Synthesis and Reactivity of a Cobalt-Supported Singlet Nitrene
Weiqing Mao, Zihan Zhang, Dominik Fehn, Sergio A. V. Jannuzzi, Frank W. Heinemann, Andreas Scheurer, Maurice van Gastel, Serena DeBeer, Dominik Munz, Karsten Meyer
Abstract
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 ](PF 6 ) (TIMMN mes = t ris -[2-(3-mesityl- im idazolin-2-ylidene)- m ethyl]ami n e) with p -methoxyphenyl azide yields a Co III imide [(TIMMN mes )Co III (NAnisole)](PF 6 ) ( 1 ). Treatment of 1 with 1 equiv of [FeCp 2 ](PF 6 ) at −35 °C affords a formal Co IV imido complex [(TIMMN mes )Co(NAnisole)](PF 6 ) 2 ( 2 ), which features a bent Co–N(imido)–C(Anisole) linkage. Subsequent one-electron oxidation of 2 with 1 equiv of AgPF 6 provides access to the tricationic cobalt imido complex [(TIMMN mes )Co(NAnisole)](PF 6 ) 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 H 2 O and t BuNH 2 add to 3 ─analogous to the parent free nitrene─in the para position of the aromatic substituent, thus, clearly corroborating singlet nitrene-type reactivity.