Enhancement of Magnetocaloric Effect through Fixation of Carbon

Mar 14, 2017 - Synopsis. Octanuclear Ln4 cluster pairs were assembled from Ln4 clusters through the uptake of CO2 from air in a more basic media, resu...
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Enhancement of Magnetocaloric Effect through Fixation of Carbon Dioxide: Molecular Assembly from Ln4 to Ln4 Cluster Pairs Jianfeng Wu,†,‡ Xiao-Lei Li,† Lang Zhao,† Mei Guo,†,‡ and Jinkui Tang*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: A series 1.Ln of tetranuclear lanthanide clusters [Ln4(μ4-O)L2(PhCOO)6]·solvent (Ln = Gd (1.Gd), Dy (1.Dy), Ho (1.Ho)) and octanuclear lanthanide Ln4 cluster pairs 2.Ln [Ln8(μ3−OH)4(CO3)2L4(PhCOO)8]·solvent (Ln = Gd (2.Gd), Dy (2.Dy), Tb (2.Tb)) were assembled by using a bi-Schiff-based ligand H2L and characterized structurally and magnetically. Interestingly, the octanuclear Ln4 cluster pairs 2.Ln are proposed to be assembled from the tetranuclear clusters 1.Ln through the uptake of CO2 from air in a more basic media. X-ray structural analyses approved the possible evolution mechanism. Magnetic studies reveal the coexistence of ferro- and anti-ferromagnetic interaction in 1.Gd and 2.Gd by simulating the direct-current magnetic susceptibility and indicate the CO32− bridges produce weak ferromagnetic interaction in 2.Gd rather than antiferromagnetic interaction by benzoate bridges in 1.Gd. The magnitude of the magnetocaloric effect has been examined and shows that complex 2.Gd exhibits larger magnetocaloric effect than 1.Gd, which could be probably ascribed to the weak ferromagnetic interaction produced by the CO32− bridges.

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INTRODUCTION Lanthanide-based coordination clusters displaying interesting magnetic properties have attracted increasing attention of physicists, chemists, and material scientists over the past two decades not only because of the beauty of their fascinating structures but also due to the potential applications in functional materials, including magnetism and electricity.1−7 Among these elements, heavy rare earths are particularly attractive and have been proposed to be used as magnetic refrigeration, high-density data storage, and quantum computer, inspiring the researches of magnetocaloric effect (MCE),8 single-molecule magnets (SMMs),9,10 and single-molecule toroics (SMTs).11−13 Generally speaking, molecules featuring large MCE should possess a large-spin ground state with negligible magnetic anisotropy, a large magnetic density, and dominant intramolecular ferromagnetic exchange interaction.14−17 Therefore, the isotropic gadolinium ions with high spin state (S = 7/2) is proposed to construct multinuclear clusters, which would be a promising candidate to perform magnificent MCE.18 In the past few years, lots of high-nuclear clusters based on GdIII and/or transition metal have been reported showing remarkable MCE,19−26 in most of which the GdIII ions bond with small ligands, such as NO3− and CO32−, resulting in a large-spin state to guarantee a high magnetic density.14,16,19−21,27,28 For an SMM or SMT, large-spin state and high axial anisotropy in proper ligand field are needed for blocking the magnetic moment in ground state.13,29−33 Thus, the anisotropic DyIII, TbIII, HoIII, and ErIII ions are the most © XXXX American Chemical Society

promising candidates to assemble SMMs with high effective energy barrier and blocking temperature.34−38 Attempts to seek efficient synthetic strategies to assemble high-nuclear lanthanide-based clusters have been made to obtain remarkable MCE, SMMs, and SMTs. In the past two decades, hundreds of polynuclear clusters showing fantastic molecular magnetic property have been reported. For example, the [Ln60] nanocages show the largest MCE with −ΔSm = 66.5 J·kg−1·K−1,39 [Dy4K2] represents the best polynuclear SMM with energy barrier of 842 K,40 and the coupled Dy3 triangles maximize the toroidal moments by strong couplings via a μ4O2− ion.41−43 As part of our research interests, we have focused on the coordination system based on Schiff-base ligands, by using which we have obtained a series of pure 4f and 3d-4f heterometallic polynuclear clusters showing fantastic magnetic properties.36,44−52 In these systems, the Schiff-base ligands are stable in strongly basic media; thus, CO2 was usually absorbed from air and produced carbonate ligands,53−55 resulting in the construction of high-nuclear clusters and cluster pairs. Herein, we successfully synthesized a series of tetranuclear lanthanide clusters of [Ln4(μ4-O)L2(PhCOO)6]·solvent (Ln = Gd (1.Gd), Dy (1.Dy), Ho (1.Ho)) by applying a bi-Schiffbased ligand H2L (Scheme 1). When the reactions were performed in a more basic media relevant octanuclear lanthanide Ln4 cluster pairs of [Ln8(μ3− Received: January 12, 2017

