Chanchal Bhaumik

Eric Manoury

Jean-Claude Daran

Pelin Sözen-Atkaş

Funda Demirhan

Rinaldo Poli

butylhydroperoxide in MeCN; implications for olefin epoxidation catalyzed by organomolybdenum and organotungsten compounds

Keywords: Molybdenum, Tungsten, Pentamethylcyclopentadienyl, Oxido ligands, Oxidation, Catalyst degradation

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

Investigation of the reaction of [Cp*2M2O5] (M=Mo, W) with hydrogen peroxide and tert-butylhydroperoxide in MeCN; implications for olefin epoxidation catalyzed by organomolybdenum and organotungsten compounds

Chanchal Bhaumik, E. Manoury, Jean-Claude Daran, Pelin Sözen-Aktaş, F.

Demirhan, Rinaldo Poli

To cite this version:

Introduction

High oxidation state organometallic compounds are often thermally unstable and hydrolytically sensitive but exceptions are known [1] such as for instance methyltrioxorhenium (MTO), [MeReO3], which is air and water stable [2][3][4]. This compound is an active oxidation catalysts and its stoichiometric interaction with H2O2 yields a bis(peroxido) derivative MeReO(O2)2, which was isolated and fully characterized [5]. Another compound behaving similarly to MTO in terms of air and water stability, [Cp*2Mo2O5], has attracted our attention for some time, particularly in terms of its behavior in an aqueous environment [6]. We have shown in early work that the compound resists hydrolytic cleavage of the Cp*-Mo bond even at the lowest pH, for at least a few days [7,8]. Although the compound (and also its tungsten analogue

[Cp*2W2O5]) maintains the dinuclear neutral structure in a variety of polar solvents including MeOH and MeCN [9], it displays an interesting speciation for the aqueous "Cp*Mo VI " fragment:

it behaves as an electrolyte in a water-rich environment with self-dissociation to other substrates by tert-butylhydroperoxide (TBHP) [11]. Stoichiometric oxidation of [Cp*MoO2Cl] by TBHP [11] or H2O2 [12] allowed isolation of a peroxido derivative, [Cp*MoO(O2)Cl], which was however found inactive as an epoxidation catalyst. Kühn and Romão later showed that [Cp*MoO2Cl] as well as analogues with differently substituted (η 5 -C5R5) ligands can be more conveniently generated in situ from the tricarbonyl complex [(η 5 -C5R5)MoCl(CO)3] and TBHP [13]. This opened the way to the generation of alkylmolybdenum(VI) analogues by oxidation of a variety of [(η 5 -ring)Mo(R)(CO)3] precursors and these compounds have been thoroughly investigated as olefin epoxidation catalysts [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28].

We have initially tested the [Cp*2Mo2O5] compound, as well as various analogues containing differently substituted cyclopentadienyl rings, (η 5 -C5R5), in the catalyzed cyclooctene epoxidation by TBHP in chloroform and found them to yield activities and kinetic profiles in close correspondence to those of the analogous [(η 5 -C5R5)MoO2Cl] precatalysts [29]. Intrigued by the previously established stability of these compounds in an aqueous environment, we also tested [Cp*2Mo2O5] in the cyclooctene epoxidation by TBHP/H2O and by aqueous H2O2 resulting in lower or no epoxide yields, respectively. Subsequently, we have applied this compound as well as the tungsten analogue, [Cp*2W2O5], for which we have meanwhile found a convenient and reproducible high-yield synthesis [30], to the catalytic oxidation of thiophene derivatives under different conditions (using aqueous H2O2 as oxidant in MeCN). We were pleased to find a clean catalytic process (well behaved first-order kinetics, high activities), with the W compound being much more active than the Mo compound (kW/kMo ~ 100) [31].

Subsequent adaptation of the same conditions to cyclooctene epoxidation revealed the same phenomena: clean kinetics and greater activity for the W compound [32]. Theoretical calculations seemed to justify the greater activity of the W compound on the basis of a mechanism closely related to one proposed by Thiel [33] and previously explored for the [Cp*MoO2Cl] system [34].

More recently, however, we have discovered a puzzling change of relative activity when changing the oxidant from aqueous H2O2 to TBHP/decane, all other conditions and notably the reaction solvent remaining the same [35]. The relative catalytic activity of [Cp*2M2O5] (M = Mo, W) for the cyclooctene epoxidation by TBHP/decane in MeCN becomes Mo >> W. This result has made us suspect that the catalyst is chemically transformed in different ways during the initial moments of the catalytic process, depending on the oxidant and metal nature. It is also relevant to mention that a study published by Colbran et al. [36], which focused on a bromide derivative with a bulky perarylcyclopentadienyl ligand, [(η 5 -C5Ph4{C6H3(OMe)2-2,5})MoO2Br],

demonstrated that this complex catalyzes cyclooctene epoxidation by TBHP but decomposes to a much more active non-cyclopentadienyl containing catalyst as the reaction proceeds. We have therefore set out to investigate the stoichiometric reaction between [Cp*2M2O5] and the oxidants used in the epoxidation process, H2O2 and TBHP, under our experimental conditions (MeCN solution) and report here the results of our findings.

Experimental section

General. All preparations and manipulations were carried out with Schlenk techniques under an argon atmosphere. Solvents were dried by standard procedures and distilled under argon prior to use. 1 H NMR spectra were recorded on a Bruker Avance 300. For the quantitative NMR analyses, the relaxation delay was incremented to 30 s (the use of the instrumental standard settings did not yield reliable quantitative intensities). Chemical shifts are expressed in ppm downfield from Me4Si. The IR spectra were recorded on Perkin Elmer Spectrum 100 FT-IR Spectrometer. The gas chromatographic analyses were completed by using Fisons 8130 instrument equipped with a DB-1MS capillary column (30 m × 0.32 mm × 0.25 µm ) and with a flame ionization detector. The elemental analyses were carried out by the analytical service of the Laboratoire de Chimie de Coordination, Toulouse. Compounds [Cp*2M2O5] (M = Mo, W) [30] and (nBu4N)2[M6O19] (M = Mo [37], W [38]) were prepared as described in the literature.

Cyclooctene (Fluka), dodecane (Aldrich), t BuOOH solution in decane (5.5 M) and H2O2 (30 wt % in water) (Aldrich) were used as received. The silica was washed by 1 ml diethyl ether. The resulting organic phase was analyzed by gas chromatography.

Oxidation experiments monitored by

Catalytic epoxidation experiments with aged solutions of [Cp*2M2O5

]. The epoxidation reactions were carried out under an argon atmosphere in Schlenk tubes equipped with a magnetic stirrer and immersed in a thermostatic bath set at the 55°C. In a typical experiment, the tube was charged with 4 ml stock solution and cyclooctene (157 µL, 132 mg, 1.2 mmol). The preparation of the stock solutions is described in the supporting information. Aliquots of the reaction mixture (0.2 ml), withdrawn at predetermined times, were worked-up and analyzed as described in the previous section.

Results and Discussion

(a) Oxidations of [Cp*2M2O5] by aqueous H2O2

Compounds [Cp*2M2O5] react within seconds at room temperature with aqueous H2O2 in CD3CN under conditions similar to those previously used in catalytic cyclooctene epoxidation (oxidant:M = 50:1), except for the absence of cyclooctene, to yield clear solutions that no longer revealed 1 H NMR resonances attributable to a metal coordinated Cp* ligand. For the Mo compound, the 1 H NMR spectrum did not reveal any resonances except for that of water at ca. δ 2.0 and the 1:2:3:2:1 solvent multiplet. For the W compound, on the other hand, the spectrum revealed the immediate formation of a main Cp*-derived product where the ring is desymmetrized (three uncoupled Me resonances in a 2:2:1 ratio at δ 1.28, 1.13 and 0.96, see Figure 1), while the water peak is dramatically shifted downfield to ca. δ 5.0, suggesting a significant alteration of the water hydrogen bonding network. This product, however, has limited stability in the strongly oxidizing environment because the observed resonances disappeared at longer reaction times. The possible nature of this species will be clarified below. It can be immediately observed that the total Cp* intensity remains constant for the 1:1 experiment (while the solution remained homogeneous), whereas it is slowly eroded over time for the 2:1 reaction (accompanied by the formation of a precipitate). The starting material resonance at δ 1.994 disappears faster for the experiment with the 2:1 ratio. 4 shows a representative spectrum in the Cp* region taken for the 2:1 reaction after 50 min. When repeating the same reaction with 5 equiv of H2O2, there was an immediate precipitation and all the Cp* resonances were already lost in the first spectrum collected after 10 min. The precipitate formed under these conditions was isolated and investigated by IR spectroscopy and elemental analysis. In addition, three compounds could be crystallized out of these solutions (with or without quenching the excess H2O2 by MnO2 at a preset reaction time) and investigated by X-ray diffraction, revealing their identity as polyoxometallates, see details in section (c).

It appears reasonable to assign the two 1:1 resonances at δ 2.007 and 2.054 to an asymmetric dinuclear compound where only one terminal oxido ligand has been transformed to a peroxido ligand, which could be produced by an oxygen transfer process from H2O2 (or from peroxyacetimidic acid, CD3C(OOH)=NH, which is known to form from H2O2 and acetonitrile) [39,40] The same experiment conducted on the W compound gave similar results, shown in Figure 5 for the 1:1 experiment and in Figure 6 for the 2:1 experiment. The number of observed products is smaller and the chemical transformations are slower, however. For the 1:1 experiment, a slight initial decrease of the total integrated Cp* intensity is followed by stability beyond 1 h. The main relative amount being greater relative to the 1:1 experiment, but always remaining the minor product in the spectrum (maximum of 7% after 40 min). A representative spectrum of this experiment is shown in Figure 7. Additional NMR experiments were conducted using a decane solution of TBHP as oxidant in CD3CN. For the Mo compound, a slow reaction could be monitored when using 2 equiv of TBHP, as shown in Figure 9. The starting material resonance at δ 1.994 decreased to ca. 65% of the initial intensity after 0.5 h and was no longer present in the final spectrum take after 18 h.

The decay of this resonance followed a first order rate law with k = 2. shows the TBHP resonance at δ 1.204, decreasing with time, and another resonance at δ 1.195, assigned to tBuOH, which continuously increases. Note that this experiment does not produce any asymmetric dinuclear intermediate, confirming the tentative assignment made in Scheme 1 because only H2O2 could produce the proposed peroxido ligand by a fast proton transfer process.

Scheme 2

When the above experiment was repeated in the presence of 100 equivalents of TBHP, the starting material resonance decayed very rapidly and disappeared after 15 min. The [Cp*MoO2(OOtBu)] intermediate at δ 2.025 was again formed, in this case up to a maximum intensity of 56% after 15 min, then rapidly decaying. Several other minor resonances were observed at δ 2.268, 2.252, 2.243, 2.229, 2.121, 2.106, 2.089 and 2.019, only those at δ 2.121 and 2.106 ever reaching a notable intensity of ca. 12% of the initial Cp* intensity, whereas the others had a maximum of 2% of the intensity. After 430 min, the highest residual Cp* resonance was that at δ 2.121 with 9% of the initial intensity. This resonance was still present with approximately the same intensity after one week. Figure in SI? For the W compound, no change was evident in the NMR spectrum over a 24 h range at room temperature in the presence of only 2 equiv of the oxidant. When 100 equivalents of TBHP were used, the starting material resonance decreased slowly according to a first order decay (based on 7 points, k = 6.3•10 -4 s -1 , t1/2 ~18 min) and numerous small Cp* resonances were produced. The main ones were observed at δ 2.167, 2.161, 2.121 and 2.090, none of them corresponding to those observed for the H2O2 reaction (Figure 7 and Figure 8) and none ever having greater than 5% of the initial Cp* intensity. The first two resonances eventually disappeared, whereas the other two remained with approximately constant intensity (4.5% and 1.5%) after 6 h. Thus, a decay process with Cp* loss also occurred in this case. decane solution) shows that the Mo complex reacts faster than the W complex and that aqueous H2O2 reacts faster than TBHP/decane. Several Cp*-containing intermediates were generated in all cases, these being relatively stable when no more than 1 equivalent of oxidant per dimer was used, but Cp* loss (faster for greater oxidant/complex ratios) eventually took place in every case and this loss was complete when aqueous H2O2 was used as oxidant. 2) in a different crystal habit. The metric parameters of the cation in our new structure of 2a are similar to those previously reported in the above mentioned salts. The single crystals of the tungsten analogue 2b were of very poor quality, but the unit cell was isomorphous with that of 2a. The data set was sufficient to identify the hexagonal W6 array but did not warrant a full structural refinement (see more details in the Supporting Information).

