Carbonyl-carbonyl inter­actions and amide [pi]-stacking as the directing motifs of the supra­molecular assembly of ethyl N-(2-acetyl­phenyl)oxalamate in a synperiplanar conformation

The title compound, C₁₂H₁₃NO₄, is one of the few examples that exhibits a syn conformation between the amide and ester carbonyl groups of the oxalyl group. This conformation allows the engagement of the amide H atom in an intra­molecular three-centred hydrogen-bonding S(6)S(5) motif. The compound is self-assembled by C=O...C=O and amide-[pi] inter­actions into stacked columns along the b-axis direction. The concurrence of both inter­actions seems to be responsible for stabilizing the observed syn conformation between the carbonyl groups. The second dimension, along the a-axis direction, is developed by soft C-H...O hydrogen bonding. Density functional theory (DFT) cal­cul­ations at the B3LYP/6-31G(d,p) level of theory were performed to support the experimental findings.

S1. Introduction

N-Acyl derivatives of 2-acetyl­aniline are useful inter­mediates; they have been transformed into quinazolines (Ferrini et al., 2007), xantate (Tran & Zard, 2014), indoles (Kothandaraman et al., 2011) and Tröger's base analogues (Slater et al., 2006). In particular, alkyl oxalamates or oxalamic acid esters, such as ethyl 2-(2-acetyl-6-meth­oxy­phenyl­amino)-2-oxo­acetate, are active as anti­allergics (Sellstedt et al., 1975).

Oxalamates, which are the amide and ester of oxalic acid, represent construction units with high in-plane hydrogen-bonding potential; they are self-complementary and capable of unidirectional inter­molecular hydrogen bonding. The NH and amide carbonyl groups are usually in a trans disposition, as also are the amide and ester carbonyl groups. Thus, the most common conformation is trans–anti. The phenyl­oxalamate group has become particularly important and is a versatile supra­molecular building block in the context of conformationally controlled molecular clefts (González-González et al., 2011) and molecular complexes with benzene-1,3-diols (González-González et al., 2014). Among several noncovalent inter­actions, hydrogen bonding is the most important element involved in the design of such self-assembled structures. Thus, knowledge of the structural factors that favour or avoid hydrogen bonding is important for the control of the molecular arrangement in the solid state. Ethyl oxalamates have been used as model compounds for the study of intra­molecular three-centre hydrogen bonding in solution (Gómez-Castro et al., 2008) and in the solid state (Gómez-Castro et al., 2014) by comparison among an N-acyl set of compounds (amide, oxalamate and oxalamide). In this context, the title compound, ethyl N-(2-acetyl­phenyl)­oxalamate, (I), is the missing structure that completes the series with an acetyl donor group ortho to the N-acyl group composed by compounds (I), N-(2-acetyl­pheny)acetamide (Slater et al., 2006) and N¹,N²-bis­(2-acetyl­phenyl)­oxalamide, (II) (Brewer et al., 2007a,b). Further reference to these structures is made in the following discussion.

S2. Experimental

S2.1. Synthesis and crystallization

Compound (I) was synthesized starting from commercial 2-acetyl­aniline and mono­ethyl­oxalyl chloride in equimolar qu­anti­ties, following a reported procedure (Gómez-Castro et al., 2008), and was crystallized from a saturated chloro­form solution.

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. The molecule of (I) is on a crystallographic mirror plane; thus, the occupancies of all non-H and some H atoms are 0.5. H atoms were included in geometrically calculated positions riding on the C and N atoms to which they are bonded. C—H bond distances were restrained to 0.93 (aromatic), 0.98 (methyl) and 0.99 Å (methyne). The N-bound H atom was found in a difference Fourier map and refined freely. H-atom displacement parameters were set at U_iso(H) = 1.2U_eq(C) for aromatic/methyne H atoms and 1.5U_eq(C) for methyl H atoms.

IR spectra were recorded neat at 298 K using a Varian 3100 FT–IR with an ATR system Excalibur Series spectrophotometer.

S2.3. Theoretical calculations

Geometry optimizations at the B3LYP/6-31G(d,p) level of theory were performed on the syn conformer of (I) without any symmetry restraints using the GAUSSIAN09 package (Frisch et al., 2009). Relaxed linear potential-energy surface scans for the C8—C9 bond rotation were performed using direct inversion of iterative subspace (GDIIS) (Farkas & Schlegel, 2002). Rotations were done in increments of 10° to complete 360°.

