Designed Construction of Cluster Organic Frameworks from Lindqvist

Aug 10, 2018 - 2 and 3 represent the second examples of cluster organic frameworks constructed from Lindqvist-type POM cluster secondary building unit...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Designed Construction of Cluster Organic Frameworks from Lindqvist-type Polyoxovanadate Cluster Xin-Xiong Li,*,†,‡ Lin-Jie Zhang,‡ Cai-Yan Cui,‡ Rui-Hu Wang,*,‡ and Guo-Yu Yang*,§ †

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State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, Fujian 350108, China ‡ State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China § MOE Key Laboratory of Cluster Science, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China S Supporting Information *

ABSTRACT: Two unprecedented examples of cluster organic frameworks (TBA)3Cu[V6O13(L)2]2·4DEF (2) (TBA)Ag[V6O13(L)2] solvent (3) (TBA = tetrabutylammonium, H3L = tris(hydroxymethyl)-4-picoline, DEF = N,N′-diethylformamide) based on Lindqvist-type polyoxometalate (POM) secondary building units (SBUs) have been constructed successfully. Compounds 2 and 3 are the second cases of cluster organic frameworks based on Lindqvist-type POM cluster SBUs. Furthermore, the cluster organic framework of 2 exhibits efficient electrocatalytic activity and strong durability in oxygen reduction reaction.

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INTRODUCTION The design and fabrication of cluster organic frameworks from preformed rigid metal clusters have raised extreme interest due to their intriguing structures and unexpected properties, which give raise to potential applications in magnetism, catalysis, photoluminescence and separation.1,2 Rigid metal clusters usually possess specific geometries and can maintain their structural integrity during the assembly process, the controllable synthesis of the desirable frameworks may be realized through the judicial selection of specific metal clusters as secondary building units (SBUs).3 Additionally, rigid metal clusters can transfer their distinct physiochemical properties to the final frameworks, greatly promoting potential application of cluster organic frameworks.4 To date, numerous efforts have been devoted in assembling cluster organic frameworks from transition-metal clusters such as [Cu2(CO2)4(H2O)2], [In3O(CO2)3], [Cr3(CO2)6O4], [Zn4(CO2)6O], [Cu4I4], and [Zr6O4(OH)4(CO2)12], resulting in the development of series of representative architectures.5 Nevertheless, the design and fabrication of cluster organic frameworks by using polyoxometalate (POM) clusters as SBUs is still in its infancy. POMs are a typical class of anionic transition-metal-oxide clusters with not only redox, adjustable acid−base and photochemical properties, but also different compositions, sizes and shapes, offering plenty of SBUs for fabricating innovative cluster organic frameworks.6 For example, Dolbecq group and Lan’s group have created a large family of porous cluster organic frameworks by {Zn4PMo12O40} SBUs for electrocatalysis.7 Our group has long been devoted in POM© XXXX American Chemical Society

based cluster organic frameworks. Earlier, we have used {Ni6PW9} SBUs and multicarboxylate ligands to make various cluster organic frameworks.8 More recently, we also reported two cluster organic frameworks made by using Anderson-type POMs as SBUs.9 It is noteworthy that the above illustrated cluster organic frameworks are built from Keggin/Andersontype POM SBUs. The exploration of new cluster organic frameworks based on Lindqvist-type POM SBUs has received relatively less concern,10 though Lindqvist-type POMs have been widely used in designing various discrete nano aggregates.11,12 The only example of cluster organic framework based on Lindqvist-type POM SBUs was reported by Hill and co-workers.10 As our continuous effort in exploring new POM-based cluster organic frameworks, here, we present a step by step synthetic strategy to fabricate two unprecedented cluster organic frameworks (TBA)3Cu[V6O13(L)2]2·4DEF (2) and (TBA)Ag[V6O13(L)2] solvent (3) (TBA = tetra-butylammonium, H3L = tris(hydroxymethyl)-4-picoline, DEF = N,N′diethylformamide) from Lindqvist-type polyoxovanadate clusters (Scheme 1). As far as we know, 2 and 3 are the second cases of cluster organic frameworks constructed from Lindqvist-type POM cluster SBUs. Received: June 5, 2018

