Marc Guerre

S M Wahidur Rahaman

Bruno Ameduri

Rinaldo Poli

Vincent Ladmiral

Bruno Améduri

Limits of Vinylidene Fluoride RAFT Polymerization

Keywords: RAFT, Vinylidene Fluoride, Reversible deactivation radical polymerization, Xanthate, DFT calculations

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INTRODUCTION

Fluoropolymers (i.e. polymers the main chain of which is fluorinated), such as polytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF) and poly(chlorotrifluoroethylene) (PCTFE) to name just a few, constitute a very important class of polymeric materials and have found numerous high added-value applications. 1,[START_REF] Ameduri | Well-Architectured Fluoropolymers: Synthesis, Properties and Applications[END_REF] Fluoropolymers are usually prepared by radical polymerization. Ironically, while fluoroolefins were among the first monomers polymerized using reversible deactivation radical polymerization (RDRP) techniques, [START_REF] Tatemoto | The First Regular Meeting of Soviet-Japanese Fluorine Chemists[END_REF] the development of well-defined architectures based on fluoropolymers is clearly lagging behind that of more widespread vinyl monomers (styrene, acrylics…). This is probably due to the fact that most fluoroolefins are gases under Standard Temperature and Pressure, thus requiring less common equipment such as pressure reactors, and to their specific reactivity due to the fluorine atoms, which exert both strong σ-inductive electron-withdrawing and π-donating effects. [START_REF] Dolbier | Structure, Reactivity and Chemistry of Fluoroalkyl Radicals[END_REF] PVDF is the second most produced fluoropolymer after PTFE. PVDF and VDF copolymers have found numerous applications, for example in filtration membranes, [START_REF] Cui | Recent progress in fluoropolymers for membranes Prog[END_REF] architectural coatings, fuel hoses, liners, wire and insulation, and their electroactive properties have the potential to mr open new high-technology markets. [START_REF] Soulestin | Importance of Microstructure Control for Designing New Electroactive Terpolymers Based on Vinyllidene Fluoride and Trifluoroethylene[END_REF][START_REF] Katsouras | The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride)[END_REF][START_REF] Ameduri | From Vinylidene Fluoride (VDF) to the Applications of VDF-Containing Polymers and Copolymers: Recent Developments and Future Trends[END_REF] Recent years have witnessed a growing interest for the development of efficient preparation methods of well-defined fluoropolymer-based architectures. [START_REF] Ameduri | Controlled radical (Co)polymerization of Fluoromonomers[END_REF] Iodine Transfer Polymerization (ITP) [START_REF] David | Use of Iodocompounds in Radical Polymerization[END_REF] and RAFT/MADIX [START_REF] Hill | Expanding the Scope of RAFT Polymerization: recent Advances and[END_REF] polymerization have emerged as the most efficient techniques to control the polymerization of fluoroolefins [START_REF] Liu | Living/controlled free radical copolymerization of chlorotrifluoroethene and butyl vinyl ether under 60 Co γ-ray irradiation in the presence of S-benzyl O-ethyl dithiocarbonate[END_REF][START_REF] Kostov | First Amphiphilic Poly(vinylidene fluoride -co-3,3,3-trifluoropropene)-b-oligo(vinyl alcohol) Block Copolymers as potential Nonpersistent Fluorosurfactants from Radical Polymerization Controlled by Xanthate[END_REF][START_REF] Asandei | Mild Temperature Mn2(CO)10-Photomediated Controlled Radical Polymerization of Vinylidene Fluoride and Synthesis of Well-defined Poly(vinylidene fluoride)block CopolymersJ[END_REF][START_REF] Girard | Direct synthesis of Vinylidene Fluoride-Based Amphiphilic Diblock Copolymers by RAFT/MADIX Polymerization M[END_REF][START_REF] Wang | Xanthate-mediated living/controlled radical copolymerization of hexafluoropropylene and butyl vinyl ether under 60 Co γ-ray irradiationand preparation of fluorinated polymrs end-capped with fluoroalkyl sulfonic acid group[END_REF][START_REF] Asandei | Visible-Light Hypervalent Iodide Carboxylate Photo(trifluoro)methylations and Controlled Radical Polymerization of Fluorinated Alkenes[END_REF][START_REF] Patil | First RAFT/MADIX copolymerization of tert-butyl 2trifluoromethacrylate with vinylidene fluoride controlled by xanthate[END_REF][START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF][START_REF] Asandei | Photomediated Controlled Radical Polymerization and Block Copolymerization of Vinylidene Fluoride[END_REF] and to prepare welldefined fluoropolymer architectures. We recently reported the results of our detailed investigations of the RAFT polymerization of VDF. [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] That study clearly showed that xanthates chain transfer agents (CTA) are very efficient for preparing PVDF with narrow molar mass distributions (Ɖ < 1.5). These well-defined xanthate-terminated PVDF can easily be converted into macromonomers, opening the way to novel fluorinated architectures. [START_REF] Guerre | One-pot synthesis of poly(vinylidene fluoride) methacrylate macromonomers via thia-Michael[END_REF] However, the radical polymerization of VDF is accompanied by a non-negligible amount of chain inversions, head-to-head (HH) VDF additions. These reverse additions are detrimental to the preparation of well-controlled PVDF chains using ITP or RAFT. It has indeed been proven that chain-ends terminated by an inversely added VDF unit accumulate in the reaction medium relatively rapidly. [START_REF] Asandei | Mild Temperature Mn2(CO)10-Photomediated Controlled Radical Polymerization of Vinylidene Fluoride and Synthesis of Well-defined Poly(vinylidene fluoride)block CopolymersJ[END_REF][START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF][START_REF] Boyer | Iodine Transfer Polymerization (ITP) of Vinylidene Fluoride (VDF). Influence of the Defect of VDF Chaining on the Control of ITP[END_REF] These PVDF chains are also believed not to be able to participate into further degenerative transfer. Asandei et al. [START_REF] Asandei | Mild Temperature Mn2(CO)10-Photomediated Controlled Radical Polymerization of Vinylidene Fluoride and Synthesis of Well-defined Poly(vinylidene fluoride)block CopolymersJ[END_REF] reported a method, based on the use of Mn2(CO)10, to reactivate unreactive iodine-capped chainends and to successfully extend PVDF chains under free radical polymerization conditions. In addition, polymerization of VDF in hydrogenated solvents is also affected by undesirable transferto-solvent reactions. This H-abstraction results in losses of CTA and chain-end functionality, and in some cases in the generation of undesired additional polymer chains. [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] Our previous report [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] gave thorough evidence for all these phenomena, but restricted the investigation to short polymer chains (target DPn < 50). The present research explores the limits of the RAFT polymerization of VDF and describes how higher-molar-mass PVDF can be prepared. In the course of these investigations, we were prompted to examine the fate of the PVDF chains that are terminated with an inversely added VDF unit. Our study, combining experimental observations and DFT calculations, proves that these reputedly inactive chains are not "dead", and shows how they are reactivated throughout the polymerization.

EXPERIMENTAL SECTION

Materials

All reagents were used as received unless stated otherwise. dimethyl carbonate (DMC), and laboratory reagent grade hexane (purity > 95%) were purchased from Sigma Aldrich and used as received.

Characterization Nuclear Magnetic Resonance

Nuclear Magnetic Resonance (NMR) spectra were recorded on a Bruker AC 400 instrument.

Deuterated acetone was used as the solvent for all samples. Coupling constants and chemical shifts are given in hertz (Hz) and part per million (ppm), respectively. The experimental conditions for recording 1 H and [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] F NMR spectra were as follows: flip angle 90° (or 30°), acquisition time 4.5 s (or 0.7 s), pulse delay 2 s (or 2 s), number of scans 128 (or 512), and a pulse width of 5 s for [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] F NMR.

Size Exclusion Chromatography

Size exclusion chromatograms (SEC) were recorded using a triple detection GPC from Agilent

Technologies with its corresponding Agilent software, dedicated to multi-detector GPC calculation. The system used two PL1113-6300 ResiPore 300 x 7.5 mm columns with DMF (containing 0.1 wt % of LiCl) as the eluent with a flow rate of 0.8 mL.min -1 and toluene as flow rate marker. The detector suite comprised a PL0390-0605390 LC light scattering detector with 2 diffusion angles (15° and 90°), a PL0390-06034 capillary viscometer, and a 390-LC PL0390-0601 refractive index detector. The entire SEC-HPLC system was thermostated at 35°C. PMMA standards were used for the calibration. The typical sample concentration was 10 mg/mL.

