Selective Hydrogenation of Aldehydes Using a Well-Defined Fe(II) PNP Pincer Complex in Biphasic Medium

Article abstract of DOI:10.1002/cctc.201800841

A biphasic process for the hydrogenation of aldehydes was developed using a well-defined iron (II) PNP pincer complex as model system to investigate the performance of various ionic liquids. A number of suitable hydrophobic ionic liquids based on the N(Tf)₂^? anion were identified, allowing to immobilize the iron (II) catalyst in the ionic liquid layer and to facilitate the separation of the desired alcohols. Further studies showed that targeted Br?nsted basic ionic liquids can eliminate the need of an external base to activate the catalyst.

Full text of DOI:10.1002/cctc.201800841

Full Papers
DOI: 10.1002/cctc.201800841
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Selective Hydrogenation of Aldehydes Using a
Well-Defined Fe(II) PNP Pincer Complex in Biphasic
Medium
[a]
[a]
[b]
[b]
¨
Stefan Weber, Julian Bru¨nig, Veronika Zeindlhofer, Christian Schroder,
[c]
[d]
[a]
[a]
¨
Berthold Stoger, Andreas Limbeck, Karl Kirchner,* and Katharina Bica*
A biphasic process for the hydrogenation of aldehydes was
developed using a well-defined iron (II) PNP pincer complex as
model system to investigate the performance of various ionic
liquids. A number of suitable hydrophobic ionic liquids based
iron (II) catalyst in the ionic liquid layer and to facilitate the
separation of the desired alcohols. Further studies showed that
targeted Brønsted basic ionic liquids can eliminate the need of
an external base to activate the catalyst.
ꢀ
on the N(Tf)2 anion were identified, allowing to immobilize the
20 Introduction
attractive candidate for the formation of well-defined base
metal catalysts.[2] While iron-based catalysts typically show a
rather low reactivity compared to noble metals catalysts, the
development of novel and finely tuned ligands systems in this
rapidly expanding research field can compensate this issue.
Nowadays, a broad variety of transformations, including the
industrially important chemoselective reduction of polar double
bonds such as aldehydes can be realized with high efficiency.[3]
A milestone in the development of highly active iron-based
catalysts for the reduction of carbonyl groups was the
implementation of pincer ligands, providing a basis for the mild
and chemoselective iron-catalyzed reduction of aldehydes,
ketones, imines but also of CO2 under benign conditions and
moderate hydrogen pressure.[4] In 2014, Kirchner and co-work-
ers developed the iron (II) pincer complex [Fe(PNPMe-iPr)(-
CO)(H)(Br)] (I) that showed remarkable reactivity and chemo-
selectivity for the reduction of aldehydes.[5] The first step of the
catalytic cycle is the activation of pre-catalyst I to form a
mixture of cis- and trans-dihydride complexes [Fe(PNPMe-
iPr)(CO)(H)2] (II) upon addition of dihydrogen in the presence of
base, with the trans isomer being the catalytically active species
(Figure 1). Key step is the regeneration of the catalytically active
21
22 The design and application of new catalysts and catalytic
23 systems is a key driver of sustainable chemistry and a constant
24 field of innovation. To date, the chemical industry ranging from
25 bulk to fine chemical production relies heavily on the use of
26 metal catalysts that are often based on rare noble metals as
27 catalytically active site, particularly in case of homogenous
28 catalysis. In the light of the limited abundance and the threat of
29 a global shortage, the development of base metal catalysts
30 relying on abundant and low cost metals with low toxicity has
31 received increasing attention.[1] Among all potential candidates,
32 iron as the most common element in the earth’s crust with its
33 broad occurrence in biological systems is
a particularly
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[a] S. Weber, J. Brꢀnig, Prof. K. Kirchner, Prof. K. Bica
Institute of Applied Synthetic Chemistry
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Vienna University of Technology
Getreidemarkt 9/163-AC
Wien A-1060 (Austria)
E-mail: karl.kirchner@tuwien.ac.at
katharina.bica@tuwien.ac.at
[b] V. Zeindlhofer, Prof. C. Schrçder
Department of Computational Biological Chemistry
University of Vienna
Faculty of Chemistry
Wꢁhringerstrasse 17
Wien A-1090 (Austria)
[c] Dr. B. Stçger
X-Ray Center
Vienna University of Technology
Getreidemarkt 9
Wien A-1060 (Austria)
[d] Prof. A. Limbeck
Institute of Chemical Technologies and Analytics
Vienna University of Technology
Getreidemarkt 9/163-AC
Figure 1. Pre-catalyst [Fe(PNPMe-iPr)(CO)(H)(Br)] (I) and active species trans-
[Fe(PNPMe-iPr)(CO)(H)2)] (II) used for the biphasic reduction of aldehydes in
these studies.
Wien A-1060 (Austria)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201800841
© 2018 The Authors. Published by Wiley-VCH Verlag GmbH& Co. KGaA. This
is an open access article under the terms of the Creative Commons Attri-
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medium, provided the original work is properly cited.
complex II which requires dissociation of the product alkoxide
to enable coordination and heterolytic cleavage of dihydrogen.
Thus, high catalytic activity was observed in the protic solvent
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EtOH (relative polarity 0.654) which facilitates alkoxide dissocia-
tion and stabilization, while no conversion was observed in
aprotic and low polarity solvents such as n-heptane (0.009).[6]
As a result of the homogenous reaction environment in
EtOH, difficulties in the separation of the formed alcohols and
catalyst from the solvent ethanol arise as it is typical for
homogenously catalyzed processes. To solve these issues, a
number of strategies have been developed that can balance
the positive aspects of homogenous and heterogeneous
Results and Discussion
A number of critical aspects need to be considered when
selecting ionic liquids for the biphasic process, including a high
solubility and stability of the pre-catalyst and the active species
in the ionic liquid, but also a low miscibility of ionic liquid and
organic phase. Based on literature data and on our previous
experience, we selected a set of hydrophobic ionic liquids
ꢀ
based on the N(Tf)2 anion for an initial screening. Apart from
10 catalysis.[7] Biphasic catalysis, in which the catalysts resides in
11 one of the two phases while the product is dissolved in the
12 second phase can facilitate problems encountered with separa-
13 tion but maintain the high activity associated with homoge-
14 nous catalysis.[8] This concept of liquid-liquid biphasic catalysis
15 is well established for different biphasic systems, i.e., aqueous-
16 organic solvent mixtures, and currently applied in several
the favorable physical properties in terms of viscosity, the very
ꢀ
weakly coordination nature of N(Tf)2 should avoid ligand
substitution reactions on the (pre-) catalyst while also providing
a higher hydrogen gas solubility compared to other hydrophilic
ꢀ
ꢀ
ionic liquids with BF4 or PF6 anion.[14] According to these
considerations, a pool of ten hydrophobic ionic liquids with
different cations and variable alkyl chain length was selected as
shown in Figure 2.
