2,3-Dieth­oxy-9,10-anthraquinone

Anthraquinone derivatives form an important class of dyes and are also known for their medicinal properties. Recently, 2,3-disubstituted anthraquinones have been shown unexpectedly to jellify various organic solvents. No information on the packing mode of these derivatives was known. Here, the first X-ray structure of a 2,3-disubstituted anthraquinone is reported, namely 2,3-dieth­oxy-9,10-anthraquinone, C₁₈H₁₆O₄. The merit of this study lies in the observation of significant differences between the packing in 9,10-anthraquinone, which displays a herring-bone arrangement, and that in the title 2,3-dieth­oxy derivative, in which the mol­ecules lie on parallel crystallographic morror planes separated by a distance of 3.4081 (1) Å, reminiscent of the graphite layer architecture.

S1. Comment

In recent decades, the field of thermoreversible physical gels formed from low molar mass organogelators (LMOGs; Abdallah & Weiss, 2000) has been the subject of renewed interest (Terech & Weiss, 1997; Weiss & Terech, 2005). A number of new systems (Terech & Weiss, 1997) belonging to a variety of functionalities have been discovered. One class of organogelators, consisting of 2,3-dialkoxy-9,10-anthraquinones, (1_n) (see scheme; 8 ≤ n ≤ 12), was found readily to form yellow gels in ethanol, heptane or acetonitrile (Clavier et al., 1998). These gels are soft materials made of a self-assembled fibrillar network imprisoning a liquid. One of the main questions regarding the structure of these aggregates is best studied by X-ray crystallography (Ostuni et al., 1996; Abdallah et al., 2000; Ballabh et al., 2003). Usually, the gel-forming molecules do not yield good crystals, as was found to be the case for (1_n) (8 ≤ n ≤ 12) [Please check added text]. However, on shortening the chain length (i.e. for n = 2), we obtained single crystals suitable for X-ray analysis. No molecular and crystal structure of any 2,3-disubstituted-9,10-anthraquinones has been determined so far. The first structure of a member of this important series, the title compound, 2,3-diethoxy-9,10-anthraquinone, (1₂), is reported here and is compared with that of the parent compound to examine how this substitution affects the packing arrangement.

The presence of the two substituents induces a change of crystal space group, from P2₁/c in the anthraquinone to C2/m in the present work. The molecule is perfectly planar, with all atoms except the H atoms of the methyl groups located in mirror planes (4i) (Fig. 1). It is noticeable that, despite their flexibility, the substituents lie in the anthraquinone substrate plane. Owing to their location in mirror planes parallel to the b axis, the molecules are stacked parallel to each other with an interplanar distance of 3.41(s.u.?) Å. Thus, the molecules form a unidirectional columnar packing, which is reminiscent of the packing of hexagonal graphite (3.40 Å; Pauling, 1945). Such packing results in π–π stacking occurring between the substituted benzene rings, whereas the non-substituted benzenes do not exhibit such an interaction (Table 1, Fig. 2).

The overlap between closest molecules occurs by symmetry around a twofold screw axis. Two consecutive molecules in the same plane exhibit a head-to-tail orientation. This is also the case for closest molecules in the stacking direction. This feature appears clearly in Fig. 2, where four molecules in the unit cell are represented. A salient feature is that the quinoid ring is superimposable on the ethoxy substituents of a neighouring molecule. The intermolecular distance between the O atom of the C═O group and the C atom of the ethoxy group O9···C31 = 3.476(s.u.?), and O10···C21 = 3.483(s.u.?) Å, whereas the distance of O9 from the O atom of the ethoxy group O9···O3 = 3.625(s.u.?) Å, and O10···O2 = 3.903(s.u.?) Å. For comparison, the same intermolecular distances between two molecules in the same plane were found to be: O atom of C═O group to C atom (CH₂) of OCH₂CH₃ = 3.554(s.u.?) Å; C atom of C═O group to O atom of the OCH₂CH₃ = 5.760(s.u.?) Å. Finally, the calculated O···O distance between the two closest carbonyl groups was found to be 3.752(s.u.?) Å. This value is smaller than the shortest equivalent distance in the anthraquinone lattice.

