Wheel-like Ln18 Cluster Organic Frameworks for Magnetic

23 hours ago - On the other hand, MOFs with metal clusters as magnetocaloric materials have been greatly developed due to their high magnetic density ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Wheel-like Ln18 Cluster Organic Frameworks for Magnetic Refrigeration and Conversion of CO2 Tian-Qun Song,† Jie Dong,‡ An-Fei Yang,‡ Xue-Jing Che,‡ Hong-Ling Gao,† Jian-Zhong Cui,*,†,§ and Bin Zhao*,‡,§ †

Department of Chemistry, National Demonstration Center for Experimental Chemistry & Chemical Engineering Education, National Virtual Simulation Experimental Teaching Center for Chemistry & Chemical Engineering Education, Tianjin University, Tianjin 300072, P. R. China ‡ Department of Chemistry, Nankai University, Tianjin 300071, P. R. China § Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: Two isostructural 2D MOFs ([Ln7(CDA)6(HCOO)3(μ3OH)6(H2O)8]n, abbreviated as 1-Gd and 2-Dy) were successfully synthesized under solvothermal conditions. The self-assembly of lanthanide(III) nitrate and 1,1′-cyclopropane-dicarboxylic acid (H2CDA) resulted in wheel-like Ln18 cluster second building units (SBU), which are further linked to six neighboring wheels to generate a 2D ordered honeycomb array. Both 1-Gd and 2-Dy exhibit high thermal stability and decompose above 330 °C. Moreover, they have good solvent stability in ten common solvents and pH stability with pH values from 1 to 13. Magnetic studies reveal that 1-Gd exhibits weak antiferromagnetic coupling between adjacent Gd3+ ions and has a large magnetocaloric effect of 47.30 J kg−1 K−1 (ΔH = 7.0 T at 2 K), while 2-Dy shows ferromagnetic interaction between adjacent Dy3+ ions. Interestingly, 1Gd and 2-Dy can catalyze the cycloaddition of CO2 to epoxides under mild conditions and can be reused at least five rounds with negligible loss of catalytic performance. applied magnetic field. However, only a few cases have the large changes necessary for application for refrigerant materials.22 To obtain a large MCE, weak superexchange interactions and large magnetic density are of great importance. Gd3+ is an excellent choice to construct refrigerant materials due to its low lying excited spin states, large spin ground state (S = 7/2), and negligible magnetic anisotropy (D).18,23,24 Besides, small ligands can impove the magnetic density, which is apt to get a large MCE. Recently, many studies show that the dimensionality seems to play a negligible role in the MCE and a compact structure with large spin density is beneficial for a large MCE.25 It is worth mentioning that various small ligands are employed to achieve high nuclear wheels. By this method, it is easy for them to capture closely situated metallic ions to generate curved building units, which contributes to the construction of wheel-like structures. Here, we selected the lightweight 1,1′-cyclopropane-dicarboxylic acid (H2CDA) as the ligand, which is expected to yield fascinating structure and a large magnetic density, and successfully synthesized two 2D frameworks based on wheel-like Ln18 second building units

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n the past few decades, great interest in the construction and study of high nuclearity clusters has emerged due to not only their structural diversity but also their potential applications in electricity, optics, magnetism, and catalysis.1−8 As we all know, the assembly of high nuclearity clusters can generate clusterbased MOFs, such as [Ln24],9 [Cu30],10 [Cd4Cu6]n,11 and [Gd3Cu12].12 An especially fascinating type within this class is the molecular wheel. Although numerous molecular wheels have been reported, the design and construction of molecular wheels are still formidable challenges owing to the uncontrollability of the nuclearity and size of the wheel. Up to now, only a few lanthanide wheel clusters like [Ln8],13 [Ln10],14 [Ln12],15 [Ln15],15 and [Ln18]16 have been synthesized. However, as lanthanide ions have not only diversified and high coordination numbers but also poor directionality,17,18 the 2D or 3D lanthanide MOFs based on wheel-like clusters are very rare; so far, only two examples of such 3D MOFs ([Ln24]9 and [Ln12]19) have been reported, and there have been almost no reports for 2D. On the other hand, MOFs with metal clusters as magnetocaloric materials have been greatly developed due to their high magnetic density and are expected to replace the rare and expensive 3He.20,21 Magnetocaloric effect (MCE) has attracted increasing attention, as it is related to the magnetic entropy change and the adiabatic temperature change with a varying © XXXX American Chemical Society

