Influence of the Ionic Liquid on the Activity of a Supported Ionic Liquid Phase FeII Pincer Catalyst for the Hydrogenation of Aldehydes

Article abstract of DOI:10.1002/ejic.201900636

The catalytic hydrogenation of different aldehydes to the corresponding alcohols was investigated using an Fe^II hydride pincer complex as catalyst in the supported ionic liquid phase (SILP) reaction mode. Two different ionic liquids of the type [X₄₄₄₁][NTf₂] with X=N or P were applied with mesoporous silica gel as support, which was coated first with a chemisorbed monolayer of the corresponding modified IL to remove acidic surface OH-groups and to prevent IL leaching. Quantitative conversion with turn-over frequencies in the order of 1000 h^{– 1} were obtained for various aromatic and heteroaromatic aldehydes and highly selective aldehyde reduction was observed also for substrates containing reducible C=C bonds. Aldehydes with longer aliphatic chains or cycloalkyl substituents, however, showed no conversion here, in contrast to a previous study with an imidazolium-based ionic liquid. These differences were ascribed primarily to differences in substrate/ionic liquid interactions. Whereas [N₄₄₄₁][NTf₂] and [P₄₄₄₁][NTf₂] gave essentially identical results for different substrates in single-batch reactions, prolonged use of the catalyst in repeated reaction cycles lead to a quick drop-off in catalyst activity in [P₄₄₄₁][NTf₂], but a continuous, quantitative conversion in [N₄₄₄₁][NTf₂].

Full text of DOI:10.1002/ejic.201900636

A Journal of
Accepted Article
Title: Influence of the Ionic Liquid on the Activity of a SILP Fe(II) Pincer
Catalyst for the Hydrogenation of Aldehydes
Authors: Zita Csendes, Julian Brünig, Nevzat Yigit, Günther
Rupprechter, Katharina Bica-Schröder, Helmuth Hoffmann,
and Karl Kirchner
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900636
Link to VoR: http://dx.doi.org/10.1002/ejic.201900636

10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Influence of the Ionic Liquid on the Activity of a SILP Fe(II)
Pincer Catalyst for the Hydrogenation of Aldehydes
Zita Csendes,1,‡ Julian Brünig,1,‡ Nevzat Yigit,2 Günther Rupprechter,2 Katharina Bica-Schröder,1 Helmuth
Hoffmann,*,1 and Karl Kirchner*,1
1Dr. Z. Csendes, J. Brünig, Prof. Dr. K. Bica-Schröder, Prof. Dr. H. Hoffmann, Prof. Dr. K. Kirchner
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060
Vienna, Austria
2Dr. N. Yigit, Prof. Dr. Rupprechter
Institute of Materials Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna,
Austria
‡These authors contributed equally.
*E-mail: karl.kirchner@tuwien.ac.at
*E-mail: helmuth.hoffmann@tuwien.ac.at.
Abstract
The catalytic hydrogenation of different aldehydes to the corresponding alcohols was investigated
using an Fe(II) hydride pincer complex as catalyst in the supported ionic liquid phase (SILP)
reaction mode. Two different ionic liquids of the type [X4441][NTf2] with X=N or P were applied
with mesoporous silica gel as support, which was coated first with a chemisorbed monolayer of
the corresponding modified IL to remove acidic surface OH-groups and to prevent IL leaching.
Quantitative conversion with turn-over frequencies in the order of 1000 h-1 were obtained for
various aromatic and heteroaromatic aldehydes and highly selective aldehyde reduction was
observed also for substrates containing reducible C=C bonds. Aldehydes with longer aliphatic
chains or cycloalkyl substituents, however, showed no conversion here, in contrast to a previous
study with an imidazolium-based ionic liquid. These differences were ascribed primarily to
differences in substrate/ionic liquid interactions. Whereas [N4441][NTf2] and [P4441][NTf2] gave
essentially identical results for different substrates in single-batch reactions, prolonged use of the
catalyst in repeated reaction cycles lead to a quick drop-off in catalyst activity in [P4441][NTf2], but
a continuous, quantitative conversion in [N4441][NTf2].
