Assembling β-Octamolybdate Clusters into New Polyoxomolybdates with Unusual Architectures Sandip Chakrabarti and Srinivasan Natarajan* Framework Solids Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India
CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 5 333-335
Received June 3, 2002
ABSTRACT: β-octamolybdate [Mo8O26] clusters have been assembled, under hydrothermal conditions, into a variety of interesting one-dimensional structures. The compositional variations, the structural diversity coupled with the interesting properties of the inorganic oxides, are one of the important reasons for the continued focus in this area.1,2 The assembly of transition metal oxides, especially the polyoxo ions, in the presence of organic amines is of contemporary interest.3-8 The assembly of the polyoxo ions appears to be crucially dependent on the conditions under which they are crystallized, though a clear understanding of the organization of these hybrid materials is still elusive. The structure-directing properties of the organic components, however, have been exploited in the preparation of many metastable solids with novel architectures. The formation of new polyoxomolybdates and related solids has been investigated in great detail, in recent years, resulting in new types of structures, including the oxo-molybdenum clusters linked via organic/ inorganic tethers.3-6 It occurred to us that it could be possible to replace the tethers by simple Mo-O linkages resulting in new types of polyoxomolybdates with novel structures, though no such compounds have been synthesized earlier. During the course of our investigations on the formation of novel zinc molybdates in the presence of organic amines, we have now isolated three new polyoxomolybdates, [NH3(CH2)3NH3]2[Mo8O26]‚H2O, I, [NH3(CH2)2NH2(CH2)2NH3]2[Mo9O30], II, and [NH3(CH2)2NH2(CH2)3NH3]2[Mo10O33], III, synthesized in the presence of 1,3diaminopropane (DAP), diethylenetriamine (DETA), and N(2-aminoethyl)1-3-diaminopropoane (AEDAP), respectively, employing hydrothermal methods. The structures of I-III are related and have identical [Mo8O26] β-octamolybdate clusters, while I has the Mo-O clusters connected together forming the one-dimensional chains, II and III have the [Mo8O26] clusters linked through a MoO6 octahedra and [Mo2O7] units, respectively, forming new polymeric oxomolybdates. Compound II has helical columns of one-dimensional [Mo9O30] chains, and III has zigzag chains of [Mo10O33] separated by the amine molecules. The arrangement of the helical chains in II resemble the double helix structure of DNA. Both the structures of II and III, to our knowledge, are new and have been encountered for the first time. The compounds I-III were the products of our attempts to make novel zinc molybdates employing hydrothermal methods, resulting in novel polyoxomolydates. A composition of ZnCl2:3MoO3‚H2O:DAP:200H2O for I, ZnCl2:3.7MoO3‚ H2O:DETA:100H2O for II and ZnCl2:4MoO3‚H2O:AEDAP: 100H2O for III, respectively, were taken in a 23-mL Teflonlined Parr autoclave. For I and III, the mixtures were heated at 150 °C for 72 h, and for II, at 150 °C for 72 h followed by at 180 °C for 24 h, resulting in large quantities * Corresponding author: e-mail:
[email protected], FAX: (+91)-80-8462766, (+91)-80-856-6581.
Figure 1. (a) The β-octamolybdate[Mo8O26] cluster. (b) Polyhedral view of the structure of I showing the one-dimensional chains. Note that the β-octamolydate clusters are linked directly (see text).
of rodlike colorless crystals (yield 60% for I, 50% for II and III). In all the cases, the initial pH value was close to 5.5 and did not show appreciable change after the reaction. Our efforts to prepare I-III in the absence of ZnCl2 in the reaction mixture were not successful, though the exact role of which is still unclear. The crystals were filtered off from the mother liquor, washed with water, and dried at room temperature. Compounds I-III were characterized by powder XRD, TGA, and IR spectroscopy. The TGA studies, in oxygen atmosphere in the range 25-550 °C, showed two losses corresponding to the loss of the water and amine molecules for I, and one mass loss corresponding to the loss of amine molecules in the case of II and III. The calcined sample was found to be pure MoO3 (JCPDS: 05 - 0508), consistent with the framework formula. IR spectrum for all the compounds exhibited a strong band at 910 cm-1 attributed to ν(M ) O), in addition to showing typical C-H and N-H bands. The single-crystal structure was determined using a Siemen’s SMART-CCD diffractometer. The structure was solved by direct methods and refined using the SHELXL-97 package of programs.9 The structure of I consists of octahedrally coordinated Mo atoms connected through their edges forming [Mo4O13]2units. Two [Mo4O13] subunits are stacked together to give rise to [Mo8O26]4- β-octamolybdate clusters, which are connected together forming infinite anionic one-dimensional chains as shown in Figure 1. The extraframework water and diprotonated DAP molecules are located between these chains and interact with the framework via O -H...O and N-H...O hydrogen bonds. β-Octamolybdate clusters of the types observed in I have also been prepared and linked
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Figure 2. Figure showing the β-octamolybdate[Mo8O26] clusters and its connectivity with (a) MoO6 in II and (b) [Mo2O7] units in III.
