Chiral rhodium complexes covalently anchored on carbon nanotubes for enantioselective hydrogenation

Article abstract of DOI:10.1039/c3dt53301h

Chiral rhodium hybrid nanocatalysts have been prepared by covalent anchorage of pyrrolidine-based diphosphine ligands onto functionalized CNTs. This work constitutes the first attempt at covalent anchoring of homogeneous chiral catalysts on CNTs. The catalysts, prepared with two different chiral phosphines, were characterized by ICP, XPS, N₂ adsorption and TEM, and have been tested in the asymmetric hydrogenation of two different substrates: methyl 2-acetamidoacrylate and α-acetamidocinnamic acid. The hybrid nanocatalysts have shown to be active and enantioselective in the hydrogenation of α-acetamidocinnamic acid. A good recyclability of the catalysts with low leaching and without loss of activity and enantioselectivity was observed. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53301h

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Chiral rhodium complexes covalently anchored on
carbon nanotubes for enantioselective
hydrogenation†
Cite this: Dalton Trans., 2014, 43,
7455
C. C. Gheorghiu,a B. F. Machado,b,c C. Salinas-Martínez de Lecea,a M. Gouygou,*b,c
M. C. Román-Martínez*a and P. Serpb,c
Chiral rhodium hybrid nanocatalysts have been prepared by covalent anchorage of pyrrolidine-based
diphosphine ligands onto functionalized CNTs. This work constitutes the first attempt at covalent anchor-
ing of homogeneous chiral catalysts on CNTs. The catalysts, prepared with two different chiral phos-
phines, were characterized by ICP, XPS, N2 adsorption and TEM, and have been tested in the asymmetric
hydrogenation of two different substrates: methyl 2-acetamidoacrylate and α-acetamidocinnamic
acid. The hybrid nanocatalysts have shown to be active and enantioselective in the hydrogenation of
α-acetamidocinnamic acid. A good recyclability of the catalysts with low leaching and without loss of
activity and enantioselectivity was observed.
Received 22nd November 2013,
Accepted 27th January 2014
DOI: 10.1039/c3dt53301h
www.rsc.org/dalton
most successful diphosphine ligands have been anchored onto
inorganic platforms, leading to comparable performance in
Introduction
Carbon materials have proved to be suitable supports for terms of enantioselectivity and efficiency than their homo-
heterogeneous catalysis, due to their high versatility in terms geneous counterparts.12–17 If some attempts to immobilize
of textural/surface properties as well as morphology, together hydrogenation catalysts on conventional carbons have been
with chemical inertness under many conditions, thermal stabi- reported,18,19 the use of CNTs as supports for the immobiliz-
lity and mechanical resistance.1,2 In particular, nanostructured ation of a homogeneous catalyst has surprisingly not been
carbon materials are preferred because of a potential confine- extensively developed, particularly if we consider the rich
ment effect.3,4 Among them, carbon nanotubes (CNTs) showed chemistry dealing with CNT surface functionalization.20–31
interesting structures and properties, leading therefore to prom- Among the different possible immobilization strategies9 the
ising applications as catalyst supports.5,6 They are usually pre- most common is the covalent one, and to the best of our knowl-
ferred to other classical carbon materials, like activated carbon, edge only a few studies have dealt with the covalent anchoring of
because they are mesoporous. Thus, the accessibility to the sup- a homogeneous catalyst on CNTs22–31 and none of chiral
ported active phase is better and the reaction is not negatively rhodium-based catalysts. The most common method for anchor-
affected by mass transfer limitations. In some cases, the ing a ligand on carbon materials consists in the formation of an
superior properties of supported CNT catalysts have been attrib- amide bond between the oxidized carbon material and amino-
uted to the existence of a strong metal–support interaction.7,8
tagged ligands. In this context, pyrrolidine-based diphosphine
When enantioselective catalysts are immobilized on solid ligands (PPM family, PPM is 2S,4S-4-diphenylphosphino-
supports, the resulting catalysts combine the advantages of 2-diphenylphosphinomethylpyrrolidine), which are efficient
both, homogeneous (selectivity, tunability and homogeneous ligands in rhodium(I) catalyzed asymmetric hydrogenation of
sites) and heterogeneous (recovery and separation) olefins,32–34 are good candidates for a covalent anchoring.35–38
catalysts.9–11 For enantioselective hydrogenation, many of the
The present work reports on the preparation of chiral
rhodium hybrid nanocatalysts by covalent anchorage of PPM-
based ligands onto functionalized CNTs (Scheme 1), and their
characterization and application in asymmetric catalysis.
