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Chem. Mater. 1996, 8, 814-818
Articles Investigations into the Engineering of Inorganic/Organic Solids: Hydrothermal Synthesis and Structure Characterization of One-Dimensional Molybdenum Oxide Polymers Yan Xu* Division of Chemistry, School of Science, Nanyang Technological University, Singapore 259756, Singapore
Li-Hua An and Lip-Lin Koh Department of Chemistry, National University of Singapore, Singapore 119260, Singapore Received May 25, 1995. Revised Manuscript Received November 21, 1995X
Two one-dimensional intercalating molybdenum oxide polymers have been synthesized using different structure-directing units under specific hydrothermal conditions. MoO-0 (NaNH4Mo3O10) was synthesized using tetramethylammonium hydroxide, and MoO-1(C6H18N2Mo4O13) was synthesized using 1,6-hexanediamine. The two crystal structures have been determined by the single-crystal X-ray diffraction method. Both consist of bundles of parallel molybdenum oxide chains interspersed with metals or organic cations which play the role of intercalating agent in the host structure. The Mo has a 6+ valence state and distorted octahedral coordination. Mo-0 has orthorhombic symmetry (Pnma, No. 62) with a ) 8.407(2) Å, b ) 7.603(2) Å, and c ) 14.350(3) Å. MoO-1 has triclinic symmetry (P1 h , No. 2) with a ) 8.267(2) Å, b ) 8.986(2) Å, c ) 12.714(3) Å, R ) 87.58(3)°, β ) 76.48(3)°, γ ) 67.97(3)°. The study suggests that the structures and properties of intercalation-type molybdenum oxide polymers can be engineered by incorporating the structure-directing units, acidity, and the hydrothermal conditions.
Introduction The applications of cooperative assembly of inorganic and organic species under hydrothermal conditions have led to the successful syntheses of many one-, two-, and three-dimensional solid materials of versatile compositions and structure types.1-6 The exploration of a generalized approach for preparation of periodic inorganic/organic composite materials using hydrothermal method by Stucky et al.7 further demonstrates the possibility of materials “engineering”. Current research interest is directed toward the development of solid materials with unusual structural complexity and a concomitant diversity of applicable properties.3,4 The chemistry of molybdenum polyoxoanions has been a challenging field due to the stoichiometric and Abstract published in Advance ACS Abstracts, March 1, 1996. (1) Haushalter, R. C.; Mundi, L. A. Chem. Mater. 1992, 4, 31. (2) Bideau, J. L.; Payen, C.; Palvadeau, P.; Bujoli, B. Inorg. Chem. 1994, 33, 4885. (3) Soghomonian, V.; Chen, Q.; Haushalter, R. C.; Zubieta, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 223. (4) Soghomonian, V.; Chen, Q.; Zhang, Y. P.; Haushalter, R. C.; O’Connor, C. J.; Tao, C. H.; Zubieta, J. Inorg. Chem. 1995, 34, 3509. (5) Maeda, K.; Akimoto, J.; Kiyozumi, Y.; Mizukami, F. J. Chem. Soc., Chem. Commun. 1995, 1033. (6) Harrison, W. T. A.; Dussack, L. L.; Jacobson, A. J. Inorg. Chem. 1995, 34, 4774. (7) Huo, Q. S.; Margolese, D. I.; Clesla, U.; Feng, P. Y.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. X
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structural complexity and reactivity of these materials.8-12 For example, molybdenum trioxide has been intensively studied for its catalytic properties in the selective oxidation of methanol to formaldehyde.13 The molybdenum polyoxoanions are generally prepared by acidifying the aqueous or aprotic media containing MoVI and heteroatoms (M) involving simple metal or organic cations.10 By careful adjustment of the acidity, the Mo:M ratio, and the temperature of the reaction, a large number of molybdenum polyoxoanions where MoO6 (MOx) polymerizes in different fashion and degree have been produced.12 However, they are relatively soluble in various solvents and some are air or light sensitive. The crystallization of products becomes difficult as the size of molybdenum polyoxoanions increases. Such molybdenum polyoxoanions are normally simple salts with restricted structual versatility and (8) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (9) Owowe, Y. Chemtech 1978, 8, 432. (10) Klemperer, W. G. Inorg. Synth. 1990, 27, 71. (11) Nugent, W. A. Tetrahedron Lett. 1978, 24, 1831. (12) Che, T. M.; Day, V. W.; Francesconi, L. C.; Fredrich, M. F.; Klemperer, W. G.; Shum, W. Inorg. Chem. 1985, 24, 4055. (13) Cheng, W. H.; Chowdhry, U.; Ferretti, A.; Firment, L. E.; Groff, R. P.; Machiels, C. J.; McCarron, E. M., III; Ohuchi, F.; Staley, R. H.; Sleight, A. W. Heterogeneous Catalysis, Proceedings of the 2nd Symposium of IUCCP of the Department of Chemistry; Texas A & M University Press: 1984; p 165.
