In Solution Identification of the Lysine-Cysteine Redox Switch with a NOS Bridge in Transaldolase by Sulfur K-Edge X-ray Absorption Spectroscopy.

A novel covalent post-translational modification (lysine-NOS-cysteine) was discovered in proteins, initially in the enzyme transaldolase of Neisseria gonorrhoeae (NgTAL) [Nature 2021, 593, 460-464], acting as a redox switch. The identification of this novel linkage in solution was unprecedented until now. We present detection of the NOS redox switch in solution using sulfur K-edge X-ray absorption spectroscopy (XAS). The oxidized NgTAL spectrum shows a distinct shoulder on the low-energy side of the rising edge, corresponding to a dipole-allowed transition from the sulfur 1s core to the unoccupied σ* orbital of the S-O group in the NOS bridge. This feature is absent in the XAS spectrum of reduced NgTAL, where Lys-NOS-Cys is absent. Our experimental and calculated XAS data support the presence of a NOS bridge in solution, thus potentially facilitating future studies on enzyme activity regulation mediated by the NOS redox switches, drug discovery, biocatalytic applications, and protein design.

Dioxygen Reduction and Bioinspired Oxidations by Non-heme Iron(II)-α-Hydroxy Acid Complexes.

ConspectusAerobic organisms involve dioxygen-activating iron enzymes to perform various metabolically relevant chemical transformations. Among these enzymes, mononuclear non-heme iron enzymes reductively activate dioxygen to catalyze diverse biological oxidations, including oxygenation of C-H and C═C bonds and C-C bond cleavage with amazing selectivity. Several non-heme enzymes utilize organic cofactors as electron sources for dioxygen reduction, leading to the generation of iron-oxygen intermediates that act as active oxidants in the catalytic cycle. These unique enzymatic reactions influence the design of small molecule synthetic compounds to emulate enzyme functions and to develop bioinspired catalysts for performing selective oxidation of organic substrates with dioxygen. Selective electron transfer during dioxygen reduction on iron centers of synthetic models by a sacrificial reductant requires appropriate design strategies. Taking lessons from the role of enzyme-cofactor complexes in the selective electron transfer process, our group utilized ternary iron(II)-α-hydroxy acid complexes supported by polydentate ligands for dioxygen reduction and bioinspired oxidations. This Account focuses on the role of coordinated sacrificial reductants in the selective electron transfer for dioxygen reduction by iron complexes and highlights the versatility of iron(II)-α-hydroxy acid complexes in affecting dioxygen-dependent oxidation/oxygenation reactions. The iron(II)-coordinated α-hydroxy acid anions undergo two-electron oxidative decarboxylation concomitant with the generation of reactive iron-oxygen oxidants. A nucleophilic iron(II)-hydroperoxo species was intercepted in the decarboxylation pathway. In the presence of a Lewis acid, the O-O bond of the nucleophilic oxidant is heterolytically cleaved to generate an electrophilic iron(IV)-oxo-hydroxo oxidant. Most importantly, the oxidants generated with or without Lewis acid can carry out cis-dihydroxylation of alkenes. Furthermore, the electrophilic iron-oxygen oxidant selectively hydroxylates strong C-H bonds. Another electrophilic iron(IV)-oxo oxidant, generated from the iron(II)-α-hydroxy acid complexes in the presence of a protic acid, carries out C-H bond halogenation by using a halide anion.Thus, different metal-oxygen intermediates could be generated from dioxygen using a single reductant, and the reactivity of the ternary complexes can be tuned using external additives (Lewis/protic acid). The catalytic potential of the iron(II)-α-hydroxy complexes in performing O2-dependent oxygenations has been demonstrated. Different factors that govern the reactivity of iron-oxygen oxidants from ternary iron(II) complexes are presented. The versatile reactivity of the oxidants provides useful insights into developing catalytic methods for the selective incorporation of oxidized functionalities under environmentally benign conditions using aerial oxygen as the terminal oxidant.

Cu4S Cluster in "0-Hole" and "1-Hole" States: Geometric and Electronic Structure Variations for the Active CuZ* Site of N2O Reductase.

The active site of nitrous oxide reductase (N2OR), a key enzyme in denitrification, features a unique µ4-sulfido-bridged tetranuclear Cu cluster (the so-called CuZ or CuZ* site). Details of the catalytic mechanism have remained under debate and, to date, synthetic model complexes of the CuZ*/CuZ sites are extremely rare due to the difficulty in building the unique {Cu4(µ4-S)} core structure. Herein, we report the synthesis and characterization of [Cu4(µ4-S)]n+ (n = 2, 2; n = 3, 3) clusters, supported by a macrocyclic {py2NHC4} ligand (py = pyridine, NHC = N-heterocyclic carbene), in both their 0-hole (2) and 1-hole (3) states, thus mimicking the two active states of the CuZ* site during enzymatic N2O reduction. Structural and electronic properties of these {Cu4(µ4-S)} clusters are elucidated by employing multiple methods, including X-ray diffraction (XRD), nuclear magnetic resonance (NMR), UV/vis, electron paramagnetic resonance (EPR), Cu/S K-edge X-ray emission spectroscopy (XES), and Cu K-edge X-ray absorption spectroscopy (XAS) in combination with time-dependent density functional theory (TD-DFT) calculations. A significant geometry change of the {Cu4(µ4-S)} core occurs upon oxidation from 2 (τ4(S) = 0.46, seesaw) to 3 (τ4(S) = 0.03, square planar), which has not been observed so far for the biological CuZ(*) site and is unprecedented for known model complexes. The single electron of the 1-hole species 3 is predominantly delocalized over two opposite Cu ions via the central S atom, mediated by a π/π superexchange pathway. Cu K-edge XAS and Cu/S K-edge XES corroborate a mixed Cu/S-based oxidation event in which the lowest unoccupied molecular orbital (LUMO) has a significant S-character. Furthermore, preliminary reactivity studies evidence a nucleophilic character of the central µ4-S in the fully reduced 0-hole state.

A Combined Spectroscopic and Computational Study on the Mechanism of Iron-Catalyzed Aminofunctionalization of Olefins Using Hydroxylamine Derived N-O Reagent as the "Amino" Source and "Oxidant".

Herein, we study the mechanism of iron-catalyzed direct synthesis of unprotected aminoethers from olefins by a hydroxyl amine derived reagent using a wide range of analytical and spectroscopic techniques (Mössbauer, Electron Paramagnetic Resonance, Ultra-Violet Visible Spectroscopy, X-ray Absorption, Nuclear Resonance Vibrational Spectroscopy, and resonance Raman) along with high-level quantum chemical calculations. The hydroxyl amine derived triflic acid salt acts as the "oxidant" as well as "amino" group donor. It activates the high-spin Fe(II) (St = 2) catalyst [Fe(acac)2(H2O)2] (1) to generate a high-spin (St = 5/2) intermediate (Int I), which decays to a second intermediate (Int II) with St = 2. The analysis of spectroscopic and computational data leads to the formulation of Int I as [Fe(III)(acac)2-N-acyloxy] (an alkyl-peroxo-Fe(III) analogue). Furthermore, Int II is formed by N-O bond homolysis. However, it does not generate a high-valent Fe(IV)(NH) species (a Fe(IV)(O) analogue), but instead a high-spin Fe(III) center which is strongly antiferromagnetically coupled (J = -524 cm-1) to an iminyl radical, [Fe(III)(acac)2-NH·], giving St = 2. Though Fe(NH) complexes as isoelectronic surrogates to Fe(O) functionalities are known, detection of a high-spin Fe(III)-N-acyloxy intermediate (Int I), which undergoes N-O bond cleavage to generate the active iron-nitrogen intermediate (Int II), is unprecedented. Relative to Fe(IV)(O) centers, Int II features a weak elongated Fe-N bond which, together with the unpaired electron density along the Fe-N bond vector, helps to rationalize its propensity for N-transfer reactions onto styrenyl olefins, resulting in the overall formation of aminoethers. This study thus demonstrates the potential of utilizing the iron-coordinated nitrogen-centered radicals as powerful reactive intermediates in catalysis.

