Acid–base properties of spinaceamine and spinacine and their complexing capacity with divalent metals

Abstract

The behaviour of spinaceamine (4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine) and spinacine (4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid) in aqueous solutions in equilibria with protons and divalent cations has been investigated potentiometrically. The compounds are cyclic homologues of histamine and histidine, respectively. The protonation constants at 25 °C, µ= 0·1 M KCl for spinaceamine are logK₁= 8·904(1), logK₂= 4·895(2) and the protonation constants at the same conditions for spinacine are logK₁= 8·663(4), logK₂= 4·936(6), and log K₃= 1·649(9). The thermodynamic functions ΔG, ΔH, ΔS for the protonation processes of both compounds have been calculated from potentiometric measurements of equilibria at temperatures between 5 and 35 °C. The values of ΔS for different association steps, as compared with those of histamine and histidine, are consistent with the rigid structure of the cyclic homologues. Spinaceamine does not form complexes in solution with divalent cations, Ni²⁺ and Cu²⁺. Spinacine forms the complexes HNiL²⁺, NiL⁺, NiL₂, with nickel and the complexes HCuL²⁺, H₂CuL₂²⁺, HCuL₂⁺ with copper; these complexes are very likely (N,O)-chelates of glycine-type. The proton of the hydrogen-complexes is probably bound to the tertiary nitrogen atom of the imidazole ring.