Unsymmetrical Bimetallic Complexes with M^II–(μ-OH)–M^III Cores (M^IIM^III = Fe^IIFe^III, Mn^IIFe^III, Mn^IIMn^III): Structural, Magnetic, and Redox Properties

Heterobimetallic cores are important units within the active sites of metalloproteins but are often difficult to duplicate in synthetic systems. We have developed a synthetic approach for the preparation of a complex with a Mn^II–(μ-OH)–Fe^III core, in which the metal centers have different coordination environments. Structural and physical data support the assignment of this complex as a heterobimetallic system. A comparison with analogous homobimetallic complexes, Mn^II–(μ-OH)–Mn^III and Fe^II–(μ-OH)–Fe^III cores, further supports this assignment

Unsymmetrical Bimetallic Complexes with M^II–(μ-OH)–M^III Cores (M^IIM^III = Fe^IIFe^III, Mn^IIFe^III, Mn^IIMn^III): Structural, Magnetic, and Redox Properties

Heterobimetallic cores are important units within the active sites of metalloproteins but are often difficult to duplicate in synthetic systems. We have developed a synthetic approach for the preparation of a complex with a Mn^II–(μ-OH)–Fe^III core, in which the metal centers have different coordination environments. Structural and physical data support the assignment of this complex as a heterobimetallic system. A comparison with analogous homobimetallic complexes, Mn^II–(μ-OH)–Mn^III and Fe^II–(μ-OH)–Fe^III cores, further supports this assignment

Unsymmetrical Bimetallic Complexes with M^II–(μ-OH)–M^III Cores (M^IIM^III = Fe^IIFe^III, Mn^IIFe^III, Mn^IIMn^III): Structural, Magnetic, and Redox Properties

Heterobimetallic cores are important units within the active sites of metalloproteins but are often difficult to duplicate in synthetic systems. We have developed a synthetic approach for the preparation of a complex with a Mn^II–(μ-OH)–Fe^III core, in which the metal centers have different coordination environments. Structural and physical data support the assignment of this complex as a heterobimetallic system. A comparison with analogous homobimetallic complexes, Mn^II–(μ-OH)–Mn^III and Fe^II–(μ-OH)–Fe^III cores, further supports this assignment

Unsymmetrical Bimetallic Complexes with M^II–(μ-OH)–M^III Cores (M^IIM^III = Fe^IIFe^III, Mn^IIFe^III, Mn^IIMn^III): Structural, Magnetic, and Redox Properties

Heterobimetallic cores are important units within the active sites of metalloproteins but are often difficult to duplicate in synthetic systems. We have developed a synthetic approach for the preparation of a complex with a Mn^II–(μ-OH)–Fe^III core, in which the metal centers have different coordination environments. Structural and physical data support the assignment of this complex as a heterobimetallic system. A comparison with analogous homobimetallic complexes, Mn^II–(μ-OH)–Mn^III and Fe^II–(μ-OH)–Fe^III cores, further supports this assignment

NaClO-Generated Iron(IV)oxo and Iron(V)oxo TAMLs in Pure Water

The unique properties of entirely aliphatic TAML activator [Fe^III{(Me₂C<i>N</i>COCMe₂<i>N</i>CO)₂CMe₂}­OH₂]⁻ (<b>3</b>), namely the increased steric bulk of the ligand and the unmatched resistance to the acid-induced demetalation, enables the generation of high-valent iron derivatives in pure water at any pH. An iron­(V)­oxo species is readily produced with NaClO at pH values from 2 to 10.6 without any observable intermediate. This is the first reported example of iron­(V)­oxo formed in pure water. At pH 13, iron­(V)­oxo is not formed and NaClO oxidizes <b>3</b> to an iron­(IV)­oxo derivative

Models for Unsymmetrical Active Sites in Metalloproteins: Structural, Redox, and Magnetic Properties of Bimetallic Complexes with M^II-(μ-OH)-Fe^III Cores

Bimetallic complexes are important sites in metalloproteins but are often difficult to prepare synthetically. We have previously introduced an approach to form discrete bimetallic complexes with M^II-(μ-OH)-Fe^III (M^II = Mn, Fe) cores using the tripodal ligand <i>N</i>,<i>N</i>′,<i>N</i>″-[2,2′,2″-nitrilotris­(ethane-2,1-diyl)]­tris­(2,4,6-trimethylbenzenesulfonamido) ([MST]^3–). This series is extended to include the rest of the late 3d transition metal ions (M^II = Co, Ni, Cu, Zn). All of the bimetallic complexes have similar spectroscopic and structural properties that reflect little change despite varying the M^II centers. Magnetic studies performed on the complexes in solution using electron paramagnetic resonance spectroscopy showed that the observed spin states varied incrementally from <i>S</i> = 0 through <i>S</i> = 5/2; these results are consistent with antiferromagnetic coupling between the high-spin M^II and Fe^III centers. However, the difference in the M^II ion occupancy yielded only slight changes in the magnetic exchange coupling strength, and all complexes had <i>J</i> values ranging from +26(4) to +35(3) cm^–1

Activation of Dioxygen by a TAML Activator in Reverse Micelles: Characterization of an Fe^IIIFe^IV Dimer and Associated Catalytic Chemistry

