Fine-Tuning of Copper(I)-Dioxygen Reactivity by 2-(2-Pyridyl)ethylamine Bidentate Ligands

Copper(I)-dioxygen reactivity has been examined using a series of 2-(2-pyridyl)ethylamine
bidentate ligands R1Py1R2,R3. The bidentate ligand with the methyl substituent on the pyridine nucleus
MePy1Et,Bz (N-benzyl-N-ethyl-2-(6-methylpyridin-2-yl)ethylamine) predominantly provided a (μ-η2:η2-peroxo)dicopper(II) complex, while the bidentate ligand without the 6-methyl group HPy1Et,Bz (N-benzyl-N-ethyl-2-(2-pyridyl)ethylamine) afforded a bis(μ-oxo)dicopper(III) complex under the same experimental conditions.
Both Cu2O2 complexes gradually decompose, leading to oxidative N-dealkylation reaction of the benzyl
group. Detailed kinetic analysis has revealed that the bis(μ-oxo)dicopper(III) complex is the common reactive
intermediate in both cases and that O−O bond homolysis of the peroxo complex is the rate-determining
step in the former case with MePy1Et,Bz. On the other hand, the copper(I) complex supported by the bidentate
ligand with the smallest N-alkyl group (HPy1Me,Me, N,N-dimethyl-2-(2-pyridyl)ethylamine) reacts with molecular
oxygen in a 3:1 ratio in acetone at a low temperature to give a mixed-valence trinuclear copper(II, II, III)
complex with two μ3-oxo bridges, the UV−vis spectrum of which is very close to that of an active oxygen
intermediate of lacase. Detailed spectroscopic analysis on the oxygenation reaction at different concentrations has indicated that a bis(μ-oxo)dicopper(III) complex is the precursor for the formation of trinuclear
copper complex. In the reaction with 2,4-di-tert-butylphenol (DBP), the trinuclear copper(II, II, III) complex
acts as a two-electron oxidant to produce an equimolar amount of the C−C coupling dimer of DBP (3,5,3‘,5‘-tetra-tert-butyl-biphenyl-2,2‘-diol) and a bis(μ-hydroxo)dicopper(II) complex. Kinetic analysis has shown that
the reaction consists of two distinct steps, where the first step involves a binding of DBP to the trinuclear
complex to give a certain intermediate that further reacts with the second molecule of DBP to give another
intermediate, from which the final products are released. Steric and/or electronic effects of the 6-methyl
group and the N-alkyl substituents of the bidentate ligands on the copper(I)-dioxygen reactivity have been
discussed.