Ionic liquid-assisted gelation of an organic solvent

Article abstract of DOI:10.1016/j.molliq.2010.08.011

Tetrafluoroborate-containing ionic liquids allow for a room temperature gelation of dimethylsulfoxide, which contains an organogelator at below the minimum gelation concentration. The gel formation has no effect on the mobility of ions, as the conductivity values of ionic liquid-DMSO gels are virtually identical to those of ionic liquid-DMSO solutions.

Full text of DOI:10.1016/j.molliq.2010.08.011

Journal of Molecular Liquids 157 (2010) 83–87
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Short Communication
Ionic liquid-assisted gelation of an organic solvent
⁎
Nicholas W. Smith, Joshua Knowles, John G. Albright, Sergei V. Dzyuba
Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetrafluoroborate-containing ionic liquids allow for a room temperature gelation of dimethylsulfoxide,
which contains an organogelator at below the minimum gelation concentration. The gel formation has no
effect on the mobility of ions, as the conductivity values of ionic liquid-DMSO gels are virtually identical to
those of ionic liquid-DMSO solutions.
Received 27 October 2009
Received in revised form 3 June 2010
Accepted 26 August 2010
Available online 31 August 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquids
Organogel
Conductivity
1. Introduction
sulfoxide (DMSO) could be gelled by a known organogelator A [7]
(Fig. 1) below A's minimum gelation concentration. Since high
Gelation of organic solvents using small molecules, i.e., low
molecular weight organogelators, holds great promise for various
areas of science and engineering [1]. Gels, as soft materials, with
unique thermo reversible electronic and/or mechanical properties
that are capable of modulating input–output responses are appealing
as sensors, membranes and molecular devices [1]. Generally, from the
structural point of view, organogels can be viewed as three-
dimensional supramolecular assemblies of the organogelator mole-
cules that “trap” solvent molecules within their network [1].
concentrations of organogelators are known to impair physical
properties of the gels, such as the ionic conductivity, for example,
minimizing the gelator's concentration, while still producing a gel is
highly advantageous. Here, we wish to report on a new ability [8] of
certain ionic liquids to induce the gelation of a molecular solvent
containing small amounts of A.
2. Materials and methods
Ionic liquids, compounds that are composed entirely of ions and
possess phase transition temperatures at or around room temperature,
have attracted considerable attention in recent years due to their wide
range of applications [2]. Primarily, ionic liquids have been used as
solvents for organic reactions and separation processes. Recently ionic
liquids have also been exploited as advanced materials and energetic
salts [3]. Gelation of ionic liquids is an interesting and potentially useful
area of research that might have implications in the areas of materials
and energy [4]. Specifically, gelled ionic liquids have been successfully
used in dye-sensitized solar cell systems and gas separations [5].
A greatnumber of organic compoundshave limited solubilityin ionic
liquids. In addition, high viscosity of ionic liquids affects a variety of
other physical properties such as conductivity, for example. These issues
significantly limit the utility of these solvents. In this light, studies on the
ionic liquid–molecular solvent mixtures are of interest [6].
2.1. General
All chemicals and solvents were from commercial sources (Sigma-
Aldrich or Acros), they were of highest grade possible, and were used
as received All ionic liquids were prepared according to literature
procedures following the general sequence shown in Scheme 1 [9].
Tetrabutylammonium tetrafluoroborate and sodium tetrafluorobo-
rate were from Aldrich, and were used as received.
2.2. Synthesis of ionic liquids
All tetrafluoroborate ionic liquids were treated with charcoal and
repetitive dissolutions into CH2Cl2 followed by filtration and removal
of CH2Cl2 to get rid of colored and inorganic impurities. Finally,
the ionic liquids were dried in vacuo to remove the residual CH2Cl2.
NMR spectra were recorded on a Varian-300 and chemical shifts are
reported in parts per million relative to internal tetramethylsilane.
During our studies on ionic liquid gelation of molecular organic
solvents, we noticed that mixtures of [C4−mim]BF4 1 and dimethyl-
[C4−mim]BF4, 1: [9b] 1H NMR (300 MHz, DMSO−d6) δ=9.07 (1H,
s), 7.73 (1H, t, J=1.8 Hz), 7.67 (1H, t, J=1.8 Hz), 4.13 (2H, t, J=7.2 Hz),
3.83 (s, 3H), 1.74 (2H, pent, J=7.7 Hz), 1.22 (2H, six, J=7.7 Hz), 0.88
(3H, t, J=7.2 Hz).
⁎
Corresponding author. Department of Chemistry, Texas Chrisrtian University, Fort
Worth, TX 76129, USA. Tel.: +1 817 257 6218; fax: +1 817 257 5851.
E-mail address: s.dzyuba@tcu.edu (S.V. Dzyuba).
0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2010.08.011