aDepartment of Inorganic Chemistry and Materials Institute, University of Alicante,
ctra San Vicente del Raspeig s/n., 03690 Alicante, Spain. E-mail: mcroman@ua.es
bCNRS, LCC (Laboratoire de Chimie de Coordination, Composante ENSIACET),
4 allée Emile Monso, BP 44362, F-31030 Toulouse Cedex 4, France.
Experimental
E-mail: maryse.gouygou@lcc-toulouse.fr
cUniversité de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3dt53301h
All commercially available reagents were used as received. The
anhydrous solvents were obtained from a Solvent Purification
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Scheme 1 Covalent grafting of chiral Rh complex on CNT surface.
System (Innovative Technologies) or distillated with the appro- overnight at room temperature and stored under an inert
priate drying agents and they were degassed prior to use. The atmosphere to avoid hydrolysis of the acyl chloride functional-
inert atmosphere reactions were run under nitrogen or argon ities. After this treatment, the support was named CNTCl.
using standard Schlenk techniques.
Preparation of chiral diphosphines
Preparation of functionalized carbon nanotubes, CNTCl
N-(tert-butoxycarbonyl)-(2S,4S)-4-diphenylphosphino-2-
The carbon nanotubes used in this work are multi-walled [(diphenylphosphino)-methyl]-pyrrolidine, (S,S)-1a. A solution
carbon nanotubes, prepared by the catalytic-CVD method.39 of N-(tert-butoxycarbonyl)-4-hydroxy-L-prolinol di-p-toluenesul-
This sample was named CNT. It has been subjected to a purifi- fonate34 (0.85 g, 1.6 mmol) in 10 mL of dry THF was added
cation treatment to remove the Fe/Al2O3 catalyst used in the under an inert atmosphere to 8.53 mL (4.26 mmol) of a solu-
synthesis process. Such a treatment is as follows: the sample tion of potassium diphenylphosphide, Ph2PK, 0.5 M in THF.
was put in contact with a 50% vol. H2SO4–H2O mixture The mixture was stirred at room temperature for 35 hours
(100 mL gCNT−1) for 3.5 h under reflux and stirring. Then, the under an inert atmosphere. Then, the solution was treated
solid was separated by filtration, thoroughly washed with dis- with methanol to remove the anion excess, filtered and con-
tilled water and dried at 120 °C for two days. The purified centrated under vacuum. The yellow viscous residue was crys-
sample was named CNTP.
tallized from 9 mL of anhydrous ethanol and the solid was
The CNTP were oxidized by treatment with a nitric acid washed with a CH2Cl2–pentane mixture (50 : 50) and then puri-
solution (65 wt%), 100 mL solution per gram of carbon, at fied by alumina column chromatography. 31P NMR (CDCl3):
140 °C, under stirring, for 4 hours. Afterwards, the mixture δ −22.57 (s, 1P), −14.94 (s, 1P).40 1H NMR (CDCl3): δ 1.43
was left to cool down and filtered, followed by washing with (s, 9H, CH3), 1.69–2.31 (m, 3H), 2.76–3.26 (m, 3H), 3.68–4.00
cold distilled water, until a stable pH of the rinsing water was (m, 2H), 7.35–7.58 (m, 20H, Ph); see ESI.1.† 13C NMR (CDCl3):
reached. The sample was then dried at 120 °C for two days. δ 28.59 (s, (CH3)3C), 35.82 (m, CH–P), 38.02 (m, CH2–P), 50.32
The oxidized carbon nanotubes were named CNTO.
