Dioxygen activation by copper complexes. Mechanistic insights into copper monooxygenases and copper oxidases

Reactions of copper(I) complexes with molecular oxygen have been examined using a series of N-alkyl-bis[2-(2-pyridyl)ethyl]amine tridentate ligands ((R')Py2(R)) and N,N-dialkyl-2-(2-pyridyl)ethylamine didentate ligands ((R)'Py1(R1,R2)) at low temperature. The tridentate ligands predominantly provide (mu-eta(2):eta(2)-peroxo)dicopper(H) complexes (side-on type peroxo complex), while the didentate ligands enhance O-O bond homolysis of the peroxo species to produce bis(mu-oxo)dicopper(III) complexes. With the (mu-eta(2):eta(2)-peroxo)dicopper(II) complexes supported by the tridentate ligand, efficient oxygenation of phenolates to the corresponding catechols has been accomplished to provide a good model reaction of tyrosinase. The bis(mu-oxo)dicopper(III) complexes, on the other hand, undergo aliphatic ligand hydroxylation as well as oxygen atom transfer to sulfides to give the corresponding sulfoxides. In the reaction of bis(mu-oxo)dicopper(III) complex with 10-methyl-9,10-dihydroacridine (AcrH(2)) and 1,4-cyclohexadiene (CHD), a new active oxygen intermediate such as a (mu-oxo)(mu-oxyl radical)dicopper(III) or a tetranuclear copper-oxygen complex has been suggested to be involved as the real active oxygen species for the C-H bond activation of the external substrates. A mixed valence bis(mu(3)-oxo) trinuclear copper(II,II,III) complex has also been assessed using the didentate ligand with the smallest N-alkyl substituent (methyl). Mechanistic details of the above reactions as well as ligand effects on the copper(I)-dioxygen reactivity are discussed systematically.