New Family of Octagonal-Prismatic Lanthanide Coordination Cages

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New Family of Octagonal-Prismatic Lanthanide Coordination Cages Assembled from Unique Ln17 Clusters and Simple Cliplike Dicarboxylate Ligands Yuan-Yuan Zhou,† Bing Geng,‡ Zhen-Wei Zhang,† Qun Guan,† Jun-Ling Lu,† and Qi-Bing Bo*,† †

Key Laboratory of Chemical Sensing and Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, and Shandong Provincial Key Laboratory of Fluorine Chemistry and Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, P. R. China ‡

S Supporting Information *

ABSTRACT: Novel high-nuclearity lanthanide clusters (Ln17) are generated in situ in the coordination-driven self-assembly. A metal-cluster-directed symmetry strategy for building metal coordination cages is successfully applied to a lanthanide system for the first time. A new family of octagonal-prismatic lanthanide coordination cages UJN-Ln, formulated as [Ln(μ3OH)8][Ln16(μ4-O)(μ4-OH)(μ3-OH)8(H2O)8(μ4-dcd)8][(μ3dcd)8]·22H2O (Ln = Gd, Tb, Dy, Ho, and Er; dcd = 3,3dimethylcyclopropane-1,2-dicarboxylate dianion), have been assembled from the unique Ln17 clusters and simple cliplike ligand H2dcd. Apart from featuring aesthetically charming structures, all of the compounds present predominantly antiferromagnetic coupling between the corresponding lanthanide ions. Additionally, the intense-green photoluminescence for UJN-Tb and magnetic relaxation behavior for UJN-Dy have been observed. Remarkably, UJN-Gd shows a large magnetocaloric effect (MCE) with an impressive entropy change value of 42.3 J kg−1 K−1 for ΔH = 7.0 T at 2.0 K due to the high-nuclearity cluster and the lightweight ligand. The studies highlight the structural diversity of multigonal-prismatic metal coordination cages and provide a new direction in the design of cagelike multifunctional materials by the introduction of lanthanide clusters and other suitable cliplike ligands.

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INTRODUCTION The chemistry of metal coordination cages (MCCs), i.e., coordination-driven from the self-assembly of labile metal cations and organic ligands, has attracted much more attention for the following reasons: (1) They feature considerably synthetic advantages such as few steps and fast and facile construction of the final products. (2) Their geometric architectures simulated from nature have been of aesthetic interest and fascination. (3) Their high symmetry means that such structures can be rationally designed to use the same cage architectures as those formerly planned. (4) Many kinds of the metal cations employed by the self-assemblies can introduce unique characteristics such as physical/chemical properties. Therefore, the related studies not only offer us a pathway to a deep understanding of the elf-assembly processes for MCCs but also provide new insight into the design of cagelike materials with structural complexity and functional improvement especially for potential applications in optics, electrics, and magnetism.1 Generally, there are three types of MCCs: the Platonic solids, the Archimedean solids, and the prisms. The prisms here denote n-gonal prismatic cages, such as the tetragonal and hexagonal ones. Up to now, the trigonal ([M3L2],2 [M6L3],3 [M6L4],4 [M6L6],5 [M6L9],6 and [M8L3],7) and tetragonal ([M2L4],8 [M4L2],9 and [M4L4]10) prismatic MCCs with © XXXX American Chemical Society

transition-metal ions are well represented in the literature. However, the pentagonal ([M 10 L 15 ] 11 ) and hexagonal ([M6L2],12 [M8L6],13 [M12L6],14 and [M12L18]15) ones are rarely reported. Recently, Yaghi et al.16 and Hong et al.1a have proposed a metal-cluster-directed symmetry approach according to the fact that multinuclear transition-metal clusters may endow high-symmetry coordination sites. Accordingly, a hexagonal-prismatic MCC has been constructed from the Ni4(μ3-OH)4 clusters and cliplike organic ligands.13 It reveals that, although the single cliplike ligand has a symmetry no higher than 2-fold, two adjacent clusters containing the inherent symmetry of the coordination sites available at a 2fold axial center can be connected by six molecular clips to form a hexagonal-prismatic MCC. It is reasonable that the combination of many of the same cliplike ligands with a high-nuclearity metal cluster at a 2-fold axial center will give rise to the multigonal-prismatic MCCs. Typically, six cliplike ligands afford a hexagonal-prismatic cage, and eight cliplike ligands afford an octagonal-prismatic cage. On the basis of the fact that the lanthanide ions generally display high coordination numbers, we reason that an alternative method of obtaining high-symmetry coordination sites available for the metal cluster Received: October 13, 2015

A

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) Ln17 cluster (data from UJN-Tb; the “ball-and-stick” mode is used for clarity). (b) Structure of the cliplike ligand H2dcd. Symmetry codes: A, 0.5 − y, 0.5 + x, z; B, −0.5 + y, 0.5 − x, z; C, −x, 1 − y, z.

