Two polymorphs of 2,6-dichloropurine, C
5H
2Cl
2N
4, have been crystallized and identified as the 9
H- and 7
H-tautomers. Despite differences in the space group and number of symmetry-independent molecules, they exhibit similar hydrogen-bonding motifs. Both crystal structures are stabilized by intermolecular N—H
N interactions that link adjacent molecules into linear chains, and by some nonbonding contacts of the C—Cl
π type and by π–π stacking interactions, giving rise to a crossed two-dimensional herringbone packing motif. The main structural difference between the two polymorphs is the different role of the molecules in the π–π stacking interactions.
Supporting information
CCDC references: 862229; 862230
Polymorph (I) (m.p. 466.05–466.75 K) was obtained unintentionally in an
attempted synthesis of
(RS)-2,6-dichloro-9-(2,3-dihydro-1,4-benzoxathiin-3-ylmethyl)-9H-purine (Díaz-Gavilán et al., 2008). Unreacted
2,6-dichloropurine was recovered using ethyl acetate as eluent. After
concentrating the solvent under reduced pressure, suitable crystals of
2,6-dichloropurine were obtained after dissolving the compound in CH2Cl2.
A vial with a screw top allowed the slow evaporation of the solvent at room
temperature to produce colourless crystals. Crystals of polymorph (II) (m.p.
467.95–468.85 K) were obtained by solvent evaporation with commercially
available 2,6-dichloropurine using ethanol as solvent. The remarkable
similarity of the crystal structures of the reported polymorphs yields minimal
differences in the shape and position of the peaks in the FT–IR spectra
(polycrystalline samples in KBr disks). Hence, the stretching mode ν(N—H)
(a weak peak at 3210 cm-1) and the in-plane deformation mode δ(N—H) (1513 cm-1) appear at the same site in (I) and (II).
H atoms on N atoms were located in difference maps and refined as riding, with
N—H = 0.86 Å and Uiso(H) = 1.2Ueq(N). Other H atoms were
positioned geometrically and treated as riding, with C—H = 0.92–0.95 Å
and Uiso(H) = 1.2Ueq(C).
For both compounds, data collection: APEX2 (Bruker, 2010); cell refinement: SAINT (Bruker, 2010); data reduction: SAINT (Bruker, 2010); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: publCIF (Westrip, 2010).
(I) 2,6-dichloro-9
H-purine
top
Crystal data top
C5H2Cl2N4 | F(000) = 752 |
Mr = 189.01 | Dx = 1.695 Mg m−3 |
Monoclinic, P21/c | Cu Kα radiation, λ = 1.54178 Å |
Hall symbol: -P 2ybc | Cell parameters from 2440 reflections |
a = 14.0867 (12) Å | θ = 3.5–65.0° |
b = 9.4898 (7) Å | µ = 7.36 mm−1 |
c = 12.2656 (9) Å | T = 296 K |
β = 115.381 (4)° | Cut block, colourless |
V = 1481.4 (2) Å3 | 0.12 × 0.10 × 0.06 mm |
Z = 8 | |
Data collection top
Bruker X8 Proteum diffractometer | 2547 independent reflections |
Radiation source: fine-focus rotating anode | 1713 reflections with I > 2σ(I) |
