catena-Poly­[[di­aqua­lithium(I)]-μ-[9H-purine-2,6(1H,3H)-dionato-O²:N⁷]]

In the title compound, [Li(C₅H₃N₄O₂)(H₂O)₂]_n, the coordinate geometry about the Li⁺ ion is distorted tetrahedral and the Li⁺ ion is bonded to N and O atoms of adjacent ligand mol­ecules forming an infinite polymeric chain with Li—O and Li—N bond lengths of 1.901 (5) and 2.043 (6) Å, respectively. Tetrahedral coordination at the Li⁺ ion is completed by two cis water mol­ecules [Li—O 1.985 (6) and 1.946 (6) Å]. The crystal structure is stabilized both by the polymeric structure and by a hydrogen-bond network involving N—H...O, O—H...O and O—H...N hydrogen bonds.

S1. Comment

3,7-Dihydro-1H-purine-2,6-dione, (I), also known as xanthine, is a well known metabolite of purine bases, guanine and adenine, derived from hypoxanthine by xanthine oxidase or from guanine by guanase (Martin, 1983). Xanthine is further converted to the final metabolite, namely uric acid. In xanthinurea, crystalline xanthine is found as a constituent of urinary calculi or in muscle tissue as a crystalline alkaline salt of uric acid in hyperuricemia or gout. Up until now, structures of chelate compounds of xanthine with the transition metal ions Cu^II, Zn^II Co^III, Ti^III and CH₃Hg^I have been determined (Dubler et al., 1992). However structural information on compounds with alkaline or alkaline earth metals is rare. The only structurally characterized compound is the sodium salt of xanthine (Mizuno et al., 1969), in which each Na⁺ cation is surrounded by six water molecules and each xanthine anion participates in hydrogen bonds without direct coordination to the metal ions.

In order to clarify the coordination modes of chelate compounds of xanthine with alkaline metals, we have analyzed the 1:1 complex, (II), of the Li⁺ ion with xanthine. \sch

The complex contains a monoanionic ligand in which deprotonation of the xanthine has taken place at the N3—H imino group (see Fig. 1). The coordination geometry about the Li⁺ ion is distorted tetrahedral (Table 1) with the Li⁺ ion bonded to N7 of one ligand and O1 of a symmetry-related ligand to yield an infinite-chain structure. This polymeric coordination mode has not been seen in any other metal compounds of xanthine. The O1—C2 bond distance of 1.268 (4) Å is significantly longer than the standard C═O double-bond distance of 1.20 Å. This suggests the presence of some negative charge at the O1 atom of the xanthine anion. The Li^I ion is also bonded to two water O atoms.

The crystal structure is also stabilized by a network of N—H···O, O—H···O and O—H···N hydrogen bonds, as detailed in Table 2. No stacking interactions between the xanthine ligands are observed.

S2. Experimental

The colorless prismatic crystal used for analysis was obtained by slow evaporation from a 50% ethanol–water solution of a mixture containing xanthine and lithium hydroxide (1:1) at room temperature.

S3. Refinement

All H atoms were clearly resolved in difference maps. Those bonded to C and N atoms were treated as riding atoms (C—H 0.93 Å and N—H 0.86 Å). The O—H distances in the water molecule were controlled by a SHELXL97 DFIX command (Sheldrick, 1997), with an overall O—H distance as a free-variable (see Table 2 for O—H distances). An overall isotropic vibration parameter for the water H atoms was initially refined as a free-variable and in the final cycles, this parameter was set at the refined value (0.054 Ų). No correction was required for extinction. With no atoms having significant anomalous dispersion being present, the few measured Friedel reflections were merged to give the reflection file used in the final refinement.

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1992); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: TEXSAN and PROCESS (Molecular Structure Corporation, 1992); program(s) used to solve structure: SIR88 (Burla et al., 1989) and DIRDIF (Beurskens et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976) and PLATON (Spek, 2000); software used to prepare material for publication: IUCr SHELXL97 template.

Figures

Fig. 1. ORTEPII (Johnson, 1976) drawing of (II) with the atomic numbering scheme. Ellipsoids for non-H atoms correspond to 50% probability. The symmetry codes are as in Table 1.

