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 FeC 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 CC bond, providing a previously unknown way for the formation of NHC transition metal complexes.