# Non-Heme Tri-Iron(II) Complex as a Precursor for Dioxygen Activation and Oxygen Reduction Reaction Catalysis

Lili Sun, Javier Gutierrez, David Gatineau, Romain Sanahuges, Mathieu Curtil, Christian Philouze, Sebastian Dechert, Serhiy Demeshko, Timothy Jackson, Franc Meyer, et al.

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Lili Sun, Javier Gutierrez, David Gatineau, Romain Sanahuges, Mathieu Curtil, et al. Non-Heme Tri-Iron(II) Complex as a Precursor for Dioxygen Activation and Oxygen Reduction Reaction Catalysis. Inorganic Chemistry, 2025, 64 (32), pp.16309-16322. 10.1021/acs.inorgchem.5c00198 . hal-05320059

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Non-Heme Tri-Iron(II) Complex as a Precursor for Dioxygen Activation and Oxygen Reduction Reaction Catalysis

<table><tr><td>Journal:</td><td>Inorganic Chemistry</td></tr><tr><td>Manuscript ID</td><td>ic-2025-00198v.R3</td></tr><tr><td>Manuscript Type:</td><td>Article</td></tr><tr><td>Date Submitted by the Author:</td><td>n/a</td></tr><tr><td>Complete List of Authors:</td><td>Sun, Lili; Univ. Grenoble Alpes, CNRS<br>Gutierrez, Javier; Université Grenoble Alpes, CIRE<br>Gatineau, David; Département de Chimie Moléculaire<br>Sanahuges, Romain; Université Grenoble Alpes, Département de Chimie Moléculaire<br>Curtil, Mathieu; Département de Chimie Moléculaire<br>Philouze, Christian; Universite Grenoble Alpes, DCM<br>Dechert, Sebastian; Georg-August-Universitat Göttingen, Institute of Inorganic Chemistry<br>Demeshko, Serhiy; Institut für Anorganische Chemie, Georg-August-Universität Göttingen<br>Jackson, Timothy; University of Kansas, Chemistry<br>Meyer, Franc; Georg-August-Universitat Göttingen, Institute of Inorganic Chemistry; Georg-August-Universität Göttingen, Institute of Inorganic Chemistry<br>Duboc, Carole; Univ. Grenoble Alpes, CNRS ;<br>Gennari, Marcello; Universite Grenoble Alpes, Département de Chimie Moléculaire</td></tr></table>
# Non-Heme Tri-Iron(II) Complex as a Precursor for Dioxygen Activation and Oxygen Reduction Reaction Catalysis

Lili Sun,ª Javier Gutierrez,ª,b David Gatineau,ª Romain Sanahuges,ª Mathieu Curtil,ª Christian Philouze,ª Sebastian Dechert,ᶜ Serhiy Demeshko,ᶜ Timothy A. Jackson,ᵇ Franc Meyer,ᶜ Carole Duboc,ª,\* and Marcello Gennariª,\*

ªUniv. Grenoble Alpes, CNRS UMR 5250, DCM, F-38000 Grenoble, France

ᵇThe University of Kansas, Department of Chemistry and Center for Environmentally Beneficial Catalysis, Lawrence, Kansas 66045, United States

ᶜUniversity of Göttingen, Institute of Inorganic Chemistry, Tammannstrasse 4, D-37077 Göttingen, Germany

marcello.gennari@univ-grenoble-alpes.fr, carole.duboc@univ-grenoble-alpes.fr

## Abstract

Non-heme iron catalysts are attractive for bio-inspired aerobic oxidations and oxygenations, as well as for the Oxygen Reduction Reaction (ORR), although their potential in ORR remains largely underexploited. This study presents a rare example of such a system, displaying O₂ activation activity and ORR catalytic selectivity towards H₂O generation. The triiron(II) complex [FeII*L]₃(BF₄)₃, where L⁻ is a thiolate-containing N3S donor bis(2-pyridylmethyl)amine derivative, was fully characterized in the solid state and in MeCN solution, where it dissociates into the mononuclear solvent adduct [FeL(MeCN)]⁺. Upon exposure to O₂ in an acetonitrile solution, [FeL(MeCN)]⁺ generates a µ-oxo diiron(III), that further evolves to an adduct featuring an original {Fe₄OF₅}³⁺ core. Besides, a µ-oxo, µ-peroxo diiron(III) was identified by ESI-mass spectrometry as a minor species, the origin of the oxygen atoms being from O₂. In the presence of 2,6-lutidinium tetrafluoroborate (proton source), and octamethylferrocene (reducing agent), [FeL(MeCN)]⁺ catalyzes the ORR with exclusive production of H₂O. This system is finally discussed with respect to a few other reported non-heme Fe-based ORR catalysts operating under chemical conditions.
Introduction

The reductive activation of dioxygen by transition-metal complexes plays a central role in
various essential biological and industrial processes. This crucial step not only triggers the
aerobic oxidation and oxygenation of small substrates, utilizing molecular oxygen as an
environmentally friendly oxidant in organic synthesis, but also initiates the proton-assisted O₂
reduction. The two-electron Oxygen Reduction Reaction (ORR) results in the production of the
chemically versatile hydrogen peroxide, while the four-electron ORR is critical for sustaining
life through aerobic respiration and powering coupled electrochemical processes in fuel cells.

In the domain of both aerobic oxidations and ORR,¹,² the challenge is to combine catalytic efficiency and selectivity with environmental, energetic, and cost sustainability. From this perspective, one strategy is to design molecular catalysts capable of activating and reducing O₂ at well-defined metal sites. More specifically, homogeneous catalysis based on 3d-metal ions emerges as a promising strategy for establishing structure-activity correlations, which are essential for identifying the key features governing reactivity.²,³ This approach relies on the fine-tuning of multiple factors to optimize catalytic performance, such as ligand design considering both steric and electronic properties, as well as the second coordination sphere. Additionally, intrinsic properties of the transition metal ion, including its nature, oxidation state, spin state, and coordination geometry, also play central roles in modulating efficiency and selectivity.

In the context of oxidation and oxygenation reactions,⁴⁻¹⁰ the strategy was mostly driven by the search to mimic the active sites of a plethora of non-heme iron enzymes,¹¹ including monooxygenases, dioxygenases, and hydroxylases, all of which initiate their unique activity by O₂ activation as a common step.¹²⁻¹⁴ As for ORR catalysts, only a handful of molecular non-heme iron complexes have been reported so far (Scheme 1). The first example is a diiron(II) complex reported in 2019 by some of our groups ([Fe₂SH]⁺ in Scheme 1), supported by a bpy-N2S2 thiolate-rich ligand.¹⁵ [Fe₂SH]⁺ exhibits a selectivity switch in ORR depending on the mode of electron delivery: H₂O₂ is primarily produced using a chemical reducing agent, while H₂O is mainly generated under electrochemical conditions. More recently, a few other non-heme catalysts have emerged, supported by N/O-donor ligands, including polypyridyl-carboxylates/phenol(ate)s (giving the FeN3O· and [FeN4Ox]2⁺ complexes shown in Scheme 1),¹⁶⁻¹⁸ tetraamido¹⁹ and bispyridine-dioxime²⁰ macrocycles (giving the [FeN⁴]⁻ and [FeN⁴·]²⁺
complexes, respectively). Most of these catalysts are selective for 4H+/4e⁻ ORR using a chemical reducing agent (see Table 1) and/or under electrochemical conditions. For FeN3O(c'),16, 17 H₂O₂ was identified as an intermediate towards H₂O production via a 2H+/2e⁻+2H+/2e⁻ mechanism, while in the case of [FeN4Ox]²⁺ a variable amount of H₂O₂ (≤ 47% with a reducing agent) was detected as a function of the ligand substituents.

