Michael Papadakis

Alexandre Barrozo

Tatiana Straistari

Nicolas Queyriaux

Anisa Putri

Jennifer Fize

Michel Giorgi

Marius Réglier

Julien Massin

Renaud Hardré

Maylis Orio
email: maylis.orio@univ-amu.fr

Ligand-based electronic effects on the electrocatalytic hydrogen production by thiosemicarbazone nickel complexes

Introduction

Along with economic growth, the demand and use of fuel has increased. The risk of depletion of fossil resources combined with environmental problems also reflects the unsustainability of the current energy model. 1 The development of renewable energy sources that are environmentally friendly and affordable is of critical importance facing the 21 st century energy challenge. 1 Hydrogen is regarded as a promising molecular fuel to address such a problem. 2,3 Up to now, platinum-based catalysts possess the best performances among catalysts able to reduce protons to hydrogen.

However, due to the scarcity and cost of this noble element, intense research has been performed to find alternative nonnoble transition metal catalysts. 4 Nature can serve as a guide for such quest, as it offers efficient H 2 evolution catalysts in the form of hydrogenases, organometallic enzymes containing nickel and/or iron sites whose catalytic performances rival commonly used platinum catalysts for hydrogen production.

5,6

Hydrogenases thus offer fascinating blueprints for the design of new molecular catalysts based on Earth-abundant metals, and several studies indeed report homo- [7][8][9] and hetero-binuclear [10][11][12] metal complexes as electrocatalysts for hydrogen evolution reactivity (HER) reproducing the structure of the active site of hydrogenase. Besides these biomimetic approaches, a large number of molecular electrocatalysts have been developed in recent years. They involve mainly the conversion of chemical into electrical energy through the electrochemical oxidation of hydrogen using a metal. Particularly, such molecular electrocatalysts are the subject of a proper chemical design for the efficient hydrogen production at low overpotential. [13][14][15][16][17][18][19] In this context, diverse ligand skeletons with cobalt and nickel-based complexes were found to generate molecular hydrogen in organic and aqueous media. 20 Metal complexes based on thiosemicarbazone ligands are emerging as a new class of electrocatalyst for HER.

21-26

Indeed, it was suggested that the introduction of proton relays in the second coordination sphere of the metal center combined with the use of redox-active ligands can be exploited to enhance or promote catalytic activity. [27][28][29] These complexes thus present some interesting features relevant to electrocatalytic proton reduction: the thiosemicarbazone ligand has already been shown to be redox active while the presence of S-donors and several N-atoms allows for protonation of the ligand and can serve as proton relays. The ability to store electrons in the ligand rather than in the metal renders a variety of mechanisms for HER. Those will give freedom to design electrocatalysts in terms of flexibility and thermodynamics (reduction potentials and pK a values). 30 In 2016, our group reported the synthesis and characterization of a mononuclear nickel complex, NiOCH 3 , an efficient electrocatalyst for HER. 24 The use of catalytic Tafel plot Please do not adjust margins Please do not adjust margins benchmark its catalytic performances with other catalysts reported in the literature demonstrated that this complex is among the best catalysts reported so far with respect to overpotential requirement. A ligand-assisted metal-centered reactivity was proposed for this system with a two-step mechanism involving initial ligand-centered reduction and protonation followed by metal-centered reduction leading to the formation of a stable hydride intermediate. Following this work, our goal is now to improve the catalytic performances for hydrogen production of the nickel complex bearing methoxy substituents (R 1 =R 2 =OCH 3 ). For that purpose, we have prepared and characterized a series of mononuclear complexes by varying the chemical nature of the R 1 and R 2 phenyl ring substituents (Figure 1). Based on Hammett  Constants, we modified our parent thiosemicarbazone ligand H 2 L by tuning the electron withdrawing/donating character of both substituents. Our study also included the synthesis of an asymmetric ligand that combines at the same time the electron withdrawing or donating abilities of two different R 1 and R 2 groups. The resulting complexes display a wide range of electron-donating abilities of their para substituents. Their capability to mediate hydrogen evolution was evaluated to determine the impact of the electronic substitution on the two parameters that are key for catalyst evaluation: the overvoltage and the rate of catalysis.

ARTICLE

Results and Discussion

Synthesis and Characterization

In this work, two types of new ligands were considered: (i) symmetric ligands built with three different substituents including thiomethyl (SCH 3 ); dimethylamino (N(CH 3 ) 2 ) and cyano (CN) groups and (ii) an asymetric ligand which combines dimethylamino and cyano (N(CH 3 ) 2 , CN) substituents mixing the two opposite strongest mesomeric effects in a molecule and therefore presenting an electronic "push-pull effect". These ligands were designed such that their properties were a function of the substituents at the para position of the phenyl ring. Indeed, the Hammett constants of the chosen substituents combine mesomeric and inductive effects induced by para-substitutions. The mesomeric effect, which is a property of functional groups used in a qualitative way, describes the electron withdrawing or donating properties of substituents based on relevant resonance structure, 35 while the inductive effect is an experimentally observed effect of the transmission of charge through a chain of atoms in a molecule, resulting in a permanent dipole in a bond.

The protonated ligands, referred here as H 2 L , were synthesized by the reaction of 2,3-dihydrazide-butane with two equivalents of isothiocyanate derivative in ethanol heated at reflux as previously reported for the parent ligand with methoxy substituents. 24 The products were obtained as yellow solids in good yields (> 56%) and were further used to generate the metal complexes. The NiL complexes were obtained in good yield (> 80 %) by adding one equivalent of nickel nitrate to a suspension of H 2 L ligands in methanol. The crude compounds precipitated as solid powders after washing with methanol and did not require any further purification (Scheme 1). The sets of ligands and complexes were fully characterized by HRMS ESI, IR and NMR spectroscopy (see Supporting Information).

Scheme 1 Synthetic route for both the ligands H2L and the metal complexes NiL.

Our attempts to obtain suitable crystals for X-ray diffraction analysis of the NiL complexes remained unsuccessful. As the PdL complex with OCH 3 substituent was previously reported to be isostructural with the nickel analogue 37 the palladium was also used for a comparative analysis of the structural features of NiL. The PdL complexes were prepared in refluxing acetonitrile by interaction of one equivalent of H 2 L ligands in DMF with one equivalent of palladium chloride. A series of four PdL complexes was obtained as crystalline precipitates and only the one having SCH 3 as R 1 and R 2 substituents was successfully crystallized.

Single crystal X-ray diffraction

Crystals suitable for X-ray diffraction analysis were obtained from DMF solution by slow evaporation of the solvent. . Both PdN 2 S 2 fragments are essentially planar with 0.006 Å distortion. However the overall complex is not flat as shown by Please do not adjust margins Please do not adjust margins the angle between the two phenyl of the methylsulfanylphenyl moieties equal to 27°. Selected bond lengths are provided in the caption of Figure 2.

•••O1 [N1-H 0.86 Å, H-O1 1.94 Å, N1•••O1 2.785 Å, N1-H•••O1 167.5°]; N6- H•••O2 [N6-H 0.86 Å, H-O2 1.97 Å, N6•••O2 2.81 Å, N6-H•••O2 165.14°].
Comparison of PdSCH 3 with its methoxy analogue 24 shows that the two palladium complexes have similar structures. At first glance, the electronic properties of R 1 and R 2 substitutions do not seem to greatly influence the geometry of the complex. However, they may likely affect their electronic structures and their spectroscopic parameters.

UV/Vis spectroscopy

The optical properties of the ligands and the nickel complexes were followed by UV/Vis spectroscopy in DMF (Figures 3 andSI-1). The evolution of the spectra upon ligand metalation allowed us to confirm the nickel binding to the ligands for the whole series of systems. All ligands display one main absorption band centered at 350 nm while the NiL complexes display one main electronic transition in the 445-515 nm range and one shoulder around 700-750 nm (Table SI-2). The absorption band found at high energy has been previously attributed to a mixed metal-ligand to metal-ligand transition 24 while the low-energy one is compatible with a d-d transition. The spectral features of the complex with the asymmetric ligand relate with the presence of different phenyl rings substituents. They are expected to affect individually the electron density distribution in the ligand upon excitation resulting in two high-energy absorption bands. Based on the spectral features reported in Figure 3, it is worth noting that the different R 1 and R 2 substitutions have an electronic impact on the optical properties that influences the position of the main absorption band. Indeed, we observe a sequential bathochromic shift of the band when going from the CN substituent to the N(CH3) 2 one which reflects the electron-donating abilities of the para substituents (N(CH 3 ) 2 >OCH 3 >SCH 3 > CN). This trend was further supported by TD-DFT calculations performed on the NiL series of complexes. Indeed, the predicted spectroscopic data provide calculated spectra that compare well with the experimental observations (Figure SI-17). Our computations adequately reproduce the energy of the key features of the experimental spectra, which highlight the electronic influence of the phenyl ring substituent on the optical properties of the systems.

Electrochemical studies

Cyclic voltammograms (CV) were recorded at room temperature at a glassy carbon electrode in Ar-deaerated anhydrous DMF containing NBu 4 PF 6 as a supporting electrolyte. The redox behaviour of the ligands was investigated in the potential range between +0.5 and -2.5 V vs. Fc +/0

. We recorded the successive cyclic voltammograms of the H 2 L ligands with and without 40 mM of trifluoroacetic acid (Figure SI-4). In the absence of TFA, the CV show one irreversible wave in the cathodic region while in its presence, no catalytic wave was detected for the whole series. This is consistent to what was previously observed, confirming that the ligands themselves cannot perform catalytic proton reduction. 24 The CVs of the five NiL complexes recorded in anhydrous DMF under argon are shown in Figure 4. We can observe two quasireversible processes in the cathodic region from 0 to -2.5 V vs. Fc +/0 . The values of the processes in cathodic directions for the four NiL complexes are presented in Table 1 and compared with the NiOCH 3 reference. The plot of the peak currents versus the square root of the scan rates for both cathodic and anodic waves were found to be linear, indicating that it is a diffusion-controlled process (Figure SI-5). A detailed analysis of the data presented in Figure 4 and Table 1 show that the R 1 and R 2 substitutions have an electronic effect influencing the redox properties of the NiL complexes. Indeed, the more positive is the σ para Hammett constant, the more positive is the potential for the first cathodic wave. These results thus This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins support that the redox properties of the systems can be tuned by the ligand, highlighting the importance of the substituent at the para position of the phenyl ring. The trend observed for the first reduction process of the four NiL derivatives matches the relative electron-donating ability of the para-substituents. This is of interest as it provides qualitative information about the degree of electronic delocalization in the complex and gives insight into the radical distribution within the ligand framework. Indeed, our results show that electron withdrawing groups tend to diminish the potential required for reduction of the system which is likely due to stabilization of electron attachment energy and can be correlated to larger delocalization within the complex.

DFT calculations

In previously published studies on thiosemicarbazone nickel complexes, 24,32,34 DFT calculations were performed to understand their HER mechanism. The results offered important insight into the protonation sites of the complex, and also the electronic structure change upon reduction, the latter confirming the non-innocence of the ligand. In this work, we chose to focus solely on the two reduction processes, in order to infer the impact of the substituent on the electronic structures of the complexes (Figure SI-15). Our computational results confirm that the first reduction process remains ligandbased and show a fair agreement when comparing our calculated values with those from electrochemical measurements (Tables 2 and SI-5-9). Regarding the second reduction process, we assessed all spin electronic configurations for the doubly reduced species by considering the closed-shell singlet state (S =0), the open-shell triplet state (S=1) and the open-shell singlet state (Ms=0), the latter being evaluated using the Broken Symmetry (BS) approach. For all complexes, the open shell singlet (Ms=0) was found to be the lowest in energy and the predicted redox potentials compare well with experiment. Analysis of the Unrestricted Corresponding Orbitals confirm that the second reduction process is metal-centered (Figure SI-16). The qualitative trend for the different substituents and their relative order is well reproduced for both reduction steps. This serves as a validation of the choice of theoretical approach, in order to understand how substituents can modulate redox potentials.

