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Two Types of Lanthanide Coordination Polymers Based on Homophthalate Ligand Exhibiting Luminescence and Significant Magnetocaloric Effect Zhong-Yi Li, Yan Chen, Xin-Ying Dong, Bin Zhai, XiangFei Zhang, Chi Zhang, Fuli Zhang, Suzhi Li, and Guangxiu Cao Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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Crystal Growth & Design

Two Types of Lanthanide Coordination Polymers Based on Homophthalate Ligand Exhibiting Luminescence and Significant Magnetocaloric Effect Zhong-Yi Li,a,b Yan Chen,a,b Xin-Ying Dong,a,b Bin Zhai,*a,b Xiang-Fei Zhang,a,b Chi Zhang,a,b Fu-Li Zhang,a,b Su-Zhi Li,a,b and Guang-Xiu Cao*a,b

a

Henan Key Laboratory of Biomolecular Recognition and Sensing, College of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu 476000, P. R. China

b

Engineering Research Center of Photoelectric Functional Material, Shangqiu Normal University, Shangqiu 476000, P. R. China

KEYWORDS: Lanthanide; Coordination Polymer; Homophthalate; Magnetic Refrigerant; Photoluminescence

ABSTRACT: Two types of lanthanide coordination polymers based on homophthalic acid (H2HPA), one-dimensional {[Ln(HPA)(NO3)(H2O)2]·H2O}n (Ln=Eu (1); Gd (2); Tb (3); Dy (4)) and two-dimensional {[Ln2(HPA)3(H2O)2]·H2O}n (Ln=Eu (5); Gd (6); Tb (7); Dy (8)), were solvothermally synthesized. Compounds 1–4 show a 1D linear chain structure consisting of

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HPA2- linker and [Ln2] cluster unit. Compounds 4–8 are crystallographically isostructural and contain 1D lanthanide-carboxylate building units [Ln4(CO2)10]n built from the adjacent Ln3+ ions and carboxyl groups of the H2HPA ligands, which can further give 2D layer structures via the link of HPA2−. The magnetic studies reveal that complexes 2 and 6 with isotropic Gd3+ ion exhibit significant cryogenic magnetocaloric effects with a maximum −∆Sm value of 35.58 and 35.41 J kg−1 K−1 at 2 K and 7 T, respectively. In addition, the solid-state photophysical properties of 1/5 and 3/7 respectively display strong characteristic Eu3+ and Tb3+ photoluminescent emission in the visible region, suggesting that Eu- and Tb-based luminescences are sensitized by the effective energy transfer from the ligand to the metal centers.

INTRODUCTION

Rescently, the lanthanide coordination polymers behaving as dual magneto-optical materials have become an attractive research field because of not only the intrinsic magnetic and luminescent properties of Ln3+ ions, but also their captivating structures and potential applications in cryogenic magnetic refrigeration, high-density information storage, MRI imaging and luminescent sensing.1-6 For magnetic properties, resulting from the different local magnetic anisotropy and the large-spin multiplicity of the spin ground-state, Ln3+ ions can be used to build either single-molecule magnets (SMMs) for highly anisotropic Tb- and Dy-based systems,7,8 or as low-temperature molecular magnetic coolers for isotropic Gd-containing examples.6,9-12 In particular, magnetic refrigerants with a significant magnetocaloric effect (MCE) have attracted


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increasing interest in recent years because of the energy-efficient and environmentally friendly advantages as well as the possibility of replacing the rare and expensive He-3 in ultralowtemperature refrigeration.13-15 The MCE represents the change of isothermal magnetic entropy (– ∆Sm) and adiabatic temperature (∆Tad) in change of applied magnetic field.16,17 To achieve large –∆Sm in a low temperature range, it is usually necessary that a molecular contains the features of a large spin ground state S, negligible magnetic anisotropy, low-lying excited spin states, a high magnetic density (or a large metal/ligand mass ratio) and weak coupling.18−20 In this regard, the combination of the Gd3+ ion and small multidentate ligands with carboxyl, oxo, hydroxo groups has been proven to be one effective strategy.16,21,22 The isotropic Gd3+ ion has a large spin value (S = 7/2) and usually displays weak superexchange interactions, and the functional groups could accumulate large number of metal to ligands and promote dense arrangements in the lattice.16,21,23 To this day, a large amount of Gd-based molecular clusters and coordination polymers with impressive MCE have been reported.9–11,16,21 Compared with Gd-based cluster complexes, multidimensional Gd-based polymers may be beneficial to obtain materials with promising MCE, when considering the enhanced magnetic density due to the sharing of bridging ligands between magnetic centers and that the nonmagnetic guest or solvent molecules are more difficult to trap in such structures.21,22 However, pure Gd-containing frameworks showing remarkable MCEs are still limited, and need to be further explored to discover their potential for cryogenic application.22,23


