João Paulo

V Damasceno

Ferdinand Hof

Olivier Chauvet

Aldo José G Zarbin
email: aldozarbin@ufpr.br

Alain Pénicaud
email: alain.penicaud@crpp.cnrs.fr

The Role of Functionalization on the Colloidal Stability of Aqueous Fullerene C 60 Dispersions Prepared with Fullerides

Keywords: Fullerene C 60, Fulleride solutions, Aqueous dispersions, Colloidal stability, Electrostatic stabilization

Aqueous dispersions of fullerene C 60 were prepared from fulleride solutions in tetrahydrofuran, followed by air oxidation, transfer to water and solvent evaporation.

Fulleride exposition to air produces charged fullerene dispersions in THF that can be transferred to water to produce aqueous fullerene dispersions, stable for months.

INTRODUCTION

Fullerenes, and more precisely buckminsterfullerene C 60 as its most well-known representative, are molecular carbon allotropes. [1,[START_REF] Krätschmer | Solid C 60 : a new form of carbon[END_REF] C 60 and other fullerenes can be solubilized as molecular species in different solvents, [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF] like toluene, benzene, and CS 2 and concentrations up to 50 g L -1 can be reached in naphthalene based solvents. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Mchedlov-Petrossyan | Fullerenes in molecular liquids. Solutions in "good" solvents: Another view[END_REF][START_REF] Ruoff | Solubility of fullerene (C 60 ) in a variety of solvents[END_REF] In contrast, C 60 is less soluble in chlorinated or polar solvents such as tetrahydrofuran (THF), acetonitrile, dimethyl sulfoxide (DMSO) and almost insoluble in methanol or water.

There are different approaches to enhance the solubility of fullerenes. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF] The most common is to functionalize C 60 with groups chosen to enhance the solubility in targeted solvents. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF]] A well-known example is [START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF][START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF]-phenyl-C 60 -butyric acid methyl ester (PCBM), a functionalized C 60 derivative, ubiquitous in organic solar cells, soluble in aromatic and chlorinated solvents. [START_REF] Machui | Determination of the P3HT:PCBM solubility parameters via a binary solvent gradient method: Impact of solubility on the photovoltaic performance[END_REF] Water solubility can also be achieved by covalently grafting hydroxyl groups [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Husebo | Fullerenols revisited as stable radical anions[END_REF][START_REF] Kokubo | Facile synthesis of highly water-soluble fullerenes more than half-covered by hydroxyl groups[END_REF][START_REF] Wu | Photoenhanced transformation of hydroxylated fullerene (fullerol) by free chlorine in water[END_REF] generating fullerenols, which are dispersible in water as nanoparticles [START_REF] Husebo | Fullerenols revisited as stable radical anions[END_REF][START_REF] Prylutskyy | On the origin of C 60 fullerene solubility in aqueous solution[END_REF][START_REF] Sayes | The differential cytotoxicity of water-soluble fullerenes[END_REF][START_REF] Chiang | Multi-hydroxy Additions onto C 60 Fullerene Molecules[END_REF][START_REF] Wang | Novel and efficient synthesis of water-soluble [60]fullerenol by solvent-free reaction[END_REF] or soluble as individual molecular species with high functionalization degree as in C 60 (OH) [START_REF] Dresselhaus | Optical properties of C 60 and related materials[END_REF] -44 [START_REF] Kokubo | Facile and scalable synthesis of a highly hydroxylated water-soluble fullerenol as a single nanoparticle[END_REF], which have been exploited in biological studies. [START_REF] Husebo | Fullerenols revisited as stable radical anions[END_REF][START_REF] Prylutskyy | On the origin of C 60 fullerene solubility in aqueous solution[END_REF][START_REF] Sayes | The differential cytotoxicity of water-soluble fullerenes[END_REF][START_REF] Chiang | Multi-hydroxy Additions onto C 60 Fullerene Molecules[END_REF][START_REF] Wang | Novel and efficient synthesis of water-soluble [60]fullerenol by solvent-free reaction[END_REF] Another method to increase the solubility in polar solvents is based on the reduction of C 60 . [START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF][START_REF] Reed | Discrete Fulleride Anions and Fullerenium Cations[END_REF][START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF][START_REF] Clancy | Charged Carbon Nanomaterials: Redox Chemistries of Fullerenes, Carbon Nanotubes, and Graphenes[END_REF] Solutions containing reduced fullerenes, commonly called fulleride solutions, are usually prepared by three methods: (i) reaction between the carbon material and alkaline or alkaline-earth metals by melting or in vapour phase, (ii) reductive dissolution using those metals in ammonia or in naphthalene solutions, (iii) electrochemical methods in organic media.

A radically different approach to stabilize fullerene in aqueous media is by generating aqueous dispersions, typically stable for months, which contain polydisperse fullerene nanoparticles, up to 200 nm in size. [START_REF] Mchedlov-Petrossyan | Colloidal dispersions of fullerene C 60 in water: Some properties and regularities of coagulation by electrolytes[END_REF][START_REF] Andrievsky | On the production of an aqueous colloidal solution of fullerenes[END_REF][START_REF] Pospísil | Search for the form of fullerene C 60 in aqueous medium[END_REF][START_REF] Dhawan | Stable colloidal dispersions of C 60 fullerenes in water: Evidence for genotoxicity[END_REF][START_REF] Deguchi | Stable Dispersions of Fullerenes, C 60 and C 70[END_REF] These dispersions can be prepared by solvent exchange methods, [START_REF] Dhawan | Stable colloidal dispersions of C 60 fullerenes in water: Evidence for genotoxicity[END_REF][START_REF] Deguchi | Stable Dispersions of Fullerenes, C 60 and C 70[END_REF][START_REF] Hilburn | Synthesizing aqueous fullerene colloidal suspensions by new solvent-exchange methods[END_REF] sonication in solvent mixtures [START_REF] Prylutskyy | On the origin of C 60 fullerene solubility in aqueous solution[END_REF][START_REF] Mchedlov-Petrossyan | Colloidal dispersions of fullerene C 60 in water: Some properties and regularities of coagulation by electrolytes[END_REF][START_REF] Andrievsky | On the production of an aqueous colloidal solution of fullerenes[END_REF][START_REF] Pospísil | Search for the form of fullerene C 60 in aqueous medium[END_REF], methods based on dialysis, [START_REF] Andreev | Study of Fullerene Aqueous Dispersion Prepared by Novel Dialysis Method: Simple Way to Fullerene Aqueous Solution, Fullerenes[END_REF] or using fulleride solutions as precursors. Xu and collaborators [START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF] reported a procedure to generate C 60 -anions and used them to prepare an aqueous dispersion by adding a THF solution of C 60 -into water, since naturally present oxygen in water causes C 60 -oxidation and aggregation. [START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF][START_REF] Wei | Selective solution-phase generation and oxidation reaction of C 60 n-(n = 1,2) and formation of an aqueous colloidal solution of C 60[END_REF] In a more recent work, Wabra et al. [START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF] described the preparation of KC 60 and K 2 C 60 and studied their reactivity with different molecules and water, and they claimed that C 60 2-reacts with water and produces a yellow solution of C 60 -that is stable for few weeks under inert conditions. However, C 60 -solutions in oxygen-free water should be red and not yellow. A yellowish colour is typically observed for dispersions of fullerenes. [START_REF] Mchedlov-Petrossyan | Colloidal dispersions of fullerene C 60 in water: Some properties and regularities of coagulation by electrolytes[END_REF][START_REF] Andrievsky | On the production of an aqueous colloidal solution of fullerenes[END_REF][START_REF] Pospísil | Search for the form of fullerene C 60 in aqueous medium[END_REF][START_REF] Dhawan | Stable colloidal dispersions of C 60 fullerenes in water: Evidence for genotoxicity[END_REF][START_REF] Deguchi | Stable Dispersions of Fullerenes, C 60 and C 70[END_REF] Herein, we report (i) the re-examination of the oxidation step of fulleride solutions that leads to the generation of metastable dispersions in THF, (ii) the transfer to water, producing, aqueous dispersions of C 60 , in analogy to aqueous dispersions of graphene [START_REF] Bepete | Surfactant-free single-layer graphene in water[END_REF][START_REF] Bepete | Eau de graphene" from a KC 8 graphite intercalation compound prepared by a simple mixing of graphite and molten potassium[END_REF] and carbon nanotubes, [START_REF] Bepete | Hydroxyl Ions Stabilize Open Carbon Nanotubes in Degassed Water[END_REF][START_REF] Damasceno | A new approach for the achievement of stable aqueous dispersions of carbon nanotubes[END_REF] (iii) that these dispersions are stable regardless of the presence or not of functionalized fullerenes (e.g. fullerenols).