not been employed to direct any self-assembly around the given metal ions so far. All of the aforementioned characteristics will make the ligand H2dcd beneficial to the assemblies of MCCs and provide new opportunities in the field of coordination-driven self-assembly. Therefore, an attempt is made in our research group to extend these hypotheses. Considering that our research interest has been focused on the synthesis, structure, and luminescent and magnetic properties for the novel lanthanide compounds, we hope that the structural complexity and functional improvement can be synchronously achieved in these lanthanide compounds. In this paper, coordination-driven self-assembly from a combination of lanthanide ions and a simple cliplike ligand (H2dcd) generates the high-nuclearity lanthanide clusters (Ln17) in situ. A new family of octagonal-prismatic LCCs, formulated as [Ln(μ3-OH)8][Ln16(μ4-O)(μ4-OH)(μ3OH)8(H2O)8(μ4-dcd)8][(μ3-dcd)8]·22H2O (here, it is denoted as UJN-Ln; Ln = Gd, Tb, Dy, Ho, and Er; dcd = 3,3dimethylcyclopropane-1,2-dicarboxylate dianion), have been assembled from the unique Ln17 clusters and the ligand H2dcd. Apart from the structural characterizations, the luminescent and magnetic properties for UJN-Ln are also discussed. Single-crystal X-ray diffraction analysis shows that UJN-Ln possesses an aesthetically charming structure. Static magnetic analysis for UJN-Ln reveals the antiferromagnetic interactions between adjacent Ln3+ ions. The intense-green photoluminescence for UJN-Tb and the magnetic relaxation behavior for UJN-Dy are also observed. Notably, because of the high-nuclearity cluster and lightweight ligand, UJN-Gd shows a high magnetocaloric effect (MCE) with an impressive entropy change value of 42.3 J kg−1 K−1 for ΔH = 7.0 T at 2.0 K, which makes it appealing for widespread magnetic refrigeration applications.39

centers may be the use of a lanthanide cluster instead of a transition-metal cluster in order to obtain the multigonalprismatic MCCs. Now, many pure high-nuclearity lanthanide− oxo clusters (larger than five) ranging from [Ln6],17 [Ln7],18 [Ln8],19 [Ln9],20 [Ln10],21 [Ln12],22 [Ln13],23 [Ln14],24 [Ln15],22a,25 [Ln19],22b [Ln20],26 [Ln22],27 [Ln24],28 [Ln26],29 [Ln36],30 [Ln38],31 [Ln48],32 [Ln60],33 to [Ln104],34 have been reported because lanthanide ions show a strong affinity toward the oxygen-atom donors. To the best of our knowledge, the self-assemblies of the pentagonal, hexagonal, and even highorder prismatic MCCs based on the lanthanide clusters have never been reported, although the trigonal ([Ce2L3],35 [Ln7L6],36 and [Ln7L9]37) and tetragonal ([Ln2L4]38) ones have appeared in the literature recently. According to Hong et al.’s strategy mentioned above, we wonder whether the multigonal-prismatic lanthanide coordination cages (LCCs) based on high-nuclearity lanthanide clusters could be conveniently synthesized by judicious choice of the organic ligands. If so, many of the related LCCs would be endowed the fascinating structures, as well as the preferably luminescent and magnetic properties, due to the typical characteristics of the lanthanide ions. As a rule, it remains difficult to find suitable organic ligands in making sure that their geometric configurations can be fairly flexible and their coordination sites can be accurately controlled to link the lanthanide clusters. 3,3-Dimethylcyclopropane-1,2-dicarboxylic acid (H2dcd) contains two carboxylic acid units connected to a cyclopropane spacer with two methyl groups (Figures 1b and S1). We select it as a potential ligand based on the following considerations: (1) H2dcd can act as a multidentate ligand via many oxygen sites, and it can be easily formed through hydrolysis of the inexpensive 6,6-dimethyl-3-oxabicyclo[3.1.0]hexane-2,4-dione (dohd). (2) By the introduction of a cyclopropane spacer between the two carboxylic acid centers, in conjunction with a lightweight ligand, H2dcd reveals a long-range pseudorigidity with some degree of rotational flexibility compared with the common aromatic ligands (such as 1,2-benzenedicarboxylic acid). (3) The angular bend (θ = 120°) between 3,3dimethylcyclopropane and the two carboxylic acid sites is very important to form the cagelike compounds. (4) The larger steric hindrance of the electron-donating −(CH3)2 substituents on the ligand H2dcd can greatly reduce the interpenetration of the coordination polymers. (5) The cliplike ligand H2dcd has

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EXPERIMENTAL SECTION

General Remarks. All of the syntheses were performed in poly(tetrafluoroethylene)-lined stainless steel autoclaves under autogenous pressure. Reagents were purchased commercially and used without further purification. Deionized water was used throughout. Physical Techniques. Elemental analyses (carbon and hydrogen) were performed on a PerkinElmer 2400 series II CHNS/O elemental analyzer. Microscopic images of the solid-state samples were taken by a Canon digital camera (IXUS 275 HS). Fourier transform infrared (FTB

