Garima Garg

Anna M Masdeu-Bultó

Norberto Farfán

Javier Ordóñez-Hernández

Montserrat Gómez
email: gomez@chimie.ups-tlse.fr

Yaocihuatl Medina-González

Yaocihuatl Medina-Gonzalez
email: yaocihuatl.medina-gonzalez@u-bordeaux.fr

Palladium Nanoparticles in Glycerol/Ionic Liquid/Carbon Dioxide Medium as Hydrogenation Catalysts

Keywords: CO2, DES, micro-viscosity, diffusion, fluorescent molecular rotors, palladium nanoparticles, hydrogenation, scCO2 extraction

In this work, the influence of carbon dioxide on the synthesis of palladium nanoparticles (PdNPs) in glycerol/ionic liquid mixtures and their further applications in catalytic hydrogenations, was studied. In the glycerol/choline tosylalaninate/CO2 medium, small and very well-dispersed metal nanoparticles were obtained (1.3 ± 0.3 nm), exhibiting a relative narrow size distribution. Palladium nanoparticles prepared in the presence of supercritical carbon dioxide (scCO2) showed higher catalytic reactivity in comparison with those prepared in its absence. In addition, we observed a beneficial effect of CO2 in the studied hydrogenations. With the aim of rationalizing this trend, measurements of viscosity of the solvent media were carried out by using fluorescent molecular rotors, allowing the determination of diffusion coefficients for the organic substrates and molecular hydrogen. This study evidenced that CO2 causes a decrease of the viscosity of the medium and an increase of the diffusion constants of reagents including H2; consequently, PdNPs in the glycerol/choline tosylalaninate/CO2 solvent showed an enhanced catalytic activity. From both process engineering and environmental points of view, it is important to highlight the efficiency of the extraction of organic products from the catalytic glycerol/ionic liquid solution by scCO2.

Introduction

The beneficial effect of CO2 in sub-or super-critical conditions in catalytic processes, in particular for hydrogenation reactions, is well-known in the literature, mainly involving supported metal-based catalysts. [START_REF] Hitzler | Selective Catalytic Hydrogenation of Organic Compounds in Supercritical Fluids as a Continuous Process[END_REF][START_REF] Hitzler | Continuous Hydrogenation of Organic Compounds in Supercritical Fluids[END_REF][START_REF] More | Effect of Supercritical CO2 as Reaction Medium for Selective Hydrogenation of Acetophenone to 1-Phenylethanol[END_REF] The full miscibility of CO2 with H2 together with the high diffusivity and good transport properties of this fluid represent the foremost physico-chemical reasons for achieving high reaction rates in comparison with hydrogenations carried out in conventional organic solvents.

ScCO2 has been also applied in the synthesis of metal-based nanomaterials. In contrast to the supercritical fluid chemical deposition methodology to prepare metal nanoparticles (MNPs) supported on solid supports, [START_REF] Aymonier | CHAPTER 10: Materials Processing and Recycling with Near-and Supercritical CO2-Based Solvents[END_REF][START_REF] Zhang | Applications of Supercritical Carbon Dioxide in Materials Processing and Synthesis[END_REF][START_REF] Wang | A Green and Efficient Route for Preparation of Supported Metal Colloidal Nanoparticles in ScCO2[END_REF][START_REF] Erkey | Preparation of Metallic Supported Nanoparticles and Films Using Supercritical Fluid Deposition[END_REF][START_REF] Hu | Green CO2-Assisted Synthesis of Mono-and Bimetallic Pd/Pt Nanoparticles on Porous Carbon Fabricated from Sorghum for Highly Selective Hydrogenation of Furfural[END_REF][START_REF] Qiao | Preparation of SBA-15 Supported Pt/Pd Bimetallic Catalysts Using Supercritical Fluid Reactive Deposition: How Do Solvent Effects during Material Synthesis Affect Catalytic Properties?[END_REF] the synthesis of support-free MNPs in scCO2 has been less studied. The pioneering work reported by Korgel and co-workers concerned the synthesis of AgNPs in neat scCO2 using perfluorodecanethiol as scCO2-soluble stabilizer. [START_REF] Shah | Steric Stabilization of Nanocrystals in Supercritical CO2 Using Fluorinated Ligands[END_REF] Based on this strategy, PdNPs and AgNPs were also prepared under flow conditions. [START_REF] Mcleod | Metallic Nanoparticle Production Utilizing a Supercritical Carbon Dioxide Flow Process[END_REF] Later on, AgNPs were prepared by the polyol approach assisted by scCO2, using ethylene glycol as both co-solvent and reducing agent, and polyvinylpyrrolidone as stabilizer. [START_REF] Chih | Supercritical Carbon Dioxide-Assisted Synthesis of Silver Nano-Particles in Polyol Process[END_REF] In all these reports, uniform and better dispersed nanoparticles were obtained, in relation to those prepared in usual organic solvents.

However, working in neat scCO2 as solvent shows some limitations such as the low solubility of some reagents and catalysts. Combining compressed gas and liquid solvents (organic solvents, ionic liquids, deep eutectic mixtures), called gas-expanded liquids, benefits of advantages of both gas and liquid partners. [START_REF] Jessop | Gas-Expanded Liquids[END_REF][START_REF] Subramaniam | Gas-Expanded Liquids for Sustainable Catalysis and Novel Materials: Recent Advances[END_REF] Furthermore, it is known that a number of ionic liquids can be dissolved in scCO2 in the presence of polar compounds (such as acetone, ethanol, ethylene glycol); this effect is very restricted when apolar compounds are involved (e.g. alkanes). 15,[START_REF] Wu | Solubility of Room-Temperature Ionic Liquid in Supercritical CO2 with and without Organic Compounds[END_REF] Concerning ionic liquids, Deep Eutectic Solvents (DES) have attracted great attention as reaction media in the last years, as an answer to the need of greener, less toxic, and non-volatile solvents. Interestingly, one of the most typical DES, i.e. choline chloride/glycerol, has been reported to be comparable in cost to some organic solvents, which comes from the large availability of choline chloride and glycerol. [START_REF] Handy | Deep Eutectic Solvents in Organic Synthesis[END_REF] DES have been proposed as solvent media in catalytic processes, mainly due to the ability to tailor their solvent capabilities by changing the constituents of the DES that can be judiciously chosen to obtain a compound of low toxicity and with the desired physicochemical properties. [START_REF] Zhang | Deep Eutectic Solvents: Syntheses, Properties and Applications[END_REF] Nevertheless, and even though the several advantages of DES, some drawbacks can interfere with their implementation in large scales such as their high viscosity and their difficult recyclability.

Concerning the viscosity, highly viscous solvents (>100 cP) can hinder mass transfer during chemical reactions, affecting kinetics and selectivity of the transformation. For instance, during hydrogenation reactions in highly viscous media, a decrease of H2 diffusion can be expected, imposing high pressures of hydrogen to obtain acceptable conversions and thus increasing risks in terms of safety. On the other hand, recyclability of DES is typically performed by biphasic extraction of products using an immiscible organic solvent or by dissolving the DES in water in order to perform the separation; both strategies need the drying of the DES, which can be highly energy and time consuming in particular if the latter strategy is used. In the case of the first strategy of separation of DES, emissions of volatile organic solvents can be an important issue.

Accordingly, extraction of products and recycling of DES represent key steps to be improved in order to develop applications of DES at industrial scale.

Curiously, few applications have been reported involving unsupported metal nanoparticles (in particular palladium-based catalysts) in the presence of CO2 for applications in hydrogenations, all of them involving water as (co)solvent (see Table S1 in the Supporting Information). [START_REF] Meric | Micelle-Hosted Palladium Nanoparticles Catalyze Citral Molecule Hydrogenation in Supercritical Carbon Dioxide[END_REF][START_REF] Meric | Pressure-Dependent Product Distribution of Citral Hydrogenation over Micelle-Hosted Pd and Ru Nanoparticles in Supercritical Carbon Dioxide[END_REF][START_REF] Yeung | Catalysis in Supercritical CO2 Using Dendrimer-Encapsulated Palladium Nanoparticles[END_REF][START_REF] Liu | Selective Hydrogenation of Citral Catalyzed with Palladium Nanoparticles in CO2-in-Water Emulsion[END_REF] In this context, high-pressure CO2-based systems appear as an appropriate approach to attain both objectives: i) decreasing viscosity to improve diffusion during hydrogenation reactions and ii) proposing a process including an easy and feasible extraction step. Specifically, we studied the glycerol/choline-based derivative/CO2 system as solvent for both the synthesis of unsupported palladium nanoparticles (PdNPs) and catalytic hydrogenations (Figure 1). With the purpose of rationalizing the observed behavior, the determination of physicochemical properties of the solvent such as viscosity is crucial. Thus, we determined the viscosity under high pressure conditions. For that, measurements using two fluorescent molecular rotors were implemented (Figure 1). [START_REF] Vyšniauskas | A Twisted Tale: Measuring Viscosity and Temperature of Microenvironments Using Molecular Rotors[END_REF][START_REF] Kuimova | Mapping Viscosity in Cells Using Molecular Rotors[END_REF][START_REF] Shimolina | Imaging Tumor Microscopic Viscosity in Vivo Using Molecular Rotors[END_REF] The use of these molecular rotors as viscosity probes, allows the study of microviscosity which can be particularly important in highly heterogeneous media, and also permits to measure the viscosity without the use of expensive high-pressure rheometers.

Experimental Section

Materials and Methods

Unless otherwise stated, all chemical reagents and 9-(2,2-dicyanovinyl)julolidine molecular rotor (DCVJ) were purchased from commercial suppliers and used without further purification.

Solvents were dried by standard methods or distilled prior to use.

Choline tosylalaninate and palladium nanoparticles (PdNPs) were prepared following reported methodologies. [START_REF] Garg | Palladium Nanoparticles Stabilized by Novel Choline-Based Ionic Liquids in Glycerol Applied in Hydrogenation Reactions[END_REF] Bulk viscosity of the solvent medium was measured using a Rheometer AR2000Ex from TA Instruments. High-pressure reactions were carried out in a Top Industrie Autoclave. GC analyses were carried out on a GC Perkin Elmer Clarus 500 with ionization flame detector, using a SGE BPX5 column composed by 5% phenylmethylsiloxane, coupled to a Perkin Elmer Clarus MS560 mass detector. NMR spectra were recorded on a Bruker Advance 300 spectrometer at 293 K (300 MHz for 1 H NMR and 75.5 MHz for [START_REF] Jessop | Gas-Expanded Liquids[END_REF] C NMR), using chloroform-d as solvent. For the synthesis of BODIPY-MR, 1 H, [START_REF] Jessop | Gas-Expanded Liquids[END_REF] C, [START_REF] Mcleod | Metallic Nanoparticle Production Utilizing a Supercritical Carbon Dioxide Flow Process[END_REF] B and [START_REF] Meric | Micelle-Hosted Palladium Nanoparticles Catalyze Citral Molecule Hydrogenation in Supercritical Carbon Dioxide[END_REF] F NMR spectra were recorded using Varian Unity Inova 300 and JEOL ECA 400 spectrometers. High-resolution mass spectra were acquired with an Agilent Technologies ESI TOF spectrometer. TEM images of PdNPs dispersed in glycerol were obtained from a JEOL JEM 1400 instrument running at 120 kV. PdNPs size distributions and average diameters were determined from TEM images applying Image-J software associated to a Microsoft Excel worksheet. Elemental ICP-AES analyses were carried out by using a Perkin Elmer 2400 series II analyzer and an iCAP 6300 ICP Spectrometer. A SEPAREX SF200 pilot (Separex Company, Nancy, France) was used to carry out the extraction tests. Melting points were determined using an Electrothermal Mel-Temp® apparatus. Infrared spectra were obtained using a Perkin-Elmer Spectrum 400 FT-IR/FT-FIR spectrophotometer.