(m, CH2), 56.20 (m, CH), 79.90 (s, (CH3)3C), 128.43–128.99
The surface carboxylic groups produced by the oxidation (m, CPh), 132.67–133.37 (m, CPh), 136.94 (m, Cipso), 137.63
treatment were further transformed into acyl chloride groups (m, Cipso), 153.99 (s, CvO); see ESI.2.†
by reaction with SOCl2 according to the following procedure:
N-(tert-butoxycarbonyl)-(2S,4S)-4-[2′,5′-diphenylphospholyl]-
under an inert atmosphere (N2), sample CNTO was mixed with 2-[(2′,5′-diphenyl-phospholyl)methyl]-pyrrolidine, (S,S)-1b. The
SOCl2 (approximately 40 mL per gram of the sample), and the synthesis of diphosphine 1b was accomplished by the same
mixture was kept at reflux temperature (80 °C) and constant procedure as described above for 1a using 2,5-diphenylphos-
stirring for 24 hours. Then, it was cooled down and the liquid pholyl lithium41 instead of PPh2PK. 31P NMR (CDCl3): δ −13.30
was removed under vacuum. The solid was vacuum dried (s, 1P), −7.69 (s, 1P). 1H NMR (CDCl3): δ 1.50 (s, 9H, CH3),
7456 | Dalton Trans., 2014, 43, 7455–7463
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1.60–2.25 (m, 3H), 3.40–3.60 (m, 3H), 4.16 (bs, 1H), 4.39 (bs, were determined as described in the literature.40 Briefly, the total
1H), 7.18–7.65 (m, 20H, Ph); see ESI.3.† 13C NMR (CDCl3): δ micropore volume (Vµt) was determined by applying the Dubinin–
28.44 (s, (CH3)3C), 37.88 (s, CH–P), 55.86 (s, CH2–P), 58.92 (s, Radushkevich (DR) equation to the N2 adsorption data. A similar
CH2), 67.43 (s, CH), 80.66 (s, CH3)3C), 126–136–44 (m, CAr), calculation on the CO2 adsorption data gives the volume of the
151.51 (s, CvO); see ESI.4.†
narrower micropores (Vnµ). The volume of supermicropores (Vsµ)
was determined by the subtraction Vµt − Vnµ. Finally, mesopore
volumes (Vmeso) were calculated as difference between the amount
Preparation of chiral diphosphine supported on CNTs, CNT1a-b
0.45 g of sample CNTCl was added to a solution containing the of nitrogen adsorbed at 0.97 and 0.2 P/P0 expressed as a liquid.40,41
1a diphosphine ligand (0.768 g, 1.38 mmol), MeOH (0.5 mL)
The study of the surface chemistry was carried out by Tempera-
and anhydrous NaI (0.19 g, 1.3 mmol) in CH3CN (6 mL). The ture Programmed Desorption (TPD), using a thermobalance SDT
mixture was stirred for 2 days at room temperature. Then, N,N- TA Instruments 2960 coupled to a mass spectrometer Blazers
diisopropylethylamine (DIPEA) (0.44 mL, 2.5 mmol) was added MSC 200 Thermostar. Approximately 10 mg of the sample were
at 0 °C, and the mixture was stirred for 24 hours at room temp- heated, at 20 °C min−1, up to 1100 °C in 20 cm3 min−1 He flow.
erature. Finally, 9 mL of degassed HCl solution (10 wt% aq.)
Samples CNTO and CNTCl were analysed by FT-IR spec-
were added to the solution to neutralize the excess of base. troscopy using a Nicolet 380 device.
The filtration procedure for the isolation of CNT1a-b was per-
Supports and catalysts were analysed by XPS using the
formed under an inert atmosphere, using a filter funnel with equipment VG-Microtech Mutilab 3000 with MgKα (1253.6 eV)
drip-tip and one on side-arm, frit porosity 25–50 μm, and with radiation. Pressure for measurements was 5 × 10−10 mbar. C1s
a 47 mm diameter 0.2 μm pore size nuclepore hydrophilic transition was adjusted to 284.6 eV.
PC (polycarbonate) filter membrane. Afterwards, the solid was
NMR spectra were recorded at 25 °C on a Bruker Avance 300
washed with a saturated aqueous solution of NaHCO3 (10 mL), or on a DPX300 spectrometer.
ethanol (2 × 10 mL) and diethyl ether (4 × 10 mL). The solid
was dried and stored under argon.
