# Inner-sphere electron transfer is rate limiting in the photoreduction of N-oxides by re-complexes

Zineb El Moqaouil, Marianne Kjellberg, Winfried Leibl, Thibault Cantat, Lucile Anthore-Dalion, Annamaria Quaranta, Emmanuel Nicolas

► To cite this version:

Zineb El Moqaouil, Marianne Kjellberg, Winfried Leibl, Thibault Cantat, Lucile Anthore-Dalion, et al. Inner-sphere electron transfer is rate limiting in the photoreduction of N-oxides by re-complexes. Organometallics, 2025, 44 (16), pp.1805-1814. 10.1021/acs.organomet.5c00158. cea-05289001

HAL Id: cea-05289001

https://cea.hal.science/cea-05289001v1

Submitted on 29 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

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

Distributed under a Creative Commons Attribution 4.0 International License
Inner-Sphere Electron Transfer is Rate-Limiting in
the Photoreduction of *N*-Oxides by Re-Complexes

Zineb El Moqaouil<sup>‡,a,b</sup> Marianne Kjellberg<sup>‡,a</sup> Winfried Leibl,<sup>b</sup> Thibault Cantat,<sup>a</sup> Lucile Anthore-Dalion,<sup>a</sup> Annamaria Quaranta<sup>b*</sup> and Emmanuel Nicolas<sup>a*</sup>

a. Université Paris-Saclay, CEA, CNRS, NIMBE, 91191 Gif-sur-Yvette, France.
emmanuel.nicolas@cea.fr.

b. Université Paris-Saclay, CEA, CNRS, I2BC, 91191 Gif-sur-Yvette, France.
annamaria.quaranta@cea.fr

‡ Equal contribution of the authors.

KEYWORDS: nitrous oxide, pyridine *N*-oxide, reductive deoxygenation, photocatalysis, spectroscopy, DFT calculations.

ABSTRACT

In this study, we investigate the key mechanistic steps of the photocatalytic deoxygenation of nitrous oxide (N₂O), a significant greenhouse gas, and pyridine *N*-oxide (PNO), an important intermediate in pharmaceutical synthesis, using rhenium(I) tricarbonyl bipyridine complex, [Re¹(bpy)(CO)₃Cl] (*Re*-Cl), in the presence of a sacrificial electron donor (DIPEA). Optimization of the reaction conditions revealed that the addition of water (3 M) enhances catalytic efficiency,
particularly for PNO reduction, suggesting a crucial role of proton availability in facilitating N–O
bond cleavage. Through a combination of techniques such as nanosecond transient absorption
spectroscopy, infrared spectroscopy, and density functional theory (DFT) calculations, we
elucidate the mechanism of this photocatalytic transformation. The reaction is initiated by
photoexcitation of the **Re-Cl** complex, followed by reductive quenching by DIPEA, generating
the singly reduced species (**1ERS**), which serves as the catalytically active species. The
deoxygenation mechanism proceeds via proton-assisted N–O bond cleavage, forming a metal-
hydroxo intermediate. The rate-limiting step is identified as the electron transfer to the substrate.
The presence of water facilitates proton transfers and ligand exchanges during PNO
photodeoxygenation but has no significant impact on N₂O reduction.

INTRODUCTION

The selective deoxygenation of N–O bonds is a critical process with broad applications,
including the synthesis of nitrogen-containing heterocycles,¹ pharmaceutical intermediates,² and
the mitigation of harmful nitrogen oxide emissions.³⁻⁵ In particular, nitrous oxide (N₂O), a by-
product of industrial processes,⁶ is a potent greenhouse gas with a global warming potential
300 times higher than CO₂ and contributes to ozone layer depletion. Thus, deoxygenation of N₂O
into nitrogen and oxygen is a key focus in environmental chemistry.⁷⁻¹¹ Similarly, the
deoxygenation of pyridine N-oxides, which are widely used as intermediates in pharmaceutical
synthesis and fine chemical production, offers a pathway for the selective functionalization of
pyridine derivatives, a process of great importance in medicinal chemistry.¹²
Often, traditional thermochemical methods for N–O bond deoxygenation face limitations such as hazardous by-products, poor selectivity, and low atom economy.<sup>13,14</sup> Recent advances have focused on electrochemical and photochemical processes, utilizing transition metal complexes (e.g., copper, iron, and rhenium) for efficient and selective deoxygenation of N<sub>2</sub>O and pyridine N-oxides under mild conditions.<sup>15–18</sup>

Mechanistic investigations of N–O bond cleavage using transition metal complexes are relatively scarce. Intermediates have only been identified in a few cases, primarily involving N<sub>2</sub>O. The different catalytic pathways proposed to date can be categorized into three main mechanisms (Scheme 1). Typically, the process begins with the coordination of N<sub>2</sub>O to a metal center, forming a metal-N<sub>2</sub>O adduct (Scheme 1a, b). The subsequent cleavage of the N–O bond can then occur either through spontaneous cleavage, resulting in the formation of a metal-oxo species and the release of N<sub>2</sub> (Scheme 1a), or through N–O bond cleavage facilitated by a proton source and resulting in the formation of a metal-hydroxo species (Scheme 1b).

<figure><img src="image_2.png" /><figcaption>(b) Proton assisted N-O cleavage forming metal-hydroxo species</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>(c) Hydride transfer to N<sub>2</sub>O</figcaption></figure>

<figure><img src="image_4.png" /></figure>

**Scheme 1.** Different mechanisms for N<sub>2</sub>O reduction with a transition metal complex (M), based on literature.
Metal-oxo intermediates (Scheme 1a) have been widely described in the case of heterogeneous
catalysis.¹¹ For instance, Co₃O₄ catalysts adsorb N₂O, causing the N–O bond to break, releasing
N₂ and adsorbed oxygen (Co–O).¹⁹ On zeolites, N₂O dissociation occurs at Fe dimers, forming N₂
and O₂, with the primary active sites being [HO–Fe–O–Fe–OH]²⁺.²⁰ A similar mechanism occurs
with dicopper sites in Cu-ZSM-5, where N₂O dissociates into N₂ and [Cu–O–Cu] centers, followed
by O₂ desorption to regenerate the active sites.²¹

Metal-oxo intermediates were also described in the case of molecular catalysts: H. Shan et al. investigated iron complexes that catalyze the oxidation of acetylene by N₂O.²² [Fe]⁺ reacts with N₂O to generate [FeO]⁺ and N₂, through a direct O-abstraction mechanism. H. Grützmacher et al. explored oxygen atom transfer from N₂O to phosphanes using zero-valent amino olefin cobalt complexes.²³ N₂O coordination to [CoPPh₃] occurs via an associative ligand exchange between N₂O and PPh₃, giving [CoN₂O]. An oxygen-transfer to the metal center then takes place via a barrierless transition state, resulting in the direct formation of N₂ and cobalt-oxo species [Co-O]. L. Anthore-Dalion et al. reported a metal-free system for N₂O reduction using disilanes. The reaction proceeds through the nucleophilic addition of a silyl anion to the central nitrogen of N₂O. The cleavage of the N–O bond results from a pseudo-Brook rearrangement to release N₂, triggered by the formation of a strong Si–O bond.²⁴

The involvement of a proton source assisting the N–O bond cleavage (Scheme 1b) was described
in studies involving ruthenium-based catalysts where N₂O was found to coordinate to the
ruthenium center via the oxygen atom.²⁵⁻²⁸ The subsequent proton transfer to the terminal nitrogen
of N₂O, facilitated by the presence of water, occurred in an inner-sphere manner and was identified
in some cases as the rate-determining step (RDS).²⁵,²⁷
A third mechanism involves the initial hydride transfer to N₂O, forming [HN₂O]⁻ and [M]⁺. The subsequent steps include the coordination of [HN₂O]⁻ to the metal center via the oxygen atom, followed by N₂ release and the formation of a metal-hydroxo species (Scheme 1c). This mechanism was reported by A. Suàrez and collaborators in the activation of N₂O by a pincer iridium complex.²⁹

