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Instability of end-on superoxocopper(II) complexes, with respect to conversion to peroxo-bridged dicopper(II) complexes, has largely constrained their study to very low temperatures. This limits their kinetic capacity to oxidize substrates. In response, we have developed a series of bulky ligands, Ar 3 -TMPA (Ar = tpb, dpb, dtbpb), and used them to support copper(I) complexes that react with O 2 o yield [Cu II (η 1 -O 2  •- )(Ar 3 -TMPA)] + species, which are stable against dimerization at all temperatures. Binding of O 2 saturates at subambient temperatures and can be reversed by warming. The onset of oxygenation for the Ar = tpb and dpb systems is observed at 25 °C, and all three [Cu II (η 1 -O 2  •- )(Ar 3 -TMPA)] + complexes are stable against self-decay at temperatures of ≤-20 °C. This provides a wide temperature window for study of these complexes, which was exploited by performing extensive reaction kinetics measurements for [Cu II (η 1 -O 2  •- )(tpb 3 -TMPA)] + using a broad range of O-H, N-H, and C-H bond substrates. This includes correlation of second order rate constants ( k  2 ) versus oxidation potentials ( E  ox ) for a range of phenols, construction of Eyring plots, and temperature-dependent kinetic isotope effect (KIE) measurements. The data obtained indicate that reaction with all substrates proceeds via H atom transfer (HAT), reaction with the phenols proceeds with significant charge transfer, and full tunneling of both H and D atoms occurs in the case of 1,2-diphenylhydrazine and 4-methoxy-2,6-di- er -butylphenol. Oxidation of C-H bonds proved to be kinetically challenging, and whereas [Cu II (η 1 -O 2  •- )(tpb 3 -TMPA)] + can oxidize moderately strong O-H and N-H bonds, it is only able to oxidize very weak C-H bonds.

Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O–H, N–H, and C–H Substrates