(c) Analysis of the isolated decomposition products

However, these results make the identity of this crystal completely unambiguous. The structure of 3 is affected by severe disorder and no reliable structural information beyond confirmation of the chemical constitution may be inferred. The Lindqvist anion [Mo6O19] 2-is reported in another > 130 structurally characterized salts according to the Cambridge structural database, whereas the [Cp*Mo6O18] 2-ion is already reported for the above mentioned polymorph of the [C5Me5O] + salt [42], for the nBu4N + salt [43] and for the Ph4P + salt [44]. A detailed discussion of these structures is not warranted. Figures and relevant metric parameters can be found in the Supporting Information.

C5Me5 -+ O2  C5Me5O + + O 2- (1)
The precipitates that were isolated from the controlled decomposition of [Cp*2M2O5] with a small excess of aqueous H2O2 have been analyzed by IR spectroscopy and by elemental analysis.

The broader bands centered at 751 and 577 cm -1 for Mo-O-Mo. The frequency of the former is close, though a bit higher, to that measured for [nBu4N]2[Mo6O19] (949 cm -1 , also reported in SI). It is closer to those reported for the [Cp*Mo6O18] -salts (957 cm -1 for the nBu4N + salt, 967 cm -1 for the nBu4P + and Ph4P + salts, and 962 cm -1 for the n-butylpyridinium salt [44]) and for 1 (966 cm -1 [10]). The IR pattern of the solid isolated from the controlled decomposition of [Cp*2W2O5] is similar, with a sharp band centered at 974 cm -1 and a broader band centered at 776 cm -1 . The former is close to those observed for [nBu4N]2[W6O19] (966 cm -1 ), for different salts of [Cp*W6O18] -(960 cm -1 for the nBu4N + , nBu4P + and Ph4P + salts), and for the W analogue of 1,

[Cp*2W6O17] (984 cm -1 [45]).

The combined evidence from these isolation and characterization studies, in combination with the NMR studies outlined in the previous sections, is that while a small excess of oxidant generates soluble products that are still characterized by a Cp*/M ratio of 1, larger and larger amounts lead to oxidation of the Cp* rings, initially to the pyrilium cation [C5Me5O] + , with formation of insoluble polyoxometallate salts. The analysis of the solids obtained with a moderate excess of H2O2 are rather consistent with the presence of 1 for the Mo sample (and its analogue for the W sample) but the presence of small amounts of other intermediates with a Cp*/Mo ratio > 2:6 is not to be excluded. Obviously, after precipitation, further oxidation of the solid material becomes slower. Thus, with a moderate excess of oxidant, less extensive Cp* removal has resulted in the crystallization of neutral 1, whereas use of a greater excess has resulted in faster and more extensive Cp* removal with formation of anionic polyoxometallates that are probably more soluble and more slowly crystallize as 2 and ultimately 3. Therefore, use of a much larger excess of oxidant (e.g. 100 equivalents of more as under catalytic conditions) likely leads to [M6O19] 2-as the only polyoxometallate species, which do not precipitate under catalytic conditions given their greater solubility and the lower concentration relative to the conditions used for crystallization. It is interesting to mention in this respect that the oxidative decarbonylation by TBHP of the tris(pyrazolyl)methane complexes of molybdenum, [{HC(3,5-Me2pz)3}Mo(CO)3], also leads to the generation of a salt of the Linqvist polyoxometallate,

[{HC(3,5-Me2pz)3}2Mo2O5][Mo6O19], which was completely characterized and found to be an active catalyst in olefin epoxidation [46].

(d) Epoxidation catalysis with the [M6O19] 2-complexes

On the basis of the considerations outlined in the above section, we considered it possible that all the [Cp*2M2O5] pre-catalyst is converted in situ to the Lindqvist structure during the catalytic experiments. In order to establish whether this species could correspond to the real epoxidation The catalytic tests revealed very poor activity and selectivity relative to the previously reported ones run with the [Cp*2M2O5] precursors. When using H2O2 as oxidant and the [Mo6O19] 2-salt as catalyst, the conversion was only 56% (32% epoxide yield) after 28 h. This performance is not dramatically poorer in terms of activity, though the selectivity is worse, relative to that of [Cp*2Mo2O5] (63% conversion and 63% epoxide yield after 24 h) [32]. On the other hand, the In each case, a stock solution containing the oxidant and the precatalyst in a 200:1 ratio was prepared and kept at room temperature, as in the NMR monitoring experiments described above.

Then, aliquots of this solution were used for cyclooctene epoxidation experiments started at different times (immediately after preparation and after 18 h, 24 h and 7 days) and run under conditions of concentrations and temperature similar to those previously reported (3:1

MeCN/toluene, 55%, oxidant/substrate = 2, i.e. the final catalytic solution contained a substrate/precatalyst ratio of 100, or 2% metal loading) [32,35]. The catalytic activity was evaluated by the initial rate method and the results are reported in Table 1. Graphs of the time evolution of the concentrations monitored by GC are available in the SI.

Table 1. Initial rates for the epoxidation of cyclooctene with aged solutions of the catalyst in the presence of oxidant. For the TBHP reaction, the initial rate method for the evaluation of the catalytic activity is rendered necessary by the fact that this reaction is affected by product inhibition [35], the reaction rate being negatively affected by the tert-butanol product and a data fit to an integrated rate law being therefore impossible. For the H2O2 reaction, the established first order decay makes it possible in principle to make use of the integrated rate law [32]. However, the experiments carried out with the solution aged for 24 and 192 hours showed that the reaction stopped after only a partial conversion of cyclooctene (ca. 70% and 55%, respectively), suggesting that the H2O2 concentration has decreased by a slow catalase process during the ageing in such a way that the amount is substoichiometric during the catalytic experiment. Thus, the initial rate provides a simpler and direct way to measure the catalytic activity. Note that the rate law for the catalyzed epoxidation by aqueous H2O2 was shown to be zero order in H2O2 [32], hence the initial rate should in principle be independent on the actual starting H2O2 concentration.

t/h [a] r(Mo-TBHP)/M•s -1 [b,c] r(W-H2O2)/M•s -1 [b,d]
As shown in Table 1, the initial rate decreases with increase ageing of the catalyst solution, but the degree of activity loss is rather small (less than 50% over one week) for both catalytic solutions. This attests to the relative robustness of the catalytically active species. On the other hand, the organometallic precatalysts are totally consumed on a much shorter timescale (within minutes for the W compound in the presence of H2O2 at the concentrations of the catalytic experiments). For the oxidations with H2O2, no Cp*-containing soluble compounds remained in solution, whereas only a minor amount of a Cp*-containing complex remained in solution for the oxidation with TBHP.

Conclusions

We have clearly established herein that compounds [Cp*2M2O5] (M = Mo, W), previously shown to be efficient epoxidation catalysts and particularly the Mo compound when the oxidizing agent is TBHP/decane in CHCl3 [29] or in MeCN [35] or the W compound when the oxidizing agent is aqueous H2O2 in acetonitrile [32], are rapidly decomposed under catalytic conditions to species that are devoid of the Cp* ligand. A number of Cp*-containing intermediates have been detected when using a small excess of oxidizing agent but full loss of the Cp* resonance occurs in the presence of H2O2, the process being faster in the order W < Mo. TBHP/decane reacts more slowly than H2O2/H2O in CD3CN, but still leads to extensive (though not quite complete) loss of the Cp* resonance. The fate of the Cp* ring was not studied in great detail. However, the formation of the pentamethyl pyrilium cation, C5Me5O + , at least as an intermediate, was demonstrated by the crystallographic study of two minor products and was also suggested by an 1 H NMR investigation.

The Cp* loss for the reaction of the W precatalyst with H2O2 was complete in a very short time (with respect to the time required for a complete catalytic epoxidation). For the reaction of the Mo precatalyst with TBHP/decane extensive loss of the Cp* resonance (>90%) occurred within a few hours. However, the initial reaction rate measured for epoxidation experiments run with differently aged solutions of both [Cp*2Mo2O5]/TBHP and [Cp*2W2O5]/H2O2 combinations decrease by less than a factor of 2 over 1 week. This clearly demonstrates that the active catalyst for the epoxidation by at least H2O2 and probably also TBHP must be a non-organometallic species, which is relatively stable under the catalytic conditions. For the [Cp*2W2O5]/H2O2 system, it is quite clear that the catalyzed transformation must entirely be promoted by a nonorganometallic species. For the [Cp*2Mo2O5]/TBHP system, on the other hand, it cannot be completely excluded that the organometallic intermediates generated in the initial stages of the [Cp*2Mo2O5] oxidative degradation process continues to be responsible for part of the catalytic activity of the aged solutions, but it seems more likely that the predominant non-organometallic degradation products are also at least in part responsible for the catalytic effect. These results are closely related to those reported by Colbran et al. [36], as already mentioned in the Introduction, and hint to the possible intervention or even dominant contribution of non-organometallic degradation products also in the previously published catalytic studies making use of organometallic precatalysts [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], most of which were carried out with compounds of molybdenum and with TBHP/decane as oxidant.

The exact nature of the catalytically active non organometallic species that form after a very short time in the presence of a large excess of H2O2 (few min) or at longer times in the presence of a large excess of TBHP (few h) is not yet elucidated. The identification of the Linqvist polyoxomolybdate, [Mo6O19] 2-, as the ultimate product of Cp* loss from the precatalysts made us initially suspect that this could be an active species, but experiments reported here allowed us to exclude this hypothesis. In this respect, the recently reported [{HC (3,3}2Mo2O5][Mo6O19] compound, produced by the oxidative decarbonylation of [{HC(3,5-Me2pz)3}Mo(CO)3] by TBHP [46], was shown to be an active, stable, and recyclable catalyst.

These results, in combination with the results reported herein on the poor performance of the Lindqvist anion (although the catalytic conditions are different), suggest the intriguing possibility that the catalytic activity is associated with the cationic species and that related cationic complexes are also formed during the oxidative degradation of [Cp*2M2O5]. Further work is ongoing in our laboratory to identify these species and to understand why the catalytic activity is greater for the epoxidation with H2O2 when M = W and for the epoxidation with TBHP when M = Mo under otherwise identical conditions.

Supporting Information

Infrared spectra of the solids isolated from the controlled oxidative decomposition of

Acknowledgements.

