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Redox-Induced Reversible Uptake-Release of Cations in Porous Ionic Crystals based on Polyoxometalate: Cooperative Migration of Electrons with Alkali Metal Ions Ryosuke Kawahara, Sayaka Uchida, and Noritaka Mizuno Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm504526z • Publication Date (Web): 24 Feb 2015 Downloaded from http://pubs.acs.org on March 1, 2015
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Chemistry of Materials
Redox-Induced Reversible Uptake-Release of Cations in Porous Ionic Crystals based on Polyoxometalate: Cooperative Migration of Electrons with Alkali Metal Ions Ryosuke Kawahara,† Sayaka Uchida,†,‡,* Noritaka Mizuno§ †
Department of Basic Sciences, School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ‡
JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
§
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 1138656, Japan. ABSTRACT: Redox-active porous ionic crystals based on polyoxometalates (POM) were synthesized. By treating the crystal with an aqueous solution of ascorbic acid (reducing reagent) and KCl, one-electron reduction of POM proceeded followed by simultaneous uptake of K+. Interestingly, the reduction did not proceed without KCl, and the molecular size of ascorbic acid was too large to enter the porous crystal lattice. The time courses of reduction and K+ uptake were monitored by UV-vis spectroscopy and atomic absorption spectrometry (AAS), respectively. Both profiles could be reproduced by the linear driving force (LDF) model with similar rate constants. The reduced crystal could be oxidized with aqueous chlorine solution followed by the release of K+, and the redox cycles were reversible. The water sorption properties of the crystals could be controlled by the types of alkali metal ions incorporated. The Cs+ uptake and the simultaneous reduction of the crystal proceeded much faster than in the case of K+, which is in line with the trends in the Gibbs energies of hydration of alkali metal ions. Complete selectivity to Cs+ was observed in the uptake of ions from an aqueous binary mixture of Cs+ and Na+. All these results suggest the cooperative migration of electrons with alkali metal ions and the redox induced ion-exchange in porous ionic crystals based on POM.
INTRODUCTION Polyoxometalates (POMs) are nano-sized anionic metaloxygen clusters of early transition metals and have stimulated research in broad fields of science.1,2 Especially, reversible redox properties of POMs have been utilized as catalysts, single-molecule magnets, cathodes, and electro-optical devices, etc. For example, Yoshikawa and Awaga et al. have reported the preparation of a molecular cluster battery utilizing Keggin-type phosphododecamolybdate [-PMo12O40]3 as a cathode: Twelve Mo6+ in [-PMo12O40]3 are reduced to Mo4+, and 24 electrons can be stored in the molecule.3 Cronin et al., have reported the syntheses and structures of a metal-oxide solid framework4 and a porous macrocycle5 based on POMs, which undergo reversible redox processes by using redox reagents. The porous macrocycle initially contains Li+ and K+ which are exchangeable with Cu2+, and the extent of ionexchange can be controlled by the degree of oxidation.5 Reversible redox property of solids is especially a key to the function of rechargeable batteries: Li+ can be electrochemically inserted into various transition metal oxides and sulfides as cathodes, and the sizes and dimensions of channels as well as the framework stability affect their function.6 Rechargeable Na-ion batteries have been researched extensively since Na is much abundant and less expensive than Li, while few solids such as manganese oxides can reversibly insert Na+ because of
the larger ionic radius.7 By utilizing the redox property of Fe2+/Fe3+ in prussian blue analogues, reversible insertion of Li+ and controlled release of much larger Cs+ have been successfully carried out.8,9 Redox property has been utilized for the post-synthetic modification of metal-organic frameworks (MOFs). In a landmark work by Hupp et al., the exposure of Zn2(NDC)2(diPyNI) (NDC = 2,6-naphthalenedicarboxylate, diPyNI = N,N’-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxy diimide) to Li metal in DMF resulted in the reduction of diPyNI and incorporation of Li+ into the micropores, which resulted in an increase in hydrogen adsorption capacity.10 In a work by Long et al., oxidation of Cu[Ni(pdt)2] (pdt2 = pyrazine-2,3-dithiolate) with I2 resulted in an enhancement of electronic conductivity.11 While most examples focus on the redox property of organic ligands, Long et al. utilized Fe-ions (Fe2+/Fe3+) for the selective binding of O2 over N2.12 There are still few reports which could utilize the reversible redox property of metal ions,12,13 since most MOFs cannot support the geometry changes in the frameworks which often accompany the redox processes. We have been working on the synthesis of functional porous ionic crystals. Ionic crystals (e.g., NaCl, CsCl) are normally non-porous because of the isotropic and long-range Coulomb interaction.14 The use of molecular ions with appropriate ele-
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ments, charges, sizes, shapes, and ligands enables the anisotropic packing and/or utilization of anisotropic interactions such as - and hydrogen-bonding interactions, resulting in the formation of voids and channels.15-20 Specific functions (guest binding, catalysis, ion-exchange etc.) that are incorporated beforehand into the ionic components can be maintained and utilized after the complexation, since they still exist as discrete molecules in the crystal lattice.15-20 POMs are potentially suited for the construction of porous ionic crystals,18-20 while prediction of the framework structure is difficult because of the high-symmetry of POMs (e.g., Td for -Keggintype POMs) and the isotropic and long-range Coulomb interactions. Therefore, post-synthesis can be a useful approach to control the structure and properties of porous ionic crystals based on POMs.21
ligands and chemical inertness due to the large crystal field stabilization energy of Cr3+ with d3 electronic configuration.22,23 The post-synthetic treatment of I-as with an aqueous solution containing ascorbic acid as a reducing reagent and KCl, resulted in a color change from yellow to blue: Oneelectron reduction of the POM to [-PMo12O40]4 occurred followed by simultaneous uptake of K+, and K2[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-red-K+] was formed. The reduced crystal (I-red-K+) could be oxidized with aqueous chlorine solution followed by the release of K+ in the aqueous solution, and K[Cr3O(OOCH)6(mepy)3]2[PMo12O40]·5H2O [I-ox-K+] was formed. The redox cycles were reversible and proceeded in a single crystal-to-single crystal manner.24
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red a
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I-as
I-red -K + I-ox -K + II-red -K +
Figure 1. Schematic illustration of the redox-induced reversible uptake-release of cations (K+). Crystal structures of (a) I-as, (b) Ired-K+, and (c) I-ox-K+. The opening of the one-dimensional channel is indicated by the blue oval in (a). The molecules in light green and blue show the [-PMo12O40]3 and [-PMo12O40]4, respectively. The molecule in orange shows the macrocation. The photograph below the crystal structures (a)(c) corresponds to the image of each compound, and the inset in each photograph shows the image of the same crystal through the redox process. (d) Void analyses of I-as with a probe diameter of 3.2 Å (void spaces are shown in brown). (e) Photo images of I-as in water, and aqueous mixtures after the redox reactions.