Autoclave

The polymerizations of VDF were performed in a 100 mL Hastelloy Parr autoclave systems (HC 276), equipped with a mechanical Hastelloy stirring system, a rupture disk (3000 PSI), inlet and outlet valves, and a Parr electronic controller to regulate the stirring speed and the heating. Prior to reaction, the autoclave was pressurized with 30 bars of nitrogen to check for leaks. The autoclave was then put under vacuum (20•10 -3 bar) for 30 minutes to remove any trace of oxygen. A degassed solution of solvent, initiator and CTAXA was introduced via a funnel under vacuum. The reactor was then cooled down using a liquid nitrogen bath and VDF was transferred by double weighing (i.e. mass difference before and after filling the autoclave with VDF). After warming up to ambient temperature, the autoclave was heated to the target temperature under mechanical stirring.

Synthesis

RAFT Homopolymerization of Vinylidene Fluoride (VDF) (Scheme 1)

Using the experimental setup described above, a typical polymerization (run 5 in Table 1) of VDF was performed as follows: A solution of Trigonox 121 (158 mg, 6.87 10 -4 mol) and CTAXA

(1.30 g, 6.25 10 -3 mol) in DMC (60 mL), was degassed by N2 bubbling during 30 min. This homogenous solution was introduced into the autoclave using a funnel, VDF gas (19.0 g, 0.297 mol) was transferred in the autoclave at low temperature, and the reactor was gradually heated to 73 °C. The reaction was stopped after 20 h. During the reaction, the pressure increased to a maximum of 25 bars and then decreased to 10 bars after 20 h.

Scheme 1. Schematic representation of the RAFT Polymerization of VDF.

The autoclave was cooled down to room temperature (ca. 20 °C), purged from the residual monomers, and the dimethylcarbonate solvent was removed under vacuum. The crude product was dissolved in 30 mL of warm THF (ca. 40 °C), and left under vigorous stirring for 30 minutes. This polymer solution was then precipitated from 400 mL of chilled hexane. The precipitated polymer (white powder) was filtered through a filter funnel and dried under vacuum (15 reverse addition), -113.09 (CH2-CF2-CF2-CH2-S-), -112.69 (-CH2-CF2-CF2-CH2-S-), -94.79 (-CH2-CH2-CF2-CH2-, TT reverse addition), -107.7 (-CF2-CH3), -93.50 (-CH2-CF2-CH2-CH(CH3)(C=O)-), -92.12 (-CH2-CF2-CH2-CF2H), -91.44 (-CH2-CH2-CF2-CH2-CF2-CH2-CF2-, regular VDF-VDFHT addition), -91.00 (-CH2-CF2-CH2-, regular VDF-VDF HT addition).

Computational details

The computational work was carried out using the Gaussian09 suite of programs. [START_REF] Frisch | Revision D.01[END_REF] The geometry optimizations were performed in the gas phase without any symmetry constraint using the B3PW91 functional in combination with the 6-31G(d,p) basis functions for all atoms. The unrestricted formulation was used for all radicals, yielding negligible spin contamination in all cases. The ZPVE, PV and TS corrections at 298 K were obtained with Gaussian09 from the solution of the nuclear equation using the standard ideal gas and harmonic approximations at T = 298.15 K, which also verified the nature of all optimized geometries as local minima or first order saddle points. A correction of 1.95 kcal/mol was applied to all G values to change the standard state from the gas phase (1 atm) to solution (1 M). 25

Determination of the VDF concentration in DMC at 73 °C under 33.1 bar

The autoclave was put under vacuum (20•10 -3 bar) for 30 minutes to remove any trace of oxygen.

Dimethylcarbonate (37 g, 34.6 mL) was introduced via a funnel under vacuum. The reactor was then cooled down using a liquid nitrogen bath and VDF (21.0 g) was transferred by double weighing. The autoclave was heated up to 73 °C under stirring. After 30 min, a constant pressure of 33.1 bar was recorded. Then the autoclave was cooled down to room temperature (ca. 20 °C), purged from monomers and solvent. Exactly the same volume of lead balls (34.6 mL) was placed in the reactor and the total weight was recorded. 17 g of VDF were then introduced in the reactor following the protocol described above, and the autoclave was heated up to 73 °C until reaching constant pressure. VDF was then progressively removed from the reactor until the pressure reached

RESULTS AND DISCUSSION

Assessment of the apparent chain transfer constant of VDF towards CTAXA

Although RAFT polymerization was shown to efficiently control the polymerization of VDF, [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] there is a clear lack of kinetic data related to this polymerization. We thus assessed the apparent transfer constant of VDF towards O-ethyl-S-(1-methoxycarbonyl)ethyldithiocarbonate (CTAXA, Scheme 1) using O'Brien and Gornick's method (Figure S1). [START_REF] O'brien | Chain Transfer in the Polymerization of Methyl Methacrylate. I. Transfer with Monomer and Thiols. The Mechanism oft he Termination reaction at 60[END_REF] This method gave a CTr(app) value of 49 at 73 °C confirming the efficient transfer of PVDF radicals to the RAFT chain transfer agent used. It is important to note that the transfer constant determination method used here does not take into account the reversibility of the addition onto CTAXA, the real transfer constant is therefore likely higher than 49. [START_REF] Moad | Living Radical Polymerization by the RAFT Process-A Second Update[END_REF] The same method used for the ITP of VDF (at 75°C) reported CTr(app) values for HCF2CF2CH2I and C6F13CH2CF2I of 0.3 and 7.4 respectively. [START_REF] Boyer | Kinetics od the iodine transfer polymerization of vinylidene fluoride[END_REF] These results confirm that CTAXA is a much better CTA than alkyl iodides.

Investigations of the ability of the RAFT process to produce high molar mass PVDF

To test the capacity of the RAFT polymerization of VDF to afford high molar mass PVDF, three polymerizations targeting different DPn (50, 100 and 200) were examined. As reported previously, [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] HH additions were shown to lead to an accumulation of -CF2-CH2-XA-terminated PVDF chains, henceforth noted as PVDFT-XA (where -XA designates the O-ethyl xanthate group:

-SC(S)OCH2CH3). Conversely, -CH2CF2-XA-terminated chains will be noted PVDFH-XA. The evolution of the PVDF chain-ends was thus carefully monitored by 1 H. [START_REF] Guerre | A Journey into the Microstructure of PVDF Made by RAFT[END_REF] for each polymerization (see Table 1, and Figure S2 for the assignment of the 1 H NMR signals), and compared to the corresponding first-order kinetic plots (Figure 1). These kinetic plots (Ln([VDF]0 / [VDF]) vs t),

(Figure 1, bottom) showed four polymerization regimes: (i) an induction period where the polymerization rate is almost negligible, 30 maybe caused by slow initiation of VDF by the R • radical, followed by (ii) polymerization at a relatively slow rate (which increases upon decreasing

[VDF]0/[CTA]0 ratios as is often observed during RAFT polymerizations), [START_REF] Stenzel | Xanthate Mediated Living Polymerization of Vinyl Acetate: A Systematic Variation in MADIX/RAFT Agent Structure[END_REF] (iii) a significant increase in the rate of polymerization, and (iv) a plateau where the polymerization progresses very slowly or even stops. This plateau, which had not been described in earlier reports, occurs at similar VDF conversions (ca. 65 %) and may be caused by the pressure drop inside the reactor below the vapor pressure (and critical pressure) of VDF, which decreases the VDF concentration in solution.

The other 3 phases observed in the RAFT polymerizations of VDF have been described before. However, the sharp increase in the rate of polymerization had been tentatively explained by an increase in the local VDF concentration caused by a change in the solubility of PVDF upon increase of its molar mass. The correlation between the evolution of PVDFH-XA chains and the polymerization kinetics presented here (Figure 1) proves this interpretation to be incorrect, even though PVDF is indeed insoluble in DMC, even at 73 °C. Figure 1 clearly shows that the sharp increase in the rate of polymerization starts exactly when all the PVDF chains have been converted

into PVDFT-XA chains (at t =17.5 h for DPtarget = 50, t = 7.5 h for DPtarget = 100 and t = 8 h for DPtarget = 200). This observation thus rather suggests that the polymerization rate increase is related to a change in the polymerization mechanism. These PVDF chains terminated by an inversely added VDF unit are believed [START_REF] Asandei | Mild Temperature Mn2(CO)10-Photomediated Controlled Radical Polymerization of Vinylidene Fluoride and Synthesis of Well-defined Poly(vinylidene fluoride)block CopolymersJ[END_REF][START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF][START_REF] Boyer | Iodine Transfer Polymerization (ITP) of Vinylidene Fluoride (VDF). Influence of the Defect of VDF Chaining on the Control of ITP[END_REF] to be quasi unreactive vis-à-vis the degenerative chain transfer process at the heart of RAFT or ITP. It follows that the degenerative chain transfer (DT) should stop (or become negligible) when the last PVDFH-XA chain has disappeared. If this were true, at this stage the remaining radicals and the newly formed radicals (from the thermal initiator) should have no choice but to polymerize the remaining VDF via free radical polymerization, thus forming PVDF chains that do not bear either R-or Z-group from the CTA.