  Bhaumik, E. Manoury, Jean-Claude Daran, Pelin Sözen-Aktaş, F. Demirhan, et al.. Investigation of the reaction of [Cp*2M2O5] (M=Mo, W) with hydrogen peroxide and tertbutylhydroperoxide in MeCN; implications for olefin epoxidation catalyzed by organomolybdenum and organotungsten compounds. Journal of Organometallic Chemistry, 2014, 760, pp.115-123. 10.1016/j.jorganchem.2013.11.029. hal-02007933

[

  Cp*MoO2(H2O)] + and [Cp*MoO3] -, each species becoming dominant at low and high pH, respectively. The neutral [Cp*MoO2(OH)] species is also present in solution in equilibrium with the charged complexes around pH 4. Partial loss of Cp*, however, occurred over an extended period of time (months) from aqueous solutions upon standing at low pH in air to yield the organometallic polyoxometallate compound [Cp*2Mo6O17] [10], which is isoelectronic and displays the same core structure as the Lindqvist anion [Mo6O19] 2-. More recently, we have investigated [Cp*2Mo2O5] as a precatalyst for olefin epoxidation. The catalytic activity of Cp*Mo VI derivatives in this reaction was first reported by Bergman, showing that compound [Cp*MoO2Cl] has a high activity for the epoxidation of cyclooctene and a few

  1 H NMR. The oxidation reactions of [Cp*2M2O5] (M = Mo, W) were carried out in an NMR tube at room temperature. For each experiment the NMR tube was charged with 0.7 mL of the deuterated acetonitrile solvent. Subsequently the compound ([Cp*2M2O5]; 10.5 µmol) was added and finally the oxidation reactions were started by the addition of oxidants and monitored by NMR with time. As examples, 1.1 µL of 30% aqueous H2O2 was used for the oxidation of the Mo compound 1 equivalent of with H2O2, whereas 3.8 µL of a 5.5 M TBHP solution in decane was used for the oxidation of the same compound with 2 equivalents of TBHP. Isolation and characterization of metallic products. (a) From the [Cp*2Mo2O5]/H2O2 reaction.

Figure 1 . 1 H

 11 Figure 1. 1 H NMR of the CD3CN solution of [Cp*2W2O5] before (a) and after (b) the addition

Figure 2 .

 2 Figure 2. Time evolution of various resonances in the 1 H NMR spectrum of the CD3CN

Figure 3 .

 3 Figure 3. Time evolution of various resonances in the 1 H NMR spectrum of the CD3CN

  (Scheme 1(a), M = Mo). The asymmetric peroxido compound could then undergo a slow ligand redistribution to regenerate starting material plus a bis(peroxido) compound (Scheme 1(b)), thus rationalizing why the starting material increases again at longer reaction times and why this phenomenon is more evident in the 1:1 experiment. The putative bis(peroxido) compound may be associated with the resonance at δ 2.112, since this resonance increases in parallel with the reappearance of starting material in the 1:1 experiment and it increases to a greater extent in the 2:1 experiment. Candidates for the other unassigned resonances are dinuclear symmetric compounds such as Cp*(O)2Mo-O-O-Mo(O)2Cp* or Cp*(O)(O2)Mo-O-O-Mo(O)(O2)Cp*, and mononuclear compounds such as Cp*MoO2(OH), Cp*MoO2(OOH), Cp*MoO(O2)(OH), etc.

Figure 4 . 1 H

 41 Figure 4. 1 H NMR spectrum of the CD3CN solution resulting from the reaction of

Figure 5 .

 5 Figure 5. Time evolution of various resonances in the 1 H NMR spectrum of the CD3CN

Figure 6 .Figure 7 . 1 HFigure 5 and Figure 6 .

 67156 Figure 6. Time evolution of various resonances in the 1 H NMR spectrum of the CD3CN

Figure 8 . 1 HFigure 5 and Figure 6 .

 8156 Figure 8. 1 H NMR spectrum of the CD3CN solution resulting from the reaction of

  Figure in SI? In conclusion of the NMR investigation of sections (a) and (b), the stoichiometric reaction between [Cp*2M2O5] and ROOH in CD3CN (M = Mo, W; R = H, aqueous solution; R = tBu,

  Scheme 3

  /4.21 for W). The IR spectra (shown in the Supporting Information) clearly display the typical M-O vibrational pattern of the Lindqvist type polyoxometallates. The solid derived from the [Cp*2Mo2O5] decomposition shows a sharp absorption centered at 957 cm -1 for Mo=O and two

  catalyst, we proceeded to verify the catalytic activity of the nBu4N + salts of the [M6O19] 2-ions, which are completely soluble under the chosen experimental conditions. The tests were carried out under the same conditions previously employed for [Cp*2M2O5] (MeCN/toluene 3:1 as solvent, [cyclooctene] ~ 0.1 M; oxidant/cyclooctene = 2, cycloctene/metal = 50, T = 55°C)[32,35].

H2O2 oxidation catalyzed by

  the [W6O19] 2-salt gave only 6% conversion (0% epoxide yield) after 4 h, vs. a quantitative conversion and epoxide yield after only 2 h for [Cp*2W2O5][32]. For the epoxidation reactions with the TBHP/decane reagent, a conversion of only 13% (2% epoxide yield) was recorded after 3.5 h with the [Mo6O19] 2-catalyst (cf. quantitative conversion and epoxide yield in less than 1 h in the presence of [Cp*2Mo2O5])[35], whereas a conversion of only 10% (2% epoxide yield) was recorded after 2.5 h with the [W6O19] 2-catalyst (cf. > 20% epoxide yield after 4 h in the presence of [Cp*2W2O5])[35]. Plots of cyclooctene conversion and epoxide yield for the epoxidations catalyzed by [nBu4N]2[M6O19] (M = Mo, W) are shown in the Supporting Information. Clearly, the oxidative decomposition of the [Cp*2M2O5] (M = Mo, W) compounds must produce also other species which are the real active and selective epoxidation catalyst, particularly for [Cp*2W2O5] in the presence of H2O2 and for [Cp*2Mo2O5] in the presence of TBHP. (e) Catalysis with aged solutions of [Cp*2M2O5] Since the results presented above establish quite clearly that both [Cp*2M2O5] (M = Mo, W) compounds are more or less rapidly decomposed with Cp* loss by a large excess of oxidant, it is important to verify the catalytic activity of aged solutions. The most interesting cases pertain to the [Cp*2Mo2O5]/TBHP and [Cp*2W2O5]/H2O2 combinations which show the highest activities.

  10 -4 [a] Time of the beginning of the catalytic run from the preparation of the catalyst/oxidant solution. [b] The rate was measured from the amount of cyclooctene oxide produced in the first 20 min. [c] Conditions: [Cp*2Mo2O5]0 = 2.5•10 -3 M; [TBHP]0 = 0.5 M; [cyclooctene]0 = 0.25 M. [d] Conditions: [Cp*2W2O5]0 = 2.6•10 -3 M; [H2O2]0 = 0.5 M; [cyclooctene]0 = 0.25 M.

[

  Cp*2M2O5], experimental details of the data collection and structural refinement for compounds 2a and 3, results of the cyclooctene epoxidation by aqueous H2O2 and by TBHP/decane catalyzed by [nBu4N]2[M6O19] (M = Mo, W), and results of the cyclooctene epoxidation with aged solutions of [Cp*2Mo2O5]/TBHP and [Cp*2W2O5]/H2O2 (12 pages).

From the [Cp*2W2O5]/H2O2 reaction. [

	and dried under vacuum. Elem. anal. Found: C, 22.50; H, 2.44%. IR in the M-O stretching region
	(cm -1 ): 957 s, 751 vs (br), 577 s (br).
	(b) Cp*2W2O5] (75 mg; 0.105 mmol) was dissolved
	in 3.0 ml of MeCN. To this solution was added 52.5 µL (0.525 mmol) of 30% aq. H2O2,
	followed by stirring for 30 min and then standing overnight. The color of the solution changed
	from colorless to yellow and a yellow precipitate formed along with a few crystals. Elem. anal.
	Found: C, 15.31; H, 1.73%. IR in the M-O stretching region (cm -1 ): 974 s, 776 vs (br), 595 m,
	556 m. A single crystal of 2b from this batch was analyzed by X-ray diffraction (see supporting
	information).
	(c)
	[Cp*2Mo2O5] (50 mg; 0.092 mmol) was dissolved
	in 2.5 ml of MeCN. To this solution was added 18.43 µL of 30% aq. H2O2 (0.184 mmol)
	followed by stirring for 15 min and then standing undisturbed overnight. The color of the
	solution changed from yellow to orange-red and an orange precipitate formed along with a few
	well formed, diamond shaped orange-red crystals. A single crystal of 1 from this batch was
	analyzed by X-ray diffraction (see supporting information). From a second reaction run under
	identical conditions, the recovered orange solid was isolated by filtration, washed with MeCN

Reaction leading to [C5Me5O]2[Cp*Mo6O18] crystals. [

	Cp*2Mo2O5] (28.9 mg, 5.33 x10 -5
	mol) was dissolved in 4.0 mL of MeCN/Toluen (3:1) mixture. To this solution was added 0.1 mL
	(5.33x10 -4 mol) TBHP (5.5M in decane). The solution was stirred at room temperature for 2
	hours. After 2 days at room temperature in air, crystal formation was observed from yellow
	solution. A single crystal of 2a from this batch was analyzed by X-ray diffaction (see supporting
	information).

(d) Reaction leading to [C5Me5O]2[Mo6O19] crystals. [

	Cp*2Mo2O5] (50 mg; 0.092 mmol)
	was dissolved in 2.5 ml of MeCN. To this solution was added 46.1 µL (0.46 mmol) of 30% aq.
	H2O2 followed by stirring for 15 min and then standing overnight. The color of the solution
	changed from yellow to red and led to the crystallization of deep-red colored, square shaped
	crystals, suitable for x-ray diffraction. A single crystal of 3 from this batch was analyzed by X-
	ray diffraction (see supporting information).

Catalytic epoxidation experiments with [nBu4N]2[M6O19]. The

	epoxidation reactions were
	carried out under an argon atmosphere in Schlenk tubes equipped with a magnetic stirrer and
	immersed in a thermostatic bath set at 55°C. In a typical experiment, the tube was charged with
	cyclooctene (130 µL, 110 mg, 1.0 mmol), the internal standard dodecane (160 µL, 120 mg, 0.7
	mmol), the catalyst ([nBu4N]2[M6O19]; 10.0 µm) and the oxidant (TBHP, as a 5.5 M solution in
	decane, 0.4 mL, 2.2 mmol or 30 wt % aqueous H2O2, 190 µL, 2.2 mmol) in 9.6 mL of
	acetonitrile/toluene (3:1). Aliquots of the reaction mixture (0.2 ml) were withdrawn at
	predetermined times, quenched by addition of manganese dioxide, diluted with 2 ml of diethyl
	ether and filtered through a Pasteur pipette filled up with silica to eliminate the residual MnO2.

  In the 1:1 experiment the intensity initially decreases to a minimum of ca. 10% of the initial value after 30 min, but then it increases again to reach ca. 17% of the initial amount after 110 min. Simultaneously, an intermediate compound which is characterized by two resonances at δ 2.007 and 2.054 in a 1:1

	039, 2.078 and 2.099
	reach a greater maximum intensity (21, 39 and 20%) on a shorter time scale (10, 10 and 30 min,
	respectively), before decreasing. Finally, the last product with the resonance at δ 2.112 increases
	to a greater relative amount (ca. 20% after 2 h). Figure

ratio grows to a maximum amount of 70% of the total Cp* intensity after 30 min and then slowly decreases. The combination of these two trends suggests that this intermediate partially reverts back to starting material. Starting at ca. 20-30 min, a number of other products, each one being characterized by a single Cp* resonance, also become visible in the 1 H NMR spectrum. Notably three products with resonances at δ 2.039, 2.078 and 2.099 reach a maximum intensity of ca. 8, 15 and 7% respectively after 40, 40 and 80 min before decreasing again, whereas a last product with a resonance at δ 2.112 continues to accumulate. In the 2:1 experiment, the starting material resonance also regains a bit of intensity after essentially disappearing (<1%) after 10 min, but to a much lesser extent relative to the 1:1 experiment. The intermediate with the two 1:1 resonances at δ 2.007 and 2.054 accumulates to a smaller extent (ca. 30% after 40 min) and then remains approximately constant. The three intermediates with resonances at δ 2.

