1358
Chem. Mater. 1993,5, 13591361
Decacyclene: A Molecular Propeller with Helical Crystals Douglas M. Ho and Robert A. Pascal, Jr.' Department of Chemistry, Princeton Uniuersity, Princeton, New Jersey 08544 Receiued May 19,1993. Reuised Manuscript Received July 6,I9930 The molecular structure of decacyclene was determined by X-ray analysis of a single crystal grown by sublimation. The molecule is twisted into a shallow molecular propeller due t o nonbonded repulsions between hydrogens of the peripheral naphthalene groups. Because decacyclene crystallizes in the chiral space group C2221with Z = 4, the molecule lies on a special position and all of the propellers have the same relative configuration. Interestingly, crystals of decacyclene also have a helical morphology which is most pronounced when grown from solutions in organic solvents. Powder X-ray diffraction studies on the solution-grown crystals (which are too small for single-crystal work) show them to have the same unit cell, space group, and chiral crystal structure. I t is likely, though not proven, that left- and right-handed helical crystals correspond to opposite enantiomers of the decacyclene propeller.
Introduction Decacyclene (1) is a large, commercially available, polycyclic aromatic hydrocarbon possessing 3-fold symmetry. First prepared near the turn of the century,' in recent years its radical cation salts have been featured in studies directed toward the preparation of organic ferr o m a g n e t ~ . However, ~,~ despite its long history and ready availability, the molecular structure of decacyclene has not been reported. Most investigators appear to regard 1 as a typically planar polycyclic aromatic hydrocarbon, but semiempirical molecular orbital calculations (AM1') suggest that 1 is a gently twisted molecular propeller due to nonbonded interactions between the hydrogens of the peripheral naphthalene groups. Prompted by a longstanding interest in helical polycyclic aromatic hydrocarbons,= we sought to determine the structure of decacyclene by single-crystal X-ray diffraction. In this paper we report not only the crystal and molecular structure of 1, which does possess a helical conformation, but also the helical morphology of its crystals, which together represent a remarkable parallel expression of helicity at the microscopic and macroscopic levels.
Figure 1. Optical micrograph of crystals of decacyclene grown from a solution in 1:l benzene-pinacolone. The image area is 0.78 mm X 0.78 mm. The crystals are yellow. but a yellowinsensitive film was used to prepare the copy negative for this photographinordertaenhance thecontrast between thecrystals and the light background.
Results and Discussion
Our initial attempts to grow large single crystals of decacyclene were attended by a serendipitous discovery: Abstract published in Advance ACS Abstracts. September 1.1993. (1) Dziewonski, K. Chem. Ber. 1903.36, 962-971. (2) Torrance.J. 9.;Bague, P. S.;Johannnen, 1.; Nazzal. A. I.; Parkin. S.S. P.; Batail, P. J. Appl. Phys. 1988.63, 2962-2965. (3) Sugano. T.;Kinoshita, M.Bull. Chem. Soe. Jpn. 1989.62.22732278.
the concentration or cooling of solutions of 1 in a variety of organic solvents deposits crystals of an unusual helical morphology. Thesecrystalsare long, thin, twistedribbons, and the pitch of the helix varies with crystallization conditions. For example, crystals formed upon the slow cooling of solutions of 1 in hot nitrobenzene are long (up to 1 cm), thin (0.05 mm X 0.01 mm) needles with a very gradual twist. At theother extremeare the twisted ribbons grown by the slow evaporation of solutions of 1 in 1:l henzenepinacolone (see Figure 1) which are approxi(4) Dewar, M.J. S.;Zoebiaeh, E.G.; Healy. E. F.;Stewart. J. J. P. J. Am. Chem. Soc. 1985,107.3902-3909. (5) Paseal, R.A., Jr.; McMillan. W.D.; Van Engen. D.; Eason. R. G. J . Am. Chem. Soe. 1987.109.466C-4665. (6) Smyth, N.; Van Engen, D.; Pascal, R.A,, Jr. J . Org. Chem. 1990, 55. 1937-1940. (7) Pascal, R.A,. Jr. Pure Appl. Chem. 1993.65. 105-110. ( 8 ) Meurer, K. P.; Vogtle, F. Top. Cum. Chem. 1985, 127, 1-76.