Tuning the stereoelectronic factors of iron(II)-2-aminophenolate complexes for the reaction with dioxygen: oxygenolytic C-C bond cleavage vs. oxidation of complex.

Oxidative C-C bond cleavage of 2-aminophenols mediated by transition metals and dioxygen is a topic of great interest. While the oxygenolytic C-C bond cleavage reaction relies on the inherent redox non-innocent property of 2-aminophenols, the metal complexes of 2-aminophenolates often undergo 1e-/2e- oxidation events (metal or ligand oxidation), instead of the direct addition of O2 for subsequent C-C bond cleavage. In this work, we report the isolation, characterization and dioxygen reactivity of a series of ternary iron(ii)-2-aminophenolate complexes [(TpPh,Me)FeII(X)], where X = 2-amino-4-tert-butylphenolate (4-tBu-HAP) (1); X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-tBu-HAP) (2); X = 2-amino-4-nitrophenolate (4-NO2-HAP)(3); and X = 2-anilino-4,6-di-tert-butylphenolate (NH-Ph-4,6-di-tBu-HAP) (4) supported by a facial tridentate nitrogen donor ligand (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate). Another facial N3 ligand (TpPh2 = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate) has been used to isolate an iron(ii)-2-anilino-4,6-di-tert-butylphenolate complex (5) for comparison. Both [(TpPh,Me)FeII(4-tBu-HAP)] (1) and [(TpPh,Me)FeII(4,6-di-tBu-HAP)] (2) undergo regioselective oxidative aromatic ring fission reaction of the coordinated 2-aminophenols to the corresponding 2-picolinic acids in the reaction with dioxygen. In contrast, complex [(TpPh,Me)FeII(4-NO2-HAP)] (3) displays metal based oxidation to form an iron(iii)-2-amidophenolate complex. Complexes [(TpPh,Me)FeII(NH-Ph-4,6-di-tBu-HAP)] (4) and [(TpPh2)FeII(NH-Ph-4,6-di-tBu-HAP)] (5) react with dioxygen to undergo 2e- oxidation with the formation of the corresponding iron(iii)-2-iminobenzosemiquinonato radical species implicating the importance of the -NH2 group in directing the C-C bond cleavage reactivity of 2-aminophenols. The systematic study presented in this work unravels the effect of the electronic and structural properties of the redox non-innocent 2-aminophenolate ring and the supporting ligand on the C-C bond cleavage reactivity vs. the metal/ligand oxidation of the complexes. The study further reveals that proper modulation of the stereoelectronic factors enables us to design a well synchronised proton transfer (PT) and dioxygen binding events for complexes 1 and 2 that mimic the structure and function of the nonheme enzyme 2-aminophenol-1,6-dioxygenase (APD).

Hydroxylamine-Derived Reagent as a Dual Oxidant and Amino Group Donor for the Iron-Catalyzed Preparation of Unprotected Sulfinamides from Thiols.

An iron catalyzed reaction for the selective transformation of thiols (-SH) to sulfinamides (-SONH2 ) by a direct transfer of -O and free -NH2 groups has been developed. The reaction operates under mild conditions using a bench stable hydroxylamine derived reagent, exhibits broad functional group tolerance, is scalable and proceeds without the use of any precious metal catalyst or additional oxidant. This novel, practical reaction leads to the formation of two distinct new bonds (S=O and S-N) in a single step to chemoselectively form valuable, unprotected sulfinamide products. Preliminary mechanistic studies implicate the role of the alcoholic solvent as an oxygen atom donor.

