Intra- and intermolecular hydrogen bonding in acetylacetone and benzoylacetone derived enaminone derivatives

• Solution state structures of enamine derivatives determined.
• Enaminones adopt localised keto-amine tautomeric form.
• Quantum chemical calculations corroborate NMR and UV data.
• Intra- and inter-molecular hydrogen bonds exist in solution state.

The structure and hydrogen bonding in solution of acetylacetone and benzoylacetone derived enaminone derivatives, 1a–1d and 2a–2d, have been studied by a combination of experimental (NMR and UV spectroscopies) and theoretical (PM6 and DFT) methods. It has been shown that all studied compounds predominantly existed in the localised keto-amine tautomeric form in solution as found also in the solid state. Significant line-broadening and down-field chemical shifts of NH and OH protons have strongly indicated that both groups formed hydrogen bonds, which has further been supported by quantum chemical calculations. Temperature and concentration dependent NMR measurements have pointed towards the fact that NH protons are engaged in strong intramolecular hydrogen bonds of the NH⋯OC type in both solvents. On the other hand, OH protons are involved in weaker intermolecular hydrogen bonding with solvent molecules in DMSO, while in chloroform intermolecular interactions between two molecules dominate. The results presented in this paper can be used for better understanding and further exploiting properties these ligands possess, especially their bioactivity.