Controllable Assembly of Vanadium-Containing Polyoxoniobate

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Controllable Assembly of Vanadium-Containing PolyoxoniobateBased Three-Dimensional Organic−Inorganic Hybrid Compounds and Their Photocatalytic Properties Jufang Hu,† Yin Wang,† Xinning Zhang, Yingnan Chi,* Song Yang, Jikun Li, and Changwen Hu* Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China S Supporting Information *

ABSTRACT: The controllable synthesis of two vanadiumcontaining polyoxoniobate-based three-dimensional organic− inorganic hybrid compounds, [Co(pn) 2 ] 4 [HPNb 10 V IV 2 O 40 (V IV O) 4 ]·17H 2 O (1) and [Co(pn)2]5[PNb12O40(VIVO)6](OH)7·15H2O (2), where pn = 1,2-diaminopropane, is realized by changing the hydrothermal temperature or adding N-(aminoethyl)piperazine as an additive. Both compounds 1 and 2 are structurally characterized by single-crystal/powder X-ray diffraction and IR and X-ray photoelectron spectroscopy. Compound 1 features a new divanadium-substituted Keggin polyoxoniobate capped by four vanadyl groups, and the polyanion in 2 exhibits the highest coordination number (10-connected) in polyoxoniobate chemistry. Moreover, the photocatalytic activities of 1 and 2 for hydrogen evolution are preliminarily assessed.

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INTRODUCTION The Keggin-type polyanion with the formula {XM12O40}n− (X = heteroatom and M = addenda atom) is one of the most thoroughly studied and widely used members in the family of polyoxometalates (POMs).1 The incorporation of Keggin POMs with transition-metal (TM) organic units not only affords a variety of extended organic−inorganic hybrid compounds but also provides POMs with wide applications ranging from environmentally benign catalysis to medicine.2 Despite the considerable advancements made in the polyoxoniobates (PONbs),3−6 the number of reported Keggin-type PONbs is still far less than that of W/Mo-based Keggin POMs. The 5+ oxidation state of Nb renders Keggin-type PONb clusters with intrinsically high negative charge, which is expected to facilitate their coordination with TM organic units.3,4b However, to the best of our knowledge, there is no report about Keggin-type [XNb12O40]n−-based organic−inorganic hybrids. One possible reason is that Keggin PONb ions prefer to attract small Na+ or K+ cations, and as a result, the accessibility of other larger electrophilic units, such as TM− organic units, is blocked.3,7 V and its neighbor Nb have similar electronegativities as well as ionic radii,8 and their hydrolysis and condensation all occur under alkaline conditions. Inspired by these similarities, in 2011 we synthesized the first V-containing PONb cluster, the bicapped Keggin-type {VNb12O40(VO)2}, and for the first time the coordination connection between a TM−organic unit and a hetero-PONb was realized.9 The V-containing PONbs probably possess both the base catalysis of PONbs10 and the excellent redox properties of vanadate11 and might become a promising catalyst. Therefore, a list of V-containing Keggin © XXXX American Chemical Society

derivatives were isolated, including {XNb12O40(VO2)2} (X = Si,12a Ge,12a P6b), {VNb12O40(VO)4},12b {XNb12O40(VO)6} (X = P,12c V12b), {XNb8V4O40(VO)4} (X = P,12d V,12d As12e), and {AsNb9V3O40(VO)4}.12e All of them have two, four, or six V caps, and most of them are TM-modified hybrids.12 It seems that the capping V atoms contribute to not only stabilizing the polyanion clusters but also activating the surface O atoms of PONbs to form organic−inorganic hybrids.12c,d In general, basic conditions are necessary for the assembly and stability of PONbs, but under such conditions, most TM ions are hydrolyzed into hydroxide precipitation.3 Because copper complexes can readily coexist with PONb anions in alkaline solution, the organic−inorganic hybrids of PONbs are dominated by Cu ions.12,13 In contrast, Co-based PONb hybrids are very rare.14 The combination of a cobalt complex with PONbs as hybrids not only provides diamagnetic PONbs with magnetic properties but also has a positive influence on the photocatalytic activity of PONbs. In this work, two Vcontaining PONb-based three-dimensional (3D) organic− inorganic hybrid compounds, [Co(pn) 2 ] 4 [HPNb 10 V IV 2 O 40 (V IV O) 4 ]·17H 2 O (1) and [Co(pn)2]5[PNb12O40(VIVO)6](OH)7·15H2O (2), where pn = 1,2-diaminopropane, are assembled. Although the two compounds were prepared from the same starting materials under hydrothermal conditions, both the polyanion structures and the connection mode between PONbs and cobalt complexes are different (Scheme 1). The control experiments display that the temperature and additive are two important factors controlling Received: April 3, 2016

