The formation of Ca(II) enolato complexes with α- and β-ketoglutarate in strongly alkaline solutions

Abstract In the present study, the proton dissociation and calcium complex formation reactions of α- and β-ketoglutarate (2- and 3-oxopentane-dioate, α-Ket 2− and β-Ket 2− , 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 log K 1,1 α  = 1.15 ± 0.02 and log K 1,1 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, p K a α and p K a 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 Ca 2+ 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 ] − , [Ca 2 (α-Ket)H −3 ] − , [Ca(β-Ket)H −1 ] − , [Ca 2 (β-Ket)H −1 ] + with the formation constants of log β 11−1 α  = −11.91 ± 0.02, log β 21−3 α  = −36.10 ± 0.06, log β 11−1 β  = −11.58 ± 0.01, log β 21−1 β  = −10.80 ± 0.03, respectively. Beside the formation of the rare Ca 2+ -enolato complexes, the deprotonation of the hydrating water molecules around the Ca 2+ ion was also invoked in some cases.