Rh nanoparticles from thiolate dimers: Selective and reusable hydrogenation catalysts in ionic liquids

Article abstract of DOI:10.1039/c8cy00227d

Thiolate-capped rhodium nanoparticles were synthesized by decomposition of the organometallic precursors [Rh(μ-SC₁₂H₂₅)(COD)]₂ (I) and [Rh(μ-SC₆H₁₁)(COD)]₂ (II) under hydrogen pressure, in imidazolium-based ionic liquids and organic solvent, without the external addition of ligands as stabilizers and were compared to RhNPs synthesized from [Rh(μ-OMe)(COD)]₂. The use of ionic liquids during the synthesis of rhodium nanoparticles resulted in homogeneously well-dispersed systems as observed by TEM, where the thiolate ligand remains intact as demonstrated by XPS analysis and gas chromatography. The thiolate-capped rhodium nanoparticles generated from II were active catalysts in hydrogenation reactions with high selectivity towards alkene, nitro and imine groups. The absence of thiolate ligand generated very active catalytic systems and the hydrogenation of carbonyl groups was possible. The effect of thiol stabilizer was studied in the reductive N-alkylation to produce N-benzylaniline through one pot synthesis from benzaldehyde and nitrobenzene, where only the thiolate-capped RhNPs showed complete conversion and high selectivity. Additionally, the thiolate-capped RhNPs permitted the reuse of the catalytic systems for ten catalytic cycles for selected hydrogenation reactions after product separation.

Full text of DOI:10.1039/c8cy00227d

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Rh nanoparticles from thiolate dimers: selective and reusable
hydrogenation catalysts in ionic liquids
Received 00th January 20xx,
Accepted 00th January 20xx
A. Serrano-Maldonado,* S. S. Rozenel, J. L. Jimenez-Santiago, I. Guerrero-Ríos* and E. Martin†
Thiolate-capped rhodium nanoparticles were synthesized by decomposition of the organometallic precursors [Rh(ꢀ-
SC12H25)(COD)]2 (I) and [Rh(ꢀ-SC6H25)(COD)]2 (II) under hydrogen pressure, in imidazolium-based ionic liquids and organic
solvent, without the external addition of ligands as stabilizers and were compared to RhNPs synthesized from [Rh(ꢀ-
OMe)(COD)]2. The use of ionic liquids during the synthesis of rhodium nanoparticles resulted in homogeneously well-
dispersed systems as observed by TEM, where the thiolate ligand remains intact as demonstrated by XPS analysis and gas
chromatography. The thiolate-capped rhodium nanoparticles generated from II were active catalysts in hydrogenation
reactions with high selectivity towards alkene, nitro and imine groups. The absence of thiolate ligand generated very active
catalytic systems and the hydrogenation of carbonyl groups was possible. The effect of thiol stabilizer was studied in the
reductive N-alkylation to produce N-benzylaniline through one pot synthesis from benzaldehyde and nitrobenzene, where
only the thiolate-capped RhNPs showed complete conversion and high selectivity. Additionally, the thiolate-capped RhNPs
permitted the reuse of the catalytic systems for ten catalytic cycles for selected hydrogenation reactions after product
separation.
DOI: 10.1039/x0xx00000x
www.rsc.org/
of stabilizers during the synthesis prevents the agglomeration of the
MNPs that leads to loss of catalytic activity.18,19 The use of ligands
Introduction
Rhodium-based catalysts are versatile systems to perform chemical
transformations, such as hydrogenation,1,2 hydroformylation,3,4 and
carbonylation reactions,5 as well as tandem reactions.6,7 Since many
industrial processes rely on rhodium, diverse methodologies have
been developed to reduce the environmental impact and to
improve the activity of this precious metal. In this context, rhodium
nanoparticles (RhNPs) have gained importance towards catalyzed
as stabilizers can modify the surface properties of the MNPs,
controlling its catalytic activity and selectivity.20,21 In particular,
voluminous P-donor ligands are preferred to stabilize RhNPs used
as hydrogenation catalysts, improving the selectivity towards
reduction of aromatic rings with moderate activity.22–26 The use of
thiolate ligands as stabilizers of MNPs have been limited in catalysis
due to the strong coordination of the thiolates to the surface,
resulting in systems with low activity27,28 or inactive as catalysts.29
Recently, the group of Medlin et. al showed that the modification of
heterogeneous platinum and palladium catalysts with thiolates
were capable to tune the steric and electronic properties of the
surface, influencing the selectivity in hydrogenation reactions.30–34
In addition, palladium nanoparticles stabilized by thiolates showed
good activity in hydrogenation and C-C coupling reactions.35–37 As
an alternative to the use of ligands as MNPs stabilizers, ionic liquids
can provide good stabilization with application in several catalytic
reactions.38–45
chemical transformations under mild conditions, favoring catalyst
8,9
recycling after product separation,
in particular when ionic
liquids are used as both, reaction media and liquid phase catalyst
immobilization support.10–13
One of the most successful strategies for the synthesis of transition
metal nanoparticles (MNPs) employed as catalysts, is the
decomposition of organometallic precursors. This strategy, first
developed for the synthesis of ruthenium nanoparticles,14 consists
in removing organic ligands, frequently alkenes, under reductive
dihydrogen atmosphere. As a result, nanoparticles with narrow size
distribution are obtained, the byproducts of the reaction are
innocuous to the metal and, in most cases, the resulting alkanes are
volatile materials that are easily removed.15 Rhodium precursors for
the synthesis of RhNPs encompass halogenated salts and rhodium
complexes.16 The use of halogen free precursors is preferred since a
rational control over the nanoparticle composition provides
reproducible catalytic systems, precluding deleterious interactions
of the anions onto the surface of the preformed RhNPs.17 Addition
Here, we report the synthesis of thiolate-capped rhodium
nanoparticles from thiolate dimers in imidazolium-based ionic
liquids and its application as recyclable catalysts for the selective
hydrogenations of alkene, nitro and imine groups. The resulting Rh-
thiolate nanoparticles were tested in the one pot synthesis of
secondary amines.