This change in the polymerization mechanism from DT to free radical polymerization should also be observed in the plots of the evolutions of molar mass and dispersity versus conversion (Figure 2), and in the evolution of the GPC traces versus conversion (Figure 3). Figure 2 does show a slope change in the molar mass evolution versus conversion after the disappearance of the last PVDFH-XA (at conversion = 45 % for DPtarget = 50; 32 % for DPtarget = 100; and 33 % for DPtarget = 200). However, this slope change is towards a slower increase of molar masses with conversion and it is not accompanied by a significant dispersity increase. If free radical polymerization was indeed taking place instead of RAFT polymerization, higher molar masses and dispersities would be expected (Run 16, Table 1). Such additional high molar mass distributions are not observed, not even as shoulders of the main distribution. These observations (or lack thereof) suggest that, beyond the transition point, the VDF polymerization proceeds neither under control by an RDRP mechanism nor as a pure free radical polymerization. To elucidate this surprising behavior, the propagation, chain inversion and reactivation rates were investigated using DFT calculations (vide infra).

Regardless of this mechanistic transition and its origin, the present study shows that higher molar mass, well-defined PVDF can be prepared using RAFT polymerization. Well-defined PVDF with DP = 41, 69 and 168 were synthesized when DP = 50, 100 and 200 were targeted respectively (Figure 1 and Table 1). However, the study clearly shows that well-controlled PVDF (i.e. with optimum chain-end functionality and containing only negligible quantities of polymer chains formed after the transition point) can only be prepared at relatively low VDF conversion and that the preparation of well-defined high molar mass PVDF can only be done at the detriment of conversion. Indeed, the disappearance of the last PVDFH-XA chain occurs at a conversion around 45 % for DPtarget = 50 and ca. 33 % for DPtarget = 100 or 200.

Study of the chain defects evolution during the RAFT polymerization of VDF

The evolution of the chain defects (head-to-head and tail-to-tail additions) proportion as a function of conversion was also monitored. Since the proportion of defects in PVDF only depends on the reaction temperature, 32 longer chains should display a higher number of intra-chain defects than shorter chains prepared at the same temperature. Indeed, the average number of monomer additions occurring per chain between two degenerative transfers increases with increasing

[VDF]0/[CTA]0 initial ratios. Figure 4 shows the plots of the proportion of intrachain and total HH additions versus conversion. As expected, the amount of intrachain VDF HH additions increases with conversion and with increasing targeted degree of polymerizations (DPtarget). The total amount of HH VDF additions (intrachain + chain-end) stabilizes to identical proportion (ca. 4.1 %) for the three polymerizations examined. This value is in good agreement with the previously reported proportion of defects for polymerizations carried out at similar temperatures. 32-

Evolution of the PVDF chains functionality made by RAFT

Finally, we monitored by NMR the evolution of all chain-end groups for the PVDF made by RAFT polymerization with DPtarget = 100. The results of this investigation are summarized in Figure 5. As observed and reported before, [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF] PVDFH-XA are progressively converted into PVDFT-XA, and the overall end-group functionality decreases due to a slowly increasing amount of transfer to DMC (H-abstraction). Whereas new polymer chains initiated by the resulting DMC radicals were not observed in our previous study, here on the contrary DMC-initiated PVDF chains were detected, but only significantly after all the PVDF chains were converted into PVDFT-XA (Figure S3). These chains give 1 H NMR signals characteristic of the CH3 (singlet at 3.73 ppm) and of the CH2 (triplet at 4.34 ppm) of the CH3-O(C=O)-O-CH2-PVDF α-end-group (Figure S2).

Remarkably, the DMC radicals seemed to add exclusively to the CH2 of VDF. The formation of these DMC-initiated PVDF chains is accompanied by a gradual disappearance of the chain-ends (loss of xanthate group) of the PVDFT-XA chains. These phenomena start with the mechanistic transition mentioned above (Figure 1 and Figure 5). The formation of increasing amounts of -CF2-CH3 end-groups after this transition (Figure S2 and S4) was also detected. These observations suggest that after the mechanism transition, transfer to DMC becomes more prevalent and that the thus formed DMC radicals not only initiate new PVDF chains, but also readily remove the xanthate groups from the PVDFT-XA chains. This transfer reaction, however, seems to be irreversible and to lead to small DMC-xanthate adducts that are removed from the final polymer upon purification by precipitation, as shown previously. [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF]

Computational study a. General considerations, choice of models and computation level.

The above described, unexpected experimental findings raised several mechanistic questions, the most important one being to understand what happens when all PVDF chains are converted into PVDFT-XA. To shed light on this phenomenon, DFT calculations were carried out on model systems. Previous studies have amply demonstrated the value of DFT for rationalizing unexpected phenomena in RDRP, particularly in RAFT, [START_REF] Coote | Ab initio Evidence for Slow Fragmentation, in RAFT Polymerization[END_REF] ATRP, [START_REF] Gillies | A DFT Study of R-X Bond Dissociation Enthalpies of Relevance to the Initiation process of Atom Radical Polymerization[END_REF] OMRP, [START_REF] Maria | Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)2] : Degenerative Transfer versus Reversible Homolytic Cleavage of an Organocobalt(III) Complex[END_REF] and NMP. [START_REF] Abreu | Nitroxide-Mediated Polymerization of Vinyl Chloride at low Temperature[END_REF] Calculations on the polymerization of VDF by the RAFT method have not been previously reported, to the best of our knowledge.

To keep the calculations manageable in terms of computational effort while obtaining results of indicative value, the polymer chains were simplified to an H atom beyond the terminal monomer unit. Thus, the PVDF-CH2CF2 • (or PVDFH • ) and PVDF-CF2CH2 • (or PVDFT • ) chains were modeled by CH3CF2 • and CHF2CH2 • , respectively. In the xanthate group, for the same reason of computational economy, the ethyl substituent was simplified to a methyl group, -SC(S)OCH3. These structural modifications are not expected to introduce any major electronic change (polarity, homolytic strength) or steric effect in the bonds that are involved in the computed processes.

It must be emphasized that the main objective of the computational approach is not that of quantitatively reproducing observed data, but rather to provide insight into the occurring chemical processes. The values of DFT-calculated energy changes associated to chemical processes rarely deviate by less than a couple of kcal/mol from the experimentally available ones. DFT calculations provide better information when comparing data for closely related systems, because energy trends are reproduced more faithfully than absolute values. For this reason, we have not made a major effort in testing a variety of different functionals and basis sets in order to choose the most suitable method for the systems of interest. The theory level selected for our study (B3PW91/6-31G(d,p)) is commonly used for high-level calculations on organic and organometallic systems. However, we have initially benchmarked this method against available experimental data. All the results obtained from the computational study (energy, enthalpy and Gibbs energy at 298.15 K, Gibbs energy extrapolated to 343.5 K, Cartesian coordinates and views of the optimized geometries) are collected in tabular form in the SI.

b. Benchmarking on VDF free radical propagation.

The values of kp/kt 1/2 (0.14 L 1/2 mol -1/2 s -1/2 in 1,1,1,3,3-pentafluorobutane at 74 °C) [START_REF] Ameduri | From Vinylidene Fluoride (VDF) to the Applications of VDF-Containing Polymers and Copolymers: Recent Developments and Future Trends[END_REF][START_REF] Boyer | Kinetics od the iodine transfer polymerization of vinylidene fluoride[END_REF] and kp (19,400 L mol -1 s -1 at 60 °C and 1000 bar in supercritical CO2) [START_REF] Siegmann | PropagationRate Coefficient for Vinylidene Fluoride Homopolymerizations[END_REF] for the VDF polymerization have been reported, but individual kp and kt values in solution are not available to the best of our knowledge. Notably, it would have been interesting to compare the calculated propagation Gibbs energy barrier (ΔGp ‡ ) with that derived from kp using the Eyring equation. However, it is experimentally known that the most probable chain carrier is PVDFH • and that the most probable monomer addition mode is HT, conserving the nature of this reactive chain end, while the HH diads are present only at a level of 3.5-6 % [START_REF] Stenzel | Xanthate Mediated Living Polymerization of Vinyl Acetate: A Systematic Variation in MADIX/RAFT Agent Structure[END_REF][START_REF] Bachmann | An infrared study of phase-III Poly(vinylidene Fluoride)[END_REF] . We have calculated the profiles for all four possible types of radical addition to monomer, as shown in Figure 6. Figure 6a illustrates the energy profiles for the PVDFH • model radical additions to VDF in both HT and HH modes, while Figure 6b shows the profiles for the TH and TT additions of the PVDFT • model radical. Of all four possible addition modes, the lowest barrier is indeed, as known from the experimental evidence, that of the HT addition (11.9 kcal/mol), while the barrier of the alternative HH addition is ca. 2.2 kcal/mol higher (14.1 kcal/mol). This Δ(ΔG) allows the prediction of a 2.4 % HH error probability at 298.15 K or 3.2 % after extrapolation of Δ(ΔG) to the polymerization temperature (73 °C). These values are quite consistent with the experiment. The energy difference between the two isomeric HT and HH transition states is carried over to the isomeric products. This difference results from the different C-C bond strengths of the CF2-CH2 and the CF2-CF2 bonds formed by HT and HH coupling, respectively, and/or from the different stabilization of the resulting CH2 • and CF2 • radicals. Concerning the PVDFT • model radical addition, the TT mode is more favored (12.3 kcal/mol) than the TH mode (16.3 kcal/mol). This is again in line with the experimental evidence, since the HT/HH/TT triad is, by far, much more frequent than the HT/HH/TH triad. This latter triad has indeed never been detected by NMR spectroscopy. Hence, generation of an inverted radical chain end is more likely followed by a TT addition (98.8 % probability according to the calculated barriers) to regenerate the preferred PVDFH • chain end. Similar results were also previously obtained, in reference to an uncontrolled VDF telomerization process, at a slightly different computational level (using the B3LYP functional) and with use of CF3 instead of H as chain end. [START_REF] Laflamme | Characterization of the telomerization reaction path for vinylidene fluoride with CCl3 radicals[END_REF] The E barriers reported here with the use of BP86 are slightly and systematically smaller (by 0.5-1.5 kcal/mol) than those obtained with B3LYP, but their relative order is the same for both investigations: ΔE ‡ p,TT < ΔE ‡ p,HT < ΔE ‡ p,HH < ΔE ‡ p,TH.

