Series of Highly Stable Lanthanide-Organic Frameworks Constructed

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Series of Highly Stable Lanthanide-Organic Frameworks Constructed by a Bifunctional Linker: Synthesis, Crystal Structures, and Magnetic and Luminescence Properties Dong Peng,†,⊥ Lei Yin,†,⊥ Peng Hu,‡ Bao Li,*,‡ Zhong-Wen Ouyang,*,† Gui-Lin Zhuang,*,§ and Zhenxing Wang*,† †

Wuhan National High Magnetic Field Center & School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ‡ Key laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China § Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310023, People’s Republic of China S Supporting Information *

ABSTRACT: By utilizing a preselected functional ligand produced by 1H-imidazole-4,5-dicarboxylic acid, three isostructural lanthanide coordination polymers (CPs), denoted as {[Ln2(OH)2(L)2]·(DMF)·(H2O)4}n (Ln = Gd (1), Eu (2), Dy (3); L = 1(4-carboxybenzyl)imidazole-4-carboxylic acid), containing a 1D infinite [Ln4(OH)4] subchain have been successfully constructed. The highly connected mode between the multifunctional ligand and 1D building units is responsible for the exceptional chemical stability of three lanthanide CPs. In addition, a study of the magnetic properties reveals that 1 displays a large magnetic entropy change (−ΔSm = 30.33 J kg−1 K−1 with T = 2 K and ΔH = 7 T). Furthermore, genetic algorithm and quantum Monte Carlo methods were combined to simulate the magnetic coupling parameters of compound 1, shedding light on the effect of linking bridges on magnetic propagation. 2 shows intense luminescence in the range of 350−710 nm. Comparably, magnetic studies of 3 reveal the existence of a metamagnetic transformation from an antiferromagnetic interaction to a ferromagnetic interaction along with a decrease in temperature. Through fitting of the results of HF-EPR measurements, a component of the g tensor is obtained, g|| = 16.4(5), indicating the large anisotropy of 3.

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molecular magnetism,6 magnetic resonance imaging,7 luminescent sensing,8 etc. Although a number of Ln-CPs have been well-documented to date, the oriented syntheses of expected Ln materials are still an arduous challenge due to the subtle synthesis conditions. The final structures could be affected not only by the different starting materials and ratios but also by the variable reaction conditions including pH value, temperature, reaction solvents, pressure, etc.9 Comparably, the reasonable selection of linkers as building blocks has been seen as one

INTRODUCTION Recently, metal−organic frameworks (MOFs) or coordination polymers (CPs), usually built with repeated metal ions and linkers connected via coordination bonds, have expanded rapidly in the past two decades because of their interesting structures and variable applications as the storage of energy molecules (H2, CH4),1 selective separation,2 catalysis,3 and sensing.4 In comparison with the transition metal, reports on lanthanide (Ln)-based CPs are relatively limited.5 Due to the unique 4f electronic configurations of Ln ions, they normally exhibit high and versatile coordination numbers, large magnetic moments, and strong spin−orbital coupling originating from them, which endows their meaningful potential applications in © XXXX American Chemical Society

