Article pubs.acs.org/IC
Three Giant Lanthanide Clusters Ln37 (Ln = Gd, Tb, and Eu) Featuring A Double-Cage Structure Yang Zhou,† Xiu-Ying Zheng,† Jing Cai, Zi-Feng Hong, Zhi-Hao Yan, Xiang-Jian Kong,* Yan-Ping Ren, La-Sheng Long,* and Lan-Sun Zheng Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China S Supporting Information *
ABSTRACT: Three homometallic high-nuclearity clusters, formulated as [(CO3)2@ Ln37(LH3)8(CH3COO)21(CO3)12(μ3-OH)41(μ2-H2O)5(H2O)40]·(ClO4)21·(H2O)100 (abbreviated as Ln37, Ln = Gd (1); Tb (2); Eu (3), LH3 = 1,2,3-cyclohexanetriol) and featuring a double cage-like structure, were obtained through the reaction of 1,2,3-cyclohexanetriol, acetate ligand, and Ln(ClO4)3. The largest odd-numbered lanthanide cluster Gd37 exhibits an entropy change (−ΔSm) of 38.7 J kg−1 K−1.
1. INTRODUCTION Over the past two decades, we have witnessed intense research activity in the field of giant metal clusters, not only for their intriguing geometrical features but also because of their fascinating physical and chemical properties arising from the interplay between spins located on different metal ions.1−3 As an important family of metal clusters, the synthesis and characterization of giant homometallic molecular aggregates is a challenging but rapidly developing research field, driven largely by their potential applications in molecular magnetism and magnetocaloric materials.4−8 Although great efforts have been made to prepare high-nuclearity lanthanide clusters, only a few giant clusters (over 30 metal ions), such as ball-like Ln36,5 cagelike Ln386 and Ln60,7 barrel-like Ln48,6 tubular-like Ln72,8 and multishell Ln104,4 have been reported. A successful strategy to prepare high-nuclearity lanthanide clusters is the ligand-controlled hydrolytic approach, which uses suitable ligands to control the growth of the metal cores.9 In such an approach, the choice of the ancillary ligands is very important, because their steric hindrance, charge, and coordination mode have important effects on the structure of the metal cluster core. Among the large number of ancillary ligands, carboxylates have been studied most extensively. For example, Ln36 and Ln48 protected by nicotinic acid, Ln38 and Ln48 supported by chloroacetic acid, Ln60 prepared through threonine, and the largest cluster, Ln104, based on the simplest ligand acetic acid, have been reported. Although some researchers have shown that amino-polyalcohol ligands are candidates to assemble high-nuclearity 3d clusters,10 it is rare to prepare high-nuclearity 4f clusters by utilizing polyalcohol ligands. © 2017 American Chemical Society
In our previous work, we successfully obtained two 3d-4f clusters Ln24M2 (M = Ni and Mn) through myo-inositol ligand11a and prepared two nonanuclear lanthanide complexes with deprotonated Chromogen I derived in situ from the starting N-acetyl-D-glucosamine.11b Recently, many efforts have used polyalcohol ligands or amino-polyalcohol ligands to connect the 3d-4f metals into larger assemblies.11c These results suggest that polyalcohol ligands may be favorable candidates to assemble lanthanide-containing clusters, because they possess multioxygen donors bonding to multilanthanide ions.11,12 Following this idea, we choose 1,2,3-cyclohexanetriol (LH3) as the chelated polyalcohol ligand to control the hydrolysis of Ln3+ ions. Here, we report three nanoscale lanthanide clusters with the formula [(CO 3 ) 2 @ Ln37(LH3)8(CH3COO)21(CO3)12(μ3-OH)41(μ2H2O)5(H2O)40]·(ClO4)21·(H2O)100 (Ln = Gd (1); Tb (2); Eu (3), LH3 = 1,2,3-cyclohexanetriol). Crystal structure analysis shows that these largest odd-numbered pure lanthanide clusters Ln37 feature a double cage-like structure encapsulating two CO32− anions.