When comparing the relative redox potentials (Figures SI-20-23) with spin density distributions, we notice that less spin density in the Ni center yields less negative redox potentials, thus reducing the potential necessary for reduction. This is evidenced by the CN substituent, which seems to draw electronic density away from the metal center (Figure 5). Conversely, for N(CH 3 ) 2 we see the most negative redox potential, and it is the substituent that causes a subsequent localization of the spin density at Ni center. Thus, addition of electron-withdrawing groups tends to decrease the reduction potential, which will have interesting implications for catalysis as HER with our systems is supposedly initiated with the electrochemical step (electron transfer).

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Electrocatalytic proton reduction

To examine the capability of the NiL complexes to mediate proton reduction catalysis, their electrochemical behaviour was studied by cyclic voltammetry in the presence of trifluoroacetic acid (TFA, pK a = 6.00 ± 0.3 in DMF) 38 Focusing on NiSCH 3 , the addition of 20 mM of TFA triggers the appearance of two proton-dependent waves with peak potentials at -1.32 and -2.18 V vs. Fc +/0

, suggesting the existence of potential-dependent mechanisms (Figure SI-6). The current response of these peak-shaped waves is directly proportional to the concentration of protons in the electrolyte solution. While the peak potential of the more negative process is gradually shifted of from -2.18 V to -2.39 V vs. Fc +/0 (Figure SI-6), the first catalytic process potential remains mostly unaffected (Figure SI-7). Comparison with control experiments performed in the absence of NiSCH 3 clearly indicates the role played by the nickel complex to in the observed current enhancement (Figure 6). As for NiOCH 3 [24] ,

we only investigated the process occurring at the more positive potentials with a midwave potential = -1.32 V.

To verify that the observed catalytic response in presence of TFA can be attributed to the complexes in solution and not due to adsorbed species on the surface electrode or degradation products, we performed a series of dip and rinse tests. After each electrochemical measurement, the working electrode was removed from the solution and immersed into a fresh DMF solution. 60 mM TFA was then added and the electrochemical response was recorded between 0 to -2.5 V vs. Fc +/0 . For all complexes, only a residual background current was observed upon sweeping the potential in the cathodic region and upon addition of TFA, only the acid blank was observed (Figure SI-8). These control experiments confirm that no active adsorbed species is formed during electrocatalysis as no catalytic current could be detected indicating that the HER activity is due to dissolved NiL complexes. It is also worth mentioning that our systems displaying ligand-based reduction steps, it will limit the formation of Ni(0) species, preventing from a possible degradation pathway. To confirm that the observed catalytic waves correspond to the electrocatalytic reduction of protons into hydrogen, we performed bulk electrolysis experiments of DMF solutions of TFA (100 mM) and NiL (1 mM). Gas products were monitored by continuous in-line GC analysis. A potential of -1.70 V versus Fc +/0 was applied to the mercury-pool working electrode resulting in the immediate formation of hydrogen. These experiments allow to rule out the possibility that proton reduction occurs as the result of deposition of nickel nanoparticles on the electrode surface since they would amalgamate within the mercury pool electrode. Consequently, inorganic coating cannot be responsible for catalysis with our NiL complexes and the catalysis is expected to be homogeneous in nature.

During the 16 hours electrolysis experiments, the four new NiL complexes were active in catalytic proton reduction. Their catalytic performances are summarized in Table 3 and graphically represented in Figure SI-9. It is worth mentioning that the maximum TON that can be obtained in our case is 50.

In similar condition, our reference catalyst (NiOCH 3 Hammett constant = -0.27) mediated hydrogen evolution during a 16 hours electrolysis with 80 % faradaic efficiency and a turnover number of 21. These data confirm that NiSCH 3 is more efficient to catalyze proton reduction when compared to the NiOCH 3 complex which would support that chemical substitution of the ligand can help in enhancing the catalytic activity of our thiosemicarbazone nickel complexes. However, it is not possible at this stage to correlate the catalytic performances of the complexes with the electron-donating properties of their phenyl ring substituents even though we could observe a nice trend when looking at their redox and spectroscopic properties. This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins The foot-of-the-wave analysis (FOWA) was tentatively used in this work to estimate the relevant HER rate constants from CV measurements. 31,[47][48][49] Linearization of the catalytic wave was achieved by applying the equation corresponding to an ECEC mechanism with the second electron transfer being easier than the first one. The slope of the linear portion is expected to provide helpful kinetic information. 49 From this point, the nature of the rate-determining step (RDS) must be examined. Considering fast and non-limiting electron transfers, kinetic limitations may arise either from the first (k 1 <<k 2 ) or the second (k 2 <<k 1 ) protonation step. In the present case, the mid-wave potentials of the series of complexes are less negative than the potential of the redox process triggering the catalytic wave, suggesting that k 2 is more likely the ratedetermining step. To confirm these findings, we estimated the maximum value of k 2 as approximately k 1 /10 50 to conform to a regime where catalysis is kinetically-controlled by the second protonation step. Under such condition, theoretical values of the catalytic enhancement ratio i/i 0 p can be estimated in regard to the nature of the rate determining step. These estimates were made for the NiL series and are reported in Figures 7 and SI-11-13 at various scan rates. .

Upon comparing the experimental data (blue squares) to the theoretical boundaries established for k 1 = k cat (red circles) and k 2 = k cat (green triangles), it is clearly observed that the experimental values for all complexes fall in the last region, precluding any insight regarding the nature of the rate determining step. Such situation thus prevents any unambiguous determination of the rate constant from FOWA (see Supporting Information).

To circumvent this limitation, the TOF values were determined from the bulk electrolyses experiments as previously reported.

51,52

For heterolytic processes, the following equation was used: (1) with F is Faraday's constant, A the surface area of the mercurypool working electrode, D the diffusion coefficient, [Cat] the concentration of the catalyst, E appl the applied potential during electrolyses, E 1/2 the midwave potential for the catalytic wave and i el the current density during bulk electrolysis (see Supporting Information for the detailed analysis). The maximal turnover frequencies (TOF max ) of the series of complexes were then evaluated using the following relation:

(

TOF max values of 115.9, 18.8, 14.3 and 10.2 s -1 were thus obtained for NiSCH 3 ; NiN(CH 3 ) 2 CN; NiN(CH 3 ) 2 and NiCN, respectively. For comparison, the TOF max value of the parent complex was also evaluated following the same procedure and a value of 89. Focusing on NiSCH 3 (red trace, Figure 7), we observe that this catalyst displays significant catalytic activity (log(TOF) > 1) for low overpotential values. It compares well with benchmark molecular catalysts like cobaloximes 14 (blue trace, Figure 8) or nickel bisdiphosphine catalysts 43,45,55 (orange trace, Figure 8)

as far as the overpotential requirement is concerned. Interestingly, NiSCH 3 features similar intrinsic catalytic performances when compared to NiOCH 3 . However, when looking at the macroscopic scale, the HER parameters (FY and TON) suggest that NiSCH 3 displays an increased long-term stability with respect to the parent thiosemicarbazone complex. Taken together, these results support the fact that thiomethyl substitution on the ligand influences the electronic and redox properties of the system which directly impact its catalytic behavior. However, it seems unclear whether there is an actual relationship between the catalytic observables and the electronic properties of the phenyl ring substituent within the NiL series of complexes. While the overpotential of the four derivatives increases when going from the CN substituent to the N(CH3) 2 one and matches the relative electron-donating ability of the para-substituents, the TOF max values seem to go in the opposite direction with smaller values being obtained for the more electron-withdrawing substituents (Figure SI-14).

Conclusions

We have reported a new series of thiosemicarbazone nickel complexes for which the R 1 and R 2 phenyl ring substituents were chosen to cover a wide range of electron-donating properties. This approach was meant to modulate the electronic properties of the catalysts based on the different Hammett Sigma Constants at the para position of the benzene. The complexes were experimentally and theoretically characterized, and shown to exhibit from moderate to high electrocatalytic activity for proton reduction to dihydrogen. Detailed analysis of the electrocatalytic data allowed to benchmark the performances of the series of complexes and catalytic Tafel plots showed that NiSCH 3 compares well with benchmark molecular catalysts as far as the overpotential requirement is concerned. Our results indicate that the chemical nature of the substituent in the para position of the ligand indeed influences the electrochemical and catalytic behaviour of the system. Further work will be necessary to decipher the actual relationship between the electrondonating ability of the phenyl ring substituent of the complexes and the HER parameters as opposite trends were obtained when looking at the overpotential and the TOF max values. This encourages us to pursue our research on improving the catalytic performances of our nickel thiosemicarbazone complexes and on unravelling the key elements that both affect and control the redox and electronic properties of these HER catalysts.

Experimental Section

All solvents were purchased from Sigma Aldrich and used without further purification, and for reagents chemical were purchased from Alfa Aesar for isothiocyanate derivative. Syntheses of isothiocyanate derivative were performed according to procedures described by Demchenko et al. 58 and Lakshmi et al. . High resolution mass spectra (HRMS) were performed on a QStar Elite (Applied Biosystems SCIEX) spectrometer equipped with atmospheric pression ionization source (API) pneumatically assisted. The samples were placed in a methanol/3 mM ammonium acetate. The results were validated by three measurements. UV/Vis spectra were recorded on a Varian Cary 50 spectrophotometer. ) used in the working compartment of the cell was 8 ml. The working electrode used was a pool of mercury, separated from the coiled platinum wire counter electrode by a porous frit. Bulk electrolysis solutions were purged witch N 2 gas for at least 20 min prior to electrolysis and stirred throughout bulk electrolysis experiment. During the experiment, the cell was continuously purged with nitrogen (5 mL.min -1

Single crystal X-ray diffraction

) and the output gas was analyzed at two-minute intervals in a Perkin Elmer Clarus 500 gas chromatographer using a previously described setup

DFT calculations

All theoretical calculations used ORCA 4.1.2. 63,64 Geometry optimizations and numerical frequency calculations were performed for all intermediates using B3LYP and BP86, with def2-TZVP as the basis set. [65][66][67][68][69][70][71][72] We chose to use 'Grid4' and 'TightSCF' for integration and convergence criteria, following ORCA notation, and CPCM as the implicit solvation model for the solvent effect of DMF. 69 Single-point Broken-Symmetry DFT calculations were performed to evaluate all possible spin configurations using the "FlipSpin" feature of ORCA. 72,73 Free energy differences for the redox potential calculations were extracted from the Gibbs free energies obtained from numerical frequencies calculations. Since experimental values are reported with respect to the ferrocene electrode in DMF, we need to shift our calculated absolute potentials. We calculated the Gibbs free energy difference for the ferrocene reduction in DMF, obtaining ∆G(BP86) = 4.87 eV and ∆G(B3LYP) = 4.36 eV. While those values differ by 0.5 eV, they inherit the behaviours of each functional, thus offering the possibility of cancelling systematic errors. Optical properties were predicted from additional single-point calculations using the CAM-B3LYP functional 74,75 and the def2-TZVP basis set.

Electronic transition energies and dipole moments for all models were calculated using time-dependent DFT (TD-DFT) [START_REF] Casida | Recent Advances in Density Functional Methods[END_REF][START_REF] Stratmann | [END_REF][78] within the Tamm-Dancoff approximation. 79,80 To increase computational efficiency, the RI approximation 80 was used in calculating the Coulomb term and at least 40 excited states were calculated in each case. All chemical structure images were generated using Chemcraft.

Journal Name 2 |

 2 J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Fig. 1

 1 Fig.1 Thiosemicarbazone nickel complexes under investigation with R1, R2 = OCH3, SCH3, N(CH3)2 or CN

Fig. 2 X

 2 Fig.2 X-ray crystal structure of [PdSCH3].2 DMF with atom labelling scheme and ellipsoids parameters at 50 % probability level. Selected bond lengths: Pd1-S2 2.28(10) Å, Pd1-S3 2.275(9) Å, Pd1-N3 1.98(3) Å, Pd1-N4 1.98(3) Å, N2-N3 1.36(4) Å, N2-C8 1.33(4) Å, N3-C9 1.31(4) Å, N4-N5 1.36(4) Å, N4-C10 1.31(4) Å, N5-C11 1.33(4) Å, C9-C19 1.49(4) Å, C10-C20 1.49(4) Å. Hydrogen bonds parameters: N1-H•••O1 [N1-H 0.86 Å, H-O1 1.94 Å, N1•••O1 2.785 Å, N1-H•••O1 167.5°]; N6-H•••O2 [N6-H 0.86 Å, H-O2 1.97 Å, N6•••O2 2.81 Å, N6-H•••O2 165.14°].

Fig. 3

 3 Fig.3 Electronic absorption spectra for the series of NiL complexes, concentration of 0.05 mM in DMF.

Fig. 4

 4 Fig.4 Cyclic voltammograms of the series of NiL complexes, concentration 1 mM at a stationary glassy carbon electrode in DMF + 0.1M NBu4PF6. Scan rate 500 mV/s.

Fig. 5 :

 5 Fig.5: Localized SOMOs and spin density plots for the one-electron reduced species of the NiL series.

  as proton source. Cyclic voltammograms of the NiL complexes were recorded under argon in DMF solutions with increased concentrations of TFA and are shown in Figures 6, SI-6 and SI-7.

Fig. 6

 6 Fig.6 Successive cyclic voltammograms of a 1 mM solution of NiSCH3 in DMF (0.1 M NBu4PF6) recorded at a glassy carbon electrode with increasing amounts of TFA at 500 mV/s (top) and with 60 mM TFA at five different scan rates (bottom).

Fig. 7

 7 Fig.7 Theoretical values for ic/ for NiSCH3 in the presence of 60 mM TFA and various scan rates plotted using equation (8). Red circles represent values were kcat = k1 = 2812 M -1 s -1 . Green triangles represent values were kcat = k2 = k1/10 = 281 M -1 s -1

Fc + /Fc 24 ) at 1 M 39 Fig. 8

 241398 Fig.8 Catalytic Tafel plots: comparison of performances for HER catalyzed by NiSCH3 in DMF in the presence of 1 M TFA (red line) and NiOCH3 24 in DMF in the presence of 1 M TFA (green line) with other catalysts reported in the literature. Black: Fe II TPP, DMF, Et3NH + 31,51 Blue: [Co II (dmgH)2py], DMF, Et3NH + 31,54 Orange: [Ni II (P2PhN2C6H4X)2] 2+ , X=CH2P(O)(OEt)2, MeCN, DMFH + . 31,55

59 1 H

 591 NMR spectra were recorded on Bruker 400 and 300 MHz Avance III nanobay. Chemical shifts for 1 H NMR spectra refer to TMS or the residual protonated solvent. IR spectra were recorded with Bruker TENSOR27 spectrometer equipped with a single-reflection DuraSamplIR diamond. ESI-MS analyses were performed using a SYNAPT G2 HDMS (Waters) spectrometer equipped with a pneumatically assisted Atmospheric Pressure Ionization (API) source. Samples were ionized by positive electrospray mode as follows: ion-spray voltage: 2.8 kV; sampling cone: 20 V for NiSCH 3 , NiCN, NiN(CH 3 ) 2 and 40 V for NiN(CH 3 ) 2 CN; nitrogen flux (nebulization): 100 L h -1

Table 1

 1 Electrochemical data for NiL complexes. Potentials (V) refer to Fc

	+/0 .

I L 2-∆E (V) vs.

NiOCH 3

				Epc*	Epc**
	NiCN	-1.40	-1.99	-0.21	-0.21
	NiN(CH3)2CN	-1.48	-2.09	-0.09	-0.13
	NiOCH3	-1.57	-2.20	-	-
	NiSCH3	-1.50	-2.13	-0.07	-0.07
	NiN(CH3)2	-1.62	-2.11	0.04	-0.08

Table 2

 2 Comparison of calculated and experimental shifts of redox potentials with respect to NiOCH3.

				∆E (V) vs. NiOCH 3 (BP86/def2-TZVP)
		SCH3		N(CH3)2	N(CH3)2CN	CN
		Calc.	Exp.	Calc.	Exp.	Calc.	Exp.	Calc.	Exp.
	Ni II L/Ni II L •-	0.02	-0.07	0.11	0.04	-0.11	-0.09	-0.33	-0.26
	Ni II L •-/Ni I L 2-	-0.08	-0.07	-0.03	-0.09	-0.20	-0.12	-0.31	-0.21

Table 3

 3 HER parameters of the series of NiL complexes.

		Hammett		Faradaic
	Complex	constant	Q (C)	efficiency	TON
				(FY, %)
	NiSCH3	0	61075	94	37
	NiN(CH3)2CN	-0.83/0.66	26169	86	14
	NiN(CH3)2	-0.83	26065	56	10
	NiCN	0.66	18617	63	8

  A suitable crystal for compound [PdSCH3].2DMF was measured on a Rigaku Oxford Diffraction SuperNova diffractometer at room temperature at the CuKα radiation (λ=1.54184 Å). Data collection reduction and multiscan ABSPACK correction were performed with CrysAlisPro (Rigaku This journal is © The Royal Society of Chemistry 20xx /Fc) = 0.52 V vs. Ag/AgCl for each measurement. All studies were performed in deoxygenated DMF containing NBu 4 PF 6 (0.1 M) as supporting electrolyte. Controlled potential electrolysis experiments were carried out in two-compartment cell. The volume of solution (DMF, 0.1 M NBu 4 PF 6 ) used in the working compartment of the cell was 2 ml. Controlled potential electrolysis experiments were carried out in twocompartment cell. The volume of solution (DMF, 0.1 M NBu 4 PF 6

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	Oxford Diffraction). Using Olex2	60 the structures were solved
	by direct methods with SHELXT	61 and SHELXL	62 was used for
	full matrix least squares refinement. All H-atoms were found
	experimentally except those for the methyl groups that were
	introduced at geometrical positions. Their coordinates and
	Uiso parameters were constraint to 1.5Ueq (parent atoms) for
	the methyl groups and to 1.2Ueq (parent atom) for the other
	carbons and the amines. The crystallographic and refinement
	parameters are given in supporting information. CCDC number
	1971128 contains the supplementary crystallographic data for
	this paper.
	Electrochemistry
	Cyclic voltammetry experiments were performed using a
	BioLogic SP300 potentiostat and a three-electrode set-up
	consisting of a glassy carbon working electrode, a platinum
	wire counter electrode and a Leakless (Ag/AgCl) reference
	electrode. Ferrocene was used as an internal standard with
	E 0 (Fc
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+

Acknowledgements

The authors gratefully acknowledge research support of this work by the French National Research Agency (ANR JCJC, CODEC, ANR-19-CE05_0030_01 and Labex program, ARCANE, ANR-11-LABX-0003-01, Graduate school of Chemistry, Biology and Health of Univ.

Grenoble Alpes, CBH-EUR-GS, ANR-17-EURE-0003).

Conflicts of interest

There are no conflicts to declare

Author contributions

M.P. and A.P. carried out the experimental work under the supervision of M.O. T.S. and N.Q. contributed to the analysis of the electrochemical data. A.B. carried out the theoretical calculations. J.F performed the measurements of gas production and M.G. performed X-ray analysis. J.M., R.H. and M.O. analysed and interpreted the experimental data. M.R. and M.O. conceived and designed the project. M.O prepared the manuscript. All authors reviewed and contributed to the manuscript.


Cyril Pieri

Anirban Bhattacharjee

Alexandre Barrozo

Bruno Faure

Michel Giorgi

Jennifer Fize

Marius Réglier

Martin Field
email: martin.field@cea.fr

Maylis Orio
email: maylis.orio@univ-amu.fr

Vincent Artero

Hydrogen evolution reaction mediated by an all-sulfur trinuclear nickel complex

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

We report the synthesis and the characterization of a trinuclear nickel complex. Solid state and solution studies using X-ray diffraction, NMR and UV-vis spectroscopy highlight the square planar geometries around the metal centers and an all-sulfur coordination sphere. It exhibits significant electrocatalytic activity for hydrogen evolution in DMF using Et 3 NHCl as the proton source. The theoretical reaction mechanism suggests that sulfur atoms act as proton relay, as proposed in [NiFe] hydrogenases.

The limited sources of fossil fuels, together with their environmental impact, render them unviable in the long run. This is aggravated by an increasing energy demand worldwide, resulting in higher prices, as well as with the lack of renewability of fossil energy at the human time scale. 1 To the contrary, dihydrogen seems to be an ideal substitute, since it is based on the most abundant atom in the universe, and its combustion yields high energy, with water as the only waste.

2,3

The main issue remains with its production. While water electrolysis appears as the most efficient way to produce hydrogen, the flexible proton exchange membrane technology requires the use of platinum, a rare and expensive metal with limited stocks. 4 In the search for an economically competitive way of producing dihydrogen with non-noble metal-based catalysts, Nature can guide us and inspiration can be found with the [NiFe] hydrogenases. This family of enzymes is a very attractive example as it is the most abundant hydrogenases among living organisms. Their active sites are composed of two metal ions, iron and nickel, the latter lying in a rare all-sulfur and distorted tetrahedral coordination geometry. [5][6][7] Many bioinspired complexes mimicking this active site can be found in literature [START_REF] Sirbu | Hydrogenases: From Biomimetic to Bioinspired Models in Bioinspired Chemistry[END_REF][9][10][11][12][13][14][15] These models generally show some degree of variation from the enzyme active site: metals, coordinating atoms, bridging and exogenous ligands, e.g. CO and CN -.

16-18

To reproduce the features of the [NiFe] hydrogenase active site, an all-sulfur nickel complex was obtained by Yamamura et al. in 1985, long before the crystal structure of the enzyme was resolved. 19 Sellman et al. also explored a certain number of allsulfur nickel complexes. 20 Once the structure of the [NiFe] hydrogenase was known, many heterobimetallic complexes were then developed. 21 Unlike the enzyme, all of these complexes suffered from a lack of nickel centred reactivity 22 and the situation only changed recently. 12, 23-28 Moreover, the hydrogen producing catalytic performances of these complexes with respect to overpotential requirement and turnover frequency (TOF) towards hydrogen production were far below those of [NiFe] hydrogenases, operating at the thermodynamic equilibrium with a TOF around 1,000 s -1 .

