N,N′-Butane-1,4-diylbis­(bromo­acetamide)

The title compound, C₈H₁₄Br₂N₂O₂, lies about an inversion centre and adopts a pleated conformation, with the C(O)—NH—CH₂—CH₂ and NH—CH₂—CH₂—CH₂ torsion angles of the butane­diamine residue being −89.5 (6) and −62.1 (7)°, respectively. These data are useful in discerning the structure of polymers containing such a unit. A skew conformation is found for the Br—CH₂—C(O)—NH torsion angle [−124.2 (4)°]. The mol­ecular packing is stabilized by strong hydrogen bonds between amide groups and also by weak CH₂...OC inter­actions. In this way, each mol­ecule inter­acts with its six closest neighbours through eight hydrogen bonds.

S1. Comment

The development of new biodegradable polymers currently receives much attention, due to their increasing number of applications in different fields such as medical technology. For instance, polyesters in general, and glycolide derivatives in particular, have been applied in surgical sutures and drug-delivery systems (Huang, 1985; Chu, 1997). Poly(ester amide)s have recently been considered due to their ability to establish intermolecular hydrogen bonds that improve mechanical properties. In this way, new materials with a sequential repeat unit made up of glycolic acid, a diamine and a dicarboxylic acid have been patented as bioabsorbable sutures (Barrows, 1985). We have recently demonstrated that these kinds of polymers can also be prepared by a condensation reaction between N,N'-bischloroacetyldiamines and dicarboxylate salts (Vera et al., 2004), which results in both high yield and high molecular weight. The driving force of these polymerizations corresponds with the formation of metal halide salts (Epple & Kirschnick, 1997).

The title compound, (I), is also a suitable monomer for the preparation of a series of poly(ester amide)s which include glycolyl and butylene diamine residues. The study of its crystalline structure provides information on the arrangement of the reactive groups. In addition, the crystallographic model compound resulting from this study is useful in discerning the structure of the related polymer fragment. Few crystallographic data on poly(ester amide)s are available. Results obtained with some polymers derived from α-amino acids indicate that structures with characteristic features of both polyesters and polyamides can be obtained (Paredes et al., 1999).

The molecule of (I) is shown in Fig. 1, and principal torsion angles are given in Table 1. The amide group is planar to within experimental error, with an r.m.s. deviation of 0.0360 Å for atoms C2, C3, O3, H4, N4 and C5 from the best plane passing through them. The molecules adopt a pleated conformation with an inversion centre symmetry [symmetry code: (1 − x, 1 − y, 1 − z)]. The tggt(-g)(-g)t sequence of torsion angles of the butanediamide moiety was unexpected. An inspection of the Cambridge Structural Database (CSD, Release date?, CONQUEST Version 1.7; Allen, 2002; Bruno et al., 2002) showed that the structures of only 30 molecules with this fragment have been solved to date, most of them corresponding to aromatic and cycloalkane derivatives, or cyclic compounds. These are not entirely representative of conformational preferences, due to packing requirements associated with end rings or with geometric restrictions associated with the ring closure. Only five linear and aliphatic compounds are relevant to the structure reported here, namely 1,4-bis(propylaminomalonylamino)butane (QUJBEC; Navarro et al., 1995), butane-1,4-diyl bis(acetamide) (ABAWOQ; Navarro et al., 1998), N,N'-bis(glycylglycine)-1,4-diaminobutane bis(hydrogen chloride) (ACEHAS; Madhavaiah et al., 2004), N,N'-bis(2-hidroxyiminopropionyl)-1,4-diaminebutane dihydrate (BIGFEC; Fritsky et al., 1999) and butane-1,4-diyl bis(formamide) (TOPPUJ; Chaney et al., 1996).