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DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Step-by-Step Synthetic Routes of 2 and 3

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CH3CN and TBA resonances. IR (KBr, cm−1): 2961 (m), 2932 (w), 2873 (m), 2851 (w), 1596 (m), 1543 (w), 1483 (m), 1458 (w), 1410 (w), 1382 (w), 1082 (s), 1069 (s), 954 (vs), 805 (s), 723 (s), 663 (m), 584 (m), 564 (w), 514 (w), 485 (w), 418 (s). Synthesis of (TBA)3Cu[V6O13(L)2]2·4DEF (2). A mixture of 1 (15 mg, 0.01 mmol), CuI (11 mg, 0.06 mmol), tetrabutyl ammonium iodide (20 mg, 0.05 mmol), N,N′-diethylformamide (DEF) (3 mL), CH3CN (1 mL), and triethylamine (0.5 mL) was placed in a 20 mL vial and stirred for 30 min. The vial was sealed, heated at 100 °C for 3 days, and cooled to room temperature, after which dark brown stick crystals of 2 were obtained. Yield: 15 mg, 30% based on 1. Elemental analysis (%) calcd for H192C104N11O42V12Cu (2943.53): C: 42.44; N: 5.23; H: 6.57. Found: C: 43.12; N: 5.85; H: 5.83. IR (KBr, cm−1): 3425 (m), 3006 (w), 2973 (w), 2893 (w), 2848 (w), 2472 (w), 1605 (s), 1537 (w), 1495 (w), 1476 (w), 1460 (w), 1440 (w), 1415 (m), 1385 (m), 1356 (w), 1308 (w), 1224 (m), 1163 (m), 1094 (vs), 1019 (vs), 1069 (vs), 1014 (m), 946 (vs), 843 (w), 821 (m), 798 (vs), 707 (m), 675 (s), 583 (m), 554 (m), 488 (w), 418 (vs), 376 (w). Synthesis of (TBA)Ag[V6O13(L)2]·Solvent (3). A methanol solution of AgNO3(0.1 mol/L) was slowly layered on the top of a N,N′-dimethylacetamide and dimethyl sulfoxide mixed solution of compound 1 (0.1 mol/L) through a buffer layer consists of N,N′dimethylacetamide and methanol (1:2) in a 10 mL glass tube. After 2 weeks without being disturbed, a small amount of light brown stick crystals suitable for SXRD were obtained. We also attempted to prepare compound 3 by a solvothermal technique; however, we were fruitless because Ag+ ions are easily reduced at high temperature. Single Crystal Structure Analysis. Single crystals of 1−3 were mounted on glass fibers for indexing and single crystal X-ray diffraction (SCXRD) data were recorded on a Rigaku Saturn 70CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å). Structures 1−3 were solved by the direct method and

