Violuric acid monohydrate: a definitive redetermination at 150 K

A redetermination at 150 K of the structure of violuric acid monohydrate, C₄H₃N₃O₄·H₂O, confirms that the space group is non-centrosymmetric Cmc2₁, despite indications from the intensity statistics and possible mol­ecular symmetry that it could be centrosymmetric Cmcm. Issues raised in the original reports [Craven & Mascarenhas (1964). Acta Cryst. 17, 407–414; Craven & Takei (1964). Acta Cryst. 17, 415–420] suggested either a disordered model or an ordered one with high thermal motion. The redetermination shows that an ordered model is correct, and the low-temperature data collection leads to normal displacement parameters. The precision of the structure is significantly improved in this new study. The violuric acid mol­ecule is entirely planar, and every atom in the structure lies on a crystallographic mirror plane. Violuric acid and water mol­ecules form hydrogen-bonded sheets.

Comment

Part of our research has concentrated on the structural chemistry of s-block metal complexes of cyanuric acid, barbituric acid and other related compounds, well known for their pharmaceutical properties. Violuric acid is a 5-sub­stituted derivative of barbituric acid, and the isonitroso sub­stit­uent gives extra scope for metal coordination and hydrogen bonding, compared with unsubstituted barbituric acid.

The crystal structure of violuric acid dihydrate (I) has already been reported from room-temperature X-ray (Craven & Mascarenhas, 1964) and neutron (Craven & Takei, 1964) diffraction studies, refined to final R values of 0.059 and 0.070, respectively. The two studies were combined to produce a single result; the positions of the non-H atoms were located from X-ray data and the positions of the H (actually D as a deuterated sample was used) atoms were located from the neutron data. In their reports the authors highlighted unusual issues with the data and the final result, some of which they were unable to resolve to a satisfactory conclusion. These included the choice of space group; extremely high atomic displacement parameters of the isonitroso group and the water mol­ecule, suggesting possible disorder; and poor bond-length precision.

With these uncertainties in mind, and encouraged by our previous research which had revealed that two other barbiturates undergo a phase transition on cooling (Nichol & Clegg, 2005a,b), we redetermined the structure of violuric acid monohydrate at 150 K for the purpose of having a reference structure for the metal complexes, also studied at 150 K. No phase transition was observed in this case, but we were able to address the issues raised in the initial 1964 studies.

The asymmetric unit of (I) is shown in Fig. 1. Systematic absence data for this structure indicated that the space group could be one of Cmcm, Cmc2₁ or Ama2 (with exchanged axes). The data set intensity statistics strongly indicated a centrosymmetric space group (mean |E² − 1| = 0.95). However, the structure could not be solved in space group Cmcm, so space group Cmc2₁ (the previously reported space group) was selected, giving an entirely satisfactory solution and refinement. The ADDSYM function of PLATON (Spek, 2003) detected potentially missed further mirror and inversion symmetry, suggesting that Cmcm was indeed the true space group. In this space group, however, the refinement is very poor, giving a final R = 0.20. The extra mirror symmetry detected by ADDSYM would bis­ect the violurate ring along the axis running through the C=N bond and the carbonyl group opposite, making the two N—H groups and the remaining carbonyl groups symmetry-equivalent. While the geometry of the ring itself is compatible with this extra mirror plane, the isonitroso group is not; this would involve disorder of the N—O bond over the mirror plane. This disorder is not compatible with the crystal packing and hydrogen bonding, so we can safely disregard the pseudo-symmetry and state with confidence that this structure is non-centrosymmetric, in space group Cmc2₁.

The originally reported X-ray crystal structure contains atoms with extremely high displacement parameters, causing the authors to consider also a model with all atoms disordered across the mirror plane of Cmc2₁; this also gave a satisfactory refinement result and they were unable to reject it conclusively. By redetermining the structure at 150 K we find the atomic displacements to be reduced appreciably and we can be confident that the structure is not disordered. The mol­ecular geometry (Table 1) is determined here with much improved precision, and some apparent anomalies in the original results are removed.

