(E,E)-2,5-Di­methoxy-1,4-bis­[2-(3,4,5-tri­methoxy­phenyl)­ethenyl]­benzene

In the title compound, C₃₀H₃₄O₈, molecular symmetry is coincident with crystallographic inversion symmetry. A three-dimensional network is generated containing both C-H...[pi] and C-H...n(O) interactions. A comparison of the geometry of this mol­ecule and the structure of a number of 2,4,6-tri­methoxy-substituted analogues is provided.

S1. Comment

The title compound, (I) (Fig. 1), is a PPV [poly(p-phenylene vinylene)] oligomer that can be used as the electroactive material in conductimetric gas sensors (De Wit et al., 1998; Vanneste et al., 1998). A recent study of the crystal packing of a series of 2,4,6-trimethoxy-substituted PPV oligomers (Vande Velde et al., 2004) prompted us to investigate further the crystal structure of (I) in order to discover whether its packing would be comparable with that of these analogues, or rather more similar to the previously published 3,4,5-trimethoxy-substituted derivative lacking the methoxy groups on the central ring [Cambridge Structural Database (CSD; Allen, 2002) refcode JACBIY; Verbruggen et al., 1988]. \sch

Compound (I) forms crystals in which crystallographic inversion symmetry is coincident with molecular symmetry. Bond distances and angles are normal, and no profound effects due to libration or substitution can be identified. The molecule displays a peculiar Z-shape, which is obvious from Fig. 2. This shape can be expressed most conveniently via the relative orientations of the central and peripheral rings and the C31—C8 and C7—C8 bonds. The least-squares (LS) plane of the peripheral ring makes an angle of 18.2 (3)° with the LS plane of the central ring. The C31—C8 and C7—C8 bonds make angles of 6.5 (5) and 2.1 (5)°, respectively, with the LS plane of the central ring, and of 5.6 (5) and 16.2 (5)°, respectively, with the peripheral ring. The related 3,4,5-trimethoxy-substituted PPV oligomer, CSD refcode JACBIY, which lacks the methoxy groups on the central ring, displays a similar Z-shape (Verbruggen et al., 1988).

In addition, the molecules in JACBIY are packed edge-to-face, with a few stabilizing C—H···ν(O) contacts, which is fairly standard for distyrylbenzenes (Bartholomew et al., 2000). In contrast, the packing of (I) displays a large number of intermolecular close contacts, some of which can also be observed in the structures of the 2,4,6-substituted analogues (Vande Velde et al., 2004). These will now be discussed in detail.

As can be seen in Fig. 3, there are –OCH₃···π contacts (shown as dashed lines), where each molecule is donor in the interactions with two other molecules, and acceptor in those with two different ones, and the four of them are related to the first via glide planes. Thus, and in contrast with what was observed for the 2,4,6-analogues, these contacts are not reciprocated by their recipients and thus cannot be the reason for the observed layering in the stacking. The contacts can be designated C9—H9A····CgAⁱ [symmetry code: (i) 1 − x, y − 1/2, 1/2 − z], with C···CgA 2.84 (2) Å, C—H····CgA 111.8 (15)° and 3.375 (9) Å perp., where CgA is the centroid of the peripheral ring A, and the distance designated by `perp.' is that of the H atom to the LS plane of the ring. All contacts given in the discussion, unless specifically noted, use the coordinates of the H atoms from the CIF, which are not normalized to 1.083 Å. These contacts also give rise to the close contacts of atom H9A with atoms C35 and C36, given by H9A····C35ⁱ 2.76 (2) Å and C9—H9A—C35 127.5 (16)°, and H9A····C36ⁱ 2.66 (2) Å and C9—H9A—C36 139.7 (17)°, which are not shown in Fig. 3.

The observed layered stacking of (I) is due to the interactions detailed in Fig. 4, given by O2····H40Cⁱⁱ 2.67 (2) Å and C9—O2···H40C 118.3 (6)° [symmetry code: (ii) 1 + x, y, z], and C8····C8ⁱⁱⁱ 3.325 (9) Å and C7—C8···C8ⁱ Should this be C8ⁱⁱⁱ? 104.1 (4)° [symmetry code: (iii) −x, −y, −z]. The first of these contacts, which can be labelled C—H···ν(O), links the molecules into strings with overlapping backbones. It is quite interesting to note that the above-mentioned Z-shape of the molecules seems to originate specifically from the latter two interactions. This suggests that they play a rather important role in the determination of the packing mode. The methoxy groups that are involved are bent out of the plane of the ring they are attached to, possibly to relieve steric strain.

