Three Types of Lanthanide Coordination Polymers with

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX-XXX

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Three Types of Lanthanide Coordination Polymers with Methylmalonate and Isonicotinate as Coligands: Structures, Luminescence, and Magnetic Properties Zhong-Yi Li,† Yuan-Qing Cao,† Jing-Yu Li,‡ Xiang-Fei Zhang,† Bin Zhai,*,† Chi Zhang,† Fu-Li Zhang,† and Guang-Xiu Cao*,† †

Henan Key Laboratory of Biomolecular Recognition and Sensing, College of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, P. R. China ‡ School of Business Administration, Zhengzhou University of Aeronautics, Zhengzhou 450046, P. R. China S Supporting Information *

ABSTRACT: Three types of lanthanide coordination polymers based on mixed methylmalonic acid (H2MMA) and isonicotinic acid (HINA), two-dimensional (2D) [Ln2(MMA)2(INA)2(H2O)3]n (Ln = Eu (1); Gd (2)) and [Ln(MMA)(INA)(H2O)2]n (Ln = Gd (3); Tb (4)) as well as one-dimensional (1D) [Dy(MMA)(INA)(H2O)2]n (5), have been hydrothermally synthesized, which can be controlled by the usage amount of H2MMA or the type of Ln3+ ion. Complexes 1 and 2 contain two kinds of 1D lanthanide chain units (wave and linear), which are alternately linked by the MMA2− and INA− ligands to form a 2D layer structure. Complexes 3 and 4 also have a 2D layer structure, consisting of 1D linear lanthanide chain units, MMA2− and INA− linkers. Compound 5 shows a 1D linear chain structure, in which the neighboring Dy3+ ions are fastened by the carboxylate groups from MMA2− and INA− ligands. The magnetic studies reveal the presence of ferromagnetic Gd···Gd coupling in the 1D chain units of 2 and 3, which display significant cryogenic magnetocaloric effects with a maximum −ΔSm value of 34.32 J kg−1 K−1 at 3 K and 36.02 J kg−1 K−1 at 2 K for ΔH = 7 T, respectively. Furthermore, the solid-state photophysical properties of 1 and 4 exhibit strong characteristic Eu3+ and Tb3+ photoluminescent emission in the visible region, indicating that Eu- and Tb-based luminescences are sensitized effectively by the energy transfer from the INA− ligand to the metal centers.

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INTRODUCTION The rational design and assembly of novel lanthanide coordination polymers (Ln-CPs) have attracted great interest over the past two decades, due to not only their various intriguing architectures, but also their potential applications such as cryogenic magnetic refrigeration, high-density information storage, luminescent sensing, and magnetic resonance imaging (MRI) contrast agents fields.1−4 For photoluminescence in both visible and near-infrared regions, most trivalent lanthanide ions are usually selected as luminescent centers because of their characteristic narrow line-like emissions for pure colors.3,5−8 However, the direct excitation of the lanthanide ions is very inefficient owing to the weak absorption coefficient of Laporte forbidden f−f transitions.8−11 Therefore, suitable organic ligands with strong absorbing chromophores are usually incorporated as adjacent antennas or sensitizers, which can stimulate the optical absorption by transferring energy to lanthanide ions to enhance their fluorescence intensity.8,10,12,13 Apart from the excellent photophysical properties, the magnetic properties of Ln-CPs are unusual because of the diverse local magnetic anisotropy and the large-spin multiplicity of the spin ground-state of Ln3+ cations, which can be used to © XXXX American Chemical Society

construct either single-molecule magnets (SMMs) and singlechain magnets (SCMs), especially for highly anisotropic Tband Dy-based systems,14−16 or as low-temperature molecular magnetic coolers for isotropic Gd-containing entities.12,17−21 Particularly, molecule-based magnetic refrigerants, as alternatives to rare and expensive He-3 in ultralow-temperature refrigeration, have drawn increasing attention in recent years because of the energy-efficient and environmentally friendly advantages.22−24 The refrigeration effect was appraised by the magnetocaloric effect (MCE), which represents the change of isothermal magnetic entropy (−ΔSm) and adiabatic temperature (ΔTad) in change of the applied magnetic field.17,25,26 To obtain larger −ΔSm, it is usually necessary that a molecular includes the features of a large spin ground state S, negligible magnetic anisotropy, low-lying excited spin states, weak coupling, and high magnetic density (or a large metal/ligand mass ratio).18,27−29 In this perspective, Gd-containing CPs with light and multidentate organic ligands may be promising candidates because the isotropic Gd3+ ion has a large spin value (S = 7/2) and usually displays weak superexchange interactions, Received: September 20, 2017 Published: October 16, 2017 A

