The crystal structures of diazine (pyridazine, pyrimidine or pyrazine)–chloranilic acid (2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone) (2/1) systems have been determined at room temperature (Ishida & Kashino, 1999a
) and recently re-determined at 110 K for the pyridazine and pyrazine compounds (Gotoh et al.
). In each compound, the diazine and chloranilic acid molecules are connected by strong N
O hydrogen bonds [N
O = 2.582 (3), 2.615 (2) and 2.590 (4) Å for the pyridazine, pyrimidine and pyrazine compounds, respectively, at room temperature] to afford a centrosymmetric 2:1 unit. H-atom motions attributable to dynamically disordered H atoms in these hydrogen bonds have been detected by 1
H NMR and 35
Cl nuclear quadrupole resonance (NQR) measurements for the pyridazine compound (Nihei et al.
), the pyrimidine compound (Ikeda et al.
) and the pyrazine compound (Nihei et al.
). Recently, measurements of 14
N NQR and multi-temperature X-ray diffraction were made for the pyridazine compound and the temperature dependence of the population ratio of two disordered sites of the H atom in the N
O hydrogen bond, i.e.
H-atom migration was determined (Seliger et al.
). On the other hand, in the crystal structures of pyridazine–chloranilic acid (1/1) (Gotoh & Ishida, 2008
) and pyrazine–chloranilic acid (1/1) (Ishida & Kashino, 1999c
), such short hydrogen bonds have not been observed. The hydrogen bonds between the base and the acid are fairly long [N
O = 2.6138 (13)–2.6621 (13) and 2.719 (3) Å for the pyridazine and pyrazine compounds, respectively] and the H atoms are ordered.
We report here the structures of two solid phases of the title compound, (I), which shows a solid–solid phase transition at about 198 K determined by 35
Cl NQR. In the 35
Cl NQR measurements in the temperature range 77–328 K, two resonance lines were observed in the high-temperature range above 198 K; the 35
Cl NQR frequencies observed at 321 K are 35.973 and 35.449 MHz. Below 198 K, the resonance lines split into four lines (36.565, 36.357, 35.974 and 36.011 MHz at 77 K). This indicates that a phase transition occurs around 198 K, accompanying a change in the number of crystallographically independent Cl atoms, i.e.
a change from two independent Cl atoms in the high-temperature phase to four in the low-temperature phase (Armstrong & van Driel, 1975
). The detailed NQR results are reported elsewhere (Asaji, 2010
). The phase transition was also detected by differential thermal analysis (DTA) operated in the range 150–295 K, which showed a broad and weak heat anomaly with onset and peak temperatures of 200 (1) and 206 (1) K, respectively, on heating, and 201 (1) and 197 (1) K on cooling.
Fig. 1 shows the asymmetric unit in the high-temperature phase determined at 225 K, where the three components are held together by O—H
O and C—H
O hydrogen bonds (Table 1). The dihedral angle between the pyrimidine ring (N1/C7/N2/C8–C10) and the anion ring (C1–C6) is 30.43 (6)°. An acid–base interaction involving a proton transfer occurs through the water molecule. The transferred proton is shared by the pyrimidine and water molecules, resulting in two disordered states, viz.
pyrimidin-1-ium–water and pyrimidine–oxonium. This disordered feature in the short N
O hydrogen bond [N1
O5 = 2.5440 (15) Å] is confirmed in a difference Fourier map (Fig. 2
), where two distinct peaks are observed between the pyrimidin-1-ium cation and the water molecule. The asymmetric units related by an inversion centre are connected by O—H
O and C—H
Cl hydrogen bonds (Table 1), forming a centrosymmetric 2+2+2 aggregate (Fig. 3). These aggregates are further connected through an O—H
O hydrogen bond between the hydrogen chloranilate anions related by a twofold screw rotation axis (Fig. 4).
Figure 1 The molecular structure in the high-temperature phase (225 K) of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary (more ...)
Table 1 Hydrogen-bond geometry (Å, °) for (I) at 225 K
Figure 2 Difference Fourier maps (a) of the high-temperature phase, (b) of the low-temperature phase and (c) associated with the NHO hydrogen bond between the pyrimidine and water molecules. Maps were calculated on the mean plane of (more ...)
Figure 4 A partial packing view of the hydrogen chloranilate anions in the high-temperature phase. The O—HO hydrogen bonds are indicated by dashed lines. [Symmetry codes: (i) −x, y − , −z + (more ...)
The asymmetric unit of the low-temperature phase determined at 120 K is shown in Fig. 5. This unit is a 2+2+2 aggregate of the three components; there are two crystallographically independent molecules for each component and the four independent Cl atoms in the crystal, being consistent with the 35
Cl NQR result. Although this phase crystallizes in the same space group and has a similar molecular packing, the centrosymmetry of the 2+2+2 aggregate (Fig. 3) and the twofold screw rotation symmetry between the anions (Fig. 4) in the high-temperature phase are lost, resulting in a doubling of the a
axis as shown in Fig. 6. For the low-temperature phase, a nonstandard setting of space group No. 14, viz.
, was selected in order to facilitate comparison of the two phases. In the 2+2+2 aggregate, the pyrimidin-1-ium cation and the water molecule are connected by short hydrogen bonds [N1
O5 = 2.5207 (15) Å and N3
O10 = 2.5285 (15) Å; Table 2]. The difference Fourier map (Fig. 2
) shows two peaks between N1 and O5. On the other hand, between N3 and O10 only one diffuse peak is observed near the centre of the N
O vector (Fig. 2
). The maximum lies closer to N3 than O10, and a long N—H bond [N3—H3 = 1.10 (3) Å] is obtained. The dihedral angle between the pyrimidine ring (N3/C17/N4/C18–C20) and the anion ring (C11–C16) is 28.30 (5)°, comparable with the angle of 30.43 (6)° in the high-temperature phase, while a larger dihedral angle of 54.12 (5)° is observed between the N1/C7/N2/C8–C10 and C1–C6 rings. The dihedral angles between the two base rings and between the two acid rings are 44.86 (6) and 1.49 (5)°, respectively. These angles indicate that the phase transition is accompanied by a considerable rotation of one pyrimidine molecule (N1/C7/N2/C8–C10) around the N
O hydrogen bond, breaking the C—H
O and C—H
Cl hydrogen bonds.
Figure 5 The molecular structure in the low-temperature phase (120 K) of the title compound, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary (more ...)
Figure 6 Packing diagrams of the high- and low-temperature phases, viewed down the b axis. The twofold screw axes and the inversion centres are shown by black symbols. The space group in the high-temperature phase is P21/c, while the space group in the low-temperature (more ...)
Table 2 Hydrogen-bond geometry (Å, °) for (I) at 120 K