Arene hydrogenation in a room-temperature ionic liquid using a ruthenium cluster catalyst

Article abstract of DOI:10.1039/a807447j

The air and moisture stable system [bmim][BF₄]-[Ru₄(η⁶-C₆H₆)₄][BF₄] {[bmim]⁺ = 1-butyl-3-methylimidazolium cation} presents a novel medium for conducting hydrogenations of arenes; the environmental problems associated with related aqueous-organic biphasic regimes are eliminated.

Full text of DOI:10.1039/a807447j

Arene hydrogenation in a room-temperature ionic liquid using a ruthenium
cluster catalyst
a
b
c
b
Paul J. Dyson, David J. Ellis, David G. Parker and Thomas Welton
a
Department of Chemistry, The University of York, Heslington, York, UK YO10 5DD. E-mail: pjd14@york.ac.uk
Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London,
UK SW7 2AY. E-mail: t.welton@ic.ac.uk
b
c
ICI Group R & T Affairs, PO Box 90, Wilton Centre, Middlesbrough, Cleveland, UK TS90 8JE
Received (in Cambridge, UK) 24th September 1998, Accepted 13th November 1998
6
6
The air and moisture stable system [bmim][BF
6 6 4 4
4
]–[Ru
4
(h -
water with [H
cursor.†
4
Ru
4
6 6 4 4 2
(h -C H ) ][BF ] as the catalyst pre-
+
C H ) ][BF ] {[bmim] = 1-butyl-3-methylimidazolium cat-
ion} presents a novel medium for conducting hydrogenations
of arenes; the environmental problems associated with
related aqueous–organic biphasic regimes are eliminated.
The turnover frequencies obtained in the ionic liquid and
aqueous regimes are similar. This would suggest that the active
catalytic species is the same in both systems. It has been clearly
6
2+
shown that [H
4
Ru
4
(h -C
6
H
H
6
)
4
]
can oxidatively add hydrogen
6
2+
The heterogenisation of homogenous catalysts is rapidly
to give [H Ru
6
4
(h -C
6
6
)
4
] , which is the hydrogenating
1
10
becoming an important area of chemistry. It is hoped that the
species. This contrasts with many cluster-based catalyst
precursors that are believed to fragment into mononuclear
species during reaction.12 Few other effective hydrogenation
catalysts are known and most of these cannot be used in biphasic
advantages of both homogeneous (greater efficiency, all metal
centres being involved in the process) and heterogeneous (ease
of catalyst separation, greater selectivity) catalysts can be
combined in one system. One increasingly attractive route to
greater catalyst separation exploits the use of biphasic liquid
reaction systems.2 Not surprisingly, aqueous–organic biphasic
systems have emerged as an important class of catalysts and
1
1,13
processes and they tend to be air and moisture sensitive.
The turnover frequencies compare well to other homogeneous
arene hydrogenation systems, such as [(h -C
Cl)]Cl which will hydrogenate benzene at 50 °C and 50 atm
with a catalytic turnover of 246 mol mol
,3
6
6
Me
6
)
2
Ru
2
(m-
2
4
21 21 14
have found several industrial applications. However, the
h . However, the
method is not without problems; it precludes the use of water
sensitive catalysts and reagents and, from an environmental
perspective, trace amounts of organic compounds in water are
notoriously difficult to remove.
main advantage of the ionic liquid–organic system arises from
the ease of separation of the catalyst from the starting material/
product stream and the subsequent purification of the solvent,
allowing different compounds to be hydrogenated without
contamination. The aqueous phase in a biphasic hydrogenation
processes cannot be used for different arenes or returned to the
environment without expensive treatment to remove trace
An alternative system that will allow the use of water
sensitive reagents and catalysts and solves some of the
containment problems associated with aqueous–organic sys-
tems is that based on biphasic ionic liquid–organic systems. The
development of a variety of air and moisture stable ionic liquids
in recent years has lead to their deployment in a number of
4
quantities of organic compounds. Since the [bmim][BF ] ionic
liquid has no vapour pressure, organic compounds (and water)
may be removed by merely placing the liquid under a high
vacuum. Using this technique we have been able to repeatedly
use the same batch of ionic liquid for the catalytic hydro-
genation of several different arenes.
4
processes. Building on our previous results with the chloro-
5
aluminate(iii) ionic liquids, we have been investigating the
+
[bmim][BF
4
] ionic liquid {[bmim] = 1-butyl-3-methylimida-
zolium cation}, which has received some attention as a solvent
for the hydrogenation of olefins using Wilkinson’s and related
catalysts. Whilst these catalysts are moderately effective they
We are currently investigating the hydrogenation of function-
alized arenes and heteroaromatics using a series of ionic clusters
in the full range of available ionic liquids.
6
are deactivated by trace quantities of chloride ions remaining in
the ionic liquid from its preparation.
The hydrogenation of arenes is an important industrial
process, particularly for the generation of cleaner diesel fuels,7
and is dominated by the use of heterogeneous catalysts. In an
attempt to develop biphasic methodologies for the hydro-
genation of arenes we describe herein the use of a
4
Table 1 Biphasic hydrogenation reactions of arenes in the [bmim][BF ]
6
ionic liquid and water with [H
precursor
4 4 6 6 4 4 2
Ru (h -C H ) ][BF ] as the catalyst
Con-
version
(%)
Catalytic
a
Reaction
Substrate system
Reaction
conditions
turnover /
mol mol h21
21
[
bmim][BF
4
]–organic system. Many catalysts, especially neu-
] and therefore
tral species, are not soluble in [bmim][BF
4
Benzene Ionic liquid 60 atm H
0 °C, 2.5 h
60 atm H
0 °C, 2.5 h
2
,
91
88
72
78
34
31
364
352
240
261
136
124
careful choice of catalyst is required. Ionic inorganic and
organometallic compounds tend to be highly soluble in
9
Water
2
,
[
bmim][BF
4
] and for this reason we chose to investigate the
9
6
cluster, [H
4
Ru
4
(h -arene)][BF
4
]
2
, which proved to be both
Toluene
Cumene
a
Ionic liquid 60 atm H2,
90 °C, 3 h
soluble and stable. Clusters of this type were first reported by
Maitlis and coworkers8 and then later by Süss–Fink and
Water
60 atm H2,
9
0 °C, 3 h
Ionic liquid 60 atm H
0 °C, 2.5 h
60 atm H
0 °C, 2.5 h
coworkers who went on to fully characterise these intriguing
electronically unsaturated cluster cations.9 Süss-Fink and
coworkers then went on to show that they could be used as
2
,
9
Water
2
,
catalyst precursors for the hydrogenation of fumeric acid and
9
arenes under aqueous–organic biphasic conditions.9
–11
This
Catalytic turnover is calculated on the assumption that the tetra-
ruthenium catalyst does not break down into monoruthenium fragments
which is entirely consistent with the data.
provides an ideal opportunity to compare the two systems
directly. Table 1 lists a number of biphasic hydrogenation
reactions of arenes employing the [bmim][BF ] ionic liquid and
4
Chem. Commun., 1999, 25–26
25

