# H-Bond Assistance and Electronic Effects Accelerate the Cleavage of the N–O Bond in the Electrochemical Reduction of N2O Catalyzed by Rhenium Bipyridine Tricarbonyl Complexes

Céline Naddour, Rana Deeba, Marianne Kjellberg, Sylvie Chardon-Noblat, Cyrille Costentin

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Céline Naddour, Rana Deeba, Marianne Kjellberg, Sylvie Chardon-Noblat, Cyrille Costentin. H-Bond Assistance and Electronic Effects Accelerate the Cleavage of the N–O Bond in the Electrochemical Reduction of N2O Catalyzed by Rhenium Bipyridine Tricarbonyl Complexes. Inorganic Chemistry, 2025, 64 (29), pp.15217-15223. 10.1021/acs.inorgchem.5c02395. [hal-05153865](https://hal-inorgchem.sciencedirect.com)

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# H-bond Assistance and Electronic Effects to Accelerate the Cleavage of N-O Bond in the Electrochemical Reduction of N₂O Catalyzed by Rhenium Bipyridine Triscarbonyl Complexes.

Céline Naddour,ª Rana Deeba,ª Marianne Kjellberg,ᵇ Sylvie Chardon-Noblat,ª* Cyrille Costentinª*

cyrille.costentin@univ-grenoble-alpes.fr; sylvie.chardon@univ-grenoble-alpes.fr

**Keywords:** nitrous oxide, reductive deoxygenation, molecular catalysis, electrochemistry, hydrogen bond

**ABSTRACT** The development of molecular catalysts for the electrochemical activation of small molecules focuses on understanding the factors that reduce overpotential and increase catalytic rate constants. Key factors include the electronic effects of substituents on ligands and the role of cofactors such as proton donors. In this study, we demonstrate that the reductive deoxygenation of nitrous oxide (N₂O), catalyzed by rhenium bipyridine triscarbonyl complexes, is accelerated in the presence of water due to hydrogen bonding, which assists the cleavage of the N–O bond. Moreover, water decreases the overpotential by accelerating the decoordination of the hydroxo ligand, facilitating the regeneration of the active site after the first turnover. We also find that modifying the bipyridine ligand with electron-donating groups (CH₃, tBu, OCH₃) has only a modest effect on the catalysis, suggesting that the electron stored on the bipyridine ligand does not significantly influence the reaction unless it is strongly trapped, as evidenced by the lack of reactivity in the complex bearing strongly electron-withdrawing groups (CO(O)CH₃).

## Introduction

The electrochemical molecular catalysis of small molecule activation is a highly active research field, not only due to its potential role in energy storage processes¹ but also for its application in the conversion of harmful nitrogen oxide waste involved in the nitrogen cycle.² Among these transformations, although thermodynamically easier than the reduction of nitrate (NO₃⁻) to nitrite (NO₂⁻) and nitrite to nitric oxide (NO), the reduction of nitrous oxide (N₂O) to N₂ is kinetically challenging. This reaction can be viewed as a simple model for N-O bond activation.³ Recently we demonstrated that [Re(bpy)(CO)₃Cl] (bpy = 2,2'-bipyridine) 1 complex acts as a molecular catalyst for the electrochemical reduction of N₂O in acetonitrile⁴ via an innersphere mechanism⁵ involving a self-modulation process due to the production of hydroxide, a strong ligand, as a co-product.⁶ Beyond the latter Re complex, molecular catalysts for N₂O reduction remain scarce,⁷ and the factors influencing their reactivity are not well understood.⁸ As seen in other examples of molecular catalysis of electrochemical reactions, such as the conversion of CO₂ to CO⁹ or oxygen reduction,¹⁰ improving the catalytic efficiency within a family of transition metal complex catalysts requires a deep understanding of how ligand modifications¹¹ and co-substrates, such as proton donors, affect the process.¹² Interestingly, rhenium bipyridine triscarbonyl complexes have also been reported to be efficient photocatalysts for N₂O reduction. Specifically, [Re(4,4'-tBu-bpy)(CO)₃Cl] (4) is more efficient than the parent compound [Re(bpy)(CO)₃Cl] (1) as well as [Re(4,4-dmbpy)(CO)₃Cl] (2) and [Re(6,6'-dmbpy)(CO)₃Cl] (3).¹³ Additionally, this family of photocatalysts can also efficiently deoxygenate N-heterocyclic and alkyl N-oxides, particularly in the presence of water, playing a key role in enhancing the reaction rate.¹⁴ This prompted us to explore the influence of electronic effects of substituents on the bipyridine