Previous non-heme ORR catalysts

<figure><img src="image_2.png" /><figcaption>$[Fe_2^{SH}]^+$</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>$[Fe^{N4}]^-$</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>$[Fe^{N4'}]^{2+}$</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>$Fe^{N3O}$</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>$Fe^{N3O'}$</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>$[Fe^{N4Ox}]^{2+}$</figcaption></figure>

**Scheme 1.** Previously described non-heme iron homogeneous ORR catalysts (R = H, Quinol, R' = H, OH).

In the quest for novel non-heme ORR catalysts with enhanced efficiency and selectivity, thiolate-rich metal complexes provide an appealing strategy since thiolates are known to promote metal-mediated O₂ activation, as observed in both biological²¹, ²² and synthetic systems,¹⁵, ²³⁻³¹ including for ORR.¹⁵,³² In parallel, the bis(2-pyridylmethyl)amine (bpa) platform is well-established in the field of aerobic oxidation with iron complexes³³⁻³⁸ and was previously extended to NₓS-type ligands containing either thiolates or disulfides. N₃S-bpa ligands were explored in the coordination chemistry of various 3d metals, including Cu,³⁹⁻⁴¹ Ni,⁴² Co,⁴³ Fe,⁴⁴ and Mo,⁴⁵ although O₂ reactivity studies have remained limited to one copper system.⁴⁰ Regarding N₄S-bpa ligands, they provided access to original metal-oxygen adducts with Fe⁴⁶ and Mn.²³, ²⁴
While the present study was in progress, a mononuclear Fe<sup>II</sup>-thiolate complex supported by the tetradentate N<sub>3</sub>S-donor ligand L<sup>-</sup> based on the bpa platform (Scheme 2, R = H) and featuring a Br<sup>-</sup> axial ligand ([LFeBr]) was reported.<sup>27</sup> This complex is able to activate O<sub>2</sub> to form a Fe<sup>III</sup>-hydroxo species, while a related complex leads to the stabilization of a mononuclear superoxide complex (Scheme 2, [L<sup>NH</sup>FeBr]).<sup>28</sup> However, no ORR catalytic was observed with either of these complexes.

We now describe the O₂ reactivity and ORR catalysis of the parent triiron(II) complex [Fe<sup>II</sup>L]₃<sup>3+</sup> ([Fe₃]<sup>3+</sup>, Scheme 2), formed in the absence of halide co-ligands. In MeCN, [Fe₃]<sup>3+</sup> reacts with O₂ under non-catalytic conditions to afford a µ-oxo diiron(III) complex. Under catalytic conditions using a chemical reducing agent, [Fe₃]<sup>3+</sup> is active for selective 4H<sup>+</sup>/4e<sup>-</sup> ORR. Notably, the ORR selectivity observed with [Fe₃]<sup>3+</sup> differs from that of the previous Fe-based thiolate-supported [Fe₂<sup>SH</sup>]<sup>+</sup> catalyst, which produces H₂O₂ in a selective manner under identical conditions.

<figure><img src="image_8.png" /><figcaption>**Scheme 2.** Family of O₂-reactive non-heme thiolate complexes reported previously<sup>27, 28</sup> (left) and in this work (right).</figcaption></figure>

## Results

**Synthesis and solid-state properties of [Fe<sup>II</sup>L]₃(BF₄)₃.** The tetradentate N₃S-donor ligand L⁻, based on the bis(2-pyridylmethyl)amine (bpa) platform, was synthesized in its air-stable acetyl-protected form, L<sup>Ac</sup> (Scheme 3). The one-step synthesis of L<sup>Ac</sup> involves the nucleophilic substitution of the bis-picolylamine with the electrophilic 2,2-dimethylthiirane, as recently reported,<sup>27</sup> followed by the in-situ protection of the thiolate group using an acetyl group. After the saponification of L<sup>Ac</sup>, the trinuclear iron(II) complex, [Fe<sup>II</sup>L]₃(BF₄)₃ ([Fe₃](BF₄)₃, see below), is obtained by reacting the in-situ generated L⁻ with Fe(BF₄)₂·6H₂O in THF-MeOH under an inert atmosphere. As expected, the addition of an excess (40 equiv. per Fe) of n-
Bu₄NBr to [Fe₃](BF₄)₃ in CD₃CN leads to the quantitative formation of previously reported
[LFeIIBr] (Figure S6).²⁷

<figure><img src="image_9.png" /></figure>

sub-stoichiometric amount

**Scheme 3.** Synthesis of L<sup>Ac</sup> and [Fe<sub>3</sub>]<sup>3+</sup> and reactivity of [Fe<sub>3</sub>]<sup>3+</sup> with dioxygen.

Single crystals of [Fe₃](BF₄)₃ were successfully obtained, enabling its structural characterization through X-ray diffraction. The structure of [Fe₃](BF₄)₃ reveals its trimetallic nature, as depicted in Figure 1a with additional crystallographic data in Tables S1-S2.
<figure><img src="image_10.png" /><figcaption>**Figure 1.** Molecular structures of (a) [Fe₃](BF₄)₃·2MeCN and (b) [Fe₂O](BPh₄)₂·1.45MeCN·2Et₂O, determined by single-crystal X-ray diffraction. The thermal ellipsoids of the metal cores are drawn at 30% probability level. All hydrogen atoms, counterions and solvent molecules are omitted for clarity.</figcaption></figure>

[Fe₃](BF₄)₃ exhibits an uncommon,⁴⁷ nearly planar central hexagonal {Fe₃(SR)₃}

metallacycle, as indicated by the distances between the six Fe and S centers and the averaged {Fe(SR)₃} plane, ranging from 0.047 to 0.210 Å. A pseudo-C3 symmetry axis passes orthogonally through the center of the metallacycle, while a pseudo-symmetry plane encloses the {FeS}₃ mean plane (Figure S1). In [Fe₃](BF₄)₃, each Fe¹¹ center adopts a distorted trigonal bipyramidal geometry, surrounded by an N3S2 donor set, with the thiolates acting as bridging ligands between two Fe units. A similar 5-coordinated geometry was previously observed in [LFe¹¹Br],²⁷ and in Fe¹¹-thiolate complexes supported by either a pentadentate bpa-N4S ligand⁴⁶ or a tetradentate bpy-N2S2 ligand.¹⁵ In the case of the bpy-N2S2 ligand ([Fe₂SH]⁺ in Scheme 1), the fifth ligand arises from a bridging thiolate, resulting in a dinuclear Fe¹¹ complex. In [Fe₃](BF₄)₃, the geometry around the Fe ions deviates from an ideal trigonal bipyramidal arrangement as evidenced by the τ₅ values:⁴⁵ 0.83 for Fe1, 0.74 for Fe2, and 0.67 for Fe3. The Fe-S distances range from 2.328(2) Å to 2.395(2) Å, consistent with bridging Fe-thiolates.¹⁵ No significant Fe···Fe interactions are observed in [Fe₃](BF₄)₃, as the shorter Fe···Fe distance is approximately 4.363 Å.
**Solution and redox properties of [Fe<sup>II</sup>L]<sub>3</sub><sup>3+</sup>.** To evaluate whether the trinuclear structure of [Fe<sub>3</sub>]<sup>3+</sup> is retained in solution, we investigated its behavior by ESI mass spectrometry (ESI-MS) and <sup>1</sup>H NMR spectroscopy.

The ESI-MS spectrum of [Fe<sub>3</sub>]<sup>3+</sup> recorded in MeCN displays a prominent peak at *m/z* = 383, assigned to a mononuclear [FeL(MeCN)]<sup>+</sup> adduct. Additional minor peaks are observed at *m/z* = 1200, corresponding to [FeL]<sub>3</sub>(BF<sub>4</sub>)<sub>2</sub><sup>+</sup>, and at *m/z* = 703, 719, and 771 correspond to the F<sup>-</sup>, Cl<sup>-</sup>, and BF<sub>4</sub><sup>-</sup> adducts of the [FeL]<sub>2</sub><sup>2+</sup> fragment (see Figure S18).

The <sup>1</sup>H NMR spectrum of [Fe<sub>3</sub>]<sup>3+</sup> in CD<sub>3</sub>CN displays a single set of paramagnetically shifted peaks corresponding to the L<sup>-</sup> unit (Figure S5), indicative of a high-spin Fe<sup>II</sup> species. However, this spectrum, along with those recorded for comparison in non-coordinating CD<sub>2</sub>Cl<sub>2</sub> (after exchanging BF<sub>4</sub><sup>-</sup> for BArF<sub>4</sub><sup>-</sup> to improve solubility) and strongly coordinating d<sub>7</sub>-DMF (Figure S6), do not provide conclusive information about the nuclearity of the complex.

To assess the nuclearity of [Fe<sub>3</sub>]<sup>3+</sup> in different solvents, <sup>1</sup>H Pulsed Gradient Spin Echo (PGSE) NMR experiments have been carried out, as it is a reliable technique<sup>48, 49</sup> even in the case of paramagnetic species.<sup>50</sup> PGSE spectra were performed in CD<sub>3</sub>CN, CD<sub>2</sub>Cl<sub>2</sub>, and d<sub>7</sub>-DMF at 20 mM [Fe]. From their analysis, the hydrodynamic radii (r<sub>H</sub>) of the complex(es) can be estimated, and these r<sub>H</sub> values compared with the van der Waals radii (r<sub>vdW</sub>) calculated for both mononuclear and trinuclear species.

