Two Types of New Three-Dimensional d–f Heterometallic

Article pubs.acs.org/crystal

Two Types of New Three-Dimensional d−f Heterometallic Coordination Polymers Based on 2‑(Pyridin-3-yl)‑1H‑Imidazole-4,5Dicarboxylate and Oxalate Ligands: Syntheses, Structures, Luminescence, and Magnetic Properties Song-Liang Cai,† Sheng-Run Zheng,*,† Zhen-Zhen Wen,† Jun Fan,† Ning Wang,‡ and Wei-Guang Zhang*,† †

School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, China School of Material Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

‡

S Supporting Information *

ABSTRACT: Two types of new d−f heterometallic coordination frameworks, [Ln2M(μ3-HPyIDC)2(ox)2(H2O)4]·4H2O [Ln = Sm, M = Co (1); Ln = Sm, M = Fe (2); Ln = Eu, M = Co (3); Ln = Eu, M = Fe (4); Ln = Gd, M = Co (5); H3PyIDC = 2-(pyridine-3-yl)-1H-4,5-imidazoledicarboxylic acid; ox = oxalate] for type I and [Ln2Ag3(μ5-PyIDC)(μ6-PyIDC)(μ2-OH)(ox)(H2O)2] [Ln = Sm (6); Gd (7); Tb (8); Dy (9)] for type II, were prepared under hydrothermal conditions and structurally characterized. Complexes 1−5 are isostructural, exhibiting three-dimensional (3D) heterometallic networks with pcu topology based on the linkages of two-dimensional (2D) lanthanide−oxalate layers and pillar-like M(HPyIDC)2 (M = Fe, Co) subunits. Complexes 6−9 are also isomorphous and display another type of 3D heterometallic coordination framework built through the connections of 2D lanthanide−oxalate layers and one-dimensional (1D) Ag2(PyIDC)2 chains. Interestingly, the oxalate ligands in complexes 6−9 are produced by the in situ decarboxylation of the H3PyIDC ligand in the presence of 4,4′-bipyridine (4,4′-bpy). The photoluminescence, thermal stabilities, and magnetic properties of the selected complexes were also investigated.

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INTRODUCTION In recent years, rational design and synthesis of lanthanidetransition metal (d−f) heterometallic coordination polymers have attracted much attention due not only to their intriguing architectures and topologies but also to their potential applications as important functional materials in the fields of magnetism,1 sensors,2 adsorption,3 catalysis,4 and ion-exchange.5 A number of structures containing d−f metals have been successfully obtained under different reaction conditions. However, the assembly of extended structures of d−f polymeric compounds, especially three-dimensional (3D) d−f heterometallic coordination polymers, is less successful and remains a challenge for chemists. Some complicated factors, especially the variable and high coordination numbers of lanthanide ions, as well as competitive reaction between lanthanide and transition metal ions, can significantly affect the self-assembly process.6 According to the hard−soft acid base theory, lanthanide ions behave as hard acids and prefer O- over N-donors. On the other hand, transition metal ions located in the d-block are borderline acids, having a strong tendency to coordinate to both N- and O-donors. Therefore, some types of ligands with N- and O-donors, such as -CN,7 carbonyl,8 amino acids,9 pyridinecarboxylate,10 pyrazine-carboxylate,11 and imidazolecarboxylate,12 have been employed to construct d−f hetero© 2012 American Chemical Society

metallic complexes with interesting structures and useful physicochemical properties. Among of them, 4,5-imidazoledicarboxylic acid (H3IDC), a planar rigid ligand containing two nitrogen and four oxygen atoms, has attracted much interest in coordination chemistry and has been proven as an excellent building block for constructing novel d−f heterometallic coordination polymers. For example, a series of 3D 4d−4f luminescent Ln(III)-Cd(II) frameworks possessing unprecedented one-dimensional (1D) helical channels and tubes have been reported by Sun et al.12a Gu et al.12e have reported a family of 3D 4d−4f Ln(III)-Zn(II) frameworks with two types of structures through employing H3IDC and oxalate ligands under similar conditions. On the basis of the previous studies on design and construction of new metal−organic frameworks (MOFs) with 4,5-imidazoledicarboxylic acid or its derivatives, we believe that 2-(pyridin-3-yl)1H-imidazole-4,5-dicarboxylate (H3PyIDC), which contains a H3IDC moiety and pyridyl group on the 2-position of the imidazole ring, is also an ideal ligand for constructing heterometallic frameworks. Some homometallic structures Received: May 7, 2012 Revised: June 22, 2012 Published: June 26, 2012 4441

dx.doi.org/10.1021/cg300613t | Cryst. Growth Des. 2012, 12, 4441−4449

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Table 1. Crystallographic Data and Structure Refinement Summary for Complexes 1−9 complex

