DOI: 10.1002/chem.201404847
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& Magnetic Properties
Spin Frustration in a Family of Pillared Kagom Layers of HighSpin Cobalt(II) Ions Long-Fei Wang,[a] Cui-Jin Li,[a, b] Yan-Cong Chen,[a] Ze-Min Zhang,[a] Jiang Liu,[a] WeiQuan Lin,[a] Yan Meng,[a] Quan-Wen Li,[a] and Ming-Liang Tong*[a]
Abstract: Based on the analogous kagom [Co3(imda)2] layers (imda = imidazole-4,5-dicarboxylate), a family of pillarlayered frameworks with the formula of [Co3(imda)2(L)3]·(L)n·xH2O (1: L = pyrazine, n = 0, x = 8; 2: L = 4,4’-bipyridine, n = 1, x = 8; 3: L = 1,4-di(pyridin-4-yl)benzene, n = 1, x = 13; 4: L = 4,4’-di(pyridin-4-yl)-1,1’-biphenyl, n = 1, x = 14) have been successfully synthesized by a hydrothermal/solvothermal method. Single-crystal structural analysis shows a significant increase in the interlayer distances synchronized with the extension of the pillar ligands,
Introduction Geometric spin frustration, which is an effective method for inducing macroscopic quantum states in electron systems, will arise when none of the nearest neighbor interactions can be satisfied simultaneously because of the special arrangement of spins.[1] The kagom lattice, which is characterized by a two-dimensional array of corner-sharing triangles, presents an ideal structure to study frustration. In theory, a S = 1/2 antiferromagnet with a kagom lattice has the greatest possibility of having an interesting magnetic ground state, which is a so-called resonating valence bond (RVB) or spin-liquid state that corresponds to no magnetic ordering, even at zero temperature.[2] However, when spin frustration is coupled with some perturbations, such as anisotropy, lattice disorder, next-nearest interactions, and higher S, exotic physical properties and ground states can be observed.[3] There have been extensive theoretical and experimental investigations into the magnetic behavior of frustrated kagom antiferromagnetic systems when consid[a] L.-F. Wang, Dr. C.-J. Li, Y.-C. Chen, Z.-M. Zhang, J. Liu, W.-Q. Lin, Y. Meng, Q.-W. Li, Prof. Dr. M.-L. Tong Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education School of Chemistry & Chemical Engineering Sun Yat-Sen University, Guangzhou, 510275 (P.R. China) E-mail: [email protected] Homepage: http://ce.sysu.edu.cn/tml/ [b] Dr. C.-J. Li College of Chemistry and Chemical Engineering Zhongkai University of Agriculture and Engineering Guangzhou, 510225 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404847. Chem. Eur. J. 2015, 21, 2560 – 2567
namely, 7.092(3) (1), 10.921(6) (2), 14.780(5) (3), and 19.165(4) (4). Despite the wrinkled kagom layers in complexes 2–4, comprehensive magnetic characterizations revealed weakening of interlayer magnetic interactions and an increase in the degree of frustration as the pillar ligand becomes longer from 1 to 4; this leads to characteristic magnetic ground states. For compound 4, which has the longest interlayer distance, the interlayer interaction is so weak that the magnetic properties observed within the range of temperature measured would correspond to the frustrated layer.
ering one or more perturbations. For example, Rao et al. performed a theoretical calculation of the low-energy magnetic spectrum of a finite kagom lattice with different magnitudes of spin carriers and found that half-odd-integer spins led to the strongest frustration.[4] As synthesized by Nocera et al., pure-phase FeIII–jarosites with the formula of AxFe3(OH)6(SO4)2 (x = 1 for A = Na + , K + , Rb + , Tl + , Ag + , and NH4 + ; x = 0.5 for A = Pb2 + ) displayed a weak field-induced ferromagnetic transition below the Nel temperature because of the role of Dzyaloshinskii–Moriya (DM) interactions, resulting in anisotropy in the systems.[5] Gao et al. presented a pillar-layered framework, [Co(N3)2(bpg)]·4/3 DMF, with a kagom layer constructed from Co2 + ions and azido ligands with interesting re-entered magnetic behavior.[6] Recently, we presented the preliminary characterization of two pillared, layered frameworks,[7] [Co3(imda)2(pyz)3]·8 H2O (1; imda = imidazole-4,5-dicarboxylate, pyz = pyrazine) and [Co3(imda)2(4,4’-bpy)3]·(4,4’-bpy)·8 H2O (2; 4,4’-bpy = 4,4’-bipyridine), with a similar frustrated kagom [Co3(imda)2] layer constructed from corner-sharing [Co3(imda)] triangles. Although there have been extensive studies on 2D magnetic systems,[8] systemic investigations on the influence of interlayer magnetic interactions on 2D frustrated layers in this fascinating family are rare. Herein, we attempt to further replace the pillars by longer N,N-pillars of 1,4-dpb (1,4-dpb = 1,4-di(pyridin-4-yl)benzene) and 4,4’-dpbp (4,4’-dpbp = 4,4’-di(pyridin-4-yl)-1,1’-biphenyl; Scheme 1), and successfully synthesize two analogous pillared, layered frameworks, [Co3(imda)2(1,4-dpb)3]·(1,4dpb)·13 H2O (3) and [Co3(imda)2(4,4’-dpbp)3]·(4,4’-dpbp)·14 H2O (4). Depending on the choice of pillar ligands, the interlayer spacing for the four structures differs from 7.092(3) to 19.165(4) . A detailed comparison of the structural and mag-
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Scheme 1. Structures of the organic ligands used as pillars herein; lengths and corresponding abbreviations are also indicated.
netic properties of the four closely related complexes demonstrates the controlling influence of interlayer distance on the degree of magnetic frustration. In addition, the combination of spin frustration and DM interactions that come from the wrinkled layers results in a variety of magnetic ground states, namely, metamagnetism for 1 and 2 with different critical fields, the coexistence of spin glass and spin canting for 3, and the absence of ordering above 1.8 K for 4.
Figure 1. An imda3 ion chelates three cobalt atoms and acts as a triangle unit (a and b), resulting in a 2D layer with a kagom lattice by sharing corners (c and d).
Results and Discussion Structural characterization Single-crystal X-ray analysis revealed that complex 1 crystallized in the hexagonal space group P6/mmm, whereas complexes 2–4 crystallized in the triclinic space group P1¯ (Table S1 in the Supporting Information). All of them are built on neutral kagom [Co3(imda)2] layers (Figure 1 c) and pillar ligands, but with notable differences. Because the structures of 1 and 2 have been previously described in detail,[7] herein we focus the discussion of complexes 3 and 4. The asymmetric unit of 3 contains 1.5 1,4-dpb linkers, 1 imda3 ion, and 3 CoII ions in distorted octahedral coordination geometry. Although the space groups of 2–4 are centrosymmetric, no inversion centers lie between the independent CoII ions, which makes asymmetric coupling possible. As shown in Figure 2 a, there are two kinds of coordination spheres for the three CoII ions. Both Co1 and Co2 are chelated by two NO bidentate sites of imda3 at the equatorial positions and two pyridyl N atoms at the axial positions; this leads to a [CoO2N4] environment (CoNimda = 2.071(4)–2.076(3), Co N1,4-dpb = 2.162(4)–2.191(4), CoO = 2.103(4)–2.109(4) ). For Co3, two OO bidentate sites of imda3 occupy its equatorial positions to form a different [CoO4N2] environment (CoN = 2.161(5), CoO = 2.042(3)–2.092(3) ]. The imda3 ion acts as a multidentate ligand, chelating 3 CoII ions with a Co···Co separation of 5.7809(4)–6.3190(4) , to form a triangular [CoII3(imda)] unit and further extend to a 2D layer on the ab plane with magnetic kagom topology (Figure 1). Chem. Eur. J. 2015, 21, 2560 – 2567
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Figure 2. Coordination geometries of the Co2 + ions in complexes 3 (a) and 4 (b). The p–p stacking between the aromatic rings of the ligands is indicated by black dotted lines.
The kagom layers are pillared by 1,4-dpb organic linkers along the c axis with a crisscross connecting model, resulting in a 3D pillar-layered framework (Figure 3 c). The interlayer distance in 3 is 14.780(5) , which is larger than 10.921(6) in 2 and 7.092(3) in 1. For 4, the asymmetric unit is isostructural with 3, except for the pillar ligands and different bond lengths and angles. The longest pillar ligand in this family, 4,4’-dpbp, results in an increase in the CoCo interlayer distance to 19.165(4) , which is rare among most pillar-layered frameworks without penetration. Analysis with the PLATON[10] program suggested that the open channels in the frameworks occupied 41.5 and 43.8 % of the crystal volumes for 3 and 4, respectively; these channels are filled with guest ligands and water molecules. To better understand the connectivity of the two structures, we performed topological analysis with the software TOPOS 4.0[9] and found the same connectivity for 3
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Figure 5. The arrangement of the CoII octahedron in the [Co3(imda)] unit for 1 (a) and 2–4 (b), and the corresponding side view of the frustrated kagom layer for 1 (c) and 2–4 (d).
