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Compounds 1 and 2 are prepared in aqueous solution by one-pot reaction of ... and tetrameric assembly of [(Mo3O8){O3PC(O)(CH2-3-C5NH5)PO3}]2– subuni...
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Assembly of Dimeric and Tetrameric Complexes of Polyoxomolybdobisphosphonates Built from [(Mo3O8){O3PC(O)(CH2‑3‑C5NH5)PO3}]2− Subunits Lu Yang,† Zhen Zhou,† Pengtao Ma,† Jingping Wang,*,† and Jingyang Niu*,†,‡ †

Henan Key Laboratory of Polyoxometalate, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, China ‡ State Key Laboratory of Coordination Chemistry, Nanjing, Jiangsu 210093, China S Supporting Information *

ABSTRACT: Two hybrid organic−inorganic organobisphosphonate polyoxomolybdates, Na4H2[(Mo3O8)2(O){O3PC(O)(CH2-3-C5NH5)PO3}2]·12H2O (1) and Na8[(Mo3O8){O3PC(O)(CH2-3-C5NH5)PO3}]4·30H2O (2), have been successfully synthesized and structurally characterized. Compounds 1 and 2 are prepared in aqueous solution by one-pot reaction of 1-hydroxo-2-(3-pyridyl) ethylidenebisphosphonate (risedronic acid) with molybdate. Structural analysis reveals that the two polyanions are composed of the same building blocks and can be described as a dimeric and tetrameric assembly of [(Mo3O8){O3PC(O)(CH23-C5NH5)PO3}]2− subunits, respectively. Elemental analysis, IR spectra, thermogravimetric analysis, and luminescence properties of the compounds have been studied in the solid state. The compounds are also characterized in the solution by multinuclear 31P NMR spectrometry, which evidences a subtle equilibrium between some species, and the results indicate that 1 presents two conformers of these hexanuclear polyanions in solution, whereas 2 is metastable, which might gradually decompose into one isomer of the dimer units. The photochromic property of 1 is determined under ultraviolet (UV) irradiation of 365 nm, and the sample develops a reddish coloration.

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INTRODUCTION Hybrid organic−inorganic polyoxometalates, as a significant class of polyoxometalate (POM) chemistry, have received considerable interests due to their structural diversity and attractive applications.1 In particular, functionalized POMs with organic or organometallic ligands, such as carboxylic acid, alkoxy, organosilicon, organotin, and organophosphonates, grafted on the polyanions have achieved a dominant position.2 Bisphosphonates (BPs), generally formularized as H2O3PC(R1)(R2)PO3H2, are good modification reagents with covalently bonded and multifunctional heterogroups and not only can construct original architectures as new building blocks but also their compounds can be utilized for various potential properties.3 Alendronate acid (R1 = OH, R2 = (CH2)3NH2; noted as Ale), as one of the examples containing N+−H groups, could incorporate into POMs and interact with the building units by the formative hydrogen bonds. Both the various structures and photochromic properties,4 catalysis,5 and biological activity against tumor cell lines6 of Ale-based POMs have been explored sufficiently and widely. In addition, some other functionalized organobisphosphonate ligands, also containing N atoms with aromatic rings, such as risedronic acid (R1 = H, R2 = CH2(C5H5N); here noted as Ris), have attracted increasing attention with the expectation of exploring more functionalized organobisphosphonates constructed POM build© 2013 American Chemical Society

ing blocks. To the best of our knowledge, the aromatic pyridyl groups are conjugated with high delocalization and act as good electron donors, easily bringing about charge transfer. These unique characteristics of these types of ligands can optimize the optical properties via molecule design and supramolecule assembly.7 To date, Ris acid associated with the hybrid POMs units have been rarely explored, besides three examples of risedronatecontaining polyoxomolybdates that reacted with a [MoV2O4]2+ dinuclear moiety reported by Kortz’s group recently.8 Two of the compounds are mixed-valent MoV/VI fragments with open {Mo4} and {Mo6} structures, whereas another is a cyclic ring of an octanuclear complex. However, no unitary MoVI POMs constructed of risedronate-functionalized ligands have been explored. Although [MoV2O4]2+ moieties are regarded as common building units in organobisphosphonate POM chemistry and many novel structures have been obtained by these blocks, for instance, the chair conformation of the cyclohexane-like ring [(MoV2O4)6(OH)6(O3PCH2PO3)6]18−,9a fluorinated-diphosphonate polyanion [{MoV 2O 4(H2 O)}4 {O3PC(CF3)(O)PO3}4]12−,9b and the C-shaped mixed-valent Received: February 21, 2013 Revised: April 18, 2013 Published: April 18, 2013 2540

