Selective hydrogenation of fluorinated arenes using rhodium nanoparticles on molecularly modified silica

Article abstract of DOI:10.1039/d0cy01716g

The production of fluorinated cyclohexane derivatives is accomplished through the selective hydrogenation of readily available fluorinated arenes using Rh nanoparticles on molecularly modified silica supports (Rh?Si-R) as highly effective and recyclable catalysts. The catalyst preparation comprises grafting non-polar molecular entities on the SiO2 surface generating a hydrophobic environment for controlled deposition of well-defined rhodium particles from a simple organometallic precursor. A broad range of fluorinated cyclohexane derivatives was shown to be accessible with excellent efficacy (0.05-0.5 mol% Rh, 10-55 bar H2, 80-100 °C, 1-2 h), including industrially relevant building blocks. Addition of CaO as scavenger for trace amounts of HF greatly improves the recyclability of the catalytic system and prevents the risks associated to the presence of HF, without compromising the activity and selectivity of the reaction.

Full text of DOI:10.1039/d0cy01716g

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Selective hydrogenation of fluorinated arenes
using rhodium nanoparticles on molecularly
Cite this: DOI: 10.1039/d0cy01716g
modified silica†
a
ab
Souha Kacem,ab Meike Emondts,bc Alexis Bordet
*
*
and Walter Leitner
The production of fluorinated cyclohexane derivatives is accomplished through the selective hydrogenation
of readily available fluorinated arenes using Rh nanoparticles on molecularly modified silica supports
(Rh@Si–R) as highly effective and recyclable catalysts. The catalyst preparation comprises grafting non-
polar molecular entities on the SiO2 surface generating a hydrophobic environment for controlled
deposition of well-defined rhodium particles from a simple organometallic precursor. A broad range of
fluorinated cyclohexane derivatives was shown to be accessible with excellent efficacy (0.05–0.5 mol% Rh,
10–55 bar H2, 80–100 °C, 1–2 h), including industrially relevant building blocks. Addition of CaO as
scavenger for trace amounts of HF greatly improves the recyclability of the catalytic system and prevents
the risks associated to the presence of HF, without compromising the activity and selectivity of the
reaction.
Received 1st September 2020,
Accepted 15th October 2020
DOI: 10.1039/d0cy01716g
rsc.li/catalysis
this transformation, largely hampering its application and
exploration so far (Scheme 1).14a In the past decades, several
Introduction
While fluorinated cycloalkanes are of great interest as
building blocks for the production of materials,1
agrochemicals2 and pharmaceuticals,3 their synthesis remains
particularly challenging. The most prominent methods for
alkane fluorination4 typically require several steps and the
use of stoichiometric quantities of toxic and difficult-to-
handle reagents such as elemental fluorine,5 cesium
fluoroxysulfate,6 anhydrous hydrofluoric acid,1b or cobalt
trifluoride for polyfluorination.7 Recent progress in catalytic
fluorination was achieved using homogeneous catalysts (e.g.
Cu,8 Mg,9 Ag10) in the presence of complex fluoride sources
studies investigated the use of heterogeneous catalysts for the
hydrogenation of fluorinated arenes to fluorinated
cyclohexanes.13a,14a–c In many cases, the C–F bond cleavage
was predominant and the catalysts were used mostly for
hydrodefluorination.14 As a notable exception, a study by
Stanger and Angelici evidenced the possibility of using non-
polar solvents to limit hydrodefluorination.14a
In heptane as solvent, the authors were able to
hydrogenate fluorobenzene to
a
mixture of 73%
fluorocyclohexane and 27% cyclohexane using large (10–15
nm) and polydisperse Rh NPs immobilized on SiO2. They
showed that the release of HF can promote the conversion of
fluorocyclohexane to cyclohexane with negative impact on
(e.g.
AgF/TBAF·3H2O,9
Selectfluor®8,10).