Absorption and fluorescence spectra under ambient conditions were obtained using a Perkin Elmer Lambda 2S UV/Vis spectrophotometer and Varian Cary Eclipse Fluorescence spectrometer, respectively. Fluorescence spectra were recorded at 25 °C in a 1 cm quartz fluorescence cuvette placed in a thermostated holder at 25 ± 0.1 °C with a recirculating water bath. High-resolution mass spectra were acquired with an Agilent Technologies ESI TOF spectrometer.

Fluorescence spectroscopy determinations at high pressure were performed by using a stainless-steel cell of 5 mL with four sapphire windows. The windows were sealed with Teflon gasket joints capable of withstanding pressures up to 350 bar. The temperature control unit for the cell consisted of heating cartridges inserted into the body, probes, and a temperature controller. A blue led light source and a fluorescent detector were placed on adjacent windows of the cell making them orthogonal to each other (see Figure S1 in the Supporting Information). A StellarInc BLUE-Wave spectrometer was used to record the detected fluorescence on a computer.

Synthesis and characterization of BODIPY-MR

10-(Di(1H-pyrrol-2-yl)methyl)-2,3,6,7-tetrahydro-1H,5H-quinolizino(9,1-gh)coumarin (0.150 g, 0.39 mmol; see synthesis methodology -Scheme S1-, NMR and UV-vis spectra -Figures S2-S4-in the Supporting Information) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (0.100 g, 0.47 mmol) were dissolved in CH2Cl2 (5 mL). The reaction mixture was stirred at room temperature for 2 h, followed by addition of triethylamine (0.81 mL, 5.8 mmol). After 10 minutes, BF3 . OEt2 (0.97 mL, 7.8 mmol) was added dropwise and the reaction mixture was stirred for 2 h and then washed with water and extracted with CH2Cl2/H2O. The organic phase was filtered over Na2SO4, the solvent was removed under reduced pressure and the crude product was further purified by column chromatography over silica gel with hexane/ethyl acetate (85:15) as eluent to give 33 mg of BODIPY-MR (20% yield) as a green solid. M.p. 90 °C (decomposition) (ATR, , cm

Calibration and measurements of viscosity using fluorescent molecular rotors.

Four solutions of different viscosity were prepared using mixtures of glycerol:ethylene glycol in the ratio of 1:0, 8:2, 6:4, and 4:6. The viscosities of these four solutions were measured using a rheometer at 60 °C and 80 °C. A known quantity of a solution of molecular rotor in acetone was added to each one of the glycerol:ethylene glycol solutions; acetone was then evaporated (concentration of DCVJ and BODIPY-MR at the end was 1.12 x 10 -4 mmolL -1 ); fluorescence spectra of these solutions were recorded using the procedure mentioned in section 2.1 at 60 °C and 80 °C.

The Förster-Hoffmann equation establishes a mathematical relationship between viscosity and quantum yield (eq. 1): 27 ln = 𝐶 + 𝑥 ln (eq. 1)

where C is a temperature-dependent constant and x is a dye-dependent constant. If the absorbed light intensity is known, the emission intensity and the quantum yield are directly and proportionally related. [START_REF] Haidekker | Molecular Rotors-Fluorescent Biosensors for Viscosity and Flow[END_REF] At low concentrations, the intensity of emission increases linearly with the concentration of the rotor. Response of rotors can be calibrated by using fluids of known viscosity, thus permitting to determine the viscosity of an unknown fluid.

In

Determination of viscosity of choline tosylalaninate/glycerol mixtures under CO2

The high-pressure cell (see Figure S1 in the Supporting Information) was completely filled with the above prepared DES and the corresponding molecular rotor solution (for both molecular rotors, DCVJ and BODIPY-MR, concentration was 1.12 x 10 -4 mmolL -1 ), then the cell was tightly closed. A magnetic stirrer was used to agitate the contents in the cell to facilitate reaching equilibrium. This experiment was done at 60 and 80 °C. For each measurement, the cell was filled with a certain pressure of CO2 and kept pressurized until an equilibrium between DES and CO2 was achieved. Fluorescence spectra were recorded by the spectrometer described in Section 2.1.

General procedure for hydrogenation using choline tosylalaninate stabilized palladium nanoparticles under CO2

In an autoclave (working from 5 to 350 bar total pressure), the appropriate substrate (1 mmol for 1 mol% of catalyst or 10 mmol for 0.1 mol% of catalyst) was added to 1 mL of preformed nanoparticles stabilized by choline tosylalaninate (1 mol%; 2.85 mg of Pd) in glycerol under argon. [START_REF] Garg | Palladium Nanoparticles Stabilized by Novel Choline-Based Ionic Liquids in Glycerol Applied in Hydrogenation Reactions[END_REF] The reaction mixture was put under vacuum and then pressurized with H2 and/or CO2 at the convenient pressure, heated up at 80 °C and stirred for the suitable time; then, cooled down to room temperature before extraction. Organic products were extracted from glycerol by a biphasic methodology, adding dichloromethane (5 × 3 mL). Organic phases were collected and solvent removed under vacuum. Conversions and yields were determined by GC using decane as internal standard. The obtained products were characterized by GC-MS and ( 1 H and 13 C)-NMR, and compared to the previously reported data.

scCO2 extractor

A SEPAREX SF200 pilot (Separex Company, Nancy, France) was used to carry out the extraction (see Figure S11 in the Supporting Information). [START_REF] Camy | Modelling and Experimental Study of Separators for Co-Solvent Recovery in a Supercritical Extraction Process[END_REF] This apparatus consisted of a 200 mL contacting vessel that can be used as a reactor and/or liquid-fluid extractor. The introduction of CO2 was done through a filter mesh screen from a commercial HPLC pump. This device allowed good dispersion of CO2 in the glycerol-based mixture. Additionally, a magnetic stirrer was added to improve the dispersion. A cascade of three 20-mL cyclonic separators was present at the outlet of the extractor. The pressure in each vessel was adjusted by depressurization valves. A volumetric pump (Milton Roy, flow rate maximum 5 kg/h) was used to pump subcooled CO2 in the system.

The extractor was then heated to the desired temperature with a continuous flow of CO2.

Experiments were carried out in open-loop or closed-loop configuration; in the latter case, CO2 was recycled at the head of the pump after condensation. Temperature and pressure were controlled in each unit of the pilot. The limitation of the pressure was 300 ± 10 bar and temperature was 80 5 °C.

Results and Discussion

Effect of scCO2 on the synthesis and catalytic behavior of PdNPs.

In the absence of scCO2, our precedent works prove that glycerol favors the dispersion of MNPs but needs the presence of a stabilizer in order to avoid the aggregation. [START_REF] Favier | Palladium Nanoparticles in Polyols: Synthesis, Catalytic Couplings, and Hydrogenations[END_REF][START_REF] Dang-Bao | Bimetallic Nanocatalysts in Glycerol for Applications in Controlled Synthesis. A Structure-Reactivity Relationship Study[END_REF][START_REF] Reina | Palladium Nanoparticles Stabilised by Cinchona-Based Alkaloids in Glycerol: Efficient Catalysts for Surface Assisted Processes[END_REF][START_REF] Chahdoura | Glycerol as Suitable Solvent for the Synthesis of Metallic Species and Catalysis[END_REF][START_REF] Chahdoura | Palladium Nanoparticles in Glycerol: A Versatile Catalytic System for C-X Bond Formation and Hydrogenation Processes[END_REF] However, choline tosylalaninate /glycerol mixture is a solvent that also acts as nanoparticle stabilizer, leading to stable solutions of size-controlled metal nanoparticles, [START_REF] Garg | Palladium Nanoparticles Stabilized by Novel Choline-Based Ionic Liquids in Glycerol Applied in Hydrogenation Reactions[END_REF] in agreement with the modelling studies carried out by Atilhan and Aparicio. [START_REF] Atilhan | Molecular Dynamics Simulations of Metal Nanoparticles in Deep Eutectic Solvents[END_REF] In the present study, we prepared PdNPs stabilized by choline tosylalaninate (IL) in glycerol under argon (PdA) following our previously reported methodology, [START_REF] Garg | Palladium Nanoparticles Stabilized by Novel Choline-Based Ionic Liquids in Glycerol Applied in Hydrogenation Reactions[END_REF] and also in the presence of scCO2 (PdB) with the aim of studying the effect of scCO2 in the morphology of the as-prepared PdNPs (Figure 2). In both cases, black colloidal solutions were obtained, constituted of well-dispersed and small spherical nanoparticles (for PdA, 1.7 ± 0.6 nm; for PdB, 1.3 ± 0.3 nm) as evidenced by TEM, being PdNPs prepared under scCO2 (PdB) clearly smaller than those prepared in its absence (PdA). Hence, the scCO2/glycerol/IL tricomponent medium was probably less viscous than that glycerol/IL di-component solvent, mainly due to the enhanced solubility of CO2 in the glycerol and IL mixture, [START_REF] Medina-Gonzalez | Phase Equilibrium of the CO2/Glycerol System: Experimental Data by in Situ FT-IR Spectroscopy and Thermodynamic Modeling[END_REF] triggering a better diffusion of the metal precursor [PdCl2(cod)] (cod = cycloocta-1,5-diene). This effect leads to a homogeneous dispersion of Pd nuclei (generated by reduction of Pd(II) molecular species by glycerol), favoring a more uniform growth of these nanoclusters, in agreement with the relative narrow size distribution of nanoparticles observed under scCO2 (1.3 ± 0.3 nm for PdB vs 1.7 ± 0.6 nm for PdA). In addition, intermolecular interactions between CO2 and glycerol/IL mixture can modify the supramolecular arrangement of the medium at least at nanoscopic level (probably no long-range effects are induced), [START_REF] Percevault | Do Deep Eutectic Solvents Form Uniform Mixtures Beyond Molecular Microheterogeneities?[END_REF][START_REF] Herrera | A Theoretical Study on Aminoacid-Based Ionic Liquids with Acid Gases and Water[END_REF] most likely causing an impact on the growth of MNP by confinement effect of PdNPs; actually, smaller size of PdNPs was obtained in the presence of scCO2 than in its absence (dmean = 1.3 nm for PdB vs dmean = 1.7 nm for PdA). The effect of metal nanoparticles size on catalytic hydrogenation reactions has been extensively studied. [START_REF] Liu | Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles[END_REF] The well-controlled synthesis of MNPs in terms of morphology and surface state permits to tune the reactivity of MNPs, leading to selective processes when poly-functional substrates are considered. [START_REF] Jiang | Particle Size Effects in the Selective Hydrogenation of Cinnamaldehyde over Supported Palladium Catalysts[END_REF] It is important to note that for the as-prepared nanoparticles, the difference in size is significant. For PdA, the most population of nanoparticles are ca. 1.9 nm; for PdB, ca. 1.1 nm (Figure 2). Considering a fcc structure of spherical nanoparticles, [START_REF] Garg | Palladium Nanoparticles Stabilized by Novel Choline-Based Ionic Liquids in Glycerol Applied in Hydrogenation Reactions[END_REF] PdNPs exhibiting a mean diameter of ca. 1.1 nm represents ca. 76% of total atoms at the surface, while for PdNPs showing a mean diameter of ca. 2.0 nm, 52% of total atoms are at the surface, clearly decreasing the number of active sites.

