CRYSTAL GROWTH & DESIGN
A One-Dimensional Coordination Polymer Based on Novel Radical Anion Ligand Generated In Situ: Notable Magnetic and Luminescence Properties
2008 VOL. 8, NO. 5 1465–1467
Guo-Ping Yong,* Shu Qiao, and Zhi-Yong Wang* Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed January 8, 2008; ReVised Manuscript ReceiVed March 21, 2008
ABSTRACT: A one-dimensional Zn(II) radical anion coordination polymer, [ZnCl(obip•)]n (Hobip ) 2,3′-biimidazo[1,2-a]pyridin-2′(3′H)-
one) (1) was synthesized by a hydrothermal reaction between Zn(ClO4)2 · 6H2O and hydrochloride salt of imidazo[1,2-a]pyridin-2(3H)-one, in which the radical anion ligand ([obip]•-) was generated in situ by aldol condensation and then deprotonation. The compound 1 presents strong antiferromagnetic coupling between paramagnetic radical species and notable greenish yellow-red fluorescence emission. The design of molecular magnetic materials has been of considerable interest in recent years. The basic strategy to design such materials is to organize paramagnetic metal centers into polynuclear or polymeric aggregates by use of bridging ligands that can efficiently propagate magnetic superexchange.1 Nevertheless, using stable radicals as bridging ligands appeared an attractive way to develop even stronger interactions relying on the unpaired electron of the radical ligand, because direct exchange interactions between the metal-radical pair can provide stronger coupling. The most studied radicals are the (nitronyl) nitroxides,2,3 nitroxides,4 and semiquinones.5 In recent years, coordination complexes of other kinds of stable radicals, such as thiazyl radical,6 thioaminyl radical,7 and verdazyl radical,8 have begun to be explored; however, these radicals have mainly led to discrete compounds. The reports of the diamagnetic metal ions with radicals are much less than that of the paramagnetic metal ions with radicals. Some diamagnetic metal ions with radicals have shown antiferro- or ferromagnetic interactions between the radicals through the diamagnetic metal ions.3d,8c,9 On the other hand, recent studies have revealed that hydrothermal synthesis under pressure and at low temperatures (100-200 °C) proves to be an effective method for in situ ligand synthesis. This strategy not only presents the opportunity to generate organic ligands that are difficult to synthesize but also represents a potential direction for the construction of new metal-organic frameworks (MOFs) through crystal engineering.10 Herein we report the synthesis, crystal structure, and magnetic and luminescent properties of a novel one-dimensional (1D) Zn(II) radical anion coordination polymer. To the best of our knowledge, this is the first reported coordination polymer with imidazo[1,2-a]pyridin-2(3H)-one derivatives that combines magnetic character and notable greenish-yellowred fluorescence emission in the solid state. As described in Scheme 1, the in situ hydrothermally reaction between hydrochloride salt of imidazo[1,2-a]pyridin-2(3H)-one and Zn(ClO4)2 · 6H2O offers a novel radical anion ligand ([obip]•-) and one red crystalline coordination polymer, [ZnCl(obip•)] (1) (Hobip ) 2,3′-biimidazo[1,2-a]pyridin-2′(3′H)-one), in which IR absorption at 1631 cm-1 is attributed to the stretching vibration of carbonyl group.11 All major peaks of experimental powder X-ray pattern (XRPD) of compound 1 match quite well with that of the simulated XRPD, indicating reasonable crystalline phase purity (see the Supporting Information, Figure S1). Thermogravimetric analysis (TGA) measurements showed that 1 has a higher thermal stability, as no strictly clean weight loss step occurred below 250 °C (see the Supporting Information, Figure S2). The 1D polymeric structure of 1 was disclosed by an X-ray single crystal diffraction study.12 As shown in Figure 1, each slightly * Corresponding authors. E-mail:
[email protected] (G.-P.Y.); zwang3@ ustc.edu.cn (Z.-Y.W.).
Figure 1. Coordination environment of Zn(II) and [obip]•- with thermal ellipsoid 50% probability of compound 1. The hydrogen atoms are omitted for clarity. Symmetry code: (A) -x + 1/2, y + 1/2, -z + 1/2.
