Pyrazine-2,3-Carboxylate Based Ag+ Homometallic and Ln3+âAg+

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Pyrazine-2,3-Carboxylate Based Ag+ Homometallic and Ln3+−Ag+ Heterometallic Coordination Frameworks: Synthesis, Structures, and Magnetic Properties Fengming Zhang,†,‡ Xiaoyan Zou,† Pengfei Yan,*,† Jingwen Sun,† Guangfeng Hou,† and Guangming Li*,† †

Key Laboratory of Functional Inorganic Material Chemistry (MOE); School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China ‡ College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, P. R. China S Supporting Information *

ABSTRACT: One Ag+ homometallic complex [Ag4(pzdc)2]·H2O (1), and five Ln−Ag heterometallic complexes, namely, [Gd2Ag6(pzdc)6(H2O)9]·8H2O (2) and [LnAg(pzdc)2(H2O)2]·2H2O [Ln = Tb (3), Dy (4), Ho (5), Er (6)] based on pyrazine2,3-dicarboxylate (pzdc2−) have been synthesized under room temperature. Complex 1 exhibits 3D metal−organic framework by Ag2 ions connecting 1D square columns consisting of Ag1 ions and pzdc2− ligands, possessing a unique trinodal net with {42· 6}{43}{45·64·83·103} topology. Complex 2 displays a 3D heterometal−organic framework with a novel 12-nodal topological net constructed by Gd1 ions linking the adjacent Ag-pzdc2− anion layers. Complexes 3−6 are isostuctural 2D Ln−Ag heterometallic coordination frameworks featuring uninodal 4-connected net with {44·62} topology. Magnetic analyses suggest that complexes 4 and 6 exhibit field-dependent slow magnetic relaxation, and complex 6 represents the first slow magnetic relaxation behavior existing in Er3+-based 2D heterometallic coordination framework.

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interesting topology.19−22 However, owing to the different nature of the lanthanide and transition metals ions, the synthesis of Ln−Ag heterometallic coordination polymers is still a challenge,23−25 because the competitive coordination between these two kinds of metal ions to one ligand possibly leads to the construction of homometallic complexes. Therefore, the selection of multifunctional ligands containing both O and N donors is the most crucial for achieving this goal. On the other hand, lanthanide complexes have rapidly expanded in the field of molecular magnetism. This is because these systems may hold the key to high anisotropic barriers of single-molecule magnets (SMMs),26−28 exhibiting the behaviors of slow magnetic relaxation at low temperature. Therefore, many SMMs incorporating Tb3+,29,30 Dy3+,31−36 Ho3+,26 Er3+,37−41 and Yb3+,42,43 ions have been synthesized. In order to improve the blocking temperature of SMMs, Dy3+ ion is

INTRODUCTION The design and construction of metal−organic frameworks (MOFs) have attracted intense interest due to their diverse structures, intriguing topologies, interesting properties, and wide applications in various aspects such as gas storage and separation, magnetism, luminescence, and catalysis.1−8 Among them, the homometallic MOFs have received much more attention than the heterometallic MOFs;9−13 especially, the reported Ln−Ag (4f−4d) heterometal frameworks are still very limited. In fact, the silver ion easily bonds to the ligand with Ndonor atoms usually exhibiting tetrahedral, trigonal, and linear coordination environments, being a good building block for assembling the framework of complexes.14,15 Lanthanide ions prefer bonding to O-donor atoms of the ligand with low stereochemical preference and high coordination number in comparison with transition metals, which are important to construct different dimensional lanthanide-containing complexes.16−18 If the two different types of ions could be combined into one coordination framework, the resulting complexes easily exhibit aesthetically pleasing architectures and © XXXX American Chemical Society

Received: November 9, 2014 Revised: December 24, 2014

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DOI: 10.1021/cg501644z Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Crystal Growth & Design

