Densely Packed Lanthanide Cubane Based 3D Metal–Organic

Article pubs.acs.org/IC

Densely Packed Lanthanide Cubane Based 3D Metal−Organic Frameworks for Efficient Magnetic Refrigeration and Slow Magnetic Relaxation Soumava Biswas, Amit Kumar Mondal, and Sanjit Konar* Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462 066, India S Supporting Information *

ABSTRACT: Two isostructural densely packed squarato-bridged lanthanidebased 3D metal−organic frameworks (MOFs) [Ln5(μ3-OH)5(μ3-O)(CO3)2(HCO2)2(C4O4)(H2O)2] [Ln = Gd (1) and Dy (2)] show giant cryogenic magnetic refrigeration (for 1) and slow magnetic relaxation (for 2). The structural analyses reveal the presence of a self-assembled crown-shaped building unit with a cubane-based rectangular moiety that leads to a special array of metal centers in 3D space in the complexes. Magnetic investigations confirm that complex 1 exhibits one of the largest cryogenic magnetocaloric effects among the molecular magnetic refrigerant materials reported so far (−ΔSm = 64.0 J kg−1 K−1 for ΔH = 9 T at 3 K). The cryogenic cooling effect (of 1) is also quite comparable with that of the commercially used magnetic refrigerant gadolinium−gallium garnet, whereas for complex 2, slow relaxation of magnetization was observed below 10 K.

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INTRODUCTION Exploration of the magnetocaloric effect (MCE) of molecular magnetic materials has attracted immense interest in recent years.1 The MCE of a magnetic material is ascribed to ΔSm, the change of the isothermal magnetic entropy, and ΔTad, the adiabatic temperature change following a change in the applied magnetic field.1 It has been claimed that magnetic refrigerant materials can potentially replace conventional compressorbased refrigerants for ultralow-temperature applications because of their environment friendliness and economic advantages.2 Gadolinium is one of the most important key components for designing efficient magnetic refrigerant materials because of its large-spin ground state, zero orbital momentum, and weak superexchange interactions.3 To date, several such exciting materials have been reported by a few groups,1,2,4 but achieving a molecular material for practical application still remains a challenging task.5 The main drawbacks of gadolinium-based molecular materials is the high atomic weight of the Gd3+ ion, which eventually lowers the magnetic entropy change per unit weight.5 Besides this, the diamagnetic components of the materials, mostly the ligands, do not have any contributions to the magnetic entropy change. Recently, a few successful attempts have been made to minimize the problem using suitable small inorganic bridging components (such as the use of oxides,4p hydroxides, or carbonates5a) and a few such complexes having a −ΔSm value above 60.0 J kg−1 K−1. Another important strategy could be the use of both short organic bridging ligands and extended metal clusters4e,i to construct a highly dense metal−organic framework (MOF) material. The advantage of the resultant materials over pure inorganic © XXXX American Chemical Society

frameworks lies in their design strategy and structural diversity. Several attempts have been made so far,4q but pushing the limit (the highest value reported to date is 55.9 J kg−1 K−1) still remains a difficult challenge, and substantial effort has been made by our group in the past few years.6 On the other hand, because of the presence of inherent magnetic anisotropy, a large number of f electrons, and significant crystal-field effects for the dysprosium ion, investigation of the magnetic behavior of dysprosium-based MOFs has also become a potential area of research interest because of their possible application as porous magnets.7 In this continuation, we report two isostructural densely packed squarato-bridged lanthanide cubane based 3D MOFs, [Ln5(μ3-OH)5(μ3-O)(CO3)2(HCO2)2(C4O4)(H2O)2] [Ln = Gd (1) and Dy (2)]. Complex 1 shows one of the highest MCE values (−ΔSm = 64.0 J kg−1 K−1) among all MOFs reported so far, and the cooling efficiency is quite comparable with the commercially available gadolinium−gallium garnet (GGG). For complex 2, slow magnetic relaxation is observed below 10 K.

