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May 10, 2016 - Synopsis. Efficient synthesis of the iridium(III)-containing polytungstate cluster K12Na2H2[Ir2Cl8P2W20O72]·37H2O (1) was achieved usi...
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Efficient Synthesis of Ir-Polyoxometalate Cluster Using a Continuous Flow Apparatus and STM Investigation of Its Coassembly Behavior on HOPG Surface Junyong Zhang,† Shaoqing Chang,‡ Bryan H. R. Suryanto,§ Chunhua Gong,† Xianghua Zeng,† Chuan Zhao,*,§ Qingdao Zeng,*,‡ and Jingli Xie*,†,# †

College of Biological, Chemical Science and Engineering, Jiaxing University, Jiaxing 314001, P. R. China CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, 11 Zhongguancun Beiyitiao, Beijing, 100190, P. R. China § School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia # State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China ‡

S Supporting Information *

ABSTRACT: Taking advantage of a continuous-flow apparatus, the iridium(III)-containing polytungstate cluster K12Na2H2[Ir2Cl8P2W20O72]·37H2O (1) was obtained in a reasonable yield (13% based on IrCl3·H2O). Compound 1 was characterized by Fourier transform IR, UV−visible, 31P NMR, electrospray ionization mass spectrometry (ESI-MS), and thermogravimetric analysis measurements. 31P NMR, ESI-MS, and elemental analysis all indicated 1 was a new polytungstate cluster compared with the reported K14[(IrCl4)KP2W20O72] compound. Intriguingly, the successful isolation of 1 relied on the custom-built flow apparatus, demonstrating the uniqueness of continuous-flow chemistry to achieve crystalline materials. The catalytic properties of 1 were assessed by investigating the activity on catalyzing the electro-oxidation of ruthenium tris-2,2′bipyridine [Ru(bpy)3]2+/3+. The voltammetric behavior suggested a coupled catalytic behavior between [Ru(bpy)3]3+/2+ and 1. Furthermore, on the highly oriented pyrolytic graphite surface, 1,3,5-tris(10-carboxydecyloxy) benzene (TCDB) was used as the two-dimensional host network to coassemble cluster 1; the surface morphology was observed by scanning tunneling microscope technique. “S”-shape of 1 was observed, indicating that the cluster could be accommodated in the cavity formed by two TCDB host molecules, leading to a TCDB/cluster binary structure.

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[PW11O39IrCl]5− have been reported by Kögerler, Sokolov, and co-workers.15−17 In 2009, Hill and co-workers showed that [(IrCl4)KP2W20O72]14− could be fabricated and explored its catalytic water oxidation activity.18 Finke and co-workers have incorporated iridium in polyoxoanions for prospective applications in catalysis.19−22 It is worth noting that most compounds were obtained under one-pot solution processing conditions or hydrothermal reaction. On the one hand, exploring novel synthetic approach may tackle the problem of laborious and time-consuming synthesis and achieve the target molecule in a feasible way. By using continuous-flow apparatus, hitherto, several impressive compounds have been obtained,

INTRODUCTION The study of polyoxometalate (POM) cluster is a burgeoning and exciting area of chemical science.1−3 Bearing with the structural diversity, this group of early transition-metal− oxygen-anion clusters has received intensive investigations for applications in biology, magnetism, optics, medicine, and green chemistry.4−12 The involvement of noble metals (the second and third transition series of groups 8−11) with POM provide sufficient basis to engage this kind of materials in a variety of materials studies, and the development of this area has been summarized by Kortz, Lefebvre, and co-workers recently.13,14 Compared to the extensive research into noble metals such as Ru, Pd, and Pt, several studies have performed using Ir metal as the heteroatom. The clusters containing one iridium atom such as [HIrIVW6O24]7−, [PW11O39Ir(H2O)]4−, and © XXXX American Chemical Society

