Modeling the cis-Oxo-Labile binding site motif of non-heme iron oxygenases: Water exchange and oxidation reactivity of a non-heme iron(IV)-Oxo compound bearing a tripodal tetradentate ligand

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe^IV(O)(^Me,HPytacn)(S)] ²⁺ (2, ^Me,HPytacn=1-(2′-pyridylmethyl)-4,7-dimethyl- 1,4,7-triazacyclononane, S=CH₃CN or H₂O) is described. Complex 2 was prepared by reaction of [Fe^II(CF₃SO ₃)₂(^Me,HPytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N₄ ligand that leaves two cis sites available for binding an oxo group and a second external ligand but, unlike the related iron(IV)-oxo species with tetradentate ligands, it is remarkably stable at room temperature (t_1/2>2a h at 288a K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism is proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen-atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (bond dissociation energy=99.3a kcala mol^-1). A powerful oxidant: A non-heme oxo-iron(IV) complex (see picture), containing a labile site cis to the terminal oxo group, has been prepared and characterized. The complex is unusually stable (t_1/2>2a h at 288a K). Water exchange, oxo transfer, and Ha abstraction reactions have been studied. Despite its thermal stability, the complex is a very powerful oxidant.