© 1996 American Chemical Society
Synthesis of Open-Framework Polymolybdates Table 1. Data Collection and Refinement Details for MoO-0 and MoO-1 MoO-0 (NaNH4Mo3O10) crystal system a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 space group Dc/Mg m-3 F(000) Z linear abs coeff/mm-1 X-radiation 2 θ min, max/deg obsd data [F > nσ(F)], n ) abs corr/Tmin, Tmax no. of parameters ∆F(max)/e Å-3 final, R, wRa/% a
MoO-1 (C6H18N2MO4O13)
Crystal Data orthorhombic 8.407(2) 7.603(2) 14.350(3)
917.2(5) Pnma 3.511 896 4 4.126 Data Collection Mo KR 3.0, 50.0 701 0.6506/0.7411 Refinement 80 0.72 2.35, 3.07
triclinic 8.267(2) 8.986(2) 12.714(3) 87.58(3) 76.48(3) 67.97(3) 850.2(3) P1 h 2.773 680 2 2.950 Mo KR 3.5, 50.0 3653 0.5617/0.7653
227 1.31 2.59, 4.40
R ) ∑||Fo ) |Fc||/∑|Fo|; wR ) [Σw(|Fo| ) |Fc|)2/∑w|Fo|2]1/2.
poor stability. Recent investigation using hydrothermal technique for the preparation of molybdates results in a series of microporous reduced molybdenum phosphates.1 We have applied the hydrothermal method to the synthesis of molybdates in acidic medium by using various templates as structure-directing units. Our studies showed that the structures of [MoxOy]n- can be “tailored” to a certain degree by altering the size, conformation, and properties of templates and the acidity of reaction system. This has resulted in a series of chain- and layer-type polymeric molybdenum oxides with intercalating cations. In this paper, we report the hydrothermal syntheses and the structures of two onedimensional polymeric molybdenum oxides, MoO-0 and MoO-1. Experimental Section MoO-0 and MoO-1 were prepared by hydrothermal synthesis in the temperature range 150-200 °C under autogenous pressures. Tetramethylammonium hydroxide (TMAOH, reagent grade, Aldrich) and 1,6-hexanediamine (1,6-HDA, reagent grade, Aldrich) were used as templates for MoO-0 and MoO-1, respectively. Molybdic acid (MoO3‚H2O, reagent grade, BDH) was used as molybdenum source. A typical preparation of MoO-0 was conducted as follows: 4.5 g of MoO3‚H2O, 0.89 g of sodium hydroxide (reagent grade, BDH), and 20 mL of distilled water were mixed and stirred for 10 minutes; 2.5 g of TMAOH was added dropwise into the mixture under moderate stirring. MoO-1 was prepared in a similar fashion. A typical preparation required 4.5 g of MoO3‚H2O, 25 mL of H2O, and 6.6 g of 1,6-HDA. The reaction mixture was acidified using 1 M HCl solution to obtain the pH of ∼5 for MoO-0 and 2-4 for MoO-1. The final yellowish slurry was transferred to a PTFE-lined stainless steel autoclave and sealed. The crystallization of MoO-0 and MoO-1 was completed after 2 days at 200 °C and autogenous pressure. White needlelike microcrystallites of MoO-0 and dark-gray crystals of MoO-1 of some 1 mm in three dimensions were recovered by washing, suctionfiltering, and drying at 80 °C in air. Mo content was analyzed using ICP, and those of C, N, and H by microanalysis. Simultaneous DT/TG analyses were carried out on a Du Pont 9900 thermal analyzer from room
