ARTICLE pubs.acs.org/crystal
Unusual Hybrid Materials Prepared by the Oxidation of a Ketone Daniel L. Holden,† Helen V. Goulding,† John Bacsa,† Neil G. Berry,† Nicholas Greeves,† Richard A. Stephenson,‡ Ross W. Harrington,§ William Clegg,§ and Andrew M. Fogg*,† †
Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, United Kingdom EPSRC X-ray Crystallography Service, School of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, United Kingdom § School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU, United Kingdom ‡
bS Supporting Information ABSTRACT: Four new layered phases have been synthesized by the hydrothermal reactions of zinc and magnesium salts with fenbufen (γ-oxo-(1,10 -biphenyl)-4-butanoic acid) as a result of the oxidation of fenbufen at the ketone forming biphenylcarboxylate. Reactions with zinc nitrate give different degrees of oxidation, yielding Zn(C16H13O3)(C13H9O2)(H2O) (1) and Zn5(OH)6(C13H9O2)4 (2), depending on the reaction temperature. Compound 2 has an unusual zinc hydroxide layer structure which contains hydroxide-centered Zn4OH(OH)4 units where the central hydroxide has two long and two short bonds to the Zn atoms. The mechanism for the ketone oxidation has been investigated by a series of DFT calculations which indicate that a concerted, metalmediated pathway is lower in energy than a two-step process. The importance of the metal cation in this oxidation reaction has been shown by demonstrating that oxidation of fenbufen also occurs during reactions with magnesium nitrate and zinc chloride. These reactions yielded fully oxidized Mg(C13H9O2)2(H2O)2 (3) and half-oxidized Zn(C16H13O3)(C13H9O2)(H2O) (4), a polymorph of 1. Reactions with ammonium salts gave only recrystallized fenbufen, providing further evidence for the role of the metal cation in these oxidation reactions.
’ INTRODUCTION Organicinorganic hybrid materials represent an extensive and diverse range of compounds, which have received significant attention in recent years because of their structural and compositional flexibility creating scope for applications in numerous fields.16 These hybrid materials can generally be grouped into two classes: coordination polymers, including metal organic frameworks (MOFs), and extended inorganic hybrids.1,2 The coordination polymers contain isolated metal centers or clusters linked by organic molecules, which are most commonly carboxylates although sulfonate710 and phosphonate1114 bridges are also well-known. Large numbers of chain and layered coordination polymers have been reported but the majority of the interest has centered on framework materials following the initial report of the porous MOF-5.15 Since this discovery, a wide range of materials displaying interesting properties including structural flexibility16,17 and gas storage18,19 have been reported. Extended inorganic hybrids have long-range inorganic connectivity, usually through MOM links, in the structure. As is observed for the coordination polymers there is a high degree of structural diversity among these hybrid materials with 1D and 2D structures being particularly common. Examples of these include zinc organophosphate chains,20 metal succinate sheets21 and zirconium phosphonate layers.2224 Three-dimensional examples are much rarer but are known in the nickel succinate25 and glutarate26 systems and for cadmium malonate.27 In this paper, we report the synthesis of new coordination polymers and extended inorganic hybrid materials which result r 2011 American Chemical Society
from the oxidation of a ketone during the hydrothermal reaction. The oxidation of ketones is an uncommon reaction and is usually achieved through a BaeyerVilliger oxidation forming an ester or lactone by reaction with stoichiometric amounts of an organic peroxy acid or through the use of a transition metal catalyst which is most often a PtII species.28 In this study the organic molecule undergoing oxidation is the nonsteroidal anti-inflammatory drug fenbufen (γ-oxo-(1,10 -biphenyl)-4-butanoic acid) leading to the formation of the corresponding acid biphenyl-4-carboxylate as shown schematically in Figure 1. DFT calculations have indicated a Zn2þ or Mg2þ metal-mediated pathway for the reaction. The oxidation of other 4-oxo-4-arylbutanoic acids, forming carboxylic acids, has previously been achieved by reaction with alkaline hexacyanoferrate(III) solutions although little mechanistic information is available.29 The in situ formation of ligands during hydrothermal or solvothermal syntheses has been observed for many other systems and can provide a useful route to ligands which are not readily available through more conventional methods.30