0897-4756/93/2805-1358$04.00/0 0 1993 American Chemical Society
Chem. Mater., Vol. 5, No. 9, 1993 1359
Decacyclene: A Molecular Propeller
Table I. Experimental Data for the Single-Crystal X-ray Analysis of Decacyclene mol formula mol w t crystal system space group
C d i 8
450.5 orthorhombic C2221 (NO.20) 3.944 (2) 25.430 (8) 20.795 (8) 2085.8 (13) 4 1.435 0.10 X 0.10 X 0.30 Mo Ka (X = 0.710 73 A)
alA b/A CIA VIA3
z
d d d g cm4 crystal sizelmm radiation
mately 1mm long, 0.03 mm in width, and no more than 0.002 mm thick, and which typically undergo a 360' twist for every 0.3 mm of length. Intermediate pitches are observed for crystals grown from solutions in other aromatic solvents; however, the crystals themselves do not contain molecules of the solvent. Crystals with a helical morphology are rare but not unknown. At least as early as 1888, Lehmann described several helical inorganic and organic crystals? and more recent (and astonishing) examples are the helical whiskers of copper reported byBrenner.lo However, we are unaware of any report of helical crystals of decacyclene,even though they must have formed in other laboratories. It is probable that this feature has been missed during casual inspections of decacyclene crystals produced when purifying the compound by recrystallization; only under a microscope is the unusual morphology obvious. In any event, the dimensions of our solution-grown crystals were inadequate for an X-ray structure determination, but the sublimation of 1 (see the Experimental Section) yielded long orange needles of the necessary thickness. These crystals had a gradual curvature, and the vast majority were layered and cracked, but a satisfactory specimen was obtained for diffraction experiments. The structure of decacyclene was successfully solved and refined in the orthorhombic space group C2221 with 2 = 4. Notably, the unit-cell dimensions are rather unequal (see Table I), and the crystals are elongated along the very short a axis. Each molecule resides on a special position and possesses crystallographic 2-fold symmetry (i.e., the idealized molecular D3 symmetry is lowered by the alignment of one of the three molecular COaxes with a crystallographic 2-fold axis parallel to b in the solid state). The molecular structure of compound 1 is illustrated in Figure 2, and as predicted by the AM1 calculations, the molecule is propeller shaped. The overall twist is modest: the angles between the mean plane of the central benzene ring and the mean planes of the two crystallographically independent naphthalene moieties are 9.3" and 7.7', respectively. The nonbonded contacts H(2)-H(2') and H(8)-H(15), which are responsible for the distortion from planarity, are 1.921 and 1.953 A, respectively, in the structural model, but the actual contacts are somewhat shorter, since the crystallographic refinement employed a riding model with C-H distances of 0.96 A. There is substantial bond alternation in the central benzene ring; the three bonds fused to the five-membered rings ("endo" bonds) average 1.444 A in length, while the other three
28 rangeldeg temp1K p(Mo Ka)/mm-l
4.0-50.0 296 0.082 936 2266 1861 1083 [F> 2a(F)1 164 0.0747 0.0686 1.15
F(000) reflections collected independent reflections observed reflections parameters refined
R
WR
S
Table 11. Atomic Coordinates (XlOr) and Equivalent Isotropic Displacement Coefficients (A* X 1P)for Decacyclene. Y
X
2
U(W)
1521(21) -1544(2) 6740(3) 44(2) C(2) 861(22) -2077(2) 6703(3) 58(3) C(3) 1987(25) -2358(2) 6156(3) 61(3) C(4) 3699(24) -2130(3) 5670(3) 65(3) 4410(26) -1583(3) 5680(3) 57(3) C(6) 6095(24) -1289(3) 5212(3) 65(3) - 756(3) 5288(3) 63(3) C(7) 6496(22) C(8) 5232(22) - 478(3) 5825(3) 57(3) - 753(2) 6288(2) 44(3) C(9) 3370(21) C(10) 3150(23) -1308(2) 6213(3) 50(3) C(11) 724(18) -1116(2) 7203(2) 39(2) C(12) 1634(22) - 627(2) 6891(2) 43(2) - 153(2) 7186(2) 45(3) C(13) 760(20) C(14) 1097(22) 397(2) 6976(3) 47(3) C(15) 1899(22) 660(2) 6410(3) 55(3) C(16) 1875(23) 1213(2) 6401(3) 65(3) C(17) 1019(26) 1511(2) 6923(3) 65(3) C(18) 0 1259(3) 7500 5W) C(19) 0 705(3) 7500 53(4) a Equivalent isotropic U defined as one-third of the trace of the orthogonalized Uij tensor. Cl17)
I
Figure 2. X-ray structure of decacyclene. The thermalellipsoids are drawn at the 50 % probability level, and hydrogen atoms are drawn as spheres of arbitrary size.
