Structure and Reactivity of Copper Complexes Supported by a Bulky Tripodal N4 Ligand: Copper(I)/Dioxygen Reactivity and Formation of a Hydroperoxide Copper(II) Complex

Abstract

A copper(I) complex, [CuI(TIPT2aped)(CH3CN)]ClO4 (1), and two copper(II) complexes, [CuII(TIPT2aped)(Cl)]PF6 (2) and [CuII(TIPT2aped)(ClO4)]ClO4 (3), were synthesized and characterized. The reaction of 1 with dioxygen in acetone at –90 °C gave an end‐on superoxide copper(II) complex 4. The superoxide copper(II) complex 4 reacted with 2,2,6,6‐tetramethylpiperidin‐1‐ol (TEMPO‐H) and 2,6‐di‐tert‐butyl‐4‐methoxyphenol (DTBMP) to generate a hydroperoxide copper(II) complex 6. These copper‐oxygen adducts were thoroughly characterized by UV/Vis, resonance Raman, EPR, and NMR spectroscopic techniques. Complex 6 was also synthesized from the copper(II) complex 3 in the presence of hydrogen peroxide and triethylamine. The reaction of 4 with TEMPO‐H showed a relatively large kinetic deuterium isotope effect (kH/kD = 5.8) and a largely negative activation enthalpy (ΔH‡). Based on the kinetic data, the reaction mechanism for the conversion of 4 to 6 is discussed.