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Acta Cryst. (2012). E68, o235-o236    [ doi:10.1107/S1600536811054626 ]

Tetra­gonal polymorph of 5,5-dichloro­barbituric acid

T. Gelbrich, D. Rossi and U. J. Griesser

Abstract top

The tetra­gonal polymorph of 5,5-dichloro­barbituric acid (m.p. 478 K), C4H2Cl2N2O3, forms an N-H...O hydrogen-bonded tape structure along [001]. Two tapes related by a twofold rotation axis are associated via Cl...O contacts [3.201 (1) Å], and four such chain pairs are arranged around a fourfold roto-inversion axis. The crystal structures of the monoclinic and ortho­rhom­bic polymorphs have been reported previously [Gelbrich et al. (2011). CrystEngComm, 13, 5502-5509].

Comment top

The polymorphic nature of 5,5-dichlorobarbituric acid (I) is already mentioned in Groth's compendium on the chemical crystallography of organic compounds, published more than a hundred years ago (Groth, 1910). As part of our wider investigation of solid state forms of barbiturates, we have recently determined the crystal structures of a monoclinic (Ia) and an orthorhombic (Ib) form (Gelbrich et al., 2011), and herein we report on the tetragonal polymorph (Ic) of the title compound. The equilibrium melting points of (Ia), (Ib) and (Ic), determined by hot-stage microscopy, are 477, 490 and 478 K, respectively. All three modifications were obtained in sublimation experiments; (Ia) as plates, (Ib) as prisms and (Ic) as long needles.

The molecular structure of (Ic) is illustrated in Fig. 1. The crystal structure of (Ic) consists of N—H···OC bonded tapes (see Fig. 2) that belong to the C-3 type in the classification scheme proposed (Gelbrich et al., 2011) for the H-bonded structures of 5,5-substituted derivatives of barbituratic acid. By contrast, all of the other five known crystal structures of 5,5-dihalogen analogues form either an N—H···OC bonded layer (L) or a framework (F) structure (DesMarteau et al., 1994; Gelbrich et al., 2011). In particular, the monoclinic polymorph (Ia) and the orthorhombic form (Ib) display the layer types L-6 and L-5, respectively (Gelbrich et al., 2011). C-3 tapes have been reported previously for solid forms of γ-methylamobarbital (Gartland & Craven, 1971), butobarbital (Gelbrich et al., 2007), quinal barbitone (Nichol & Clegg, 2007) and alphenal (Zencirci et al., 2009).

In the crystal structure of (Ic), a single tape consists of two parallel strands. Neighbouring molecules forming a single strand are N—H···OC bonded to one another via their C6 carbonyl groups. Two strands of a tape are linked together by a second set of N—H···OC interactions in which the C2 carbonyl group is involved. These interactions result in two independent R33(12) rings (Etter et al., 1990; Bernstein et al., 1995). The molecules of a single strand are related to one another by a translation along [001]. Additionally, the tape possesses a glide mirror plane that is oriented perpendicular to its mean plane. The C4 carbonyl group is not involved in hydrogen bonding.

The cross section of the H-bonded tape structure is somewhat bent, so that the mean planes of the two strands from an angle of 23.7 (2)°. Two neighbouring H-bonded tapes, which are related to one another by a twofold rotation, form an assembly exhibiting short intermolecular Cl2···O4(–y+1/2, –x+1/2, z+1/2) distances of 3.201 (1) Å. As illustrated in Fig. 3, four such two-tape assemblies are situated around the fourfold roto-inversion axis in such a way that the Cl1 sites of four neighbouring molecules are the vertices of an almost ideal tetrahedron, whose edges are the intermolecular contacts Cl1···Cl1(–y+1, x, –z+1) and Cl1···Cl1(–x+1, –y+1, z) of 3.3873 (8) Å and 3.4462 (9) Å, respectively.

