# Non-Covalent Immobilization of Chiral Rhodium
Catalysts on Carbon Nanotubes for Asymmetric
Hydrogenation

Zinnia Arora, Meriem Rais, Vasile I Pârvulescu, Karine Philippot, Jérôme

Durand, Maryse Gouygou

► To cite this version:

Zinnia Arora, Meriem Rais, Vasile I Pârvulescu, Karine Philippot, Jérôme Durand, et al. Non-Covalent Immobilization of Chiral Rhodium Catalysts on Carbon Nanotubes for Asymmetric Hydrogenation. ChemNanoMat, 2025, 11 (2), pp.e202400125. 10.1002/cnma.202400125. hal-04931851

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# Non-Covalent Immobilization of Chiral Rhodium Catalysts on Carbon Nanotubes for Asymmetric Hydrogenation

Zinnia Arora,[a, b] Meriem Rais,[a] Vasile I. Pârvulescu,[b] Karine Philippot,[a] Jérôme Durand,[*a] and Maryse Gouygou*[a]

Supported chiral catalysts were prepared by ionic anchoring of two cationic rhodium complexes (A and B) onto functionalized carbon nanotubes (CNTs). The immobilization of the Rh complexes was done by applying two ionic anchoring strategies: i) electrostatic interaction with the negatively charged surface of carboxylate-functionalized CNTs (CNT-COO⁻); ii) electrostatic interaction through a heteropoly acid (HPA) spacer previously grafted at the CNTs surface (CNT-HPA). The characterization of the obtained CNT-COO⁻@complex A, CNT-COO⁻@complex B, CNT-HPA@complex A, and CNT-HPA@com-

plex B materials by ICP, TEM and XPS showed the successful anchoring of the Rh complexes onto the supports. The HPA spacer was found to increase the catalyst's stability and to limit the effect of the CNT surface on the catalytic performances of the immobilized Rh complexes. CNT-HPA@complexes performed the hydrogenation of dimethyl itaconate into methyl succinate with high conversions (83–100%) and good enantioselectivities (58–63%). The recyclability tests showed the catalysts were less active but still enantioselective after recycling.

## Introduction

The metal-catalyzed enantioselective hydrogenation of olefins being one of the most powerful methods for the synthesis of optically active compounds, it has been widely implemented in academy[1-6] and industry.[7-12] Efforts in the ligand design, catalyst development and rationalization of the metal-ligand-solvent systems allowed a significant progress. However, both the product separation and catalyst recycling remain two major issues for the development of the homogeneous catalytic processes although several approaches have been investigated to find solutions. Among these, the immobilization of a chiral homogeneous catalyst onto a solid support has become one of the most promising strategy to improve the recovery and reuse of expensive homogeneous complexes, as well as to avoid the product contamination.[13-18] Related to these, carbon-based materials provide an attractive alternative to conventional supports, mainly oxides. Among these carbon materials[19] carbon nanotubes (CNTs) have elicited a high interest due to several advantages including: i) tailored pore structure; ii) large external surface and reduced micro-porosity; iii) reduced

number of defects and impurities and iv) specific mechanisms of adsorption. In addition, the conductive properties of CNTs allow managing the exothermicity of the hydrogenation reaction and therefore limiting the induction of hot spots that can be detrimental to both catalyst stability and reaction enantioselectivity. Despite the significant progress in the synthesis of CNT-based catalysts, only a few examples of asymmetric hydrogenation reactions catalyzed over CNT-supported chiral catalysts have been described to date. Examples on the asymmetric hydrogenation of model substrates such as α-acetamidocinnamic acid by chiral [Rh-(2S,4S)-BPPM][20] and [Au-(2S,4S)-BPPM] (BPPM = (2S,4S)-(-)-2-(diphenylphosphinomethyl)-4-(diphenylphosphino)-N-(t-butoxycarbonyl)-pyrrolidine) complexes covalently anchored on functionalized CNTs[21] reported this approach imposes modifications of both the CNTs and the ligand, typically requiring multistep syntheses. In consequence, non-covalent interaction appeared as an efficient, simple and low-cost strategy to access the supported catalysts as such interaction allows keeping intact the support structure and therefore its properties. Zhou and co-workers described the use of adsorbed pyrene-modified Rh-catalyst onto CNTs, involving $π-π$ stacking interaction, for the hydrogenation of various β-dehydroaminoesters.[22] Román-Martínez and coworkers[23] reported the non-covalent immobilization of the RhDuphos complex onto CNTs by electrostatic and $π-π$ interactions for the hydrogenation of methyl 2-acetamidoacrylate.