Received: December 14, 2017

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DOI: 10.1021/acs.inorgchem.7b03142 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (SBU), [Gd7(CDA)6(HCOO)3(μ3-OH)6(H2O)8]n (1-Gd) and [Dy7(CDA)6(HCOO)3(μ3-OH)6(H2O)8]n (2-Dy). Then, we structurally and magnetically characterized the frameworks. Magnetic study shows that 1-Gd has a larger magnetic entropy change with −ΔSmmax = 47.30 J kg−1 K−1 (ΔH = 7 T at 2 K). Furthermore, considering the high stability and Lewis acidic sites of 1-Gd and 2-Dy, they can act as heterogeneous catalysts to be employed in CO2 conversion with epoxides and can be reused at least five times with negligible loss of catalytic ability. Therefore, 1-Gd and 2-Dy are potential magnetic and catalytic materials.

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EXPERIMENTAL SECTION

Preparation of [Gd7(CDA)6(HCOO)3(μ3-OH)6(H2O)8]n (1-Gd). H2CDA (0.2 mmol), Gd(NO3)3·6H2O (0.15 mmol), and NaOH (0.2 mmol) were mixed in H2O (7 mL) and DMF (3 mL). The mixture was sealed in a Teflon-lined stainless-steel vessel and heated at 120 °C for 3 days. After the reaction temperature was slowly decreased to 30 °C, colorless block single crystals were obtained. The yield was 79% (based on Gd(NO3)3·6H2O). Elemental analysis (%) calcd: C 17.60, H 2.18. Found: C 17.22, H 2.13. IR (cm−1) (Figure S1 in the Supporting Information): 3395 (s), 2799 (w), 1569 (vw), 1544 (vs), 1451 (s), 1420 (vw), 1352 (w), 1255 (m), 1209 (m), 1044 (w), 972 (m), 931 (m), 874 (m), 787 (s), 731 (vs), 571 (s), 509 (w), 478 (s). Preparation of [Dy7(CDA)6(HCOO)3(μ3-OH)6(H2O)8]n (2-Dy). The synthesis of 2-Dy was similar to that of 1-Gd, only using Dy(NO3)3·6H2O instead of Gd(NO3)3·6H2O. Colorless block single crystals were obtained, and the yield was 68% (based on Dy(NO3)3· 6H2O). Elemental analysis (%) calcd: C 17.32, H 2.14. Found: C 16.76, H 2.17. IR (cm−1): 3370 (s), 2804 (w), 1564 (vw), 1544 (vs), 1456 (s), 1410 (s), 1342 (w), 1255 (m), 1219 (m), 1044 (w), 977 (m), 937 (m), 880 (m), 797 (s), 735 (vs), 571 (s), 509 (w), 478 (s).

Figure 1. (a) The coordinated environments of Gd3+. (b) The hexagonal prism structure of a Gd18 wheel second building unit (SBU) along the c axis. (c) The Gd18 wheel second building unit (SBU) and the 2D honeycomb array of 1-Gd. All hydrogen atoms were omitted for clarity. Cyan: Gd. Red: O. Gray: C.

are two identical planes composed of three Gd1 and three Gd3, which are parallel to the plane of Gd2 (Figure S4). The ligand has a cyclopropane ring alternately above or below the plane of the wheel due to steric hindrance. Three HCOO− anions are trapped in the inside of the wheel, which play a significant role in stabilizing the lanthanide wheel.32 Each Gd18 wheel can be viewed as a centrosymmetrical second building unit (SBU) which is surrounded by six neighboring, as well as equivalent, wheels by sharing −(Gd1) 2 −(μ3-OH)−Gd2−(μ3-OH)− (Gd3)2− bridges, generating a 2D ordered honeycomb array (Figure 1c).9 In addition, these honeycomb layers are further stacked along the c axis to generate a 3D supramolecular structure (Figure S5). Comparatively, to the best of our knowledge, there are a few wheel-like Ln cluster or clusterbased MOFs with larger nuclei, and they are listed in Table S3. Therefore, 1-Gd is a rare example of a 2D wheel-like Ln18 organic framework. Powder X-ray Diffraction (PXRD) and Thermogravimetric Analysis (TGA). The PXRD analyses of 1-Gd and 2Dy were carried out, and corresponding patterns are shown in Figure S6. The peaks of as-synthesized samples matched well with simulated ones, showing that 1-Gd and 2-Dy were presented as pure phases. To study the pH stability and solvent stability of 1-Gd and 2-Dy, the as-synthesized samples were immersed in ten common solvents for about 14 h and a series of solutions with pH values from 1 to 14 for about 5 h, respectively. The PXRD analyses reveal that both samples were stable in ten solvents and solutions with pH values between 1 and 13 (Figure S7). The peaks disappeared partly under pH = 14, indicating that the samples became amorphous. The thermal stability of 1-Gd and 2-Dy was studied by TGA under a N2 atmosphere (Figure S8). The TGA plots have similar curves since 1-Gd and 2-Dy are isomorphous. A weight loss of 6.50% for 1-Gd and 6.22% for 2-Dy could be seen from 110 to 210 °C, owing to the loss of coordinated H2O molecules (calcd: 6.40% (1-Gd) and 6.30% (2-Dy)). Then, 1-Gd and 2Dy were decomposed gradually above 330 °C. Magnetic Properties. Magnetic properties of 1-Gd and 2Dy were studied in order to understand magnetic interactions.