In
contrast,
fluorinated arenes are widely accessible through a variety of
very efficient routes,11 especially thanks to recent advances in
late stage fluorination techniques.12 Consequently, the
hydrogenation of fluorinated arenes appears as an attractive
and benign approach to access fluorocycloalkane
derivatives.13 However, the competing hydrodefluorinating
pathway significantly reduces the efficiency and selectivity of
a Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470
Mülheim an der Ruhr, Germany. E-mail: alexis.bordet@cec.mpg.de,
walter.leitner@cec.mpg.de
b Institut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Worringerweg 2, 52074 Aachen, Germany
c DWI-Leibniz Institute for Interactive Materials, Forckenbeckstrasse 50, 52056
Aachen, Germany
Scheme 1 (a) Scheme illustrating the catalytic hydrogenation of
fluorinated arenes and the competing hydrodefluorination pathway.
Examples of applications for fluorinated cyclohexane derivatives: (b)
5-HT2 (c) receptor modulator,3b (c) potassium channel modulator.3d
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy01716g
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selectivity. More recently, Glorius et al. reported the use of a
Rh-cyclic(alkyl)(amino)carbene complex for the selective
of HF over the course of the reaction which is a serious issue
with regards to safety as well as catalyst and product stability.
The potential of this catalytic system is demonstrated for the
synthesis of a broad range of fluorinated cycloalkanes from
readily available substrates including building blocks used
for the production of fine chemicals and pharmaceuticals.
hydrogenation
of
various
fluoroaromatics
to
fluorocycloalkanes using hexane as solvent and molecular
sieves or SiO2 as additives.15 While the present study was in
progress, the Glorius group reported that the Rh–carbene
complex acts as pre-catalyst (in agreement with studies by
Bullock et al.),16 decomposing under reaction conditions to Results and discussion
form polydisperse Rh NPs (2–10 nm) on SiO2.17 It was
demonstrated that the presence of the carbene ligand in the
precursor was required to obtain materials exhibiting high
chemoselectivity. High yields of fluorinated cyclohexanes
were obtained using the resulting Rh@SiO2 material at Rh
loading of 1–5 mol% (relative to substrate) and H2 pressure
of 50 bar within 24 h. The stability and recyclability of the
catalytic system was not yet addressed in this study.17
Despite these promising advances, the development of
catalytic systems possessing high activity, selectivity, stability,
and recyclability for the hydrogenation of fluoroarenes to
The design and synthetic methodology for the catalyst
materials is depicted in Fig. 1a. The preparation of
molecularly functionalized silica supports (Si–R) was
accomplished through the condensation of commercial alkyl-
triethoxysilanes on dehydroxylated SiO2 (see ESI† for full
synthetic details).19 The hydrophobicity of the surface was
modulated by variation of the length of the alkyl chains
between C3 to C10 including also a perfluorinated C10-chain.
The generation of Rh NPs on the Si–R supports to produce
Rh@Si–R catalysts was accomplished by adapting a method
previously reported for NPs@SILP materials.20 This involved
the wet impregnation of the Si–R supports (0.50 g) with a
solution of [Rh(allyl)3] (11.3 mg, 0.05 mmol) in
dichloromethane (DCM) (2 mL). After evaporation of the
solvent under vacuum, the impregnated powder was
subjected to an atmosphere of H2 (50 bar) at 100 °C for 16 h.
Under these conditions, the bright yellow powder turned
black indicating the reduction of the organometallic
precursor and the formation of Rh NPs. The same method
was used to synthesize Rh NPs on the unmodified silica,
Rh@SiO2, for direct comparison.
fluorocyclohexanes remains
a major challenge. In this
context, working with metal NPs immobilized on molecularly
modified supports seems particularly attractive since these
materials were demonstrated to be versatile and suitable for
the production of catalytic systems with tailor-made reactivity
for challenging hydrogenation and hydrodeoxygenation
reactions.18 We report here the rational design of Rh
nanoparticles on hydrophobic silica supports (Rh@Si–R),
exploiting the molecular control over the NPs environment at
the support as determining factor to tune the selectivity of
the reaction. In addition, we address the formation of traces
Fig. 1 (a) Design and synthetic methodology for the preparation of Rh@Si–R materials with R = Prop (n-C3H7); Oct (n-C8H19), Dec (n-C10H21) and
F-Dec (n-(C2H4)C8F19); (b) schematic representation of Rh@Si–Dec, and (c) scanning transmission electron microscopy with high angle annular dark
field (STEM-HAADF) image of Rh@Si–Dec and the corresponding particle size distribution histogram.
