XAS characterization of end-on and side-on peroxoiron(iii) complexes of the neutral pentadentate N-donor ligand N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine

Abstract

Peroxo intermediates are implicated in the catalytic cycles of iron enzymes involved in dioxygen metabolism. X-ray absorption spectroscopy has been used to gain insight into the iron coordination environments of the low-spin complex [Fe(III)(Me-TPEN)(eta(1)-OOH)](2+)(1) and the high-spin complex [Fe(III)(Me-TPEN)(eta(2)-O(2))](+)(2)(the neutral pentadentate N-donor ligand Me-TPEN =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) and obtain metrical parameters unavailable from X-ray crystallography. The complexes exhibit relatively large pre-edge peak areas of approximately 15 units, indicative of iron centers with significant distortions from centrosymmetry. These distortions result from the binding of peroxide, either end-on hydroperoxo for 1 (r(Fe-O)= 1.81A) or side-on peroxo for 2 (r(Fe-O)= 1.99 A). The XAS analyses of 1 strongly support a six-coordinate low-spin iron(III) center coordinated to five nitrogen atoms from Me-TPEN and one oxygen atom from an end-on hydroperoxide ligand. However, the XAS analyses of 2 are not conclusive: Me-TPEN can act either as a pentadentate ligand to form a seven-coordinate peroxo complex, which has precedence in the DFT geometry optimization of [Fe(III)(N4Py)(eta(2)-O(2))](+)(the neutral pentadentate N-donor ligand N4Py =N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), or as a tetradentate ligand with a dangling pyridylmethyl arm to form a six-coordinate peroxo complex, which is precedented by the crystal structure of [Fe(2)(III)(Me-TPEN)(2)(Cl)(2)(mu-O)](2+).

Oxygen Activation at Mononuclear Nonheme Iron Centers: A Superoxo Perspective

“…1B) is proposed to be an iron(III)-peroxo species that derives from one-electron reduction of the initial Fe–O 2 adduct by the nearby reduced Rieske Fe 2 S 2 cluster. The O–O bond distance is 1.46 Å and the Fe–O distances are 1.7 and 2.0 Å,25 consistent with an iron(III)-η 2 -peroxo assignment 28, 29. This difference in Fe–O bond lengths suggests that the more distal oxygen atom may be protonated.…”

Acid‐Triggered O−O Bond Heterolysis of a Nonheme FeIII(OOH) Species for the Stereospecific Hydroxylation of Strong C−H Bonds

“…[46][47][48][49] Furthermore, EXAFS analysis of 3 shows as ix-coordinate metal center with 0. [43,46] Ad etailed kinetic study on the formation and disappearance of 3 in acetonitrile solutionw as carried out using ac ryostopped-flow instrumentw ith ad iode-array detector.W hen 1 reacts with an excess of H 2 O 2 ab iphasic kinetic behavior is observed. In particular, the 1.82-scatterer found for 3 can be assigned to the O-atom of an HOO ligand, as observed at 1.76 and1 .81 in the EXAFS analysis of two relatedl owspin Fe III (OOH) complexes.…”

Mössbauer Investigation of Peroxo Species in the Iron(III)–EDTA–H2O2 System

“…Brausam and van Eldik [1] have suggested that iron is seven-coordinate in this complex, which means dechelation of one carboxylate arm (pentadentate EDTA), although they cited theoretical calculations in the literature that were in favor of sixfold coordination. [40] XAS measurements on peroxoiron() complexes with pentadentate Ndonor ligands by Koehntop et al [42] were not always conclusive either regarding the exact coordination number of iron (side-on or end-on bonding of O 2 2-). Our Mössbauer data support minimum sevenfold coordination on the basis of the large isomer shift.…”