As part of our continuing interest in heterogeneous enantioselective hydrogenation,[20,21] we report herein the non-covalent immobilization of two chiral cationic rhodium complexes onto CNTs and their application in the enantioselective hydrogenation of dimethyl itaconate (DMI) as a model substrate. The immobilization of the Rh complexes on the CNT supports was investigated through an ionic anchoring strategy. This strategy takes advantage of an electrostatic interaction

<footnote>[a] Z. Arora, M. Rais, K. Philippot, J. Durand, M. Gouygou
Laboratoire de Chimie de Coordination du CNRS, Université de Toulouse,
UPS, Toulouse-INP, 205 route de Narbonne, BP 44099, F-31077 Toulouse
Cedex 4, France
E-mail: jerome.durand@ensiacet.fr
maryse.gouygou@lcc-toulouse.fr</footnote>

<footnote>[b] Z. Arora, V. I. Pârvulescu
Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, 4-12 Regina Elisabeta Avenue, S3, 030018, Bucharest, Romania</footnote>

<footnote>Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cnma.202400125</footnote>

<footnote>© 2024 The Authors. ChemNanoMat published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>
between the cationic rhodium complexes and the negatively charged CNT surface. As the surface of CNTs can affect the catalytic behavior of the immobilized complexes,[14,24] we also used a spacer-type anchoring agent, i.e. a heteropoly acid (HPA)[25,26] to have an electrostatic interaction via a linker that keeps the complexes away from the CNT surface. This second approach relies on: i) an acid-base interaction between the support and the HPA and ii) an ionic interaction between the HPA and the cationic complexes, as in the Kobayashi's

method.[27] Two rhodium complexes, Rh-(S,S)-BPPM (complex A)[28] and Rh-(S,S)-Et-Ferrocelane (complex B)[29] (Figure 1), already reported for the hydrogenation of DMI, have been selected for our study. The catalytic performances of the immobilized rhodium complexes CNT-COO⁻ @complex A, CNT-COO⁻ @complex B, CNT-HPA@complex A, and CNT-HPA@complex B (Scheme 1) have been evaluated in the enantioselective hydrogenation of DMI.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Ionic anchoring of chiral rhodium complexes onto the CNT surface: a) direct electrostatic interaction; b) electrostatic interaction through a HPA spacer and c) rhodium (I) complexes investigated in this work.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Immobilization of chiral Rh complexes A and B onto the CNTs surface.</figcaption></figure>
# Experimental Section

All commercially available reagents, i.e. dimethyl itaconate (99%, Sigma Aldrich), Bis(1,5-cyclooctadiene)rhodium(I) tetrafluoroborate (98 + %, Strem Chemicals), (2S,4S)-(-)-2-(diphenylphosphinomethyl)-4-(diphenylphosphino)-N-(t-butoxycarbonyl)pyrrolidine (Strem Chemicals), tungstosilicic acid hydrate (Sigma Aldrich), sodium hydroxide (NaOH) (pellets, purum, Chimica), 3-aminopropyl triethoxysilane (3-APTES) (99%, Sigma Aldrich), nitric acid (HNO₃) (65%, VWR), sulfuric acid (H₂SO₄) (95%, VWR), methanol (Carlo Erba), 2-propanol (Sigma Aldrich), acetone (Carlo Erba) were used as received. The anhydrous solvents: dichloromethane (DCM), pentane, diethyl ether and THF were achieved from a Solvent Purification System (Innovative Technologies). H₂ (99.99%) and Ar (99.99%) were purchased from Linde. Rh complex A was prepared from commercially available (2S,4S)-(-)-2-(diphenylphosphinomethyl)-4-(diphenylphosphino)-N-(t-butoxycarbonyl)-pyrrolidine according to a previously described procedure.[28] Commercially available 1,1-Bis((2S,5S)-2,5-diethylphospholano)ferrocene-(cyclooctadiene)rhodium(I)-tetrafluoroborate, complex B (Strem Chemicals), was used as received.

## Preparation of Functionalized Carbon Nanotubes

**CNTs** – Multi-walled carbon nanotubes prepared by Chemical Vapor Deposition (CVD)[30] using AlFeCoO₄ as catalyst were utilized as starting material. The CNTs were purified as follows: CNTs were mixed with a 50% vol. H₂SO₄-H₂O mixture (100 mL/gCNT) and then heated under reflux and stirred for 3 h. The solid was then separated by filtration, thoroughly washed with distilled water and dried at 120 °C for 2 days (m = 10 g).

**CNT-O** – The oxidation of CNTs was achieved by an acidic treatment with a nitric acid solution (65 wt.%). 100 mL of HNO₃ solution were used per gram of CNTs and the mixture was stirred at 140 °C for 3 h. The mixture was then left to cool down, filtered and washed with distilled water until observing a stable pH of the rinsing water. Finally, the CNT-O were dried at 120 °C for 3 days (m = 2.8 g).

**CNT-COO⁻Na⁺** – CNT-O (2 g) were mixed with 100 mL of a sodium hydroxide solution (0.05 M) for 24 h at RT to generate sodium carboxylate groups onto the CNTs. The solid was then separated by filtration and washed thoroughly with distilled water until pH 7 before drying at 120 °C for 2 days (m = 1.6 g).