Based on these considerations, we synthesized a redoxactive porous ionic crystal of (mepyH)[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O (mepy = 4-methylpyridine, mepyH+ = 4-methylpyridinium ion) [I-as] with -Keggin-type phosphododecamolybdate [-PMo12O40]3 as a macroanion and Cr(III)-carboxylate [Cr3O(OOCH)6(mepy)3]+ as a macrocation. Cr(III)carboxylates with a general formula of [Cr3O(OOCR)6(L)3]+ have been studied as building blocks of solid materials because of the versatile selection of bridging (R) and terminal (L)
EXPERIMENTAL SECTION Synthesis. [Cr3O(OOCH)6(mepy)3](ClO4)•nH2O25 (0.41 g, 0.5 mmol) was dissolved in 50 mL of 1,2-dichloroethane and the solution was filtered to remove the impurities (solution A). H3[-PMo12O40]•nH2O (0.46 g, 0.25 mmol) was dissolved in 20 mL of methanol (solution B). Solution B was layered on solution A, and yellow crystals of I-as were formed at the interface within few days. Post-synthesis modification of I-as (i.e., syntheses of reduced forms) was carried out as follows:
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To 0.33 g (0.1 mmol) of I-as, ascorbic acid (0.0176 g, 0.1 mmol), KCl (0.74 g, 10.0 mmol), and 50 mL of water were added consecutively, and the solution was stirred at r.t. for 24 h. Dark blue crystals of I-red-K+ were obtained. Postsynthesis modification of I-as in the presence of other metal ions were carried out with the same method by using the chlorides. Oxidation of I-red-K+ was carried out as follows: To 0.34 g (0.1 mmol) of I-red-K+, 50 mL of aqueous chlorine solution (0.3%) was added, and the aqueous solution was stirred at r.t. for 30 min. Yellow crystals of I-ox-K+ were obtained. The amount of chlorine in I-ox-K+ was 0.12% and negligible. Single Crystal X-ray Diffraction (XRD). X-ray diffraction data of I-as, I-red-K+, and I-ox-K+ were collected at 93 K with a CCD 2D detector by using a Rigaku Mercury diffractometer with graphite-monochromated MoK radiation ( = 0.71069 Å). The structures were solved by direct methods (SHELX97), expanded by Fourier techniques, and refined by full-matrix least-squares against F2 with the SHELXL package. Crystal data for I-as: Monoclinic P21/c, a = 29.59(6), b = 25.19(5), c = 13.39(2), = 91.23(4), V = 9979(32), Z = 4, R1 = 0.1660, wR2 = 0.4461, GOF = 1.092. While it was possible to locate mepyH+ with its characteristic hexatomic ring structure in the one-dimensional channel of I-as, the shift/error value in the least-squares refinement was high, and the structure did not converge. This result suggests that the mepyH+ molecules are highly disordered in the crystal lattice. Therefore, mepyH+ was not located, and the SQUEEZE program of PLATON was used to take into account the high residual electron density. Crystal data for I-red-K+: Triclinic P-1, a = 13.362(4), b = 20.179(6), c = 20.186(7), = 76.11(4), = 72.33(3), = 72.24(3), V = 4873(3), Z = 2, R1 = 0.1164, wR2 = 0.3477, GOF = 1.043. Three crystallographically independent K+ were located with an occupancy of 2/3. Crystal data for I-ox-K+: Triclinic P-1, a = 13.527(8), b = 20.304(13), c = 20.367(12), = 75.84(3), = 72.18(3), = 72.14(3), V = 4998(6), Z = 2, R1 = 0.2332, wR2 = 0.5534, GOF = 1.784. Two crystallographically independent K+ were located with an occupancy of 1/2. CCDC 10237741023776 contain the supplementary crystallographic data for this paper. Time courses of the reduction and uptake of metal ions. The time courses of the reduction of I-as were monitored as follows: To 0.33 g (0.1 mmol) of I-as, ascorbic acid (0.0176 g, 0.1 mmol), KCl (0.74 g, 10 mmol) or CsCl (1.6 g, 10 mmol), and 50 mL of water were added consecutively, and the solution was stirred at r.t. for 24 h. The solid was periodically fractionated from the solution and ca. 3.4 mg of the solid was dissolved into 10 mL of DMSO, followed by UV-vis spectra measurements in the range of 12500-40000 cm1 with a V-560 UV-vis spectrometer (JASCO). A calibration curve was prepared by the absorbance of 13900 cm1 (720 nm) (IVCT band) of DMSO solutions with specified amounts of I-red-K+ or Ired-Cs+ (correlation coefficient > 0.997). In order to monitor the time courses of the uptakes of metal ions (K+ or Cs+), the solid was periodically fractionated from the solution, conc. HNO3 (1 mL), NH3aq. (2 mL), and conc. HNO3 (2 mL) were added consecutively to ca. 10 mg of the solid to dissolve the solid completely into water, and AAS were measured with a Hitachi Z-2000 or ZA3000 Zeeman atomic absorption spectrophotometer (Hitachi).
