In the crystal structure of the title compound, C18H14O4, there are two independent molecules, both of which are exactly centrosymmetric and therefore have a transoid arrangement of the ester substituents. The planes of these are inclined at 63.90 (4) and 61.02 (5)° to the anthracene central ring in the two molecules because of steric interactions, preventing electronic delocalization. The observed conformation is in agreement with molecular-modelling calculations for the isolated molecule, indicating no major influence from crystal-packing forces.
Supporting information
CCDC reference: 147666
A stirred mixture of 9,10-dibromoanthracene (2.0 g, 5.96 mmol) in anhydrous diethyl ether (20 ml) was cooled in an ice bath, then n-butyllithium (9.76 ml of a 1.6 M solution in hexane, 15.6 mmol) was added dropwise from a syringe. The mixture was allowed to warm to room temperature and stirred for 30 min. After cooling again in ice, dry carbon dioxide (from evaporation of solid through a silica gel trap) was passed over the mixture for 1 h. Water (10 ml) and diethyl ether (10 ml) were then added, the aqueous phase separated, washed with two more portions of diethyl ether (2 ml), acidified with 1 M sulfuric acid (pH 2.0), and the resulting yellow product was filtered off and dried in vacuo. Thionyl chloride (100 ml) was added to the dried product (1.26 g) and the resulting mixture heated at reflux overnight. The thionyl chloride was then removed in vacuo. Anhydrous methanol (50 ml) was added, and the solution was heated at reflux for 2.5 h and then evaporated to dryness. Recrystallization by slow evaporation of dichloromethane/petrol (60–80) yielded 0.92 g (52% overall yield) of golden plates. Subsequent recrystallization of a small sample under the same conditions gave golden needles that were used in this study; m.p. 441–443 K (Literature 448–449 K from MeOH; Duerr et al., 1988); 1H NMR (CDCl3) δ 7.98 (dd, 4H, J 6.8, 3.4 Hz, Ar—H), 7.54 (dd, 4H, J 6.8, 3.4 Hz, Ar—H), 4.15 (s, 6H, OCH3); 13C NMR (CDCl3) δ 169.7 (C═O), 130.7 (C9, C10), 127.7 (Ar—C), 127.0 (Ar—CH), 125.4 (Ar—CH), 52.9 (MeO).
Hydrogen atoms were placed geometrically and refined with a riding model (including free rotation about C—C bonds), and with Uiso constrained to be 1.2 (1.5 for methyl groups) times Ueq of the carrier atom.
Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 1994); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.
9,10-diacetylanthracene
top
Crystal data top
C18H14O4 | F(000) = 616 |
Mr = 294.29 | Dx = 1.405 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
a = 18.4434 (14) Å | Cell parameters from 7819 reflections |
b = 6.8590 (5) Å | θ = 2.3–28.6° |
c = 11.5334 (9) Å | µ = 0.10 mm−1 |
β = 107.488 (2)° | T = 160 K |