  80% of the initial Cp* amount) is again an asymmetric dinuclear intermediate as for the Mo experiment, characterized in this case by two 1:1 resonances at δ 2.175 and 2.160. The only other observed product in minor amounts (< 3%) shows a resonance at δ 2.112. The starting material (δ 2.140) is not completely consumed with ca. 10% remaining after 90 min, probably because of an imperfect stoichiometry. In comparison with the Mo system, it seems that this asymmetric W intermediate does not decompose as rapidly by a ligand redistribution process (e.g. according to Scheme 1). The reaction carried out in a 2:1 ratio, like in the Mo case, resulted in a progressive degradation of the total Cp* intensity with >35% being lost after 3 h and in the formation of a precipitate. The starting material was consumed faster relative to the 1:1 reaction (30 min instead of ca. 80 min) and yielded a lower amount of the asymmetric dinuclear

	product (ca.	H 2 O		solvent
	2,3	2,2	2,1	2	1,9
			δ/ppm

intermediate (maximum amount of ca. 65% at 30 min, instead of 80%). After 30 min, the amount of this intermediate decreased to ca. 40% while a new resonance at δ 2.180 increased. The latter resonance appears therefore attributable to a double oxygenation product such as that proposed in Scheme 1. The minor product with resonance at δ 2.112 is also observable in this case, its

  water impurity that is present in the solvent and/or in the oxidant solution. While the δ 2.025 and 2.261 signals disappear from the final spectrum, the signal at δ 2.112 further grew and remained by far the major signal in the spectrum after 18 h, with an intensity corresponding to

	due to the 23% of the initial Cp* intensity. Therefore, although the reaction of [Cp*2Mo2O5] with
	TBHP/decane is slower than that with aqueous H2O2, it also results in loss of Cp*.
					solvent
	2,3	2,2	2,1	2		1,9
			δ/ppm		18 h
					0.5 h
					h
	2,3	2,2	2,1	2	1,9	1,8
			δ/ppm
	Figure 9. Representative 1 H NMR spectra of the CD3CN solution resulting from the reaction of
	[Cp*2W2O5] with TBHP/decane (2 equiv).
	The major intermediate at δ 2.025 does not correspond to any of the compounds observed for
	the oxidation with H2O2, whereas the minor intermediate at δ 2.261 and the signal at δ 2.112
	which is assigned to the dinuclear symmetric peroxido complex [{Cp*MoO(O2)}2(µ-O)]) are
	also observed at the same positions in the H2O2 experiment (cf. Figure 4). The new intermediate,
	therefore, likely contains the TBHP reagent, as confirmed by the appearance of a new tBu
	6•10 -4 s -1 (based on 5 resonance at δ 1.236 integrating correctly for 9 protons vs the 15 protons of the Cp* resonance at
	points; t1/2 ~ 45 min). After 0.5 h from the TBHP addition, the spectrum showed a major δ 2.025. Furthermore, these two resonances initially increase and then decrease proportionally.
	This intermediate is therefore interpreted as the tert-butylperoxido derivative

intermediate characterized by a single resonance at δ 2.025 (29% of the initial Cp* intensity), plus other minor resonances at δ 2.112 and 2.261. The broader resonance observed at δ 2.173 is [Cp*MoO2(OOtBu)], as shown in Scheme 2. The tBu region of the 1 H NMR spectrum also

  C,H analyses gave results in slight excess relative to the expected values for the formulations determined by X-ray diffraction (C/H for Mo: 22.50/2.44 vs. 21.28/2.70 for 1, 20.88/2.62 for 2 and 20.32/2.56 for 3; for W: 15.31/1.73 vs. 14.60/1.84 for 1, 14.32/1.80 for 2 and 14.05/1.77 for 3) but much closer to these formulations than to those of compounds with a 1:1 Cp*/M ratio (e.g. for the starting material [Cp*2M2O5]: 44.29/5.58 for Mo and 33.45

We are grateful to the Centre National de la Recherche Scientifique (CNRS), to the Institut Universitaire de France (IUF) and to Celal Bayar University (project FBE 2010-081) for support of this work and to the Région Midi-Pyrénées for the post-doctoral fellowship of CB.


Yaozhong Zhang

Kristin Schröder

Yungwan Kwak

Pawel Krys

Aurélie N Morin

Tomislav Pintauer

Rinaldo Poli

Krzysztof Matyjaszewski

Reversible-Deactivation Radical Polymerization of Methyl Methacrylate and Styrene Mediated by Alkyl Dithiocarbamates and Copper Acetylacetonates

come   L'archive ouverte pluridisciplinaire

diethyldithiocarbamate (MAN-DC) or 2-(N,N-diethyldithiocarbamyl) ethyl isobutyrate (EMA-DC) initiators/transfer agents in the absence of additional reducing agents. Linear first order kinetic plots were obtained for the polymerization of MMA in the presence of [Cu(hfa)2] or [Cu(acac)2] and MAN-DC. [Cu(hfa)2] provided better control than [Cu(acac)2] for the polymerization of MMA, leading to PMMA with narrow molecular weight distribution, Mw/Mn ~ 1.1. Polymerization of St was successfully carried out with either MAN-DC or EMA-DC in the presence of [Cu(hfa)2], also resulting in polymers with low Mw/Mn values. In the absence of alkyl dithiocarbamates or copper acetylacetonates, the polymerizations resulted in only trace amounts of polymers or polymers with high values of Mw/Mn. Thus, the combination of alkyl dithiocarbamates and copper(II) acetylacetonates provides a convenient way to prepare wellcontrolled PMMA and PSt. NMR analysis of low MW polymers reveals the presence of DC groups as chain ends. DFT calculations show that DC group transfer from a H-MMA-DC model of the growing chain to the Cu(II) complexes is energetically accessible and more favorable than Br atom transfer, thus rationalizing the need for the Cu(II)/dithiocarbamate combination for successful control and suggesting that the process takes place by reversible DC group transfer involving a Cu II /Cu III couple. Attempts to synthesize complexes [Cu(acac)2(DC)] and [Cu(hfa)2(DC)], in combination with DFT calculations, suggests that these complexes are thermodynamically unstable relative to the bis(diketonate)copper(II) and dithiuram disulfide, but this does not preclude the involvement of the Cu(III) species as a spin trap in RDRP controlled by DC group transfer.

INTRODUCTION.

Reversible-deactivation radical polymerizations (RDRP) procedures are powerful techniques for synthesizing functional homo-and block (co)polymers with different architectures under mild conditions. (I don't like the word procedure. It is a methodology or technique). [START_REF] Matyjaszewski | Handbook of Radical Polymerization[END_REF] Typical RDRP processes include stable free radical polymerization (SFRP), 2 atom transfer radical polymerization (ATRP), 3 and reversible addition-fragmentation chain transfer (RAFT), an example of a degenerative transfer (DT) process 4 (Scheme 1). All RDRP processes rely on a dynamic equilibrium between dormant species and propagating radicals, which plays a pivotal role enabling formation of polymers with narrow molecular weight distribution (MWD) and predetermined molecular weights and additionally allows design and precise control of molecular architecture and topology. The first SFRP procedure employed iniferters that "induce a radical polymerization that proceeds via initiation, propagation, primary radical termination, and transfer to initiator". 6 Originally dithiocarbamates were used for polymerization of styrene (St) and methyl methacrylate (MMA) yielding relatively poorly controlled polymers with broad MWD. This was due to a very slow exchange process between the propagating radicals and the dormant species.

Subsequently, more efficient SFRP processes were invented, including nitroxide-mediated polymerizations, 2a and organometallic mediated radical polymerization based e.g. on cobalt systems, such as cobalt porphyrins and cobalt acetylacetonate derivatives. 7 ATRP, in which alkyl halides or pseudohalides act as initiators/dormant species that are intermittently activated by transition metal complexes, 3a,8 enables the synthesis of a wide range of polymers with predetermined structures. The dynamic reversible equilibrium between dormant alkyl halides (Pn-X) and lower oxidation state transition metal complexes (Mt n /L), and the respective propagating radicals (Pn•) and higher oxidation state transition metal complexes (X-Mt n+1 /L) is maintained during the ATRP process, as indicated or shown in Scheme 1. Both SFRP and ATRP are based on the persistent radical effect. 9 Recent improvements in ATRP have focused on diminishing the catalyst loading to a few parts per million, as discussed in activators regenerated by electron transfer (ARGET) ATRP, 10 initiators for continuous activator regeneration (ICAR) ATRP, 11 eATRP 12 and photoATRP. 13 RAFT polymerization employs a fast degenerative exchange between various dithio compounds, serving as dormant species, and radicals that must be continuously supplied to the system. 4b,4c Recently, concurrent ATRP/RAFT processes were reported which involved the direct activation of chain transfer agents (CTA)/ (alkyl pseudohalides) initiators with a Cu I X/L (X = Br or Cl; L = N-donor ligand) catalyst that allowed the reaction to proceed without addition of conventional radical initiators, As part of a continued search for new Cu species for this process, we explored the viability of using Cu II acetylacetonates. Some metal acetylacetonates, such as Cu(acac)2, Co(acac)3, and Mn(acac)3, have been reported as free radical initiators 16 for the polymerization of vinyl chloride, 17 St, 18 and MMA, 19 yielding polymers with high MW and broad MWD. Cu(acac)2 has also been employed as a catalyst for other processes, such as ring-opening cycloisomerization, 20 or cyclopropanation. 21 Herein, we report the synthesis of well-controlled PMMA and PSt, using 1-cyano-1methylethyldiethyldithiocarbamate (MAN-DC) or 2-(N,N-diethyldithiocarbamyl) ethyl isobutyrate (EMA-DC) as initiators/transfer agents in the presence of oxidatively stable copper(II) acetylacetonates at 80 and 120°C (Scheme 2) (or between 80 and 120 oC).

Surprisingly, these reactions did not require any reducing agents to convert Cu II species to Cu I , used previously in concurrent ATRP/RAFT. [Cu(acac)2] was studied, targeting a degree of polymerization (DP) of 400, as shown in Figure S1. The resulting PMMA had broad MWD (Mw/Mn ~1.5) and experimental molecular weights (Mn,exp) were higher than theoretical ones (Mn,theo) (Entries 1-4 in Table 1, Figure S1b).

Furthermore, Mn did not increase with conversion, indicating inefficient deactivation and a similarity of the reaction with conventional radical polymerization. Nevertheless, the presence of a higher concentration of [Cu(acac)2], as in Figure S1a, resulted in an increase of the polymerization rate, suggesting a catalytic process for radical generation, even though an oxidatively stable Cu II species was used rather than Cu I , and no reducing agent was added to the reaction.

Subsequently, the polymerization of MMA mediated by [Cu(hfa)2] was studied. Figure 1 and entries 5-9 in Table 1 present the results of a polymerization conducted in bulk at 80 °C. Linear first order kinetic plots were obtained, which indicated a constant concentration of propagating radicals in the system (Figure 1a). The employed ratio of [Cu(hfa)2]0/[MAN-DC]0 was similar to [Cu(acac)2] system; an increase in polymerization rates was observed at higher copper loadings.

The apparent propagation rate constant (kapp) scaled to [Cu(hfa)2]0 0.64 , as shown in Figure S2. In contrast to [Cu(acac)2], the [Cu(hfa)2] system showed excellent correlation between Mn,exp and Mn,theo, indicating high initiation efficiency for the polymerization (Figure 1b). The MWD of PMMA synthesized with [Cu(hfa)2] decreased with monomer conversion, as expected for a wellcontrolled polymerization, and reached values as low as 1.1. After 15 h reaction, a chain extended PMMA with Mn = 57600, Mw/Mn = 1.18 was prepared, indicating an initiation efficiency Ieff = 96%, without any detectable unreacted macroinitiator remaining (Figure 2). This result demonstrates that the synthesized PMMA maintains the ability for highly efficient chain-extension reactions, which is a requirement for well-controlled reversible-deactivation radical polymerization processes. Although PSt samples with narrow MWD (Mw/Mn < 1.2) were obtained, Mn,exp values of the resulting polymers were higher than the theoretical values regardless of initiator type, such as, MAN-DC or EMA-DC (Entries 10-11 in Table 1). This could suggest decomposition of some of the initiators at elevated temperatures (120 o C).

Effect of the Structure of Initiator and Cu Compound.