With every atom constrained to lie on a crystallographic mirror plane, the crystal packing consists of stacked sheets with a very close spacing of 3.0377 (6) Å, half the a-axis length. Fig. 2 shows a projection along the a axis, with all the mol­ecules of one sheet coloured blue and all the mol­ecules of another coloured red; it can be seen that there is no ring-stacking between the violuric acid mol­ecules in adjacent sheets, as their relative displacement along the c axis means that the water mol­ecule overlaps the violurate ring in the next sheet.

The hydrogen-bonding arrangement within each sheet, shown in Fig. 3, is slightly unusual in that all the carbonyl groups are acceptors; it is far more commonly observed in the packing of barbiturate derivatives that one group is not involved in hydrogen bonding (Lewis et al., 2005). A familiar R₂²(8) hydrogen-bonding graph-set motif (Bernstein et al., 1995) links the violurate rings together, while the water mol­ecule is neatly hydrogen-bonded to the third carbonyl group and to the oxygen atom of the isonitroso group. As noted by Craven & Takei (1964), one of the water H atoms acts as a bifurcated donor. While this is now a fairly common observation, in 1964 it was very unusual, and the authors devoted some discussion, including examination of an alternative centrosymmetric model with pseudo-tetra­hedral water hydrogen bonding, to this now commonly accepted inter­action.

Figure 1 The asymmetric unit of (I), with 50% displacement ellipsoids and H atoms as small spheres of arbitrary size.

Figure 2 A view along the a axis of the packing of (I). One sheet is coloured blue and the other sheet is red, to show the relative displacement along the c axis of mol­ecules in the two sheets, preventing ring stacking.

Figure 3 The hydrogen bonding (dashed lines) observed in a single sheet of (I), viewed along the a axis.

Experimental

Commercially available violuric acid (1 mmol) was dissolved in a small amount of distilled water with gentle heating. Storage overnight at 278 K resulted in large octa­hedral colourless crystals of (I).

Crystal data

• C₄H₃N₃O₄·H₂O

• M_r = 175.11

• Orthorhombic, C m c 2₁

• a = 6.0754 (11) Å

• b = 14.343 (3) Å

• c = 7.5288 (13) Å

• V = 656.1 (2) ų

• Z = 4

• D_x = 1.773 Mg m⁻³

• Mo Kα radiation

• Cell parameters from 2281 reflections

• θ = 2.2–28.3°

• μ = 0.17 mm⁻¹

• T = 150 (2) K

• Octahedron, colourless

• 0.50 × 0.50 × 0.50 mm

Data collection

• Bruker SMART 1K CCD diffractometer

• Thin–slice ω scans

• Absorption correction: none

• 2856 measured reflections

• 468 independent reflections

• 448 reflections with I > 2σ(I)

• R_int = 0.018

• θ_max = 28.3°

• h = −8 → 8

• k = −18 → 19

• l = −9 → 9

Refinement

• Refinement on F²

• R[F² > 2σ(F²)] = 0.028

• wR(F²) = 0.085

• S = 1.10

• 468 reflections

• 87 parameters

• Only H-atom coordinates refined

• w = 1/[σ²(F_o²) + (0.0763P)² + 0.0025P] where P = (F_o² + 2F_c²)/3

• (Δ/σ)_max < 0.001

• Δρ_max = 0.39 e Å⁻³

• Δρ_min = −0.26 e Å⁻³

• Extinction correction: SHELXL97

• Extinction coefficient: 0.008 (2)

Table 1
Selected geometric parameters (Å, °)

O1—C1	1.209 (3)
O2—C2	1.224 (3)
O3—C3	1.210 (4)
O4—N3	1.349 (3)
N1—C1	1.385 (3)
N1—C2	1.378 (4)
N2—C2	1.367 (4)
N2—C3	1.382 (4)
N3—C4	1.293 (3)
C1—C4	1.478 (4)
C3—C4	1.485 (4)

