Zirconium Phosphate Supported MOF Nanoplatelets - Inorganic

May 13, 2016 - ZrP nanoplatelets, used as seeds for the growth of metal−organic frameworks, directed the MOF to form with a plate-like morphology. T...
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Zirconium Phosphate Supported MOF Nanoplatelets Yuwei Kan and Abraham Clearfield* Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *

ABSTRACT: We report a rare example of the preparation of HKUST-1 metal−organic framework nanoplatelets through a step-by-step seeding procedure. Sodium ion exchanged zirconium phosphate, NaZrP, nanoplatelets were judiciously selected as support for layer-by-layer (LBL) assembly of Cu(II) and benzene-1,3,5-tricarboxylic acid (H3BTC) linkers. The first layer of Cu(II) is attached to the surface of zirconium phosphate through covalent interaction. The successive LBL growth of HKUST-1 film is then realized by soaking the NaZrP nanoplatelets in ethanolic solutions of cupric acetate and H3BTC, respectively. The amount of assembled HKUST-1 can be readily controlled by varying the number of growth cycles, which was characterized by powder X-ray diffraction and gas adsorption analyses. The successful construction of HKUST-1 on NaZrP was also supported by its catalytic performance for the oxidation of cyclohexene.

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INTRODUCTION Smectite-type layered inorganic materials have received considerable attention in the past decades owing to their two-dimensional structure, tunable particle size, high stability, and a variety of applications, particularly in ion exchange,1 catalysis,2 thin films,3 and biological applications.4 Among these materials, zirconium phosphates (ZrP), a family of twodimensional (2D) layered materials, have been extensively studied for ion exchange,5 polymer nanocomposite,6 drug delivery,4 and lubricant additive.7 The best characterized ZrP is the α-phase with the formula Zr(O3POH)2·H2O (α-ZrP) and interlayer distance of 7.6 Å. The single-crystal structure of αZrP was reported by Troup and Clearfield in 1977 (Figure 1).8 The tetravalent zirconium cations are arranged at the corners of a parallelogram with Zr(IV) ions alternately above and below the mean plane of the layer. The phosphate groups sit

alternately above and below the mean plane of the layer with three oxygen atoms of the phosphate group bonding to three of the Zr(IV) ions forming a triangle in half of the parallelogram. Each Zr(IV) ion is coordinated with six oxygen atoms from six different phosphate groups in adjacent parallelograms. The P− OH groups form a double layer in the interlayer space. The protons on hydroxyl groups can be easily replaced by other positively charged species, resulting in the formation of new phases.1b The diameter of α-ZrP particles varies in the range from 50 nm to 2 μm, which can be controlled by varying the synthetic conditions.9 To improve its applicability, the surface of α-ZrP can be functionalized with various organic functional groups, such as organic silanes,10 isocyanate,11 and epoxide groups.12 As a family of multifunctional porous material, metal− organic frameworks (MOFs) have been well-studied for their structures and diverse applications in the past two decades.13 The combination of MOFs with other materials have shown improved properties for given applications, especially in separations and catalysis.14 Although many applications have been suggested for MOFs, a considerable problem is their stability.15 To improve the stability of MOFs, there is a demand to develop controllable synthesis of MOFs on solid supports. For practical applications, processing and formulation into specific configurations is normally needed.16 In the case of MOFs, the way to realize this is to prepare composite materials, where MOFs are supported on substrates or constructed at the interfaces to form composites.16 Such interfaces are referred to as self-assembled monolayers (SAMs). The first ones were

Figure 1. Structural representation of α-ZrP viewed along the b-Axis.