Figure 2. A view of the 1D helical chain of compound 1 running along the b axis.
Scheme 1
distorted tetrahedral Zn(II) center coordinates to two obip radical anions (Zn1-N 2.015(3) (N1) and 1.978(3) (N3A) Å, and Zn1-O1 1.946(3) Å), and one terminal chlorine atom (Zn1-Cl1 2.3635(10) Å). The dihedral angle between the two obip rings coordinated to one Zn(II) ion is 50.47°. In addition, the dihedral angle between two imidazo[1,2-a]pyridine rings of one [obip]•- is 13.76° showing that they are nearly coplanar. Each obip acts as N, O, N-tripodal ligand and links two Zn(II) atoms through the chelating and bridging mode, giving rise to the formation of a 1D polymeric helical chain structure along the b axis (Figure 2), in which the intrachain Zn · · · Zn distance is 5.001 Å. 1D helical chains further assemble into a 2D network through strong offset π-π staking interactions between fused heterocycles with a centroid · · · centroid distance of 3.555 Å. It is worth noting that the neighboring helixes assembled by π-π staking interactions show opposite handedness (see the
10.1021/cg800025y CCC: $40.75 2008 American Chemical Society Published on Web 04/16/2008
1466 Crystal Growth & Design, Vol. 8, No. 5, 2008
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Scheme 2
Figure 3. Temperature dependence of χMT for compound 1.
Supporting Information, Figure S3), and as a result, the whole crystal crystallizes in the centrosymmetric space group P2(1)/n. Under hydrothermal conditions, Hobip was synthesized through an aldol condensation mechanism from two molecules of imidazo[1,2-a]pyridin-2(3H)-one (see the Supporting Information, Scheme S1). In compound 1, the charge of (ZnCl)+ ion needs be balanced by one anion, which comes from the deprotonation of Hobip (Scheme 2). Generally, the ketone form (II) is more stable than the enolate form (I).13 After deprotonation, the anion of enolate form (III) should be more stable than that of ketone form (IV). To our knowledge, the typical C-O bond length in aromatic alcohols is ≈1.36 Å, while the typical length of CdO bonds is ≈1.23 Å. The coordinated CdO bonds have lengths in the range of 1.25 to 1.30 Å.14 The results of the C-O distance (1.284(4) Å) and IR spectrum (1631 cm-1) of carbonyl group indicated that ketone form anion (IV) seems to exist in compound 1. Because that carbanion is not stable, the negative charge should delocalize in the 2-oxoimidazole ring. Interestingly, compound 1 exhibits an ESR signal and unusual magnetic behavior as discussed in detail later, which demonstrate that 1 probably contains a paramagnetic radical ligand, because the Zn(II) ion presents diamagnetic behavior. The radical form of the obip ligand probably derives from oxidization of the ligand in the presence of Zn(II) ion.8g Therefore, obip acts as a radical anion, which can be demonstrated by the crystal structure and magnetic behavior of compound 1. Compound 1 exhibits a sharp isotropic ESR signal centered at g ) 2.00 (line width 0.82 mT) at room temperature, attributable to the aromatic radical (see the Supporting Information, Figure S4). 1 is an extended 1D chain structure where two radicals are bound through a diamagnetic Zn(II) ion. As such, the magnetic property of 1 deserves particular attention. The temperature dependence of the magnetic susceptibility was measured at 1 kOe in the range of 4-300 K. The χMT vs T plot is shown in Figure 3. χMT value of 1 at 300 K is 0.377 emu K mol-1, very close to the theoretical value 0.375 emu K mol-1 for one isolated spin of a radical (S ) 1/2, g ) 2.00). On cooling, the χMT value decreases rapidly, which clearly indicates strong antiferromagnetic coupling between paramagnetic radical species.15 The result shows that the diamagnetic metal ion provides an orbital pathway for interradical exchange. Below 115 K, χMT decreases more rapidly and reaches a final value of 0.00444 emu K mol-1 at 4 K. The antiferromagnetic exchange seems to be consistent with the intrachain radical-radical interactions. The crystal structure also shows that 1 is made of chain of radicals linked by diamagnetic Zn(II) ions. Generally, antiferro-
Figure 4. Antiparallel π-stacking (dash line) of 2-oxoimidazo[1,2a]pyridine radical units between adjacent chains.
