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Generation and Catalytic Applications of High-Valent Iron Species in Metal-Organic Frameworks

Weijian Yu , Fuguo Qiu , Fei Wang , Chongchen Wang

Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (9) : 1599 -1609.

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Progress in Chemistry ›› 2026, Vol. 38 ›› Issue (9) : 1599-1609 DOI: 10.7536/PC20260121
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Generation and Catalytic Applications of High-Valent Iron Species in Metal-Organic Frameworks
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Abstract

Metal-organic frameworks（MOFs）， due to their structural tunability， provide a unique platform for stabilizing and generating high-valent iron-oxo intermediates like highly active， high-spin Fe（Ⅳ）＝O species. This review summarizes the mechanisms of forming high-valent iron species in several MOFs through the activation of N2O， O2， and persulfates， as well as their applications in the selective oxidation of alkanes and efficient degradation of pollutants. The key roles of MOF structural features including but not limited to weak-field coordination， site isolation， and dynamic flexible frameworks in regulating reaction pathways and product selectivity are highlighted. These characteristics demonstrate the significant potential of MOFs to mimic the catalytic functions of natural metalloenzymes. Current research offers a theoretical foundation and directional guidance for designing the next generation of efficient and green biomimetic catalytic materials. The ability to engineer MOF structures at the molecular level allows for precise control over active site environments， facilitating the generation of otherwise unstable high-valent iron intermediates under mild conditions. Studies on N2O activation reveal how framework rigidity and isolated sites enable selective alkane hydroxylation. Remarkably， the activation of O2 in MOFs has been achieved without external oxidants or photo-assistance， replicating key enzymatic steps. Furthermore， dynamic structural adaptations in certain MOFs enhance peroxydisulfate activation， leading to superior pollutant degradation via a dominant Fe（IV）=O pathway. These advances underscore MOFs as versatile and stable solid catalysts that bridge the gap between homogeneous enzyme mimics and practical heterogeneous systems. The insights gained from the limited current research not only deepen the understanding of high-valent iron chemistry in confined spaces but also pave the way for applications in sustainable chemical synthesis and advanced environmental remediation technologies.

Contents

1 Introduction

2 Activation of N2O to form high-valent iron species for selective oxidation of alkanes

3 Activation of O2 to form high-valent iron species for selective oxidation of alkanes

4 Activation of persulfates to form high-valent iron species for the degradation of pollutants

5 Conclusion and outlook

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Keywords

metal-organic frameworks / Fe（Ⅳ）＝ O intermediates / selective catalytic oxidation / advanced oxidation processes

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Weijian Yu, Fuguo Qiu, Fei Wang, Chongchen Wang. Generation and Catalytic Applications of High-Valent Iron Species in Metal-Organic Frameworks. Progress in Chemistry, 2026, 38(9): 1599-1609 DOI:10.7536/PC20260121