There are more of these C—H···ν(O) interactions present in the structure of (I), also indicated by dotted lines in Fig. 4, and these are given by O33····H39A^iv 2.67 (3) Å and C41—O33···H39A Should this be H39A^iv? 111.3 (7)°, and O2····H40B^iv 2.53 (3) Å and C2—O2···H40B Should this be H40B^iv? 116.5 (7)° [symmetry code: (iv) −x, 1/2 + y, 1/2 − z].

These C—H···ν(O) contacts link molecules through the central ring of the oligomer and thus diagonally through the unit cell. In this way, they form T-shaped contacts, in which the C39 and C40 methoxy groups exchange intermolecular C—H···ν(O) interactions with atoms O2 and O33. The O2····H40B Should this be H40B^iv? contact is about 0.2 Å shorter than the sum of the van der Waals radii of the participating atoms, which could explain why only the C40 terminal methoxy groups are bent so far out of the plane of the ring they are attached to: atom O34 lies 0.187 (4) Å out of the LS plane of the ring.

The last remaining short contact in the structure of (I) is given by H9B····H41A^v 2.37 (5) Å and C9—H9B···H41A Should this be H41A^v? 113 (2)° [symmetry code: (v) 1 + x, 1/2 − y, 1/2 + z]. This contact represents yet another way in which the molecules link up by means of their methoxy groups. It is quite likely that this close contact is of purely steric origin.

It is instructive to look also at the intramolecular C—H···ν(O) contacts, which are always present in these types of methoxy-substituted PPV oligomers. We have demonstrated previously that they are attractive contacts and contribute to the observed conformation and planarity of the molecule (Wu et al., 1996). In (I), there is only one ortho-methoxy group in the asymmetric unit, so there is only one such contact, given by O2····H7 2.3806 Å and C9—O2···H7 157°. After normalization of the C—H distance to 1.083 Å, O2····H7 becomes 2.344 Å. This contact fixes the central ring with respect to the double bond. The lack of this contact for the peripheral 3,4,5-trimethoxy-substituted ring may be a second important contributing factor to the Z-shape of the molecule. Indeed, when comparing the solid-state geometry with that obtained from a DFT/B3LYP/6–31G* calculation (Frisch et al., 2001) on an isolated molecule, we find that bond distances and angles are quasi-equal, but that the Z-shape has disappeared from the gas-phase structure. Furthermore, when we study the calculated angles between the C31—C8 and C7—C8 bond vectors and the LS plane of the central ring, we find values of 0.82° and 6.06°, respectively. The corresponding angles with the peripheral ring are 0.18° and 10.17°, respectively. The LS planes of the central and peripheral rings themselves are at an angle of 20.68°.

The introduction of 3,4,5-trimethoxy- instead of 2,4,6-trimethoxy-substitution on the peripheral rings appears to have a twofold effect. While it allows the peripheral rings to twist about 20° away from the co-planar arrangement, it also increases the end-to-end flexibility of the molecule, and this results in its deformation to the observed Z-shape by the packing environment. To support this theory, we utilized the CSD [version 1.6 (Allen, 2002) with the November 2003, and April and July 2004 updates], and found 20 compounds containing 29 different (3,4,5-trimethoxyphenyl)ethenyl moieties, and five compounds containing a (2,4,6-trimethoxyphenyl)ethenyl moiety [ten including our recent data (Vande Velde et al., 2004, and unpublished data]. It is clear that the angle the double-bond vector makes with the plane of the peripheral ring is generally closer to planar in the 2,4,6-trimethoxy-substituted moieties (mean 85.7°) than in the 3,4,5-substituted moieties (81.2°) (Fig. 5a), further corroborating the attractive nature of the C—H···ν(O) interaction and the role it plays in keeping the distyrylbenzene skeleton planar in these highly substituted derivatives. The angle of the C31—C8 bond vector with respect to the ring plane was also evaluated in the same compounds (Fig. 5 b), but it is in all cases smaller than or equal to 5°, which, combined with the small number of compounds that were evaluated, makes it difficult to draw conclusions. In short, in a 3,4,5-trimethoxy-substituted distyrylbenzene, the peripheral ring is more free to adapt to the other packing needs of the structure, and able to twist up to 20° away from the plane of the double bond, without insurmountable energetic requirements. This favours a C—H···ν(O) network instead of a layered OCH₃···π network for (I).

