Spectroscopic and Theoretical Study of Spin-Dependent Electron Transfer in an Iron(III) Superoxo Complex

Abstract

It was shown previously (J. Am. Chem. Soc. 2014, 136, 10846) that bubbling of O2 into a solution of Fe(II)(BDPP) (H2BDPP = 2,6-bis[[(S)-2-(diphenylhydroxymethyl)-1-pyrrolidinyl]methyl]pyridine) in tetrahydrofuran at -80 °C generates a high-spin (SFe = (5)/2) iron(III) superoxo adduct, 1. Mössbauer studies revealed that 1 is an exchange-coupled system, [Formula: see text], where SR = (1)/2 is the spin of the superoxo radical, of which the spectra were not well enough resolved to determine whether the coupling was ferromagnetic (S = 3 ground state) or antiferromagnetic (S = 2). The glass-forming 2-methyltetrahydrofuran solvent yields highly resolved Mössbauer spectra from which the following data have been extracted: (i) the ground state of 1 has S = 3 (J < 0); (ii) |J| > 15 cm(-1); (iii) the zero-field-splitting parameters are D = -1.1 cm(-1) and E/D = 0.02; (iv) the major component of the electric-field-gradient tensor is tilted ≈7° relative to the easy axis of magnetization determined by the MS = ±3 and ±2 doublets. The excited-state MS = ±2 doublet yields a narrow parallel-mode electron paramagnetic resonance signal at g = 8.03, which was used to probe the magnetic hyperfine splitting of (17)O-enriched O2. A theoretical model that considers spin-dependent electron transfer for the cases where the doubly occupied π* orbital of the superoxo ligand is either "in" or "out" of the plane defined by the bent Fe-OO moiety correctly predicts that 1 has an S = 3 ground state, in contrast to the density functional theory calculations for 1, which give a ground state with both the wrong spin and orbital configuration. This failure has been traced to a basis set superposition error in the interactions between the superoxo moiety and the adjacent five-membered rings of the BDPP ligand and signals a fundamental problem in the quantum chemistry of O2 activation.

Structure and Reactivity of a High-Spin, Nonheme Iron(III)- Superoxo Complex Supported by Phosphinimide Ligands

“…Similar total spin states can be envisioned for (L Ad H)FeO2, the relative energies of which dictate the observed Boltzmann distribution at elevated temperatures. The parallel mode X-band EPR spectrum of (L Ad H)FeO2 at 5 K contains an intense resonance at g = 9.32 (Fig 2C ), similar to that observed in other synthetic 35 and enzymatic Fe-O2 species. 4 This feature can be observed at temperatures up to 65 K and no other resonances emerge over this temperature range.…”

Dioxygen Activation by Nonheme Diiron Enzymes: Diverse Dioxygen Adducts, High-Valent Intermediates, and Related Model Complexes

“…The use of the glass forming 2-MeTHF solvent significantly improved the resolution of the high-field Mössbauer spectra and allowed the exchange coupling to be determined as ferromagnetic, that is, 35 has a S = 3 ground spin state. 243…”

Activation of Dioxygen by Iron and Manganese Complexes: A Heme and Nonheme Perspective

“…An analysis of the Mössbauer data suggested a ferromagnetic spin-coupling interaction between the Fe III and O 2 − ligand, which would result in an S = 3 ground state. 78 The reactivity of [Fe III (O 2 − )(BDDP)] toward H-atom abstraction was examined. 77 Treatment of the Fe III (O 2 − ) complex with excess 9,10-dihydroanthracene (bond dissociation energy, BDE C–H = 78 kcal/mol) at −70 °C resulted in the formation of anthracene in good yield, with a 1:1 reaction stoichiometry.…”