CrIII-Substituted Heteropoly-16-Tungstates [CrIII2(B-Î²-XIVW8O31)2]14

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CrIII-Substituted Heteropoly-16-Tungstates [CrIII2(B‑β‑XIVW8O31)2]14− (X = Si, Ge): Magnetic, Biological, and Electrochemical Studies Wenjing Liu,† Rami Al-Oweini,†,$ Karen Meadows,† Bassem S. Bassil,†,‡ Zhengguo Lin,† Jonathan H. Christian,§ Naresh S. Dalal,§ A. Martin Bossoh,∥,⊥ Israel̈ M. Mbomekallé,∥ Pedro de Oliveira,∥ Jamshed Iqbal,# and Ulrich Kortz*,† †

Jacobs University, Department of Life Sciences and Chemistry, P.O. Box 750561, 28725 Bremen, Germany Department of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box 100, Tripoli, Lebanon § Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States ∥ Laboratoire de Chimie-Physique, UMR8000 CNRS, Université Paris-Sud, Orsay F-91405, France ⊥ Université Félix Houphouët-Boigny, 01 BP V34 Abidjan 01, Ivory Coast # Center for Advanced Drug Research, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan ‡

S Supporting Information *

ABSTRACT: The dichromium(III)-containing heteropoly-16tungstates [Cr III 2 (B-β-Si IV W 8 O 31 ) 2 ] 14− (1) and [Cr III 2 (B-βGeIVW8O31)2]14− (2) were prepared via a one-pot reaction of the composing elements in aqueous, basic medium. Polyanions 1 and 2 represent the first examples of CrIII-containing heteropolytungstates comprising the octatungstate unit {XW8O31} (X = Si, Ge). Magnetic studies demonstrated that, in the solid state, the two polyanions exhibit a weak antiferromagnetic interaction between the two CrIIIcenters with J = −3.5 +/− 0.5 cm−1, with no long-range ordering down to 1.8 K. The ground-state spin of polyanions 1 and 2 was thus deduced to be 0, but the detection of a complex set of EPR signals implies that there are thermally accessible excited states containing unpaired spins resulting from the two S = 3/2 CrIII ions. A comprehensive electrochemistry study on 1 and 2 in solution was performed, and biological tests showed that both polyanions display significant antidiabetic and anticancer activities.

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Cr-containing POMs are known to date.5 In particular, the Lunk group has pioneered this field.5a,c,e−g,o The main obstacle for the development of Cr-POM chemistry is the kinetic inertness of [Cr(OH2)6]3+ toward ligand exchange in aqueous media.6 Nonetheless, two or more paramagnetic CrIII ions incorporated in a POM framework can provide a viable platform for studying magnetic phenomena such as exchange coupling at the molecular level. A survey of Cr-containing POMs is shown in Table 1 along with their respective magnetic behavior. Recently, we reported two mono-CrIII-containing heteropolytungstates, [CrIII(HXVW7O28)2]13− (X = P, As), prepared by direct reaction of the composing elements, which exhibit exceptionally large magnetic anisotropy.5n Using the same synthetic strategy, we have now succeeded in adding two members to the CrIII-containing heteropolytungstate family, which are reported here.

INTRODUCTION Polyoxometalates (POMs) are discrete metal oxides of early transition metals in high oxidation states (e.g., WVI, VV). Lacunary (vacant) POMs can be considered as all-inorganic, polydentate ligands, allowing for incorporation of d-block metal ions, and the resulting derivatives frequently have intriguing properties leading to interest in many different areas such as magnetism, catalysis, medicine, and nanoscience.1 The rational design of fundamentally novel POM structures is difficult, as the formation process is usually affected by various parameters, such as pH, countercations, ionic strength, solvent, temperature, and concentration of reagents.2 In recent decades many novel POM structures were prepared via different synthetic pathways such as conventional vs hydrothermal, ionic liquid, solid state, liquid phase, microwave, photochemical, and electrochemical.3 In general, preformed lacunary POMs are used in the reactions, but composing elements can also be reacted directly with each other. Various examples of sandwich-type POMs with nuclearities of the encapsulated metal ions ranging from 1 to 7 have been prepared over the past few years.4 However, only a handful of © XXXX American Chemical Society

Received: June 17, 2016

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DOI: 10.1021/acs.inorgchem.6b01458 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Chromium(III)-Containing POMs and Coordination Complexes and Their Magnetic Behavior formula

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metal core

magnetic behavior

ref

Cr-Containing Heteropolytungstates [A-α-SiW9O34Cr3(OH)3(H2O)3]4− Cr3 NA [{A-α-SiW9O34Cr3(OH)3}2(OH)3]11‑ 2Cr3 antiferromagnetic [γ-SiW10O36Cr2(OH)(OOCCH3)2(H2O)2]5− Cr2 antiferromagnetic [(γ-SiW10O36)2(Cr(OH)(H2O))3]10‑ Cr3 antiferromagnetic [(γ-SiW10O36)2(Cr(OH)(H2O))3(La(H2O)7)2]5− Cr3 NA [Cr(HXVW7O28)2]13− (X = P, As) Cr paramagnetic (large D) Cr-Containing Heteropolymolybdates [Cr(OH)6Mo6O18]3− Cr antiferromagnetic [Cr2(AsMo7O27)2]12‑ Cr2 antiferromagnetic [CrFe(AsMo7O27)2]12‑ CrFe antiferromagnetic Polynuclear Cr-Containing Coordination Complexes [(CrCl)6O{MeC(CH2O)3}4]2− 6Cr antiferromagnetic

