A nonheme peroxo-diiron(III) complex exhibiting both nucleophilic and electrophilic oxidation of organic substrates

Hydrogen evolution driven by heteroatoms of bidentate N-heterocyclic ligands in iron(II) complexes

While Pt is considered the best catalyst for the electrocatalytic hydrogen evolution reaction (HER), it is evident that non-noble metal alternatives must be explored. In this regard, it is well known that the binding sites for non-noble metals play a pivotal role in facilitating efficient catalysis. Herein, we studied Fe(II) complexes with bidentate 2-(2'-pyridyl)benzoxazole (LO), 2-(2'-pyridyl)benzthiazole (LS), 2-(2'-pyridyl)benzimidazole (LNH), and 2-2'-bipyridyl (Lpy) ligands - by adding trifluoroacetic acid (TFA) to their acetonitrile solution - in order to examine how their reactivity towards protons under reductive conditions could be impacted by the non-coordinating heteroatoms (S, O, N, or none). By applying this ligand series, we found that the reduction potentials relevant for HER correlate with ligand basicity in the presence of TFA. Moreover, DFT calculations underlined the importance of charge distribution in the ligand-based LUMO and LUMO+1 orbitals of the complexes, dependent on the heterocycle. Kinetic studies and controlled potential electrolysis - using TFA as a proton source - revealed HER activities for the complexes with LNH, LO, and LS of kobs = 0.03, 1.1, and 10.8 s-1 at overpotentials of 0.81, 0.76, and 0.79 V, respectively, and pointed towards a correlation between the kinetics of the reaction and the non-coordinating heteroatoms of the ligands. In particular, the activity was associated with the [Fe(LS/O/NH)2(S)2]2+ form (S = solvent or substrate molecule), and the rate-determining step involved the formation of [Fe(H-H)]+, during the weakening of Fe-H and CF3CO2-H bonds, according to the experimental and DFT results.

Effect of Substituted Pyridine Co-Ligands and (Diacetoxyiodo)benzene Oxidants on the Fe(III)-OIPh-Mediated Triphenylmethane Hydroxylation Reaction

Iodosilarene derivatives (PhIO, PhI(OAc)2) constitute an important class of oxygen atom transfer reagents in organic synthesis and are often used together with iron-based catalysts. Since the factors controlling the ability of iron centers to catalyze alkane hydroxylation are not yet fully understood, the aim of this report is to develop bioinspired non-heme iron catalysts in combination with PhI(OAc)2, which are suitable for performing C-H activation. Overall, this study provides insight into the iron-based ([FeII(PBI)3(CF3SO3)2] (1), where PBI = 2-(2-pyridyl)benzimidazole) catalytic and stoichiometric hydroxylation of triphenylmethane using PhI(OAc)2, highlighting the importance of reaction conditions including the effect of the co-ligands (para-substituted pyridines) and oxidants (para-substituted iodosylbenzene diacetates) on product yields and reaction kinetics. A number of mechanistic studies have been carried out on the mechanism of triphenylmethane hydroxylation, including C-H activation, supporting the reactive intermediate, and investigating the effects of equatorial co-ligands and coordinated oxidants. Strong evidence for the electrophilic nature of the reaction was observed based on competitive experiments, which included a Hammett correlation between the relative reaction rate (logkrel) and the σp (4R-Py and 4R'-PhI(OAc)2) parameters in both stoichiometric (ρ = +0.87 and +0.92) and catalytic (ρ = +0.97 and +0.77) reactions. The presence of [(PBI)2(4R-Py)FeIIIOIPh-4R']3+ intermediates, as well as the effect of co-ligands and coordinated oxidants, was supported by their spectral (UV-visible) and redox properties. It has been proven that the electrophilic nature of iron(III)-iodozilarene complexes is crucial in the oxidation reaction of triphenylmethane. The hydroxylation rates showed a linear correlation with the FeIII/FeII redox potentials (in the range of -350 mV and -524 mV), which suggests that the Lewis acidity and redox properties of the metal centers greatly influence the reactivity of the reactive intermediates.