**CNT-HPA** – In a first step, dried CNT-O (1 g) were dispersed in 260 mL of absolute ethanol *via* ultrasonication for 25–30 min. Then 3-aminopropyl triethoxysilane (3-APTES) was added (1.0 mmol) and the reaction mixture stirred at 70 °C for 4 h. The solid was then filtered and washed thoroughly with distilled water and acetone. The obtained silanized CNT-Si were dried under vacuum at 80 °C for 24 h. In the second step, 1 mmol of silicotungstic acid (HPA) in 5 mL of 2-propanol was slowly added to CNT-Si (1 g) previously dispersed in 2-propanol (10 mL). The reaction mixture was then stirred at RT for 15 h. The solid was collected by filtration and washed with 2-propanol (25 mL) and THF (10 mL). The resulted CNT-HPA were finally dried under vacuum at RT for 3 h (m = 0.8 g).

## General Procedure for the Immobilization of the Chiral Rh-Complexes onto the Surface of CNT-COO⁻Na⁺ and CNT-HPA

A suspension of CNTs (200 mg) in MeOH (4 mL) was maintained under ultrasonication for 30 min. A solution of complex A (yellow) or B (orange) (2 wt.%) in MeOH (4 mL) was then added *via* a cannula under inert atmosphere and the resulting suspension was stirred for 24 h at RT. The solid was filtered and washed with MeOH

until getting a colorless filtrate before drying under vacuum for 20 h.

## Characterization Techniques

**CNTs, CNT-O, CNT-COO⁻Na⁺ and CNT-HPA** were analyzed by several techniques. Thermogravimetry techniques allowed to study their reactivity in air and to determine the ash amount (related to impurities). Thermogravimetry Differential Thermal Analyses (TG/DTA) were carried out on a Shimadzu apparatus applying a method that consisted in heating the carbon materials in a synthetic air flow (100 cm³ min⁻¹) up to 1000 °C, at 10 °C min⁻¹. Raman spectroscopy analysis was carried out with a Horiba JobinYvon-Labram HR UV-Visible-NIR Raman microscope with the excitation wavelengths of 633 nm, in the 150–4000 cm⁻¹ spectral region. FTIR analysis was conducted for the CNT-O sample using a Brucker Tensor II spectrometer fitted with a Harrick Praying Mantis Diffusion Reflection Accessory. BET surface areas of CNT, CNT-O, CNT-COO⁻Na⁺, and CNT-HPA were measured using a Micromeritics ASAP2020 Surface Area and Porosity Analyzer. The samples were outgassed under vacuum for 24 h at 120 °C.

The immobilized complexes A and B (CNT-COO⁻@complex A, CNT-COO⁻@complex B, CNT-HPA@complex A, and CNT-HPA@complex B) were also analyzed by X-Ray Photoelectron Spectroscopy (XPS) using a Bin a Kratos AXIS Ultra DLD system equipped with a hemispherical analyzer with a reference intensity of over 50,000 counts per second (cps) and a spectral resolution of 0.6 eV, using Al K-alpha1 (1486.74 eV) radiation produced by a monochromatized X-Ray source with a spot size of 0.7 mm. The operating power was 192 W (12 kV×16 mA) and the spectra were recorded with a 160 eV pass energy for surveys and a 40 eV pass energy for high resolution measurements, with hybrid lens mode and slot aperture. The partial charge compensation was achieved using a neutralizer flood gun (filament current of 1.8 A, charge balance of 2.7 V, filament bias of 1.02 V). The pressure in the analysis chamber was 10⁻⁹ mbar. The above parameters were optimized in order to fix the C 1s band of the carbon contamination at 284.60 ± 0.01 eV.

Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) analyses were performed to study the morphology of the CNT-HPA support and of the immobilized A and B complexes, after deposition on a holey copper grid. TEM analyses were performed with a JEOL JEM 1011 CX T electron microscope operating at 100 kV with a spot resolution of 4.5 Å and a JEOL JEM 1400 electron microscope operating at 120 kV. HR-TEM analyzes were performed using a JEOL JEM 2100 F fitted with a field emission gun (FEG) operating at 200 kV with a spot resolution of 2.3 Å and a JEOL JEM-ARM200F cold FEG equipped with a STEM HAADF system (Scanning Transmission Electron Microscopy High Angle Annular Dark Field) and an EDX (Energy Dispersive X-ray Spectroscopy) system operating at 200 kV with a point resolution > 1.9 Å.

The loading of rhodium in the catalysts was determined by ICP-AES at Marion technologie.

**CNT-COO⁻@complex A:** Rh = 0.92%; **CNT-COO⁻@complex B:** Rh = 0.68%, **CNT-HPA@complex A:** Rh = 0.9%, **CNT-HPA@complex B:** Rh = 0.62%. As expected, and in agreement with previously reported results on the electrostatic immobilization of cationic species, the final Rh loading depends on the nature of the complex.

**Conversion and Enantiomeric excess (ee)** were determined on a Perkin Elmer Clarus 500 chromatograph equipped with chiral column, Beta DEX™ 225 (30 m×0.25 mm×0.25 µm) using decane as an internal standard. Retention times for (S) and (R)-methyl
succinate products were 7.8 and 8.2 min, respectively and 11.4 min for the dimethyl itaconate.

