Rana Deeba

Sylvie Chardon-Noblat

Cyrille Costentin

Importance of Ligand Exchange in the Modulation of Molecular Catalysis: Mechanism of the Electrochemical Reduction of Nitrous Oxide with Rhenium Bipyridyl Carbonyl Complexes

Keywords: Molecular Catalysis, Nitrous Oxide, Electrochemistry, Mechanism, Self-modulation

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Molecular catalysis of electrochemical reactions often involves transition metal complexes as catalysts due to their ability to bind and activate the substrate to be transformed. The current intense activity in the field of small molecules activation, in particular reduction of protons, 1 O2, 2 CO2 3 or more recently N2 [START_REF] Forrest | Nitrogen Fixation via Splitting into Nitrido Complexes[END_REF], offers a myriad of examples. A common feature of such processes is the necessity to get a free metal coordination site allowing the substrate to bind. The corollary is thus that among the various steps of such a catalytic process, ligand exchanges at the metal center are involved, often triggered by variation of the metal oxidation state. One consequence is that the formation of a reaction product or co-product being a strong coordinating ligand might bind the metal and then block the regeneration of a free coordination site required to sustain catalysis. The homogeneous electrochemical reduction of CO2 to CO by the molecular catalyst [Ni(cyclam)] 2+ provides a typical example in which the rate-limiting step during catalysis is CO decoordination from the deactivated intermediate [Ni(cyclam)(CO)] + as shown by Kubiak and coworkers. [START_REF] Froehlich | The Homogeneous Reduction of CO2 by [Ni(cyclam)] + : Increased Catalytic Rates with the Addition of a CO Scavenger[END_REF] Even more drastic is the case N2 splitting by electrogenerated reduced rhenium or molybdenum complexes leading to stable nitride complexes, which then would require additional overpotential to release NH3 via three proton-coupled electron transfers . [START_REF] Lindley | Mechanism of Chemical and Electrochemical N2 Splitting by a Rhenium Pincer Complex[END_REF][START_REF] Van Alten | Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex[END_REF][START_REF] Van Alten | Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes[END_REF][START_REF] Merakeb | Molecular Electrochemical Reductive Splitting of Dinitrogen with A Molybdenum Complex[END_REF][START_REF] Bruch | Mechanism of Electrochemical N2 Splitting by a Molybdenum Pincer Complex[END_REF][START_REF] Ibrahim | Electrocatalytic Nitrogen Reduction on a Molybdenum Complex Bearing a PNP Pincer Ligand[END_REF] In other cases, catalysis is not inhibited but only self-modulated as shown for the reductive cleavage of chloroacetonitrile catalyzed Co(I) cobalamins and cobinamides where the production of free chloride, as catalysis goes on, shifts the generation of the Co(I) active species toward more negative potentials thus slowing down catalysis. [START_REF] Argüello | Role of Protonation and of Axial Ligands in the Reductive Dechlorination of Alkyl Chlorides by Vitamin B12 Complexes. Reductive Cleavage acknowledged for providing facilities for NMR and MS analyses. The service d'analyses, de mesures physiques et de spectroscopie optique from the Fédération de Chimie Le Bel -FR 2010-CNRS -Université de Strasbourg is acknowledged for elemental analysis. The authors thank Florian Molton for his technical assistance. REFERENCES of Chloroacetonitrile by Co(I) Cobalamins and Cobinamides[END_REF] Due to their ubiquity, it is of prime importance to identify and analyze these phenomena in order to fully understand catalytic processes and, more importantly, provide strategies to avoid or delay catalyst deactivation [START_REF] Froehlich | The Homogeneous Reduction of CO2 by [Ni(cyclam)] + : Increased Catalytic Rates with the Addition of a CO Scavenger[END_REF] or even design new reactions. [START_REF] Fritz | Photoelectrochemical Conversion of Dinitrogen to Benzonitrile: Selectivity Control by Electrophileversus Proton-Coupled Electron Transfer[END_REF] In that context, we recently reported that rhenium carbonyl complexes catalyze nitrous oxide (N2O) electroreduction. [START_REF] Deeba | Effective Homogeneous Catalysis of Electrochemical Reduction of Nitrous Oxide to Dinitrogen at Rhenium Carbonyl Catalysts[END_REF] N2O is a greenhouse gas responsible for 4% of the additional greenhouse effect observed since the start of the industrial era. [START_REF] Solomon | Contribution of Working Group I to the fourth Assessment report of the Intergovernmental Panel on climate Change[END_REF] Its reduction to dinitrogen is thus of interest to minimize its release in the atmosphere. Additionally, deoxygenation of N-O bonds is currently relying on thermochemical methods and sacrificial reducing agents. 16 It is thus timely to elaborate selective, clean, and energy thrifty processes for deoxygenation reactions targeting N-oxides. Interestingly, rhenium bi-pyridyl tri-carbonyl complexes able to catalyze the electrochemical reduction of N2O have also proved to be efficient photocatalysts for pyridine N-oxide deoxygenation. [START_REF] Kjellberg | Photocatalytic deoxygenation of N-O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides[END_REF] This suggests that an innersphere mechanism via metal-oxygen binding might be at work as opposed to an activation through a nucleophilic attack of the reduced metal to the electrophilic central nitrogen of N2O proposed from the electronic analogy between N2O and CO2. [START_REF] Rana | Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis[END_REF] We have thus investigated in detail the mechanism of N2O electroreduction catalyzed by [Re(bpy)(CO)3(X)] n+ complexes (bpy= bipyridine; X = chloride, hydroxide or acetonitrile, n depends on the X ligand). These complexes are indeed known for a long time to be efficient catalysts for CO2 to CO conversion and mechanistic studies have shown that the formation of the bi-reduced species [Re 0 (bpy •-)(CO)3] -is required for CO2 to bind the rhenium center and trigger catalysis. [START_REF] Smieja | Re(bipy-tBu)(CO)3Cl-imporoved Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical and Mechanisti Studies[END_REF] This involves via loss of the X ligand to generate a free coordinated site (scheme 1). As opposed to the case of nickel cyclam complexes, after the C-O bond breaking, CO is easily released allowing regenerating the vacant site. Scheme 1. Proposed principle for CO2 and N2O activation with rhenium bipyridyl carbonyl complexes. [START_REF] Rana | Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis[END_REF][START_REF] Smieja | Re(bipy-tBu)(CO)3Cl-imporoved Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical and Mechanisti Studies[END_REF] Activation of N2O has also been proposed to be initiated by its reaction with [Re 0 (bpy •-)(CO)3] -but, on the timescale of cyclic voltammetry, no substantial catalysis occurs at a potential where [Re 0 (bpy •-)(CO)3] -is electrogenerated. [START_REF] Deeba | Effective Homogeneous Catalysis of Electrochemical Reduction of Nitrous Oxide to Dinitrogen at Rhenium Carbonyl Catalysts[END_REF][START_REF] Rana | Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis[END_REF] This is the first result of the present work which reveals that formation of [Re(bpy)(CO)3(OH)] is responsible for this lack of catalysis. This prompted us to investigate the role of the labile ligand in the catalytic process and uncover unexpected slower catalytic pathways but operating at a lower overpotential without requiring the generation of the bi-reduced species [Re 0 (bpy •-)(CO)3] -. Kinetic analysis will show that ligand exchanges on the rhenium center will prove to be crucial in directing the reaction toward innersphere or slower outersphere pathways and hence modulate the catalysis.