Recently, it has been shown that rhenium complexes are competent in the catalytic reduction of N₂O under electro-¹⁷ and photoactivation.¹⁸ The same complexes show promises for the photochemical reduction of pyridine *N*-oxide (PNO) derivatives,¹⁸,³⁰ more difficult to reduce (E₁/₂ (PNO/pyridine) = -1.28 V vs SCE).³¹ In this study, we aim to highlight the key mechanistic steps in the photocatalytic process for the deoxygenation of N₂O and pyridine *N*-oxide (PNO) using rhenium tricarbonyl bipyridine complexes [Re¹(bpy)(CO)₃Cl] (**Re-Cl** hereafter, bpy = 2,2'-bipyridine, Scheme 2) in the presence of *N*,*N*-diisopropylethylamine (DIPEA, Hünig's base) employed as sacrificial electron donor (SED).¹⁸ Optimization of the reaction conditions revealed that using blue Kessil® lamps (440 nm, 40 W) significantly enhanced the reaction rate.³⁰ Moreover, the addition of water (3 M) to the reaction medium further improved catalytic efficiency (Figure S2), suggesting a crucial role of proton availability in facilitating N–O bond cleavage.³⁰ The present study aims to disclose the mechanism of the N–O bond deoxygenation by identifying the active species of the catalyst, understanding its interaction with *N*-oxide substrates, and exploring the influence of proton sources. Additionally, we aim to determine competitive and parallel pathways, as well as possible deactivation processes. In order to do that, we have used nanosecond transient absorption and infrared spectroscopies coupled with DFT calculations to interpret our experimental findings.
<figure><img src="image_5.png" /><figcaption>**Scheme 2.** Nitrous oxide (N₂O) and pyridine N-oxide (PNO) photocatalytic deoxygenation using rhenium tricarbonyl bipyridine complexes (**Re-Cl**) as catalyst and diisopropylethylamine (DIPEA) as sacrificial electron donor in acetonitrile at 20 °C.</figcaption></figure>

The mechanism for the photocatalytic cleavage of N–O bonds was investigated in the case of **Re-Cl**, for which, as a model and well-studied complex, a wealth of spectroscopic data is available in the literature.³²⁻³⁹ Herein, we demonstrate that this reaction proceeds *via* a proton-assisted photocatalytic mechanism (Scheme 1b), as follows: (i) excitation and reduction of **Re-Cl**, (ii) interaction of the reduced species with the substrate (PNO or N₂O) and subsequent electron transfer, (iii) proton transfer from a donor, (iv) generation of the product (pyridine or N₂) and regeneration of the active species. The study was performed first in the case of the reduction of PNO to pyridine and then extended to the reduction of N₂O.

## EXPERIMENTAL

### Safety Warning

The photochemical reactor described here is equipped with powerful lamps, and its use requires eye protection (inactinic films around the experimental set-up and UV-filter glasses for experimenters).

### General remarks
Unless otherwise stated, all reactions were performed in a recirculating MBraun LabMaster DP inert atmosphere (Ar) drybox and/or using Schlenk lines. Glassware was dried overnight at 120 °C prior to use. All reagents were purchased from usual suppliers (Aldrich, Acros, Alfa Aesar, TCI, Strem) and degassed prior to use when necessary. Nitrous oxide was purchased from Air Liquide (purity ≥ 99,998%).

*Synthesis of Re-Cl, [ReMeCN](PF<sub>6</sub>), Re-OH*

Complexes Re-Cl, [ReMeCN](PF<sub>6</sub>), and Re-OH were prepared according to reported procedures, and NMR data are in agreement with those reported in the literature.<sup>40–42</sup>

*Photoaccumulation followed by UV-visible spectroscopy*

Photoaccumulation experiments were conducted in a 1 cm optical path quartz cuvette hermetically closed with a septum. An Agilent Technologies Cary 60 UV-Vis spectrophotometer was used. During the measurement, the sample is irradiated with a blue LED light source (463 nm, SugarCube). The CaryWin software is used for spectra acquisition, in the Scanning Kinetics mode. A scanning rate of 9600 nm/min was applied.

*Photoaccumulation followed by IR spectroscopy*

FTIR absorption spectra were measured on a Nicolet 6700 FTIR spectrometer, equipped with an ATR silicon crystal on top of which a glass chamber was mounted onto a home-made PTFE cell and the optical fiber was tightened inside the inlet. The junction between the glass chamber and the IR cell was wrapped with PTFE tape to improve airtightness. The gas chamber was degassed with an argon or N<sub>2</sub>O flow for 20 minutes prior to use. Then, the Ar or N<sub>2</sub>O-saturated sample was injected onto the prism using a gas-tight syringe. The solution layer was ca. 2 mm to ensure light penetration all the way to the layer in contact with the prism. Irradiation was performed
with a blue LED (463 nm, 100 W/m², SugarCube). The signal was detected with a liquid N₂-cooled MCT detector. Typically, 100 spectra with a resolution of 4 cm⁻¹ were averaged.

## Laser Flash Photolysis

An Edinburgh Instruments LP920 Laser Flash Photolysis Spectrometer system was used. It incorporates a Continuum Surelite OPOs laser for sample excitation at 420 nm (6 ns pulse duration, energy 10 mJ, unless otherwise stated). The LP920 system is equipped with a 450 W pulsed xenon arc lamp and a tungsten lamp as probes for the transient absorption measurements. Detection in the LP920 system is performed either via a Czerny-Turner blazed 500 nm monochromator (bandwidth: 1–5 nm) coupled with a Hamamatsu R928 photomultiplier tube (kinetics mode), or via a 500 nm blazed spectrograph (bandwidth: 5 nm) coupled with a water-cooled ICCD camera (Andor DH720) (spectral mode).

## Photocatalysis of N₂O followed by µGC

Photocatalysis experiments were performed in 41.5 mL glass vials hermetically closed with a septum and containing 6 mL of solution and an 8 mm stir bar. The reaction mixture was first purged with pure N₂O (1 bar) for 15 minutes before being irradiated with a blue LED light source (SugarCube) at an intensity of 100 W/m² while being continuously stirred with a magnetic stirring bar throughout the experiment. The blue light was filtered through a water cell to prevent heat from reaching the solution. The gas phase was automatically injected and analyzed by a Micro GC Fusion Gas chromatograph (Inficon). The analysis was carried out via two columns: in the first one, module A (PLOT column with molecular sieve and 10 m length), a 9 s backflush is applied to separate H₂, N₂, and O₂ (injector at 90 °C, column at 110 °C and 30 psi, TCD at 85 °C). The second column, module B (RT-Q-bond column of 12 m), was used for the detection of heavier molecules, such as N₂O and H₂O vapor (injector at 90 °C, column at 70 °C and 22.5 psi, TCD at
85 °C). The setup is equipped with an Ar carrier gas with 99.999 % purity. To quantify the obtained products, manual calibrations were performed by adding known volumes of different gases (ranging from 100 to 600 µL) into the reactor containing the initial purged solution under pure argon (1 bar). The calibration curves obtained for N₂O and N₂ are shown in Figure S1.