(Yexon)average 1.384 A. This degree of alternation is significantly greater than that observed in the related compound tricyclopentabenzene (endo, 1.395A; exo 1.378 A).11J2 In addition, the central ring is distorted into a shallow chair conformation, but with no atom deviating from the mean plane of the ring by more than 0.02 A. Most interesting, however, is the fact that C2221 is a chiral space group, and all of the decacyclene molecules in the unit cell have the same relative c~nfiguration.~s Since crystallization of decacyclene from solution yields both right- and left-handed helical crystals, it is tantalizing
~~
(9) Lehmann, 0. Molekularphysik; Wilhelm Engelmann: Leipzig, 1888; pp 374-371. (10) Brenner, S. S. "Metals" in The Art and Science of Growing Crystals; Gilman, J. J., Ed.; Wiley: New York, 1963; p 47.
(11) Boyko, E. R.; Vaughan, P. A. Acta Crystallogr. 1964,17,152-158. (12) For a review and discussion of bond alternation in triannelatad benzenes, see: Baldridge, K. K.; Siegel,J. S. J. Am. Chem. SOC.1992,114, 9583-9587.
1360 Chem. Mater., Vol. 5, No. 9,1993 Table 111. Bond Lengths (A) and Angles (deg) for Decacvclene C(l)-C(lO) 1.406 (9) C(l)-C(2) 1.382 (8) C(2)-C(3) 1.415 (9) C(l)-C(ll) 1.486 (7) C(4)-C(5) 1.418 (9) C(3)-C(4) 1.348 (10) C(5)-C(10) 1.401 (9) C(5)-C(6) 1.396 (10) C(7)-C(8) 1.411 (9) C(6)-C(7) 1.376 (10) C(9)-C(lO) 1.424 (8) C(8)-C(9) 1.397 (9) C(ll)-C(12) 1.449 (7) C(9)-C(12) 1.464 (8) C(12)-C(13) 1.395 (7) C(ll)-C(ll’) 1.361 (10) C(13)-C(13’) 1.435 (10) C(13)-C(14) 1.472 (7) C(14)-C(19) 1.411 (7) C(14)-C(15) 1.390 (8) C(16)-C(17) 1.368 (9) C(15)-C(16) 1.405 (8) C(18)-C(19) 1.408 (11) C(17)-C(18) 1.418 (7) symmetry code: ’ (-Z, y, 1.5 - Z ) 135.6(6) C(P)-C(l)-C(lO) 117.5(5) C(2)-C(l)-C(ll) 106.8(5) C(l)-C(2)-C(3) C(lO)-C(l)-C(ll) 118.7(6) 120.7(6) C@)-C(3)-C(4) 122.9(6) C(3)-C(4)-C(5) 115.5(6) C(4)-C(S)-C(6) 127.4(6) C(4)-C(5)-C(lO) 120.1(6) C(6)-C(S)-C(lO) 117.0(6) C(5)-C(6)-CU) 118.7(6) C(6)-C(7)-C(8) 123.0(6) C(7)-C(8)-C(9) 136.6(6) C(8)-C(9)-C(10) 116.9(6) C(8)4(9)4(12) C(lO)-C(9)-C(12) 106.4(5) C(l)-C(lO)C(5) 124.5(5) 124.0(6) C(l)-C(lO)-C(S) 111.5(5) C(5)-C(lO)-C(9) 106.7(4) C(l)-C(ll)-C(ll’) 132.5(3) C(l)-C(ll)-C(l2) 108.2(4) C(l2)-C(ll)-C(ll’) 120.7(3) C(9)-C(12)-C(ll) 133.0(5) C(ll)-C(l2)-C(13) 118.8(5) C(9)-C(12)-C(13) C(12)4(13)-C(14) 131.8(5) C(12)4(13)-C(13’) 120.2(3) C(l4)-C(13)4(13’) 108.0(3) C(13)-C(14)-C(15) 137.0(5) 117.16) C(13)-C(14)-C(19) 105.7(5) C(15)-C(14)-C(19) C(14)4(15)-C(16) 119.5(5) C(l5)-C(16)4(17) 123.1(6) 119.3(6) C(17)-C(18)-C(19) 116.9(4) C(16)-C(17)-C(18) C(17)-C(l8)-C(17’) 126.2(7) C(14)-C(19)-C(18) 123.8(3) C(14)-C(19)-C(14’) 112.5(7) symmetry code: ’ = (-x, y, 1.5 - z )
