Introduction

High-valent iron-oxo species (FeIV=O) are prevalent in metalloenzymes such as Rieske dioxygenase, cytochrome P450, playing a crucial role in various naturally biological processes for the selective C–H or C = C bond activation during the metabolites synthesis and xenobiotic degradation1,2,3,4. These metalloenzyme-catalyzed processes involve the formation of FeIV = O with a low-lying unoccupied antibonding orbital, which effectively transfer its axial oxo moiety to substrates with unpaired electrons5,6,7. These processes have inspired many metalloenzymes-like chemical syntheses via oxygen atom transfer of artificial FeIV = O, strongly depending on the efficient synthesis of highly active FeIV = O species. Although great efforts have been made in the synthesis of homogeneous FeIV = O species through mimicking the enzymatic iron centers, their fragile organic ligands disfavor the robust catalysis in an oxidized environment8,9,10, and their homogeneous nature also increases the difficulty in separation. Recently, well-tailored iron carriers, such as zero-valent iron (nZVI) and iron-based zeolites11,12, have been employed to synthesize surface FeIV = O species (≡FeIV=O) coordinated with simple inorganic ligands by using common oxidants (e.g., perhydrol, persulfate, peracetic acid) as the oxygen source13,14,15. Generally, these robust and heterogonous ≡FeIV = O possess an octahedral structure with four planar symmetric ligands in an Fe-O4 configuration, whose localized electron distribution results in their strong “acceptance–backdonation” interactions between the 3d orbitals of Fe and the 2p orbitals of axial oxo, preventing ≡FeIV = O from accepting electrons from substrates and thus slowing the subsequent oxygen atom transfer processes16.

This localized electron distribution around the axial Fe = O bond might be destroyed by breaking its planar symmetric coordination structure via substituting one or more planar atoms with more electronegative ones, which can weaken the strength of axial ligand through the electron transfer from the axial bond to the planar one17. In theory, this electron redistribution can build a strong local electric field around the central metal atom to enhance the electron delocalization around axial Fe = O bond, thus favoring the electron transfer process from the substrates to the central metal atom18,19, which was recently verified by the more efficient reduction of uranyl(VI) to uranium(IV) after partial removal of planar Cl ligand to disrupt the symmetry of planar atomic structure20.

Pyrite (FeS2), characterized by abundant surface ≡FeII sites and easily-substituted planar S ligands, is more suitable for planar asymmetric ≡FeIV = O synthesis than commonly-used iron carriers of strong Fe-O bonds21,22,23,24. Chlorite (ClO2−), with decent oxidation capacity, safe storage and less environmental risks than other oxidants (e.g., H2O2 and Peroxymonosulfate), is found to be a desirable oxidant in ≡FeIV = O synthesis with high selectivity11. In this study, we demonstrate the synthesis of planar asymmetric ≡FeIV = O with FeS2 and chlorite, given their reaction can simultaneously produce ≡FeIV = O on the surface of FeS2 and chlorine dioxide (ClO2)25,26, while the in-situ generated ClO2 might oxidize the planar coordinated sulfur atoms of symmetric ≡FeIV = O to substitute S by Cl27. The synthesis process was carefully checked by the Extended X-ray absorption fine structure (EXAFS), Mössbauer spectroscopy, and in-situ Raman spectra. The electrophilicity of axial Fe = O bond and its OAT performance were subsequently investigated with theoretical calculations, including electron spin density, electron localization function (ELF), density of states (DOS), and transfer energy barrier of oxygen atoms. Finally, planar asymmetric ≡FeIV = O was employed to trigger OAT reactions such as selective methane (CH4) conversion to methanol (CH3OH), triphenylphosphonate oxidation and styrene epoxidation, showcasing its potential for selective oxidation.