ligand in the electrochemical reduction of N₂O catalyzed by rhenium bipyridine triscarbonyl complexes 1 to 6 (Scheme 1). We also examined the effect of water as a co-substrate and found that its role in boosting the catalysis is twofold: hydrogen bonding assists the cleavage of the N-O bond and water accelerates the decoordination of the hydroxo ligand, which facilitates the regeneration of the catalytic active intermediate after the first catalytic turnover.

## Results and Discussion

*Cyclic voltammetry and IR characteristics of catalysts under argon*

Cyclic voltammograms (CV) of compounds 1 to 6 have been recorded in CH₃CN + 0.1 M *n*-Bu₄NPF₆ under argon to assess the electronic effect of the substituents on the bpy ligand (Figure 1). In all cases except 6 CVs show a first reversible one electron wave followed by an irreversible one electron wave. As already reported,¹⁵,¹⁶ the first wave corresponds to a reduction on the bpy ligand, whereas the second wave is metal centered and coupled with the decoordination of a chloride ligand. (steps (1) and (2) in Scheme 2). The intermediate formed after the first reduction, [Re⁰(*R*-bpy*)*(CO)₃Cl]⁻, may also lose a chloride leading then to [Re⁰(*R*-bpy)(CO)₃] which then dimerizes.⁶ However this competition process is slow on the timescale of the CV but, still, it gives rise to a small amount of dimer which redox features can be seen via shoulders on the reverse traces of the CVs (Figure 1).¹⁵,⁶

**Scheme 1.** [Re(*R*-bpy)(CO)₃Cl] catalysts 1 to 6 and 1(OH)

<footnote>ª Univ Grenoble Alpes, DCM, CNRS, 38000 Grenoble, France.</footnote>

<footnote>ᵇ Université Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France.</footnote>
<figure><img src="image_2.png" /></figure>

Electron donating groups (CH₃ on 4,4' positions (2), CH₃ on 6,6' positions (3), tBu (4) and OCH₃ on 4,4' positions (5)) induce a standard potential for the first reduction slightly more negative than that of the regular bipyridine, ca. 70-80 mV (Table 1). It is in line with a destabilization of the reduced species [Re¹(R-bpy⁺)(CO)₃Cl]⁻ by these electron donating groups. The peak potentials of the second reduction waves are also shifted to more negative potentials compared to the unsubstituted bpy compound 1. This shift results from a combination of the antagonistic electronic effects of substituents on the standard potential of the [Re¹(R-bpy⁺)(CO)₃Cl]⁻/[Re⁰(R-bpy⁺)(CO)₃Cl]²⁻ redox couples (expected to shift cathodically) and on the rate constants of the chloro ligand coordination, expected to increase with electron donating groups (and therefore to shift peak potentials anodically).¹⁷ We can thus conclude that the thermodynamic destabilization of the bi-reduced species [Re⁰(R-bpy⁺)(CO)₃Cl]²⁻ by electron donating groups has a stronger effect on the peak potential than the acceleration of the rate constant of chloro ligand coordination.

The behavior of compound 6 bearing electron withdrawing substituents is different. It exhibits two reversible waves (Figure 1f) which standard potentials are shifted anodically compared to the parent compound 1. As already described elsewhere,¹⁸ it is due to a delocalization of the added electrons on the -C(O)OCH₃ groups. Delocalization of the electrons on the substituents groups strongly stabilizes the reduced species and also prevent chloro ligand coordination of the bi-reduced species 6²⁻. As shown in the case of CO₂ reduction catalyzed by such a complex, this absence of chloro ligand coordination prevents catalysis.¹⁸ It is therefore anticipated to be similar for N₂O reduction (vide infra).

