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Hydrothermal Synthesis, Crystal Structure and Characterization of a Microporous 3D Pillared-Layer 3d-4f Copper-Holmium Heterometallic Coordination Polymer
Hydrothermal Synthesis, Crystal Structure and Characterization of a Microporous 3D Pillared-Layer 3d-4f Copper-Holmium Heterometallic Coordination Polymer
Bulletin of the Korean Chemical Society. 2014. Jun, 35(6): 1841-1844
Copyright © 2014, Korea Chemical Society
  • Received : December 23, 2013
  • Accepted : February 08, 2014
  • Published : June 20, 2014
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About the Authors
Le-Qing Fan
Ji-Huai Wu
Yun-Fang Huang
Jian-Ming Lin
Yue-Lin Wei

Abstract
Keywords
Experimental Section
Materials and Characterization Methods. All of the reagent-grade reactants were commercially available and employed without further purification. Powder X-ray diffr-action (PXRD) datum was measured on a DMAX2500 diffr-actometer. Solid infrared (IR) spectrum was obtained from a Nicolet Nexus 470 FT-IR spectrometer between 400 and 4000 cm –1 using KBr pellet. Element analyses of carbon, hydrogen and nitrogen were performed with a Vario EL III element analyzer. Thermogravimetric analysis (TGA) was performed on a Netzsch Sta449C thermoanalyzer under N 2 atmosphere in the range of 30–600 °C at a heating rate of 10 °C/min. Variable-temper-ature magnetic susceptibilities were performed with a PPMS-9T magnetometer over the temperature range of 2–300 K under a magnetic field of 1000 Oe. A diamagnetic correction was estimated from Pascal’s constants. 5 The crystal structure was determined by a Rigaku Mercury CCD area-detector diffractometer and SHELXL crystallographic software of molecular structure.
Synthesis of {Ho2Cu4Br4(IN)4(OAc)2(H2O)2⋅H2O}n (1). A mixture of Ho 2 O 3 (0.189 g, 0.5 mmol), CuBr 2 (0.223 g, 1 mmol), HIN (0.246 g, 2 mmol), malonate (0.208 g, 2 mmol) and H 2 O (10 mL) was stirred at room temperature until a homogeneous mixture was obtained. The mixture was trans-ferred into a Teflon-line autoclave (23 mL) and heated at 170 °C for 7 days and then cooled at rate of 2 °C h −1 to room temperature. Yellow block crystals of 1 were recovered by filtration, washed with distilled water, and dried in air (36% yield based on Ho). Anal. Calcd. for 1 (dried) (%): C, 21.50; H, 1.80; N, 3.58. Found: C, 21.63; H, 1.91; N, 3.37. Selected IR data (KBr pellet, cm −1 ): 3463, 1621, 1540, 1412, 769, 688. PXRD pattern for the bulk product is in fair agreement with the pattern based on single-crystal X-ray solution in position, indicating the phase purity of the as-synthesized sample of 1 (Figure S1). The difference in reflection inten-sities between the simulated and experimental patterns was due to the variation in preferred orientation of the powder sample during collection of the experimental PXRD data.
X-ray Crystal Determination. The crystallographic data for 1 were collected on a Rigaku Mercury CCD area-detector equipped with a graphite-monochromated Mo radiation ( λ = 0.71073 Å) at 293(2) K using an ω -2 θ scan mode. Absorption correction was performed by the CrystalClear program. 6 This structure was solved by direct methods using SHELXS-97 program and refined by full-matrix least-squares refinement on F 2 with the aid of SHELXL-97 pro-gram. 7 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to carbon were placed in geo-metrically idealized positions and refined using a riding model. Hydrogen atoms on water molecules were located from difference Fourier maps and were also refined using a riding model. The Cu(2) and Br(2) atoms in 1 are disorder-ed, and occupy 74.3% and 83.5% of the corresponding sites, respectively. Some refinement details and crystal data of 1 are summarized in Table S1. Selected bond lengths and bond angles of 1 are shown in Table S2.
Results and Discussion
The compound 1 was synthesized from the reaction mix-ture of Ho 2 O 3 , CuBr 2 , HIN and malonate with the mole ratio of 1:2:4:4 in water at 170 °C by the hydrothermal technique. And it was found that crystals suitable for X-ray single-crystal analysis were obtained only with this ratio. However, isomorphous compounds with other Ln 3+ having larger or smaller ion radii are not obtained. Series of experiments using Cl and I ions as halide sources in place of Br ion have been carried out to prepare compounds similarly struc-tural to this compound, but unfortunately, we were un-successful. The main reason may be that Cl and I have smaller and larger ion radii than Br−, respectively, which do not favor proper coordination numbers that benefit to give rise to such a microporous 3D pillared-layer network. On the other hand, during the hydrothermal reaction, in-situ de-carboxylation of molanate gives OAc ligand. So, experi-ments using NaOAc instead of molanate have been carried out to prepare the compound 1 and isomorphous compounds with other Ln 3+ . But no crystal was produced. It is suggested that the reaction condition was changed for replacement of molanate by NaOAc.