Diaquabis[3,7-dihydro-1H-purine-2,6-dione]lithium(I)

Crystal data

[Li(C5H3N4O2)(H2O)2]	Z = 1
Mr = 194.08	F(000) = 100
Triclinic, P1	Dx = 1.632 Mg m^−3
a = 5.4017 (5) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.3903 (7) Å	Cell parameters from 25 reflections
c = 4.6325 (4) Å	θ = 12.7–14.5°
α = 102.237 (7)°	µ = 0.14 mm^−1
β = 93.676 (8)°	T = 293 K
γ = 104.154 (6)°	Prism, colorless
V = 197.46 (3) Å^3	0.13 × 0.12 × 0.10 mm

Data collection

Rigaku AFC-5R diffractometer	Rint = 0.018
Radiation source: fine-focus sealed tube	θmax = 27.5°, θmin = 2.6°
Graphite monochromator	h = −7→7
ω–2θ scans	k = −10→10
1015 measured reflections	l = 0→6
903 independent reflections	3 standard reflections every 150 reflections
722 reflections with I > 2σ(I)	intensity decay: 0.1%

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)] = 0.032	Hydrogen site location: diff-map
wR(F^2) = 0.096	H atoms treated by a mixture of independent and constrained refinement
S = 0.87	w = 1/[σ^2(Fo^2)]
903 reflections	(Δ/σ)max < 0.001
140 parameters	Δρmax = 0.18 e Å^−3
7 restraints	Δρmin = −0.22 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
Li1	0.5000 (10)	0.5000 (6)	0.5001 (13)	0.0304 (12)
O1	0.2554 (4)	−0.3906 (3)	0.3795 (6)	0.0326 (6)
O2	0.6462 (4)	0.1030 (3)	0.1529 (5)	0.0325 (6)
O3	0.7090 (5)	0.6592 (3)	0.8618 (5)	0.0341 (6)
H31	0.751 (10)	0.592 (6)	0.963 (11)	0.054*
H32	0.854 (7)	0.695 (6)	0.797 (11)	0.054*
O4	0.7537 (4)	0.4451 (3)	0.2435 (5)	0.0309 (5)
H41	0.913 (6)	0.482 (6)	0.320 (11)	0.054*
H42	0.727 (10)	0.336 (3)	0.194 (13)	0.054*
N1	0.4430 (5)	−0.1383 (3)	0.2833 (7)	0.0256 (6)
H1	0.5356	−0.1904	0.1748	0.031*
N3	0.1145 (5)	−0.1657 (3)	0.5998 (6)	0.0260 (6)
N7	0.3050 (6)	0.2707 (4)	0.5623 (7)	0.0304 (6)
N9	0.0275 (5)	0.1067 (4)	0.7833 (6)	0.0300 (6)
H9	−0.0906	0.0779	0.8926	0.036*
C2	0.2662 (5)	−0.2344 (4)	0.4279 (7)	0.0245 (7)
C4	0.1513 (5)	0.0030 (4)	0.6187 (7)	0.0223 (6)
C5	0.3236 (5)	0.1062 (4)	0.4826 (7)	0.0222 (6)
C6	0.4839 (6)	0.0325 (4)	0.2973 (7)	0.0218 (6)
C8	0.1259 (7)	0.2641 (4)	0.7419 (8)	0.0320 (7)
H8	0.0721	0.3579	0.8315	0.038*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Li1	0.029 (3)	0.019 (2)	0.047 (3)	0.009 (2)	0.010 (2)	0.012 (2)
O1	0.0289 (11)	0.0161 (11)	0.0547 (15)	0.0070 (9)	0.0093 (11)	0.0101 (10)
O2	0.0335 (12)	0.0251 (12)	0.0436 (14)	0.0088 (9)	0.0271 (11)	0.0105 (10)
O3	0.0390 (14)	0.0328 (14)	0.0336 (14)	0.0109 (11)	0.0156 (11)	0.0100 (11)
O4	0.0280 (11)	0.0222 (11)	0.0432 (14)	0.0048 (9)	0.0143 (10)	0.0081 (10)
N1	0.0255 (11)	0.0191 (12)	0.0345 (14)	0.0082 (9)	0.0143 (11)	0.0059 (10)
N3	0.0255 (13)	0.0207 (13)	0.0348 (14)	0.0053 (10)	0.0121 (11)	0.0117 (11)
N7	0.0357 (14)	0.0232 (14)	0.0361 (15)	0.0099 (11)	0.0189 (12)	0.0083 (12)
N9	0.0303 (13)	0.0261 (13)	0.0372 (15)	0.0084 (11)	0.0217 (12)	0.0089 (11)
C2	0.0228 (14)	0.0198 (15)	0.0323 (17)	0.0054 (11)	0.0073 (13)	0.0082 (13)
C4	0.0220 (14)	0.0240 (14)	0.0229 (15)	0.0086 (11)	0.0095 (13)	0.0048 (11)
C5	0.0230 (14)	0.0180 (13)	0.0266 (15)	0.0045 (11)	0.0105 (13)	0.0066 (12)
C6	0.0217 (13)	0.0183 (14)	0.0270 (15)	0.0056 (11)	0.0107 (12)	0.0062 (11)
C8	0.0385 (17)	0.0228 (16)	0.0387 (18)	0.0123 (13)	0.0197 (15)	0.0064 (13)