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RESULTS AND DISCUSSION Structure Description. Crystallographic analysis reveals that 1-Gd and 2-Dy are isomorphous in the hexagonal space group P63/m, and the structure of 1-Gd is described as an example. The asymmetric unit of 1-Gd contains three independent Gd ions, Gd1, Gd2, and Gd3, with different occupancies of 1/3, 1/2, and 1/2, respectively. As shown in Figure 1a, Gd1 is coordinated by ten oxygen atoms: three oxygen atoms (O1, O1a, O1b) from μ3-OH groups, one oxygen atom (O2) from a terminal H2O molecule, and six oxygen atoms (O3, O4, O3a, O4a, O3b, O4b) from three CDA2− ligands. The coordination mode of Gd3 is similar to that of Gd1, except for a lack of a coordinated H2O molecule. Gd2 is nine-coordinated, surrounded by two oxygen atoms (O1b, O6b) from μ3-OH groups, one oxygen atom (O9) of a HCOO− anion derived from the decomposition of DMF under solvothermal conditions,26−29 two oxygen atoms (O5, O5a) from terminal H2O molecules, and four oxygen atoms (O4, O7, O4c, O7c) from two CDA2− ligands. The Gd···Gd separations range from 3.472 to 4.041 Å (Figure S2). The Gd−O bond lengths fall in the range of 2.387−2.777 Å, and the Gd−O−Gd angles are in the range of 92.6(2)−115.6(2)° (Table S2). Besides, Gd−O bond distances are slightly longer than that of Dy−O, which can be attributed to the effect of the lanthanide contraction.30,31 Interestingly, Gd ions are interlinked by μ3OH groups and stabilized by CDA2− ligands (coordination mode: μ3-κO3, O4: κO4, O7: κO7, O8) to assemble into a nanosize Gd18 wheel structure with a diameter of 18.17 Å (Figure S3). Along the c axis, 18 Gd ions form a hexagonal prism whose plane is composed of five Gd ions (Figure 1b). In the ac plane, six Gd2 are in the same plane. Meanwhile, there B

DOI: 10.1021/acs.inorgchem.7b03142 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry The magnetic susceptibility measurements were performed from 2.0 to 300 K in an applied field of 1 kOe. As shown in Figure 2, the χMT value is 55.26 cm3 K mol−1 at 300 K for 1-

Figure 3. Field-dependent experimental magnetization plots for (a) 1Gd and (b) 2-Dy at 0−8 T between 2 and 10 K.

Figure 2. Plots of temperature-dependent χMT under an applied field of 1 kOe between 2.0 and 300 K for (a) 1-Gd and (b) 2-Dy. (■)