Graded multilayer optics monochromator | Rint = 0.085 |
ϕ and ω scans | θmax = 66.0°, θmin = 5.8° |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | h = −16→16 |
Tmin = 0.377, Tmax = 0.753 | k = −11→11 |
18199 measured reflections | l = −14→13 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.051 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.154 | H-atom parameters constrained |
S = 1.10 | w = 1/[σ2(Fo2) + (0.0735P)2 + 0.6074P] where P = (Fo2 + 2Fc2)/3 |
2547 reflections | (Δ/σ)max < 0.001 |
199 parameters | Δρmax = 0.27 e Å−3 |
0 restraints | Δρmin = −0.29 e Å−3 |
Crystal data top
C5H2Cl2N4 | V = 1481.4 (2) Å3 |
Mr = 189.01 | Z = 8 |
Monoclinic, P21/c | Cu Kα radiation |
a = 14.0867 (12) Å | µ = 7.36 mm−1 |
b = 9.4898 (7) Å | T = 296 K |
c = 12.2656 (9) Å | 0.12 × 0.10 × 0.06 mm |
β = 115.381 (4)° | |
Data collection top
Bruker X8 Proteum diffractometer | 2547 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | 1713 reflections with I > 2σ(I) |
Tmin = 0.377, Tmax = 0.753 | Rint = 0.085 |
18199 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.051 | 0 restraints |
wR(F2) = 0.154 | H-atom parameters constrained |
S = 1.10 | Δρmax = 0.27 e Å−3 |
2547 reflections | Δρmin = −0.29 e Å−3 |
199 parameters | |
Special details top
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell esds are taken
into account individually in the estimation of esds in distances, angles
and torsion angles; correlations between esds in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell esds is used for estimating esds 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 > 2sigma(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 (Å2) top | x | y | z | Uiso*/Ueq | |
N1 | 0.6775 (3) | 0.4986 (3) | 0.6268 (3) | 0.0531 (8) | |
C2 | 0.6641 (3) | 0.6302 (5) | 0.5827 (4) | 0.0532 (10) | |
Cl2 | 0.71871 (10) | 0.76088 (13) | 0.68964 (11) | 0.0745 (4) | |
N3 | 0.6148 (3) | 0.6744 (3) | 0.4701 (3) | 0.0485 (8) | |
C4 | 0.5742 (3) | 0.5666 (4) | 0.3941 (3) | 0.0449 (9) | |
C5 | 0.5820 (3) | 0.4237 (4) | 0.4254 (3) | 0.0455 (9) | |
C6 | 0.6369 (3) | 0.3951 (4) | 0.5480 (3) | 0.0501 (10) | |
Cl6 | 0.65201 (10) | 0.22623 (12) | 0.60104 (11) | 0.0710 (4) | |
N7 | 0.5292 (3) | 0.3429 (3) | 0.3227 (3) | 0.0506 (8) | |
C8 | 0.4929 (3) | 0.4362 (4) | 0.2358 (4) | 0.0536 (10) | |
H8 | 0.4539 | 0.4114 | 0.1555 | 0.064* | |
N9 | 0.5165 (3) | 0.5715 (3) | 0.2720 (3) | 0.0514 (8) | |
H9 | 0.4989 | 0.6455 | 0.2271 | 0.062* | |
N11 | 0.8212 (3) | 0.2753 (4) | 0.9484 (3) | 0.0600 (9) | |
C12 | 0.8426 (3) | 0.4071 (5) | 0.9271 (4) | 0.0569 (11) | |
Cl12 | 0.79181 (11) | 0.53758 (13) | 0.98390 (12) | 0.0801 (4) | |
N13 | 0.8973 (3) | 0.4514 (3) | 0.8684 (3) | 0.0560 (9) | |
C14 | 0.9317 (3) | 0.3431 (4) | 0.8255 (4) | 0.0510 (10) | |
C15 | 0.9166 (3) | 0.2009 (4) | 0.8412 (4) | 0.0521 (10) | |