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DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

introduced as an additive. The pH was elevated from ∼10.7 to ∼11.4 after the addition of N-(aminoethyl)piperazine. Synthesis of Compound 2. Compound 2 was prepared following a similar procedure, except that the reaction temperature was raised to 160 °C. Block-shaped dark-brown crystals of 2 were isolated in 46% yield based on Nb. Anal. Calcd for C30H124Co5N20Nb12O58PV6: C, 10.48; H, 3.63; N, 8.14; P, 0.90; Co, 8.57; V, 8.89; Nb, 32.41. Found: C, 10.31; H, 3.43; N, 8.01; P, 0.88; Co, 8.34; V, 8.71; Nb, 32.29. IR (KBr, cm−1): 3414(s), 3321(s), 3245(s), 3145(m), 2961(w), 2933(w), 2875(w), 1638(m), 1593(s), 1459(m), 1395(w), 1380(m), 1337(w), 1306(w), 1270(w), 1197(w), 1035(s), 1016(s), 951(s), 877(s), 768(m), 695(s), 617(s), 522(w), 457(w). Single-Crystal X-ray Crystallography. Complete data for compounds 1 and 2 were collected at 296(2) K on a Bruker APEXII CCD detector with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELX program.17 All non-H atoms were refined anisotropically. The H atoms on the C and N atoms were fixed at the calculated positions. The H atoms on water were located from difference Fourier maps. Although 2 water molecules for 1 and 12 for 2 were found by single-crystal X-ray diffraction, according to the TGA results, we concluded that there are a total of 17 and 15 lattice water molecules in 1 and 2, respectively. The crystallographic data are summarized in Table 1.

Scheme 1. Controllable Syntheses of Compounds 1 and 2

their formation. It is worth mentioning that the divanadiumsubstituted {PNb10V2O40(VO)4} in 1 is a novel cluster and the 10-connected mode of {PNb12O40(VO)6} in 2 represents the highest coordination number to date in PONb chemistry. Furthermore, the preliminary photocatalytic properties of 1 and 2 for hydrogen evolution were investigated.

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EXPERIMENTAL SECTION

Table 1. Crystallographic Data and Structure Refinement for 1 and 2a

Materials and Methods. All common laboratory chemicals were reagent grade, were purchased from commercial sources, and were used without further purification. The hexaniobate K7HNb6O19·8H2O and vanadyl phosphate VOPO4·2H2O were prepared according to literature methods,15,16 and both were identified by IR spectroscopy and thermogravimetric analysis (TGA). Elemental analyses (C, H, and N) were measured on an Elementar Vario EL cube Elmer CHN elemental analyzer; Nb, V, Co, and P were determined by a Thermo iCAP 6000 atomic emission spectrometer. IR spectra were collected (as KBr-pressed pellets) on a Nicolet 170SXFT-IR spectrophotometer in the range 400−4000 cm−1. Powder X-ray diffraction (PXRD) data on samples were recorded on a Bruker instrument equipped with graphite-monochromatized Cu Kα radiation (λ = 0.154060 nm; scan speed of 8° min−1; 2θ = 5−50°) at room temperature. Diffusereflectance spectra were collected on a Cary 5G spectrometer (Varian) equipped with a 60-nm-diameter integrating sphere, and computer control uses the “Scan” software. The diffuse reflectance was measured from 200 to 800 nm with a 2 nm step using Teflon as the reference (100% reflectance). X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250 spectrometer using Al Kα radiation as the X-ray source (1486.7 eV) with a pass energy of 30 eV. The pressure inside the analyzer was maintained at 10−9 Torr. TGA of the samples was performed using a Shimadzu DTG-60AH thermal analyzer under nitrogen at a heating rate of 10 °C min−1. Synthesis of Compound 1. Method 1. To a solution of K7HNb6O19·8H2O (0.15 g, 0.12 mmol) in 8 mL of water were successively added VOPO4·2H2O (0.10 g, 0.51 mmol) and 2CoCO3· 3Co(OH)2·H2O (0.29 g, 0.55 mmol), followed by the addition of 0.5 mL of 1,2-propanediamine. The final pH value was ∼10.7. The mixture was stirred for 30 min at room temperature. The resulting solution was transferred to a Teflon-lined stainless steel autoclave (23 mL), kept in an oven at 140 °C for 24 h, and then cooled to room temperature at a rate of 5 °C h−1. Block-shaped dark-brown crystals of 1 were isolated in 60% yield based on Nb. Anal. Calcd for C24H84Co4N16Nb10O46PV6: C, 10.17; H, 2.99; N, 7.91; P, 1.09; Co, 8.32; V, 10.78; Nb, 32.78. Found: C, 10.23; H, 2.72; N, 7.81; P, 1.21; Co, 8.22; V, 10.83; Nb, 32.24. IR (KBr, cm−1): 3672(s), 3414(s), 3332(s), 3261(s), 3142(m), 2960(w), 2939(w), 2876(w), 1641(m), 1585(s), 1460(m), 1393(w), 1376(m), 1336(w), 1298(w), 1193(w), 1102(w), 1045(s), 1012(s), 959(s), 871(s), 777(m), 693(s), 617(s), 517(w), 451(w). Method 2. Compound 1 can also be prepared by a procedure similar to that of method 1, where the reaction temperature was elevated to 160 °C and N-(aminoethyl)piperazine (1 mL) was

formula fw (g mol−1) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) abs coeff (mm−1) F(000) measd reflns indep reflns reflns used Rint R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data)a wR2 (all data)b GOF on F2 a