Received: March 22, 2016

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.6b00710 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry based on gold platforms.17 Gold-supported SAMs are not practical due to the fragility of the gold−SAM interface at higher temperatures.15b,17 To increase the interaction, pre SAMs were immersed into solutions containing metal ions and ligands. Therefore, SAMs containing −COOH or −OH groups were developed to facilitate the layer-by-layer (LBL) growth processes.18 ZrP is a family of material different from SAMs. The amorphous particles are nanosized, and the layers grow slowly to form crystals. In so doing the ZrP pass through a series of stages in which the ion exchange curves change from a normal stage where the pH increases, as more cation is taken up displacing protons, to a stage where the system has no degrees of freedom and the pH is fixed at ∼2.0.19 Powder X-ray diffraction (PXRD) patterns show a gradual change from a poorly crystalline to a highly crystalline phase.19 Thus, the arrangement of the P−OH groups must change as the particles increase in crystallinity. Although MOFs can grow as thin films on substrate or in polymers to foster their applications,14a,20 to the best of our knowledge, MOFs growth on the surface of 2D layered materials have not been reported.18b,c,21 In this work we show that a MOF can be easily deposited on small ZrP particles. To prove this concept, LBL assemblies of Cu(II) and 1,3,5-benzenetricarboxylic acid (H3BTC) on fully sodium exchanged ZrP particles is selected as a model. The ZrP supported HKUST-122 has the following advantages: First, the thickness of the growing film can be controlled by governing the reaction cycles via LBL assembly. Second, the Cu(II) ions are directly bonded to the P−OH groups on ZrP surfaces, which prevents the MOF material from dissociation with the support and increases the stability. Third, the 2D nature of ZrP forces HKUST-1 to grow as nanoplatelets, which is difficult to achieve through a one-pot solvothermal reaction. Last but not the least, with careful control, the resulted ZrP⊂HKUST-1 possesses higher crystallinity and porosity compared to those grown on porous α-alumina support.23

deposited on the surface of ZrP. The Zr/Cu atomic ratio is 1:0.052, while the Zr/Na atomic ratio changed from 1:1.86 to 1:1.75 after copper deposition (see Supporting Information). Combining thermogravimetric analysis (TGA) with WDS results, it suggests that 0.11 mol of Na(I) on the surface of NaZrP were exchanged by Cu(II) in 2:1 ratio. The quantity of Cu(II) was compared with Cs(I) titrated ZrP, because it was reported before that cesium ions only exchange surface protons of ZrP in one-to-one ratio due to its large size, which has been utilized to quantify the amount of surface protons for a given ZrP material.24 ZrP nanoplatelets with diameter of ∼150 nm were found to contain 0.11 mol/mol surface protons by using Cs(I) titration,1c which is in good agreement with the fact that there are 0.11 mol/mol less of Na(I) after the deposition of Cu(II). Recently, we have shown that the surface of α-ZrP nanoplatelets can be decorated by tetravalent metal ions, which are capable of binding phosphonic acids for further surface modifications.24 However, the assembled layers are amorphous due to the randomness of metal−phosphonate coordination modes,25 which makes it difficult to characterize by PXRD technique. In contrast, MOFs are highly crystalline materials that, in most circumstances, retain their crystallinity when assembled on surfaces.18d As mentioned above, the surface sodium ions can be readily exchanged with Cu(II) to achieve the copper-decorated NaZrP denoted as Cu/NaZrP. To construct HKUST-1, fresh Cu/NaZrP nanoplatelets were treated with ethanolic solution of H3BTC for 30 min and washed thoroughly with ethanol to complete the first cycle deposition. The same decoration procedure was repeated to achieve the growth of multiple layers of HKUST-1. As we expected, after the second cycle of deposition, a small peak appeared in the PXRD pattern, which we suspect belongs to HKUST-1. Indeed, after the fourth cycle, more peaks start to appear, which are in good agreement with the characteristic PXRD pattern of HKUST-1. As illustrated in Figure 2, the