Butane-1,4-diyl bis(acetamide) (Navarro et al., 1998) was specifically investigated to gain insights into the structure of aliphatic polyamides. Ab initio quantum-mechanical calculations showed an energy minimum when the CONH—CH₂CH₂ (ϕ) and CH₂CH₂—NHCO (ϕ') torsion angles tended to 90° and −90°, respectively, while the other main-chain torsion angles remained close to 180°. By contrast, the crystal structure of that compound showed a quasi-all-trans conformation and a packing which resembled the characteristic sheet structure of aliphatic polyamides. Only a slight deviation from 180° was observed for the ϕ and ϕ' torsion angles (±7.5°), which suggests the need to consider similar distortions in the structural modelling of polyamides. An all-trans conformation was also found in N,N'-bis(2-hydroxyiminopropionyl)-1,4-diaminebutane dihydrate (Fritsky et al., 1999) and butane-1,4-diyl bis(formamide) (Chaney et al., 1996), a compound that, in addition to the characteristic hydrogen bonds between amide groups, has short contacts between the formyl H and the carbonyl O atoms.

Other structures have been postulated in polyamides when the specific chemical sequence (i.e. nylons derived from diamines and diacids with an odd number of C atoms) and the indicated conformation give rise to weak intermolecular interactions. Thus, a good hydrogen-bond geometry and an energetic stabilization can be attained by modifying the ϕ and ϕ' torsion angles towards skew conformations. This feature is clearly observed in 1,4-bis(propylaminomalonylamino)butane (Navarro et al., 1995), a model compound for nylons derived from butanediamine and malonic acid. The ϕ and ϕ' torsion angles appear rather variable in aromatic derivatives. Thus, values close to trans [173.4 and 169.5° for p,p'-dimethoxy-N,N'-tetramethylenedibenzamide (Brisson et al., 1989) and benzoic Acid? (Harkema et al., 1980) derivatives], skew [120.0° for p,p'-dicyano-N,N'-tetramethylenedibenzamide (Brisson et al., 1989)] and gauche [79.8° for p,p'-tert-butyl-N,N'-tetramethylenedibenzamide (Brisson et al., 1989)] conformations have been experimentally observed. A pleated conformation with an unusual t(-s)st(-s)st sequence of torsion angles has been reported for only one linear and aliphatic compound, N,N'-bis(glycylglycine)-1,4-diaminobutane bis(hydrogen chloride) (Madhavaiah et al., 2004).

The BrCH₂—C(═O)NH torsion angle of (I) has a skew [−124,2(4)°] conformation that prevents an intramolecular electrostatic interaction between the Br atom and the H atom of the amide group. Intramolecular N—H···Cl hydrogen bonds have been reported for some related chloroacetamides (Urpí et al., 2003; Rao & Mallikarjunan, 1973; Kalyaranaman et al., 1978), a cis conformation being clearly stabilized in these cases. However, inspection of the CSD showed that this torsion angle is rather variable in bromoacetamide derivatives. Thus, only five compounds of a total of 17 bromoacetamide fragments have been reported to date. Trans, gauche, skew and cis conformations have been observed in one, five, seven and four fragments, respectively. In the last case, N—H···Br and N···Br distances close to 2.5 and 3.0 Å, respectively, were reported (Ozawa et al., 1974; Yavorskii et al., 1990; Amrhein et al., 2002).

The packing in (I) is characterized by the establishment of strong intermolecular hydrogen bonds between amide groups along a single direction (Fig. 2). In this way, each molecule interacts with its four closest neighbours via c-glide plane operations. The hydrogen bonds are essentially linear and have normal geometry (Table 2). In addition, weak intermolecular CH₂···OC hydrogen bonds (Fig. 2 and Table 2) link inversion-related molecules. This kind of CH₂···OC interaction has also been found in related ester compounds (Urpí et al., 2004; Martínez-Palau et al., 2005). The quasi-perpendicular orientation between the pleated tetramethylene segment and the terminal bromoacetamide fragments should be emphasized. The molecular conformation and packing clearly differ from those of related butane-1,4-diyl(halogenoacetate) compounds (Urpí et al., 2004; Martínez-Palau et al., 2005), where strong hydrogen-bond interactions cannot be established.