5e 5f 5g 5m 5m 5n 5b, h 5j 5k 5m

corrections were applied. Multiscan absorption corrections were performed using SADABS.8 Direct methods (SHELXS) successfully located the tungsten atoms, and successive Fourier syntheses (SHELXL) revealed the remaining atoms.8 Refinements were fullmatrix least squares against |F|2 using all data. In the final refinement, the heavy atoms (Cr, W, Si, Ge, Na) were refined anisotropically, whereas the O atoms were refined isotropically. Crystallographic data are summarized in Table 2. Electrochemistry. Pure water was obtained from a Milli-Q Integral 5 purification set. All reagents were of high-purity grade and were used as purchased without further purification: H2SO4 (SigmaAldrich), CH3COOH (Glacial, ProlaboNormapur), Li2SO4·H2O (Acros Organics), and LiCH3COO·2H2O (Acros Organics). The compositions of the various media were as follows: for pH 1, 2, and 3, 0.5 M Li2SO4 + H2SO4; for pH 4, 5, and 6, 1.0 M LiCH3COO + CH3COOH. The stability of polyanions 1 and 2 in solution as a function of pH and time was studied by monitoring the evolution of their UV−visible spectra for at least 24 h. Such a duration is long enough for the electrochemical characterization of each compound and its possible application in electrocatalytic processes. Polyanions 1 and 2 were found to be stable in media having pH values ranging from 1 to 7 at least. The UV−visible spectra were recorded on a PerkinElmer 750 spectrophotometer with 10−4 M solutions of the polyanion. Matched 2.00 mm optical path quartz cuvettes were used. Electrochemical data were obtained using an EG&G 273A potentiostat driven by a PC with the M270 software. A onecompartment cell with a standard three-electrode configuration was used for cyclic voltammetry (CV) experiments. The reference electrode was a saturated calomel electrode (SCE), and the counter electrode was a platinum gauze of large surface area; both electrodes were separated from the bulk electrolyte solution via fritted compartments filled with the same electrolyte. The working electrode was a 3 mm diameter edge plane pyrolytic graphite (EPG) disk or a ca. 10 × 10 × 2 mm3 glassy-carbon stick electrode (Mersen, France). The pretreatment of the first type of working electrode before each experiment was adapted from a method described elsewhere.9 The second type of working electrode was polished twice with SiC paper, grit 500 (Struers). After each polishing step, which lasted for about 5 min, the electrode was rinsed and sonicated twice in Millipore water for a total of 10 min. Prior to each experiment, solutions were thoroughly deaerated for at least 30 min with pure Ar. A positive pressure of this gas was maintained during subsequent work. All CVs were recorded at a scan rate of 10 mV s−1, and potentials are quoted against a saturated calomel electrode (SCE) unless stated otherwise. The polyanion concentration was 2 × 10−4 M. All experiments were performed at room temperature, which was controlled and fixed for the laboratory at 20 °C. Results were very reproducible from one experiment to the other, and slight variations observed over successive runs are attributed to the uncertainty associated with the detection limit of our equipment (potentiostat, hardware, and software) and not

EXPERIMENTAL SECTION

Synthesis. All chemicals were commercially available and were used without further purification. Na14[CrIII2(B-β-SiIVW8O31)2]·45H2O (Na-1). A sample of Cr(NO3)3· 9H2O (0.10 g, 0.25 mmol) was dissolved in 20 mL of sodium acetate solution (1 M, pH 6) followed by addition of Na2SiO3 (0.07 g, 0.60 mmol) and Na2WO4·2H2O (1.32 g, 4 mmol). The mixture was stirred for 1 h at 80 °C and then cooled to room temperature, followed by filtration (the pH was 8.7). Slow evaporation of the solution at room temperature led to the formation of green crystals of Na-1 within 3 weeks (yield 0.05 g, 8% based on Cr). IR (cm−1): 981 (w), 935 (m), 866 (s), 809 (m), 751 (s), 671 (w), 530 (m), 438 (w), 404 (w). Anal. Calcd (found): Na, 6.16 (6.14); Si, 1.07 (1.17); W, 56.28 (55.91); Cr, 1.99 (2.11). Mw = 5226.1 g/mol. Na14[CrIII2(B-β-GeIVW8O31)2]·50H2O (Na-2). A sample of Cr(NO3)3· 9H2O (0.10 g, 0.25 mmol) was dissolved in 20 mL of sodium acetate solution (1 M, pH 6) followed by addition of GeO2 (0.05 g, 0.50 mmol) and Na2WO4·2H2O (1.32 g, 4 mmol). The mixture was stirred for 1 h at 60 °C and then cooled to room temperature, followed by filtration (the pH was 8.7). The product Na-2 was obtained as a pure crystalline phase after evaporation of the solution at room temperature within 1 week (yield 0.4 g, 58% based on Cr). This material was dissolved in a minimum amount of sodium acetate solution (1 M, pH 6) at 50 °C. The solution was then cooled to room temperature and kept in a closed vial in the refrigerator (7 °C), leading to dark green crystals of Na-2 suitable for X-ray diffraction within 48 h. IR (cm−1): 936 (m), 778 (s), 732 (m), 657 (w), 505 (w), 458 (m), 420 (w). Anal. Calcd (found): Na, 5.95 (5.79); Ge, 2.69 (2.87); W, 54.42 (54.15); Cr, 1.92 (2.28). Mw = 5405.3 g/mol. Instrumentation. Infrared spectra (Figure S1 in the Supporting Information) were recorded on a Nicolet Avatar 370 FT-IR spectrophotometer using KBr pellets. The following abbreviations were used to assign the peak intensities: w = weak; m = medium; s = strong; br = broad. Thermogravimetric analyses were carried out on a TA Instruments SDT Q600 thermobalance with a 100 mL/min flow of nitrogen; the temperature was ramped from 20 to 800 °C at a rate of 5 °C/min. Elemental analysis was performed by the CNRS, Service Central d’Analyze, Solaize, France. Magnetization data were obtained using a Quantum Design MPMS-XL 7 SQUID magnetometer over a temperature range of 300−1.8 K at a measuring field of 100 G. X-band (9.4 GHz) and Q-band (34.5 GHz) electron paramagnetic resonance (EPR) measurements were performed on a Bruker Elexsys E 500 spectrometer. High magnetic field high-frequency EPR was measured at the EPR facility of the National High Magnetic Field Laboratory in Tallahassee, FL, USA. X-ray Crystallography. Single crystals of Na-1 and Na-2 were mounted on a Hampton cryoloop in light oil for data collection at 100 K. Indexing and data collection were carried out on a Bruker D8 SMART APEX II CCD diffractometer with κ geometry (graphite monochromator, λ(Mo Kα) = 0.71073 Å). Data integration was performed using SAINT. 7 Routine Lorentz and polarization B


