Lanthanide-Based Porous Coordination Polymers - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Lanthanide-Based Porous Coordination Polymers: Syntheses, Slow Relaxation of Magnetization, and Magnetocaloric Effect Chinmoy Das,† Apoorva Upadhyay,†,⊥ Kamal Uddin Ansari,† Naoki Ogiwara,‡ Takashi Kitao,‡ Satoshi Horike,*,‡,§,∥ and Maheswaran Shanmugam*,† †

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, Maharashtra, India Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan § Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Institute for Advanced Study, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ∥ AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ‡

S Supporting Information *

ABSTRACT: Two lanthanide-containing structurally analogous porous coordination polymers (PCPs) have been isolated with the general molecular formula [Ln2(L1)2(H2O)4(ox)]n.4nH2O (where L1 = fumarate, ox = oxalate; Ln = Dy (1), Gd (2)). Thermogravimetric analysis (TGA) and TG-MS measurements performed on 1 and 2 suggest that not only the solvated water molecules in the crystal lattice but also the four coordinated water molecules on the respective lanthanides in 1 and 2 are removed upon activation. Due to the removal of the waters, 1 and 2 lost their crystallinity and became amorphous, as confirmed by powder X-ray diffraction (PXRD). We propose the molecular formula [Ln2(L1)2(ox)]n for the amorphous phase of 1 and 2 (where Ln = Dy (1′), Gd (2′)) on the basis of XANES, EXAFS, and other experimental investigations. Magnetization relaxation dynamics probed on 1 and 1′ reveal two different relaxation processes with effective energy barriers of 53.5 and 7.0 cm−1 for 1 and 45.1 and 6.4 cm−1 for 1′, which have been rationalized by detailed ab initio calculations. For the isotropic lanthanide complexes 2 and 2′, magnetocaloric effect (MCE) efficiency was estimated through detailed magnetization measurements. We have estimated −ΔSm values of 52.48 and 41.62 J kg1− K−1 for 2′ and 2, respectively, which are one of the largest values reported for an extended structure. In addition, a 26% increase in −ΔSm value in 2′ in comparison to 2 is achieved by simply removing the passively contributing (for MCE) solvated water molecule in the lattice and coordinated water molecules.

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INTRODUCTION

two-coordinate Dy(III) ion reported by Chilton and coworkers recently with a record high TB value of 60 K,3 while a pseudolinear complex (seven-coordinated pentagonal-bipyramidal (five weak equatorial ligands and two strong axial ligands)) of Dy(III) reported by Murugavel and co-workers registered the second highest blocking temperature.4 The importance of a suitable ligand field around various lanthanides to block the magnetization vector was greatly exemplified by Long and co-workers earlier.5 This was elegantly proven experimentally in various symmetric and unsymmetric

The intrinsic large spin−orbit coupling and the ligand field interaction associated with lanthanide ions make them interesting and ideal magnetic materials to investigate singlemolecule magnet (SMM) or single-ion magnet (SIM) behavior.1 Slow relaxation of magnetization phenomena below a certain temperature (blocking temperature; TB) are shown by oligonuclear or mononuclear lanthanide complexes, called SMMs or SIMs, respectively.2 Although ligand field interaction with the valence electrons of lanthanide is quite weak, it does have a non-negligible influence in determining the anisotropic barrier, which in turn has a direct correlation with the blocking temperature. This can be evidently visualized in a © XXXX American Chemical Society

Received: March 17, 2018

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DOI: 10.1021/acs.inorgchem.8b00720 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

pointed out by them, smaller and lighter linkers are preferred over longer organic spacers, because the latter passively contribute to MCE and hence reduce the magnetic cooling efficiency. Nevertheless, a 3D network with smaller and lighter linker (fluoride ion) with a record high magnetic density has been unveiled already in the literature.20 Therefore, an alternate synthetic methodology is sought after to reveal better magnetic coolants. Keeping in mind all these factors said above, we intend to employ a rather large multitopic aliphatic dicarboxylic acid (fumaric acid, H2L1) organic spacer to reveal a new Dy(III)and Gd(III)-based 3D network. We report in this article two structurally analogous 3D extended structures, which were characterized by single-crystal X-ray diffraction. The porous nature and stability of the networks were checked by TGA, PXRD TG-MS, EXAFS, XANES, etc. An amorphous phase of the crystalline network was identified by PXRD for both Dy(III) and Gd(III) ions upon activation of PCPs (vide infra). Field-induced slow relaxation of the magnetization phenomenon of the anisotropic Dy(III) network and magnetic coolant efficiency of the isotropic Gd(III) network were investigated on both phases. Two different relaxation times have been extracted experimentally for the Dy(III) network, which are rationalized by detailed ab initio calculations. The amorphous phase of the isotropic Gd(III) network shows one of the highest −ΔSm values (52.8 J kg K−1) reported in the literature; in addition we noticed a 26% increase in −ΔSm value in its amorphous phase in comparison to its crystalline phase. On the basis of this observation, we have proposed an alternate strategy to overcome the hurdle of identifying the smallest and lightest linker to increase MCE efficiency.

lanthanide complexes by several research groups, including by us,6 and was firmly supported by ab initio calculations.7 Among the lanthanide-based SMMs or SIMs reported in the literature, more than 90% of the reports were dominated by Dy(III) ions, whose D4d geometry is observed to be one of the ideal geometries to stabilize uniaxial anisotropy.8 However, enforcing an ideal square-antiprismatic geometry is quite a challenging task for the chemist. Despite cognizant efforts taken to increase TB on the majority of the complexes, it could not be increased significantly due to quantum tunneling of magnetization (QTM).9 Due to the distortion from ideal D4d geometry, nonsuitable ligand field, hyperfine interaction, dipolar spin− spin interaction, and transverse internal field are the predominant factors mainly triggering QTM in lanthanidebased SMMs or SIMs.10 By employing an isotopically labeled lanthanide with a nuclear spin of zero11 or by magnetic dilution methodology,12 one can control the under-barrier relaxation mechanism due to hyperfine interaction, dipolar spin−spin interaction, and the transverse internal field. But again, enriching isotopically labeled lanthanides and identifying an ideal diamagnetic matrix are demanding exercises in the isolation of discrete SIMs or SMMs.11 QTM can be arrested completely or suppressed to a maximum extent by enhancing exchange interaction between lanthanides, which was exemplified by Long and co-workers,2a,b Murray and co-workers,13 and us14 independently. Due to the shielded nature of the valence electron in lanthanide(III) ions, however, increasing the exchange interaction is an enormously uphill process. The aforementioned problems (QTM triggered by various factors) demand a new approach to reveal new generation of SMMs or SIMs. This is where a lanthanide-based extended network (1D, 2D, or 3D) facilitated by organic spacers can be effectively employed to tune the dipolar spin−spin and transverse field interaction between the lanthanides. Moreover, investigations related to slow magnetization of relaxation studies are still in their infancy, although lanthanide-based porous coordination polymers (PCPs) have been investigated for several other applications such as gas storage, gas separation, catalysis, proton conduction, luminescence, ion exchange, drug delivery, sensing, etc.15 For the construction of lanthanide PCPs, various multitopic aromatic carboxylic acid spacers have predominantly been utilized considering the oxophilic nature of the lanthanide ions along with secondary building block ligands such as oxalate, bipyridine, etc.16 In comparison to the slow relaxation magnetization phenomenon of the extended network of anisotropic lanthanides, investigations of magnetic refrigeration phenomena of its corresponding isotropic structurally analogous complexes remain largely unexplored. The magnetic cooling efficiency of a material is utterly governed by its intrinsic magnetocaloric effect (MCE; material that changes its temperature by the application of an external magnetic field), as it is an energy efficient and environmentally benign technology.17 The thrust in this area of research is due to the experimental observation that these magnetic refrigeration materials could outperform the well-established cryogenic coolants such as gadolinium gallium garnet (GGG)18 at liquid helium temperature. The change in magnetic entropy (−ΔSm) and change in adiabatic temperature (ΔTad) are the two major factors which govern the MCE efficiency of a material. For better MCE efficiency, the metal/ligand ratio should be high with a large number of unpaired electrons along with other recipes that enhance MCE, discussed elsewhere by Evangelisti and co-workers and others.19 Among the various factors

