Diethyl piperazine-1,4-diyldioxalate

The ethyl oxamate group, N-C(O)-C(O)-OEt, in the title compound, alternatively called diethyl N,N':N,N'-bis(ethylene)dioxamate, C₁₂H₁₈N₂O₆, can be considered as being composed of two singly bonded amide and ester functionalities. The ethyl oxamate group is not planar. The two carbonyl groups are almost perpendicular, with an oxalyl O=C-C=O torsion angle of -111.34 (17)°. The mol­ecule is located on an inversion centre. Infinite supramolecular tapes, propagating along the b axis, are formed through soft C-H...O interactions which form a centrosymmetric R(12) motif.

S1. Comment

Alkyl oxamates have been used as intermediates in the synthesis of oxamides (Toda et al., 1986) and oxamic acid derivatives which exhibit inhibitory protein tyrosine phosphatase activity (Andersen et al., 2002). Recently, oxamates derived from primary amines have been used as molecular models for the study of three-centred hydrogen bonding (Martínez-Martínez et al., 1998; Padilla-Martínez et al., 2001) and in the design of molecular clefts (Martin et al., 2002; Padilla-Martínez et al., 2003). However, oxamates derived from secondary amines are scarce in the literature (Cambridge Structural Database, Version of April 2004; Allen, 2002). In this context, the molecular and supramolecular structures of the title compound, (I), are reported. \sch

Compound (I) forms monoclinic centrosymmetric crystals (P2₁/c, Z = 2). Thus, only one half of the molecule is found in the asymmetric unit and the other half is generated by symmetry. The molecular structure of (I) and the atom-numbering scheme are shown in Figure 1. Selected bond lengths and angles are listed in Table 1.

The piperazine ring exhibits a chair conformation, with bond lengths and angles in the standard ranges, with a mean N—C bond length of 1.48 (2) Å. The angles around the N atom sum to almost 360° and the N1—C2 bond length is 1.3603 (19) Å, in accordance with Nsp²—Csp² amide character [1.355 (14) Å; Allen et al., 1987]. The oxalyl C2—C3 bond length is exactly 1.541 (2) Å, the value usually assumed for a Csp³—Csp³ single bond (Dewar & Schmeizing, 1968). Therefore, the ethyl oxamate moiety, –N—C(O)—C(O)-OCH₂CH₃, can be considered as composed of two singly bonded functionalities, an amide [–N1—C2(O2)] and an ester [–C3(O3)—O4—Et]. The ethyl oxamate group in (I) is not planar. The two carbonyl groups are almost perpendicular, with an O2—C2—C3—O3 torsion angle of −111.37 (17)°.

The steric hindrance between the amide carbonyl and the ethoxy O atom is released by the adoption of the conformation exhibited by compound (I), in which the O2···O4 contact distance of 2.893 (2) Å is longer than the equivalent distance of 2.640 (2) Å observed in the antiperiplanar conformation (Padilla-Martínez et al., 2003), but is nevertheless shorter than the sum of the van der Waals radii (3.04 Å; Bondi, 1964).

It is worth noting that the almost perpendicular conformation between the two carbonyl groups in (I) seems to be preferred in oxamates derived from secondary amines (Venkatramani et al., 1994), in contrast with the antiperiplanar arrangement usually found in those derived from primary amines, probably due to its stabilization by strong intramolecular hydrogen-bonding interactions (Padilla-Martínez et al., 2001; García-Báez et al., 2003).

