In the present study, the proton dissociation and calcium complex formation reactions of α- and β-ketoglutarate (2- and 3-oxopentane-dioate, α-Ket2− and β-Ket2−, respectively) in neutral and alkaline aqueous solutions were explored. From potentiometric measurements, the equilibrium constants of the [Ca(α-Ket)]0 and [Ca(β-Ket)]0 complexes formed in neutral medium were determined to be logK1,1 α = 1.15 ± 0.02 and logK1,1 β = 0.78 ± 0.02, respectively. In alkaline medium, it was found for both ligands that the methylene group underwent deprotonation yielding the carbanion-enolate anion. The deprotonation constants, pKa α and pKa β, were found to be 15.16 ± 0.01 and 14.41 ± 0.01, respectively. The calcium complexing behaviour of the two ligands was also studied in strongly alkaline medium. Under these conditions, the Ca2+ ion was capable of promoting the deprotonation of the ligands and forming various enolato-type complexes. The formation of both mono- and binuclear species were deduced, their formulae are suggested to be [Ca(α-Ket)H−1]−, [Ca2(α-Ket)H−3]−, [Ca(β-Ket)H−1]−, [Ca2(β-Ket)H−1]+ with the formation constants of log β11−1 α = −11.91 ± 0.02, log β21−3 α = −36.10 ± 0.06, log β11−1 β = −11.58 ± 0.01, log β21−1 β = −10.80 ± 0.03, respectively. Beside the formation of the rare Ca2+-enolato complexes, the deprotonation of the hydrating water molecules around the Ca2+ ion was also invoked in some cases.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry