Concomitant polymorphism and a temperature-dependent phase change in (E)-[1-(4-methoxyphenyl)-3-phenyl-2-propenylideneamino]oxyacetic acid.

The title compound, C(18)H(17)NO(4), crystallizes from ethanol at ambient temperature as two concomitant polymorphs (I) and (II), both monoclinic P2(1)/c with Z' = 1. The less abundant form (I) undergoes a reversible phase-transition at ca 173 K to a third monoclinic polymorph (III), P2(1)/n, with Z' = 2, while the more abundant polymorph (II) is unchanged down to 120 K. In each polymorph of (I)-(III) the molecules are linked by pairs of O-H...O hydrogen bonds into cyclic dimers which are crystallographically centrosymmetric in (I) and (II), and approximately, but not crystallographically, centrosymmetric in (III). There are no direction-specific interactions between the hydrogen-bonded dimers in polymorph (I); in polymorph (II) the dimers are linked into sheets by C-H...N and C-H...pi(arene) hydrogen bonds; in polymorph (III) the dimers are linked into chains by a C-H...pi(arene) hydrogen bond. The interconversion of polymorphs (I) and (III) is a simple displacive phase transition.