Results

Chemical reduction of N2O by electrogenerated

[Re 0 (bpy •-)(CO)3] -.
Starting from [Re I (bpy)(CO)3Cl], a solution of the bi-reduced species [Re 0 (bpy •-)(CO)3] -in acetonitrile is obtained by a two-step electrolysis (see Supporting Information (SI) for details). The first electrolysis is performed at -1.36 V (all potentials are given vs. SCE) and leads to the formation of a metal-bonded Re-Re dimer, [Re 0 (bpy)(CO)3]2, as attested by the dark green color of the solution (see Figure S1). Further reduction of the dimer at -1.82 V gives the bi-reduced species [Re 0 (bpy •-)(CO)3] -as a major product (see Figure S1). Under inert atmosphere, the electrogenerated solution of bi-reduced species (0.6 mM) is stable. [Re 0 (bpy •-)(CO)3] - exhibits a large absorption band at 570 nm. A N2O saturated solution of acetonitrile is then added so as to get 1.5 mM of N2O and the evolution of the bi-reduced species is monitored in situ by UV-vis. Rapid decay is observed: half of the absorbance at 570 nm is lost after 0.5 s (Figure 1). This indicates a fast reaction between [Re 0 (bpy •-)(CO)3] - and N2O. The reaction can be controlled by mass transport, i.e. mixing of N2O, but the rapid decay indicates that a lower limit of the pseudo-first order rate constant can be evaluated as ap k  1.4 s -1 . Therefore, for a saturated N2O solution (i.e. [N2O] = 280 mM, see SI for determination of the concentration), a lower limit of the pseudo-first order corresponding rate constant is ca. An IR spectrum was collected few minutes after the addition of N2O to the bi-reduced species (Figure 2a) and reveals that a mixture of [Re I (bpy •-)(CO)3(OH)] - and [Re I (bpy)(CO)3(OH)] is obtained. This conclusion was obtained from comparison of the IR spectra of authentic samples of [Re I (bpy)(CO)3(OH)] and [Re I (bpy •-)(CO)3(OH)] - gathered from a synthesized [Re I (bpy)(CO)3(OH)] complex (Figure 2b and Figure S3). Cyclic voltammograms (CV) recorded after the addition of N2O shows a reversible system A (570 nm)

