Formation of Highly Strained N-Heterocycles via Decomposition of Iron N-Heterocyclic Carbene Complexes: The Value of Labile Fe-C Bonds

An unusual, highly-strained annulated 2,2′-biimdazole was isolated as decomposition product of the outer-sphere one-electron oxidation of an iron(II) N-heterocyclic carbene (NHC) complex bearing a tetradentate bis(NHC)-bis- (pyridine) ligand (NCCN). Reductive elimination leading to the 2,2′-biimdazole is a consequence of the lability of the Fe-C bonds in the transient species and also extends to complexes with modified ligands but the same coordination geometry. Closely related by a two-electron redox step to a family of less-strained tetraazafulvalenes, the obtained 2,2′-biimidazolium salts were studied electrochemically. Introduction of methyl substituents at the methylene tether significantly increased the reversibility of the electrochemical reduction. Furthermore, the reactivity of the 2,2′-biimidazolium salt was examined by oxidative addition of [Ni(cod)₂] to the central C-C bond, providing a previously unknown way for the formation of NHC transition metal complexes.