V = 1391.58 (18) Å3 | Plate, colourless |
Z = 4 | 0.80 × 0.50 × 0.08 mm |
Data collection top
Siemens SMART CCD diffractometer | 2719 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.017 |
Graphite monochromator | θmax = 28.6°, θmin = 2.3° |
Detector resolution: 8.192 pixels mm-1 | h = −23→24 |
ω rotation with narrow frames scans | k = −9→8 |
11472 measured reflections | l = −15→15 |
3322 independent reflections | |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.038 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.105 | H-atom parameters constrained |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0602P)2 + 0.2247P] where P = (Fo2 + 2Fc2)/3 |
3322 reflections | (Δ/σ)max < 0.001 |
201 parameters | Δρmax = 0.46 e Å−3 |
0 restraints | Δρmin = −0.24 e Å−3 |
Crystal data top
C18H14O4 | V = 1391.58 (18) Å3 |
Mr = 294.29 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 18.4434 (14) Å | µ = 0.10 mm−1 |
b = 6.8590 (5) Å | T = 160 K |
c = 11.5334 (9) Å | 0.80 × 0.50 × 0.08 mm |
β = 107.488 (2)° | |
Data collection top
Siemens SMART CCD diffractometer | 2719 reflections with I > 2σ(I) |
11472 measured reflections | Rint = 0.017 |
3322 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.038 | 0 restraints |
wR(F2) = 0.105 | H-atom parameters constrained |
S = 1.06 | Δρmax = 0.46 e Å−3 |
3322 reflections | Δρmin = −0.24 e Å−3 |
201 parameters | |
Special details top
Experimental. The data collection nominally covered over a hemisphere of reciprocal space, by a combination of three sets of exposures; each set had a different ϕ angle for the crystal and each exposure covered 0.3° in ω. The data set is essentially complete for θ<26°. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
O1 | 0.69402 (5) | 0.31652 (14) | 0.53100 (8) | 0.0324 (2) | |
O2 | 0.67027 (5) | 0.28670 (13) | 0.70950 (7) | 0.0258 (2) | |
C1 | 0.53546 (6) | 0.13812 (17) | 0.40767 (10) | 0.0219 (2) | |
H1 | 0.5847 | 0.0826 | 0.4391 | 0.026* | |
C2 | 0.48173 (7) | 0.04270 (17) | 0.31942 (10) | 0.0233 (2) | |
H2 | 0.4934 | −0.0799 | 0.2910 | 0.028* | |
C3 | 0.40847 (7) | 0.12399 (17) | 0.26926 (11) | 0.0243 (3) | |
H3 | 0.3719 | 0.0575 | 0.2057 | 0.029* | |
C4 | 0.38981 (6) | 0.29643 (17) | 0.31113 (10) | 0.0220 (2) | |
H4 | 0.3401 | 0.3481 | 0.2771 | 0.026* | |
C9 | 0.57347 (6) | 0.42198 (16) | 0.54702 (10) | 0.0194 (2) | |
C11 | 0.44409 (6) | 0.40134 (16) | 0.40587 (10) | 0.0189 (2) | |
C12 | 0.51921 (6) | 0.32140 (16) | 0.45424 (10) | 0.0188 (2) | |
C13 | 0.65273 (6) | 0.33748 (16) | 0.59329 (10) | 0.0206 (2) | |
C14 | 0.74505 (7) | 0.20168 (19) | 0.76033 (12) | 0.0309 (3) | |
H14A | 0.7478 | 0.0782 | 0.7191 | 0.046* | |
H14B | 0.7543 | 0.1778 | 0.8474 | 0.046* | |
H14C | 0.7836 | 0.2916 | 0.7489 | 0.046* | |
