Kinetics of ligand exchange between 2-thenoyltrifluoroacetone and zirconium-lactate solution in weakly acidic lactate buffers

Zirconium(IV) (total Zr = TZr) in excess lactate (total lactate = TL) buffer forms “zirconium-lactate”, which reacts with 2-thenoyltrifluoroacetone (HT) resulting in UV spectral changes sufficient for monitoring the kinetics of the reaction. The ligand HT undergoes a very fast keto enolate equilibration: HK⇌H++E-HK⇌H++E-, for which the equilibrium constant was spectrophotometrically evaluated as KT = 7.15 × 10−7. In lactate buffers, under the conditions, THT ≪ TZr ≪ TL, the reaction follows first-order kinetics with respect to both THT and TZr. The first-order rate constant, k0 increases with decreasing TL and increasing pH. These simple kinetic observations, and the associated spectral changes, can be interpreted by the sequence: (a) fast, reversible aquation of “zirconium-lactate” with concomitant loss of only one lactate (L) (eqm. const. KA), (b) rapid hydrolysis (eqm. const. KB) of the aquated product and loss of a proton followed by, (c) reversible, rate determining (k1, k−1) entry of the enolate form (E−) of HT into the coordination sphere of ZrIV. Kinetic studies yielded k1KAKB = 2.62 × 10−6 mol−2dm6 s−1 and k−1 = 4.9 × 10−3 s−1. The spectrally evaluated equilibrium constant for the overall reaction is K = 3.2. These constants are related as: KAKBk1k-1=KABk1k-1=KKa; and yielded pKa (3.8) in close agreement with the reported value (3.86).

Under the conditions, THT ≪ TZr ≪ TL, “zirconium-lactate” (Zr2L6) reacts with 2-thenoyltrifluoroacetone (HT) to produce Zr2L5E and L; the equilibrium constant K = 3.2: k−1 = 4.9 × 10−3 s−1, k1KAKB = 2.62 × 10−6 dm6 mol−2 s−1 at T = 25.0 °C.Figure optionsDownload as PowerPoint slide