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Synthesis, Characterization, and Rotational Dynamics of Crystalline Molecular Compasses with N-Heterocyclic Rotators Braulio Rodriguez-Molina,†,§ Ma. Eugenia Ochoa,† Norberto Farfan,*,‡ Rosa Santillan,*,† and Miguel A. Garcı´ a-Garibay*,§ † Departamento de Quı´mica, Centro de Investigaci on y Estudios Avanzados del IPN, M exico D.F. 07360, anica, Universidad Nacional Aut onoma de M exico, M exico, ‡Facultad de Quı´mica, Departamento de Quı´mica Org M exico D.F. 04510, M exico, and §Chemistry and Biochemistry Department, University of California, Los Angeles, California 90095
[email protected];
[email protected];
[email protected] Received July 3, 2009
We describe in this paper the synthesis, crystallization, and solid-state NMR dynamics of molecules intended to emulate the structure and function of macroscopic compasses. The desired structures consist of polar pyridine (2) and pyridazine (3) groups as well as their corresponding N-oxides (2O and 3O), each linked axially to two bulky triphenyl methyl groups by 1,4-triple bonds. The structures are such that the central polar heterocycles may rotate about the dialkyne axle while being sterically shielded by the two trityl groups. In addition to the synthesis of samples with natural isotopic abundance, we describe the preparation of 2-d2 and 3-d30, one labeled with deuteria in the pyridine rotator and the other fully deuterated in the two trityl groups in the stator. Crystal structures of 2 and 3 revealed packing motifs analogous to those previously reported for samples prepared with substituted phenylene rotators. While solid-state NMR measurements by 13C CPMAS NMR revealed insufficient chemical shift dispersion for a dynamic characterization of the rotation of compounds 2 and 3 (including 3-d30), the use of quadrupolar echo 2H NMR methods with 2-d2 revealed a rotational site exchange with a barrier of ca. 8.5 kcal/mol (35.5 kJ mol-1) for the pyridine group in 2.
Introduction The synthesis of molecules that emulate the structure and motion of macroscopic devices has been a subject of great interest over the last few years.1 While most studies are carried
out with molecules in solution, recent efforts have addressed the potential function of molecules bound to surfaces2,3 and in the
(1) (a) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72. (b) Marsella, M. J.; Rahbarnia, S.; Wilmont, N. Org. Biomol. Chem. 2007, 5, 391. (c) Skopek, K.; Hershberger, M.C.; Gladysz, J.A. Coord. Chem. Rev. 2007, 251, 1723. (d) Shirai, Y.; Morin, J.-F.; Sasaki, T.; Guerrero, J. M.; Tour, J. M. Chem. Soc. Rev. 2006, 35, 1043. (e) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl Chem. Rev. 2005, 105, 1281. (f) Balzani, V.; Margherita, V.; Credi, A. Molecular Devices and Machines; Wiley-VCH: Weinheim, 2003. (g) Feringa, B. L.; Koumura, N.; Delden, R. A.; TerWiel, M. K. Appl. Phys. A 2002, 75, 301. (h) Sestelo, J. P.; Kelly, T. R. Appl. Phys. A 2002, 75, 337. (i) Balzani, V.; Credi, A.; Raymo, F.; Stoddart, F. Angew. Chem., Int. Ed. 2000, 39, 3349.
