Unusual Rearrangement of an N-Donor-Functionalized N-Heterocyclic Carbene Ligand on Group 8 Metals

We report an unexpected rearrangement of a deprotonated picolyl-functionalized N-heterocyclic carbene (NHC) ligand from N,C-chelate to N,N-chelate in three-legged piano-stool Fe(II) and Ru(II) complexes. The reaction mechanism has been explored for one of the Fe(II) complexes. Experimental and computational studies suggest an unusual intermediate featuring a four-membered chelate ring, where the NHC and the α-carbon of one of the N-substituents coordinate to the Fe(II) center. A possible Fe–alkylidene intermediate has also been predicted by computations.