Results

Synthesis and characterization

The Fe-S coordination environment coupled with abundant surface FeII sites (≡FeII) on FeS2, as revealed by transmission electron microscopy, elemental mapping, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), favors the construction of PS-≡FeIV = O on FeS2 (Supplementary Figs. 1–3). First, we employed in-situ surface-enhanced Raman spectroscopy to investigate the molecular processes associated with ClO2– dissociation and subsequent PS-≡FeIV = O formation. The introduction of ClO2– into the FeS2-containing solution immediately resulted in the emergence and gradual increase of a characteristic Raman peak of the δ(H···Cl–O) bending mode (387.5 cm−1), suggesting that the terminal O of ClO2– could interact with neighboring H2O molecules to form hydrogen bonds (H···Cl–O), thereby significantly stretching the Cl–O bond for its subsequent cleavage11 (Fig. 1a). Along with the oxygen atom transfer from ClO2– to ≡FeII sites of FeS2, PS-≡FeIV = O was then formed in an octahedral structure with a planar symmetric Fe-S4 configuration, as reflected by the enhanced Raman peak at 920.7 cm−1 (Eq. 1, Fig. 1a and Supplementary Figs. 4 and 5) and the presence of a seven-split signal in electron paramagnetic resonance (EPR) spectra using 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) as the spin-trapping agent28 (Supplementary Fig. 6). The appearance of four peaks with the same 17 G interval in the EPR spectra indicated the simultaneous generation of ClO2 during the PS-≡FeIV = O formation29 (Fig. 1b), which was supported by the increasing absorbance of ClO2 at 350 nm along with the decreasing absorbance of ClO2− at 262 nm, indicative of the conversion of ClO2− to ClO2 (Fig. 1c). Regarding that ClO2 can be generated through two possible pathways including the activation of ClO2− via the electron transfer from ClO2− to ≡FeIV = O (Eq. 2) and the direct disproportionation of ClO2–26,30 (Eq. 3 and Supplementary Figs. 7 and 8), we then quantified the production of ClO2 by quenching ≡FeIV = O with dimethyl sulfoxide (DMSO), and found that the ClO2 generation rate was only reduced by ~10% (Supplementary Figs. 9 and 10), suggesting that ClO2 was mainly generated through the disproportionation of ClO2– and thereby ensuring the coexistence of PS-≡FeIV = O and ClO2 for subsequent PA-≡FeIV = O synthesis.

$$\equiv {{{{\rm{Fe}}}}}^{{{{\rm{II}}}}}+{{{{\rm{ClO}}}}}_{2}^{-}\to \equiv {{{{\rm{Fe}}}}}^{{{{\rm{IV}}}}}={{{\rm{O}}}}+{{{{\rm{ClO}}}}}^{-}$$
(1)
$$\equiv {{{{\rm{Fe}}}}}^{{{{\rm{IV}}}}}={{{\rm{O}}}}+{{{{\rm{ClO}}}}}_{2}^{-}\to \equiv {{{{\rm{Fe}}}}}^{{{{\rm{III}}}}}+{{{{\rm{ClO}}}}}_{2}$$
(2)
$${{{{\rm{H}}}}}^{+}+{{{{\rm{ClO}}}}}_{2}^{-}\to {{{{\rm{Cl}}}}}^{-}+{{{{\rm{ClO}}}}}_{2}$$
(3)
Fig. 1: Synthesis and characterization of PA-≡FeIV = O.
figure 1

a In-situ Raman spectra to investigate the formation of δ(H···Cl–O) and the generation process of ≡FeIV = O with FeS2 and ClO2−. b EPR spectra to determine ClO2 generated by FeS2 and ClO2– without any trapping agent. c UV–vis spectra to detect the transformation from ClO2– (262 nm) to ClO2 (350 nm). d XPS spectra of Cl 2p scan on R-FeS2-ClO2 and R-FeS2. e Raman spectra of the Fe-Cl bond and Fe-S bond signals in R-FeS2-ClO2, R-FeS2, original FeS2 and FeCl3. EXAFS spectra and corresponding fitting lines of (f) R-FeS2-ClO2 and g R-FeS2. h Illustration of the transformation process from Fe-S to Fe-Cl bond for PA-≡FeIV = O synthesis with ClO2 as the inducer. The blue ball refers to the Fe atom, the yellow ball refers to the S atom, the green ball refers to the Cl atom and the red ball refers to the O atom. Experiment conditions: [FeS2]0 = 1.0 g/L (if not specified), [ClO2−]0 = 1.0 mmol/L (if not specified), [Na2S2O3]0 = 1.0 mmol/L (if not specified).