’ EXPERIMENTAL SECTION Synthesis. All of the organicinorganic hybrid materials described in this paper were prepared via a hydrothermal synthesis. In a typical reaction 10 mL of a 1 M aqueous solution of MX2 3 6H2O (M = Zn, Received: March 7, 2011 Revised: May 6, 2011 Published: May 17, 2011 3013
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Figure 1. Schematic for the oxidation of fenbufen to biphenyl-4-carboxylate. Mg; X = NO3, Cl) was added to either 0.0255 (for Mg2þ) or 0.0212 (for Zn2þ) g of fenbufen (γ-oxo-(1,10 -biphenyl)-4-butanoic acid) in a Teflon-lined 23 mL autoclave. The resulting mixture was treated hydrothermally at either 135 or 165 °C for 48 h. The resulting product was then filtered, and washed with deionized water and ethanol before being dried in air at room temperature. Four distinct products were obtained. Reactions with Zn(NO3)2 3 6H2O yielded Zn(C16H13O3)(C13H9O2)(H2O) (1) at 135 °C and Zn5(OH)6(C13H9O2)4 (2) at 165 °C, those with Mg(NO3)2 3 6H2O gave Mg(C13H9O2)2(H2O)2 (3) at all temperatures, while those with ZnCl2 formed a second polymorph of Zn(C16H13O3)(C13H9O2)(H2O) (4) at 165 °C. Characterization. Powder X-ray diffraction patterns were recorded with Cu KR1 radiation on a Stoe Stadi-P diffractometer in either BraggBrentano or DebyeScherrer geometry. A combination of thermogravimetric analysis (TGA) and elemental analysis was used to determine the stoichiometry of the materials. TGA was performed using a Perkin-Elmer STA6000 Simultaneous Thermal Analyzer and the sample was heated to 1000 °C in an atmosphere of nitrogen at a rate of 5 °C/min. ICP analysis, to determine the metal content of the samples, was carried out on a Ciros CCD optical emission spectrometer following complete dissolution of the samples in dilute HNO3, and CHN analysis was performed on a FlashEA 1112 instrument. Fourier transform infrared (FTIR) spectra were obtained using a Perkin-Elmer Spectrum 100 spectrometer fitted with the Spectrum 100 Universal Diamond/ZnSe ATR. 1H NMR spectra of the reactions performed in D2O were recorded on a Bruker ARX 250 spectrometer. Mass spectrometry of the reaction solutions following filtration was performed on a Micromass LCT Mass Spectrometer. X-ray Crystallography. The crystal structures of two of the zinc phases, Zn(C16H13O3)(C13H9O2)(H2O) (1) and Zn5(OH)6(C13H9O2)4 (2), were determined in the Department of Chemistry, University of Liverpool. For these samples the crystals grew as thin, flat plates. A suitable, good quality crystal was selected from the sample, secured on a glass fiber and placed in a cold stream at 100 K. Singlecrystal X-ray diffraction data were collected on a Bruker D8 diffractometer with an APEX CCD detector, and 1.5 kW graphite-monochromated MoKR radiation. Unit cell refinement and data reduction were performed with SAINT version 6.45a.31 A semiempirical absorption correction using multiple and symmetry-equivalent reflections was carried out using the program SADABS V2008132 and the structure was solved and refined with SHELX97.33 The crystal structure of the magnesium compound, Mg(C13H9O2)2(H2O)2 (3), was determined at the EPSRC X-ray Crystallography Service, School of Chemistry, University of Southampton. Single-crystal X-ray diffraction analysis of 3 was performed using a Bruker APEXII CCD diffractometer mounted at the window of a Bruker FR591 rotating anode (λMoKR = 0.71073 Å) and equipped with an Oxford Cryosystems Cryostream device (data collected at 120 K). Data were processed using the Collect34 package and unit cell parameters were refined against all data. An empirical absorption correction was carried out using SADABS.32 The structures were solved by direct methods using SHELXS9733 and refined on Fo2 by full-matrix least-squares refinements using SHELXL97.33 All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were added at calculated positions and refined using a riding model. Figures were
