# H-Bond assisted cleavage of N-O bond in the electrochemical reduction of N_2O catalyzed by iron tetraphenylporphyrin

Céline Naddour, Rana Deeba, Camille Chartier, Emmanuel Nicolas, Sylvie Chardon-Noblat, Cyrille Costentin

<figure><img src="image_1.png" /></figure>

## To cite this version:

Céline Naddour, Rana Deeba, Camille Chartier, Emmanuel Nicolas, Sylvie Chardon-Noblat, et al. H-Bond assisted cleavage of N-O bond in the electrochemical reduction of N_2O catalyzed by iron tetraphenylporphyrin. ACS Catalysis, 2025, 15 (11), pp.9393-9400. 10.1021/acscatal.5c01965. hal-05061360

HAL Id: hal-05061360

https://hal.science/hal-05061360v1

Submitted on 9 May 2025

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# H-Bond Assisted Cleavage of N-O Bond in the Electrochemical Reduction of N₂O Catalyzed by Iron Tetraphenylporphyrin.

Céline Naddour,‡,ª Rana Deeba,‡,ª Camille Chartier,ª Emmanuel Nicolas,ᵇ Sylvie Chardon-Noblatª and Cyrille Costentinª,*,

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

**ABSTRACT:** Reductive deoxygenation reactions play a crucial role in electrocatalytic processes relevant to energy and environmental challenges, including the reduction of CO₂, O₂, and N₂O. In particular, N₂O reduction is essential for closing the nitrogen cycle and preventing its accumulation in the atmosphere. These reactions are also significant in chemical processes, such as the reduction of phosphine or sulfur oxides. In this context, we demonstrate that the molecular catalysis of N₂O to N₂ reduction by iron tetraphenyl porphyrin occurs predominantly via an inner-sphere mechanism. We then reveal that acids of varying strengths, including water, ethanol, trifluoroethanol, phenol, and acetic acid, can accelerate the N–O bond cleavage. Surprisingly, the effect of acid $pK_a$ is minimal, suggesting that the acceleration arises from hydrogen-bond-assisted N–O bond cleavage rather than direct conventional protonation. The process begins with N₂O coordination to the low-valent iron tetraphenyl porphyrin, activating the bond. At high acid concentrations, this binding step becomes rate-determining. Density functional theory (DFT) calculations support the proposed mechanism, highlighting the importance of a dual activation strategy: bond activation by a low-valent transition metal and hydrogen-bond assistance from an acid acting as an H-bond donor. These findings provide valuable insights for designing more effective catalysts for N–O bond activation and, more broadly, for advancing reductive deoxygenation reactions.

## Introduction

Proton transfers are ubiquitous and play a well-recognized role in the reductive cleavage of chemical bonds. ¹ In particular, they are crucial for activating small molecules such as dioxygen, ² carbon dioxide ³ and the more challenging dinitrogen. ⁴ Specifically, in the molecular catalysis of the electrochemical reductions of O₂ and CO₂ using low valent transition metal complexes, proton transfers can dictate the selectivity (e.g., H₂O₂ vs. H₂O for O₂ electroreduction) ⁵ or be part of the rate determining step of the C-O bond cleavage for CO₂ to CO conversion. ⁶ The underlying principle of these reactivities follows a push-pull mechanism. In that case, the substrate molecule, coordinated to the transition metal center undergoes activation (reduction) via formal electron transfers from the low valent metal center of the complex leading to the cleavage of a O-O ⁷ or C-O ⁶ bond while the leaving oxygen is stabilized by a proton transfer (Scheme 1). The proton transfer may be concerted with bond cleavage or occur in a separate step. ⁸ More broadly, in catalytic reductive deoxygenation reactions, it is anticipated that the cleavage of the oxygen containing bond is the rate determining step. Gaining insights into this process and developing strategies to accelerate it is therefore essential.