The three polymorphs of (I) can be readily distinguished from each other by their FT—IR spectra, which are depicted in Fig. 4. The calculated densities (Mg m-3) at -100 K for the three polymorphs (Ia), (Ib) and (Ic) are 1.984, 1.842 and 1.963, respectively. Therefore, the order of decreasing densities is (Ia) > (Ic) >> (Ib). The density of the tetragonal form (Ic) is 1% lower than that of the monoclinic form (Ia) and 6% higher than that of the orthorhombic polymorph (Ib), which is also the form of (I) with the most complex H-bonded structure.

Related literature top

The polymorphic nature of 5,5-dichlorobarbituric acid was mentioned in Groth's compendium on the chemical crystallography of organic compounds, published more than a hundred years ago (Groth, 1910). For the monoclinic and orthorhombic polymorphs, see: Gelbrich et al. (2011). For related structures, see: Gartland & Craven (1971); Gelbrich et al. (2007, 2010, 2010a,b); Nichol & Clegg (2007); Zencirci et al. (2009, 2010); DesMarteau et al. (1994). For a description of the synthesis, see: Ziegler et al. (1962). For hydrogen-bond motifs, see: Bernstein et al. (1995); Etter et al. (1990).

Experimental top

Needle-shaped crystals of (Ic) were obtained in a sublimation experiment carried out at 473 K. On heating, (Ic) undergoes a transformation into (Ib). However, the melting of (Ic) can be observed in a thermomicroscopic experiment if the crystals are placed on a hot stage that is preheated to just below the melting temperature of (Ic).

The FT—IR spectrum of (Ic) (see Fig. 4) shows a strong and sharp N—H vibration at 3258 cm-1 and a weak one at 3152 cm-1. In the CO region the spectrum exhibits a weaker band at 1756 cm-1 with a shoulder at about 1769 cm-1 and a stronger band at 1729 cm-1. These characteristics are consistent with the G5b-type spectrum in the IR classification schmeme for barabiturates (Zencirci et al., 2009). This type indicates the presence of the H-bonded tape connectivity C-3. Previous G5b examples include form I of alphenal and the metastable polymorph VIII of phenobarbital (Zencirci et al., 2009).

Refinement top

The NH H-atoms were located in a difference Fourier map. They were refined with a distance restraint: N—H = 0.88 (1) Å, with Uiso(H) = 1.2Ueq(N).

