Three-Dimensional Lanthanide Thiophenedicarboxylate Framework with an Unprecedented (4,5)-Connected Topology Jin-Gen Wang,† Chang-Cang Huang,*,† Xi-He Huang,† and Dong-Sheng Liu†,‡ Department of Chemistry, Fuzhou UniVersity, Fuzhou, Fujian, 350002, P. R. China, and Department of Chemistry, Jinggangshan UniVersity, Ji’an, Jiangxi, 343009, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 795–798
ReceiVed NoVember 17, 2007; ReVised Manuscript ReceiVed December 18, 2007
ABSTRACT: Three novel three-dimensional metal-organic frameworks [Ln2(tdc)3(H2O)4]n, Ln ) Dy(1), Ho(2), Er(3) (H2tdc ) thiophene2,5-dicarboxylic acid) have been synthesized and characterized by single crystal X-ray diffraction (XRD), powder XRD and IR spectra. X-ray crystallography reveals that the three complexes are isostructural and exhibit an unprecedented (4,5)-connected (42 · 64)(42 · 67 · 8) topology, in which the lanthanide ions act as unusual 5-connected nodes. The thermal stability and UV–vis absorption spectra of the title complexes have also been studied.
Introduction The rational design of metal-organic frameworks (MOFs) has attracted considerable attention in supramolecular and materials chemistry due to their enormous variety of interesting structural topologies and wide potential applications as functional materials.1,2 Through the replacement of monatomic anions by polyatomic organic ligands as linkers and the use of the well-defined coordination geometries of metal centers as nodes, a large number of MOFs with mineral topologies, such as CdSO4, NbO, Pt3O4, pyrite, quartz, rutile, halite, and sodalite, have been artificially reproduced.3–9 Among these reported topologies, the major structural types are based on 3-, 4-, or 6-connected examples of coordination frameworks; 5-connected and highly connected networks are relatively scarce. In particular, only several examples of frameworks with three-dimensional (3D) 5-connected networks have been reported to date.10 Generally, there are two developed routes for highly connected coordination polymers. One is based on the use of metal cluster entities as nodes because the enhanced coordination numbers and reduced steric hindrances of metal clusters are helpful to generate highly connected coordination polymers. As a result, several 8-, 9-, and 12-connected networks have been synthesized.11,12 The other is to combine the f-block metal centers with carboxylate containing ligands, by which 7- and 8-connected coordination polymers have been obtained by Schröder and co-workers.3a,b,13–15 Because of the unique nature of lanthanide ions, such as their large radius, usually high and variable coordination numbers, and some of the available coordination sites often blocked by solvent molecules, lanthanide-based frameworks might also be considered as an effective route to construct novel 3D 5-connected topology networks. Champness et al. have reported two lanthanide coordination polymers with unprecedented 5-connected (46 · 62 · 82) and (44 · 66) topology networks10d and one lanthanide-based bilayer structure with 5-connected topology.10f We report herein the first examples of MOFs with unprecedented (4,5)-connected (42 · 64)(42 · 67 · 8) topology, three remarkable lanthanide coordination frameworks, namely, [Ln2(tdc)3(H2O)4]n, Ln ) Dy(1), Ho(2), Er(3) (H2tdc ) thiophene-2,5-dicarboxylic acid), by using thiophenedicarboxylic acid as a bridging ligand. Complexes 1-3 are isostructural, in which the lanthanide ions act as 5-connected nodes and tdc ligands act as 4-connected nodes. Complexes 1-3 were obtained from the same reaction mixture by mixing Ln2O3, H2tdc, and HClO4 with water and ethanol under hydrothermal conditions.16 The formations of complexes 1-3 are * To whom correspondence
[email protected]. † Fuzhou University. ‡ Jinggangshan University.
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Figure 1. View of the coordination environment of Ho3+ in framework 2 at 50% probability level with labeling schedule. Symmetry codes: (a) x, 1 - y, -0.5 + z; (b) x, -y, -0.5 + z; (c) 0.5 - x, -0.5 + y, 0.5 - z; (d) -x, y, 0.5 - z.
Figure 2. View of the coordination environment of tdc-1 ligand (up) and tdc-2 ligand (bottom).
identified by single-crystal X-ray diffraction (XRD), elemental analysis, and IR spectroscopy. The powder X-ray diffraction studies confirm the pure phase of complexes 1-3. The similarity of IR spectroscopy and powder XRD of complexes 1-3 indicate that these three complexes are isostructural, and these conclusions are further proved by the single crystal X-ray analysis results. The characteristics and structure of 2 is described here in detail. The IR spectra of 2 (see Figure S1, Supporting Information) show the characteristic stretching vibrations of the carboxylato groups, of which the absorbance peaks in the range of 1517–1549 cm-1 and 1373–1395 cm-1 are attributed to the asymmetric stretching and symmetric stretching, respectively. The absence of the peak around 1661 cm-1, which is observed in the starting H2tdc material,
10.1021/cg7011342 CCC: $40.75 2008 American Chemical Society Published on Web 02/08/2008
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Figure 3. View of crystal packing of 1 along the b-axis. All the isolated water molecules and hydrogen atoms are omitted for clarity.