The crystal structure of the anthraquinone (2) has been determined and redetermined a number of times in order to carry out different analyses, and better refinements have been obtained recently (Fu & Brock, 1998; Slouf, 2002, and references therein). It was found to crystallize in the monoclinic system (space group P2₁/c) with two molecules per unit cell. Each molecule is arranged around an inversion centre located in the middle of the quinone ring. As seen in Fig. 3, the packing shows a herring-bone arrangement where the two closest molecules are slightly staggered relative to each other and overlap at a distance of 3.49(s.u.?) Å, which is completely different from the title compound. One observes a mutual overlap between the quinoid (central) ring of one molecule and the benzenoid (lateral) ring of the other which results in the formaton of a slipped π–π stacking arrangement (Table 1). In the unit cell, the calculated distances between the closest O atoms are 3.90(s.u.?), 5.14(s.u.?) and 5.19(s.u.?) Å, respectively.

Please give s.u.s for all quoted distances.

Thus, our work has showed that the presence of two ethoxy substituents along the long axis of the 9,10-anthraquinone substrate induces a fundamental change in the molecular arrangement in the crystal, significantly affecting the intermolecular parameters. It seems to be worth examining the influence of chain lengthening on these properties and the possible analogy of (1₂) with graphite.

Related literature

For related literature, see: Abdallah & Weiss (2000); Ballabh et al. (2003); Clavier et al. (1998); Desvergne et al. (2005); Fu & Brock (1998); Ostuni et al. (1996); Pauling (1945); Slouf (2002); Terech & Weiss (1997); Weiss & Terech (2005).

S2. Experimental

The preparation of (1₂) (m.p. 445 K [343 K in CIF data tables - please clarify]) was described by Desvergne et al. (2005). Single crystals were obtained by slow evaporation of a solution of (1₂) in a mixture of ethanol and dichloromethane (Solvent ratio?).

S3. Refinement

H atoms were generated geometrically and treated as riding on their parent atoms, with C—H = 0.93 (aromatic), 0.96 (methyl) or 0.97 Å (methylene) and with U_iso(H) = 1.2U_eq(C) or 1.5U_eq(methyl C).

Computing details

Data collection: COLLECT (Nonius, 1999); cell refinement: DENZO (Otwinowski & Minor, 1997); data reduction: SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEPIII (Burnett & Johnson, 1996), ORTEP-3 for Windows (Farrugia, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: publCIF (Westrip, 2008).

Figures

	Fig. 1. The molecular structure of (12), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are represented as small spheres of arbitrary radii.
	Fig. 2. A partial packing view of (12), showing the head-to-tail arrangement of the molecules and the π–π interactions which develop along the b axis. H atoms have been omitted for clarity.
	Fig. 3. A partial packing view of 9,10-anthraquinone, (2), showing the herring-bone arrangement. H atoms have been omitted for clarity.

2,3-diethoxy-9,10-anthraquinone

Crystal data

C18H16O4	F(000) = 624
Mr = 296.30	Dx = 1.310 Mg m^−3
Monoclinic, C2/m	Melting point: 445 K
Hall symbol: -C 2y	Mo Kα radiation, λ = 0.71073 Å
a = 17.8007 (3) Å	Cell parameters from 2934 reflections
b = 6.8163 (2) Å	θ = 2–27.5°
c = 13.9992 (3) Å	µ = 0.09 mm^−1
β = 117.809 (1)°	T = 293 K
V = 1502.42 (6) Å^3	Plate, yellow
Z = 4	0.37 × 0.10 × 0.10 mm

Data collection

Nonius Kappa CCD diffractometer	1835 independent reflections
Radiation source: fine-focus sealed tube	1376 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.015
Detector resolution: 9 pixels mm^-1	θmax = 27.4°, θmin = 4.2°
CCD scans	h = −22→20
Absorption correction: empirical (using intensity measurements) SCALEPACK (Otwinowski & Minor 1997)	k = 0→8
Tmin = 0.959, Tmax = 0.989	l = 0→18
3353 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.049	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.149	H-atom parameters constrained
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0912P)^2 + 0.1475P] where P = (Fo^2 + 2Fc^2)/3
1835 reflections	(Δ/σ)max < 0.001
135 parameters	Δρmax = 0.23 e Å^−3
0 restraints	Δρmin = −0.19 e Å^−3

