DOI: 10.1021/cg901312t
Metal-Organic Framework Based on [Zn4O(COO)6] Clusters: Rare 3D Kagome Topology and Luminescence
2010, Vol. 10 44–47
Qi Yue, Qian Sun, Ai-Ling Cheng, and En-Qing Gao* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China Received October 22, 2009; Revised Manuscript Received November 28, 2009
ABSTRACT: A 2-fold interpenetrated metal-organic framework displaying intense yellow fluorescence is constructed from [Zn4O(COO)6] clusters and a new dicarboxylate ligand bearing the fluorene chromophore fluorene-2,7-dicarboxylic acid. The [Zn4O(COO)6] secondary building unit exhibits unusual coordination features and leads to the rare 3D pillared kagome net topology (324865). Metal-organic frameworks (MOFs) have attracted intense attention for their potential applications.1 Much work has been devoted to the rational design of MOFs with specific structures and topologies.2 A very effective design strategy is to replace the “vertices” in a network by metal-carboxylate clusters as SBUs (secondary building units) connected by multicarboxylate ligands as linkers.3 Perhaps the most eminent SBU is the [Zn4O(COO)6] cluster, which acts as a 6-connected octahedral vertex, and particularly, the combination of such SBUs with different linear dicarboxylate ligands has afforded a large family of isoreticular frameworks (IRMOFs) with the same primitive cubic (pcu) topology but with tunable pore sizes, shapes, and functionalities.3b In fact, the majority of 3D MOFs with 6-connected vertices adopt the pcu net (the “default” net).2c The pcu net can be viewed as 4-connected 44 2D subnets being pillared in a third direction. This consideration is very useful from the viewpoint of net design. New topologies may be predicted if the intersheet pillaring fashion is changed. For instance, Champness et al. have analyzed some 3D nets derived by pillaring 44 subnets.2d We recently demonstrated that the crossing of the pillars between 44 subnets led to a rare 6-connected 3D net.4 Another approach to derive new topologies is to replace the 44 subnets by other 2D nets. If constricted to 4-connected vertices, there are three basic uniform Archimedean 2D nets built of regular polygons, including the Kagome net (triangles and hexagons) and the “bounce” net (triangles, squares, and hexagons), besides the 44 net (squares).5 Solid networks with the Kagome topology have evoked great interest for their interesting magnetic properties.6 Interesting 3D nets could be constructed by pillaring Kagome nets. However, MOFs with such topologies are still rare.7 Here, we report a new MOF (1) derived from fluorene-2,7dicarboxylic acid (H2FDC). Although a number of dicarboxylic acids with different rigid spacers (e.g., benzene, naphthalene, biphenyl, stilbene) have been used for the design of MOFs, the rather simple H2FDC ligand has not been explored, which contains the fluorene chromophore. Fluorene compounds have a wide variety of applications in photophysics and photochemistry.8 Notably, the connection of the octahedral [Zn4O(COO)6] SBUs by the fluorene spacers leads to a rare pillared kagome net for 1, instead of the default pcu net. The N2 adsorption and luminescent properties were examined. Compound 1 was obtained by the solvothermal reaction of Zn(OAC)2 with H2FDC9 in DMSO and ethanol,10 and it was characterized by single-crystal X-ray analyses.11 The compound is formulated as [Zn(DMSO)6]1/6[(Zn4O)(FDC)3(DMSO)3/2*To whom correspondence should be addressed. E-mail: eqgao@chem. ecnu.edu.cn. pubs.acs.org/crystal
Published on Web 12/10/2009
(OH)1/3(H2O)1/6][(Zn4O)(FDC)3(DMSO)2] 3 15DMSO 3 17H2O.11 The structure contains two independent tetranuclear clusters, [Zn4(μ4-O)(COO)6(DMSO)2] and [Zn4(μ4-O)(COO)6(DMSO)3/2(OH)1/3(H2O)1/6]-1/3. In the former cluster (Figure 1), two Zn ions (Zn2 and Zn3) reside in tetrahedral environments, each coordinated by three carboxylate oxygen atoms from different FDC2ligands and a μ4-O atom at the center of the cluster, and the coordination of the Zn1 ion is similar but with an additional oxygen atom from a DMSO molecule, affording a five-coordinated trigonal bipyramidal geometry. The Zn4 ion adopts an octahedral coordination geometry surrounded by the μ4-O atom, three carboxylate oxygen atoms, and another two oxygen atoms from DMSO molecules. Each pair of neighboring Zn ions is bridged by a syn-syn carboxylate group and a μ4-O atom, and there is an additional μ2-O bridge from DMSO between Zn1 and Zn4 ions. The four Zn ions form a distorted tetrahedron around μ4-O, and the distances from μ4-O to Zn range from 1.925 to 1.974 A˚, and the distances of Zn 3 3 3 Zn range from 3.105 to 3.245 A˚, which fall in the usual range for similar clusters.3b The Zn-O-Zn angles for μ4-O are revealed to range from 105.54° to 113.12° and deviate