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Characterization of Rh@Si–Dec by STEM-HAADF revealed
the formation of small, uniform, and well-dispersed NPs with
a mean diameter of 0.9 ( 0.1) nm (Fig. 1). The Rh loading on
Rh@Si–Dec was determined to be 0.92 wt% by ICP-AAS, well
in agreement with the theoretical value (1 wt%). Surface area
as determined by BET analysis was with 311 m2 g−1 somewhat
lower than for the starting SiO2 (342 m2 g−1), as expected due
to the chemisorption of the alkylsilanes. Characterization of
the other catalysts gave similar results with comparable
nanoparticles sizes, showing no significant variation of the
NPs size depending on the alkyl chain length. (Fig. S1–S4 and
Table S1†). Solid-state 29Si NMR of Si–Dec and Rh@Si–Dec
(Fig. S5†) showed the presence of two types of Si species: (1)
tetra-functionalized (Q) signals at −109 ppm (Q4 = Si(OSi)4)
and −101 ppm (Q3 = Si(OSi)3OH); and (2) tri-functionalized
signals at −56 ppm (T2 = R-Si(OSi)2OR′) and – 49 ppm (T1 = R-
Si(OSi)(OR′)2). The T2 and T1 signals correspond to the Si
atoms of the alkyl-triethoxysilanes bound to the SiO2 surface
and thus provide evidence for the covalent attachment of the
alkyl chains on the silica support.
The catalytic activity of Rh@SiO2 and Rh@Si–R catalysts
was tested using fluorobenzene (1) as a model substrate
(Table 1). A preliminary screening of the reaction parameters
using Rh@Si–Dec as catalyst (Tables S2–S5†) resulted in the
definition of a standard set of conditions: 10 bar H2, 80 °C, 1
h, 500 rpm in heptane. In agreement with previous literature
reports,14a,15 heptane was found to be the most suitable
among the solvents tested. Using Rh@SiO2, a fairly good
conversion of 1 (75%) and selectivity (84%) to the fluorinated
cycloalkane 1a were observed corresponding to a yield of
63% for the desired product (entry 1) for a 1 h reaction. Full
conversion was reached after extending the reaction time to 2
h, with 79% selectivity for the desired product (entry 2). The
molecularly modified Rh@Si–R catalysts proved to be more
active, reaching full conversion in 1 h in all cases at
significantly higher selectivity. The selectivity toward the
formation of 1a increased significantly when increasing
length size of the alkyl chain from n-propyl to n-octyl and
n-decyl (entry 3–5). Using SiO2 modified with 1H,1H,2H,2H-
perfluorodecyltriethoxysilane as support did not lead to
improvement of the selectivity (entry 6). An excellent yield of
1a of 92% was obtained with Rh@Si–Dec, exceeding the
values obtained previously with any other catalyst.14,15 The
superior performance of the molecularly modified support
was confirmed also using ethylfluorobenzene (2), for which
Rh@SiO2 gave much lower selectivity than the Rh@Si–R
catalysts (Table S6†). Again, Rh@Si–Dec showed with 85%
the highest selectivity at full conversion, while the use of
Rh@SiO2 resulted in the lowest selectivity (65%, 61% yield).
These results demonstrate that the use of molecularly
modified supports allows control of the NPs environment to
promote
the
selective
hydrogenation
over
the
hydrodefluorination. Therefore, Rh@Si–Dec was selected as
catalyst for further studies.