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filtrate was left to evaporate slowly at room temperature. Colorless crystals were collected after 2 weeks. Yield: 0.12 g (46.3% based on risedronic acid). Anal. Found (%): Mo, 33.82; P, 7.31; Na, 6.01; C, 10.06; N, 1.41; H, 2.62. Calcd.: Mo, 33.58; P, 7.22; Na, 5.36; C, 9.81; N, 1.63; H, 2.23. IR (KBr pellets): ν (cm−1) = 1637 (m), 1468 (w), 1159 (s), 1099 (s), 1075 (m), 1051 (m), 1033 (m), 925 (vs), 895 (vs), 802 (w), 745 (s), 703 (s), 646 (s), 585 (m), 525 (m). Synthesis of Na 8 [(Mo 3 O 8 ){O 3 PC(O)(CH 2 -3-C 5 NH 5 )PO3}]4·30H2O (2). A 0.330 g (0.265 mmol) portion of (NH4)6Mo7O24·4H2O and 0.210 g (0.741 mmol) of risedronic acid were dissolved in 10 mL of 1 M sodium acetate buffer solution (pH = 4.0) with stirring, and then 24 μL of N2H4·2H2O was added. The mixture was heated at 60 °C for 3 h. The dark green solution was filtered off, and the filtrate was left to evaporate slowly at room temperature. Dark green crystals were collected after 2 days. Yield: 0.06 g (11.9% based on Mo). Anal. Found (%): Mo, 33.76; P, 7.24; Na, 5.48; C, 9.09; N, 1.87; H, 3.15. Calcd.: Mo, 32.80; P, 7.06; Na, 5.24; C, 9.58; N, 1.60; H, 2.70. IR (KBr pellets): ν (cm−1) = 1636 (m), 1398 (m), 1382 (m), 1154 (s), 1093 (s), 1063 (vs), 1033 (s), 915 (vs), 884 (s), 776 (s), 706 (s), 683 (s), 659 (s), 588 (m), 523 (m). X-ray Crystallographic Analyses. X-ray structure analysis on single crystals was performed on a Bruker CCD Apex-II diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 296K. The structures of compounds 1 and 2 were solved by direct methods and further refined by full-matrix least-squares refinements on F2 using the SHELXL-97 software, and an absorption correction was performed using the SADABS program.20 In all structures, it was difficult to estimate all the disordered water molecules and the alkali metal counterions. Moreover, NH4+ and lattice H2O could not be distinguished based on electron densities, and we thus determined the lattice water molecules and NH4+ ions by elemental analysis. For 1 and 2, the hydrogens of all the C atoms were added in calculated positions and were refined isotropically as a riding mode by the default SHELXL parameters. Non-H atoms were refined with anisotropic displacement parameters. Crystallographic data and structural refinements for 1 and 2 are summarized in Table 1. CCDC 916890 (1) and 916891 (2)

structure [(MoV2O4)(MoVI2O6)2{O3PC(O)(CH3)PO3}2]8−,9c the field of MoVI chemistry has proven to be more active than MoV or mixed-valent MoV/VI compounds.6c,10 In addition, most potential properties, as mentioned above, are determined by BP polyoxomolybdate complexes in the state of MoVI. For example, we have recently synthesized a family of lanthanidecontaining an organobisphosphonate (H2O3PCCH3OHPO3H2, etidronic acid) and polyoxomolybdates (MoVI), which creates a cage with a small cavity and exhibits high photocatalytic activities for the degradation of rhodamine B (RhB) dye.11 It follows that the design of assembling MoVI polyoxomolybdates with Ris acid is feasibly expected to have a major contribution to enrich the structures and functionalizations of this system. On the basis of the above consideration, we thus decided to focus our study on MoVI/BP polyoxomolybdates based on Ris ligands. Herein, two compounds with a dimeric and tetrameric assembly of [(Mo3O8){O3PC(O)(CH2-3-C5NH5)PO3}]2− subunits have been isolated in aqueous solution, which represent the first examples of unitary MoVI compounds constructed of Ris. Given the presence of the pyridyl group in Ris, the luminescence behaviors of 1 and 2 have been studied. We also describe the 31P NMR spectroscopic characterization of the two compounds, both dissolved in D2O and 1 M sodium acetate buffer. The optical properties of compound 1 in the ground state under 365 nm UV irradiation have been examined, and the color of 1 shifts from white to reddish.