magnetic exchange between radicals chelated to Zn(II) ion is weak;16 however, strong antiferromagnetic coupling of 1 suggests the presence of significant intermolecular contributions to the observed magnetic behavior. Recent studies have shown both experimentally and theoretically that π-π stacking can mediate very strong exchange interaction in radical-based materials.15,17 In the present case, the strong antiferromagnetic exchange coupling is probably mediated by the π-π stacking interactions (centroid · · · centroid of 3.560 Å) between the antiparallel spin-containing 2-oxoimidazo[1,2-a]pyridine ring units of adjacent chains (Figure 4). Below 115 K, More rapid decrease in the χMT vs T plot implies the magnetic property of 1 may be unusual and require further studies. The solid-state luminescent spectrum at room temperature was recorded and depicted in Figure 5. In compound 1, there are four fluorescence emission peaks (at about 515 nm, 560 nm, 595 nm, and 645 nm (shoulder)) upon excitation at 290 nm. It is interesting to note that the broad emission bands in the range of 515-645 nm with λmax at 515 and 595 nm were observed. The long-wavelength emission of 1 may be attributed to metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT) in nature, even though the detailed photoluminescent mechanism is not clear in
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Crystal Growth & Design, Vol. 8, No. 5, 2008 1467
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Figure 5. The solid-state emission spectrum of compound 1 (upon excitation at 290 nm) at room temperature.
the present status. Very recently, the luminescent properties of metal complexes based on imidazo[1,2-a]pyridine derivatives have received attention in the field of electroluminescence devices.18 Our previous work had shown that imidazo[1,2-a]pyridine derivative, 3,3-bis(carboxymethyl)imidazo[1,2-a]pyridin-2-one, could not only construct novel coordination networks, such as a two-dimensional basket weave network, but also obtain increasing blue fluorescence emissions while coordinated to metal centers.19 The striking greenish-yellow-red fluorescence emission and high stability of 1 suggest that it may be used as an emitter for an electroluminescence device or as a potential material for a light-emitting diode device. In summary, a novel radical anion ligand ([obip]•-) and its 1D Zn(II) coordination polymer was synthesized by a hydrothermal method. This coordination polymer shows strong antiferromagnetic coupling between paramagnetic radical species, and broad greenishyellow-red fluorescence emission, further demonstrating the potential of the [obip]•- ligand as a candidate for multifunctional materials. The further extension of this work to other metals, and its electroluminescent property is in progress in our group.
Acknowledgment. We appreciate the financial support from the National Nature Science Foundation of China (20472078).
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Supporting Information Available: The formation mechanism of the radical anion ligand, experimental and simulated XRPDs of the compound 1, thermogravimetric analysis and ESR spectrum for compound 1, and additional figure; CIF file for the compound 1. This information is available free of charge via the Internet at http:// pubs.acs.org.