Received: March 17, 2016

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DOI: 10.1021/acs.inorgchem.6b00670 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry because this technique could precisely tune diffusion, mixing, and mass and heat transport, and consequently it could finely control reaction rate, speed up and scale up the synthesis, and deliver products with better yields.23−28 On the other hand, exploring the assembly process of different POM clusters at the atomic level could help unveil the intrinsic relationship between crystalline characters and observed nanostructures, thus enabling the optimization of intermolecular interaction, which has been identified as an important field in supramolecular science.29,30 In this section, the clusters’ assembly with the host molecule on the surface down to the nanoscale remained less exploited.31−37 Taking advantage of two-dimensional (2D) networks as the host, the surface morphology of cluster, acting as guest molecule, would advance the understanding of self-assembly of functional compounds. It is envisaged that ordered arrays of POM materials could be observed because of their chemical robustness and stability. Therefore, 2D hydrogen-bonded networks may immobilize POM clusters, and the unique organization process of host−guest system could help to develop potential nanodevices.38,39 Among the various host molecules, the intermolecular hydrogen-bonded network of 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) is an important supramolecular template,40 which allows a large variety of guest molecules to be trapped into their cavities, including recently reported tetrathiafulvalene-supported triple-decker phthalocyaninato dysprosium complex,41 1,1′-bi-2-naphthol,42 etc. Herein we report the iridium(III)-containing polytungstate cluster K12Na2H2[Ir2Cl8P2W20O72]·37H2O (1) could be successfully synthesized using a continuous-flow apparatus. Furthermore, the target molecule was deposited on highly oriented pyrolytic graphite (HOPG) surface, and its assembled pattern was investigated by scanning tunneling microscope (STM) technique. In doing so, well-ordered arrays are obtained, thus providing a unique route to elucidate the relationship between crystalline characters and the observed nanostructures.

Figure 1. (a) Use the flow chemistry reactor to synthesize 1. (b, top) The reported batch-type reaction.18 (b, bottom) The successful flowtype reaction in this work.

reaction rates, as [PW9O34]9−/IrCl3 can undergo rapid heat/ mass transfer; obviously, the custom-built apparatus shows its effectiveness and reliability for the synthesis of Ir-POM cluster (Figure 1b). Compared with previous reports,23−28 the fabrication of 1 by flow-type reactor proves to be a viable method to achieve those crystalline materials that are difficult to be obtained by using traditional methods. This may account for the process intensification when the reaction is conducted in microfluidic geometries, which is sufficiently different from those conventional batch-type reactors. Compound 1 crystallizes in the orthorhombic space group Pnnm, and the crystal structure consists of polyoxoanion [Ir2Cl8P2W20O72]16−, K+, Na+, and H+ cations. The polyoxoanion of 1 possesses an S-shaped structure based on two trivacant Keggin [PW9O34]9− building blocks, two {IrCl4} fragments, and two {WVIO2} segments (Figure 1). All tungsten atoms exhibit the octahedral coordination geometry. Similar to the reported [(IrCl4)KP2W20O72]14−, the two {WVIO2} segments that connect two [PW9O34]9− building blocks together to form an S-shaped structure in 1. There are two iridium atoms hanging at two [PW9O34]9−, and the Ir centers exhibit six-coordination geometry, formed by two terminal oxygen atoms from [PW9O34]9− fragments and four chloride ions. The Ir−O bond length is 2.012(12) Å, and Ir−Cl bond lengths are in the range of 2.344(5)−2.359(8) Å. Noticeably, although the unit cell parameters of 1 are very close to that of K14[(IrCl4)KP2W20O72] reported by Hill,18 those two compounds are different based on the following observations: (1) the 31P NMR spectrum of 1 was recorded, and only one peak at −10.92 ppm was observed (Figure S4), whereas the reported [(IrCl4)KP2W20O72]14− has two peaks at −11.05 and −12.75 ppm, indicating the existence of the only kind of P atom in 1; (2) the electrospray ionization (ESI) mass spectrum (in the negative ion mode) of 1 was performed (Figure 2). Prominent envelopes centered at 900.4 provide supporting characterization: experimental m/z 900.4; calculated for [H2Ir2Cl2PW9O34·H2O]3−: 901.6 (2704.88/3);43,44 (3) there are two IrCl4 units per polyanion unit that derived from the elemental analysis. Bond valence sum calculations indicate that the oxidation states of all W and Ir centers exhibit +6 and +3,

