Rapid Iron(III)-Fluoride-Mediated Hydrogen Atom Transfer

Abstract

We anticipate high-valent metal-fluoride species will be highly effective hydrogen atom transfer (HAT) oxidants because of the magnitude of the H-F bond (in the product) that drives HAT oxidation. We prepared a dimeric Fe^III (F)-F-Fe^III (F) complex (1) by reacting [Fe^II (NCCH₃ )₂ (TPA)](ClO₄ )₂ (TPA=tris(2-pyridylmethyl)amine) with difluoro(phenyl)-λ³ -iodane (difluoroiodobenzene). 1 was a sluggish oxidant, however, it was readily activated by reaction with Lewis or Brønsted acids to yield a monomeric [Fe^III (TPA)(F)(X)]⁺ complex (2) where X=F/OTf. 1 and 2 were characterized using NMR, EPR, UV/Vis, and FT-IR spectroscopies and mass spectrometry. 2 was a remarkably reactive Fe^III reagent for oxidative C-H activation, demonstrating reaction rates for hydrocarbon HAT comparable to the most reactive Fe^III and Fe^IV oxidants.

Keywords: biomimetic chemistry; fluoride oxidant; high-valent oxidants; nonheme iron; proton-coupled electron transfer.

Figure 1

ORTEP of the tricationic species 1. ClO₄ ⁻ anions and H atoms have been omitted for clarity. Ellipsoids are drawn at 50 % probability level.

Scheme 1

Synthesis of compounds 1 and 2, anions omitted for clarity.

Figure 2

UV/Vis spectra for the addition of Sc^III(OTf)₃ to a solution of 1 (black trace, 0.1 mM, CH₃CN, 0 °C) to yield 2 (blue trace). Inset: Time trace of absorbance change at λ=375 nm during the sequential titration of Sc^III(OTf)₃ (1 equiv. per titer). *=the point when each titer of Sc^III(OTf)₃ was added.

Figure 3

X‐band EPR spectrum of 2 (black trace) obtained from the reaction of 1 (20 mM) and Sc^III(OTf)₃ in CH₃CN. Measured at 77 K, 9.2 GHz microwave frequency, 1.99 mW microwave power, and 0.3 mT modulation amplitude. Simulated spectrum for 2 (red trace; g_x=2.70, g_y =2.40, g_z =1.53).

Figure 4

Electronic absorption spectra changes upon addition of CHD (350 equiv.) to 2 (0.1 mM, CH₃CN, at 0 °C). The inset shows the time profile of the feature at λ=400 nm.

Figure 5

Plot of ΔG ^≠ vs. BDE_C−H. ΔG ^≠ was calculated using the Eyring equation from the corresponding k ₂ values. We have plotted the BDE_C−H instead of bond dissociation free energy (BDFE_C−H) as the latter values for all the substrates in CH₃CN are not available.