Exceptionally Large Difference in Bond Length among Conformational

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CRYSTAL GROWTH & DESIGN

Exceptionally Large Difference in Bond Length among Conformational Isomorphs of a Hexaphenylethane Derivative with a Dispiropyracene Skeleton†

2005 VOL. 5, NO. 6 2256-2260

Hidetoshi Kawai, Takashi Takeda, Kenshu Fujiwara, Tamotsu Inabe, and Takanori Suzuki* Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan Received February 22, 2005;

Revised Manuscript Received April 2, 2005

ABSTRACT: A large difference of 0.064(3) Å for a chemically equivalent Csp3-Csp3 bond was observed among crystallographically independent molecules with different conformations (conformational isomorphs) of a hexaphenylethane derivative, dispiro[(10-methylacridan)-9,1′-pyracene-2′,9′′-(10-methylacridan)] (1): triclinic, P1 h, a ) 17.699(3) Å, b ) 18.111(3) Å, c ) 18.844(3) Å, R ) 111.183(2)°, β ) 93.828(1)°, γ ) 102.130(2)°, V ) 5438.5(14) Å3, T ) -180 °C, Z ) 8 (four independent molecules: A, A′, B, and B′). While B and B′ adopt the skewed conformation to reduce steric repulsion in 1, in other positions of the crystal, molecules A and A′ adopt the eclipsed conformation, which induces a much longer bond length [1.771(3) and 1.758(3) Å versus 1.712(2) and 1.707(2) Å]. Introduction Covalent bond lengths, such as 1.54 Å for Csp3-Csp3, are basic parameters in chemistry. Compounds that show remarkable deviation from standard values have attracted the attention of chemists. Hexaphenylethane (HPE) derivatives1 are representative examples, in which the polyarylated central C-C bond is elongated by steric repulsion2,3 among the bulky aryl substituents. Since the bond energy decreases with an increase in bond length,4 the long bonds are prone to easily undergo bond fission under homolytic1 or mesolytic5 conditions. The latter point underlies our novel design for electrochromic systems6 based on the dynamic redox properties7 of HPE-type electron donors. During the course of our continuing studies using the spiroacridan skeleton,8 we have found that dispiro[(10-methylacridan)-9,1′pyracene-2′,9′′-(10-methylacridan)] (1) has a very long C1-C2 bond [1.771(3) Å]. Further examination of the X-ray structure showed that two of the four crystallographically independent molecules of 1 adopt a skewed conformation whereas the other two adopt an eclipsed conformation. The largest difference [0.064(3) Å] in bond length is seen for the central C-C bond among the conformational isomorphs. We describe here the detailed structural features of 1 and discuss the origin of this phenomenon based on a series of low-temperature X-ray structural analyses. Experimental Section Material. Dispiropyracene 1 was prepared by reductive cyclization of acenaphthene-5,6-diylbis(10-methylacridinium).8c Single-crystalline samples of 1 were obtained by recrystallization from AcOEt/n-hexane. Depending on the crystallization conditions, the solvated crystal of 13‚AcOEt2‚n-hexane1 was obtained. When THF or benzene was used as a good solvent instead of AcOEt, other solvated crystals (13‚solvent2‚n-hexane0.5)9 were obtained. † Dedicated to Prof. J. Michael McBride on the occasion of his 65th birthday. * Corresponding author: e-mail, [email protected].

Chart 1. Formulas for Dispirobis(10-methylacridan) Derivatives

X-ray Structural Analyses. The crystal data of 1 (P1 h, Z ) 8) at different temperatures (-40, -93, -113, -150, and -180 °C) are summarized in Table 1. All diffraction data were collected with a Rigaku CCD apparatus equipped with a liquidnitrogen flow system. The structures were solved by the direct method and refined by the full matrix least-squares method on F2 with anisotropic temperature factors for non-hydrogen atoms. Hydrogens were located at the calculated positions. The estimated standard deviations (esd’s) of bond lengths and angles are 0.002-0.005 Å and 0.1-0.3°, respectively, for the structure of 1 at -180 °C. Diffraction data measured at -40 °C were also analyzed by assuming a half-volume cell (V ) 2755 Å3; P1 h , Z ) 4), but similar analyses did not give satisfactory results for the data measured at lower temperatures. Details on the crystal structural analyses are presented in the supplemental CIF files (Supporting Information).