1

2

3

4

5

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V/Å3 Z D/g cm−3 μ/mm−1 T/K Ra/wRb total/unique/Rint complex

C24H26CoN6O24Sm2 1142.14 triclinic P1̅ 10.5614(12) 12.7686(15) 14.1557(16) 91.9710(10) 110.1040(10) 102.5550(10) 1737.3(3) 2 2.183 3.914 298(2) 0.0422/0.1020 8871/6133/0.0245 6

C24H26FeN6O24Sm2 1139.06 triclinic P1̅ 10.5890(13) 12.8173(16) 14.2610(18) 92.008(2) 110.5310(10) 102.872(2) 1753.4(4) 2 2.157 3.819 298(2) 0.0432/0.1034 8792/6095/0.0256

C24H26CoN6O24Eu2 1145.36 triclinic P1̅ 10.5406(9) 12.7755(11) 14.1480(13) 91.8010(10) 110.0120(10) 102.8610(10) 1733.2(3) 2 2.195 4.154 298(2) 0.1333/0.3121 7561/5742/0.0267

C24H26FeN6O24Eu2 1142.28 triclinic P1̅ 10.5283(16) 12.771(2) 14.211(2) 91.875(2) 110.438(2) 103.068(2) 1731.0(5) 2 2.192 4.099 298(2) 0.1722/0.3777 7218/5501/0.0246 8

C24H26CoN6O24Gd2 1155.94 triclinic P1̅ 10.5121(9) 12.7592(11) 14.1244(12) 91.6990(10) 109.9520(10) 102.9840(10) 1723.5(3) 2 2.227 4.386 298(2) 0.0915/0.2655 7748/5565/0.0370 9

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V/Å3 Z D/g cm−3 μ/mm−1 T/K Ra/wRb total/unique/Rint a

C22H13Ag3N6O15Sm2 1225.69 monoclinic P21/c 8.4700(6) 23.5866(17) 15.4231(9) 90 117.274(3) 90 2738.7(3) 4 2.973 6.419 298(2) 0.1122/0.2607 18596/4612/0.0466

7 C22H13Ag3N6O15Gd2 1239.49 monoclinic P21/c 8.4073(4) 23.5633(12) 15.3911(6) 90 117.149(2) 90 2713.1(2) 4 3.035 7.039 298(2) 0.1072/0.2251 17956/4483/0.0375

R1 = ∑[||F0| − |Fc||/∑|F0|. bωR2 = [∑[ω(F02 − Fc2)2]/∑ω(F02)2]1/2.

based on H3PyIDC ligands have been reported recently by our group and the others, including several mononuclear complexes,13a−c a 3D Nd(III) compound with an ant topology,13d a two-dimensional (2D) Ag(I) ammine layer,13e a family of 2D Ln(III) networks,13f two chiral and one achiral coordination polymers containing 1D helical chains,13g and a highly connected 3D Pb(II) framework.13h However, until now, no heterometallic coordination polymer has been documented to the best of our knowledge. In this work, a series of 3d-4f heterometallic coordination frameworks were successfully synthesized and structurally characterized for the first time. The frameworks were [Ln2M(μ3-HPyIDC)2(ox)2-(H2O)4]·4H2O [Ln = Sm, M = Co (1); Ln = Sm, M = Fe (2); Ln = Eu, M = Co (3); Ln = Eu, M = Fe (4); Ln = Gd, M = Co (5); ox = oxalate] and a family of 4d−4f heterometallic complexes [Ln2Ag3(μ5-PyIDC)(μ6PyIDC)-(μ2-OH)(ox)(H2O)2] [Ln = Sm (6); Gd (7); Tb (8); Dy (9)]. Their solid-state photoluminescence, thermal stabilities, luminescence, and magnetic properties were also investigated.

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C22H13Ag3N6O15Tb2 1242.83 monoclinic P21/c 8.367(2) 23.540(12) 15.388(10) 90 117.11(5) 90 2698(2) 4 3.060 7.405 298(2) 0.0364/0.0842 14677/5294/0.0863

C22H13Ag3N6O15Dy2 1249.99 monoclinic P21/c 8.4705(10) 23.577(3) 15.4220(14) 90 117.257(5) 90 2737.9(5) 4 3.032 7.589 298(2) 0.1127/0.2578 19038/4642/0.0420