Figure 4. View of (42 · 64) (42 · 67 · 8) network. (purple: 5-connected node, yellow: 4-connected node).
indicates that all carboxyl groups of organic moieties in 2 are deprotonated. The presence of a strong broadband centered at 3437 cm-1 and the weak band at 1643 cm-1 implies the existing of hydrogen bonds. Complexes 1-3 are air-stable and insoluble in water and common organic solvents. Single crystal X-ray analysis shows that complex 2 is a neutral 3D coordination polymer.17 There are one 8-coordinated Ho3+ center, one and one-half tdc ligands, and two coordinated water molecules in the asymmetric unit, of which one tdc ligand is located on the 2-fold axis (denoted as tdc-1), the other tdc ligand (denoted as tdc-2), as well as the Ho3+ center and water molecules are in the general position. The Ho3+ ion is coordinated with eight oxygen atoms from four tdc-2 ligands through bridging carboxylate groups (O1a, O2b, O3c, and O4), one chelating bidentate carboxylate groups (O5, O6) of tdc-1 ligand, and two water molecules (Ow1 and Ow2) (Figure 1), to form a distorted triangular dodecahedral coordination polyhedron. The bridging carboxylic O-Ho bond distances range from 2.267(2) to 2.327(2) Å, all of which are considerably shorter than the Ho-Ow distances (2.386(2) and 2.392(3) Å) and the chelating carboxylic O-Ho bonds (2.415(2) and 2.516(2) Å). The coordination environment of the two crystallographically independent tdc ligands is different from each other. As shown in Figure 2, the tdc-2 ligands adopt a (κ1-κ1)-(κ1-κ1)-µ4-bridging mode connecting with four Ho centers, resulting a [Ho(H2O)2(tdc)]nn+ cationic double-layer substructure parallel to the bc plane. The double-layer substructure can be described by the combination of (6,3) plane nets linked through the fourth coordinated O3 atom of the tdc-2 ligands. Such an interesting configuration might be attributed to the distinctive coordination mode of tdc-2 ligand, of which the two bidentate bridging carboxylate groups show distinct differences in the inclination away from the thiophene group with a dihedral angle of 1.6(9)° (for O1C1O2 group) and 26.5(6)° (for O3C6O4 group), respectively. Meanwhile, the tdc-1 ligand adopts a
Figure 5. UV–vis absorption spectra of (a) complexes 1–3 and H2tdc, (b) 1, (c) 2, (d) 3, respectively.
(κ2)-(κ2)-µ2-bridging mode connecting the adjacent [Ho(H2O)2(tdc)]nn+ cationic double layers, constructing a 3D lanthanide thiophenedicarboxylate framework (Figure 3). Extensive hydrogen bonds formed among the carboxylate groups and coordinated water molecules with the O · · · O range from 2.830(3) to 2.963(4) Å, which further enhanced the stabilization of the crystal structure. Furthermore, there is a strong aromatic π-π interaction between the two adjacent thiophene rings of two tdc-2 ligands. The two thiophene rings are parallel to each other with a distance of 3.39(1) Å and slight offset (the centroid-to-centroid distance of the two thiophene rings is 3.586(3) Å). Topologically, each tdc-2 ligand is connected to four adjacent Ho centers through four Ho-O bonds, as well as each Ho3+ ion links with four tdc-2 ligands and one tdc-1 ligand. Thus, the tdc-2 ligand and Ho3+ ion can be defined as a 4- and 5-connected node, and the µ2-bridging tdc-1 ligand can be considered as a linker between two 5-connected nodes. On the basis of this simplification, the structure of 2 can be described as a unprecedented binodal (42 · 64)(42 · 67 · 8) topological network (Figure 4). The “long” Schla¨fli symbol for this network is (4 · 4 · 62 · 64 · 64 · 64)(4 · 4 · 6 · 62 · 62 · 62 · 63 · 63 · 64 · 810). Although several (4,5)-connected networks have been identified and categorized by O’Keeffe et al., such as bnn-a, ctn-x, ffa, ffb, gar-a, iac-a, ibd-a, mcf-d, nia-a, ocu-a, rtw, scu-f, sqp-a, tcs, and toc-a, to the best of our knowledge, no MOFs with (4,5)connected topology have been reported to date.18 Attempts to synthesize more isostructural complexes of 1-3 with other lanthanide centers were unsuccessful, which may due to the effect of lanthanide contraction.19 The Ln-O distances of the oxolanthanide coordination spheres are in the range of 2.280(3)-2.530(3) Å for 1, and 2.263(3)-2.516(3) Å for 3, respectively, showing obvious lanthanide contraction. Complexes 1-3 exhibit well-resolved UV–vis absorption spectra (Figure 5). The strong absorption below 350 nm is ascribed to the
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Crystal Growth & Design, Vol. 8, No. 3, 2008 797 be employed as powerful building blocks to construct attractive MOFs with novel topologies.