From our calculated ΔG ‡ p,HT, a kp,HT value of 1.1•10 4 L mol -1 s -1 can be derived at 298.15, which becomes 2.3•10 4 L mol -1 s -1 after extrapolation of ΔG ‡ p,HT to the polymerization temperature. This value is actually quite close to that reported by Beueurmann et al. (1.94•10 4 L mol -1 s -1 ) in scCO2. [START_REF] Siegmann | PropagationRate Coefficient for Vinylidene Fluoride Homopolymerizations[END_REF] It also appears reasonable when compared for instance with those reported for tetrafluoroethylene (7.4•10 3 L mol -1 s -1 at 40 °C) 41 and ethylene (470 L mol -1 s -1 at 83 °C in benzene). [START_REF] Litia | The kinetic of radical polymerization of ethylene. II. Determination of the Kinetic coefficients[END_REF] This agreement encouraged us to continue the computational exploration using this level of theory.

c. Bond strengths.

The calculated homolytic bond dissociation enthalpy (BDE, ΔH°298.15), in kcal/mol, is 54.5 for CH3CF2-XA and 60.7 for CHF2CH2-XA, confirming the common perception that the bond is stronger in the T-terminated dormant species obtained after an inverted (HH) monomer addition.

The difference between the BDE of the T-and H-terminated dormant chain models almost exactly matches the energy difference of the isomeric radicals: the calculated standard enthalpy of CHF2CH2 • is 6.6 kcal/mol higher than that of CH3CF2 • . Consequently, the isomeric CTA models are essentially isoenergetic (CH3CF2-XA is more stable than CHF2CH2-XA by 0.8 kcal/mol, see Figure 7).

Discussion

The lower barrier for the PVDFH • degenerative chain transfer (9.0 kcal/mol) relative to HT propagation (11.9 kcal/mol, cf. Figures 8 and6) agrees with the experimental observation of efficient control in degenerate transfer polymerization, so long as the growing chain remains PVDFH • . The calculated rate constant ratio kCT/kp is 1.3•10 2 at 298.15 K according to the Eyring equation. After extrapolation of the two Gibbs energy barriers to the polymerization temperature (73 °C), this ratio is reduced to 70 (kCT increases less than kp at higher temperatures because the chain transfer activation barrier is characterized by a greater entropy decrease relative to the propagation activation barrier). Actually, the parameters to be compared are the rates, not the rate constants. Since vCT = kCT[R • ][CTA] and vp = kp[R • ][M], the rate ratio depends not only on the rate constant ratio but also on the concentration ratio: vCT/vp = (kCT/kp)•([CTA]/[M]). A problem in VDF polymerization is that the system is biphasic. The gaseous monomer is only partly dissolved in the polymerization solvent (DMC) where the CTA and macroradical react, and the monomer solubility in this condensed phase was not previously reported. Therefore, we have measured the concentration under conditions close to those of the polymerization (DMC at 70 °C and 33.1 bars, see details in the Experimental Section) and found it to be 5.2 mol/L. Thus, although this is a significant concentration, it is unlikely that all the monomer is completely dissolved in the reaction medium, hence [CTA]/[M] > 0.01 for a target DP of 100. The calculated rate ratio would predict comparable rates of chain transfer and propagation, in agreement with a reasonably controlled system.

The major point of interest in this study is the question of the PVDFT-XA reactivation after an inverted monomer addition. This can in principle occur by exchange with either the more abundant PVDFH • radicals or with the PVDFT • radicals. The former has a calculated barrier of 15.1 kcal/mol for the model system. This does not mean that this reactivation cannot occur because this barrier yields a rate constant of 54 M -1 s -1 under standard conditions, by application of the Eyring relationship. However, after the endothermic non-degenerative head-tail chain transfer process (ΔG°CT,HT = 6.1 kcal/mol at 298.15 K, KCT,HT = 3.4•10 -5 from the van't Hoff equation) the resulting free tail radicals can be back-trapped with a lower barrier (9.0 kcal/mol) than the subsequent addition to monomer (ΔG ‡ p,TT = 12.3 kcal/mol for the more favorable TT addition) needed to reenter the regular PVDFH • chain growth process. The Gibbs energy span is thus (ΔG°CT,HT + ΔG ‡ p,TT = 18.4 kcal/mol), which can be extrapolated to 20.1 kcal/mol at 343.15 K, leading to the prediction of an effective activation rate constant kExch,TH (corresponding to Kkp,TT) for the PVDFT-XA chains in VDF homopolymerization of 1.2 M -1 s -1 . Thus, in contrast to what is commonly believed, PVDFT-XA chains cannot be considered dead, because the rate constant of their reactivation is significant. Using the estimated [VDF] given above (5.2 M), for instance, a pseudo-first order decay of ≈ 6 s -1 can be calculated for a half-life of ≈ 0.1 s, which naturally increases as the [VDF] is lowered at higher conversions. Although not negligible, this reactivation rate is nevertheless slower by 4 orders of magnitude than the reactivation of the PVDFH-XA chains by the degenerative transfer and also slower than the rate at which the PVDFH • chains are converted to PVDFT • by the head-to-head propagation (Figure 6): kp,HH = 6.1•10 2 M -1 s -1 ; kExch,TH/kp,HH = 2•10 -3 . This is in agreement with the accumulation of the PVDFT-XA macroCTA as conversion increases.

This relatively slow reactivation would also explain the loss of control observed experimentally when all xanthate-terminated PVDF chains have been converted into PVDFT-XA. However, as alluded to above, the PVDFT-XA dormant chains may also be reactivated by the less numerous PVDFT • radicals by a degenerative tail-tail process with a lower activation barrier (9.5 kcal/mol), see Figure 8, for which the estimated rate constant at the polymerization temperature is kExch,TT = Given that the monomer concentration is greater than the PVDFT-XA concentration, even when the monomer is partially consumed and 100% of the macroCTA is in the PVDFT-XA form, this calculation predicts a much faster propagation than the rate of radical exchange, thus loss of control.

Scheme 2 summarizes all the findings of this study. The VDF RAFT polymerization in DMC gradually leads to the accumulation of PVDFT-XA macroCTA (equations ( 4) and ( 7), Scheme 2) and undergoes a mechanistic transition from a controlled DT mechanism to an uncontrolled DT mechanism (a DT mechanism too slow to provide control) when no more PVDFH-XA macroCTA remains. During the uncontrolled phase of the polymerization, DT still occurs, but only between the PVDFT-XA macroCTA and the PVDFT • radicals, which are present at low concentration (equation ( 8), Scheme 2). The non-DT exchange (equation ( 7), Scheme 2), although theoretically non negligible, likely does not occur due to a much lower rate compared to the DT exchange between HH-terminated PVDF chains. In addition, during that phase, transfer to DMC (equations ( 9) and ( 10), Scheme 2) becomes more prominent and reactions such as chain initiation by DMC radical (equation ( 13), Scheme 2) or irreversible exchange between DMC radical and PVDF-XA (equations ( 11) and ( 12), Scheme 2), which were almost insignificant during the controlled phase of the polymerization, occur more frequently with a growing impact on the PVDF functionality.