Chem. Mater., Vol. 8, No. 4, 1996 815 Table 2. Atomic Coordinates (×104) and Equivalent Isotropic Displacement Coefficients of Selected Non-Hydrogen Atoms (Å2 × 103)a atom
x
y
z
U(eq)
Mo1 Mo2 O1 O2 O3 O(4) O5 O6 O7 Na1 N1
3012(1) 1145(1) -1120(7) -3919(7) -2248(7) 616(7) 1320(4) 3144(5) 561(5) -271(4) 3631(10)
MoO-0 2500 -16(1) 2500 2500 2500 2500 -121(5) 343(6) -311(5) 2500 2500
4248(1) 1069(1) -717(4) 358(4) 1942(4) 938(4) -511(3) 1134(3) 2189(3) 3089(3) -1041(6)
10(1) 9(1) 11(1) 16(2) 16(2) 14(2) 12(1) 19(1) 19(1) 20(1) 27(2)
Mo1 Mo2 Mo3 Mo4 O1 O2 O3 O(4) O(5) O6 O7 O8 O9 O10 O(11) O12 O13 C1 C2 C3 C4 C5 C6 N1 N6
7016(1) 9226(1) 4662(1) 6924(1) 10530(3) 7303(3) 5626(3) 3223(3) 8424(3) 6303(3) 8892(3) 8479(3) 4798(3) 10776(3) 6913(3) 5124(3) 6796(3) 2648(5) 2390(5) 2658(6) 2105(5) 2489(5) 1326(5) 2268(4) 1940(4)
MoO-1 1240(1) 2640(1) 3183(1) 4695(1) 3727(3) 1741(3) 212(3) 2227(3) 5623(3) 4507(3) 2562(3) 2641(3) 3522(3) 542(3) 1577(3) 3407(3) 4580(3) 4418(5) 2872(4) 2154(5) 728(5) -93(4) 984(4) 5128(4) 318(4)
6586(1) 4388(1) 4925(1) 2715(1) 4036(2) 7797(2) 6907(2) 5302(2) 2362(2) 1555(2) 5808(2) 2945(2) 3541(2) 3957(2) 4905(2) 6500(2) 4517(2) 11105(3) 11137(3) 10010(3) 10044(3) 8930(3) 8235(3) 12221(3) 7099(3)
14(1) 13(1) 12(1) 14(1) 22(1) 24(1) 26(1) 23(1) 24(1) 26(1) 17(1) 16(1) 18(1) 21(1) 15(1) 16(1) 14(1) 33(2) 28(1) 34(2) 33(2) 27(1) 27(1) 28(1) 29(1)
a Equivalent isotropic U defined as one-third of the trace of the orthogonalized Uij tensor.
temperature to 1000 °C in N2 stream (10 °C min-1). XRD data were collected at room temperature for the as-synthesized and calcined precursors of both compounds for 2θ [3.6-60°] on a Rigaku D/Max IIIA diffractometer with Cu KR. Single crystals suitable for structure determination (0.05 × 0.05 × 0.20 mm for MoO-0 and 0.30 × 0.30 × 0.40 mm for MoO-1) were mounted on a Siemens R3M/V2000 diffractometer with Mo KR radiation by ω scan (3.0° < 2θ < 50.0° for MoO-0; 3.5° < 2θ < 50.0° for MoO-1). The constants of the unit cell were determined from the positions of 20 centered reflections for both MoO-0 and MoO-1 (5.0° < 2θ < 25.0°) and optimized by least-squares refinement. Absorption correction was made by a ψ scan using applied program14 (Tmin/Tmax: 0.6506/0.7411 for MoO-0 and 0.5617/0.7653 for MoO-1). The crystal structures were solved by direct methods using the XS program of SHELXTL-PLUS15 and refined by full-matrix leastsquares analysis on F with XLS program of SHELXS-76. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at calculated positions with fixed isotropic thermal parameters. This gives the final values of R ) 2.35% and wR ) 3.07% for MoO-0, and R ) 2.59% and wR ) 4.40% for MoO-1. The details of data collection and refinement of both compounds are summarized in Table 1, and atomic coordinates of selected atoms are listed in Table 2.
