Biomimetic Iron Complex Research Articles

Electrocatalytic alcohol oxidation by a molecular iron complex.

An efficient electrochemical method for the selective oxidation of alcohols to their corresponding aldehydes/ketones using a biomimetic iron complex, [(bTAML)FeIII-OH2]-, as the redox mediator in an undivided electrochemical cell with inexpensive carbon and nickel electrodes using water as an oxygen source is reported. The substrate scope also includes alcohols that contain O and N heteroatoms in the scaffold, which are well tolerated under these reaction conditions. Mechanistic studies show the involvement of a high-valent FeV(O) species, [(bTAML)FeV(O)]-, formed via PCET (overall 2H+/2e-) from [(bTAML)FeIII-OH2]- at 0.77 V (vs. Fc+/Fc). Moreover, electrokinetic studies of the oxidation of C-H bonds indicate a second-order reaction, with the C-H abstraction by FeV(O) being the rate-determining step. The overall mechanism, studied using linear free energy relationships and radical clocks, indicates a "net hydride" transfer, leading to the oxidation of the alcohol to the corresponding aldehyde or ketone. When the reaction was carried out at pH > 11, the reaction could be carried out at a ∼500 mV lower potential than that at pH 8, albeit with reduced reaction rates. The reactive intermediate involved at pH > 11 is the corresponding one-electron oxidized [(bTAML)FeIV(O)]2- species.

Probing the Mechanism for 2,4'-Dihydroxyacetophenone Dioxygenase Using Biomimetic Iron Complexes.

In this study, we report the synthesis and characterization of [Fe(T1Et4iPrIP)(2-OH-AP)(OTf)](OTf) (2), [Fe(T1Et4iPrIP)(2-O-AP)](OTf) (3), and [Fe(T1Et4iPrIP)(DMF)3](OTf)3 (4) (T1Et4iPrIP = tris(1-ethyl-4-isopropyl-imidazolyl)phosphine; 2-OH-AP = 2-hydroxyacetophenone, and 2-O-AP- = monodeprotonated 2-hydroxyacetophenone). Both 2 and 3 serve as model complexes for the enzyme-substrate adduct for the nonheme enzyme 2,4'-dihydroacetophenone (DHAP) dioxygenase or DAD, while 4 serves as a model for the ferric form of DAD. Complexes 2-4 have been characterized by X-ray crystallography which reveals T1Et4iPrIP to bind iron in a tridentate fashion. Complex 2 additionally contains a bidentate 2-OH-AP ligand and a monodentate triflate ligand yielding distorted octahedral geometry, while 3 possesses a bidentate 2-O-AP- ligand and exhibits distorted trigonal bipyramidal geometry (τ = 0.56). Complex 4 displays distorted octahedral geometry with 3 DMF ligands completing the ligand set. The UV-vis spectrum of 2 matches more closely to the DAD-substrate spectrum than 3, and therefore, it is believed that the substrate for DAD is bound in the protonated form. TD-DFT studies indicate that visible absorption bands for 2 and 3 are due to MLCT bands. Complexes 2 and 3 are capable of oxidizing the coordinated substrate mimics in a stoichiometric and catalytic fashion in the presence of O2. Complex 4 does not convert 2-OH-AP to products under the same catalytic conditions; however, it becomes anaerobically reduced in the presence of 2 equiv 2-OH-AP to 2.

Computational Study on O–O Bond Formation on a Mononuclear Non‐Heme Iron Center

Cleavage and formation of the O–O bond in dioxygen is among the most important metal‐dependent processes in Nature. The former process has been mimicked structurally and functionaly with a number of biomimetic iron complexes. The reverse process, namely the dioxygen bond formation, however, appears unfavorable with iron. Over the years many attempts have been made to oxidize water on a mononuclear iron center through reactions with peroxide. In this work we present a computational study focused on the mechanism of dioxygen formation on two mononuclear non‐heme iron(IV)‐oxo species upon addition of peracid (m‐chloroperbenzoic acid, mCPBA). A range of test reactions and sequences are analyzed with thermochemical cycles with respect to the thermodynamic and kinetic feasibility of product formation. The work shows that iron‐borne O–O bond formation through peracid attack at FeIVO indeed is feasible through two competing pathways. These are initiated by homolytic inner‐sphere splitting of the RC(O)OO‐H and the RC(O)O‐OH bond in the peracid, respectively.

Exploring Hydrogen Evolution Accompanying Nitrogen Reduction on Biomimetic Nitrogenase Analogs: Can Fe-N xH yIntermediates Be Active Under Turnover Conditions?