TLS tensor analysis (Shomaker & Trueblood, 1968) implemented in PLATON (Spek, 2003) shows that the translation and screw components are close to zero, as expected for a structure that is centrosymmetric, and that the libration along the axis parallel to the long axis of the molecule is about one order of magnitude larger than those along the other axes (L1 = 20.05 °², L2 = 0.96 °², L3 = 0.63 °²); its orientation deviates only 2.97° from the long axis. In JACBIY (Verbruggen et al., 1988), this same phenomenon was attributed to the large librational motion of the middle ring only, an explanation which is far less likely for (I), as the methoxy groups on the central ring display a number of stabilizing contacts comparable with those on the peripheral rings. Furthermore, the bond distances and angles of the central ring display no unusual deviations. It would be more logical that, in (I), the entire molecule librates along its long axis.

In conclusion, although the packing mode of (I) is similar to that of the previously described 2,4,6-trimethoxy-substituted derivatives, the most important intermolecular interactions in (I) are of the C—H···ν(O) type, as also observed by Bartholomew et al. (2000), and not of the –OCH₃···π type, as observed by us (Vande Velde et al., 2004) in the 2,4,6-trimethoxy derivatives. The reason for this must be found in the competition that exists between these weak interactions, which is fierce in an environment that is entirely controlled by them. Thus, a small difference in stabilization energy can make a very large difference in the resulting packing. The change from 2,4,6-trimethoxyphenyl to 3,4,5-trimethoxyphenyl, apart from steric considerations (in particular, the methoxy group in the 4-position of the peripheral ring standing perpendicular to the plane of this ring), has its main effect on the molecular geometry, by twisting the peripheral rings out of the plane formed by the central ring and the ethenylic bonds. This causes the molecule to adopt a Z-shape.

S2. Experimental

Compound (I) was prepared by the Wittig reaction between 3,4,5-trimethoxybenzaldehyde and 2,5-dimethoxy-p-xylylene-bis(triphenylphosphonium bromide), under conditions identical to those described for the 2,4,6-trimethoxy derivatives (Nowaczyk et al., 2004). Crystals of (I) were grown from a hot ethanol solution, and a single fragment was used for the diffraction experiment.

S3. Refinement

The H atoms were placed in calculated positions. Methoxy groups were allowed to rotate and the C—H distances to refine, but the angles between the H atoms were kept close to 109.5°. The final range of C—H distances was 0.9843–1.0311 Å and U_iso(H) values were constrained to 1.5U_eq(C). All other H atoms were treated as riding, with C—H = 0.93 Å and U_iso(H) = 1.2U_eq(C). Please check amended text. Software-specific language is to be avoided, for preference. The crystal is rather weakly diffracting, and this causes R_int to be higher than usual, due to the large amount of weak data. As the other quality indicators are satisfactory, we do not judge this to be truly problematic.

Computing details

Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Version 1.2.1; Bruno, 2002); software used to prepare material for publication: WinGX (Version 1.64; Farrugia, 1999) and PLATON (Spek, 2003).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacment ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A view of a molecule of (I), demonstrating its Z-shape in the crystal.
	Fig. 3. A view of the cell of (I), showing the OCH3···π interactions (dashed lines) between the middle-ring methoxy groups and the outer aromatic rings.
	Fig. 4. The C—H···ν(O) interactions (dotted lines) in the structure of (I).
	Fig. 5. Graphical representations of the population of known compounds with a trimethoxyphenylethenyl fragment (a) versus the angle their outer rings make with the double-bond vector and (b) versus the angle between the ring and the extra-annular single-bond vector.

(E,E)-2,5-Dimethoxy-1,4-bis[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene

Crystal data

C30H34O8	F(000) = 556
Mr = 522.57	Dx = 1.321 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 25 reflections
a = 8.778 (3) Å	θ = 7.1–13.6°
b = 11.520 (5) Å	µ = 0.10 mm^−1
c = 14.717 (6) Å	T = 293 K
β = 118.05 (3)°	Block, yellow
V = 1313.5 (9) Å^3	0.3 × 0.2 × 0.2 mm
Z = 2

Data collection

Enraf-Nonius MACH3 diffractometer	Rint = 0.113
Radiation source: Enraf Nonius FR590	θmax = 25.0°, θmin = 2.4°
Graphite monochromator	h = −10→0
non–profiled ω/2θ scans	k = −13→13
5016 measured reflections	l = −15→17
2308 independent reflections	3 standard reflections every 60 min
1149 reflections with I > 2σ(I)	intensity decay: 2%

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)] = 0.053	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.137	H atoms treated by a mixture of independent and constrained refinement
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0533P)^2] where P = (Fo^2 + 2Fc^2)/3
2308 reflections	(Δ/σ)max < 0.001
180 parameters	Δρmax = 0.19 e Å^−3
0 restraints	Δρmin = −0.24 e Å^−3