As expected, in non-coordinating CD<sub>2</sub>Cl<sub>2</sub>, the measured r<sub>H</sub> = 8.8±0.4 Å is roughly consistent with the trinuclear formulation ([Fe<sub>3</sub>](BArF<sub>4</sub>)<sub>3</sub>, r<sub>vdW</sub> = 10.7 Å). In contrast, in the coordinating solvents CD<sub>3</sub>CN and d<sub>7</sub>-DMF, the observed r<sub>H</sub> values of 5.2±0.4 Å and 5.0±0.3 Å, respectively, closely match those calculated for the mononuclear adducts [FeL(MeCN)]BF<sub>4</sub> and [FeL(DMF)]BF<sub>4</sub> (r<sub>vdW</sub> = 5.6 Å and 5.8 Å, respectively), indicating predominant dissociation into mononuclear species. However, the presence of a minor amount of [Fe<sub>3</sub>]<sup>3+</sup> in equilibrium with [FeL(MeCN)]<sup>+</sup> in MeCN is necessary to account for the crystallization of the trimer from this solvent.

This behavior contrasts with that of a previously reported Fe<sub>3</sub>-thiolate metallacycle, which fully retains its trinuclear structure in MeCN solution.<sup>47</sup>

The magnetic molar susceptibility of 0.0743 cm<sup>3</sup>·mol<sup>-1</sup> determined for [FeL(MeCN)]BF<sub>4</sub>, measured in CD<sub>3</sub>CN at 298 K through the Evans method,<sup>51</sup> corresponds to an effective magnetic moment of 4.29 µ<sub>B</sub>, slightly below the expected value for a spin-only *S* = 2 system (4.90 µ<sub>B</sub>), in agreement with the predominance of the paramagnetic mononuclear high-spin Fe<sup>II</sup> species in MeCN.
Consistently, the zero-field Mössbauer spectrum recorded at 80 K on a frozen solution of
[Fe₃](BF₄)₃ dissolved in MeCN (68 mM [Fe], Figure 2a) exhibits a single doublet with δ = 0.94
mm·s⁻¹ and ΔE_Q = 3.08 mm·s⁻¹, characteristic of high-spin FeII. These values are close to those
reported for similar thiolate-ligated FeII complexes such as [LFeIIBr] (δ = 0.93 mm·s⁻¹ and ΔE_Q
= 3.29 mm·s⁻¹)²⁷ and [Fe₂SH]⁺ (δ = 0.87 mm·s⁻¹ and ΔE_Q = 3.76 mm·s⁻¹),¹⁵ see Scheme 1 and
2.

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

**Figure 2.** Zero-field <sup>57</sup>Fe Mössbauer spectrum recorded at 80 K on a frozen solution of [Fe<sub>3</sub>](BF<sub>4</sub>)<sub>3</sub> dissolved in MeCN (68 mM [Fe]), before (a) and (b) after exposure to air to generate [Fe<sub>2</sub>O]<sup>2+</sup>.

The redox properties of [Fe₃](BF₄)₃ were investigated by cyclic voltammetry (CV, Figure 3a) in a 0.1 M *n*-Bu₄PF₆ solution (1 mM [Fe]) of MeCN, in which the main species is [FeL(MeCN)]⁺. All potentials are referenced to the Fc⁺/Fc redox couple. The CV exhibits a
reduction process at $Ep_c$ = -2.02 V and a first oxidation one at $Ep_a$ = -0.05 V, both being irreversible. The first reduction, which becomes partially reversible at higher scan rates (Figure S10), is proposed to occur at the iron-coordinated pyridine groups based on the potential value. Regarding oxidation, it is challenging to identify whether it takes place on the metal ($Fe^{II}$ to $Fe^{III}$) or on the sulfur (thiolate to disulfide).

<figure><img src="image_13.png" /><figcaption>**Figure 3.** Cyclic voltammetry (CV) of $[Fe_3](BF_4)_3$ dissolved in MeCN (a) before and (b) after exposure to $O_2$ to generate $[Fe_2O]^{2+}$ (1 mM [Fe], 0.1 M $n$-Bu$_4$NPF$_6$, 100 mV·s$^{-1}$ scan rate, 3 mm diameter GC electrode).</figcaption></figure>

**Reactivity with dioxygen.** Upon exposure to $O_2$, either at room temperature or at -40 °C, the yellow solution obtained by dissolving $[Fe_3](BF_4)_3$ in MeCN undergoes an instantaneous color change to red. A single main product is formed, characterized by (i) the presence of a major broad absorption band at 529 nm ($ε$ ≈ 1100 cm$^{-1}$M$^{-1}$ per Fe) in the corresponding UV-vis spectrum (Figure 4), and (ii) a distinct pattern in the 10-25 ppm range of the $^1H$ NMR spectrum recorded on $CD_3CN$ (Figure S5). This new complex is stable when stored in a glove box using freshly distilled solvents. However, it exhibits gradual decomposition in the presence of adventitious water.
<figure><img src="image_14.png" /><figcaption>**Figure 4.** UV-vis absorption spectrum of $[Fe_3(BF_4)_3]$ dissolved in MeCN at room temperature (1 mM [Fe]) before (---) and after (___) exposure to dry air (1 cm pathlength).</figcaption></figure>

The O₂-derived product could be isolated by precipitating it with excess diethyl ether from the acetonitrile solution. Single crystals suitable for X-ray diffraction were obtained through anion exchange from BF₄⁻ to BPh₄⁻ prior to O₂ reactivity in a 3:1 mixture of THF/MeCN and slow vapor diffusion of diethyl ether. The corresponding structure (Figure 1b) reveals the formation of an unsupported µ-oxo diiron(III) complex, $[Fe^{III}_2L_2O](BPh_4)_2$ ($[Fe_2O](BPh_4)_2$). Notably, the nature and nuclearity of the iron-oxygen adduct differ from those observed when $[LFe^{II}Br]$ is used as the starting material, which led to the formation of a mononuclear Fe³II-hydroxo species.²⁷ In $[Fe_2O](BPh_4)_2$, both Fe³II ions are pentacoordinated and surrounded by a N3SO donor set in distorted trigonal bipyramidal geometries (τ₅ = 0.74 and 0.83 for Fe1 and Fe2, respectively).⁵² The Fe-O distances (Fe1-O1= 1.760(7) Å, Fe2-O1 = 1.807(8) Å) and Fe1-O1-Fe2 angle (162.6(4) °) are in the reported ranges for µ-oxo di-Fe³II complexes.⁵³

The zero-field Mössbauer spectrum of the in-situ generated $[Fe_2O]^{2+}$, recorded on a frozen MeCN solution at 80 K, exhibits a predominant (89%) doublet ($\delta$ = 0.41 mm·s⁻¹ and $\Delta E_Q$ = 1.52 mm·s⁻¹), typical of high-spin (S = 5/2) Fe³II ions (Figure 2b). The minor doublet (11%) is assigned to a small amount of unreacted initial complex. As expected, the two iron centers of $[Fe_2O]^{2+}$ are indistinguishable by Mössbauer spectroscopy. The $\delta$ and $\Delta E_Q$ values of $[Fe_2O]^{2+}$ are in line with those obtained for a similar unsupported µ-oxo Fe³III-thiolate dinuclear complex previously reported by some of us and featuring pentacoordinated Fe³III ions in a N2S3 donor set ($\delta$ = 0.42 mm·s⁻¹ and $\Delta E_Q$ = 1.22 mm·s⁻¹).⁵⁴ At room temperature, $[Fe_2O]^{2+}$ remains
paramagnetic with an effective magnetic moment of 3.48 µB determined by the Evans method in CD₃CN. This value is well below the spin-only effective magnetic moment expected for two non-interacting high-spin Fe<sup>III</sup> ions (S = 5/2 each, 8.37 µB), thus consistent with an antiferromagnetically coupled µ-oxo Fe<sup>III</sup> dimer.

The redox properties of [Fe<sub>2</sub>O]<sup>2+</sup> were studied through CV measurements in MeCN (Figure 3b). An irreversible redox event is observed in the anodic scan at E<sub>pA</sub> ≈ 0.87 V vs Fc<sup>+</sup>/Fc. The cathodic scan reveals two consecutive irreversible one-electron reduction processes, at E<sub>pC1</sub> = -0.78 V and E<sub>pC2</sub> = -1.31 V. While the oxidation process is most likely ligand-based, corresponding to the thiolate or tertiary amine oxidation, the two reduction events are attributed to the successive Fe<sup>III</sup>Fe<sup>III</sup> → Fe<sup>II</sup>Fe<sup>III</sup> and Fe<sup>II</sup>Fe<sup>III</sup> → Fe<sup>II</sup>Fe<sup>II</sup> redox processes.