Synthesis of Complexes 2−6. Complexes 2−6 were prepared in a similar manner. In a typical synthesis of 2, a water solution (10 mL) of Gd(NO3)3·6H2O (45.2 mg, 0.1 mmol) and Ag(NO3)3 (17.0 mg, 0.1 mmol) was layered on a water solution (10 mL) of H2pzdc (33.6 mg, 0.2 mmol) and NaOH (16.0 mg, 0.4 mmol) in a long tube. The tube was sealed and placed in darkness under room temperature for 3 weeks. Colorless crystals of 2 suitable for X-ray analysis were obtained. [Gd2Ag6(C6N2O4H2)6(H2O)9]·8H2O (2). Yield: 27.2 mg, 72%. Anal. Calcd for C36H46N12O41Ag6Gd2 (2264.52): C, 19.09; H, 2.05; N, 7.42%. Found: C, 19.08; H, 2.06; N, 7.49%. IR (KBr, cm−1): 3434 (s), 1606 (s), 1437 (w), 1390 (s), 1357 (s), 1164 (w), 1118 (m), 891 (w), 841 (w), 746 (w). [TbAg(C6N2O4H2)2(H2O)2]·2H2O (3). Yield: 47.6 mg, 70%. Anal. Calcd for C12H12N4O12AgTb (671.04): C, 21.48; H, 1.80; N, 8.35%. Found: C, 21.40; H, 1.83; N, 8.40%. IR (KBr, cm−1): 3520 (s), 1606 (s), 1548 (s), 1394 (s), 1366 (s), 1167 (w), 1125 (m), 842 (w), 748 (w). [DyAg(C6N2O4H2)2(H2O)2]·2H2O (4). Yield: 45.9 mg, 68%. Anal. Calcd for C12H12N4O12AgDy (674.63): C, 21.36; H, 1.79; N, 8.31%. Found: C, 21.33; H, 1.81; N, 8.45%. IR (KBr, cm−1): 3523 (s), 1607 (s), 1549 (s), 1394 (s), 1367 (s), 1166 (w), 1119 (m), 842 (w), 748 (w). [HoAg(C6N2O4H2)2(H2O)2]·2H2O (5). Yield: 46.0 mg, 68%. Anal. Calcd for C12H12N4O12AgHo (677.04): C, 21.29; H, 1.79; N, 8.28%. Found: C, 21.32; H, 1.81; N, 8.44%. IR (KBr, cm−1): 3518 (s), 1605 (s), 1549 (s), 1394 (s), 1367 (s), 1166 (w), 1119 (m), 842 (w), 748 (w). [ErAg(C6N2O4H2)2(H2O)2]·2H2O (6). Yield: 47.6 mg, 70%. Anal. Calcd for C12H12N4O12AgEr (679.37): C, 21.21; H, 1.78; N, 8.25%. Found: C, 21.30; H, 1.80; N, 8.41%. IR (KBr, cm−1): 3523 (s), 1607 (s), 1550 (s), 1396 (s), 1368 (s), 1163 (w), 1119 (m), 842 (w), 748 (w). X-ray Crystallography. Single-crystal X-ray data of 1−6 were collected on a Siemens SMART CCD diffractometer using graphitemonochromated Mo−Kα radiation (λ = 0.71073 Å). Data processing was accomplished with the SAINT processing program.55 The structures were solved by direct methods and Patterson methods and refined by full matrix least-squares on F2, which were performed using the SHELXTL-97 software package.56 The solvent molecules in complex 2 are highly disordered and not able to be refined by using conventional discrete-atom models; thus, the contribution of partial solvent electron densities was removed by the SQUEEZE routine in PLATON.57,58 The final chemical formula was estimated from the SQUEEZE result combined with the TGA results. The topological analyses were performed with TOPOS.59 The crystallographic data and structure refinement for complexes 1−6 are summarized in Table 1. The selected bond lengths and angles of complexes 1−6 are listed in Tables S1−6 (Supporting Information). CCDC 1003369−1003374 are for complexes 1−6, respectively.

usually chosen for its large magnetic anisotropy originating from the 6H15/2 state. Thus, many pure Dy3+ and Dy3+containing heterometallic complexes have been prepared and their magnetic properties have been explored.31−36 However, most reported Dy3+-containing heterometallic systems exhibiting SMM behaviors are mainly about discrete clusters, while the reports about heterometallic coordination frameworks were seldom known.7,44,45 In addition, the highly anisotropic Er3+ ion could also exhibit SMM behavior if it locates at a proper anisotropic ligand field. Nevertheless, the limited reports of Er3+-based SMMs are mainly associated with mononuclear37−41 and polynuclear46,47 complexes. No extended 2D and 3D heterometallic coordination polymers have been reported up to now. Our group is interested in the structure and magnetic properties of new lanthanide-based MOFs.48−50 To explore the synthesis of novel 4f−4d heterometallic coordination frameworks, pyrazine-2,3-dicarboxylic acid was selected as the organic ligand, since it contains both pyrazine and carboxyl group as functional units, possessing the potential to incorporating f and d metal ions to form a heterometallic complex.51−54 Upon the reaction of H2pzdc with the Ag+ ions in the presence of sodium hydrate under room temperature, a homometallic MOF of [Ag4(pzdc)2]·H2O has been facilely isolated. In contrast, when the reaction of H2pzdc with the Ln3+ and Ag + ions in the presence of NaOH, five novel heterometallic complexes, [Gd2Ag6(pzdc)6(H2O)9]·8H2O and [LnAg(pzdc)2(H2O)2]·2H2O (Ln = Tb, Dy, Ho, and Er) featuring 3D and 2D heterometallic coordination frameworks have been isolated (Scheme 1). Furthermore, their magnetic properties have been investigated and the field-dependent slow magnetization relaxations have been observed and studied. Scheme 1. Schematic Representation of the Ag+ Homometallic and Ln3+−Ag+ Heterometallic Systems