S3. Results and discussion

The whole molecule of (I) (Fig. 1) is on a crystallographic mirror plane, so the molecule is completely planar. Both carbonyl groups of the oxalyl group are in a syn disposition with respect to each other. This conformation is not observed frequently in oxalamic acid derivatives. Among the 30 organic oxalamates deposited in the Cambridge Structural Database (CSD, Version 5.34, November 2012; Groom & Allen, 2014), only 20% adopt the syn disposition and 77% adopt the anti disposition, while only 3% are perpendicular (Fig. 2). However, the syn conformation is observed in both N¹,N²-1,2-phenyl­enebis(oxalamic acid ethyl ester) (Martín et al., 2002; Muñoz et al., 2010) and its acid derivative (Souza et al., 2012), probably due to steric constraints.

In (I), the geometry of the oxalamate group (Table 2) is in agreement with a typical amide group (N7—C8═O8) and an ester (O9═C9—O10) group bonded by a single C8—C9 bond as in other phenyl­oxalamates (Martínez-Martínez et al., 1998). The C1—N7 bond [1.398 (2) Å] is longer than C8—N7 [1.350 (2) Å], indicating that the N7 lone pair is delocalized to the carbonyl amide group. The first distance is sensitive to the electronic properties of the ortho substituent on the phenyl ring: it is shorter when the group is electron-withdrawing [NO₂ = 1.378 (3) Å (Yin et al., 2003); OR (R = H, Me) = 1.41 (1) Å (Padilla-Martínez et al., 2001); H = 1.418(2 )Å (García-Báez et al., 2003)]. Thus, the value in (I) is in agreement with the electron-withdrawing character of the acetyl group and very similar to that observed in ethyl N-(2-benzoyl­phenyl)­oxalamate, (III) [1.399 (2) Å; Gómez-Castro et al., 2014].

Compound (I) forms an intra­molecular three-centre one-donor hydrogen bond, also known as a bifurcated hydrogen bond, constituted by three adjacent rings of S(6)S(5)S(6) type [see Bernstein et al. (1995) for graph-set notation]. The geometric parameters associated with hydrogen bonding are listed in Table 3. The geometries of the intra­molecular N7—H7···O13 [S(6)] and N7—H7···O10 [S(5)] hydrogen bonds, as well as the co-operative C6—H6···O8 inter­action [S(6)], are similar to those observed in compounds (II) and (III). It is worth noting that, as a consequence of the syn disposition between the amide and ester carbonyl groups in the oxalyl group, the three-centre O13(sp²)···H7···O10(sp³) hydrogen bond is formed instead of the expected O13(sp²)···H7···O9(sp²), when the conformation is anti, such as happens in (III). In spite of the differences in hybridization between the acceptor O atoms, the strengths of these three-centre hydrogen bonds seem to be similar to each other, judging by the NH-stretching IR frequency values of 3267 cm^-1 in (I) and 3274 cm^-1 in (III).

The involvement of the amide H atom in the three-centred hydrogen bond makes it unavailable for inter­molecular hydrogen bonding. Thus, the crystal packing of (I) is achieved by dipolar carbonyl–carbonyl (n→ π*) and amide–phenyl (π→ π*) inter­actions. The O atom of the amide carbonyl group acts as a donor of electron density to the C atom of the acetyl group, forming a C8═O8···C13ⁱⁱ dipolar inter­action [O8···C13 = 3.496 (2) Å, C8═O8···C13 = 99.6 (2)° and C13═O13···C8 = 50.5 (2)°; symmetry code: (ii) -x + 1, y - 1/2, -z + 1]. Given the geometry, this C═O···C═O inter­action is in agreement with the sheared-parallel type of motif.

Reported theoretical calculations estimate an inter­action energy effective even at large range, of -3.95 kcal mo1^-1 [1 kcal mol^-1 = 4.184 kJ mol^-1] at a distance of 3.62 Å (Allen et al., 1998). Furthermore, the amide–phenyl inter­action contributes to the stabilization of the crystal packing. The geometry of this last inter­action is in agreement with the π–π-type motif [Cg1···Cg2ⁱⁱ = 3.295 (2) Å, N7—C8···Cg1 = 74.1 (2)° and C8—N7···Cg1 = 82.8 (2)°; Cg1 is the centroid of the C1/C6 ring and Cg2 is the centroid of the N7—C8 bond]. It is worth noting that the Cg1···Cg2 distance measured for (I) is shorter than the values measured in other molecules, such as halogen-substituted phenyl­pyrazinamides (Khavasi & Tehrani, 2013) and amides (Cheng et al., 2014). This inter­action also participates in the supra­molecular structure of the P2₁/n space group polymorph of (II) (Brewer et al., 2007a), but it is absent in the other polymorph (P1 space group; Brewer et al., 2007b), whose supra­molecular strucuture is driven by C═O···A (A = CO, Ph) inter­actions. The inter­action energy of -1.52 kcal mol^-1 has been estimated elsewhere by ab initio molecular orbital calculations using the formamide–benzene complex as a model of amide–π inter­actions (Imai et al., 2009). The sum of the inter­action energies of the C═O···C═O and amide···π inter­actions of -5.47 kcal mol^-1 is enough to overcome the rotational barrier of the C8—C9 single bond between syn and anti conformations, estimated herein by ab initio theorical calculations to be 4.74 kcal mol^-1. The energy difference between the anti and syn conformers of (I) is only 1.63 kcal mol^-1 in favour of the anti conformer. The energy profile between the syn and anti conformers is displayed in Fig. 3.