2. EXPERIMENTAL SECTION Materials and Physical Measurements. Caution! Perchlorates are potentially explosive. Only a small amount should be used, and they should be handled with great care. All reagents were of commercial origin and were used as received. Aqueous solutions of lanthanide perchlorates were prepared by digesting lanthanide oxides in concentrated perchloric acid. A suitable concentration was achieved Received: November 9, 2016 Published: February 6, 2017 2037
DOI: 10.1021/acs.inorgchem.6b02714 Inorg. Chem. 2017, 56, 2037−2041
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Inorganic Chemistry by dilution of the concentrated solution with deionized water. The C, H, and N microanalyses were performed with a CE instruments EA 1110 elemental analyzer. The infrared spectra were recorded on a Nicolet AVATAR FT−IR360 spectro-photometer with pressed KBr pellets. The thermogravimetric analysis (TGA) curves were prepared on an SDT_Q600 Thermal Analyzer. Magnetic susceptibility was measured by a Quantum Design MPMS superconducting quantum interference device (SQUID). Syntheses of Compounds 1−3. [(CO3)2@ Gd37(LH 3)8(CH3 COO)21(CO3)12(μ3-OH)41(μ2 -H2O)5(H 2O)40]·21ClO4· 100H2O (1). 1,2,3-Cyclohexanetriol (132.2 mg, 1.0 mmol), sodium acetate (136.0 mg, 1.0 mmol), and Gd(ClO4)3 (3.0 mL, 3.0 mmol) were mixed together in 5 mL of anhydrous ethanol and 3 mL of acetonitrile. Then, a freshly prepared NaOH solution (aq 1.0 M) was added dropwise to the mixture to the point of incipient but permanent precipitation while heating and stirring the mixture solution at ∼80 °C. After that, the mixture was maintained under reflux for 2 h. Then, the resulting solution was filtered and evaporated under ambient conditions. After approximately one month, colorless crystals were obtained (yield 20% based on Gd 3+ ). Anal. Calcd for Gd37C104O378H490Cl21 (FW = 14353.53): C, 8.70; H, 3.51. Found: C, 8.72; H, 2.72%. IR (KBr, cm−1): 3402 (s), 2943 (w), 2018 (w), 1563 (s), 1435 (s), 1331 (w), 1144 (m), 1113 (s), 1083 (m), 979 (w), 938 (w), 623 (s). [(CO3)2@Tb37(LH3)8(CH3COO)21(CO3)12(μ3-OH)41(μ2-H2O)5(H2O)40]· 21ClO4·100H2O (2). Compound 2 was prepared using the same procedure as described above for the synthesis of 1 but using Tb(ClO4)3 in place of Gd(ClO4)3. Colorless crystals were obtained in 20% yield based on Tb3+. Anal. Calcd For Tb37C104O378H490Cl21 (FW = 14415.52): C, 8.66; H, 3.49. Found: C, 8.40; H, 2.71%. IR (KBr, cm−1): 3402 (s), 2943 (w), 2018 (w), 1563 (s), 1435 (s), 1331 (w), 1144 (m), 1113 (s), 1083 (m), 979 (w), 938 (w), 623 (s). [(CO3)2@Eu37(LH3)8(CH3COO)21(CO3)12(μ3-OH)41(μ2-H2O)5(H2O)40]· 21ClO4· 100H2O (3). Compound 3 was prepared using the same procedure as described above for the synthesis of 1 but using Eu(ClO4)3 in place of Gd(ClO4)3. Colorless crystals were obtained in 20% yield based on Eu3+. Anal. Calcd For Eu37C104O378H490Cl21 (FW = 14157.95): C, 8.82; H, 3.49. Found: C, 8.59; H, 2.99%. IR (KBr, cm−1): 3402 (s), 2943 (w), 2018 (w), 1563 (s), 1435 (s), 1331 (w), 1144 (m), 1113 (s), 1083 (m), 979 (w), 938 (w), 623 (s). X-ray Crystallography. Data for compound 1 were collected on an Agilent Technologies Supernova Micro Focus detector with Mo Kα radiation (λ = 0.710 73 Å) at 100 K. Data for compound 2 were collected on an Oxford Gemini S Ultra CCD area detector with monochromatic Mo Kα radiation (λ = 0.710 73 Å) at 173 K. Absorption corrections were applied by using the multiscan program CrysAlis Red.13 Data for compound 3 were collected on a MarCCD mx300 in the National Center for Protein Sciences Shanghai at the Shanghai Synchrotron Radiation Facility. The structures were solved by direct methods, and non-hydrogen atoms were refined anisotropically by least-squares on F2 using the SHELXTL-97 program.14 The hydrogen atoms of the organic ligand were generated geometrically (C−H, 0.96 Å). Crystal data, as well as data collection and refinement details for the complexes, are summarized in Table S1. Selected bonds are shown in Tables S1−S3. According to the charge balance, there are 21 ClO4− counteranions per formula unit. On the basis of the elemental analysis (EA) and TGA, there are ca. 100 guest water molecules per formula unit. Additional crystallographic information is available in the Supporting Information.