1

2

C24H84Co4N16Nb10O46PV6 2834.50 tetragonal I4/m 16.9087(17) 16.9087(17) 16.6617(17) 90 90 90 4763.7(11) 2 1.976 2.481

C30H124Co5N20Nb12O58PV6 3439.66 triclinic P1̅ 13.3030(11) 14.1260(12) 14.3554(13) 77.1710(10) 80.0380(2) 63.0880(10) 2337.3(3) 1 2.442 2.957

2758 11767 2192 1657 0.1161 0.0482 0.1329

1686 11295 8234 4982 0.0302 0.0612 0.1588

0.0611 0.1402

0.1099 0.1986

0.996

1.024

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

Magnetic Measurement. Direct-current (dc) magnetic susceptibility measurement was carried out on the crystalline samples under an applied field of 1000 Oe using a Quantum Design MPMS-XL7 magnetometer operating between 2 and 300 K. The samples were compacted and immobilized into a c ylindrical poly(tetrafluoroethylene) sample holder. Diamagnetic corrections were applied using Pascal constants and diamagnetisms of the sample holder. B

DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Procedure of Photocatalytic Hydrogen Production. Photocatalytic hydrogen production was carried out using a CEL-SPH2N device. The photocatalytic reactions were performed in 50 mL of an aqueous solution containing 10% methanol as the sacrificial agent, 50 mg of POM as the photocatalyst, and 1% H4PtCl6 as the cocatalyst. The above mixture was irradiated by a 300 W xenon lamp, and the hydrogen produced was analyzed using a GC SP7800 instrument. The durability tests were carried out with a time interval of 5 h for each cycle. After each cycle, the catalyst was collected by centrifugation, washed by deionized water, and dried under vacuum conditions. Then additional cocatalyst and sacrificial agent were added before the next cycle. In order to confirm that compound 2 acted as a heterogeneous catalyst, the following two leaching tests were designed and conducted. The leach liquor A after 5 h of stirring without irradiation and B after the first run of the photocatalytic experiment were collected and used in the hydrogen evolution experiment. Photocurrent Experiment. The photocurrents were measured using an electrochemical analyzer (CHI 660C Instruments) in a standard three-electrode system with the as-prepared samples as the working electrode, a platinum wire as the counter electrode, and Ag/ AgCl (saturated KCl) as the reference electrode. A Na2SO4 aqueous solution (0.2 M) was used as the electrolyte. The working electrode was prepared as follows: 4 mg of compound 1 or 2 was suspended in 1 mL of a solution including 700 μL of water, 270 μL of ethanol, and 30 μL of a 5 wt % Nafion solution, and then 5 μL of the above resulting colloidal dispersion was dropped onto a glassy carbon electrode (diameter 3 mm).

Figure 1. Crystal structures of 1. (a) Ball-and-stick representation of the polyanion {PNb10V2O40(VO)4}. (b) Hydrogen-bonding interactions (represented by red dashed lines) between the lattice water molecules and {PNb10V2O40(VO)4}. The H atoms of the lattice water molecules are disordered. (c) Combined polyhedral and ball-and-stick drawing of the 3D structure. Color code: Nb, cyan; V, pink; Nb/V, light cyan; Co, olive; P, green; O, red; N, blue.

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RESULTS AND DISCUSSION Synthesis and Structures. Hydrothermal treatment of a mixture of potassium hexaniobate, vanadyl phosphate, and cobalt carbonate hydroxide in the presence of 1,2-diaminopropane at 140 °C gave rise to compound 1. Single-crystal X-ray diffraction reveals that compound 1 crystallizes in the tetragonal I4/m space group. The polyanion {PNb10V2O40(VO)4} can be described as a V2-substituted {PNb10V2O40} capped by four vanadyl groups (Figure 1a and Scheme 2). {PNb10V2O40(VO)4} in 1 represents a new kind of V-substituted PONb cluster following the isolation of tetrasubstituted {XNb8V4O40(VO)4} (X = P, V, As),12d,e and trisubstituted {AsNb9V3O40(VO)4}.12e The polyanion exhibits a site-occupancy disorder on the Nb2/V2 centers, and the lightcyan balls in Figure 1a represent half Nb and half V. The bond lengths are 1.635−1.755 Å for Nb−Ot (Ot = terminal oxygen), 1.913−2.024 Å for Nb−Ob (Ob = bridge oxygen), and 2.540− 2.627 Å for Nb−Oc (Oc = central oxygen). The bond lengths of V−Ot are 1.633−1.635 Å, and those of V−Ob are 1.948−1.993 Å. Interestingly, the remaining two square windows of {Nb4} are capped by two crystallographic water molecules (their H atoms are disordered) via hydrogen bonds with a O···O distance of 2.986 Å (Figure 1b). The central P atom lies on an inversion center and is surrounded by eight O atoms with a partial occupancy of 0.5. The P−Oc bond length is 1.561 Å. In this work, vanadyl phosphate was used as both V and P precursors instead of the most commonly used sodium metavanadate and phosphoric acid. The successful formation of a P-centered PONb rather than a V-centered one suggests that a P-centered PONb is thermodynamically more stable than a V-centered one, which is consistent with the conclusion of Casey et al.6b There is only one crystallographically independent Co center (Co1) in 1. The Co center is hexacoordinated with octahedral geometry and coordinates with four N atoms from two pn ligands and two Ot atoms of {NbO6} from two adjacent PONbs. The bond lengths are 2.119−2.155 Å for Co−N and