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1 Introduction

Metal-organic frameworks (MOFs), as a class of crystalline porous materials formed by the self-assembly of metal ions or clusters with organic ligands through coordination bonds, feature highly tunable pore structures, enormous specific surface areas, and functionalizable active sites, demonstrating immense potential in fields such as gas storage, separation, catalysis, and environmental remediation.[1-2]. In catalytic oxidation technologies, MOFs, as emerging high-efficiency catalysts, are becoming a key class of materials for treating gaseous and liquid pollutants.[3-6]. Zhang et al.[7]prepared defective MIL-88B(Fe) nanorods, whose one-dimensional hexagonal channels facilitate the contact of formaldehyde and oxygen with abundant coordinatively unsaturated iron sites. The Lewis acid-base interaction between formaldehyde and iron sites accelerates the Fe3+/Fe2+cycle, promoting Fenton-like activation of oxygen to generate reactive oxygen species such as superoxide radicals, hydroxyl radicals, and singlet oxygen, ultimately achieving deep mineralization of formaldehyde into CO2at room temperature, with a mineralization efficiency exceeding 80%. Lan et al.[8]employed a cobalt-based MOF material containing polyoxovanadate clusters, Co2(V4O12)(bpy)2, to activate peroxymonosulfate (PMS) for the efficient degradation of organic pollutants such as ofloxacin. Among them, the exposed terminal oxygen atoms in [V4O12]4-form hydrogen bonds with the terminal hydrogen atoms of PMS, realizing a multi-channel electron transfer mechanism that generates singlet oxygen and surface-bound sulfate radicals. Li et al.[9]prepared amorphous ZIF-62(Co) glass via a melt-quenching process for activating peroxymonosulfate to degrade micropollutants, combining the advantages of an amorphous structure, good water stability, and ease of immobilization and recovery. Chu et al.[10-12]further employed batch-prepared MIL-88A(Fe) to activate hydrogen peroxide under low-power light synergy, generating hydroxyl radicals and superoxide radicals to disrupt the protective layer of algal cells, thereby causing irreversible damage to the photosynthetic system (chlorophyll a, phycobiliproteins) and the antioxidant system (superoxide dismutase), ultimately leading to cell lysis and effective mineralization of released microcystin-LR, reducing the risk of secondary pollution.
In the catalytic process, compared with traditional reactive oxygen species represented by hydroxyl radicals (·OH) and sulfate radicals (SO4-·)[13-14], high-valent iron-oxygen species possess unique reaction characteristics and application advantages. Firstly, high-valent metal (iron, cobalt, manganese, etc.) species mainly react through well-defined pathways such as hydrogen atom abstraction and oxygen atom transfer, which facilitates the selective transformation of specific functional groups[15-16]; whereas radical reaction pathways are diverse and product distributions are complex. Secondly, high-valent iron species preferentially oxidize electron-rich organic pollutants[17], and can avoid the generation of over-oxidized by-products; whereas radicals exhibit non-selective oxidation characteristics. Thirdly, high-valent iron species demonstrate strong resistance to interference from common inorganic ions and natural organic matter in water, maintaining catalytic activity even under complex water quality conditions[18]; while radicals are easily quenched and deactivated by the aforementioned components[19]. Based on these characteristics, high-valent iron species are particularly suitable for scenarios such as selective catalytic oxidation (e.g., alkane C—H bond functionalization)[20]and the efficient degradation of trace pollutants in complex water bodies.
Due to their tunable structures, isolated active sites, and controllable coordination environments, MOFs provide an ideal platform for regulating the generation and reaction pathways of high-valent iron species. They can not only optimize electron transfer to promote advanced oxidation reactions but also offer a unique spatial and electronic structural platform for modulating highly active, high-spin high-valent iron-oxygen intermediates (such as Fe(IV)=O), thereby simulating enzymatic catalysis at the molecular level to achieve efficient and selective alkane conversion and pollutant degradation. Although related work is scarce, it demonstrates the distinct advantages of generating high-valent iron within MOFs.[21]This paper will analyze the structural characteristics, related performance, and underlying mechanisms of MOFs capable of generating high-valent iron species based on the limited existing literature, aiming to help researchers in related fields gain insights into new directions for MOF design and application.

