Reactive Microporous Rare-Earth Coordination Polymers that Exhibit Single-Crystal-to-Single-Crystal Dehydration and Rehydration Adonis Michaelides* and Stavroula Skoulika*
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 529-533
Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece Received August 5, 2004;
Revised Manuscript Received December 8, 2004
ABSTRACT: Two isostructural coordination polymers of formula [Ln2(ADC)3(H2O)6]‚2H2O, (ADC ) acetyledicarboxylato dianion, Ln3+ ) La3+ or Ce3+) have been prepared at room temperature. The structure is three-dimensional, possessing one-dimensional channels filled with lattice and coordinated water molecules. The presence of three water molecules in the coordination sphere of the metal creates a dense hydrogen-bonded network. The lattice water molecules were removed from the cerium compound at 100 °C with retention of the structure. This transformation was also possible in a crystal-to-crystal manner. The structure of the dehydrated solid was determined and found to present only minor modifications compared to the parent structure. The departure of the coordinated water molecules is carried out in three steps, with retention of the structure up to 150 °C. Beyond this temperature, dehydration is accompanied by polymerization, due to the existence of an infinite chain of short acetylene-acetylene contacts. Introduction The construction of microporous metal-organic frameworks (MOFs) attracts considerable interest in view of potential applications in separation science,1 catalysis,2 and gas storage.3 The assembly of these materials is usually conducted by using d-block metal ions and multifunctional organic ligands.4 The analogous chemistry of rare-earth ions has been developed in the last few years, mainly by using polycarboxylate ligands.5 Indeed, the ability of the carboxylate group to bridge adjacent metal centers is expected to enhance the stability of the crystal framework, which is of utmost importance in this class of materials. In addition, the chelate coordination mode of the carboxylate group may also contribute to increase the framework stability.6 Finally, the concept of hard acid/hard base was invoked recently by James4c to explain the high decomposition temperatures observed in some rare-earth carboxylates. In previous work, we used some aliphatic acids5b,7 and fumaric acid8 in an attempt to construct robust microporous rare-earth carboxylate polymers. In most cases, the crystal framework was severely deformed upon departure of the solvent (water), resulting in narrowing of the channels. In this context, the choice of a ligand such as acetylenedicarboxylic acid (H2ADC) seems quite reasonable since the presence of a carboncarbon triple bond would confere additional stability to the framework. Moreover, as many organic ligands are usually arranged around a rare-earth cation, the probability of observing short acetylene-acetylene contacts increases. In this case, it is possible to observe “in situ” polymerization along the chain of the short contacts, with formation of novel conjugated polymers.9 In this work, we synthesized two coordination polymers of acetylenedicarboxylic acid (H2ADC), with La3+, 1, and Ce3+, 2. Crystal structure analysis showed that * To whom correspondence should be addressed.
[email protected] (S.S.);
[email protected] (A.M.).
E-mail:
these compounds are isostructural to the Gd3+ complex with H2ADC, described by Yan et al.,10 in a succinct structure report. In the present work, we describe in detail the crystal packing of these phases. In the case of compound 2 we also studied its unique thermal behavior including single-crystal-to-single-crystal dehydration and solid-state polymerization. Experimental Section [La2(ADC)3(H2O)6]‚2H2O (1). An aqueous solution of 0.35 M lanthanum nitrate was allowed to diffuse in a silica gel column containing 0.5 M H2ADC at pH ) 6. Crystals suitable for X-ray analysis were formed after about five months. Elemental analysis calcd (%) for 1: C 19.00, H 2.11; found: C 19.11, H 2.23. [Ce2(ADC)3(H2O)6]‚2H2O (2). Crystals suitable for X-ray analysis were prepared as described for 1. Elemental analysis calcd (%) for 1: C 18.94, H 2.10; found: C 19.12, H 2.28. Crystals were also prepared by mixing 30 mL of an aqueous solution of 0.25 M H2ADC (pH ) 6) with 50 mL of an aqueous solution of 0.25 M cerium nitrate. The resulting solution was distributed in small Petri dishes. Many crystals were formed within only a few days and were identified from XRPD data. Physical Measurements. Combined TG/DTA analysis was performed on a Schimadzu 60 apparatus under an air stream (5 mL/min). FT-IR spectra were recorded on a Perkin-Elmer spectrum GX FT-IR spectrophotometer, using the KBr technique. UV-Vis spectra were obtained on a Jasco UV/Vis/NIR V570 spectrophotometer. X-ray Crystallography. The X-ray diffraction intensities were collected at room temperature on a Bruker P4 diffractometer, employing graphite monochromated MoKR radiation (λ ) 0.71073 Å). The structure was solved by direct methods and refined on F2 using SHELXL97.11 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were located from difference Fourier maps and refined isotropically. XRPD data were collected at room temperature on a Bruker D8 Advanced system with monochromatic CuKR radiation (λ ) 1.54056 Å). SCHAKAL-97 software12 was used for the drawings.
