Sterically Stabilized End-On Superoxocopper(II) Complexes and Mechanistic Insights Into Their Reactivity With O-H, N-H and C-H Substrates.

Instability of end-on superoxocopper(II) complexes, with respect to conversion to the corresponding peroxo-bridged complexes, has largely constrained their study to very low temperatures (< -80°C). This limits their kinetic capacity to oxidize substrates. In response, we have developed a series of ligand systems bearing bulky aryl substituents that are primarily directed away from the metal centre, Ar3-TMPA (Ar = tpb, dpb, dtbpb), and used them to support [Cu(Ar3-TMPA)(NCMe)]+ copper(I) complexes. Solutions of all three react with O2 to yield [Cu(η1-O2•−)(Ar3-TMPA)]+ complexes that are stable against dimerization at all temperatures. Full binding of O2 is observed at sub-ambient temperatures and can be reversed by warming. The onset of oxygenation is ligand dependent, but can be observed at 25°C in the case of Ar = tpb and dpb. Furthermore, all three [Cu(η1-O2•−)(Ar3-TMPA)]+ complexes are stable against self-decay at temperatures ≤ -20°C. This provides a wide temperature window over which these complexes can be studied, which was exploited by performing extensive reaction kinetics measurements for [Cu(η1-O2•−)(tpb3-TMPA)]+ with a broad range of O-H, N-H, and C-H bond substrates. This includes correlation of second order rate constants (k2 values) versus oxidation potentials (Eox) for a range of phenols (i.e., a Marcus plot), construction of Eyring plots, and temperature-dependent kinetic isotope effect (KIE) measurements. The data obtained indicates that reaction with all substrates proceeds via H-atom transfer (HAT) to [Cu(η1-O2•−)(tpb3-TMPA)]+. In addition, evidence suggests that HAT reaction with the phenols studied proceeds with significant charge transfer, and that it involves full tunelling of both H and D atoms in the case of 1,2-diphenylhydrazine (DPH) and 4-methoxy-2,6-di-tert-butylphenol (MeO-ArOH). Consistent with expectations for HAT, large entropic barriers (ΔS‡) were measured for the substrates MeO-ArOH, DPH, triphenylhydrazine (TPH), and 1-benzyl-1,4-dihydronicotinamide (BNAH). Despite having the lowest X-H bond dissociation energy (BDE) amongst these substrates, the C-H substrate BNAH exhibits both the largest ΔS‡ and the second largest enthalpic barrier (ΔH‡) to reaction. This is congruent with the expectation that oxidation of C-H bonds is kinetically challenging and the experimental observation that [Cu(η1-O2•−)(tpb3-TMPA)]+ is only able to oxidize very weak C-H bonds, whereas it can oxidize moderately strong N-H bonds.