A Novel Pillar-Layered Organic-Inorganic Hybrid Based on Lanthanide Polymer and Polyomolybdate Clusters: New Opportunity toward the Design and Synthesis of Porous Framework Jian Lu¨, Enhong Shen, Yangguang Li, Dongrong Xiao, Enbo Wang,* and Lin Xu Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China Received August 2, 2004;
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Revised Manuscript Received September 9, 2004
ABSTRACT: An unprecedented microporous lanthanum polymolybdate, [{La(H2O)5(dipic)}{La(H2O)(dipic)}]2{Mo8O26}‚ 10H2O (H2dipic ) pyridine-2,6-dicarboxylic acid) 1, which is constructed from lanthanum-organic coordination polymer sheets pillared by individual octamolybdate clusters, is reported. Self-assembly of porous metal-organic frameworks (MOFs) through the choice of metals and the numerous choice and design of ligands has attracted considerable attention since the 1990s.1,2 Most of the work on metalorganic frameworks employs selected or designed organic ligands with different dimensions as spacers to achieve porosity, which endows these solid materials with intriguing topologies and fascinating properties in new chemical separations, molecular selection, ion-exchange, and selective sensor materials.2,3 Typically, the as-synthesized porous frameworks can be divided into three groups, namely, metal-organic frameworks having multidimensional channels, pillar-layered architectures, and three-dimensional (3-D) nanotubular structures, as illustrated in Scheme 1. Among them, pillar-layered architectures, which have been proven to be an effective and controllable route to design 3-D frameworks with large channels, seized much research attention because of their distinct structural feature of changeable pillaring fragments in the interlamellar regions,4 especially when a burgeoning research target has recently been put forward on chemical design strategies to isolate desired novel porous frameworks through exploitation of functional secondary building blocks (SBUs) as spacers to create functional open frameworks.5 It soon become a very attractive research area because of the merits it possesses compared with the traditional methods: (i) the properties distinction between metalorganic polymer moieties and functional SBUs can effectively prevent the occurrence of lattice interpenetration, which is the most plaguing problem encountered in the preparation of porous MOFs; (ii) the resulting porous frameworks can stabilize the functional groups by isolating their spacial positions; (iii) immobilization of functional groups into porous hosts can enhance their functionalities.6 It should be noted that polyoxometalates (POMs),7 known as their potential applications in catalysis, biology, magnetism, nonlinear optics, and medicine,8 are an outstanding class of functional SBUs for the design and construction of interesting functional porous materials. What is more important, POMs have similar capabilities with organic ligands to coordinate to metal centers to achieve its combination with a metal-organic skeleton and display various discrete structures of definite nanoscopic sizes9,10 and diversified shapes which make them possible to be used as spacers to control the sizes of the cavities. On the other hand, lanthanide ions, in contrast to the widely used 3d-metals, generally adopt coordination num* To whom correspondence should be addressed. E-mail: wangenbo@ public.cc.jl.cn.
bers higher than six; therefore, they can become important choices in designing novel MOFs.11 Inspired by the aforementioned considerations, our current synthetic strategy is to acquire predictable functional porous MOFs via linking lanthanide-organic layers with POM pillars on the basis of rational design by solving these exigent problems below concerned with the integration of lanthanideorganic moieties and POMs: (i) the variable and large coordination numbers of 4f metals allow less prediction to design rational lanthanide complexes or polymers to construct target products; (ii) low stereochemical preferences of 4f metals limit their selective linkage with POM building blocks; (iii) the ratio between coordination numbers and volume of the organic components, which directly affect the extension of polymeric structures, along with their coordination fashions, are key factors for the integration of lanthanide-organic units and POM moieties. A remarkable multidentate ligand, pyridine-2,6-dicarboxylic anion (dipic2-), which has recently shown interesting properties in constructing extended structures with 4f metals,12,13 seized our attention. It possesses relatively high coordination numbers, small volume, and versatile coordination behaviors, which can cause appreciable polymeric metal-organic moieties fitting for combination POM subunits. By the introduction of this distinctive polycarboxylic ligands via hydrothermal technology, we got a novel compound, [{La(H2O)5(dipic)}{La(H2O)(dipic)}]2{Mo8O26}‚ 10H2O (H2dipic ) pyridine-2,6- dicarboxylic acid) 1, which is built from lanthanum-organic coordination polymer sheets pillared by individual octamolybdate clusters via covalent bonds forming a 3-D microporous framework with two types of channels, and the “guest” water molecules residing in the channels. To the best of our knowledge, no 3-D architectures based on lanthanide polymers and POM subunits have been documented to date. X-ray crystallography14 reveals that the structure of 1 is constructed from the cationic two-dimensional (2-D) coordination polymer sheets, [{La(H2O)5(dipic)}{La(H2O)(dipic)}]24+, pillared by anionic β-{Mo8O26}4- clusters into a 3-D microporous framework containing “guest” water molecules. The basic building block of compound 1 is shown in Figure 1. The well-known β-{Mo8O26}4- cluster1 consists of eight distorted corner- and/or edge-sharing {MoO6} octahedra with Mo-O distances in the range 1.682-2.452 Å and bond angles in the range 71.64-174.02°. The crystallographically unique La(1) atom, residing in a slightly distorted monocapped square antiprismatic environment, is defined by a nitrogen atom from dipic, three carboxyl oxygen atoms and five terminal aqua groups. The ten-coordinated La(2) links to one nitrogen atom of dipic, two oxygen atoms from two adjacent octamolybdate clus-
10.1021/cg049732j CCC: $30.25 © 2005 American Chemical Society Published on Web 10/08/2004
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Scheme 1.
