Synthesis and Reactivity of a Cobalt-Supported Singlet Nitrene

The synthesis, characterization, and reactivity of a series of cobalt terminal imido complexes supported by an N-anchored tripodal tris(carbene) chelate is described, including a Co-supported singlet nitrene. Reaction of the Co^I precursor [(TIMMN^mes)Co^I](PF₆) (TIMMN^mes = -[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with p-methoxyphenyl azide yields a Co^III imide [(TIMMN^mes)Co^III(NAnisole)](PF₆) (1). Treatment of 1 with 1 equiv of [FeCp₂](PF₆) at -35 °C affords a formal Co^IV imido complex [(TIMMN^mes)Co(NAnisole)](PF₆)₂ (2), which features a bent Co-N(imido)-C(Anisole) linkage. Subsequent one-electron oxidation of 2 with 1 equiv of AgPF₆ provides access to the tricationic cobalt imido complex [(TIMMN^mes)Co(NAnisole)](PF₆)₃ (3). All complexes were fully characterized, including single-crystal X-ray diffraction (SC-XRD) analyses, infrared (IR) vibrational, ultraviolet/visible (UV/vis) electronic absorption, multinuclear NMR, X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XAS). Quantum chemical calculations provide additional insight into the electronic structures of all compounds. The dicationic Co^IV imido complex 2 exhibits a doublet ground state with considerable imidyl character as a result of covalent Co-NAnisole bonding. At room temperature, 2 readily converts to a Co^II amine complex involving intramolecular C-H bond amination. Electronically, tricationic complex 3 can be understood as a singlet nitrene bound to Co^III with significant Co^IV imidyl radical character. Verifying the pronounced electrophilicity, nucleophiles such as H₂O and BuNH₂ add to 3─analogous to the parent free nitrene─in the para position of the aromatic substituent, thus, clearly corroborating singlet nitrene-type reactivity.