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DOI: 10.1021/acs.inorgchem.7b00094 Inorg. Chem. XXXX, XXX, XXX−XXX

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diffraction analysis were collected after one week. Yield in 40−60%. C h em i ca l f or mu l a s a n d I R sp e c t r a: 2.Gd , [G d 8 ( μ 3 − OH)4(CO3)2L4(PhCOO)8]·2H2O; 2.Dy, [Dy8(μ3− OH)4(CO3)2L4(PhCOO)8]·4CH3OH·2H2O, Selected IR (cm−1): 3655.05(w), 2900.72(br), 1611.72(br), 1564.99(m), 1537.46(m), 1466.53(s), 1403.32(s) 1339.19(w), 1295.74(w), 1249.66(w), 1216.78(m), 1075.41(w), 998.75(w), 968.25(w), 853.52(w), 719.65(m), 682.36(m), 652.47(w); 2.Tb, [Tb8(μ3− OH) 4 (CO 3 ) 2 L 4 (PhCOO) 8 ]·2CH 3 CN, selected IR (cm −1 ): 3652.56(w), 2900.98(br), 1609.89(br), 1567.80(m), 1536.87(m), 1465.55(s), 1402.65(s) 1337.72(w), 1294.87(w), 1248.96(w), 1215.33(m), 1169.41(w), 998.76(w), 968.39(w), 852.75(w), 720.32(m), 681.94(m), 652.31(w). Crystallography. Single-crystal X-ray data of the titled complexes were collected on a Bruker Apex II CCD diffractometer equipped with graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å) at 293(2) K. The structures were solved by direct methods and refined by full-matrix least-squares methods on F2 using SHELXTL-2014.58 All non-hydrogen atoms in the whole structure were refined with anisotropic displacement parameters. Hydrogen atoms were introduced in calculated positions and refined with fixed geometry with respect to their carrier atoms. The large solvent voids were refined using solvent masking routine in the Olex2 package.59 Crystallographic data are listed in Table S2. Additional crystallographic information is available in the Supporting Information. Magnetic Measurements. Magnetic susceptibility measurements were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer equipped with a 7 T magnet. Direct-current (dc) magnetic susceptibility measurements were performed on polycrystalline samples of 1.Ln and 2.Ln in the temperature range of 2−300 K, in an applied field of 1000 Oe. The variable-temperature magnetization was measured in the temperature range of 1.9−300 K with an external magnetic field of 1000 Oe. The dynamics of the magnetization was investigated from the alternating-current (ac) susceptibility measurements in the zero static fields and a 3.0 Oe ac oscillating field. Diamagnetic corrections were made with the Pascal’s constants for all the constituent atoms as well as the contributions of the sample holder.60

Scheme 1. Schematic Drawing of the Binding Modes of Ligand H2L Indicated by the Harris Notation56

OH)4(CO3)2L4(PhCOO)8]·solvent (Ln = Gd (2.Gd), Dy (2.Dy), Tb (2.Tb)) were obtained. X-ray structural analyses revealed the possible evolution mechanism of 2.Ln. Magnetic studies indicated that the gadolinium analogues 1.Gd and 2.Gd exhibit significant cryogenic MCE, and the dysprosium derivatives 1.Dy and 2.Dy display slow relaxation of magnetization. Magnetic interaction analyses suggested the introduction of CO32− bridges probably enhanced the cryogenic MCE in 2.Gd.