O1A | 0.19855 (5) | 0.34901 (15) | 0.16181 (8) | 0.0346 (2) | |
O2A | 0.17708 (5) | 0.34455 (13) | −0.03986 (8) | 0.0291 (2) | |
C1A | 0.10685 (7) | 0.74061 (17) | −0.06854 (10) | 0.0234 (2) | |
H1A | 0.1588 | 0.7037 | −0.0482 | 0.028* | |
C2A | 0.08370 (7) | 0.91022 (18) | −0.12953 (11) | 0.0261 (3) | |
H2A | 0.1196 | 0.9904 | −0.1513 | 0.031* | |
C3A | 0.00653 (7) | 0.96816 (18) | −0.16079 (10) | 0.0255 (3) | |
H3A | −0.0089 | 1.0870 | −0.2033 | 0.031* | |
C4A | −0.04564 (7) | 0.85557 (17) | −0.13057 (10) | 0.0232 (2) | |
H4A | −0.0972 | 0.8963 | −0.1528 | 0.028* | |
C9A | 0.07609 (6) | 0.44138 (16) | 0.02973 (10) | 0.0200 (2) | |
C11A | −0.02406 (6) | 0.67632 (16) | −0.06566 (10) | 0.0198 (2) | |
C12A | 0.05403 (6) | 0.61651 (16) | −0.03435 (10) | 0.0196 (2) | |
C13A | 0.15805 (7) | 0.37350 (17) | 0.06075 (10) | 0.0233 (2) | |
C14A | 0.25457 (8) | 0.2819 (2) | −0.02043 (14) | 0.0407 (4) | |
H14D | 0.2894 | 0.3826 | 0.0246 | 0.061* | |
H14E | 0.2634 | 0.2602 | −0.0991 | 0.061* | |
H14F | 0.2635 | 0.1603 | 0.0263 | 0.061* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0249 (4) | 0.0449 (6) | 0.0300 (5) | 0.0065 (4) | 0.0122 (4) | 0.0009 (4) |
O2 | 0.0224 (4) | 0.0297 (5) | 0.0237 (4) | 0.0036 (3) | 0.0044 (3) | 0.0032 (3) |
C1 | 0.0235 (6) | 0.0218 (5) | 0.0217 (6) | 0.0030 (4) | 0.0087 (5) | 0.0012 (4) |
C2 | 0.0297 (6) | 0.0199 (5) | 0.0232 (6) | −0.0005 (4) | 0.0121 (5) | −0.0021 (4) |
C3 | 0.0251 (6) | 0.0250 (6) | 0.0230 (6) | −0.0064 (5) | 0.0076 (5) | −0.0032 (5) |
C4 | 0.0189 (5) | 0.0247 (6) | 0.0227 (6) | −0.0023 (4) | 0.0070 (4) | −0.0004 (4) |
C9 | 0.0188 (5) | 0.0218 (5) | 0.0186 (5) | 0.0006 (4) | 0.0073 (4) | 0.0020 (4) |
C11 | 0.0194 (5) | 0.0204 (5) | 0.0180 (5) | −0.0009 (4) | 0.0076 (4) | 0.0013 (4) |
C12 | 0.0199 (5) | 0.0203 (5) | 0.0177 (5) | 0.0005 (4) | 0.0077 (4) | 0.0019 (4) |
C13 | 0.0215 (5) | 0.0184 (5) | 0.0224 (6) | −0.0023 (4) | 0.0073 (4) | −0.0022 (4) |
C14 | 0.0225 (6) | 0.0324 (7) | 0.0321 (7) | 0.0047 (5) | −0.0006 (5) | 0.0028 (5) |
O1A | 0.0226 (4) | 0.0481 (6) | 0.0291 (5) | 0.0036 (4) | 0.0016 (4) | 0.0031 (4) |
O2A | 0.0241 (4) | 0.0339 (5) | 0.0312 (5) | 0.0063 (4) | 0.0111 (4) | 0.0015 (4) |
C1A | 0.0207 (5) | 0.0266 (6) | 0.0221 (6) | −0.0036 (5) | 0.0055 (4) | −0.0016 (5) |
C2A | 0.0299 (6) | 0.0251 (6) | 0.0244 (6) | −0.0072 (5) | 0.0098 (5) | −0.0014 (5) |
C3A | 0.0342 (6) | 0.0213 (6) | 0.0201 (6) | 0.0004 (5) | 0.0070 (5) | 0.0024 (4) |
C4A | 0.0241 (6) | 0.0240 (6) | 0.0200 (6) | 0.0026 (4) | 0.0044 (4) | −0.0001 (4) |
C9A | 0.0193 (5) | 0.0230 (5) | 0.0163 (5) | 0.0001 (4) | 0.0034 (4) | −0.0018 (4) |
C11A | 0.0218 (5) | 0.0208 (5) | 0.0154 (5) | −0.0008 (4) | 0.0037 (4) | −0.0021 (4) |
C12A | 0.0197 (5) | 0.0223 (5) | 0.0156 (5) | −0.0017 (4) | 0.0036 (4) | −0.0026 (4) |