Single crystal X-ray diffraction study revealed that 1 is a 3D pillared-layer PCP based on the linkages of 2D Ho-carboxylate layers and 1D Cu 4 Br 4 inorganic chains by IN pillars. An ORTEP view of 1 is shown in Figure 1 . The asymmetric unit of 1 contains one unique Ho 3+ ion, two (1 + 0.5 + 0.5) Cu + ions, two (1 + 0.5 + 0.5) Br ions, two IN ligands, one OAc ligand, one coordinated water molecule and half an uncoordinated water molecule. Ho(1) center is eight-coordinated and displays distorted bicapped trigonal-prism coordination environment: four oxygen atoms from four IN ligands, three oxygen atoms from two OAc ligands, one oxygen atom from coordinated water molecule. The Ho–O bond lengths vary from 2.299(4) to 2.463(4) Å, and the O–Ho–O bond angles are in the range of 53.78(14)– 153.99(14)°, thus being in the normal range observed in other compounds. 8 Cu(1) center exhibits tetrahedral confor-mation, being coordinated by one nitrogen atom from IN ligand and three Br ions. But, Cu(2) and Cu(3) centers present trigonal geometry: two nitrogen atoms from two IN ligands and one Br ion for Cu(2); three Br ions for Cu(3). The Cu–N bond lengths are 2.038(5) and 1.955(6) Å, and Cu–Br bond lengths range from 2.372(2) to 2.8256(15) Å, all within the range of those observed for other Ln–Cu compounds. 9 In this structure, the IN ligand has only one coordination mode: behaving as a bridging ligand to coordi-nate two Ho 3+ ions and one Cu + ion (Scheme S1a). The OAc ligand also has only one coordination mode: acting as a bridging ligand to coordinate two Ho 3+ ions (Scheme S1b). It is noted that the OAc− ligand came from the in-situ decarboxylation of malonate in the hydrothermal reaction. The decarboxylation reaction in the transformation of 2,5-pyridinedicarboxylic acid into nicotinic acid was also found in the preparation of Ln–Cu coordination polymer under hydrothermal condition. 10 The reason may be that pressure under hydrothermal condition is a necessary factor for the decarboxylation reaction. Although Cu 2+ ions were used as starting materials in 1 , the Cu centers have an oxidation state of +1, attributed to a reduction reaction involving the IN ligands, 9a 11 and is consistent with the geometry of the Cu + ions and evidenced by the yellow color of crystals.
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Lager Image
ORTEP plot of the asymmetric unit of 1 (30% prob-ability ellipsoids). All H atoms and uncoordinated water molecule are omitted for clarity. Symmetry codes: A = –x, 1 – y, 1 – z; B = –1/2 + x, y, 3/2 – z; C = 1/2 + x, y, 1/2 – z; D = x, 3/2 – y, –1 + z; E = x, 3/2 – y, z.
Two Ho(1) centers are connected by four carboxylate groups from two different IN ligands and two different OAc ligands to form Ho 2 dinuclear unit with Ho⋯Ho distance of 3.8361(5) Å ( Figure 2 ). Adjacent Ho 2 dinuclear units are bridged by four IN ligands to produce novel 2D wavelike Ho-carboxylate layers extending along ac plane (Figure S2 and Figure S3). As shown in Figure 3(a) , two adjacent Cu(1)Br 3 N tetrahedra are connected by common edge (Br(1)-Br(1)) to form a Cu 2 Br 4 N 2 dimer, which further links one Cu(3)Br 3 triangle through sharing vertices (Br(3)) to generate Cu 3 Br 5 N 2 trimer, where Cu(1)–Cu(1) and Cu(1)– Cu(3) distances are 2.8805(19) and 2.7549(17) Å, respec-tively, which is comparable with the double van der Waals radius of the Cu + ion (1.4 Å), implying relatively strong Cu–Cu interactions. The phenomena of Cu–Cu interactions have been observed for other Ln–Cu compounds. 9b 12 The Cu 3 Br 5 N 2 trimer joins Cu(2)BrN 2 triangle via sharing vertex (Br(2)) to engender Cu 4 Br 5 N 4 tetra-mer. Neighboring Cu 4 Br 5 N 4 tetramers link each other by common vertices (Br(1)) to form unusual 1D Cu 4 Br 4 inorganic chains in centipede-type struc-ture along a axis ( Figure 3(b) ). As a consequence of the connectivity of CuBr 3 N tetrahedral, and CuBr 3 and CuBrN 2 triangles in 1 , the Br ions act as µ 2 (Br(2), Br(3)) and µ 4 (Br(1)) ligands. 2D Ho-carboxylate layers and 1D Cu 4 Br 4 inorganic chains are connected by IN pillars to give birth to such a novel 3D pillared-layer network (Figure S4), which contains 1D channels with dimensions of about 7.5 × 11.6 Å (based on Ho⋯Ho separations) along the b axis ( Figure 4 ), providing an example of microporous 3D pillared-layer 3d–4f coordination polymer. As shown in Figure 4 , the channels in 1 impenetrate not only Ho-carboxylate layers but also Cu-Br inorganic chains almost locating at a plane, which benefits from the peculiar centipede-type structure of the latter. The uncoordinated water molecules situate in the channels by O–H⋯Br hydrogen bondings (Table S3).
