Metal-stabilized phenoxyl radicals appear to be important intermediates in a variety of enzymatic oxidations. We report that transition metal coordination also supports an aminyl radical, resulting in a stable crystalline complex: [Rh(I)(trop2N.)(bipy)]+OTf- (where trop is 5-H-dibenzo[a,d]cycloheptene-5-yl, bipy is 2,2'-bipyridyl, OTf- is trifluorosulfonate). It is accessible under mild conditions by one-electron oxidation of the amide complex [Rh(I)(trop2N)(bipy)], at a potential of -0.55 volt versus ferrocene/ferrocenium. Both electron paramagnetic resonance spectroscopy and density functional theory support 57% localization of the unpaired spin at N. In reactions with H-atom donors, the Rh-coordinated aminyl behaves as a nucleophilic radical.

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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