FromPyrazolate-Based Binuclear Copper(I) Complexes to Octanuclear σ-Mesityl-Bridged μ4-Oxo-Cuprocuprates: Controlled Dioxygen Splitting by Organocopper Scaffolds

Organocopper chemistry has attracted considerable interest over the last few decades, which is attributed to its high and versatile potential in synthetic chemistry. This is, for instance, reflected by the unique role of copper in the activation of dioxygen, as demonstrated by important transformations such as the catalytic oxidation of ethylene to ethylene glycol, the Wacker process, and the oxidative coupling of acetylenes and of biaryls. 4 Another large area of importance is represented by organocuprates and their stoichiometric and catalytic applications as selective transfer reagents that generate valuable organic building blocks. Although organocopper reagents provide this broad range of useful synthetic methodologies, relatively little is known about the specific role of such species in oxidatively induced C C coupling reactions and cuprate-type transformations. To elucidate the structural role of organocuprates in the latter types of reactions, valuable approaches were made by employing mesitylcopper as a well-defined and well-characterized precursor. Its use is convenient for the synthesis of a variety of unusual yet relatively stable organocopper frameworks which serve as potential intermediates in cuprate-involving reactions. This has been demonstrated earlier for the unique cuprocuprate [(DPPE)2Cu][CuMes2] (DPPE = 1,2-bis(diphenylphosphino)ethane) by Leoni, Pasquali, and Ghilardi, and later for magnesium organocuprates bearing arenethiolate coligands by van Koten et al., and recently again for several intriguing lithium and magnesium organocuprates byDavies et al. A real breakthrough en route to a deeper insight into oxidative copper-mediated C C couplings was also achieved with mesitylcopper, namely its controlled oxygenation resulting in the formation of bimesityl and the unusual complex array [Cu10O2Mes6] (Chart 1). 10 According to an X-ray structure determination, this remarkable compound consists of ten Cu centers which are held together by two μ4-O bridges and six μ-σ-mesityl linkers. Further spectroscopic investigations of this highly sensitive framework still remain a challenging goal and have not been reported so far. We have recently communicated an exceptionally stable octanuclear σ-mesitylcopper complex with a related structure