A Reactive, Photogenerated High-Spin (S = 2) FeIV(O) Complex via O2 Activation

Generation and Reactivity of a High-Spin Iron(IV)-Oxo Complex That Is Stable at Ambient Temperatures

Nature operates a variety of challenging oxidation reactions through intermediates bearing tetravalent iron centers bound to a terminal oxo ligand. The high-spin (S = 2) electronic configuration is believed to be particularly important in C-H activation reactions mediated by iron(IV)-oxo species. Coordination environments that promote high-spin ground states obviate the need for spin-state crossing to access this state and can promote rapid oxidation reactivity. As a result, however, synthetic iron(IV)-oxo species with S = 2 ground states tend to exhibit poor thermal stabilities, which has hampered a broader elucidation of their reactivity profiles. In this work, we report the synthesis of a remarkably stable high-spin iron(IV)-oxo complex that localizes the Fe═O unit within a rigid organic macrocycle. This design results in essentially unlimited stability at ambient temperatures and a half-life of 21 h at 70 °C in CH3CN, endowing this compound with the highest thermal stability for a high-spin FeIV═O complex reported to date. The ligand's steric profile shuts down intermolecular reactivity with potential O atom acceptors and hydrocarbons bearing weak C-H bonds, but proton-coupled electron transfer reactivity with 2,4,6-tri-tert-butylphenol (TTBP) occurs readily at room temperature despite its steric bulk.

Bridging the Oxo Wall: A New Perspective on High-valent Metal-Oxo Species and Their Reactivity in Mn, Fe, and Co Complexes

The "oxo-wall" is a well-established concept in the area of bioinorganic chemistry, which refers to the instability of the terminal metal-oxo complexes in the +4 oxidation state, with tetragonal C4v symmetry beyond group 8 elements. This leads to a diverse and highly reactive chemistry of Co-oxo complexes, as evidenced in the literature, ranging from challenging C-H bond activation to efficient water oxidation. Despite extensive research on first-row terminal metal-oxo complexes and the "oxo-wall" concept, studies correlating the reactivity of these species across the periodic table remain scarce. In this work, using a combination of DFT and ab initio CASSCF calculations, we have explored the structure, bonding, and reactivity of [MIV/V(15-TMC)(O)(CH3CN)]m+ (M= Mn, Fe and Co) species. Our study reveals several intriguing outcomes: (i) while existing literature typically indicates the presence of either CoIV=O or CoIII-O• species beyond the wall, we propose a quantum mechanical mixture of these two species (termed as CoIV=O CoIII-O•), with the per cent of mixing dictated by ligand architecture and symmetry considerations; (ii) we observe that the oxyl radical character increases beyond the wall, correlating with larger Ntrans-M-O tilt angles; and (iii) we identify an inverse relationship between the percentage of M-O• character and the kinetic barriers for C-H bond activation. These findings offer a new perspective on the roles of oxidation states, spin states, and the nature of the metal ion in reactivity.

Sulfur Bridge Geometry Boosts Selective FeIV═O Generation for Efficient Fenton-Like Reactions

High-valent iron-oxo species (FeIV═O) is a fascinating enzymatic agent with excellent anti-interference abilities in various oxidation processes. However, selective and high-yield production of FeIV═O remains challenging. Herein, Fe diatomic pairs are rationally fabricated with an assisted S bridge to tune their neighbor distances and increase their loading to 11.8 wt.%. This geometry regulated the d-band center of Fe atoms, favoring their bonding with the terminal and hydroxyl O sites of peroxymonosulfate (PMS) via heterolytic cleavage of O─O, improving the PMS utilization (70%), and selective generation of FeIV═O (>90%) at a high yield (63% of PMS) offers competitive performance against state-of-the-art catalysts. These continuous reactions in a fabricated device and technol-economic assessment further verified the catalyst with impressive long-term activity and scale-up potential for sustainable water treatment. Altogether, this heteroatom-bridge strategy of diatomic pairs constitutes a promising platform for selective and efficient synthesis of high-valent metal-oxo species.

A 5,000-fold increase in the HAT reactivity of a nonheme FeIV=O complex simply by replacing two pyridines of the pentadentate N4Py ligand with pyrazoles

A pentadentate [N5] ligand (N2Py2Pz) based on the classic N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework has been synthesized by replacing the two pyridylmethyl arms with corresponding (N-methyl)pyrazolylmethyl units to form [N-bis(1-methyl-2-pyrazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L1). The oxidation of the iron(II) precursor (N2Py2Pz)FeII(OTf)2 (1) with (tBuSO2)C6H4IO at 298 K leads to the formation of the [FeIV(O)(N2Py2Pz)]2+ intermediate (2) with a near-IR band at 750 nm (εM = 250 M-1cm-1) and a t1/2 ~ 2 min at 298 K. The introduction of the less basic pyrazolylmethyl ligands in place of two pyridylmethyl units generates FeIV=O intermediate 2 that exhibits a cyclohexane oxidation rate of 0.29 s-1 at 298 K, which is 5,000-fold faster than that observed for the classic FeIV(O)N4Py parent complex and 40,000-fold more reactive than the least reactive FeIV(O)N2Py2Q' complex in this series (Py = pyridine, Q' = isoquinoline) recently reported by Nordlander.