## RESULTS

**Re-Cl** absorbs visible light due to the presence of a metal-to-ligand charge transfer (MLCT $d\pi \to \pi^*$) transition⁴³ (the ground state absorption spectrum of **Re-Cl** is shown in Figure S3) and, in the photocatalytic system developed, acted simultaneously as photosensitizer and catalyst. In the triplet excited state ³MLCT (formed quantitatively from ¹MLCT),³³ a reductive quenching by DIPEA is thermodynamically favorable by 0.35 eV (8.1 kcal.mol⁻¹).§ Possible interactions between both substrates, PNO and N₂O, and the catalyst ground or excited states were excluded considering that the absorption spectrum, the excited-state differential absorption spectrum, and the emission lifetime of **Re-Cl** were not altered in the presence of either substrate (Figures S3-S5). These results confirm that the initial step in the photocatalytic process involves the interaction of the excited photocatalyst with SED and the generation of the one-electron reduced species (**1ERS**).

### Generation of 1ERS

When an argon-saturated solution containing 420 µM **Re-Cl**, 1 M DIPEA and 3 M H₂O was irradiated, the characteristic spectral changes associated with the formation of the **1ERS-Cl**, including a peak at 360 nm and a broad band in the visible with a maximum at 505 nm,³⁴,³⁶ were observed 1 µs after excitation (Figure 1a, black curve). However, some modifications appeared in
the spectrum taken at 20 µs (Figure 1a, red curve): the initial maximum shifted to 497 nm, and a well-defined peak appeared at 462 nm. We attribute these features to the loss of Cl⁻ and its replacement with a solvent molecule to give the species [Re¹(bpy)(CO)₃(MeCN)]⁺ (1ERS-MeCN). Evidence for this was provided by the coincidence of the 20 µs trace (red) in Figure 1a and the transient spectrum obtained for a solution containing the synthetically prepared [Re¹(bpy)(CO)₃(MeCN)(PF₆)] (Re-MeCN) complex and DIPEA (Figure S6). The halide loss upon reduction has previously been reported for the Re-Cl photocatalysis of CO₂ reduction.³²,³⁷

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Transient absorption spectrum taken 1 µs and 20 µs after laser excitation at 420 nm (a), and kinetics at 505 nm (b and c) for a CH₃CN solution containing 420 µM Re-Cl, 1 M DIPEA and 3 M H₂O. Argon-saturated solution. Kinetics at 505 nm were fitted by mono-exponential decays (red curves).</figcaption></figure>

Inspection of the kinetics at 505 nm (Figure 1b, 1c) gives insight into the details of the photoinduced electron transfer process: the shift of the maximum from 505 to 497 nm, observed in the transient absorption, coincides with a small decay at 505 nm (Figure 1c) occurring with an
apparent rate constant of $1.1 \times 10^5$ s$^{-1}$. Critically, in the absence of water, even after 100 ms the spectrum retains the peak at 505 nm (Figure S7), hinting at a beneficial effect of water on the ligand displacement process.

Looking at 505 nm kinetics on a shorter timescale (Figure 1b), the formation of **1ERS-Cl** appears to occur in two steps. The initial vertical rise corresponding to the electron transfer from DIPEA to $^3\text{Re-Cl*}$ is consistent with the previously reported quenching rate $k_I = 2 \times 10^8 \text{ M}^{-1}\text{ s}^{-1}$,<sup>18</sup> similar to the data reported in the literature for amines quenching of $[\text{Re(CO)}_3(\text{bpy})\text{Cl}](\text{Re-Cl})$.<sup>34</sup> A subsequent growth occurring with an apparent rate $k_{app} = 1.8 \times 10^6 \text{ s}^{-1}$ is then observed. We attribute the latter to a thermal electron transfer from the radical resulting from DIPEA oxidation to a ground-state **Re-Cl** molecule (cf ESI for details). The thermal electron transfer occurs with a rate constant $k_2 = 4 \times 10^9 \text{ M}^{-1}\text{ s}^{-1}$,<sup>*</sup> close to the diffusion limit.

*Interaction of 1ERS with substrates*

In the absence of substrates, the amplitude of the signal at 505 nm is rather stable on a timescale of 1 s (Figure 2, black curves), indicating that the formation of the monoreduced species is irreversible on this timescale. The small decay observed is attributed to the interaction of the monoreduced species **1ERS-MeCN** with residual oxygen in solution. However, in the presence of increasing concentrations of PNO, the **1ERS-MeCN** decay accelerates (Figure 2a), suggesting an interaction between the substrate and the singly reduced catalyst, further confirmed in a steady-state experiment where the addition of PNO resulted in the disappearance of 497 nm peak of **1ERS-MeCN** (Figure S8). Of note, when anhydrous acetonitrile was used, no decay of **1ERS-Cl** was observed even in the presence of 200 mM PNO (Figure S9). This suggests that water, facilitating the chloride loss, promotes the binding of the substrate and/or the advancement in the
catalytic cycle. This is in line with the stronger solvation of chloride ions in mixtures of water and acetonitrile rather than in pure acetonitrile.⁴⁴

Interaction of **1ERS-MeCN** with N₂O (Figure 2b) seems to be, in contrast, much slower even in a solution saturated in N₂O (the solubility of N₂O has been reported to be 280 mM in pure acetonitrile),²⁹ suggesting that N₂O binding on **1ERS-MeCN** is less favorable. The slow rate of interaction of **1ERS-MeCN** with N₂O (k < 2.5 s⁻¹) is consistent with the rate found in electrocatalysis,⁴⁵ and is reflected in the small initial turnover frequency (TOF) that was found in photocatalysis.¹⁸

In summary, the interaction of both substrates with **1ERS** has been observed, although it is slow in the case of N₂O, confirming a less favorable binding for this substrate.⁴⁶,⁴⁷

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Kinetic traces at 505 nm obtained for an Ar-purged solution containing 420 µM **Re-Cl**, 1 M DIPEA, 3 M H₂O and (a) different concentrations of PNO; (b) N₂O-saturated.</figcaption></figure>

### PNO deoxygenation mechanism

In order to get more information on the dynamics of PNO deoxygenation, we fitted the curves in Figure 2a using a 3-exponential decay. This is consistent with a sequential mechanism involving two intermediates having negligible ΔA at 505 nm.
Fits results, given in SI (Figure S10, Table S1), suggest a faster decay of the three apparent lifetimes with increasing PNO concentrations.

We hypothesized one intermediate to be an adduct between **1ERS** and PNO. This assumption is based on the fact that an outer-sphere electron transfer between **1ERS-MeCN** and PNO would be energetically uphill of +1.1 eV (25.4 kcal.mol⁻¹) given the reduction potentials of the two reacting species (E(**Re-MeCN**/**1ERS-MeCN**) = -1.20 V vs SCE³⁵) and **PNO** (E(PNO/PNO⁺) = -2.30 V vs SCE⁴⁸). Such an adduct, however, could not be identified spectroscopically: even in the presence of 500 mM of PNO, we could not observe any new clear spectral feature (Figure S11). Density functional theory (DFT) calculations were thus performed to infer the structure of the hypothesized intermediate (**Re-PNO**), rationalize the experimental findings, and elucidate the remaining steps of the mechanism (Scheme 4).

