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,H)Pytacn)(S)](2+) (2, (Me,H)Pytacn=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-(CF3SO3)(2)((Me,H)Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N-4 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) > 2h 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-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 =99.3 kcal mol(-1)).