Through Raman and ESI-MS labeling experiments using <sup>18</sup>O<sub>2</sub> and <sup>16</sup>O<sub>2</sub>, we successfully identify O<sub>2</sub> as the oxygen source for the µ-oxo bridge in [Fe<sub>2</sub>O]<sup>2+</sup>. The isotope-difference Resonance Raman spectrum (<sup>16</sup>O<sub>2</sub> - <sup>18</sup>O<sub>2</sub>) obtained from frozen MeCN solutions of [Fe<sub>2</sub>O]<sup>2+</sup> prepared with either <sup>16</sup>O<sub>2</sub> or <sup>18</sup>O<sub>2</sub> is shown in Figure 5 (λ<sub>exc</sub> = 532 nm). The vibration at 833 cm<sup>-1</sup> (<sup>16</sup>O<sub>2</sub>) that downshifts to 802 cm<sup>-1</sup> (<sup>18</sup>O<sub>2</sub>) can be assigned to an antisymmetric Fe-O-Fe stretching vibration (ν<sub>as</sub>),<sup>55</sup> typically appearing in the 750-900 cm<sup>-1</sup> region.<sup>56</sup> Based on a previous study evidencing a correlation between the Fe-O-Fe angle and its vibrational properties,<sup>56</sup> the position of the ν<sub>as</sub> band aligns with an Fe-O-Fe angle of 162.6(4)°) determined in the X-ray diffraction structure of [Fe<sub>2</sub>O]<sup>2+</sup>. The ν<sub>as</sub>(<sup>16</sup>O<sub>2</sub>-<sup>18</sup>O<sub>2</sub>) isotopic shift of 31 cm<sup>-1</sup> is within the expected range for such systems (30-45 cm<sup>-1</sup>),<sup>56</sup> thus confirming the origin of the oxo-bridge in [Fe<sub>2</sub>O]<sup>2+</sup> from O<sub>2</sub>.
<figure><img src="image_15.png" /><figcaption>**Figure 5.** Baseline-corrected and normalized Raman spectra recorded on frozen solutions of pure [Fe<sub>2</sub>O]<sup>2+</sup> (40 mM Fe) in MeCN prepared in situ with either <sup>16</sup>O<sub>2</sub> (blue) or <sup>18</sup>O<sub>2</sub> (clear orange); λ<sub>exc</sub> = 532 nm. The inset shows the isotope-difference Raman spectrum (<sup>16</sup>O<sub>2</sub> - <sup>18</sup>O<sub>2</sub>).</figcaption></figure>

In the ESI mass spectra recorded on [Fe<sub>2</sub>O]<sup>2+</sup> generated in situ with <sup>16</sup>O<sub>2</sub>/<sup>18</sup>O<sub>2</sub>, several isotopically-shifted peaks are observed (Figures 6 and S19) at *m/z* = 787, 719 (+2 shift under <sup>18</sup>O<sub>2</sub>), and 350 (doubly charged, +1 shift under <sup>18</sup>O<sub>2</sub>), and assigned to the [Fe<sub>2</sub>(L)<sub>2</sub>O(BF<sub>4</sub>)]<sup>+</sup>, [Fe<sub>2</sub>(L)<sub>2</sub>OF]<sup>+</sup> and [Fe<sub>2</sub>(L)<sub>2</sub>O]<sup>2+</sup> ions, respectively.

<figure><img src="image_16.png" /><figcaption>**Figure 6.** a) ESI-MS spectrum of [Fe<sub>3</sub>](BF<sub>4</sub>)<sub>3</sub> dissolved in MeCN (1.3 mM Fe) after exposure to <sup>16</sup>O<sub>2</sub> with b) enlargement of the 751 *m/z* ion (blue line) and its isotopic simulation (orange dashed line);<sup>57</sup> c) enlargement of the ion at 755 *m/z* (blue line) obtained after exposure to <sup>18</sup>O<sub>2</sub></figcaption></figure>
and its isotopic simulation (orange dashed line) with 44% of $[Fe_2(L)_2^{18}O_3F]^+$ and 50% of $[Fe_2(L)_2^{18}O_2^{16}OF]^+$. Inset: structure assigned to the 751 m/z peak.

Remarkably, an additional prominent peak is observed at m/z = 751 (Figure 6b). This peak is attributed to a diiron species containing three oxygen atoms, $[Fe_2(L)_2O_3F]^+$ (exact mass measured at 751.12663, calculated at 751.12825 for $[C_{32}H_{40}FN_6Fe_2S_2O_3]^{+}$). Upon reaction with $^{18}O_2$, an isotopic shift of this peak is observed, consistent with a mixture of diiron-trioxygen isotopomers (Figure 6c), including $[Fe_2(L)_2^{18}O_3F]^+$ (44%) and $[Fe_2(L)_2^{18}O_2^{16}OF]^+$ (50%). These data indicate that all three oxygen atoms of $[Fe_2(L)_2O_3F]^+$ originate from $O_2$, even though one oxygen atom can partially be exchanged with adventitious $H_2^{16}O$. This ion is proposed to be a µ-oxo, µ-peroxo diiron(III) complex $[Fe^{III}_2(LSSL)(\mu-O)(\mu-O_2)F]^+$ ($[Fe_2^{O3,ss}]^{2+}$, inset of Figure 6), in which the ligand is in its oxidized disulfide form (LSSL), accounting for the overall charge of the complex. This complex is present in trace amounts in MeCN alongside the main product $[Fe_2^O]^{2+}$, since it remains undetectable by experimental techniques other than mass spectrometry.

**Time evolution of $[Fe^{III}_2L_2O](BF_4)_2$.** When we left to stand for a few days a freshly prepared solution of $[Fe_2^O](BF_4)_2$, single crystals of a follow-up product were obtained under vapor-diffusion of $Et_2O$. The X-ray diffraction data reveal a tetrametallic structure corresponding to $[Fe^{III}_2Fe^{II}_2OF_5(LSSL)_2](BF_4)_3$, $[Fe_4^{OF_5}](BF_4)_3$ (LSSL is the disulfide form of L⁻, Figure 7). The unprecedented adamantane-like $\{Fe^{III}_2Fe^{II}_2OF_5\}^{3+}$ core, including one µ-oxo and five µ-F bridges, is decorated by two disulfide-containing LSSL ligands. The fluoride co-ligands are proposed to arise from the activation and cleavage of the B-F bonds of $BF_4^-$ in the $[Fe_2^O]^{2+}$ adduct. The Fe-O distances in the 1.871(2)-1.872(2) Å range, along with the Fe-O-Fe angle of 129.3(1)°, are consistent with a µ-oxo bridging $Fe_2$ unit. Moreover, the longer Fe-F distances (from 1.939 to 2.080 Å) fall within the typical range for Fe-F bonds of µ-fluorido ligands.

The ESI-Mass spectrum of $[Fe_4^{OF_5}]^{3+}$ in MeCN confirms the presence of the $\{Fe^{III}_2Fe^{II}_2OF_5\}^{3+}$ core, with a peak at m/z = 493 attributed to the $[Fe^{III}_2Fe^{II}_2OF_5(LSSL)_2]^{3+}$ ion (Figure S20). The $^1H NMR spectrum of $[Fe_4^{OF_5}]^{3+}$ in CD$_3CN$ displays features ranging from 135 to -25 ppm, consistent with the paramagnetic nature of the product and its formal $Fe^{II}_2Fe^{III}_2$ formulation (Figure S7). The presence of $Fe^{II}$ ions in $[Fe_4^{OF_5}]^{3+}$ is in line with the thiolate-to-disulfide oxidation process in the ligand, resulting in the formation of LSSL from two L⁻ units. Such thiolate-based reactivity is common in complexes bearing N3S-bpa ligands, known for their propensity to form multinuclear metallic compounds with S-S bridges.$^{39, 41, 43, 58-60}$
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<figure><img src="image_17.png" /><figcaption>**Figure 7.** (a) Molecular structure of [Fe<sub>4</sub><sup>OF5</sup>](BF<sub>4</sub>)<sub>3</sub>·2MeCN, determined by single-crystal X-ray diffraction. The thermal ellipsoids of the metal cores are drawn at 30% probability level. All hydrogen atoms, counterions (three BF<sub>4</sub><sup>-</sup>), and solvent molecules are omitted for clarity. A zoom on the {Fe<sub>4</sub>OF<sub>5</sub>}<sup>3+</sup> core is provided in (b).</figcaption></figure>