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RESULTS AND DISCUSSION Given the fact that α-ZrP is an excellent ion exchanger, we sought to use Cu(II) to replace protons on α-ZrP surface. The interlayer distance of α-ZrP is 7.6 Å, which corresponds to the peak at 11.98° in PXRD patterns. However, when treated with Cu(II), this peak shifted to 9.63°, which indicates that the interlayer distance changed to 9.3 Å, suggesting that significant amount of Cu(II) intercalated into α-ZrP interlayers, which makes it difficult to quantify how much Cu(II) was deposited on the surface. To avoid Cu(II) ions from entering between ZrP layers, as the first step, a sodium ion titration was performed for ZrP to displace the P−OH protons. The ZrP particles are now Zr(NaPO4)2 (denoted as NaZrP) with an interlayer spacing of 8.3 Å, which corresponds to the peak at 10.73° in PXRD (Figure S1). Subsequently, the Na(I) on the external NaZrP surfaces were displaced by treating with a stoichiometric amount of copper acetate in ethanol. Nice pale blue particles were obtained. The presence of Cu(II) was characterized by several techniques. PXRD patterns confirmed that only the Na(I) on the external surface of ZrP nanoparticles are replaced by Cu(II), as there is no observed peak position change after the treatment of Cu(II), suggesting that the interlayer distance of NaZrP nanoplatelets remained intact during Cu(II) ion exchange (Figure S1). Moreover, wavelength-dispersive X-ray spectroscopy (WDS) quantitative analysis was conducted to quantify how much Cu(II) was

Figure 2. PXRD of ZrP⊂HKUST-1 with 0, 2, 4, 8, 10, 15 cycles of deposition and simulated HKUST-1.

intensity of HKUST-1 PXRD peaks are being enhanced, which corresponds to increased cycles of Cu(II) and BTC deposition. Notably, the intensity of the major peak of NaZrP at 10.73° diminished from the PXRD pattern, as more layers of highly crystalline HKUST-1 are constructed on NaZrP. This is mainly because the portion of low crystalline NaZrP in the particle is being reduced, while more layers of HKUST-1 are formed. After the 15 cycles, the PXRD pattern is clearly that of B

DOI: 10.1021/acs.inorgchem.6b00710 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry HKUST-1 with a very small shoulder peak at ∼11.0°, which belongs to that of NaZrP. We have demonstrated that the composition of a given ZrP material can be derived from TGA measurements and electron microprobe WDS quantitative analysis.24 A sample of the sodium ion exchanged ZrP displayed a total weight loss of 8.03% when subjected to TGA up to 850 °C. Surface water was lost at ∼50 °C, and total dehydration occurred at 120 °C. In the case of the ZrP⊂HKUST-1 samples, there are two major weight changes: the first weight loss in the range from 50 to 150 °C is attributed to residual solvent and intercalated water, and the second weight loss between 250 and 300 °C is believed to be the decomposition of BTC ligands. On the basis of TGA and electron microprobe WDS results, the molar ratio between BTC ligand, Cu, Na, and Zr can be calculated. The molecular formula of ZrP⊂HKUST-1 compounds with selected cycles were calculated and are shown in Table 1.

retained properties of the given MOF. Field emission scanning electron microscopy (FE-SEM) images in Figure 3 show that the morphology of ZrP⊂HKUST-1 nanoplatelets after four, eight, and ten cycles of deposition maintained the hexagonal shape of the parent NaZrP. However, the change of morphology was observed after 15 cycles of growth, which can be attributed to particle aggregation. More importantly, we did not observe any isolated HKUST-1 crystals in SEM images, indicating that HKUST-1 prefers to grow on ZrP surfaces to form thin films. The homogeneous distribution of Cu(II) in ZrP⊂HKUST-1 nanoplatelets was further confirmed by backscattered electron images from electron microprobe (see Supporting Information). It is worth noting that, as WDS suggested, the normalized ratio of Na/Zr decreases from 1.57 (two cycles) to 1.33 (eight cycles), which indicated that the Na(I) in the interlayer region can also be exchanged through the deposition process. As discussed above, under the experimental conditions, it is difficult for copper ions to intercalate between NaZrP layers, especially after deposition of HKUST-1 layers. We hypothesized that it is the protons from H3BTC ligands that partially exchanged the interlayer sodium ions. The MOF growth on the edge of nanoplatelets can also occur, which is supported by the SEM images of ZrP⊂HKUST-1 after 15 cycles of deposition. Note that the ZrP platelet has two surfaces, and thus there are 15 layers of HKUST-1 on each side of NaZrP, along with HKUST-1 on edges. For porous materials, besides the PXRD measurements, it is also very important to evaluate the porosity and surface area by means of nitrogen adsorption−desorption isotherm.27 To confirm the successful grafting of HKUST-1 thin films on NaZrP, N2 adsorption−desorption studies were performed for the activated hybrid materials at 77 K. As expected, because of its nonporous nature, there is nearly no nitrogen uptake for the starting NaZrP material. As Figure 4 illustrates, after two cycles of growth, the material shows a type I isotherm with 60 cm3·g−1 (STP) N2 uptake. Significant increase of N2 uptake was