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EXPERIMENTAL SECTION

General Synthetic Considerations. All chemicals and solvents were commercially obtained and used as received without any further purification. FTIR spectra were measured using a Nicolet 6700 Flex FTIR spectrometer equipped with smart iTR attenuated total reflectance (ATR) sampling accessory in the range from 500 to 4000 cm−1 (Figure S1). Elemental analyses for C, H, and N were performed on a PerkinElmer 2400 analyzer (Table S1). Ligand H2L was synthesized by the same procedure as described previously.57 In short, H2L was obtained by the condensation of 3-methoxysalicylaldehyde and 1,2-diaminopropane in the molar ratio of 2:1 in 50 mL of ethanol under heating at 80 °C for 12 h. Yield in 96%. Syntheses of 1.Ln. Ln(PhCOO)3·6H2O (0.15 mmol) was added to a solution of H2L (0.15 mmol) in a 20 mL mixture of methanol and acetonitrile (1:3, v/v), and then triethylamine (0.4 mmol) was added. The resultant solution was stirred for 3 h and subsequently filtered. The filtrate was exposed to air to allow the slow evaporation of the solvent. Yellow crystals of 1.Ln suitable for X-ray diffraction analysis were collected after one week. Yield in 65−75%. Chemical formulas and IR spectra: 1.Gd, [Gd4(μ4-O)L2(PhCOO)6]·H2O; 1.Dy, [Dy4(μ4O)L2(PhCOO)6]·2CH3OH·H2O, selected IR (cm−1): 3297.07(w), 3059.73(br), 2941.06(br), 1629.07(m), 1594.98(s), 1541.38(s), 1493.77(w), 1446.35(m), 1407.30(s), 1375.49(m), 1285.32(s), 1238.94(w), 1221.94(s), 1168.98(w), 1070.40(m), 1024.30(w), 999.59(w), 965.66(w), 854.01(m), 712.18(s), 684.56(w), 625.04(w); 1.Ho, [Ho4(μ4-O)L2(PhCOO)6], selected IR (cm−1): 3061.84(w), 2887.59(br), 1630.37(s), 1595.47(s), 1545.39(s), 1447.26(m), 1407.93(s) 1376.03(m), 1286.89(s), 1238.81(w), 1221.66(s), 1169.19(w), 1070.86(w), 967.29(w), 853.91(w), 711.93(m), 684.36(w), 625.32(w). Syntheses of 2.Ln. Complexes 2.Ln were prepared by the similar procedure as 1.Ln with the extra addition of 0.2 mmol of triethylamine. A small quantity of needle crystal was obtained in 20% yield. Alternatively, we performed the reaction under a continuous CO2 flow with a detailed procedure given as follows. Ln(PhCOO)3·6H2O (0.15 mmol) was added to a solution of H2L (0.15 mmol) in a 20 mL mixture of methanol and acetonitrile (1:1, v/ v) under a continuous CO2 flow, and then triethylamine (0.6 mmol) was added. The resultant solution was stirred for 3 h and subsequently filtered. The filtrate was exposed to air atmosphere to allow the slow evaporation of the solvent. Yellow crystals of 2.Ln suitable for X-ray

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RESULTS AND DISCUSSION Crystal Structures of 1.Ln. The reactions of H2L with Ln(PhCOO)3·6H2O and triethylamine in the mole ratio 3:3:8 in the mixture of methanol and acetonitrile produced complexes 1.Ln, [Ln4(μ4-O)L2(PhCOO)6]·solvent (Ln = Gd (1.Gd), Dy (1.Dy), Ho (1.Ho)). Full structure determinations were performed for these complexes (Table S2). By contrasting the structural cores, unit cells, and IR spectra of 1.Gd, 1.Dy, and 1.Ho, we find that these complexes are isostructural, differing only in the cocrystallized solvent molecules as observed in their molecular formulas. Therefore, the structural description of the complex 1.Gd will only be given here for the sake of brevity. Single-crystal X-ray studies reveal that 1.Gd is tetranuclear (Figures 1 and S2), which is similar to the complex of [Dy4(μ4-O)L′2(C6H5COO)6] reported recently.61 The asymmetric unit consists of the full neutral molecule: two H2L ligands, six-coordinated benzoate ligands, one μ4-O bridged Gd4 core, and one cocrystallized water molecule in lattice. Each ligand H2L is completely deprotonated and coordinates with three GdIII ions in the binding mode of 3.11212223131212, forming a crescent Gd3 plate (Scheme 1). The two crescent Gd3 plates share the two terminal GdIII ions and bond with the μ4-O bridge, resulting in the construction of μ4O bridged Gd4 core (Figure 1). Six benzoate ligands finish the coordination sphere of GdIII ions in three binding modes (Figure 1) and provide six negative charges to balance the positive Gd4 core. Herein, all the GdIII ions are eightB

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Figure 2. Structure of 2.Gd (top) and the relevant bonding modes of the ligands (bottom left) with parameter labels of C−O distance in the carbonate ligand; solvents were omitted for clarity.