Figure 3. The 3D structures of the four complexes constructed with different pillar ligands and frustrated cobalt–imda layers are shown along with the corresponding interlayer distances for a) 1, b) 2, c) 3, and d) 4.
also in the wave-like [Co3(imda)2] layer for complexes 2–4 (Figure 5 and Table 1). However, they still belong to the frustrated 2D kagom lattice owing to the corner-sharing triangular connection, while the pillar ligands are responsible for the interlayer interactions as the perturbation of frustration and determine the final magnetic behavior.
and 4 with a new point symbol, (63)2(62.103.12)(64.8.10)2 (Figure 4). Two marked structural differences are observed among the four complexes. First, the interlayer distances in the four structures are successfully controlled from 7.1 in 1 to 19.2 in 4,
Table 1. The dihedral angles [8] for complexes 1–4 between the coordinative equatorial plane of each CoII ion and the crystal ab plane.
1 2 3 4
Co1
Co2
Co3
0.0 14.9(1) 19.1(1) 17.2(2)
– 15.8(1) 19.6(1) 19.7(2)
– 30.93(8) 29.11(7) 31.8(1)
Direct (dc) and alternating current (ac) magnetic measurements
Figure 4. Simplified topological structure of complexes 3 and 4; the gray and black spheres represent imda3 ions as three-connected nodes and CoII ions as six-connected nodes, respectively, and the pillar ligands are simplified as the dark gray rods.
which is the direct effect of the increasing lengths of the pillar ligands. Second, the longer pillars also enable structural variation of the kagom layers. In complex 1, the short and rigid ligands are perfectly perpendicular to the flat kagom layers on the ab plane. When the layers are pillared by the extended ligands in 2–4, the p–p interactions, along with the allowable bend of the pillars, result in the unparalleled linking mode and Chem. Eur. J. 2015, 21, 2560 – 2567
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The magnetic behavior of 1–4 was investigated by using a Quantum Design MPMS XL-7 SQUID magnetometer. In addition to the preliminary magnetic measurements for complexes 1 and 2 in our previous report, herein we performed comprehensive magnetic measurements for all four complexes to investigate and compare their frustrated behavior. As shown in Figure 6 a, the cMT value of 1 is 8.33 cm3 mol1 K at 300 K per Co3 unit, which is significantly higher than the spin-only value (5.63 cm3 mol1 K) expected for three uncoupled octahedral CoII ions because of the orbit contribution. With a decrease in temperature, the cMT value decreases gradually and reaches a minimum of 0.28 cm3 mol1 K at 1.8 K. The decrease in the cMT plot upon cooling at high temperature is attributed to spin-orbit coupling effects and possible antiferromagnetic interactions. A Curie–Weiss fitting (Figure S1a in the Supporting Information) of cM versus T data in the range of 50–300 K gives C = 9.77 cm3 mol1 K and q = 53.99 K. The significantly negative Weiss constant indicates dominant antiferromagnetic interactions and spin-orbit coupling in the system. The irreversibility of the ZFC and FC molar susceptibilities under 100 Oe for 1 (Figure 6 b), as well as a sharp peak of the ac susceptibility at about 8.5 K in both real and imaginary com-
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Figure 6. Magnetic measurements of complex 1. a) cM versus T (black) and cMT versus T (gray) at 100 Oe, b) zero-field-cooled (ZFC) and field-cooled (FC) magnetization at 100 Oe, c) ac measurements between 1.8 and 20 K, and d) M–H plots at various temperatures.
ponents (Figure 6 c) not only indicate the onset of long-range magnetic ordering, but also exhibit the occurrence of a net moment in 1. Furthermore, the M–H plot (Figure 6 d) at 2 K has a weak jump when the field is above 40 kOe; this indicates metamagnetic behavior, which is likely to be due to strong interlayer interactions in 1. For 2, the decrease in the cMT plot (Figure 7 a) in the hightemperature region is similar to that of 1. The Curie–Weiss fitting of the 1/cMT data between 50 and 300 K gives C = 9.69 cm3 mol1 K and q = 46.24 K (Figure S1b in the Supporting Information). Owing to the long distance between the layers, the negative Weiss constant mainly represents the intralayer CoCo antiferromagnetic interactions in addition to the spin-orbit coupling of octahedral CoII ions. The magnetic susceptibility of 2 increased upon cooling from room temperature and a clear cusp was observed at approximately 4.4 K, which indicated long-range antiferromagnetic ordering. When the temperature was further decreased, the cM value tended to be constant below 3.7 K. The ZFC–FC curve (Figure 7 b) was measured at 10 Oe. The FC susceptibility follows the ZFC curve from the high-temperature region to form a distinct peak, which suggests a Nel temperature of 4.5 K; this is further confirmed by the AC susceptibility with the frequency-independent peaks of the real part of the susceptibility (c’) at around 4.4 K and the absence of the imaginary part (c’’) (Figure 7 c). Interestingly, the field-induced spin flip can be clearly observed below TN, as demonstrated by the FC curves involving larger fields (Figure 7 d). In addition, the M–H curves for 2 exhibit a sudden increase during magnetization (Figure 7 e), which indicate that 2 is a field-induced metamagnet below TN and the critical field is calculated from the dM/dH curves to be approximately 5 kOe (Figure 7 f). The cMT versus T curve of complex 3 is shown in Figure 8 a. Different from 1 and 2, upon cooling, the cMT value of 3 gradually decreases to a minimum around 5.2 K and then abruptly increases to a maximum at 2.4 K. Simultaneously, the cM value Chem. Eur. J. 2015, 21, 2560 – 2567
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Figure 7. Magnetic measurements of complex 2. a) cM versus T (black) and cMT versus T (gray) at 100 Oe; b) ZFC and FC magnetization at 10 Oe; c) the temperature dependence of ac measurement at 10 and 499 Hz, respectively, which shows a frequency-independent peak at real component, d) cM versus T at various fields. M–H plots at various temperatures (e) and the corresponding differential curves (f).
of 3 increases gradually with decreasing temperature from 300 K, but rises abruptly below 3.5 K and reaches 25.72 cm3 mol1 at T = 1.8 K without dropping; this suggests the weak ferromagnetic behavior of 3. Above 50 K, the magnetic behavior can be fitted by the Curie–Weiss law (Figure S1c in the Supporting Information) to give C = 8.33 cm3 mol1 K and q = 59.64 K. Considering the negative Weiss constant and weak ferromagnetic behavior of 3, the canted antiferromagnetic behavior may be ascribed to 3; this is further confirmed by the cM versus T measurements at various fields (Figure 8 c). The M–H curves (Figure 8 d) at various temperatures are also in accordance with the canted behavior and a narrow hysteresis loop is observed at 2 K with a small coercive field of 25 Oe, which indicates soft magnetic behavior. To determine the phase transition temperature, the temperature dependence of the FC and ZFC curves (Figure 8 b) were measured under a dc field of 50 Oe to give Tc = 2.6 K. However, the subsequent ac susceptibility measurement (Figure 8 e) is beyond our expectations. Under a zero dc field and a 5 Oe oscillating field in the frequency range of 1–1488 Hz, both the real and imaginary parts of ac susceptibility reveal a peak at about 2.3 K, which is slightly frequency dependent. This frequency dependence can be quantitatively studied by the equation f= (DTp/Tp)/Dlog w, in which Tp is the peak temperature and w is the frequency. The estimated value of f is 0.016, which is of the same magni-
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Figure 9. Magnetic measurements of complex 4. a) cM versus T (black) and cMT versus T (gray) at 100 Oe, b) ZFC and FC magnetization at 50 Oe field, c) the temperature dependence of ac susceptibility at various frequencies, and d) M–H plots at various temperatures.
Discussion of the magnetic behavior Figure 8. Magnetic measurements of complex 3. a) cM versus T (black) and cMT versus T (gray) at 100 Oe, b) ZFC and FC magnetization at 50 Oe, c) cM versus T at various fields, d) magnetic hysteresis loop at various temperatures, e) the temperature dependence of ac susceptibility at various frequencies, and f) M–H plots at various temperatures.
Among the four complexes, the intralayer antiferromagnetic interactions will undoubtedly lead to spin frustration on the ab plane because of the characterized kagom lattice. On the other hand, the critical temperature (Tc) for complexes 1–4 shifts monotonically to lower temperatures as the interlayer distance increases with the employment of longer N,N-pillars (Table 2). Such an observation is consistent with a reduction in interlayer magnetic coupling owing to extension of the p-conjugated aromatic bridges[12] between the cobalt–imda layers, leading to more separated 2D kagom layers. To estimate the degree of frustration for each complex, a widely used empirical parameter, f, provided by Ramirez[1a] is calculated by using Equation (1):
tude with spin-glass.[11] Such behavior indicates a cooperative magnetic state of spin-glass and spin canting for complex 3. Between 1.8 and 300 K, the cMT value (Figure 9 a) of complex 4 gradually decreases to a minimum at 2.6 K and then a tiny increase is observed, such behavior is similar to that of 3. The 1/cM versus T curve fitted by the Curie–Weiss law between 50 and 300 K gave the following parameters: C = 9.87 cm3 mol1 K and q = 58.20 K (Figure S1d in the Supporting Information). ZFC–FC measurement (Figure 9 b) under 50 Oe did not show f ¼ jqj=T c ð1Þ any furcation of the two curves down to 1.8 K, which indicates the absence of a magnetic Table 2. Magnetic properties of complexes 1–4 and selected CoII metal–organic frameworks with kagom lattices. phase transition. Further ac susceptibility of 4 (Figure 9 c) was f Magnetic ground state Interlayer q Complex[a] Tc measured at a zero dc field and [K] distance [K] a 5 Oe oscillating field at a fre[] quency of 1 and 1488 Hz. The 1 7.092(3) 53.99 9.3 5.8 metamagnetism (Hc = 40 kOe) below real part increases gradually and TN 2 10.921(6) 46.24 4.4 10.5 metamagnetism (Hc = 5 kOe) below the imaginary part has no signal, TN which proves the absence of 3 14.780(5) 59.64 2.6 22.9 coexistence of spin-glass and spin magnetic ordering in 4 between canting 1.8 and 300 K. 4 19.165(4) 58.20 < 1.8 > 32.3 no ordering above 1.8 K 14.169(5) 80.33 [Co3(m3-OH)2(1,2-chdc)2][13a] 35.5 [Co3F6(SO4)2]·[H2N(CH2)4NH2]·[NH4]2[13b] 8.521(2) 13.416(4) 165.8 [Co3(N3)6(bpg)3]·4 DMF[6a]
11 2.8 16
7.3 canted antiferromagnetic ordering 12.7 antiferromagnetic ordering 10.4 canted antiferromagnetic ordering with a second Tc
[a] 1,2-chdc = trans-1,2-cyclohexane-dicarboxylate, bpg = meso-a,b-bi(4-pyridyl) glycol.