t (s) + N 2 O
with a standard potential of -1.43 V as well as a new irreversible wave at ca. -2 V (Figure 2c). The reversible wave matches the redox behavior of the synthesized [Re I (bpy)(CO)3(OH)] complex (Figure S3). [START_REF] Gibson | Synthesis and Reactions of fac-[Re(dmbpy)(CO)3X] (dmbpy: 4,4′-Dimethyl-2,2′-bipyridine; X: COOH, CHO) and Their Derivative[END_REF] Successive application of an oxidative potential at -1.22 V and then a reductive potential at -1.52 V to the final solution with UV-vis and IR characterization after each steps shows identical features (Figure 2d) of authentic samples of [Re I (bpy)(CO)3(OH)] and [Re I (bpy •-)(CO)3(OH)] - respectively (Figure 2b) further confirming the formation of both species upon addition of N2O to [Re 0 (bpy We thus propose the reaction scheme depicted in scheme 2a with consideration that the second step might not be completed. Moreover, the source of water is assumed being the residual water contained in acetonitrile. The key conclusion at this stage is that the bi-reduced species is indeed able to quickly react with N2O. However, as observed on the cyclic voltammogram of [Re(bpy)(CO)3Cl] under N2O (Figure 3), at a potential where [Re 0 (bpy •-)(CO)3] -is generated (second wave), there is no substantial catalysis, the wave corresponding to a maximum of 1.5 electron per rhenium center as attested by linear scan voltammetry on a rotating disk electrode (Figure 3c). Our results reveal that the reason for lack of efficient catalysis is that the labile position is coordinated by hydroxide after the first turnover (equation (1) in scheme 2). Although [Re I (bpy)(CO)3(OH)] is reduced at this potential, either at the electrode or in solution by [Re 0 (bpy •-)(CO)3] -(equation ( 4) and ( 2) respectively in scheme 2b), hydroxide is partially decoordinated and released in solution on the timescale of CV.

•-)(CO)3] -.

Scheme 2.

a) Reaction between N2O and [Re 0 (bpy

•-)(CO)3] -in solution b) Electrochemical reduction of N2O in the presence of [Re I (bpy •-)(CO)3Cl] -
The overall reaction at the level of the second CV wave thus corresponds to a net reduction of 0.5 N2O per rhenium complex and a ligand exchange on the rhenium center (equation ( 5) in scheme 2b). Fast release of hydroxide requires a further reduction at more negative potential (third wave in figure 3a). This additional reduction regenerates the bi-reduced species [Re 0 (bpy •-)(CO)3] -via a "Proton-Coupled Electron transfer"type reaction ("PCET", vide infra in the discussion section) and catalysis can be sustained as summarized on scheme 3. This process parallels the required PCETs to release NH3 from metal nitride complexes in nitrogen reduction reaction catalyzed by molecular catalysts. [START_REF] Lindley | Mechanism of Chemical and Electrochemical N2 Splitting by a Rhenium Pincer Complex[END_REF][START_REF] Van Alten | Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex[END_REF][START_REF] Van Alten | Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes[END_REF][START_REF] Merakeb | Molecular Electrochemical Reductive Splitting of Dinitrogen with A Molybdenum Complex[END_REF][START_REF] Bruch | Mechanism of Electrochemical N2 Splitting by a Molybdenum Pincer Complex[END_REF][START_REF] Ibrahim | Electrocatalytic Nitrogen Reduction on a Molybdenum Complex Bearing a PNP Pincer Ligand[END_REF] The established mechanism is at variance with the previously

A (a.u.)  (cm -1 )
suggested mechanism that was proposed on the sole basis of CV. [START_REF] Deeba | Effective Homogeneous Catalysis of Electrochemical Reduction of Nitrous Oxide to Dinitrogen at Rhenium Carbonyl Catalysts[END_REF] Our results show that formation of hydroxide as a byproduct of N2O reduction is detrimental to catalysis because it is a strong coordinating ligand. Displacement of hydroxide is thus a requirement to get catalysis at lower overpotential. Moreover, the potential at which the initial reductive active species [Re 0 (bpy •-)(CO)3] -is formed is related to the labile ligand X present on the initial complex [Re(bpy)(CO)3X]. Therefore, we will now investigate the role of the labile ligand in the catalytic process. [START_REF] Stor | Spectroelectrochemical (IR, UV/Vis) Determination of the Reduction Pathways for a Series of [Re(CO)3(-diimine)L'] 0/+ (L' = Halide, OTf -, THF, MeCN, n-PrCN, PPh3, P(OMe)3) Complexes[END_REF] The first and second reductions of the dimer occur at -1.52 V and -1.68 V respectively (waves 3/3' and 4/4' in figure 4a). Generation of the bi-reduced species [Re 0 (bpy •-)(CO)3] - occurs at less negative potential than with [Re(bpy)(CO)3Cl] (shift by ca. 300 mV) because decoordination of acetonitrile is easier than decoordination of chloride and also because the overall charge is neutral for the singly reduced solvato complex and anionic for the singly reduced chloro complex. When the solution is saturated with N2O, important changes are observed on CV (Figure 4b). The first wave becomes almost irreversible and slightly increases in intensity. However it remains mono-electronic (Figure 4c) thus excluding a pathway via the bi-reduced species which formation requires an additional 100 mV overpotential. It is then followed by a reversible one-electron wave (Figure 4b) with a standard potential equal to -1.43 V. Finally a catalytic wave is obtained at the same position than the one obtained when starting from either [Re I (bpy)(CO)3Cl] or [Re I (bpy)(CO)3(OH)] (Figure 4d). These observation strongly suggest that N2O reacts with the mono-reduced species [Re I (bpy •-)(CO)3(CH3CN)] (or presumably [Re I (bpy •-)(CO)3]) on the timescale of CV. An exhaustive electrolysis of [Re(bpy)(CO)3(CH3CN)](PF6) was thus performed at -1.22 V in the presence of a large excess of N2O (see SI). The solution product was characterized by UVvis, IR and CV (Figure S4). It unambiguously shows that the obtained product is [Re I (bpy)(CO)3(OH)] which reduction corresponds to the reversible second wave observed on CV (Figure 4b). We also note that no dimer was formed during electrolysis (UV-vis) indicating that the reaction of N2O with [Re I (bpy •-)(CO)3] is faster than dimerization. It is smaller than the corresponding value for the reaction of [Re 0 (bpy •-)(CO)3] -with N2O (ca. 250 s -1 , vide supra). Importantly, the stoichiometry of the first wave remains one electron per rhenium center. This implies that the transformation of [Re I (bpy)(CO)3(CH3CN)] + to [Re I (bpy)(CO)3(OH)] is associated with the net reduction of 0.5 equivalent of N2O and substitution of acetonitrile by hydroxide on the rhenium center (scheme 4). Importantly, the oxidation state of the rhenium complex (Re(I)) is not modified at the end of the experiment. It means that the reaction can be potentially catalytic if the hydroxide ligand could be displaced. Since it is not possible when starting with [Re I (bpy)(CO)3(CH3CN)] + because acetonitrile cannot displace hydroxide, we will now explore this possibility of a catalytic process when starting from [Re I (bpy)(CO)3(OH)] and [Re I (bpy)(CO)3(Cl)].

