1H-Pyrazole (pzH) is both a hydrogen-bond donor and acceptor molecule, owing to its NH and N centers. Consequently, pyrazole moieties of the parent compound or Csubstituted analogues form hydrogen bonds to each other in the corresponding crystal structures, and similarly to imidazole, have higher melting and boiling points than other fivemembered cyclic aromatic molecules lacking either the hydrogen-bond acceptor (pyrrole), the hydrogen-bond donor (N-methyl derivatives, furan, isoxazole, oxazole, thiophene, isothiazole, thiazole) or both centers (cyclopentadiene) (Fig. 1). The proximity of the hydrogen-bond donor and acceptor centers in pz allows for the formation of either Comparison of the structures of five-membered aromatic heterocycles and their corresponding melting and boiling points ( C) according to the CRC Handbook of Chemistry and Physics (Rumble, 2022) or the literature. Those with no melting points reported are liquids at room temperature.
discreet hydrogen-bonded motifs, such as dimers, trimers, tetramers and hexamers, or polymeric catemers depending on the substituents (Bertolasi et al., 1999;Foces-Foces et al., 2000;Claramunt et al., 2006;Alkorta et al., 2006), whereas imidazole only forms catemers (Cammers & Parkin, 2004).
As shown in Fig. 2, the asymmetric unit contains two symmetry-independent 4-fluoro-1H-pyrazole moieties (Z 0 = 2; P1) , similarly to 1H-pyrazole (Z 0 = 2; P2 1 cn/Pna2 1 ; La Cour & Rasmussen, 1973;Sikora & Katrusiak, 2013) but unlike the chloro and bromo analogues (Z 0 = 1.5; Pnma) (Rue & Raptis, 2021;Foces-Foces et al., 1999). Structures with Z 0 >1 result when two or more intermolecular interactions, such as optimal shape packing, optimization of hydrogen bonds and aromatic interactions, are in conflict (Steed & Steed, 2015). Therefore, the Z 0 value larger than 1 observed in these pyrazole structures emphasizes the importance of hydrogen bonding in their solid-state structures.
The two crystallographically independent 4-Fpz moieties, which are identical within experimental error, are planar with deviations from the C 3 FN 2 mean-plane of less than 0.004 and 0.008 A ˚, respectively. Table 1 presents a comparison of bond lengths determined by X-ray diffraction for the parent pzH at 150 K (Sikora & Katrusiak, 2013), 4-FpzH at 150 K, 4-ClpzH at 150 K (Rue & Raptis, 2021) and 4-BrpzH at room temperature (Foces-Foces et al., 1999). All structures contain two symmetry-independent moieties. In the case of pzH and 4-FpzH, the NH and N centers of the pz rings are distinct, unlike in the case of 4-Cl/BrpzH. Therefore, in the former case two distinct sets of C-N and C-C bond distances are observed. Similarly to pzH, in 4-FpzH the C-N bond adjacent to N is shorter than the one adjacent to NH [by 0.008 (1)/ 0.010 (1) A ˚], whereas the C-C bond adjacent to N is longer than the one adjacent to NH [by 0.019 (1)/0.018 (1) A ˚]. In general, the N-N, C-N and C-C bond distances in 4-RpH are consistent across the R = H, F, Cl and Br series.
The pz N-H proton donates an N-HÁ Á ÁN hydrogen bond to a neighbouring pz unit on one side, while the pz N atom accepts an N-HÁ Á ÁN hydrogen bond from another pz unit on the opposite side (Table 2, Fig. 3). Thus, pz units in the resulting 4-FpzH catemer form dihedral angles of 59. 351 (2) 1.344 (3) 1.326 (3) 1.366 (3) 1.389 (3) 4-FpzH b 1.3484 (9) 1.3473 (10) 1.3391 (10) 1.3729 (11) 1.3922 (11) 1.3513 (10) 1.3476 (10) 1.3375 (10) 1.3742 (10) 1.3924 (10) 4-ClpzH c 1.346 (2) 1.335 (2 on alternate sides (Fig. 4). As a result, the pz units are arranged in a herringbone pattern within the crystal (Fig. 5).
Although the parent pyrazole (pzH) also forms a hydrogenbonded catemer in the crystal packing, its structure is quite different from the one formed by 4-FpzH. As illustrated in Fig. 6, pairs of pz units are found in two different geometries in the catemer of pzH: one in which they form dihedral angles of 5.45 (9) with centroid-centroid distances of 5.1659 (15) A ˚, and another with dihedral angles of 74.64 (9) and centroidcentroid distances of 5.0504 (15) A ˚. In contrast, pz units in 4-Cl/BrpzH form discreet hydrogen-bonded trimers. While the presence/nature of the 4-halo substituent leads to very different outcomes in terms of the overall packing and hydrogen-bonded motifs in 4-RpzH (R = H, F, Cl/Br), it has little effect on the corresponding hydrogen bonding parameters (Table 2). Interestingly, the structure of the catemer in 4-FpzH is essentially identical to the one found in the lattice of 4-acetyl-1H-pyrazole (monoclinic P2 1 /n; Frey et al., 2014), with dihedral angles of 57.28 (7) and centroid-centroid distances of 4.9501 ( 13
4-Fluoro-1H-pyrazole was synthesized according to a published procedure, from sodium fluoroacetate (WARNING: highly toxic!) by reaction with oxalyl chloride and dimethylformamide, followed by treatment with base and then hydrazine (England et al., 2010). Single crystals were obtained from the powder by slow isothermal sublimation inside a sealed vial under ambient conditions.
Crystal data, data collection and structure refinement details are summarized in Table 3. C-H bond distances were constrained to 0.95 A ˚and these H atoms were refined as riding. Positions of N-bound H atoms were freely refined.
U iso (H) values were set to 1.2 times U eq (C/N) for all H atoms.
Crystal data T min = 0.678, T max = 0.747 15049 measured reflections 2875 independent reflections 2635 reflections with I > 2σ(I) R int = 0.024 Extinction correction: SHELXL2018/3 (Sheldrick 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.040 (10)
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.