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

Reaction of the Co(I) complex [(TIMMN(mes))Co(I)](PF(6)) (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)](PF(6)) (2). Oxidation of 2 with [FeCp(2)](PF(6)) provides access to a rare Co(III) imidyl [(TIMMN(mes))Co(NMes)](PF(6))(2) (3). Single‐crystal X‐ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S= [Formula: see text] 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.