Because of the symmetry of the crystal packing, infinite (···O8···C13···)_n and (···Cg1···Cg2···)_n stacked columns propagate along the a-axis direction (Fig. 4). The second dimension is developed by a soft C5—H5···O9ⁱ inter­action that links the stacked molecules along the a-axis direction (Fig. 5).

The formation of the intra­molecular three-centre hydrogen-bond motif S(6)S(5)S(6) in (I) was demonstrated based on the geometric parameters associated with the molecular structure. The syn conformation between the amide and ester carbonyl groups leads to the involvement of the O atom of the eth­oxy group in hydrogen bonding, instead of the ester carbonyl group. The combined effect of the N7—H7···O13, N7—H7···O10 and C6—H6···O8 inter­actions flattens the molecular structure, leading to an intra­molecular three-centred Osp²···H···Osp³ hydrogen bond which is as strong as the more often observed Osp²···H···Osp² hydrogen bond. It is thus capable of avoiding further inter­molecular hydrogen-bonding involvement of the amide H atom. Nevertheless, the supra­molecular architecture of (I) in one and two dimensions is developed by the concurrence of much weaker dipolar inter­actions, such as C═O···C═O, amide···π and C—H···O, and these inter­actions might be responsible for stabilizing the observed syn conformation between the carbonyl groups. Knowledge of the structures of syn-oxalamic acid derivatives and an understanding of the factors that stabilize them are important for controlling the supra­molecular assembly of these molecules in the context of crystal engineering.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and WinGX (Farrugia, 2012).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Dashed lines indicate intramolecular hydrogen bonding and the graph sets are indicated. [Added text OK?]
	Fig. 2. Possible conformers adopted by the oxalyl group as a result of single O═C—C═O bond rotation.
	Fig. 3. The energy profile of the C8—C9 single-bond rotation.
	Fig. 4. (a) A crystal packing diagram for (I), along the b-axis direction, showing carbonyl–carbonyl C8═O8···C13 dipolar interactions. (b) A view of the stacked columns in the ac plane, showing the amide–phenyl π–π*-type interactions [dashed lines]. [Are parts (a) and (b) the wrong way round? Part (a) shows Cg···Cg interactions. Part (b) does not seem to match either caption. Please check..
	Fig. 5. A crystal packing diagram for (I), along the a-axis direction. Dashed lines indicate hydrogen bonding. [Added text OK?]

Ethyl N-(2-acetylphenyl)oxalamate

Crystal data

C12H13NO4	Dx = 1.413 Mg m^−3
Mr = 235.23	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 600 reflections
a = 16.667 (3) Å	θ = 20–25°
b = 6.480 (1) Å	µ = 0.11 mm^−1
c = 10.241 (2) Å	T = 100 K
V = 1106.1 (3) Å^3	Block, colourless
Z = 4	0.44 × 0.18 × 0.14 × 0.15 (radius) mm
F(000) = 496

Data collection

Bruker APEXII CCD area-detector diffractometer	984 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.037
ϕ and ω scans	θmax = 25.2°, θmin = 2.3°
Absorption correction: for a sphere (Dwiggins, 1975)	h = −20→19
Tmin = 0.861, Tmax = 0.862	k = −7→7
10441 measured reflections	l = −12→12
1099 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.032	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.083	w = 1/[σ^2(Fo^2) + (0.0426P)^2 + 0.354P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max < 0.001
1099 reflections	Δρmax = 0.19 e Å^−3
108 parameters	Δρmin = −0.20 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc* = kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0051 (11)

