A second orthorhombic polymorph of 4-{[(1 <i>E</i> ,2 <i>E</i> )-3-(4-methoxyphenyl)prop-2-en-1-ylidene]amino}-1,5-dimethyl-2-phenyl-1 <i>H</i> -pyrazol-3(2 <i>H</i> )-one

The second ortho-rhom-bic polymorph of the title compound, C₂₁H₂₁N₃O₂, (<b>I</b>), crystallizes in space group <i>Pna</i>2₁, compared to <i>Pbca</i> for the first ortho-rhom-bic polymorph (<b>Iⁱ</b> ) [Obasi <i>et al.</i> (2016 ▸). <i>J. Mol. Struct</i>. <b>1120</b>, 180-186]. The difference in the structure of the two polymorphs resides in the orientation of the 4-meth-oxy moiety of the (4-meth-oxy-phen-yl)allyl-idene unit with respect to the phenyl ring to which it is attached. Compound (<b>I</b>) also exhibits rotational disorder of the phenyl ring of the 4-amino-anti-pyrine moiety. In the crystal of (<b>I</b>), the mol-ecules are linked by C-H⋯O and C-H⋯N hydrogen bonds, forming a three-dimensional network. The conformations, Hirshfeld surfaces, and two-dimensional fingerprint plots of the two polymorphs and closely related structures are compared.