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Diagram of Ho2 dinuclear unit.
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Lager Image
(a) Diagram of Cu4Br5N4 tetramer. (b) Diagram of 1D Cu4Br4 inorganic chain in centipede-type structure.
In the IR spectrum of 1 (Figure S5), the strong and broad absorption band in the range of about 3463 cm −1 is assigned as characteristic peak of OH vibration. The strong vibrations appearing at 1621 and 1412 cm −1 correspond to the asym-metric and symmetric stretching vibrations of carboxylate group, respectively. The absence of strong bands in the range of 1690–1730 cm −1 indicates that all carboxyl groups of HIN are deprotonated. 13
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Lager Image
Polyhedral diagram of 1 viewed approximately down the [010] direction. All uncoordinated water molecules, and IN and OAc ligands are omitted for clarity.
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Lager Image
Plots of χM−1 ( ■ ) and χMT ( O ) vs. T of 1 over the temperature of 2–300 K at the field of 1000 Oe.
To study the thermal stability of 1 , TGA was performed on polycrystalline sample of this complex in N2 atmosphere from 30 to 600 °C (Figure S6). The lattice-water and coordi-nated water molecules are gradually lost in the temperature ranging 25–235 °C (calcd/found: 3.45/3.71%). Thereafter 1 is stable to ca . 270 °C. Above this temperature, the weight loss is due to the decomposition of the organic ligand and the collapse of the whole framework.
The magnetic susceptibilities of 1 have been measured from ground crystals under a constant magnetic field of 1000 Oe over the temperature range of 2–300 K. The data are presented as plots of χ M −1 vs . T and χ M T vs . T ( χ M being molar magnetic susceptibility per Ho 3+ ion) in Figure 5 . The observed χ M T at room temperature is 13.82 cm 3 K mol −1 , which is close to the theoretical value of 14.07 cm 3 K mol −1 on the basis of a independent Ho 3+ ion in the 5I8 ground state (g = 5/4). The χ M T decreases slowly from room temperature to 50 K, and then decreases abruptly to 10.78 cm 3 K mol −1 at 2 K. The χ M −1 vs. T plot obeys the Curie-Weiss law, χ M = C /( T θ ), over the temperature range from 2 to 300 K with Curie constant C = 13.96 cm 3 K mol −1 , Weiss constant θ = –2.17 K. However, the trend of the χ M T value and the negative value of θ cannot unambiguously confirm the ex-istence of antiferromagnetic coupling between two adjacent Ho 3+ ions because of the strong spin-orbit coupling for Ln 3+ ions and the progressive thermal depopulation of the Ln 3+ Stark components. 14
In conclusion, a microporous 3D pillared-layer 3d–4f (Cu + –Ho 3+ ) coordination polymer based on the linkages of 2D wavelike Ho-carboxylate layers and 1D Cu 4 Br 4 inor-ganic chains in centipede-type structure by IN pillars has been obtained. Furthermore, the magnetic properties of this complex have been investigated. Our results provide an intriguing example of 3D 3d–4f PCPs and further demon-strate that the pillared-layer approach can be used for constructing novel 3D 3d–4f PCPs.
Acknowledgements
Supplementary Maerial.Crystallographic data for the structure reported here have been deposited with CCDC (No. CCDC-954960). These data can be obtained free of chargeviahttp://www.ccdc.cam.ac.uk/conts/retrieving.html or from CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, E-mail: deposit@ccdc.cam.ac.uk
References
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