Synthesis of a New Tetrapyridyl Ligand and the Characterization of Three Distinct Metal- and Hydrogen-Bonding Conformations Eric Bosch* and Nate Schultheiss
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 2 263-266
Department of Chemistry, Southwest Missouri State University, Springfield, Missouri 65804
Nigam Rath Department of Chemistry, University of Missouri, St. Louis, Missouri 63121
Marcus Bond Department of Chemistry, Southeast Missouri State University, Cape Girardeau, Missouri 63701 Received October 5, 2002;
Revised Manuscript Received December 24, 2002
ABSTRACT: The design, synthesis, and complexation characteristics of the ligand 1,2,4,5-tetrakis-(2′-pyridylethynyl)benzene are described. Three distinct essentially planar conformations are demonstrated. Conformational flexibility allows the ligand to adopt an “in-in” conformation and complex silver(I) cations. On self-assembly with resorcinol, the ligand adopts an “out-out” conformation to accommodate the larger resorcinol molecule with pyridylhydroxyl hydrogen bonding. In contrast, an “in-out-in-out” conformation is adopted on complexation with 2,7dinitrofluorenone through weak hydrogen bonds. Introduction The engineering of supramolecules based on the selfassembly of small building blocks has become a major research area over the past decade.1 The major goal of this research, the controlled synthesis of predictably ordered crystalline solids, is driven by the direct relationship between the structure of a solid and the properties of the solid. Potential applications of engineered solids include molecular electronics2 and magnetic and optical devices.3 Two of the most common methods of organizing the smaller building blocks within crystalline solids are self-assembly through coordination of metal cations with polydentate N-based ligands4 or self-assembly of complementary H-bond donors and acceptors.5,6 It is common for polypyridyl ligands to be used in both types of networks. Many studies have focused on the coordination chemistry and hydrogen-bonding motifs generated using commercially available components. There is a growing need to expand this arsenal of ligands with the ultimate goal of custom designing ligands to generate materials with predictable properties. Clearly then, there is a connection between molecular design and design of crystalline functional materials. In this paper, we disclose the structure of a novel tetrapyridyl ligand and demonstrate its ability to adopt three different, essentially coplanar, conformations in coordination complexes and H-bonded structures. Experimental Section General. All chemicals were obtained from Aldrich and used without further purification. * To whom correspondence should be addressed. E-mail: erb625f@ smsu.edu.
Synthesis. 1,2,4,5-Tetraethynylbenzene7 was prepared in 63% overall yield by Sonogashira coupling of 1,2,4,5-tetrabromobenzene with trimethylsilylacetylene followed by basic deprotection.8 1H NMR: δ 7.64 (s, 2H), 3.42 (s, 4H). 13C NMR: δ 139.70, 128.42, 86.62, 83.63. Argon was bubbled through a mixture of 1,2,4,5-tetraethynylbenzene (0.271 g, 1.5 mmol), 2-bromopyridine (1.183 g, 7.0 mmol), bis(triphenylphosphine)palladium(II) dichloride (59 mg), copper(I) iodide (6 mg), and triphenyl phosphine (53 mg) in triethylamine (5 mL) and toluene (5 mL) for 10 min. The flask was then sealed and heated at 50 °C for 30 h. The mixture was cooled and diluted with dichloromethane (200 mL) and washed with aqueous sodium bicarbonate (150 mL). The organic layer was washed with water and dried over sodium carbonate, and the solvent was evaporated. The crude residue was subjected to flash chromatography with a mixture of hexane and ethyl acetate to yield tetrakis-1,2,4,5(2′-pyridylethynyl)benzene as an offwhite solid (260 mg, 36%). 