<figure><img src="image_8.png" /><figcaption>**Scheme 4.** DFT-calculated mechanism for the DIPEA/H₂O assisted N–O bond cleavage in PNO. Gaussian16, MN15L/Def2TZVP/W06 / SMD(Acetonitrile).</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 3.** Structures of 2, 3, 4, TS<sub>3-5</sub> and TS<sub>4-5</sub>.</figcaption></figure>

The results support a multi-step initiation pathway, starting with the conversion of **1ERS-Cl** to **1ERS-MeCN** (1) via ligand exchange from chloride to acetonitrile. This step is essential for entry into the catalytic cycle, as explained earlier. In the first step of the catalytic cycle, acetonitrile is decoordinated and replaced by PNO in an exergonic, dissociative, and barrierless manner, to form *O*-coordinated PNO complex **2** (ΔG = -6.7 kcal.mol⁻¹). The proton will then be transferred to cleave the N–O bond. In that regard, multiple proton sources are present in the reaction medium: H₂O, DIPEA, for which a model can be iPr₂EtNH⁺. As described in Scheme S1, after giving an electron, DIPEA can readily give a proton that can be trapped by the excess of DIPEA present in solution. Two complexes have thus been computed, **3** and **4**, that correspond respectively to the coordination of H₂O and iPr₂EtNH⁺ through H-bonding on the O atom of the PNO. Both complexes are slightly higher in energy than **2** (ΔΔG = +2.2 and +2.7 kcal.mol⁻¹ for **3** and **4**, respectively). The actual N–O bond cleavage then occurs through transition states **TS**<sub>3-5</sub> or **TS**<sub>4-5</sub> at +10.9 and +15.6 kcal.mol⁻¹, respectively.
Once the N–O bond is cleaved, free DIPEA and water are released, and hydroxyl complex 5 is formed, which features a pyridine coordinated to the hydroxyl moiety through H-bonding. The release of pyridine then takes place via another exergonic step (ΔG = -2.16 kcal mol⁻¹), leading to the formation of free hydroxyl complex 6 (ΔΔG = -2.1 kcal.mol⁻¹).

The overall energetic demand of the N–O cleavage for PNO is defined by 2 and TS₃-5, at 17.6 kcal.mol⁻¹, compatible with the fast rate of reaction in catalytic conditions (a few minutes of reaction at room temperature). When TS₄-5 is considered, the energetic demand is slightly higher (22.3 kcal.mol⁻¹), which is still compatible with a rather fast reaction at room temperature. The respective structures of the computed complexes provide further insight into the process of the reduction and cleavage of the N–O bond. Indeed, the N–O bond distance and the angle between the N–O bond and the pyridine ring do not change upon water or iPr₂EtNH⁺ coordination (N–O between 1.330 and 1.341 Å; O–N–Py angle between 178.0 and 178.6 °). It is only at the transition state that the actual electron transfer occurs, which is apparent by the sudden increase in the N–O bond distance to 1.721 or 1.687 Å and the change in the O–N–Py angle to 142.1 or 136.4 ° for TS₃-5 and TS₄-5, respectively. Interestingly, the O–H bond distances around the transferred H atom are barely modified, and even at the transition states, the H atom is much closer to the O atom of H₂O or N atom of DIPEA than that of the PNO. This should point to a very low kinetic isotope effect (KIE),# and indeed, it was computed at 1.01 and 1.19 from the DFT calculations for TS₃-5 and TS₄-5, respectively, which are not compatible with a primary KIE.⁴⁹,⁵⁰ Moreover, in transient absorption spectroscopy, no significant difference could be found when the reaction was run in H₂O or D₂O (Figure S12). Interestingly, an additional TS combining iPr₂EtNH⁺ as the donor and H₂O as a proton relay could be computed and found at an intermediate value of +15.6 kcal.mol⁻¹ (See ESI for details). This result indicates that depending on the pH of the solution, the
concentration of relative proton sources and the actual donor, the transition state energy value may vary, but should be found between TS<sub>3-5</sub> and TS<sub>4-5</sub>. Moreover, this finding points out that the presence of a proton source is necessary for the electron transfer and subsequent N–O bond cleavage to occur, but the formation of the O–H bond is not involved in the limiting step.

*N₂O deoxygenation mechanism*

N₂O differs from PNO as substrate since it has a more positive one-electron reduction potential (E⁰(N₂O/N₂O⁺ ~ -1.1 V vs Ag⁺/Ag, estimated in ref 51), which makes an outer-sphere reduction from **1ERS** thermodynamically feasible. However, due to the inertness of the molecule, the process is expected to be kinetically slow, as confirmed by the small decay of **1ERS-MeCN** observed in N₂O saturation conditions (Figure 2b). Such decay, nevertheless, hints at an interaction between the two species. This was corroborated by the finding that under continuous irradiation a smaller amount of **1ERS-MeCN** is formed in a N₂O-saturated solution containing **Re-Cl** and DIPEA (Figure S13) in comparison with an Ar-saturated one.

As in the case of PNO, DFT calculations were performed to understand the interaction between **1ERS** and N₂O (Scheme 5). Similarly, MeCN is first decoordinated from **1** to form **1ERS**, on which N₂O can coordinate to form **Re–ON₂** complex (7). However, in this case, the coordination goes through a high energy barrier, **TS**<sub>1ERS-7</sub>, 23.9 kcal.mol<sup>-1</sup> higher in energy than **1** and N₂O. This transition state is correlated with significant electron transfer from the rhenium complex to the N₂O ligand. In complex **7**, the O–N–N angle is 125.7 ° (vs. 180.0 ° in free N₂O). Moreover, the SOMO in **7** is mainly located on the N₂O ligand (Figure 4a), while on PNO complex **2**, it was mainly located on the bipyridine moiety (Figure 4b). Finally, the N–O bond in **7** is also much longer than in free N₂O: 1.384 vs. 1.182 Å.
<figure><img src="image_10.png" /><figcaption>**Scheme 5.** DFT-calculated mechanism for the DIPEA-assisted N–O bond cleavage in N₂O. Gaussian16, MN15L/Def2TZVP/W06 / SMD(Acetonitrile).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 4.** SOMO of complex 7 (a). SOMO of complex 2 (b)</figcaption></figure>

The following steps involve first the coordination of HNiPr₂Et⁺ to 7 in a slightly endergonic (ΔΔG = +1.9 kcal mol⁻¹), barrierless way to form adduct 8. The proton transfer to the O atom then occurs through TS₈-₆ (ΔG‡ = +6.2 kcal mol⁻¹ from 1 and N₂O), liberating N₂ and free DIPEA, with a very low energy barrier, and forming hydroxyl complex 6 in a highly exergonic step (ΔG = -43.2 kcal mol⁻¹ from 1 and N₂O). In a similar fashion to PNO, H₂O can also be considered as the proton source. In this case, adduct 9 is formed, slightly higher in energy than 8
($\Delta\Delta G = +3.0 \text{ kcal mol}^{-1}$), followed by the low-energy transition state $\text{TS}_{9-6}$ ($\Delta G^{\ddagger} = +8.6 \text{ kcal mol}^{-1}$ from 1 and $N_2O$) that corresponds to the concomitant proton transfer and $N_2$ release. Both proton sources have to be considered since they present similar energy demands, but neither will be limiting.

We can already note that while both mechanisms follow a similar pathway, the rate-determining steps (RDS) are very different. For the reduction of PNO, the RDS is the cleavage of the N–O bond and requires the assistance of water, while for $N_2O$, the RDS is the coordination of $N_2O$. We can note that in both cases, it is linked to the electron transfer from the Re complex to the N–O bond. The N–O bond cleavage requires, in both cases, the concomitant arrival of a proton, but it is only in the case of PNO that water helps the electron transfer.