Results Since the initial report of very long bonds by X-ray analysis of 1,1,2,2-tetraphenylbenzocyclobutene3c or -naphthocyclobutene2 derivatives, bond lengths greater than 1.70 Å have been considered “normal” for conventional Csp3-Csp3 bonds,10a even though they are significantly longer than the standard (1.54 Å).10b On the basis of the careful examination of thermal ellipsoids for longbonded atoms or the measurement of diffraction data at low temperatures, they have been proven to be intrinsic by being distinguished from crystallographic artifacts.11 The nearly eclipsed arrangement of phenyl substituents (“front strain”1b) in these compounds is most responsible for the observed bond expansion and can also account for the difference between the central Csp3-Csp3 bond lengths of dispiroacenaphthene 2 [1.696-

10.1021/cg050064r CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005

Different Bond Lengths in Conformational Isomorphs

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Table 1. Unit Cell Parameters and Other Crystallographic Data of 1 (C40H30N2, M ) 538.69, Space Group P1 h) T (°C) a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm-3) µ (cm-1) unique reflns obs reflns (F2 > 2σF2) ref/param ratio R GOF a

-40a 9.438(2) 17.227(4) 17.804(4) 74.978(9) 85.936(13) 80.331(12) 2754.9(11) 4 1.299 0.075 11695 7238 8.85 0.0670 0.732

-40a 17.814(5) 18.219(6) 18.881(6) 111.085(5) 94.033(5) 102.044(3) 5519.9(28) 8 1.296 0.075 23426 8113 4.97 0.0710 0.569

-93 17.767(4) 18.158(5) 18.879(5) 111.220(5) 93.872(4) 101.834(2) 5489.5(24) 8 1.303 0.075 23329 13225 8.09 0.0580 0.588

-113 17.752(4) 18.146(5) 18.871(5) 111.246(5) 93.850(4) 101.827(2) 5478.4(23) 8 1.306 0.076 23287 13699 8.38 0.0560 0.587

-150 17.719(2) 18.116(2) 18.852(3) 111.270(2) 93.800(2) 102.006(2) 5449.0(12) 8 1.313 0.076 23197 14221 8.70 0.0590 0.576

-180 17.699(3) 18.111(3) 18.844(3) 111.183(2) 93.828(1) 102.130(2) 5438.5(14) 8 1.316 0.076 23025 16581 10.2 0.0520 0.721

The same image-data set was reindexed and integrated to give two sets of reflection data.