C13A | 0.0280 (6) | 0.0201 (5) | 0.0205 (6) | −0.0065 (4) | 0.0053 (5) | 0.0000 (4) |
C14A | 0.0297 (7) | 0.0477 (8) | 0.0501 (9) | 0.0146 (6) | 0.0199 (6) | 0.0091 (7) |
Geometric parameters (Å, º) top
O1—C13 | 1.2022 (14) | O1A—C13A | 1.1922 (14) |
O2—C13 | 1.3273 (14) | O2A—C13A | 1.3243 (15) |
O2—C14 | 1.4491 (14) | O2A—C14A | 1.4439 (15) |
C1—C2 | 1.3566 (16) | C1A—C2A | 1.3601 (17) |
C1—C12 | 1.4337 (15) | C1A—C12A | 1.4353 (16) |
C1—H1 | 0.9500 | C1A—H1A | 0.9500 |
C2—C3 | 1.4146 (16) | C2A—C3A | 1.4161 (17) |
C2—H2 | 0.9500 | C2A—H2A | 0.9500 |
C3—C4 | 1.3603 (16) | C3A—C4A | 1.3583 (17) |
C3—H3 | 0.9500 | C3A—H3A | 0.9500 |
C4—C11 | 1.4341 (15) | C4A—C11A | 1.4323 (16) |
C4—H4 | 0.9500 | C4A—H4A | 0.9500 |
C9—C11i | 1.4051 (15) | C9A—C12A | 1.4049 (16) |
C9—C12 | 1.4067 (15) | C9A—C11Aii | 1.4082 (16) |
C9—C13 | 1.5129 (15) | C9A—C13A | 1.5182 (16) |
C11—C9i | 1.4051 (15) | C11A—C9Aii | 1.4082 (16) |
C11—C12 | 1.4377 (15) | C11A—C12A | 1.4354 (15) |
C14—H14A | 0.9800 | C14A—H14D | 0.9800 |
C14—H14B | 0.9800 | C14A—H14E | 0.9800 |
C14—H14C | 0.9800 | C14A—H14F | 0.9800 |
| | | |
C13—O2—C14 | 115.32 (9) | C13A—O2A—C14A | 114.79 (10) |
C2—C1—C12 | 121.08 (10) | C2A—C1A—C12A | 121.05 (11) |
C2—C1—H1 | 119.5 | C2A—C1A—H1A | 119.5 |
C12—C1—H1 | 119.5 | C12A—C1A—H1A | 119.5 |
C1—C2—C3 | 120.62 (11) | C1A—C2A—C3A | 120.47 (11) |
C1—C2—H2 | 119.7 | C1A—C2A—H2A | 119.8 |
C3—C2—H2 | 119.7 | C3A—C2A—H2A | 119.8 |
C4—C3—C2 | 120.66 (11) | C4A—C3A—C2A | 120.68 (11) |
C4—C3—H3 | 119.7 | C4A—C3A—H3A | 119.7 |
C2—C3—H3 | 119.7 | C2A—C3A—H3A | 119.7 |
C3—C4—C11 | 120.91 (10) | C3A—C4A—C11A | 121.01 (11) |
C3—C4—H4 | 119.5 | C3A—C4A—H4A | 119.5 |
C11—C4—H4 | 119.5 | C11A—C4A—H4A | 119.5 |
C11i—C9—C12 | 121.55 (10) | C12A—C9A—C11Aii | 121.99 (10) |
C11i—C9—C13 | 120.30 (10) | C12A—C9A—C13A | 119.77 (10) |
C12—C9—C13 | 118.13 (10) | C11Aii—C9A—C13A | 118.24 (10) |
C9i—C11—C4 | 122.56 (10) | C9Aii—C11A—C4A | 122.62 (10) |
C9i—C11—C12 | 119.08 (10) | C9Aii—C11A—C12A | 118.88 (10) |
C4—C11—C12 | 118.36 (10) | C4A—C11A—C12A | 118.49 (10) |
C9—C12—C1 | 122.28 (10) | C9A—C12A—C1A | 122.57 (10) |
C9—C12—C11 | 119.37 (10) | C9A—C12A—C11A | 119.13 (10) |
C1—C12—C11 | 118.33 (10) | C1A—C12A—C11A | 118.29 (10) |
O1—C13—O2 | 124.48 (11) | O1A—C13A—O2A | 125.62 (12) |
O1—C13—C9 | 123.60 (10) | O1A—C13A—C9A | 124.09 (11) |
O2—C13—C9 | 111.91 (9) | O2A—C13A—C9A | 110.29 (9) |
O2—C14—H14A | 109.5 | O2A—C14A—H14D | 109.5 |
O2—C14—H14B | 109.5 | O2A—C14A—H14E | 109.5 |
H14A—C14—H14B | 109.5 | H14D—C14A—H14E | 109.5 |
O2—C14—H14C | 109.5 | O2A—C14A—H14F | 109.5 |
H14A—C14—H14C | 109.5 | H14D—C14A—H14F | 109.5 |
H14B—C14—H14C | 109.5 | H14E—C14A—H14F | 109.5 |
| | | |
C12—C1—C2—C3 | 1.28 (17) | C12A—C1A—C2A—C3A | −0.03 (18) |