## General Procedure for the Asymmetric Hydrogenation with the Immobilized Rh Complexes

Hydrogenation reactions with the immobilized Rh complexes were performed in a stainless-steel autoclave, using 0.16 g (1 mmol) of dimethyl itaconate, 0.09 g of CNT-COO⁻@complex A, or 0.05 g of CNT-COO⁻@complex B that corresponds to 0.83 mg of Rh (0.00804 mmol) and 0.34 mg of Rh (0.0033 mmol), respectively and 4 mL of MeOH. The reactor was pressurized with H₂ to 5.5 bar and the reaction mixture was stirred (1100 rpm) at RT for an appropriate time. After the reaction, the hydrogen pressure was carefully released and the catalyst was recovered via filtration under a nitrogen atmosphere and then washed with fresh solvent.

Hydrogenation catalysis with CNT-HPA@complex A and CNT-HPA@complex B was performed in the same conditions as above using 0.16 g (1 mmol) of substrate, 0.06 g of CNT-HPA@complex A or CNT-HPA@complex B corresponding to 0.54 mg (0.0052 mmol) and 0.37 mg (0.0036 mmol) of Rh, respectively.

## General Procedure for the Asymmetric Hydrogenation with the Non-Immobilized Rh Complexes

The catalytic experiments with the non-immobilized Rh complexes were carried out in a stainless-steel autoclave using 0.16 g (1 mmol) of dimethyl itaconate, 0.01 g (0.012 mmol) of complex A or 0.01 g (0.01 mmol) of complex B in 4 mL of MeOH. The reactor was pressurized with H₂ to 5.5 bar and the reaction mixture was stirred (1100 rpm) at RT for an appropriate time. Then the hydrogen pressure was carefully released.

## Results and Discussion

### Characterization of the Carbon Supports

FTIR analysis of the CNT and the oxidized CNT-O, i.e. the starting materials for the preparation of the CNT-HPA and CNT-COO⁻Na⁺ supports, indicated bands characteristic of the CNT skeletal with C=C plane vibration at 1628 cm⁻¹. CNT-O presented additional bands at 1714 cm⁻¹ and 1120 cm⁻¹ for the C=O and C-O stretching, respectively, corresponding to the carboxylic acid functions and a band at 3443 cm⁻¹ associated to the hydroxyl groups. This confirmed the presence of the carboxylic acid groups onto the CNT surface (Figure 2). After deprotonation, the FT-IR spectrum of CNT-COO⁻Na⁺ does not present anymore the band at 1714 cm⁻¹, evidencing the generation of carboxylate functions (see Supporting Information, Figure S13).

Textural characterization was performed by BET and N₂ adsorption-desorption analysis. The results indicated no difference in the surface areas of the CNT and CNT-O (Table 1). However, the treatment with NaOH and the modification of the carbon nanotubes by 3-APTES and silicotungstic acid, respectively, changed the texture characteristics (Figure 3 and Table 1 entry 1 vs entries 3 and 4). The CNT-HPA functionalized support displayed a lower surface area compared to the pristine CNT that could be attributed to the close packing of the carbon

<figure><img src="image_5.png" /><figcaption>Figure 2. FTIR spectrum of CNT-O.</figcaption></figure>

<table><caption>Table 1. Surface area of the CNT, CNT-O, CNT-COO⁻Na⁺ and CNT-HPA carbon supports.</caption><thead><tr><td>Entry</td><td>Carbon material</td><td>S<sub>BET</sub> (m<sup>2</sup> g<sup>-1</sup>)</td></tr></thead><tbody><tr><td>1</td><td>CNT</td><td>180</td></tr><tr><td>2</td><td>CNT-O</td><td>207</td></tr><tr><td>3</td><td>CNT-COO<sup>-</sup>Na<sup>+</sup></td><td>225</td></tr><tr><td>4</td><td>CNT-HPA</td><td>118</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Figure 3. N₂ adsorption-desorption isotherms of CNT-O, CNT-COO⁻Na⁺ and CNT-HPA.</figcaption></figure>

nanotubes and the blockage of the channels entrance by the silicotungstic acid (Table 1, entry 4).

According to the IUPAC classification, the adsorption isotherms of the functionalized multi-walled CNTs correspond to the type IIb (Figure 3)³¹ indicating the presence of inner hollow cores and pores.

The thermal stability of the carbon nanotubes was determined by TG analysis in air. The results indicated a slight
variation of the thermal stability in the order CNT > CNT-COO⁻Na⁺ > CNT-O that correlates to the presence of the oxygenated functionalities (Figure 4). However, the stability of these three types of carbon nanotubes was pretty high (around 400°C). Differently, CNT-HPA was less stable (decomposition

temperature around 290 °C) which could be associated to the presence of tungsten and the decomposition of the silicotungstic acid.

The Raman spectra of all carbon supports are shown in Figure 5. The lines at ~1330 cm⁻¹ (D line) relate to the presence of defects, while that at ~1570 cm⁻¹ (G line) account for a graphitic arrangement. The intensity ratio of the D and G lines serves as an indicator of this graphitic character,[³²] a high I<sub>D</sub>/I<sub>G</sub> ratio indicating a larger population of defects. The I<sub>D</sub>/I<sub>G</sub> ratio increased from 1.23 for CNTs to 1.38 for CNT-COO⁻Na⁺ indicating more defects in the latter. This result is in accordance with the higher surface area observed for CNT-COO⁻Na⁺ compared to CNTs. The I<sub>D</sub>/I<sub>G</sub> ratio of CNT-HPA (1.28) showed fewer defects when compared to CNT-O and CNT-COO⁻Na⁺.