1H NMR: δ 7.24-7.31 (m, 4H), 7.64-7.76 (m, 8H), 7.89 (s, 2H), 8.66 (td, J ) 1.2, 4.8 Hz, 4H). 13C NMR: δ 86.44, 95.20, 123.24, 125.70, 128.03, 135.50, 136.17, 143.03, 150.15. Anal. calcd for C34H18N4‚0.5 H2O: C, 83.10; H, 3.90; N, 11.40. Found: C, 82.50; H, 3.97; N, 11.45%. Complexation. (a) Silver(I). A solution of the ligand 2 (9.6 mg, 0.02 mmol) and silver trimethanesulfonate (10.7 mg, 0.04 mmol) was heated in a mixture of dichloromethane (2 mL) and nitrobenzene (2 mL) until a homogeneous solution was obtained. The solution was allowed to cool, and rod-shaped crystals formed over the course of several days. The solution was filtered to yield 15.7 mg (77%) of off-white crystalline solid, 5. Anal. calcd for C34H18N4‚C6H5NO2: C, 45.06; H, 2.07; N, 6.25. Found: C, 45.28; H, 2.13; N, 6.25%.g (b) Resorcinol. A mixture of the ligand 2 (9.6 mg, 0.02 mmol) and resorcinol (4.5 mg, 0.04 mmol) were heated in nitromethane until a homogeneous solution was obtained. After it was cooled, yellow block-shaped crystals were formed and 11.7 mg (83%) was harvested after 3 days (see Table 1). 1H NMR analysis indicated a 2:1 ratio of resorcinol:2. Anal. calcd for C46H30N4O4: C, 78.62; H, 4.30; N, 7.97. Found: C, 78.36; H, 4.35; N, 8.09%. (c) 2,7-Dinitrofluorenone. A mixture of 2 (9.7 mg, 0.02 mmol) and 2,7-dinitrofluorenone (11.1 mg, 0.04 mmol) in
10.1021/cg0255974 CCC: $25.00 © 2003 American Chemical Society Published on Web 01/23/2003
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Table 1. Crystallographic Data CCDC deposit no. formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z µ (mm-1) R(F0) Rw(F0)
5
6
7
CCDC201327 C36H18Ag2F6N4O6S2 996.40 triclinic P1 h 6.3142(3) 11.5015(5) 13.13220(6) 80.170(3) 78.343(3) 78.581(3) 907.00(7) 2 31.278 0.0478 0.1257
CCDC201328 C44H48N4O4 696.892 triclinic P1 h 8.58590(10) 10.2506(2) 11.8619(2) 108.6048(12) 96.7034(11) 113.7929(8) 868.31(2) 1 0.09 0.038 0.068
CCDC201326 C60H30N8O10 1022.92 triclinic P1 h 7.1305(2) 11.1981(3) 15.2304(4) 93.824(2) 101.110(2) 96.865(2) 1179.74(6) 1 0.101 0.0592 0.0934
Figure 3. View of the 2:1 complex of silver(I) triflate with ligand 2 showing the weak Ag-O and O- - -H-C contacts.
Figure 1. trans-Coordinating bipyridyl ligand 1,2-bis-(2′pyridylethynyl)benzene (1) and the complex with silver(I) triflate.
Figure 4. Two orthogonal views of the stacking of adjacent layers of the silver complexes.
Scheme 1 Figure 2. Potential planar conformations of tetrapyridyl ligand 2. nitromethane was heated until a homogeneous solution was obtained. Yellow irregular crystals formed in 73% yield (20.8 mg). 1H NMR analysis indicated a 2:1 ratio of 2,7-dinitrofluorenone:2.
Results and Discussion We had previously reported the design, synthesis, and complexation characteristics of the trans-coordinating dipyridyl ligand 1 shown in Figure 1A.9 That ligand was shown to form trans complexes with palladium(II) and silver(I) as shown in Figure 1B with the pyridyls in the “in-in” conformation. Thummel et al. independently prepared palladium complexes of 1.10 We also observed nonplanar conformations of 1 in the formation of a onedimensional coordination polymer on self-assembly with copper(I) iodide.11 Interestingly, Bunz et al. also formed a coordination polymer on treatment of the dimethoxy analogue of 1 with rhodium(II) dicarboxylate.12 In their polymer, the ligand adopted the “out-out” conformation shown in Figure 1C to accommodate the larger dicationic unit.