S2. Experimental

In a round-bottomed flask provided with a magnetic stirrer, butane-1,4-diamine (0.05 mol) and sodium hydroxide (0.1 mol) were dissolved in water (50 ml). Bromoacetyl bromide (0.10 mol) was dissolved in ethyl ether (20 ml) and added dropwise. The resulting two-phase mixture was stirred for 1 h in an ice-cooled bath. Drops of a 1 M NaOH solution were also added in order to keep the pH close to 10–11 and to neutralize the hydrobromic acid produced during the reaction. The white solid which precipitated was filtered off, washed copiously with water and ethyl ether, and then recrystallized from ethanol to give colourless crystals of (I) (yield 85%; m.p. 408 K). ¹H NMR (DMSO, δ, p.p.m.): 8.21 (t, 2H, NH), 3.77 (s, 4H, BrCH₂CO), 3.01 (m, 4H, NHCH₂), 1.35 (m, 4H, NHCH₂CH₂); ¹³C NMR (DMSO, δ, p.p.m.): 166.02 (CO), 38.83 (NHCH₂), 29.72 (BrCH₂CO), 26.38 (NHCH₂CH₂).

S3. Refinement

The amide H atoms, which are involved in hydrogen bonds, were located in difference Fourier maps and refined isotropically. The remaining H atoms were placed in calculated positions, with C—H = 0.97 Å, and refined isotropically riding on their attached C atoms. The displacement parameter of the two H atoms of each CH₂ group was refined as a free variable.

Computing details

Data collection: CAD-4 Software (Kiers, 1994); cell refinement: CAD-4 Software; data reduction: WinGX-PC (version 1.64.05; Farrugia, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 and WinGX-PC.

Figures

Fig. 1. A drawing of the molecule of (I), with the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Primed atoms are at the symmetry position (−x, 2 − y, −z).

Fig. 2. The crystalline packing of (I). A view along the crystallographic a axis has been selected in order to show both the conformation of the tetramethylene segment and the network of hydrogen-bond interactions (dashed lines). Atoms marked with an asterisk (*) or a hash (#) are at the symmetry positions (x, 3/2 − y, z + 1/2) and (x, y, 1 + z), respectively. Atom H2A makes a hydrogen bond with atom O3 at (1 − x, 1 − y, −z); this has been omitted for clarity.

Butane-1,4-diyl bis(bromoacetamide)

Crystal data

C8H14Br2N2O2	F(000) = 324
Mr = 330.01	Dx = 1.842 Mg m^−3
Monoclinic, P21/c	Melting point: 135 K
Hall symbol: -P 2ybc	Mo Kα radiation, λ = 0.71073 Å
a = 8.7065 (18) Å	Cell parameters from 25 reflections
b = 7.7518 (13) Å	θ = 12–21°
c = 9.333 (4) Å	µ = 6.79 mm^−1
β = 109.15 (2)°	T = 293 K
V = 595.1 (3) Å^3	Plate, colourless
Z = 2	0.4 × 0.2 × 0.04 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	837 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.029
Graphite monochromator	θmax = 30.0°, θmin = 2.5°
ω/2θ scans	h = −12→11
Absorption correction: gaussian (PLATON; (Spek, 2003)	k = 0→10
Tmin = 0.228, Tmax = 0.761	l = 0→13
1816 measured reflections	1 standard reflections every 120 min
1729 independent reflections	intensity decay: none

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)] = 0.060	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.159	H atoms treated by a mixture of independent and constrained refinement
S = 1.02	w = 1/[σ^2(Fo^2) + (0.0771P)^2] where P = (Fo^2 + 2Fc^2)/3
1729 reflections	(Δ/σ)max < 0.001
72 parameters	Δρmax = 0.85 e Å^−3
0 restraints	Δρmin = −0.54 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.

