Ammonium violurate: a compact structure with extensive hydrogen bonding in three dimensions

The title compound [systematic name: ammonium pyrimidine-2,4-5,6(1H,3H)-tetrone 5-oximate], NH₄⁺·C₄H₂N₃O₄^-, crystallizes from water in the triclinic space group P and is isomorphous with a known rubidium complex [Gillier (1965). Bull. Soc. Chim. Fr. pp. 2373-2384]. The principal feature of the structure is hydrogen bonding; each ammonium H atom acts as a bifurcated donor and three of the four violurate O atoms are bifurcated acceptors, with the fourth acting as a trifurcated acceptor. The pattern of hydrogen bonding around the cation is very similar to the rubidium coordination environment in the related structure. The violurate anions pack as hydrogen-bonded crinkled tapes, which are linked and separated by the ammonium cations to give a compact three-dimensional structure.

S1. Comment

Violuric acid is a derivative of barbituric acid, having an isonitroso substituent at the 4-position on the barbiturate ring. We are interested in the structures of barbituric acid and its derivatives (Nichol & Clegg 2005a,b, 2007) and of their s-block coordination complexes (Nichol & Clegg, 2006b). Additionally we have reported a definitive redetermination of violuric acid monohydrate (Nichol & Clegg, 2005d), a second polymorph of violuric acid monohydrate (Guille et al., 2007) and the structure of violuric acid methanol solvate (Nichol & Clegg, 2005c).

Complexes of violuric acid with potassium, rubidium (Gillier, 1965) and strontium (Hamelin, 1976) are already known. The isonitroso substituent allows greater scope for coordination when compared with unsubstituted barbituric acid, and it also introduces two extra hydrogen-bonding acceptors. We have also been investigating the influence of weak hydrogen bonding in organic complexes of these barbiturate derivatives using a counter-ion that forms no intermolecular classical hydrogen bonds (Nichol & Clegg, 2006a), and we wished to investigate the hydrogen bonding in an organic complex in which classical hydrogen bonding is expected to be dominant. The ammonium salt of violuric acid, (I), is a logical choice for study since the NH₄⁺ cation has four hydrogen-bond donor sites. Additionally the crystal structure of ammonium barbiturate has already been reported (Craven, 1964), giving us a useful comparison structure.

The asymmetric unit of (I) is presented in Fig. 1. The salt crystallizes from aqueous solution in space group P1 and has Z' = 1. A search of the Cambridge Structural Database (version 5.28 plus two updates; Allen, 2002) shows that this compound is isostructural with the rubidium complex determined by Gillier (1965), the ammonium and rubidium cations being approximately the same size. The C, N and O atoms of the violurate ring form a planar skeleton (r.m.s. deviation 0.0075 Å) and the isonitroso group is almost coplanar with the ring (the r.m.s. deviation for all non-H atoms is 0.1965 Å). The isonitroso group is ordered, disorder of this group being occasionally seen in violurate crystal structures. Bond lengths are listed in Table 1 and are largely unexceptional. At 1.351 (3) Å, the C═N bond is longer than that observed in neutral violuric acid monohydrate (1.295 Å; Nichol & Clegg, 2005d), whilst the N—O bond, at 1.265 (3) Å, is shorter than that observed for N—OH (1.346 Å); this is consistent with what would be expected upon deprotonation of the OH group. The two C—C bond lengths are similar to those found in neutral violuric acid. The ammonium cation is regular tetrahedral as expected, and the freely refined N—H distances range from 0.91 (4) to 0.95 (3) Å, with no significant angular distortions from ideal geometry.

The violurate anion has five potential hydrogen-bonding acceptor sites and two donor sites, with an additional four donor sites in the ammonium cation. Indeed this hydrogen-bonding arrangement is a rather good match for the rubidium coordination in the known isostructural complex. Table 2 gives details of the hydrogen-bonding geometry, Fig. 2 gives a pictorial representation of how the ammonium cation interacts with the violurate anion, and Fig. 3 shows both the hydrogen bonding in (I) and the rubidium coordination in the related structure. This shows that all the hydrogen bonding interactions are replicated as Rb—O contacts in the coordination complex, two additional Rb—O and Rb—N contacts (one each) probably resulting from the nondirectional Coulombic nature of the metal–ligand bonds, which are not found in the more directional hydrogen-bonding interactions.