Experimental
General
Depto. de Química Inorgánica, Facultad de Química, Universidad Nacional Autónoma
de México, Av. Universidad 3000, 04510 CDMX., Mexico.
E-mail: itzelgr@unam.mx; a.serrano@comunidad.unam.mx
† In loving memory.
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All manipulations were carried out in a dry, oxygen-free nitrogen
s frames, followed by spot integration and least-squares
atmosphere using standard Schlenk techniques. Organic solvents refinement. A data collection strategy was computed with COSMO
were distilled under nitrogen atmosphere from Na/benzophenone to ensure a redundant and complete data set, and the raw data
and were degassed. Ionic liquids were dried under vacuum (0.05 were integrated using SAINT.53 The data were corrected for Lorentz
mmHg) for 18 h at 60°C prior to use. All reagents were acquired and polarization effects, but no correction for crystal decay was
from commercial suppliers and used as received. Ionic liquid 1- applied. An empirical absorption correction based on comparison of
butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) (BASF redundant and equivalent reflections was applied using SADABS.54
grade) was purchased from Sigma-Aldrich. The compound [Rh(ꢀ- XPREP55 was used to determinate the space group. The structures
46
OMe)(COD)]2 and the ionic liquid 1-hexyl-3-methylimidazolium were solved using SIR-9756 and refined on all data by full-matrix
tetrafluoroborate [HMIM][BF4]47 were synthesized following least-squares with SHELXL-97.57 The structures were refined as a
literature procedures. Infrared spectra were recorded on a Perkin- two-component twin using the corresponding twinning law and the
1
Elmer FTIR/FIR Spectrum 400 spectrometer. H NMR and {1H}COSY disorder was modeled. Thermal parameters for all non-hydrogen
were obtained on a Varian Unity Nova 300 spectrometer at 299.7 atoms were refined anisotropically, except for those atoms for
MHz for 1H. 13C{1H} NMR, 2D 1H-13C HSQC and 2D 1H-13C HMBC were which twinning and disorder did not allow it, in this case the atoms
obtained on a Varian VNMRS 400. Chemical shifts (δ, ppm) were were refined isotropically. Hydrogen atoms were placed based on
calibrated relative to residual solvent peak. Mass spectra (MS-FAB) idealized positions. ORTEPs were created using ORTEP-32.58 WinGX
and high resolution mass spectra for [Rh(ꢀ-SC12H25)(COD)]2 (I) were software package was used to prepare the publication material.58
acquired using a Thermo DFS mass spectrometer. High resolution
Synthesis of [Rh(ꢀ-SC12H25)(COD)]2 (I).
A
solution of 1-
mass spectra for [Rh(ꢀ-SC6H11)(COD)]2 (II) was acquired on an
Agilent Technologies LC/MSD-TOF mass spectrometer coupled to an
Agilent Technologies 1100 HPLC. Transmission electron microscopy
images were obtained on a JEOL JEM-2010 microscope (200 kV).
Nanoparticle diameter was determined counting at least 400
individual nanoparticles with the software package Digimizer
4.6.1.48 Catalytic conversions and ligand fragmentation were
determined on a Varian 3800 gas chromatograph with a capillary
column DB-Wax (30 m x 0.32 mm x 0.25 mm) coupled to a FID
detector, using decane as internal standard. Synthesis of
nanoparticles and catalytic reactions were performed on a Parr
Multi Reactor 500 system. ICP-MS experiments were performed on
a Bruker Aurora M90 equipment.
dodecanethiol (0.4 mmol, 95 ꢀl) in THF (3 mL) was added dropwise
to a solution of [Rh(ꢀ-OMe)(COD)]2 (100mg, 0.2 mmol) in THF
(12mL). The resulting orange solution was stirred 1 h at room
temperature. Solvent and organic byproducts were removed under
reduced pressure resulting in an orange powder (Yield 85%). M. P.