DOI: 10.1021/acs.cgd.7b01341 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinement of 1−5 formula Mr T (K) cryst. system space group a/Å b/Å c/Å α/° β/° γ/° V (Å3) Z dcalcd., g/cm3 μ (mm−1) F(000) reflections collected/unique R(int) GOF on F2 R1a (I > 2σ(I)) wR2b (all data) a

1

2

3

4

5

C20H22Eu2N2O15 844.90 299(2) monoclinic Pc 12.4771(11) 11.0531(9) 9.5062(8) 90 110.918(4) 90 1224.60(18) 2 2.291 5.450 808 42709/5647 0.1009 1.054 0.0247 0.0580

C20H22Gd2N2O15 834.32 298(2) monoclinic Pc 12.4830(15) 11.0884(13) 9.5317(12) 90 110.787(4) 90 1233.5(3) 2 2.246 5.119 804 22341/5186 0.0452 1.064 0.0254 0.0602

C10H12GdNO8 431.46 298(2) monoclinic P2(1)/c 13.2754(6) 10.7721(6) 9.3473(5) 90 93.383(2) 90 1334.37(12) 4 2.148 5.007 828 23164/3063 0.0527 1.038 0.0301 0.0651

C10H12TbNO8 433.13 299(2) monoclinic P2(1)/c 13.213(2) 10.771(2) 9.3696(17) 90 93.329(6) 90 1331.2(4) 4 2.161 5.349 832 18759/2303 0.0253 1.037 0.0162 0.0487

C10H12DyNO8 436.71 298(2) monoclinic P2(1)/c 11.4560(5) 13.0070(6) 9.1208(4) 90 98.958(2) 90 1342.50(10) 4 2.161 5.602 836 19274/2360 0.0500 1.024 0.0245 0.0560

R1 = ∑(||Fo| − |Fc||)/∑ |Fo|. bwR2 = {∑w [(Fo2 − Fc2)]/∑w [(Fo2)2]}0.5.

and the light ligands could favor a large metal/ligand mass ratio.28,30 Up to now, a large amount of Gd-based molecular magnetorefrigerants have been built under this principle.17−30 Compared with Gd-based cluster complexes and one-dimensional (1D) CPs, the Gd-based two- (2D) and threedimensional (3D) CPs may be better for obtaining materials with promising MCEs, when considering the improved magnetic density because of the sharing of bridging ligands between magnetic centers and that the nonmagnetic guest or solvent molecules are more difficult to trap in such structures.23,31 However, the high-dimensional Gd-based CPs with remarkable MCEs are still limited, and further systematic investigation to discover their potential applications is very necessary.6,23,26 As mentioned above, to build high-dimensional Ln-CPs behaving as dual magneto-optical materials, an effective synthetic strategy may be the selection of mixed flexible small-size and rigid multidentate organic ligands containing carboxylate groups. On the one hand, besides comparable stability, the multifarious coordination and bridging modes of carboxylate groups can result in interesting topological architectures, magnetic coupling, and fluorescence properties.4,32 On the other hand, the introduction of a rigid ligand as a multidentate connector can be more beneficial to form porous 2D or 3D CPs, on which the rigid ligand may also endow rigidity and stability as well as improved fluorescence.33 Flexible methylmalonic acid and rigid isonicotinic acid both could display diverse bridging modes and have been demonstrated to be excellent ligands for building CPs.34,35 Herein, on the basis of mixed methylmalonic acid (H2MMA) and isonicotinic acid (HINA), three types of lanthanide coordination polymers, 2D [Ln2(MMA)2(INA)2(H2O)3]n (Ln = Eu (1); Gd (2)) and [Ln(MMA)(INA)(H2O)2]n (Ln= Gd (3); Tb (4)) as well as 1D [Dy(MMA)(INA)(H2O)2]n (5), were successfully prepared, and their structures and magnetic and photophysical properties were discussed.