With the purpose of evaluating the catalytic behavior of PdA and PdB, we chose 4-phenylbut-3-en-2-one (1) due to the presence on the same substrate of different unsaturated groups prone to be reduced (C=O, C=C, aromatic ring). The reactions were carried out under 20 bar of H2 at 80 °C for 18 h, using 1 mol% Pd (Table 1). In contrast to non-aromatic C=C bonds, carbonyl groups are more demanding to be hydrogenated with Pd-based catalysts. [START_REF] Garg | Palladium Nanoparticles Stabilized by Novel Choline-Based Ionic Liquids in Glycerol Applied in Hydrogenation Reactions[END_REF][START_REF] Simon | Electronic and Catalytic Properties of Palladium Nanoparticles Supported on Poly(Ionic Liquid)[END_REF] Under these conditions both catalysts gave full conversion, obtaining a mixture of 4-phenylbutan-2-one (1a) and 4phenylbutan-2-ol (1b). PdB led to a higher amount of 1b than PdA (entry 2 vs 1, Table 1), pointing to its higher reactivity towards the reduction of the carbonyl group. Thus, smaller nanoparticles exhibited higher activity (PdA vs PdB, see above Figure 2). This behavior can be explained by the fact that C=C are hydrogenated on the faces of PdNPs (favored for larger PdNPs [START_REF] Wilson | Effect of Pd Nanoparticle Size on the Catalytic Hydrogenation of Allyl Alcohol[END_REF] ) while for the activation of C=O group, palladium atoms exhibiting more electrophilic character are needed, [START_REF] Kuai | Titania Supported Synergistic Palladium Single Atoms and Nanoparticles for Room Temperature Ketone and Aldehydes Hydrogenation[END_REF] being those occupying sites with low coordination number (such as edges and vertexes); the relative atoms in these latter positions decrease with the increase of PdNPs size. After catalysis, the extraction of organic products was carried out by a biphasic system using a non-miscible organic solvent with glycerol like dichloromethane, cyclohexane, ethyl acetate or heptane. With the purpose of replacing the use of volatile organic solvents, we were also interested in the extraction process using scCO2, as well-known for the extraction of different organic compounds. [START_REF] Erkey | Supercritical Carbon Dioxide Extraction of Organics from Soil[END_REF][START_REF] Serbanovic | Osmium Catalyzed Asymmetric Dihydroxylation of Methyl Trans-Cinnamate in Ionic Liquids, Followed by Supercritical CO2 Product Recovery[END_REF][START_REF] Chang | Supercritical Carbon Dioxide Extraction of Turmeric Oil from Curcuma Longa Linn and Purification of Turmerones[END_REF][START_REF] Rovetto | Supercritical Carbon Dioxide Extraction of Cannabinoids from Cannabis Sativa L[END_REF] We carried out the hydrogenation of 4-phenylbut-3-en-2-one under reaction conditions which permit the exclusive reduction of the C=C bond by working at lower pressure, only obtaining 4-phenylbutan-2-one (see Scheme S2 in the Supporting Information). The extraction of 4-phenylbutan-2-one from the catalytic mixture using scCO2 was successfully carried out with the SEPAREX extractor (see above, section 2), obtaining up to 90% of 1a at 200 bar of CO2 and 60 °C. In the extracted product, neither leaching of Pd (checked by ICP-AES) nor extraction of glycerol or IL (checked by NMR) were detected, in agreement with the known low solubility of glycerol in scCO2. [START_REF] Medina-Gonzalez | Phase Equilibrium of the CO2/Glycerol System: Experimental Data by in Situ FT-IR Spectroscopy and Thermodynamic Modeling[END_REF][START_REF] Gu | Glycerol as a Sustainable Solvent for Green Chemistry[END_REF] Additionally, TEM analysis of the PdNPs after scCO2 extraction showed a very good dispersion without any sign of agglomeration, preserving the mean size (1.6 ± 0.5 nm for 1799 particles) compared to the PdNPs before being used in catalysis (1.7 ± 0.6 nm, see Figure 2), in accordance with the beneficial effect of scCO2 in the dispersion of PdNPs (see Figure S12 in the Supporting Information). [START_REF] Chen | Uniform Dispersion of Pd Nanoparticles on Carbon Nanostructures Using a Supercritical Fluid Deposition Technique and Their Catalytic Performance towards Hydrogen Spillover[END_REF] This behavior permits to envisage a plausible recycling of the catalytic phase, study that is outside the scope of this work.

With the aim of increasing the solubility of H2 in the choline IL/glycerol medium, the challenging hydrogenation of the C=O bond of the ketone 1a was carried out in the presence of CO2 (Table 2), known as carrier of H2 in different media. [START_REF] Yoshida | Liquid Phase Hydrogenation of Phenylacetylene over Pd and PdZn Catalysts in Toluene: Effects of Alloying and CO2 Pressurization[END_REF][START_REF] Nanao | Furfuryl Alcohol and Furfural Hydrogenation over Activated Carbon-Supported Palladium Catalyst in Presence of Water and Carbon Dioxide[END_REF][START_REF] Kriaa | 2-Butyne-1,4-Diol Hydrogenation in Supercritical CO2: Effect of Hydrogen Concentration[END_REF] In the absence of CO2, up to 40% conversion was reached under 40 bar H2 (entry 2, Table 2); only 23% conversion under 20 bar H2

(entry 1, Table 2). The benefic effect of CO2 could clearly be stated when the reaction was carried out under the same total pressure (40 bar) but using four times less amount of H2 (entry 2 vs 3, Table 2), evidencing that CO2 increased the efficiency of the hydrogenation reaction. However, conversion could not be increased beyond 40% even at higher pressures (entry 4, Table 2). The reason behind this behavior might be the catalyst deactivation after a certain time, probably due to CO metal surface poisoning, which can be generated as consequence of the reverse water-gas shift reaction. [START_REF] Arunajatesan | Fixed-Bed Hydrogenation of Organic Compounds in Supercritical Carbon Dioxide[END_REF][START_REF] Burgener | Supercritical Carbon Dioxide: An Inert Solvent for Catalytic Hydrogenation?[END_REF] Determined by GC using decane as internal standard. c TON calculated as the ratio between converted mol of 1 and mol of total Pd (converted mol of 1/mol of total Pd).

Rationalization of CO2 effect in choline tosylalaninate/glycerol system.

Viscosity is one of the key parameters that controls the diffusion rate of molecular species in a solvent medium and hence affects the reaction rates of diffusion-controlled processes on the microscopic level. [START_REF] Shimolina | Imaging Tumor Microscopic Viscosity in Vivo Using Molecular Rotors[END_REF] Molecular rotors have emerged as one the most innovative methods to measure the viscosity at the micro-scale. [START_REF] Vyšniauskas | A Twisted Tale: Measuring Viscosity and Temperature of Microenvironments Using Molecular Rotors[END_REF][START_REF] Kuimova | Mapping Viscosity in Cells Using Molecular Rotors[END_REF][START_REF] Shimolina | Imaging Tumor Microscopic Viscosity in Vivo Using Molecular Rotors[END_REF] Artificial molecular rotors, inspired in natural motor proteins, are "molecular machines" capable of continuous directional motions under an energy input. [START_REF] Kassem | Artificial Molecular Motors[END_REF] In particular, in the case of fluorescent molecular rotors (FMR), the rotation of bonds linked to a fluorescent π-conjugated moiety is overcome by increasing the viscosity of the reaction medium, triggering a fluorescence intensity increase [START_REF] Lee | Fluorescent Molecular Rotors for Viscosity Sensors[END_REF][START_REF] Koumura | Light-Driven Monodirectional Molecular Rotor[END_REF] One of the most commonly known FMR is DCVJ. Reed et al. have described DCVJ as a fluorescent dye whose intramolecular rotational relaxation depends on the nature of the solvent. [START_REF] Kung | Microviscosity Measurements of Phospholipid Bilayers Using Fluorescent Dyes That Undergo Torsional Relaxation[END_REF][START_REF] Kung | Fluorescent Molecular Rotors: A New Class of Probes for Tubulin Structure and Assembly[END_REF] Intramolecular rotation caused within the FMR is governed by the molecular-free volume of the solvent; in general, the higher the viscosity, the smaller molecular-free volume, hence the fluorescence intensity increases. [START_REF] Sasaki | Recent Advances in Twisted Intramolecular Charge Transfer (TICT) Fluorescence and Related Phenomena in Materials Chemistry[END_REF] In this part of the study, an effort has been made to rationalize the role of CO2 in the choline tosylalaninate/glycerol/CO2 system in order to understand its effect on the synthesis of PdNPs and its improved catalytic activity as well as in the extraction of products by using scCO2. Some of these queries can be answered by understanding the mass transfer and diffusion properties of various compounds in choline tosylalaninate/glycerol mixture.