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RESULTS AND DISCUSSION Needle-like black crystals of K12Na2H2[Ir2Cl8P2W20O72]· 37H2O (1) were synthesized from Na8[HPW9O34] and IrCl3· H2O with the help of a flow apparatus (Figure 1a and Figure S1). Intriguingly, we found that the flow-type technique is crucial for the efficient and successful synthesis of 1 in reasonable yield (∼13%) and isolation of large single crystals (Figure S2). The reproducible manner of this approach was verified by two researchers (independently). With the identical experimental parameters such as pH value, stoichiometry, reaction temperature and time, etc., the control experiment, by using traditional batch-type process in round-bottom flask, suggests that no crystalline 1 could be achieved, which was verified by two researchers. The powdery product from the batch-type reaction was analyzed by powder X-ray diffraction (PXRD), and the resulting pattern was different than that of pure crystalline 1 (Figure S3). The similar scenario has been noticed by Hill and co-workers; their attempts to react [PW9O34]9− and IrCl3 directly were always unsuccessful. This was due to the hydrolytic degradation of [PW9O34]9− before its reaction with inert Ir3+ ions. By exploiting the in situ generation of [PW9O34]9− from K10[P2W17O61]/K2WO4, [(IrCl4)KP2W20O72]14− cluster could be obtained under batch-type reaction.18 In this work, the flow-type process could accelerate B

DOI: 10.1021/acs.inorgchem.6b00670 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) CV of 1 (2 mM) at 100, 80, 50, 20 mv/s, in 1 M NaH2PO4, pH = 4.20 and (b) CV of 0.5 mM [Ru(bpy)3]2+ in 0.5 M NaPB solution, with consecutive additions of 0.03 μmol of 1. Potentials are vs Ag/AgCl. Scan rate: 25 mV s−1.

Figure 2. (a) The ESI mass spectrum (negative ion mode) of water solutions of compound 1 in the m/z = 700−1400 range. (b) Simulation of the expected spectrum matches that of the observed peak at 900.4 m/z.

highly unstable and reduced rapidly, which results in simultaneous oxidation of water. The catalytic effect of 1 on the redox chemistry [Ru(bpy)3]2+ is studied by CV and shown in the Figure 3b. The CV obtained in 0.5 mM [Ru(bpy)3]2+ in 0.5 M sodium phosphate buffer solution (NaPB, pH = 7.0) shows a pair of reversible peaks for the [Ru(bpy)3]2+/3+ process with a Ep,a value of 1.10 V and Ep,c value of 1.00 V, consistent to previous study.47 Following the oxidation peak of [Ru(bpy)3]2+ at 1.14 V, an onset of anodic currents contributed by the water oxidation process was observed.48 Shown in Table 1, with the addition of 0.03

respectively. Research endeavors by Hill and co-workers could not lead to the diiridium-substituted product, and we believe compound 1 is a new Ir-POM compound; its efficient synthesis could be attributed to the employed flow-type synthetic approach. The electrochemical properties of 1 were studied by cyclic voltammograms (CVs), which were recorded in a 2.0 mM solution of 1 in 1 M NaH2PO4 solution (pH = 4.20). The voltammograms (Figure 3a) show one pair of quasi-reversible peaks in the range from 0 to 1.2 V, with the corresponding E1/2 located at 0.72 V, attributed to the IrIII/IrIV and redox transformation within the polyanion [Ir2Cl8P2W20O72]16−. The peak currents for the both processes are proportional to the square root of the scan rates (not shown), indicating electrode reactions are diffusion-controlled. The diffusion coefficient D = 1.60 × 10−4 cm2 s−1 can be calculated for 1 according to Randles−Sevcik equation. Moreover, the catalytic properties of 1 were also assessed by investigating its activity on catalyzing the electro-oxidation of ruthenium tris-2,2′-bipyridine [Ru(bpy)3]2+/3+. The ruthenium ion complex has been established as an excellent water oxidation photocatalyst.45,46 It is known that visible light absorbed by [Ru(bpy)3]2+ forming metal-to-ligand chargetransfer (MLCT) complex state of [Ru(bpy)3]2+*, which later will be oxidized into [Ru(bpy)3]3+ in the presence of sacrificial electron acceptor such as S2O82− ion. The [Ru(bpy)3]3+ is

