Chiral Functionalization of a Zirconium Metal−Organic Framework

Nov 8, 2017 - the previously reported data).40,41 Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis shows zirconium contents...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Chiral Functionalization of a Zirconium Metal−Organic Framework (DUT-67) as a Heterogeneous Catalyst in Asymmetric Michael Addition Reaction Khoa D. Nguyen, Christel Kutzscher, Franziska Drache, Irena Senkovska,* and Stefan Kaskel* Department of Inorganic Chemistry, Technische Universität Dresden, Bergstraße 66, 01062 Dresden, Germany S Supporting Information *

ABSTRACT: DUT-67, an 8-connected zirconium and 2,5-thiophenedicarboxylate-based metal−organic framework (MOF), was postsynthetically functionalized by L-proline via solvent-assisted linker incorporation to obtain a chiral base catalyst. The parent monocarboxylate could be almost completely exchanged by L-proline after 5 days of treatment. The resulting chiral DUT-67 was demonstrated to be a promising heterogeneous catalyst for the asymmetric Michael addition of cyclohexanone to trans-β-nitrostyrene with excellent yield (up to 96%) and enantioselectivity comparable to that of L-proline in homogeneous reaction (ee of approximately 38%). The Zr-MOF could be reused at least five times without substantial degradation in the crystallinity or catalytic activity. No leaching of catalytically active species into the liquid phase was detected over five cycles.

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INTRODUCTION The asymmetric Michael addition of ketones to nitroalkenes is one of the most effective methods for C−C bond formation and thus has been widely applied in natural products and pharmaceutical compounds synthesis.1,2 In recent years, a significant improvement in performance has been made using small chiral amines as organocatalysts, typically L-proline (pro) and its derivatives.3−5 The search for heterogeneous catalysts, which enable facile separation of catalysts from liquid-phase products, resulting in a more economical industrial process, is always favorable.5−9 Therefore, the development of chiral heterogeneous catalysts by anchoring chiral molecules onto insoluble supports such as polymers,9−11 silica materials,9,12 and inorganic nanoparticles13−15 has recently attracted significant attention. Metal−organic frameworks (MOFs) provide an attractive platform as heterogeneous catalysts or catalyst supports because of uniform and well-defined pore geometry, high specific surface area, and a wide variability of metal cations and organic groups as their constituents, providing often active-site uniformity.16−19 Indeed, MOFs have been widely applied for a broad range of organic reactions including C−C coupling, C− N coupling, ring opening, and oxidation.20,21 However, only a few reports on chiral MOFs used as heterogeneous catalysts for asymmetric organic transformations have been given to date.22−24 One widely used approach to incorporate chiral catalytic sites into MOFs is the integration of natural amino acids from the chiral pool, among other L-proline, by direct grafting to the linker backbone,25 postsynthetic modification of the linker,26−31 or coordination to the open metal sites of the cluster.32 © XXXX American Chemical Society

UiO-66, as a representative of zirconium-based MOFs (ZrMOFs), has been widely investigated because of its high thermal and chemical stability.33,34 Recently, solvent-assisted linker incorporation (SALI) has been reported to be an effective strategy to functionalize Zr-MOFs with reduced cluster connectivity. For example, in the 8-connected [Zr6(μ3OH)8(OH)8]8+ node of NU-1000, containing eight terminal hydroxyl groups in addition to linker molecules, all −OH groups could be substituted by organic molecules containing carboxylic groups.35,36 Thus, fluoroalkanes, dye molecules, and iridium pincer complexes could be attached to the zirconium cluster of NU-1000.37−39 A similar strategy was applied to modify the hydrophilic/hydrophobic properties of the inner surface of DUT-67 (DUT = Dresden University of Technology).40,41 However, decoration of the zirconium node by chiral molecules for heterogeneous asymmetric catalysis remains an unexplored field and offers great potential because of its simplicity in comparison to the incorporation of chiral linkers. DUT-67 is a representative of Zr-MOFs based on 8connected clusters ([Zr6(μ3-O)6(μ3-OH)2]10+) and has a network with reo topology.40−42 In this work, we demonstrate the decoration of DUT-67 with L-proline and application of this chiral material as a heterogeneous catalyst for asymmetric Michael addition involving trans-β-nitrostyrene and cyclohexanone. Received: November 8, 2017