C16 | 0.8593 (3) | 0.1724 (4) | 0.9055 (4) | 0.0564 (10) | |
Cl16 | 0.83484 (10) | 0.00077 (12) | 0.93271 (12) | 0.0798 (4) | |
N17 | 0.9637 (3) | 0.1194 (4) | 0.7842 (3) | 0.0592 (9) | |
C18 | 1.0046 (4) | 0.2121 (4) | 0.7380 (4) | 0.0618 (11) | |
H18 | 1.0419 | 0.1866 | 0.6941 | 0.074* | |
N19 | 0.9884 (3) | 0.3483 (3) | 0.7590 (3) | 0.0590 (9) | |
H19 | 1.0095 | 0.4224 | 0.7353 | 0.071* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
N1 | 0.058 (2) | 0.054 (2) | 0.0489 (18) | −0.0015 (17) | 0.0237 (16) | −0.0004 (17) |
C2 | 0.054 (2) | 0.058 (3) | 0.056 (2) | −0.003 (2) | 0.030 (2) | −0.006 (2) |
Cl2 | 0.0826 (8) | 0.0732 (8) | 0.0662 (7) | −0.0190 (6) | 0.0304 (6) | −0.0211 (6) |
N3 | 0.060 (2) | 0.0436 (18) | 0.0478 (19) | −0.0046 (16) | 0.0283 (16) | −0.0047 (15) |
C4 | 0.055 (2) | 0.040 (2) | 0.046 (2) | −0.0004 (17) | 0.0274 (19) | 0.0028 (17) |
C5 | 0.051 (2) | 0.037 (2) | 0.053 (2) | −0.0030 (17) | 0.0269 (19) | −0.0018 (17) |
C6 | 0.053 (2) | 0.054 (2) | 0.049 (2) | 0.0043 (19) | 0.0267 (19) | 0.012 (2) |
Cl6 | 0.0859 (8) | 0.0571 (7) | 0.0740 (7) | 0.0127 (6) | 0.0379 (7) | 0.0202 (6) |
N7 | 0.066 (2) | 0.0346 (17) | 0.0542 (19) | 0.0030 (15) | 0.0284 (17) | 0.0011 (15) |
C8 | 0.073 (3) | 0.038 (2) | 0.052 (2) | −0.0025 (19) | 0.029 (2) | −0.0052 (18) |
N9 | 0.072 (2) | 0.0366 (17) | 0.0465 (18) | 0.0016 (15) | 0.0267 (17) | 0.0026 (14) |
N11 | 0.068 (2) | 0.054 (2) | 0.065 (2) | −0.0005 (18) | 0.035 (2) | −0.0045 (18) |
C12 | 0.056 (3) | 0.059 (3) | 0.055 (2) | 0.007 (2) | 0.023 (2) | −0.007 (2) |
Cl12 | 0.0956 (9) | 0.0677 (8) | 0.0907 (9) | 0.0121 (7) | 0.0528 (8) | −0.0122 (6) |
N13 | 0.062 (2) | 0.0447 (19) | 0.063 (2) | 0.0015 (16) | 0.0287 (19) | −0.0038 (16) |
C14 | 0.051 (2) | 0.041 (2) | 0.061 (2) | 0.0010 (18) | 0.024 (2) | −0.0018 (19) |
C15 | 0.056 (2) | 0.040 (2) | 0.057 (2) | 0.0015 (18) | 0.021 (2) | −0.0019 (18) |
C16 | 0.058 (3) | 0.052 (2) | 0.063 (3) | −0.001 (2) | 0.030 (2) | 0.004 (2) |
Cl16 | 0.0988 (10) | 0.0541 (7) | 0.1062 (10) | −0.0107 (6) | 0.0627 (8) | 0.0022 (6) |
N17 | 0.071 (2) | 0.0411 (19) | 0.077 (2) | 0.0010 (17) | 0.042 (2) | −0.0021 (17) |
C18 | 0.070 (3) | 0.046 (2) | 0.077 (3) | 0.001 (2) | 0.039 (3) | −0.005 (2) |
N19 | 0.069 (2) | 0.0418 (18) | 0.078 (2) | −0.0035 (17) | 0.044 (2) | −0.0012 (17) |
Geometric parameters (Å, º) top
N1—C6 | 1.324 (5) | N11—C16 | 1.327 (5) |
N1—C2 | 1.341 (5) | N11—C12 | 1.339 (5) |
C2—N3 | 1.320 (5) | C12—N13 | 1.328 (5) |
C2—Cl2 | 1.728 (4) | C12—Cl12 | 1.720 (4) |
N3—C4 | 1.336 (5) | N13—C14 | 1.337 (5) |
C4—N9 | 1.365 (5) | C14—N19 | 1.367 (5) |
C4—C5 | 1.401 (5) | C14—C15 | 1.392 (5) |
C5—N7 | 1.387 (5) | C15—C16 | 1.376 (6) |
C5—C6 | 1.392 (5) | C15—N17 | 1.387 (5) |
C6—Cl6 | 1.708 (4) | C16—Cl16 | 1.726 (4) |
N7—C8 | 1.310 (5) | N17—C18 | 1.307 (5) |
C8—N9 | 1.352 (5) | C18—N19 | 1.356 (5) |
C8—H8 | 0.9300 | C18—H18 | 0.9300 |