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For photoluminescence, most trivalent lanthanide ions are commonly used as luminescent centers and can display characteristic narrow emissions in the visible to near-infrared region of the optical spectrum.4–6,23,24 However, the direct excitation of the metals is very inefficient because of the weak light absorption for the forbidden f–f transitions.4,5 Upon incorporation into coordination polymers, suitable organic ligands with strongly absorbing chromospheres capable of transmitting the energy to the metal center could successfully resolve the obstacles, which is so-called “luminescence sensitization” or “antenna effect”.2,4,5,25 Therefore, it is crucially important and a great challenge to construct bifunctional magneto-optical materials to legitimately choose the functional organic connectors to effectively incorporate with the lanthanide ions.4 To continue the structural-property researches along the bifunctional materials, commercially available homophthalic acid (H2HPA) was selected as functional ligands to react hydrothermally with different lanthanide ions. The unsymmetric H2HPA contains both aromatic and aliphatic carboxylate functionalities, which could display various bridging modes to metal ions,26,27 even so, has rarely been documented to construct lanthanide coordination polymers.28 As a result of our previous continuous investigation on the H2HPA ligand, we present here two types of lanthanide coordination polymers, one-dimensional (1D) {[Ln(HPA)(NO3)(H2O)2]·H2O}n (Ln=Eu (1); Gd (2); Tb (3); Dy (4)) and two-dimensional (2D) {[Ln2(HPA)3(H2O)2]·H2O}n (Ln=Eu (5); Gd (6); Tb (7); Dy (8)). The magnetic and photoluminescent properties have been discussed.


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EXPERIMENTAL SECTION Materials and Physical measurements. All chemicals were obtained from commercial sources and used without further purification. Powder X-ray diffraction (PXPD) measurements were carried out on a Bruker D8 ADVANCE X-ray Diffractometer using Cu Kα (λ = 1.5418 Å) at room temperature. Elemental analyses were determined by a Vario EL III elemental analyzer. FT-IR spectra were recorded in the range of 4000-400 cm-1 on a JASCO FT/IR-430 spectrometer with KBr pellets. Solid state luminescence properties were carried out using a F-7000 FL Spectrophotometer. Thermogravimetric analyses were performed 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 1–4. Complexes 1–4 were synthesized under the same conditions. 1 mL Ln(NO3)3 (2 M, 2 mmol; Ln=Eu; Gd; Tb; Dy) aqueous solution, 0.036 g H2HPA (0.2 mmol) and 1 mL 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 4.0 under stirring. The vial was sealed and heated at 90°C in an oven for 24 h, then cooled to room temperature. Colourless crystals of the products were obtained.

{[Eu(HPA)(NO3)(H2O)2]·H2O}n (1): Yield, 72% based on H2HPA. Anal. Calcd. for C9H12EuNO10: C, 24.23; H, 2.71; N, 3.14%. Found: C, 24.18; H, 2.70; N, 3.16%. IR (KBr pellet,


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cm−1): 3616 m, 3411 s, 1610 s, 1493 s, 1411 s, 1228 s, 1165 w, 1028 w, 932 w, 816 w, 747 m, 665 w, 507 w.

{[Gd(HPA)(NO3)(H2O)2]·H2O}n (2): Yield, 69% based on H2HPA. Anal. Calcd. for C9H12GdNO10: C, 23.94; H, 2.68; N, 3.10%. Found: C, 23.86; H, 2.73; N, 3.13%. IR (KBr pellet, cm−1): 3616 w, 3459 s, 1610 s, 1452 s, 1411 s, 1336 s, 1157 w, 1048 w, 939 w, 816 w, 733 m, 665 w, 501 w.