Bioinspired Olefin cis-Dihydroxylation and Aliphatic C-H Bond Hydroxylation with Dioxygen Catalyzed by a Nonheme Iron Complex.

A mononuclear iron(II)-α-hydroxy acid complex [(TpPh,Me)FeII(benzilate)] (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate) of a facial tridentate ligand has been isolated and characterized to explore its catalytic efficiency for aerial oxidation of organic substrates. In the reaction between the iron(II)-benzilate complex and O2, the metal-coordinated benzilate is stoichiometrically converted to benzophenone with concomitant reduction of dioxygen on the iron center. Based on the results from interception experiments and labeling studies, different iron-oxygen oxidants are proposed to generate in situ in the reaction pathway depending upon the absence or presence of an external additive (such as protic acid or Lewis acid). The five-coordinate iron(II) complex catalytically cis-dihydroxylates olefins and oxygenates the C-H bonds of aliphatic substrates using O2 as the terminal oxidant. The iron(II) complex exhibits better catalytic activity in the presence of a Lewis acid.

Aliphatic C-H Bond Halogenation by Iron(II)-α-Keto Acid Complexes and O2: Functional Mimicking of Nonheme Iron Halogenases.

α-Ketoglutarate-dependent nonheme halogenases catalyze the halogenation of aliphatic C-H bonds in the biosynthesis pathway of many natural products. An iron(IV)-oxo-halo species has been established as the active oxidant in the halogenation reactions. With an objective to emulate the function of the nonheme halogenases, two iron(II)-α-keto acid complexes, [(phdpa)Fe(BF)Cl] (1) and [(1,4-tpbd)Fe2(BF)2Cl2] (2) (where phdpa = N,N-bis(2-pyridylmethyl)aniline, 1,4-tpbd = N,N, N',N'-tetrakis(2-pyridylmethyl)benzene-1,4-diamine, and BF = benzoylformate), have been prepared. The iron complexes are capable of carrying out the oxidative halogenation of aliphatic C-H bonds using O2 as the terminal oxidant. Although the complexes are not selective toward C-H bond halogenation, they are the only examples of nonheme iron(II)-α-keto acid complexes mimicking the activity of nonheme halogenases. The dinuclear complex (2) exhibits enhanced reactivity toward C-H bond halogenation/hydroxylation.

Electrocatalytic Reduction of CO2 to Formate by an Iron Schiff Base Complex.

The synthesis, structural characterization, and reactivity of an iron(III) chloride compound of 6,6'-di(3,5-di-tert-butyl-2-hydroxybenzene)-2,2'-bipyridine (Fe(tbudhbpy)Cl), under electrochemically reducing conditions is reported. In the presence of carbon dioxide (CO2) under anhydrous conditions in N,N-dimethylformamide (DMF), this complex mediates the 2e- reductive disproportionation of two equivalents of CO2 to carbon monoxide (CO) and carbonate (CO32-). Upon addition of phenol (PhOH) as a proton source under CO2 saturation, catalytic current is observed; product analysis from controlled potential electrolysis experiments shows the majority product is formate (68 ± 4% Faradaic efficiency), with H2 as a minor product (30 ± 10% Faradaic efficiency) and minimal CO (1.1 ± 0.3% Faradaic efficiency). On the basis of data obtained from cyclic voltammetry and infrared spectroelectrochemistry (IR-SEC), the release of CO from intermediate Fe carbonyl species is extremely slow and undergoes competitive degradation, limiting the activity and lifetime of this catalyst. Mechanistic studies also indicate the phenolate moieties coordinated to Fe are sensitive to protonation in the reduced state, suggesting the possibility of cooperative pendent proton interactions being involved in CO2 reduction.

Aliphatic C-C Bond Cleavage in α-Hydroxy Ketones by a Dioxygen-Derived Nucleophilic Iron-Oxygen Oxidant.

