Structure and Reactivity of Copper Complexes Supported by a Bulky Tripodal N-4 Ligand: Copper(I)/Dioxygen Reactivity and Formation of a Hydroperoxide Copper(II) Complex

A copper(I) complex, [Cu-I(TIPT(2)aped)(CH3CN)]ClO4 (1), and two copper(II) complexes, [Cu-II(TIPT(2)aped)(Cl)]PF6 (2) and [Cu-II(TIPT(2)aped)(ClO4)]ClO4 (3), were synthesized and characterized. The reaction of 1 with dioxygen in acetone at -90 degrees 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 (k(H)/k(D) = 5.8) and a largely negative activation enthalpy (Delta H-double dagger). Based on the kinetic data, the reaction mechanism for the conversion of 4 to 6 is discussed.