Scheme 2. Schematic View of the Structures of {PNb10V2O40(VO)4} in 1 and {PNb12O40(VO)6} in 2a

a

Color mode: NbO6, cyan octahedra; VO5, pink octahedra; VO6, yellow octahedra; PO4, green octahedra.

2.167 Å for Co−O t . In 1, each {PNb 10 V 2 O 40 (VO)4 } coordinates with eight neighboring cobalt complexes, and the cobalt complexes and polyanions are connected into a 3D organic−inorganic architecture (Figure 1c). Compound 1 represents the first 3D PONb-based hybrid, which is assembled only by coordination bonds. When the hydrothermal temperature was raised to 160 °C, compound 2 was prepared using the same starting materials. Compound 2 crystallizes in the triclinic P1̅ space group. The polyanion {PNb12O40(VO) 6} features a typical Keggin {PNb12O40} capping six vanadyl groups on all six square windows (Scheme 2). The corresponding bond lengths are 1.672−1.711 Å for Nb−Ot, 1.938−1.998 Å for Nb−Ob, 2.508− C

DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX

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Compared with 1, the additional coordination O atoms in 2 are from two {VO5} caps. Therefore, we speculate that raising temperature is likely to activate the Ot atoms of {VO5}. Alternatively, compound 1 could be prepared at 160 °C when N-(aminoethyl)piperazine was used as the additive. After the addition of N-(aminoethyl)piperazine, the pH of the resulting solution increased from ∼10.7 to ∼11.4. In order to investigate whether the improved pH value could affect the product, the following control experiments were performed. Without N(aminoethyl)piperazine, even if the pH value was adjusted to ∼11.4, only compound 2 was obtained instead of 1. The above results indicate that the steric hindrance rather than the pH value controls the assembly of the final product. The large steric hindrance of N-(aminoethyl)piperazine prevents the accessibility of cobalt complexes. The controllable synthesis in our experiments is helpful to the rational design of PONb-based compounds. The phase purity of 1 and 2 was confirmed by PXRD (Figure S3). In the IR spectra (Figure S4), the P−O vibrations are clearly observed at 1012 and 1016 cm−1 for 1 and 2, respectively. Peaks corresponding to NbOt and Nb−Ob are at 874 and 693 cm−1 for 1 and at 876 and 693 cm−1 for 2. We assign the strong bands at 960 cm−1 in both 1 and 2 to the vibration mode of VO.19 Peaks of the ligand are in the ranges of 1040−1460 and 2870−2970 cm−1. Information about the oxidation states of the metal ions was provided by XPS. In XPS, peaks at 516.48 eV for 1 and at 516.65 eV for 2 are attributable to VIV (Figure S8).20 Previous studies also mentioned that under hydrothermal conditions VV in the starting materials is easily reduced by organoamine to VVI. Well-resolved peaks of XPS (781.53 and 796.58 eV for 1; 781.37 and 796.47 eV for 2) together with satellite structures support 2+ of cobalt (Figure S9).21 The oxidation state determination is in accordance with the electronic spectra (Figure S5). Magnetic Properties. Variant temperature-dependent magnetic studies for 1 and 2 were performed. The dc magnetic properties were measured under an external magnetic field of 1000 Oe in the temperature range of 2−300 K (Figure 3). At room temperature, the χMT values (11.08 cm3 K mol−1 for 1 and 13.25 cm3 K mol−1 for 2) are higher than the expected ones (9.72 cm3 K mol−1 for 1 and 11.595 cm3 K mol−1 for 2). For 1, upon cooling, the χMT value slowly decreases to 9.66 cm3 K mol−1 at 100 K and then rapidly decreases, reaching a value of 5.55 cm3 K mol−1 at 2 K. For 2, the χMT value declines

2.636 Å for Nb−Oc, 1.594−1.670 Å for V−Ot, and 1.947− 2.031 Å for V−Ob. Similar to 1, the central P atom of 2 lies on an inversion center and is coordinated with eight O atoms with an occupancy of 0.5. The P−Oc bond lengths are in the range of 1.486−1.601 Å. There are four crystallographically independent Co centers in 2 (Figure 2a). All of the Co centers in 2 are hexacoordinated

Figure 2. Crystal structures of 2. (a) Connections between {PNb12O40(VO)6} and cobalt complexes. (b) Combined polyhedral and ball-and-stick drawing of the 3D framework. Color code: Nb, cyan; V, pink; Co, olive; P, green; O, red; N, blue.

