Metal Activation
Whereas the advantages of using molecular oxygen (O2) are well recognized for the synthesis of organic molecules, O2 is infrequently used for the oxidation of metals or the synthesis of metal-organic materials. This stems from the low solubility, high lattice energy, and high bond dissociation energies of metal oxides, which are largely inert under conventional conditions of organometallic synthesis. More common metal oxidants include molecular chlorine (Cl2) and nitric acid (HNO3), which afford metal chlorides or nitrates that are mainstays of organometallic synthesis. The environmental cost of using these high-energy, toxic and corrosive reagents is large, creating opportunities for more benign alternatives that retain desirable function.
Our group has designed a quinone-catechol redox couple capable of mediating aerobic metal oxidation. Quinones readily oxidize most transition metals or main-group elements by 1 or 2 electrons to the corresponding semiquinone or catecholate complexes. Substitution of the catecholate or semiquinone with an exogenous anionic ligand affords the corresponding functionalized metal, and releases a free catechol, which can be recovered. In a subsequent step, the catechol can be oxidized to the ortho-quinone by O2, completing a redox cycle in which metal oxidation and O2 reduction are compartmentalized. This allows metal-oxidation to be linked to the terminal reduction of O2, while avoiding metal oxide formation. Using this platform, we have successfully oxidized a range of transition metals and main-group elements, and demonstrated that semiquinones or catecholates can serve as versatile chloride or nitrate surrogates. We have also demonstrated that mixtures of metals can be selectively activated, creating opportunities to extract metals from mineral ores or postconsumer mixtures.
Representative Publications
1) Glavinović, M.; Qi, F.; Katsenis, A. D.; Friščić, T.; Lumb, J.-P. “Redox-Promoted Associative-Assembly of Metal Organic Materials.” Chem. Sci. 2016, 7, 707-712.
2) Glavinović, M.; Krause, M.; Baines, K.; Friščić, T.; Lumb, J.-P. “A Chlorine-Free Protocol for Processing Germanium.” Sci. Adv. 2017, 3, e1700149 (DOI: 10.1126/sciadv.1700149).