One-electron reduction of [Fe(NO)-(N3PyS)]BF4 (1) leads to the production of the metastable nonheme {FeNO}8 complex, [Fe(NO)(N3PyS)] (3). Complex 3 is a rare example of a high-spin (S = 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 3 involves addition of Piloty’s acid, an HNO donor, to an FeII precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases.

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, [FeIV(O)-(N4Py2Ar1)](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 FeIV(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, N4Py2Ar2 (L2, Ar2 = −2,6-difluoro-4-methoxyphenyl) and N4Py2Ar3 (L3, Ar3 = −2,6-difluoro-3-methoxyphenyl), with –OMe substituents in the meta or ortho/para positions with respect to the C–F bonds. FeII complexes [Fe(N4Py2Ar2)(CH3CN)]-(ClO4)2 (2) and [Fe(N4Py2Ar3)(CH3CN)](ClO4)2 (3) reacted with isopropyl 2-iodoxybenzoate to give the C–F hydroxylated FeIII–OAr products. The FeIV(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 FeII(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.

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 S = 1/2 ground state is populated at 0–150 K, while both low-spin S = 1/2 and high-spin S = 3/2 states are populated at T > 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, [FeIII(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 FeIII–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 FeIII-superoxo and FeIII-peroxynitrite intermediates and takes into account the experimental observations. The stability of the FeIII-nitrite complex is limited, and decay of [FeIII(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.

Reaction of the mononuclear nonheme complex [FeII(CH3CN)(N3PyS)]BF4 (1) with an HNO donor, Piloty’s acid (PhSO2NHOH, P.A.), at low temperature affords a high-spin (S = 2) FeII-P.A. intermediate (2), characterized by 57Fe Mössbauer and Fe K- edge X-ray absorption (XAS) spectroscopies, with interpretation of both supported by DFT calculations. The combined methods indicate that P.A. anion binds as the N-deprotonated tautomer (PhSO2NOH−) to [FeII(N3PyS)]+, leading to 2. Complex 2 is the first spectroscopically characterized example, to our knowledge, of P.A. anion bound to a redox-active metal center. Warming of 2 above −60 °C yields the stable {FeNO}7 complex [Fe(NO)(N3PyS)]BF4 (4), as evidenced by 1H NMR, ATR-IR, and Mössbauer spectroscopies. Isotope labeling experiments with 15N-labeled P.A. confirm that the nitrosyl ligand in 4 derives from P.A. In contrast, addition of a second equivalent of a strong base leads to S-N cleavage and production of an {FeNO}8 species, the deprotonated analog of an Fe-HNO complex. This work has implications for the targeted delivery of HNO/ NO−/NO‧ to nonheme Fe centers in biological and synthetic applications, and suggests a new role for nonheme FeII complexes in the assisted degradation of HNO donor molecules.

The synthesis of a new nonheme iron NO binding complex, [FeII(CH3CN)­(N3Py2PhSEtCN)]­(BF4)2 (1), is reported. Complex 1, which contains two sterically encumbering phenyl substituents, exhibits a high-spin (hs) FeII (S = 2) ground state in contrast to the S = 0 ground state for unsubstituted [FeII(CH3CN)­(N3PySEtCN)­(BF4)2. Reaction of 1 with NO(g) in CH3CN yields an {FeNO}7 (S = 3/2) complex 2, which slowly decays at 25 °C with loss of NO• to regenerate 1. One-electron reduction of 2 with Cr­(C6H6)2 at −40 °C yields the metastable, S = 1 {FeNO}8 complex 3. The nitrosyl moieties in thioether-ligated 2 and 3 are significantly less activated than in thiolate-ligated [Fe­(NO)­(N3PyS)]+/0, a structurally analogous pair of hs {FeNO}7/8 complexes. Calculations reveal that reduction of 2 is iron-centered, which may be a general property of hs {FeNO}7/8 complexes.