10-Fold Increase in Hydrogen Atom Transfer Reactivity for a Series of S = 1 FeIV═O Complexes Over the S = 2 [(TQA)FeIV═O]2+ Complex via Entropic Lowering of Reaction Barriers by Secondary Sphere Cycloalkyl Substitution

Nonheme iron enzymes utilize S = 2 iron(IV)-oxo intermediates as oxidants in biological oxygenations. In contrast, corresponding synthetic nonheme FeIV═O complexes characterized to date favor the S = 1 ground state that generally shows much poorer oxidative reactivity than their S = 2 counterparts. However, one intriguing exception found by Nam a decade ago is the S = 1 [FeIV(O)(Me3NTB)]2+ complex (Me3NTB = [tris((N-methyl-benzimidazol-2-yl)methyl)amine], 1O) with a hydrogen atom transfer (HAT) reactivity that is 70% that of the S = 2 [FeIV(O)(TQA)]2+ complex (TQA = tris(2-quinolylmethyl)amine, 3O). In our efforts to further explore this direction, we have unexpectedly uncovered a family of new S = 1 complexes with HAT reaction rates beyond the currently reported limits in the tripodal ligand family, surpassing oxidation rates found for the S = 2 [FeIV(O)(TQA)]2+ complex by as much as an order of magnitude. This is achieved simply by replacing the secondary sphere methyl groups of the Me3NTB ligand with larger cycloalkyl-CH2 (R groups in 2OR) moieties ranging from c-propylmethyl to c-hexylmethyl. These 2OR complexes show Mössbauer data at 4 K and 1H NMR spectra at 193 and 233 K that reveal S = 1 ground states, in line with DFT calculations. Nevertheless, they give rise to the most reactive synthetic nonheme oxoiron(IV) complexes found to date within the tripodal ligand family. Our DFT study indicates transition state stabilization through entropy effects, similar to enzymatic catalysis.

Coordination Induced Spin State Transition Switches the Reactivity of Nickel (II) Porphyrin in Hydrogen Evolution Reaction

Electron spin plays a critical role in chemical processes, particularly in reactions involving metal complexes with unpaired electrons. However, more definitive state-to-state experiments are needed to better elucidate the role of electronic spin. Herein, we chose nickel (II) 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin 1 as a catalyst, which allows switching from a low spin to a high spin state of Ni (II) center through an axial pyridine coordination, for electrocatalytic hydrogen evolution reaction (HER). When pyridine is present, we observed β-hydrogenation of porphyrin through electron transfer followed by proton transfer. In contrast, hydrogen evolution mainly occurs via the concerted proton-coupling electron transfer without pyridine coordination. Similar distinct spin-dependent selectivity was also observed in chemical reduction of 1 by CoCp2 with subsequent addition of pyridinium p-toluenesulfonate. Computational calculations using density functional theory demonstrated that the transition from low spin to high spin state enriches the ligand's electron density after one-electron reduction, leading to preferential protonation of β-periphery rather than meso-position or metal center.

New Frontiers in Nonheme Enzymatic Oxyferryl Species

Non-heme mononuclear iron dependent (NHM-Fe) enzymes exhibit exceedingly diverse catalytic reactivities. Despite their catalytic versatilities, the mononuclear iron centers in these enzymes show a relatively simple architecture, in which an iron atom is ligated with 2-4 amino acid residues, including histidine, aspartic or glutamic acid. In the past two decades, a common high-valent reactive iron intermediate, the S=2 oxyferryl (Fe(IV)-oxo or Fe(IV)=O) species, has been repeatedly discovered in NHM-Fe enzymes containing a 2-His-Fe or 2-His-1-carboxylate-Fe center. However, for 3-His/4-His-Fe enzymes, no common reactive intermediate has been identified. Recently, we have spectroscopically characterized the first S=1 Fe(IV) intermediate in a 3-His-Fe containing enzyme, OvoA, which catalyzes a novel oxidative carbon-sulfur bond formation. In this review, we summarize the broad reactivities demonstrated by S=2 Fe(IV)-oxo intermediates, the discovery of the first S=1 Fe(IV) intermediate in OvoA and the mechanistic implication of such a discovery, and the intrinsic reactivity differences of the S=2 and the S=1 Fe(IV)-oxo species. Finally, we postulate the possible reasons to utilize an S=1 Fe(IV) species in OvoA and their implications to other 3-His/4-His-Fe enzymes.