with octahedral geometries. Co1, Co2, and Co4 coordinate with four N atoms from two pn ligands and two Ot atoms from {NbO6} (Figure S2a). Co3 coordinates with four N atoms from two ligands and two Ot atoms: one from the {VO5} cap and the other from {NbO6} (Figure S2b). The bond lengths are 2.090− 2.209 Å for Co−N and 2.135−2.227 Å for Co−Ot. Each {PNb12O40(VO)6} connects with 10 adjacent cobalt complexes, and the surrounding cobalt complexes extend the polyanions into a 3D organic−inorganic architecture (Figure 2b). The 10connected mode of {PNb12O40(VO)6} in 2 represents the highest coordination connection in PONb chemisry. In the construction of POM-based organic−inorganic hybrids besides starting materials, some chemical stimuli such as the pH value and additive also affect the formation of hybrids.18 Generally, the synthesis of hetero-PONb clusters is performed under hydrothermal conditions. Because the hydrothermal reaction is known as a “black box”, the final products are usually unpredicted. As shown in Scheme 1, compounds 1 and 2 with different structural motifs were synthesized from the same starting materials, and the hydrothermal temperature is the key factor distinguishing their formations. Compound 1 was obtained at relatively lower temperature, 140 °C, while compound 2 was isolated at 160 °C. The polyanion in 1 is an 8-connected note, but that in 2 is a 10-connected one.

Figure 3. Temperature dependence of XMT and XM under 1000 Oe applied dc field at 2−300 K for 1 (a) and 2 (b). D

DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Photocatalytic hydrogen evolution using different catalysts. Conditions: xenon lamp (300 W), 50 mL of a 10% (v/v) methanol aqueous solution, catalyst (50 mg), 1% platinum cocatalyst. (b) Current density−time curve of 1 and 2 under a 300 W xenon lamp during ON/OFF cycles in an aqueous sodium sulfate solution (0.2 M) as the supporting electrolyte at an applied potential (0.1 V vs Ag/AgCl).

photocurrent is light-dependent; that is, when light is on, the photocurrent is observed immediately, but when it is off, the photocurrent nearly disappears at once. As shown in Figure 4b, the transient photocurrent of 2 is obviously higher than that of 1. The order of the photocurrent test is consistent with that of hydrogen evolution. As is commonly recognized, a higher photocurrent means a higher e−−h+ separation efficiency, leading to a higher photocatalytic activity.27 The different activities of 1 and 2 are possibly related to their structural differences.

gradually with a decrease of the temperature, reaching a minimum of 7.38 cm3 K mol−1 at 15 K, and then abruptly increases to a peak of 7.81 cm3 K mol−1 at 14 K, followed by a drop to 5.47 cm3 K mol−1 at 2 K. The data above 50 K follow the Curie−Weiss law with C = 11.75 emu K mol−1 and θ = −21.59 K for 1 and C = 14.44 emu K mol−1 and θ = −32.94 K for 2 (Figure S10). These characteristics clearly suggest the antiferromagnetic interactions within the given temperature range.22 In 2, the abrupt increase of χMT at low temperature suggests the occurrence of weak ferromagnetism, which may arise from the presence of spin canting. Photocatalytic Hydrogen Evolution. Photocatalytic water splitting for hydrogen production is a long-term attractive issue.23 In the past decade, POMs as a kind of light-driven water-splitting photocatalyst have been widely explored because they combine the stability of a heterogeneous catalyst and the activity of a homogeneous catalyst.24,25 Inspired by this work, the photocatalytic activities of 1 and 2 as heterogeneous catalysts for hydrogen evolution were evaluated. During the selection of a sacrificial agent, we found that compound 2 is unstable when triethanolamine is used as the sacrificial agent (Figure S15), so methanol was chosen. As shown in Figure 4a, under UV-light irradiation, hydrogen continuously evolved at a rate of 19.25 μmol g−1 h−1 for 1 and 29.25 μmol g−1 h−1 for 2. Both exhibit a better performance than homogeneous catalysts K7HNb6O19 and Na16Nb12O40 (Figure 4a). Negligible activity was observed in the absence of a sacrificial agent or a catalyst (Figure 4a). Because cobalt-based compounds can effectively catalyze water splitting,26 Co(ethylenediamine)2Cl2 as a homogeneous catalyst was used in order to explore the role of the cobalt complex in the photocatalytic experiment. The result displays that the catalytic activity of Co(ethylenediamine)2Cl2 is much lower than those of 1 and 2. We speculate that the synergistic effect between PONbs and cobalt complexes might contribute to the photocatalytic activity of 1 and 2. The heterogeneity of the catalytic system was further confirmed by the leaching test, where no hydrogen could be detected when a leach liquor of 2 was used (Figure S11). In addition, the catalytic activity of 2 did not decrease after three cycles (Figure S12). The PXRD pattern and IR spectrum after the photocatalytic experiments revealed that the structure of 2 was maintained (Figures S13 and S14). To further explain the photocatalytic activity of compounds 1 and 2, the working electrode consisting of 1 or 2 was made and the generating photocurrents were recorded. We find that the

