Variable Dimensionality in the UO2(CH3CO2) - American Chemical

Apr 16, 2003 - South Park Road, Oxford, OX1 3QR, United Kingdom. Received November 19, 2002. Revised Manuscript Received February 5, 2003...
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Chem. Mater. 2003, 15, 1970-1975

Variable Dimensionality in the UO2(CH3CO2)2·2H2O/HF/ Isonicotinic Acid System: Synthesis and Structures of Zero-, One-, and Two-Dimensional Uranium Isonicotinates Jong-Young Kim, Alexander J. Norquist, and Dermot O’Hare* Inorganic Chemistry Laboratory, Department of Chemistry, Oxford University, South Park Road, Oxford, OX1 3QR, United Kingdom Received November 19, 2002. Revised Manuscript Received February 5, 2003

Four new uranium isonicotinate framework solids were synthesized in the UO2(CH3CO2)‚ 2H2O/HF/isonicotinic acid system under hydrothermal conditions. The isolated uranium isonicotinates span a range of dimensionalities from zero-dimensional UO2(C5H5NCO2)(CH3CO2)2 (1), one-dimensional [UO2F2][C5H5NCO2] (2) and [UO2F3][C5H6NCO2]‚0.5H2O (3), and two-dimensional [UO2F2]2[C5H5NCO2]‚H2O (4). Regions of phase stability were determined using composition space diagrams, which were used to demonstrate the dependence of product composition on the initial reagent concentrations. The molecular zero-dimensional UO2(C5H5NCO2)(CH3CO2)2 (1) consists of UO8 hexagonal bipyramids in which the [UO2]2+ center is coordinated by two acetate and one isonicotinate ligands. The first one-dimensional compound, [UO2F2][C5H5NCO2] (2), contains a chain of edge-sharing [UO3F4] pentagonal bipyramids with isonicotinate ligands separating the chains. The second one-dimensional compound, [UO2F3][C5H6NCO2]‚0.5H2O (3), is constructed from edge-sharing chains of [UO2F5] pentagonal bipyramids and a hydrogen-bonding network of isonicotinic acid. [UO2F2]2[C5H5NCO2]‚H2O (4) contains uranium oxyfluoride layers consisting of edge-sharing dimers of UO3F4 and the corner-sharing UO3F4 chains, where the UO3F4 dimer is cornerlinked with UO3F4 chains.

Introduction There has been considerable efforts expended on the preparation of porous metal-organic framework materials by the coordination of metal ions to an organic linker since zeolitic materials were successfully applied to shape-selective catalysis,1 ion exchange,2 molecular sieving,3 and gas separation.4 Extensive studies have been carried out on the design and synthesis of these functional coordination polymers5 because of their diverse structural properties and potential applications in the field of shape-selective catalysis,6 enantioselective separation,7 gas adsorption/storage,8 and host-guest chemistry.9 Although the strategic design of tailored * To whom correspondence should be addressed. E-mail: Dermot. [email protected]. (1) (a) Venuto, P. B. Microporous Mater. 1994, 2, 297. (b) Jones, C. W.; Tsuji, K.; Davis, M. E. Nature 1998, 393, 52. (2) Clearfield, A. Chem. Rev. 1988, 88, 125. (3) (a) Breck, D. W. Zeolite Molecule Sieves: Structure, Chemistry, and Use; Wiley and Sons: London, 1974. (b) Szostak, R. Molecule Sieves: Principles of Synthesis and Identification; Van Nostrand Reinold: New York, 1989. (4) Gaffney, T. R. Curr. Opin. Solid State Mater. Sci. 1996, 1, 69. (5) (a) Zaworotko, M. J. Angew. Chem., Int. Ed. 2000, 39, 3052. (b) Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319. (c) Zaworotko, M. J. Nature 1999, 402, 242. (d) Robson, R.; Batten, S. R. Angew. Chem., Int. Ed. 1998, 37, 1460. (6) (a) Seo, S. J.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.; Kim. K. Nature 2000, 404, 982. (b) Kepert, C. J.; Prior, T. J.; Rosseinsky, M. J. J. Am. Chem. Soc. 2000, 122, 5158-5168. (7) Xiong, R.; You, X.; Abraham, B. F.; Xue, Z.; Che, C. Angew. Chem., Int. Ed. 2001, 40, 4422.