C1—C2—C3—C4 | −2.02 (18) | C1A—C2A—C3A—C4A | −0.05 (18) |
C2—C3—C4—C11 | 0.83 (17) | C2A—C3A—C4A—C11A | 0.47 (18) |
C3—C4—C11—C9i | −178.84 (11) | C3A—C4A—C11A—C9Aii | 178.13 (11) |
C3—C4—C11—C12 | 1.02 (16) | C3A—C4A—C11A—C12A | −0.78 (16) |
C11i—C9—C12—C1 | −178.09 (10) | C11Aii—C9A—C12A—C1A | 178.20 (10) |
C13—C9—C12—C1 | 3.51 (15) | C13A—C9A—C12A—C1A | −2.50 (16) |
C11i—C9—C12—C11 | 0.30 (17) | C11Aii—C9A—C12A—C11A | −0.84 (18) |
C13—C9—C12—C11 | −178.10 (10) | C13A—C9A—C12A—C11A | 178.46 (10) |
C2—C1—C12—C9 | 178.98 (11) | C2A—C1A—C12A—C9A | −179.34 (11) |
C2—C1—C12—C11 | 0.58 (16) | C2A—C1A—C12A—C11A | −0.30 (17) |
C9i—C11—C12—C9 | −0.29 (17) | C9Aii—C11A—C12A—C9A | 0.81 (17) |
C4—C11—C12—C9 | 179.85 (10) | C4A—C11A—C12A—C9A | 179.76 (10) |
C9i—C11—C12—C1 | 178.16 (10) | C9Aii—C11A—C12A—C1A | −178.27 (10) |
C4—C11—C12—C1 | −1.70 (15) | C4A—C11A—C12A—C1A | 0.68 (15) |
C14—O2—C13—O1 | 0.27 (17) | C14A—O2A—C13A—O1A | −1.32 (18) |
C14—O2—C13—C9 | 179.14 (9) | C14A—O2A—C13A—C9A | 179.24 (10) |
C11i—C9—C13—O1 | −116.06 (13) | C12A—C9A—C13A—O1A | 119.48 (13) |
C12—C9—C13—O1 | 62.36 (15) | C11Aii—C9A—C13A—O1A | −61.19 (16) |
C11i—C9—C13—O2 | 65.06 (13) | C12A—C9A—C13A—O2A | −61.07 (13) |
C12—C9—C13—O2 | −116.52 (11) | C11Aii—C9A—C13A—O2A | 118.26 (11) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x, −y+1, −z. |
Experimental details
Crystal data |
Chemical formula | C18H14O4 |
Mr | 294.29 |
Crystal system, space group | Monoclinic, P21/c |
Temperature (K) | 160 |
a, b, c (Å) | 18.4434 (14), 6.8590 (5), 11.5334 (9) |
β (°) | 107.488 (2) |
V (Å3) | 1391.58 (18) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.10 |
Crystal size (mm) | 0.80 × 0.50 × 0.08 |
|
Data collection |
Diffractometer | Siemens SMART CCD diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 11472, 3322, 2719 |
Rint | 0.017 |
(sin θ/λ)max (Å−1) | 0.673 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.038, 0.105, 1.06 |
No. of reflections | 3322 |
No. of parameters | 201 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.46, −0.24 |
Selected geometric parameters (Å, º) topO1—C13 | 1.2022 (14) | O1A—C13A | 1.1922 (14) |
O2—C13 | 1.3273 (14) | O2A—C13A | 1.3243 (15) |
O2—C14 | 1.4491 (14) | O2A—C14A | 1.4439 (15) |
C9—C13 | 1.5129 (15) | C9A—C13A | 1.5182 (16) |
| | | |
C13—O2—C14 | 115.32 (9) | C13A—O2A—C14A | 114.79 (10) |
O1—C13—O2 | 124.48 (11) | O1A—C13A—O2A | 125.62 (12) |
O1—C13—C9 | 123.60 (10) | O1A—C13A—C9A | 124.09 (11) |
O2—C13—C9 | 111.91 (9) | O2A—C13A—C9A | 110.29 (9) |
| | | |
C14—O2—C13—C9 | 179.14 (9) | C14A—O2A—C13A—C9A | 179.24 (10) |
C11i—C9—C13—O1 | −116.06 (13) | C12A—C9A—C13A—O1A | 119.48 (13) |
C12—C9—C13—O1 | 62.36 (15) | C11Aii—C9A—C13A—O1A | −61.19 (16) |