a

b

<figure><img src="image_3.png" /><figcaption>**Figure 1.** CVs in CH₃CN + 0.1 M n-Bu₄NPF₆ under Ar at 0.1 V/s on a 3 mm diameter GCE of **1** (a), **2** (b), **3** (c), **4** (d), **5** (e), **6** (f) c ≈ 1.5 mM.</figcaption></figure>

<table><caption>**Table 1.** Thermodynamic and kinetic parameters</caption><thead><tr><th></th><th>R</th><th>E<sub>1</sub><sup>0</sup><br>(V)<sup>a</sup></th><th>E<sub>p,2</sub><br>(V)<sup>b</sup></th><th>σ<sub>p</sub><sup>d</sup></th><th>k<sub>ap</sub><br>(M<sup>-1</sup>s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1</td><td>H</td><td>-1.64<sub>5</sub></td><td>-2.01<sub>7</sub></td><td>0</td><td>2010</td></tr><tr><td>2</td><td>CH<sub>3</sub> (4,4')</td><td>-1.71<sub>5</sub></td><td>-2.03<sub>0</sub></td><td>-0.17</td><td>2200</td></tr><tr><td>3</td><td>CH<sub>3</sub> (6,6')</td><td>-1.71<sub>5</sub></td><td>-2.04<sub>6</sub></td><td>-0.17</td><td>1510</td></tr><tr><td>4</td><td>tBu (4,4')</td><td>-1.72<sub>5</sub></td><td>-2.11<sub>5</sub></td><td>-0.20</td><td>1800</td></tr><tr><td>5</td><td>OCH<sub>3</sub><br>(4,4')</td><td>-1.72<sub>6</sub></td><td>-2.04<sub>5</sub></td><td>-0.27</td><td>1960</td></tr></tbody></table>
<table><tr><td>6</td><td>C(O)OCH<sub>3</sub><br>(4,4')</td><td>-1.22<sub>8</sub></td><td>-1.48<sub>7</sub><sup>c</sup></td><td>-</td><td>-</td></tr></table>

a all potentials are referred to Ag⁺/Ag reference electrode throughout the paper; see SI to report the potential vs. SCE). b 2nd cathodic peak potential c standard potential d benzene para-substituted Hammet parameter.

The C≡O stretching region in the IR spectra of 1 to 6 shows one sharp and one broad absorption peaks attributed to two overlapping bands, for a total of three CO stretching vibrational modes (Figure 2). The presence of electron donating groups (2 to 5) induces a very small shift (1-6 cm⁻¹) of the CO bands to lower energies corresponding to slightly more π-back-bonding from carbonyl ligands to the rhenium center. Conversely, the electron withdrawing effect of the substituents on 6 induce a shift to higher energies.¹⁹

<figure><img src="image_4.png" /><figcaption>**Figure 2.** IR spectra (C≡O region) of CH₃CN solutions of 1 to 6.</figcaption></figure>

Altogether, these electrochemical and spectroscopic characteristics confirm the expected effect of electron donating and withdrawing groups on the bipyridine ligand. Whereas electron donating effects appear to be modest, the electron withdrawing effect of -C(O)OCH₃ groups is important on the reduced forms of compound 6 inducing a large shift of the standard potentials and, importantly, a large decrease of the chloro ligand lability on the bi-reduced species 6².

### Cyclic voltammetry under N₂O – effect of water

Under N₂O, the CVs of compounds 1 to 5 show two important changes compared to CVs under argon as already described for 1:⁶ (i) the second irreversible wave becomes bi-electronic (Figure S1) and (ii) a catalytic wave develops at more negative potential than the 2nd wave (Figure 3). Previous investigation of the mechanism of N₂O reduction catalyzed by 1 showed that these observations can be rationalized considering the mechanism depicted in Scheme 2.⁶ After the first monoelectronic wave corresponding to step (1), the second wave corresponds to a two-electron reduction of one N₂O molecule corresponding to step (2) to (4) where the rhenium complex is converted from [Re¹(R-bpy⁻)(CO)₃Cl]⁻ to [Re¹(R-bpy⁻)(CO)₃(OH)]⁻. No catalytic process is triggered because, at the level of this second wave, hydroxo is a strong ligand and

cannot be displaced by N₂O. Catalysis is indeed triggered at a more negative potential where [Re¹(R-bpy⁻)(CO)₃(OH)]⁻ is reduced and regenerates [Re⁰(R-bpy⁻)(CO)₃]⁻ with a free coordination site (step 5) in Scheme 2). The intensity of the catalytic wave is slightly different depending on the catalyst (1 to 5) and is analyzed to extract kinetic information (vide infra).