To further study the effect of the iniferter initiator, polymerizations with typical ATRP initiator ethyl α-bromophenylacetate (EBPA) were examined (Entries 12-15 in Table 1). Polymerizations of MMA with [Cu(hfa)2] resulted in trace amount of PMMA. However, polymerizations with [Cu(acac)2] showed higher conversions than with [Cu(hfa)2]. These results points to a faster radical producing process for [Cu(acac)2] than for [Cu(hfa)2]. Yields were higher for styrene polymerization, presumably due to its thermal self-initiation at higher temperature and longer reaction times (25 h). Lower molecular weights indicate that additional chains were formed by thermal self-initiation.

Regardless, all polymers had very broad molecular weight distributions (Mw/Mn >2.0).

In order to test the role of the copper species, MMA was polymerized in the presence of only alkyl dithiocarbamate, without Cu species (Entry 16 in Table 1 End-Group Analysis. To confirm the role of the dithiocarbamate moiety as (pseudo)halide transferring group, shorter polymer chains were prepared and examined by end-group analysis.

Reaction conditions for the polymerization of St and MMA were similar to those used previously (Table 1, entries 9-11). The reactions were stopped at early monomer conversions and the resulting PSt and PMMA were purified and analyzed by GPC, 1 H NMR, 19 F NMR and MALDI-TOF MS. [START_REF] Matyjaszewski | Handbook of Radical Polymerization[END_REF] H NMR spectra (Figure S5) confirmed characteristic DC end-group signals (diethylamine end groups -N-CH2-CH3, σ (ppm): 3.6-4.0). DP calculated by integration signals of these end-groups in 1 H NMR agrees well with DP obtained from GPC measurements (Table 2). Furthermore, MALDI-TOF MS of PSt (Table 2, entries 1-2) was investigated. The spectra (Figure S6) confirmed the dominant presence of cyanoisopropyl or ethyl isobutyrate initiated polymer chains (with Ag or Na cations). In addition, a second minor series was observed in both spectra. They could be assigned to either [(St)n + Ag + ], resulting from thermally self-initiated chains at 120 o C, or to [hfa -(St)n-2 + Ag + ]. However, 19 F NMR of PSt samples showed no signals, eliminating this possibility and incorporation of hfa groups into polymers.

(b) Mechanistic Investigations

As mentioned before, ATRP is typically mediated by Cu I species as activators and Cu II species act as deactivators. Thus, the latter must be first reduced to initiate ATRP. This can happen by light, electrical current, ligands or even monomer. 13-14,14d,22 It is possible that Cu II acetylacetonates are not stable at elevated temperatures and are in situ reduced to some active Cu I species, assisted by dithiocarbamates (Scheme 3A). Indeed, decomposition of [Cu(acac)2] assisted by water vapors under harsh conditions, resulting in the oxidized acac ligand and Cu or Cu2O was reported. 23 Also, the homolysis of metal-oxygen bonds in metal acetylacetonates, producing acetylacetonate radicals, was proposed. 16a,18 Decomposition of γ-substituted (but-1enyl) [Cu(acac)2] resulted in polymerization of MMA. 19,24 Thus, an attempt to observe the decomposition [Cu(acac)2] via UV-Vis was conducted. However, the decrease in the UV signal intensity of [Cu(acac)2] at 650 nm in DMSO solution was very limited, only 7.4% after 23 h, while no noticeable decrease of [Cu(hfa)2] signal at 860 nm in DMSO solution was observed at T = 80 o C for nearly one day, as shown in Table S1 & S2. Subsequently, polymerization of MMA with [Cu(acac)2] but without alkyl dithiocarbamate initiator was conducted (Table S3). The [Cu(acac)2] system resulted in only 9% monomer conversion after 44 h, and the [Cu(hfa)2] system produced nearly no polymer at [MMA]0/[Cu II ]0 = 200/1. Thus, plausibly, copper(II) acetylacetonate could react with alkyl dithiocarbamate and form reversibly Cu III species or could decompose slowly at elevated temperatures and produce activators in situ (Scheme 3). Me2C(COOMe)-X reagent where the Me2C(COOMe) group models the polymer radical chain PMMA • (i.e., the polymer chain beyond the ultimate monomer unit is replaced with an H atom), allowing simplification of the calculations of the reaction energetics. This simplification is likely to introduce a systematic error related to the penultimate effect, known to be important for the MMA polymerization 29 , and previously estimated by DFT calculations as 2-3 kcal/mol in ΔH for MMA at room temperature depending on the nature of X (Cl or Br) 30 . As the effect is steric and should be stronger in X-CMe2(COOMe) than in • CMe2(COOMe), the equilibria will be calculated as too endothermic by only a few kcal/mol. However, this approximation will affect all the equilibria in a similar way and will have no dramatic effect on the conclusions of the study, which are based on the relative energy values for the different equilibria. The X atom/group in Scheme 5 was either Br, to investigate the likelihood of a suitable ATRP We now turn our attention to the energy changes related to the group transfer equilibria illustrated in Scheme 5 (Table 3). All equilibria are endothermic. For each compound, the cost of the atom or group transfer is lower when the product has a triplet state configuration, since all Cu III products are predicted as more stable as triplets at the chosen level of theory. However, the endothermicity is lower for the DC group transfer than for the Br atom transfer. This signifies that the DC group transfer from the RAFT transfer agent should occur more easily than the Br atom transfer from a classical ATRP initiator/dormant species. Thus, these results are consistent with the presence of controlled polymer growth in the presence of a dithiocarbamate initiator and with the absence of polymerization (for the hfa system) in the presence of a bromide initiator.

Discussion

The combined experimental and computational mechanistic investigations point toward a rationalization based on DC group transfer and involving a Cu II /Cu III redox pair for the controlled polymerization of styrene and MMA by the [Cu(acac)2] or [Cu(hfa)2]/MAN-DC system, in the absence of external radical sources or reducing agents. This is made possible by the lower energy involved for the DC group transfer from the dithiocarbamate initiator and from the end of the growing chain to the Cu II , relative to a Br atom transfer in ATRP.

The fact that the proposed [Cu III (acac)2(DC)] and [Cu III (hfa)2(DC)] are thermodynamically unstable relative to the corresponding Cu II and DC-DC does not preclude the in situ generation of such species by group transfer and their function as spin traps. The reason for this is that the energy required to accomplish the group transfer is kinetically compatible with the polymerization rate. Once the Cu III complex is formed together with the active radical chain, it has the choice of either deactivating the radical or decompose by reduction and formation of DC-DC. However, whereas the former process is first order in Cu III (and first order in radical), the latter one is second order in Cu III and will consequently be disfavoured at low concentrations. This is the same principle governing the preferred spin trapping of the reactive radical chain relative to the bimolecular radical-radical couplings and disproportionations. This principle, according to which the spin trapping species in controlled radical polymerization need not be a stable species, is not unprecedented.

Scheme 2 .

 2 Scheme 2. Reversible-deactivation radical polymerization of MMA and styrene with MAN-

[Figure 1 .

 1 Figure 1. (a) Kinetic plots of ln([M]0/[M]) vs. time; (b) plot of Mn and Mw/Mn values vs. conversion for MMA/MAN-DC/[Cu(hfa)2] = 400/1/X at 80 o C in bulk.

  bulk; b Target DP = 400; c Target DP = 200; d [AIBN]0:[MAN-DC]0 = 0.5; e [Cu(hfa)2]0:[AIBN]0 = 2. Chain Extension Experiments and Polymerization of St. The living character of the synthesized PMMA was demonstrated by chain extension with MMA from a PMMA macroinitiator (Mn = 6000, Mw/Mn = 1.39, prepared by a polymerization with [Cu(hfa)2]). Chain extension was carried out at 80 o C with a ratio of MMA/macroinitiator/[Cu(hfa)2] = 800/1/1.

Figure 2 .

 2 Figure 2. Chain extension of PMMA as a macroinitiator with MMA and [Cu(hfa)2].

Scheme 3 .Scheme 4 .

 34 Scheme 3. Proposed putative mechanisms for the RDRP of MMA and St with copper(II)

Scheme 5 .

 5 Scheme 5. Model equilibria investigated by the DFT calculations.

k t P n -P m P m k t P n -P m P m k t P n -P m P n k ini Initiator R Scheme

		4g,5
					P n -P m
				P m
					k t
	SFRP	P n -T	k a k d	P n + R	T
				k p
	ATRP	P n -X	Mt/L	k a k d	P n + R	X-Mt n+1 /L
					k p
	DT	P n -X	+ R P m	k ex k ex	P n + R	P m -X
			k p		k p

1. Examples of RDRPs: SFRP, ATRP, and DT polymerization.

Table 1 .

 1 Results for the polymerization of MMA and St with [Cu(acac)2] or [Cu(hfa)2]

	Entry Monomer Initiator	Cu-species a	[Cu]0 [Initiator]0 /	T ( o C)	Time (h)	Conv. (%)	Mn,theo Mn,GPC	Mw/Mn
	b	MMA	MAN-DC	1	0.01	80	8	28	11,200 28,000 1.57
	b	MMA	MAN-DC	1	0.02	80	8	36	14,400 27,500 1.51
	b	MMA	MAN-DC	1	0.05	80	8	46	18,400 27,300 1.54
	b	MMA	MAN-DC	1	0.2	80	8	55	22,000 23,800 1.53
	b	MMA	MAN-DC	2	0.05	80	8	21	8,400 9,200	1.26
	b	MMA	MAN-DC	2	0.1	80	8	27	10,800 10,800 1.22
	b	MMA	MAN-DC	2	0.2	80	8	42	16,800 26,600 1.12
	b	MMA	MAN-DC	2	0.5	80	8	64	25,600 21,300 1.12
	b	MMA	MAN-DC	2	1	80	8	69	27,600 23,400 1.09

a General polymerization conditions: Monomer with [Cu(acac)2] (1) or [Cu(hfa)2] (2) and various initiators in

  ). High molecular weight polymer with a broad MWD (Mw/Mn ~ 2.1) excludes a SFRP/DT mechanism for control, as previously reported. 6b To test the possibility of a RAFT type process, polymerization of MMA with MAN-DC and 2,2′-azobisisobutyronitrile (AIBN) was carried out (Entry 17 in Table 1) (should be 2,2'). High molecular weight polymers with broad MWD were obtained already after one hour, indicating typical behavior of conventional radical polymerization with insufficient contribution of exchange. Polymerization with AIBN and [Cu(hfa)2] behaved in a similar manner (see entry 18 in Table 1). These results clearly indicate that the presence of both MAN-DC/EMA-DC and [Cu(acac)2]/[Cu(hfa)2] is necessary to achieve RDRP behavior.

Table 2 .

 2 Characterization of shorter polymer chains

	Entry Monomer Initiator	Mn,GPC a Mw/Mn,GPC a DP,GPC a DP,NMR b
	1	St	MAN-DC 6.53*10 3	1.08	~63	~57
	2	St	EMA-DC 4.71*10 3	1.13	~45	~47
	3	MMA	MAN-DC 6.06*10 3	1.28	~60	~59
	a Determined by size exclusion chromatography in THF using polystyrene standards,
	b Determined by 1 H NMR in CDCl3

Table 3 .

 3 Calculated equilibrium parameters (kcal/mol) for the atoms transfer or group transfer equilibria for the MMA polymerization mediated by [Cu(acac)2] or [Cu(hfa)2] and dithiocarbamate (Scheme 5).