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We interestingly found that the amount of chlorine-containing species in solution decreased from 1.0 mmol L−1 to 0.92 mmol L−1 after 2 h of reaction, indicative of partial chlorine binding onto the FeS2 surface (Supplementary Figs. 11–13). To deeply understand this process, we quenched ClO2 with sodium thiosulfate (Na2S2O3) or ascorbic acid (C6H8O6) utilizing their easy oxidation by ClO2 and slow reaction with ClO2–, and observed a sharp increase of SO42− concentration in the presence of ClO2, owing to the oxidation of some planar coordinated S atoms in PS-≡FeIV = O into aqueous SO42− by ClO2 (Eq. 4 and Supplementary Figs. 14–18). Subsequently, 34S isotope labeling further confirmed ClO2− induced S oxidation in FeS2 (Supplementary Fig. 19). In-situ attenuated total reflectance Fourier transform infrared spectrometer (ATR-FTIR) analysis displayed a left-shifted SO42– peak (1089 cm−1 for Fe34S2 compared to 1106 cm−1 for Fe32S2) upon ClO2 exposure, with signal disappearance post-ClO2 quenching, verifying FeS2 as the S source and underscoring the critical role of ClO2 (Supplementary Fig. 20). Aligning well with these findings, inductively coupled plasma-Mass Spectrometry detected 3.15 mg L−1 34S and 2.85 mg L−1 32S with ClO2, but negligible sulfur signals without ClO2 (Supplementary Fig. 21). Meanwhile, the appearance of a strong XPS signal of Fe-Cl bonds at 198.5 eV for the FeS2 reacted with ClO2 (R-FeS2-ClO2), was in sharp contrast to the ignorable signals of Fe-Cl bonds on the reacted FeS2 (R-FeS2) when ClO2 was quenched (Fig. 1d and Supplementary Fig. 22). Moreover, Raman shifts at 212 cm−1 and 279 cm−1 further evidenced the formation of Fe-Cl bonds on the R-FeS2-ClO2, different from the observed Fe-S bonds on the R-FeS2 (Fig. 1e and Supplementary Fig. 23).

$${{{\rm{Fe}}}}-{{{\rm{S}}}}+{{{{\rm{ClO}}}}}_{2}+{{{{\rm{H}}}}}_{2}{{{\rm{O}}}}\to {{{\rm{Fe}}}}-{{{\rm{Cl}}}}+{{{{\rm{SO}}}}}_{4}^{2-}+{{{{\rm{H}}}}}^{+}$$
(4)

We then checked the role of other Cl species (i.e., ClO2− and Cl–) in the synthesis of PA-≡FeIV = O. The Cl atom with a higher electrophilicity (0.855) in ClO2 was more positively charged than that of ClO2− (−0.153), and thus more powerful to oxidize the negatively charged S atom (Supplementary Figs. 24 and 25 and Supplementary Tables 1 and 2), consistent with the significantly lower LUMO orbital energy of ClO2 than that of ClO2– (Supplementary Figs. 26 and 27). The contribution of Cl– was excluded by the negligible Raman signal of Fe-Cl bonds on FeS2 reacted with Cl– for 2 h (Supplementary Fig. 28). Consequently, we conclude that ClO2 plays a crucial role in transforming the planar Fe-S bond in PS-≡FeIV = O to the Fe-Cl one and distorting the original symmetric Fe-S4 configuration, rather than ClO2– and Cl–.

