Amer Al Nafiey

Ahmed Addad

Brigitte Sieber
email: brigitte.sieber@univ-lille1.fr

Guillaume Chastanet

Alexandre Barras

Sabine Szunerits

Rabah Boukherroub
email: rabah.boukherroub@iemn.univ-lille1.fr

Reduced

Keywords: Co3O4, reduced graphene oxide, 4-nitrophenol, reduction, Cr(VI), dyes, adsorption

teaching and research institutions in France or abroad, or from public or private research centers.

Reduced graphene oxide decorated with Co3O4 nanoparticles (rGO-Co3O4) nanocomposite: A reusable catalyst for highly efficient reduction of 4-nitrophenol, and Cr(VI) and dye removal from aqueous solutions

Amer Al-Nafiey, Ahmed Addad, Brigitte Sieber, Guillaume Chastanet, Alexandre Barras, Sabine Szunerits, Rabah Boukherroub

To cite this version:

Amer Al-Nafiey, Ahmed Addad, Brigitte Sieber, Guillaume Chastanet, Alexandre Barras, et al.. Reduced graphene oxide decorated with Co3O4 nanoparticles (rGO-Co3O4) nanocomposite: A reusable catalyst for highly efficient reduction of 4-nitrophenol, and Cr(VI) and dye removal from aqueous solutions. Chemical Engineering Journal, 2017, 322, pp.375-384. [START_REF] Ikedo | Spatial inhomogeneity of magnetic moments in the cobalt oxide spinel Co3O4[END_REF].1016/j.cej.2017.04.039. hal-

Introduction

With the rapid industrialization, the discharge and release of heavy metal ions and synthetic organic dyes from the textile, paper, and leather industries into the environment represent a serious issue to be addressed since these substances are potentially carcinogenic and mutagenic.

To meet environmental regulations and limit the amount of these substances, effluents or water contaminated with these organic and inorganic species must be treated before discharge [START_REF] Santhosh | Role of nanomaterials in water treatment applications: A review[END_REF].

Chromium is widely used in industries as for instance, mining, and pigment manufacturing.

In aqueous media, Cr exists in two different oxidation states i.e. trivalent Cr(III) and hexavalent Cr(VI) forms. While Cr(III) is less toxic, Cr(VI) is extremely mobile in the environment and carcinogen to living organisms [START_REF] Depault | Genotoxic effects of chromium(VI) and cadmium(II) in human blood lymphocytes using the electron microscopy in situ end-labeling (EM-ISEL) assay[END_REF]. Due to its bioaccumulation through the food chain, Cr(VI) can cause harm to the human body even at low concentrations. Therefore, it is necessary to reduce the Cr(VI) concentration to a level as low as possible before discharging the industrial waste matter into the environment. Synthetic dyes represent also an important class of water pollutants. They originate from various sources such as textile, dyeing, printing, paper and pulp, tannery and paint industries.

Dyes discharged in water can cause problems to aquatic life and some dyes are even toxic to human life [START_REF] Santhosh | Role of nanomaterials in water treatment applications: A review[END_REF]. Most of synthetic dyes are easily dissolved in water in cationic or anionic forms.

The cationic dyes include rhodamine B (RhB), methylene blue (MB), methyl violet (MV), malachite green (MG), and the anionic dyes consist of methyl orange (MO) and rose Bengal.

For example, wastewater from textile industries is highly colored by dissolved organic dyes.

Color removal from these wastewaters is one of the most difficult tasks, because synthetic dyes and pigments are resistant to biodegradation, and thus remain for a long period of time in the environment. Therefore, from an environmental point of view, the disposal and treatment of organic dyes are of great concern as well as the development of cost-effective methods to remove colored substances from wastewater [START_REF] Santhosh | Role of nanomaterials in water treatment applications: A review[END_REF].

To provide solutions for the above-mentioned environmental problems, various approaches and technologies to remove Cr(VI) and synthetic dyes have been described in the literature. These strategies include adsorption, photocatalysis, electro-chemical precipitation, reverse osmosis, and ion exchange [START_REF] Santhosh | Role of nanomaterials in water treatment applications: A review[END_REF][START_REF] Wang | Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials[END_REF][START_REF] Ren | Enhanced electrokinetic technologies with oxidizationreduction for organically-contaminated soil remediation[END_REF][START_REF] Ezzatahmadi | Clay-supported nanoscale zero-valent iron composite materials for the remediation of contaminated aqueous solutions: A review[END_REF][START_REF] Gómez-Pastora | Recent progress and future challenges on the use of high performance magnetic nano-adsorbents in environmental applications[END_REF]. Among these methods, only adsorption is a fast, highly efficient, inexpensive, and a widely applicable technique with no discharge of harmful substances.

Even though, adsorption represents an appealing method for environmental remediation, the lack of full recovery of the adsorbent from wastewater may cause a secondary pollution.

For that, a magnetic adsorbent is highly desirable, because it allows removal of toxic pollutants (metal ions, dyes) and the adsorbent itself can be recovered easily from water through application of an external magnet.

Despite its short development history, graphene-metal oxide nanocomposites have attracted increasing attention for their applications in environmental and energy-related areas, and GO/rGO-based composites have been successfully applied to treat different types of organic dyes and heavy metal ions [START_REF] Wang | Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials[END_REF]. As discussed above, organic dyes exist in ionic (anionic or cationic) forms in water, while GO/rGO are negatively charged, so dye adsorption on GO/rGO is believed to be dominated by electrostatic interactions (anionic-cationic). However, other interactions such as H-bonding and π-π stacking are not excluded during adsorption processes of organic dyes on GO/rGO-based composites [START_REF] Wang | Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials[END_REF]. On the one hand, negatively charged graphene oxide (GO) possesses various oxygen containing groups such as OH and COOH groups. As a consequence, it exhibits high adsorption capacity for basic compounds and metal cations. Reduced graphene oxide (rGO), on the other hand, displays a hydrophobic surface and this presents high adsorption capacity for dyes through strong π-π stacking and hydrophobic interactions [START_REF] Wang | Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials[END_REF].

An enhanced adsorption ability can be obtained by a modification of GO or rGO with metal oxides or organics. Recent reports have shown that GO/rGO-based composites have an excellent adsorption ability for Cr(VI) [START_REF] Wang | Facile synthesis of polypyrrole decorated reduced graphene oxide-Fe3O4 magnetic composites and its application for the Cr(VI) removal[END_REF][START_REF] Vu | Magnetite graphene oxide encapsulated in alginate beads for enhanced adsorption of Cr(VI) and As(V) from aqueous solutions: Role of crosslinking metal cations in pH control[END_REF] and of dyes (methyl orange, methylene blue, rhodamine B) and as such they represent promising adsorbents for water purification [START_REF] Wang | Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials[END_REF][START_REF] Bai | Synthesis of waterdispersible graphene-modified magnetic polypyrrole nanocomposite and its ability to efficiently adsorb methylene blue from aqueous solution[END_REF].

For instance, cobalt oxide Co3O4 is a magnetic p-type semiconductor with a cubic spinel crystal structure in which Co II and Co III atoms occupy the tetrahedral sites and octahedral sites, respectively [START_REF] Ikedo | Spatial inhomogeneity of magnetic moments in the cobalt oxide spinel Co3O4[END_REF]. It is a transition metal oxide with interesting optical, electronic, electrochemical, catalytic and electrocatalytic properties [START_REF] Li | Preparation and application of Co3O4 nanostructures with various morphologies[END_REF]. However, the Co3O4 outstanding properties are decreased or even lost, because severe particle aggregation takes place during their processing. To limit Co3O4 aggregation, their hybridization with carbon-based supports seems to be a very effective approach. For instance, the integration of Co3O4 onto graphene matrices endows the resulting materials with enhanced performance [START_REF] Xia | Three-dimensional graphene and their integrated electrodes[END_REF]. This has also been demonstrated in a few examples for environmental remediation. Co3O4/graphene have been successfully applied for heterogeneous activation of peroxymonosulfate (oxone) for the degradation of Orange II [START_REF] Shi | Co3O4 nanocrystals on graphene oxide as a synergistic catalyst for degradation of Orange II in water by advanced oxidation technology based on sulfate radicals[END_REF][START_REF] Wang | Synergistic effect of Co3O4 nanoparticles and graphene as catalysts for peroxymonosulfate-based Orange II degradation with high oxidant utilization efficiency[END_REF][START_REF] Shi | Supported cobalt oxide on graphene oxide: Highly efficient catalysts for the removal of Orange II from water[END_REF][START_REF] Shi | Synergistic catalysis of Co3O4 and graphene oxide on Co3O4/GO catalysts for degradation of Orange II in water by advanced oxidation technology based on sulfate radicals[END_REF], phenol [START_REF] Yao | Hydrothermal synthesis of Co3O4-graphene for heterogeneous activation of peroxymonosulfate for decomposition of phenol[END_REF] by advanced oxidation technology.