(2) (a) Horinek, D.; Michl, J. Proc. Nat. Acad. Sci. U.S.A. 2005, 102, 14175. (b) Vacek, J.; Michl, J. Adv. Funct. Mater. 2007, 17, 730. (c) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540. (d) Balzani, V.; Credi, A.; Venturi, M. ChemPhysChem 2008, 9, 202. (3) (a) Shirai, Y.; Morin, J. F.; Sasaki, T.; Guerrero, J. M.; Tour, J. M. Chem. Soc. Rev. 2006, 35, 1043. (b) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Yao, Y.; Saudan, L.; Yang, H.; Yu-Hung, C.; Alemany, L. B.; Sasaki, T.; Morin, J. F.; Guerrero, J. M.; Kelly, K. F.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 4854. (c) Sasaki, T.; Osgood, A. J.; Alemany, L. B.; Kelly, K. F.; Tour, J. M. Org. Lett. 2008, 10, 229. (4) (a) Karlen, S. D.; Garcia-Garibay, M. A. Top. Curr. Chem. 2006, 262, 179. (b) Khuong, T.A.; Nu~ nez, J.E.; Godinez, C.E.; Garcı´ a-Garibay, M.A. Acc. Chem. Res. 2006, 39, 413. (c) Garcia-Garibay, M. A. Proc. Nat. Acad. Sci. U.S.A. 2005, 102, 10771.
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DOI: 10.1021/jo901261j r 2009 American Chemical Society
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FIGURE 1. (a) Line formula of a molecular compass with a polar pyridine rotator as a representative of other nitrogen heteroaromatics and (b) representation of a crystalline array of molecular compasses and the suggested analogy to (c) an array of macroscopic compasses.
solid-state.4-6 In that context, our group has been interested in exploring the properties of crystals made up of molecular rotors7 intended to emulate the structures and function of macroscopic compasses and gyroscopes (Figure 1).4,8 With structures consisting of substituted phenylene rotators linked by triple bonds to bulky triarylmethanes, we have described the formation of crystals that support an internal rotation consisting of a site exchange between energy minima related by angular displacements of 180°.4,9 With structural elements that are relatively static linked to others that are highly mobile, we suggested the term amphidynamic crystals to highlight the dynamic properties of these materials.4 Using metal organic frameworks10 and molecular crystals with nonpolar rotators, we have shown that variations in the size and topology of the stator11 and on the size and rotational symmetry order of the stator12 have a deep influence on their internal dynamics in the solid state. However, it has (5) (a) Akutagawa, T.; Koshinaka, H.; Sato, D.; Takeda, S.; Noro, S.-I.; Takahashi, H.; Kumai, R.; Tokura, Y.; Nakamura, T. Nature Mater. 2009, 8, 342. (b) Trolliet, C.; Poulet, G.; Tuel, A.; Wuest, J. D.; Sautet, P. J. Am. Chem. Soc. 2007, 129, 3621. (c) Horike, S.; Matsuda, R.; Tanaka, D.; Matsubara, S.; Mizuno, M.; Endo, K.; Kitagawa, S. Angew. Chem., Int. Ed. 2006, 43, 7226. (d) Sato, D.; Akutagawa, T.; Takeda, S.; Noro, S.; Nakamura, T. Inorg. Chem. 2007, 46, 363. (e) Sato, N.; Nishikiori, S.I. Dalton Trans. 2007, 1115. (f) Zhou, W.; Yildirim, T. Phys. Rev. B 2006, 74, 180301. (6) Sokolov, A. N.; Swenson, D. C.; MacGillivray, L. R. Proc. Nat. Acad. Sci. U.S.A. 2008, 105, 1794. (7) Adopting the nomenclature suggested in ref 1e, we will use the term molecular rotor to refer to the entire molecule, in this case as a synonym of the term “molecular compass”. A molecular rotor itself is an assembly that consists of a stator and rotator that are linked by an axle. (8) For an overview of molecular structures that emulate certain aspects of macroscopic gyroscopes, see: Skopek, K.; Hershberger, M. C.; Gladysz, J. A. Coord. Chem. Rev. 2007, 251, 1723. (9) (a) Dominguez, Z.; Dang, H.; Strouse, M. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124, 2398. (b) Godinez, C. E.; Zepeda, G.; GarciaGaribay, M. A. J. Am. Chem. Soc. 2002, 124, 4701. (10) Gould, S. L.; Tranchemontagne, D.; Yaghi, O. M.