Geometric parameters (Å, º)

Li1—O1^i	1.901 (5)	N1—C2	1.395 (4)
Li1—O3	1.985 (6)	N1—H1	0.86
Li1—O4	1.946 (6)	N3—C2	1.326 (4)
Li1—N7	2.043 (6)	N3—C4	1.363 (4)
O1—C2	1.268 (4)	N7—C8	1.314 (5)
O1—Li1^ii	1.901 (5)	N7—C5	1.382 (4)
O2—C6	1.247 (4)	N9—C8	1.357 (4)
O3—H31	0.86 (3)	N9—C4	1.362 (4)
O3—H32	0.86 (3)	N9—H9	0.86
O4—H41	0.87 (3)	C4—C5	1.388 (4)
O4—H42	0.86 (3)	C5—C6	1.415 (4)
N1—C6	1.382 (4)	C8—H8	0.93
O1^i—Li1—O3	105.5 (3)	C8—N9—C4	106.9 (3)
O1^i—Li1—O4	120.9 (3)	C8—N9—H9	126.5
O1^i—Li1—N7	108.3 (3)	C4—N9—H9	126.5
O4—Li1—O3	104.1 (3)	O1—C2—N3	122.4 (3)
O4—Li1—N7	103.6 (2)	O1—C2—N1	116.2 (3)
O3—Li1—N7	114.9 (3)	N3—C2—N1	121.4 (3)
C2—O1—Li1^ii	129.8 (3)	N9—C4—N3	126.5 (3)
H31—O3—H32	100 (5)	N9—C4—C5	105.4 (3)
Li1—O4—H41	117 (4)	N3—C4—C5	128.1 (3)
Li1—O4—H42	109 (4)	N7—C5—C4	110.3 (3)
H41—O4—H42	104 (5)	N7—C5—C6	131.4 (3)
C6—N1—C2	126.3 (3)	C4—C5—C6	118.3 (3)
C6—N1—H1	116.8	O2—C6—N1	119.9 (3)
C2—N1—H1	116.8	O2—C6—C5	127.8 (3)
C2—N3—C4	113.6 (3)	N1—C6—C5	112.3 (3)
C8—N7—C5	104.1 (3)	N7—C8—N9	113.2 (3)
C8—N7—Li1	119.3 (3)	N7—C8—H8	123.4
C5—N7—Li1	136.3 (3)	N9—C8—H8	123.4
O1^i—Li1—N7—C8	−54.2 (4)	Li1—N7—C5—C4	173.2 (3)
O4—Li1—N7—C8	176.3 (3)	C8—N7—C5—C6	−179.2 (4)
O3—Li1—N7—C8	63.4 (4)	Li1—N7—C5—C6	−6.1 (7)
O1^i—Li1—N7—C5	133.5 (4)	N9—C4—C5—N7	−0.1 (3)
O4—Li1—N7—C5	4.0 (5)	N3—C4—C5—N7	−178.9 (4)
O3—Li1—N7—C5	−108.9 (4)	N9—C4—C5—C6	179.3 (3)
Li1^ii—O1—C2—N3	−112.4 (4)	N3—C4—C5—C6	0.5 (4)
Li1^ii—O1—C2—N1	69.2 (5)	C2—N1—C6—O2	−178.9 (3)
C4—N3—C2—O1	−178.6 (3)	C2—N1—C6—C5	0.4 (4)
C4—N3—C2—N1	−0.3 (5)	N7—C5—C6—O2	−2.1 (6)
C6—N1—C2—O1	178.5 (3)	C4—C5—C6—O2	178.6 (3)
C6—N1—C2—N3	0.1 (5)	N7—C5—C6—N1	178.6 (3)
C8—N9—C4—N3	178.9 (3)	C4—C5—C6—N1	−0.6 (4)
C8—N9—C4—C5	0.1 (4)	C5—N7—C8—N9	−0.1 (4)
C2—N3—C4—N9	−178.5 (3)	Li1—N7—C8—N9	−174.6 (3)
C2—N3—C4—C5	0.0 (5)	C4—N9—C8—N7	0.0 (4)
C8—N7—C5—C4	0.1 (4)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H31···O4^iii	0.86 (3)	1.98 (3)	2.814 (3)	163 (5)
O3—H32···N3^iv	0.86 (3)	2.00 (3)	2.820 (4)	159 (5)
O4—H41···O1^iv	0.87 (3)	1.86 (3)	2.689 (3)	159 (5)
O4—H42···O2	0.86 (3)	1.86 (3)	2.719 (3)	170 (6)
N1—H1···O3^v	0.86	2.13	2.985 (4)	171
N9—H9···O2^vi	0.86	1.94	2.760 (3)	158

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