In the hydrogen-bonding analysis, each H atom of the ammonium cation acts as a bifurcated donor whilst the O atoms of the violurate anion are a mixture of bifurcated and trifurcated acceptors. Hydrogen bonding consists of a combination of short strong, almost linear interactions with long, weak and very bent interactions. This is in contrast to ammonium barbiturate, where the hydrogen bonding is much more conventional in length and direction. Firstly there are the 'standard' hydrogen bonds found in the R²₂(8) motif (Bernstein et al., 1995) involving N—H···O interactions about a crystallographic inversion centre, which link the violurate anions to form a tape. This is a very common motif in the packing of molecules of this type and they are indicated by dotted lines (shown in blue in the online version of the journal). Secondly there are the longer interactions between the ammonium cation and the oxygen acceptors of the violurate anion. These are denoted by five-dash lines (orange in the online version) and range from the reasonably short and linear (N4—H1···O4ⁱⁱⁱ) to the incredibly long (N4—H4···O1ⁱ) and the extremely bent (N4—H3···O4ⁱⁱ) (symmetry codes as in Table 2). Most are of the form D(3), although the bifurcated atom H3 forms an R²₁(6) motif with atoms O3 and O4 as acceptors. Finally, there is a long N4–H1···N3ⁱⁱⁱ interaction which is denoted by a long-dashed line (black in the online version). The relative orientation suggests minimal interaction between the H atom and the acceptor lone pair, and so this is a very weak interaction, forming an R²₁(3) interaction with atom O4 as the other acceptor. Fig. 4 shows an a-axis projection revealing the web of hydrogen bonds that hold the violurate anions together into tapes and which link these tapes via the ammonium cation to form the three-dimensional structure.

The crystal packing of (I) shares some similarities with the crystal packing of ammonium barbiturate. A projection along the ab diagonal is given in Fig. 5. The ammonium ion lies out of the plane of the violurate tapes and keeps the tapes approximately 4.8 Å apart, which is much larger than the separation of 3.25 Å seen in ammonium barbiturate; there is thus no π–π stacking in ammonium violurate. Hydrogen bonding of the ammonium cations to the violurate tapes completes the three-dimensional framework in this crystal structure.

Related literature

For related literature, see: Allen (2002); Bernstein et al. (1995); Craven (1964); Gillier (1965); Guille et al. (2007); Hamelin (1976); Nichol & Clegg (2005a, 2005b, 2005c, 2005d, 2006a, 2006b, 2007).

S2. Experimental

Ammonium carbonate (0.08 g, 1 mmol) was added to a pale-yellow solution of violuric acid (0.17 g, 1 mmol) dissolved in 10 ml of hot distilled water. The solution instantly turned purple. Overnight standing at room temperature resulted in a powder-like mass of very small violet crystals of (I) (mass 0.123 g, yield 70%). CHN analysis found: C 27.33, H 3.54, N 31.68%; C₄H₆N₄O₄ requires: C 27.58, H 3.45, N 32.18%.

S3. Refinement

All hydrogen atoms were located in a difference map and were freely refined. N–H distances are in the range 0.90 (3)–0.96 (3) Å. All U_iso(H) values were freely refined.

Computing details

Data collection: COLLECT (Hooft, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT (Hooft, 1998); data reduction: DENZO (Otwinowski & Minor, 1997) and COLLECT (Hooft, 1998); program(s) used to solve structure: SHELXTL (Sheldrick, 2001); program(s) used to refine structure: SHELXTL (Sheldrick, 2001); molecular graphics: DIAMOND (Brandenburg & Putz, 2004); software used to prepare material for publication: SHELXTL (Sheldrick, 2001) and local programs.

Figures

	Fig. 1. The asymmetric unit of (I), with 50% probability displacement ellipsoids and H atoms as small spheres.
	Fig. 2. The hydrogen-bonding environment of one ammonium cation (top) and one violurate anion (bottom) in (I). Dotted lines (blue in the online version of the journal) indicate the hydrogen bonds linking the violurate anions into a tape, five-dash lines (orange in the online version) indicate the N—H···O bonds from ammonium, and long-dashed lines (black in the online version) indicate the N—H···N bonds.
	Fig. 3. A comparison of the hydrogen-bonding environment of (I) with the coordination environment of the known rubidium complex. The two are very similar.
	Fig. 4. A projection along the a axis of (I).
	Fig. 5. A projection along the ab diagonal of (I). Ammonium N—H···O interactions are indicated by short-dashed lines [orange in the online version of the journal) and the N—H···N interaction is indicated by long-dashed lines (black).

