End-on (η1) mononuclear primary copper(I)-dioxygen adducts, i.e., cupric superoxide (CuII-O2–) complexes, are key intermediates in the catalytic cycles of the enzymes peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and lytic polysaccharide monooxygenases (LPMO’s). These enzymes are responsible for the hydroxylation of organic substrates containing strong C–H bonds (85-100 kcal mol-1) and thus are of great interest to researchers. The (ligand)CuII/(ligand)CuI redox potential has been shown to affect the stability of Cu-O2 species; however isolating these effects from steric or geometric changes in ligand scaffold can be difficult, and little is known about how the redox potential effects the hydrogen atom transfer (HAT) reactivity of the superoxide. To study the effects of redox potential on cupric superoxide reactivity, while keeping other conditions constant, a series of ligands based on the tris-(2-pyridylmethyl)amine (TMPA) framework were synthesized substituting either electron withdrawing (Cl) or electron donating (dimethyl methoxy or dimethylamino) substituents on one, two, or all three pyridyl groups of the TMPA based ligand. The (ligand)CuII/(ligand)CuI redox potential range increases in a stepwise fashion from -300 (3ClTMPA) to -700 (3DMATMPA) mV vs Fc+/Fc while the coordination geometry with respect to copper remains constant. While the complexes’ stability with regard to temperature varies across this range, the thermodynamic reactivity of cupric superoxide towards phenolic substrate is unchanged. All complexes are capable of performing HAT reactions with 2,6-di-tert-butyl-4-methoxyphenol (BDEOH = 80.8 kcal mol-1) but not with 2,4,6-tri-tert-butylphenol (BDEOH = 82.7 kcal mol-1) at -135 °C in MeTHF. Further studies are currently underway to examine the kinetics of substrate reaction with these complexes, to measure the redox potential and basicity of the superoxide moiety itself, and to investigate the effects of temperature on these aspects of cupric superoxides.

Investigation of redox potential effects on the reactivity of primary copper(I)-dioxygen adducts

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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