The morphology of the CNT-COO⁻Na⁺ and CNT-HPA was studied by transmission electron microscopy (TEM) as shown on images a and b, respectively in Figure 6. The micrographs indicate that the functionalization by HPA did not change the structure of the initial CNTs. The HPA groups are clearly visible at the CNT surface (see dark dots in Figure 6b). An analysis by high-resolution transmission electron microscopy (HR-TEM) (Figure 7) coupled with Energy-Dispersive X-ray spectroscopy (EDX) on the CNT-HPA sample confirmed the presence of Si and W atoms at the surface of the CNTs (See Supporting Information, Figures S1 and S2, respectively). These results thus confirmed the successful functionalization process *via* the grafting of the heteropoly acid groups (HPA) onto the CNT surface.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** Mass loss determined by TG in air as a function of temperature for CNT, CNT-O, CNT-COO⁻Na⁺ and CNT-HPA carbon-based supports.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Raman spectra of the CNTs, CNT-O, CNT-COO⁻Na⁺ and CNT-HPA.</figcaption></figure>

## Characterization of the Supported Chiral Rh-Complexes

The two A and B complexes were immobilized onto the CNT-COO⁻Na⁺ and CNT-HPA carbon supports following the procedure described in the Experimental Section (Scheme 1). In brief, it consisted in the addition of the complexes (A or B) to a MeOH suspension of the support (CNT-COO⁻Na⁺ or CNT-HPA) previously dispersed under ultrasounds, and stirring of the resulting suspension for 24 h at RT. The solids were then separated from the non-immobilized complexes by filtration and washed with MeOH until observing a colorless filtrate that ensured the immobilization of the Rh complex. The obtained solids were then dried under vacuum. The loading of rhodium was determined by ICP-AES analysis: 0.92 wt.% for CNT-

<figure><img src="image_9.png" /><figcaption>**Figure 6.** TEM micrographs of a) CNT-COO⁻Na⁺ and b) CNT-HPA supports (red arrows indicate the presence of W from HPA).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7.** HR-TEM micrographs of CNT-HPA support at two different magnifications.</figcaption></figure>

**COO⁻ @complex A**, 0.68 wt % for **CNT-COO⁻ @complex B**, 0.90 wt. % for **CNT-HPA@complex A** and 0.62 wt. % for **CNT-HPA@complex B**.

The TEM analysis of the **CNT-COO⁻ @complexes A** and **B** revealed the absence of nanoparticles or rhodium agglomeration (see Supporting Information, Figures S3 and S4).

EDX-coupled HR-TEM analysis of the **CNT-HPA@complexes** confirmed the presence of rhodium onto the **CNT-HPA** support. For the analysis of the **CNT-HPA@complex A**, a line mapping analysis validated the coexistence of rhodium (Rh) from **complex A** near tungsten (W) originating from the HPA on the support. For the **CNT-HPA@complex B**, the EDX analysis confirmed the presence of all elements from **complex B** and the **CNT-HPA** support (See Supporting Information, Figures S5 and S6, respectively).

XPS analysis was performed to identify the oxidation states of the major elements present in the **CNT-COO⁻ @complexes** (C, O, P, Rh) and **CNT-HPA@complexes** (C, O, P, Rh, W, Si) (see Supporting Information, Figures S7–S10). The two **CNT-COO⁻ @complexes** systems exhibited rather similar functionalities. In both cases, XPS spectra revealed C1s peaks of high intensity located at 284.6 eV (graphitic carbon, sp²-carbon), 285.3 eV (aliphatic C=C=O, sp³-carbon), 286.3 eV (carbon in ketoenolic form), and at 287 eV (keto group). The peaks at 291.2 eV (**complex A**) and 291.3 eV (**complex B**) eV are assigned to carbonates. These functionalities were also confirmed by the intensity of the O1s peaks located at 531.7 eV (C=O), 533.3 eV (C–O of ester), and 536.0 eV (oxygen of water).[33] Binding energies in the range of 130.8–132.8 eV that correspond to P2p level were observed which can be attributed to phosphorus attached to phenyl groups in organo-phosphorus compounds. Finally, spectra collected in the $Rh3d_{5/2}$ binding energy region were found very similar (308.5–315.0 eV) in both complexes and in good agreement with rhodium in the (I) oxidation state. The deconvolution of these bands revealed two components located at 309.8 and 311.0 eV that were assigned to the metal in two different surroundings, namely, in an agglomerated complex and a highly dispersed one.[34–37] No Rh (0) was detected for these materials. Additionally, the XPS analysis showed a Rh/P ratio higher than those corresponding to the theoretical value (0.5) (Supporting Information, Table S1). These results indicate a partial decoordination of the diphosphine

ligand. TEM, HR-TEM, and XPS characterization data did not indicate the presence of Rh(0) nanoparticles for **CNT-COO⁻ @complexes**. The higher Rh/P ratio may correspond to Rh(I) species coordinated to the carboxylate groups of the **CNT-COO⁻** support.