Behavior of [Re(bpy)(CO)3(OH)] and [Re(bpy)(CO)3Cl] under N2O upon monoelectronic reduction

On the timescale of CV (at 0.1 V/s), the mono-reduced complexes [Re I (bpy •-)(CO)3(OH)] -and [Re I (bpy •-)(CO)3Cl] -are stable both under argon and N2O atmospheres as shown by the corresponding monoelectronic reversible waves (Figure S7). Under constant potential electrolysis conditions and stirring of the solution, the situation is different. At -1.52 V under argon,

[Re(bpy

)(CO)3(OH)] is converted to [Re(bpy •-)(CO)3(OH)] -with a characteristic cell time con- stant  = cell V t SD
250 s, where V is the cell volume, S the electrode surface area, D the diffusion coefficient and  the size of the diffusion layer set by the stirring of the solution (Figure S8). Under N2O, a prolonged current is observed leading to the passage of 12 C after 4 h of electrolysis (Figure 6a), corresponding to 9.2 moles of electron per mole of rhenium complex in solution. UV-vis monitoring of the solution shows that [Re(bpy)(CO)3(OH)] is slowly converted in the bulk (Figure 6b and SI). This indicates that, once formed at the electrode surface, [Re I (bpy •-)(CO)3(OH)] -reacts with N2O on its way to the bulk and [Re I (bpy)(CO)3(OH)] is regenerated. However, after ca 15 min (see figure S10), [Re I (bpy)(CO)3(OH)] starts to be converted in the bulk into a new compound with a broad absorption band in the visible (dark yellow spectrum in figure 6c). Such a band may suggest a decarbonylation of the rhenium complex [START_REF] Schindler | Aerobically stable and substitutionally labile -diimine rhenium dicarbonyl complexes[END_REF] but we cannot exclude formation of other "degraded" catalysts. The reactivity of [Re I (bpy •-)(CO)3(OH)] -with N2O was further confirmed via the electrogeneration of [Re I (bpy •-)(CO)3(OH)] -under inert atmosphere by constant potential electrolysis of [Re I (bpy)(CO)3(OH)] at -1.52 V and then addition of a small excess of N2O. As shown in figure S9, the time evolution of the UV-vis spectrum after addition of N2O shows at 508 nm an initial fast decay that we attribute to the reaction of some bi-reduced species resulting from a slight overoxidation of [Re I (bpy)(CO)3(OH)] (see SI). Then a slow decay is observed corresponding to the reaction of [Re I (bpy •-)(CO)3(OH)] -with N2O. From this decay a rough evaluation of the pseudo first order rate constant for the reaction of [Re I (bpy •-)(CO)3(OH)] -with a saturated solution of N2O is , = mono OH ap k 0.04 s -1 . We thus conclude from these results that the electrochemical reduction of N2O is catalyzed by [Re(bpy)(CO)3(OH)] at a potential corresponding to the formation of the mono-reduced species but it is a slow process, probably limited by the unfavorable displacement of hydroxide by N2O in [Re I (bpy •-)(CO)3(OH)] -. A concurrent decarbonylation of the catalyst may occur that is eventually detrimental to catalysis. Starting from [Re(bpy)(CO)3Cl], the process is more complex. Constant potential electrolysis at -1.42 V under argon shows the formation of [Re(bpy •-)(CO)3Cl] -together with the dimer [Re 0 (bpy)(CO)3]2. Analysis of the time evolution of UV-vis spectra of the solution (see SI for details) indicates that [Re(bpy •-)(CO)3Cl] -initially builds up in the bulk of the solution and then decays at the expense of the dimer (Figure 7a). It is in line with a slow loss of chloride by [Re(bpy •-)(CO)3Cl] - and the observation of a reversible monoelectronic CV wave for the reduction of [Re(bpy)(CO)3Cl] at 0.1 V/s (Figure S7). The transformation of [Re(bpy)(CO)3Cl] to the dimer [Re(bpy)(CO)3]2 is completed after 20 min and the consumption of 1.05 C corresponding to one electron per rhenium center. Under N2O, application of a constant potential at -1.37 V leads to a prolonged current and the consumption of 3 C in 1.5 h (Figure 7b). The analysis of the time evolution of UV-vis spectra of the solution (see SI) shows that [Re(bpy)(CO)3Cl] concentration only slowly decreases over time and that no dimer is formed as well as no intermediate [Re(bpy •-)(CO)3Cl] - (Figure 7c). This again indicates that [Re(bpy •-)(CO)3Cl] - formed at the electrode surface at -1.37 V does not accumulate in the solution bulk but it reacts with N2O with regeneration of [Re(bpy)(CO)3Cl]. This reaction prevents evolution toward formation of the dimer seen under argon. The reaction is however slow enough for not taking place on the timescale of CV. Interestingly, we observe that, in the first 15 min, the decrease of [Re(bpy)(CO)3Cl] concentration in solution is concomitant with the formation of [Re(bpy)(CO)3(OH)] (red trace in figure 7c) and then the appearance of a new compound with a broad absorption band in the visible attributed to the non-identified "degraded" catalyst (Figure 7d) previously observed (figure 6c). Finally, electrolysis at -1.37 V performed in a tight cell under N2O atmosphere in the presence of [Re(bpy)(CO)3Cl] as catalyst allows analysis of the gas in the headspace and reveals that after passing a charge of 10 C the faradaic yield for N2O to N2 conversion is 90% (see SI). These results support the homogeneous reduction of N2O by the electrogenerated [Re(bpy •-)(CO)3(Cl)] -leading to hydroxide as co-product which accumulates in the solution and then progressively binds preferentially the rhenium center, slowing down catalysis and leading eventually to the degradation of the catalyst. for reaction of [Re(bpy •-)(CO)3Cl] -with N2O is not possible because the reaction is too slow to take place on the CV timescale and the lifetime of [Re I (bpy •-)(CO)3Cl] -is too small to prepare it as a bulk solution by electrolysis and then add N2O. However a lower limit of