For that, viscosity of the choline tosylalaninate/glycerol with increasing amounts of CO2 was studied. We implemented a method used in the literature by using fluorescent molecular rotors to determine the viscosity of the system. [START_REF] Haidekker | Molecular Rotors-Fluorescent Biosensors for Viscosity and Flow[END_REF][START_REF] Mustafic | Apparent Shear Sensitivity of Molecular Rotors in Various Solvents[END_REF][START_REF] Haidekker | Sensing of Flow and Shear Stress Using Fluorescent Molecular Rotors[END_REF] We used two molecular rotors (Figure 1), being DCVJ our benchmark molecule. Figure 3 collects the values of viscosity measured by using DCVJ at 60 °C and 80 °C. As reported in the literature for liquids in general, [START_REF] Jessop | Gas-Expanded Liquids[END_REF][START_REF] Li | Solvation and Solvatochromism in CO2-Expanded Liquids. 1. Simulations of the Solvent Systems CO2 + Cyclohexane, Acetonitrile, and Methanol[END_REF][START_REF] Jin | Physical Properties of Ionic Liquids Consisting of the 1-Butyl-3-Methylimidazolium Cation with Various Anions and the Bis(Trifluoromethylsulfonyl)Imide Anion with Various Cations[END_REF][START_REF] Granero-Fernandez | CO2-Expanded Alkyl Acetates: Physicochemical and Molecular Modeling Study and Applications in Chemical Processes[END_REF] a monotonous decrease in viscosity of the DES (choline tosylalaninate/glycerol mixture with a 1/100 ratio) was expected by increasing CO2 pressure. However, we observed an increase in the viscosity at pressures lower than 50 bar and a viscosity decrease at higher pressures. Data presented in Figure 3 shows an increase of viscosity at pressures up to 50 bar; these observations are in accordance with previous measurements of viscosity in DES. [START_REF] Crespo | Thermodynamic Characterization of Deep Eutectic Solvents at High Pressures[END_REF] In our study, viscosity reaches a maximum at 50 bar after which a clear decrease could be observed; this trend is observable at both temperatures studied.

The behavior of viscosity as function of CO2 pressure can be explained in terms of the high excess molar volume (Vm E ) of the solution, which is the difference between the molar volume (𝑉 𝑚 ) and the ideal volume (𝑉 𝑚 𝑖𝑑𝑒𝑎𝑙 ) of the mixture DES-CO2, and the high increase in density of the system, at some concentrations of CO2. V. A. Bloomfield and R. K. Dewan have related the free excess volume of mixing to the viscosity through equation 2 (for a detailed treatment of thermodynamic equations, see references [START_REF] Bloomfield | Viscosity of Liquid Mixtures[END_REF] ), and iii) differences of internal pressures and differences of reduced volumes of the components (𝑉 𝑚 𝑃 * 𝐸 ), the P* parameter corresponds to the internal pressure contribution (eq. 3). Equation 4 used to describe the increase in viscosity of choline chloride-based DES with different hydrogen bond donor partners such as ethylene glycol, glycerol or urea. [START_REF] Kumar | Non-Ideal Behaviour of a Room Temperature Ionic Liquid in an Alkoxyethanol or Poly Ethers at T =[END_REF] Some authors have proposed the filling of free volume between molecules of DES by CO2, which would hinder mass transfer, increasing viscosity followed by a decrease in viscosity when free volume is completely filled and CO2 continues to be added. [START_REF] Li | Solvation and Solvatochromism in CO2-Expanded Liquids. 1. Simulations of the Solvent Systems CO2 + Cyclohexane, Acetonitrile, and Methanol[END_REF][START_REF] Sih | Viscosity Measurements on Gas Expanded Liquid Systems-Methanol and Carbon Dioxide[END_REF] Furthermore, Luo et al.

explained that due to the formation of hydrogen bonds during CO2 captured by amino-tailored cation, an increase in viscosity can be observed. [START_REF] Luo | Decreasing the Viscosity in CO2 Capture by Amino-Functionalized Ionic Liquids through the Formation of Intramolecular Hydrogen Bond[END_REF] In our case, this behavior was observed with the two molecular rotors used in this study. Figure 4 shows the measurement of viscosity using a BODIPY-MR (see above Figure 1). With this data, it is clearly proven that the viscosity of the choline tosylalaninate/glycerol mixture decreases with increasing CO2 pressure. These results help us to understand the mass transfer behavior of the system. one) in the glycerol/choline tosylalaninate/CO2 mixture at 80 °C with increasing pressures, were calculated (Figure 5). The diffusion constant is a crucial thermodynamic property to be considered for the extraction of solutes. [START_REF] Blanchard | Recovery of Organic Products from Ionic Liquids Using Supercritical Carbon Dioxide[END_REF] In this work, diffusion constants were calculated using the Stokes-Einstein equation (eq. 5). [START_REF] Kaintz | Solute Diffusion in Ionic Liquids, NMR Measurements and Comparisons to Conventional Solvents[END_REF]

𝐷 =

𝑘 𝐵 𝑇 6𝜋𝜂𝑟 (eq. 5)

where, kB is the Boltzmann constant (1.38 × 10-23 m 2 kg s -2 K -1 ), T is the temperature in K, ƞ is the viscosity (values taken from the data obtained from measurements with molecular rotors), and r is defined as the radius of a sphere of volume equal to the van der Waals volume of the diffusing molecule; r was calculated using the Bondi method described by Zhao et al. [START_REF] Zhao | Fast Calculation of van Der Waals Volume as a Sum of Atomic and Bond[END_REF] Viscosity (Pa . We also calculated the diffusion constant of H2 in the medium at 60 °C and at 80 °C (Figure 6). According to our results, the decrease in viscosity of the solvent medium increased the diffusion constants of organic compounds (1, 1a) and H2, which favors the increase of H2 solubility. These physico-chemical data may explain the results obtained when CO2 was used in the catalytic hydrogenation of 1 (Table 2), observing a higher catalytic activity when CO2 was present (entry 3, Table 2). Furthermore, the use of CO2 permitted to decrease the partial pressure of H2 used for the hydrogenation reactions (entry 3 vs 2, Table 2).

Conclusions

To sum up, in this work, we conceived a new catalytic system constituted of small palladium nanoparticles homogeneously dispersed in a choline tosylalaninate/glycerol/scCO2 mixture, in the absence of any other nanoparticle stabilizer, proving its efficiency in hydrogenation processes.

These PdNPs exhibited higher catalytic activity than those prepared in the absence of scCO2 in hydrogenation reactions, probably due to their smaller size (1.7 nm vs 1.3 nm for PdNPs prepared in choline tosylalalinate/glycerol and choline tosylalalinate/glycerol/scCO2 mixture, respectively).

Moreover, extraction of organic products from the choline tosylalalinate/glycerol/PdNPs catalytic system was successfully carried out by using scCO2, without addition of any volatile organic cosolvent.

Furthermore, when hydrogenation reactions were carried out in the presence of CO2, a beneficial effect was observed. With the aim of rationalizing this behavior, we carried out a study concerning both viscosity and mass transfer. Concerning viscosity, we could prove the efficiency of fluorescent molecular rotors used as viscosity probes in environments where measurements by mechanical rheometers are complicated or expensive. We used two molecular rotors, i.e. DCVJ (commercially available, used as benchmark rotor) and BODIPY-MR (new molecular rotor prepared by a multi-step methodology). Although these rotors exhibit a different backbone and functionalization, both presented the same tendency: when CO2 pressure was increased up to ca.

50 bar, viscosity increased; under higher pressure, a decrease in viscosity by half was observed.

This behavior is in accordance with other reported studies concerning DES in contact with CO2. [START_REF] Crespo | Thermodynamic Characterization of Deep Eutectic Solvents at High Pressures[END_REF] In the same sense, calculations concerning diffusion coefficients of H2 and reactants in the catalytic medium indicated that diffusion increased to values of almost twice in relation to the initial values, due to the viscosity decrease of the medium.

In conclusion, we determined the micro-viscosity for a complex catalytic system in the presence of CO2 pressure and thus rationalized its catalytic behavior. We observed a viscosity decrease of the medium at high pressures; this effect triggered an increase of the diffusion coefficients of reactants and consequently a better efficiency of the catalytic system.

Acknowledgments

The

Figure 1 .

 1 Figure 1. Scheme that represents the main objective of this work.

  [START_REF] Jessop | Gas-Expanded Liquids[END_REF] C NMR(75 MHz, CDCl3, δ, ppm):160.5 (C-11), 152.6 (C-12), 147.8 (C-14), 146.5 (C-9), 143.9 (C-21), 142.0 (C-17), 135.2 (C-18), 130.4 (C-19), 126.2 (C-20), 119.4 (C-10), 118.2 (C-8), 112.2 (C-15), 107.6 (C-16), 106.3 (C-13), 50.3 (N-CH2), 49.9 (N-CH2), 27.6 (CH2), 21.3 (CH2), 20.4 (CH2). 11 B NMR (160.4 MHz, CDCl3, δ, ppm):-0.76 (t, JB-F = 28.8 Hz) 19 F NMR (282.4 MHz, CDCl3, δ, ppm): -142.51 (dq, JF-F=105 Hz, JB-F =28.8 Hz), -147.82 (dq, JF-F=105 HZ, JB-F=28.8 Hz). HRMS (ESI-TOF): calcd. for [C24H20BF2N3O2+H + ] 432.1689, found 432.1691 (0.46 ppm).

Figure 2 .

 2 Figure 2. Synthesis of PdNPs under argon (PdA) 26 or under scCO2 (PdB) (top). TEM images in

65 35 a

 6535 Results from duplicated experiments. Reaction conditions: 1 mmol of 1 and 1 mL of the corresponding catalytic solution of PdA or PdB (10 -2 mol L -1 , 0.01 mmol of total Pd), 80 °C for 18 h. b See Figure2for the synthesis of PdNPs in glycerol/IL. d Determined by GC using decane as internal standard. d TON calculated as the ratio between converted mol of 1 and mol of total Pd (converted mol of 1/mol of total Pd).

a

  Results from duplicated experiments. Reaction conditions: 1 mmol of 1a and 1 mL of the corresponding catalytic glycerol solution of PdNPs (10 -2 mol L -1 , 0.01 mmol of total Pd). b

Figure 3 .

 3 Figure 3. Viscosity measurements of choline tosylalaninate/glycerol mixture (1/100) using DCVJ

  to viscosity deviations from ideal behavior, f(v) is a characteristic function of the free volume, and G R is the residual energy of mixing which is closely related to the excess energy of mixing and the molar volumes of the compounds in the mixture (R represents the ideal gas constant and T, the temperature). Thus, free volume and viscosity are closely and related through the energy of mixing; from this equation an inflection point is possible if the second part of the equation becomes greater than the first. Furthermore, and according to the Prigogine-Flory-Patterson (PFP) theory, three contributions to the excess volume of mixing can be taken into consideration: i) interactional (𝑉 𝑚 𝑖𝑛𝑡𝑒𝑟 𝐸 ), ii) free volume contribution (𝑉 𝑚 𝑓𝑟𝑒𝑒 𝑣𝑜𝑙 𝐸

Figure 4 .

 4 Figure 4. Viscosity measurements of choline tosylalaninate/glycerol mixture (1/100) by using

Figure 5 .

 5 Figure 5. Diffusion constant of 1 (left) and 1a (right) in choline tosylalaninate/glycerol/CO2

Figure 6 .