To the best of our knowledge, the utilization of rGO-Co3O4 nanocomposite for synthetic dyes and metal ions adsorption has been investigated only in a few reports [START_REF] Yavuz | A graphene/Co3O4 nanocomposite as a new adsorbent for solid phase extraction of Pb(II), Cu(II) and Fe(III) ions in various samples[END_REF]. Indeed, Yavuz et al. successfully applied Co3O4/graphene as an effective adsorbent for the retention/separation of Pb(II), Cu(II) and Fe(III) ions from environmental water [START_REF] Yavuz | A graphene/Co3O4 nanocomposite as a new adsorbent for solid phase extraction of Pb(II), Cu(II) and Fe(III) ions in various samples[END_REF]. In this paper, we took benefit of the high surface area and of the magnetic properties of rGO-Co3O4 nanocomposite to improve the catalytic and environmental efficiencies of the process. rGO-Co3O4 nanocomposites was used for two environmental applications: Cr(VI) and organic dyes removal from aqueous solution. The description of these applications is preceded with the catalytic reduction 4-nitrophenol to 4-aminophneol by rGO-Co3O4 nanocomposite using sodium borohydride (NaBH4); this model reaction is commonly used to evaluate the catalytic performance of metal or metal oxide nanoparticles.

Experimental

Preparation of graphene oxide (GO)

GO was synthesized from natural graphite powder according to [START_REF] Fellahi | Silicon nanowire arrays-induced graphene oxide reduction under UV irradiation[END_REF]. A homogeneous GO suspension (0.5 mg/mL) in water was achieved by ultrasonication for 3 h.

Preparation of reduced graphene oxide/Co3O4 nanocomposite

Different routes have been developed for the synthesis of rGO-Co3O4 nanocomposites.

Generally, they all require a two-step synthesis with the use of hazard agents and high temperatures (80-600°C) to reduce graphene oxide and anchor cobalt oxide NPs. In the present study, the rGO-Co3O4 nanocomposite was synthesized using a one-step, low-cost and simple preparation method based on simultaneous reduction of GO and CoCl2 using borohydride NaBH4 as reducing agent at room temperature [START_REF] Panwar | Magnetic Co3O4/reduced graphene oxide nanocomposite as a superior heterogeneous catalyst for one-pot oxidative esterification of aldehydes to methyl esters[END_REF]. rGO-Co3O4 nanocomposite was prepared by simultaneous chemical reduction of GO and CoCl2 with sodium borohydride (NaBH4). Typically, to 5 mL of 1 mg/mL GO suspension in Milli-Q water, was added 50 mg of CoCl2. The resulting mixture was sonicated for 10 min. Then 5 mL of 0.1 M sodium borohydride aqueous solution was added to the mixture at room temperature. A black precipitate was formed spontaneously, which was separated by centrifugation and washed repeatedly with water [START_REF] Panwar | Magnetic Co3O4/reduced graphene oxide nanocomposite as a superior heterogeneous catalyst for one-pot oxidative esterification of aldehydes to methyl esters[END_REF].

Reduction of 4-nitrophenol to 4-aminophenol catalyzed by rGO-Co3O4

The reduction of 4-nitrophenol (4-NP) to 4-aminophenol was carried out at room temperature under stirring. To 3 mL of freshly prepared 0.1 mM 4-nitrophenol aqueous solution was added 300 µL of 0.1 M NaBH4 and 200 µL of 0.1 mg/mL rGO-Co3O4 in water. Following NaBH4 addition, the solution turned yellow green due to the formation of 4-nitrophenolate. This is accompanied by a change of the maximum absorption from 317 nm (4-nitrophenol) to 400 nm (4-nitrophenolate). The progress of 4-nitrophenolate reduction to 4-aminophenol catalyzed by rGO-Co3O4 was monitored by the decrease of the absorption band at 400 nm using UV-vis spectrophotometry.

Adsorption studies

The batch adsorption mode was used to study Cr2O7 2-and dyes adsorption. The % of removal of Cr(VI) and dyes is calculated using:

Removal (%) = (C0 -Cf) / C0 x 100
where C0 is the initial concentration and Cf the final concentration of Cr(VI) ions or dyes.

Results and discussion

Characterization of rGO-Co3O4 nanocomposite

The morphology and microstructures of the as-synthesized samples were investigated by FE-SEM (Fig. 1). Figure 1a displays the SEM image of the initial GO, consisting of crumpled sheets. Similarly, the SEM image of the as-synthesized rGO-Co3O4 nanocomposite comprises aggregated and crumpled sheets of many graphene sheets along with Co3O4 nanoparticles (Fig. 1b).

The TEM image of the prepared rGO-Co3O4 nanocomposite displays Co3O4 NPs of 2-10 nm in size distributed on the rGO sheets (Fig. 1c). Additionally, the HR-TEM image of a single Co3O4 NP exhibits and interplanar distance between adjacent planes of 0.47 nm, corresponding to the interplanar spacing of (111) plane of Co3O4 (Inset Fig. 1c) [START_REF] Panwar | Magnetic Co3O4/reduced graphene oxide nanocomposite as a superior heterogeneous catalyst for one-pot oxidative esterification of aldehydes to methyl esters[END_REF]. The diffraction rings of the selected-area electron diffraction (SAED) pattern consists of five diffraction rings corresponding to the (220), ( 311), ( 400), (440), and (511) planes of Co3O4 (Fig. 1d). The as-synthesized rGO-Co3O4 nanocomposite contains only cobalt, carbon, and oxygen, as revealed by TEM-EDX analysis (Fig. S1), consistent with the chemical composition of the material. Some of the carbon signal may originate from the carbon grid onto which the sample was deposited for TEM and EDX analysis.

The cobalt content in the synthesized rGO-Co3O4 nanocomposite was estimated by ICP-AES analysis, and the results showed a Co loading of 50.4 g in 100 g of composite. NaBH4 reduction of a mixture of GO and CoCl2 aqueous solution induces also a red-shift of the absorption band of GO from 225 to 260 nm as well as the appearance of a new strong band at 433 nm due to Co3O4 NPs (Fig. 2, black trace) [START_REF] Farhadi | Simple preparation of ferromagnetic Co3O4 nanoparticles by thermal dissociation of the [CoII(NH3)6](NO3)2 complex at low temperature[END_REF][START_REF] Zhang | Co3O4 quantum dots: reverse micelle synthesis and visible-light-driven photocatalytic overall water splitting[END_REF]. The results suggest that GO nanosheets have been reduced to rGO and the sp 2 network within the GO nanosheets was partially restored upon reaction with cobalt chloride and NaBH4. The presence of a new band at 433 nm evidences that the cobalt oxide nanoparticles have been anchored onto the rGO sheets, suggesting a strong interaction between rGO and Co3O4 NPs. Co3O4 NPs are 2-5 nm in size as evaluated by X-ray diffraction (XRD) and transmission electron microscopy (TEM)

analysis [START_REF] Panwar | Magnetic Co3O4/reduced graphene oxide nanocomposite as a superior heterogeneous catalyst for one-pot oxidative esterification of aldehydes to methyl esters[END_REF].

The FTIR spectra show remarkable differences between GO and rGO-Co3O4 (Fig. 3). For GO, a strong and broad absorption ascribed to O-H vibration mode, and the C=O stretching mode of COOH groups situated at the edges of GO sheets are observed at 3404 and 1740 cm - 1 , respectively. The vibration mode at 1422 cm -1 may be due to bending vibrations of tertiary C-OH groups or or to bending vibration of C-H bonds. But, due to the presence of OH groups on rGO-Co3O4 composites (Fig. 3) and to the usual existence of hydroxyls and carboxyl group on rGO, we suggest that the 1422 cm -1 vibration modes are more likely due to C-OH groups. The band at 1023 cm -1 is attributed to stretching vibrations of C-O groups, while the band at 1623 cm -1 is attributed to C=C stretching vibration as part of the ring breathing mode in the GO skeleton [START_REF] Liu | Graphene oxide enhances the Fenton-like photocatalytic activity of nickel ferrite for degradation of dyes under visible light irradiation[END_REF]. By contrast, for the rGO-Co3O4 nanocomposite, the O-H absorption band is shifted to 3424 cm -1 , while the skeletal vibration band C=C at 1623 cm -1 is shifted to 1627 cm -1 .

Moreover, the stretching band of C=O at 1740 cm -1 disappears, suggesting that GO was reduced to rGO during the chemical process [START_REF] Hosu | Cobalt phthalocyanine tetracarboxylic acid modified reduced graphene oxide: a sensitive matrix for the electrocatalytic detection of peroxynitrite and hydrogen peroxide[END_REF].

The rGO-Co3O4 NPs hybrid exhibits typical ferromagnetic behavior with a saturation magnetization (Ms) of 0.1 emu.g -1 and a hysteresis of 115 Oe at 300 K (27°C), as shown by the magnetic hysteresis loop in Fig. 4a. (red) and 4K (dark).

The coercive force increases as the temperature is decreasing, to 495 Oe at 2 K; similarly, the saturation magnetization (Ms) increased to 40 emu.g -1 (Fig. 4b). This value can facilitate the recycling of the nanocomposite catalyst by a low energy-saving magnetic separation process. Then a mixture of 3 mL of 4-NP and 0.5 mL of NaBH4 was prepared from these solutions.

During this process, the color changed from transparent to yellow, because 4-nitrophenol was converted to 4-nitrophenolate anion by NaBH4 (Figs. 5 and6). To this mixture, 100 µL of rGO-Co3O4 nanocomposite (0.5 mg mL -1 ) catalyst was added at room temperature; the yellow-green color disappeared (Fig. 6).

NaBH4 rGO-Co3O4 NPs

Figure 6: Images illustrating the catalytic reduction of 4-NP to 4-AP using rGO-Co3O4 nanocomposite as catalyst and its separation at the end of the process using an external magnet.

The progress of the reduction reaction was monitored by UV-vis spectroscopy (Fig. 7). The UV-vis absorption spectrum of 4-NP exhibits an absorbance peak at 317 nm (Fig. 7, black curve). Upon addition of NaBH4, the absorption maximum shifts to 400 nm accompanied by a color change from transparent to light yellow due to the formation of p-nitrophenolate ion (Fig.

7, red curve). In absence of rGO-Co3O4

NPs catalyst, the peak due to 4-nitrophenolate ion at 400 nm is stable and remains unaltered even after two days, confirming the catalytic activity of the nanocomposite. Thus, the catalytic reduction of 4-NP to 4-AP can be monitored by following the decay of the characteristic absorption peak of 4-nitrophenoxide at 400 nm [START_REF] Wang | One-step synthesis of Au nanoparticles-graphene composite using tyrosine: Electrocatalytic and catalytic properties[END_REF]. The recyclability of catalysts is an important aspect for a more economical process. Thus recycling catalysis experiments were conducted for the rGO-Co3O4 nanocomposite (0.5 mg/ mL) catalyzed the reduction of 4-NP (0.1 mM) with NaBH4 (0.1 M). At the end of the reaction, the rGO-Co3O4 catalyst was easily recovered from the reaction medium using an external magnet (1 Tesla) (Fig. 6), and the supernatant was analyzed using UV-vis spectrophotometry.

The same procedure was repeated at least ten times in order to explore the reusability and activity of the nanocatalyst. The performance of the as-synthesized rGO-Co3O4 catalyst in repeated cycles of reaction is exhibited in Fig. S2. It reveals that the rGO-Co3O4 catalyst has a good stability and activity since it maintains a high performance for ten cycles.

We can consider that the decrease of the reduction time of 4-NP to 4-AP catalyzed by rGO-Co3O4 nanocomposite is attributable to the following factor: rGO sheets have high adsorption capacity for 4-NP via π-π stacking interactions. As a consequence, high concentration of 4-NP is present near the Co3O4 nanoparticles on rGO, leading to better contact between them; the electron transfer from rGO to Co3O4 nanoparticles increases the local electron density, improving the uptake of electrons by 4-NP molecules. It is clearly seen that in the presence of rGO-Co3O4 the absorption band of Cr2O7 2-at 348 nm shifts to 367 nm. This new peak decreases and disappears within 12 h. Also, as noted previously, the color of the solution changes from deep yellow to colorless, confirming the complete adsorption of Cr(VI) by rGO-Co3O4.

Cr(VI

To check the influence of Cr(VI) concentration in solution on the removal efficiency, the as-synthesized rGO-Co3O4 composite (2 mg) was dispersed in 2 mL of Cr(VI) solution at various Cr(VI) concentrations (pH=3) using a shaking speed of 400 rpm for 12 h. The solution was extracted by using a magnet to separate the rGO-Co3O4 and the supernatant was analyzed by UV-Vis spectrophotometry in order to determine the residual concentration of Cr(VI). A linear relationship between the Cr(VI) concentration and the absorbance at 367 nm was established (Fig. S3).

Adsorption isotherm

The Langmuir model was used to simulate adsorption isotherm of Cr(VI). If the adsorbent exhibits an ideal homogeneous adsorption surface with all the adsorption sites having the same sorption energy independent of surface coverage, the Langmuir model applies. The Cr(VI) removal capacity is thus calculated using:

Q = (C0 -Ce) V/W
where Q (mg/g) is the adsorption capacity, C0 (mg/L) the initial concentration of Cr(VI) ions, Ce (mg/L) the equilibrium concentration of Cr(VI) ions, V (L) the initial volume of Cr(VI) solution, W (g) the mass of the as-prepared rGO-Co3O4.

The removal amount of Cr(VI) ions was deduced from a calibration curve obtained from UVvis absorption measurements from Cr(VI) (0.5-300 mg/L) solutions, as shown in Fig. S4.

From the results in Fig. 10, it is clear that Cr(VI) ions are favorably adsorbed on the rGO-Co3O4 with a maximum adsorption capacity of 208.8 mg.g -1 , which is much higher than that obtained using many other magnetic adsorbents such as Fe3O4@mTiO2@GO (118 mg.g -1 ), graphene oxide decorated with magnetic cyclodextrin (MCGN, 120.0 g.g -1 ), graphene oxide functionalized with magnetic cyclodextrin-chitosan (CCGO, 67.66 mg.g -1 ) and iron nanoparticle decorated graphene (G-nZVI, 162.0 mg.g -1 ), Table 1. Table 1. Adsorption capacity of Cr(VI) from wastewater using different adsorbents.

The pH of the solution is known to be one of the most important parameters affecting the adsorption characteristics of ionic adsorbates. The relation between Cr(VI) adsorption capacity and initial pH of solution is depicted in Fig. S5. The higher adsorption capacity observed at a lower solution pH could be explained by the higher surface charge of the adsorbent, but also by the degree of ionization of the adsorbate. Chromium occurs in different anionic forms in aqueous solution (i.e., H2Cr2O7, Cr2O7 2-, HCrO4 -, CrO4 2-and HCr2O7 -), depending on the solution pH and concentration [START_REF] Dubey | Green synthesis of graphene sand composite (GSC) as novel adsorbent for efficient removal of Cr(VI) ions from aqueous solution[END_REF]. The following equilibria exist in an aqueous solution (1-

3):

H2CrO4 ↔ H + + HCrO4 - (1)
HCrO4 -↔ H + + CrO4 2- (2) 2 HCrO4 -↔ Cr2O7 2-+ H2O (3)
The results in Fig. S5 clearly reveal that the maximum adsorption of Cr(VI) is achieved in acidic media. This may be due to the fact that in the aqueous solution several forms of Cr(VI) ions exist such as Cr2O7 2-and hydrogen chromate (HCrO4 -), which are the dominant species in acidic media. In this case, the negatively charged HCrO4 -can be adsorbed on the rGO-Co3O4 through electrostatic interaction. In acidic medium, high concentration of H + ions is present which are able to neutralize the negatively charged adsorbent surface, thereby enhancing dichromate ions diffusion at low pH [START_REF] Wang | Graphene-decorated porous ceramics for efficient removal of Cr(VI)[END_REF]. In alkaline media, surface protonation decreases gradually and only chromate (CrO4 2-) ions remain as stable species. Therefore, when the solution pH is changed, the adsorption capacity of the rGO-Co3O4 for Cr(VI) is dominated by the existing form of Cr(VI) ions in solution.

