A Cationic Zinc–Organic Framework with Lewis Acidic and Basic

Aug 23, 2018 - A Cationic Zinc–Organic Framework with Lewis Acidic and Basic Bifunctional Sites as an Efficient Solvent-Free Catalyst: CO2 Fixation ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Cationic Zinc−Organic Framework with Lewis Acidic and Basic Bifunctional Sites as an Efficient Solvent-Free Catalyst: CO2 Fixation and Knoevenagel Condensation Reaction Cheng Yao,† Shaolin Zhou,† Xiaojing Kang, Yang Zhao, Rui Yan, Yan Zhang, and Lili Wen* Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, 430079, China

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S Supporting Information *

ABSTRACT: A novel two-dimensional cationic framework [Zn2(TCA)(BIB)2.5]·(NO3) (1) (H3TCA = tricarboxytriphenyl amine, BIB = 1,3-bis(imidazol-1-ylmethyl)benzene) was successfully achieved. Compound 1 not only presents a moderate affinity toward CO2 molecules, but it also displays good catalytic performance and substrate selectivity toward both CO2 conversion with epoxides and Knoevenagel condensation under solvent-free environments, taking advantage of the Lewis acidity endowed by lower four-coordinated Zn(II) centers and Lewis basicity originated from the amines within TCA3−. More importantly, the bifunctional heterogeneous catalyst compound 1 shows easy recovery and reuse without an obvious decrease of activity. Strikingly, compound 1 exhibits good catalytic efficiency for CO2 coupled with propylene oxide forming propylene carbonate even at ambient temperature under 1 atm pressure. To the best of our knowledge, compound 1 is presented to be the first cationic MOF holding great promise as a heterogeneous solvent-free catalyst toward both CO2 epoxidation and Knoevenagel condensation reaction.



INTRODUCTION Carbon dioxide (CO2) is suspected to be the primary greenhouse gas causing global warming and a consequent series of environmental problems over the past decades.1 In this regard, CO2 activation and efficient conversion into value-added chemicals are imperative and attractive, given that CO2 is recognized to be a nontoxic, economic, and renewable C1 building block.2 Particularly, the coupling of epoxides with CO2 into cyclic carbonates is regarded as one of the most promising protocols for CO2 utilization in terms of green chemistry and atomic economy.3 The target cyclic carbonates have extensive applications as chemical intermediates and battery electrolytes.4 Meanwhile, Knoevenagel condensation, as an important C−C coupling reaction, has been extensively utilized for the preparation of fine chemicals and pharmaceutical products.5 Owing to permanent porosities, facile tunability,6 and functional pore surface, metal−organic frameworks (MOFs) might be ideal catalysts in a heterogeneous system.7,8 Up to now, even though a variety of MOFs were demonstrated as fine catalysts for CO2 chemical fixation9−13 or Knoevenagel condensation,14−18 there is still a need to rationally construct highly effective MOFs as catalysts for such transformations especially under mild reaction conditions, so as to lower the energy consumption and production costs. In principle, a promising heterogeneous catalyst for CO2 epoxidation features selective CO2 capture capacity as well as abundant Lewis/ Brønsted acid or base to activate epoxide/CO2. In the case of © XXXX American Chemical Society

Knoevenagel reaction, the Lewis basic sites would support the formation of a C−C bond between aldehydes or ketones and a methylene group containing active hydrogen atoms. Moreover, chemical and thermal durability is a prerequisite for heterogeneous catalysts in practical implementation. Taking inspiration from the above consideration, by employing the ligands tricarboxytriphenyl amine (H3TCA, Scheme S1) and 1,3-bis(imidazol-1-ylmethyl)benzene (BIB) to assemble with Zn(II) ions, a novel two-dimensional (2D) cationic framework [Zn2(TCA)(BIB)2.5]·(NO3) (1) was successfully achieved. Remarkably, compound 1 incorporates lower fourcoordinated Zn(II) centers functioning as Lewis acidic sites and the accessible N-donor of triphenylamine acting as Lewis basic centers and CO2-philic groups. Impressively, as an ideal heterogeneous Lewis acid/base bifunctional catalyst, compound 1 offers great performance and substrate selectivity as well as satisfactory recyclability for both the chemical transformation of CO2 to cyclic carbonates and Knoevenagel condensation reactions under mild conditions. More interestingly, 1 is able to catalyze the cycloaddition of propylene oxide with CO2 even under ambient pressure and temperature with good efficiency. Until now, compound 1 represents the first cationic MOF-based heterogeneous solvent-free catalyst toward both CO2 epoxidation and Knoevenagel condensation. Received: June 21, 2018