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Selected Crystallographic Data and Refinement Parameters for UJN-Ln UJN-Gd empirical formula fw temperature/K wavelength/Å crystal system space group a/Ǻ b/Ǻ c/Ǻ α/deg β/deg γ/deg volume/Ǻ 3 Z ρcalcd/)g cm−3) μ/mm−1 F (000) reflns collected indep reflns GOF final R indices [I > 2σ(I)] R indices (all data)

C112H205Gd17O112 6017.01 293(2) 0.71073 tetragonal P4212 25.2485(5) 25.2485(5) 15.9652(6) 90 90 90 10177.6(5) 2 1.963 5.545 5722 34392 8987 [Rint = 0.0524] 0.934 R1 = 0.0388, wR2 = 0.0900 R1 = 0.0507, wR2 = 0.0979

UJN-Tb

UJN-Dy

C112H205Tb17O112 6045.40 293(2) 0.71073 tetragonal P4212 25.1752(5) 25.1752(5) 15.9129(4) 90 90 90 10085.5(4) 2 1.991 5.966 5756 39437 8900 [Rint = 0.0542] 1.080 R1 = 0.0348, wR2 = 0.0870 R1 = 0.0417, wR2= 0.0911

C112H205Dy17O112 6106.26 293(2) 0.71073 tetragonal P4212 25.3946(5) 25.3946(5) 15.9393(5) 90 90 90 10279.0(4) 2 1.973 6.185 5790 28718 9104 [Rint = 0.0543] 1.072 R1 = 0.0336, wR2 = 0.0826 R1 = 0.0380, wR2 = 0.0850

IR) spectra were recorded in the range of 400−4000 cm−1 on a PerkinElmer FT-IR spectrometer using KBr pellets. Thermogravimetric (TG) analysis was performed on a PerkinElmer Diamond TG/ DTA instrument in flowing air with a heating rate of 10 °C min−1. The powder X-ray diffraction (PXRD) patterns were recorded by a Bruker D8-Focus Bragg−Brentano X-ray powder diffractometer equipped with a copper sealed tube (λ = 1.54178 Å) at a scan rate of 0.5 s deg−1. Synthesis of Compounds. Synthesis of UJN-Gd. A total of 0.5 mmol of Gd(NO3)3·6H2O, 0.50 mmol of dohd, and 1.0 mmol of NaOH were mixed in 10.0 mL of H2O and stirred for 15 min at room temperature, generating a transparent solution (the pH value of the mixture was about 5−6). Then, the transparent solution was transferred to a Teflon-lined stainless-steel vessel (23 mL) and heated at 140 °C for 4 days under autogenous pressure. After the reaction mixture was slowly cooled to room temperature, colorless rod-shaped single crystals of UJN-Gd were filtered off, washed with distilled water, and dried in air (Figure S2). Elem anal. Calcd for C112H205Gd17O112 (6017.01): C, 22.36; H, 3.43. Found: C, 22.45; H, 3.31. IR (KBr pellets, cm−1): 3527 vs, 3445 vs, 2963 w, 2930 w, 2871 w, 1635 w, 1580 s, 1539 s, 1463 s, 1384 m, 1297 s, 1215 w, 1127 w, 1049 w, 985 w, 961 w, 877 m, 853 w, 833 w, 714 m, 562 w, 518 w. Synthesis of UJN-Tb. The synthesis of UJN-Tb is similar to that of UJN-Gd but using Tb(NO3)3·6H2O. Colorless rod-shaped crystals of UJN-Tb were obtained (Figure S2). Elem anal. Calcd for C112H205Tb17O112 (6045.40): C, 22.25; H, 3.42. Found: C, 22.36; H, 3.39. IR (KBr pellets, cm−1): 3507 vs, 3435 vs, 2969 w, 2936 w, 2872 w, 1638 w, 1583 s, 1539 s, 1460 s, 1379 w, 1294 s, 1212 w, 1122 m, 1069 w, 1049 w, 981 w, 961 w, 877 m, 853 w, 833 w, 719 m, 562 w, 518 w. Synthesis of UJN-Dy. The synthesis of UJN-Dy is similar to that of UJN-Gd but using Dy(NO3)3·6H2O. Colorless rod-shaped crystals of UJN-Dy were obtained (Figure S2). Elem anal. Calcd for C112H205Dy17O112 (6106.26): C, 22.03; H, 3.38. Found: C, 22.17; H, 3.31. IR (KBr pellets, cm−1): 3507 vs, 3442 vs, 2962 w, 2930 w, 2873 w, 1641 w, 1583 s, 1539 s, 1463 s, 1384 w, 1294 s, 1212 w, 1125 m, 1069 w, 1037 w, 982 w, 961 w, 877 m, 853 w, 833 w, 722 w, 562 w, 518 w. Synthesis of UJN-Ho. The synthesis of UJN-Ho is similar to that of UJN-Gd but using Ho(NO3)3·6H2O. Flaxen rod-shaped crystals of UJN-Ho were obtained (Figure S2). Elem anal. Calcd for