CONCLUSION

This study demonstrates that RAFT polymerization of VDF can be used to efficiently prepare well-defined PVDF with high end-group functionality and over a relatively large range of molar masses in spite of the unavoidable accumulation of less reactive PVDFT-XA chains formed by head-to-head VDF addition. However, well-defined high-molar-mass PVDF can only be prepared at relatively low conversions (< ca. 30 %). Indeed, this work shows that controlled polymerization, via a DT mechanism with a sufficiently fast radical exchange, only occurs while regioregularly ended PVDFH-XA chains are still present. When all the xanthate-terminated PVDF chains have been converted into PVDFT-XA chains, the degenerative transfer mechanism slows down dramatically, although it does not stop completely, resulting in a significant loss of polymerization controllability, as the xanthate group exchange rate becomes slower than the propagation rate. DFT calculations suggest that, contrarily to common belief, the PVDFT-XA dormant chains are not dead and can be reactivated at a non-negligible rate. They further indicate that this reactivation occurs primarily via degenerative transfer with the PVDFT • radicals, in spite of their lower concentrations.

In addition, the present study suggests that chain transfer to the solvent, DMC, may be responsible for additional detrimental reactions after the full conversion of all the PVDF active chains into the less reactive PVDFT-XA chains. The resulting DMC radicals seem to both readily initiate new PVDF chains and exchange with the PVDFT-XA chains, thus decreasing the overall end-group functionality of the final PVDF. In summary, the RAFT polymerization of VDF proceeds through two consecutive regimes marked by a sharp kinetic transition. In the first regime both PVDFH-XA and PVDFT-XA coexist and polymerize at different rates. PVDFH-XA chains are involved in fast DT mechanism which leads to PVDF with low dispersity, while the PVDFT-XA chains polymerize

33. 1

 1 bars. The reactor was then weighed again. The mass of VDF contained in the free volume of the reactor at P = 33.1 bars and T = 73 °C was calculated to be 9.5 g. The solubility of VDF in DMC at P = 33.1 bars and T = 73 °C was obtained as (21 -9.5) / (64.02 x 34.6 10 -3 ) = 5.2 mol L - 1 .

  Reactions conditions: Chain Transfer Agent (CTA) = CTAXA, Initiator (I) = Trigonox 121, [I]/[CTA] = 0.1 (runs 1-15) [I]/[VDF] = 0.005 (run 16), T = 73 °C, Solvent = DMC. a Determined gravimetrically. b Determined by 1 H NMR using equations S1 to S6. c Calculated using yield as conversion. d Calculated from DP(NMR). e Determined by SEC. f Point where all PVDFH-XA have disappeared. n.d stands for not determined. n.a. stands for not applicable.

Figure 1 .

 1 Figure 1. Correlation between the evolution vs time of the proportion of PVDFH-XA chains (-

Figure 2 .

 2 Figure 2. Correlation between the evolution of the proportion of PVDFH-XA chains (top), of the

Figure 3 .

 3 Figure 3. DMF GPC chromatograms of PVDF prepared by RAFT and targeting three degrees of

Figure 4 .

 4 Figure 4. Evolution of the proportion of intrachain (bottom) and total (top) HH VDF additions vs

Figure 5 .

 5 Figure 5. Evolution of the PVDF chains end-group functionality (-CH2-CF2-XA, -CF2-CH2-XA)

Figure 6 .

 6 Figure 6. Energy profiles for (a) the HT and HH additions of the PVDFH • model radical, and (b) the TH and TT additions of the PVDFT • model radical to VDF. The reported values are ΔG°298.15

Figure 7 .

 7 Figure 7. Relative energies and views of the optimized geometries of the isomeric CHF2CH2 • and CH3CF2 • radicals, the (MeO)C(S)S • radical, and the corresponding adducts. The values are the

Figure 8 .

 8 Figure 8. Energy profiles for the degenerative (H/H and T/T) and non-degenerative (H/T) radical

4. 6 •

 6 10 5 M -1 s -1 . The relative rates of TT degenerative exchange and TH non-degenerative exchange can be calculated as vExch,TT/vExch,HT = (kExch,TT/kExch,HT) ([PVDFT • ]/[PVDFH • ]), where the kExch,TT/kExch,HT rate constant ratio is 3.8•10 5 and [PVDFT • ]/[PVDFH • ] can be estimated as 0.04, considering a proportion of HH addition of 4 % at 343 K. Thus, this calculation yields vExch,TT/vExch,HT = 1.5•10 4 . Counterintuitively, and in spite of the non-negligible rate constant of the non-degenerate reactivation of PVDFT-XA by PVDFH • radicals, the computational investigation suggests that the PVDFT-XA chains are actually reactivated much faster by the minority PVDFT • radicals. The controlled nature of the VDF polymerization can be appreciated from the comparison of the rate of propagation and the rate of exchange. The rate of propagation would still be dominated by the HT addition, after liberation of the dormant PVDFT chain which is converted to a PVDFH • radical by the first TT addition: vp,HT = (1.9•10 4 M -1 s -1 )[PVDFH • ][VDF]. The faster rate of exchange is vExch,TT = (4.6•10 5 M -1 s -1 )[PVDFT • ][PVDFT-XA]. By introducing [PVDFT • ] = 0.04[PVDFH • ], we derive vp,HT/vExch,TT ~ 1.0•[VDF]/[PVDFT-XA].

  [START_REF] Guerre | Deeper Insight into the MADIX polymerization of Vinylidene Fluoride[END_REF]

Table 1 .

 1 Experimental

Conditions and Results for the RAFT polymerization of VDF in the presence of CTAXA.

The authors thank Arkema (Pierre Bénite, France) for providing VDF, and the French Ministry of Science and Technology for the Ph.D. grant attributed to MG.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: vincent.ladmiral@enscm.fr; rinaldo.poli@lcc-toulouse.fr.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT


Weili Wang

Jean-Claude Daran

Rinaldo Poli

Dominique Agustin
email: dominique.agustin@iut-tlse3.fr.

OH-Substituted Tridentate ONO Schiff base ligands and related molybdenum(VI) complexes for solvent-free (ep)oxidation catalysis with TBHP as oxidant

Keywords: Molybdenum, solvent-free, epoxidation, DFT, X-ray structures

Molybdenum complexes with ONO tridentate Schiff base ligands based on salicylideneaminophenolato (SAP) backbone with OH substitution on the salicyl arene ring have been synthesized and characterized. The molecular structures of the full series of OH substituted molybdenum complexes were determined by X-ray crystallography as monomers stabilized by one solvent molecule, [MoO2L(D)]. All dimeric complexes [MoO2L]2 were tested for the epoxidation of cyclooctene and cyclohexene under organic solvent-free conditions using aqueous TBHP as oxidant. The position of the OH on the SAP-modified ligand influenced the catalytic activity of the respective complexes. DFT calculations for the catalytic cycle yield energy spans in agreement with the experimentally observed activity trend.

Introduction

Epoxides are useful reagents for different applications in fine chemistry and polymer chemistry [1]. They are generally obtained through classical organic reactions using organic oxidants (metachloroperbenzoic acid or m-CPBA, CH3CO3H or PhIO), with the use of organic solvents (often chlorinated) or ionic liquids [2]. The organic by-products produced by these oxidants and the solvents can be considered as waste, increasing the E factor [START_REF] Sheldon | [END_REF]4]. In addition, certain chlorinated solvents are potentially toxic and their use in several industrial sectors is not recommended [5]. Therefore, using a lower amount of solvent or a safer one or, even better, no organic solvent at all represents a challenging benefit for green chemistry. Catalysis brings several advantages to green chemistry in general, and in the particular case of epoxidation processes also through the possibility to replace the above mentioned oxidants with others having lower environmental impact (e.g. tertbutylhydroperoxide or TBHP, H2O2, O2). There are, however, economic and safety issues about the amount (cost) of the needed catalyst and its toxicity. Molybdenum is the most commonly used transition metal in academic and industrial applications of epoxidation catalysis [6], in combination with many different supporting ligands. We and others have been interested in the use of tridentate ONO and ONS Schiff base ligands in combination with molybdenum in organic solvents [7] and without solvent [8,9]. We have recently shown high catalytic efficiencies at relatively low metal loadings for different types of molybdenum and vanadium complexes [8,10 ] as well as for polyoxometalates [11] for the epoxidation of cyclooctene as a model substrate and of bio-sourced compounds [12] using aqueous TBHP as oxidant and no added organic solvent. Several advantages can be pointed out for these processes. The by-product and waste product (tBuOH and water) are nontoxic and can be recycled, tBuOH being also used as additive in the gasoline industry as well as for the synthesis of methyl tert-butyl ether (MTBE) [ 13 ]. The salicylideneaminophenolato (SAP) and salicylideneaminothiophenolato (SATP) ligands have yielded good results in terms of reaction efficiency and catalyst stability [8] under these organic solvent-free conditions. Using DFT calculations, we have also established that the intimate mechanism of the catalytic cycle involves activation of TBHP by coordination to the catalytic metal without proton transfer followed by oxygen transfer to the olefin in a way that closely resembles the mechanism proposed by Bartlett for the stoichiometric epoxidation with peracids [14]. Slight improvements of these results could be achieved by tuning the SAP ligand through the addition of electron donor (NEt2) and/or acceptors (NO2) substituents on the ligand backbone [15]. Therefore, we have wondered whether the introduction of simple OH groups on the SAP ligand backbone could also positively affect the catalyst performance.