Received: November 22, 2017

A

DOI: 10.1021/acs.inorgchem.7b02969 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry convenient way to design Ln-CPs exhibiting the anticipated topological structures and properties. The phenomenon of the magnetocaloric effect (MCE) was reported in metallic iron by Warburg in 1881.10 Recently, CPbased MCE materials have been persistently investigated and have been attracting great attention in the last 5 years due to their potential applications as magnetic refrigerant materials.11 MCE has been termed as one sort of phenomenon of magneto thermodynamics, and the relationship between the change in adiabatic temperature (ΔTad) and entropy (ΔSM) in a varying magnetic field has been characterized. Normally, the molecular refrigerant must possess several factors, such as low-lying excited spin states, negligible magnetic anisotropy, large spin ground state S, high magnetic density, and dominant ferromagnetic interaction.12 Directed by these essential elements, most of the reported models have been concentrated on versatile oligo-cluster 1D or 2D coordination complexes. However, investigations of 3D MCE materials, especially LnCPs, have rarely been documented.11 Herein, we report a series of Ln-CPs, {[Ln2(OH)2(L)2]· (DMF)·(H2O)4}n (Ln = Gd (1), Eu (2), Dy (3)), constructed by a dicarboxylate ligand, 1-(4-carboxybenzyl)imidazole-4carboxylic acid (L). The utilization of the bifunctional derivative was based on the following two considerations. (1) The benzoate group was decorated onto the imidazoledicarboxylate template to increase the connection sites, which is very important for constructing a highly stable coordination network with lanthanide ions. (2) The decorated extended group and N−O chelating site tend to construct 3D coordination networks.13 Consistent with our assumption, a series of Ln coordination complexes has been constructed, and their synthesis, crystal structures, analysis of topology, corresponding magnetic properties, and luminescent properties have been described in detail. In addition, high-field EPR as a new effective measurement for magnetic complexes14 and theoretical calculation of the magnetic properties of the Gd complex have been carried out to present further investigation on the structure−activity relationship.

Figure 1. (a) View of the asymmetric unit in 1 with atom labeling. (b) View of the 1D Gd-hydroxyl ladderlike chain along the a axis direction. (c) View of the octacoordination environment of two asymmetric Gd ions (asymmetric code: (A) x, −y + 1, z + 1/2; (B) −x + 1, −y + 1, −z + 2; (C) x, y, z + 1; (D) −x + 1, y, −z + 5/2; (E) −x + 1, −y + 1, −z + 2; (F) x + 1/2, −y + 3/2, z + 3/2; (G) x + 1/2, y − 1/2, z + 2). (d) Partial perspective view of the 3D packing structure of 1 along the c axis direction.

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RESULTS AND DISCUSSION Crystal Structure of 1. The isostructures of 1 and 2 could be determined by single-crystal and powder X-ray diffraction studies, and herein the structural characteristics of 1 have been described as the model. The monoclinic space group C2/c is assigned for complex 1, and the asymmetric unit contains one gadolinium ion, one ligand generated in situ, and one μ3hydroxyl group (Figure 1a and Table 1). The central Gd(III) ion is octacoordinated and exhibits a slightly distorted squareantiprismatic coordination geometry, which consists of three OH groups, four carboxyl groups from different ligands generated in situ, and one N atom from the imidazole ring, located on the centrosymmetric positions. The distances between each of two square planes and the central Gd(1) ion are 1.377 and 1.342 Å, and the dihedral angle of the two square planes is 8.25°. The typical N−O chelating mode has been observed around the coordination environments of the central Gd atom. The Gd−N and −O bond lengths fall in the range of 2.347(2)−2.505(1) Å, similar to the values of previously reported Gd(III) polymers. Each μ3-OH group ligates three Gd ions to form the typical trinuclear node and is further elongated to a 1D Gd-hydroxyl ladderlike chain by sharing the edges of adjacent triangles along the crystallographic c axis (Figure 1b). The predesigned imidazole tricarboxylic ligand generated in situ

Table 1. Crystallographic Data and Structural Refinement Details for 1−3 formula formula mass cryst syst a/Å b/Å c/Å β/deg V/Å3 space group Z Rint R1 (I > 2σ(I)) wR2(F2) (I > 2σ(I)) R1 (all data) wR2(F2) (all data)

1

2

3

C12H8GdN2O5 417.45 monoclinic 23.4807(6) 18.6160(5) 7.4167(2) 102.729(10) 3162.29(15) C2/c 8 0.0166 0.0435 0.1282

C12H9EuN2O5 413.18 monoclinic 23.543(2) 18.6543(16) 7.4513(6) 102.609(10) 3193.5(5) C2/c 8 0.0167 0.0158 0.0502