To gain a better understanding on the adsorption mechanism, we have measured the zeta potential of rGO-Co3O4 in water (Fig. S6). A zeta potential of 17.0±1.5 mV was recorded.

According to Eqs (1-3), Cr(VI) exists mostly in its anionic form in acidic pH, suggesting that electrostatic interactions are dominant for Cr(VI) adsorption on rGO-Co3O4 although one cannot exclude Cr(VI) interaction with the oxygen-containing groups on the nanocomposite surface.

We have used the Langmuir isotherm to describe the equilibrium established between adsorbed Cr(VI) ions on the rGO-Co3O4 adsorbent (Qe) and Cr(VI) ions remaining in solution (Ce) at a constant temperature. Adsorption isotherms of the type Qe vs Ce was used to verify the isotherm model. The equilibrium adsorption isotherm was fitted with the Langmuir model using the following equation:

where Ce (mg/L) is the equilibrium concentration of Cr (VI) ions, Qe (mg/g) the amount of adsorbed Cr(IV) ions, Qmax (mg/g) the maximum adsorption capacity, and b (L/mg) the Langmuir isotherm coefficient.

The Langmuir isotherm parameters are obtained from the linear fit shown in Fig. S7. The Langmuir isotherm model gives a maximum adsorption capacity of 222.22 mg.g -1 for Cr(VI), which is very close to the experimental value of 208.8 mg.g -1 .

Dye removal from aqueous solution by rGO-Co3O4 nanocomposite

The removal ability of rGO-Co3O4 nanocomposite of organic dyes in aqueous solution has been investigated for three different dyes (one cationic dye, Rhodamine B and two anionic dyes, Methyl Orange and Rose Bengal) at three different concentrations for each dye. For the adsorption experiments, 2 mg of rGO-Co3O4 composite was added to 2 mL of an aqueous solution of RhB, MO or RB (5, 10 or 15 µM) at room temperature and pH 7.

Dye removal efficiency was calculated by recording the dye concentration before and after adsorption using UV-vis spectrophotometry. S1) with the benefit of being easily separated by applying an external magnet.

Removal by rGO: adsorption mechanism

To gain a better understanding on the important parameters that govern the adsorption process, we have applied rGO as adsorbent of RhB, MO and RB from aqueous solution (Fig. 13). Although rGO was efficient for dye removal, the adsorption process was quite longer than with rGO-Co3O4 nanocomposite; it took 20 min to remove 92% of RhB and 6 min to remove 75 % of MO. Surprisingly, rGO was not efficient for RB adsorption with only 8% removal after 50 min. Interestingly, under otherwise identical experimental conditions, rGO-Co3O4 nanocomposite removed 96%, 91% and 92% of RhB, MO and RB after 2, 1 and 2 min, respectively. From these results, we can conclude that hybridization of rGO with Co3O4 NPs has improved significantly its adsorption capacity for the three investigated dyes with the big advantage that they can be easily separated by an external magnet. The mechanism of dye adsorption is controlled by various parameters such as the physical and/or chemical properties of the adsorbent. rGO nanosheets are capable of interacting with a dye molecule through π-π conjugation, H-bonding, electrostatic and hydrophobic interactions [START_REF] Lin | One-step green synthesis of arginine-capped iron oxide/reduced graphene oxide nanocomposite and its use for acid dye removal[END_REF][START_REF] Gong | Adsorptive removal of methyl orange and methylene blue from aqueous solution with finger-citron-residue-based activated carbon[END_REF]. Moreover, dye adsorption is strongly influenced by the surface area and surface functionality of adsorbents [START_REF] Wang | Adsorptive remediation of environmental pollutants using novel graphene-based nanomaterials[END_REF]. The surface structure of graphene has a strong influence on its adsorption capability. The surface of rGO-Co3O4 is positively charged under our experimental conditions (zeta potential of 17±1.5 mV in water, Fig. S6). This implies that, for anionic dyes, electrostatic interactions play a major role even though aromatic interactions (π-π stacking) are well recognized in adsorption processes of aromatic molecules on graphene surface. Moreover, the homogeneous dispersion of Co3O4 NPs over the rGO support results in enhanced surface area and improved adsorption ability. For cationic dyes, the interaction between the nanocomposite and the dyes is believed to be governed by π-π interactions rather than binding at the oxygen groups of the nanocomposite [START_REF] Qin | RhB Adsorption Performance of Magnetic Adsorbent Fe3O4/RGO Composite and Its Regeneration through A Fenton-like Reaction[END_REF].

Our results prove that rGO-Co3O4 nanocomposite is an excellent, very efficient and fast dye adsorbent which works at neutral pH. Moreover, the rGO-Co3O4 hybrid is superparamagnetic at room temperature. Therefore, it can be easily and rapidly recovered from the solution through magnetic separation.

Conclusion

We have demonstrated that superparamagnetic rGO-Co3O4 nanocomposite exhibited good catalytic properties for 4-nitrophenol reduction to 4-aminophneol by NaBH4 with a full reduction in less than 1 min. The nanocomposite can be reused for 10 consecutive catalytic cycles without any apparent loss of its activity, indicating a good stability. Furthermore, the nanocomposite displayed high adsorption capacity for Cr(VI) at room temperature. The adsorption process can be well described by the Langmuir model, suggesting monolayer coverage sorption of Cr(VI) on the surface of rGO-Co3O4. The adsorption process of rGO-Co3O4 for Cr(VI) ions is pH dependent, reaching a higher adsorption capacity at low pH values.

Finally, the nanocomposite was successfully applied for rhodamine B, methyl orange and rose Bengal (5-15 μM) removal in less than 1 min at neutral pH. More importantly, the magnetic properties of the nanocomposite permit its simple separation from the liquid phase by application of an external magnet (1 T). These results evidence that rGO-Co3O4 holds promise for potential applications for the removal of heavy metal ions and organic dyes from wastewater, but also for other catalytic transformations in the field of environmental remediation.

  A stock potassium dichromate (K2Cr2O7) aqueous solution (500 mg/mL) was prepared. Aqueous solutions of different Cr concentrations (25, 50, 100, 200, 300 and 400 mg/L) were prepared by dilution of the stock solution with Milli-Q water. The pH of the Cr solutions was adjusted to 3 with 0.1 M HCl aqueous solution. The removal ability of rGO-Co3O4 nanocomposite of organic dyes in aqueous solutions was investigated for three different dyes (rhodamine B, methyl orange and rose Bengal) at three different concentrations for each dye. For the adsorption experiments, 2 mg of rGO-Co3O4 nanocomposite was added to 2 mL of an aqueous solution of rhodamine B (RhB), methyl orange (MO) or rose Bengal (RB) (5, 10 or 15 µM) at room temperature at pH 7.

Figure 1 .

 1 Figure 1. FE-SEM images of the (a) GO and (b) rGO-Co3O4 nanocomposite. Red arrows point toward the folds which connect the different GO/rGO sheets. The white arrows point toward Co3O4 nanoparticles; (c) TEM image of Co3O4 nanoparticles on rGO sheet. The inset is a HR-TEM image of a nanoparticle, (d) SAED of rGO/Co3O4.

Figure 2

 2 Figure 2 depicts the UV-vis absorption spectra of GO, rGO and rGO-Co3O4

Figure 2 .

 2 Figure 2. UV-visible absorption spectra of GO before (red), rGO (blue) and rGO-Co3O4 nanocomposite (black).

Figure 3 :

 3 Figure 3: Infrared spectra of GO (blue) and rGO-Co3O4 nanocomposite (red).

Figure 4 .

 4 Figure 4. Magnetic hysteresis loops of rGO-Co3O4 nanocomposite at a) 300 K and b) 2K

1 .