Figure 2. Crystal structure of Zn(C16H13O3)(C13H9O2)(H2O) (1). produced using ORTEP3 for Windows.35 3 crystallizes as extremely thin plates. As a result only very weak data were obtained at high angles leading to the poor statistics. The result presented is the best that was obtained after extensive screening of the crystals and clearly shows that the fenbufen has been oxidized during this reaction. The single-crystal X-ray diffraction analysis of 4 was performed on data collected on Beamline I19 of the Diamond Light Source, using a Crystal Logics kappa-geometry diffractometer and a Rigaku Saturn 724þ CCD detector with a Cryostream cooler (at 120 K); Rigaku CrystalClear was used to record images,36 Bruker APEX2 for data integration,37 and SHELXTL for structure solution and refinement,33 with procedures similar to those for 13. The synchrotron X-ray wavelength was 0.6889 Å. Computational Chemistry. All structures were optimized at the density functional theory (DFT) level by using the B3LYP functional as implemented in PCGAMESS.38 All geometry optimizations used the 6-31** basis set for all atoms and were performed with no symmetry restrictions. Vibrational frequency calculations at the B3LYP/6-31G** level of theory were used to derive zero-point-energy and entropy contributions at 438.15K using unscaled frequencies. Energy minima were confirmed by the lack of imaginary frequencies. To locate transition 3014
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Table 1. Summary of the Crystallographic Information for 14 compound number empirical formula
1 C29H24O6Zn
2 C52H42O14Zn5
formula weight
533.85
1217.71
450.71
533.85
temperature (K)
100
100
120
120
wavelength (Å) crystal system
0.71073 orthorhombic
0.71073 monoclinic
0.71073 monoclinic
synchrotron, 0.6889 Å monoclinic
space group
Pna21
C2/c
C2/c
Cc
a (Å)
8.0336(11)
48.786(9)
45.229(14)
52.27(3)
b (Å)
5.5785(7)
5.6211(11)
6.5476(19)
5.618(3)
c (Å)
51.750(8)
16.530(3)
7.2194(18)
8.091(4)
3 C26H18MgO6
4 C29H24O6Zn
unit cell dimensions
β (deg)
96.074(3)
90.32(2)
94.773(6)
volume (Å3) Z
2319.2(5) 4
4507.7(15) 4
2137.9(10) 4
2368(2) 4
density (calculated) (g cm3)
1.529
1.794
1.400
1.498
absorption coefficient (mm1)
1.104
2.696
0.125
1.001
crystal size (mm3)
0.6 0.4 0.04
0.30 0.10 0.03
0.16 0.08 0.01
0.05 0.02 0.01
θ range for data collection (deg)
2.4 25.4
1.7 25.4
3.0 24.1
1.524.2
reflections collected
7175
10 774
9710
8703
independent reflections, Rint
3104, 0.0475
4112, 0.0468
1691, 0.0971
3885, 0.0831
max. and min transmission restraints/refined parameters
0.745 and 0.553 334/313
0.924 and 0.499 805/409
0.999 and 0.980 0/152
0.990 and 0.950 4/331
goodness-of-fit on F2
1.153
1.018
2.341
1.054
R (F, F2 > 2σ)
0.0708
0.0426
0.2054
0.0608
Rw (F2, all data)
0.1517
0.1058
0.5659
0.1312
max, min electron density (e Å3)
1.4, 2.6
0.95, 0.66
0.73, 0.65
0.74, 0.75
states certain interatomic distances were fixed (e.g., nucleophilic oxygen to carbonyl carbon, 1.9 Å) and the rest of the molecule optimized and the Hessian calculated. This structure was then subjected to a saddle point calculation with no constraints. Transition states were confirmed by the appearance of only one imaginary frequency with associated atomic motion consistent with the mechanism and IRC calculations confirmed that the appropriate starting material and product(s) formed. The free energy G(gas) has been calculated as follows: G(gas) = H(gas) TS(gas), H(gas) = H(SCF) þ ZPE. G(gas) = free energy in gas phase, H(gas) = enthalpy in gas phase, T = temperature, 438.15 K, S(gas) = entropy in gas phase, H(SCF) = self-consistent field energy, ZPE = zero point energy assuming harmonic oscillator approximation. Structures were visualized using Molekel. All calculations were performed on University of Liverpool linux clusters.
