A Nonheme, High-Spin {FeNO}⁸ Complex that Spontaneously Generates N₂O

One-electron reduction of [Fe­(NO)-(N3PyS)]­BF₄ (<b>1</b>) leads to the production of the metastable nonheme {FeNO}⁸ complex, [Fe­(NO)­(N3PyS)] (<b>3</b>). Complex <b>3</b> is a rare example of a high-spin (<i>S</i> = 1) {FeNO}⁸ and is the first example, to our knowledge, of a mononuclear nonheme {FeNO}⁸ species that generates N₂O. A second, novel route to <b>3</b> involves addition of Piloty’s acid, an HNO donor, to an Fe^II precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases

Photoinitiated Reactivity of a Thiolate-Ligated, Spin-Crossover Nonheme {FeNO}⁷ Complex with Dioxygen

The nonheme iron complex, [Fe­(NO)­(N3PyS)]­BF₄, is a rare example of an {FeNO}⁷ species that exhibits spin-crossover behavior. The comparison of X-ray crystallographic studies at low and high temperatures and variable-temperature magnetic susceptibility measurements show that a low-spin <i>S</i> = 1/2 ground state is populated at 0–150 K, while both low-spin <i>S</i> = 1/2 and high-spin <i>S</i> = 3/2 states are populated at <i>T</i> > 150 K. These results explain the observation of two N–O vibrational modes at 1737 and 1649 cm^–1 in CD₃CN for [Fe­(NO)­(N3PyS)]­BF₄ at room temperature. This {FeNO}⁷ complex reacts with dioxygen upon photoirradiation with visible light in acetonitrile to generate a thiolate-ligated, nonheme iron­(III)-nitro complex, [Fe^III(NO₂)­(N3PyS)]⁺, which was characterized by EPR, FTIR, UV–vis, and CSI-MS. Isotope labeling studies, coupled with FTIR and CSI-MS, show that one O atom from O₂ is incorporated in the Fe^III–NO₂ product. The O₂ reactivity of [Fe­(NO)­(N3PyS)]­BF₄ in methanol is dramatically different from CH₃CN, leading exclusively to sulfur-based oxidation, as opposed to NO· oxidation. A mechanism is proposed for the NO· oxidation reaction that involves formation of both Fe^III-superoxo and Fe^III-peroxynitrite intermediates and takes into account the experimental observations. The stability of the Fe^III-nitrite complex is limited, and decay of [Fe^III(NO₂)­(N3PyS)]⁺ leads to {FeNO}⁷ species and sulfur oxygenated products. This work demonstrates that a single mononuclear, thiolate-ligated nonheme {FeNO}⁷ complex can exhibit reactivity related to both nitric oxide dioxygenase (NOD) and nitrite reductase (NiR) activity. The presence of the thiolate donor is critical to both pathways, and mechanistic insights into these biologically relevant processes are presented

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 (<b>1-O</b>, 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 <b>1-O</b> indicates an intermediate-spin (<i>S</i> = 1) ground state. The Mössbauer parameters for <b>1-O</b> 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 (<b>L2</b>, Ar₂ = −2,6-difluoro-4-methoxyphenyl) and N4Py^2Ar₃ (<b>L3</b>, Ar₃ = −2,6-difluoro-3-methoxyphenyl), with −OMe substituents in the <i>meta</i> or <i>ortho</i>/<i>para</i> positions with respect to the C–F bonds. Fe^II complexes [Fe­(N4Py^2Ar₂)­(CH₃CN)]­(ClO₄)₂ (<b>2</b>) and [Fe­(N4Py^2Ar₃)­(CH₃CN)]­(ClO₄)₂ (<b>3</b>) reacted with isopropyl 2-iodoxybenzoate to give the C–F hydroxylated Fe^III–OAr products. The Fe^IV(O) intermediates <b>2-O</b> and <b>3-O</b> were trapped at low temperature and characterized. Complex <b>2-O</b> displayed a C–F hydroxylation rate similar to that of <b>1-O</b>. In contrast, the kinetics (via stopped-flow UV–vis) for complex <b>3-O</b> 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 (<b>4</b>) was prepared independently to investigate the possibility of a nucleophilic aromatic substitution pathway, but the stability of <b>4</b> rules out this mechanism. Taken together the data fully support an electrophilic C–F hydroxylation mechanism