Crystal structure of 1-ferrocenyl-2-(4-nitrophenyl)ethyne

The title ferrocene derivative including an alkyne and a para-nitrophenyl substitution crystallizes in the P21/n space group. In the ferrocene unit, the pentadienyl (Cps) rings are in an eclipsed conformation. Strong intermolecular π–π-stacking, CH(Cp)—C(Cp), and O(p-nitrophenyl)—HC(Cp) interactions consolidate the crystal structure.

The title ferrocene derivative, [Fe(C 5 H 5 ) 2 (C 8 NO 2 )], including an alkyne bonded to a para-nitrophenyl substituent, which was synthesized from a copper-free Sonogashira cross-coupling reaction between ethynylferrocene and 4-bromo-1nitrobenzene, crystallizes in the P2 1 /n space group. In the ferrocene unit, the pentadienyl (Cps) rings are in an eclipsed conformation. The angle of rotation between the substituted cyclopentadienyl ring and the p-nitrophenyl group is 6.19 (10) , yielding a quasi-linear extension of the ferrocenyl substitution. Important intermolecular interactions arise fromstacking between the Cp rings and the p-nitrophenyl, from corners of the Cp rings that are perpendicularly aligned, and between the O atoms from the nitro substituent and carbons at the corners of the Cp rings, propagating along all three crystallographic axes.

Chemical context
Recent efforts in the field of medicinal organometallic chemistry have been driven by a high interest in the synthesis of metal ethynyl complexes, particularly because of their biological activity (Gö rmen et al., 2012). In addition, phenylethyne-derived compounds display active electrochemical properties such as the generation of stable redox forms, regeneration at low potentials and good electrochemical reversibility (Gasser & Metzler-Nolte, 2012). 1-Ferrocenyl-2-(4-nitrophenyl)ethyne has previously been prepared in moderate-to-high yields (52-92%) by applying Sonogashira coupling reactions. However, all of them used 4-iodo-1nitrobenzene or 4-triflate-1-nitrobenzene and a variety of solvents, catalysts and conditions, under an inert atmosphere. The reaction time varied from 25 min to 4 h (Torres et al., 2002;Shoji et al., 2014;Li et al., 2009;Fu et al., 2008;Coutouli-Argyropoulou et al., 2003). Other approaches involved the use of iodoferrocene and 4-ethynyl-1-nitrobenzene (Kulhá nek et al., 2013). Our approach focuses on performing copper-free Sonogashira coupling between ethynylferrocene and 4-bromo-1-nitrobenzene without the need of inert atmosphere protocols and obtaining moderate-to-high yields, by following green chemistry protocols.   Macrae et al., 2020) shows the molecular structure of the title compound, which crystallizes in space group P2 1 /n. The substituted ferrocene (Fc) system is linked to a p-nitrobenzene moiety by an acetylenic bridge between C11 and C12 with a bond distance of 1.202 (2) Å , which is comparable to those in similar complexes, e.g. 1.202 (2) Å (Misra et al. 2014), 1.197 (3) Å (Fu et al., 2008), and 1.193 (2) Å (Zora et al. 2006). The unit cell is comprised of four molecules with one molecule present per asymmetric unit. The substituted Cp and phenyl rings are almost parallel to each other, subtending a dihedral angle of 6.19 (10) , in contrast to (phenyl-ethynyl)ferrocene (Zora et al., 2006), which has no substituent in the para position and exhibits a nearly perpendicular dihedral angle of 89.06 (3) . The distances of the Fe1 atom from the centroids of the substituted and unsubstituted Cp rings are 1.6461 (8) and 1.6584 (8) Å , respectively. The Cg1-Fe1-Cg2 angle is 179.27 , where Cg1 and Cg2 are the centroids of substituted and unsubstituted Cp rings, respectively. The Cp rings in the ferrocene system are thus almost parallel, since the angle between the Cp ring planes is 1.03 (13) . In addition, the Cp rings display a nearly eclipsed conformation with a slight deviation, as demonstrated by the average C-Cg1-Cg2-C torsion angle of 12.26 . The C-C bond distances in the Cp rings range from 1.417 (2) to 1.436 (2) Å , while the Fe-C bond lengths range between 2.038 (2) and 2.055 (2) Å .

Supramolecular features
The title compound exhibitsstacking interactions between one of the Cp rings from the Fc moiety and the p-nitrophenyl substituent, allowing the formation of a zigzag structure; atom pairs involved relate C6(Cp) and C7(Cp) to C17(p-nitrophenyl) and C18(p-nitrophenyl) of a neighboring molecule, with short contacts of 3.340 (2) and 3.397 (2) Å , respectively. This interaction can be described as pairs of molecules being interrupted by two C3(Cp)Á Á ÁH8-C8(Cp) interactions from a different interconnected pair of perpendicularly oriented Fc moieties with short contact distances of 2.83 Å each. Short contacts from neighboring molecules establishing a distinctive interconnected pair between a corner of the Cp ring and one of the oxygen atoms from the p-nitrophenyl substituent yield a closed arrangement of atoms. Short contacts involve H6-C6(Cp)Á Á ÁO1(p-nitrophenyl) at a distance of 3.461 (2) Å . Another interconnection is found between adjacent p-nitrophenyl groups, yielding a ring arrangement involving pairs from H17-C17(p-nitrophen- Acta Cryst. (2020). E76, 1403-1406 research communications Figure 1 Molecular structure of the title compound, including atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx; Ày þ 1; Àz þ 1; (ii) Àx À 1; Ày þ 1; Àz þ 1; (iii) Àx; Ày þ 1; Àz.

Figure 2
Crystal packing of the title compound along the a axis with short-contact interactions shown as dashed lines.

Hirshfeld Surface Analysis
CrystalExplorer17 ( Table 2 gives the dihedral angles for previously reported compounds; our compound having the second lowest dihedral angle and a nearly parallel conformation. Exchanging the hydrogen atoms in the methyl group for fluorine atoms shifts the dihedral angle from 1.01 to 90.00 in the case of methyl and trifluoromethyl substituents, respectively.

Synthesis and crystallization
The Full (a) and individual (b)-(j) two-dimensional fingerprint plots showing the nine intermolecular contacts present in the crystal structure.

Table 2
The effect of the substituent on the dihedral angle ( ) between the substituted Cp ring and the phenyl ring in compounds containing a 1ferrocenyl-2-phenylethyne backbone and a para-substituted phenyl ring.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were included in geometrically calculated positions, C-H = 0.93 Å , and refined as riding on their parent C atom with U iso (H) = 1.2U eq (C).

Funding information
The

Special details
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.