Ammonium violurate

Crystal data

NH4^+·C4H2N3O4^−	Z = 2
Mr = 174.13	F(000) = 180
Triclinic, P1	Dx = 1.800 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 4.8087 (6) Å	Cell parameters from 1410 reflections
b = 7.4678 (10) Å	θ = 2.9–27.5°
c = 9.5748 (11) Å	µ = 0.16 mm^−1
α = 101.734 (13)°	T = 120 K
β = 93.643 (14)°	Shard, light purple
γ = 105.869 (11)°	0.06 × 0.04 × 0.01 mm
V = 321.22 (8) Å^3

Data collection

Bruker-Nonius KappaCCD diffractometer	1466 independent reflections
Radiation source: Bruker-Nonius FR591 rotating anode	1102 reflections with I > 2σ(I)
10cm confocal mirrors monochromator	Rint = 0.067
ϕ and ω scans	θmax = 27.6°, θmin = 4.0°
Absorption correction: multi-scan (SADABS; Sheldrick, 2003)	h = −6→6
Tmin = 0.920, Tmax = 0.998	k = −9→9
7149 measured reflections	l = −12→12

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)] = 0.054	Hydrogen site location: difference Fourier map
wR(F^2) = 0.133	All H-atom parameters refined
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0555P)^2 + 0.3524P] where P = (Fo^2 + 2Fc^2)/3
1466 reflections	(Δ/σ)max < 0.001
133 parameters	Δρmax = 0.56 e Å^−3
0 restraints	Δρmin = −0.26 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.

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	0.6589 (3)	0.4998 (2)	0.83350 (17)	0.0198 (4)
O2	0.9327 (3)	0.6859 (2)	0.43065 (17)	0.0198 (4)
O3	0.3806 (4)	1.0013 (2)	0.67959 (17)	0.0202 (4)
O4	0.2018 (4)	0.8773 (2)	0.91461 (18)	0.0232 (4)
N1	0.7901 (4)	0.5956 (3)	0.6332 (2)	0.0162 (4)
H1N	0.883 (7)	0.496 (5)	0.616 (3)	0.035 (8)*
N2	0.6600 (4)	0.8417 (3)	0.5606 (2)	0.0177 (4)
H2N	0.657 (6)	0.904 (4)	0.489 (3)	0.023 (7)*
N3	0.3458 (4)	0.7580 (3)	0.8907 (2)	0.0203 (5)
N4	0.8913 (5)	0.7542 (3)	0.1278 (2)	0.0229 (5)
H1	1.000 (7)	0.796 (5)	0.057 (4)	0.044 (9)*
H2	0.729 (7)	0.650 (5)	0.080 (3)	0.040 (9)*
H3	0.826 (8)	0.847 (6)	0.180 (4)	0.053 (10)*
H4	1.003 (9)	0.717 (6)	0.192 (5)	0.066 (12)*
C1	0.6478 (5)	0.6099 (3)	0.7552 (2)	0.0159 (5)
C2	0.8026 (5)	0.7064 (3)	0.5363 (2)	0.0160 (5)
C3	0.5026 (5)	0.8746 (3)	0.6749 (2)	0.0158 (5)
C4	0.4962 (5)	0.7557 (3)	0.7762 (2)	0.0160 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0207 (9)	0.0211 (8)	0.0197 (8)	0.0059 (7)	0.0026 (7)	0.0093 (7)
O2	0.0235 (9)	0.0226 (9)	0.0185 (8)	0.0121 (7)	0.0073 (7)	0.0074 (7)
O3	0.0264 (9)	0.0220 (9)	0.0187 (8)	0.0141 (7)	0.0092 (7)	0.0075 (7)
O4	0.0238 (9)	0.0243 (9)	0.0224 (9)	0.0075 (7)	0.0080 (7)	0.0047 (7)
N1	0.0168 (9)	0.0173 (9)	0.0168 (10)	0.0075 (8)	0.0026 (7)	0.0060 (8)
N2	0.0205 (10)	0.0207 (10)	0.0171 (10)	0.0093 (8)	0.0067 (8)	0.0100 (8)
N3	0.0170 (10)	0.0220 (10)	0.0198 (10)	0.0042 (8)	0.0026 (8)	0.0021 (8)
N4	0.0245 (11)	0.0245 (11)	0.0181 (11)	0.0032 (9)	0.0062 (9)	0.0057 (9)
C1	0.0133 (10)	0.0172 (11)	0.0156 (10)	0.0020 (8)	0.0003 (8)	0.0039 (9)
C2	0.0137 (10)	0.0156 (11)	0.0183 (11)	0.0037 (8)	0.0009 (8)	0.0040 (9)
C3	0.0124 (10)	0.0182 (11)	0.0158 (11)	0.0023 (8)	0.0026 (8)	0.0039 (9)
C4	0.0133 (10)	0.0178 (11)	0.0160 (11)	0.0033 (9)	0.0014 (8)	0.0040 (9)

