Anion-Induced Formation of Lanthanide-Organic Chains From 3D Framework Solids. Anion Exchange in a Crystal-to-Crystal Manner Adonis Michaelides* and Stavroula Skoulika* Department of Chemistry, UniVersity of Ioannina, 45110 Ioannina, Greece
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ReceiVed October 13, 2008; ReVised Manuscript ReceiVed March 5, 2009
ABSTRACT: A water-mediated transformation reversibly converts the neutral 3D microporous coordination polymers [Ln2(adipate)3-
(H2O)4] · 6H2O (Ln3+ ) Pr3+) and [Ln2(adipate)3(H2O)4]xH2O (Ln3+ ) Gd3+, Sm3+), into the ionic 1D polymer [Ln(adipate)(H2O)5]XH2O (with X ) NO3-, Cl- and Ln3+ ) Pr3+, Gd3+, Sm3+). The 1D polymer consists of positively charged hydrogen-bonded lanthanide-adipate chains enclathrating NO3- or Cl- anions acting as templates. The hydrogen bonds linking the chains between them are so strong that it is possible to exchange NO3- for Cl- and vice versa in a crystal-to-crystal manner. The anion exchange reactions were directly observed on crystals by using Raman microprobe spectroscopy, single-crystal X-ray diffraction, and scanning electron microscopy. In the recent years, the role of anions as templates for the assembly of supramolecular entities has been receiving increasing attention.1 Although early work was mainly devoted to aniondirected synthesis of finite assemblies (molecular cages, rotaxanes, etc.),1,2a significant number of reports dealing with infinite coordination solids have been published in the last few years.3,4 The role of anions in the assembly of infinite coordination solids is not always easy to be assessed because, in many structures, the anion may accomplish various functions in the crystal lattice (complexation of the metal, charge balance, bridging, etc.).4a,5 In this work, we present a water-mediated transformation that converts the 3D compounds [Ln2(adipate)3(H2O)4]6H2O (Ln3+ ) Pr3+), 1, and [Ln2(adipate)3(H2O)4]xH2O (for Ln3+ ) Gd3+, x ) 0, and for Ln3+ ) Sm3+, x ) 1.3), 2, into the novel 1D polymers [Ln(adipate)(H2O)5]NO3 H2O, where Ln3+ ) Pr3+ (3 · NO3), Sm3+ (4 · NO3), Gd3+ (5 · NO3), and [Ln(adipate)(H2O)5]Cl H2O, where Ln3+ ) Pr3+ (3 · Cl), Sm3+ (4 · Cl), Gd3+ (5 · Cl). The structures of compounds 1 and 2 were reported in a previous work6 (Figure 1 and Figures 1S and 2S in the Supporting Information). The novel 1D polymers consist of positively charged metal-organic chains organized, through hydrogen bonding, around noncoordinated NO3or Cl- anions. The reaction is reversible; as the starting 3D polymers do not contain anions in their structure, we suggest that the conversion is induced by the NO3- or Cl- anions. The transformation reaction takes place when attempting to prepare crystals of 1 or 2 by slow evaporation of unstirred, saturated aqueous solutions of LnX3 (X ) Cl-, NO3-) in the presence of sodium adipate.7 Prismatic crystals of 1 and 2 are initially formed. When the volume of the saturated solution is significantly reduced, conversion of these crystals into crystals of the 1D polymers 3 · X, 4 · X,and 5 · X takes place. The transformation is very easy to observe because the morphology of the starting crystals (compds 1 and 2) differs substantially from that of the novel 1D polymers (see Figure 3S in the Supporting Information). This observation was further confirmed by measuring the unit cell of some representative crystals. These results suggest that a very high anion concentration is needed to trigger the transformation reaction. The six 1D polymers prepared are isostructural as was established by a straightforward atom-atom correspondence of the lanthanum adipate framework.8,9 Their structure consists of zigzag metalorganic chains directed along the [101] direction (Figure 1). Each metal is nine-coordinated by four carboxylate oxygen atoms and five water molecules. The chains are organized in layers perpendicular to b by strong quadruple hydrogen bonds, involving the carboxylate oxygen atoms, the coordinated and lattice water molecules (see text below), and one nitrate oxygen (O1N)9 or Cl* Corresponding author. E-mail:
[email protected] (S.S.);
[email protected] (A.M.).