We designed the tetradentate ligand 2 reasoning that the two distinct essentially planar conformationssthe “all in” and “all out” conformations shown in Figure 2A,Bswould distinguish between small guests, typically transition metal cations, and larger organic guests. In the absence of appropriate guests, the third essentially planar “in-out-in-out” conformation shown in Figure 2C may be favored by the formation weak intramolecular CH- - -N hydrogen bonds between adjacent pyridyl rings. The ligand 2 was prepared in moderate yield from 1,2,4,5-tetrabromobenzene by repeated Sonogashira coupling as shown in Scheme 1. Thus, palladiumcatalyzed coupling of the tetrabromobenzene with excess
Synthesis of a New Tetrapyridyl Ligand
Figure 5. Perspective view of the 2:1 complex of resorcinol with the ligand 2.
trimethylsilylacetylene followed by base-promoted deprotection yielded 1,2,4,5-tetraethynylbenzene (4). Subsequent palladium-catalyzed coupling of the tetraethynylbenzene with excess 2-bromopyridine yielded 2 in moderate yield. The ligand 2 was then allowed to self-assemble with a variety of metal salts. A complex, 5, with 1:2 stoichiometry was obtained from solutions containing 1 equiv of 2 mixed with 2 equiv of silver(I) triflate. Indeed, X-ray quality crystals were obtained for the silver(I) triflate complex on slow cooling of a dichloromethane solution of the components. The structure shown in Figure 3 shows two transcoordinated silver cations. The asymmetric unit consists of one-half of the ligand and the complexed silver cation along with the triflate anion. The ligand remains essentially planar with the pyridyl rings slightly twisted out of coplanarity with the central benzene ring with torsional angles of 3.2 and 9.8°. The alkynyl moieties are almost linear with all angles in the range of 174180°. The N(I)-Ag(1)-N(2) angle is slightly bent at
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158.80° with a triflate oxygen-silver interaction of O(2)-Ag(1) of 2.737 Å. The Ag(1)-N(1) and Ag(1)-N(2) distances of 2.184(3) and 2.187(3) Å are slightly longer than the 2.152(2) Å observed with the complex of ligand 1 with silver(I) triflate. There are multiple weak C-H- - O (triflate) interactions with adjacent ligands. The H- - O distances range from 2.321 to 2.855 Å with C- - -O distances in the range of 3.220-3.711 Å. On the basis of the interatomic distances, these secondary interactions may be classified as weak interactions according to Jeffrey.13 The butterfly-shaped ligand-silver complexes are slip-stacked along the a-axis. The offset overlap or stairlike arrangement of the ligands is shown in the two orthogonal views in Figure 4. With this arrangement, the triflate anions effectively step up the staircase by complexing to silver atoms in overlaying complexes. The silver cation in one complex lies over the central benzene ring of the ligand in the adjacent layer; however, the silver does not make contact with the benzene ring with a distance of 3.67 Å between the silver atom and the benzene-centroid. The adjacent planes of complexes are close-packed with an interplanar distance of approximately 3.4 Å. We reasoned that a larger hydrogen bond donor would fit in the cavity formed in the all out conformation shown in Figure 2B. Indeed, Moore had earlier described the synthesis of the series of related ligands based on 1,3-bis(2′-pyridylethynyl)benzene as receptors for isophthalic acid.14 Self-assembly with resorcinol in nitromethane solvent yielded brick-shaped yellow crystals of the 2:1 resorcinol:ligand complex, 6, that were suitable for X-ray crystallography, and the crystal structure of the complex in Figure 5 reveals that the ligand does adopt the expected out-out conformation.
Figure 6. View of a section of the two-dimensional sheet formed on self-assembly of 2,7-dinitrofluorenone with ligand 2.