	L	X	S ΔE	ΔH	ΔG	L	X	S ΔE	ΔH	ΔG
	acac Br	0	52.9 51.5 48.9 hfa	Br	0	57.9	56.4	53.6
	acac Br	1	44.5 42.9 39.7 hfa	Br	1	49.3	47.1	45.2
	acac DC 0	50.8 48.8 44.1 hfa	DC	0	39.1	37.4	33.4
	acac DC 1	38.5 36.8 32.8 hfa	DC	1	31.3	29.7	25.0

  The same phenomenon was previously demonstrated for the ATRP of styrene with control by [Mo III I3(PMe3)3], where the proposed [Mo IV I3Br(PMe3)3] product of atom transfer was shown to be a thermodynamically unstable species with respect to [Mo III I2Br(PMe3)3] and I2 36 . CONCLUSIONS. Well-controlled radical polymerization of MMA and styrene were conducted with halogen-free alkyl dithiocarbamate initiators (MAN-DC, EMA-DC) and oxidatively stable copper(II) acetylacetonate derivatives. PMMA and PSt with narrow MWD were obtained (Mw/Mn < 1.1 for PMMA and Mw/Mn < 1.2 for PSt). It was found that the polymerization system containing [Cu(hfa)2] provided better control than that with [Cu(acac)2].Chain extension of a PMMA macroinitiator prepared with the [Cu(hfa)2] catalyst with MMA exhibited high initiation efficiency (96%) and formed a higher MW polymer with a narrow MWD, Mw/Mn ~ 1.2. Mechanistic studies indicate that the polymerization proceeds via a reversible DC group transfer process generating Cu III -DC species, which are competent radical chain deactivators in spite of their thermodynamic instability. This constitutes the first reported example of a controlled radical polymerization involving the Cu II /Cu III redox pair.For Table of Contents Use Only Reversible-Deactivation Radical Polymerization of Methyl Methacrylate and Styrene Mediated by Alkyl Dithiocarbamate and Copper Acetylacetonates Yaozhong Zhang, Kristin Schröder, Yungwan Kwak, † Pawel Krys, Aurélie N. Morin,

‡ Tomislav Pintauer, ‡,± Rinaldo Poli, ‡ and Krzysztof Matyjaszewski* .

AUTHOR INFORMATION

Corresponding Author

* E-mail: km3b@andrew.cmu.edu.

Present Addresses † Silberline Manufacturing Co., Inc., 36 Progressive Avenue, Tamaqua, PA, 18252.

Author Contributions

All authors have given approval to the final version of the manuscript.


Béatrice Guérin

Daniel Mesquita Fernandes

Jean-Claude Daran

Dominique Agustin

Rinaldo Poli

Investigation of induction times, activity, selectivity, interface and mass transport in solvent-free epoxidation by H2O2 and TBHP: a study with organic salts of the [PMo12O40]3-anion

come

Investigation of induction times, activity, selectivity, interface and mass transport in solvent-free epoxidation by H2O2 and TBHP: a study with organic salts of the [PMo12O40]3-anion

Introduction

Among the Green chemistry principles, catalysis plays a key role. [START_REF] Anastas | Green Chemistry: Theory and Practice[END_REF][START_REF] Tundo | [END_REF][3] In addition to obvious economical reasons related to catalyst loss and to separation and/or regeneration costs, efficient catalyst recovery and recycling also has a positive impact on the environment. The catalyst must be active and stable under catalytic conditions, its leaching should be minimal and its recovery must be facile. Both economical and environmental concerns are also leading industry to consider the implementation of solvent-free operating conditions. [START_REF] Regulation | 2006 of the European Parliament and Of the Council[END_REF] Oxidation reactions carried out in the absence of organic solvents have received so far little attention. [5][6][7][8][9][10][11][12] We are currently investigating solvent-free epoxidation. Our investigations have so far addressed catalysts based on molybdenum(VI) and vanadium(V) oxo-complexes surrounded by tridentate ligands. 13- vanadium complexes, [MoO2L 2 ]2 and [VOL 2 ]2O (L 2 = Schiff base ONO tridentate ligand derived from aminoalcohols) were also found to be highly active. Although the [MoO2(SAP)] fragment (SAP = salicylideneaminophenolato) is stable during the process, other ONO tridentate Schiff base ligands suffered from hydrolysis under the reaction conditions. It was interesting to explore more deeply this model reaction using "old" but very stable and relatively simple compounds, the polyoxometalates (POMs), which do not have surrounding ligands that could potentially suffer from decomposition processes under the aggressive reaction conditions.

POMs have been known for quite a long time [16][17][18][19] and have attracted interest as chromogenic devices and for applications to biological processes and to catalysis, including heterogeneous and homogeneous industrial processes. [20][21][22] They are very 50 efficient catalysts for pulp bleaching or for the transformation of cellulose. [23][24][25] Oxidation reactions catalyzed by POMs usually work with several oxidants (H2O2, H2O2-urea, TBHP, O2 26,27 ) and transform different substrates such as alcohols 5, [28][29][30][31][32] sulphurcontaining compounds [33][34][35][36] and olefins. [37][38][39][40][41][42] Due to their 55 robustness under a variety of experimental conditions, the most often used POMs in catalysis are the Keggin type, [43][44][45][46][47] particularly those containing tungsten since they generally show greater activity. 48 The stoichiometric oxidation of tungsten containing POMs leads to the generation of peroxido derivatives that are 60 believed to be the active forms in catalysis. One of the most active systems is the so called Venturello catalyst, i.e. a peroxopolyoxotungstate of formula [PW4O24] 3-obtained from [PW12O40] 3-with excess H2O2. This anion has been used as a catalyst under phase transfer conditions in the presence of 65 substituted imidazolium cations as transfer agents. [49][50][51][52] These catalytic reactions were mainly performed in the presence of an organic solvent, including Pickering emulsion systems. 42 One of the best systems is known as the Ishii-Venturello catalytic system, indicated by W VI /P V /H2O2/CHCl3/PTC (PTC = phase 70 transfer catalyst). 53,54 Recently, solvent-free conditions were reported to yield good activity for the epoxidation of different olefins, especially cyclooctene, in the presence of a peroxopolyoxotungstate-based room temperature ionic liquid. 12 Other peroxotungstates exhibited 75 interesting results in the presence of ethylacetate as solvent, which is less toxic than the chlorinated solvents that are more

2 | Journal Name, [year], [vol], 00-00
commonly used in this process. 55,56 The molybdenum equivalent of formula [PMo4O24] 3-was shown to be an active catalyst under homogeneous conditions. 57 However, although it has been used in many catalytic applications such as oxygen transfer from sulfoxides, 58 methacrolein oxidation, 59 or the Meyer-Schuster rearrangement of propargyl alcohols, 60 the epoxidation of olefins in the presence of this catalyst has been poorly investigated. 61 We present in this article the application of various salts of the [PMo4O24] 3-ion at low catalytic loading and under solvent-free conditions to the epoxidation of cyclooctene by both aqueous TBHP and H2O2. The determination of the kinetic profile and the evolution of the selectivity allow interesting considerations on the chemical environment of the catalytic transformation. The IR and NMR properties of the new BP salts are as expected (see Experimental section). In addition, a single crystal obtained for the (BP)3[PMo12O40] salt was analyzed by X-ray diffraction.

Results and Discussion

The molecular unit is composed of the well known Keggin

[PMo12O40] 3-anion surrounded by three butylpyridinium cations (see Figure 1). All the bonding parameters in the trianion agree with those found in the structures of other salts containing the same POM anion and in the hydrated heteropolyacid. [63][64][65][66][67] There are no short contacts between cations and anion, the former lacking any active proton.

(b) Catalytic studies

All synthesized salts have been tested as catalysts for the epoxidation of cyclooctene as model substrate, using either hydrogen peroxide (35 wt% in water) or TBHP (70 wt% in water) as oxidant (Scheme 2), with emphasis on the less extensively investigated Mo species. The reaction was carried out at 80°C in 50 the absence of any extra organic solvent, hence under aqueous biphasic conditions. As previously established, the TBHP oxidant partitions mainly in favour of the substrate phase under these conditions, 15 whereas H2O2 remains in the aqueous phase. The influence of cation and oxidant has been investigated by 60 following the conversion and selectivity at a relatively low precatalyst loading (0.1 mol % vs. substrate) for 24 h, using an oxidant/substrate ratio of 1.5. The amount of catalyst is expressed in terms of POM content rather than metal content, since the number of catalytically active metal atoms in the anion is unclear. 65 The POM salts appear totally insoluble in the reaction medium at room temperature, but partially dissolve at the reaction temperature in the substrate phase (see below for more details). The results of the kinetic monitoring are shown in Figure 2 (conversion) and Figure 3 (selectivity). All catalytic solutions 70 gave a rapid phase separation when stirring was stopped, allowing the clean withdrawal of samples of the supernatant organic phase.

Most other reports of epoxidations catalyzed by POM compounds and notably those carried out under heterogeneous or 75 homogeneous biphasic conditions report only the final conversion and selectivity data. As evidence in Figure 2, the profile under our conditions depends on oxidant and cation nature. An induction period was observed when using H2O2, longer for the TBA salt, intermediate for BP and not detectable for CP, whereas 80 no induction period could be detected for any of the three salts when using TBHP. On the other hand, the activity for the oxidations with TBHP decreases in the order BP > CP > TBA.

Figure 3 shows that the selectivity also depends on the nature of the oxidant and cation. For the oxidations with TBHP, the 85 epoxide selectivity is the range of 65-75% for all Q3[PMo12O40] Journal Name, [year], [vol], 00-00 | 3 catalysts. The only other observed product is the transcyclooctane-1,2-diol and no isomeric cis-diol was detected. For the H2O2 oxidations, on the other hand, the TBA and BP catalysts yield low selectivities (20-30%) from the very beginning of the reaction, the only other observed product being once again the trans-diol. The CP catalyst, on the other hand, yields an initially high selectivity, comparable to that of the TBHP oxidation, then decreasing at higher conversion toward values comparable to those obtained with the other catalysts. 1 (same labels and lines as in Figure 1).

It should be remarked that the reactant and products were only analyzed in the organic layer. Cyclooctene and cyclooctene oxide are not strongly water soluble, therefore the determination of the 20 epoxide selectivity (epoxide yield/cyclooctene conversion) from this measurement should be accurate. The diol product, on the other hand, partitions in both phases as also suggested by indirect observations (see later). Therefore, mass balance in the organic phase is not expected and indeed not observed (see fuller details below). We also cannot exclude the formation of other byproducts that are entirely confined in the water phase or that do not travel through the GC column.

Where does the catalysis take place? 30 The rationalization of the trends shown in Figure 2 and Figure 3 requires an understanding of where the catalytic transformation takes place. The two most reasonable possibilities are a homogenous process catalyzed by the dissolved POM salt within the bulk of the reactant phase, and a heterogeneous process on the 35 surface of the undissolved catalyst, since the solubility of both catalyst and substrate in water is negligible. The question is not a trivial one, because numerous reports exist of oxidation reactions (of olefin, alcohols, diols, ..) in organic solvents such as chloroform, benzene, tert-butanol, etc. by aqueous H2O2 run 40 under phase-transfer conditions, which is generally considered as a homogeneous phase process. This includes the use of (CP)3[PMo12O40] as oxidation catalyst. 54,61,[68][69][70] However, a recent report on olefin epoxidation (including cyclooctene) by aqueous H2O2 using toluene as solvent and 45 (C12H25NMe3)3[PW12O40] as catalyst emphasized a heterogeneous process occurring at the water-organic interface of Pickering emulsions where the catalyst is located in the form of solid nanoparticles. 42 A significant solubility difference may of course exist between the dodecyltrimethylammonium salt of the 50 phosphotunsgate Keggin ion in toluene and the cetylpyridinium salt of the phosphomolybdate Keggin ion in neat cyclooctene.

