The dramatic acceleration effect of imidazolium ionic liquids on electron transfer reactions

Article abstract of DOI:10.1039/b708044a

Imidazolium ionic liquids (ILs) exhibited a dramatic acceleration effect on the electron transfer from metal complexes such as (C₅Me ₅)₂Fe(ii) and (C₅Me₅) ₂Co(ii) to the oxygen molecule; this acceleration effect can be ascribed to the stabilization of the oxygen radical anions by coordinating with the acidic C2-H of imidazolium ILs. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708044a

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COMMUNICATION
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The dramatic acceleration effect of imidazolium ionic liquids on
electron transfer reactions{
Doo Seong Choi,a Dong Hyun Kim,a Ueon Sang Shin,a Ravindra R. Deshmukh,a Sang-gi Leeb and
Choong Eui Song*a
Received (in Cambridge, UK) 29th May 2007, Accepted 11th July 2007
First published as an Advance Article on the web 31st July 2007
DOI: 10.1039/b708044a
imidazolium proton. Hence, it could be assumed that the rate of
aerobic oxidation of metal complexes may be largely dependent on
the counter anion of imidazolium ILs.
Imidazolium ionic liquids (ILs) exhibited a dramatic accelera-
tion effect on the electron transfer from metal complexes such
as (C5Me5)2Fe(II) and (C5Me5)2Co(II) to the oxygen molecule;
this acceleration effect can be ascribed to the stabilization of the
oxygen radical anions by coordinating with the acidic C2-H of
imidazolium ILs.
In order to investigate whether imidazolium ILs can really
promote the one-electron transfer reaction, we first carried out the
aerobic oxidation of Fe(II) complex 1 in the presence and absence
of ILs 2 and the progress of the reaction was monitored periodi-
cally by UV-Vis spectroscopy. Upon mixing 1 (20 mg) with
[bmim][X] 2 (9.2 mmol; bmim 5 1-butyl-3-methylimidazolium
cation; X 5 SbF6, PF6, NTf2, BF4 and Cl) in CH2Cl2 (10 mL), the
color changed from yellow to green. As shown in the UV-Vis
spectra in Fig. 1 (curve a–e), in the presence of [bmim][X] 2, a
strong red shift was observed in the specific absorption of Fe(II)
complex 1 from 423 nm to 779 nm of Fe(III)-X 3 complex. In
contrast to these results, no oxidation was observed in the absence
of ILs 2 (curve f) indicating that IL presence is crucial for the
aerobic oxidation of the metal complex. As expected, the oxidation
rate was highly dependent on the anion’s structure and thus
decreased in the order of SbF62 . PF62 . NTf22 y BF42 & Cl2
which reflected the relative acidity of the C2-H.
In nature, electron transfer plays a fundamental role in most
important processes such as photosynthesis, respiration and
nitrogen fixation. Transition metals such as copper and iron are
known to be crucial in electron transport as one-electron redox-
active centers within proteins.1 Mimicking these biological systems
enables the aerobic oxidation of metal complexes, which are
promoted by Brønsted and Lewis acids. Mechanistically these
aerobic oxidations proceed through electron transfer from metal
complexes to an oxygen molecule, thereby affording an oxygen
radical anion intermediate, which can be stabilized by coordinating
with an acid, consequently lowering the activation energy.2 In this
paper, we present our finding of the remarkable ability of
imidazolium-based ionic liquids (ILs) to promote electron transfer
reactions such as aerobic oxidation of metal complexes.