Geometric parameters (Å, º)

O1—C1	1.229 (3)	N2—C3	1.389 (3)
O2—C2	1.228 (3)	N3—C4	1.351 (3)
O3—C3	1.237 (3)	N4—H1	0.93 (4)
O4—N3	1.265 (3)	N4—H2	0.95 (3)
N1—H1N	0.96 (3)	N4—H3	0.91 (4)
N1—C1	1.392 (3)	N4—H4	0.93 (4)
N1—C2	1.356 (3)	C1—C4	1.456 (3)
N2—H2N	0.90 (3)	C3—C4	1.439 (3)
N2—C2	1.359 (3)
H1N—N1—C1	116.1 (18)	O1—C1—N1	118.0 (2)
H1N—N1—C2	118.0 (18)	O1—C1—C4	126.4 (2)
C1—N1—C2	125.84 (19)	N1—C1—C4	115.55 (19)
H2N—N2—C2	114.1 (18)	O2—C2—N1	122.4 (2)
H2N—N2—C3	120.0 (18)	O2—C2—N2	120.9 (2)
C2—N2—C3	125.61 (19)	N1—C2—N2	116.68 (19)
O4—N3—C4	118.1 (2)	O3—C3—N2	117.42 (19)
H1—N4—H2	107 (3)	O3—C3—C4	126.4 (2)
H1—N4—H3	113 (3)	N2—C3—C4	116.20 (19)
H1—N4—H4	111 (3)	N3—C4—C1	114.1 (2)
H2—N4—H3	109 (3)	N3—C4—C3	125.7 (2)
H2—N4—H4	110 (3)	C1—C4—C3	120.12 (19)
H3—N4—H4	107 (3)
C2—N1—C1—O1	−179.3 (2)	O4—N3—C4—C3	−1.7 (3)
C2—N1—C1—C4	1.2 (3)	O3—C3—C4—N3	2.7 (4)
C1—N1—C2—O2	180.0 (2)	O3—C3—C4—C1	179.1 (2)
C1—N1—C2—N2	−0.8 (3)	N2—C3—C4—N3	−176.8 (2)
C3—N2—C2—O2	178.9 (2)	N2—C3—C4—C1	−0.4 (3)
C3—N2—C2—N1	−0.4 (3)	O1—C1—C4—N3	−3.2 (3)
C2—N2—C3—O3	−178.6 (2)	O1—C1—C4—C3	−180.0 (2)
C2—N2—C3—C4	1.0 (3)	N1—C1—C4—N3	176.19 (19)
O4—N3—C4—C1	−178.29 (19)	N1—C1—C4—C3	−0.6 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···O2^i	0.96 (3)	1.81 (3)	2.772 (2)	175 (3)
N2—H2N···O3^ii	0.90 (3)	1.91 (3)	2.805 (3)	172 (3)
N4—H1···O4^iii	0.93 (4)	1.84 (4)	2.765 (3)	178 (3)
N4—H1···N3^iii	0.93 (4)	2.41 (4)	3.249 (3)	150 (3)
N4—H2···O1^iv	0.95 (3)	2.20 (3)	2.898 (3)	130 (3)
N4—H2···O1^v	0.95 (3)	2.36 (3)	3.010 (3)	125 (2)
N4—H3···O3^ii	0.91 (4)	2.06 (4)	2.947 (3)	165 (3)
N4—H3···O4^ii	0.91 (4)	2.45 (4)	3.007 (3)	120 (3)
N4—H4···O2	0.93 (4)	2.38 (4)	3.048 (3)	129 (3)
N4—H4···O1^i	0.93 (4)	2.58 (4)	3.293 (3)	134 (3)

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