Table 1. Effect of the Addition of 1 on the Electrochemical Parameters of [Ru(bpy)3]2+/3+ Redox Couple in pH = 7.0, 0.5 M NaPB amount of 1 (μmol)

net ip,a (μA)

net ip,c (μA)

net ip,a/ip,c

Ep‑a (V)

Ep‑c (V)

0 0.03 0.06 0.09 0.12

2.00 2.26 3.20 4.31 4.68

2.27 2.63 2.23 2.36 2.30

0.88 0.86 1.43 1.83 2.03

1.07 1.07 1.09 1.07 1.07

1.00 1.00 1.00 1.00 1.00

μmol of 1, the oxidation peak current from [Ru(bpy)3]2+ to [Ru(bpy)3]3+ is significantly enhanced, while the reduction peak current on the cathodic scan decreases, resulting in a net ip,a/ip,c = 0.86. With increasing amount of 1, the oxidation peak current of [Ru(bpy)3]2+ increases gradually with the Ep,a remaining unchanged. This voltammetric behavior strongly suggests a coupled parallel catalytic behavior of [Ru(bpy)3]3+/2+ C

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nm, and the width d1 is 1.9 ± 0.1 nm. Those values are slightly larger than the values of 1.7 and 1.6 nm, which are revealed from the crystal structure analysis (Figure S8a), indicating that flexible packing may be achieved on the surface within the host system. The measured unit cell parameters are a = 4.4 ± 0.1 nm, b = 2.2 ± 0.1 nm, and α = 73 ± 2°. A model of the 2D host−guest unit is proposed in Figure 4c. To compare the surface morphology of 1 and the starting material Na8[HPW9O34] on HOPG surface, STM investigations on the 2D assembly of Na8[HPW9O34]/TCDB are presented in Figure 5. The bright semicircle spots represent

in the presence of 1, and it is characteristic for an ErCi mechanism (reversible electrochemical reaction Er followed with irreversible chemical reaction Ci),49 as described below: IrPOM

[Ru(bpy)3 ]2 + ⎯⎯⎯⎯⎯⎯→ [Ru(bpy)3 ]3 + + e−

Thus, 1 can potentially be used to optimize the Ru-complex assisted water oxidation process by accelerating the oxidation of [Ru(bpy)3]2+/3+, hence providing more of the active [Ru(bpy)3]3+, which in turn leads to more efficient photocatalytic water oxidation. TCDB has been known as a molecular template for forming 2D networks with nearly tetragonal cavities (the inner cavity size is 2.8 nm × 1.7 nm) on the HOPG surface,40,41 and the coadsorption of organic molecules such as the coronene and phthalocyanine could tune the size and shape of the TCDB network cavities because of the flexibility of long alkyl chains (decyl). Noticeably, this host system could be used not only to accommodate guest molecules but also to control the chemical reactions as spatial confiners, to separate different molecules as molecular sieves, and to detect ions as molecular sensors.40 So exploring the assembly process of polyoxometalates cluster with the TCDB molecule on the surface down to the nanoscale will help the development of nanodevices. Figure 4 displays the

Figure 5. (a) A large-scale STM image of the 2D self-assembled structure of Na8[HPW9O34]/TCDB adsorbed on the HOPG surfaces. iset = 289.9 pA; Vbias = 699.2 mV. (b) A higher-resolution STM image of Na8[HPW9O34]/TCDB on the HOPG surface. iset = 289.9 pA; Vbias = 648.2 mV. (c) A proposed molecular model shows the STM image of the Na8[HPW9O34]/TCDB complex. Color codes, C: blue; H: white; O: red for TCDB molecule. P: pink; W: gray; O: red for Na8[HPW9O34]. Note: the image was taken after the film was dried.