**Oxygen Reduction Reaction Catalysis.** The ORR catalytic ability of the [Fe₃](BF₄)₃ complex was explored in MeCN (where the [FeL(MeCN)]⁺ form is predominant) using 2,6-lutidinium tetrafluoroborate (LutHBF₄) as a proton source and octamethylferrocene (Me₈Fc, *E*₁/₂ = -0.41 V vs Fc⁺/Fc) as a monoelectronic chemical reducing agent. When a catalytic amount of complex (0.1-0.6 mM [Fe]) is added to an air-saturated MeCN solution containing Me₈Fc (2 mM) and LutH⁺ (15 mM), the progressive oxidation of Me₈Fc to Me₈Fc⁺ is observed by UV-vis spectroscopy (λmax = 750 nm, ε = 420 M⁻¹·cm⁻¹, Figure 8a). After 40 s of reaction, up to 95(±2) % of Me₈Fc (for 0.6 mM [Fe]) is converted to Me₈Fc⁺.
<figure><img src="image_18.png" /><figcaption>**Figure 8.** (a) UV-vis spectral changes observed during ORR catalyzed by $[Fe_3]^{3+}/[FeL(MeCN)]^+$ (0.2 mM [Fe]) with LutH$^+$ (15 mM) and $Me_8Fc$ (2.0 mM) in air-saturated MeCN (1 cm path length, $\Delta t = 0.3$ s). (b) Kinetic profiles for $Me_8Fc^+$ formation at 750 nm with varying catalyst concentrations (0–0.6 mM [Fe]). (c–f) Plots of $k_{obs}$ vs. [Fe], [LutH$^+$], $[O_2]$, and $[Me_8Fc]$, respectively, for the ORR<sup>fast</sup> regime with the concentrations of the other co-substrates held constant.</figcaption></figure>

Note that no generation of $Me_8Fc^+$ is observed within the same time frame in the absence of the catalyst (Figure 8b, green trace) or the absence of LutH$^+$ (Figure S17). In addition, when replacing $[Fe_3]^{3+}$ with the in-situ generated $[Fe_2O]^{2+}$, $4H^+/4e^-$ ORR activity is maintained (Figure S16).

The selectivity of the ORR process was determined by quantifying $H_2O_2$ production through a spectrophotometric assay (Figure S14).<sup>61, 62</sup> No traces of $H_2O_2$ are detected within the 10–150 s timeframe, indicating exclusive $H_2O$ production through a $4H^+/4e^-$ ORR process (Eq. 2):

$$O_2 + 4Me_8Fc + 4LutH^+ \rightarrow 2H_2O + 4Me_8Fc^+ + 4Lut \quad (\text{Eq. 2})$$

Kinetic data (Figures 8b and S15) reveal two different regimes. An initial fast phase, dominant at high [Fe] and associated with the early catalytic steps, is followed by a slower steady-state regime. We focused our investigation on the faster phase by varying the concentrations of $Me_8Fc$, LutH$^+$, $O_2$, (Figure 8c-f). A clear linear dependence of $k_{obs}$ on [Fe] was observed, while $[Me_8Fc]$, [LutH$^+$], or $[O_2]$ showed no impact.<sup>63</sup> This led to the derivation of the following rate law for the fast regime:

$$d[Me_8Fc^+]/dt=k_{\text{fast}}[\text{Fe}], \quad \text{with } k_{\text{fast}}=1.3(\pm 0.2)\times 10^3 \text{ s}^{-1} \quad (\text{Eq. 1})$$
In addition, the amplitude of the fast phase (A<sup>750 nm</sup>) scales linearly with [Fe] and, based on the extinction coefficient of Me<sub>8</sub>Fc<sup>+</sup> (ε = 383 M<sup>-1</sup>·cm<sup>-1</sup>),<sup>26</sup> the linear plot of [Me<sub>8</sub>Fc<sup>+</sup>] vs [Fe] yields a slope of 3.1±0.1 (Figure 9). This indicates that three equivalents of Me<sub>8</sub>Fc<sup>+</sup> per iron center are generated during this initial step, corresponding to three electron-transfer events (see below).

<figure><img src="image_19.png" /><figcaption>**Figure 9.** Amplitude (A<sup>750 nm</sup>) of the initial rapid phase vs [Fe] and the corresponding [Me<sub>8</sub>Fc]<sup>+</sup> vs [Fe] plot.</figcaption></figure>

**Discussion and conclusion**

In this study, we report a rare example of a non-heme iron complex, [Fe<sub>3</sub>](BF<sub>4</sub>)<sub>3</sub>, capable of activating O<sub>2</sub> and catalyzing the ORR using a chemical reducing agent.

Even though thiolate-bridged metal complexes are well-documented, the solid-state trinuclear structure of [Fe<sub>3</sub>]<sup>3+</sup> supported by the bis(2-pyridylmethyl)amine/thiolate ligand is original for its rare quasi-planar {Fe<sub>3</sub>(SR)<sub>3</sub>} core.<sup>47, 64, 65</sup> In a few other examples, where an additional chalcogenide is present at the center of the {Fe<sub>3</sub>(SR)<sub>3</sub>} motif, a drastic loss of planarity is observed.<sup>66-68</sup>

In MeCN solutions, the [Fe<sub>3</sub>]<sup>3+</sup> complex mostly dissociates to form the solvent adduct [FeL(MeCN)]<sup>+</sup>. Its reaction with O<sub>2</sub> in MeCN likely occurs through an inner-sphere reactivity and results in the generation of an unsupported µ-oxo diiron(III) complex [Fe<sub>2</sub>O]<sup>2+</sup>, Scheme 3. Interestingly, during the O<sub>2</sub> activation process, traces of a bio-inspired<sup>37, 38</sup> µ-oxo, µ-peroxo diiron(III) complex were detected by mass spectrometry (Figure 6), in which it was unambiguously evidenced that the three oxygen atoms arise from dioxygen, supporting the inner-sphere reactivity. This reactivity under non-catalytic conditions contrasts with that of the
parent [LFeIIBr] complex, which affords a mononuclear FeIII-hydroxo adduct instead of an oxo-bridging dinuclear species,²⁷ thus highlighting the role of the Br⁻ co-ligand in modulating the reaction outcome. Only a few examples of thiolate-bound µ-oxo diiron(III) complexes were previously generated from the reaction between FeII precursors and O₂.⁴⁶, ⁶⁷, ⁶⁹, ⁷⁰ Over a longer timescale, [Fe₂O]²⁺ activates the B-F bonds of BF₄⁻ counterions, leading to the formation of [Fe₄OF₅]³⁺ (Scheme 3) that features an {Fe₄OF₅}³⁺ core.

Regarding ORR catalysis with [Fe₃]³⁺/[FeL(MeCN)]⁺, a selectivity for the production of H₂O is observed. Kinetic analysis reveals an initial fast process involving a three-electron transfer, followed by a slower steady-state regime corresponding to the final reduction step required to complete the overall four-electron ORR. Based on these data, a tentative mechanistic proposal is presented in Scheme 4.

<figure><img src="image_20.png" /><figcaption>**Scheme 4.** A tentative mechanistic proposal for ORR catalyzed by [Fe₃]³⁺/[FeL(MeCN)]⁺.</figcaption></figure>

Consistent with the involvement of an initial three-electron reduction process, we propose a monometallic pathway starting from the predominant [FeL(MeCN)]⁺ species in solution. The mechanism begins with MeCN decoordination, enabling O₂ binding and formation of a FeIII-superoxo intermediate. This intermediate is then proposed to undergo three successive proton/electron transfers, yielding a FeIII-hydroxo species analogous to a previously isolated complex bearing a related ligand.²⁸ This hydroxo intermediate is subsequently converted to the active FeII species in the global rate-limiting step of the cycle, allowing turnover. On the other hand, the rate-determining step within the initial phase (kfast) is proposed to be the cleavage of the Fe-MeCN bond in [FeL(MeCN)]⁺, consistent with the experimentally observed first-order dependence on [Fe].
It is worth noting that an alternative bimetallic pathway, involving the homolytic cleavage of an O-O bond of a μ-peroxo diiron(III) (related to that detected by ESI-MS), would not be compatible with the observed $3e^{-} + 1e^{-}$ sequence.