{[Tb(HPA)(NO3)(H2O)2]·H2O}n (3): Yield, 70% based on H2HPA. Anal. Calcd. for C9H12TbNO10: C, 23.86; H, 2.67; N, 3.09%. Found: C, 23.88; H, 2.69; N, 3.11%. IR (KBr pellet, cm−1): 3616 w, 3465 s, 1617 s, 1452 s, 1411 s, 1336 s, 1157 w, 1049 w, 939 w, 816 w, 733 m, 665 w, 501 w.

{[Dy(HPA)(NO3)(H2O)2]·H2O}n (4): Yield, 66% based on H2HPA. Anal. Calcd. for C9H12DyNO10: C, 23.67; H, 2.65; N, 3.08%. Found: C, 23.66; H, 2.67; N, 3.09%. IR (KBr pellet, cm−1): 3616 m, 3465 m, 1623 s, 1452 s, 1411 s, 1336 s, 1157 w, 1048 w, 939 w, 816 w, 733 m, 665 w, 501 w.

Synthesis of 5–8. The following is the general progress for the preparation of 5–8. 0.6 mL Ln(NO3)3 (2 M, 1.2 mmol; Ln=Eu; Gd; Tb; Dy) aqueous solution, 0.036 g H2HPA (0.2 mmol) and 8 mL 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 3.5 under stirring. The vial was


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sealed and heated at 90°C in an oven for 24 h, then cooled to room temperature. Colourless sheet crystals of the products were obtained.

{[Eu2(HPA)3(H2O)2]·H2O}n (5): Yield, 54% based on H2HPA. Anal. Calcd. for C27H24Eu2O15: C, 36.34; H, 2.71%. Found: C, 36.32; H, 2.68%. IR (KBr pellet, cm−1): 3389 m, 1562 s, 1411 s, 1309 w, 1164 w, 1049 w, 953 w, 870 w, 747 m, 665 w, 562 w.

{[Gd2(HPA)3(H2O)2]·H2O}n (6): Yield, 56% based on H2HPA. Anal. Calcd. for C27H24Gd2O15: C, 35.91; H, 2.68%. Found: C, 35.89; H, 2.67%. IR (KBr pellet, cm−1): 3390 m, 1555 s, 1411 s, 1315 w, 1164 w, 1049 w, 953 w, 870 w, 747 w, 665 w, 576 w.

{[Tb2(HPA)3(H2O)2]·H2O}n (7): Yield, 49% based on H2HPA. Anal. Calcd. for C27H24Tb2O15: C, 35.78; H, 2.67%. Found: C, 35.79; H, 2.66%. IR (KBr pellet, cm−1): 3383 m, 1562 s, 1411 s, 1309 w, 1165 w, 1042 w, 953 w, 870 w, 747 m, 665 w, 562 w.

{[Dy2(HPA)3(H2O)2]·H2O}n (8): Yield, 46% based on H2HPA. Anal. Calcd. for C27H24Dy2O15: C, 35.50; H, 2.65%. Found: C, 35.54; H, 2.63%. IR (KBr pellet, cm−1): 3390 m, 1562 s, 1411 s, 1315 w, 1165 w, 1042 w, 953 w, 870 w, 747 m, 665 w, 562 w.

X-ray Crystallography. Crystallographic data were collected on a Bruker D8 Quest CMOS area detector system with graphite-monochromated Mo-Kα (λ = 0.71073 Ǻ) radiation. Data reduction and unit cell refinement were performed with Smart-CCD software. The structures were solved by direct methods and refined by full-matrix least squares methods using SHELXL-2016.29 For 1


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Table 1. Crystal data and structure refinement of 1–6.

Formula Mr. T (K) Cryst. system Space group a /Å b /Å c /Å

α/° β/° γ/°

1

2

3

4

5

6

C9H12EuNO10

C9H12GdNO10

C9H12TbNO10

C9H12DyNO10

C27H24Eu2O15

C27H24Gd2O15

446.16

451.45

453.12

456.70

892.38

902.96

298(2)

298(2)

298(2)

298(2)

298(2)

298(2)

Monoclinic

Monoclinic

Monoclinic

Monoclinic

Triclinic

Triclinic

P2(1)/c

P2(1)/c

P2(1)/c

P2(1)/c

P-1

P-1

6.9000(6)

6.8896(3)

6.8785(4)

