N,N'-Bis­(methoxy­carbonyl­methyl)­terephthal­amide

Molecules of the title compound, di­methyl N,N'-(1,4-benzene­dicarbox­amido)­di­acetate, C₁₄H₁₆N₂O₆, lie on inversion centres and are hydrogen bonded along a single direction that runs parallel to the crystallographic b axis. Glycine residues adopt a conformation which deviates slightly from that characteristic of the polyglycine II structure. An angle close to 27° is found between the planar amide groups and the plane of the aromatic ring.

S1. Comment

Poly(ester amide)s derived from natural amino acids have recently been suggested as a potential family of biodegradable polymers (Paredes et al., 1998). In order to get data for the determination of the polymer structures, different model compounds (Urpi et al., 1998, 1999) have been solved by direct methods. The title compound, (I), has been chosen for the study of polymers derived from terephthalic acid, glycine and different diols, since it may be a model for the common sequence: –OCOCH₂NHCOC₆H₄CONHCH₂COO–. A schematic representation of the model molecule is shown in Fig. 1. Selected rotation angles and hydrogen-bond geometry are reported in Tables 1 and 2. \scheme

The amide and ester groups, and the benzene ring are planar within experimental accuracy, with a root-mean-square distance of the atoms from the best planes defined by them of 0.011, 0.034 and 0.014 Å for C3/N1/C4/O3/C5, C1/O1/C2/O2/C3 and C4/C5/C6/C7/C7'/C6'/C5'/C4', respectively. The molecule is centrosymmetric and consequently the torsion angles of its two halves are equal but with opposite signs. The glycine residues are characterized by the torsion angles ϕ (C2—C3—N1—C4) and ψ (O1—C2—C3—N1), the values of which are very close to those found in the polyglycine II structure (75 and -145°, respectively; Crick & Rich, 1955). The molecular conformation is also characterized by the N1—C4—C5—C6 torsion angle of 156.09 (13)°, which clearly deviates from 180°. Thus, a displacement of the planar amide group out of the plane of the benzene ring (27°) is produced. This departure from a planar structure (favoured by resonance energy of the conjugate system) could be explained taking into account the combination of two factors: (i) steric hindrances between the H and O atoms of the amide groups, and the nearest H atoms of the aromatic ring; (ii) the establishment in the crystal of intermolecular hydrogen bonds between amide groups of adjacent molecules. Similar values in the 20–30° interval for the internal rotation angle have been found for different model compounds of aromatic polyamides (Blake & Small, 1972; Palmer & Brisse, 1980; Harkema & Gaymans, 1977) and poly(ester amide)s (Cesari et al., 1976). The molecular packing (Fig. 2) is characterized by the establishment of hydrogen bonds along a single direction. A standard geometry is found between the hydrogen-bonded molecules, which are not shifted along its molecular axis direction. A twofold screw axis parallel to the b axis relates the non-hydrogen-bonded molecules of the unit cell. The aromatic rings of these two molecules adopt a disposition close to perpendicular and a distance of 5.13 Å can be measured between the centers of the two rings. This geometry is in agreement with recent calculations on benzene dimers (Chipot et al., 1996) that show the T-shaped disposition as more stable than the stacked one. In the same sense, a T-shaped disposition of aromatic rings seems to be preferred in proteins (Hunter et al., 1991).

Related literature

For related literature, see: Blake & Small (1972); Cesari et al. (1976); Chipot et al. (1996); Crick & Rich (1955); Harkema & Gaymans (1977); Hunter et al. (1991); Palmer & Brisse (1980); Paredes, Rodriguez-Galan & Puiggali (1998); Paredes, Rodriguez-Galan, Puiggali & Peraire (1998); Urpi et al. (1999); Urpi, Rodriguez-Galan & Puiggali (1998, 1998).

S2. Experimental

The title compound was synthesized by the reaction of a solution of glycine methyl ester hydrochloride (0.02 mol) and triethylamine (0.04 mol) in chloroform (30 ml) with a solution of terephthaloyl chloride (0.01 mol) in chloroform (20 ml), which was was added slowly while maintaining the temperature at 273 K. After 2 h at room temperature, the solution was evaporated yielding a yellow powder that was recrystallized from water (yield 75%, m.p. 435 K). Colorless prismatic crystals were obtained by vapor diffusion (293 K) of a 91:9 (v/v) water/2-propanol solution (concentration 3.6 mg ml^-1) against 100% water used as precipitant.

