Noninnocent Ligand in Rhodium(III)-Complex-Catalyzed C-H Bond Amination with Tosyl Azide

Six-coordinate rhodium(III) complexes coordinated by a planar trianionic ligand (L^3-) are synthesized. One of the axial positions is occupied by chloride (Cl^-), bromide (Br^-), or iodide (I^-), and another axial position is coordinated by a solvent molecule such as acetonitrile (AN), water (H₂O), tetrahydrofuran (THF), or pyridine (PY) to complete an octahedral rhodium(III) center; [Rh^III(L^3-)(X)(Y)]^- (1^X/Y; X = Cl^-, Br^-, or I^-, Y = AN, H₂O, THF, or PY). Coordination of the AN, H₂O, and THF ligands to the metal center is rather weak, so that these solvent molecules are easily replaced by PY to give [Rh^III(L^3-)(Cl)(PY)]^-. In the electrochemical measurements, all complexes have two reversible redox couples based on the ligand-centered oxidation L^3- to L^•2- and to L^-, as reflected by the very similar redox potentials regardless of the different axial ligands. The rhodium(III) complexes catalyze C-H bond amination of xanthene with tosyl azide (TsN₃). Because the yields of the aminated product are nearly the same among the complexes, replacement of the axial solvent ligands with TsN₃ readily occurs to give a nitrene-radical-bound rhodium(III) complex, [Rh^III(L^•2-)(N^•Ts)(X)]^-, as an active oxidant, which is generated by one-electron transfer from the trianionic L^3- to the nitrene nitrogen atom. Generation of such a diradical intermediate was substantiated by the direct reaction of 1^Cl/AN with TsN₃ in the absence of the substrate (xanthene). In this case, a rhodium(III) iminosemiquinone complex, 2, was generated by the intramolecular reaction between the nitrene-radical moiety and the radical moiety of the ligand L^•2-.