2 Activated N2O forms high-valent iron species for selective oxidation of alkanes

Simulating the high efficiency and specificity of natural metalloenzymes is the core objective of selective catalytic oxidation of hydrocarbons[22-23]. MOFs demonstrate unique advantages in simulating natural metalloenzymes. Their highly ordered and tunable crystal structures enable precise design and reproduction of the coordination microenvironment of enzyme active centers, thereby mimicking the catalytic functions of metalloenzymes. Meanwhile, MOFs possess chemical and thermal stability far exceeding that of natural enzymes. Due to their porous characteristics and solid-state form, they are not only easy to recover and reuse but also capable of integrating multiple catalytic sites within confined spaces to achieve biomimetic synergistic catalysis.
Xiao et al.[24]investigated the pathway for activating N2O using the metal-organic framework Fe2(dobdc) (dobdc = 2,5-dihydroxyterephthalic acid) and its derivatives to selectively oxidize ethane to ethanol. Fe2(dobdc), also known as Fe-MOF-74[25-26], features regular five-coordinate high-spin Fe(II) sites arranged within one-dimensional hexagonal channels in its structure (Figure 1(a)). These open sites exhibit reversible, weak binding characteristics toward N2O (Figure 1(b)), laying the foundation for subsequent controlled activation. Under mild heating conditions, N2O undergoes heterolytic dissociation, with the released oxygen atom binding to the Fe(II) center to form an Fe(IV)=O intermediate. Although this intermediate cannot be directly captured due to its extreme reactivity, its existence is corroborated by a series of indirect evidence and theoretical calculations. The stable product isolated after the reaction is Fe2(OH)2(dobdc), a process consistent with the typical decay pathway of high-valent metal-oxo intermediates undergoing hydrogen atom abstraction (HAA). Notably, in the presence of ethane, this intermediate can directly attack C—H bonds (bond dissociation energy 423 kJ/mol) to generate oxygenated products. This reaction process demonstrates that the rigid framework of MOFs and the unique coordination environment provided by weak-field ligands (carboxylate/aryloxy) can mimic enzymatic active centers, stabilizing the formation of high-spin (S=2) Fe(IV)=O species that are extremely unstable and difficult to achieve in homogeneous systems, thereby realizing biomimetic activation of inert alkane C—H bonds (Figure 1(c)).
However, intrinsic Fe2(dobdc) materials, when catalyzing the oxidation of ethane, yield a mixture of products including ethanol, acetaldehyde, and diethyl ether, exhibiting poor selectivity. This indicates that in catalytic reactions, merely generating highly active Fe(IV)=O species is insufficient; reaction selectivity must also be addressed. Xiao et al.[24]found that the high density and close proximity of Fe sites within the Fe2(dobdc) channels (with a spacing of 3.1–3.2 Å, where 1 Å = 0.1 nm) are the structural roots causing over-oxidation and coupling side reactions. To address this, they synthesized a magnesium-diluted mixed-metal MOF—Fe0.1Mg1.9(dobdc). In this material, redox-inert Mg(II) ions highly dilute and isolate the active Fe(II) sites within the framework, effectively blocking synergy between multiple active sites. Experimental results demonstrate that this approach achieved a leap in selectivity. Under identical reaction conditions, Fe0.1Mg1.9(dobdc) almost exclusively catalyzes the oxidation of ethane to produce ethanol and acetaldehyde, with ethanol becoming the overwhelmingly dominant product.
Density functional theory (DFT) and multireference (CASSCF/CASPT2) calculations indicate that in the weak-field environment of Fe2(dobdc), the generated Fe(IV)=O intermediate has a quintet state (S=2) as its ground state, with an electronic structure similar to the "iron-oxygen" centers in many highly active non-heme iron enzymes. This high-spin state is considered the electronic structural basis for its powerful C—H bond activation capability. The entire catalytic reaction can be summarized as: (1) N2O undergoes reversible adsorption and activation at isolated Fe(II) sites to generate high-spin Fe(IV)=O; (2) Fe(IV)=O abstracts a hydrogen atom from ethane, forming Fe(III)-OH and an ethyl radical; (3) A subsequent "oxygen rebound" step produces ethanol and regenerates the active center via reduction (or partially converts it into inert Fe(III)-OH, leading to catalyst deactivation). In the diluted material Fe0.1Mg1.9(dobdc), isolated sites ensure that the radical intermediates generated in step (2) can only undergo "rebound" with the original site or acquire hydrogen from the environment, without migrating to adjacent sites to trigger coupling or further oxidation, thereby anchoring the product to the primary product of single oxygen insertion, ethanol.