58°C. IR (KBr, cm-1): 3006(m, CH), 2920(s, CH3), 2844(s, CH2),
1460(m, CH2), 1320(s, CH3), 845(w, SC), 717(s, (CH2)n; n>4). 1H NMR
(C6D6, 299.7 MHz): δ 4.3 (s, 4H, CH), 2.35 (m, 4H, CH2), 2.15 (t, 2H, J
= 6.9 Hz, CH2), 1.94 (m, 4H, CH2), 1.64 (q, 2H, J = 6.9 Hz, CH2), 1.28
(m, 18H, (CH2)9), 0.92 (t, 2H, J = 6.9 Hz, CH3). 13C{1H} NMR (C6D6,
100.5 MHz) δ: 78.68 (CH), 33.39 (CH2), 32.37 (CH2), 31.93 (CH2),
30.16 (CH2), 30.13 (CH2), 30.1 (CH2), 30.04 (CH2), 29.85 (CH2), 29.81
(CH2), 29.7 (CH2), 25.57 (CH2), 23.16 (CH2), 14.41 (CH3). MS HR-FAB+:
m/z: 824.3305 (calc. 824.3337 for C40H74Rh2S2+), error (ppm) = -
3.83.
XPS Analysis. XPS measurements were performed using a PHI 5000
VersaProbe II (from Physical Electronics), with an Al-Kα X-ray source
(hν = 1486.6 eV), a beam size of 100 ꢀm, power of 25 W and X-ray
voltage of 25 KV. XPS spectra were obtained at 45° from the normal
surface in the constant pass energy mode (CAE), E0 = 117.3 eV and
23.5 eV for survey and high resolution narrow scan with a step size
of 1 eV and 0.2 eV respectively. Peak positions were referenced to
the Shirley background Ag 3d5/2 core level at 368.20 eV, Au 4f7/2 at
84.00 eV and C 1s hydrocarbon groups at 284.00 eV central peaks.
Analyses were performed in ultra-high vacuum (10-8 mbar). XPS
spectra were fitted with the program CasaXPS.49 The assignment of
chemical components of core level S 2p and Rh 3d were made using
the reference rhodium precursor II (figure S25ǂ), and Rh(0) and
Synthesis of [Rh(ꢀ-SC6H11)(COD)]2 (II).
A
solution of
cyclohexanethiol (0.4 mmol, 49 ꢀl) in THF (3 mL) was added
dropwise to a solution of [Rh(ꢀ-OMe)(COD)]2 (100mg, 0.2 mmol) in
THF (12mL). The resulting solution was stirred 1 h at room
temperature. Solvent and organic byproducts were removed under
reduced pressure resulting in an orange powder (yield 87%). M. P.
85°C. IR (KBr, cm-1): 3021(m, CH), 2922(s, CH2), 2849(s, CH2),
1
1475(m, CH2), 1444(m, (CH2)cycle), 1327(s, CH2), 847(w, SC). H NMR
(C6D6, 299.7 MHz) δ: 4.61(s, 4H, CH), 2.88 (m, 1H, CH-cycle), 2.34 (s,
4H, CH2), 2.09 (m, 2H, CH2), 1.92 (m, 4H, CH2), 1.62 (m, 2H, CH2),
1.44 (m, 2H, CH2), 1.39(m, 1H, CH2), 1.2 (c, 2H, J = 9.3 Hz, CH2), 1.05
(t, 1H, J = 9.3 Hz, CH2). 13C{1H} NMR (C6D6, 100.5 MHz) δ: 79.43 (CH),
45.40 (CH-cycle), 38.94 (CH2), 32.07 (CH2), 26.94 (CH2), 26.01 (CH2).
Rh(III) components were referenced using the values reported in
50,51
the literature.
For the deconvolution analysis the uncertainty
was estimated at 5% of the binding energy.
MS HR-ESI+: m/z: 652.1145 (calc. 652.1148 for C28H46S2Rh2 ), error
+
Crystallographic Analysis. A fragment of yellow plate-like crystals of
.
[Rh(ꢀ-SC12H25)(COD)]2 (I) and [Rh(ꢀ-SC6H11)(COD)]2 0.5(C6H14) (II),
(ppm) = 0.3038.
were mounted on a Kapton loop using Paratone N hydrocarbon oil.
They were transferred to a Bruker APEX D8 Venture diffractometer
with CCD area detector,52 centered in the beam, and the crystal was
cooled during data collection by a nitrogen flow low-temperature
apparatus previously calibrated by a thermocouple placed at the
same position as the crystal (T = 100 K). Preliminary orientation
matrixes and cell constants were determined by collection of 30 10-
General synthesis of RhNPs in ionic liquids. The desired rhodium
precursor (0.025 mmol) and ionic liquid (5 mL) were placed in a 25
mL stainless steel reactor vessel. The mixture was magnetically
stirred and heated for 15 min at 70°C, followed by pressurization
with H2 (15 bar for RhNPs-A and B and 10 bar for RhNPs-C) and
stirring for 18 h at 80°C. The reactor vessel was cooled to room
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temperature and depressurized. The resulting black homogeneous mmol). The reactor was pressurized with H2 at the desired pressure,
solution of dispersed nanoparticles was washed with hexanes (3 x 5 temperature and time. After the reaction was completed, the
mL) to remove organic byproducts and dried under vacuum (0.05 reactor vessel was cooled to room temperature and depressurized.
mmHg) for 18 hours.
Products were extracted with a solvent mixture of hexanes/toluene
10:1 (5 x 3 mL) and analyzed by gas chromatography by comparison
with the commercial product. In the case of the hydrogenation
reactions of 4-nitroacetophenone and p-benzoquinone, and the
reductive N-alkylation, products were analyzed by 1H-NMR after
removal of volatiles under reduced pressure.
