Hirshfeld Surface Investigation of Structure-Directing Interactions within Dipicolinic Acid Derivatives.

Six compounds based on dipicolinic acid esters have been synthesized and Hirshfeld surfaces used to investigate the structure-directing effects of functional groups in controlling their solid-state behavior. Compounds 1-4 are 4-bromo dipicolinic acid esters substituted with methyl, ethyl, propyl, and benzyl groups, respectively. The main structure-directing motif within 1-3 is a pairwise O···H interaction involving two carbonyl oxygen atoms and two aromatic H atoms. The introduction of bulky benzyl groups in 4 forces a significant change in the position of this interaction. Compounds 2 and 4 were used in Suzuki coupling reactions to prepare extended analogues 5 and 6, respectively, and their solid-state behavior was also studied using Hirshfeld surfaces. Extension of these dipicolinic acid esters results in the complete loss of the pairwise O···H interaction in 5, where the dominant structure-directing motifs are π-based interactions. However, the pairwise O···H interaction reappears for the more flexible 6, demonstrating control of the solid-state structure of these dipicolinic acid derivatives through the choice of functional groups.