 1 Catalytic reduction of 4-nitrophenolThe catalytic activity of the rGO-Co3O4 nanocomposite was evaluated for 4-nitrophenol (4-NP) reduction to 4-aminophenol (4-AP) in the presence of excess NaBH4. This model reaction is commonly used in catalytic processes to examine the catalytic performance of metal or metal oxide nanoparticles (Fig.5).

Figure 5 :

 5 Figure 5: Schematic illustration of 4-nitrophenol reduction by NaBH4 catalyzed by rGO-Co3O4 nanocomposite.

Figure 7 : 7 ,

 77 Figure 7: UV-visible spectra of 4-NP (a), 4-NP before (b) and after (c) addition of 100 µL of rGO-Co3O4 nanocomposite (0.5 mg/mL).

) removal from aqueous solution by rGO-Co3O4 nanocomposite 3 . 2 . 2 . 1 .

 3221 Adsorption experimentsCr(VI) aqueous solutions of different Cr concentrations (0.5-300 mg/L) were prepared through successive dilutions of a stock K2Cr2O7 solution (500 mg/mL) with Milli-Q water. The pH value of the Cr solutions was adjusted to 3 with 0.1 M HCl aqueous solution. After the adsorption process, rGO-Co3O4 composite was conveniently recovered by magnetic separation, as shown in Fig.8.

Figure 8 :

 8 Figure 8: Images of Cr(VI) adsorption by rGO-Co3O4 nanocomposite and magnetic recovery of the catalyst by application of an external magnet (1 Tesla).

Fig. 9

 9 Fig. 9 shows the UV-vis spectrum in the range of 200-500 nm of a Cr(VI) aqueous solution

Figure 9 :

 9 Figure 9: UV-visible absorption spectral evolution with time during adsorption of Cr(VI) ions by rGO-Co3O4; Cr (VI) ion concentration: 50 mg/L, adsorbent dosage: 1 mg/mL, temperature: 25°C, pH: 3.0.

Figure 10 :

 10 Figure 10: Cr(VI) adsorption isotherms on the rGO-Co3O4 at pH 3.0 (Cr(VI) concentration: 0.5-300 mg/L, rGO-Co3O4 dosage: 1 mg/mL, pH 3.0, temperature: 25°C, contact time: 12 h).

Figure 11 :

 11 Figure 11: UV-Vis spectra of the a) RhB, b) MO, c) Rose Bengal solutions before and after adsorbent treatment. The initial concentration of dyes is 5 µM, of rGO-Co3O4: 1 mg/mL, T=25°C).

Fig. 12

 12 Fig. 12 displays photos of the adsorption process; the formation of transparent solutions

Figure 12 :

 12 Figure 12: Photographs of (A) RhB, (B) MO and (C) Rose Bengal aqueous solutions (5 μM) before and after treatment with 1 mg/ml of rGO-Co3O4 nanocomposite. External magnet of 1 Tesla.

Figure 13 :

 13 Figure 13: UV-Vis spectra of the original RhB, MO and Rose Bengal aqueous solutions before and after adsorbent treatment (initial concentration of dyes: 5 µM, rGO: 1 mg/mL, T=25°C).

Acknowledgements

The authors gratefully acknowledge financial support from the Centre National de la Recherche Scientifique (CNRS), the University Lille 1, and the Hauts-de-France region. The SEM facilities in Lille (France) are supported by the Conseil Régional des Hauts-De-France, the European Regional Development Fund (ERDF) and the CNRS.


Agnès Labande

Nathalie Debono

Alix Sournia-Saquet

Jean-Claude Daran

Rinaldo Poli

Oxidation-Promoted Activation of a Ferrocene C-H Bond by a Rhodium Complex

The oxidation of a rhodium(I) complex containing a ferrocene-based heterodifunctional phosphine Nheterocyclic carbene (NHC) ligand produces a stable, planar chiral rhodium(III) complex with an unexpected C-H activation on ferrocene. The oxidation of rhodium(I) to rhodium(III) may be accomplished by initial oxidation of ferrocene to ferrocenium and subsequent electron transfer from rhodium to ferrocenium. Preliminary catalytic tests showed that the rhodium(III) complex is active for the Grignard-type arylation of 4-nitrobenzaldehyde via C-H activation of 2-phenylpyridine.

Introduction

Cyclometalation via C-H activation has been very well known for decades, especially with the use of pincer ligands on a large variety of metals. [START_REF][END_REF] The mechanisms for C-H activation involve principally oxidative addition on a low oxidation state metal, electrophilic bond activation or -bond metathesis. [1e] C-H activation on rhodium complexes has been widely studied, [2][3][START_REF] Colby | For catalytic applications[END_REF] however, only two examples in the literature describe the C-H activation by oxidative addition on rhodium(I) complexes with PCP pincer ligands bearing a ferrocenyl unit in place of a phenyl. [5] One example of C-H activation has been observed on an iridium(I) complex bearing a heterodifunctional ferrocenylphosphine-pyridine ligand, although the corresponding rhodium(I) complex only shows a Rh  HC agostic interaction. [6] Activation on rhodium(III) complexes covers Caryl-H [7] as well as Calkyl-H [8] bonds. In this context, half-sandwich rhodium(III) complexes such as [Cp*RhCl2]2 and [Cp*Rh(MeCN)3]X2 proved particularly useful for the catalyzed functionalization of C-H bonds, [START_REF] Colby | For catalytic applications[END_REF], [9] but very few contributions deal with chiral complexes [8g,h] and we only found one example reporting catalytic C-H activation with a non half-sandwich complex. [10] Our group has a strong interest in the coordination chemistry and catalytic activity of functionalized N-heterocyclic carbene (NHC) ligands and among them redox-active ferrocenyl ligands. [11] Based on the pioneering work of Wrighton [12] and Mirkin [13] and more recent one of Long [14] and Bielawski [15] on the influence of redox-active ligands on the reactivity of the metal center, we have investigated the behavior of rhodium(I) complex 1 [11a] (Figure 1) upon oxidation. The phosphine group in the ferrocene-based heterodifunctional phosphine-NHC ligand is directly linked to the ferrocenyl unit, whereas the NHC is moved away from it by a methylene spacer. As the phosphine group is connected to both ferrocene and rhodium, we envisioned that the electronic changes on ferrocene would be efficiently transmitted to the metal center, [12c,16,17] making it more electrophilic upon oxidation. In the process of studying the redox chemistry of compound 1, we have discovered an unprecedented C-H activation of a ferrocene C-H bond on Rh triggered by ferrocene oxidation, eventually leading to a planar chiral rhodium(III) 50 complex and we present the results of this investigation in this contribution.

Results and discussion

55

A cyclic voltammetry (CV) analysis of 1 in CH2Cl2 showed a reversible redox wave for the ferrocene unit at E°1/2 = 0.35 V vs.

[FcH/FcH + ] [START_REF]Redox potentials were given vs the ferrocene/ferrocenium couple, as recommended by IUPAC. In our hands[END_REF] and a second irreversible process at Ep,a = 0.95 V vs. [FcH/FcH + ] that could be assigned to the oxidation of Rh(I) (Figure 2, top). A square wave voltammetry analysis at the 60 potential of the first oxidation wave (0.35 V vs. [FcH/FcH + ]) gave a total of 0.91 F/mol, which indicated the transfer of one electron per molecule. [19] The CV of 1 in acetonitrile showed a different behavior. Besides a shift of the two waves to less positive potentials (E°1/2 65 = 0.31 V and Ep,a = 0.8 V vs. [FcH/FcH + ] for the reversible ferrocene and the irreversible Rh(I) oxidations, respectively), two additional reduction waves, not present in CH2Cl2, now appear at 0.53 V and -0.47 V (see Figure 2, bottom).

The controlled potential electrolysis of This is a first indication that ferrocene can act as an electron relay for oxidation of the Rh center. When, on the other hand, the same electrolysis was continued beyond the consumption of 2 F/mol of Rh complex, decomposition occurred resulting in partial loss of the ferrocene wave. 20 The isolation and characterization of the product were then attempted by chemical oxidation experiments. The oxidants 1,1'diacetylferrocenium tetrafluoroborate [FcAcH][BF4] (E°1/2 = 0.49 V vs. [FcH/FcH + ] in MeCN) and thianthrenium tetrafluoroborate [Th][BF4] (E°1/2 = 0.86 V vs. [FcH/FcH + ] in MeCN) were selected because of their suitable oxidation potentials, their "innocent" character and the expected ease of purification of the oxidized Rh complex. [START_REF] Connelly | [END_REF] As calculated from the voltammogram of 1 in MeCN, [FcAcH][BF4] should oxidize only ferrocene while [Th][BF4] should be capable of oxidizing ferrocene and rhodium.