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Introduction
Over the past 15 years, supported ionic liquid phase (SILP) catalysis has emerged as a
powerful hybrid technology of classical homogeneous and heterogeneous reaction techniques.1
Therein, the catalyst is dissolved in an ionic liquid and is applied as a thin film coated onto a high
surface area support material. This liquid-impregnated support can be applied just like a
heterogeneous catalyst either as a powder, a packed cartridge or a monolithic structure and contains
the catalyst in an easily separable and reusable phase. The catalyst itself is dissolved in the ionic
liquid and is believed to retain its chemical nature, its activity, and selectivity analogous to a
classical, homogeneous environment. Previous studies have shown, however, that this assumption
is rarely fulfilled and that the unique multiphase environment of a SILP catalyst entails several
peculiarities which influence the catalyst’s performance: First, ionic liquids are highly polar
liquids, which tend to display complex structural patterns at or near interfaces such as domain
formation, surface segregation, solvent cages etc.2 The ionic liquid layer of a SILP catalyst which
is typically only a few nanometers thick can therefore hardly be considered as a bulk solution phase
and an inert reaction medium like the solvents that are typically used in homogeneous catalytic
reactions. Second, physical support properties like pore size, pore volume, and wetting properties
determine how the ionic liquid film spreads across the support surface and how well it adheres to
the support.3 Third, unfavorable cross solubilities of catalyst and ionic liquid in the reactant
solution and of substrate and product in the ionic liquid layer can shift the reaction equilibrium
and slow down or stop the reaction. Fourth, functional groups at the support surface might interact
or react with the catalyst and influence its activity.4 The knowledge of how the catalyst and the
reaction mechanism are influenced by these parameters is still very limited and requires more
comparative studies of catalytic reactions under homogeneous and SILP reaction conditions.
We have recently started to expand our studies on base metal pincer catalysts in
homogeneous solution5 to IL-based multiphase reaction techniques such as liquid-liquid biphasic6
and SILP catalysis.7 As a test reaction, the reduction of aldehydes to alcohols using Fe(PNPMe-
iPr)(CO)(H)(Br)] as catalyst was used. In the biphasic mode,6 the catalyst was dissolved in various
ionic liquids A+ [NTf2]- with the weakly coordinating [NTf2]- anion together with different types
of cations A+ (ammonium, phosphonium, imidazolium, pyridinium, picolinium as well as targeted
Bronsted acidic species) and was brought in contact with a second, immiscible organic solvent
phase which contained the substrate. In the SILP mode,7 the catalyst was dissolved in 1-butyl-2,3-
dimethyl-imidazolium bis(trifluoromethylsulfonyl)imide ([bm2im][NTf2]) and was applied as a
thin layer coated onto two different support materials: silica7a and carbon.7b
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
In general, the chemoselectivity of this catalyst for the reduction of aldehydes was
preserved and C=C double bonds or C=O ketone groups were not reduced in both the biphasic and
the SILP mode. However, the catalytic activity and, in particular, the reaction rates (TOF numbers)
were lower compared to the homogeneous reactions and depended strongly on the type of
substrate, the support pretreatment and the particular ionic liquid used.
Silica supports had to be coated first with a chemisorbed layer of ionic liquid in order to
remove acidic surface hydroxyl groups which destroyed the catalyst. Carbon supports could be
used without pretreatment but showed 4-10 times lower reaction rates due to their smaller pore
sizes and the thereby reduced active area of the ionic liquid film. The influence of the ionic liquid
on the catalytic activity was initially investigated in the biphasic reaction mode. Isolated yields
were in the range from < 1% to > 99 % and showed that the ionic liquid has a tremendous influence
on the catalytic function. The best results were obtained with [P4441][NTf2] which yielded complete
conversion at a turn over frequency of 1330 h-1 compared to 2000 h-1 for the homogeneous reaction
mode under otherwise equal conditions. One drawback of the biphasic mode, however, is the
relatively large amount of ionic liquid that is needed, which dissolves a substantial part of the
produced alcohol. The higher solubility of the formed alcohol compared to the starting materials
complicated work-up, and quantitative recovery of the reaction product, therefore, requires
subsequent extraction of the IL phase and prevents a continuous operation in this biphasic
configuration.
In the SILP mode, in comparison, the total amount of ionic liquid is much smaller and
product losses due to dissolution in the ionic liquid phase are usually negligible. For this work, we,
therefore, chose two ionic liquids - [P4441][NTf2] and [N4441][NTf2] - which gave the best results
in the biphasic mode and tested them for SILP catalysis. By comparison with the biphasic data and
with previous SILP data using an imidazolium-based ionic liquid, we were hoping to gain deeper
insights into the role of the ionic liquid in this model reaction.
