A Comparison of Ruthenium-Catalysed Arene Hydrogenation Reactions in Water and 1-Alkyl-3-methylimidazolium Tetrafluoroborate Ionic Liquids

Article abstract of DOI:10.1002/adsc.200390015

The hydrogenation of benzene and other arene substrates under biphasic conditions is evaluated using the catalyst precursor Ru(η⁶-C ₁₀H₁₄)(pta)Cl₂ (pta = 1,3,5-triaza-7- phosphaadamantane) immobilised in water and 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids. The effect that contamination of the 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids with chloride has on the hydrogenation reaction has also been examined. Of the immobilisation solvents tested the optimum solvent was found to be chloride-free 1-butyl-3-methylimidazolium tetrafluoroborate. Catalytic turnovers in this solvent are highest, and in general, turnovers for the hydrogenation reactions follow the trend: chloride-free 1-butyl-3-methylimidazolium tetrafluoroborate > water > chloride-contaminated 1-butyl-3-methylimidazolium tetrafluoroborate.

Full text of DOI:10.1002/adsc.200390015

FULL PAPERS
A Comparison of Ruthenium-Catalysed Arene Hydrogenation
Reactions in Water and 1-Alkyl-3-methylimidazolium
Tetrafluoroborate Ionic Liquids
a,
b
c
a
¬
Paul J. Dyson, * David J. Ellis, William Henderson, Gabor Laurenczy
a
À
¬
¬ ¬
Institut de chimie moleculaire et biologique, Ecole polytechnique federale de Lausanne, EPFL BCH, CH-1015
Lausanne, Switzerland
Fax: ( ‡ 41)-21-693-98-85, e-mail Paul.Dyson@epfl.ch
b
c
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
Department of Chemistry, University of Waikato, Private Bag3105, Hamilton, New Zealand
Received: June 3, 2002; Accepted: October 5, 2002
Abstract: The hydrogenation of benzene and other butyl-3-methylimidazolium tetrafluoroborate. Cata-
arene substrates under biphasic conditions is eval- lytic turnovers in this solvent are highest, and in
uated usingthe catalyst precursor Ru( h6- general, turnovers for the hydrogenation reactions
C10H14)(pta)Cl2 (pta ˆ 1,3,5-triaza-7-phosphaadaman- follow the trend: chloride-free 1-butyl-3-methyl-
tane) immobilised in water and 1-alkyl-3-methylimi- imidazolium tetrafluoroborate > water > chloride-
dazolium tetrafluoroborate ionic liquids. The effect contaminated 1-butyl-3-methylimidazolium tetra-
that contamination of the 1-alkyl-3-methylimidazoli- fluoroborate.
um tetrafluoroborate ionic liquids with chloride has
on the hydrogenation reaction has also been exam-
ined. Of the immobilisation solvents tested the Keywords: arenes; biphasic catalysis; hydrogenation;
optimum solvent was found to be chloride-free 1- ionic liquids; ruthenium; water as solvent
Introduction
faced by those usingionic liquids composed of tetra-
fluoroborate anions is that the purity of ionic liquids has
Ionic liquids were originally developed as highly con- hampered research.[4] Tetrafluoroborate-based ionic
ductingelectrolytes for electrochemical use, [1] but sub- liquids are most frequently prepared via a metathesis
sequently, it was found that they have a number of reaction between the appropriate dialkylimidazolium
properties that make them ideally suited to immobilisa- halide salt and sodium tetrafluoroborate.[5] This method
tion of catalysts for biphasic synthesis. These properties provides a relatively cheap and effective way to produce
have been described in detail elsewhere,[2] but in brief:
ionic liquids of a moderate purity. Alternative reagents
to sodium tetrafluoroborate have been explored[6] and
completely different routes have also been reported.[7]
Any organic impurities can be removed via a combina-
tion of high vacuum and filtration through a column of
activated alumina, but it proves extremely difficult to
remove final traces of chloride impurities from the ionic
liquid. It is well known that halide ions can coordinate to
transition metal centres so their presence in a catalyst
support solvent is undesirable as they can block reactive
co-ordination sites which can lead to side reactions and
catalyst deactivation.[8] In addition, trace amounts of
chloride impurities, which may be present at levels
They are good solvents for a range of both inorganic
and organic species.
Although they are polar (due to their ionic nature),
the component ions are poorly co-ordinatingand they
are unlikely to interfere chemically with any solute.
A number of organic solvents display little or very
limited miscibility with these liquids.
