ja9b09015_si_002.cif (4.36 MB)

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

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posted on 10.10.2019, 13:03 by Yunjung Baek, Elisabeth T. Hennessy, Theodore A. Betley
We report the catalytic C–H amination mediated by an isolable CoIII 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) CoI 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 CoIII imide (TrL)­Co­(NAd) as confirmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute Co–Nimido–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 [kH/kD = 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.

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