A flexible germanate structure containing 24-ring channels and with very low framework density.

Oxide materials with open structures have been the subject of intense research for decades, because of their economically important applications including catalysis, separations, waste removal, and water softening. The now classical materials are the aluminosilicate zeolites whose structures are based on frameworks of corner-sharing metal-oxygen tetrahedra.1 The pores in such materials are characterized by the size of the rings defining them, which in turn are measured by the number of metal atoms in the ring. Traditionally, rings of 8, 10, or 12 metal atoms have been denoted as small, medium, and large. The discovery2 of an aluminophosphate with 18-membered rings (“extra-large”) has directed attention3 to oxide materials with even larger pores, and examples are now known of materials with 20-membered4 and 24-membered5,6 rings. Even larger rings are found inside the caVity of a framework with 16-membered-ring windows.7 All of the materials mentioned so far are phosphates,5 and contain a metal such as Al, V, Fe, Ni, Zn, or Ga in 5and/or 6-coordination, with the exception of a 24-membered-ring germanate reported recently.6 Here we report a new germanate framework, of unusually low density, with pores based on 24-membered rings. Density in zeolite materials is often measured as FD (framework density) ) the number of metal atoms per nm3. The lowest density zeolite is cloverite4a with FD ) 11.1 nm-3. Germanate analogues of zeolites with all tetrahedral frameworks are rare,8a,d but there are also a number of materials with mixed 4-, 5-, and/or 6-coordination.8 In the compound formulated as Ge14O29F4[(CH3)2NH2]6‚xH2O (ASU-12),9 a novel cluster formed by condensation of four GeX4 tetrahedra, two GeX5 trigonal bipyramids, and one GeX6 octahedron (X ) O or F) was found. This material, which contains pores outlined by 16-membered rings, is one of the least dense germanates prepared to date with 12.0 Ge atoms nm-3 (comparable to the least dense aluminosilicate zeolites). ASU-12 was genuinely porous as shown by exchange studies. We have now found that by using a different base, diaminobutane (DAB), but otherwise similar synthesis conditions, that a material (ASU-16) with the same building units, but with a different topology and very much lower framework density (8.6 Ge atoms nm-3), can be prepared. In fact, it has a lower FD than any of the other materials referred to in this paper, or indeed of any that we are aware of, although a few others come close.10 ASU-16 was synthesized as a pure germanate under hydrothermal conditions for 4 days at 160 °C from a mixture of germanium dioxide, water, 1,4-diaminobutane (99%, Aldrich), pyridine, and hydrofluoric acid (48 wt %) with a typical molar ratio of GeO2:70 H2O:12 DAB:40 pyridine:2 HF. The resulting product was washed and filtered before drying at room temperature. ASU-16 crystallizes as spherical bundles of tightly packed needles. The length of the needles rarely exceeds 100 μm. No other phases can be detected in the X-ray powder diffraction patterns. The structure of ASU-16 was solved by single-crystal X-ray microdiffraction at the APS synchrotron source (Argonne, IL).11 It is made up of two crystallographically independent clusters, identical in composition. The clusters are composed of seven germanium atoms with mixed coordination: four tetrahedral GeO4, two trigonal bipyramidal GeO4F, and one octahedral GeO5F (Figure 