Structural Determinants in the Oxovanadium Diphosphonate System

Chapter 28

Structural Determinants in the Oxovanadium Diphosphonate System

Downloaded by MICHIGAN STATE UNIV on August 10, 2013 | http://pubs.acs.org Publication Date: August 30, 2007 | doi: 10.1021/bk-2007-0974.ch028

Wayne Ouellette and Jon Zubieta Department of Chemistry, Syracuse University, Syracuse, NY 13244

Oxovanadium organodiphosphonates are prototypical organic— inorganic hybrid materials. While these phases are generally characterized by V - P - O layers separated by organic domains, the structural details are complex, reflecting structural determinants such as the length and identity of the organic spaces, the presence of organic or complex inorganic cationic components, and the incorporation of fluoride into the V-P-O substructure.

The widespread contemporary interest in metal oxide based solid phases is related to their significant applications to areas of chemistry and materials science as diverse as catalysis, sorption, molecular electronics, ceramics, energy storage, fuel cells and optical materials (1-8). This diversity of properties reflects a vast compositional range, which allows variations in covalency, geometry, and oxidation states, and a versatile crystalline architecture, which may provide diverse pore structures, coordination sites, and juxtapositions of functional groups. The ubiquity of inorganic oxide phases in both the geosphere and the biosphere (9-13) suggests that naturally occurring oxides may provide useful guidelines for the preparation of synthetic phases and the modification of oxide microstructures. It is noteworthy that many of the remarkable oxide materials fashioned by nature contain mixtures of inorganic oxides coexisting with organic molecules which dramatically influence the microstructure of the inorganic and allow higher order organization of hierarchical structures (14-16). 392

© 2007 American Chemical Society

In Vanadium: The Versatile Metal; Kustin, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.


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Complex structures (17), based on a molecular scale composite of inorganic and organic components, provide the potential for the design of novel functional materials for technological applications (18). A n inorganic material may provide useful magnetic, dielectric or optical properties, mechanical hardness, and thermal stability, while organic compounds offer processability, structural diversity, a range of polarizabilities and luminescent properties (19). Consequently, the combination of the characteristics of the organic and inorganic components offers an opportunity to conflate useful properties within a single composite, providing access to a vast area of complex, multifunctional materials (20). Inorganic-organic hybrid materials (21) are extended arrays o f metal atoms or clusters bridged by polyfunctional organic molecules. A n important subclass of this family of materials are the hybrid metal oxides, which contain metaloxygen-metal (M-O-M) arrays as part of their structures. In such materials, the inorganic oxide contributes to the increased complexity, and hence functionality, through incorporation as one component in multilevel structural materials where there is a synergistic interaction between organic and inorganic components. Metal organophosphonates are prototypical composite materials, which can exhibit a range of structures, including molecular clusters, chains, layers and three-dimensional frameworks. A n important subclass of these materials are the oxovanadium organophosphonates, whose structures are often characterized by a two-dimensional network of V-P-0 layers separated by hydrophobic organic domains. However, the detailed structural chemistry may be exceedingly complex, reflecting a variety of structural determinants. In an attempt to elaborate the structural systematics of these materials, we have investigated the oxovanadium organodiphosphonate system, focusing on a number of variables, specifically: (i) the length and identity of the organic tether of the diphosphonates, (ii) the introduction of organic or metal complex cations, and (iii) the incorporation of fluoride anions into the V-P-0 substructure.

Variations in tether length of a , oo-alkyldiphosphonates

While the prototypical structural motif of V-P-0 layers buttressed by organic linkers is retained throughout the series of neutral network materials, [V O {0 P(CH2)nP03} ], significant changes evolve as the tether length increases from n = 2 to n = 11. Thus, for n = 2-5, the three dimensional structure [V 02(H20){0 P(CH ) P03}] (1) of Figure l a is maintained. In contrast, at n = 6 through n = 8, the two-dimensional structure of [V 02(H20)4{0 P(CH )nP0 }] (2), shown in Figure 1c is observed, wherein the organic chains are sandwiched between V-P-0 layers. Further expansion of the tether to n = 9 results in the 3-D structure of [V 0 (H 0){H0 P(CH ) P0 H} ] x

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Figure 1. Polyhedral representations of the structures of compounds of the type (1), (2), and (3), viewed both parallel to the V-P-O plane and normal to the VP-O plane: (a and b) 1; (c and d) 2; (e and f) 3.

