Geometry of the 2-aminoheterocyclic-carboxylic acid R2(2)(8) graph set: implications for crystal engineering.

The geometry of the R2(2)(8) graph set formed between a 2-aminoheterocyclic ring containing an Nsp2 atom (in the 1-position of the ring) and a carboxylic acid has been studied. Collating data from known co-crystal structures containing five- and six-membered heterocyclic rings from the Cambridge Structural Database revealed unexpected differences between two kinds of non-hydrogen contact distances, and between specific bond distances and angles of the heterocycle. Not only were the interatomic non-hydrogen distances between the N atoms (heterocycle) and O atoms (carboxylate) asymmetric, but also the 2-amino N atom (N21) to the heterocyclic C atom (C2) bond was shorter than the C2 to N1sp2 bond. However, this shortening of the C2-N21 bond was not observed in the examples where N21 was substituted with a non-H atom. For the six-membered rings the data also showed that as the C2-N21 bond shortened the N1-C2-N21 bond angle increased.