The synthesis and characterization of the modified systems as well as the catalytic test on different substrates will be discussed herein.

Results and discussion

Synthesis and characterization

Scheme 1. Access routes to the H2L pro-ligands and molybdenum complexes.

Synthesis

Reaction of the previously reported [16,17,18,19] H2L 1-3 molecules with [MoO2(acac)2] in refluxing alcohol gave, by analogy with the parent H2SAP ligand [8a, 20] and with other related systems [ 20 ], either mononuclear alcohol adducts, [MoO2L(ROH)], or oligonuclear [MoO2L]n depending on the nature of the ligands and on the reaction conditions (see Scheme 1). The reaction of H2L 1 in EtOH and H2L 2 in MeOH yielded the new mononuclear complexes [MoO2L 1 (EtOH)] and [MoO2L 2 (MeOH)], respectively, which could be transformed to the alcohol free systems upon drying under vacuum at high temperatures. The recovered product of the reaction of H2L 3 and [MoO2(acac)2] was dried directly to yield [MoO2L 3 ]n.

IR Characterizations

The most relevant IR spectroscopic properties of the new complexes are collected in Table 1 and compared with those of the previously reported ligands and [MoO2(SAP)]2 complex. The key feature in the IR spectrum of the H2SAP and H2L 1-3 molecules is the C=N stretching vibration around 1600-1630 cm -1 . Bonding of L 2-to the {MoO2 2+ } fragment is accompanied by a small red shift of this band to the 1600-1610 cm -1 region. The vibration pattern of the {MoO2 2+ } fragment observed between 750 and 1100 cm -1 gives additional information about the form of the isolated complex, particularly those associated to the terminal Mo=O bonds that appear in the 850-950 cm -1 region. For the solvent stabilized monomeric species of general formula [MoO2L(D)], two sharp bands are observed around 910 and 940 cm -1 , whereas the dimeric or oligomeric [MoO2L]n feature only one Mo=O vibration in this region, plus another band corresponding to a Mo-O-Mo vibration at lower frequency (760-820 cm -1 ) [21].

Table 1. Selected IR vibrations (ν, cm -1 ) of the H2SAP, H2L 1-3 [17] 1618 --H2L 2 [18] 1603 --H2L

, [MoO2L n (ROH)] (n = 1,2), [MoO2(SAP)]2 and [MoO2L 1-3 ]2 compounds. C=N Mo=O Mo-O-Mo Pro-ligands H2SAP 1633 - - H2L 1

NMR Characterizations

The 1 H NMR spectrum of the H2L 1-3 molecules in DMSO-d 6 shows distinct resonances for the three OH groups of the substituted salicylidene ring (see SI), indicating their slow mutual exchange.

Complexation to molybdenum leads to the disappearance of two of the three OH resonances in each case and to a weak shift of the imine resonance. Note that, since all NMR measurements of the dioxomolybdenum complexes were carried out in DMSO-d 6 , the observed spectrum of each product, whether mononuclear alcohol adduct or alcohol-free oligomer, corresponds to the [MoO2L(DMSO)] adduct.

TG analyses

The thermogravimetric analyses of the isolated [MoO2L 1-2 (ROH)] complexes show loss of the alcohol molecule below 200°C, followed by loss of the Schiff base ligand at higher temperatures (up to 600°C). The second process is also observed for the alcohol free oligomers, leaving MoO3 as a residue as previously observed with similar ligands [22]. The relevant data are collected in

X-ray structures

All L n systems (n = 1, 2, 3) have given crystals suitable for a structural determination by X-ray diffraction in the form of solvent-stabilized mononuclear complexes. For n = 1 and 3, crystals were obtained from DMSO as [MoO2L n (DMSO)], whereas for ligand L 2 the crystal was obtained from an acetonitrile/water mixture as the water adduct, [MoO2L 2 (H2O)]. The complex [MoO2L 2 (DMSO)] was also crystallized but its structure could not be satisfactorily refined because of a severe disorder problem. The common feature of all these complexes is a pseudo-octahedral geometry around the molybdenum center and a cis arrangement for the {MoO2} 2+ fragment, which is surrounded by an ONO tridentate ligand in a mer arrangement and by a sixth donor molecule D (D being DMSO or H2O), in accordance with the structures of similar compounds [15, 20h, 22, 23]. Views of the molecules are shown in Figure 1. The [MoO2L 1 (DMSO)] complex crystallizes as an H-bonded dimer, the interaction being established between the free OH group of the ligand (O61 atom) of one molecule as a proton donor with one of the oxido ligands (O2 atom) of the second one as a proton acceptor, see Figure 2 and relevant bonding parameters in the supporting information (Table S1). These dimers are further linked together in a 3-dimensional network via additional H-bonds involving the water molecule (O1) of one complex as a proton donor and the OH of a complex in another dimer (O51) as a proton acceptor (Figure 4 and Table S3 in SI). 3. Interatomic distances around the molybdenum atom are very similar for all complexes. Some slight differences occur for the angles. The cis conformation of the MoO2 moiety is observed with O=Mo=O angles of 103.99° and 103.88° for L 1 and L 3 respectively, whereas a slightly wider angle of 105.2(1)° is observed for the L 2 complex (see) Table 3 and in accordance with similar structures [21e,g,h, 24]. The L ligands deviate slightly from coplanarity, with very similar dihedral angles for the L 1 (10.6°) and L 3 (10.4°) complexes, and even smaller for the L 2 complex (5.5°).

Table 3. Relevant bond lengths around the molybdenum. cyclooctene by aqueous TBHP (Scheme 2) at 80°C without added organic solvent. Given the high concentration (70% w/w) of the aqueous TBHP and the higher affinity of TBHP for cyclooctene, the reaction mixture consists of a majority of organic phase containing cyclooctene and TBHP and a small proportion of essentially neat water. The complexes are only slightly soluble in cyclooctene at room temperature but appear to dissolve completely in the organic phase after addition of TBHP at the reaction temperature. The organic phase is colored during the entire reaction procedure, whereas the water phase is colorless or very faintly colored. Standard catalytic tests were carried out using two equivalents of TBHP per cyclooctene and a 0.5% Mo/substrate ratio.

[MoO2L 1 (DMSO)] [MoO2L 2 (H2O)] [MoO2L 3 (DMSO)] Relevant Bonds (Å) Mo=O(2) 1.714(2) 1.731(3) 1.7082(15) Mo=O(5) 1.698(2) 1.696(3) 1.7038(15) Mo-N 2.277(2) 2.252(3) 2.2752(15) Mo-O(3) 1.957(2) 1.948(3) 1.9673(13) Mo-O(4) 1.943(2) 1.944(3) 1.9302(14) Mo-O(1) 2.285(2) 2.325(3) 2.289(3) Relevant angles (°) O(2)-Mo-O(
Scheme 2. Epoxidation of cyclooctene.

Introduction of a free OH group in the SAP system produced the effects illustrated in Figure 5.

The selectivity and activity data are detailed in Table 4. While the OH addition at the ortho (L 1 ) and para (L 3 ) positions of the salicyl ring (particularly the former) increased the catalytic activity relative to the reference SAP catalyst, substitution at the meta (L 2 ) position had essentially no effect. The selectivity towards cycloctene oxide is high (>85%) with all catalysts. In the case of the L 2 ligand, there is no significant difference between the ethanol-stabilized mononuclear and the oligomeric systems, suggesting that the two pre-catalysts may generate the same active species. This result is in agreement with our previous mechanistic work on the non-substituted SAP system, where it was concluded that the active catalyst is the 5-coordinate mononuclear [MoO2(SAP)] species, with growing concentrations of added MeOH having only minor negative effect on the activity [8a]. The observed differences could be intuitively attributed to either a different catalyst solubility or at an electronic effect of the OH substituent. However, all complexes appeared totally soluble at the reaction temperature. The major acceleration when the free OH group lies in ortho position, followed by the OH substitution at the para position, seems to indicate a favorable electronic effect and suggests a transmission of this effect by a combination of the π delocalization and inductive mechanisms.

[MoO2(SAP)]2 (), [MoO2L 1 ]2 (),[MoO2L 2 ]2 (),[MoO2L 2 (EtOH)] () and [MoO2L 3 ]2 ( ).
For the reaction conditions, see Table 4. The catalytic runs were also carried out with different catalyst loadings down to 0.1% for [MoO2(SAP)]2 and 0.05% for [MoO2L 1 ]2 (Figure 6). The selectivity remained approximately constant in the 92-95% range (see details in Table S5 in the SI) upon catalyst dilution for the experiments run with [MoO2L1]2, whereas it slightly deteriorated from 93% at 1% catalyst to 73% at 0.1% catalyst for the experiments run with [MoO2(SAP)]2. The observed conversion after 4 h was approximately the same in the high catalyst concentration range and slightly decreased only when using very low loadings (< 0.2%), while the TOF correspondingly increased. This phenomenon is not expected for a stable and fully soluble catalyst, for which the rate is expected to be first order in catalyst and the TOF should not dependent on the catalyst concentration. The constant conversion in the higher catalyst concentration range suggests a saturation effect, which seems however contradicted by the visual evidence of full catalyst dissolution in the reaction medium. a negative effect of water on the catalysis (for full details, see Table S6 in SI). Therefore, the dramatic TOF increase observed in Figure 6 at lower catalyst loading (greater water/[Mo] ratios) cannot be attributed to a chemical activation of the catalyst by water.