Chem. Eur. J. 2015, 21, 2560 – 2567
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Full Paper in which q is the Weiss constant, which reflects not only the magnitude of the magnetic interactions, but also the anisotropy of the high-spin six-coordinate CoII ions that will vary in the series presented herein; Tc is the critical temperature for ordering in the system; the strength of frustration is in direct proportion to the value of f and a value of f > 10 signifies strongly frustrated behavior.[1a, b] Herein, in our family of complexes, the f values show a drastic increase from 1 to 4 (Table 2), which suggests strengthening of frustration owing to weakening of the interlayer coupling. For 4, such strongly frustrated behavior, f > 32.3, is rare in most CoII-frustrated materials with a kagom lattice (Table 2). In addition to strengthening of the magnetic frustration from complexes 1–4, further magnetic measurements revealed a change in the magnetic ground states for these four complexes. For complex 1, the layers are linked by the shortest pillar, pyz, which can be used as a magnetically active ligand[15] because of the relatively short length and conjugated aromatic structure. Therefore, the relatively strong interlayer superexchange coupling, compared with the other three compounds, through pyz hinders frustration and results in the metamagnetic behavior of 1 with a large critical field with Hc = 40 kOe. When pillar pyz was replaced with a longer ligand to construct the other three compounds, there were two clear changes for complexes 2–4 that would remarkably influence the magnetic behavior of each compound. First, extending the distance between the neighboring frustrated layer weakens the superexchange coupling and the dominant interlayer interactions will be dipolar interactions.[14] Second, the kagom layer of the other three compounds has a clear wrinkling compared with the flat structure of 1 (Figure 5). Such a structural change results in the absence of a symmetric center between the neighboring octahedral CoII ions; this is in agreement with the lower space group of these three compounds compared with that of 1. Therefore, DM[16] interactions will be induced because of the asymmetric coupling[17] and lead to a net moment between antiferromagnetically coupled spins. For complex 2, although the antiferromagnetic interlayer coupling leads to long-range ordering at TN = 4.4 K, DM interactions may cause the spins to rotate out of the layer and produce a net magnetic moment for each layer. When the applied field exceeds the critical field, the ferromagnetic arrangement of the net magnetic moments should be observed. Such behavior has been commonly found in some layered cobalt compounds,[18] but for a frustrated magnet with a 2D kagom lattice, as far as we know, there is only one family of inorganic FeIII–jarosite materials that displays similar magnetic behavior to that of complex 2.[5] In addition, the smaller critical field (5 kOe) for 2 compared with 1 (40 kOe) is reasonable because of weaker interlayer interactions. The kagom layer of complex 3 is similar to that of complex 2; thus DM interactions also exist; however, the coexistence of spin-glass and spin canting in 3 is quite exotic, and can be regarded as a median state called “order with disorder”.[19] In theory, the coexistence of magnetic order and disorder can occur when one or more sublattices are ordered below the critical temperature, Tc, whereas at least one sublattice remains Chem. Eur. J. 2015, 21, 2560 – 2567
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disordered at all temperatures. For complex 3, the kagom layer is more separated (14.780(5) ); thus the interlayer coupling for 3 is too weak to completely suppress frustration. However, the presence of DM interactions within layers partially relieves frustration and prefers the canted long-range magnetic order. As a result, when the competition between longrange order and quantum spin fluctuations caused by strong spin frustration cannot be suppressed by each other, such exotic magnetic behavior as that shown by complex 3 will be observed. There are some materials[20] that show similar magnetic behavior to that of 3; however, a theoretical investigation is difficult because of the complicated frustrated structure and competitive interactions; therefore, complex 3 offers a simpler model for investigating the mechanism of the coexisting behavior in frustrated systems. For complex 4, DM interactions still exist because of the same wrinkled layer as those in 2 and 3. However, the weakest interlayer interaction caused by the longest pillar, 4,4’-dpbp, leads to the strongest spin frustration, which is in agreement with the critical temperature below 1.8 K. The magnetic ground state of 4 in the lower temperature region would be interesting because a perfect 2D kagom layer would maintain frustration without ordering until 0 K, as predicted by Anderson.[2]
Conclusion We reported a family of pillar-layered 3D cobalt(II) coordination polymers with analogous kagom [Co3(imda)2] layers. Their structures and magnetic properties could be tuned through the choice of different N,N-pillars. In general, increasing the interlayer distance leads to weakening of the interlayer magnetic coupling and the strengthening of the degree of frustration, whereas the introduction of DM interactions from wrinkled layers partially relieved frustration to generate competition. For complexes 1 and 2, the interlayer interaction was non-negligible and resulted in long-range magnetic ordering with metamagnetic behavior; however, the critical field for both compounds had a significant difference, Hc = 40 and 5 kOe for 1 and 2, respectively, which indicated weakening of interlayer interactions from 1 to 2. By choosing longer pillar, 1,4-dpb, in 3, competition of the interlayer magnetic interactions with spin frustration was evident, which led to exotic magnetic behavior. Finally, the degree of frustration in 4 was strengthened by the longest pillar, 4,4’-dpbp, which eliminated any magnetic ordering above 1.8 K, and thus, complex 4 might be more attributed to the real 2D magnet because of the long interlayer distance. The pillar-layered structure is a feasible way to investigate the perturbations of frustrated 2D systems trapped in 3D frameworks, in which the pillar ligands can be systematically replaced and diverse magnetic behaviors can be obtained and fine-tuned.
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Full Paper Experimental Section Material and physical measurements All of the starting materials employed were commercially available and used as received without further purification. The synthetic methods for complexes 1 and 2 were reported previously. The thermogravimetric (TG) analyses were performed on a PerkinElmer TGA7 thermogravimetric analyzer in a flow of N2 with a heating rate of 10 K min1. The corresponding TG curves for complexes 3 and 4 are presented in Figure S4a and b in the Supporting Information, respectively. The C, H, and N microanalyses were carried out with an Elementar Vario EL CHNS elemental analyzer. Powder XRD diffraction intensities for polycrystalline samples were measured at 293 K on a Bruker D8 Advance diffractometer (CuKa, l = 1.54056 ; Figure S2 in the Supporting Information).
reflections measured, 6834 independent reflections (Rint = 0.0345), R1 = 0.0984 (I > 2s(I)), wR2 = 0.2908 (all data), final GooF = 1.145. Crystal data as well as details of data collection and refinements for 3 and 4 are summarized in Table S1 in the Supporting Information. CCDC 1019076 (3) and 1019077 (4) 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.
Magnetic measurements The magnetic measurements were performed on polycrystalline samples by using a Quantum Design MPMS XL-7 SQUID magnetometer with experimental diamagnetic correction. The samples were mixed with Vaseline to prevent orientation in magnetic fields. The ZFC and FC magnetic susceptibilities were measured with the ultra-low field option to provide a remnant field < 0.1 Oe.
Synthesis of 3 A mixture of CoCl2·6 H2O (0.018 g, 0.075 mmol), H3imda (0.012 g, 0.075 mmol), 1,4-dpb (0.01725 g, 0.075 mmol), and triethylamine (0.023 g, 0.023 mmol) was added to deionized water (15 mL) and stirred for 10 min. The resulting solution was heated in a 25 mL stainless-steel reactor with a Teflon liner at 160 8C for 12 h. After a period of approximately 20 h of cooling to room temperature, orange prismatic crystals of 3 were obtained, isolated by filtration, and washed with water. Yield: 87 %; elemental analysis (%) calcd for C74H76N12O21Co3 : C 54.00, H 4.65, N 10.21; found: C 54.00, H 4.25, N 10.35.