Acknowledgment. This work was supported financially by the Education Foundation of Fujian Province (JB06049) and Fuzhou University (XRC0645, 2007-XQ-09). Supporting Information Available: X-ray crystallographic data in CIF format, experimental section, crystal data, IR spectra, simulated and experimental powder XRD patterns for complexes 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 6. TGA diagrams of complexes 1-3.
Figure 7. Experimental (b) and simulated (c) powder X-ray diffraction patterns for the sample of 2 as prepared and experimental pattern for the dehydrated sample of 2 (a).
organic linker’s π-π* transitions, while the absorption bands in the range of 350–800 nm can be readily assigned to the corresponding electronic transitions of Dy3+ from the ground state 6H15/2 to the excited states 4G11/2, 4I15/2, 4F9/2, 6F1/2, 6F3/2 for complex 1, of Ho3+ from 5I8 to 5G5, 5G6, 3K8, 5F2, 5F4, 5F5 for complex 2, and of Er3+ from 4I15/2 to 2H9/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2 for complex 3, respectively. The thermogravimetric analysis (TGA) data show that complexes 1-3 undergo two weight loss processes under 700 °C (Figure 6). The coordinated water molecules are removed in a single step in the temperature range of 210–290 °C, 210–270 °C, and 210–250 °C for complexes 1, 2, and 3, respectively. The release of the water molecules up to a high temperature is ascribed to the strong hydrogen-bonding interaction with a descending trend following the lanthanide contraction. A further sharp weight loss was observed from 450 to 560 °C, implying that the complexes decompose. The powder XRD patterns of the dehydrated samples of 2, by heating the crystal samples at 240 °C for 4 h, still retain strong diffraction peaks (Figure 7), indicating the good crystallinity of the dehydrated products and a certain structural change or distortion occurs after removal of the coordinated water molecules. Attempts to obtain the precise structures from the dehydrated crystals were unsuccessful, which was attributed to their poor crystal quality after water molecules were removed from the lattice. In summary, three new 3D lanthanide thiophenedicarboxylate frameworks had been prepared and characterized. The network of these complexes exhibits unprecedented (42 · 64)(42 · 67 · 8) topology and represents the first example of MOF with a (4,5)-connected network. The success in production of unusually 5-connected complexes 1-3 indicates once again that the lanthanide ions can
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798 Crystal Growth & Design, Vol. 8, No. 3, 2008 (7 mL) and ethanol (3 mL) at room temperature. After the solution was stirred for 20 min, 0.2 mL of 0.6 mol/mL HClO4 was added dropwise to the initial mixture and then heated at 145 °C for 5 days under hydrothermal conditions. Block crystals of 1–3 were obtained in 57% (1), 54% (2), and 48% (3) yield (based on Ln2O3), respectively. The colors of the as-synthesized crystals were colorless, light yellow, and pink for 1, 2 and 3, respectively. Anal. Calcd for C18H14O16S3Dy2 (1): C, 23.82; H, 1.55. Found: C, 23.87; H, 1.59. Anal. Calcd for C18H14O16S3Ho2 (2): C, 23.70; H, 1.55. Found: C, 23.73; H, 1.59. Anal. Calcd for C18H14O16S3Er2 (3): C, 23.58; H, 1.54. Found: C, 23.62; H, 1.57. (17) Crystal data of 2: C18H14O16S3Ho2, M ) 912.33, monoclinic, space group C2/c, a ) 25.1320(7), b ) 5.7534(2), c ) 18.7976(5) Å, β ) 124.1530(13)°, V ) 2249.28(12) Å3, Z ) 4, Dc ) 2.694 Mg/m3, µ(Mo
Communications KR) ) 7.350 mm-1, F(000)) 1728, GOF)1.039. A total of 7153 reflections were collected, and 2400 are unique (Rint ) 0.0338). R1 and wR2 are 0.0211 and 0.0520, respectively, for 202 parameters and 2238 reflections [I > 2σ(I)]. Crystal data of 1 and 3 are available as Supporting Information. (18) More than 1000 nets have been collected and described in detail at the Web site of the O’Keeffe group at Arizona State University; see http://rcsr.anu.edu.au/. (19) (a) Abram, U.; Dell’Amico, D. B.; Calderazzo, F.; Porta, C. D.; Englert, U.; Marchetti, F.; Merigo, A. Chem. Commun. 1999, 2053. (b) Pan, L.; Huang, X.; Li, J.; Wu, Y.; Zheng, N. Angew. Chem., Int. Ed. 2000, 39, 527.
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