29,30

The high activity of hydrogenase depends on the presence of proton relays at the vicinity of the active site. In [NiFe] hydrogenases, terminal cysteinates have been assigned this role. 22 While there are several examples of FeFe hydrogenase models with terminal thiolate ligands prone to protonation, 31,32 there is only one known example of a NiFe complex, and this was only observed under strongly acidic conditions, not pertinent for catalysis. 33 In this context, we have developed a bio-inspired nickel complex bearing an allsulfur and constrained ligand that resembles the Ni-center of the enzyme active site with its cysteine-thiolate ligands implicated as bases in proton transfer reactions. Inspired by the seminal work of Bosnich and Pickett, [34][35][36] our strategy to prepare the trithiol 3 was to start the synthesis from the thiol 1 easily accessible according to the Bosnich procedure. 34, 37 Compound 2 was then obtained by S-alkylation of 1 with 2-bromoethyl-trityl-sulfide. Removal of the trityl substituent in dry acidic condition (CF 3 COOH) and the reduction of the disulfide moiety by triethyltin hydride led to the trithiol 3. This overall process provided access to the trithiol 3 which was used for the synthesis of Ni(II) complexes. Thus, the reaction of the ligand 3 with 1 eq. of [Ni(CH 3 CN) 6 ](BF 4 ) 2 in acetonitrile with K 2 CO 3 as base led to the formation of a soluble complex that rapidly evolved by the formation of a brown precipitate. Filtration followed by liquid chromatography with CH 2 Cl 2 as eluant allowed us to get the pure Ni(II) complex 4 in 45% yield (Scheme 1). Suitable crystals were obtained from a saturated solution of the complex in CH 3 CN. The complex crystallizes as a symmetric trimer with the central Ni-atom lying on the crystallographic two-fold axis (Figure 1).

The asymmetric unit is thus composed of half the complex. The coordination geometries of the metals are square planar with small deviations from planar (defined by the Ni and the four coordinated S atoms, Figure S6 and Table S1): the maximum out-of-plane distances are close to 0.15 Å and 0.19 Å (S1 in plane P1 including Ni2 and S1 in plane P2 including Ni1, respectively). The dihedral angle between P1 and P2 is equal to 73.0697(4)°, while the dihedral angle between P1 and the symmetric plane P1' (symmetry code ': 1-x, y, 3/2-z) is equal to 33.8471 (8). The structure of compound 4 is therefore very compact and folded with a bowl shape and a small accessible cavity toward the central Ni-atom (mean width and depth equal to 3.4 Å and 4.9 Å, respectively, Table S2).

To check that the structure of 4 corresponds to the one found in solution, we performed the UV-vis titration of the ligand 3 by nickel perchlorate in the presence of triethylamine in DMF. Upon addition of [Ni(H 2 O) 6 ](ClO 4 , the UV-vis spectrum displays three absorption bands at 290 nm, 410 nm and 540 nm (Figure S3). Plotting the molecular extinction coefficient as a function of the number of Ni(II) equivalents for each absorption band allowed us to determine the stoichiometry of the complex (Figure S4). Analysis of the titration curves for both the 410 and 540 nm bands support the formation of a trinuclear species like 4 with a 2:3 nickel to ligand ratio, e.g. 2 ligands for 3 metal ions.

To support the above assignment, we used timedependent DFT to calculate the theoretical UV-Vis spectrum of 4 (Figure S9 and Table S5) in the trimer form revealed by X-ray diffraction. Our computations support that its UV-vis spectrum is dominated by three main absorption bands of different intensity (Figure S10). The low-energy absorption is assigned to a d-d transition while the two other spectral features are attributed to charge transfers from the ligand to metal (Figure S11 and Table S4). Both the shape and relative intensities of the calculated spectrum are in pretty fair agreement with the spectrum measured in solution.

The electrochemical study of 4 was performed by cyclic voltammetry between +0.5 and -2.5 V vs. Fc +/0 (Figure 2). In the cathodic region, two one-electron redox systems were observed: one quasi-reversible at -1.65 V (½(E pc *+E pa *)) and another irreversible at -2.09 V vs. Fc +/0 (E pc **). The plot of the peak current versus the square root of the scan rates for the reversible cathodic wave was found to be linear, indicating that it is a diffusion-controlled process (Figure S7). In the anodic direction, the CV revealed one oxidative process with an irreversible wave observed at +0.22 V vs. Fc +/0 . In an effort to gain insight into the nature of the species observed during cyclic voltammetry experiments, we calculated the structures of the putative species resulting from one and two-electron reduction of 4 (Figures S12-14 and Tables S6-8), and their electronic properties. Upon reduction, Please do not adjust margins Please do not adjust margins our results indicate that most of the spin density is distributed at one nickel center (Figure S15). The first reduction process of 4 occurs at the central metal site leading to the formation of a Ni(II)-Ni(I)-Ni(II) complex. The two-electron reduction of 4 results in a triplet ground spin state S = 1 with a stabilization energy of 24 kJ.mol -1 over the singlet state S = 0. This result supports a metal-based reduction occurring on another nickel center (Figure S16) and leading to the formation of a Ni(I)-Ni(II)-Ni(I) complex. We can notice a subsequent electronic rearrangement between the first and second reduction, which explains the irreversibility observed experimentally for the second redox system. Redox potential calculations were conducted to fully assign the electrochemical events (Tables SI-3). The computed values for the one-and two-electron reduction processes are in good agreement with experiment which indicates that the two cathodic systems correspond to a monoelectronic processes. and support the above assignments of the reduction loci in 4.

To examine the capability of complex 4 to mediate proton reduction catalysis, its electrochemical behaviour was studied by cyclic voltammetry in the presence of Et 3 NHCl (pK a = 9.2in DMF) [START_REF] Izutsu | Acid-Base Dissociation Constants in Dipolar Aprotic Solvents[END_REF][START_REF] Fourmond | [END_REF] as proton source. Cyclic voltammograms of 4 were recorded under argon in DMF solutions with increased amounts of Et 3 NHCl (Figure 3) and show a large irreversible catalytic wave with a mid-wave potential of -1.70 ± 0.03 V vs. Fc +/0 , i.e. close to the reduction potential of 4. The current response of this peak-shaped wave is directly proportional to the concentration of protons in the electrolyte solution (Figure S8). A value of 590 mV was calculated for the overpotential requirement of 4 for proton reduction in the presence of Et 3 NHCl in DMF. 39 To confirm that the observed catalytic waves correspond to the electrocatalytic reduction of protons into hydrogen, we performed a bulk electrolysis experiment of a DMF solution of Et 3 NHCl in excess (200 mM) and 4 (1 mM). Gas products were monitored by continuous in-line GC analysis. A potential of -1.60 V versus Fc +/0 was applied to the mercury-pool working electrode resulting in the immediate formation of hydrogen. This experiment rules out the possibility that proton reduction occurs as the result of deposition of nickel nanoparticles on the electrode surface since they would amalgamate within the mercury pool electrode. Consequently, under these conditions catalysis mediated by 4 is expected to be homogeneous in nature. During the 15 h experiment, a turnover number of 5 was achieved with a moderate Faradaic yield of 30%; indicating some degradation of the complex during turn-over. when using the B86 and B3LYP functionals, respectively (Table S3).

We then investigated the protonation of the reduced complex S9). This reaction is calculated to be almost isergonic. Another isomer, with higher energy, is obtained when protonation occurs at the Ni site thus yielding a hydride derivative with a distorted square-based pyramidal geometry (Figure S18 and Table S10). Reduction of the two isomers of [HNi 3 'S4' 2 ] yielding [HNi 3 'S4' 2 ] -is calculated to occur at potentials close to the one calculated for the first reduction of [Ni 3 'S4' 2 ]. We note that the stability order of the two isomers is inverted after this reduction step: in that case the sulfur-protonated species (Figure S19 and Table S11) is less stable (by 76 and 24 kJ.mol -1 with BP86/B3LY, respectively) than the Ni-hydride square-This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins planar isomer (Figure S20 and Table S12) in which one thioether ligand has been eliminated from the Ni coordination sphere and replaced by the hydride ligand. (Figure S21 and Table S13). This reaction is calculated to be thermodynamically favorable. The structure is reminiscent of a dihydrogen bond between nickel and sulfur that has been proposed as a key intermediate in the route to H 2 evolution at the active site of [NiFe] hydrogenases (Ni-R in Scheme 3).

44,45

From this dihydrogen complex, liberation of H 2 is strongly and entropically favored. In this work, we have reported the experimental and theoretical study of an original trinuclear nickel complex that displays an all-sulfur coordination sphere and exhibits a significant catalytic activity with a moderate overpotential requirement for hydrogen production in DMF using Et 3 NHCl as the proton source. Detailed analysis of the electrocatalytic data combined with DFT calculations allowed us to propose an ECEC catalytic pathway involving Ni-bound terminal thiolate ligand as proton relay mimicking the role of terminal cysteinate ligand in the active site of [NiFe] hydrogenases. Further work is currently in progress in our group to develop new bio-inspired mimics of the [NiFe] hydrogenase and to design original polynuclear metal complexes with thiolate ligands capable of mediating efficient hydrogen evolution.

Figure 1 X

 1 Figure 1 X-ray molecular structure of 4 with atom labelling scheme and ellipsoid parameters at 50% probability level. Hydrogen atoms and carbon labels are omitted for clarity.

Figure 2 1 .Figure 3

 213 Figure 2 Cyclic voltammogram of 4, concentration 0.3 mM at a stationary glassy carbon electrode (3 mm diameter) in DMF + 0.1M NEt4ClO4. Scan rate 100 mV.s - 1 .

  DFT calculations were finally conducted to decipher the reaction mechanism of hydrogen evolution by 4 (Scheme 2). We propose the catalytic cycle for H 2 evolution presented in Scheme 2 and based on the ECEC sequence, classically found for cobalt and nickel complexes. 40-42 UV/Vis measurements performed in the absence and presence of Et 3 NHCl (Figure SI-5) display almost identical features, indicating that the structure of 4 is unaffected by the addition of a proton source of such strength. From the cyclic voltammetry measurements in the presence of acid, it can be inferred that the reaction proceeds from the one-electron reduced species with a midwave electrocatalytic potential equal to that of the first reduction in 4 denoted [Ni 3 'S4' 2 ]. The first reduction at the Ni site was calculated at potentials of -1.84 and -2.02 V vs Fc +/0

[

  Ni' 3 S4' 2 ] -with Et 3 NH + yielding [HNi 3 'S4' 2 ]. Two distinct protonation sites could be found. The most stable isomer (by 28 and 61 kJ/mol -1 with BP86/B3LYP, respectively) is protonated on a terminal thiolate ligand coordinated on a peripheral Ni ion that remains in a squareplanar environment (Figure S17 and Table

Scheme 2

 2 Scheme 2 Proposed DFT computed reaction mechanism for hydrogen evolution mediated by 4. Finally, protonation of the reduced hydride species [HNi 3 'S4' 2 ] leads to formation of a dihydrogen complex [(H 2 )Ni 3 'S4' 2 ] (Figure S21 and TableS13). This reaction is calculated to be thermodynamically favorable. The structure is reminiscent of a dihydrogen bond between nickel and sulfur that has been proposed as a key intermediate in the route to H 2 evolution at the active site of [NiFe] hydrogenases (Ni-R in Scheme 3).

