4-[(3-Hydroxy-5-phenyl-1H-pyrazol-4-yl)-methyl]-5-phenyl-1H-pyrazol-3(2H)-one

The solid-state structure of the title compound, C19H16N4O2, is dominated by the keto-enol tautomerization of its two 1H-pyrazol-3-one moieties. Since all H atoms could be located in a difference Fourier synthesis, it was possible to distinguish the enol form from the keto form unambiguously. As a result of this tautomerization, an intramolecular hydrogen bond embedded in an eight-membered ring is formed. The two-dimensional hydrogen-bonding system results from three additional intermolecular hydrogen bonds of different strengths, all involved in eight-and ten-membered ring systems.