UJN-Ho C112H205Ho17O112 6147.57 293(2) 0.71073 tetragonal P4212 25.1018(3) 25.1018(3) 15.8777(3) 90 90 90 10004.6(2) 2 2.041 6.728 5824 29627 8844 [Rint = 0.0374] 1.091 R1 = 0.0315, wR2 = 0.0742 R1 = 0.0360, wR2 = 0.0770

UJN-Er C112H205Er17O112 6187.18 293(2) 0.71073 tetragonal P4212 25.1663(8) 25.1663(8) 15.8693(9) 90 90 90 10050.7(8) 2 2.044 7.103 5858 38927 8884 [Rint = 0.1161] 1.052 R1 = 0.0684, wR2 = 0.1638 R1 = 0.0843, wR2 = 0.1792

C112H205Ho17O112 (6147.57): C, 21.88; H, 3.36. Found: C, 21.79; H, 3.30. IR (KBr pellets, cm−1): 3507 vs, 3435 vs, 2969 w, 2929 w, 2875 w, 1635 w, 1585 s, 1539 s, 1460 s, 1387 m, 1297 s, 1212 w, 1127 m, 1069 w, 1046 w, 985 w, 961 w, 880 m, 853 w, 836 w, 722 w, 565 w, 518 w. Synthesis of UJN-Er. The synthesis of UJN-Er is similar to that of UJN-Gd but using Er(NO3)3·6H2O. Pink rod-shaped crystals of UJNEr were obtained (Figure S2). Elem anal. Calcd for C112H205Er17O112 (6187.18): C, 21.74; H, 3.34. Found: C, 21.67; H, 3.29. IR (KBr pellets, cm−1): 3514 vs, 3448 vs, 2956 w, 2930 w, 2874 w, 1589 s, 1545 s, 1454 s, 1384 m, 1297 s, 1212 w, 1125 m, 1069 w, 1040 w, 982 w, 958 w, 877 m, 853 w, 833 w, 722 w, 564 w, 518 w. Single-Crystal X-ray Crystallography. Suitable single crystals of UJN-Ln (Ln = Gd, Tb, Dy, Ho, and Er) were selected for singlecrystal X-ray diffraction analysis. Crystal data were collected on an Xcalibur, Eos, Gemini diffractometer (Mo Kα radiation, λ = 0.71073 Å). Data reduction was accomplished by the CrysAlisPro (version 1.171.33.55, Oxford Diffraction Ltd.) program. The structures were solved by direct methods and refined by a full matrix least-squares technique based on F2 using the SHELXL 97 program.40 All of the non-hydrogen atoms were refined anisotropically. The organic hydrogen atoms were generated geometrically, and the aqua hydrogen atoms were located from difference maps and refined with isotropic temperature factors. The structural pictures of the compounds were drawn with the Diamond program.41 Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre. CCDC 1062237 (UJN-Gd), 1062239 (UJN-Tb), 1062234 (UJN-Dy), 1062238 (UJN-Ho), and 1062235 (UJN-Er) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Photoluminescent Measurements. Solid-state emission and excitation spectra were measured with a Horiba Scientific FluoroMax-4 spectrofluorometer using a JX monochromator and a R928P PMT detector at room temperature. Fluorescence lifetime measurements are recorded and detected on the same system using the time-correlated single-photon-counting (TCSPC) method with the FM-2013 accessory and a TCSPC hub from Horiba Jobin Yvon. Analysis of the photoluminescence decay curve was performed with the software C

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Self-assembly details of a Tb17 cluster considering the eight-coordinated Tb1 ion as a structure-directing “code” (the “ball-and stick” mode is used for clarity): (a) geometrically prefixed metal Tb3+ ion; (b) Tb(μ3-OH)8; (c) Tb9(μ3-OH)8; (d) Tb9(μ3-OH)16(μ4-O)(μ4-OH); (e) Tb17(μ3OH)16(μ4-O)(μ4-OH). Symmetry codes: A, 0.5 − y, 0.5 + x, z; B, −0.5 + y, 0.5 − x, z; C, −x, 1 − y, z. (The coordinate vector represents the interaction between the coordinate site and metal.)