Figure 1. Structure of 1.Gd (top) and the relevant bonding modes of the ligands (bottom); solvents were omitted for clarity.

coordinated and bound by the center μ4-O bridge with Gd−O bonds and Gd−O−Gd angles in the ranges of 2.31−2.36 Å and 97.9−125.9° (Table S3). Crystal Structures of 2.Ln. Complexes 2.Ln, [Ln8(μ3− OH)4(CO3)2L4(PhCOO)8]·solvent (Ln = Gd (2.Gd), Dy (2.Dy), Tb (2.Tb)), were prepared by the similar procedure as 1·Ln with the extra addition of 0.2 mmol of triethylamine. Single-crystal X-ray studies reveal that complex 2.Gd crystallizes in the triclinic space group P1̅ with Z = 1 (Figure S3) and complexes 2.Dy and 2.Tb crystallize in the monoclinic space group P2(1)/n with Z = 2, while the structural cores, IR spectra, and asymmetric units of these complexes are almost the same, differing only in the cocrystallized solvent molecules. Therefore, the structural description of the complex 2.Gd will only be given here for the sake of brevity. Complex 2.Gd is octanuclear Gd4 cluster pair with the asymmetric unit consisting of one Gd4 cluster (half of the neutral molecule): two H2L ligands, four-coordinated benzoate ligands, two μ3− OH bridges, and one carbonate-bridged Gd4 core. In the Gd4 cluster, both of the ligands H2L are completely deprotonated and coordinate to three GdIII ions with different binding modes, one of which is in 3.11212223131212 mode and the other in 3.11212223131212 mode (Scheme 1), forming two edge-shared Gd3 plates (Figures 2 and 3). The two μ3−OH bridges cap the Gd3 plates from the convex faces and the carbonate anion coordinates to the four GdIII ions from the internal concave face of the Gd3 plates in 3.114123414 mode (Figure 2). Two benzoate ligands coordinate to the center GdIII ions of each plate, and the other two benzoate ligands bridge the Gd4 clusters, forming the octanuclear Gd4 cluster pair. Herein, all the GdIII ions in the center of the plates are nine-coordinated, and the GdIII ions in terminal are eight-coordinated. The Gd−O distances and Gd− O−Gd angles between GdIII ions and the center O atom are in the ranges of 2.539−2.641 Å and 89.64−179.49°. Further insight into the structures, we can find that complexes 1.Gd and 2.Gd are closely related to each other. First, both of the Gd4 clusters in 1.Gd and 2.Gd contain two completely deprotonated H2L ligands with different coordina-

Figure 3. Coordination motifs of the ligand L2− in complexes 1.Gd (left) and 2.Gd (right).

tion motifs and orientations. As shown in Figure 3, the two ligands in 1.Gd are in the same coordination motif. In the case of 2.Gd, one ligand is almost in the same coordination motif and orientation with 1.Gd, while the other rotates along the Gd−O direction and breaks the Gd−O bonds between methoxyl groups and two GdIII ions. Second, both clusters contain the same μ4-O atom. In 1.Gd, the μ4-O atom resides in the center of Gd4 core and provides two negative charges, while in 2.Gd the μ4-O atom connects with CO2 group forming the CO32−, bonding with the GdIII ions in the concave face of the Gd3 plates. Herein, the formation of the CO32− would be explained by the well-known CO2 uptake from air in basic media. We suppose that complex 2.Gd possibly evolves from 1.Gd; that is, with the addition of extra base and the presence of CO2 in the reaction of 1.Gd, one of the benzoate ligands with syn−syn bridging mode is replaced by the μ3−OH, and the other is substituted by CO2. As a result one of the H2L ligands rotates along the Gd−O direction and breaks the Gd−O bond between methoxyl groups and GdIII ions, and the coordinated CO2 reacts with μ4-O2− forming the CO32− bridge. To probe this assumption, the C−O distances within the carbonato bridge are taken into account (Figure 2). The C−Oμ4 (Oμ4 represents the μ4-O in the CO32−) distance of 1.36 Å is longer C