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Each resorcinol is held by a unique arrangement with two pyridyl-hydroxyl hydrogen bonds and one CH-π interaction. The pyridyl groups are slightly puckered to form hydrogen bonds to the resorcinol molecules that are almost perpendicular to the plane of the tetrapyridyl ligand to accommodate the π-bonding to C(1)H. The N- - -H and N- - -O distances of 1.735, 1.842, and 2.7751(11) and 2.7799(12) Å are typical for hydrogen bonds between pyridines and resorcinol. The resorcinolcentroid C(1) distance is 3.552 Å, and the H-centroid distance is 2.625 Å with a centroid-H-C(1) angle of 177.29° that is well within the values for C-H- - -π interactions recently discussed by Desiraju in his review.15 The third binding mode was observed on self-assembly of 2 with the acceptor molecule 2,7-dinitrofluorenone. We initially wondered if the ligand, as a planar hydrocarbon electron rich molecule, would form electron donor-acceptor complexes as reported for the analogous planar tetrakis(phenylethynyl)ethene.16 Yellow/orange irregular-shaped crystals were obtained on self-assembly of the two components in nitromethane or dichloromethane. 1H NMR analysis indicated a 2:1 complex, 7, and the crystal structure revealed a hydrogen-bonded complex. The hydrogen-bonded network must be favored due to the multiple synergistic interactions while electron donor-acceptor complexation may not be favored due to the π-deficient nature of the ligand. In the structure, the ligand adopted the in-outin-out conformation shown in Figure 2C with an intramolecular pyridyl-HC hydrogen bond labeled A in Figure 6. The intramolecular hydrogen bond is accommodated by a bent alkynyl group with angles of 172174°. The pyridyl N- - -H-C distance is 3.443 Å with a N-H distance of 2.54 Å and would be classified as weak according to Jeffrey.13 A two-dimensional network is set up with the ligands bridged by pairs of head-to-head hydrogen-bonded fluorenone dimers. The sheetlike structure is held together by a combination of cooperative “weak” hydrogen bonds as shown in A-G of Figure 6. Thus, the fluorenone molecules are held together by two weak C-H- - -OdC hydrogen bonds, B in Figure 6, with an O- - -H distance of 2.507 Å, a C- - -O distance of 3.446 Å, and a CdO-H angle of 154.28°. The dinitrofluorenone molecules are then also held to the ligand 2 by a combination of weak hydrogen bonds. Thus, each dinitrofluorenone molecule has three weak C-H hydrogen bonds to the ligand 2. There is a bifurcated hydrogen bond from both the nitro group oxygen atoms, O3 and O4, to the H27 on the pyridyl ring that is labeled as C in Figure 6. The H- - -O distances are 2.619 and 2.684 Å with O- - -C distances of 3.528 and 3.548 Å. The oxygen atom O3 also has a close contact to the hydrogen atom, H22, on the central benzene ring of the ligand shown as D in the figure. The interatomic distances are 2.423 Å for O- - -H and 3.287 Å for O- - -C. The pyridyl moiety with the out conformation has a weak hydrogen bond to H6 on the fluorenone with N- - -H and N- - -C distances of 2.516 and 3.446 Å, respectively (E in Figure 6). It is noteworthy that the strands of ligand 2:dinitrofluorenone molecules form two-dimensional sheets that are separated by approximately 4.1 Å. Adjacent
Bosch et al.
