Synthesis of Zinc Oxalate Coordination Polymers via Unprecedented Oxidative Coupling of Methanol to Oxalic Acid Owen R. Evans and Wenbin Lin* Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454
CRYSTAL GROWTH & DESIGN 2001 VOL. 1, NO. 1 9-11
Received August 7, 2000
ABSTRACT: The zinc oxalate coordination polymers Zn(ox)(Py)2 (1) and [methylpyridinium]2[Zn2(ox)3] (2) have been synthesized via unexpected oxidation of ethanol and oxidative coupling of methanol under hydro(solvo)thermal conditions. 1 adopts a 1D zigzag chain structure in which the Zn centers are linked by exo-tetradentate oxalate groups, while 2 exhibits an interesting layered architecture with 12-membered honeycomblike openings (composed of 6 Zn atoms and 6 oxalates). There is intense research interest in the design and synthesis of extended solid frameworks1-4 as potential zeolitic,5-8 magnetic,9-12 conducting,13 and nonlinear optical materials14-17 over the past decade. Although most of these insoluble solids have been synthesized by controlled (slow) mixing of suitable soluble molecular components,1-8 hydro(solvo)thermal conditions have witnessed increasing success in providing alternative pathways to the preparation of single-crystalline supramolecular solids, including metalorganic coordination networks14-19 and hydrogen-bonded systems.20 Hydro(solvo)thermal syntheses carried out in superheated solvent systems provide ideal conditions for the crystal growth owing to the enhanced transport ability of the solvents.21,22 Alcohols are typically employed as solvents or cosolvents for hydro(solvo)thermal syntheses of coordination networks because the starting metal salts and organic ligands tend to be both soluble in alcoholic solvents to afford a homogeneous solution from which X-ray-diffraction-quality single crystals can be readily grown. Herein we wish to report our unexpected discovery of the synthesis of two new zinc oxalate coordination polymers via facile oxidation of ethanol and oxidative coupling of methanol to oxalic acid under hydro(solvo)thermal conditions. Reaction of Zn(NO3)2‚6H2O and pyridine in ethanol at 140 °C over 14 days resulted in colorless crystals with the formula of Zn(ox)(Py)2 (1) in a modest yield along with a brown powder of nanocrystalline zinc oxide;21 the oxalate ligands in 1 have evidently resulted from the five-electron oxidation of the ethanol molecules. Even more surprisingly, when the reaction of Zn(NO3)2‚6H2O and pyridine was carried out in methanol at 140 °C for 5 days, colorless crystals of [methylpyridinium]2[Zn2(ox)3] (2) were isolated along with a brown powder of nanocrystalline zinc oxide.22 The oxalate ligands in 2 have presumably resulted from unprecedented oxidative coupling of methanol molecules. Since the hydro(solvo)thermal reactions were carried out under oxygen-free conditions and we have failed to carry out the same reactions with Zn(ClO4)2‚6H2O in place of Zn(NO3)2‚6H2O, we believe that nitrate groups act as oxidants for both oxidation of ethanol and oxidative coupling of methanol to oxalic acid. Consistent with this, nanocrystalline zinc oxide (zincite) isolated from the above reactions is presumably a byproduct of the reduction of Zn(NO3)2‚ 6H2O.23,24 We believe that the insolubility of both 1 and 2 under hydro(solvo)thermal conditions is at least in part responsible for the present facile oxidation of ethanol and oxidative coupling of methanol molecules to oxalic acid. Control experiments using KNO3 as the oxidant have failed * To whom correspondence should be addressed. Tel: (781) 736-2508. E-mail:
[email protected].
Figure 1. Zigzag chain structure of 1. The asymmetric unit is shown with ellipsoids. Key bond distances: Zn1-O1, 2.091(2) Å, Zn1-N1, 2.125(9) Å. Only one of the two orientations of disordered pyridine molecules is shown.
Figure 2. Interchain interdigitation of coordinated pyridine molecules in 1. The open circles with increasing sizes represent C, N, O, and Zn, respectively.
to oxidize ethanol or to oxidatively couple methanol to oxalic acid under identical conditions, presumably because of insufficient driving force for the oxidation processes due to good solubility of the expected product K2C2O4.25 A single-crystal X-ray diffraction study of 1 reveals an infinite 1D zigzag chain structure that crystallizes in the C2/c space group.26 The asymmetric unit contains one zinc
10.1021/cg005508s CCC: $20.00 © 2001 American Chemical Society Published on Web 11/01/2000
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Figure 3. ORTEP plot of 2. The asymmetric unit (except hydrogen) is shown with ellipsoids at 50% probability.
