Direct Observation of a Nonheme Iron(IV)–Oxo Complex That Mediates Aromatic C–F Hydroxylation

Abstract

The synthesis of a pentadentate ligand with strategically designed fluorinated arene groups in the second coordination sphere of a nonheme iron center is reported. The oxidatively resistant fluorine substituents allow for the trapping and characterization of an FeIV(O) complex at −20 °C. Upon warming of the FeIV(O) complex, an unprecedented arene C–F hydroxylation reaction occurs. Computational studies support the finding that substrate orientation is a critical factor in the observed reactivity. This work not only gives rare direct evidence for the participation of an FeIV(O) species in arene hydroxylation but also provides the first example of a high-valent iron–oxo complex that mediates aromatic C–F hydroxylation.

Aromatic C–F Hydroxylation by Nonheme Iron(IV)–Oxo Complexes: Structural, Spectroscopic, and Mechanistic Investigations

“…The 4.2 K/53 mT Mössbauer spectrum of 1-O displays a quadrupole doublet with parameters ( δ = 0.03 mm/s and Δ E Q = 0.54 mm/s) identical to those previously reported. 33 While the isomer shift is typical of ferryl complexes, the absolute magnitude of the quadrupole splitting parameters is smaller than those observed for other ferryl complexes (Δ E Q ≈ 1.2 mm/s). 36 However, spectroscopic parameters calculated using density functional theory (DFT) methods on geometry-optimized structures of 1-O and the extensively characterized intermediate-spin ferryl complex with 1,4,8,11-tetramethylcyclam ligand [(TMC)Fe IV (O)(CH 3 CN)] 2+ are in good agreement with the experimentally observed values (Table S1).…”

“…33 Upon warming, this complex readily converted to the arene hydroxylated product 1-OAr as shown in Scheme 1, and time-dependent studies confirmed 1-O was the reactive intermediate in this rare C–F hydroxylation reaction. Given the novel reactivity of this Fe IV (O) complex, we were motivated to obtain the complete structural and electronic characterization of this complex by X-ray crystallography and variable-field Mössbauer spectroscopy.…”

“…An 57 Fe-enriched sample of 1-O was prepared from combining 57 Fe II (BF 4 ) 2 and free ligand in CH 3 CN, followed by addition of the oxidant at −20 °C, as previously described. 33 Analysis of the spectra (Figure 2, left panel, black vertical bars) reveals that ~60% of the total intensity of the spectrum is attributable to 1-O . The remaining ~40% of the total intensity emanates from one or more ferric complexes and can be reasonably well approximated by the experimental spectra of a duplicate sample that was allowed to decay for 40 min at room temperature (green lines).…”

“…The tethered aromatic ring was fluorinated at the site of oxidation. 33 Density functional theory (DFT) calculations suggested that this Fe IV (O) complex not only had a triplet ground state but also indicated that a quintet excited state may be close in energy. The presumed 6-coordinate geometry of this complex was consistent with a triplet state, although there were recent reports of quintet Fe IV (O) accommodated by a weak 6-coordinate ligand field.…”

“…15,33 In one case, we directly characterized the Fe IV (O) intermediate [Fe IV (O)(N4Py 2Ar 1 )] 2+ (Ar 1 = −2,6-difluorophenyl), and observed the intramolecular arene hydroxylation reaction for this system by UV–vis, Mössbauer, and cold-spray ionization mass spectrometry (CSIMS). This study provided only the second example of a well-characterized Fe IV (O) complex that could mediate arene hydroxylation.…”