Oxidative Dissolution of Copper and Zinc Metal in Carbon Dioxide with tert-Butyl Peracetate and a β-Diketone Chelating Agent

A series of β-diketone ligands, R¹COCH₂COR² [tmhdH (R¹ = R² = C(CH₃)₃); tfacH (R¹ = CF₃; R² = CH₃); hfacH (R¹ = R² = CF₃)], in combination with tert-butyl peracetate (t-BuPA), have been investigated as etchant solutions for dissolution of copper metal into carbon dioxide solvent. Copper removal in CO₂ increases in the order tfacH < tmhdH < hfacH. A study of the reactions of the hfacH/t-BuPA etchant solution with metallic copper and zinc was conducted in three solvents:  scCO₂ (supercrical CO₂); hexanes; CD₂Cl₂. The etchant solution/metallic zinc reaction produced a diamagnetic Zn(II) complex, which allowed NMR identification of the t-BuPA decomposition products as tert-butyl alcohol and acetic acid. Gravimetric analysis of the amount of zinc consumed, together with NMR studies, confirmed the 1:1:2 Zn:t-BuPA:hfacH reaction stoichiometry, showing t-BuPA to be an overall two-electron oxidant for Zn(0). The metal-containing products of the copper and zinc reactions were characterized by elemental analysis, IR spectroscopy, and, as appropriate, NMR spectroscopy and single-crystal X-ray diffraction [trans-M(hfac)₂(H₂O)(CH₃CO₂H) (1, M = Cu; 2, M = Zn)]. On the basis of the experimental results, a working model of the oxidative dissolution reaction is proposed, which delineates the key chemical variables in the etching reaction. These t-BuPA/hfacH etchant solutions may find application in a CO₂-based chemical mechanical planarization (CMP) process.