This work, through Fe2(dobdc) and Fe0.1Mg1.9(dobdc) comparative study, reveals two core principles for designing efficient alkane selective oxidation catalysts in MOFs. (1) Coordination-unsaturated metal sites constructed using the rigid framework and weak-field ligands of MOFs serve as an ideal platform for stabilizing highly active, high-spin, high-valent metal-oxygen intermediates (such as S=2 Fe(IV)=O). This is the electronic structure prerequisite for achieving biomimetic activation of inert C—H bonds. (2) Precise spatial isolation of active sites via crystal engineering of MOFs (such as heterometallic doping) is key to controlling the reaction process and guiding the selective formation of target products. This work, through material design and mechanistic studies, clarifies how to rationally construct and stabilize highly active high-valent iron-oxygen intermediates within such highly ordered porous crystals as MOFs, and how to precisely control the spatial density and distribution of active sites through MOF crystal engineering to regulate reaction pathways, suppress secondary reactions, and achieve high selectivity.
Verma et al.[27]employed M06-L density functional theory and multi-reference wavefunction methods to construct an 88-atom cluster model simulating the catalytic active site, thereby deeply revealing the metal-organic framework (MOF) material Fe0.1Mg1.9(dobdc) in the presence of N2O for the complete reaction mechanism of catalyzing the selective oxidation of ethane to ethanol (Figure 2(a)). (1) The decomposition of N2O at the Fe(II) sites in Fe0.1Mg1.9(dobdc) leads to the formation of a high-spin (S=2) Fe(IV)=O species. This reaction step is the rate-determining step, with a corresponding activation energy of 82 kJ/mol (Figure 2(b)). (2) The C—H bond activation of ethane occurs via a σ-attack pathway, following the single-state reactivity (SSR) mechanism, proceeding entirely on the quintet surface without spin crossing. (3) Compared to methane, the C—H bond activation barrier for ethane is lower (58 kJ/mol vs. 69 kJ/mol), explaining why Fe0.1Mg1.9(dobdc) can catalyze the hydroxylation of ethane but not methane. (4) The barriers for competing pathways (dehydrogenation, dissociation) after ethanol formation are all higher than that for hydroxylation, ensuring the selective generation of ethanol. (5) The barrier for the non-catalytic reaction is as high as 280 kJ/mol, demonstrating the key catalytic role of Fe(IV)=O. This work provides an important theoretical basis for designing efficient catalysts for selective alkane oxidation and showcases the potential of MOF materials in mimicking enzymatic catalysis.
Wang et al.[15]investigated the catalytic oxidation mechanism of ethane and ethanol by high-valent iron (oxoiron(IV)) species in Fe-MOF-74 (i.e., Fe2(dobdc)) using multi-scale QM/MM calculations. The computational results indicate that ethane hydroxylation proceeds via a high-spin quintet rebound mechanism, with a hydrogen abstraction barrier of only 65.3 kJ/mol (15.6 kcal/mol), which is far lower than the homolytic dissociation energy of ethane (421.3 kJ/mol or 100.7 kcal/mol); this is attributed to the exchange-enhanced effect of the iron center during the reaction. In contrast, the hydrogen abstraction barrier for the Fe(III)OH species is as high as 146.0 kJ/mol (34.9 kcal/mol), rendering it non-reactive. For the conversion of ethanol to acetaldehyde, although the O—H bond strength is higher than that of the C—H bond, calculations show that the barrier for O—H bond cleavage (30.1 kJ/mol or 7.2 kcal/mol) is actually lower. This benefits from the stabilization of the transition state by proton-coupled electron transfer (PCET), favoring O—H hydrogen abstraction. These results are consistent with experimental observations, providing a reasonable explanation for the pathway of selective ethanol oxidation in Fe-MOF-74. This work reveals that the weak ligand field environment in MOFs favors the stabilization of high-spin high-valent iron species, and through multi-scale modeling, clarifies their unique reactivity in catalysis compared to enzymes and synthetic complexes, offering a theoretical basis for designing efficient biomimetic catalysts.
Based on Fe0.1Mg1.9(dobdc) in the presence of N2O catalyzes the selective oxidation of ethane to ethanol; Vogiatzis et al.[28]proposed a computational high-throughput screening method for rapidly identifying metal-organic framework materials with oxidative catalytic potential. The study focused on the stabilization and formation of high-spin Fe(IV)=O intermediates in weak ligand field environments and their effective activation of alkane C—H bonds. From a database containing over 5,000 known MOFs, 46 structures with unsaturated iron coordination sites were screened, and 7 candidates were further selected for in-depth analysis via density functional theory calculations. Theoretical predictions indicated that 3 of these materials could form high-spin Fe(IV)=O intermediates through N2O dissociation. Experimental validation demonstrated that the Fe-BTT (BTT = 1,3,5-benzenetristetrazolate) material can selectively oxidize ethane to ethanol at 120 °C without producing over-oxidized products. However, further spectroscopic and cycling experiments revealed that the catalytic activity did not originate from the theoretically predicted framework sites but was closely related to defect sites within the material. Cation exchange experiments also ruled out the contribution of extra-framework iron sites. This work not only demonstrates the practicality of computational screening in accelerating the discovery of catalytic materials but also highlights the critical role of defect sites in MOF catalysis, providing important directions for future research on defect engineering and precise regulation of active sites.