From an environmental perspective they exhibit
negligible vapour pressure allowing volatile organic
contaminants to be easily removed, but also prevent-
ingtheir escape to the atmosphere by evaporation.
There is considerable interest in the use of room between 0.1 0.5 mol kgÀ1, have significant effects on
temperature ionic liquids in biphasic catalysis and the physical properties of the ionic liquid such as
many elegant examples of ionic liquid-organic catalysis viscosity and density. Increases in viscosity are a
have been reported.[3] Although ionic liquids offer particular concern in biphasic processes as the forma-
certain advantages in biphasic catalysis they are not tion of an emulsion and hence the interface of the two
without their own problems. One of the main problems phases is effected.
216
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Adv. Synth. Catal. 2003, 345, No. 1+2

Ru-Catalysed Arene Hydrogenation Reactions in Water and Ionic Liquids
FULL PAPERS
We previously described the biphasic hydrogenation hexane that are relevant to this work. The ionic liquids
of arene substrates usinga cationic ruthenium cluster exhibit similar polarities to one another and are similar
catalyst supported in 1-butyl-3-methylimidazolium tet- to the polarities of lower alcohols.[13] The polarity of
rafluoroborate [bmim][BF4] ionic liquid.[9] Duringan
benzene is intermediate between that of dichlorome-
extension to these studies we noticed that certain thane (a miscible solvent) and diethyl ether (an immis-
catalysed reactions carried out in tetrafluoroborate cible solvent) and exhibits a limited degree of misci-
ionic liquids showed a variance in turnovers of Æ 10% bility. Cyclohexane, on the other hand, has a polarity
which depended on the batch of ionic liquid used. With lower than that of diethyl ether. Clearly, the difference
this in mind, we set about establishingthe optimum 1-
alkyl-3-methylimidazolium tetrafluoroborate ionic liq- facilitate biphasic arene hydrogenation reactions using
uid for conductingarene hydrogenation reactions, ex- these ionic liquids since the substrate will be more
in polarity between benzene and cyclohexane could
plored the effect of chloride contamination on the soluble in the ionic liquid thereby increasingthe rate of
catalyst and compared the ionic liquids with water as an the catalytic reaction and the product should be less
immobilisation solvent. In this paper we describe the soluble makingextraction more straightforward.
outcome of these studies.
The concentration of benzene and cyclohexane in a
range of ionic liquids (Scheme 1) has been determined
at 20 8C and the results from these experiments are
summarised in Figure 1. As the length of the alkyl group
increases from a methyl group in [mmim][BF4] to an
Results and Discussion
Many catalysts are soluble in ionic liquids, especially octyl group [omim][BF4] the solubility of benzene
ionic compounds, although neutral species, such as rapidly increases. In contrast, the solubility of cyclo-
Wilkinson×s catalyst, are also soluble to some extent.[10] hexane, which has a lower polarity than benzene,
While the solubility of the catalyst is clearly important, remains effectively constant. Since the solubility of
in a biphasic process the nature of the interaction benzene is greatest in [omim][BF4] and the solubility of
between the catalyst solution and the substrates and cyclohexane is almost as low as it is for [mmim][BF4]
products are also significant. In the ideal process, the then selection of [omim][BF4] for hydrogenation reac-
substrates will be completely miscible with the catalyst tions seems reasonable. In this solvent the interface
support solution and the products should be completely between a benzene substrate and an ionic liquid is
immiscible. Such a situation is very rare although the
synthesis of butyraldehyde from propylene, CO and H2
involves an aqueous-organic biphasic process that
−
BF4
operates under such conditions.[11] In this process the
+
N
N
substrates are all gases under normal conditions, but in
general, if the substrates are more soluble than the
products in a catalyst support solvent then the process
may still be advantageous.
[mmim][BF4]
Dialkylimidazolium tetrafluoroborate ionic liquids
are miscible with polar organic solvents such as chlori-
nated solvents and alcohols, and immiscible with non-
polar solvents such as ethers and alkanes.[12] Solvents
with intermediate polarity, however, have a degree of
miscibility. Table 1 compares polarities of some dialkyl-
imidazolium tetrafluoroborate ionic liquids with con-
ventional organic solvents including benzene and cyclo-
−
BF4
+
N
N
[bmim][BF4]
−
BF4
+
Table 1. Solvatochromic shifts of the dye Nile Red in various
N
N
solvents. Data for ionic liquids are taken from ref.[13]
Solvent
lmax [nm]
[hmim][BF4]
[bmim][BF4]
[hmim][BF4]
[omim][BF4]
methanol
dichloromethane
benzene
550.8
551.9
549.5
549.6
535.2
525.4
504.4
487.6
−
BF4
+
N
N
diethyl ether
cyclohexane
[omim][BF4]
Scheme 1.