Results and Discussion
Surface modification of silica gel
The hydride ligand in the catalyst precursor 1, as well as the hydride ligands of the in situ
generated active catalyst [Fe(PNPMe-iPr)(CO)(H)2] (2), are strongly basic and thus sensitive to
acidic functionalities such as the OH groups of the silica gel. To avoid catalyst decomposition by
this group and to make the surface of silica gel “IL-philic” to prevent IL leaching, the silica gel
was covalently coated, in a three-step procedure, with the corresponding IL (Figure 1). In the first
step, 3-iodopropyltrimethoxysilane was covalently grafted yielding SILP[I]. Then, SILP[I] was
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
reacted with tributylamine or tributylphosphine to yield N-SILP[I] and P-SILP[I], respectively.
Finally, N-SILP[I] and P-SILP[I] were reacted with Li[NTf2] to obtain N-SILP[NTf2] and P-
SILP[NTf2].
From elemental analysis of the nitrogen content, the ionic liquid loadings of the
ammonium- and phosphonium-functionalized silica gels were calculated as 0.22 mmol/g (N-
SILP[NTf2]) and 0.25 mmol/g (P-SILP[NTf2]). With the corresponding BET surface areas (Table
1) the mean surface coverage is about 4.1013 molecules/cm2 for N-SILP[NTf2] and around 6.1013
molecules/cm2 for P-SILP[NTf2]. These values are about one order of magnitude lower than the
densest packing of 5.1014 molecules/cm2 found in long-chain n-alkylsiloxane monolayers on
silica.8 This must be ascribed to the rather bulky ammonium and phosphonium cations in the
present system.
The surface modification sequence depicted in Figure 1 was studied by 29Si{1H}CP-MAS
NMR spectroscopy (Figure 2). Pristine SG (Figure 2A) displays three resonances at –110, –100
and –91 ppm which are characteristic of Si(OSi)4 (Q4), Si(OSi)3OH (Q3) and Si(OSi)2(OH)2 (Q2)
species of the silica framework, respectively.9 Anchoring 3-iodopropyltrimethoxysilane results in
a reduction of the Q3 and Q2 bands (Figure 2B), and a concomitant relative increase of the Q4 peak.
This indicates the reaction of singular and geminal silanol groups with the trimethoxysilane
moiety. The spectrum of SILP[I] in Figure 2B also displays two signals at –60 and –53 ppm, which
clearly confirm that the organic molecule is covalently grafted to the silica surface via either two
RSi(OSi)2(OMe)(T2) or one RSi(OSi)(OMe)2 (T1) siloxane bonds, respectively.
O
O
O Si OH
O Si O
O
toluene
OMe
Si
+
+
O
I
Si
(MeO)3
I
X
OH
O
Si
reflux
48 h
O
O
Si
O
O
SILP[I]
O
O
O Si O
OMe
Si
O Si O
O
Li[NTf2]
X+
OMe
Si
toluene
X+
O
-
Si
I-
CH2Cl2
24 h
O
O
reflux
48 h
Si
NTf2
O
O
O
O
=
=
=
=
X
X
N
P
N-SILP[I]
P-SILP[I]
X
X
N
P
N-SILP[NTf2
]
P-SILP[NTf2
]
Figure 1. Step by step functionalization of silica gel.
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
No resonances appeared above –50 ppm, indicating the absence of physically adsorbed
silane molecules. The solid state 29Si NMR spectrum of N-SILP[I] (Figure 2C) revealed that
reacting SILP[I] with tributylamine further promoted the direct condensation of the grafted siloxy
functionalized moiety with silica since the T1 signal at –53 ppm disappeared while the T3
(RSi(OSi)3) signal at –68 ppm appeared in the spectrum. It means that the functionalized ionic
liquid is grafted via two or three siloxane bonds to the surface of the support. The 29Si NMR
spectrum of P-SILP[I] (Figure 2D), on the other hand, shows only a decrease of the T1 peak
compared to SILP[I], but no T3 signal of triple-bonded RSi(OSi)3 moieties, indicating that
tributylphosphine did not promote the above mentioned further condensation as efficiently as
tributylamine. This observation can be explained by the higher basicity of N(Bu)3 (pKa = 10.9)10
compared to P(Bu)3 (pKa = 8.4).11 Similar effects were observed previously with trimethylamine
and pyridine as base catalysts of surface silanol condensation.12
2
T
1
D
C
4
T
Q
3
Q
2
T
3
T
1
3
T
Q
2
T
B
4
Q
2
Q
A
50
0
-50
-100
-150
-200
δ(ppm)
Figure 2. Solid-state 29Si CP-MAS NMR spectra of (A) pristine SG, (B) SILP[I], (C) N-SILP[I]
and (D) P-SILP[I].