3 Activated O2Formation of high-valent iron species for selective oxidation of alkanes

In nature, many α-ketoglutarate-dependent dioxygenases (such as TauD) can utilize oxygen to form a high-spin (S=2) Fe(IV)=O intermediate at an iron center (Figure 3(a)), thereby efficiently catalyzing the hydroxylation of C—H bonds[29-30]. However, in artificial synthetic systems, generating similar high-spin Fe(IV)=O species using only O2has not yet been achieved; previously, most methods relied on strong oxidants or photoexcitation. Huo et al.[31]used a Zn2Cl4(btdd)3framework with an MFU-4 structure as the parent framework. Through post-synthetic metal ion exchange and ligand substitution, they successfully synthesized a novel metal-organic framework material Fe1.8Zn3.2(prv)4(btdd)3(Hprv = pyruvic acid; H2btdd = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i]dibenzo[1,4]dioxin))[31]. The iron(II) sites in this MOF possess a local coordination environment similar to that of the TauD enzyme, namely a pseudo-tetrahedral geometry and a triazole coordination mode. At low temperatures (e.g., 100 K), Fe1.8Zn3.2(prv)4(btdd)3can react with O2, accompanied by the decarboxylation of α-keto acid ligands (such as pyruvate), to generate Fe(IV)=O species in situ (Figure 3(b)). The study systematically characterized the reaction intermediates using various advanced in situ spectroscopic techniques: In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) showed a characteristic Fe(IV)=O vibrational peak at 831 cm-1; when using18O2, it shifted to 796 cm-1, confirming that the oxygen atom originates from ligand decarboxylation rather than O2(Figure 3(c)). Mössbauer spectroscopy revealed that the isomer shift and quadrupole splitting values of the Fe(IV)=O species are consistent with a high-spin state (S=2), distinguishing it from the common intermediate-spin state (S=1). Variable-field Mössbauer spectroscopy and Fe Kβ X-ray emission spectroscopy further confirmed its high-spin ground state, consistent with the properties of the TauD-J intermediate in enzymes. Nuclear resonance vibrational spectroscopy revealed the vibrational modes of the Fe(IV)=O species and its ligand environment, agreeing with theoretical calculations. This work, for the first time in a purely synthetic system, used O2(without added oxidants or light assistance) to successfully generate and characterize high-spin Fe(IV)=O species. Meanwhile, the rigid structure of the MOF stabilized the active intermediate, avoiding common dimerization or ligand oxidation side reactions observed in solution. This work not only reproduces the key steps of oxygen activation in nature but also lays an important foundation for designing new generations of heterogeneous catalytic systems and achieving selective functionalization of alkanes under mild conditions.
Börgel et al.[32]used the "radical clock" molecule—norcarane—as a probe substrate, with FeZn4(prv)4(btdd)3(Fe-1), Fe1.8Zn3.2(prv)4(btdd)3(Fe1.8-1), and FeZn4(moba)4(btdd)3(Fe-2) and other MOFs as platforms for site-isolated "caged active centers" to investigate how spatial confinement and electronic spin states jointly regulate the reaction mechanism of C–H bond oxidation. After undergoing hydrogen atom transfer to generate a radical, the cyclopropane structure of the norcarane molecule undergoes rapid, characteristic ring-opening rearrangement; the rearrangement rate is directly related to the radical lifetime. By analyzing the ratio of different rearranged products to unrearranged products, the lifetime of radical intermediates generated in the reaction can be quantitatively measured like a "stopwatch." Experimental results show that the intermediate species generated by the interaction of three iron-based MOFs (Fe-1, Fe1.8-1, and Fe-2) with oxygen exhibit radical lifetimes in the low nanosecond range (4–12 ns) during norcarane oxidation. This lifetime value is far longer than that of freely diffusing photolytic radicals in solution but significantly shorter than that of certain enzymatic systems utilizing higher oxidation state intermediates. This result provides, for the first time, quantitative data on radical kinetics within such MOF-confined spaces, indicating that although the framework pores do not completely imprison the radicals, they effectively restrict their rapid escape, making them more prone to undergo a "rebound" step near the generation site, combining with iron(III)–hydroxo species or adjacent ligands to form the final oxidation products.
This work, for the first time, observed the key ligand rebound product within a MOF system, accurately reproducing the side reaction pathway found in natural enzymes (Figure 4(a)). For instance, in the reaction corresponding to FeZn4(moba)4(btdd)3(Fe-2), the detected ligand rebound cyclic and ring-opened products are norbornan-2-yl pivalate and 2-cyclohexen-1-ylmethyl pivalate, respectively. Through structural modeling, this work also revealed the mechanism by which the reaction is spatially directed by the MOF channels (Figure 4(b)). In FeZn4(moba)4(btdd)3(Fe-2), the large pores with an inner diameter of 19 Å serve as the primary channel for the substrate molecule norbornene to approach the Fe(IV)=O active site and undergo hydrogen atom transfer (HAT) reactions (Figure 4(c, d)). Conversely, in Fe-2, the small pores with an inner diameter of 8 Å prevent the substrate from effectively approaching the Fe(IV)=O species to initiate HAT reactions due to steric hindrance (particularly from the tert-butyl group of the pivalate) (Figure 4(e)). However, this channel may be related to substrate diffusion and the geometric accessibility of the ligand rebound reaction. Researchers also designed comparative experiments for this reaction in two solvents, dichloromethane and acetonitrile, to understand the fine-tuning effect of the solvent on the active site. The results indicate that acetonitrile significantly suppresses the formation of ligand rebound products. This is likely because acetonitrile molecules can competitively coordinate to the iron active center, thereby altering the binding mode of the carboxylate ligands, making them spatially less accessible to attack by substrate radicals.
The aforementioned work revealed for the first time that in a purely synthetic system, using only oxygen, a high-spin iron(IV)-oxo species with an electronic structure identical to that of a key enzymatic intermediate (TauD-J) was generated within a solid-state material. This not only mimics enzymatic function but also precisely reproduces its core active unit at the molecular level, providing a stable, well-defined, and tunable platform for studying such highly reactive species, no longer limited to unstable solution-phase molecular complexes. Meanwhile, Börgel et al.[32]were the first to quantitatively determine the lifetime of cage-confined radicals in a solid-state biomimetic system and observed the ligand rebound pathway, which is crucial for understanding enzymatic catalytic selectivity. The study revealed how the rigid pores of metal-organic frameworks, much like enzyme pockets, precisely guide the approach direction of substrates through steric hindrance, and demonstrated that even within a solid-state framework, the local coordination environment retains a tunable "flexibility." This provides a concrete experimental model for understanding "reaction dynamics within confined spaces."