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EXPERIMENTAL SECTION

Materials and General Methods. Unless otherwise mentioned, all of the reactions were carried out under aerobic conditions. All of the chemicals were purchased from commercially available sources (Sigma-Aldrich, Alfa Aesar, and TCI Chemicals) and were used without further purification. The elemental analyses (CHN) were carried out on a Thermoquest microanalyzer. The powder X-ray diffraction (PXRD) data were recorded on powdered samples on a Rigaku X-ray diffractometer (RINT). Thermogravimetric analysis (TGA) was performed on the solid powdered samples in a Rigaku Smart Loader THERMO PLUS EVO (TG 8120) instruement at a heating rate of 10 °C/min under nitrogen. TGA combined with mass spectrometry was performed on a Rigaku Thermo plus EVO II instruement equipped with a ThermoMass Photo/S apparatus by using an electron impact ionization method. The measurements were performed from 25 to 500 °C with a heating rate of 10 °C min−1 under a flow of helium. The magnetic susceptibility measurements were obtained with the use of an MPMS XL SQUID magnetometer equipped with 70 kOe superconducting magnet. SQUID measurements were performed on polycrystalline samples, and the magnetic data were corrected for the sample holder and diamagnetic contribution. Gd LIII-edge and Dy LIII-edge X-ray absorption spectra (XAS) of these materials were collected at the BL5S1 beamline at the Aichi Synchrotron Radiation Center (AichiSR; Aichi Science and Technology Foundation, Aichi, Japan). The XAS of their powder samples were recorded in transmission mode under ambient conditions, using a Si(111) double-crystal monochromator. The data were processed with Athena. Fourier transformation was k2-weighted in the k range from 2.5 to 9.0 Å−1. X-ray single-crystal data was collected for complex 1 on a Rigaku Micromax-007HF High Intensity Microfocus Rotating Anode X-ray Generator using a graphite monochromator (Mo Kα, λ = 0.71073 Å) at 133 K. The selected crystals were mounted on the tip of a glass pin using mineral oil and placed in the cold flow produced with an Oxford Cryo-cooling device. B


Article

Inorganic Chemistry Scheme 1. General Synthetic Procedure Followed To Isolate both 1 and 2

Figure 1. (A) Asymmetric unit of complex 1 (symmetrically generated atoms are represented in transparent view). (B) Crystal structure of the dimeric repeating unit found in the three-dimentional extended structure of 1. (C, D) Views of the coordination modes of the oxalate and fumarate ligands found in 1 (also in 2). Color code: sky blue, Dy(III); red, O; gray, C. ions in the repeating dimeric unit of 1 were computed by fitting the experimental data within the Lines model26 using the POLY_ANISO module.27 Synthesis of [Dy2(L1)2(H2O)4(ox)]n·4nH2O (1). A mixture of Dy(NO3)3·5H2O (0.175 g, 0.40 mmol), fumaric acid (H2L1) (0.139 g, 1.2 mmol), oxalic acid (H2ox) (0.018 g, 0.20 mmol), and H2O (15 mL) was taken in a sealed Teflon-lined stainless-steel vessel (25 mL). The entire contents were heated at 150 °C for 2 days. After completion of the reaction, the mixture was cooled to room temperature at a rate of 2.5 °C/h. Prism-shaped single crystals were isolated that were suitable for single-crystal X-ray diffraction. Yield: 142 mg (78%, based on Dy(III)). Anal. Calcd (air-dried sample): C, 15.27; H, 2.57. Found: C, 15.19; H, 2.47. FT-IR (KBr pellet): 3410 cm−1 (br, ν(O−H)), 1670 cm−1 (s, νas(ox-COO)), 1408 cm−1 (vs, νs(ox/fum-COO)), 1652 cm−1 (s, ν(fum-CC)), 1562 cm−1 (vs, νas(fum-COO)). Synthesis of [Gd2(L1)2(H2O)4(ox)]n·4nH2O (2). A synthetic procedure was used similar to that followed for 1 to isolate 2, but with a stoichiometric equivalent of Gd(NO3)3·5H2O (0.137 g, 0.40 mmol) used in place of Dy(NO3)3·5H2O. At this juncture, we point out that the sample used for the X-ray structure of 2 was synthesized by an alternative synthetic method, which is reported elsewhere.28 Yield: 133 mg (72%, based on the Gd(III)). Anal. Calcd (air-dried sample): C, 15.47; H, 2.58. Found: C, 15.33; H, 2.47. FT-IR (KBr pellet): 3405 cm−1 (br, ν(O−H)), 1642 cm−1 (s, νas(ox-COO)), 1405 cm−1 (vs, νs(ox/fum-COO)), 1663 cm−1 (s, ν(fum-CC)), 1535 cm−1 (vs, νas(fum-COO)). Activation of Porous Coordination Polymers 1 and 2 and Formation of Amorphous [Ln2(L1)2(ox)]n (Where Ln = Dy (1′), Gd (2′)). The amorphous compounds were prepared when the crystalline 3D PCPs (complexes 1 and 2) were heated overnight under dynamic vacuum (10−3 bar) at 120 °C. The resultant material

Complete hemispheres of data were collected using ω and φ scans (0.3°, 16 s per frame). The unit cell determination and data reduction were performed using Rigaku CrystalClear-SM Expert 2.1 software. The structures were solved by direct methods and completed by iterative cycles of ΔF syntheses and refined by least-squares procedures on F2 with the SHELXTL package. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed on geometrically calculated positions and refined as a riding model (CCDC number for 1: 1830577). Ab initio calculations were performed on the Dy(III) ions using MOLCAS 8.0 suite software.7 The anisotropy of a single Dy(III) ion was calculated on the basis of the X-ray structure. We have adopted the fragmentation approach by replacing the neighboring Dy(III) ion with a diamagnetic Lu(III) ion. Relativistic effects were taken into account on the basis of the Douglas−Kroll Hamiltonian.21 The spin-free eigenstates were achieved by the complete active space self-consistent field (CASSCF) method.22 We have employed the [ANO-RCC···8s7p5d3f2g1h] basis set23 for Dy, [ANO-RCC···7s6p4d2f1g] Lu atoms, the [ANO-RCC···3s2p] basis set for C atoms, the [ANO-RCC···2s] basis set for H atoms, and the [ANO-RCC···3s2p1d] basis set for O atoms. The CASSCF calculations were performed by considering 9 electrons across 7 4f orbitals of the Dy(III) ion. With this active space, 21 sextet roots were computed for complex 1. After computing all these excited states, we mixed all 21 sextet states using the RASSI-SO24 module to compute spin−orbit coupled states. These computed SO states and the ab initio computed matrix elements of the angular momentum were further utilized to compute the g tensors and local magnetic properties of Dy(III) ions by the SINGLE_ANISO module.25 The Cholesky decomposition for two electron integrals is employed throughout in the calculations to reduce the disk space. Crystal-field parameters have been extracted using the SINGLE_ANISO code, as implemented in MOLCAS 8.7a The exchange interactions between the two Dy(III) C