Article

Inorganic Chemistry to the working electrode pretreatment or to possible fluctuations in temperature. Biological Materials. All chemicals were purchased commercially and used as received without further purification. p-Nitrophenyl α-Dglucopyranoside (p-NPG), p-nitrophenyl β-D-glucopyranoside (pNPG), α-glucosidase from Saccharomyces cerevisiae, and β-glucosidase from sweet almonds were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Commercially available tissue specific alkaline phosphatase enzyme extracted from calf intestine (c-IAP) was obtained from Calbiochem (Germany), and tissue nonspecific alkaline phosphatase from bovine (b-TNAP) was purchased from Calzyme Laboratories, Inc. (United States). Lung carcinoma (H157) (ATCC CRL-5802) and kidney fibroblast (BHK-21) (ATCC CCL-10) cell lines and Leishmania major (ATCC 30012D) were purchased from ATCC, and African green monkey kidney normal cell line (Vero) (ATCC CCL-81) was acquired from RIKEN Bio Resource Center (Japan). Tris-HCl, MgCl2, ZnCl2, RPMI-1640, fetal bovine serum (FBS; Thermo scientific), (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), D-MEM/F-12 medium (Gibco BRL), glutamine, penicillin, streptomycin, and phosphate-buffered saline (PBS) were purchased from Sigma-Aldrich (Steinheim, Germany). Enzyme Inhibition Assays. Glucosidase Isoenzyme Inhibition Assays. For this purpose, the previously described assay methods of the α-glucosidase10 and β-glucosidase11 enzymes were followed. Briefly, the solutions of α-glucosidase and β-glucosidase enzymes and their substrate p-NPG were prepared in 0.07 M phosphate buffer (pH 6.8). The total assay volume of 100 μL contained 70 μL of assay buffer and 10 μL of the tested compound, followed by the addition of 10 μL of α-glucosidase (0.25 U/well) or 10 μL of β-glucosidase (0.2 U/well). The mixture was preincubated for 5 min at 37 °C, and the absorbance was measured at 405 nm as preread using a microplate reader (Bio-Tek Instruments, Inc. ELx 800TM, Winooski, VT, USA). The reaction was initiated by the addition of 10 μL of p-NPG (1 mM/ well) to each well, and the assay mixture was further incubated at 37 °C for 30 min. The reaction was stopped by adding 80 μL of 0.2 M Na2CO3 solution. The change in absorbance was measured as afterread. The percent inhibition of each compound was calculated by comparing the results with total activity control (having no inhibitor at all). The percent inhibition was calculated using the equation

showed ≥50% inhibition of either b-TNAP activity or c-IAP activity, full concentration inhibition curves were produced to extract IC50 values. For this purpose, seven to nine serial dilutions of each compound spanning 3 orders of magnitude were prepared in an assay buffer and their dose−response curves were obtained by adopting the same methods used for the initial screening. All experiments were performed in triplicate. The Cheng−Prusoff equation was used to calculate the Ki values from the IC50 values, determined by the nonlinear curve-fitting program PRISM 5.0 (GraphPad, San Diego, CA, USA). Anticancer Activity. Cell Lines and Cell Cultures. Lung carcinoma (H157) (ATCC CRL-5802) and kidney fibroblast (BHK21) (ATCC CCL-10) cell lines and African green monkey kidney normal cell line (Vero) (ATCC CCL-81) were kept in RPMI-1640 (having heat-inactivated fetal bovine serum (10%), glutamine (2 mM), pyruvate (1 mM), 100 U/mL of penicillin, and 100 μg/mL of streptomycin) in T 75 cm2 sterile tissue culture flasks in a 5% CO2 incubator at 37 °C.13 For the experiments, all cell lines were grown in different 96-well plates by inoculating 5 × 104 cells with 100 μL/well and plates were incubated at 37 °C for 24h in a CO2 (5%) incubator. Within 24 h, a uniform monolayer was formed which was used for experiments. Cytotoxicity Analysis by Sulforhodamine B (SRB) Assays. The cytotoxicity assay with H157, BHK-21, and Vero cells was performed with slight modifications of a previously described method.14 Briefly, cells were cultured in different 96-well plates for 24 h. The compounds in different concentrations (10, 1, 0.1, 0.01, and 0.001 μM) were inoculated in test wells, while control and blank wells were also prepared containing vincristine (VCN) and culture media, respectively. The plates were then incubated for 48 h. Afterward, the cells were fixed with 50 μL of 50% ice-cold TCA solution at 4 °C for 1 h. The plates were washed five times with PBS and air-dried. Fixed cells were further treated with 0.4% w/v sulforhodamine B dye prepared in 1% acetic acid solution and left at room temperature for 30 min. Then the plates were rinsed with 1% acetic acid solution and allowed to dry. In order to solubilize the dye, the dried plates were treated with 10 mM Tris base solution for 10 min at room temperature. The absorbance was measured at 490 nm with subtraction of the background measurement at 630 nm.15 All experiments were performed in triplicate. The following formula was used in order to determine the percent inhibition:

percent inhibition (%) = [1 − (absorbance of sample/absorbance of control)] × 100

percent inhibition (%)

Dose−response curves of potential inhibitors (≥50%) were obtained, and IC50 values were determined with the help of nonlinear regression analysis of the program PRISM 5.0 (GraphPad, San Diego, CA, USA). Alkaline Phosphatase Inhibition Assays. The derivatives were tested against tissue nonspecific alkaline phosphatase from bovine (bTNAP) and tissue specific alkaline phosphatase enzyme extracted from calf intestine (c-IAP). The conditions of the assay were optimized and carried out with slight modifications in the method previously described by Iqbal.12 For each enzyme, the solutions were prepared in an enzyme buffer containing 50 mM Tris-HCl (pH 9.5), 5 mM MgCl2, 0.1 mM ZnCl2, and 50% glycerol. The solution of pnitrophenyl phosphate (p-NPP) substrate and the test compounds were prepared in an assay buffer containing 50 mM Tris-HCl (pH 9.5), 5 mMMgCl2, and 0.1 mM ZnCl2. Initial screening of the test compounds was carried out at a final concentration of 10 μM. The total volume of 100 μL contained 75 μL of assay buffer and 10 μL of test compound, followed by addition of 5 μL of b-TNAP (0.05 U/ well) or 5 μL of c-IAP (0.015 U/well). The mixture was preincubated for 10 min at 37 °C, and the absorbance was measured at 405 nm as preread using a microplate reader (Bio-Tek Instruments, Inc. ELx 800TM Winooski, VT, USA). Then 10 μL of substrate (5 mM p-NPP) was added to initiate the reaction and the assay mixture was incubated for an additional 30 min. The change in absorbance of released pnitrophenolate was measured as after-read. The percent inhibition of each compound was calculated by comparing the results with total activity control (having no inhibitor at all). For compounds that

