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

Abstract

We report the catalytic C-H amination mediated by an isolable Co m imido complex (L-Tr)Co(NR) supported by a sterically demanding dipyrromethene ligand (L-Tr = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (L-Tr)Li with CoCl2 in THE afforded a high-spin (S = 3/2) threecoordinate complex (L-Tr)CoCI. Chemical reduction of (L-Tr)CoCl with potassium graphite yielded the high-spin (S = 1) Co-1 synthon (L-Tr)Co which is stabilized through an intramolecular eta(6)-arene interaction. Treatment of (L-Tr)Co with a stoichiometric amount of 1-azidoadamantane (AdN(3)) furnished a three-coordinate, diamagnetic Co-III imide (L-Tr)-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)degrees. Exposure of 1-10 mol % of (L-Tr)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 (L-Ar)Co(NR) (L-Ar = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin), (L-Tr)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, (L-Tr)Co(NAd) exhibits an extremely low one-electron reduction potential (E degrees (red )= -1.98 V vs [Cp2Fe](+/0)) indicating that the highly basic terminal imido unit contributes to the driving force for H-atom abstraction.