Novel 3D Alkali–Lanthanide Heterometal–Organic Frameworks with

Mar 3, 2014 - Three novel alkali–lanthanide heterometal–organic frameworks, namely, [K5Ln5(pztc)5(H2O)19]·7H2O [Ln = Dy (1), Ho (2), and Yb (3); ...
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Novel 3D Alkali−Lanthanide Heterometal−Organic Frameworks with Pyrazine-2,3,5,6-tetracarboxylic Acid: Synthesis, Structure, and Magnetism Fengming Zhang,†,‡ Pengfei Yan,*,† Xiaoyan Zou,† Juwen Zhang,† Guangfeng Hou,† and Guangming Li*,† †

Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, People’s Republic of China ‡ Harbin University of Science and Technology, Harbin 150080, People’s Republic of China S Supporting Information *

ABSTRACT: Three novel alkali−lanthanide heterometal− organic frameworks, namely, [K5Ln5(pztc)5(H2O)19]·7H2O [Ln = Dy (1), Ho (2), and Yb (3); H4pztc = pyrazine-2,3,5,6tetracarboxylic acid], have been facilely synthesized. X-ray crystallographic analysis reveals that complexes 1−3 are isostructural, featuring unique 3D open frameworks, which possess a rare (4,8)-connected net with the Schläfli symbol of (415·612·8)(45·6)2. The 3D coordination framework involves a ladder-like square column structure by pztc4− ligands bridging metal ions, exhibiting 1D channels along the b axis. The magnetic analysis suggests that complexes 1 and 3 exhibit frequency-dependent out-of-phase signals in alternating current magnetic susceptibility measurements, indicating their slow magnetic relaxation characteristics. It is the first report of slow magnetic relaxation behavior existing in Yb3+-based HMOFs.



INTRODUCTION The design and construction of heterometal−organic frameworks (HMOFs) is a rapidly growing field in coordination chemistry and supramolecular chemistry, which stems not only from their intriguing molecular topologies1 but also from their potential applications in various areas, such as magnetism,2 catalysis,3 and photochemical sensors.4 Up to now, a number of HMOFs with fascinating structures have been prepared. Among them, the majority are associated with 3d−4f mixed metal ions,5 while the alkali−lanthanide HMOFs are still very limited. In fact, alkali-metal cations can serve as favorable components in MOFs assembly because of their distinct coordinative admissibility, low polarizability, and unique affinity for various basic molecules (either strong or weak).6 Examples include the [Na/KLn(bta)(H2O)3]·nH2O (H4bta = 1,2,4,5-benzenetetracarboxylic acid) complexes,7 {K[Ln(pdtc)(H 2 O)]·H 2O} n (pdtc4− = pyridine-2,3,5,6-tetracarboxylate) complexes,8 and the {KDy(C2O4)2(H2O)4}n complex.9 In these systems, the introduction of Na+/K+ cations can be viewed as a viable heterometallic strategy to change the size and geometry of the MOFs. It is undoubtedly significant to understand the role of alkali-metal cations in directing the assemblies of MOFs with diverse and aesthetically pleasing architectures. On the other hand, lanthanide complexes have recently attracted much attention in the field of molecular magnetism. © 2014 American Chemical Society

Indeed, these systems may hold the key to high anisotropic barriers of single-molecule magnets (SMMs),10 which exhibit supermagnet-like behavior of slow magnetic relaxation at low temperature. To improve the application temperature of SMMs, Dy3+ ion is particularly preferred in order to exploit the potential of its large magnetic anisotropy arising from the 6 H15/2 state. In this regard, many pure Dy3+ and Dy3+containing heterometallic systems have been explored.11 Nevertheless, most Dy3+-containing heterometallic compounds with SMM behaviors focused on the discrete clusters, and the investigations of 2D and 3D HMOFs featuring the slow magnetic relaxation were less developed.5b,12 In addition, it is known that the highly anisotropic Yb3+ ion could also lead to SMM behavior if an appropriate ligand system is employed.13 However, only rare mononuclear14 and polynuclear clusters15 of Yb3+-based magnets have been reported so far. No extended 3D systems have been reported to date. We are interested in the synthesis and magnetism of new lanthanide-based MOFs.16 To synthesize high-dimensional alkali−lanthanide frameworks, the high symmetrical and rigid pyrazine-2,3,5,6-tetracarboxylatic acid was chosen as the organic Received: January 24, 2014 Revised: February 21, 2014 Published: March 3, 2014 2014