anion (Figure 2). The layers are linked between them by hydrogen bonds involving the coordinated water molecules and the carboxylate oxygens (see Figure 4S in the Supporting Information). The nitrate or chloride anions and the lattice water molecules are located in channels formed along the c axis (Figure 3 and Figure 5S in the Supporting Information). We emphasize that the transformation we report herein is not another case of the well-known solvent mediated transformation.10 Indeed, the present transformation is composed of at least two steps: The first step involves departure of one adipate ligand from 1 or 2 and addition of three water ligands in the coordination sphere of the metal, preserving the coordination number of nine. This step is crucial because the transformation does not take place with Ln3+ ) La3+or Ce3+ (although isostructural to 1), i.e., with cations of high crystal radius, thereby lower metal-water bond strength.11 The second step is a template reaction involving wrapping of the positively charged metal-organic chain around chloride or nitrate anions to produce stable ionic channeled solids (Figure 3). As we already pointed out, this step should necessitate a very high anion concentration. Indeed, it was possible to prepare compounds 4 · Cl and 5 · Cl directly without performing the transformation reaction, just by adding a great amount of NaCl to a saturated solution of GdCl3 or SmCl3 and adipate anions.12 The existence of strong hydrogen bonds linking the chains between them prompted us to investigate whether it is possible to proceed to anion exchange.3a,13 For this purpose, a crystal of [Pr(adipate)(H2O)5]NO3 H2O, 3 · NO3, was mounted on a glass fiber as for a diffraction experiment and put in contact with some drops of a 3 M NaCl solution, for about 30 h. Precaution was taken in order to avoid rapid evaporation of the solution. The solution was renewed after about 3 h. It was observed that the anion-exchanged crystal retained its external morphology, although it became less transparent. The crystal was then cut in three pieces in order to examine its surface and different cross-sections, and each piece was subjected to SEM/EDX analysis. It was found that in all cases about 75% of the Cl- anions had been exchanged. During the exchange process, we observed no modification of the size or the morphology of the crystal. The SEM images show no evidence for dissolution/recrystallization (see Figure 6S in the Supporting Information), thereby indicating that the anion exchange is a solidstate diffusion process.14 The exchange process was further investigated by single-rystal X-ray diffraction. For this, a crystal of [Gd(adipate)(H2O)5]Cl H2O, 5 · Cl, was subjected to anion exchange, as described above, by immersion in a 3 M NaNO3 solution for 10, 18, and 26 h. In each case the three strong reflections, 1-3-1, -6-2-1, and -131, were recorded. For each peak, we observed the appearance of a shoulder (see Figure 7S in the Supporting Information) whose intensity was increasing with
10.1021/cg801138e CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
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Figure 1. 3D lanthanide-adipate polymers (1) and (2) transform reversibly into the 1D polymers.
Figure 2. View of the structure of compound 5 · NO3, approximately along b.The lanthanide-adipate chains are linked by quadruple hydrogen bonds to form layers perpendicular to b.
Figure 3. Projection of the structure of compound 5.NO3 along c. For clarity, only two nitrate anions are shown inside a channel.
immersion time. The approximate difference in θ value between the main peak and its shoulder was similar to that calculated from the unit cells of 5 · NO3 and 5 · Cl. Upon reimmersion of the same crystal in a 3 M NaCl solution, for about 30 h, the shoulder disappears, indicating the reversibility of the process (see Figure 7S in the Supporting Information). Furthermore, in another series of experiments, we investigated the selectivity and reversibility of the ion-exchange process by single-crystal Raman microprobe spectroscopy. For this purpose single crystals of 5 · NO3 and 5 · Cl were immersed in 1 M NaCl or
1 M NaNO3 solutions, respectively, and the 1040 cm-1 strong band of NO3- was used to monitor the ion-exchange process (Figure 4). We observed that under these conditions, there is no anion exchange in the case of 5 · NO3, whereas the NO3- anions do diffuse in the lattice of 5 · Cl. In the latter case, it became possible to reintroduce Cl- anions in the partially exchanged crystals of 5 · Cl, by employing more concentrated NaCl solutions (2 M). This observation confirms the greater affinity of the novel 1D polymers for NO3- anions, in line with the Hoffmeister series. As the size of the two anions is practically the same,15 it is clear that this is a case of shape-selective
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Figure 4. Raman spectra of compound 5.Cl; before any exchange (black), after partial exchange of Cl- by NO3- (red), after reintroduction of Cl- anions in the partially exchanged crystal (green).