Communications oxalate groups thus link adjacent Zn centers to form 1D zigzag chains.27 The zigzag chains pack efficiently via the interdigitation of coordinated pyridine molecules (Figure 2). Compound 2 crystallizes in the monoclinic space group P21/c.28 The asymmetric unit of 2 contains one zinc atom, one and a half oxalate ligands, and one methylpyridinium cation (Figure 3). Unlike 1, the Zn centers in 2 coordinate to three different oxalate groups to adopt a highly distorted octahedral coordination geometry. The Zn-O distances range from 2.075(3) to 2.094(3) Å, while the O-Zn-O angles range from 79.9(1) to 171.49(1)°. The Zn centers are linked by oxalate groups to form 2D [Zn2(ox)3]2- macroanions which have large channels along the a axis formed by six Zn centers and six oxalate groups (Figure 4a). These open channels have been occupied by methylpyridinium cations that stack along the channels via π-π interactions (with a centroid-to-centroid distance of 3.70 Å) (Figure 4b). A similar zinc oxalate open-framework structure with the diprotonated 1,3-propylenediamine counterion has recently been synthesized by Rao et al. via an entirely different route,29 while an analogous Cr(III) oxalate 2D framework has been reported by Farrell et al.30 In summary, we have synthesized two new zinc oxalate coordination polymers via the facile oxidation of ethanol and oxidative coupling of methanol to oxalic acid under hydro(solvo)thermal conditions. These results demonstrate that alcohols can act as more than just innocent solvents during hydro(solvo)thermal syntheses and can provide an interesting route to the synthesis of oxalate-based coordination networks. Work is in progress on extending this method to the synthesis of other oxalate-based coordination networks that cannot be prepared via conventional routes. Acknowledgment. We acknowledge the NSF (Grant No. DMR-9875544) and the Petroleum Research Fund, administered by the American Chemical Society, for financial support. We also thank Dr. Scott R. Wilson and the Materials Chemistry Laboratory at the University of Illinois at Urbana-Champaign for X-ray data collections and help with the structure solution of 1. W.L. is an Alfred P. Sloan Fellow, an Arnold and Mabel Beckman Young Investigator, and a Cottrell Scholar of Research Corp. Supporting Information Available: Listings of CIFs and XRPD of zincite. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 4. (a, top) Stacking of methylpyridinium cations within the open channels alng the a axis. The open circles with increasing sizes represent C, N, O, and Zn, respectively. (b, bottom) Spacefilling diagram showing the open channels as viewed down the a axis. Methypyridinium cations have been omitted.
atom, half of an oxalate group, and a coordinated pyridine molecule that is disordered over two positions (Figure 1). The Zn1 center lies on a 2-fold axis. The Zn centers exhibit distorted-octahedral geometry by coordinating to four oxygen atoms of two oxalate ligands and to two pyridyl nitrogen atoms in cis configuration. The exo-tetradentate
(1) Zaworotko, M. J. Chem. Soc. Rev. 1994, 283-288. (2) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2638-2684. (3) Batten, S. R.; Robson, R. Angew. Chem., Int. Ed. Engl. 1998, 37, 1461-1494. (4) Yaghi, O. M.; Li, H.; Davis, C.; Richardson, D.; Groy, T. L. Acc. Chem. Res. 1998, 31, 474-484. (5) Janiak, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 14311434. (6) Munakata, M.; Wu, L. P.; Kuroda-Sowa, T. Adv. Inorg. Chem. 1999, 46, 173-304. (7) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148-1150. (8) Kiang, Y.-H.; Gardner, G. B.; Lee, S.; Xu, Z.; Lobkovsky, E. B. J. Am. Chem. Soc. 1999, 121, 8204-8215. (9) Entley, W. R.; Girolami, G. S. Science 1995, 268, 397-400. (10) Mallah, T.; Thiebaut, S.; Verdaguer, M.; Veillet, P. Science 1993, 262, 1554-1557. (11) Sato, O.; Iyoda, T.; Fujishama, A.; Hashimoto, K. Science 1996, 271, 49-51. (12) Kahn, O.; Martinez, C. J. Science 1998, 279, 44-48. (13) Munakata, M.; Ning, C. L.; Kuroda-Sowa, T.; Maekawa, M.; Suenaga, Y.; Horino, T. Inorg. Chem. 1998, 37, 5651-5656.