We recently investigated the reductive deoxygenation of nitrous oxide (N₂O), ⁹ a kinetically inert greenhouse gas, ¹⁰ which can be seen as a model example of a reductive N-O bond cleavage. Among the few known molecular catalysts for N₂O to N₂ electrochemical reduction, iron tetraphenylporphyrin (TPPFe) has demonstrated activity at both the formal

$Fe(0)$¹¹ and the Fe(I) redox states, though the latter operates at significantly slower rate. ¹² In the Fe(0) state, it has been reported that, in THF solvent, water facilitates the reaction as a proton donor. It was suggested that the rate determining step could be proton transfer to N₂O concerted with electron transfer and N-O bond breaking ¹¹ similar to the mechanism observed in CO₂ reduction catalyzed by TPPFe(0) (Scheme 1). ⁶

**Scheme 1.** Push-pull electron transfer-proton transfer bond cleavage mechanism

<figure><img src="image_3.png" /></figure>

Herein, we demonstrate that the reductive deoxygenation of N₂O to N₂ by TPPFe(0) in dimethylformamide (DMF) occurs predominantly via an innersphere process and can proceed with acids of varying strength including water, ethanol, trifluoroethanol (TFE), phenol or acetic acid. A kinetic analysis using cyclic voltammetry (CV) provides insights into the mechanism of the N-O bond cleavage. Surprisingly, while the

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

<footnote>ᵇNIMBE (UMR 3685), CEA, CNRS, Université Paris-Saclay, CEA Paris-Saclay, 91191 CEDEX Gif-sur-Yvette, France.</footnote>

<footnote>‡Equal contribution</footnote>
reaction is clearly acid-assisted, the effect of the acid pKₐ is minimal, suggesting a H-bond type assistance of the bond cleavage. The proposed mechanism is supported by DFT calculations.

## Results and Discussion

*Initial assessment of reliable conditions in cyclic voltammetry*

Figure 1a presents the CV of TPPFeCl in DMF + 0.1 M n-Bu₄NPF₆ under argon revealing three reversible waves, corresponding to the formal Fe(III)/Fe(II), Fe(II)/Fe(I) and Fe(I)/Fe(0) couples. Under N₂O, and in the absence of added proton source, a catalytic wave emerges at the level of the formal Fe(I)/Fe(0) couple. Since TPPFe(0) is known to react with n-Bu₄N⁺ cation from the supporting electrolyte,¹³ we first ensure that this alkylation does not interfere with the CV analysis. The chemical reversibility of the Fe(0)/Fe(I) couple indicates that this reaction does not occur on the CV timescale. The rate constant for the reaction between electrogenerated TPPFe(0) (0.5 mM) in DMF + 0.1 KPF₆ and n-Bu₄N⁺ (see Supporting Information (SI) for details; Figure S1) is determined to be 0.7 M⁻¹s⁻¹, leading to a pseudo first order rate constant of 0.07 s⁻¹ in 0.1 M n-Bu₄NPF₆. This value is sufficiently low to avoid interference with the catalytic process under N₂O, which can be confirmed once the rate constant for N₂O reduction by TPPFe(0) is determined (vide infra). Despite the large excess of N₂O, the catalytic CV does not exhibit an S-shaped response, suggesting that the process is limited, maybe by a side process that could be the lack of protons or a partial deactivation of the catalyst.¹⁴

<figure><img src="image_4.png" /><figcaption>**Figure 1.** CVs in DMF at 0.1 V/s on a 3 mm diameter GCE under Ar (black) and under N₂O (blue). TPPFeCl (1 mM) + (a) 0.1 M n-Bu₄NPF₆, (b) 0.1 M KPF₆.</figcaption></figure>

Interestingly, when the reaction is conducted in dry DMF + 0.1 KPF₆, a more intense catalytic wave is observed (Figure 1b). However, the current abruptly drops, resulting in a catalytic wave shape characteristic of an irreversible surface inhibition process,¹⁵ which is confirmed by the complete disappearance of the catalyst's reoxidation waves in the reverse scan. Both the increase rising current, compared to the CV in DMF + 0.1 M n-Bu₄NPF₆, and the inhibition behavior can be explained by the dual role of the K⁺ electrolyte cation. As a Lewis acid, K⁺ enhances N₂O reduction, but its involvement also leads to the formation of insoluble potassium oxide/hydroxide, which passivates the electrode surface.¹⁶ To mitigate electrode passivation and considering that the side reaction between TPPFe(0) and n-Bu₄N⁺ does not occur, the study of the role of different added acids, described below, was conducted

using n-Bu₄NPF₆ as supporting electrolyte.

