Kinetic and Equilibrium Studies on the Esterification of Monomeric and μ-Oxo Dimeric Vanadium(V) Complexes with Butyl Alcohols in Chlorobenzene

Abstract

Abstract Kinetic and equilibrium studies were made on the esterification of the μ-oxo dimeric vanadium (V) complexes with monoanionic bidentate ligands(HL) such as 8-quinolinol, 5-chloro-8-quinolinol, 5,7-dibromo-8-quinolinol, and 4-isopropyltropolone by butyl alcohols (ROH) in chlorobenzene. The stoichiometry and the rate equation are expressed as follows: V2O3L4+2ROH\ightleftharpoons2VO(OR)L2+H2O:KDE; −\fracd[V2O3L4]dt=\frac12×\fracd[VO(OR)L2]dt=(k1+k2[H2O])[V2O3L4][ROH]. The more basic the bound ligand L−, the lower the water-independent rate constant k1 is. For less sterically crowded alcohols, k1 is higher, giving the reactivity order: n->i->s->t-. Water, presumably hydrogen bonded to the terminal oxo group of the complexes, accelerates the esterification(k2 path). The equilibrium constant KDE is linearly correlated with the rate constant k1. The esterification kinetics of the monomeric complex with 2-methyl-8-quinolinol(HL′), which corresponds to the proposed reactive intermediate in the esterification of the μ-oxo dimeric complexes, is also investigated and found the rate equation to be expressed as −\fracd[VO(OH)L′2]dt=\fracd[VO(OR)L′2]dt=kM,1[VO(OH)L′2[ROH]. k M,1 is 5 times higher than the estimated for the corresponding dimeric complex. Large negative values of ΔS1\eweq and ΔSM,1\eweq point to an associative nucleophilic attack of alcohol to the vanadium center of both dimeric and monomeric complexes. The difference of the reactivity of complexes and alcohols and the detailed mechanism will be discussed.