Bis(6-thioxo-1,6-dihydro­purinium) tetra­chloro­zincate(II)

The title salt, (C₅H₅N₄S)₂[ZnCl₄], consists of two 6-thioxo-1,6-dihydro­purinium (6mpH₂⁺) cations (A and B) and a tetra­chloro­zincate anion, which are held together by N-H...Cl and C-H...Cl inter­actions. There is an anion-[pi] inter­action between one Cl atom of the [ZnCl₄]^- anion and the pyrimidine ring of the 6mpH₂⁺(B) cation. Inter­molecular [pi]-[pi] stacking inter­actions allow 6mpH₂⁺(A) cations to form anti­parallel pairs. One inter­esting structural feature is the double N-H...N inter­molecular hydrogen bonds between two 6mpH₂⁺(A) cations. This kind of inter­action, mimicking that of natural nucleobases, can be very valuable in designing new therapeutic purine derivatives.

S1. Comment

6-Mercaptopurine (6 m pH; 1,7-dihydro-6H-purine-6-thione) exists in solid and liquid states in the 6-thione form. It has also a tautomeric imine proton equilibrium between positions 7 and 9 of the imidazole ring (Pazderski et al., 2006), being found in the N9—H form in anhydrous 6 m pH (Gyr, 1991) but in the N7—H form in 6-mercaptopurine monohydrate (see the first scheme below) (Sletten et al., 1969; Brown, 1969). The 6-thioxo-1,6-dihydropurinium cation (6 m pH₂⁺), in which both the N7 and the N9 positions are protonated and the positive charge is shared by the imidazole N atoms, was previously confirmed by single-crystal X-ray diffraction and vibrational spectroscopy of 6-thioxo-1,6-dihydropurinium chloride (Perez-Ruiz et al., 1998).

Preparation and structural characterization of complexes of biologically active metals, such as zinc, with purines is one of our recent interests. In an attempt to prepare the zinc analogue of bis(6-mercaptopurinato)mercury(II) (Lavertue et al., 1976), we obtained to the title salt, (I).

There are two crystallographically independent, planar 6 m pH₂⁺ cations and one tetrachlorozincate anion in the asymmetric unit (Fig. 1). The pyrimidine H atoms of the 6 m pH₂⁺ cations A and B are involved in N1—H···Cl3 and N21—H···Cl2 hydrogen bonds, the latter being much weaker (Desiraju & Steiner, 1999), as seen from Table 2. There is also a weak, short C—H···Cl contact between C2 and Cl2, generating an R₂²(7) graph-set motif (Bernstein et al., 1995) with an N1—H···Cl2 hydrogen bond.

The 6 m pH₂⁺ cations can form pairs in the crystalline state, regardless of the electrostatic force between their positive charges. Repulsion of similar charges is reduced by antiparallel geometry, and the cations are held together by π–π stacking and possibly also by cation–π interactions (Meyer et al., 2003). Such a pair of antiparallel displaced 6 m pH₂⁺(A) cations can be observed in Fig. 2 around (1/2, 1/2, 1) with an interplanar distance of 3.447 (3) Å. The centroid-to-centroid distance of the six-membered aromatic rings is 3.584 (3) Å. Cation–π interactions in this pair may also exist, but because of the partial overlap of the 6 m pH₂⁺(A) cations with positive charges at two ends, such interactions are not favoured. There is no 6 m pH₂⁺(B) pair that has a rational centroid-to-centroid distance, but the 6 m pH₂⁺(B) cation participates in anion–π interaction with atom Cl1 (Fig. 1). The distance between the centroid of the pyrimidine ring and atom Cl1 is 3.247 (3) Å, being slightly longer than that observed in the 1,3,5-triazine complex (Demeshko, et al., 2004). The angle between the Cl1···centroid axis and the plane of the pyrimidine ring is 85°, and the Cl1···centroid axis points away from atoms N21 and N23.

Previous studies of tetrachlorozincates have shown that the Zn—Cl bond lengths are not identical (Parvez & Sabir, 1998; García-Raso et al., 1999). The environment around the anion, as well as hydrogen bonds or repulsions, influences appreciably the Zn—Cl bond lengths and Cl—Zn—Cl bond angles. In (I), however, no clear trend of such influences was detected (Table 1). For example, the fact that the Zn—Cl4 bond is the longest does not meet the expectations derived from hydrogen bonding effects (Table 2), because atom Cl4 accepts only one, rather weak, N—H···Cl contact.

