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

摘要

Zirconium(IV) (total Zr = T-Zr) in excess lactate (total lactate = T-L) 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 reversible arrow H+ + E-, for which the equilibrium constant was spectrophotometrically evaluated as K-T = 7.15 x 10(-7). In lactate buffers, under the conditions, T-HT T-Zr T-L, the reaction follows first-order kinetics with respect to both T-RT and T-Zr. The first-order rate constant, k(0) increases with decreasing T-L 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. K-A), (b) rapid hydrolysis (eqm. const. K-B) of the aquated product and loss of a proton followed by, (c) reversible, rate determining (k(1), k(-1)) entry of the enolate form (E-) of HT into the coordination sphere of Zr-IV. Kinetic studies yielded k(1)K(A)K(B) = 2.62 x 10(-6) mol(-2) dm(6) s(-1) and k(-1) = 4.9 x 10(-3) s(-1). The spectrally evaluated equilibrium constant for the overall reaction is K=3.2. These constants are related as: K(A)K(B)k(1)/k(1) = K(AB)k(1)/k(1) = KKa; and yielded pK(a) (3.8) in close agreement with the reported value (3.86).