For **CNT-HPA@complexes**, the XPS survey analysis confirmed the presence of all expected elements into the **CNT-HPA** (Si, W, N, O, and C), as well as that of Rh, N, and P from complexes **A** and **B** (Supporting Information, Figures S9 and S10). The oxidation state (I) of rhodium was unequivocally established through the values of the measured binding energies of $Rh3d_{5/2}$ at 309.2 and 308.7 eV, respectively. Binding energies of the P2p signal are observed at similar values as for the **CNT-COO⁻ @complexes**. The binding energy of $W4f_{7/2}$ corresponded well to a typical W(VI) in a tungstate environment.[38] Furthermore, the Rh/P ratio for the two **CNT-HPA@complexes** is very close to the theoretical value (0.5) (Supporting Information, Table S2).

## Enantioselective Hydrogenation of Dimethyl Itaconate

The enantioselective hydrogenation of dimethyl itaconate (DMI) was carried out using all the immobilized rhodium catalysts (**CNT-COO⁻ @complex A**, **CNT-COO⁻ @complex B**, **CNT-HPA@-complex A** and **CNT-HPA@complex B**) as well as with the unsupported **Rh complexes A** and **B** for comparison purpose (Scheme 2). As presented in Table 2, the hydrogenation of dimethyl itaconate (in MeOH, RT, 5.5 bar of hydrogen) with **complexes A** and **B**, occurred with very high activities (complete conversion in 30 min) and good enantioselectivities (ca. 70% in (S) or (R)-methyl succinate), being in a good accordance with previous reports.[39]

The results of the catalytic tests (5.5 bar of $H_2$, RT, 4 mL of MeOH) carried out with the immobilized Rh complexes are reported in Table 3. Blank tests performed with **CNT-COO⁻ Na⁺** and **CNT-HPA** supports alone under the same catalytic conditions indicated that the rhodium-free supports displayed no activity in the hydrogenation of dimethyl itaconate. **CNT-COO⁻ @complex A** afforded a moderate activity (60% conversion after 24 h at RT) and a low enantioselectivity for the hydrogenation of DMI (8% ee towards (S) methyl succinate
<figure><img src="image_11.png" /><figcaption>**Scheme 2.** Enantioselective hydrogenation of dimethyl itaconate.</figcaption></figure>

<table><caption>**Table 2.** Catalytic performances of complexes A and B for the hydrogenation of dimethyl itaconate under homogeneous conditions.[a]</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>DMI/Rh</th><th>Reaction time (min)</th><th>Conversion (%)<sup>b)</sup></th><th>ee (%)<sup>b)</sup></th></tr></thead><tbody><tr><td>1</td><td rowspan="3">complex A</td><td rowspan="3">80</td><td>10</td><td>50</td><td>72 (S)</td></tr><tr><td>2</td><td>30</td><td>100</td><td>72 (S)</td></tr><tr><td>3</td><td>60</td><td>100</td><td>72 (S)</td></tr><tr><td>4</td><td rowspan="3">complex B</td><td rowspan="3">100</td><td>10</td><td>35</td><td>69 (R)</td></tr><tr><td>5</td><td>30</td><td>100</td><td>71 (R)</td></tr><tr><td>6</td><td>60</td><td>100</td><td>70 (R)</td></tr></tbody></table>

[a] Reaction conditions: 5.5 bar H₂, RT, 4 mL of MeOH. [b] Determined by chiral GC using a Beta DEX™ 225 column.

<table><caption>**Table 3.** Catalytic performances of the Rh-complexes immobilized onto CNT-COO⁻ or CNT-HPA for the hydrogenation of dimethyl itaconate.[a]</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>DMI/Rh</th><th>Run</th><th>Reaction time (h)</th><th>Conversion<sup>b)</sup><br>(%)</th><th>Ee<sup>b)</sup><br>(%)</th></tr></thead><tbody><tr><td>1</td><td>CNT-COO<sup>-</sup>@complex A</td><td>124</td><td>1</td><td>24</td><td>60</td><td>8 (S)</td></tr><tr><td>2</td><td rowspan="4">CNT-COO<sup>-</sup>@complex B</td><td rowspan="4">303</td><td>1</td><td>7</td><td>50</td><td>64 (R)</td></tr><tr><td>3</td><td>1</td><td>15</td><td>97</td><td>65 (R)</td></tr><tr><td>4</td><td>1</td><td>24</td><td>100</td><td>64 (R)</td></tr><tr><td>5</td><td>2</td><td>24</td><td>100</td><td>0</td></tr><tr><td>6</td><td rowspan="3">CNT-HPA@complex A</td><td rowspan="3">192</td><td>1</td><td>7</td><td>100</td><td>57 (S)</td></tr><tr><td>7</td><td>2</td><td>7</td><td>100</td><td>54 (S)</td></tr><tr><td>8</td><td>3</td><td>7</td><td>95</td><td>47 (S)</td></tr><tr><td>9</td><td>4</td><td>7</td><td>7</td><td>88</td><td>43 (S)</td></tr><tr><td>10</td><td rowspan="4">CNT-HPA@complex B</td><td rowspan="4">277</td><td>1</td><td>15</td><td>100</td><td>63 (S)</td></tr><tr><td>11</td><td>1</td><td>7</td><td>83</td><td>54 (R)</td></tr><tr><td>12</td><td>1</td><td>15</td><td>100</td><td>58 (R)</td></tr><tr><td>13</td><td>2</td><td>15</td><td>25</td><td>27 (R)</td></tr></tbody></table>