Scheme 3

 3 Scheme 3 Catalytic cycle of the [NiFe] hydrogenases involving the Ni-SIa, Ni-R, and Ni-C states.

Acknowledgements

The authors gratefully acknowledge research support of this work by the French National Research Agency (CODEC, ANR-19-CE05_0030_01; CatH 2 , ANR-07-BLAN-0298; NiFe-Cat, ANR-2010-BLAN-711-1; NiFemim, ANR-DFG-2016-NLE; Labex ARCANE; Graduate school of Chemistry, Biology and Health of Univ. Grenoble Alpes, CBH-EUR-GS, ANR-17-EURE-0003).

Conflicts of interest

There are no conflicts to declare.


Mohammad S Askari

Farshid Effaty

Federica Gennarini

Maylis Orio

Nicolas Le Poul
email: nicolas.lepoul@683univ-brest.fr

X Ottenwaelder

Tuning Inner-Sphere Electron Transfer in a Series of Copper/ Nitrosoarene Adducts

A series of copper/nitrosoarene complexes were created that mimic several steps in biomimetic O 2 activation by copper(I). The reaction of the copper(I) complex of N,N,N′,N′tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet-visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η 2 -NO and dinuclear μ-η 2 :η 1 for ArNO •-, and dinuclear μ-η 2 :η 2 for ArNO 2-. 15 N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm -1 for κN-ArNO, 1226 cm -1 for η 2 -ArNO •-, 1133 cm -1 for dinuclear μ-η 2 :η 1 -ArNO •-, and 875 cm -1 for dinuclear μ-η 2 :η 2 for ArNO 2-). The 15 N NMR signal disappears for the ArNO •-species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O 2 analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion.

usually too oxidative to be stable above -60 °C. By contrast, 48 Cu/ArNO adducts have been shown to have geometric and 49 electronic structures very similar to those in Cu/O 2 adducts 50 but were advantageously characterized at ambient temper-51 ature. 17,23-25 52 Owing to the asymmetric structure of ArNO in comparison 53 with that of O 2 , the structural variety of metal/nitrosoarene 54 complexes exceeds that of metal/O 2 compounds. Some of the 55 main bonding modes of ArNO to metal ions are shown in 56 s1 Scheme 1, 13,14 with the most common one being through the 57 N atom (κN). The other bonding modes are thought to be 58 more prevalent when the ArNO moiety is reduced to the 59 mono-or dianion. 60 The NO bond length in metal/ArNO complexes depends on 61 the bonding mode, nature, and oxidation state of the metal and 62 the supporting ligands but alone is insufficient to characterize

■ INTRODUCTION

The interaction of nitrosoarenes (ArNO) with metal centers has drawn much attention because of its relevance to biological pathways [START_REF]Scheme 6) 83 with a temperature correction, the value of 486 E 1/2 C for 3 R taken as E pc C[END_REF][2][3][START_REF]H 2 O < 0.1 ppm) or using standard Schlenk techniques. Solvents were 528 dried by standard procedures, degassed, and stored over 4 Å 529 molecular sieves in the glovebox. N,N,N′,N′-Tetramethyl-1,3-530 propanediamine (TMPD) was distilled over CaH 2 under nitrogen and stored in the glovebox. The copper salt [(MeCN) Cu](TfO) was prepared by adapting the Kubas procedure using[END_REF][5][START_REF]arene-Based Zinc and Copper Complexes: Structure, Properties, and O2 Binding[END_REF][7] and catalytic C-N bond formation processes. [8][9][10][11][12] Chemists now have a good understanding of the geometric structure of transition metal/nitrosoarene complexes. 13,14 In addition, ArNO species are redox-noninnocent ligands, [15][16][17] which portends a large landscape of electronic structures and reactivity types upon interaction with redox-active metal ions.

Because ArNO species are isovalent with O 2 , the reduction of ArNO by a transition metal is akin to the reduction of O 2 to the superoxide ion (O 2

•-, 1e reduction) or the peroxide ion (O 2 2-, 2e reduction). Therefore, metal/ArNO adducts are often regarded as surrogates for metal/O 2 adducts. In particular, and with relevance to the present paper, the activation of O 2 by Cu(I) centers is paramount in the biological world. This process fuels enzymes such as dopamine-β-hydroxylase, tyrosinase, and particulate methane monooxygenase, to name but a few. 18,19 This has inspired numerous biomimetic studies in which an electron-rich Cu(I) species is reacted with O 2 . [20][START_REF] Lewis | Reactivity of Dioxygen-Copper Systems[END_REF][START_REF] Mirica | Structure and Spectroscopy of Copper-Dioxygen Complexes[END_REF] Without protection of the protein backbone, however, the ensuing Cu/O 2 complexes are arylnitrosyl radical anion) was confirmed by magnetic measurements and vibrational and computational studies. 17,[START_REF] Effaty | Ligand and electronic effects on copper-arylnitroso self-assembly[END_REF] (iii) Dinuclear μ-η 1 :η 1 (end-on) complexes present varying degrees of ArNO reduction: by 0e (NO = 1.257-1.32 Å), [START_REF] Stephens | Cyclopentadienyliron complexes of nitrosobenzene: Preparation, structure and reactivity with olefins[END_REF][START_REF] Dey | Electronically Triggered Switchable Binding Modes of the C-Organonitroso (ArNO) Moiety on the {Ru(acac)2} Platform[END_REF][START_REF] Krinninger | Dinuclear μ2-(η2-NO)-Nitrosoaniline-Bridged Complexes of Rhenium of the Type [{(CO)3Re(μ-X)}2ONC6H4NR2] (X = Cl, Br, I; R = Me, Et)[END_REF][START_REF] Krinninger | Absence of Dichroism in Dinuclear Rhenium Complexes with[END_REF][40][41] 1e (1.33-1.35 Å), 42 and 2e (1.37-1.49 Å; Scheme 2c). 43,44 (iv) In η 2 -NO complexes, the NO bond length (1.323-1.432 Å) 16,[START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF][START_REF] Barnett | Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals[END_REF][45][46][47][48][49][50][51][52] is significantly longer than that in free nitrosoarenes. 1e reduction of the ArNO moiety has been confirmed in Cu and Ni complexes (Scheme 2d). [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF]52 Further reduction of the Ni complex led to a doubly reduced PhNO 2- moiety. 52 2e reduction of the ArNO moiety was also confirmed in a square-planar Pd(II) species upon reaction of a Pd 0 species with TolNO. 16 (v) Alongside several ArNO 2-examples (N-O = 1.40-1.53 Å), [START_REF] Barnett | Solution Dynamics of Redox Noninnocent Nitrosoarene Ligands: Mapping the Electronic Criteria for the Formation of Persistent Metal-Coordinated Nitroxide Radicals[END_REF][53][54][55][56][START_REF] Hoard | Chemistry of [Cp*Rh(.mu.-Cl)]2 and its dioxygen and nitrosobenzene insertion products[END_REF][START_REF] Sharp | Rhodium(II) complex with a highly reactive rhodium-rhodium bond: insertion of dioxygen and nitrosobenzene[END_REF] dinuclear μ-η 2 :η 1 complexes have been found in the solid-state structures of Cu complexes with shorter NO bond lengths (1.322-1.375 Å). [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF][START_REF] Effaty | Ligand and electronic effects on copper-arylnitroso self-assembly[END_REF] Typically, the 1e-reduced ArNO •-moiety binds η 2 to a Cu(II) center and η 1 to a Cu(I) center (Scheme 2e). These species are thought to be in equilibrium with the mononuclear form [Cu II (η 2 -ArNO •-)] in solution. [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF][START_REF] Effaty | Ligand and electronic effects on copper-arylnitroso self-assembly[END_REF] (vi) Dinuclear μ-η 2 :η 2 complexes are quite rare, and only a few examples with Rh, [START_REF] Hoard | Chemistry of [Cp*Rh(.mu.-Cl)]2 and its dioxygen and nitrosobenzene insertion products[END_REF] Zr, 59 Hf, 59 Ni, [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF] and Cu [START_REF] Askari | Controlled nitrene transfer from a tyrosinase-like arylnitroso-copper complex[END_REF] are reported in the literature. With an NO bond length in the range of single bonds (1.422-1.500 Å), these complexes possess a doubly reduced ArNO 2-moiety. In the case of the Cu complex (Scheme 2f), this 2e reduction was made possible by using a very electron-poor nitrosoarene bearing a p-NO 2 substituent. More on this complex will be discussed below. To summarize, 1e reduction of the ArNO moiety is usually indicated by NO bond lengths in the range 1.29-1.37 Å and NO stretching frequencies in the range 1000-1300 cm -1 (Scheme 2). Reduction by 2e is revealed by NO bond lengths of 1.36-1.46 Å and NO stretching frequencies below 950 cm -1 . When no reduction occurs, the NO bonds are short [1.261(4) and1.268(4) Å for free PhNO] and the NO stretching frequency is high (1506 cm -1 for free PhNO), although these values can be modified significantly when backbonding is present. 16 Last, 4e reduction of PhNO, with complete NO bond cleavage, is possible with very electron-rich metal complexes such as cobalt(I) β-diketiminate species. [START_REF] Dai | Me2NN]Co(η6-toluene): OO, NN, and ON Bond Cleavage Provides β-Diketiminato Cobalt μ-Oxo and Imido Complexes[END_REF] Noting that these examples comprise different supporting ligands and metal ions, we aimed at providing a systematic study of the degree of inner-sphere ArNO reduction by using a single Cu(I) precursor. Thus, in the present study, we report on adducts 3 R (R = NMe 2 , H, Cl, Br, NO 2 ) formed upon intermolecular reaction of para-substituted nitrosobenzenes 2 R with the Cu(I) complex of N,N,N′,N′-tetramethyl-1,3-s3 propanediamine (TMPD), 1 (Scheme 3), for which analogous Cu/O 2 chemistry is known. [START_REF] Mahadevan | Exogenous Substrate Reactivity with a [Cu(III)2O2]-2+ Core: Structural Implications[END_REF][START_REF] Herres-Pawlis | Phenolate Hydroxylation in a Bis(μ-oxo)dicopper(III) Complex: Lessons from the Guanidine/Amine Series[END_REF][START_REF] Large | Selective oxidation of exogenous substrates by a bis-Cu(III) bis-oxide complex: Mechanism and scope[END_REF] ■ RESULTS AND DISCUSSION Synthesis and Crystallography. The slow addition of a [(MeCN) 4 Cu](TfO) (MeCN = acetonitrile) solution to a solution of TMPD and nitrosoarene, in a 1:1:1 ratio in tetrahydrofuran (THF) at 25 °C (1:1:2 for R = NO 2 ), results in the formation of deeply colored complexes that remain stable under inert conditions. Crystallization of the complexes by the slow diffusion of pentane into the reaction mixtures at -30 °C afforded crystals suitable for X-ray diffraction analysis. Several binding motifs consisting of mono-and dinuclear complexes are obtained depending on the para substituent of f1 the nitrosoarene (Figure 1 and Table S1). consistent with a 1.5 bond order. [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF]52 Cu sits in a square-164 pyramidal environment with a TfO -anion as a weak axial 2.828 Å). Species 3 H ′ is therefore well described as formed from the association of the minor green component 3 H with s4 one molecule of 1 (Scheme 4). Such architecture and association were already described in the literature. [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF][START_REF] Effaty | Ligand and electronic effects on copper-arylnitroso self-assembly[END_REF] Because the mononuclear complex prevails in solution (the solution is green, and a Job titration confirms a 1:1 stoichiometry; Figure S14), formation of the dinuclear compound is an artifact of crystallization.

