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

Abstract

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [FeIV(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 [FeII(CF3SO3)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 (t1/2>2 h at 288 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[BOND]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[BOND]H bond of cyclohexane (bond dissociation energy=99.3 kcal mol−1)