General synthesis of RhNPs in THF. A solution of the desired
rhodium precursor (0.025 mmol) in THF (15 mL) were introduced in
a 25 mL stainless steel reactor vessel. The mixture was magnetically
stirred and heated for 15 min at 70°C, followed by pressurization
with H2 (10 bar) and stirring for 18 h at 80°C. The reactor vessel was
cooled to room temperature and depressurized. The resulting black Recycling experiments. After the extraction of products described
homogeneous solution was transferred to a Schlenk flask. Organic above, the ionic liquid phase containing the rhodium nanoparticles
solvent was removed under reduced pressure to obtain a black (1 mL of RhNPs-B) was dried under vacuum at 80°C for 3 hours.
powder of nanoparticles. RhNPs were washed with hexanes (3 x 5 Additional substrate and internal standard were added and the
mL) to remove organic byproducts and dried under vacuum (0.05 same procedure for selected catalytic reaction was followed.
mmHg) for 18 h.
Thiolate fragmentation studies of RhNPs in ionic liquids: Synthesis Results and discussion
of RhNPs were carried out using
a similar methodology as
Synthesis of rhodium thiolate complexes
previously described, adding 10 L of decane as internal standard.
ꢀ
The black homogeneous solution of dispersed nanoparticles was
washed with diethylether (3 x 5 mL) and the resulting organic phase
was analyzed by gas chromatography. GC method for dodecane:
110°C(4.1’), 30°C/min to 180°C, 180°(8.57’). GC method for
cyclohexane: 70°C(15’).
Dimeric rhodium precursors with thiolate bridges were synthesized
by a substitution reaction, adding two equivalents of selected thiol
to
a solution of [Rh(ꢀ-OMe)(COD)]2 in THF (scheme 1).
Dodecanethiol and cyclohexylthiol were selected as ligands to
evaluate steric effects.
Thiolate fragmentation studies of RhNPs in THF: Synthesis of
C12H25
CH3
RhNPs were carried out using a similar methodology as previously
S
THF
O
+2C12H25SH
Rh
Rh
Rh
Rh
Rh
described, adding 10
ꢀL of n-decane as internal standard. The
20°C, 1h
S
O
resulting solution was filtered over celite to remove RhNPs and the
organic phase was analyzed by gas chromatography. Fragmentation
percentage was determined with a calibration curve, correlating
cyclohexane and dodecane with n-decane (internal standard). GC
method for RhNPs-A/THF: 110°C(4.1’), 30°C/min to 180°C,
180°(8.57’). GC method for RhNPs-B/THF: 70°C(15’).
C12H25
I
CH3
CH3
O
C6H11
S
THF
Rh
Rh
Rh
S
2C H11SH
+
6
O
20°C, 1h
CH3
C6H11
II
General procedure for catalytic hydrogenation with RhNPs.
Dispersed RhNPs in ionic liquid (1 mL, 0.01 mmol of Rh) were placed
in a 25 mL stainless steel reactor vessel together with the selected
Scheme 1. Synthesis of rhodium thiolate complexes I and II
substrate(s) (1 mmol) and n-decane as internal standard (97 ꢀl, 0.5
(I)
(II
Figure 1. ORTEP of [Rh(ꢀ-SC12H25)(COD)]2 (I); ORTEP of [Rh(ꢀ-SC6H11)(COD)]2 (II)
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Complexes [Rh(
ꢀ
-SC12H25)(COD)]2 (I) and [Rh(
ꢀ
-SC6H11)(COD)]2 (II)
were characterized by IR, HRMS, H NMR, 13C NMR, 2D HSQC NMR
and 2D HMBC NMR (figures S1-S14ǂ). Suitable crystals for an X-ray
diffraction study of I were grown from a concentrated solution in
hexane, slowly evaporated at 0°C. For complex II, suitable crystals
were obtained from a concentrated solution in hexane cooled at -
40°C. Both X-ray structures presented twinning, being more
troublesome for I, where the twinning was over a symmetry plane
and the SC12H25 and COD (cycloocta-1,5-diene) ligands were
disordered. Both X-ray structures showed dimeric complexes, with
the metal centres displaying distorted square planar geometries
and the thiolate ligands bridging the two Rh atoms (Figure 1). As
expected, elongated double bond distances in the COD ligands are
observed in comparison to free COD, due to donation to the metal
center.59 The Rh-Rh distance found in II (2.9481(6) Å) is similar to
those found in the literature for related complexes,60,61 and it is
indicative of no metal bond between the two metal centers. For
complex I, the Rh-Rh distance found is considerable shorter
(2.875(18) Å) than that observed in II, implying metal-metal
interaction. The S-S distance in II (2.835(1) Å) is also shorter than
that in I (3.058(2) Å), indicating that steric effects have a major
influence in the Rh-Rh and S-S distances observed. All the Rh-S
bonds in the two molecules are within 0.073 Å of each other and
are unexceptional. For I, the SC12H25 ligands have an exo, exo
conformation, with the dodecyl chains of adjacent molecules being
parallel to each other, thus, allowing stacking. For II, the thiolate
ligands appeared in an endo, exo configuration.