(3) (Figure le), where once again slabs defined by two V - P - O networks sandwich the organic tethers. However, in this instance, the V-P-O networks of adjacent slabs link through bridging aqua ligands to produce a double layer substructure and the resultant three-dimensional connectivity. This structural diversity is reflected in the connectivities o f the component V-P-O networks of 1-3 shown in Figures l b , d and e. Compound 1 exhibits a layer substructure constructed from face-sharing pairs o f V(IV) octahedra and phosphorus tetrahedra. In contrast, the V-P-O layer of 2 does not exhibit V - O - V linkages, but rather consists of isolated { V 0 } octahedra, each of which exhibits cw-oriented aqua ligands. In the case o f 3, the double layer results from aqua bridging of pairs of corner-sharing V(IV) octahedra. 6

Aromatic diphosphonates As shown in Figure 2, the structures o f [ V O { H 0 P ( l , 4 - C H 4 ) P 0 3 } J (4) and [VO{H0 P(C6H4)2P03}] (5) conform to the buttressed layer prototype. However, the V - P - O layer is quite distinct from those o f the a , ooalkyldiphosphonate materials 1-3. The network is constructed from six coordinate V(V) sites, each bonding to oxygen donors from five phosphonate ligands and a terminal oxo-group. The V - P - O layer is three polyhedra thick 3

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Figure 2. Polyhedral representations of the structures of (a) (4) and (b) (5); (c) a view of the V-P-O layer of 4 and 5.

with vanadium octahedra sandwiched between phosphonate tetrahedra. One phosphonate terminus bonds to three vanadium sites while the second bonds to two, leaving a pendant - O H group.

Figure 3. (a) A view of the structure of (4); (b) the V-P-O layer of 4; (c) and (d) views of the structure of (5).

O f course, the relative location o f the {P0 } groups can be moved about the central ring o f the benzene-diphosphonate. The structural consequences are quite dramatic. Thus, [V202(H20)2{0 P(1,2-C H4)P03}] (6) (Figure 3a) is twodimensional, with the ligand adopting a chelating role. Curiously, there are two unique V(IV) sites. The square pyramidal geometry o f the first is defined by four oxygen donors from four diphosphonate groups in the basal plane and an 3

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apical oxo-group. In contrast, the second site exhibits two aqua ligands in the basal plane and two diphosphonate oxygen donors. In contrast, the structure of [ V z O i ^ O ^ ^ P O ^ Q R O P O a } ] (7) is three dimensional (Figure 3c). Once again there are two distinct V(IV) sites. One consists of square pyramids defined by two aqua ligands and two diphosphonate oxygen donors in the basal plane and an apical oxo-group. The second consists of chains of trans corner-sharing octahedra with the common {V=0 -V=0} short-long alternation of V - O bonds along the chain axis.

Introduction of cationic components The introduction of organonitrogen cations results in dramatic structural perturbations. Thus, while [H N(CH2)„NH3][V404(OH)2{03P(CH2)3P03}2]• x H 0 (8) retains the prototypical "pillared" layer architecture (Figure 4a) for n = 2-8, the V-P-O substructure is quite distinct from those o f 1-3. A s shown in Figure 4b, the network exhibits corner-sharing pairs of V(IV) square pyramids. It is noteworthy that the content of water of crystallization x decreases as n increases; that is, the water is "squeezed" out by the cations. 3

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Figure 4. Polyhedral representations of the structures of materials of the types (8), (9) and (10), viewed both parallel and normal to the V-P-O planes: (a and b) 8; (c and d) 9; (e and f) 10.

When n = 5, the two-dimensional phase [H N(CH )2NH3][V404(OH)2(H20) { 0 P ( C H 2 ) 5 P 0 } 2 ] (9) is obtained (Figure 4c). The tether groups of the diphosphonate ligands project from a single face of the V - P - O network, 3

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397 resulting in a slab-like structure with the organic tethers sandwiched between two V - P - O layers. Substitution o f piperazinium cations for the simple alkyl chain cations of 8 and 9 provides the 2-D structure of [H N(CH CH ) NH ][ V 0 2 { 0 3 P ( C H 2 ) n P 0 H } ] (10) (Figure 4e). The undulating structure of 10 is a consequence of the different environments o f the phosphorus termini o f the diphosphonate ligand: the first bridges three vanadium sites, while the second bonds to a single site. Curiously, when the diphosphonate tether length is reduced to n = 2, the three-dimensional [H N(CH ) NH3][V303{0 P(CH ) P03} ] (11) (Figure 5) is observed. The structure exhibits large channels o f dimensions 10.4A x 8A defined by a 30-membered { V P O | C } ring and encapsulating the cation. 2

