Kinetics and mechanisms of the reactions of vanadyl ion with β-diketones

Abstract

The kinetics and equilibrium constants of the reactions of vanadyl ion with 4,4,4-trifluoro-1-(2-thienyl)butane-1,3-dione (Htftbd), 1,1,1-trifluoropentane-2,4-dione (Htfpd), and pentane-2,4-dione (Hpd) to form the mono complexes have been investigated in aqueous solution at 25 °C and I= 1.0 mol dm^–3. The values of the equilibrium constants, K₁, for the reaction [VO]²⁺+ HL ⇌[VO(L)]⁺+ H⁺ where HL represents the undissociated form of the β-diketone are 0.55 ± 0.09, 0.32 ± 0.05, and 0.11 ± 0.02 for Hpd, Htftbd, and Htfpd respectively. The kinetic data are consistent with a mechanism in which the vanadyl ion reacts with the enol forms of Htftbd and Htfpd by parallel acid-independent and inverse-acid paths. The rate constants for reaction of [VO]²⁺ with HE, the undissociated enol tautomers of Htftbd and Htfpd, are 7.6 and 3.5 dm³ mol^–1 s^–1 respectively. The [VO]²⁺ ion reacts with the enolate ion of Htftbd and Hftpd with rate constants of 3.6 × 10³ and 2 × 10² dm³ mol^–1 s^–1 respectively, while it reacts with the keto-tautomer of Hpd with a rate constant of 4.4 dm³ mol^–1 s^–1.