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 [Fe IV (O)( Me,H Pytacn)(S)] 2+ (2, Me,H Pytacn = 1-(2′-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S = CH 3 CN or H 2 O) is described. 2 has been prepared by reaction of [Fe II (CF 3 SO 3 ) 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 related iron(IV)-oxo of 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 has been 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 (BDE = 99.3 kcal·mol −1 ).

Evidence for a Precursor Complex in CH Hydrogen Atom Transfer Reactions Mediated by a Manganese(IV) Oxo Complex

“…does not undergo protonation of the oxo ligand even at pH % 1, [15] indicating that the corresponding pK a < 1. These values show that in an analogous chemical environment, the Mn IV =O unit is at least 7 pKa units more basic than Fe IV =O.…”

Spectroscopic Characterization and Reactivity of Triplet and Quintet Iron(IV) Oxo Complexes in the Gas Phase

“…Note that this is in agreement with the previous results obtained by Mçssbauer spectroscopy. [17] Thee xpected Fe = Os tretch is predicted in the computed spectra of 3 2+ in frozen acetonitrile solution (831 cm À1 ). [17] Theexperimentally determined Fe = Os tretches of iron(IV) complexes in condensed phase expand from 780 to 900 cm À1 and as pin correlation with the spin state cannot be established.…”

“…[17] Thee xpected Fe = Os tretch is predicted in the computed spectra of 3 2+ in frozen acetonitrile solution (831 cm À1 ). [17] Theexperimentally determined Fe = Os tretches of iron(IV) complexes in condensed phase expand from 780 to 900 cm À1 and as pin correlation with the spin state cannot be established. [24] TheI RPD spectra do not give strong evidence for which isomers are formed.…”

“…Mçssbauer spectroscopy has demonstrated that this reaction sequence leads to the generation of triplet state iron(IV) complexes (S = 1). [17] Using electrospray ionization (ESI) we were able to transfer the [(PyTACN)Fe(O)(OTf)] + ions to the gas phase and on addition of HNO 3 ,t he [(PyTACN)Fe(O)(NO 3 …”

Spectroscopic Characterization and Reactivity of Triplet and Quintet Iron(IV) Oxo Complexes in the Gas Phase

“…Mössbauer spectroscopy has demonstrated that this reaction sequence leads to the generation of triplet state iron(IV) complexes ( S =1) 17. Using electrospray ionization (ESI) we were able to transfer the [(PyTACN)Fe(O)(OTf)] + ions to the gas phase and on addition of HNO 3 , the [(PyTACN)Fe(O)(NO 3 )] + ions ( m / z  382) were also detected (Figure S5 in the Supporting Information).…”