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EXPERIMENTAL SECTION

Materials and Physical Measurements. All chemicals were obtained from commercial sources and used without further purification. Elemental analyses were determined by a Vario EL III elemental analyzer. Fourier transform infrared (FT-IR) spectra were collected in the range of 4000−400 cm−1 on a JASCO FT/IR-430 spectrometer with KBr pellets. Powder X-ray diffraction (PXPD) measurements were executed on a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα (λ = 1.5418 Å) at room temperature. Solid state luminescence properties were performed using a F-7000 FL spectrophotometer. Thermogravimetric analyses were carried out under a flow of nitrogen (40 mL/min) at a ramp rate of 10 °C/min, using a NETZSCH STA 449F3 instrument. Magnetic measurements were performed on a Quantum Design SQUID magnetometer MPMS XL-7. The data were corrected for the sample holder and the diamagnetic contributions. Synthesis of 1 and 2. Complexes 1 and 2 were synthesized under the same conditions. A total of 0.4 mL of Ln(NO3)3 (1 M, 0.4 mmol; Ln = Eu; Gd) aqueous solution, 0.2 mL of H2MMA (1 M, 0.2 mmol) aqueous solution, 0.049 g of HINA (0.4 mmol), and 2 mL of deionized water were placed in a 15 mL vial. 1 M NaOH aqueous solution was added dropwise to adjust the pH value of the resulting solution to about 5.0 under stirring. The vial was sealed and heated at 90 °C in an oven for 2 days and then cooled to room temperature. Block colorless crystals of the products were obtained. [Eu2(MMA)2(INA)2(H2O)3]n (1). Yield, 22% based on HINA. Anal. Calcd for C20H22Eu2N2O15: C, 28.79; H, 2.66; N, 3.36%. Found: C, 28.75; H, 2.69; N, 3.32%. IR (KBr pellet, cm−1): 3339 s, 1596 s, 1408 s, 1326 m, 1225 w, 1125 w, 1071 w, 939 w, 875 w, 779 w, 679 w, 550 w. [Gd2(MMA)2(INA)2(H2O)3]n (2). Yield, 20% based on HINA. Anal. Calcd for C20H22Gd2N2O15: C, 28.43; H, 2.62; N, 3.31%. Found: C, 28.45; H, 2.58; N, 3.33%. IR (KBr pellet, cm−1): 3344 s, 1596 s, 1406 s, 1326 m, 1227 w, 1127 w, 1074 w, 941 w, 875 w, 776 w, 683 w, 550 w. Synthesis of 3 and 4. The following is the general synthetic progress for complexes 3 and 4. 0.4 mL Ln(NO3)3 (1 M, 0.4 mmol; Ln = Gd; Tb) aqueous solution, 0.4 mL H2MMA (1 M, 0.4 mmol) aqueous solution, 0.049 g of HINA (0.4 mmol) and 2 mL of deionized water were placed in a 15 mL vial. 1 M NaOH aqueous solution was added dropwise to adjust the pH value of the resulting solution to B



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about 5.0 under stirring. The vial was sealed and heated at 90 °C in an oven for 2 days and then cooled to room temperature. Spindly block colorless crystals of the products were collected. [Gd(MMA)(INA)(H2O)2]n (3). Yield, 21% based on HINA. Anal. Calcd for C10H12GdNO8: C, 27.84; H, 2.80; N, 3.24%. Found: C, 27.81; H, 2.78; N, 3.26%. IR (KBr pellet, cm−1): 3183 s, 1591 s, 1413 s, 1286 m, 1227 w, 1061 w, 1001 w, 923 w, 710 m, 676 w, 544 w. [Tb(MMA)(INA)(H2O)2]n (4). Yield, 22% based on HINA. Anal. Calcd for C10H12TbNO8: C, 27.73; H, 2.79; N, 3.23%. Found: C, 27.75; H, 2.77; N, 3.25%. IR (KBr pellet, cm−1): 3163 s, 1552 s, 1412 s, 1286 m, 1227 w, 1067 w, 1001 w, 921 w, 710 w, 683 w, 557 w. Synthesis of 5. A total of 0.2 mL of Dy(NO3)3 (1 M, 0.4 mmol) aqueous solution, 0.4 mL of H2MMA (1 M, 0.4 mmol) aqueous solution, 0.049 g of HINA (0.4 mmol), and 2 mL of deionized water were placed in a 15 mL vial. 1 M NaOH aqueous solution was added dropwise to adjust the pH value of the resulting solution to about 5.0 under stirring. The vial was sealed and heated at 90 °C in an oven for 2 days and then cooled to room temperature. Spindly block colorless crystals of the products were prepared. [Dy(MMA)(INA)(H2O)2]n (5). Yield, 18% based on HINA. Anal. Calcd for C10H12DyNO8: C, 27.50; H, 2.77; N, 3.21%. Found: C, 27.52; H, 2.79; N, 3.20%. IR (KBr pellet, cm−1): 3149 s, 1591 s, 1425 s, 1287 m, 1227 w, 1067 w, 1007 w, 922 w, 710 m, 676 w, 550 w. X-ray Crystallography. Crystallographic data of complexes 1−5 were collected on a Bruker D8 Quest CMOS area detector system with graphite-monochromated Mo−Kα (λ = 0.71073 Å) radiation. Data reduction and unit cell refinement were performed with SmartCCD software. The structures were solved by direct methods and refined by full-matrix least-squares methods using SHELXL-97.36 For 1−5, All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on organic ligands were placed in idealized positions and refined using a riding model. Hydrogen atoms on the terminal water molecules were initially found on Fourier difference maps and then restrained by using the DFIX instruction. A summary of the important crystal and structure refinement data of 1−5 is given in Table 1. Selected bond lengths and angles for 2, 3, and 5 were listed in Tables S1, S2, and S3, respectively.