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CONCLUSION In summary, two PONb-based 3D organic−inorganic hybrid compounds were controllably synthesized by merely tuning the hydrothermal temperature or adding an additive. For the first time, a V2-substituted phosphoniobate, {PNb10V2O40(VO)4}, was observed in 1 and a 10-connected {PNb12O40(VO)6} cluster was found in 2. Moreover, 2 exhibits a better performance than 1 in both the photocurrent and photocatalytic hydrogen evolution experiments. The successful isolation of the two reported compounds not only helps the controllable synthesis of PONb-based hybrids but also lays the foundation for the development of PONb-based materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00823. Some structural figures, selected bond lengths, PXRD patterns, IR, UV−vis, and XPS spectra, and the photocatalytic hydrogen production of leaching tests and recycle tests (PDF) X-ray crystallographic file in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-10-68912631. Fax: ++86-10-68912631. *E-mail: [email protected]. Tel: +86-10-68912631. Fax: ++8610-68912631. Author Contributions †

J.H. and Y.W. contributed equally.

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DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Notes

(13) (a) Bontchev, R. P.; Nyman, M. Angew. Chem., Int. Ed. 2006, 45, 6670−6672. (b) Niu, J. Y.; Chen, G.; Zhao, J. W.; Ma, P. T.; Li, S. Z.; Wang, J. P.; Li, M. X.; Bai, Y.; Ji, B. S. Chem. - Eur. J. 2010, 16, 7082− 7086. (c) Guo, G. L.; Xu, Y. Q.; Cao, J.; Hu, C. W. Chem. - Eur. J. 2012, 18, 3493−3497. (d) Huang, P.; Qin, C.; Wang, X. L.; Sun, C. Y.; Yang, G. S.; Shao, K. Z.; Jiao, Y. Q.; Zhou, K.; Su, Z. M. Chem. Commun. 2012, 48, 103−105. (14) Niu, J. Y.; Wang, G.; Zhao, J. W.; Sui, Y. X.; Ma, P. T.; Wang, J. P. Cryst. Growth Des. 2011, 11, 1253−1261. (15) Flynn, C. M.; Stucky, G. D. Inorg. Chem. 1969, 8, 332−334. (16) Dietl, N.; Hockendorf, R. F.; Schlangen, M.; Lerch, M.; Beyer, M. K.; Schwarz, H. Angew. Chem., Int. Ed. 2011, 50, 1430−1434. (17) Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal Structure; University of Göttingen: Göttingen, Germany, 1997. (18) (a) Zheng, P. Q.; Ren, Y. P.; Long, L. S.; Huang, R. B.; Zheng, L. S. Inorg. Chem. 2005, 44, 1190−1192. (b) Sha, J. Q.; Peng, J.; Lan, Y. Q.; Su, Z. M.; Pang, H. J.; Tian, A. X.; Zhang, P. P.; Zhu, M. Inorg. Chem. 2008, 47, 5145−5153. (c) Niu, J. Y.; Li, F.; Zhao, J. W.; Ma, P. T.; Zhang, D. D.; Bassil, B.; Kortz, U.; Wang, J. P. Chem. - Eur. J. 2014, 20, 9852−9857. (19) Wendt, M.; Warzok, U.; Nather, C.; van Leusen, J.; Kogerler, P.; Schalley, C. A.; Bensch, W. Chem. Sci. 2016, 7, 2684−2694. (20) Nguyen, T. D.; Do, T. O. Langmuir 2009, 25, 5322−5332. (21) Tufts, B. J.; Abrahams, I. L.; Caley, C. E.; Lunt, S. R.; Miskelly, G. M.; Sailor, M. J.; Santangelo, P. G.; Lewis, N. S.; Roe, A. L.; Hodgson, K. O. J. Am. Chem. Soc. 1990, 112, 