N9—H9 | 0.8600 | N19—H19 | 0.8600 |
| | | |
C6—N1—C2 | 117.0 (3) | C16—N11—C12 | 116.5 (4) |
N3—C2—N1 | 129.6 (4) | N13—C12—N11 | 129.3 (4) |
N3—C2—Cl2 | 115.4 (3) | N13—C12—Cl12 | 115.5 (3) |
N1—C2—Cl2 | 114.9 (3) | N11—C12—Cl12 | 115.2 (3) |
C2—N3—C4 | 111.3 (3) | C12—N13—C14 | 111.2 (3) |
N3—C4—N9 | 127.9 (3) | N13—C14—N19 | 127.6 (4) |
N3—C4—C5 | 126.2 (3) | N13—C14—C15 | 126.1 (4) |
N9—C4—C5 | 105.9 (3) | N19—C14—C15 | 106.3 (3) |
N7—C5—C6 | 134.9 (4) | C16—C15—N17 | 134.8 (4) |
N7—C5—C4 | 109.8 (3) | C16—C15—C14 | 115.6 (4) |
C6—C5—C4 | 115.3 (3) | N17—C15—C14 | 109.7 (4) |
N1—C6—C5 | 120.7 (4) | N11—C16—C15 | 121.3 (4) |
N1—C6—Cl6 | 118.3 (3) | N11—C16—Cl16 | 118.0 (3) |
C5—C6—Cl6 | 121.0 (3) | C15—C16—Cl16 | 120.7 (3) |
C8—N7—C5 | 103.6 (3) | C18—N17—C15 | 103.8 (3) |
N7—C8—N9 | 114.9 (4) | N17—C18—N19 | 114.7 (4) |
N7—C8—H8 | 122.6 | N17—C18—H18 | 122.7 |
N9—C8—H8 | 122.6 | N19—C18—H18 | 122.7 |
C8—N9—C4 | 105.9 (3) | C18—N19—C14 | 105.6 (3) |
C8—N9—H9 | 127.0 | C18—N19—H19 | 127.2 |
C4—N9—H9 | 127.0 | C14—N19—H19 | 127.2 |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N9—H9···N7i | 0.86 | 1.96 | 2.785 (4) | 161 |
N19—H19···N17ii | 0.86 | 1.94 | 2.768 (5) | 160 |
Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) −x+2, y+1/2, −z+3/2. |
(II) 2,6-dichloro-7
H-purine
top
Crystal data top
C5H2Cl2N4 | F(000) = 376 |
Mr = 189.01 | Dx = 1.787 Mg m−3 |
Orthorhombic, P212121 | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: P 2ac 2ab | Cell parameters from 4413 reflections |
a = 5.5716 (9) Å | θ = 2.6–27.4° |
b = 9.5820 (16) Å | µ = 0.85 mm−1 |
c = 13.159 (2) Å | T = 120 K |
V = 702.5 (2) Å3 | Plate, colourless |
Z = 4 | 0.12 × 0.10 × 0.08 mm |
Data collection top
Bruker SMART APEX diffractometer | 1156 reflections with I > 2σ(I) |
Radiation source: fine-focus sealed tube | Rint = 0.041 |
ϕ and ω scans | θmax = 25.0°, θmin = 2.6° |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | h = −6→6 |
Tmin = 0.905, Tmax = 0.935 | k = −11→11 |
6847 measured reflections | l = −15→15 |
1243 independent reflections | |
Refinement top
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.033 | H-atom parameters constrained |
wR(F2) = 0.071 | w = 1/[σ2(Fo2) + (0.0253P)2] where P = (Fo2 + 2Fc2)/3 |
S = 1.38 | (Δ/σ)max = 0.006 |
1243 reflections | Δρmax = 0.33 e Å−3 |
100 parameters | Δρmin = −0.22 e Å−3 |
0 restraints | Absolute structure: Flack (1983), with how many Friedel pairs? |
Primary atom site location: structure-invariant direct methods | Absolute structure parameter: 0.03 (10) |
Crystal data top
C5H2Cl2N4 | V = 702.5 (2) Å3 |
Mr = 189.01 | Z = 4 |
Orthorhombic, P212121 | Mo Kα radiation |
a = 5.5716 (9) Å | µ = 0.85 mm−1 |
b = 9.5820 (16) Å | T = 120 K |
c = 13.159 (2) Å | 0.12 × 0.10 × 0.08 mm |
Data collection top
Bruker SMART APEX diffractometer | 1243 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2001) | 1156 reflections with I > 2σ(I) |
Tmin = 0.905, Tmax = 0.935 | Rint = 0.041 |
6847 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.033 | H-atom parameters constrained |
wR(F2) = 0.071 | Δρmax = 0.33 e Å−3 |
S = 1.38 | Δρmin = −0.22 e Å−3 |
1243 reflections | Absolute structure: Flack (1983), with how many Friedel pairs? |
100 parameters | Absolute structure parameter: 0.03 (10) |
0 restraints | |
Special details top
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell esds are taken
into account individually in the estimation of esds in distances, angles
and torsion angles; correlations between esds in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell esds is used for estimating esds 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 > 2sigma(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 (Å2) top | x | y | z | Uiso*/Ueq | |
N1 | −0.1017 (5) | 0.6250 (3) | 0.56651 (17) | 0.0259 (6) | |
C2 | −0.0199 (6) | 0.4957 (3) | 0.5851 (2) | 0.0264 (7) | |
Cl2 | −0.18790 (15) | 0.36192 (8) | 0.53068 (6) | 0.0325 (2) | |
N3 | 0.1656 (5) | 0.4541 (3) | 0.63990 (19) | 0.0262 (6) | |
C4 | 0.2858 (6) | 0.5621 (3) | 0.6803 (2) | 0.0234 (7) | |
C5 | 0.2224 (6) | 0.7022 (3) | 0.6650 (2) | 0.0237 (7) | |
C6 | 0.0226 (6) | 0.7281 (3) | 0.6077 (2) | 0.0262 (7) | |
Cl6 | −0.08278 (14) | 0.89604 (7) | 0.59069 (6) | 0.0293 (2) | |
N7 | 0.3879 (4) | 0.7792 (3) | 0.71888 (17) | 0.0254 (6) | |
H7 | 0.3936 | 0.8775 | 0.7233 | 0.030* | |
C8 | 0.5365 (6) | 0.6864 (3) | 0.7618 (2) | 0.0251 (7) | |
H8 | 0.6678 | 0.7132 | 0.8035 | 0.030* | |
N9 | 0.4858 (5) | 0.5539 (2) | 0.7414 (2) | 0.0262 (6) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
N1 | 0.0277 (14) | 0.0230 (13) | 0.0270 (13) | −0.0014 (12) | 0.0019 (11) | 0.0000 (10) |
C2 | 0.0312 (18) | 0.0212 (16) | 0.0266 (16) | −0.0056 (14) | 0.0030 (16) | −0.0006 (14) |
Cl2 | 0.0378 (5) | 0.0234 (4) | 0.0363 (4) | −0.0044 (4) | −0.0058 (4) | −0.0034 (4) |
N3 | 0.0295 (15) | 0.0203 (14) | 0.0289 (14) | −0.0013 (12) | 0.0012 (13) | 0.0002 (11) |
C4 | 0.0266 (16) | 0.0208 (17) | 0.0229 (15) | −0.0018 (14) | 0.0037 (14) | 0.0010 (13) |
C5 | 0.0285 (18) | 0.0213 (17) | 0.0213 (14) | −0.0035 (14) | 0.0042 (14) | −0.0008 (13) |
C6 | 0.0321 (18) | 0.0203 (16) | 0.0261 (17) | 0.0026 (14) | 0.0078 (15) | 0.0041 (14) |
Cl6 | 0.0328 (4) | 0.0217 (4) | 0.0335 (4) | 0.0030 (3) | −0.0002 (4) | 0.0008 (3) |
N7 | 0.0302 (15) | 0.0183 (13) | 0.0277 (13) | −0.0011 (12) | 0.0014 (13) | 0.0017 (11) |
C8 | 0.0266 (19) | 0.0273 (17) | 0.0215 (15) | −0.0025 (14) | 0.0004 (14) | 0.0003 (14) |
N9 | 0.0337 (16) | 0.0170 (13) | 0.0278 (14) | 0.0002 (12) | −0.0018 (13) | 0.0004 (12) |
Geometric parameters (Å, º) top
N1—C6 | 1.323 (4) | C5—C6 | 1.367 (4) |
N1—C2 | 1.342 (4) | C5—N7 | 1.378 (4) |
C2—N3 | 1.322 (4) | C6—Cl6 | 1.728 (3) |