Results and Discussion Crystal Structure Analysis of 1 and 2. The two compounds are isostructural. The drawings shown are
10.1021/cg049725v CCC: $30.25 © 2005 American Chemical Society Published on Web 02/08/2005
530
Crystal Growth & Design, Vol. 5, No. 2, 2005
Michaelides and Skoulika Table 2. Coordination and Bond Lengths (Å) for Compounds 1-3
Figure 1. View of a part of the structure of compound 2 with the atom labeling scheme.
exclusively for compound 2. Each metal cation is coordinated by nine oxygen atoms, belonging to five different organic ligands and three water molecules. The structure is characterized by the formation of infinite metal chains containing two types of metal “dimers”, one linked exclusively by bridging carboxylate groups (dimer 1) and the other linked exclusively by chelatingbridging carboxylate groups (dimer 2) (Figure 1). Alternate metal “dimers” with the same type of linking were observed previously in rare-earth adipates.7 For compound 1, the metal-metal distance is 4.961 Å (dimer 1) and 4.522 Å (dimer 2). The corresponding distances for compound 2 are 4.981 and 4.480 Å. The crystallographic data and the coordination geometry are given in Tables 1 and 2, respectively. The La-oxygen distances vary from 2.467 to 2.720 Å, while the Ce-oxygen ones vary from 2.438 to 2.693 Å. These distances are close to those encountered in the similarly linked nine-coordinated rare-earth adipates.5b,7 There are two crystallographically independent acetylenedicarboxylato dianions, of which one is centrosymmetric (ligand A) and acts exclusively in the chelatebridging mode of coordination. This ligand is planar and roughly perpendicular to the bc plane (Figure 2). The noncentrosymmetric ligand (ligand B) binds with one carboxylate group in the bridging mode and with the other in the monodentate mode. The “free” oxygen atom (O4 in Figure 1) is engaged in hydrogen bonding as explained below. While the various bridging or chelating bridging modes are common in rare-earth polymers, the
M-O(6) M-O(5) M-O(3) M-O(1) M-O(7) M-O(8) M-O(9) M-O(2) M-O(1)i O8-H‚‚‚O9ii O7-H‚‚‚O3i O9-H‚‚‚O4 O9-H‚‚‚O10iii O9-H‚‚‚O2iii O7-H‚‚‚O10 O8-H‚‚‚O4iv a
1 M ) La
2 M ) Ce
3 M ) Ce
2.467(3) 2.503(3) 2.529(3) 2.537(2) 2.547(3) 2.560(3) 2.594(3) 2.642(3) 2.720(2) 2.795(4) 2.807(4) 2.792(4)a 3.009(6)a 2.831(4) 2.752(6) 2.824(4)
2.438(4) 2.474(3) 2.501(3) 2.513(3) 2.517(4) 2.535(4) 2.565(3) 2.619(3) 2.693(3) 2.798(5) 2.804(5) 2.777(5)a 3.012(7)a 2.843(5) 2.750(7) 2.845(5)
2.446(8) 2.486(9) 2.488(10) 2.526(8) 2.523(9) 2.519(7) 2.575(8) 2.618(6) 2.716(7) 2.796(12) 2.768(12) 2.738(12)b 2.779(12)b 2.757(12)
b
Bifurcated H-bonds. Only the hydrogen atoms of O9 have been found. Symmetry codes: i-x, -y, -z; ii-x, -y, -z + 1; iii-x + 1, -y, -z + 1; ivx, y + 1, z.
