The interplay of hydrogen bonds and halogen bonds in the structure of NH-pyrazoles bearing C-aryl and C-halogen substituents

The behavior in solution and in the solid state of 3(5)-phenyl-1H-pyrazole (7), 3(5)-phenyl-4-chloro-1H-pyrazole (6), 3(5)-phenyl-4-bromo-1H-pyrazole (1), and 3(5)-p-chlorophenyl-4-bromo-1H-pyrazole (8) is discussed in relation to their 3-phenyl (a)/5-phenyl (b) annular tautomerism. Two new X-ray structures are reported: a new polymorph of 1 and the structure of 6. The new polymorph is a 3-phenyl-1H-pyrazole 1a′ trimer while the new structure is a 5-phenyl-1H-pyrazole 6b trimer. The combined use of NMR at low temperature and DFT calculations allows to discuss the tautomerism of the first three pyrazoles and to predict that the fourth one should be a tetramer formed by both tautomers, 8a and 8b.

Graphical abstractThe X-ray structures of two hydrogen bonded trimers, a new polymorph of 3(5)-phenyl-4-bromo-1H-pyrazole and that of 3(5)-phenyl-4-chloro-1H-pyrazole, are reported. The combined use of solid state and solution NMR with DFT calculations permits to discuss the 3-phenyl versus 5-phenyl annular tautomerism comparatively to other cases.Figure optionsDownload full-size imageDownload as PowerPoint slide