A nucleophilic iron-oxygen oxidant, formed in situ in the reaction between an iron(II)-benzilate complex and O2 , oxidatively cleaves the aliphatic C-C bonds of α-hydroxy ketones. In the cleavage reaction, α-hydroxy ketones without any α-C-H bond afford a 1:1 mixture of carboxylic acid and ketone. Isotope labeling studies established that one of the oxygen atoms from dioxygen is incorporated into the carboxylic acid product. Furthermore, the iron(II) complex cleaves an aliphatic C-C bond of 17-α-hydroxyprogesterone affording androstenedione and acetic acid. The O2 -dependent aliphatic C-C bond cleavage of α-hydroxy ketones containing no α-C-H bond bears similarity to the lyase activity of the heme enzyme, cytochrome P450 17A1 (CYP17A1).

Substrate-dependent aromatic ring fission of catechol and 2-aminophenol with O2 catalyzed by a nonheme iron complex of a tripodal N4 ligand.

The catalytic reactivity of an iron(ii) complex [(TPA)Fe(II)(CH3CN)2](2+) (1) (TPA = tris(2-pyridylmethyl)amine) towards oxygenative aromatic C-C bond cleavage of catechol and 2-aminophenol is presented. Complex 1 exhibits catalytic and regioselective C-C bond cleavage of 3,5-di-tert-butylcatechol (H2DBC) to form intradiol products, whereas it catalyzes extradiol-type C-C bond cleavage of 2-amino-4,6-di-tert-butylphenol (H2AP). The catalytic reactions are found to be pH-dependent and the complex exhibits maximum turnovers at pH 5 in acetonitrile-phthalate buffer. An iron(iii)-catecholate complex [(TPA)Fe(III)(DBC)](+) (2) is formed in the ring cleavage of catechol. In the extradiol-type cleavage of H2AP, an iron(iii)-2-iminobenzosemiquinonate complex [(TPA)Fe(III)(ISQ)](2+) (3) (ISQ = 4,6-di-tert-butyl-2-iminobenzosemiquinonate radical anion) is observed in the reaction pathway. This work shows the importance of the nature of 'redox non-innocent' substrates in governing the mode of ring fission reactivity.

Hydroxylation versus Halogenation of Aliphatic C-H Bonds by a Dioxygen-Derived Iron-Oxygen Oxidant: Functional Mimicking of Iron Halogenases.

An iron-oxygen intermediate species generated in situ in the reductive activation of dioxygen by an iron(II)-benzilate complex of a monoanionic facial N3 ligand, promoted the halogenation of aliphatic C-H bonds in the presence of a protic acid and a halide anion. An electrophilic iron(IV)-oxo oxidant with a coordinated halide is proposed as the active oxidant. The halogenation reaction with dioxygen and the iron complex mimics the activity of non-heme iron halogenases.

Oxygenation of Organoboronic Acids by a Nonheme Iron(II) Complex: Mimicking Boronic Acid Monooxygenase Activity.

Phenolic compounds are important intermediates in the bacterial biodegradation of aromatic compounds in the soil. An Arthrobacter sp. strain has been shown to exhibit boronic acid monooxygenase activity through the conversion of different substituted phenylboronic acids to the corresponding phenols using dioxygen. While a number of methods have been reported to cleave the C-B bonds of organoboronic acids, there is no report on biomimetic iron complex exhibiting this activity using dioxygen as the oxidant. In that direction, we have investigated the reactivity of a nucleophilic iron-oxygen oxidant, generated upon oxidative decarboxylation of an iron(II)-benzilate complex [(Tp(Ph2))Fe(II)(benzilate)] (Tp(Ph2) = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate), toward organoboronic acids. The oxidant converts different aryl/alkylboronic acids to the corresponding oxygenated products with the incorporation of one oxygen atom from dioxygen. This method represents an efficient protocol for the oxygenation of boronic acids with dioxygen as the terminal oxidant.