Imidazole atoms N7 and N9 in the 6 m pH₂⁺ cation share the positive charge (Perez-Ruiz et al., 1998). Therefore, the bonds involving atoms N7 and N9, as well as N27 and N29, in (I) are expected to be of about equal length. In fact, the N7—C8 and N27—C28 bonds (Table 1) are comparable to the N7—C8 distance in 6 m pH₂Cl (Pazderski et al., 2006), and the N9—C8 and N29—C28 bonds are slightly longer than the N7—C8 and N27—C28 bonds. However, all of these bond lengths in (I) are closer to one another than the analogous bonds in 6 m pH·H₂O and in anhydrous 6 m pH (Pazderski et al., 2006; Gyr, 1991). One interesting structural feature involving the imidazole N atoms is the double N9—H9···N3 intermolecular hydrogen-bond system between two 6 m pH₂⁺(A) cations (shown at the centre of Fig. 2), generating an R₂²(8) graph-set motif. These cations are in the same plane and related by an inversion centre.

Sulfur is a weak atomic acceptor and it is often superseded in hydrogen-bond formation because of the presence of stronger acceptors (Desiraju & Steiner, 1999). Certainly, some such interactions, e.g. N—H···S (Dubler & Gyr, 1988) and C—H···S (Cini et al., 2000), have been reported for 6 m pH compounds, but they are rather unusual. In (I), there is possibly a very weak C28—H28···S26^v interaction (Table 2). Atom S26 also forms a short contact, 3.3929 (12) Å, to Cl4 at (−x, 1/2 + y, 3/2 − z), which is shorter than the sum of van der Waals radii (3.55 Å).

S2. Experimental

6-Mercaptopurine monohydrate (119 mg, 0.70 mmol) and zinc chloride (48 mg, 0.35 mmol) were dissolved in water (50 ml). The mixture was stirred at room temperature for one hour, then aqueous 2M HCl (20 ml) was added and the resulting solution was left to stand. After one week, some crystals and a yellow [or white?] powder were formed, and these were removed from the solution; the powder was found to be mostly 6-thioxo-1,6-dihydropurinium chloride, and the crystals were found to be 6-mercaptopurine monohydrate. After slow evaporation of the solution at room temperature over a period of three months, only a few colourless crystals of (I) were obtained. A similar synthetic route was also tested with cobalt(II) and cadmium(II) chlorides without any success.

S3. Refinement

All H atoms were found in difference density maps, but were placed in ideal calculated positions and allowed to ride on their parent atoms at distances of 0.88 (N—H) and 0.95 Å (C—H), with U_iso(H) of 1.2U_eq(C,N).

Computing details

Data collection: Collect (Bruker, 2004); cell refinement: DENZO–SMN (Otwinowski & Minor, 1997); data reduction: DENZO–SMN; program(s) used to solve structure: SIR2002 (Burla, et al., 2003); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The 6 m pH2^+ and [ZnCl4]^2− components of (I), showing the atom-labelling scheme. Displacement ellipsoids were drawn at the 50% probability level and non-covalent interactions are illustrated with dashed lines.
	Fig. 2. The packing of (I), viewed along the b axis. Hydrogen bonds are illustrated with dashed lines. Symmetry code: (i) −x + 1, y − 1/2, −z + 3/2.

Bis(6-thioxo-1,6-dihydropurinium) tetrachlorozincate(II)

Crystal data

(C5H5N4S)2[ZnCl4]	F(000) = 1024
Mr = 513.55	Dx = 1.851 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 16093 reflections
a = 17.1397 (5) Å	θ = 0.4–28.3°
b = 6.8500 (2) Å	µ = 2.15 mm^−1
c = 16.1142 (5) Å	T = 173 K
β = 103.021 (2)°	Block, colourless
V = 1843.27 (10) Å^3	0.25 × 0.15 × 0.15 mm
Z = 4