EXPERIMENTAL SECTION

Materials and Measurements. The ligand H3PyIDC was synthesized according to the literature method.14 Lanthanide nitrate hydrates and lanthanide chloride hydrates were prepared by dissolving the respective lanthanide oxides (99.5%) with concentrated HNO3 and concentrated HCl, respectively, and then evaporating at 100 °C until the crystals were formed. Other materials were reagent grade obtained from commercial sources and used without further purification; solvents were dried by standard procedures. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets. X-ray powder diffraction measurements were measured by using a Bruker D8 Advance diffractometer at 40 kV, 40 mA with a Cu-target tube and a graphite monochromator. Thermogravimetric analyses were performed on Perkin-Elmer TGA7 analyzer with a heating rate of 10 °C/min in flowing air atmosphere. The solid-state photoluminescent spectra were recorded on Hitachi F2500 and Edinburgh FLS-920 with a xenon arc lamp as the light source at room temperature. Magnetic susceptibility data for polycrystalline samples were collected at an external field of 1000 Oe on a MPMS XL7 magnetometer (Quantum Design SQUID) in the 2−300 K temperature range. The output data were corrected for the diamagnetism of the sample holder and of the samples calculated from their Pascal constants. 4442

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Figure 1. (a) Coordination environments of the Sm(III) and Co(II) ions in 1. All H atoms and uncoordinated water molecules are omitted for clarity. (b) Schematic presentation of four-connected 2D network for 1 based on [Sm2O2] SBUs. (c) Polyhedral view of the 3D pillar-layered framework of 1. (d) Schematic presentation of the six-connected pcu type 3D framework in 1. Synthesis of Type I Complexes [Ln 2 M(μ 3 -HPyIDC) 2 (ox)2(H2O)4]·4H2O [Ln = Sm, M = Co (1); Ln = Sm, M = Fe (2); Ln = Eu, M = Co (3); Ln = Eu, M = Fe (4); Ln = Gd, M = Co (5)]. A mixture of LnCl3·6H2O (0.40 mmol), CoCl2·6H2O (0.30 mmol)/ FeCl2·4H2O (0.30 mmol), H3PyIDC (0.50 mmol), K2C2O4·H2O (0.50 mmol), and H2O (8 mL) was sealed into a 15 mL Teflon-lined stainless autoclave and heated to 170 °C for 72 h under autogenous pressure. After cooling to room temperature at a rate of 5 °C/h, block crystals suitable for X-ray diffraction analysis were collected manually, washed with water, and dried in the air. Complex 1, yield: 42% (based on the Sm). Calc. for C24H26CoN6O24Sm2: C, 25.24, H, 2.29, N, 7.36. Found: C, 25.17, H, 2.32, N, 7.41%. IR (KBr, ν/cm−1): 3428s, 3098w, 2924w, 2852w, 1684s, 1611s, 1569s, 1533w, 1480w, 1452m, 1432w, 1393m, 1362w, 1314m, 1270w, 1239w, 1138w, 1119w, 1052w, 985w, 876w, 834w, 814w, 791m, 724w, 702w, 638w, 551w. Complex 2, yield: 47% (based on the Sm). Calc. for C24H26FeN6O24Sm2: C, 25.31, H, 2.30, N, 7.38. Found: C, 25.25, H, 2.34, N, 7.43%. IR (KBr, ν/cm−1): 3417m, 3104w, 2924w, 2847w, 1684s, 1608s, 1566s, 1530w, 1480w, 1452s, 1429w, 1393m, 1362m, 1317m, 1276w, 1239w, 1138w, 1116w, 982w, 876w, 831w, 811w, 789w, 724w, 702w, 638w, 551w. Complex 3, yield: 55% (based on the Eu). Calc. for C24H26CoN6O24Eu2: C, 25.17, H, 2.29, N,7.34. Found: C, 25.22, H, 2.25, N, 7.38%. IR (KBr, ν/cm−1): 3423s, 3098w, 2924m, 2852w, 1684s, 1611s, 1569s, 1452s, 1432w, 1393m, 1362m, 1317m, 1273w, 1242w, 1138w, 1119w, 1049w, 876w, 831w, 811w, 789m, 727w, 702w, 638w, 548w. Complex 4, yield: 45% (based on the Eu). Calc. for C24H26FeN6O24Eu2: C, 25.24, H, 2.29, N, 7.36. Found: C, 25.27, H, 2.30, N, 7.39%. IR (KBr, ν/cm−1): 3429s, 3098w, 2969w, 1687s, 1614s, 1569s, 1533w, 1480w, 1454s, 1432w, 1393m, 1362m, 1317m, 1273w, 1242w, 1119w, 1138w, 1116w, 1052w, 985w, 929w, 878w, 834w, 814w, 791m, 727m, 702w, 638w, 551w.