Scheme 1. Schematic Representation of the Synthetic Procedures for 1−5

might be favorable for the formation of the structure of type I, which has a tighter arrangement with the eight- and ninecoordinate Ln3+ ions. In contrast, the Ln3+ ion with a comparative small radius (such as Dy3+) is suitable for the construction of the 1D analogue, in which the Ln3+ ions are eight-coordinate. In contrast, the Ln3+ ions with a middle radii (such as Tb3+) may promote the formation of the 2D structure of type II, in which the Ln3+ ions are nine-coordinate. Interestingly, the Gd3+ ion with a proper radius can be used to construct the 2D structure of type I and II by controlling the usage amount of H2MMA over the synthetic process. Crystal Structures of 1 and 2. Single crystal X-ray diffraction analyses reveal that complexes 1 and 2 are isostructural, and only the structure of 2 is discussed in detail. 2 crystallizes in the monoclinic Pc space group and has a 2D layer structure. As shown in Figure 1a, the asymmetric unit of 2 contains two Gd3+ ion, two MMA2− ligands, two INA− ligands, and three coordinated water molecules. The two Gd3+ ions display distinct different coordination environments. The coordination geometry of the nonacoordinate Gd1 ion can be described as a monocapped square antiprism, featuring coordination by nine oxygen atoms (O1, O2A, O3, O4, O4B, O6B, O8, O13, and O14) from two INA− ligands, three MMA2− ligands, and two terminal water molecules (Figure 1b). In contrast, Gd2 ion is eight-coordinated and has a distorted square antiprismatic geometry, completed by eight oxygen atoms (O5, O7C, O9, O9C, O10, O11, O12D, and O15) from two INA− ligands, three MMA2− ligands, and one terminal water molecule. The bond lengths of Gd−O and the angles of O−Gd−O are in the range of 2.312(4)−2.819(4) Å and 66.38(14)−154.36(17)°, respectively, which are comparable to those in the reported Gdcontaining compounds.12,17,18,37 In the structure, the two symmetric independent MMA2− ligands present similar μ3-η1:η2:η1:η1 coordination modes (Scheme 2, I and II), which uses one tridentate bridging carboxylic group and one bidentate bridging carboxylic group to connect three Gd3+ ions. However, one links two Gd1 and one Gd2 ions (mode I), and the other links one Gd1 and two Gd2 ions (mode II). Around every Gd1 or Gd2 ion, there are three MMA2− ligands. The neighboring Gd1 ions are connected together by one tridentate bridging carboxy and

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RESULTS AND DISCUSSION Synthesis. On the basis of flexible or aromatic multicarboxylate ligands, our group has designed and synthesized series of Ln-CPs with interesting luminescent and magnetic properties.17,18 As a continuous work, in this context, flexible H2MMA and rigid HINA were chosen as coligands to synthesize Ln-CPs because of their various bridging modes and excellent affinity to metal ions.34,35 As a result, the 2D [Ln2(MMA)2(INA)2(H2O)3]n (Ln = Eu (1); Gd (2)), [Ln(MMA)(INA)(H2O)2]n (Ln = Gd (3); Tb (4)), and 1D [Dy(MMA)(INA)(H2O)2]n (5) were successfully synthesized under hydrothermal conditions. After the synthetic processes of these systems were deeply researched, the following findings were obtained and are detailed in Scheme 1. (1) Gd(NO3)3, H2MMA, and HINA with a molar ratio of 2:1:2 can produce the 2D structure of type I, while a 2:2:2 molar ratio results in the 2D architecture of type II. Given the same pH value of about 5.0, the usage amount of H2MMA may play an important role in the formation of the two Gd-containing systems. (2) Tb(NO3)3, H2MMA, and HINA with a molar ratio of 2:2:2 can lead to the 2D structure of type II. However, pure 2D structure of type I cannot be gained after a thorough study of the synthesis condition. (3) For the Eu- and Dy-containing analogues, the 2D structure of type I and the 1D structure can be obtained solely, respectively, regardless of whether the molar ratio of 2:1:2 or 2:2:2 was used. The results indicate that the formation of the three family polymers is sensitive to the radii of Ln3+ ions. A large radius C



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Figure 1. (a) The asymmetric unit of 2. (b) Coordination environments of the Gd3+ ions in 2. Symmetry codes: A, x, 2 − y, 0.5 + z; B, x, 2 − y, −0.5 + z; C, x, 1 − y, 0.5 + z; D, x, 1 − y, −0.5 + z. (c) View of the 2D layer structure unit built from Gd3+ ions and MMA2− ligands in 2 along the a axis. (d) View of the 2D structure in 2 along the b axis.