6.8725(6)

7.9295(4)

7.9250(3)

18.3269(15)

18.2823(8)

18.2330(10)

18.1921(15)

12.9146(6)

12.9706(5)

11.1714(10)

11.1419(5)

11.1240(7)

11.1000(10)

14.3489(7)

14.3433(6)

90

90

90

90

84.121(2)

83.770(2)

97.425(3)

97.392(2)

97.309(2)

97.329(3)

86.421(2)

86.326(2)

90

90

90

90

89.390(2)

89.1620(10)

1400.8(2)

1391.74(11)

1383.79(14)

1376.4(2)

1458.83(12)

1462.61(10)

4

4

4

4

2

2

2.115

2.155

2.175

2.204

2.032

2.050

4.527

4.816

5.161

5.480

4.334

4.569

864

868

872

876

864

868

21078 / 2460

20954 / 2449

20613 / 2436

20244 / 2420

22132 / 5125

21717 / 5143

0.0262

0.0403

0.0531

0.0455

0.1106

0.0349

1.038

1.039

1.032

1.049

1.004

1.066

0.0177

0.0172

0.0215

0.0210

0.0383

0.0246

0.0745

0.0408

0.0485

0.0530

0.0514

0.0598

3

V (Å ) Z 3

dcalcd., g/cm µ(mm-1) F(000)

Reflections collected/ unique R(int) GOF on F2 R1a(I > 2σ (I)) b

wR2 (all data ) a

R1= ∑(||Fo|-|Fc||) / ∑ |Fo| , bwR2 = {∑w [(F2o − F2c)] / ∑w [(F 2o ) 2]}0.5

−6, All non-hydrogen atoms were refined anisotropically except O15 in 5 and 6, which is disordered and the occupancy factors are 0.59 (1) and 0.41 (1) for C15A and C15B in 5, 0.78 (1) and 0.22 (1) for C15A and C15B in 6, respectively. Hydrogen atoms on organic ligands were


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placed in idealised positions and refined using a riding model. Hydrogen atoms on the terminal and free water molecules were initially found on Fourier difference maps and then restrained by using the DFIX instruction. Exceptionally, no attempts were made to locate the hydrogen atoms on the free water molecule in 5 and 6. A summary of the important crystal and structure refinement data of 1−6 was given in Table 1. Selected bond lengths and angles for 2 and 6 were listed in Table S1 and S2, respectively. Crystal data of complexes 7 and 8 were also collected. Because of the weak intensities, only preliminary structures can be observed. Compounds 7 and 8 are isomorphous with 6, as also confirmed by powder XRD patterns. Their cell parameters are listed in Table S3.

RESULTS AND DISCUSSION Synthesis and physical characterization. The asymmetric H2HPA ligand bearing one aromatic carboxylate and one flexible acetate arm could display various bridging modes, based on which some transition metal complexes have been reported,26,27 however, the lanthanide one has been rarely discussed.28 Here, by using H2HPA as ligand, the one-dimensional {[Ln(HPA)(NO3)(H2O)2]·H2O}n (Ln=Eu (1); Gd (2); Tb (3); Dy (4)) and two-dimensional {[Ln2(HPA)3(H2O)2]·H2O}n (Ln=Eu (5); Gd (6); Tb (7); Dy (8)) coordination polymers have been prepared under hydrothermal conditions. Compared with the synthetic process of the two systems, the formation of 2D isologues is more easily. The 2D samples can be obtained under a