<figure><img src="image_5.png" /><figcaption>**Figure 3.** CVs in CH₃CN + 0.1 M n-Bu₄NPF₆ on a 3 mm diameter GCE of 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), 6 (f) (1 mM) at 0.1 V/s under argon (black) and under N₂O without added proton source (blue) and with 0.05 (magenta), 0.1 (red), 0.2 (green), 0.3 (dark yellow) M H₂O. The current, i, is normalized by the peak current of first one electron wave $i_p^0$.</figcaption></figure>

As anticipated, compound 6 has a singular behavior (Figure 3f). The second wave does not become bi-electronic but the initial mono-electronic reduction is split, presumably because of the sluggish displacement of the chloro ligand by N₂O forming a small amount of hydroxo complex which is reduced at a slightly more negative potential. At more negative potential, a small catalytic wave is observed triggered by the further
reduction of the corresponding hydroxide complex. Due to the inefficient catalysis, in terms of rate, 6 will not be further investigated, albeit its reduced overpotential compared to the other catalysts.

**Scheme 2.** Mechanism of N₂O reduction catalyzed by [Re(R-bpy)(CO)₃Cl] complexes

<figure><img src="image_6.png" /></figure>

Addition of water has two noticeable effects on the catalytic wave as already reported for 1:⁵ (i) an increase of the catalytic current and (ii) a shift of the catalytic wave to less negative potentials. We also note that the first reversible wave remains unchanged upon addition of water and that the position of the second wave is not significantly modified (Figure S2); we only observe that addition of water is required to fully reach a two-electron stoichiometry for the second wave. These observations can be rationalized in the framework of the mechanism shown in Scheme 2 via a rigorous kinetic analysis that will reveal the role of proton donor in the activation of N₂O as well as the possible effect of substituents on the bipyridine ligand.

### Kinetic analysis

Cyclic voltammetry is a convenient technique to extract kinetic information of an electrochemical reaction catalyzed by a homogeneous molecular catalyst provided a rigorous analysis is performed. In pure kinetic conditions, i.e., when catalysis is fast enough, a steady state is established due to compensation of diffusion and chemical reaction for the active species of the catalyst. If, additionally, there is no substrate and co-substrate consumption and no side phenomena, (such as catalyst degradation, electrode fooling etc...) an S-shape CV independent on scan rate is obtained.¹⁷ Kinetic information can thus be obtained from the plateau current. In the present case, in the framework of the mechanism shown in Scheme 2, the apparent rate constant $k_{ap}^{H_2O}$ can be obtained from the plateau current according to equation (6):¹⁷

$$ \frac{i_{pl}}{i_p^0} = 2.24 \sqrt{\frac{RT}{F_v}} \sqrt{2k_{ap}^{H_2O}} \quad (6) $$

where $i_p^0$ is the peak current of the first reversible wave of the catalyst, v the scan rate, T the temperature, R the gas constant and F the Faraday constant. As seen in Figure 3, CVs exhibit catalytic waves not far from ideal S-shaped CVs. Additionally, the catalytic waves are only slightly sensitive to the variation of the scan rate (Figure S3) indicating that pure kinetics conditions are met and that the plateau current can be safely used to extract kinetic informations. $k_{ap}^{H_2O}$ have thus been obtained for catalysts **1(OH)**, **2**, **3**, **4**, and **5** with various concentrations of added water (Figure 4). **1(OH)** gives the same catalytic wave as **1** (Figure S4) in agreement with the mechanism showing that the chloro ligand is replaced by an hydroxo ligand and upon the first turnover of the reaction (Scheme 2). **1(OH)** was thus used instead of **1** to extract the kinetic information.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Plot of $k_{ap}^{H_2O}$ as function of the added water for **1(OH)** (black), **2** (blue), **3** (red), **4** (green), **5** (magenta). Lines: fitting according to equation (7).</figcaption></figure>