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DOI: 10.1021/acs.inorgchem.7b02854 Inorg. Chem. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION

Catalyst Synthesis and Characterization. The parent DUT-67-fa, containing formate as modulator, was obtained in 85% yield using a previously reported synthesis procedure.41 To incorporate chiral catalytic sites into the cluster, the formate anions were exchanged against L-proline by SALI by suspending DUT-67-fa in a L-proline solution for up to 5 days, giving DUT67-pro (Scheme 1). Scheme 1. Exchange Reaction between the Formate in DUT67-fa and L-Proline Anions To Produce DUT-67-pro

According to 1H NMR of the digested polycrystalline product, L-proline could be successfully incorporated directly, without additional intermediate steps, such as HCl treatment (for more details, see section 1 in the Supporting Information, SI). The amount of L-proline incorporated per zirconium cluster could be adjusted by controlling the exposure time of DUT-67-fa to the L-proline solution. After a 12 h induction period, the exchange rate suddenly increases, resulting in an almost complete exchange (4 mol of L-proline per Zr6) after 4 days (Figure 1 and section 1 in the SI). This functionalization method was also found to well preserve the crystallinity of DUT-67. The powder X-ray diffraction (XRD) pattern of DUT-67-pro matches perfectly the XRD pattern of the parent DUT-67-fa, with only tiny changes in the relative intensity of the peaks (Figure 2a). Moreover, according to scanning electron microscopy (SEM) analysis (Figure 3), the crystal’s size remains below ca. 550 nm during exposure to the L-proline solution (in agreement with the previously reported data).40,41 Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis shows zirconium contents of 3.78 and 3.48 mmol/g in DUT-67-fa and DUT-67-pro, respectively, which are close to the calculated values (3.72 mmol/g for DUT-67-fa and 3.41 mmol/g for

Figure 2. (a) XRD patterns of as-made DUT-67-fa, as-made DUT-67pro, and DUT-67-pro after two catalytic cycles. (b) Nitrogen physisorption isotherms of as-made DUT-67-fa (black diamonds), as-made DUT-67-pro (blue triangles), and DUT-67-pro after two catalytic cycles (red circles). The filled symbols represent the adsorption branch and the empty symbols the desorption branch.

DUT-67-pro). The nitrogen adsorption experiment at 77 K shows that the porosity of L-proline-functionalized DUT-67 depends on the functionalization degree (Figure 2b). The specific Brunauer−Emmett−Teller (BET) surface area and total pore volume decrease from 1040 m2/g and 0.44 cm3/g in the case of the parent DUT-67-fa to 730 m2/g and 0.31 cm3/g for DUT-67-pro possessing approximately four L-proline molecules on the zirconium node. A similar decrease was also

Figure 1. (a) Exchange rate of formate (triangles) by L-proline (diamonds). (b) 1H NMR spectra of digested DUT-67-pro after exposure of the samples to the L-proline solution during different time periods. B

DOI: 10.1021/acs.inorgchem.7b02854 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Performance of Various Catalysts in the Michael Reaction with IPA as the Solvent

Figure 3. SEM images of (a) as-made DUT-67-fa and (b) as-made DUT-67-pro (after soaking of DUT-67-fa in the L-proline solution for 5 days).

entry

catalyst

reaction time (h)

