Introducing a Metal−Metal Multiply Bonded Group as an “Axial Ligand” to Iron: Synthetic Design of a Linear Cr−Cr···Fe Framework

In natural systems, reactivity of iron is regulated by interactions with axial donor ligands. We have designed a new synthetic route to a novel iron complex, CrCrFe(dpa)4Cl2 (1, dpa = 2,2‘-dipyridylamide), that features axial interactions between the Fe atom and a coaxial Cr−Cr quadruply bonded group. This linear Cr−Cr···Fe complex has been characterized by X-ray crystallography in two distinct crystal forms, as well as by elemental analysis, mass spectrometry, UV−vis and IR spectroscopy, and magnetic susceptibility, which firmly establish the presence of a high-spin Fe(II) ion in 1. Redox properties of the Fe(II) ion in this unique coordination environment have been probed by cyclic voltammetry, where we observe that 1 is significantly easier to oxidize than corresponding iron complexes having axial N-donor ligands. This observation leads us to propose that the electron-rich Cr−Cr quadruply bonded unit may act as an “axial ligand” to the Fe atom in 1.