EXPERIMENTAL SECTION

Materials and General Methods. Tris(hydroxymethyl)-4-picoline (H3L) was synthesized as previously described.13 All other reactants were acquired from commercial sources and were used without further purification. Infrared (IR) spectra were measured on an Opus Vertex 70 FT-IR infrared spectrophotometer in the range of 450−4000 cm−1 with pressed KBr pallets. Elemental analyses (C, H, and N) were performed on a Vario EL III elemental analyzer. Thermal analyses were carried out on a NETZSCH STA449C thermal analyzer under a dynamic nitrogen atmosphere with a heating rate of 10 °C/min in the temperature region of 30−800 °C. Powder X-ray diffraction (PXRD) patterns were obtained by using a Rigaku DMAX 2500 diffractometer with Cu Kα radiation (λ = 1.54056 Å) in the range of 5−45°. All 1H NMR and 13C spectra were recorded on a Bruker AVANCE III NMR spectrometer at 400 and 100 MHz, respectively, using deuterated DMSO as locking solvent. The electrospray ionization mass spectroscopy (ESI-MS) was measured on a Thermo Scientific Exactive Plus mass spectrometer (German) and processed on a Bruker Data Analysis (Version 4.0) software. The theoretical peak was simulated on a Bruker Isotope Pattern software. Synthesis of TBA2[V6O13(L)2]·2CH3CN (1). A mixture of [nBu4N]3[H3V10O28]14 (6.24 g, 3.70 mmol) and H3L (2.03 g, 11.1 mmol) in acetonitrile (170 mL) was refluxed for 10 h. The dark green mixture was cooled slowly to room temperature and large amount of red crystals were formed after 10 h. The crystals were filtrated, washed with acetonitrile and diethyl ether, and dried in air. Yield: 4.62 g, 52% based on [n-Bu4N]3[H3V10O28]. Anal. Calcd For H98C54N6O19V6 (1441.03): C: 45.00; N: 5.83; H: 6.85. Found: C: 44.63; N: 5.96; H: 6.56. 1H NMR δ = 8.55 (d, J = 5.80 Hz, 4H), 7.37 (d, J = 5.92 Hz, 4H), 5.22 (s, 12H), 3.19−3.15 (m, 16H), 1.61−1.53 (m, 16H), 1.34− 1.29 (m, 16H), 0.94 (t, J = 7.24 Hz, 24H). 13C NMR δ = 150.5, 157.9, 121.8, 118.2, 84.8, 57.9, 41.4, 23.7, 19.6, 13.9, 1.6, in addition to the B

DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. X-ray Crystallographic Data for 1−3 compound empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z F(000) ρcalcd (g cm−3) temperature (K) μ (mm−1) refln collected independent refln parameters GOF on F2 final R indices (I = 2σ(I))a R indices (all data)a

1 C54H98N6O19V6 1441.02 monoclinic P2(1)/c 10.831(4) 17.590(5) 18.044(6) 90 103.536(5) 90 3342.1(18) 2 1508 1.432 293(2) 0.875 25130 7505 386 1.037 R1 = 0.0446 wR2 = 0.1220 R1 = 0.0570 wR2 = 0.1361

2 C104H192CuN11O42V12 2943.51 tetragonal P4(2)/n 20.8997(4) 20.8997(4) 9.8147(4) 90 90 90 4287.0(2) 2 3068 2.280 293(2) 1.608 31792 4897 258 1.057 R1 = 0.0579 wR2 = 0.1528 R1 = 0.0604 wR2 = 0.1549

3 C18H20AgN2O19V6 981.87 trigonal R3̅ 36.42(3) 36.42(3) 10.729(14) 90 90 120 12325(22) 9 4310 1.191 293(2) 1.376 13903 6005 211 1.035 R1 = 0.0738 wR2 = 0.1390 R1 = 0.1318 wR2 = 0.1570

R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo2− Fc2)2/∑w(Fo2)2]1/2; w = 1/[σ2(Fo2) + (xP)2 + yP], P = (Fo2 + 2Fc2)/3, where x = 0.0745, y = 1.475300 for 1; x = 0.077000, y = 18.391499 for 2, x = 0.0409, y = 0 for 3. a

Figure 1. ESI-MS of 1, showing that the experimental peak of [V6O13(L)2]2− is in good consistent with the theoretical value. refined on F2 by full-matrix least-squares methods using the SHELX97 program package.15 All hydrogen atoms attached to carbon atoms were geometrically placed and refined isotropically as a riding mode. Because of the high symmetry of the porous framework structures, the charge balancing cations in 2 and 3 cannot be mapped by SCXRD studies, which are common in porous crystal structures.5 The residual electron density in the different Fourier map that could not sensibly be assigned as solvents or cations were eliminated by the SQUEEZE function of PLATON. The final formulas of 2 and 3 were defined by the combination of the results of the elemental analysis and thermogravimetric analysis and charge balance considerations.