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EXPERIMENTAL SECTION

Materials. Lanthanide salts and squaric acid were obtained from Sigma-Aldrich Chemical Co. All of the other reagents and solvents were commercially available and were used as supplied without further purification. Received: October 28, 2015

A

DOI: 10.1021/acs.inorgchem.5b02486 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Single-Crystal X-ray Diffraction. Suitable single crystals of complexes 1 and 2 were mounted on a Bruker APEX-II diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å, 120 K) radiation. Data collection was performed using φ and ω scans. The structures were solved using direct methods followed by fullmatrix least-squares refinements against F2 (all data HKLF 4 format) using SHELXTL.8 Subsequent difference Fourier synthesis and leastsquares refinement revealed the positions of the remaining nonhydrogen atoms. Determinations of the crystal system, orientation matrix, and cell dimensions were performed according to the established procedures. Lorentz polarization and multiscan absorption correction were applied. Non-hydrogen atoms were refined with independent anisotropic displacement parameters, and hydrogen atoms were placed geometrically and refined using the riding model. All calculations were carried out using SHELXL 97,9 PLATON 99,10 and WinGX, version 1.64.11 The final molecular formula was calculated from elemental analysis, thremogravimetric analysis (TGA), and crystal data. Data collection and structure refinement parameters are summarized in Table 1, and selected bond lengths and angles for 1 and 2 are given Table S2 in the Supporting Information (SI).

1493(s), 1180(m), 1153(m), 1101(m), 1066(m), 1026(s), 951(m), 904(m), 840(w), 758(s), 700(s), 538(s). Synthesis of 2. Complex 2 was synthesized following the same synthetic procedure as that of 1 but using DyCl3·H2O (188 mg, 0.5 mmol) instead of GdCl3·6H2O. Colorless crystals were separated and washed with cold methanol. Yield: 26% (based on dysprosium). Elem anal. Calcd (found) for C8H11Dy5O22: C, 7.56 (7.71); H, 0.87 (0.64). Selected IR data (4000−400 cm−1, KBr pellets): 3422(b), 3024(s), 2929(s), 2355(s), 1521(b), 1490(s), 1153(m), 1101(m), 1019(s), 947(m), 907(m), 832(w), 761(s), 695(s), 543(s). Physical Measurements. Magnetic measurements were performed using a Quantum Design PPMS-EVERCOOL magnetometer and a SQUID vibrating sample magnetometer for complexes 1 and 2, respectively. The measured values were corrected for the contribution of the sample holder, and the derived susceptibilities were corrected for the diamagnetic contribution of the sample, estimated from Pascal’s tables.12 Elemental analysis was performed on an Elementar Vario Micro Cube instrument. IR spectrometry was performed on a PerkinElmer spectrometer with KBr pellets.

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RESULTS AND DISCUSSION Reactions of hydrated lanthanide chloride salts and squaric acid under solvothermal conditions yielded the complexes. In situ generated carbonate and formate were obtained either from thermal cleavage of squaric acid or from hydrolysis of DMF at solvothermal conditions. TGA suggests a weight loss of ∼3% (calcd 2.8%) in the temperature range of 68−140 °C for both complexes. The weight loss corresponds to the loss of two coordinated water molecules (Figure S1 in the SI), and the dehydrated frameworks are stable up to around 350 °C. The bulk phase purity was confirmed by powder X-ray diffraction (PXRD) patterns (Figure S2 in the SI). Structural Descriptions of 1 and 2. The complexes crystallized in the I4̅2m space group, and because they are isostructural, only the structure of 1 is described in detail. The relevant structural refinement parameters of both complexes are listed in Table 1. There are two crystallographically independent Gd3+ centers (Gd1 and Gd2) present in the structure (Figure S3 in the SI), and both are eight-coordinated (Figure S4 in the SI). Systematic analysis of the coordination geometries around the metalsusing SHAPE 2.113 reveals that the eight-coordinated Gd1 and Gd2 centers adopt distorted triangular-dodecahedral and biaugmented trigonal-prismatic geometries with minimum CShM values of 0.186 and 3.996, respectively (Table S1 in the SI). The molecular entity is closely related to a reported yttrium complex except one of the formate ions is replaced by a μ3-O group for both complexes.14 The relevant bond parameters around each lanthanide center are listed in Table S2 in the SI. The framework consists of layers formed by densely arranged gadolinium cubanes, which are further bridged by formate and carbonate ligands (Figure 1). The presence of formate in the resultant structure is possibly due to hydrolysis of the DMF solvent at high-temperature solvothermal conditions.14,15 On the other hand, thermal hydrolysis of squaric acid forms oxalic acid, which further decomposes to generate carbon dioxide. So, the reaction of carbon dioxide with water could be a possible source of carbonate in the framework14 (Scheme S1 in the SI). Now, each of the four adjacent Gd3+ cubanes is well organized in a rectangular fashion around a common Gd3+ ion and represents the wings of a butterfly (Figure 1d). Moreover, the centrally situated Gd3+ ion in the rectangular unit is connected to the cubanes through μ2-bridged formate and carbonate ions, and the general bridging mode of the

