5-Methyl­sulfanyl-3H-1,2,3-triazolo[4,5-d]pyrimidin-7(6H)-one (2-methyl­thio-8-aza­xanthine) monohydrate

The title compound, C₅H₅N₅OS·H₂O, crystallizes as the monohydrate. Disorder of the H atoms that participate in the hydrogen bonds implies that two different tautomers are present in the crystal structure, one of them with both acidic H atoms attached to the imidazole ring and the other with one acidic H atom on each ring.

S1. Comment

Triazolopyrimidine compounds exhibit a wide spectrum of biological activities and versatile coordination properties. The interaction of metal ions with 1,2,4-triazolo[1,5-a]pyrimidine derivatives has been extensively investigated by our research group and a number of papers have been reported (Salas et al., 1999). Recently, we have started to explore the coordination possibilities of other triazolopyrimidines, such as 1,2,4-triazolo[4,3-a]pyrimidines, which differ from the [1,5-a] derivatives by the position of one N atom in the triazole ring (Salameh et al., 2005). 1,2,3-Triazolo[4,5-d]pyrimidines, which contain three contiguous N atoms in the imidazole ring, represent yet another way of joining a triazole and a pyrimidine ring. These compounds may also be regarded as purine derivatives, replacing the external C atom of the imidazole ring with a N atom. For this reason, these compounds may also be named as 8-azapurines, using a biochemical instead of a systematic IUPAC numbering scheme.

Several crystal structures have been reported for this family of ligands (Sánchez et al., 1995) as well as for some of their metal complexes (Ravichandran et al., 1986; Sheldrick & Bell, 1986). Most of these results were published prior to 1990, with very few of them having been published in the last 25 years. Revisiting this research area, this paper describes the crystal structure of a member of the family which has a methylmercapto group at position 5, namely 6,7-dihydro-5-methylmercapto-7-oxo-1,2,3-triazolo-[4,5-d]pyrimidine, (I), which may also be named as a substituted purine using the biochemical numbering scheme, i.e. 2-methylthio-8-azaxanthine. However, the systematic IUPAC name and numbering scheme have been used throughout this paper and are displayed in Figs. 1 and 2.

Compound (I) crystallizes as the monohydrate and the asymmetric unit is made up of just of one organic moiety and a water molecule. A hydrogen bond is formed between two crystallographically equivalent water molecules related by a binary axis [O1W···O1Wⁱ = 2.717 (5) Å; symmetry code: 1 − x, y, −z + 1/2 Please check added symmetry code]. The corresponding H atom, which was located in a ΔF map, has been forcibly disordered between two equivalent positions close to the binary axis. In one position the water molecule acts as an H-atom donor and in the other it acts as an acceptor of the O1W···O1Wⁱ hydrogen bond. The water molecule also forms hydrogen bonds with atom N6 in the same asymmetric unit and atom N1 of an adjacent molecule at (1/2 − x, −1/2 + y, 1/2 − z) [distances of 2.768 (4) and 2.844 (4) Å, respectively]. Two weak peaks were found in each of these two O···N regions, one close to O and the other close to N. This has been rationalized using two superimposed hydrogen-bonding schemes. When the water molecule acts as a donor of the O1W···O1Wⁱ hydrogen bond, it also acts as donor for the O1W···N1 hydrogen bond and acceptor of the O1W···N6 hydrogen bond. The scheme is reversed when the water molecule acts as the acceptor of the O1W···O1Wⁱ hydrogen bond.

This implies that the organic molecule of (I) is disordered between two tautomers, one with an H atom attached to atom N6 and the other with an H atom attached to atom N1. These two tautomers are depicted in Figs. 1 and 2, respectively. Both hydrogen-bonding schemes, including the corresponding tautomers for the heterocycle, and the relation between them are represented in Fig. 3. In both cases, the remaining acidic H atom is clearly located on atom N3, forming a hydrogen bond with the carbonyl O atom of the molecule at (x, 1 − y, 1/2 + z) [distance 2.692 (3) Å]. To our knowledge, the N1 tautomer is the only example with two acidic H atoms on the imidazole ring for this family of compounds.

Tautomerism is an interesting feature of these compounds, examples with one acidic H atom at each of the three imidazole N atoms having been found previously (Sánchez et al., 1995). The presence of an H atom is reflected by an opening of the corresponding endocyclic angle, comparable with the change observed when a substituent such as a methyl group is attached to an imidazole N atom (Cline & Hodgson, 1980). Endocyclic bond angles in the imidazole ring in compound (I) (Table 1) follow this trend, with the values at N1 and N3 analogous to those of compounds with H atoms or substituents at these positions and the value at N2 analogous to those of compounds with a naked N atom at that position (Cline & Hodgson, 1980; Sánchez et al., 1995). The methylmercapto group is almost coplanar with the heterocycle [torsion angle N4—C5—S—CH₃ = 0.8 (3)°].