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Figure 5. Two views of the structure of (11). 4+

When metal complex cations, such as {Cu (bisterpy)} (bisterpy = 2,2':4\4'':2 \2 -quaterpyridyl-6\6''-di-2-pyridine), are used, the V - P - O networks common to structures 1-6 are largely disrupted and one-dimensional and cluster V - P - O substructures are observed. Coordination o f the Cu(II) centers to vanadium and/or phosphorus oxygen atoms prevents aggregation of the V - P - O component into higher dimensionality substructures. Thus, [{Cu (bisterpy)(H 0) }V 04{0 P(CH ) P03} { H 0 P ( C H ) P 0 H } ] (12) and [{Cu (bisterpy)}V 0 {0 P(CH ) P03} ] (13) exhibit V - P - O chains, while [{Cu (bisterpy)}V0 {0 P(CH )3P03}{H03P(CH )3P03H }] (14) is constructed 2

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from cyclic [V0 {0 P(CH )3P03}] 2 " clusters and [{Cu (bisterpy)}V 0 (OH) . { 0 P ( C H ) P 0 } ] (15) contains embedded [ V 0 ( O H ) {0 P(CH ) P0 }] " subunits (Figure 6). 2

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Fluoride incorporation The introduction of fluoride into the V - P - O substructure of these materials has profound structural consequences, generally related to the reduction of one



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Figure 6. Polyhedral representations of the structures of (a) (12), (b) (13), (c) (14) and (d) (15).

or more vanadium sites to the V(III) oxidation state. Since vanadium(III) adopts more or less regular octahedral geometry with no short multiply-bonded {V=0} units, considerable structural variety is possible. Several structures are reminiscent o f those previously discussed. Thus, [H2pip]rV4F402(H 0)2{0 P(CH )3P0 }2] (16) exhibits the prototypical "pillared" layer structure (Figure 7a). However, the V-P-O-F layer, shown in Figure 5b, contains pairs of edge-sharing V ( I Y ) and V(III) octahedra linked through bridging fluoride ligands. In contrast, [ H N C H C H N H ] [ V 2 0 2 F 2 ( H 2 0 ) 2 { 0 P ( C H 2 ) 4 P 0 } ] (17) (Figure 7c) exhibits pairs of V-P-O-F layers linked through the alkyl chains of the diphosphonate ligands with the cations separating adjacent slabs, in a fashion reminiscent of compound 9. While the theme o f two-dimensional V-P-O-F slabs with cations occupying the interlamellar domain is reiterated in [H NCH CH NH ] [V F (H20)2{0 P(CH )5P0 }{H0 P(CH )5P0 } ] (18) (Figure 7e), the V-P-O-F network of 18 is constructed from chains of V(III) octahedra (Figure 7b). A s noted for the structure of compound 11, shortening the diphosphonate tether length to n = 2 allows the entertainment of the organic tether within V - P 2

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Figure 7. The structures of (16), (17) and (18) viewed both parallel and normal to the V-P-O-Fplanes: (a and b) 16; (c and d) 17; (e andf) 18.

O or V-P-O-F frameworks o f expanded dimensionality. Thus, both [ H N C H C H N H 3 ] 2 [ V 0 F 4 ( H 0 ) { 0 3 P ( C H ) P 0 } 4 ] (19) and ( H 0 ) [ V F ( H 0 ) { 0 P ( C H ) P 0 } ] (20) are three-dimensional, albeit with quite distinct vanadium substructures (Figure 8). 3

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Figure 8. The three-dimensional structures of (a) (19) and (b) (H 0) (20). 3

When metal complex cations are introduced to provide charge compensation, as well as an additional coordinating component, an un usually broad range of structures is observed. The tendency of Cu(II) coordination to the oxygen atoms o f the V-P-O-F substructure to reduce the dimensionality o f this subunit is evident in the one-dimensional structure o f



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Figure 9. Views of the structures of (a) (21); (b) (22); (c) (23); and (d) (24).