coordination polymers with “predictable” structurebased properties is still in an early stage, the ability to achieve extra-large pores and high pore volume opens up valuable application possibilities. Moreover, the metal-organic framework approach is well-suited for the functionalization of organic component and construction of a wide range of chiral porous materials. One of the major challenges to porous metal-organic zeolite analogues is the retention of permanent microporosity even in the absence of guests, yielding reversible molecular sorption.8 It was recently demonstrated that the construction of metal carboxylate clusters provided an effective strategy to form robust hosts possessing porosity.6,8,10 Another approach involves the use of rigid building blocks such as pyridine dicarboxylate in the framework to enhance the robustness. Pyridine dicarboxylate is a readily available ligand (8) (a) Eddaoudi, M.; Li, H.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 1391. (b) Eddaoudi, M.; Kim, J. H.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 469-472. (c) Noro, S.; Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem., Int. Ed. 2000, 39, 2081. (d) Tabares, L. C.; Navarro, J. A. R.; Salas, J. M. J. Am. Chem. Soc. 2001, 123, 383. (e) Chen, B.; Eddaoudi, M.; Hyde, S. T.; O’Keeffe, M.; Yaghi, O. M. Science 2001, 291, 1021. (f) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (g) Seki, K. Chem. Commun. 2001, 1496-1497. (h) Kondo, M.; Okubu, T.; Asami, A.; Noro, S.; Yoshitomi, T.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Seki, K. Angew. Chem., Int. Ed. 1999, 38, 140. (9) (a) Holman, K. R.; Pivovar, A. M.; Swift, J. A.; Ward, M. D. Acc. Chem. Res. 2001, 34, 107. (b) Kasai, K.; Aoyagi, M.; Fujita, M. J. Am. Chem. Soc. 2000, 122, 2140. (c) Kiang, Y.-H.; Garner, G. B.; Lee, S.; Xu, Z.; Lobkovsky, E. B. J. Am. Chem. Soc. 1999, 121, 8204.