This work was supported by the “973 Project” (2012CB821704 and 2014CB845602), the NSFC (grant nos. 21371183, 91122032, and 21121061), the NSF of Guangdong (S2013020013002), and the Program for Changjiang Scholars and Innovative Research Team in University of China. Keywords: cobalt · magnetic properties · metal–organic frameworks · layered compounds · spin frustration
Synthesis of 4 A mixture of CoCl2·6 H2O (0.019 g, 0.008 mmol), H3imda (0.006 g, 0.004 mmol), and 4,4’-dpbp (0.025 g, 0.008 mmol) was added to a solution of H2O/glycol (14:1; 15 mL) and stirred for 10 min. The resulting solution was heated in a 25 mL stainless-steel reactor with a Teflon liner at 180 8C for 6 days. After a period of approximately 20 h of cooling to room temperature, orange prismatic crystals of 4 were obtained, isolated by filtration, and washed with water. Yield: 64 %; elemental analysis (%) calcd for C98H94N12O22Co3 : C 59.79, H 4.81, N 8.54; found: C 59.53, H 4.63, N 8.62.
X-ray crystallography Diffraction data for complex 3 were collected on a Rigaku R-AXIS SPIDER IP diffractometer by using MoKa radiation (l = 0.71073 ) at 150 K. Diffraction data for complex 4 were recorded on an Oxford Diffraction Gemini R CCD diffractometer with CuKa, l = 1.54056 , at 150 K. The structures of 3 and 4 were solved by direct methods, and all non-hydrogen atoms were refined anisotropically by leastsquares methods on F2 by using the SHELXTL program.[19] Hydrogen atoms on organic ligands were generated by the riding mode. Some of the severely disordered water molecules for both 3 and 4 were removed by SQUEEZE in the structural refinement and their amounts were determined by elemental analysis and TG analysis (Figure S3 in the Supporting Information). For 3: C74H76N12O21Co3, M = 1646.25, triclinic, space group P1¯, a = 11.5618(8) , b = 11.8493(8) , c = 15.7206(12) , a = 76.325(2)8, b = 71.246(2)8, g = 65.330(2)8, V = 1840.1(2) 3, Z = 1, 1 = 1.372 g cm3 ; m(MoKa) = 0.752 mm1, 12 772 reflections measured, 7023 independent reflections (Rint = 0.0615), R1 = 0.0812 (I > 2s(I)), wR2 = 0.2612 (all data), final GooF = 1.038. For 4: C98H94N12O22Co3, M = 1968.64, triclinic, space group P1¯, a = 11.4712(6) , b = 11.6147(7) , c = 19.8781(10) , a = 81.156(5)8, b = 74.921(5)8, g = 65.987(10)8, V = 2332.5(2) 3, Z = 1, 1 = 1.222 g cm3 ; m(CuKa) = 4.795 mm1, 11 367 Chem. Eur. J. 2015, 21, 2560 – 2567
Acknowledgements
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[1] a) A. P. Ramirez, Ann. Rev. Mater. Sci. 1994, 24, 453 – 480; b) J. E. Greedan, J. Mater. Chem. 2001, 11, 37 – 53; c) S. H. Lee, C. Broholm, W. Ratcliff, G. Gasparovic, Q. Huang, T. H. Kim, S. W. Cheong, Nature 2002, 418, 856 – 858; d) Z. Lin, M.-L. Tong, Coord. Chem. Rev. 2011, 255, 421 – 450. [2] a) P. W. Anderson, Mater. Res. Bull. 1973, 8, 153 – 160; b) P. W. Anderson, Science 1987, 235, 1196 – 1201. [3] a) M. Elhajal, B. Canals, C. Lacroix, Phys. Rev. B 2002, 66, 014422; b) K. Damle, T. Senthil, Phys. Rev. Lett. 2006, 97, 067202; c) D. G. Nocera, B. M. Bartlett, D. Grohol, D. Papoutsakis, M. P. Shores, Chem. Eur. J. 2004, 10, 3850 – 3859; d) S. K. Pati, C. N. R. Rao, Chem. Commun. 2008, 39, 4683 – 4693. [4] a) S. K. Pati, C. N. R. Rao, J. Chem. Phys. 2005, 123, 234703; b) J. N. Behera, C. N. R. Rao, J. Am. Chem. Soc. 2006, 128, 9334 – 9335. [5] a) D. Grohol, D. G. Nocera, D. Papoutsakis, Phys. Rev. B 2003, 67, 064401; b) B. M. Bartlett, D. G. Nocera, J. Am. Chem. Soc. 2005, 127, 8985 – 8993; c) T. Fujita, M. Hagiwara, H. Yamaguchi, S. Kimura, T. Kashiwagi, K. Matan, D. G. Nocera, Y. S. Lee, D. Grohol, J. Low Temp. Phys. 2013, 170, 242 – 247; d) K. Matan, B. M. Bartlett, S. Helton, V. Sikolenko, S. Matˇasˇ, K. Prokesˇ, Y. Chen, J. W. Lynn, D. Grohol, T. J. Sato, M. Tokunaga, D. G. Nocera, Y. S. Lee, Phys. Rev. B 2011, 83, 214406. [6] a) X.-Y. Wang, L. Wang, Z.-M. Wang, S. Gao, J. Am. Chem. Soc. 2006, 128, 674 – 675; b) W. Li, S.-S. Gong, Y. Zhao, S.-J. Ran, S. Gao, G. Su, Phys. Rev. B 2010, 82, 134434. [7] C.-J. Li, S. Hu, W. Li, C.-K. Lam, Y.-Z. Zheng, M.-L. Tong, Eur. J. Inorg. Chem. 2006, 1931 – 1935. [8] a) P. Rabu, P. Janvierb, B. Bujoli, J. Mater. Chem. 1999, 9, 1323 – 1326; b) A. Rujiwatra, C. J. Kepert, M. J. Rosseinsky, Chem. Commun. 1999, 2307 – 2308; c) G. Rogez, C. Massobrio, P. Rabu, M. Drillon, Chem. Soc. Rev. 2011, 40, 1031 – 1058. [9] a) V. A. Blatov, Struct. Chem. 2012, 23, 955 – 963; b) E. V. Alexandrov, V. A. Blatov, A. V. Kochetkova, D. M. Proserpio, CrystEngComm 2011, 13, 3947 – 3958. [10] A. L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, 2003. [11] J. A. Mydosh, Spin Glass: An Experimental Introduction, Taylor & Francis, London, 1993.
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Full Paper [12] a) M. Castellano, F. R. Fortea-Prez, A. Bentama, S.-E. Stiriba, M. Julve, F. Lloret, G. D. Munno, D. Armentano, Y.-l. Li, R. Ruiz-Garca, J. Cano, Inorg. Chem. 2013, 52, 7645 – 7657; b) E. Pardo, J. Faus, M. Julve, F. Lloret, M. C. MuÇoz, J. Cano, X. Ottenwaelder, Y. Journaux, R. Carrasco, G. Blay, I. Femndez, R. Ruiz-Garca, J. Am. Chem. Soc. 2003, 125, 10770 – 10771. [13] a) Y.-Z. Zheng, M.-L. Tong, W.-X. Zhang, X.-M. Chen, Chem. Commun. 2006, 2, 165 – 167; b) J. N. Behera, G. Paul, A. Choudhury, C. N. R. Rao, Chem. Commun. 2004, 4, 456 – 457. [14] M. Drillon, P. Panissod, J. Magn. Magn. Mater. 1998, 188, 93 – 99. [15] a) K.-L. Zhang, Z. Wang, H. Huang, Y. Zhu, X.-Z. You, J. Mol. Struct. 2004, 693, 193 – 197; b) R. Podgajny, D. Pinkowicz, T. Korzeniak, W. Nitek, M. Rams, B. Sieklucka, Inorg. Chem. 2007, 46, 10416 – 10425; c) Y. Ma, G.-F. Xu, X. Yang, L.-C. Li, J.-K. Tang, S.-P. Yan, P. Cheng, D.-Z. Liao, Chem. Commun. 2010, 46, 8264 – 8266. [16] a) I. Dzyaloshinskii, J. Phys. Chem. Solids 1958, 4, 241 – 320; b) L. M. Sandratskii, Adv. Phys. 1998, 47, 91 – 160.
Chem. Eur. J. 2015, 21, 2560 – 2567
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[17] a) D.-F. Weng, Z.-M. Wang, S. Gao, Chem. Soc. Rev. 2011, 40, 3157 – 3181; b) X.-Y. Wang, Z.-M. Wang, S. Gao, Chem. Commun. 2008, 48, 281 – 294. [18] a) A. Rujiwatra, C. J. Kepert, J. B. Claridge, M. J. Rosseinsky, H. Kumagai, M. Kurmoo, J. Am. Chem. Soc. 2001, 123, 10584 – 10594; b) X.-N. Cheng, W. Xue, J.-H. Huang, X.-M. Chen, Dalton Trans. 2009, 29, 5701 – 5707. [19] a) P. Azaria, H. T. Diep, H. Giacomini, Phys. Rev. Lett. 1987, 59, 1629 – 1632; b) R. Quartu, H. T. Diep, Phys. Rev. B 1997, 55, 2975 – 2980. [20] a) Q. Chen, W. Xue, B.-Y. Wang, M. H. Zeng, X.-M. Chen, CrystEngComm 2012, 14, 2009 – 2014; b) L. Hou, W.-X. Zhang, J.-P. Zhang, W. Xue, Y.-B. Zhang, X.-M. Chen, Chem. Commun. 2010, 46, 6311 – 6313. [21] SHELXTL 6.10, Bruker Analytical Instrumentation, Madison, WI (USA), 2000.