Computing details top

Data collection: CrysAlis PRO (Oxford Diffraction, 2003); cell refinement: CrysAlis PRO (Oxford Diffraction, 2003); data reduction: CrysAlis RED (Oxford Diffraction, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures top
[Figure 1] Fig. 1. The molecular structure of the title compound, (Ic), with atom numbering. Displacement ellipsoids are drawn at the 50% probability level, with hydrogen atoms shown as spheres of arbitrary size.
[Figure 2] Fig. 2. A view along the b axis of the H-bonded C-3 tape structure of (Ic). H, O and N atoms directly involved in N—H···O interactions (dashed lines) are drawn as balls.
[Figure 3] Fig. 3. A portion of the crystal structure of (Ic), view along [001] (translation direction of the C-3 tapes) with four pairs of tapes. The N—H···O bonds and short intermolecular Cl···Cl and Cl···O contacts are indicated.
[Figure 4] Fig. 4. FT—IR spectra of the polymorphs (Ia), (Ib) and (Ic) of 5,5-dichlorobarbituric acid.
5,5-dichlorobarbituric acid top
Crystal data top
C4H2Cl2N2O3Dx = 1.963 Mg m3
Mr = 196.98Melting point: 478 K
Tetragonal, P421cMo Kα radiation, λ = 0.71073 Å
Hall symbol: P -4 2nCell parameters from 5173 reflections
a = 13.8883 (3) Åθ = 2.9–29.3°
c = 6.9126 (2) ŵ = 0.92 mm1
V = 1333.34 (6) Å3T = 173 K
Z = 8Needle, colourless
F(000) = 7840.20 × 0.05 × 0.05 mm
Data collection top
Oxford Diffraction Xcalibur Ruby Gemini ultra
1310 independent reflections
Radiation source: Enhance Ultra (Mo) X-ray Source1242 reflections with I > 2σ(I)
Mirror monochromatorRint = 0.041
ω scansθmax = 26.0°, θmin = 2.9°
Absorption correction: multi-scan
(CrysAlis RED; Oxford Diffraction, 2003)
h = 1717
Tmin = 0.837, Tmax = 0.955k = 1617
11025 measured reflectionsl = 78
Refinement top
Refinement on F2Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: fullAll H-atom parameters refined
R[F2 > 2σ(F2)] = 0.020 w = 1/[σ2(Fo2) + (0.0266P)2 + 0.2183P]
where P = (Fo2 + 2Fc2)/3
wR(F2) = 0.050(Δ/σ)max < 0.001
S = 1.07Δρmax = 0.20 e Å3
1310 reflectionsΔρmin = 0.16 e Å3
107 parametersExtinction correction: SHELXL97 (Sheldrick, 2008), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
2 restraintsExtinction coefficient: 0.0046 (8)
Primary atom site location: structure-invariant direct methodsAbsolute structure: Flack (1983), 541 Friedel pairs
Secondary atom site location: difference Fourier mapAbsolute structure parameter: 0.08 (7)
Crystal data top
C4H2Cl2N2O3Z = 8
Mr = 196.98Mo Kα radiation
Tetragonal, P421cµ = 0.92 mm1
a = 13.8883 (3) ÅT = 173 K
c = 6.9126 (2) Å0.20 × 0.05 × 0.05 mm
V = 1333.34 (6) Å3
Data collection top
Oxford Diffraction Xcalibur Ruby Gemini ultra
1310 independent reflections
Absorption correction: multi-scan
(CrysAlis RED; Oxford Diffraction, 2003)
1242 reflections with I > 2σ(I)
Tmin = 0.837, Tmax = 0.955Rint = 0.041
11025 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.020All H-atom parameters refined
wR(F2) = 0.050Δρmax = 0.20 e Å3
S = 1.07Δρmin = 0.16 e Å3
1310 reflectionsAbsolute structure: Flack (1983), 541 Friedel pairs
107 parametersAbsolute structure parameter: 0.08 (7)
2 restraints
Special details top

