Nonheme Oxoiron(IV) Complexes of Pentadentate N5 Ligands: Spectroscopy, Electrochemistry, and Oxidative Reactivity

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 [Fe(IV)(O)(L(N5))](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 Fe=O 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 Fe=O 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(Fe=O) modes at 825-841 cm(-1). Spectropotentiometric experiments in acetonitrile with 0.1 M water reveal that the supporting pentadentate ligands modulate the E(1/2)(IV/III) redox potentials with values ranging from 0.83 to 1.23 V vs. Fc, providing the first electrochemical determination of the E(1/2)(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 L(N5) ligand and in the orientations of the pyridine donors relative to the Fe=O 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 Fe(IV)=O complexes with different ligand frameworks and sheds some light on the complexities of the reactivity of the oxoiron(IV) unit.

Figure 1

Electronic spectra of oxoiron(IV) complexes 1 (black dashed line), 2 (black solid line), 3 (red dashed line), 4 (red solid line) and 5 (blue solid line) recorded in CH₃CN at room temperature.

Figure 2

Unfiltered EXAFS spectra (dotted lines) and corresponding best fits (solid lines) of 1, 3, 4, and 5. The fits shown are given in bold italics in Table S1 in the Supporting Information.

Figure 3

Difference spectra derived from spectropotentiometric titrations for complexes 2 (A), 3 (B), and 5 (C)at 25 °C in CH₃CN containing 0.1 M H₂O. The iron concentration used for all three experiments was 0.25 mM. For each panel, the increase in the absorbance at ~740 nm is plotted as a function of the applied potential to show a sigmoidal shape that can be fit well with the Nernst equation.

Figure 4

Plot of E_p,c (black circles) and ν(Fe=O) values (blue squares) versus E_1/2(IV/III) vs. E_1/2(IV/III) for the Fe^IV=O complexes in this study (data from Tables 2 and 1, respectively). The red line represents the best linear fit for the E_pc vs. E_1/2(IV/III) correlation with the red dot indicating an estimation of the E_1/2(IV/III) value for 4 from the linear correlation.

Figure 5

Plots of the logarithms of the second-order rate constants for the oxidation of thioanisole at –10 C (filled black squares), CHD at –40 °C (filled red circles), PhCH₂OH at 25 °C (open blue triangles) and DHA at 25 °C (open black diamonds) in CH₃CN vs. the E_1/2(IV/III) value measured by spectropotentiometry for oxoiron(IV) complexes 1 – 5. The red solid line is defined by the rates of thioanisole oxidation by 1 – 5, while the red dashed lines are defined by the rates of H-atom abstraction from various substrates by 1 – 3. The red dashed lines show that the H-atom abstraction rates associated with 4 and 5 are about an order of magnitude lower than predicted.

Scheme. 1

Structures of oxoiron(IV) complexes 1 – 5 studied in this work. Numerical designations 1a – 5a refer to corresponding [Fe^II(L_N5)(NCCH₃)]²⁺ complexes, while 1b – 5b refer to corresponding [Fe^III(OH)(L_N5)]²⁺ species

Scheme. 2