ORGANIC LETTERS
[1.1](3,3′)-Azobenzenophane: Novel Crystal Structure and Cis−Trans Isomerization of Distorted Azobenzene
2002 Vol. 4, No. 22 3907-3910
Yasuo Norikane, Kogorou Kitamoto, and Nobuyuki Tamaoki* Institute for Materials and Chemical Process, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1-1, Higashi, Tsukuba, Ibaraki, 305-8565, Japan
[email protected] Received August 21, 2002
ABSTRACT
[1.1](3,3′)-Azobenzenophane, in which two azobenzenes are cyclically connected by −CH2− chains at the meta positions, has been synthesized. The crystal structures of all isomers have been revealed. This is the first report on the crystal structure of the cis isomer of macrocyclic azobenzenes. The trans,trans isomer was slightly distorted, the trans,cis isomer highly deformed, and the cis,cis isomer unstrained. The thermal stability of cis isomers in solutions are deducible from the crystal structures.
Azobenzene is one of the typical photochromic compounds whose reversible cis-trans isomerization causes drastic changes in the structure and chemical properties. To switch the functions of materials by light, azobenzenes have been chemically introduced or dispersed into functional molecules and organized molecular systems such as host molecules,1 polymers,2 liquid crystals,3 and so on. In these systems, the cis-trans isomerization reactions are often affected by the steric environment such as free volume or macrocyclic structure.4,5 For example, in glassy polymeric matrixes, the thermal cis-trans or photochemical trans-cis isomerization
of azobenzene do not always follow simple first-order kinetics.4 For the azobenzenophanes, in which two azobenzenes are cyclically connected by relatively short chemical chains at the para positions, thermal isomerization in solvents from the trans,cis isomer to the trans,trans isomer is faster than that of the cis,cis isomer to the trans,cis isomer.5a-g These facts are explained by assuming that the strain in the cis form enhances the isomerization rate; however, the evidence for such a strained structure of cis-azobenzene moiety is limited.6 In addition, no X-ray crystal structures of cis isomers of any macrocyclic azobenzenes have been
(1) (a) Shinkai, S.; Minami, T.; Kasano, Y.; Manabe, O. J. Am. Chem. Soc. 1983, 105, 1851-1856. (b) Vo¨gtle, F. In Supramolecular Chemistry: An Introduction; Wiley & Sons: England, 1989; Chapter 7. (2) (a) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817-1845. (b) Hagen, R.; Bieringer, T. AdV. Mater. 2001, 13, 1805-1810. (c) Pieroni, O.; Fissi, A.; Angelini, N.; Lenci, F. Acc. Chem. Res. 2001, 34, 9-17. (3) (a) Ichimura, K. Chem. ReV. 2000, 100, 1847-1873. (b) Tamaoki, N. AdV. Mater. 2001, 13, 1135-1147. (4) (a) Yoshii, K.; Machida, S.; Horie, K. J. Polym. Sci. Part B: Polym. Phys. 2000, 38, 3098-3105. (b) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558-563. (c) Victor, J.; Torkelson, J. Macromolecules 1987, 20, 2241-2250.
(5) (a) Rau, H.; Lu¨ddecke, E. J. Am. Chem. Soc. 1982, 104, 1616-1620. (b) Tamaoki, N.; Koseki, K.; Yamaoka, T. Angew. Chem., Int. Ed. Engl. 1990, 29, 105-106. (c) Tamaoki, N.; Ogata, K.; Koseki, K.; Yamaoka, T. Tetrahedron 1990, 46, 5931-5942. (d) Tamaoki, N.; Yamaoka, T. J. Chem. Soc., Perkin. Trans. 2 1991, 873-878. (e) Tamaoki, N.; Yoshimura, S.; Yamaoka, T. Thin Solid Films 1992, 221, 132-139. (f) Rau, H.; Ro¨ttger, D. Mol. Cryst. Liq. Cryst. 1994, 246, 143-146. (g) Ro¨ttger, D.; Rau, H. J. Photochem. Photobiol. A: Chem. 1996, 101, 205-214. (h) Tauer, E.; Machinek, R. Liebigs Ann. 1996, 1213-1216. (i) Funke, U.; Gru¨tzmacher, H.-F. Tetrahedron 1987, 43, 3787-3795. (6) Bunce, N. J.; Ferguson, G.; Forber, C. L.; Stachnyk, G. J. J. Org. Chem. 1987, 52, 394-398.