Special details

Experimental. Absorption correction: Interpolation using Int. Tab. Vol. C (1992) p. 523, Tab. 6.3.3.3 for values of muR in the range 0–2.5, and Int. Tab. Vol. II (1959) p. 302; Table 5.3.6 B for muR in the range 2.6–10.0. The interpolation procedure of C. W. Dwiggins Jr [Acta Cryst. (1975) A31, 146–148] is used with some modification.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
C1	0.51761 (9)	0.7500	0.48326 (16)	0.0174 (4)
C2	0.51132 (10)	0.7500	0.34538 (16)	0.0186 (4)
C3	0.58236 (10)	0.7500	0.27291 (16)	0.0207 (4)
H3	0.5791	0.7500	0.1803	0.025*
C4	0.65707 (10)	0.7500	0.33116 (17)	0.0231 (4)
H4	0.7044	0.7500	0.2795	0.028*
C5	0.66185 (10)	0.7500	0.46594 (17)	0.0233 (4)
H5	0.7130	0.7500	0.5070	0.028*
C6	0.59338 (10)	0.7500	0.54146 (16)	0.0202 (4)
H6	0.5978	0.7500	0.6339	0.024*
C8	0.43998 (10)	0.7500	0.68912 (16)	0.0185 (4)
C9	0.35202 (10)	0.7500	0.73565 (15)	0.0199 (4)
C11	0.21605 (10)	0.7500	0.67083 (17)	0.0229 (4)
H11A	0.2026	0.8741	0.7228	0.027*	0.50
H11B	0.2026	0.6259	0.7228	0.027*	0.50
C12	0.17035 (10)	0.7500	0.54468 (17)	0.0262 (4)
H12A	0.1866	0.8691	0.4920	0.039*	0.50
H12B	0.1127	0.7583	0.5629	0.039*	0.50
H12C	0.1818	0.6226	0.4966	0.039*	0.50
C13	0.43310 (10)	0.7500	0.27513 (16)	0.0203 (4)
C14	0.43319 (11)	0.7500	0.12857 (17)	0.0290 (4)
H14A	0.4545	0.6184	0.0967	0.044*	0.50
H14B	0.4669	0.8633	0.0967	0.044*	0.50
H14C	0.3782	0.7683	0.0966	0.044*	0.50
N7	0.44734 (8)	0.7500	0.55788 (13)	0.0173 (3)
O8	0.49322 (7)	0.7500	0.77012 (11)	0.0240 (3)
O9	0.33456 (7)	0.7500	0.84921 (11)	0.0277 (3)
O10	0.30085 (7)	0.7500	0.63694 (11)	0.0201 (3)
O13	0.36892 (7)	0.7500	0.33342 (11)	0.0254 (3)
H7	0.4022 (12)	0.7500	0.514 (2)	0.030*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0188 (8)	0.0127 (8)	0.0207 (9)	0.000	−0.0004 (6)	0.000
C2	0.0215 (8)	0.0137 (7)	0.0206 (8)	0.000	−0.0012 (7)	0.000
C3	0.0265 (9)	0.0172 (8)	0.0185 (9)	0.000	0.0023 (7)	0.000
C4	0.0208 (9)	0.0201 (9)	0.0286 (10)	0.000	0.0043 (7)	0.000
C5	0.0184 (8)	0.0208 (9)	0.0308 (10)	0.000	−0.0045 (7)	0.000
C6	0.0225 (8)	0.0182 (8)	0.0198 (8)	0.000	−0.0026 (7)	0.000
C8	0.0239 (9)	0.0135 (8)	0.0182 (8)	0.000	−0.0033 (7)	0.000
C9	0.0274 (9)	0.0162 (8)	0.0161 (9)	0.000	−0.0018 (7)	0.000
C11	0.0181 (8)	0.0283 (9)	0.0222 (9)	0.000	0.0039 (7)	0.000
C12	0.0200 (8)	0.0330 (10)	0.0255 (10)	0.000	0.0012 (7)	0.000
C13	0.0248 (9)	0.0153 (8)	0.0208 (9)	0.000	−0.0023 (7)	0.000
C14	0.0277 (10)	0.0390 (11)	0.0205 (9)	0.000	−0.0031 (7)	0.000
N7	0.0167 (7)	0.0197 (7)	0.0154 (7)	0.000	−0.0021 (5)	0.000
O8	0.0244 (6)	0.0294 (7)	0.0180 (6)	0.000	−0.0044 (5)	0.000
O9	0.0295 (7)	0.0377 (7)	0.0159 (6)	0.000	0.0015 (5)	0.000
O10	0.0183 (6)	0.0267 (6)	0.0154 (6)	0.000	0.0007 (4)	0.000
O13	0.0198 (6)	0.0360 (7)	0.0203 (6)	0.000	−0.0020 (5)	0.000