Results and Discussion Synthesis and General Features of MoO-0 and MoO-1. MoO-0 composes of Na[NH4]‚[Mo3O10]. The (14) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr. 1968, A24, 351. (15) Simens Analytical X-ray Instruments, Inc., Madison, WI, 1990.
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Figure 1. Crystal structure of MoO-0. (a, top left) View of alternating (Mo3O10)2- anionic chains and Na+ cations along the b axis. (b, bottom) Polyhedral view of the one-dimensional chains and the locations of Na+ and NH4+ cations along the a axis. (c, top right) Structural moiety of the (Mo3O10)2- chains.
presence of NH4+ in MoO-0 is strongly supported by a TG/DT study which shows a sharp endothermic change between 300 and 370 °C with a corresponding weight loss of ∼3.65% and typical odor of NH3 (In theory (NH4)/ (NaNH4‚Mo3O10) ) 3.68% in wt %). It is believed that NH4+ is generated from the decomposition of TMAOH during crystallization,16,17 which together with Na+ cation plays the role as the templating agent. NH4+ is exchangeable in solution. The crystal structure of MoO-0 remains intact below 800 °C. MoO-1 consists of [1,6-HDAH2]‚[Mo4O13] units. The thermal behavior of MoO-1 is as follows: two major weight losses of ∼16.5% and 13% appear at temperatures of 300-350 700-800 °C, respectively. The first (16) Parise, J. B. Acta Crystallogr. 1984, C40, 1641. (17) Xu, R. R.; Chen, J. S.; Feng, S. H. Chemistry of Microporous Crystals; Inui, T., Namba, S., Tatsumi, T., Eds.; Kodansha-Elsevier: 1990; p 63.
one is due to the complete removal of 1,6-HDA, which weighs 16.6% of the total weight. At this temperature, the crystal structure of MoO-1 no longer remains as indicated by the XRD pattern acquired after it is heated at 300 °C for 10 min. This suggests that in MoO-1, the 1,6-HDA molecule is associated strongly to its host structure. Both MoO-0 and MoO-1 show poor electronic conductivities at room temperature and atmospheric pressure. A variety of amine templates including RNH2, R2NH, R3N, R4N+, and diamines have been applied to organize crystal structures and induce crystallization. This has resulted in a series of 1-D and 2-D intercalation-type molybdenum oxide polymers. The observation shows that the structures of this series of intercalation compounds are very template specific, and they have a strong preference to diamine templates. The study indicates that the crystal structures and properties of
Synthesis of Open-Framework Polymolybdates
Chem. Mater., Vol. 8, No. 4, 1996 817
Figure 2. Crystal structure of MoO-1. (a, top left) View of alternating (Mo4O13)2- anionic chains and (1,6-HDAH2)2+ cations along [1 -1 0] direction. (b) Polyhedral view of the one-dimensional chains and the locations of (1,6-HDAH2)2+ cations. (b-1, bottom left) along the b axis; (b-2, top right) along the a axis. (c, bottom right) Structural moiety of the (Mo4O13)2chains.