13C CP-MAS NMR spectra also confirm the successful modification of the surface of silica
gel (Figure 3). The spectrum of SILP[I] displays peaks at –0.5 (d), 4.6 (b), 19.2 (c) and 42.1 (a)
ppm that are assigned as (d) the carbon next to I, (b) the carbon next to Si, (c) the center carbon of
the C3 chain, and (a) the carbon in the methoxy group. The peaks marked with asterisks are
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
attributed to ethanol residue. The shift of the peak at –0.5 (d) ppm to the 4–24 ppm region in the
spectra of N-SILP[I] (Figure 3B) and P-SILP[I] (Figure 3C) proves the displacement of iodine13
and thus the reaction of the SiO2-supported propyl iodide with tributylamine and tributylphosphine
under formation of a monolayer of ammonium- and phosphonium-based ionic liquid on the surface
of the support. The decrease of the peak intensity of the methoxy carbon in the spectra 3B and 3C
also confirms that further condensation/hydrolysis took place catalyzed by the basic N(Bu)3 and
P(Bu)3 reactants. The peaks of the propyl and butyl carbon atoms between 4–24 ppm in the spectra
of N-SILP[I] and P-SILP[I] are broadened and not well-resolved. The ion exchange of I- to [NTf2]-
did not change the NMR spectra (not shown). 31P CP-MAS NMR spectroscopy also confirmed the
successful formation of the phosphonium-based IL layer on the surface since the corresponding
peak at 30 ppm can be found in the spectrum (not shown).
P+
I-
P+
I-
MeO Si
O
O
SiO2
MeO Si
N+
I-
O
O
MeO Si
O
O
SiO2
C
B
SiO2
I
MeO Si
O
O
SiO2
N+
I-
MeO Si
O
O
c
SiO2
*
*
I
b
a
d
c
d
b
MeO Si
a
O
O
A
SiO2
200
150
100
50
0
-50
δ(ppm)
13
Figure 3. Solid-state C CP-MAS NMR spectrum of (A) SILP[I], (B) N-SILP[I], and (C) P-
SILP[I].
Additional evidence for the successful modification of the silica support is provided by the
infrared spectra shown in Figure 4. Silica gel pretreated at 400°C yields a spectrum with just one,
highly characteristic absorption at 3748 cm-1 in Figure 4A due to the OH stretching vibration of
isolated surface hydroxyl groups. This absorption disappears, as expected, upon adsorption of
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
iodopropyl trimethoxysilane (Figure 4B), which indicates the removal of these acidic OH-groups.
Concurrently, the ν(CH) stretching absorptions of the propyl group between 3035 cm-1 and 2875
cm-1 appear in the spectrum of Figure 4B together with a characteristic single peak at 2850 cm-1
due to the ν(CH3) vibration of the OCH3 groups.14 The latter absorption indicates incomplete
hydrolysis and condensation of the –Si(OCH3)3 anchor groups, in agreement with the previously
discussed 29Si and 13C NMR spectra. The subsequent coupling of N(Bu)3 and P(Bu)3, respectively,
to the surface (spectra 4C and 4E) yields changes in the ν(CH) stretching absorptions around 3000
cm-1 and the δ(CH) deformation modes (1400 cm-1-1500 cm-1) and, most noticeably, the
disappearance of the methoxy ν(CH3) absorption at 2850 cm-1, indicating a base-catalyzed
progression of methoxy group hydrolysis and condensation. This has also been attested before in
the 29Si and 13C NMR spectra. Finally, the ion exchange step [I]– → [NTf2]– is evidenced by the
intense absorption of νas(SO2) at 1348 cm-1 and νas(CF3) at 1196 cm–1.15
υ (SO2)
as
F
E
υ (SO2)
as
D
C
υ(CH)
alkyl
υ(CH)
methoxy
B
υ(OH)
A
3600
3200
1800
1600
1400
-1
Wavenumber (cm )
Figure 4. FT-IR spectrum of (A) SG, (B) SILP[I], (C) N-SILP[I], (D) N-SILP[NTf2], (E) P-
SILP[I], and (F) P-SILP[NTf2].
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Impregnation of modified silica gel with ionic liquid/catalyst
In order to find the optimum pore filling, N-SILP[NTf2] and P-SILP[NTf2] were
impregnated with 10, 20, 30, and 40 wt% of 1 dissolved in ([N4441][NTf2]) and ([P4441][NTf2])
producing N-SILP10, N-SILP20, N-SILP30, N-SILP40 and P-SILP10, P-SILP20, P-SILP30, P-
SILP40, respectively. These materials were characterized by N2 adsorption-desorption (Figure 5).
