The electron attachment effect on the structure and properties of ortho-hydroxyaryl Schiff and Mannich bases – the hydrogen/proton transfer processes†
The attachment of electrons is known to significantly influence some chemical and biological processes. The chemical differences between Schiff and Mannich bases are characterized by strong intramolecular hydrogen bonds, resulting from the presence of, respectively, single or double carbon–nitrogen bonds in the chelate rings. Differences are especially visible in the hydrogen transfer processes from molecular (O–H⋯N) to the proton transfer (O⋯H–N) forms. The reaction in a Schiff base occurs as an ordinary hydrogen transfer from a donor to an acceptor, while in a Mannich base the transfer of hydrogen occurs simultaneously with a C–N bond scission leading to an intermolecular complex. The attachment of electrons preserves the overall structural topology of the reactants; however, due to differences in electron affinities, reactions switch from endothermic to exothermic and reaction rates in the anionic systems are significantly higher. The difference in electron affinities for particular reactants comes from the fundamental differences in electron binding mechanisms, leading to the valence-bound or dipole-bound states. The observed mechanisms are closely related to the nature and size of the LUMOs of the parent molecules. The transition state of the Mannich base corresponds to the σ and π orbital conversion and possesses the characteristics of the valence-bound state and the dipole-bound electronic state.