Umpolung in a Pair of Cobalt(III) Terminal Imido/Imidyl Complexes

Abstract

Reaction of the Co^I complex [(TIMMN^mes )Co^I ](PF₆ ) (1) (TIMMN^mes =tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with mesityl azide yields the Co^III imide [(TIMMN^mes )Co^III (NMes)](PF₆ ) (2). Oxidation of 2 with [FeCp₂ ](PF₆ ) provides access to a rare Co^III imidyl [(TIMMN^mes )Co(NMes)](PF₆ )₂ (3). Single-crystal X-ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S= $1/2$ ground state. ENDOR, X-ray absorption spectroscopy and computational analyses indicate a ligand-based oxidation; thus, an imidyl-radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the Co^I N-heterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the Co^II congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.

Keywords: Cobalt; Imidyl Radical; N-Heterocyclic Carbene; Terminal Imido; Umpolung.