Figure 1. Ball-and-stick view of the cationic cluster [(CO3)2@Gd37(μ3OH)41(μ2-H2O)5(LH3)8(CH3COO)21(CO3)12(H2O)40]21+.
Figure 2. (a) Ball-and-stick view of unit I, cubane-like [Gd4(μ3OH)4]8+; (b) unit II, a triangle-like structure [Gd3(μ3-OH)5]4+; (c) the Gd-centered trigonal prismatic unit [Gd7(μ3-OH)3]18+; (d) the ringlike unit [(CO3)@Gd12(μ3-OH)11(CO3)3]17+; (e) the cage-like unit [(CO3)@Gd15(μ3-OH)16(CO3)3]21+; and (f) the cationic core [(CO3)2@Gd37(μ3-OH)41(CO3)12]42+.
(H2O)40]21+ (Figure 1), 21 ClO4− and ∼100 guest water molecules. The carbonate was generated by atmospheric CO2 fixation in these structures.15 The role of carbonate in forming high-nuclearity metal clusters is to not only act as a template to induce the formation of the cluster frameworks but also to balance the positive charges of the metal ions as negative charges to stabilize the cluster. The cationic cluster core of 1 consists of a peanut-like core [(CO3)2@Gd37(μ3-OH)41(CO3)12]42+, eight protected 1,2,3cyclohexanetriol ligands, 21 acetate ligands, and 40 coordinated water molecules. The peanut-like cationic core can be viewed as
3. RESULTS AND DISCUSSION Crystal Structural Analysis. The Ln37 clusters were obtained by the reaction of Ln(ClO4)3 (Ln = Gd/Tb/Eu), 1,2,3-cyclohexanetriol, and sodium acetate in a mixed solution of EtOH and MeCN. Single-crystal X-ray diffraction shows that the three compounds 1−3 are isostructural. As a representative example, we only illustrated the structural features of 1 in detail. Compound 1 consists of one cationic cluster of [(CO3)2@ Gd 37( μ 3 -OH) 41 (μ 2 -H 2 O) 5 (LH 3 ) 8 (CH 3 COO) 21 (CO 3 ) 12 2038
DOI: 10.1021/acs.inorgchem.6b02714 Inorg. Chem. 2017, 56, 2037−2041
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Inorganic Chemistry
Table 1. −ΔSmax Data Based on ΔH at a Given Temperature for Some Gd-Based Giant Clusters
a
Figure 3. (a) Ball-and-stick view of the metal skeleton of Gd37.
complex
−ΔSmax (J kg−1 K−1)
−ΔSmax (mJ cm−3 K−1)
T, K
ΔH (kOe)
[Gd24]9d [Gd27]15 [Gd36]5 [Gd37]a [Gd38]6 [Gd48]6 [Gd104]4
46.1 41.8 39.7 38.7 37.9 43.6 46.9
89.9 120.4 91.3 100.0 102 120.7 137.2
2.5 2.0 2.5 2.0 1.8 1.8 2.0
70 70 70 70 70 70 70
This work.