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Table 1. Crystal Data and Structure Refinement for Complexes 1−3

a

complexes

1

2

3

formula fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm−3) μ (mm−1) F(000) R1 [I > 2σ(I)]a wR2 [I > 2σ(I)]b R1 (all data) wR2 (all data) GOF on F2

C40N10H52O66K5Dy5 2736.92 orthorhombic Pnma 15.2339(4) 33.4083(10) 14.3529(4) 90 90 90 7304.7(4) 4 2.489 5.480 5260 0.0444 0.0919 0.0538 0.0966 1.231

C40N10H52O66K5Ho5 2749.07 orthorhombic Pnma 15.203(3) 33.371(7) 14.395(3) 90 90 90 7303(3) 4 2.500 5.783 5280 0.0316 0.0729 0.0485 0.0814 1.029

C40N10H52O66K5Yb5 2789.62 orthorhombic Pnma 15.0646(3) 33.0834(8) 14.4068(3) 90 90 90 7180.2(3) 4 2.581 6.885 5340 0.0466 0.0894 0.0565 0.0936 1.165

R1 =∑∥Fo| − |Fc∥/|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. [K5Ho5(C8N2O8)5(H2O)19]·7H2O (2). Yield 4.1 mg, 75%. Anal. Calcd for C40N10H52O66K5Ho5 (2749.01): C, 17.48; H, 1.91; N, 5.10%. Found: C, 17.52; H, 1.93; N, 5.32%. IR (KBr, cm−1): 3422 (m), 1607 (s), 1471 (w), 1414 (w), 1313 (m), 1165 (m), 887 (w), 643 (w). UV− vis [MeOH, λ]: 239, 296 nm. [K5Yb5(C8N2O8)5(H2O)19]·7H2O (3). Yield 4.0 mg, 72%. Anal. Calcd for C40N10H52O66K5Yb5 (2789.56): C, 17.22; H, 1.88; N, 5.02%. Found: C, 17.26; H, 1.90; N, 5.30%. IR (KBr, cm−1): 3399 (m), 1607 (s), 1463 (w), 1420 (m), 1308 (m), 1163 (m), 884 (w), 633 (w). UV−vis [MeOH, λ]: 239, 296 nm. X-ray Crystallography. Suitable single crystals of 1−3 were selected for X-ray diffraction analysis. Structural analysis was performed on a Siemens SMART CCD diffractometer using graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å). Data processing was accomplished with the SAINT processing program.18 All data were collected at a temperature of 20 ± 2 °C. The structure was solved by direct methods, and all non-hydrogen atoms are anisotropically refined by full-matrix least-squares on F2 using the SHELXTL-97 program.19 The topological analysis was performed with TOPOS.20 The crystallographic data are summarized in Table 1, and the selected bond lengths of these three complexes are listed in Table S1 (Supporting Information).

building blocks for its versatile coordination directions in space, and mixed O and N donors make the design and prediction of the 3D networks possible. As a result, upon the reaction of H4pztc with the lanthanide ions in the presence of KOH under room temperature, three Ln−K HMOFs, [K5Ln5(pztc)5(H2O)19]·7H2O (Ln = Dy, Ho, and Yb), have been facilely isolated. Their crystal structures have been determined, and their magnetism has been investigated.