(3) Å]8b,12 and dispirodihydrophenanthrene 3 [1.635(2) Å]8a,12 (Chart 1). We envisaged that dispiropyracene 1 would exhibit a greater bond length than 2, since it is well-known that the ethano bridge over the peri positions (C1, C8) of naphthalene causes the wider separation of atoms attached at positions (C4, C5) opposite the bridged peri positions.13 The crystal structure of 1 was first analyzed by measuring the diffraction data with a CCD camera at -40 °C. The triclinic cell (V ) 2755 Å3) contains four molecules of 1 with a space group of P1 h , so that there are two crystallographically independent molecules (A and B) (Figure 1). Despite the rigid nature of the five-membered ring fused with the naphthalene unit, molecule B adopts a significantly skewed conformation with an inner torsion angle (θ1 ) ∠C8a-C1-C2-C2a) of 24.0(2)° to reduce the front strain between the spiroacridan units (Table 2, Chart 2). The other two torsion angles (θ2 ) ∠CAr-C1-C2-CAr) are slightly larger than θ1 [32.8(2) and 32.4(2)°]. The central C1-C2 bond length (d1) of 1.712(3) Å in B is slightly longer than that in 2, as expected. Interestingly, molecule A adopts quite a different conformation from that of B (conformational isomorphism14). The two spiroacridan units in A are nearly eclipsed [θ1 ) 3.3(2)°; θ2 ) 3.9(2) and 5.2(3)°]. Hence, it suffers from much more severe front strain, which must result in the exceptionally large value of d1 [1.772(3) Å]. The flanking bond lengths (d2 and d3) are in the range 1.500(4)-1.523(3) Å for A and 1.509(3)-1.531(3) Å for B, which are close to the standard value (1.51 Å for Csp3-CAr).10 Thus, the through-bond orbital interaction cannot account for the bond elongation observed here.11b Although the thermal ellipsoids for the long-bonded carbons in A are small and round, the fused benzene rings in one of the spiroacridan units exhibit large inplane displacements of anisotropic temperature factors. This observation may be related to the positional disorder of atoms, the averaging of superstructure, or the intramolecular motion.15 To confirm that the extremely long bond in A and the large difference (0.06 Å) in d1 between A and B are intrinsic for the structure of 1 in crystal, the same data set was reanalyzed by assuming a doubled unit cell, since a reciprocal view of the diffraction data suggested longer periodicity (superstructure) along the shortest axis (9.483 Å).17 The new cell (P1 h , Z ) 8; V ) 5520 Å3) contains four crystallographically independent molecules (A1, A2, B1,

Figure 1. ORTEP drawings of two crystallographically independent molecules of 1 obtained by analyzing diffraction data at -40 °C by assuming a half unit cell (Z ) 4): (a) molecule A with an eclipsed conformation [d2 ) 1.523(3) and 1.519(3) Å; d3 ) 1.507(3), 1.505(3), 1.504(3), and 1.500(4) Å, respectively]; (b) molecule B with a skewed conformation [d2 ) 1.531(3) and 1.524(3) Å; d3 ) 1.527(4), 1.518(3), 1.509(3), and 1.509(3) Å, respectively]. Note the difference in conformation between A and B as well as the large thermal ellipsoids in one of the two acridan units in A.

and B2). The latter two adopt a skewed geometry as in molecule B [θ1 ) 23.7(3)° and 24.0(3)° for B1 and B2, respectively], and the structural parameters, including the central bond length [d1 ) 1.708(5) Å and 1.710(5) Å, respectively], are nearly identical. On the other hand, the other two molecules derived from A showed significant differences: the values of d1 and θ1 are 1.781(6) Å and -0.1(3)° for A1, and 1.766(3) Å and 6.6(3)° for A2, respectively.18 Thus, the large in-plane displacement

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Table 2. Selected Structural Parameters of 1 Determined by X-ray Analysis molecule A (-40 °C)a molecule B (-40 °C)a molecule A1 (-40 °C)a molecule A2 (-40 °C)a molecule B1 (-40 °C)a molecule B2 (-40 °C)a molecule A1 (-93 °C) molecule A2 (-93 °C) molecule B1 (-93 °C) molecule B2 (-93 °C) molecule A1 (-113 °C) molecule A2 (-113 °C) molecule B1 (-113 °C) molecule B2 (-113 °C) molecule A1 (-150 °C) molecule A2 (-150 °C) molecule B1 (-150 °C) molecule B2 (-150 °C) molecule A1 (-180 °C) molecule A2 (-180 °C) molecule B1 (-180 °C) molecule B2 (-180 °C) a

d1 (Å)

θ1 (deg)

1.772(3) 1.712(3) 1.781(6) 1.766(6) 1.708(5) 1.710(5) 1.771(3) 1.761(3) 1.717(3) 1.709(3) 1.774(3) 1.760(3) 1.715(3) 1.710(3) 1.772(3) 1.758(3) 1.712(3) 1.709(3) 1.771(3) 1.758(3) 1.712(2) 1.707(2)