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RESULTS AND DISCUSSION Synthesis and Characterization. Complex [Ag4(pzdc)2]· H2O (1) has been easily synthesized by the reaction of H2pzdc with single Ag+ ions in the presence of NaOH under room temperature. When the Ln3+ ions were introduced to the reactions of H2pzdc with Ag+ ions, five novel heterometallic frameworks [Gd2Ag6(pzdc)6(H2O)9]·8H2O (2) and [LnAg(pzdc)2(H2O)2]·2H2O [Ln = Tb (3), Dy (4), Ho (5), and Er (6)] featuring 3D and 2D heterometallic frameworks were isolated. The IR spectra of complexes 1−6 and H2pzdc ligand have been measured (Figure S1, Supporting Information). For all complexes, the strong and broad bands between 3300 and 3500 cm−1 are assigned as the characteristic peaks of O−H vibration from the water molecules. For complexes 1−6 asymmetric and symmetric stretching vibrations of the carboxylate groups are observed in the range of 1548−1607 cm−1 and 1394−1437 cm−1, respectively. The absence of the characteristic band

EXPERIMENTAL SECTION

Synthesis of Complex [Ag4(C6N2O4H2)2]·H2O (1). A water solution (10 mL) of Ag(NO3)3 (34.0 mg, 0.2 mmol) was layered on a water solution (10 mL) of H2pzdc (16.8 mg, 0.1 mmol) and NaOH (8.0 mg, 0.2 mmol) in a long tube. Then, the tube was sealed and placed in darkness under room temperature for 2 weeks. Yellow crystals of 1 suitable for X-ray analysis were obtained. Yield: 29.3 mg, 75%. Anal. Calcd for C12H6Ag4N4O9 (781.67): C, 18.44; H, 0.77; N, 7.17%. Found: C, 18.42; H, 0.77; N, 7.25%. IR (KBr, cm−1): 3391 (s), 1592 (s), 1430 (s), 1384 (s), 1352 (s), 1200 (w), 1160 (m), 1108 (s), 882 (m), 835 (m), 787 (m), 739 (m), 603 (w). B


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Crystal Growth & Design Table 1. Crystal Data and Structure Refinement for Complexes 1−6

a

Complexes

1

2

3

4

5

6

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 date) GOF on F2

C12H6N4O9Ag4 781.69 monoclinic C2/c 11.2970(11) 9.6083(10) 14.7567(15) 90 91.536(9) 90 1601.2(3) 4 3.243 4.885 1464 0.0375 0.0917 0.0469 0.0970 1.182

C36H46N12O41Ag6Gd2 2264.57 triclinic P1̅ 13.8475(7) 15.1719(8) 16.4770(8) 75.798(4) 81.449(4) 63.074(5) 2989.3(3) 2 2.536 4.208 2008 0.0440 0.1071 0.0581 0.1161 1.018

C12H12N4O12AgTb 671.05 triclinic P1̅ 6.6739(4) 10.0822(6) 14.9383(8) 99.695(5) 101.871(5) 105.780(5) 919.34(9) 2 2.424 4.954 640 0.0455 0.1244 0.0475 0.1268 1.046

C12H12N4O12AgDy 674.63 triclinic P1̅ 6.6630(7) 10.0416(9) 14.9312(14) 99.561(8) 101.678(9) 105.824(9) 914.92(15) 2 2.449 5.197 642 0.0447 0.1139 0.0488 0.1186 1.049

C12H12N4O12AgHo 677.06 triclinic P1̅ 6.641(2) 10.095(3) 14.902(5) 99.44(3) 101.82(3) 106.00(3) 913.9(5) 2 2.460 5.444 644 0.1432 0.3284 0.2387 0.4722 1.054

C12H12N4O12AgEr 679.39 Triclinic P1̅ 6.6298(4) 10.0444(10) 14.8892(15) 99.418(8) 101.793(7) 105.825(7) 908.11(14) 2 2.485 5.742 646 0.0893 0.2360 0.0993 0.2489 1.089

R1 = ∑∥F0| − |Fc∥/|F0|. bwR2 = [∑w (F02 − Fc2)2/∑w(F02)2]1/2.