’ RESULTS AND DISCUSSION New organicinorganic hybrid phases have been synthesized following the oxidation of fenbufen molecules during the course of hydrothermal reactions with the nitrate and chloride salts of zinc and magnesium. Crystal structures have been determined for all of these materials and they have been further characterized by powder XRD, TGA, FTIR, and elemental analysis, which is included in the Supporting Information. In each case it has been observed that fenbufen has been either fully or partially oxidized at the ketone to form biphenyl-4-carboxylate. In the case of the reaction between fenbufen and zinc nitrate two different products were obtained depending on the reaction temperature. The first of these materials, Zn(C16H13O3)(C13H9O2)(H2O) (1), is obtained as a pure phase at 135 °C. 1 crystallizes with an orthorhombic structure (space group
Pna21) which is shown in Figure 2 with full crystallographic details summarized in Table 1. From Figure 2 it can be seen that it is a highly unusual structure containing two different organic ligands, both fenbufen and its oxidation product biphenyl-4carboxylate, bound to each Zn atom. The structure comprises layers with the composition Zn(C16H13O3)(C13H9O2)(H2O) which gives rise to a bilayer of the two different organic molecules between the inorganic components. Each Zn atom is 4-coordinate with a distorted tetrahedral geometry, binding to one water, one biphenyl-4-carboxylate and two fenbufen molecules. Each fenbufen bridges two Zn centers while biphenyl-4-carboxylate coordinates through one O atom to a single metal atom. The purity of this phase was confirmed by a combination of elemental analysis, TGA and powder XRD, the data for which are included in the Supporting Information (Figure S1 and S2). The FTIR spectrum of 1 is also included in the Supporting Information (Figure S3) and clearly shows a ketone stretch at 1684 cm1 along with other bands characteristic of both fenbufen and biphenyl-4-carboxylate. When the reaction is performed at the higher temperature of 165 °C a second phase is obtained in which the fenbufen has been fully oxidized. It has not been possible to isolate this phase pure as some amorphous inorganic material is always formed under these conditions and this becomes the dominant reaction product at even higher temperatures. This phase, Zn5(OH)6(C13H9O2)4 (2), crystallizes with a monoclinic structure (space group C2/c) which is shown in Figure 3a with full crystallographic details given in Table 1. The structure comprises zinc hydroxide layers separated by a bilayer of coordinated biphenyl4-carboxylate anions. Within these layers (Figure 3b) there are three crystallographically distinct Zn atoms. Two of these have a 5-coordinate trigonal bipyramidal geometry while the third 3015
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Figure 3. (a) Crystal structure and (b) layer structure of Zn5(OH)6(C13H9O2)4 (2). (c) The hydroxide-centered Zn4OH(OH)4 unit.
Zn atom has an out-of-center, distorted octahedral environment, with an unusually long ZnO bond. The five-coordinate zinc atom Zn(1) is bound to the four hydroxides and one carboxylate O atom, while Zn(2) is coordinated by three hydroxides and O atoms from two different biphenyl-4-carboxylate anions. Both five-coordinate zinc atoms are associated with a nearly linear OZnO angle, corresponding to the bipyramid axis and the
bipyramids are elongated along this direction. The coordination geometries can be more accurately described as trigonal bipyramidal with some distortion toward square pyramidal geometry. The value of τ, a geometric parameter which gives a measure of the amount of distortion in these 5-coordinate systems, is 0.78 for Zn(1) and 0.94 for Zn(2), indicating that they are closer to ideal trigonal bipyramidal geometry than the square pyramidal one.39 3016
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Figure 4. Crystal structure of Mg(C13H9O2)2(H2O)2 (3).
Zn(3) is surrounded by three hydroxides, two carboxylate O atoms from different anions, with a longer bond of 2.5163(2) Å to a fourth hydroxide anion. Each of the biphenyl-4-carboxylate anions in the structure bridges between two Zn cations. The layer also contains an unusual hydroxide-centered Zn4OH(OH)4 unit. Unlike the other OH groups which are 3-coordinate, the coordination environment of the central hydroxide anion has two long (2.558(3) and 2.530(3) Å) and two short bonds (2.018(7) and 2.053(7) Å) to the zinc atoms. This OH group is located on a crystallographic 2-fold rotation axis and must therefore have an even number of off-axis Zn neighbors. However, two bonds to Zn are too few to satisfy the bonding requirements of this hydroxide and four too many, so the coordination environment is distorted to give an effective coordination number of three from two short and two long interactions. The central hydroxide (O7A) is split into two different positions each with a 50% site occupancy. Attempts to refine a single site anisotropically produced an extremely prolate ellipsoid indicating that the disordered model is the correct one. This arrangement of ZnO bonds ensures that the OH group contributes a total of 1.0 vu to the Zn atoms and the valence sum of ZnO bonds is 2.40 The FTIR spectrum of 2 does not show a characteristic ketone stretch but only bands expected by comparison with the spectrum of biphenyl-4-carboxylate (see Supporting Information Figure S5). It is apparent from the crystal structures of 1 and 2 that fenbufen is being oxidized at the ketone group during the course of the hydrothermal reaction. In order to determine whether or
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Figure 5. Crystal structure of Zn(C16H13O3)(C13H9O2)(H2O) (4).