A

DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry



EXPERIMENTAL SECTION

Synthesis of [Zn2(TCA)(BIB)2.5]·(NO3) (1). A mixture of H3TCA (4.9 mg, 0.01 mmol), BIB (11.90 mg, 0.05 mmol), Zn(NO3)·6H2O (14.8 mg, 0.10 mmol), N,N′-dimethylacetamide (DMA, 2 mL), and EtOH (1 mL) was placed in a 5 mL glass vial, which was heated at 110 °C for 2 days. After cooling to ambient temperature, yellowish block crystals were collected with a yield of 42% (based on H3TCA). FT-IR (KBr, cm−1): 3431m, 3128w, 1599s, 1523w, 1378s, 1315s, 1276m, 1169m, 1093m, 1023w, 948w, 840w, 784m, 721w, 658m, 524w, 442w.



RESULTS AND DISCUSSION Crystal Structure. Compound 1 crystallizes in the orthorhombic space group Pnnm (Table 1). The asymmetric

Table 1. Crystallographic Data for Compound 1 1 empirical formula formula weight cryst syst space group a (Å) b (Å) c (Å) V (Å3) Z Dc (g cm−3) μ (mm−1) no. of reflns collected/unique R1 [I > 2σ (I)] wR2 (all data)

C56H47N12O9Zn2 1162.84 orthorhombic Pnnm 23.3735(9) 25.8030(11) 22.9869(9) 13863.5(10) 8 1.114 0.746 68557/12540 0.0623 0.1823

unit of 1 includes two crystallographically independent ZnII ions, one TCA3− anion, two and a half BIB ligands, as well as two NO3− counteranions respective with a half occupancy (Figure S1a). Both ZnII ions exhibit a similar tetrahedral coordination sphere but different coordination environments. As shown in Figure 1a, the Zn1 was surrounded by two carboxylate O atoms from different TCA3− moieties and two N atoms from separate BIB bridges, whereas the Zn2 was coordinated by one carboxylate O atom and three N atoms from two distinct BIB linkers. The Zn−O and Zn−N bond distances are in the range 1.963(3)−1.982(3) and 1.984(4)−2.036(4) Å, respectively (Table S1). Each TCA3− anion is attached to three ZnII ions in monodentate coordination mode, and each BIB is linked to two ZnII centers with two terminal imidazole moieties taking a cisconfiguration, affording a 2D cationic skeleton charge-balanced by free NO3− anions (Figure 1b). Compound 1 has open channels along the a axis of ca. 6.5 × 7.9 Å 2 (regardless of the van der Waals radii), which were filled with guest NO3− anions and solvent molecules. The total solvent-accessible space accounts for ∼29.5% (4084.5 Å3) per unit cell by PLATON analysis.19 By simplification of TCA3− moieties and Zn2II as well as Zn1II centers as 3-, 3-, and 4-connected nodes, respectively, the skeleton of 1 adopts a (3, 3, 4) trinodal net with the topological20 point symbol of (63)(62.8)(64.8.10) (Figure 1c). The resulting adjacent 2D framework is further cross-linked via π···π interactions between the benzene rings of BIB linkers, with separations of 3.589(4) and 3.765(4) Å, which might contribute to the stabilization of compound 1 (Figure S1b). Gas Adsorption. The PXRD profile of the fresh sample at ambient temperature is coincident with the simulated one from the single-crystal data, indicative of good phase purity of bulk 1

Figure 1. Compound 1: (a) the coordination environment around ZnII (the H atoms are omitted for clarity); (b) the 2D cationic framework; (c) the topological representation: cyan, purple, and blue spheres represent Zn1, Zn2, and N1 nodes, respectively. Symmetry code: #1 −1/2 + x, 3/2 − y, 3/2 − z.