We would like to thank to thank The Royal Society for a
University Research Fellowship and ICI (Wilton) for financial
support (D. J. E.).
2 Aqueous-Phase Organometallic Catalysis, Concepts and Applications,
ed. B. Cornils and W. A. Herrmann, Wiley–VCH, Weinheim, 1998.
3
4
P. J. Dyson, D. J. Ellis and T. Welton, Platinum Met. Rev., in press.
Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem., Int. Ed. Engl.,
1
995, 34, 2698.
5
P. J. Dyson, M. C. Grossel, N. Srinivasan, T. Vine, T. Welton, D. J.
Williams, A. J. P. White and T. Zigras, J. Chem. Soc., Dalton Trans.,
1997, 3465.
Notes and references
†
The ionic liquid [bmim][BF
Ru (h-C )][BF is very soluble and stable in this ionic liquid and is
4
] was prepared using the literature method.6
[H
4
4
6
H
6
4
]
2
6 P. A. Z. Suarez, J. E. L. Dullis, S. Einloft, R. F. de Souza and J. Dupont,
Polyhedron, 1996, 15, 1217.
7 A. Stanislaus and B. H. Cooper, Catal. Rev.-Sci. Eng., 1994, 36, 73.
8 J. A. Cabeza, A. Nutton, B. E. Mann, C. Brevard and P. M. Maitlis,
Inorg. Chim. Acta, 1986, 115, L47.
9 G. Meister, G. Rheinwald, H. Stoecki-Evans and G. Süss-Fink, J. Chem.
Soc., Dalton Trans., 1994, 3215.
10 L. Plasseraud and G. Süss-Fink, J. Organomet. Chem., 1997, 539,
163.
11 E. G. Fidalgo, L. Plasseraud and G. Süss-Fink, J. Mol. Catal. A, 1998,
132, 5.
12 B. F. G. Johnson, Transition metal clusters, Wiley, Chichester, 1980.
13 I. P. Rothwell, Chem. Commun., 1997, 1331 and references therein.
14 M. A. Bennett, T.-N. Huang and T. W. Turney, J. Chem. Soc., Chem.
Commun., 1979, 312.
1
readily characterised in the ionic liquid using H NMR spectroscopy which
revealed and spectrum similar to that in conventional solvents.
Hydrogenations were carried using a Parr stainless steel autoclave (300
6
ml) fitted with either a glass or PTFE liner. The catalyst [H
)][BF was added together with the required amount of [bmim][BF
ionic liquid. The autoclave was then sealed and purged with hydrogen gas
99.9995% purity) and the appropriate reaction pressure was then set at
4 4
Ru (h -
C
6
H
6
4
]
2
4
]
(
room temperature. The autoclave was then sealed and heated to the required
reaction temperature and stirred for the time period required. After reaction
the contents were then separated into organic and ionic liquid phases and the
1
products analysed by H NMR spectroscopy and GC. The only products
observed were the perhydrogenated cycloalkanes, there was no evidence for
the formation of partially hydrogenated products or polymeric by-
products.
1
J. H. Clark and D. J. Macquarrie, Chem. Commun., 1998, 853.
Communication 8/07447J
26
Chem. Commun., 1999, 25–26