As shown in Figure 4, the apparent rate constant $k_{ap}^{H_2O}$ varies linearly with the concentration of added water according to equation (7):

$$ k_{ap}^{H_2O} = k_0 + k_{ap} [H_2O] \quad (7) $$

with $k_0$ being the apparent rate constant in the absence of added proton donor. It presumably corresponds to catalysis occurring with residual water in the solvent and supporting electrolyte which concentration can be estimated to be ca. 15-40 mM from extrapolation of the linear fits of Figure 4. The linear fits indicate that the chemical reduction of N₂O by the reduced species [Re⁰(R-bpy•⁻)(CO)₃]⁻ is first order in water with apparent rate constants $k_{ap}$ gathered in Table 1. Importantly, we note that $k_0$ is of the same order of magnitude as the apparent rate constant obtained from the homogeneous reaction between N₂O and electrogenerated [Re⁰(bpy•⁻)(CO)₃]⁻.⁶ It indicates that the rate determining step controlling that plateau current is the chemical N₂O activation sequence gathered as step (3) in Scheme 2 and not coordination of the hydroxo ligand which is thus assumed to be faster (step (5)).

We first observe that there is no obvious correlation between $k_{ap}$ and the Hammet coefficient $\sigma_p$ (Figure S5). We can thus
conclude that there is no important electronic effect of the substituents on the rate constant, with moderate electron donating groups. This confirms that the reactivity occurs on the rhenium center and that, the electron initially stored on the bipyridine ligand and formally transferred to N₂O for its reduction, does not play a determining role.

<figure><img src="image_8.png" /></figure>

**Figure 5.** (a) CVs in CH₃CN + 0.1 M *n*-Bu₄NPF₆ on a 3 mm diameter GCE of **1(OH)** at 0.1 V/s under argon (black) and under N₂O atmosphere with added proton source: 0.05 (magenta), 0.1 (red), 0.2 (green), 0.3 (dark yellow) M. Full lines: H₂O; Dashed lines: D₂O. The current, *i*, is normalized by the peak current of first one electron wave *i*₀ₚ. (b) same as (a) but only forward trace and zoom in on a smaller potential range.

To get more insights into the role of water as proton source in the reduction process of N₂O by [Re�0(R-bpy*-)(CO)₃]⁻, isotopic effect has been evaluated by comparing addition of H₂O and D₂O. Experiments have been performed with **1(OH)** as catalyst. As shown in Figure 5, very similar plateau currents are obtained with both H₂O and D₂O indicating no kinetic isotope effect and therefore no direct proton transfer at the transition state. Combined with the unity reaction order shown in Figure 4, this observation leads us to propose that a water molecule is assisting N-O bond cleavage, in the reductive process, via a hydrogen bond as depicted in Scheme 3. The actual proton transfer is proposed to occur late in the reaction path leading to the products. The proposed reaction scheme also assumes a pre-equilibrated thermodynamically unfavorable binding of N₂O as no experimental evidence suggest a stable adduct between [Re0(bpy*-)(CO)₃]⁻ and N₂O. Notably the proposed mechanism bears similarities with the recently deciphered mechanism with iron tetraphenyl porphyrin as a catalyst.²⁰ Additionally, binding of N₂O to the rhenium center is assumed to occur with the oxygen atom according to DFT calculations reported in reference 14.