C2—Cl2 | 1.741 (3) | N7—C8 | 1.340 (4) |
N3—C4 | 1.343 (4) | N7—H7 | 0.9442 |
C4—N9 | 1.376 (4) | C8—N9 | 1.327 (3) |
C4—C5 | 1.403 (4) | C8—H8 | 0.9500 |
| | | |
C6—N1—C2 | 115.9 (3) | N1—C6—C5 | 121.1 (3) |
N3—C2—N1 | 130.0 (3) | N1—C6—Cl6 | 117.7 (2) |
N3—C2—Cl2 | 115.0 (2) | C5—C6—Cl6 | 121.1 (2) |
N1—C2—Cl2 | 115.0 (2) | C8—N7—C5 | 105.9 (2) |
C2—N3—C4 | 112.0 (3) | C8—N7—H7 | 128.0 |
N3—C4—N9 | 126.3 (3) | C5—N7—H7 | 126.0 |
N3—C4—C5 | 123.7 (3) | N9—C8—N7 | 114.7 (3) |
N9—C4—C5 | 110.0 (3) | N9—C8—H8 | 122.6 |
C6—C5—N7 | 137.0 (3) | N7—C8—H8 | 122.6 |
C6—C5—C4 | 117.2 (3) | C8—N9—C4 | 103.6 (2) |
N7—C5—C4 | 105.7 (3) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
N7—H7···N9i | 0.94 | 1.88 | 2.774 (3) | 158 |
Symmetry code: (i) −x+1, y+1/2, −z+3/2. |
Experimental details
| (I) | (II) |
Crystal data |
Chemical formula | C5H2Cl2N4 | C5H2Cl2N4 |
Mr | 189.01 | 189.01 |
Crystal system, space group | Monoclinic, P21/c | Orthorhombic, P212121 |
Temperature (K) | 296 | 120 |
a, b, c (Å) | 14.0867 (12), 9.4898 (7), 12.2656 (9) | 5.5716 (9), 9.5820 (16), 13.159 (2) |
α, β, γ (°) | 90, 115.381 (4), 90 | 90, 90, 90 |
V (Å3) | 1481.4 (2) | 702.5 (2) |
Z | 8 | 4 |
Radiation type | Cu Kα | Mo Kα |
µ (mm−1) | 7.36 | 0.85 |
Crystal size (mm) | 0.12 × 0.10 × 0.06 | 0.12 × 0.10 × 0.08 |
|
Data collection |
Diffractometer | Bruker X8 Proteum diffractometer | Bruker SMART APEX diffractometer |
Absorption correction | Multi-scan (SADABS; Bruker, 2001) | Multi-scan (SADABS; Bruker, 2001) |
Tmin, Tmax | 0.377, 0.753 | 0.905, 0.935 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 18199, 2547, 1713 | 6847, 1243, 1156 |
Rint | 0.085 | 0.041 |
(sin θ/λ)max (Å−1) | 0.593 | 0.594 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.051, 0.154, 1.10 | 0.033, 0.071, 1.38 |
No. of reflections | 2547 | 1243 |
No. of parameters | 199 | 100 |
H-atom treatment | H-atom parameters constrained | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.27, −0.29 | 0.33, −0.22 |
Absolute structure | ? | Flack (1983), with how many Friedel pairs? |
Absolute structure parameter | ? | 0.03 (10) |
Hydrogen-bond geometry (Å, º) for (I) top
D—H···A | D—H | H···A | D···A | D—H···A |
N9—H9···N7i | 0.86 | 1.96 | 2.785 (4) | 160.9 |
N19—H19···N17ii | 0.86 | 1.94 | 2.768 (5) | 160.4 |
Symmetry codes: (i) −x+1, y+1/2, −z+1/2; (ii) −x+2, y+1/2, −z+3/2. |
Hydrogen-bond geometry (Å, º) for (II) top
D—H···A | D—H | H···A | D···A | D—H···A |
N7—H7···N9i | 0.94 | 1.88 | 2.774 (3) | 157.7 |
Symmetry code: (i) −x+1, y+1/2, −z+3/2. |
2,6-Dichloropurine is an important pharmaceutical intermediate (Schaefer et al., 1978) used in the preparation of purine nucleosides and nucleotides, as well as other purine derivatives of great importance, owing to their biological properties (Nair & Pal, 1998; Rao Kode & Phadtare, 2011).
