X-ray crystallographic structures of enamine and amine Schiff bases of pyridoxal and its 1: 1 hydrogen-bonded complexes with benzoic acid derivatives: Evidence for coupled inter- and intramolecular proton transfer

Crystal structures of Schiff bases containing pyridoxal (PL), N-(pyridoxylidene)-tolylamine, C₁₅H₁₆N₂O ₂ (I), N-(pyridoxylidene)-methylamine, C₉H ₁₂N₂O₂ (III), and their 1:1 adduct with 2-nitrobenzoic acid, (I)⁺ C₇H₄NO (II), and 4-nitrobenzoic acid, (III)⁺ C₇H₄NO (IV), serve as models for the coenzyme pyridoxal-5′-phosphate (PLP) in its PLP-dependent enzymes. These models allow the study of the intramolecular OHN hydrogen bond of PL/PLP Schiff bases and the H-acceptor properties of their pyridine rings. The free base (I) forms hydrogen-bonded chains involving the hydroxyl side groups and the rings of adjacent molecules, whereas (III) forms related hydrogen-bonded cyclic dimers. The adducts (II)/(IV) consist of 1:1 hydrogen-bonded complexes, exhibiting strong intermolecular bonds between the carboxylic groups of the acids and the pyridine rings of (I)/(III). In conclusion, the proton in the intramolecular O - H⋯N hydrogen bond of (I)/(III) is located close to oxygen (enolamine form). The added acids protonate the pyridine ring in (II)/(IV), but only in the latter case does this protonation lead to a shift of the intramolecular proton towards the nitrogen (ketoimine form). All crystallographic structures were observed in the open form. In contrast, the formation of the pyridinium salt by dissolving (IV) leads to the cyclic aminal form.