The generated aqueous dispersions exhibit concentrations up to 200 mg L -1 , mean particle sizes between 25 and 70 nm and zeta potentials from -10 to -50 mV depending on the precursor used and on the specific ratio between fulleride solution and water. The degree of C 60 functionalization is higher when higher charged fullerenes (K 6 C 60 ) are used in the dissolution step. Thus, at lower charge ratios (KC 60 ), metastable dispersions can be prepared that barely have any content of functionalized derivatives, and are not re-dispersible in water once dried due to their lyophobic nature. In contrast, dispersions exhibiting higher content of functionalized fullerenes can be dried and re-dispersed, evidencing the coexistence of C 60 nanoparticles and fullerenols, which dramatically alters the physicochemical properties of fullerenes. Moreover, a sample of fullerenol containing no nanoparticles has been isolated and exhibits about 15 functional groups in average and is barely water soluble as expected for this low number of hydroxyl functions. These observations highlight the complexity and the coexistence of water based aqueous dispersions of C 60 and fullerenol solutions and the need for discriminating between the effects of these different chemical species. until melting of the potassium was observed. Afterwards, the solid was heated for 55 min more minutes at 180 °C and stirred each 10 to 15 min. Afterwards, the sample was cooled to room temperature to yield the respective salt KC 60 (1a, see Figure 1) and K 6 C 60 (2a).

Dissolution of KC 60 (1b) and K 6 C 60 (2b), oxidation and preparation of aqueous dispersion

For all dissolutions, absolute THF (see preparation above) was used. The dissolution was carried out by placing 4 mg of KC 60 or K 6 C 60 in a 21 mL glass vial and adding 20 mL of absolute THF. Subsequently, the mixture was shaken by hand and kept for about 15 min inside the glove box, and was additionally shaken by hand each few minute. Both salts are soluble in THF and spontaneously produce fulleride solutions, KC 60 as a dark red solution and K 6 C 60 as a light orange one. The supernatant was isolated in each case, yielding fulleride solution (1b) from KC 60 and (2b) from K 6 C 60 .

Oxidation of the fulleride solution and preparation of the respective dispersion (1c;

2c)

The solutions (1b) and (2b) were removed from the glovebox, the vials opened at ambient atmosphere and air was bubbled four times with a 1.5 cc glass Pasteur pipette through the solution to promote oxidation of fulleride solutions. A color change was visible from dark red (1b) or light orange (2b) to brown (1c) or light gray (2c). Both dispersions (1c) and (2c) were isolated and were stable for at least two months, without solvent evaporation and stored in contact with minimum amount of air as possible (no free volume inside the glass flask).

Preparation of the aqueous dispersions (1d; 2d)

Aqueous dispersions were prepared by mixing the dispersions (1c) and (2c) with deionized water in different mass proportions. 20 mL glass vial was filled with a weighted amount of deionized water and the vial was placed on a balance. Afterwards, the respective dispersion, directly after the oxidation step, was added by means of a glass Pasteur pipette.

Then, the vial was kept in a hood for 2 days to allow THF evaporation. Aqueous dispersions of (1d) and (2d) were obtained having mass ratios of 1:8, 1:4, 1:2 or 1:1 respectively by combining 2.00, 3.75, 6.00 or 8.00 g of the respective oxidized fulleride solution (1c, or 2c) with 16.00, 15.00, 12.00 or 8.00 g of water to achieve the above mentioned mass proportions.

The generated aqueous dispersions (1d; 2d) are slight to dark orange and are stable for at least 6 months.

RESULTS AND DISCUSSION

The aqueous dispersions of C 60 reported here, (see Figure 1, details in Experimental Section and in ESI), were prepared starting from reduced fullerenes, (1a and 2a, stoichiometry KC 60 and K 6 C 60 respectively) which can be directly dissolved in polar solvents such as tetrahydrofuran (THF) (solutions 1b and 2b). By oxidation of the solutions (1b, 2b)

with air, the total charge in both systems is significant lowered, which causes aggregation within the samples, leading to the formation of dispersions of C 60 nanoparticles in THF (1c, 2c). These dispersions in THF can be added to water, and, after evaporation of THF, aqueous dispersions containing C 60 nanoparticles are obtained that are stable for months (1d, 2d). The initial amount of charge in 2a is much higher than in 1a, and is lowered significantly by the oxidation step and simultaneous production of significant amount of fullerenols (3) by side reactions.

The formation of KC 60 and K 6 C 60 by the direct mixing of molten potassium and fullerene has been performed in analogy to the synthesis of reduced carbon nanotubes and intercalated graphite [START_REF] Bepete | Eau de graphene" from a KC 8 graphite intercalation compound prepared by a simple mixing of graphite and molten potassium[END_REF][START_REF] Hof | New basic insight into reductive functionalization sequences of single walled carbon nanotubes (SWCNTs)[END_REF] and the resulting salts have been studied by XRD patterns (see Fig. S1, ESI) and Raman spectroscopy (see Fig. S2). The data suggest that the salts are predominantly KC 60 and K 6 C 60 with minor contamination of C 60 in the latter case. [START_REF] Krätschmer | Solid C 60 : a new form of carbon[END_REF][START_REF] Dresselhaus | Raman Scattering in Fullerenes[END_REF][START_REF] Kuzmany | Raman Scattering in C 60 fullerenes and fullerides[END_REF][START_REF] Poirier | KC 60 fulleride phase formation: An x-ray photoemission study[END_REF][START_REF] Erwin | Electronic structure of crystalline K 6 C 60[END_REF] Upon addition of THF to the C 60 salts (1a, 2a), a spontaneous dissolution can be observed that produces dark red (1b) or light orange (2b) fullerenide solutions in minutes without the need for stirring. UV-Vis-NIR spectrum of 1b exhibits bands at 991 and 1074 nm, typical of the C 60 anion, [START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF][START_REF] Reed | Discrete Fulleride Anions and Fullerenium Cations[END_REF][START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF][START_REF] Wei | Selective solution-phase generation and oxidation reaction of C 60 n-(n = 1,2) and formation of an aqueous colloidal solution of C 60[END_REF]] and bands at 827 and 985 nm, characteristic of C 60 2-dianion, [START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF][START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF][START_REF] Wei | Selective solution-phase generation and oxidation reaction of C 60 n-(n = 1,2) and formation of an aqueous colloidal solution of C 60[END_REF] suggesting that a mixture between the two fullerene anions is present in solution (see Figure 2A). UV-Vis-NIR spectrum of 2b shows a band at 335 nm with low intensity characteristic of fullerene electronic transition, also seen in the spectrum of solution 1b. According to literature, the electronic states of C 60 are not too much affected by anion formation, [START_REF] Dresselhaus | Optical properties of C 60 and related materials[END_REF] so some electronic transitions of pristine fullerene can also be seen in doped C 60 . Both fulleride solutions were then bubbled with air to promote oxidation and a visible colour change was observed in each case (see Fig. S3). This oxidation step lowers the overall charge density, leading to a destabilization of the solutions and the formation of C 60 nanoparticles in THF. The change from solution (1b) to dispersion (1c) can be visualized by the Tyndall effect: the formation of aggregates of size close to the wavelength of an incident radiation provokes scattering (see insert in Figure 2A). This is confirmed by DLS measurements that show particles with mean diameter equal to 200 nm for sample 1c (see Table S1 in ESI). This behaviour change, from solution to dispersion, does not depend on the initial electron density as in both cases metastable THF dispersions (1c, 2c) could be generated. Alternatively, if solution 1b, not oxidized, is added to oxygen-free water, there is no color change and the solutions remains stable as long as protected from air (see Fig. S4 in ESI). Indeed, only oxygen and not water can oxidize C 60 -in agreement with reports [START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF][START_REF] Wei | Selective solution-phase generation and oxidation reaction of C 60 n-(n = 1,2) and formation of an aqueous colloidal solution of C 60[END_REF][START_REF] Stinchcombe | Buckminsterfulleride(1-) Salts: Synthesis, EPR, and the Jahn-Teller Distortion of C 60[END_REF] on related systems.

The formation of metastable dispersions (1c, 2c) in organic media that are stable for weeks after exposition of fulleride solutions to air has not been reported in the literature to the best of our knowledge, although fullerenes are known to produce dispersions in polar solvents. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Alargova | Stable colloidal dispersions of fullerenes in polar organic solvents[END_REF][START_REF] Mchedlov-Petrossyan | The peculiar behavior of fullerene C 60 in mixtures of 'good' and polar solvents: Colloidal particles in the toluene-methanol mixtures and some other systems, Colloids Surfaces A Physicochem[END_REF][START_REF] Deguchi | Top-down preparation of dispersions of C 60 nanoparticles in organic solvents[END_REF] This interpretation can be corroborated by UV-Vis-NIR spectra of 1c and 2c, as in both cases the characteristic absorption bands between 700 and 1300 nm of the respective anions vanish and the peaks below 400 nm characteristic of neutral C 60 broaden and increase (see Figure 2A and2B).

Just after the oxidation step, THF dispersions 1c and 2c were added to water in different proportions (1:1, 1:2, 1:4, 1:8, (dispersion:water)) avoiding transferring any suspended or precipitated material, especially in the case of K 6 C 60 that is less soluble in THF.

The mixtures were placed in a fume hood and left opened for 2 days to allow THF evaporation and formation of aqueous dispersions 1d and 2d. Dispersions 1d and 2d are stable at least for several months and they are yellow to red-brown, depending on the concentration and on the precursor salt (see pictures inserted in Figure 3 (A) and (B)). The concentration of the respective sample ranges from 2.7 to 200 mg L -1 (in relation to C 60 , see Table S1 and details in ESI) and agrees with the proportion used in the synthesis. The colloidal nature pointed out by the Tyndall effect shown in Fig. S5 as well as the yellowbrownish colour are in accordance with other reports on C 60 aqueous dispersions. [START_REF] Mchedlov-Petrossyan | Colloidal dispersions of fullerene C 60 in water: Some properties and regularities of coagulation by electrolytes[END_REF][START_REF] Deguchi | Stable Dispersions of Fullerenes, C 60 and C 70[END_REF] Figure 3 shows UV-Vis spectra of the dispersions 1d (A) and 2d (B) and for both sets of The overall total absorption is related to the number of dispersed nanoparticles and increases with the ratio between fulleride dispersion and water used. In the corresponding spectra, (Figure 3) absorption bands at 216, 260, and 337 nm can be seen that are linked to electronic transitions of fullerene, present in C 60 solutions. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Deguchi | Stable Dispersions of Fullerenes, C 60 and C 70[END_REF] The same bands can be observed in C 60 saturated solution in THF (see Fig. S6). The broad band between 400 and 500 nm is characteristic of C 60 dispersions [START_REF] Pospísil | Search for the form of fullerene C 60 in aqueous medium[END_REF][START_REF] Deguchi | Stable Dispersions of Fullerenes, C 60 and C 70[END_REF] and is observed in both set of experiments.

These bands are not present in spectra of fullerene solutions (1b, 2b), but are also visible in the THF dispersions (1c, 2c) (see Figure 2) and are responsible for the dispersions color.

In order to understand the chemical nature of those samples, THF dispersions (1c, 2c)

have been dried and brown powders have been retained. The solid residues of 1c, 2c and C 60 have been analysed by infrared spectroscopy (IR) (Figure 4). C 60 has four active IR vibrations, [START_REF] Krätschmer | Solid C 60 : a new form of carbon[END_REF] two modes with maxima at 1180 and 1426 cm -1 , and the other two next to 530 and 580 cm -1 that are not shown. The residue material of 1c has a similar IR profile to pristine fullerene, except for a band next to 2800 cm -1 that may be attributed to C-H stretching vibrations. Peaks that may be attributed to C-H vibrations are also present in the residue materials of 2c, at about 2900 cm -1 . More importantly, sample 2c has a distinct profile differing from the other two, with intense and broad absorption bands at 1000, 1363, 1608, and 3150 cm -1 , which are attributed to C-OH bending, and C-O, C=O and O-H stretching respectively. [START_REF] Pospísil | Search for the form of fullerene C 60 in aqueous medium[END_REF][START_REF] Wu | Photo-Oxidation of Hydrogenated Fullerene (Fullerane) in Water[END_REF] This different spectral profile indicates that residue materials of 2c has a higher content of oxidized fullerene species, fullerenols, than the the residue material of sample 1c. Elemental analysis showed 88.0 % and 56.7 % of carbon and 5.7 % and 20.1 % of oxygen for the residue materials of 1c and 2c respectively, corroborating the IR results for a higher degree of functionalization for sample 2c.

The residue sample of 1c and 2c have been extracted by toluene exhibiting the characteristic purple colour of C 60 in this solvent [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF] for both samples (see insert in Figure 4, II and IV). However, only the residue solid of 2c and not that of 1c can be redispersed in water in appreciable extent, as the characteristic brown colour can only be seen in flask (III) and not in flask (I) from Figure 4. This observation corroborates the evidence for surface modification and the higher content of fullerenol for samples with higher initial charging ratios. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Husebo | Fullerenols revisited as stable radical anions[END_REF][START_REF] Pospísil | Search for the form of fullerene C 60 in aqueous medium[END_REF] Both residue samples of 1c and 2c have been dissolved / re-dispersed in deuterated toluene and deuterated water and 13 C NMR analysis were performed (see Fig. S7). Consistent with the purple color of the toluene dissolution, 13 C NMR spectrum shows the characteristic signal of C 60 at 143 ppm.

Raman spectra of the residue samples of 1c and 2c exhibit a similar spectral profile to pristine fullerene with the A g mode at 1463 cm -1 , suggesting that a significant content of C 60 is preserved in each case after the oxidation step (see Fig. S8). Interestingly, the presence of fullerenol in the aqueous dispersion is crucial for repeated re-dispersibility from the powder and is only possible for sample 2c. However, redispersed sample from residue of 2c is composed by nanoparticles of mean diameter close to 380 nm (see Table S1), suggesting that those fullerenol nanoparticles are formed by a core made mostly of pristine C 60 and a shell enriched in fullerenols. Thus, a purification has been performed, based on Soxhlet extraction followed by column chromatography (details see Figure 1 and ESI) and a molecular fullerenol sample (3) has been extracted. The first fraction contains non-functionalized fullerenes, indicated by the deep purple colour and analysed by mass spectrometry, UV-Vis, and NMR (see Fig. S9-11). The remaining fractions contain differently functionalized fullerenols, although no symmetric or pure product could be obtained. This observation highlights the complex oxidation process and shows the demand to understand the fundamental reaction steps occurring in more detail.

The molecular reaction mechanism for fullerenols formation is complex as evidenced by the heterogeneity of the sample composition. Similar charged carbon materials (potassium salts of carbon nanotubes), also sensitive towards oxygen and water from atmosphere, have been studied extensively. [START_REF] Hof | New basic insight into reductive functionalization sequences of single walled carbon nanotubes (SWCNTs)[END_REF] A similar mechanism is probably occurring for fullerides. Upon discharging of the carbon lattice, the C 60 molecules start to form nanoparticles, because the electrostatic repulsion is diminished. When oxygen is used as oxidant, single electron transfer is occurring and forming superoxide radical anions. [START_REF] Stinchcombe | Buckminsterfulleride(1-) Salts: Synthesis, EPR, and the Jahn-Teller Distortion of C 60[END_REF] Those can either react with the exposed surface of the nanoparticles, which leads to the generation of fullerenols or they can disproportionate in water, which does not lead to functionalization. Thus, the larger the amount of potassium, the larger the amount of produced fullerenols.

Aqueous dispersions 1d and 2d are composed of polydisperse nanoparticles (See Fig. S12) with mean diameters between 25 and 70 nm as measured by DLS (see Table S1).

Interestingly, the final size of the particles in aqueous dispersions (1d, 2d) diminishes when the amount of water increases vs that of THF dispersion (1c, 2c). Most of these nanoparticles are formed by pristine C 60 as shown by IR, Raman and redissolution experiments, with more functional groups in the case of 2d dispersions. The nanoparticles exhibit zeta potentials from -10 to -50 mV as shown in Table S1, which is higher for aqueous dispersions obtained with higher content of fulleride solutions (1:1). Aqueous dispersions 1d and 2d are homogeneous in the whole volume and no aggregation was observed after one year of storage in closed glass flasks.

TEM images for aqueous dispersions (1d and 2d, 1:2 proportion) are shown in Fig. S13 (a,b). In both samples, a size distribution is visible spanning from a few nanometers up to about 60 nm, and a fraction of larger particles coexists with a fraction of smaller particles.

The apparent distribution is agreement with the average particle size distribution by DLS, considering that the latter emphasizes larger particles. HR-TEM images for are shown in Fig. S14 (1d) and S15 (2d). Crystalline nanoparticles can be found in aqueous dispersion 1d (Fig. Nanoparticles observed for dispersion 2d are significantly less ordered as evidenced by the FFT analysis (Fig. S15 c) of the respective particle (Fig. S15 b).