anisotropy and/or the depopulation of the Dy3+ Stark sublevels.36 Dynamic Magnetic Property. To explore the magnetic dynamics of 2-Dy, the temperature dependence of the alternating current (ac) susceptibility data was collected at a 3 Oe ac magnetic field with a zero direct current (dc) field (Figure S10). Unfortunately, no out-of-phase ac signal is observed, indicating that there is no slow magnetic behavior for 2-Dy. Magnetocaloric Effect. Considering the low Mw/NGd (Mw is molecular mass and NGd is the number of Gd3+ ions per mole of 1-Gd) ratio of 321, 1-Gd is a good candidate for magnetocaloric materials. In order to estimate the MCE, magnetic entropy changes ΔSm are obtained from the magnetization data according to the Maxwell equation ΔSm(T) = ∫ [∂M(T, H)/∂T]H dH.37−39 As shown in Figure 4a, the −ΔSm value increases when the magnetic field increases and temperature decreases. Under a mild field change with ΔH of 3.0 T at 2 K for 1-Gd, the maximum magnetic entropy change is 34.57 J kg−1 K−1 (94.45 mJ cm−3 K−1). With ΔH = 7.0 T at 2 K, the maximum magnetic entropy change is 47.30 J kg−1 K−1 (129.2 mJ cm−3 K−1), which is smaller than the theoretical value of 53.80 J kg−1 K−1 due to the existence of weak antiferromagnetic interactions between Gd3+ ions.40,41 However, 1-Gd still has a large magnetic entropy change compared with the recently reported wheel clusters, 2D MOFs, and 3D macrocycle frameworks as magnetocaloric materials (Table 1). Therefore, 1-Gd is a possible magnetic refrigerant to be used in practice. By comparison, the −ΔSm value rises gradually from 10.0 to 5.0 K and then slowly drops with cooling above 1.0 T for 2-Dy (Figure 4b). With ΔH = 3.0 T at 5.0 K, the maximum value of −ΔSm is 13.31 J kg−1 K−1 (37.65 mJ cm−3 K−1). When ΔH =

Gd, which is in good agreement with the expected value (55.16 cm3 K mol−1) for seven free Gd3+ ions (8S7/2, g = 2). The χMT value stays almost constant from 300 to 55 K and then decreases rapidly to reach a minimum of 42.93 cm3 K mol−1 at 2 K, indicating antiferromagnetic interactions between adjacent Gd3+ ions. The curve of χM−1 versus T from 2.0 to 300 K can be well fitted according to the Curie−Weiss law, giving C = 55.34 cm3 K mol−1 and θ = −0.123 K (Figure S9). The negative value of θ further indicates a weak antiferromagnetic coupling interaction between adjacent Gd3+ ions.33,34 As for 2-Dy, the χMT value of 98.75 cm3 K mol−1 at 300 K is close to the expected value (99.19 cm3 K mol−1) for seven isolated Dy3+ ions (6H15/2, g = 4/3). When the temperature decreases, the χMT value gradually reduces to a minimum value of 83.47 cm3 K mol−1 at 12 K. Then, below 12 K, the χMT value increases rapidly to a maximum value of 99.17 cm3 K mol−1 at 2.0 K. The phenomenon indicates the coexistence of ferromagnetic interaction and thermal depopulation of the Stark sublevels of Dy3+ ions in the system.35 When the temperature is above 12 K, the thermal depopulation of the Stark sublevels of Dy3+ ions is dominant, leading to the slight decrease of the χMT value. On the contrary, ferromagnetic interaction plays a crucial role below 12 K, which results in the sharp rise of the χMT value. The magnetization data of 1-Gd and 2-Dy were measured at 0−8 T between 2 and 10 K (Figure 3). The M increases with H and reaches the maximum value (48.52 Nβ) for 1-Gd at 2.0 K and 8 T, which is approximate to the theoretical value (49 Nβ) for seven isolated Gd3+ ions, while the M value of 47.68 Nβ at 8 T for 2-Dy does not reach the saturation value for seven isolated Dy3+ ions (70 Nβ), indicating the presence of magnetic C

DOI: 10.1021/acs.inorgchem.7b03142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

kg−1 K−1, which is mainly attributed to the significant magnetic anisotropy in 2-Dy.2 The large difference in the −ΔSm value between 1-Gd and 2-Dy results from negligible magnetic anisotropy in 1-Gd and the significant one in 2-Dy, the large ground-state spin (Gd3+, S = 7/2 and g = 2.0), as well as a large number of low lying excited spin states of Gd3+. Catalytic Properties. Considering the high stability and Lewis acidic sites of 1-Gd and 2-Dy, we focused catalytic experiments on the conversion of CO2 with epoxides, and the initial substrate was styrene oxide (Table 2). The yield of the desired epoxide was 99% (TON 32.1) when using 2 mmol of styrene oxide, about 3 mol % 1-Gd (based on Gd), 2.5 mol % cocatalyst TBAB (tetrabutylammonium bromide) under solvent-free condition, and a CO2 pressure of 0.1 MPa at 80 °C for 12 h (Table 2, entry 3), which is higher than the yields achieved at 60 and 70 °C (Table 2, entries 1 and 2), showing that higher temperatures contribute to the cycloaddition reaction of CO2. Comparably, the yield was 98% (TON 33.3) using 3 mol % 2-Dy (based on Dy), 2.5 mol % cocatalyst TBAB, and a CO2 pressure of 0.1 MPa at 80 °C for 12 h, which shows catalytic ability similar to that of 1-Gd (Table 2, entry 4). Here, 1-Gd was chosen for further investigation. When the reaction time was reduced to 9 h, the yield became 68% (TON 21.9) (Table 2, entry 5). Therefore, the optimized condition was 20 mg of 1-Gd and 2.5 mol % TBAB at 80 °C for 12 h. Additionally, under the optimized reaction conditions, the yield was only 67% when 1-Gd was absent (Table 2, entry 6), indicating that 1-Gd can largely improve the yield of cycloaddition reactions and will be an effective catalyst. The cycloaddition of CO2 and other epoxide substrates was studied to evaluate the generality of 1-Gd. As shown in Table 3, 1-Gd showed excellent catalytic ability for the transformation of CO2 with various epoxides in high yields (81−99%), and

