With regard to the hydrogen bonding in complexes of pyridylureas, less is more. a role for shape complementarity and CH...O interactions?

A pyridylurea.tetraazaanthracenedione complex with three hydrogen bonds is more stable than an analogous complex with four hydrogen bonds. An X-ray analysis and modeling suggests a steric mismatch destabilizing the latter and a CH...O contact enhancing the stability of the former.