Synthesis, crystal structure and Hirshfeld surface analysis of 2-(perfluorophenyl)acetamide in comparison with some related compounds

The title compound was synthesized by a new method at the interphase of aqueous solutions of LiOH and pentafluorophenylacetonitrile. In the crystal, hydrogen bonds and π–halogen interactions connect the molecules into double layers. The Hirshfeld surfaces of analogues of the title compound were compared and the effect of perfluorination on the crystal packing was shown.

The molecular and crystal structures of the title compound, C 8 H 4 F 5 NO, were examined by single-crystal X-ray diffraction and Hirshfeld surface analysis. The title compound was synthesized by a new method at the interface of aqueous solutions of LiOH and pentafluorophenylacetonitrile. In the crystal, hydrogen bonds and -halogen interactions connect the molecules into double layers. Analysis of the Hirshfeld surface showed that the most important contributions to the crystal packing are made by FÁ Á ÁF (30.4%), CÁ Á ÁF/FÁ Á ÁC (22.9%), OÁ Á ÁH/ HÁ Á ÁO (14.9%), HÁ Á ÁF/FÁ Á ÁH (14.0%) and HÁ Á ÁH (10.2%) contacts. The Hirshfeld surfaces of analogues of the title compound were compared and the effect of perfluorination on the crystal packing was shown.

Chemical context
The development of effective methods for the formation of an amide bond CONR 2 is of great importance because of the high synthetic value of amides, their industrial applications and pharmacological interest (Massolo et al., 2020). The addition of functional groups such as -F, -Cl etc. can improve the catalytic or biological activity of the corresponding coordination compounds (Naumann, 2003).

Structural commentary
The title compound crystallizes in the space group P2 1 /c with four molecules in the unit cell. All H atoms in the phenyl ring are replaced by fluorine atoms. The asymmetric unit is illustrated in Fig. 1. Carbon atom C7 is in the plane of the imidazole ring. The acetamide group is twisted relative to the Ph-ring with a C2-C1-C7-C8 torsion angle of 107.61 (14) .

Supramolecular features
The hydrogen-bond system is shown in Fig. 2. In the structure, there are three hydrogen bonds. Two relatively strong hydrogen bonds are formed between the amino group and the oxygen atom of the carbonyl group. The shortest hydrogen bond N2-H2BÁ Á ÁO1 ii [symmetry code: (ii) x, Ày + 3 2 , z À 1 2 ] is 2.8795 (14) Å (Table 1). The structure also contains one short contact of the type C-HÁ Á ÁF with a C7-H7BÁ Á ÁF1 iii [symmetry code: (iii) x, Ày + 1 2 , z À 1 2 ] distance of 3.3764 (15) Å , but this cannot be called a hydrogen bond (Howard et al., 1996). The structure contains a short contact between the fluorine atoms F4 and F4 iv [symmetry code: (iv) Àx + 1, Ày + 2, Àz + 1], whose length of 2.6649 (18) Å is shorter then the sum of the van der Waals radii (Mantina et al., 2009). However, according to the recommendations of Cavallo et al. (2016), it cannot be considered to be a halogen bond.

Figure 3
Crystal packing of 1 showing the double layers.

Figure 1
Molecular structure of the title compound, including atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Hirshfeld surface analysis
Crystal Explorer 21 was used to calculate the Hirshfeld surfaces and two-dimensional fingerprint plots (Spackman et al., 2021). The donor-acceptor groups are visualized using a standard (high) surface resolution and the d norm surfaces are mapped over a fixed colour scale of À0.542 (red) to 1.121 (blue) a.u., as illustrated in Fig. 4a. The most important hydrogen bonds, N2-H2AÁ Á ÁO1 i are N2-H2BÁ Á ÁO1 ii , are shown by red spots on the surface. A weak red spot may indicate the presence of a -interaction between the C atom of the ring and the F atom of another ring. There are nostacking interactions of the molecules, which can be seen from the absence of characteristic triangles in Fig. 4b. However, a bright spot on the shape-index surface may indicate the presence of a -halogen interaction. The overall two-dimensional fingerprint map for the title compound is shown in Fig. 5a. The fingerprint plots show that the FÁ Á ÁF contacts (30.4%) make the largest contribution to the overall packing of the crystal. Contacts of the CÁ Á ÁF/FÁ Á ÁC type also make a significant contribution (22.9%). This can also be related to the presence of a -F interaction in the structure. The short distances C1Á Á ÁF2 iii [symmetry code: (iii) x, Ày + 1 2 , z À 1 2 ] and C4Á Á ÁF5 v [symmetry code: (v) x, Ày + 3 2 , z + 1 2 ], which are 3.079 (2) and 3.110 (2) Å , respectively, confirm this fact (Novikov et al., 2021;Zhuo et al., 2014). The OÁ Á ÁH/HÁ Á ÁO and HÁ Á ÁF/FÁ Á ÁH hydrogen bonds also make a significant contribution to the Hirshfeld surface area (28.9% in total). In addition, van der Waals interactions (HÁ Á ÁH) contribute 10.2%. The contribution of other contacts is less than 8% in total and is not discussed in this work.
An analogous methodology was applied to construction of Hirshfeld surfaces for similar benzenamide derivatives with one Cl substituent and an unsubstituted six-membered ring. The effect of perfluorination is evident from the comparison made in Fig. 6. On going from the title compound 1 to compound 3, the halogen bonds disappear. Moreover, if only one hydrogen atom in the ring is replaced by chlorine, the proportion of halogen and -halogen bonds is significantly reduced. However, in the transition from compound 1 to 3, the contribution of van der Waals interactions increases.     compound with a fluorine-substituted phenyl ring similar to the title compound. In 3 (SAWHAC; Skelton et al., 2017), the H atoms in the phenyl ring are not substituted, and this compound crystallizes in space group C2/c different from 1. In 2 (OCETAT; Ma et al., 2011) the hydrogen atom in the paraposition to the acetamide group is substituted with chlorine. FIXCEV (Davis et al., 2005) contains a chlorine atom at the meta-position and a hydroxo-group at the para-position to the amido group. However, as a result of the presence of a hydroxo group, a different system of hydrogen bonds and packing is present in the structure, as is the case in the structure of BHXPAM10 (McMillan et al., 1975), where there are two bromine atoms and two hydroxo groups.

Synthesis and crystallization
A saturated aqueous solution of LiOH (0.5 mL, at 298 K) and 2,3,4,5,6-pentafluorophenylacetonitrile (0.5 mL) were placed in a 1.5 mL screw-neck vial. The closed vial was shaken in a water bath at 383 K until the organic phase turned dark red (30 min). The closed vial with the resulting two-phase system was left for three days at 298 K, and slow growth of the crystal phase at the interface of water-2,3,4,5,6-pentafluorophenylacetonitrile was observed. The obtained crystals were identi-fied as 2-(pentafluorophenyl)acetamide by X-ray structural analysis.

2-(2,3,4,5,6-Pentafluorophenyl)acetamide
Crystal data 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.