Figure 2. Projection of the structure of compound 2 along the a axis.
monodentate mode is rarely observed. Channels filled with lattice and coordinated water molecules are formed along the a axis (Figure 2). It is interesting to note that, for each metal, two of the coordinated water molecules point into the channels, while the third one lies between the metal centers. This difference will have some consequences on the thermal behavior of these materials, as will be described below. No channels with appreciable size are found along the two other directions
Table 1. Crystal and Structure Refinement Data for Compounds 1-3 formula crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z T (K) dx (g cm-3) Ntot/Nind R1 (I > 2σ(I)) wR2 (I > 2σ(I)) GOF Fres (e Å-3)
1
2
3
[La2(C4O4)3(H2O)6]‚2H2O triclinic P1 h 8.307(1) 8.769(1) 9.204(1) 95.63(1) 115.61(1) 110.26(1) 542.31(9) 2 293 2.321 2017/1873 [R(int) ) 0.0191] 0.0203 0.0532 1.131 0.668
[Ce2(C4O4)3(H2O)6]‚2H2O triclinic P1 h 8.304(1) 8.728(1) 9.177(1) 95.59(1) 115.70(1) 110.26(1) 537.59(11) 2 293 2.349 2292/1866 [R(int) ) 0.0241] 0.0254 0.0636 0.924 0.753
[Ce2(C4O4)3(H2O)6] triclinic P1 h 8.279(2) 8.731(3) 9.098(3) 94.83(4) 115.54(1) 111.93(2) 526.3(3) 2 293 2.286 1241/1241 0.0432 0.1096 1.080 1.184
Reactive Microporous Rare-Earth Coordination Polymers
Figure 3. A chain of acetylenedicarboxylato ligands (A centrosymmetric and B noncentrosymmetric) of compound 2 presenting short acetylene-acetylene contacts. The values in parentheses refer to the evacuated compound 3.
Crystal Growth & Design, Vol. 5, No. 2, 2005 531
Figure 5. TG (solid line)/DTA (dotted line) curves for compound 2.
Figure 6. XRPD patterns of compound 2 at (a) room temperature, (b) 100 °C, (c) 130 °C, (d) 150 °C.
Figure 4. Part of the structure of compound 2 showing the hydrogen-bonded network.
of space and, thus, the dehydration/rehydration process (described below) is essentially one-dimensional. An aspect of the packing that merits some further discussion concerns the acetylene-acetylene contacts within the crystal. Interestingly, we found that there is an infinite chain of acetylene-acetylene contacts in the sequence ‚‚‚A‚‚‚B‚‚‚B‚‚‚A‚‚‚ (Figure 3). Ligands B are related by a center of symmetry, and consequently their -CtC- bonds (distant by 3.898 Å for 1 and 3.872 Å for 2) are strictly parallel between them. In the case of contacts A-B the triple bonds (distant by 3.432 Å for 1 and 3.411 Å for 2) are rotated by 89.2 for 1 and 88.8 ° for 2, with respect to each other. The angle formed between the triple bond of ligand B and the plane of ligand A is 57.1° for 1 and 56.6° for 2. These metric properties strongly suggest that these materials may polymerize to afford conjugated polymers.9 An interesting feature of the structure concerns the hydrogen-bonded network (Table 2), shown in Figure 4. We observe that in the metal pair bridged by ligand B (see also Figure 1), the coordinated water molecules O8 and O9 and their symmetry related ones form cyclic patterns with the R22(8) graph-set notation. Furthermore, O9 is further linked to the carboxylate oxygen atoms O2 and O4 as well as to the lattice water O10 (not shown in Figure 4), through a three-center (bifurcated) bond (Table 2). The sum of the three angles around the hydrogen atom participating in this bond (353.38°) is close to 360°, satisfying well the criteria used for the existence of this type of bond.13 In the other metal pair, connected by the centrosymmetric ligand A,
there is also a centrosymmetric cyclic pattern with graph-set notation R22(8), involving the carboxylate O3 atom and the third coordinated water molecule O7. Thermal Analysis, Structure of the Dehydrated Phases, and Solid-State Reactivity of 2. Simultaneous TG/DTA experiments were carried out in air on a powdered sample of 2 (Figure 5). It is clear that, apart decomposition that occurs beyond 250 °C, there are four distinct thermal events. The first one, between 40 and 115 °C, is endothermic and corresponds to loss of the two lattice water molecules per formula unit (calc. 4.74%, exp. 4.89%). The next two, not well-resolved, endothermic