Coordination and electronic structures

We thereafter characterized the coordination structures of PA-≡FeIV = O and PS-≡FeIV = O with the Fe K-edge EXAFS spectra and the wavelet transformation contour plots (Fig. 1f, g and Supplementary Fig. 29). In the fitted EXAFS spectra, the coordination numbers of Cl and S in the first shell of Fe in R-FeS2-ClO2 were calculated to be 1.2 ± 0.1 and 5.0 ± 0.1 respectively. In contrast, only a coordination number of 5.9 ± 0.1 was determined for S in the R-FeS2 (Supplementary Table 3). These fitting results suggested that one of the four planar Fe-S bonds in PS-≡FeIV = O was transformed to the Fe-Cl bond, generating a planar asymmetric Fe-Cl1S3 configuration in PA-≡FeIV = O (Fig. 1h, Supplementary Figs. 30–36 and Supplementary Table 4). Different from the planar symmetric Fe-S4 configuration with uniform electron distribution, this planar asymmetric Fe-Cl1S3 configuration might disrupt the original electron structure due to the stronger electron-withdrawing ability of Cl, as evidenced by the electrostatic potential analysis of more negatively charged Cl (Fig. 2a). Therefore, we investigated the electronic structures of PA-≡FeIV = O and PS-≡FeIV = O, and found that the Fe K-edge X-ray absorption near edge structure (XANES) spectra of R-FeS2-ClO2 experienced a right-shift of ~0.2 eV in comparison with pristine FeS2 and R-FeS2, indicative of a slightly increased Fe valence state in PA-≡FeIV = O (Fig. 2b and Supplementary Fig. 37), as supported by the left-shift in the Fe 2p XPS of R-FeS2-ClO231,32 (Fig. 2c). To further clarify the differences of electronic properties between PA-≡FeIV = O and PS-≡FeIV = O, we measured their 57Fe Mössbauer spectra, where the isomer shift (δ) reflects the electron density around the 57Fe nucleus in PA-≡FeIV = O or PS-≡FeIV = O33 (Supplementary Table 5 and Supplementary Fig. 38). Although the fitted parameters suggested both PS-≡FeIV = O and PA-≡FeIV = O possessed the high valence state and high spin state (S = 2), the δ value (0.05 mm s−1) in the doublet of PA-≡FeIV = O was much smaller than that of PS-≡FeIV = O (0.08 mm s−1), confirming the decreased electron density around the Fe center of PA-≡FeIV = O (Fig. 2d), because Fe atom of PA-≡FeIV = O transferred more electrons to its ligands due to the higher electronegativity of Cl (χ = 3.16) than that of S (χ = 2.58)34,35,36 (Fig. 2a), as revealed by the higher work function of PA-≡FeIV = O (4.744 eV) than that of the PS-≡FeIV = O (4.695 eV) (Fig. 2e). This transfer of more electrons from Fe center to adjacent ligands led to the downshift of the Fermi level of FeIV = O in PA-≡FeIV = O, which might facilitate its acceptance of electrons from substrates to boost the oxygen atom transfer process.

Fig. 2: Comparison of coordination and electronic structures between PA-≡FeIV = O and PS-≡FeIV = O.
figure 2

a Electrostatic potential in Fe, O, S, and Cl atoms in PA-≡FeIV = O and PS-≡FeIV = O. The blue ball refers to the Fe atom, the yellow ball refers to the S atom, the green ball refers to the Cl atom and the red ball refers to the O atom. b Fe K-edge XANES spectra of R-FeS2-ClO2, R-FeS2, original FeS2 and FeCl3. c XPS spectra on Fe 2p scan of R-FeS2-ClO2 and R-FeS2. d Mössbauer spectra to differentiate the electronic structure of PA-≡FeIV = O and PS-≡FeIV = O. e Schematic illustration of the work functions of PA-≡FeIV = O and PS-≡FeIV = O. Ef represents the Fermi level. Experiment conditions: [FeS2]0 = 1.0 g/L (if not specified), [ClO2–]0 = 1.0 mmol/L (if not specified), [Na2S2O3]0 = 1.0 mmol/L (if not specified).