Complex 5, yield: 52% (based on the Gd). Calc. for C24H26CoN6O24Gd2: C, 24.94, H, 2.27, N, 7.27. Found: C, 25.01, H, 2.33, N, 7.25%. IR (KBr, ν/cm−1): 3423m, 3104w, 2924w, 2857w, 1687s, 1612s, 1569s, 1480w, 1454s, 1432w, 1393m, 1365m, 1317m, 1272w, 1242w, 1192w, 1138w, 1119w, 1049w, 985w, 878w, 834w, 811w, 789m, 727w, 702w, 638w, 551w. Synthesis of Type II Structures [Ln2Ag3(μ5-PyIDC)-(μ6PyIDC)(μ2-OH)(ox)(H2O)2] [Ln = Sm (6); Gd (7); Tb (8); Dy (9)]. An aqueous mixture (8 mL) of Ln(NO3)3·6H2O (0.40 mmol), AgNO3 (0.30 mmol), H3PyIDC (0.50 mmol), and 4,4′-bpy (0.50 mmol) was placed in a 15 mL Teflon-lined stainless autoclave and heated to 170 °C for 72 h and then cooled to room temperature at a rate of 5 °C/h. Yellow block crystals were obtained and dried in the air. Complex 6, yield: 32% (based on the Sm). Calc. for C22H13Ag3N6O15Sm2: C, 21.56, H, 1.07, N, 6.86. Found: C, 21.47, H, 1.09, N, 6.91%. IR (KBr, ν/cm−1): 3681m, 3311s, 1673s, 1583m, 1530s, 1488w, 1452w, 1407w, 1376s, 1334w, 1267m, 1231s, 1125w, 1105w, 1055w, 1001w, 876w, 859w, 817s, 797w, 727w, 691w, 649w, 607w, 520w, 444w. Complex 7, yield: 28% (based on the Gd). Calc. for C22H13Ag3N6O15Gd2: C, 21.32, H, 1.06, N, 6.78. Found: C, 21.35, H, 1.08, N, 6.74%. IR (KBr, ν/cm−1): 3681m, 3300s, 1672s, 1583m, 1529s, 1488w, 1452w, 1404w, 1376s, 1334w, 1267m, 1231s, 1124w, 1105w, 1055w, 1004w, 873w, 859w, 820s, 800w, 730w, 694w, 652w, 610w, 520w, 445w. Complex 8, yield: 25% (based on the Tb). Calc. for C22H13Ag3N6O15Tb2: C, 21.26, H, 1.05, N, 6.76. Found: C, 21.22, H, 1.02, N, 6.78%. IR (KBr, ν/cm−1): 3681w, 2924w, 2852w, 1700s, 1583s, 1527s, 1502w, 1485w, 1449s, 1404w, 1376m, 1334w, 1264w, 1228m, 1122w, 873w, 856w, 816s, 797m, 724w, 691w, 442w. Complex 9, yield: 33% (based on the Dy). Calc. for C22H13Ag3N6O15Dy2: C, 21.14, H, 1.05, N, 6.72. Found: C, 21.17, H, 1.04, N, 6.68%. IR (KBr, ν/cm−1): 3681m, 3294s, 1673s, 1583m, 1530s, 1488w, 1452w, 1404w, 1376s, 1331w, 1264m, 1231s, 1124w, 4443

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1105w, 1057w, 1004w, 878w, 859w, 817s, 800w, 730w, 694w, 649w, 607w, 520w, 448w, 406w. X-ray Data Collection and Structure Refinement. Data collections were performed at 298 K on a Bruker Apex II Smart CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) for compounds 1−9. Multiscan absorption corrections were applied by using the program SADABS.15 Structural solutions and full-matrix least-squares refinements based on F2 were performed with the SHELXS-9716 and SHELXL-9716 program packages, respectively. Anisotropic thermal parameters were used to refine all non-H atoms. The hydrogen atoms for C−H were placed in idealized positions. The hydrogen atoms on water molecules were located from difference Fourier maps and were also refined using a riding model. Details of the crystal parameters, data collections, and refinements for compounds 1−9 are summarized in Table 1. Selected bond lengths and angles are shown in Table S1. Further details are provided in Supporting Information. CCDC nos. 877878, 877879, 877880, 877891, 877881, 877882, 877883, 877884, and 877885 are for compounds 1−9, respectively.

octahedral geometry. The Co−N bond lengths span from 2.171(7) to 2.215(7) Å, while the Co−O distances are in the range of 2.076(6)−2.158(7) Å. The HPyIDC2− ligands in 1 adopt a μ3-kN,O:kO′,O″:kO″,O‴ coordinated mode to bridge one Co(II) ion in N,O-chelating and two Sm(II) ion in O,Ochelating fashion (Scheme 1a). The pyridyl group remains Scheme 1. Coordination Modes of the H3PyIDC and ox Ligands in Complexes 1−9