Article

Inorganic Chemistry

Figure 2. Crystal structure of 1 showing the three-dimensional extended network along with the guest water molecules (green spheres) in the cavity.

links the Dy(III) dimeric units in a classical μ,η1-η1 bridging mode across the ab plane (O11 and O12 and its symmetrically related atoms), resulting in a three-dimensional network (Figure 2). The closest Ln···Ln distances along the ab plane are found to be 4.567(4) and 4.582(2) Å for 1 and 2, respectively. In addition, two terminal water molecules (O14 and its symmetrically related atom) are bound on each Dy(III) ion in the dimeric unit. Thus, the eight-coordinated Dy(III) ion (two from water, two from oxalate, and four from fumarate) exists in a distortedsquare-antiprismatic geometry. The geometry around the Dy(III) ion was confirmed by continuous shape measurement (CShM) software29 (Table S2 in the Supporting Information). As mentioned earlier, complex 2 has been reported already by Ruiz-Perez et al, where the authors claimed that the complex possesses, in addition to the bis-bidentate bridging oxalate (as in 1 or 2), a bis-bidentate fumarate bridge as well.28 However, careful analysis of the CIF file (structural analogue of 2) reported by Ruiz-Perez et al. does not show bis-bidentate fumarate bridging, and the coordination modes of the fumarate ligand are exactly similar to those in 1.28 The average Dy1−O bond lengths vary from 2.313(4) to 2.453(4) Å in 1. The Dy1− O(fumarate) bond oriented along the b axis is the shortest (2.313(4) Å) of all other bond lengths. On the other hand, the Dy1−O(fumarate) bond oriented along the a axis of the unit cell is found to have the largest distance (2.453(4) Å) among all. The average Dy1−O(oxalate) and Dy1−O(water) bond distances are observed to be 2.408(5) and 2.433(6) Å, respectively. The cavity generated within the crystal lattice of 1 (as well as in 2) is occupied by four water molecules which mediate H-bonding with the protons of the water molecules (O14−H14B···O1W_$1 = 2.768 Å (D···A), where $1= −0.5 + x, 0.75 − y, 1.25 − z) that are coordinated with the Dy(III) ions (Figure 2). There are reports available where similar kinds of extended structures are known for other lanthanide ions, which are pillared by malonate and oxalate ligands.30 Thermogravimetric Analysis (TGA) and Thermogravimetry−Mass Spectrometry (TG-MS). To check the thermal stability of the extended network complexes 1 and 2, TGA was performed under a N2 atmosphere (Figure S1 in the Supporting Information). The TGA profiles of both 1 and 2 are observed to be very similar to each other, suggesting that they are structurally similar to each other; not surprisingly, similar physical behavior is also observed. Gradual weight loss for both 1 and 2 is noted upon increasing temperature. At 120 °C,

completely lost its crystallinity, as confirmed by PXRD (vide infra). Anal. Calcd for 1′: C, 18.73; H, 0.63. Found: C, 18.66; H, 0.59. Calcd for 2′: C, 19.05; H, 0.64. Found: C, 18.97; H, 0.59. FT-IR (KBr pellet): for 1′, 3078 cm−1 (vw, ν(O−H)), 2965 cm−1 (vw, ν(C−H)), 1654 cm−1 (s, νas(ox-COO)), 1402 cm−1 (s, νs(ox/fum-COO)), 1650 cm−1 (s, ν(fum-CC)), 1558 cm−1 (s, νas(fum-COO)); for 2′, 3082 cm−1 (vw, ν(O−H)), 2955 cm−1 (vw, ν(C−H)), 1631 cm−1 (s, νas(oxCOO)), 1404 cm−1 (s, νs(ox/fum-COO)), 1656 cm−1 (s, ν(fum-C C)), 1528 cm−1 (s, νas(fum-COO)). Rehydration of the Compounds [Ln2(L1)2(ox)]n (Where Ln = Dy (1′), Gd (2′)). The rehydration of the amorphous compound was performed upon immersing 1′ and 2′ in degassed Milli-Q water for 7 days. The resultant crystalline material was used for the powder X-ray diffraction studies (vide infra).

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RESULTS AND DISCUSSION Crystal Structure Description. The reaction of the corresponding lanthanide precursor with fumaric acid and oxalic acid in water under solvothermal conditions yields prismshaped single crystals, which are of excellent quality for singlecrystal X-ray diffraction studies (Scheme 1). Single-crystal X-ray diffraction reveals that complexes 1 and 2 are structurally analogous to each other, which is further evidenced by the similar unit cell parameters observed (Table S1 in the Supporting Information). Further, as mentioned already, the crystal structure of 2 was reported elsewhere by Ruiz-Perez et al.,28 while 1 is reported for the first time. Therefore, the structural description for 1 is detailed below. Complex 1 crystallized in the orthorhombic Fddd space group. The asymmetric unit of 1 consists of only one-fourth of a molecule (one Dy(III) ion, one water molecule, and onefourth of oxalate and fumarate fragments) and the remaining three-dimensional network was generated by rotation and inversion symmetry. For an asymmetric unit there is one water solvent molecule residing in the crystal lattice. A detailed structural analysis reveals that two of the Dy(III) ions are bridged by an oxalate ligand in a μ-η2-η2 binding mode (Figure 1). Thus, the Dy(III) dimer that formed spread across the c axis of the unit cell. The Ln···Ln distance across the oxalate bridge is found to be 6.237(2) Å for complex 1 and 6.298(1) Å for 2. Each of the Dy(III) ions in the dimeric unit coordinated to four more oxygen donors which are derived from four fumarate ligands (see the green and magenta traces in Figure 1B). These ligands are located perpendicular to the c axis around Dy(III) ions. The carboxylate functionality of these fumarate ligands D


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Inorganic Chemistry

Figure 3. Powder X-ray diffraction patterns of as-prepared (dark blue), amorphous (sky blue), and rehydrated (magenta) samples of 1 (A) and 2 (B) and their simulated patterns (red).

Figure 4. (A) Dy LIII-edge XANES for 1 and 1′ and (B) Gd LIII-edge XANES for 2 and 2′.