To confirm the role of acidity of the C2-proton of ILs 2 in the
oxidation process, we also carried out the same oxidation of 1
(20 mg) in CH2Cl2 (10 mL) in the presence of three different types
of SbF6 salts (4.6 mmol),7 i.e., [Bu4N][SbF6], 2-SbF6 and [N-
butylthiazolium][SbF6]. As anticipated, the oxidation-promoting
effect profile of these salts was consistent with their relative acidity
scale. Thus, in the presence of [Bu4N][SbF6], which does not
Imidazolium ILs have recently received a great deal of attention
as potential new media for organic synthesis, catalyst support, and
nanostructure construction materials.3 The physicochemical prop-
erties of ILs such as hydrophilicity and hydrophobicity can easily
be varied by changing the counter anions. We have recently
demonstrated that the anion-directed, switchable properties of the
ILs could be transferred to the self-assembled monolayer surfaces
and carbon nanotubes.4 During our on-going study on IL
chemistry,5 we expected, as depicted in Scheme 1, that the aerobic
oxidation of a metal complex such as (C5Me5)2Fe(II) (1) might also
be promoted by employing imidazolium ILs since the imidazolium
ring contains the acidic C2-proton,6 for which the acidity scale
largely depends on the degree of hydrogen bonding between C2-H
and anion [X].6b The more basic anions such as Cl2 can be
strongly coordinated to the C2-proton in the imidazolium cation
via hydrogen bonding, thereby weakening the acidity of the
aDepartment of Chemistry, Institute of Basic Science, Sungkyunkwan
University, Suwon 440-746, Korea. E-mail: s1673@skku.edu;
Fax: (+82) 31-290-7075; Tel: (+82) 31-290-5964
bDivision of Nano Sciences (BK21)/Department of Chemistry, Ewha
Womans University11-1 Deahyun-dong, Seodaemun-gu, Seoul, 120-750,
Korea
{ Electronic supplementary information (ESI) available: compound
characterization data for 3-SbF6 (X-ray data) and N-butylthiazolium
[SbF6] (1H-, 13C- and 19F-NMR) and UV-Vis spectra of a mixture of
(C5Me5)2Co with ILs. See DOI: 10.1039/b708044a
Scheme 1 Proposed mechanism for aerobic oxidation of 1 to 3-X
promoted by imidazolium ILs 2.
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Fig. 1 UV-Vis spectra of a mixture of 1 (0.06 mmol) either with or
without ILs 2 (9.2 mmol) in CH2Cl2 (10 mL) (after 1 h).
Fig. 3 Single X ray crystal structure of (C5Me5)2Fe(III)-SbF6 (3-SbF6)
with the 50% probability thermal ellipsoids.
possess the Brønsted proton, the oxidation reaction proceeded
sluggishly (curve c of Fig. 2), whereas, [N-butylthiazolium][SbF6]
which bears a more acidic proton than does 2-SbF6,8 accelerated
the oxidation rate more dramatically than did 2-SbF6 (compare
curve a and b of Fig. 2). All these observations strongly support
our contention that the formed oxygen radical anion can be
stabilized by coordinating with the acidic C2-proton of the
imidazolium ring, whereas such a stabilization effect is impossible
in conventional solvents.
reactions, we tried to isolate the oxidized Fe(III)-SbF6 complex
3-SbF6. Fortunately, we could isolate green colored, single crystals
of Fe(III)-SbF6 complex 3-SbF6 by standing the reaction mixture
of 1 and [bmim][SbF6] in CH2Cl2 at room temperature for two
weeks and identify its structure by single crystal X-ray crystallo-
graphy (Fig. 3) (ESI{).9 According to our aforementioned
assumption, NHCs should also be formed in situ during the
aerobic oxidation of 1 in the presence of ILs 2. However, despite
our intensive efforts, we failed to isolate and identify NHC and its
derivatives. Therefore, to prove the in-situ NHC formation during
the electron transfer reaction from 1 to oxygen in the presence of
imidazolium ILs 2, we carried out transesterification reaction
under O2 atmosphere, since nucleophilic NHCs are known to be
efficient catalysts for this reaction.10 For this experiment, benzyl
alcohol (0.2 mL, 9.6 mmol), vinyl acetate (0.22 mL, 4.8 mmol),
and 20 mol% (based on benzyl alcohol) of [bmim][SbF6] were
added to a flask loaded with 20 mol% of 1 (Scheme 2). The
resulting green colored reaction mixture was then stirred under O2
atmosphere at room temperature for 24 h. Surprisingly, the
reaction proceeded smoothly and thus the acylated product
(benzyl acetate) was isolated in about 50% yield, although the
sterically non-hindered NHCs such as 4 may have been highly
unstable. However, in the absence of ILs 2, the same reaction
barely occurred with a product yield of less than 2%. The results
presented here combine to form strong evidence that imidazolium
ILs 2 promoted the electron transfer reactions. It should also be
here noted that the base-free generation of NHCs could also be
possible through the aerobic oxidation of metal complex with the
imidazolium ILs.11
Following these encouraging results, we extended the IL-
promoted electron transfer to other metallic system such as
(C5Me5)2Co(II) (5) to gain generality of this protocol. The aerobic
oxidation of 5 in CH2Cl2 was carried out in the presence and
absence of ILs ([N-butylthiazolium][SbF6], 2-PF6, 2-NTf2 and
2-OTf). As expected, in the presence of ILs, the dark-brown
Co-complex 5 was rapidly oxidized to the yellow Co(III)-X
complex. Moreover, similar to the case of Fe complex 1, oxidation
rate was dependent on the relative acidity of the C2-H of ILs
([N-butylthiazolium][SbF6] . 2-PF6 . 2-NTf2 . 2-OTf; UV-Vis
spectra, see ESI{).