For 3 Cl , only green crystals of [TMPDCu II (η 2 -2 Cl•-)(TfO)] were formed. The molecular structure is very similar to that of 3 H , with a 1.5 N-O bond order (1.3315 Å), except that the TfO -anion and the aromatic ring are on the same side of the CuNO plane.

The complex with the most electron-poor ArNO moiety, 3 NO 2 , was characterized in a previous communication. [START_REF] Askari | Controlled nitrene transfer from a tyrosinase-like arylnitroso-copper complex[END_REF] It is a dinuclear species of the formula [TMPDCu II (μ-η 2 :η 2 -PhNO 2-)(μ-TfO)Cu II TMPD](TfO), where 2 NO 2 is reduced by 2e (N-O = 1.456 Å) and both Cu centers are in the 2+ oxidation state.

Overall, the crystallographic study concludes on an increased degree of electron transfer from 1 to 2 R inasmuch as the p-R substituent is made more electron-poor: 0e in 3 NMe 2 , 1e in 3 H / 3 H ′ and 3 Cl , and 2e in 3 NO 2 . The lability of Cu complements the self-assembly process by allowing TfO -or extra Cu(I) coordination when necessary.

IR Properties. Vibrational analysis by IR spectroscopy was conducted on 2 R precursors and 3 R complexes, where the N atom of the nitroso moiety is either 14 N or 15 N. Synthesis of the 15 N-labeled 2 R precursors is provided in the Supporting Information. Isotopic labeling enables one to isolate the vibrations near the nitroso moiety from the rest of the molecule. In parallel, density functional theory (DFT) calculations were conducted to identify the nature of the modes observed (especially NO vs CN stretches in the ArNO moiety).

Comparing the IR properties of the organic precursors 2 R is tentative because they have different structures in the solid state: monomeric for 2 NMe 2 , syn dimeric for 2 H , and anti t1 dimeric for 2 Br (Tables 1 andS2 andFigures S1-S6). Still, the correlation between the experimental and calculated spectra is excellent, providing confidence that the calculations can enable us to locate the NO stretch accurately in the complexes.

Drastic changes in the NO stretching frequency are seen in 3 R complexes, consistent with NO bond weakening upon electron transfer (Tables 1 andS3 and Figures S7-S12). While the symmetry of the complexes is different and some complexes have multiple vibrational modes involving the NO stretch, the NO stretching energy decreases from 1315/1392 cm -1 for 3 NMe 2 to 1226 cm -1 for 3 Br to 875 cm -1 for 3 NO 2 . [START_REF] Askari | Controlled nitrene transfer from a tyrosinase-like arylnitroso-copper complex[END_REF] This trend, supported by DFT calculations, is fully consistent species 3 H ′, the NO stretch is lowered from 3 Br by about 70- S47). For 3 H , 3 Cl , and 3 Br , this 230 indicates a singlet ground state, as was observed for similar η 2 -231 ArNO complexes. [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF][START_REF] Williams | Copper C-Nitroso Compounds: Activation of Hydroxylamines and NO Reactivity[END_REF][START_REF] Effaty | Ligand and electronic effects on copper-arylnitroso self-assembly[END_REF] By analogy with structurally similar 232 η 2 -superoxocopper(II) species, this ground-state singlet is 233 expected to be highly delocalized. [START_REF] Chen | Spectroscopic and Electronic Structure Studies of the Diamagnetic Side-On CuII-Superoxo Complex Cu(O2)[HB(3-R-5-iPrpz)3: Antiferromagnetic Coupling versus Covalent Delocalization[END_REF] This situation also DFT Calculations. DFT calculations have been undertaken 273 on the 3 R complexes to gain insight into the nature of the 274 species observed experimentally and to correlate their 275 electronic structures to the experimental data. The structures 276 of the 3 R species were subjected to geometry optimization, and their electronic properties were investigated. A good agreement is found upon a comparison of the molecular geometries with the X-ray crystallographic data (Figure S13). The calculated NO bond lengths are 1.243 Å for 3 NMe 2 , 1.287-1.288 Å for 3 H , 3 Cl , and 3 Br , 1.313 Å for 3 H ′, and 1.405 Å for 3 NO 2 . While these values are all underestimated (up to 0.05 Å), they lie within the typical error range of DFT and provide a fair trend along the series, being thus informative on the redox state of the ArNO moiety. The DFT-optimized structures are very close to those observed experimentally, with root-mean-square deviations of 0.543, 0.427, 0.407, 0.419, 0.444, and 0.338 from the crystal molecular structures of 3 NMe 2 , 3 H , 3 Cl , 3 Br , 3 H ′, and 3 NO 2 , respectively. Time-dependent DFT (TD-DFT) calculations were performed on the 3 R complexes, and the predicted spectroscopic data provide calculated spectra that compare well with the experimental observations (Tables S4-S8 andFigures S15-S20). Our computations support that the UV-vis spectra of 3 H , 3 Cl , 3 Br , and 3 NO 2 are similar and dominated by two main absorption bands of different intensities, while that of 3 NMe 2 displays two intense electronic transitions. For the latter, the band at 538 nm is assigned to a metal-to-ligand charge transfer (MLCT), and the band at 426 nm is attributed to a ligand-toligand charge transfer (LLCT). For both transitions, the acceptor states mainly involve the nitroso moiety (Figure S16).

The electronic transitions for 3 NO 2 were already analyzed. [START_REF] Askari | Controlled nitrene transfer from a tyrosinase-like arylnitroso-copper complex[END_REF] The 345 and 644 nm bands correspond to MLCT transitions involving the μ-η 2 :η 2 -NO 2-moiety, in a very similar manner to the transitions in the (μ-η 2 :η 2 -O 2 2-)Cu II 2 cores that mimic the active sites of oxytyrosinase and oxyhemocyanin. [START_REF] Mirica | Structure and Spectroscopy of Copper-Dioxygen Complexes[END_REF] = -1.40 V (process A) and E 1/2 B = -1.86 V versus Fc +/0 t4 (process B) (Figure S21 and Table 4). An irreversible oxidation peak is also detected at 0.63 V on the backscan after reduction at -1.90 V (Figure S22). Variation of the scan rate v induces a significant modification of the redox behavior (Figure S23), which is typical of two successive electrontransfer processes coupled to a chemical reaction, namely, an ECE mechanism (E = electrochemical and C = chemical; Scheme 5). In agreement with previous electrochemical studies, [START_REF] Mugnier | Electrochemically induced chain reactions: the electrochemical behavior of nitrosobenzene in the presence of proton donors in tetrahydrofuran[END_REF][START_REF] Asirvatham | Electron-transfer processes: The electrochemical and chemical behavior of nitrosobenzene[END_REF] process A corresponds to the monoelectronic reduction of 2 H (Scheme 5), while process B corresponds to 1e reduction of the azoxybenzene formed in situ by reaction of the radical anion with residual water. This dimerization hypothesis is supported by the ratio of the cathodic peak currents, i pc B /i pc A ≈ 0.5 (assuming similar diffusion coefficients). In addition, the value of E 1/2 B is in good agreement with the standard potential values for the azoxy species in organic solvents. [START_REF] Mugnier | Electrochemically induced chain reactions: the electrochemical behavior of nitrosobenzene in the presence of proton donors in tetrahydrofuran[END_REF][START_REF] Asirvatham | Electron-transfer processes: The electrochemical and chemical behavior of nitrosobenzene[END_REF][START_REF] Nuńẽz-Vergara | Nitrosobenzene: electrochemical, UV-visible and EPR spectroscopic studies on the nitrosobenzene free radical generation and its interaction with glutathione[END_REF] The processes described in Scheme 5 occur for 2 R with different para substituents (R = NMe 2 , H, Cl, Br, NO 2 ) but at different redox potentials (Figure S21 and Table 4). Under the same experimental conditions, 2 H , 2 Cl , and 2 Br display almost the same redox pattern, i.e., one quasi-reversible redox system at ca. E 1/2 A = -1.3 V and a second one at ca.

E 1/2 B = -1.8 V. For 2 NMe 2 , E 1/2
A is shifted negatively by ca. 300 mV with respect to E 1/2 A (2 H ), and the (2 NMe 2 ) •-radical anion is relatively unstable because process A occurs irreversibly for v < 0.2 V s -1 (Figure S21a). For 2 NO 2 , E 1/2

A and E 1/2 B are shifted positively by ca. 500 mV (Figure S21b). Thus, a span of +760 mV is observed for E 1/2

A upon NMe 2 / NO 2 substitution, consistent with the electron-donating/ withdrawing properties of the substituents. Fittingly, plots of E 1/2

A versus the σ para Hammett parameter follow a linear trend, indicating that the value of the redox potential is mainly controlled by electronic effects (Figure S29).

[Cu(TMPD)(ArNO)](OTf) Complexes 3 R . CV studies of the 3 R complexes (R = NMe 2 , H, Br, NO 2 ) were performed under the same experimental conditions as those for 2 R ligands f4t5 (Figure 4 and Table 5). Adding 2 R to a solution of 1 under CV monitoring led to the same conclusions as those below (Figure S25). All 3 R complexes display a first irreversible reduction 381 peak E pc C (process C) extending from -1.05 V for 3 NMe 2 to 382 -0.72 V for 3 NO 2 (Figure 4 and Table 5). As was the case with Table 5. Electrochemical Data of 3 R Complexes a S30).

E pc C (3 R ) E 1/2 D (3 R ) E 1/2 E (3 R ) σ para b 3 NMe2 -1.
Exhaustive electrolyses at E pc C and coulometric measurements confirm that system C is a 1e process per mole of 3 R . For example, electrochemical reduction of 3 NMe 2 leads to its disappearance, while a new wave appears at E 1/2 D (Figure S31), together with a significant color change of the solution (purple to orange). A new system also appears in oxidation at -0.2 , +0.45, and +0.65 V, suggesting a release of TMPD (Figure S25a).

In a general manner, 1e reduction of the 3 R complexes is accompanied by fast chemical processes that lead to partial decomplexation and release of the TMPD ligand and/or 2 R . The transient electron-reduced species may thus be implicated in several reactions: radical dimerization and simple decoordination, which themselves seem dependent on R.