5123−5136. (22) (a) Vallejo, J.; Castro, I.; Ruiz-Garcia, R.; Cano, J.; Julve, M.; Lloret, F.; De Munno, G.; Wernsdorfer, W.; Pardo, E. J. Am. Chem. Soc. 2012, 134, 15704−15707. (b) Zhu, Y. Y.; Cui, C.; Zhang, Y. Q.; Jia, J. H.; Guo, X.; Gao, C.; Qian, K.; Jiang, S. D.; Wang, B. W.; Wang, Z. M.; Gao, S. Chem. Sci. 2013, 4, 1802−1806. (23) (a) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010, 110, 6503−6570. (b) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253−278. (c) Kubacka, A.; Fernandez-Garcia, M.; Colon, G. Chem. Rev. 2012, 112, 1555−1614. (24) (a) Liu, X.; Li, Y.; Peng, S.; Lu, G.; Li, S. Int. J. Hydrogen Energy 2012, 37, 12150−12157. (b) Zhang, Z.; Lin, Q.; Zheng, S. T.; Bu, X.; Feng, P. Y. Chem. Commun. 2011, 47, 3918−3920. (c) Matt, B.; Fize, J.; Moussa, J.; Amouri, H.; Pereira, A.; Artero, V.; Izzet, G.; Proust, A. Energy Environ. Sci. 2013, 6, 1504. (d) Lv, H.; Song, J.; Zhu, H.; Geletii, Y. V.; Bacsa, J.; Zhao, C.; Lian, T.; Musaev, D. G.; Hill, C. L. J. Catal. 2013, 307, 48−54. (e) Suzuki, K.; Tang, F.; Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Chem. Lett. 2014, 43, 1429−1413. (f) Lv, H.; Guo, W.; Wu, K.; Chen, Z.; Bacsa, J.; Musaev, D. G.; Geletii, Y. V.; Lauinger, S. M.; Lian, T.; Hill, C. L. J. Am. Chem. Soc. 2014, 136, 14015−14018. (g) Sumliner, J. M.; Lv, H.; Fielden, J.; Geletii, Y. V.; Hill, C. L. Eur. J. Inorg. Chem. 2014, 2014, 635−644. (h) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. Science 2010, 328, 342−325. (i) Lv, H.; Geletii, Y. V.; Zhao, C.; Vickers, J. W.; Zhu, G.; Luo, Z.; Song, J.; Lian, T.; Musaev, D. G.; Hill, C. L. Chem. Soc. Rev. 2012, 41, 7572−7589. (25) (a) Zhang, Z.; Lin, Q.; Kurunthu, D.; Wu, T.; Zuo, F.; Zheng, S. T.; Bardeen, C. J.; Bu, X.; Feng, P. Y. J. Am. Chem. Soc. 2011, 133, 6934−6937. (b) Huang, P.; Qin, C.; Wang, X. L.; Sun, C. Y.; Jiao, Y. Q.; Xing, Y.; Su, Z. M.; Shao, K.-Z. ChemPlusChem 2013, 78, 775− 779. (c) Huang, P.; Qin, C.; Su, Z. M.; Xing, Y.; Wang, X. L.; Shao, K. Z.; Lan, Y. Q.; Wang, E. B. J. Am. Chem. Soc. 2012, 134, 14004−14010. (d) Wang, Z. L.; Tan, H. Q.; Chen, W. L.; Li, Y. G.; Wang, E. B. Dalton Trans. 2012, 41, 9882−9884. (e) Niu, J.; Li, F.; Zhao, J.; Ma, P.; Zhang, D.; Bassil, B.; Kortz, U.; Wang, J. Chem. - Eur. J. 2014, 20, 9852−9857. (26) (a) Burke, M. S.; Kast, M. G.; Trotochaud, L.; Smith, A. M.; Boettcher, S. W. J. Am. Chem. Soc. 2015, 137, 3638−3648. (b) Huang, J.; Chen, J.; Yao, T.; He, J.; Jiang, S.; Sun, Z.; Liu, Q.; Cheng, W.; Hu, F.; Jiang, Y.; Pan, Z.; Wei, S. Angew. Chem., Int. Ed. 2015, 54, 8722− 8727. (c) Liao, L.; Zhang, Q.; Su, Z.; Zhao, Z.; Wang, Y.; Li, Y.; Lu, X.; Wei, D.; Feng, G.; Yu, Q.; Cai, X.; Zhao, J.; Ren, Z.; Fang, H.; RoblesHernandez, F.; Baldelli, S.; Bao, J. Nat. Nanotechnol. 2013, 9, 69−73.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grants 21231002, 21276026, 21271023, and 21371024) and 973 Program (Grant 2014CB932103).