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2008, 130, 3246. (11) (a) Khuong, T.A.V.; Zepeda, G.; Ruiz, R.; Kahn, S. I.; GarciaGaribay, M.A. Cryst. Growth Des. 2004, 4, 15. (b) Godinez, C. E.; Zepeda, G.; Mortko, C. J.; Dang, H.; Garcia-Garibay, M. A. J. Org. Chem. 2004, 69, 1652. (c) Karlen, S. D.; Godinez, C. E.; Garcia-Garibay, M. A. Org. Lett. 2006, 8 3417. (d) Karlen, S. D.; Khan, S. I.; Garcia-Garibay, M. A. Mol. Cryst. Liq. Cryst. 2006, 456, 221. (e) Khuong, T.-A. V.; Dang, H.; Jarowski, P. D.; Emily, F.; Maverick; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2007, 129, 839. (f) Jarowski, P.D.; Houk, K. N.; Garcia-Garibay, M.A. J. Am. Chem. Soc. 2007, 129, 3110. (g) Nu~ nez, J. E.; Natarajan, A.; Khan, S. I.; Garcia-Garibay, M. A. Org. Lett. 2007, 9, 3559. (h) Gould, S. L.; Rodriguez, R. B.; Garcia-Garibay, M A. Tetrahedron 2008, 64, 8336. (12) Karlen, S. D.; Ortiz, R; Chapman, O. L.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2005, 127, 6554.
been suggested that optimal structures of dipolar arrays formed by the introduction of polar substituents on the rotator will provide opportunities for a new class of materials based on physical properties that change with the orientation of reorienting dipoles as a function of internal and external fields.4,5a,13,14 Among the structural features expected to influence the dynamic and electric behavior of such materials are their barriers to rotation, the magnitude of their dipole moments, as well as their distances, relative orientations, and lattice symmetries.13 Other things being equal, smaller rotators are expected to have fewer steric interactions and lower rotational barriers and greater dipoles are expected to have stronger interactions with neighboring dipoles and external fields. We previously reported a set of molecular compasses with monosusbtituted phenylene rotators having fluoro (1b), amino (1c), cyano (1d), and nitro (1e) substituents (Scheme 1).9,15,16 Compounds 1b-e have calculated dipole moments between 1.49 and 4.74 D and molecular volumes that increase modestly by ca. 0.96 to 4.1% as compared to that of 1a.17 In spite of their reduced molecular symmetries, these compounds were shown to have molecular and packing structures analogous to those of the parent hydrocarbon 1a. Crystals were obtained in a solvent-free environment grown from CH2Cl2 and in a solvent chlathrate in the presence of benzene, both in the triclinic space group P-1.9,15 A third packing structure in the monoclic space group P21/c was obtained with compound 1d from a mixture of hexane and EtOAc.15 The two types of triclinic crystals were characterized by having only one molecule per unit cell with the polar (13) (a) Michl, J.; Sykes, C.H. ACS Nano 2009, 3, 1042. (b) Horinek, D.; Michl, J. J. Am. Chem. Soc. 2003, 125, 11900. (c) de Jonge, J.J.; Ratner, M.A.; de Leeuw, S.W. J. Phys. Chem. C 2007, 111, 3770. (14) (a) Salech, B. E. A; Teich, M. C Fundamentals of Photonics; WileyInterscience: New York, 1991. (b) Weber, M. J. Handbook of Optical Materials; CRC Press: Boca Raton, 2002. (c) Setian, L. Applications in Electrooptics; Prentice Hall: New York, 2001. (15) Dominguez, Z.; Khuong, T. A. V.; Sanrame, C. N.; Dang, H.; Nu~ nez, J. E.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2003, 125, 8827. (16) Compounds with 2,3-diamino and 2-nitro-5-amino substituents were also reported in ref 15. (17) The volume and dipole moment values calculated in this study for structures minimized with the AM1 method using the program Spartan are as follows: 1a, 606 A˚3 and 0 D; 1b, 611.8 A˚3 and 1.49 D; 1c, 618.4 A˚3 and 1.47 D; 1d, 625.6 A˚3 and 3.02 D, 1e, 630.9 A˚3 and 4.74 D; 1f, 617.2 A˚3 and 2.37 D; 2, 602.8 A˚3 and 1.81 D; 3, 599.2 A˚3 and 3.28 D; 2O, 611.5 A˚3 and 3.78 D; 3O, 607.8 A˚3 and 4.35 D.