(Figure S2). Moreover, compound 1 still retains single crystallinity after solvent treatment (cyclohexane, toluene, CH2Cl2, isopropanol, acetone, EtOH, CH3OH, CH3CN, DMF, and H 2 O) for 72 h under room temperature. Furthermore, no obvious loss of crystallinity for compound 1 was found in a wide pH range of 2−14 (Figure S3). The high chemical stability of compound 1 may benefit from the increased B

DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry coordinative bond strength between the ZnII and imidazolebased BIB ligand.21 The as-synthesized 1 was soaked in methanol, and then, it was treated by supercritical CO 2 activation. The desolvated sample’s PXRD pattern suggested that the pristine structure was preserved after the activation process (Figure S2). As shown in Figure 2, the CO2 sorption isotherm at 195 K of the activated 1 featured a typical type I adsorption isotherm, verifying the permanent microporous

nature. The Brunauer−Emmett−Teller surface area of 1 was fitted to be ∼504 m2/g. For compound 1, the maximum CO2 uptakes are 72.8 (273 K) and 48.4 cm3/g (298 K) under 1 bar, as well as very little N2 capture under the same conditions (Figure 2b). In order to gain more insights into the interaction between the adsorbate and the framework, the adsorption heat of CO2 (Qst) for 1 at zero coverage was found to be 29.0 kJ/mol by a virial method,22 based on the sorption isotherms at 273 and 298 K (Figure 2c). The observed value surpassed those of recently reported cationic MOFs, such as [Zn2(L)(bpb)2]·(NO3)·(DMF)3·(H2O)4 (23.5 kJ/mol), [Zn2(L)(dpe)2](NO3)·(DMF)3·(H2O)2 (20.5 kJ/ mol)23 (L = 1,3-bis(3,5-dicarboxyphenyl)imidazolium, bpb = 1,4-bis(4-pyridyl)benzene, dpe = 1,2-di(4-pyridyl) ethylene), and FJU-14-BF4 (18.8 kJ/mol).24 The strong charge-induced forces between the positively charged host and guest CO2 molecules25 alongside the interactions between the Lewis basic amines26 involved in the framework of 1 and the acidic CO2 molecules within the pores are critical for the considerable uptake of CO2. Catalytic Cycloaddition of CO2 to Epoxides. Noticeably, despite no open metal sites in the channels, the lower fourcoordinated Zn(II) centers within the cationic framework of 1 could behave as available Lewis acidic sites. On account of the high affinity to CO2 and the incorporation of Lewis acid sites of Zn(II) ions as well as the superior chemical stability of the MOF, detailed catalytic performance of compound 1 in CO2 chemical fixation was exploited both at 1 and 10 atm, respectively. The CO2 cycloaddition with epichlorohydrin at atmospheric pressure has been utilized as a model reaction to screen the optimal conditions. After detailed investigations, the optimum conditions were set at 80 °C for 4 h with compound 1 (0.01 mmol) and tetra-n-tertbutylammonium bromide (nBu4NBr, 0.02 mmol) as cocatalysts under a solvent-free environment (Table S2). It should be noted that almost no cyclic carbonates were detected when pure 1 was chosen as catalyst (Table S3, entry 1), while nBu4NBr alone led to a low catalytic efficiency with 34% yield (Table S3, entry 2) under the employed experimental conditions. Strikingly, in the presence of compound 1 together with n-Bu4NBr as a binary catalytic system, epichlorohydrin was almost completely converted to the corresponding carbonate (>99% yield, Table S3, entry 3), highlighting the remarkable synergistic effect of 1 and nBu4NBr in the promoting of the CO2 epoxidation reaction under mild conditions. Except for the cyclic carbonates, no other side products are found, suggesting the exclusively high selectivity of compound 1. The catalytic generality over compound 1 can be further extended to several typical epoxide substrates with variable dimensions (Table 2). Under the optimized conditions, the chemical fixation of CO2 to the smaller substrates, such as epibromohydrin and 1,2-epoxyhexane, gave decent yields comparable to that of epichlorohydrin (>99% yield, Table 2, entries 1−3). Nevertheless, in the case of bulky substrates, such as 1,2-epoxyethylbenzene and glycidyl phenyl ether, the yields of the desired products were sharply reduced to 51.3 and 59.5%, respectively (Table 2, entries 4 and 5). The poor catalytic efficiency toward larger substrates may be attributed to the higher steric hindrance of the reactants with limited diffusion through the channels of compound 1 during the reaction.27 These results manifest that compound 1 exhibits a size-selective catalysis to a certain degree. Notably, as illustrated in Figure 3, significantly improved yields of the respective carbonates for the