Measurements. Water vapor (298 K) and CO2 (198 K) sorption isotherms were measured using a Belsorp-max volumetric adsorption apparatus (BEL Japan). Prior to the measurement, about 0.1 g of crystals were ground and treated in vacuo at 298 K for > 3 h. Sorption equilibrium was judged by the following criteria: ±0.3% of pressure change in 300 s. Thermogravimetry-differential thermal analysis (TG-DTA) were conducted with a Thermo Plus 2 thermogravimetric analyzer (Rigaku) with -Al2O3 as a reference under a dry N2 flow (100 mL min1) in the temperature range of 293773 K with an increasing rate of 10 K min1. The temperature was held at 303 K for > 1 h before the temperature increase. Powder XRD patterns were measured with a New advance D8 Xray diffractometer (Bruker) by using Cu K radiation ( = 1.54056 Å, 40 kV-40 mA) at 1.8 deg min1. The lattice parameters of I-ox-K+ were calculated using a Materials Studio software (Accelrys): Unit cell indexing and space group determination were performed by X-cell26 followed by the peak profile fitting using the Pawley method27 to refine the parameters. Diffuse-reflectance UV-vis spectra were measured in the range of 1250040000 cm1 (800250 nm) with a V-560 UVvis spectrometer (JASCO). The samples were prepared by grinding and diluting the compounds (ca. 1 mg) with ca. 100 mg of NaCl. 1H-NMR spectra were recorded with a Bruker Avance III (500 MHz) instrument (Bruker). Aqueous mixtures (50 mL) of I-as (0.33 g, 0.1 mmol), acsorbic acid (0.0176 g, 0.1 mmol or 0.0088 g, 0.05 mmol), and CsCl (1.6 g, 10 mmol) were prepared and stirred for 24 h. The solutions were filtered and the filtrates were concentrated in vacuo for 24 h followed by dilution with DMSO-d6. FT-IR spectra were measured by transmission method using a JASCO FT/IR 4100 instrument (JASCO). The pelletized samples were prepared by grinding and diluting the compounds (ca. 1 mg) with ca. 100 mg of KBr followed by compressing at 100 kgf cm2.
RESULTS AND DISCUSSION A porous ionic crystal I-as was obtained as yellow crystals (Figure 1a) from H3[-PMo12O40]·nH2O and [Cr3O(OOCH)6(mepy)3](ClO4) in a mixed solvent of methanol and 1,2-dichloroethane. The POMs and macrocations were lined up along the c-axis forming struts. The distances between adjacent mepy ligands were 3.53.8 Å indicating - interaction. The struts were assembled in the ab-plane to form one-dimensional channels along the c-axis with a minimum aperture of ca. 3.2 Å (Figure 1d).28 CO2 (molecular diameter: 3.3 Å)29 was used instead of larger N2 (molecular diameter: 3.643.80 Å)29 to evaluate the porosity of I-as. The CO2 sorption isotherm at 198 K (Figure S1) was type-I of the IUPAC classification, which is characteristic of microporous materials,30 and the BET surface area was 52 m2 g1. While the positions of mepyH+ could not be located by single crystal XRD because of disordering, the existence of mepyH+ was suggested by the charge balance and elemental analysis (Table 1 entry 1). The post-synthetic treatment of I-as with an aqueous solution containing ascorbic acid as a reducing reagent and KCl for 24 h resulted in a color change from yellow to blue, suggesting the reduction of Mo6+ in the POM.31 A strong broad band centered around 14000 cm1 appeared in the diffusereflectance UV-vis spectrum after the treatment (Figure 2a).
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Table 1. Chemical formulae of ionic crystals Formula Elemental analysis (%) exp. (calc.) 1
(mepyH)[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-as] C: 18.82 (18.80), H: 2.06 (2.10), N: 2.48 (2.84), Cr: 9.50 (9.04), K: 0.0 (0.0), Mo: 35.40 (33.38), P: 0.95 (0.90).
2
K2[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-red-K+] C: 17.44 (16.79), H: 1.97 (1.88), N: 2.31 (2.45), Cr: 9.10 (9.09), K: 2.04 (2.28), Mo: 34.60 (33.53), P: 0.94 (0.90).
I‐ox‐K+
+
K[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-ox-K ] C: 17.80 (16.98), H: 1.98 (1.90), N: 2.04 (2.48), Cr: 9.53 (9.19), K: 0.85 (1.15), Mo: 35.45 (33.92), P: 1.23 (0.91).
4
a
5
Rb2[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-red-Rb+] C: 16.45 (16.35), H: 1.92 (1.83), N: 2.30 (2.38), Cr: 8.96 (8.85), P: 0.91 (0.88), Mo: 33.12 (32.65), Rb: 4.28 (4.85).
I‐as
K2[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [V-red-K+] C: 16.43 (16.79), H: 2.03 (1.88), N: 2.28 (2.45), Cr: 9.10 (9.09), K: 2.25 (2.28), Mo: 33.70 (33.53), P: 0.93 (0.90).