slightly from the ideal value for the tetrahedral [Zn4O] motif. The [Zn4(μ4-O)(COO)6(DMSO)3/2(OH)1/3(H2O)1/6]-1/3 cluster is very similar to the former, but the μ2-O bridge comes from disordered DMSO, H2O, and OH- with occupancies of 1/2, 1 /6, and 1/3, respectively (see the Supporting Information). We note that the clusters in 1 are different from the usual [Zn4O(COO)6] clusters in the previous IRMOF-n series.3 In these previous series, all Zn ions are tetrahedrally coordinated by three carboxylate oxygens and a central μ4-O atom, as found for Zn2 and Zn3 in 1, and each pair are doubly bridged by a μ4-O atom and a μ2-carboxylate, with no solvent molecules involved in coordination. In 1, however, the solvent molecules are incorporated into a cluster as both bridging and terminal ligands, leading to distortion of the cluster from tetrahedral. A similar cluster, [Zn4(μ4-O)(COO)6(DMF)2], has been recognized in a MOF with 1,10 -binaphthyl-derived dicarboxylate ligands,12 where the DMF molecules take the place of DMSO in 1. As usually recognized, the tetranuclear clusters act as 6-connecting octahedral SBUs, and the connection of such SBUs by FDC2- ligands in the μ4-bis(bridging) mode yields a 3D framework. However, the net topology of 1 is different from the default pcu net observed for the IRMOF-n series. Topological analyses on 1 suggest a kag net with Schl€ afli symbol 324865, in which adjacent 2D Kagome sheets are pillared in an eclipsing fashion (Figure S2). To the best of our knowledge, this topology is very rare in the MOF regime, albeit some 3D systems containing Kagome sheets have been reported.13 A previous example of kag is [Zn2(BDC)2(DABCO)] (BDC = 1,4-benzenedicarboxylic acid, r 2009 American Chemical Society
Communication
Figure 1. View of the [Zn4(μ4-O)(COO)6(DMSO)2] cluster in 1.
Figure 2. Views showing the evolution of the 3D kag net (e) and the default pcu net (f) from the 2D kag subnet (c) and the 44 subnet (d) constructed by the Zn4 SBU (a, b) in 1 and IRMOF-1, respectively. The intra- and interlayer linkers are colored in blue and purple, respectively.
DABCO = 1,4-diazabicyclo[2.2.2]octane), in which 2D Kagome networks based on paddle-wheel [Zn2(COO)4] clusters are pillared by DABCO.7b 1 represents the first example of 3D pillared Kagome nets based on [Zn4O(COO)6] clusters, in which the intraand interlayer linkers are chemically identical. It is worthwhile to make a comparison between the kag (in 1) and pcu nets based on similar 6-connected SBUs. For convenience, we distinguish two kinds of spacers between the octahedral SBUs: the intralayer linker (blue rods in Figure 2) and the interlayer pillars (purple rods). It should be noted that the two kinds of linkers are topologically identical in a pcu net but
Crystal Growth & Design, Vol. 10, No. 1, 2010
45
independent in a 1 net. The difference between kag and pcu arises from the intralayer connection, which is closely related to the relative orientations of the intralayer linkers. In pcu, the intralyer linkers around each SBU are orthogonal, and this naturally leads to a 2D square net. By contrast, as clearly shown in Figure 2c, the intralayer linkers in 1 deviate significantly from the orthogonal positions with alternating small (close to 60°) and large (close to 120°) angles between neighboring linkers. Such an arrangement of linkers is in favor of the Kagome net. The deviations of the linkers are mainly related to the incorporation of DMSO molecules into the clusters. As shown in Figure 2a, the S(CH3)2 moieties are inserted between two interlayer linkers and thus push the linkers apart from each other. Interestingly, the previous compound based on similar [Zn4(μ4-O)(COO)6(DMF)2] clusters adopts the common pcu net.12 Close inspection (Figure S3) suggests that the N(CH3)2 groups of the DMF molecules in the previous compound are displaced out of the layer plane and far from the interlayer linkers, imposing little influence on the interlinker angle. The single 3D framework of 1 possesses a large interconnected void space. The hexagonal window of the layer has an average diagonal separation of about 30.22 A˚, measured between two nearest Zn ions from diagonal clusters, and an average edgeto-edge separation of about 26.17 A˚, measured between two nearest Zn ions from opposite edges. The triangular window has a lateral length of about 14.44 A˚, measured between the nearest Zn ions from different clusters. To occupy the space, a pair of identical frameworks, related by 3 symmetry, is mutually interpenetrated with each other (Figure 3). The layers from the two nets are not equally separated, with a short interlayer distance of 5.83 A˚ and a long one of 11.13 A˚ (Figure S4). As a result of the interpenetration, half of the triangular windows from the two nets are