Expression, purification, kinetics, and crystallization of non-heme mononuclear iron enzymes: Biphenyl, Phthalate, and Terephthalate dioxygenases

Non-heme iron oxygenases constitute a versatile enzyme family that is crucial for incorporating molecular oxygen into diverse biomolecules. Despite their importance, only a limited number of these enzymes have been structurally and functionally characterized. Surprisingly, there remains a significant gap in understanding how these enzymes utilize a typical architecture and reaction mechanism to catalyze a wide range of reactions. Improving our understanding of these catalysts holds promise for advancing both fundamental enzymology and practical applications. This chapter aims to outline methods for heterologous expression, enzyme preparation, in vitro enzyme assays, and crystallization of biphenyl dioxygenase, phthalate dioxygenase and terephthalate dioxygenase. These enzymes catalyze the dihydroxylation of biphenyl, phthalate and terephthalate molecules, serving as a model for functional and structural analysis of other non-heme iron oxygenases.

NMR and Mössbauer Studies Reveal a Temperature-Dependent Switch from S = 1 to 2 in a Nonheme Oxoiron(IV) Complex with Faster C-H Bond Cleavage Rates

S = 2 FeIV═O centers generated in the active sites of nonheme iron oxygenases cleave substrate C-H bonds at rates significantly faster than most known synthetic FeIV═O complexes. Unlike the majority of the latter, which are S = 1 complexes, [FeIV(O)(tris(2-quinolylmethyl)amine)(MeCN)]2+ (3) is a rare example of a synthetic S = 2 FeIV═O complex that cleaves C-H bonds 1000-fold faster than the related [FeIV(O)(tris(pyridyl-2-methyl)amine)(MeCN)]2+ complex (0). To rationalize this significant difference, a systematic comparison of properties has been carried out on 0 and 3 as well as related complexes 1 and 2 with mixed pyridine (Py)/quinoline (Q) ligation. Interestingly, 2 with a 2-Q-1-Py donor combination cleaves C-H bonds at 233 K with rates approaching those of 3, even though Mössbauer analysis reveals 2 to be S = 1 at 4 K. At 233 K however, 2 becomes S = 2, as shown by its 1H NMR spectrum. These results demonstrate a unique temperature-dependent spin-state transition from triplet to quintet in oxoiron(IV) chemistry that gives rise to the high C-H bond cleaving reactivity observed for 2.

Synchronously improved permeability, selectivity and fouling resistance of Fe-N-C functionalized ceramic catalytic membrane for effective water treatment: The critical role of Fe

Constructing catalytic membrane simultaneously displaying high permeability, selectivity and antifouling performance in water treatment remains challenging. Herein, the surface and pore channels of the ceramic membrane were co-functionalized with nitrogen doped carbon supported Fe catalyst (CN-F), and the Fe content was varied to investigate its effect on performance of CN-F coupled with peroxymonosulfate (PMS) activation (CN-F/PMS) for water treatment. Results confirmed the introduced Fe (in Fe-N coordination form) greatly enhanced the permeability, selectivity and fouling resistance of CN-F. Optimal CN-F3/PMS achieved 96.5% removal and 52.1% mineralization of sulfamethoxazole in short retention duration (2.7 min), whose performance was 5.4 and 6.7 times higher than that of nitrogen doped carbon functionalized ceramic catalytic membrane (CN/PMS) and CN-F3 filtration alone, respectively. CN-F3/PMS also efficiently inhibited fouling on both surface and pores with 2.8 and 2.4 times lower flux loss than that of CN/PMS and CN-F3 filtration alone, respectively. Moreover, CN-F3/PMS displayed superior performance in long-term treatment of real coking wastewater. The outstanding performance of CN-F was mainly attributed to the dual role of supported Fe, which served as hydrophilic site for enhanced water permeation and major active site for PMS adsorption and reduction into reactive species (mainly high-valent Fe(IV)=O species) towards pollutant elimination.

Preparations of trans- and cis-μ-1,2-Peroxodiiron(III) Complexes

The iron(II) complex with cis,cis-1,3,5-tris(benzylamino)cyclohexane (Bn3CY) (1) has been synthesized and characterized, which reacted with dioxygen to form the peroxo complex 2 in acetone at -60 °C. On the basis of spectroscopic measurements for 2, it was confirmed that the peroxo complex 2 has a trans-μ-1,2 fashion. Additionally, the peroxo complex 2 was reacted with benzoate anion as a bridging agent to give a peroxo complex 3. The results of resonance Raman and 1H-NMR studies supported that the peroxo complex 3 is a cis-μ-1,2-peroxodiiron(III) complex. These spectral features were interpreted by using DFT calculations.

Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kβ x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Highly selective synthesis of surface FeIV=O with nanoscale zero-valent iron and chlorite for efficient oxygen transfer reactions

High-valent iron-oxo species (FeIV=O) has been a long-sought-after oxygen transfer reagent in biological and catalytic chemistry but suffers from a giant challenge in its gentle and selective synthesis. Herein, we propose a new strategy to synthesize surface FeIV=O (≡FeIV=O) on nanoscale zero-valent iron (nZVI) using chlorite (ClO2-) as the oxidant, which possesses an impressive ≡FeIV=O selectivity of 99%. ≡FeIV=O can be energetically formed from the ferrous (FeII) sites on nZVI through heterolytic Cl-O bond dissociation of ClO2- via a synergistic effect between electron-donating surface ≡FeII and proximal electron-withdrawing H2O, where H2O serves as a hydrogen-bond donor to the terminal O atom of the adsorbed ClO2- thereby prompting the polarization and cleavage of Cl-O bond for the oxidation of ≡FeII toward the final formation of ≡FeIV=O. With methyl phenyl sulfoxide (PMS16O) as the probe molecule, the isotopic labeling experiment manifests an exclusive 18O transfer from Cl18O2- to PMS16O18O mediated by ≡FeIV=18O. We then showcase the versatility of ≡FeIV=O as the oxygen transfer reagent in activating the C-H bond of methane for methanol production and facilitating selective triphenylphosphine oxide synthesis with triphenylphosphine. We believe that this new ≡FeIV=O synthesis strategy possesses great potential to drive oxygen transfer for efficient high-value-added chemical synthesis.

Highly efficient peroxymonosulfate activation on Fe-N-C catalyst via the collaboration of low-coordinated Fe-N structure and Fe nanoparticles for enhanced organic pollutant degradation

Supporting Fe catalysts on N doped carbon (Fe-N-C) renders a promising way towards peroxymonosulfate (PMS) activation for water decontamination, but constructing high-efficiency Fe-N-C remains challenging due to the insufficient understanding of the structure-performance relationship. Herein, the N doped carbon nanotube supported Fe catalysts (Fe-NCNT) were prepared towards PMS activation for organic pollutants removal, in which the Fe-N coordination number and Fe species were tuned through changing the pyrolysis temperature to study their roles in PMS activation. Results showed increasing the pyrolysis temperature converted the Fe-N₄ structure in Fe-NCNT to low-coordinated Fe-N₃ structure and produced Fe nanoparticles (FeNP, encapsulated in carbon). The Fe-NCNT with Fe-N₃ and FeNP exhibited a remarkably high specific activity (0.119 L min^-1 m^-2), which was 1.8 times higher than that of Fe-NCNT with only Fe-N₄ and obviously outperformed those of the state-of-the-art PMS activators. The low-coordinated structure and FeNP promoted the PMS reduction on Fe²⁺ of Fe-N_x for •OH and SO₄^•- production, which served as major oxidants for pollutants degradation. The experimental results and theoretical calculation corroborated the low-coordinated structure and FeNP jointly enhanced the PMS adsorption and electron density on Fe center, which accelerated electron transfer from Fe center to PMS for radical production.

Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O2 Binding, and H-Atom Abstraction Reactivity

A five-coordinate, disiloxide-ligated cobalt(II) (S = 3/2) complex (1) was prepared as an oxygen-ligated analogue to the previously reported silanedithiolate-ligated CoII(Me3TACN)(S2SiMe2) (J. Am. Chem. Soc., 2019, 141, 3641-3653). The structural and spectroscopic properties of 1 were analyzed by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR), and NMR spectroscopies. The reactivity of 1 with dioxygen was examined, and it was shown to bind O2 reversibly in a range of solvents at low temperatures. A cobalt(III)-superoxo complex, CoIII(O2·-)(Me3TACN)((OSi2Ph)2O) (2), was generated, and was analyzed by UV-vis, EPR, and resonance Raman spectroscopies. Unlike its sulfur-ligated analogue, complex 2 can thermally release O2 to regenerate 1. Vibrational assignments for selective 18O isotopic labeling of both O2 and disiloxide ligands in 2 are consistent with a 6-coordinate, Co(η1-O2·-)("end-on") complex. Complex 2 reacts with the O-H bond of 4-methoxy-2,2,6,6-tetramethylpiperidin-1-ol (4-MeO-TEMPOH) via H-atom abstraction with a rate of 0.58(2) M-1 s-1 at -105 °C, but it is unable to oxidize phenol substrates. This bracketed reactivity suggests that the O-H bond being formed in the putative CoIII(OOH) product has a relatively weak O-H bond strength (BDFE ∼66-74 kcal mol-1). These thermodynamic and kinetic parameters are similar to those seen for the sulfur-ligated Co(O2)(Me3TACN)(S2SiMe2), indicating that the differences in the electronic structure for O versus S ligation do not have a large impact on H-atom abstraction reactivity.