Herringbone array of hydrogen-bonded ribbons in 2-ethoxy­benzamide from high-resolution X-ray powder diffraction

In 2-ethoxy­benzamide, C₉H₁₁NO₂, the amide substituents are linked into centrosymmetric head-to-head hydrogen-bonded dimers. Additional hydrogen bonds between adjacent dimers give rise to ribbon-like packing motifs, which extend along the c axis and possess a third dimension caused by twisting of the 2-ethoxy­phenyl substituent with respect to the hydrogen-bonded amide groups. The ribbons are arranged in a T-shaped herringbone pattern and cohesion between them is achieved by van der Waals forces.

S1. Comment

2-Ethoxybenzamide, (I), also called ethenzamide, is a compound with pharmaceutical applications as an analgesic and antipyretic (Kawano et al., 1978; Darias et al., 1992) in cold medications (Okamoto et al., 2005). Besides its medicinal use, it is interesting from the crystal engineering point of view for information regarding the usual packing modes of benzamides, which often involve the formation of head-to-head hydrogen-bonded dimers (Braga et al., 1999).

A search of the Cambridge Structural Database (CSD, Version 5.30; Allen, 2002) yielded 245 benzamide hits. Often having no, or rather small substituents, like benzamide, halobenzamides, o- and m-methylbenzamide and o-nitrobenzamide, the amide dimers arrange themselves into hydrogen-bonded ribbons using the remaining two H atoms of the two –NH₂ groups in each dimer and the unused oxygen lone pairs of the two carbonyl groups. In almost all cases, these ribbons run parallel to a cell axis with a dimension of about 5 Å (exceptionally 10 or 15 Å), given approximately by the projections of the donor–acceptor hydrogen-bond distances between dimers, plus the intramolecular amide N···O distance, onto the corresponding lattice direction. Ocassionally, the hydrogen-bonded amide groups are not coplanar, forming the core of a `ribbon', but rather in zigzag orientations, and the hydrogen-bonded packing motif produced forms the core of a flat object similar to a sheet, e.g. in m-chlorobenzamide (Hattori et al., 1975). Only exceptionally does the packing not involve hydrogen-bonded dimers, but rather a hydrogen-bonded chain, e.g. p-nitrobenzamide (Jones et al., 2002) and m-bromobenzamide (Kato et al., 1967). Occasionally, the dimers are isolated, e.g. in p-chlorobenzamide (Hayashi et al., 1980); other examples with isolated dimers include cases with rather bulky substituents (Aakeroy et al., 2007), in chlathrates (Reddy et al., 2002) and coordination compounds (Aakeroy et al., 2005). More complex hydrogen-bonding arrangements form if additional hydrogen-bonding donors or acceptors are present, e.g. in o-acetamidobenzamide (Errede et al., 1981).

It is interesting to note that the parent compound, benzamide, shows three polymorphic forms. This polymorphism was first reported by Wöhler & von Liebig (1832), who reported the existence of two polymorphic forms. The crystal structure of the thermodynamically stable form I was solved in 1959 (Penfold & White, 1959), and form II was solved in 2005 (Bladgen et al., 2005; David et al., 2005) from synchrotron X-ray powder diffraction. In addition, the conversion of form II to form I was reported. The structure of form III was reported in 2007 (Thun et al., 2007). In all structures, the benzamide molecules form head-to-head hydrogen-bonded dimers, which are also arranged in ribbons as described above, extending along the shortest axis of their unit cells (around 5 Å) but differently packed (Thun et al., 2007). The three forms present shifted π–π stacks and T-shaped interactions. In form II, the two molecules in the dimer are not related by inversion symmetry and considerable disorder is present in one of the molecules (Bladgen et al., 2005; David et al., 2005).

This work reports the crystal structure of 2-ethoxybenzamide, (I), obtained from its room-temperature high-resolution X-ray powder diffraction pattern. Bond distances and angles in (I) are as expected from the chemical bonding. Fig. 1 shows the molecular structure, the refined isotropic displacement parameters and the labelling scheme used.

The crystal structure of (I) is composed of centrosymmetric hydrogen-bonded amide dimers, located around the inversion centrs at (1/2, 0, 0) and (0, 1/2, 1/2). Lattice translations generate two ribbon-like packing motifs, oriented differently and both extending along the c axis, of 5.05009 (6) Å, wherein the dimers are connected into ribbons by additional hydrogen bonds formed between the carbonyl O atom and the –NH₂ H atoms of adjacent amide groups, in a nearly coplanar array of amide C, N and O. The hydrogen-bonded array and the ribbon-like packing motif are shown in Fig. 2. Hydrogen-bonding parameters are summarized in Table 1.

Cohesion between the ribbons in the crystal structure of (I) is achieved by van der Waals forces. The packing of the ribbons to extend the structure perpendicular to the c axis can be described as a T-shaped arrangement, wherein the `edges' of one ribbon point to the `planes' of adjacent ribbons, forming the herringbone array shown in Fig. 3.

