Synthesis and Characterization of a Bis(imino)-N-heterocyclic Carbene Analogue to Bis(imino)pyridine Iron Complexes

New bis(imino)-N-heterocyclic carbene ligands have been synthesized, and the corresponding iron complexes have been isolated and characterized. Whereas imidazole-derived complexes exhibited exclusively bidentate binding modes, 4,5,6-trihydropyrimidylidene-based ligands adopted a tridentate pincer conformation analogous to complexes of 2,6-bis(imino)pyridines. Bonding in the five-coordinate bis(imino)-N-heterocyclic carbene complex displayed considerably contracted iron ligand bond distances in comparison to those in the analogous bis(imino)pyridine iron complex. Of particular note was an extraordinarily short iron carbene carbon bond distance (1.812(2) angstrom). In addition to demonstrating differences in bonding, the isoelectronic structures have different electrochemical properties that reflect the ability of the carbene to stabilize both oxidized and reduced forms of the parent organometallic species.