S3. Refinement

H atoms were placed in calculated positions and refined riding upon the atom to which they are attached (N—H = 0.86 Å and C—H 0.93–0.97 Å) with a fixed isotropic displacement parameter.

Computing details

Data collection: CAD-4 Software (Kiers, 1994); cell refinement: CAD-4 Software; data reduction: Local program; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976).

Figures

	Fig. 1. A view of (I) with the atom-numbering scheme for the non-H atoms. Displacement ellipsoids are shown at the 50% probability level and H atoms are drawn as circles with arbitrarii radii.
	Fig. 2. The crystal packing of (I). The views are normal to the (a) ac and (b) bc planes. Hydrogen bonds (not shown) are established along a single direction that runs practically parallel to the a axis.

Bis(Methylglycyl)terephthalamide

Crystal data

C14H16N2O6	Dx = 1.363 Mg m^−3
Mr = 308.29	Melting point: 435 K
Monoclinic, P21/a	Cu Kα radiation, λ = 1.54178 Å
a = 8.9889 (10) Å	Cell parameters from 20 reflections
b = 4.977 (2) Å	θ = 8–35°
c = 16.790 (4) Å	µ = 0.92 mm^−1
β = 90.90 (1)°	T = 293 K
V = 751.1 (4) Å^3	Prism, colourless
Z = 2	0.2 × 0.1 × 0.05 mm
F(000) = 324

Data collection

Enraf-Nonius CAD4 diffractometer	Rint = 0.031
Radiation source: fine-focus sealed tube	θmax = 65.8°, θmin = 5.3°
Graphite monochromator	h = −10→10
ω scans	k = 0→5
2237 measured reflections	l = −19→19
1172 independent reflections	3 standard reflections every 60 min
1029 reflections with I > 2σ(I)	intensity decay: 0.5%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.069	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.175	Text
S = 1.49	w = 1/[σ^2(Fo^2) + (0.1P)^2] where P = (Fo^2 + 2Fc^2)/3
1172 reflections	(Δ/σ)max < 0.001
102 parameters	Δρmax = 0.31 e Å^−3
0 restraints	Δρmin = −0.29 e Å^−3

Special details

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
O3	0.86488 (14)	0.1651 (2)	0.68138 (8)	0.0442 (5)
C4	0.86983 (15)	0.3965 (3)	0.65560 (10)	0.0275 (5)
N1	0.80684 (14)	0.6007 (2)	0.69587 (9)	0.0356 (5)
H1	0.8075	0.7609	0.6767	0.043*
C6	1.04855 (16)	0.2899 (3)	0.54652 (9)	0.0287 (5)
H6	1.0814	0.1469	0.5778	0.034*
C5	0.93734 (15)	0.4582 (3)	0.57553 (9)	0.0243 (5)
O1	0.78791 (14)	0.3036 (4)	0.88695 (9)	0.0692 (6)
C3	0.73784 (18)	0.5470 (4)	0.77195 (11)	0.0422 (6)
H3A	0.6629	0.4083	0.7650	0.051*
H3B	0.6885	0.7084	0.7902	0.051*
C7	1.11225 (15)	0.3297 (3)	0.47158 (10)	0.0309 (5)
H7	1.1866	0.2156	0.4538	0.037*
C2	0.8477 (2)	0.4595 (3)	0.83286 (12)	0.0389 (6)
O2	0.97471 (16)	0.5213 (4)	0.83301 (13)	0.0818 (7)
C1	0.8864 (3)	0.1853 (7)	0.9437 (2)	0.0984 (11)
H1A	0.9622	0.0868	0.9166	0.148*
H1B	0.8319	0.0654	0.9773	0.148*
H1C	0.9317	0.3236	0.9757	0.148*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O3	0.0731 (9)	0.0092 (7)	0.0506 (9)	0.0031 (5)	0.0120 (7)	0.0038 (5)
C4	0.0314 (8)	0.0080 (8)	0.0431 (10)	−0.0008 (6)	0.0020 (6)	0.0020 (6)
N1	0.0501 (9)	0.0093 (8)	0.0476 (10)	0.0035 (6)	0.0067 (7)	0.0039 (6)
C6	0.0341 (9)	0.0107 (9)	0.0411 (10)	0.0002 (5)	−0.0080 (7)	0.0047 (6)
C5	0.0302 (8)	0.0032 (8)	0.0391 (10)	−0.0029 (5)	−0.0072 (7)	−0.0016 (5)
O1	0.0528 (9)	0.0955 (14)	0.0595 (10)	−0.0118 (7)	0.0076 (7)	0.0431 (9)
C3	0.0476 (10)	0.0260 (11)	0.0531 (12)	0.0090 (7)	0.0101 (9)	−0.0032 (8)
C7	0.0322 (9)	0.0120 (8)	0.0484 (10)	0.0052 (6)	−0.0031 (7)	−0.0003 (7)
C2	0.0419 (10)	0.0248 (10)	0.0502 (12)	−0.0081 (7)	0.0063 (7)	−0.0010 (7)
O2	0.0559 (11)	0.0897 (15)	0.0993 (16)	−0.0219 (8)	−0.0177 (9)	0.0370 (12)
C1	0.0812 (17)	0.136 (3)	0.0779 (19)	0.0207 (17)	−0.0022 (15)	0.047 (2)