Communications (14) Evans, O. R.; Xiong, R.-G.; Wang, Z.; Wong, G. K.; Lin, W. Angew. Chem., Int. Ed. 1999, 38, 536-538. (15) Lin, W.; Evans, O. R.; Xiong, R.-G.; Wang, Z. J. Am. Chem. Soc. 1998, 120, 13272-13273. (16) Lin, W.; Wang, Z.; Ma, L. J. Am. Chem. Soc. 1999,121, 11249-11250. (17) Lin, W.; Ma, L.; Evans, O. R. Chem. Commun., in press. (18) Shan, Y. K.; Huang, R. H.; Huang, S. D. Angew. Chem., Int. Ed. 1999, 38, 1751-1754. (19) Lu, J. Y.; Lawandy, M. A.; Li, J.; Yuen, T.; Lin, C. L. Inorg. Chem. 1999, 38, 2695-2704. (20) Ranganathan, A.; Pedireddi, V. R.; Rao, C. N. R. J. Am. Chem. Soc. 1999, 121, 1752-1753. (21) A mixture of Zn(NO3)2‚6H2O (0.50 mmol, 0.1485 g), pyridine (0.3 mL), and ethanol (0.5 mL) was thoroughly mixed in a heavy-walled Pyrex tube. The Pyrex tube was sealed under vacuum and heated at 140 °C. Colorless platelike crystals and brown powders of zincite were obtained after 14 days. Yield for 1: 0.018 g (11.6%). Yield for zincite: 0.017 g (41.9%). IR for 1 (cm-1, KBr): 3107 (w), 1676 (s), 1610 (s), 1449 (s), 1316 (m), 1219 (w), 1159 (w), 796 (m) 795 (m), 702 (m), 629 (w). Thermal analysis: 1 exhibits a 48.3% weight loss by 230 °C corresponding to a loss of two pyridine molecules per formula unit (calcd 50.8%). (22) A mixture of Zn(NO3)2‚6H2O (0.50 mmol, 0.1485 g), pyridine (0.3 mL), and methanol (0.5 mL) was thoroughly mixed in a heavy-walled Pyrex tube. The Pyrex tube was sealed under vacuum and heated at 140 °C. Colorless platelike crystals and brown powders of zincite were obtained after 5 days. Yield for 2: 0.013 g (8.9%). Yield for zincite: 0.009 g (22.1%). IR for 2 (cm-1 , KBr): 3065 (m), 1615 (s), 1362 (w), 1313 (s), 1183 (w), 769 (s), 773 (m), 682 (s), 493 (w). Thermal analysis: 2 exhibits no weight loss until 430 °C.
Crystal Growth & Design, Vol. 1, No. 1, 2001 11 (23) Nitrate groups have probably been reduced to nitrite or/ and other reduced nitrogen-containing species. (24) X-ray powder diffraction studies have unequivocally identified the brown powders as zincite. From the line broadening of the X-ray diffraction peaks, we have estimated the average particle size of 8.5 nm according to the Scherer equation (Supporting Information). See: Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed.; Wiley-Interscience: New York, 1974; p 635. (25) The yields for 1 and 2 did not increase when excess of KNO3 was added as co-oxidants during the synthesis of 1 and 2. (26) Crystal data for 1: ZnC12H10N2O4, formula weight 311.61, monoclinic, space group C2/c (No. 15), with a ) 15.018(1) Å, b ) 9.211(1) Å, and c ) 9.457(1) Å, β ) 94.562(2)°, V ) 1304.1(2) Å3, Z ) 4, T ) 198 K, µ ) 18.9 cm-1 (Mo KR). Least-squares refinements based on 987 reflections with I > 2σ(I) (out of 1335 unique reflections) led to a final value of R1 ) 0.036 and wR2 ) 0.069. (27) The X-ray structure of a 1D zigzag chain of Zn(ox)(Meimidazole)2 was previously reported. See: Cryst. Struct. Commun. 1979, 8, 499-505. (28) Crystal data for 2: ZnC9H8NO6, formula weight 291.60, monoclinic, space group P21/c (No. 14), with a ) 7.545(1) Å, b ) 16.578(2) Å, and c ) 9.105(1) Å, β ) 104.863(2)°, V ) 1100.8(2) Å3, Z ) 4, T ) 198 K, µ ) 22.5 cm-1 (Mo KR). Least-squares refinements based on 1157 reflections with I > 2σ(I) (out of 2625 unique reflections) led to a final value of R1 ) 0.049 and wR2 ) 0.108. (29) Vaidhyanathan, R.; Natarajan, S.; Cheetham, A. K.; Rao, C. N. R. Chem. Mater. 1999, 11, 3636-3642. (30) Farrell, R. P.; Hambley, T. W.; Lay, P. A. Inorg. Chem. 1995, 34, 757-758.
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