A New Organically Templated Zinc Phosphite Synthesized in Phosphorous Acid Flux and Its Hydrothermal Analogue Zhi-En Lin, Wei Fan, Feifei Gao, Naotaka Chino, Toshiyuki Yokoi, and Tatsuya Okubo* Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2435-2437
ReceiVed July 19, 2006; ReVised Manuscript ReceiVed September 18, 2006
ABSTRACT: A new organically templated zinc phosphite, (H2bbp)2‚Zn6(HPO3)8 (1) (bbp ) 1,3-bis(4-pyridyl)propane), has been synthesized in a water-free flux of H3PO3. This compound possesses a new layered structure different from the analogue obtained in the corresponding hydrothermal synthesis, Zn(bbp)0.5(HPO3)‚0.5H2O (2). Crystal data: 1, monoclinic, P21/c, a ) 9.3364(5) Å, b ) 14.8119(7) Å, c ) 33.6668(8) Å, β ) 91.004(3)°, V ) 4655.1(4) Å3, Z ) 4; 2, monoclinic, P2/n, a ) 11.392(4) Å, b ) 5.199(2) Å, c ) 15.863(6) Å, β ) 106.329(5)°, V ) 901.5(6) Å3, Z ) 4. Microporous materials have been extensively studied due to their rich structural chemistry and potential applications in catalysis, adsorption, and ion-exchange processes.1 Since the discovery of aluminophosphate molecular sieves in 1982,2 widespread enthusiasm has been spurred in making non-aluminosilicate-based zeolitic materials, such as microporous metal phosphates,3 arsenates,4 germanates,5 etc. More recently, metal phosphite frameworks are of special interest since they possess characteristic pseudo-pyramidal HPO32- groups, which can reduce the M-O-P connectivity and generate more open interrupted frameworks.6 A notable example is [(C4H12N)2][Zn2(HPO3)4] containing extra-large 24-ring channels.7 Generally, metal phosphites are prepared in a system with the composition of H3PO3, metallic cation, organic templating agent, and solvent. A great deal of attention has been paid to the structuredirecting role of the organic species and the structural effect of various coordinated metallic cations under hydrothermal conditions. We have been interested in the development of new synthetic methods to prepare open-framework metal phosphites templated by organic molecules. Herein we report the facile synthesis of a new organically templated zinc phosphite, (H2bbp)2‚Zn6(HPO3)8 (1) [bbp ) 1,3-bis(4-pyridyl)propane], without any addition of water. Compared to conventional hydrothermal and solvothermal syntheses, the new synthetic approach possesses several advantages.8 For example, it reduces one reaction parameter in the starting composition, avoids the competition between solvent-framework and template-framework interactions, and might give insight into the roles of solvent molecules in the formation of microporous materials. Furthermore, the low pressure presented in the system will eliminate safety concerns associated with high hydrothermal pressure. In a typical synthesis, a mixture of Zn(OAc)2‚2H2O (0.548 g), bbp (0.496 g), and H3PO3 (0.411 g) in a molar ratio of 1:1:2 was sealed in a Teflon-lined steel autoclave and heated at 160 °C for 3 days and then cooled to room temperature. The resulting product was washed with deionized water to remove the residual H3PO3, and the colorless prismlike crystals of 1 with dimensions around 0.4 × 0.3 × 0.06 mm were recovered as the only solid phase (78.9% yield based on Zn). Chemical analysis confirms the stoichiometry: Found: Zn, 26.95; P, 17.01; C, 20.73; H, 2.72; N, 3.86%. Calcd: Zn, 27.38; P, 17.30; C, 21.80; H, 2.81; N, 3.91%. The thermogravimetric analysis shows that compound 1 has an onset temperature for the decomposition of organic amines above 250 °C (observed: 27.22%; expected: 27.96%). Single-crystal structural analysis reveals that 1 crystallizes in the monoclinic space group P21/c (No. 14) with a ) 9.3364(5) Å, b ) 14.8119(7) Å, c ) 33.6668(8) Å and β ) 91.004(3)°.9 The structure consists of macroanionic zinc phosphite layers and diprotonated * To whom correspondence should be addressed. Tel: +81-3-5841-7348. Fax: +81-3-5800-3806. E-mail:
[email protected].