On the oxidation side, a quasi-reversible system (process E) is detected at E 1/2 E (Table 5), with varying peak potential and intensity values as R is varied. E 1/2 E is in the same range as that reported by Warren et al. (E pa = +0.48 V in MeCN) for a similar side-on arylnitrosylcopper(II) complex with a diketiminate ligand. [START_REF] Wiese | Nitric Oxide Oxidatively Nitrosylates Ni(I) and Cu(I) C-Organonitroso Adducts[END_REF] The reduction data obtained for the 3 R complexes can be compared with the few redox processes reported for Ni n / t6 ArNO and Cu n /O 2 analogues (Table 6). The side-on arylnitrosyl 3 H species (entry 1) gets reduced at a potential similar to that of Warren's side-on arylnitrosylnickel(II) complex. 52 When ArNO binds in a 1,2-fashion (end-on) between two Ni(II) centers, the potential for ArNO 2-/•- conversion is decreased by ca. 650 mV (entry 3). 44 Comparing ArNO with O 2 complexes would be interesting, but so far there is no reported redox data for monocopper superoxo species that would be similar in structure to 3 H . The exception is the recent work by Reinaud et al., which showed by spectroelectrochemistry that an in situ generated calix [6]amino-tren endon superoxo complex could not be reduced above -0.90 V versus Fc at -60 °C (113 K) in acetone. [START_REF] De Leener | Two-Story[END_REF] On the other hand, a few dicopper peroxo and superoxo species have been well characterized by electrochemistry with the help of lowtemperature approaches (entries 4-7). Here, the irreversible 1e reduction of 3 R is comparable to the monoelectronic and reversible electron-exchange reactions detected for the end-on superoxo/peroxo pyrazolate-and xylO-based complexes (entries 4 and 5). [START_REF] Kindermann | Hydrogen Atom Abstraction Thermodynamics of a μ-1,2-Superoxo Dicopper-(II) Complex[END_REF][START_REF] Loṕez | Direct Determination of Electron-Transfer Properties of Dicopper-Bound Reduced Dioxygen Species by a Cryo-Spectroelectrochemical Approach[END_REF] Interestingly, the reduction potential of 3 NO 2 (entry 8) is close to that of Kodera's side-on peroxodicopper(II) species (entry 7), although the latter is a 2e process. [START_REF] Loṕez | O-O bond cleavage via electrochemical reduction of a side-on peroxo dicopper model of hemocyanin[END_REF] Overall, using such comparisons to make educated assignments of the electrochemical processes remains tentative given the large variety of ligands, charge, nuclearity, and bonding topology of the reported complexes. While data for μ-hydroxodicopper complexes that are reminiscent of Cu n / O 2 species is readily available, [START_REF] Halvagar | Hydroxo-Bridged Dicopper(II,III) and -(III,III) Complexes: Models for Putative Intermediates in Oxidation Catalysis[END_REF][START_REF] Ali | Thermodynamics of a μ-oxo Dicopper(II) Complex for Hydrogen Atom Abstraction[END_REF][START_REF] Isaac | Room-Temperature Characterization of a Mixed-Valent μ-Hydroxodicopper-(II,III) Complex[END_REF][START_REF] Thibon-Pourret | Effect of Monoelectronic Oxidation of an Unsymmetrical Phenoxido-Hydroxido Bridged Dicopper(II) Complex[END_REF][START_REF] Gennarini | Influence of Asymmetry on the Redox Properties of Phenoxo-and Hydroxo-Bridged Dicopper Complexes: Spectroelectrochemical and Theoretical Studies[END_REF][START_REF] Shearer | Substrate Oxidation by Copper-Dioxygen Adducts: Mechanistic Considerations[END_REF] comparisons with the 3 R complexes would be even more tentative.

HAT Reactivity. We evaluated the reactivity of 3 Cl , 3 Br , and Computational Details. All theoretical calculations were performed with the ORCA program package. 92 Full geometry optimizations were carried out for all complexes using the generalized gradient approximation functional BP86 [93][94][95] in combination with the TZV/P 96 basis set for all atoms and by taking advantage of the resolution of the identity (RI) approximation in the Split-RI-J variant [START_REF] Schafer | Fully optimized contracted 1085 Gaussian basis sets of triple zeta valence quality for atoms Li to Kr[END_REF] with the appropriate Coulomb fitting sets. [START_REF] Neese | An improvement of the resolution of the identity 1088 approximation for the formation of the Coulomb matrix[END_REF] Increased integration grids (Grid4 in the ORCA convention) and tight selfconsistent-field convergence criteria were used. IR spectra were obtained from numerical frequency calculations performed on DFToptimized structures. Isotope shift effects ( 14 N/ 15 N) were taken into account using the orca_vib utility program, and vibrational normal modes were visualized with Chemcraf t 99 software. Solvent effects were accounted for according to the experimental conditions. For that purpose, we used the CH 2 Cl 2 (e = 9.08) solvent within the framework of the conductor-like screening (COSMO) dielectric continuum approach. 100 Single-point optical properties were predicted from additional single-point calculations using the same functional/basis set as that employed previously. Electronic transition energies and dipole moments for all models were calculated using TD-DFT 101-103 within the Tamm-Dancoff approximation. 104,105 To increase the computational efficiency, the RI approximation 106 was used to calculate the Coulomb term. At least 40 excited states were calculated in each case, and difference transition density plots were generated for each transition. For each transition, difference density plots were generated using the ORCA plot utility program and visualized with the Chemcraft program. The same procedure was also employed to generate and visualize spin-density plots as well as molecular orbitals.

Synthetic Procedures. General Procedure for the Synthesis of 3 R Complexes (R = NMe 2 , H, Cl, Br). [START_REF] Askari | Controlled nitrene transfer from a tyrosinase-like arylnitroso-copper complex[END_REF] To a stirring solution of TMPD (0.28 mmol, 1.1 equiv) and the corresponding nitrosobenzene 2 R (0.27 mmol, 1.05 equiv) in 5 mL of THF was added dropwise at 25 °C a solution of [(MeCN) 4 Cu](TfO) (0.26 mmol, 1 equiv) in 2 mL of THF. The solution was stirred for 15 min and then cooled to -30 °C. Dropwise addition of the solution to 15 mL of swirling pentane previously cooled to -30 °C resulted in the precipitation of a solid. The solid was isolated, washed with diethyl ether and pentane, and dried in vacuo (yields typically 70-85%). Crystals suitable for Xray structure determination were grown through the slow layered diffusion of pentane into a concentrated solution of the complex in THF at -30 °C.

[(TMPDCu)(TfO)(κN-p-NMe 2 -C 6 H 4 NO)](TfO) (3 NMe 2 ). Yield: dark purple solid. [START_REF]Scheme 6) 83 with a temperature correction, the value of 486 E 1/2 C for 3 R taken as E pc C[END_REF] H NMR (500 MHz, CDCl 3 ): δ ppm 1.76 (m, 2H), 2.51 (s, 12H), 2.84 (m, 4H), 3.22 (s, 6H), 6.81 (br, 2H), 9.09 (very br, 2H). 13 C NMR (125 MHz, CDCl 3 ): δ ppm 22.89, 40.80, 48.69, 61.75, 112.2, 122.05, 156.26, 158.23. 15 H, 5.72; N, 11.36; S, 6.50. Found: C, 37.86; H, 5.82; N, 11.13; S, 6.61. [(TMPDCu) 2 (μ-TfO)(μ-η 2 :η 1 -PhNO)](TfO) (3 H ′). Yield: brown solid. 1 H NMR (500 MHz, CDCl 3 ): δ ppm 1.72 (br, 4H), 2.55 (br, 24H), 2.69 (br, 8H), 7.49 (t, 2H), 7.67 (t, 1H), 8.09 (d, 2H). 13 C NMR (125 MHz, CDCl 3 ): δ ppm 22.36, 48.59, 60.59, 120.97 (d upon 15 N labeling, J( 13 C-15 N) = 3 Hz), 130.94 (d upon 15 N labeling, J( 13 C-15 N) = 2 Hz), 131.78, 160.93. Anal. Calcd for C 22 H 41 Cu 2 F 6 N 5 O 7 S 2 : C, 33.33; H, 5.21; N, 8.83; S, 8.09. Found: C, 31.28; H, 5.35; N, 8.37; S, 8.19 (precision is lacking because this compound contains a minor quantity of 3 H in the solid state).

[(TMPDCu)(TfO)(η 2 -PhNO)](TfO) (3 H ). Yield: prepared in situ (green solution). [START_REF]Scheme 6) 83 with a temperature correction, the value of 486 E 1/2 C for 3 R taken as E pc C[END_REF] H NMR (500 MHz, CDCl 3 ): δ ppm 1.69 (br, 2H), 2.47 (12H), 2.63 (4H), 7.43 (t, 2H), 7.63 (t, 1H), 7.97 (d, 2H). 13 C NMR (125 MHz, CDCl 3 ): δ ppm 20. 37, 46.59, 59.24, 119.34 (d upon 15 N labeling, J( 13 C-15 N) = 5 Hz), 127.97 (d upon 15 N labeling, J( 13 C-15 N) = 2 Hz), 130. 78, 160.67 (d upon 15 N labeling, J( 13 C-15 N) = 6 Hz). 15 N NMR (50.7 MHz, CDCl 3 ): not observed.

[(TMPDCu)(TfO)(η 2 -p-ClC 6 H 4 NO)](TfO) (3 Cl ). Yield: green solid. [START_REF]Scheme 6) 83 with a temperature correction, the value of 486 E 1/2 C for 3 R taken as E pc C[END_REF] H NMR (500 MHz, acetone-d 6 ): δ ppm 2.42 (br, 2H), 3.06 (s, 12H), 3.45 (br, 4H), 7. 77 (d, 2H), 7.97 (d, 2H). 13 C NMR (125 MHz, acetoned 6 ): δ ppm 20.17, 43.19, 55.50, 122.16, 126.10, 130.01, 166.47. Anal. Calcd for C 14 H 22 ClCuF 3 N 3 O 4 S: C, 34.71; H, 4.58; N, 8.67; S, 6.62. Found: C, 34.27; H, 4.49; N, 8.16; S, 6.38. [(TMPDCu)(TfO)(η 2 -p-Br-C 6 H 4 NO)](TfO) (3 Br ). Yield: green solid. 2.78 (br, 4H), 7.54 (d, 2H), 7.91 (d, 1H). 13 C NMR (125 MHz, CDCl 3 ): δ ppm 22.28, 48.52, 60.29, 122.81 (d upon 15 N labeling, J( 13 C-15 N) = 5 Hz), 126.01, 133.55 (d upon 15 N labeling, J( 13 C-15 N) = 2.5 Hz), 160.59 (d upon 15 N labeling, J( 13 C-15 N) = 5 Hz). 15 N NMR (50.7 MHz, CDCl 3 ): not observed. Anal. Calcd for C 14 H 22 BrCuF 3 N 3 O 4 S: C, 31.80; H, 4.19; N, 7.95; S, 6.06. Found: C, 31.29; H, 4.49; N, 7.81; S, 6.37. X-ray data for 3 NMe 2 , 3 H , 3 H ′, and 3 Cl are available as CCDC 1959040-1959043, respectively. Note that 3 NO 2 is CCDC 1029423.

We thank the Capobianco lab and Biofins platform Inorganic Chemistry

147

  The structure of 3 NMe 2 is that of a copper(I) arylnitroso 148 complex, i.e., mere κN coordination of 1 NMe 2 onto the [TMPD-149 Cu I ] + complex 1. The N-O bond lengths in the two 150 independent molecules, 1.281 and 1.293 Å, are typical for 151 NO double bonds. The trigonal-planar ligand field of Cu is 152 consistent with a Cu(I) oxidation state. In the solid state, the 153 species dimerizes via two weak Cu•••O interactions (2.242 and 154 2.278 Å) between two crystallographically related 3 NMe 2 155 cations. This fact, coupled with the back-bonding of Cu into 156 the nitroso π* orbital, could explain the slight elongation of the 157 N-O bond compared with a true double bond. 16,37 158 Two types of crystals were obtained in the same 159 crystallization pot with R = H. The minor component, of 160 green color, is the mononuclear [TMPDCu II (η 2 -PhNO •-)-161 (TfO)] species (3 H ). This complex displays an η 2 -NO 162 coordination with an elongated N-O bond of 1.327 Å, 163

  Scheme 3. Formation of the 3 R Adducts a

Figure 1 .

 1 Figure 1. ORTEP at 50% probability of 3 NMe 2 (one of two independent molecules), 3 H , 3 H ′, 3 Cl , and 3 NO 2 , with relevant N-O bond lengths. Uncoordinated TfO -anions (3 NMe 2 and 3 NO 2 ) and H atoms were omitted for clarity.

