A second monoclinic polymorph of 2-(diformyl-methyl-idene)-3,3-dimethyl-2,3-dihydro-1H-indole.

The crystal structure of the title compound, C13H13NO2, is a polymorph of the structure first reported by Helliwell et al. [Acta Cryst. (2006), E62, o737-o738]. It is also monoclinic (space group P21/c), but with completely different cell constants. The mol­ecular conformations of these polymorphs differ by a 180° rotation of one formyl group. The present mol­ecule is planar [maximum deviation 0.089 (2) A] with the exception of the two methyl groups which lie on either side of the plane. There are strong intra- and inter­molecular N—H⋯O hydrogen bonds. The latter link pairs of mol­ecules across crystallographic centers of symmetry. Two aldehyde O atoms are brought close together [2.896 (4) A in this arrangement but are not hydrogen bonded. In the earlier polymorph, one formyl group is rotated by 180° to yield inter­molecular hydrogen bonding and an infinite polymeric chain. The other formyl group is involved in the same intra­molecular hydrogen bonding as has been found here.