Enantiomeric excess (ee) was determined by chiral GC using
the equipment Agilent Technologies 7820 A with a flame ioniza-
The preparation of CNT1b was accomplished following the tion detector (FID) and the capillary column CP-1Chirasil-L-Val
same procedure as described above for CNT1a, using 0.072 g (25 m × 250 μm × 0.12 μm) with decane as an internal standard.
of CNTCl and 0.072 g (0.110 mmol) of 1b.
General procedure for the asymmetric hydrogenation
Preparation of chiral Rh-complex supported on CNTs, CNT1a-
b-Rh. 200 mg of CNT1a were impregnated with a solution of
anhydrous dichloromethane (7 mL) containing 15 mg of [Rh-
(COD)2]BF4 (0.037 mmol). These amounts correspond to 2 wt%
rhodium on the solid catalyst. The mixture was stirred at room
temperature for 24 h under an inert atmosphere, and then, the
catalyst was filtered (under an inert atmosphere, via cannula) and
washed first with a mixture of CH2Cl2–pentane (50 : 50) (3 × 8 mL)
and then with CH2Cl2 (5 × 5 mL). Subsequently, the catalyst was
dried under an inert atmosphere at room temperature for 24 h.
The synthesis of CNT1b-Rh was performed in the same way
using 50 mg of CNT1b and 4 mg of [Rh(COD)2]BF4 complex
(0.01 mmol).
To a stainless steel Parr reactor was added a mixture of 80 mg
(0.560 mmol) of substrate, 30 mg of CNT1a-b-Rh in 7 mL anhy-
drous methanol. We used 30 mg of the hybrid catalyst that cor-
respond to 0.32 mg Rh or 0.18 mg Rh when using the catalyst
CNT1a-Rh or CNT1b-Rh, respectively. The reactor was pressur-
ized with H2 to 5.5 bar at room temperature and the reaction
mixture was stirred (1100 rpm) for an appropriate time. After
reaction, the hydrogen pressure was carefully released and the
catalyst was recovered via filtration under a nitrogen atmo-
sphere and then washed with fresh solvent. Afterwards it was
used in a new catalytic run under the same conditions. The
conversion and ee values of products were determined by GC
chromatography after the acids had been transformed into the
corresponding esters for compound 3b.42
Characterization techniques
TEM analysis was used to get information about the mor-
phology of the investigated samples. It was performed with a
JEOL JEM-2010 equipment.
The homogeneous catalytic experiments were performed with
[Rh(COD)PPM]BF4 (3.5 mg, 4.7 μmol) prepared in situ by adding
2.3 mg (5 μmol) of (S,S)-PPM ligand to a methanol solution con-
taining 1.9 mg (4.7 μmol) of [Rh(COD)2]BF4. After 1 hour of stir-
ring at room temperature, 175 mg (1.2 mmol) of 2a substrate
were added and the resulting solution was directly transferred
into a stainless steel autoclave and the reaction was started under
the same experimental conditions as those described above.
Samples CNT, CNTP and CNTO were analyzed by thermogra-
vimetry (TG) with the purpose of studying their reactivity in air
and to determine the ash amount (related to impurities). For
the experiments, carried out in a thermobalance SDT TA
Instruments 2960, the samples were heated, at 10 °C min−1
up to 1000 °C in synthetic air flow (100 cm3 min−1).
,
The textural properties of the original and oxidized carbon
nanotubes were analyzed by gas adsorption: N2 at −196 °C and
CO2 at 0 °C, using the automatic volumetric apparatus Auto-
sorb-6B. The samples were previously degassed at 250 °C for
Results and discussion
Synthesis of chiral Rh-complexes supported on CNTs
4 h. The textural properties of the samples (BET surface area, For anchoring diphosphine ligands, the CNTs were firstly
pore volumes of different size range, pore size distributions) modified to generate surface acyl groups.43
two-step
A
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Fig. 1 TEM micrographs of sample CNTO.
procedure gave rise to the functionalized carbon nanotubes
CNTCl (Scheme 1). In order to perform the coupling reaction
under mild conditions, because of the sensitivity of diphos-
phine compounds, we have chosen the procedure reported by
Nazih et al.,44 which allows the direct conversion of an N-pro-
tected amine into an amide species. This procedure involved
the synthesis of diphosphine ligands tailored for immobiliz-
ation. The amino-tagged chiral diphosphines 1a–b were syn-
Fig. 2 Thermogravimetric profiles obtained for samples CNT, CNTP and
CNTO.
micrographs of pristine CNT and CNTP are shown in ESI.5.†
No significant difference in terms of morphology was noticed
for the three samples, except for some CNT tip opening in
sample CNTO (see below).
thesized
according
to
slightly
modified
reported
procedures.32,34 Then, the reaction of CNTCl with ligands 1a–b
was conducted at room temperature in CH3CN in the presence
of MeOH and NaI (Scheme 1). This one-pot procedure implies
the in situ (i) cleavage of the Boc group (tert-butoxycarbonyl
group) with HI generated from the reaction of MeOH with the
acyl group in the presence of NaI and (ii) amide bond formation
in the presence of a base. After treatment, filtration and purifi-
cation, the resulting CNT tethered diphosphines, CNT1a-b,
were analysed by XPS (vide supra). In the last step, the immobi-
lized diphosphines, CNT1a-b, were reacted with the rhodium (I)
complex [Rh(COD)2]BF4 in CH2Cl2 at room temperature to
produce the hybrid catalysts CNT1a-b-Rh (Scheme 1), which
were characterized by ICP and XPS (vide supra).
Samples CNT, CNTP and CNTO, were analysed by thermo-
gravimetry (TG). Fig. 2 shows the obtained thermograms.
The reactivity in air of samples CNT and CNTP is similar;
the differences observed for the oxidized sample CNTO are due
to the decomposition of surface oxygen groups.45 Regarding
the ash content, these data show that the original CNT sample
contains 6.35 wt%, that is reduced to 2.82 wt% after the purifi-
cation treatment and to 0.92 wt% after the HNO3 treatment
(sample CNTO). Fig. 3a shows the N2 adsorption isotherms
obtained for samples CNT and CNTO.
These isotherms are of type IIb according to the subdivision
of the IUPAC classification presented by Rouquerol et al.,46
and are indicative of capillary condensation. It is considered
that the porosity of multi-walled CNTs consists mainly of the
inner hollow cores and pores formed by bundles of
nanotubes.47–49 The presence of hysteresis can be related to
the presence of pores with both ends open.50,51 Sample CNTO
shows an increase of the adsorption capacity compared to the
original sample that reveals CNT tip opening upon such a
Characterization of carbon nanotubes
Fig. 1 shows TEM micrographs of CNTO. These images show
that CNTO present few compartments (bamboo like structure)
and that they are characterized by outer and inner diameters
of 20–25 nm and 7–10 nm, respectively. For comparison TEM
Fig. 3 (a) N2 adsorption isotherms at −196 °C of samples CNT and CNTO and (b) TEM micrograph of an opened CNT.
7458 | Dalton Trans., 2014, 43, 7455–7463
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Table 1 Textural properties of the original and oxidized CNTs
Table 2 Quantification of TPD profiles: CO and CO2 evolved, oxygen
weight percentage, and amount of carboxylic acid type groups (calcu-
lated by deconvolution of the CO2 desorption profile)
SBET
Vμt
Vnμ
Vsμ
Vmeso
Sample
[m2 g−1
]
[cm3 g−1
]
[cm3 g−1
]
[cm3 g−1
]
[cm3 g−1
]
CO
CO2
O
[%]
Carboxylic acid
[μmol g−1
]
[μmol g−1
]
[μmol g−1
]
CNT
CNTO
222
323
0.09
0.13
0.041
0.070
0.047
0.061
0.54
0.73
Sample
CNT
847
697
1742
533
100
150
1076
870
1.7
1.6
6.2
3.6
0
0
425
126
SBET, BET surface area; Vμt, total micropore volume; Vnμ, narrow
micropore volume; Vsμ supermicropore volume; Vmeso mesopore
volume (calculated between 0.2 and 0.97 P/P0).
CNTP
CNTO
CNTCl
,
,
treatment (Fig. 3b).52 Table 1 includes the surface area and
porosity parameters determined from the adsorption data.
The data show that the purification and oxidation treat-
ments produce an increase of the BET surface area and of the
pore volumes in the whole range of porosity. Fig. 4 shows the
CO and CO2 evolution profiles obtained during TPD experi-
ments for samples CNT, CNTP, CNTO and CNTCl.