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indicates the intramolecular magnetic interactions. We fitted the χMT versus T in the temperature range of 150−300 K and gave χTIP = 0.0592 cm3·mol−1 (Figure 4). We then intend to investigate the magnetic interactions within these complexes. Because of the lack of orbital contribution to the magnetic moment of the GdIII ions, the magnetic interactions in 1.Gd and 2.Gd are easy to be analyzed. The magnetic susceptibility data of complex 1.Gd can be fitted by using PHI package63 on the basis of the following simplified spin Hamiltonian:

than the C−Omc (Omc represent the two monodentatecoordinated O atoms in the CO32−) distances of 1.22 and 1.26 Å, indicating a relatively weak bonding between CO2 group and the μ4-O2−. To probe this further, detailed comparison on the benzoate ligands in 1.Gd and 2.Gd are investigated. In complex 1.Gd, the benzoate ligands coordinate with Gd1 and Gd2 in monodentate-coordinated modes (Figure 1), which has the ability to chelate with neighboring cluster. Indeed, the benzoate ligands coordinated with Gd1 and Gd2 bridge the neighboring Gd2 and Gd1, forming the cluster pair of 2.Gd. The benzoate ligands coordinted with Gd3 and Gd4 in complexes 1.Gd and 2.Gd are in almost same coordination motifs and orientations (Figure 2), which provides the evidence that the relevant orientations of the Gd3 and Gd4 did not change when ligand H2L rotated along the Gd−O direction and constructed the cluster pair of 2.Gd. Magnetic Properties. Direct-current (dc) magnetic susceptibility measurements were performed on polycrystalline samples of these complexes in the temperature range of 2−300 K, in applied fields of 1000 Oe. Magnetic susceptibility in the form of the χMT (χM = molar magnetic susceptibility) versus T plots were investigated and shown in Figure 4. The room-

̂ SGd3 ̂ + J SGd1 ̂ SGd4 ̂ + J SGd1 ̂ SGd2 ̂ + J SGd3 ̂ SGd4 ̂ Ĥ = −(J1SGd1 2 3 4 ̂ SGd4 ̂ + J SGd2 ̂ SGd3 ̂ ) + J5SGd2 6

(1)

The best fit is obtained with parameters of J1 = −0.0438 cm−1, J2 = 0.0479 cm−1, J3 = −0.274 cm−1, J4 = −0.0620 cm−1, J5 = −0.0421 cm−1, J6 = 0.0454 cm−1. The coexistence of negative and positive J values confirms the mixed antiferromagnetic and ferromagnetic magnetic interactions between the GdIII ions. To avoid the overparametrization problem, we regard J1 and J5, J2 and J6 the same (Figure 5), not only because

Figure 4. Temperature dependence of χMT product at 1 kOe, between 2 and 300 K for 1.Ln and 2.Ln.

Figure 5. Magnetic interaction models of 1.Gd (top) and 2.Gd (bottom).

temperature χMT product of 30.53, 55.69, and 53.96 cm3·K· mol−1 for complexes 1.Gd, 1.Dy, and 1.Ho are in good agreement with the values for each GdIII (7.88 cm3·K·mol−1), DyIII (14.17 cm3·K·mol−1), and HoIII (14.07 cm3·K·mol−1) ions, indicating the free-ion approximation applies. When cooled, the χMT product gradually decreases and reaches 8.55, 39.82, and 38.07 cm3·K·mol−1 at 2 K, respectively. The decrease of χMT can be possibly attributed to the dominant anti-ferromagnetic interaction. For complexes 2.Gd and 2.Dy, the χMT product at room temperature is 63.01 and 108.31 cm3·K·mol−1, which is slightly smaller than the expected value for each GdIII and DyIII ion, and gradually decreases when decreasing the temperature, reaching 23.17 and 32.46 cm3·K·mol−1 at 2 K, respectively. The case in complex 2.Tb is different; that is, the χMT product is 110.45 cm3·K·mol−1 at room temperature, which is much larger than the value of 94.56 cm3·K·mol−1 for eight TbIII ions (11.82 cm3·K·mol−1 for each TbIII) and decreases linearly until 150 K, followed by a gradual decrease reaching 37.79 cm3·K·mol−1 at 2 K. The linear decrease suggests the presence of temperatureindependent paramagnetism (χTIP), which arises from the second-order Zeeman effect between the nonmagnetic ground state and higher magnetic states,62 and the final decrease

they are close to each other but also due to Gd1 and Gd2, Gd3 and Gd4 are in similar coordination environments with similar distances (Gd1···Gd3 = Gd2···Gd4, Gd1···Gd4 = Gd2···Gd3, see Table S3), and then fit the plot using the following equation: ̂ SGd3 ̂ + SGd2 ̂ SGd4 ̂ ) + J (SGd1 ̂ SGd4 ̂ + SGd2 ̂ SGd3 ̂ ) Ĥ = −[J1(SGd1 2 ̂ SGd2 ̂ + J SGd3 ̂ SGd4 ̂ ] + J3SGd1 4 −1