anion exchange. Indeed, inspection of the crystal structures shows that the NO3- anions are more strongly bound, by hydrogen bonding, than the Cl- ones (see Figure 5S in the Supporting Information). In a recent publication, P. Suh et al. described a reversible reaction in which a 1D coordination solid, containing coordinated NO3- anions, was converted into a 2D solid containing NO3- anions as guests.3a In this reaction, the stoichiometry was conserved and the conversion reaction was explained by considering simple movements following the departure of the NO3- anion from the coordination sphere of the metal. A solid-to-solid transformation (without dissolution prior to transformation) upon slow but not complete evaporation of the solvent has also been recently described by Moorthy et al.16 In this case, too, the counteranion is present in the structure of both initial and final solids. The reaction we describe herein appears more complex because it involves departure of a bridging organic ligand, followed by hydration of the metal and reorganization of the new structure around the NO3- and Cl- anions, which were not part of the initial solid.17 We suggest that this result gives some insight into how anions preorganize the metal-organic assemblies in solution, thereby influencing the nature of the solid to be precipitated.18 In conclusion, we presented a unique complex solvent-mediated transformation that reversibly converts 3D MOFs into ionic 1D coordination solids. Moreover, we provided evidence that an anion exchange process takes place in a crystal-to-crystal manner.19 Although the anion exchange experiments were carried out for some of the complexes prepared, we presume that the results are valid for all six isostructural compounds. As far as we know, this is the first example ever reported of crystal-to-crystal anion exchange in molecule-based materials.
Acknowledgment. We are grateful to Prof. Karakasides for recording the Raman spectra and to Dr. A. Katsoulides for the SEM/ EDX experiments. Also, we thank the Authorities of the region of Epirus for the purchase of the X-ray equipment. Supporting Information Available: Figures 1S-9S showing the structures of compounds 1 and 2, the H-bonding interactions of Cland NO3- anions in case of compounds 5 · Cl and 5 · NO3, the morphology of the crystals of compounds 5 · NO3 and 2, SEM image of compound 3 · Cl the profiles of three reflections of compound 5 · Cl and IR spectra of compounds 3 · Cl, 4 · Cl, 5 · Cl, 3 · NO3, 4 · NO3, 5 · NO3 (PDF). Cif files of 3 · Cl, 4 · Cl, 5 · Cl, 3 · NO3, 4 · NO3, and 5 · NO3. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Vilar, R. Angew. Chem., Int. Ed. 2003, 42, 1460–1477. (b) Gale, P. A. Coord. Chem. ReV. 2003, 240, 191–221. (c) Gale, P. A.; Quesada, R Coord. Chem. ReV. 2006, 250, 3219–3244. (d) Filby, M. H.; Steed, J. W. Coord. Chem. ReV. 2006, 250, 3200–3218.
