DOI: 10.1021/cg1014626
Hydrothermal Syntheses, Crystal Structures, and Magnetic Properties of a Series of Unique Three-Dimensional Lanthanide-Organic Coordination Frameworks with a N-Protonated 2,6-Dihydroxypyridine4-Carboxylate Tecton
2011, Vol. 11 811–819
Shao-Ming Fang,*,† E. Carolina Sa~ nudo,§ Min Hu,† Qiang Zhang,† Song-Tao Ma,† Li-Ran Jia,† Cong Wang,† Jia-You Tang,† Miao Du,*,‡ and Chun-Sen Liu*,† †
Zhengzhou University of Light Industry, Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou, Henan 450002, P. R. China, ‡College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecule, Tianjin Normal University, Tianjin 300387, anica i Institut de Nanoci encia i Nanotecnologia, P. R. China, and §Departament de Quı´mica Inorg Universitat de Barcelona, Diagonal, 647, 08028-Barcelona, Spain Received November 5, 2010; Revised Manuscript Received December 9, 2010
ABSTRACT: A series of novel three-dimensional (3-D) lanthanide-organic frameworks (LnOFs), with the general formula [Ln2(H-L)3(H2O)4]¥ (Ln = DyIII for 1, GdIII for 2, TbIII for 3, EuIII for 4, and HoIII for 5; H-L = N-protonated 2,6dihydroxypyridine-4-carboxylate), have been synthesized under hydrothermal conditions. X-ray structural analysis reveals that complexes 1-5 are isostructural and show the unique 3-D coordination framework with a trinodal (3,4,5)-connected (4.62)(42.6)(42.84)(43.6.86)(42.65.83) net topology, in which the H-L ligands adopt different μ4- and μ3-bridging forms. Variable-temperature magnetic susceptibility studies reveal that complexes 1-3 and 5 show ferromagnetic behaviors. Notably, complex 1 (DyIII) is the first instance leading to ferromagnetic coupling for a lanthanide ion through a spin-polarization mechanism. The alternating current (ac) signal observed for 1 and 5 should originate from the long-range ferromagnetic ordering, or it could reflect the splitting of the J multiplet for DyIII or HoIII under a low symmetry crystal field, which would give rise to slow relaxation of the magnetization. Additionally, thermal stability of these crystalline materials has also been investigated by thermogravimetric analysis of mass loss.
*Corresponding author. E-mail:
[email protected] (S.M.F.); dumiao@ public.tpt.tj.cn (M.D.);
[email protected] (C.S.L.).
3,5-dihydroxybenzoic acid,16 have been widely used in the preparation of LnIII coordination complexes.13-16 In comparison with H3ptc and H3hpdc, however, 2,6-dihydroxypyridine-4-carboxylic acid (H3L, see Scheme S1 in the Supporting Information) has not been explored so far, according to our latest Cambridge Structural Database (CSD) search. This may arise from its poor solubility in conventional solvents. In this work, H3L was selected to construct LnOFs on the basis of the following considerations: (1) similar to H3ptc and H3hpdc, the rigidity and the higher symmetry of H3L may usually lead to the formation of ordered structures upon metal complexation; (2) two ionized hydroxyl groups on 2- and 6-positions around the pyridyl ring prefer to coordinate to the Ln-ions, and furthermore, the 4-positional carboxylate group on the pyridyl ring can further interlink the metal ions into multidimensional LnOFs by using different coordination modes. On the other hand, it is unexpected that the H3L ligand is present in the resulting LnOFs in a negative bivalent H-L style (see Chart 1), and normally the protonated pyridyl group will not be involved in metal coordination. As a result, the familiar η3 chelating coordination mode13,14 for ptc and hpdc ligands (via two 2-/6-positional oxygen atoms and one pyridyl nitrogen) is not available for H-L, and this feature will also affect the chemometrics ratio of the Ln3þ and ligand (H-L) components, which leads to the formation of completely different coordination frameworks compared with those based on ptc and hpdc trianionic tectons. From the viewpoint of magnetism, the large unquenched orbital angular momentum and large spin moments of most LnIII ions associated with the internal nature of the valence f orbitals make such ions