Geometric parameters (Å, º)

O3—C4	1.232 (2)	O1—C1	1.418 (3)
C4—N1	1.350 (2)	C3—C2	1.476 (3)
C4—C5	1.515 (2)	C3—H3A	0.9700
N1—C3	1.454 (2)	C3—H3B	0.9700
N1—H1	0.8600	C7—C5^i	1.389 (2)
C6—C5	1.398 (2)	C7—H7	0.9300
C6—C7	1.405 (2)	C2—O2	1.183 (2)
C6—H6	0.9300	C1—H1A	0.9600
C5—C7^i	1.388 (2)	C1—H1B	0.9600
O1—C2	1.316 (2)	C1—H1C	0.9600
O3—C4—N1	120.70 (16)	N1—C3—H3B	109.2
O3—C4—C5	121.24 (15)	C2—C3—H3B	109.2
N1—C4—C5	117.95 (14)	H3A—C3—H3B	107.9
C4—N1—C3	119.31 (14)	C5^i—C7—C6	119.08 (15)
C4—N1—H1	120.3	C5^i—C7—H7	120.5
C3—N1—H1	120.3	C6—C7—H7	120.5
C5—C6—C7	122.10 (13)	O2—C2—O1	123.84 (19)
C5—C6—H6	119.0	O2—C2—C3	124.05 (18)
C7—C6—H6	119.0	O1—C2—C3	112.12 (13)
C7^i—C5—C6	118.83 (15)	O1—C1—H1A	109.5
C7^i—C5—C4	122.01 (14)	O1—C1—H1B	109.5
C6—C5—C4	119.10 (13)	H1A—C1—H1B	109.5
C2—O1—C1	116.84 (17)	O1—C1—H1C	109.5
N1—C3—C2	111.91 (12)	H1A—C1—H1C	109.5
N1—C3—H3A	109.2	H1B—C1—H1C	109.5
C2—C3—H3A	109.2
O3—C4—N1—C3	2.5 (2)	N1—C4—C5—C6	156.09 (13)
C5—C4—N1—C3	178.49 (12)	C4—N1—C3—C2	65.2 (2)
C7—C6—C5—C7^i	0.4 (2)	C5—C6—C7—C5^i	−0.4 (2)
C7—C6—C5—C4	177.72 (11)	C1—O1—C2—O2	−6.1 (4)
O3—C4—C5—C7^i	149.30 (15)	C1—O1—C2—C3	173.7 (2)
N1—C4—C5—C7^i	−26.7 (2)	N1—C3—C2—O2	27.7 (3)
O3—C4—C5—C6	−27.9 (2)	N1—C3—C2—O1	−152.04 (17)

Symmetry code: (i) −x+2, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O3^ii	0.86	2.08	2.868 (2)	152

Symmetry code: (ii) x, y+1, z.