It can be observed that the purification treatment produces
no significant changes in the surface chemistry of the supports
(only a slight decrease of groups which decompose as CO).
However, the oxidation treatment with nitric acid produces an
important amount of surface oxygen groups and further treat-
ment with SOCl2 leads to a significant reduction in the evol-
ution of both CO and CO2.
The quantification of the TPD profiles as the amount of CO
and CO2 evolved (in μmol g−1) and the calculated oxygen
weight percentage are shown in Table 2. The amount of car-
boxylic acid groups, necessary to create the –COCl functional-
ities for the covalent bond with the diphosphine ligands, has
Fig. 5 FT-IR spectra of CNTO and CNTCl samples.
been determined by deconvolution of the CO2 evolution groups, visible in both samples. The spectrum of sample
CNTCl additionally exhibits a shoulder at 1760 cm−1 corre-
profile, considering that this kind of groups decompose
between 130 °C and 350 °C.53,54 After the acylation treatment, sponding to the acetyl chloride. A decrease in the intensity of
the amount of carboxylic acid groups (first peak) is consider- the peak at 3450 cm−1 corresponding to O–H stretch from car-
boxylic acid groups in sample CNTCl is noticeable. This obser-
ably reduced (Table 2), indicating that the transformation of
the carboxylic acid groups occur.
vation, together with the variation of the band at 1200 cm−1, is
The FT-IR spectra for samples CNTO and CNTCl (Fig. 5) indicative of the transformation of carboxylic acid groups. The
appearance of a characteristic peak at 592 cm−1, corresponding
show the peak at 1567 cm−1, assigned to CvC stretching,
which originates from the inherent structure of CNTs and also to C–Cl stretch in acetyl chloride, confirms the formation of
the peaks at 1720 and 1200 cm−1 arising from CvO and C–O acyl chloride groups. Signal assignments were performed
according to literature data.55–57
stretching, respectively, indicating the existence of carboxylic
Fig. 4 TPD desorption profiles for the original, purified and functionalized CNTs: (a) CO and (b) CO2 evolutions.
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These data confirmed the effective transformation of car-
boxylic into acyl chloride groups. However, in agreement with
the TPD data, not all the COOH groups were converted, as
signals due to carboxyl groups are still present in the spectrum
of sample CNTCl. This might be due to partial hydrolysis of the
surface acetyl chloride functionalities during air exposure.
Scheme 2 Enantioselective hydrogenation of methyl 2-acetamidoacry-
late and α-acetamidocinnamic acid.
Characterization of the tethered chiral diphosphine ligands
Samples CNT1a and CNT1b have been analysed by XPS (see
spectra in ESI.6 and ESI.7†) and the obtained results are sum-
marized in Table 3. For comparison purposes, data for the
(2S,4S)-4-diphenylphosphino-2-[(diphenylphosphino)methyl]-
pyrrolidine ligand, PPM, have been also included. It can be
observed that the binding energy of P 2p in the carbon teth-
ered diphosphines CNT1a and CNT1b is higher than in the
free PPM ligand. This can be attributed to a modification of
the P electronic state due to the interaction with the support
surface.58,59 The binding energy found for N 1s (around
400 eV) is characteristic of nitrogen in amine form.60,61 The
P/N ratio is close to the theoretical one (P/N = 2).
Table 4 Enantioselective hydrogenation of methyl 2-acetamidoacry-
late, 2aa
Time Conversionb
Entry Catalysts
S/C
Run (h)
(%)
eeb
/
1
2
3
4
5
6
7
8
[Rh(COD)2]BF4 140
1
1
1
2
3
3c
1
2
42
2
20
3
3
3
90
20
100
100
100
70
(S,S)-PPM-Rh
CNT1a-Rh
CNT1a-Rh
CNT1a-Rh
CNT1a-Rh
CNT1b-Rh
CNT1b-Rh
250
200
7 (R)
11 (R)
12 (R)
10 (R)
/
16 (R)
15 (R)
300
4
4
100
100
a Reaction conditions: 5.5 bar H2, at RT in MeOH. b Determined by
chiral GC using a CP-1 Chirasil-L-Val column with decane as an
internal standard. c Catalytic run using liquid phase.