Special details

Experimental. A yellow crystal 0.37 × 0.10 × 0.10 mm was mounted on an Nonius KappaCCD diffractometer and analysed using monochromated Mo Kα X-ray radiation (λ = 0.7073 Å). Crystal is monoclinic; space group C2/m. All non-H atoms were refined anisotropically in F². H atoms were generated geometrically and treated as riding on their parent atoms with C—H = 0.93 Å(aromatic), 0.96Å (methyl) and 0.97 Å(methylene) with U_iso(H)= 1.2U_iso(C) or U_iso(H)= 1.5Uiso(Cmethyl).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.17213 (10)	0.0000	0.44422 (13)	0.0528 (4)
H1	0.1137	0.0000	0.4191	0.063*
C2	0.22587 (10)	0.0000	0.55355 (12)	0.0507 (4)
O2	0.20059 (7)	0.0000	0.63146 (9)	0.0611 (4)
C3	0.31442 (10)	0.0000	0.59182 (12)	0.0498 (4)
O3	0.36107 (7)	0.0000	0.70053 (8)	0.0622 (4)
C4	0.34706 (10)	0.0000	0.51911 (12)	0.0512 (4)
H4	0.4055	0.0000	0.5442	0.061*
C5	0.29888 (13)	0.0000	0.13948 (15)	0.0712 (5)
H5	0.3572	0.0000	0.1635	0.085*
C5a	0.29184 (10)	0.0000	0.40798 (12)	0.0479 (4)
C6	0.24355 (15)	0.0000	0.02997 (15)	0.0794 (6)
H6	0.2649	0.0000	−0.0193	0.095*
C6a	0.20454 (10)	0.0000	0.37048 (12)	0.0494 (4)
C7	0.15788 (16)	0.0000	−0.00623 (15)	0.0847 (7)
H7	0.1209	0.0000	−0.0800	0.102*
C8	0.12629 (14)	0.0000	0.06676 (15)	0.0798 (6)
H8	0.0679	0.0000	0.0418	0.096*
C9	0.14435 (11)	0.0000	0.25381 (13)	0.0573 (4)
O9	0.06755 (8)	0.0000	0.22070 (10)	0.0790 (5)
C9a	0.18047 (11)	0.0000	0.17694 (13)	0.0579 (4)
C10	0.32791 (10)	0.0000	0.33156 (13)	0.0546 (4)
O10	0.40435 (8)	0.0000	0.36335 (10)	0.0758 (4)
C10a	0.26744 (11)	0.0000	0.21405 (12)	0.0550 (4)
C21	0.11112 (11)	0.0000	0.59751 (14)	0.0616 (5)
H21A	0.0846	−0.1156	0.5545	0.074*	0.50
H21B	0.0846	0.1156	0.5545	0.074*	0.50
C22	0.10190 (13)	0.0000	0.69965 (16)	0.0741 (6)
H22A	0.0447	−0.0342	0.6825	0.111*	0.50
H22B	0.1148	0.1282	0.7317	0.111*	0.50
H22C	0.1404	−0.0940	0.7496	0.111*	0.50
C31	0.45175 (11)	0.0000	0.74686 (13)	0.0670 (5)
H31A	0.4712	0.1156	0.7245	0.080*	0.50
H31B	0.4712	−0.1156	0.7245	0.080*	0.50
C32	0.48472 (13)	0.0000	0.86741 (14)	0.0849 (7)
H32A	0.5438	−0.0349	0.9024	0.127*	0.50
H32B	0.4534	−0.0935	0.8861	0.127*	0.50
H32C	0.4780	0.1284	0.8905	0.127*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0448 (8)	0.0744 (10)	0.0428 (8)	0.000	0.0234 (6)	0.000
C2	0.0534 (9)	0.0656 (10)	0.0423 (8)	0.000	0.0300 (7)	0.000
O2	0.0525 (7)	0.0978 (9)	0.0415 (6)	0.000	0.0291 (5)	0.000
C3	0.0500 (8)	0.0645 (9)	0.0370 (7)	0.000	0.0221 (6)	0.000
O3	0.0502 (6)	0.1033 (10)	0.0349 (5)	0.000	0.0214 (5)	0.000
C4	0.0483 (8)	0.0688 (10)	0.0410 (7)	0.000	0.0246 (6)	0.000
C5	0.0710 (11)	0.1041 (15)	0.0497 (9)	0.000	0.0376 (8)	0.000
C5a	0.0509 (8)	0.0594 (9)	0.0392 (7)	0.000	0.0259 (7)	0.000
C6	0.0900 (15)	0.1143 (17)	0.0465 (10)	0.000	0.0424 (10)	0.000
C6a	0.0493 (8)	0.0625 (9)	0.0401 (7)	0.000	0.0240 (7)	0.000
C7	0.0858 (14)	0.1293 (19)	0.0383 (9)	0.000	0.0284 (9)	0.000
C8	0.0631 (11)	0.1304 (18)	0.0426 (9)	0.000	0.0219 (8)	0.000
C9	0.0498 (8)	0.0803 (11)	0.0425 (8)	0.000	0.0220 (7)	0.000
O9	0.0493 (7)	0.1367 (13)	0.0489 (7)	0.000	0.0211 (6)	0.000
C9a	0.0584 (9)	0.0783 (11)	0.0392 (8)	0.000	0.0246 (7)	0.000
C10	0.0537 (9)	0.0733 (10)	0.0438 (8)	0.000	0.0284 (7)	0.000
O10	0.0525 (7)	0.1321 (12)	0.0505 (7)	0.000	0.0306 (6)	0.000
C10a	0.0582 (10)	0.0705 (11)	0.0415 (8)	0.000	0.0277 (8)	0.000
C21	0.0531 (9)	0.0890 (12)	0.0532 (9)	0.000	0.0336 (7)	0.000
C22	0.0720 (12)	0.1062 (15)	0.0638 (11)	0.000	0.0482 (10)	0.000
C31	0.0499 (9)	0.1098 (15)	0.0408 (8)	0.000	0.0206 (7)	0.000
C32	0.0628 (11)	0.145 (2)	0.0406 (9)	0.000	0.0188 (8)	0.000