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Theoretical investigations

We thus carried out theoretical calculations to further investigate the oxygen atom transfer capacity of PA-≡FeIV = O and PS-≡FeIV = O. When one planar S atom in PS-≡FeIV = O was replaced by Cl, the originally localized electrons around the axial Fe = O were induced to redistribute among the adjacent ligand atoms, which significantly enhanced the electron spin density of the ligand atoms surrounding the axial Fe = O in PA-≡FeIV = O, despite negligible changes in the electron spin state of Fe itself (Fig. 3a). In this case, the delocalized electrons around PA-≡FeIV = O facilitated the electron transfer across different atoms, thus enhancing oxygen atom transfer process. To further elucidate the enhanced electron-withdrawing and the corresponding oxygen atom transfer processes in PA-≡FeIV = O, we then analyzed the ELF cross-sections near the Fe atom and its ligands, and found that the Fe and O atoms in PA-≡FeIV = O respectively carried a more positive charge of +1.829 e and a more negative charge of −0.820 e than those of PS-≡FeIV = O (+1.264 e, and −0.699 e)37,38,39 (Fig. 3b). Obviously, this greater electronic delocalization and stronger ionic characteristic significantly weakened the axial Fe = O bond in PA-≡FeIV = O, as reflected by the lower oxygen release energy barriers of PA-≡FeIV = O (0.156 eV and 3.25 eV) than PS-≡FeIV = O (0.576 eV and 3.41 eV) in the two electron transfer steps of oxygen transfer process (Fig. 3c). The calculated delocalization in Fe = O bond and the lowered oxygen transfer energy barrier were further corroborated by in-situ Raman spectroscopy. The progressive right-shift of the Fe = O stretching frequency from 920.7 cm−1 (PS-≡FeIV = O) to 929.2 cm−1 (PA-≡FeIV = O) directly reflected the weakening of the Fe = O bond due to electron delocalization (Supplementary Fig. 39). Furthermore, DOS analysis revealed distinct electronic configurations between PA-≡FeIV = O and PS-≡FeIV = O (Fig. 3d, e). PA-≡FeIV = O exhibited continuous Fe states near the Fermi level, indicating enhanced electron delocalization along Fe = O, which reduced the bandgap and facilitated efficient electron transfer. In contrast, PS-≡FeIV = O possessed more discrete states with a larger bandgap, reflecting localized electrons and lower reactivity. This divergence stemmed from ligand field effects that stronger electronic confinement in PS-≡FeIV = O stabilized uniform electron distribution, which is unfavorable for delocalization. Overall, the planar asymmetric structure of PA-≡FeIV = O could delocalize and weaken its axial Fe = O bond to enhance its oxygen atom transfer ability.

Fig. 3: Theoretical analysis on the enhanced oxygen atom transfer capacity of PA-≡FeIV = O.
figure 3

a Comparison of electron spin density between PA-≡FeIV = O and PS-≡FeIV = O. b Electron localization function (ELF) analysis of PA-≡FeIV = O and PS-≡FeIV = O. The blue ball refers to Fe atom, the yellow ball refers to S atom, the green ball refers to Cl atom and the red ball refers to the O atom. c Differences of oxygen atom transfer energy barrier in the two steps between PA-≡FeIV = O and PS-≡FeIV = O. d Density of states analysis on PS-≡FeIV = O. e Density of states analysis on PA-≡FeIV = O.

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Oxygen atom transfer reactions

We first chose oxidation of CH4 to investigate the influence of planar coordination environment on the oxygen atom transfer efficiency of ≡FeIV = O species, regarding that direct CH4 oxidation to CH3OH offers a promising route to access various fundamental feedstocks for the chemical industry, but the high C − H dissociation energy (~439 kJ mol−1) and the robust non-polar structure of CH4 significantly challenges its efficient and selective conversion40,41,42. Although CH4 can be exclusively oxidized to CH3OH by ≡FeIV = O under ambient conditions, the sluggish oxygen transfer of transitional ≡FeIV = O resulted in low CH3OH yield rate43,44. As expected, with a selectivity exceeding 95%, the PA-≡FeIV = O could efficiently oxidize CH4 to CH3OH with a yield rate of 312.67 μmol h−1 gpyrite−1, far superior to the PS-≡FeIV = O counterpart (188.42 μmol h−1 gpyrite−1) (Fig. 4a and Supplementary Figs. 40–42) as well as most records of traditional methods including light irradiation, heating, pressurizing and multiple combined one (Fig. 4b and Supplementary Table 6). Under the same reaction conditions, CH3OH was not detected during the control experiments with using FeS2, ClO2− or ClO2 alone, demonstrating the indispensable contribution of ≡FeIV = O to the CH4 oxidation (Supplementary Fig. 40).

Fig. 4: Oxygen atom transfer reactions.
figure 4

a Comparison of CH3OH yield by PA-≡FeIV = O and PS-≡FeIV = O. b Comparison of CH3OH yield rates in this PA-≡FeIV = O driven system with those in light- or thermal-driven CH4 activation systems. c Illustration of CH4 oxidation process by ≡FeIV = O. d EPR spectra to identify the ·CH3 intermediate after H abstraction by PA-≡FeIV = O and PS-≡FeIV = O. e CH4 consumption and CH3OH generation by PA-≡FeIV = O through in-situ Fourier Transform infrared spectroscopy. f Energy barriers of CH4 conversion process by PA-≡FeIV = O and PS-≡FeIV = O. g MPS and h Ph3P oxidation efficiencies by PA-≡FeIV = O and PS-≡FeIV = O in comparison. i Comparison of efficiencies over PA-≡FeIV = O and PS-≡FeIV = O for styrene epoxidation to styrene oxide. Experiment conditions: [FeS2]0 = 0.02 g (for CH4 oxidation), [FeS2]0 = 0.1 g (for other reactions), [ClO2–]0 = 1.0 mmol L−1 (if not specified), aqueous CH4-containing solution volume = 20 mL, other solution volume = 100 mL. The error bars represent the standard deviation derived from two repeated experiments.

