DOI: 10.1021/cg100843k
Lanthanide-Organic Framework of a Rigid Bis-Gd Complex: Composed by Carbonate Ions Spacers
2010, Vol. 10 4235–4239
Chidambaram Gunanathan,† Yael Diskin-Posner,‡ and David Milstein*,† † Department of Organic Chemistry, and ‡Unit of Chemical Research Support, The Weizmann Institute of Science, Rehovot 76100, Israel
Received June 25, 2010; Revised Manuscript Received August 15, 2010
ABSTRACT: A new lanthanide-organic framework (LOF) based on a long (21.2 A˚) and rigid bis-Gd complex synthon (complex 1)
and carbonate anion spacers was discovered. Complex 1 is comprised of two Gd3þ ions complexed with a pyridine tetracarboxylate ligand and a bridging linear 1,4-diethynylbenzene unit. Two water molecules are coordinated to each Gd ion in order to satisfy its high coordination number. Upon crystallization of 1 in ethanolic aqueous sodium bicarbonate, the carbonate anions replace the coordinated water molecules, partially or fully, resulting in the construction of two network types (type A and B). All three oxygen atoms of the carbonate anion are utilized in its function as a spacer between two synthons (complex 1) in the networks, but the chelating oxygen atoms (to Gd3þ ions) and their coordination modes differ in each of the two networks. In addition, the carbonate anions also serve as acceptors for hydrogen bonding (O-H 3 3 3 O) with water molecules coordinated to Gd3þ ions in both the networks. While all the synthons adopt a planar conformation in network A, they are backed alternatingly in plane and vertical conformation in network B. Overall, in this new LOF the synthons are packed in cris-cross fashion and tethered by carbonate ion spacers, creating a crystal lattice perforated by 10.6 15.6 A˚ and 18.6 15.6 A˚ wide-open channels. Molecular self-assembly leading to metal-organic frameworks (MOFs) is an attractive approach to the design and development of new supramolecular systems with specific properties.1 This process involves recurring packing patterns adapted by certain functional groups, and the resulting new materials have found applications in various fields such as selective catalysis,2 optoelectronics,3 and gas storage.4 Despite the exponential growth in MOFs, lanthanide-organic framework (LOF) have had limited success thus far, owing to the higher coordination numbers and the versatile chemistry associated with lanthanides, leading to complex molecular solids that are difficult to crystallize. However, recent successes in the construction of LOFs based on well established principles of supramolecular design have provided new materials.5 LOFs are of great interest due to their unique physical and chemical properties, structural flexibility, thermodynamic stability, and magnetic and photoluminescent properties with interesting applications.5,6 In order to satisfy the higher coordination number, LOFs predominantly contain ancillary ligands such as water molecules. High-spin gadolinium(III) complexes have displayed an assortment of applications as contrast agents for MRI.7 A few years ago, we developed Gd3þ complexes of a pyridine tetracarboxylate ligand (embedded with 17R-ethynyl estradiol, [estradiol-17Rylethynyl(pyridin-2,6-diyl)bis(methylenenitrilo)]tetrakis(aceticacid)) as selective MRI contrast agents for human breast cancer cells.8 This ligand system was further extended to form the rigid bis-Gd3þ complex (1, Figure 1) in a study that demonstrates the potential use of such systems as spin labels for EPR experiments.9 Upon crystallization of the bis-Gd3þ complex 1, a novel LOF with large pore sizes was obtained. Although metal carbonates have been well studied, thus far there are only a few metal and lanthanide carbonate networks known, and the carbonate ions were used as building blocks.10 We are not aware of systems in which carbonate anions function as spacers of metallo-organic coordination complexes to create a MOF. Herein, we report that the rigid bis-Gd3þ complex 1 forms a framework with pore sizes of 10.6 15.6 A˚ and 18.6 15.6 A˚ in which the bisGd3þ synthons are tethered by carbonate ion spacers.
Figure 1. Bis-Gd3þ complex 1.
*To whom the correspondence should be addressed. E-mail: david.
[email protected]. Tel: 972-8-9342599. Fax: 972-8-9344142.