Epoxidation of cyclohexene

Cyclooctene oxide is quite robust compared to cyclohexene oxide relative to ring opening in the presence of water. Thus, cyclohexene oxidation by TBHP was also investigated with same catalysts.

The catalytic procedure was carried out under the same conditions as for cyclooctene (at 80°C with 2 equivalent of aqueous TBHP) except for the use of a catalyst loading of only 0.1% mol vs. substrate, the catalysts being less soluble in this reaction mixture. The kinetic profiles for the substrate conversion are shown in Figure 7 and the main activity and selectivity results are collected in Table 5.

Three main products detected within the organic phase for the oxidation of cyclohexene are ]2, the catalyst is totally dissolved, giving similar bright orange solutions. The [MoO2L 3 ]2 catalyst dissolves slowly but is eventually fully dissolved in the reaction mixture. Qualitatively, the trend of the dissolution rate is

[MoO2L 1 ]2 ~ [MoO2(SAP)]2 > [MoO2L 2 ]2 > [MoO2L 3 ]2.
Like for the epoxidation of cyclooctene, the highest activity is for the epoxidation of cyclohexene is exhibited by [MoO2L 1 ]2 (see Figure 7). The slower conversion observed with the other catalysts, especially at the beginning of the reaction, might be due to the slow catalyst dissolution in the organic phase. The activity trend among the OH substituted compounds is the same as for the cyclooctene. The catalysts affording the highest cyclohexene conversion lead to a greater extent of ring opening. Indeed, [MoO2L 1 ]2 and [MoO2L 3 ]2 give less CHO than CHD, while [MoO2L 2 ]2 and [MoO2(SAP)]2 give more CHO than CHD.

DFT calculations

A cycle for the epoxidation process catalyzed by the [MoO2L] system with L = tridentate Schiff base has already been proposed on the basis of DFT calculations for the SAP system, using ethylene as model olefin [8a]. On the basis of this cycle, which is recalled in Scheme 4, the resting state is the TBHP adduct [MoO2L(TBHP)] (I) and the rate-determining transition state (TS) corresponds to transfer of the TBHP O α -atom to the outer-sphere olefin, for an energy span of 22.5 kcal/mol on the ΔH scale. We have recently communicated the greater activity of the SATP system (L 5 ) and have

shown by DFT calculations that the same proposed cycle gives a lower energy span of 22.3 kcal/mol [8b]. We have also recently reported a beneficial effect of an electron-withdrawing substituent on the aminophenol ring (NO2) and a negative effect of an electron donating substituent on the salicyl ring (NEt2) on the catalytic activity of modified SAP catalysts; the DFT calculations, based again on the same mechanism, provided variation of the energy spans in qualitative agreement with the experimental results (21.0 kcal/mol for the former, 23.8 kcal/mol for the latter) [15]. We have therefore wished to verify whether this mechanism is also able to reconcile the observed activity trend of the [MoO2L 1-3 ] systems. All the key structures involved in the catalytic cycle have been recalculated for the L 1 , L 2 and L 3 systems; optimized geometries and energies are reported in the SI (Tables S7 andS8). Relative to the 5-coordinate [MoO2L], TBHP coordination gives very similar stabilization ranging from 8.3 to 8.5 kcal/mol on the ΔH scale for all systems (SAP, -8.4; L 1 , -8.5; L 2 , -8.5; L 3 , -8.3). The relative TS enthalpy (G) is also very similar for all systems (SAP, 14.1; L 1 , 13.9; L 2 , 14.4; L 3 , 14.3). The result for the enthalpy span, ESH = ΔH(TS)-ΔH(I), is roughly consistent with the experimentally observed activity differences when cyclooctene is the substrate: ESH(L 1 ) (22.4) < ESH(SAP) (22.5) < ESH(L 3 ) (22.6) < ES(L 2 )H (22.9).

Conclusions

The effect of OH substituents at different positions of the salicyl ring of the SAP ligand has been evaluated for the epoxidation of cyclooctene and cyclohexene by aqueous TBHP catalyzed by dimeric molybdenum complexes [MoO2L]2 under organic solvent-free conditions. The position of the OH substituent and the nature of the substrate have a slight influence on the catalytic activity. DFT calculations on the catalytic cycle give a trend of energy spans in accordance with the experimental results. Further ligand tuning remains of interest for several purposes. The optimization of catalytic activity and selectivity are to be improved, as well suitable groups for grafting on solid supports and heterogeneization of the most stable and active complexes in order to allow efficient catalyst recovery.

Experimental part

Materials and methods

All preparations were carried out in air. Water was distilled before use. Organic solvents (ethanol, methanol, diethylether and acetrontrile) and organic compounds (TBHP (70% in water, ACROS), cyclooctene (98%, Aldrich), cyclooctene oxide (98%, Aldrich), cyclohexene (purity, supplier), cyclohexene oxide (purity, supplier), trans-cyclohexanediol (purity, supplier), 2-cyclohexen-1-ol (purity, supplier), 2-aminophenol (99%, Aldrich), 2,5-dihydroxybenzaldehyde (98%, Aldrich), 2,3dihydroxybenzaldehyde (97%, Aldrich), 2,4-dihydroxybenzaldehyde (98%, Aldrich), salicylaldehyde (98%, Aldrich) were used as received without any purification. The H2SAP and H2L 1-3 compounds were synthesized and characterized according to literature (see SI) [8a,17-19]]. [MoO2(acac)2] was synthesized as previously described [26] and used freshly prepared. Complex [MoO2(SAP)]2 was prepared as described earlier [8a]. The thermogravimetric analyses were performed on a SETARAM TGA 92-16.18 thermal analyzer. The sample was placed into a nickel/platinum alloy crucible and heated at 0.83 °C s -1 in a reconstituted air flow from 15°C to 700°C. An empty crucible was used as a reference. Infrared spectra were recorded in KBr matrices at room temperature with a Mattson Genesis II FTIR spectrometer. 1 H spectra were recorded at 200.1 MHz on a Bruker Avance DPX-200 spectrometer. Catalytic reactions were followed by gas chromatography on an Agilent 6890A chromatograph equipped with FID detector, a HP5-MS capillary column (0.30 m x 0.25 mm x 0.25 m)

and automatic sampling, or on a Fisons GC 8000 chromatograph equipped with FID detector and with a SPB-5 capillary column (30 m x 0.32 mm x 0.25 m). The GC parameters were quantified with authentic samples of the reactants and products. The conversion of cis-cyclooctene, cyclohexene and the formation of cyclooctene oxide, cyclohexene oxide and cyclohexanediol were calculated from calibration curves (r 2 = 0.999) relatively to an internal standard. [27]) and refined by least-squares procedures on F 2 using SHELXL-2013 [28].

In both compounds, the DMSO molecule linked to the molybdenum is disordered over two positions related by a pseudo mirror plane. This disorder has been modelised using the tools available in SHELXL-2013. The two disordered fragments were included in two different parts. Then the occupancy factor for the two sites was refined using the free variable restraining the sum of the occupancy factors to be equal to 1. The ratio of the occupancy is 8/2. All H atoms attached to carbon were introduced in calculation in idealised positions and treated as riding models . The drawing of the molecules was realised with the help of ORTEP32 [29]. Crystal data and refinement parameters are shown inTable 6. Routine experiments. In a typical experiment, cyclooctene or cyclohexene (1 equiv) and catalyst (x equiv, see tables) were mixed together and then stirred in air in a round bottom flask. Acetophenone was added as internal standard. The temperature was regulated at 80°C (see Tables 4,5,S4 and S5) and then THBP (70% in water, y equiv, see tables) was added, starting the reaction. Samples of the organic phase (0.1 mL) were periodically withdrawn. The reaction was quenched by addition of MnO2

(2 mg), followed by addition of diethylether (2.5 mL) and removal of the manganese oxide and residual water by filtration through silica before GC analysis.

Water effect. The effect of water was tested for the catalytic runs with [MoO2(SAP)]2 and [MoO2L 1 ]2 using the same experimental conditions as for the routine experiments except for adding additional water (seeTable S6). The transition states were optimized using a preliminary scan of a relevant internal coordinate, followed by full optimization of the TS guided by knowledge of such coordinate. All optimized geometries were confirmed to be stationary points and local minima (for stable molecules and reaction intermediates) or first order saddle points (for the TS's) by frequency analyses. For the TS's, analysis of the imaginary frequency confirmed the expected motion along the reaction coordinate. The calculated frequencies were also used to derive the thermochemical parameters at 298 K according to the standard ideal gas approximation.