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wide molar ratio of Ln : H2HPA from 5 : 1 to 8 : 1, while only 9 : 1 and 10 : 1 could produce the 1D analogues. The thermal stabilities of 1−8 were researched on the crystalline samples under the N2 atmosphere from 25 to 800 °C (Figure S3 and S4). The thermogravimetric (TG) curves show that complexes 1−4 and 5−8 have the similar weight loss processes, respectively. The weight losses of 1−4 in the range of 25−250 °C are 12.08%, 11.30%, 10.34% and 10.84%, respectively, corresponding to the release of one free and two coordinated water molecules for per formula unit (calculated 12.11% for 1, 11.97% for 2, 11.93% for 3 and 11.83% for 4). After 250 °C, the continuous decline of the curves can be ascribed to the collapse of the frameworks. The weight of 5−8 lose 6.22%, 6.00%, 5.79% and 5.63% in the range of 25−290 °C, which could be attributed to the loss of one free and two coordinated water molecules for every formula unit (calculated 6.06% for 5, 5.99% for 6, 5.96% for 7 and 5.92% for 8). After 290 °C, the weight losses may be due to the complete decomposition of the polymers. The experimental and computer simulated PXRD patterns of compounds 1−4 and 5−8 are shown in Figure S5 and S6. The PXRD patterns of the bulk samples are in good agreement with their simulated patterns from the single crystal structures, indicating the phase purity. Crystal Structures of 1−4. Single crystal X-ray diffraction analyses reveal that complexes 1−4 are isomorphic, so only the structure of 2 is discussed in detail. 2 crystallizes in the monoclinic P2(1)/c space group and has a 1D linear chain structure built from HPA2- linker and [Gd2] node. As shown in Figure 1a, the asymmetric unit of 2 consists of one Gd3+ ion, one HPA2-


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ligand, one nitrate anion, two coordinated and one lattice water molecules. The Gd3+ ion is ninecoordinated and has a distorted monocapped square antiprismatic geometry, completed by five oxygen atoms (O1, O2A, O3B, O4B, and O4C) from four HPA2- ligands, two oxygen atoms (O5 and O7) from one nitrate anion, and two oxygen atoms (O8 and O9 ) from coordinated water molecules. The bond lengths of Gd−O and the angles of O−Gd−O fall in the range of 2.340(2)−2.583(2) Å and 50.03(7)−151.24(7)°, respectively, which are comparable to those in the reported Gd-containing compounds.4,5,8

(a)

(b)

(c)

(d)

Figure 1. (a) Coordination environment of the Gd3+ ion in compound 2. Symmetry codes: A, 1-x, 1-y, 1-z; B, 1+x, y, z; C, -x, 1-y, 1-z. (b) Coordination environment of the [Gd2] unit. (c) View of the 1D chain structure in 2 (H bonding: light orange dotted lines). (d) Supramolecular 2D arrangement in 2 (π–π interaction: green dotted lines).


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The HPA2- ligand in 2 uses its tridentate bridging acetate group and bidentate bridging carboxylate group to connect four Gd3+ ions (Scheme 1a). The coordination mode could be described as µ4-η2: η1: η1: η1. Each pair neighboring Gd3+ ions are fixed by two carboxylate and two acetate groups from four HPA2- ligands to form a [Gd2] cluster unit with the Gd···Gd distance of 3.8869(3) Å (Figure 1b). The adjacent [Gd2] units are linked together by the other carboxy of the four HPA2- ligands to result in a 1D linear chain structure, which is further firmed by the intrachain O8−H8D···O5 hydrogen bonds with H8D···O5 separation of 2.057 Å (Figure 1c, Table S4). The intercluster Gd···Gd distance is 6.8896(3) Å. In the crystal packing, there is an interchain π–π interaction within the phenyl rings of interchain adjacent HPA2- ligands which is responsible for the locking of the adjacent 1D chains to give a supramolecular 2D layer arrangement (Figure 1d, Table S5). The neighboring 2D structures are further connected together by the O8−H8D···O5 hydrogen bonds to give a supramolecular 3D arrangement (Figure 2), to which the free water molecules are binded by hydrogen bonds of O8-H8C···O10, O9H9B···O10, O10-H10A···O3, and O10-H10B···O6 (Figure S7).

I (a)

II

III

(b)

Scheme 1. Coordinate modes of HPA2- ligands in compounds 2 (a) and 6 (b).


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Figure 2. Supramolecular 3D arrangement in 2 (π–π interaction: green dotted lines, H bonding: light orange dotted lines).

Crystal Structures of 5−8. Compounds 5−8 are also isomorphic and only the structure of 6 will be discussed in detail. 6 crystallizes in the triclinic P-1 space group and has a 2D layer structure. As shown in Figure 3a, the asymmetric unit of 6 includes two Gd3+ ions, three HPA2ligands, two coordinated and one lattice water molecules. The two Gd3+ ions display two different coordination environments (Figure 3b). The coordination geometry of the octacoordinate Gd1 ion can be described as a square antiprism, featuring coordination by eight oxygen atoms (O1, O2A, O3B, O3C, O4D, O5, O7, and O9) from five HPA2- ligands. In contrast, Gd2 ion is nine-coordinated and bears a {O9} monocapped square antiprismatic donor set, completing by seven oxygen atoms (O5E, O6E, O8, O10E, O11, O11F, and O12F) from three HPA2- ligands and two oxygen atoms (O13 and O14) from two terminal water molecules. The Gd−O bond lengths and the O−Gd−O angles is in the range of 2.301(3)−2.574(3) Å and 50.68(11)−155.07(12)°, respectively, which are in agreement with those in 2.