1 2 3 4 5 6 7

no catalyst DUT-67-fa DUT-67-pro L-proline DL-proline L-histidine L-tyrosine

240 240 168 24 24 240 240

a

observed in the case of perfluoroalkane functionalization of DUT-67 and NU-1000.35,41 Catalytic Studies. After SALI, the functionalized DUT-67pro samples were soaked in ethanol (EtOH) for removal of unreacted reagents. The catalytic activity of chiral DUT-67-pro with L-proline coordinated to the cluster was studied in the asymmetric Michael addition of cyclohexanone to trans-βnitrostyrene, yielding four stereoisomers of 2-(2-nitro-1phenylethyl)cyclohexanone as products (Scheme 2), including

yield (%)a syn:antib 0 0 72 100 100 trace trace

95:5 93:7 90:10

ee (%)b

35 42 0

Determined by GC. bDetermined by HPLC.

Table 2. Effect of the Solvents on the Performance in the Michael Reaction Carried Out at 50 °C entry

solvent

reaction time (h)

yield (%)a

8 9 10 11 12 13 14 15 16

IPA EtOH IPA/EtOH (1:1) THF 1,4-dioxane DMSO chloroform toluene cyclohexanone

168 168 168 240 240 240 240 240 240

72 67 96 5 8 36 trace trace trace

Scheme 2. Michael Addition Reaction of Cyclohexanone to trans-β-Nitrostyrene and the Product Isomers a

syn:antib

ee (%)b

95:5 92:8 94:6

35 32 38

91:9

25

Determined by GC. bDetermined by HPLC.

such as tetrahydrofuran (THF) and 1,4-dioxane, affording yields of only 5% and 8%, respectively (Table 2, entries 11 and 12). Only 36% yield and approximately 25% ee were detected if dimethyl sulfoxide (DMSO) was used as the solvent. Alcohols as solvents are known to show the best performance for this homogeneous catalytic reaction system.43−46 Such protic solvents tend to stabilize the proline enamine intermediates (Figure 4) formed during the catalytic cycle.1,43 The same behavior was also observed for DUT-67-pro as the catalyst. The reaction carried out in EtOH provided 67% yield and 32% ee, while IPA was found to be more efficient, with a 5% increase in

two pairs of enantiomers originating via syn and anti addition. The reaction was studied in isopropyl alcohol (IPA) as the solvent, with a nitrostyrene/cyclohexanone molar ratio of 1:20, in the presence of 5−20 mol % catalyst (based on L-proline) at different temperatures (Table S2). In the absence of the catalyst as well as upon utilization of DUT-67-fa as the catalyst, no adduct was formed after 240 h of reaction. A 72% yield was obtained in the case of DUT-67-pro (15 mol % based on L-proline) after 168 h at 50 °C, demonstrating that the catalytic activity of DUT-67-pro clearly originates from immobilized L-proline molecules and not from the parent MOF. Remarkably, the enantiomeric excess (ee) achieved in the heterogeneous Michael addition is close to that obtained with the homogeneous L-proline catalyst under the same reaction conditions (35% vs 42%, respectively). The diastereomeric ratio is even slightly higher (95:5 for DUT-67-pro vs 90:10 for L-proline; Table 1). Obviously, L-proline coordinated to the zirconium cluster still retains its enantiopurity, which is not necessarily the case if proline is incorporated into the framework backbone as a side group of the linker molecule.31 The catalytic performance of DUT-67-pro could even be improved by variation of the solvent composition (Table 2). Nonpolar solvents such as toluene and chloroform inhibit the reaction completely, and only traces of Michael adducts could be detected after 240 h of reaction (Table 2, entries 14 and 15). The reaction also proceeded slowly in polar aprotic solvents,

Figure 4. Proposed mechanism of the Michael reaction using DUT67-pro as the catalyst.1,47 C

DOI: 10.1021/acs.inorgchem.7b02854 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry yield and 3% increase in enantioselectivity. Interestingly, these values could even be significantly improved to 96% yield and 38% ee if an EtOH/IPA mixture (molar ratio 1:1) was used as the solvent (Table 2, entry 10). The pure product [2-(2-nitro1-phenylethyl)cyclohexanone] could be isolated as a white solid by using flash chromatography with 93% yield, confirming the yield estimated by gas chromatography (GC) analysis. The yield of 2-(2-nitro-1-phenylethyl)cyclohexanone in the Michael addition reaction was found to be directly proportional to the amount of L-proline (Figure S11) and to the functionalization degree of the cluster (Figure 5).