CCDC 1445407, 1445408, and 1846895 contain the supplementary crystallographic data for 1−3, and parameters are outlined in Table 1. Electrochemical Analysis. Cyclic voltammetry (CV) and linearsweep voltammetry (LSV) were used to investigate the electrochemical properties through a conventional three-electrode system. The working electrode is a rotating ring-disk electrode (RRDE, Φ = 5.61 mm) equipped on a Pine instrument, with the saturated Ag/ AgCl and Pt foil as the corresponding reference electrode and counter electrode, respectively. First, 1 mg of compound 2 was sonicated in a 3:1 water−isopropanol mixture (10 mL) containing 10 μL of Nafion solution (5 wt %). Then, this catalyst slurry was drop-coated onto the disk electrode surface of RRDE and dried under a lamp to form a C

DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry uniform thin film (ca. 100 μg cm−2) as the working electrode. Phosphate-buffered aqueous solutions (0.2 M, pH 7.0), respectively saturated with N2 and O2, were used as the electrolytes. In order to reach a stable electrochemical surface, the working electrode was repeatedly scanned between the potential of −1.0 and 0.2 V at a scan rate of 100 mV s−1 for more than 20 cycles prior to the electrochemical tests. The ring electrode potential of RRDE was set at +0.50 V for the oxidation of any hydrogen peroxide intermediates that passed on the ring surface. All polarization currents had been processed background subtraction and normalized by dividing the geometric area of disk electrode of RRDE. All measurements were conducted under the constant ambient temperature (25 ± 2 o̲C). The number of electrons transferred (n) and the HO2− intermediate production percentage (HO2− (%)) during the electrocatalytic ORR process were determined based on the following equations, respectively: n=4×

Id Id + (Ir /N )

HO2− (%) = 200 ×

Ir /N Id + (Ir /N )

where Id and Ir represent the current recorded from the disk and ring electrodes, respectively, and N is the collection coefficient, which was calculated to be ca. 0.38 using the [Fe(CN)6]3−/4− as redox probe.

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RESULTS AND DISCUSSION Syntheses. So far, organic ligand functionalized Lindqvisttype POMs have been well-studied in material sciences.11,12 Inspired by these results, we first introduced a tripodal alcohol ligand H3L to the Lindqvist-type POM cluster and obtained a new hybrid lindqvist-type POM TBA2[V6O13(L)2]·2CH3CN (1). 1 was prepared with high yield through refluxing (TBA)3[H3V10O28] precursor and H3L in dry acetonitrile (Supporting Information). The successful incorporation of L into the POM cluster can be deduced from 1H NMR spectrum (Figure S1). Furthermore, high-resolution electrospray ionization mass spectrometry (ESI-MS) also confirms the identity of 1. The peak at m/z 436.87 is assigned as [V6O13(L)2]2− (1a), which is in good consistent with the theoretical peak of 1a (Figure 1). It has been reported that tripodal alcohol ligands functionalized Lindqvist-type structures usually possess transand cis-configurations.16 Here, the expected trans-configuration in 1a is proved by SCXRD analysis. The structural analysis indicates that 1 consists of one L-grafted cluster 1a, two TBA cations, and two guest acetonitrile molecules. 1a can be depicted as two deprotonated L ligands capped on two opposite sides of a Lindqvist-type hexavanadate cluster, and the Lindqvist-type anion rests on an inversion center. The length of the whole polyanion measured between the two pyridyl nitrogen atoms is around 1.69 nm (Figure 2a), which is a little longer than that in a L-grafted Anderson-type POM [MnMo6O18(L)2]3− (1.47 nm) reported by us previous.9 Compared with POMs functionalized with flexible pyridyl groups,16b 1a is preferable for further assembly because its contains rigid pyridyl groups which hold more definite directional information. What’s more, the coordination sites of 4-pyridyl groups in 1a are easily accessible without any steric hindrance from the POM cluster, thus providing new opportunities for further assembly with transition metal ions. Using 1 as a precursor to react with CuI under solvothermal conditions, dark brown crystals of 2 were obtained. When silver salts were used to instead CuI under similar conditions, no crystal forms and black silver precipitate was observed, mainly because Ag+ ions are easily reduced under high