The use of d6-MeCN as solvent allowed 1 H and 31 P NMR monitoring. 1 0,1 0,3 0,5 0,7 0,9 1,1 1, When using [FcAcH][BF4], only minute amounts of 3 were 50 detected by 31 P NMR as a doublet at ca. δ 41 (JRh-P = 138 Hz) after 12 h. [START_REF]A blank experiment was carried out by dissolving complex 1 in d6-MeCN. After 1 week[END_REF] Greater insight was obtained from the experiment carried out with 1.5 equivalents of [Th][BF4]. The 31 P NMR spectrum after 10 min showed a large signal at δ 28.5 that evolved after 4h 15 min to a well-defined major doublet at δ 28.7 The orange product 3 was isolated in 75% yield by repeated precipitation and washings with diethyl ether and characterized by NMR and mass spectrometry. [22] It proved stable at room temperature and even air-stable as a solid. To our surprise, 1 H and 13 C NMR analyses revealed only seven proton resonances for the ferrocene unit when we expected eight. Additional 13 C NMR and 2D experiments (COSY, HSQC and HMBC) confirmed the seven C-H bonds and 13 C NMR revealed the presence of three quaternary carbon signals in the ferrocene region, one of which appeared as a doublet of doublets with a large coupling constant of 30 Hz to the Rh atom. [23] This suggested that ferrocene C-H activation and creation of a C-Rh bond has occurred, hence generating planar chirality at ferrocene. The carbenic carbon was found at 147.2 ppm, almost 30 ppm upfield relative to 1 (176.9 ppm), indicating binding of the NHC to a rhodium(III) center. [START_REF] Poyatos | [END_REF] Additional NMR data acquired in CD2Cl2 account for three MeCN molecules, one of which is in rapid exchange with free acetonitrile. Complex 2 could not be isolated, since it evolved spontaneously to 3 even in the solid state, but its 1 H and 13 C NMR properties indicate the presence of eight ferrocene C-H bonds. Hence, intermediate 2 contains the unactivated ferrocenyl ligand while the COD ligand is no longer present.

Scheme 1 Oxidation of rhodium(I) complex 1 to rhodium(III) complex 3.

The crystallization of 3 proved unsuccessful whatever the solvent/technique, but the addition of one equivalent of 2,2'bipyridine (bipy) to this complex in CH2Cl2 gave an orange-red solid after purification. Slow evaporation of a MeCN/C6F6 solution of the latter led to X-ray quality crystals of 4, where one bipy and one Cl -ligand (coming from the solvent) occupy the three MeCN coordination positions in 3 (Scheme 2 and Figure 6). Selected bond distances and angles are shown in Table 1.

The Rh(1)-C(1) distance is in the range of previously described Rh complexes with pincer-type σ-ferrocenyl ligands, [2a,5b,7a] and the other distances are within the expected range for this type of complex, yet this structure displays a few remarkable features. Firstly, it confirms the presence of a Rh-C bond at ferrocene with a fac arrangement of the tridentate ligand. A second interesting aspect is the generation of both planar chirality at ferrocene and central chirality at the metal, hence the potential production of up to three diastereoisomers in this case (given the rigidity of the tridentate ligand and the strength of the bonds with the rhodium center, imposing the fac stereochemistry). Finally, the tridentate ligand adopts a severely 50 distorted geometry to accommodate the octahedral coordination of rhodium. Indeed, the octahedral coordination is also distorted (see Figure 6). The metal coordination forces the phosphorus atom to deviate significantly from the plane of the Cp ring to which it is attached by 0.357(1) Å, and tilts the two ferrocene 55 cyclopentadienyl units to a dihedral angle of 10.9(2)°. A solution of the orange-red solid in acetone-d6 displays two 31 P NMR doublets at δ 35.8 (JRh-P = 128 Hz) and δ 31.5 (JRh-P = 127 Hz) in a 4:3 ratio, and two sets of 1 H NMR signals, which we 65 attributed to the existence of two diastereoisomers. Hence, one of the three possible diastereoisomers is not observed. The carbenic

ARTICLE TYPE

[journal], [year], [vol], 00-00 | 4 Fig. 7 Relative electronic energies (in kcal/mol) and figures of the optimized structures for the three possible isomers of the crystallographically characterized compound 4.

carbon atom in both isomers appears around 154.4 ppm. However, the newly formed Rh-bonded quaternary carbon atom is observed at δ 62.94 for the major isomer and at δ 76.61 for the minor one. This important shift may be imputed to a different trans influence of the Cl -and bipy ligands. [2e] Finally, a ROESY experiment showed dipolar coupling between a bipyridyl proton and two ferrocenyl protons (one on each Cp) for the major species, which does not fit the geometrical features of the structure represented in figure 6. Therefore, we tentatively attribute the minor species to the crystallographically characterized diastereoisomer 4 and the major species to the diastereoisomer bearing the Cl -ligand trans to the Cp ligand, 5.

Scheme 2 Synthesis of complexes 4 and 5.

In order to confirm this assignment, DFT calculations were carried out on the three possible isomers, optimizing first the geometry of 4 on the basis of the experimental structure as a 20 starting geometry, and then the other two possible isomers by transposing the positions of the Cl and bipy ligands. The suitability of the computational level (see Supporting Information) is verified by the relatively good match between experimental and computed geometric parameters of 4, as shown 25 in Table 1. The computed distances between the metal and the donor atoms are slightly longer, within 0.1 Å of the experimental value except for the P donor for which the difference is 0.13 Å, as is commonly found for this level of theory. The match in the angular parameters is less satisfactory with a deviation up to ca. 30 7° for the P-Rh-Cl angle, whereas all the other angles are in better agreement.

The energy results are shown in Figure 7. For isomer 5 with the Cl atom placed trans to the metallated Cp ring, a lower energy was obtained in agreement with the NMR assignment. For 35 the third isomer (6), containing the Cl ligand trans to the P donor, construction of the starting geometry immediately revealed a severe steric limitation caused by the short van der Waals contacts between the NHC methyl substituent and the o-H atom of the bipy ligand. The geometry optimization converged to a 40 final structure where the NHC is bent away from the ferrocene Cp plane, on one side of the Rh equatorial coordination plane, while the adjacent bipy ring is bent away in the opposite direction (see Figure 7), resulting in significant distortion of the preferred octahedral coordination geometry at the metal and of the 45 preferred coplanarity of the bipy ligand and in a much higher energy minimum, 8.4 kcal/mol higher than the most stable isomer 5. The difference between the calculated electronic energies of 4 and 5 (3.7 kcal/mol) is too high to rationalize the observed relative 4:3 population of the two isomers, for which a ΔG of 50 only 0.17 kcal/mol is expected. However, the prediction of relative stability is in the right direction. More sophisticated calculations including the zero point vibrational energy and the thermal corrections to enthalpy and entropy were not carried out because prohibitive in this case, given the large size of the calculation (all atoms were treated quantomechanically).

The mechanism of formation of 3 probably involves initial oxidation of rhodium(I) in 1 to rhodium(III), facilitating the release of the COD ligand and leading to the tricationic intermediate 2 (Scheme 3), which is stabilized by solvent coordination. The rhodium center becomes thus very electrophilic and promotes electrophilic aromatic substitution at the ferrocene 10 ring to give the stable complex 3. [13c,25] The C-H activation step is slower than for previously described ferrocenyl PCP pincer ligands, [5] consistent with the high strain generated in the new rhodium(III) complex.

Scheme 3 Proposed mechanism for the formation of complex 3.

An interesting point is that, although the oxidation of rhodium(I) to rhodium(III) may be accomplished directly by electrolysis at 1.19 V/[FcH/FcH + ] or by chemical oxidation with [Th][BF4], the same transformation is also realized by initial 20 oxidation of ferrocene to ferrocenium by electrolysis at 0.49 V vs.