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To give deep insights into the superior oxygen atom transfer performance of PA-≡FeIV = O, we then investigated the mechanism of selective CH4 oxidation at the molecular scale. As illustrated by the widely accepted CH4 oxidation mechanism mediated by ≡FeIV = O (Fig. 4c), the C–H bond of CH4 is significantly stretched upon its adsorption onto ≡FeIV = O site, leading to its cleavage into a crucial radical intermediate (•CH3), along with the formation of ≡FeIII–OH through an H-abstraction process (Supplementary Figs. 43 and 44)45,46. Given that the formation of •CH3 is regarded as the rate-determining step for the CH4 oxidation, we thus carried out EPR experiments with DMPO as a spin-trapping agent to dynamically monitor the generation of •CH3 in the CH4-saturated aqueous solutions with PA-≡FeIV = O and PS-≡FeIV = O. As expected, a stronger DMPO–•CH3 signal was detected in the PA-≡FeIV = O system (Fig. 4d and Supplementary Fig. 45), verifying the enhanced electron-accepting ability of PA-≡FeIV = O, and also consistent with the results of XPS and Mössbauer spectra as well as the corresponding theoretical calculations. This •CH3 then captures the axial OH of ≡FeIII–OH via an oxygen-rebound step for the selective CH3OH formation rather than the C–C couple to generate CH3-CH3 as a result of the strong adsorption interactions between •CH3 and ≡FeIII–OH (Supplementary Fig. 46). The enhanced oxygen transfer ability of PA-≡FeIV = O was further confirmed by the stronger C–O bonds vibrations of CH3OH in the in-situ ATR-FTIR spectra with PA-≡FeIV = O, accounting for its higher CH3OH yields (Fig. 4e and Supplementary Fig. 47), as evidenced by the much lower transition state energy barrier (0.205 eV) of PA-≡FeIV = O than that of PS-≡FeIV = O (1.050 eV) (Fig. 4f).

Besides selective CH4 oxidation, PA-≡FeIV = O exhibited better oxygen atom transfer performance than PS-≡FeIV = O for methyl phenyl sulfide (MPS) oxidation, triphenylphosphine (Ph3P) oxidation and styrene epoxidation. The MPS to MPSO oxidation rate of PA-≡FeIV = O reached 0.30 mmol L−1 h−1 (Fig. 4g and Supplementary Fig. 48), much higher than that of PS-≡FeIV = O ( ~ 0.18 mmol L−1 h−1). Impressively, PA-≡FeIV = O stoichiometrically converted 0.5 mmol L−1 Ph3P to triphenylphosphine oxide (Ph3PO) in 120 min with an impressive conversion rate of 97.5%, which was merely ~70% for PS-≡FeIV = O (Fig. 4h and Supplementary Fig. 48). Furthermore, PA-≡FeIV = O enabled the epoxidation of styrene to value-added styrene oxide with a yield rate of 0.18 mmol L−1 h−1, much better than PS-≡FeIV = O (0.10 mmol L−1 h−1) (Fig. 4i and Supplementary Fig. 48). Promisingly, the efficiencies of MPS, Ph3P and styrene oxidation by PA-≡FeIV = O surpassed many previous reports, indicating the prominent advantages of PA-≡FeIV = O in selective oxidation reactions (Supplementary Table 7). These results showcase the great potential of PA-≡FeIV = O on selective synthesis of various value-added chemicals via a feasible oxygen atom transfer process.