Figure 1. (a) Polyhedral view of the zinc phosphite layer of 1 constructed from caplike Zn3P4 building block. (b) View of the structure along the [100] direction showing the inorganic layers intercalated with organic cations. Color code: ZnO4 tetrahedra, yellow; HPO3 pseudo pyramids, green; N, blue; C, gray.
bbp cations. The inorganic layers are built from the caplike building blocks as shown in Figure 1. The building block of the framework contains three ZnO4 tetrahedra and four HPO3 pseudo-pyramids. It can be understood from the following procedure. First, the three ZnO4 tetrahedra and three HPO3 pseudo-pyramids are linked alternatively by sharing vertices and formed a six-membered ring. Then, the fourth HPO3 pseudo-pyramid is capped above the sixmembered ring by sharing vertices with the three ZnO4 tetrahedra to give rise to the caplike Zn3P4 cluster. The Zn-O bond lengths are in the range 1.879(3)-1.971(3) Å, and P-O bond distances are between 1.478(4)-1.533(3) Å. The existence of P-H bonds is confirmed by the characteristic band of the HPO32- anion [ν(HP) 2375 cm-1] in the IR spectrum.
10.1021/cg060466e CCC: $33.50 © 2006 American Chemical Society Published on Web 10/05/2006
2436 Crystal Growth & Design, Vol. 6, No. 11, 2006 The six-membered ring of each building unit is connected to four adjacent Zn3P4 clusters to form a layered framework with eightmembered rings. The capping [HPO32-] units are situated alternatively up and down the inorganic layer. The adjacent inorganic layers are stacked in an ABCDABCD sequence along the [001] direction. Strikingly, we found that the modification of the molar ratio of starting source would affect the resulting phase in the synthesis. In our attempt to obtain compound 1 without excess phosphorous acid in the reaction mixture, a mixture of bbp, Zn(OAc)2‚2H2O, and H3PO3 in a molar ratio of 1:3:4 was sealed in a Teflon-lined steel autoclave and heated at 160 °C for 3 days. Elemental analysis reveals that the resulting compound possesses the chemical composition of H2bbp‚Zn3(HPO3)4,10 which is identical to compound 1. However, powder XRD and IR spectrum of the resulting compound are quite different from those of compound 1, indicating that the structure of the resulting compound is different from that of compound 1. It should be noted here that although the synthesis of compound 1 was carried out without any additional water, the zinc source Zn(OAc)2‚2H2O contains two H2O molecules, which may act as the potential solvent in the synthesis of compound 1. To solve this problem, ZnO was selected as the zinc source to replace Zn(OAc)2‚ 2H2O in the synthesis. Power X-ray diffraction (XRD) analysis reveals that the reaction of ZnO, bbp, and H3PO3 in a molar ratio of 1:1:2 at 160 °C for 3 days could also generate a phase-pure compound 1. This result confirms that it is indeed possible to prepare open-framework metal phosphites under water-free conditions. The role of phosphorous acid in the crystallization process is subtle and merits some attention. It is well-known that phosphorous acid possesses a low melting point at about 73 °C, and thus it will form a flux at a reaction temperature well above its melting point. We believe that phosphorous acid does not just play the role of reagent; it may also play an important part in the transport of reagents for crystallization. The metal phosphite frameworks known so far have been synthesized invariably by the use of water or various alcohols as the solvent.6,7 The nature of the interaction between the solvent and the reacting species is critical to the outcome of the framework material synthesis. On the basis of this consideration, an interesting question arises: that is, what would be the product if large quantities of water are added into the synthetic system of compound 1? To resolve this question, studies on the synthetic system H2O-Zn(OAc)2‚2H2O-bbp-H3PO3 should be carried out. In a typical synthesis, the reaction of H2O, Zn(OAc)2‚2H2O, bbp, H3PO3 in a molar ratio of 95:1:1:1 at 160 °C for 3 days could generate a new 2D zinc phosphite, Zn(bbp)0.5(HPO3)‚0.5H2O (2). Powder XRD analysis confirms the phase-purity of as-synthesized compound. Single-crystal X-ray analysis indicates that compound 2 crystallizes in the monoclinic space group P2/n (No. 13) with a ) 11.392(4) Å, b ) 5.199(2) Å, c ) 15.863(6) Å and β ) 106.329(5)°.11 The framework consists of ZnO3N tetrahedra, HPO3 pseudo-pyramids and bbp ligand, as shown in Figure 2. The two-dimensional (2D) framework can be conceptually built by the following procedures. First, the connectivity of the strictly alternating ZnO3N tetrahedra and HPO3 pseudo-pyramids results in infinite ladders running along the [010] direction. Then, the bbp ligands are directly coordinated to the zinc phosphite ladders and connect the inorganic ladders into a 2D framework. The bridged Zn‚‚‚Zn distance along the bbp ligand is ca. 13.9 Å. The inorganic-organic hybrid zinc phosphite layers are stacked along the [001] direction in an ABCABC sequence, with the guest water molecules located between the hybrid sheets. It is of interest to examine the different roles of the organic amine in the formation of the two layered zinc phosphites. In the case of 1, the organic amine resides in the interlayer region of the inorganic framework and interacts with the inorganic framework through hydrogen bonds. It acts as the structure-directing agent and also the charge balancing cation. It is noteworthy that the distance between the least-squares planes of adjacent pyridine rings of the
Communications
Figure 2. View of the layered structure of 2 constructed from 1D zinc phosphite ladders and bridging organic ligands. Color code: ZnO3N tetrahedra, yellow; HPO3 pseudo pyramids, green.
organic molecules is less than 3.5 Å, indicating the presence of intermolecular π-π interactions. Such supramolecular self-assembled molecules as the structure-directing agents are rarely found in open-framework crystalline materials.12 In the case of 2, it is not surprising to find that the organic molecule is incorporated into the framework because the synthesis was carried out in aqueous medium and the rigid pyridine-based organic molecule has a great tendency to coordinate with metal centers under hydrothermal conditions.13 The organic species serves as a bidentate ligand rather than a structure-directing agent. It not only completes the polyhedral coordination of metal center but also extends the one-dimensional inorganic ladders into a 2D hybrid framework. We have also investigated the use of other organic templates to prepare open-framework zinc phosphites without any additional water. Preliminary results show that the reaction of Zn(OAc)2‚2H2O, ethylenediamine, and H3PO3 in a molar ratio of 1:1:2 under the same conditions as used for 1 gives rise to a phase-pure openframework zinc phosphite, namely, (C2N2H10)Zn2(HPO3)3.14 Given the large variety of amines that can be used in this synthetic system, and the range of elements observed in metal phosphites, further work on this subject appears to be very inspiring. In summary, a water-free approach has been developed for the synthesis of a new open-framework zinc phosphite. The reaction product is prepared in a high yield and completely different from the analogue obtained in the corresponding hydrothermal synthesis. This novel synthetic approach is convenient in the growth of highquality large single crystals and might be a route for the preparation of many novel microporous materials with previously unknown frameworks. Acknowledgment. This work was supported by the Japan Society for the Promotion of Science (JSPS). Supporting Information Available: X-ray data in CIF format, ORTEP view of asymmetric unit of 1 and 2, IR spectra, TGA curves, and powder XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org.
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Crystal Growth & Design, Vol. 6, No. 11, 2006 2437 (8) (a) Althoff, R.; Unger, K.; Schu¨ff, F. Microporous Mater. 1994, 2, 557. (b) Deforth, U.; Unger, K. K.; Schu¨ff, F. Microporous Mater. 1997, 9, 287. (9) Crystal data for 1: C26H40N4O24P8Zn6: M ) 1432.60, monoclinic, space group P21/c (No. 14), a ) 9.3364(5), b ) 14.8119(7), c ) 33.6668(8) Å, β ) 91.004(3)°, V ) 4655.1(4) Å3, Z ) 4, Dc ) 2.044 g cm-3, µ ) 3.405 mm-1, 2.18 < θ < 27.48°. A total of 28 105 reflections were measured at 293 K. Final agreement indices were R1(wR2) ) 0.0496 (0.1272) for 645 parameters and 10337 reflections with I g 2σ(I) [R(int) ) 0.0366]. CCDC-604213. (10) Found: Zn, 26.87; P, 17.06; C, 21.04; H, 2.73; N, 3.84%. Calcd for H2bbp‚Zn3(HPO3)4: Zn, 27.38; P, 17.30; C, 21.80; H, 2.81; N, 3.91%. (11) Crystal data for 2: C6.5H9NO3.5PZn: M ) 253.49, monoclinic, space group P2/n (No. 13), a ) 11.392(4), b ) 5.199(2), c ) 15.863(6) Å, β ) 106.329(5) °, V ) 901.5(6) Å3, Z ) 4, Dc ) 1.868 g cm-3, µ ) 2.877 mm-1, 1.96 < θ < 28.27°. A total of 5249 reflections were measured at 293 K. Final agreement indices were R1(wR2) ) 0.0385 (0.0958) for 151 parameters and 2072 reflections with I g 2σ(I) [R(int) ) 0.0332]. CCDC-294488. (12) Corma, A.; Rey, F.; Rius, J.; Sabater, M. J.; Valencia, S. Nature 2004, 431, 287. (13) (a) Xie, J.; Bu, X.; Zheng, N.; Feng, P. Chem. Commun. 2005, 4916. (b) Shi, Z.; Li, G.; Zhang, D.; Hua, J.; Feng, S. Inorg. Chem. 2003, 42, 2357. (c) Huang, L.-H.; Kao, H.-M.; Lii, K.-W. Inorg. Chem. 2002, 41, 2936. (d) Halasyamani, P. S.; Drewitt, M. J.; O’Hare, D. Chem. Commun. 1997, 867. (14) Lin, Z.-E.; Zhang, J.; Zheng, S.-T.; Yang, G.-Y. Solid State Sci. 2004, 6, 371.
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