An intramolecular soft C—H···O hydrogen-bonding interaction (Desiraju, 1996) exists between C8—H8B and the amidic carbonyl O2 atom [C8···O2 2.42 Å 2.842 (2) Å in Table 2? and C8—H8B···O2 106°], forming an S(5) ring motif, according to graph-set notation (Bernstein et al., 1995). The hydrogen-bonding geometry is listed in Table 2. Infinite supramolecular tapes, which propagate along the b direction (Fig. 2), are formed through a second soft hydrogen-bonding interaction along C7—H7B···O2ⁱ [C7···O2 3.425 (2) Å and C7—H7B···O2 156°; symmetry code: (i) 1 + x, y, z], which forms a twelve-membered ring described by the graph-set descriptor R₂²(12). The rotation energy barrier around the oxalyl C2—C3 bond should be low enough that these two hydrogen bonds fix the molecule of (I) in a symmetrical conformation in the solid state. On the other hand, the four C atoms of the piperazine ring are observed as four different signals, at 45.5, 44.7, 40.7 and 40.0 p.p.m. by ¹³C NMR, revealing the predominance of a non-symmetrical conformation in solution.

S2. Experimental

The title compound was prepared from piperazine (0.5 g, 5.8 mmol) and ethyl chlorooxoacetate (1.3 ml, 11.6 mmol) in tetrahydrofuran, in the presence of triethyl amine (1.6 ml, 11.6 mmol). After filtering, evaporation of the solvent, several washings with distilled water and drying, compound (I) was obtained as a colourless solid (1.33 g, 80% yield; m.p. 393–394 K). IR spectroscopy (neat solid, ν, cm⁻¹): 3286 (NH), 1780 (CO); ¹H NMR (300.08 MHz, DMSO-d₆, δ, p.p.m.): 4.29, 4.28 (q, 2H each, 2CH₂), 3.55, 3.42 (m, 4H each, 4NCH₂), 1.26 (t, 6H, 2CH₃); ¹³C NMR (75.46 MHz, DMSO-d₆, δ, p.p.m.): 162.4, 159.8 (4CO), 62.0 (2CH₂), 45.5, 44.7, 40.7, 40.0 (4NCH₂), 13.8 (2CH₃). Crystals of (I) suitable for X-ray analysis were obtained after slow crystallization from a solution in ethyl alcohol.

S3. Refinement

All H atoms were refined as riding on their parent atoms, with C—H distances in the range 0.96–0.97 Å and with U_iso(H) = 1.2U_eq(C). Please check added text.

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000); software used to prepare material for publication: SHELXL97 and WinGX2003 (Farrugia, 1999).

Figures

	Fig. 1. A view of the molecular structure of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small spheres of arbitrary radii. The molecule is located on an inversion centre.
	Fig. 2. The supramolecular arrangement of (I), showing the tapes formed by the R2^2(12) motif along the b direction [symmetry codes: (i) 1 + x, y, z; (ii) 1 − x, 1 − y, −z; (iii) 2 − x, 1 − y, −z; (iv) x − 1, y, z; (v) −x, 1 − y, −z].

Diethyl N,N':N,N'-bis(ethylene)dioxamate

Crystal data

C12H18N2O6	F(000) = 304
Mr = 286.28	Dx = 1.311 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 600 reflections
a = 6.2258 (15) Å	θ = 20–25°
b = 13.660 (3) Å	µ = 0.11 mm^−1
c = 8.656 (2) Å	T = 293 K
β = 99.807 (4)°	Block, colourless
V = 725.4 (3) Å^3	0.42 × 0.36 × 0.30 mm
Z = 2

Data collection

Bruker SMART CCD area-detector diffractometer	1473 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.032
Graphite monochromator	θmax = 27.2°, θmin = 2.8°
ϕ and ω scans	h = −7→7
7773 measured reflections	k = −17→17
1581 independent reflections	l = −10→10

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)] = 0.051	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.118	H-atom parameters constrained
S = 1.11	w = 1/[σ^2(Fo^2) + (0.0466P)^2 + 0.4689P] where P = (Fo^2 + 2Fc^2)/3
1581 reflections	(Δ/σ)max < 0.001
91 parameters	Δρmax = 0.35 e Å^−3
0 restraints	Δρmin = −0.21 e Å^−3

Special details

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All e.s.d.'s are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles

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
O2	0.06263 (19)	0.45470 (8)	0.21364 (15)	0.0260 (4)
O3	0.3298 (2)	0.24711 (9)	0.23533 (16)	0.0311 (4)
O4	0.01101 (17)	0.26405 (8)	0.06638 (14)	0.0198 (3)
N1	0.3783 (2)	0.45017 (9)	0.10382 (16)	0.0174 (4)
C2	0.2040 (2)	0.40967 (11)	0.15561 (18)	0.0176 (4)
C3	0.1934 (2)	0.29694 (11)	0.15493 (18)	0.0182 (4)
C5	−0.0192 (3)	0.15697 (11)	0.0696 (2)	0.0216 (5)
C6	−0.2481 (3)	0.13523 (12)	−0.0141 (2)	0.0251 (5)
C7	0.5456 (3)	0.39694 (11)	0.03207 (19)	0.0185 (4)
C8	0.4227 (3)	0.55547 (11)	0.12582 (19)	0.0197 (4)
H5A	−0.00004	0.13379	0.17694	0.0259*
H5B	0.08678	0.12447	0.01702	0.0259*
H6A	−0.27423	0.06602	−0.01211	0.0377*
H6B	−0.26395	0.15708	−0.12082	0.0377*
H6C	−0.35125	0.16881	0.03769	0.0377*
H7A	0.50096	0.32929	0.01391	0.0278*
H7B	0.68316	0.39749	0.10415	0.0278*
H8A	0.55302	0.56492	0.20342	0.0295*
H8B	0.30191	0.58678	0.16390	0.0295*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O2	0.0222 (6)	0.0208 (6)	0.0384 (7)	−0.0022 (5)	0.0145 (5)	−0.0034 (5)
O3	0.0227 (6)	0.0215 (6)	0.0447 (8)	−0.0029 (5)	−0.0068 (5)	0.0117 (5)
O4	0.0178 (6)	0.0136 (5)	0.0270 (6)	−0.0004 (4)	0.0006 (5)	0.0012 (4)
N1	0.0165 (7)	0.0134 (6)	0.0228 (7)	−0.0013 (5)	0.0050 (5)	−0.0009 (5)
C2	0.0167 (7)	0.0168 (7)	0.0189 (8)	−0.0015 (6)	0.0022 (6)	0.0008 (6)
C3	0.0156 (7)	0.0181 (8)	0.0215 (8)	−0.0012 (6)	0.0049 (6)	0.0020 (6)
C5	0.0206 (8)	0.0140 (7)	0.0300 (9)	−0.0005 (6)	0.0041 (7)	0.0006 (6)
C6	0.0247 (9)	0.0183 (8)	0.0309 (10)	−0.0028 (7)	0.0008 (7)	−0.0027 (7)
C7	0.0164 (7)	0.0133 (7)	0.0267 (9)	0.0013 (5)	0.0061 (6)	0.0005 (6)
C8	0.0211 (8)	0.0148 (7)	0.0246 (8)	−0.0026 (6)	0.0082 (6)	−0.0040 (6)

Geometric parameters (Å, º)

