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

[Image: see text] Nearly 50 years have passed since the classic studies by Larson and Lister [ D. W. Larson; M. W. ListerCan. J. Chem.1968, 46, 823]and Hay and Leong [ R. W. Hay; K. N. LeongJ. Chem. Soc. A1971, 0, 3639]on the copper-catalyzed decarboxylation of acetonedicarboxylic acid (H(3)Acdica). 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 CH(2) groups in [Cu(II)(H(2)Acdica)(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)(H(2)Acdica)(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.