viewed as one central Gd3+ linking three Gd2 units by three μ3OH− bridges, producing the one Gd-centered trigonal prismatic topology of [Gd7(μ3-OH)3]18+ (Figure 2c). As shown in Figure 2f, two cage-like [(CO3)@Gd15(μ3OH)16(CO3)3]21+ units and one type-III unit [Gd7(μ3OH)3]18+ are joined together through six CO32− anions and six μ3-OH, forming the peanut-like core [(CO3)2@Gd37(μ3OH)41(CO3)12]42+. The peanut-like core is further stabilized by eight 1,2,3-cyclohexanetriols, 21 acetates as organic ligands, five bridged waters, and 40 aqua ligands. The metal skeleton of compound 1 is shown in Figure 3a. Three tetrahedral (12 Gd3+) and two triangles (6 Gd3+) were connected together, forming the framework of one cage (Figure S2a). Two cages link together by one centered Gd3+, generating the peanut-like metal skeleton of 1. The inner metal structure containing 31 Gd3+ ions consists of 12 pentagonal and 20 triangular faces (Figure 3b). Each pentagonal face is templated by one CO32− anion. The ranges of the Gd···Gd distances are 3.6008(12)−3.9856(18) Å, close to the previously reported values in Gd-clusters.16
being constructed from three different types of cluster units, a cubane-like [Gd4(μ3-OH)4]8+ unit (I, Figure 2a), a triangular structural unit [Gd3(μ3-OH)5]4+ (II, Figure 2b), and one Gdcentered trigonal prismatic cluster of [Gd7(μ3-OH)3]18+ (III, Figure 2c). In the type I unit [Gd4(μ3-OH)4]8+, four Gd3+ atoms connect with each other through four μ3-OH, forming a common cubane-like configuration.15 As shown in Figure 2d, three units I connect together by three CO32−, forming a ringlike unit [(CO3)@Gd12(μ3-OH)11(CO3)3]17+ in which one μ3OH group is replaced by one CO32−. Type II, formulated as [Gd3(μ3-OH)5]4+, can be viewed as a triangular Gd3 linked by five OH−, in which two OH− cap the trimetallic plane and three OH− bridge three edges of the triangle. The type II unit [Gd3(μ3-OH)5]4+ and the ring-like unit [(CO3)@Gd12(μ3OH)11(CO3)3]17+ connect together by sharing the coordination of three OH− from [Gd3(μ3-OH)5]4+ and the CO32− from the ring-like unit, generating the cage-like [(CO3)@Gd15(μ3OH)16(CO3)3]21+ unit (Figure 2e). Type III unit can be
Figure 4. (a) Plot of temperature dependence of χMT for 1 and 2 under a 1000 Oe dc field between 2 and 300 K; (b) magnetization vs H/T for 1 and 2 at 2 K and at indicated fields; (c) field dependence of the magnetization plots for 1 at the indicated temperatures; and (d) ΔSm calculated by using the magnetization data at various fields and temperatures. 2039
DOI: 10.1021/acs.inorgchem.6b02714 Inorg. Chem. 2017, 56, 2037−2041
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Inorganic Chemistry Magnetic Properties. We measured the variable-temperature magnetic susceptibility of compounds 1 and 2 over the temperature ranges of 2 to 300 K with an applied direct-current (dc) magnetic field of 1000 Oe (Figure 4a). At room temperature, the χMT values are 290.0 cm3 K mol−1 for 1 and 436.17 cm3 K mol−1 for 2, in agreement with the theoretical expected values of 291.37 cm3 K mol−1 for 37 uncoupled Gd(III) ions (J = 7/2, g = 2) and 437.06 cm3 K mol−1 for 37 uncoupled Tb(III) ions (J = 6, g = 3/2), respectively. For 1, the χMT value remains basically constant between 125 and 300 K and then decreases gradually with temperature to 20 K. When further cooled, the χMT value starts to decrease sharply to the minimum value of 168.40 cm3 K mol−1 at 2 K. For 2, the χMT value remains essentially constant above 100 K and then decreases slowly to the minimum of 298.10 cm3 K mol−1 at 2 K. The variable-temperature magnetic susceptibility data suggest that 1 and 2 have both shown intracluster anti-ferromagnetic interaction.17 In the temperature range from 2 to 300 K, the data can be fitted with the Curie− Weiss law (Figure S8), resulting in C = 292.39 cm3 K mol−1 and θ = −1.75 K for 1 and C = 442.47 cm3 K mol−1 and θ = −3.53 K for 2, respectively, which further confirm the antiferromagnetic interactions