C24H18Dy2N4O10 847.42 monoclinic 7.6140(15) 19.274(4) 23.581(5) 95.24(3) 3446.1(12) P21/c 4 0.1119 0.0727 0.2171

0.0467 0.1305

0.0176 0.0525

0.0765 0.2234

a new dicarboxylic ligand. The high reaction temperature along with the catalytic role of central metal ions must be responsible for the resulting new ligands, which links four Gd ions from two B

DOI: 10.1021/acs.inorgchem.7b02969 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

planes of the Dy(2)III ion consist of O1, O9, O9C, O10 and O5, N3C, O8C, O3D, respectively. The typical N−O chelate mode is presented in each Dy environment. The distances between the two square planes and the central Dy(2)III ion are 1.402 and 1.367 Å, and the dihedral angle of the two square planes is 8.19°. The Dy−N and −O bond lengths fall in the range of 2.381(2)−2.554(1) Å, similar to the values of previously reported DyIII polymers. Each μ3-OH group ligates three Dy ions to form the typical trinuclear node and further is elongated to a 1D Dy-hydroxyl ladderlike chain by sharing the edges of adjacent triangles along the crystallographic a axis (Figure 2b). The new dicarboxylic ligand was generated in situ from the predesigned imidazole tricarboxylic ligand, which links four Dy ions from two 1D chains by utilizing the syn-syn benzoate and imidazolecarboxylic acid. The dihedral angles of the two types of ligands are 79.99(2) and 75.85(2)°. Furthermore, each of the fold ligands binds two 1D Gd chains to build the 3D structure with 1D channels along the c axis direction. The solvent-accessible surface left in the final packing structure of 1 was calculated as 30.9% by utilizing the PLATON routine. Air and Chemical Stabilities of 1. The three Ln-CPs exhibit high thermal stability due to the 1D La chains and bifunctional ligands being highly interconnected with each other. Herein, the chemical stability of 1 is described as an example. The framework of 1 could maintain its integrity on exposure to the open environment for up to 3 months. Moreover, we have put it into the acidic and alkaline solvents with various pH values. To our excitement, we found that the complex preserved the original framework in aqueous solutions with different pH values from 1 to 13 for at least 1 week, which could be validated by powder X-ray diffraction (Figure 3). The

1D chains by utilizing the syn-syn benzoate and imidazolecarboxylic acid. The dihedral angle of the ligand is 76.49(2)°. Furthermore, each of the fold ligands binds two 1D Gd chains to build the 3D structure with 1D channels along the c axis direction. The solvent-accessible surface left in the final packing structure of 1 was calculated as 30.2% by utilizing the PLATON routine. The absorption amount of N2 at 77 K and 0.99 kPa could be as high as 63.5 cm3 g−1. Crystal Structure of 3. The monoclinic space group P21/c was assigned for complex 3, and the asymmetric unit contains two dysprosium atoms, two ligands generatedin situ, and two μ3-hydroxyl groups (Figure 2a and Table 1). Each metal ion is

Figure 2. (a) View of the asymmetric unit in 3 with atom labeling. (b) View of the 1D Dy-hydroxyl ladderlike chain. (c) View of the octacoordination environment of two asymmetric Dy ions (asymmetric code: (A) x − 1, y, z; (B) −x + 1, −y + 1, −z + 1; (C) −x + 1, y + 1/2, −z + 1/2; (D) −x + 2, y + 1/2, −z + 1/2). (d) Partial perspective view of the 3D structure of 3 along the a axis direction.

octacoordinated and exhibits a slightly distorted square antiprismatic coordination geometry, which consists of three OH groups, four carboxyl groups from different ligands generated in situ, and one N atom from the imidazole ring. All of the central Dy ions are located on the centrosymmetric positions, where the two bottom planes in the square antiprism are constructed by O6D, O8C, O9, O10A and O1A, N1A, O6, O4C (Figure 2c), respectively. The distances between each of the two square planes and the central Dy(1)III ion are determined as 1.370 and 1.361 Å, and the dihedral angle of the two square planes is 8.07°. Similarly, the two bottom square