Despite the high selectivity achieved with Rh@Si–Dec,
small amounts of HF will still be released due to the
competing hydrodefluorination.14a For practical applications,
this raises concerns regarding safety aspects and material
compatibility of the reactors (stainless steel autoclaves).
Furthermore, it has been noted that this can lead to acid
catalyzed side-reactions.14a Importantly, HF can also react
with the support and/or the Rh-particles18d altering and
ultimately depleting the catalyst performance. Indeed, it was
found that catalyst stability was limited using the Rh@Si–Dec
under standard conditions. Upon recycling the catalyst
material, 77% of selectivity for the fluorinated product (1a)
were obtained during the first two runs, after which a
significant drop to 74% (third run) and 61% (fourth run)
were noted (Fig. 2a). Analysis by STEM-HAADF of the spent
catalyst after the second run revealed a significant increase
in the particle size (2.1 0.3 nm) with the formation of some
aggregates (Fig. S6†). After the fourth run, the Rh NPs were
found completely aggregated (Fig. 2b), and characterization
by solid-state 29Si NMR showed major changes with the
almost complete disappearance of the tri-functionalized T1
and T2 signals observed on the starting Rh@Si–Dec material,
indicating a loss of the molecular modifiers (Fig. S7a†).
In order to address these limitations in a straightforward
manner, we attempted to use catalytic amounts of CaO as
additive to the catalytic system. While CaO is well established
Table 1 Hydrogenation of fluorobenzene (1) using Rh@SiO2 and Rh@Si–R catalysts
#
Catalyst
X (%)
S1a (%)
Y1a (%)
Y1b (%)
1
Rh@SiO2
Rh@SiO2
Rh@Si–Prop
Rh@Si–Oct
Rh@Si–Dec
Rh@Si–Fdec
75
>99
99
>99
>99
>99
84
79
69
81
92
82
63
79
68
81
92
82
12
21
31
19
8
2a
3
4
5
6
18
Reaction conditions: catalyst (5 mg, 0.0005 mmol Rh), fluorobenzene (0.2 mmol, 19.0 mg, 400 eq.), n-heptane (750 mg, ≈1 mL), 10 bar H2, 80
a
°C, 1 h, 500 rpm. 2 h. Conversion and selectivity determined by GC-FID using tetradecane as an internal standard (X: conversion/S:
selectivity/Y: yield).
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Fig.
2 Study of the stability of Rh@Si–Dec through recycling
experiments. a) Catalytic results, b) STEM-HAADF of Rh@Si–Dec after 4
cycles. Reaction conditions: Rh@Si–Dec (20 mg, 0.002 mmol Rh),
fluorobenzene (2 mmol, 192 mg, 1000 eq.), n-heptane (1300 mg, ≈2
mL), 10 bar H2, 80 °C, 1 h, 500 rpm. Catalyst decanted, washed with
heptane (1 mL) and dried under argon flow for 1 h between each run.
The first cycle was performed using a fresh catalyst. Product yield
determined by GC-FID using tetradecane as an internal standard.
Fig. 3 Time profile for the conversion of fluorobenzene (1) using
Rh@Si–Dec in the presence of CaO. Reaction conditions: Rh@Si–Dec (6
mg, 0.0006 mmol Rh, 0.05 mol% relative to substrate), fluorobenzene
(1.2 mmol, 115 mg, 2000 eq.), n-heptane (750 mg, ≈1 mL), CaO (4.5
mol% relative to substrate), 10 bar H2, 80 °C, 500 rpm. Conversion and
selectivity determined by GC-FID using tetradecane as an internal
standard. TOFs calculated based on the total amount of Rh as well as
on the estimated percentage of Rh atoms at the surface of the NPs,
see ESI† for details.
as sorbent for SO2 capture in many industrial processes,21
a
few reports also highlighted its efficiency as scavenger for HF
in waste streams22 leading to formation of CaF2 and water.
