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

The synthesis and reactivity of a series of mononuclear nonheme iron complexes that carry out intramolecular aromatic C-F hydroxylation reactions is reported. The key intermediate prior to C-F hydroxylation, [Fe^IV(O)(N4Py^2Ar₁)](BF₄)₂ (1-O, Ar₁ = -2,6-difluorophenyl), was characterized by single-crystal X-ray diffraction. The crystal structure revealed a nonbonding C-H···O=Fe interaction with a CH₃CN molecule. Variable-field Mössbauer spectroscopy of 1-O indicates an intermediate-spin (S = 1) ground state. The Mössbauer parameters for 1-O include an unusually small quadrupole splitting for a triplet Fe^IV(O) and are reproduced well by density functional theory calculations. With the aim of investigating the initial step for C-F hydroxylation, two new ligands were synthesized, N4Py^2Ar₂(L2, Ar₂ = -2,6-difluoro-4-methoxyphenyl) and N4Py^2Ar₃(L3, Ar₃ = -2,6-difluoro-3-methoxyphenyl), with -OMe substituents in the meta or ortho/para positions with respect to the C-F bonds. Fe^II complexes [Fe(N4Py^2Ar₂)(CH₃CN)](ClO₄)₂ (2) and [Fe(N4Py^2Ar₃)(CH₃CN)](ClO₄)₂ (3) reacted with isopropyl 2-iodoxybenzoate to give the C-F hydroxylated Fe^III-OAr products. The Fe^IV(O) intermediates 2-O and 3-O were trapped at low temperature and characterized. Complex 2-O displayed a C-F hydroxylation rate similar to that of 1-O. In contrast, the kinetics (via stopped-flow UV-vis) for complex 3-O displayed a significant rate enhancement for C-F hydroxylation. Eyring analysis revealed the activation barriers for the C-F hydroxylation reaction for the three complexes, consistent with the observed difference in reactivity. A terminal Fe^II(OH) complex (4) was prepared independently to investigate the possibility of a nucleophilic aromatic substitution pathway, but the stability of 4 rules out this mechanism. Taken together the data fully support an electrophilic C-F hydroxylation mechanism.