Dinuclear copper–dioxygen intermediates supported by polyamine ligands

Abstract

Reactivity of the dicopper(I) and dicopper(II) complexes supported by novel polyamine ligands L1 (1,11-bis(6-methylpyridin-2-yl)-2,6,10-triaza-2,6,10-tribenzylundecane) and L2 (5-benzyl-1,9-bis(6-methylpyridin-2-yl)-2,8-bis(6-methylpyridin-2-ylmethyl)-2,5,8-triazanonane) towards O2 and H2O2, respectively, has been investigated in order to shed light on the ligand effects on Cu2/O2 chemistry. The dicopper(I) complex of L1 (1a) readily reacted with O2 in a 2:1 ratio at a low temperature (–94 °C) in acetone to afford a mixture of (μ–η2:η2-peroxo)dicopper(II) and bis(μ-oxo)dicopper(III) complexes. The formation of these species has been confirmed by the electron spin resonance (ESR) silence of the solution as well as their characteristic absorption bands in the UV–visible region [λmax=350 and 510 nm due to the peroxo complex and ∼400 nm due to the bis(μ-oxo) complex] and the resonance Raman bands at 729 cm−1 [Δν (16O2–18O2) = 38 cm−1] due to the peroxo complex and at 611 and 571 cm−1 [Δν (16O2–18O2) = 22 and 7 cm−1, respectively] due to the bis(μ-oxo) complex. The peroxo and bis(μ-oxo) complexes were unstable even at the low temperature, leading to oxidative N-dealkylation at the ligand framework. The dicopper(I) complex of L2 (2a) also reacted with O2 to give (μ-hydroxo)dicopper(II) complex (2bOH) as the product. In this case, however, no active oxygen intermediate was detected even at the low temperature (–94 °C). With respect to the copper(II) complexes, treatment of the (μ-hydroxo)dicopper(II) complex of L1 (1bOH) with an equimolar amount of H2O2 in acetone at −80 °C efficiently gave a (μ-1,1-hydroperoxo)dicopper(II) complex, the formation of which has been supported by its ESR-silence as well as UV–vis (370 and 650 nm) and resonance Raman spectra [881 cm−1; Δν (16O2–18O2) = 49 cm−1]. The (μ-1,1-hydroperoxo)dicopper(II) intermediate of L1 also decomposed slowly at the low temperature to give similar oxidative N-dealkylation products. Kinetic studies on the oxidative N-dealkylation reactions have been performed to provide insight into the reactivity of the active oxygen intermediates.