*Outersphere electron transfer pathway*

The catalytic wave observed in Figure 1a clearly demonstrates that TPPFe(0) efficiently reduces N₂O even in the absence of an intentionally added proton source, as previously reported.¹¹,¹² The first question we address is whether this reaction follows an outersphere or an innersphere mechanism by showing that the kinetics are much faster than predicted for an outersphere mechanism and considering that TPPFe(0) might be better described as a (TPP²⁻)Fe(II) species with both added electrons localized onto the porphyrin ring.¹⁷ To investigate this, we evaluated the rate constant for N₂O reduction using two electrogenerated aromatic radical anions, benzophenone and 1-naphthonitrile, whose standard potentials ($E_{P/P*}^0 = -2.13$ V and -2.22 V respectively; all potentials are reported vs. Ag⁺/Ag throughout the paper) are close to that of TPPFe ($E_{Fe(I)/Fe(0)}^0 = -2.08$ V). Both electrogenerated aromatic radical anions act exclusively as outersphere electron donors toward N₂O, as previously observed in a different medium (acetonitrile + 0.1 M n-Bu₄NPF₆).¹⁸ In the presence of N₂O, a catalytic wave emerges upon electrochemical generation of the radical anions, even without added proton source (Figure 2a-b). These waves are largely unaffected by water addition but do not exhibit an S-shaped profile. However, for 1-naphthonitrile, increasing the scan rate up to 10 V/s results in canonical behavior (pure kinetics conditions with an excess of substrate¹⁹) producing S-shaped CVs that remain insensitive to water addition (Figure 2c). For benzophenone, increasing the scan rate does not achieve pure kinetics conditions, yet the CVs do not revert to a non-catalytic wave. This allows extraction of kinetic information on the catalytic process, with the wave remaining independent of water addition. The CVs can be analyzed in the framework of the mechanism depicted in Scheme 2. The proposed mechanism begins with an initial outersphere electron transfer from the electrogenerated radical anion to N₂O forming N₂O radical anion. This intermediate is then further reduced homogeneously by P*⁻. This second step is assumed to be fast, ultimately yielding the product N₂. The initial step is rate determining, characterized by a pseudo first order rate constant $k = k_{ET} [N₂O]$, where $[N₂O]$ is the concentration of a saturated N₂O solution in DMF, i.e., 0.25 M assuming solubility similar to that in acetonitrile, as it is the case for CO₂.²⁰

**Scheme 2.** N₂O reduction by radical anions (P*⁻)

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a

b
<figure><img src="image_6.png" /><figcaption>**Figure 2.** CVs in DMF + 0.1 M n-Bu₄NPF₆ on a 3 mm diameter GCE. (a) of naphthonitrile (1 mM) at 0.1 V/s under Ar (black) and under N₂O without added proton source (violet) and with 0.05 (purple), 0.2 (blue), 0.3 M (cyan) H₂O; (b) of benzophenone (1 mM) at 0.1 V/s under Ar (black) and under N₂O without added proton source (violet) and with 0.02 (wine), 0.05 (purple), 0.1 (blue), 0.2 M (cyan) H₂O (c) of naphthonitrile (1 mM) at 10 V/s under N₂O atmosphere without added proton source (violet) and with 0.05 (purple), 0.2 (blue), 0.3 M (cyan) H₂O. CVs corrected from capacitive current. Red line: fitting according to equation (1) (d) of benzophenone (1 mM) at 10 V/s under Ar (black) and under N₂O without added proton source (violet) and with 0.02 (wine), 0.05 (purple), 0.1 (blue), 0.2 M (cyan) H₂O (e) Rate constants (in s⁻¹) as function of the standard potential of the reductant E⁰ (in V vs. Ag⁺/Ag) for the homogeneous catalysis of reduction of N₂O. Blue data: reduction by electrogenerated radical anions or dianions in acetonitrile (adapted from reference 18 with permission). The blue line corresponds to the prediction for outersphere electron transfers from reference 18 (Reproduced from ref. 18. Available under a CC-BY XX license. Copyright 2021). Red data: in DMF with 1-naphthonitrile and benzophenone as catalysts (see text).</figcaption></figure>