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RESULTS AND DISCUSSION Syntheses. Hydrothermal synthesis has been widely employed to form novel d−f heterometallic coordination frameworks with diverse structural architectures, due to unusual conditions available to minimize the problems associated with ligand solubility and enhancing the reactivity of reactants. All complexes 1−9 were synthesized by hydrothermal techniques at 170 °C. Though most of the yields were relatively low, all of them can be reproduced easily in the same reaction conditions. It should be noted that coordinated oxalate anions were observed in two types of complexes, but they were obtained from different components: in complexes 1−5, they were directly derived from potassium oxalate, while in complexes 6− 9, they may have came from in situ decarboxylation of the H3PyIDC ligand induced by 4,4′-bpy. Interestingly, we tried to repeat synthesis of complexes 1−5 after replacing potassium oxalate with 4,4′-bpy, and only a few small crystalline powders were obtained. In addition, if adding potassium oxalate instead of 4,4′-bpy molecules in the starting materials for complexes 6− 9, it also failed to produce suitable crystals for X-ray diffraction analysis. These results indicated that the starting reactants of potassium oxalate and 4,4′-bpy played important roles in the formation of two types of complexes. Crystal Structure of Type I. Single-crystal X-ray structural analyses reveal that complexes 1−5 crystallize in the triclinic space group P1̅ and possess 3D coordination frameworks consisting of 2D lanthanide−oxalate layers pillared by M(HPyIDC)2 subunits. These compounds are isomorphous (type I), so only the structure of 1 is described in detail. As illustrated in Figure 1a, there are two Sm(III) ions, one Co(II) ion, two HPyIDC2− anions, three oxalate anions with 1, 1/2, and 1/2 site occupancy for each, four coordinated water molecules, as well as five lattice water molecules in the asymmetric unit of 1. Sm1 and Sm2 ions are nine-coordinated and have a distorted tricapped trigonal prism coordination geometry, which is composed of four oxygen atoms from two HPyIDC2− ligands, four oxygen atoms from two oxalate ligands, and one oxygen atom from one water molecule. The Sm−O bond lengths range from 2.350(5)−2.606(5) Å, and the O−Sm−O bond angles vary from 50.45(17)° to 151.4(2)°, all of which are within the range of those observed for the other samarium-carboxylate complexes.17 The Co(II) ion is sixcoordinated with two nitrogen and two oxygen atoms from two individual HPyIDC2− ligands in the equatorial plane, and two water molecules in axial position, forming a slightly distorted

uncoordinated, but may bring steric hindrance to the N,Ochelating site and make it more suitable for the low-coordinated Co(II) ions than the high-coordinated Ln(III) ions. Each ox anion in 1 acts as a bis(chelating) ligand and coordinates to two Sm(III) centers in O,O-chelating coordination (Scheme 1d). The dihedral angles between the carboxyl group and the imidazole ring of the HPyIDC2− ligands are in the range from 3.647° to 17.980°, respectively, while the imidazole ring and pyridyl ring have a dihedral angle of 28.2° and 14.6°, respectively. In 1, two crystallographically independent Sm(III) ions are bridged through the O2 and O7 atoms from two different HPyIDC2− ligands to form a dinuclear [Sm2O2] unit with an Sm···Sm separation of 4.252 Å, further linked by ox ligands to generate a 2D lanthanide−oxalate layer framework with 44 lattice topology (Figure 1b, Figure S1). In addition, the Co(HPyIDC)2 units constructed from the HPyIDC2− ligands and Co(III) centers act as pillar-like linkers to connect the adjacent layers aforementioned into a 3D pillar-layered framework (Figure 1c). Lattice water molecules occupy the channels along the a-axial direction. If they are removed, complex 1 has 229.3 Å3 potential solvent volume (13.2%) estimated by PLATON.18 To better understand the structure of 1, the topological analysis approach was employed, which is a standard procedure for reducing multidimensional structures to a simple node-andlinker. Each dinuclear [Sm2O2] unit connects six adjacent dinuclear units through four ox anions and two Co(HPyIDC)2 units, which can be defined as a 6-connected node. Both ox ligand and Co(HPyIDC)2 unit act as the bridges that have not been considered in the topological analysis. On the basis of this simplification, the resulting structure of 1 can be described as a six-connected pcu type 3D network with the Schläfli symbol of (412·63) analyzed by TOPOS19 (Figure 1d). Crystal Structure of Type II. Complexes 6−9 are also isomorphous; thus, only the structure of 8 is discussed in detail. Structure determination shows that complex 8 crystallizes in the monoclinic form with space group P21/c and features a 3D Tb(III)−Ag(I) heterometallic organic framework which is made up of 2D lanthanide−oxalate layers connected by 1D [Ag3(PyIDC)2] chains. As presented in Figure 2a, the asymmetric unit of 8 contains two crystallographically independent Tb(III) ions, three Ag(I) ions, one ox ligand, one μ2-OH group, and two PyIDC3− ligands with two distinctly 4444