(2) (a) Bashall, A; Bond, A. D.; Doyle, E. L.; Garcia, F.; Kidd, S.; Lawson, G. T.; Parry, M. C.; McPartlin, M.; Woods, A. D.; Wright, D. Chem.sEur. J. 2002, 8, 3377–3385. (b) Hasenknopf, B.; Lehn, J.M.; Boumediene, N.; Leize, E.; Van Dorsselaer, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 3265–3268. (3) (a) Min, S. K.; Suh, M. P. J. Am. Chem. Soc. 2000, 122, 6834–6840. (b) Yi, L.; Yang, X.; Lu, T.; Cheng, P. Cryst. Growth Des. 2005, 5, 1215–1219. (c) Hannon, M. J.; Painting, C. L.; Plummer, E. A.; Childs, L. J.; Alcock, N. W. Chem.sEur. J. 2002, 8, 2226–2238. (d) Jung, O.-S.; Kim, Y. J.; Lee, Y.-A.; Park, J. K.; Chae, H. K. J. Am. Chem. Soc. 2000, 122, 9921–9925. (e) Lopez, S.; Keller, S. W. Inorg. Chem. 1999, 38, 1882–1888. (f) Zheng, L.-M.; Wang, Y.; Wang, X.; Korp, J. D.; Jacobson, A. J. Inorg. Chem. 2001, 40, 1380–1385. (g) Zhang, X.; Zhou, X.-P.; Li, D. Cryst. Growth Des. 2006, 6, 1440–1444. (h) Sarkar, M.; Biradha, K. Cryst. Growth Des. 2007, 7, 1318–1331. (i) Shi, Q.; Sun, Y.; Sheng, L.; Ma, K.; Hu, M.; Hu, X.; Huang, S. Cryst. Growth Des. 2008, 8, 3401–3407. (j) Huang, Y.-Q.; Zhao, X.-Q.; Shi, W.; Liu, W.-Y.; Chen, Z.-L.; Cheng, P.; Liao, D.-Z.; Yan, S P. Cryst. Growth Des. 2008, 8, 3652–3660. (4) (a) Gimeno, N.; Vilar, R. Coord. Chem. ReV. 2006, 250, 3161–3189. (b) Daz, P.; Benet-Buchholz, J.; Vilar, R.; White, A. Inorg. Chem. 2006, 45, 1617–1626. (c) Xu, N.; Shi, W.; Liao, D-Z.; Yan, S-P.; Cheng, P. Inorg. Chem. 2008, 47, 8748–8756. (d) Biswas, C.; Mukherjee, P.; Drew, M.; Gomez-Garcia, C.; Clemente-Juan, J.; Ghosh, A. Inorg. Chem. 2007, 46, 10771–10780. (e) Awaleh, M.; Badia, A.; Brisse, F.; Bu, X-H. Inorg. Chem. 2006, 45, 1560–1574. (f) Awaleh, M.; Badia, A.; Brisse, F. Cryst. Growth Des. 2005, 5, 1897–1906. (g) Lin, S-H.; Zhou, X-P.; Li, D.; Ng, S. Cryst. Growth Des. 2008, 8, 3879–3881. (h) Byrne, P.; Lloyd, G.; Anderson, K.; Clarke, N.; Steed, J. Chem. Commun. 2008, 3720–3722. (5) He, Z.; Gao, E.-Q.; Wang, Z.-M.; Yan, C.-H; Kurmoo, M. Inorg. Chem. 2005, 44, 862–876. (6) (a) Dimos, A.; Tsaousis, D.; Michaelides, A.; Skoulika, S.; Gohlen, S.; Ouahab, L.; Didierjean, C.; Aubry, A. Chem. Mater. 2002, 14, 2616–2622. (b) Kiritsis, V.; Michaelides, A.; Skoulika, S.; Gohlen, S.; Ouahab, L. Inorg. Chem. 1998, 37, 3407–3410. (7) (a) Synthesis of 3 · Cl: A suspension of adipic acid (0.2923 g, 2 mmol) in water (20 mL), was brought to pH 6 by means of NaOH pellets. The resulting solution was then mixed with a solution of PrCl3 · 6H2O (0.4975 g, 1.4 mmol) in 20 mL of water. The precipitate was filtered and the supernatant solution was distributed in small petri dishes (5 mL in each dish), semicovered with aluminium foil, and allowed to evaporate. Small prismatic crystals of 1 were formed within 2-3 days. Upon almost complete evaporation (time needed was about 1 week), the crystals began to disappear while new, well-formed, transparent crystals of 3 · Cl of quite distinct morphology appeared. Usually, the transformation reaction is completed within 3 days. The conversion reaction is reversible because upon addition of water, the initial phase reappears. Elemental anal. (%). Calcd for PrC6H20O10Cl: H, 4.67; C, 16.80. Found: H, 4.50; C, 16.65. IR (KBr, cm-1): 3275s, 1653m, 1524s, 1462s, 1446s, 1422s, 1322s, 1305s, 1142s, 931m, 814s, 650s. (b) Synthesis of 4 · Cl: Same as above but with SmCl3 · 6H2O (0.5107 g, 1.4 mmol) as the lanthanide salt. In this case, the initial phase formed is compound 2 (Ln ) Sm3+). Elemental anal. (%). Calcd for SmC6H20O10Cl: H, 4.57; C, 16.44. Found: H, 4.52; C, 16.20. IR (KBr, cm-1): 3279s, 1654m, 1527s, 1463s, 1447s, 1424s, 1323m, 1306m, 1142m, 932w, 815m, 651s. (c) Synthesis of 5 · Cl: Same as above with the only difference of the lanthanide salt used GdCl3.6H2O (0.5204 g, 1.4 mmol). In this case, too, the initial phase