Crystal structures of two C,N-disubstituted acetamides: 2-(4-chlorophenyl)-N-(2-iodophenyl)acetamide and 2-(4-chlorophenyl)-N-(pyrazin-2-yl)acetamide

A combination of N—H⋯O and C—H⋯O hydrogen bonds together with C—Cl⋯π(arene) and C—I⋯π(arene) interactions links the molecules of 2-(4-chlorophenyl)-N-(2-iodophenyl)acetamide into twofold interwoven sheets, and the molecules of 2-(4-chlorophenyl)-N-(pyrazin-2-yl)acetamide are linked into complex sheets built solely from hydrogen bonds.


Structural commentary
The molecular conformations of compounds (I) and (II), illustrated in Figs. 1 and 2, respectively, can be defined in terms of the torsional angles N1-C1-C2-C21, 141.8 (3) and 129.22 (18) respectively, and by the dihedral angles between the central spacer unit, atoms N1,C1,O1,C2, and the two independent rings. The dihedral angles to the chlorinated ring (C21-C26) are 80.02 (11) and 61.74 (6) in (I) and (II); those to the iodinated ring in (I) and the pyrazinyl ring in (II) are 67.48 (11) and 5.86 (11) , respectively. This difference is probably associated with the participation in the intermolecular hydrogen bond of both N atoms of the pyrazinyl ring in (II), as discussed below. The molecules of (I) and (II) do not therefore exhibit any internal symmetry, so that they are conformationally chiral: the centrosymmetric space groups ISSN 2056-9890 confirm that each compound has crystallized as a conformational racemate.
In the pyrazine ring of compound (II) the four independent C-N distances span a range of only ca 0.01 Å , indicating that this ring is fully aromatic.
Because the repeat unit of this sheet in the [010] direction spans two unit cells, there are in fact two such sheets present, related to one another by a unit translation along [010]: the deep puckering of the sheets (Fig. 5) means that the two independent sheets are interwoven. The structure of (I) also contains a short IÁ Á ÁO contact with dimension I12Á Á ÁO1 i The molecular structure of compound (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level.

Figure 2
The molecular structure of compound (II), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I).
Cg2 is the centroid of the C21-C26 ring.

Figure 3
Part of the crystal structure of compound (I) showing the formation of a hydrogen-bonded chain of rings running parallel to the [001] direction. Hydrogen bonds are shown as dashed lines and, for the sake of clarity, the H atoms bonded to the C atoms which are not involved in the motif shown have been omitted. The atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions (x, 1 2 À y, 1 2 + z) and (x, 1 2 À y, À 1 2 + z), respectively.
The hydrogen-bonded supramolecular assembly in compound (II) is more complex than that in compound (I): molecules of (II) are linked into complex sheets by a combination of N-HÁ Á ÁN, C-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds, weakly augmented by two C-HÁ Á Á(arene) hydrogen bonds (Table 2): hydrogen bonds of N-HÁ Á ÁO type, often observed in the structures of amides, are absent, however. The formation of this structure can readily be analysed in terms of two simple sub-structures in one-and two-dimensions (Ferguson et al., 1998a,b;Gregson et al., 2000). In the simpler of the sub-structures, a combination of N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonds links molecules which are related by the 2 1 screw axis along (x, 3 4 , 1 2 ) into a C(4)C(5)[R 2 2 (7)] chain of rings running parallel to the [100] direction ( Fig. 6). A more complex one-dimensional sub-      Part of the crystal structure of compound (II) showing the formation of a hydrogen-bonded chain of rings running parallel to the [010] direction and built from N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonds, shown as dashed lines. For the sake of clarity, the C-bound H atoms which are not involved in the motifs shown have been omitted. The atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions ( 1 2 + x, 3 2 À y, 1 À z) and (À 1 2 + x, 3 2 À y, 1 À z), respectively. structure results from the combination of the N-HÁ Á ÁN, C-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds, in the form of a ribbon containing alternating R 2 2 (7) and R 4 4 (22) rings (Fig. 7). The combination of these two chains along [100] and [010] generates a sheet lying parallel to (001) in the domain 1 4 < z < 3 4 , and a second such sheet, related to the first by inversion, lies in the domain 3 4 < z < 5 4 . The C-HÁ Á Á(arene) interactions both lie within the sheet.

Synthesis and crystallization
For the synthesis of compounds (I) and (II), equimolar quantities (1.0 mmol of each component) of (4-chlorophenyl)acetic acid and either 2-iodoaniline for (I), or 2-aminopyrazine for (II), were dissolved in dichloromethane (20 ml) in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (0.01 mol) and triethylamine (0.02 mol) at 273 K. The mixtures were stirred at 273 K for 3 h, and then poured with stirring into an excess of aqueous hydrochloric acid (4 mol dm À3 ). The aqueous mixtures were exhaustively extracted with dichloromethane and in each case, the combined organic extracts were washed first with saturated aqueous sodium hydrogencarbonate solution and then with brine. The solutions were dried with anhydrous sodium sulfate and then the solvent was removed under reduced pressure, to give the products.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were located in Part of the crystal structure of compound (II) showing the formation of a hydrogen-bonded ribbon of R 2 2 (7) and R 4 4 (22) rings running parallel to the [100] direction and built from N-HÁ Á ÁN, C-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds, shown as dashed lines. For the sake of clarity, the C-bound H atoms which are not involved in the motifs shown have been omitted. Table 2 Hydrogen-bond geometry (Å , ) for (II).
difference Fourier maps. The C-bound H atoms were then treated as riding atoms in geometrically idealized positions with C-H distances 0.93 Å (aromatic and hetero-aromatic) or 0.97 Å (CH 2 ) and with U iso (H) = 1.2U eq (C). For the H atoms bonded to N atoms in compound (II), the atomic coordinates were refined with U iso (H) = 1.2U eq (N) giving the N-H distance shown in Table 2; an attempt to refine similarly the corresponding H-atom coordinates in compound (I) led to an unsatisfactorily low value, 0.74 (3) Å for the N-H distance, possibly associated with the presence of the strongly scattering iodene atom: accordingly this distance was thereafter fixed at 0.86 Å . A small number of low-angle reflections, which had been attenuated by the beam stop [ (100) and (200)

(I) 2-(4-Chlorophenyl)-N-(2-iodophenyl)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.