Scheme 2. Coordinate Modes of MMA2− and INA− Ligands in 2.

one η1-O atom of the bidentate bridging carboxy of the MMA2− ligand (mode I) to result in a 1D wave Gd1 chain. The Gd1··· Gd1 distance is 4.84(1) Å and the Gd1−Gd1−Gd1 angle is 158.68(1)°. Similarly, the 1D linear Gd2 chain is also formed by the adjacent Gd2 ions and the MMA2− ligand (mode II) with a Gd2···Gd2 distance of 4.78(1) Å and Gd2−Gd2−Gd2 angle of 168.71(1)°. As shown in Figure 1c, the Gd1 and Gd2 chains are alternately linked by the other η1-O atoms of the bidentate bridging carboxys of the MMA2− ligands (mode I and II) to lead to a 2D layer structure unit with the shortest interchain Gd1···Gd2 distance of 5.94(1) Å. The 2D layer unit is further fastened by the INA− ligands (Scheme 2, III and IV) to produce a 2D structure (Figure 1d). The INA− ligands bear two similar bridging μ2-η1:η1 modes. However, one INA− ligand bridges two Gd1 ions, while the other bridges two Gd2 ions. The neighboring 2D structures further link to each other by the O15−H15A···N1 and O14−H14B···N2 hydrogen bonds to give a supramolecular 3D arrangement (Figure 2 and Table S4). Crystal Structures of 3 and 4. Complexes 3 and 4 are also isostructural. 3 crystallizes in the monoclinic P2(1)/c space group and processes a 2D layer structure. As shown in Figure 3a, the asymmetric unit of 3 consists of one Gd3+ ion, one MMA2− ligand, one INA− ligand, and two coordinated water molecules. The Gd3+ ion is nine-coordinated and displays a distorted monocapped square antiprismatic coordination

Figure 2. Supramolecular 3D arrangement in 2 viewed along the c axis (H bonding: light orange dotted lines).

geometry, completed by nine oxygen atoms (O1, O2A, O3, O3A, O4, O5B, O6B, O7, and O8) from two INA− ligands, three MMA2− ligands and two terminal water molecules. The bond lengths of Gd−O and the angles of O−Gd−O fall in the range of 2.315(3)−2.788(3) Å and 49.09(9)−153.91(11)°, respectively, which are close to those in 2. D



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Figure 3. (a) Coordination environment of the asymmetric Gd3+ ion in 3. Symmetry codes: A, x, 1.5 − y, 0.5 + z; B, 1 − x, 0.5 + y, 0.5 − z. (b) Coordination modes of MMA2− and INA− ligands in 3. (c) View of the 2D layer structure unit built from Gd3+ ions and MMA2− ligands in 3 along the a axis. (d) View of the 2D structure in 3 along the b axis.

Figure 4. (a) Coordination environment of the asymmetric Dy3+ ion in 5. Symmetry codes: A, x, 0.5 − y, −0.5 + z; B, x, 0.5 − y, 0.5 + z. (b) Coordination modes of MMA2− and INA− ligands in 5. (c) View of the 1D linear chain structure of 5.

settled by the μ2-η1:η1 INA− ligands (Figure 3d). The neighboring 2D structures are connected with each other by the O8−H8A···N1 hydrogen bond to result in a supramolecular 3D framework (Figure S1 and Table S5). Crystal Structure of 5. Compound 5 crystallizes in the similar monoclinic P2(1)/c space group as 3, but has a distinct 1D linear chain structure (Figure 4). The asymmetric unit of 5 includes one Dy3+ ion, one MMA2− ligand, one INA− ligand, and two coordinated water molecules (Figure 4a). The Dy3+ ion is eight-coordinated and has a distorted square antiprismatic {O8} donor set, completed by eight oxygen atoms (O1, O2A, O3, O3B, O4B, O6, O7, and O8) from two INA−

