Ratio-Controlled Precursors of Anderson–Evans Polyoxometalates

Mar 19, 2018 - A series of triol ligand [CH3C(CH2OH)3] covalently decorated polyoxometalates (POMs), which could be ascribed to the primary complexes ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Ratio-Controlled Precursors of Anderson−Evans Polyoxometalates: Synthesis, Structural Transformation, and Magnetic and Catalytic Properties of a Series of Triol Ligand-Decorated {M2Mo6} Clusters (M = Cu2+, Co2+, Ni2+, Zn2+) Yang Wang,†,‡ Xueping Kong,† Wei Xu,§ Fengrui Jiang,† Bao Li,† and Lixin Wu*,† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry and Institute of Theoretical Chemistry, and State Key Laboratory of Inorganic Synthesis & Preparative Chemistry, Jilin University, Changchun 130012, P. R. China ‡ Department of Chemistry, School of Food Engineering, Harbin University, Harbin 150086, P. R. China §

S Supporting Information *

ABSTRACT: A series of triol ligand [CH3C(CH2OH)3] covalently decorated polyoxometalates (POMs), which could be ascribed to the primary complexes with structural formulas {M2[Mo2O4(CH3C(CH2O)3)2]3}2− (M = Cu2+, Co2+, Ni2+, Zn2+), have been synthesized in organic solvents. Single-crystal X-ray structural analysis reveals that the synthesized polyanionic clusters are comprised of three {Mo2} units and two divalent transition-metal ions connecting to each other in an alternating style, where all {Mo2} blocks were covalently decorated by two triol ligands in the trans conformation. The 1/3 molar ratio of M/Mo in the prepared complexes was higher than those ratios in typical Anderson−Evans, Wells−Dawson, and Keggin POMs. With a decrease in the M/Mo molar ratio of a Mo-contained reactant to 1/6 and/or the addition of acetic acid to the reaction solution, the primary complexes acting as precursors transformed continuously into the corresponding triol-liganddecorated Anderson−Evans POMs. Detailed investigations were conducted by using different isopolymolybdates in various solvent environments, and several Anderson−Evans POMs in different triol-ligand-decorated fashions were obtained from the primary complexes. In addition, we also realized the transformation between the Anderson−Evans clusters in different decoration fashions by simply controlling the acidity in solution. Magnetic measurement showed a general property, but the catalytic experiments demonstrated that CoII- and Zn II-containing POMs displayed a higher efficiency for the selective oxidation of thioanisole to sulfoxide.

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INTRODUCTION Polyoxometalates (POMs) can be basically treated as the assemblies that collect metal (such as Mo, W) oxide building units, together with the absence or presence of other centered metals or nonmetal oxides, through corner-, edge-, and, less commonly, face-sharing style.1 A suitable combination of starting materials under controlled conditions, such as the reaction temperature, pH, solvents, and reaction additives, leads to the target-directed synthesis for diverse cluster structures. As one of the important architecture types, Anderson−Evans POMs possess the general formula of [Xn+M6O24Hm](12−n−m)− (X = heteroatom; M = Mo or W).2,3 Partially or fully protonated O atoms surrounding the heteroatom of Anderson−Evans POMs in the B type have been demonstrated to be replaced by organic ligands with multiple hydroxyl groups, forming organically decorated hybrids4−7 toward important building units for self-assemblies both in solution and at the interface.8−12 The triol-ligand-decorated Anderson−Evan POMs also have valuable applications in the fields of battery structures,13 nanostructure fabrication,14 and protein crystallization.15,16 In contrast to the quick development of the organic-modified structure fashions, however, the formation © XXXX American Chemical Society

approaches of these types of hybrids and their structure transformations are rarely studied.17−20 When high-resolution mass spectrometry was employed, a real-time observation of the assembly process of triol-ligand decoration on MnIII-centered Anderson−Evans POMs with TBA4Mo8O26 (TBA = tetrabutylammonium cation) was carefully characterized,17 in which some subclusters derived from decomposition of the reactant were proposed as intermediate fragments, including a partially decorated {Mo2O10} cluster by the triol ligand. We also used TBA2Mo2O7 as the initial cluster to obtain different types of triol-ligand-decorated fashions on Anderson−Evans POMs, which further supported the existence of the intermediate products.18,19 However, it is still unknown whether there are organically modified primary complexes or intermediates during the reactions and whether they could be separated out from the reactions. As far as we know, no relevant publications have dealt with the utilization of such kinds of subclusters with triol ligand modification for the final synthesis of Anderson− Evans POMs. Received: November 24, 2017

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DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Schematic Illustration for the Synthesis of Triol-Ligand-Decorated {M2Mo6} (M = Cu2+, Co2+, Ni2+, Zn2+) Primary Complexes and Their Transformations to the Corresponding Anderson−Evans POMs under Various Conditions

atoms in a methanol solution. The present study not only discloses novel structured POMs but also provides a new route for the synthesis of targeted Anderson−Evans POMs.