Table 1. Crystallographic Data for 1 and 2 1 CCDC empirical formula fw (g mol−1) size (mm3) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z temperature (K) μ(Mo Kα) (mm−1) Dc (g cm−3) F(000) h, k, l ranges collected reflns indep reflns GOF on F2 R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b a

2

1411791 C8H11Gd5O22

1429463 C8H11Dy5O22

1245.41 0.47 × 0.38 × 0.25 tetragonal I4̅2m 10.925(2) 10.925(2) 20.461(5) 90 90 90 2442.1(11) 4 120(2)

1271.66 0.42 × 0.35 × 0.27 tetragonal I4̅2m 10.7012(14) 10.7012(14) 20.408(3) 90 90 90 2337.0(7) 4 120(2)

13.481

15.885

3.3734 2200.0 −14 ≤ h ≤ 14, −14 ≤ k ≤ 14, −26 ≤ l ≤ 26 1511

3.594 2232.0 −12 ≤ h ≤ 12, −12 ≤ k ≤ 12, −24 ≤ l ≤ 24 1132

1510 1.1494 0.0190

1044 1.119 0.0187

0.0461

0.0507

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = |∑w(|Fo|2 − |Fc|2)|/∑|w(Fo)2|1/2.

Synthesis of 1. A total of 0.5 mmol (186 mg) of GdCl3·6H2O and 0.5 mmol (57 mg) of squaric acid were added to a solvent mixture of 5 mL of water and 5 mL of N,N-dimethylformamide (DMF). After 30 min of stirring, the solution became clear. Then the whole solution was transferred to a Teflon vessel sealed in a stainless steel container and heated at 170 °C for 96 h, followed by cooling to room temperature (∼30 °C) over a period of 12 h at a rate of 0.20 °C min−1. Colorless block-shaped crystals were separated and washed with cold methanol. Yield: 30% (based on gadolinium). Elem anal. Calcd (found) for C8H11Gd5O22: C, 7.72 (7.92); H, 0.89 (0.77). Selected IR data (4000− 400 cm−1, KBr pellets): 3419(b), 3028(s), 2920(s), 2356(s), 1517(b), B

DOI: 10.1021/acs.inorgchem.5b02486 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Representations of (a) the gadolinium cubane, bridging modes of (b) carbonate and (c) formate ions, and (d) a rectangularshaped building unit found in the 2D layer of complex 1.

Figure 3. Crown-shaped building block of the overall 3D framework found in complex 1.

carbonate ion can be represented as 1κ2O,O′:2κO′:3κO″:4κO″:5κO (Figure 1b). The overall shape of the rectangular unit can be considered as a butterfly-like building block (Figure S5 in the SI). Interestingly, removal of all of the atoms except Gd3+ centers and joining them by imaginary lines result in edge-sharing tetrahedral and face-sharing triangular arrangements, as shown in Figure 2. Careful investigation shows that the metal ions

Figure 4. Illustration of the packing view of the framework along the b axis.

along the layer (Figure S7 in the SI). Further several supramolecular interactions assist their arrangement in the lattice. Magnetic Property Studies. The direct-current (dc) magnetic susceptibility data for the polycrystalline samples were collected at an applied field of 1000 Oe in the temperature range of 2−300 K (Figure 5). For complex 1, the room temperature χMT (χM = molar magnetic susceptibility) value Figure 2. Polyhedral view of the gadolinium core in the 2D layer.