Figure 3. Self-assembly details of the nested square-antiprism arrays considering the eight-coordinated Tb1 ion as a structure-directing “code” (the “ellipsoids” mode is used for clarity): (a) one isolated square antiprism; (b) two nested square antiprisms; (c) three nested square antiprisms; (d) four nested square antiprisms. Symmetry codes: A, 0.5 − y, 0.5 + x, z; B, −0.5 + y, 0.5 − x, z; C, −x, 1 − y, z. Origin (version 8.0), and the quality of the fitted curve was assessed by the R2 value. For measurement of the solid-state emission quantum yield, an Horiba Jobin Yvon F-3018 integrating sphere was equipped to the FluoroMax-4 spectrofluorometer, and the specpure BaSO4 was used as a reflecting standard. Data analysis for the external quantum yield was performed with the PLQY measurement software, provided by Horiba Jobin Yvon. Magnetic Measurements. Magnetic susceptibility measurements were performed on a Quantum Design SQUID MPMS XL-7 instrument within the temperature range of 2.0−300 K under an applied magnetic field of 1000 Oe. Field-dependent magnetization for UJN-Gd and UJN-Dy was measured at 2.0−10.0 K within the field range of 0.0−7.0 T. The alternating-current (ac) susceptibility

measurements of UJN-Ln were performed at the zero static field with an oscillation of 3.0 Oe in the temperature range of 2.0−20 K. The diamagnetic corrections for all of the compounds were estimated using Pascal’s constants,42 and magnetic data were corrected for the diamagnetic contributions of the sample holder.

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RESULTS AND DISCUSSION Description of the Crystal Structures. Single-crystal Xray diffraction analysis revealed that all of the compounds UJNLn are isostructural and crystallize in the high-symmetry tetragonal space group P4212. The crystallographic data and refinement parameters for UJN-Ln are given in Table 1. D

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The asymmetric unit for each of the compounds contains five crystallographically unique lanthanide ions. Ln1 is coordinated by eight μ3-OH ligands. Both Ln2 and Ln3 are nine-coordinate: one μ4-OH (or μ4-O), four μ3-OH groups, two carboxylate oxygen atoms (COAs) from two bridging dcd ligands, and two COAs from one dcd ligand. Similar to Ln1, both Ln4 and Ln5 are also coordinated by eight oxygen atoms: one μ3-OH group, one terminal aqua ligand, two COAs from two bridging dcd ligands, and four COAs from two dcd ligands. It is noticeable that the central Ln1 forms a regular square-antiprism configuration with the top and bottom planes defined by eight hydroxy oxygen atoms. Viewed as a right structuredirecting “code”, the eight-coordinated Ln1 also induces a novel Ln17 cluster containing the nested square-antiprism arrays, as shown in Figures 1−3. Actually, the Ln17 cluster is built upon one central lanthanide ion (resided on a crystallographic 2-fold axis) and 16 peripheral lanthanide ions with the help of hydroxo and oxo bridges. Sixteen peripheral lanthanide ions can be divided into four groups, and each of them involves four lanthanide ions. The two groups, i.e., (Ln2, Ln2A, Ln2B, and Ln2C) and (Ln3, Ln3A, Ln3B, and Ln3C), are held together by the μ4-OH and μ4-O ligands, respectively. The other two groups, i.e., (Ln4, Ln4A, Ln4B, and Ln4C) and (Ln5, Ln5A, Ln5B, and Ln5C), are separated by eight μ3-OH ligands. It is interesting that 17 lanthanide ions are bridge-linked together by one μ4−OH, one μ4-O and 16 μ3−OH ligands, giving a unique {[Ln(μ3− OH)8][Ln16(μ4-O)(μ4-OH)(μ3-OH)8]}32+ cluster featuring the nested arrays (Figure 3). The nested arrays consist of three classes of regular square antiprisms and one bicapped square antiprism via the apical Ln1 ion. For each of the square antiprisms, the upper and bottom regular squares are twisted at 45° to each other. The selected bonds Ln1−O18 and Ln1− O20, together with the shortest separations of Ln1···O19, Ln1···O21, Ln2···Ln2, Ln3···Ln3, Ln4···Ln4, Ln5···Ln5, O18··· O18, O20···O20, O17···O17, and O22···O22, are tabulated in Table S1. It should be pointed out that most of the coordination sites of the lanthanide ions in this Ln17 cluster are occupied by hydroxo and oxo bridges, and the remnant unsaturated ones from 16 lanthanide ions are further linked by peripheral cliplike dcd ligands, giving rise to a new family of octagonal-prismatic LCCs with the decoration of two bowl-like brackets along the b axis (Figures 4 and S3). Closer inspection reveals that the two coordination modes of the dcd ligands with lanthanide ions are very important in the formation of the LCCs (Figure S1). The peripheral position of