1
Figure 2. TEM micrograph of RhNPs-A (a); size distribution of RhNPs-
A with mean diameter of 2.7±0.2 nm for 400 nanoparticles (b);
HRTEM of RhNPs-A (c); magnification of circled zone (d), interplanar
distances of 2.2 Å (111); crystallographic plane spots observed by FFT
of (d) (0.455 Å-1) (e).
Thiolate-stabilized RhNPs in ionic liquids
Thiolate-capped RhNPs were synthesized by decomposition of the
organometallic precursors [Rh(ꢀ-SC12H25)(COD)]2 (I) and [Rh(ꢀ-
SC6H11)(COD)]2 (II) under reductive H2 atmosphere, using
imidazolium-based ionic liquids as solvents, 1-hexyl-3-
methylimidazolium
tetrafluoroborate
and
1-butyl-3-
methylimidazolium tetrafluoroborate ([BMIM][BF4]), respectively
(scheme 2). The ionic liquids were selected in accordance to the
solubility of the rhodium precursors: I in [HMIM][BF4] and II in
[BMIM][BF4]. A 15 bar of hydrogen pressure was required to carry
out the complete reduction of Rh-thiolate precursors overnight.
R
S
H2 (15 bar)
Rh
Rh
RhNPs/SR/IL+ C8H16
IL
S
R
80°C, 18h
Figure 3. TEM micrograph of RhNPs-B (a); size distribution of RhNPs-B
with mean diameter of 2.8±0.3 nm for 400 nanoparticles (b); HRTEM of
one nanoparticle of RhNPs-B (c); magnification of circled zone,
interplanar distances of 2.2 Å (111) (d); crystallographic plane spots
observed by FFT of (d) (0.455 Å-1) (e).
I, II
RhNPs-A: R=C12H25; IL= [HMIM][BF ]
4
RhNPs-B: R=C6H11; IL= [BMIM][BF ]
4
Scheme 2. Synthesis of thiolate-capped rhodium nanoparticles (RhNPs-A
and RhNPs-B) in ionic liquids ([Rh] = 10-2 mol L-1).
RhNPs-A with a narrow size distribution and a mean diameter of
2.7±0.2 nm for 400 nanoparticles (figure 2, top). In the analysis by
high resolution transmission electron microscopy (HRTEM), the
interplanar distances measured (2.2 Å) correspond to the
crystallographic planes (111) of Rh with fcc structure (figure 2,
Both Rh nanoparticles systems were characterized by transmission
electron microscopy (TEM), revealing spherical nanoparticles for
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bottom). In the case of RhNPs-B, pseudospherical nanoparticles stabilizers of Pd nanoparticles in THF, resulting in the formation of
were obtained with a mean diameter of 2.8±0.3 nm for 400 sulfide species in the surface.63 Employing the same thioether-
nanoparticles (figure 3, top). The shorter alkyl length of the phosphine ligand as stabilizers of Pd nanoparticles with ionic liquid
cyclohexane-thiolate ligand with less steric effect, caused the as a solvent, only 6% of ligand fragmentation was observed.64
formation of larger nanoparticles. HRTEM images for these Rhodium nanoparticles (RhNPs) from precursors I and II were
nanoparticles showed the same interplanar distances of 2.2 Å synthesized in THF, to evaluate the effect of the solvent in the
(figure 3, bottom). EDX analysis evidenced for both thiolate-capped synthesis. TEM micrographs of RhNPs-A/THF showed spherical
nanoparticles the presence of Rh and S (figures S15, S16ǂ).
dispersed nanoparticles with a mean diameter of 2.8±0.3 nm for
To observe the influence of thiolate ligands in the shape and size of 400 nanoparticles (figure S17ǂ), similar to RhNPs-A. RhNPs-B/THF
nanoparticles during the synthesis, RhNPs from the precursor presented nanoparticles with a narrow size distribution and a mean
diameter of 2.3±0.3 for 400 nanoparticles (figure S18ǂ), a shorter
[Rh(ꢀ-OMe)(COD)]2 in the ionic liquid [BMIM][BF4] were
diameter than in RhNPs-B.
synthesized (RhNPs-C). In this case, 10 bar of hydrogen pressure
were enough to completely reduce the rhodium precursor.
TEM micrographs of RhNPs-C showed disperse and spherical well-
defined shape nanoparticles, with a mean diameter of 5.2±0.8 nm
for 400 nanoparticles (figure 4). Thus, the thiolate ligand clearly
affects the size of nanoparticles, presenting a considerable decrease
of 2.42 nm in the case of RhNPs-B when ligand is present. However,
in the absence of thiolate ligand, RhNPs-C have a better-defined
shape.
Figure 4. TEM micrograph of RhNPs-C (a); size distribution of RhNPs-C
with mean diameter of 5.2±0.8 nm (b).