[FcH/FcH + ] or more sluggishly by chemical oxidation with [FcAc][BF4]. This probably involves a relay mechanism where one or two electrons are transferred from rhodium to ferrocenium. This phenomenon has already been observed on rhodium(I) complexes bearing ferrocenyl ligands. [17] The difference between the oxidation potentials of ferrocene and rhodium (ca. 0.5 V) means that, after generating the Fe(III)-Rh(I) oxidation product which presumably involves minimal structural rearrangement, the electron transfer from Rh(I) to Fe(III) is endoergic by ca. 11.5 kcal/mol, an activation barrier than can be easily overcome at room temperature. It therefore seems likely that the slow step of the process is this endoergic electron transfer yielding a Fe(II)-Rh(II) intermediate that can subsequently be more easily oxidized with intervention of the MeCN coordination. It is known that, in the presence of structural rearrangement, a second electron transfer can be facilitated. [26] The second electron transfer may occur either before or after losing the COD ligand. Therefore, the transformation of 1 to 2 should be a relatively fast process in agreement with the cyclic voltammogram shown in Figure 2. The 40 intervention of the solvent before the second electron transfer is suggested by the reversibility of the CV in CH2Cl2. Evidently, the absence of stabilization of the Fe(II)-Rh(II) intermediate in CH2Cl2 further raises the activation barrier for the oxidative decomposition of compound 1. Unfortunately, it was not possible 45 to determine whether a rhodium(II) species was indeed involved in the process, as RPE experiments conducted at 113K immediately after addition of the oxidant to complex 1 did not show any signal corresponding to a metal-centered radical.

The electronic structure of the new complex 3 is similar to that 50 of [Cp*Rh(MeCN)3](BF4)2, which is known to be a good Csp2-H activation catalyst. It was therefore tested for the Grignard-type arylation of 4-nitrobenzaldehyde via the C-H activation of 2phenylpyridine (Scheme 4). [START_REF] Yang | [END_REF] To our delight, preliminary catalytic tests with complex 3 showed 33% conversion into the 55 expected alcohol after 24h at 65°C in THF ( 1 H NMR measurement, non-optimised conditions). The reaction was also carried out in 1,2-dichloroethane at 60°C and gave 24% conversion after 24h.

Conclusions

In summary, the oxidation of rhodium(I) complex 1 produced a stable, planar chiral rhodium(III) complex with an unexpected C-H activation on ferrocene that shows some activity for the 65 catalytic C-H activation of 2-phenylpyridine. More work will be done to optimize the reaction conditions and expand the substrate scope. Given the growing importance of rhodium(III)-catalyzed C-H activation/functionalization and the fact that there are very few examples of chiral rhodium(III) complexes bearing three 70 available coordination sites, our efforts will also focus on the resolution of planar chiral complex 3 to give an enantiopure catalyst.

Fig. 1

 1 Fig.1 Rhodium(I) complex 1 studied in this work.

Fig. 3 Fig. 4

 34 Fig.3 Square-wave voltammograms recorded during the electrolysis of 35

Fig. 5

 5 Fig.5 Chemical oxidants [FcAcH][BF4] (left) and [Th][BF4] (right).

55 assigned to the intermediate 2 ,

 2 along with some starting material (doublet at δ 19.5) and a small doublet at δ 41.6 (JRh-P = 140 Hz, complex 3). The NMR monitoring showed gradual disappearance of 2 in favor of the growth of 3. The transformation was complete after 48 h, with ca. 25% of 1 remaining unreacted. The 60 observation of these 31 P resonances suggests that compounds 2 and 3 are diamagnetic. The final 1 H NMR spectrum also shows the resonances of reduced thianthrene and free 1,5cyclooctadiene. The latter is released during the first step of the process leading from 1 to 2. Use of an excess amount (2.1 equivalents) of[Th][BF4] led to initial broadening of the 31 P resonance of 2 and did not allow the detection of the resonance of 3, presumably because of overoxidation and rapid self-exchange between 3 and 3 + .

Fig. 6

 6 Fig.6 ORTEP view of the cation in compound 4. Ellipsoids are shown at the 50% probability level. All hydrogens are omitted for clarity.

60Scheme 4

 4 Scheme 4Reaction of 2-phenylpyridine with 4-nitrobenzaldehyde.

E (V) vs [Fc/Fc + ]

		interrupting the current flow at this stage, 2 evolved
		spontaneously and slowly to product 3 (0.32 V vs. [FcH/FcH + ])
		which contains ferrocene in its reduced state, as indicated by its
		electrochemical properties (Figure 3, plain curve). When the
		electrolysis was carried out at 1.19 V vs. [FcH/FcH + ] (thus
		beyond the oxidation potential of rhodium) and interrupted after
		counting 2 F/mol, the same intermediate species 2 was again
		generated selectively after only 50 min, followed by conversion
		to 3, which was almost complete 20 h after interruption of
	10	electrolysis (Figure 4).
				2,50E-06
				2,00E-06
				1,50E-06
		i (A)	1,00E-06
				5,00E-07
				3,00E-21
				-0,7	-0,5	-0,3	-0,1	0,1	0,3	0,5	0,7	0,9	1,1
				-5,00E-07
			2.0E-06
			1.5E-06
			1.0E-06
		i (A)	5.0E-07
			0.0E+00
			-5.0E-07
			-1.0E-06
				-1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5
											70	1 in MeCN at 0.49 V
											vs. [FcH/FcH + ], a potential once again sufficient to oxidize the
											ferrocene unit selectively, resulted in transfer of greater charge.
											After 165 min of electrolysis, 2 F/mol had passed through the cell
											and 1 was completely transformed into a new species 2, which
											shows a redox process at 0.58 V vs. [FcH/FcH + ] (Figure 3). Upon
		2 | Journal Name, [year], [vol], 00-00

E (V) vs [Fc/Fc + ] Fig.2 Cyclic voltammograms on a Pt microelectrode of complex 1, top

	: 1
	mM in CH2Cl2 with nBu4NBF4 (0.1M) at a scan rate of 0.1 V s -1 ; bottom :
	1 mM in MeCN with nBu4NBF4 (0.1M) at a scan rate of 0.2 V s -1 .

Table 1

 1 Selected

		X-Ray	DFT
		4	4	5	6
	Distances (Å)
	Rh(1)-Cl(1)	2.4313(6)	2.499	2.577	2.474
	Rh(1)-P(1)	2.3210(6)	2.454	2.463	2.482
	Rh(1)-N(3)	2.104(2)	2.116	2.153	2.237
	Rh(1)-N(4)	2.156(2)	2.226	2.112	2.140
	Rh(1)-C(1)	2.028(2)	2.042	2.041	2.046
	Rh(1)-C(11)	2.022(2)	2.061	2.062	2.073
	Angles (°)
	Cl(1)-Rh(1)-P(1) 101.98(2)	94.94	95.06	175.27
	Cl(1)-Rh(1)-N(3) 82.78(5)	83.63	80.56	83.52
	Cl(1)-Rh(1)-N(4) 83.56(5)	80.19	84.07	83.63
	Cl(1)-Rh(1)-C(1) 87.57(7)	89.76	168.20	91.07
	Cl(1)-Rh(1)-C(11) 168.68(7)	173.51	99.85	86.33
	P(1)-Rh(1)-N(3) 175.08(5)	178.09	101.02	100.82
	P(1)-Rh(1)-N(4) 101.58(6)	101.50	178.64	99.24
	P(1)-Rh(1)-C(1)	87.00(7)	88.88	91.69	84.94
	P(1)-Rh(1)-C(11) 88.92(7)	91.47	87.75	90.95
	N(3)-Rh(1)-N(4) 77.56(8)	77.03	77.82	74.98
	N(3)-Rh(1)-C(1) 94.51(8)	92.37	88.67	169.26
	N(3)-Rh(1)-C(11) 86.40(8)	89.99	171.17	101.26
	N(4)-Rh(1)-C(1) 168.75(8)	166.07	88.99	95.24
	N(4)-Rh(1)-C(11) 97.38(8)	99.60	93.43	169.62
	C(1)-Rh(1)-C(11) 89.97(9)	89.29	90.06	87.59
	C(6)-P(1)-Rh(1) 113.85(8)	112.75	110.65	109.76

bond distances (Å) and angles (°) for the experimental 60 structure of 4 and for the optimized geometries of 4, 5 and 6.

