| Summary: | 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<sup>III</sup> 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 H<sub>2</sub>O as nucleophile. Seven well-defined aryl-Cu<sup>III</sup> complexes featuring different ring size and different electronic properties have been reacted with water in basic conditions to produce final bis-phenoxo-Cu<sup>II</sup><sub>2</sub> 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<sup>III</sup> center, subsequent reductive elimination with H<sub>2</sub>O as nucleophile to form phenoxo products, and finally air oxidation of the Cu<sup>I</sup> produced to form the final bis-phenoxo-Cu<sup>II</sup><sub>2</sub> complexes, whose enhanced stability acts as a thermodynamic sink and pushes the reaction forward. Furthermore, the corresponding triazamacrocyclic-Cu<sup>I</sup> complexes react with O<sub>2</sub> to undergo 1e<sup>−</sup> oxidation to Cu<sup>II</sup> and subsequent C-H activation to form aryl-Cu<sup>III</sup> species, which follow the same fate towards bis-phenoxo-Cu<sup>II</sup><sub>2</sub> complexes. This work further highlights the ability of the triazamacrocyclic-Cu<sup>III</sup> platform to undergo aryl-OH formation by reductive elimination with basic water, and also shows the facile formation of rare bis-phenoxo-Cu<sup>II</sup><sub>2</sub> complexes.
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