Received: August 13, 2014 Published online on December 5, 2014
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& Magnetic Properties
Spin Frustration in a Family of Pillared Kagom Layers of HighSpin Cobalt(II) Ions Long-Fei Wang,[a] Cui-Jin Li,[a, b] Yan-Cong Chen,[a] Ze-Min Zhang,[a] Jiang Liu,[a] WeiQuan Lin,[a] Yan Meng,[a] Quan-Wen Li,[a] and Ming-Liang Tong*[a]
Abstract: Based on the analogous kagom [Co3(imda)2] layers (imda = imidazole-4,5-dicarboxylate), a family of pillarlayered frameworks with the formula of [Co3(imda)2(L)3]·(L)n·xH2O (1: L = pyrazine, n = 0, x = 8; 2: L = 4,4’-bipyridine, n = 1, x = 8; 3: L = 1,4-di(pyridin-4-yl)benzene, n = 1, x = 13; 4: L = 4,4’-di(pyridin-4-yl)-1,1’-biphenyl, n = 1, x = 14) have been successfully synthesized by a hydrothermal/solvothermal method. Single-crystal structural analysis shows a significant increase in the interlayer distances synchronized with the extension of the pillar ligands,
Introduction Geometric spin frustration, which is an effective method for inducing macroscopic quantum states in electron systems, will arise when none of the nearest neighbor interactions can be satisfied simultaneously because of the special arrangement of spins.[1] The kagom lattice, which is characterized by a two-dimensional array of corner-sharing triangles, presents an ideal structure to study frustration. In theory, a S = 1/2 antiferromagnet with a kagom lattice has the greatest possibility of having an interesting magnetic ground state, which is a so-called resonating valence bond (RVB) or spin-liquid state that corresponds to no magnetic ordering, even at zero temperature.[2] However, when spin frustration is coupled with some perturbations, such as anisotropy, lattice disorder, next-nearest interactions, and higher S, exotic physical properties and ground states can be observed.[3] There have been extensive theoretical and experimental investigations into the magnetic behavior of frustrated kagom antiferromagnetic systems when consid[a] L.-F. Wang, Dr. C.-J. Li, Y.-C. Chen, Z.-M. Zhang, J. Liu, W.-Q. Lin, Y. Meng, Q.-W. Li, Prof. Dr. M.-L. Tong Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education School of Chemistry & Chemical Engineering Sun Yat-Sen University, Guangzhou, 510275 (P.R. China) E-mail: [email protected] Homepage: http://ce.sysu.edu.cn/tml/ [b] Dr. C.-J. Li College of Chemistry and Chemical Engineering Zhongkai University of Agriculture and Engineering Guangzhou, 510225 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404847. Chem. Eur. J. 2015, 21, 2560 – 2567
namely, 7.092(3) (1), 10.921(6) (2), 14.780(5) (3), and 19.165(4) (4). Despite the wrinkled kagom layers in complexes 2–4, comprehensive magnetic characterizations revealed weakening of interlayer magnetic interactions and an increase in the degree of frustration as the pillar ligand becomes longer from 1 to 4; this leads to characteristic magnetic ground states. For compound 4, which has the longest interlayer distance, the interlayer interaction is so weak that the magnetic properties observed within the range of temperature measured would correspond to the frustrated layer.
ering one or more perturbations. For example, Rao et al. performed a theoretical calculation of the low-energy magnetic spectrum of a finite kagom lattice with different magnitudes of spin carriers and found that half-odd-integer spins led to the strongest frustration.[4] As synthesized by Nocera et al., pure-phase FeIII–jarosites with the formula of AxFe3(OH)6(SO4)2 (x = 1 for A = Na + , K + , Rb + , Tl + , Ag + , and NH4 + ; x = 0.5 for A = Pb2 + ) displayed a weak field-induced ferromagnetic transition below the Nel temperature because of the role of Dzyaloshinskii–Moriya (DM) interactions, resulting in anisotropy in the systems.[5] Gao et al. presented a pillar-layered framework, [Co(N3)2(bpg)]·4/3 DMF, with a kagom layer constructed from Co2 + ions and azido ligands with interesting re-entered magnetic behavior.[6] Recently, we presented the preliminary characterization of two pillared, layered frameworks,[7] [Co3(imda)2(pyz)3]·8 H2O (1; imda = imidazole-4,5-dicarboxylate, pyz = pyrazine) and [Co3(imda)2(4,4’-bpy)3]·(4,4’-bpy)·8 H2O (2; 4,4’-bpy = 4,4’-bipyridine), with a similar frustrated kagom [Co3(imda)2] layer constructed from corner-sharing [Co3(imda)] triangles. Although there have been extensive studies on 2D magnetic systems,[8] systemic investigations on the influence of interlayer magnetic interactions on 2D frustrated layers in this fascinating family are rare. Herein, we attempt to further replace the pillars by longer N,N-pillars of 1,4-dpb (1,4-dpb = 1,4-di(pyridin-4-yl)benzene) and 4,4’-dpbp (4,4’-dpbp = 4,4’-di(pyridin-4-yl)-1,1’-biphenyl; Scheme 1), and successfully synthesize two analogous pillared, layered frameworks, [Co3(imda)2(1,4-dpb)3]·(1,4dpb)·13 H2O (3) and [Co3(imda)2(4,4’-dpbp)3]·(4,4’-dpbp)·14 H2O (4). Depending on the choice of pillar ligands, the interlayer spacing for the four structures differs from 7.092(3) to 19.165(4) . A detailed comparison of the structural and mag-
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Scheme 1. Structures of the organic ligands used as pillars herein; lengths and corresponding abbreviations are also indicated.
netic properties of the four closely related complexes demonstrates the controlling influence of interlayer distance on the degree of magnetic frustration. In addition, the combination of spin frustration and DM interactions that come from the wrinkled layers results in a variety of magnetic ground states, namely, metamagnetism for 1 and 2 with different critical fields, the coexistence of spin glass and spin canting for 3, and the absence of ordering above 1.8 K for 4.
Figure 1. An imda3 ion chelates three cobalt atoms and acts as a triangle unit (a and b), resulting in a 2D layer with a kagom lattice by sharing corners (c and d).
Results and Discussion Structural characterization Single-crystal X-ray analysis revealed that complex 1 crystallized in the hexagonal space group P6/mmm, whereas complexes 2–4 crystallized in the triclinic space group P1¯ (Table S1 in the Supporting Information). All of them are built on neutral kagom [Co3(imda)2] layers (Figure 1 c) and pillar ligands, but with notable differences. Because the structures of 1 and 2 have been previously described in detail,[7] herein we focus the discussion of complexes 3 and 4. The asymmetric unit of 3 contains 1.5 1,4-dpb linkers, 1 imda3 ion, and 3 CoII ions in distorted octahedral coordination geometry. Although the space groups of 2–4 are centrosymmetric, no inversion centers lie between the independent CoII ions, which makes asymmetric coupling possible. As shown in Figure 2 a, there are two kinds of coordination spheres for the three CoII ions. Both Co1 and Co2 are chelated by two NO bidentate sites of imda3 at the equatorial positions and two pyridyl N atoms at the axial positions; this leads to a [CoO2N4] environment (CoNimda = 2.071(4)–2.076(3), Co N1,4-dpb = 2.162(4)–2.191(4), CoO = 2.103(4)–2.109(4) ). For Co3, two OO bidentate sites of imda3 occupy its equatorial positions to form a different [CoO4N2] environment (CoN = 2.161(5), CoO = 2.042(3)–2.092(3) ]. The imda3 ion acts as a multidentate ligand, chelating 3 CoII ions with a Co···Co separation of 5.7809(4)–6.3190(4) , to form a triangular [CoII3(imda)] unit and further extend to a 2D layer on the ab plane with magnetic kagom topology (Figure 1). Chem. Eur. J. 2015, 21, 2560 – 2567
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Figure 2. Coordination geometries of the Co2 + ions in complexes 3 (a) and 4 (b). The p–p stacking between the aromatic rings of the ligands is indicated by black dotted lines.
The kagom layers are pillared by 1,4-dpb organic linkers along the c axis with a crisscross connecting model, resulting in a 3D pillar-layered framework (Figure 3 c). The interlayer distance in 3 is 14.780(5) , which is larger than 10.921(6) in 2 and 7.092(3) in 1. For 4, the asymmetric unit is isostructural with 3, except for the pillar ligands and different bond lengths and angles. The longest pillar ligand in this family, 4,4’-dpbp, results in an increase in the CoCo interlayer distance to 19.165(4) , which is rare among most pillar-layered frameworks without penetration. Analysis with the PLATON[10] program suggested that the open channels in the frameworks occupied 41.5 and 43.8 % of the crystal volumes for 3 and 4, respectively; these channels are filled with guest ligands and water molecules. To better understand the connectivity of the two structures, we performed topological analysis with the software TOPOS 4.0[9] and found the same connectivity for 3
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Figure 5. The arrangement of the CoII octahedron in the [Co3(imda)] unit for 1 (a) and 2–4 (b), and the corresponding side view of the frustrated kagom layer for 1 (c) and 2–4 (d).