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Figure 2. (a) View of the coordination environments of the Tb(III) and Ag(I) ions in complex 8. All H atoms and noncoordinated water molecules are omitted for clarity. (b) View of the 1D Ag3(PyIDC)2 chain constructed by binuclear ring SBUs in 8 (one SBU was marked as black for clarity). (c) The infinite 1D chain constructed from [Tb2O2] dimers with bridging μ2-O2 atoms and the schematic presentation of 2D network built by 1D chains with oxalate ligands. (d) Polyhedral view of the 3D chain-layer heterometallic coordination framework of 8.

kO″:kN′:kN″ fashion (Scheme 1c). The carboxyl groups of H3PyIDC ligands are completely deprotonated, in agreement with the IR analysis. As illustrated in Figure 2b, the Ag1 and Ag3 atoms are bridged by two PyIDC3− anions with the Nimidazol and Npyridyl atoms to generate a binuclear ring with a Ag···Ag separation of 4.660 Å, and these binuclear rings are further connected by Ag2 atoms, resulting in a 1D Ag3(PyIDC)2 chain running along the c axis. In addition, the short Ag···Ag contact of 3.046(4) Å link two neighboring 1D Ag3(PyIDC)2 chains into a new 1D chain. On the other hand, a dinuclear [Tb2O2] unit with a Tb···Tb separation of 3.875 Å is formed through the connection of carboxylate oxygen atom (O7) from the μ6-PyIDC3− ligand and the μ2-OH group, which are further linked by the carboxylate oxygen atom (O2) from the μ5-PyIDC3− ligand to generate an infinite 1D chain (Figure 2c). Then, the oxalate ligands as bis(chelating) ligands connect the adjacent 1D [Tb2O2] chains into a 2D lanthanide−oxalate layer (Figure 2c). From the viewpoint of network topology, if each dinuclear [Tb2O2] SBU is regarded as a four-connected node, this 2D lanthanide− oxalate layer can also be simplified as a (4,4) topological network similar to that in 1 (Figure S2). Finally, the 2D lanthanide−oxalate layers and the 1D Ag3(PyIDC)2 chains are further extended by Tb−O coordination bonds, thus giving rise

different coordination modes, and two ligated water molecules. Both Tb(III) ions adopt distorted bicapped trigonal prism geometry. Tb1 ion is eight-coordinated by four oxygen atoms from two PyIDC3− ligands, two oxygen atoms from one oxalate anion, one oxygen atom from one μ2-OH group, and one water oxygen atom. Tb2 ion coordinates to four oxygen atoms from three individual PyIDC3− ligands, two oxygen atoms from one oxalate anion, one oxygen atom from one μ2-OH group, and one oxygen atom from one water molecule. The Tb−O bond lengths range from 2.244(5) to 2.488(5) Å, and the O−Tb−O bond angles vary from 47.04(17) to 153.30(19)°, all of which are within the normal range.20 Three Ag(I) ions are threecoordinated, showing a T-shaped coordination geometry, and all of them are defined by two nitrogen atoms and one carboxylate oxygen atom from two different PyIDC3− ligands. The Ag−O bond lengths are 2.573(6), 2.668(5), and 2.542(5) Å, respectively, and the Ag−N distances span from 2.115(7) to 2.177(6) Å, all of which are comparable to those observed for the other Ag(I) complexes.21 Two PyIDC3− ligands in complex 8 exhibit two kinds of coordination modes: one coordinates to two Tb(III) ions and three Ag(I) ions in a μ5-kN,O:kO,O′:kO′,O″:kO‴,N′: kN″ mode (Scheme 1b), while the other bonds with three Tb(III) ions and three Ag(I) ions in a μ6-kN,O:kO:kO′,O″:4445

dx.doi.org/10.1021/cg300613t | Cryst. Growth Des. 2012, 12, 4441−4449

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Above 325 °C, the second weight loss is attributable to the decomposition of the organic ligands and the collapse of the whole framework. The final residue may be a mixture of Dy2O3 and Ag2O (observed 54.6%, calculated 56.9%). Photoluminescent Properties. Because of the excellent luminescent properties of Eu(III) and Tb(III) ions, the solidstate photoluminescence of complexes 3, 4, and 8 was investigated at room temperature. Compound 3 exhibits very weak emission (Figure S5). When being excited at 319 nm, complex 4 displays interesting luminescence with emission not only in the violet-blue-green region (λmax = 380 and 470 nm), but also in the red region as the characteristic f−f transition of a Eu(III) ion (λmax = 593 and 616 nm), as illustrated in Figure 4.