Nitrogen reduction reaction (N2RR) carried out on biomimetic catalytic systems is considered to be a promising alternative for the traditional Haber-Bosch ammonia synthesis. Unfortunately, the selectivity of the currently known biomimetic catalysts is poor, as they also catalyze the unproductive hydrogen evolution reaction (HER). In the present computational study, we examine the HER activity of early N2RR intermediates in EP3 (E = B, Si) ligated single-site biomimetic iron complexes by calculating and comparing the activation Gibbs free energies of HER and N2RR elementary steps. We find that, in contrast to previous suggestions, early N2RR intermediates are not likely sources of HER under turnover conditions, as the barriers of the competing N2RR steps are significantly lower. Consequently, future research should focus on preventing other potential HER mechanisms, e.g., hydride formation, rather than accelerating the consumption of early N2RR intermediates as proposed earlier to design more efficient biomimetic catalysts.

Mechanistic insights into the biomimetic catalytic hydroxylation of arenes by a molecular Fe(NHC) complex

The mechanism of the hydroxylation of benzene by an iron(II) bis(NHC) complex, [FeII(NCCN)(CH3CN)2](PF6)2 (1; NCCN=bis(o-imidazol-2-ylidenepyridine)methane), in the presence of hydrogen peroxide was investigated. The invariance of the catalytic system towards the presence of radical scavengers and an inverse intermolecular kinetic isotope effect (KIE) allow to rule out a hydrogen abstraction pathway. Additionally, using 1,3,5-[D3]benzene as substrate, NIH-shift products were detected, resembling the reactivity of naturally occurring enzymes and some molecular biomimetic iron complexes. Supported by density functional theory calculations, the aromatic hydroxylation reaction is proposed to proceed via an iron-arene σ-complex as intermediate species.

Development of a Continuous-Flow Microreactor for Asymmetric Epoxidation of Electron-Deficient Olefins

Asymmetric epoxidation of electron-deficient olefins catalyzed by a biomimetic iron complex in a continuous-flow microreactor is described. The reaction conducted in the microreactor is characterized by rapid (<4 min) epoxidation of trisubstituted enones in high yields (up to 90%) and excellent enantioselectivities (up to 92% ee); compared to its batchwise counterpart, the microreactor reaction also allows shorter reaction times, safer operation, and more precise control over the reaction conditions.

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.

Enhanced Reactivity of a Biomimetic Iron(II) α‐Keto Acid Complex through Immobilization on Functionalized Gold Nanoparticles

Immobilized biomimetic iron complex: An iron(II) benzoylformate complex of a thiol-appended N4 ligand immobilized on gold nanoparticles activates dioxygen to carry out oxidative decarboxylation of benzoylformic acid to benzoic acid catalytically.

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.

An Iron(III) Iodosylbenzene Complex: A Masked Non‐Heme FeVO

Coordinative flexibility and ligand bifunctionality play important roles in a biomimetic iron complex of a hexadentate ligand (see picture). This system can bind an auxiliary iodosylbenzene ligand to give the first structurally characterized metal complex of this important oxidizing agent. The complex is a protected precursor for FeVO species, and the selective high-yielding catalytic oxygenation of a sulfide is demonstrated.

DFT Study on the Catalytic Reactivity of a Functional Model Complex for Intradiol-Cleaving Dioxygenases

The enzymatic ring cleavage of catechol derivatives is catalyzed by two groups of dioxygenases: extradiol- and intradiol-cleaving dioxygenases. Although having different oxidation state of their nonheme iron sites and different ligand coordinations, both groups of enzymes involve a common peroxy intermediate in their catalytic cycles. The factors that lead to either extradiol cleavage resulting in 2-hydroxymuconaldehyde or intradiol cleavage resulting in muconic acid are not fully understood. Well-characterized model compounds that mimic the functionality of these enzymes offer a basis for direct comparison to theoretical results. In this study the mechanism of a biomimetic iron complex is investigated with density functional theory (DFT). This complex catalyzes the ring opening of catecholate with exclusive formation of the intradiol cleaved product. Several spin states are possible for the transition metal system, with the quartet state found to be of main importance during the reaction course. The mechanism investigated provides an explanation for the observed selectivity of the complex. First, a bridging peroxide is formed, which decomposes to an alkoxy radical by O-O homolysis. In contrast to the subsequent barrier-free intradiol C-C bond cleavage, the extradiol pathway proceeds via the formation of an epoxide, which requires an additional activation barrier.