Discussion

In conclusion, we have demonstrated the synthesis of planar asymmetric ≡FeIV = O confined on pyrite with chlorite as the oxidant, where the concomitant ClO2 was able to oxidize a planar Fe-S bond of ≡FeIV = O to Fe-Cl, along with conversion of planar symmetric Fe-S4 configuration into planar asymmetric Fe-Cl1S3 counterpart, as substantiated by various spectroscopic techniques including XAFS, Mössbauer and XPS spectra. Theoretical calculations revealed that the planar asymmetry structure formation delocalized the electron distribution around ≡FeIV = O because of the stronger electron-withdrawing capacity of Cl, and then weakened the orbital overlapping of axial Fe = O bond, thus reducing the oxygen atom transfer energy barrier and facilitating the electron transfer from substrates to PA-≡FeIV = O. This novel PA-≡FeIV = O possessed superior reactivity towards selective methane oxidation, styrene epoxidation, and sulfides/phosphides oxidation, showcasing high potential for efficient high-valued chemical production via its superior oxygen transfer ability.

Methods

Chemicals

Chemicals purchased for this research were of at least analytical grade without further operation and the information of them was listed in Supplementary Method 1.

Experimental procedures

The PA-≡FeIV = O synthesis with FeS2 and ClO2− was carried out in 100-mL conical flasks in a shaker (180 rpm) under ambient pressure and at room temperature (25 °C) (Supplementary Fig. 49). To start the reaction, FeS2 (1.0 g L−1) and ClO2− (0.1 to 1.0 mmol L−1) were introduced into the flask with 100 mL deionized water. In the quenching tests, 10.0 mmol L−1 DMSO or 1.0 mmol L−1 Na2S2O3 was first mixed with the deionized water before reaction. The reacted FeS2 underwent three steps of filtration, freeze-drying, and vacuum preservation for characterizations including Raman, XPS, and XAFS. (Supplementary Method 2).

CH4-to-CH3OH conversion experiments were conducted in a well-designed reactor (300 mL, Supplementary Fig. 50). Initially, the reactor was charged with 20 mg of FeS2, then it was sealed and purged with CH4 gas (containing 2%, Ar as a balance gas) for a duration fof 30 min to purge out the air and make CH4 saturated in the reactor. Subsequently, the deionized water saturated with CH4 (purged with CH4 for 30 min) was introduced into the reactor and combined with FeS2. To trigger the reaction, ClO2− was injected into the reactor, followed by placing the reactor in a shaker operating at 60 rpm. This rotating speed was selected to endure the sufficient contact between CH4 and ≡FeIV = O while prevent destroying the adsorption of C–H bond on the ≡FeIV = O. Of note, all reactions were conducted at room temperature (25 °C). For the experiment for PS-≡FeIV = O induced CH4 oxidation, scavenger of ClO2 was added into the solution before ClO2−. MPS and Ph3P oxidation and styrene epoxidation experiments were performed in 100-mL conical flasks in a shaker (180 rpm) with FeS2 and ClO2– dosed in sequence.

Analytical methods and spectroscopic characterizations

The transformation from ClO2− to ClO2 was verified by the UV–vis spectrophotometer (UV-2550, Shimadzu, Japan). The quantification of ClO2 was realized through a DPD UV–vis method (UV-2550, Shimadzu, Japan, Supplementary Method 3). Except for ClO2, the concentrations of chlorine-containing species were detected by high-performance ion chromatography (Dionex ICS-900, Thermo). ≡FeIV = O and •CH3 were captured by EPR (Bruker EMXnano, German, Supplementary Method 4) with DMPO as the trapping agent. The spin state and electronic structure of PA-≡FeIV = O and PS-≡FeIV = O were recognized and compared by 57Fe Mössbauer measurements (Wissel MS-500, Germany, Supplementary Method 5). The concentrations of MPS, Ph3P, styrene and their corresponding oxidation products were detected using high-performance liquid chromatography (Dionex Ultimate 3000, Thermo, USA, Supplementary Method 6). The methane-to-methanol conversion rates were examined by gas chromatography (GC) equipped with a GC capillary column (30 m × 0.32 mm × 0.5 μm) and a flame ionization detector. The generation of ≡FeIV = O from FeS2 and ClO2− was checked by in-situ surface enhanced Raman spectroscopy (Renishaw inVia Qontor, UK, Supplementary Method 7). The signals of CH4 consumption and CH3OH generation by PA-≡FeIV = O or PS-≡FeIV = O were recorded through in-situ ATR-FTIR (Nicolet iS50, Thermo, USA, Supplementary Method 8).

Theoretical calculations

The theoretical calculations were carried out using Vienna Ab initio Simulation Package and Gaussian 09 software based on the DFT (Supplementary Method 9).