  225 100 cm -1 , consistent with the electron density being 226 delocalized onto the additional Cu(I) center. 227 NMR Properties. In CDCl 3 , CD 2 Cl 2 , or acetone-d 6 228 solutions, all 3 R species display diamagnetic 1 H and 13 C 229 NMR spectra (Figures S37-

  234contrasts with the end-on topology, where end-on 235 superoxocopper(II) complexes have a S = 1 ground 236 state,[START_REF] Ginsbach | Correlation of the Electronic and Geometric Structures in Mononuclear Copper(II) Superoxide Complexes[END_REF][START_REF] Woertink | Spectroscopic and Computational Studies of an End-on Bound Superoxo-Cu(II) Complex: Geometric and Electronic Factors That Determine the Ground State[END_REF][START_REF] Noh | Synthesis, characterization and reactivity of non-heme 1st row transition metal-superoxo intermediates[END_REF][START_REF] Solomon | Dioxygen Binding, Activation, and Reduction to H2O by Cu Enzymes[END_REF] as do Cu II (κO-ArNO •-) complexes when Cu-237 O-N-C Ar is coplanar. 17

238

  Scheme 4. Mononuclear/Dinuclear Equilibrium in 3 H Species a

Figure 2 .

 2 Figure 2. 15 N NMR data (50.7 MHz) of the 3 R species (R = NMe 2 , H, Br, NO 2 ), 15 N-enriched at the NO position, in CDCl 3 at 25 °C. Species 3 H was made in situ by mixing equimolar amounts of TMPD, [Cu(MeCN) 4 ](TfO), and 2 H .

  For 3 H , 3 Cl , and 3 Br , the absorptions near 320 nm are due to a combination of MLCT and LLCT, with the acceptor state involving the NO moiety, while the transitions in the visible around 610 nm display a mixed character with similar contributions from the metal and the nitroso moiety in both donor and acceptor states (Figures S17-S19). Our TD-DFT calculations thus adequately reproduce the energy of the key features of the experimental spectra, which further support the geometries and electronic properties of the 3 R complexes. Vibrational analysis also confirmed the experimental observations (vide supra). Consequently, 3 NMe 2 can be described as a Cu(I) complex, while 3 H , 3 Cl , and 3 Br are Cu II -(ArNO •-) species. The dimer 3 H ′ is assigned to Cu II -Cu I -(ArNO •-) species, while 3 NO 2 was previously shown to be a Cu(II)-Cu(II) complex with a ArNO 2-moiety (2e-reduced ArNO). Electrochemical Studies. Because this work aims at tuning the redox properties by simple substitution, we studied the electrochemical behavior of both precursors 2 R and complexes 3 R for the whole series of R substituents (NMe 2 , H, Cl, Br, and NO 2 ). The goal was to correlate the electrochemical properties with the reactivity (0e, 1e, or 2e transfer) observed upon reaction with the [(TMPD)Cu I ] + complex 1 and the H-atom-transfer (HAT) reactivity of the 3 R complexes (see below). The data were also compared to existing records for analogous ArNO and O 2 complexes. Cyclic voltammetry (CV) studies were performed at a glassy-carbon working electrode in dry CH 2 Cl 2 with 0.1 M NBu 4 OTf as the supporting electrolyte. In what follows, all potentials are referenced to the Fc +/0 couple. Substituted Nitrosoarenes, 2 R . CV studies of the free s5 nitrosoarenes, 2 R , led to the summary in Scheme 5. 2 H was first studied for a comparison with the literature. 70-72 When scanned negatively, it displays two reversible responses at E 1/2 A

Figure 3 .

 3 Figure 3. UV-vis spectra of 3 R species in CH 2 Cl 2 at 25 °C.

  FigureS27a), hence excluding an ECE process.

a

  In CH 2 Cl 2 containing 0.1 M NBu 4 OTf at 25 °C (glassy-carbon working electrode); scan rate v = 0.1 V s -1 , E/V versus Fc +/0 (ΔE p / mV). b σ para Hammett parameters. c Determined at v = 0.5 V s -1 . d Not determined. e An intermediate wave at -1.17 V was observed at faster scan rates.

Figure 4 .

 4 Figure 4. CV cycles at a glassy-carbon working electrode (E/V vs Fc +/0 ; v = 0.1 V s -1 ) of 3 R (1.0 mM) in dry CH 2 Cl 2 and 0.1 M NBu 4 OTf: (a) R = Br; (b) R = NO 2 .

  s6 3 NO 2 for HAT reactivity (Scheme 6). In previous work, Warren et al. reported a Ni II -(η 2 -ArNO •-) that converts into the related Ni II -(η 2 -ArN(H)O) complex (protonated hydroxylamine) upon reaction with 9,10-dihydroanthracene (DHA). 52 Conversely, Meyer et al. reported a dinuclear Ni II 2 -(μ-η 1 :η 1 -467 ArN(H)O) species that released its H atom to a phenoxyl 468 radical to form the related Ni II 2 -(μ-η 1 :η 1 -ArNO •-) species and 469 evaluated a bond dissociation energy BDE(N-H) of around

Electrochemistry.

  Room temperature electrochemical studies of the nitrosoarene ligands and their copper complexes were performed in a glovebox (Jacomex; O 2 < 1 ppm and H 2 O < 1 ppm) with a homedesigned three-electrode cell (WE, glassy carbon or platinum; RE, platinum wire in a Fc + /Fc solution; CE, platinum or graphite rod). Ferrocene was added at the end of the experiments to determine the redox potential values. The potential of the cell was controlled by an AUTOLAB PGSTAT 100 (Metrohm) potentiostat monitored by the NOVA 1.11 software. Dichloromethane (Acros) was distilled over CaH 2 under an inert atmosphere and stored in a glovebox. The supporting salt NBu 4 PF 6 was synthesized from NBu 4 OH (Acros) and HPF 6 (Aldrich). It was then purified, dried under vacuum for 48 h at 100 °C, and then kept under argon in the glovebox. NBu 4 OTf (Aldrich, 99%) was stored as received in the glovebox. Electrolytic solutions were prepared in the glovebox and dried for a few days under molecular sieves (3 Å) to remove traces of water before use.

1 H

 1 NMR (500 MHz, CDCl 3 ): δ ppm 1.74 (br, 2H), 2.65 (br, 12H),

722(

  Concordia) for help with IR spectral measurement and 723 analysis and Alexey Denisov (Concordia) for help with 15 N 724 NMR.

  Scheme 1. Some Bonding Modes in Metal/Nitrosoarene Complexes, with Typical NO Bond Lengths 13

Scheme 2. Confirmed Examples of Group 10 and 11 Complexes in Which ArNO Gets Reduced by 1e or 2e upon Reaction a a Asterisks indicate calculated values.

Table 1 .

 1 NO Stretching Frequencies a Measured at 25 °C on species labeled with 14 N and 15 N on the NO moiety. Full data are given in the Supporting Information.

	species	ν (Δ)/cm -1	species	ν (Δ)/cm -1
	2 NMe2	1365 (12), 1340 (19)	3 NMe2	1392 (14), 1315 (6)
	2 H b	1388 (27)	3 H ′ c	1162 (10), 1133 (23)
	2 Br d	1286 (4), 1256 (24)	3 Br	1226 (6)
	2 NO2 d	1238 (20)	3 NO2	875 (15)

a b Syn ArN(O)N(O)Ar dimer. c Contains a small amount of mononuclear species 3 H . d Anti ArN(O)N(O)Ar dimer.

Table 2 .

 2 15 N NMR Data a

	species	δ( 15 N)/ppm	species	δ( 15 N)/ppm
	2 NMe2	787.58	3 NMe2	668.58
	2 H	885.83	3 H	not observed
	2 Br	878.67	3 Br	not observed
	2 NO2	913.23	3 NO2	363.67
	a Measured in CDCl 3 at 25 °C on a 500 MHz instrument; ν( 15 N) =
	50.7 MHz.

Table 3 .

 3 UV-Vis Data of 3 R Complexes aMeasured in CH 2 Cl 2 at 25 °C. b λ max /nm (ε/mM -1 cm -1 ).

	species		λ max (ε) b
	3 NMe2	426 (32.8)	538 (34.7)	690 (2.3)
	3 H	313 (8.0), 338 (7.4)	440 (sh)	609 (1.9)
	3 Cl	319 (10.2), 350 (7.3)	440 (sh)	614 (1.1)
	3 Br	325 (10.4), 350 (8.2)	440 (sh)	614 (1.2)
	3 NO2	345 (19.4)	440 (sh)	644 (1.5)

a

Table 5

 5

, the potential value at E D is Scheme 5. Reduction Steps of 2 R Species

Table 4 .

 4 Electrochemical Data of the 2 R Nitrosoarenes a

	species	E 1/2 A (2 R )	E 1/2 B (2 R )	σ para	b
	2 NMe2	-1.69 (120) c	d	-0.83
	2 H	-1.40 (90)	-1.86 (100)	0
	2 Cl	-1.32 (110)	-1.79 (90)	0.227
	2 Br	-1.30 (90)	-1.75 (90)	0.232
	2 NO2	-0.93 (90)	-1.33 (100) e	0.78

  on the substituting group R, meaning that the chemical species or complex contains the ArNO moiety. Possibly, reduction of the complex induces breaking of the Cu-ArNO bond, liberating 2 R and explaining the similarity of the E D and E A values. Such a hypothesis is verified in all cases except for 3NO 2 (Figures S28 and

		05	-1.75	-0.36 (140)	-0.83
	3 H	-0.92	-1.45 c	0.28	0.00
	3 Br	-0.82	-1.22 (180)	0.08 (130)	0.23
	3 NO2	-0.72	-1.17 (145)	0.11 (120)	0.78

F

highly dependent

Table 6 .

 6 Electrochemical Data of 3 R and Related Ni n /ArNO and Cu n /O 2 Complexes C S, cis-end-on peroxo; S P, sideon peroxo; O, bis(μ-oxo). b Potential versus Fc +/0 . c Irreversible cathodic peak. d Reversible reduction. e Reversible oxidation. f Converted from the value versus saturated calomel electrode (SCE) using E 1/2 (Fc +/0 ) = 560 mV versus SCE in these conditions.

	Scheme 6. Thermodynamic Analysis of HAT Reactivity

a The reaction is written in a way it was carried out either as an oxidation or as a reduction. Abbreviations: n/c, compound not characterized; C P, cis-end-on peroxo;

  N NMR (50.7 MHz, CDCl 3 ): δ ppm 668.58 (NO moiety). Anal. Calcd for C 16 H 28 CuF 3 N 4 O 4 S: C, 38.98;

with reduction of the bond order upon inner-sphere electron

transfer from the Cu center(s). For mixed-valent dinuclear

https://dx.doi.org/10.1021/acs.inorgchem.9b03175Inorg. Chem. XXXX, XXX, XXX-XXX

a In CH 2 Cl 2 containing 0.1 M NBu 4 OTf at 25 °C; scan rate v = 0.1 V s -1 . E/V versus Fc +/0 (ΔE p /mV). b σ para Hammett parameters. c Irreversible cathodic peak.

https://dx.doi.org/10.1021/acs.inorgchem.9b03175Inorg. Chem. XXXX, XXX, XXX-XXX H

https://dx.doi.org/10.1021/acs.inorgchem.9b03175Inorg. Chem. XXXX, XXX, XXX-XXX K

https://dx.doi.org/10.1021/acs.inorgchem.9b03175Inorg. Chem. XXXX, XXX, XXX-XXX L

Author Contributions