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(a) Metal-Organic Frameworks Having Multidimensional Channels, (b) Pillar-Layered Architecture, (c) 3-D Nanotubular Structure
ters, six carboxyl oxygen atoms, and one aqua molecule to finish its bicapped square antiprismatic coordination environment. The La-N distances are 2.645 and 2.732 Å, respectively. The La(1)-O bond lengths vary from 2.518
Figure 1. ORTEP drawing of the fundamental building block of 1, showing the coordination environment around La and Mo with thermal ellipsoids at 50% probability. Only parts of atoms are labeled, and all the hydrogen atoms and the noncoordinated water molecules are omitted for clarity.
to 2.628 Å, while La(2)-O bonds possess one shorter 2.450 Å, one longer 2.836 Å exceeding the range of 2.517-2.665 Å. There are also two crystallographically independent dipic ligands adopting two different coordination modes (Scheme 2).
lanthanide dimers to form a regular two-dimensional (2D) network parallel to the ac plane with hexa-membered and tetra-membered ring arrays (see Figure 2). Interest-
Figure 2. View of the individual 2-D lanthanum-organic network in 1; H atoms are omitted for clarity.
ingly, the pyridine rings of the dipic ligands chelated to La(1) atoms extend into the hexa-membered rings formed by two La(1) atoms and two La(2) dimmers within the 2-D layers, whereas the pyridine rings of the dipic ligands chelated to La(2) project above and below the 2-D sheets. The extended structure is realized through the covalent bonding of terminal oxo-groups of the octamolybdate clusters to the La(2) sites of the dimmers (as shown in Figure 3). In a word, the 2-D sheets are pillared by the
Scheme 2 Coordination Modes of dipic Ligands in 1
The extension of the structure can be described as follows: two La(2) atoms link to each other by sharing two µ3-O atoms of dipic liangds to form a lanthanide binuclear dimer, then the lanthanide dimer bridges to six ninecoordinated La(1) atoms through the carboxyl groups of dipic ligands. Meanwhile, each La(1) atom connects three
Figure 3. Projection of the 3D framework down the c axis in 1; all the hydrogen atoms and the water molecules trapped into the channels are omitted for the sake of clarity.
β-{Mo8O26}4- subunits to form a pillar-layered framework having two types of channels along the a and c axis with dimensions of about 3.8 × 4.7 Å and 5.3 × 4.7 Å, respectively.15 Five crystallographically independent water
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molecules reside in the void of the pillaring regions. It is worth mentioning that the crystal lattice remains intact after removal of “guest” water molecules by heating the sample at 100 °C for 12 h, which can be proved by the powder X-ray diffraction patterns (see Figure S2, Supporting Information). In short, we have prepared and structurally characterized a novel 3-D pillar-layered organic-inorganic hybrid lanthanum polymolybdate, which features an open-framework structure constructed from lanthanum-organic polymer sheets pillared by octamolybdate clusters. The successful preparation of 1 indicates that use of POMs as functional SBUs is a feasible route for the construction of functional porous solid-state materials. The novelty represented by simultaneous occurrence of lanthanideorganic polymers and polyoxometalate clusters can dramatically lead to research efforts in the system of novel organic-inorganic hybrids. Future research may focus on attempting to explore the size effects of POM subunits on the reaction systems.
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Acknowledgment. The authors thank the National Natural Science Foundation of China (20371011) for financial support.
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Supporting Information Available: X-ray crystallographic files for compound 1 in CIF format, experimental details, TG spectrum, and X-ray powder diffraction patterns. These materials are available free of charge via the Internet at http://pubs.acs.org.
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[email protected]). The channel sizes are estimated from the van der Waals radii of constituent atoms.
CG049732J