not the oxidation reaction is metal-mediated the synthesis was repeated using either Mg(NO3)2 3 6H2O or NH4NO3. In the latter case only recrystallized fenbufen was obtained from the reaction, suggesting that the metal cation is essential for the oxidation process. This hypothesis is further confirmed by the product from the reaction with Mg(NO3)2, Mg(C13H9O2)2(H2O)2 (3), which contains only biphenyl-4-carboxylate. In this case this product was obtained at all temperatures investigated. 3 crystallizes in the monoclinic space group C2/c and contains octahedral Mg cations bound to four biphenyl-4-carboxylate anions and two water molecules. The organic anions bridge between two Mg cations, giving rise to the layered structure shown in Figure 4. 3 was obtained phase pure and the characterizing data are given in the Supporting Information (Figures S68). As for 2 no ketone stretch is observed in the FTIR spectrum. Although the crystal structure of 3 is of only low precision, the result demonstrates unambiguously the nature of the product of this reaction. To further confirm the role of the metal cation in these reactions the possibility of nitrate being the oxidizing agent was ruled out by the use of ZnCl2 in place of the nitrate salt in the hydrothermal reaction. In this case Zn(C16H13O3)(C13H9O2)(H2O) (4), a polymorph of 1, was obtained and the structure is shown in Figure 5. 4 crystallizes in the monoclinic space group Cc and, as for 1, has a distorted tetrahedral coordination Zn environment comprising two fenbufen, one biphenyl-4-carboxylate and one water molecule. The differences between 1 and 4 arise from 3017
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the stacking of the Zn(C16H13O3)(C13H9O2)(H2O) layers which in 4 have an AAAA stacking sequence while in 1 they are arranged ABAB where A and B are related by a 180° rotation. As for 1 a ketone stretch is observed at 1684 cm1 in the FTIR spectrum (Supporting Information Figure S10). The oxidation of 4-oxo-4-arylbutanoic acids is not unprecedented and alkaline hexacyanoferrate(III) solutions have been reported to be suitable oxidizing agents; however, detailed
Figure 6. Concerted (left) and stepwise (right) pathways for the cationmediated oxidation of fenbufen modeled using DFT.
Table 2. Activation Energies Relative to Starting Material of the Two Pathways for Febufen Oxidation stepwise (kcal/mol) concerted TS (kcal/mol)
TS1
TS2
Fenbufen Zn
40.80
48.72
12.98
Fenbufen Mg
45.74
57.34
18.06
mechanistic information for the reaction is not known.29 DFT calculations have been used to investigate the possible mechanism of oxidation in these reactions using 4-oxo-4-phenylbutanoic acid as a model compound. Two mechanistic pathways were investigated for this unusual oxidation process. The first is a concerted single-step process and the second a stepwise twostage process, both of which are outlined in Figure 6. Both pathways were studied with the Zn2þ, Mg2þ, and NH4þ cations. In order to facilitate the modeling the calculations were performed for species in vacuo. As can be seen from the activation energies summarized in Table 2, the concerted pathway is considerably lower in energy than the stepwise process for both Zn2þ and Mg2þ. In this mechanism, the metal cation is coordinated by both the carboxylate and the ketone O atoms leading to the formation of a cyclic intermediate from which cyclopropanone is eliminated. The transitions states for the two mechanisms are shown in Figure 7. When NH4þ was used as the cation, no transition states for either the concerted or stepwise pathways that gave the oxidation product were found. Instead, proton transfer was observed between NH4þ and the carboxylate group. These results closely mirror the experimental findings in suggesting that the oxidation process has a viable reaction profile when mediated by either Zn2þ or Mg2þ but no oxidation pathway is present when NH4þ is employed. To test the predicted mechanism the reaction between fenbufen and Zn(NO3)2 at 135 °C was repeated in D2O to permit the identification of other organic products. The 1H NMR spectrum of the resulting solution is complex but showed a resonance at 0.88 ppm which can be assigned to cyclopropanediol (Supporting Information Figure S12) which would be expected to result from the hydration of cyclopropanone.41 Mass spectrometry confirms the complex nature of the system indicating the presence of much higher molecular weight species in the solution (Supporting Information Figure S13). The isotope pattern is strongly suggestive of the presence of zinc, implying that residual zinc complexes are present in the solution.