Figure 2. (a) Structure of 1a; (b) coordination environment of Cu+ ion in 2; (c) view of the 3D anionic framework based on fourconnected Cu+ ions and 1a anions; (d) diamondoid cavity built from 12 1a anions and 12 Cu+ ions in 2; (e) view of the 6-fold interpenetration framework in 2. Color codes in (a−d): VO6: green.

temperature. Therefore, a mild method was used for the reactions of silver salts with 1. Light brown stick crystals of 3 were grown by layering methanolic AgNO3 onto a mixed solution of N,N′-dimethylacetamide and dimethyl sulfoxide that contains 1 after several days. Due to the very low yield, we only report the crystal structure of 3 in this work. Crystal Structural Description of 2. SCXRD analysis indicated that 2 crystallizes in the tetragonal P42/n space group and features a 3D porous anionic framework based on 4D

DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry connected Cu+ ions and 1a anions. The asymmetric unit contains one-quarter Cu+ ion and half of [V6O13(L)2]2− cluster. The Cu+ ion employs a slightly distorted tetrahedral geometry and is ligated by four pyridyl nitrogen atoms from different 1a anions with N−Cu−N bond angles of about 104.46(15) and 112.04(8)° (Figure 2b). The Cu−N bond length is 2.035(3) Å, which is comparable with the reported Cu+-based frameworks.17 Every 1a anion acts as a linear bridge to join two neighboring Cu+ ions, and the Cu−Cu distance is about 2.09 nm. Such linkage between tetrahedral Cu+ ions and linear 1a anions lead to the creation of a 3D anionic framework with diamond topology (Figures 2c and S2). It is noteworthy that Hill and co-workers made the first Lindqvist-type POMbased cluster organic framework Tb[V6O13{(OCH2)3C(NH 2 CH 2 C 6 H 4 -4-CO 2 )}{(OCH 2 ) 3 C(NHCH 2 -C 6 H 4 -4CO2)}2] (Tb1).10 However, an auxiliary ligand 4,4-bis(pyridine-N-oxide) is indispensable in their assembly process for the formation of the framework structure. Comparatively, no additional auxiliary ligand is needed during the formation of the framework of 2. The size of tetragonal channels based on opposite atoms and polyhedra in 2 is about 2.43 × 4.24 nm2. The whole size of the diamondoid cage is 5.88 × 4.21 nm2, and the inner diameter is about 3.04 nm (Figure 2d). Such extremely porous characteristic gives rise to 6-fold interpenetration of six symmetry-related frameworks (Figure 2e). Although the final structure is 6-fold interpenetrated, there is still a large irregular channels with size of about 0.90 × 0.90 nm2 in the ab plane. The disordered TBA cations and solvent molecules which play the role of charge balancing and framework stabilization occupied those approximate nanoscale channels. PLATON calculation indicates that the total empty volume of the 6-fold interpenetrated anionic framework is 1764.8 Å3, corresponding to 41.2% of the total crystal volume (4287.0 Å3). Crystal Structural Description of 3. SCXRD analysis revealed that 3 crystallizes in the centrosymmetric space group R3̅ and exhibits a 3D structure. There are half a Ag+ ion and half a [V6O13(L)2]2− cluster in the asymmetric unit. Different from Cu+ ion in 2, the Ag+ ion in 3 has a square-planar geometry and is surrounded by two nitrogen atoms from two pyridyl groups and two bridge oxygen atoms of two Linqvisttype clusters (Figure 3a). The Ag−N and Ag−O bond distances are 2.174(5) and 2.811(5) Å, respectively. In 3, the Ag+ ions are linked by [V6O13(L)2]2− polyanions to give rise to an infinite 1D cluster chain with Ag−Ag distance of 2.13 nm (Figure 3a). The most intriguing structural feature in 3 is further assembly of these 1D cluster chains. Three discrete chains are aligned in a crossed mode to generate a unique layer containing a three-membered window in the ab plane (Figure S3). The adjoining layer is rotated by 60° along the vertical axis in the center of the three-membered window, giving rise to a larger six-membered hole and a smaller three-membered hole along the c axis (Figures 3b,c and S4). Furthermore, the adjacent layers are interconnected by Ag−O bonds of two intersected chains from different layers, resulting in the formation of a 3D open framework with two kinds of channels with smaller triangular and large hexagonal shapes (Figure 3d). As is well known, such structure is not common in cluster organic framework materials. The size of the larger hexagonal channel is about 1.52 nm. The total potential accessible volume of the framework is about 50.6% based on PLATON calculation, which is a slightly higher than that of 2.