present in the Gd3+ cubanes are arranged in a distorted tetrahedral manner. Furthermore, two adjacent tetrahedra are connected through a corner-sharing triangular plane. Additionally, this plane shares the remaining corner with another adjacent plane. Therefore, the overall view of the 2D layer displays a nice topological feature of the sole arrangement of the metal centers (Figure 2). The pillar of the 3D framework is made up of an infinite 1D array of squarate linkers having a μ4bridging mode16 (Scheme S2 in the SI), and Gd3+ cubanes are bound to two squarates in an up-and-down manner (Figure S6 in the SI), where the dihedral angles between two adjacent squarate rings are 90°. The rectangular building units are assembled through four squarate ions to form a crown-shaped building block (Figure 3), which undergoes a long-range selfassembly to form a 3D framework (Figure 4). Importantly, the overall arrangement of the squarate groups creates pseudovoids

Figure 5. Temperature dependence of the dc susceptibility for 1 and 2 at 0.1 T. C

DOI: 10.1021/acs.inorgchem.5b02486 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (39.18 cm3 K mol−1) is well consistent with the expected spinonly value [39.4 cm3 K mol−1 for five uncoupled Gd3+ ions (8S7/2 and g = 2)]. Upon cooling, the values remain almost constant down to 65 K and then decrease rapidly to reach the minimum of 21.13 cm3 K mol−1 at 2 K. The decrease in χMT at low temperature is due to not only antiferromagnetic interaction but also the zero-field-splitting (ZFS) effect of the Gd3+ ion (although this ion is rather isotropic, it presents a small ZFS effect due to second-order spin−orbit coupling). The dc magnetic data were fitted using the Curie−Weiss equation, which gives the best-fitting values of C = 39.37 cm3 K mol−1 and θ = −1.64 K (Figure S8 in the SI). A low negative θ value indicates the presence of weak overall antiferromagnetic exchange interactions throughout the framework. For complex 2, the experimental room temperature χMT value is 70.66 cm3 K mol−1 (calculated χMT = 70.85 cm3 K mol−1 for five uncoupled Dy3+ ions (6H15/2 and g = 1.33; Figure 5). This χMT value slowly decreases to around 70 K and then decreases rapidly to a minimum of 47.17 cm3 K mol−1 at 2 K. The dc susceptibility magnetic data were fitted using the Curie−Weiss equation, which gives the best-fitting values of C = 70.87 cm3 K mol−1 and θ = −2.11 K (Figure S9 in the SI). However, the shape of the plot and small negative θ value do not surely imply the presence of antiferromagnetic interactions between the Dy3+ centers. The reason behind the nature of the plot is due to the presence of strong spin−orbit coupling and thermal depopulation of the excited Stark sublevels.17 The isothermal magnetization data for both complexes were measured in the temperature range of 2−10 K. For complex 1, the magnetization value reached to a saturation of 34.87NμB at 9 T and 2 K (Figure 6).

which is equivalent to 69.5 J K−1 kg−1 of the complex. However, experimentally, the maximum value of −ΔSm at 3 K for ΔH = 9 T is found to be 64.0 J kg−1 K−1 (Figure 7).

Figure 7. Temperature dependences of the magnetic entropy change (−ΔSm) for complex 1, as obtained from the magnetization data.

The difference between the theoretical and experimentally calculated magnetic entropy change is due to not only the antiferromagnetic interactions but also the diamagnetic contributions of the ligands. To best of our knowledge, this value (64.0 J kg−1 K−1) of the maximum entropy change for an applied magnetic field of 9 T is the second largest one reported so far (Table 2). It is worth mentioning that only four Table 2. Comparison of Magnetic Entropy Changes of Some Magnetocaloric Materials compound Gd(OH)3 Gd2O(OH)4(H2O)2 [Gd(HCOO)3]n [Gd4]a [Gd(OH)CO3]n [Mn(glc)2(H2O)2] GGG complex 1 (Gd5)

ΔH (T)

−ΔSm (J K−1 kg−1)

−ΔSm (mJ cm−3 K−1)

7 7 7 7 7 7 7 3 9

62 59.1 55.9 51.5 66.4 60.3 38.4 24 64.0

346 217 216 190.5 355 112 273 173 215.8

7

60

202

3

35.5

119.8

ref 4p 4p 4q 4r 5 18 19 this work this work this work

Figure 6. M/NμB versus H plots for complex 1 at the indicated temperatures.