Adv. Synth. Catal. 2003, 345, 216 221
217

Paul J. Dyson et al.
FULL PAPERS
Scheme 2.
Figure 1. Graph showingthe miscibilty of benzene and
cyclohexane in [Cx-mim][BF4] ionic liquids (where x ˆ
1 mmim; x ˆ 4 bmim; x ˆ 6 hmim and x ˆ 8 omim). Measure-
ments were conducted at 20 8C on pure ionic liquids without
catalyst present since the catalyst is present in low concen-
tration and therefore should not effect solubility.
greatest allowing optimum mixing of the benzene
substrate and catalysts without compromisingthe effi-
ciency of the extraction of the cyclohexane product after
reaction. We have found, however, that differences in
turnovers for reactions carried out in [bmim][BF4],
[hmim][BF4] and [omim][BF4] are indistinguishable and
Figure 2. Relationship between catalytic turnover and stirring
rate for the hydrogenation of benzene to cyclohexane using
[H4Ru4(h6-C6H6)][BF4]2. Reaction conditions: catalyst (30 mg)
this is because at the temperature of the reaction, viz. in solvent (10 mL), substrate (2 mL), H2 (60 atm) 90 8C, 1 h.
90 8C, all the solutions are essentially homogeneous.
We have previously shown that the cluster pre-catalyst
[H4Ru4(h6-C6H6)][BF4]2, (Scheme 2) originally devel- in the ionic liquid that deactivated the catalyst. From the
oped for biphasic aqueous-organic arene hydrogenation literature,[4] the most likely culprit is chloride, known to
reactions,[14] operates effectively in [bmim][BF4].[9] At be present at levels between 0.1 0.5 mol kgÀ1, and also
the time we ascribed slight improvements in catalytic known to deactivate homogeneous catalysts. The im-
activity to increased solubility of hydrogen gas in the portance of the purity of ionic liquids has been discussed
ionic liquid; the Henry coefficient solubility constant of previously,[17] and much work has been devoted to
hydrogen in [bmim][BF4] is 3.0 Â 10À3 mol LÀ1 atmÀ1 studyingthe effect of chloride impurities on the physical
which is much higher than water.[15] Based on the above properties ionic liquids and in designing ways in
resultsit seems likelythat theimprovement is also due to removingall the residual chloride from the metathesis
increased solubility of the benzene substrate in the synthesis of [bmim][BF4] and related ionic liquids. We
[bmim][BF4] ionic liquid, although other factors such as used a modification of a synthesis described previously
viscosity, which is higher for the ionic liquid, are in which 1-butylimidazole is methylated with the
important. The effect of the increased solubility of simultaneous formation of the tetrafluoroborate anion
benzene in the ionic liquid has been confirmed by usingtrimethyloxonium tetrafluoroborate, Me OBF4,
3
comparingthe effect of stirrer speed on turnover in the
which avoids halide precursors.[18] The original synthesis
two solvents. The results are presented in Figure 2 which is conducted in an organic solvent, whereas we con-
demonstrates that the ionic liquid solution is less ducted the reaction under solvent-free conditions at low
sensitive to stirrer rate exhibitingclose to the maximum
turnover at a lower stirrer rate than the analogous
reaction in water.
temperature.
Benzene hydrogenation using Ru(h6-C10H14)(pta)Cl2
as the pre-catalyst was carried out in the −halide free×
The complex Ru(h6-C10H14)(pta)Cl2 (pta ˆ 1,3,5-tri- [bmim][BF4] and the results are displayed in Table 2
aza-7-phosphaadamantane) has also been shown to be a together with comparisons with water, and
pre-catalyst for the hydrogenation of arenes.[16] Since it [bmim][BF4], [hmim][BF4] and [omim][BF4] prepared
is soluble in ionic liquids we screened it in arene by the metathesis route. The catalytic activity in water is
hydrogenation reactions and found it to be less effective greater than that obtained for the ionic liquids made by
in [bmim][BF4] than in water. Surprised by this result we the metathesis route. However, this trend is reversed
proposed that the poorer result for was due to impurities when the chloride-free [bmim][BF4] ionic liquid is
218
Adv. Synth. Catal. 2003, 345, 216 221

Ru-Catalysed Arene Hydrogenation Reactions in Water and Ionic Liquids
FULL PAPERS
Table 2. Hydrogenation
of
benzene
using
Ru(h6-
C10H14)(pta)Cl2 or [H4Ru4(h6-C6H6)][BF4]2 in various sol-
vents.
