The present study provides mechanistic details of a mild aromatic C-H activation effected by a copper(II) center ligated in a triazamacrocylic ligand, affording equimolar amounts of Cu III -aryl species and Cu I as reaction products. At low-temperatures the Cu II complex 1 forms a 3-center 3-electron C-H⋯Cu II interaction, identified by pulse-EPR spectroscopy and supported by density functional theory (DFT) calculations. C-H bond cleavage is coupled with copper oxidation, as a Cu III -aryl product 2 is formed. This reaction proceeds to completion at 273 K within minutes through either a copper disproportionation reaction or, alternatively, an even faster reaction with one-equivalent of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), quantitatively yielding 2. Kinetic studies of both reactions strongly implicate a rate-limiting proton coupled electron transfer (PCET) as the key C-H activation step, a mechanism that does not conform to either the C-H activation mechanism in a Ni II analogue or to any previously proposed C-H activation mechanisms.

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