Figure 1. (a) Coordination environments of Ag+ ions in 1. (b) Coordination mode of the pzdc2− ligand in 1. Symmetry codes: i = x − 0.5, y + 0.5, z; ii = −x + 1.5, −y + 1.5, − z + 1; iii = −x + 1.5, y + 0.5, −z + 0.5; iv = −x + 1, y, −z + 0.5; v = x + 0.5, y − 0.5, z; vi = −x + 1.5, y − 0.5, −z + 0.5.

around 1725 cm−1 from the carboxylic groups of H2pzdc ligand indicates that all carboxylate groups in these complexes are completely deprotonated and coordinate to the metal ions in 1−6. TG−DSC analyses for 1−6 are shown in Figures S2−7 (Supporting Information). For complex 1, the weight loss of 2.0% between 30 and 280 °C is ascribed to the loss of lattice H2O molecules (calcd 2.3%). The decomposition of complex 1 occurs above 280 °C. For complex 2, the weight loss of 13.1% in the range of 30−260 °C is attributed to the loss of all H2O molecules (calcd 13.5%). TG−DSC curves of complexes 3−6 are similar. In a typical curve of 3, the first weight loss of 6.2% in the range of 30−110 °C corresponds to the loss of lattice H2O molecules (calcd 5.4%); the second weight loss of 11.9% occurred between 250 and 302 °C is attributed to the loss of lattice and coordinated H2O molecules in crystal (calcd 10.7%). The powder X-ray diffraction patterns (PXRD) of complexes 1−6 match those simulated patterns of single-crystal X-ray data, indicating that the pure phases are obtained (Figures S8 and S9, Supporting Information). Structural Description for Complex 1. X-ray crystallographic analysis reveals that complex 1 crystallizes in the monoclinic system with space group of C2/c, featuring 3D coordination framework (Figure 1a). The Ag1 ion is 4-

coordinated in the coordination geometry of a tetrahedron by two N atoms and two carboxylate oxygen atoms from three coplane/parallel pzdc2− ligands, while the Ag2 ion is 3coordinated in the coordination geometry of a plane triangle by three carboxylate oxygen atoms from three different pzdc2− ligands in which one pzdc2− ligand is perpendicular to the others. A pair of Ag2 ions is bridged by bidentate carboxylate groups with a Ag···Ag distance of 2.8395(11) Å. It is worth mentioning that this separation is shorter than the Ag···Ag distance of 2.88 Å in the metallic state.60 The Ag−N and Ag−O bond lengths are in the range of 2.311(5)−2.342(5) Å and 2.184(4)−2.544(4) Å, respectively, which are close to that reported previously.19,23,54 Noticeably, the fully deprotonated pzdc2− ligand adopts a novel μ6-(η2-N,O),N′,O,O′,O″,O″ coordination mode connecting six Ag+ ions (Figure 1b). It is different from that in the reported analogue of [Ag3(C6H2N2O4)(C6H3N2O4)] in which the fully deprotonated pzdc 2 − ligand acts as a μ 6 -(η 2 -N,O),(η 2 O,O′),N′,O″,O‴,O‴ coordination mode.61 As shown in Figure 2a, Ag1 ions are linked by nitrogen atoms and O1 atoms from pzdc2− ligands to form a 1D square column. Further, the 1D square column is extended to 3D coordination framework by the Ag2 ion coordinated by three C


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Figure 2. (a) 1D square column in complex 1. (b) 3D framework of 1 built by Ag2 ions connecting 1D square columns composed of Ag1 ions and pzdc2− ligands. (c) 3D framework of 1 with the connection of Ag1-pzdc2− represented as square columns. (d) 1D channels of complex 1 viewed along the c axis. (e) View of the (3,3,6)-connected {42·6}{43}{45·64·83·103} topology of 1. Silver atoms are shown in teal and the ligand centroids in pink.

Figure 3. (a) View of the asymmetric unit in 2. The lattice water molecules are omitted. (b) Coordination modes of type A−F ligands in 2. Symmetry codes: i = x, y − 1, z; ii = −x + 1, −y + 1, −z + 1; iii = −x + 1, −y + 2, −z + 1; iv = −x + 1, −y + 1, −z; v = −x + 2, −y + 1, −z + 1; vi = x, y + 1, z − 1; vii = x, y + 1, z; viii = x, y − 1, z + 1.