(2)

giving J1 = −0.0432 cm , J2 = 0.0466 cm , J3 = −0.274 cm−1, J4 = −0.0618 cm−1, which are in good agreement with the values above. For complex 2.Gd, the magnetic interaction is more complex due to the eight GdIII ions would produce 14 exchange J values, which could not be fitted by PHI at once. Therefore, we just evaluate the magnetic interaction within the asymmetric unit of 2.Gd. In the asymmetric unit, Gd1 and Gd2, Gd3 and Gd4 are in similar coordination environments with similar Gd1···Gd4 and Gd2···Gd4, Gd1···Gd3 and Gd2···Gd3 distances (Table S3); thus, we regard J1 and J6, J2 and J5 the same (Figure 5), to avoid the overparametrization problem, and fit the χMT versus T plot using the following equation: D

−1

DOI: 10.1021/acs.inorgchem.7b00094 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ̂ SGd ̂ 4 + SGd2 ̂ SGd4 ̂ ) + J (SGd1 ̂ SGd3 ̂ + SGd2 ̂ SGd3 ̂ ) Ĥ = −[J1(SGd1 2 ̂ SGd2 ̂ + J SGd3 ̂ SGd4 ̂ ] + J3SGd1 4

(3)

giving J1 = −0.261 cm−1, J2 = 0.0393 cm−1, J3 = −0.117 cm−1, J4 = 0.245 cm−1. The positive J4 suggests ferromagnetic interaction between Gd3 and Gd4 through the CO32− bridge in 2.Gd rather than anti-ferromagnetic interaction through the benzoate bridge in 1.Gd. Generally speaking, the magnetic interaction in 1.Gd and 2.Gd are comparable; therefore, we could qualitatively analyze the magnetic interaction between the Gd4 cluster pair in 2.Gd by subtracting twofold χMT of 1.Gd from the χMT of 2.Gd to obtain the temperaturedependent difference ΔχMT = χMT(2.Gd) − 2χMT(1.Gd). The upturn in ΔχMT versus T plot suggests that the Gd4 cluster pair is dominated by ferromagnetic interaction. The field dependence of the magnetization for 1.Ln and 2.Ln were performed at low-temperature region in the field range of 0−70 kOe (Figures S4−S9). The magnetization versus field plots for 1.Dy, 1.Ho, 2.Dy, and 2.Tb reveal quick increase at low-field region and slow increase at high-field region without saturation until 70 kOe, indicating magnetic anisotropy possibly presenting at the lanthanide ions. While the magnetizations for 1.Gd and 2.Gd show steady increase in the whole field region, especially at high temperature, reaching 27.7 and 53.5 Nβ for an applied field of 70 kG and at 2 K, which are in good agreement with the saturation values of 28 and 56 Nβ, respectively. The relatively large magnetization values are in favor of large magnetocaloric effect. Therefore, we use the magnetization data to estimate the magnetic entropy change ΔSm by using the Maxwell relation ΔSm = ∫ [∂M(T,H)/ ∂T]HdH.64 The −ΔSm values reach maximum of 22.88 and 28.38 J·kg−1·K−1 at ΔH = 7 T for 1.Gd and 2.Gd, respectively (Figure 6). Assuming SGd = 7/2 for each uncorrelated GdIII ion at low temperature, the calculated −ΔSm values based on −ΔSm = nR ln(2SGd + 1) are 8.32R and 16.64R for 1.Gd and 2.Gd, which correspond to the values of 33.41 and 36.29 J·kg−1·K−1. The theoretical values for both complexes are much larger than the experimental values, which can be ascribed to the dominant anti-ferromagnetic interactions in these complexes. The larger MCE observed in 2.Gd than 1.Gd can be mainly attributed to the large metal/ligand mass ratio of 0.49 for 2.Gd (0.44 for 1.Gd), which helps to achieve a high spin density in this highnuclearity metal cluster pair. What’s more, the weak ferromagnetic interactions between Gd3 and Gd4 induced by the CO32− bridges, as well as the ferromagnetic interaction between the Gd4 clusters, would also contribute to the MCE of 2.Gd. Therefore, the absorption of CO2 not only produces the transformation of the structure but also makes an enhancement of the MCE. Alternating current (ac) experiments were also performed at low temperature under an oscillating field of 3.0 Oe to probe the dynamics of the magnetic relaxation. However, no out-ofphase (χ″) susceptibility is observed above 1.9 K for these complexes except the DyIII-based complexes 1.Dy and 2.Dy (Figure S10). Temperature-dependent χ″ signals for 1.Dy and 2.Dy are observed below 15 and 8 K, respectively (Figure 7), suggesting slow magnetic relaxation behavior. When cooled, the χ″ increase quickly without temperature-dependent χ″ peaks, indicating the presence of fast quantum tunneling of magnetization (QTM), which is a shortcut of the magnetic relaxation. The SHAPE software is used to quantify the coordination geometry of the DyIII centers (Table S4) and reveals that all the