Figure 3. The 3D structures of the four complexes constructed with different pillar ligands and frustrated cobalt–imda layers are shown along with the corresponding interlayer distances for a) 1, b) 2, c) 3, and d) 4.
also in the wave-like [Co3(imda)2] layer for complexes 2–4 (Figure 5 and Table 1). However, they still belong to the frustrated 2D kagom lattice owing to the corner-sharing triangular connection, while the pillar ligands are responsible for the interlayer interactions as the perturbation of frustration and determine the final magnetic behavior.
and 4 with a new point symbol, (63)2(62.103.12)(64.8.10)2 (Figure 4). Two marked structural differences are observed among the four complexes. First, the interlayer distances in the four structures are successfully controlled from 7.1 in 1 to 19.2 in 4,
Table 1. The dihedral angles [8] for complexes 1–4 between the coordinative equatorial plane of each CoII ion and the crystal ab plane.
1 2 3 4
Co1
Co2
Co3
0.0 14.9(1) 19.1(1) 17.2(2)
– 15.8(1) 19.6(1) 19.7(2)
– 30.93(8) 29.11(7) 31.8(1)
Direct (dc) and alternating current (ac) magnetic measurements
Figure 4. Simplified topological structure of complexes 3 and 4; the gray and black spheres represent imda3 ions as three-connected nodes and CoII ions as six-connected nodes, respectively, and the pillar ligands are simplified as the dark gray rods.
which is the direct effect of the increasing lengths of the pillar ligands. Second, the longer pillars also enable structural variation of the kagom layers. In complex 1, the short and rigid ligands are perfectly perpendicular to the flat kagom layers on the ab plane. When the layers are pillared by the extended ligands in 2–4, the p–p interactions, along with the allowable bend of the pillars, result in the unparalleled linking mode and Chem. Eur. J. 2015, 21, 2560 – 2567
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The magnetic behavior of 1–4 was investigated by using a Quantum Design MPMS XL-7 SQUID magnetometer. In addition to the preliminary magnetic measurements for complexes 1 and 2 in our previous report, herein we performed comprehensive magnetic measurements for all four complexes to investigate and compare their frustrated behavior. As shown in Figure 6 a, the cMT value of 1 is 8.33 cm3 mol1 K at 300 K per Co3 unit, which is significantly higher than the spin-only value (5.63 cm3 mol1 K) expected for three uncoupled octahedral CoII ions because of the orbit contribution. With a decrease in temperature, the cMT value decreases gradually and reaches a minimum of 0.28 cm3 mol1 K at 1.8 K. The decrease in the cMT plot upon cooling at high temperature is attributed to spin-orbit coupling effects and possible antiferromagnetic interactions. A Curie–Weiss fitting (Figure S1a in the Supporting Information) of cM versus T data in the range of 50–300 K gives C = 9.77 cm3 mol1 K and q = 53.99 K. The significantly negative Weiss constant indicates dominant antiferromagnetic interactions and spin-orbit coupling in the system. The irreversibility of the ZFC and FC molar susceptibilities under 100 Oe for 1 (Figure 6 b), as well as a sharp peak of the ac susceptibility at about 8.5 K in both real and imaginary com-
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Figure 6. Magnetic measurements of complex 1. a) cM versus T (black) and cMT versus T (gray) at 100 Oe, b) zero-field-cooled (ZFC) and field-cooled (FC) magnetization at 100 Oe, c) ac measurements between 1.8 and 20 K, and d) M–H plots at various temperatures.
ponents (Figure 6 c) not only indicate the onset of long-range magnetic ordering, but also exhibit the occurrence of a net moment in 1. Furthermore, the M–H plot (Figure 6 d) at 2 K has a weak jump when the field is above 40 kOe; this indicates metamagnetic behavior, which is likely to be due to strong interlayer interactions in 1. For 2, the decrease in the cMT plot (Figure 7 a) in the hightemperature region is similar to that of 1. The Curie–Weiss fitting of the 1/cMT data between 50 and 300 K gives C = 9.69 cm3 mol1 K and q = 46.24 K (Figure S1b in the Supporting Information). Owing to the long distance between the layers, the negative Weiss constant mainly represents the intralayer CoCo antiferromagnetic interactions in addition to the spin-orbit coupling of octahedral CoII ions. The magnetic susceptibility of 2 increased upon cooling from room temperature and a clear cusp was observed at approximately 4.4 K, which indicated long-range antiferromagnetic ordering. When the temperature was further decreased, the cM value tended to be constant below 3.7 K. The ZFC–FC curve (Figure 7 b) was measured at 10 Oe. The FC susceptibility follows the ZFC curve from the high-temperature region to form a distinct peak, which suggests a Nel temperature of 4.5 K; this is further confirmed by the AC susceptibility with the frequency-independent peaks of the real part of the susceptibility (c’) at around 4.4 K and the absence of the imaginary part (c’’) (Figure 7 c). Interestingly, the field-induced spin flip can be clearly observed below TN, as demonstrated by the FC curves involving larger fields (Figure 7 d). In addition, the M–H curves for 2 exhibit a sudden increase during magnetization (Figure 7 e), which indicate that 2 is a field-induced metamagnet below TN and the critical field is calculated from the dM/dH curves to be approximately 5 kOe (Figure 7 f). The cMT versus T curve of complex 3 is shown in Figure 8 a. Different from 1 and 2, upon cooling, the cMT value of 3 gradually decreases to a minimum around 5.2 K and then abruptly increases to a maximum at 2.4 K. Simultaneously, the cM value Chem. Eur. J. 2015, 21, 2560 – 2567
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Figure 7. Magnetic measurements of complex 2. a) cM versus T (black) and cMT versus T (gray) at 100 Oe; b) ZFC and FC magnetization at 10 Oe; c) the temperature dependence of ac measurement at 10 and 499 Hz, respectively, which shows a frequency-independent peak at real component, d) cM versus T at various fields. M–H plots at various temperatures (e) and the corresponding differential curves (f).
of 3 increases gradually with decreasing temperature from 300 K, but rises abruptly below 3.5 K and reaches 25.72 cm3 mol1 at T = 1.8 K without dropping; this suggests the weak ferromagnetic behavior of 3. Above 50 K, the magnetic behavior can be fitted by the Curie–Weiss law (Figure S1c in the Supporting Information) to give C = 8.33 cm3 mol1 K and q = 59.64 K. Considering the negative Weiss constant and weak ferromagnetic behavior of 3, the canted antiferromagnetic behavior may be ascribed to 3; this is further confirmed by the cM versus T measurements at various fields (Figure 8 c). The M–H curves (Figure 8 d) at various temperatures are also in accordance with the canted behavior and a narrow hysteresis loop is observed at 2 K with a small coercive field of 25 Oe, which indicates soft magnetic behavior. To determine the phase transition temperature, the temperature dependence of the FC and ZFC curves (Figure 8 b) were measured under a dc field of 50 Oe to give Tc = 2.6 K. However, the subsequent ac susceptibility measurement (Figure 8 e) is beyond our expectations. Under a zero dc field and a 5 Oe oscillating field in the frequency range of 1–1488 Hz, both the real and imaginary parts of ac susceptibility reveal a peak at about 2.3 K, which is slightly frequency dependent. This frequency dependence can be quantitatively studied by the equation f= (DTp/Tp)/Dlog w, in which Tp is the peak temperature and w is the frequency. The estimated value of f is 0.016, which is of the same magni-
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Figure 9. Magnetic measurements of complex 4. a) cM versus T (black) and cMT versus T (gray) at 100 Oe, b) ZFC and FC magnetization at 50 Oe field, c) the temperature dependence of ac susceptibility at various frequencies, and d) M–H plots at various temperatures.
Discussion of the magnetic behavior Figure 8. Magnetic measurements of complex 3. a) cM versus T (black) and cMT versus T (gray) at 100 Oe, b) ZFC and FC magnetization at 50 Oe, c) cM versus T at various fields, d) magnetic hysteresis loop at various temperatures, e) the temperature dependence of ac susceptibility at various frequencies, and f) M–H plots at various temperatures.
Among the four complexes, the intralayer antiferromagnetic interactions will undoubtedly lead to spin frustration on the ab plane because of the characterized kagom lattice. On the other hand, the critical temperature (Tc) for complexes 1–4 shifts monotonically to lower temperatures as the interlayer distance increases with the employment of longer N,N-pillars (Table 2). Such an observation is consistent with a reduction in interlayer magnetic coupling owing to extension of the p-conjugated aromatic bridges[12] between the cobalt–imda layers, leading to more separated 2D kagom layers. To estimate the degree of frustration for each complex, a widely used empirical parameter, f, provided by Ramirez[1a] is calculated by using Equation (1):
tude with spin-glass.[11] Such behavior indicates a cooperative magnetic state of spin-glass and spin canting for complex 3. Between 1.8 and 300 K, the cMT value (Figure 9 a) of complex 4 gradually decreases to a minimum at 2.6 K and then a tiny increase is observed, such behavior is similar to that of 3. The 1/cM versus T curve fitted by the Curie–Weiss law between 50 and 300 K gave the following parameters: C = 9.87 cm3 mol1 K and q = 58.20 K (Figure S1d in the Supporting Information). ZFC–FC measurement (Figure 9 b) under 50 Oe did not show f ¼ jqj=T c ð1Þ any furcation of the two curves down to 1.8 K, which indicates the absence of a magnetic Table 2. Magnetic properties of complexes 1–4 and selected CoII metal–organic frameworks with kagom lattices. phase transition. Further ac susceptibility of 4 (Figure 9 c) was f Magnetic ground state Interlayer q Complex[a] Tc measured at a zero dc field and [K] distance [K] a 5 Oe oscillating field at a fre[] quency of 1 and 1488 Hz. The 1 7.092(3) 53.99 9.3 5.8 metamagnetism (Hc = 40 kOe) below real part increases gradually and TN 2 10.921(6) 46.24 4.4 10.5 metamagnetism (Hc = 5 kOe) below the imaginary part has no signal, TN which proves the absence of 3 14.780(5) 59.64 2.6 22.9 coexistence of spin-glass and spin magnetic ordering in 4 between canting 1.8 and 300 K. 4 19.165(4) 58.20 < 1.8 > 32.3 no ordering above 1.8 K 14.169(5) 80.33 [Co3(m3-OH)2(1,2-chdc)2][13a] 35.5 [Co3F6(SO4)2]·[H2N(CH2)4NH2]·[NH4]2[13b] 8.521(2) 13.416(4) 165.8 [Co3(N3)6(bpg)3]·4 DMF[6a]
11 2.8 16
7.3 canted antiferromagnetic ordering 12.7 antiferromagnetic ordering 10.4 canted antiferromagnetic ordering with a second Tc
[a] 1,2-chdc = trans-1,2-cyclohexane-dicarboxylate, bpg = meso-a,b-bi(4-pyridyl) glycol.