Figure 2. (a) CO2 adsorption isotherm for 1 at 195 K. (b) CO2 and N2 sorption isotherms of 1 at 273 and 298 K. (c) Isosteric heat of adsorption for CO2 in 1. C

DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(0.1 mmol) with 10 atm pressure of CO2 at 80 °C. As illustrated in Table 3, the substrates epichlorohydrin and epibromohydrin were completely transformed to the corresponding carbonates within 4 h with a turnover number (TON) of 250. Strikingly, the 1,2-epoxyethylbenzene and glycidyl phenyl ether (Table 3, entries 4 and 5) were also catalyzed over compound 1 in high conversion rates of 95.5 and 96.1% after 4 h, with a TON of 225 and 230, respectively. Remarkably, as compared to the CO2 epoxidation that happened at 1 atm (Table S4), about 4 times of the TON values for 1,2-epoxyethylbenzene and glycidyl phenyl ether were reached with the same amount of catalyst 1 at 10 atm, revealing that the increasing CO2 pressure was beneficial for the epoxidation reaction. Easy separation and activation as well as recyclability are of paramount importance for heterogeneous catalyst. Catalyst compound 1 recovered from the reaction mixture was thoroughly washed with CH2Cl2 and air-dried and then used as a catalyst without additional activation treatments. Further, the reusability of compound 1 was examined, using epichlorohydrin as the example at both 1 and 10 atm. Within four catalytic runs, high yields of 99.5−98% (1 atm) and 99.5−87% (10 atm) were maintained, suggesting retention of the activity (Figure S7). Moreover, PXRD analyses evidence the structural integrity of compound 1 after at least four times (Figure S8). Remarkably, at 1 atm of CO2 atmosphere and ambient temperature, compound 1 reveals great catalytic performance for CO2 coupled with propylene oxide forming propylene carbonate, with a yield of 83% for 48 h and a TON of 207 (Figure 4), which is high and comparable to that for USTC-253TFA28 with Lewis/Brønsted acid sites under similar conditions but lesser amounts of catalyst in our case. The results showed 1 not only could efficiently catalyze the CO2 chemical fixation under high temperature and a high/atmospheric pressure environment, but it also presents high performance under ambient conditions, originating from the existence of Lewis acid sites and high CO2 enrichment capability in 1. As far as we know, only rare MOFs are capable of accomplishing CO2 cycloaddition conversion under ambient temperature and pressure to date.29 As illustrated in Scheme S2, a tentative synergistic catalytic mechanism is proposed for the cycloaddition of epoxide and CO2 into cyclic carbonate catalyzed by compound 1 and nBu4NBr. Knoevenagel Condensation Reaction. Interestingly, the Lewis base triphenylamine sites are uniformly located in the framework of compound 1, which make it applicable for the heterogeneous Knoevenagel condensation reaction. For a typical experiment, benzaldedyde (1 mmol) and malononitrile (2 mmol) along with the catalyst compound 1 (0.003 mmol) proceeded at 60 °C for 1 h under solvent-free conditions, giving a 99% yield of 2-benzylidenemalononitrile as the only product with a high turnover frequency (TOF) of 167 h−1 (Table 4, entry 1). In parallel studies, when the reaction proceeded without compound 1, only trace conversion was achieved (Table S6, entry 2). Besides, poor yields of 2-benzylidenemalononitrile were achieved with Zn(NO3)2·6H2O and ligands H3TCA or BIB as catalysts (Table S6, entries 3−5). Those control experiments clearly reveal that compound 1 played a crucial role in the Knoevenagel condensation reaction. To further investigate the substrate scope, a series of benzaldehyde derivatives with malononitrile were expanded under identical conditions. It is generally accepted that the electron-deficient benzaldehyde derivatives are more reactive than those with electron-rich groups in the Knoevenagel