r 2011 American Chemical Society
Published on Web 01/18/2011
Introduction Recently, the realm of crystal engineering has been flourishing1 and the crystal engineering technique has been widely used for rational design and controlled synthesis of lanthanide-organic frameworks (LnOFs),2,3 also known as lanthanide coordination polymers (LCPs), owing to their fascinating structural topologies4 and potential applications as functional materials in many areas, such as luminescence,5,6 magnetism,7-9 catalysis,10 nonlinear optics (NLO),11 and electroluminescence in light emitting diodes (LEDs).12 In this field, several strategies have been developed to prepare such crystalline materials with desired structures and properties,1h,2a,2d,4f among which the appropriate choice of predesigned organic bridging ligands and lanthanide ions or clusters is one of the most effective ways.2-12 Lanthanide ions tend to coordinate with the O-donor ligands in higher and flexible coordination numbers (CNs) compared with d-block transition metal (TM) ions. In this context, carboxylic acids can generally cater to the oxophilic nature of Ln-ions, and their ability to take diverse binding forms fits well with the irregular coordination geometries of 4f metals.4f As for the aromatic carboxylic acid ligands, those with three carboxyl and/or hydroxyl groups arranged in 1,3,5- or metafashion around the central aromatic ring, such as pyridine2,4,6-tricarboxylic acid (H3ptc, see Scheme S1 in the Supporting Information),13 4-hydroxypyridine-2,6-dicarboxylic acid (H3hpdc),14 5-hydroxy-1,3-benzenedicarboxylic acid,15 and
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very appealing for the synthesis of magnetic materials.7,8 Since the orbital contributions of most 4f electrons and the influence of the crystal field effect must be considered all together, the investigation of magnetostructural correlations for LnIII complexes will be quite difficult and challenging, not only in the theory of magnetism but also in exploiting new magnetic materials.8d Considering all the aspects stated above, the ligand H3L with a 1,3,5- or meta- carboxyl/hydroxyl fashion was applied in this research to construct LnOFs with novel topological structures and potential properties, by taking the advantage of its carboxyl and/or hydroxyl bridging coordination. Five new LnIII-H-L coordination frameworks were prepared under hydrothermal conditions, with the general formula [Ln2(HL)3(H2O)4]¥ (Ln=DyIII for 1, GdIII for 2, TbIII for 3, EuIII for 4, and HoIII for 5), and their crystal/topological structures, magnetic properties, and thermal stability have also been investigated in detail.
Experimental Section Materials and General Methods. All reagents and solvents were commercially available and used as received. Distilled water was used throughout. Fourier transform infrared (FT-IR) spectra (KBr pellets) were recorded in the range of 4000-400 cm-1 on a Tensor 27 OPUS (Bruker) FT-IR spectrometer. Elemental analyses (C, H, and N) were performed on a Vario EL III elemental analyzer. Thermogravimetric (TG) analysis experiments were carried out on a PerkinElmer Diamond SII thermal analyzer in the temperature range of 25-800 C at a heating rate of 10 C/min under N2 atmosphere. Magnetic Studies. The variable-temperature magnetic susceptibilities were carried out in the Unitat de Mesures Magnetiques (Universitat de Barcelona) on crushed polycrystalline samples (ca. 30 mg) with a Quantum Design SQUID MPMS-XL magnetometer equipped with a 5 T magnet. The data were corrected for temperatureindependent paramagnetism (TIP) and the diamagnetic corrections were evaluated from Pascal’s constants for all the constituent atoms. An experimental correction for the sample holder was also applied. All the magnetic susceptibility data of complexes 1, 2, 3, and 5 were collected in the 2-300 K temperature range at an applied field of 0.3 T. Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) patterns of 1-5 were recorded on a Bruker D8 Advance diffractometer (Cu-KR, λ = 1.54056 A˚) at 40 kV and 30 mA, by using a Cu-target tube and a graphite monochromator. The powder samples were prepared by crushing the crystals and the intensity data were recorded by continuous scan in a 2θ/θ mode from 3 to 80 with a step size of 0.02 and a scan speed of 8/min. Simulation of the PXRD patterns was carried out by the single-crystal data and diffraction-crystal module of the Mercury (Hg) program. Synthesis of Complexes 1-5. [Dy2(H-L)3(H2O)4]¥ (1). A mixture of Dy2O3 (0.5 mmol, 0.1865 g), H3L (0.3 mmol, 0.0465 g), and H2O (15 mL) was sealed in a 23-mL Teflon-linear autoclave, heated in an oven at 170 C for 5 days, and then cooled to room temperature at a rate of 5 C h-1. Yellow block crystals suitable for X-ray structural
Chart 1
Table 1. Crystallographic Data and Structure Refinement Summary for 1-5 empirical formula formula weight crystal system space group a/A˚ b/A˚ c/A˚ V/A˚3 Z D/g cm-3 μ/mm-1 Rint GOF T/K R1a/wR2b [I > 2σ(I)] Fmax/Fmin (e A˚-3) empirical formula formula weight crystal system space group a/A˚ b/A˚ c/A˚ V/A˚3 Z D/g cm-3 μ/mm-1 Rint GOF T/K R1a/wR2b [I > 2σ(I)] Fmax/Fmin (e A˚-3) a
1
2
3
C18H17Dy2N3O16 856.35 orthorhombic Pbca 14.7990(3) 15.7385(3) 19.0145(5) 4428.74(17) 8 2.569 6.791 0.0448 0.938 294(2) 0.0277/0.0418 0.856/-1.009
C18H17Gd2N3O16 845.85 orthorhombic Pbca 14.8594(2) 15.8477(2) 19.0553(3) 4487.28(11) 8 2.504 5.954 0.0348 1.078 294(2) 0.0198/0.0444 0.903/-0.751
C18H17Tb2N3O16 849.19 orthorhombic Pbca 14.8267(7) 15.7817(3) 19.0579(7) 4459.4(3) 8 2.530 6.386 0.0259 1.026 294(2) 0.0222/0.0477 0.567/-1.103
4
5
C18H17Eu2N3O16 835.27 orthorhombic Pbca 14.8656(16) 15.9425(18) 19.175(2) 4544.4(9) 8 2.442 5.562 0.0577 0.969 294(2) 0.0404/0.0682 1.482 /-0.794
C18H17Ho2N3O16 861.21 orthorhombic Pbca 14.7597(5) 15.7023(6) 18.9885(9) 4400.8(3) 8 2.600 7.234 0.0240 1.004 294(2) 0.0192/0.0340 0.864/-0.616
R1 = Σ(||Fo| - |Fc||)/Σ|Fo|. b wR2 = [Σw(|Fo|2 - |Fc|2)2/Σw(Fo2)2]1/2.
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Table 2. Selected Bond Distances (A˚) and Angles (deg) for 1a Dy1-O8#1 Dy1-O3#2 Dy1-O1 Dy1-O13 Dy2-O12#4 Dy2-O9 Dy2-O15 Dy1#6-O3 Dy1#6-O6 Dy2#5-O10 O8#1-Dy1-O5 O5-Dy1-O3#2 O5-Dy1-O6#2 O8#1-Dy1-O1 O3#2-Dy1-O1 O8#1-Dy1-O14 O3#2-Dy1-O14 O1-Dy1-O14 O5-Dy1-O13 O6#2-Dy1-O13 O14-Dy1-O13 O10#3-Dy2-O4#5 O10#3-Dy2-O9 O4#5-Dy2-O9 O12#4-Dy2-O2 O9-Dy2-O2 O12#4-Dy2-O15 O9-Dy2-O15 O10#3-Dy2-O16 O4#5-Dy2-O16 O2-Dy2-O16
2.243(4) 2.283(4) 2.313(4) 2.375(4) 2.243(5) 2.297(5) 2.368(4) 2.283(4) 2.305(4) 2.220(5) 163.25(17) 83.81(16) 101.39(16) 84.95(16) 147.23(14) 79.91(19) 68.16(16) 144.31(15) 82.21(16) 153.01(15) 69.46(16) 127.82(16) 150.58(16) 77.21(16) 83.89(16) 71.92(15) 169.00(16) 88.10(16) 72.03(17) 68.90(16) 146.04(15)
Dy1-O5 Dy1-O6#2 Dy1-O14 Dy2-O10#3 Dy2-O4#5 Dy2-O2 Dy2-O16 Dy2#3-O4 Dy1#7-O8 Dy2#8-O12 O8#1-Dy1-O3#2 O8#1-Dy1-O6#2 O3#2-Dy1-O6#2 O5-Dy1-O1 O6#2-Dy1-O1 O5-Dy1-O14 O6#2-Dy1-O14 O8#1-Dy1-O13 O3#2-Dy1-O13 O1-Dy1-O13 O10#3-Dy2-O12#4 O12#4-Dy2-O4#5 O12#4-Dy2-O9 O10#3-Dy2-O2 O4#5-Dy2-O2 O10#3-Dy2-O15 O4#5-Dy2-O15 O2-Dy2-O15 O12#4-Dy2-O16 O9-Dy2-O16 O15-Dy2-O16
2.274(5) 2.305(4) 2.371(5) 2.220(5) 2.264(4) 2.338(4) 2.371(5) 2.264(4) 2.243(4) 2.243(5) 112.63(16) 86.07(16) 76.99(15) 82.23(15) 76.94(15) 104.64(16) 133.27(16) 84.47(16) 129.92(15) 77.09(14) 96.64(17) 110.31(16) 86.21(17) 79.26(15) 145.04(15) 83.78(16) 77.48(15) 85.40(14) 81.85(18) 137.12(17) 108.61(16)
a Symmetry codes: #1 = -x þ 1, y þ 1/2, -z þ 1/2; #2 = x - 1/2, y, -z þ 1/2; #3 = x þ 1/2, -y þ 3/2, -z þ 1; #4 = -x þ 1/2, y þ 1/2, z; #5 = x - 1/2, -y þ 3/2, -z þ 1; #6 = x þ 1/2, y, -z þ 1/2; #7 = -x þ 1, y 1/2, -z þ 1/2; #8 = -x þ 1/2, y - 1/2, z.