The obtained isotherms are type IV isotherms according to the IUPAC classification with capillary
condensation taking place between p/p0 =0.5 and p/p0 =0.8, characteristic of mesoporous solids
with pore diameters between 2 nm and 50 nm. N-SILP40 and P-SILP40 could not be measured
due to the low surface area.
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Figure 5. N2 adsorption–desorption isotherms of N-SILP and P-SILP catalysts with different ionic
liquid loadings ε. A) SILP[NTf2], ε = 0 , B) SILP10, ε = 10%, C) SILP20, ε = 20%, D) SILP30, ε
= 30%.
Structural parameters derived from N2 adsorption-desorption isotherms showed that the
pore volume continuously decreased upon loading with IL/catalyst solution due to the filling of
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
the pores (Table 1). The pore filling degree (α) increases with the IL loading up to 100% for
SILP40s.
Table 1. Structural parameters calculated from the N2 adsorption-desorption isotherms
sample
BET
surface
areaa
(m2/g)
325
pore
average
pore
diameterc
(nm)
5.4
α
(%)d
volumeb
(cm3/g)
N-SILP[NTf2]
N-SILP10
N-SILP20
N-SILP30
N-SILP40
P-SILP[NTf2]
P-SILP10
0.50
0.26
0.15
0.02
138
4.8
17
66
5.2
38
4
14.0
65
too low to measure
~100
269
112
55
0.39
0.21
0.12
0.01
4.3
4.7
5.0
16.9
21
P-SILP20
48
P-SILP30
2
83
P-SILP40
too low to measure
~100
b
c
aCalculated by the BET equation. BJH pore desorption volume. Desorption average pore
diameter. dPore filling degree (IL volume/pore volume)*100.
Catalytic aldehyde hydrogenation
The hydrogenation of 4-fluorobenzaldehyde to 4-fluorobenzyl alcohol was used as a test
reaction to investigate the performance of SILP catalysis with two different ionic liquids
[N4441][NTf2] and
[P4441][NTf2]. 50 bar H2 pressure, 5 mol% of DBU (1,8-diaza-
bicyclo[5.4.0]undec-7-ene) as base in n-heptane at 25oC were applied. The results are shown in
Table 2. The reaction times were chosen to achieve quantitative conversion for each entry. No
conversion was obtained without catalyst. The N-SILP and the P-SILP systems showed essentially
equal performance. An approximately linear decrease of reaction time and a corresponding
increase of TOFs with IL loading was observed (entries 1-4 and 6-9). The SILP 40 systems,
however, which gave the highest TOF values (entries 4 and 9), showed substantial leaching of both
the catalyst and the ionic liquid into the n-heptane solution, as evidenced from ICP-OES and
19F{1H} NMR measurements. In that case, a partial homogeneous hydrogenation in the n-heptane
solution is likely the reason for the high reaction rates.
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Table 2. SILP hydrogenation of 4-fluorobenzaldehyde in different ionic liquids: [N4441][NTf2] (N-
SILP), [P4441][NTf2] (P-SILP) and [bm2im][NTf2] (Im-SILP)
H
Me
N
PiPr2
CO
cat
H
N
Fe
,
2
OH
O
DBU mol%)
(5
N PiPr2
1
25o
F
F
n
C
Me
-heptane,
Br
entry conditionsa S/C
P
time
conversion
TON TOF
(h-1)
IL leaching
(bar)
(min) (%)
1
2
3
4
5
6
N-SILP10 200
N-SILP20 200
N-SILP30 200
N-SILP40 200
N-SILP20 1000
P-SILP10 200
50
50
50
50
50
50
20
15
15
5
>99
>99
>99
>99
>99
>99
200
200
200
200
1000
200
600
no
no
no
yes
no
no
800
800
2400
1333
600
45
20
7
P-SILP20 200
P-SILP30 200
P-SILP40 200
P-SILP20 1000
Im-SILP20 1000
50
50
50
50
50
15
10
10
45
15
>99
>99
>99
>99
>99
200
800
no
no
yes
no
no
8
200
1200
1200
1333
4000
9
200
10
11
1000
1000
aConditions: entry 1,6: 2 mmol substrate, 1000 mg SILP10 (5 mg 1, 95 mg IL, 900 mg SILP[NTf2]); entry 2,7: 2 mmol
substrate, 500 mg SILP20 (5 mg 1, 95 mg IL, 400 mg SILP[NTf2]); entry 3,8: 2 mmol substrate, 333 mg SILP30 (5
mg 1, 95 mg IL, 233 mg SILP[NTf2]); entry 4,9: 2 mmol substrate, 250 mg SILP40 (5 mg 1, 95 mg IL, 150 mg
SILP[NTf2]); entry 5,10: 10 mmol substrate, 500 mg SILP20 (5 mg 1, 95 mg IL, 400 mg SILP[NTf2]); entry 11: taken
from Ref. [7a].