84
N.W. Smith et al. / Journal of Molecular Liquids 157 (2010) 83–87
2.4. Gel preparation for the conductivity measurements
The gel electrolytes were formed by mixing organogelator A,
DMSO, and ionic liquid in a vial followed by heating with a heat gun
until a homogeneous solution was obtained. The mixture was
transferred into the cell by syringe while hot followed by the insertion
of the electrodes. The measurement was performed after the gel was
formed.
Fig. 1. Structures of ionic liquid 1 and organogelator A.
2.5. Tgel measurements
[Bn−mim]BF4, 2: [10] 1H NMR (300 MHz, DMSO−d6) δ=9.18
Tgel were determined using test-tube-tilting method: [16] a vial
containing the gel was inversely (or horizontally, i.e., on its side)
immersed into a water-bath and the temperature was raised at 1 °C/
min. The Tgel was recorded as the temperature when the gelled mass
started to flow downward.
(1H, s), 7.77 (1H, t, J=1.8 Hz), 7.70 (1H, t, J=1.8 Hz), 5.40 (2H, s), 3.84
(3H, s).
[HO(CH2)2−mim]BF4, 3: [11] 1H NMR (300 MHz, DMSO−d6)
δ=9.04 (1H, s), 7.69 (1H, t, J=1.8 Hz), 7.65 (1H, t, J=1.8 Hz), 5.14
(1H, s), 4.19 (2H, t, J=4.8 Hz), 3.84 (3H, s), 3.70 (2H, m).
[C12−mim]BF4, 1: [9b] 1H NMR (300 MHz, DMSO−d6) δ=9.06
(1H, s), 7.73 (1H, t, J=1.8 Hz), 7.67 (1H, t, J=1.8 Hz), 4.13 (2H, t,
J=7.5 Hz), 3.83 (3H, s), 1.75 (2H, m), 1.22 (18H, m), 0.84 (3H, t,
J=6.6 Hz).
[C4−py]BF4, 5: [12] 1H NMR (300 MHz, DMSO−d6) δ=9.06 (2H,
d, J=5.6 Hz), 8.59 (1H, d, J=7.9 Hz), 8.14 (2H, t, J=6.9 Hz), 4.59 (2H,
t, J=7.7 Hz), 1.88 (2H, pent, J=7.7 Hz), 1.27 (2H, six, J=7.7 Hz), 0.89
(3H, t, J=7.2 Hz).
[Bn−py]BF4, 6: [12] 1H NMR (300 MHz, CDCl3) δ=9.64 (2H, d,
J=5.6 Hz), 8.43 (1H, t, J=7.9 Hz), 8.01 (2H, t, J=6.7 Hz), 7.68 (2H,
m), 7.31 (3H, m), 6.30 (2H, 1 s).
[HO(CH2)2−py]BF4, 7: [13] 1H NMR (300 MHz, DMSO−d6)
δ=8.99 (2H, d, J=5.4 Hz), 8.60 (1H, tt, J=7.5, 0.9 Hz), 8.15 (2H, t,
J=6.6 Hz), 5.20 (1H, s), 4.64 (2H, t, J=5.1 Hz), 3.84 (2H, t, J=5.1 Hz).
3. Results and discussion
A was reported to gelate a variety of molecular solvents [7]. 10 mg/ml
is a minimum gelator concentration for A to gel DMSO [7]. In order to test
the ability of [C4−mim]BF4 to assist in the gelation of DMSO, we prepared
a 2 mg/ml solution of A in DMSO and, as expected, no gelation of DMSO
was observed. However, upon addition of [C4−mim]BF4 (20% v/v) an
instantaneous formation of a stable transparent gel occurred at room
temperature. The gel formation was thermoreversible. We then examined
whether 1, which contained 2 mg/ml of A, could be gelled by the addition
of various amounts of DMSO. However, no gelation was observed when
DMSO was added to the solution of Ain 1. In this case the dissolution of the
ionic liquid into DMSO took place, leading to the precipitation of A. The gel
formation of the obtained heterogenoues mixture was only observed
upon subjecting it to a heating/cooling procedure, i.e., the vial was heated
with the heat gun until a homogeneous mixture was obtained and then
allowed to cool to room temperature. The gel formation was observed
visually by the inverse tube method.
In order to gain an insight into the role of 1 in the gelation process,
we probed 1-DMSO-A interactions using NMR spectroscopy, using
DMSO−d6 in place of DMSO. Chemical shifts of ionic liquids are
known to be environment and concentration dependent [17].
Therefore, if the ionic liquid is trapped within the gel, one would
expect the chemical shifts to be distinct from those observed in the
solution. Our experiments revealed that all chemical shifts of 1 (both
1H and 19F nuclei resonances) were identical in the gel state and in the
DMSO solution, i.e., in the presence and absence of A, respectively.
These experiments indicated that the ionic liquid is neither a part of
the gel network nor does it undergoes any aggregation processes
upon gelation. Due to low gelator concentration, it was not possible to
estimate the behavior of A in neither solution nor the gel states.
Collectively, the observations presented above indicated that the ionic
[C12−py]BF4, 8: [14] 1H NMR (300 MHz, CDCl3) δ=8.54 (2H, d,
J=5.8 Hz), 8.48 (1H, t, J=8.0 Hz), 8.05 (2H, t, J=7.2 Hz), 4.64 (2H, t,
J=7.4 Hz), 2.00 (2H, m), 1.24 (18H, m), 0.87 (3H, t, J=6.6 Hz).
2.3. Conductivity measurements
The specific conductance values were obtained by the direct
current four-electrode method using a custom-made glass cell (Fig. 2)
and Ag-electrodes at 25.00 0.01 °C. The measurements were
performed using: Hewlett Packard 34401A multimeter (Estd), Keithley
182 sensitive digital voltmeter (Ecell), AMEL instructions general
purpose potentiostat model 2049 (constant current source) and
Leed&Northrup 1000 Ohm D–C resistance standard (Rstd). The
conductance cell was calibrated using KCl solutions, according to
literature procedures [15].
Scheme 1. Synthesis of ionic liquids.