analysis were isolated by filtration, washed with ethanol and ether, and dried in air. Yield: ca. 30% (based on H3L). Anal. Calcd for C18H17Dy2N3O16 (856.35): C, 25.25; H, 2.00; N, 4.91%. Found: C, 25.37; H, 2.11; N, 4.77%. IR (cm-1): 3234(m,br), 2341(w), 1634(vs), 1574(vs), 1454(s), 1412(m), 1396(w), 1370(w), 1304(s), 1248(m), 1187(m), 1097(w), 1047(w), 997(w), 864(w), 835(w), 781(s), 714(w), 623(w), 564(w), 481(w). [Gd2(H-L)3(H2O)4]¥ (2). The same procedure as that for 1 was used, with the exception of using Gd2O3 (0.5 mmol, 0.1813 g) instead of Dy2O3. Yellow block crystals suitable for X-ray structural analysis were obtained (yield: ca. 40% based on H3L). Anal. Calcd for C18H17Gd2N3O16 (845.85): C, 25.56; H, 2.03; N, 4.97%. Found: C, 25.41; H, 2.11; N, 4.76%. IR (cm-1): 3234(m,br), 2001(w), 1638(vs), 1578(vs), 1453(s), 1414(m), 1396(w), 1372(w), 1311(s), 1248(m), 1186(m), 1100(w), 1047(w), 998(w), 960(w), 841(w), 781(s), 716(w), 682(w), 665(w), 622(w), 563(w), 480(w). [Tb2(H-L)3(H2O)4]¥ (3). The same procedure as that for 1 was used with the exception of using Tb(NO3)3 (0.5 mmol, 0.1725 g) instead of Dy2O3. Yellow block crystals suitable for X-ray structural analysis were obtained (yield: ca. 40% based on H3L). Anal. Calcd for C18H17Tb2N3O16 (849.19): C, 25.46; H, 2.02; N, 4.95%. Found: C, 25.34; H, 2.17; N, 5.07%. IR (cm-1): 3233(m,br), 2341(w), 1635(vs), 1577(vs), 1452(s), 1412(m), 1395(w), 1371(w), 1306(m), 1248(m), 1188(m), 1118(w), 1048(w), 997(w), 961(w), 912(w), 864(w), 835(w), 780(s), 716(w), 663(w), 623(w), 564(w), 478(w). [Eu2(H-L)3(H2O)4]¥ (4). A mixture of Eu(NO3)3 (0.5 mmol, 0.1689 g), H3L (0.3 mmol, 0.0465 g), and H2O (15 mL) was sealed in a 23-mL Teflon-linear autoclave, heated in an oven at 140 C for 72 h, and then cooled to room temperature at a rate of 5 C h-1. Yellow block crystals suitable for X-ray structural analysis were isolated by filtration, washed with ethanol and ether, and dried in air. Yield: ca. 40% (based on H3L). Anal. Calcd for C18H17Eu2N3O16 (835.27): C, 25.88; H, 2.05; N, 5.03%. Found: C, 25.97; H, 2.14; N, 4.95%. IR (cm-1): 3230(m,br), 1633(vs), 1576(vs), 1453(s), 1412(m), 1395(w), 1369(w), 1309(m), 1247(m), 1188(m), 1116(w), 1092(w), 1044(w), 996(w), 941(w), 866(w), 780(m), 714(w), 624(w), 563(m), 480(w).