EXPERIMENTAL SECTION

Materials and Instrumentation. The H4pztc ligand was synthesized according to the literature.17 Ln(NO3)3·6H2O were prepared by reactions of lanthanide oxide and nitric acid. All the other reagents were of analytical grade and used without further purification. Elemental analysis for C, H, and N was performed on a PerkinElmer 2400 analyzer. FT-IR data were collected on a PerkinElmer 100 spectrophotometer by using KBr pellets in the range of 4000−400 cm−1. UV−vis spectra were recorded on a PerkinElmer Lambda 35 spectrometer. Thermal analysis was conducted on a PerkinElmer STA 6000 with a heating rate of 10 °C·min−1 in a temperature range from 30 to 800 °C under atmosphere. Powder X-ray diffraction (PXRD) data were recorded on a Rigaku D/Max-3B X-ray diffractometer with Cu Kα as the radiation source (λ = 0.15406 nm) in the angular range of θ = 5−50° at room temperature. Magnetic susceptibilities were performed on a Quantum Design MPMS XL-7 SQUID-VSM magnetometer. Diamagnetic corrections were made with Pascal’s constants for all the constituent atoms and sample holders. Synthesis of Complexes 1−3. Complexes 1−3 were prepared in a similar manner, and crystals were grown by the layer diffusion method. In a typical synthesis of 1, a methanol solution (10 mL) of Dy(NO3)3·6H2O (4.6 mg, 0.01 mmol) was layered on an aqueous solution (10 mL) of H4pztc (2.5 mg, 0.01 mmol) and KOH (2.2 mg, 0.04 mmol) in a long tube. The tube was sealed and allowed to stand at room temperature. Colorless crystals of 1 suitable for X-ray analysis were obtained about 1 month. [K5Dy5(C8N2O8)5(H2O)19]·7H2O (1). Yield 4.1 mg, 75%. Anal. Calcd for C40N10H52O66K5Dy5 (2736.86): C, 17.55; H, 1.92; N, 5.12%. Found: C, 17.57; H, 1.94; N, 5.33%. IR (KBr, cm−1): 3419 (m), 1607 (s), 1471 (w), 1414 (w), 1314 (m), 1165 (m), 887 (w), 645 (w). UV− vis [MeOH, λ]: 239, 296 nm.



RESULTS AND DISCUSSION Synthesis and Characterization. Complexes 1−3 were formed by Ln(NO3)3·6H2O reacting with H4pztc in the presence of KOH at room temperature. It is well-known that metal ions play a crucial role in coordination assemblies. In this work, our synthetic strategy uses KOH as deprotonation agent; at the same time, K+ ions act as structure-directing agents incorporated into the resulting crystalline materials. Infrared spectra for complexes 1−3 are similar (Figure S1, Supporting Information). In a typical spectrum of 1, a broad strong band at 3419 cm−1 is newly generated, which is from the O−H stretching vibration of water molecules. The asymmetric and symmetric stretching vibrations of the carboxylate groups are observed at 1607, 1471, and 1414 cm−1. The absence of the characteristic band around 1726 cm−1 indicates a complete deprotonation of carboxylate groups and coordination to the metal ions.21 All UV−vis absorption spectra of 1−3 and H4pztc 2015

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split into two peaks at 280 and 296 nm, resulting from the changes in the energy levels of the ligand orbitals upon the coordination of the metal ions. TG−DSC analyses for complexes 1−3 are shown in Figures S2−S4 (Supporting Information). For the complex 1, the first weight loss (5.2%) in the range of 30−88 °C is related to the removal of the lattice H2O molecules (calcd 4.6%). The second weight loss, 15.1%, occurred between 135 and 178 °C, which corresponds to the loss of all H2O molecules in the crystal. Above 500 °C, the weight loss is due to the collapse of the whole framework. The TG−DSC curve of 2 is similar to that of 1; but for complex 3, the process of losing lattice H2O molecules is not obvious. The powder X-ray diffraction patterns (PXRD) of complexes 1−3 all match those simulated from single-crystal X-ray data, clearly indicating that the pure phases were obtained (Figure S5, Supporting Information). Crystal Structure. X-ray crystallographic analysis reveals that complexes 1−3 are isostructural, crystallizing in the orthorhombic system with a space group of Pnma, featuring 3D open frameworks. In a typical structural unit of complex 1 (Figure 2a), each Dy3+ ion is 9-coordinated by eight O atoms and one N atom from the tridentate chelate ONO atoms, one chelating bidentate carboxylate group, two monodentate carboxylate groups, and two H2O molecules, indicating four pztc4− ligands around each Dy3+ ion (Figure S6, Supporting Information). However, the Dy1 and Dy2 ions adopt the similar coordination geometries of a tricapped trigonal prism (Figure 2b,c), while the Dy3 ion exhibits a distorted single-

in methanol solutions at room temperature were recorded and are shown in Figure 1. For the ligand, there are two

Figure 1. UV−vis spectra for complexes 1−3 and H4pztc in methanol solution.

characteristic absorption peaks at 203 and 282 nm, due to the K-band and B-band appearing separately, both corresponding to the π−π* transitions.21 The absorption bands for complexes 1−3 are similar, but the K-band at 217 nm redshifted as compared to that of the free ligand, and the B-bond