The addition of CaO to the reaction mixture did not affect
the product distribution in the hydrogenation of 1. This shows
that the performance of Rh@Si–Dec is not influenced negatively
by the presence of this fluoride scavenger (Table S7†). This is in
contrast to other bases such as HCOOK and K2CO3 which have
been shown to promote hydrodefluorination by facilitating the
formation of HF in homogeneous and heterogeneous catalyzed
processes.23 Characterization by XRD of the mixture after
catalysis evidenced diffraction peaks characteristic of CaF2,
confirming the ability of CaO to trap HF under these conditions
(Fig. S8†). Thus, CaO was systematically evaluated as additional
stabilization component in the catalytic system.
material, indicating that the molecular modification of the
surface was conserved under these conditions (Fig. S7b†). To
the best of our knowledge, this is the first catalytic system
To get further insight into the reaction pathway, a time
profile was recorded for the conversion of 1 in the presence
of CaO. For that, the conditions were slightly modified to
slow down the reaction and allow the collection of sufficient
data points (Fig. 3). The results show a mixture of 1a (44%)
and 1b (5%) after 30 min, and full conversion with 91% yield
of 1a after 120 min, with high TOF. Thus, the hydrogenation
and hydrodefluorination occur in parallel whereby the
hydrodefluorination takes place either before or during the
hydrogenation of the aromatic ring. In addition, no
consecutive hydrodefluorination of the fluorocyclohexane 1a
to 1b was observed.
Most importantly, the addition of CaO greatly increased the
stability of the Rh@Si–Dec catalyst allowing for effective
recycling. As shown on Fig. 4, the Rh@Si–Dec/CaO catalyst
system was able to produce fluorocyclohexane (1a) in high
Fig.
4 Study of the stability of Rh@Si–Dec through recycling
yield and constant selectivity for at least
5
cycles.
experiments in the presence of CaO. (a) Catalytic results, (b) particle
size distribution histogram corresponding to (c) TEM of Rh@Si–Dec
after 5 cycles. Reaction conditions: Rh@Si–Dec (20 mg, 0.002 mmol
Rh), fluorobenzene (2 mmol, 192 mg, 1000 eq.), CaO (36 mg, 0.64
mmol), n-heptane (1300 mg, ≈2 mL), 10 bar H2, 80 °C, 1 h, 500 rpm.
Catalyst decanted, washed with heptane (1 mL) and dried under argon
flow for 1 h between each run. The first cycle was performed using a
fresh catalyst. Product yield determined by GC-FID using tetradecane
as an internal standard.
Characterization of Rh@Si–Dec by TEM after 5 cycles showed
that the Rh NPs remained small and well dispersed on the
support, despite a small increase in size (1.8
0.6 nm).
Elemental analysis by ICP-AAS did not evidence any leaching
of the metal during the reaction (Table S1†). Characterization
by solid-state 29Si NMR showed similar tri-functionalized T1
and T2 signals as observed on the starting Rh@Si–Dec
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possessing high activity (0.1 mol% catalyst) and selectivity for
the hydrogenation of fluoroarenes that can be effectively
reused over several cycles.
The scope of potential applications of Rh@Si–Dec was
studied for a selection of fluorinated aromatic substrates
(Table 2). All the substrates considered were effectively
hydrogenated under optimized reaction conditions, giving
fluorinated cyclohexane derivatives in good to excellent yields
with stereoselectivities in the range of what is typically
observed for arene hydrogenation.
The catalyst was found tolerant to various functional
groups. In some cases, an increase of the temperature to 100
Table 2 Hydrogenation of fluorobenzene (1a) using Rh@Si–R catalysts
Entry
1
Substrate
Eq.