Acknowledgements

We

Figure 1 .

 1 Figure 1. Molecular view of compound [MoO2L 1 (DMSO)] (left), [MoO2L 2 (H2O)] (center) and [MoO2L 3 (DMSO)] (right) with the atom labelling scheme. Displacement ellipsoids are drawn at the

Figure 2 .

 2 Figure 2. View of the dinuclear unit formed by intermolecular hydrogen bonds between two [MoO2L 1 (DMSO)] molecules. Symmetry code: (i) -x+2,-y,-z.

Figure 3 .

 3 Figure 3. View of compound [MoO2L 3 (DMSO)].2 DMSO showing the hydrogen bond (dashed line) between the complex and one of the DMSO solvent molecule. Displacement ellipsoids are drawn at the 50% probability level. H atoms are represented as small spheres of arbitrary radii. The disordered part is omitted for clarity.

Figure 4 .

 4 Figure 4. View of the intermolecular H-bonding network for compound [MoO2L 2 (H2O)]•CH3CN.

Figure 5 .

 5 Figure 5. Cyclooctene conversion vs. time with different ONO based catalysts:[MoO2(SAP)]2 (), [MoO2L 1 ]2 (),[MoO2L 2 ]2 (),[MoO2L 2 (EtOH)] ()and [MoO2L 3]2 ( ). For the reaction conditions, see Table4.

Figure 6 .

 6 Figure 6. Conversion after 4 h ( ) and initial TOF () vs. [Mo]/cyclooctene ratio for the processes catalyzed by [MoO2(SAP)]2 (a) and [MoO2L 1 ]2 (b).

Figure 7 .

 7 Scheme 3. Epoxidation of cyclohexene.

Scheme 4 .

 4 Scheme 4. Proposed cycle for the olefin epoxidation catalyzed by [MoO2L] [8].

[MoO2L 1 (

 1 MeOH)] and [MoO2L 1 ]2. In a 100 mL Erlenmeyer flask, H2L 1 (0.35 g, 1.53 mmol) was dissolved in 20 mL of methanol and then [MoO2(acac)2] (0.50 g, 1.53 mmol) was added. The mixture was refluxed under magnetic stirring for 4 hours. The resulting red solution was left to stand overnight, leading to the separation of orange needles characterized as [MoO2L 1 (MeOH)] (80%). These needles were treated under reduced pressure at 80°C for 12 hours, leading to [MoO2L 1 ]2 as brown solid. Addition of MeOH to this solid led a color change indicating the regeneration of [MoO2L 1 (MeOH)]. [MoO2L 1 (MeOH)]: TGA: exp(theo) Δm1 over 20-120 °C = 8.3 (9.8), Δm2 over 250-600 °C = 62.8 (63.4) %. IR (KBr): 908, 933 (Mo=O) 1612 (C=N) cm -1 . [MoO2L

This 1 H 7 (

 17 NMR spectrum in DMSO-d6 is the same as that for [MoO2L 1 (MeOH)], suggesting that for both compounds the stable species in solution is [MoO2L 2 (DMSO)].13 C NMR (300 MHz, DMSO-d6, δ(ppm)): 160.3 (Cq-O), 157.3 (CH-N), 150.3 (Cq-O), 147.0 (Cq-O), 135.9 (Cq-N), 130.5 (CH-Ar), 125.CH-Ar), 122.9 (Cq), 121.5 (CH-Ar), 121.4 (CH-Ar), 120.7 (CH-Ar), 117.7 (CH-Ar), 116.8 (CH-Ar). [MoO2L 2 (EtOH)] and [MoO2L 2 ]2. In a 100 mL Erlenmeyer flask, 0.70 g of H2L 2 (3.05 mmol) was dissolved in 25 mL of ethanol and 1.00 g of [MoO2(acac)2] (3.06 mmol) was added. The mixture was refluxed under magnetic stirring over night. The resulting precipitate was separated by filtration giving orange needles of [MoO2L 2 (EtOH)]. When dried under reduced pressure at 120°C for 12 hours, [MoO2L 2 ]2 is obtained as a red-brown solid. Addition of EtOH to this solid led a color change indicating the regeneration of [MoO2L 2 (EtOH)]. [MoO2L 2 (EtOH)]: TGA: exp(theo) Δm1 over 20-200 °C = 11.6 (11.5) %, Δm2 over 200-600 °C = 62.8 (63.4) %. IR (KBr): 908, 945 (Mo=O), 1600 (C=N) cm -1 . [MoO2L 2 ]2: TGA: exp(theo) Δm over 200-600 °C = 61.6 (59.5) %. IR (KBr): 821(Mo-O-Mo), 904 (Mo=O), 1598 (C=N) cm -1

4. 5 .

 5 Computational Details.All input geometries were adapted from the X-ray structures of [MoO2(SAP)(EtOH)][22] and optimized without any symmetry constraint using the Gaussian 03 program suite[ 30 ]. The calculations used the standard B3LYP three-parameter functional[ 31 ] in conjunction with the 6-31G** basis set[32] for the C, H, N, and O atoms and the CEP-31G* basis set[33] for molybdenum.

  acknowledge the Centre National de la Recherche Scientifique (CNRS) and the Institut Universitaire de France (IUF) for financial support, the Université Paul Sabatier and its Institut Universitaire Technologique for the facilities, and the China Scholarship Council for the fellowship of W.W. (No.201208370239). This work was granted access to the HPC resources of CINES under the allocation 2010-086343 made by GENCI (Grand Equipement National de Calcul Intensif) and to the resources of the CICT (Centre Interuniversitaire de Calcul de Toulouse, project CALMIP).

Table 2

 2

	, in

  All synthesized complexes have been tested for the epoxidation of the model substrate

	2.3. Catalysis studies
	2.3.1. Epoxidation of cyclooctene
	5)	103.98(11)	105.2(1)	103.90(8)
	O(2)-Mo-N(1)	164,32(10)	163.0(1)	164.78(7)
	O(3)-Mo-O(4)	151,30(9)	152.6(1)	152.29(6)
	O(5)-Mo-O(1)	169.11(10)	169.5(1)	172.14(10)

Table 4 .

 4 Results of cyclooctene epoxidation by aqueous TBHP catalyzed by different molybdenum complexes. a TOF is calculated on the time interval with maximum slope for the conversion plot.

		Conversion	Selectivity	TOF b	TON
		[%]	[%]	[h -1 ]
	[MoO2(SAP)]2	76	93	70	153
	[MoO2L 1 ]2	92	93	251	184
	[MoO2L 2 ]2	73	85	71	146
	[MoO2L 2 (EtOH)]	74	90	68	148
	[MoO2L 3 ]2	81	91	127	163
	a Conditions : Mo/TBHP/cyclooctene = 0.5/100/200; T = 80°C; t
	= 4 h.

b

Table 5 .

 5 Results of cyclohexene oxidation by [MoO2(SAP)]2 and [MoO2L1-3 ]2 .

	catalyst	Conversion (%)	Selectivity (%)/ oxide	Yield (%) CHO CHD CHol	TON	TOF (h -1 )
	[MoO2(SAP)]2	48.8	41.6	20.3	8.7	1.1	447 150 a
	[MoO2L 1 ]2	68.8	22.8	15.7 21.8	2.1	687 377 b
	[MoO2L 2 ]2	53.4	35.8	19.1	8.1	1.0	535 187 b
	[MoO2L 3 ]2	61.0	22.7	13.9 21.2	1.8	612 165 a
	a 1 h, b 0.5 h
	In the case of the reactions catalyzed by [MoO2(SAP)]2 and [MoO2L

  1

Table 6 .

 6 Crystal data and structure refinement parameters for all compounds.

	Formula	[MoO2L 1 (DMSO)]	[MoO2L 2 (H2O)]•CH3CN [MoO2L 3 (DMSO)]• 2DMSO
	Empirical formula	C15H15MoNO6S	C15H14 MoN2O6		C19H27MoNO8S3
	Formula weight	436.30	414.22		589.54
	Temperature, K	173(2)	180(2)		180(2)
	Wavelength, Å	0.71073	0.71073		0.71073
	Crystal system	Monoclinic	Monoclinic		Monoclinic
	Space group	P 21/c	I 2/a		P 21/n
	a, Å	7.4084(4)	21.202(5)		15.1752(4)
	b, Å	20.8805(12)	6.599(5)		7.5571(2)
	c, Å	10.8957(6)	24.110(5)	.	22.2331(6)
	, °	90.0	90.0		90.0
	Β, °	99.961(2)	106.582(5)		105.963(2)
	, °	90.0	90.0		90.0
	Volume, Å3	1660.06(16)	3233(3)		2451.39(11)