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EXPERIMENTAL SECTION

General Methods and Materials. All chemicals and solvents were used as purchased without further purification. C, H, and N elemental analyses were performed by using a PerkinElmer 2400-II CHNS/O analyzer. Inductively coupled plasma (ICP) spectra (P and Mo) were obtained on a PerkinElmer Optima 2000 ICP-OES spectrometer. The amount of Na was further determined by an atomic absorption spectrum (AAS) on a HITACHI Z-2000. The infrared spectra (using KBr in pellets) were recorded on a Bruker VERTEX 70 IR spectrometer (4000−400 cm−1). The TG analyses were measured under the nitrogen gas atmosphere on a MettlerToledo TGA/SDTA851e instrument with a heating rate of 10 °C/min from 25 to 1000 °C. XPS were recorded by an Axis Ultra (Kratos, U.K.) photoelectron spectroscope with Al Kα (1486.7 eV) irradiation. The photoluminescence property was determined on a HITACHI F7000 fluorescence spectrophotometer in the solid state at the room temperature. 31P NMR spectra were detected in 5 mm tubes with 1H decoupling on a Bruker AV-400 model spectrometer operating at 400 MHz. 31P chemical shifts were referenced to the 85% H3PO4 as external standard. For the two compounds, about 15 mg of sample was dissolved in D2O (700 μL). Thus, the concentrations varied with each other. EPR experiments were performed on a BrukerER-2000DSRC10 spectrometer at the X-band at 300 and 110 K. Diffuse reflectivity spectra were collected on a HITACHI U-4100 UV−vis spectrometer with a 60 mm diameter integrating sphere at room temperature. Diffuse reflectivity was measured from 250 to 1000 nm (i.e., from 5 to 1.24 eV) with a 2 nm step using nonabsorbing BaSO4 powder as reference. The samples were irradiated with a ZSZ-6-B type ultraviolet lamp (λexc = 365 nm, P = 12 W). Synthesis of Na4H 2[(Mo3O8) 2(O){O3PC(O)(CH 2-3-C5NH5)PO3}2]·12H2O (1). Method (I): A 0.330 g (0.265 mmol) portion of (NH4)6Mo7O24·4H2O and 0.210 g (0.741 mmol) of risedronic acid were dissolved in 10 mL of 1 M sodium acetate buffer solution (pH = 4.0) with stirring. The mixture was heated at 60 °C for 3 h and filtered off the solution to evaporate the filtrate slowly at room temperature. Method (II): A 0.436 g (1.80 mmol) portion of Na2MoO4·2H2O and 0.085 g (0.300 mmol) of risedronic acid were dissolved in 10 mL of distilled water with stirring, and then 2 mL of DMF was added. The mixture was heated at 60 °C for 3 h after the pH value was adjusted to 4.0 with 1 M H2SO4. The colorless solution was filtered off, and the

Table 1. Crystallographic Data and Structural Refinements for 1 and 2 compounds empirical formula formula weight T/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalc/Mg m−3 μ/mm−1 F(000) Rint data/parameters GOF R [I > 2σ(I)]a R indices (all data)b Δρmax,min/e·Å−3 a

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1

2

C14H40Na4O43N2P4Mo 1715.94 296(2) monoclinic P21/c 18.686(7) 17.255(7) 18.508(7) 90 111.687(8) 90 5545(4) 4 2.066 1.567 3344 0.0502 9823/730 1.047 R1 = 0.0585 wR2 = 0.1670 R1 = 0.0811 wR2 = 0.1807 2.187/−1.036

C28H94Na8O90N4P8Mo1 3510.01 296(2) triclinic P1̅ 17.799(3) 17.815(3) 19.653(3) 92.461(3) 90.371(2) 117.609(2) 5514.4(15) 2 2.122 1.582 3458 0.0276 19314/1432 1.065 R1 = 0.0491 wR2 = 0.1342 R1 = 0.0728 wR2 = 0.1465 1.518/−0.878

