The thermodynamics of protonation and copper(II) complex formation of a-aminomalonic acid

The thermodynamic functions (ΔG⊖, ΔH⊖, and ΔS⊖) for the protonation of α-aminomalonic acid were determined in aqueous solution (25°C, l = 0.1 mol dm-3 NaCl). The basicity constants and enthalpy changes were evaluated from potentiometric and calorimetric data for both the primary amino and the carboxylate groups. On the basis of the thermodynamic changes a closed structure is hypothesized with the protonated amino nitrogen hydrogen-bonded to both the carboxylate groups to form two five-membered rings. The protonation of the CO2- groups provokes opening of the rings. A relatively high stability constant for complexation with Cu2+ was determined, however in a narrow range of low pH. The co-ordination to the copper(II) occurs both with nitrogen and the two oxygen donor atoms, to form two five-membered chelate rings.