Increasing the S/C ratio from 200 to 1000 resulted in an increase of TON and TOF values
to 1000 and 1333 h–1, respectively, with quantitative conversion within 45 min (entries 5 and 10).
In our previous study7a, this reaction was carried out under the same conditions using an
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
imidazolium-based IL modified silica gel as support and ([bm2im][NTf2]) as IL. In that case, three-
fold faster conversion was observed (entry 11).
In order to test the recyclability of the catalyst in this reaction, we reused the N-SILP20
and P-SILP20 systems by repeated addition of aldehyde after each reaction cycle (Figure 6). This
procedure was carried out four times every 7 minutes. It was found that N-SILP20 could be
recycled yielding a TON of 1000 and a TOF of 1714 h-1, while P-SILP20 lost almost completely
its activity after the first cycle. This apparent poisoning of the catalyst dissolved in [P4441]][NTf2]
might be related to the presence of residual surface OH groups, which was indicated by a
comparison of the 29Si NMR spectra of P-SILP20 and N-SILP20 (Figure 2). A similar effect was
also observed in our previous study on the SILP-catalyzed continuous formation of cyclic
carbonates, were residual surface OH were responsible for the rapid loss of activity over time.16
Figure 6. Catalyst recycling for the SILP hydrogenation of 4-fluorobenzaldehyde in different ionic
liquids. (A) [N4441][NTf2] and (B) [P4441][NTf2]. Conditions: 500 mg of N-SILP20 (A) and P-
SILP20 (B), 2 ml n-heptane, 2 mmol 4-fluorobenzaldehyde/step, 0.5 mol % of catalyst 1, 5 mol %
of DBU, 25°C, 50 bar H2.
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
N-SILP20 and P-SILP20 have also been tested with other substrates (Table 3) in the
presence of 0.5 mol % of catalyst, 5 mol % of DBU at 25 °C under 50 bar hydrogen pressure and
compared to the corresponding Im-SILP20 system.7a It was found that under these reaction
conditions, all three SILP20s could efficiently catalyze the hydrogenation of aromatic and
heteroaromatic aldehydes bearing both electron-withdrawing halogen groups such as F, Cl, and Br
in 4-halobenzaldehydes or electron-donating substituent such as OMe in 4-anisaldehyde.
Table 3. Achieved conversions in SILP hydrogenations of different aldehydes using catalyst 1
dissolved in different ionic liquids [N4441][NTf2] (N-SILP), [P4441][NTf2](P-SILP) and
[bm2im][NTf2] (Im-SILP)a
N-SILP20, P-SILP20, Im-SILP20
R
O
R
OH
O
N
O
S
O
O
O
O
R
R
=
Br,
Cl,
OMe
H, F,
>98|>98|>98b
>98|>98|>98b
>98|>98|>98b
>98|>98|>98b
O
>98|>98|>98b
OH
O
O
O
O
>98|>98|>98b
>98|>98|>98b
90|90|>98b
O
O
O
O
0|0|>98b
0|0|>98b
0|0|>98b
O
O
O
O
O
0|0|>98b
0|0|>98b
0|0|>98b
0|0|>98b
a Conditions: 500 mg SILP20, 2 mL n-heptane, 2 mmol substrate, 0.5 mol% 1, 5 mol% DBU, 25
b
oC, 50 bar H2, 1 h. Yields for N-SILP20, P-SILP20 and Im-SILP20 determined by calibrated
GC/MS and 1H NMR spectroscopy with mesitylene as internal standard.
High chemoselectivity was observed for the challenging α,β-unsaturated cinnamaldehyde
since its C=C double bond was not hydrogenated. Aldehydes with longer aliphatic chains or
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
cycloalkyl substituents, however, which were quantitatively reduced using Im-SILP20,7a showed
no conversion with either the N-SILP20 or the P-SILP20 catalyst. This result is believed to be
related to solubility differences and/or specific substrate/ionic liquid interactions. [bm2im][NTf2],
for example, can interact through its aromatic [bm2im]+ cation with aromatic aldehydes via π – π
bonding and with the nonaromatic aldehydes in Table 3, which all contain double bonds, via is
currently further investigated in our group using experimental in-situ spectroscopic methods in
combination with semi-empirical models (COSMO-RS, Kamlet-Taft).