Figure 6. Leaching test carried out at 323 K. Kinetic of the reaction with the catalyst (red) and after filtering of the catalyst after 72 h (black) of reaction.

could be detected according to analysis of the samples after two catalytic cycles (see Tables S1 and S3 and Figure S8). Highly important for the application is also the possibility of recovering and reusing the catalyst without deactivation. Therefore, the chiral DUT-67-pro catalyst was applied in five successive catalytic cycles. For it, the DUT-67-pro was separated from the reaction mixture after 10 days, washed with N,N-dimethylformamide (DMF) and EtOH, dried under vacuum at 120 °C overnight, and reused in the next reaction run. It was found that DUT-67-pro could be reused several times without degradation in the catalytic activity. Indeed, the same yield and selectivity was achieved in the fifth run compared with the first run (95% vs 96% for yield and 33% vs 38% for enantioselectivity; Figure 7). According to XRD analysis, the DUT-67-pro maintains its crystallinity throughout the reaction process (Figure 2).

Figure 5. Effect of the degree of L-proline functionalization.

To investigate the impact of the functionalization degree, four different samples (containing one, two, three, or four Lproline molecules per cluster) were prepared and applied in the catalysis. If the amount of L-proline was kept unchanged (15% mol), only 22% yield of 2-(2-nitro-1-phenylethyl)cyclohexanone was obtained after 10 days of reaction with material containing ca. one L-proline molecule per cluster, while the reaction could proceed to a yield of 43% with materials containing two L-proline molecules per zirconium cluster. A yield as high as 96% was reached in the presence of approximately four molecules of L-proline per zirconium node. Obviously, the formation of product in the asymmetric Michael addition reaction catalyzed by DUT-67-pro significantly rely on the degree of L-proline functionalization. This phenomenon could be rationalized because the acidic sites (including the coordinatively unsaturated zirconium as well as the hydroxyl groups of DUT-67) were found to be inactive on this type of reaction as mentioned earlier, but their Lewis acidity could inhibit the active amine sites of connected Lproline and hence limit the accessibility of cyclohexanone to the catalytic active center. Consequently, the material DUT-67-pro possessing a lower L-proline concentration would tend to be less active in catalyzing the Michael addition reaction. In addition, the increasing amount of L-proline per cluster does not significantly affect the ee value (Figure 5). If protected N(tert-butoxycarbonyl)-L-proline immobilized on DUT-67 (DUT-67-pro-Boc) is applied, only traces of adducts are obtained after 10 days, pointing to the secondary amine group of incorporated L-proline as the active catalytic site in the Michael addition reaction investigated (Figure 5). The truly heterogeneous character of the DUT-67-procatalyzed reaction was proven in a leaching test. Hence, the MOF catalyst was separated from the reaction mixture by centrifugation after the first 72 h of reaction (Figure 6). It was found that no further product formation takes place in solution. Thus, the reaction proceeds under real heterogeneous catalysis conditions. Moreover, no L-proline and no zirconium leaching

Figure 7. Catalyst recycling.