ˆ
17 industrially relevant processes such as the Rhone-Poulenc
18 Ruhrchemie process for hydroformylation.[9] More recently ionic
19 liquids broadened the application range for liquid-liquid
20 biphasic catalysis, as they can avoid limitations of aqueous
21 systems.[10] Compatibility issues of organometallic species with
22 water can be often overcome using ionic liquids instead.
23 Moreover, many classical transition-metal catalyst precursors
24 are readily soluble in ionic liquids rendering the synthesis of
25 specially designed ligands obsolete as it would be required for
26 aqueous, fluorinated, or supercritical fluid-based catalytic
27 processes.[11]
28
We recently demonstrated that I can also be conveniently
29 immobilized on precoated silica in a supported ionic liquid
30 phase (SILP) approach using the hydrophobic ionic liquid [C4m2
31 im]N(Tf)2.[12] Reasonable high turnover frequencies (TOF) and
32 turnover numbers (TON) could achieved with this catalyst
33 system. For an extension of this approach to other SILP systems,
34 i.e. other ionic liquids and other support materials, the wetting
35 properties of a particular IL/support system play a key role. A
36 strong affinity between the ionic liquid and the support surface
37 is important both for the impregnation of the support with IL/
38 catalyst solution, i.e. an even thin-film coverage of the inner
39 pore surface, and its resistance against IL/catalyst leaching.
40 Bordes et al. have shown[13] in a recent study of the wettability
41 of graphite by ionic liquids, that the solid/liquid interfacial
42 energy gSL is the best experimentally accessible parameter for
43 the affinity of ionic liquids toward a solid surface: the lower gSL,
44 the lower the liquid contact angle and the higher the adhesion
45 of a liquid film on a solid surface. Since gSL depends on both
46 the solid surface energy gSV and the liquid surface tension gLV
47 via Young‘s equation, the most suitable liquid with the
48 strongest affinity must be found separately for each solid
49 support material, i.e. each SILP system must be individually
50 optimized and a pool of suitable ionic liquids must be available.
51 For this purpose, we utilize here the iron (II) catalyzed reduction
52 of aldehydes under biphasic conditions as model reaction and
53 investigate reactivity, substrate dependence and catalyst recy-
54 cling with a variety of different ionic liquids in order to create a
55 pool of functional systems for their later application and
56 optimization in SILP catalysis.
Figure 2. Hydrophobic ionic liquids used for the biphasic reduction of
aldehydes.
Initial investigations on the catalytic properties were
performed with 4-fluorobenzaldehyde as model system due to
our previous research and its facile analysis via 19F{1H} NMR
spectroscopy (Scheme 1). In order to minimize the required
Scheme 1. Catalytic reduction of 4-fluorobenzaldehyde in ionic liquid/n-
heptane biphasic medium.
amount of ionic liquid, the biphasic system was composed of
1.5 ml of n-heptane and 250 mg ionic liquid, which was
sufficient to dissolve 0.5 mol% of the Fe-PNP pre-catalyst (S/C
200) used in all cases.
To activate the pre-catalyst I, DBU (1,8-diaza-bicyclo [5.4.0]
undec-7-ene) (5 mol%) was added, and hydrogenations were
routinely run at 10 bar hydrogen pressure for 60 min reaction
time. With the exception of pyridinium and picolinium-based
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ionic liquids 8 and 9, all ionic liquids were suitable for the
biphasic reduction of 4-fluorobenzaldehyde, and the corre-
sponding alcohol was formed as sole product in high yields
(Figure 3).
Apart from undesired leaching of ionic liquid into the
organic layer, the distribution of substrate, catalyst and product
is a key aspect that will influence the efficiency of the reaction
as well as separation of the products in biphasic catalysis. In
general, the reduction of aldehydes is not an ideal situation for
catalysis in organic/ionic liquid biphasic systems, as the more
polar alcohol formed during the reactions is more likely to
migrate to the ionic liquid phase. This unfavorable situation can
be partially compensated when using a large volumetric excess
of n-heptane compared to ionic liquid. We further studied
concentration effects of 4-fluorobenzaldehyde 11 and 4-
fluorobenzylalcohol 12 in a biphasic model system composed
of ionic liquid [P4441]N(Tf)2 and n-heptane similar to the ratio
and condition used for catalysis. These substrates were
separately added to a biphasic model system composed of
1.5 ml of n-heptane and 250 mg of ionic liquid [P4441]N(Tf)2 4,
stirred for 60 min at 258C. 19F{1H} NMR measurements were
carried out in the presence of an external standard (fluoroben-
zene) to quantify the distribution of substrate and product
between the two phases. As expected, the less polar aldehyde
11 had a higher affinity for the organic phase compared to the
product, and 0.62 mmol (corresponding to 31%) 4-fluoroben-
zaldehyde 11 were found in the n-heptane layer. The more
polar product 4-fluorobenzylalcohol 12 is more likely to migrate
to the ionic liquids phase, and only 0.32 mmol (16%) were
detected in the organic phase of the pure model system.
However, the efficiency of the ionic liquid-heptane biphasic
system is evident when addressing the distribution of the Fe(II)-
PNP catalyst between ionic liquid and organic phase. 31P{1H}
NMR analysis of n-heptane could not detect any traces of
catalyst. Moreover, ICP-MS analysis revealed an iron content of
<10 ng in the organic phase which clearly shows that no
leaching of the catalyst into the n-heptane solution takes place.
Likewise, we did not detect DBU in the organic phase. It has to
be noted that DBU is predominantly present in its protonated
cationic form after activation of the pre-catalyst I and is thus
immobilized in the polar ionic liquid rather than in the apolar n-
heptane layer.