between metal ions.18 The field dependence of the magnetization for 1 and 2 was also measured at 2 K and 0−7 T (Figure 4b). The magnetization value of 265.09 NμB for 1 is close to the saturation value of 259.00 NμB, suggesting a lack of significant anisotropy and low-lying excited states. For 2, the magnetization value of 181.59 NμB at 2 K and 7 T is slower than the saturation value of 333.00 NμB, indicating strong anisotropy and zero-field splitting.19 Because of the isotropy of Gd3+ ion and the high magnetic density of the high-nuclearity metal cluster, the magnetocaloric effect of compound 1 was studied. As shown in Figure 4c,d, the magnetic entropy change (ΔSm) is 38.7 J·kg−1·K−1 at 2 K for ΔH = 7 T by using the Maxwell equation of ΔS m(ΔH) = ∫ [∂M(T, H)/∂T] H dH. 20 The experimental value is smaller than the calculated value of 45 J·kg−1·K−1 for 37 uncorrelated Gd3+ ions (S = 7/2) based on the equation −ΔSm = nR ln(2S + 1) (where R is the gas constant, and S is the spin state),21 which may have resulted from the anti-ferromagnetic interaction. The observed value of −ΔSm value is close to the values of other high-nuclearity lanthanide clusters.22 As shown in Table 1, the large volumetric magnetic entropy change of 100 mJ cm−3 K−1 for 1 is comparable to the large value attained for reported highnuclearity lanthanide clusters. For compound 2, temperature dependencies of the alternating-current magnetic susceptibility were collected at Hdc = 0 Oe between 10 and 1000 Hz. As shown in Figure S9, compound 2 does not display frequencydependent out-of-phase signals, due to the fast quantum tunneling of the magnetization.23
lanthanide-containing clusters. We are attempting to synthesize other high-nuclearity 4f-clusters by adopting chelated polyalcohols as ligands.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02714. Additional figures of the structures (PDF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF) Crystallographic data in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. (X.J.K.) *E-mail:
[email protected]. (L.S.L.) ORCID
Xiang-Jian Kong: 0000-0003-0676-6923 La-Sheng Long: 0000-0002-0398-4709 Author Contributions †
These authors (Y. Z. and X.-Y. Z.) contributed equally to this work. Notes
The authors declare no competing financial interest. CCDC contains the supplementary crystallographic data for this paper with deposition numbers of 1511373 for 1, 1511374 for 2, and 1524473 for 3. The crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
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ACKNOWLEDGMENTS This work was supported by the 973 Project (Grant No. 2014CB845601) from the Ministry of Science and Technology of China and by the National Natural Science Foundation of China (Grant Nos. 21422106, 21371144, 21431005, and 21390391), the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (201219) for financial support, and the Fok Ying Tong Education Foundation (151013). We thank the staffs from the BL17B beamline of National Center for Protein Sciences Shanghai at Shanghai Synchrotron Radiation Facility for assistance during data collection.
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REFERENCES
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4. CONCLUSION In summary, three giant homometallic peanut-like clusters Gd37, Tb37, and Eu37 were obtained through a ligand-controlled hydrolytic approach with the polyalcohol 1,2,3-cyclohexanetriol ligand. These 37 metal ions are arranged into a double cage-like structure, encapsulating two CO32− anion templates. Notably, the present Ln37 cluster represents the largest odd-numbered homometallic cluster to date. Magnetization studies indicated that Gd37 displays large magnetic entropy changes. The present work suggests that polyalcohol ligands featuring multidentate coordinated sites are favorable candidates to assemble 2040
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DOI: 10.1021/acs.inorgchem.6b02714 Inorg. Chem. 2017, 56, 2037−2041