4 Activation of persulfate to form high-valent iron species for degrading emerging contaminants

High-valent metal species (such as high-valent iron Fe(IV)=O, Fe(V)=O) and conventional reactive oxygen species represented by hydroxyl radicals (·OH) and sulfate radicals (SO4-·) follow distinctly different reaction mechanisms during pollutant degradation; the two exhibit fundamental differences in reaction pathways, kinetic characteristics, and selectivity.[33-36]. (1) Differences in reaction mechanisms exist. Radicals primarily react with substrates via pathways such as electron transfer, radical addition, and hydrogen atom abstraction; they exhibit extremely high reactivity but poor selectivity. In contrast, high-valent iron species possess more diverse reaction mechanisms, mainly including: hydrogen atom abstraction, oxygen atom transfer, electrophilic addition, electron transfer, and nucleophilic attack. Among these, direct oxygen atom transfer is a characteristic reaction pathway distinguishing high-valent iron species from radicals: Fe(IV)=O can directly insert an oxygen atom into electron-rich substrates such as sulfides and alkenes to generate sulfoxides or epoxides, whereas radical pathways struggle to achieve such selective oxidation. This property gives high-valent iron species unique advantages in the synthesis of fine chemicals and the selective transformation of specific pollutants. (2) Differences in kinetic characteristics exist. The redox potential (E0(Fe(IV)/Fe(III)) ≈ 2.00 V, vs SHE) of Fe(IV)=O species, although slightly lower than that of ·OH (1.89~2.72 V, vs SHE) and SO4-· (2.5~3.1 V, vs SHE)[37-38], their lifetime in aqueous environments is significantly longer: the lifetime of Fe(IV) can reach 7~10 s, whereas the lifetimes of ·OH and SO4-· are only about 40 μs and 300 μs, respectively.[39-40]. Correspondingly, the steady-state concentration of high-valent iron species is approximately 1×10-9 mol/L, which is 3~6 orders of magnitude higher than that of radicals (10-12~10-15 mol/L). This enables high-valent iron species to continuously react with pollutants in actual water bodies, overcoming the problem of low utilization efficiency caused by the short lifetime and easy quenching of radicals.[41-42]. (3) Differences in selectivity and matrix resistance exist. High-valent iron species exhibit distinct electrophilic characteristics, preferentially reacting with organic pollutants containing electron-rich groups (such as phenolic hydroxyl, amino, sulfide, and unsaturated bonds). Studies show that the lower the ionization potential of an organic substrate, the faster its reaction rate with high-valent iron species. This selective reaction characteristic gives high-valent iron species unique advantages in complex water matrices: common radical scavengers such as Cl-, HCO3-, phosphates, and natural organic matter have a significantly smaller impact on high-valent iron species, whereas radical pathways are severely inhibited.[38-43].
The above analysis indicates that high-valent iron species (Fe(IV)=O) possess significant advantages over traditional radical pathways in terms of reaction mechanisms, kinetic characteristics, and selectivity, particularly regarding their selective oxidation capability for electron-rich pollutants in complex water matrices and their strong tolerance against interference from background matrices.[38]. However, how to controllably generate highly active and selective high-valent iron species within heterogeneous catalytic systems remains a key challenge in this field. In recent years, metal-organic frameworks have demonstrated significant potential in the field of advanced oxidation processes for water treatment, and their structural designability provides broad space for regulating catalytic active sites and reaction pathways.[44]. However, most studies have focused on optimizing electronic structures through static means such as ligand modification[45-46], metal doping[11], constructing composites[44], or defect engineering[47], often overlooking the dynamic structural evolution that catalysts may undergo during the reaction process and its profound impact on the reaction mechanism. Wang et al.