermore, antitumor study indicates that 1−4 have high antitumor activity against SGC7901 cells and HepG2 cells. F

dx.doi.org/10.1021/cg401227g | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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dx.doi.org/10.1021/cg401227g | Cryst. Growth Des. XXXX, XXX, XXX−XXX