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Full Paper in which q is the Weiss constant, which reflects not only the magnitude of the magnetic interactions, but also the anisotropy of the high-spin six-coordinate CoII ions that will vary in the series presented herein; Tc is the critical temperature for ordering in the system; the strength of frustration is in direct proportion to the value of f and a value of f > 10 signifies strongly frustrated behavior.[1a, b] Herein, in our family of complexes, the f values show a drastic increase from 1 to 4 (Table 2), which suggests strengthening of frustration owing to weakening of the interlayer coupling. For 4, such strongly frustrated behavior, f > 32.3, is rare in most CoII-frustrated materials with a kagom lattice (Table 2). In addition to strengthening of the magnetic frustration from complexes 1–4, further magnetic measurements revealed a change in the magnetic ground states for these four complexes. For complex 1, the layers are linked by the shortest pillar, pyz, which can be used as a magnetically active ligand[15] because of the relatively short length and conjugated aromatic structure. Therefore, the relatively strong interlayer superexchange coupling, compared with the other three compounds, through pyz hinders frustration and results in the metamagnetic behavior of 1 with a large critical field with Hc = 40 kOe. When pillar pyz was replaced with a longer ligand to construct the other three compounds, there were two clear changes for complexes 2–4 that would remarkably influence the magnetic behavior of each compound. First, extending the distance between the neighboring frustrated layer weakens the superexchange coupling and the dominant interlayer interactions will be dipolar interactions.[14] Second, the kagom layer of the other three compounds has a clear wrinkling compared with the flat structure of 1 (Figure 5). Such a structural change results in the absence of a symmetric center between the neighboring octahedral CoII ions; this is in agreement with the lower space group of these three compounds compared with that of 1. Therefore, DM[16] interactions will be induced because of the asymmetric coupling[17] and lead to a net moment between antiferromagnetically coupled spins. For complex 2, although the antiferromagnetic interlayer coupling leads to long-range ordering at TN = 4.4 K, DM interactions may cause the spins to rotate out of the layer and produce a net magnetic moment for each layer. When the applied field exceeds the critical field, the ferromagnetic arrangement of the net magnetic moments should be observed. Such behavior has been commonly found in some layered cobalt compounds,[18] but for a frustrated magnet with a 2D kagom lattice, as far as we know, there is only one family of inorganic FeIII–jarosite materials that displays similar magnetic behavior to that of complex 2.[5] In addition, the smaller critical field (5 kOe) for 2 compared with 1 (40 kOe) is reasonable because of weaker interlayer interactions. The kagom layer of complex 3 is similar to that of complex 2; thus DM interactions also exist; however, the coexistence of spin-glass and spin canting in 3 is quite exotic, and can be regarded as a median state called “order with disorder”.[19] In theory, the coexistence of magnetic order and disorder can occur when one or more sublattices are ordered below the critical temperature, Tc, whereas at least one sublattice remains Chem. Eur. J. 2015, 21, 2560 – 2567
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disordered at all temperatures. For complex 3, the kagom layer is more separated (14.780(5) ); thus the interlayer coupling for 3 is too weak to completely suppress frustration. However, the presence of DM interactions within layers partially relieves frustration and prefers the canted long-range magnetic order. As a result, when the competition between longrange order and quantum spin fluctuations caused by strong spin frustration cannot be suppressed by each other, such exotic magnetic behavior as that shown by complex 3 will be observed. There are some materials[20] that show similar magnetic behavior to that of 3; however, a theoretical investigation is difficult because of the complicated frustrated structure and competitive interactions; therefore, complex 3 offers a simpler model for investigating the mechanism of the coexisting behavior in frustrated systems. For complex 4, DM interactions still exist because of the same wrinkled layer as those in 2 and 3. However, the weakest interlayer interaction caused by the longest pillar, 4,4’-dpbp, leads to the strongest spin frustration, which is in agreement with the critical temperature below 1.8 K. The magnetic ground state of 4 in the lower temperature region would be interesting because a perfect 2D kagom layer would maintain frustration without ordering until 0 K, as predicted by Anderson.[2]
Conclusion We reported a family of pillar-layered 3D cobalt(II) coordination polymers with analogous kagom [Co3(imda)2] layers. Their structures and magnetic properties could be tuned through the choice of different N,N-pillars. In general, increasing the interlayer distance leads to weakening of the interlayer magnetic coupling and the strengthening of the degree of frustration, whereas the introduction of DM interactions from wrinkled layers partially relieved frustration to generate competition. For complexes 1 and 2, the interlayer interaction was non-negligible and resulted in long-range magnetic ordering with metamagnetic behavior; however, the critical field for both compounds had a significant difference, Hc = 40 and 5 kOe for 1 and 2, respectively, which indicated weakening of interlayer interactions from 1 to 2. By choosing longer pillar, 1,4-dpb, in 3, competition of the interlayer magnetic interactions with spin frustration was evident, which led to exotic magnetic behavior. Finally, the degree of frustration in 4 was strengthened by the longest pillar, 4,4’-dpbp, which eliminated any magnetic ordering above 1.8 K, and thus, complex 4 might be more attributed to the real 2D magnet because of the long interlayer distance. The pillar-layered structure is a feasible way to investigate the perturbations of frustrated 2D systems trapped in 3D frameworks, in which the pillar ligands can be systematically replaced and diverse magnetic behaviors can be obtained and fine-tuned.
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Full Paper Experimental Section Material and physical measurements All of the starting materials employed were commercially available and used as received without further purification. The synthetic methods for complexes 1 and 2 were reported previously. The thermogravimetric (TG) analyses were performed on a PerkinElmer TGA7 thermogravimetric analyzer in a flow of N2 with a heating rate of 10 K min1. The corresponding TG curves for complexes 3 and 4 are presented in Figure S4a and b in the Supporting Information, respectively. The C, H, and N microanalyses were carried out with an Elementar Vario EL CHNS elemental analyzer. Powder XRD diffraction intensities for polycrystalline samples were measured at 293 K on a Bruker D8 Advance diffractometer (CuKa, l = 1.54056 ; Figure S2 in the Supporting Information).
reflections measured, 6834 independent reflections (Rint = 0.0345), R1 = 0.0984 (I > 2s(I)), wR2 = 0.2908 (all data), final GooF = 1.145. Crystal data as well as details of data collection and refinements for 3 and 4 are summarized in Table S1 in the Supporting Information. CCDC 1019076 (3) and 1019077 (4) 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.
Magnetic measurements The magnetic measurements were performed on polycrystalline samples by using a Quantum Design MPMS XL-7 SQUID magnetometer with experimental diamagnetic correction. The samples were mixed with Vaseline to prevent orientation in magnetic fields. The ZFC and FC magnetic susceptibilities were measured with the ultra-low field option to provide a remnant field < 0.1 Oe.
Synthesis of 3 A mixture of CoCl2·6 H2O (0.018 g, 0.075 mmol), H3imda (0.012 g, 0.075 mmol), 1,4-dpb (0.01725 g, 0.075 mmol), and triethylamine (0.023 g, 0.023 mmol) was added to deionized water (15 mL) and stirred for 10 min. The resulting solution was heated in a 25 mL stainless-steel reactor with a Teflon liner at 160 8C for 12 h. After a period of approximately 20 h of cooling to room temperature, orange prismatic crystals of 3 were obtained, isolated by filtration, and washed with water. Yield: 87 %; elemental analysis (%) calcd for C74H76N12O21Co3 : C 54.00, H 4.65, N 10.21; found: C 54.00, H 4.25, N 10.35.