pzdc2− ligands from two different square columns (Figure 2b). The most striking feature of the 3D framework is that all square columns are parallel/perpendicular to each other, and the Ag2 ions locate in the junction point of two perpendicular square columns (Figure 2c). The 3D framework exhibits very narrow 1D channels along the c axis with dimensions of ∼3.5 × 5.5 Å2 (Figure 2d). A better insight into the nature of this intricate

framework can be achieved by the application of a topological approach. As depicted above, each Ag+ ion bonds to three pzdc2− ligands, regarded as a 3-connected node. Every pzdc2− ligand links six Ag+ ions acting as a 6-connecting node. On the basis of the simplification principle, the structure of complex 1 presents a trinodal (3,3,6)-connected net with a new {42· 6}{43}{45·64·83·103} topology (Figure 2e). D


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Figure 4. (a) Infinite 1D chain in 2. (b) View of the 2D layer structure in 2. (c) 3D framework of 2 by Gd1 ions linking the adjacent Ag-pzdc2− anion layers. The H2O molecules are omitted for clarity. (d) Polyhedral view of the 3D heterometallic MOFs with 1D channels. Yellow and light blue polyhedra are Gd3+ and Ag+ centers, respectively. (e) Topological view of the 3D structure of 2. Gadolinium atoms are shown in deep yellow, silver atoms in teal, and the ligand centroids in purple.

modes of pzdc2− ligands coexisting in one framework have not been observed in pzdc2− based complexes. As shown in Figure 4a, two neighboring Ag+ ions are bridged by pzdc2− ligand using its two N atoms and monodentate carboxylate group to form a 1D chain. Further, the 1D chain is extended to be a 2D layer through carboxylate groups coordinating to Ag+ ions from the neighboring chains (Figure 4b). Meanwhile, the adjacent layers are connected by Gd1 ions coordinated to three pzdc2− ligands (two from the same layer, the other one from the neighboring layer) to form a 3D heterometallic MOFs (Figure 4c and d). In the framework, the Gd2 ions just hanging on both sides of the 2D layer do not play the role of connecting the adjacent layers. The 3D framework contains large 1D channels occupied by H2O molecules with dimensions of ∼10.56 × 8.94 Å2. To the best of our knowledge, such heterometallic MOFs assembled by Ln3+ ion connecting the adjacent Ag+-ligands layers has not been observed in previously reported Ln−Ag heterometallic complexes. To better understand the structure of complex 2, the topological approach is employed. As depicted above, the Gd1 ion is coordinated by three pzdc2− ligands, can be regarded as a 3-connected node. The Ag1, Ag3, and Ag4 ions are coordinated by three pzdc2− ligands, respectively, each of which can be regarded as a 3-connected node. Similarly, the Ag5 and Ag6 ions act as 4-connected nodes, respectively. The Gd2 and Ag2 ions are, respectively, connected by two pzdc2− ligands and can be regarded as a line. Each pzdc2− ligand links four metal ions acting as a 4-connecting node. In this way, the resulting structure of complex 2 is an unique 12-nodal (3,3,3,3,4,4,4,4,4,4,4,4)-connected net with Schläfli symbol of {3·4·5·6 2 ·7}{3·4·5 2·6·7}{3·4 2·52 ·6}{4·12 2 }{4·5·62 ·7·8}{4·5· 6}{4·62·123}{4·62}{42·5·63}{42·64}{43·5·62}{63} (Figure 4e). Structural Description for Complexes 3−6. Complexes 3−6 are isostuctural, crystallizing in the triclinic system with space group of P1̅. Thus, only the structure of complex 3 is representatively described here. A typical asymmetric unit of

Structural Description for Complex 2. Complex 2 crystallizes in the triclinic system with space group of P1̅, featuring 3D heterometallic MOFs. The asymmetric unit contains two crystallographically independent Gd3+ ions, six Ag+ ions, six pzdc2− ligands, nine coordinated H2O molecules, and eight lattice H2O molecules (Figure 3a). The Gd1 ion is 8coordinated in the coordination geometry of square-antiprism by eight oxygen atoms from four monodentate carboxylate groups of three pzdc2− ligands and four coordinated water molecules (Figure S10, Supporting Information). The Gd2 ion is similar 8-coordinated geometry of square-antiprism by eight oxygen atoms from two pzdc2− ligands and five water molecules. The Gd−O bond lengths range from 2.262(9) to 2.511(10) Å which are comparable to previously reported Gd− O bond lengths.62,63 Six Ag+ ions exhibit three different coordination geometries. The Ag1, Ag3, and Ag4 ions are 4coordinated in the similar coordination geometry of tetrahedron by two N atoms and two O atoms from three pzdc2− ligands. The Ag2 ion is 3-coordinated by one O atom and two N atoms from two pzdc2− ligands adopting the coordination geometry of a plane triangle. The Ag5 and Ag6 ions are 5coordinated in the coordination geometry of distorted trigonal bipyramid by two N atoms and three O atoms from four pzdc2− ligands. The Ag−N and Ag−O bond lengths are in the range of 2.208(6)−2.339(6) Å and 2.313(5)−2.678(12) Å, respectively. The Ag−N bond lengths are comparable to that in complex 1, while the Ag−O bond lengths are slightly longer. It is noteworthy that the six unique pzdc2− ligands in 2, namely, type A−F, exhibit four different μ4-coordination modes (Figure 3b). The type A and F ligands behave as μ4-(η2-N,O), (η2-O′,O″),N′,O″ coordination mode, type B and E ligands act as μ4-(η2-N,O),N′,O,O′ mode, while type C ligand features a μ4-(η2-N,O),N′,O′,O″ mode, each of them connecting one Gd3+ and three Ag+ ions. However, type D ligand is coordinating to four Ag+ ions by adopting a different μ4coordination mode. Such flexible and complicated coordination E