As the simplest assembly of isopolymolybdates, a bare {Mo2O10} cluster composed of two {MoO6} octahedra in edgesharing style cannot exist steadily because of deregulation of the Lipscomb Principle. That means that each {MoO6} octahedron possesses no more than two unshared terminal O atoms.21−23 However, these types of clusters can be synthesized by coordination with organic ligands to reduce the number of unshared terminal O atoms24,25 and finally assemble into a ring architecture.26 The triol ligand is demonstrated to have an ideal coordination form to stabilize the {Mo2O10} cluster through its three O atoms, forming a doubly triol-ligand-decorated {Mo2O10} cluster.27,28 The modification makes it possible for this cluster to serve as a building block for further assembly with other metal ions.29 Following our previous studies on the decoration fashions of Anderson−Evans cluster hybrids by changes in the starting materials,18,19 we speculate that there should be triol-ligand-decorated {Mo2O10} clusters during the preparation process, and they could act as independent precursors for coordination with metal ions. With these isolated reactants, it is also possible to create novel organic modification fashions that could not be obtained with known reactants. Therefore, controlling the molar ratios among the starting components becomes an essential part in identifying the formation route of triol-ligand-decorated Anderson−Evans POMs. We herein report the synthesis of a series of triolligand-decorated {M2Mo6} primary complexes and their transformations to Anderson−Evans POMs, as shown in Scheme 1, in which the structure formulas can be described as TBA2{M2[Mo2O4(CH3C(CH2O)3)2]3} (abbreviated as {M2Mo6}, where M = Cu2+, Co2+, Ni2+, and Zn2+). With the triol-ligand-decorated {M2Mo6} clusters as the reactants, the corresponding double-sided χ/χ-isomer Anderson−Evans POMs, where two triol ligands replace two μ3-O (triplebridging O atom) and one μ2-O (double-bridging O atom) atoms symmetrically on each side, are synthesized successfully in the presence of acetic acid and TBA4Mo8O26 as the additive Mo-contained reactant. In addition, further decoration by methanol molecules on the other one or two unreacted μ3-O atoms is realized as well. By addition of acetic acid, the singlemethanol-containing triol-ligand-decorated χ/δ isomer transforms to a χ/δ isomer without methanol in acetonitrile and a χ/ χ isomer with two methanol molecules anchored to the μ3-O

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EXPERIMENTAL SECTION

Instrumentations. The 1H NMR spectrum was performed on a Bruker Avance 500 spectrometer in CD3CN. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vertex 80v spectrometer equipped with a DTGS detector (32 scans) with a resolution of 4 cm−1 using KBr pellets. Elemental analyses for C, H, and N were carried out on a Vario MICRO cube of Elementar Company. Elemental analyses for Cu, Co, Ni, Zn, and Mo were taken on a PLASMA-SPEC (I) inductively coupled plasma atomic emission spectrometer. Thermogravimetric analysis (TGA) was carried out on a TA Instruments Q500 thermal analyzer with a flow of nitrogen, and the temperature was set from 30 to 900 °C under a heating rate of 10 °C min−1. Magnetic susceptibility measurements were carried out on a Quantum Design MPMS XL-5 SQUID system in the temperature range of 2−300 K. Electrochemical experiments were recorded on a CHI 660C electrochemical workstation at room temperature. The conversions in the catalytic experiments were determined by highperformance liquid chromatography (HPLC) on a Shimadzu LC-20A chromatograph equipped with a Chiralcel OD-H column (4.6 × 250 mm) from Daicel Chemical Industries Ltd. X-ray Crystallography. Single-crystal X-ray diffraction indexing and data collection were performed on a Rigaku R-AXIS RAPID imaging-plate diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) at 293 K. The empirical absorption correction based on equivalent reflections was applied. All complex crystals were solved by direct methods and refined by full-matrix least-squares fitting on F2 using SHELXTL-97 software.30 All non-H atoms, except some lattice solvent molecules, were refined with anisotropic thermal parameters. Synthesis. All general chemicals, including solvents used in the reactions, were purchased from Aladdin and used directly except those noted specifically. The two starting molybdate clusters, TBA4Mo8O26 and TBA2Mo2O7, were prepared according to reported procedures.31,32 TBA2{Cu2[Mo2O4(CH3C(CH2O)3)2]3}·2CH3CN (1). Cu(CH3COO)2· 2H2O (0.10 g, 0.50 mmol), TBA4Mo8O26 (0.41 g, 0.19 mmol), and CH3C(CH2OH)3 (0.18 g, 1.50 mmol) were added into 20 mL of acetonitrile, and the mixture was refluxed for 12 h. The formed bluegreen solution was cooled to room temperature, and green crystals formed in 2−3 days with a yield of 57.8% based on Mo. Elem anal. Calcd for Cu2Mo6C66H132N4O30: Cu, 5.87; Mo, 26.59; C, 36.62; H, 6.15; N, 2.59. Found: Cu, 5.60; Mo, 26.63; C, 36.58; H, 6.13; N, 2.58. IR (KBr, cm−1): 3460, 2959, 2932, 2872, 1632, 1481, 1461, 1398, B

DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Summary of the Crystal Data and Structure Refinements for 1−4, 7, and 9 chemical formula fw space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) F(000) reflns collected/ unique Rint GOF on F2 R1a [I > 2σ(I)] wR2b (all data) a