Olefin cis-Dihydroxylation and Aliphatic C-H Bond Oxygenation by a Dioxygen-Derived Electrophilic Iron-Oxygen Oxidant.

Many iron-containing enzymes involve metal-oxygen oxidants to carry out O2-dependent transformation reactions. However, the selective oxidation of C-H and C=C bonds by biomimetic complexes using O2 remains a major challenge in bioinspired catalysis. The reactivity of iron-oxygen oxidants generated from an Fe(II)-benzilate complex of a facial N3 ligand were thus investigated. The complex reacted with O2 to form a nucleophilic oxidant, whereas an electrophilic oxidant, intercepted by external substrates, was generated in the presence of a Lewis acid. Based on the mechanistic studies, a nucleophilic Fe(II)-hydroperoxo species is proposed to form from the benzilate complex, which undergoes heterolytic O-O bond cleavage in the presence of a Lewis acid to generate an Fe(IV)-oxo-hydroxo oxidant. The electrophilic iron-oxygen oxidant selectively oxidizes sulfides to sulfoxides, alkenes to cis-diols, and it hydroxylates the C-H bonds of alkanes, including that of cyclohexane.

Oxygenative aromatic ring cleavage of 2-aminophenol with dioxygen catalyzed by a nonheme iron complex: catalytic functional model of 2-aminophenol dioxygenases.

2-Aminophenol dioxygenases catalyze the oxidative ring cleavage of 2-aminophenol to 2-picolinic acid using O2 as the oxidant. Inspired by the reaction catalyzed by these nonheme iron enzymes, a biomimetic iron(III)-2-amidophenolate complex, [(tBu-L(Me))Fe(III)(4,6-di-tBu-AP)](ClO4) (1a) of a facial tridentate ligand (tBu-L(Me) = 1-[bis(6-methyl-pyridin-2-yl)-methyl]-3-tert-butyl-urea and 4,6-di-tBu-H2AP = 2-amino-4,6-di-tert-butylphenol) bearing a urea group have been isolated. The complex reacts with O2 to cleave the C-C bond of 4,6-di-tBu-AP regioselectively and catalytically to afford 4,6-di-tert-butyl-2-picolinic acid. An iron(II)-chloro complex [(tBu-L(Me))Fe(II)Cl2(MeOH)] (1) of the same ligand also cleaves the aromatic ring of 4,6-di-tBu-AP catalytically in the reaction with O2. To assess the effect of urea group on the ring cleavage reaction of 2-aminophenol, two iron complexes, [(BA-L(Me))2Fe(II)2Cl4] (2) and [(BA-L(Me))Fe(III)(4,6-di-tBu-AP)](ClO4) (2a), of a tridentate ligand devoid of urea group (BA-L(Me) = benzyl-[bis(6-methyl-pyridin-2-yl)-methyl]-amine) have been isolated and characterized. Although the iron complexes (1 and 1a) of the ligand with urea group display catalytic reaction, the iron complexes (2 and 2a) of the ligand without urea group do not exhibit catalytic aromatic ring fission reactivity. The results support the role of urea group in directing the catalytic reactivity exhibited by 1 and 1a.

Reactivity of an iron-oxygen oxidant generated upon oxidative decarboxylation of biomimetic iron(II) α-hydroxy acid complexes.