T °C
P (bar)
X (%)
>99
Product a
Ya (%)
cis : trans
Yb (%)
1000
80
10
92
—
8
2
3
4
5
400
500
400
400
80
100
80
10
10
10
10
>99
>99
>99
>99
85
77
84
75
77 : 23
88 : 12
91 : 9
15
23
16
25
80
75 : 25
6
7
400
800
100
80
10
10
>99
>99
85
92
73 : 27
89 : 11
15
8
8
100
200
80
80
55
55
>99
>99
88
75
79 : 21
71 : 29
12
25
9a
10a
11a
200
250
100
80
55
55
>99
>99
70
70
80 : 20
78 : 22
30
30
Reaction conditions: Rh@Si–Dec (5 mg, 0.0005 mmol Rh), n-heptane (750 mg, ≈1 mL), substrate (0.05–0.5 mmol), CaO (4.5 mol% relative to
a
substrate), 80–100 °C, 10–55 bar H2, 1 h, 500 rpm. 2 h. Eq: refers to the equivalence number. Conversion and yield determined by GC-FID
using tetradecane as an internal standard. X: conversion/Y: yield.
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°C was necessary to reach higher selectivity for the
fluorinated product (substrates 3, 6 and 10). No significant
change in stereoselectivity was observed when tuning the
temperature in the 80–100 °C range. A low H2 pressure of 10
bar was sufficient for the selective conversion of substrates 1
to 7. Substrates 8–11 could only be effectively converted when
raising the pressure to 55 bar.
390919832. Furthermore, the authors thank Yasmin
Eisenmann for her support with the experiments, Alina
Jakubowski, Annika Gurowski, Justus Werkmeister, Norbert
Pfänder (MPI-CEC, Mülheim/Ruhr), Adrian Schlüter, Claudia
Weidenthaler, Jan Ternieden (MPI-Kofo, Mülheim/Ruhr) for
their support with the analytics. Open Access funding
provided by the Max Planck Society.
To the best of our knowledge, this is the first report of the
selective hydrogenation of substrates 2, 4–7 and 9 (see ESI†
for characterization). In particular, this new single-step
pathway for the production of 4-fluorocyclohexane carboxylic
acid (9a) is noteworthy since 9a is a key intermediate for the
preparation of various bio-active molecules.24 Interestingly,
9a, the (TMS)-protected alcohol (10a) and the (Boc)-protected
amine (11a) fluorinated cyclohexane could be also isolated
and characterized by NMR (see ESI†). These compounds are
important building blocks used in the synthesis of
pharmaceuticals3e including those highlighted in Scheme 1
that serve as 5-HT2(c) receptor modulator drug3b and
potassium channel modulators,3d respectively. Moreover, 11a
is used in the synthesis of analogs of the anticancer drug
lomustine and of the mucolytic agent bromhexine.15
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Conclusions
In conclusion, Rh nanoparticles (NPs) on molecularly
modified silica supports (Rh@Si–R) were developed as
catalysts for selective hydrogenation of fluorinated arenes to
fluorocyclohexanes. Our results demonstrate that controlling
the hydrophobicity of the intimate environment of the NPs is
of critical importance to favor the hydrogenation over the
hydrodefluorination pathway. The molecular approach to
material preparation offers rational control over the
properties of the support and the generation of well-defined
metal particles. Adding CaO as HF scavenger to the catalytic
system was found highly effective to prevent side-reactions
and catalyst degradation, thus improving the recyclability of
the catalytic system while avoiding the risks associated to the
presence of HF. Overall, the Rh@Si–Dec (Dec = n-C10H21)
catalyst combines high activity, selectivity and stability/
recyclability for the hydrogenation of a large scope of
fluorinated arenes, providing access to a broad range of
fluorinated cyclohexane derivatives. We believe that this
approach combining NPs on molecularly modified supports
and HF scavenger will facilitate the development of next-
generation catalysts for this challenging transformation.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors acknowledge financial support by the Max Planck
Society and by the Deutsche Forschungsgemeinschaft (DFG,
German Research Foundation) under Germany's Excellence
Strategy – Exzellenzcluster 2186 “The Fuel Science Center” ID:
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