[a] Reaction conditions: 5.5 bar of H₂, RT, 4 mL of MeOH. [b] Determined by chiral GC using a Beta DEX™ 225 column.

(Table 3, entry 1)). In contrast, the CNT-COO⁻@complex B provided higher conversions (50 to 100%) and a better enantioselectivity towards the (R) methyl succinate (64–65%) (Table 3, entries 2–4). These differences in the ee compared to the values collected with the unsupported complexes indicated that the support affected the catalytic reaction. Actually, if the CNT-COO⁻ support strongly affected the enantioselectivity of complex A it affected only slightly that of complex B, without inversion of the chiral induction in any case. One hypothesis may be a modification of the geometry of the ligand that would limit the enantioselective hydrogenation of DMI and/or the partial decoordination of the chiral ligand, in agreement with the XPS results (vide supra).

The recyclability of the CNT-COO⁻@complex B was evaluated as well. The catalyst was separated from the reaction

mixture by filtration and then washed with MeOH to remove any trace of the catalytic mixture. The ICP analysis of the obtained liquid phase indicated no leaching of Rh, thus highlighting an efficient recovery of the catalyst. This result was confirmed by the recycling tests where the conversion of the substrate was reproduced (Table 3, entry 5 vs 4). However, if the conversion remained total the enantioselectivity was depleted to zero (Table 3, entry 5). A TEM analysis of the recycled catalyst (Figure 8) evidenced the presence of metal dots (with an average size of ca. 1.7 nm) that can be attributed to Rh nanoparticles. This indicates that a decomposition of the rhodium complex B occurred during the recovery protocol. However, if one can account that these Rh nanoparticles provided a high catalytic activity towards methyl succinate it was without enantioselectivity.
<figure><img src="image_12.png" /><figcaption>**Figure 8.** TEM micrographs of the recycled CNT-COO⁻ @complex B catalyst.</figcaption></figure>

<figure><img src="image_13.png" /></figure>

HR-TEM analysis of the spent catalyst also showed the decomposition of **complex B** into metal nanoparticles during the catalytic process as visible through the prominent bright dots on Figure 9. The formation of rhodium nanoparticles was unequivocally confirmed by the EDX results (See Supporting Information, Figure S11). The lack of ligand was evidenced as no indication of the presence of P element close to the nanoparticle surface was detected by EDX analysis.

The catalytic tests performed in the same conditions with the immobilized rhodium complexes onto CNT-HPA support, CNT-HPA@complexes, also showed high conversions in the hydrogenation of DMI (83–100%) (Table 3, entries 6, 11). In terms of enantioselectivity, the CNT-HPA@complex A generated an ee of 63% in the succinate stereoisomer S which was higher than that generated by the CNT-HPA@complex B (58%) to *R* methyl succinate (Table 3, entries 10 vs 12). In both cases, the ee obtained for the immobilized complexes were lower than those achieved with the homogeneous complexes. It is worth noting that the same enantiomers were obtained with both homogeneous and heterogeneous complexes. This result indicates that the support has no effect on the control of the enantiomeric induction.

Recycling tests were also performed with the two CNT-HPA supported catalysts following the same recovery protocol as described above. For the CNT-HPA@complex A, a 100% conversion was maintained till the second catalytic run. A slight decrease in the activity and enantioselectivity was observed for runs 3 and 4 (Table 3, entries 6–9). For the CNT-HPA@complex

B, both the conversion and enantioselectivity decreased after 15 h (Table 3, entry 13). The characterization by ICP-AES of the spent CNT-HPA@complex A catalyst indicated no Rh leaching in the filtrate. The TEM images of the spent CNT-HPA@complex A catalyst after the catalytic run 2 did not reveal the formation of Rh nanoparticles, thus indicating that complex A was stable against decomposition (Figure 10). This observation may strongly suggest that the Rh remains mainly dispersed on the CNT-HPA support, which may better stabilize it than CNT-COO⁻ material. XPS analysis of the fresh and spent CNT-HPA@complex A showed no change in the oxidation state of rhodium (see Supporting Information, Figure S12). Consequently, it can be concluded that complex A remained stable on the CNT-HPA support in the catalysis conditions applied. Nevertheless, some decomposition leading the rhodium agglomeration cannot be completely ruled out, especially after run 3 (Table 3, entries 8–9), that could be responsible for the slight decrease of both the catalytic activity and the enantioselectivity.