Acknowledgements

This work was supported by the Agence Nationale de la 75 Recherche (ANR-07-JCJC-0041, postdoctoral grant to ND).

Notes and references

a CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France. Fax: (+33) 561553003; Tel: (+33) Albrecht, G. van Koten, Angew. Chem. Int. Ed., 2001, 40, 3750; (c)


Chiara Dinoi

Rinaldo Poli

Lionel Perrin

Laurent Maron

Mechanistic insights into β-oxygen

come

hydroperoxido O α atom. The reasons for this unusual pathway are discussed.

Alkene epoxidation is one of the fundamental and straightforward reactions in functionalising hydrocarbons as well as rapidly building selective functionality into a wide 15 range of molecules. 1,2 High scale production of epoxides needs the development of new synthetic methods that employ safe oxidants and produce as little waste as possible. In that perspective, hydrogen peroxide is an attractive oxygen source, both on environmental and economic grounds. 3,4 It is cheap, 20 readily available and gives water as the only by-product. Among the different transition metal catalysts used to carry out this transformation, high oxidation state cyclopentadienyl Mo-and W-oxido compounds have attracted great attention due to their rich aqueous chemistry [5][6][7] as well as to their ability to catalyse the olefin epoxidation in the presence of H2O2 in both organic and aqueous media. [8][9][10] As previously reported by our groups, in particular, the Cp*2W2O5 complex efficiently catalyzes the cyclooctene (COE) epoxidation with aqueous H2O2 in acetonitrile/toluene, showing a much greater catalytic activity than its corresponding Mo analogue. 9,10 The calculated Mimoun profile leads to prohibitive activation energies and will not be further discussed here. Concerning the external pathway, both mechanisms II) and III) have been computed (Fig. 2). Both pathways are exoergonic and lead, through trasition states ts45a and ts45b (respectively 26. Interestingly, the energy barrier corresponding to the O α transfer (ts45a) is ca. 5 kcal mol -1 higher than that of O β (ts45b), indicating that the latter mechanism is the most favourable pathway in the case of [Cp*WO(OH)2] + .

In order to assess solvation effects, we have computed the profile reported in Fig. 2 The W complex, therefore, differs from the analogue Mo complex, for which the computed activation barriers of these 45 two pathways were close in energy. 8 It also differs from other previously reported hydroperoxido derivatives of Ti, Re, and Mo. 16 The DFT studies carried out on Ti(OH)3(OOH), 17 Re(CH3)(O2)2O(H2O) 18 and Mo(NH3)(O2)O(OH)(OOH), 19 in fact, revealed the kinetic preference for the attack of the O α of 50 the hydroperoxido group over the attack of the O β .

To better understand the reason of this difference and to investigate the influence of the metal electron density on the oxygen transfer mechanism, the two energy profiles (II and III) have also been recomputed for the neutral Cp*WO2Cl 55 complex at the same level of theory (ESI, FigS3 and Fig. 3). Also for the latter system, both pathways are exoergonic, with activation barriers of 36.6 (ts45c) and 45.5 kcal mol -1 (ts45d) Differently from the cationic system 1, however, the external attack to the O α center (ts45c) is ca. 9 kcal mol -1 more favourable than that to the O β atom (ts45d), indicating that, within a series of similar complexes containing the same metal, the electron density at the metal center plays an 85 important role on the operative oxygen transfer mechanism.

The Cp*WO2Cl compound is a 16 electrons system while the corresponding [Cp*WO2] + ion is a 14 electrons system. Due to its lower electron density, the cationic In order to validate this idea, we computed the olefin

Fig. 1

 1 Fig. 1 Free enthalpy profile (kcal mol -1 ) for the H2O2 activation by [Cp*WO(OH)2] + . Cp* omitted for clarity.75

85 90 Scheme 1 . 2 | 15 Fig. 2

 9012152 Scheme 1. Cp* omitted for clarity.The reaction continues with the approach of the olefin followed by the oxygen transfer from the activated oxidant to 95

  6 and 22.2 kcal mol -1 relative to 3), to [Cp*WO(OH)2] + •H2O (6) and C2H4O. Optimised geometries of the transition states are shown in the ESI, Fig.S2.

  in water and CH3CN (ESI, Fig S3) using implicit solvent model. At this level of modeling, transition states ts45a and ts45b define energy barriers relative to 1 of 28.2 and 23.1 kcal mol -1 in water and of 27.6 and 22.8 kcal mol -1 in CH3CN. These values are respectively 40 reported in Fig. 2 in parenthesis and brackets. Though the energy barriers are modified, the trend between α-versus βoxygen transfer remains unchanged.

Fig. 3

 3 Fig.3Free enthalpy profile (kcal mol -1 ) for the oxygen atom transfer from 3_Cl to ethylene. Cp* omitted for clarity.

  80

Fig. 4 15 20Fig. 5

 4155 Fig. 4 Optimised geometries for the 3_Cl and 3 systems. In order to quantify the W-O α bond strength, we analysed the W-O α bond length in both neutral [Cp*WO(OH)(OOH)Cl] and cationic [Cp*WO(OH)(OOH)] + •H2O systems (3_Cl and 100

  30epoxidation mechanism catalysed by the cationic [Cp*W(O2)(OH)2] + peroxido complex (more stable than the corresponding water adduct by 8.5 and 2.4 kcal mol -1 , in gas phase and in water respectively). (Fig.5and Fig.S5in the ESI). Here again, both pathways are exoergonic and lead, through trasition states ts45e and ts45f (respectively 18.5 and 21.1 kcal mol -1 relative to 3_O2) to [Cp*W(O2)(OH)2] + •H2O (6_O2) and C2H4O (for the geometries of the transition states see the ESI, FigS6). The most favourable pathway corresponds to the O β attack (ts45e), the activation barrier related to the O α transfer (ts45f) being ca. 3 kcal mol -1 higher. The profile in shown in Fig.5was also computed in water and CH3CN (ESI Fig.S8). Energy barriers associated to ts45e and ts45f are of 21.5 and 25.9 kcal mol -1 in water and 21.3 and 25.4 kcal mol -1 in CH3CN relatively to 1_O2. These values are respectively reported in Fig. 5 in parenthesis and in brackets. Once again, the trend between α-versus β-oxygen transfer remaining unchanged. According to the O β transfer, in addition, the computed W-O α O β H and WO α -O β H homolytic bond dissociation energy 50 (BDE) were 46.9 and 7.6 kcal mol -1 , respectively, indicating, once again, that the high energy of the W-O α O β H bond cleavage drives the system toward the O β atom transfer. Conclusions 55 Through a DFT study, we have investigated the olefin epoxidation mechanism catalysed by the [Cp*WO(OH)2] + and [Cp*W(O2)(OH)2] + complexes with H2O2. Inspection of the different transition states reveals that the transfer of the hydroperoxido O β to the olefin is considerably more 60 favourable than that of the hydroperoxido O α . This O β atom transfer pathway has not been previously considered as a competitive epoxidation mechanism to the best of our knowledge and has been rationalized here on the basis of the metal electron density as well as of the W-O α O β H and WO α -65 O β H bond energies. The stronger the interaction between the W center and the O α , the more favourable the O β transfer process. With the present work, we have therefore shown that within a series of similar complexes containing the same W VI metal, the oxygen transfer pathway depends on the metal 70 charge and electron density, providing further insights into the comprehension of the oxygen transfer process for the design of efficient olefin epoxidation W catalysts. The authors thank the Institut Universitaire de France and CALMIP, CINES for grants of computing time. 75 Notes and references a Laboratoire de Physique et Chimie des Nano-objets (LPCNO), UMR 5215, Université de Toulouse-CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France. Fax. 33 561 559 697; Tel. 33 561 559 664; E-mail: lionel.perrin@insa-toulouse.fr 80 b CNRS; LCC (Laboratoire de Chimie de Coordination); Université de Toulouse; UPS, INPT; 205, Route de Narbonne, F-31077 Toulouse. c Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France. 85 † Electronic Supplementary Information (ESI) available: additionnal figures and cartesian coordinates and energies of all the stationnary points optimized in gas and implicit solvents. See DOI: 10.1039/b000000x/ 1 G. Sienel, R. Rieth and K. T. Rowbottom, Ullmann's Encyclopedia of