events between 115 and 150 °C are due to removal of four coordinated water molecules (calc. 14.2%, exp. 13.7%). We verified that samples heated separately for 2 h at 90, 130, and 150 °C show approximately the weight losses indicated by the TG analysis of the initial sample. The XRPD patterns (Figure 6) show that, up to 150 °C, the framework structure is retained. Furthermore, a sample of 2 heated at 150 °C and rehydrated shows practically the same TG/DTA curves as 2, indicating that the dehydration process is reversible. As each metal center is coordinated to three water molecules (Figure 1), it is clear that, at 150 °C, it retains one water molecule. We believe that this is O9 because it forms four strong hydrogen bonds (Table 2, Figure 4), while O8 and O7 form only two. In addition, O8 and O7 point into the channels (Figure 2), fulfilling the topological and energetic criteria for easy removal from the lattice.14 Further insight into the dehydration process was obtained by determining the crystal structure of 2 after removal of the lattice water molecules (compound 3). For this, a single crystal of 2 was slowly heated in an
532
Crystal Growth & Design, Vol. 5, No. 2, 2005
Figure 7. FT/IR spectra of compound 2 heated at (a) room temperature, (b) 130 °C, (c) 150 °C, (d) 200 °C.
oven up to 100 °C. To avoid as much as possible initial rapid uptake of water, the evacuated crystal was covered with epoxy glue. The cell volume shows a decrease of only 12 Å3 (2%) as compared to the initial one, indicating zeolite-like behavior. A very slight increase in the cell volume (0.6 Å3) occurs essentially at the beginning of data collection, and consequently, a consistent data set was obtained by omitting the first 700 reflections and working with the final cell dimensions (Table 1). The structure is essentially the same as that of 2. We observe that the A-B acetyleneacetylene contact is slightly decreased, while the B-B contact is slightly increased (Figure 3). In the case of contacts A-B, the triple bonds are rotated by 89.0° with respect to each other, while the angle formed between the triple bond of ligand B and the plane of ligand A is 55.9° The departure of the lattice water (O10) and the subsequent displacement of the ligands bring about some modifications in the hydrogen-bonded network (Table 2). Shortening is observed for all hydrogen bonds involving carboxylate oxygens, while the water-water interaction (O8-O9) remains as in 2. We may observe that the coordinated water molecule O9 is still the most tightly linked, and this reinforces the hypothesis that it is the remaining water at 150 °C. The reversibility of this dehydration process was also checked by leaving an evacuated crystal in air without covering it with epoxy glue. The cell volume increases and reaches its initial value within 24 h. Further heating of the crystal showed that the external morphology is retained, at least up to 125-130 °C, but beyond 120 °C we noted the appearance of a pale yellow color. The unit cell dimensions were essentially those of 3, but it was not possible, up to now, to obtain a cell accurate enough to proceed to data collection. We verified however that the dehydration process is still reversible. The weight loss beyond 150 °C is due to departure of the last coordinated water molecule, but, in this case, the thermal event is exothermic (Figure 5) and is accompanied by the appearance of a brown color in the solid residue. The XRPD pattern showed that this phase is amorphous. These results suggest that a polymerization reaction accompanies the last dehydration step.9b As the polymerization reaction is strongly exothermic and the dehydration is endothermic, the overall thermal effect is exothermic. We checked this hypothesis by recording the FT-IR spectra of powdered samples of 2 heated at 130, 150, and 200 °C for 2 h (Figure 7). After heat treatment, the samples were stored in a desiccator
Michaelides and Skoulika
Figure 8. UV-Vis absorption spectrum of an aqueous acid solution of the conjugated polymer. The spectrum shown in the inset was taken from a more concentrated solution.