Due to the low degree of functionalization and to their non-re-dispersibility after water addition (see Figure 4), dispersions 1d are lyophobic and the colloidal stability of those is provided by electrostatic repulsion. [START_REF] Mchedlov-Petrossyan | Colloidal dispersions of fullerene C 60 in water: Some properties and regularities of coagulation by electrolytes[END_REF][START_REF] Damasceno | Electrostatic stabilization of multi-walled carbon nanotubes dispersed in nonaqueous media[END_REF][START_REF] Rodgers | Electrostatic Stabilization of Graphene in Organic Dispersions[END_REF][START_REF] Mchedlov-Petrossyan | Interaction of C 60 aggregates with electrolytes in acetonitrile, Colloids Surfaces A Physicochem[END_REF][START_REF] Mchedlov-Petrossyan | Towards better understanding of C 60 organosols[END_REF] So, these dispersions can be considered as model to apply DLVO theory for carbon-based systems, [START_REF] Damasceno | A new approach for the achievement of stable aqueous dispersions of carbon nanotubes[END_REF] as has been done for fullerene dispersions made by other routes. [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF][START_REF] Mchedlov-Petrossyan | Colloidal dispersions of fullerene C 60 in water: Some properties and regularities of coagulation by electrolytes[END_REF][START_REF] Mchedlov-Petrossyan | Interaction of C 60 aggregates with electrolytes in acetonitrile, Colloids Surfaces A Physicochem[END_REF][START_REF] Mchedlov-Petrossyan | Towards better understanding of C 60 organosols[END_REF][START_REF] Mchedlov-Petrossyan | Formation and ageing of the fullerene C 60 colloids in polar organic solvents[END_REF] Figure 5A presents a graphic of total interaction energy (U T ) as a function of distance for two identical spherical particles calculated based on DLVO theory, [START_REF] Shaw | Introduction to Colloid and Surface Chemistry[END_REF][START_REF] Hunter | Foundations of Colloid Science[END_REF] considering the experimental radius (half of the diameters shown in Table S1) and zeta potentials for 1d dispersions, an approximate value for Hamaker constant (1 x 10 -19 J) from literature, [START_REF] Mchedlov-Petrossyan | Fullerenes in liquid media: An unsettling intrusion into the solution chemistry[END_REF] and 1:1 electrolyte (1 . 10 -1 mmol L -1 ). The kinetic barriers against coagulation increase with concentration for dispersions 1d because concentrated systems have bigger nanoparticles with higher zeta potentials as shown in Table S1. These zeta potentials provide electrostatic stabilization for aqueous dispersions 1d. For the most concentrated system, a kinetic barrier higher than 45k B T is achieved, which is enough to keep the system stable for long periods. [START_REF] Shaw | Introduction to Colloid and Surface Chemistry[END_REF][START_REF] Overbeek | Recent developments in the understanding of colloid stability[END_REF] Dispersions 2d exhibit a higher contribution of solvation by water due to the presence of fullerenols. Thus, their colloidal behaviour is more complex, and a combination of different effects is expected. It has been shown that fullerenols with about 15 hydroxyl groups are barely water soluble and only with 36 or more functional groups, water solubility could be reached. [START_REF] Kokubo | Facile and scalable synthesis of a highly hydroxylated water-soluble fullerenol as a single nanoparticle[END_REF] The degree of functionalization of dispersions 1d and 2d and fullerenol sample 3 is way lower than this threshold value, as evidenced by mass spectroscopy (see Fig. S9), and thus the stabilization of the dispersions is mainly provided by the electrical charges on dispersed nanoparticles evidenced by zeta potential measurements rather than by functional groups. However, the re-dispersibility of the residue sample of 2c is strongly connected with the presence of functional groups. Therefore, it is important to identify the origin of the charges in these dispersions leading to negative zeta potentials, especially on low functionalized systems because reports about stable radicals are related with higher content of fullerenols. [START_REF] Husebo | Fullerenols revisited as stable radical anions[END_REF] Thus, Electron Spin Resonance (ESR) measurements where conducted on the samples 1b, 1c and 1d (Figure 5B-D) prepared following the same procedure, but with higher material concentration (five times the concentration of 1b used before).

ESR spectrum from solution 1b shown in Figure 5B has a broad signal (ΔH pp = 35 G) at g = 1.9996 ± 0.0003, characteristic of C 60 -anion radicals [START_REF] Reed | Discrete Fulleride Anions and Fullerenium Cations[END_REF][START_REF] Wu | A new method for facile and selective generation of C 60 and C 60 2in aqueous caustic/THF (or DMSO)[END_REF][START_REF] Stinchcombe | Buckminsterfulleride(1-) Salts: Synthesis, EPR, and the Jahn-Teller Distortion of C 60[END_REF][START_REF] Paul | Artifacts in the electron paramagnetic resonance spectra of C 60 fullerene ions: Inevitable C 120 O impurity[END_REF] with an integrated intensity of about 1.6 x 10 -9 emu cm -3 (8 x 10 17 equivalent S=1/2 Curie spins cm -3 ). A second and much narrower signal is present at a slightly higher g = 2.0003 ± 0.0003, but with a linewidth ΔH pp ~ 3 G and a relative intensity 100 times smaller. This sharp signal is likely to arise from C 120 O n-impurities (n odd). [START_REF] Paul | Artifacts in the electron paramagnetic resonance spectra of C 60 fullerene ions: Inevitable C 120 O impurity[END_REF] After oxidation by air-exposure, the ESR spectra are significantly modified. The spectrum of dispersion 1c is at g = 2.0020 ± 0.0003, with ΔH pp ~ 1.8 G and the total intensity is decreased by almost two orders of magnitude down to 3.2 x 10 -11 emu cm -3 . The main line is slightly asymmetric due to an unresolved axial powder spectrum lineshape. The small bumps on the wings are characteristic of a badly resolved triplet spectrum with g ~ 2.0023 ± 0.0003 and D ~ 5.8 G. The spectrum of dispersion 1d was difficult to obtain because of the water transfer, however it is very similar to the spectrum of dispersion 1c (g = 2.0023 ± 0.0003, ΔH pp ~ 1.2 G, intensity ~2.0 x 10 -11 emu cm -3 ). Most charges have been lost upon oxidation from 1b to 1c, as expected. The remaining sharp signal is not significantly different from the impurities arising from off the shelf C 60 , [START_REF] Paul | Artifacts in the electron paramagnetic resonance spectra of C 60 fullerene ions: Inevitable C 120 O impurity[END_REF] and should probably be ascribed to those.

CONCLUSIONS

The developed protocol for the synthesis of dispersions in THF or water is versatile and concentrations up to 200 mg L -1 of fullerene C 60 in water can be reached. Oxidation of fulleride organic solutions with oxygen leads to fullerene nanoparticles dispersed in THF, due to charge loss of most fullerene molecules. This leads to aggregation into nanostructures, forming a metastable dispersion. This dispersion can be transferred to water in different proportions and after evaporation of the THF, aqueous fullerene dispersions composed by size calibrated nanoparticles are generated. The content of functionalized species, i.e. fullerenols, can be tuned by the amount of initial charges during the reduction and dissolution steps.

Those initial charges have a direct influence on the re-dispersibility of the C 60 dispersions.

Samples with a content of fullerenols can be redispersed, while those exhibiting almost no fullerenols cannot. However, it should be pointed out that even barely functionalized dispersions remain stable due to electrostatic repulsion between individual nanoparticles.

Colloidal stability has been checked for months, and are still stable after one year, exceeding by far the minimum demand for processability.

ASSOCIATED CONTENT

Supporting information

The Supporting Information has all experimental procedures, samples characterization, some complementary figures, pictures, and table (PDF).

Conflicts of interest

There are no conflicts to declare.

Figure 1 .

 1 Figure 1. Preparation of aqueous dispersions containing fullerene nanoparticles by two

Figure 2 .

 2 Figure 2. UV-Vis-NIR spectra of samples 1b and 1c (A); Inserted: pictures of samples 1c and

Figure 3 .

 3 Figure 3. UV-Vis spectra of aqueous dispersions 1d (A) and 2d (B) prepared with different

Figure 4 .

 4 Figure 4. Infrared spectra of pristine C 60 and dried residues from samples 1c and 2c; Inset:

  S14 a,b), evidenced by the FFT analysis (Fig.S14 c) of the respective particle (Fig.S14 b).

Figure 5 .

 5 Figure 5. Graphic of total interaction energy in terms of thermal energy in function of

2. EXPERIMENTAL SECTION 2.1 Materials and chemicals

	2.2 Safety remark
	The possible user of the reported synthesis protocol is reminded of the specific hazards
	This work demonstrates a pathway to produce in large quantities, concentrated related to the use of potassium metal. Safety precautions are advised for storage, handling and
	aqueous dispersions of C 60 composed by small nanoparticles. Additionally, it forms a basis waste treatment. Potassium metal is extremely dangerous in contact with water or moisture,
	for further studies to understand the electronic band structure and the nature of electrical releasing hydrogen with enough heat to cause ignition or explosion. Peroxide formation may
	charges in analogous dispersed systems such as carbon nanotubes and graphene. Furthermore, occur in containers that have been opened and remained in storage. May produce corrosive
	this study demonstrates that functionalized fullerenes and dispersed fullerenes can co-exist solutions on contact with water.
	and urges caution for explaining specific phenomena or observed behaviour of those or
	related systems in biological, medical, and material science applications by considering either
	solely fullerenols or only dispersed fullerenes.
	Fullerene C 60 was acquired from Merck (>99.998 %), metallic potassium (98 %),
	toluene (≥ 99.8 %) and tetrahydrofuran (THF) from Sigma-Aldrich (≥ 99.8 %, HLPC, non-
	stabilized). Deuterated toluene was acquired from Sigma-Aldrich (99% deuterated) and
	deuterated water from Sigma-Aldrich (99.9 % deuterated). THF was purified by a Pure Solv
	400-4-MD solvent purification system (Inert Corporation) that is attached directly to the glove
	box (Model: S1-DL, Inert Corporation), equipped with an oxygen purifier and H 2 O and O 2
	sensors. The as purified THF was distilled over NaK alloy prior to use inside the glove box.
	Typical O 2 concentrations during manipulation inside the glove box are ~0.4 ppm O 2 /H 2 O.
	Water was deionized in a Millipore equipment, model Elix 10, and the resistivity was equal to
	18.2 MΩ. Stirring plates were acquired from Velp Scientifica, model MST Magnetic Stirrer.