Figure 3. (a) View of the coordination environment of Ag+ ion in 3; (b and c) schematic presentation of the packing arrangement for the 1D coordination chains; (d) view of the 3D anionic framework of 3.

PXRD Patterns, Thermalgravimetric Analysis, and IR Spectra. In powder X-ray diffraction (PXRD) of 2, the good consistence between the experimental pattern of the assynthesized sample and the simulated pattern based on SCXRD result indicates the phase purity of 2 (Figure 4).

Figure 4. Simulated and experimental variable-temperature PXRD patterns of 2. E

DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. (a) Typical CVs on the 2-modified electrode and bare electrode in N2- and O2-saturated 0.20 M pH 7.0 phosphate-buffered at a scan rate of 10 mV s−1. (b) LSVs respectively recorded on the 2-modified electrode and bare electrode in O2-saturated 0.20 M pH 7.0 phosphate buffer with electrode rotation speed of 1600 rpm and scan rate of 5 mV s−1; the dotted curve represents the ring current density on 2-modified electrode. (c) Variations in the electron-transfer number and HO2− yield at various disk electrode potentials during the ORR on 2-modified RRDE; (d) durability test of 2-modified electrode at −0.80 V in O2-saturated 0.20 M pH 7.0 phosphate buffer with electrode rotation speed of 1600 rpm, inset being the LSVs measured before and after the durability test.

electrode is ca. 100 mV, while the onset potential on bare electrode is almost 300 mV, further identifying that 2 is an efficient electrocatalyst for ORR. To further evaluate the ORR activity of 2, linear sweep voltammogram (LSV) measurements at a rotating speed of 1600 rpm were recorded. As shown in Figure 5b, on bare electrode, the rate of ORR is really sluggish, with an apparent current response at almost −0.60 V and a final reach of 1.0 mA cm−2 at −1.0 V. As for 2-modified electrode, discernible ORR current is observed at above −0.25 V and the current quickly increases with the potential sweeping to more negative direction, with a final current density approaching to 2.5 mA cm−2 at −1.0 V. It should be mentioned that there is an almost constantly a huge potential gap of ca. 300 mV required for the bare electrode to reach the same current density as that on 2modified electrode, although ORR on both electrodes are far from reaching a diffusion-controlled process with the appearance of current plateaus. Obviously, the positive shift of the potentials further suggests the efficient electrocatalytic activity of 2. We have also investigated the number of electrons transferred (n) per O2 molecule on 2 during the ORR process based on the disk and ring currents recorded on 2-modified RRDE (Figure 5b). As illustrated in Figure 5c, an average value of 3.46 in a wide range of potentials from −0.25 to −1.0 V is determined, and the corresponding HO2− yield is only at 20− 30%, demonstrating a highly desired four-electron-dominant ORR pathway on 2-modified RRDE, which is very close to the theoretical value for ideal one-step direct reduction of O2 with high selectivity for H2O production. Although the mechanism