Characterization of the hybrid catalysts, CNT1a-Rh and
CNT1b-Rh
The amount of rhodium determined by ICP-OES in the
CNT1a-Rh fresh catalysts is 1.05 wt%, corresponding to experiments. As the first reference, we used [Rh(COD)2]BF4 as
a catalyst without the diphosphine ligand. As can be observed
145 μmol of complex per gram of sample, while for catalyst
CNT1b-Rh the amount is 0.6 wt% corresponding to 83 μmol of in Table 4 (entry 1), the conversion obtained after 42 h of reac-
complex per gram of sample. XPS data are shown in Table 3. It tion (90%) shows the low activity of this catalyst. The second
reference was the homogeneous catalyst (S,S)-PPM-Rh, pre-
can be observed that the binding energies of P 2p and N 1s
almost do not change upon rhodium coordination. The pared in situ from the (S,S)-PPM ligand and the [Rh(COD)2]BF4
binding energy of Rh 3d5/2 corresponds to rhodium(I), indicat- precursor. This catalyst shows also low activity and selectivity
under reaction conditions (Table 4, entry 2). The stability of
ing that the electronic state of rhodium in the complex is not
modified upon heterogenisation. The Rh/P ratio, higher than the hybrid catalysts under reaction conditions (24 h) was evalu-
the theoretical value (0.5), can be interpreted considering the ated performing a run without the presence of the substrate.
In this case, no leaching was observed. The hybrid catalysts
CNT1a-b-Rh proved to be active but moderately enantioselective
presence of a certain amount of rhodium not coordinated to
the phosphine ligands.
Enantioselective hydrogenation. The catalytic performances for the hydrogenation of substrate 2a (Table 4, entries 3–8). The
moderate increase in selectivity observed compared to the
metric hydrogenation of 2-methylacetamidoacrylate (2a) and homogeneous catalyst is probably the consequence of the acyla-
of the hybrid catalysts CNT1a-b-Rh were explored in the asym-
α-acetamidocinnamic acid (2b) (Scheme 2).
tion of the secondary amino group of the PPM ligand as already
observed.33,35
In the first set of experiments, the hydrogenation reaction
of 2-methylacetamidoacrylate (2a) was investigated at room
The recyclability of hybrid CNT1a-b-Rh catalysts was evalu-
temperature under 5.5 bar of dihydrogen (Table 4). The results ated in the hydrogenation of 2a. After the first run, the CNT1a-
Rh was recovered from the reaction mixture by filtration,
washed with MeOH to remove traces of the previous mixture
and engaged in a new catalytic run. A complete conversion of
the substrate was obtained with the same enantioselectivity
(Table 4, entry 4). Furthermore, we observed that the CNT1a-
Rh catalyst becomes more active after having been used; for
comparison the conversion was only 25% after 3 h with the
fresh CNT1a-Rh. Interestingly, CNT1a-Rh still exhibited a high
catalytic activity after 3 consecutive cycles without loss of
enantioselectivity (Table 4, entry 5). At this stage, the liquid
phase was engaged in another catalytic reaction after the
addition of a fresh substrate. The conversion of the substrate
were compared with those obtained from two reference
Table 3 XPS data of the carbon-tethered diphosphine and hybrid
catalysts
Binding energy [eV]
Atomic ratio
Sample
P 2p
N 1s
C 1s
Rh 3d5/2
P/N
Rh/P
PPM
CNT1a
CNT1a-Rh
CNT1b
CNT1b-Rh
130.8
133.0
132.9
132.4
132.2
399.0
399.8
400.3
400.0
400.1
284.7
284.6
284.5
284.5
284.6
—
—
308.5
—
309.1
2.3
2.7
1.5
1.3
0.9
—
—
0.7
—
1.1
7460 | Dalton Trans., 2014, 43, 7455–7463
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was 70% but no enantioselectivity could be measured (Table 4,
entry 6). Similarly, CNT1b-Rh catalyst can be reused without
loss of activity and enantioselectivity in two consecutive runs
(Table 4, entries 7 and 8).
Encouraging results in terms of enantioselectivity were
obtained in the hydrogenation of α-acetamidocinnamic acid,
2b (Table 5). Indeed, the hybrid catalysts CNT1a-b-Rh are able
to perform the hydrogenation reaction with good conversions
(71–75%) producing enantiomeric excess in the range 54–63%.