Figure 3). This amorphous phase of complexes 1 and 2 will be designated as 1′ and 2′, respectively, hereafter. The amorphization of both 1′ and 2′ is likely due to the removal of the guest solvent molecules and the coordinated water molecules from the Dy(III) or Gd(III) ions. Precedents are known in the literature that the majority of the amorphous porous coordination polymers (PCPs) preserve their backbone structural frame without the cleavage of the coordination bonds between metal ions and organic ligands, even though they loses crystallinity.15c,e,f To check that 1′ and 2′ retain these 3D networks, we have rehydrated both 1′ and 2′ and PXRD was recorded. As shown in Figure 3, the PXRD patterns of the rehydrated samples (1′ and 2′; see magenta trace in Figure 3) are almost identical with the corresponding PXRD profiles generated from their corresponding single-crystal data. We also observed that the data do not have PXRD patterns corresponding to the metal-free fumaric acid and/or oxalic acid fragments in the rehydrated samples.31 This suggests that both 1′ and 2′ do not have a long-range structural periodicity but preserve their backbone 3D structural framework. X-ray Absorption Spectroscopy. The PXRD patterns of 1′ and 2′ revealed that removal of the guest water molecules and the coordinated water molecules induced a phase transition from a crystalline state to an amorphous state. However, there is still a poor understanding of the structures of 1′ and 2′ in an amorphous state. To obtain insight, we have performed LIIIedge X-ray absorption spectroscopy (XAS) for crystalline and amorphous samples of both 1 and 2. The observed X-ray absorption near-edge structure (XANES) spectra are shown in Figure 4. The sharp lines correspond to the electron transition from Ln(III) (Ln = Dy (1), Gd (2)) 2p3/2 to outer unoccupied 5d orbitals, and the positions of 1′ and 2′ are identical with those of 1 and 2, respectively, indicating that the oxidation state

18.39% (for 1) and 18.46% (for 2) weight loss is observed, which is attributed to the removal of all of the guest water molecules and the coordinated water molecules. The remaining framework is stable up to 400 °C under a N2 atmosphere, beyond this temperature, the framework decomposes completely. To further validate that the weight loss observed at 120 °C is exclusively due to the water molecules, we performed TGMS analysis and the m/z value of 18 g/mol unambiguously confirms that there are no other fragments of the framework released except water molecules (Figure S2 in the Supporting Information). Nevertheless, the removal of the four guest water molecules alone (expected weight loss 9.25% only) does not account for the entire weight loss (18.39% (for 1) and 18.46% (for 2)) observed at 120 °C. As proven already through TGMS analysis, no other fragments except water molecules are removed at this temperature, and therefore, the remaining weight loss (apart from the guest molecule weight loss) can be attributed to the loss of four more coordinated water molecules on the repeating dimeric unit of both Dy(III) and Gd(III) ions in 1 and 2, respectively. Powder X-ray Diffraction Studies. To check the bulk phase purity of 1 and 2 and to confirm whether crystallinity was retained even after the removal of the coordinated water molecules on both Dy(III) and Gd(III) ions, we have performed PXRD measurements at room temperature (Figure 3). The PXRD profiles of the as-synthesized bulk complexes (1 and 2) show excellent agreement with the PXRD patterns generated from their respective single-crystal data. This confirms the crystalline phase purity of the complexes. PXRD has been measured for samples of 1 and 2 that were kept under dynamic vacuum overnight at 120 °C. The activated samples of 1 and 2 totally lost their crystallinity, and the PXRD profile resembles that of an amorphous powder (see sky blue trace in E


Article

Inorganic Chemistry

Figure 5. LIII-edge extended X-ray absorption fine structure (EXAFS) spectra (k2 weighted) for compounds 1 and 1′ (A) and 2 and 2′ (C). Radial distribution functions (RDFs) from Fourier-transformed EXAFS for 1 and 1′ (B) and 2 and 2′ (D), displayed in the r space contacting both the magnitude of the Fourier transform (solid line) and real components (dashed lines).

Table 1. Experimental χMT Values of Compounds 1, 1′, 2, and 2′ and Their Expected χMT Values along with Their Crystal Field Term Symbolsa theory complex 1 1′ 2 2′ a

exptl χMT (cm3 K mol−1) 27.91 28.04 15.79 15.69

g 4/3 4/3 2 2

S

L

5/2 5/2 7/2 7/2

5 5

term 6

H15/2 H15/2 8 S 8 S 6

χMT (cm3 K mol−1)

gfit

J/kB (K)

28.34 28.34 15.75 15.75

2.0 2.0

+0.012 −0.028

Spin Hamiltonian parameters were extracted from the fit of the experimental magnetic data of 2 and 2′.

Gd(III) ions. Further, the elemental analysis performed on 1′ and 2′ shows excellent agreement between the calculated and experimental data. On the basis of the TGA, TG-MS, PXRD (rehydration experiment), XANES, EXAFS, and elemental analysis, we propose a molecular formula of the amorphous sample of 1 and 2 of [Ln2(L1)2(ox)]n, with Ln = Dy (1′, molecular weight 641.25 g/mol) or Gd (2′, molecular weight 630.76 g/mol), where the guest and coordinated molecules are removed while the 3D network is maintained, pillared by fumarate and oxalate ligands. dc Magnetic Susceptibility Measurement. Temperature-dependent direct current magnetic susceptibility measurements were performed for both the crystalline and amorphous samples of 1 and 2 in the presence of an external magnetic field of 1 kOe in the temperature range of 2−300 K. For 1 and 1′ the observed χMT value is slightly lower than the expected value, while for 2 and 2′, the observed values are in close agreement with the theoretical expected value for the magnetically uncorrelated spins of Dy(III) and Gd(III) ions, respectively (see Table 1). For 1, the χMT value decreases steadily upon decreasing temperature from room temperature to 75 K. The progressive decrease in χMT value is attributed to the depopulation of the mj levels and/or weak antiferromagnetic coupling between the Dy(III) ions, which could be

of Dy/Gd in amorphous 1′ and 2′ is preserved as a trivalent state as with crystalline 1 and 2.32 On the other hand, the peak intensities of 1′ and 2′ are slightly diminished relative to those of 1 and 2, indicating that the local coordination geometry of Dy(III)/Gd(III) ions is changed in the structural transformation from a crystalline state to an amorphous state. Figure 5 represents the radial distribution functions (RDFs) obtained from Fourier transforms of extended X-ray absorption fine structure (EXAFS). The first atomic shell in the range of 1.5−2.5 Å is attributable to oxygen atoms coordinated to Dy(III)/Gd(III) ions. We observed the decline in the peak intensity of amorphous 1′ and 2′ relative to that of crystalline 1 and 2, suggesting that a transformation from the crystalline state to the amorphous state would give rise to a decrease in the coordination number of the Dy(III)/Gd(III) ions, in line with the change of local coordination geometry of Dy(III)/Gd(III) observed in XANES. Taken together with the results of TGA and TGMS, we postulate that the decrease in coordination number may be due to removal of the coordinated water molecules from the Dy(III)/Gd(III) ions. We note that a quantitative calculation of the coordination number of the Dy(III)/Gd(III) ions in 1′ and 2′, respectively, was not feasible due to the complexity of the coordination environment of the Dy(III)/ F


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Inorganic Chemistry

the χMT value increases for 2 to 16.65 cm3 K mol−1 and decreases for 2′ to 10.73 cm3 K mol−1 at 2 K. The temperatureindependent nature of the magnetic susceptibility from 300 to 30 K for both 2 and 2′ signifies that both Gd(III) ions behave like paramagnets. The increase in χMT value for 2 and decrease in χMT value for 2′ are attributed to the ferromagnetic and antiferromagnetic exchange coupling between the Gd(III) dimeric repeating units, respectively. The presence of dominant ferro- and antiferromagnetic interactions in 2 and 2′, respectively, is further supported by the positive (Θ = +0.032 K) and negative (Θ = −0.28 K) Curie−Weiss constant (Figure S3 in the Supporting Information). On the basis of the crystal structure of 2, the exchange interaction between the Gd(III) centers must be mediated through (1) fumarate ligand and (2) oxalate ligand. However, it is observed that the M···M distances via oxalate and fumarate are 6.298(1) and 4.582(2) Å, respectively. Since the Gd(III)···Gd(III) distance is significantly larger via oxalate (compared to fumarate), we consider that the exchange interaction between the metal centers is mainly dominated by the fumarate ligand only. On the basis of this assumption, the magnetic data of the isotropic 2 and 2′ were modeled by using the following regular chain model with the spins SGd = 7/2 using the Langevin function28,33

masked due to the depopulation effect. Below 75 K, χMT drops drastically and reached a final value of 18.69 cm3 K mol−1 at 2.0 K. This could be due to the combined effect of magnetic anisotropy, intermolecular antiferromagnetic interaction, dipolar interaction, etc. Like 1, 1′ shows nearly similar temperaturedependent magnetic behavior between 2 and 300 K, but the rate at which the depopulation effect is observed in 1′ is larger than that in 1 (Figure 6A). In contrast to the case for 1 or 1′, 2 and 2′ χMT values remains unchanged from 300 to 30 K; below this temperature