Table 1. Formulas of ZrP⊂HKUST-1 Compounds with Selected Cycles of Cu(II) and BTC Deposition As Determined by Thermogravimetric Analysis and Microprobe Analysis no. of cycles

TGA total wt loss%

calculated formula

0 2 8

8.03% 14.20% 18.93%

ZrNa1.86 (PO4)2 ZrNa1.57Cu0.31(PO4)2(BTC)0.2 ZrNa1.33Cu1.04(PO4)2(BTC)0.69

It is known that the properties of nanoparticles can be tuned by controlling their morphologies through different synthetic routes. Nano MOFs have attracted increasing interest due to their unique properties compared with large crystals. Specifically, nano MOFs have been extensively studied for biological applications.26 Constructing MOF nanoparticles with different morphology is an intriguing topic. However, to the best of our knowledge, MOF with nanoplatelets shape have never been reported. By depositing MOF thin films on ZrP nanoplatelets, we anticipate to obtain MOF nanoplatelets with

Figure 3. FE-SEM images of ZrP⊂HKUST-1 with (a) 4, (b) 8, (c) 10, (d) 15 cycles of deposition. Scale bar represents 500 nm. C

DOI: 10.1021/acs.inorgchem.6b00710 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 2. Conversion and Yield of Each Product for the Oxidation Reaction of Cyclohexene by Using 6.5 Cycles of ZrP⊂HKUST-1 as Catalyst

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yield

total conversion

32.6% 20.9% 2.8% 3.3%

59.5%

CONCLUSION In sum, we have successfully synthesized the first example of ZrP⊂HKUST-1 nanoplatelets through a step-by-step seeding procedure. The amount of surface supported HKUST-1 can be controlled through successive LBL process. The surface area as well as nitrogen uptake correspond to the HKUST-1/ZrP ratio. We also demonstrated that HKUST-1 forms thin films grafted on ZrP instead of isolated crystals. The HKUST-1 thin film preserved not only its crystallinity and porosity but also its high catalytic activity and selectivity in the aerobic oxidation of cyclohexene. The construction of MOF nanoplatelets in such a seeded route controls the morphology of MOF nanoparticles, which endows new properties to functional nanoMOFs.

Figure 4. N2 adsorption−desorption isotherms of NaZrP (cyan ◇) and ZrP⊂HKUST-1, 2 cycles (black □), 4 cycles (red ○), 8 cycles (green △), 10 cycles (blue ⬠), 15 cycles (pink ○), and pristine HKUST-1 (purple ☆). Ads stands for adsorption, and Des stands for desorption.

observed when more layers are added, 75 cm3 g−1 after four cycles, 140 cm3·g−1 after eight cycles, 200 cm3·g−1(STP) after ten cycles, and 325 cm3·g−1(STP) after 15 cycles, whereas pristine HKUST-1 possesses nitrogen uptake of 400 cm3·g−1 (STP). Notably, the gravimetric N2 uptake is related to the sample weight; therefore, the low N2 uptake of these ZrP⊂HKUST-1 materials can be attributed to the existence of the nonporous NaZrP. To further confirm the successful construction of HKUST-1 thin films, the resulted ZrP⊂HKUST-1 material was examined as a catalyst for the solvent-free oxidation of cyclohexene. It is known that cyclohexene can be readily oxidized to produce 2cyclohexen-1-ol, 2-cyclohexen-1-one, epoxycyclohexane, and cyclohexene hydroperoxide in the presence of copper catalyst and oxygen, Scheme 1.28 The hybrid material ZrP⊂HKUST-1