[ { C u ( b i s t e i p y ) } V F 0 { 0 P C H P 0 } { H 0 P C H P 0 } ] (21) and, the twodimensional structures o f [ { C u ( b i s t e r p y ) } V F 0 ( H 0 ) } H 0 3 P ( C H ) 2 P O j } ] (22), t { C u ( b i s t e r p y ) ( H 0 ) } V F 0 { 0 P ( C H ) P 0 3 } { H 0 P ( C H ) P 0 H } ] (23) and [{Cu (bisterpy)}V F 04(OH)(H 0){0 P(CH )5P0 } { H 0 P ( C H ) P 0 } ] (24) , all o f which are constructed from V-P-O-F chains and {Cu (bisterpy)} clusters (Figure 9). However, the overall three-dimensional structure of [ {Cu (bisterpy)(H 0)} V F40 (OH)4 { H 0 P ( C H ) P 0 H } { 0 P ( C H ) P 0 } ] (25) exhibits a two-dimensional V-P-O-F substructure, while [{Cu (bisterpy)}V F 0 {0 P(CH ) P0 } ] (26) exhibits a [V^CMOjP2

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(CH ) P0 } ] "pillared" framework component (Figure 10). As noted in Table 1, the introduction of {Cu (bisterpy)} rods as building units generally correlates with reduced complexity of the V - P - O or V-P-O-F substructure. Thus, for the sixteen structures reported to date, thirteen exhibit cluster of chain V-P-O(F) substructures. Only one material possesses a twodimensional V-P-O-F subunit, which is so common in the absence of the secondary metal-ligand component. On the other hand, two of these materials are characterized by three-dimensional V-P-0(F) substructures. 2

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Structures and Properties The structural diversity of these vanadium-diphosphonate materials is reflected in their properties, such as magnetism and thermal stability. While the



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[ {Cu (bisterpy)} V F 0 {H0 PCH P0 } {0 PCH P0 } ] [{Cu (bisterpy)}V 0 {H0 PCH P0 } ] [{Cu (bisterpy)}V F 0 (H 0) (H0 P(CH ) P0 } ].2H 0 [{Cu (bisterpy)]V F 0 {H0 P(CH ) P0 H}] [{Cu (bisterpy)(H 0) }V 0 {H0 P(CH ) P0 } ] [{Cu (bisterpy)(H O) V F 0 {0 P(CH ) PO }{HO P(CH ) P03}] [{Cu (bisterpy)} V F O {HO P(CH ) PO } ]-0.8H O [{Cu (bisterpy)}V O {0 P(CH ) P0 } ]-4H 0 [{Cu (bisterpy)(H 0)}V0 {0 P(CH ) P0 }{H0 P(CH ) P0 H }] [{Cu (bisterpy)}V 0 {H0 P(CH ) P0 } ] [{Cu (bisterpy)} V O (OH) (H0 P(CH ) P0 } {0 P(CH ) P0 } ] [{Cu (bisterpy)}V F 0 {0 P(CH ) P0 } ] [{Cu (bisterpy)}V 0 (OH) {0 P(CH ) P0 }]«4H 0 [{Cu (bisterpy)}V F 0 (OHXH 0){H0 P(CH ) P0 }{0 P(CH ) P0 }] [{Cu (bisterpy)} V F 0 (OH) {H0 P(CH ) P0 H} {0 P(CH )sP0 } ] [{Cu (bisterpy)}V 0 {H0 P(CH ) P0 } ]»7«3H 0

Compound

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1-D 1-D 2-D 1-D 2-D 2-D 3-D 2-D 1-D 3-D 3-D 3-D 2-D 2-D 3-D 3-D

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V-P-O or V-P-O-F Substructure Chains Clusters Chains Clusters Chains Chains Clusters Chains Clusters Chains Clusters Framework Clusters Chains Layer Framework

No No No No No No Yes,X = O No No No No Yes, X = F No Yes, X = OH X = O and X = F No

V-X-V Linkage

No No No Yes No No Yes Yes No Yes Yes Yes No Yes Yes Yes

V-O-Cu Linkage

Table I. Summary of structural characteristics for materials containing anionic V-P-O or V-O-O-F components and {Cu (bisterpy)} cations.