= [100 − (absorbance of sample/absorbance of control)] × 100 IC50 values of potential inhibitors (≥50%) were determined with the help of nonlinear regression analysis of the program PRISM 5.0 (GraphPad, San Diego, CA, USA). Antileishmanial Activity. Parasite and Culture. Leishmania major promastigotes were cultured at 25 ± 1 °C to logarithmic phase in D-MEM/F-12 medium (Gibco BRL) without phenol red, supplemented by 10% heat inactivated fetal bovine serum (FBS), 100 U/mL of penicillin, and 100 μg/mL of streptomycin. Then the parasites were washed three times with phosphate-buffered saline (PBS) followed by centrifugation at 1500 rpm for 10 min at room temperature. Finally, the parasites were resuspended at a concentration of 2.5 × 106 parasites/mL in the medium. Antileishmanial Activity Assays (MTT Assay). In vitro antileishmanial activity of the compounds was evaluated against the promastigote forms of Leishmania major by using an MTT (3-(4,5-dimethylthiazol2-yl)-2,5-diphenyltetrazolium bromide)-based cell viability assay. This MTT assay was originally described by Mosmann16 and was further modified by Nikš and Otto.17 A stock solution of MTT (Sigma Chemical Co., St. Louis, MO, USA) was prepared at a concentration of 5 mg/mL in phosphate-buffered saline (PBS) and then stored in the dark at 4 °C. For the antileishmanial activity, a total volume of 100 μL of the culture, which contained 2.5 × 106 parasites/mL, was seeded per well in 96-well flat-bottom plates. Then, 10 μL of the compounds of C


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Inorganic Chemistry

Figure 1. Polyhedral (left) and ball-and-stick (right) representations of polyanions 1 and 2. Color code: balls, X = Si/Ge (pink), W (blue), Cr (green), O (red); polyhedra, WO6 (blue).

Table 2. Crystal Data for Na-1 and Na-2 formula formula wt cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dcalcd, g/cm3 abs coeff F(000) cryst size, mm θ range for data collection, deg no. of rflns collected no. of indep rflns R(int) no. of obsd rflns (I > 2σ(I)) goodness of fit on F2 R1 (I > 2σ(I))a wR2 (all data)b a

Na-1

Na-2

Na14[Cr2(SiW8O31)2]·45H2O 5226.35 triclinic P1̅ 11.840(3) 13.405(5) 16.785(4) 110.892(19) 93.430(13) 108.697(10) 2311.4(12) 1 3.755 20.270 2360 0.2 × 0.2 × 0.1 3.448−28.282 128034 11411 0.0567 10276 1.002 0.0404 0.1096

Na14[Cr2(GeW8O31)2]·50H2O 5405.30 triclinic P1̅ 11.7963(9) 13.3898(10) 16.8498(13) 110.934(4) 93.251(4) 108.541(3) 2312.7(3) 1 3.881 20.878 2446 0.15 × 0.1 × 0.04 3.445−30.034 239622 13500 0.067 11895 1.004 0.0433 0.1257

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.

various concentrations (10, 1, 0.1, 0.01, 0.001 μM) was added to the respective wells, and plates were incubated for 72 h at 25 ± 1 °C. A well containing only 100 μL of the culture medium without any compound, drug, or parasite was taken as the blank. Amphotericin B was used as a standard drug. At the end of incubation, 10 μL of MTT was added to each well and plates were incubated for 3 h at 25 ± 1 °C. Then 100 μL of stopping reagent (50% isopropyl alcohol and 10% sodium dodecyl sulfate in 0.1 N HCl) was added to stop the reaction. The plates were incubated for an additional 30 min with agitation at room temperature. The absorbance was measured at 570 nm, with subtraction of the background measurement at 690 nm, using a 96well microplate reader (Bio-TekELx 800TM, Instruments, Inc. Winooski, VT, USA). The absorbance of the formazan produced by

the action of mitochondrial dehydrogenases of metabolically active cells is shown to correlate with the number of viable cells. All experiments were repeated in triplicate. The following formula was used in order to determine the percent inhibition:

percent inhibition (%) = [100 − (absorbance of sample/absorbance of control)] × 100 IC50 values of potential inhibitors (≥50%) were determined with the help of nonlinear regression analysis of the program PRISM 5.0 (GraphPad, San Diego, CA, USA). D


Article

Inorganic Chemistry

Figure 2. Temperature dependence of μeff for Na-1 and Na-2. The solid red line is the best-fit simulation with the parameters listed in the text. The blue line is a fit using the same parameters but with positive J.