Effect of Redox Potential on Diiron-Mediated Disproportionation of Hydrogen Peroxide

Heme and nonheme dimanganese catalases are widely distributed in living organisms to participate in antioxidant defenses that protect biological systems from oxidative stress. The key step in these processes is the disproportionation of H₂O₂ to O₂ and water, which can be interpreted via two different mechanisms, namely via the formation of high-valent oxoiron(IV) and peroxodimanganese(III) or diiron(III) intermediates. In order to better understand the mechanism of this important process, we have chosen such synthetic model compounds that can be used to map the nature of the catalytically active species and the factors influencing their activities. Our previously reported μ-1,2-peroxo-diiron(III)-containing biomimics are good candidates, as both proposed reactive intermediates (Fe^IVO and Fe^III₂(μ-O₂)) can be derived from them. Based on this, we have investigated and compared five heterobidentate-ligand-containing model systems including the previously reported and fully characterized [Fe^II(L^1-4)₃]²⁺ (L¹ = 2-(2'-pyridyl)-1H-benzimidazole, L² = 2-(2'-pyridyl)-N-methyl-benzimidazole, L³ = 2-(4-thiazolyl)-1H-benzimidazole and L⁴ = 2-(4'-methyl-2'-pyridyl)-1H-benzimidazole) and the novel [Fe^II(L⁵)₃]²⁺ (L⁵ = 2-(1H-1,2,4-triazol-3-yl)-pyridine) precursor complexes with their spectroscopically characterized μ-1,2-peroxo-diiron(III) intermediates. Based on the reaction kinetic measurements and previous computational studies, it can be said that the disproportionation reaction of H₂O₂ can be interpreted through the formation of an electrophilic oxoiron(IV) intermediate that can be derived from the homolysis of the O-O bond of the forming μ-1,2-peroxo-diiron(III) complexes. We also found that the disproportionation rate of the H₂O₂ shows a linear correlation with the Fe^III/Fe^II redox potential (in the range of 804 mV-1039 mV vs. SCE) of the catalysts controlled by the modification of the ligand environment. Furthermore, it is important to note that the two most active catalysts with L³ and L⁵ ligands have a high-spin electronic configuration.

Catalytic and Stoichiometric Baeyer-Villiger Oxidation Mediated by Nonheme Peroxo-Diiron(III), Acylperoxo, and Iodosylbenzene Iron(III) Intermediates

In this paper we describe a detailed mechanistic studies on the [Fe^II(PBO)₂(CF₃SO₃)₂] (1), [Fe^II(PBT)₂(CF₃SO₃)₂] (2), and [Fe^II(PBI)₃](CF₃SO₃)₂ (3)-catalyzed (PBO = 2-(2′-pyridyl)benzoxazole, PBT = 2-(2′-pyridyl)benzthiazole, PBI = 2-(2′-pyridyl)benzimidazole) Baeyer–Villiger oxidation of cycloketones by dioxygen with cooxidation of aldehydes and peroxycarboxylic acids, including the kinetics on the reactivity of (μ-1,2-peroxo)diiron(III), acylperoxo- and iodosylbenzene-iron(III) species as key intermediates.

Generation of a μ-1,2-hydroperoxo FeIIIFeIII and a μ-1,2-peroxo FeIVFeIII Complex