 6 Figure 6. Diffusion constants of H2 in the reacting medium at 60 °C (left) and 80 °C (right).

  Université Fédérale de Toulouse (program IDEX Recherche -Actions Thématiques Stratégiques), the Centre National de la Recherche Scientifique (CNRS) and the Université Toulouse 3-Paul Sabatier are gratefully acknowledged for their financial support. A. M.-B. thanks the Ministerio de Economía y Competitividad and AEI/FEDER UE (CTQ2016-75016-R) and Departament d'Economia i Coneixement (2017 SGR 1472) for the financial support. N. F. acknowledges financial support from CONACYT, PAIP and PAPIIP IN222819. Authors greatly acknowledge Severine Camy for her help on extraction experiments, Christian Pradel for TEM analyses and Isabelle Favier for her assistance in hydrogenation reactions. G. G. thanks Université Fédérale de Toulouse for PhD scholarship through the IDEX program.

  -1 ): 3129, 2954, 2932, 2856, 1715, 1619, 1590, 1559, 1522, 1389, 1308, 1261, 1114, 1078, 954. UV-Vis absorption. (CH2Cl2): λ = 567, 508, 404 nm. UV-Vis emission (CH2Cl2): λ = 520, 453 nm. 1 H NMR (300 MHz, CDCl3, δ, ppm): 7.86 (bs, 2H, H-21), 7.71 (s, 1H, H-9), 7.09 (bs, 2H, H-19), 6.92 (s, 1H, H-8), 6.49 (bs, 2H, H-20), 3.34 (q, J=5.2 Hz, 4H, H-3, H-5), 2.91 (t, J=6.4 Hz, 2H, H-1), 2.77 (t, J=6.4 Hz, 2H, H-7), 2.00-1.98 (m, 4H, H-2, H-6).

  this context, calibration curves were generated from the fluorescence data measured for different solutions prepared by varying concentrations of ethylene glycol and glycerol at different

temperatures (see Figures S5-S7 in Supporting Information); their viscosity was measured by the rheometer. Calibration curves at different temperatures were then used to calculate the viscosity using the fluorescence intensity (see Section 2.4). For DCVJ, values of parameter x are reported to be close to 0.6,

[START_REF] Haidekker | Molecular Rotors-Fluorescent Biosensors for Viscosity and Flow[END_REF]

which is in agreement with the values obtained in this work during the calibration experiments (see Figures S8-S10 in Supporting Information).

Table 1 .

 1 Hydrogenation of 4-phenylbut-3-en-2-one (1) catalyzed by PdA and PdB. a

	Entry	Catalyst (PdNPs) b	Conv. c (%) [TON] d	Selectivity d (%)
			1a	1b
			100
	1	PdA	82	18
			[100]
			100
	2	PdB
			[100]

Table 2 .

 2 Hydrogenation of 4-phenylbutan-2-one in glycerol catalyzed by PdA in the presence of CO2. a

	p(H2)	p(CO2)	Conv. (yield) b (%)
	Entry
	(bar)	(bar)	[TON] c

  permits to calculate Vm E from density of the system (𝜌), densities of both compounds present in the solution, 𝜌 1 and 𝜌 2 respectively, and molecular weight of both compounds, 𝑀 1 and 𝑀 2 respectively. EGMME systems,[START_REF] Crespo | Thermodynamic Characterization of Deep Eutectic Solvents at High Pressures[END_REF] while the Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state used in combination of Free Volume Theory has been successfully

	negative to positive denotes the inflection in the behavior of viscosity, from increase to decrease
	of this property. From a physicochemical point of view, a negative 𝑉 𝑚 𝐸 comes from two
	phenomena: a) mixtures of compounds (such as a DES) exhibit stronger intermolecular
	interactions than neat molecular compounds; and b) CO2 molecules fills interstitial free volume of
	the liquid phase increasing the density of the system, suggesting the formation of clusters in the
	DES-CO2 mixture at low concentrations of CO2. A similar behavior of 𝑉 𝑚 𝐸 with the concentration
	of one of the components has previously been reported for systems constituted of [BMIM][PF6]-
	EGMME (BMIM = 1-buty-3-methyl imidazolium; EGMME = ethylene glycol monomethyl ether)
	at different temperatures. Actually, PFP theory accurately predicts this behavior in particular for
	Ionic Liquids-
	𝑉 𝑚 𝐸 = 𝑉 𝑚 𝑖𝑛𝑡𝑒𝑟 𝐸	+ 𝑉 𝑚 𝑓𝑟𝑒𝑒 𝑣𝑜𝑙 𝐸	+ 𝑉 𝑚 𝑃 * 𝐸				(eq. 3)
	𝑉 𝑚 𝐸 = 𝑉 𝑚 -𝑉 𝑚 𝑖𝑑𝑒𝑎𝑙 =	𝑥 1 𝑀 1 +𝑥 2 𝑀 2 𝜌	-[	𝑥 1 𝑀 1 𝜌 1	+	𝑥 2 𝑀 2 𝜌 2	]	(eq. 4)
	By analyzing equation 4, a qualitative analysis can be done as Vm E can change from negative
	to positive values if density of one of the compounds becomes high enough; the second part of the
	equation becomes then smaller than the first part, which can happen at high pressures of CO2 at
	different temperatures (see Figure S13 in the Supporting Information). 69 Change of 𝑉 𝑚 𝐸 values from

Funding Sources

The Université Fédérale de Toulouse (program IDEX Recherche -Actions Thématiques Stratégiques), the Centre National de la Recherche Scientifique (CNRS), the Université Toulouse 3-Paul Sabatier, the Ministerio de Economía y Competitividad and AEI/FEDER UE (CTQ2016-75016-R), the Departament d'Economia i Coneixement (2017 SGR 1472) and the Consejo Nacional de Ciencia y Tecnología (CONACYT).

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: Table S1; Schemes S1-S2; Figures S1-S13; synthesis and characterization of intermediates for the synthesis of BODIPY-MR; characterization of organic products obtained in hydrogenation reactions and temperatures.

AUTHOR INFORMATION

Corresponding Authors

*Authors to whom correspondence should be addressed. Electronic mail: yaocihuatl.medinagonzalez-ext@solvay.com, gomez@chimie.ups-tlse.fr

Author Contributions

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


Lucas Thevenin

Jean-Claude Daran

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Christophe Fliedel
email: christophe.fliedel@lcc-toulouse.fr

Cobalt complexes of an OSNSO-tetrapodal pentadentate ligand: Synthesis, structures and reactivity

Keywords: Cobalt complexes, Bis(phenolato) ligands, Thioether ligands, Salen analogues, Crystal structures, NMR

The novel pentadentate tetrapodal proligand 2,6-bis [(2-hydroxyphenyl)sulfanylmethyl]pyridine (1•H2) and its cobalt(II) complex [Co(1)] (2) were synthesized and characterized by several analytical (EA, ESI-MS) and spectroscopic methods (NMR or EPR, FT-IR), including X-ray crystallography for 1•H2. Cyclic voltammetry studies showed that 2 undergoes a reversible metal-based oxidation (Co II /Co III ). Complex 2 was designed to be applied to organometallic mediated radical polymerization (OMRP), however it exhibited an extremely poor solubility in non-coordinating solvents and several vinyl monomers (styrene, vinyl acetate and tert-butyl acrylate), which hampers its potential as moderator. Complex 2 has a high affinity towards Lewis bases, such as pyridine, leading to the clean formation of the monopyridine adduct 2•py, as confirmed by X-ray crystallography. In 2•py, ligand 1 is pentacoordinated to the Co II center, with the two thioether-phenolate (S,O) moieties oriented anti to each other, and the only free coordination site of the octahedron is completed by the additional pyridine, trans to the central pyridine linker of 1. The equilibrium between 2 and 2•py could be studied by 1 H NMR. Complex 2 could be cleanly and quantitatively oxidized to its diamagnetic iodo cobalt(III) analog [Co(1)I] (3), by simple reaction with iodine. The latter could then be subjected to a halide abstraction reaction, mediated by K[B(C6F5)], affording the cationic complex [Co(1)][B(C6F5)], 4.

Highlights

• Cobalt(II) complexes of a bis(thioether)-bis(phenolato)-pyridyl-based OSNSO-ligand.

• Neutral (OSNSO)-cobalt(III) iodide and cationic (OSNSO)-cobalt(III) complexes.

• XRD, NMR, CV and EPR studies.

• Evaluation in radical polymerization.

Introduction

Transition metal catalysis is an essential tool in many fields, such as the synthesis of fine chemicals, natural products and pharmaceuticals. While this area has been dominated by catalysts based on noble metals such as Pd, Pt, Rh and Ir, it is now urgent to replace them by catalysts derived from non-precious metals, such as Mn, Fe and Ni, which are less-expensive, more abundant and sustainable and sometimes less-toxic [START_REF] Hapke | Cobalt catalysis in organic synthesis: Methods and reactions[END_REF][START_REF] Klein Gebbink | Non-noble metal catalysis: Molecular approaches and reactions[END_REF]. Iron and cobalt complexes are now well established as efficient (pre)catalysts for many transformations [START_REF] Hapke | Cobalt catalysis in organic synthesis: Methods and reactions[END_REF][START_REF] Klein Gebbink | Non-noble metal catalysis: Molecular approaches and reactions[END_REF][START_REF] Wen | [END_REF], particularly hydrogenation reactions [4][5][6].

Another important field of application of iron and cobalt complexes is polymer synthesis, particularly by metal-mediated radical polymerization. While iron complexes are gaining more and more attention in atom transfer radical polymerization (ATRP) [7][8][9] as an alternative to copper catalysts for the above-mentioned reasons, cobalt complexes are the compounds of choice to mediate organometallic mediated radical polymerization (OMRP) [10][11][12][13][14][15][16]. Both techniques of controlled radical polymerization make use of a transition metal complex ([L/Mt]), available in two successive oxidation states (x and x+1), as moderator to control the active/propagating radical species (P • ) concentration, thereby favoring a controlled chain growth and limiting undesired (irreversible) termination reactions (Scheme 1) [17][18][19][20]. However, ATRP and OMRP differ by the way in which the complex operates during the process. While the low valent metal complex ([L/Mt x ]) acts as activator of the halogen-capped dormant chain (P-Y) in ATRP, leading to the active radical species (P • ) and the deactivator ([L/Mt x+1 -Y]), it serves as deactivator in OMRP, via direct carbon/polymer-metal bond formation, resulting in an organometallic dormant species ([L/Mt x+1 -P], Scheme 1). Recently, we focused our attention on the development of iron-based moderating systems for the ATRP of reactive monomers such as methyl methacrylate (MMA) [21][22][23], and on manganese- [24,25] and cobalt-based systems [26][27][28][29][30][31][32][33] for the OMRP of less activated monomers (LAMs = monomers leading to poorly stabilized radicals) such as vinyl acetate (VAc) or vinylidene fluoride (VDF).

The common point for the development of more efficient systems, both for fine chemicals or polymeric materials production, relies on the understanding of the reaction mechanisms and the relationship between the catalysts structure and its activity [34,35]. Over the last decade, our group has highlighted, through both experimental and computational investigations, how subtle changes in the complex/ligand architecture may affect its performances in radical polymerization [36][37][38][39].

Several ligands have already been evaluated in Co-mediated OMRP. It was recently shown that the ligand structure and the nature of the donor set, such as N4 (porphyrin-type), N2O2 (salen-type) and (O2)2 (acac-type), influences the equilibrium constant of the moderating processes [34]. Therefore, in the present study, we designed a new pentadentate tetrapodal ligand, inspired by salen/salalen/salan ligands in which the N donor is replaced by a S (thioether) donor and incorporating a capping pyridine donor (Figure 1). Such a combination of donor atoms was only reported once, for the synthesis of a copper(II) complex, as model of the active site of the enzyme galactose oxidase [40]. The OSNSO-cobalt(II) complex isolated after coordination presents only one coordination site available for radical coordination under OMRP condition, and should therefore avoid the contribution of any other phenomena (monomer chelation, coordinating solvent interaction…) to the OMRP equilibrium [41]. Accessibility of this coordination site and of the +III oxidation state were investigated by paramagnetic 1 H NMR spectroscopy and cyclic voltammetry, respectively.

Experimental procedures 1.General considerations

Only the polymerization tests and the synthesis of complex 2 were carried out under inert atmosphere (Ar), in flame-dried glassware and with dried and/or distilled monomers and solvents (see below). All other operations were carried out in air and using non-dried glassware and solvents.

Materials

Compounds cobalt(II) acetylacetonate (Acros Organics, 99%), potassium tetrakis pentafluorophenyl borate (Alfa Aesar, 99%), 2,6-bis(hydroxymethyl)pyridine (Sigma Aldrich, 98%), 2-mercaptophenol (Acros Organics, 98+%), iodine (Acros Organics, 99.5%), pyridine (Acros Organics, 99+%), sodium hydroxide (Carlo Erba, 97%), 2,2'-Azobis(4-methoxy-2.4-dimethyl valeronitrile) (V-70, Wako Chemicals), azobisisobutyronitrile (AIBN, Sigma Aldrich, 98%), 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, Acros Organics, 98%), mesitylene (Sigma Aldrich, >99.9 %), d6-DMSO (99.8%D, Euriso-top), d5-pyridine (99.5%D, Euriso-top), CD2Cl2 (99.9%D, Euriso-top) and CDCl3 (99.8%D, Euriso-top) were used as received, without prior purification. Laboratory Reagent grade (≥99.5%) ethanol, dichloromethane, acetone, pentane and methanol were purchased from VWR Chemicals and used as received, without prior purification. Toluene and THF were dried using an MBraun SPS system. Vinyl acetate (VAc, >99 %, Sigma Aldrich), tert-butyl acrylate (tBa, Sigma Aldrich, 98%) and styrene (St, Sigma Aldrich, >99%) were dried over calcium hydride, distilled under argon, degassed by several freeze/pump/thaw cycles and stored under argon. 2,6-bis(chloromethyl)pyridine was prepared following a previously reported procedure [42].

Instrumentation

The NMR spectra were recorded on a Bruker Avance III 300 or 400 MHz spectrometer at ambient temperature. 1 H and 13 C chemical shifts (δ) are reported in ppm vs. SiMe4 and were determined by reference to the residual 1 H and 13 C solvent peaks. The coupling constants are reported in Hertz. The IR spectra were recorded in the 4000-100 cm -1 region on a PerkinElmer Frontier FT-IR spectrometer (ATR mode, diamond crystal). The electrospray mass spectra (ESI-MS) were recorded on a Q-Tof Premier (Waters) instrument using nitrogen as drying agent and nebulizing gas and MeCN as solvent. The elemental analyses for all compounds were carried out by the analytical service of the LCC-Toulouse using a PerkinElmer 2400 CHNS/O Series II System (100V). Voltammetric measurements were carried out with an Autolab PGSTAT100 potentiostat. Experiments were performed at room temperature in a homemade airtight three-electrode cell connected to a vacuum/argon line. The reference electrode consisted of a saturated calomel electrode (SCE) separated from the solution by a bridge compartment. The counter electrode was a platinum wire of ca 1cm² apparent surface. The working electrode was a Pt microdisk (0.5 mm diameter). The supporting electrolyte [ n Bu4N][PF6] (Fluka, 99% puriss electrochemical grade) was used as received and simply degassed under argon. Dichloromethane was purified under nitrogen atmosphere using solvent purification equipment (MBraun) prior to use. The solutions used during the electrochemical studies were typically 10 -3 M in complex compound and 0.1 M in supporting electrolyte. Before each measurement, the solutions were degassed by bubbling Ar and the working electrode was polished with a polishing machine (Presi P230). Compound 2 was very poorly soluble in dichloromethane, however the addition of pyridine (8 µL in 10 mL of DCM) improved the solubility, supposedly promoting the formation of compound 2•py. The EPR spectrum was obtained at 4.13 K on an Elexsys E 500 Bruker spectrometer, operating at a microwave frequency of approximately 9.5 GHz, using a microwave power of 20 mW across a sweep width of 150 mT (centered at 310 mT) with modulation amplitude of 0.5 mT. UV-Visible spectra were recorded on a PerkinElmer Lambda 35 spectrophotometer, using a quartz Hellma Analytics high precision cell with a light path of 10 mm.

X-ray structural analyses

A single crystal of each compound was mounted under inert perfluoropolyether at the tip of a glass fiber and cooled in the cryostream of either a Rigaku Oxford Diffraction GEMINI EOS diffractometer for 2•py or a Bruker Nonius APEXII diffractometer for 1•H2. The structures were solved by direct methods (SHELXt) [43] and refined by least-squares procedures on F 2 using SHELXL-2014 [44]. All H atoms attached to carbon were introduced in the calculations in idealized positions and treated as riding models. In compound 2•py, the unit cell contains a certain amount of pentane. These pentane molecules appear to be highly disordered and it was difficult to model their positions and distribution reliably. Therefore, the SQUEEZE function of PLATON [45] was used to eliminate the contribution of the electron density in the solvent region from the intensity data, and the solvent-free model was employed from the final refinement. The drawing of the molecules was realized with the help of ORTEP32 [46,[START_REF] Burnett | Report ORNL-6895[END_REF] and POV-Ray version 3.7. Crystal data and refinement parameters are shown in Table 1. Selected distances and angles are reported below Figures 1 and3. All bond lengths and angles (Tables S1 andS2) and hydrogen interactions (Table S3) are given in the ESI.

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. 2034822-2034823. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Synthesis of complex 3

Solid I2 (90 mg, 0.39 mmol, 0.5 eq) was added all at once to a stirred suspension of complex 2 (320 mg, 0.77 mmol, 1 eq) in 10 mL of methanol at room temperature. . UV-Vis: λmax(CH2Cl2)/nm: 417 (ε/dm 3 mol -1 cm -1 : 2 440), 535sh.

Synthesis of complex 4

A CH2Cl2 solution (1 mL) of K[B(C6F5)] (22 mg, 30.6 µmol, 1 eq) was added dropwise to a stirred CH2Cl2 solution (3 mL) of complex 3 (16.5 mg, 30.6 µmol, 1 eq) at room temperature, and the resulting mixture was stirred for 16 h. After evaporation to dryness, washing with pentane (2 x 5 mL) and drying under vacuum, a dark powder was isolated (33.4

Polymerization tests

All polymerization tests were carried out in bulk monomer or with addition of pyridine, under a protective argon atmosphere, in flame-dried glassware and using distilled and degassed monomers.

As a representative procedure, a Schlenk tube equipped with a magnetic stirrer was charged with the cobalt complex (2 or 2•py, 1 equiv.), freshly distilled monomer (St, tBA, VAc, 100-1000 equiv.), pyridine in some cases (see main text) and 0.3 mL of mesitylene as internal reference. The mixture was degassed by three freeze/pump/thaw cycles. The initiator (V-70 for tests at 30°C or AIBN for tests at 65 and 80°C, 0.6 equiv. in all cases) was added under an argon flow on the top of the frozen mixture before the last pump cycle. The Schlenk tube was immersed in an oil bath at the desired temperature (starting time t0). Whenever samples were withdrawn during the polymerization, they were collected under an argon flow by a syringe followed by quenching of the polymerization by adding TEMPO. Conversions were determined by 1 H NMR.

Results and discussion

Synthesis and characterization of the OSNSO pro-ligand 1•H2

Pro-ligand 1•H2 was synthesized in two steps with a global yield of 64%. In a first step, commercially available 2,6-bis(hydroxymethyl)pyridine was converted into 2,6-bis(chloromethyl)pyridine, in 79% yield, using thionyl chloride in dry THF, following a previously reported procedure [42]. Reaction between 2,6-bis(chloromethyl)pyridine and 2 equiv. of commercially available 2-hydroxythiophenol in the presence sodium hydroxide (2 equiv.) afforded the desired pro-ligand 1•H2 in 81% yield after recrystallization from an acetone/H2O mixture (Scheme 2) [40]. The latter was characterized by elemental analysis (EA), by mass spectrometry (ESI-MS), and by infrared (FT-IR) and multi-nuclear NMR spectroscopic techniques (see Exp. Section). Both 1 H and 13 S3). No X-ray structures of free (pro)ligands presenting the OSNSO skeleton are available for comparison. However, the structural parameters of 1•H2 may be compared with those of related compounds incorporating the characteristic aryl 2-picolyl thioether scaffold [START_REF] Song | [END_REF][49][50][51]. In particular, both CH2-S (aver. S1 for the list of bond lengths and angles for 1•H2) [START_REF] Song | [END_REF][49][50][51]. 16), N1-C5 1.3425( 15), O1-C112 1.3597( 14), S1-C111 1.7707( 12), S1-C11 1.8298( 13), O2-C212 1.3589( 13), S2-C211 1.7770( 12), S2-C51 1.8189( 13), C11-S1-C111 99.17( 6), C51-S2-C211 103.16(6).

Synthesis and characterization of cobalt(II) complexes (2 and 2•py)

The neutral OSNSO-cobalt(II) complex 2 was synthesized in good yield (75%) by reaction between proligand 1•H2 and an equimolar amount of [Co(acac)2], acting both as metal source and base. Complex 2 is extremely poorly soluble in non-coordinating solvents, however its composition could be confirmed by EA and ESI-MS. The latter spectrum revealed the [M + H] + ion (m/z = 413.0) as the most intense peak, with the expected isotopic distribution (see ESI -Figure S3). Alternatively, complex 2 could be synthesized by reaction between pro-ligand 1•H2, CoCl2 and NEt3 in a 1:1:3.3 molar ratio, but the yield of this reaction was lower (30%) and the procedure more time consuming (separation of 2 from the ammonium salt). Unfortunately, all attempts to obtain single crystals of 2 were unsuccessful. The low solubility of 2 in non-coordinating solvents such as chloroform or dichloromethane did not allow to obtain spectroscopic information in such solvents. On the other hand, the addition of increasing amounts of pyridine to 2 in CD2Cl2 led to increased solubility and to the appearance of paramagnetically shifted resonances in the 1 H NMR spectrum, which continuously shifted as a function of the py/Co ratio, as shown in Figure 3. This observation suggests the coordination of one pyridine molecule to complex 2, favoring its dissolution by stabilization of a mononuclear form (Scheme 3). Indeed, the structure of 2 is most likely dimeric with a diphenoxide-bridged bioctahedral geometry, as previously established for several other related compounds (see e.g. [52][53][54][55][56][57]). There are, however, no examples of dinuclear cobalt(II) compounds with an O3NS2 coordination environment in the Cambridge Structural Database. 3), two of ligand 1 and two of pyridine, could be analyzed as a function of the py/Co ratio in order to extract the equilibrium constant of the ligand addition process (equation 1). The p-py resonance could not be used for this purpose because its chemical shift is insensitive to the py/Co ratio (δ = 7.65 ppm).