1

2

3

4

7

9

Cu2Mo6C66H132N4O30

Co2Mo6C66H132N4O30

Ni2Mo6C66H132N4O30

Zn2Mo6C66H132N4O30

NiMo6C48H106N4O26

CuMo6C65H143N5O26

2164.48 Pbca 18.115(4) 19.703(4) 25.515(5) 90 90 90 9107(3) 4 1.579 4424 81513/10410

2155.26 Pbca 18.267(4) 19.375(4) 25.824(5) 90 90 90 9140(3) 4 1.566 4408 77965/10443

2154.82 Pbca 18.271(4) 19.390(4) 25.935(5) 90 90 90 9188(3) 4 1.558 4416 74575/10523

2155.26 Pbca 18.380(4) 19.348(4) 25.657(5) 90 90 90 9124(3) 4 1.578 4432 78880/10416

1789.72 P1̅ 12.448(3) 12.829(3) 13.517(3) 106.41(3) 102.54(3) 112.83(3) 1773(1) 1 1.676 910 17298/8040

2050.02 C2 26.473(5) 14.205(3) 24.772(5) 90 91.64(3) 90 9312(3) 4 1.462 4228 43764/19330

0.0627 1.041 0.0410 0.1113

0.0488 1.059 0.0635 0.2039

0.0641 1.028 0.0509 0.1547

0.0574 1.041 0.0624 0.1287

0.0350 1.136 0.0489 0.1288

0.0598 1.053 0.0623 0.1774

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = ∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]1/2. based on Mo, 0.25 g, 73.5%). In route 2, compound 1 (0.10 g, 0.05 mmol), TBA4Mo8O26 (0.09 g, 0.04 mmol), and 0.10 mL of acetic acid were dissolved in 20 mL of acetonitrile. The solution was heated to reflux for 12 h and then allowed to cool to room temperature to obtain blue crystals (yield 71.8% based on Mo, 0.12 g) in several days. Both products here were confirmed by single-crystal X-ray analysis to have the same structure as that in the literature.18 TBA2{CoMo6O16(OH)2[CH3C(CH2O)3]2}·4CH3CN (6). This compound was also first reported by our group,19 and could be prepared from the precursor compound 2 (0.10 g, 0.05 mmol) in the presence of TBA4Mo8O26 (0.09 g, 0.04 mmol) and 0.10 mL of acetic acid in 20 mL of acetonitrile. The reaction solution was heated to reflux for 12 h and then cooled to generate orange crystals in several days (yield 65.9% based on Mo). The structure was proved by single-crystal X-ray analysis. TBA2{NiMo6O16(OH)2[CH3C(CH2O)3]2}·2C3H7NO (7). The polyanionic part of this compound was first reported by Hasenknopf et al.,8 but no single-crystal X-ray data were involved. Here compound 3 (0.10 g, 0.05 mmol) was used as the starting material, together with TBA4Mo8O26 (0.09 g, 0.04 mmol) and 0.10 mL of acetic acid, to be added into 20 mL acetonitrile. The solution was heated to reflux for 12 h and then cooled to obtain a crude product. Dissolving in N,Ndimethylformamide (DMF) gives highly qualified pale-blue crystals (yield 54.2% based on Mo). Elem anal. Calcd for NiMo6C48H106N4O26: Ni, 3.28; Mo, 32.16; C, 32.21; H, 5.97; N, 3.13. Found: Ni, 3.27; Mo, 32.19; C, 32.24; H, 6.01; N, 3.11. IR (KBr, cm−1): 3418, 2963, 2931, 2872, 1663, 1471, 1460, 1396, 1383, 1117, 1040, 919, 902, 735, 671, 557, 431. TBA2{ZnMo6O16(OH)2[CH3C(CH2O)3]2}·4CH3CN (8). This compound was initially reported by Hasenknopf et al.,8 and a different synthetic route was conducted by mixing compound 4 (0.10 g, 0.05 mmol) with TBA4Mo8O26 (0.09 g, 0.04 mmol) and 0.10 mL of acetic acid in 20 mL of acetonitrile and refluxing for 12 h. Colorless crystals were obtained in several days (yield 60.2% based on Mo, 0.10 g), and the structure was verified by single-crystal X-ray analysis. TBA3{CuMo6O17(CH3O)[CH3C(CH2O)3]2}·2C3H7NO (9). Compound 1 (0.10 g, 0.05 mmol) and TBA2Mo2O7 (0.12 g, 0.15 mmol) were added into 20 mL of methanol. The solution was heated to reflux for 12 h and then cooled for several days to obtain a crude product. Recrystallization from DMF gave blue crystals suitable for singlecrystal X-ray analysis in a yield of 47.3% based on Mo. Elem anal. Calcd for CuMo6C65H143N5O26: Cu, 3.10; Mo, 28.08; C, 38.08; H, 7.03; N, 3.42. Found: Cu, 3.02; Mo, 28.01; C, 38.54; H, 7.15; N, 3.55.

1384, 1125, 1052, 1028, 918, 903, 631, 610, 575, 561, 523, 503, 466, 452. TBA2{Co2[Mo2O4(CH3C(CH2O)3)2]3}·2CH3CN (2). Co(CH3COO)2· 2H2O (0.10 g, 0.40 mmol), TBA4Mo8O26 (0.33 g, 0.15 mmol), and CH3C(CH2OH)3 (0.15 g, 1.20 mmol) were dissolved in 20 mL of acetonitrile, and the mixture was refluxed for 12 h. The formed purple solution was allowed to cool to room temperature and stand for 2−3 days until purple crystals were obtained in a yield of 79.6% based on Mo. Elem anal. Calcd for Co2Mo6C66H132N4O30: Co, 5.47; Mo, 26.71; C, 36.78; H, 6.17; N, 2.60. Found: Co, 5.51; Mo, 26.68; C, 36.75; H, 6.15; N, 2.59. IR (KBr, cm−1): 3453, 2959, 2931, 2873, 1628, 1484, 1460, 1401, 1381, 1125, 1051, 1023, 921, 901, 636, 619, 610, 578, 557, 513, 480, 450, 426. TBA2{Ni2[Mo2O4(CH3C(CH2O)3)2]3}·2CH3CN (3). Ni(CH3COO)2· 2H2O (0.10 g, 0.40 mmol), TBA4Mo8O26 (0.33 g, 0.15 mmol), and CH3C(CH2OH)3 (0.15 g, 1.20 mmol) were added into 20 mL of acetonitrile, and the mixture was refluxed for 12 h. The obtained blue solution was cooled to room temperature slowly, and blue crystals formed after 2−3 days in a yield of 86.0% based on Mo. Elem anal. Calcd for Ni2Mo6C66H132N4O30: Ni, 5.45; Mo, 26.71; C, 36.79; H, 6.17; N, 2.60. Found: Ni, 5.43; Mo, 26.70; C, 36.76; H, 6.18; N, 2.62. IR (KBr, cm−1): 3463, 2958, 2932, 2874, 1633, 1481, 1460, 1398, 1382, 1124, 1052, 1020, 921, 902, 636, 622, 609, 576, 559, 522, 482, 451, 430. TBA2{Zn2[Mo2O4(CH3C(CH2O)3)2]3}·2CH3CN (4). Zn(CH3COO)2· 2H2O (0.10 g, 0.47 mmol), TBA4Mo8O26 (0.37 g, 0.17 mmol), and CH3C(CH2OH)3 (0.16 g, 1.40 mmol) were added into 20 mL of acetonitrile, and the mixture was heated to reflux for 12 h. The obtained colorless solution was cooled and kept at room temperature for 2−3 days to give colorless crystals (yield based on Mo, 0.29 g, 78.4%). Elem anal. Calcd for Zn2Mo6C66H132N4O30: Zn, 6.03; Mo, 26.55; C, 36.56; H, 6.14; N, 2.58. Found: Zn, 5.99; Mo, 26.65; C, 36.66; H, 6.15; N, 2.60. IR (KBr, cm−1): 3460, 2960, 2930, 2872, 1629, 1483, 1460, 1402, 1380, 1124, 1050, 1025, 917, 902, 636, 618, 608, 579, 556, 510, 477, 449, 430. TBA2{CuMo6O16(OH)2[CH3C(CH2O)3]2}·4CH3CN (5). This compound was first reported in our previous work,18 and it was obtained in this study again to demonstrate the present motivation through different synthetic routes. In route 1, Cu(CH3COO)2·2H2O (0.10 g, 0.50 mmol), TBA4Mo8O26 (0.41 g, 0.19 mmol), CH3C(CH2OH)3 (0.18 g, 1.50 mmol), and 0.5 mL of acetic acid were added into 20 mL of acetonitrile. After 12 h of reflux, the solution was then cooled to room temperature for several days to generate blue crystals (yield C

DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry IR (KBr, cm−1): 3437, 2961, 2931, 2876, 1674, 1483, 1461, 1384, 1113, 1056, 1031, 932, 913, 710, 662, 601, 414. TBA2{CuMo6O16(CH3O)2[CH3C(CH2O)3]2}·H2O (10). This compound was first reported in our previous work18 and also obtained in this study via different synthetic routes by using different primary complex precursors. In route 1, compound 1 (0.10 g, 0.05 mmol), TBA4Mo8O26 (0.09 g, 0.04 mmol), and 0.10 mL of acetic acid were added to a mixed solvent containing 18 mL of methanol and 2 mL of DMF. The solution was heated to reflux for 12 h and then cooled to room temperature. Blue product crystals were obtained within several days (yield 70.5% based on Mo, 0.11 g). In route 2, compound 9 (0.10 g, 0.05 mmol) and 0.10 mL acetic acid were added to 20 mL of methanol. The solution was heated to reflux for 12 h. After cooling to room temperature, blue-green crystals were obtained in several days (yield 36.1% based on Mo, 0.03 g). The structures of both products were verified by single-crystal X-ray analysis. TBA3{CuMo6O17(OH)[CH3C(CH2O)3]2}·CH3C(CH2OH)3·CH3CN (11). This compound was first reported in our previous work,18 and it was obtained in this study through another synthetic route. Compound 9 (0.10 g, 0.05 mmol) and 0.10 mL of acetic acid were added to 20 mL of acetonitrile. The solution was heated to reflux for 12 h and then was cooled to room temperature to obtain blue-green crystals in several days (yield, 64.7% based on Mo, 0.06 g). The structure was verified by single-crystal X-ray analysis. Except compounds 5, 6, 8, 10, and 11, having been reported in the literature, a summary of the crystallographic data and structural refinements of all other products is listed in Table 1, and the detailed assignments of IR spectra and TGA are listed in Table S1 and Figures S1−S7.

atom, the {M2Mo6} primary complexes show an interesting characteristic to transform into the corresponding Anderson− Evans structures. By the addition of TBA4Mo8O26 and acetic acid (their amounts are described in the experimental part) to an acetonitrile solution of compounds 1−4, we obtained decorated double-sided χ/χ isomers of Anderson−Evans POMs, 5−8, as shown in the middle route of Figure 1. Compounds 5−8 have the same architecture except central heterometal ions, from Cu2+ to Co2+ to Ni2+ to Zn2+.

Figure 1. Schematic routes for the controllable synthesis of triolligand-decorated Anderson−Evans POMs in a double-sided χ/χ isomer. The top and bottom routes indicate a one-step method, while the middle route represents a two-step method, where {M2Mo6} (M = Cu2+, Co2+, Ni2+, and Zn2+) clusters are obtained as precursors that then transform into the corresponding Anderson−Evans POMs.

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RESULTS AND DISCUSSION Synthesis and Transformation of {M2Mo6} Primary Complexes. To demonstrate the component-controlled formation of triol-ligand-decorated divalent-metal-ion-centered Anderson−Evans POMs, we separate the reaction into two isolated steps by modulating the molar ratio of the reactant components. Typically, in the first step, the reaction of TBA4Mo8O26 and M(CH3COO)2 (M = Cu2+, Co2+, Ni2+, and Zn2+) in an acetonitrile solution in the presence of CH3C(CH2OH)3 is carried out under reflux, in which the molar ratio of M/Mo is controlled at ca. 1/3, double the molar ratio used in the preparation of Anderson−Evans polyanions. Unexpectedly, four primary complexes, compounds 1−4 decorated by triol ligands in a chemical formula of [M2Mo6C66H132N4O30] (abbreviated as {M2Mo6}), are synthesized. Through this ratio-controlling strategy, we successfully prepare one type of stable primary complex precursor, which is helpful for understanding the formation mechanism of triolligand-decorated Anderson−Evans POMs. This method is not only suitable for the transition-metal ions used in 1−4 but also may work for other divalent transition metals, such as Fe2+, Mn2+, and Cd2+, although at present the transition-metal ions in the 1+ and 3+ states are not applicable under the same condition. Several important factors dominate the intermediate products. First, the molar ratio of M/Mo maintained at ca. 1/3 is necessary because the lower M/Mo ratio will directly cause the direct formation of triol-ligand-decorated Anderson−Evans POMs. Second, the reaction temperature is also very important because a temperature lower than the refluxing state in acetonitrile will not lead to the primary product even if the reaction is allowed to continue for a very long time. Finally, a suitable acidic environment such as acetic acid is requisite because the extra acidity like hydrochloric acid would benefit for the formation of Anderson−Evans POMs. As the second step, in the presence of an extra isopolymolybdate for increasing the molar ratio of the Mo