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(a)

(c)

(b)

(d)

Figure 3. (a) View of the asymmetric unit of structure 6. (b) Coordination environments of the Gd3+ ions. (c) The 1D chain-shaped building unit [Gd4(CO2)10]n built from the adjacent Gd3+ ions and carboxyl groups of the H2HPA ligands in 6. (d) View of the 2D layer structure of 6. The hydrogen atoms have been omitted for clarity.

In 6, the HPA2- ligands adopt three different coordination modes (I−III, Scheme 1b), µ4-η2: η1: η1: η1, µ3-η1: η1: η2: η1 and µ4-η1: η2: η1: η1. For mode I, the HPA2- ligand has one tridentate bridging acetate group and one bidentate bridging carboxylate group to link four Gd1 ions. The Gd1-O3-Gd1 angle is 101.81(11)°. For mode II, the HPA2- ligand uses its bidentate bridging acetate group and tridentate bridging carboxylate group to connect one Gd1 and two Gd2 ions. For mode III, the tridentate bridging acetate group of the HPA2- ligand bridges two Gd2 ions and


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the bidentate bridging carboxylate group bridges one Gd1 and one Gd2 ion with the Gd2-O11Gd2 angle of 101.81(11)°. It is noted that two symmetry-related Gd1 ions are fastened by two tridentate and two bidentate carboxyl groups (mode I), while two symmetry-related Gd2 ions are bridged by two tridentate carboxyl groups (mode III). The Gd1 and Gd2 ion pairs further connect each other by one tridentate carboxyl group (mode II) and one bidentate carboxyl group (mode III) to result in one 1D [Gd4(CO2)10]n chain unit (Figure 3c). The neighboring chain units are linked together by the other carboxylate groups of the HPA2- ligands with modes I-III to form one 2D layer structure (Figure 3d). Between the layers, the lattice water molecules are filled (Figure S8). The Gd1···Gd1, Gd2···Gd2 and Gd1···Gd2 distances are 3.8603(4), 4.2073(1) and 4.5863(2) Å, respectively, which are different from the identical intra- and inter-cluster Gd···Gd distances of 3.8869(3) and 6.8896(3) Å in 2, suggesting the system may show more complicated magnetic properties. Magnetic Properties. The magnetic susceptibilities of 2−4 and 6−8 have been investigated in the temperature range of 2−300 K with an applied direct current (dc) magnetic field of 1000 Oe (Figure 4 and 5). At room temperature, the χMT values of 2−4 are 7.93, 11.94, and 13.99 cm3 mol-1 K, which are in agreement with the expected values, 7.88 cm3 mol-1 K for 2 (one isolated Gd3+ (S =7/2, g = 2) ion ), 11.81 cm3 mol-1 K for 3 ( one Tb3+ (S = 3, L = 3, g = 3/2) ion), and 14.18 cm3 mol-1 K for 4 (one Dy3+ (S = 5/2, L = 5, g = 4/3) ion). With lowering the temperature, for 2, the χMT values remain nearly constant between 300 and 85 K, and then decrease gradually


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to 7.73 cm3 mol-1 K at 20 K before decreasing quickly to 6.14 cm3 mol-1 K at 2 K. The data over the temperature range of 2–300 K fit the Curie–Weiss law well with C = 8.00 cm3 K mol–1 and θ = -0.54 K (Figure S9). These characteristics indicate dominated antiferromagnetic interactions between the metal centers in the 1D linear chain of 2.8,30 The χMT values of 3 remains constant between 300 and 100 K before decreasing continuously to 4.68 cm3 mol-1 K at 2 K, while the χMT values of 4 decrease gradually to 13.02 cm3 mol-1 K at 50 K before decreasing abruptly to 6.09 cm3 mol-1 K at 2 K. The decrease of χMT values may be attributed to a combination of the antiferromagnetic interactions between the metal ions, the progressively thermal depopulation of the ground-state Ln3+ sublevels as well as the magnetic anisotropy.31,32

Figure 4. Temperature dependence of the χMT values for 2−4 at 1000 Oe dc magnetic field.