C11i—C9—C13—O2 | 65.06 (13) | C12A—C9A—C13A—O2A | −61.07 (13) |
C12—C9—C13—O2 | −116.52 (11) | C11Aii—C9A—C13A—O2A | 118.26 (11) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x, −y+1, −z. |
The conflicting demands of electronic conjugation and steric interference are well known in many types of organic molecules, particularly aromatic compounds with appropriate unsaturated substituents. A classic case is that of biphenyl, which appears to adopt an essentially planar conformation in the crystalline solid state (Trotter, 1961; Charbonneau & Delugeard, 1976, 1977), but is twisted by ca 45° about the central C—C bond in the gas phase (Almenningen et al., 1985; Bastiansen & Samdal, 1985); the steric interaction of ortho-hydrogen atoms is unfavourable for a coplanar arrangement of the two rings, but this is advantageous for delocalization between the rings, and the balance between the two factors is tipped in favour of planarity in the solid state by intermolecular interactions. A carboxyl substituent on a benzene ring, as in carboxylic acids, esters and other derivatives, tends to lie approximately in the plane of the ring, provided there are no adverse steric interactions with other substituents. This can be illustrated by a search in the Cambridge Structural Database (Allen & Kennard, 1993) for 1,4-dicarboxyl-substituted benzene with no other substituents, for example; these are terephthalic acid itself and its salts, esters, metal complexes and other molecules incorporating the terephthalate unit. Of the 55 structures reported, only three have the carboxyl group rotated more than 20° out of the ring plane, and a large majority of dihedral angles are under 10°. By contrast, higher polycarboxylates involving substituent groups on adjacent ring carbon atoms, such as derivatives of pyromellitic acid with a 1,2,4,5-tetrasubstituent pattern, can not adopt a fully planar configuration and have large dihedral angles.
In the case of 9,10-disubstituted anthracenes with unsaturated substituents such that a double bond X═Y might be conjugated to the central ring, steric interference may occur between outer ring hydrogen atoms and groups attached to X and Y and also the atom Y itself. Port et al. (1997) have reported the synthesis and structural characterization of anthracenes substituted at both the 9 and 10 positions with either tertiary butyl ketone or tertiary butyl imine groups. For these compounds, the steric interactions are so great that there is highly restricted rotation about the substituent bonds to the anthracene nucleus and distinct cisoid and transoid isomers can be isolated. The crystal structures of the two isomeric imines were determined, and both have the substituents almost perpendicular to the central ring plane.