40000 (250)
6
Cs2[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-red-Cs+] C: 16.14 (15.92), H: 1.74 (1.78), N: 2.26 (2.32), Cr: 8.28 (8.62), Cs: 7.10 (7.34), Mo: 32.41 (31.79), P: 1.17 (0.86).
7
Na(mepyH)[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-redNa+] C: 18.70 (18.68), H: 2.10 (2.09), N: 2.74 (2.82), Cr: 9.24 (8.98), Mo: 33.56 (33.16), Na: 0.99 (0.66), P: 1.21 (0.89).
8
Ba0.5(mepyH)[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O [I-redBa2+] C: 18.12 (18.44), H: 1.97 (2.06), N: 2.64 (2.79), Ba: 1.97 (1.95), Cr: 8.28 (8.87), Mo: 32.41 (32.72), P: 1.07 (0.88).
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mation of one-electron reduced species [-PMo12O40]4.36-38 The possible formation of two-electron reduced species [PMo12O40]5 can be excluded by the results of the elemental analysis (ionic charges should be neutralized in a compound) and IR spectroscopy, since further redshift of the as(Mo=O) band to 940 cm1 has been reported in the in-situ IR spectrum of [-PMo12O40]5, which was electrochemically generated in acetonitrile.37
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Cs2[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O C: 15.57 (15.92), H: 1.82 (1.78), N: 2.18 (2.32), Cr: 8.74 (8.62), Cs: 7.00 (7.34), Mo: 32.50 (31.79), Na: 0.0 (0.0), P: 1.14 (0.86). c
KNa[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O C: 17.90 (16.87), H: 1.98 (1.89), N: 2.56 (2.46), Cr: 9.56 (9.13), K: 0.76 (1.14), Na: 0.42 (0.67), Mo: 35.20 (33.69), P: 0.90 (0.91). d
K0.1(mepyH)0.9[Cr3O(OOCH)6(mepy)3]2[-PMo12O40]·5H2O C: 18.84 (18.62), H: 1.98 (2.08), N: 2.72 (2.81), Cr: 9.90 (9.06), K: 0.13 (0.11), Mo: 36.28 (33.43), P: 1.29 (0.90).
a
The reduced form after five consecutive redox cycles. b,cCompounds after the competitive uptake of ions from binary mixtures: bNa+/Cs+ = 1 and c Na+/K+ = 1. dThe compound after the treatment of I-as in an aqueous solution containing excessive amount of KCl (i.e., simple ion-exchange). The degree of ion-exchange was ca. 10%.
This band can be assigned to the intervalence charge transfer (IVCT) among Mo5+ and Mo6+ characteristic of reduced Keggin-type POMs.32,33 The elemental analysis (Table 1 entry 2) of the compound after the treatment showed that K+ was exchanged with mepyH+ and incorporated additionally. In the IR spectra (Figure 2b), the as(Mo=O) band shifted from 958 to 950 cm1 by the treatment, which agrees with the decrease in the force constant of the Mo=O stretching mode by the for-
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Figure 2. (a) Diffuse reflectance UV-vis and (b) FTIR spectra of I-as, I-red-K+, and I-ox-K+. In (b), the position of the as(Mo=O) band of [-PMo12O40]3 (958 cm1) is indicated by the broken line.
The crystal structure of I-red-K+ was successfully analyzed by single crystal XRD (Figure 1b). The framework structure was essentially retained by the reduction, while the lattice volume slightly decreased (2.3%) probably because of the increase in Coulomb interactions between the oppositely charged molecular ions by the increase in the negative charge of the POM from 3 to 4. Two K+ per formula were located in the one-dimensional channels, which agreed with the formula. The powder XRD pattern of I-red-K+ well agreed with that calculated with the single crystal data (Figures 3b and c), indicating that the structure shown in Figure 1b reasonably represented the whole bulk solid. Notably, the molecular size of the reducing reagent (i.e., ascorbic acid) is too large to enter the one-dimensional channels of I-as, and the oxidized product of ascorbic acid was detected in the reaction solution (1H-NMR, Figure S2) and not in the bulk solid (IR, Figure S3).39 When the reduction of I-as was carried out by adding stoichiometric amount of ascorbic acid, 1H-NMR (Figure S2) showed that ascorbic acid was completely consumed. These results suggest that one-electron
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reduction of the bulk solid homogeneously proceeded without incorporating the reducing reagent. Interestingly, the reduction of I-as did not take place without the addition of KCl as suggested by the retention of the color of the reaction mixture (Figure S4). Moreover, a simple ion-exchange between mepyH+ and K+ hardly proceeded (Table 1, entry 11).
The elemental analysis (Table 1 entry 3) showed that K+ was released by the oxidation.45 The crystal structure of I-ox-K+ was analyzed by single crystal and powder XRD (Figure S5).46 The framework structure was essentially retained by the oxidation, and one K+ per formula could be located in the onedimensional channel (Figure 1c). Compound I-ox-K+ can be reduced with aqueous ascorbic acid containing KCl, and IIred-K+ (the roman numeral stands for the number of redox cycles) was formed.
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Amount [mol mol-1]
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Figure 3. Powder XRD patterns of (a) I-as (exp.), (b) I-red-K+ (calc.), (c) I-red-K+ (exp.), (d) I-ox-K+ (exp.), and (e) V-red-K+ (exp.)