Conclusions
In the present study, the catalytic reduction of aldehydes to alcohols in the SILP mode - using an
Fe(II) PNP pincer complex dissolved in ionic liquid and applied as a thin film on silica gel - was
+
investigated. Two different ionic liquids containing the same anion but different cations - [N4441
]
in N-SILP and [P4441]+ in P-SILP - were applied. The catalytic performance in terms of TON and
TOF values for single reactions as well as for repeated reaction cycles with the same, recycled
catalysts was determined and compared to a previous study of the same reactions using an
imidazolium-based ionic liquid (Im-SILP). Aldehydes with aromatic or heteroaromatic
substituents showed quantitative conversions with all three ionic liquids, although an about three
times lower reaction rate for N-SILP and P-SILP compared to Im-SILP. Aldehydes with larger
alkyl or cycloalkyl groups, on the other hand, gave no conversion with N-SILP and P-SILP in
contrast to quantitative conversion with Im-SILP. Solubility effects and/or specific substrate/ionic
liquid interactions are believed to be the main reason for these differences, although further studies
are needed here. The N-SILP and the P-SILP systems gave very similar results for single reactions.
Catalyst recycling, however, by repeated aldehyde addition worked well only with the N-SILP
catalyst, whereas the P-SILP catalyst became inactive after the first reaction cycle, probably cause
by catalyst poisoning by residual surface hydroxyl groups of the silica support. The trends which
were observed in the biphasic catalytic reactions, where [P4441][NTf2] as the solvent gave the
highest reaction rates followed by [bm2im][NTf2] and [N4441][NTf2], were not replicated in the
SILP mode, where 3-times higher reaction rates were observed in [bm2im][NTf2] compared to
[P4441][NTf2] and [N4441][NTf2]. As mentioned in the introduction, a complex interplay of different
factors, which are still poorly understood till today, determines the performance of a SILP catalyst.
Further studies and molecular information of the interactions between catalyst, substrates, and
products at the reaction interface ionic liquid/organic solution are needed to understand and
optimize this powerful catalytic technique.
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Experimental
Materials
All operations were performed under argon atmosphere by using Schlenk techniques or in an
MBraun inert-gas glovebox. The solvents were purified according to standard procedures.17 The
deuterated solvents were purchased from Aldrich and dried over 4 Å molecular sieves. As support
material for the SILP catalysts, silica gel (SG; silica gel 60, Carl Roth, 400-230 mesh) was used.
The ionic liquids (tributylmethylammonium bis(trifluoromethanesulfonyl)imide ([N4441][NTf2])
and tributylmethylphosphonium bis(trifluoromethanesulfonyl)imide ([P4441][NTf2]) were dried for
at least 24 h at room temperature and 0.01 mbar before use and were stored under an argon
atmosphere; resulting water contents were typically <500 ppm. The complex [Fe(PNPMe-
iPr)(CO)(H)(Br)] (1) was prepared according to the literature.5b All aldehyde substrates were
obtained from commercial sources and purified by distillation prior to use. Hydrogen (99.999%
purity) was purchased from Messer Austria and used as received.
Synthesis of SILPs
N-SILP[NTf2] and P-SILP[NTf2]
As a first step, silica gel was treated at 400°C in an oven for 16h, carefully evacuated and stored
in the glovebox. 5 g of the treated silica gel, 2 g 3-iodopropyltrimethoxysilane, and 15 mL toluene
were refluxed for 48 h. The solvent was carefully removed under reduced pressure. The resulting
SILP[I] was added to an extraction hull in the glovebox. It was brought to an evacuated and flushed
with argon gas 100 mL Soxhlet extractor under a slight flow of argon. The SILP[I] was extracted
with anhydrous ethanol for 16 h. The powder was dried under reduced pressure afterwards. Then,
2 g tributylamine or tributylphosphine in 15 mL toluene was added to 5 g SILP[I] and refluxed for
48 h to prepare N-SILP[I] and P-SILP[I], respectively. The materials were extracted with
anhydrous ethanol for 16 h. For ion exchange, the N-SILP[I] and P-SILP[I] were reacted with 2 g
Li[NTf2] in 10 mL of CH2Cl2 at room temperature for 24 h. They were again extracted with a
Soxhlet extractor as described before. N-SILP[NTf2] and P-SILP[NTf2] were dried under reduced
pressure and stored in the glovebox.
N-SILP10-40 and P-SILP10-40
5 mg of 1, specific amount of N-SILP[NTf2] or P-SILP[NTf2] and 2 mL of anhydrous ethanol were
added to a round bottom flask equipped with a small stirring bar and a vacuum valve in the
glovebox. The reaction mixture was stirred for 1 h. The solvent was carefully removed under
reduced pressure. The powder was dried for an additional 5 min.