2.3 Synthesis of KC 60 (1a) and K 6 C 60 (2a)

	Preparation and dissolution of fullerene salts were performed in a glove box filled
	with argon (5 mbar above atmospheric pressure). KC 60 and K 6 C 60 were prepared by molten
	potassium intercalation based on procedures from literature on nanotubes[31] and
	graphite.[28] Briefly, in a 2 mL glass vial, 8.8 mg (0.23 mmol) of cleaned metallic potassium
	(free of oxide layer) and 162.2 mg (0.23 mmol) of fullerene C 60 were mixed for KC 60 and
	29.7 mg (0.76 mmol) of potassium and 91.2 mg (0.13 mmol) of C 60 for K

[START_REF] Wabra | Exohedral Addition Chemistry of the Fullerenide Anions C 60 2and C 60[END_REF]

C 60 . The solid mixture was heated on a hot plate at 180 °C and stirred with a stainless-steel spatula for 5 min

ACKNOWLEDGMENTS

JPVD acknowledges CAPES for the fellowship and funding to stay in CRPP at Bordeaux, France (CAPES-PrInt). We thank CAPES, CNPq, Fundação Araucária and INCT Nanocarbonos for research funding and CAPES-COFECUB (grant PH-C931/19 -42002PK) for collaboration grant. FH and AP thank the French ANR (EdgeFiller project). FH thanks CNRS (Momentum grant). We thank Christelle Absalon for the results of mass spectrometry.

Author Contributions

JPVD and FH have performed all the experiments, sample analysis, interpretation, and discussion of results. OC has performed the ESR experiments. AJGZ and AP have collaborated in the interpretation and discussion of results. All authors have contributed on writing the manuscript.


Chiara Dinoi

Petr Prikhodchenko

Funda Demirhan

Jenny Gun

Ovadia Lev

Jean-Claude Daran

Rinaldo Poli
email: poli@lcc-toulouse.fr

Jean- Claude Daran

Reduction of [Cp*2Mo2O5] by mercaptopropionic acid in an aqueous medium. Isolation and characterization of a dinuclear oxo-and 3-sulfidoproprionato(2-)-bridged molybdenum(IV) compound

Keywords: Molybdenum, aqueous organometallic chemistry, alkylsulfido ligands, carboxylato ligands, pentamethylcyclopentadienyl ligands

teaching and research institutions in France or abroad, or from public or private research centers.   Reduction of [Cp*2Mo2O5] by mercaptopropionic acid in an aqueous medium. Isolation and characterization of a dinuclear oxo-and 3-sulfido-proprionato(2-)-bridged molybdenum(IV) compound

Introduction

Aqueous organometallic chemistry is attracting growing interest because of its ''green'' impact [1][2][3] and because of its potential in catalytic [4,5] and biomedical applications [6].

Although this area is widely explored for low-valent systems, usually upon appending hydrophilic functionalities to the ligand periphery in order to increase solubility in an aqueous environment [7][8][9], studies of high oxidation organometallics are still rather scarce [10]. For redox-active metals, these studies are particularly interesting as they may open the way to electrocatalytic applications [10].

In a recent contribution [11], we have reported the result of the interaction between compound [Cp*2Mo2O5] and mercaptoacetic (or thioglycolic) acid, HSCH2COOH, which resulted in the isolation and characterization of the dinuclear Mo IV product [Cp*2Mo2(-S)(-SCH2COO)2]. In this reaction, the mercaptoacetic acid exerts three simultaneous functions: reducing agent, ligand, and substrate for the reductive cleavage of the S-C bond (leading to the incorporation of the sulphur atom in the isolated product as a bridging sulfido ligand). This phenomenon, which is common to other Mo-mediated C-S bond cleavage reactions [12][13][14][15][16][17], mimics a step of the hydrodesulfurization process, which is in fact typically catalyzed by molybdenum-sulfur compounds [18][19][20][21]. We were interested in probing the generality of this reaction and proceeded to explore the reaction with mercaptopropionic acid, HSCH2CH2COOH, which contains an additional methylene unit between the acid and the mercapto functions relative to mercaptoacetic acid. The outcome of this reaction, which is unexpectedly different from that of the lighter analogue, is reported and discussed in this contribution.

Experimental Section

General procedures.

All preparations and manipulations were carried out with Schlenk techniques under an oxygen-free argon atmosphere. All glassware was oven-dried at 120°C. Solvents were dried by standard procedures and distilled under dinitrogen prior to use.

X-ray diffraction study.

A single crystal of the compound was mounted under inert perfluoropolyether on the tip of glass fibre and cooled in the cryostream of the Oxford-Diffraction XCALIBUR CCD diffractometer. Data were collected using the monochromatic MoK radiation (= 0.71073). The structure was solved by direct methods (SIR97) [25] 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 attached to carbon and oxygen (OH) were introduced in calculation in idealised positions and treated as riding on their parent atoms. The drawing of the molecules was realised with the help of ORTEP32 [START_REF] Farrugia | [END_REF]. Crystal data and refinement parameters are shown in Table 1

Results and Discussion

The reaction between Cp*2Mo2O5 and HSCH2CH2COOH qualitatively appeared to follow the same course as the related, previously described reaction with the related substrate HSCH2COOH [11], under identical experimental conditions (room temperature in a 1: that the same transformation also occurs cleanly in the gas phase. An MS/MS study of the isolated product peaks (Figure 1) reveals various decay patterns, the major ones corresponding to a single and a double loss of 72 mass units. This is easily interpreted as the loss of CH2CH2CO2 units from the mercaptopropionato(2-) ligand, leaving a sulfido bridge on the dinuclear complex. MS/MS/MS on the peak resulting from the first CH2CH2CO2 loss (at m/z 615), also shown in Figure 1, mostly yields the second CH2CH2CO2 expulsion product.

The presence of two envelopes 16 m/z units lighter than the above fragments (at m/z 599 and 529, respectively) is unlikely due to subsequent oxygen atom expulsion. Rather, these peaks could correspond to minor fragmentation pathways involving the elimination of SCH2CH2CO

(m/z 88), thereby leaving an oxo bridge on the dinuclear complex. As was the case for the related mercaptoacetic acid reaction, the product obtained with the mercaptopropionic acid co-crystallized together with a molecule of the disulfide byproduct, thereby providing direct evidence for its formation (Equation 1). A view of the dinuclear Mo compound is shown in Figure 2. The coordination geometry around each Mo center can best be described as a four-legged piano stool, the four legs being defined by the Cp*2Mo2Cl2(-O)(-Cl)2 [28], and [Cp*2Mo2Cl2(-Cl)3] + [32]. The overall geometry is identical to that recently described for the mercaptoacetic acid product [11], the bridging sulphur atom in the latter being replaced by a bridging oxygen atom. The other geometric parameters are sensibly identical in the two compounds. As already found for the [Cp*2Mo2(-S)(-SCH2COO)2] structure [11], the dinuclear [Cp*2Mo2(-O)(-SCH2CH2COO)2] product establishes H-bonds with the disulfide by-product. They yield a pseudo dimer by associating two dinuclear complexes and two disulfide by-products, with a graph-set motif of R4 4 (42) [33,34], as highlighted in Figure 3. Relevant metric parameters are collected in Table 3. The presence of the disulfide molecule in the crystal is also revealed by the 1 H NMR study. However, an ordinary spectrum does not allow the clear-cut identification of all protons in the organometallic dinuclear species. Each pair of CH2 protons is diastereotopic and yields an ABCD system for the CH2CH2 tether, which partially overlaps with the stronger triplets of the free disulfide molecule at  2.99 and 2.81. An unambiguous identification of the four mutually coupled protons of the ABCD system was possible by a 2D-COSY experiment (see Supporting Information). The two stronger triplet resonances only show, as expected, coupling cross-peaks with each other, leading to the straightforward identification of the 4 resonances of the ABCD system from the additional cross-peaks.

The title compound was also investigated by cyclic voltammetry. As shown in Figure 4a, the compound undergoes a reversible oxidation wave in CH2Cl2 solution at E1/2 = 0.06 V vs. ferrocene. This indicates that the mixed-valence Mo2 IV,V product enjoys a certain stability.

Conversely, the reduction behaviour is less well defined. A major irreversible wave is observed at Ep,c = -1.96 V, with an associated oxidation peak of reduced intensity. This suggests that a chemical process is coupled to the electrochemical reduction. Two less intense reduction waves at less negative potential probably originate from the co-crystallized disulfide by-product.