For 6−8, the χMT values at 300 K are 15.57, 23.57, and 28.58 cm3 mol-1 K, close to the expected values of 15.76 cm3 mol-1 K (calculated for two uncoupled Gd3+ (S =7/2, g = 2) ions), 23.62 cm3 mol-1 K (calculated for two spin-only Tb3+ (S = 3, L = 3, g = 3/2) ions), and 28.36 cm3 mol-1 K (calculated for two isolated Dy3+ (S = 5/2, L = 5, g = 4/3) ions). Upon cooling, for 6, the


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χMT value increases gradually to a maximum of 15.87 cm3 mol-1 K at 140 K, and then decrease slightly to 15.76 cm3 mol-1 K at 20 K before decreasing quickly to 15.22 cm3 mol-1 K at 2 K (Figure S10). The data over the temperature range of 50–300 K fit the Curie–Weiss law well with C = 15.48 cm3 K mol–1 and θ = 3.55 K (Figure S11). The positive θ value and the increase of χMT values above 100 K suggest the presence ferromagnetic interactions between the metal ions ， while the decrease in the low-temperature region may be ascribed to the dominant antiferromagnetic coupling between the Gd3+ ions and the zero-field splitting of the ground states.4,33 For 7 and 8, the χMT values decrease gradually to 21.76 and 26.76 cm3 mol-1 K at 50 K before decreasing quickly to 15.07 and 20.57 cm3 mol-1 K at 2 K, respectively, which may be ascribed to a combination of the crystal-field effect of Ln3+ ions and the antiferromagnetic interactions between the metal centers.32,34

Figure 5. Temperature dependence of the χMT values for 6−8 at 1000 Oe dc magnetic field.


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(a)

(b)

(c)

(d)

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

Magnetization measurements for 2 and 6 were carried out at a field of 0−7 T between 2 and 7 K (Figure 6a and 6b). The M versus H data show a steady increase in magnetization to reach a maximum value of 6.99 Nβ for 2 and 14.01 Nβ for 6 at 7 T and 2 K, which are extremely approximate with the expected value of 7 and 14 Nβ for one and two uncoupled Gd3+ (S = 7/2, g = 2) ions, respectively. To evaluate the MCE, the magnetic entropy change of 2 and 6 can be calculated from the magnetization change as a function of applied field and temperature (Figure 6c and 6d) by using the Maxwell equation ∆Sm(T) = ∫[∂M(T,H)/∂T]HdH.6,35 For 2, the resulting


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maximum −∆Sm is 35.58 J K−1 kg−1 for ∆H = 7 T at 2.0 K, which is considerable large value among molecular magnetic coolants, and comparable with those for the reported impressive Gdbased polymers.19−21,30,31 Theoretically, the full entropy change per mole of compound corresponding to one Gd3+ ion is 38.30 J K−1 kg−1, as calculated from the equation Rln(2S+1) with S = 7/2. The difference of −∆Sm between the theoretical and experimental values may be ascribed to the MW/NGd ratio of 415 (where MW is the molecular mass of 415.45 g mol−1 and NGd is the number of Gd3+ ion present per mole of 2) and the antiferromagnetic interaction in 2.8,36 For 6, the maximum value of −∆Sm is 35.41 J K−1 kg−1 at 2.0 K and ∆H = 7 T, smaller than the expected maximum −∆Sm of 38.29 J K−1 kg−1, calculated from 2Rln(2S + 1). Additionally, this value is similar to that of 2, which may be due to their nearly same MW/NGd ratio (415).8,20

Figure 7. Solid-state photoluminescence spectra of H2HPA ligand, 2 and 6 (Gd complexes) at room temperature.

