Cocrystallization of Two Tautomers: 1-Phenyl-3-(propan-2-yl)-1,2-dihydropyrazol-5-one and 1-Phenyl-3-(propan-2-yl)-1H-pyrazol-5-ol

Two different tautomeric forms, namely the keto [1-phenyl-3-(propan-2-yl)-1,2-dihydro-pyrazol-5-one] and the enol form, [1-phenyl-3-(propan-2-yl)-1H-pyrazol-5-ol], C12H14N2O·2C12H14N2O, are present in the crystal in a 1:2 ratio. During crystallization, 1-phenyl-3-(propan-2-yl)-1,2-dihydro-pyrazol-5-one undergoes tautomerization to afford a corresponding enol via proton transfer reaction. The compound crystallizes in the triclinic space group P1 with the following unit-cell parameters: a = 11.1593(3)Å, b = 11.2247(3)Å, c = 14.1140(4)Å, α = 73.333(3)°, β = 88.286(2)°, γ = 82.767(2)°, Z = 2. The crystal structure was solved by direct methods and refined by full-matrix least-squares procedures to a final R-value of 0.0405 for 4611 observed reflections. The dihedral angles between the mean planes through the phenyl ring and the pyrazole ring are: 28.04(5)°, 47.38(5)° and 49.32(6)° for molecules I, IIA, IIB, respectively. The crystal structure is stabilized by intermolecular N-H…N, O-H…O, O-H…N, C-H…O and C-H…π hydrogen bonds.