In order to learn more about the catalyst solubility in the organic phase at the reaction temperature, recovery and recycling experiments were carried out, as well as catalytic runs at lower 55 catalyst loading. From the experiments run under the conditions of Figure 2 (0.1% catalyst loading), the catalyst was recovered at the end of the reaction by filtration from the crude solutions. In the case of BP and TBA, > 85% of the catalyst was recovered from both the H2O2 and the TBHP oxidation processes. A lower 60 amount of solid (ca. 70%), on the other hand, was recovered from the experiments with the CP salt. This suggests a greater catalyst solubilisation of the CP salt in the organic phase, which appears consistent with the longer hydrophobic chain in the CP cation. The different catalyst solubility and the hypothesis of a 65 homogeneous process would seem consistent with the higher activity of the CP catalyst for the epoxidation with H2O2, although it is slightly surpassed by the BP salt for the epoxidation with TBHP. The TBA salt, on the other hand, results in the lowest activity with both oxidants. Weak activity of TBA salts 70 was already reported with other types of polyanions and directly linked to their lower solubility. 47 In order to rationalize the results on the basis of a heterogeneous process, it would be necessary to invoke a difference in active surface related to the granule size of the solid catalyst and/or in a counterion steric effect. 75 Catalytic runs for the epoxidation by TBHP with different catalyst loadings were carried out with the more active BP and CP salts by measuring the conversion and selectivity after 24 h at 80°C. The results are reported in Table 1 and illustrated in Figure 4. This investigation shows that significant conversions can be 80 obtained even at extremely low catalyst loadings (down to 0.0002% or 2 ppm for the CP salt) for turnover frequencies up to 66000 h -1 (18 s -1 ). A control experiment run under the same conditions without catalyst shows only a 6.5% conversion in 24 h with 29% epoxide selectivity. The positive effect of the catalyst, 85 even at 2 ppm level, on activity and selectivity is therefore demonstrated. Figure 4 shows that the epoxide yield after 24 h is relatively independent on catalyst loading down to certain cutoff value, which is much lower for the BP salt (0.002%) than for the CP salt (0.03%). The study confirms that the maximum conversion at 24 h is slightly greater for the BP salt (cf. Figure 1). These results do not seem consistent with a fully 10 heterogeneous process, for which the efficiency should be proportional to the active surface area and the yield would therefore be expected to reach 100% at higher loadings. Rather, they seem consistent with a saturation effect and a mostly homogeneous process. At loadings lower than the cutoff value, when all catalyst is presumably dissolved, the activity remains greater for the BP salt. This phenomenon may be caused by the cation steric effect since the dissolved catalyst presumably remains associated in the form of ion pairs at the higher concentrations. The two profiles, however, tend to converge at 20 the lowest catalyst concentrations as may be expected for an equilibrated ion pair dissociation and catalytic action by the free POM ion.

A brief recycling study, run with catalytic loadings greater than the cut-off value of Figure 4, shows that the recovered catalyst is still active for all salts (Table 2). Incidentally, spectroscopic analyses (IR, 1 H and 31 P NMR) on the solids recovered at the end of the reactions confirmed the catalyst stability. The decreasing amount of solid catalyst recovered and used in subsequent recycles confirms the partial catalyst solubility, while the approximately constant conversion confirms the saturation effect discussed above.

Rationalization of the selectivity

Turning now our attention on the question of the epoxide selectivity (Figure 3), the relatively low observed selectivity, even for the epoxidation by TBHP, contrasts with the high selectivity generally reported for reactions conducted in an 40 organic solvent, for instance > 99% for both the phase transfer conditions with Venturello's system 71 and the Pickering emulsion system. 42 However, epoxides can be ring-opened in the presence of water, especially under acidic conditions. The pH of the aqueous phase during our catalytic runs was ca. 4 and did not 45 change the course of the reaction. It is well known that the impact of the epoxide ring opening reaction is reduced when the epoxide is protected in a hydrophobic solvent where the water solubility is low and when the time of contact with the aqueous phase is limited. 49 No cis isomer that can potentially be obtained by 50 concerted dihydroxylation could be detected; all diol formed under our conditions has the trans stereochemistry, therefore clearly resulting from the epoxide ring opening process.

The lower selectivity observed for the H2O2 epoxidations can be rationalized by the greater amount of water (the aqueous H2O2 55 solution has a lower concentration of oxidant -30% -than the aqueous TBHP solution -70%), leading to faster ring opening. In addition, the TBHP epoxidation produces tBuOH whereas the H2O2 epoxidation produces additional water. The relatively constant selectivity with conversion observed in the TBHP case 60 (Figure 3) probably results from compensation between additional epoxide formation and epoxide ring opening. Leaving the final reaction mixture for a longer time after complete cyclooctene conversion would certainly result in further ring opening with decrease of epoxide selectivity. For the oxidations 65 with H2O2, the slower initial epoxidation with the TBA and BP salts also results in compensation of epoxide production and ring opening rates resulting in the observed relatively constant selectivity (Figure 3). On the other hand, the faster conversion obtained with the CP salt results in initially greater epoxide 70 selectivity, similar to that of the epoxidations by TBHP, followed by a loss of selectivity when the ring opening process becomes more important. It is clear from these results that solvent-free conditions are not suitable when the desired product is the epoxide, but may provide advantages when the desired product is 75 the trans-diol.

Further relevant information is provided by a more detailed analysis of the reaction conversion and selectivity at short reaction times for the reaction with H2O2. Figure 5 shows that formation of the epoxide product does not start immediately, whereas some cyclooctene is slowly converted even in the initial induction phase. However, trans-cyclooctane-1,2-diol is produced immediately, even though this is detected in very small amounts in the organic phase because most of it is retained in the aqueous phase. The continuous decrease with time of the total measured mass indicates the increasing production of water soluble product(s). More notable epoxide formation begins only at ca. 2 h from the start. Our interpretation of this phenomenon is that the initial slow cyclooctene consumption occurs by 10 heterogeneous catalysis at the organic-water interface where, given the large presence of water, the epoxide is mostly opened to the diol. Subsequently, as more H2O2 is able to migrate toward the bulk of the organic phase, the epoxide selectivity increases by virtue of the homogeneous process (see also next section).

Rationalization of the induction period

The final question of interest related to the results of Figure 2 concerns the induction period and its dependence on the type of oxidant and cation. Clearly, the absence of an induction period for the TBHP epoxidations, where oxidant and catalyst are present in the reactant phase from the very beginning of the process, and its presence for the H2O2 epoxidation, where the oxidant is initially present only in the aqueous phase, reinforces the idea that the catalyzed reaction mostly occurs in the bulk of the organic phase. Therefore, initial difficulty for the epoxidation by H2O2 consists in bringing the oxidant into the organic phase. The presence of an induction period may be rationalized under the hypothesis that the products of the reaction (the epoxide and/or the diol) facilitate the H2O2 transfer from the aqueous to the organic phase through a compatibilization effect.

In order to test this hypothesis, two additional catalytic experiments were carried out under the same conditions of the initial experiments for Figure 2 The results are shown in Figure 6 in comparison with those of 50 the parent system (100 % cyclooctene). As can be seen from the kinetic profiles, the initial rate is slower in the presence of epoxide than in the presence of diol, but both transformations occur without a notable induction period. Hence, we can conclude that the induction period is indeed related to a 55 compatibilization problem, rather than to the need for a catalyst activation phase. The reaction products, particularly the diol, have indeed a positive effect in increasing the concentration of the polar oxidant H2O2 (and thus presumably also the water solvent) in the organic phase. Note that a rapid catalyst 60 deactivation occurs in the presence of diol after ca. 1 h of reaction, before the conversion reaches the expected maximum of 50% (conversion is plotted with respect to the total number of moles, cyclooctene + additive, in order to directly compare the different runs). The conversion continues beyond this time, on the 65 other hand, for the reaction carried out in the presence of the epoxide. Thus, the diol formation in the parent experiments of Figure 2 and Figure 3 has several effects, one being the acceleration of cyclooctene conversion through the oxidant transport into the cyclooctene phase, a second one being the 70 acceleration of ring opening by also increasing the water concentration in the organic phase, and a third one being the catalyst deactivation.

Additional observations of interest are related to the selectivity and mass balance. The results of the monitoring of all 75 concentrations in the organic phase for the two experiments are presented in Figure 7 for the experiment run in the presence of epoxide and in Figure 8 for the experiment run in the presence of diol. In the presence of epoxide, the diol formation starts once again immediately, but the epoxide production also occurs 80 without induction time and to a much greater extent than that of the diol, with the initial epoxide selectivity being nearly 100%. The selectivity is calculated from the amount of epoxide produced in the reaction (and not the total amount) relative to the consumed cyclooctene. The mass balance is much better in this 85 case relative to the experiment without additive (cf. with Figure 6 | Journal Name, [year], [vol], 00-00 5), probably because the large amount of epoxide changes the solvent properties of the organic phase making it a better solvent for the diol relative to water. In the presence of the diol, on the other hand, the mass balance is terrible, with a large amount of diol disappearing initially. This confirms that the diol affinity in a cyclooctene/water biphasic 20 system is largely in favor of water. As the reaction progresses, on the other hand, the diol concentration in the organic phase increases again while the epoxide starts to form. The epoxide selectivity in this catalytic run is much lower, the reaction producing nearly only diol at the beginning and then increasing as 25 the reaction progresses. In summary, the above experiments confirm the tendency of the epoxide to produce diol by ring opening in the presence of a large amount of diol and its little tendency when the organic phase is rich in epoxide and poor in diol, consistent with the greater efficiency of the diol as a vector 30 for bringing water into the organic phase.

The above described experiments rationalize the presence of an induction period for the H2O2 epoxidation catalyzed by the TBA and BP salts, but does not explain why this phenomenon is absent when the same reaction is catalyzed by the CP salt (Figure 35 2). A possible explanation, which is supported by visual evidence, is that the longer hydrophobic chains of the CP cation are better able to stabilize micelles containing the substrate phase in the core and having the POM anions at the surface. Indeed, the reaction medium in the presence of the (CP)3[PMo12O40] catalyst 40 has the aspect of an emulsion, which phase separates much more slowly when stirring is stopped relative to the mixtures containing the TBA and BP catalysts. Therefore, the surface contact between the two phases is greatly increased and mass transport is accelerated relative to the stirred bulk phases in the 45 presence of the other salts. The role of salts of POM anions as micelle stabilizing surfactants has also been described in the case of alcohol oxidation when performed in water/organic solvent biphasic systems. 72 50

Influence of transition metal: Mo vs. W

In several processes involving Keggin polyanions, the tungsten salts have been described as more active than the molybdenum counterpart in the presence of H2O2, although the opposite trend has been reported for oxidations with TBHP. 70,73 We wished to 55 establish whether the same trend remains valid under solvent free conditions. Hence, analogous epoxidation processes to those described in Figure 2 were also run with both H2O2 and TBHP using the BP salt of the [PW12O40] ion. As shown in Figure 9, the (BP)3[PW12O40] salt indeed 65 performs more poorly than the Mo analogue in combination with TBHP and better than the Mo analogue in combination with H2O2. The differences, however, are not dramatic. A similar induction effect appears to be present for the H2O2 epoxidation catalyzed by the W species.

Conclusions

We have described in this article the use of three salts of the [PMo12O40] 3-ion with tetrabutylammonium (TBA), butylpyridinium (BP), and cetylpyridinium (CP), as well as the BP salt of the analogous tungsten complex [PW12O40] 3-, in the epoxidation of cyclooctene under solvent-free conditions. The phosphomolybdate compounds, particularly those with the BP and CP cations, show very high catalytic activity, the highest measured TOF being in excess of 60000 h -1 at 2 ppm catalyst loading for the CP salt and can be recycled when used in 10 sufficiently large amount to exceed the solubility limit. The investigation gives compelling evidence in favour of a homogeneous process occurring in the bulk of the organic phase. However, the use of water as the carrier of the oxidizing reagent (and also formed as a co-product when using H2O2) lead to significant epoxide ring opening to yield the trans-diol. Thus, solvent free conditions in combination with aqueous solution of the oxidant are not recommended when high epoxide selectivities are desired. The observed induction period for the H2O2 oxidations has been demonstrated to be related to mass transport 20 limitations for the H2O2 reagent, these being attenuated by the presence of the epoxide product and especially by the diol product of further ring opening, which act as phase compatibilizers. The (BP)3[PW12O40] salt yield results quite comparable with those of the Mo analogue, being slightly less efficient for the epoxidation by TBHP and more efficient when using H2O2.

Experimental part.