10.1021/cm021722n CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003

Variable Dimensionality in Uranium Isonicotinates

that has been observed in coordination polymer networks that exhibit properties such as polarity,11-13 porosity,14-16 or magnetism.17 We have recently synthesized new organically templated uranium fluorides (UFO-n and MUF-1),18 phosphites and phosphates (MUPH-1 and UPO-n),19 and molybdates20 with varying dimensionalities from molecular zero-dimensional (0D) to microporous threedimensional (3D) frameworks under hydrothermal conditions. Actinide elements such as uranium exhibit interesting topologies and coordination chemistry because of higher coordination numbers with respect to main group elements, and the structural diversity of an anionic building block depends on the detailed reaction conditions and structure-directing organic template. Composition space was employed to gain understanding of the phase stability of the UFO-n series for the UO2(CH3CO2)‚2H2O/HF/piperazine system. In this work, we report the syntheses and structure of novel zero-, one-, and two-dimensional uranium isonicotinate frameworks, UO2(C5H5NCO2)(CH3CO2)2 (1), [UO2F2][C5H5NCO2] (2), [UO2F3][C5H6NCO2]‚0.5H2O (3), and [UO2F2]2[C5H5NCO2]‚H2O (4). Experimental Section Materials and Methods. The uranium reagent, UO2(CH3CO2)2‚2H2O, was synthesized as previously reported.20 The isonicotinic acid (4-pyrindinecarboxylic acid; 99%, Aldrich) and HF (40% solution, BDH) were also used as received. The reagents listed below were placed in separate 23-mL Teflonlined autoclaves in stainless steel vessels, then heated at 120 °C for 24 h, and cooled slowly at 3 °C/h to room temperature. 1, 2, 3, and 4 were synthesized with phase purity in 10%, 68%, 31%, and 60% yields, respectively. Powder X-ray diffraction patterns on the synthesized phases are in excellent agreement with the generated patterns using the single-crystal data (see Supporting Information). Synthesis. UO2(C5H5NCO2)(CH3CO2)2 (1). UO2(CH3CO2)2‚ 2H2O (0.424 g, 1.0 × 10-3 mol), HF(aq) (6.25 × 10-3 g, 1.25 × 10-4 mol), isonicotinic acid (1.54 × 10-2 g, 1.25 × 10-4 mol), and 1 g of H2O. Elemental analysis. Calcd: U, 46.56; C, 23.49; H, 2.17; N, 2.74. Exptl: U, 45.6; C, 23.5; H, 2.07; N, 2.98. (10) (a) Dong, Y.-B.; Smith, M. D.; zur Loye, H.-C. Angew. Chem., Int. Ed. 2000, 39, 4271. (b) Pan, L.; Huang, X.; Li, J.; Wu, Y.; Zheng, N. Angew. Chem., Int. Ed. 2000, 39, 527. (c) Reineke, T. M.; Eddaoudi, M.; Moler, D.; O’Keeffe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 4843. (d) Chen, B.; Eddaoudi, M.; Reineke, T. M.; Kampf, J. W.; O’Keefe, M.; Yaghi, O. M. J. Am. Chem. Soc. 2000, 122, 11559. (11) Lin, W.; Evans, O. R.; Xiong, R. G.; Wang, Z. Y. J. Am. Chem. Soc. 1998, 120, 13272. (12) Evans, O. R.; Lin, W. J. Chem. Soc., Dalton Trans. 2000, 3949. (13) Evans, O. R.; Lin, W. Chem. Mater. 2001, 13, 3009. (14) Evans, O. R.; Wang, Z. Y.; Xiong, R. G.; Foxman, B. M.; Lin, W. Inorg. Chem. 1999, 38, 2969. (15) Ayyappan, P.; Evans, O. R.; Lin, W. Inorg. Chem. 2001, 40, 4627. (16) Evans, O. R.; Lin, W. Inorg. Chem. 2000, 39, 2189. (17) Lin, W.; Evans, O. R.; Yee, G. T. J. Solid State Chem. 2000, 152, 152. (18) (a) Francis, R. J.; Halasyamani, P. S.; O’Hare, D. Angew. Chem. Int. Ed. 1998, 37, 2214. (b) Francis, R. J.; Halasyamani, P. S.; O’Hare, D. Chem. Mater. 1998, 10, 3131. (c) Simon, A.; Barlow, S.; Halasyamani, P. S.; Mosselmans, J. F. W.; O’Hare, D.; Walker, S. M.; Walton, R. I. Inorg. Chem. 2000, 39, 3791. (d) Walker, S. M.; Halasyamani, P. S.; Simon, A.; O’Hare, D. J. Am. Chem. Soc. 1999, 121, 10513. (e) Francis, R. J.; Halasyamani, P. S.; Bee, J. S.; O’Hare, D. J. Am. Chem. Soc. 1999, 121, 1609. (f) Halasyamani, P. S.; Walker, S. M.; O’Hare, D. J. Am. Chem. Soc. 1999, 121, 7415. (19) (a) Doran, M.; Walker, S. M.; O’Hare, D. Chem. Comm. 2001, 1988. (b) Francis, R. J.; Drewitt, M. J.; Halasyamani, P. S.; Ranganathachar, C.; O’Hare, D.; Clegg, W.; Teat, S. J. Chem. Commun. 1998, 279. (20) Halasyamani, P. S.; Francis, R. J.; Walker, S. M.; O’Hare, D. Inorg. Chem. 1999, 38, 271.