Na8[HPW9O34] molecules. Around the bright semicircle spots are the smaller bright spot structures, which can be attributed to the benzene cores of TCDB. The measured length L2 is 1.6 ± 0.1 nm, and the width d2 is 1.2 ± 0.1 nm. Similar to the previous observation, those values are larger than the values of 1.0 and 0.9 nm, which are given by the structural analysis (Figure S8b). A unit cell is placed upon the images with a = 4.2 ± 0.1 nm, b = 2.2 ± 0.1 nm, and α = 82 ± 1°. The drive for hypothesis-led research about surface morphology of the POM cluster with the TCDB host is that it will provide the relationship between the crystalline characters and observed nanostructures. In principle, S-shape of 1 and semicircle shape of Na8[HPW9O34] have been explored by STM technique, indicating that (1) POM cluster could be accommodated in the cavity that formed by two TCDB host molecules, leading to an observable TCDB/cluster binary structure; (2) the images are different due to their distinctive crystalline structures, and they could be distinguished in the region of nanoscale. Figure 6 presented the proposed molecular model shows the STM images of TCDB/ cluster host−guest system.

Figure 4. (a) A large-scale STM image of the 2D self-assembled structure of 1/TCDB adsorbed on the HOPG surface. iset = 289.9 pA; Vbias = 699.2 mV. (b) A higher resolution STM image of 1/TCDB on the HOPG surface. iset = 289.9 pA; Vbias = 648.2 mV. (c) A proposed molecular model shows the STM image of the 1/TCDB complex. Color codes, C: blue; H: white; O: red for TCDB molecule. Ir: dark blue; Cl: green; P: pink; W: gray; O: red for cluster 1. Note: the image was taken after the film was dried.

STM images of 1/TCDB adsorbed on the HOPG surface. As marked in Figure 4b, each bright feature (drawn in S-shape) corresponds to complex 1, and the details of the entrapped molecules are visible. The overall S-shaped features may include clockwise and counterclockwise directions, but this subtle phenomena could not be observed. The dispersed small spots around S-shaped features can be ascribed to the benzene cores of the TCDB molecule. It can be seen that one molecule of 1 is entrapped in one cavity. The measured length L1 is 2.1 ± 0.1 D

DOI: 10.1021/acs.inorgchem.6b00670 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. A proposed molecular model shows the STM image of (a) the TCDB/1 complex and (b) the TCDB/Na8[HPW9O34] complex.

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the primary 5 m tubular reactor (external temperature: 60 °C). After 0.20 g of KCl was added, the final solution was collected in an open beaker and allowed to crystallize. Black needles of 1 (0.12 g, yield: 13% based on IrCl3·H2O) were collected two weeks later. Anal. Calcd for K12Na2H76Ir2Cl8O109P2W20 (6742.50): K, 6.96; Na, 0.68; H, 1.14; Ir, 5.70; Cl, 4.21; O, 25.90; P, 0.92; W, 54.53%. Found: K, 7.00; Na, 0.62; H, 1.31; Ir, 5.65; Cl, 4.15; O, 26.20; P, 0.90; W, 50.10%. 31P NMR (162 M Hz) spectrum: −10.92 ppm. ESI mass spectrum (negative ion mode): experimental m/z 900.4; calculated for [H2Ir2Cl2PW9O34· H2O]3−: 901.6 (2704.88/3). IR (KBr pellet): 686(m), 795(s), 944(m), 1016(m), 1073(s), 1615(s), 3435(s). UV−vis spectrum (λ = 250 nm, ε = 1580 L mol−1 cm−1). Note: By using the continuous-flow apparatus, we did the similar experiment, and the only difference was that no KCl was added in the last step. The reaction system was set up for crystallization, and after one week, the black crystalline precipitates were isolated from the mother liquor. X-ray crystallographic data were collected, and it showed that this experiment also led to the compound containing cluster ion [Ir2Cl8P2W20O72]16−. Methods. 31P Nuclear Magnetic Resonance Spectroscopy. The 31 P NMR spectrum was measured on an VARIAN 400 M Hz spectrometer with D2O as the solvent. Electrospray Ionization Mass Spectrometry. ESI mass spectrum was recorded on a Bruker solariX FT-ICR mass spectrometer in the negative ion mode. Fourier-Transform Infrared Spectroscopy. The IR (KBr pellet) spectrum was recorded (400−4000 cm−1 region) on a VARIAN CARY5000 IR spectrometer. Wavenumbers (ν) are given in inverse centimeters. Ultraviolet−Visible Absorption Spectroscopy. UV−vis absorption spectrum was recorded on UV-1650 PC spectrophotometer (double cell mode, baseline correction was performed a wavelength scan from 800 to 200 nm). Electrochemistry. Electrochemical measurements were performed using a CHI 760 Electrochemical Workstation (CH Instrument, Texas, USA). All electrochemical data were obtained from conventional three electrodes in a cell consisting of glassy carbon (GC) working electrode, reference electrode, and a counter electrode. In the measurements, a Ag/AgCl (3 M KCl) was used as the reference electrode and Pt wire as the counter electrode. Before each measurement, the GC working electrode was thoroughly polished on a polishing pad (Buehler, USA) with 0.05 μm alumina slurry. After each polishing the working electrode is thoroughly rinsed with milli-Q water and acetone. Sodium phosphate buffer solution (NaPB) pH = 7.0, 1.0 M was prepared by mixing 57.7 mL of Na2HPO4 (Ajax Finechem, Australia) and 42.3 mL of NaH2PO4 (Ajax Finechem, Australia), and the mixture was diluted using Milli-Q water to obtain 0.5 M NaPB pH = 7.0. Single Crystal X-ray Diffraction. X-ray crystallographic data of 1 were collected on an Xcalibur, Eos, Gemini diffractometer with graphite-monochromated Cu Kα radiation (λ = 1.541 84 Å) at 150 K. Using Olex2,50 the structure was solved with the Superflip51 structure