molybdenum oxide intercalating polymers are controlled by the properties of templates, the pH of the medium, and hydrothermal conditions. Crystal Structures of MoO-0 and MoO-1. MoO-0 and MoO-1 are examples of one-dimensional transition oxide polymers where the Mo atom is octahedrally coordinated. Both structures consist of infinite zigzag chains of (Mo3O10)2- and (Mo4O13)2- moieties stretching along b in MoO-0 and [1 -1 0] direction in MoO-1 as shown in Figures 1a and 2a, respectively. The molybdenum polyanion chains in each case are immediately surrounded by six chains of the same type forming a cylindrical unit. In MoO-0, the cylindrical unit is symmetrical whereas in MoO-1 it is elongated due to the nonspherical shape of 1,6-HDA molecules. In the crystal structures of MoO-0 and MoO-1, cations occupy the interstrand regions of molybdenum oxide chains playing a role as intercalating pillars. In MoO0, Na+ cations are nonexchangeable, and they are staggered along molybdenum polyanion chains with Na-O distances ranging from 2.339 to 2.502 Å and a Na-Na distance of ∼4.5 Å. NH4+ cations hang in the pockets of the puckered chain (Figure 1b), and they are exchangeable. The N atom is H-bonded to two O atoms
at N-O distances of 2.876 and 2.886 Å. In MoO-1, (1,6HDAH2)2+ cations stack one upon another along the [1 -1 0] direction as shown in Figure 2a,b with the 1,6HDA molecules lying nearly perpendicular to the direction of polyanion chain. Charge interaction between [-MoO-‚‚‚(-NH3)+] is recognized based on the N-H‚‚‚O distances varying between 2.812 and 2.965 Å (Table 3). In (1,6-HDAH2)2+, the C-C bond lengths are between 1.480 and 1.535 Å, which are shorter than the paraffinic C-C single bond (1.54 Å), whereas C-N bonds (1.4891.494 Å) are lengthened compared to the C-N single bond (1.47 Å). This may be attributed to the H bonds formed between [MoO-‚‚‚(-NH3)+] as well as the steric constraint due to their location. The molybdate chain in MoO-0 is constructed from basic (Mo3O10)2- moieties. Within a (Mo3O10)2- moiety, the three octahedra share two edges (Figure 1c). Each moiety is then joined up with another symmetry related moiety by sharing two edges of an octahedron to form a unit of six octahedra. Translational repetition of the six octahedra units, through sharing edges, results in an infinite Mo oxide chain along the b axis. Of the seven independent O atoms, four are bonded to only one Mo atom with average bond length of 1.71 Å. The other
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Table 3. Selected Geometric Parameters of MoO-0 and MoO-1 selected bond distances (Å) Mo1-O2a Mo1-O3a Mo1-O5a
1.719(6) 1.721(6) 1.925(4)
Mo1-O4a Mo1-O1a Mo2-O7 Mo2-O6 Mo2-O1a Mo2-O4 Mo2-O5c Mo2-O5
2.206(5) 2.231(5) 1.696(4) 1.705(5) 1.955(2) 1.973(2) 2.224(4) 2.273(4)
Mo1-O2 Mo1-O3 Mo1-O7 Mo1-O11 Mo1-O12 Mo1-O10A Mo2-O1 Mo2-O7 Mo2-O8 Mo2-O10 Mo2-O11 Mo2-O13 Mo3-O4 Mo3-O9 Mo3-O11 Mo3-O12 Mo3-O13 Mo3-O13A Mo4-O5 Mo4-O6 Mo4-O8 Mo4-O9 Mo4-O13 Mo4-O12A C1-C2 C2-C3 C3-C4 C4-C5 C5-C6 C1-N1 C6-N6
1.715(2) 1.699(3) 2.315(3) 2.163(2) 2.007(2) 1.919(2) 1.686(3) 1.765(2) 2.068(3) 1.856(2) 2.394(3) 2.085(2) 1.688(3) 1.758(2) 1.875(2) 2.155(3) 2.477(3) 1.974(2) 1.707(3) 1.705(3) 1.870(2) 2.395(3) 2.269(2) 1.990(2) 1.480(7) 1.530(6) 1.509(7) 1.535(5) 1.505(5) 1.495(5) 1.485(5)
N6-O3a N1-O1b N1-O9c N6-O8d N6-O11e N1-O2f
2.965(7) 2.921(6) 2.906(6) 2.812(5) 2.884(5) 2.964(7)
selected bond angles (deg) MoO-0 O1a-Mo1-O5a (O5b) O3a-Mo1-O5a (O5b) O5a-Mo1-O5b O1a-Mo1-O3a O2a-Mo1-O4a O1a-Mo2-O5 O1a-Mo2-O7 O7-Mo2-O4 O4-Mo2-O5 O5-Mo2-O7 O1a-Mo2-O4 O5c-Mo2-O6