Chem. Mater., Vol. 15, No. 10, 2003 1971 [UO2F2][C5H5NCO2] (2). UO2(CH3CO2)2‚2H2O (0.424 g, 1.0 × 10-3 mol), HF(aq) (0.475 g, 9.5 × 10-3 mol), isonicotinic acid (0.246 g, 2.0 × 10-3 mol), and H2O (1 g). Elemental analysis. Calcd: U, 55.21; C, 16.72; H, 1.17; N, 3.25. Exptl: U, 53.21; C, 16.82; H, 1.18; N, 3.30. [UO2F3][C5H6NCO2]‚0.5 H2O (3). UO2(CH3CO2)2‚2H2O (0.424 g, 1.0 × 10-3 mol), HF(aq) (0.425 g, 8.5 × 10-3 mol), isonicotinic acid (0.369 g, 3.0 × 10-3 mol), and H2O (1 g). Elemental analysis. Calcd: U, 51.73; C, 15.7; H, 1.53; N, 3.04. Exptl: U, 50.28; C, 15.8; H, 1.58; N, 3.12. [UO2F2]2[C5H5NCO2]‚H2O (4). UO2(CH3CO2)2‚2H2O (0.424 g, 1.0 × 10-3 mol), HF(aq) (0.100 g, 2.0 × 10-3 mol), isonicotinic acid (1.54 × 10-2 g, 1.25 × 10-4 mol), and H2O (1 g). Elemental analysis. Calcd: U, 62.87; C, 9.52; H, 0.93; N, 1.85. Exptl: U, 59.05; C, 10.1; H, 1.23; N, 2.04. X-ray Crystallographic Studies. Data were collected using an Enraf Nonius FR 590 Kappa CCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.71073 Å). Crystals were mounted on a glass fiber using N-Paratone oil and cooled in situ, using an Oxford Cryostream 600 Series, to 150 K for data collection. Frames were collected, indexed, and processed using Denzo SMN and the files scaled together using HKL GUI within Denzo SMN.21 The heavy atom positions were determined using SIR97.22 All other non-hydrogen atom were located from Fourier difference maps. All non-hydrogen sites were refined with anisotropic thermal parameters using fullmatrix least-squares procedures on Fo2 with I > 3σ(I). All calculations were performed using CRYSTALS23 and CAMERON.24 Selected crystallographic data for each compound are given Table 1. Complete tables of fractional atomic coordinates, thermal parameters, and lists of pertinent bond lengths and angles are listed in the Supporting Information. Thermogravimetric Analyses. TGA measurements were performed on a Rheometric Scientific STA 1500H thermal analyzer. The samples were contained within platinum crucibles and heated at a rate of 10 °C min-1 from room temperature to 900 °C under flowing argon. Elemental Analyses. C, H, and N analyses were conducted using an Elementar Vario EL Analyzer. U compositions were determined by ICP using a Thermo Jarrel Ash Scan 16 instrument.

Result and Discussion Synthesis, Structural, and Thermal Characterization of Uranium Isonicotinates. UO2(C5H5NCO2)(CH3CO2)2, (1) is a molecular (zero-dimensional) phase consisting of UO8 hexagonal bipyramids in which UVI is coordinated by two acetate ligands and an isonicotinate ligand (Figure 1). The UVI center is bound axially to two oxygens, forming a nearly linear uranyl unit (OdUdO bond angle: 179.5(2)°) with a UdO bond length of 1.777(3) Å. Equatorially, uranium is coordinated to six bridging oxides of two acetate and one isonicotinate ligands. Equatorial U-O bond lengths range from 2.435 to 2.479 Å. In connectivity terms, each uranium is considered an [UO2/1O6/2]2- anion. N‚‚‚H-O hydrogen-bonding interactions between isonicotinate and acetate ligands are observed (N-H‚‚‚O(3): 2.862 Å). [UO2F2][C5H5NCO2] (2) contains a one-dimensional structure with a uranium oxyfluoride chain built up from edge-sharing [UO3F4] pentagonal bipyramid poly(21) Otwinowski, Z. In Data Collection and Processing, Proceedings of the CCP14 Study Weekend; Otwinowski, Z., Ed.; Daresbury Laboratory: Warrington, UK, 1993. (22) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1994, 27, 343. (23) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.; Cooper, R. I. CRYSTALS Issue 11; Chemical Crystallography Laboratroy: Oxford, UK, 2001. (24) Watkin, D. J.; Prout, C. K.; Pearce, L. J. CAMERON; Chemical Crystallography Laboratroy: Oxford, UK, 1996.

