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On the Noninnocent Nature of 1 3-Dialkylimidazolium Ionic Liquids.

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Highlights
Ionic Liquids
On the Noninnocent Nature of 1,3-Dialkylimidazolium
Ionic Liquids**
Jairton Dupont* and John Spencer
Keywords:
carbenes · homogeneous catalysis · ionic liquids ·
solvent effects · synthetic methods
1,3-Dialkylimidazolium
salts are
one of the most popular and investigated classes of the vast family of roomtemperature ionic liquids (RTILs). In
particular, those resulting from the association of the 1-n-butyl-3-methylimidazolium (bmim) cation with relatively
weakly coordinating anions such as
tetrafluoroborate,
hexafluorophosphate,[1] and trifluoromethanesulfonate
display unique physical-chemical properties: they are liquids over a large range
of temperatures (down to 80 8C), possess high thermal and chemical stability,
a large electrochemical window, high
density, relatively low viscosity, and
negligible vapor pressure. These materials are very popular and have been used
in various domains of physical sciences
such as fluids in synthesis, catalysis,
spectroscopy, electrochemistry, nanomaterials, extraction and separation
processes.[2] It is usually assumed that
these liquids are entirely innocent and
noncoordinating solvents. However, in
older and more recent examples such
innocuous behavior was not always
observed, and a certain degree of caution should be exercised when ionic
liquids are chosen as solvents.
[*] Prof. Dr. J. Dupont
Laboratory of Molecular Catalysis
Institute of Chemistry, UFRGS
Av. Bento Gon5alves 9500
Porto Alegre 91501-970 RS (Brazil)
Fax: (+ 55) 513-316-7304
E-mail: dupont@iq.ufrgs.br
Dr. J. Spencer
James Black Foundation
68 Half Moon Lane
Dulwich, SE24 9JE (UK)
[**] This work was supported by CNPq and
CENPES-PETROBRAS (Brazil).
5296
There is no doubt that most 1,3dialkylimidazolium ILs are stable towards many organic and inorganic substances, but under certain reaction conditions both the cation and anion can
undergo “undesirable” transformations.
In some cases the anions of imidazolium
ILs can easily undergo hydrolysis, particularly those containing AlCl4 and PF6
anions. In the case of the hexafluorophosphate anion, phosphate and HF are
formed, and 1,3-dialkylimidazolium
phosphates and transition-metal fluorides have been isolated during reactions and purification procedures.[3] The
hydrolysis of the PF6 anion may be more
pronounced in reactions involving metals, which can catalyze this decomposition. Cation metathesis was also observed with highly negatively charged
complexes such as Na3[Co(CN)5] and
Na2[{(UO2)(NO3)2}2(m4-C2O4)] dissolved
in ionic liquids, and the the respective
coordination complexes associated with
the imidazolium cation precipitated.[4]
The reactivity of imidazolium cations mainly stems from the relatively
high acidity (pKa = 21–23) of the H2
hydrogen of the imidazolium nucleus,
which has been found to be roughly
intermediate between the acidities of
acetone (pKa = 19.3) and ethyl acetate
(pKa = 25.6).[5] It is well known from the
seminal work of Arduengo that deprotonation at the C2 position of the
imidazolium salt generates N-heterocyclic carbene ligands.[6] Not surprisingly,
the formation of metal–carbene complexes has been observed in Pd-catalyzed Heck-type reactions performed in
ionic liquids. In these cases the side
reaction has a beneficial effect since the
carbenes most probably stabilize the
catalytically active species.[7] For exam-
condensation reactions under basic conditions are also often hampered by the
deprotonation at C2 of the imidazolium
ion. Additional quantities of base are
required for reuse of the IL, and it has
been proposed that in order to obviate
these problems C2-substituted ILs
might be more suitable media for some
base-catalyzed processes.[6b] This clearly
indicates that when ionic liquids are
employed under basic conditions, carbenes are likely to form in the mixture
with either detrimental or beneficial
results.
An even more striking result was
recently reported. Under “less basic”
conditions the C2 H bond of the imid-
DOI: 10.1002/anie.200460431
Angew. Chem. Int. Ed. 2004, 43, 5296 –5297
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ple, under drastic conditions (200 8C/
50 atm C2H4) reaction of the ionic liquid
ethylmethylimidazolium chloride/AlCl3
(1.3:1) with PtCl2/PtCl4 led to cis[PtCl2-(C2H4)(1-ethyl-3-methylimidazol-2-ylidene)].
Moreover, organic adducts have also
been isolated when ILs are utilized in
the presence of organic electrophiles
such as aldehydes, as in the Baylis–
Hillman reaction[8] (Scheme 1). Related
Scheme 1. Examples of side reactions of the
bmim cation under basic conditions (X = Br,
Cl). DABCO = 1,4-diazabicyclo[2.2.2]octane.