1). One oxygen at the core of the cluster is tricoordinated † Arizona State University. ‡ University of Minnesota. § University of Michigan. (1) Thomas, J. M. Angew. Chem. 1999, 38, 3588-3628. (2) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J. M.; Crowder, C. Nature 1988, 331, 698-699. (3) (a) Davis, M. E. Chem. Eur. J. 1997, 3, 1745-1750. (b) Cheetham, A. K.; Férey, G.; Loiseau, T. Angew. Chem., Int. Ed. Engl. 1999, 38, 32693292. (4) (a) Estermann, M.; McCusker, L. B.; Baerlocher, Ch.; Merrouche, A.; Kessler, H. Nature 1991, 352, 320-323. (b) Jones, R. H.; Thomas, J. M.; Chen, J. S.; Xu, R. R.; Huo, Q. S.; Li, S. G.; Ma, Z. G.; Chippindale, A. M. J. Solid State Chem. 1993, 102, 204-208. (c) Lii, K. H.; Huang, Y. F. Chem. Commun. 1997, 839-840. (d) Chippindale, A. M.; Peacock, K. J.; Cowley, A. R. J. Solid State Chem. 1999, 145, 379-386. (e) Walton, R. I.; Millange, F.; Loiseau, T.; O’Hare, D.; Férey, G. Angew. Chem., Int. Ed. 2000, 39, 45524555. (5) (a) Yang, G.-Y.; Sevov, S. C. J. Am. Chem. Soc. 1999, 121, 83898390. (b) Guillou, N.; Gao, Q.; Noguès, M.; Morris, R. E.; Hervieu, M.; Férey, G.; Cheetham, A. K. C. R. Acad. Sci. Paris, Ser. II 1999, 2, 387-392. (c) Zhu, J.; Bu, X.; Feng, P.; Stucky, G. D. J. Am. Chem. Soc. 2000, 122, 1156311564. (d) Lin, C.-H.; Wang, S.-L.; Lii, K.-H. J. Am. Chem. Soc. 2001, 123, 4649-4650. (e) Guillou, N.; Gao, Q.; Forster, P. M.; Chang, J.-S.; Noguès, M.; Park S.-E.; Férey, G.; Cheetham, A. K. Angew. Chem., Int. Ed. 2001, 40, 2831-2834. (6) Zhou, Y.; Zhu, H.; Chen, Z.; Chen, M.; Xu, Y.; Zhang, H.; Zhao, D. Angew. Chem., Int. Ed. 2001, 40, 2166-2168. (7) Khan, M. I.; Meyer, L. M.; Haushalter, R. C.; Schweitzer, A. L.; Zubieta, J.; Dye, J. T. Chem. Mater. 1996, 8, 43-53. (8) For a review of germanates and comparison with silicates see: (a) O’Keeffe, M.; Yaghi, O. M. Chem. Eur. J. 1999, 5, 2796-2801. Recent papers include: (b) Cascales, C.; Gutiérrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C. Angew. Chem., Int. Ed. Engl. 1999, 38, 2436-2439. (c) Cascales, C.; Gutiérrez-Puebla, E.; Iglesias, M.; Monge, M. A.; Ruiz-Valero, C.; Snejko, N. Chem. Commun. 2000, 2145-2146. (d) Conradsson, T.; Dadachov, M. S.; Zou, X. D. Microporous Mesoporous Mater. 2000, 41, 183191. (e) Dadachov, M. S.; Sun, K.; Conradsson, T.; Zou, X. Angew. Chem., Int. Ed. 2000, 39, 3674-3676. (f) Li, H.; Eddaoudi, M.; Plévert, J.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 12409-12410. (9) Li, H.; Eddaouddi, M.; Richardson, D. A.; Yaghi, O. M. J. Am. Chem. Soc. 1998, 120, 8567-8568. (10) The previous record low was claimed to be FD ) 9.10 nm-3 in a guanidinium zinc phosphate: Harrison, W. T. A.; Phillips, M. L. F. Chem. Mater. 1997, 9, 1837-1846. Other low FD are 9.13 nm-3 for Ge2ZrO6F2(H2DAB)‚H2O (ASU-15),8f 9.16 nm-3 for another zinc phosphate,5c and 9.29 nm-3 for “vanadium phosphate I”.7 For ASU-16 we use the volume as prepared, rather than the slightly smaller volume of the material used for crystal structure determination. (11) Crystal data for ASU-16: space group I222, a ) 16.9109(8) Å, b ) 24.267(2) Å, c ) 30.210(3) Å, Z ) 8, λ ) 0.5594 Å, 0.10 × 0.01 × 0.01 mm3. A total of 50669 reflections (10948 independent) were measured at 120 K. Final agreement indices are R1 ) 3.85%, wR2 ) 9.24%, GoF ) 0.967. Figure 1. The building units of ASU-16 composed of seven germanate groups with coordination varying from four to six: tetrahedra green, trigonal bipyramids yellow, and octahedra red. 12706 J. Am. Chem. Soc. 2001, 123, 12706-12707