Data collection

Bruker Kappa APEX-II diffractometer	4528 independent reflections
Radiation source: fine-focus sealed tube	3408 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.087
Detector resolution: 9 pixels mm^-1	θmax = 28.3°, θmin = 2.4°
ϕ and ω scans	h = −22→22
Absorption correction: multi-scan (MULABS in PLATON; Blessing, 1995; Spek, 2003)	k = −9→8
Tmin = 0.603, Tmax = 0.724	l = −19→21
27322 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)] = 0.046	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.084	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.0145P)^2 + 3.7658P] where P = (Fo^2 + 2Fc^2)/3
4528 reflections	(Δ/σ)max = 0.001
226 parameters	Δρmax = 0.44 e Å^−3
0 restraints	Δρmin = −0.44 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
Zn1	0.23591 (2)	0.37451 (6)	0.71080 (3)	0.02477 (11)
Cl1	0.17815 (5)	0.39452 (13)	0.57077 (5)	0.02776 (19)
Cl2	0.24644 (5)	0.66457 (13)	0.78154 (5)	0.02746 (19)
Cl3	0.35860 (5)	0.23304 (13)	0.72793 (6)	0.0311 (2)
Cl4	0.15987 (5)	0.17928 (13)	0.77642 (6)	0.0285 (2)
S6	0.59798 (6)	0.22584 (14)	0.88019 (6)	0.0321 (2)
S26	−0.00637 (5)	0.77810 (14)	0.62897 (6)	0.0281 (2)
N1	0.48598 (16)	0.5044 (4)	0.84906 (18)	0.0265 (7)
H1	0.4609	0.4281	0.8076	0.032*
N3	0.47771 (16)	0.8030 (4)	0.91970 (19)	0.0258 (6)
N7	0.66257 (16)	0.5891 (4)	1.01923 (17)	0.0238 (6)
H7	0.7004	0.4996	1.0277	0.029*
N9	0.59655 (16)	0.8543 (4)	1.03268 (18)	0.0252 (6)
H9	0.5839	0.9673	1.0521	0.030*
N21	0.14076 (17)	0.8882 (4)	0.61649 (18)	0.0250 (6)
H21	0.1491	0.9172	0.6710	0.030*
N23	0.19977 (16)	0.8842 (4)	0.49758 (18)	0.0239 (6)
N27	0.00114 (16)	0.7200 (4)	0.42570 (17)	0.0223 (6)
H27	−0.0473	0.6876	0.4306	0.027*
N29	0.10400 (16)	0.7714 (4)	0.37104 (17)	0.0238 (6)
H29	0.1344	0.7781	0.3338	0.029*
C2	0.4499 (2)	0.6740 (5)	0.8601 (2)	0.0274 (8)
H2	0.4004	0.7025	0.8218	0.033*
C4	0.55008 (19)	0.7493 (5)	0.9677 (2)	0.0221 (7)
C5	0.59081 (18)	0.5804 (5)	0.9593 (2)	0.0207 (7)
C6	0.5592 (2)	0.4381 (5)	0.8969 (2)	0.0244 (7)
C8	0.66458 (19)	0.7552 (5)	1.0615 (2)	0.0263 (8)
H8	0.7078	0.7978	1.1057	0.032*
C22	0.2022 (2)	0.9159 (5)	0.5783 (2)	0.0262 (8)
H22	0.2512	0.9621	0.6126	0.031*