χM =

Ng 2β 2S(S + 1) ⎛ 1 + u ⎞ ⎜ ⎟ ⎝1 − u ⎠ 3kBT

and ⎤ ⎡ J S(S + 1) ⎤ ⎡ k T B ⎥ ⎥−⎢ u = coth⎢ 1 ⎦ ⎣⎢ J1S(S + 1) ⎥⎦ ⎣ kBT

where N, g, β, kB, and T have their usual meanings and J1 is the exchange interaction between the neighboring spins. The Hamiltonian is represented in eq 1. ∞

Ĥ = −J1 ∑ Sî ·Sî + 1 i=1

(1)

The parameters are extracted from fitting compiled in Table 1. Very weak exchange interactions between the Gd(III) centers were witnessed by looking at the exchange interaction values extracted for 2 (J/kB = +0.012 K) and 2′ (J/kB = −0.028 K), respectively. The change in nature of exchange interaction from ferro (in 2) to antiferro (in 2′) interaction is perhaps due to the change in the electronic structure and geometry of the Gd(III) ion upon removal of coordinated terminal water molecules from 2. Isothermal field dependent magnetization measurements were performed on a polycrystalline samples of 1, 1′, 2, and 2′ up to 70 kOe (Figure 6 and Figures S4 and S5 in the Supporting Information). It is been observed that both 1 and 1′ and similarly 2 and 2′ show similar M(H) curves; therefore, only a representative example is discussed below. Complexes 1 and 2 both show a sharp increase in the magnetic moment as soon as the external magnetic field is turned on, implying that nondiamagnetic ground state levels are populated. On a further sweep of the magnetic field, 1 does not show any sign of saturation of magnetic moment and exhibits a simple linear response to the external magnetic field. The magnetic moment reaches a value of 11.59 NμB at 70 kOe, which is significantly lower than the expected magnetic moment value (20 NμB) for 1. Such behavior is not uncommon for complexes containing anisotropic ions such as Dy(III). The presence of magnetic

Figure 6. Temperature-dependent direct current magnetic susceptibility measurements performed on polycrystalline samples of 1, 1′ (A) and 2, 2′ (B and C, respectively) measured at 1 kOe. Isothermal field dependent magnetization measurements were performed on polycrystalline samples of 1 and 1′ (inset in (A)) measured from 0 to 70 kOe at 2.0 K. The solid red trace in (A) represents the simulation of experimental magnetic data using the computed (CASSCF+RASSI) magnetic susceptibility and magnetization from the crystal structure of complex 1. The solid red lines in (B) and (C) represent the best fit obtained for the experimental data using the parameters described in the main text and Table 1. G


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Inorganic Chemistry

Figure 7. Field induced frequency-dependent in-phase (χM′) (A) and out-of-phase (χM″) (B) ac susceptibility measurements performed on a polycrystalline sample of 1 at the indicated temperatures. (C, D) Cole−Cole plot and Arrhenius plot of 1 derived from the ac relaxation dynamics for various relaxation processes. The red solid traces in (C) and red and magenta traces in (D) represent the best fits obtained for all of the experimental data points using the parameters described in the main text.

Figure 8. Field induced frequency-dependent in-phase (χM′) (A) and out-of-phase (χM″) (B) ac susceptibility measurements performed on a polycrystalline sample of 1′ at the indicated temperatures. (C, D) Cole−Cole plot and Arrhenius plot of 1′ derived from the ac relaxation dynamics for various relaxation processes. The red solid traces in (C) and red and magenta traces in (D) represent the best fits obtained for all the experimental data points using the parameters described in the main text.

anisotropy is further confirmed by the nonsuperimposable nature of reduced magnetization curves for compounds 1 and

1′ (Figures S4C,D in the Supporting Information). In contrast to 1, 2 shows saturation of the magnetic moment value of 14.29 H


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Inorganic Chemistry ⎛ −U ⎞ 1 1 = + CT n + τ0−1 exp⎜ eff ⎟ τ τQTM ⎝ KBT ⎠

NμB (2.0 K) at 70 kOe, which is in close agreement with the expected magnetic moment (Figures S5 in the Supporting Information). Magnetization Relaxation Dynamics Investigation of 1 and 1′. In order to understand slow relaxation magnetization behavior and to investigate the influence of the guest molecule in the cavity on the magnetization dynamics, ac magnetic susceptibility measurements were performed for both 1 and 1′ in the presence of 3.5 Oe oscillating ac field. Neither 1 nor 1′ shows a frequency-dependent out-of-phase susceptibility signal (χM″) in the absence of an external magnetic field. This suggests that an under barrier tunneling mechanism is much more dominant through the ground state mj level than the temperature-dependent Orbach process. In order to find out the optimum external magnetic field where quantum tunneling of magnetization (QTM) is quenched/suppressed significantly, field sweep ac susceptibility measurements were performed on polycrystalline and amorphous samples of both 1 and 1′, respectively (Figures S6 in the Supporting Information). The plot of χM″ as a function of frequency of both 1 and 1′ markedly reveals that more than one temperature-dependent relaxation process exists. This is well corroborated by the Cole−Cole plot of 1 and 1′, where unmistakably the asymmetry in the χM″= f(χM′) curve can be discerned (Figure 7). The Cole−Cole curves were fitted34 by considering two different relaxation process using the modified Debye equation given by eq 2 χac (ω) = χS + χS + 1

2

χT − χS 1

1

1 + (iωτ1)1 − α1

+

χT − χS 2

(3)

The parameters extracted from linear and nonlinear fits of Arrhenius curves of the relaxation processes of complexes 1 and 1′ are shown in Table 2. From Table 2 it is evident that the Table 2. Parameters Extracted from Arrhenius Plots for 1 and 1′ Raman compound 1 (slow relaxation) 1 (fast relaxation) 1′ (slow relaxation) 1′ (fast relaxation)

Ueff (cm−1)

τ0 (s)

C (s−1 K−n)

n

QTM (τQTM) (s)