C1—N1—C2	126.1 (2)
C2—N2—C3	126.9 (3)
O4—N3—C4	115.9 (2)
N1—C1—C4	115.6 (2)
N1—C2—N2	116.4 (2)
N2—C3—C4	115.2 (3)
C1—C4—C3	119.70 (18)

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O4—H4⋯O5	0.98 (4)	1.58 (4)	2.552 (2)	172 (4)
O5—H5A⋯O1^i	0.86 (5)	1.91 (5)	2.744 (3)	161 (5)
O5—H5B⋯O3^ii	0.90 (5)	2.11 (5)	2.789 (3)	131 (4)
O5—H5B⋯O4^ii	0.90 (5)	2.15 (5)	2.978 (3)	152 (4)
N1—H1N⋯O2^iii	0.90 (5)	2.08 (5)	2.972 (3)	173 (4)
N2—H2N⋯O2^iv	0.76 (5)	2.30 (5)	3.060 (3)	180 (5)
Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

All H atoms were located in a difference Fourier map and were freely refined, except that water H atoms were assigned U_iso(H) = 1.2U_eq(O); refined bond lengths are 0.86 (5) and 0.90 (5) Å for water O—H, 0.76 (5) and 0.90 (5) Å for amide N—H atoms, and 0.94 (4) Å for hydr­oxy O—H. In the absence of significant anomalous scattering, Friedel pairs were merged.

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXTL (Sheldrick, 2001); molecular graphics: DIAMOND (Brandenburg & Putz, 2004); software used to prepare material for publication: SHELXTL and local programs.

Supporting information

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SIR92 (Altomare et al., 1993); program(s) used to refine structure: SHELXTL (Sheldrick, 2001); molecular graphics: DIAMOND (Brandenburg & Putz, 2004); software used to prepare material for publication: SHELXTL and local programs.

Violuric acid monohydrate

Crystal data

C4H3N3O4·H2O	F(000) = 360
Mr = 175.11	Dx = 1.773 Mg m^−3
Orthorhombic, Cmc21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C 2c -2	Cell parameters from 2281 reflections
a = 6.0754 (11) Å	θ = 2.2–28.3°
b = 14.343 (3) Å	µ = 0.17 mm^−1
c = 7.5288 (13) Å	T = 150 K
V = 656.1 (2) Å^3	Octahedron, colourless
Z = 4	0.50 × 0.50 × 0.50 mm

Data collection

Bruker SMART 1K CCD diffractometer	448 reflections with I > 2σ(I)
Radiation source: sealed tube	Rint = 0.018
Graphite monochromator	θmax = 28.3°, θmin = 2.8°
thin–slice ω scans	h = −8→8
2856 measured reflections	k = −18→19
468 independent reflections	l = −9→9

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.028	Only H-atom coordinates refined
wR(F^2) = 0.085	w = 1/[σ^2(Fo^2) + (0.0763P)^2 + 0.0025P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max < 0.001
468 reflections	Δρmax = 0.39 e Å^−3
87 parameters	Δρmin = −0.26 e Å^−3
1 restraint	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.008 (2)