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Crystal Growth & Design complex 3 contains one Tb3+ ion, one Ag+ ion, two pzdc2− ligands, two coordinated H2O molecules, and two lattice H2O molecules (Figure 5a). The Tb3+ ion is an 8-coordinated

Ag+ ion is 5-coordinated geometry of trigonal bipyramid by three carboxylate oxygen atoms and two nitrogen atoms from four different pzdc2− ligands. The Ag−O bond lengths range from 2.505(5) to 2.668(4) Å and the Ag−N bond lengths are 2.259(5) Å and 2.306(6) Å. As shown in Figure 5c, the pzdc2− ligands exhibit two different coordination modes. Type a ligand acts as μ4-(η2-N,O)(η2-O′,O″),O′,O″ coordination mode linking one Tb3+ ion and three Ag+ ions, while type b ligand adopts μ4-(η2-N,O),N′,O′,O″ coordination mode connecting three Tb3+ ions and one Ag+ ion. The Tb3+ ions and Ag+ ions are alternately arrayed through the bridge of pzdc2− ligands to form a 1D chain (Figure 6a). Notably, the 1D chain is further extended to a 2D layer by the connection of the bidentate and tridentate carboxylate groups from type b and a ligands, respectively (Figure 6b), whereas the Tb···Tb and Ag···Ag distances bridged by carboxylate groups are 6.739(6) and 6.739(10) Å, respectively. From the viewpoints of topology, the metal center and ligand can be regarded as 4-connected and 4-connecting nodes, respectively. Thus, the 2D coordination framework is considered to be a simplified uninodal 4-connected net with {44·62} topology, as depicted in Figure 6c. In the structure of the Ag+ homometallic complex 1, the pzdc2− ligands adopt only one μ6-coordination mode (Figure 1b). The pzdc2− ligands link the Ag1 ions to form a 1D square column which is extended to 3D MOF by the connections of the Ag2 ions between the two perpendicular square columns. However, upon the introduction of the lanthanide ions, the structures of heterometallic complexes 2−6 were essentially affected. As the preparation procedures of complexes 2−6 are extremely similar, two different types of coordination frameworks obtained provide a fair assessment of the critical influence of lanthanide contraction. In complex 2, the Gd3+ ion has larger ionic radius than other Ln3+ ions, which enable the Gd1 ion connecting the neighboring Ag-pzdc2− anion layers to form 3D heterometallic MOFs possible (Figure 4c) by adopting an 8-coordinated geometry of square-antiprism (Figure S10, Supporting Information). Strikingly, six unique pzdc2− ligands exhibiting four different coordination modes and six Ag+ ions adopting three different coordination geometries

Figure 5. (a) View of the asymmetric unit in 3. (b) Coordination geometry of Tb3+ ion in 3. (c) Coordination modes of the ligands pzdc2− in 3. Symmetry codes: i = −x, −y, −z; ii = x − 1, y, z; iii = −x + 1, −y + 1, −z + 1; iv = x + 1, y, z.

geometry of triangle dodecahedron by two coordinated H2O molecules, five carboxylate oxygen atoms, and one nitrogen atom from four pzdc2− ligands with monodentate (O5 and O6i), OO-chelated (O1, O2), and NO-chelated (N4iii, O8iii) modes (Figure 5b and Figure S11, Supporting Information). The Tb−O bond lengths range from 2.302(4) to 2.472(5) Å, and the Tb−N distance is 2.625(5) Å, which fall in the range of Tb−O and Tb−N bond lengths reported previously.64−66 The