Three biomimetic iron(II) α-hydroxy acid complexes, [(Tp(Ph2))Fe(II)(mandelate)(H2O)] (1), [(Tp(Ph2))Fe(II)(benzilate)] (2), and [(Tp(Ph2))Fe(II)(HMP)] (3), together with two iron(II) α-methoxy acid complexes, [(Tp(Ph2))Fe(II)(MPA)] (4) and [(Tp(Ph2))Fe(II)(MMP)] (5) (where HMP = 2-hydroxy-2-methylpropanoate, MPA = 2-methoxy-2-phenylacetate, and MMP = 2-methoxy-2-methylpropanoate), of a facial tridentate ligand Tp(Ph2) [where Tp(Ph2) = hydrotris(3,5-diphenylpyrazole-1-yl)borate] were isolated and characterized to study the mechanism of dioxygen activation at the iron(II) centers. Single-crystal X-ray structural analyses of 1, 2, and 5 were performed to assess the binding mode of an α-hydroxy/methoxy acid anion to the iron(II) center. While the iron(II) α-methoxy acid complexes are unreactive toward dioxygen, the iron(II) α-hydroxy acid complexes undergo oxidative decarboxylation, implying the importance of the hydroxyl group in the activation of dioxygen. In the reaction with dioxygen, the iron(II) α-hydroxy acid complexes form iron(III) phenolate complexes of a modified ligand (Tp(Ph2)*), where the ortho position of one of the phenyl rings of Tp(Ph2) gets hydroxylated. The iron(II) mandelate complex (1), upon decarboxylation of mandelate, affords a mixture of benzaldehyde (67%), benzoic acid (20%), and benzyl alcohol (10%). On the other hand, complexes 2 and 3 react with dioxygen to form benzophenone and acetone, respectively. The intramolecular ligand hydroxylation gets inhibited in the presence of external intercepting agents. Reactions of 1 and 2 with dioxygen in the presence of an excess amount of alkenes result in the formation of the corresponding cis-diols in good yield. The incorporation of both oxygen atoms of dioxygen into the diol products is confirmed by (18)O-labeling studies. On the basis of reactivity and mechanistic studies, the generation of a nucleophilic iron-oxygen intermediate upon decarboxylation of the coordinated α-hydroxy acids is proposed as the active oxidant. The novel iron-oxygen intermediate oxidizes various substrates like sulfide, fluorene, toluene, ethylbenzene, and benzaldehyde. The oxidant oxidizes benzaldehyde to benzoic acid and also participates in the Cannizzaro reaction.

Catalytic and regiospecific extradiol cleavage of catechol by a biomimetic iron complex.

An iron(III)-catecholate complex of a facial tridentate ligand reacts with dioxygen in the presence of ammonium acetate-acetic acid buffer to cleave the aromatic C-C bond of 3,5-di-tert-butylcatechol regiospecifically resulting in the formation of an extradiol product with multiple turnovers.

Functional models of α-keto acid dependent nonheme iron oxygenases: synthesis and reactivity of biomimetic iron(II) benzoylformate complexes supported by a 2,9-dimethyl-1,10-phenanthroline ligand.

Two biomimetic iron(II) benzoylformate complexes, [LFe(II)(BF)(2)] (2) and [LFe(II)(NO(3))(BF)] (3) (L is 2,9-dimethyl-1,10-phenanthroline and BF is monoanionic benzoylformate), have been synthesized from an iron(II)-dichloro complex [LFe(II)Cl(2)] (1). All the iron(II) complexes have been structurally and spectroscopically characterized. The iron(II) center in 2 is coordinated by a bidentate NN ligand (2,9-dimethyl-1,10-phenanthroline) and two monoanionic benzoylformates to form a distorted octahedral coordination geometry. One of the benzoylformates binds to the iron in 2 via both carboxylate oxygens but the other one binds in a chelating bidentate fashion via one carboxylate oxygen and the keto oxygen. On the other hand, the iron(II) center in 3 is ligated by one NN ligand, one bidentate nitrate, and one monoanionic chelating benzoylformate. Both iron(II) benzoylformate complexes exhibit the facial NNO donor environment in their solid-state structures. Complexes 2 and 3 are stable in noncoordinating solvents under an inert atmosphere, but react with dioxygen under ambient conditions to undergo oxidative decarboxylation of benzoylformate to benzoate in high yields. Evidence for the formation of an iron(IV)-oxo intermediate upon oxidative decarboxylation of benzoylformate was obtained by interception and labeling experiments. The iron(II) benzoylformate complexes represent the functional models of α-keto acid dependent oxygenases.