3.3(2) 24.0(2) -0.1(3) 6.6(3) 23.7(3) 24.0(3) -3.5(2) 8.9(2) 23.2(2) 24.5(2) -3.7(2) 9.3(2) 23.2(2) 24.7(2) -4.1(2) 9.7(2) 23.4(2) 24.7(2) -3.4(2) 9.4(2) 23.4(1) 24.7(1)

θ2 (deg) 3.9(2) 32.8(2) 0.0(3) 9.6(4) 33.1(5) 32.6(5) -3.7(2) 12.8(2) 32.4(2) 34.4(3) -4.1(2) 12.9(2) 32.3(3) 34.2(3) -3.9(2) 13.4(2) 32.0(3) 34.1(3) -3.7(2) 12.9(2) 32.0(2) 34.3(2)

5.2(3) 32.4(2) -2.1(4) 10.9(4) 33.8(4) 31.0(4) -6.3(2) 13.9(2) 31.3(2) 32.7(2) -6.3(2) 14.3(2) 31.3(2) 32.7(2) -6.8(2) 14.3(2) 31.1(2) 32.7(2) -6.2(2) 14.5(2) 31.5(2) 32.5(2)

Derived from the same image-data set measured at -40 °C.

Chart 2.

Designation of Selected Bond Lengths and Torsion Angles in 1

observed in molecule A in the smaller cell was mainly caused by averaging crystallographically independent molecules of A1 and A2. Again, molecule A1 with the more eclipsed conformation has the larger value of d1. A smaller but still noticeable ellipsoidal deformation remains in molecules A1 and A2, which may be due to thermal in-plane motion (Figure 2a). Thus, diffraction data were collected at -93, -113, -150, and -180 °C. Several specimens were used in obtaining diffraction data to confirm the sample independence. The reciprocal views clearly indicate the presence of longer periodicity, and the unit cell parameters were essentially identical to those with a doubled unit cell measured at -40 °C (Table 1). Marginally smaller values (by less than 2%) for a, b, c, and V may be due to the suppression of thermal vibration at lower temperatures. The structural parameters for molecules B1 and B2 did not show any important temperature dependence. As shown in Figure 2b, the anisotropic temperature factors in the spiroacridan units in A1 and A2 become smaller and are comparable with those in other atoms at -180 °C without changing the extremely long bond lengths (d1). The data set measured at -180 °C gave the geometrical parameters with the smallest esd’s, which are considered the final structural data of 1 (Table 2). The longest bond [d1 ) 1.771(3) Å] was found in A1 with the most eclipsed conformation [θ1 ) -3.4(2)°] while the most skewed conformer B1 [θ1 ) 24.7(1)°] exhibited the smallest value of d1 [1.707(2) Å] among the conformational isomorphs. The scattering plots of d1 against θ1 or θ2 obtained using all of the data show a linear correlation with a negative slope (Figure 3), indicating that the front strain between the two spiroacridan units

Figure 2. ORTEP drawings of the molecular arrangement in the crystal (Z ) 8) showing four crystallographically independent molecules: (a) at -40 °C; (b) at -180 °C.

Figure 3. Scattering plots of d1 vs θ1 (filled circles) and d1 vs θ2 (open circles) for conformational isomorphs of 1 obtained by X-ray analysis at several temperatures.

is an important factor that accounts for expansion of the central C-C bond.9b Discussion There are three cases of notable differences in bond length among crystallographically independent mol-