The bis-Gd3þ complex 1 is composed9 of two units of a pyridine-based tetracarboxylate gadolinium complex, covalently bound to 1,4-diethynylbenzene. Each Gd3þ ion is coordinated to three nitrogen atoms and four carboxylate oxygen atoms, and the remaining two coordination sites are occupied by two water molecules. Dissolving complex 1 in an ethanolic aqueous sodium bicarbonate solution yielded crystals suitable for X-ray diffraction studies which revealed that a new LOF is formed, in which complex 1 acts as the basic synthon unit (repetitive building block) and the carbonate ions function as spacers. Two water molecules coordinated to Gd3þ ions (of complex 1) are partially or fully replaced by CO32- ions in two different modes of coordination, leading to two types of networks (types A and B). In addition to the differences arising from coordinating carbonate ions and water molecules, the synthons are packed in different combinations in each direction of the network. The detailed ball illustration and simplified schematic representations of the coordination polymer A and B (Figure 2) clearly depicts the different conformations adopted by the synthons, carbonate spacers, and water molecules. In the type A network of coordination polymer, all three oxygen atoms of each carbonate ions are coordinated with Gd3þ ions. While the two anionic oxygen atoms of CO32- are coordinated to two Gd3þ ions of different synthons, the third oxygen atom (CdO of CO32-) is also chelated to one of the Gd3þ ions of a synthon unit in the chain (two free coordination sites left on the Gd3þ ion are occupied, leaving no free site for coordination of water molecules). One free coordination site left on the other Gd3þ ion is occupied by a water molecule. As a result, the
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Figure 2. Ball and schematic representations of the synthons in LOF (Gd - purple; O - red; C - black; N - blue; hydrogen atoms are not drawn for clarity). Blue dashed lines mark metal ligand coordination bonds and hydrogen bonds. (a) Network A. (b) Network B.
water molecules are coordinated to alternating Gd3þ ions (i.e., one water molecule per synthon), and they occupy the same (cis to each other) conformation throughout the networks. All the synthons in network type A adopt a planar conformation (Figure 2a). In the type B network, all three of its oxygen atoms are coordinated to two Gd3þ ions of different synthons, as described in network A. However, unlike in type A network, where the chelation by CO32- occurs alternatively on each Gd3þ ion, in type B chelation by CO32- takes place alternatingly on each synthon. Thus, when two anionic oxygen atoms of CO32- are chelated to one Gd3þ ion, another Gd3þ ion of the same synthon also exhibits similar chelation by a CO32- spacer. The third CdO oxygen atom of CO32- is coordinated to a Gd3þ ion of the next synthon, which leaves one coordination site free on Gd3þ ions, occupied by a water molecule. While similar coordination modes by water molecules and CdO oxygen of CO32- spacers are observed with both the Gd3þ ions of a single synthon, the water molecules (on both Gd3þ ions of a single synthon) adopt trans conformation relative to each other. Note that in network B the alternating synthons also differ in their conformations, as schematically shown in Figure 2b. The differences in the synthon conformations are also apparent in the central phenyl ring conformations. These differences arise from the content of the asymmetric unit cell. There is one complete synthon of network A, and two half synthons (having only one Gd3þ unit) of network B. In both networks A and B the water molecules coordinated to Gd3þ are also involved in hydrogen bonding with the carbonate spacer, as described in Figure 3. In addition, the indicated C-O bond lengths of the carbonate spacers in Figure 3 corroborate the differences in the chelating oxygen atoms of CO32- with Gd3þ ions in networks A and B. The bis-Gd3þ complex is 21.2 A˚ long (interatomic distance between two Gd3þ ions in a single synthon) (see Table 1) and possesses a nonpolar aromatic ring at the center and polar pyridine tetracarboxylate units complexed to Gd3þ ions at the ends. The polar carboxylate ends are enveloped by external water molecules and eight sodium ions, which are bonded in a complex pattern as shown in Figure 4.
Figure 3. Coordination modes of CO32- spacers and water molecules with Gd3þ ions, and hydrogen bonding between water and carbonate spacers, found in networks A and B. Bond lengths of C-O in carbonate spacers are indicated.