Regarding the molecular conformation of (I) in the solid state, the ethoxy substituent is almost coplanar with the phenyl ring, forming an angle of 8.2 (11)° (H-atom positions excluded), whereas the amide–phenyl dihedral angle is 50.4 (14)°. This latter feature gives the ribbon a bounded third dimension, since the two phenyl groups in each dimer protrude at this angle from the plane of the hydrogen-bonded amide groups, with the 2-ethoxy substituents pointing in opposite directions (Fig. 2). It can be noted that the ribbons in form I of benzamide have a lesser three-dimensional character, since the angle between the amide and phenyl groups is 26° (Penfold & White, 1959). Similar considerations apply to forms II and III.

Similar packing in P2₁/a, with a shortest axis of around 5 Å, was found in o-nitrobenzamide (Fujimori et al., 1972).

S2. Experimental

A powder sample of (I) was obtained from Sigma–Aldrich and used as received. The crystallites had a needle-like habit, with the longest axis ranging from 50–140 µm, as seen under the optical microscope.

S3. Refinement

The powder diffraction pattern was indexed without impurity peaks using the program DICVOL04 (Boultif & Louër, 2004) to a monoclinic lattice with a = 14.2227 Å, b = 12.0525 Å, c = 5.0527 Å, β = 96.552°, V = 860.47 ų and M₂₀ = 35.7. A Le Bail fit (Le Bail et al., 1988) was carried out using the program GSAS (Larson & Von Dreele, 2004) and confirmed the lattice found. The Le Bail agreement factors are R_wp = 0.0553 and χ² = 1.6. The space group P2₁/n was determined from the observation of the systematic absences and Z was estimated to 4 from the estimated density (around 1.3 Mg m^-3). The molecular geometry used was optimized using the program MOPAC2009 (Stewart, 2009).

The crystal structure was solved using the simulated annealing global optimization algorithm (Kirkpatrick et al., 1983) implemented in the computer code PSSP (Powder Structure Solution Program; P. W. Stephens & S. Pagola; https://powder.physics.sunysb.edu/programPSSP/pssp.html), using the correlated integrated intensities of the first 53 Bragg reflections.

The atomic positions of the H atoms were calculated using WinGX (Farrugia, 1999). All atomic positions were refined using soft bond-length and bond-angle restraints. The isotropic displacement parameters of all non-H atoms were refined independently, whereas those of the H atoms were constrained to 1.2 times the value of the parent atom. At an intermediate Rietveld refinement stage, the identities of the amide O and N atoms were switched and the bond-distance restraints changed accordingly, in order to obtain all positive displacement parameters.

The following parameters were refined: scale factor, background coefficients, lattice parameters, 2θ zero error, peak profile parameters, atomic coordinates and isotropic displacement parameters. The standard uncertainties of the atomic coordinates were corrected using the procedure described by Scott (1983). The final Rietveld refinement graph is shown in Fig. 4.

Computing details

Data collection: X16C beamline software; cell refinement: GSAS (Larson & Von Dreele, 2004); data reduction: GSAS (Larson & Von Dreele, 2004); program(s) used to solve structure: PSSP (https://powder.physics.sunysb.edu/programPSSP/pssp.html)'; program(s) used to refine structure: GSAS (Larson & Von Dreele, 2004); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and Mercury (Version 2.2; Macrae et al., 2006); software used to prepare material for publication: publCIF (Westrip, 2009).

Figures

	Fig. 1. The molecular structure of (I), showing the isotropic displacement parameters determined from the powder diffraction experiment at 50% probability level. H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. The amide hydrogen-bonding pattern (dashed lines) forming a ribbon-like packing motif in (I).
	Fig. 3. The herringbone array of hydrogen-bonded ribbons in (I), viewed approximately along [001].
	Fig. 4. Rietveld refinement of the powder diffraction pattern of (I). Observed intensities are represented with dots, the calculated profile is shown with a solid line and the difference plot is at the bottom. Tick marks represent allowed peak positions.

2-ethoxybenzamide

Crystal data

C9H11NO2	Z = 4
Mr = 165.19	F(000) = 352.0
Monoclinic, P21/n	Dx = 1.278 Mg m^−3
Hall symbol: -P 2yn	Synchrotron radiation, λ = 0.69847 Å
a = 14.2141 (2) Å	µ = 0.11 mm^−1
b = 12.0446 (1) Å	T = 298 K
c = 5.05009 (6) Å	Particle morphology: needles (fine powder)
β = 96.5811 (8)°	white
V = 858.89 (2) Å^3	cylinder, 8 × 1.5 mm

Data collection

Huber adapted diffractometer	Scan method: step
Specimen mounting: glass capillary	2θmin = 1.0°, 2θmax = 35.0°, 2θstep = 0.005°
Data collection mode: transmission

Refinement

Least-squares matrix: full	96 parameters
Rp = 0.053	60 restraints
Rwp = 0.063	11 constraints
Rexp = 0.046	Secondary atom site location: inferred from neighbouring sites
RBragg = 0.055	H-atom parameters constrained
R(F^2) = 0.07312	Weighting scheme based on measured s.u.'s
χ^2 = 2.103	(Δ/σ)max = 0.07
? data points	Background function: GSAS background function number 2 (cosine Fourier series) with 36 terms.
Excluded region(s): 1-3.5° (diffraction peaks not present)	Preferred orientation correction: none
Profile function: pseudo-Voight (Thompson et al., 1987) with asymmetry correction (Finger et al., 1994)

Special details

Experimental. sample as received from Sigma-Aldrich loaded into a glass capillary.