### *Closure of the catalytic cycle*

Upon reaching the radical cation hydroxyl complex **6**, which is identical in both PNO and $N_2O$ reduction mechanisms, the transfer to the system of a second electron and a second proton is still needed to close the catalytic cycle. Complex **6** can be reduced by a molecule of DIPEA, leading to the neutral hydroxyl complex **Re-OH 10** (Scheme 6). Given the peak potential for the couple $E_p([\text{Re-OH}]^+/\text{Re-OH}) = +1.79 \text{ V vs SCE})$, the one-electron transfer from DIPEA to $[\text{Re-OH}]^+$ should indeed be exergonic. Subsequently, $\text{DIPEAH}^+$ transfers a proton to the hydroxyl radical to form aqua complex **11** in an almost thermoneutral way. A simple, barrierless, dissociative ligand exchange can then occur to form a cationic complex $[\text{ReMeCN}]^+ 13$ that can undergo a further catalytic cycle through light excitation.
<figure><img src="image_12.png" /><figcaption>**Scheme 6.** DFT-calculated mechanism for the generation of the active species from 6. Gaussian16, MN15L/Def2TZVP/W06 / SMD(Acetonitrile).</figcaption></figure>

Seeking experimental evidence for the catalytic cycle resulting from DFT calculations, we prepared and characterized [Re(bpy)(CO)₃OH] (10) and [Re (bpy)(CO)₃MeCN]PF₆ ([13](PF6)) (cf ESI), which are both presumed intermediates in the cycle. Both complexes were found to be photocatalytically competent (Figures S14-S15) although less efficient than **Re-Cl**, confirming that they are plausible intermediates. In order to get further insight, we further characterized spectroscopically these complexes. While [13](PF6) exhibits a blue-shifted absorption, which may possibly explain the slightly lower efficiency in photocatalysis, **Re-OH** ground state absorption is similar to **Re-Cl** (Figure S16).♦ For the latter complex, however, the excited state lifetime is significantly shorter than that of **Re-Cl** (SI, Section 2.14). This might be the reason for the observed lower efficiency. Upon formation of one of the species [Re-OH]⁺, Re-OH, [11] and [13](PF6) after interaction with the substrate, no spectral changes would be observed in the transient absorption spectra, in agreement with our experimental results.

Evidence for the presence of the intermediate [ReMeCN]⁺ 13, speculated to close the catalytic cycle, was obtained from the FTIR spectra recorded in photocatalytic conditions (Figure 5). Under continuous illumination, the initial **Re-Cl** carbonyl vibrations at 202, 1916 and 1900 cm⁻¹, ³⁵,⁴⁵ decrease over time, and new bands appear related to the species **1ERS-Cl** (1996, 1880, 1868 cm⁻¹),³⁴ **1ERS-MeCN** (2011 cm⁻¹ and 1895 cm⁻¹)³⁵ and [ReMeCN]⁺ (shoulders at 2038 and
1941 cm⁻¹).³⁵,⁴⁵ The latter is, however, much less evident in the case of N₂O reduction. Bands related to **1ERS-MeCN** were also observed in the absence of PNO (Figure S17) and are due to the photoredox process with DIPEA.

<figure><img src="image_13.png" /><figcaption>**Figure 5.** FTIR absorption spectra of an argon-saturated CH₃CN solution containing **Re-Cl** (2.5 mM), DIPEA (1 M) and 63 mM PNO (a), or N₂O saturated (b) under CW irradiation with a blue LED (460 nm, 100 W/m²). Water traces are present in the solvent.</figcaption></figure>

DISCUSSION

The mechanistic studies performed on the photocatalytic system developed for PNO and N₂O deoxygenation allowed us to propose a catalytic cycle, and to provide a clear picture of the role of each component, namely, **Re-Cl**, H₂O, and DIPEA. The collected data support the ability of **Re-Cl** to perform the photocatalytic deoxygenation of PNO and N₂O through an inner-sphere process, although some differences, inherent to the nature of the substrate (PNO difficult to reduce, N₂O kinetically inert), are present. The initial step consists of the reductive quenching of **Re-Cl*** by DIPEA, resulting in the formation of the **1ERS-MeCN** species, which, upon loss of the MeCN molecule, is able to bind either substrate through the oxygen atom. With respect to this step, however, a striking difference is the rate of interaction between the active state and the substrate, found to be much smaller for N₂O. This observation has been rationalized by the high transition
state found for the coordination of N₂O to the Re center, while no barrier was observed for PNO.
This step involves significant electron transfer to N₂O upon coordination, which is not the case for
PNO. The next steps are similar: a proton transfer from the iPr₂EtNH⁺ occurs, which induces the
cleavage of the N–O bond. The resulting hydroxyl complex 10 can then be further reduced to
regenerate the active catalyst. In both cases, the RDS corresponds to the single electron transfer
from the Re center to the N–O bond, but it is coupled to a proton arrival only in the case of PNO,
while for N₂O it occurs upon coordination.

The mechanism of N₂O catalytic electroreduction has also recently been investigated by Costentin *et al.*⁴⁵,⁵² They reported a fast catalysis from the doubly reduced species **2ERS**, and a slower one involving the **1ERS**. In both cases, an inner-sphere mechanism was demonstrated, in which the substrate binds after the displacement of the labile chloride ligand. Formation of [Re(bpy)(CO)₃OH] (**Re-OH**, **10**) and [Re(bpy)(CO)₃OH]⁻ species was found to hamper and ultimately inhibit the catalytic process.⁴⁵ Under photocatalytic conditions, the formation of **2ERS** was not observed, suggesting that catalysis mainly occurs with the **1ERS**. Another interesting difference is the absence of accumulation of the **Re-OH** species (having IR bands at 2005, 1894, 1875 cm⁻¹)⁴⁵. Under photoirradiation conditions, higher energy species can be formed, such as DIPEA⁺ or **Re-Cl***, that promote the final reduction of **Re-OH**. This hydroxyl complex is thus not limiting the photoactivated process.

We also observed that the presence of water plays a critical role in enhancing the catalytic rate,
especially in the case of PNO deoxygenation.³⁰ Experimental data has shown that H₂O acts at
different levels: (i) it facilitates the displacement of the axial Cl⁻ resulting in the formation of
**1ERS-MeCN**, which is replaced by PNO in an exergonic step; (ii) **1ERS-MeCN** decay is
accelerated in the presence of H₂O, presumably because H₂O can enhance the proton transfer rate,
and thus the N-O bond cleavage in the case of PNO; (iii) finally, it is likely that the exchanges of ligand to regenerate the active species are influenced by the presence of water (H₂O vs. MeCN vs. Cl⁻). This process may lead to variations, during catalysis, in the relative concentrations of the species **Re-Cl** and [**ReMeCN**]⁺, which possess different spectroscopic properties from **Re-Cl**. For instance, [**ReMeCN**]⁺ absorption coefficient at 420 nm is four times smaller, while **Re-OH** excited state lifetime is considerably shorter. These different properties explain why both complexes exhibit a smaller efficiency in the generation of **1ERS** species. In agreement with this, it has previously been observed that the presence of Cl⁻ ions has a beneficial effect in photocatalysis.⁵³,⁵⁴

Finally, some considerations can be made on the role of DIPEA employed as SED, and its contribution to catalysis. Transient absorption kinetics results show that DIPEA•, generated after SED oxidation, is able to thermally reduce **Re-Cl**, thereby increasing the quantum yield of **1ERS**.