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CONCLUSIONS The Zr-MOF DUT-67 was demonstrated to be an ideal platform for the incorporation of chiral monocarboxylates at the zirconium nodes by SALI. The great advantage of the proposed approach is the simplicity of the synthetic procedure, making it possible to synthesize an enantiomerically pure MOF by exchanging the monocarboxylic ligand directly by soaking in the L-proline solution. We believe that the procedure can be applied to the majority of 8- or 6-connected Zr-MOFs. The catalytic activity of DUT-67-pro was confirmed in the asymmetric Michael reaction of cyclohexanone to trans-βnitrostyrene to form the enantiomers of 2-(2-nitro-1phenylethyl)cyclohexanone, with excellent yield (up to 96%) and enantioselectivity (approximately 38% ee) comparable to that of the homogeneous catalyst L-proline. In addition, DUT67-pro can be separated easily from the products and reused D

DOI: 10.1021/acs.inorgchem.7b02854 Inorg. Chem. XXXX, XXX, XXX−XXX

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NMR (500 MHz, DMSO-d6/DCl): δ 7.67 (s, 2H, TDC), 4.19 (t, 1H, 3.15 (m, 2H, L-proline), 2.22, 1.86 (m, 4H, L-proline). Catalytic Studies. trans-β-Nitrostyrene (0.0375 g, 0.25 mmol), 3 mL of a solvent mixture (1:1 IPA/EtOH), n-hexadecane as an internal standard (0.05 mL, 0.17 mmol), and DUT-67-pro (0.017 g, 15 mol % L-proline based on trans-β-nitrostyrene) were placed in a 10 mL Schlenk tube. After the mixture was stirred for 3 min, 20 equiv of cyclohexanone (0.50 mL, 5 mmol) was added. The resulting mixture was stirred at 50 °C. The reaction progress was monitored by GC as follows: Aliquots (0.20 mL) were sampled from the reaction mixture after defined time intervals, diluted with IPA (1.00 mL), and dried over Na2SO4. The resulting samples were analyzed by GC. Yields were calculated by comparing the area of the product peak (tmajor = 16.242 min and tminor = 16.334 min) with the area of the n-hexadecane peak (t = 13.935 min). The product, 2-(2-nitro-1-phenylethyl)cyclohexanone, was isolated by column chromatography (silica gel, ethyl acetate/ hexane = 1:4) as a white solid. For the recycling test, DUT-67-pro was collected by centrifugation, washed intensively with DMF and EtOH to remove any undesired compounds, activated in a vacuum at 120 °C for 24 h, and then reused for a new catalytic run. To ensure the heterogeneity of the catalyst, the MOF was removed after 72 h of reaction. The remaining reaction solution was then stirred for a further 5 h. The reaction progress, if any, was monitored by GC as previously described.

many times without substantial degradation in the catalytic activity. Thus, chiral DUT-67-pro is a promising catalyst in the asymmetric Michael addition reaction of unmodified ketones as cyclohexanone to trans-β-nitrostyrene.

