Nonheme oxoiron(iv) complexes of pentadentate N5 ligands: spectroscopy, electrochemistry, and oxidative reactivity
- 25 October 2012
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Chemical Science
- Vol. 4 (1), 282-291
- https://doi.org/10.1039/c2sc21318d
Abstract
Oxoiron(IV) species have been found to act as the oxidants in the catalytic cycles of several mononuclear nonheme iron enzymes that activate dioxygen. To gain insight into the factors that govern the oxidative reactivity of such complexes, a series of five synthetic S = 1 [FeIV(O)(LN5)]2+ complexes has been characterized with respect to their spectroscopic and electrochemical properties as well as their relative abilities to carry out oxo transfer and hydrogen atom abstraction. The FeO units in these five complexes are supported by neutral pentadentate ligands having a combination of pyridine and tertiary amine donors but with different ligand frameworks. Characterization of the five complexes by X-ray absorption spectroscopy reveals FeO bonds of ca. 1.65 Å in length that give rise to the intense 1s → 3d pre-edge features indicative of iron centers with substantial deviation from centrosymmetry. Resonance Raman studies show that the five complexes exhibit ν(FeO) modes at 825–841 cm−1. Spectropotentiometric experiments in acetonitrile with 0.1 M water reveal that the supporting pentadentate ligands modulate the E1/2(IV/III) redox potentials with values ranging from 0.83 to 1.23 V vs. Fc, providing the first electrochemical determination of the E1/2(IV/III) redox potentials for a series of oxoiron(IV) complexes. The 0.4 V difference in potential may arise from differences in the relative number of pyridine and tertiary amine donors on the LN5 ligand and in the orientations of the pyridine donors relative to the FeO bond that are enforced by the ligand architecture. The rates of oxo-atom transfer (OAT) to thioanisole correlate linearly with the increase in the redox potentials, reflecting the relative electrophilicities of the oxoiron(IV) units. However this linear relationship does not extend to the rates of hydrogen-atom transfer (HAT) from 1,3-cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA), and benzyl alcohol, suggesting that the HAT reactions are not governed by thermodynamics alone. This study represents the first investigation to compare the electrochemical and oxidative properties of a series of S = 1 FeIV O complexes with different ligand frameworks and sheds some light on the complexities of the reactivity of the oxoiron(IV) unit.Keywords
This publication has 84 references indexed in Scilit:
- Nuclear Resonance Vibrational Spectroscopy on the FeIVO S=2 Non‐Heme Site in TMG3tren: Experimentally Calibrated Insights into ReactivityAngewandte Chemie, 2011
- Evidence for a High-Spin Fe(IV) Species in the Catalytic Cycle of a Bacterial Phenylalanine HydroxylaseBiochemistry, 2011
- 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 LigandChemistry – A European Journal, 2011
- Understanding Hydrogen Atom Transfer: From Bond Strengths to Marcus TheoryAccounts of Chemical Research, 2010
- Formation, Structure, and EPR Detection of a High Spin FeIV—Oxo Species Derived from Either an FeIII—Oxo or FeIII—OH ComplexJournal of the American Chemical Society, 2010
- A Synthetic High‐Spin Oxoiron(IV) Complex: Generation, Spectroscopic Characterization, and ReactivityAngewandte Chemie, 2009
- Substrate-Triggered Formation and Remarkable Stability of the C−H Bond-Cleaving Chloroferryl Intermediate in the Aliphatic Halogenase, SyrB2Biochemistry, 2009
- A Two‐State Reactivity Model Explains Unusual Kinetic Isotope Effect Patterns in C-H Bond Cleavage by Nonheme Oxoiron(IV) ComplexesAngewandte Chemie, 2009
- Proton‐ and Reductant‐Assisted Dioxygen Activation by a Nonheme Iron(II) Complex to Form an Oxoiron(IV) IntermediateAngewandte Chemie, 2008
- Direct Spectroscopic Evidence for a High-Spin Fe(IV) Intermediate in Tyrosine HydroxylaseJournal of the American Chemical Society, 2007