Each of the MMA2− ligands in 3 uses one tridentate bridging carboxylic group and one chelating bidentate bridging carboxylic group to connect three Gd3+ ions (Figure 3b). The coordination mode could be described as μ3-η1: η2: η1: η1. Around every Gd3+ ion, there are three MMA2− ligands. The adjacent Gd3+ ions are linked together by one tridentate bridging carboxy of the MMA2− ligand to result in a 1D linear chain with a intrachain Gd···Gd distance of 4.68(1) Å and the Gd−Gd−Gd angle of 175.59(1)°. The neighboring chains are connected by the other carboxy of the MMA2− ligands to give a 2D layer structure unit with the shortest interchain Gd···Gd distance of 6.86(1) Å (Figure 3c). The 2D structure is further E



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15.96 and 8.04 cm3 mol−1 K at 2 K. The increase of χMT values in low-temperature region suggests the existence of dominant weak ferromagnetic interactions between adjacent Gd 3+ ions.17,37 To evaluate the exchange coupling, the dominant interaction pathways in 2 and 3 should be identified. According to the crystal structure of 3 described above, the Gd···Gd distance within the uniform chain is 4.68(1) Å, shorter than those between adjacent chains through the MMA2− bridge (longer than 7.2 Å). Thus, the main exchange interactions can be assumed to be 1D chain model, and the following expressions deduced by Fisher could be used to quantitatively analyze the interaction between adjacent Gd3+ ions with S = 7/2.3,38,39

ligands, two MMA2− ligands, and two terminal water molecules. The bond lengths of Dy−O and the angles of O−Dy−O are in the range of 2.262(3)−2.632(3) Å and 50.86(10)− 152.55(16)°, respectively. Every MMA2− ligand in 5 adopts a μ2-η1: η2: η1 coordination mode and bridges two Dy3+ ions by using its tridentate bridging and monodentate bridging carboxylic groups (Figure 4b). While the INA− ligand adopts a similar μ2-η1:η1 bridging mode as that in 3 to connect two Dy3+ ions. Around every Dy3+ ion, there are two MMA2− and two INA− ligands. The neighboring Dy3+ ions are linked together by the MMA2− and INA− ligands to result in a 1D linear chain structure (Figure 4c). The intrachain Dy···Dy distance is 4.62(1) Å, and the Dy−Dy−Dy angle is 161.02(1)°. Thermal Analysis and PXRD Patterns. The thermal stabilities of 1−5 were investigated on the crystalline samples under the N2 atmosphere from 25 to 900 °C (Figure S3). Their thermogravimetric (TG) curves are similar, and two mass loss steps are observed. For 1 and 2, the first weight losses in the range of 25−260 °C are 6.78% and 6.29%, respectively, corresponding to the release of three coordinated water molecules for per formula unit (calcd.: 6.40% for 1 and 6.48% for 2). Above 260 °C, the weight losses may be ascribed to the collapse of the frameworks. For 3−5, the first weight losses are 8.18% and 8.36% for 3 and 4 in the range of 25−235 °C, and 8.39% for 5 in the range of 25−250 °C, which are ascribed to the release of two coordinated water molecules for per formula unit (calcd.: 8.35% for 3, 8.32% for 4 and 8.25% for 5). Then, the following weight losses may be attributed to the complete decomposition of the polymers. The purity of the polycrystalline powder samples of 1−5 was determined by the PXRD data (Figure S4−S6), and the results suggested that the experimental data are in agreement with the simulated data from their single-crystal structures. Magnetic Properties. The magnetic susceptibilities of 2−5 have been studied in the temperature range of 2−300 K under an applied direct current (dc) magnetic field of 1000 Oe (Figure 5). At 300 K, the χMT values of 2 and 3 are 15.83 and 7.92 cm3 mol−1 K, which are in agreement with the expected value of 15.76 cm3 mol−1 K (calculated for two spin-only Gd3+ (S = 7/2, g = 2) ions) and 7.88 cm3 mol−1 K (calculated for one Gd3+ (S = 7/2, g = 2) ion), respectively. With lowering the temperature, the χMT values of 2 and 3 remain almost constant before 44 and 38 K, respectively, and then increase slightly to

Ng 2β 2 1 + u S(S + 1) 3kT 1 − u

(1)

where u = cth(JS(S + 1)/kT) − kT/JS(S + 1) χchain χM = 1 − (zJ ′χchain /Ng 2β 2)

(2)