Figure 6. (a) Infinite 1D chain in 3. (b) View of the 2D layer in 3. (c) Topological view of the 2D heterometallic coordination framework in 3. F


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consistent with the crystal structure in which no carboxylate groups bridge the neighboring Gd3+ ions in complex 2. For complexes 3−6, the χmT values at room temperature are 11.73, 13.95, 13.55, and 10.92 cm3 K mol−1, respectively, which are comparable to the theoretical values of Tb3+, Dy3+, Ho3+, and Er3+ ions (Tb3+, 7F6, S = 3, L = 3, g = 3/2, χT = 11.82 cm3 K mol−1; 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; Er3+, 4I15/2, S = 3/2, L = 6, g = 6/5, χT = 11.48 cm3 K mol−1). Upon decreasing the temperature, χmT products almost remain constant until ∼75 K and then drop suddenly to the minimums of 8.23, 8.94, 6.22, and 4.93 cm3 K mol−1 at 1.8 K for complexes 3−6, respectively. These decreases could be ascribed to a combination of antiferromagnetic interactions and thermal depopulation of Stark sublevels.49 Magnetization (M) data for complexes 2−6 were recorded between 0 and 70 kOe at 1.8 K (Figure S13, Supporting Information). For complex 2, the M versus H data exhibits near saturation at M = 13.99 NμB with an external field of 70 kOe. The value is comparable to the theoretical value of 14.00 NμB. For complexes 3−6, the M versus H data first rapidly increase at low magnetic fields and then smoothly increase without saturation until 70 kOe. The magnetization values are 5.17, 4.96, 5.63, and 4.88 NμB at 70 kOe for complexes 3−6, respectively, which are lower than the calculated values for one Ln3+ magnetic moments (9, 10, 10, and 9 NμB for Tb3+, Dy3+, Ho3+, and Er3+, respectively). The lack of saturation of the M versus H data and the nonsuperimpositon of the M versus H/T data (Figure 7 inset, S13 and S14, Supporting Information) on a single master curve suggest the presence of large magnetic anisotropy an/or low-lying excited states in these systems.71−75 The magnetization dynamics of 3−6 were carried out by alternating current (ac) susceptibility measurements in the frequency range of 1−1000 Hz. For complex 3 and 5, there are no frequency-dependent signals in in-phase (χ′) and no obvious signal in out-of phase (χ″) observed under zero dc field and 2000 Oe field. For complex 4, it displays clear frequency-dependent χ″ signals at low temperatures under zero dc magnetic field (Figure S15, Supporting Information), suggesting the existence of slow magnetic relaxation behavior.76 The absence of any frequency-dependent peaks may be due to the quantum tunneling of magnetization (QTM).76,77 Hence, ac susceptibility measurements were obtained under an applied dc field to suppress the effects of QMT. Under 2000 Oe field, complex 4 shows frequency-dependent peaks in both χ′ and χ″

exist in complex 2 (Figure 3). In contrast, the heavier lanthanide ions Tb3+−Er3+ are of the smaller ionic radius and adopt 8-coordinated geometries of triangle dodecahedron in complexes 3−6, which are different from that in complex 2. Thus, the different coordination geometries of the lanthanide ions, caused by the lanthanide contraction, give rise to the different structures of complex 2 and complexes 3−6. This is in agreement with previously reported 4f−4d22,67 and 4f−3d68 heterometallic systems. Magnetic Properties. Variable-temperature magnetic susceptibility measurements were performed for microcrystalline samples of 2−6 at an external field of 100 Oe between 1.8 and 300 K (Figure 7). For complex 2, χmT takes a value of

Figure 7. Temperature dependence of χmT at 100 Oe for 2−6. Inset: plots of M versus H/T at different temperatures in the field range of 0−70 kOe for 3.

15.72 cm3 K mol−1 at room temperature, which is close to the theoretical value of 15.76 cm3 K mol−1 expected for two magnetically isolated Gd3+ ions (Gd3+, 8S7/2, S = 7/2, L = 0, g = 2, χT = 7.88 cm3 K mol−1). χmT remains almost constant as T is lowered while slightly decreasing at 1.8−3 K, indicating the absence of any obvious magnetic interaction between Gd3+ ions.69 The χm−1 versus T data for complex 2 in the range of 1.8−300 K obeys the Curie−Weiss law, with Curie constant C = 15.5 cm3 K mol−1 and Weiss constant θ = −0.04 K (Figure S12, Supporting Information). These results indicate that complex 2 shows simple paramagnetic behavior,70 which is