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Figure 3. Results of the biphasic reduction of 4-fluorobenzaldehyde with
various hydrophobic ionic liquids.
In case of pyridinium and picolinium salts, drastically lower
33 yields (<25%) were observed presumably due to decomposi-
34 tion of the catalyst in these ionic liquids. While the ionic liquid
35 layer typically had an orange-red color after the reduction,
36 pyridinium-based systems rapidly turned black clearly suggest-
37 ing an undesired interaction between the catalyst and these
38 particular ionic liquid cations. Further analysis via NMR and GC-
39 MS did not reveal any evidence for alteration of the pyridinium
40 and picolinium-based ionic liquids indicating that an unin-
41 tended reduction of the ionic liquids was not responsible for
42 the failure of the reaction. It is interesting to note that there is
43 little difference between the performance of imidazolium-based
44 cations [C4mim]N(Tf)2 1 and [C4m2im]N(Tf)2 2. This suggests that
45 the presence of an acidic proton in the C-2 position and the
46 ability to form an N-heterocyclic carbene (NHC)-ligand does not
47 interfere with the reaction as it is often observed when reactive
48 organometallic species are involved.[15] The alkyl chain length
49 had limited impact on the catalytic performance and compara-
50 ble yields were found for the more viscous long alkyl chain
51 ionic liquids [C12mim]N(Tf)2 3, [P66614]N(Tf)2 5 and [N8881]N(Tf)2 7
52 and the corresponding short chain derivatives [C4mim]N(Tf)2 1,
53 [P4441]N(Tf)2 4 and [N4441]N(Tf)2 6. However, from NMR analysis of
54 the n-heptane layer it was found that leaching of these more
55 hydrophobic ionic liquids into the organic phase took place.
56 Such a behavior was not encountered in case of the short-chain
57 derivatives.
Due to the necessity of a base such as DBU during the iron-
catalyzed hydrogenation with Kirchner’s catalyst, special atten-
tion was dedicated to Brønsted basic ionic liquids with dual
function. In general, Brønsted basic ionic liquids contain at least
one Brønsted basic group either on the cation or on the
anion.[16] Basic ionic liquids can be used as an alternative to
commonly used (in) organic bases, and several applications in
organic reactions such as esterifications,[17] Michael additions[18]
or Aldol reactions[19] have been reported. Non-volatile organic
salts with amine functionalities were also used to immobilize
transition metal catalysts, for example palladium species[20] or,
more recently well-defined ruthenium catalysts that have been
employed for the elegant continuous flow hydrogenation of
carbon dioxide to formic acid.[21]
We therefore expanded the pool of ionic liquids with four
Brønsted basic ionic liquids to mimic typical organic bases
(Figure 4). Ionic liquids with 4-(dimethylamino) pyridine
(DMAP), 1,8-diazabicycloundecen-7-ene (DBU) and 1,4-diazabi-
cyclo-[2.2.2] octane (DABCO) structural motif (13–15) were
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found for the phosphonium-based ionic liquid [P4441]N(Tf)2 4,
and complete conversion was observed in only 9 min reaction
time. In case of the imidazolium-based ionic liquid [C4m2
im]N(Tf)2 2 slightly longer reaction times of 12 minutes were
required; however, the maximum TON was higher (Table 1,
entries 5–6). In contrast, considerably lower TOF and TON
values were determined for Brønsted-basic ionic liquids
[C4DBU]N(Tf)2 13 and [C4DMAP]N(Tf)2 14 (Table 1, entries 1–2).
The lower catalytic activity might be a result of a reduced
electron density on the free nitrogen in the Brønsted-basic ionic
liquid cations (see Figure 5) compared to the free base.
Figure 4. Brønsted-basic ionic liquids for the biphasic reduction of alde-
hydes. The inset displays the X-ray structure of the intermediate [C4DMAP]Cl
(see ESI for more details).
18 prepared via selective alkylation of diamines and successive ion
19 exchange. In case of the DMAP-based ionic liquid 13, crystals
20 were grown from the intermediated chloride salt that sup-
21 ported its structure. This set was complemented with an
22 amino-functionalized imidazolium derivative as N,N-diisopropy-
23 lethylamine (DIPEA) analogue (16) that has been previous used
24 for palladium catalyzed cross coupling reactions. An interesting
25 pattern was observed when studying the catalytic activity of
26 [Fe(PNPMe-iPr)(CO)(H)(Br)] (I) in Brønsted basic ionic liquids 13–
27 16 without additional base. The DIPEA and DABCO-based ionic
28 liquids were not suitable and failed, indicating that the pre-
29 catalyst was not activated in these ionic liquids (Table 1,
30
Figure 5. Molecular orbital description of the non-binding HOMO orbitals of
the alkaline ionic liquid cations. The bold numbers represent the correspond-
ing N(Tf)2-based ionic liquids.
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Table 1. Yields and kinetic data for the biphasic reduction of 4-fluoroben-
zaldehyde in selected conventional and Brønsted-basic ionic liquids.
Table 2. Computational analysis of the basicity of the nitrogen atoms of
the cations and the corresponding bases. The bold numbers represent the
corresponding N(Tf)2-based ionic liquid.
[b]
Entry
Ionic liquid[a]
Yield [%][a]
TOF [hꢀ1
]
TON[c])
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35
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39
40
41
42
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1
2
3
4
5
6
[C4DMAP]N(Tf)2 13
[C4DBU]N(Tf)2 14
98
97
<1
<1
>99
>99
800
480
n.d.
n.d.
1332
1008
220
240
n.d.
n.d.