to a 3D chain-layer heterometallic coordination framework as displayed in Figure 2d. Obviously, some small amount of organic ligand has converted into the oxalate anion during the hydrothermal formation of type II. Although it needs to be further explored, we may suppose that the oxalate anions are generated from decarboxylation of 2-(pyridine-3-yl)-1H-4,5-imidazoledicarboxylic acid, followed by CO2 coupling, as similar to the reported reaction system.22 We also found that complexes 6−9 cannot be obtained in the absence of 4,4′-bpy or replacing 4,4′-bpy with potassium oxalate in the reaction systems, which means that the 4,4′-bpy may have an important influence on the in situ reaction.22 XRPD and TGA. In order to confirm whether the crystal structures are truly representative of the bulk materials, X-ray powder diffraction (XRPD) experiments were carried out for compounds 1−7 at room temperature. As illustrated in Figures S3 and S4, the peaks displayed in the measured patterns closely match those in the simulated patterns generated from singlecrystal diffraction data, thus indicating that the bulk synthesized materials and the measured single crystals are the same. XPRD patterns of type I and type II are obviously different from each other, while the XPRD patterns within type I and type II are identical, indicating that the compounds type I and type II are isomorphous. To estimate thermal stability of the two types of complexes, thermogravimertric analyses (TGA) of the selected compounds 3 and 9 were carried out from room temperature to 700 °C at a heating rate of 10 °C/min under an air atmosphere (Figure 3).

Figure 4. The solid-state emission spectrum for 4 at room temperature (excited at 319 nm).

The band at 616 nm can be assigned to the electric-dipolar 5D0 → 7F2 transition, and the other at 593 nm can be ascribed to the magnetic-dipolar 5D0 → 7F1 transition. The intensity of the 5 D0 → 7F2 transition is ca. 2.2 times stronger than that of the 5 D0 → 7F1 transition, indicating that Eu(III) ions have a low symmetric coordination environment,23 which is confirmed by the X-ray crystal structure of 4. The presence of the ligandbased emission suggests that the energy transfer from the ligand to the Eu(III) center is inefficient under the experimental conditions. What’s more, the very weak emission of 3 indicates a heavy-atom quenching induced by the Co(II) ion. Complex 8 emits intense green light when excited at 286 nm, and the emission spectrum exhibits the characteristic transition of 5D4 → 7FJ (J = 3−6) of the Tb(III) ion (Figure 5). Two intense emission bands at 490 and 545 nm correspond to 5 D4 → 7F6 and 5D4 → 7F5 transition, while the weaker emission bands at 586 and 623 nm originate from the 5D4 → 7F4 and 5 D4 → 7F3 transition, respectively. Different from complex 3, the ligand-based emission is absent in 8, thus suggesting that the ligand-to-terbium energy transfer is very efficient. Magnetic Properties. The solid-state dc magnetic susceptibility measurements of compounds 1, 3, 5, 6, and 7 have been performed in the range 2−300 K at a magnetic field of 1000 G. The results are illustrated in Figures 6−10, respectively, as plots of χM−1 versus T and χMT versus T, where χM is the molar magnetic susceptibility. The χMT value of 1 is 4.15 cm3 K mol−1 at 300 K, larger than the calculated value of 2.06 cm3 K mol−1 that is expected for one CoII (S = 3/2, g = 2) and two SmIII (J = 5/2, gJ = 2/7) ions, which may be due to the spin−orbit coupling of CoII and/or

Figure 3. TGA curves for compounds 3 and 7.

It can be seen from the TGA curve that compound 3 lost the uncoordinated and coordinated water molecules below 230 οC, and the weight loss of 12.1% was consistent with that calculated (12.6%). The 3D framework was stable up to about 350 οC, and the gray amorphous residue may be a mixture of Eu2O3 and CoO (observed 38.5%, calculated 37.3%). For complex 9, there is no obvious weight loss until the temperature reached 265 °C, suggesting that the framework is thermally stable up to 265 °C. The first weight loss of 3.8% in the temperature range from 265 to 325 °C is ascribed to the removal of two coordinated water molecules (calculated 2.9%). In general, water molecules in coordination frameworks are removed below 250 °C. But compound 9 loses the coordinated water molecules at a relatively high temperature (∼325 °C), which may be probably to strong hydrogen bonding attractions between water molecules and carboxylate oxygen atoms of organic ligands. 4446

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Crystal Growth & Design

Article

Figure 8. Plots of the χMT vs T and χM−1 vs T for 5. The red line represents the best-fit curve.

Figure 5. The solid-state emission spectrum for 8 at room temperature (excited at 286 nm).

Figure 9. Plots of the χMT vs T and χM−1 vs T for 6. Figure 6. Plots of the χMT vs T and χM−1 vs T for 1. The red line represents the best-fit curve.