μ-1,2-Peroxo-diferric intermediates (P) of non-heme diiron enzymes are proposed to convert upon protonation either to high-valent active species or to activated P′ intermediates via hydroperoxo-diferric intermediates. Protonation of synthetic μ-1,2-peroxo model complexes occurred at the μ-oxo and not at the μ-1,2-peroxo bridge. Here we report a stable μ-1,2-peroxo complex {Fe^III(μ-O)(μ-1,2-O₂)Fe^III} using a dinucleating ligand and study its reactivity. The reversible oxidation and protonation of the μ-1,2-peroxo-diferric complex provide μ-1,2-peroxo Fe^IVFe^III and μ-1,2-hydroperoxo-diferric species, respectively. Neither the oxidation nor the protonation induces a strong electrophilic reactivity. Hence, the observed intramolecular C-H hydroxylation of preorganized methyl groups of the parent μ-1,2-peroxo-diferric complex should occur via conversion to a more electrophilic high-valent species. The thorough characterization of these species provides structure-spectroscopy correlations allowing insights into the formation and reactivities of hydroperoxo intermediates in diiron enzymes and their conversion to activated P′ or high-valent intermediates.

Iron coordination complexes can be used to gain insight on biologically relevant iron-oxygen compounds generated in iron metalloenzymes. Here, the authors characterise a μ-1,2-hydroperoxo Fe^IIIFe^III and a μ-1,2-peroxo Fe^IVFe^III, and study their reactivity in C-H activation.

Nonheme Diiron Oxygenase Mimic That Generates a Diferric-Peroxo Intermediate Capable of Catalytic Olefin Epoxidation and Alkane Hydroxylation Including Cyclohexane

Herein are described substrate oxidations with H2O2 catalyzed by [FeII(IndH)(CH3CN)3](ClO4)2 [IndH = 1,3-bis(2'-pyridylimino)isoindoline], involving a spectroscopically characterized (μ-oxo)(μ-1,2-peroxo)diiron(III) intermediate (2) that is capable of olefin epoxidation and alkane hydroxylation including cyclohexane. Species 2 also converts ketones to lactones with a decay rate dependent on [ketone], suggesting direct nucleophilic attack of the substrate carbonyl group by the peroxo species. In contrast, peroxo decay is unaffected by the addition of olefins or alkanes, but the label from H218O is incorporated into the the epoxide and alcohol products, implicating a high-valent iron-oxo oxidant that derives from O-O bond cleavage of the peroxo intermediate. These results demonstrate an ambiphilic diferric-peroxo intermediate that mimics the range of oxidative reactivities associated with O2-activating nonheme diiron enzymes.

Disproportionation of H2O2 Mediated by Diiron-Peroxo Complexes as Catalase Mimics

Heme iron and nonheme dimanganese catalases protect biological systems against oxidative damage caused by hydrogen peroxide. Rubrerythrins are ferritine-like nonheme diiron proteins, which are structurally and mechanistically distinct from the heme-type catalase but similar to a dimanganese KatB enzyme. In order to gain more insight into the mechanism of this curious enzyme reaction, non-heme structural and functional models were carried out by the use of mononuclear [Fe^II(L_1-4)(solvent)₃](ClO₄)₂ (1-4) (L₁ = 1,3-bis(2-pyridyl-imino)isoindoline, L₂ = 1,3-bis(4'-methyl-2-pyridyl-imino)isoindoline, L₃ = 1,3-bis(4'-Chloro-2-pyridyl-imino)isoindoline, L₄ = 1,3-bis(5'-chloro-2-pyridyl-imino)isoindoline) complexes as catalysts, where the possible reactive intermediates, diiron-perroxo [Fe^III₂(μ-O)(μ-1,2-O₂)(L₁-L₄)₂(Solv)₂]²⁺ (5-8) complexes are known and well-characterized. All the complexes displayed catalase-like activity, which provided clear evidence for the formation of diiron-peroxo species during the catalytic cycle. We also found that the fine-tuning of iron redox states is a critical issue, both the formation rate and the reactivity of the diiron-peroxo species showed linear correlation with the Fe^III/Fe^II redox potentials. Their stability and reactivity towards H₂O₂ was also investigated and based on kinetic and mechanistic studies a plausible mechanism, including a rate-determining hydrogen atom transfer between the H₂O₂ and diiron-peroxo species, was proposed. The present results provide one of the first examples of a nonheme diiron-peroxo complex, which shows a catalase-like reaction.