New Insight into and Characterization of the Aqueous Metal-Enol(ate) Complexes of (Acetonedicarboxylato)copper

Nearly 50 years have passed since the classic studies by Larson and Lister [Larson D. W.; Lister M. W.Can. J. Chem.1968, 46, 823]and Hay and Leong [Hay R. W.; Leong K. N.J. Chem. Soc. A1971, 0, 3639]on the copper-catalyzed decarboxylation of acetonedicarboxylic acid (H₃Acdica). Although the authors laid the foundations for what we know about this reaction; still very little information exists regarding the underlying aqueous metal-enol(ate)s of (acetonedicarboxylato)copper. In this study, UV-visible titrations revealed three pK values, pK _[Cu(H2A)], pK _[Cu(HA)], and pK _[Cu(A)]. We associated the first two with ionization of α-carbon CH₂ groups in [Cu^II(H₂Acdica)_keto]¹⁺ and [Cu^II(HAcdica)_keto]⁰ to form unstable metal-enolates, {[Cu^II(HAcdica)_enolate]} and {[Cu^II(Acdica)_enolate]}, which through β-carbonyl oxygen protonation can form metal-enols [Cu^II(H₂Acdica)_enol]¹⁺ and [Cu^II(HAcdica)_enol]⁰. The square-planar Cu^II center (electron paramagnetic resonance results) plays a dual role of stabilizing negative electron density at the β-carbonyl oxygen and as an electron sink in [[Cu^I(HAcdica)_enolate]⁰]^‡ and [[Cu^I(Acdica)_enolate]^1-]^‡ (confirmed through cyclic voltammetry as two single 1e ^- transfers). The π → π* transition associated with [Cu^II(HAcdica)_enol]⁰ was used to determine pK _[Cu(A)] (deprotonation of enol OH) and enolization rate constant (stopped-flow spectroscopy) but also exhibited a time-dependent decrease in absorbance (on the order of min^-1), suggesting a new method to possibly obtain experimental values for the estimated "k _CuL" decarboxylation rate constant of metal-enolate [CuL]^1- calculated by Larson and Lister. On the basis of our results, we postulate that decarboxylation takes place primarily through {[Cu^II(HAcdica)_enolate]} and [Cu^II(HAcdica)_enol]⁰. These results add to our understanding of aqueous metal-enol(ate)s, which contain underlying Cu^II/I redox chemistry, "active methylenes" and enol tautomers and enolate anions, which play roles in many catalytic reactions of interdisciplinary importance.