Figure 4. Calculated magnetic entropy changes for (a) 1-Gd and (b) 2-Dy at 0−7 T between 2 and 10 K.

7.0 T at 5.0 K, the maximum value of −ΔSm is 13.92 J kg−1 K−1 (39.4 mJ cm−3 K−1), far from the theoretical value of 52.93 J

Table 1. Comparison of −ΔSmmax Values among 1-Gd, Wheel Cluster, and 2D and 3D Gd-Based Complexes That Are Potential Magnetic Coolers magnetic interaction 0D wheel clusters {Gd24Zn6} {Gd10} {Gd24Co16} {Gd18} {Gd24Cu36} {Gd6Cu12} 2D frameworks Gd(HCOO)3 Gd2(OH)5Cl·1.5H2O [Gd(C2O4)(H2O)3Cl]n 1-Gd [Gd(oxa)(H2PO2)(H2O)2] [Gd2(C2O4)3(H2O)6·0.6H2O] [Gd(HCOO)(OAc)2(H2O)2]n [Gd(pda)(ox)0.5(H2O)]n [Gd(C4O4)(OH)(H2O)4]n [Gd(cit)(H2O)]n {[Gd36(NA)36(OH)49(O)6(NO3)6 (N3)3(H2O)20]Cl2·28H2O}n 3D macrocycle frameworks {[Gd2(IDA)3]·2H2O}n {[Gd(fum)(ox)0.5(H2O)2]·2H2O}n [Ln7(DPA)5(NA)3(μ3-OH)8(H2O)3]·2.5H2O D

Mw/Nmetal ratio

−ΔSm (J kg−1 K−1) (ΔH) (7 (7 (7 (7 (7 (7

−ΔSm (mJ cm−3 K−1)

ref

AF AF AF AF AF AF

436 582 592 476 727 755

30.0 26.6 26.0 25.9 21.0 14.0

T) T) T) T) T) T)

44.8 45.8 55.3 40.0 32.4 25.5

AF AF AF AF

292 231 335 321

56.0 (7 T) 51.9 (7 T) 48.0 (7 T) 47.3 (7 T)

215.7 − 144.0 129.2

AF ferro AF ferro AF ferro AF

346 348 356 339 350 363 347

46.6 46.6 45.9 45.0 43.8 43.6 39.7

T) T) T) T) T) T) T)

134.3 75.9 110.0 128.1 104.5 115.2 91.4

47 48 49 this work 50 51 25 52 23 53 17

AF ferro AF

315 387 414

40.6 (7 T) 37.1 (7 T) 34.1 (7 T)

100.6 93.4 74.5

54 55 56

(7 (7 (7 (7 (7 (7 (7

42 43 44 16 45 46

DOI: 10.1021/acs.inorgchem.7b03142 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

electron-donating group in the structure of the substrate.8,57 Hence, 1-Gd has great generality to various epoxides. Here, we evaluated the recyclability of 1-Gd. As shown in Figure 5, 1-Gd can be used at least five times with negligible

Table 2. Cycloaddition Reaction of CO2 with Styrene Oxide under Various Reaction Conditionsa

entry

catalyst

TBAB (mol %)

T (°C)

t (h)

TONb

TOFc (h−1)

yield (%)d

1 2 3 4 5 6 7 8

1-Gd 1-Gd 1-Gd 2-Dy 1-Gd 0 1-Gd Gd(NO3)3

2.5 2.5 2.5 2.5 2.5 2.5 0 2.5

60 70 80 80 80 80 80 80

12 12 12 12 9 12 12 12

16.1 21.9 32.1 33.3 21.9 − − 8

1.3 1.8 2.7 2.8 2.4 − − 0.7

50 68 >99 98 68 67