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

摘要

Abstract Reaction of the CoI complex (TIMMNmes)CoI(PF6) (1) (TIMMNmes=tris‐2‐(3‐mesityl‐imidazolin‐2‐ylidene)‐methylamine) with mesityl azide yields the CoIII imide (TIMMNmes)CoIII(NMes)(PF6) (2). Oxidation of 2 with FeCp2(PF6) provides access to a rare CoIII 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 CoI N‐heterocyclic imine (4) within 12?h. Conversely, it takes merely 0.5?h for 3 to transform to the CoII 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.