793
1258
3
2
˚
˚
cation/base
q
N [e]
aN [A ]
PA [eV]
SASA [A ]
[C4DMAP]+ (13)
DMAP
ꢀ0.147
ꢀ0.323
ꢀ0.213
ꢀ0.396
ꢀ0.507
ꢀ0.635
ꢀ0.840
ꢀ0.836
1.66
1.51
1.19
1.19
1.11
1.22
1.57
1.50
10.30
11.21
9.54
11.07
11.06
11.97
11.50
12.06
2.0
1.5
0.7
0.7
11.8
11.4
0.0
0.0
[C8DABCO]N(Tf)2 15
[iPr2N(CH2)2mim]N(Tf)2 16
[P4441]N(Tf)2 4 [d]
[C4DBU]+ (14)
DBU
[C4m2im]N(Tf)2 2 [b]
[C8DABCO]+ (15)
DABCO
[a] Performed with 2 mmol aldehyde and 0.5 mol% pre-catalyst in 250 mg
ionic liquid/1.5 ml n-heptane for 60 min. Yield determined via 19F NMR
spectroscopy of organic and ionic liquid phase using fluorobenzene as
external standard; [b] performed with 2 mmol aldehyde and 0.5 mol% pre-
catalyst in 250 mg ionic liquid/1.5 ml n-heptane. TOF reported as amount
of substrate [mmol]/(amount of catalyst [mmol]ꢁreaction time for full
conversion [h]); [c] performed with 20 mmol aldehyde and 0.05 mol% pre-
catalyst in 250 mg ionic liquid/1.5 ml n-heptane for 80 h. Maximum TON
reported as amount of product [mmol]/amount of catalyst [mmol]; [d]
5 mol% DBU added.
[iPr2N(CH2)2mim]+ (16)
DIPEA
Although the actual values of the partial charge qN in Table 2
are generally not an ideal measure for the basicity of the
nitrogen atoms,[22,23] relative trends for the basicity can still be
observed when altering moieties of the compound. Except for
the isopropyl amine compounds the charge density of the
alkaline nitrogen is reduced by roughly 0.15e in the cationic
compound compared to the pure base which probably
corresponds to lower basicities of the charged compounds. This
fact is confirmed by the respective proton affinities (PA) in
Table 2. Lower proton affinities are usually correlated with lower
basicity of the nitrogen atoms.[23,24]
50 entries 3–4). In contrast, excellent yields of 95 and 97% were
51 found with DBU and DMAP analogue in a biphasic n-heptane/
52 ionic liquid system, thereby eliminating the necessity of base
53 addition in this reaction (Table 1, entries 1–2).
54
Further studies on the catalytic performance were per-
55 formed with four selected ionic liquids, including [C4m2im]N(Tf)2
56 2 and [P4441]N(Tf)2 4 that gave the highest yields as well as the
57 Brønsted basic ionic liquids 13 and 14. The highest TOF was
However, arguments solely based on the Brønsted basicity
cannot explain the failure of the compounds 15 and 16 as the
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alkaline nitrogen atoms in these compounds should have a
higher basicity compared to those in 13 and 14. The flexibility
of the electron cloud at the alkaline nitrogen is characterized by
its polarizability aN. Consequently, the ability to donate the lone
pair to a Lewis electron acceptor should scale with the atomic
polarizability on the nitrogen. Unfortunately, the nitrogen
polarizabilities in Table 2 show no clear trend.
poor yield of 12%. To increase the yield, the catalyst containing
phase was successively extracted with diethyl ether to collect
any remaining product. This resulted in an improved isolated
yield of 93%. While we did not observe any leaching of the
iron-based catalysts due to its extremely low solubility in diethyl
ether, the increased solubility of phosphonium-based ionic
liquid [P4441]N(Tf)2 4 resulted in trace contamination of the
diethyl ether extract. Consequently, an additional filtration step
over a batch of silica was performed to remove traces of ionic
liquids and to obtain a pure product. This extraction with
diethyl ether was routinely performed in the substrate screen-
ing to provide reliable results, since all products and - in case of
uncomplete reaction – aldehyde starting materials could be
extracted with diethyl ether.
The majority of Lewis acid-base reaction involve nꢀs* or
nꢀp* molecular orbital interactions.[22] The corresponding non-
10 bonding HOMO of the alkaline ionic liquid cations are depicted
11 in Figure 5. In contrast to the other cations, the n-orbital of
12 [C8DABCO]+ has a deformed shape for a non-bonding molec-
13 ular orbital which may indicate less effective interaction with
14 the s* or p* molecular orbital of the Lewis acid and thereby
15 reducing the obtained yield in Table 2.
In general, good to excellent yields were achieved for
aromatic systems, even in the presence of coordinating groups.
Similar results were found for the heterocyclic substrates
pyridine-2-carboxaldehyde, furfural and thiophene-2-carboxal-
dehyde with high isolated yields, whereas the reduction failed
with N-methyl-pyrrol-2-carboxaldehyde aldehyde. This is in
accordance with our previous observations using ethanol as
solvent, indicating an unfavorable interaction between the
Fe(II) PNP catalyst and pyrrole-based heterocycles. The reduc-
tion of aliphatic aldehydes was more challenging and gave
variable results. While the branched long-chain aliphatic
aldehyde 2-ethylhexanal was not reduced, moderate yields
were observed with octanal. Moderate to high yields were
obtained for selected unsaturated aldehydes. It is particularly
worth noticing that the chemoselective behavior for the
reduction of aldehydes was preserved in the biphasic set-up,
since unsaturated aldehydes such as the challenging a,b-
unsaturated substrates cinnamic aldehyde and citral were
selectively reduced to unsaturated alcohols that are of partic-
ular importance for the flavor and fragrance industries.
16
In addition to the electrostatic and energetic properties as
17 well as the molecular orbitals discussed so far, steric reasons
18 may also play a role for the interaction of the catalyst with the
19 base. As visible from the solvent accessible surface (SASA) of
20 the alkaline nitrogen in Table 2 the nitrogen of [C4DMAP]+ is
21 more accessible compared to [C4DBU]+ which may explain the
22 higher TOF and TON values in Table 1. In contrast, the alkaline
23 nitrogen atom in [iPr2N(CH2)2mim]+ and DIPEA seems to be
24 sterically more hindered, thereby prohibiting any contact of the
25 nitrogen and the catalyst.
26
After identifying the most suitable ideal ionic liquid for the
27 biphasic reaction set-up, we addressed substrate scope and
28 application range for a set of (hetero-)aromatic and aliphatic
29 aldehydes (Figure 6). To separate product and catalyst, the
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Apart from the benefits of simple product separation,
liquid-liquid biphasic catalysis also offers a powerful tool for
catalyst recovery and recycling. However, initial attempts to
recover the Fe(II) PNP catalyst after product extraction failed.