<table><caption>**Table 1.** Comparison between the different homogeneous non-heme Fe ORR catalysts reported in the literature (spectrochemical conditions; structures depicted in Scheme 1).</caption><thead><tr><th>Catalyst</th><th>Solvent/<br>O<sub>2</sub><br>source</th><th>Reductant</th><th>Acid</th><th>Selectivity</th><th>Kinetic parameters</th><th>Proposed mechanism</th></tr></thead><tbody><tr><td>[Fe<sub>3</sub>]<sup>3+</sup>/<br>[FeL(MeCN)]<sup>+</sup></td><td>MeCN,<br>air</td><td>Me<sub>8</sub>Fc</td><td>LutH<sup>+</sup></td><td>100% H<sub>2</sub>O<sup>b</sup></td><td>k<sub>fast</sub> = 1.03(±2) × 10<sup>3</sup> s<sup>-1</sup></td><td>Direct O-O cleavage</td></tr><tr><td>[Fe<sub>2</sub><sup>SH</sup>]<sup>+</sup></td><td>MeCN,<br>air</td><td>Me<sub>8</sub>Fc</td><td>LutH<sup>+</sup></td><td>~95% H<sub>2</sub>O<sub>2</sub></td><td>TOF<sub>initial</sub> = 8 × 10<sup>3</sup> h<sup>-1</sup></td><td>Protonation of an Fe-<br>peroxo</td></tr><tr><td>[Fe<sup>N4</sup>]<sup>-</sup></td><td>Acetone,<br>air</td><td>Me<sub>n</sub>Fc,<br>n=8,10</td><td>CH<sub>3</sub>COOH</td><td>H<sub>2</sub>O (<2.5%<br>H<sub>2</sub>O<sub>2</sub>)</td><td>-</td><td>Direct O-O cleavage</td></tr><tr><td>[Fe<sup>N4</sup>]<sup>2+</sup></td><td>MeCN,<br>O<sub>2</sub><sup>a</sup></td><td>Me<sub>10</sub>Fc</td><td>CF<sub>3</sub>COOH</td><td>100% H<sub>2</sub>O<sup>b</sup></td><td>k<sub>cat</sub> = 6.07 × 10<sup>4</sup> M<sup>-2</sup>s<sup>-1</sup></td><td>Direct O-O cleavage</td></tr><tr><td>Fe<sup>N3O</sup><sup>-</sup></td><td>MeCN,<br>O<sub>2</sub><sup>a</sup></td><td>Me<sub>10</sub>Fc</td><td>CH<sub>3</sub>COOH</td><td>H<sub>2</sub>O (<1.1%<br>H<sub>2</sub>O<sub>2</sub>)</td><td>TOF = 3.3 h<sup>-1</sup></td><td>2+2 mechanism</td></tr><tr><td>Fe<sup>N3O</sup><sup>16</sup></td><td>MeCN,<br>O<sub>2</sub><sup>a</sup></td><td>Me<sub>10</sub>Fc</td><td>CH<sub>3</sub>COOH</td><td>100% H<sub>2</sub>O<sup>b</sup></td><td>k<sub>cat</sub> = 1.13 × 10<sup>9</sup> M<sup>-2</sup>s<sup>-1</sup></td><td>2+2 mechanism</td></tr><tr><td>[Fe<sup>N4Ox</sup>]<sup>2+</sup></td><td>MeCN,<br>O<sub>2</sub><sup>a</sup></td><td>Me<sub>10</sub>Fc</td><td>CH<sub>3</sub>COOH</td><td>from <5% to<br>47% H<sub>2</sub>O<sub>2</sub></td><td>-</td><td>Competition between<br>O-O cleavage and<br>protonation</td></tr></tbody></table>

Table 1 summarizes the main catalytic properties of all reported non-heme homogeneous ORR catalysts (Scheme 1) when using a chemical reducing agent. However, it is challenging to compare their kinetic performance due to variations in experimental conditions and different levels of analysis across these studies. Regarding the ORR selectivity, $[Fe_3]^{3+}/[FeL(MeCN)]^+$ is selective for H<sub>2</sub>O production, as most of the reported non-heme Fe complexes supported by N/O ligands.<sup>3, 16, 17, 20</sup> In contrast, the previously reported $[Fe_2^{SH}]^+$ catalyst, which also features a thiolate-based ligand, is selective for H<sub>2</sub>O<sub>2</sub> production. This discrepancy in selectivity for these iron-thiolate complexes is possibly related to a change in mechanism, specifically the involvement of monometallic pathway (see Scheme 4) rather than a bimetallic one.

In conclusion, this study highlights the interest of the bpa platform, which, combined with a thiolate, provides access to an efficient non-heme Fe-based ORR catalyst. Further efforts will be directed toward adjusting the N/S ratio of the ligand in an attempt to elucidate key factors governing the ORR mechanism, thereby improving ORR performance and fine-tuning selectivity.

<footnote><sup>a</sup> use of various concentrations of O<sub>2</sub>, <sup>b</sup> no H<sub>2</sub>O<sub>2</sub> detected</footnote>
Experimental section

*General.* All reactants were used as received. Solvents were distilled and degassed with argon prior to use. The synthesis, isolation, and crystallization of $[Fe_3](BF_4)_3$ was carried out under argon (in a glove box with less than 5 ppm of $O_2$). The sodium methoxide solution in MeOH was prepared by reacting a precise amount of sodium metal with a precise volume of MeOH inside the argon glove box.

$^1$H/$^{13}$C/$^{15}$N-gHSQC NMR spectra were recorded on a Bruker Avance III 400 or 500 MHz spectrometer in suitable solvent, and spectra were referenced to residual solvent. For paramagnetic compounds, only signals observed in the paramagnetic region are reported. The infrared spectra were recorded as neat solids on a Thermo Scientific Nicolet iS10 FT-IR spectrometer equipped with ATR accessory. The electronic absorption spectra were recorded on an AvaSpec-ULS2048CL-EVO UV-vis CCD spectrophotometer from Avantes in quartz cells (optical pathlength: 1 cm).

*Synthesis of L<sup>Ac</sup>.* In a 10-mL flask, bis-picolylamine (2.18 g, 10.95 mmol) and 2,2-dimethylthiirane (0.93 g, 10.57 mmol) were stirred at 65°C for 20 h. Acetic anhydride (2 g, 20 mmol) was then added and the mixture was stirred at 65°C overnight. The crude reaction mixture was diluted with ethyl acetate (100 mL) and washed with 50 mL of a saturated $NaHCO_3$ solution. The organic phase was dried over sodium sulphate and concentrated to give 2.73 g of crude material that were chromatographed in a $CH_2Cl_2$-acetone gradient (rf = 0.6 in $CH_2Cl_2$/acetone 1/1) to produce pure L<sup>Ac</sup> (1.53 g, 4.65 mmol, 44%) of pure thiolacetate. IR (cm<sup>-1</sup>, neat oil): 3070(w), 3059(w), 3048(w), 3009(m), 2963(s), 2925(s), 2864(m), 2828(m), 2358(m), 2336(w), 2305(w), 2295(vw), 1679(vs), 1590(s), 1568(m), 1471(m), 1436(s), 1355(m), 1314(w), 1261(w), 1149(w), 1109(s), 1045(m), 993(m), 946(m), 762(s), 646(s), 614(s). $^1$H NMR (400 MHz, $CDCl_3$): $\delta$ 8.52 (d, *J* = 4.7 Hz, 2H), 7.65 (td, *J* = 7.7, 1.6 Hz, 2H), 7.50 (d, *J* = 7.8 Hz, 2H), 7.14 (m, 2H), 3.92 (s, 4H), 3.11 (s, 2H), 2.16 (s, 3H), 1.32 (s, 6H). $^{13}$C NMR (101 MHz, $CDCl_3$) $\delta$ 196.92 (C), 159.61 (C), 149.12 (CH), 136.41 (CH), 123.55 (CH), 122.10 (CH), 62.87 ($CH_2$), 62.02 ($CH_2$), 53.77 ($CH_2$), 31.56 ($CH_3$), 25.85 ($CH_3$). $^{15}$N NMR (40.5 MHz, $CDCl_3$, gHSQC) $\delta$ 38, 311. ESI-MS (0.05 mM in $CH_3OH$, m/z, I%): 330.25, 100, [L<sup>Ac</sup>H]<sup>+</sup>.
High resolution ESI-MS (0.05 mM in CH₃OH, *m/z*): calculated for C₁₈H₂₄N₃SO ([L**Ac**H]⁺), 330.16401, found: 330.16349.