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
O1	0.5000	0.75071 (16)	0.8953 (3)	0.0286 (5)
O2	0.5000	1.01444 (10)	0.5700 (3)	0.0196 (4)
O3	0.5000	0.74307 (13)	0.2641 (3)	0.0280 (5)
O4	0.5000	0.58881 (13)	0.4565 (3)	0.0231 (5)
H4	0.5000	0.526 (3)	0.507 (5)	0.023 (8)*
O5	0.5000	0.41962 (13)	0.5613 (3)	0.0229 (4)
H5A	0.5000	0.374 (3)	0.487 (7)	0.027*
H5B	0.5000	0.396 (3)	0.672 (7)	0.027*
N1	0.5000	0.88054 (15)	0.7296 (3)	0.0160 (5)
H1N	0.5000	0.916 (3)	0.827 (7)	0.023 (10)*
N2	0.5000	0.87691 (16)	0.4198 (4)	0.0177 (6)
H2N	0.5000	0.904 (3)	0.332 (6)	0.023 (9)*
N3	0.5000	0.64146 (18)	0.6049 (3)	0.0183 (6)
C1	0.5000	0.78444 (16)	0.7481 (4)	0.0166 (6)
C2	0.5000	0.92913 (16)	0.5717 (5)	0.0151 (4)
C3	0.5000	0.7807 (2)	0.4079 (4)	0.0166 (6)
C4	0.5000	0.73073 (17)	0.5806 (4)	0.0146 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0586 (12)	0.0164 (10)	0.0109 (9)	0.000	0.000	0.0028 (8)
O2	0.0297 (7)	0.0118 (7)	0.0173 (9)	0.000	0.000	0.0016 (9)
O3	0.0565 (11)	0.0150 (8)	0.0124 (12)	0.000	0.000	−0.0003 (9)
O4	0.0413 (8)	0.0105 (9)	0.0175 (11)	0.000	0.000	−0.0002 (8)
O5	0.0408 (8)	0.0109 (8)	0.0169 (9)	0.000	0.000	−0.0003 (9)
N1	0.0285 (10)	0.0099 (10)	0.0097 (12)	0.000	0.000	0.0002 (8)
N2	0.0273 (10)	0.0158 (14)	0.0101 (12)	0.000	0.000	0.0034 (9)
N3	0.0278 (8)	0.0146 (9)	0.0125 (16)	0.000	0.000	−0.0004 (8)
C1	0.0258 (11)	0.0102 (12)	0.0138 (14)	0.000	0.000	0.0032 (10)
C2	0.0213 (8)	0.0120 (9)	0.0120 (10)	0.000	0.000	−0.0023 (10)
C3	0.0217 (10)	0.0173 (15)	0.0110 (15)	0.000	0.000	−0.0001 (9)
C4	0.0218 (8)	0.0124 (9)	0.0097 (10)	0.000	0.000	0.0026 (12)

Geometric parameters (Å, º)

O1—C1	1.209 (3)	N1—C1	1.385 (3)
O2—C2	1.224 (3)	N1—C2	1.378 (4)
O3—C3	1.210 (4)	N2—H2N	0.76 (5)
O4—H4	0.98 (4)	N2—C2	1.367 (4)
O4—N3	1.349 (3)	N2—C3	1.382 (4)
O5—H5A	0.86 (5)	N3—C4	1.293 (3)
O5—H5B	0.90 (5)	C1—C4	1.478 (4)
N1—H1N	0.90 (5)	C3—C4	1.485 (4)
H4—O4—N3	101 (2)	N1—C1—C4	115.6 (2)
H5A—O5—H5B	108 (4)	O2—C2—N1	121.0 (3)
H1N—N1—C1	119 (3)	O2—C2—N2	122.6 (3)
H1N—N1—C2	115 (3)	N1—C2—N2	116.4 (2)
C1—N1—C2	126.1 (2)	O3—C3—N2	120.2 (3)
H2N—N2—C2	116 (3)	O3—C3—C4	124.6 (3)
H2N—N2—C3	117 (3)	N2—C3—C4	115.2 (3)
C2—N2—C3	126.9 (3)	N3—C4—C1	113.3 (2)
O4—N3—C4	115.9 (2)	N3—C4—C3	127.0 (3)
O1—C1—N1	119.4 (3)	C1—C4—C3	119.70 (18)
O1—C1—C4	125.0 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O4—H4···O5	0.98 (4)	1.58 (4)	2.552 (2)	172 (4)
O5—H5A···O1^i	0.86 (5)	1.91 (5)	2.744 (3)	161 (5)
O5—H5B···O3^ii	0.90 (5)	2.11 (5)	2.789 (3)	131 (4)
O5—H5B···O4^ii	0.90 (5)	2.15 (5)	2.978 (3)	152 (4)
N1—H1N···O2^iii	0.90 (5)	2.08 (5)	2.972 (3)	173 (4)
N2—H2N···O2^iv	0.76 (5)	2.30 (5)	3.060 (3)	180 (5)

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