Different Bond Lengths in Conformational Isomorphs

ecules: (1) crystals containing differently charged molecules;19 (2) crystals containing tautomers;20 and (3) crystals containing more than two conformers with different structural requirements.21 The first case includes many cation-radical salts of tetrathiafulvalene (TTF) derivatives studied in the field of organic electroconducting materials.22 Most of these salts contain more than two molecules of TTF derivative per closedshell counteranion in the crystal, so that the positive charge is equally or unequally shared by TTFs. Since the π-bond order of each bond in the TTF skeleton is determined by the amount of positive charge,23 the crystallographically independent molecules in these salts often exhibit considerable bond length differences, as large as 0.050(7) Å,19a due to the unequal charge distribution. N-Salicylideneaniline derivatives20a are representative examples of the second case. The geometries of the two tautomers are so close to each other that both can occupy the same site in a crystal. Even when crystallographically independent sites are occupied by a pair of tautomers in different ratios, the observed differences in bond length between these sites are much smaller than those in the first case. The third case is related to conformational isomorphism,14 which has rarely been reported24 probably because different conformers usually crystallize as polymorphs.25 In general, conformational isomorphs differ considerably in torsion angles but not in bond lengths because modification of bond lengths requires much more energy than modification of torsion angles, which cannot be compensated by the crystal packing force. Hence, only a few cases21 have been reported, in which the bond length differs among conformational isomorphs. The crystal of 1 studied here belongs to the third category, and the difference of 0.064(3) Å is the largest ever reported for a chemically equivalent Csp3-Csp3 bond length even though the crystallographically independent molecules of 1 do not differ in the charge they possess (case 1) or in atom connectivity, as in tautomers (case 2). On the basis of low-temperature measurements and by following the gradual changes in parameters depending on the temperature, there is no doubt that the present HPE 1 can adopt the eclipsed or skewed comformation9b,26 in a crystal. An outstanding feature is the extremely long bond length [d1 ) 1.771(3) Å] found in the eclipsed conformer, which is surely due to steric repulsion among the aryl substituents attached to this bond. As can be seen by the negative correlation between d1 and θ1 (Figure 3), HPE 1 can relieve the front strain by expansion of the central C-C bond (large d1) and/or by skewing the five-membered ring fused to the naphthalene nucleus (large θ1). The former structural perturbation seems to cost too much energy in ordinary molecules. However, as predicted previously,27 the prestrained bonds are more susceptible to external effects for geometrical deformation. This should be the case for HPE 1, and the elongated bond length can be further expanded by the small energy that can be compensated by the crystal packing force. Conclusion We have described here a case in which chemically equivalent Csp3-Csp3 bonds in crystallographically in-