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2. dx.doi.org/10.1021/cg400292g | Cryst. Growth Des. 2013, 13, 2540−2547

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contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/date_request/cif.

the valence of Mo atoms based on the BVS (bond valence sum) calculations. The results were further confirmed by XPS (X-ray photoelectron spectroscopy). In addition, we have tried to change the source of molybdate into Na2MoO4·2H2O for 2 at the same condition, but no similar crystals were obtained. Thus, for 2, the starting reaction source can be regarded as another significant factor in this system. With the successive isolation of compound 2, the color of the solution gradually turned to shallow, and then some white crystals of 1 and white unknown powders were formed. We investigated a wide condition of the system, in an effort to avoid the cocrystallization of compounds 1 and 2 during this section, but we failed to obtain the pure crystalline phase of 2. As a result, we finally collected the dark green crystals by filtering the solution as soon as they appeared. Structural Descriptions. Single-crystal X-ray analysis reveals that 1 crystallizes in the monoclinic system with space group P21/c and 2 in the triclinic system with space group P1̅. A common structural feature of polyanions 1 and 2 (Figure 1) is the assembly of equivalent [(Mo3O8){O3PC(O)(CH2-3C5NH5)PO3}]2− subunits, composed of a Mo−trimeric species and a bisphosphonate group grafted on the trimeric through P−O bonds. Therefore, 1 and 2 can be described as dimeric and tetrameric of the subunits connected via μ2 oxygen atoms, respectively (represented in Figure 2). Each subunit of the Mo−trimeric is composed of a dimer of face-sharing {MoO6} octahedra and another {MoO6} octahedron connected by a common vertex oxygen atom; the hexadentate risedronic ligands graft to the terminal O atom inside the trimeric. The Mo−O bond lengths range from 1.679(5) to 2.385(5) Å, while the P−O bond lengths are between 1.490(6) and 1.572(5) Å. The BVS calculations indicate that all the Mo atoms in compounds 1 and 2 are in the oxidation state of +6, and the calculations of Mo atoms are given in Tables S1 and S2 (in the Supporting Information), while all the P atoms from bisphosphonate ligands are +3. These results are further indicated by XPS spectra (Figures S2 and S3 in the Supporting Information). In addition, the phase purity was confirmed by Xray powder diffraction (Figures S4 and S5 in the Supporting Information). Compound 1 consists of two [(Mo3O8){O3PC(O)(CH2-3C5NH5)PO3}]2− fragments linked by sharing one μ2 oxygen atom, which has the approximate C2h symmetry (Figure 1a). All the six MoVI cations are basically located in the same plane only

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RESULTS AND DISCUSSION Syntheses. Compounds 1 and 2 were synthesized in the aqueous solution by the one-pot reaction of risedronic acid with molybdate. The synthetic routes followed for the formation of 1 and 2 are summarized in Scheme 1. Compound 1 could be

Scheme 1. Synthetic Routes of 1 and 2 and the Representation of the Bisphosphonate Ligands Used in This Work

prepared by two different methods. On one hand, Na2MoO4·2H2O was used as a starting material by the dropwise addition of 1 M H2SO4 solution to adjust the pH value to 4.0. The obtained colorless crystals were suitable for the singlecrystal X-ray diffraction. On the other hand, 1 could also be synthesized by dissolving the risedronic acid and (NH4)6Mo7O24·4H2O in 1 M sodium acetate buffer solution (pH = 4.0) at different stoichiometric ratios of these two reactants, compared with the former method, which was judged by the IR spectroscopy, but with poor crystal quality and low yield. Crystals of 2 were isolated only by the second method in the presence of reducing agent N2H4·2H2O. After an amount of parallel experiments, it is shown that only compound 1 can be obtained without adding N2H4·2H2O. We could conclude that N2H4·2H2O played a curial role in synthetic procedures of 2. However, the color of 2 is dark green, mainly due to the reduction of Mo atoms as the effect of N2H4·2H2O, and we can see the delocalized electron signals from the EPR (electron paramagnetic resonance, Figure S10 in the Supporting Information) spectrum. However, it is not enough to affect

Figure 1. Wireframe representation of compounds 1 (a) and 2 (b). The countercations and hydrogen atoms are omitted for clarity. Mo, sky blue; P, yellow; C, gray; N, blue; O, red. 2542