53.52

1.85 × 10−6

0.0471

3.32

0.152

7.12

1.28 × 10−4

45.18

2.5 × 10−5

0.33657

2.28

0.114

6.45

−4

2.2 × 10

anisotropic barrier extracted for 1′ is smaller than that for 1. This is conceivable by considering a reduced coordination number of 6 from 8 upon removal of coordinated water. It is been already very well witnessed in the literature that a sixcoordinate Dy(III) complex is not ideally suited to design single-molecule magnets, which has been elegantly exemplified by both experimental and theoretical research groups.37 Computational Calculations for Complex 1. To understand the origin of relaxation processes in 1 and the plausible relaxation mechanism of the magnetization and the electronic structure of the complex, detailed ab initio calculations were performed. Calculations were performed on an individual metal fragment of the repeating dimeric unit in 1 where the neighboring lanthanide ion was replaced by a closed-shell Lu(III) ion without altering any other structural parameters. As pointed out earlier, both Dy(III) ions in the repeating dimeric unit are symmetrically related to each other; hence for the sake of clarity, the Dy(III) ion present in the asymmetric unit of 1 is labeled Dy1 and its symmetrically generated ion bridged by an oxalate ligand is labeled Dy1A. Initially the local magnetic properties and plausible relaxation mechanism of the individual metal centers (Dy1 and Dy1A) will be discussed; subsequently the exchange-coupled energy spectrum of these ions will be described. Results of ab initio calculations reveal that both Dy1 and Dy1A are strongly axial but are not Ising in nature. This is clearly visible from its g tensors (Table 3 and also Tables S5 and S6 in the Supporting Information) computed for the ground Kramers state (KD) level. The ground state g tensors of both Dy1 (gxx = 0.004, gyy = 0.008, gzz = 19.84) and Dy1A (gxx = 0.006, gyy = 0.01, gzz = 19.75) are similar to each other, reiterating that both Dy(III) ions are crystallographically equivalent. The transverse component gradually increases for all of the KDs up to third excited state for Dy1 and Dy1A, beyond which the transverse component decreases further and reaches axiality again for the eighth KD. The observation of such mirror symmetry in KDs of both Dy1 and Dy1A is in absolute contrast to purely axial systems (see Tables S5 and S6 in the Supporting Information). The relative energies computed for all the KDs of Dy1 and Dy1A are given in Tables S5 and S6, which span 0− 571.23 and 0−559.76 cm−1, respectively. The principal

2

1 + (iωτ2)1 − α2 (2)

The parameters extracted by fitting the curves of both complexes are compiled in Tables S3−S4 in the Supporting Information. Tables S3 and S4 reveal that the α value ranges 0.19−0.65 (α1) and 0.11−0.53 (α2) for 1 and 0.18−0.89 (α1) and 0.22−0.55 (α2) for 1′ suggest a broad distribution of relaxation. Literature precedents are known already where multiple relaxation processes are witnessed even for a mononuclear Dy(III) ion. For example, a monomeric [DyIII(DOTA)] complex exhibits Raman and Direct processes apart from Orbach process, as reported by Sessoli and coworkers.35 In addition, the interaction beyond the first coordination sphere (H-bonding and other supramolecular interactions) around the Dy(III) ion possesses non-negligible contribution to dictating the easy axis of magnetization orientation, as elegantly shown by Sessoli and co-workers.36 The two different relaxation processes (fast relaxation (τ1) and slow relaxation (τ2)) with distinctly different relaxation times extracted by fitting the Cole−Cole plots of 1 and 1′ was used to construct the Arrhenius plot (ln τ = f(1/T)) for both complexes (Figures 7 and 8). In both complexes, the fast relaxation process (ln τ = f(1/T) curve) observed shows a linear dependence in the temperature range measured. In contrast, the slow relaxation process observed in both 1 and 1′ shows a nonlinear behavior of the ln τ = f(1/T) curve below 5 and 6 K, respectively, implying that other relaxation processes are still active, in addition to the temperaturedependent Orbach process, even in the presence of an optimum external magnetic field. The nonlinear ln τ = f(1/ T) curve was fitted in the entire temperature range by considering QTM, Raman, and Orbach processes which are respectively represented on the right-hand side of eq 3 I


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Inorganic Chemistry Table 3. Energy and Computed g Tensors for the Ground and First Excited Kramers State (KD) of Dy1 and Dy1A sites of 1a Dy1

gxx gyy gzz energy (cm−1) θ (deg)

Dy1A

ground state KD

1st excited KD

ground state KD

1st excited KD

0.004 0.008 19.84 0

0.06 0.1 17.44 113.29

0.006 0.01 19.75 0

0.06 0.1 17.46 109.01

0

18.47

0

19.32

θ represents the angle between the gzz orientation of the ground and the first excited KD. a

magnetization axes of the lowest energy Kramers doublets for both Dy(III) ions are shown in Figure 9, which undisputedly shows that the gzz orientation of the ground state KD of both Dy1 and Dy1A is in the opposite direction. To shed light on the plausible mechanism of relaxation in Dy1 and Dy1A, the computed data need to be analyzed beyond ground state KDs on each site. In general, magnetization relaxation occurs via three major pathways in the absence of intermolecular interactions: (1) magnetization reversal via ground state QTM due to the presence of transverse g tensors, (2) Orbach/Raman processes via the excited KDs due to the noncollinear orientation of the gzz axis in comparison to the ground state KD orientation, and (3) thermally assisted QTM (TA-QTM) via the excited state KDs due to their non-Ising nature. The plausible magnetization relaxation mechanism is shown in Figure 9 for both Dy1 and Dy1A sites. A detailed analysis of the ground state KD of Dy1 and Dy1A suggests that QTM is comparable in Dy1 (0.2 × 10−2 μB) and Dy1A (0.3 × 10−2 μB); however, the observed tunneling probability is relatively large for both single-ion sites in 1. This is also evidently reflected in the g tensors computed for both Dy1 and Dy1A. Therefore, 1 is not expected to show zero field SMM behavior, as tunneling via ground state KD is observed to be the predominant pathway. This prediction is consistent with the experimental observation that 1 does not show χM″ signals in the absence of an external magnetic field. It has already been witnessed in the literature that, by enhancing the exchange interaction in lanthanide-containing complexes, QTM via ground state mj levels can be effectively quenched, which enforces the magnetization vector relaxation via higher excited mj levels.2a,b,13,14 Such an approach certainly facilitates an increase in the blocking temperature to 14 K in a Tb(III) dimer bridged by an N23− radical ligand.2a The absence of zero field SMM behavior in 1 implies that the exchange interactions between the repeating Dy(III) dimeric units are likely not strong; in other words, the weak exchange interaction is likely to open up the under-barrier relaxation mechanism effectively. Application of an external bias field in 1 enables quenching of the ground state QTM to some extent and hence slow relaxation of the magnetization observed. The analysis of the local g tensors of various KDs of both Dy1 and Dy1A reveal the presence of spin−phonon relaxation, TA-QTM (due to transverse component of g tensors), and noncollinearity of the gzz orientation of the first excited KD with respect to the ground state KD suggests that the magnetization vectors relax back to the ground state mj level via the first excited KD.

Figure 9. SINGLE_ANISO computed blocking barrier of magnetization reversal for (A) Dy1 and (B) Dy1A metal centers in complex 1. Blue/green dotted lines represent the Orbach and Raman processes. Red dotted lines represent QTM/TA-QTM. (C) Ball and stick representation of complex 1 showing the SINGLE_ANISO computed direction of the gzz tensor of the ground state. The values in (C) represent the charge densities observed on the coordinated atoms.