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dependent molecules have significantly different lengths. Conformational differences can account for this phenomenon. The huge steric hindrance across this bond in 1 has to be relieved either by expansion of the bond or by skewing the hexaphenylethane moiety.28 The former is generally an energy-consuming process, yet the polycyclic structure of 1 also makes the latter process difficult. Hence, there is only a small difference in energy between the eclipsed conformer with an extremely long C-C bond and the skewed conformer with a not-so-long C-C bond.26 On the other hand, as suggested by facile solvate formation, it is difficult for this HPE 1 to show proper packing due to the threedimensionally expanded structure. This situation may make it possible for the unsolvated crystal of 1 to accommodate different conformers in the same crystal. The present case clearly shows that once the Csp3Csp3 bonds are expanded beyond 1.7 Å, they can be further expanded much more easily. Further studies on novel reactivities and intriguing phenomena concerning this “soft” Csp3-Csp3 bond are now underway. Acknowledgment. This work was supported by grants from the Ministry of Education, Science, and Culture, Japan (Nos. 16750024, 15350019, and 14654139). H.K. thanks Sekisui Chemical Co., Ltd., for financial support. Supporting Information Available: Crystallographic information files (CIF) of 1 analyzed using diffraction data measured at various temperatures (-180, -150, -113, -93, and -40 °C). ORTEP drawings (PDF) of crystallographically independent molecules (A1, A2, B1, and B2) obtained by analyzing data measured at -180 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) (a) McBride, J. M. Tetrahedron 1974, 30, 2009. (b) Hounshell, W. D.; Dougherty, D. A.; Hummel, J. P.; Mislow, K. J. Am. Chem. Soc. 1977, 99, 1916. (c) Stein, M.; Winter, W.; Rieker, A. Angew. Chem., Int. Ed. Engl. 1978, 17, 692. (d) Kahr, B.; Engen, D. V.; Mislow, K. J. J. Am. Chem. Soc. 1986, 108, 8306. (e) Yannoni, N,; Kahr, B.; Mislow, K. J. J. Am. Chem. Soc. 1988, 110, 6670. (2) (a) Toda, F.; Tanaka, K.; Stein, Z.; Goldberg, I. Acta Crystallogr., C 1996 52, 177. (b) Baldridge, K. K.; Kasahara, Y.; Ogawa, K.; Siegel, J. S.; Tanaka, K.; Toda, F. J. Am. Chem. Soc. 1998, 120, 6167. (c) Toda, F.; Tanaka, K.; Watanabe, M.; Tamura, K.; Miyahara, I.; Nakai, T.; Hirotsu, K. J. Org. Chem. 1999, 64, 3102. (d) Toda, F. Eur. J. Org. Chem. 2000, 1377. (e) Tanaka, K.; Takamoto, N.; Tezuka, Y.; Kato, M.; Toda, F. Tetrahedron 2001, 57, 3761. (3) (a) Baldridge, K. K.; Battersby, T. R.; VernonClark, R.; Siegel, J. S. J. Am. Chem. Soc. 1997, 119, 7048. (b) O Ä sawa, S.; Sakai, M.; O Ä sawa, E. J. Phys. Chem. A 1997, 101, 1378. (c) Kammermeier, S.; Jones, P. G.; Herges, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 1757. (d) Kaupp, G.; Boy, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 48. (e) Suzuki, T.; Ono, K.; Nishida, J.; Takahashi, H.; Tsuji, T. J. Org. Chem. 2000, 65, 4944. (f) Suzuki, T.; Ono, K.; Kawai, H.; Tsuji, T. J. Chem. Soc., Perkin Trans. 2 2001, 1798. (4) Zavitsas, A. A. J. Phys. Chem. A 2003, 107, 897. (5) Maslak, P.; Narvaez, J. N.; Vallombroso, T. M., Jr. J. Am. Chem. Soc. 1995, 117, 12373. (6) (a) Suzuki, T.; Nishida, J.; Tsuji, T. Angew. Chem., Int. Ed. Engl. 1997, 36, 1329. (b) Nishida, J.; Suzuki, T.; Ohkita, M.; Tsuji, T. Angew. Chem., Int. Ed. 2001, 40, 3251. (c) Suzuki, T.; Yamamoto, R.; Higuchi, H.; Hirota, E.; Ohkita, M.; Suzuki, T. J. Chem. Soc., Perkin Trans. 2 2002, 1937. (d) Suzuki, T.; Tanaka, S.; Higuchi, H.; Kawai, H.; Fujiwara, K. Tetrahedron Lett. 2004, 45, 8563.