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SCHEME 1
group positionally disordered with 50% occupancies in sites related by a crystallographic center of inversion, which coincides with a molecular center of inversion only in the case of structure 1a (Scheme 1). With the bulky trityl groups determining the packing structures, an increase in the size of the polar substituent resulted in higher rotational barriers and rotational rates that are too small to measure for substituents with sizes larger than that of fluorine. Notably, the activation energies for rotation determined with a combination of variable-temperature (VT) 13C CPMAS NMR and quadrupolar echo 2H NMR line shape analysis showed that the barrier in the solvent-free samples increases by ca. 1 kcal/ mol (4.184 kJ mol-1), from 12.89,15 to 13.718 to 14.819 kcal/ mol (53.5, 57.3, and 61.9 kJ mol-1, respectively), as the rotator varies from phenylene (1a) to 2-fluorophenylene (1b) to 2,3-difluorophenylene (1f). In this paper, we report our analysis of nitrogen heteroaromatics 2, 3, 2O, 3O, and 3O2 as potential polar molecular rotators. Our motivation comes from the fact that pyridine and pyridazine rotators in 2 and 3 have greater dipole moments (1.81 and 3.28 D)17 than those of fluorobenzene and 2,3-difluorobenzene in 1a and 1f (1.49 and 2.37 D). An additional incentive comes from the fact that pyridine and pyridazine are smaller than phenylene by ca. 0.5% and 1.1%, respectively,17 and might display faster rotational motion if they adopt isomorphous packing structures. Furthermore, the nitrogen heterocycles in 2 and 3 may be transformed into the corresponding N-oxides 2O and 3O, and perhaps the N,N-dioxides 3O2, which have greater dipole moments with sizes that are very similar to those of the mono- and difluorinated 1b and 1f.17 In the sections that follow, we describe the synthesis and characterization of the two heterocyclic rotors. While the pyridine and pyridazine oxides could be prepared by oxidation with m-CPBA, only 3O was reasonably persistent. The pyridazine N,N-dioxide 3O2 could not be obtained under similar reaction conditions. We report the crystal structures of 2, 3, and 3O and a detailed solid-state 13C CPMAS NMR characterization of polycrystalline samples of 2 and 3. Having discovered that compound 3 lacks the chemical shift resolution needed to detect (18) Santillan, R.; Karlen, S. D.; Dang, H.; Garcia-Garibay, M. A. J. Mex. Chem. Soc. 2008, 52, 125. (19) Horansky, R. D; Clarke, L; Winston, E; Price, J. C; Karlen, S. D; Jarowski, P. D; Santillan, R; Garcia-Garibay, M. A. Phys. Rev. B 2006, 74, 054306.