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Scheme 1. Coordination Modes of H-L Ligands in 1-5: (a) μ4-Bridging; (b) μ3-Bridging
[Ho2(H-L)3(H2O)4]¥ (5). The procedure is the same as that in 4 except that Ho2O3 (0.5 mmol, 0.1889 g) was used instead of Eu(NO3)3. Yellow block crystals suitable for X-ray structural analysis were obtained (yield: ca. 30% based on H3L). Anal. Calcd for C18H17Ho2N3O16 (861.21): C, 25.10; H, 1.99; N, 4.88%. Found: C, 25.24; H, 2.11; N, 4.97%. IR (cm-1): 3236(m,br), 1635(vs), 1579(vs), 1454(s), 1412(m), 1371(w), 1304(m), 1248(m), 1189(m), 1117(w), 1095(w), 1048(w), 998(w), 937(w), 867(w), 781(s), 713(w), 624(w), 565(m), 483(w). X-ray Data Collection and Structure Determination. X-ray singlecrystal diffraction data for complexes 1-5 were collected on a Bruker Smart 1000 CCD area-detector diffractometer with Mo-KR radiation (λ = 0.71073 A˚) by ω scan mode. The program SAINT17 was used for integration of the diffraction profiles, and semiempirical absorption corrections were applied using SADABS.18 All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by fullmatrix least-squares methods with SHELXL.19 Metal ions were located from the E-maps, and the other non-H atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. Generally, C-bound hydrogen atoms were determined theoretically and refined with isotropic thermal parameters riding on their parents. H-atoms of water and the protonated pyridyl group were first located by difference Fourier E-maps and then treated isotropically as riding. Further details for crystallography are summarized in Table 1. Selected bond parameters are listed in Table 2 for 1 and Tables S1-S4 (Supporting Information) for 2-5.
Results and Discussion Synthesis Consideration and General Characterization. Because of the poor solubility of the H3L ligand in common solvents, the hydrothermal synthesis method20 was applied to prepare complexes 1-5. In this work, reactions of H3L with Ln(NO3)3 by trial and error can only form suitable single-crystal products for 3 (TbIII) and 4 (EuIII), whereas for others, only polycrystalline precipitates are obtained. Generally, when lanthanide oxide was used in the synthetic system, the lanthanide ions will be released more slowly via in situ Lewis acid-base reactions with the acidic organic ligands, which may be helpful for the single-crystal growth of products. Considering this point, we used Ln2O3 instead of Ln(NO3)3 to react with H3L under hydrothermal conditions, and as expected, well-shaped single crystals for 1 (DyIII), 2 (GdIII), and 5 (HoIII) were successfully isolated in satisfying yields. Complexes 1-5 are air stable and insoluble in common organic solvents and water. All general characterizations were carried out by using the crushed single-crystal samples. The IR spectra usually show features attributable to each component of the complexes.21 In the IR spectra of 1-5, the broad bands centered at ca. 3230 cm-1 (3234 cm-1 for 1, 3234 cm-1 for 2, 3233 cm-1 for 3, 3230 cm-1 for 4, and 3236 cm-1 for 5) indicate the O-H stretching vibration of water ligands. As a matter of fact, the IR absorption of carboxylate
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group is very complicated due to its coordination diversity.21,22 The characteristic bands of carboxylate groups in 1-5 appear in the usual regions at 1638-1574 cm-1 (antisymmetric stretching vibrations) and at 14521454 cm-1 (symmetric stretching vibrations).22 The Δν [νasym(COO-) - νsym(COO-)] values are 120 and 180 cm-1 for 1, 125 and 185 cm-1 for 2, 125 and 183 cm-1 for 3, 123 and 180 cm-1 for 4, and 125 and 181 cm-1 for 5. These results indicate that the carboxylate groups in 1-5 are coordinated to the LnIII ions in different fashions,22 as observed in their crystal structures (see Scheme 1). Also, the absence of characteristic bands at around 1700 cm -1 confirms the complete deprotonation of carboxyl for the ligands in 1-5,23 being in good agreement with their solid state structural features. Description of the Crystal Structures for 1-5. [Ln2(H-L)3(H2O)4]¥. Single-crystal X-ray analysis of 1-5 indicates that they are isomorphous and crystallize in space group Pbca (see Table 1). Thus, only the crystal structure of complex 1 will be described herein as a representative example. The structure of 1 is a very complicated 3-D coordination network with two crystallographically independent DyIII centers (Dy1 and Dy2; see Figure 1), three H-L ligands (one μ4-bridging and two μ3-bridging H-L, as designated infra with H-LA, H-LB, and H-LC; see Scheme 1 and Figure 2), and four water ligands in the asymmetric unit. Both DyIII centers are seven-coordinated and take the distorted pentagonal-bipyramidal geometries,24
Figure 1. Local coordination environments of DyIII in 1 (symmetry codes: x þ 1/2, -y þ 3/2, -z þ 1 for A; x - 1/2, -y þ 3/2, -z þ 1 for B; x - 1/2, y, -z þ 1/2 for C; -x þ 1, y þ 1/2, -z þ 1/2 for D; -x þ 1/2, y þ 1/2, z for E).
