Crystal and molecular structure of 4-fluoro-1H-pyrazole at 150 K

Abstract

Only two 4-halo-1H-pyrazole crystal structures are known to date (chloro and bromo, the structure of 4-iodo-1H-pyrazole has not been reported yet). The triclinic structure of 4-fluoro-1H-pyrazole, C3H3FN2 (P\overline{1}), reported here is not isomorphous with those of the chloro and bromo analogues (which are isomorphous, orthorhombic Pnma). To avoid sublimation during the measurement, diffraction data were collected at 150 K. Two crystallographically unique 4-fluoro-1H-pyrazole moieties linked by an N—H...N hydrogen bond are found in the asymmetric unit. Unlike the trimeric supramolecular motifs found in the structures of the chloro and bromo analogues, 4-fluoro-1H-pyrazole forms one-dimensional chains by intermolecular hydrogen bonding in the crystal.

Completion of Crystallographic Data for the Series of 4-Halogenated-1H-pyrazoles: Crystal Structure Determination of 4-Iodo-1H-pyrazole and Spectroscopic Comparison

“…1 H NMR and solid state infrared spectra were recorded on a 400 MHz Bruker Avance NMR spectrometer and an Agilent Advanced Cary-660 FT-IR spectrometer, respectively. The crystal structure of 4-bromo-1H-pyrazole (4-Br-pzH) was published in 1999 [16], 4-chloro-1H-pyrazole (4-Cl-pzH) in 2021 [17], and, most recently, 4-fluoro-1H-pyrazole (4-F-pzH) in 2023 [18]. The bromo and chloro analogs are isostructural and form trimeric H-bonding motifs; however, the fluoro analog forms catemeric chains.…”

“…Figure 3c shows the thermal ellipsoid plot and numbering scheme of the P21ab structure and refinement details can be found in the supplemental information (Table S1). Like 4-F-pzH [18], 4-I-pzH forms a catemeric H-bonded network that extends along the a-axis (Figure 4). However, the fluoro and iodo analogs are not isostructural as the trimeric motifs of 4-Br-pzH [16] and 4-Cl-pzH [17] are.…”

“…The N(H)…N distance of 2.87 (3) in 4-I-pzH (N1A(H)…N2A) is comparable to the average corresponding distance of 2.889 (1) Å in 4-F-pzH (N1_1(H)…N2_2 = 2.876 (1) Å and N1_2(H)…N2_1 = 2.902 (1) Å). Counter to chemical intuition, the electronegativity differences among the halogen substituents are not reflected in the H-bonded N(H)…N distances in the solid state structures of pyrazole and the set of its 4-halogenated derivatives Like 4-F-pzH [18], 4-I-pzH forms a catemeric H-bonded network that extends along the a-axis (Figure 4). However, the fluoro and iodo analogs are not isostructural as the trimeric motifs of 4-Br-pzH [16] and 4-Cl-pzH [17] are.…”

“…Infantes and Motherwell successfully predicted the H-bonding motif of substituted pyrazoles into one of the groups based on the accessible surface of the nitrogen atoms [9]; however, further prediction within each group is still not possible, largely due to limited crystallographic data. The crystal structure of 4-bromo-1H-pyrazole (4-Br-pzH) was published in 1999 [16], 4-chloro-1H-pyrazole (4-Cl-pzH) in 2021 [17], and, most recently, 4-fluoro-1H-pyrazole (4-F-pzH) in 2023 [18]. The bromo and chloro analogs are isostructural and form trimeric Hbonding motifs; however, the fluoro analog forms catemeric chains.…”