C(sp2)–H Amination Reactions Mediated by Metastable Pseudo-O(h) Masked Aryl-Co(III)-nitrene Species

[Image: see text] Cobalt-catalyzed C–H amination via M-nitrenoid species is spiking the interest of the research community. Understanding this process at a molecular level is a challenging task, and here we report a well-defined macrocyclic system featuring a pseudo-O(h) aryl-Co(III) species that reacts with aliphatic azides to effect intramolecular C(sp2)–N bond formation. Strikingly, a putative aryl-Co=NR nitrenoid intermediate species is formed and is rapidly trapped by a carboxylate ligand to form a carboxylate masked-nitrene, which functions as a shortcut to stabilize and guide the reaction to productive intramolecular C(sp2)–N bond formation. On one hand, several intermediate species featuring the C(sp2)–N bond formed have been isolated and structurally characterized, and the essential role of the carboxylate ligand has been proven. Complementarily, a thorough density functional theory study of the C(sp2)–N bond formation mechanism explains at the molecular level the key role of the carboxylate-masked nitrene species, which is essential to tame the metastability of the putative aryl-Co(III)=NR nitrene species to effectively yield the C(sp2)–N products. The solid molecular mechanistic scheme determined for the C(sp2)–N bond forming reaction is fully supported by both experimental and computation complementary studies.