Directed Amination as a Strategy for C-N Bond Formation by Iron Imido Complexes

The stabilization of low-coordinate and high-spin iron complexes plays a central role in enabling novel pathways for C-H functionalization and catalysis of organic reactions. Previous work from our group demonstrated that the bulky bis(carbene)borate ligand Ph2B(tBuIm)2- supports the three-coordinate iron(II) imido complex [Ph2B(tBuIm)2Fe=NDipp]-. In this study, we extend this platform by employing an even larger, more electron-withdrawing imido ligand to generate Ph2B(tBuIm)2Fe(N(terph)) (terph = 2,6-C6H5(C6H3)). This iron(III) imido complex was isolated and fully characterized, including by single-crystal X-ray diffraction. The complex reacts with 2-picoline to afford C-N bond functionalization at the benzylic position of the substrate. Mechanistic investigations by density functional theory calculations support a pathway in which intramolecular hydrogen atom transfer from the coordinated substrate to the imido ligand provides a carbon-centered radical. Subsequent intramolecular radical rebound forms a new C-N bond. Together, these two steps define a mechanistic sequence reminiscent of enzymatic C-H functionalization pathways at a well-defined, low-coordinate iron complex. In addition, DFT provides insight into the directing effect of the picoline ligand.