10.1021/ol026760r CCC: $22.00 Published on Web 10/02/2002
© 2002 American Chemical Society
reported. We now report the first example of the X-ray structures of the trans,cis isomer, in which the cis form is deformed due to the ring strain, as well as the slightly deformed trans,trans isomer and the nondeformed cis,cis isomer of an azobenzenophane. The effect of the ring strain on the isomerization among these three isomers is discussed. trans,trans-[1.1](3,3′)-Azobenzenophane (2(t,t)) was synthesized by a method similar to that in the literature.5c A dilute solution of bis(3-nitrophenyl)methane7 (1) was added dropwise to a suspension of LiAlH4 in dry 1,4-dioxane under a nitrogen atmosphere at 100 °C (Scheme 1). The reaction
Scheme 1.
Synthesis of Azobenzenophanes
Figure 1. Chemical structure (left) and one of the two crystallographically independent molecules in 2(t,t) (right) with displacement ellipsoids shown at the 50% probability level. The other molecule in 2(t,t) is similar.
N8-C9 are 108.6 and 117.6°, respectively. The reported Nd N-C angle in trans-azobenzene is 114.1°.10 Upon exposure to 436 or 366 nm light, 2(t,t) isomerized to give the trans,cis isomer (2(t,c)) followed by the isomerization to the cis,cis isomer (2(c,c)) (Scheme 2). These two mixture contained macrocycles with two (2(t,t)), three, and four azobenzene units, larger rings, and polymers. Compound 2(t,t) was isolated8 by column cromatography (silica gel) and gel permeation chromatography (GPC) and recrystallized from a dichloromethane/hexane mixed solvent. Only the alltrans isomer was produced by the reaction. An X-ray crystal structure analysis revealed that there are two different conformers in a unit cell of 2(t,t), while there are four molecules in a unit cell (Z ) 4). One of the X-ray crystal structures of 2(t,t) is shown in Figure 1.9 The two conformers are similar to each other, and both form a conelike structure, in which benzene rings are tilted to the inside of the cavity. The torsion angles of the NdN double bonds are -177.9 and 168.3° for C1-N7-N8-C9 and C15N21-N22-C23, respectively. The planes of the phenyl rings are twisted around the NdN-C planes. The torsion angles of the NdN-CdC bonds are -173.4, -10, -164.3, and -16° for N8-N7-C1-C6, N7-N8-C9-C14, N22-N21C15-C20, and N21-N22-C23-C28, respectively. These values indicate that all the azobenzene units in 2(t,t) are not planar. The bond angles of NdN-C are bent toward the inside of the cavity; for example, N8-N7-C1 and N7(7) Gattermann, L.; Ru¨dt, H. Chem. Ber. 1894, 27, 2293-2297. (8) 1H NMR (270 MHz, CDCl3): δ ) 8.17 (s, 4H, ArH), 7.66 (d, J ) 7.2 Hz, 4H, ArH), 7.38 (t, J ) 7.2 Hz, 4H, ArH), 7.35 (d, J ) 7.2 Hz, 4H, ArH), 4.19 (s, 4H, CH2). 13C NMR (150 MHz, CDCl3): δ ) 153.56, 140.76, 130.99, 129.12, 125.90, 118.85, 40.26. ESI-MS calcd for C26H21N4 [M + H]+ ) 389.18, found 389.12. (9) Crystal data for 2(t,t): C26H20N4, M ) 388.46, monoclinic, P21, a ) 10.409(2), b ) 16.274(2), c ) 12.299(3) Å, β ) 101.256(9)°, V ) 2043.3(6) Å3, Z ) 4, Dcalcd ) 1.263 g/cm3, crystal size 0.27 × 0.17 × 0.07 mm. Diffraction data were collected at 296 K on an Enraf-Nonius CAD4 diffractometer with Cu KR radiation. The structure was solved by direct methods and refined by a full-matrix least-squares analysis. All calculations were performed using the teXsan crystallographic software package. R1 ) 0.081 and Rw ) 0.203 (based on F2). 3908
Scheme 2.