**Scheme 3.** Water assisted reduction of N₂O by [Re⁰(bpy*-)(CO)₃]⁻

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

Another noticeable effect of water is the shift of the catalytic wave toward less negative potentials (Figure 3) which can be quantified by *E*<sub>pl/2</sub> corresponding to the potential when the current is *i*<sub>pl</sub>/2. This shift is more pronounced with H₂O compared to D₂O (Figure 5 and Figure S6). According to the proposed mechanism (Scheme 2), the shift of the catalytic wave upon addition of water results from a shift of the redox couple at which the catalytic wave is pinned, i.e. [Re⁴(R-bpy*-)(CO)₃(OH)]⁻/[Re⁰(R-bpy*-)(CO)₃]⁻ which is responsible for regeneration of a free coordination site on the catalyst. The redox event is not a simple electron transfer but it is coupled with hydroxo ligand decoordination. It is indeed confirmed by running CVs under argon of **1(OH)** in the presence of increasing amount of water (Figure 6). Whereas the first reversible wave is insensitive to the water content with a standard potential equal to -1.675 V (corresponding to step (4) in Scheme 2), the second wave is not reversible and is shifting toward less negative potentials as the amount of water is increasing (Figure 6). It is also observed that more reversibility of the first wave is recovered with water.

<figure><img src="image_10.png" /></figure>

**Figure 6.** CVs in CH₃CN + 0.1 M *n*-Bu₄NPF₆ on a 3 mm diameter GCE of **1(OH)** at 0.1 V/s under argon without (black) and with added proton source: 0.05 (magenta), 0.1 (red), 0.2 (green), 0.3 (dark yellow) M. The current, *i*, is normalized by the peak current of first one electron wave *i*₀ₚ.

Considering that step (5) in Scheme 2 is an EC process (Scheme 4), the features of the CVs shown in Figure 6, indicate that water is probably increasing the rate constants for both hydroxo ligand decoordination (shift of the cathodic peak) and hydroxide coordination (no modification of the plateau type oxidation current indicated by an arrow in Figure 6).
A simple shift of the standard potential would not modify the reversibility of the first wave (see SI for details).

**Scheme 4.** Reductive decoordination of $[Re^{l}(bpy^{•-})(CO)_{3}(OH)]^{-}$ in the presence of water.

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

Overall, both this EC reductive decoordination and the irreversible sequence of N₂O reduction (step (3) in Scheme 2) are more favorable in the presence of water making the catalysis more efficient both in terms of overpotential and rate. Paradoxically, in the first case (decrease of overpotential), the improvement appears to be due to an improvement of the kinetics of a chemical step coupled to the redox event (in line with an isotopic effect) whereas in the second case (increase of catalysis rate), the improvement appears to be due to a thermodynamic effect (H-bonding, no substantial isotopic effect).

## Experimental section

All experimental details are given in the Supporting Information.

## Conclusion

This study demonstrates the influence of electronic substituents on the bipyridine ligand and the presence of water on the catalytic properties of rhenium complexes for N₂O reduction. Electron-donating substituents cause modest changes in electrochemical behavior, while electron-withdrawing groups (as in compound 6) have a more pronounced effect which drastically slow down the catalytic reaction. Water plays a key role

1. Zhang, W.; Lai, W.; Cao, R. Energy-Related Small Molecules Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems. *Chem. Rev.* **2017**, *117*, 3717-3797.

2. Lehnert, N.; Dong, H. T.; Harland, J. B.; Hunt, A. P.; White, J. C. Reversing nitrogen fixation. *Nat. Rev. Chem.* **2018**, *2*, 278-289.

3. Pauleta, S. R.; Carepo, M. S. P.; Moura, I. Source and Reduction of Nitrous Oxide. *Coord. Chem. Rev.* **2019**, *387*, 436-449.

4. Deeba, R.; Molton, F.; Chardon-Noblat, S.; Costentin, C. Effective Homogeneous Catalysis of Electrochemical Reduction of Nitrous Oxide to Dinitrogen at Rhenium Carbonyl Catalysts. *ACS Catal.* **2021**, *11*, 6099-6103.

5. Deeba, R.; Chardon-Noblat, S.; Costentin, C. Homogeneous molecular catalysis of the electrochemical reduction of N₂O to N₂: redox vs. chemical catalysis. *Chem. Sci.* **2021**, *12*, 12726-12732.