Special details

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
C1	0.6081 (3)	0.5330 (2)	0.4575 (2)	0.0493 (7)
C2	0.4435 (3)	0.4904 (2)	0.3959 (2)	0.0513 (7)
C31	0.9746 (3)	0.6622 (2)	0.4116 (2)	0.0491 (7)
C32	1.1119 (3)	0.7329 (3)	0.4719 (2)	0.0532 (7)
C33	1.2034 (3)	0.7896 (2)	0.4303 (2)	0.0494 (7)
C34	1.1569 (3)	0.7746 (2)	0.3267 (2)	0.0480 (7)
C35	1.0245 (3)	0.6997 (2)	0.2669 (2)	0.0501 (7)
C36	0.9325 (3)	0.6421 (2)	0.3085 (2)	0.0508 (7)
C39	0.8432 (4)	0.6281 (4)	0.0975 (2)	0.0888 (12)
C40	1.3994 (4)	0.7987 (3)	0.3023 (3)	0.0817 (11)
C41	1.3997 (4)	0.8758 (4)	0.5892 (2)	0.0932 (13)
C6	0.6621 (3)	0.5423 (2)	0.5621 (2)	0.0537 (8)
C7	0.7165 (3)	0.5706 (2)	0.4104 (2)	0.0569 (8)
C8	0.8710 (4)	0.6146 (3)	0.4581 (2)	0.0574 (8)
C9	0.2354 (4)	0.4276 (3)	0.2282 (2)	0.0825 (11)
O2	0.3919 (2)	0.48319 (19)	0.29166 (15)	0.0694 (6)
O33	1.3367 (2)	0.86466 (19)	0.48269 (15)	0.0683 (6)
O34	1.2347 (2)	0.83976 (17)	0.28148 (15)	0.0640 (6)
O35	0.9928 (3)	0.6887 (2)	0.16696 (15)	0.0711 (6)
H32	1.1433	0.7427	0.5412	0.064*
H36	0.8443	0.5909	0.2685	0.061*
H39A	0.8483	0.5479	0.1218	0.133*
H39B	0.8369	0.627	0.0289	0.133*
H39C	0.740	0.6670	0.0933	0.133*
H40A	1.4805	0.8090	0.3761	0.123*
H40B	1.4411	0.8428	0.2608	0.123*
H40C	1.3917	0.7155	0.2844	0.123*
H41A	1.302	0.904	0.6038	0.140*
H41B	1.499	0.9354	0.6180	0.140*
H41C	1.444	0.7965	0.6241	0.140*
H6	0.7717	0.5711	0.6052	0.064*
H7	0.6693	0.5619	0.3394	0.068*
H8	0.9220	0.6162	0.5296	0.069*
H9A	0.2398	0.3455	0.2521	0.124*
H9B	0.2159	0.4276	0.1553	0.124*
H9C	0.1384	0.4698	0.2318	0.124*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0488 (16)	0.0517 (16)	0.0670 (19)	0.0035 (14)	0.0435 (14)	0.0056 (15)
C2	0.0588 (17)	0.0522 (16)	0.0602 (18)	−0.0002 (15)	0.0424 (15)	0.0023 (15)
C31	0.0502 (15)	0.0546 (16)	0.0601 (17)	0.0068 (15)	0.0404 (14)	0.0107 (15)
C32	0.0509 (16)	0.0659 (19)	0.0563 (17)	0.0013 (15)	0.0363 (14)	0.0032 (16)
C33	0.0452 (15)	0.0532 (16)	0.0600 (18)	0.0013 (14)	0.0331 (14)	0.0039 (16)
C34	0.0497 (15)	0.0517 (17)	0.0569 (18)	−0.0014 (14)	0.0370 (14)	0.0080 (14)
C35	0.0562 (16)	0.0575 (18)	0.0497 (17)	0.0017 (15)	0.0358 (14)	0.0061 (15)
C36	0.0452 (15)	0.0535 (17)	0.0627 (18)	−0.0046 (14)	0.0327 (14)	0.0034 (15)
C39	0.087 (2)	0.123 (3)	0.059 (2)	−0.026 (2)	0.0364 (18)	−0.015 (2)
C40	0.079 (2)	0.088 (2)	0.116 (3)	−0.0060 (19)	0.078 (2)	0.008 (2)
C41	0.079 (2)	0.134 (4)	0.068 (2)	−0.025 (2)	0.0365 (18)	−0.030 (3)
C6	0.0448 (15)	0.0584 (18)	0.069 (2)	−0.0029 (13)	0.0361 (15)	0.0051 (15)
C7	0.0554 (17)	0.0631 (19)	0.0690 (19)	−0.0021 (15)	0.0433 (15)	0.0074 (17)
C8	0.0571 (16)	0.0685 (19)	0.0666 (18)	0.0029 (16)	0.0456 (15)	0.0101 (17)
C9	0.074 (2)	0.109 (3)	0.075 (2)	−0.031 (2)	0.0437 (19)	−0.017 (2)
O2	0.0678 (13)	0.0901 (16)	0.0682 (14)	−0.0236 (13)	0.0469 (11)	−0.0073 (13)
O33	0.0671 (13)	0.0820 (15)	0.0662 (14)	−0.0200 (12)	0.0399 (11)	−0.0125 (12)
O34	0.0666 (12)	0.0691 (13)	0.0750 (14)	−0.0076 (11)	0.0488 (11)	0.0138 (11)
O35	0.0723 (14)	0.0965 (17)	0.0537 (12)	−0.0221 (12)	0.0373 (10)	−0.0031 (12)