Therefore, it is likely that the homogeneous one-electron reduction of the bulk solid proceeds cooperatively with the uptake of K+.40 It is already known that protons and electrons in POM salts as redox catalysts or hydrogen storage materials migrate rapidly and cooperatively through the whole bulk solid.41,42 The time course of the reduction of I-as was monitored by the increase in the intensity of the IVCT band of POM in the UV-vis spectra. The experimental profile could be fairly reproduced by the LDF model as shown in eqs. (1) and (2), which has been well adapted to various (ad)sorption processes,43,44 dM k M M t (1) dt
Mt 1 exp kt M
(2)
where Mt and M∞ are the amounts of sorption at time t and equilibrium, respectively, and k is the rate constant. As shown in Figure 4a, the experimental profile (square plots) was well reproduced with kred = 8.0 × 102 h1. Next, the time course of the uptake of K+ in I-as in the reduction process was monitored by AAS. The experimental profile (circle plots) was well produced with kion = 9.0 × 102 h1. The rate constants (kred and kion) were similar to each other, suggesting that the uptake of K+ proceeded cooperatively with the reduction. The treatment of I-red-K+ in aqueous chlorine solution as an oxidizing reagent resulted in a color change from blue to the original yellow color (Figure 1c) suggesting the oxidation of POM, which was confirmed by the absence of the IVCT band in the diffuse-reflectance UV-vis spectrum (Figure 2a).
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12 18 24 30 36 42 48 Time [h]
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0
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Figure 4. Time courses of the reduction of POM (■) and uptake of alkali metal ions (●) by the addition of aqueous ascorbic acid and (a) KCl or (b) CsCl.
The photo images of the reversible color changes in the redox cycles are summarized in Figure 1e. Note that the reaction solutions are colorless, which exclude the possibility of leaching. The photo images of the same crystal through the redox process are shown in Figures 1ac (inset). The morphology of the crystal was basically unchanged, confirming that I-red-K+ and I-ox-K+ were not formed by a dissolution-recrystallization mechanism. Furthermore, the reversibility of the redox cycles was demonstrated by the elemental analysis (Table 1 entry 4)47 and the powder XRD pattern of the reduced compound after five consecutive redox cycles (V-red-K+) (see Figures 3c and e). It is well known especially in zeolites that metal ions in the pores function as guest binding sites.48 The water sorption isotherms (298 K) of I-as, I-red-K+, and I-ox-K+ after the pretreatment at 298 K in vacuo for > 3 h are shown in Figure 5a.49 The amount of water sorption in I-as increased almost linearly with the vapor pressure. In contrast, the amount of water sorption in I-red-K+ steeply increased at the low vapor pressure. The amount of water sorption reduced almost to half by the oxidation of I-red-K+ to form I-ox-K+, which agreed with the release of about half of K+ by this treatment. No sig-
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nificant increase in the amount of water sorption was observed by the treatment of I-as with only ascorbic acid (Figures S750). 8
(a)
Amount [mol mol1]
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Figure 5. Water sorption isotherms (298 K). In (a), circles, squares, and triangles show the data of I-as, I-red-K+, I-ox-K+, respectively. In (b), squares, circles, triangles, and diamonds show the data of I-red-K+, I-red-Na+, I-red-Rb+, and I-red-Cs+, respectively.
It is also well known that the host-guest interaction can be controlled by the ionic potential (z/r, where z and r are the charge and radius of the ion) of the ion in the host. For example, the amounts of water sorption in zeolites increase with the increase in the ionic potentials (i.e., decrease of ionic radius or increase in charge) of the counter cations.48 In this context, the uptakes of other alkali metal ions in I-as were investigated. When RbCl or CsCl were added to the aqueous solution containing I-as and ascorbic acid, the color change from yellow to blue was also observed, suggesting that the POM was reduced (Figure S4). The results of the elemental analyses showed that Rb+ and Cs+ were exchanged with mepyH+ and incorporated additionally into the solid bulk (Table 1 entries 5 and 6). The water sorption isotherms of I-red-Rb+ and I-red-Cs+ are shown in Figure 5b. While the total amounts of sorption were similar, the amounts at low vapor pressures decreased in the order of I-red-K+ > I-red-Rb+ > I-red-Cs+. This trend is in line with the increase in ionic radii (i.e., decrease in ionic potential) K+ (r = 1.38 Å) < Rb+ (r = 1.52 Å) < Cs+ (r = 1.67 Å),51 suggesting that alkali metal ions function as water sorption sites. On the other hand, when LiCl was added to the solution, little color change was observed (Figure S4). Elemental analyses showed that no Li+ and only one Na+ was incorpo-