To assess the integrity of thiolate ligands and identify species at the
surface of nanoparticles, XPS analysis was performed for the system
RhNPs-B. This system was selected considering its better catalytic
performance (see below). XPS analysis evidenced the presence of
Rh and S at the surface (figure S21ǂ). HR-XPS for Rh showed only
the presence of Rh(0) and Rh(I) species due to the thiolate binding
in the surface (figure 5, top). The presence of Rh(I) was
corroborated by comparison with the XPS spectrum of the starting
precursor II (figure S25ǂ). In the HR-XPS spectrum for S, only the
spin-orbit doublet for S2p (2p3/2 and 2p1/2) doublet corresponding
to Rh-thiolate was observed. No oxides or sulfide species were
detected (figure S22ǂ).
Figure 5. High resolution XPS spectra for Rh 3d binding region: RhNPs-B
(top) and RhNPs-B/THF (bottom).
The XPS and EDX survey spectra of RhNPs-B/THF showed the
presence of Rh, S, C and O (figure S23ǂ). HR-XPS deconvolution
analysis evidenced the presence of Rh(0), Rh(I) and Rh(III) species,
where the latter corresponds to sulfide species (figure 5, bottom).
In the HR-XPS spectrum for S, two doublets corresponding to Rh-
thiolate and Rh-sulfide were observed (figure S24ǂ). These results
indicate the cleavage of the C-S bonds, generating Rh-sulfide
species in the surface of the nanoparticles. Further evidence was
obtained by analyzing the organic phase of RhNPs-A/THF and
RhNPs-B/THF by GC, revealing the presence of the alkanes
dodecane and cyclohexane in 33% and 54%, respectively. However,
when ionic liquids were used as solvent, the thiolate ligand was
preserved, and no C-S bond cleavage was observed.
The synthetic strategy chosen for the synthesis of RhNPs in this
work, is based on the decomposition of organometallic precursors
under hydrogen pressure, which can lead to modifications of the
stabilizer through bond cleavage. Chaudret and collaborators
showed the formation of disulfide species when octanethiol was
used as ligand stabilizer of ruthenium nanoparticles synthesized
under H2 in THF,62 and in our group, C-S bond cleavage was
observed when thioether-phosphine ligands were used as
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Catalytic results
Table 2. Cyclohexene (6), cyclohex-2-ene-1-one (7) and p-benzoquinone
(8) hydrogenation catalyzed by RhNPs-B and RhNPs-C
The catalytic activity of RhNPs was explored for hydrogenation
reactions and the results are summarized in Table 1. Styrene (1) is
completely converted to ethylbenzene (1i) when RhNPs-B and
RhNPs-C are used as catalysts (entry 1, table 1), where RhNPs-A
Main
Prod.
Conv. (sel.) (%)a
RhNPs-B
Conv. (sel.) (%)a
RhNPs-C
1 h
Entry
Subs.
1 h
2 h
1
2
9 (>99)
14 (>99)
>99 (>99)
6
7
6i
7i
Table 1. RhNPs-catalyzed hydrogenation in ionic liquids
Conv.
(sel.)
Conv.
(sel.)
Conv.
(sel.)
Main
Product
59 (>99)
19 (>99)b
80 (>99)
56 (>99)b
>99 (96)
Entry
Substrate
(%)a
(%)a
(%)a
RhNPs-A
RhNPs-B
RhNPs-C
>99
(>99)
1
2
78 (>99)
0
>99 (92)
82 (72)
3
65 (>99)
1
2
3
1i
2i
3i
8i
8
0
0
Results from duplicated experiments. Reaction conditions: 1 mmol of
substrate and 1 mL of the catalytic solution of RhNPs (10-2 mol L-1, 0.01
mmol of total Rh), H2 (50 bar), 80°C. a Determined by GC using decane as
internal standard. b Determined by 1H NMR (see Figure S32ǂ).
3
4
0
23 (90)
the hydrogenation of carbonyl groups and aromatic rings in
substrates 2 and 3 with poor selectivity (figures S28 and S29ǂ). At
the end of the reaction with RhNPs-C, a black precipitate due to the
complete agglomeration of the nanoparticles was observed, as well
as in the hydrogenation of styrene case (1).
25 (>99)
78 (>99)
>99 (>79)
4i
4
5
6
10 (>99)b 15 (>99)b
99 (95)b
The substrate 4-phenyl-3-buten-2-one (4) was also evaluated. The
systems RhNPs-A and RhNPs-B were completely selective to alkene
hydrogenation (entry 4, table 1), with 78% conversion for RhNPs-B
and 25% conversion for RhNPs-A in 1 h. This behavior agrees with
results observed for styrene (1) (see above), where RhNPs-A
showed lower activity compared to RhNPs-B due to the greater
steric hindrance of the thiolate ligand. The system without ligand
RhNPs-C (entry 4), gave complete conversion, with moderate
selectivity towards 4i (79%), and reduction of the ketone (11 %) and
the aromatic ring (10 %) were observed as byproducts. After 2 h of
reaction, the hydrogenation of 4 catalyzed by RhNPs-A reaches 55%
conversion, while the reaction catalyzed by RhNPs-B was
completed, preserving the selectivity towards alkene hydrogenation
(table S2ǂ). The catalytic system RhNPs-B was reused for 10 cycles
after product extraction at 2 h of reaction, without significant losses
of activity and selectivity (table S3ǂ). The catalytic system RhNPs-C
was not reused due to the formation of Rh agglomerates.