Geometric parameters (Å, º)

O35—C35	1.368 (3)	C40—H40A	0.9875
O35—C39	1.412 (4)	C40—H40B	0.9875
O34—C34	1.378 (3)	C40—H40C	0.9875
O34—C40	1.411 (3)	C34—C33	1.391 (4)
C32—C31	1.378 (4)	C7—C8	1.302 (4)
C32—C33	1.382 (3)	C7—H7	0.9300
C32—H32	0.9300	C8—C31	1.477 (3)
O33—C33	1.365 (3)	C8—H8	0.9300
O33—C41	1.402 (3)	C6—C2^i	1.386 (3)
O2—C2	1.382 (3)	C6—H6	0.9300
O2—C9	1.401 (3)	C41—H41A	1.0311
C36—C35	1.390 (3)	C41—H41B	1.0311
C36—C31	1.402 (4)	C41—H41C	1.0311
C36—H36	0.9300	C9—H9A	1.0032
C1—C2	1.384 (4)	C9—H9B	1.0032
C1—C6	1.386 (4)	C9—H9C	1.0032
C1—C7	1.481 (3)	C39—H39A	0.9843
C2—C6^i	1.386 (3)	C39—H39B	0.9843
C35—C34	1.381 (4)	C39—H39C	0.9843
C35—O35—C39	118.3 (2)	C7—C8—C31	127.4 (3)
C34—O34—C40	113.7 (2)	C7—C8—H8	116.3
C31—C32—C33	120.6 (3)	C31—C8—H8	116.3
C31—C32—H32	119.7	C1—C6—C2^i	121.6 (3)
C33—C32—H32	119.7	C1—C6—H6	119.2
C33—O33—C41	118.2 (2)	C2^i—C6—H6	119.2
C2—O2—C9	118.3 (2)	O33—C33—C32	124.8 (2)
C35—C36—C31	119.0 (3)	O33—C33—C34	115.5 (2)
C35—C36—H36	120.5	C32—C33—C34	119.6 (3)
C31—C36—H36	120.5	C32—C31—C36	120.0 (2)
C2—C1—C6	117.5 (2)	C32—C31—C8	117.7 (3)
C2—C1—C7	119.9 (2)	C36—C31—C8	122.2 (3)
C6—C1—C7	122.6 (3)	O33—C41—H41A	109.5
O2—C2—C1	116.4 (2)	O33—C41—H41B	109.5
O2—C2—C6^i	122.6 (3)	H41A—C41—H41B	109.5
C1—C2—C6^i	121.0 (3)	O33—C41—H41C	109.5
O35—C35—C34	115.3 (2)	H41A—C41—H41C	109.5
O35—C35—C36	124.2 (3)	H41B—C41—H41C	109.5
C34—C35—C36	120.5 (2)	O2—C9—H9A	109.5
O34—C40—H40A	109.5	O2—C9—H9B	109.5
O34—C40—H40B	109.5	H9A—C9—H9B	109.5
H40A—C40—H40B	109.5	O2—C9—H9C	109.5
O34—C40—H40C	109.5	H9A—C9—H9C	109.5
H40A—C40—H40C	109.5	H9B—C9—H9C	109.5
H40B—C40—H40C	109.5	O35—C39—H39A	109.5
O34—C34—C35	119.5 (2)	O35—C39—H39B	109.5
O34—C34—C33	120.3 (3)	H39A—C39—H39B	109.5
C35—C34—C33	120.1 (2)	O35—C39—H39C	109.5
C8—C7—C1	126.7 (3)	H39A—C39—H39C	109.5
C8—C7—H7	116.7	H39B—C39—H39C	109.5
C1—C7—H7	116.7

Symmetry code: (i) −x+1, −y+1, −z+1.