currents of the two main peaks are directly proportional to the
square root of the scan rate; this could be a sign of a diffusion
controlled solution phase process within the ionic liquid (no
adsorption to the electrode surface). However in light of the
previous proposal of formation of a passivating layer of the
electrode surface, it is also possible that the peak current to scan
rate relationship could be attributable to a diffusion controlled
process within such a film on the electrode surface. The relative
sizes of the two main peaks indicate that this is probably an EC
process, i.e. an electrochemical oxidation followed by a
chemical process occurring on the same time scale as the sweep
time of the voltammetry. In this way less oxidised material is
available for re-reduction. The potential window of the material
is extremely large as expected for an ionic liquid. Reductive
decomposition seems to occur after about 23 V. The positive
limit (although the response is noisy) is above +4 V. The width
of the anodic peak increases dramatically with time (with a
slight peak current increase), presumably as the ionic liquid
starts to decompose.
We would like to thank the Royal Society for a University
Research Fellowship (to P. J. D.) and the EPSRC and University
of York for financial support.
Notes and references
1
L. Mond, C. Langer and F. Quincke, J. Chem. Soc. (London), 1890,
49.
7
2 For example, see: Transition Metal Clusters, ed. B. F. G. Johnson,
Wiley, Chichester, 1980; J. Lewis and P. R. Raithby, J. Organomet.
Chem., 1995, 500, 227; Transition Metal Carbonyl Cluster Chemistry,
P. J. Dyson and J. S. McIndoe, Gordon and Breach Science Publishers,
Amsterdam, 2000.
3
4
J. E. Ellis, Adv. Organomet. Chem., 1990, 31, 1.
C. Bach, H. Willner, C. Wang, S. J. Rettig, J. Trotter and F. Aubke,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1974; S. M. Ivanov, S. M.
Miller, O. P. Anderson, K. A. Solntsev and S. H. Strauss, Inorg. Chem.,
1
999, 38, 3756.
5 Catalysis by Di- and Polynuclear Metal Cluster Complexes, ed. R. D.
Adams and F. A. Cotton, Wiley-VCH, New York, 1998.
6 Transition Metals in Organic Synthesis, ed. S. E. Gibson, Oxford
University Press, Oxford, 1997.
Ionic liquids based on imidazolium cations have recently
attracted much interest as green solvents for catalysis.13 Two
main methodologies have been explored in catalysis. First,
where the ionic liquid is itself catalytically active, for example
those with chloroaluminate anions have been shown to catalyse
Friedel–Craft reactions14 and depolymerisation. Second,
where the ionic liquid is essentially inert and is used to dissolve/
support homogeneous catalysts. This latter technique has been
7
8
B. F. G. Johnson, J. Chem. Soc., Dalton Trans., 1997, 1473.
Iron Compounds in Organic Synthesis, A. J. Pearson, Academic Press,
London, 1994.
9
A suspension of [bmim]Cl (14.0 g) in propanone was slowly added to a
15
4 2
flask containing Na[Co(CO) ] (12.0 g) under an atmosphere of N with
continuous stirring. After a short time, the colour of the solution
changed from pale yellow to blue and precipitation of sodium chloride
commenced. After stirring for 24 h the solution was filtered and the
solvent was removed under reduced pressure. The resulting deep blue
liquid was then further dried and degassed under high vacuum over a
period of 24 h before storage at 220 °C under an inert atmosphere.
1
6
17
used in reactions such as hydrogenation, hydroformylation,
C–C coupling18 and many others. The anion [Co(CO)
generated from Co (CO) , and NaOH, has previously been
shown to catalyse dehalogenation and coupling reactions.