S2. Experimental

The synthesis of (I) was carried out according to the published method of Nübel & Pfleiderer (1965). Glacial acetic acid (2 mmol) and solid NaNO₂ (2 mmol) were added to an aqueous solution (Volume?) of 5,6-diamino-4-hydroxy-2-methylthiopyrimidine (1 mmol). The mixture was allowed to evaporate at room temperature and, after 5 d, yellow prismatic crystals of the title compound were obtained. Elemental analysis data reveal that the compound crystallizes as the monohydrate. Analysis, found: C 29.20, H 3.44, N 34.98%; calculated: C 29.85, H 3.51, N 34.81%.

S3. Refinement

Methyl H atoms were idealized, allowing for free rotation around the N—C bond. H atoms attached to the N atoms in positions 3, 1 and 6 were introduced in ideal positions, the first with full occupancy and the last two with half occupancy (peaks in the ΔF maps with appropriate intensity appeared before the introduction of these atoms). Four residual peaks around the water O atom with correct positions for hydrogen bonding and reasonable angles for considering the water molecule as disordered between two positions were introduced as half-occupancy H atoms and refined with restrained O—H [0.84(s.u.?) Å] and H—H [1.33(s.u.?) Å] distances. Isotropic displacement parameters of all H atoms were fixed to 1.2 times the U_eq of their parent atoms.

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SMART; data reduction: SAINT (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Xtal_GX (Hall & du Boulay, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the asymmetric unit of (I), considered as the N6—H tautomer (see text). Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. A view of the asymmetric unit of (I), considered as the N1—H tautomer (see text). Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 3. A disorder scheme for the hydrogen bonds. Both water molecules in the centre of the drawing are related by a crystallographic binary axis. One of the alternative positions for the H atoms is represented on the left-hand side of the drawing (including the N6—H tautomer) and the other on the right-hand side (including the N1—H tautomer).

5-Methylsulfanyl-6,7-dihydro-3H-1,2,3-triazolo[4,5-d]pyrimidin-7(8H)-one monohydrate

Crystal data

C5H5N5OS·H2O	F(000) = 832
Mr = 201.22	Dx = 1.616 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 2960 reflections
a = 17.1216 (12) Å	θ = 2.6–27.8°
b = 7.3269 (5) Å	µ = 0.37 mm^−1
c = 14.3494 (10) Å	T = 100 K
β = 113.259 (1)°	Prismatic, yellow
V = 1653.8 (2) Å^3	0.53 × 0.25 × 0.17 mm
Z = 8

Data collection

Bruker SMART APEX CCD area-detector diffractometer	1886 independent reflections
Radiation source: fine-focus sealed tube	1799 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
Detector resolution: 8.26 pixels mm^-1	θmax = 28.1°, θmin = 2.6°
ϕ and ω scans	h = −22→21
Absorption correction: multi-scan (SADABS; Bruker, 1999)	k = −5→9
Tmin = 0.806, Tmax = 0.940	l = −16→18
5011 measured reflections

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)] = 0.059	Hydrogen site location: mixed
wR(F^2) = 0.126	H atoms treated by a mixture of independent and constrained refinement
S = 1.28	w = 1/[σ^2(Fo^2) + (0.015P)^2 + 7P] where P = (Fo^2 + 2Fc^2)/3
1886 reflections	(Δ/σ)max = 0.008
131 parameters	Δρmax = 0.48 e Å^−3
6 restraints	Δρmin = −0.45 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	Occ. (<1)
N1	0.17097 (17)	0.5962 (3)	0.42191 (17)	0.0273 (5)
H1	0.1312	0.6476	0.3690	0.033*	0.50
N2	0.17104 (17)	0.5831 (3)	0.51291 (18)	0.0299 (6)
N3	0.24344 (17)	0.4950 (3)	0.57277 (17)	0.0281 (6)
H3	0.2575	0.4689	0.6372	0.034*
C3a	0.29059 (19)	0.4526 (4)	0.5199 (2)	0.0248 (6)
N4	0.36755 (16)	0.3678 (3)	0.55353 (17)	0.0275 (5)
C5	0.39526 (19)	0.3474 (4)	0.4811 (2)	0.0256 (6)
S5	0.49211 (5)	0.24298 (10)	0.50147 (6)	0.0326 (2)
C51	0.5288 (2)	0.1846 (5)	0.6340 (2)	0.0411 (8)
H511	0.5362	0.2961	0.6743	0.049*
H512	0.5834	0.1207	0.6547	0.049*
H513	0.4873	0.1050	0.6450	0.049*
N6	0.35275 (15)	0.4045 (3)	0.38212 (16)	0.0244 (5)
H6	0.3758	0.3804	0.3385	0.029*	0.50
C7	0.27547 (18)	0.4976 (4)	0.3461 (2)	0.0234 (6)
O7	0.24264 (13)	0.5540 (3)	0.25832 (14)	0.0281 (5)
C7a	0.24310 (18)	0.5167 (4)	0.4235 (2)	0.0237 (6)
O1W	0.41641 (16)	0.3504 (5)	0.2344 (2)	0.0586 (9)
H11W	0.4691 (9)	0.336 (10)	0.254 (6)	0.070*	0.50
H12W	0.395 (4)	0.262 (8)	0.195 (6)	0.070*	0.50
H13W	0.396 (5)	0.366 (12)	0.278 (4)	0.070*	0.50
H14W	0.394 (5)	0.430 (10)	0.190 (5)	0.070*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0387 (14)	0.0245 (12)	0.0220 (11)	−0.0058 (11)	0.0153 (10)	0.0003 (9)
N2	0.0439 (15)	0.0255 (12)	0.0254 (12)	−0.0072 (11)	0.0192 (11)	−0.0024 (10)
N3	0.0454 (15)	0.0232 (12)	0.0181 (11)	−0.0088 (11)	0.0153 (11)	−0.0017 (9)
C3a	0.0393 (16)	0.0173 (13)	0.0181 (12)	−0.0090 (12)	0.0116 (12)	−0.0032 (10)
N4	0.0383 (14)	0.0209 (12)	0.0190 (11)	−0.0072 (11)	0.0066 (10)	−0.0001 (9)
C5	0.0327 (15)	0.0161 (12)	0.0227 (13)	−0.0095 (11)	0.0052 (11)	−0.0020 (10)
S5	0.0330 (4)	0.0236 (4)	0.0341 (4)	−0.0032 (3)	0.0058 (3)	0.0023 (3)
C51	0.0410 (19)	0.0364 (18)	0.0343 (17)	−0.0032 (15)	0.0025 (15)	0.0060 (14)
N6	0.0281 (12)	0.0249 (12)	0.0191 (11)	−0.0043 (10)	0.0081 (9)	−0.0030 (9)
C7	0.0289 (14)	0.0224 (13)	0.0200 (12)	−0.0063 (11)	0.0111 (11)	−0.0032 (10)
O7	0.0315 (11)	0.0367 (12)	0.0168 (9)	0.0009 (9)	0.0104 (8)	0.0028 (8)
C7a	0.0332 (15)	0.0200 (13)	0.0178 (12)	−0.0050 (11)	0.0098 (11)	−0.0012 (10)
O1W	0.0256 (12)	0.094 (2)	0.0408 (15)	0.0175 (14)	−0.0029 (11)	−0.0339 (15)