*Synthesis of [Fe<sup>II</sup>L]₃(BF₄)₃ ([Fe₃](BF₄)₃).* A freshly-prepared solution of NaOMe in MeOH (3.14 M, 100 µL, 0.314 mmol) was added to one of L<sup>Ac</sup> (115 mg, 0.349 mmol) in MeOH (2 mL). No apparent changes were observed. After 30 min of stirring, the solvent was removed in vacuo and the residue was suspended in THF (2 mL). A solution of Fe(BF₄)₂·6H₂O (113 mg, 0.335 mmol) in THF (3 mL) was added dropwise leading to a transient red color of the mixture then turning into clear yellow-greenish. The mixture was stirred for an additional 10 minutes, and the obtained precipitate was filtered, washed with THF (3 mL), dried, and collected as a yellow-greenish powder (140 mg, 0.109 mmol, 97 %). Anal. Calcd. for C₄₈H₆₀Fe₃N₉S₃B₃F₁₂·3NaBF₄ (1617.20): C, 35.66; H, 3.74; N, 7.80; S, 5.95; Found: C, 35.39; H, 3.72; N, 7.68; S, 6.18. IR (cm⁻¹, neat solid): 3603(br), 3096(w), 2957(s), 2933(s), 2868(s), 1607(s), 1573(m), 1482(m), 1458(m), 1443(s), 1429(m), 1363(w), 1299(m), 1290(s), 1165(s), 1050(vs), 1024(vs), 943(s), 882(m), 857(w), 840(w), 821(w), 782(s), 723(w), 650(m). ¹H NMR (CD₃CN, 500 MHz): δ 74.1, 72.7, 66.5, 50.2, 45.0, 36.0, 15.0, -8.5. ¹H NMR (CD₂Cl₂, 500 MHz): δ 73.6, 72.3, 68.5, 50.0, 44.1, 40.1, 16.4, -9.3. High resolution ESI-MS (0.075 mM in CH₃CN, *m/z*): calculated for C₁₆H₂₀FeN₃S ([FeL]⁺), 342.07274, found: 342.07146. X-ray suitable dark-green crystals of [Fe₃](BF₄)₃·2CH₃CN were obtained by slow diffusion of diethyl ether into a solution of [Fe₃](BF₄)₃ in MeCN.

*Synthesis of [Fe<sup>III</sup>₂L₂O](BF₄)₂ ([Fe₂O](BF₄)₂).* [Fe<sup>II</sup>L]₃(BF₄)₃ (5.0 mg, 0.0039 mmol) was dissolved in CH₃CN (1 mL), then excess O₂ was bubbled through it. The color of the solution changed from yellow-green to deep red. A red precipitate was obtained by addition of Et₂O (6 mL). The product was dried and collected as a red powder (3.2 mg, 0.0037 mmol, 63 %). IR (cm⁻¹, neat solid): 3543(br), 2974(w), 2963(w), 2943(w), 2933(w), 2923(w), 2914(w), 2909(w), 2886(w), 2880(w), 2865(w), 1608(s), 1575(w), 1486(w), 1445(s), 1305(w), 1288(w), 1051(vs), 1020(vs), 822(s), 765(s), 655(w). ¹H NMR (CD₃CN, 500 MHz): δ 22.9, 16.8, 16.1, 13.8, 11.3, 9.7, 5.4. ¹⁹F NMR (CD₃CN, 470 MHz): δ -149.2, -149.3 (BF₄⁻). ESI-MS (CH₃CN, *m/z*): 719 [Fe₂L₂OF]⁺, 751 [Fe₂L₂O₃F]⁺, 787 [Fe₂L₂OBF₄]⁺. UV-vis (CH₃CN): 529 nm (ε ≈ 1100 cm⁻¹ M⁻¹ per Fe).
Generation of [LFe<sup>II</sup>Br] from [Fe<sup>II</sup>L]<sub>3</sub>(BF<sub>4</sub>)<sub>3</sub>. [Fe<sup>II</sup>L]<sub>3</sub>(BF<sub>4</sub>)<sub>3</sub> (5.0 mg, 0.0039 mmol) and n-Bu<sub>4</sub>Br (150 mg, 0.4653 mmol) were dissolved in CD<sub>3</sub>CN (0.5 mL), affording an orange solution. *In situ* characterization by ¹H NMR confirmed the exclusive formation of the previously reported [LFe<sup>II</sup>Br], with spectral data consistent with literature values.<sup>27</sup> ¹H NMR (CD<sub>3</sub>CN, 500 MHz): δ 135.6, 101.0, 96.0, 76.9, 57.4, 40.5, -9.8.

Crystallization of [Fe<sub>2</sub>O](BPh<sub>4</sub>)<sub>2</sub> and [Fe<sub>4</sub>OF<sub>5</sub>](BF<sub>4</sub>)<sub>3</sub>. Black single crystals of [Fe<sub>4</sub>OF<sub>5</sub>](BF<sub>4</sub>)<sub>3</sub>·2MeCN were obtained by vapor diffusion of Et<sub>2</sub>O at room temperature onto a solution of [Fe<sub>3</sub>](BF<sub>4</sub>)<sub>3</sub> in MeCN, which was previously exposed to excess O<sub>2</sub> to yield a deep red solution. In the case of [Fe<sub>2</sub>O](BPh<sub>4</sub>)<sub>2</sub>·1.45MeCN·2Et<sub>2</sub>O crystallization, a preliminary anion exchange from BF<sub>4</sub><sup>-</sup> to BPh<sub>4</sub><sup>-</sup> was performed on [Fe<sub>3</sub>]<sup>3+</sup> by treatment with 3 equiv. of NaBPh<sub>4</sub> in a 3:1 mixture of THF/MeCN. Subsequently, the resulting solution was exposed to excess O<sub>2</sub> to yield a deep red solution, and successive vapor diffusion of Et<sub>2</sub>O yielded black single crystals of [Fe<sub>2</sub>O](BPh<sub>4</sub>)<sub>2</sub>·1.45MeCN·2Et<sub>2</sub>O.

*X-ray Crystallography.* Single-crystal diffraction data for [Fe₃](BF₄)₃·2CH₃CN, [Fe₂O](BPh₄)₂·1.45MeCN·2Et₂O, and [Fe₄OF₅](BF₄)₃·2MeCN were measured on a Nonius-Bruker 4 circles diffractometer with an APEXII CCD detector and an Incoatec high brilliance microsource with multilayer mirrors to monochromatize the MoKα radiation (λ = 0.71073Å) at 200 K. Eval, Sadabs and Xprep (Nonius-Bruker) were used for cell refinements, integration, absorption correction and data reduction. Subsequent steps were run under Olex2. The molecular structures were solved by intrinsic phasing with SHELXT and refined on *F*² by full matrix least-squares techniques using SHELXL.<sup>71</sup> All non-hydrogen atoms were refined anisotropically for the three complexes. All hydrogen atoms were set geometrically and constrained to ride on their carrier atoms. Concerning [Fe₂O](BPh₄)₂·1.45MeCN·2Et₂O, the crystal used was a twin. Despite the fact that the twin law was found with the matrix : 1 0 0 0 - 1 0 -1 -0.209 1.002 the overlaps of the data are so tight that we did not obtain a successful deconvolution to provide a better resolution. CCDC- 2346905-2346907 contain the crystallographic data for this paper; the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/?

*Mössbauer spectroscopy.* Mössbauer spectra were recorded with a <sup>57</sup>Co source in a Rh matrix using an alternating constant acceleration Wissel Mössbauer spectrometer operated in the transmission mode and equipped with a *Janis* closed-cycle helium cryostat. Isomer shifts are
given relative to iron metal at ambient temperature. Simulation of the experimental data was
performed with the *mf2.SL* program using Lorentzian line doublets: E. Bill, Max-Planck
Institute for Chemical Energy Conversion, Mülheim/Ruhr, Germany.

*Resonance Raman spectroscopy.* The Raman spectra of frozen solutions (liquid N₂) of in-situ generated [Fe₂O³]²⁺ in MeCN were measured using a Horiba Jobin Yvon LabRAM HR 800 Vis with a 532 nm excitation wavelength.

*Electrochemistry.* Cyclic Voltammetry (CV) experiments were performed on [Fe₃](BF₄)₃ in CH₃CN solution under an argon-saturated atmosphere (in a glove box with less than 5 ppm of O₂). The sample was then exposed to dry dioxygen outside the glove box. The supporting electrolyte tetrabutylammonium hexafluorophosphate (n-Bu₄NPF₆) was used as received and stored in glove box. CV experiments were carried out by using a CompactStat.h potentiostat/galvanostat from Ivium technologies. A standard three-electrode electrochemical cell was used. Potentials were referred to an Ag/0.01 M AgNO₃ reference electrode in CH₃CN + 0.1 M Bu₄NPF₆ and measured potentials were calibrated through the use of an internal Fc/Fc⁺ standard. The working electrode was a vitreous carbon disk (3 mm in diameter) polished with 2 µm diamond paste (Mecaprex Presi) (Eₚ,a, anodic peak potential; Eₚc, cathodic peak potential; E₁/₂ = (Eₚa + Eₚc)/2; ΔEₚ = Eₚa - Eₚc). The auxiliary electrode was a Pt wire in CH₃CN + 0.1 M Bu₄NPF₆.

