CATALYSIS OF THE DEPROTONATION OF BETA-DIKETONES DURING FORMATION OF THE 1-1 METAL-COMPLEXES

Catalytic effects on the deprotonation rate constants of beta-diketones were observed during kinetic studies of the formation reactions of the 1:1 complexes of these ligands. When vanadium(IV), chromium(III), iron(III), cobalt(II), nickel(II), copper(II), and uranium(VI) react with pentane-2,4-dione, heptane-3,5-dione, 1,1,1-trifluoropentane-2,4-dione, 4,4,4-trifluoro-1-(2-thienyl)-butane-1,3-dione, 1-phenylbutane-1,3-dione, and 2-acetylcyclohexanone a significant catalytic effect on the rate of ionization of the keto tautomer of the ligand was observed. A considerable enhancement of the rate of ionization of the enol tautomers of these ligands was observed during the complex formation reactions with Cu2+. A mechanism is proposed that accounts for these catalytic effects. In the case of Cu2+ the increased rate of proton removal from the ligands may be rationalized in terms of the relatively large reduction potential of this metal ion.