Chiral versus achiral crystal structures of 4-benzyl-1H-pyrazole and its 3,5-diamino derivative

Abstract

The crystal structures of 4-benzyl-1H-pyrazole (C10H10N2, 1) and 3,5-diamino-4-benzyl-1H-pyrazole (C10H12N4, 2) were measured at 150 K. Although its different conformers and atropenantiomers easily interconvert in solution by annular tautomerism and/or rotation of the benzyl substituent around the C(pyrazole)—C(CH2) single bond (as revealed by 1H NMR spectroscopy), 1 crystallizes in the non-centrosymmetric space group P21. Within its crystal structure, the pyrazole and phenyl aromatic moieties are organized into alternating bilayers. Both pyrazole and phenyl layers consist of aromatic rings stacked into columns in two orthogonal directions. Within the pyrazole layer, the pyrazole rings form parallel catemers by N—H...N hydrogen bonding. Compound 2 adopts a similar bilayer structure, albeit in the centrosymmetric space group P21/c, with pyrazole N—H protons as donors in N—H...π hydrogen bonds with neighboring pyrazole rings, and NH2 protons as donors in N—H...N hydrogen bonds with adjacent pyrazoles and other NH2 moieties. The crystal structures and supramolecular features of 1 and 2 are contrasted with the two known structures of their analogs, 3,5-dimethyl-4-benzyl-1H-pyrazole and 3,5-diphenyl-4-benzyl-1H-pyrazole.

Keywords

• pyrazole
• crystal structure
• hydrogen-bonding motifs
• chirality