Independent of the ionic liquid, we did not observe any
conversion when fresh starting material was added. Further
studies on long-term stability of the catalyst in ionic liquids
showed that the activated dihydride species is considerable less
stable (see ESI Figure 1). 31P{1H} NMR spectroscopy indicated
the rapid formation of several intractable decomposition
products when the pre-formed dihydride complex was kept in
[P4441]N(Tf)2 4. This is a considerable difference and drawback
compared to pre-catalyst I, which is stable in [P4441]N(Tf)2 4 even
for one week according to 31P{1H} NMR spectroscopy. It was
however possible to show a certain reusability of the catalyst
immobilized in [P4441]N(Tf)2 4 via a semi-continuous addition of
substrate (Figure 7). After 10 min reaction time a fresh batch of
4-fluorobenzaldehyde (2 mmol) was added to prevent decom-
position of the active. With this strategy, the catalyst remained
active for four consecutive runs and a total of 5.2 mmol starting
material can be converted. Similar experiments with the
Brønsted-basic ionic liquids [C4DMAP]N(Tf)2 13 and
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Figure 6. Substrate scope and limitation for a set of aromatic and aliphatic
aldehydes. The reported yields refer to isolated yields of the pure
corresponding alcohols.
54 heptane phase was simply removed via decantation. Although
55 the product was obtained in high purity without traces of
56 starting material, catalyst, base or ionic liquid, evaporation of n-
57 heptane gave the product 4-fluorobenzylalcohol 12 only in
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catalyst in the absence of hydrogen could not be avoided.
Eventually, the design of targeted Brønsted basic ionic liquids
can eliminate the necessity of base addition and facilitate the
reaction set-up towards the future design of continuous flow
processes.
6
7
8
9
Experimental Section
All used reagents and solvents were purchased from commercial
suppliers and directly used without further purification, if not
stated otherwise. Anhydrous CH2Cl2, Et2O, n-heptane, MeOH, THF
and toluene were dried over molecular sieve and/or via Na/K alloy
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
and degassed via pump freezing. H, 19F{1H} and 31P{1H} NMR were
1
recorded in acetonitrile-d6, chloroform-d, methylene chloride-d2,
dimethylsulfoxide-d6 or methanol-d4 solution on a Bruker Avance
200 (200 MHz) or Bruker Avance 250 (250 MHz). All chemical shifts
1
(d) are reported in ppm, using tetramethylsilane for H, trichloro-
fluoromethane for 19F and H3PO4 (85%) for 31P NMR spectra. All
coupling constants (J) are reported in Hertz (Hz).
Determination of Fe concentrations was done 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.
Figure 7. Cumulative turnover of 4-fluorobenzaldehyde in biphasic reaction
media using [P4441]N(Tf)2 4, [C4DMAP]N(Tf)2 13 or [C4DBU]N(Tf)2 14 to
immobilize the iron-based pre-catalyst [Fe(PNPMe-iPr)(CO)(H)(Br)] (I). Per-
formed with 2 mmol aldehyde each run and 0.5 mol% pre-catalyst in
250 mg ionic liquid/1.5 ml n-heptane.
X-ray diffraction data of [C4DMAP]Cl·CHCl3 [CCDC entry 1825413]
were collected at T=100 K in a dry stream of nitrogen on a Bruker
Kappa APEX II diffractometer system using graphite-monochromat-
˚
ized Mo-Ka radiation (l=0.71073 A) and fine sliced f- and w-scans.
Data were reduced to intensity values with SAINT and an
absorption correction was applied with the multi-scan approach
implemented in SADABS.[25] The structure was solved by the dual
space method implemented in SHELXT[26] and refined against F2
with JANA2006.[27] Non-hydrogen atoms were refined anisotropi-
cally. H atoms were placed in calculated positions and thereafter
refined as riding on the parent C atoms. A chloroform solvent
molecule was modelled as disordered about three positions.
Molecular graphics were generated with the program MERCURY.[28]
Crystal data and experimental details are given in Table S1 in ESI.
28 [C4DBU]N(Tf)2 14 were less successful as the catalytic activity
29 ceased already after the first run.
30
31
32 Conclusions
33
34 In summary, we showed that hydrophobic ionic liquids in
35 combination with n-heptane can be used as biphasic reaction
36 media for the iron (II) catalyzed hydrogenation of aldehydes.
37 The presented liquid-liquid biphasic process presents a novel
38 and improved methodology compared to the homogenous
39 reaction in terms of product isolation and catalyst separation.
Quantum-chemical geometries of the compounds were optimized
with GAUSSIAN09[24] using B3LYP/6-311+ +G(2d,2p) and a polar-
izable continuum model (PCM) with the dielectric constants eð0Þ=
14.0 and eð1Þ=2.1 mimicking solvation in the ionic liquids [C4m2
im]N(Tf)2 or [C4mim]N(Tf)2. Partial charges were determined by
chelpg[23] using wB97XD/aug-cc-pVTZ/PCM to account for polar-
izable and dispersion effects. Atomic polarizabilities were obtained
by applying a electric field of 0.0008 au in positive and negative x-
,y- and z-direction using M06-2X/Sadlej.[29] Computational proton
affinities (PA)[29,30,31,32,33]
40
The iron (II) PNP pincer complex catalyst was efficiently
41 immobilized in the ionic liquid phase without requiring any
42 derivatization of the ligand, and no leaching in the organic
43 phase could be detected. Hydrogenation of a set of aliphatic
44 and aromatic aldehydes including unsaturated species was
45 performed under comparatively mild conditions (258C, 1 h,
46 10 bar H2). However, while the products were typically obtained
47 in high purities without contamination from catalyst, ligand or
48 base, the necessity of an addition extractions step with diethyl
49 ether in order to obtain high isolated yields of the formed
50 alcohols reduction the utility of the biphasic system for this
51 particular application. Clearly, a reduced amount of ionic liquid
52 as used in our previous study with supported ionic liquid
53 phases (SILPs) is beneficial and can overcome this issue.[12] As
54 for the use of supported catalysts, the facile separation of the
55 catalyst containing ionic liquid allowed for a certain recovery
56 and reuse of the catalyst for three runs, although losses in the
57 catalytic activity due to partial decomposition of the Fe(II)
PA ¼ ðEðAÞ þ ZPEðAÞÞ ꢀ ðEðHAþÞ þ ZPEðHAþÞÞ
ð1Þ
were evaluated from the difference of the total energy of the
protonated species HA+ and the neutral species A. Since the
protonation adds additional vibrational degrees of freedom, both
total energies were corrected for their respective zero-point vibra-
tional energy (ZPE). The molecular orbitals were visualized using
VMD[34] with an iso value of 0.08. The solvent accessible surface
˚
(SASA) was calculated by VMD using a probe radius of 1.4 A.