For 1-naphthonitrile, once canonical behavior is established (Figure 2c), the CV can be described by equation (1):

$$i = \frac{FSC_{cat}^{0} \sqrt{D} \sqrt{2k}}{1 + \exp\left[\frac{F}{RT}(E - E_{cat}^{0})\right]} \quad (1)$$

where C₀ₜ is the catalyst concentration, E₀ₜ its standard potential, S the electrode surface area and D the catalyst diffusion coefficient. Taking into account the peak current under argon, i₀ₚ = 0.446FSC₀ₜ√DFv/RT, the rate constant can be extracted using equation (1). We obtain k = kₘₚtho[N₂O] = 250 s⁻¹. For benzophenone, CVs recorded at 10 V/s were simulated using Digielch software based on the mechanism depicted in Scheme 2, assuming no N₂O consumption (see Figure S2). These simulations account for non pure kinetics conditions, resulting in an estimated rate constant of k = kₘₚzo[N₂O] = 50 s⁻¹. As shown in Figure 2e, the extracted rate constants are comparable to those previously measured in acetonitrile, under the assumption that N₂O solubility is similar in both solvents. The activation-driving force remains consistent in both DMF and acetonitrile for outersphere electron transfers to N₂O. Based on this trend, we predict that a reductant with a standard potential of E₀ₘₚzo[Fe(I)/Fe(0)] = -2.08 V would reduce N₂O via outersphere electron transfer with an apparent pseudo first order rate constant of ca. 30 s⁻¹ (Figure 2e), independent of the acid content.

### Effect of acid addition and kinetic analysis of the innersphere pathway

Figure 3 illustrates the effect of addition of acids with pKₐ values ranging from 31.5 to 13.3²¹ (Table 1) on the CVs of TPPFeCl under an N₂O atmosphere at 0.1 V/s. The reduction waves corresponding to TPPFe(III)/TPPFe(II) and TPPFe(II)/TPPFe(I) couples remain unaffected, while the catalytic wave associated with the TPPFe(I)/TPPFe(0) redox couple increases in intensity. On the reverse scan, oxidation waves shift anodically, suggesting that the catalytic reaction generates a species that stabilizes higher oxidation states of TPPFe (Figure S3). This species is likely the conjugate base of the acid, which may coordinate the iron metallic center in axial position. For the strongest acid used, AcOH, a significant catalytic wave appears under argon, attributed to AcOH reduction leading to H₂ generation (Figure 3f).²² Thus, under N₂O (Figure 3e), the catalytic wave corresponds to both N₂O reduction and partial hydrogen evolution reaction (HER), making the kinetic analysis with AcOH as proton donor unreliable. In contrast, PhOH exhibits minimal HER activity in the range of PhOH concentration used (Figure S4), allowing its contribution to be neglected under N₂O, and this holds even more for weaker acids. To confirm this, we demonstrated that N₂O reduction remains dominant when PhOH is used as proton donor. A controlled potential electrolysis at -2.10 V (1 mM TPPFeCl in the presence of 115 mM PhOH under 1 atm of N₂O) on a carbon felt electrode produced a stable catalytic current of 14 mA for 76 min leading to a linear increase of charge passed over time (Figure S5). Gas chromatography analysis of the gas headspace at the end of the electrolysis
quantified 286.3 µmol of N₂, yielding a 85% Faradaic efficiency (FY) for N₂O to N₂ conversion. No H₂ gas was detected, confirming the high selectivity of the process for N₂O reduction.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** CVs of TPPFeCl (1 mM) in DMF + 0.1 M n-Bu₄NPF₆ on a 3 mm diameter GCE at 0.1 V/s under Ar (black) and (red) under N₂O with addition of (a) H₂O 0.1, 0.2, 0.5 1 M; (b) EtOH 17, 34, 68, 137 mM; (c) TFE 14, 42, 97, 209 mM; (d) PhOH 10, 20, 50, 100 mM. (e) AcOH 17.5, 35, 70, 140 mM; (f) under Ar with addition of AcOH 17.5, 35, 70, 140 mM.</figcaption></figure>

