Catalytic oxidation with dinuclear Cu(I) macrocyclic dioxygen complexes as intermediates

Abstract

The investigation of dinuclear copper complexes which interact with dioxygen has been extensive because of their relevance to biological systems and oxygen active multicopper sites are found in hemocyanin, tyrosinase and polynuclear copper oxidases. Oxygenated model copper dinuclear complexes have characteristics similar to those of the dinuclear copper sites of oxytyrosinase and oxyhemocyanin. In this paper the dinuclear Cu(I) complexes of macrocyclic ligands are described and the reactivities of their dioxygen complexes for the oxidation of various substrates will be examined. Macrocyclic ligands were prepared by the 2 + 2 condensation of an aromatic dialdehyde with diethylenetriamine and with ditrimethylenetriamine. The dinuclear Cu(I) complex of the macrocyclic ligand formed by the 2 + 2 condensation of isophthaldehyde and diethylenetriamine, forms a dioxygen adduct which rapidly hydroxylates one of the benzene rings of the macrocyclic ligand. Oxygen insertion was not possible for the Cu(I) dioxygen complex of the macrocyclic ligand with furane bridging groups, and the oxygen complex was found to be stable at room temperature. In this complex the coordinated dioxygen is activated so that it readily oxidizes various substrates, such as phenols and catechols. When the Cu(II) complex thus formed oxidizes the same substrate, a catalytic system results. The substrate was oxidized by the Cu(I) dioxygen complex, which was converted to a Cu(II) complex. The latter was in turn reduced to the Cu(I) complex by oxidation of the substrate, which then combined with oxygen to continue the catalytic cycle. Similar results were obtained with the Cu(I) Cu(II) complexes of the ligand prepared by the 2 + 2 condensation of pyridine-2,6-dialdehyde and diethylenetriamine. Results obtained with analogous dinuclear-Cu(I) Cu(II) complexes with larger macrocyclic rings are described. Examples of substrates that undergo catalytic oxidation, with turnovers from 2 to 30, are 2,6-dimethoxyphenol, 2,6-ditertiarybutylphenol, hydroquinone, tertiarybutylhydroquinone and 3,5-ditertiarybutylcatechol. Schemes illustrating catalytic cycles for the oxidation of 2,6-ditertiarybutylphenol and 3,5-ditertiarybutylcatechol by molecular oxygen with the dinuclear Cu(I) Cu(II) complexes of macrocyclic ligands are presented. An interesting aspect of the present work is that the catalytically active Cu(I) dioxygen complexes with macrocyclic ligands are stable enough at room temperature, and have long enough lifetimes, to carry out two-electron oxidation of various phenolic and catecholic substrates. These models of tyrosinase are unique in that they function at room temperature. Copper dioxygen complexes reported previously decompose above −70°C with very few exceptions. Also, the Cu(I) dioxygen complexes described in this paper are the first tyrosinase models to oxidize substrates catalytically, a process that requires that the Cu(II) complexes also oxidize the same substrates.