C24	0.12665 (19)	0.8192 (5)	0.4555 (2)	0.0208 (7)
C25	0.06210 (19)	0.7868 (5)	0.4905 (2)	0.0208 (7)
C26	0.06553 (19)	0.8180 (5)	0.5776 (2)	0.0216 (7)
C28	0.0278 (2)	0.7130 (5)	0.3550 (2)	0.0263 (8)
H28	−0.0026	0.6724	0.3009	0.032*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0183 (2)	0.0318 (2)	0.0238 (2)	−0.00040 (16)	0.00371 (15)	−0.00279 (18)
Cl1	0.0256 (4)	0.0327 (5)	0.0237 (4)	−0.0028 (3)	0.0028 (3)	0.0016 (4)
Cl2	0.0213 (4)	0.0329 (5)	0.0277 (5)	0.0026 (3)	0.0043 (3)	−0.0056 (4)
Cl3	0.0189 (4)	0.0365 (5)	0.0366 (5)	0.0019 (3)	0.0035 (4)	−0.0123 (4)
Cl4	0.0224 (4)	0.0381 (5)	0.0254 (4)	−0.0039 (3)	0.0059 (3)	−0.0010 (4)
S6	0.0323 (5)	0.0271 (5)	0.0374 (6)	0.0008 (4)	0.0091 (4)	−0.0047 (4)
S26	0.0294 (5)	0.0320 (5)	0.0255 (5)	−0.0036 (4)	0.0113 (4)	0.0005 (4)
N1	0.0203 (15)	0.0323 (17)	0.0257 (16)	−0.0027 (12)	0.0025 (12)	−0.0077 (13)
N3	0.0184 (14)	0.0300 (16)	0.0274 (16)	0.0016 (11)	0.0017 (12)	−0.0017 (13)
N7	0.0185 (14)	0.0274 (16)	0.0233 (15)	0.0029 (11)	0.0002 (11)	0.0016 (12)
N9	0.0210 (15)	0.0256 (16)	0.0275 (16)	0.0028 (11)	0.0020 (12)	−0.0025 (13)
N21	0.0279 (16)	0.0316 (16)	0.0158 (14)	−0.0023 (12)	0.0056 (12)	−0.0018 (13)
N23	0.0197 (14)	0.0274 (16)	0.0245 (15)	−0.0012 (11)	0.0045 (11)	0.0019 (13)
N27	0.0163 (13)	0.0260 (15)	0.0238 (15)	−0.0017 (11)	0.0030 (11)	0.0010 (12)
N29	0.0213 (14)	0.0297 (16)	0.0210 (15)	−0.0009 (12)	0.0062 (11)	−0.0007 (12)
C2	0.0163 (17)	0.037 (2)	0.0271 (19)	−0.0022 (14)	0.0006 (14)	−0.0028 (16)
C4	0.0192 (16)	0.0247 (18)	0.0227 (18)	0.0002 (13)	0.0050 (13)	0.0005 (14)
C5	0.0155 (16)	0.0297 (19)	0.0162 (16)	0.0000 (12)	0.0022 (12)	0.0006 (14)
C6	0.0271 (18)	0.0243 (18)	0.0240 (18)	−0.0040 (13)	0.0107 (14)	0.0002 (14)
C8	0.0203 (17)	0.0316 (19)	0.0258 (19)	0.0018 (14)	0.0026 (14)	−0.0003 (16)
C22	0.0216 (17)	0.0280 (19)	0.0272 (19)	−0.0037 (14)	0.0016 (14)	−0.0001 (15)
C24	0.0196 (16)	0.0219 (17)	0.0205 (17)	0.0003 (12)	0.0037 (13)	0.0002 (14)
C25	0.0189 (16)	0.0233 (17)	0.0200 (17)	−0.0004 (12)	0.0041 (13)	0.0000 (14)
C26	0.0244 (17)	0.0170 (16)	0.0226 (17)	0.0016 (12)	0.0035 (14)	0.0031 (13)
C28	0.0250 (18)	0.0306 (19)	0.0215 (18)	−0.0020 (14)	0.0015 (14)	−0.0040 (15)