The equations and the procedure used for the data fitting are described in the ESI. It is worth noting that, in case the pyridine-free complex (2) is a pentacoordinated mononuclear species, different equations and a simpler procedure for the data fitting would apply. This model, the full description of which is also available in the ESI, gave results of slightly lower quality. For the dinuclear model, all four independent fits gave an equilibrium constant in a narrow range, for an average K = 32.8±3.3, whereas the mononuclear model yielded constants in a different range from the data fitting of the ligand and pyridine resonances, for an overall average K = 58±19. This analysis is therefore in support of the dinuclear structural assignment to 2 (Scheme 3). The yellow powder isolated by precipitation from a solution of compound 2 in neat pyridine was characterized by elemental analysis, ESI-MS, FT-IR and UV-Vis, supporting the formation the monoadduct 2•py (see experimental section and ESI -Figure S4 andS14). Slow diffusion of pentane into a saturated pyridine solution of 2 afforded dark single crystals suitable for X-ray diffraction analysis. The solid-state structure, depicted in Figure 4, confirmed the formation of the mono-pyridine adduct 2•py under these conditions. The complex adopts a distorted octahedral geometry around the cobalt(II) center. Ligand 1 acts as a pentadentate tetrapodal ligand, with the two thioether-phenolate (S,O) moieties oriented anti to each other, and the connecting pyridine (of 1) located trans to the additional pyridine ligand. A similar ligand arrangement was observed in the case of a closely related pyridine adduct of a OSNSO-copper(II) complex [40] The 1 H NMR spectrum obtained for a solution of 2 in neat d5-pyridine (see Exp. section and ESI -Figure S5), on the other hand, is different from that observed in CD2Cl2 at high py/Co ratio. In addition to the residual solvent resonances, seven resonances are clearly shown with approximate relative intensity 2:2:2:2:4:1:2. This is consistent with the number of different protons in the ligand in C2v symmetry, whereas the C2 symmetry observed in the solid state would predict a larger number of resonances. In particular, the methylene protons in the CH2S tether should be diastereotopic, thus no resonance with a relative intensity corresponding to 4 protons should be present. This suggest the presence of yet another equilibrium, resulting in the decoordination of the labile sulfur donors and generation of a tris(pyridine) adduct in the presence of a large pyridine concentration, as suggested in Scheme 3. Very small resonances corresponding to those observed in neat d5-pyridine could indeed be detected also in the CD2Cl2 solutions at high py/Co ratios, by a fuller exploration of this pyridine addition equilibrium was not carried out.

The species present in pyridine solution was EPR silent down to liquid nitrogen temperature, but a spectrum could be detected at 4.13 K (ESI -Figure S6). It exhibits a characteristic signal for a cobalt(II) species with a tetragonal g-tensor (g̝ ┴ = 5.00 and g|| = 3.88) and hyperfine coupling that could not be properly modelled [61,62]. Indeed, the expected eight lines signal, corresponding to 59 Co (I = 7/2) hyperfine splitting, may be further split into five lines due to the coupling with the two transcoordinated pyridine nitrogen atoms [61].

No cyclic voltammetric (CV) data could be obtained for complexes 2 and 2•py in pure dichloromethane (DCM) nor acetonitrile due to their extremely low solubility in these solvents. Therefore, the CV analyses were performed in DCM with addition of 8 µL of pyridine, in order to solubilize the complexes. Under these conditions, the cyclic voltammograms of 2 and 2•py were found very similar, exhibiting a reversible redox wave at E1/2 = 0.071 and 0.064 V vs SCE, respectively, and with separations between the anodic and cathodic peaks of 0.074 and 0.103 V respectively, which could be assigned to the cobalt(II)/cobalt(III) redox couple (Figure 5) [61].

Synthesis of the cobalt(III) complexes 3 and 4

The cobalt(II) center of complex 2 could be smoothly and selectively oxidized to cobalt(III) by reaction with iodine (Co/I2 1:0.5 molar ratio) in MeOH (Scheme 4, top). The cobalt(III) iodide complex 3 was isolated in good yields (77%) and characterized by 1 H and 13 C{ 1 H} NMR, FT-IR, ESI-MS, UV-Vis and EA. The NMR analyses confirmed the diamagnetic nature of the compound (d 6 configuration). In d6-DMSO, the chemical shift of the signal attributed to the CH2 groups (4H) is downfield shifted by ca. 1 ppm from the position found for the free ligand, (ESI -compare Figures S7 andS1) and the 13 C NMR signal of the same CH2 group is also strongly downfield shifted to 55.6 ppm in 3 vs. 37.5 ppm in 1•H2 (ESI -compare Figures S8 andS2). Notably, the CH2 protons resonate as a second-order AB system ( 2 J = 17 Hz) confirming the κ 5 -coordination mode of ligand 1 and formation of a stereogenic sulfur center upon coordination. In CDCl3, the diastereotopic methylene H atoms are more greatly separated, yielding an AX spin system centered at 4.73 and 5.19 ppm with a 2 J of 19 Hz (ESI -Figures S9) [START_REF] Pellissier | Chiral sulfur ligands: Asymmetric catalysis[END_REF]. The signals of the aromatic H, on the other hand, are only slightly affected by the coordination process. The UV-Vis spectrum of complex 3 , in CH2Cl2, revealed an intense absorption band centered at 417 nm, along with a shoulder around 535 nm (see ESI -Figure S14). Despite numerous attempts, no crystals of 3, suitable for an X-ray diffraction analysis, could be grown.

Compound 3 could further be engaged in an halide abstraction reaction, using an equimolar amount of KB(C6F5)4 in dichloromethane at room temperature (Scheme 4, bottom). The resulting cationic complex 4 could be isolated in good yields (90%) and characterized by various spectroscopic and analytical techniques (see Exp. Section and ESI). S12 andS13) [64,65]. The formation of 4 from 3 was accompanied by a shift of the strong absorption band in the UV-Vis spectrum (in CH2Cl2) from 417 to 401 nm and by the disappearance of the shoulder at 535 nm (see ESI -Figure S14). To the best of our knowledge, no example of cationic pentacoordinated (OSNSO)-cobalt(III) complexes were reported. However, several examples of six-coordinated cationic cobalt(III) complexes, supported by one (hexadentate) or two (tridentate) related ligands based of N, S, and O-donors were structurally characterized [66][67][68][69]. It is therefore reasonable to speculate that compound 4 exists more likely as an O-or S-bridged dimeric species or as solvent adduct in coordinating solvents, such as acetone. Scheme 4. Oxidation of the cobalt(II) complex 2 to the cobalt(III) iodo complex 3 (top) and formation of the cationic complex 4 by halide abstraction (bottom).

Polymerization tests

Complexes 2 and 2•py were evaluated as moderators in radical polymerization of styrene, tert-butyl acrylate and vinyl acetate under various reaction conditions, but in all cases no control of the polymerization was observed (see ESI -Table S4 for details). The polymerization kinetics were similar to that of the free radical polymerization (absence of cobalt), meaning that the presence of the cobalt complex did not provide efficient trapping of the active radical species. Moreover, the recovered polymer samples exhibited much higher molecular weights (MW) than expected for a controlled process and broad dispersities. The lack of control was attributed to the non-solubility of 2 in pure monomer and to the competition between radical trapping and Lewis base coordination under homogeneous conditions, the latter being achieved only in the presence of 90 (at 80°C) or 330 (at 65°C) equiv pyridine.

Conclusion

In the present study, we reported the synthesis of a new OSNSO-tetrapodal pentadentate proligand and its complete characterization, including by X-ray crystallography, as a potential alternative to salen/salan/salalen ligands for applications in cobalt mediated radical polymerization. The corresponding cobalt(II) complex 2 was obtained via a one-step procedure in high yield. CV studies confirmed that the required (for OMRP) Co II /Co III couple is accessible with this ligand. Unfortunately, the poor solubility of 2 prevented any control of radical polymerization processes. The reactivity of complex 2 was further investigated, establishing that it has a high affinity towards Lewis bases, such as pyridine, and the mono-pyridine adduct 2•py could be isolated and characterized, including via paramagnetic 1 H NMR and X-ray crystallography. Complex 2 was cleanly oxidized to its iodo cobalt(III) analog 3 and the latter could then be subjected to an halide abstraction reaction, affording the cationic complex 4 in high yield. The high-valent complexes 3 and 4 may be of interest for further applications in ring-opening copolymerization of CO2 and epoxides for the production of polycarbonates, as their salen analogs. Ligand modifications are currently under investigation, in order to improve the solubility of the resulting complexes.

Scheme 1 .

 1 Scheme 1. ATRP (left), OMRP (right) equilibria. M = monomer, [L/Mt] = transition metal complex, x and x+1 = oxidation states, Pn = propagating polymer chain, Y = halogen.

Figure 1 .

 1 Figure 1. Representation of the targeted cobalt(II) complex, supported by the newly developped pentadentate tetrapodal OSNSO ligand.

  NMR (300 MHz, d6-DMSO): δ (ppm) 5.12 (4H, d, 4 J = 6.1 Hz, CH2), 6.46 (2H, d, 3 J = 8.4 Hz, Harom), 6.56 (2H, pseudo t, 3 J = 7.4 Hz, Harom), 6.85 (2H, pseudo t, 3 J = 7.7 Hz, Harom), 7.60 (2H, d, 3 J = 7.7 Hz, Harom), 7.79 (1H, d, 3 J = 7.4 Hz, Harom), 7.82 (2H, d, 3 J = 8.4 Hz, Harom). 1 H NMR (400 MHz, CDCl3): δ (ppm) 4.73 and 5.19 (4H, AX spin system, 2 J = 16 Hz, CH2), 6.64 (4H, m, Harom), 6.92 (2H, pseudo t, 3 J = 7.6 Hz, Harom), 7.39 (2H, d, 3 J = 7.6 Hz, Harom), 7.62 (1H, pseudo t, 3 J = 7.6 Hz, Harom), 7.67 (2H, d, 3 J = 6.8 Hz, Harom). 13 C{ 1 H} NMR (101 MHz, d6-DMSO): δ (ppm) 55.6 (CH2), 113.6 (Cquat), 115.5 (CHarom), 120.1 (CHarom), 122.2 (CHarom), 130.4 (CHarom), 131.4 (CHarom), 138.5 (CHarom, para-N), 159.6 (Cquat), 173.8 (Cquat)

Scheme 2 .