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the pyridazine signals for dynamic studies using variabletemperature 13C NMR, we prepared a sample of 3-d30 with a fully deuterated trityl stator in order to remove all the interfering 13C signals in a CPMAS experiment. Unfortunately, 13C CPMAS NMR measurements with 3-d30 showed a broad pyridazine carbon signal with insufficient resolution or chemical shift dispersion to observe splitting indicative of site exchange. Similarly, while samples of polycrystalline 2 showed 13C CPMAS NMR spectra with line shape changes consistent with an exchange process, insufficient chemical shift dispersion was not conducive to useful rate determinations by line-shape analysis. Fortunately, samples of 3,4-d22,5-dibromopyridine became available by base-catalyzed 2H exchange and were used to prepare pyridine-labeled 2-d2, which revealed a barrier of ca. 8.5 kcal/mol (35.5 kJ mol-1) by analysis of their variable-temperature (VT) quadrupolar echo 2H NMR spectra. Remarkably, this barrier is about 4-6 kcal/mol (16.7-25.1 kJ mol-1) lower than those of the parent hydrocarbon 1a and its fluorinated derivatives 1b and 1f. Unfortunately, we were not successful in our attempts to prepare samples of deuterium-labeled pyridazine for analogous studies with compound 3. Results and Discussion Synthesis of Molecular Rotors. Compounds 2 and 3 were synthesized by Pd(0)-catalyzed cross coupling between the appropriate 3,3,3-triphenyl-1-propyne 4 and the corresponding disubstituted heteroaromatic rotator in a fashion analogous to that in a previously described methodology (Scheme 2). Samples of 3,3,3-triphenyl-1-propyne 4 were obtained in 95% yield by reaction of trityl chloride with 4 equiv of ethynylmagnesium bromide in dry benzene under reflux. Samples of 3,3,3-triphenyl-1-propyne-d15 (compound 4-d15, Scheme 3) were obtained by Friedel-Crafts reaction of benzene-d6 with CCl4 and AlCl3 affording chlorotris(phenyl-d5)methane as previously described.20 The halogenated product was treated under reflux with an excess of ethynylmagnesium bromide in benzene to give the desired product. Samples of 2O and 3O were obtained by m-chloroperbenzoic acid (m-CPBA) oxidation of 2 and 3. The pyridine-containing molecular rotor 2 was prepared in 80% yield through Sonogashira cross coupling between (20) Karlen, S. D.; Garcia-Garibay, M. A. Chem. Comm. 2005, 189.
Rodriguez-Molina et al. SCHEME 2
SCHEME 3
commercially available 2,5-dibromopyridine 5 (Scheme 2) and trityl acetylene 4. All of the Pd(0)-coupling reactions were carried out under Sonogashira conditions by reflux with Pd2Cl2(PPh3)2, CuI, and i-Pr2NH under N2 or Ar atmosphere in freshly distilled THF.21 Coupling products were often obtained with a small amount of hexaphenylbutadyine 8 (Scheme 4) as a side product,22 and purifications were carried out by column chromatography with a ternary mixture of hexanes, CH2Cl2, and EtOAc as the eluent using silica gel (for compounds 2, 2-d2, 3, and 3-d30) or alumina (21) (a) Chinchilla, R.; Najera, C. Chem. Rev. 2007, 107, 874 and references cited therein. (b) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 50, 4467. (22) (a) Li, D. F.; Yin, K.; Li, J.; Jia, X. S. Tetrahedron Lett. 2008, 49, 5918. (b) Batsanov, A. S.; Collings, J. C.; Fairlamb, I. J. S.; Holland, J. P.; Howard, J. A. K.; Lin, Z. Y.; Marder, T. B.; Parsons, A. C.; Ward, R. M.; Zhu, J. J. Org. Chem. 2005, 70, 703.