74.6(2) 102.4(1) 139.9(2) 168.2(2) 167.7(2) 73.1(2) 96.6(2) 98.8(2) 87.3(2) 163.6(2) 155.4(2) 158.5(2)
MoO-1 O2-Mo1-O10A O2-Mo1-O12 O11-Mo1-O12 O11-Mo1-O10A O2-Mo1-O11 O3-Mo1-O7 O12-Mo1-O10A O7-Mo2-O13 O8-Mo2-O13 O8-Mo2-O10 O7-Mo2-O10 O1-Mo2-O11 O7-Mo2-O8 O10-Mo2-O13 O9-Mo3-O11 O9-Mo3-O13A O11-Mo3-O12 O12-Mo3-O13A O4-Mo3-O13 O9-Mo3-O12 O11-Mo3-O13A O6-Mo4-O8 O6-Mo4-O12A O8-Mo4-O13 O13-Mo4-O12A O5-Mo4-O9 O6-Mo4-O13 O8-Mo4-O12A
102.0(1) 93.3(1) 72.2(1) 85.4(1) 154.0(1) 168.9(1) 154.2(1) 91.9(1) 73.3(1) 85.1(1) 100.8(1) 169.1(1) 153.4(4) 152.8(1) 102.2(1) 97.5(1) 74.9(1) 73.1(1) 175.6(1) 156.0(1) 140.0(1) 105.8(1) 103.0(1) 72.9(1) 70.3(1) 169.6(1) 155.8(1) 141.5(1)
H Bonds
a x, y, z. b -1 + x, y, 1 + z. c x, y, 1 + z. x, -y, 1 - z. f 1 - x, 1 - y, 2 - z.
d
1 - x, -y, 1 - z. e 1 -
three O atoms are each bonded to three Mo atoms with average O-Mo distance of 2.08 Å. The MoO6 octahedra are therefore distorted. In the case of MoO-1, the basic moiety is (Mo4O13)2which is made up of four MoO6 octahedra sharing edges
(Figure 2b,c). Another symmetry-related moiety is joined up by sharing an edge to form a unit of eight octahedra. Translational repetition of the eight-octahedra units, through sharing vertexes, results in infinite Mo oxide chain along the crystallographic [1 -1 0] direction. There are 13 crystallographically independent O atoms falling into four groups which are 1-coordinated (terminal), 2-coordinated, 3-coordinated, and 4-coordinated. The average O-Mo distances are 1.70, 2.00, 2.10, and 2.20 Å, respectively. The 1- and 3-coordinated O-Mo distances are similar to those in MoO0. Again, the different O-Mo distances lead to distortion of the MoO6 octahedra. In the structures of MoO-0 and MoO-1, (MoxOy)nmoieties are connected into 1-D chains. These chains are bonded by cations into bundles that are interleaved by Na+, NH4+, or protonated (1,6-HDAH2)2+ cations (Figures 1b and 2b). Similar structure organization has been observed in 1-D reduced molybdenum phosphates.1 Moieties in MoO-0 and MoO-1 are connected by sharing O atoms, whereas in the 1-D reduced molybdenum phosphates, moieties are joined by either PO4 tetrahedra or metal cations. Similar structural features have been found in a lately discovered oxovanadyl organodiphosphonates where puckered chains are formed as a result of the long-range axial interactions between adjacent (VO) sites through an alternating short-long (VdO‚‚‚VdO‚‚‚VdO) zigzag motif.3 The chains are interleaved by the protonated piperazinium cations. The study also indicates that vanadyl organophosphonates encompassing a wide structural range can be designed and tailored by combining the hydrothermal synthesis methods, appropriate templates, organic spacers, and suitable pH of solution. The mechanism of the formation of different Mo oxide polymers such as MoO-0 and MoO-1 is yet to be understood. However, it appears that the appropriate combination of structure-directing agents, acidity, and hydrothermal synthetic conditions can lead to the formation of molybdenum oxide polymer structures with different geometrical and structural characteristics. Acknowledgment. We would like to thank the National University of Singapore for financial support (Grant No RP920603). We are also grateful to the technical staff in the department for their supporting services. Supporting Information Available: Crystal structure data (11 pages); observed and calculated structure factors (5 pages). Ordering information is given on any current masthead page. CM9502333
