Direct Manipulation of Metal Imido Geometry: Key Principles to Enhance C-H Amination Efficacy

We report the catalytic C-H amination mediated by an isolable Co^III imido complex (^TrL)Co(NR) supported by a sterically demanding dipyrromethene ligand (^TrL = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (^TrL)Li with CoCl₂ in THF afforded a high-spin (S = 3/2) three-coordinate complex (^TrL)CoCl. Chemical reduction of (^TrL)CoCl with potassium graphite yielded the high-spin (S = 1) Co^I synthon (^TrL)Co which is stabilized through an intramolecular η⁶-arene interaction. Treatment of (^TrL)Co with a stoichiometric amount of 1-azidoadamantane (AdN₃) furnished a three-coordinate, diamagnetic Co^III imide (^TrL)Co(NAd) as confirmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute Co-N_imido-C angle 145.0(3)°. Exposure of 1-10 mol % of (^TrL)Co to linear alkyl azides (RN₃) resulted in catalytic formation of substituted N-heterocycles via intramolecular C-H amination of a range of C-H bonds, including primary C-H bonds. The mechanism of the C-N bond formation was probed via initial rate kinetic analysis and kinetic isotope effect experiments [k_H/k_D = 38.4(1)], suggesting a stepwise H-atom abstraction followed by radical recombination. In contrast to the previously reported C-H amination mediated by (^ArL)Co(NR) (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin), (^TrL)Co(NR) displays enhanced yields and rates of C-H amination without the aid of a cocatalyst, and no catalyst degradation to a tetrazene species was observed, as further supported by the pyridine inhibition effect on the rate of C-H amination. Furthermore, (^TrL)Co(NAd) exhibits an extremely low one-electron reduction potential (E°_red = -1.98 V vs [Cp₂Fe]^+/0) indicating that the highly basic terminal imido unit contributes to the driving force for H-atom abstraction.