O2—C2	1.2474 (18)	C5—H5A	0.9695
O3—C3	1.211 (2)	C5—H5B	0.9705
O4—C3	1.3350 (18)	C6—H6A	0.9600
O4—C5	1.4756 (19)	C6—H6B	0.9597
N1—C2	1.3603 (19)	C6—H6C	0.9601
N1—C7	1.490 (2)	C7—H7A	0.9701
N1—C8	1.471 (2)	C7—H7B	0.9699
C2—C3	1.541 (2)	C8—H8A	0.9698
C5—C6	1.514 (3)	C8—H8B	0.9703
C7—C8^i	1.556 (2)
O2···O4	2.8932 (17)	C7···O3	3.146 (2)
O2···C5^ii	3.376 (2)	C7···O3^viii	3.332 (2)
O3···C8^iii	3.169 (2)	C8···O3^ix	3.169 (2)
O3···C7	3.146 (2)	C3···H7A	2.4795
O3···N1	3.0328 (19)	H5A···O3	2.5524
O3···C7^iv	3.332 (2)	H5A···O2^vi	2.6757
O4···O2	2.8932 (17)	H5A···H8B^vi	2.5932
O4···C7^v	3.389 (2)	H5B···O3	2.7736
O2···H8B	2.4245	H5B···O2^viii	2.8203
O2···H5B^iv	2.8203	H6B···H7B^x	2.4626
O2···H7B^v	2.5155	H6C···H7A^v	2.3728
O2···H5A^ii	2.6757	H7A···O3	2.6009
O3···H8A^iii	2.6221	H7A···C3	2.4795
O3···H7A	2.6009	H7A···H6C^vii	2.3728
O3···H7A^iv	2.6766	H7A···O3^viii	2.6766
O3···H5A	2.5524	H7B···O2^vii	2.5155
O3···H5B	2.7736	H7B···O4^vii	2.7970
O4···H7B^v	2.7970	H7B···H6B^xi	2.4626
N1···O3	3.0328 (19)	H8A···O3^ix	2.6221
N1···N1^i	2.882 (2)	H8B···O2	2.4245
C5···O2^vi	3.376 (2)	H8B···H5A^ii	2.5932
C7···O4^vii	3.389 (2)
C3—O4—C5	114.92 (12)	H5A—C5—H5B	108.51
C2—N1—C7	126.33 (12)	C5—C6—H6A	109.46
C2—N1—C8	119.86 (13)	C5—C6—H6B	109.49
C7—N1—C8	113.70 (13)	C5—C6—H6C	109.44
O2—C2—N1	126.11 (14)	H6A—C6—H6B	109.49
O2—C2—C3	117.49 (12)	H6A—C6—H6C	109.47
N1—C2—C3	116.18 (12)	H6B—C6—H6C	109.48
O3—C3—O4	125.82 (14)	N1—C7—H7A	109.34
O3—C3—C2	122.26 (13)	N1—C7—H7B	109.36
O4—C3—C2	111.72 (12)	H7A—C7—H7B	107.99
O4—C5—C6	107.34 (13)	C8^i—C7—H7A	109.35
N1—C7—C8^i	111.39 (13)	C8^i—C7—H7B	109.34
N1—C8—C7^i	110.12 (13)	N1—C8—H8A	109.66
O4—C5—H5A	110.28	N1—C8—H8B	109.61
O4—C5—H5B	110.22	H8A—C8—H8B	108.14
C6—C5—H5A	110.28	C7^i—C8—H8A	109.66
C6—C5—H5B	110.22	C7^i—C8—H8B	109.62
C3—O4—C5—C6	170.75 (13)	C8—N1—C2—C3	−169.32 (13)
C5—O4—C3—O3	−0.5 (2)	C7—N1—C2—C3	6.7 (2)
C5—O4—C3—C2	−175.47 (12)	C8—N1—C2—O2	5.1 (2)
C2—N1—C7—C8^i	128.91 (16)	O2—C2—C3—O3	−111.34 (17)
C7—N1—C2—O2	−178.94 (15)	O2—C2—C3—O4	63.85 (18)
C2—N1—C8—C7^i	−129.30 (16)	N1—C2—C3—O3	63.5 (2)
C8—N1—C7—C8^i	−54.87 (18)	N1—C2—C3—O4	−121.26 (14)
C7—N1—C8—C7^i	54.22 (18)	N1—C7—C8^i—N1^i	52.85 (18)

Symmetry codes: (i) −x+1, −y+1, −z; (ii) −x, y+1/2, −z+1/2; (iii) −x+1, y−1/2, −z+1/2; (iv) x, −y+1/2, z+1/2; (v) x−1, y, z; (vi) −x, y−1/2, −z+1/2; (vii) x+1, y, z; (viii) x, −y+1/2, z−1/2; (ix) −x+1, y+1/2, −z+1/2; (x) x−1, −y+1/2, z−1/2; (xi) x+1, −y+1/2, z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C7—H7B···O2^vii	0.97	2.52	3.425 (2)	156
C8—H8B···O2	0.97	2.42	2.842 (2)	106

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