Figure 2. Structure unit in complex 1 (a). Coordination geometries of the Dy1 (b), Dy2 (c), and Dy3 (d) ions. Coordination modes for type A ligand (e), type B ligand (f), and type C ligand (g). Symmetry codes: i = −x + 1, −y, −z; ii = x + 0.5, y, −z + 0.5; iii = x − 0.5, y, −z − 0.5; iv = x, −y + 0.5, z; v = x + 0.5, −y + 0.5, −z − 0.5; vi = x + 0.5, y, −z + 0.5; vii = x, −y + 0.5, z; viii = x − 0.5, y, −z + 0.5; ix = x + 0.5, y, −z − 0.5; x = x − 0.5, −y + 0.5, −z + 0.5. 2016

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Figure 3. (a) Infinite 1D ladder-like square column structure in 1. (b) View of 2D layer structure by the connections of μ3-η2:η2-tridentate carboxylate group along the c axis. (c) View of the 2D layer structure in 1 along the b axis. The H2O molecules are omitted, and the pyrazine rings are represented as planes.

Figure 4. 3D framework of 1: (a) View of the structure along the c axis. (b) View of 1D channels in the 3D framework along the b axis. The H2O molecules are omitted, and the pyrazine rings are represented as planes.

structural unit consists of one type A, two type B, and two type C ligands, arranging symmetrically along the A type ligand. Thus, the ratios of metal ions in the complex 1 are Dy1:Dy2:Dy3 = 2:2:1 and K1:K2:K3 = 1:2:2. Further, the adjacent Dy3+ and K+ ions are bridged (up and down) by two carboxylate groups perpendicular to the pyrazine plane, respectively, to form a 1D ladder-like square column extending along the b axis (Figure 3a). In the ladder structure, there are two Dy3+ metallic chains and two K+ metallic chains where the distances of Dy···Dy range from 6.628(5) to 6.747(5) Å. Notably, the 1D square column is further extended to a 2D layer by the connection of one of the μ3-η2:η2-tridentate carboxylate groups from the ligand (Figure 3b,c). Then, in the vertical direction, another μ3-η2:η2-tridentate carboxylate group from the ligand in the layer bounds to the metal ions from the neighboring layers to generate a 3D framework (Figure 4). The 3D framework exhibits 1D channels occupied by H2O

capped square antiprism coordination geometry (Figure 2d). The Dy−O bond and Dy−N bond lengths range from 2.346(5) to 2.576(5) Å and 2.486(6) to 2.508(6) Å, respectively, which are comparable to previously reported Dy−O bond and Dy−N bond lengths.8,22 Interestingly, K+ ions exhibit flexible coordination numbers in the framework. The K1 ion is 9coordinated by oxygen atoms from two chelating bidentate carboxylate groups, two monodentate carboxylate groups, and three H2O molecules (Figure S6, Supporting Information). The K2 ion is 7-coordinated by one chelating bidentate carboxylate group, three monodentate carboxylate groups, and two H2O molecules, while the K3 ion is 8-coordianted by two chelating bidentate carboxylate groups, two monodentate carboxylate groups, and two H2O molecules. The K−O bond lengths range from 2.608(6) to 3.268(12) Å, which fall in the range of K−O bond lengths reported previously.8 Notably, there are three crystallographically independent pztc4− ligands in complex 1, namely, type A, type B, and type C (Figure 2e,g). The 2017

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molecules along the b axis direction with the dimensions of ∼5.5 × 5.7 Å. A better insight into the nature of this intricate framework can be achieved by a topological approach. In the structure of complex 1, both the Dy3+ and the K+ ions are integral in constructing the framework, which can be regarded as 4connected nodes. If we only consider the Dy-pztc4− or K-pztc4− anion framework, every pztc4− ligand links four Dy3+/K+ ions to act as a 4-connected node and yields the polyanionic frameworks with the same Schläfli symbol of (42·63·8) (Figure 5a,b). Each pztc4− ligand in the heterometallic framework links

Figure 6. Temperature dependence of χmT at 100 Oe for 1−3. Inset: plots of M versus H/T in the field range of 0−70 kOe at different temperatures for 1.

