Tetra­gonal polymorph of 5,5-di­chloro­barbituric acid

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].

Related literature

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

Crystal data

• C₄H₂Cl₂N₂O₃

• M_r = 196.98

• Tetragonal,

• a = 13.8883 (3) Å

• c = 6.9126 (2) Å

• V = 1333.34 (6) ų

• Z = 8

• Mo Kα radiation

• μ = 0.92 mm⁻¹

• T = 173 K

• 0.20 × 0.05 × 0.05 mm

Data collection

• Oxford Diffraction Xcalibur Ruby Gemini ultra diffractometer

• Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2003) T_min = 0.837, T_max = 0.955

• 11025 measured reflections

• 1310 independent reflections

• 1242 reflections with I > 2σ(I)

• R_int = 0.041

Refinement

• R[F² > 2σ(F²)] = 0.020

• wR(F²) = 0.050

• S = 1.07

• 1310 reflections

• 107 parameters

• 2 restraints

• All H-atom parameters refined

• Δρ_max = 0.20 e Å⁻³

• Δρ_min = −0.16 e Å⁻³

• Absolute structure: Flack (1983), 541 Friedel pairs

• Flack parameter: −0.08 (7)

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N3—H3⋯O6^i	0.87 (1)	2.07 (1)	2.923 (2)	167 (2)
N1—H1⋯O2^ii	0.86 (1)	2.05 (1)	2.881 (2)	165 (2)
Symmetry codes: (i) x, y, z-1; (ii) .

Data collection: CrysAlis PRO (Oxford Diffraction, 2003); cell refinement: CrysAlis PRO; 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).

Supporting information

S1. Comment

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···O═C 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···O═C 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···O═C bonded to one another via their C6 carbonyl groups. Two strands of a tape are linked together by a second set of N—H···O═C 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

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).

S2. Experimental

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 C═O 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).

S3. Refinement

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 U_iso(H) = 1.2U_eq(N).

Computing details

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

	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.
	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.
	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.
	Fig. 4. FT—IR spectra of the polymorphs (Ia), (Ib) and (Ic) of 5,5-dichlorobarbituric acid.

5,5-dichlorobarbituric acid

Crystal data

C4H2Cl2N2O3	Dx = 1.963 Mg m^−3
Mr = 196.98	Melting point: 478 K
Tetragonal, P421c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P -4 2n	Cell parameters from 5173 reflections
a = 13.8883 (3) Å	θ = 2.9–29.3°
c = 6.9126 (2) Å	µ = 0.92 mm^−1
V = 1333.34 (6) Å^3	T = 173 K
Z = 8	Needle, colourless
F(000) = 784	0.20 × 0.05 × 0.05 mm

Data collection

Oxford Diffraction Xcalibur Ruby Gemini ultra diffractometer	1310 independent reflections
Radiation source: Enhance Ultra (Mo) X-ray Source	1242 reflections with I > 2σ(I)
Mirror monochromator	Rint = 0.041
ω scans	θmax = 26.0°, θmin = 2.9°
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2003)	h = −17→17
Tmin = 0.837, Tmax = 0.955	k = −16→17
11025 measured reflections	l = −7→8

Refinement

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	All H-atom parameters refined
R[F^2 > 2σ(F^2)] = 0.020	w = 1/[σ^2(Fo^2) + (0.0266P)^2 + 0.2183P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.050	(Δ/σ)max < 0.001
S = 1.07	Δρmax = 0.20 e Å^−3
1310 reflections	Δρmin = −0.16 e Å^−3
107 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
2 restraints	Extinction coefficient: 0.0046 (8)
Primary atom site location: structure-invariant direct methods	Absolute structure: Flack (1983), 541 Friedel pairs
Secondary atom site location: difference Fourier map	Absolute structure parameter: −0.08 (7)