JOC Article (for N-oxides 2O and 3O). Samples of 2,5-dibromopyridined2 5-d2 for the synthesis of molecular compass 2-d2 were prepared by deuterium exchange of 5 with NaOD in D2O (Scheme 3b). The reaction proceeded in a reasonable yield of 65% with selective (>90%) deuterium exchange at positions C-3 and C-4 of the heteroaromatic ring, as determined by 1H and 13C NMR. Samples of 2,5-dibromopyridine and its deuterated analogue 5 and 5-d2 afforded products 7 and 7-d2 by C-C coupling at the carbon ortho to the nitrogen. This observation is in agreement with previous results, which have shown that analogous reactions can be accelerated by the electron withdrawing effects of the heteroatom.23 Molecular compasses 2 and 2-d2 were obtained in similar yields in two steps by separating and purifying the monocoupling product followed by a second coupling reaction or by adding an excess of the alkyne and increasing the reaction time. The pyridazine-containing structure 3 was obtained in 75% yield from 3,6-pyridazinediol ditriflate 6 (Scheme 3c). The latter was readily accessible from a stirred solution of commercial 3,6-pyridazinediol and triflic anhydride in dichloromethane at 0 °C. Compound 3-d30 was synthesized by reaction of 2.5 equiv of 3,3,3-triphenyl-1-propyne-d15 (4-d15)24 with 3,6-pyridazinediol ditriflate 6. Pure samples of molecular compasses 2 and 3 were treated with m-chloroperbenzoic acid to afford pyridine N-oxide rotor 2O, which rapidly decomposes in solution, and the more stable pyridazine N-oxide rotor 3O, respectively.25 Attempts to prepare pyridazine N,N-dioxide 3O2 from samples of purified 3O led to complex product mixtures, and the synthesis of this compound was not pursued further. Spectroscopic Characterization. While spectroscopic analysis by 1H and 13C NMR of molecular compasses with pyridine (2, 2-d2), pyridazine (3, 3-d30), and pyridazine N-oxide (3O) gave results that conformed to our expectations, we found that samples of pyridine N-oxide 2O decompose very rapidly in solution. High-resolution ESI-TOF mass spectra gave abundant parent ions for all compounds, and analysis by FT-IR showed relatively weak stretching bands for the disubstitued triple bonds. Single crystals suitable for X-ray diffraction analysis were obtained for the pyridine rotor 2 and the pyridazine rotor 3. The single crystal of the pyridazine N-oxide rotor 3O was of lower quality, but a reliable structure could be obtained. A summary of the 1H and 13C NMR data is included in Tables 1 and 2. The atom numbering used in these tables is different from that used in the crystal structures to reflect their different symmetries and magnetically different nuclei. While the presence of a nitrogen atom in compound 2 makes the two halves of the molecule different and carbons C1-C19 in Table 1 magnetically nonequivalent, the crystal structure shows the two sides to be crystallographically equivalent by an average inversion center that results from the disorder of the nitrogen atom with a 50% occupancy over the two inversion related sites (vide infra). Conversely, molecular rotor 3 has an average mirror plane with magnetically equivalent carbons C1-C9 in solution, but the two halves of the molecule are crystallographic nonequivalent in (23) (a) Tilley, J. W.; Zawoiski, S. J. Org. Chem. 1988, 53, 386. (b) Singh, R.; Just, G. J. Org. Chem. 1989, 54, 4453. (24) Karlen, S.D.; Garcia-Garibay, M.A. Chem. Commun. 2005, 189. (25) Caron, S.; Do, N. M.; Sieser, J. E. Tetrahedron Lett. 2000, 41, 2299.
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TABLE 1.
1
H NMR Signals from Central Rotators of Molecular Compasses 2, 3, and 3O in CDCl3
a Data determined with NMR spectrometer operating at 500 MHz. bData determined with NMR spectrometer operating at 300 MHz. cData determined with NMR spectrometer operating at 400 MHz.
the solid state (vide infra). The atom numbering for molecular compass 3O was kept in agreement with that of 3 in order to highlight their differences in chemical shifts. The 1H NMR signals from the central rings in compounds 2, 3, and their deuterated isotopologs, 2-d2, and 3-d30 are summarized in Table 1 along with those of 3O. In the case of pyridine rotator 2, the signal of H1 appears at δ 7.72 ppm as a doublet due to the 3J=8.2 Hz coupling with H10, which in turn appears at δ 7.39 ppm as a doublet of doublets with 3J=8.2 Hz and 4J= 2.0 Hz due to further coupling with H19. As expected from its neighboring relation to the nitrogen atom, the latter is the most deshielded signal with a chemical shift δ 8.70 ppm. After deuterium exchange in C1 and C10 of the pyridine ring, the chemical shift of the signal at H19 remains unaltered in rotor 2-d2, but the scalar coupling disappears from the spectrum. Weak residual peaks in 1H NMR (