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Table 1. Selected Crystallographic Data for 1, 2, 3, and 4 formula f.w. crystal color, shape crystal size (mm) temperature (K) crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3), Z Fcalc (g cm-3) µ (mm-1), Mo KR reflections collected reflections independent reflections observed R(F)a Rw(F2)b a

UO8C10H11N (1)

UO4F2C6H5N (2)

511.22 yellow, block 0.10 × 0.10 × 0.10 150 monoclinic C21/c (No. 15) 10.9552(3) 11.6754(4) 10.1378(3) 90 101.233(3) 90 1271.8, 4 2.670 12.801 2300 1431 1346 [I > 3σ(I)] 0.0198 0.0486

431.14 yellow, block 0.10 × 0.10 × 0.10 150 orthorhombic Pmcn (No. 62) 7.3637(3) 8.6292(4) 13.1367(7) 90 90 90 834.7, 4 3.430 19.467 1834 1020 836[ I > 3σ(I)] 0.0195 0.0452

U2O9F6C12H14N2 (3)

U2O7F4C6H7N (4)

910.24 yellow, needle 0.02 × 0.02 × 0.30 150 orthorhombic Fddd (No. 70) 7.3473(3) 19.3676(8) 27.7936(14) 90 90 90 3955.0, 8 3.057 16.459 2250 1129 856[I > 3σ(I)] 0.0447 0.0553

755.16 yellow, needle 0.02 × 0.04 × 0.16 150 monoclinic P21/n (No. 14) 8.2483(2) 8.2304(2) 19.4872(6) 90 98.684(1) 90 1307.8, 4 3.835 24.814 5442 2957 2406 [I > 3σ(I)] 0.0269 0.0671

R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑w(|Fo2| - |Fc2|)2/∑w(Fo2)2]1/2.

Figure 1. Thermal ellipsoid plot (50% probability) of 1.

hedra (Figure 2). The uranium chains run along the [100] direction and are separated by isonicotinates. Within the chain, each uranium is coordinated to two axial oxygens, four equatorial fluorines, and one equatorial oxygen shared with the carboxylate group of isonicotinate unit. Thus, each uranium pentagonal bipyramid contains four bridging fluorides and two terminal and one bridging oxides and can be described in connectivity terms as a [UO2/1F4/2O1/2]- anion. As with 1 and other U(VI)-oxo complexes, a nearly linear uranyl unit is observed with an OdUdO angle of 179.9(3)° and UdO distances of 1.758(5) and 1.763(6) Å. Equatorial U-F and U-O distances range from 2.322(3) to 2.371(6) Å. Hydrogen bonding interactions are observed between nitrogen of the isonicotinate ligand and the uncoordinated oxide of the carboxylate (N-H‚‚‚O(4): 2.761 Å), which binds the structure together and creates a hydrogen-bonding network (Figure 2). [UO2F3][C5H6NCO2]‚0.5H2O (3) exhibits another onedimensional chain structure consisting of the edgesharing chain of [UO2F5] pentagonal bipyramids and a hydrogen-bonding network of isonicotinic acid (Figure 3). Within the uranium chain, uranium is bound axially to two oxygens, which forms a linear uranyl unit (Od UdO angle: 179.2 (5)°; UdO distances: 1.761(5) Å) and equatorially to four bridging and one terminal fluoride.

