Low-temperature crystal structure of 4-chloro-1H-pyrazole

The crystal structure of 4-chloro-1H-pyrazole has been determined at 170 K, showing a hydrogen-bonded trimeric molecular assembly that is isostructural to its bromo analogue, 4-bromo-1H-pyrazole.

Also disordered in this structure are the C and N atoms. Without disorder, pyrazoles have one 'pyrrole-like' side and one 'pyridine-like' side, as was discussed in the neutron diffraction study of 1H-pyrazole (Krebs Larsen et al., 1970). The C-N, C-C, and C-H bonds on either side of the molecule are crystallographically distinct and resemble either pyrrole or pyridine. However, due to the disorder of the N-H protons in the current structure, only average positions of the C and N atoms have been obtained. Therefore, the C-N, C-C, and C-H bonds on either side of the molecule are indis-tinguishable. This is most apparent in the C-N bonds. In the current structure, the C-N bonds are 1.335 (2), 1.334 (2), and 1.334 (2) Å for C1-N1, C3-N2, and C4 -N3, respectively. In the 1H-pyrazole structure, the C-N bond lengths are 1.356 and 1.350 Å for the 'pyrrole-like' side and the 'pyridine-like' side, respectively. The C-N bond lengths of the current structure are in agreement with those previously reported in the 4-bromo analogue with C-N bond lengths of 1.343 (10), 1.331 (10), and 1.327 (10) Å (Foces-Foces et al., 1999). Furthermore, these bond lengths are also in agreement with the 4-chloro-1H-pyrazol-2-ium chloride salt, which exhibits C-N bond lengths of 1.334 (2) and 1.331 (2) Å (Farmiloe et al., 2019). The N-N bonds of the present molecule are 1.346 (2) and 1.345 (3) Å , for N1-N2 and N3-N3 i , respectively, and are similar to those of 4-methyl-1H-pyrazole (which is refined without proton disorder in Pca2 1 ) with N-N bond lengths of 1.343 (2), 1.344 (2), and 1.349 (2) Å (Goddard et al., 1999). However, the N-N bonds of the 4-bromo analogue are slightly shorter at 1.335 (9) Å each (Foces-Foces et al., 1999).

Figure 1
Perspective view of the asymmetric unit of 4-chloro-1H-pyrazole. Displacement ellipsoids are shown at 50% probability and half the disordered protons are removed for clarity.

Figure 3
Packing of 4-chloro-1H-pyrazole, viewed parallel to the b axis, showing the formation of a herringbone motif.
These values are in agreement with other intermolecular hydrogen-bond interactions of pyrazoles. Just as with the 4-bromo derivative, 4-chloro-1H-pyrazole packs in a herringbone arrangement when viewed down the b axis (Fig. 3). The current structure exhibits no -stacking interactions.

Synthesis and crystallization
4-Chloro-1H-pyrazole was purchased commercially and crystals were grown from the slow evaporation of a methylene chloride solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The data were collected at 170 K on a Bruker D8 CMOS diffractometer equipped with a Photon II detector. CrysAlis PRO was used for scaling and absorption correction. All C-H protons were refined freely while the N-H protons were fixed using an AFIX command and constrained to half occupancy due to the proton disorder.
To remove the proton disorder, an attempt was made to refine the molecule in the non-centrosymmetric Pn2 1 a space group, which is also consistent with the systematic absences. In Pn2 1 a, hydrogen atoms were refined in both possible positions -on the odd-labeled N atoms or on the even-labeled N atoms. However, as Foces-Foces et al. (1999) found with the 4-bromoanalogue, refinement in the lower symmetry space group did not improve the refinement. For the trial Pn2 1 a structure with protons on the even-labeled N atoms, the R 1 and wR 2 values slightly increased to 3.67% and 9.67%, respectively, and the shifting of the carbon atoms could not be consolidated. For the trial Pn2 1 a structure with protons on the odd-labeled N atoms, the R 1 and wR 2 values increased even more to 3.71% and 9.69%, respectively, and the coordinates of the non-protonated N atoms could not converge. Therefore, the best refined model was chosen to be in the centrosymmetric Pnma space group with NH protons disordered over two positions.   Special details 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.