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the POM units with distances of donor and acceptor in the range of 2.69−2.89 Å (Table S3 in the Supporting Information) have been found. However, no significant π−π stacking interactions exist between the two parallel planes of the aromatic rings because of the distances beyond the range. The presence of countercations Na+ and the hydrogen bonds can make the structure form a three-dimentional framework. Polyanion 2 is composed of four [(Mo3O8){O3PC(O)(CH23-C5NH5)PO3}]2− subunits linking through four bridging oxygen atoms, as shown in Figure 1b. The windmill-type structure displays a coplanar configuration and has C 2 symmetry. It is very interesting to find that the four fragments are arranged clockwise, and the −CH2−C5H5N species of the risedronic acid drifting out of the building block is located up and down alternately, unlike the −CH2−C5H5N species in 1, which stay at the same layer (Figures 3b and 2). Compared with the reported windmill-type tetramer [Na{(Mo3O8)NH3CH2CH2CH2C(O)(PO3)2}4]7−,12,14 one significant difference is that the former has a Na+ cation acting as the template ion, while there is a square cavity in compound 2 (Figure S7 in the Supporting Information). Two anions are connected by a {Na4(H2O)10} fragment, which is located in the interlayer of them, and the structure further extends to a two-dimensional network (Figure 4). 31 P NMR Spectroscopic Characterization. To study whether the structures characterized in the crystal state were stable in solution, the 31P NMR spectroscopy was used as a powerful and easy-handling tool for measuring the nature of the species. Both 1 and 2 are soluble in water and measured at room temperature. However, the tetramer is less soluble than the dimer so that the signals in spectrum 2 are less intense than those of 1. The spectrum of 1 presents two signals at 19.59 and 19.71 ppm with relative intensities of 3.7:1.0, respectively. It is reported that there is an equilibrium in solution between the A and B conformers of this type of hexanuclear polyanion, so the two signals are corresponding to the presence of these two species.14 However, it is hard to distinguish the species from the two signals. In contrast, because 2 possesses C2 symmetry, there are two types of P that exist in the structure.15 As is shown in Figure 5a, the spectrum of 2 exhibits one singlet and one doublet resonance located at 19.59, 19.97, and 20.15 ppm, respectively. The doublet resonances, with almost equal intensity in the low field, are attributed to the two types of P atoms. The other singlet close to the expected ones has been placed in the similar chemical shift, and thus, we can deduce that compound 2 might decompose gradually into the dimer units. To further confirm this conjecture, we have also measured the 31P NMR spectroscopy of 2 after the slight heating of the solution for a while. It can be clearly seen that

Figure 2. Polyhedral representation of compounds 1 and 2 built up from two and four subunits grafted by Ris ligands. Sky blue octahedral, MoO6; yellow tetrahedral, PO3C.

with 0.53 Å of mean deviation from crystallography, and the central oxo bridge is positioned on a pseudo-inversion center. The two subunits are placed against each other, displaying an Sshaped structure (Figure S6 in the Supporting Information), and two Ris ligands are also situated on each side owing to the steric hindrance. The similar structure of hexanuclear polyanions constructed of other bisphosphonate ligands has been reported by Sergienko,12 Wang,13 and Mialane,14 and they observed two conformers: The A conformer, in which the six MoVI ions are almost coplanar, is the most frequently encountered. The B conformer exhibits a rotation around the central oxygen atom of a trimeric unit, leading to a twisted perpendicular plane. However, we have not found the B conformer of 1 after an amount of experiments. Intermolecular hydrogen bonds N−H···O between the conjugate groups and

Figure 3. Model of −CH2−C5H5N species of the risedronic acid located up and down alternately: (a) for 1 and (b) for 2. 2543

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Figure 4. Packing modes for 2 and the representation of a {Na4(H2O)10} fragment linking between the two polyanions.

Figure 6. 31P NMR spectra of free Ris ligands (top) and compound 1 (bottom) dissolved in 1 M sodium acetate buffer (pH = 4.0) at room temperature.