, mono
Cl ap k can be estimated from the absence of observation of [Re(bpy •-)(CO)3Cl] -in the bulk under N2O (Figure 7c). We obtain

, mono Cl ap k > 0.5 s -1 (see SI for details).
From the observation that catalytic reduction of N2O by the monoelectronic reduction of [Re(bpy)(CO)3Cl] fades due to the chloride decoordination (equation ( 3)) and hydroxide coordination (equation ( 1)), we surmise that addition of an excess of free chlorides in the solution might allow sustaining catalysis by preventing hydroxide coordination to the rhenium center and/or displace hydroxide. Constant potential electrolysis at -1.37 V of a [Re(bpy)(CO)3Cl] under N2O atmosphere in the presence of 100 mM NEt4Cl gives similar results as in the absence of NEt4Cl but with higher current (Figure 8a). Unfortunately, after a while [Re(bpy)(CO)3(OH)] accumulates in the solution, presumably because the concentration of hydroxide built from N2O to N2 conversion is high enough to prevent regeneration of [Re(bpy)(CO)3Cl] (Figure 8b). At the same time, we again observe apparition of a new compound with a broad absorption band in the visible, the "degraded" catalyst, tentatively attributed to a decarbonylation product (Figure 8c). [START_REF] Schindler | Aerobically stable and substitutionally labile -diimine rhenium dicarbonyl complexes[END_REF] Nonetheless, it appears that the presence of free chloride in large concentration in the solution boosts the initial reduction of N2O.

Discussion

We have first shown that fast catalysis of N2O electroreduction by [Re(bpy)(CO)3X] n+ complexes involves the formation of a bi-reduced species [Re 0 (bpy •-)(CO)3] -as homogeneous reductant. A lower limit of the evaluated pseudo-first order rate constant in the presence of excess of N2O (saturated in acetonitrile) and no specific addition of a proton donor, is = bi ap k 250 s -1 (step (1) in scheme 2). The corresponding expected catalytic plateau current in cyclic voltammetry considering pure kinetics conditions and scheme 3 as a mechanism is given by