Geometric parameters (Å, º)

Zn1—Cl1	2.2547 (9)	N9—H9	0.8800
Zn1—Cl2	2.2776 (10)	N21—C22	1.348 (4)
Zn1—Cl3	2.2750 (9)	N21—C26	1.386 (4)
Zn1—Cl4	2.2861 (9)	N21—H21	0.8800
S6—C6	1.647 (4)	N23—C22	1.310 (4)
S26—C26	1.655 (3)	N23—C24	1.359 (4)
N1—C2	1.347 (4)	C24—C25	1.368 (4)
N1—C6	1.392 (4)	N27—C25	1.378 (4)
N1—H1	0.8800	N27—C28	1.321 (4)
N3—C2	1.313 (4)	N27—H27	0.8800
N3—C4	1.357 (4)	N29—C28	1.334 (4)
C4—C5	1.374 (4)	N29—C24	1.368 (4)
C5—C6	1.418 (5)	N29—H29	0.8800
N7—C5	1.383 (4)	C2—H2	0.9500
N7—C8	1.323 (4)	C8—H8	0.9500
N7—H7	0.8800	C22—H22	0.9500
N9—C8	1.339 (4)	C25—C26	1.408 (5)
N9—C4	1.368 (4)	C28—H28	0.9500
Cl1—Zn1—Cl3	109.44 (4)	N1—C2—H2	117.4
Cl1—Zn1—Cl2	114.54 (4)	N3—C4—N9	126.4 (3)
Cl3—Zn1—Cl2	110.12 (3)	N3—C4—C5	126.3 (3)
Cl1—Zn1—Cl4	109.33 (3)	N9—C4—C5	107.3 (3)
Cl3—Zn1—Cl4	107.60 (4)	C4—C5—N7	106.6 (3)
Cl2—Zn1—Cl4	105.54 (4)	C4—C5—C6	121.8 (3)
C2—N1—C6	126.2 (3)	N7—C5—C6	131.5 (3)
C2—N1—H1	116.9	N1—C6—C5	108.8 (3)
C6—N1—H1	116.9	N1—C6—S6	123.0 (3)
C2—N3—C4	111.7 (3)	C5—C6—S6	128.2 (3)
C8—N7—C5	108.2 (3)	N7—C8—N9	109.7 (3)
C8—N7—H7	125.9	N7—C8—H8	125.1
C5—N7—H7	125.9	N9—C8—H8	125.1
C8—N9—C4	108.1 (3)	N23—C22—N21	125.2 (3)
C8—N9—H9	126.0	N23—C22—H22	117.4
C4—N9—H9	126.0	N21—C22—H22	117.4
C22—N21—C26	125.8 (3)	N23—C24—N29	126.7 (3)
C22—N21—H21	117.1	N23—C24—C25	126.0 (3)
C26—N21—H21	117.1	N29—C24—C25	107.3 (3)
C22—N23—C24	111.7 (3)	C24—C25—N27	106.7 (3)
C28—N27—C25	108.3 (3)	C24—C25—C26	122.0 (3)
C28—N27—H27	125.8	N27—C25—C26	131.3 (3)
C25—N27—H27	125.8	N21—C26—C25	109.2 (3)
C28—N29—C24	108.1 (3)	N21—C26—S26	123.6 (3)
C28—N29—H29	125.9	C25—C26—S26	127.2 (3)
C24—N29—H29	125.9	N27—C28—N29	109.6 (3)
N3—C2—N1	125.1 (3)	N27—C28—H28	125.2
N3—C2—H2	117.4	N29—C28—H28	125.2
C4—N3—C2—N1	2.6 (5)	C24—N23—C22—N21	0.2 (5)
C6—N1—C2—N3	−1.5 (6)	C26—N21—C22—N23	1.4 (6)
C2—N3—C4—N9	177.3 (3)	C22—N23—C24—N29	−179.7 (3)
C2—N3—C4—C5	−1.6 (5)	C22—N23—C24—C25	−0.6 (5)
C8—N9—C4—N3	−177.5 (3)	C28—N29—C24—N23	179.6 (3)
C8—N9—C4—C5	1.6 (4)	C28—N29—C24—C25	0.4 (4)
N3—C4—C5—N7	178.1 (3)	N23—C24—C25—N27	−179.4 (3)
N9—C4—C5—N7	−1.1 (4)	N29—C24—C25—N27	−0.2 (4)
N3—C4—C5—C6	−0.5 (5)	N23—C24—C25—C26	−0.4 (5)
N9—C4—C5—C6	−179.7 (3)	N29—C24—C25—C26	178.7 (3)
C8—N7—C5—C4	0.1 (4)	C28—N27—C25—C24	−0.1 (4)
C8—N7—C5—C6	178.5 (3)	C28—N27—C25—C26	−178.9 (4)
C2—N1—C6—C5	−0.8 (5)	C22—N21—C26—C25	−2.2 (5)
C2—N1—C6—S6	178.6 (3)	C22—N21—C26—S26	177.3 (3)
C4—C5—C6—N1	1.7 (4)	C24—C25—C26—N21	1.7 (4)
N7—C5—C6—N1	−176.6 (3)	N27—C25—C26—N21	−179.7 (3)
C4—C5—C6—S6	−177.7 (3)	C24—C25—C26—S26	−177.8 (3)
N7—C5—C6—S6	4.1 (6)	N27—C25—C26—S26	0.8 (6)
C5—N7—C8—N9	1.0 (4)	C25—N27—C28—N29	0.3 (4)
C4—N9—C8—N7	−1.6 (4)	C24—N29—C28—N27	−0.5 (4)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···Cl3	0.88	2.34	3.180 (3)	159
N21—H21···Cl2	0.88	2.76	3.245 (3)	116
N7—H7···N23^i	0.88	2.01	2.811 (4)	151
N9—H9···N3^ii	0.88	2.00	2.857 (4)	163
N21—H21···Cl4^iii	0.88	2.45	3.216 (3)	146
N27—H27···Cl1^iv	0.88	2.31	3.184 (3)	174
N29—H29···Cl2^v	0.88	2.30	3.134 (3)	159
C2—H2···Cl2	0.95	2.58	3.432 (3)	149
C8—H8···Cl4^vi	0.95	2.61	3.540 (3)	165
C28—H28···S26^v	0.95	2.78	3.556 (4)	140

Symmetry codes: (i) −x+1, y−1/2, −z+3/2; (ii) −x+1, −y+2, −z+2; (iii) x, y+1, z; (iv) −x, −y+1, −z+1; (v) x, −y+3/2, z−1/2; (vi) −x+1, −y+1, −z+2.