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 (Å2) top
Cl10.40679 (3)0.41812 (3)0.32982 (7)0.01953 (13)
Cl20.26242 (3)0.27671 (3)0.22840 (7)0.01938 (13)
O20.53042 (10)0.10721 (9)0.1061 (2)0.0225 (3)
O40.37178 (9)0.39086 (9)0.0966 (2)0.0178 (3)
O60.44620 (11)0.22096 (11)0.4808 (2)0.0260 (4)
N10.48923 (11)0.16639 (11)0.1869 (2)0.0158 (3)
N30.44480 (11)0.24555 (11)0.0993 (2)0.0127 (3)
C20.49068 (12)0.16895 (13)0.0125 (3)0.0134 (4)
C40.40209 (13)0.32207 (12)0.0102 (3)0.0120 (4)
C50.38637 (12)0.30971 (12)0.2083 (3)0.0131 (4)
C60.44390 (12)0.22935 (13)0.3078 (3)0.0148 (4)
H10.5202 (13)0.1184 (10)0.233 (3)0.018*
H30.4508 (14)0.2465 (15)0.2243 (14)0.018*
Atomic displacement parameters (Å2) top
Cl10.0247 (2)0.0140 (2)0.0199 (2)0.0036 (2)0.0065 (2)0.00722 (19)
Cl20.0156 (2)0.0263 (2)0.0162 (2)0.00433 (19)0.00163 (18)0.00387 (19)
O20.0286 (8)0.0197 (7)0.0193 (7)0.0098 (6)0.0067 (6)0.0037 (6)
O40.0212 (7)0.0149 (7)0.0172 (7)0.0031 (5)0.0002 (6)0.0056 (6)
O60.0414 (9)0.0266 (8)0.0100 (7)0.0112 (7)0.0030 (6)0.0013 (6)
N10.0221 (8)0.0124 (7)0.0128 (8)0.0072 (6)0.0019 (7)0.0002 (7)
N30.0165 (8)0.0151 (8)0.0065 (7)0.0004 (6)0.0010 (6)0.0011 (7)
C20.0122 (9)0.0134 (9)0.0148 (10)0.0017 (7)0.0013 (8)0.0001 (8)
C40.0091 (9)0.0132 (9)0.0136 (9)0.0027 (7)0.0006 (8)0.0006 (7)
C50.0133 (8)0.0130 (8)0.0129 (9)0.0006 (7)0.0007 (7)0.0046 (7)
C60.0173 (9)0.0140 (9)0.0130 (10)0.0002 (7)0.0021 (7)0.0006 (8)
Geometric parameters (Å, º) top
Cl1—C51.7471 (18)N1—H10.856 (9)
Cl2—C51.7868 (18)N3—C41.364 (2)
O2—C21.208 (2)N3—C21.378 (2)
O4—C41.203 (2)N3—H30.868 (9)
O6—C61.202 (2)C4—C51.536 (3)
N1—C61.363 (2)C5—C61.535 (2)
N1—C21.379 (2)
C6—N1—C2127.07 (17)N3—C4—C5114.75 (16)
C6—N1—H1120.1 (14)C6—C5—C4116.60 (15)
C2—N1—H1112.9 (14)C6—C5—Cl1109.07 (12)
C4—N3—C2127.31 (17)C4—C5—Cl1110.70 (13)
C4—N3—H3118.6 (14)C6—C5—Cl2106.27 (12)
C2—N3—H3113.6 (14)C4—C5—Cl2104.00 (12)
O2—C2—N3121.74 (17)Cl1—C5—Cl2109.88 (10)
O2—C2—N1121.60 (18)O6—C6—N1122.37 (18)
N3—C2—N1116.65 (16)O6—C6—C5122.00 (17)
O4—C4—N3123.15 (18)N1—C6—C5115.60 (16)
O4—C4—C5121.84 (17)
Hydrogen-bond geometry (Å, º) top
N3—H3···O6i0.87 (1)2.07 (1)2.923 (2)167 (2)
N1—H1···O2ii0.86 (1)2.05 (1)2.881 (2)165 (2)
Symmetry codes: (i) x, y, z1; (ii) y+1/2, x1/2, z+1/2.

Experimental details

Crystal data
Chemical formulaC4H2Cl2N2O3
Crystal system, space groupTetragonal, P421c
Temperature (K)173
a, c (Å)13.8883 (3), 6.9126 (2)
V3)1333.34 (6)
Radiation typeMo Kα
µ (mm1)0.92
Crystal size (mm)0.20 × 0.05 × 0.05
Data collection
DiffractometerOxford Diffraction Xcalibur Ruby Gemini ultra
Absorption correctionMulti-scan
(CrysAlis RED; Oxford Diffraction, 2003)
Tmin, Tmax0.837, 0.955
No. of measured, independent and
observed [I > 2σ(I)] reflections
11025, 1310, 1242
(sin θ/λ)max1)0.617
R[F2 > 2σ(F2)], wR(F2), S 0.020, 0.050, 1.07
No. of reflections1310
No. of parameters107
No. of restraints2
H-atom treatmentAll H-atom parameters refined
Δρmax, Δρmin (e Å3)0.20, 0.16
Absolute structureFlack (1983), 541 Friedel pairs
Absolute structure parameter0.08 (7)

Computer programs: CrysAlis PRO (Oxford Diffraction, 2003), CrysAlis RED (Oxford Diffraction, 2003), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008), publCIF (Westrip, 2010).

Hydrogen-bond geometry (Å, º) top
N3—H3···O6i0.868 (9)2.070 (11)2.923 (2)167.0 (19)
N1—H1···O2ii0.856 (9)2.045 (11)2.881 (2)165 (2)
Symmetry codes: (i) x, y, z1; (ii) y+1/2, x1/2, z+1/2.
Acknowledgements top

TG gratefully acknowledges financial support from the Lize Meitner Program of the Austrian Science Fund (FWF, project M 1135-N17). We thank Clemens Häfele for providing a sample of 5,5-dichlorobarbituric acid and Professor Volker Kahlenberg for access to the X-ray instrument used in this study.