Crystal structure of (E)-1,2-diferrocenyl-1,2-bis(furan-2-yl)ethene

The title compound is the product of a new synthetic route towards tetraaryl/hetaryl-substituted ethenes that reduces the occurrence of side-products. In the crystal, the molecule is centrosymmetric and the cyclopentadienyl rings are nearly coplanar and aligned slightly closer to a staggered conformation than to an eclipsed one.

The title compound, [Fe 2 (C 5 H 5 ) 2 (C 20 H 14 O 2 )], is the product of a new synthetic route towards tetraaryl/hetaryl-substituted ethenes that reduces the occurrence of side-products. In the crystal, the molecule is centrosymmetric and the cyclopentadienyl (Cp) rings are nearly coplanar and aligned slightly closer to a staggered conformation than to an eclipsed one. The ethene plane is tilted by 32.40 (18) with respect to that of the substituted Cp ring and by 63.19 (19) with respect to that of the furan ring. C-HÁ Á Á interactions link the molecules into a three-dimensional supramolecular framework.

Chemical context
Tetrasubstituted ethenes bearing aryl, hetaryl or ferrocenyl groups are of current interest, as many of them find applications as novel materials for photooptics, electronics, crystal engineering and as new medications (Astruc, 2017). Ethene derivatives with a ferrocenyl unit on one or both C atoms of the alkene deserve special attention. Prominent representatives of the first type are ferrocifene {1-[4-(2-dimethylaminoethoxy)phenyl]-1-phenyl-2-ferrocenylbut-1-ene} and its di-OH analogue, which are known as potent, organometallic antitumor drugs (Jaouen et al., 2015;Resnier et al., 2017). On the other hand, dimethyl (Z)-2,3-diferrocenylbut-2-enedioate displays interesting redox and solvatochromic properties (Solntsev et al., 2011). As typical procedures for the preparation of tetrasubstituted ethenes containing a ferrocenyl substituent, conversions of the corresponding ketones under the McMurry reaction conditions (Top et al., 1997) or reductive coupling using low-valent titanium agents are recommended (Dang et al., 1990). In both cases, the reported yields are satisfactory to good, but a serious disadvantage is the formation of side-products. Recently, we reported a new approach to tetraaryl/hetaryl-substituted ethenes via desilylation of 2-(trimethylsilyl)-4,4,5,5-tetraaryl/hetaryl-1,3-dithiolanes, obtained from diaryl/hetaryl thioketones by treatment with (trimethylsilyl)diazomethane (TMS-CHN 2 ) at low temperature (Mlostoń et al., 2017). The mechanism of this unusual conversion was explained by the assumption that the in situ-generated 1,3-dithiolane anion undergoes a spontaneous cycloelimination ([3 + 2]-cycloreversion) to give the dithioformate anion and the corresponding tetrasubstituted ethene derivative. The same method was applied for the preparation of some ferrocenyl/hetaryl-substituted ethenes (Mlostoń et al., 2018). ISSN 2056-9890 Here we report the analogous synthesis and crystal structure of the known title compound, (E)-1, with m.p. 485-487 K. For the previously described synthesis of this product (Dang et al., 1990), a m.p. of 489-491 K and a yield of 17% were reported and the authors tentatively assigned the (E)-configuration to the obtained compound. In our case, single crystals of (E)-1 were grown from hexane/CH 2 Cl 2 and used for an X-ray diffraction analysis, from which the previous tentatively postulated structure of the obtained isomer could be confirmed.