a

In the plot of M/NμB versus H/T (Figure S10 in the SI), isotherm magnetization curves almost collapse on the same master curve, which designates the overall isotropic nature of the Gd3+ centers. Taking the very low Mw/NGd ratio (where the molecular mass is Mw = 1245.41 g mol−1 and NGd= number of gadolinium ions present per mole of complex 1) of 249 into account, 1 can be considered as a dense magnetic framework. The magnetic entropy change (ΔSm) was calculated from the magnetization data using the Maxwell relationship ΔSm(T) = ∫ [∂M(T,H)/ ∂T]H dH.18 Theoretically, the full entropy change per mole of complex containing five Gd3+ ions will be 86.5 J mol−1 K−1 (calculated from the equation 5R ln(2S + 1), where S = 7/2),

complexes having maximum entropy changes of higher than 60 J kg−1 K−1 are reported in the literature, and none of them is a MOF material.4p,5,19 The volumetric entropy change for 1 is 215.8 mJ cm−3 K−1, which is also one of the largest among the molecule-based magnetic cooler (Table 2). It is interesting to note that the cryogenic efficiency of 1 is quite higher than the commercially used GGG [−ΔSm = 24 J K−1 kg−1 (173 mJ cm−3 K−1) for ΔH = 3 T] even in a low-field regime.20 Now, for complex 2, the magnetization plot does not show saturation even at the high magnetic field (7 T) and 2 K (the magnetization value is 26.12 NμB) because of the presence of magnetic anisotropy and crystal-field effects (Figure S11 in the SI). In the plot of M/NμB versus H/T (Figure S12 in the SI), D

Check the reference for the complete molecular formula.

DOI: 10.1021/acs.inorgchem.5b02486 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry all isotherm magnetization curves do not collapse on the same master curve, which further indicates the anisotropic nature of Dy3+. The maximum magnetic entropy change (−ΔSm) for complex 2 was found to be 18.6 J K−1 kg−1 at 5 K for ΔH = 7 T (Figure S13 in the SI). To examine the magnetic dynamics of 2, the frequency and temperature dependences of the alternating-current (ac) susceptibilities were collected under a 3.5 Oe ac magnetic field (Figure S14 in the SI). At zero dc field, the observance of out-of-phase frequency-dependent signals indicates the presence of slow relaxation of magnetization21 (Figure 8).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-755-6692339. Fax: +91755-6692392. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.B. and A.K.M. thank IISER Bhopal and UGC for an SRF fellowship, respectively. The authors thank Dr. H. S. Jena for helpful scientific discussions. The authors are also thankful to Vijay Singh Parmar for designing the Abstract graphic. S.K. thanks CSIR, Government of India, and IISER Bhopal for generous financial and infrastructural support.

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REFERENCES

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Figure 8. Temperature dependence of the out-of-phase (χ″) ac susceptibility component for 2 at the indicated frequencies and in zero field.

However, because of fast quantum tunneling of magnetization (QTM),21 the peak maximum of the out-of-phase ac susceptibility signals is absent at zero dc field. To minimize the effect of QTM, ac measurements were performed at some applied dc magnetic field (Figures S15 and S16 in the SI). However, still no peak maxima were obtained, indicating the presence of very fast QTM (Figures S17−S19 in the SI).

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CONCLUSIONS In summary, two isostructural 3D MOFs containing densely packed squarato-bridged lanthanide cubane have been reported. The complexes feature fascinating crown-shaped building units made from cubane-based rectangular moieties, which lead to a special array of metal centers in the 3D space. The gadoliniumbased complex is the first MOF to show an MCE of over 60 J K−1 kg−1, and that is very close to the highest value reported for any material. Furthermore, the cryogenic cooling effect of this framework is also quite comparable with the commercially used refrigerant GGG. Slow magnetic relaxation is observed for the dysprosium analogue.

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X-ray crystallographic data in CIF format for 2 (CIF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02486. PXRD patterns of the compounds, TGA profiles, and some additional figures and tables (PDF) X-ray crystallographic data in CIF format for 1 (CIF) E

DOI: 10.1021/acs.inorgchem.5b02486 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.5b02486 Inorg. Chem. XXXX, XXX, XXX−XXX