χchain =

In the equation, N is Avogadro’s number, β is the Bohr magnetron, k is the Boltzmann constant, J is the exchange coupling parameter between adjacent intrachain spins, and the interchain interaction (zJ′) is treated by the molecular field approximation. The best-fit parameters are g = 1.99(1), J = 0.0209(1) cm−1, zJ′ = −0.0324(1) cm−1, and R = 2.41 × 10−5, where R is calculated from Σ[(χMT)obsd − (χMT)calcd]2/ Σ[(χMT)obsd]2. The positive and small J value is in good agreement with the reported values for other carboxyl-bridged Gd-containing complexes,3,17,37,39 suggesting the presence of weak Gd−Gd ferromagnetic coupling interaction in the 1D chain in 3. In contrast, the magnetic interaction pathways in 2 are too complicated to find a proper 2D model to fit the data accurately. However, as shown in Scheme 3, because of the similar Gd···Gd distances in (between) the alternating Gd1 and Gd2 chains, the intrachain Gd−Gd exchange coupling parameter can be treated as J1, and the interchain ones as J2. Therefore, the following equations for a simplified 2D layer model could be used to quantitatively analyze the magnetic interaction.38,40,41 χchain =

Ng 2β 2 (1 + u1) (1 + u 2) S(S + 1) 3kT (1 − u1) (1 − u 2)

(3)

where, u1 = cth(J1S(S + 1)/kT) - kT/J1S(S + 1), u2 = cth(J2S(S + 1)/kT) - kT/J2S(S + 1) χchain χM = 1 − (zJ ″χchain /Ng 2β 2) (4) The zJ″ represents the interplayer interaction treated by the molecular field approximation. The best fitting results give g = 2.02(1), J1 = 0.0552(1) cm−1, J2 = −0.0548(1) cm−1, zJ″ = −0.0323(1) cm−1, and R = 2.92 × 10−5 (R is calculated from Σ[(χMT)obsd − (χMT)calcd]2/Σ[(χMT)obsd]2), indicating that the intra- and interchain Gd−Gd coupling are ferromagnetic and antiferromagnetic, respectively. When considering the magnetostructural relationship, the ferromagnetic behaviors in the 1D chain units of 2 and 3, which are bridged by the η2-O atom of the carboxy of the MMA2− ligand, may be mainly ascribed to the larger Gd−O−Gd angle (ca. 135.46° for 2 and 127.84(1)° for 3) and Gd−Gd bond length (ca. 4.81 Å for 2 and 4.68(1) Å

Figure 5. Temperature dependence of the χMT values for 2−5 at 1000 Oe dc magnetic field. The red solid lines represent the best fit to the data. F



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Scheme 3. View of the Magnetic Exchange Pathways in the Simplified 2D Layer Model

Figure 6. Field dependence of the magnetization plots of 2 (a) and 3 (b) at the indicated temperatures. −ΔSm calculated from the magnetization data of 2 (c) and 3 (d) at various fields and temperatures.

the metal ions, the progressively thermal depopulation of the ground-state Ln 3+ sublevels as well as the magnetic anisotropy.17,31,43 Magnetization measurements for 2 and 3 were performed at a field of 0−7 T between 2 and 7 K (Figure 6a,b). The M versus H data show a steady increase in magnetization to come up to a maximum value of 13.97 Nβ for 2 and 7.01 Nβ for 3 at 7 T and 2 K, which are close to the expected value of 7 and 14 Nβ for two and one uncoupled Gd3+ (S = 7/2, g = 2) ions, respectively. To evaluate the MCE, the magnetic entropy change of 2 and 3 can be obtained from the magnetization change as a function of applied field and temperature (Figure 6c,d) by using the Maxwell equation ΔSm(T) = ∫ [∂M(T,H)/

for 3), as reported that a large Gd−O−Gd angle (>110°) and Gd···Gd distance (>4.0 Å) usually tend to a ferromagnetic coupling interaction for the chain-like polymers.3,37,42 For 4 and 5, the χMT values at room temperature are 11.68 and 14.24 cm3 mol−1 K, which are essentially consistent with the expected values, 11.81 cm3 mol−1 K for 4 (one isolated Tb3+ (S = 3, L = 3, g = 3/2) ion) and 14.18 cm3 mol−1 K for 5 (one Dy3+ (S = 5/2, L = 5, g = 4/3) ion). Upon cooling, for 4, the χMT values decrease gradually to 9.97 cm3 mol−1 K around 25 K and then decrease abruptly to 5.92 cm3 mol−1 K at 2 K. For 5, the χMT values decrease continuously to 10.80 cm3 mol−1 K at 2 K. The decrease of χMT values may be attributed to a combination of the antiferromagnetic interactions between G