In the case of N₂O, this radical could thermodynamically reduce N₂O (E(N₂O/N₂O•⁻) = -1.1 V vs Ag/Ag⁺)⁵², but this pathway does not significantly contribute to N₂O reduction. In order to estimate the extent to which SED can directly reduce N₂O, we selected phenazine as photosensitizer, which is able to generate DIPEA• but not able to catalytically reduce N₂O.⁵² Upon irradiation of a N₂O-saturated solution, a minimal amount of N₂ was observed (Figure S18), indicating that DIPEA• does not contribute significantly to the catalysis. Another step of the catalytic cycle, in which DIPEA and DIPEA• play a significant role, is the generation of **Re-OH**, formed through the reduction of [**Re-OH**]⁺ either by the large excess of DIPEA or by DIPEA•. In this step, the reduction by DIPEA would generate another DIPEA• able to interact either with **Re-Cl** or [**ReMeCN**]⁺ to give a new **1ERS** species, thereby propagating a radical chain mechanism (cf. Scheme S2). In order to examine this possibility, a solution containing 1 mM **Re-Cl** was irradiated in the presence of DIPEA until the signature of **1ERS-MeCN** appeared (Scheme 7). At this point,
the light was switched off, and 137 equivalents of PNO were added to the sample. The solution was then kept in the dark, and analyzed by GC-MS (cf. Figure S19). We found that 2 equiv. of pyridine were produced, corresponding to TON> 1. This result indicates that some pyridine may be produced through radical chain reactions. However, this process is not likely to be the main mechanism at play since upon light-on-light-off cycles, no product was obtained in the dark,¹⁸ in contrast to what is expected if long radical chains were present.

<figure><img src="image_14.png" /><figcaption>**Scheme 7.** Evaluation of the presence of radical chains.</figcaption></figure>

A further contribution of DIPEA to the catalytic cycle comes from the DIPEAH⁺ species, generated upon deprotonation of DIPEA⁺ by another DIPEA molecule. DIPEAH⁺ can supply the protons needed for the deoxygenation process.

## CONCLUSIONS

In this study, we demonstrated that the photocatalyzed reduction of PNO and N₂O by Re-Cl complexes proceeds through a proton-assisted N-O bond cleavage, forming a metal-hydroxo intermediate (Scheme 1b). The active species corresponds to the singly reduced species 1ERS, resulting from the reductive quenching of the excited Re-species by the sacrificial electron donor. The rate-limiting step with both substrates is linked to the electron transfer to the substrates, which
occurs upon coordination for N₂O and is concomitant to the N–O bond cleavage for PNO, assisted
by proton transfer. We have also demonstrated that the addition of water facilitates the proton
transfers and ligand exchanges in the case of PNO photodeoxygenation, but has no significant
effect on N₂O reduction.

ASSOCIATED CONTENT

**Supporting Information.** The following files are available free of charge.

Photocatalysis, photophysical characterization, DFT calculations, additional references.

AUTHOR INFORMATION

Corresponding Author

*emmanuel.nicolas@cea.fr; *annamaria.quaranta@cea.fr

Author Contributions

Conceptualization : ZEM, MK, TC, LAD, AQ, EN; Data curation : ZEM, MK, LAD, AQ, EN;

Formal analysis : ZEM, MK, LAD, AQ, EN; Funding acquisition : WL, TC, LAD, AQ, EN;

Investigation : ZEM, MK, LAD, AQ, EN; Methodology : ZEM, MK; Project administration :

WL, TC, LAD, AQ, EN; Resources : WL, TC, LAD, AQ, EN; Supervision : WL, TC, LAD, AQ,
EN; Visualization : ZEM, MK, LAD, AQ, EN; Writing – original draft : ZEM, LAD, AQ, EN.

The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript. ‡These authors contributed equally.

Notes

<footnote>§The free energy change was calculated using
ΔG = E(DIPEA*+/DIPEA) - E₁/₂(Re-Cl/1ERS-Cl) - E_T, where</footnote>
$E(\text{DIPEA}^+/\text{DIPEA}) = 0.68 \text{ V vs SCE},^{55}E_{1/2}(\text{Re-Cl}/1\text{ERS-Cl}) = -1.35 \text{ V vs SCE}^{33,40} \text{ and}$
$E_T = 2.38 \text{ eV}^{56} (54.9 \text{ kcal.mol}^{-1})$

*Obtained using a pseudo first-order approximation for 8 µM DIPEA• interacting with 400 µM Re-Cl, Figure S6

#KIE was computed with the Kinisot package as provided by the Paton group:
https://doi.org/10.5281/zenodo.595243

Cyrille Costentin, personal communication

♦The spectrum of Re-OH excited state could not be observed with our set-up, hinting to a short-lived species (< 5 ns). This may explain the small initial TOF observed in photocatalytic conditions. As the photocatalysis proceeds, Re-OH should convert to Re-MeCN⁺ resulted to be more efficient. This may explain the initial linear slope in the time evolution of the product.

ACKNOWLEDGMENT

This work was supported by CEA, CNRS, ANR (PRC Grant no ANR-22-CE07-0028), and
French Infrastructure for Integrated Structural Biology (FRISBI) (ANR-10-INSB-005). MK was
supported by a PhD fellowship from ADEME/CEA. This work was performed using HPC
resources from GENCI (Grants 2024- AD010814129R1, 2023-A0140814129, and 2022-
AD010806494R1)

ABBREVIATIONS

PNO, pyridine N-oxide; DFT, density functional theory; 1ERS, one-electron reduced species; RDS, rate-determining step; DIPEA, *N*,*N*-diisopropylethylamine; SED, sacrificial electron donor; MLCT, metal-to-ligand charge transfer; Re-Cl, [Re(CO)₃(bpy)Cl]; Re-MeCN, [Re'(bpy)(CO)₃(MeCN)(PF₆)]; KIE, kinetic isotope effect.

REFERENCES

(1) Wang, Y.; Zhang, L. Recent Developments in the Chemistry of Heteroaromatic N-Oxides. *Synthesis* **2015**, *47*, 289–305. https://doi.org/10.1055/s-0034-1379884.
(2) Vitaku, E.; Smith, D. T.; Njardarson, J. T. Analysis of the Structural Diversity, Substitution Patterns, and Frequency of Nitrogen Heterocycles among U.S. FDA Approved Pharmaceuticals. *J. Med. Chem.* **2014**, *57* (24), 10257–10274. https://doi.org/10.1021/jm501100b.

(3) Hayhurst, A. N.; Lawrence, A. D. The Reduction of the Nitrogen Oxides NO and N₂O to Molecular Nitrogen in the Presence of Iron, Its Oxides, and Carbon Monoxide in a Hot Fluidized Bed. *Combustion and Flame* **1997**, *110* (3), 351–365. https://doi.org/10.1016/S0010-2180(97)00085-0.

(4) Odenbrand, C. U. I.; Andersson, L. A. H.; Brandin, J. G. M.; Lundin, S. T. Catalytic Reduction of Nitrogen Oxides. *Applied Catal.* **1986**, *27* (2), 363–377. https://doi.org/10.1016/S0166-9834(00)82931-0.

(5) Lewine, H. R.; Teigen, A. G.; Trausch, A. M.; Lindblom, K. M.; Seda, T.; Reinheimer, E. W.; Kowalczyk, T.; Gilbertson, J. D. Sequential Deoxygenation of CO₂ and NO₂⁻ via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine. *Inorg. Chem.* **2023**, *62* (37), 15173–15179. https://doi.org/10.1021/acs.inorgchem.3c02323.

(6) Chen, S.; Perathoner, S.; Ampelli, C.; Centi, G. Chapter 2 - Electrochemical Dinitrogen Activation: To Find a Sustainable Way to Produce Ammonia. In *Studies in Surface Science and Catalysis*; Albonetti, S., Perathoner, S., Quadrelli, E. A., Eds.; Horizons in Sustainable Industrial Chemistry and Catalysis; Elsevier, 2019; Vol. 178, pp 31–46. https://doi.org/10.1016/B978-0-444-64127-4.00002-1.