Compared to homogeneous catalytic systems reported in the
literature (entries 1 and 2),33 CNT1a-b-Rh proved to be more
enantioselective but slightly less active.
Fig. 6 TEM micrographs of the spent catalyst CNT1a-Rh (3 catalytic runs).
Characterization of used catalysts. The amount of Rh
remaining on the support after their use in several catalytic
runs of 2-methyl-acetamidoacrylate hydrogenation was ana-
lysed by ICP (Table 6). The results showed that leaching is rela-
tively low (less than 15%).
XPS analysis of the spent catalysts gives the following
binding energies: Rh 3d5/2 310 eV, P 2p 133 eV, N 1s 400 eV
and C 1s 284.5 eV. These data suggest that the electronic state
of the anchored complex has not been modified. However, the
P/N and Rh/P atomic ratios are lower than in the fresh catalysts
(0.30 and 0.42, respectively), meaning that some alteration of
the complex structure has taken place.
the tethered phosphines) and this small amount of Rh(0) is not
detected by XPS, or that the Rh complex decomposition takes
place under the electron beam of the TEM. Such a decompo-
sition could be favoured with the spent catalyst since the
complex has been altered, being less robust. The possible for-
mation of low amounts of Rh(0) could explain the higher cataly-
tic activity of the spent catalyst.24 The fact that the ee are not
significantly affected upon recycling could come from the fact
that the R(0) nanoparticles are stabilized by the chiral ligand.62
More analyses and experiments are needed to confirm this
hypothesis.
The hybrid catalyst CNT1a-Rh was also analysed by TEM
after 3 runs of 2-methyl acetamidoacrylate hydrogenation.
Fig. 6 shows some of the micrographs obtained, where the pres-
ence of small metallic nanoparticles can be observed (size
between 1 and 3 nm). It is expected that the same will happen
after use in the hydrogenation of α-acetamidocinnamic acid,
because reaction conditions are similar. The observation of Rh
particles is surprising because Rh(0) was not detected by XPS,
and also because the spent catalysts are still enantioselective.
This means either that only a small part of the rhodium
complex has been reduced (likely, the species not coordinated to
Conclusions
Chiral rhodium complexes have been successfully grafted on
CNTs for the first time. The grafting strategy consists of the
synthesis and tether of the diphosphine ligands PPM on
CNTs, followed by the reaction with a Rh precursor. The
grafted complexes are active in the hydrogenation of methyl
2-acetamidoacrylate, with a modest enantioselectivity, and are
fully reusable. Leaching was low and the ee did not decrease
upon consecutive runs. The hybrid catalysts showed a very
good activity for the hydrogenation of α-acetamidocinnamic
acid and a better enantioselectivity than the homogeneous
counterpart. Thus, this work opens prospects for the develop-
ment of hybrid enantioselective catalysts on carbon
nanostructures.
Table 5 Asymmetric hydrogenation of α-acetamidocinnamic acid, 2ba
Entry Catalysts
S/C
Time (h) Conversionb (%) eeb
1
2
3
4
(S,S)-PPM-Rhc
100 20
100
100
75
6 (S)
30 (R)
63 (R)
54 (R)
(S,S)-BPPM-Rhc 100 20
CNT1a-Rh
CNT1b-Rh
200 24
200 24
71
Acknowledgements
a Reaction conditions: 5.5 atm H2, at RT in MeOH. b Determined by
chiral GC using a CP-1 Chirasil-L-Val column with decane as an
internal standard. c Data from the literature, see ref. 33.
The authors acknowledge the financial support from the Insti-
tut National Polytechnique de Toulouse (ENSIACET), the
Centre National de la Recherche Scientifique, MICINN, Project
MAT2012-32832, GVA and FEDER, Project Prometeo 2009/047,
and MEC for the FPU scholarship of C.C.G.
Table 6 Amount of Rh remaining in catalyst CNT1a-Rh after several
catalytic runs
% Rh (mg) remaining
in the catalyst
Leaching
(%)
Catalysts
Run
References
CNT1a-Rh
CNT1a-Rh
CNT1a-Rh
CNT1a-Rh
0
1
2
3
1.05 (0.347 mg)
0.94 (0.312 mg)
0.898 (0.297 mg)
0.897(0.296 mg)
/
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