to think that there may be a direct relationship between the macroscopic handedness of the ribbon-like crystals and the absolute configuration of the decacyclene molecules contained within.l4 We therefore carried out a powder X-ray diffraction study of a decacyclene sample recrystallized from benzenepinacolone (Table IV). The refined unit-cell parameters were identical within experimental error to those of the sublimed single crystal, and the measured powder pattern was in good agreement with a simulation of the powder pattern based upon the single-crystal data. We note that Guet and Tchoubar have previously carried out a powder X-ray diffraction study of decacyclene, and they report a monoclinic unit cell with a = 12.77 A, b = 20.74 A, c = 3.95 A, and fl = 99O.l’ However, the centering condition was missed in this earlier study, and the space group was misassigned as monoclinic P 2 1 (No. 4) which is a maximal nonisomorphic subgroup of C2221 (No. 20). The vectors [0,0,11, [2,0,11, [0,1,01describea C-centeredorthorhombic cell with a = 3.95 A, b = 25.23 A, and c = 20.74 A. The b axis derived from their monoclinic cell appears to differ significantly from our single-crystal value; unfortunately, no estimated standard deviations were reported. Our redetermination of the decacyclene powder pattern, how(13) The figures in this paper show right-handed decacyclene molecules;however,the absolute c o n f i a t i o n of the decacyclene crystal used for the X-ray structure determination hae not been determined. Such a determination would not be likely to yield reliable results given the absence of heavy atoms in the crystal. (14) One might hope to observe helical morphology in the crystals of the helicenes,and a searchof the CambridgeStructuralDatabase16yielded eight structuresof heliceneewhichhad crystallized in chiral space groups.16 However, there is no mention of helical crystal morphology in any of these publications. (15) Allen, F. H.; Kennard, 0.; Taylor, R. Acc. Chem. Res. 1983,16, 146-153.
Ho and Pascal Table IV. X-ray Powder Diffraction Data for Decacyclene‘ 12.7339 57 0 2 0 10.8531 100 0 2 1 10.4030 78 0 0 2 8.0502 43 0 2 2 6.3571 56 0 4 0 6.0874 85 0 2 3 5.2002 7 0 0 4 4.8121 10 0 2 4 4.2388 4 0 6 0 4.1521 6 0 6 1 3.9223 1 0 6 2 3.8348 2 1 1 1 3.6518 20 1 1 2 3.3994 21 1 1 3 3.2852 1 0 6 4 1 1 3 3 3.1802 3.1417 2 0 8 1 3.0419 1 0 4 6 2.8930 2 0 2 7 2.8434 2 1 5 3 2.6422 3 0 10 0 2.2149 2 0 6 8
* Unit celk A28
a = 3.948 (1)A,
200b - 28dc.
6.936 8.140 8.493 10.982 13.919 14.539 17.037 18.423 20.941 21.383 22.652 23.176 24.354 26.194 27.122 28.035 28.386 29.338 30.884 31.437 35.277 40.703
6.948 8.146 8.498 10.985 13.922 14.545 17.044 18.425 20.948 21.383 22.642 23.180 24.351 26.193 27.128 28.039 28.387 29.331 30.890 31.433 35.273 40.692
-0.012 -0,006
-0.006 -0.003 -0.003 -0.006
-0.007 -0.002 -0.007 O.Oo0 0.010 -0.004 0.003 0.001 -0.006
-0.004 -0.004 0.007 -0.006
0.004 0.004
0.011
b = 25.424 (24) A, c = 20.793 (16) A.
