Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights into Their Reactivity with O–H, N–H, and C–H Substrates
Sebastian Y. Quek, Suman Debnath, Shoba Laxmi, Maurice van Gastel, Tobias Krämer, Jason England
Abstract
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, Ar3-TMPA (Ar = tpb, dpb, dtbpb), and used them to support copper(I) complexes that react with O2 to yield [CuII(η1-O2•–)(Ar3-TMPA)]+ species, which are stable against dimerization at all temperatures. Binding of O2 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 [CuII(η1-O2•–)(Ar3-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 [CuII(η1-O2•–)(tpb3-TMPA)]+ using a broad range of O–H, N–H, and C–H bond substrates. This includes correlation of second order rate constants (k2) versus oxidation potentials (Eox) 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-tert-butylphenol. Oxidation of C–H bonds proved to be kinetically challenging, and whereas [CuII(η1-O2•–)(tpb3-TMPA)]+ can oxidize moderately strong O–H and N–H bonds, it is only able to oxidize very weak C–H bonds.