to avoid uptake of water from the ambient moisture. In all cases, in agreement with the crystal structure results, a large band between 3500 and 3000 cm-1 indicates the presence of hydrogen-bonded water molecules. In the region of 1600-1350 cm-1, we note severe overlapping of the carboxylate stretching vibration peaks, reflecting well the fact that several coordination modes of the carboxylate group are found in the structure of 2.9b In the particular case of the sample heated at 200 °C, there is practically only one broad band in this region because of the additional contribution of the alternate double bonds in the conjugate chain.15 The relatively strong narrow band at about 1020 cm-1 was assigned to the ν≡C-C stretching vibration.16 This band disappears completely at 200 °C, showing clearly that, at that temperature, polymerization is practically quantitative. The Raman spectra show also intense νC≡C bands up to 150 °C, but at higher temperatures the appearance of a very intense fluoresence signal prevented further detailed study of the polymerization process. In addition, the electronic spectrum (Figure 8) of a dilute acidified aqueous solution of the brown solid obtained at 200 °C, shows a maximum absorption at 237 nm and a broad shoulder around 300 nm tailing into the visible up to 800 nm, accounting well for the color observed. These features are also compatible with the presence of a conjugated polycarboxylic polymer.9b Finally, we measured by light scattering the molecular weight of the brown solid obtained by heating compound 2 at 200 °C for about 2 h, followed by dissolution in a dilute nitric acid solution, which was found to be ca. 7000 g/mol. This result fully confirms the polymerization hypothesis. Our calculations show that only 60% of the last water content is evacuated up to 250 °C, beyond which decomposition occurs. This may be explained by the fact that the departure of water is accompanied by displacement of the ligands and chemical reaction, thereby causing narrowing of the channels. As the water molecules are hydrogen bonded to the ligands, a part of them remains in the structure, possibly “trapped” in small cavities. We observed such a “trapping” recently in another example of solid-state reaction.17 Comments and Conclusions. We showed that isostructural robust microporous MOFs were constructed by using La3+ and Ce3+ cations and acetylenedicarboxylic acid. The presence of three water molecules in the coordination sphere of the metal and of lattice water
Reactive Microporous Rare-Earth Coordination Polymers
Crystal Growth & Design, Vol. 5, No. 2, 2005 533
molecules allowed the formation of a dense hydrogenbonded network, reinforcing the framework stability. The thermal behavior was studied by taking as representative example compound 2. The lattice water was evacuated in a first dehydration step with retention of the crystal structure. The framework is so robust that this step also takes place in a crystal-to-crystal manner. There are some examples of such transformations in metal-organic compounds,4c,18 but this is the first one reported for a rare-earth framework. Two water molecules could also be removed from the coordination sphere of the metal in a two-step mechanism, creating vacant Lewis-acidic metal sites. The crystal structure was still retained, but beyond 150 °C the departure of the third water molecule was accompanied by polymerization reaction, due to the existence of an infinite chain of acetylene-acetylene short contacts. The presence of conjugate polymer chains near arrays of rareearth cations might lead to interesting optical or magnetic properties. We intend to address this issue soon.