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The {B-β-SiW8} fragment was reported for the first time in 2005 by our group in the Co15-containing 24-tungsto-3-silicate [Co6(H2O)5{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}]5−,19 followed by Mialane’s group in the Cu9-containing 24-tungsto-3silicate [{Cu3(SiW8O31) (OH)(H2O)2(N3)}3(N3)].20 Later, our group reported the Co6-containing 34-tungsto-4-silicate [{Co3(B-β-SiW9O33(OH))(B-β-SiW8O29(OH)2)}2]5− containing the same {B-β-SiW8O31} fragment,21 but with two μ2hydroxo (W−OH−W) bridges. On the other hand, the [B-βGeW8O31]10− fragment was first observed in [Cu5(2,2′bpy)6(H2O)][GeW8O31], which was prepared under hydrothermal conditions,22 and the phosphorus analogue [B-βPW8O31]9− is present in [{Cu8(2,2′-bpy)8}(PW8O31)2]2−.23 It should also be mentioned that POMs containing the {B-βSiW8O31} moiety were synthesized by using [γ-SiW10O36]8− as a starting reagent, which undergoes a structural transformation in aqueous media. Table S1 in the Supporting Information shows examples of polyanions containing the {B-β-XW8} fragment. Figures S2 and S3 show the bonding of the counter cations in the solid-state structures of Na-1 and Na-2. Recently, we reported the mononuclear chromium(III)containing polyanions [CrIII(HXVW7O28)]13− (X = P, As), starting respectively with H3PO4 and As2O5, under otherwise the same reaction conditions as for 1 and 2, for which the reagent ratio was adjusted for better yield.6n It has to be noted that although a successful synthesis requires careful control of pH and temperature, the stoichiometry indicated by the formation equation is not strictly necessary, which means that an excess of the chromium salts or heteroatom does not influence the nature of the final products. The chemical influence of the heteroatom on the formation of heteropolytungstates is obvious in the case of the chromiumcontaining title products 1 and 2 (group IV heteroatoms) versus our reported [CrIII(HXVW7O28)]13− (group V heteroatoms), where the main effect can be seen in the sandwiching lacunary unit ({XW7} vs {XW8}). The effect of group IV vs V heteroatoms on the chemical and structural properties of heteropolytungstates is well-known.1 Magnetic Studies. Magnetic susceptibility data were corrected for the sample holder and diamagnetism using Pascal’s constants.24 The temperature dependence of the effective magnetic moment (μeff) per Cr2 unit for the complexes is shown in Figure 2. The μeff values at room temperature are 5.71 and 5.67 μB for Na-1 and Na-2, respectively. Below 50 K, these values decrease sharply, suggesting there is either zerofield splitting of the spin states or magnetic exchange between the two CrIII ions. These data were simulated using the magnetic susceptibility equation for a binuclear complex under

RESULTS AND DISCUSSION Synthesis and Structure. Direct reaction of chromium(III) nitrate, sodium tungstate, and sodium metasilicate at suitable stoichiometric ratios in sodium acetate solution (pH 6.0) at 80 °C resulted in the sandwich-type dichromiumcontaining 16-tungsto-2-silicate [CrIII2(B-β-SiIVW8O31)2]14− (1). Replacing sodium metasilicate with germanium dioxide under otherwise almost identical conditions resulted in the isostructural germanium derivative [CrIII2(B-βGeIVW8O31)2]14− (2) (see Figure 1). The successful formation of the title compounds required heating in the range of 60−95 °C, as lower temperatures did not lead to the desired products. Polyanions 1 and 2 consist respectively of two equivalent octatungstosilicate{B-β-SiW8} and {B-β-GeW8} moieties linked by two CrIII ions, leading to a sandwich-type structure with C2h point group symmetry, having the C2 axis passing through the two CrIII centers and a perpendicular σh mirror plane passing through the heteroatoms (Figure 1). Each POM half-unit consists of two {W3O13} triads linked each through a corner oxo bridge to two edge-shared {WO6} octahedra (which links the triads together), all connected by the central SiO4 or GeO4 hetero group. The two structurally equivalent CrIII ions in 1 and 2 have an almost ideal octahedral coordination environment comprising four corner-shared μ2 -O atoms (Cr−O−W) from the tungsten−oxo framework with Cr−O distances of 1.946(9)− 2.009(9) Å in 1 vs 1.954(7)−1.968(6) Å in 2 and two μ3-O (GeCrCr′) atoms with Cr−O distances of 2.002(9) and 2.009(9) Å in 1 vs 1.990(6) and 1.996(6) Å in 2, respectively. The four kinds of W−O bond lengths fall within the anticipated ranges for the different types of oxo ligands with the expected trend: terminal (1.719(10)−1.748(12) Å in 1 vs 1.726(8)− 1.756(8) Å in 2), μ2 bridging (1.811(10)−1.984(11) Å in 1 vs 1.818(8)−2.101(8) Å in 2), μ3 bridging (2.255(10)−2.260(9) Å in 1 vs 2.211(7)−2.235(7) Å in 2), and central μ4 bridging (2.325(9)−2.338(9) Å in 1 vs 2.277(6)−2.302(7) Å in 2). The central heteroatom X (Si, Ge) is connected to two μ4-O atoms from two {W3O13} triads, a μ3-O atom from the two edgeshared {WO6} octahedra, and a μ3-O atom from two chromium centers. The X−O bond lengths are in the range of 1.627(10)− 1.651(10) Å (1) vs 1.732(7)−1.760(6) Å (2). Bond valence sum (BVS) calculations for 1 and 2 suggest that the polyanions are not protonated.18 Crystallographic structural analysis revealed that Na-1 and Na-2 are isomorphous, both crystallizing in the triclinic P1̅ space group with similar unit cell parameters (see Table 2). E


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Inorganic Chemistry

Table 3. Cathodic Peak Potentials, Epc, Anodic Peak Potentials, Epa, and Midpoint Redox Potentials, E°′ = (Epa + Epa)/2, for the W Center Reduction Steps of Na-1 and Na-2 in 0.5 M Li2SO4 + H2SO4, from pH 1 to 3, and in 1.0 M LiCH3COO + CH3COOH, from pH 4 to 6a