Therefore, the energy gap between the ground state mj level and that of the first excited KD of Dy1 (Ucal = 113.29 cm−1) and Dy1A (Ucal = 109.01 cm−1) is the computed anisotropic barrier. The computed energy barrier is overestimated in comparison to the experimental effective energy barrier, which could be likely due to the nonincorporation of intermolecular interaction and hyperfine interactions etc. A similar scenario has J


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Inorganic Chemistry been witnessed in several other cases reported in the literature.6c,38 Since both Dy1 and Dy1A possess similar computed energy barriers, both metal ions are expected to follow similar relaxation mechanisms (Figure 9); therefore, the origin of the second relaxation is likely due to the different microenvironment generated by solvate water molecules involved in hydrogen bonding with ligand fragments that are coordinated to the metal centers. The influence of such supramolecular interaction in the gz orientation of monomeric Dy(III) complexes has already been witnessed in the literature.35 We have also computed the crystal field parameters to better understand the relaxation dynamics (see Tables S7 and S8 in the Supporting Information) for Dy1 and Dy1A. The corresponding crystal field Hamiltonian can be given by the eq 4, where Bqk is the crystal field parameter and Õ qk is the Stevens operator. q

Ĥ CF =

∑∑ k =−q

Figure 10. Exchange-coupled energy spectrum of 1 constructed using POLY_ANISO. The computed exchange states are placed on the diagram according to their magnetic moments (bold black lines). The red arrows show the tunneling transitions (Δtun) within each doublet state. The blue arrows show the possible relaxation pathways through Orbach relaxation.

q Bkq Õk

(4)

The axial terms (Bqk where q = 0, k = 2, 4, 6) are large and negative in comparison to the nonaxial terms, indicative of uniaxial anisotropy associated with that. This is also clearly reflected in the g tensor of a ground state KD of Dy1 and Dy1A (see Tables S7 and S8 in the Supporting Information). In order to shed light on the coupled electronic structure on the magnetization relaxation dynamics of 1, exchange interaction between the Dy(III) ions within the dimeric unit was computed within the Lines model,26 using the POLY_ANISO routine.27 The exchange-coupled anisotropy barrier was computed using the lowest energy state of individual metal fragments (Dy1 and Dy1A) and their corresponding wave functions. Employing the exchange coupled energy spectrum and the wave functions of the polynuclear complex, temperature and field-dependent magnetic properties were computed. Reasonably good agreement between the simulated and experimental magnetic data (χMT(T) and M(H))) was observed by considering JDy−Dy = −0.028 cm−1 and zJ = −0.040 cm−1 (Figure 6A and see also Figure S7 in the Supporting Information), revealing the reliability of the computed parameters. The energies of lowest exchange coupled states, gzz values, and corresponding tunneling gaps are given in Table S9 in the Supporting Information. The computed exchange coupling values are very weak and antiferromagnetic in nature, which eventually leads to a singlet ground state with several excited states close to the ground state (see Figure 10). Under this circumstance, the complex is not expected to show zero field SIM behavior. This prediction is in line with the experimental observation, as there is no out-of-phase signal in the absence of external magnetic field. On the other hand, if the strength of an applied dc magnetic field exceeds the magnitude of the computed exchange coupling value (which is the case in 1), slow relaxation of magnetization can be expected from 1, which is of single-ion origin. In addition, the applied external magnetic field will facilitate in quenching the QTM within the ground Kramers state. As anticipated, experimentally, 1 indeed shows slow relaxation of magnetization in the presence of external magnetic field.39 Estimation of Magnetocaloric Effect (MCE). Detailed field dependent magnetization data were collected between 2 and 15 K for the isotropic complexes 2 and 2′ to extract the MCE efficiency (Figure 11). The change in magnetic entropy

(−ΔSm) of 2 and 2′ was estimated from a thermodynamic Maxwell equation which is given in eq 5 −ΔSm(T , H ) =

Hf

∫H

i

⎡ ∂M(T , H ) ⎤ ⎢ ⎥ dH ⎣ ⎦H ∂T

(5)

Hi and Hf denote the initial and final applied magnetic fields, respectively. Using the detailed magnetization measurements, −ΔSm of 2 is estimated to be 41.6 J kg−1 K−1 at 3.0 K for ΔH = 70 kOe (Figure 11C). Theoretically, the full entropy content calculated per mole of Gd(III) involved is R ln 8 = 17.3 J mol−1 K−1 = 44.61 J kg−1 K−1, which is expected from R ln(2S + 1) and S = 7/2 (where R is the universal gas constant). From the detailed isothermal field dependent magnetization data collected for 2′ from 2 to 15 K the change in magnetic entropy (−ΔSm) was estimated. It is observed that the −ΔSm value of 2′ increases with increasing ΔH upon a decrease in temperature. The −ΔSm value of 2′ increased by 26% upon removal of the eight passively contributing water molecules (four lattice and four coordinated water molecules) in comparison to 2: i.e., 52.48 J kg−1 K−1 at 3.0 K for ΔH = 70 kOe (Figure 11D). Although smaller linkers are excellent for enhancing the exchange interaction, they can mediate strong exchange interactions, which could potentially affect the MCE efficiency. Tong and co-workers elegantly showed this recently in manganese glycolate extended structures.40 Nevertheless, the Gd···Gd distance in 2′ is likely significantly larger (due to larger linker) and the weak AF interaction is likely to have little influence on −ΔSm value. In addition, we observed that the −ΔSm value of 2′ surpassed the values reported for the majority of the discrete 3d−4f- and/or 4fbased magnetic coolant molecules.41 Further, the −ΔSm value of 2′ is comparable to the values reported for some of the densest materials of 3D PCPs and some of the discrete gadolinium complexes (Table S10 in the Supporting Information). As mentioned earlier, ideally the magnetic density (metal/ ligand ratio) should be high for better MCE efficiency; in other words the organic linkers or diamagnetic constituents contribute passively to MCE, which will reduce the cooling efficiency. Tong and co-workers20 and Evangelisti and coK


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Inorganic Chemistry

Figure 11. Change in magnetic entropy (−ΔSm) estimated from magnetization measurement for 2 (A) and 2′ (B) at the indicated ΔH external magnetic field between 2 and 15 K. The red lines are the simulated patterns of the experimental data of 2 and 2′.

were satisfactorily substantiated by various experimental techniques (TGA, TG-MS, EXAFS, XANES, PXRD, and elemental analysis). Magnetization relaxation dynamics performed on 1 and 1′ show two different frequency-dependent out-of-phase susceptibility signals (slow relaxation and fast relaxation processes) in the presence of an external magnetic field. The SINGLE_ANISO calculation performed on the individual Dy(III) fragment reveals a large tunneling probability associated with the ground Kramers state, which is responsible for the absence of zero field SIM behavior in 1. Further, POLY_ANISO calculations suggest that a weak antiferromagnetic coupling between the Dy(III) ions lead to a singlet ground state with various excited states close to the ground state of 1. This scenario triggers the under-barrier relaxation mechanism and rationalizes the absence of zero field SIM behavior in 1. Magnetocaloric effect (MCE) efficiency was measured on the isotropic analogues of 1 in its crystalline and amorphous phases (2 and 2′). The MCE efficiency of 2′ (52.48 J kg−1 K−1 at 3.0 K for ΔH = 70 kOe) is 26% larger than that for 2 (41.61 J kg−1 K−1 at 3.0 K for ΔH = 70 kOe), discloses a new synthetic strategy. This approach is distinctly different from the other proposed methods in the literature to increase MCE efficiency: i.e., removal of atoms or molecules that passively contribute to MCE. This new finding will pave the way to reveal new generation of magnetic coolants with improved MCE efficiency.

workers have been elegantly shown this phenomenon in unrelated reports.17d The largest MCE value reported for a dense [GdF3]n PCP (71.6 J kg1− K−1 (ΔH = 70 kOe)), where all the Gd(III) centers are linked through the lightest and smallest fluoride linker, which is responsible for the unprecedented magnetic density observed among the MCE materials reported so far. Since the size of the linker reached its limit already, the present work indeed shows an alternative strategy to improve the MCE efficiency of isotropic metal complexes containing a 3D network. Although 2′ still possesses other passively contributing diamagnetic linkers (fumarate and oxalate), enhancing MCE efficiency through amorphization (removal of passively contributing terminal neutral ligands coordinated to the metal along with solvent molecules in the crystal lattice) appears to be a promising strategy and presents an alternate approach to address the problem of finding linkers smaller and lighter than fluoride.