To obtain kinetic information, CVs were analyzed within the framework of the general mechanism depicted in Scheme 3, where *k* represents an apparent pseudo-first order rate constant. The reduction of TPPFe(II)(OH), formed upon reduction of N₂O with electrogenerated TPPFe(0),¹² is assumed to occur in homogeneous solution rather than at the electrode surface. This assumption is supported by the fact that the catalytic process is not fast enough for the latter to be dominant. The apparent rate constant *k* was determined from the CVs as

function of acid concentration. To achieve this, CVs were recorded at various scan rates until a plateau current independent of scan rate was reached (Figure S6). As shown in Figure 4, for each added acid at a concentration equal or close to 0.1 M (except for AcOH, 0.018 M), a quasi-S-shaped CV was obtained at 10 V/s with all forward traces fitted with equation (1). CVs at all acid concentrations are provided in the SI (Figure S7).

<figure><img src="image_8.png" /><figcaption>**Figure 4.** CVs of TPPFeCl (1 mM) in DMF + 0.1 M n-Bu₄NPF₆ on a 3 mm diameter GCE at 10 V/s (capacitive current subtracted) under N₂O with addition of (a) H₂O 0.100 M (b) EtOH 0.137 M. (c) TFE 0.097 M. (d) PhOH 0.100 M. (e) AcOH 0.018 M. Red lines correspond to fitting with equation</figcaption></figure>
(1) taking into account ohmic drop with a 70 Ω uncompensated resistance in the case of H₂O and PhOH.

It is important to note that ohmic drop compensation is crucial for accurate agreement between the experimental data and equation (1) at all potentials. However, it does not affect the plateau current value (Figure S7a, d, e). Even when the positive feedback compensation implemented in the potentiostat was insufficient, leaving some substantial uncompensated resistance, rate constants could still be reliably extracted (Figure 5). Additionally, all extracted rate constants are sufficiently large to prevent deactivation of the catalyst via reaction of TPPFe(0) with n-Bu₄N⁺ (see Figure S8). Finally, using 0.1 M H₂O, as a proton donor and variable amount of N₂O, kinetic analysis of the foot-of-the-wave¹⁴ shows that the reaction is first order in N₂O (Figure S9).

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Apparent rate constant *k* for the reduction of N₂O (saturated solution in DMF + 0.1 M n-Bu₄NPF₆) as function of the amount of added acid (a) H₂O, (b) EtOH, (c) TFE, (d) PhOH, (e) AcOH. Lines correspond to the fitting with equation (2). (f) Plot of log(k₂K₁[N₂O]) as function of the acid</figcaption></figure>

pKₐ.

Figure 5 shows that for all investigated acids, the apparent rate constant *k* increases with acid concentration before reaching a plateau. These variations, along with the observed first-order dependence on N₂O, suggest a reaction mechanism in which TPPFe(0) first forms a pre-association complex with N₂O (Scheme 4). This adduct then evolves into the product (N₂) via a reaction involving the acid (AH) as a first order reactant. At high acid concentrations, this step becomes sufficiently fast compared to adduct dissociation, rendering the initial pre-association effectively irreversible and rate determining (Scheme 4). Based on this mechanism and applying a steady-state approximation for the intermediate adduct between TPPFe(0) and N₂O, the apparent rate constant can be expressed as:

$$k = k_0 + \frac{k_2 k_1 [\text{N}_2\text{O}][\text{AH}]}{k_{-1} + k_2 [\text{AH}]} \quad (2)$$

where *k*₀ is the apparent rate constant in the absence of an added proton donor, determined to be ca. 300 s⁻¹ and it is certainly related to the residual water in DMF + 0.1 M n-Bu₄NPF₆ which concentration was determined to be 13 mM. The other rate constants are defined in Scheme 4. Notably, *k*₀ is significantly higher than the estimated rate for outersphere electron transfer under similar conditions (30 s⁻¹). This suggests that, in the absence of intentionally added acid, the catalytic process involves both a minor contribution (ca. 10%) from outersphere electron transfer by TPPFe(0) and a predominant contribution from an innersphere mechanism. The latter likely involves residual proton donors present in the solvent (DMF) and the supporting electrolyte (n-Bu₄NPF₆), presumably water.

<figure><img src="image_10.png" /><figcaption>**Figure 6.** Plot of k₁[N₂O] as function of the acid pKₐ. Dashed red line: rate constant kET[N₂O] corresponding to an outersphere process (see text). Dashed black line: apparent rate constant k₀ in the absence of added acid.</figcaption></figure>

<table><caption>**Table 1.** Thermodynamic and kinetic constants</caption><thead><tr><th>AH</th><th>pK<sub>a</sub><sup>21</sup></th><th>k<sub>1</sub>[N<sub>2</sub>O] (s<sup>-1</sup>)</th><th>k<sub>2</sub>K<sub>1</sub>[N<sub>2</sub>O] (M<sup>-1</sup>s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>H<sub>2</sub>O</td><td>31.5</td><td>3050</td><td>2.0 10<sup>4</sup></td></tr><tr><td>EtOH</td><td>30.1</td><td>2250</td><td>6.4 10<sup>4</sup></td></tr><tr><td>TFE</td><td>24.0</td><td>2650</td><td>5.3 10<sup>4</sup></td></tr></tbody></table>
<table><tr><td>PhOH</td><td>18.8</td><td>3000</td><td>6.0 10<sup>4</sup></td></tr><tr><td>AcOH</td><td>13.3</td><td>3050</td><td>43.6 10<sup>4</sup></td></tr></table>

At high acid concentrations, the apparent rate constant *k* plateaus, reaching a value corresponding to $k_1[N_2O] \gg k_0$ that is largely independent of the proton donor's nature (Table 1 and Figure 6). Given that the N₂O concentration in DMF is ca. 0.25 M, the resulting rate constant is $k_1 \approx 10^4 M^{-1}s^{-1}$. While N₂O is generally considered a poor coordinating ligand,²³ this value suggests that its weak coordination here is due to thermodynamic rather than kinetic factors. For comparison the rate constant for the OH⁻ binding to TPPFe(I) is only 0.1 M⁻¹s⁻¹.²⁴ Fitting the variation of apparent rate constants with acid concentration using equation (2) provides the apparent second order rate constant $k_2K_1[N_2O]$ when the reaction is first order in acid, with an initial pre-equilibrium step (Table 1). Interestingly, $k_2K_1[N_2O]$ values remain largely unaffected by the acid's $pK_a$ value (Figure 5f), except in the cases of AcOH and, to a lesser extent, H₂O. As already indicated, an additional contribution from HER may influence the apparent rate constant in the AcOH case.

<figure><img src="image_11.png" /><figcaption>**Scheme 4.** Acid assisted reduction of N₂O by TPPFe(0)</figcaption></figure>