This work was supported by the “973 Project” (2012CB821704 and 2014CB845602), the NSFC (grant nos. 21371183, 91122032, and 21121061), the NSF of Guangdong (S2013020013002), and the Program for Changjiang Scholars and Innovative Research Team in University of China. Keywords: cobalt · magnetic properties · metal–organic frameworks · layered compounds · spin frustration
Synthesis of 4 A mixture of CoCl2·6 H2O (0.019 g, 0.008 mmol), H3imda (0.006 g, 0.004 mmol), and 4,4’-dpbp (0.025 g, 0.008 mmol) was added to a solution of H2O/glycol (14:1; 15 mL) and stirred for 10 min. The resulting solution was heated in a 25 mL stainless-steel reactor with a Teflon liner at 180 8C for 6 days. After a period of approximately 20 h of cooling to room temperature, orange prismatic crystals of 4 were obtained, isolated by filtration, and washed with water. Yield: 64 %; elemental analysis (%) calcd for C98H94N12O22Co3 : C 59.79, H 4.81, N 8.54; found: C 59.53, H 4.63, N 8.62.
X-ray crystallography Diffraction data for complex 3 were collected on a Rigaku R-AXIS SPIDER IP diffractometer by using MoKa radiation (l = 0.71073 ) at 150 K. Diffraction data for complex 4 were recorded on an Oxford Diffraction Gemini R CCD diffractometer with CuKa, l = 1.54056 , at 150 K. The structures of 3 and 4 were solved by direct methods, and all non-hydrogen atoms were refined anisotropically by leastsquares methods on F2 by using the SHELXTL program.[19] Hydrogen atoms on organic ligands were generated by the riding mode. Some of the severely disordered water molecules for both 3 and 4 were removed by SQUEEZE in the structural refinement and their amounts were determined by elemental analysis and TG analysis (Figure S3 in the Supporting Information). For 3: C74H76N12O21Co3, M = 1646.25, triclinic, space group P1¯, a = 11.5618(8) , b = 11.8493(8) , c = 15.7206(12) , a = 76.325(2)8, b = 71.246(2)8, g = 65.330(2)8, V = 1840.1(2) 3, Z = 1, 1 = 1.372 g cm3 ; m(MoKa) = 0.752 mm1, 12 772 reflections measured, 7023 independent reflections (Rint = 0.0615), R1 = 0.0812 (I > 2s(I)), wR2 = 0.2612 (all data), final GooF = 1.038. For 4: C98H94N12O22Co3, M = 1968.64, triclinic, space group P1¯, a = 11.4712(6) , b = 11.6147(7) , c = 19.8781(10) , a = 81.156(5)8, b = 74.921(5)8, g = 65.987(10)8, V = 2332.5(2) 3, Z = 1, 1 = 1.222 g cm3 ; m(CuKa) = 4.795 mm1, 11 367 Chem. Eur. J. 2015, 21, 2560 – 2567
Acknowledgements
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[1] a) A. P. Ramirez, Ann. Rev. Mater. Sci. 1994, 24, 453 – 480; b) J. E. Greedan, J. Mater. Chem. 2001, 11, 37 – 53; c) S. H. Lee, C. Broholm, W. Ratcliff, G. Gasparovic, Q. Huang, T. H. Kim, S. W. Cheong, Nature 2002, 418, 856 – 858; d) Z. Lin, M.-L. Tong, Coord. Chem. Rev. 2011, 255, 421 – 450. [2] a) P. W. Anderson, Mater. Res. Bull. 1973, 8, 153 – 160; b) P. W. Anderson, Science 1987, 235, 1196 – 1201. [3] a) M. Elhajal, B. Canals, C. Lacroix, Phys. Rev. B 2002, 66, 014422; b) K. Damle, T. Senthil, Phys. Rev. Lett. 2006, 97, 067202; c) D. G. Nocera, B. M. Bartlett, D. Grohol, D. Papoutsakis, M. P. Shores, Chem. Eur. J. 2004, 10, 3850 – 3859; d) S. K. Pati, C. N. R. Rao, Chem. Commun. 2008, 39, 4683 – 4693. [4] a) S. K. Pati, C. N. R. Rao, J. Chem. Phys. 2005, 123, 234703; b) J. N. Behera, C. N. R. Rao, J. Am. Chem. Soc. 2006, 128, 9334 – 9335. [5] a) D. Grohol, D. G. Nocera, D. Papoutsakis, Phys. Rev. B 2003, 67, 064401; b) B. M. Bartlett, D. G. Nocera, J. Am. Chem. Soc. 2005, 127, 8985 – 8993; c) T. Fujita, M. Hagiwara, H. Yamaguchi, S. Kimura, T. Kashiwagi, K. Matan, D. G. Nocera, Y. S. Lee, D. Grohol, J. Low Temp. Phys. 2013, 170, 242 – 247; d) K. Matan, B. M. Bartlett, S. Helton, V. Sikolenko, S. Matˇasˇ, K. Prokesˇ, Y. Chen, J. W. Lynn, D. Grohol, T. J. Sato, M. Tokunaga, D. G. Nocera, Y. S. Lee, Phys. Rev. B 2011, 83, 214406. [6] a) X.-Y. Wang, L. Wang, Z.-M. Wang, S. Gao, J. Am. Chem. Soc. 2006, 128, 674 – 675; b) W. Li, S.-S. Gong, Y. Zhao, S.-J. Ran, S. Gao, G. Su, Phys. Rev. B 2010, 82, 134434. [7] C.-J. Li, S. Hu, W. Li, C.-K. Lam, Y.-Z. Zheng, M.-L. Tong, Eur. J. Inorg. Chem. 2006, 1931 – 1935. [8] a) P. Rabu, P. Janvierb, B. Bujoli, J. Mater. Chem. 1999, 9, 1323 – 1326; b) A. Rujiwatra, C. J. Kepert, M. J. Rosseinsky, Chem. Commun. 1999, 2307 – 2308; c) G. Rogez, C. Massobrio, P. Rabu, M. Drillon, Chem. Soc. Rev. 2011, 40, 1031 – 1058. [9] a) V. A. Blatov, Struct. Chem. 2012, 23, 955 – 963; b) E. V. Alexandrov, V. A. Blatov, A. V. Kochetkova, D. M. Proserpio, CrystEngComm 2011, 13, 3947 – 3958. [10] A. L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University, Utrecht, 2003. [11] J. A. Mydosh, Spin Glass: An Experimental Introduction, Taylor & Francis, London, 1993.
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Full Paper [12] a) M. Castellano, F. R. Fortea-Prez, A. Bentama, S.-E. Stiriba, M. Julve, F. Lloret, G. D. Munno, D. Armentano, Y.-l. Li, R. Ruiz-Garca, J. Cano, Inorg. Chem. 2013, 52, 7645 – 7657; b) E. Pardo, J. Faus, M. Julve, F. Lloret, M. C. MuÇoz, J. Cano, X. Ottenwaelder, Y. Journaux, R. Carrasco, G. Blay, I. Femndez, R. Ruiz-Garca, J. Am. Chem. Soc. 2003, 125, 10770 – 10771. [13] a) Y.-Z. Zheng, M.-L. Tong, W.-X. Zhang, X.-M. Chen, Chem. Commun. 2006, 2, 165 – 167; b) J. N. Behera, G. Paul, A. Choudhury, C. N. R. Rao, Chem. Commun. 2004, 4, 456 – 457. [14] M. Drillon, P. Panissod, J. Magn. Magn. Mater. 1998, 188, 93 – 99. [15] a) K.-L. Zhang, Z. Wang, H. Huang, Y. Zhu, X.-Z. You, J. Mol. Struct. 2004, 693, 193 – 197; b) R. Podgajny, D. Pinkowicz, T. Korzeniak, W. Nitek, M. Rams, B. Sieklucka, Inorg. Chem. 2007, 46, 10416 – 10425; c) Y. Ma, G.-F. Xu, X. Yang, L.-C. Li, J.-K. Tang, S.-P. Yan, P. Cheng, D.-Z. Liao, Chem. Commun. 2010, 46, 8264 – 8266. [16] a) I. Dzyaloshinskii, J. Phys. Chem. Solids 1958, 4, 241 – 320; b) L. M. Sandratskii, Adv. Phys. 1998, 47, 91 – 160.
Chem. Eur. J. 2015, 21, 2560 – 2567
www.chemeurj.org
[17] a) D.-F. Weng, Z.-M. Wang, S. Gao, Chem. Soc. Rev. 2011, 40, 3157 – 3181; b) X.-Y. Wang, Z.-M. Wang, S. Gao, Chem. Commun. 2008, 48, 281 – 294. [18] a) A. Rujiwatra, C. J. Kepert, J. B. Claridge, M. J. Rosseinsky, H. Kumagai, M. Kurmoo, J. Am. Chem. Soc. 2001, 123, 10584 – 10594; b) X.-N. Cheng, W. Xue, J.-H. Huang, X.-M. Chen, Dalton Trans. 2009, 29, 5701 – 5707. [19] a) P. Azaria, H. T. Diep, H. Giacomini, Phys. Rev. Lett. 1987, 59, 1629 – 1632; b) R. Quartu, H. T. Diep, Phys. Rev. B 1997, 55, 2975 – 2980. [20] a) Q. Chen, W. Xue, B.-Y. Wang, M. H. Zeng, X.-M. Chen, CrystEngComm 2012, 14, 2009 – 2014; b) L. Hou, W.-X. Zhang, J.-P. Zhang, W. Xue, Y.-B. Zhang, X.-M. Chen, Chem. Commun. 2010, 46, 6311 – 6313. [21] SHELXTL 6.10, Bruker Analytical Instrumentation, Madison, WI (USA), 2000.
Received: August 13, 2014 Published online on December 5, 2014
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