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∂T]H dH.3,44 For 2, the resulting maximum −ΔSm is 34.32 J K−1 kg−1 for ΔH = 7 T at 3.0 K, which is a considerable large value among molecular magnetic coolants, and comparable with those for the reported impressive Gd-based polymers.28,29,37 Theoretically, the full entropy change per mole of complex corresponding to two Gd3+ ions is 40.92 J K−1 kg−1, as calculated from eq 2 R ln(2S + 1) with S = 7/2. The difference of −ΔSm between the theoretical and experimental values may be due to the MW/NGd ratio of 422 (where MW is the molecular mass of 844.90 g mol−1 and NGd is the number of Gd3+ ion in per mole of 2) and the weak magnetic interaction in 2.18,29,45 For 3, the maximum value of −ΔSm is 36.02 J K−1 kg−1 at 2.0 K and ΔH = 7 T, smaller than the expected maximum −ΔSm of 40.07 J K−1 kg−1, calculated from R ln(2S +1). Interestingly, this value is slightly bigger than that of 2, although 3 has a greater MW/NGd ratio (431). Considering the similar dominant weak ferromagnetic coupling between the Gd3+ ions in 2 and 3, this phenomenon may be related to the fact that the structure of 3 has higher symmetry. For the Tb- and Dy-containing complexes 4 and 5, alternating-current susceptibility measurements were measured at zero direct-current (dc) field (Figures S8 and S9); however, no frequency-dependent out-of-phase signal was observed, which suggests that none of them exhibits slow relaxation of the magnetization. Luminescent Properties. The luminescence spectra of complexes 1, 4, and 5 were investigated in the solid state at room temperature to explore the photophysical behavior in the visible range under ultraviolet (UV) irradiation and are shown in Figures 7, 8 and S10. Upon excitation at 395 nm, complex 1

Figure 8. Solid-state photoluminescence spectrum of 4 excited at 370 nm. Inset: photograph of green emissive 4 excited at 265 nm.

to the 4F9/2 → 6HJ (J = 15/2, 13/2, and 11/2) transitions of Dy3+ ions (Figure S10).8,43 Obviously, the characteristic emission for the INA− ligand was not observed in the three complexes, suggesting that the INA− as an antenna can effectively transfer the energy to Ln3+ centers during photoluminescence.18,27 Thus, the photoluminescence studies of the three complexes suggest that the HINA can be used as an excellent sensitive reagent for effectively sensitizing the luminescence of Ln3+, especially for Eu3+ and Tb3+ ions, which may be promising candidates for photoluminescent materials.10,18,46

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CONCLUSION

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ASSOCIATED CONTENT

In summary, three types of lanthanide-based coordination polymers with 2D layer (1/2 and 3/4) or 1D chain (5) structures have been successfully prepared based on mixed MMA2− and INA− ligands under hydrothermal conditions. Magnetic studies suggest that the Gd-containing complexes 2 and 3 display ferromagnetic Gd···Gd coupling in the chain units and significant cryogenic MCEs with a maximum −ΔSm value of 34.32 at 3 K and 36.02 J kg−1 K−1 at 2 K for ΔH = 7 T, respectively. Additionally, the Eu- and Tb-based polymers exhibit strong characteristic Ln-centered emission in the visible region. These results suggest that the mixed-ligand strategy may be a promising way to construct Ln-CPs with interesting magnetic and luminescent properties.

Figure 7. Solid-state photoluminescence spectrum of 1 excited at 395 nm. Inset: photograph of red emissive 1 excited at 265 nm.

S Supporting Information *

exhibits red luminescence with characteristic Eu3+ bands at 593, 618, 652, and 698 nm (Figure 7), which are ascribed to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions of Eu3+ centers. The most intense emission is centered at 618 nm and corresponds to the hypersensitive transition 5D0 → 7F2, which is consistent with the Eu3+ complexes reported previously.8,12 When excited at 370 nm, 4 displays a strong Tb3+ characteristic emission with four typical narrow peaks at 491, 545, 584, and 622 nm, which corresponds to the 5D4 → 7FJ (J = 6, 5, 4, and 3) transitions of the Tb3+ centers.27,38 The emission spectrum is dominated by the most intense band at 544 nm assigned to 5D4 → 7F5, giving in the complex strong bright-green emission (inset of Figure 8). On excitation at 352 nm, compound 5 displays three relatively weak emissions at 485, 574, and 662 nm, which are attributed

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01341. Crystal data, additional crystallographic diagrams, magnetic diagrams, IR spectra, TG curves, and PXRD patterns (PDF) Accession Codes

CCDC 1567439−1567443 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. H



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (B.Z.). *E-mail: [email protected]. (G.-X.C.). ORCID

Zhong-Yi Li: 0000-0003-0597-9054 Bin Zhai: 0000-0002-2866-1121 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 (NSFC) (Grant Nos. 21401126, 21371114, 21571123, 21601119, 21501117), Scientific and Technological Projects of Science and Technology Department of Henan province (172102210437).

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Three Types of Lanthanide Coordination Polymers with

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