Figure 2. (a) A view of the structure of 2 along the [100] direction. (b) One-dimensional chain of 2 showing the backbone of uranium polyhedra and pendant isonicotinates.

Thus, in connectivity terms, each uranium is described as [UO2/1F4/2F1/1]-. Equatorial U-F distances range from 2.257(7) to 2.335(4) Å, of which the terminal bond is the shortest (U-F(1) ) 2.257(7) Å), as expected. Solvent water molecules are hydrogen-bonded to the fluoride of the [UO2F3]- chain (O-H‚‚‚F(1): 2.723 Å). Unlike 2, the isonicotinic acid is unconnected to the uranium chain. Instead they are hydrogen-bonded to

Variable Dimensionality in Uranium Isonicotinates

Figure 3. (a) Structure of 3 viewed along the [100] direction, showing the chain of uranium polyhedra and the hydrogenbonding network of isonicotinic acid, which is disordered over two orientations. (b) Thermal ellipsoidal representation of the edge-sharing uranium chain of 3 (50% probability).

each other to form a zigzag chain structure. Hydrogenbonding interaction between the carboxylate and nitrogen ends of the isonicotinic acid serves to link each other, in which the isonicotinic acid in 3 is disordered in two orientations ((N/C)(2))-H‚‚‚O(2): 2.489 Å). Isonicotinic acid is well-known to form a rigid network by single or multiple hydrogen-bond interactions. A single-crystal X-ray diffraction study of [UO2F2]2[C5H5NCO2]‚H2O (4) revealed a two-dimensional layered structure containing two unique uranium pentagonal bipyramids (Figure 4). Within the layers, each uranium is axially bonded to two oxygens, forming nearly linear uranyl units (U(1)dO: 1.765(6) Å, 1.779(7) Å; OdU(1)dO angle: 179.1 (3)°; U(2)dO: 1.765(6) Å, 1.769(6) Å; OdU(2)dO angle: 179.2 (3) °). Equatorially, each uranium is bonded to four fluorides and one oxide shared with the carboxylate group of the isonicotinate unit. The equatorial U-F and U-O bond lengths for U(1) and U(2) range from 2.281(5) to 2.350(6) Å and from 2.314(5) to 2.373(6) Å, respectively. In connectivity terms, both uraniums can be described as [UO2/1F4/2O1/2]with two terminal (uranyl) oxides, four bridging fluorides, and one bridging oxide sharing with the carboxylate group. One of the uranium centers, [U(1)O3F4], shares an edge to form a dimer, whereas the other type, [U(2)O3F4], corner-share with each other to form an infinite chain along the [010] direction. As shown in Figure 4b, the dimer of [U(1)O3F4] pentagonal bipyramids is cornerlinked with adjacent polyhedra of the [U(2)O3F4] chains, which creates an elliptical eight-membered aperture of dimensions 5.9 × 2.0 Å within the layer (defined by the shortest oxide-oxide or fluoride-fluoride contacts using ionic radii from R.D. Shannon).25 The pyridine moiety

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Figure 4. (a) Layer structure of 4 viewed along the [010] direction, showing that isonicotinate residues are pointing toward the interlayer space. (b) The structure of the uranium isonicotinate layer in 4 viewed perpendicular to the layers along the [100] direction. The layer contains the eightmembered ring aperture and the isonicotinate groups above and below the aperture.