CONCLUSIONS In conclusion, efficient synthesis of the iridium(III)-containing polytungstate cluster K12Na2H2[Ir2Cl8P2W20O72]·37H2O (1) has been successfully achieved with the help of a continuousflow apparatus. Noticeably, the overwhelming majority of inorganic compounds are produced by one-pot solution processing conditions, which usually suffers from long operation time, low reaction efficiency, and inconsistent product properties from batch to batch. The results in this contribution demonstrate the uniqueness of flow chemistry and highlight its great potential for the inorganic synthesis. We aim to use this emerging technique to synthesize other POM systems incorporating different noble metals. The catalytic properties of 1 were assessed by investigating its activity on catalyzing the electro-oxidation of ruthenium tris2,2′-bipyridine [Ru(bpy)3]2+/3+. It can potentially be used to optimize the water oxidation with Ru-complex by accelerating the oxidation of [Ru(bpy)3]2+/3+. On the basis of the STM observations, it can be concluded that the cavities of TCDB are capable of accommodating 1 and Na8[HPW9O34], which are entrapped into the networks as guest molecules. It is noteworthy that we could not observe the assembling structures of both guest molecules without TCDB template. The information gained from the assembly behavior on the surface could provide direct insight into the relationship between the structural pattern and the assembly process in the region of nanoscale, and it could help to develop promising molecular materials. Additionally, the TCDB supramolecular network could be used as spatial confiners to control chemical reactions.40 In future, we aim to use it as nanoreactor and to monitor the in situ reaction of POM cluster (such as redox reaction).

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EXPERIMENTAL SECTION

Materials. All chemicals were reagent grade and used as supplied from Aladdin-Reagent without further purification. Synthesis of Compound 1. The flow reactor apparatus was shown in Figure S1. The aqueous stock solutions were prepared as follows: Solution A: A sample of IrCl3·H2O (0.088 g, 0.28 mmol) was added to NaH2PO4−Na2HPO4 (5 mL, 0.25 mol/L) solution under stirring, then the pH was adjusted to 6.8 with 4 mol/L HCl. Finally, the volume of the mixture was added to 6 mL with deionized water. Solution B: The pH of the solution of Na8[HPW9O34] (1.42 g, 0.59 mmol) in 10 mL of deionized water was adjusted to 6.8, then the solution was added to 12 mL with deionized water. The experiment was performed using the following method: solutions A and B were transferred into two different syringes, which were connected to two pumps; then, two solutions were mixed at room temperature (flow rate for A: 0.02 mL/min; B: 0.04 mL/min) and started to flow around E