Utilization of immobilized biomimetic iron complexes within nanoreactors of Al-MCM-41 as cyclohexane oxidation catalyst

Biomimetic iron(III) complexes such as [Fe(en) 2Cl 2]Cl, [Fe(bpy) 2Cl 2]Cl, [Fe(salen)Cl], [Fe(TPP)Cl] and [Fe(TMC)Cl] in which en, bpy, salen, TMC, and TPP refer to ethylenediamine, 2,2′-bipyridine, N,N′-bis(salicylidene)ethylenediamine), 5,7,12,14-tetramethyl-1,4,8,11 tetraazacyclotetradeca 4,6,11,13-tetraene and meso-tetraphenylporphyrin were immobilized within nanoreactors of Al-MCM-41. The structures of immobilized complexes were characterized by powder X-ray diffraction (XRD), nitrogen adsorption–desorption, FTIR and UV–vis spectroscopy. The pore volume, surface area and pore diameter of Al-MCM-41 were found to decrease after immobilization of iron complexes. It was found that immobilized iron complexes within Al-MCM-41 with the order of [Fe(bpy) 2Cl 2] > [Fe(TPP)Cl] > [Fe(TMC)Cl] ∼ [Fe(salen)Cl] > [Fe(en) 2Cl 2]Cl catalyze the oxidation of cyclohexane to cyclohexanol and cyclohexanone with hydrogen peroxide in refluxing acetonitrile.

Bio-inspired arenecis-dihydroxylation by a non-haem iron catalyst modeling the action of naphthalene dioxygenase

Reported in this paper is the first example of a biomimetic iron complex, ([Fe(II)(TPA)(NCMe)(2)](2+) (TPA = tris(2-pyridylmethyl)amine), that catalyses the cis-dihydroxylation of an aromatic double bond, mimicking the action of the non-haem iron enzyme naphthalene dioxygenase and shedding light on its possible mechanism of action.

Quantum Chemical Modeling of the Oxidation of Dihydroanthracene by the Biomimetic Nonheme Iron Catalyst [(TMC)FeIV(O)]2+

Hybrid density functional theory has been employed to model the oxidation of dihydroanthracene (DHA = C14H12) to anthracene (C14H10) by the biomimetic iron complex [(TMC)FeIV(O)]2+ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). Experimentally, the reaction has been studied in a solution of the reactants and the counterion trifluoroacetate (CF3C ) in the solvent acetonitrile (CH3CN). Depending on the concentration of trifluoroacetate, different coordination situations have been observed by NMR spectroscopy. The complexity of the chemical environment offers a challenging modeling problem, and five different models were initially considered. The effects of the coordination of either a counterion or a solvent molecule were found to be rather small. The reaction was found to be a two-step process in which the first step is rate-limiting. The free energy of activation (ΔG⧧) for the first H-abstraction was found to be between 14.5 and 16.9 kcal mol-1 depending on the model, in reasonable agreement with experimental data. The second step has a much lower free-energy barrier, found to be completely entropic in origin. In all models, the system is found to have a triplet ground state in the FeIV(O) reactant. A spin-crossing of the triplet and quintet potential energy surfaces occurs before the first transition state, and the system is found to end up in the FeII quintet state, releasing a water molecule and the anthracene product. Because of the formation of the aromatic anthracene molecule, the reaction is very exothermic.

Immobilized Complexes of Metals with Amino Acid Ligands − A First Step toward the Development of New Biomimetic Catalysts

The natural amino acids histidine and glutamic acid were immobilized on the surface of functionalized mesoporous silica supports. Histidine and glutamic acid alone and combinations of both were covalently bound to the amino groups on the surface of the support by the formation of amide bonds. The immobilized amino acids were used as ligands for Fe(II). The obtained biomimetic iron complexes were tested as catalysts for cyclohexane oxidation at mild reaction conditions with hydrogenperoxide as oxidant. This stepwise preparation method allowed the synthesis of materials with varying nature and loading of immobilized amino acids. The observed trends in catalytic activity show that both the type and the surface density of immobilized amino acid have an influence on the catalytic performance during cyclohexane oxidation and also on the structure of the immobilized Fe complex.

Analysis of spin states, spin barriers, and trans-effects involved in the coordination and stabilization of dinitrogen by biomimetic iron complexes

Coordination of dinitrogen to Sellmann-type iron (II) complexes in a sulfur-dominated coordination sphere, which emulates the environment of iron centers in the FeMo-cofactor of nitrogenase, is analyzed with respect to spin states, spin barriers, and the effect of trans-ligands. Such detailed investigations became only recently feasible when the reliability of density functional methods, which are the only quantum chemical methods capable of describing large transition metal complexes, could significantly be improved for the calculation of energies for states of different spin. It is found that the actual binding energy of dinitrogen is of sufficient magnitude for a reasonably strong fixation of N2 by Sellmann-type coordination compounds. However, potential fixation is determined by additional factors which reduce the binding energy. One factor is the change in spin state of the N2-free metal fragment, which lowers the total energy and quenches the thermodynamic stabilization effect of the binding energy. In addition, the metal fragment rearranges and gains even more stabilization energy for the un-coordinated state. Apart from these thermodynamical effects, the existence of spin barriers, which must be overcome upon binding of dinitrogen, leads to kinetical effects, which cannot be neglected.