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L-proline),

EXPERIMENTAL SECTION

Materials and Instrumentation. All reagents and starting materials were purchased from Sigma-Aldrich and used as received without any further purification. Nitrogen physisorption measurements were conducted on a BELSORP-max (MicrotacBEL, Japan) apparatus at 77 K up to 1 bar using high-purity nitrogen gas (99.999%). Prior to the adsorption experiments, the samples were heated in a vacuum at 120 °C for 24 h. 1H and 13C NMR spectra were recorded on a Bruker DRX 500 P spectrometer. Chemical shifts (ppm) were referenced to tetramethylsilane (0.00 ppm). The 1H NMR spectra of digested DUT-67 samples were recorded by a Bruker spectrometer at 500 MHz. The samples were digested using a small amount of CsF (ca. 15 mg) with 5 drops of deuterated hydrochloric acid (DCl, 37%) in deuterated DMSO (1.0 mL) for 6 h. SEM images were recorded on a Zeiss DSM-982 Gemini microscope. Powder XRD patterns were collected on a STOE STADI P diffractometer with Cu Kα1 radiation (λ = 1.5405 Å) at room temperature. The zirconium content in the samples was determined by ICP-OES using an Optima 7000 DV instrument. GC analyses were performed using a Shimadzu GC 17 A analyzer equipped with a flame ionization detector and a BPX5 column (30 m length, 0.25 mm inner diameter, and 0.25 mm film thickness). The following temperature program for GC analysis was used: the samples were heated from 40 to 48 °C at a heating rate of 4 °C/min and then from 48 to 100 °C at 15 °C/min and held at 100 °C for 4 min; after that, they were continuously heated from 100 to 270 °C at 30 °C/min and held at 270 °C for 4 min. Inlet and detector temperatures were set constant at 270 °C. n-Hexadecane was used as an internal standard to calculate reaction yields. Mass spectra were collected using a Shimadzu GCMS-QP5000 spectrometer and compared with the database of the NIST library. The ee of 2-(2-nitro-1-phenylethyl)cyclohexanone in the product mixture was determined by high-performance liquid chromatography (HPLC; ELITE LACHROM system from VWR/Hitachi using a UV L2400 detector) using a Lux Amylose I chiral column: 0.5 μm, 98:2 (v/v) hexane/IPA, 1 mL/min, λ = 225 nm, retention time t{(S)2[(R)-2-nitro-1-phenylethyl]cyclohexan-1-one} = 48.437 min (major), t{(R)-2[(S)-2-nitro-1-phenylethyl]cyclohexan-1-one} = 31.120 min, t{(S)-2[(S)-2-nitro-1-phenylethyl]cyclohexan-1-one} = 40.887 min, and t{(R)-2[(R)-2-nitro-1-phenylethyl]cyclohexan-1-one} = 33.123 min. Synthesis of DUT-67 Analogues. Synthesis of DUT-67-fa. In a typical procedure, ZrCl4 (1.38 g, 6.00 mmol) was dissolved in 75.0 mL of a 1:1 DMF/N-methyl-2-pyrrolidone mixture in the presence of formic acid (fa; 26.8 mL, 120 equiv, 720 mmol).41 A solution of 2,5thiophenedicarboxylic acid (TDC; 0.66 g, 4.00 mmol) in DMF/NMP (75.0 mL) was added, and the mixture was sonicated for 5 min. The resulting mixture was placed in an oven for 48 h at 120 °C. The product was filtered, washed three times with DMF and EtOH, and dried under vacuum at 120 °C for 48 h to give activated DUT-67-fa [Zr6O6(OH)2(tdc)4(HCOO)2(DMF)(H2O)5] as a white solid. Yield: 1.35 g (84% based on zirconium). 1 H NMR (500 MHz, DMSO-d6/DCl): δ 8.10 (s, 1H, fa), 7.89 (s, 1H, DMF), 7.66 (s, 2H, TDC), 2.83, 2.66 (s, 6H, DMF). Synthesis of DUT-67-pro. DUT-67-fa (0.15 g) was suspended in 5 mL of a DMF solution, containing 0.14 g of L-proline (1.25 mmol, 12 equiv) and 0.01 mL of HCl (1.00 mmol, 1.00 equiv) at room temperature for 5 days. The product (Zr6O6(TDC)4(OH)4−x(Pro)x) was collected by decantation and washed several times with DMF and EtOH. Elem anal. Calcd for Zr6O6(TDC)4(OH)0.08(Pro)3.92: C, 29.5; H, 2.22; S, 7.23; N, 3.11. Found: C, 29.4; H, 2.05; S, 7.09; N, 3.01. 1H

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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.7b02854. NMR data of DUT-67 as well as the products of the reaction, effect of the catalyst amount and additives on the catalytic performance, thermogravimetric analysis data, ICP-OES, and HPLC chromatograms (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: +49 351 463-33632. Fax: +49 351 463-37287. ORCID

Irena Senkovska: 0000-0001-7052-1029 Stefan Kaskel: 0000-0003-4572-0303 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.D.N. thanks the Vietnamese Government for a doctoral fellowship (Project 911). Support by the DFG is also gratefully acknowledged (Grant KA 1698/19-1).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02854 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02854 Inorg. Chem. XXXX, XXX, XXX−XXX