 2 Scheme 2. Synthesis of the OSNSO pro-ligand 1•H2. The X-ray diffraction analysis of a single crystal grown by diffusion of water into a saturated solution of 1•H2 in confirmed its structure, as depicted in Figure 2. Compound 1•H2 crystallizes in the monoclinic P21/n space group with four independent molecules in the unit cell. The phenol O2-H group is involved as proton donor in an H-bond with the proton acceptor pyridine N1 atom [N1•••O2 2.708(1) Å] and, at the same time, as proton acceptor towards the second phenol O1-H moiety [O1•••O2 2.808(1) Å] (see ESI -TableS3). No X-ray structures of free (pro)ligands presenting the OSNSO skeleton are available for comparison. However, the structural parameters of 1•H2 may be compared with those of related compounds incorporating the characteristic aryl 2-picolyl thioether scaffold[START_REF] Song | [END_REF][49][50][51]. In particular, both CH2-S (aver. 1.825 Å) and Carom-S (aver. 1.774 Å) bond lengths and the CH2-S-Carom angles (99.2° and 103.2°) in 1•H2 are in the same ranges found for the aforementioned compounds, i.e. CH2-S [1.804(2)-1.835(18) Å], Carom-S [1.759(4)-1.779(4) Å] and CH2-S-Carom [99.91(8)-105.05(9)°] (see ESI -TableS1for the list of bond lengths and angles for 1•H2)[START_REF] Song | [END_REF][49][50][51].

  1.825 Å) and Carom-S (aver. 1.774 Å) bond lengths and the CH2-S-Carom angles (99.2° and 103.2°) in 1•H2 are in the same ranges found for the aforementioned compounds, i.e. CH2-S [1.804(2)-1.835(18) Å], Carom-S [1.759(4)-1.779(4) Å] and CH2-S-Carom [99.91(8)-105.05(9)°] (see ESI -Table

Figure 2 .

 2 Figure 2. View of the molecular structure of 1•H2 with ellipsoids set at the 50% probability level. H atoms are represented as small spheres of arbitrary radii. Hydrogen bond interactions are represented as dashed bond. Selected bond lengths (Å) and angles (deg.): N1-C1 1.3455(16), N1-C5 1.3425(15), O1-C112 1.3597(14), S1-C111 1.7707(12), S1-C11 1.8298(13), O2-C212 1.3589(13), S2-C211 1.7770(12), S2-C51 1.8189(13), C11-S1-C111 99.17(6), C51-S2-C211 103.16(6).

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectra obtained for complex 2 in CD2Cl2 in the presence of variable amounts of pyridine. The resonance marked with S is the solvent resonance. Only one average resonance for each type of proton was observed at all pyridine concentrations, showing that the coordination/decoordination process (equation 1) is fast on the NMR timescale. Therefore, the observed position of each signal depends on the chemical shift values of all species involved in the equilibrium and on their relative concentrations, which depend on the equilibrium constant. The resonances of the coordinated ligand (1), equilibrating between two paramagnetic complexes, converged at high [py] to the resonances of the adduct, [Co(1)(py)] or 2•py, whereas the resonances of pyridine, equilibrating between the paramagnetic 2•py complex and the diamagnetic free ligand, converged at high [py] toward the resonances of free pyridine. Besides the three pyridine resonances (o, m and p protons in a 2:2:1 ratio), the intensities of which grew proportionally to py/Co, only two additional resonances attributable to the OSNSO ligand were clearly visible at δ ca. 96 and 29 in a 1:1 ratio, thus their structural assignment is uncertain. The chemical shift dependence of four resonances (marked with red arrows in Figure3), two of ligand 1 and two of pyridine, could be analyzed as a function of the py/Co ratio in order to extract the equilibrium constant of the ligand addition process (equation 1). The p-py resonance could not be used for this purpose because its chemical shift is insensitive to the py/Co ratio (δ = 7.65 ppm).

Scheme 3 .

 3 Scheme 3. Equilibria between complex 2 and its pyridine adducts.

  . As expected, the Co-L (L = donor atom) bond lengths respect the following order: Co-O [2.009(3) Å] < Co-N [2.150(5) Å for N2 and 2.165(5) Å for N1] < Co-S [2.4502(11) Å] (see ESI -Table S2 for the list of bond lengths and angles for 2•py). The bite angle values of the S,O-and S,N-chelates are 84.44(9)° and 81.85(3)°, respectively, and the other angles around the Co II center were found in the range 89.40(8)-98.15(3)°. No X-ray structure of other cobalt complexes with a coordination sphere composed of two phenolate, two thioether and two pyridine donors are available for comparison. However, the structural parameters of complex 2•py could be compared to other cobalt(II) complexes presenting an O2S2N2 coordination sphere, with O = phenolate, alkoxide or carboxylate, S = thioether and N = pyridyl or neutral amine donors [58-60]. The ranges of the Co-O [1.952(3)-2.033(2) Å], Co-N [2.134(3)-2.198(3) Å] and Co-S [2.478(2)-2.657(1) Å] bond lengths in these compounds encompass those observed for 2•py (see above).

Figure 4 .

 4 Figure 4. View of the molecular structure of 2•py with ellipsoids set at the 50% probability level. H atoms omitted for clarity. Symmetry code: (') -x+1,y,-z+1/2. Selected bond lengths (Å) and angles (deg.): Co1-O1 2.009(3), Co1-N1 2.165(5), Co1-N2 2.150(5), Co1-S1 2.4502(11), O1-Co1-O1' 178.80(17), O1-Co1-N1 90.60(8), O1-Co1-N2 89.40(8), O1-Co1-S1 95.73(9), N1-Co1-N2 180.0, N1-Co1-S1 81.85(3), N2-Co1-S1 98.15(3), S1-Co1-S1' 163.70(6).

Figure 5 .

 5 Figure 5. Cyclic voltammograms of complex 2 (red) and 2•py (blue) in dry CH2Cl2 (1 mM) with addition of 8µL of pyridine, at a scan rate of 200 mV s -1 .

  The 1 H NMR spectrum of complex 4 in d6-acetone (Figures S10) resembles that of its precursor 3 (ESI -Figures S11), nevertheless both signals of the CH2 linkers (δ 5.03 (4) and 5.15 (3) ppm) and the para-H of the pyridyl groups (δ 7.91 (4) and 7.80 (3) ppm) were significantly shifted. Both 19 F{ 1 H} and 11 B{ 1 H} NMR spectra confirmed the presence of the B(C6F5)4 counter ion and the absence of any cation-anion interaction (ESI -Figures

Table 1 .

 1 Crystal data for 1•H2 and 2•py.Complex 2 (50 mg, 0.12 mmol) was dissolved in pyridine (8 mL) at room temperature to afford a yellow solution, which was stirred overnight at room temperature. Addition of pentane (20 mL) induced precipitation of the pyridine adduct 2•py, which was isolated as a light brown powder after decanting off the mother liquor, further washing with pentane (2 x 10 mL) and drying under vacuum(56.

	Identification code	1•H2 (CCDC 2034822)	2•py (CCDC 2034823)
	Empirical formula	C19H17NO2S2	C24H20CoN2O2S2
	Formula weight	355.45	491.46
	Temperature, K	173(2)	173.2(3)
	Wavelength, Å	0.71073	1.54184
	Crystal system	Monoclinic	Orthorhombic
	Space group	P21/n	Pbcn
	a, Å	9.5890(5)	7.8069(4)
	b, Å	9.1824(5)	15.4402(7)
	c, Å	19.2580(11)	22.4263(12)
	α, °	90.0	90.0
	β, °	92.262(2)	90.0
	γ, °	90.0	90.0

1.5. Synthesis of pro-ligand 1•H2

Pure 2-mercaptophenol (4.51 g, 3.6 mL, 35.8 mmol, 2 eq) was slowly added to a solution of 2,6di(chloromethyl)pyridine (3.15 g, 17.9 mmol, 1 eq) in 180 mL of deoxygenated ethanol. Sodium hydroxide (1.43 g, 35.8 mmol, 2 eq) and water (18 mL) were then added and the resulting solution was stirred overnight at room temperature. The resulting colorless precipitate was isolated by filtration and dried under reduced pressure. Recrystallization from an acetone/H2O mixture afforded 1•H2 as colorless needles (5.15 g, 81% yield). Anal. Calcd for C19H17NO2S2 (355.47): C, 64.20; H, 4.82; N, 3.94; S, 18.04%. Found: C, 63.88; H, 4.66; N, 3.94; S, 17.61%. MS (ESI): m/z 356.0 [M + H] + . FTIR: νmax(solid)/cm - 1

: 3480br, 3137br, 1593m, 1574m, 1492w, 1467s, 1453s, 1401m, 1356w, 1293s, 1251s, 1223w, 1217w, 1173m, 1152s, 1027m, 1011m, 945w, 938w, 888m, 865m, 834m, 808m, 753vs, 732sh, 713m, 677m, 611m.

1

H NMR (400 MHz, d6-DMSO): δ (ppm) 4.21 (4H, s, CH2), 6.71 (2H, dt, 3 J = 7.6 Hz, 4 J = 1.2 Hz, Harom), 6.84 (2H, dd, 3 J = 8.0 Hz, 4 J = 1.6 Hz, Harom), 7.04 (2H, dt, 3 J = 8.0 Hz, 4 J = 1.6 Hz, Harom), 7.20 (2H, dd, 3 J = 8.0 Hz, 4 J = 1.6 Hz, Harom), 7.25 (2H, d, 3 J = 7.6 Hz, Harom), 7.69 (1H, t, 3 J = 8.0 Hz, Harom), 9.98 (2H, s, br, OH).

13

C{ 1 H} NMR (101 MHz, d6-DMSO): δ (ppm) 37.5 (CH2), 115.5 (CHarom), 120.0 (CHarom), 121.8 (Cquat), 122.1 (CHarom), 128.0 (CHarom), 130.4 (CHarom), 138.7 (CHarom), 155.9 (Cquat), 157.3 (Cquat).

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

The authors thank the Centre National de la Recherche Scientifique (CNRS, France) and the Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation, France (Ph.D. fellowship to L.T.) for support of this work. Alix Sournia-Saquet and Lionel Rechignat are gratefully acknowledged for the CV and EPR measurements, respectively.

Appendix A. Supplementary data

NMR spectra of compounds 1•H2, 2•py, 3 and 4; ESI-MS spectra of complex 2 and 2•py; UV-Vis spectra of complexes 2•py, 3 and 4; EPR spectrum of 2•py; details about the study of the equilibrium between complex 2 and the pyridine adduct 2•py; tables summarizing all bond lengths, angles and hydrogen interactions in the solid-state structures of compounds 1•H2 and 2•py are available in the Electronic Supporting Information.