All ionic liquids 1–10 were synthesized according to standard
methodologies, and analytical data was in accordance with
literature. The ionic liquids were dried for at least 48 h with stirring
at 10ꢀ2 bar and 508C before use and stored under argon
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Full Papers
atmosphere in a glove box. The water content was measured via
Karl-Fischer titration and was typically below <500 ppm.
filtrated and the solvent was removed. Remaining volatile traces
1
2
3
4
5
were removed under high vacuum with stirring at 508C to yield
1
[C4DBU]N(Tf)2 14 as orange liquid (18.8 g, 85%). H NMR (250 MHz,
For Brønsted basic ionic liquids 13–15 a modified protocol relying
on selective alkylation of diamines and successive ion exchange
was performed as indicated below.
chloroform-d) d=3.71–3.37 (m, 8H), 2.88–2.75 (m, 2H), 2.13 (t, J=
5.9 Hz, 2H), 1.91–1.69 (m, 8H), 1.62 (quint, J=15.5, 7.5 Hz, 2H, ),
1.37 (sext, J=7.3 Hz, 2H), 0.98 ppm (t, J=7.2 Hz, 3H).
6
7
8
9
Synthesis of 1-butyl-4-(dimethylamino) pyridine-1-ium
Chloride ([C4DMAP]Cl)
Synthesis of 1-octyl-1,4-diazabicyclo [2.2.2] octan-1-ium
Bromide [C8DABCO]Br
N,N-Dimethylpyridine-4-amine (11.4 g, 93 mmol) was dissolved in
anhydrous acetonitrile and freshly distilled 1-chlorobutane (16.9 g,
180 mmol) was added in one batch. The reaction mixture was
refluxed for four days. During the reaction, the product precipitated
from the reaction mixture as colorless solid. The solid was
separated from the solution via filtration, washed three times with
diethylether and dried under vacuum to yield [C4DMAP]Cl as
colorless solid (19.2 g, 96%). 1H NMR (200 MHz, methylene chloride-
d2) d=8.44 (d, J=7.8 Hz, 2H), 6.92 (d, J=7.8 Hz, 2H), 4.27 (t, J=
7.3 Hz, 2H), 3.16 (s, 6H), 1.77 (q, J=7.5 Hz, 2H), 1.39–1.18 (m, 2H),
0.88 ppm (t, J=7.3 Hz, 3H).
1,4-Diazabicyclo [2.2.2] octane (5.5 g, 49.1 mmol) was dissolved in
anhydrous ethyl acetate and 1-bromooctane (9.5 g, 49.1 mmol)
dissolved in ethyl acetate was added dropwise. The reaction
mixture was refluxed for three days. During the reaction, the
product precipitated from the reaction mixture as light yellow solid.
The solid was removed via filtration, washed several times with
ethyl acetate and anhydrous tetrahydrofuran and dried under
vacuum to yield [C8DABCO]Br as light yellow solid (5.9 g, 39%). H
NMR (250 MHz methylene chloride-d2) d=3.36–3.09 (m, 14H), 1.8–
1.68 (m, 2H), 1.46–1.20 (m, 13H), 0.98 ppm (t, J=7.1 Hz, 3H).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
1
Synthesis of 1-octyl-1,4-diazabicyclo [2.2.2] octan-1-ium bis
(trifluoromethane) Sulfonylimide ([C8DABCO]N(Tf)2) 15
Synthesis of 1-butyl-4-(dimethylamino) pyridin-1-ium bis
(trifluoromethane) Sulfonylimide ([C4DMAP]N(Tf)2) 13
[C8DABCO]Br (4.88 g, 16 mmol) as dissolved in H2O and lithium bis
(trifluoromethane) sulfonylimide (4.82 g, 16.8 mmol) in H2O was
added dropwise. The mixture was stirred for one hour at room
temperature. The biphasic reaction mixture was extracted three
times with CH2Cl2. The combined organic phases were washed
repeatedly with MilliQ-grade H2O until no chloride anions could be
detected. The organic phase was dried over Na2SO4, filtrated and
the solvent was removed. Remaining volatile traces were removed
under high vacuum with stirring at 508C to yield [C8DABCO]N(Tf)2
14 as light yellow liquid (7.44 g, 92%). 1H NMR (250 MHz methylene
chloride-d2) d=3.36–3.09 (m, 14H), 1.8-1.68 (m, 2H), 1.46–1.20 (m,
13H), 0.98 ppm (t, J=7.1 Hz, 3H).
[C4DMAP]Cl (7.0 g, 32.7 mmol) was dissolved in H2O and lithium bis
(trifluoromethane) sulfonylimide (10.3 g, 35.9 mmol) in H2O was
added dropwise. The mixture was stirred for one hour at room
temperature. The biphasic reaction mixture was extracted three
times with CH2Cl2. The combined organic phases were washed
repeatedly with MilliQ-grade H2O until no more chloride anions
could be detected. The organic phase was dried over Na2SO4,
filtrated and the solvent was removed. Remaining volatile traces
were removed under high vacuum with stirring at 508C to yield
[C4DMAP]N(Tf)2 13 as colorless liquid (12.9 g, 86%). 1H NMR
(200 MHz, methylene chloride-d2) d=8.44 (d, J=7.8 Hz, 2H), 6.92
(d, J=7.8 Hz, 2H), 4.27 (t, J=7.3 Hz, 2H), 3.16 (s, 6H), 1.77 (q, J=
7.5 Hz, 2H), 1.39–1.18 (m, 2H), 0.88 ppm (t, J=7.3 Hz, 3H).
Synthesis of 1-(2-(diisopropylamino)
ethyl)-3-methyl-imidazolium Chloride
Synthesis of 1-butyl-2,3,4,6,7,8,9,10-octahydro-pyrimido
[1,2-a] azepin-1-ium chloride [C4DBU]Cl
A round bottom flask was charged with 2-(diisopropylamino)
ethylchloride hydrochloride (14.01 g, 70 mmol) and 1-methylimida-
zol (8.0 g, 97 mmol). The mixture was suspended in anhydrous
ethanol and refluxed for 2 days. The solid was collected via
filtration, washed repeatedly with anhydrous THF and dried under
vacuum to yield [iPr2N(CH2)2mim]N(Tf)2 as colorless solvent (17.10 g,
99%). 1H NMR (250 MHz, methylene chloride-d2) d=8.49 (s, 1H),
7.25 (t, J=1.8 Hz, 1H), 7.15 (t, J=1.8 Hz, 1H), 4.04 (d, J=6.1 Hz, 2H),
3.84 (s, 3H), 2.92 (hept, J=6.6 Hz, 2H), 2.72 (t, J=5.5 Hz, 2H),
0.81 ppm (d, J=6.6 Hz, 12H).