These results showed that, compared to their CNT-COO⁻ counterparts, the Rh complexes immobilized on CNT-HPA are still enantioselective in the hydrogenation of DMI after recycling. This indicates that with CNT-HPA support the chiral ligand of the complex is less affected by the proximity of the carbon nanotube as it still enables the hydrogenation of DMI in an enantioselective way, contrarily to the CNT-COO⁻ support. This difference highlights the interest of adding a spacer such HPA between the Rh complex and the CNT support to limit the

<figure><img src="image_14.png" /><figcaption>**Figure 9.** HR-TEM images of the spent CNT-COO⁻ @complex B catalyst at two different magnifications.</figcaption></figure>

<figure><img src="image_15.png" /></figure>
<figure><img src="image_16.png" /><figcaption>**Figure 10.** TEM images of the spent CNT-HPA@complex A catalyst.</figcaption></figure>

<figure><img src="image_17.png" /></figure>

effect of the CNT surface environment on the catalytic behavior of the immobilized complexes.

## Conclusions

We have studied the non-covalent immobilization of two chiral cationic rhodium complexes, Rh-(S,S)-BPPM (complex A) and Rh-(S,S)-Et-Ferrocelane (complex B) by using ionic anchoring strategies at the surface of functionalized carbon nanotubes (CNTCOO⁻Na⁺ and CNT-HPA) and their application in the enantioselective hydrogenation of dimethyl itaconate as a model substrate.

Electrostatic interaction between the cationic Rh complexes and the negatively charged surface of carboxylate-functionalized CNTs (CNT-COO⁻Na⁺) occurs via partial decoordination of the chiral diphosphine ligand and probably partial coordination of Rh (I) to the carboxylate groups on the CNTs surface. As a result, CNT-COO⁻@complex A showed low activity and low enantioselectivity in the asymmetric hydrogenation of DMI substrate. CNT-COO⁻@complex B was found to be more efficient, displaying good activity and enantioselectivity, reaching an enantiomeric excess of 65%. However, decomposition of the catalyst into Rh nanoparticles was observed in the recovery conditions applied.

The ionic anchoring strategy that is based on the electrostatic interaction between the cationic Rh complexes and a heteropoly acid spacer (HPA) previously grafted at the CNT surface allowed successful immobilization of complexes A and B. The catalytic tests with the CNT-HPA@complex A and CNT-HPA@complex B showed high conversions in the hydrogenation of DMI (83–100%). In terms of enantioselectivity, the CNT-HPA@complex A generated a high ee of 63% that was much higher than that generated by the CNT-COO⁻@complex A (8%). CNT-HPA@complex B achieved a 58% ee to (R) methyl succinate. Recyclability tests with the CNT-HPA@complexes showed they were less active but still enantioselective in the hydrogenation of DMI after recycling, showing a different behavior from their CNT-COO⁻ counterparts. This highlights the role of a spacer such as HPA between the Rh complex and the CNT support to limit the effect of the CNT surface environment on the catalytic performance of the immobilized Rh complexes.

In summary, we have developed an efficient method for the immobilization of metal complexes onto carbon nanotubes by

ionic anchoring *via* a spacer. In addition, the spacer allowed minimizing the influence of the CNT support on the catalytic properties of the metal centers, which were performant in terms of both activity and enantioselectivity. This work opens up prospects for immobilizing other catalysts on carbon nanostructures for enantioselective hydrogenation and maybe also for other reactions.

## Acknowledgements

This work has received funding from the European Union's Horizon2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 860322 for the ITN-EJD "Coordination Chemistry Inspires Molecular Catalysis" (CCIMC). We are grateful to the CNRS (Centre National de la Recherche Scientifique), the INP-ENSIACET (Institut National Polytechnique de Toulouse) and the University of Bucharest for providing access to facilities.

## Acknowledgements

The authors warmly thank the technical assistance provided by Vincent Collière (LCC-CNRS, Toulouse) for TEM analyses, Popescu Dana (University of Bucharest) for XPS analyses and Jérôme Esvan (CIRIMAT, Toulouse) for fruitful scientific discussions. The Raimond Castaing Micro-characterization Center (UAR 3623, Toulouse) is also thanked for the access to electron microscopes.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.
**Keywords:** Supported catalysts · Rhodium complexes · Carbon nanotubes · Enantioselective hydrogenation

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Version of record online: ■■, ■■
RESEARCH ARTICLE

Electrostatic interactions were exploited to immobilize chiral rhodium catalysts onto the surface of carbon nanotubes, evidencing the beneficial influence of the use a heteropoly acid as spacer on both the catalyst stability and its catalytic performances in the enantioselective hydrogenation of dimethyl itaconate.

<figure><img src="image_18.png" /></figure>

Z. Arora, M. Rais, V. I. Pârvulescu, K. Philippot, J. Durand*, M. Gouygou*

1 – 11

Non-Covalent Immobilization of
Chiral Rhodium Catalysts on Carbon
Nanotubes for Asymmetric Hydroge-
nation

<figure><img src="image_19.png" /></figure>