Figure 5. (a) 31P NMR spectra of compounds 1 and 2 dissolved in D2O at room temperature. (b) 31P NMR spectrum of compound 2 after the slight heating of the solution for a while.

the color of the solution has turned to colorless completely. The spectrum shows only one sharp singlet at 19.84 ppm, which is mainly attributed to the minor signal of one isomer of compound 1 (Figure 5b). In addition, we can note that all the peaks are within the range of those previously reported.16 Besides, there is no resonance corresponding to the free bisphosphonate ligands,8 which have poor solubility in D2O but can dissolve well in 1 M sodium acetate buffer (pH = 4.0) at room temperature (Figure 6, top). The sharp signal shifting from 16.64 to 19.41 ppm is the result of the different medium and pH value. Compared with the two spectra of compound 1 dissolved in D2O and buffer (Figure 6, bottom), there is no obvious change of the peaks, but with a slight shift toward lowfield values 22.59 and 22.67 ppm, which can also reveal the presence of the two conformers. Nevertheless, we have not obtained the spectrum of compound 2 in sodium acetate buffer because of its poor solubility. The results are in agreement with their symmetry considerations. Solid-State Luminescent Properties. The solid-state luminescence properties of the π-conjugated pyridyl group in compounds 1 and 2 have been detected in the solid state at room temperature. As shown in Figure 7, free Ris ligands and their two constructed compounds show approximately the

Figure 7. Solid-state emission spectra of free risedronic acid and compounds 1 and 2 at room temperature.

same emission upon excitation at 500 nm. Three main broad emission bands are observed in the ranges of 572−682, 682− 737, and 737−777 nm with different intensities, respectively. The CN bonds and the conjugate group presented in pyridyl rings exhibit good charge transporting characteristics and are regarded as strong electron donors. Therefore, these luminescence emissions, which are centered at visible light and NIR regions, are mainly due to the intraligand π → π* charge transfer.17 Compared with the three curves, the free ligand presents much stronger than those in compounds 1 and 2544

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2. The difference in the intense emission results from the different coordinate environments, which suggests that the metal−ligand coordination has an important effect on the luminescence emission. Optical Properties. 1 exhibits a white coloration in the ground state, whereas the crystals of 2 are isolated in dark green. Hence, only compound 1 is examined to explore the optical properties in ambient conditions by diffuse reflectance spectroscopy. The optical band gap is 385 nm (3.22 eV) for the {Mo6} inorganic core materials, which perfectly matches with those recently reported for Mo6−Ale, Mo12−Ale, and their derivatives.4,14 Under 12 W UV excitation at 365 nm (3.4 eV), a strong photochromic response of 1 is shown, and the obvious coloration contrast of the material is observed by the eyes after 5 min of irradiation. The color begins to shift from white to reddish with increasing of the irradiation time and can be stable for a certain time (Figure 8a). At approximately 60 min of

The proposed mechanism of this photochromic process relies on the presence of intermolecular N−H···O POM interactions between the two polyoxoanions of 1 (Table S3 in the Supporting Information). The conjugate group of the pyridyl ring is beneficial for charge transfer and the hydrogen bonds, acting as a bolt, conducting the electron that was produced via the d−d transitions of photoreduction from MoVI (4d0) to MoV (4d1).18 As a result, the POM units create a {MoV(OH)O5} site, as described previously, and the coloration is induced.19 The analysis follows the classical mechanism of solid-state photochromism, which was initially developed by the Dessapt’s group.19a,20 We have also superficially analyzed the kinetics of this coloration process. Figure 9a shows the

Figure 8. (a) The coloration change of compound 1 after 0, 5, 10, 20, 30, and 60 min of UV irradiation at 365 nm. (b) Kubelka−Munk transformed reflectivity of 1 vs irradiation times of 0, 2, 6, 8, 10, 20, 25, 30, 40, 50, 60, 70, 90, 110, 120, and 140 min.

irradiation, the color of the sample changes to saturated and cannot be further detected by the naked eyes. This photochromic process can also be taken under sunlight irradiation. After cutting off the UV excitation, the reddish color is gradually faded and the process is completed after keeping the sample in the dark for 1 day, which reveals a good reversibility. XPRD of the fresh sample and the sample after irradiating for 60 min has been performed, which reveals no significant structural change taking place during the photochromic process (Figure S4 in the Supporting Information). Figure 8b shows the reflectivity values transformed by the Kubelka−Munk function with different irradiation times in the 300−800 nm range. It shows an increasing strong absorption band peaking at 506 nm (2.45 eV) and a second quite lesser band slowly rises up at 780 nm (1.59 eV) associated with the irradiation time. The results of both the photogenerated absorption band and the reddish color are in agreement with the materials reported by Mialane’s group mentioned above.14

