Hydrogen‐Atom Transfer (HAT) Initiated by Intramolecular Ligand–Metal Electron Transfer

Hydrogen‐atom transfer (HAT) is of key importance for several catalytic and biological processes, and provides an elegant access to C−H activation. In synthetic chemistry, a photoactivated metal complex is often employed to an oxygen‐ or nitrogen‐bound hydrogen, and the as‐generated oxygen‐ or nitrogen‐centered radical is the hydrogen‐atom acceptor for HAT. Here, we report the first examples for HAT processes initiated by one‐electron oxidation of urea azines. A further novelty is that the HAT‐initiating oxidation can be realized by intramolecular ligand–metal electron transfer in copper(II)‐urea azine complexes. These complexes are first characterized in the solid state, in which they are stable. Electron‐transfer‐initiated HAT processes are observed upon dissolving the complexes in organic solvents, and the kinetics of these processes varies with the solvent polarity. The carbon‐centered radicals formed by HAT can either be trapped with 2,2,6,6‐tetramethylpiperidinyloxyl (TEMPO) or undergo radical recombination reactions with itself, yielding diamagnetic end‐products. CuII complexes with redox‐active urea azine ligands undergo ligand–metal electron transfer reactions in solutions. The resulting radical monocationic urea azines with nitrogen‐centered radical react further via HAT to carbon‐centered radicals, and finally to diamagnetic products.