References (1) (a) Pei, Y.; Verdaguer, M.; Kahn, O.; Sletten, J.; Renard, J.-P. J. Am. Chem. Soc. 1986, 108, 7428. (b) Tamaki, H.; Zhong, Z.-J.; Matsumoto, N.; Kida, S.; Koikawa, M.; Achiwa, N.; Hashimoto, Y.; Okawa, H. J. Am. Chem. Soc. 1992, 114, 6974. (c) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. Nature 1995, 378, 701. (2) (a) Caneschi, A.; Gatteschi, D.; Rey, P. Prog. Inorg. Chem. 1991, 39, 331. (b) Bogani, L.; Sangregorio, C.; Sessoli, R.; Gatteschi, D. Angew. Chem., Int. Ed. 2005, 44, 5817. (c) Caneschi, A.; Gatteschi, D.; Lalioti, N.; Sangregorio, C.; Sessoli, R.; Venturi, G.; Vindigni, A.; Rettori, A.; Pini, M. G.; Novak, M. A. Angew. Chem., Int. Ed. 2001, 40, 1760. (d) Bernot, K.; Bogani, L.; Caneschi, A.; Gatteschi, D.; Sessoli, R. J. Am. Chem. Soc. 2006, 128, 7947. (3) (a) Luneau, D.; Rey, P. Coord. Chem. ReV. 2005, 249, 2591. (b) Lhotel, E.; Simonet, V.; Ressouche, E.; Canals, B.; Amabilino, D. B.; Sporer, C.; Luneau, D.; Veciana, J.; Paulsen, C. Phy. ReV. B 2007, 75, 104429. (c) Rajadurai, C.; Ostrovsky, S.; Falk, K.; Enkelmann, V.; Haase, W.; Baumgarten, M. Inorg. Chim. Acta 2004, 357, 581. (d) Wang, L.-Y.; Ma, L.-F.; Wang, Y.-F.; Zhao, B.; Jiang, Z.-H.; Liao, D.-Z.; Yan, S.P. Inorg. Chem. Commun. 2007, 10, 212. (4) (a) Kumagai, H.; Inoue, K. Angew. Chem., Int. Ed. 1999, 38, 1601. (b) Sakane, S.; Kumada, H.; Karasawa, S.; Koga, N.; Iwamura, H. Inorg. Chem. 2000, 39, 2891. (c) Kanewaga, S.; Karasawa, S.; Nakano,
(13)
(14) (15) (16) (17) (18)
(19) (20)
M.; Koga, N. Chem. Commun. 2004, 1750. (d) Yao, M.; Asakura, S.; Abe, M.; Inoue, H.; Yoshioka, N. Cryst. Growth Des. 2005, 5, 413. (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331. (b) Caneschi, A.; Dei, A.; Gatteschi, D.; Poussereau, S.; Sorace, L. Dalton Trans. 2004, 1048. (c) Shultz, D. A.; Fico, R. M.; Bodnar, S. H., Jr.; Kumar, R. K.; Vostrikova, K. E.; Kampf, J. W.; Boyle, P. D. J. Am. Chem. Soc. 2003, 125, 11761. (d) Caneschi, A.; Dei, A.; Gatteschi, D.; Tangoulis, V. Inorg. Chem. 2002, 41, 3508. (e) Magnetism: Molecules to Materials; Miller, J. S., Drillon, M., Eds.; Wiley-VCH: Weinheim, 2001; Vol. III. Fujita, W.; Awaga, K. J. Am. Chem. Soc. 2001, 123, 3601. Miura, Y.; Kato, I.; Teki, Y. Dalton Trans. 2006, 961. (a) Hicks, R. G.; Lemaire, M. T.; Thompson, L. K.; Barclay, T. M. J. Am. Chem. Soc. 2000, 122, 8077. (b) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K.; Xu, Z. Chem. Commun. 2002, 1688. (c) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K. Inorg. Chem. 2003, 42, 2261. (d) Lemaire, M. T.; Barclay, T. M.; Thompson, L. K.; Hicks, R. G. Inorg. Chim. Acta 2006, 359, 2616. (e) Koivisto, B. D.; Ichimura, A. S.; McDonald, R.; Lemaire, M. T.; Thompson, L. K.; Hicks, R. G. J. Am. Chem. Soc. 2006, 128, 690. (f) Gilroy, J. B.; Ferguson, M. J.; McDonald, R.; Patrickc, B. O.; Hicks, R. G. Chem. Commun. 2007, 126. (g) Pointillart, F.; Train, C.; Herson, P.; Marrotb, J.; Verdaguer, M. New J. Chem. 2007, 31, 1001. (a) Zhang, D.; Ding, L.; Xu, W.; Hu, H.; Zhu, D.; Huang, Y.; Fang, D. Chem. Commun. 