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
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.