complexes 1−3 at room temperature are 69.42, 67.94, and 11.75 cm3 K mol−1, respectively, which are close to the theoretical values of 70.85, 70.35, and 12.85 cm3 K mol−1 of five paramagnetic Ln3+ ions (Dy3+, 6H15/2, S = 5/2, L = 5, g = 4/3, χT = 14.17 cm3 K mol−1; Ho3+, 5I8, S = 2, L = 6, g = 5/4, χT = 14.07 cm3 K mol−1; Yb3+, 2F7/2, S = 1/2, L = 3, g = 8/7, χT = 2.57 cm3 K mol−1). For complexes 1 and 3, the χmT gradually decreases to reach the minimums of 36.07 and 4.84 cm3 K mol−1 at 1.8 K, respectively. Upon decreasing the temperature, χmT product almost remains a constant until 75.0 K and then drops rapidly to a minimum of 19.69 cm3 K mol−1 at 1.8 K for complex 2. The decreases are most likely attributed to a combination of antiferromagnetic interactions and thermal depopulation of Stark sublevels.16b Magnetization (M) data for all complexes were collected in the range of 0−70 kOe below 5 K. The M versus H data at 1.8 K show a relatively rapid increase at low magnetic fields and slow linear increase without complete saturation up to 70 KOe (Figure S7, Supporting Information). For complexes 1−3, their magnetization values are 25.48, 25.98, and 9.47 NμB at 70 KOe, respectively, which are lower than the calculated values for five uncorrelated Ln3+ magnetic moments (50, 50, and 20 NμB for five Dy3+, Ho3+, and Yb3+ ions, respectively). The lack of saturation of the M versus H data and the nonsuperimpositon of the M versus H/T data (Figures 6, inset, S8 and S9, Supporting Information) on a single master curve may be explained by the presence of large magnetic anisotropy and/or low-lying excited states in these systems.11b,16c,27 To explore the dynamics of the magnetization, alternating current (ac) susceptibility measurements of 1−3 were carried out under zero direct current (dc) magnetic field in the frequency range of 1−900 Hz. As shown in Figure 7, complex 1 displays clear frequency-dependent out-of-phase (χ″) signals at low temperatures, suggesting the existence of slow magnetic relaxation behavior.28 The case is similar to the reported 3D {[Ln4(CPOA)6(H2O)4]·H2O}12a and [Dy2Co2(2,5-pydc)6(H2O)4]n·nH2O complexes.29 The absence of any frequencydependent peaks suggests that fast quantum tunneling of magnetization (QTM) may be an efficient pathway above 1.8 K.12a,28 Hence, ac susceptibility measurement under an applied dc field was performed in order to suppress the possible tunneling effects. When a static field of 2000 Oe was applied, complex 1 shows a peak in in-phase (χ′) around 3 K and also no frequency-dependent peak in χ″ (Figure S10, Supporting

Figure 5. (a) Topological view of the Dy-pztc4− polyanionic framework. (b) Topological view of the K-pztc4− polyanionic framework. (c) Topological view of the (4,8)-connected binodal net in complex 1. Dysprosium ions are shown in green, potassium ions in pink, and the ligand centroids in blue.

four Dy3+ ions and four K+ ions to act as an 8-connected node. According to the simplification principle, the resulting structure of complex 1 is a (4,8)-connected net with the Schläfli symbol of (415·612·8)(45·6)2 (Figure 5c), which is seldom reported23 and different from the known flu and scu.24 It should be noticed that, although some coordination polymers constructed by H4pztc ligands have been reported, most of them focused on transition metals25 and few of lanthanide complexes.21,26 Especially, only one heterometallic complex [EuK(pztc)(H2O)4] with the pztc4− ligand has been reported,26a in which all pztc4− ligands and Eu3+ and K+ ions are crystallographically equivalent. However, in complexes 1−3, three crystallographically independent pztc4− ligands adopt two different coordination modes, three Ln3+ ions exhibit two types of coordination geometries, and three K+ ions feature different coordination numbers. Thus, the structure of complexes 1−3 is extremely different from that of the reported complex [EuK(pztc)(H2O)4]. The structural difference is probably attributed to the different synthetic routes that the reported complex [EuK(pztc)(H2O)4] was synthesized under hydrothermal conditions, while complexes 1−3 in this work were afforded by solution reactions at room temperature. Magnetic Properties. The temperature dependence of the magnetic susceptibility is recorded for microcrystalline samples of 1−3 at an applied magnetic field of 100 Oe over the temperature range of 1.8−300 K (Figure 6). The χmT values for 2018

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were not observed as the result of the fast quantum tunneling.12a,28 Fitting the experimental χ″/χ′ data to the equation, ln(χ″/χ′) = ln(ωτ0) + Ea/kBT, yields the energy barrier Ea/kB ≈ 5.78 K and the relaxation time τ0 ≈ 1.71 × 10−6 s under the field of 2000 Oe (Figure 9). It is comparable to that

Figure 7. Temperature dependence of the in-phase (χ′) (inset) and out-of phase (χ″) ac susceptibility of 1 under zero field in the frequency range of 1−900 Hz.