Angewandte
Chemie
azolium nucleus can oxidatively add to
electron-rich Ni0 or Pd0 complexes to
generate stable (carbene)metal hydride
compounds (Scheme 2).[9] The 1,3-dimethylimidazolium cation also oxidatively
plex B. Although B was considered to be
a catalytic intermediate, it was found to
be a poor catalyst for C C bond-forming reactions, whereas A was significantly more effective. Complexes related to
Scheme 2. Reactions of the imidazolium cation under neutral conditions.
adds to [Pt(PPh3)4] as observed by
31
P NMR spectroscopy, although yields
of the product cis-[PtH(1,3-dimethylimidazolin-2-ylidene)(PPh3)2]BF4 were
rather poor.[10] Tetralkylammonium salts
are known to undergo thermally and
chemically induced dealkylation.[11] The
dealkylation of the imidazolium nucleus
(Hofmann elimination) was also observed in the catalytic hydrodimerization of butadiene by PdII compounds
immobilized in ILs (Scheme 2).[12]
Under sonochemical conditions, decomposition of the IL has been observed, which is linked to the creation of
hot spots in the solvent. For example,
when [bmim]Cl was sonified at 135 8C
and the headgas analyzed, chloromethane and chlorobutane (from SN2 processes) were detected as well as imidazole decomposition products.[13] This
could clearly limit the scope of ILs in
ultrasound-assisted chemistry as well as
in other thermal processes including
microwave-induced chemistry.
In an RTIL-relevant area, more insight into the complexity of the chemistry involving the generation of palladium–carbene complexes from imidazolium salts is provided by a very recent
example depicted in Scheme 3.[14] An
imidazolium salt reacted with a PdII
derivative under base-free conditions,
yielding the unusual complex A and
none of the expected symmetrical com-
Angew. Chem. Int. Ed. 2004, 43, 5296 –5297
Scheme 3. An unusual N-heterocyclic carbene–Pd complex formed from an imidazolium salt. Mes = 2,4,6-trimethylphenyl.
A may indeed be catalytically relevant
intermediates, and this demonstrates
that both H2 and H5 in imidazolium
salts can be activated under neutral
conditions.
Although a tremendous amount of
work has been carried out on applications of ionic liquids and a reasonable
amount of knowledge about their structure and physical-chemical properties
has been accumulated over recent years,
ionic liquids still represent a wide,
largely unexplored territory. In particular, much work is still needed order to
predict and utilize the intrinsic chemistry and properties of these intriguing
liquids.
[1] Y. Chauvin, L. Mussmann, H. Olivier,
Angew. Chem. 1995, 107, 2941 – 2943;
Angew. Chem. Int. Ed. Engl. 1995, 34,
2698 – 2700.
[2] Recent review: J. Dupont, R. F. de Souza, P. A. Z. Suarez, Chem. Rev. 2002,
102, 3667 – 3691.
[3] a) R. P. Swatloski, J. D. Holbrey, R. D.
Rogers, Green Chem. 2003, 5, 361 – 363.
b) G. S. Fonseca, A. P. Umpierre, P. F. P.
Fichtner, S. R. Teixeira, J. Dupont,
Chem. Eur. J. 2003, 9, 3263 – 3269.
[4] a) A. E. Bradley, J. E. Hatter, M. Nieuwenhuyzen, W. R. Pitner, K. R. Seddon,
R. C. Thied, Inorg. Chem. 2002, 41,
1692 – 1694; b) P. A. Z. Suarez, J. E. L.
Dullius, S. Einloft, R. F. de Souza, J.
Dupont, Inorg. Chim. Acta 1997, 255,
207 – 209.
[5] T. L. Aymes, S. T. Diver, J. P. Richard,
F. M. Rivas, K. Toth, J. Am. Chem. Soc.
2004, 126, 4366 – 4374.
[6] Reviews: a) A. J. Arduengo III, Acc.
Chem. Res. 1999, 32, 913 – 921; b) P.
Formentin, H. Garcia, A. Leyva, J.
Mol. Catal. A 2004, 214, 137 – 142.
[7] a) L. J. Xu, W. P. Chen, J. L. Xiao, Organometallics 2000, 19, 1123 – 1127; b) C. J.
Mathews, P. J. Smith, T. Welton, A. J. P.
White, D. J. Williams, Organometallics
2001, 20, 3848 – 3850; c) M. Hasan, I. V.
Kozhevnikov, M. Rafiq H. Siddiqui, C.
Femoni, A. Steiner, N. Winterton, Inorg.
Chem. 2001, 40, 795 – 800.
[8] a) V. K. Aggarwal, I. Emme, A. Mereu,
Chem. Commun. 2002, 1612 – 1613;
b) ILs can be used under basic conditions, for example, for N-indole and Onaphthol
deprotonation/alkylations:
M. J. Earle, P. B. McCormac, K. R. Seddon, Chem. Commun. 1998, 2245 – 2246.
[9] N. D. Clement, K. J. Cavell, C. Jones,
C. J. Elsevier, Angew. Chem. 2004, 116,
1297 – 1299; Angew. Chem. Int. Ed.
2004, 43, 1277 – 1279.
[10] D. S. McGuinness, K. J. Cavell, B. F.
Yates, Chem. Commun., 2001, 355 – 356.
[11] a) M. Amirnasr, M. K. Nazeeruddin, M.
GrLtzel, Thermochim. Acta 2000, 31,
105 – 114; b) M. R. R. Prasad, V. N.
Krishnamurthy, Thermochim. Acta
1991, 22, 1 – 10.
[12] J. E. L. Dullius, P. A. Z. Suarez, S. Einloft, R. F. de Souza, J. Dupont, J. Fischer,
A. De Cian, Organometallics 1998, 17,
815 – 819.
[13] J. D. Oxley, T. Prozorov, K. S. Suslick, J.
Am. Chem. Soc. 2003, 125, 11 138 –
11 139.
[14] H. Lebel, M. K. Janes, A. B. Charette,
S. P. Nolan, J. Am. Chem. Soc. 2004, 126,
5046 – 5047.
Published Online: September 21, 2004
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5297
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