■

CONCLUSION To conclude, employing fumarate (L1) and oxalate as pillared ligands, two three-dimensional PCPs based on lanthanide ions (Dy(III) and Gd(III)) were isolated as single crystals and their crystal structures were characterized by single-crystal X-ray diffraction. Single-crystal X-ray diffraction reveals the molecular formula of the complexes as [Ln2(L1)2(H2O)4(ox)]n·4nH2O (where Ln = Dy (1), Gd (2)). Consistent with the same unit cell parameters obtained for both complexes, they are structurally analogous to each other. The loss of solvent water molecules in the crystal lattice and the coordinated water molecules on the respective lanthanide ions results in an amorphous phase of 1 and 2 upon activation of PCPs. The amorphous phase molecular formula [Ln2(L1)2(ox)]n (where Ln = Dy (1′), Gd (2′)) and the stability of the 3D network

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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.8b00720. L


Article

Inorganic Chemistry

(3) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular magnetic hysteresis at 60 K in dysprosocenium. Nature 2017, 548 (7668), 439. (4) Gupta, S. K.; Rajeshkumar, T.; Rajaraman, G.; Murugavel, R. An air-stable Dy(III) single-ion magnet with high anisotropy barrier and blocking temperature. Chem. Sci. 2016, 7 (8), 5181. (5) Rinehart, J. D.; Long, J. R. Exploiting single-ion anisotropy in the design of f-element single-molecule magnets. Chem. Sci. 2011, 2 (11), 2078. (6) (a) Chen, Y.-C.; Liu, J.-L.; Ungur, L.; Liu, J.; Li, Q.-W.; Wang, L.F.; Ni, Z.-P.; Chibotaru, L. F.; Chen, X.-M.; Tong, M.-L. SymmetrySupported Magnetic Blocking at 20 K in Pentagonal Bipyramidal Dy(III) Single-Ion Magnets. J. Am. Chem. Soc. 2016, 138 (8), 2829. (b) Liu, J.; Chen, Y.-C.; Liu, J.-L.; Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer, W.; Gao, S.; Chen, X.-M.; Tong, M.-L. A Stable Pentagonal Bipyramidal Dy(III) Single-Ion Magnet with a Record Magnetization Reversal Barrier over 1000 K. J. Am. Chem. Soc. 2016, 138 (16), 5441. (c) Upadhyay, A.; Vignesh, K. R.; Das, C.; Singh, S. K.; Rajaraman, G.; Shanmugam, M. Influence of the Ligand Field on the Slow Relaxation of Magnetization of Unsymmetrical Monomeric Lanthanide Complexes: Synthesis and Theoretical Studies. Inorg. Chem. 2017, 56 (22), 14260. (d) Das, C.; Upadhyay, A.; Vaidya, S.; Singh, S. K.; Rajaraman, G.; Shanmugam, M. Origin of SMM behaviour in an asymmetric Er(III) Schiff base complex: a combined experimental and theoretical study. Chem. Commun. 2015, 51 (28), 6137. (7) (a) Aquilante, F.; Autschbach, J.; Carlson, R. K.; Chibotaru, L. F.; Delcey, M. G.; De Vico, L.; Fdez. Galvan, I.; Ferre, N.; Frutos, L. M.; Gagliardi, L.; Garavelli, M.; Giussani, A.; Hoyer, C. E.; Li Manni, G.; Lischka, H.; Ma, D.; Malmqvist, P. Å.; Mueller, T.; Nenov, A.; Olivucci, M.; Pedersen, T. B.; Peng, D.; Plasser, F.; Pritchard, B.; Reiher, M.; Rivalta, I.; Schapiro, I.; Segarra-Marti, J.; Stenrup, M.; Truhlar, D. G.; Ungur, L.; Valentini, A.; Vancoillie, S.; Veryazov, V.; Vysotskiy, V. P.; Weingart, O.; Zapata, F.; Lindh, R. Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table. J. Comput. Chem. 2016, 37 (5), 506. (b) Aquilante, F.; Pedersen, T. B.; Veryazov, V.; Lindh, R. MOLCAS-a software for multiconfigurational quantum chemistry calculations. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2013, 3 (2), 143. (8) (a) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.-Y.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125 (29), 8694. (b) Ishikawa, N.; Sugita, M.; Wernsdorfer, W. Nuclear Spin Driven Quantum Tunneling of Magnetization in a New Lanthanide SingleMolecule Magnet: Bis(Phthalocyaninato)holmium Anion. J. Am. Chem. Soc. 2005, 127 (11), 3650. (c) Ishikawa, N.; Sugita, M.; Wernsdorfer, W. Quantum tunneling of magnetization in lanthanide single-molecule magnets: Bis(phthalocyaninato)terbium and bis(phthalocyaninato)dysprosium anions. Angew. Chem., Int. Ed. 2005, 44 (19), 2931. (9) Gatteschi, D.; Sessoli, R. Quantum tunneling of magnetization and related phenomena in molecular materials. Angew. Chem., Int. Ed. 2003, 42 (3), 268. (10) Fukuda, T.; Matsumura, K.; Ishikawa, N. Influence of Intramolecular f-f Interactions on Nuclear Spin Driven Quantum Tunneling of Magnetizations in Quadruple-Decker Phthalocyanine Complexes Containing Two Terbium or Dysprosium Magnetic Centers. J. Phys. Chem. A 2013, 117 (40), 10447. (11) Pointillart, F.; Bernot, K.; Golhen, S.; Le Guennic, B.; Guizouarn, T.; Ouahab, L.; Cador, O. Magnetic Memory in an Isotopically Enriched and Magnetically Isolated Mononuclear Dysprosium Complex. Angew. Chem., Int. Ed. 2015, 54 (5), 1504. (12) (a) Jiang, S.-D.; Wang, B.-W.; Su, G.; Wang, Z.-M.; Gao, S. A mononuclear dysprosium complex featuring single-molecule-magnet behavior. Angew. Chem., Int. Ed. 2010, 49 (41), 7448. (b) Habib, F.; Lin, P.-H.; Long, J.; Korobkov, I.; Wernsdorfer, W.; Murugesu, M. The use of magnetic dilution to elucidate the slow magnetic relaxation effects of a Dy2 single-molecule magnet. J. Am. Chem. Soc. 2011, 133 (23), 8830. (c) Meihaus, K. R.; Long, J. R. Magnetic Blocking at 10 K

Crystallographic parameters, TGA analysis, TG-MS data, supporting dc and ac magnetic data, computed crystal field parameters, and energies of the computed KDs of SINGLE_ANISO and POLY_ANISO (PDF) Accession Codes

CCDC 1830577 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.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for S.H.: [email protected]. *E-mail for M.S.: [email protected]. ORCID

Satoshi Horike: 0000-0001-8530-6364 Maheswaran Shanmugam: 0000-0002-9012-743X Present Address ⊥

Department of Chemistry Texas A University, College Station, Texas 77843, USA

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S. thanks the DST-SERB (EMR/2015/000592), IIT Bombay, for financial assistance. M.S. and S.H. extend their special thanks to the Indo-Japan bilateral research programme (DST/INT/JSPS/P-171/2014) and “Aichi Synchrotron BL5S1 beamline for XAS”.

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