5i
6i
5
6
<5
9 (>99)
>99 (>99)
Results from duplicated experiments. Reaction conditions: 1 mmol of
substrate and 1 mL of the catalytic solution of RhNPs (10-2 mol L-1, 0.01
a
mmol of total Rh), H2 (50 bar), 80°C, 1 h. Determined by GC using
decane as internal standard, selectivity to main product in parenthesis. b
Determined by 1H NMR.
reaches 78% conversion. The thiolate stabilizer of RhNPs-A presents
a greater steric hindrance, thus, minimizing the availability of active
sites over the nanoparticle surface and reducing its activity. When
RhNPs-C was used, 8% of aromatic ring hydrogenation was
observed, and formation of Rh agglomerates at the end of the
reaction occurs preventing the reuse of the catalytic system after
product separation. When the reaction was performed at a lower
H2 pressure (15 bar), the catalytic system RhNPs-A showed an
important decrease in activity, whereas RhNPs-B showed 90% of
conversion (figure S27ǂ).
Interestingly, the hydrogenation of 4-nitroacetophenone (5),
resulted in the exclusive reduction of the nitro group for all the
RhNPs tested (entry 6, table 1). The presence of a thiolate ligand in
RhNPs-A and RhNPs-B resulted in low conversion after 1 h, whereas
the absence of ligand in RhNPs-C achieved good conversion to 4-
aminoacetophenone (5i) with 95% selectivity and only 5% of ketone
hydrogenation. Complete conversion of 5 to 5i was achieved after 6
h for the systems bearing thiolate ligands RhNPs-A and RhNPs-B
(figure S30ǂ).
The selective hydrogenation of styrene (1) to ethylbenzene (1i)
catalyzed by RhNPs-B, could be recycled for up to 10 cycles, with no
significant loss of activity and selectivity (table S1ǂ). TEM analysis of
RhNPs-B after 10 catalytic runs showed dispersed nanoparticles
with no significant increase in the mean diameter (figure S19ǂ).
To further evaluate the selectivity of RhNPs towards aromatic ring
and carbonyl groups, the hydrogenation reactions of acetophenone
(2) and benzaldehyde (3) (entries 2 and 3, table 1) were performed.
The carbonyl group and the aromatic ring were not reduced by the
thiolate capped-RhNPs. However, RhNPs-C were active catalysts for
In the hydrogenation of cyclohexene (6) (entry 6, table 1), systems
RhNPs-A and RhNPs-B showed conversions below 10%, whereas
RhNPs-C gave complete conversion. The low conversion observed
when thiolate stabilizers were present correlates to the minimized
availability of active sites occupied by the thiolate ligand. Increasing
6 | J. Name., 2012, 00, 1-3
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the reaction time to 2 h and employing RhNPs-B as the catalysts for condensation of 9i with 3 to form N-benzylidenaniline (10); and c)
the hydrogenation of 6, gave a 14% conversion to 6i (entry 1, table the hydrogenation of 10 to N-benzylaniline (10i).
2). The use of cyclohex-2-ene-1-one (7) is known to favor the The first step of the one pot synthesis of N-benzylaniline (10i)
conversion to cyclohexanone (7i) by thiolate-capped PdNPs, due to (nitrobenzene (9) to aniline (9i)) was completed after 3 h at 100oC
and 50 bar H2 pressure (table 3). The second and third step
(reaction (b) and (c)) were followed over time at 100°C and 50 bar
H2 pressure, reaching 60% conversion to 10i after 4 h, 89%
conversion after 8 h (table S5ǂ), and 97% conversion after 18 h,
with exclusive hydrogenation of the condensation product N-
benzylidenaniline (10).
the formation of di-σ-bonded species on the metal nanoparticle
surface.37 When RhNPs-B were used as catalysts for the
hydrogenation of 7, 59% conversion was reached in 1 h, and 80%
conversion in 2 h, with complete selectivity towards cyclohexanone
(7i) (entry 2, table 2). In addition, the hydrogenation of p-
benzoquinone (8) to 1,4-cyclohexanodione (8i) was observed with a
conversion of 56% in 2 h (entry 3, table 2), demonstrating the
Table 4. RhNPs-B catalyzed one-pot multi-step synthesis of N-benzylaniline
positive effect of di-σ-bonded species to RhNPs surface in the
catalytic activity. When RhNPS-C were used as catalysts in similar
reactions, we observed complete conversion of 7 into 7i (96%) and
cyclohexanol (4%) in 1 h; for substrate 8, a 65% conversion to 8i is
reached within 1 h (table 2).
Entry
Catalytic
system
Time (h)
Conv. (%)a
Selectivity (%)
10/10i
To discard the influence of electron withdrawing groups in the
hydrogenation of 4-nitroacetophenone (5), we evaluated the
reduction of nitrobenzene (9) employing RhNPs-B. At similar
reaction conditions, moderate conversion (56%) to aniline (9i) was
achieved after 3 h (table 3). Increasing the temperature to 100°C
afforded complete conversion after 3 h with 9i as the sole product.
The absence of ligand in RhNPs-C afforded complete conversion to
9i at 80 and 100°C within one hour.