An electrochemical investigation of the related thioglycolate product, [Cp*2Mo2(-S)(-SCH2CO2)2], was not previously carried out [11], therefore we proceeded to examine this behaviour for comparison purposes with the title compound, see Figure 4b.

This compound shows a reversible oxidation at E1/2 = 0.32 V and a reduction at Ep,c = -1.54 V.

Negligible electrochemical activity is observed in this case at intermediate potentials, since this sample was recovered as a powder after washing away the disulfide by-product with pentane. The shape of the reduction wave of this compound is similar to that of the related mercaptopropionate compound. Therefore, the thioglycolate derivative also gives a non persistent reduction product. Both waves are shifted to less positive potentials on going from the thioglycolato to the mercaptoptopionato derivative ( = -0.26 V for the oxidation process and -0.42 V for the reduction process). Since the coordination mode of the two mercaptoacids is identical, the effect of the additional CH2 group in the ligand backbone is probably not having a large effect on the redox potentials. The potential shifts are mostly related to the change of the single atom bridge from sulphur to oxygen. The negative shift is consistent with the harder and stronger -donating character of the lighter group 16 element, which is then better able to stabilize higher oxidation states (easier oxidation process), whereas the heavier and softer sulphur atom is better able to stabilize lower oxidation states (easier reduction process).

- The two compounds were also investigated in a mixed MeOH-H2O solution (50:50 ratio) at various pH values in the 2-8 range. For both compounds, the oxidation process is pH-independent, whereas the reduction process shifts toward more negative potentials as the pH increases. A figure showing representative data for the mercaptoacetic complex are given in the Supporting Information. This is consistent with the molecular structure of the two compounds, since the one-electron oxidation product does not have active protons and therefore cannot be deprotonated. Conversely, the one-electron reduction product can accept a proton from the medium, most likely on the single-atom brigde. The half-wave of the pHindependent oxidation process is 0.49 V vs. ferrocene in MeOH-H2O (vs. 0.32 V in CH2Cl2)

2 -1 0 1 E/V vs. ferrocene (a) (b)
for the sulfido-bridged thioglycolate compound and 0.30 V (vs. 0.06 V in CH2Cl2) for the oxo-bridged mercaptopropionate compound. Thus, the negative shift observed for the potentials in CH2Cl2 on going from the S-bridged to the O-bridged complex is also reproduced in the mixed MeOH-H2O solvent, although the difference is less ( = -0.19 V in MeOH-H2O, vs. -0.26 V in CH2Cl2). Solvation differences in the different solvents, especially for the reduced charged species, are the probable cause of this effect.

The major point of interest in this study is the different outcome of the reactions of Cp*2Mo2O5 with two closely related substrates, HS(CH2)xCOOH (x = 1 or 2), to yield two closely related products, Cp*2Mo2(-X)[-S(CH2)xCO2], which however surprisingly differ in the nature of the bridging X ligand: S for x = 1 (previously described) [11] and O for x = 2

(described here for the first time). While we do not currently have sufficient information to outline the intimate details of the reaction mechanism, we note that thioglycolates are more facile substrates than alkyl mercaptans for the hydrodesulfurization process. For instance, HSCH2COX (X = OMe, OEt, Me, NMe2) are hydrodesulfurized efficiently and with high selectivities to the corresponding CH3COX under H2 (20 bar) with the Cp2Mo2(CO)4 catalyst, whereas HSCH2CH2COOMe, n-octylmercaptan and thiophenol are much less reactive.

Compound Cp2Mo2(-S)2(-SCH2COOEt)2 was isolated from the catalytic mixture and structurally characterized [35]. It is therefore likely that mercaptoacetic molybdenum complexes having suitable coordination geometries and/or oxidation states are more susceptible to C-S bond cleavage than longer-chain analogues. The authors of the above catalytic study formulated the hypothesis that the S-C bond is activated in a bimetallic Mo-SCH2CO(X)-Mo intermediate [35]. The same is probably true for out system. Thus, although

Conclusion

The interaction of Cp*2Mo2O5 with HSCH2CH2COOH in an aqueous environment at room temperature quite readily leads to reduction to Mo IV with incorporation of the doubly deprotonated substrate as a ligand, but contrary to the previously reported process with the closely related HSCH2COOH substrate, a C-S bond cleavage does not occur. This difference relates to a previously established more facile hydrodesulfurization of mercaptoacetic acid with respect to mercaptopropionic acid and to simpler alkylthiols.

1 H

 1 NMR spectra were recorded on a Bruker AM 250, operating at 250 MHz. Chemical shifts are expressed in ppm downfield from Me4Si. Coupling constants are given in Hertz. The 2D-COSY spectrum were recorded on a Bruker Avance 500, operating at 500 MHz. Mass spectra were recorded with a Finnigan (San Francisco, USA) LCQ quadrupole ion-trap mass spectrometer equipped with an electrospray-ionization (ESI) interface. The ESI was operated in the positive-ion mode with a spray voltage of 4.0 kV, a capillary voltage of 5.5 V, and a source temperature of 100 °C. Mass spectra were obtained by scanning the mass analyzer from m/z 100 to 2000 with a total of 3 microscans. The maximum injection time into the ion trap was 25 ms. The analyzer was operated at a background pressure of 2•10 -5 Torr. The compounds were isolated in the ion trap with isolation width of 15 m/z units and were activated by using increased collision energy to obtain the sequential product-ion fragmentation by collision-energy-dissociation[22,23].The starting compound,[Cp*2Mo2O5], was prepared as described in the literature[24]. The 3-mercaptopropionic acid (99%, ACROS) and the trifluoroacetic acid (99%, ACROS) were used as received. Cyclic voltammograms were recorded with an EG&G 362 potentiostat connected to a Macintosh computer through MacLab hardware/sofware. The electrochemical cell was fitted with an Ag-AgCl(KCl sat) reference electrode, a platinum disk working electrode for aprotic solvents or glassy carbon disk electrode for H2O-MeOH solution, and a platinum wire counterelectrode. [Bu4N]PF6 (ca. 0.1 M) was used as supporting electrolyte in CH2Cl2 whereas NH4Ac (0.

  Figure S1 (Supporting Information). The observed peaks have one mass unit greater than the

Figure 1 . 1 [

 11 Figure 1. MS n studies of the fragmentation pattern of compound [Cp*2Mo2(-O)(-SCH2CH2COO)2]+H + (peak at m/z = 687). Relative collision energy at every step is 15%.

  three bridging atoms (two S and one O) and by a terminally bonded second O atom from the carboxylate function. It is a member of the relatively large family of triply-bridged dinuclear (CpMo IV ) compounds, other members of which are Cp*2Mo2Br2(-Br)(-O)(-SMe) [31],

Figure 2 .Figure 3 .

 23 Figure 2. ORTEP view of compound [Cp*2Mo2(-O)(-SCH2CH2COO)2]. Hydrogen atoms have been omitted for clarity

Figure 4 .

 4 Figure 4. Cyclic voltammetry in CH2Cl2 solution of : (a) compound [Cp*2Mo2(-O)(-SCH2CH2CO2)2]•(HO2CCH2CH2SSCH2CH2COOH); (b) compound [Cp*2Mo2(-S)(-SCH2CO2)2].

  the products obtained with HS(CH2)xCOOH (x = 1 or 2) are structurally related, they could be formed by quite different mechanisms.It is highly unlikely that C-S bond cleavage for the mercaptoacetic acid reaction occurs after the formation of an oxo-bridged product, because we see no evidence (by ESI MS) for the formation of an oxo-bridged intermediate in the synthesis of the Cp*2Mo2(-S)(-SCH2CO2)2 product. In addition, compound Cp*2Mo2(-O)(-SCH2CH2CO2)2 does not react with an excess of mercaptoacetic acid under the same conditions that lead to its synthesis (50:50 MeOH-H2O mixture, acidified by CF3COOH, at room temperature). No reaction was detected even after warming the mixture for 1 h at 100°C. Conversely, no reaction took place under the same conditions when mixing compound Cp*2Mo2(-S)(-SCH2CO2)2 with an excess of mercaptopropionic acid.