(7) Reay, D. S.; Davidson, E. A.; Smith, K. A.; Smith, P.; Melillo, J. M.; Dentener, F.; Crutzen, P. J. Global Agriculture and Nitrous Oxide Emissions. *Nature Clim Change* **2012**, *2* (6), 410–416. https://doi.org/10.1038/nclimate1458.

(8) *Climate Change 2014: Synthesis Report*; Pachauri, R. K., Mayer, L., Intergovernmental Panel on Climate Change, Eds.; Intergovernmental Panel on Climate Change: Geneva, Switzerland, 2015.

(9) Masson-Delmotte, V.; Zhai, P.; Pirani, A.; Connors, S. L.; Pean, C.; Berger, S.; Caud, N.; Chen, Y.; Goldfarb, L.; Gomis, M. I.; Huang, M.; Leitzell, K.; Lonnoy, E.; Matthews, J. B. R.; Maycock, T. K.; Waterfield, T.; Yelekçi, O.; Yu, R.; Zhou, B. *IPC 2021. Summary for Policymakers. Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change*.

(10) Portmann, R. W.; Daniel, J. S.; Ravishankara, A. R. Stratospheric Ozone Depletion Due to Nitrous Oxide: Influences of Other Gases. *Phil. Trans. R. Soc. B* **2012**, *367* (1593), 1256–1264. https://doi.org/10.1098/rstb.2011.0377.

(11) Wu, X.; Du, J.; Gao, Y.; Wang, H.; Zhang, C.; Zhang, R.; He, H.; Lu, G. (Max); Wu, Z. Progress and Challenges in Nitrous Oxide Decomposition and Valorization. *Chem. Soc. Rev.* **2024**, *53* (16), 8379–8423. https://doi.org/10.1039/D3CS00919J.

(12) Singh, J.; Patel, R. I.; Sharma, A. Visible-Light-Mediated C-2 Functionalization and Deoxygenative Strategies in Heterocyclic N-Oxides. *Adv. Synth. Catal.* **2022**, *364* (14), 2289–2306. https://doi.org/10.1002/adsc.202200200.

(13) Bull, J. A.; Mousseau, J. J.; Pelletier, G.; Charette, A. B. Synthesis of Pyridine and Dihydropyridine Derivatives by Regio- and Stereoselective Addition to N-Activated Pyridines. *Chem. Rev.* **2012**, *112* (5), 2642–2713. https://doi.org/10.1021/cr200251d.

(14) Severin, K. Synthetic Chemistry with Nitrous Oxide. *Chem. Soc. Rev.* **2015**, *44* (17), 6375–6386. https://doi.org/10.1039/C5CS00339C.
(15) Martinez, J. L.; Schneider, J. E.; Anferov, S. W.; Anderson, J. S. Electrochemical Reduction of N₂O with a Molecular Copper Catalyst. *ACS Catal.* **2023**, *13* (19), 12673–12680. https://doi.org/10.1021/acscatal.3c02658.

(16) 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* (19), 12617–12622. https://doi.org/10.1021/acscatal.3c02707.

(17) 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* (10), 6099–6103. https://doi.org/10.1021/acscatal.1c01197.

(18) Kjellberg, M.; Ohleier, A.; Thuéry, 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.* **2021**, *12* (30), 10266–10272. https://doi.org/10.1039/D1SC01974K.

(19) Xiong, Y.; Zhao, Y.; Qi, X.; Qi, J.; Cui, Y.; Yu, H.; Cao, Y. Strong Structural Modification of Gd to Co3O4 for Catalyzing N2O Decomposition under Simulated Real Tail Gases. *Environ. Sci. Technol.* **2021**, *55* (19), 13335–13344. https://doi.org/10.1021/acs.est.1c05052.

(20) Zhang, T.; Qiu, Y.; Liu, G.; Chen, J.; Peng, Y.; Liu, B.; Li, J. Nature of Active Fe Species and Reaction Mechanism over High-Efficiency Fe/CHA Catalysts in Catalytic Decomposition of N₂O. *J. Catal.* **2020**, *392*, 322–335. https://doi.org/10.1016/j.jcat.2020.10.015.

(21) Farhan, K. M.; Thabassum, A. N. K.; Ismail, T. M.; Sajith, P. K. Theoretical Investigation into the Effect of Water on the N₂O Decomposition Reaction over the Cu-ZSM-5 Catalyst. *Catal. Sci. Technol.* **2022**, *12* (5), 1466–1475. https://doi.org/10.1039/D1CY01883C.

(22) Zhao, L.; Wang, Y.; Guo, W.; Shan, H.; Lu, X.; Yang, T. Theoretical Investigation of the Fe+-Catalyzed Oxidation of Acetylene by N₂O. *J. Phys. Chem. A* **2008**, *112* (25), 5676–5683. https://doi.org/10.1021/jp800511h.

(23) Gianetti, T. L.; Rodríguez-Lugo, R. E.; Harmer, J. R.; Trincado, M.; Vogt, M.; Santiso-Quinones, G.; Grützmacher, H. Zero-Valent Amino-Olefin Cobalt Complexes as Catalysts for Oxygen Atom Transfer Reactions from Nitrous Oxide. *Angew. Chemie Int. Ed.* **2016**, *55* (49), 15323–15328. https://doi.org/10.1002/anie.201609173.

(24) Anthore-Dalion, L.; Nicolas, E.; Cantat, T. Catalytic Metal-Free Deoxygenation of Nitrous Oxide with Disilanes. *ACS Catal.* **2019**, *9* (12), 11563–11567. https://doi.org/10.1021/acscatal.9b04434.

(25) Luque-Urrutia, J. A.; Poater, A. The Fundamental Noninnocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-Based Catalysts. *Inorg. Chem.* **2017**, *56* (23), 14383–14387. https://doi.org/10.1021/acs.inorgchem.7b02630.

(26) Zeng, R.; Feller, M.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. CO Oxidation by N₂O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism. *J. Am. Chem. Soc.* **2018**, *140* (23), 7061–7064. https://doi.org/10.1021/jacs.8b03927.

(27) Xie, H.; Zhang, Y.; Xiang, C.; Li, Y.; Fan, T.; Lei, Q.; Fang, W. Non-Innocent PNN Ligand Is Important for CO Oxidation by N2O Catalyzed by a (PNN)Ru–H Pincer Complex: Insights from DFT Calculations. *Dalton Trans.* **2018**, *47* (43), 15324–15330. https://doi.org/10.1039/C8DT03304H.

(28) Escayola, S.; Solà, M.; Poater, A. Mechanism of the Facile Nitrous Oxide Fixation by Homogeneous Ruthenium Hydride Pincer Catalysts. *Inorg. Chem.* **2020**, *59* (13), 9374–9383. https://doi.org/10.1021/acs.inorgchem.0c01252.
(29) Ortega-Lepe, I.; Sánchez, P.; Santos, L. L.; Lara, P.; Rendón, N.; López-Serrano, J.; Salazar-Pereda, V.; Álvarez, E.; Paneque, M.; Suárez, A. Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes. *Inorg. Chem.* **2022**, *61* (46), 18590–18600. https://doi.org/10.1021/acs.inorgchem.2c02963.

(30) Joly, N.; Kjellberg, M.; Bouchet, A.; Nicolas, E.; Cantat, T.; Anthore-Dalion, L. Water-Mediated Rhenium-Catalyzed Photodeoxygenation of Polyfunctional Organic N-Oxides. *Synthesis* https://doi.org/10.1055/a-2649-0086.

(31) Kubota, T.; Miyazaki, H. The Effect of Substituents on the Half-Wave Potential of the Polarographic Reduction of Pyridine N-Oxide Derivatives. *BCSJ* **1966**, *39* (10), 2057–2062. https://doi.org/10.1246/bcsj.39.2057.