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surrounded by two carboxylate and three hydroxyl O donors from five distinct H-L ligands (two H-LA and three H-LB for Dy1; two H-LA and three H-LC for Dy2; see Figure 1) as well as two oxygen atoms from water ligands. The bond lengths and angles around Dy1 and Dy2 are similar (Dy1-O= 2.243(4)2.375(4) A˚ and O-Dy1-O=68.16(16)-163.25(17); Dy2-O= 2.243(5)-2.368(4) A˚ and O-Dy2-O=68.90(16)-169.00(16); see Table 2), which are also comparable to those observed in other reported DyIII complexes.25,26 For the H-L ligands in 1, two types of coordination forms exist (see Scheme 1 and Figure 2). The first type H-LA uses all of its four oxygen atoms to bridge four DyIII centers (two Dy1 and two Dy2) by a μ4bridging mode (see Scheme 1a and Figure 2, left). The second type H-LB or H-LC employs one carboxylate and two ionized hydroxyl oxygen atoms to bridge three Dy1 or Dy2 centers by a μ3-bridging fashion (see Scheme 1b and Figure 2, middle and right). Thus, such linkages between the DyIII ions and H-L components lead to the formation of a 3-D coordination framework (see Figure 3). It is worth mentioning that the 3-D framework in 1 contains three types of DyIII-H-L coordination layers (see Figure 3). The Dy1 and Dy2 ions are bridged in turn by the 4-connected H-LA ligands to form an undulated (4,4) sheet parallel to the ac plane (see Figure 3, top). Along this line of structural analysis, another two (6,3) Dy1-H-LB and (4.82) Dy2-H-LC layers parallel to the ab plane can also be observed with Dy1 and Dy2 regarded as the 3-connected nodes (see Figure 3, bottom right and left), which are alternately arranged along the [001] axis and further interlinked to the above-mentioned (4,4) layers via sharing the Dy1 and Dy2 ions to result in the final 3-D network. Analysis of network topology is a convenient tool in designing and understanding the complicated crystal structures such as coordination polymers or H-bonding networks.4 Such structures can usually be simplified into reduced networks of nodes and links with different connectivity. In 1, both Dy1 and Dy2 centers can be viewed as the 5-connected nodes that are connected to five H-L ligands, while H-LA or H-LB/H-LC ligands can be regarded as the 4-connected or 3-connected nodes (see Figure 4). As a consequence, the resulting 3-D structure can be rationalized as an unusual trinodal (3,4,5)-connected topological network with the Schl€ afli symbol of (4.62)(42.6)(42.84)(43.6.86)2 5 3 (4 .6 .8 ) [representing H-LC/H-LB/H-LA/Dy2/Dy1 nodes, respectively] (see Figure S1 in the Supporting Information). According to a careful examination of the RCSR, EPINET,
Figure 2. Coordination modes of the H-L ligands in 1: μ2-η1:η1-syn-syn bridging mode for O3-C1-O4 carboxylate group, μ1-η1: η0-monodentate mode for O7-C7-O8 and O11-C13-O12 carboxylate groups, and μ1-monodentate mode for O1, O2, O5, O6, O9, and O10 hydroxyl groups. Symmetry codes: x þ 1/2, -y þ 3/2, -z þ 1 for A; x - 1/2, -y þ 3/2, -z þ 1 for B; x þ 1/2, y, - z þ 1/2 for F; -x þ 1, y 1/2, -z þ 1/2 for G; -x þ 1/2, y - 1/2, z for H.
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Figure 3. The 3-D coordination network (middle), viewed along the a axis, constructed by three types of Dy-H-L metal-oganic layers, namely, an undulated (4,4) Dy1/2-H-LA layer (top) running parallel to the ac plane as well as a planar (6,3) Dy1-H-LB layer (bottom right) and a planar (4.82) Dy2-H-LC layer (bottom left) running parallel to the ab plane, which are arranged alternately along the a axis.
Figure 4. Schematic representation of the trinodal (3,4,5)-connected topological net with the Schl€ afli symbol of (4.62)(42.6)(42.84)(43.6.86)(42.65.83), around which the connectivity between two types of DyIII nodes (cyan spheres: Dy1; pink spheres: Dy2) and three types of H-L nodes (yellow spheres: H-LA; red spheres: H-LB; green spheres: H-LC) are shown.
and TOPOS databases,27 the topological net of 1 is unknown so far. Magnetic Properties. Variable-temperature and variablefield magnetic measurements have been carried out for complexes 1, 2, 3, and 5.