Iron TAML activators of peroxides are functional catalase-peroxidase mimics. Switching from hydrogen peroxide (H₂O₂) to dioxygen (O₂) as the primary oxidant was achieved by using a system of reverse micelles of Aerosol OT (AOT) in <i>n</i>-octane. Hydrophilic TAML activators are localized in the aqueous microreactors of reverse micelles where water is present in much lower abundance than in bulk water. <i>n</i>-Octane serves as a proximate reservoir supplying O₂ to result in partial oxidation of Fe^III to Fe^IV-containing species, mostly the Fe^IIIFe^IV (major) and Fe^IVFe^IV (minor) dimers which coexist with the Fe^III TAML monomeric species. The speciation depends on the pH and the degree of hydration <i>w</i>₀, viz., the amount of water in the reverse micelles. The previously unknown Fe^IIIFe^IV dimer has been characterized by UV–vis, EPR, and Mössbauer spectroscopies. Reactive electron donors such as NADH, pinacyanol chloride, and hydroquinone undergo the TAML-catalyzed oxidation by O₂. The oxidation of NADH, studied in most detail, is much faster at the lowest degree of hydration <i>w</i>₀ (in “drier micelles”) and is accelerated by light through NADH photochemistry. Dyes that are more resistant to oxidation than pinacyanol chloride (Orange II, Safranine O) are not oxidized in the reverse micellar media. Despite the limitation of low reactivity, the new systems highlight an encouraging step in replacing TAML peroxidase-like chemistry with more attractive dioxygen-activation chemistry

A “Beheaded” TAML Activator: A Compromised Catalyst that Emphasizes the Linearity between Catalytic Activity and p<i>K</i>_a

Studies of the new tetra-amido macrocyclic ligand (TAML) activator [Fe^III{(Me₂C<i>N</i>COCMe₂<i>N</i>CO)₂CMe₂}­OH₂]⁻ (<b>4</b>) in water in the pH range of 2–13 suggest its pseudo-octahedral geometry with two nonequivalent axial H₂O ligands and revealed (i) the anticipated basic drift of the first p<i>K</i>_a of water to 11.38 due to four electron-donating methyl groups alongside (ii) its counterintuitive enhanced resistance to acid-induced iron­(III) ejection from the macrocycle. The catalytic activity of <b>4</b> in the oxidation of Orange II (S) by H₂O₂ in the pH range of 7–12 is significantly lower than that of previously reported TAML activators, though it follows the common rate law (<i>v</i>/[Fe^III] = <i>k</i>_I<i>k</i>_II[H₂O₂]­[S]/(<i>k</i>_I[H₂O₂] + <i>k</i>_II[S]) and typical pH profiles for <i>k</i>_I and <i>k</i>_II. At pH 7 and 25 °C the rate constants <i>k</i>_I and <i>k</i>_II equal 0.63 ± 0.02 and 1.19 ± 0.03 M^–1 s^–1, respectively. With these new values for p<i>K</i>_a, <i>k</i>_I and <i>k</i>_II establishing new high and low limits, respectively, the rate constants <i>k</i>_I and <i>k</i>_II were correlated with p<i>K</i>_a values of all TAML activators. The relations log <i>k</i> = log <i>k</i>⁰ + α × p<i>K</i>_a were established with log <i>k</i>⁰ = 13 ± 2 and 20 ± 4 and α = −1.1 ± 0.2 and −1.8 ± 0.4 for <i>k</i>_I and <i>k</i>_II, respectively. Thus, the reactivity of TAML activators across four generations of catalysts is predictable through their p<i>K</i>_a values

Structure and Spectroscopy of Alkene-Cleaving Dioxygenases Containing an Atypically Coordinated Non-Heme Iron Center

Carotenoid cleavage oxygenases (CCOs) are non-heme iron enzymes that catalyze scission of alkene groups in carotenoids and stilbenoids to form biologically important products. CCOs possess a rare four-His iron center whose resting-state structure and interaction with substrates are incompletely understood. Here, we address this knowledge gap through a comprehensive structural and spectroscopic study of three phyletically diverse CCOs. The crystal structure of a fungal stilbenoid-cleaving CCO, CAO1, reveals strong similarity between its iron center and those of carotenoid-cleaving CCOs, but with a markedly different substrate-binding cleft. These enzymes all possess a five-coordinate high-spin Fe­(II) center with resting-state Fe–His bond lengths of ∼2.15 Å. This ligand set generates an iron environment more electropositive than those of other non-heme iron dioxygenases as observed by Mössbauer isomer shifts. Dioxygen (O₂) does not coordinate iron in the absence of substrate. Substrates bind away (∼4.7 Å) from the iron and have little impact on its electronic structure, thus excluding coordination-triggered O₂ binding. However, substrate binding does perturb the spectral properties of CCO Fe–NO derivatives, indicating proximate organic substrate and O₂-binding sites, which might influence Fe–O₂ interactions. Together, these data provide a robust description of the CCO iron center and its interactions with substrates and substrate mimetics that illuminates commonalities as well as subtle and profound structural differences within the CCO family

Reactivity of an Fe^IV-Oxo Complex with Protons and Oxidants

High-valent Fe-OH species are often invoked as key intermediates but have only been observed in Compound II of cytochrome P450s. To further address the properties of non-heme Fe^IV-OH complexes, we demonstrate the reversible protonation of a synthetic Fe^IV-oxo species containing a tris-urea tripodal ligand. The same protonated Fe^IV-oxo species can be prepared via oxidation, suggesting that a putative Fe^V-oxo species was initially generated. Computational, Mössbauer, XAS, and NRVS studies indicate that protonation of the Fe^IV-oxo complex most likely occurs on the tripodal ligand, which undergoes a structural change that results in the formation of a new intramolecular H-bond with the oxido ligand that aids in stabilizing the protonated adduct. We suggest that similar protonated high-valent Fe-oxo species may occur in the active sites of proteins. This finding further argues for caution when assigning unverified high-valent Fe-OH species to mechanisms