Luminescent properties. Functional ligands in the lanthanide coordination polymers can serve as excellent sensitizer to enhance the lanthanide luminescence.4,36 Thus, the solid-state


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photoluminescence spectra of 1−3 and 5−7 as well as free H2HPA ligand were investigated at room temperature to explore the photophysical behavior in the UV−visible region. As shown in Figure 7, upon excitation at 360 nm, the free H2HPA ligand and the Gd-containing complexes 2 and 6 display broad emissions at around 442, 430 and 436 nm, respectively, which can be attributed to the intraligand π*→π or π*→n transition.4,5 As compared with the free H2HPA organic, the apparent shifts of the emission spectra of 2 and 6 may be due to the nephelauxetic effect. The absence of the typical 4f→4f emission of 2 and 6 indicate that the energy level from the intraligand electron transfer is too low to meet the requirement of the lowest resonant energy level of Gd3+ ion.4

Figure 8. Solid-state photoluminescence spectra of 1 and 5 (Eu complexes) at room temperature.

On excitation at 396 nm (Figure 8), complexes 1 and 5 exhibit red luminescence with characteristic Eu3+ bands at ca 593, 618 and 698 nm, which are ascribed to the 5D0→7FJ (J = 1, 2, 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+ compounds reported


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previously.5,37 Excitation of 3 and 7 at 352 nm results in green luminescence with characteristic Tb3+ bands at 490, 545, 585, and 622 nm arising from 5D4→7F6, 5D4→7F5, 5D4→7F4, and 5

D4→7F3 transitions of the Tb3+ center, respectively.5,8 Clearly, the characteristic emission for the

ligand was not observed in the four complexes, suggesting that the H2HPA can effectively transfer the energy to Eu3+ and Tb3+ centers during photoluminescence.4 Consequently, the photoluminescence studies of the four aforementioned complexes indicate that the H2HPA could be used as an excellent sensitive reagent for effectively sensitizing the luminescence of Eu3+ and Tb3+ ions, which may be promising candidates for photoluminescent materials.4,36

Figure 9. Solid-state photoluminescence spectra of 3 and 7 (Tb complexes) at room temperature. CONCLUSION In summary, two series of lanthanide-based coordination polymers with 1D chain (1–4) or 2D layer structures (5–8) have been successfully prepared based on unsymmetric dicarboxylate ligand HPA2+ under hydrothermal conditions. Magnetic studies show that the Gd-containing


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complexes 2 and 6 both exhibit significant cryogenic MCEs, with the maximum −∆Sm values of 35.58 and 35.41 J kg−1 K−1 at 2 K and 7 T, respectively. Additionally, the Eu- and Tb-based polymers display strong characteristic Ln-centered emission in the visible region, indicating that the carboxylate-functionalized aromatic ligands can convert energy efficiently to the lanthanide centers. ASSOCIATED CONTENT

Supporting Information. Crystal data, additional crystallographic diagrams and magnetic diagrams, IR spectra, TG curves and PXRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for 1−6 have been assigned the CCDC numbers 1541791-1541794, 1543039 and 1541795 by the Cambridge Crystallographic Data Centre.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (B.Z.)

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

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (NSFC) (grant numbers 21401126, 21371114, 21571123, 21601119, 21501117, 21471095), Scientific and Technological Projects of Science and Technology Department of Henan province (172102210437). REFERENCES (1) (a) Tranchemontagne, D. J.; Mendoza–Cortés, J. L.; O’Keeffe, M.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1257–1283. (b) Feng, J.; Zhang, H. J. Chem. Soc. Rev. 2013, 42, 387–410. (c) Heffern, M. C.; Matosziuk, L. M.; Meade, T. J. Chem. Rev. 2014, 114, 4496–4539. (d) Cui, Y. J.; Yue, Y. F.; Qian, G. D.; Chen, B. L. Chem. Rev. 2012, 112, 1126–1162. (e) Aromı, G.; Aguila, D.; Gamez, P.; Luis, F.; Roubeau, O. Chem. Soc. Rev. 2012, 41, 537−546.

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Insert Table of Contents Graphic and Synopsis Here

Four 1D chain and four 2D layer lanthanide coordination polymers have been successfully prepared based on asymmetric homophthalic acid under hydrothermal conditions. The Gdcontaining complexes exhibit significant cryogenic MCEs, and the Eu- and Tb-based polymers display strong characteristic lanthanum luminescence, which are sensitized by the effective energy transfer from the ligand to the metal centers.


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Two Types of Lanthanide Coordination Polymers ... - ACS Publications

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