The χT product for [Dy2(H-L)3(H2O)4]¥ (1) is shown in Figure 5 (circles). It has a value of 29.0 cm3 K mol-1 at 300 K, being in good agreement with the expected value for two non-interacting DyIII ions (S = 5/2, L=5, 6H15/2, χT=13.6 cm3 K mol-1 for one DyIII ion at 300 K). As temperature decreases, the χT product remains nearly constant until below 50 K and the χT product drops to a value of 19.1 cm3 K mol-1 at 2 K. This drop is a combination of the depopulation of Stark sublevels of the 6H15/2 term and the magnetic coupling between the DyIII centers. The magnetization vs field behavior (see Figure 6, circles) indicates that the magnetization saturates rapidly as expected for ferromagnetically coupled DyIII centers at 2 K, which indicates weak ferromagnetic interactions between the DyIII ions in 1. The magnetic susceptibility data for [Gd2(H-L)3(H2O)4]¥ (2) are shown in Figure 5 (squares) as χT vs T plot. The χT product has a value of 16.2 cm3 K mol-1 at 300 K, agreeing well with the expected value of 15.75 cm3 K mol-1 for two non-interacting GdIII ions with S = 7/2 and g = 2.0. As temperature decreases, so does the χT value, until a sharp decrease is found below 30 K. The GdIII-GdIII lengths in 2 are from 6.015 A˚ to 7.499 A˚, which are longer than those observed for DyIII-DyIII in 1. The dipolar contribution in this case should also be ferromagnetic, but the metals are too far apart and are practically uncoupled in the temperature range studied, which can be clearly seen in a Curie plot depicted in Figure S6, Supporting Information. The solid line is the best fitting of the experimental data to the Curie law, with C=15.9 cm3 mol-1. In the GdIII ion, the spin-orbit coupling effect is absent in the first order for the 8S7/2 ground state of the 4f7 core, and the influence of the ligand field can be safely neglected. Clearly, the data for 2 tend to saturation at a value of 13 at 5 T. This can be well modeled with the
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Figure 5. χT vs T plots for 1 (circles), 2 (squares), 3 (triangles), and 5 (diamonds).
Figure 6. Magnetization vs field plots at 2 K for 1 (circles), 2 (squares), 3 (triangles), and 5 (diamonds). The solid line is the Brillouin function for two non-interacting GdIII ions with g = 2.0 and S = 7/2.
Brillouin function for two non-interacting GdIII ions with S =7/2 and g=2.0 (see Figure 6, squares and the solid line). As shown in Figure 5 (triangles), the χT value for [Tb2(HL)3(H2O)4]¥ (3) is 23.3 cm3 K mol-1 at 300 K, as expected for two non-interacting TbIII ions (S=3, L=3, 7F6, χT=11.5 cm3 K mol-1 for one TbIII ion at 300 K). As the temperature decreases, the χT product remains nearly constant until below 50 K and the χT product drops to a value of 10.4 cm3 K mol-1 at 2 K. The magnetization vs field (see Figure 6, triangles) saturates rapidly at 2 K as expected for ferromagnetically coupled TbIII ions, being similar to that observed for 1. However, there is no evidence of slow-relaxation for 3 down to 1.8 K, as no out-of-phase signal is observed in the ac magnetic susceptibility. The χT product for [Ho2(H-L)3(H2O)4]¥ (5) has a value of 34.5 cm3 K mol-1 at 300 K (see Figure 5, diamonds) and is larger than the expected value for two non-interacting HoIII ions (S=2, L=6, 5I8, χT=13.6 cm3 K mol-1 for one HoIII ion at 300 K). As the temperature decreases, the χT product remains nearly constant until below 50 K and the χT product drops to a value of 20.5 cm3 K mol-1 at 2 K. The magnetization vs field (see Figure 6, diamonds) saturates rapidly at 2 K as expected for ferromagnetically coupled HoIII ions, and this behavior is also similar to that of 1 and 3. The LnIII ions in 1-5 are bridged by the tetradentate O-donor ligand H-L, and the shortest Ln-Ln distance is
Fang et al.
Figure 7. Out-of-phase ac magnetic susceptibility plot for 1 at 75, 202, 551, and 1400 Hz frequencies.
Figure 8. Out-of-phase ac magnetic susceptibility plot for 5 at 50 and 1000 Hz frequencies.