Since $k_1$ represents the binding constant of N₂O to TPPFe(0), it is not expected to depend on the acid $pK_a$, as indeed seen for $k_1$. Consequently, the low sensitivity of $k_2K_1[N_2O]$ to $pK_a$ suggests that $k_2$ is largely independent of acid strength. The step associated with the rate constant $k_2$ involves N-O bond cleavage (deoxygenation reaction). Despite its significant large driving force, this step is not diffusion-limited but rather controlled by activation energy. We propose that this bond breaking process is initiated by a formal electron transfer from the iron porphyrin (TPPFe(0)) to the N₂O ligand, facilitated by the acid through H-bond stabilization rather than direct proton transfer (Scheme 5). An attempt to correlate the kinetics with a H-bond descriptor, i.e., the α hydrogen bond donor (HBD) parameter which quantifies a molecule's ability to denote hydrogen bonds²⁵ shows that, besides water, a trend is observed indicating a faster rate with increasing H-bonding

capabilities of AH (Figure S10). We propose that the actual proton transfer occurs late in the reaction path (Scheme 4). In other words, the N-O bond cleavage step has such a large driving force that the transition state is reactant-like and therefore the proton does not substantially move toward O at the transition state. Nonetheless the proton probably transfers in a barrierless process so that no formal TPPFe(II) oxo is formed. Unlike in CO₂ reduction by TPPFe(0), where proton transfer and bond cleavage are concerted⁶ the weak dependence of the rate-determining step on $pK_a$ suggests that, in this case, AH primarily assists bond cleavage via H-bonding. This conclusion contrasts with previous claims that water facilitates the reaction as a proton donor, based on a reported primary kinetic isotope effect (KIE) of 1.8 in THF.¹¹ However, that KIE was derived from rate constants obtained via CVs where the current appeared independent of acid concentration and may not reflect catalysis under canonical conditions. To clarify this, we recorded CVs of TPPFe under N₂O in the presence of either H₂O or D₂O (0.1 M) within the acid-dependent regime (cf. Figure 5a) and at 5 V/s to satisfy canonical conditions. The resulting quasi-plateau currents (Figure S11) allowed us to determine a KIE of 1.18 (see SI for details). This small KIE aligns with the idea that acids assist bond cleavage via H-bonding to the oxygen atom rather than through direct proton transfer.

<figure><img src="image_12.png" /><figcaption>**Scheme 5.** Acid assisted N-O bond cleavage</figcaption></figure>

Finally, DFT calculations were carried out at the ωB97M-V/Def2TZVPP//B3LYP/Def2-SVP level, for three different starting TPPFe complexes (Fe(II), Fe(I), Fe(0)), and considering two proton donors, H₂O and PhOH. For each combination, the computed mechanism follows the sequence: N₂O coordination via the oxygen atom, approach of the proton donor to the oxygen atom of coordinated N₂O, and concomitant proton transfer and N-O bond cleavage, leading to the departure of N₂, and formation of the hydroxide complex together with the conjugated base of the proton donor, as depicted in Scheme 4. First, we see that in all six cases, the coordination of N₂O and of the proton donor is slightly endergonic, the resulting TPPFe(ON₂) complexes being 1.0 to 5.8 kcal.mol⁻¹ higher in energy than the starting TPPFe complexes. Coordination of the proton donor HA (HA = H₂O or PhOH) is also endergonic, leading to the formation of [TPPFe(ON₂)(HA)], 8 to 10 kcal.mol⁻¹ higher than the starting complexes. The N-O bond cleavage step occurs through a transition state whose energy is highly dependent on the redox state of iron (Table 2). Indeed, the energies are ranging from 20.6 kcal.mol⁻¹ (Fe(0) / PhOH) to 40.5 kcal.mol⁻¹ (Fe(II) / H₂O), thus confirming that, experimentally, substantial catalysis requires generation of TPPFe(0) although slower catalysis can occur from TPPFe(I).¹² While we can observe a small influence of the $pK_a$ of the proton donor, in both H₂O and PhOH cases, the energies for a given redox state are similar (Table 2).