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rated (Table 1 entry 7). To summarize above, the numbers of alkali metal ions incorporated in the reduced crystals (in parentheses) increased in the order of Li+(0) < Na+(1) < K+(2) ≈ Rb+(2) ≈ Cs+(2). This trend can be interpreted as follows. It is well known that ions are hydrated in aqueous solutions and the hydration radii52 (Li+ 3.82 Å, Na+ 3.58 Å, K+ 3.31 Å, Rb+ 3.29 Å, Cs+ 3.29 Å) are much larger than the ionic radii51 (Li+ 0.76 Å, Na+ 1.02 Å, K+ 1.38 Å, Rb+ 1.52 Å, Cs+ 1.67 Å). The degree of hydration is larger for smaller ions since an ion with higher ionic potential can interact more strongly with polar water molecules. The minimum aperture of the one-dimensional channels of I-as (ca. 3.2 Å) is much smaller than the hydration diameters of alkali metal ions (6.67.6 Å) and comparable to the ionic diameters (1.53.3 Å). Therefore, when the alkali metal ions enter and diffuse through the one-dimensional channels, the water of hydration should be at least partially removed. The Gibbs energy of hydration for alkali metal ions are reported to increase in the order of Li+ (529 kJ mol1) < Na+ (424 kJ mol1) < K+ (352 kJ mol1) < Rb+ (329 kJ mol) < Cs+ (306 kJ mol1).53 Therefore, it is more facile to remove the water of hydration from Cs+ than Li+, so that Cs+ can more easily enter and diffuse through the one-dimensional channels of I-as. According to the above idea, the time course of the reduction of I-as with the addition of CsCl instead of KCl was monitored by UV-vis spectroscopy (square plots in Figure 4b). The reduction apparently proceeded faster, and the rate constant was kred = 1.5 × 101 h1. The uptake of Cs+ in I-as also apparently proceeded faster, and the experimental profile (circle plots in Figure 4b) could be fairly reproduced with kion = 3.9 × 101 h1.54 The large differences in the rate constants between K+ and Cs+ can also be explained by the trends in the Gibbs energies of hydration. According to the crystal structures and kinetic analyses of the reduction processes, the mechanism of the cooperative migration of alkli metal ions with electrons in our system was considered. It is well known that proton-coupled electron transfer (PCET) is ubiquitous in energy conversion and storage reactions concerning proteins, enzymes, and even metal oxides,55,56 and these processes are analogous to the addition of both Li+ and electron upon charging a Li-ion secondary battery. In the crystal structures of I-as and I-red-K+, minimum distances between the adjacent POMs (distances between the terminal oxygen atoms) along the one-dimensional channel were 3.06 and 2.95 Å, respectively, and comparable to the diameter of O2 (2.80 Å)51. Therefore, adjacent POMs were close enough to provide paths to the cations diffusing through the one-dimensional channel. In fact, some of the K+ (K2 in the cif file) in I-red-K+ were located in the vicinity of two adjacent POMs with K+-O distances of 3.12 and 3.32 Å. Given that the cation uptake rates were faster than the reduction rates (Figure 4), the migration of cation is probably accompanied by the reduction of POM for the charge neutralization. Since the reduced crystals could incorporate two alkali metal ions per formula at maximum, the uptakes of group two metal ions were investigated. Out of the four ions (Mg2+, Ca2+, Sr2+, Ba2+), only Ba2+ with the smallest Gibbs energy of hydration was incorporated (Table 1 entry 8). Next, the competitive uptakes of alkali metal ions from binary aqueous mixtures were investigated: The uptakes from a mixture containing
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equimolar amounts of Na+ and Cs+ (or K+) showed 0% (or 50%) selectivity to Na+ (Table 1 entries 9 and 10). These results show that the ionic crystals are potentially suitable for the selective uptake of radioactive Cs+ in sea water, and the release of Cs+ can be controlled by the oxidation.57
CONCLUSION Redox-active porous ionic crystals based on POM were synthesized. By treating the crystal with an aqueous solution of ascorbic acid (reducing reagent) and KCl, one-electron reduction of POM proceeded followed by simultaneous uptake of K+. Interestingly, the reduction did not proceed without KCl, and the molecular size of ascorbic acid was too large to enter the porous crystal lattice. The time courses of reduction and K+ uptake were monitored by UV-vis spectroscopy and AAS, respectively. Both profiles could be reproduced by the LDF model with similar rate constants. The reduced crystal could be oxidized with aqueous chlorine solution followed by the release of K+, and the redox cycles were reversible. The water sorption properties of the crystals could be controlled by the types of alkali metal ions incorporated. The Cs+ uptake and the simultaneous reduction of the crystal proceeded much faster than in the case of K+, which is in line with the trends in the Gibbs energies of hydration of alkali metal ions. Complete selectivity to Cs+ was observed in the uptake of ions from an aqueous binary mixture of Cs+ and Na+. All these results suggest the cooperative migration of electrons with alkali metal ions and the redox induced ion-exchange in porous ionic crystals based on POM.
ASSOCIATED CONTENT Supporting Information. Figure S1: CO2 sorption isotherm (198 K) of I-as. Figure S2: 1H-NMR spectra (DMSO-d6) of the filtrates of the reaction mixtures. Figure S3: FTIR spectra. Figure S4: Time courses of the photo images of the reaction mixtures. Figure S5: Pawley analysis of the powder XRD pattern of I-ox-K+. Figure S6: Thermogravimetry. Figure S6: Additional water sorption isotherms (298 K). X-ray data of I-as, I-red-K+, and I-ox-K+ in cif format. This information is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Notes The authors declare no competing interests.
ACKNOWLEDGMENT This work was supported by JST-PRESTO and Grant-in-Aids for Scientific Research from the Ministry of Education, Culture, Science, Sports, and Technology of Japan. Dr. Kosuke Suzuki (Univ. of Tokyo) and Dr. Takeru Ito (Tokai Univ.) are acknowledged for the helpful comments. Prof. Keiichiro Ogawa (Univ. of Tokyo) is acknowledged for providing access to the UV-vis spectrometer. Prof. Motoyuki Matsuo and Dr. Katsumi Shozugawa (Univ. of Tokyo) are acknowledged for providing access to the ICP instrument.