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
Cl1	0.40679 (3)	0.41812 (3)	0.32982 (7)	0.01953 (13)
Cl2	0.26242 (3)	0.27671 (3)	0.22840 (7)	0.01938 (13)
O2	0.53042 (10)	0.10721 (9)	−0.1061 (2)	0.0225 (3)
O4	0.37178 (9)	0.39086 (9)	−0.0966 (2)	0.0178 (3)
O6	0.44620 (11)	0.22096 (11)	0.4808 (2)	0.0260 (4)
N1	0.48923 (11)	0.16639 (11)	0.1869 (2)	0.0158 (3)
N3	0.44480 (11)	0.24555 (11)	−0.0993 (2)	0.0127 (3)
C2	0.49068 (12)	0.16895 (13)	−0.0125 (3)	0.0134 (4)
C4	0.40209 (13)	0.32207 (12)	−0.0102 (3)	0.0120 (4)
C5	0.38637 (12)	0.30971 (12)	0.2083 (3)	0.0131 (4)
C6	0.44390 (12)	0.22935 (13)	0.3078 (3)	0.0148 (4)
H1	0.5202 (13)	0.1184 (10)	0.233 (3)	0.018*
H3	0.4508 (14)	0.2465 (15)	−0.2243 (14)	0.018*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0247 (2)	0.0140 (2)	0.0199 (2)	0.0036 (2)	−0.0065 (2)	−0.00722 (19)
Cl2	0.0156 (2)	0.0263 (2)	0.0162 (2)	−0.00433 (19)	0.00163 (18)	0.00387 (19)
O2	0.0286 (8)	0.0197 (7)	0.0193 (7)	0.0098 (6)	0.0067 (6)	−0.0037 (6)
O4	0.0212 (7)	0.0149 (7)	0.0172 (7)	0.0031 (5)	0.0002 (6)	0.0056 (6)
O6	0.0414 (9)	0.0266 (8)	0.0100 (7)	0.0112 (7)	−0.0030 (6)	0.0013 (6)
N1	0.0221 (8)	0.0124 (7)	0.0128 (8)	0.0072 (6)	−0.0019 (7)	0.0002 (7)
N3	0.0165 (8)	0.0151 (8)	0.0065 (7)	0.0004 (6)	0.0010 (6)	−0.0011 (7)
C2	0.0122 (9)	0.0134 (9)	0.0148 (10)	−0.0017 (7)	0.0013 (8)	0.0001 (8)
C4	0.0091 (9)	0.0132 (9)	0.0136 (9)	−0.0027 (7)	−0.0006 (8)	−0.0006 (7)
C5	0.0133 (8)	0.0130 (8)	0.0129 (9)	0.0006 (7)	0.0007 (7)	−0.0046 (7)
C6	0.0173 (9)	0.0140 (9)	0.0130 (10)	−0.0002 (7)	−0.0021 (7)	−0.0006 (8)

Geometric parameters (Å, º)

Cl1—C5	1.7471 (18)	N1—H1	0.856 (9)
Cl2—C5	1.7868 (18)	N3—C4	1.364 (2)
O2—C2	1.208 (2)	N3—C2	1.378 (2)
O4—C4	1.203 (2)	N3—H3	0.868 (9)
O6—C6	1.202 (2)	C4—C5	1.536 (3)
N1—C6	1.363 (2)	C5—C6	1.535 (2)
N1—C2	1.379 (2)
C6—N1—C2	127.07 (17)	N3—C4—C5	114.75 (16)
C6—N1—H1	120.1 (14)	C6—C5—C4	116.60 (15)
C2—N1—H1	112.9 (14)	C6—C5—Cl1	109.07 (12)
C4—N3—C2	127.31 (17)	C4—C5—Cl1	110.70 (13)
C4—N3—H3	118.6 (14)	C6—C5—Cl2	106.27 (12)
C2—N3—H3	113.6 (14)	C4—C5—Cl2	104.00 (12)
O2—C2—N3	121.74 (17)	Cl1—C5—Cl2	109.88 (10)
O2—C2—N1	121.60 (18)	O6—C6—N1	122.37 (18)
N3—C2—N1	116.65 (16)	O6—C6—C5	122.00 (17)
O4—C4—N3	123.15 (18)	N1—C6—C5	115.60 (16)
O4—C4—C5	121.84 (17)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N3—H3···O6^i	0.87 (1)	2.07 (1)	2.923 (2)	167 (2)
N1—H1···O2^ii	0.86 (1)	2.05 (1)	2.881 (2)	165 (2)

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