Sodium hydroxide is soluble in the [bmim][Co(CO) ] ionic
13
2
4
] ,
2
8
19
2
1
1
2 2
0 Spectroscopic data: IR (uCO, CH Cl ) 1890 cm . Electrospray mass
4
+
3
spectrum (CHCl ): Positive ion: 139 [bmim] , Negative ion: 171
liquid and the solution was found to catalyse the debromination
of 2-bromo-2A-acetonaphthone and 2-bromoacetophenone to
their corresponding ketones (see Scheme 1). The melting point
of 2-bromo-2A-acetonaphthone is ca. 83 °C and was therefore
dissolved in benzene. In contrast, the lower melting point of
2 2
4 3 2
] , 143 [Co(CO) ] . UV-VIS (Me CO) 664, 645 (sh), 605
[
(
6
Co(CO)
1
sh), 275 (sh), 230 nm. H NMR (neat): d 9.49 (br s, 1H), 6.17 (br s, 1H),
13
.05 (br s, 2H), 3.88 (br s, 1H), 3.19 (br s, 1H), 3.44 (br s, 2H). C NMR
(neat): d 133.08, 121.00, 119.63, 47.15, 34.13, 29.07, 16.37, 10.46. The
carbonyl carbon atoms were not observed.
2-bromoacetophenone (250 °C) meant it could be added
11 P. S. Braterman, Metal Carbonyl Spectra, Academic Press, London,
1
975.
directly to the catalyst without the need for organic solvents or
other reagents. Extraction of the product into an organic solvent
also removed the bromine generated during the reaction from
the ionic liquid, which was recovered with the characteristic
blue colour with the same spectroscopic properties.
1
2 Electrochemistry was performed in a small volume cell with a platinum
disc working electrode, a platinum counter electrode and a platinum
pseudo-reference electrode with current densities quoted. The electro-
chemistry was performed on the neat ionic liquid. In such a set-up, with
no recognised background electrolyte or redox standard, the potential
versus the platinum pseudo-reference is difficult to compare with
standard potentials, however, in such unusual conditions it is the
qualitative nature of the electrochemistry that is important.
1
3 T. Welton, Chem. Rev., 1999, 99, 2071; J. D. Holbrey and K. R. Seddon,
J. Chem. Soc., Dalton Trans., 1999, 2133; P. Wasserscheid and W.
Keim, Angew. Chem., Int. Ed., 2000, 39, 3773.
1
4 C. J. Adams, M. J. Earle, G. N. Roberts and K. R. Seddon, Chem.
Commun., 1998, 2097; P. J. Dyson, M. C. Grossel, N. Srinivasan, T.
Vine, T. Welton, D. J. Williams, A. J. White and T. Zigras, J. Chem.
Soc., Dalton Trans., 1997, 3465.
1
1
5 C. J. Adams, M. J. Earle and K. R. Seddon, Green Chem., 2000, 21.
6 For example, see: Y. Chauvin, L. Mussmann and H. Olivier, Angew.
Chem., Int. Ed. Engl., 1995, 34, 2698; P. A. Z. Suarez, J. E. L. Dullius,
S. Einloft, R. F. de Souza and J. Dupont, Polyhedron, 1996, 15, 1217:
P. J. Dyson, D. J. Ellis, D. G. Parker and T. Welton, Chem Commun.,
1999, 25: C. J. Adams, M. J. Earle and K. R. Seddon, Chem. Commun.,
Scheme 1
We
bmim][Mn(CO)
bmim][Co(CO)
have
prepared
] in an analogous manner to that described for
] and they are also intensely coloured and
[bmim][HFe(CO)
4
]
and
1
999, 1043.
[
[
5
1
1
7 C. C. Brasse, U. Englert, A. Salzer, H. Waffenschmidt and P.
Wasserscheid, Organometallics, 2000, 19, 3818.
4
liquid at room temperature, although much more viscous than
the cobalt liquid. Presumably, the colour is due to charge
8 C. J. Matthews, P. J. Smith and T. Welton, Chem. Commun., 2000,
1
1
249; J. Ross, W. Chen, L. Xu and J. Xiao, Organometallics, 2001, 20,
38.
transfer from the transition metal carbonyl anion to the bmim
2
cation. The [HFe(CO)
4
]
anion is an active hydroformylation
1
9 H. Alper, K. D. Logbo and H. des Abbayes, Tetrahedron Lett., 1977,
catalyst20 and we will report on these ionic liquids in due
course.
2861.
20 N. Kutepow and H. Kindler, Angew. Chem., 1960, 72, 802.
Chem. Commun., 2001, 1862–1863
1863