2 == bi pl ap i FS Dk
337 µA that is of the order of magnitude of the catalytic current observed experimentally (Figure 3a). This indicates that the homogeneous reduction of N2O by electrogenerated [Re 0 (bpy •-)(CO)3] -is rate determining. However, as previously shown and recalled in figure S13, addition of water has two effects on the catalytic wave: (i) cathodic shift of the onset of the catalytic current noting that the shallow increase of the current is indicative of partial limitation by an electron transfer process and (ii) increase of the plateau current. In the framework of the established mechanism corresponding to scheme 3, these experimental features confirm that: (i) the reduction of [Re I (bpy •-)(CO)3(OH)] -to generate the homogeneous reductive species [Re 0 (bpy •-)(CO)3] -involves water and thus corresponds to a "PCET"-type process (scheme 3) and (ii) the chemical step of the fastest catalytic process involves a proton donor such as water. Hence, this excludes that the chemical reduction of N2O by [Re 0 (bpy •-)(CO)3] -is a simple outersphere electron transfer process. Moreover, if it were a simple outersphere electron transfer process the dimer [Re 0 (bpy)(CO)3]2 would be formed from [Re I (bpy •-)(CO)3]. That it is not the case is attested by the final UV-vis spectrum showing no absorption at 590 and 780 nm (Figure 1a). Finally, based on the activa-tion-driving force relationship previously uncovered for homogeneous outersphere reduction of N2O, [START_REF] Rana | Homogeneous molecular catalysis of the electrochemical reduction of N2O to N2: redox vs. chemical catalysis[END_REF] the evaluated rate constant = bi ap k 250 s -1 would correspond to an electron donor with a standard potential of ca. -1.97 V (Figure 9), i.e. much more negative than the oxidation potential of [Re 0 (bpy •-)(CO)3] -evaluated above to be ca. -1.36 V (Figure 4a). Our results confirm the innersphere character of the activation of N2O by [Re 0 (bpy •-)(CO)3] -in line with the requirement for the displacement of hydroxide from the pre-catalyst to trigger catalysis. It appears that water plays a crucial role in both the displacement of hydroxide (via the "PCET"-type step, equation ( 6)) and in the innersphere activation of the N-O bond (equation ( 1)). That the product formed from innersphere activation of N2O by [Re 0 (bpy •-)(CO)3] -is the hydroxide complex confirms the suggestion that N2O binds the rhenium center via the oxygen atom and not the central electrophilic nitrogen. 24 It is in line with the report that rhenium bipyridyl tri-carbonyl complexes are efficient photocatalysts for pyridine N-oxide deoxygenation. [START_REF] Kjellberg | Photocatalytic deoxygenation of N-O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides[END_REF] We have then revealed that reaching the bi-reduced species [Re 0 (bpy •-)(CO)3] -is not mandatory to activate and reduce N2O. We have indeed shown that [Re I (bpy •-)(CO)3] reduces N2O albeit with a slower rate than [Re 0 (bpy •-)(CO)3] -, ,  mono ACN ap k 10 s -1 (pseudo-first order rate constant in N2O saturated acetonitrile). Given the reductive power of [Re I (bpy •-)(CO)3], corresponding to a standard potential of -1.23 V (Figure 4a), we can conclude that the reduction of N2O by [Re 0 (bpy •-)(CO)3] is an innersphere process because it is much faster than predicted for an outersphere electron transfer (Figure 9). Using [Re I (bpy)(CO)3(CH3CN)] + as a pre-catalyst was crucial as acetonitrile is a labile ligand allowing to generate a vacant site on the rhenium center after a monoreduction of the complex. This contention is confirmed by our results showing that [Re I (bpy •-)(CO)3(OH)] -is able to reduce N2O (Figure 6) but with a small rate constant , = mono OH ap k 0.04 s -1 (pseudo-first order rate constant in N2O saturated acetonitrile). Taking into account the standard potential of [Re(bpy)(CO)3(OH)] 0/-, -1.43 V, it appears that the rate constant , mono OH ap k aligns on the activation-driving force correlation predicted for an outersphere electron transfer (Figure 9). Therefore, the strong ligand hydroxide, not able to be displaced by N2O in [Re I (bpy •-)(CO)3(OH)] -, prevents an innersphere activation of N2O. This confirms that an innersphere catalytic activation of N2O requires the regeneration of a free coordination site on the rhenium and the formation of the strong ligand hydroxide as a co-product of N2O reduction induces a self-inhibition of the catalysis. We have also shown that electrogeneration of [Re I (bpy •-)(CO)3Cl] -allows reducing N2O (Figure 7b) with a pseudo-first order rate constant , mono Cl ap k > 0.5 s -1 . This rate constant does not correspond to a single event because it involves loss of chloride and reaction with N2O. Knowing that the standard potential of [Re(bpy)(CO)3Cl] 0/-is -1.38 V, this lower limit of the rate constant indicates that the reaction is an innersphere process and not an outersphere electron transfer from [Re I (bpy •-)(CO)3Cl] -to N2O (Figure 9). This indicates that chloride can be displaced by N2O once the complex is monoreduced whereas hydroxide cannot be displaced by N2O. It is in line with chloride being more labile than hydroxide as [Re I (bpy •-)(CO)3(OH)] -is stable in solution (Figure S8) whereas [Re I (bpy •-)(CO)3Cl] -evolves toward formation of a dimer [Re(bpy)(CO)3]2 (Figure 7a). Interestingly, we have finally shown that, in the presence of large quantity of free chloride in solution, [Re I (bpy •-)(CO)3(OH)] -can be converted back to [Re I (bpy •-)(CO)3Cl] -via displacement of hydroxide by the free chloride, provided that the concentration of free hydroxide produced by the reduction of N2O is not too large (Figure 8).

