Planar asymmetric surface FeIV = O synthesis with pyrite and chlorite for efficient oxygen atom transfer reactions

Abstract

Surface high-valent iron-oxo species (≡Fe^IV=O) are reliable and green oxygen atom transfer reagents, but the ability is seriously inhibited by the maximal orbital overlap of axial Fe = O double bond in a symmetric planar coordination environment. Herein, we report the synthesis of planar asymmetric surface Fe^IV = O (PA-≡Fe^IV = O) on pyrite using chlorite as the oxidant, where the in-situ generated ClO₂ can transform a planar Fe-S bond to Fe-Cl by oxidizing and subsequently substituting planar sulfur atoms. Different from planar symmetric surface Fe^IV = O (PS-≡Fe^IV = O) with electron localization around axial Fe = O, PA-≡Fe^IV = O delocalizes electrons among Fe, axial oxo moiety and its planar ligands owing to the stronger electron-withdrawing capacity of Cl, which effectively weakens the orbital overlap of axial Fe = O bonding and thus facilitates the rapid electron transfer from the substrates to the unoccupied antibonding orbital of PA-≡Fe^IV = O, realizing more efficient oxygen atom transfer oxidation of methane, methyl phenyl sulfide, triphenylphosphonate and styrene than PS-≡Fe^IV = O. This study offers a facile approach for the synthesis of planar asymmetric surface Fe^IV = O, and also underscores the importance of planar coordination environment of high-valent metal-oxo species in the oxygen atom transfer reactions.

Fig. 1. Synthesis and characterization of PA-≡Fe^IV = O.

a In-situ Raman spectra to investigate the formation of δ(H···Cl–O) and the generation process of ≡Fe^IV = 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-≡Fe^IV = 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).

Fig. 2. Comparison of coordination and electronic structures between PA-≡Fe^IV = O and PS-≡Fe^IV = O.

a Electrostatic potential in Fe, O, S, and Cl atoms in PA-≡Fe^IV = O and PS-≡Fe^IV = 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-≡Fe^IV = O and PS-≡Fe^IV = O. e Schematic illustration of the work functions of PA-≡Fe^IV = O and PS-≡Fe^IV = O. E_f 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).

Fig. 3. Theoretical analysis on the enhanced oxygen atom transfer capacity of PA-≡Fe^IV = O.

a Comparison of electron spin density between PA-≡Fe^IV = O and PS-≡Fe^IV = O. b Electron localization function (ELF) analysis of PA-≡Fe^IV = O and PS-≡Fe^IV = 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-≡Fe^IV = O and PS-≡Fe^IV = O. d Density of states analysis on PS-≡Fe^IV = O. e Density of states analysis on PA-≡Fe^IV = O.

Fig. 4. Oxygen atom transfer reactions.

a Comparison of CH₃OH yield by PA-≡Fe^IV = O and PS-≡Fe^IV = O. b Comparison of CH₃OH yield rates in this PA-≡Fe^IV = O driven system with those in light- or thermal-driven CH₄ activation systems. c Illustration of CH₄ oxidation process by ≡Fe^IV = O. d EPR spectra to identify the ·CH₃ intermediate after H abstraction by PA-≡Fe^IV = O and PS-≡Fe^IV = O. e CH₄ consumption and CH₃OH generation by PA-≡Fe^IV = O through in-situ Fourier Transform infrared spectroscopy. f Energy barriers of CH₄ conversion process by PA-≡Fe^IV = O and PS-≡Fe^IV = O. g MPS and h Ph₃P oxidation efficiencies by PA-≡Fe^IV = O and PS-≡Fe^IV = O in comparison. i Comparison of efficiencies over PA-≡Fe^IV = O and PS-≡Fe^IV = 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.