ever, eliminates any doubt that the b axes are the same and that the compound is better described in the orthorhombic space group C2221. We conclude from the powder data that the unusual helical crystals of compound 1have the same microscopic structure as the crystals formed by sublimation. However, there is no necessary correspondence between the macroscopic configuration of the helices and the configuration of the decacyclene propellers within. To establish such a relationship, one must separate the left- and right-handed helixes (A la Pasteur), dissolve them, and measure the optical rotation of the resulting solutions. Unfortunately, the barrier to enantiomerization of 1is estimated by AM1 to be no more than 3 kcal/mol. Thus this experiment cannot succeed at normal temperatures, and, for the present a t least, the final link between the macroscopic and microscopic structures of decacyclene cannot be established.18 We have shown that the neutral decacyclene molecule is a three-bladed molecular propeller in the solid state. Many salts of decacyclene cations have been prepared,ZJ but none have been structurally characterized. Notably, however, Sugano and Kinoshita3 state that tris(decacyclene) bis(hexafluorophosphate) also crystallizes as long, thin needles in a chiral space group-the hexagonal group P6322-but no structural details have been reported. (16) (a) Hexahelicene: de Rango, C.; Tsoucari~,G.; Declerq, J. P.; Germain,G.; Putseys, J. P. Cryst. Struct. Commun. 1973,2,189-192. (b) Heptahelicene: Beurskene,P. T.; Beurekens, G.; van den Hark, T. E. M. Cryst. Struct. Commun. 1976,5,241-246. (c) [lOIHelicene: Le Baa, G.; Navaza, A.; Mauguen, Y.; de Rango, C. Cryst. Struct. Commun. 1976,5, 357-361. (d) [llIHelicene: Le Bas, G.; Navaza, A.; Knwsow, M.; de Rango, C. C r y s t . S t r u c t . C o m m u n . 1976, 5 , 713-718. (e) 2-Bromohexahelicene: Lightner,D. A.; Hefelfiiger, D. T.;Powers, T. W.; Frank, G. W.; Trueblood, K. N. J. Am. Chem. SOC.1971,94,3492-3497. (02-Methylhexahelicene: Frank, G. W.; Hefelfiier, D. T.; Lightner, D. A . A c t a C r y s t a l l o g r . , S e c t . E 1973, B29, 223-230. ( g ) 1-(1-Hydroxyethy1)hexahelicene: Van Meerssche, M.; Declerq, J. P.; Germain, G.; Soubrier-Payen,B.; Lindner, H. J.; Kitechke, B. Bull. SOC. Chim.Belg. 1984,93,445-448. (h)Tetrathia[7lhetamhelicene:Nakagawa, H.; Obata, A.; Yamada, K.; Kawazura, H.; Konno, M.; Miyamae, H. J. Chem. SOC.,Perkin Tram. 2 1985, 1899-1903. (17) Guet, J. M.; Tchoubar, D. Carbon 1984,22,543-649. (18) One might hope to measure the optical rotation of the crystals themselves, but the birefringence and small size of the crystals makes this highly impractical.