the effects of Heisenberg intra-cluster magnetic exchange (−2JS1·S2) with S1 = 3/2, S2 = 3/2.25 The magnetic susceptibility data for both compounds were adequately described using essentially the same parameters of S1 and S2 = 3/2, J = −3.3 cm−1, and g1 and g2 = 2.10 for 1 and S1 and S2 = 3/2, J = −4.1 cm−1, and g1 and g2 = 2.08 for 2, as indicated by the acceptable fit of the solid line to the experimental data (circles) as shown in Figure 2. Also shown are simulations with J = +3.3 cm−1 for 1 and +4.1 cm‑1 for 2. It is evident that the simulations with negative J values are significantly better, implying that the magnetic interaction between the dimer ions is weak but antiferromagnetic. The small negative J value indicates the presence of a weak antiferromagnetic spin-exchange interaction between CrIII centers. It also indicates the presence of many low-lying excited states. The J values of Na-1 and Na-2 are comparable with those determined for several other CrIII dimers.26 For example, the J value of an alkoxo-bridged CrIII dimer has been reported at −3.67 cm−1.26a As the exchange coupling is small, and the dimer is diamagnetic in its lowest state, one expects a multitude of excited states that should be thermally accessible and paramagnetic in nature. EPR measurements were thus undertaken to probe these excited states. Furthermore, while it was not necessary to add a zero-field splitting term (D) to our susceptibility simulations, recent work from our group has shown that CrIII in mononuclear polyoxotungstates exhibits exceptionally large magnetic anisotropy.5n EPR measurements on powder samples were performed at 9.4, 34.5, and 240 GHz. Despite a significant effort, our EPR investigations were unfortunately not as successful as desired, since the spectra were found to be exceedingly complex. No satisfactory simulation was obtained for any spin, including S = 3/2, with the standard g and D parameters. Some typical spectra and our effort at their computer simulations are presented in the Supporting Information. As shown in Figures S4−S8 in the Supporting Information, at all measured frequencies, the spectra of powdered samples of Na-1 and Na-2 are exceedingly complex, likely due to the existence of several low-lying excited states. Electrochemistry. The electrochemical characterization of polyanions 1 and 2 was carried out in 0.5 M Li2SO4 + H2SO4 media at pH 1, 2, and 3 and in 1.0 M LiCH3COO + CH3COOH media at pH 4, 5, and 6. The CVs obtained in these different media exhibit the same overall features for the two polyanions 1 and 2: namely, a single reversible redox wave assigned to the reduction of WVI centers present in these species. Beyond this single redox process, a reduced form of the polyanions reaches the solvent limit, which corresponds to the electrocatalytic reduction of protons. This reduction of the polyanions is irreversible and encompasses the modification of the electrode surface, rendering it slightly more active toward the hydrogen evolution reaction (HER, see Figure S14 in the Supporting Information). As expected, the midpoint redox potential values, E°′ = (Epa + Epc)/2 (Epa and Epc being the anodic and the cathodic peak potentials, respectively), decrease as the electrolyte pH increases (Table 3).27 Figure 3 shows the CVs of 2 obtained at all six pH values selected for the present study (see Figure S11 in the Supporting Information for 1). The redox waves shift toward negative potentials as the pH rises from 1 to 6. This behavior indicates that the reduction of the two polyanions encompasses proton uptake, as is the case for the majority of POMs in aqueous solution. The reduction is facilitated (less negative E°′ values) when the proton

pH

Epc Epa E°′

1 −0.480 −0.440 −0.460

2 −0.570 −0.530 −0.550

Epc Epa E°′

−0.440 −0.400 −0.420

−0.550 −0.510 −0.530

Na-1 3 −0.660 −0.630 −0.650 Na-2 −0.660 −0.620 −0.640

4 −0.750 −0.700 −0.720

5 −0.850 −0.800 −0.830

6 −0.960 −0.880 −0.920

−0.780 −0.690 −0.730

−0.880 −0.800 −0.840

−0.970 −0.890 −0.940

a

Epc, Epa, and E°′ values are all given in units of V vs SCE. The CVs were recorded at a scan rate of 10 mV s−1. The working electrode was EPG. Potentials are quoted against the SCE reference electrode.

concentration is higher (lower pH values) and becomes more and more difficult as protons in the medium become scarce (higher pH values). The existence of a mechanism composed of an electrochemical step, E (electron transfer), coupled with a chemical reaction, C (proton exchange), is further confirmed by the relative reduction peak current intensities at the different pH values. In fact, the peak currents should be very close, if not similar, irrespective of the pH, since the polyanion concentrations were the same for all experiments. However, when the cathodic peak current intensities, Ipc, obtained at the selected pH values are compared, an obvious trend was observed: the absolute value of Ipc increases when the electrolyte pH decreases (i.e., when the proton concentration increases) on going from Ipc = −3.4 μA at pH 6 to Ipc = −4.4 μA at pH 1, proving that protons are implicated in the process. This constitutes another evidence of the fact that POM reduction follows an EC mechanism: i.e., an electrochemical reaction is coupled with a chemical reaction. Nevertheless, we would expect the shape of the redox waves on the CVs to change as they keep shifting toward negative potentials upon an increase in the pH, which is not really the case. It is known that, for EC processes for which protonation is the coupled chemical reaction, a drastic decrease of the proton concentration or the eventual absence of protons in the medium renders the electron transfer energetically more demanding (higher energy input required; therefore, the reduction occurs at more negative potentials) and also kinetically slower (less reversible). Hence, for high pH values, the redox waves are thought to lose some of their reversibility (slower electron transfer kinetics). In addition, for multielectron waves, as is the case for the present compounds,27 a progressive splitting of the main peak into several peaks, possibly via intermediate steps exhibiting shoulders, was the expected behavior. Reversible, single-peak redox waves at low pH values would evolve into less reversible, composite waves and finally into an ensemble of several waves more or less separated from each other. In the present case, it is surprising that over such a large pH range (1−6) the sole multielectron wave observed does not undergo any pronounced evolution in its shape, which might be attributed to a slowing of the electron transfer, nor is there any hint of a peak splitting phenomenon, even at a small scan rate (2 mV s−1). For small pH values, F


Article

Inorganic Chemistry

Figure 3. CVs of Na-2 (a) in different 0.5 M Li2SO4 + H2SO4 media (pH 1 (black), pH 2 (red), and pH 3 (blue)) and (b) in different 1.0 M LiCH3COO + CH3COOH media (pH 4 (black), pH 5 (red), and pH 6 (blue)). Each scan was started in the sense of the negative potentials, and the waves are associated with the redox response of the WVI centers. Conditions: scan rate, 10 mV s−1; working electrode, EPG; counter electrode, Pt; reference electrode, SCE.