of the isonicotinate group is located below and above the aperture with the pyridine ends pointing toward the interlayer region with a 45° angle to the uranium oxyfluoride layer. In the interlayer region, there is a channel of approximate dimension 3.7 × 3.5 Å along the [010] direction, which is surrounded by uranium pentagonal bipyramids and an isonicotinate group pointing toward interlamellar space. Solvent water molecules occupy the channel and form hydrogen bonds to the nitrogen of the isonicotinate group (N-H‚‚‚O: 2.718 Å) and the oxygen of the uranyl unit (O‚‚‚H-O: 2.871 Å). 1 and 2 are thermally stable until 200 °C, whereas the thermal analysis data for 3 and 4 show a gradual mass loss up to 200 °C, owing to the loss of water (see Supporting Information). The mass losses due to dehydration for 3 and 4 are 4.3 and 2.5%, respectively, which is consistent with the calculated values of 3.8 and 2.4%, respectively. Beyond 200 °C, a further mass loss owing to the decomposition of the organic component occurs. The observed losses up to 600 °C are 42.0, 32.0, 32.2, and 24.8% for 1, 2, 3, and 4, respectively, which is consistent with the calculated values of 45.1, 33.7, 34.1, and 24.4% for 1, 2, 3, and 4, respectively. For 1, 2, and 3, two distinct stages are observed, that is, 220-370 and 370-600 °C, whereas a substantial mass loss for 4 begins around 400 °C. A detailed study on the mechanism of thermal decomposition is in progress. (25) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.

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Figure 5. Composition space for the UO2(CH3CO2)‚2H2O/HF/isonicotinic acid system. The lines A and B represent the syntheses under constant isonicotinic acid (0.1) and a constant ratio of UO2(CH3CO2)‚2H2O:HF of 1:1.

Composition Space. The stabilities of the isolated uranium isonicotinate phases will be discussed based on the composition space diagram for UO2(CH3COO)2‚ 2H2O/HF/isonicotinic acid. Composition space diagrams provide an easy way to describe the stability region of specific phases.26 Although composition space is graphically similar to a ternary phase diagram, it cannot be treated as such because solution phases as well as amorphous phases are disregarded. Another important distinction is that phase rule is not obeyed in a composition space diagram. Even if the molar ratio of the reactants excluding excess water is held constant, the increase or decrease in the number of moles changes the synthetic result. This is because a larger or lesser amount of reagents is placed in a finite volume with a fixed amount of water, which changes both the solubility of the reagents and the total pressure. To construct a composition space, 30 separate reactions were performed using 10-3 mol (0.424 g) of UO2(CH3COO)2‚2H2O in each reaction and 1 g of water. In the present composition space, four single-crystalline and one polycrystalline phases are found27 (Figure 5). The predominant phase observed throughout the prism is the one-dimensional compound 2, [UO2F2][C5H5NCO2]. This material can be synthesized in phase-pure form, represented as 0, over a wide range of the composition space. The molecular phase 1, UO2(C5H5NCO2)(CH3CO2)2, is found in the uranium acetate-rich corner of the composition space, represented by ×. From the uranium acetate-rich corner toward the HF-rich corner, two-dimensional phase 4, [UO2F2]2[C5H5NCO2]‚ (26) (a) Harrison, W. T. A.; Dussack, L. L.; Jacobson, A. J. J. Solid State Chem. 1996, 125 (2), 234. (b) Norquist, A. J.; Heier, K. R.; Stern, C. L.; Poeppelmeier, K. R. Inorg. Chem. 1998, 37, 6495. (27) No crystallographic data could be obtained on the polycrystalline product. Elemental analysis results imply that the ratio of uranium to isonicotinate is 1:1. (Exptl: U, 49.85; C, 16.33; H, 1.41; N, 3.20.)

