Bis-Phenoxo-Cu^II₂ Complexes: Formal Aromatic Hydroxylation via Aryl-Cu^III Intermediate Species

Ullmann-type copper-mediated arylC-O bond formation has attracted the attention of the catalysis and organometallic communities, although the mechanism of these copper-catalyzed coupling reactions remains a subject of debate. We have designed well-defined triazamacrocyclic-based aryl-Cu^III complexes as an ideal platform to study the C-heteroatom reductive elimination step with all kinds of nucleophiles, and in this work we focus our efforts on the straightforward synthesis of phenols by using H₂O as nucleophile. Seven well-defined aryl-Cu^III complexes featuring different ring size and different electronic properties have been reacted with water in basic conditions to produce final bis-phenoxo-Cu^II₂ complexes, all of which are characterized by XRD. Mechanistic investigations indicate that the reaction takes place by an initial deprotonation of the NH group coordinated to Cu^III center, subsequent reductive elimination with H₂O as nucleophile to form phenoxo products, and finally air oxidation of the Cu^I produced to form the final bis-phenoxo-Cu^II₂ complexes, whose enhanced stability acts as a thermodynamic sink and pushes the reaction forward. Furthermore, the corresponding triazamacrocyclic-Cu^I complexes react with O₂ to undergo 1e^- oxidation to Cu^II and subsequent C-H activation to form aryl-Cu^III species, which follow the same fate towards bis-phenoxo-Cu^II₂ complexes. This work further highlights the ability of the triazamacrocyclic-Cu^III platform to undergo aryl-OH formation by reductive elimination with basic water, and also shows the facile formation of rare bis-phenoxo-Cu^II₂ complexes.