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

1.9846 (0.0399) x + 7.5093 (0.0174) y − 1.5628 (0.1165) z = 5.7555 (0.0046)

* −0.0431 (0.0103) C2 * 0.0134 (0.0052) C3 * 0.0282 (0.0128) O3 * −0.0140 (0.0154) N4 * 0.0559 (0.0235) H4 * −0.0404 (0.0090) C5

Rms deviation of fitted atoms = 0.0360

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
Br1	0.64331 (7)	0.81767 (9)	0.12018 (8)	0.0742 (3)
O3	0.3007 (5)	0.6707 (5)	−0.0964 (4)	0.0576 (10)
N4	0.2009 (5)	0.7307 (6)	0.0921 (5)	0.0440 (11)
H4	0.223 (5)	0.752 (6)	0.179 (6)	0.027 (12)*
C2	0.4839 (6)	0.6656 (7)	0.1575 (6)	0.0501 (13)
H2A	0.5177	0.5467	0.1555	0.060 (11)*
H2B	0.4767	0.6889	0.2572	0.060 (11)*
C3	0.3202 (6)	0.6918 (6)	0.0393 (5)	0.0403 (11)
C5	0.0342 (6)	0.7510 (7)	−0.0048 (6)	0.0493 (13)
H5A	−0.0372	0.7165	0.0509	0.061 (11)*
H5B	0.0142	0.6743	−0.0911	0.061 (11)*
C6	−0.0083 (6)	0.9349 (7)	−0.0625 (6)	0.0564 (14)
H6A	0.0621	0.9689	−0.1194	0.093 (16)*
H6B	−0.1194	0.9370	−0.1314	0.093 (16)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0539 (4)	0.0898 (6)	0.0738 (5)	0.0013 (3)	0.0139 (3)	0.0174 (4)
O3	0.070 (2)	0.076 (3)	0.0275 (17)	0.0139 (19)	0.0177 (16)	−0.0014 (18)
N4	0.050 (2)	0.055 (3)	0.027 (2)	0.008 (2)	0.0122 (18)	0.000 (2)
C2	0.051 (3)	0.061 (4)	0.040 (3)	0.014 (2)	0.015 (2)	0.005 (3)
C3	0.053 (3)	0.041 (3)	0.028 (2)	0.007 (2)	0.0139 (19)	0.002 (2)
C5	0.043 (3)	0.052 (3)	0.051 (3)	0.001 (2)	0.012 (2)	0.000 (3)
C6	0.053 (3)	0.060 (4)	0.047 (3)	0.002 (3)	0.005 (2)	−0.007 (3)

Geometric parameters (Å, º)

Br1—C2	1.938 (6)	C2—H2B	0.97
O3—C3	1.232 (5)	C5—C6	1.526 (8)
N4—C3	1.322 (6)	C5—H5A	0.97
N4—C5	1.445 (6)	C5—H5B	0.97
N4—H4	0.78 (5)	C6—C6^i	1.514 (11)
C2—C3	1.503 (7)	C6—H6A	0.97
C2—H2A	0.97	C6—H6B	0.97
C3—N4—C5	122.8 (4)	N4—C5—C6	113.6 (4)
C3—N4—H4	119 (3)	N4—C5—H5A	108.8
C5—N4—H4	118 (3)	C6—C5—H5A	108.8
C3—C2—Br1	110.4 (3)	N4—C5—H5B	108.8
C3—C2—H2A	109.6	C6—C5—H5B	108.8
Br1—C2—H2A	109.6	H5A—C5—H5B	107.7
C3—C2—H2B	109.6	C6^i—C6—C5	113.6 (6)
Br1—C2—H2B	109.6	C6^i—C6—H6A	108.9
H2A—C2—H2B	108.1	C5—C6—H6A	108.9
O3—C3—N4	123.6 (4)	C6^i—C6—H6B	108.9
O3—C3—C2	120.9 (4)	C5—C6—H6B	108.9
N4—C3—C2	115.4 (4)	H6A—C6—H6B	107.7
C5—N4—C3—O3	0.5 (8)	Br1—C2—C3—N4	−124.2 (4)
C5—N4—C3—C2	−176.6 (5)	C3—N4—C5—C6	−89.5 (6)
Br1—C2—C3—O3	58.7 (6)	N4—C5—C6—C6^i	−62.1 (7)

Symmetry code: (i) −x, −y+2, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N4—H4···O3^ii	0.78 (5)	2.07 (5)	2.853 (6)	173 (5)
C2—H2A···O3^iii	0.97	2.49	3.367 (6)	150

Symmetry codes: (ii) x, −y+3/2, z+1/2; (iii) −x+1, −y+1, −z.