Kinetics and mechanism of ligand isotopic exchange of tris(acetylacetonato)vanadium(III) in acetylacetone and in organic solvents

Article abstract of DOI:10.1021/ic50218a030

Tris(acetylacetonato)vanadium(III) undergoes ligand exchange in acetylacetone (Hacac) at 25-40°C without side reactions. The exchange rate is expressed by rate = (k_a + k_b[H₂O])[complex] ([complex] ≈ 0.005 M, [H₂O] = 0.01-0.2 M; M = mol dm^-3); k_a = (1.36 ± 0.12) × 10^-4 s^-1 and k_b = (6.6 ± 1.5) × 10^-4 M^-1 s^-1 at 25°C. The ΔH? and ΔS? are 17.5 ± 0.4 and 18.0 ± 1.3 kcal mol^-1 and -17.5 ± 1.3 and -12.6 ± 1.3 cal K^-1 mol^-1, respectively (cal = 4.18 J). No deuterium isotope effect was observed on the rate. Linear plot of k₀ vs. the concentration of free Hacac in acetonitrile and in neat Hacac suggests the predominant participation of the enol tautomer in the exchange. The rate-determining step seems to correspond to the formation of an intermediate containing a one-ended acac^- and an Hacac (enol) or a water molecule for the k_a or k_b path, respectively, by the associative mechanism. The rate of exchange in dimethyl sulfoxide, acetonitrile, chloroform, methanol, and ethanol increased in this sequence by a factor of 10³. The acceptor number of these solvents appears to be responsible for the different rates of exchange.