Freshly distilled 2,3,4,6,7,8,9,10-octahydropyrimido [1,2-a] azepine
(12.4 g, 82.0 mmol) was dissolved in anhydrous acetonitrile and
freshly distilled 1-chlorobutane (13.8 g, 152.0 mmol) was added
dropwise. The reaction mixture was refluxed for four days to reach
1
full conversion as detected by H NMR. The solvent was removed
and the remaining orange oil was washed repeatedly with diethyl
ether and ethyl acetate. Remaining volatiles were removed under
high vacuum to yield [C4DBU]Cl as orange oil (11.1 g, 56%). 1H-NMR
(250 MHz, chloroform-d) d=3.71–3.37 (m, 8H), 2.88–2.75 (m, 2H),
2.13 (t, J=5.9 Hz, 2H), 1.91–1.69 (m, 8H), 1.62 (quint, J=15.5,
7.5 Hz, 2H), 1.37 (sext, J=7.3 Hz, 2H), 0.98 ppm (t, J=7.2 Hz, 3H).
Synthesis of 1-(2-(diisopropylamino)
ethyl)-3-methyl-imidazolium bis (trifluoromethane)
Sulfonylimide 16
Synthesis of 1-butyl-2,3,4,6,7,8,9,10-octahydro-pyrimido
[1,2-a] azepin-1-ium bis (trifluoromethane)-sulfonylimide
([C4DBU]N(Tf)2) 14
[iPr2N(CH2)2mim]Cl (5.0 g, 17.2 mmol) was suspended in dichloro-
methane. Sodium hydroxide (0.7 g, 17.2 mmol) dissolved in water
and added and the biphasic system was stirred for ten minutes at
room temperature. Lithium bis (trifluoromethane) sulfonylimide
(5.3 g 18.6 mmol) dissolved in water was added dropwise. The
mixture was stirred for one hour at room temperature. The biphasic
reaction mixture was extracted three times with CH2Cl2. The
combined organic phases were washed repeatedly with MilliQ-
grade H2O until no more chloride anions could be detected. The
[C4DBU]Cl (11.1 g, 45.3 mmol) was dissolved in H2O and lithium bis
(trifluoromethane) sulfonylimide (14.3 g, 49.9 mmol) in H2O was
added dropwise. The mixture was stirred for one hour at room
temperature. The biphasic reaction mixture was extracted three
times with CH2Cl2. The combined organic phases were washed
repeatedly with MilliQ-grade H2O until no more chloride anions
could be detected. The organic phase was dried over Na2SO4,
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organic phase was dried over Na2SO4, filtrated and the solvent was
removed. Remaining volatile traces were removed under high
vacuum with stirring at 508C to yield [iPr2N(CH2)2mim]N(Tf)2 16 as
colorless liquid (7.7 g, 88%). 1H NMR (250 MHz, methylene chloride-
d2) d=8.49 (s, 1H), 7.25 (t, J=1.8 Hz, 1H), 7.15 (t, J=1.8 Hz, 1H),
4.04 (d, J=6.1 Hz, 2H), 3.84 (s, 3H), 2.92 (hept, J=6.6 Hz, 2H), 2.72
(t, J=5.5 Hz, 2H), 0.81 ppm (d, J=6.6 Hz, 12H).
4-Chlorbenzylalcohol (isolated yield 92%):1H NMR (250 MHz,
chloroform-d) d=7.34–6.24 (m, 3H), 4.61 (s, 2H), 2.69 ppm (s, 1H).
1
2
3
4
5
6
7
8
9
Pyridin-2-ylmethanol (isolated yield 87%): 1H NMR (250 MHz,
chloroform-d) d=8.48 (d, J=4.9 Hz, 1H), 7.62 (td, J=7.7, 1.8 Hz,
1H), 7.28–7.05 (m, 2H), 4.70 (s, 2H), 4.02 ppm (s, 1H).
Furan-2-ylmethanol (isolated yield 87%): 1H NMR (250 MHz,
chloroform-d) d=7.42 (d, J=2.6 Hz, 1H), 6.53–6.28 (m, 2H), 4.61 (s,
2H), 2.10 ppm (s, 1H).
General Procedure for the Hydrogenation of Aldehydes on
the Example of 4-fluorobenzadehyde 11
Thiophen-2-ylmethanol (isolated yield 89%): 1H NMR (250 MHz,
chloroform-d) d=7.37–7.27 (m, 1H), 7.02 (ddd, J=8.4, 4.7, 3.4 Hz,
2H), 4.87 (s, 2H), 1.74 ppm (s, 1H).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
All hydrogenation reactions were carried out in a Roth steel
autoclave using a Tecsis manometer. The glass tube of the steel
autoclave was charged with ionic liquid (250 mg) and pre-catalyst I
(0.01 mmol) in a glove box and placed into the autoclave. The
autoclave was evacuated and flushed with argon three times
before 4-flurobenzaldehyde 11 (2 mmol), DBU (0.1 mmol) and n-
heptane (1.5 ml) were added. After flushing three times with
hydrogen, the desired hydrogen pressure (10 bar) was established
and the reaction was stirred at 258C for 60 min. before the pressure
was released. The organic and ionic liquid phase was separately
analyzed by 19F{1H} NMR spectroscopy. The absolute amounts of
aldehyde and alcohol were determined via integration using 10 ml
fluorobenzene as external standard, and the yield was calculated
accordingly.