1
2
RhNPs-B
RhNPs-B
RhNPs-C
18
24
24
94
98
50
10/90
2/98
65/35
3 b
Results from duplicated experiments. Reaction conditions: 1 mmol of
nitrobenzene, 1 mmol of benzaldehyde and 1 mL of the catalytic solution of
RhNPs-B (10-2 mol L-1, 0.01 mmol of total Rh), H2 (50 bar), 100°C.
a Determined by GC using decane as internal standard. b benzaldehyde was
hydrogenated to benzylic alcohol (conv. = 50%)
The overall reductive N-alkylation of nitrobenzene (9) with
benzaldehyde (3) catalyzed by RhNPs-B reached 94% conversion to
N-benzylaniline (10i) after 18 h (entry 1, table 4), and 98%
Furthermore, the catalytic system RhNPs-B was tested in the
hydrogenation of imines, in particular, N-benzylidenaniline (10). At
80oC and 50 bar of H2 pressure, RhNPs-B showed moderate activity
conversion after 24
h (entry 2, table 4). Following product
extraction, the catalytic system was reused for up to eight catalytic
runs (figure 6). The recycled experiments showed that the
hydrogenation of 9 and the condensation of aniline (9i) with 3
proceed to completion in the eight experiments. However, the
hydrogenation of N-benzylidenaniline (10) was completed only on
the first catalytic run and gradually decreased to 57% conversion on
the eighth catalytic cycle (figure 6). The catalytic system RhNPs-B
showed a gradual change of color, turning into a dark orange
solution after the eighth cycle, suggesting the formation of inactive
molecular species. TEM analysis of RhNPs-B after the eighth
catalytic cycle, showed a change in the original RhNPs through the
formation of agglomerates (figure S20ǂ). ICP-MS analysis of the
organic extracts showed absence of rhodium and the loss of activity
due to rhodium leaching was discarded.
Table 3. Nitrobenzene (9) hydrogenation catalyzed by RhNPs-B and
RhNPs-C
Conv. (%)a
RhNPs-B
2 h
Conv. (%)a
RhNPs-C
1 h
Temp.
(°C)
Entry
1 h
7
3 h
56
1
2
80
27
88
>99
>99
100
67
>99
Results from duplicated experiments. Reaction conditions: 1 mmol of 9
and 1 mL of the catalytic solution of RhNPs (10-2 mol L-1, 0.01 mmol of
total Rh), H2 (50 bar). aDetermined by GC using decane as internal
standard.
towards the product (21% conversion) after 2 h (table S4ǂ).
Increasing the temperature to 100°C, showed 27% conversion in 2
h, reaching complete conversion after 8 h (table S4ǂ).
Rh-catalyzed one-pot synthesis of N-benzylaniline
Considering the hydrogenation selectivity of thiolate-capped
RhNPs-B towards alkene, nitro and imine groups and the stability of
the catalytic systems for further recycling, we aimed for the one pot
synthesis of N-benzylamine (10i) through the reductive N-alkylation
of nitrobenzene (9) and benzaldehyde (3). The reaction proceeds
through three steps: a) the hydrogenation of 9 to aniline (9i); b) the
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the synthesis of secondary amines through the reductive N-
alkylation of nitrobenzene, where thiol-free RhNPs-C are poorly
selective and active towards the formation of N-benzylaniline.
Additionally, these catalytic systems are highly stable and can be
reused several times without loss of activity and selectivity in the
hydrogenation of olefins (styrene and 4-phenyl-3-buten-2-one) and
nitrobenzene. The fine tuning of the reactivity of well-defined
RhNPs with stable thiolate ligands in ionic liquids, offers novel
synthetic tools towards selective hydrogenations of olefins, imines
and nitro groups with low catalyst loading and high recyclability,
which are complementary to the classical Rh reactivity towards the
reduction of aromatic rings.
Figure 6 Recycling experiments of the catalytic system RhNPs-B in the one
pot synthesis of N-benzylaniline (10i).
Acknowledgements
This work was financially supported by the projects DGAPA-PAPIIT
IN226616 and CONACyT CB-167443. A.S.-M. thanks CONACyT for a
PhD grant. S.S.R. thanks CONACyT for postdoctoral scholarship
#600045. Authors thank Dr. Salas-Martin for technical support, and
Dr. Leyva and Dr. Jancik for their help in X-ray crystallography.
We further studied the catalytic system in the absence of thiolate
ligand with RhNPs-C. In this case, the hydrogenation of
nitrobenzene (9) was completed to form aniline (9i), whereas the
condensation reaction of 9i with benzaldehyde (3) was limited due
to partial hydrogenation of 3, resulting in the corresponding alcohol
(3i) with 50% conversion (entry 3, table 4). The remaining 50% of 3
reacted with 9i to form the N-benzylidenaniline (10), that was
poorly hydrogenated (35%) to N-benzylaniline (10i) (entry 3, table
4). These results clearly indicate the influence of thiolate stabilizers
in the selectivity towards nitro and imine groups that affects the
conversion to the desired product 10i. In addition to the selectivity
drawbacks using RhNPs-C, the stability of nanoparticles was
affected by the reaction conditions, resulting in the formation of
rhodium agglomerates and preventing the reuse of the catalytic
system.
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