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REFERENCES

(1) (a) Weinstock, I. A. Chem. Rev. 1998, 98, 113−170. (b) Wang, S. S.; Yang, G. Y. Chem. Rev. 2015, 115, 4893−4962. (c) Mirzaei, M.; Eshtiagh-Hosseini, H.; Alipour, M.; Frontera, A. Coord. Chem. Rev. 2014, 275, 1−18. (2) (a) Ma, F. J.; Liu, S. X.; Sun, C. Y.; Liang, D. D.; Ren, G. J.; Wei, F.; Chen, Y. G.; Su, Z. M. J. Am. Chem. Soc. 2011, 133, 4178−4181. (b) Han, Q. X.; He, C.; Zhao, M.; Qi, B.; Niu, J. Y.; Duan, C. Y. J. Am. Chem. Soc. 2013, 135, 10186−10189. (c) Wang, S. S.; Yang, G. Y. Chem. Rev. 2015, 115, 4893−4962. (d) Song, Y. F.; Tsunashima, R. Chem. Soc. Rev. 2012, 41, 7384−7402. (3) Nyman, M. Dalton Trans. 2011, 40, 8049−8058. (4) (a) Nyman, M.; Bonhomme, F.; Alam, T. M.; Rodriguez, M. A.; Cherry, B. R.; Krumhansl, J. L.; Nenoff, T. M.; Sattler, A. M. Science 2002, 297, 996−998. (b) Nyman, M.; Bonhomme, F.; Alam, T. M.; Parise, J. B.; Vaughan, G. M. B. Angew. Chem., Int. Ed. 2004, 43, 2787− 2792. (c) Bonhomme, F.; Larentzos, J. P.; Alam, T. M.; Maginn, E. J.; Nyman, M. Inorg. Chem. 2005, 44, 1774−1785. (d) Hou, Y.; Nyman, M.; Rodriguez, M. A. Angew. Chem., Int. Ed. 2011, 50, 12514−12517. (e) Hou, Y.; Alam, T. M.; Rodriguez, M. A.; Nyman, M. Chem. Commun. 2012, 48, 6004−6006. (f) Hou, Y.; Zakharov, L. N.; Nyman, M. J. Am. Chem. Soc. 2013, 135, 16651−16657. (5) Tsunashima, R.; Long, D. L.; Miras, H. N.; Gabb, D.; Pradeep, C. P.; Cronin, L. Angew. Chem., Int. Ed. 2010, 49, 113−116. (6) (a) Son, J. H.; Ohlin, C. A.; Larson, E. C.; Yu, P.; Casey, W. H. Eur. J. Inorg. Chem. 2013, 2013, 1748−1753. (b) Son, J. H.; Ohlin, C. A.; Johnson, R. L.; Yu, P.; Casey, W. H. Chem. - Eur. J. 2013, 19, 5191−5197. (c) Son, J. H.; Ohlin, C. A.; Casey, W. H. Dalton Trans. 2012, 41, 12674−12677. (d) Son, J. H.; Ohlin, C. A.; Casey, W. H. Dalton Trans. 2013, 42, 7529−7533. (e) Son, J. H.; Wang, J. R.; Osterloh, F. E.; Yu, P.; Casey, W. H. Chem. Commun. 2014, 50, 836− 838. (f) Son, J. H.; Casey, W. H. Chem. Commun. 2015, 51, 1436− 1438. (g) Son, J. H.; Casey, W. H. Chem. Commun. 2015, 51, 12744− 12747. (h) Son, J. H.; Park, D. H.; Keszler, D. A.; Casey, W. H. Chem. Eur. J. 2015, 21, 6727−6731. (7) Antonio, M. R.; Nyman, M.; Anderson, T. M. Angew. Chem., Int. Ed. 2009, 48, 6136−6140. (8) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (9) Guo, G. L.; Xu, Y. Q.; Cao, J.; Hu, C. W. Chem. Commun. 2011, 47, 9411−9413. (10) (a) Kinnan, M. K.; Creasy, W. R.; Fullmer, L. B.; SchreuderGibson, L. S.; Nyman, M. Eur. J. Inorg. Chem. 2014, 2014, 2361−2367. (b) Guo, W. W.; Lv, H.-J.; Sullivan, K. P.; Gordon, W. O.; Balboa, A.; Wagner, G. W.; Musaev, D. G.; Bacsa, J.; Hill, C. L. Angew. Chem., Int. Ed. 2016, 55, 7403−7407. (11) (a) Nakagawa, Y.; Kamata, K.; Kotani, M.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2005, 44, 5136−5141. (b) Chen, B. K.; Huang, X. Q.; Wang, B.; Lin, Z. G.; Hu, J. F.; Chi, Y. N.; Hu, C. W. Chem. - Eur. J. 2013, 19, 4408−4413. (12) (a) Zhang, Y.; Shen, J. Q.; Zheng, L. H.; Zhang, Z. M.; Li, Y. X.; Wang, E. B. Cryst. Growth Des. 2014, 14, 110−116. (b) Zhang, X.; Liu, S. X.; Li, S. J.; Wang, X. N.; Jin, H. Y.; Cui, J. L.; Liu, Y. W.; Liu, C. Z. Chem. J. Chinese Univ. 2013, 34, 2046−2050. (c) Shen, J. Q.; Zhang, Y.; Zhang, Z. M.; Li, Y. G.; Gao, Y. Q.; Wang, E. B. Chem. Commun. 2014, 50, 6017−6019. (d) Shen, J. Q.; Wu, Q.; Zhang, Y.; Zhang, Z. M.; Li, Y. G.; Lu, Y.; Wang, E. B. Chem. - Eur. J. 2014, 20, 2840−2848. (e) Lan, Q.; Zhang, Z. M.; Li, Y. G.; Wang, E. B. RSC Adv. 2015, 5, 44198−44203. F

DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (27) (a) Wei, D.; Yao, L.; Yang, S.; Cui, Z.; Wei, B.; Cao, M.; Hu, C. ACS Appl. Mater. Interfaces 2015, 7, 20761−20768. (b) Shao, M.; Ning, F.; Wei, M.; Evans, D. G.; Duan, X. Adv. Funct. Mater. 2014, 24, 580−586. (c) Liu, Y.; Yu, Y. X.; Zhang, W. D. J. Phys. Chem. C 2013, 117, 12949−12957. (d) Chen, P.; Lu, J.-Q.; Xie, G.-Q.; Hu, G.-S.; Zhu, L.; Luo, L.-F.; Huang, W.-X; Luo, M.-F. Appl. Catal., A 2012, 433-434, 236−242.

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DOI: 10.1021/acs.inorgchem.6b00823 Inorg. Chem. XXXX, XXX, XXX−XXX