On substituted pyrazole derivatives. I. 3-Methyl-4-[(Z)-2-(4-methylphenyl)-hydrazin-1-ylidene]-1-(3-nitrophenyl)-1H-pyrazol-5(4H)-one and 3-methyl-4-[(Z)-2-(4-methylphenyl)hydrazin-1-ylidene]-1-[4-(trifluoromethyl)-phenyl]-1H-pyrazol-5(4H)-one

The title substituted pyrazole derivatives, C₁₇H₁₅N₅O₃ and C₁₈H₁₅F₃N₄O, share most of their molecular features, in particular the hydrazinylidene (-HN-N=) rather than the diazene (-N=N-) tautomeric form, and differ only in the substituents (NO₂ and CF₃) on one of the outer phenyl rings. The molecular units are basically planar, with the rotation of the phenyl rings being hindered by the presence of two intramolecular hydrogen bonds having the keto O atom as acceptor. In both structures, the packing is governed by weak C - H⋯O, C - H⋯π and π-π interactions. The subtle way in which minor structural differences lead to rather different supramolecular structures is analysed.