The triol-ligand-decorated double-sided χ/χ-isomer Anderson−Evans POMs can also be synthesized directly from TBA4Mo8O26 and M(CH3COOH)2 (M = Cu2+, Co2+, Ni2+, and Zn2+) by controlling the M/Mo molar ratio at ca. 1/6, as illustrated in the bottom route in Figure 1. The lower ratio leads to the simple [Mo6O19]2− cluster due to the little transition-metal ion in the reactions. When the M/Mo ratio of the starting materials lies in the range of 1/3 to 1/6, we still obtain the same final products directly in the presence of acetic acid, as shown in the top route in Figure 1. It is noted that, without modulation of acetic acid, the product mixture of both primary complexes and final Anderson−Evans POMs is obtained at this molar ratio range because a much higher molar ratio of M/Mo in the reaction mixture could not lead to the highly efficient synthesis. On the basis of these reaction behaviors, precise control for the ratio of the initial transitionmetal ion to the Mo-containing reactant as well as the triol ligand is crucial in the synthesis of the primary complexes. We failed to get Mo and Zn NMR spectra, and, therefore, the 1 H NMR spectrum is used to demonstrate the existing state of the synthesized primary complexes in solutions. Because of the paramagnetic property of compounds 1−3, only the 1H NMR spectrum of Zn2+-containing compound 4 in acetonitrile is observed to have a higher resolution, as presented in Figure 2. According to the molecular symmetry and coordinating positions on {Mo2O10}, six triol ligands are divided into three groups. The first two triol ligands coordinating on the middle {Mo2O10} have the same chemical environment, but the three methylene groups in each of them are in two states, in which one coordinates to the μ2-O (Zn and Mo) atom and the other two connect to μ3-O (Zn, Mo, and Mo) atoms. Therefore, the protons on the terminal methyl group appear in a single peak, D

DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. 1H NMR spectrum of compound 4 in CD3CN.

Figure 3. Polyanionic structures of (a) compound 1 and (b) its skeleton presentation without an organic part with thermal ellipsoids as the atoms at a 30% probability level. All H atoms are omitted for clarity.

{M2Mo6} clusters have the same polyanionic framework structure with different divalent transition-metal ions, Cu2+, Co2+, Ni2+, and Zn2+ for 1−4. Taking 1 as an example to illustrate the characteristic structures, one can see that it crystallizes in the orthorhombic system with space group Pbca. As shown in Figure 3a,b, the central symmetric polyanion is composed of three triol-ligand-decorated {Mo2O10} clusters linked by two Cu2+ ions through Cu−O−Mo bonds, forming an interlaced structure. Each {Mo2O10} cluster consists of two edge-sharing {MoO6} octahedra in addition to two anchored triol ligands. Taking the {Mo2O10} cluster as a line segment, the three {Mo2O10} clusters can be divided into two types that are perpendicular from each other via a common transverse axis, one coordinating with two Cu2+ ions at the middle and the other two parallel segments on both side positions, as shown in Figure S8. The coordination environments of Mo1, Mo2, and Mo3 (Figure S9) and the detailed description and analysis can be found in the Supporting Information. As the linker of two {Mo2O10} clusters, the Cu2+ ion adopts a distorted octahedral coordination environment. Because of the Jahn−Teller effect,34 the mean length of the Cu−O bonds at the axis, at a value of 2.319 Å, is much longer than others with a mean value of 1.999 Å. It should be noted that the other three compounds, 2−4, show no obvious Jahn−Teller effect. We also analyze possible coordination positions in the structure. As shown in Figure S10, there are five types of O atoms in the cluster of 1. Except the terminal O atoms at the end of the MoO bonds, all of the other ones, including the μ2-O and μ3-O atoms to the bridging metal ions, are connected to the triol ligands.

which is well discerned at 0.46 ppm, while the protons on three methylene feet display two separate single peaks at 4.55 and 4.78 ppm with a proton ratio of 1/2, corresponding to the above bridging O atoms. The triol ligands coordinating on one side {Mo2O10} have the same chemical environment as the ones on the other side, but the two triol ligands show a little bit of difference from each other because the slightly split chemical shifts of the methyl groups emerge near 0.63−0.64 ppm. In addition, the chemical shifts belonging to six methylene groups from 3.80 to 4.90 ppm are found to split doubly. The clear proton assignments of the triol ligands indicate the maintained states of the primary complexes in acetonitrile. However, small satellite proton perks with integrals of less than 10% corresponding to those assigned chemical shifts are observed. The proton ratio of the total triol ligands, including the satellite peaks, is strictly proportional to the whole TBA, implying partial changes of the complexes. Considering that the peak at about 2.12 ppm could be ascribed to the protons from dissociated groups, it is possible that one methylene group on each of two triol ligands dissociates symmetrically from the middle {Mo2O10} cluster, forming partially disconnected coordination. Such a speculation is consistent with the published results, in which one foot of triol ligand locates at the free state in the crystal structure of triol-modified Anderson−Evans clusters.33 Meanwhile, no free triol ligand, as determined from the NMR spectrum, further supports the assignment. Crystal Structure Characterization of {M2Mo6} Primary Complexes. Single-crystal X-ray analysis reveals that the four E

DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Illustration of {Cu2Mo6} cluster transformation to Anderson−Evans POMs in different architectures under various reaction conditions and the controlled transformation between different triol-ligand-decoration fashions. The solid lines display the results obtained in this work, while the dashed lines indicate the results reported in our previous work.18

acidic environment. On the other hand, the heteroatom of Anderson−Evans POMs has a significant influence on the decoration style of the triol ligand, especially for the second ligand’s decoration. For example, in the triol-ligand-decorated Cu-centered Anderson−Evan POM, a methanol molecule can be attached on the μ3-O position,18 while it binds to the μ2-O atom during its grafting to the Co-centered POM.19 In contrast to the fact that {M2Mo6} clusters encounter multiple degradation and M−O bond breaking for the formation of Anderson−Evans clusters with the added TBA4Mo8O26 cluster, the reverse transformation from Anderson−Evans clusters to {M2Mo6} clusters cannot be realized by the addition of extra central hetero-transition-metal ions and triol ligands. This irreversible change indicates that the Anderson−Evans-type cluster is more stable than the {M2Mo6} clusters. In the transformation to Anderson−Evans clusters, both the {M 2 Mo 6 } cluster and the additive iospolymolybdates should contribute to the formation of an Anderson−Evans cluster because if only one starting component reaches the final product, the maximum yield would be no more than 50% according to eq 1 in the Supporting Information, while, in fact, the observed yields for the final Anderson−Evans-type products based on 1−4 are 71.8%, 65.9%, 54.2%, and 60.2%, respectively. This conclusion could also be supported by the fact that the {M2Mo6} cluster itself cannot transform to the Anderson−Evans cluster under the same reaction condition. It should be noted that {M2Mo6} clusters are not a classical type of precursor because they possess more than the required transition-metal ions and organic ligands with respect to the final products. One extra metal ion and four excess triol ligands in a {M2Mo6} cluster must be compensated for or taken off, and therefore the additive TBA4Mo8O26 utilizes the extra precursors via formation of the Anderson−Evans cluster,

The triol-ligand-decorated {Mo2O10} cluster has been used to integrate with other metal ions to construct larger cluster assemblies. For example, one or two additional {MoO6} octahedra can be attached to the triol-ligand-decorated {Mo2O10} cluster to form tri- or tetranuclear architectures.35,36 {Mn(CO)3} and Cu-containing complexes also show similar decoration behavior to construct heterotri- or tetranuclear POMs.27,37 However, all of these cluster assemblies are built on one {Mo2O10} cluster and, therefore, compounds 1−4 represent different types of transition-metal-ion-bridged triolligand-decorated {Mo2O10} clusters. Compounds 5, 6, and 8 synthesized through a one-step method have been reported in the literature.18,19 Compound 7 was also reported,8 but its single-crystal structure had not been presented until now. As shown in Figure S11, it has a classical Anderson−Evans architecture with two μ3-O and one μ2-O atoms replaced by the triol ligand on each side, affording a double-sided χ/χ isomer. Transformative from {M2Mo6} to Final Clusters. From the cluster structures of primary complexes, one can see that the starting material TBA4Mo8O26 passes through a degradation process. The formed {Mo2O10} fragment cluster acts as a basic building unit to be linked by transition-metal ions, forming {M2Mo6} complexes. Thus, it is obvious that a rearrangement happens during the transformation from {M2Mo6} clusters to Anderson−Evan POMs. By a comparison of the number of triol ligands on the primary complexes before and after the reaction, it is deduced that a dropping out of the triol ligand from the {Mo2O10} dimer is almost ineluctable, accompanied by disruption of the Cu−O bonds. This speculation is strongly supported by analysis for the degradation of additive TBA4Mo8O26 to several subclusters, for instance, {Mo2}, {Mo3} or {Mo4} clusters,17 which benefit the formation of an unexpected architecture, especially in an F

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Inorganic Chemistry

molecules. When all three transformations from the {Cu2Mo6} cluster to Anderson−Evans POMs are summarized, it can be concluded that both the additional Mo-contained starting materials and the acidity of the solvents play crucial roles. Because TBA2Mo2O7 was prepared by simply adding TBA·OH to the solution of TBA4Mo8O26 (see eq 2 in the Supporting Information), the former provides a less acidic environment compared with the latter, which results in the formation of a double-sided χ/δ isomer.18 By an increase in the acidity of the reaction system, the double-sided χ/χ isomer becomes preferential because of the activation of μ2-O atom. On the basis of an understanding of the transformation conditions of primary complexes, the acidity and solvent control of the reaction environment are also used in modulating triol-ligand-decorated fashions on Anderson−Evans POMs. In the presence of acetic acid, compound 9 transforms into compound 10 in a methanol solution, as pointed out by the solid arrow at the bottom-left of Figure 4. The triol ligands connect the POM cluster through three μ3-O atoms in 9, which shifts one of its feet to the μ2-O position, followed by occupation of a methanol molecule at the position, forming a double-sided χ/χ isomer, while two methanol molecules are grafted symmetrically on both sides. When the solvent is changed to acetonitrile, however, the triol ligand does not encounter such a positional change. Instead, the grafted methanol molecule dissociates from the mother compound 9, yielding compound 11, as pointed out by the solid line at the top-left. Thus, with the {Cu2Mo6} cluster as the starting material, we realize a circular modification change of Anderson−Evans POMs with a methanol molecule. In previous work,18 we realized the transformation from 11 to 5 in an acetonitrile solution in the presence of acetic acid. In addition, compound 10 was also confirmed to transform to compound 5 by simply adding acetic acid to the acetonitrile solution. Of course, the reverse reaction can also be conducted by refluxing compound 5 in methanol. All of the transformations presented here show that, in an acidic environment, the triol ligands tend to attach to the Anderson−Evans POM to form the doublesided χ/χ isomer, whereas the double-sided χ/δ isomer needs to be prepared in a less acidic environment. When methanol is used as the solvent, it serves as the second ligand to replace one or two of the μ3-O atoms and can be dissociated by acetic acid. Magnetic and Electrochemical Properties of {M2Mo6} Clusters. Because there are no single electrons in the Zn2+ ion due to its full state of the valence shell, only the variabletemperature magnetic susceptibilities of compounds 1−3 with transition-metal ions of Cu2+, Co2+, and Ni2+ were investigated. The temperature dependence in the range of 2−300 K on the magnetic susceptibility of 1 is illustrated in Figure 6a, where χm denotes the molar magnetic susceptibility per {Cu2Mo6} unit

although several factors may affect the process. Two excess triol ligands should be left in the reaction solution and may serve as a ligand group to bind to {Mo2}, {Mo3}, or {Mo4} clusters, such as those found in publications.27,28 Cu-Centered Primary Complex for Diverse Triol Ligand Decoration of Anderson−Evans POMs. As shown in Figure 4, in addition to the transformation of compound 1 into double-sided χ/χ isomer 5 in an acetonitrile solution of TBA4Mo8O26 and acetic acid, compound 1 transforms into a novel double-sided χ/δ isomer 9 in methanol, while TBA2Mo2O7 is used instead. One triol ligand connects to three μ3-O atoms on one side, while another ligand binds to two μ3-O and one μ2-O atoms on the other side of the Anderson−Evans POM. Importantly, a methanol molecule connects to the residue μ3-O atom, forming the triol-ligand/ methanol-codecorated Anderson−Evans structure shown in Figure 5. The anchored methanol molecule is at an angle of

Figure 5. Polyanionic structure of compound 9 with thermal ellipsoids representing the atoms at a 30% probability level. All H atoms are omitted for clarity.