Figure 10. Plots of the χMT vs T and χM−1 vs T for 7. The red line represents the best-fit curve. Figure 7. Plots of the χMT vs T and χM−1 vs T for 3. The red line represents the best-fit curve.

well fit by the Curie−Weiss Law, giving rise to the following parameters: for 1, C = 5.26 cm3 K mol−1 and θ = −85.74 K; for 3, C = 7.54 cm3 K mol−1 and θ = −114.83 K. The larger negative Weiss constants indicate antiferromagnetic coupling between the paramagnetic cations within the structure, and it should be mainly attributed to the coupling between the rare earth cations, Sm(for 1)/Eu(excited states, for 3), owing to the shorter coupling route of Ln−O−Ln than Ln−O−C−O−Co. For compound 5, the recorded room-temperature χMT value is 19.47 cm3 K mol−1, larger than the spin-only value, 17.63 cm3 K mol−1, expected for one CoII (S = 3/2, g = 2) and two GdIII (S = 7/2, g = 2) cations. As the temperature decreased from 300 to 25 K, the χMT value decreased gradually from 19.47 cm3 K mol−1 to 18.18 cm3 K mol−1. Whereas along with the

the thermal populated SmIII excited states. As the temperature cooled down, the χMT values steadily decreased and then reached the minimum value of 1.63 cm3 K mol−1 at 2 K. Similar magnetic behavior can be observed from compound 3, whereas the χMT value of 3 at 300 K is 5.51 cm3 K mol−1, which disagrees with the spin-only value of 1.88 cm3 K mol−1 for one CoII (S = 3/2, g = 2) and two EuIII (J = 0, gJ = 5, 7F0) ions. The disagreement may be due to the presence of thermally populated excited states as is well-known for EuIII complexes and/or the spin−orbit coupling of CoII. Between 140 and 300 K for 1, 100 and 300 K for 3, the χM−1 versus T curves can be 4447

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Crystal Growth & Design temperature continuously lowering to 2 K, the χMT value sharply increased to a value of 21.07 cm3 K mol−1, thus indicating that the antiferromagnetic coupling between GdIII and CoII ions leads to the ferrimagnetic behavior in this compound. At 25−300 K, the observed susceptibility data were well-fitted to the Curie−Weiss Law with C = 19.45 cm3 K mol−1 and θ = −3.21 K. The negative θ value also gives the evidence of antiferromagnetic interactions existing in compound 5. For compound 6, the plot of χMT vs T is almost linear over the whole temperature range and is similar to that for the reported usual samarium complex;24 the χMT is equal to 0.823 cm3 K mol−1 at 300 K, and decreases rapidly with the temperature to a value of 0.064 cm3 K mol−1 at 2 K, which is smaller than the calculated value of 0.180 cm3 K mol−1 for two SmIII (S = 5/2, g = 2/7) and one diamagnetic Ag+ ions. This difference may be because the 6H5/2 ground state of SmIII is split into three Kramers doublets.25 Such behavior suggests that an antiferromagnetic interaction possibly exists between SmIII pairs at low temperature, although it is very weak. Similar to compound 6, χMT vs T plot of compound 7 is also nearly linear at 300−2 K, while the χMT value is 19.38 cm3 K mol−1 at 300 K, which is much higher than the spin-only value of 15.75 cm3 K mol−1 for two GdIII (S = 7/2, g = 2) and one diamagnetic Ag+ ions. As the temperature is lowered, the χMT value decreases gradually and reaches the minimum value of 15.55 cm3 K mol−1 at 2 K. Curie−Weiss fitting of the magnetic data over the whole temperature range 2−300 K results in a Curie constant C = 19.08 cm3 K mol−1 and a Weiss constant θ = −6.21 K, characteristic of a weak overall antiferromagnetic interaction by taking into account the fact that the orbital momentum of the Gd(III) ion is completely quenched in the ground state.26 In summary, two types of novel 3D d−f heterometallic coordination frameworks were hydrothermally synthesized by employing H3PyIDC with ox anions. Type I compounds exhibit 3D pcu topologies based on the linkages of 2D lanthanide−oxalate layers and M(HPyIDC)2 pillars, whereas type II complexes exhibit 3D heterometallic coordination frameworks constructed from the connections of 2D lanthanide−oxalate layers and 1D Ag2(PyIDC)2 chains. The successful construction of type II indicates that in situ ligand synthesis provides a robust method to obtain some unprecedented heterometallic coordination frameworks that cannot be accessible from a conventional direct reaction between metal ions and ligands.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of P. R. China (Grant Nos. 21003053 and 21171059), and the Natural Science Foundation of Guangdong Province (Grant No. 10451063101004667).

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ASSOCIATED CONTENT

S Supporting Information *

Additional structural figures for the related compounds, Tables of selected bond lengths and angles, PXRD, as well as X-ray crystallographic files in CIF format for compounds 1−9. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-20-39310383. Fax: +86-20-39310187. E-mail: [email protected] (S.-R.Z.); [email protected] (W.-G.Z.). Notes

The authors declare no competing financial interest. 4448

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