Figure 6. Temperature dependences of the magnetic entropy change (−ΔSm) for 1.Gd (top) and 2.Gd (bottom), as obtained from the magnetization data.

Figure 7. Frequency-dependent out-of-phase χ″ signals for 1.Dy and 2.Dy at indicated temperatures, under zero dc field.

DyIII ions in 1.Dy are eight-coordinated and reside in distorted triangular dodecahedron coordination geometry (D2d). For complex 2.Dy, Dy1 and Dy2 are also eight-coordinated with similar coordination geometry as the DyIII ions in 1.Dy, while Dy3 and Dy4 are nine-coordinated in distorted capped square E

DOI: 10.1021/acs.inorgchem.7b00094 Inorg. Chem. XXXX, XXX, XXX−XXX

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antiprism geometry (C4v). The low and distorted coordination symmetry probably disfavors the axial anisotropy of the DyIII ions and therefore would induce the fast QTM relaxation in 1.Dy and 2.Dy. To roughly evaluate the energy barriers, we employed the relationship based on ln(χ″/χ′) = ln(ωτ0) + Ea/ (kBT).65−67 The best fitting of ln(χ″/χ′) versus 1/T plots give the energy barrier of ∼6.69 and ∼8.89 K and the characteristic time τ0 of 1.78 × 10−5 and 2.84 × 10−6 s for 1.Dy and 2.Dy, respectively (Figure S11).

CONCLUSION In conclusion, a series of tetranuclear lanthanide clusters of 1.Ln (Ln = Gd (1.Gd), Dy (1.Dy), Ho (1.Ho)) and octanuclear lanthanide Ln4 cluster pair of 2.Ln (Ln = Gd (2.Gd), Dy (2.Dy), Tb (2.Tb)) have been obtained by applying a bi-Schiff-based ligand H2L. The X-ray structure analyses reveal that the octanuclear Ln4 cluster pairs 2.Ln probably evolve from the tetranuclear clusters 1.Ln by the uptake of CO2 from air in a more basic media. Magnetic studies reveal that the anti-ferromagnetic interactions are dominant in these complexes. Magnetic interactions within the gadolinium derivatives are simulated by using PHI software, indicating the coexistence of ferro- and anti-ferromagnetic interaction in 1.Gd and 2.Gd. In addition, complexes 1.Gd and 2.Gd exhibit significant cryogenic MCE, and complexes 1.Dy and 2.Dy display slow relaxation of magnetization. Magneto-structural analyses indicate that the introduction of CO32− bridges could enhance the cryogenic MCE of 2.Gd. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00094. Plots of natural logarithm of χ″/χ′ versus 1/T, plots of temperature-dependent ac signals, plots of molar magnetization versus magnetic field at indicated temperatures, illustrated packing model, tabulated calculated CShM values, tabulated selected bond angles and distances, tabulated crystallographic data, elemental analysis results, IR spectra (PDF) X-ray crystallographic information (CIF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jinkui Tang: 0000-0002-8600-7718 Notes

The authors declare no competing financial interest. CCDC Nos. 1507070−1507075 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.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grant Nos. 21525103, 21371166, 21521092, and 21331003) for financial support. J.T. gratefully acknowledges support of the Royal Society-Newton Advanced Fellowship (NA160075). F

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