78.2° with the cluster plane and extends to the outside edge because of steric hindrance of the triol ligand. Such an asymmetric structure cannot be synthesized through other published approaches and can be attributed to the intermediate state between compounds 10 and 11 in our previous results,18 in which the former is a triol-ligand-decorated χ/χ isomer with both μ3-O atoms replaced by methanol molecules and the latter has the same architecture as that of 9 in a double-sided χ/δ isomer except the μ3-O atom in the free state. Besides the two types of triol-ligand-decorated Anderson−Evans POMs, the reaction of compound 1 and TBA4Mo8O26 in a methanol solution with the addition of acetic acid generates 10 in the χ/χ isomer, while two free μ3-O atoms are connected by methanol

Figure 6. Temperature dependence of the magnetic susceptibility (black) and inverse susceptibility (blue) of compounds (a) 1, (b) 2, and (c) 3. G

DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and T is the temperature. χmT has a value of 0.88 emu K mol−1 at 300 K, which is close to the predicted value of 0.86 emu K mol−1 (one electron with g = 2.15),38 suggesting no obvious magnetic exchange for two isolated Cu2+ ions. With a decrease of the temperature, the χmT value reduces quickly to 0.68 emu K mol−1 at 2 K, the low-temperature limit of the measurements. In addition, from the best linear fit of χm−1 versus T above 120 K, compound 1 yields a Weiss constant of −53 K. For compound 2, as depicted in Figure 6b, the χmT value at 300 K (7.08 emu K mol−1) is larger than the predicted spin-only value, which is consistent with the large first-order orbital angular momentum contributions commonly seen in high-spin octahedral Co2+-containing compounds.39 The χmT versus T plot is different from that of compound 1, in which χmT has a gentle decrease from 300 to 150 K, and then the curve undergoes a fast decline to 2.63 emu K mol−1 at 2 K. The corresponding χm−1 versus T plot of 2 gives a Weiss constant of −33 K. As shown in Figure 6c, the χmT versus T curve of compound 3 expresses a large platform from 300 to 30 K with a slight decrease. After that, the curve decreases sharply due to the additional zero-field splitting (ZFS).40 The χmT value changes from 2.45 emu K mol−1 at 30 K to 1.08 emu K mol−1 at 2 K. The χm−1 plot of 3 versus temperature T shows this compound obeying Curie−Weiss law in almost the entire measured range, and the Weiss constant is found to be −29 K. We also investigated the electrochemical behaviors of compounds 1−4 by examining their cyclic voltammetry performances. As shown in Figure S12, no obvious electrochemical signals belonging to Mo centers are detected in all four compounds. Instead, only the redox peaks of the transition-metal ions for 1−3 are observed at high potential positions. For compound 4, no useful information was obtained in the present experimental conditions. Catalytic Properties of {M2Mo6} Primary Complexes. The catalytic properties of POMs are often improved by the grafted transition metals in various oxidation reactions.41 We evaluate the catalytic capability of synthesized {M2Mo6} clusters as an independent type of POM cluster for the selective oxidation of organic substrates,42 thioanisole, as shown in the following reaction equation. In a general procedure, thioanisole (47 μL, 0.4 mmol), 30% H2O2 (83 μL, 0.8 mmol), and 4.0 × 10−4 mmol of {M2Mo6} clusters are dissolved in 5 mL of acetonitrile, and the reaction solution is stirred at 40 °C. The species are taken out of the reaction at the same time intervals to monitor the reaction proceeding by HPLC. The detailed catalytic activities of compounds 1−4 are summarized in Table 2.

Table 2. Summary of the Catalytic Activities of Synthesized Compounds 1−4 for the Selective Oxidation of Thioanisolea compound

timeb (min)

conversionc (%)

selectivityd (%)

turnover number

turnover frequency (h−1)

1 2 3 4

20 15 130 20

17.1 98.2 94.6 98.1

100 98.7 95.5 94.1

171 982 946 981

518 3928 436 2973

All reactions are performed in 5 mL of CH3CN at 40 °C at a molar ratio of catalyst to substrate of 1/1000. bThe time is stopped when the formed sulfoxide reaches its largest amount. cThe conversion is calculated based on the consumed substrate. dThe selectivity is calculated from SO/(SO + SO2), in which SO and SO2 represent sulfoxide and sulphone, respectively. a