In the course of studies on 9,10-difunctionalized anthracene derivatives we prepared the title compound, (I), by a modification of the method of Duerr et al. (1988) and obtained suitable crystals for X-ray diffraction anaylsis. We have determined the structure for comparison with other anthracenes having unsaturated substituents in the 9 and 10 positions. \sch
The structure contains two symmetry-independent molecules, each lying on a crystallographic inversion centre, so that the asymmetric unit is two half-molecules. They are shown in Figure 1 and have essentially identical geometry. Intermolecular interactions are of standard van der Waals type only, with no significant face-to-face stacking of aromatic rings. Since the molecules are strictly centrosymmetric, they have a transoid configuration of the two substituents. The mean planes of the esters (excluding hydrogen atoms) lie at dihedral angles of 63.90 (4) and 61.02 (5)° to the anthracene nucleus for the two molecules. In keeping with the steric interactions and lack of conjugation between the anthracene nucleus and the substituents, the C9—C13 and C9A—C13A bonds are relatively long for bonds between sp2 carbon atoms.
A search of the Cambridge Structural Database (Allen & Kennard, 1993) for molecules with two X═ Y double bonds (with any substituents allowed) attached to the 9 and 10 positions of an otherwise unsubstituted anthracene molecule produces just seven structures, including the two isomers of the diimine reported by Port et al. (1997). The substituents range from nitro (Trotter, 1959) to an imidazolyl hydrazone in two different salts with anti-cancer properties (Bates et al., 1986; Pierpont & Lang, 1986) and include two ketones (Harlow et al., 1975; Sundell & Becker, 1994), but there are no previously reported esters. With the exception of the cisoid isomer of the diimine, all these molecules have crystallographic inversion symmetry (including two independent molecules in each of three cases) and are transoid. Most of the dihedral angles between the central aromatic ring and the substituents lie in the range 40–90°, indicating a strong steric interference which prevents effective conjugation. The one exception is a dihedral angle of only 4.6° for one of the two independent molecules in a hydrazone (Bates et al., 1986), for which the other molecule has a dihedral angle of 41.3°. The other salt of this same cationic hydrazone (Pierpont & Lang, 1986) has a dihedral angle of 48.7°, and these values are significantly smaller than the dihedral angles for the remaining compounds. The hydrazone, for which X = CH and Y = NR with only one substituent directed away from the anthracene nucleus, clearly suffers much less steric interaction than the other molecules, particularly so in comparison with the ketones and the imines, for all of which the dihedral angles exceed 75°. The dinitro compound has a value of 61.8° for the dihedral angle, in keeping with the intermediate steric demands in this case, and a very similar result is found in the ester reported here, for which the substituents have comparable steric requirements.
The question arises whether the barrier to rotation about the substituent bonds is sufficiently high for a cisoid form of the title compound to be isolated as a separate isomer from the transoid form. Molecular modelling, with augmented MM3 parameters and an optimized map generated by searching the dihedral angle of the ester group at 15° intervals (Oxford Molecular, 1999), gave four possible low energy conformations, two of which were cisoid and two transoid. The energies of these forms were similar, and the activation energy for interconversion between cisoid and transoid forms was around 9 kcal mol−1. This is substantially less than the 25 kcal mol−1 value observed for the analogous di-tert-butylimines (Port et al., 1997). No evidence could be obtained, however, for the presence of a second isomer in the 1H NMR spectra in the temperature range 193 to 373 K in toluene solution. Further experiments are being carried out in order to investigate the existence of the cisoid form of this molecule. Subsequent global energy minimization of the lowest energy conformation of the transoid form gave dihedral angles of 65.9 and 64.4° for the two ester groups, which are in very good agreement with the observed conformation in the crystalline state. The dihedral angles of the second low energy conformer are 64.4 and 67.2°.