strands are held together by two cooperative O- - -HC bonds between the nitro oxygen atoms and the adjacent hydrogens on the pyridyl group shown as F (O-H distances of 2.621 and 2.741 Å and O-C distances of 3.515 and 3.472 Å). Note that one of the nitro oxygen atoms has a weak bond to the neighboring ligand pyridyl with O- - -HC of 2.601 Å and C- - -O of 3.347 Å, respectively, that is shown as G in Figure 6. In summary, we have described the synthesis of a new tetradentate ligand and demonstrated metal-cation complexation and two different hydrogen-bonding conformations. Acknowledgment. We thank the Graduate College at SMSU for funding and the National Science Foundation for funding an upgrade of our NMR at SMSU (CCLI Grant No. 9950853) and for funding the X-ray diffractometer at SEMO (CCLI Grant No. 9951348). We thank an anonymous reviewer for various helpful suggestions. References (1) (a) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinham, 1995. (b) Comprehensive Supramolecular Chemistry; Lehn, J.-M., Ed.; Pergammon Press: Oxford, 1995. (2) (a) Miyasaka, H.; Matsumoto, N.; Okawa, H.; Re, N.; Gallo, E.; Floriani, C. J. Am. Chem. Soc. 1996, 118, 981. (b) McCleverty, J. A.; Ward, M. D. Acc. Chem. Res. 1998, 31, 842. (c) Ciurtin, D. M.; Pschirer, N. G.; Smith, M. D.; Bunz, U. H. F.; zur Loye, H.-C. Chem. Mater. 2001, 13, 2743. (3) Chen, C.-T.; Suslick, K. S. Coord. Chem. Rev. 1993, 128, 293. (4) (a) Sauvage, J.-P. Transition Metals in Supramolecular Chemistry; Wiley: New York, 1999. (b) Constable, E. C. Prog. Inorg. Chem. 1994, 42, 67. (5) (a) Subramanian, S.; Zaworotko, M. J. Coord. Chem. Rev. 1994, 137, 357. (b) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48. (c) Sherrington, D. C.; Taskinen, K. A. Chem. Soc. Rev. 2001, 30, 83. (6) Combinations of hydrogen bonding and coordination chemistry have been employed. (a) Aakeroy, C. B.; Leinen, D. S. In Crystal Engineering: From Molecules and Crystals to Materials; Kluwer: Dordrecht, 1999; p 89. (b) Brammer, L.; Rivas, J. C. M.; Atencio, R.; Fang, S.; Pigge, F. C. J. Chem. Soc., Dalton Trans. 2000, 3855. (c) Desiraju, G. R. J. Chem. Soc., Dalton Trans. 2000, 3745. (d) Qin, Z.; Jennings, M. C.; Phuddephatt, R. J. Chem. Commun. 2001, 2676. (7) Berris, B. C.; Hovakeemian, G. H.; Vollhardt, P. C. J. Chem. Soc., Chem. Commun. 1983, 9, 502. See also Leininger, S.; Sdtang, P. J.; Huang, S. Organometallics 1998, 17, 3981. (8) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 16, 4467. (9) Bosch, E.; Barnes, C. L. Inorg. Chem. 2001, 40, 3097. (10) Hu, Y. Z.; Chamchoumis, C.; Grebowicz J. S.; Thummel, R. P. Inorg. Chem. 2002, 41, 2296. (11) Bosch, E.; Barnes, C. L. J. Coord. Chem. 2003, in press. (12) Fiscus, E. J.; Shotwell, S.; Layland, R. C.; Smith, M. D.; zur Loye, H. C.; Bunz, U. H. F. Chem. Commun. 2001, 2674. (13) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (14) Bielawski, C.; Chen, Y. S.; Peng Z.; Prest, P. J.; Moore, J. S. Chem. Commun. 1998, 1313-1314. (15) (a) Desiraju, G. R. Acc. Chem. Res. 1996, 29, 441. See also (b) Gu, Y.; Kar, T.; Scheiner, S. J. Am. Chem. Soc. 1999, 121, 9411. (c) Braga, D.; Grepioni, F. Acc. Chem. Res. 1997, 30, 81. (16) (a) Philp, D.; Gramlich, V.; Seiler, P.; Diedrich, F. J. Chem. Soc., Perkin Trans. 2 1995, 875. (b) Taniguchi, H.; Hayashi, K.; Nishioka, K.; Hori, Y.; Shiro, M.; Kitamura, T. Chem. Lett. 1994, 1921.
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