(3) (a) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469; (b) Noro, S.-i.; Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem., Int. Ed. 2000, 39, 2082. (4) Some reviews in the field: (a) Yaghi, O. M.; O’Keeffe, M.; Ockwing, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705; (b) Ferey, G. Chem. Mater. 2001, 13, 3084; (c) James, S. L. Chem. Soc. Rev. 2003, 32, 276; d) Janiak, C. Dalton Trans. 2003, 2781; (e) Rao, C. N. R.; Natarajan, S.; Vaidhyanathan, R. Angew. Chem., Int. Ed. 2004, 43, 1466. (5) (a) Reineke, T. M.; Eddaoudi, M.; Fehr, M.; Kelley, D.; Yaghi, O. M. J. Am. Chem. Soc. 1999, 121, 1651; (b) Kiritsis, V.; Michaelides, A.; Skoulika, S.; Gohlen, S.; Ouahab, L. Inorg. Chem. 1998, 37, 3407; (c) Serpaggi, F.; Ferey, G. J. Mater. Chem. 1998, 8, 2737; (d) Cao, R.; Sun, D.; Liang, Y.; Hong, M.; Tatsumi, K.; Shi, Q. Inorg. Chem. 2002, 41, 2087; (e) Pan, L.; Huang, X.; Li, J.; Wu, Y.; Zheng, N. Angew. Chem., Int. Ed. 2000, 39, 527; (f) Ma, B.-Q.; Zhang, D. S.; Gao, S.; Jin, T.-Z.; Yan, C.-X.; Xu, G. X. Angew. Chem., Int. Ed. 2000, 39, 3644; (g) Pan, L.; Adams, M. K.; Hernandez, H. E.; Wang, X.; Zheng, C.; Hattori, Y.; Kaneko, K. J. Am. Chem. Soc. 2003, 125, 3062; (h) Deluzet, A.; Mandez, W.; Daiguebonne, C.; Guillou, O. Cryst. Growth Des. 2003, 3, 475; (i) Paz, F. A. A.; Klinowski, J. J. Chem. Commun. 2003, 1484. (6) Abrahams, B. F.; Coleiro, J.; Hoskins, B. F.; Robson, R. Chem. Commun. 1996, 603. (7) Dimos, A.; Tsaousis, D.; Michaelides, A.; Skoulika, S.; Gohlen, S.; Ouahab, L.; Didierjean, C.; Aubry, A. Chem. Mater. 2002, 14, 2616. (8) Michaelides, A.; Skoulika, S.; Bakalbassis, E. G.; Mrozinski, J. Cryst. Growth Des. 2003, 3, 487. (9) (a) Brodkin, J. S.; Foxman, B. M.; Clegg, W.; Cressey, J. T.; Harbron, D. R.; Hunt, P. A.; Straugham, B. P. Chem. Mater. 1996, 8, 242; (b) Skoulika, S.; Dallas, P.; Siskos, M. G.; Delligiannakis, Y.; Michaelides, A. Chem. Mater. 2003, 15, 4576. (10) Xing, Y.; Jin, Z.-S.; Duan, Z.-B.; Ni, J.-Z. Chin. J. Struct. Chem. 1995, 14, 1. (11) Sheldrick, G. M. SHELX-97; University of Gottingen: Germany, 1997. (12) Keller, E. SCHAKAL 97; Kristallographishes Institut der Universitat: Freiburg, Germany, 1997. (13) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: UK, 1997. (14) (a) Petit, S.; Coquerel, G. Chem. Mater. 1996, 8, 2247; (b) Perrier, P. R.; Byrn, S. R. J. Org. Chem. 1982, 47, 4671. (15) Wojtkowcak, B.; Chabanel, M. Spectrochimie Moleculaire; Technique et Documentation: Paris, 1977. (16) Delabre, J.-L.; Maury, L.; Bardet, L. J. Raman Spectrosc. 1986, 17, 373. (17) Michaelides, A.; Skoulika, S.; Siskos, M. G. Chem. Commun. 2004, 2418. (18) Rather, B.; Zaworotko, M. G. Chem. Commun. 2003, 830.
Acknowledgment. We wish to thank Dr. N. Kourkoumelis for recording the XRPD patterns, Dr. J. Plakatouras for recording the FT-IR spectra and for the thermal analysis measurements, Dr. S. Hadjikakou for recording the UV-VIS spectra and the Microanalytical Laboratory of the Chemistry Department of the University of Ioannina for the elemental analyses, and Dr. A. Avgeropoulos for the light scattering experiments. We also thank the authorities of the region of Epirus for the purchase of the X-ray equipment. Supporting Information Available: X-ray crystallographic information file (CIF) is available for compounds 1-3. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Yaghi, O. M.; Li, G.; Li, H. Nature 1995, 378, 703; (b) Kondo, M.; Yoshitomi, T.; Seki, K.; Matsuzaka, H.; Kitagawa, S. Angew. Chem., Int. Ed. Engl. 1997, 36, 1725; (c) Tabares, L. C.; Navarro, J. A. R.; Salas, J. M. J. Am. Chem. Soc. 2001, 123, 383. (2) (a) Eddaoudi, M.; Li, H.; Reineke, T.; Fehr, M.; Kelley, D.; Groy, T. L.; Yaghi, O. M. Top. Catal. 1999, 9, 105; (b) Fujita, M.; Kwon, Y.; Washidzu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 116, 1151; (c) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jin, Y.; Kim, K. Nature 2000, 404, 982.
CG049725V