Geometric parameters (Å, º)

C1—C2	1.375 (2)	C7—H7	0.9300
C1—C6a	1.398 (2)	C8—C9a	1.386 (2)
C1—H1	0.9300	C8—H8	0.9300
C2—O2	1.3602 (18)	C9—O9	1.222 (2)
C2—C3	1.408 (2)	C9—C9a	1.489 (2)
O2—C21	1.4339 (19)	C9a—C10a	1.384 (2)
C3—O3	1.3515 (18)	C10—O10	1.2186 (19)
C3—C4	1.388 (2)	C10—C10a	1.487 (2)
O3—C31	1.432 (2)	C21—C22	1.513 (2)
C4—C5a	1.400 (2)	C21—H21A	0.9700
C4—H4	0.9300	C21—H21B	0.9700
C5—C6	1.382 (3)	C22—H22A	0.9600
C5—C10a	1.396 (2)	C22—H22B	0.9600
C5—H5	0.9300	C22—H22C	0.9600
C5a—C6a	1.389 (2)	C31—C32	1.506 (2)
C5a—C10	1.482 (2)	C31—H31A	0.9700
C6—C7	1.364 (3)	C31—H31B	0.9700
C6—H6	0.9300	C32—H32A	0.9600
C6a—C9	1.477 (2)	C32—H32B	0.9600
C7—C8	1.377 (3)	C32—H32C	0.9600
C2—C1—C6a	120.62 (14)	C6a—C9—C9a	117.64 (15)
C2—C1—H1	119.7	C10a—C9a—C8	119.56 (16)
C6a—C1—H1	119.7	C10a—C9a—C9	120.87 (14)
O2—C2—C1	125.01 (14)	C8—C9a—C9	119.56 (17)
O2—C2—C3	115.17 (13)	O10—C10—C5a	121.49 (14)
C1—C2—C3	119.82 (13)	O10—C10—C10a	120.83 (14)
C2—O2—C21	117.78 (12)	C5a—C10—C10a	117.68 (14)
O3—C3—C4	125.35 (14)	C9a—C10a—C5	119.19 (15)
O3—C3—C2	114.76 (12)	C9a—C10a—C10	121.37 (13)
C4—C3—C2	119.89 (13)	C5—C10a—C10	119.44 (16)
C3—O3—C31	118.73 (12)	O2—C21—C22	106.26 (14)
C3—C4—C5a	119.87 (14)	O2—C21—H21A	110.5
C3—C4—H4	120.1	C22—C21—H21A	110.5
C5a—C4—H4	120.1	O2—C21—H21B	110.5
C6—C5—C10a	120.17 (18)	C22—C21—H21B	110.5
C6—C5—H5	119.9	H21A—C21—H21B	108.7
C10a—C5—H5	119.9	C21—C22—H22A	109.5
C6a—C5a—C4	120.12 (14)	C21—C22—H22B	109.5
C6a—C5a—C10	120.80 (14)	H22A—C22—H22B	109.5
C4—C5a—C10	119.08 (14)	C21—C22—H22C	109.5
