2-(2-Pyridyl)ethylamine (Pye) ligands in copper(I)-dioxygen chemistry

Abstract This mini-review article summarizes the O2-reactivity of copper(I) complexes supported by a series of 2-(2-pyridyl)ethylamine (Pye) based ligands, which was developed by Karlin and his coworkers for biomimetic copper-oxygen chemistry. Oxygenation reaction of the copper(I) complexes supported by simple bis[2-(2-pyridyl)ethyl]amine tridentate ligands (R′Pye2R) mainly provided (μ-η2:η2-peroxido)dicopper(II) complex 5, but the reaction rates were significantly altered by the ligand substituents R and R′. The peroxide complex 5 induced aliphatic hydroxylation at the benzylic position of the ligand sidearm, when R = –CH2CH2Ph. Mechanistic studies on the aliphatic ligand hydroxylation revealed that the real reactive intermediate was a bis(μ-oxido)dicopper(III) speceis 6 generated in situ by the O–O bond homolysis of 5. On the other hand, peroxide complex 5 induced direct hydroxylation of phenolate derivatives to the corresponding catechols via electrophilic aromatic substitution mechanism, which can be regarded as a model reaction of tyrosinase. When the reaction of copper(I) complexes and O2 was conducted by using 2-(2-pyridyl)ethylamine didentate ligands (R″Pye1R,R′), not only the (μ-η2:η2-peroxido)dicopper(II) complex 5, but also the bis(μ-oxido)dicopper(III) speceis 6 and a mixed-valent bis(μ3-oxido)tricopper(II,II,III) complex 7 were generated, the product type of which was also controlled by the ligand substituents (R, R′, and R″). Reactivities of the generated complexes supported by the didentate ligands R″Pye1R,R′ were also explored. Notably, a tridentate ligand consisting with 2-(2-pyridyl)ethylamine (Pye1) and 8-membered cyclic diamine (RL8Pye) gave a mononuclear copper(II) endo-on superoxide complex 1, when the supported copper(I) complex and O2 was reacted at a low temperature. Such a unique ligand effect of the RL8Pye system were examined using a series of related tridentate ligands. Furthermore, asymmetric (μ-η1:η2-peroxido)dicopper(II) complex 8 was successfully obtained by using a newly designed penta-pyridine ligand (Pye2PyPye2).