Figure 4. CVs of Na-1 (blue) and Na-2 (red) in (a) 0.5 M Li2SO4 + H2SO4/pH 2 and in (b) 1.0 M LiCH3COO + CH3COOH/pH 5. Each scan was started in the sense of the negative potentials, and the waves are associated with the redox response of the WVI centers. Conditions: scan rate, 10 mV s−1; working electrode, EPG; counter electrode, Pt; reference electrode, SCE. (c) Dependence of the midpoint potential, E°′, on the electrolyte pH (from pH 1 to 6) for compound Na-1 (blue squares) and for compound Na-2 (red squares). The values were obtained from the CVs recorded with an EPG working electrode at a scan rate of 10 mV s−1.

G


Article

Inorganic Chemistry especially at pH 1, the CVs of the two polyanions show a slight shoulder after the reduction peak, which is possibly due to a partial decay of the two polyanions in this medium. This shoulder progressively disappears upon increasing the pH, and beyond pH 3 (Figure 3b) there is a perfectly defined reversible redox wave, exhibiting a single reduction peak during the forward scan and a single oxidation peak during the reverse scan. Controlled-potential coulometry did not allow determination of the total number of exchanged electrons during the redox process. In fact, even at pH 6, a medium where protons are relatively scarce, during an electrolysis carried out at a potential of −0.94 V vs SCE (i.e., before the cathodic peak), the beginning of the reduction process also encompasses the HER. Under these conditions, the total charge that crosses the circuit is 10-fold the charge recovered upon reoxidizing at a potential of 0.0 V vs SCE. A similar behavior was observed in a previous study concerning a NiII-containing heteropolytungstate.28 A comparison of the CVs of 1 and 2 recorded under the same experimental conditions reveals that, for pH values below 3 (Figure 4a), the reduction of the tungsten centers of 2 takes place before that of 1, as expected.29 Indeed, recent studies have shown that there is a general trend for a series of POMs having the same structure and charge and differing just in the nature of the central heteroelement (e.g., Si vs Ge or P vs As). The species reduced first is that containing the heteroelement with the larger atomic radius (Si, r ≈ 1.10 Å, Ge, r ≈ 1.25 Å).30 These observations have been confirmed and rationalized by DFT calculations. However, in the present case and for pH values above 3, there is an inversion of the expected trend, with 1 being reduced before 2 (Figure 4b). This phenomenon may be explained by the pKa values for the two polyanions, which must be around 4, matching the redox potential inversion when this pH is reached. In both polyanions 1 and 2, the Cr centers are in the +3 oxidation state. It is possible, then, to observe their conversion to higher oxidation states, such as CrV, by cyclic voltammetry and make use of them in order to carry out electrocatalytic oxidation processes. In the case of the monosubstituted Dawson-POM derivatives α2-[P2W17O61CrIII(OH2)]7− and [PW11O39CrIII(OH2)]4−, Anson et al. described an irreversible oxidation wave attributable to the CrV/CrIII couple.31 However, in other cases, namely in sandwich-type compounds possessing several Cr centers,5m it is hard to idetify clearly this oxidation wave of CrIII, because it takes place at rather positive potentials and may be concealed by the solvent limit: i.e., the electrocatalytic oxidation of water. In order to avoid this interference, we decided to work at pH 6, where the CrIII oxidation occurs at potential values that are not so positive. In fact, in the CV of polyanion 1 obtained at pH 6 there is a hump at around +0.870 V vs SCE assigned to the oxidation of CrIII, followed by the electrocatalytic oxidation of water, which takes place at around +1.100 V vs SCE and on which the POM has no effect (Figure 5 and Figure S12 in the Supporting Information). The oxidation process of CrIII is totally irreversible, with its corresponding reduction wave appearing at −0.480 V vs SCE. The electrode surface is modified upon oxidation of the CrIII centers, which renders it less reactive. Indeed, when two consecutive cycles were recorded, the reduction of the WVI centers upon the second cycle was shifted by 70 mV toward negative potentials. Similar results were observed in the CVs of 2 recorded under the same conditions (Figure S13 in the Supporting Information). A working

Figure 5. CVs of Na-1 in 1.0 M 1.0 M LiCH3COO + CH3COOH/pH 6. The scan (two cycles) was started in the direction of negative potentials, followed by the direction of positive potentials, in order to assess the redox behavior of the waves attributed to the CrIII centers. Conditions: working electrode, EPG; counter electrode, Pt; reference electrode, SCE.

electrode fouling related to the oxidation of the CrIII centers took place for the six media used in the present study. This may be rationalized by the formation of a film of insoluble oxides that covers the electrode surface and is not totally removed upon the reduction scan. Despite the fact that polyanions 1 and 2 are slightly active toward the HER (Figure S14 in the Supporting Information), even at relatively high pH values (pH 6), the potentials required to trigger the phenomenon are rather negative (−1.1 V vs SCE at pH 6) for the POMs to be considered good electrocatalysts for hydrogen production. Likewise, 1 and 2 are inefficient for the electrocatalytic oxidation of water. Enzymatic Assays. In many chronic metabolic diseases, especially diabetes and its complications, POMs have been identified as potential inhibitors of glucosidase, where they play an important role in lowering the blood glucose level.11 Keeping in mind the significance of POMs as glucosidase inhibitors, we tested polyanions 1 and 2 against glucosidases (α and β) to evaluate their antidiabetic potential. The germaniumcontaining 2 showed a good inhibition of both enzymes with IC50 values of 3.58 ± 0.67 μM (α-glucosidases) and 5.87 ± 0.12 μM (β-glucosidases), whereas the isostructural silicon analogue 1 exhibited less inhibition with IC50 values of 12.1 ± 2.26 μM (α-glucosidases) and 17.4 ± 2.45 μM (β-glucosidases) (see Table 4). Alkaline phosphatases (APs; E.C.3.1.3.1) belong to a large family of ecto-nucleotidases and are importantly involved in the hydrolysis of phospho-monoesters with release of inorganic phosphate (Pi) and alcohol. They are present throughout the human body and are categorized as tissue-nonspecific alkaline phosphatase (TNAP) and three tissue-specific alkaline phosphatases: i.e., IAP, PLAP and GCAP. The overexpression of alkaline phosphatase isozymes can cause impaired mineralization, especially in old-age chronic diseases including osteoporosis, osteoarthritis, cardiovascular tissue calcification, and bone metastasis.32 Therefore, small-molecule inhibitors of TNAP have potential for studying the causative mechanisms or treating the pathology of diseases caused by medial calcification such as arterial calcification, idiopathic infantile, end-stage renal H