Figure 3. Simulated and experimental pXRD patterns of 1 immersed in aqueous solutions with different pH values from 1 to 13 for 1 week.

preselected ligand possesses a long backbone and multiple carboxyl groups and could be partially or completely deprotonated. The versatile coordination modes between metal centers and organic linkers could obviously strengthen the framework stability. Magnetic Properties of 1 and 3. For 1 and 3, the magnetic susceptibilities were measured in the temperature C

DOI: 10.1021/acs.inorgchem.7b02969 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry range of 2−300 K and under a direct current (dc) field of 0.1 T. For 1, the χmT value at 300 K is 7.89 cm3 K mol−1, close to the typical spin-only value for a free Gd(III) ion (7.88 cm3 K mol−1, 8S7/2, g = 2). The value of χmT remains constant with a decrease in temperature from 300 to 100 K. At this point, the value decreases gradually to 4.071 cm3 K mol−1 from 100 to 2 K. The Curie−Weiss fitting results in the Weiss temperature Θ = −1.62 K, indicating antiferromagnetic coupling among the Gd(III) ions. To further discuss the mechanism of magnetic coupling in 1, theoretical fittings to the χmT curve were carried out. By inspecting the structure of 1, we can find that the magnetic properties of 1 are governed by an infinite laddershaped {Gd(COO)}n chain (Figure 1). Thus, it is difficult to investigate magnetic coupling parameters by the traditional irreducible tensor operators (ITO) method. Referring to the Hamilton operator (eq 1), we developed the GA-QMC fitting program (see the Supporting Information) based on the LOOP module of ALPS project.15

Figure 4. (a) Experimental plot of χmT vs T (denoted ○) measured at 0.1 T and the QMC fit (red line) for 1. (b) Reliability factor (R) as a function of generation. Insert: distribution sketch of Gd(III) ions. (c) Field dependence of isothermal magnetizations for 1 at T = 2−20 K and H = 0−7 T. (d) Entropy change (−ΔSm) calculated by using the magnetization data of 1 at different fields and temperatures.

N

H=−

∑ i = 0,

[J1(S1, iS1, i + 1+S2, iS2, i + 1)

i=i+2

+ J2 (S1, iS2, i + 1 + S2, i + 1S1, i + 2)]

(1)

Along the extending direction of the ladder, each Gd(III) ion connects with adjacent ions by use of syn-syn carboxylate, μ3OH, and μ2-O (carboxylate) bridges (coupling parameters J1), while on the ladder site, each Gd(III) ion links with neighboring ions by two μ3-OH bridges (coupling parameters J2). A weak interaction (zj) constant was taken into account to describe the interaction of two adjacent chains. Concerning the huge number of Gd(III) ions, temperature-independent paramagnetism (TIP) was also considered. In this simulation, we initially set the values of J1 and J2 in the range from −5.0 to 5.0 cm−1 on the basis of previous literature.16 Considering the low reliability factor (R), we combined genetic algorithm and quantum Monte Carlo (GA-QMC) methods to search globally the best parameters toward the whole multidimensional space among 1500 generations. After the 66th generation the reliability factor reached convergence with the minimum, as shown in Figure 1b. The obtained best parameters were J1 = −0.054 cm−1, J2 = 7.69 × 10−4 cm−1, zj = −0.036 cm−1, g = 1.99, TIP = 4.9 × 10−4 cm3 mol−1, and R = 2.04 × 10−5 (R = ∑[(χMT)obs − (χMT)calcd]2/∑[(χMT)obs]2). The J1 value of −0.054 cm−1 uncovers the two Gd(III) ions extending the direction of ladder feature antiferromagnetic exchange associated with integrated interaction of the syn-syn carboxylate, μ3-OH, and μ2-O (carboxylate) bridges, while the 7.69 × 10−4 cm−1 value of J2 exhibits the interaction of ferromagnetic coupling with μ3−OH bridges connecting 4f valence orbit of Gd(III) ions. It is found that the value of J1 was derived from antiferromagnetic-coupling syn-syn carboxylate16 and ferromagnetic coupling μ3-OH bridges and μ2-O (carboxylate) bridge (d(O(carboxylate)−Gd) = 2.469 and 2.480 Å).17 Moreover, the −0.036 cm−1 value of zj exhibits very weak antiferromagnetic interaction between {Gd(COO)}n chains. To the best of our knowledge, this is the f irst model combining genetic algorithm and quantum Monte Carlo methods ever reported to study the exchange interactions in large magnetic CPs. As shown in Figure 4c, the field dependence of magnetizations for 1 was measured under the conditions of a temperature range of 2−20 K and field range of 0−7 T. The