[48]utilized terephthalic acid and 4,4'-bipyridine as ligands and divalent iron ions as building blocks to synthesize BUC-95, a well-defined and tunable homointerpenetrated dynamic iron-based MOF, via a microwave method. Due to the small crystal size of the microwave-synthesized BUC-95, which precluded structural elucidation using conventional single-crystal X-ray diffraction, this study employed microcrystal electron diffraction to successfully resolve its single-crystal structure (Figure 5(a)). The structural resolution results indicate that BUC-95 consists of two identical three-dimensional frameworks interpenetrating each other. Within each framework, dinuclear iron units are connected by carboxylate and bipyridine ligands to form a three-dimensional network. This homointerpenetrated topology results in non-covalent interactions between the frameworks, endowing the entire assembly with reversible expansion capabilities, i.e., "dynamic stretching" behavior. This characteristic was intuitively confirmed by in situ PXRD: in the presence of H2O or PDS (peroxydisulfate), the diffraction peaks of BUC-95 exhibited significant shifts, indicating reversible changes in its lattice parameters. In contrast, its structural analog BUC-96 (where the terephthalic acid ligand in BUC-95 is replaced by 2,5-dihydroxyterephthalic acid) exhibits strong hydrogen bonds (e.g., O…H bonds of 2.509 Å) between DMF molecules in the framework pores and the ligands, acting like "molecular rivets" to lock the relative positions of the frameworks, thereby suppressing their dynamic stretching properties. Therefore, the structural feature of BUC-95 lies in its controllable sliding freedom between frameworks, providing a structural basis for adapting to the reaction environment and regulating active sites during the catalytic process.
In the BUC-95 framework, dynamic dual-iron sites act as Lewis acid sites that preferentially adsorb water molecules, activating and dissociating them to generate ·OH radicals and form Fe—OH intermediates. Theoretical calculations indicate that this surface hydroxylated state (BUC-95-OH) can significantly reduce the energy barriers for PDS activation and O—O bond cleavage (Figure 5(b)). Notably, dynamic stretching behavior influences the electronic structure of Fe sites by modulating their local coordination field. PDOS analysis shows (Figure 5(c)) that co-adsorption of —OH and PDS induces a rearrangement of Fe 3d orbitals from a tetrahedral field to an octahedral field, enhancing the hybridization between Fe 3d and O 2p orbitals and promoting electron transfer from the Fe dz² orbital to the antibonding orbital of PDS, thereby efficiently driving the generation of Fe(IV)=O. BUC-95 exhibits efficient, broad-spectrum pollutant degradation capability dominated by Fe(IV)=O. At extremely low PDS concentrations, the degradation rate constants of BUC-95 for various pollutants such as ofloxacin, enrofloxacin, and tetracycline are more than an order of magnitude higher than those of traditional MIL-series Fe-MOFs, with good cycling stability. Mechanistic studies reveal that the contribution of traditional radical pathways is negligible, while chemical probe experiments, Mössbauer spectroscopy, and XPS analysis collectively confirm that Fe(IV)=O is the dominant active species in the system.
Comparative studies with BUC-96, which has an identical structure, reveal that the dynamic stretching of BUC-95 breaks the singular dependence of catalytic performance on electron transfer efficiency. Electrochemical characterization (CV, EIS) indicates that BUC-96, with its rigid framework, actually possesses a larger electrochemically active surface area and lower charge transfer resistance, demonstrating superior electron transfer capability compared to BUC-95. However, its catalytic degradation performance is far inferior to the latter. This "contradictory" phenomenon highlights the advantage of dynamic stretching in BUC-95: its excellent catalytic performance does not stem from intrinsic electron conduction, but rather benefits from the structural functionality conferred by its framework's dynamic stretching ability (Figure 5(d)). This dynamic stretching enables BUC-95 to achieve the following objectives. (1) Dynamic stretching microenvironment: Under the presence of H2O and PDS, reversible dynamic stretching occurs, optimizing mass transport and orientation of reactants near active sites. (2) Regulating reaction coordinates: Through framework displacement, it indirectly modulates the geometric and electronic structure of Fe sites, creating a local environment more favorable for Fe-OH formation and O—O bond cleavage. (3) Achieving energy barrier shunting: It shares part of the activation energy demand from the highly demanding electron transfer process to the thermodynamically more favorable structural rearrangement process. In contrast, due to hydrogen bond locking, BUC-96 loses this capacity for structural dynamic stretching; even though its electron transfer is easier, it cannot effectively promote the subsequent critical step of Fe(IV)=O generation. This work transcends the conventional approach of enhancing catalytic performance by modifying local electronic structures, revealing the critical role of reversible mechanical dynamics at the MOF framework scale in advanced oxidation reactions.