Preparation of [Cp*Mo(-SCH2CH2COO)]2(-O). HSCH2CH2COOH

	added to each solution and measured at the end of the experiments. EPR measurements were
	carried out at the X-band microwave frequency on a Bruker ESP300 spectrometer. The
	spectrometer frequency was calibrated with diphenylpicrylhydrazyl (DPPH, g = 2.0037).
	(0.392 g, 3.69
	mmol) and 9 drops of CF3COOH were added to a solution of Cp*2Mo2O5 (0.200 g, 0.369
	mmol) in MeOH:H2O (1:1, 5 ml), resulting in a color change from yellow to red-brown. After
	3 h of stirring, all solvents were removed in vacuum and the oily residue was washed with 2 x
	5 ml of oxygen-free distilled water and with 2 x 5 ml of pentane. The resulting red-brown
	residue was extracted with THF, followed by addition of pentane to yield a brown precipitate
	which was filtered and isolated (0.127 g, 38% yield). The product is soluble in CH2Cl2 and
	THF, insoluble in Et2O and saturated hydrocarbons. The NMR of the isolated product
	showed only the presence of the title compound, but the presence of a paramagnetic impurity,
	which could not be crystallized in pure form, was revealed by EPR spectroscopy in CDCl3 (g
	= 1.97, aMo = 41.6 G). The 1 H NMR spectrum (CDCl3) revealed extensive overlap between
	the methylene protons of the ligand and those in a co-crystallized
	HOOCCH2CH2SSCH2CH2COOH molecule (also revealed by the X-ray analysis on a single
	crystal). The assignment was aided by a 2D-COSY analysis (see Results section). For the
	dinuclear Mo complex:  2.96 (m, 1H, Ha), 2.65 (m, 1H, Hb), 2.59 (m, 1H, Hc), 1.93 (s, 15H,
	Cp*), 1.87 (m, 1H, Hd). For the disulfide by-product:  2.99 (t, J = 7.1 Hz), 2.81 (t, J = 7.2
	Hz). Subsequent recrystallizations of the crude material did not completely remove the
	paramagnetic impurity (as shown by EPR spectroscopy), a possible reason for the imperfect
	analytical results: calculated for C32H48Mo2O9S4 (896.86): C, 42.8; H, 5.4. Found: C, 42.1; H,
	standard, which was

1 M) was used as a supporting electrolyte in addition to buffer solutions at different pH in MeOH-H2O. All potentials are reported relative to the ferrocene 4.8. A single crystal for the X-ray analysis was obtained by diffusion of a pentane layer into a brown CH2Cl2 solution of the crude reaction product at room temperature.

Table 2 .

 2 Selected

	Empirical formula	C32 H48 Mo2 O9 S4
	Formula weight	896.82
	Temperature	180(2) K
	Wavelength	0.71073 Å
	Crystal system	Triclinic
	Space group	P -1
	Unit cell dimensions	a = 8.6819(9) Å	= 85.526(7)°.
		b = 14.2934(12) Å	= 73.494(8)°.
		c = 15.8121(14) Å	 = 86.910(7)°.
	Volume	1874.6(3) Å 3
	Z	2
	Density (calculated)	1.589 Mg/m 3
	Absorption coefficient	0.941 mm -1
	F(000)	920
	Crystal size	0.42 x 0.11 x 0.063 mm 3

and selected bond distances and bond angles in Erreur ! Source du renvoi introuvable.. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 632621. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Table 1. Selected crystallographic and refinement parameters for compound [Cp*2Mo2(-O)(-SCH2CH2CO2)2]•(HO2CCH2CH2SSCH2CH2COOH).

Table 3 .

 3 Relevant

	hydrogen bonding parameters for the structured of compound
	[Cp*2Mo2(-O)(-SCH2CH2CO2)2]•(HO2CCH2CH2SSCH2CH2COOH). a
		O-H (Å)	H…O (Å)	O….O (Å)	O-H…H(°)
	O432-H432…O62	0.84	1.82	2.605(6)	154
	O332-H332…O32 i	0.84	1.78	2.587(5)	160
	a Symmetry code: (i) -x, -y, -z+1.

Acknowledgements

We are grateful to the European Commission for funding of this work through the AQUACHEM Research Training Network (Project n° MRTN-CT-2003-503864). Supplemental support by a Bosphorus bilateral Programme of Integrated Actions, cosponsored by French Ministry of Foreign Affairs in France and by TUBITAK in Turkey (TBAG-U/142(105T265)) is also gratefully acknowledged. The Israeli group is grateful to the Israel Science foundation for support.


Agnès Labande
email: agnes.labande@lcc-toulouse.fr

Jean-Claude Daran

Eric Manoury

Rinaldo Poli

New (1-Phosphanylferrocen-1'-and -2-yl)methyl-Linked Diaminocarbene Ligands: Synthesis and Rhodium(I) Complexes

Keywords: Carbene ligands, Phosphane ligands, Ferrocenes, Rhodium

come

The chemistry of N-Heterocyclic carbenes (NHCs) has experienced a very rapid development over the past ten years. [1] They have found many applications in catalysis, since the corresponding transition metal complexes have proven to be very active, robust and generally air stable. [2] Ligands associating a NHC and a phosphine have also shown very interesting properties, [3] in particular for C-C coupling reactions catalysed by palladium or nickel. [4] To date, there are only two synthetic methods allowing the preparation of ferrocenyl phosphine-NHC ligands with an aryl (or ferrocenyl) substituent on the imidazole moiety. [5], [6] However, the carbon situated in  to the imidazolium has a methyl substituent in both cases. To our knowledge, there is no precedent in the literature for 1,1'-disubstituted ferrocenyl phosphine-imidazolium ligands. On the other hand, the fundamental difference between our 1,2-disubstituted ligands and previously reported ligands is that the former possess only planar chirality, whereas the latter have both planar and central chirality. [5], [6] Thus it becomes possible to study the specific effect of planar chirality in asymmetric catalytic reactions. We present here a new synthetic method for 1,1'and racemic 1,2-disubstituted ferrocenyl phosphine-imidazolium ligands, which are precursors of phosphine-NHCs.

The precursors for the introduction of the imidazolium moiety are alcohols 3 (Scheme 1) and 6 (Scheme 2). The synthesis of racemic (as well as enantiomerically pure) 1,2-alcohol 6 is already well known. [7,8] We have adapted this method to prepare the new 1,1'-alcohol 3. It was successfully obtained in two steps from known 1'-diphenylphosphino-1-bromoferrocene, [9] with satisfactory yields.

Procedures for introducing a N-substituted imidazole on a ferrocenyl unit can involve the displacement of a chloride, [10] an acetate [6], [11] or a dimethylamino group. [5] Bolm et al. described the efficient conversion of ferrocenyl alcohols into corresponding imidazoliums by a two-step procedure. [12,13] However, this method has not been applied so far to introduce aryl or tertiary alkyl groups. Here the imidazolium functionality has been introduced in one pot from the alcohol, following a protocol developed earlier in our group for the introduction of various nucleophiles on 1,2-disubstituted ferrocenyl phosphines. [14] The -carbocation is generated with a strong acid and reacts with the N-R-imidazole (a: R = Me; b R = 2,4,6-Me3C6H2 or Mes). This method allowed us to introduce equally well imidazolium groups bearing a primary alkyl or an aryl substituent. The reactions are clean, very rapid and high-yielding (Schemes 1 and 2). [START_REF]other methods (generation of chloride, bromide or acetate leaving groups from alcohol 3 failed to give the expected compound, or the reactions were nor selective neither effective[END_REF] The structure of the 4a and 7a intermediates was confirmed by X-ray diffraction methods (see Fig. 1). The structure of 7a

shows two independent molecules in the asymmetric unit, the parameters of which are essentially identical. Only one of them is represented in Fig. 1. The bond lengths and angles in both compounds are all within the expected range. The imidazolium moiety is exo with respect to the ferrocenyl unit, and slightly tilted towards the diphenylphosphino group in the case of 7a. In 4a, the packing is governed by C-H--- interactions involving the H(64) of the mesityl group and the centroid of the symmetry-related Cp ring bearing the mesityl group [C(64)-H(64)… Cg2 i : C(64)-H(64) 0.95Å; H(64)…Cg2 i 2.74 Å; C(64)…Cg2 i 3.562(5) Å; C(64)-H(64)…Cg2 i 144.8° (symmetry code (i): -x+1,-y+1,-z+1], leading to the formation of centrosymmetric pseudo dimer.

The two molecules within the asymmetric unit in 7a are also connected through weak C-H--- interactions involving the H( 14) atom of the substituted Cp and the centroid of the C(211)-C( 216) phenyl ring [C( 14)-H( 14)…Cg3: C( 14)-H( 14 3) Å] and an offset angle of 24.6°[18.7°]. The sulfur atom in 7a is pointing towards the iron, as has been seen with similar 1,2disubstituted ferrocenyl ligands. [8] Mild reaction conditions were then required for the phosphine deprotection, compatible with the imidazolium moiety. After several attempts, we found that the use of Raney nickel in acetonitrile gave the best results. [START_REF] Liu | [END_REF] Carbenic rhodium(I) complexes were obtained by deprotonation of the imidazolium salts in the presence of potassium tert-butoxide and [RhCl(COD)]2 in THF, followed by chloride abstraction (Scheme 3). The latter step could be carried out with either NaBF4 in CH2Cl2/H2O, or AgBF4 in CH2Cl2. The silver reagent results in a faster abstraction process while it does not oxidize the ferrocene unit. Room temperature conditions for the deprotonation step resulted in good yields (75%) for product 9a, but less satisfactory results were obtained in other cases (i.e. 32% for 10b).