the Ln17 cluster core is mechanistically hooked by eight tetradentate ligands (μ4-dcd, denoted as the green parts in Figure 4) from top to bottom, resulting in a primordial coordination cage with eight arrises. With further clamping coordination, the other eight tridentate ligands (μ3-dcd, denoted as the rufous parts in Figure 4) serve to decorate the primordial coordination cage, generating two bowl-like brackets with a diameter of 1.7 nm along the b axis. The final remnant unsaturated coordination sites of the lanthanide ions (namely, Ln4, Ln4A, Ln4B, Ln4C, Ln5, Ln5A, Ln5B, and Ln5C) are occupied by another eight terminal aqua ligands pointing toward the outside of the coordination cage, as shown in Figures 4 and S3. The charming concentric metal rings in the coordination cage are also presented along the c axis (Figure 4b). In addition, the absence of any counterions in the refined structure proves the neutrality of the LCC. Therefore, a neutral discrete octagonal-prismatic LCC with two bowl-like brackets can be formulated as [Ln(μ3-OH)8][Ln16(μ4-O)(μ4-OH)(μ3OH)8(H2O)8(μ4-dcd)8][(μ3-dcd)8]·22H2O. Here, three components enclosed in square brackets represent a structuredirecting template anion, an octagonal-prismatic coordination cage, and the attached two bowl-like brackets, respectively. Attentively, the discrete LCC displays a nanoscale dimension (ca. 1.7 nm × 1.9 nm along the b axis for UJN-Tb). Although the hydrophobic dimethyl groups in the periphery of the dcd ligands prevent the discrete LCC units from aggregating into an infinite cage-based framework, an intricate network of hydrogen bonds among the carboxylate groups of the dcd ligands, μ3-OH, μ4-OH groups, and coordinated and lattice water molecules, induces the packing arrangement of the octagonal-prismatic LCCs into a 3D supramolecular structure (Figure 5). Along the c axis, the 3D packing structure contains a great deal of concentric metal rings and numerous pores (the diameter is ca. 9.6 Å for UJN-Tb) filled with water molecules. By virtue of the PLATON analysis,43 approximately 34.7% of the crystal volume for UJN-Tb is occupied by water molecules (3495.5 Å3 out of 10085.5 Å3 in each cell unit). FT-IR, TG, and PXRD Analysis. The FT-IR spectra for compounds UJN-Ln show similar characteristic peaks due to the same organic ligands (Figure S4). More specifically, these compounds exhibit broad absorption bands at 3530−3440 cm−1, which can be assigned to the water molecules in the structures. The strong bands at 1380−1600 cm−1 are characteristic of the asymmetric and symmetric stretching vibrations of the carboxylate groups. Also, the absence of the absorption bands at 1690−1730 cm−1 indicates complete deprotonation of the ligand H2dcd. The absorption bands at 2871−2970 cm−1 indicate the existence of the asymmetric and symmetric stretching vibrations of the methyl groups in the organic ligand. The results of the FT-IR spectra reveal the presence of water molecules and deprotonated carboxylate groups, in agreement with the solid-state structures of all of the compounds. To examine the thermal stabilities of compounds UJN-Ln, TG analysis was performed on these crystal samples. UJN-Ln show similar thermal behaviors because of their same structures. Here, only one representative compound UJN-Gd is discussed. As shown in Figure S5, the TG curve of UJN-Gd exhibits three main steps of weight loss. The first weight loss (6.22%) between room temperature and 110 °C is in good agreement with the release of 22 solvent water molecules existing in the structure (calcd loss: 6.58%). The second weight loss (2.28%) in the temperature range of 110−200 °C corresponds to the loss of the remaining eight coordinated

Figure 4. Octagonal-prismatic LCCs with two bowl-like brackets (data from UJN-Tb; the hydroxyl hydrogen atoms are omitted, and the modes of “ball-and stick” and “wires/sticks” are used for clarity): (a) view along the b axis; (b) view along the c axis. E

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Space-filling diagrams for UJN-Tb (the solvent molecules are removed for clarity): (a) discrete coordination cage with a nanoscale dimension of ca. 1.7 nm × 1.9 nm along the b axis; (b) discrete coordination cage showing concentric metal rings with a diameter of ca. 1.7 nm along the c axis; (c) 3D supramolecular structure showing numerous pores with a diameter of ca. 9.6 Å along the c axis.

G5, 7F6 → 5G6, and 7F6 → 5D3 transitions, respectively.44 Among different excitation transitions, 7F6 → 5G6 (367 nm) is more prominent. Upon excitation at λex = 367 nm, the emission spectrum of UJN-Tb exhibits four sharp lines at 491, 544, 585, and 622 nm, which can be ascribed to the 5D4 → 7F6, 5D4 → 7 F5, 5D4 → 7F4, and 5D4 → 7F3 electronic transitions, respectively.45 The most prominent line is observed at 544 nm. Apart from the excitation and emission spectra, the solidstate luminescent lifetime and external quantum yield for UJNTb were also measured at room temperature. As shown in Figure S11, the 5D4 emission decay curve was monitored within the 5D4 → 7F5 transition under an excitation wavelength of 367 nm, which can well be fitted to a single-exponential function with a good approximation (0.99996), giving a 5D4 lifetime value of 1.29 ms. The solid-state emission quantum yield of UJN-Tb was measured by means of an integrating sphere. Data analysis shows that the external quantum yield is 8.76% with an absolute error of 0.081%. The detailed information for measuring the external quantum yield of UJN-Tb can be found in Figures S12 and S13. Note that the emissions of the free organic ligand H2dcd and compounds UJN-Ln (Ln = Gd, Dy, Ho, and Er) cannot be detected in the visible spectrum under the same experimental conditions. Magnetic Properties. Static Magnetic Properties. It is known that magnetic interactions should arise because of the multiple short distances between the lanthanide ions in the cluster. Therefore, the magnetic properties for all of the compounds are studied in order to gain possible access to multifunctional materials. The direct-current (dc) magnetic susceptibility studies are first performed on UJN-Ln in the range of 2.0−300 K under an applied field of 1 kOe. As shown in Figure 7 (denoted in red), the results plotted as the χMT product versus T reveal that each of the compounds displays similar behavior; i.e., the value of the χMT product decreases gradually with decreasing temperature first; upon a further 5