Chem. Mater., Vol. 5, No. 9, 1993 1361
Decacyclene: A Molecular Propeller
Although the deformation of neutral 1 from planarity is not very large, one might expect its cations to be more nearly planar in order to better delocalize the positive charge through the aromatic a-system, and it will be interesting to compare the structure of 1with those of its ions as the latter become available. Experimental Section Decacyclene was obtained from Aldrich Chemical Co., and it was recrystallized twice from nitrobenzene before use. Sublimation of Decacyclene. Compound 1 (-1 g) was placed in a 50-mL round-bottom flask fitted with a stopcock. The flask was evacuated to le2Torr, the stopcock was closed, and the bottom of the flask was immersed in a heated Wood's metal bath to a depth of 1cm. The temperature of the metal bath was held at 370 'C for 12 h. After cooling to room temperature, the flask was vented, and the long orange needles of 1, which were distributed all over the flask and stopcock, were collected. The sublimed material melted at 394-396 "C (1it.l mp 387 00. X-ray Crystallographic Analyses of Decacyclene. An orange needle of 1, prepared by sublimation and cut to an appropriate size, was used for single-crystalX-ray measurements. The structure was determined using a Siemens P4 diffractometer. Unit cell parameters were determined from the setting angles of 25 centered reflections having 14' 5 20 5 24'. The crystal data and experimental details are summarized in Table I. Axial photographs and preliminary peak scans through reciprocalspace revealed symmetry and systematic absences consistent with the chiral orthorhombic space group C2221. Accordingly, two octants of data (+h,+k,+l and -h,-k,-l) were collected. An w scan mode was used with scan speeds of 2.00-8.08' min-l. Three standards (040,113,205) were measured for every 97 reflections during the data collection period; no significant deviations from their mean intensities were observed. All data were corrected for Lorentz and polarization effects, but not for absorption or extinction. A total of 2266 reflections were measured, merging of equivalents gave 1861 unique reflections (Rht = 0.02121, of which 1083 were considered observed [F> 2a(F)1. The structure was solved by direct methods and refined by using the SHELXTL PLUS program package.lO All of the non-hydrogen atoms were refined with anisotropic displacement coefficients,hydrogen atoms were included with a riding model [C-H = 0.96 A, U(H) = 1.2U(C)l, + 0.0008F. and the weighting scheme employed was w l = The refinement converged to R(F) = 0.0747, wR(F) = 0.0686, and
-
(19) Sheldrick, G.M. SHELXTL PLUS 4.21 for Siemens Crystallographic Research Systems; Siemens Analytical Instruments, Inc., 1990.
S = 1.15 with 164 variables and 6.6 reflections per refined parameter. The maximum A/u in the final cycle of least squares was 0.001, and the peaks on the final Ap map ranged from -0.46 toO.44 e A3. Scattering factorswere taken from thelnternational Tables for Crystallography.20 The final atomic coordinates are listed in Table 11,and bond lengths and angles are given in Table 111. Powder X-ray diffraction measurements and analyses were performed on a Scintag PAD-V diffractometer equipped with a Ge solid-state detector and DMS 2000 v2.64 software. The decacyclene sample (recrystallizedfrom benzene-pinacolone) was mounted on a zero-background sample holder and step scanned from 3.0' to 53.0' in 20. The step size and step time parameters were 0.02°/step and 2.OOs/step, respectively. Cu K a (A = 1.540 60 A) radiation was used for the measurements, the mean temperature was 23 'C, and silicon was employed as an external standard. The final 20 values were obtained by profile fitting of the diffraction peaks using a pseudo Voight function. The crystal data and refined crystallographic constants were as follows: orthorhombic, space group C2221; a = 3.948 (1)A, b = 25.424 (24) A, c = 20.793 (16) A, V = 2087 (4) As,2 = 4, D a d = 1.434 g/cma. The Smith-SnydeP and de WolfP2 figures of merit for the analysis of the powder diffraction data were Fzz = 56 (0.005,76) and M N = 60, respectively.
Acknowledgment. This work was supported in part by National Science Foundation Grant CHE-9106903. We thank Mark Rodriguez and Corine Gerardin for assistance with the powder diffraction study. Supplementary Material Available: Single-crystal structure report for decacyclene (including full experimental details, atomic coordinates, bond lengths and angles, anisotropic displacement Coefficients, H atom coordinates, bond lengths and angles involving H atoms, torsion angles, least squares planes, and several figures) and the X-ray powder diffraction summary for decacyclene (22 pages); list of observed and calculated structure factors (10 pages). Ordering information is given on any current masthead page. (20) (a) Creagh, D. C.; McAuley, W. J. International Tables for Crystallography: Mathematica1,Physical and Chemical Tables;Wileon, A. J. C., Ed.; Kluwer: Dordrecht, The Netherlands, 1992; Vol. C, pp 206-222. (b) Maslen, E. N.; Fox,A. G.; OKeefe, M. A. International Tables for Crystallography: Mathematical, Physical and Chemical Tables; Wilson,A. J. C., Ed.; Kluwer: Dordrecht, The Netherlands, 1992; Vol. C, pp 476-516. (21) Smith, G.S.;Snyder, R. L. J. Appl. Cryst. 1979,12, 60-65. (22) de Wolff, P.M. J.Appl. Crystallogr. 1968,1, 108-113.