C7—C6—C5	120.44 (17)	H22A—C22—H22C	109.5
C7—C6—H6	119.8	H22B—C22—H22C	109.5
C5—C6—H6	119.8	O3—C31—C32	105.97 (14)
C5a—C6a—C1	119.69 (14)	O3—C31—H31A	110.5
C5a—C6a—C9	121.63 (14)	C32—C31—H31A	110.5
C1—C6a—C9	118.69 (14)	O3—C31—H31B	110.5
C6—C7—C8	119.80 (17)	C32—C31—H31B	110.5
C6—C7—H7	120.1	H31A—C31—H31B	108.7
C8—C7—H7	120.1	C31—C32—H32A	109.5
C7—C8—C9a	120.8 (2)	C31—C32—H32B	109.5
C7—C8—H8	119.6	H32A—C32—H32B	109.5
C9a—C8—H8	119.6	C31—C32—H32C	109.5
O9—C9—C6a	121.70 (15)	H32A—C32—H32C	109.5
O9—C9—C9a	120.65 (15)	H32B—C32—H32C	109.5
C6a—C1—C2—O2	180.0	C5a—C6a—C9—C9a	0.0
C6a—C1—C2—C3	0.0	C1—C6a—C9—C9a	180.0
C1—C2—O2—C21	0.0	C7—C8—C9a—C10a	0.0
C3—C2—O2—C21	180.0	C7—C8—C9a—C9	180.0
O2—C2—C3—O3	0.0	O9—C9—C9a—C10a	180.0
C1—C2—C3—O3	180.0	C6a—C9—C9a—C10a	0.0
O2—C2—C3—C4	180.0	O9—C9—C9a—C8	0.0
C1—C2—C3—C4	0.0	C6a—C9—C9a—C8	180.0
C4—C3—O3—C31	0.0	C6a—C5a—C10—O10	180.0
C2—C3—O3—C31	180.0	C4—C5a—C10—O10	0.0
O3—C3—C4—C5a	180.0	C6a—C5a—C10—C10a	0.0
C2—C3—C4—C5a	0.0	C4—C5a—C10—C10a	180.0
C3—C4—C5a—C6a	0.0	C8—C9a—C10a—C5	0.0
C3—C4—C5a—C10	180.0	C9—C9a—C10a—C5	180.0
C10a—C5—C6—C7	0.0	C8—C9a—C10a—C10	180.0
C4—C5a—C6a—C1	0.0	C9—C9a—C10a—C10	0.0
C10—C5a—C6a—C1	180.0	C6—C5—C10a—C9a	0.0
C4—C5a—C6a—C9	180.0	C6—C5—C10a—C10	180.0
C10—C5a—C6a—C9	0.0	O10—C10—C10a—C9a	180.0
C2—C1—C6a—C5a	0.0	C5a—C10—C10a—C9a	0.0
C2—C1—C6a—C9	180.0	O10—C10—C10a—C5	0.0
C5—C6—C7—C8	0.0	C5a—C10—C10a—C5	180.0
C6—C7—C8—C9a	0.0	C2—O2—C21—C22	180.0
C5a—C6a—C9—O9	180.0	C3—O3—C31—C32	180.0
C1—C6a—C9—O9	0.0