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

Despite the facile inter­conversion of its conformers in solution, 4-benzyl-1H-pyrazole adopts a chiral crystal structure (space group P2₁). Its 3,5-di­amino derivative, however, crystallizes in the centrosymmetric space group P2₁/c. In both crystal structures, the aromatic moieties are organized in alternating bilayers in which they stack in parallel columns in two orthogonal directions, and the pyrazole units form catemer motifs by N—H⋯N and N—H⋯π hydrogen bonding, respectively.

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.