5 Conclusion and Outlook

MOF materials are easily tunable in terms of chemical principles to generate high-valent iron species. (1) MOFs can provide a smaller d-orbital splitting energy for the iron center by selecting weak-field ligands such as carboxylates, aryloxides, and triazoles, thereby stabilizing high-spin (S=2) Fe(IV)=O intermediates, making their electronic structure highly consistent with that of highly active non-heme iron enzymes. (2) Through isomorphic doping with redox-inert metal ions, MOFs can dilute active iron sites with atomic precision, blocking the coupling and over-oxidation of radical intermediates to achieve highly selective catalysis. (3) MOFs with interpenetrating topologies (such as BUC-95) can undergo reversible lattice expansion under reactant stimulation; this geometric deformation adjusts the coordination field and electronic structure of the iron center, shifting part of the activation energy from electron transfer to structural rearrangement, thereby lowering the energy barrier for Fe(IV)=O formation. (4) The regular channels of MOFs can precisely guide the orientation of substrate approach to the active center through steric hindrance and regulate the subsequent transformation pathways of reaction intermediates, mimicking enzymatic catalysis. (5) Using the MOF as a basic framework, precise replication of the coordination microenvironment of the enzyme active center can be achieved via metal ion exchange and ligand substitution without destroying the framework, enabling the use of only O2to generate high-spin Fe(IV)=O for biomimetic catalysis. The synergistic action of the above mechanisms makes MOFs an ideal platform for currently regulating the generation of high-valent iron species.
Based on the limited existing work, this paper introduces the mechanisms and applications of MOFs in activating N2O, O2, and persulfate oxidants to generate high-valent iron species, revealing the critical role of MOF structural features (such as weak-field coordination, coordinatively unsaturated sites, and dynamic stretching) in regulating the formation and selective catalysis of high-valent iron species. Research indicates that through precise MOF crystal engineering, it is possible not only to mimic the catalytic behavior of natural metalloenzymes but also to achieve selective oxidation of inert C—H bonds and efficient degradation of pollutants, demonstrating broad potential in energy conversion, green synthesis, and environmental remediation. Although MOFs exhibit significant advantages in regulating the generation of high-valent iron species, this field is still in its infancy and faces several key limitations and challenges. (1) Fe(IV)=O intermediates have high reactivity and short lifespans, making them difficult to capture directly under ambient temperature and pressure; currently, they are mainly indirectly confirmed via in situ spectroscopy (such as Mössbauer spectroscopy, X-ray emission spectroscopy, and nuclear resonance vibrational spectroscopy) and theoretical calculations. However, most studies lack cross-validation using multiple techniques, and controversies remain regarding the true structure and evolution pathways of these intermediates. (2) Most iron-based MOFs lack chemical stability under neutral or alkaline pH conditions, prone to hydrolysis or ligand leaching, leading to the loss of active sites. (3) Currently reported MOF systems still face issues such as excessive dosing of oxidants and low yield of active species when activating O2or persulfate to generate high-valent iron species; in alkane hydroxylation reactions, target products (such as ethanol) are easily over-oxidized to aldehydes or acids or undergo coupling side reactions, making it difficult to achieve both high selectivity and high conversion simultaneously. (4) The scaled synthesis and practical application of MOFs face challenges. Existing research mostly employs laboratory-scale synthesis methods such as solvothermal and microwave techniques, resulting in low yields and poor batch stability; when loading MOFs onto macroporous supports or shaping them into continuous-flow reactors, how to maintain active site exposure and mass transfer efficiency still lacks systematic engineering solutions. (5) Key scientific issues, such as the electronic structure correlation between MOF structural characteristics and high-valent iron generation, the quantitative impact of pore confinement on the kinetics of radical intermediates, and the competitive mechanisms between multi-nuclear synergistic sites and isolated sites in reaction pathway selection, require deeper multi-scale theoretical calculations and in situ experiments for clarification. Future research should focus on the following directions: (1) Leveraging machine learning[49-51], to develop more efficient MOF systems capable of highly efficient activation and high-selectivity conversion of green oxidants such as O2, promoting their application in hydrocarbon functionalization and fine chemical synthesis; (2) deeply investigating the generation mechanisms and kinetic processes of high-valent iron species in MOFs, combining in situ spectroscopic techniques with theoretical calculations to reveal their catalytic behavior during dynamic structural evolution; (3) expanding the application of MOFs in water treatment by enhancing their catalytic activity and cyclic stability through dynamic structural design, thereby promoting their use in continuous-flow reactions and industrial water treatment. Furthermore, how to scale up the synthesis of stable MOF catalysts with dynamic stretching structures and achieve their long-term operation and regeneration in actual complex water environments remains a key issue that future research must prioritize overcoming.

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Funding

National Natural Science Foundation of China(22176012)

"Jiebang Guashuai" Project from Institute of Advanced Materials（BUCEA）(EFM202501)

National Natural Science Foundation of China(52370025)

Science and Technology Project of Beijing Municipal Ecology and Environment(BJST20250207)

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