However, carrying out the deprotonation at -78°C raised the yield of 10b to 77%. This shows that the bulky imidazolium substituent does not negatively affect the coordination process. Good yields (81%) were also obtained for 9b when using the low temperature deprotonation protocol, whereas the yield of 9a was lowered (62%). Whereas the products with the 1,1'-disubstituted ligands, 9a,b, were obtained selectively, the reactions with the 1,2-disubstituted ligands yielded by-products. The selectivity for 10b was almost total [START_REF]Along with the doublet attributed to 10b in 31 P NMR, a second, weak doublet can be observed[END_REF] , however, complex 10a was obtained along with another carbene-phosphine complex in a 85:15 ratio. [18][19] It is reasonable to think that, in the case of 1,2disubstituted ligands, the selectivity is affected by the steric encumbrance of the imidazolium substituent. All complexes are stable in air and could be purified by a filtration on silicagel. The structure of complex 9a was confirmed by X-ray diffraction methods (see Fig. 2). The structure shows a square-planar geometry with the carbene and phosphine donors in a cis arrangment. The bond lengths and angles are again within the expected range for this kind of complexes. The Rhcarbene distance is in accordance with what Seo obtained with a very similar complex, although the Rh-P distance is slightly longer in our case (2.3345(8) Å instead of 2.2935 (10) Å in Seo's Rh(I) complex, which has two NHC-phosphine ligands coordinated to the metal centre). [5] However, we do not observe any significant lenghtening of the Rh-C bonds trans to the carbene (Rh(1)-CG1), with respect to the other Rh-C bonds (Rh(1)-CG2), as is commonly observed for bifunctional NHC ligands. [5,13] Finally, no tilting of the Cp rings can be observed upon complexation of the ligand to the rhodium centre (the angles between the two least-squares Cp planes are 4.66(29)° for 4a and 3.15(24)° for 9a).

Preliminary tests have been carried out to evaluate the activity of complexes 9a, 9b and 10b (racemic mixture) for the hydrosilylation of ketones. The reaction of diphenylsilane with acetophenone was carried out in dichloromethane or THF at room temperature, with 2 mol% catalyst (Table 1). The first results show that THF is a better solvent, despite the low solubility of complexes 9a and 9b, and that the complex bearing a 1,2-disubstituted ligand is more active. All complexes show a moderate activity, compared with other literature benchmarks. [13,20] In conclusion, we have prepared new phosphine/imidazolium ferrocenyl ligands by a general and effective method. Further tests will be carried out to evaluate the activity of the Rh(I) complexes in various catalytic reactions. Enantiomerically pure 1,2 ligands will also be prepared and tested in the asymmetric version of the reactions. Results will be published in due course.

Experimental Section

All reactions were carried out under a dry argon atmosphere using Schlenk glassware and vacuum line techniques. Solvents for syntheses were dried and degassed by standard methods before use. N,N-Dimethylformamide (DMF) was purified by distillation from CaH2. Spectra were recorded on a Brüker AM250, a Brüker AV300 or a Brüker AV500 spectrometer. All spectra were recorded in CDCl3, unless otherwise stated. Mass spectra were obtained from acetonitrile solutions on a TSQ7000 instrument from ThermoElectron. All new compounds described were fully characterised by 1 H NMR, 13 C NMR, 31 P NMR, elemental analysis and mass spectrometry. 1,1'dibromoferrocene and 1'-diphenylphosphino-1-bromoferrocene were prepared according to literature procedures. [9] General Procedures for the Preparation of Imidazolium Salts: To a solution of 3 (100 mg, 0.23 mmol) in degassed dichloromethane (5 mL) was quickly added HBF4 (35 L, 54% wt in Et2O), immediately followed by a N-substituted imidazole (0.35 mmol). The mixture was washed with 2M aq. HCl, water, sat. aq. NaHCO3 and water again. The organic phase was dried (MgSO4), filtered and concentrated in vacuo. Methyl-substituted salts : the residue was taken up into dichloromethane (1 mL), diethyl ether was added and the orange precipitate was filtered, washed with ether and dried in vacuo to give a yellow-orange solid. Mesityl-substituted salts: the residue was purified by column chromatography on silicagel (eluent: CH2Cl2/acetone: 9/1) to give a yellow-orange solid.

Similar yields were obtained starting from 500 mg of 3.

Imidazolium salt 4a: 86 mg, 64% yield. Monocrystals suitable for X-ray diffraction studies were obtained by slow evaporation of a dichloromethane solution. C27H26BF4FeN2PS (584.21) Calcd: C 55.51,H 4.49,N 4.80%;found: C 54.98,H 4.16,N 4.55%;1 H NMR (300 MHz,CD2Cl2,25 °C):  = 8.65 (s, 1H, NCHN + ), 7.79-7.72 (m, 4H, PPh2), 7.59-7.47 (m, 6H, PPh2)

N S N R Fe PPh 2 N N R (i) (ii) (iii) 1

  ) 0.93 Å; H(14)…Cg3 3.13 Å; C(14)…Cg3 3.755(3) Å; C(14)-H(14)…Cg3 126.5°]. In 7a, the imidazolium and one of the phenyl rings [C(121)-C(126); C(221)-C(226)] interact through an offset - stacking with a plane-to-plane distance of 3.566(2)Å[3.438(2) Å], a centroid-to-centroid distance of 3.9217(3) Å[3.6301(

  , 7.29 (s, 1H, HC=C Im + ), 7.27 (s, 1H, HC=C Im + ), 5.09 (s, 2H, Cp), 4.69 (s, 2H, Cp), 4.54 (s, 2H, Cp or CH2Im + ), 4.50 (s, 2H, Cp or CH2Im + ), 4.16 (s, 2H, Cp), 3.88 (s, 3H, CH3Im + ); 13 C { 1 H} NMR (75.5 MHz, CD2Cl2, 25 °C):  = 135.5 (NCN + ), 134.2 (d, JP,C = 87.0 Hz, 2 × quat PPh2), 131.55 (2 × PPh2), 131.53 (d, JP,C = 10.6 Hz, 4 × PPh2), 128.4 (d, JP,C = 12.5 Hz, 4 × PPh2), 123.6 (C=C Im + ), 121.8 (C=C Im + ), 80.4 (quat Cp C ), 76.2 (d, JP,C = 96.9 Hz, quat Cp P ), 74.1 (d, JP,C = 12.4 Hz, 2 × Cp P ), 73.1 (d, JP,C = 10.0 Hz, 2 × Cp P ), 71.7 (2 × Cp C ), 71.3 (2 × Cp C ), 49.1 (CH2Im + ), 36.3 (CH3Im + ); 31 P { 1 H} NMR (81.0 MHz, CD2Cl2, 25 °C):  = 41.0; m/z (ESI) 497 (M + , 70%); 415 (M + -C4H6N2, 100).

Scheme 1 .FeScheme 2 Scheme 3 .

 123 Scheme 1. Synthesis of 1,1'-ferrocenyl ligands. i. a) n-BuLi, THF, -25°C, b) DMF, -25°C, c) S8, CH2Cl2, 40°C (70%); ii. NaBH4, toluene/NaOH, 0°C (88%); iii. a) HBF4, CH2Cl2, rt, b) N-R imidazole; iv) Raney Ni, MeCN, rt or 80°C.

Figure 1

 1 Figure 1 Molecular views of compounds 4a (left) and 7a (right) with atom labelling scheme. Ellipsoids are plotted at the 50% level. H atoms have been omitted for clarity. Selected bond lengths (Å) and bond angles (°), 4a: C(1)-P(1) 1.790(3), C(62)-N(1) 1.324(5), C(62)-N(2) 1.313(5), C(63)-C(64) 1.330(6); N(2)-C(62)-N(1) 108.6(3); 7a: C(21)-P(2) 1.801(3), C(6)-N(21) 1.316(3), C(6)-N(22) 1.323(3), C(7)-C(8) 1.344(4); N(21)-C(6)-N(22) 109.4(3).

Table 1

 1 Hydrosilylation of acetophenone with diphenylsilane. Conds: 1 eq. acetophenone, 1.1 eq. diphenylsilane, 2mol% catalyst, room temperature. Determined by 1 H NMR after hydrolysis of the silylated intermediate.

	Imidazolium salt 4b: 96 mg, 60% yield. C35H34BF4FeN2PS (688.36) Calcd: C 61.07, H 4.98, N
	4.07%; found: C 60.31, H 4.71, N 3.91%; 1 H NMR (300 MHz, CD3CN, 25 °C):  = 8.54 (s, 1H,

a

H-,13 C-and 31 P NMR data, mass spectral data and elemental analysis for all new compounds.Crystal data for 4a, 7a and 9a.CCDC 616649-616651 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/datarequest/cif.

Acknowledgements

We thank the CNRS for support of this work and Yannick Coppel for NMR analysis of the rhodium complexes.

Graphical abstract

New, bifunctional 1,1'and 1,2-phosphine-NHC ferrocenyl ligands have been accessed by a general and simple method from the corresponding 1,1'and 1,2-thiophosphine-alcohols and used to prepare Rh I complexes.

Keywords: Carbene ligands / Phosphane ligands / Ferrocenes / Rhodium