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(7) Suzuki, T.; Higuchi, H.; Tsuji, T.; Nishida, J.; Yamashita, Y.; Miyashi, T. Chemistry of Nanomolecular Systems. In Dynamic Redox Systems; Nakamura, T., Matsumoto, T., Tada, H., Sugiura, K., Eds.; Springer-Verlag: Heidelberg, 2003; Chapter 1, p 3. (8) (a) Suzuki, T.; Migita, A.; Higuchi, H.; Kawai, H.; Fujiwara, K.; Tsuji, T. Tetrahedron Lett. 2003, 44, 6837. (b) Kawai, H.; Takeda, T.; Fujiwara, K.; Suzuki, T. Tetrahedron Lett. 2004, 45, 8289. (c) Preparation, structure, and electrochromic behavior of 1, 2, and related compounds will be reported elsewhere: Kawai, H.; Takeda, T.; Fujiwara, K.; Suzuki, T. Manuscript in preparation. (9) (a) In the case of the solvated crystals, the positional disorder of the solvent molecules prevents us from obtaining precise structural parameters (esd’s of bond lengths ca. 0.01 Å), so that the structural data (CCDC 267567-267569) were not included in this paper. (b) The molecules of 1 in the solvated crystals adopt conformations intermediate between those of molecules A and B in the unsolvated crystals. Therefore, the values of d1 beyond 1.70 Å as well as the skewed geometry of 1 are not caused by a special crystal packing force in the unsolvated crystal of 1 but rather result from the intrinsic nature of 1 itself. (10) (a) Chemical bonds beyond 2.9 Å were indicated to exist in the dimeric form of TCNE•-, which is in a new category of C-C bond: Novoa, J. J.; Lafuente, P.; Del Sesto, R. E.; Miller, J. S. Angew. Chem., Int. Ed. 2001, 40, 2540. (b) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1. (11) (a) Harada, J.; Ogawa, K.; Tomoda, S. Chem. Lett. 1995, 751. (b) Battersby, T. R.; Gantzel, P.; Baldridge, K. K.; Siegel, J. S. Tetrahedron Lett. 1995, 36, 845. (12) (a) The torsion angle around the central bond in 2 [∠C8aC1-C2-C2a ) 18.1(3)°] is much smaller than that in 3 [∠C9a-C9-C10-C10a ) 47.1(1)°], so that the front strain in the former is much larger than that in the latter. (b) 1,1,2,2Tetraarylacenaphthenes with a framework similar to 2 have also been shown to have a long C1-C2 bond of 1.701(3), 1.670(3), or 1.633(3) Å: Wang, H.; Gabbaı¨, F. P. Angew. Chem., Int. Ed. 2004, 43, 184. Wang, H.; Gabbaı¨, F. P. Org. Lett. 2005, 7, 283. (13) (a) Balasubramaniyan, V. Chem. Rev. 1966, 66, 567. (b) Clough, R. L.; Kung, W. J.; Marsh, R. E.; Roberts, J. D. J. Org. Chem. 1976, 41, 3603. (14) (a) Bernstein, J. Organic Solid State Chemistry; Elesevier: Amsterdam, 1987; pp 471-518. (b) Bilton, C.; Howard, J. A. K.; Madhavi, N. N. L.; Nangia, A.; Desiraju, G. R.; Allen, F. H.; Wilson, C. Chem. Commun. 1999, 1675. (c) Kumar, V. S. S.; Addlagatta, A.; Nanigia, A.; Robinson, W. T.; Broder, C. K.; Mondel, R.; Evans, I. R.; Howard, J. A. K.; Allen, F. H. Angew. Chem., Int. Ed. 2002, 41, 3848. (d) Zhang, Z.-Q.; Uth, S.; Sandma, D. J.; Foxma, B. M. J. Phys. Org. Chem. 2004, 17, 769. (15) It is unlikely that the thermal motion in the crystal is related to the observed very long bond length in molecule A. As discussed in detail in ref 16, the observed bond should have been shorter than the real length when in-plane motion causes anomalies. (16) (a) Ogawa, K.; Sano, T.; Yoshimura, S.; Takeuchi, Y.; Toriumi, K. J. Am. Chem. Soc. 1992, 114, 1041. (b) Harada, J.; Ogawa, K.; Tomoda, S. J. Am. Chem. Soc. 1995, 117, 4476. (17) (a) According to the IUCr recommendation for a three-axis definition with a < b < c in the triclinic unit cell, the doubled a axis in the smaller cell becomes the new c axis in the

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(19)

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(21) (22) (23)

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(26) (27) (28)

(29)