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EXPERIMENTAL SECTION

Chemicals. Zirconyl chloride octahydrate (>99.0%) was purchased from Fluka. Phosphoric acid (85 wt % in H2O) was purchased from Fisher-Scientific. Sodium nitrate (>99.0%) and copper(II) acetate monohydrate (>98%) were purchased from Sigma-Aldrich. 1,3,5Benezenetricarboxylic acid (98%) was purchased from ACROS organics. All chemicals were used without further purification. Synthesis of Na(I) Intercalated α-ZrP. The α-ZrP nanoplatelets were synthesized by the reflux method reported by Sun and coworkers with slight modifications.9 In general, 3.2 g of ZrOCl2·8H2O was dissolved in Milli-Q water to make a 200 mL aqueous solution that was preheated to 94 °C, and then 200 mL of 6.0 M phosphoric acid was added dropwise to the ZrOCl2 solution and was kept at 94 °C for 48 h. The white powdery product (α-ZrP) was thoroughly washed with Milli-Q water and dried in an oven at 70 °C for 24 h. 500 mg of the resulting α-ZrP was well-dispersed in a 0.5 N NaNO3 solution, and then the α-ZrP/NaNO3 solution was titrated with 0.1 N NaOH/ NaNO3 (1:1) until the pH reaches 8.0 and maintains this value at least for 12 h. The product was rinsed by large amounts of Milli-Q water to remove excess sodium ions and dried in an oven at 70 °C for 24 h before using. The Growth of HKUST-1 Layers on NaZrP Surface. Approximately 500 mg (1.54 mmol) of dry NaZrP, obtained from the previous step (confirmed by PXRD and elemental analysis), was suspended in ∼75 mL of ethanol in a round-bottom flask. A 0.154 mmol copper acetate ethanolic solution was added to the NaZrP dispersion and allowed to stir for 30 min. The product was washed thoroughly with ethanol and dispersed in ethanol again. A stoichiometric amount of H3BTC was added to the Cu-NaZrP solution and allowed to stir for another 30 min. One cycle is completed, and the resulting ZrP⊂HKUST-1 was rinsed with a large amount of ethanol to ensure the removal of excess acid. Successive cycles were applied by repeating the same experimental procedure. Catalytic Reaction Procedure of Cyclohexene Oxidation. Oxidation of cyclohexene was performed by mixing 50 mg of designated ZrP⊂HKUST-1 catalyst in 5 mL of cyclohexene in a Schlenk flask connected with an oxygen balloon. The reaction was performed at 80 °C for 20 h. The reaction mixture was then centrifuged to separate the solid catalyst from the liquid mixture. A small amount of the brownish liquid was transferred into an NMR tube, and CDCl3 was added before taking 1H and 13C NMR spectra. Instrumentation. PXRD experiments were performed from 4° to 40° (2θ angle) using a Bruker-AXS D8 short arm diffractometer

Scheme 1. Catalytic Oxidationa of Cyclohexene by Using ZrP⊂HKUST-1

a

product 2-cyclohexene-1-one 2-cyclohexene-1-ol cyclohexene oxide cyclohexene hydroperoxide

After 6.5 cycles of deposition. Reaction conditions: 80 °C, 20 h.