Thermogravimetric analysis of 2 exhibits a weight loss of 7.83% from 30 to 200 °C approximately corresponding to the removal of free DEF molecules (calcd 7.38%, Figure S5). Variable-temperature PXRD patterns prove that 2 can keep its crystallinity up to 230 °C (Figure 4), which is superior than that of Tb1 (decomposition above 200 °C). In the IR spectra of 1 and 2 (Figure S6), the typical vibrational peaks of V−Ot and V−Ob (Ot, terminal oxo, and Ob, bridge oxo) on the Lindqvist-type skeleton are situated at 1016−856 cm−1 and 810−700 cm−1,12 respectively. The characteristic stretching vibrations of TBA cations can be found at 1370−1460 and 3059−2844 cm−1.18 Electrocatalytic Study. Considering rich redox chemistry with stable redox states and multiple electron transfer steps of POM,19 as well as particular oxygen reduction activity of the Cu−Nx moiety,20 we investigated the electrocatalytic activity of 2 using oxygen reduction reaction (ORR) in fuel cells as a model reaction. The test was conducted in 0.2 M neutral phosphate-buffered solution (pH 7.0) by using 2-modified RRDE as the working electrode. PXRD patterns (Figure S7) proved that 2 is stable in neutral phosphate-buffered solution. As depicted in Figure 5a, clearly different from the cases in N2saturated electrolyte, cyclic voltammograms (CV) recorded on 2-modified electrode and bare electrode (i.e., no 2 modification) in O2-saturated electrolyte exhibit steep rise in the cathodic oxygen reduction current. The ORR current on 2modified electrode is significantly larger across the whole potential range than that on bare electrode, indicating 2 is beneficial for ORR. The ORR onset potential on 2-modified F

DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry underlying the nearly four-electron electrocatalytic reduction of O2 on 2 still remains to be investigated in our future studies, the above results may shed some light on the design and synthesis of POM-based ORR electrocatalysts with more superior activity. The stability of 2 under catalytic conditions was further tested with chronoamperometric measurements. As depicted in Figure 5d, the current−time (i−t) curve for 2-modified electrode at the constant potential of −0.8 V exhibits a small attenuation and high current retention (>95%) after a nearly 10 h of durability test. Furthermore, the LSV curve on 2modified electrode measured after the durability test shows very little deviation from the one measured before the durability test. These promising results strongly conclude that 2 is also a highly robust ORR catalyst in the natural media.

Lin-Jie Zhang: 0000-0003-3011-3834 Rui-Hu Wang: 0000-0002-6209-9822 Guo-Yu Yang: 0000-0002-0911-2805 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundations of China (No. 21671040), Natural Science Foundations of Fujian Province (No. 2016J05055) and Projects from State Key Laboratory of Structural Chemistry of China (No. 20160020).

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CONCLUSIONS In summary, we have successfully constructed two unprecedented cluster organic frameworks (TBA)3Cu[V6O13(L)2]2· 4DEF (2) and (TBA)Ag[V6O13(L)2] solvent (3) based on Lindqvist-type POM SBUs. Structural analysis reveal that 2 is a 3D 6-fold interpenetrated framework structure with diamond topology, while 3 is a noninterpenetrated 3D framework structure with nanoscale 1D hexagonal channels. Compounds 2 and 3 represent the second examples of cluster organic frameworks built from Lindqvist-type POM SBUs. In addition, the framework of 2 shows good thermal stability and can catalyze ORR with efficient activity and strong durability. The present study not only demonstrates a feasible strategy to construct stable and functional POM-based cluster organic framework materials but also confirms the large potential for development new classes of cluster organic frameworks by using various POM clusters as SBUs. Now that we have successfully extended our exploration systems from Kegginand Anderson-type to Lindqvist-type, more investigations based on POM clusters such as Dawson-, Waugh-, and Silverton-types are currently underway in our group.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01528. Experimental details, crystallographic data for 1−3, additional structural figures, additional characterizations such as NMR spectra, TG curves, IR spectra (PDF) Accession Codes

CCDC 1445407−1445408 and 1846895 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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

Xin-Xiong Li: 0000-0002-9903-2699 G

DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b01528 Inorg. Chem. XXXX, XXX, XXX−XXX