Table 2. Cycloaddition of CO2 and Epoxides Catalyzed by Compound 1 at 1 atma

a

Reaction conditions: epoxide (2 mmol), compound 1 (0.01 mmol), and nBu4NBr (0.02 mmol), CO2 (1 atm), 80 °C.

Figure 3. Yields of corresponding carbonates after 4 h (green) and 12 h (red) of reactions under 1 atm with compound 1 applied as catalyst.

larger substrates of 1,2-epoxyethylbenzene and glycidyl phenyl ether were achieved, up to 90 and 96% with the unchanged quantity of catalyst and reaction temperature but the reaction time extending to 12 h. As depicted in Figure S6, no further reaction was observed after catalyst compound 1 was filtered off right after reaction for 1 h at 80 °C, confirming the catalytically active sites for CO2 conversion into cyclic carbonates located on 1. Additionally, inductively coupled plasma (ICP) results revealed no leaching of ZnII in this reaction system occurred, further verifying a typical heterogeneous catalyst nature. The CO2 epoxidation over compound 1 was also explored under high pressure. In a typical procedure, the catalytic reaction was carried out in an autoclave reactor using epoxides (10 mmol) as substrates, compound 1 (0.02 mmol), and nBu4NBr D

DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Cycloaddition of CO2 and Epoxides Catalyzed by Compound 1 at 10 atma

a Reaction conditions: epoxide (10 mmol), compound 1 (0.02 mmol), and nBu4NBr (0.1 mmol), CO2 (10 atm), 80 °C. bMoles of epoxides consumed per mole of Zn(II).

8−11). The substrates of hexanaldehyde and cyclohexanecarbaldehyde afforded the corresponding products in good yields of 93 and 90%, whereas 4-phenylbenzaldehyde and cinnamaldehyde only gave moderate yields of 77 and 58%. Moreover, after 15 min of the condensation reaction of benzaldehyde and malononitrile, compound 1 was separated from the reaction mixture, resulting in the complete shutdown of the reaction (Figure S9). The observation indicates that the catalysis over 1 is heterogeneous. Further, the recyclability of compound 1 was tested by utilizing benzaldehyde as a substrate. Compound 1 was simply collected by filtration after the completeness of the condensation reaction and washed using methanol, which can be used for the successive run. Notably, over 95% conversion of benzaldehyde was kept even after four recycles. The results revealed that compound 1 possesses easy recovery without losing obvious activity (Figure S10a). At the same time, the PXRD patterns corroborate that the skeleton of compound 1 remained unaltered after recycles (Figure S10b). To our knowledge, only scarce MOFs have been reported to be capable of efficiently catalyzing Knoevenagel condensation under solvent-free conditions hitherto.14,31 Inspiringly, compound 1 presents superior catalytic activity with very limited loading of catalyst under solvent-free environments (Table 5), which could be a new valuable MOF family member to catalyze Knoevenagel condensation reactions.