magnetization became larger steadily with an increase in the dc field and reaches 6.95 NμB at 7 T and 2 K, consistent with the typical value of 7 NμB for one Gd(III) ion (S = 7/2, g = 2). The magnetic entropy changes (−ΔSm) could be estimated according to the Maxwell equation18 ΔSm(T ) =

∫ [∂M(T , H)/∂T ]H dH

(2)

The magnetic entropy changes (−ΔSm) at different temperatures has been presented in Figure 4d. The −ΔSm value reaches a maximum value of 30.33 J kg−1 K−1 with ΔH = 7 T and T = 2 K. This value is far smaller in comparison to the expected value of 70.45 J kg−1 K−1, which was obtained from the formula R ln(2S + 1) with S = 7/2. The existence of antiferromagnetic interactions between the adjacent Gd(III) ions as discussed above must be responsible for the smaller value. For 3, the χmT value is 13.93 cm3 K mol−1 at 300 K, similar to the typical spin-only value for a free Dy(III) ion (14.17 cm3 K mol−1, 6H15/2, gJ = 4/3). The χmT value remains almost constant along with a decrease in temperature from 300 to 150 K. Then, the value decreases gradually to 11.88 cm3 K mol−1 from 150 to 7 K, mainly due to a combination of the crystal field splitting of the 6H15/2 ground multiplet of Dy(III) and an antiferromagnetic interaction among the Dy(III) ions (Θ = −1.43 K). Below 7 K, there is a sharp increase to 13.19 cm3 K mol−1 at 1.8 K, manifesting the existence of a metamagnetic transformation from an antiferromagnetic interaction at high temperatures to a ferromagnetic interaction at low temperatures.19 Alternating current (ac) susceptibilities of 3 were also tested in order to detect its dynamic magnetization behavior. Out-ofphase ac signals (χ″) were observed in the field range of 0− 1400 Oe (Figure S3). Interestingly, two sets of relaxation phases could be seen, consistent with the low- and highfrequency signal ranges. This indicates that 3 is probably a single-molecule magnet incorporated into a 3D framework.20 D

DOI: 10.1021/acs.inorgchem.7b02969 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Detailed studies of the relaxation behaviors for 3 will be reported separately. HF-EPR measurements of a polycrystalline sample of 3 were performed at 2 K over the frequency range of 60−472 GHz, as shown in Figure 5b. The EPR signals are extremely broad and

Figure 6. Emission spectrum of a crystal sample of 2 (excitation wavelength at 350 nm). Inset: excitation spectrum of a crystal sample of 2 (excitation wavelength at 613 nm) showing the transitions 7F0 → 5 D4 (a), 7F1 → 5D4 (b), 7F0 → 5G4 (c), 7F0 → 5G2 (d), 7F1 → 5G2 (e), 7 F0 → 5L6 (f), and 7F1 → 5D3 (g).