Structural commentary
The molecule of (E)-1 sits across a crystallographic centre of inversion and is shown in Fig. 1. Within the asymmetric unit, the Fe atom sits very well centred between the cyclopentadienyl (Cp) rings with all Fe-C distances in the range 2.0352 (17)-2.0712 (16) Å . The Cp C-C bond lengths [mean 1.435 (2) Å ] involving the substituted C atom, C6, are very slightly elongated compared with the other C-C distances [mean 1.418 (3) Å ]. Other bond lengths and angles are unremarkable. The two Cp rings are aligned slightly closer to a staggered conformation than to an eclipsed one, with the ring rotation from perfectly eclipsed being 20.6 (2) (18 is the halfway point between eclipsed and staggered). The dihedral angle between the planes of the two Cp rings in the ferrocenyl entity is only 4.08 (11) and ethene atom C1 is coplanar with the Cp ring to which it is bonded. However, the ferrocenyl entity is tilted with respect to the ethene plane, with a dihedral angle between the plane of the substituted Cp ring and that of the ethene plane of 32.40 (18) . The dihedral angle between the substituted Cp ring and the adjacent furan ring is 53.46 (11) , while that between the plane of the furan ring and the ethene plane is 63.19 (19) . The planes of the two furan rings are necessarily parallel because of the centre of inversion.

Supramolecular features
There are no significant C-HÁ Á ÁO orinteractions, but some weak C-HÁ Á Á interactions are present (Table 1). C8-H of the substituted Cp ring has an edge-on intermolecular interaction with the unsubstituted Cp ring at x + 1 2 , Ày + 3 2 , z À 1 2 . The extension of this interaction through the molecular centre of inversion leads to sheets of molecules, which lie parallel to the (101) plane (Fig. 2). The furan ring, via C3-H, has an edge-on intermolecular C-HÁ Á Á interaction with the substituted Cp ring at x, y, z + 1. This interaction leads to double-stranded chains or ladders, in which the molecule acts as the ladder rungs; the chains run parallel to the [001] Acta Cryst. (2018). E74, 625-629 research communications Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
The sheets of molecules lying parallel to the (101) plane formed by the C8-HÁ Á Á interactions.

Figure 1
The molecular structure of the title compound, (E)-1, showing the atomnumbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code: (i) Àx + 1, Ày + 1, Àz + 1. direction (Fig. 3). Finally, C10-H of the substituted Cp ring interacts intramolecularly with the -system of the furan ring at Àx + 1, Ày + 1, Àz + 1 on the opposite side of the molecule. This latter interaction is quite short, but has a sharp angle at the H atom (Table 1), so the arrangement might just be a consequence of the molecular conformation. The molecular inversion symmetry, in combination with the two types of intermolecular interactions, links the molecules into a threedimensional supramolecular framework.

Synthesis and crystallization
The title compound was prepared according to the reaction sequence presented in the scheme below. A solution of thioketone 2 (297 mg, 1 mmol; prepared according to Mlostoń et al., 2015) in THF (3 ml) was cooled to 198 K (acetone/dry ice). Then, TMS-CHN 2 was added portion-wise to the mixture until the green colour of the starting thioketone disappeared. The magnetically stirred reaction mixture was allowed to warm slowly to ca 268 to 273 K and at this temperature a commercially available solution of tetrabutylammonium fluoride (TBAF, 1 ml, 1 M) was added in small portions. Stirring was continued for 20 min, and after warming to room temperature, the solvent was evaporated under vacuum. The crude product was analyzed by 1 H NMR spectroscopy, which revealed the presence of two isomeric ethenes in a ratio of ca 10:1. After column chromatography (SiO 2 , CH 2 Cl 2 /hexane 3:7), the major product was isolated, contaminated with a small admixture of the minor one, as an analytically pure sample (78% yield). After additional crystallization from a hexane/CH 2 Cl 2 mixture, 285 mg (54%) of pure (E)-1 were isolated as orange crystals with m.p. 485-487 K. From this material, crystals suitable for the X-ray diffraction measurements were separated without additional recrystallization.

Special details
Experimental. Data collection and full structure determination done by Prof. Anthony Linden: anthony.linden@chem.uzh.ch Solvent used: hexane / dichloromethane Cooling Device: Oxford Instruments Cryojet XL Crystal mount: on a glass fibre Frames collected: 1290 Seconds exposure per frame: 10.0 Degrees rotation per frame: 1602.0 Crystal-detector distance (mm): 55.0 Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq