A temperature-controlled reversible ionic liquid - Water two phase - Single phase protocol for hydrogenation catalysis

Article abstract of DOI:10.1139/v01-084

An ionic liquid - water system that undergoes a reversible two phase - single phase transformation dependent upon temperature has been used as a novel medium for the transition-metal-catalyzed hydrogenation of a water soluble substrate. At room temperature, the ionic liquid 1-octyl-3-methylimidizolium tetrafluoroborate, containing [Rh(η⁴-C₇H₈)(PPh₃) ₂][BF₄] catalyst, forms a separate layer to water containing 2-butyne-1,4-diol. In a stirred autoclave the mixture was pressurized with hydrogen to 60 atm (1 atm = 101.325 kPa) and heated to 80°C giving a homogeneous single phase solution. On cooling to room temperature, two phases reform, with the ionic liquid phase containing the catalyst and the aqueous phase containing a mixture of 2-butene-1,4-diol and butane-1,4-diol products that can be simply removed without catalyst contamination.

Full text of DOI:10.1139/v01-084

705
A temperature-controlled reversible ionic liquid –
water two phase – single phase protocol for
hydrogenation catalysis
Paul J. Dyson, David J. Ellis, and Thomas Welton
Abstract: An ionic liquid – water system that undergoes a reversible two phase – single phase transformation depend-
ent upon temperature has been used as a novel medium for the transition-metal-catalyzed hydrogenation of a water sol-
uble substrate. At room temperature, the ionic liquid 1-octyl-3-methylimidizolium tetrafluoroborate, containing [Rh(η4-
C7H8)(PPh3)2][BF4] catalyst, forms a separate layer to water containing 2-butyne-1,4-diol. In a stirred autoclave the
mixture was pressurized with hydrogen to 60 atm (1 atm = 101.325 kPa) and heated to 80°C giving a homogeneous
single phase solution. On cooling to room temperature, two phases reform, with the ionic liquid phase containing the
catalyst and the aqueous phase containing a mixture of 2-butene-1,4-diol and butane-1,4-diol products that can be sim-
ply removed without catalyst contamination.
Key words: ionic liquids, biphasic catalysis, hydrogenation, rhodium.
Résumé : Un système liquide ionique – eau soumis à une transformation réversible de deux à une phase suivant la
température est utilisé comme nouveau milieu pour l’hydrogénation, catalysée par des métaux de transition, de substrats
solubles dans l’eau. À la température ambiante, le liquide ionique tétrafluoroborate de 1-méthyl-3-méthylimidizolinium
contenant le catalyseur [Rh(η4-C7H8)(PPh3)2][BF4] forme une couche séparée de l’eau qui contient du but-2-yne-1,4-
diol. Dans un autoclave sous agitation, le mélange est soumis à une pression d’hydrogène de 60 atm (1 atm =
101.325 kPa) et chauffé à 80°C pour donner une solution homogène comportant une seule phase. Par refroidissement à
la température ambiante, les deux phases se reforment avec la phase du liquide ionique qui contient le catalyseur et la
phase aqueuse qui contient un mélange de but-2-ène-1,4-diol et de but-2-yne-1,4-diol qui peut être enlevé sans conta-
mination par le catalyseur.
Mots clés : liquides ioniques, catalyse biphasique, hydrogénation, rhodium.
[Traduit par la Rédaction] 708
Introduction
Dyson et al.
in the reviews cited in ref. 1. Specific examples include hy-
drogenation (4) and hydroformylation, (5) Heck, (6) Trost–
Tsuji (7), and Sukuzi (8) coupling reactions, the Diels–Alder
reaction (9), alkylation (10), and polymerization (11) reac-
tions.
Room temperature ionic liquids are emerging as excellent
alternatives to organic solvents in homogeneous and biphasic
processes and several excellent reviews are available (1). For
example, liquids composed of various imidazolium cations
with aluminium chloride based anions have replaced organic
solvents in Friedel–Crafts reactions (2) and in polyethylene
cracking (3) (both being catalyzed by aluminium chloride).
More inert ionic liquids, also composed of imidazolium cat-
ions, but with counter anions such as tetrafluoroborate or
hexafluorophosphate, have been used as catalyst supports in
biphasic processes for a wide range of reactions as outlined
While comparisons between ionic liquid – biphasic sys-
tems and others such as water and fluorous biphasic systems
have been widely discussed from an environmental perspec-
tive, detailed assessment of the influence of the solvent re-
gime on the catalyst has not been widely made (1). One of
the most impressive aspects of some ionic liquids is that the
liquid is essentially non-nucleophilic and as a consequence
extends the lifetime of supported catalysts giving higher
turnover numbers, clearly demonstrated for the hydro-
formylation of methyl-3-pentenoate using rhodium-based
catalysts supported in the ionic liquid 1-butyl-3-
methylimidazolium hexafluorophosphate (5b). When the re-
action is carried out in organic solvents such as dichloro-
methane and toluene deactivation of the catalysts takes place
rapidly, whereas in the reactions conducted in the ionic liq-
uid, the catalyst remains active for considerably longer peri-
ods with turnover numbers typically 600% greater. This
feature is especially important since concerns have been ex-
pressed that homogeneous catalysts tend to have short life-
times and therefore there is little point in trying to recover
them.
Received September 9, 2000. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on July 6, 2001.
This paper is dedicated to Professor Brian James whose
energy and enthusiasm for chemistry has inspired us.
P.J. Dyson1 and D.J. Ellis. Department of Chemistry, The
University of York, Heslington, York YO10 5DD, U.K.
T. Welton. Department of Chemistry, Imperial College of
Science, Technology and Medicine, South Kensington,
London SW7 2AY, U.K.
1Corresponding author (telephone: +(0)1904-434076;
fax: +(0)1904-432516; e-mail: pjd14@york.ac.uk).
Can. J. Chem. 79: 705–708 (2001)
DOI: 10.1139/cjc-79-5/6-705
© 2001 NRC Canada

706
Can. J. Chem. Vol. 79, 2001
Chart 1.
Fig. 1. Graphic representation of the relative components ob-
tained from a consecutive series of hydrogenations of 2-butyne-
1,4-diol (10 g of substrate was introduced into each batch).
[BF ]-
+
4
1.4
1.2
1.0
[BF ]-
+
+
4
Butyne
Butane
Butene
0.8
0.6
0.4
0.2
0
[BF ]-
4
Scheme 1.
HO
1
2
3
4
5
OH
OH
Reaction number
+
+
[Rh( 4-C7H8)(PPh3)2][BF4]
[omim][BF4]-H2O
HO
HO
HO
OH
ous – organic catalysis (for recent examples see ref. 16) with
very little contamination of the catalyst observed in the or-
ganic phase after reaction. We have found that complexes
containing P(C6H4-m-SO3Na)3 ligands are also reasonably
soluble in ionic liquids but they are clearly unsuitable for the
ionic liquid – aqueous regime described herein due to pref-
erential solubility in water. Since the triphenylphosphine
complex [Rh(η4-C7H8)(PPh3)2][BF4] is a hydrophobic salt,
with high solubility in [omim][BF4] and other ionic liquids
(4a), it is ideally suited to the [omim][BF4]–water system.
Furthermore, cationic rhodium(I) compounds of formula
[Rh(diene)P2]+ (where P2 represents two monophosphines or
a single diphosphine) have been used extensively as hydro-
genation catalysts (17) and show excellent activity and have
OH
In this paper we describe an ionic liquid – water biphasic
system employing a cationic rhodium catalyst that has been
applied to the hydrogenation of a water soluble substrate. At
higher temperatures, the ionic liquid and water form a single
phase allowing truly homogeneous catalysis to be carried
out, whereas at lower temperatures the ionic liquid and wa-
ter form two phases allowing easy separation of the product
in the aqueous phase from the catalyst supported in the ionic
liquid. As far as we are aware this represents the first exam-
ple of an ionic liquid to be employed in such a process al-
though a related process has been used in fluorous phase –
organic systems (12).
even been used in ionic liquid
– organic biphasic
hydrogenations (18). The P(C6H4-m-SO3Na)3 derivative has
been used in aqueous – organic biphasic catalysis (19).
Based on the above criteria [Rh(η4-C7H8)(PPh3)2][BF4]
was used to catalyze the hydrogenation of maleic acid and 2-
butyne-1,4-diol in [omim][BF4]–water at 80°C. The succinic
acid product obtained quantitatively from the hydrogenation
of maleic acid was not significantly more water soluble than
ionic liquid soluble and presented separation problems that
offer no advantage over other methodologies (20). In con-
trast, the hydrogenation of 2-butyne-1,4-diol afforded prefer-
entially water soluble 2-butene-1,4-diol and butane-1,4-diol
products (see Scheme 1) that were easily separated from the
ionic liquid – catalyst phase after cooling the reaction mix-
ture to room temperature. The results obtained from a series
of consecutive batch hydrogenation reactions of 2-butyne-
1,4-diol are presented graphically in Fig. 1.
Over the series of five reactions the catalyst decomposes
slightly, as evidenced by the formation of a black precipitate
and slight decoloration of the ionic liquid phase. The pres-
ence of a black precipitate often indicates the active catalyst
species is colloidal, however, tests using mercury as a selec-
tive poison proved this not to be the case. Figure 1 shows
that over the five batch reactions the conversion not only re-
mains high, but also the recovery of the products remains
high.
Results and discussion
The
tetrafluoroborate ([bmim][BF4]), 1-hexyl-3-methylimidizolium
tetrafluoroborate ([hmim][BF4]), and 1-octyl-3-
ionic
liquids
1-butyl-3-methylimidizolium
methylimidizolium tetrafluoroborate ([omim][BF4]) (see
Chart 1) reported previously (13) have been shown to exhibit
pH dependent reversible partitioning with water (14). These
liquids also exhibit temperature-dependent reversible-phase
separation with water. The temperature at which the
[omim][BF4] ionic liquid and water form a homogeneous
system (established by measuring w/w) is 80°C which is
ideally suited to chemical reactions. At room temperature
(20°C) the [omim][BF4] ionic liquid contains 12% water
(w/w). This provides a potential method of conducting ho-
mogeneously catalyzed reactions at 80°C with the combined
advantages of biphasic separation at 20°C (or below).
The solubility of numerous catalysts in the ionic liquid –
water systems has been examined. To effectively dissolve a
catalyst into an ionic liquid the catalyst must be a salt (re-
cently a new cobaltacenium-based diphosphine ligand was
specifically made with this in mind) (5c). To induce water
solubility hydrophilic groups must be present and a wide
range of water soluble phosphine ligand have been devel-
oped over the years (15). One of the most widely used hy-
drophilic groups is sulphate, and for example, P(C6H4-m-
SO3Na)3 has been extensively employed in biphasic aque-
The initial relatively low recovery level of the product,
stems from the fact that 2-butyne-1,4-diol and the hydroge-
nation products (as well as water) show some degree of solu-
bility in the [omim][BF4] ionic liquid. Once the ionic liquid
© 2001 NRC Canada

Dyson et al.
707
solution becomes saturated with these compounds higher re-
covery is observed.
approach, could help to overcome product contamination by
toxic metal residues.
Acknowledgements
Experimental
We would like to thank the Royal Society for a University
Research Fellowship (PJD) and the University of York for a
studentship (DJE).
The ionic liquids 1-butyl-3-methylimidizolium tetra-
fluoroborate, 1-hexyl-3-methylimidizolium tetrafluoroborate,
and 1-octyl-3-methylimidizolium tetrafluoroborate (13) and
the catalyst [Rh(η4-C7H8)(PPh3)2][BF]4 (21) were prepared
according to the literature methods. 2-Butyne-1,4-diol was
purchased from Aldrich and used without further purifica-
tion; water was distilled before use.
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Hydrogenations were carried in a Parr stainless steel auto-
clave (300 mL) fitted with a PTFE liner. The catalyst was
dissolved in ionic liquid and added directly to the autoclave.
The autoclave was then sealed and purged with nitrogen.
The reaction substrate was then dissolved in water and
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(99.9995% purity) and the pressure was then set at room
temperature to 60 atm (1 atm = 101.325 kPa). The autoclave
was then heated to 10°C and stirred for 2 h. Once the de-
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1
The products were identified by H NMR spectroscopy on
a JEOL JNM-EX270 FT-NMR instrument and quantified gas
chromatographic analysis using a Varian gas chromatograph
with a capillary carbowax column (30 m) using injection,
oven and detector temperatures 10–30°C above the boiling
points of the substrate–product being studied. Such condi-
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cis and trans isomers of the 2-butene-1,4-diol could not be
resolved.
Concluding remarks
We have demonstrated the use of a temperature-controlled
reversible ionic liquid – water partitioning system for the
catalytic hydrogenation of 2-butyne-1,4-diol. However, the
process still requires some optimization as it operates under
fairly harsh conditions (cf. the hydrogenation of cyclohexene
with a related rhodium(I) catalyst in a similar ionic liquid
occurs at 10 atm (1 atm = 101.325 kPa) H2 compared to
60 atm (1 atm = 101.325 kPa) H2 used here) and some cata-
lyst decomposition takes place. Furthermore, since the
amount of water in the ionic liquid at lower temperatures re-
mains relatively high (and visa versa) some contamination of
the aqueous product phase with the catalyst or catalyst de-
composition products is observed. However, it is likely that
this problem could be largely overcome by selection of an
alternative ionic liquid. In fact, by simply using the PF6¯ an-
ion in place of BF4¯ in the ionic liquid the solubility with
water drops considerably. Despite these problems the ease of
catalyst–substrate separation combined with the isolation of
product largely uncontaminated by the catalyst does indicate
that this type of method could become highly desirable for
certain processes. For example, the synthesis of
pharmaceuticals, many of which are water soluble, using this
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
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