REFERENCES
(1) Hill, C. L. Ed. Thematic Issue on Polyoxometalates, Chem. Rev. 1998, 98, 1-390. (2) Cronin, L.; Müller, A. Eds. Thematic Issue on Polyoxometalates, Chem. Soc. Rev. 2012, 22, 7325-7648. (3) Wang, H.; Hamanaka, S.; Nishimoto, Y.; Irle, S.; Yokoyama, T.; Yoshikawa, H.; Awaga, K. J. Am. Chem. Soc. 2012, 134, 49184924. (4) Ritchie, C.; Streb, C.; Thiel, J.; Mitchell, S. G.; Miras, H. N.; Long, D-L.; Boyd, T.; Peacock, R. D.; McGlone, T.; Cronin, L. Angew. Chem. Int. Ed. 2008, 47, 6881-6884. (5) Mitchell, S. G.; Streb, C.; Miras, H. N.; Boyd, T.; Long, D-L.; Cronin, L. Nature Chem. 2010, 2, 308-312. (6) Inaguma, Y.; Liquan, C.; Itoh, M.; Nakamura, T.; Uchida, T.; Ikuta, H.; Wakihara, M. Solid State Commun. 1993, 86, 689-693. (7) Cao, Y.; Xiao, L.; Wang, W.; Choi, D.; Nie, Z.; Yu, J.; Saraf, L. V.; Yang, Z.; Liu, J. Adv. Mater. 2011, 23, 3155-3160. (8) Okubo, M.; Asakura, D.; Mizuno, Y.; Kim, J. D.; Mizokawa, T.; Kudo, T.; Honma, I. J. Phys. Chem. Lett. 2010, 1, 2063-2071. (9) Tatsuma, T.; Kuroiwa, Y.; Ishii, K.; Kudo, K.; Sakoda, A. Chem. Lett. 2014, 43, 1281-1283. (10) Mulfort, K. L.; Hupp, J. T. J. Am. Chem. Soc. 2007, 129, 9604-9605. (11) Kobayashi, Y.; Jacobs, B.; Allendorf, M. D.; Long, J. R. Chem. Mater. 2010, 22, 4120-4122. (12) Bloch, E. D.; Murray, L. J.; Queen, W. L.; Chavan, S.; Maximoff, S. N.; Bigi, J. P.; Krishna, R.; Peterson, V. K.; Grandjean, F.; Long, G. J.; Smit, B.; Bordiga, S.; Broan, C. M.; Long, J. R. J. Am. Chem. Soc. 2011, 133, 14814-14822. (13) Xiao, D. J.; Bloch, E. D.; Mason, J. A.; Queen, W. L.; Hudson, M. R.; Planas, N.; Borycz, J.; Dzubak, A. L.; Verma, P.; Lee, K.; Bonino, F.; Crocellá V.; Yano, J.; Bordiga, S.; Truhlar, D. G.; Gagliardi, Brown, C. M.; Long, J. R. Nature Chem. 2014, 6, 590-595. (14) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry, 2nd Edition, Oxford University Press, Oxford, 1994. (15) Bennett, M. V.; Beauvais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2001, 123, 8022-8032. (16) Takamizawa, S.; Akatsuka, T.; Ueda, T. Angew. Chem. Int. Ed. 2008, 47, 1689-1692. (17) Cheng, X. N.; Xue, W.; Lin, J. B.; Chen, X. M. Chem. Commun. 2010, 46, 246-248. (18) Uchida, S.; Hashimoto, M.; Mizuno, N. Angew. Chem. Int. Ed. 2002, 41, 2814-2817. (19) Eguchi, R.; Uchida, S.; Mizuno, N. Angew. Chem. Int. Ed. 2012, 51, 1635-1639. (20) Zhang, Z.; Sadakane, M.; Murayama, T.; Sakaguchi, N.; Ueda, W. Inorg. Chem. 2014, 53, 7309-7318. (21) Uchida, S.; Takahashi, E.; Mizuno, N. Inorg. Chem. 2013, 52, 9320-9326. (22) Vimont, A.; Goupil, J. M.; Lavalley, J. C.; Daturi, M.; Surble, S.; Serre, C.; Millange, F.; Férey, G.; Audebrand, N. J. Am. Chem. Soc. 2006, 128, 3218-3227. (23) Fujihara, T.; Aonahata, J.; Kumakura, S.; Nagasawa, A.; Murakami, K.; Ito, T. Inorg. Chem. 1998, 37, 3779-3784. (24) The calculated weight and volume of I-as (anhydride) are 3360 g mol1 and 1502 cm3 mol1, respectively, and I-as can store and discharge 1 mol mol1 of electron. The gravimetric and volumetric capacities can be calculated as follows by using the following relationships, 1 mAh = 103 C s × 3600 s = 3.6 C and 1 F = 96500 C mol1: 96500 ÷ 3360 × 1 ÷ 3.6 = 8.0 mAh g1, 96500 ÷ 1502 × 1 ÷ 3.6 = 18 mAh cm3. (25) Uchida, S.; Eguchi, R.; Mizuno, N. Angew. Chem. Int. Ed. 2010, 49, 9930-9934. (26) Neumann, M. A. J. Appl. Cryst. 2003, 36, 356-365. (27) Pawley, G. S. J. Appl. Crystallogr. 1981, 14, 357-361. (28) The framework structure of I-as (Figure 1a) was similar to those of K2[Cr3O(OOCH)6(etpy)3]2[-SiW12O40]19 (etpy = 4ethylpyridine) and K2[Cr3O(OOCH)6(mepy)3]2[-SiW12O40]25.