water molecules (calcd loss: 2.39%). Then the third weight loss occurs gradually between 200 and 700 °C, due to decomposition of the main framework. The final residue can be viewed as Gd2O3 (obsd 53.35%; calcd 51.21%) for UJN-Gd. Furthermore, the phase purities of all of the compounds are supported by the PXRD patterns of the bulk samples (Figures S6−S10). The patterns of the bulk samples match those calculated from their single-crystal structure data well, which demonstrates that each of the compounds is presented as a single phase. Attentively, the differences in the intensity between the measured and simulated patterns may be due to the preferred orientation of the crystalline powder samples during collection of the experimental PXRD data. Photoluminescent Properties. The photoluminescent properties for all of the compounds were investigated in the solid state at room temperature. The excitation and emission spectra for UJN-Tb are shown in Figure 6. The excitation spectrum obtained by monitoring the 5D4 → 7F5 emission line at 544 nm consists of several discrete sharp transition lines. Obviously, the sharp lines at 341, 350, 358, 367, and 378 nm are related to the intraconfigurational 4f−4f transitions of Tb3+ ions and can be ascribed to the 7F6 → 5G2, 7F6 → 5D2, 7F6 →

Figure 6. Excitation (left, λem = 544 nm) and emission (right, λex = 367 nm) spectra of UJN-Tb. F

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Plots of temperature-dependent χMT (red □) and χM−1 (blue ○) under a 1 kOe dc field between 2.0 and 300 K for (a) UJN-Gd, (b) UJNTb, (c) UJN-Dy, (d) UJN-Ho, and (e) UJN-Er. The green line is the Curie−Weiss law fitting (inset: fitting parameters).

Table 2. Observed and Calculated χMT Values at Room Temperature for UJN-Ln and Relevant Parameters for Calculations

lowering of the temperature, the value drops abruptly with a finally minimum value at 2.0 K. This phenomenon may be ascribed to a combination of the antiferromagnetic interaction between adjacent Ln3+ ions and the zero-field splitting of the ground states for the 4fn configuration.46,47 The nonzero χMT values at 2.0 K suggest possible antiferromagnetic dipole−dipole interactions between the molecules.48 It is noticeable that the room temperature χMT values are very close to the theoretical ones expected for 17 noninteracting Ln3+ ions. The correspondingly observed and calculated χMT values at room temperature, together with the relevant parameters for calculations,46 are summarized in Table 2. Additionally, as shown in Figure 7 (denoted in blue), the curves of χM−1 versus T ranging from 2.0 to 300 K can be nicely fitted following the Curie−Weiss law 1/χM = (T − θ)/C, giving the Curie constants C = 134.55, 202.74, 241.71, 243.56, and 198.90 cm3 K mol−1 and the Weiss constants θ = −2.61, −3.87, −3.64, −7.08, and −8.67 K for UJN-Gd, UJN-Tb, UJN-Dy, UJN-Ho, and UJN-Er, respectively. Similar to the observed χMT values at room temperature (Table 2), the C values for UJN-Gd, UJN-Tb, UJN-Dy, UJN-Ho, and UJN-Er are also consistent with the expected values for 17 magnetically isolated Gd3+ (S = 7/2 and g = 2.0), Tb3+ (S = 3 and g = 3/2), Dy3+ (S = 5 /2 and g = 4/3), Ho3+ (S = 2 and g = 5/4), and Er3+ (S = 3/2 and g = 6/5) ions, respectively. Besides the profile of the χMT value

UJN-Gd S (Ln3+)a L (Ln3+)a J (Ln3+)a g (Ln3+)a χM T (Ln3+)b χM T (sample calcd)b χM T (sample obsd)b

UJN-Tb

UJN-Dy

UJN-Ho

UJN-Er

7

/2 0 7 /2 2 7.88

3 3 6 3 /2 11.82

5

/2 5 15 /2 4 /3 14.17

2 6 8 5 /4 14.07

3

/2 6 15 /2 6 /5 11.48

7.88 × 17 = 133.96

11.82 × 17 = 200.94

14.17 × 17 = 240.89

14.07 × 17 = 239.19

11.48 × 17 = 195.16

134.4

201.89

241.51

238.74

194.89

a

S, L, and J are the quantum numbers for the total spin angular momentum, total orbital angular momentum, and total angular momentum of the ground multiplet, respectively; g is the Landé factor. bValues of χM T are given in cm3 K mol−1.

