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

Reaction of the Co-I complex [(TIMMNmes)Co-I](PF6) (1) (TIMMNmes=tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with mesityl azide yields the Co-III imide [(TIMMNmes)Co-III(NMes)](PF6) (2). Oxidation of 2 with [FeCp2](PF6) provides access to a rare Co-III imidyl [(TIMMNmes)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.

Automated summary

This study synthesized a series of cobalt complexes through oxidation reactions of the reactants, and characterized and analyzed them using experimental methods and computational methods, revealing their structures and reaction mechanisms.