5.828 A˚ for complex 2 (with GdIII), rising to 5.843 A˚ for complex 3 (with TbIII), and then decreasing again to 5.824 A˚ and 5.810 A˚ for 1 (with DyIII) and 5 and (with HoIII), due to lanthanide contraction, which causes the trend along the LnIII series of larger, though always weak, magnetic couplings. The three donor groups on H-L are arranged in a 1,3,5- or meta- fashion around the central pyridyl ring. This usually leads to weak ferromagnetic interactions for complexes with 3d-block metals through a spin polarization mechanism.28,29 Complex 1 (DyIII) is the first instance of this mechanism leading to weak ferromagnetic coupling for a LnIII ion. The ac magnetic susceptibilities for 1 and 5 (see Figures 7 and 8) show the tail of an out-of-phase signal below 2 K, but the frequency dependence of this cannot be assessed since the maximum falls below 1.8 K (the lowest available temperature for a commercial Quantum Design SQUID magnetometer). Thus far, several SMMs30 and SCMs31 containing lanthanide ions (in most cases, DyIII ions) have been reported. The more interesting examples are the doubledecker monomeric DyIII complexes reported by Ishikawa and co-workers, which also display slow relaxation of magnetization and SMM like behavior.32 It has been proposed that the crystal field splitting of the ground multiplet levels with the largest or second largest Jz levels, being the lowest in energy (as is the case for TbIII, DyIII, and HoIII), corresponds to the spin-up and spin-down orientation in the J multiplet. This means that the DyIII complex possesses a large axial anisotropy, which is a requirement to observe the SMM
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behavior. Gatteschi et al. have studied the magnetic anisotropy of single DyIII ions in low symmetry environments,33 as observed in 1. The ac signal observed for complexes 1 and 5 could be due to long-range ferromagnetic ordering, with a Tc < 1.8 K, or it could reflect the splitting of the J multiplet for DyIII and HoIII under a low symmetry crystal field, which would give rise to slow relaxation of the magnetization. The fact that the lanthanide ions in complexes 1 and 5 are not isolated, but form a 3-D solid could explain the shift to temperatures below 1.8 K. To distinguish between these two possible origins of the out-of-phase signal, low temperature magnetic studies are necessary, which however are not within the author’s possibilities and the scope of this paper. Thermal Stability of 1-5. Thermogravimetric analysis (TGA) experiments were conducted to examine the thermal stability of 1-5 (see Figure S7 in the Supporting Information). Complexes 1-5 exhibit similar thermal behavior, probably due to their isostructural nature, and thus, only the thermal stability of 1 was discussed in detail. The TGA curve of 1 (see Figure S7a, Supporting Information) suggests two consecutive steps of weight loss in the temperature range of 100-300 C (peaking at 180 and 247 C), corresponding to the gradual removal of four coordinated water molecules (found: 8.47%; calcd: 8.41%). The host framework starts to decompose and undergo a rapid weight loss after 380 C (peaking at 511 C), which can be attributed to the elimination of H-L ligands. The residue gradually decomposes, which does not stop upon further heating to 800 C. PXRD Results. To confirm whether the crystal structures of 1-5 are truly representative of the bulk materials, PXRD experiments have been performed. Although the experimental patterns exhibit a few unindexed diffraction lines and some peaks are slightly broadened in comparison with the simulated patterns (see Figure S8 in the Supporting Information), it can still be concluded that the bulk synthesized materials and the single crystals are homogeneous for 1-5. Conclusion and Perspectives Five new 3-D lanthanide-organic coordination polymers based on 2,6-dihydroxypyridine-4-carboxylic acid were successfully synthesized under hydrothermal conditions, which show an unusual (3,4,5)-connected (4.6 2)(42.6)(42.84)(43.6.86)(42.65.83) topological net. The N-protonated H-L ligands in these complexes adopt two types of μ4- and μ3 -bridging coordination modes to extend the lanthanide ions to form the final 3-D inorganic-organic frameworks. Weak magnetic interactions are observed in complexes 1, 3, and 5, which arise from a spin-polarization mechanism. Furthermore, the out-of-phase ac signal observed for 1 and 5 suggests a slow-relaxation process, the origin of which can be ascribed to the long-range ferromagnetic ordering, or the splitting of the J multiplet for DyIII or HoIII under a low symmetry crystal field. Our present findings will further enrich the crystal engineering strategy and provide the possibility of controlling the formation of the desired lanthanide-organic network structures. Following this lead, further work on the H-L-based coordination polymers with other light rare-earth ions from LaIII to SmIII across the lanthanide period is underway in our laboratory, for developing more interesting functional coordination polymers with higher-connected topology and potential properties.
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Acknowledgment. This work was supported by the National Natural Science Fund of China (Grant Nos. 20801049 and 21071129), Henan Outstanding Youth Science Fund (to C.S.L.), and Tianjin Normal University (to M.D.). E.C.S. acknowledges the financial support from the Spanish Government (Grant CTQ2009-06959 and Ram on y Cajal contract). We also thank Mr. Xiao-Gang Yang for analyzing the topological structures. Supporting Information Available: Crystallographic information files (CIFs) for 1-5, additional structural figures for 1-5 (Figures S1-S5), Curie plot for 2 (Figure S6), TGA plots for 1-5 (Figure S7), and PXRD patterns for 1-5 (Figure S8), as well as tables of selected bond parameters for 2-5 (Tables S1-S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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