Refinement. The refinement of the atomic positions was carried out subjected to bond distance and angle restraints. A total of 23 bond lengths and 37 bond angles were restrained to the values obtained in the geometry optimization. The weight factors were sequentially decreased from 1 to 0.001 for angles and 0.5 for distances, respectively. The isotropic thermal displacement parameters of non-H atoms were refined independently, whereas the corresponding displacement parameters of bonded H atoms were constrained to 1.2 times the value of the bonded non-H atom. The error between phenyl and amide or ethoxy least square planes was estimated from the difference between the values measured with the program Mercury from the refinement reported here and other fit previously calculated. The background coeficients were not refined in the last refinement cycle.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
C1	0.7113 (7)	0.8338 (9)	0.079 (2)	0.026 (6)*
C2	0.7309 (7)	0.7457 (10)	−0.088 (2)	0.029 (6)*
C3	0.8167 (9)	0.6874 (11)	−0.044 (3)	0.045 (6)*
C4	0.8811 (8)	0.7174 (11)	0.174 (3)	0.037 (6)*
C5	0.8663 (8)	0.8104 (11)	0.328 (3)	0.047 (7)*
C6	0.7788 (8)	0.8633 (9)	0.293 (2)	0.029 (6)*
C7	0.6257 (7)	0.9043 (11)	0.0377 (19)	0.031 (6)*
C8	0.6670 (8)	0.6167 (9)	−0.432 (3)	0.054 (7)*
C9	0.5761 (8)	0.6136 (11)	−0.619 (3)	0.049 (6)*
O1	0.5946 (7)	0.9399 (9)	−0.1830 (19)	0.030 (4)*
O2	0.6577 (7)	0.7189 (8)	−0.2791 (19)	0.036 (4)*
N1	0.5737 (8)	0.9198 (11)	0.249 (2)	0.044 (5)*
H1A	0.5283	0.9674	0.2351	0.0526*
H1B	0.5773	0.8710	0.374	0.0526*
H3	0.8257	0.6230	−0.140	0.0543*
H4	0.9380	0.6790	0.206	0.0438*
H5	0.9110	0.8298	0.469	0.0560*
H6	0.7721	0.9309	0.378	0.0353*
H8A	0.6734	0.5550	−0.3073	0.0642*
H8B	0.7237	0.6149	−0.5219	0.0642*
H9A	0.5827	0.5606	−0.758	0.0582*
H9B	0.5635	0.6857	−0.696	0.0582*
H9C	0.5247	0.5916	−0.5225	0.0582*

Geometric parameters (Å, º)

C1—C2	1.403 (15)	C5—H5	0.93
C1—C6	1.406 (15)	C4—H4	0.93
C1—C7	1.479 (15)	C3—H3	0.93
C2—C3	1.403 (17)	C6—H6	0.93
C2—O2	1.374 (14)	C8—H8A	0.97
C3—C4	1.40 (2)	C8—H8B	0.97
C5—C6	1.391 (16)	C9—H9A	0.96
C4—C5	1.393 (19)	C9—H9B	0.96
O2—C8	1.467 (15)	C9—H9C	0.96
C8—C9	1.511 (18)	N1—H1A	0.86
O1—C7	1.228 (14)	N1—H1B	0.86
N1—C7	1.379 (14)
C2—C1—C6	119.2 (9)	C5—C6—H6	119
C2—C1—C7	124.5 (9)	C2—C3—H3	120
C6—C1—C7	116.2 (9)	C4—C3—H3	120
C1—C2—C3	120.8 (10)	C6—C5—H5	120
C1—C2—O2	114.4 (9)	C4—C5—H5	120
C3—C2—O2	124.6 (11)	C3—C4—H4	119
C1—C6—C5	120.2 (10)	C5—C4—H4	119
C2—C3—C4	118.4 (12)	O2—C8—H8A	107.8
C6—C5—C4	119.2 (12)	O2—C8—H8B	113.1
C3—C4—C5	121.4 (12)	C9—C8—H8A	113.1
C1—C7—O1	122.2 (10)	C9—C8—H8B	113.8
C1—C7—N1	118.3 (9)	C8—C9—H9A	109
O1—C7—N1	119.1 (11)	C8—C9—H9B	109
C2—O2—C8	117.4 (10)	C8—C9—H9C	110
O2—C8—C9	103.2 (9)	C7—N1—H1A	120
C1—C6—H6	119	C7—N1—H1B	119

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1A···O1^i	0.86	2.06	2.917 (16)	171
N1—H1B···O1^ii	0.86	2.37	2.860 (14)	116

Symmetry codes: (i) −x+1, −y+2, −z; (ii) x, y, z+1.