Enzymes containing Cu(I) can catalyze the oxidation of small molecules via CunO2 complexes at their active sites. For some of these enzymatic active sites (e.g. peptidylglycine α-hydroxylating monooxygenase (PHM) and galactose oxidase (GO)) end-on cupric superoxide species have been proposed as intermediates during the oxidations of small molecules. The present study uses a tris(2-aminoethyl)amine (tren)-based tris-phosphinimine ligand, designed to incorporate cationic character in the secondary coordination sphere through the phosphinimine phosphorous centers. Here, the cationic electrostatic field is being investigated for its ability to alter the binding and reactivity of small molecules, as is known to occur in a range of enzyme active sites. This ligand, when treated with Cu(I) sources, forms a 1:1 complex (denoted [Cu(I)-L4]). With 20 mol % loading of [Cu(I)-L4] in the presence of oxygen at 50 °C in acetonitrile, catalytic oxidation of aromatic, benzylic, and aliphatic alcohols is observed. [Cu(I)-L4] can also catalyze the oxidation of 2,4,6-substituted phenols, such as 2,4,6-tri-tert-butylphenol, which oxidizes to 2,6-di-tert-butyl-1,4-benzoquinone. With an interest in identifying reactive CunO2 species, we found that [Cu(I)-L4] can be treated with O2 to form a green, paramagnetic precipitate (denoted [CuO2]). Stoichiometric reactions with [CuO2] have yielded promising results as to the nature of the complex. Treatment of [CuO2] with benzoin provides a quantitative yield of benzil after 30 minutes at room temperature, with hydrogen peroxide as an identifiable byproduct. Additionally, treatment of [CuO2] with triphenylmethane at RT yields a putative cupric hydroperoxide, which upon treatment with acid produces hydrogen peroxide. Preliminary spectroscopic studies, including UV/vis, MS, and EPR have been conducted on [CuO2], which along with the stoichiometric reactivity, support the formation of a room temperature stable end-on cupric superoxide.

Copper(I)-dioxygen chemistry supported by a tetrapodal ligand with cationic character in the secondary coordination sphere

254th National Meeting (2017)

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