with 6.5 cycles of treatment was used as catalyst for the oxidation reaction. In a typical reaction, 5 mL of cyclohexene and 50 mg of ZrP⊂HKUST-1 were charged into a flask under oxygen environment. The reaction mixture was heated at 80 °C for 20 h. A small portion of the isolated liquid mixture was dissolved in CDCl3 for NMR analysis. The conversion and yield of different products were calculated based on the integration of the peak area in the 13C NMR spectra; commercially available pure samples were used as references (see Supporting Information for details). As shown in Table 2, under the given reaction condition, the conversion was calculated to be 59.5%, which is much higher than the reported conversion of 14.6%.28 It is also worth noting that, in a previous report,28 2cyclohexen-1-ol was shown to be the dominant product by using HKUST-1. However, we found that more 2-cyclohexen1-one was produced by using ZrP⊂HKUST-1 nanoplatelets. D

DOI: 10.1021/acs.inorgchem.6b00710 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry equipped with a multiwire lynx eye detector using Cu (Kα, λ = 1.542 Å) and operated at a potential of 40 kV and a current of 40 mA. TGA experiments were performed on a TGA Q500 TA Instrument. Samples were heated from room temperature to 1000 °C at a heating rate of 5 °C/min under N2. Elemental analysis work employed WDS. Analyses were performed after standardization using very well-characterized compounds or pure elements. Pressed powder micropellets were prepared by pressing a few milligrams of powder between the highly polished surfaces (0.25 μm) of hardened steel dies and transferring the pellets onto double-sided conductive carbon tape. The pellets were carbon-coated before analysis to make them electrically conductive. Analyses of pressed powder pellets were performed with a 20 μm diameter beam, while the stage was being moved 20 μm every 2 s, repeated over a 10 spot traverse. This ensured representative sampling and minimized possible thermal damage to the samples. The transmission electron micrographs (TEM) of the samples were acquired using a JEOL 2010 transmission electron microscope at an acceleration voltage of 200 kV. Samples were prepared using gold grids from Ted Pella. Scanning electron microscopy (SEM) images were acquired on a JEOL JSM-7500F (FE-SEM). Low pressure nitrogen adsorption−desorption isotherm measurements were performed on an ASAP 2020 with extra-pure quality gases. The as-synthesized material was soaked in ethanol for 24 h and then centrifuged and dried at 60 °C for 3 h in an oven. The resulted blue powder was then transferred into a BET tube and activated at 80 °C at 133 microbar for 12 h. The color of the material was noticed to change from blue to purple upon activation. NMR spectra were acquired on a Mercury 300 spectrometer (300 and 75.5 MHz for 1H and 13C, respectively) at a temperature of 298 K. All 1H NMR spectra are reported in parts per million downfield of tetramethylsilane and were measured relative to the signals at 7.26 ppm (CDCl3). All 13C NMR spectra were reported in parts per million relative to 77.16 ppm (CDCl3) and were obtained with 1H decoupling. Standard materials for cyclohexene, 2-cyclohexene-1-one, 2-cyclohexene-1-ol, and cyclohexene oxide were purchased from SigmaAldrich and were used without further purification. Since cyclohexene hydroperoxide is not commercially available, the 1H and 13C NMR of cyclohexene hydroperoxide is predicted. In 1H NMR spectra, the chemical shifts between different products are very similar, and there are significant couplings between these protons. Thus, it is difficult to distinguish each product by 1H NMR due to the peak complexity and significant overlap. In contrast, the 13C NMR spectra of each product can be readily recognized, which can be used for identifying products in the reaction mixture. The ratio of products was calculated according to the integration of a fingerprint 13C NMR peak for each product. Reference peaks of 127.21, 150.23, 65.3, 51.92, and 77.7 ppm were selected for cyclohexene, 2-cyclohexene-1-one, 2-cyclohexene-1-ol, cyclohexene oxide, and cyclohexene hydroperoxide, respectively.

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Laboratory at Texas A&M Univ. for the use of the PXRD facilities.

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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.6b00710. PXRD, TGA, WDS analysis, TEM, NMR analysis, and N2 adsorption isotherms. (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: clearfi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation Grant No. A-0673, for which grateful acknowledgement is made. We would like to acknowledge the X-ray Diffraction E

DOI: 10.1021/acs.inorgchem.6b00710 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00710 Inorg. Chem. XXXX, XXX, XXX−XXX