Abstract
We report the catalytic C-H amination mediated by an isolable Co(III) imido complex ((Tr)L)Co(NR) supported by a sterically demanding dipyrromethene ligand ((Tr)L = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of ((Tr)L)Li with CoCl(2) in THF afforded a high-spin (S = 3/2) three-coordinate complex ((Tr)L)CoCl. Chemical reduction of ((Tr)L)CoCl with potassium graphite yielded the high-spin (S = 1) Co(I) synthon ((Tr)L)Co which is stabilized through an intramolecular η(6)-arene interaction. Treatment of ((Tr)L)Co with a stoichiometric amount of 1-azidoadamantane (AdN(3)) furnished a three-coordinate, diamagnetic Co(III) imide ((Tr)L)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 ((Tr)L)Co to linear alkyl azides (RN(3)) 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 ((Ar)L)Co(NR) ((Ar)L = 5-mesityl-1,9-(2,4,6-Ph(3)C(6)H(2))dipyrrin), ((Tr)L)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, ((Tr)L)Co(NAd) exhibits an extremely low one-electron reduction potential (E°(red) = -1.98 V vs [Cp(2)Fe](+/0)) indicating that the highly basic terminal imido unit contributes to the driving force for H-atom abstraction.