Geometric parameters (Å, º)

N1—N2	1.309 (3)	C51—H511	0.9801
N1—C7a	1.358 (4)	C51—H512	0.9799
N1—H1	0.8800	C51—H513	0.9799
N2—N3	1.360 (4)	N6—C7	1.394 (4)
N3—C3a	1.345 (4)	N6—H6	0.8797
N3—H3	0.8799	C7—O7	1.231 (3)
C3a—N4	1.361 (4)	C7—C7a	1.430 (4)
C3a—C7a	1.381 (4)	O1W—H11W	0.839 (10)
N4—C5	1.311 (4)	O1W—H12W	0.840 (10)
C5—N6	1.380 (3)	O1W—H13W	0.838 (10)
C5—S5	1.743 (3)	O1W—H14W	0.837 (10)
S5—C51	1.803 (3)
N2—N1—C7a	108.0 (2)	S5—C51—H513	109.8
N2—N1—H1	126.0	H511—C51—H513	109.5
C7a—N1—H1	126.0	H512—C51—H513	109.5
N1—N2—N3	107.8 (2)	C5—N6—C7	124.5 (2)
C3a—N3—N2	110.7 (2)	C5—N6—H6	118.0
C3a—N3—H3	124.6	C7—N6—H6	117.5
N2—N3—H3	124.7	O7—C7—N6	121.8 (2)
N3—C3a—N4	128.2 (2)	O7—C7—C7a	127.0 (3)
N3—C3a—C7a	103.9 (3)	N6—C7—C7a	111.3 (2)
N4—C3a—C7a	128.0 (3)	N1—C7a—C3a	109.5 (2)
C5—N4—C3a	111.9 (2)	N1—C7a—C7	131.2 (3)
N4—C5—N6	124.9 (3)	C3a—C7a—C7	119.3 (3)
N4—C5—S5	122.4 (2)	H11W—O1W—H12W	104.6 (17)
N6—C5—S5	112.7 (2)	H11W—O1W—H13W	118 (8)
C5—S5—C51	101.17 (16)	H12W—O1W—H13W	114 (8)
S5—C51—H511	109.5	H11W—O1W—H14W	118 (9)
S5—C51—H512	109.1	H12W—O1W—H14W	94 (8)
H511—C51—H512	109.5	H13W—O1W—H14W	105.4 (17)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N3—H3···O7^i	0.88	1.86	2.692 (3)	157
N6—H6···O1W	0.88	1.89	2.768 (4)	173
N1—H1···O1W^ii	0.88	2.03	2.844 (4)	154
O1W—H11W···O1W^iii	0.84 (1)	1.91 (3)	2.717 (5)	162 (9)
O1W—H12W···N1^iv	0.84 (1)	2.02 (2)	2.844 (4)	166 (9)
O1W—H13W···N6	0.84 (1)	1.93 (1)	2.768 (4)	178 (9)

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