The distances between the Gd3þ ions of a single synthon differ slightly in each of the networks, as described in Table 1. The crystal structure is an organized and well-defined architecture, with a very long synthon (over 20 A˚), many diverse atoms, together with low crystallographic symmetry. Although there is a wide number and variety of different counterions, all these counterions find a designated place in the overall structure. The overall structure contains void channels with 38% of the crystal volume being solvent accessible, containing disordered water molecules. There are massive barriers composed of the Gd3þ ion cores wrapped with carboxylates, water molecules, carbonate spacers, and sodium ions. These barriers are about
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Table 1. Gd-Gd Distances in Single Synthons According to Calculation in SHELXL synthon type
atom name 1 (symmetry)
atom name 2 (symmetry)
bond distance (A˚)
error (A˚)
Network A Network B1 Network B2
Gd1 (x, y, x) Gd65 (x, y, z) Gd97 (x, y, z)
Gd64 (x, y, z) Gd65 (1-x, -y, 1-z) Gd97 (1-x, 1-y, 1-z)
21.2135 21.2462 20.9674
0.0007 0.0009 0.0009
Figure 4. Ball presentation of the complex supramolecular arrangement that the external eight sodium ions form with water molecules to bridge between the different synthons in the (a) ac plane, (b) ab plane. Thin dashed blue lines mark hydrogen bonds and metal ion coordination interaction with water molecules, CO32- ions and carboxylates. For clarity, the bis-Gd3þ complex is cut in the middle and Na43 is omitted from (a). (Gd - purple; Na - yellow; O - red; C - black; N - blue; hydrogen atoms are not drawn for clarity).
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Figure 5. Ball presentation of the overall structure. (Gd - purple; Na - yellow; O - red; C - black; N - blue; hydrogen atoms are not drawn for clarity). (a) The ab plane. The angle between network A and B in this plane is 38.2°. Na45 is hiding just behind the point where synthon A and B appears to cross each other’s way. (b) The ac plane. The angle between networks A and B in this plane is 30.3°. Thin dashed blue lines mark hydrogen bonds and metal ion coordination interaction with water molecules, CO32- ions, and carboxylates. (c) The ac plane, but the atom Na45 is buried deep inside the barrier (as is in 17.5% of the cases) leaving the channels 18.6 15.6 A˚ wide open.
Communication 11.2 A˚ wide and are connected by thin bars made of the aromatic lipophilic part of the synthon. The thin bars of the synthons are packed criss-cross in the crystal as shown in Figure 5a. Networks type A and B interwinds inside this layer of bridging water and sodium ions and extend diagonally relatively to each other. The distance between two parallel bars is 8.7 A˚. The criss-cross packed nonpolar thin bars give rise to hollow channels in the structure that are 10.6 15.6 A˚ wide-open at the narrowest point and are present throughout the crystal (Figure 5b). Two sodium ions (Na45 and it is symmetry related atom), each bonded to two water molecules, are present in every channel, which blocks the channels to some extent. The Na45 is also splits into two positions; 82.5% of the time it occupies the space in the channel as shown in Figure 5b. However, 17.5% of the time it is buried inside the counterion mass in a small empty pocket, leaving a void 18.6 15.6 A˚ wide in the channel (Figure 5c). Turning the crystal view in about 90° and looking at the ab plane allows us to notice the triangular channels with a base size of about 6.2 A˚ and height 15.5 A˚. The crystal lattice of this material is perforated by wide-open channels, which propagate in all directions. Yet, in spite of the fact that the bis-Gd3þ complex 1 occupies only 62% of the crystal volume (water molecules that were not coordinated directly to either Naþ or Gd3þ counterions were excluded), the presence of no other species intercalated between the bis-Gd3þ complex 1 networks could be identified from the diffraction data. Only very diffuse residual electron density observed in the open galleries provide evidence for their partial occupation by disordered solvent. The structural refinement of this “hollow” crystalline system converged at a relatively low R1 value of 14.1%, which confirms a high degree of certainty for this determination. The very loose crystal packing is also reflected in the rotational disorder of the phenyl ring at the center of the ligand molecule, about the main ligand axis, as well as in the relatively large thermal displacement parameters of some of the ligand carboxylate oxygens. In summary, the bis-Gd3þ complex 1 creates a very complex supramolecular structure formed by connecting equivalent units of synthon 1 through two types of coordination modes exhibited by carbonate ion spacers. The crystal lattice of the material is perforated by wide-open channels, which propagate between thin bars of synthon 1 in different directions in a criss-cross fashion. The key features to this successful construction of novel LOF turned out to be the use of carbonate ions that function as spacers in conjunction with a long, rigid bis-Gd synthon. Acknowledgment. This research was supported by the Israel Science Foundation, the MINERVA Foundation, and the Helen and Martin Kimmel Center for Molecular Design. D.M. is the Israel Matz Professor. Supporting Information Available: Experimental procedures and X-ray data (CIF). This information is available free of charge via the Internet at http://pubs.acs.org.
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