Article

Inorganic Chemistry Table 4. Biological Activity of Polyanions 1 and 2 and Some Reference Compounds IC50 (μM) ± SEM

Ki (μM) ± SEM

IC50 (μM) ± SEM

inhibitor

α-glucosidase

β-glucosidase

b-TNAP

c-IAP

BHK-21

H157

anti-Leishmania

1 2 acarbose levamisole L-phenylalanine vincristine amphotericin B

12.1 ± 2.26 3.58 ± 0.67 108.8 ± 12.3

17.4 ± 2.45 5.87 ± 0.12

1.96 ± 0.12 1.09 ± 0.15

8.06 ± 1.52 1.54 ± 0.02

7.37 ± 1.04 2.07 ± 0.38

5.16 ± 1.87 1.01 ± 0.08

1.34 ± 0.42 0.32 ± 0.08

1.08 ± 0.09

1.03 ± 0.04

25 ± 2 100 ± 3 0.29 ± 0.05

magnetic, biological, and electrochemical properties. We have also prepared other Cr-containing POMs, which will be reported elsewhere.

disease, and diabetes. Keeping in mind the importance of alkaline phosphatase inhibitors, we hence decided to evaluate polyanions 1 and 2 for alkaline phosphatases (b-TNAP and cIAP). Here as well, polyanion 2 exhibited excellent inhibitory against alkaline phosphatases with Ki values of 1.09 ± 0.15 μM (b-TNAP) and 1.54 ± 0.02 μM (c-IAP), whereas 1 showed slightly lower inhibition of b-TNAP (Ki = 1.96 ± 0.12 μM) and significantly lower inhibition of c-IAP (Ki = 8.06 ± 1.52 μM) (see Table 4). Anticancer Studies. In the present study we investigated the toxic effects of polyanions 1 and 2 (in a dose-dependent manner) against kidney fibroblast (BHK-21) and lung carcinoma (H157) cells using an MTT assay. A comparison was made with the standard drug vincristine. Polyanions 1 and 2 were also tested on Vero cells to evaluate their safety for noncancerous cells. The cytotoxicity tests on 2 resulted in IC50 values of 2.07 ± 0.38 μM (BHK-21) and 1.01 ± 0.08 μM (H157). These results were comparable with those of the standard drug vincristine, which showed inhibition on both cell lines with an IC50 value of 1.08 ± 0.09 μM. Polyanion 2 showed minimum inhibition of Vero cells (18 ± 2%), exhibiting its safety. On the other hand, polyanion 1 showed lower inhibition with IC50 values of 7.37 ± 1.04 μM (BHK-21) and 5.16 ± 1.87 μM (H157) (see Table 4). We also decided to investigate the antiparasitic activity of polyanions 1 and 2. Hence, we tested 1 and 2 against Leishmania, and both polyanions showed excellent inhibition with IC50 values of 0.32 ± 0.08 and 1.34 ± 0.42 μM, respectively. These results are quite similar to those of amphotericin B (IC50 = 0.29 ± 0.05 μM), which is being used as a standard drug (see Table 4).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01458. Crystallographic data for 1 (CIF) Crystallographic data for 2 (CIF) FT-IR spectra, crystal packing, EPR spectra, TGA data, cyclic voltammograms, and a list of POM structures containing the {B-β-XW8} (X = Si, Ge, P) unit (PDF)

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AUTHOR INFORMATION

Corresponding Author

*U.K.: fax, (+49) 421-200-3229; e-mail, [email protected]; web, http://ukortz.user.jacobs-university.de/. Present Address $

Department of Chemistry, Faculty of Science, Beirut Arab University, P.O. Box 11 50 20, Riad El Solh 1107 2809, Beirut, Lebanon. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS U.K. thanks the German Research Council (DFG KO-2288/ 20-1), Jacobs University, and CMST COST Action CM1203 (PoCheMoN) for support. W.L. thanks the China Scholarship Council (CSC) for a doctoral fellowship allowing her to pursue Ph.D. studies at Jacobs University, Germany. A.M.B., P.d.O., and I.M.M. thank the Centre National de la Recherche Scientifique (CNRS) and the Université Paris-Sud for financial support. A.M.B. thanks the Service de Coopération et d’Action Culturelle (SCAC) of the French Embassy in Ivory Coast and Dr. B. Avo Bile from the University Félix Houphouët-Boigny for cosupervision. J.I. thanks his co-workers Syeda Abida Ejaz, Hamid Saeed Shah, and Sumera Zaib for help with experimental work. J.I. is also thankful to the Organization for the Prohibition of Chemical Weapons (OPCW), The Hague, The Netherlands, and the Higher Education Commission of Pakistan for the financial support through Project No. 20-3733/NRPU/R&D/14/520. We thank Hans van Tol for the high-field EPR studies at NHMFL, which is supported by the NSF and the state of Florida via Cooperative Agreement NSF-DMR 0654118. Figure 1 was generated using Diamond version 3.2 software (copyright Crystal Impact GbR).

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CONCLUSIONS We have reported on the direct synthesis (reaction of comprising elements, without the need for a POM precursor) and structures of the two dimeric, di-CrIII -containing heteropoly-16-tungstates [CrIII2(B-β-SiIVW8O31)2]14− (1) and [CrIII2(B-β-GeIVW8O31)2]14− (2). Polyanions 1 and 2 represent the first examples of Cr-containing POMs comprising an {B-βXW8O31} (X = Si, Ge) octatungstate unit. Polyanion 2 is the first chromium(III)-containing tungstogermanate. Susceptibility measurements were performed on the salts Na-1 and Na-2, allowing for determination of the exchange coupling constants. The electrochemical studies demonstrated that 1 and 2 are stable in water or other aqueous media at pH 1−7 at least and that the CrIII center may be irreversibly oxidized without decomposition of the polyanion. The present polyanions also exhibit antiparasitic activity. In summary, in our efforts to expand upon the small number of known Cr-containing polyanions, we have synthesized and characterized the two new members 1 and 2, which exhibit interesting structural, I


Article

Inorganic Chemistry

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