Photochemical and Thermal Isomerization of 2
isomers, 2(t,c) and 2(c,c), are stable enough for analyses and isolation by HPLC. The isomer ratios at the photostationary state after 436 nm irradiation in acetonitrile determined by HPLC were 45, 49, and 6% for 2(t,t), 2(t,c), and 2(c,c), respectively. Compound 2(t,c) was isolated by silica gel column chromatography from the solution at the 436 nm photostationary state. On the other hand, upon irradiation at 366 nm, the isomer ratios were 2, 11, and 87% in acetonitrile. Compound 2(c,c) was precipitated upon the 366 nm irradiation of a highly concentrated methanol solution of 2(t,t). Both 2(t,c) and 2(c,c) were recrystallized from a methanol/ dichloromethane mixed solvent. There are two different conformers in a unit cell of 2(t,c), while there are eight molecules in a unit cell (Z ) 8). The molecular configurations of the two conformers are almost mirror images of each other. One of the X-ray crystal structures of 2(t,c) is shown in Figure 2.11 In the transazobenzene unit, the dihedral angles of the C-NdN-C (C1-N7-N8-C9) bond is -172°, and the dihedral angles (10) Bouwstra, J. A.; Schouten, A.; Kroon, J. Acta Crystallogr. 1983, C39, 1121-1123. (11) Crystal data for 2(t,c): C26H20N4, M ) 388.46, orthorhombic, P212121, a ) 11.096(3), b ) 40.91(2), c ) 9.121(2) Å, V ) 4140(2) Å3, Z ) 8, Dcalcd ) 1.246 g/cm3, crystal size 0.30 × 0.07 × 0.03 mm. Diffraction data were collected at 243 K on a Rigaku AFC7R diffractometer with Mo KR radiation. The structure was solved by direct methods and refined by a full-matrix least-squares analysis. All calculations were performed using the teXsan crystallographic software package. R1 ) 0.044 and Rw ) 0.154 (based on F2). Org. Lett., Vol. 4, No. 22, 2002
Figure 2. Chemical structure (left) and one of the two crystallographically independent molecules in 2(t,c) (right) with displacement ellipsoids shown at the 50% probability level. The other molecule in 2(t,c) is almost a mirror image of the displayed molecule.
of the NdN-CdC bonds are 1 and 25° for N8-N7-C1C6 and N7-N8-C9-C10, respectively. The bond angles of NdN-C of the trans-azobenzene unit are bent toward the inside of the cavity; N8-N7-C1 and N7-N8-C9 are 118 and 109°, respectively. In the cis-azobenzene unit, the dihedral angle of the C-NdN-C bond was -1° (C23N22-N21-C15). The torsion angles of the NdN-CdC bonds are -51 and -94° for N22-N21-C15-C20 and N21-N22-C23-C24, respectively. It is notable that one benzene ring in the cis-azobenzene unit is perpendicular to the NdN-C plane. The NdN-C bond angles of the cisazobenzene unit are bent toward the outside of the cavity; N22-N21-C15 and N21-N22-C23 are 125 and 123°, respectively. The NdN-C bond angle in cis-azobenzene has been reported to be 121.9°.12 The X-ray structure of 2(c,c) is shown in Figure 3.13 There was only one conformer in the crystal. The molecule is highly symmetric and has D2 symmetry with three 2-fold rotation axes. The shapes of the two cis-azobenzene units are identical in a single molecule. The torsion angle of the C-NdN-C bond is 8.6°. The planes of the phenyl groups are rotated around the NdN-C planes; the dihedral angle of NdNCdC was 57.8°. The angle of the NdN-C bond was 121.6°. The crystal structure of cis-azobenzene unit is close to that of cis-azobenzene in the literature; the torsion angles of C-NdN-C and NdN-CdC are 8.0° and 53.3°, respectively.12 Thus, a cis-azobenzene unit in 2(c,c) is not as distorted as that in 2(t,c) in the crystal phase. The thermal cis-trans isomerization reactions proceeded stepwise from 2(c,c) to 2(t,c) and from 2(t,c) to 2(t,t). The lifetimes (1/k) of 2(c,c) and 2(t,c) at 298 K in acetonitrile (12) Mostad, A.; Rømming, C. Acta Chem. Scand. 1971, 25, 35613568. (13) Crystal data for 2(c,c): C26H20N4, M ) 388.46, orthorhombic, Fddd, a ) 17.009(2), b ) 27.519(3), c ) 8.500(2) Å, V ) 3978.5(10) Å3, Z ) 8, Dcalcd ) 1.297 g/cm3, crystal size 0.20 × 0.20 × 0.10 mm. Diffraction data were collected at 243 K on a Rigaku AFC7R diffractometer with Mo KR radiation. The structure was solved by direct methods and refined by a fullmatrix least-squares analysis. All calculations were performed using the teXsan crystallographic software package. R1 ) 0.038 and Rw ) 0.121 (based on F2). Org. Lett., Vol. 4, No. 22, 2002
Figure 3. Chemical structure (left) and crystal structure of 2(c,c) (right) with displacement ellipsoids shown at the 50% probability level. The molecule has D2 symmetry with three 2-fold rotation axes.