D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions, respectively. In addition, it is well-known that the strongest 5D0 → 7F2 transition is sensitive to the coordination environment around Eu(III) center and could be increased along with a symmetric decrease in the central ion, which is responsible for the red luminescence. The 5D0 → 7F1 transition related with the crystal field strength of intensity is weaker than that of 5D0 → 7F2. The calculated intensity proportion of 5D0 → 7F2/5D0 → 7F1 is approximately close to 5. The ratio value indicates that the Eu(III) atom in the structure of 2 should form a noncentrosymmetric coordination environment without an inversion center, consistent with the results of structural analysis. 5

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CONCLUSION In summary, by carefully predesign of the bifunctional ligand, three similar Ln-CPs containing a 1D infinite [Ln4(OH)4] subchain have been successfully constructed. In accordance with our assumption, the highly connected mode between the multifunctional ligand and 1D building units must be responsible for the exceptional chemical stability of the three lanthanide-CPs. In addition, the analysis of magnetic properties reveal that 1 displays large magnetic entropy changes. The −ΔSm value of 1 reaches a maximum value of 30.33 J kg−1 K−1 under the conditions of T = 2 K and ΔH = 7 T. Furthermore, genetic algorithm and quantum Monte Carlo (GA-QMC) methods were combined together to simulate the magnetic coupling parameters of compound 1, shedding light on the effect of linking bridges on magnetic propagation. 2 shows intense luminescence in the range of 350−710 nm. Comparably, magnetic studies of 3 reveal the existence of a metamagnetic transformation from an antiferromagnetic interaction to a ferromagnetic interaction along with a decrease in temperature. Through fitting of the results of HF-EPR measurements, a component of the g tensor is obtained, g|| = 16.4(5), indicating the large anisotropy of 3.

Figure 5. (a) Experimental plot of χmT vs T for 3 measured at 0.1 T. (b) HF-EPR spectra of 3 recorded at 2 K and at different frequencies. Black dots indicate the resonance fields for each spectrum, and the black solid line is the linear fit.

spread across almost the whole field range. The low-field edge moves to higher fields as the microwave frequency increases, while the high edge is not clearly seen and could possibly be out of the field range. By fitting of the resonance fields of the low-field edge as a function of frequency, a component of the g tensor is obtained, g|| = 16.4(5), and g⊥ is too small to be detected in current measurements, indicating the large anisotropy of 3.21 Fluorescent Properties of 2. The Eu(III) ion usually exhibits intense fluorescent properties, and each Eu-CP deserves investigation of its corresponding photoluminescent properties. Therefore, for 2, the measurement of the luminescence of a solid sample at room temperature was carried out (Figure 6). The typical red luminescent spectra of europium ions in 2 have been observed, which exhibits an intense f−f transition with an excitation wavelength of 350 nm. The four emission peaks at 592, 613, 652, and 702 nm could be assigned to the magnetic dipolar 5D0 → 7F1 and electric dipolar E

DOI: 10.1021/acs.inorgchem.7b02969 Inorg. Chem. XXXX, XXX, XXX−XXX

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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.7b02969. Experimental details, TGA measurement, IR spectra, ac susceptibility, and theoretical calculations (PDF) Accession Codes

CCDC 1570757−1570759 contain 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 [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 *E-mail *E-mail *E-mail

for for for for

B.L.: [email protected]. Z.-W.O.: [email protected]. G.-L.Z.: [email protected]. Z.W.: [email protected].

ORCID

Bao Li: 0000-0003-1154-6423 Zhenxing Wang: 0000-0003-2199-4684 Author Contributions ⊥

D.P. and L.Y. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21471062, 21701046). The Analytical and Testing Center at the Huazhong University of Science and Technology is thanked for the spectral measurements and related analysis.

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DOI: 10.1021/acs.inorgchem.7b02969 Inorg. Chem. XXXX, XXX, XXX−XXX