(32) Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. Development of an Efficient Photocatalytic System for CO₂ Reduction Using Rhenium(I) Complexes Based on Mechanistic Studies. *J. Am. Chem. Soc.* **2008**, *130* (6), 2023–2031. https://doi.org/10.1021/ja077752e.

(33) Caspar, J. V.; Meyer, T. J. Application of the Energy Gap Law to Nonradiative, Excited-State Decay. *J. Phys. Chem.* **1983**, *87* (6), 952–957. https://doi.org/10.1021/j100229a010.

(34) Kalyanasundaram, K. Luminescence and Redox Reactions of the Metal-to-Ligand Charge-Transfer Excited State of Tricarbonylchloro-(Polypyridyl)Rhenium(I) Complexes. *J. Chem. Soc., Faraday Trans. 2* **1986**, *82* (12), 2401. https://doi.org/10.1039/F29868202401.

(35) 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* (15), 3374–3387. https://doi.org/10.1021/om960044+

(36) Lee, Y. F.; Kirchhoff, J. R.; Berger, R. M.; Gosztola, D. Spectroelectrochemistry and Excited-State Absorption Spectroscopy of Rhenium(I)α,A'-Diimine Complexes. *J. Chem. Soc., Dalton Trans.* **1995**, No. 22, 3677–3682. https://doi.org/10.1039/DT9950003677.

(37) Hayashi, Y.; Kita, S.; Brunschwig, B. S.; Fujita, E. Involvement of a Binuclear Species with the Re–C(O)O–Re Moiety in CO₂Reduction Catalyzed by Tricarbonyl Rhenium(I) Complexes with Diimine Ligands: Strikingly Slow Formation of the Re–Re and Re–C(O)O–Re Species from Re(Dmb)(CO)₃S (Dmb = 4,4'-Dimethyl-2,2'-Bipyridine, S = Solvent). *J. Am. Chem. Soc.* **2003**, *125* (39), 11976–11987. https://doi.org/10.1021/ja035960a.

(38) Kutal, C.; Weber, M. A.; Ferraudi, G.; Geiger, D. A Mechanistic Investigation of the Photoinduced Reduction of Carbon Dioxide Mediated by Tricarbonylbromo(2,2'-Bipyridine)Rhenium(I). *Organometallics* **1985**, *4* (12), 2161–2166. https://doi.org/10.1021/om00131a016.

(39) Kutal, Charles.; Corbin, A. Joan.; Ferraudi, Guillermo. Further Studies of the Photoinduced Reduction of Carbon Dioxide Mediated by Tricarbonylbromo(2,2'-Bipyridine)Rhenium(I). *Organometallics* **1987**, *6* (3), 553–557. https://doi.org/10.1021/om00146a020.

(40) 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**, No. 20, 1414–1416. https://doi.org/10.1039/C39850001414.

(41) Gibson, D. H.; Yin, X.; He, H.; Mashuta, M. S. Synthesis and Reactions of Fac-[Re(Dmbpy)(CO)₃X] (Dmbpy = 4,4'-Dimethyl-2,2'-Bipyridine; X = COOH, CHO) and Their Derivatives. *Organometallics* **2003**, *22* (2), 337–346. https://doi.org/10.1021/om020677q.
(42) Morimoto, T.; Nakajima, T.; Sawa, S.; Nakanishi, R.; Imori, D.; Ishitani, O. $CO_2$ Capture by a Rhenium(I) Complex with the Aid of Triethanolamine. *J. Am. Chem. Soc.* **2013**, *135* (45), 16825–16828. https://doi.org/10.1021/ja409271s

(43) Giordano, P. J.; Wrighton, M. S. The Nature of the Lowest Excited State in Fac-Tricarbonylhalobis(4-Phenylpyridine)Rhenium(I) and Fac-Tricarbonylhalobis(4,4'-Bipyridine)Rhenium(I): Emissive Organometallic Complexes in Fluid Solution. *J. Am. Chem. Soc.* **1979**, *101* (11), 2888–2897. https://doi.org/10.1021/ja00505a016

(44) Sakamoto, I.; Ito, A.; Okazaki, S. Solvation of the Chloride Ion in Mixtures of Acetonitrile with Water and Ethanol. *J. Electroanal. Chem.* **1989**, *262* (1), 105–118. https://doi.org/10.1016/0022-0728(89)80015-4

(45) 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* (12), 8262–8272. https://doi.org/10.1021/acscatal.3c01495

(46) Trogler, W. C. Physical Properties and Mechanisms of Formation of Nitrous Oxide. *Coord. Chem. Rev.* **1999**, *187* (1), 303–327. https://doi.org/10.1016/S0010-8545(98)00254-9

(47) Tolman, W. B. Binding and Activation of $N_2O$ at Transition-Metal Centers: Recent Mechanistic Insights. *Angew. Chem. Int. Ed.* **2010**, *49* (6), 1018–1024. https://doi.org/10.1002/anie.200905364

(48) Kubota, T.; Miyazaki, H. The Effect of Substituents on the Half-Wave Potential of the Polarographic Reduction of Pyridine N-Oxide Derivatives. *Bull. Chem. Soc. Japan* **1966**, *39* (10), 2057–2062. https://doi.org/10.1246/bcsj.39.2057

(49) Simmons, E. M.; Hartwig, J. F. On the Interpretation of Deuterium Kinetic Isotope Effects in C-H Bond Functionalizations by Transition-Metal Complexes. *Angew. Chem. Int. Ed.* **2012**, *51* (13), 3066–3072. https://doi.org/10.1002/anie.201107334

(50) Paton, R. Kinisot Version 2.0.2, 2023. https://doi.org/10.5281/zenodo.10403662

(51) Deeba, R.; Chardon-Noblat, S.; Costentin, C. Homogeneous Molecular Catalysis of the Electrochemical Reduction of $N_2O$ to $N_2$: Redox vs. Chemical Catalysis. *Chem. Sci.* **2021**, *12* (38), 12726–12732. https://doi.org/10.1039/D1SC03044B

(52) Deeba, R.; Chardon-Noblat, S.; Costentin, C. Homogeneous Molecular Catalysis of the Electrochemical Reduction of $N_2O$ to $N_2$: Redox vs. Chemical Catalysis. *Chem. Sci.* **2021**, *12* (38), 12726–12732. https://doi.org/10.1039/D1SC03044B

(53) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Efficient Photochemical Reduction of $CO_2$ to CO by Visible Light Irradiation of Systems Containing Re(Bipy)(CO)₃X or Ru(Bipy)₃²⁺–Co²⁺ Combinations as Homogeneous Catalysts. *J. Chem. Soc., Chem. Commun.* **1983**, No. 9, 536–538. https://doi.org/10.1039/C39830000536

(54) Kjellberg, M.; Ohleier, A.; Thuéry, 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.* **2021**, *12* (30), 10266–10272. https://doi.org/10.1039/D1SC01974K

(55) Pischel, U.; Zhang, X.; Hellrung, B.; Haselbach, E.; Muller, P.-A.; Nau, W. M. Fluorescence Quenching of n,Π*-Excited Azoalkanes by Amines: What Is a Sterically Hindered Amine? *J. Am. Chem. Soc.* **2000**, *122* (9), 2027–2034. https://doi.org/10.1021/ja992508b

(56) Schanze, K. S.; Brent MacQueen, D.; Perkins, T. A.; Cabana, L. A. Studies of Intramolecular Electron and Energy Transfer Using the Fac-(Diimine)ReI(CO)₃
Chromophore. *Coord. Chem. Rev.* **1993**, *122* (1), 63–89. https://doi.org/10.1016/0010-8545(93)80042-4.