25

The exchange OH -/Cl - [Re I (bpy •-)(CO)3Cl] -is confirmed by addition of excess of chlorides to an electrogenerated solution of [Re I (bpy •-)(CO)3(OH)] -showing first formation of [Re I (bpy •-)(CO)3Cl] -and then evolution toward formation of the dimer [Re(bpy)(CO)3]2 (figure S15). Therefore the catalysis of N2O reduction via an innersphere process induced by the mono-reduction of [Re I (bpy)(CO)3(Cl)] can be prolonged before switching to a slower outersphere process due to the formation of [Re(bpy)(CO)3(OH)]. It is revealed by the time evolution of the charge which is initially fast (innersphere process) and then keeps increasing at a slower pace (outersphere process) when [Re(bpy)(CO)3(OH)] is dominant in the bulk after 30 min electrolysis as seen in Figure 8. This illustrates the role of ligand exchange on the catalysts in the modulation of the rate of catalysis. Scheme 5. General mechanism for the reduction of N2O by [Re(bpy)(CO)3(X)] n+ complexes.

Based on all the results gathered in the present study, the general mechanism for N2O electroreduction catalyzed by [Re(bpy)(CO)3(X)] n+ complexes (X = CH3CN, Cl -, n = 0 or 1) can be summarized in scheme 5.

Conclusion

The electrochemical reduction of N2O to N2 is catalyzed by rhenium bipyridyl triscarbonyl complexes [Re(bpy)(CO)3X] n+ (X = CH3CN, Cl -). A requirement for efficient catalysis is that the bi-reduced [Re 0 (bpy •-)(CO)3] -species is electrogenerated. However, the reaction generates hydroxide, a strong ligand making the potential needed to regenerate [Re 0 (bpy •-)(CO)3] - more negative as the reaction progresses, amounting to a selfinhibition process. If the requirement is not fulfilled, a less efficient reduction is nonetheless taking place when monoreduced species [Re I (bpy •-)(CO)3(X)] (n-1)+ are electrogenerated. An innersphere reduction of N2O is triggered due to the lability of both acetonitrile and chloride allowing creating a free coordination site for N2O binding. However, as in the case of the bi-reduced species pathway, generation of hydroxide as a co-product induces formation of [Re(bpy)(CO)3(OH)].

Its reduction leads to [Re I (bpy •-)(CO)3(OH)] -which is only competent for a slower outersphere reduction of N2O due to the lack of lability of hydroxide. This can be overcome if a large concentration of free chloride is present enabling to displace hydroxide. These results emphasize the critical role of ligand exchange in the modulation of the rate of catalysis. Self-modulation of catalysis by ligand exchange is a phenomenon likely to take place in many electrochemical reactions catalyzed by transition metal complexes, in particular in the context of small molecules activation. We think that this study will be an incentive to bring more attention to this phenomenon in the endeavor to understand efficiency of molecular catalysis.

Figure 1 .

 1 Figure 1. (a) Time evolution of UV-vis spectra (scans were taken every 0.5 s, l = 1 mm) of a 0.6 mM solution of electrogenerated [Re 0 (bpy •-)(CO)3] -in CH3CN + 0.1 M NBu4PF6 with addition of 1.5 mM of N2O. Spectra in light magenta: from t = 0 (addition) to 2.5 s. Spectra for t < 0 are in purple and t > 2.5s are in red. (b) Time evolution of the absorbance at 570 nm characteristic of [Re 0 (bpy •-)(CO)3] -absorption.

Figure 2 .

 2 Figure 2. (a) Ex-situ IR spectrum of [Re 0 (bpy •-)(CO)3] -0.6 mM in CH3CN + 0.1 M NBu4PF6 before (purple) and after (red) addition of 1.5 mM of N2O. (b) IR spectrum of [Re I (bpy)(CO)3(OH)] (red) and [Re I (bpy •-)(CO)3(OH)] -(orange) in CH3CN + 0.1 M NBu4PF6. (c) CV of a solution of [Re 0 (bpy •-)(CO)3] -0.6 mM after addition of 1.5 mM of N2O in CH3CN + 0.1 M NBu4PF6 at 0.1 V/s on a 3 mm-diameter GCE (d) Ex-situ IR spectrum of a [Re 0 (bpy •-)(CO)3] -0.6 mM + 1.5 mM N2O solution after exhaustive oxidative electrolysis at -1.22 V (red) and then exhaustive reductive electrolysis at -1.52 V (orange).

a b c Figure 3 [Scheme 3 .

 c33 Figure 3 [Re(bpy)(CO)3Cl] 1 mM in CH3CN + 0.1 M NBu4PF6 (a) CV at 0.1 V/s on a 3 mm-diameter GCE. under N2O (black) and under argon (blue). Dashed line: direct reduction of N2O in the absence of [Re(bpy)(CO)3Cl] (b) Same as (a); the red CV is the second part of the black curve obtained by reversing the reverse scan at -1.25 V. (c) Linear scan voltammetry at 10 mV/s on a 5-mm diameter GC rotating disk electrode at 1000 rpm under N2O. Vertical black arrows correspond to a stoichiometry of one electron per rhenium center. Scheme 3. Catalytic reduction of N2O via [Re 0 (bpy •-)(CO)3] -

Figure 4 .