Geometric parameters (Å, º)

C1—C6	1.396 (2)	C9—O9	1.199 (2)
C1—N7	1.398 (2)	C9—O10	1.3226 (19)
C1—C2	1.416 (2)	C11—O10	1.455 (2)
C2—C3	1.397 (2)	C11—C12	1.500 (2)
C2—C13	1.489 (2)	C11—H11A	0.9900
C3—C4	1.381 (2)	C11—H11B	0.9900
C3—H3	0.9500	C12—H12A	0.9800
C4—C5	1.383 (2)	C12—H12B	0.9800
C4—H4	0.9500	C12—H12C	0.9800
C5—C6	1.379 (2)	C13—O13	1.225 (2)
C5—H5	0.9500	C13—C14	1.501 (2)
C6—H6	0.9500	C14—H14A	0.9800
C8—O8	1.2146 (19)	C14—H14B	0.9800
C8—N7	1.350 (2)	C14—H14C	0.9800
C8—C9	1.542 (2)	N7—H7	0.88 (2)
C6—C1—N7	121.61 (15)	O10—C11—H11A	110.4
C6—C1—C2	119.51 (14)	C12—C11—H11A	110.4
N7—C1—C2	118.87 (14)	O10—C11—H11B	110.4
C3—C2—C1	117.83 (15)	C12—C11—H11B	110.4
C3—C2—C13	119.03 (15)	H11A—C11—H11B	108.6
C1—C2—C13	123.14 (15)	C11—C12—H12A	109.5
C4—C3—C2	122.32 (15)	C11—C12—H12B	109.5
C4—C3—H3	118.8	H12A—C12—H12B	109.5
C2—C3—H3	118.8	C11—C12—H12C	109.5
C3—C4—C5	118.90 (15)	H12A—C12—H12C	109.5
C3—C4—H4	120.5	H12B—C12—H12C	109.5
C5—C4—H4	120.5	O13—C13—C2	121.95 (15)
C6—C5—C4	120.82 (15)	O13—C13—C14	119.22 (15)
C6—C5—H5	119.6	C2—C13—C14	118.83 (15)
C4—C5—H5	119.6	C13—C14—H14A	109.5
C5—C6—C1	120.61 (15)	C13—C14—H14B	109.5
C5—C6—H6	119.7	H14A—C14—H14B	109.5
C1—C6—H6	119.7	C13—C14—H14C	109.5
O8—C8—N7	127.85 (15)	H14A—C14—H14C	109.5
O8—C8—C9	118.92 (14)	H14B—C14—H14C	109.5
N7—C8—C9	113.22 (13)	C8—N7—C1	128.34 (14)
O9—C9—O10	125.80 (16)	C8—N7—H7	115.5 (13)
O9—C9—C8	122.06 (15)	C1—N7—H7	116.2 (13)
O10—C9—C8	112.15 (13)	C9—O10—C11	116.36 (13)
O10—C11—C12	106.73 (13)
C6—C1—C2—C3	0.0	O8—C8—C9—O10	180.0
N7—C1—C2—C3	180.0	N7—C8—C9—O10	0.0
C6—C1—C2—C13	180.0	C3—C2—C13—O13	180.0
N7—C1—C2—C13	0.0	C1—C2—C13—O13	0.0
C1—C2—C3—C4	0.0	C3—C2—C13—C14	0.0
C13—C2—C3—C4	180.0	C1—C2—C13—C14	180.0
C2—C3—C4—C5	0.0	O8—C8—N7—C1	0.0
C3—C4—C5—C6	0.0	C9—C8—N7—C1	180.0
C4—C5—C6—C1	0.0	C6—C1—N7—C8	0.0
N7—C1—C6—C5	180.0	C2—C1—N7—C8	180.0
C2—C1—C6—C5	0.0	O9—C9—O10—C11	0.000 (1)
O8—C8—C9—O9	0.0	C8—C9—O10—C11	180.0
N7—C8—C9—O9	180.0	C12—C11—O10—C9	180.0

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N7—H7···O10	0.88 (2)	2.11 (2)	2.5723 (17)	113 (2)
N7—H7···O13	0.88 (2)	1.93 (2)	2.6443 (18)	137 (2)
C6—H6···O8	0.95	2.23	2.876 (2)	124
C5—H5···O9^i	0.95	2.51	3.445 (2)	170

Symmetry code: (i) x+1/2, −y+3/2, −z+3/2.