4-{[(1E)-(2-Hydroxyphenyl)methylidene]amino}-1,5-dimethyl-2-phenyl-2,3-dihydro-1H-pyrazol-3-one

In the title compound, C18H17N3O2, a strong intramolecular O-H . . .N hydro gen bond [N . . .O 2.607 (3), O-H 0.97 (3) and H . . .N 1.71 (3) Angstrom, and O-H . . .N 153 (2)degrees] was observed, which leads to a unique phenol -imine tautomerism in the solid state. The C=N imine bond distance and the C-N-C bond angle [1.287 (2) Angstrom and 121.7 (1)degrees, respectively] in dicate the existence of this phenol-imine tautomer. In solution, the phenol -imine tautomer of the title free Schiff base ligand is dominant in both po lar and non-polar solvents, as supported by H-1 NMR and UV-visible spectros copic data.