’ CONCLUSIONS Four new layered phases have been synthesized and structurally characterized following the oxidation of a ketone during hydrothermal reaction. The reactions between fenbufen and zinc nitrate resulted in the formation of either Zn(C16H13O3)(C13H9O2)(H2O) (1), in which half of the fenbufen molecules have been oxidized to biphenylcarboxylate, or Zn5(OH)6(C13H9O2)4 (2) which contains only the oxidized organic anions, depending on the reaction temperature. DFT calculations indicate that the energetically favorable reaction pathway is
Figure 7. Calculated structures of the concerted TS for 1 (left), and stepwise TSs (middle and right) for the Zn-mediated oxidation of fenbufen (carbon green, hydrogen white, oxygen red, zinc cyan. Dotted gray line indicates partial bond with key distances indicated in Ångstroms. Coordinates of all TSs are provided in the Supporting Information.). 3018
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Crystal Growth & Design via a concerted, metal-mediated mechanism. Further reactions between fenbufen and magnesium nitrate or zinc chloride provide support for this mechanism as they result in the formation of Mg(C13H9O2)2(H2O)2 (3) which contains only the biphenylcarboxylate oxidation product and Zn(C16H13O3)(C13H9O2)(H2O) (4), a polymorph of 1, respectively. Recrystallized fenbufen is the only product if the reactions are repeated with ammonium salts.
’ ASSOCIATED CONTENT
bS
Supporting Information. Further details of the structural refinements, crystal structures and additional characterizing data (powder XRD, FTIR, and elemental analysis) for all the materials are provided. This information is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*Tel: 44 151 794 2047. Fax: 44 151 794 3587. E-mail: afogg@ liverpool.ac.uk.
’ ACKNOWLEDGMENT AMF thanks the Royal Society for a University Research Fellowship and we thank The Wellcome Foundation for a vacation scholarship (DLH), EPSRC for funding the X-ray Crystallography Service (both laboratory and synchrotron components), and Diamond Light Source for access to synchrotron facilities. ’ REFERENCES (1) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Chem. Commun. 2006, 4780. (2) Maspoch, D.; Ruiz-Molina, D.; Veciana, J. Chem. Soc. Rev. 2007, 36, 770. (3) Morris, R. E.; Wheatley, P. S. Angew. Chem., Int. Ed. 2008, 47, 4966. (4) Bradshaw, D.; Claridge, J. B.; Cussen, E. J.; Prior, T. J.; Rosseinsky, M. J. Acc. Chem. Res. 2005, 38, 273. (5) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334. (6) James, S. L. Chem. Soc. Rev. 2003, 32, 276. (7) Deacon, G. B.; Gitlits, A.; Zelesny, G.; Stellfeldt, D.; Meyer, G. Z. Anorg. Allg. Chem. 1999, 625, 764. (8) Gandara, F.; Garcia-Cortes, A.; Cascales, C.; Gomez-Lor, B.; Gutierrez-Puebla, E.; Iglesias, M.; Monge, A.; Snejko, N. Inorg. Chem. 2007, 46, 3475. (9) Snejko, N.; Cascales, C.; Gomez-Lor, B.; Gutierrez-Puebla, E.; Iglesias, M.; Ruiz-Valero, C.; Monge, M. A. Chem. Commun. 2002, 1366. (10) Song, J. L.; Lei, C.; Mao, J. G. Inorg. Chem. 2004, 43, 5630. (11) Groves, J. A.; Stephens, N. F.; Wright, P. A.; Lightfoot, P. Solid State Sci. 2006, 8, 397. (12) Groves, J. A.; Miller, S. R.; Warrender, S. J.; Mellot-Draznieks, C.; Lightfoot, P.; Wright, P. A. Chem. Commun. 2006, 3305. (13) Liu, X. G.; Zhou, K.; Dong, J.; Zhu, C. J.; Bao, S. S.; Zheng, L. M. Inorg. Chem. 2009, 48, 1901. (14) Song, S. Y.; Ma, J. F.; Yang, J.; Cao, M. H.; Zhang, H. J.; Wang, H. S.; Yang, K. Y. Inorg. Chem. 2006, 45, 1201. (15) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276. (16) Mellot-Draznieks, C.; Serre, C.; Surble, S.; Audebrand, N.; Ferey, G. J. Am. Chem. Soc. 2005, 127, 16273.
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