Polymorphism, the ability of a given molecule to crystallize in different crystal structures, is a phenomenon often observed for organics (Bernstein, 2011; Brittain, 2011). The term `tautomeric polymorphs' refers to those tautomers of a given compound that crystallize in different crystal structures and are very rarely observed (Cruz Cabeza et al., 2011). We present here the crystal structure of the 9H-, (I), and 7H-, (II), tautomers of 2,6-dichloropurine.
The molecular geometric parameters in the two presented polymorphs are similar, but the structures differ in the finer details of their crystal packing. As shown in Fig. 1, polymorph (I) crystallizes with two independent molecules in the asymmetric unit (A and B, top and middle) as the 9H- tautomer, while polymorph (II) crystallizes with one symmetry-independent molecule (bottom) as the 7H- tautomer.
The effect of the different –N(H) position in the tautomeric forms (N9 or N7) gives rise to subtle differences between the relevant bond lengths and angles in both structures in the imidazole ring. In (I), the N═Csp2 bond corresponds to N7—C8 [1.310 (5) Å] and N17—C18 [1.307 (5) Å] for molecules A and B, respectively, while in (II) it involves N9—C8 [1.327 (3) Å]. These N═Csp2 bond lengths are comparable with those in related structures with 9H- (Mahapatra et al., 2008; Trávníček & Rosenker, 2006; Soriano-Garcia & Parthasarathy, 1977) and 7H- tautomers (Bo et al., 2006; Ikonen et al., 2009; Watson et al., 1965). The –N(H) tautomeric position is also evident from the greater ring angle at the site where the H atom is attached, namely N9 [C4/N9/C8 105.9 (3)°] and N19 [C14/N19/C18 105.6 (3)°] for (I) and N7 [C5/N7/C8 105.9 (2)°] for (II), compared with the ring angle involving the –N═C– bond [for (I), C5—N7—C8 = 103.6 (3)° and C15—N17—C18 = 103.8 (3)°; for (II), C4—N9—C8 = 103.6 (2)°]. In both polymorphs, the 2,6-dichloropurine molecules form linear chains along the b axis through intermolecular N—H···N interactions with C(4) motifs (Bernstein et al., 1995).
In polymorph (I), chains built by molecules of type A are linked by intermolecular face-to-face π–π stacking interactions involving ring N1/C2/N3/C4–C6 and a symmetry-related ring (symmetry code: -x + 1, y + 1, -z + 1), with a centroid-to-centroid distance of 3.493 (3) Å. Molecules of type B are not involved in π–π stacking interactions. There are also C—Cl···π interactions involving atom Cl16 and ring N1/C2/N3/C4–C6 (Cg1I), with a Cl···centroid distance of 3.468 (2) Å and a C16—Cl16···Cg1iI angle of 113.46 (17)° [symmetry code: (i) x, -y + 1/2, z + 1/2], and atom Cl6 and ring N11/C12/N13/C14–C16 (Cg2I), with a Cl···centroid distance of 3.664 (2) Å and a C6—Cl6···Cg2I angle of 94.54 (13)°, resulting in a structure containing a two-dimensional herringbone-like motif of constituent molecules.
The crystal structure of polymorph (II) is similar to that of polymorph (I). Despite the fact that π–π stacking interactions are not observed in (II), the supramolecular structure features a similar herringbone motif to that in (I), due to the presence of C—Cl···π interactions between atom Cl6 and ring N1/C2/N3/C4–C6 (Cg1II), with a Cl···centroid distance of 3.3471 (15) Å and a C6—Cl6···Cg1iiII angle of 108.45 (10)° [symmetry code: (ii) x - 1/2, -y + 3/2, -z + 1]. As a consequence of the absence of stacking interactions in (II), the width of the herringbone motif (12.352 Å) is greater than that of (I) (11.797 Å) (Fig. 2).