Information). Thus, the energy barrier and relaxation time cannot be obtained by the Arrhenius equation. On the basis of the assumption that a single relaxation process exists in complex 1, the energy barrier and relaxation time can be roughly estimated from fitting the ac susceptibility by adopting the Debye model and equation, ln(χ″/χ′) = ln(ωτ0) + Ea/ kBT,12b,30 yielding the energy barriers Ea/kB ≈ 0.937 and 5.69 K as well as the relaxation times τ0 ≈ 3.945 × 10−6 and 6.53 × 10−6 s under zero field and 2000 Oe field, respectively (Figure S11, Supporting Information). It suggests that the QTM is suppressed somewhat at 2000 Oe field and enhanced the energy barrier and relaxation time.30,31 In comparison with the energy barrier of Dy3+-based SMMs, the energy barrier for complex 1 is low. However, it is close to those for reported Dy3+-based MOFs.12a,27e For complex 2, there are no frequency-dependent in χ′ and no signal in χ″ detected above 1.8 K under zero dc field and 2000 Oe field. In the case of complex 3, it shows a frequencyindependent in χ′ accompanied by no signal in χ″ below 20 K under zero field (Figure S12, Supporting Information). As shown in Figure 8, both the frequency-dependent phenomena in χ′ and χ″ terms can be detected with an external field of 2000 Oe below 4.5 and 7 K, respectively, which indicates the presence of slow magnetic relaxation. Unfortunately, the peaks

Figure 9. Plots of natural logarithm of χ″/χ′ versus 1/T for 3 under 2000 Oe. The solid line represents the fitting results in the range of 1.8−5.9 K.

for the reported mononuclear Yb3+ complex.14b Noticeably, only a few Yb 3+-based mononuclear14a,32 and polynuclear14b,15a,b complexes exhibiting the behavior of slow magnetic relaxation were previously reported; no 3D Yb3+based complexes exhibiting the behavior of slow magnetic relaxation have been reported to date. Thus, the presence of slow magnetic relaxation in complex 3 represents the first Yb3+based HMOF exhibiting the behavior of slow magnetic relaxation. Obviously, complexes 1−3 display different magnetic behaviors, although they are isostructural. Taking the crystal structure of complexes 1−3 into account, the large and different magnetic anisotropy and complicated Stark energy levels of lanthanide ions in 1−3 from the splitting of individual 2S+1LJ states should be responsible for the significant differences of magnetic behaviors in complexes.5b,9 The spin coupling between the lanthanide should be ignored since the distances of Ln···Ln in the range of 6.6−6.7 Å are too long.



CONCLUSIONS Three pyrazine-2,3,5,6-tetracarboxylate alkali−lanthanide HMOFs with novel 3D open frameworks have been facilely synthesized. Structural analysis verifies that these isostructural complexes feature a rare (4,8)-connected net with (415·612· 8)(45·6)2 topology by way of pztc4− ligands coordinating to the Ln3+ and K+ ions. Magnetic studies reveal that complexes 1 and 3 show clear frequency-dependent out-of-phase ac susceptibility originating from the single-ion effect of the Ln3+ ion, indicating the presence of the slow relaxation behaviors in these complexes. To the best of our knowledge, frequency-dependent magnetic phenomena are relatively rare among the numerous lanthanide-based HMOFs, especially for the Yb3+-based HMOF. This approach may develop new and more SMMs of HMOFs.

Figure 8. Temperature dependence of the in-phase (χ′) (inset) and out-of phase (χ″) ac susceptibility of 3 under 2000 Oe in the frequency range of 1−900 Hz. 2019

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectra, TG−DTA curves, PXTD patterns, magnetic data, selected bond lengths, and X-ray crystallographic files (CIF) for 1−3. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.Y.). *E-mail: [email protected]. Fax: 86-451-86673647. (G.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (No. 51272069). REFERENCES

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dx.doi.org/10.1021/cg5001254 | Cryst. Growth Des. 2014, 14, 2014−2021

Crystal Growth & Design

Article

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dx.doi.org/10.1021/cg5001254 | Cryst. Growth Des. 2014, 14, 2014−2021