6. Deeba, R.; Chardon-Noblat, S.; Costentin, C. Importance of Ligand Exchange in the Modulation of Molecular Catalysis: Mechanism of the Electrochemical Reduction of Nitrous Oxide with Rhenium Bipyridyl Carbonyl Complexes. *ACS Catal.* **2023**, *13*, 8262-8272.

in improving both the rate and the efficiency of the catalytic process. Our kinetic data indicate that water assists the cleavage of the N-O bond via hydrogen bonding and that water also accelerates the decoordination of the hydroxo ligand, which facilitates the regeneration of the active species after the first catalytic turnover. These finding may be used to design better molecular catalysts for deoxygenation reactions.

## ASSOCIATED CONTENT

### Supporting Information

Experimental details: Chemicals, synthesis, methods and instrumentation. Additional data: catalysis in the presence of N₂O; effect of water on pre-catalytic waves; effect of scan rate on the catalytic waves; comparison of the catalytic wave with **1** and **10H** catalysts; correlation Hammet parameters-rate constants; effect of water on the catalytic wave position; effect of water on the catalyst. (PDF)

## ACKNOWLEDGMENT

This work was supported by the Agence Nationale de la Recherche (DeNOSOr project ANR-22-CE07-0028-01), and Labex ARCANE (CBH-EUR-GS, ANR-17-EURE-0003). The MITI (Mission pour les Initiatives Transverses et Interdisciplinaires) program of the CNRS is gratefully acknowledged for Rana Deeba doctoral financial support. ANR is gratefully acknowledged for Céline Naddour doctoral financial support. ADEME is gratefully acknowledged for Marianne Kjellberg doctoral financial support. Dr. Lucile Anthore-Dalion, Dr. Emmanuel Nicolas and Dr. Thibault Cantat (CEA-Saclay) are warmly thanked for helpful discussions.

### Notes

The author declares no competing financial interest.

## REFERENCES

7. (a) Collman, J. P.; Marrocco, M.; Elliott, C. M.; L'Her, M. Electro catalysis of nitrous oxide reduction. Comparison of several porphyrins and binary "face-to-face" porphyrins. *J. Electroanal Chem.* **1981**, *124*, 113-131.
   (b) Taniguchi, I.; Shimpuku, T.; Yamashita, K.; Ohtaki, H. Electrocatalytic Reduction of Nitrous Oxide to Dinitrogen at a Mercury Electrode using Ni<sup>II</sup> Complexes of Macrocyclic Polyamines. *J. Chem. Soc., Chem. Commun.* **1990**, 915-917.
   (c) Zhang, J.; Tse, Y-H.; Lever, A. B. P.; Pietro, W. J. Electrochemical Reduction of Nitrous Oxide (N<sub>2</sub>O) catalyzed by Tetraaminophthalocyanocobalt(II) Adsorbed on a Graphite Electrode in Aqueous Solution. *J. Porphyrins Pthalocyanines* **1997**, *1*, 323-331.
   (d) Stanley, J. S.; Wang, X. S.; Yang, J. Y. Selective Electrocatalytic Reduction of Nitrous Oxide to Dinitrogen with an Iron Porphyrin Complex. *ACS Catal.*, **2023**, *13*, 12617-12622.

8. Chartier, C. ; Deeba, R. ; Collard, A.; Chardon-Noblat, S.; Costentin, C. Iron(I) Tetraphenylporphyrin Is an Active Catalyst in Reductive Deoxygenation when Switching from CO₂ to Isoelectronic N₂O. *ACS Catal.* **2014**, *14*, 14509-14516.

9. (a) Costentin C.; Savéant J-M. Towards an intelligent design of molecular electrocatalysts. *Nat. Rev. Chem.* **2017**, *1*, 0087 (b) Derrick, J. S.; Loipersberger, M.; Nistanaki, S. K.; Rothweiler, A. K. ;
Head-Gordon, M.; Nichols, E. M.; Chang, C. J. Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins. *J. Am. Chem. Soc.* **2022**, *144*, 11656-11663. (c) Amanullah, S.; Gotico, P.; Sircoglou, M.; Leibl, W.; Llansola-Portoles, M. J.; Tibiletti, T.; Quaranta, A.; Halime, Z.; Aukauloo, A. Second Coordination Sphere Effect Shifts CO₂ to CO Reduction by Iron Porphyrin from Fe⁰ to Fe¹. *Angew. Chem. Int. Ed.* **2023**, e202314439.