together as well as by amine-ligated transition metal ions giving rise to one- and two-dimensional structures.3 The structure of II is built up of similar [Mo8O26] β-octamolybdate cluster units. Unlike in I, the linkages between the [Mo8O26] cluster units in II is through MoO6 octahedra, as shown in Figure 2a. The MoO6 octahedra occupy a special position, sitting on the inversion center. The one-dimensional chains of the formula [Mo9O30] that result from the connectivity of the MoO6 octahedra with the Mo-O clusters have helicity with both the right-handed and the left-handed units present in adjacent chains (Figure 3). These helical chains run along the 2-fold screw axis. The projection of the helical chain units in the ab plane resemble the double-helix structure of the DNA, though the helical chains in II are not intertwinned. This is indeed a unique architecture and has not been encountered before. The amine molecule, DETA, employed in the synthesis occupies the interchain spaces, and it is to be noted that it is not unsymmetrical. Encouraged by the formation of this unusual structure, we decided to investigate the formation of similar phases in the presence of unsymmetrical amine molecules, such as AEDAP. Our efforts resulted in the formation of III. The structure of III is made up from the connectivity between the [Mo8O26] β-octamolybdate cluster units and [Mo2O7] units as shown in Figure 2b. The resulting onedimensional chain of [Mo10O33] has a zigzag character, unlike the helical chains of II. The organic amine molecules occupy the interchain region as shown in Figure 4. The amine molecule is disordered with respect to the central nitrogen and carbon atom. The Mo-O bond distances are in the range 1.689-2.448 Å (av. 1.974 for I, 1.975 for II, and 1.981 Å for III), and the O-Mo-O bond angles are in the range 68.5-179.1° (av. 103.1 for I, 103.3 for II, 102.9 Å for III). These values are in the range expected for the octahedrally coordinated Mo atoms with small differences in the extend of distortion from the Oh geometry, and in agreement with those in the previously reported molybdate strctures.3-8 The successful isolation of I-III provides novel examples of assembling the oxomolybdate clusters under hydrother-
Figure 3. The structure of II in the ab plane showing two adjacent helical chains of [Mo9O30] distinguished by different colors.
mal conditions. The variations in the structures arises mainly due to the differences in the connectivity between the [Mo8O26] clusters. Compounds I-III provide novel examples of the retention of the geometry of the molecular building blocks in the synthesis of solid-state materials, and may be used as molecular precursors in the preparation of new materials. The variations of the composition of the polyoxoanions and its bridging inorganic “ligand” may be employed, under appropriate synthetic conditions, to give rise to a vast variety of solids. This synthetic approach would provide a facile method for modification of the structures of the metal oxides, and ultimately for tuning of electronic, magnetic, and optical properties of these oxide phases.
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Figure 4. The structure of III along the bc plane showing the arrangement of zigzag chains along with the amine molecule. Hydrogen atoms of the amine molecule are not shown. Dotted lines represent possible hydrogen bond interactions.
Acknowledgment. The authors thank Prof. C. N. R. Rao, FRS, for his support and encouragement. S.N. thanks the Council of Scientific and Industrial Research (CSIR), Govt. of India, for the award of a research grant.
References (1) Clearfield, A. Chem. Rev. 1988, 88, 125. (2) Cox, P. A. Transition Metal Oxides; Clarendon Press: Oxford, 1995.
(3) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2638 and the references therein. (4) Xu, Y. Curr. Opin. Solid State Mater. Sci. 1999, 4, 133 and the references therein. (5) Guillou N.; Ferey, G. J. Solid State Chem. 1997, 132, 224. (6) Chen, L.; Wang, Y.; Hu, C.; Feng, L.; Wang, E.; Hu N.; Jia, H. J. Solid State Chem. 2001, 161, 173. (7) Mu¨ller, A.; Beckmann, E.; Bogge, H.; Schmidtmann, M.; Dress, A. Angew. Chem., Int. Ed. 2002, 41, 1162. (8) Mu¨ller, A.; Reuter, H.; Dillinger, S. Angew. Chem., Int. Ed. 1995, 34, 2328 and the references therein. (9) Crystal data for I: [NH3(CH2)3NH3]2[Mo8O26]‚H2O, M ) 1371.84, monoclinic, space group P21/c, a ) 7.8511(4), b ) 15.1901(8), c ) 12.7077(7) Å, β ) 96.930(1)°, U ) 1504.44(14) Å3, Z ) 4, T ) 293(2) K, Dc ) 3.028 Mg m-3, µ (Mo KR) ) 3.333 mm-1, F(000) ) 1304, crystal size ) 0.20 × 0.16 × 0.12 mm. A total of 6109 reflections (2.10 e θ g 23.31°) were collected, of which 2157 unique reflections were used for structural elucidation (Rint ) 0.0284). The final R1 was 0.025 (all data). Crystal data for II: [NH3(CH2)2NH2(CH2)2NH3]2[Mo9O30], M ) 1555.86, monoclinic, space group C2/c, a ) 21.4235(4), b ) 9.22529(1), c ) 19.2590(4) Å, β ) 120.258(2)°, U ) 3297.60(10) Å3, Z ) 8, T ) 293(2) K, Dc ) 3.134 Mg m-3, µ (Mo KR) ) 3.421 mm-1, F(000) ) 2960, crystal size ) 0.24 × 0.16 × 0.12 mm. A total of 6641 reflections (2.20 e θ g 23.30°) were collected, of which 2373 unique reflections were used for structural elucidation (Rint ) 0.0603). The final R1 was 0.0266 (all data). Crystal data for III: [NH3(CH2)2NH2(CH2)3NH3]2[Mo10O33], M ) 1731.84, monoclinic, space group C2/c, a ) 17.5975(2), b ) 11.3496(2), c ) 18.8161(4) Å, β ) 90.801(1)°, U ) 3757.69(11) Å3, Z ) 8, T ) 293(2) K, Dc ) 3.061 Mg m-3, µ (Mo KR) ) 3.336 mm-1, F(000) ) 3296, crystal size ) 0.24 × 0.12 × 0.12 mm. A total of 7722 reflections (2.14 e θ g 23.28°) were collected, of which 2693 unique reflections were used for structural elucidation (Rint ) 0.0500). The final R1 was 0.0804 (all data).
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