2002, 44. (b) Lee, C. J.; Huang, C. H.; Wei, H. H.; Liu, Y. H.; Lee, G. H.; Wang, Y. J. Chem. Soc., Dalton Trans. 1998, 171. (c) Lee, C. J.; Wei, H. H. Inorg. Chim. Acta 2000, 310, 89. (d) Yamamoto, Y.; Suzuki, T.; Kaizaki, S. J. Chem. Soc., Dalton Trans. 2001, 2943. (a) Chen, X.-M.; Tong, M.-L. Acc. Chem. Res. 2007, 40, 162. (b) Zhang, X.-M. Coord. Chem. ReV. 2005, 249, 1201. (c) Owen, R. E.; Xiong, R.-G.; Wang, Z.; Wong, G.-K.; Lin, W. Angew. Chem., Int. Ed. 1999, 38, 536. (d) Lu, J.-Y. Coord. Chem. ReV. 2003, 246, 327. The preparation of compound 1: a mixture of hydrochloride salt of imidazo[1,2-a]pyridin-2(3H)-one (0.15 mmol) and Zn(ClO4)2 · 6H2O (0.15 mmol) was placed in a heavy-walled Pyrex tube containing DMF (0.3 mL) and H2O (0.3 mL). The tube was frozen in liquid N2, sealed under a vacuum, and then heated at 120 °C for 2 days; red crystals of 1 were produced (yield: 35% based on hydrochloride salt of imidazo[1,2-a]pyridin-2(3H)-one). Anal. Calcd for C14H9ClN4OZn: C, 47.99; H, 2.57; N, 15.99. Found: C, 47.32; H, 2.59; N, 15.57. IR (KBr pellet, cm-1): 3438s, 1631s, 1560s, 1497s, 1454m, 1364m, 1284w, 1235m, 1153w, 1131w, 1103w, 1000w, 928w, 758w, 743m, 704w, 540m, 440w. CAUTION! Zn(ClO4)2 · 6H2O is potentially explosive and should be used with care. Crystal data for 1: C14H9ClN4OZn, Mr ) 350.07, monoclinic, space group P2(1)/n, a ) 11.250(2), b ) 8.0324(16), c ) 15.068(3) Å, β ) 94.24(3)°, V ) 1357.9(5) Å3, Z ) 4, F(000) ) 704, Dc ) 1.712 g cm-3, µ ) 2.008 mm-1, T ) 293(2) K, collected/unique ) 9652/ 3000 (Rint ) 0.0192). R 1 ) 0.0474, wR 2 ) 0.1326 (I > 2σ(I), R 1 ) 0.0531, wR 2 ) 0.1374 (all data) and GOF ) 1.073, R indices based on 2667 reflections with I > 2σ(I) (refinement on F2), 190 parameters. The structure was solved and refined using the SHELXTL crystallographic software package.20 (a) Cotton, F. A.; Fanwick, P. E.; Niswander, R. H.; Sekutowski, J. C. J. Am. Chem. Soc. 1978, 100, 4725. (b) Kessissoglou, D. P.; Kirk, M. L.; Bender, C. A.; Lah, M. S.; Pecoraro, V. L. J. Chem. Soc., Chem. Commun. 1989, 84. Zheng, S.-L.; Zhang, J.-P.; Chen, X.-M.; Huang, Z.-L.; Lin, Z.-Y.; Wong, W.-T. Chem. Eur. J. 2003, 9, 3888. ¨ hrstro¨m, L.; Richardson, J. F.; Hicks, R. G.; Lemaire, M. T.; O Thompson, L. K.; Xu, Z.-Q. J. Am. Chem. Soc. 2001, 123, 7154. Wang, L.-Y.; Zhang, C.-X.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P. J. Mol. Struct. 2003, 657, 1. Jornet, J.; Deumal, M.; Ribas-Arin˜o, J.; Bearpark, M. J.; Robb, M. A.; Hicks, R. G.; Novoa, J. J. Chem. Eur. J. 2006, 12, 3995. (a) Takizawa, S.; Nishida, J.; Tsuzuki, T.; Tokito, S.; Yamashita, Y. Inorg. Chem. 2007, 46, 4308. (b) Takizawa, S.; Nishida, J.; Tsuzuki, T.; Tokito, S.; Yamashita, Y. Chem. Lett. 2005, 34, 1222. (c) Takizawa, S.; Nishida, J.; Yamashita, Y.; Tsuzuki, T.; Tokito, S. Mol. Cryst. Liq. Cryst. 2006, 455, 381. Yong, G.-P.; Qiao, S.; Xie, Y.; Wang, Z.-Y. Eur. J. Inorg. Chem. 2006, 4483. (a) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (b) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Solution; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
CG800025Y