(E)-3-Phenylprop-2-en-1-ol (isolated yield 88%):1H NMR (250 MHz,
chloroform-d) d=7.67–7.08 (m, 5H), 6.76–6.48 (m, 1H), 6.47–6.19
(m, 1H), 4.41–4.16 (m, 2H), 1.80 ppm (s, 1H).
2,2-Diphenylethan-1-ol (isolated yield 21%): 1H NMR (250 MHz,
chloroform-d) d=7.45–7.19 (m, 10H), 4.33–4.14 (m, 3H), 1.6 ppm (s,
1H).
3-(Benzo [d][1,3] dioxol-5-yl)-2-methylpropan-1-ol (isolated yield
1
91%): H NMR (250 MHz, chloroform-d) d=6.82–6.57 (m, 3H), 5.94
(s, 2H), 3.60–3.42 (m, 2H), 2.69 (dd, J=13.5, 6.3 Hz, 1H), 2.37 (dd, J=
13.5, 8.0 Hz, 1H), 1.90 (dq, J=13.1, 6.5 Hz, 1H), 1.77 (s, 1H),
0.93 ppm (d, J=6.7 Hz, 3H).
1
Octan-1-ol (isolated yield 34%): H NMR (250 MHz, chloroform-d)
In case of product isolation, a different work-up strategy was
followed. After the hydrogen pressure was released, the organic
phase was separated and the ionic liquid phase was extracted four
more times with diethyl ether (2 ml each). The combined organic
layers were filtered over silica and the solvent was evaporated. The
obtained crude products were purified via flash column chroma-
tography to yield the desired alcohols in spectroscopically pure
form.
d=3.66 (t, J=6.6 Hz, 3H), 1..68–1.51 (m, 3H), 1.46–1.23 (m, 16 H),
0.9 ppm (t, J=6.7 Hz 3H).
Cyclohex-3-en-1-ylmethanol (isolated yield 28%): 1H NMR
(250 MHz, chloroform-d) d=5.69 (d, J=2.4 Hz, 2H), 3.55 (dd, J=6.0,
1.7 Hz, 3H), 2.23–2.02 (m, 6H), 1.91–1.66 (m, 4H), 1.38–1.20 ppm (m,
1H).
3,7-Dimethylocta-2,6-dien-1-ol (isolated yield 45%): 1H NMR
(250 MHz, chloroform-d) d=5.15–5.05 (m, 1H), 3.75–3.59 (m, 2H),
2.14–1.84 (m, 2H), 1.79–1.58 (m, 6H), 1.58–1.36 (m, 2H), 1.36–1.07
(m, 2H), 0.91 ppm (d, J=6.6 Hz, 3H).
Procedure for Catalyst Recycling via semi-continuous
Substrate Addition
(E)-3,7-dimethylocta-2,6-dien-1-ol (isolated yield 53%): 1H NMR
(250 MHz, chloroform-d) d=5.43 (t, J=6.4 Hz, 1H), 5.11 (t, J=6 Hz,
1H), 4.17 (d, J=6.9 Hz, 2H), 2.19–1.99 (m, 5H), 1.7 (s, 6H), 1.62 ppm
(s, 3H).
After running the hydrogenation according to the general
procedure given above for 10 minutes, the pressure of the
autoclave was released. Fresh 4-fluorobenzaldehyde 11 (2 mmol) in
n-heptane (0.5 ml) was added and the autoclave was flushed with
hydrogen three times. The hydrogen pressure was adjusted to
10 bar, and the reaction was run for further 10 min. This procedure
was done independently with 2, 3 4 and 5 consecutive addition
steps. For each experiment, the overall yield was determined after
the final run via 19F{1H} NMR spectroscopy as described above.
Acknowledgements
4-Fluorobenzylalcohol (isolated yield 95%): 1H NMR (200 MHz,
chloroform-d) d=7.37–7.15 (m, 2H), 7.06–6.86 (m, 2H), 4.58 (s, 2H),
1.76 ppm (s, 1H).19F NMR (235 MHz, chloroform-d) d=ꢀ113.4 ppm
Benzylalcohol (isolated yield 90%):1H NMR (200 MHz, chloroform-
d) d=7.40–7.14 (m, 5H), 4.63 (s, 2H), 1.6 ppm (s, 1H).
Financial support by the Austrian Science Fund (FWF) is gratefully
acknowledged (Project No. P28866-N34 and P29146-N34).
Conflict of Interest
4-Tolylalcohol (isolated yield 93%):1H NMR (250 MHz, chloroform-
d) d=7.28 (d, J=8.0 Hz, 2H), 7.20 (d, J=8.4 Hz, 1H), 4.66 (s, 2H),
2.38 (s, 3H), 1.80 ppm (s, 1H).
The authors declare no conflict of interest.
1
4-Methoxybenzylalcohol (isolated yield 89%): H NMR (250 MHz,
Keywords: Hydrogenation
·
aldehydes iron
·
pincer
chloroform-d) d=7.32 (d, J=8.3 Hz, 2H), 6.92 (d, J=7.3 Hz, 2H),
4.64 (s, 2H), 3.85 (s, 3H), 1.62 ppm (s, 1H).
complexes · ionic liquids · biphasic catalysis
2-Hydroxybenzylalcohol (isolated yield 79%):1H NMR (250 MHz,
chloroform-d) d=7.23–6.88 (m, 3H), 6.78 (td, J=7.8, 1.0 Hz, 2H),
4.76 ppm (s, 2H).
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Manuscript received: May 23, 2018
Version of record online: &&&, &&&&
ChemCatChem 2018, 10, 1–10
www.chemcatchem.org
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ꢀ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Full Papers
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FULL PAPERS
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Biphasic catalysis: A biphasic
process for the hydrogenation of
aldehydes was developed using a
well-defined iron (II) PNP pincer
complex in various ionic liquids as
reaction media.
S. Weber, J. Bru¨nig, V. Zeindlhofer,
¨
¨
Prof. C. Schroder, Dr. B. Stoger, Prof. A.
Limbeck, Prof. K. Kirchner*, Prof. K.
Bica*
1 – 10
Selective Hydrogenation of
Aldehydes Using a Well-Defined
Fe(II) PNP Pincer Complex in
Biphasic Medium
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