H2O (f), one-dimensional phase 3, [UO2F3][C5H6NCO2]‚ 0.5 H2O (O), and unknown polycrystalline phase (+) are observed. Therefore, if we examine a series of reactions conducted at 0.1 mol % of isonicotinic acid, depicted as line A in Figure 5, an interesting change of structures and dimensionalities is observed. Following the line from the UO2(CH3CO2)2‚2H2O-rich corner to the HFrich corner at a constant mole fraction of isonicotinic acid (0.1), we observe a change in phase, UO2(C5H5NCO2)(CH3CO2)2 (1) f [UO2F2]2[C5H5NCO2]‚H2O (4) f [UO2F3][C5H6NCO2]‚0.5 H2O (3). The synthesis of 1 is observed only in the UO2(CH3COO)2‚2H2O-rich end, in which more acetate than fluoride is involved in the reaction. As the concentration of uranium acetate is decreased and the concentration of HF is increased, 4 is formed. Continuing toward the HF-rich corner, the reaction product changes from 4 to 3. Excluding an unknown polycrystalline phase, the change in product from 0D (1) f 2D (4) f 1D (3) can be understood by examining the local coordination and charge on each uranium. In 1, each uranium pentagonal bipyramid is [UO2/1O6/2]2-, which means each uranium contains a uranyl unit and six bridging oxides. As the concentration of HF is increased, the bridging oxides are replaced by fluorides, subsequently reducing local charge on each uranium bipyramid. For 4, each uranium pentagonal bipyramid is a [UO2/1F4/2O1/2]- anion with four bridging fluorides and one bridging oxide. Increasing the HF concentration further results in the synthesis of 3, in which the local coordination of the uranium pentagonal bipyramid is [UO2/1F4/2F1/1]-. That is, all of the bridging ligands are fluoride and a terminal fluoride is added. Therefore, it is clearly shown that there is direct relationship between HF concentration and astructural evolution (local coordination and charge of uranium

Variable Dimensionality in Uranium Isonicotinates

pentagonalbipyramid),thatis,[UO2/1O6/2]2- f[UO2/1F4/2O1/2]f [UO2/1F4/2F1/1]-. In addition to the changes observed at constant isonicotinic acid concentration, the product also changes with respect to isonicotinic acid concentration at a 1:1 ratio of UO2(CH3COO)2‚2H2O to HF (line B in Figure 5). In the acid corner (g60% HF), 3, a 1D-compound, of which the bridging ligands are all fluoride, is observed, whereas, outside the acid corner, two products with the same local coordination of [UO2/1F4/2O1/2]-, that is, 2 and 4, are observed. However, 2 and 4 differ in the ratio of uranium to isonicotinic acid, that is, U:isonicotinic acid ) 1:1 (2) and 2:1 (4). From the isonicotinic acid-rich corner to the central region of the composition space, 2 is observed, whereas 4 is mainly synthesized in the isonicotinic acid-deficient region, which can be expected from the ratio of uranium to isonicotinic acid. As the concentration of isonicotinic acid is increased, the dimensionality is reduced from 2D (4) to 1D (2). According to dimensional reduction theory,28 in the MXx + nAaX f AnaMXx+n reaction, the incorporation of additional X atoms into the M-X framework reduces the dimensionality of the framework. Assuming M ) uranium and X ) isonicotinate, oxide, and fluoride, it can be deduced that the increase of the incorporated isonicotinate results in the reduced dimensionality from 2D to 1D.

Chem. Mater., Vol. 15, No. 10, 2003 1975

Conclusions New uranium isonicotinates with zero-, one-, and twodimensionalities were synthesized under hydrothermal conditions. By utilizing a composition space diagram, the stability regions of the isolated crystalline phases are determined by the reactant concentrations. The evolution of the local coordination and charge of each uranium center can also be understood by the composition space diagram. A direct relationship between HF concentration and structural evolution, [UO2/1O6/2]2- f [UO2/1F4/2O1/2]- f [UO2/1F4/2F1/1]- is clearly shown in the diagram, which resulted in the change of dimensionality, 0D f 2D f 1D. The dimensionalities of the isonicotinates with the same local coordination of [UO2/1F4/2O1/2]- also change with respect to isonicotinic acid concentration, 2D f 1D, at a constant ratio of uranyl acetate to HF. Acknowledgment. The authors thank the EPSRC for support. Supporting Information Available: Tables of atomic coordinates and selected bond lengths for 1-4. Simulated and experimental powder X-ray diffraction pattern for 1-4. Thermogravimetric analysis data for 1-4 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM021722N (28) Tulsky, E. G.; Long, J. R. Chem. Mater. 2001, 13, 1149.