Figure 8. (a) In-phase (inset) and out-of phase ac susceptibility of 4 under 2000 Oe between 1 and 1000 Hz. (b) Frequency dependence of ac χM″ of 4 fitted by Arrhenius law, the solid red line representing the least-squares fitting of the experimental data to the Arrhenius equation. G


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Figure 9. (a) In-phase (inset) and out-of phase ac susceptibility of 6 under 2000 Oe between 1 and 900 Hz. (b) Plots of natural logarithm of χ″/χ′ versus 1/T for 6 under 2000 Oe. The solid line represents the fitting results in the range of 1.8−4.9 K.

further demonstrates that the complicated Stark energy levels and intrinsic magnetic anisotropy of different lanthanide ions lead to the obvious difference of the magnetic behavior.7,44,45

above 100 Hz (Figure 8a). Moreover, the value of the relaxation time of 4, which is calculated from the maximum of χM″ at given frequency (τ = 1/2πν), follows the Arrhenius law, τ = τ0 exp(Ea/kBT). The solid line in Figure 8b shows the result of a least-squares fit of the ac susceptibility relaxation data between 500 and 1000 Hz to the Arrhenius equation, giving the energy barrier Δ/kB = 25.45 K and the relaxation time τ0 = 6.22 × 10−7 s. The obtained value of the energy barrier is higher compared with that in the reported Dy3+-containing 2D44 and 3D7,45 heterometallic complexes, but comparable to those reported for Dy3+-based SMMs.78−80 In the case of complex 6, a frequency-independent signal is exhibited in χ′ but no obvious signals in χ″ in the absence of a static field (Figure S16, Supporting Information). As shown in Figure 9a, by applying an external field of 2000 Oe, both χ′ and χ″ show frequency dependence below 10 and 5 K, respectively. These phenomena indicate that complex 6 can display slow magnetic relaxation under an applied field. However, for the fast quantum tunneling the peaks of frequency dependence were not observed.76,77 By fitting the χ″/χ′ data to the equation, ln(χ″/χ′) = ln(ωτ0) + Ea/kBT, yields the energy barrier Ea/kB ≈ 8.65 K and the relaxation time τ0 ≈ 1.08 × 10−6 s under 2000 Oe (Figure 9b). It is close to reported energy barriers of Er3+ mononuclear complexes.39,81 Noticeably, previously reported Er 3+ -based SMMs are relatively rare;37−41,46,47 no Er3+-based 2D heterometallic coordination frameworks displaying the slow magnetic relaxation of magnetization have been reported to date. Thus, complex 6 represents the first Er3+-based 2D heterometallic coordination framework showing the behavior of slow magnetic relaxation. The Ln···Ln distances of about 6.7 Å in complexes 3−6 are too long for the spin coupling between the adjacent Ln3+ ions. Hence, the magnetic behaviors for complexes 4 and 6 should result from the single-ion behaviors of the Ln3+ ions. To give a highly anisotropic ground state with a large ± mJ, the Dy3+ ion should be located in an axially elongated coordination environment, maximizing the anisotropy of an oblate ion. Inversely, the coordinating atoms located around the equatorial or axially compressed positions are preferable for Er 3+ ion.40,82−84 In the structure of complexes 4 and 6, the Ln3+ ions are located in the 8-coordinated geometry of dodecahedron with low D2d symmetry providing both axial and equatorial coordinated atoms, which should contribute the slow relaxation behavior and energy barriers against spin reversal in those complexes.35 However, the isomorphic complexes 3−6 exhibit different magnetic behavior, which

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CONCLUSIONS One pyrazine-2,3-dicarboxylic acid Ag+ homometallic complex possessing a unique 3D MOF and five Ln−Ag heterometallic complexes featuring 3D and 2D heterometallic frameworks have been easily isolated under room temperature. Structural analysis verifies that when lanthanide ions are introduced, the lanthanide contraction dominates the coordination geometries of the lanthanide ions, which results in various coordination modes of pzdc2− ligands leading to structural change from 3D to 2D heterometallic coordination frameworks in the Ln−Ag heterometallic system. Magnetic studies demonstrate that complexes 4 and 6 exhibit field-dependent slow magnetic relaxation behavior originating from the single-ion nature of the Ln3+ ions, which are seldom known among the numerous lanthanide-based 2D heterometallic complexes, especially for the Er3+-based 2D heterometallic coordination frameworks. This approach provides a feasible synthesis of heterometallic coordination frameworks exhibiting field-dependent slow magnetic relaxation behaviors.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

X-ray crystallographic files (CIF), materials and instrumentation, magnetic data, FT-IR spectra, TG−DTA curves, PXTD patterns, and selected bond lengths and angles for 1−6. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 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, 21272061, 21272061 and 21471051).

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