*ESI mass spectrometry.* Low resolution electrospray ionization (ESI) mass spectra were recorded using a LCMS8060 triple-quadrupole mass spectrometer (Shimadzu Corporation, Kyoto, Japan) in positive ionization mode by direct infusion in the ESI source. Mass spectra data were acquired using *LabSolutions* software (version 5.114, Shimadzu Corporation, Kyoto, Japan). Desolvated ions were obtained using a capillary voltage at 3.5 kV, a source temperature of 100 °C and nitrogen as the desolvation and nebulizing gas. Solutions were injected at a concentration of 4.5·10⁻⁴ M into the ion source using a syringe pump at a flow rate of 1 mL/hr. The high-resolution mass spectra were recorded on a LTQ Orbitrap XL Thermo Scientific spectrometer equipped with an ESI source. Solutions were injected at a concentration of 4.5·10⁻⁴ M into the ion source using a syringe pump at a flow rate of 5 µL/min.

*Evans method.* Magnetic susceptibility measurements on solution samples were performed following Evans method⁵¹, ⁷² using precision-made coaxial tubes. Tetramethylsilane (~5% by volume) was used as the internal standard in CD₃CN. Molar susceptibilities (χ) were calculated according to Equation (1), in which χ₀ is the mass susceptibility of the pure solvent (cgs units),
$\Delta\nu$ is the paramagnetic shift (Hz), $\nu_0$ is the operating RF frequency of the spectrometer (Hz), c is the concentration (mol L⁻¹) of the paramagnetic molecule, and M is its molecular weight.

$$ \chi \approx M(\chi_0 + 3000\Delta\nu/4\pi\nu_0 c M) \qquad (1) $$

The values were corrected for the ligand diamagnetic susceptibility.⁷³ The magnetic moments ($\mu_{eff}$) were then calculated according to Equation (2).

$$ \mu_{eff} = (8 \times \chi T)^{1/2}, \text{ in which } T = 298 \text{ K} \qquad (2) $$

PGSE experiments. ¹H PGSE NMR measurements were performed for [Fe₃]X₃ (X= BF₄⁻, (BArF₄)⁻) using the standard Bruker sequence ledbpgp2s in different solvent (CD₂Cl₂, CD₃CN and d₇-DMF) at 20 mM [Fe]. The PGSE analysis was carried out on the same signal from the paramagnetic target complex (44.13–45.52 ppm range) in all three solvents, as it exhibited the longest T₁ relaxation times (8–13 ms), enabling reliable diffusion measurements.

The dependence of the resonance intensity (I) on the gradient strength (g) is described by the following equation:

$$ I = I_0 e^{-D\gamma^2 g^2 \delta^2 (\Delta - \delta/3)} $$

where I is the observed intensity (attenuated signal intensity), $I_0$ is the reference intensity (unattenuated signal intensity), D is the diffusion coefficient, $\gamma$ is the nucleus gyromagnetic ratio, g is the gradient strength, $\delta$ is the gradient duration, and $\Delta$ is the diffusion delay. The parameters $\delta$ and $\Delta$ were kept constant during the experiments, whereas g varied from 2 to 98 % in 20 steps.

PGSE data were processed by applying a reported procedure,⁴⁹ using the internal standard tris-(tert-butyl)benzene (7.22-7.27 ppm range) with $r_H^{st}$ (hydrodynamic radius) $\approx r_{vdW}^{st}$ (van der Waals radius, see below) = 5.19 Å.

Nonlinear regression on I and g2 data was performed with the Origin 2017 software to obtain the coefficients $m = D\gamma^2\delta^2(\Delta - \delta/3)$ for both the sample and the corresponding internal standard signals ($m^{sa}$ and $m^{st}$, respectively). The following expression (on the basis of the Stokes-Einstein equation) was applied and numerically resolved to get the hydrodynamic radius of each sample ($r_H^{sa}$):

$$ \frac{c^{st} r_H^{st}}{c^{sa} r_H^{sa}} = \frac{m^{sa}}{m^{st}} $$

The coefficients $c^{sa}$ and $c^{st}$ can be estimated from the semiempirical equation:⁷⁴

$$ c^x = \frac{6}{(1 + 0.695 \frac{r^{solv}}{r_H^x})^{2.234}} $$
where $x = sa$ or $st$, and $r^{\text{solv}} \approx$ van der Waals radius of the solvent (2.67 Å for CD₃CN, 2.72 Å for CD₂Cl₂ and 3.26 Å for d₇-DMF). Uncertainties were obtained by error propagation on *m*. The van der Waals radii ($r_{\text{vdW}}$) was calculated from the respective van der Waals volumes ($V_{\text{vdW}}$) : $V_{\text{vdW}} = 4/3\pi(r_{\text{vdW}})^3$. The $V_{\text{vdW}}$ were calculated from the Van der Waals Radii of Elements.⁷⁵

*Chemical catalysis, kinetic measurements, and detection of H₂O₂*. The oxidation of Me₈Fc by O₂ in the presence of [Fe₃]³⁺ ([Fe₂O]²⁺) and LutHBF₄ was carried out in MeCN at room temperature. In a typical experiment, an air-saturated solution of LutHBF₄ (19 µL, 2.0 M) was added to an air-saturated solution of Me₈Fc (364 µL, 13.75 mM), in presence of air (1 atm, 0.21 atm O₂), in a septum-sealed 1 cm quartz cuvette. After stirring for 5 s, an Ar-saturated solution of [Fe₃]³⁺ (273 µL, 1.83 mM per Fe) was added to the sample under stirring (air-saturated, 2.0 mM Me₈Fc, 15.0 mM LutHBF₄, 200 µM per Fe, 2.5 ml MeCN). The increase in the absorbance of a band at 750 nm (corresponding to the formation of the Me₈Fc⁺ ion) was monitored with time, by using an AvaSpec-ULS2048CL-EVO UV-vis CCD spectrophotometer from Avantes (Δt = 0.3 s). The corresponding control experiment was performed in the same conditions, by adding only MeCN (273 µL) instead of Fe catalyst solution.

For kinetic analysis, the dependence of the catalytic rate on substrate concentrations was investigated. The concentration of one substrate was varied ([Fe₃]³⁺: 0.1-0.6 mM [Fe]; [Me₈Fc]: 1.0-4.0 mM; [LutHBF₄]: 10-30 mM; [O₂]: 0.15-1.70 mM), while the concentrations of the other three substrates were kept constant (for example: [Fe₃]³⁺: 0.1-0.6 mM [Fe]; [Me₈Fc]: 2.0 mM; [LutHBF₄]: 15 mM; [O₂]: 1.70 mM). Acetonitrile solutions with varying oxygen concentrations were prepared by mixing the required volume of air saturated MeCN and deoxygenated MeCN solutions. All the experiments were repeated 3 times, obtaining highly reproducible data (in the 5% range).

The pseudo-first-order rate constant ($k_{\text{obs}}$) values for the faster catalytic component (ORR<sup>fast</sup>, see main text) were determined by exponential fitting of a plot of absorbance at 750 nm vs. time (s). A first-order dependence of $k_{\text{obs}}$ on [Fe] was observed, while the $k_{\text{obs}}$ values were found to be independent from [Me₈Fc], [LutHBF₄], and [O₂]. The value of $k_{\text{cat}}$ was determined by linear regression of the $k_{\text{obs}}$ vs. [Fe] plot. The derived kinetic equation is presented in Eq. 1.

The potential formation of hydrogen peroxide in the catalytic samples was assessed at various time intervals by spectroscopic titration with an acidic solution of [TiO(pypH₄)]⁴⁺ complex (Ti-TPyP reagent) by following a previously detailed procedure.¹⁵, ⁶¹, ⁶²
**Supporting Information.** Supporting Information: Additional crystallographic, NMR, electrochemical, ESI-Mass and catalytic data (PDF).

**Acknowledgements.** The authors gratefully acknowledge research support of this work by the French National Agency for Research in the framework of the "Investissements d'avenir" program (ANR-15-IDEX-02), the ManOx project (ANR-20-CE07-0007), the Labex ARCANE, CBH-EUR-GS (ANR-17-EURE-0003), the PRC program, SelectO2 (ANR-22-CE07-0045), the China Scholarship Council (LS), and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation; project no. 445916766, Me1313/18-1) in the framework of the Research Unit "Bioinspired Oxidation Catalysis with Iron Complexes" (FOR 5215)). T.A.J. acknowledges support from the US NSF from CHE-2154955 and CHE-1900384. The NanoBio ICMG (FR 2607), is acknowledged for providing facilities for NMR analyses.

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