<caption>**Table 2.** Energies for the N-O cleavage transition state for each redox state of Fe and H donor (in kcal.mol⁻¹).</caption>
<table><thead><tr><th></th><th>H<sub>2</sub>O</th><th>PhOH</th></tr></thead><tbody><tr><td>Fe(II)</td><td>40.5</td><td>39.9</td></tr><tr><td>Fe(I)</td><td>27.8</td><td>26.0</td></tr><tr><td>Fe(0)</td><td>21.6</td><td>20.6</td></tr></tbody></table>

Structurally, this is reflected in the evolution of bond distances and angles throughout the course of the reaction (Table 3). For the most favored case, TPPFe(0), N₂O is barely impacted upon coordination to the Fe center and remains linear (179.2 °), with a short N–O bond (1.182 Å). At the TS for the N–O bond cleavage, the O–N bond is significantly elongated, to 2.427 Å with H₂O and 2.517 Å for PhOH. The O–N–N angle is also affected (155.7 and 158.4 ° for H₂O and PhOH respectively). However, the O–H bond distance in the donor barely changes: elongations of 0.003 Å in H₂O and 0.006 Å in PhOH are observed in the computed structures. This corroborates the fact that while coordination of an H-donor to the O atom in N₂O significantly improves the catalytic efficiency, the *pK*ₐ of the donor has a low influence on the energy of the TS.

<table><caption>**Table 3.** Selected bond distances and angles for the reaction between TPPFe(0), N₂O, and a proton donor (H₂O and PhOH)</caption><thead><tr><th>Structure</th><th>O–N<br>(Å)</th><th>O–N–N<br>(°)</th><th>O<sub>N2O</sub>–H<br>(Å)</th><th>O<sub>EOH</sub>–H<br>(Å)</th></tr></thead><tbody><tr><td>Fe(0)–ON<sub>2</sub><br>+ H<sub>2</sub>O</td><td>1.182</td><td>179.2</td><td></td><td>0.969</td></tr><tr><td>TS (H<sub>2</sub>O)</td><td>2.427</td><td>155.7</td><td>2.062</td><td>0.972</td></tr><tr><td>Fe(0)–ON<sub>2</sub><br>+ PhOH</td><td>1.182</td><td>179.2</td><td></td><td>0.971</td></tr><tr><td>TS (PhOH)</td><td>2.517</td><td>158.4</td><td>1.866</td><td>0.977</td></tr></tbody></table>

## Conclusion

We have demonstrated that, once activated through N₂O binding to TPPFe(0), the N-O bond cleavage is facilitated by acids of various strength, with minimal effect of the acid *pK*ₐ. This suggests that the acid's role is primarily through H-bond interactions rather than direct proton transfer, and this was confirmed by DFT calculations. At high acid concentrations, this H-bonding assistance becomes sufficiently effective for the preceding N₂O binding/activation step by TPPFe(0) to become rate determining, with a rate constant of approximately 10⁴ M⁻¹s⁻¹. Additionally, we identified a competing

outersphere process with a rate constant of about 120 M⁻¹s⁻¹ from TPPFe(0). Though two orders of magnitude slower than the N₂O binding to TPPFe(0) it can contribute modestly to the catalysis (ca. 10%) in the absence of an added proton donor. This is because N-O bond cleavage, when activated solely by binding to TPPFe(0), lacks sufficient driving force unless assisted by H-bonding.

This study highlights the importance of a synergistic approach, where N-O bond activation by a low-valent transition metal is complemented by H-bond assistance from an acid. These insights provide valuable guidelines for designing improved catalysts for N-O bond activation and, more broadly, for deoxygenation reactions.

## Materials and methods

Experimental details are given in the supporting information.

## ASSOCIATED CONTENT

### Supporting Information

Experimental details. Additional data. Simulations. Details on DFT calculations.

## 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). Ecole Normale Supérieure de Lyon is acknowledged for financial support for Camille Chartier. 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. Alexandra Collard is thanked for performing initial experiments and Laure Lavernot (UGA, CNRS, Grenoble INP - LEPMI - UMR5279) for her technical assistance (K-F titration). This work was performed using HPC/AI resources from GENCI-TGCC (Grant 2023 - A0140814129, 2024 - AD010814129R1).

## AUTHOR INFORMATION

### Corresponding Author

cyrille costentin@univ-grenoble-alpes.fr

### Notes

The author declares no competing financial interest.

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