Catalyst
Solvent
Turnover*
[mol molÀ1 hÀ1]
Ru(h6-C10H14)(pta)Cl2
Ru(h6-C10H14)(pta)Cl2
Ru(h6-C10H14)(pta)Cl2
Ru(h6-C10H14)(pta)Cl2
Ru(h6-C10H14)(pta)Cl2
Ru(h6-C10H14)(pta)Cl2
H2O
170
140
137
141
[bmim][BF4][a]
[hmim][BF4]
[omim][BF4]
[bmim][BF4][b] 206
CH2Cl
54
352
364
[H4Ru4(h6-C6H6)][BF4]2 H2O
[H4Ru4(h6-C6H6)][BF4]2 [bmim][BF4][a]
Figure 3. Reuse of Ru(h6-C10H14)(pta)Cl2 for the hydrogena-
tion of benzene to cyclohexane in water, [bmim][BF4] and
halide-free [bmim][BF4] solution. Reaction conditions: cata-
lyst (30 mg) in solvent (10 mL), substrate (2 mL), H2 (60 atm)
90 8C, 1 h.
[H4Ru4(h6-C6H6)][BF4]2 [bmim][BF4][b] 406
Reaction conditions: catalyst (30 mg) in solvent (10 mL),
substrate (2 mL), H2 (60 atm) 90 8C, 1 h. Product in each
case is cyclohexane.
* Turnovers are quoted in number of moles of substrate
converted per mole of catalyst per hour.
[a]
[b]
1
the sequential reuse of the catalyst to hydrogenate
benzene and the results are presented in Figure 3. Over
a series of five batch reactions the turnovers remain
effectively constant in all three solvents, although in
water there are signs of a slight decrease in activity.
Ru(h6-C10H14)(pta)Cl2 has also been used as a pre-
catalyst for the hydrogenation of some arene substrates
in water and chloride-containingand chloride-free
[bmim][BF4] and the results are presented in Table 3.
For each arene substrate the turnover is lowest when
immobilised in [bmim][BF4] prepared by the metathesis
route and greatest in chloride-free [bmim][BF4]. This
trend is once again likely to be due to catalyst
deactivation by chloride ions present in the former
solvent. The decrease in turnover number as the number
of alkyl groups or chlorines attached to the ring
increases is typical for homogeneous catalysed reduc-
tions of arenes. In water the active catalyst generated
from Ru(h6-C10H14)(pta)Cl2 is believed to be a triruthe-
nium cluster.[16] Mass spectrometric and spectroscopic
studies would suggest that a similar species is generated
in ionic liquids.
Made via metathesis route, [Cl ] 0.2 mol kg .
Made via alkylimidazolium methylation, chloride content
zero.
Table 3. The hydrogenation of various arene substrates using
Ru(h6-C10H14)(pta)Cl2
immobilised
water
and
in
[bmim][BF4].
Substrate
Reaction Conditions
Turnover*
(mol molÀ1 hÀ1)
Toluene
Toluene
Toluene
Ethylbenzene
Ethylbenzene
Ethylbenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
water
130
54
136
122
53
145
11
6
[bmim][BF4][a]
[bmim][BF4][b]
water
[bmim][BF4][a]
[bmim][BF4][b]
water
[bmim][BF4][a]
[bmim][BF4][b]
18
Reaction conditions: catalyst (30 mg) in solvent (10 mL),
substrate (1 mL), H2 (60 atm) 90 8C, 1 h. Products formed
were completely hydrogenated cyclohexane or alkane analo-
gues of substrate.
* Turnovers are quoted in number of moles of substrate
converted per mole of catalyst per hour.
Conclusions
[a]
[b]
1
Made via metathesis route, [Cl ] 0.2 mol kg .
