Isotopic Exchange Kinetics between Trisacetylacetonatocobalt(III) and Acetylacetone-14C in Organic Solvents

The isotopic exchange proceeds faster than the decomposition of the complex at more than 80°C and below 120°C. McKay plot gives a linear diagram at the initial stage of isotopic exchange. The rate is expressed by R=[complex](k1+k2[acacH]), where acacH stands for free acetylacetone. The Arrhenius activation energy decreases in the order anisole (25 kcal per mole), chlorobenzene (21), and toluene (18) and is subject to acid catalysis with a strong acid as trichloroacetic. Comparison of the data with those of racemization of this complex, and isomerization and inversion of tris(benzoylacetonato)cobalt(III) suggested that k1 represents dissociation mechanism through a five coordinate intermediate containing a unidentate acetylacetonate anion, and the k2 an association mechanism involving association of free acacH with the intermediate.