Figure 9. (a) Reflectivity R(t) vs t plots for 1 measured at 506 nm for 0, 2, 6, 8, 10, 20, 25, 30, 40, 50, 60, 70, 90, 110, 120, and 140 min of UV irradiation at 365 nm. Inset: The relative parameter related to the coloration kinetics of 1. (b) The linear relationship of [R506(t) − R506(∞)]−1 and irradiation time (min).

reflectivity values at the maximum wavelength of the absorption (Rλmax) versus irradiation time (t) in the range of 300−800 nm. The Rλmax values are decreasing sharply at first and then tend to flatten under UV excitation of 365 nm, which is attributed to the decrease of concentration of photoreducible MoVI, on the basis of a pseudo-second-order kinetic law. It has been found that the curve of R506(t) vs t can be fitted by the function R506(t) = a/(bt + 1) + [R506(t) − a], and the relative parameters 2545

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Crystal Growth & Design related to the coloration kinetics of the sample are listed in the inset in the figure. In addition, to obtain the coloration speed of 1, the linear [R506(t) − R506(∞)]−1 (R506(∞) stands for the reflectivity value in the infinitely long irradiation time at the maximum wavelength of 1) as a function of irradiation time has been calculated (Figure 9b). The coloration speed is signified by t1/2 (coloration kinetic half-life time), and the value calculates as 14.89 min, respectively. Compared with the other two species of structures of Mialane’s group, which are all made up of the same [(Mo3O8)4(O3PC(R)(R′)PO3)4]8− subunits, the t1/2 of 1 is lower than that of the [(Mo3O8)4(O3 PC(C 3H 6NH(CH3 ) 2)(O)PO 3) 4 ]8− and [(Mo 3O 8) 2 O(O3PC(C3H6NH2(CH3))(O)PO3)2]6−, similar with that of [(Mo3O8)2O(O3PC(C3H6NH(CH3)2)(O)PO3)2]6−, and higher than that of the rest.14 These studies indicate that this material could be regarded as the efficient photochromic hybrid POMs as well.

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REFERENCES

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CONCLUSIONS In conclusion, we have successfully synthesized two new hybrid POMs, Na 4 H 2 [(Mo 3 O 8 ) 2 (O){O 3 PC(O)(CH 2 -3-C 5 NH 5 )PO3}2]·12H2O (1) and Na8[(Mo3O8){O3PC(O)(CH2-3C5NH5)PO3}]4·30H2O (2), which represent the first risedronic acid/MoVI-based POM complexes by the one-pot reaction in aqueous solution. The common feature of 1 and 2 is that they are composed of the [(Mo3O8){O3PC(O)(CH2-3-C5NH5)PO3}]2− subunits. Polyanion 1 is the dimer, whereas 2 is the tetramer of the subunits. The compounds show luminescent behaviors due to the pyridine group from the bisphosphonate ligands, and the spectra of compounds 1 and 2 are wellconsistent with the free ligands, which is mainly attributed to the intraligand charge transfer. 31P NMR studies indicate that 1 presents two conformers of these hexanuclear polyanions in solution and 2 is metastable, which might gradually decompose into one isomer of the dimer units. The optical study of 1 shows the reversible photochemical response of color change under 365 nm UV irradiation, which mainly results from the interaction of N−H···O hydrogen bonds and the reduction of POM species. The Ris/MoVI system can be regarded as a kind of efficient photochromic hybrid material. More versatile geometric structures and potent properties of this family of hybrid POMs are under study. ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic details (CIF), partical atomic labeling scheme, and bond valence summations for compounds 1 and 2. The representations of XPRD paterns, IR spectra, XPS spectra, and thermogravimetric (TG) analysis are listed. Some additional structural figures for the two compounds are also included. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The authors thank the National Natural Science Foundation of China, the Foundation of Education Department of Henan Province, and the Natural Science Foundation of Henan Province for financial support.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.W.), [email protected] (J.N.). Tel: (+86) 378 3886876. Fax: (+86) 378 3886876. Notes

The authors declare no competing financial interest. 2546

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Crystal Growth & Design

Article

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