stacked in a staggered fashion, and the other half of the triangular windows of one net are placed over the hexagonal windows of the other net. The interpenetration reduces the empty space, but there are still remaining large irregular voids that are interconnected through windows or apertures. PLATON analysis gave a total effective free volume of 42692.5 A˚3 per cell (65.3% of the crystal volume. The coordinated DMSO molecules were taken as framework components in the calculations). The voids are occupied by [Zn(DMSO)6]2þ ions and solvent molecules (15 DMSO and 17 water molecules per formula11). The solvated Zn ion has a 3 symmetry and is sandwiched between the two staggered triangular windows from different nets. TG and powder X-ray diffraction analyses on 1 suggested that the loss of the DMSO and H2O molecules upon heating at 150 °C under vacuum leaves an apparently amorphous phase (for details, see Figure S5 in the Supporting Information). The sample obtained by exchanging the guest molecules with acetone or dichloromentane remains crystalline but also becomes amorphous upon evacuation. However, N2 sorption measurements indicated that the evacuated material still retains some degree of porosity. The material after acetone exchange and room-temperature evacuation exhibits a Langmuir surface area of 457.3 m2/g, with an estimated all pore volume of 0.179 cm3/g (Figure 4). The photoluminescence spectra of 1 and the sodium salt of the ligand (Na2FDC) were measured, as shown in Figure 5. The sodium salt exhibits a relatively weak blue emission band at λmax = 450 nm upon excitation at λex = 315 nm, due to the fluorene moiety. This band is quenched in 1, which emits bright yellow light upon ultraviolet radiation and displays an intense emission band at λmax = 553 nm. The excitation scan suggests the luminescence intensity is maximized around λex = 369 nm. It has been testified that the [Zn4O(COO)6] cluster in IRMOF-1 (with BDC as linkers) behaves as a ZnO-like quantum dot (QD) and that efficient energy transfers from the BDC linker as a photon trap to the cluster as a photon emitter.14 This may also occur in 1. The emission of 1 is red-shifted by about 28 nm with respect to
46
Crystal Growth & Design, Vol. 10, No. 1, 2010
Yue et al.
Figure 3. The 2-fold interpenetration of the 3D kag frameworks in 1 viewed along the c axis.
fluorene chromophores. The framework exhibits the 3D pillared kagome topology, which is unusual for the well-known octahedral SBUs. It displays intense yellow fluorescence and has potential optical applications. Acknowledgment. This work is funded by NSFC (Grant 20771038) and the Shanghai Leading Academic Discipline Project (B409). Supporting Information Available: Supplementary crystallographic details, TGA data, XRPD data, and an X-ray crystallographic file in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.
References Figure 4. N2 sorption isotherm measured at 77 K. Adsorption data are shown as closed patterns and desorption as open patterns. Activation of the porous materials was achieved by exchanging the included solvent molecules with acetone and then removing this labile guest by evacuation ( 2σ(I)], GOF = 1.006, reflns collected/unique 134839/ 27032 [Rint = 0.1746], data/params 27032/1200. Diffraction data were collected on a SATURN70 CCD area detector equipped with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). The crystals scatter very weakly even at low temperature. The best diffraction data set after several attempts is still of limited quality. The structure has a large fraction of voids containing a number of residual density peaks, which may be attributed to heavily
Crystal Growth & Design, Vol. 10, No. 1, 2010
(12) (13)
(14) (15)
47
disordered solvent molecules but could not be satisfactorily modeled. The SQUEEZE routine in the PLATON software package15 was applied to subtract the solvent contribution. According to the crystallographic analysis, elemental analytic data, IR spectra, and TGA analyses (see the Supporting Information), the compound is formulated as [Zn(DMSO)6]1/6[(Zn4O)(FDC)3(DMSO)3/2(OH)1/3(H2O)1/6][(Zn4O)(FDC)3(DMSO)2] 3 15DMSO 3 17H2O. Kesanli, B.; Cui, Y.; Smith, M. R.; Bittner, E. W.; Bockrath, B. C.; Lin, W. Angew. Chem., Int. Ed. 2005, 44, 72–75. (a) Barthelet, K.; Marrot, J.; Ferey, G.; Riou, D. Chem. Commun. 2004, 520–521. (b) Mahata, P.; Sen, D.; Natarajan, S. Chem. Commun. 2008, 1278–1280. (c) Volkringer, C.; Meddouri, M.; Loiseau, T.; Guillou, N.; Marrot, J.; Ferey, G.; Haouas, M.; Taulelle, F.; Audebrand, N.; Latroche, M. Inorg. Chem. 2008, 47, 11892–11901. Bordiga, S.; Lamberti, C.; Ricchiardi, G.; Regli, L.; Bonino, F.; Damin, A.; Lillerud, K.-P.; Bjorgen, M.; Zecchina, A. Chem. Commun. 2004, 2300–2301. Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