According to our proposed mechanism (Scheme 1), the aerobic
oxidation of 1 promoted by imidazolium ILs 2 should afford
Fe(III)-X 3 and the in-situ formation of N-heterocyclic carbene
(NHC) 4 (or its derivatives). Thus, to obtain more direct evidence
for the promoting ability of imidazolium ILs 2 for electron transfer
In conclusion, the dramatic acceleration effect of imidazolium
ILs on the electron transfer from metal complexes such as
(C5Me5)2Fe(II) (1) and (C5Me5)2Co(II) (5) to oxygen molecule was
found which can be ascribed to stabilization of the oxygen radical
Fig. 2 UV-Vis spectra of a mixture of 1 (0.06 mmol) with a different type
of SbF6-salts (4.6 mmol) in CH2Cl2 (10 mL) (after 2 h).
Scheme 2 Transesterification catalyzed by base-free generated NHC.
This journal is ß The Royal Society of Chemistry 2007
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4 (a) B. S. Lee, Y. S. Chi, J. K. Lee, I. S. Choi, C. E. Song, S. K.
Namgoong and S.-g. Lee, J. Am. Chem. Soc., 2004, 126, 480; (b)
Y. S. Chi, S. Hwang, B. S. Lee, J. Kwak, I. S. Choi and S.-g. Lee,
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I. S. Choi and S.-g. Lee, Chem. Mater., 2006, 18, 1546.
5 Our recent examples: (a) C. E. Song, W. H. Shim, E. J. Roh and
J. H. Choi, Chem. Commun., 2000, 1695; (b) C. E. Song and E. J. Roh,
Chem. Commun., 2000, 837; (c) C. E. Song, W. H. Shim, E. J. Roh,
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D. W. Kim, C. E. Song and D. Y. Chi, J. Org. Chem., 2003, 68, 4281; (f)
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anions by coordinating with the acidic C2-H of imidazolium ILs.
Moreover, the oxidation rate can be controlled by simple switching
of the IL anion. We expect the present results to open up many
applications in the research area of electron transfer reactions. For
example, specific catalytically active oxidation states of transition
metal complexes can be stabilized in ILs against their reduction.
Moreover, the aerobic oxidation of metal complexes promoted by
imidazolium IL may be also utilized as a base-free generation
method of N-heterocyclic carbenes able to catalyze many organic
reactions.
This work was supported by a Korea Research Foundation
Grant (KRF-2005-005-J11901 for C. E. Song) funded by
MOEHRD, and by grants R01-2006-000-10426-0 (KOSEF for
C. E. Song and S.-g. Lee), R11-2005-008-00000-0 (SRC program
of MOST/KOSEF for C. E. Song), and CMDS at KAIST
(S.-g. Lee).
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7 To compare the promoting effect of these salts on this oxidation more
accurately, half the amounts (4.6 mmol) of salts were used as compared
to those (9.2 mmol) used in Fig. 1. When the same amounts of salts were
used as in Fig. 1, in the cases of 2-SbF6 and [N-butylthiazolium][SbF6],
the oxidations proceeded too fast and thus were completed within 1 h.
8 (a) F. G. Bordwell and A. V. Satish, J. Am. Chem. Soc., 1991, 113, 985;
(b) T. L. Amyes, S. T. Diver, J. P. Richard, F. M. Rivas and K. Toth,
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9 CCDC number of the single crystal of (C5Me5)2Fe(III)SbF6 3-SbF6 is
646677. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b708044a.
10 (a) G. A. Grasa, R. M. Kissling and S. P. Nolan, Org. Lett., 2002, 4,
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11 To generate the NHCs from imadazolium salts, a strong base is usually
needed which can cause side reactions.
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Chem. Commun., 2007, 3467–3469 | 3469