formed is compound 2 (Ln ) Gd3+). Elemental anal. (%). Calcd for GdC6H20O10Cl: H, 4.50; C, 16.19. Found: H, 4.31; C, 16.00. IR (KBr, cm-1): 3275s, 1655m, 1528s, 1463s, 1447s, 1424s, 1323m, 1306m, 1143m, 934w, 816m, 653s. (d) Synthesis of 3 · NO3: Same as for 3 · Cl with the only difference being that the lanthanide salt used was Pr(NO3)36H2O (0.6090 g, 1.4 mmol). Elemental anal. (%). Calcd for PrC6H20O13N: H, 4.40; C, 15.82; N, 3.08. Found: H, 4.52; C, 15.73; N, 3.15. IR (KBr, cm-1): 3365s, 1663m, 1526s, 1461s, 1434s, 1385vs, 1304s, 1142m, 1036w, 929w, 812m, 649s. (e) Synthesis of 4 · NO3: Same as for 4 · Cl with the only difference being that the lanthanide salt used was Sm(NO3)3 · 6H2O (0.6222 g, 1.4 mmol). Elemental anal. (%). Calcd for SmC6H20O13N: H, 4.31; C, 15.50; N, 3.01. Found: H, 4.13; C, 15.31; N, 2.92. IR (KBr, cm-1): 3355s, 1665m, 1531s, 1463s, 1435s, 1384s, 1304s, 1143m, 1035m, 931m, 812s, 650s. (f) Synthesis of 5 · NO3: Same as for 5 · Cl with the only difference being that the lanthanide salt used was Gd(NO3)3 · 6H2O (0.6319 g, 1.4 mmol). Elemental anal. (%). Calcd for GdC6H20O13N: H, 4.24; C, 15.28; N, 2.97. Found: H, 4.09; C, 15.15; N, 2.85. IR (KBr, cm-1): 3373s, 1668m, 1532s, 1463s, 1434s, 1385vs, 1305s, 1144m, 1036w, 931w, 812m, 651s.
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(8) For discussion on isostructurality see Fabian, L.; Kalman, A. Acta Crystallogr., Sect. B 1999, 55, 1099–1108. (9) Crystal structure determination: Single crystals of [Ln(Adipate)(H2O)5]NO3 H2O, where Ln3+ ) Pr3+ (3 · NO3), Sm3+ (4 · NO3), Gd3+ (5 · NO3), and [Ln(adipate)(H2O)5]Cl H2O, where Ln3+ ) Pr3+ (3 · Cl), Sm3+ (4 · Cl), Gd3+ (5 · Cl), were measured at 298 K on a Bruker P4 single-crystal X-ray diffractometer with MoKa radiation. The six compounds are isostructural and crystallize in the monoclinic system, space group C2/c. 3 · NO3: a ) 12.594(1) Å, b ) 16.166(1) Å, c ) 8.910(1) Å, β ) 122.59(1)°, V ) 1528.4(2) Å3, Z ) 8, Fcalcd ) 1.978 g cm-3, µ ) 3.251 mm-1, F(000) ) 904, R1 ) 0.0289, wR2 ) 0.1062 for all data. 4 · NO3: a ) 12.525(2) Å, b ) 16.039(2) Å, c ) 8.853(1) Å, β ) 122.59(1)°, V ) 1498.4(3) Å3, Z ) 8, Fcalcd ) 2.059 g cm-3, µ ) 3.984 mm-1, F(000) ) 916, R1 ) 0.0276, wR2 ) 0.0773 for all data. 5 · NO3: a ) 12.474(1) Å, b ) 15.978(1) Å, c ) 8.829(1) Å, β ) 122.58(1)°, V ) 1482.8(2) Å3, Z ) 8, Fcalcd ) 2.112 g cm-3, µ ) 4.538 mm-1, F(000) ) 924, R1 ) 0.0222, wR2 ) 0.0693 for all data. 3 · Cl: a)12.534(1) Å, b ) 15.641(1) Å, c ) 9.075(1) Å, β ) 123.63(1)°, V)1481.3(2) Å3, Z ) 8, Fcalcd ) 1.922 g cm-3, µ ) 3.506 mm-1, F(000) ) 848, R1 ) 0.0315, wR2 ) 0.1047 for all data. 4 · Cl: a ) 12.452(3) Å, b ) 15.513(2) Å, c ) 9.030(4) Å, β ) 123.60(2)°, V ) 1452.9(8) Å3, Z ) 8, Fcalcd ) 2.003 g cm-3, µ ) 4.263 mm-1, F(000) ) 860, R1 ) 0.0358, wR2 ) 0.0998 for all data. 5 · Cl: a ) 12.410(1) Å, b ) 15.460(3) Å, c ) 9.006(1) Å, β ) 123.52(1)°, V ) 1440.5(3) Å3, Z ) 8, Fcalcd ) 2.052 g cm-3, µ ) 4.827 mm-1, F(000) ) 868, R1 ) 0.0244, wR2 ) 0.0732 for all data. The structures were solved by direct methods and refined by full-matrix least-squares techniques using the SHELX97 package. The lattice water molecule together with the NO3- and Cl- anions were not found to obey the full crystal symmetry. In all structures, the lattice water molecule occupies half of the sites predicted by crystal symmetry (only noncentrosymmetrically related ones), whereas the four remaining equivalent positions are occupied, in case of compounds 3 · NO3, 4 · NO3, and 5 · NO3, by one oxygen atom (O1N) of the NO3- anion. The best structure refinement is obtained when the lattice water molecule is placed exactly on the position that would be occupied, in the absence of disorder, by the (O1N) atom. Consequently, this atom is refined with an occupational factor 1, whereas the remaining atoms of the NO3 group with occupational factor 0.5. In case of compounds 3 · Cl, 4 · Cl, and 5 · Cl, the four remaining symmetry operations describe the position of the Cl- anion. In both structures, hydrogen atoms of the adipate anion have been placed on calculated positions and refined isotropically using the riding model. Hydrogen atoms of the coordinated water molecules have been determined by difference
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(10) (11) (12)
(13)
(14)
(15) (16) (17)
(18) (19)
Fourier maps. Their positional parameters only have been refined with restrained O-H distance. Because of the above-mentioned disorder, the hydrogen atoms of the lattice water molecules have not been determined. Empirical absorption correction using program DELREFABS has been applied on the data of all structures except that of 3 · NO3. Davey, R. J.; Blagden, N.; Righini, S.; Alison, H.; Ferrari, E. S. J. Phys. Chem. B 2002, 106, 1954–1959. Cotton, F. A.; Wilkinson, G. AdVanced Inorganic Chemistry, 4th ed.; Wiley: New York, 1980; p 1188. Alternative Syntheses of 4 · Cl and 5 · Cl: The syntheses were conducted as above (refs 7b and c) except that 1.8 g of NaCl was added in the petri containing 5 mL of saturated solution. The crystallizer was completely covered. Crystals of 4 · Cl or 5 · Cl only were formed within 48 h. Noro, S.-i.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. J. Am. Chem. Soc. 2002, 124, 2568–2583. (b) Choi, H. J.; Suh, M. P. Inorg. Chem. 2003, 42, 1151–1157. (c) Muthu, S.; Yip, J. H. K.; Vittal, J. J. Dalton Trans. 2002, 4561–4568. (d) Hamilton, B. H.; Kelly, K. A.; Wagler, T. A.; Espe, M. M.; Ziegler, C. J. Inorg. Chem. 2004, 43, 50–56. (e) Shu, M.; Tu, W.; Jin, H.; Sun, J. Crystal Growth Des. 2006, 6, 1890–1896. (f) Kumar Maji, T.; Matsuda, R.; Kitagawa, S. Nat. Mater. 2007, 6, 142–148. (a) Klobystov, A. N.; Champness, N. R.; Roberts, C. J.; Tendler, S. J.B.; Thompson, C.; Schroder, M. CrystEngComm. 2002, 4, 426– 431. (b) Thompson, C.; Champness, N. R.; Khlobystov, A. N.; Roberts, C. J.; Schroder, M.; Tendler, S. J. B.; Wilkinson, M. J J. Microsc. 2004, 214, 261–271. Steed, J.; Atwood, J. Supramolecular Chemistry, 1st ed.; Wiley: New York, 2000; p 199. Moorthy, J. N.; Natarajan, R.; Savitha, G.; Suchopar, A.; Richards, R. M. J. Mol. Struct. 2006, 796, 216–222. For neutral molecules acting as templates see:(a) Tanaka, D.; Kitagawa, S. Chem. Mater. 2008, 20, 922–931. (b) de Lill, D. T.; Gunning, N. S.; Cahill, C. L. Inorg. Chem. 2005, 44, 258–266. (c) de Lill, D. T.; Bozzuto, D. J.; Cahill, C. L. Dalton Trans. 2005, 2111–2115. Yam, V. W-W.; Chan, K.H.-Y.; Wong, K.M.-C.; Zhu, N. Chem.sEur. J. 2005, 11, 4535–4543. Cation exchange in natural single crystals of zeolites has been reported: Yang, P.; Stolz, J.; Armbruster, T.; Gunter, M. E. Am. Mineral. 1997, 82, 517.
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