min. In addition, the ratio of sulfoxide in the products is observed to have no obvious decrease from 15 to 35 min. Generally, catalysis of thioethers targeting their intermediate oxide is usually accompanied by a final complete oxidation to sulphone, and it is difficult to maintain the reaction at the intermediate step. Therefore, compound 2 performs a very high conversion efficiency as a gentle catalyst for the incomplete oxidation of thioether-containing molecules. In the case of compound 3 serving as the catalyst (Figure 7c), thioanisole is oxidized to sulfoxide with the highest conversion in ca. 160 min, much longer than the time by compound 2, although the content of sulfoxide can also reach up to 95.5% in the final products. Just like that in 2, as shown in Figure 7d, compound 4 also exhibits good catalytic activity, and the reaction with 100% conversion finishes at about 25 min with the highest percentage of sulfoxide of 94.1% in the products in 20 min. However, in contrast to 2, the initially yielded sulfoxide further converts into sulphone quickly, which does not benefit collection of the intermediate product. In general, the order of the oxidation capability of all prepared POM complexes for the incomplete oxidation product is 2, 4 > 3 > 1. Considering the identical framework structures, the difference of the catalytic oxidation should be derived from the influence of central heteroatoms on the formation of peroxide bridging on Mo atom and the following oxidation to the substrate. Of course, the regularity has to be supported by collecting more reaction examples in a later study. On the other hand, the oxidation capability of all of the named intermediates of the Anderson−Evans clusters should be lower because less conversion of the sulfone products was observed, although all of the complexes display the potential as the specified catalyst for the preparation of sulfoxides from thioether derivatives via a H2O2 oxidant. Both compounds 2 and 4, which have good catalytic performances on the selective oxidation from thioanisole to sulfoxide, are further examined for their recoverability in catalytic reactions within 20 min for each species. As shown in Figure 8, conversion of the two compounds decreases to 84− 85% after five catalytic cycles. As shown in Figure S13, the IR spectra of the primary complex catalysts after five catalytic cycles indicate similarity to the characteristic vibrations of fresh samples, showing the small structure changes of the two catalysts during the catalytic reactions. The main reason for the decreased activity may be ascribed to the small weight loss of the catalysts after each recovery. Gentle oxidation in many cases is very important because the intermediate oxidation state of thioethers such as sulfoxides is

As shown in Figure 7a, when compound 1 is used as the catalyst, its catalytic efficiency at 40 °C is pretty low and only about 17% of the substrate has been oxidized to the intermediate-oxidation-state sulfoxide. After the highest conversion state is reached, increasing the reaction time does not lead to any further increase of the reaction conversion, indicating the poor capability of 1 for oxidation of the substrate under a lower temperature. In contrast, compound 2 displays good catalytic behavior (Figure 7b) because thioanisole is oxidized completely in less than 25 min under the same conditions and the percentage of intermediate oxidized product, sulfoxide, reaches the highest point of 98.7% in 15 H

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Figure 7. Content in molar ratio versus time profiles of the oxidation of thioanisole (0.4 mmol) with 30% H2O2 (0.8 mmol) using 0.4 μmol of (a) 1, (b) 2, (c) 3, and (d) 4 as catalysts in acetonitrile (5 mL) at 40 °C, respectively.

provides strong competition for selective oxidation and shows the following merits. First, the reaction time is shorter and completes within 15 min, while the reaction temperature could be a bit higher, but the selectivity still stays very high. Second, the reaction conditions are pretty mild for both the solvent and reaction temperature. Third, the POM catalysts are easier to be prepare with a one-pot route in high yield. Fourth, the conversion and selectivity to sulfoxide are both higher than 98%. All of these advantages demonstrate that {M2Mo6} clusters, especially {Co2Mo6}, are unique types of catalysts for the incomplete oxidation of thioanisole with high performance.

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Figure 8. Curves of conversion ratio versus cycle times for the oxidation of thioanisole with 2 (black) and 4 (blue) as catalysts.

CONCLUSION By control of the molar ratio of the central heteroatom M (M = Cu2+, Co2+, Ni2+, and Zn2+) to the coordination atom Mo in the reactants, a series of triol ligands covalently decorating primary complex clusters {M2Mo6} were synthesized and characterized in detail. The coordination architectures of these clusters are isostructural, and they are composed of three {Mo2O10} dimers, linked by two divalent transition-metal ions separately through M−O−Mo bonds, forming a structure in an interlaced fashion between the adjacent Mo-bearing subunits. The obtained {M2Mo6} clusters not only represent a novel combination type of small Mo clusters with transition-metal ions but also act as the primary complex to be used for the synthesis of known and new triol-ligand-decorated Anderson−Evans clusters. Systematic studies show that the acidity in the reaction solution is one of the key factors for the selection of triol-ligand-decorated positions on the Anderson−Evans polyanion, in which the acidic environment is favorable for formation of the doublesided χ/χ isomer, while reduced acidity results in the doublesided χ/δ isomer. In addition, through control of the acidity of the reaction solution, secondary modification of the methanol

usually required for the preparation of drug molecules.43 The POM catalysts with strong oxidation activity for the full oxidation of thio compounds and other substrates can be found in many publications.44,45 However, the higher selectivity of incomplete oxidation is less reported, and the known strategy is to incorporate POMs into the bone of metal−organic framework compounds for synergistic catalysis with the framework environment, while the corresponding POM alone cannot afford such a higher catalytic conversion and selectivity for sulfoxide products.46 Using electrostatic interaction, organic cations substitute for the counterions of POMs to form supramolecular complexes, which also afford increased activity for the oxidation of thioanisole to chiral sulfoxide when chiral ligands are employed.47,48 More examples concerning the incomplete oxidation products of thioethers are listed in Table S2 for an intuitive contrast, where the catalysts are clusters or metallic oxides. In comparison to the listed prominent catalysts, the {Co2Mo6} cluster among the series of synthesized products I

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Inorganic Chemistry molecules on one or two μ3-O positions, as well as their dissociation, can be operated. The catalytic experiments demonstrate that the independent {Co2Mo6} clusters possess a high efficiency for the selective oxidation of thioanisole to the intermediate oxidized product sulfoxide, which indicates its potential application for incompletely oxidized reactions.

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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.7b02996. TGA curves and IR spectra of compounds 1−4, 7, and 9, a detailed structural description of 1, as well as the coordination environments of Mo and Cu in the polyanion, polyanionic structure of 9, detailed catalytic activities and electrochemical curves of 1−4, IR spectra of 2 and 4 before and after five cycles of catalytic reaction, and proposed reaction equations (PDF) Accession Codes

CCDC 1552557−1552562 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bao Li: 0000-0003-0727-9764 Lixin Wu: 0000-0002-4735-8558 Notes

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 21574057, 21774050, and 21773090) and Changbaishan Distinguished Professor Funding from Jilin Province, China.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02996 Inorg. Chem. XXXX, XXX, XXX−XXX