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

• Published in: Journal of the American Chemical Society 2021-12, Vol.143 (47), p.19731-19747
• Main Authors: Quek, Sebastian Y, Debnath, Suman, Laxmi, Shoba, van Gastel, Maurice, Krämer, Tobias, England, Jason
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.1c07837

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 (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-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.