402

Figure 10. The three-dimensional structures of (25) and (26): (a) a view of the structure of 25 in the be plane; (b) the vanadophosphonate substructure of 25; (c) a view of 26; (d) the V-P-O-F layers of 26.

majority of the materials conform to simple Curie-Weiss behavior, several exhibit more complicated magnetic properties. For example, the magnetic susceptibility o f [V 02(H20){03P(CH2)nP0 }] (1) is best described by the Heisenberg dimer model with S = 1 (Figure 11), while that of [H3N(CH2)2NH3][V20 F2(H 0)2{03P(CH2)4P03}] (17) is described by the Heisenberg linear antiferromagnetic chain model for V(IV) (Figure 12). A noteworthy observation is that the oxyfluorovanadium/diphosphonate frameworks are thermally robust and are retained well past the dehydration temperature. This is evident for [ H N C H C H N H 3 ] [ V 0 F 4 ( H 0 ) _ { 0 P ( C H ) P 0 } ] * 7 H 0 (19) which loses water in the 50-250°C range and exhibits partial decomposition of the cation between 250-400°C. However, the thermodiffraction profile is unchanged to 450°C, indicating that the V-P-O-F framework is thermally robust and persistent to 450°C (Figure 13). 2

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Conclusions Hydrothermal chemistry provides a facile route to the synthesis of a large number of materials of the vanadium-organodiphosphonate family. The emerging structural systematics for this class of materials reveals several significant structural determinants. These include the polyhedral and oxidation state variability of vanadium, the promiscuous possibilities for vanadium and phosphorus polyhedral connectivities, the flexibility of P-O(H) bond distances and V-O-P bond angles, variable protonation of {P0 } groups and vanadium oxo-sites, coordination of aqua ligands, incorporation of varying ratios of fluoride to vanadium, organic tether lengths, the presence of cationic compounds, and the coordination of secondary metals, such as copper. 3

Acknowledgment This work was funded by a grant from the National Science Foundation, CHE-0604527.

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(a) Noguera, C., Physics and Chemistry at Oxide Surfaces, Cambridge University Press: Cambridge, U K 1996; (b) Kung, H.H., Transition Metal Oxides: Surface Chemistry and Catalysis, Elsevier: Amsterdam 1989. Bruce, D.W.; O'Hare, D., eds., Inorganic Materials, Wiley: Chichester 1992. Cheetham, A . K . , Advanced inorganic materials: an open horizon, Science 1994, 264, 794-795. Büchner, W.; Schliebs, R.; Winter, G.; Büchel, K . H . , Industrial Inorganic Chemistry, V C H , New York 1989. McCarroll, W.H., Oxides: solid state chemistry, Encyclopedia of Inorganic Chemistry. R . B . King, ed., John Wiley and Sons, New York 1994, vol. 6, 2903-2946. Newsam, J . M . , Zeolites, Solid State Compounds, A . K . Cheetham and P. Day eds., Clarendon Press, Oxford 1992, 234-280. Wells, A.F., Structural Inorganic Chemistry, 6th Ed., Oxford University Press: New York 1987. Greenwood, N . N . ; Earnshaw, A . , Chemistry of the Elements, 2nd Ed., Butterworth-Heinemann, Oxford, England 1997. Hench, L . L . , Inorganic Biomaterials, Materials Chemistry, an Emerging Discipline, L . V . Interrante, L . A . Casper, A . B . Ellis, eds., A C S Series 245, chapter 21, pp. 523-547, 1995. Smyth, J.R.; Jacobsen, S.D.; Hazen, R . M . , Comparative crystal chemistry of dense oxide minerals, Rev. Mineral. Geochem. 2001, 41, 157-186. Mason, B., Principles of Geochemistry, 3rd ed., Wiley, New York 1966. Lowenstan, H . A . ; Weiner, S., On Biomineralization, Oxford University Press, New York, 1989. Cölfen, H . ; Mann, S., High order organization by mesoscale selfassembly and transformation of hybrid nanostructures, Angew. Chem., Int. Ed. Engl. 2003, 42, 2350-2365. Soler-Illia, G.J. de A . A . ; Sanchez, C.; Lebean, B . ; Patarin, J., Chemical strategies to design textured materials: from microporous and mesoporous oxides to nanonetworks and hierarchical structures, Chem. Rev. 2002, 102, 4093-4138. Complexity is a subject of significant and general scientific interest. Complexity in chemistry refers to the description and manipulation of systems of molecules, as in living cells and materials. In the latter context, organic-inorganic hybrid structures partake of the chemical complexity of materials, with the attendant complications of predictability and rational design. See, for example: Whitesides, G . M . ; Ismagilov, R.F., Complexity in chemistry, Science 1999, 284, 89-92. The relationship between complexity and functionality is abundantly evident in biological systems. Chemists may learn from biology and make the creative leap to the design of inorganic materials whose structures are influenced by organic