 4 Figure 4. CV in CH3CN + 0.1 M NBu4PF6 at 0.1 V/s on a 3 mmdiameter GCE at 0.1 V/s. (a) [Re(bpy)(CO)3(CH3CN)](PF6) 1 mM under argon (b) [Re(bpy)(CO)3(CH3CN)](PF6) 1.6 mM under argon (blue) and under N2O (red and black). (c) Linear scan voltammetry at 10 mV/s on a 5 mm-diameter GC rotating disk electrode at 1000 rpm of [Re(bpy)(CO)3(CH3CN)](PF6) 1 mM in CH3CN + 0.1 M NBu4PF6 under N2O (black) and under argon (blue). Vertical black arrows correspond to a stoichiometry of one electron per rhenium center. (d) Normalized CV of [Re(bpy)(CO)3(CH3CN)](PF6) 1 mM under N2O (blue), [Re(bpy)(CO)3Cl] 1 mM under N2O (black), [Re(bpy)(CO)3(OH)] 1 mM under N2O (red). 0 p i is the peak current corresponding of a first one-electron wave of each catalyst under N2O.We have thus shown that reaction of N2O with [Re I (bpy •-)(CO)3] takes place on the timescale of CV at 0.1 V/s. Increasing the scan rate partially restores reversibility of the first CV wave (FigureS5) indicating that the pseudo first order rate constant is of the order of ,

Scheme 4 .Figure 5 .

 45 Scheme 4. Reduction of [Re(bpy)(CO)3(CH3CN)] + under N2O.

a b c Figure 6 .

 c6 Figure 6. (a) Charge consumed upon electrolysis at -1.52 V of [Re(bpy)(CO)3(OH)] 0.69 mM in 18 mL CH3CN + 0.1 M NBu4PF6 under N2O. (b) Time evolution of [Re(bpy)(CO)3(OH)] concentration in the bulk solution calculated from the absorbance at 380 nm and 700 nm (see SI for details). (c) UV-vis of [Re(bpy)(CO)3(OH)] 0.69 mM under N2O before electrolysis (red) and after 4h electrolysis at -1.52V (dark yellow), l = 1 mm.

Figure 7 .

 7 Figure 7. (a) Time evolution of [Re(bpy)(CO)3Cl] (black) and [Re(bpy •-)(CO)3Cl] -(green) and [Re(bpy)(CO)3]2 (magenta) concentrations in the bulk solution calculated from UV-vis absorbance (see SI for details) during electrolysis at -1.42 V of

Figure 8 .

 8 Figure 8. (a) Charge consumed upon electrolysis at -1.37 V of [Re(bpy)(CO)3Cl] 0.88 mM in 18 mL CH3CN + 0.1 M NBu4PF6 under N2O with 100 mM NEt4Cl. (b) Time evolution of [Re(bpy)(CO)3Cl] (black) and [Re(bpy)(CO)3(OH)] (red) concentrations in the bulk solution calculated from UV-vis absorbance (see SI for details) at -1.37 V of [Re(bpy)(CO)3(Cl)] 0.88 mM in 18 mL CH3CN + 0.1 M NBu4PF6 under N2O with 100 mM NEt4Cl. Dark yellow: degradation product. (c) UV-vis spectrum of [Re(bpy)(CO)3Cl] before electrolysis (black) and after 120 min electrolysis at -1.37 V under N2O (dashed red/dark yellow) with 100 mM NEt4Cl.

Figure 9 .

 9 Figure 9. Rate constants (in s -1 ) as function of the standard potential of the reductant E 0 (in V vs. SCE) for the homogeneous catalysis of reduction of N2O. Blue data: reduction by electrogenerated anion radicals or dianions (from reference 18). The blue line corresponds to the prediction for outersphere electron transfers (see reference 18). Reduction by: [Re 0 (bpy •-)(CO)3] -(black star), [Re I (bpy •-)(CO)3] (black triangle), [Re I (bpy •-)(CO)3(OH)] -(black diamond ), [Re I (bpy •-)(CO)3Cl] -(black square). Partially adapted with permission from reference 18. Copyright 2021 RSC.

ACKNOWLEGMENT

The MITI (Mission pour les Initiatives Transverses et Interdisciplinaires) program of the CNRS is gratefully acknowledged for RD doctoral financial support. This work was partially supported by the Agence Nationale de la Recherche: Labex ARCANE, CBH-EUR-GS, ANR-17-EURE-0003 and ANR-22-CE07-0028-01. The Initiatives de Recherche à Grenoble Alpes (IRGA program) is acknowledged for financial support. The NanoBio ICMG (UAR 2607), is

SUPPORTING INFORMATION

AUTHOR INFORMATION

Corresponding Authors

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

Notes

The author declares no competing financial interest.