DOI: 10.1021/acs.inorgchem.6b00670 Inorg. Chem. XXXX, XXX, XXX−XXX

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solution program using Charge Flipping and refined with the olex2.refine refinement package using Gauss−Newton minimization. Crystallographic data and structure refinements for K12Na2H2[Ir2Cl8P2W20O72]·37H2O (1), K12Na2H76Ir2Cl8O109P2W20, Mr = 6742.50 g mol−1; black needle. Orthorhombic; space group Pnnm, a = 19.3909(6), b = 18.7556(7), c = 14.7692(5) Å, V = 5371.4(3) Å3; Z = 4; ρ = 4.003 g cm−3; λ(Cu Kα) = 1.541 84 Å; 5402 reflections measured, 4508 unique (Rint = 0.0433), 261 refined parameters; GOF = 1.088; final R1 = 0.0896, wR2 = 0.2177 (all data). Additional crystallographic information is available in the Supporting Information. Note: In the packing arrangement, the adjacent polyanions of 1 are linked together by K+ ions to form a three-dimensional open framework with one-dimensional narrow tunnels. The bond lengths of K−O are in the range of 2.725(13)−3.33(2) Å. The narrow void is formed by four adjacent polyanions on the [110] plane. In each unit cell, the solvent accessible void is ca. 1999.1 Å3 estimated with PLATON software. Approximately 37 lattice water molecules were determined based on the single-crystal X-ray diffraction analysis, TG and elemental analyses. Powder X-ray Diffraction. PXRD data were collected on a DX2600 Powder diffractometer employing a scanning rate of 0.05° s−1 in the 2θ range from 2° to 50°. Thermogravimetric Analysis. TGA was performed using an STA409PC Synchronous thermal analyzer with a heating rate of 0.5 °C min−1. Scanning Tunneling Microscope measurement. The STM measurements were performed with a Nanoscope IIIa scanning probe microscope system (Bruker, USA) at room temperature. Tips were mechanically cut from Pt/Ir wires (80/20). All STM images were recorded using the constant current mode of operation. TCDB and Na8[HPW9O34] were dissolved in heptanoic acid. K12Na2H2[Ir2Cl8P2W20O72]·37H2O (1) was dissolved in water. The concentration of all samples was less than 1 × 10−4 M. A droplet (0.4 μL) of the 1:1 mixture of the heptanoic acid containing TCDB and the aqueous solution containing cluster 1 was deposited onto a freshly cleaved surface (5 mm × 5 mm) of HOPG. After the film was dried, it was studied by STM measurements. At the beginning, the dried film was studied by STM at a positive bias. After imaging the ordered arrays, the bias was changed to a negative bias, and the STM images were captured again.

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ACKNOWLEDGMENTS The authors thank Profs. C. Hu and J. Cao (Beijing Institute of Technology) for their kind assistance on ESI-MS measurements and Ms. Y. Wang for the initial contribution to this work. Financial support from Qianjiang Talent Program of Zhejiang Province (No. QJD1402007), the National Natural Science Foundation of China (Nos. 21371078, 21401077, 21472029, 51173031, and 91127043), the National Key Basic Research Program of China (Nos. 2011CB932303 and 2013CB934200), and the Australian Research Council (No. DP150101861) is gratefully acknowledged.

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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.6b00670. Further details of the crystal structure investigation could be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: (+49)7247−808−666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data. html) on quoting the CSD No. 429642. P NMR and IR/UV spectra, PXRD pattern, the flow reactor apparatus, crystallographic data for 1, TGA analysis, structure of 1 and [PW9O34]9−. (PDF) X-ray crystallographic data. (CIF) 31

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (C.Z.) *E-mail: [email protected]. (Q.Z.) *Phone: (+86 0573) 8364 3264. E-mail: [email protected]. (J. X.) Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.6b00670 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00670 Inorg. Chem. XXXX, XXX, XXX−XXX