were 19.7 days (k1 ) 5.87 × 10-7 s-1) and 6.1 days (k2 ) 1.89 × 10-6 s-1), respectively. The lifetimes of both isomers were longer than that of cis-azobenzene (4.7 days).14 The thermodynamic parameters of each process were determined using the Eyring equation15 by measuring the first-order rate constants at different temperatures. In the isomerization process of 2(c,c) to 2(t,c), ∆H‡ ) 22.6 kcal/mol and ∆S‡ ) -11.4 cal K-1 mol-1 were obtained. On the other hand, in the isomerization process of 2(t,c) to 2(t,t), ∆H‡ ) 25.0 kcal/ mol and ∆S‡ ) -0.6 cal K-1 mol-1. Despite the fact that the ∆H‡ value of the former process is lower than that of the latter process, the lifetime of 2(c,c) (19.7 days) is about three times longer than that of 2(t,c) (6.1 days) at room temperature. Thus, the main factor that governs the isomerization rate is the ∆S‡ value, which reflects the difference in the degree of freedom between the ground and transition states. The relatively smaller value of ∆S‡ can be reasonably explained by assuming that the isomerization of 2(t,c) starts from a structure that has a smaller degree of freedom because of the strain by the cyclic structure. The relative stability of 2(c,c) and 2(t,c) is deducible from the X-ray crystal structures, where the cis-azobenzene unit in 2(t,c) has more ring strain than that of 2(c,c). Previous studies on p- and o-azobenzenophanes, consisting of two azobenzene units connected by two or three atomic chains, indicate that the rate of thermal cis-trans isomerization is affected by the chain length and the position where the chain is connected. When the azobenzene units are connected at the para positions, the trans,cis isomer becomes the most unstable and the lifetime decreases to seconds to minutes. The lifetime is 6.25 min when the linkers are -CH2-S-CH2-.5a It becomes even shorter (22 s) when the linkers are -CH2-CH2-.5b-e The benzene rings of the cisazobenzene unit in the trans,cis form seem to be forced toward the outside of the molecular cavity. In these compounds, the isolation of the trans,cis isomers was virtually impossible because of their thermal instability. On the other (14) Asano, T.; Okada, T.; Shinkai, S.; Shigematsu, K.; Kusano, Y.; Manabe, O. J. Am. Chem. Soc. 1981, 103, 5161-5165. (15) Eyring, H. Chem. ReV. 1935, 17, 65-77. 3909
hand, the lifetime of the trans,cis isomer becomes longer (∼6 weeks) when the azobenzene units are bridged by -CH2-CH2- chains at the ortho positions.5h It seems that the cis-azobenzene unit in the trans,cis isomer is stabilized by the force toward the inside of the cavity because the carbon chains on the cis-azobenzene unit are fixed close to each other by the connection with the trans-azobenzene unit on the ortho positions. As for the m-azobenzenophane, the ring strain on the trans,cis isomer is intermediate between that of the p- and o-azobenzenophanes. The lifetime of 2(t,c) (6.1 days) is between them, though its chain length is the shortest. Although the X-ray crystal structure analysis revealed that the benzene rings are forced toward the outside of the cavity, the ring strain is relatively small compared to the para-substituted azobenzenophanes. Thus, the stability of the trans,cis isomers depend strongly on the position of the linkers as well as the chain length. In cis,cis isomers, on the other hand, the lifetime is not as dependent on the structure of azobenzenophanes (2-20 days5a-h) as trans,cis isomers. The two azobenzene units in a single molecule have the same cis geometry. Steric distortion seems to be relatively small because the positions where the linkers are connected in each azobenzene unit are close to each other and the linkers can reach from one azobenzene unit to the other
3910
without causing substantial steric hindrance, regardless of the position or the length of the linkers. Herein, we have presented the crystal structures of all the isomers of a macrocyclic azobenzene dimer, which has the shortest linkers between the azobenzene units in reported azobenzenophanes. This is the first report of the crystal structure of the cis isomer in macrocyclic azobenzenes. The thermal cis-trans isomerization was affected by the distorted ring structure, which was revealed by the X-ray crystal analysis. Photochemical isomerization processes may also be affected by the ring strain. Our study is currently along this line, and evaluations on the host-guest properties of 2 are also underway. Acknowledgment. We thank Ms. Midori Goto for help with the X-ray crystallography. Y.N. thanks New Energy and Industrial Technology Development Organization (NEDO) for a fellowship. Supporting Information Available: Crystal data for 2(t,t), 2(t,c), and 2(c,c). This material is available free of charge via the Internet at http://pubs.acs.org. OL026760R
Org. Lett., Vol. 4, No. 22, 2002