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
Material characterization
The C, H, N, S content was double-determined utilizing a Vario Macro elemental analyzer (CHNS-
29
13
31
mode, WLD). Solid-state Si, C, and P CP-MAS NMR measurements were recorded on a
Bruker AVANCE 300 DPX spectrometer, equipped with a 5 mm broadband inverse probe head.
Magic Angle Spinning was performed at 4 kHz spinning rate at 59.57 MHz for 29Si, at 75.40 MHz
for 13C, and at 121.38 MHz for 31P NMR measurements. Textural properties were analyzed by N2
physisorption (ASAP 2020 Micrometics GmbH) using the Brunauer−Emmett−Teller (BET)
theory for determining the surface area and the Barret−Joyner−Halenda (BJH) method for
obtaining the pore size distribution. Infrared spectra were recorded with a Bruker Vertex 80 FTIR
spectrophotometer using a narrow band MCT detector measuring diffuse reflectance. 256 Scans
were collected for each spectrum with 4 cm−1 resolution. 1H, 19F{1H}, and 31P{1H} NMR spectra
1
were recorded on Bruker AVANCE-250 spectrometer. H spectra were referenced internally to
residual protic-solvent resonances and are reported relative to tetramethylsilane (δ = 0 ppm).
19
31P{1H} NMR spectra were referenced externally to H3PO4 (85%) (δ = 0). F{1H} spectra were
referenced externally to trifluorotoluene (0.05%) (δ = 0 ppm). GC-MS analyses were recorded on
an ISQ LT Single quadrupole MS (Thermo Fisher) directly connected to a TRACE 1300 Gas
Chromatograph (Thermo Fisher), using a Rxi-5Sil MS (30 m, 0.25 mm ID) cross-bonded dimethyl
polysiloxane capillary column. Fe leaching was monitored using an inductively coupled plasma
(ICP) optical emission spectrometer PerkinElmer OPTIMA 8300 equipped with an SC-2 DX
FAST sample preparation system. A customized single-element (Merck, Roth) standard was used
for the calibration. All samples were extracted using ethanol and methanol (two times each),
followed by solvent-evaporation and acid-digestion (HNO3 and H2O2 at a 2:1 ratio).
General procedure for the hydrogenation of aldehydes
All hydrogenation reactions were carried out in a Carl Roth 100 mL stainless steel autoclave
containing a 20 mL glass vial equipped with a stirring bar. In the glovebox, freshly prepared
SILP20 was added to the glass vial which was sealed with a septum afterwards. 2 mmol substrate,
5 mol% DBU and 2 mL n-heptane were mixed and taken up in a syringe. The vial was added to
the autoclave, the septum was removed and the autoclave was evacuated and flushed 3 times with
argon gas. The solution was injected into the autoclave under a slight flow of argon. The autoclave
was then purged three times with hydrogen gas before the final pressure was adjusted to the
specified value and the reaction was carried for the stated time. Afterward, the hydrogen gas was
released and the vial was taken out of the autoclave. The reaction mixture was three times extracted
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10.1002/ejic.201900636
European Journal of Inorganic Chemistry
with 1.5 mL diethyl ether, decanted and filtered over aluminum oxide. The solvent was then slowly
1
removed under reduced pressure. The residue was analyzed by H and/or 19F{1H} NMR
spectroscopy and yields were determined by integration of the aldehyde and alcohol signals and
referenced to mesitylene. If the yields were detected by GC-MS, one drop of the extracted reaction
solution was taken out and filled up with 1 mL of CH2Cl2 and injected via a 4 mm syringe filter
(PTFE membrane, 0.2 µm pore size) into a GC-MS vial which was then sealed with a septum.
Acknowledgments
J.B., Z.C., N.G., and K.K. thank the Austrian Science Fund (FWF) for the financial support
through projects M 2068-N28 (Z.C.) and P 28866-N34 (J.B., N.G., K.K.). N.Y. and G.R. received
support by the Austrian Science Fund (FWF; DK+ Solids4Fun W1243).
Keywords: Hydrogenation, Iron complex, Supported catalysts, Supported ionic liquid phase,
Sustainable chemistry
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European Journal of Inorganic Chemistry
Graphical Abstract
Key Topic: Chemoselective Hydrogenation
TOC Text:
Ammonium- and phosphonium-based ionic liquids containing supported ionic liquid phase (SILP)
catalysts were prepared using an Fe(II) hydride pincer complex as catalyst and mesoporous silica
gel as support. Both catalysts showed high activity and chemoselectivity in the hydrogenation of
various aromatic and heteroaromatic aldehydes.
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