Made via alkylimidazolium methylation, chloride content
Arene hydrogenation is an important reaction in
organic synthesis, the preparation of cleaner fuels and
in paper production.[19] In this work we have shown that
the mononuclear ruthenium complex Ru(h6-
zero.
employed as the immobilisation solvent; resultingin a
ca. 50% increase in the activity for the catalytic hydro- C10H14)(pta)Cl2 acts as pre-catalyst for the hydrogena-
genation of benzene. The cluster pre-catalyst [H4Ru4(h6- tion of arene substrates. It dissolves in water and ionic
C6H6)][BF4]2 is not expected to be prone to poisoningby
liquids, but unless the ionic liquid is free from halide
halide impurities, but there is still a significant increase impurities the ionic liquids offer little advantage, in
in turnover when the cluster is immobilised in the terms of activity and reusability, over water. This is in
−halide free× [bmim][BF4].
spite of the advantageous substrate-product solubility
The stability of Ru(h6-C10H14)(pta)Cl2 in different properties offered by the ionic liquid. Other pre-
immobilisation solvents has been studied by exploring catalysts used in hydrogenations have been shown to
Adv. Synth. Catal. 2003, 345, 216 221
219

Paul J. Dyson et al.
FULL PAPERS
then heated to the required reaction temperature and stirring
was commenced for the period required. Once the desired time
period had elapsed, the stirringwas stopped and the autoclave
allowed to cool before releasingthe pressure. Substrate and
catalyst layers were separated in a separatingfunnel.
be less active in ionic liquids than water, but in contrast
to the pre-catalyst used here, these catalysts react with
water to form the active catalyst.[20]
Experimental Section
The ionic liquids containingchloride impurities were made
accordingto the literature method [4] and then filtered through
neutral alumina prior to use. Deionised water was used without
the addition of pH buffers or phase-transfer reagents. The
chloride-free [bmim][BF4] was made usinga modified liter-
ature method (see below). The complexes [H4Ru4(h6-
Acknowledgements
We would like to thank the Royal Society fora Univesrity
Research Fellowship (PJD), the University of York for financial
support (DJE) and the University of Waikato for a travel grant
(WH).
and Ru(h6-C10H14)(pta)Cl2 were made ac-
[13a]
[21]
C6H6)][BF4]2
cordingto the literature procedures. All other reagents were
supplied by Aldrich and were used without further purifica-
tion.
References and Notes
Electrospray mass spectra were obtained a VG Autospec
instrument. NMR spectra were recorded on a Bruker DRX-
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1
400 spectrometer with H at 400.13, 31P at 161.98 and 13C at
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(CDCl3, d ˆ 7.29), 31P{1H} NMR spectra are reported in ppm
downfield of an external 85% solution of phosphoric acid. All
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chromatography using a Varian gas chromatograph with a
capillary carbowax column (30 m) usinginjection, oven and
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1-Butylimidazole (5 mL) was added dropwise to a rapidly
stirred sample of trimethyloxonium tetrafluoroborate (1 mol
equiv.) cooled to À 78 8C over a period of 10 min. After the
initial vigorous evolution of gas had ceased, the reaction
mixture was allowed to warm to room temperature. Any
residual dimethyl ether by-product was removed under high
vacuum. The product was passed through activated charcoal
and alumina columns to afford colourless [bmim][BF4] ionic
liquid in quantitative yield.
‡
Spectroscopic data for [bmim][BF4]: ESMS : m/z ˆ 139
‡
[bmim] ; ESMSÀ: m/z ˆ 87 [BF4]À; 1H NMR (CDCl3): d ˆ 8.76
(s, 1H), 7.51 (m, 2H), 4.24 (q, J ˆ 4.24 Hz, 2H), 3.99 (s, 3H), 1.90
(quintet, J ˆ 7.28 Hz, 2H), 1.39 (sextet, J ˆ 7.28 Hz, 2H), 0.97 (t,
J ˆ 7.28 Hz, 3H); 13C NMR (DEPT, CDCl3): d ˆ 135.46 (CH),
123.23 (CH), 121.94 (CH), 49.92 (CH2), 35.50 (CH3), 31.31
(CH2), 18.86 (CH2), 12.72 (CH3) ppm; anal.: found (calcd.) C
41.44 (42.51) H 7.33 (6.69) N 12.03 (12.39) Cl 0.00 (0.00)%.
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All hydrogenations were carried out in a Parr stainless steel
autoclave (300 mL) fitted with a PTFE liner. The catalyst was
added directly to the autoclave and the solvent was added. The
autoclave was then sealed and purged with nitrogen and the
reaction substrate was then added through the liquid inlet port
via a syringe. The autoclave was then purged thoroughly with
hydrogen gas (99.9995% purity) and the appropriate reaction
pressure was then set at room temperature. The autoclave was
220
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Ru-Catalysed Arene Hydrogenation Reactions in Water and Ionic Liquids
FULL PAPERS
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Adv. Synth. Catal. 2003, 345, 216 221
221