10. (a) Martin, D., J.; Wise, C. F.; Pegis, M. L.; Mayer, J. M. Developing Scaling Relationships for Molecular Electrocatalysis through Studies of Fe-Porphyrin-Catalyzed O₂ Reduction. *Acc. Chem. Res.* **2020**, *53*, 1056-1065. (b) Bhunia, S.; Ghatak, A.; Dey, A. Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems. *Chem. Rev.* **2022**, *122*, 12370-12426.

11. Bullock M. R.; Dey, A. Introduction: Catalysis beyond the First Coordination Sphere. *Chem. Rev.* **2022**, *122*, 11897-11899.

12. Costentin, C.; Drouet, S.; Passard, G.; Robert, M.; Savéant, J-M. Proton-Coupled Electron Transfer Cleavage of Heavy-tom Bonds in Electrocatalytic Processes. Cleavage of a C-O Bond in Catalyzed Electrochemical Reduction of CO₂. *J. Am. Chem. Soc.* **2013**, *135*, 9023-9031.

13. Kjellberg, M. ; Ohleier, A. ; Tuéry, P. ; Nicolas, E. ; Anthore-Dalion, L. ; Cantat, T. Photocatalytic deoxygenation of N-O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides. *Chem. Sci.* **2022**, *12*, 10266-10272.

14. Joly, N. ; Kjellberg, M. ; Bouchet, A. ; Nicolas, E. ; Cantat, T. ; Anthore-Dalion, L. A Versatile and Fast Photocatalyzed Deoxygenation of Organic N-oxides with a Rhenium Complex in the Presence of H₂O. *10.26434/chemrxiv-2025-1fhhv* (accessed 2025-05-02).

15. Sullivan, B. P.; Bolinger, C. M; Conrad, D.; Vining, W. J.; Meyer, T. J. One- and Two-electron Pathways in the Electrocatalytic Reduction of CO₂ by *fac*-Re(bpy)(CO)₃Cl (bpy = 2,2'-bipyridine) *J. Chem. Soc., Chem. Commun.* **1985**, 1414-1416.

16. Clark, M. L.; Cheung, P. L.; Lessio, M.; Carter, E. A.; Kubiak, C. P. Kinetic and Mechanistic Effects of Bipyridine (bpy) Substituent, Labile Ligand and Bronsted Acid on Electrocatalytic CO₂ Reduction by Re(bpy) Complexes. *ACS Catal.* **2018**, *8*, 2021-2029.

17. Savéant, J-M.; Costentin, C. *Elements of Molecular and Biomolecular Electrochemistry*. 2nd Ed. Wiley 2019.

18. Guyot, M.; Lallop, M.-N.; Aguirre-Araque, J. S. ; Rogez, G. ; Costentin, C. ; Chardon-Noblat, S. Rhenium Carbonyl Molecular Catalysts for CO₂ Electroreduction: Effects on Catalysis of Bipyridine Substituents Mimicking Anchorage Functions to Modify Electrodes. *Inorg. Chem.* **2022**, *61*, 16072-16080.

19. (a) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J. Electrocatalytic reduction of CO₂ using the complexes [Re(bpy)-(CO)₃L]n (n = +1, L = P(OEt)₃, CH₃CN; n = 0, L = Cl⁻, Otf⁻; bpy = 2,2'-bipyridine; Otf⁻ = CF₃SO₃) as catalyst precursors: Infrared spectroelectrochemical investigation. *Organometallics* **1996**, *15*, 3374-3387. (b) Timney, J. A.; Turner, J. J. The calculation of energy-factored force constants for fac-Re(CO)3(N-N)(X) molecules and related complexes. *Vibrational Spectroscopy* **2022**, *120*, 103374.

20. Naddour, C.; Deeba, R.; Chartier, C.; Nicolas, E.; Chardon-Noblat, S.; Costentin, C. H-Bond-Assisted Cleavage of N-O Bond in the Electrochemical Reduction of N₂O Catalyzed by Iron Tetraphenylporphyrin. *ACS Catal.* **2025**, *15*, 9393-9400.