larger unit cell. (b) Diffraction data sets measured above 0 °C did not show the presence of superstructure, and the structure can be solved only with a triclinic cell of Z ) 4 (two independent molecules). The observed temperature dependence is reversible and may be related to the orderdisorder transition. The minus sign for the torsion angle in molecule A1 indicates that the direction of skewing is opposite to that in molecule A2. The difference in skewing angles is more noticeable at a lower temperature [θ1 ) -3.4(2)° for A1 and 9.4(2)° for A2 at -180 °C, respectively]. (a) Mori, T.; Inokuchi, H. Bull. Chem. Soc. Jpn. 1988, 61, 591. (b) Guionneau, P.; Kepert, C. J.; Rosseinsky, M.; Chasseau, D.; Gaultier, J.; Kurmoo, M.; Hursthouse, M. B.; Day, P. J. Mater. Chem. 1998, 8, 367. (c) Turner, S. S.; Day, P.; Gelbrich, T.; Hursthouse, M. B. J. Solid State Chem. 2001, 159, 385. (a) Ogawa, K.; Kasahara, Y.; Ohtani, Y.; Harada, J. J. Am. Chem. Soc. 1998, 120, 7107. (b) Nazir, H.; Yildiz, M.; Yilmaz, H.; Tahir, M. N.; U ¨ lku¨, D. J. Mol. Struct. 2000, 524, 241. (c) Kurahashi, M. Bull. Chem. Soc. Jpn. 1976, 49, 2927. Li, P.-C.; Wang, T.-S.; Lee, G.-H.; Liu, Y.-H.; Wang, Y.; Chen, C.-T.; Chao, I. J. Org. Chem. 2002, 67, 8002. Yamada, J.; Sugimoto, T. TTF Chemistry; KodanshaSpringer: Tokyo, 2004. (a) Kobayashi, H.; Nakayama, J. Bull. Chem. Soc. Jpn. 1981, 54, 2408. (b) Emge, T. J.; Wiygul, F. M.; Chappell, J. S.; Bloch, A. N.; Ferraris, J. P.; Cowan, D. O.; Kistenmacher, T. J. Mol. Cryst. Liq. Cryst. 1982, 87, 137. (c) Guionneau, P.; Kepert, C. J.; Chasseau, D.; Truter, M. R.; Day, P. Synth. Met. 1997, 86, 1973. (a) Montiel-Ortega, L. A.; Rojas-Lima, S.; Otazo-Sanches, E.; Villagome-Ibarra, R. J. Chem. Crystallogr. 2004, 34, 89. (b) Bhattacharya, S.; Dastidar, P.; Guru Row, T. N. Chem. Mater. 1994, 6, 531. (c) Hambley, T. W.; Parthasarathi, V.; Rickards, R. W.; Robertson, G. B. Aust. J. Chem. 1991, 44, 655. (d) Weichsel, A.; Lis, T. Carbohydr. Res. 1989, 194, 63. (e) Cullen, W. R.; Rettig, S. J.; Trotter, J.; Wickenheiser, E. B. Can. J. Chem. 1988, 66, 2007. (f) Lindeman, S. V.; Timofeeva, T. V.; Shklover, V. E.; Struchkov, Y. T.; Turuta, A. M.; Kamernitskii, A. V. Steroids 1984, 43, 125. (g) Del Buttero, P.; Maiorana, S.; Andreetti, G. D.; Bocelli, G.; Sgarabotto, P. J. Chem. Soc., Perkin Trans. 2 1975, 809. (a) Dunitz, J. D.; Bernstein, J. Acc. Chem. Res. 1995, 28, 193. (b) Hagler, A. T.; Bernstein, J. J. Am. Chem. Soc. 1978, 100, 6349. According to ab initio calculations, the energy difference between the two conformers < 1 kcal mol-1 (ref 8c). O Ä sawa, E.; Ivanov, P. M.; Jaime, C. J. Org. Chem. 1983, 48, 3990. Another solution to overcome the steric congestion would be bond dissociation. For example, the parent 1,1,2,2tetraphenylbenzocyclobutene transforms into the closedshell 7,7,8,8-tetraphenyl-o-quinodimethane very easily by bond fission (ref 29). However, bond dissociation in 1 would produce the acenaphthene-5,6-diyl derivative, which cannot adopt the closed-shell structure. This may prevent this bond from homolytic rupture. Quinkert, G.; Wiersdorff, W.-W.; Finke, M.; Opitz, K.; von der Haar, F.-G. Chem. Ber. 1968, 101, 2302.

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