406 molecules. See, for example: Kiss, I.Z.; Hudson, J.L., Chemical complexity: spontaneous and engineered structures, AICLE Journal 2003, 49, 22342241; Lehn J.M., Toward complex matter: supramolecular chemistry and self-organization, Proc. Natl. Acad. Sci. 2002, 99, 4763-4768; Lehn, J.M., Toward self-organization and complex matter, Science, 2002, 295, 24002403; Förster, S.; Plantenberg, T., From self-organizing polymers to nanohybrid and biomaterials, Angew. Chem., Int. Ed. Engl. 2002, 41, 688714; Miller, A . D . , Order for free: molecular diversity and complexity promote self-organization, Chem. Biochem. 2002, 3, 45-46. 18. Janiak, C., Engineering coordination polymers towards applications, Dalton Trans. 2003, 2781-2804, and references therein. 19. Mitzi, D . B . . Templating and structural engineering in organic-inorganic perovskites, Dalton Trans. 2001, 1-12. 20. The properties of inorganic-organic hybrids, specifically metal-organic frameworks (MOF) have been extensively elaborated in recent years. For example, porosity: (a) Chen, B.; Ockwig, N.W.; Millward, A.R.; Contreras, S.D.; Yaghi, O.M., High H2 adsorption in a microporous metal-organic framework with open-metal sites, Angew. Chem. Int. Ed. Eng. 2005, 44, 4745-4749; (b) Bradshaw, D.; Claridge, J.B.; Cussen, E.J.; Prior, T.J.; Rosseinsky, M.J., Design, chirality, and flexibility in nanoporous moleculebased materials, Chem. Res. 2005, 38, 273-282; (c) Ohmori, O.; Kawano, M . ; Fujita, M., A two-in-one crystal: uptake of two different guests into two distinct channels of a biporous coordination network, Angew. Chem., Int. Ed. 2005, 44, 1962-1964; (d) Wu, C-D.; L i n , W., Highly porous, homochiral metal-organic frameworks: solvent-exchange-induced singlecrystal to single-crystal transformations, Angew. Chem., Int. Ed. 2005, 44, 1958-1961. Gas sorption: (a) Sudik, A . C . ; Millward, A.R.; Ockwig, N . W . ; Cote, A.P.; Kim, J.; Yaghi, O.M., Design, synthesis, structure, and gas ( N , Ar, CO2, CH4, and H ) sorption properties of porous metal-organic tetrahedral and heterocuboidal polyhedra, J. Am. Chem. Soc. 2005, 127, 7110-7118; (b) Kitaura, R.; Kitagawa, S.; Kubtoa, Y.; Kobayashi, T.C.; Kindo, K . ; Mita Y . ; Matsuo, A . ; Kobayashi, M.; Chang H-C.; Ozawa, T.C.; Suzuki, M.; Sakata, M.; Takata, M.; Formation of a one-dimensional array of oxygen in a microporous metal-organic solid, Science 2002, 298, 23582361. a. Chiral separations: Bradshaw, D.; Prior, T.J.; Cussen, E.J.; Claridge, J.B.; Rosseinsky, M.J., Permanent microporosity and enantioselective sorption in a chiral open framework, J. Am. Chem. Soc. 2004, 126, 6106-6114. Molecular sensing: Haider, G.J.; Kepert, C.J.; Moubaraki, B.; Murray, K.S.; Cashion, J.D., Guest-dependent spin crossover in a nanoporous molecular framework material, Science (Washington, DC, US) 2002, 298, 1762-1765. 2

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21. The literature on organic-inorganic hybrid materials is voluminous. Some recent reviews and representative articles include: (a) Kitagawa, S.; Noro, S., Coordination polymers: infinite systems, Comprehensive Coordination Chemistry II 2004, 7, 231-261; (b) Rao, C.N.R.; Natarajan, S.; Vaidhyanathan, R., Metal carboxylates with open architectures, Angew. Chem., Int. Ed. 2004, 43, 1466-1496; (c) Yaghi, O . M . ; O'Keeffe, M.; Ockwig, N.W.; Chae, H.K.; Eddaoudi, M.; Kim, J., Reticular synthesis and the design of new materials, Nature 2003, 423, 705-714.


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