Starting materials

All

Instrumentation

Infrared spectra were recorded in KBr matrices at room temperature with a Mattson Genesis II FTIR spectrometer or using the ATR mode with a Nicolet FTIR spectrometer. [START_REF] Anastas | Green Chemistry: Theory and Practice[END_REF] H and 45 31 P NMR spectra were recorded on a Bruker Avance DPX-200 spectrometer. Catalytic reactions were followed by gas chromatography on an Agilent 6890A chromatograph equipped with FID detector, a capillary column (either an HP5-MS, 30 m  0.25 mm  0.25 μm; or an SE-30, 30 m  0.32 mm  0.25 μm) and automatic sampling. The GC parameters were quantified with authentic samples of the reactants and products. The conversion of cis-cyclooctene and the formation of cyclooctene oxide were calculated from calibration curves (r 2 = 0.999) relatively to an internal standard.

Synthesis of (BP)3[PMo12O40]

Using To 2.00 g (0.87 mmol) H3

Synthesis of (BP)3[PW12O40]

Using the same method B described above for the BP salt of [PMo12O40] 3-, H3

Crystal Structure determination

A single crystal was mounted under inert perfluoropolyether at the tip of a glass fibre and cooled in the cryostream of an Oxford-Diffraction XCALIBUR CCD diffractometer. The data were 95 collected using monochromatic MoK radiation (=0.71073 Å). The structure was solved by direct methods (SIR97) 74 and refined by least-squares procedures on F 2 using SHELXL-97. [START_REF] Sheldrick | SHELXL97. Program for Crystal Structure refinement[END_REF] All H atoms were introduced at idealized positions and treated as riding on their parent C atoms. The drawing of the molecules was 100 realised with the help of ORTEP32. [START_REF] Burnett | ORTEPIII[END_REF][START_REF] Farrugia | [END_REF] Crystal data and refinement parameters are shown in In a typical experiment, cyclooctene (1 equiv) and Q3[PM12O40] (x equiv, see tables) were mixed together in a round bottom flask and stirred in air. Acetophenone was added as internal standard.

The reaction temperature was regulated at 80°C and then 1.5 equiv of aqueous THBP or aqueous H2O2 was added to the mixture under stirring, setting the zero reaction time. Samples of the organic phase were periodically withdrawn after stopping stirring and phase separation (liquid/liquid/solid), which was rapid for the TBA and BP salts, slower but still allowing withdrawal of the clear organic phase for the CP salt. For each aliquot, the reaction was quenched by addition of MnO2, followed by the addition of diethylether and removal of the manganese oxide and residual water by filtration through silica before GC analysis.

(b) Catalytic runs in the presence of additives

With cyclooctene oxide. The procedure was identical to that described in the previous section, except for use of 0.55 equiv cyclooctene and 0.45 equiv of cyclooctene oxide, with 0.01 equiv of (BP)3[PMo12O40]. The experiment was run with 1.5 equiv of

H2O2.

With trans-cyclooctane-1,2-diol. The same procedure was again followed, this time starting from 0.57 equiv of cyclooctene and 0.43 equiv of trans-cyclooctane-1,2-diol.

(c) Recovery Procedure

At the end of a typical catalytic run as described above run with a 0.1% POM/substrate ratio, the solid was recovered by direct filtration of the reaction mixture at room temperature. The solid was then rinsed with water, ethanol and diethyl ether, and then 35 dried and weighted (see Results). For each salt, the spectroscopic analyses of the recovered solids revealed no change relative to the introduced material.

assistance.

Notes and references

( a )

 a Scheme 1-X-Ray Structure of (BP)3PMo12O40. Hydrogen atoms were omitted for clarity.

55

 55

Figure 1 -

 1 Figure 1-X-Ray Structure of (BP)3PMo12O40. Ellipsoids were drawn at the 30% probability level and hydrogen atoms were omitted for clarity.

Figure 2 - 15 Figure 3 -

 2153 Figure 2 -Time dependence of the cyclooctene conversion in the presence of Q3[PMo12O40]. Q = BP () TBA (◊) CP (). Oxidant = H2O2 (dotted lines), TBHP (plain lines). Conditions: cyclooctene/oxidant/POM = 1000/1500/1. T= 80°C.

Figure 5 -

 5 Figure 5-Time evolution of cyclooctene oxide yield (), transcyclooctane-1,2-diol yield (, x50), residual cyclooctene (◊),total (x), and cyclooctene oxide selectivity (• and dashed line) measured in the organic phase for the reaction of cyclooctene with H2O2 (1.5 equiv) 20

45 Figure 6 -

 456 Figure 6-Cyclooctene conversion vs. time for the (BP)3[PMo12O40]catalyzed epoxidation by aqueous H2O2. Conditions: (cyclooctene + additive)/H2O2/POM = 1000/1500/1. T= 80°C. Additive: none () (same as in Figure 2), cyclooctene oxide (), trans-cyclooctane-1,2-diol (◊).

Figure 7 -Figure 8 -

 78 Figure 7-Time evolution of cyclooctene oxide yield (),

60 Figure 9 -

 609 Figure 9-Cyclooctene conversion vs time for the cyclooctene epoxidation in presence of (BP)3[PM12O40]. M = Mo (), W (); oxidant = H2O2 (dotted lines) TBHP (plain lines). Conditions: cyclooctene/oxidant/POM = 1000/1500/1, T = 80°C.

Table 1 .

 1 Cyclooctene epoxidation by TBHP with different catalyst loadings. a

		Catalyst	% cat.	%	%	TON	TOF
				conv.	sel.		(h -1 )
	(BP)3[PMo12O40]	0.099	90.7	71.6	910	470
			0.010	85.2	72.1	8200	2500
			0.005	83.5	75.7	15000	5600
			0.002	79.4	75.3	40000	11000
			0.001	65.3	65.6	53000	16000
			0.0005	52.0	44.5	100000	35000
	(CP)3[PMo12O40]	0.149	77.7	78.0	520	140
			0.080	77.0	78.6	960	180
			0.030	82.0	79.4	2700	340
			0.004	66.7	67.5	15000	4100
			0.0005	51.2	47.1	100000	31000
			0.0002	44.4	53.1	220000	66000
		-	-	6.5	29.0	-	-
		100
	Cyclooctene conversion/%	25 50 75
		0
		1.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02
			catalyst/substrate ratio
	Figure 4-Cyclooctene epoxide yield after 24h vs. catalyst/cyclooctene
		ratio (data from

a Conditions: TBHP/cyclooctene = 1.5, 24 h, 80°C.

Table 1

 1

). Q= BP ()CP ()

Table 2 .

 2 Recycle experiments for the cyclooctene epoxidation by TBHP. a

	Complex	Run % cat. % conv. % sel.
	(TBA)3[PMo12O40]	1	0.151	47.2	38.7
		2	0.098	50.7	38.5
		3	0.052	57.8	32.3
	(BP)3[PMo12O40]	1	0.099	90.7	71.6
		2	0.052	89.0	76.4
		3	0.025	85.5	74.8
	(CP)3[PMo12O40]	1	0.149	77.7	78.0
		2	0.101	82.3	74.1

a Conditions as in Table

2

.

35

  [PMo12O40]•26H2O in 4 mL was added (BP)Br (0.56 g, 2.62 mmol) in 2 mL water. A yellow precipitate formed very rapidly. The mixture was left stirring a few hours and then the precipitate was recovered by filtration,

	60
		washed with water, diethyl ether and dried under vacuum at 70°C
		overnight. Yield: 1.89 g (97%). Yellow crystals were grown from
		a DMSO solution upon standing at room temperature over one
		week. 1 H NMR (d6-DMSO): δ 9.12 (d, 2H), 8.64 (t, 1H), 8.18 (t,
	65	2H), 4.63 (t, 2H), 1.93 (q, 2H), 1.32 (m, 2H), 0.94 (t, 3H). 31 P
		NMR (d6-DMSO): δ -2,88. IR (cm -1 ): 1064 (P=O) 962, 956
		(Mo=O) 875, 791, 760 (Mo-O-Mo) 680 (pyridinium) Anal. Calcd
		for C27H42Mo12N3O40P (Mr= 2230.87): C, 14.5; H, 1.9; N, 1.9
		Found: C, 14.6; H, 1.6; N, 1.7.
	70	Synthesis of (CP)3[PMo12O40]
		This preparation was carried out by the same procedure described
		above for the BP salt, except for using ethanol as the solvent
		carrier of (CP)Br. H3[PMo12O40].26H2O (2.00 g, 0.87 mmol) in
		10 mL water and (CP)Br (1.05 g, 2.62 mmol) in 15 mL ethanol
	75	yielded 2.06 g (86%) of product as a yellow powder. 1 H NMR
		(d6-DMSO): δ 9.11 (d, 2H), 8.63 (t, 1H), 8.18 (t, 2H), 4.62 (t,
		2H), 2,01-1,94 (m, 2H), 2,26 (s, 26H), 0,88 (t, 3H). 31 P NMR (d6-
		DMSO): δ -2,86. IR (cm -1 ): 1060 (P=O) 968, 950 (Mo=O) 875,
		794, 764 (Mo-O-Mo), 677 (pyridinium). Anal. Calcd for
		C63H114Mo12N3O40P (Mr= 2735.82): C, 27.6; H, 4.2; N, 1.5

80

Found: C, 27.6; H, 4.3; N, 1.5.

Table 3 .

 3 Fractional atomic coordinates are given in TableS1and bond distances and angles in TableS2of the Supplementary Section. Crystallographic data (excluding structure factors) for the structure reported in this

105

paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC 938809. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 IEZ, UK [Fax: int. Code +44(1223)336-033; E-mail: deposit@ccdc.cam.ac.uk].

Table 3 .

 3 Crystal data for (BP)3[PMo12O40]

		Empirical formula	[Mo12 O40 P] (C9 H14 N)3
		Formula weight	2230.89
		Temperature, K	180(2)
		Wavelength, Å	0.71073
		Crystal system	Monoclinic
		Space group	P 21/n
		a, Å	11.9672(3)
		b, Å	18.6128(4)
		c, Å	24.1420(5)
		,°	90.0
		,°	96.754(2)
		,°	90.0
		Volume, Å 3	5340.1(2)
		Z	4
		Density (calc), Mg/m 3	2.775
		Abs. coefficient, mm -1	2.854
		F(000)	4256
		Crystal size, mm 3	0.447 x 0.311 x 0.254
		Theta range, °	2.90 to 27.48
		Reflections collected	54250
		Indpt reflections (Rint)	12008 (0.0297)
		Completeness, %	97.9
		Absorption correction	Multi-scan
		Max. and min.	1.0 and 0.668
		transmission
		Refinement method	F 2
		Data	12008 / 0 / 751
		/restraints/parameters
		Goodness-of-fit on F 2	1.153
		R1, wR2 [I>2(I)]	0.0311, 0.0653
		R1, wR2 (all data)	0.0362, 0.0676
		Residual density, e.Å -3	1.854 / -1.231
		Catalytic Procedures
	5	(a) Standard catalytic runs

  CNRS; LCC (Laboratoire de Chimie de Coordination); Université de Toulouse; UPS, INPT, 205, route de Narbonne, F-31077 Toulouse, France Fax: +33-561553003 Email : rinaldo.poli@lcc-toulouse.fr 50 b Université de Toulouse; Institut Universitaire de Technologie Paul Sabatier -Département de Chimie, Av. Georges Pompidou, BP 20258, F-81104 Castres Cedex, France.E-mail: dominique.agustin@iut-tlse3.fr. c Institut Universitaire de France 103, bd Saint-Michel, 75005 Paris, France

	55

a

One particular molybdenum complex, [MoO2L 1 (MeOH)],where L 1 is a Schiff base ONS-type tridentate ligand derived from pyridoxal thiosemicarbazone, was found to be very active thus leading to satisfactory conversions at very low metal content (0.05 mol% [Mo] vs. substrate). Other molybdenum and

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| Journal Name, [year], [vol], 00-00

Acknowledgements

We acknowledge the Centre National de la Recherche 40 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 European Union for the ERASMUS mobility fellowships of D.M.F. We thank Audric Michelot and Weili Wang for technical