versus T, the negative value of the Weiss constant θ further demonstrates predominantly antiferromagnetic coupling between adjacent Ln3+ ions. Also, compared with the other compounds UJN-Ln (Ln = Tb, Ho, and Er), the smaller negative Weiss constants for UJN-Gd (θ = −2.61) and UJN-Dy G

DOI: 10.1021/acs.inorgchem.5b02367 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. (a) Field-dependent experimental magnetization plots for UJN-Gd at the indicated temperatures. (b) Calculated changes of the magnetic entropy for UJN-Gd at various fields and temperatures.

Figure 9. (a) Field-dependent experimental magnetization plots for UJN-Dy at the indicated temperatures. (b) Calculated changes of the magnetic entropy for UJN-Dy at various fields and temperatures.

(θ = −3.64) may be related to the weak magnetic interactions between the corresponding lanthanide ions. Because of weak magnetic interactions, the field dependences of the magnetic measurements were carried out in the range of 0.0−7.0 T at 2.0−10.0 K for both UJN-Gd and UJN-Dy. As shown in Figures 8a and 9a, the plot of M versus H for each of them displays a gradual increase with increasing field. Figure 8a shows that the M value for UJN-Gd reaches a saturation value of 118.0 Nβ at 2.0 K and 7.0 T (where N is the Avogadro constant and β is the Bohr magneton), which is almost identical with the theoretical value of 119.0 Nβ for 17 individual Gd3+ (7 Nβ for each Gd3+ ion with J = 7/2 and g = 2). For UJN-Dy, the M value reaches a maximum value of 84.84 Nβ at 2.0 K and 7.0 T without any sign of saturation (Figure 9a), which is half of the expected saturation value of 170.0 Nβ (10 Nβ for each Dy3+ ion with J = 15/2 and g = 4/3), confirming again the occurrence of predominantly antiferromagnetic interactions for UJN-Dy. Indeed, the observed maximum value of 84.84 Nβ at 2.0 K and 7.0 T is close to the expected theoretical value of 88.4 Nβ (17 × 5.2) for 17 uncorrelated Dy3+ ions with a value of 5.2 Nβ per Dy3+ ion assuming the presence of considerable ligand-field effects.49 Furthermore, the M versus HT−1 data for UJN-Dy at high fields do not saturate or overlay onto a single master curve (Figure S14). The lack of saturation on the M versus H data, in conjunction with the nonsuperposition for the M versus HT−1 plot, can be attributed to the presence of significant magnetic anisotropy and/or ligand-field-induced splitting of the Stark level of Dy3+ in UJNDy, which is often seen in Dy3+-based complexes.50 More importantly, the magnetic density of UJN-Gd is very high, which is reflected by the low Mw/NGd ratio of 353.94. The weak Gd3+···Gd3+ magnetic coupling and relatively low Mw/ NGd ratio are helpful for enhanced MCEs and will make UJN-

Gd a promising candidate for low-temperature magnetic cooling. The magnetic entropy change ΔSm, as a key parameter in evaluating MCE,51 can be calculated from the experimental magnetization data and the Maxwell equation ΔSm(T)ΔH = ∫ [∂M(T,H)/∂T ]H dH.52 As shown in Figure 8b, the observed −ΔSm values for UJN-Gd increase with increasing magnetic field and decreasing temperature, reaching an impressive value of 42.3 J kg−1 K−1 at 2.0 K with an applied field change (ΔH) of 7.0 T. The −ΔSm value of UJN-Gd is smaller than the theoretical value of 48.8 J kg−1 K−1 for 17 uncoupled Gd3+ ions [calculated using the function of −ΔSm = nR ln(2S + 1) = 17R ln 8 = 35.35R, where R is the gas constant, n is the degeneracy, and S is the ground-state spin] because of the presence of an antiferromagnetic exchange among the paramagnetic Gd3+ ions. However, the experimentally obtained −ΔSm value (42.3 J kg−1 K−1) for UJN-Gd at 2.0 K and ΔH = 7.0 T is comparable to that of 46.9 J kg−1 K−1 at 2.0 K and ΔH = 7.0 T in the case of the largest Gd104 cluster compound recently reported by Kong and co-workers.34 Considering the weak magnetic coupling between the Gd3+ ions, the large MCE of UJN-Gd primarily originates from the high magnetic density determined by a high-nuclearity cluster (Gd17) and the simple bridge-linking ligand with small molecular weight. Comparatively, as shown in Figure 9b, the −ΔSm versus T plot for UJN-Dy demonstrates a gradual increase with increasing ΔH and decreasing temperature from 10.0 to 2.0 K at low magnetic fields (