Figure 4. Catalytic conversion of CO2 coupled with various epoxides over compound 1. Reaction conditions: epoxide (20.0 mmol), compound 1 (0.04 mmol), nBu4NBr (2.00 mmol), CO2 (1 atm), 25 °C, 48 h.

condensation because nucleophilic addition is considered as a rate-determining step.30 As anticipated, benzaldehyde derivatives bearing electron-withdrawing groups (−Cl, −CN, −NO2) reached nearly full conversion (>99%) with the highest TOF value of 167 h−1 (Table 4, entries 2−4). In comparison, electrondonating benzaldehyde derivatives (−CH 3 , −CH 2 CH 3 , −OCH3) obtained relatively lower yields of the corresponding condensation products (Table 4, entries 5−7) varying from 95 to 78 to 60%. Evidently, the benzaldehyde derivatives gave a decreased conversion in the order of −OCH3 < −CH2CH3 < −CH3, which is reasonable, since −OCH3 is known to be of the strongest electron-donating nature among them. Moreover, hexanaldehyde, cyclohexanecarbaldehyde, 4-phenylbenzaldehyde, and cinnamaldehyde were also evaluated (Table 4, entries



CONCLUSIONS In summary, by the self-assembly of the ligands H3TCA and BIB with Zn(II) ions, a novel 2D cationic framework [Zn2(TCA)(BIB)2.5]·(NO3) (1) was successfully fabricated. Remarkably, compound 1 features Lewis acid Zn(II) and Lewis base triphenylamine on the internal surfaces as well as good CO2 enrichment capability, thus enabling both CO2 epoxidation and E

DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 4. Knoevenagel Condensation Reaction of Benzaldehyde Derivatives with Malononitrile Catalyzed by Compound 1a

Reaction conditions: aldehydes (1 mmol), malononitrile (2 mmol), compound 1 (0.003 mmol), 1 h, 60 °C. bMoles of condensation product per mole of tricarboxytriphenyl amine in compound 1 per hour. a

Table 5. Summary of the Catalytic Activity toward Knoevenagel Condensation Reaction over Various MOFs MOFs

catalyst (mol %)

compound 1 [Cd3(tipp)(bpdc)2]·(DMA)·(H2O)9 FJI-C2 UiO-66-NH-RNH2 [Zn2(L)(H2O)2]·(DMF)5·(H2O)4] UiO-67-diamine

0.3 0.6 3 1 1 6

solvent

temperature (°C)

time (h)

yield (%)

ref

toluene toluene CH2Cl2 DMF

60 60 35 23 25 25

1 1 6 2 2 2

>99 >99 80.9 97 90 99

this work 14 15 32 33 34

Knoevenagel condensation reaction with high activity and substrate selectivity under solvent-free environments. Moreover, the recyclability and truly heterogeneous nature of the catalyst for the reactions have also been illuminated. Strikingly,

compound 1 herein can efficiently catalyze CO2 coupled with propylene epoxide to afford propylene carbonate with a high yield even under ambient conditions. To our knowledge, compound 1 is the first cationic MOF as a promising F

DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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heterogeneous solvent-free catalyst toward both CO2 epoxidation and Knoevenagel condensation reaction.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01713. Additional crystal structure figures, TGA, PXRD, catalytic property tests, and proposed catalytic mechanism (PDF) Accession Codes

CCDC 1849778 contains 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lili Wen: 0000-0002-8639-2978 Author Contributions †

C.Y. and S.Z. contributed equally.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was granted financial support from the National Nature Science Foundation of China (grants 21771072 and 21371065), Self-Determined Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (grant CCNU17QN0020), and the 111 Project (grant B17019).



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DOI: 10.1021/acs.inorgchem.8b01713 Inorg. Chem. XXXX, XXX, XXX−XXX