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(29) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Chem. Soc. Rev. 2009, 38, 1477-1504. (30) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area, and Porosity, Academic Press, London, 1982. (31) Reduction of the compounds mentioned in ref. 28 do not proceed under the same reaction conditions. (32) Fang, W.; Zhang, T.; Liu, Y.; Lu, R.; Guan, C.; Zhao, Y.; Yao, J. Mater. Chem. Phys. 2002, 77, 294-298. (33) The molar absorption coefficients M1 cm1 of the IVCT bands (13900 cm1 or 720 nm) of I-red-K+ and I-red-Cs+ were calculated from the UV-vis spectra of the DMSO solutions (the solids are insoluble in water and common organic solvents). The values were 3.54 × 10 and 3.60 × 10 for I-red-K+ and I-red-Cs+, respectively, and fairly agreed with those reported for reduced POMs ( ∼103).34,35 (34) Sanchez, C.; Livage, J.; Launay, J. P.; Fournier, M.; Jeannin, Y. J. Am. Chem. Soc. 1982, 104, 3194-3202. (35) Miras, H. N.; Richmond, C. J.; Long, D. L.; Cronin, L. J. Am. Chem. Soc. 2012, 134, 3816-3824. (36) Fournier, M.; Rocchiccioli-Deltcheff, C.; Kazansky, L. P. Chem. Phys. Lett. 1994, 223, 297-300. (37) Sun, H. R.; Zhang, S. Y.; Xu, J. Q.; Yang, G. Y.; Shi, T. S. J. Electroanal. Chem. 1998, 455, 57-68. (38) Akutagawa, T.; Endo, D.; Imai, H.; Noro, S.; Cronin, L.; Nakamura, T. Inorg. Chem. 2006, 45, 8628−8637 (39) Potassium iodide (KI) and hydrazine (N2H4), which are relatively small in size and common reducing reagents, were tested instead of ascorbic acid. When an excess amount of KI was added to the heterogeneous mixture of water and I-as, little color change was observed in the solid, suggesting that the reduction was incomplete. When N2H4 was added, the colorless and transparent solution turned brown almost immediately, suggesting the dissolution of the solid. Therefore, only ascorbic acid was effective as a reducing reagent. (40) Multi-electron reduction did not occur even by the addition of an excess (10 times) amount of ascorbic acid (Figure S3f, IR spectra). The calculated void volume of I-as (Figure 1d) is ca. 603 Å3 per unit cell (Z = 4), and the calculated volume of K+ (r = 1.38 Å) is ca. 11 Å3. Assuming the closest packing of K+ in the pore (74% occupation), 603 ÷ 4 × 0.74 ÷ 11 = 10 mol mol1 is the maximum theoretical value for the amount of K+ in I-as. Since closest packing of ions is not possible because of the repulsion, realistic values should be much lower than 10 mol mol1. This calculation implies that the increase in void volume without changing the channel aperture in I-as is a way to carry out multi-electron reduction and to incorporate more alkali metal ions. (41) Mizuno, N.; Watanabe, T.; Misono, M. J. Phys. Chem. 1985, 89, 80-85. (42) Itagaki, S.; Yamaguchi, K.; Mizuno, N. Chem. Mater. 2011, 23, 4102-4104. (43) Reid, C. R.; Thomas, K. M. Langmuir 1999, 15, 3206-3218. (44) Fletcher, A. J.; Cussen, E. J.; Bradshaw, D.; Rosseinsky, M. J.; Thomas, K. M. J. Am. Chem. Soc. 2004, 126, 9750-9759. (45) The concentration of potassium in the solute was 9.5 ppm (calc. 11.4 ppm for the release of one K+ per formula). (46) The powder XRD pattern of I-ox-K+ was slightly different from those of I-as and I-red-K+. Because of the poor quality of the single crystal X-ray data (R1 = 0.2332, wR2 = 0.5534), the lattice parameters of I-ox-K+ were refined with the powder XRD pattern by the Pawley method27 (Figure S5, Rw = 14.80%). (47) The amounts of potassium (2.04 2.25wt%) and carbon (17.44 16.43wt%) slightly increased and decreased, respectively, by repeating the redox cycles (see entries 2 and 4 in Table 1). This is probably because the exchange of mepyH+ with K+ is slightly incomplete after the first reduction and completes by repeating the redox cycles. (48) Dzhigit, O. M.; Kiselev, A. V.; Mikos, K. N.; Muttik, G. G.; Rahmanova, T. A. Trans. Faraday Soc. 1971, 67, 458-467. (49) Themogravimetry of I-as, I-red-K+, and I-ox-K+ (Figure S6) showed that ca. 3 (weight loss of 1.11.8wt%) out of 5 mol mol1 of
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the water of crystallization desorbed by the treatment in vacuo or under dry N2 flow at 298303 K. While complete desorption could be attained at > 423 K, irreversible removal of mepy ligands and change in the crystal structures occurred. Therefore, the pretreatment was carried out at 298 K in vacuo. (50) Figure S7 shows that the amounts of water sorption slightly increased with the repetition of redox cycles. This is in line with the slight increase in the amount of K+ by repeating the redox cycles (see ref. 47). (51) Shannon, R. D. Acta Crystallogr. Sect. A 1976, 32, 751-767. (52) Nightingale Jr., E. R. J. Phys. Chem. 1959, 63, 1381-1387. (53) Fawcett, W. R. J. Phys. Chem. B 1999, 103, 11181-11185. (54) In contrast to the case of K+, the uptake of Cs+ (kion) proceeded faster than the reduction of the bulk solid (kred). While simple ionexchange hardly proceeded between mepyH+ and K+ (ca. 10%), simple ion-exchange between mepyH+ and Cs+ partially proceeded (ca. 30%) probably because of the smaller Gibbs energy of hydration for Cs+. Therefore, the uptake of Cs+ was probably the sum of ‘simple’ ion-exchange and ‘reduction-induced’ ion-exchange processes. (55) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer T. J. Chem. Rev. 2012, 112, 4016-4093. (56) Schrauben, J. N.; Hayoun, R.; Valdez, C. N.; Braten, M.; Fridley, L.; Mayer, J. M. Science 2012, 336, 1298-1301. (57) While Keplerate POM macroions {Mo72Fe30} also exhibit competitive recognition of alkali metal ions based on hydration radii,58 redox of POM is not utilized in this system. (58) Pigga, J. M.; Teprovich, J. A.; Flowers II, R. A.; Antonio, M. R.; Liu, T. Langmuir 2010, 26, 9449-9456.
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