N.W. Smith et al. / Journal of Molecular Liquids 157 (2010) 83–87
85
Fig. 2. Set-up (left) and the photograph of the cell (right) for the conductivity measurements.
liquid had a crucial, yet complementary role in the overall gelation
process, while A was primarily responsible for the gel formation.
To further explore the gelating ability of [C4−mim]BF4, we prepared
a set of 1/DMSO mixtures (Fig. 3) with various concentrations of 1,
which contained a fixed amount (2 mg/ml) of A. A gradual increase, i.e.,
from 5 to 10 to 15% (v/v) of 1, led to a visual increase of the mixture's
viscosity, and a formation of the stable transparent gel was observed
once the amount of 1 reached 20% (Fig. 3). Increasing the amount of the
ionic liquid up to 70% resulted in a transition from transparent to non-
transparent, off-white gels. Mixtures containing N75% of 1 were viscous
heterogeneous solutions. The gel–sol transition temperature (Tgel) of
the gels appeared to be dependent on the ionic liquid content (Fig. 3).
on the ability of 1 to assist in the gelation of DMSO, we prepared a gel
that contained A (2.0 mg/ml), DMSO (80% v/v) and 1 (20% v/v).
Incremental addition of water up to 1% (v/v), followed by heating/
cooling cycles, after each portion of water was added produced the
gel. This indicated that the gel formation was unaffected by these
amounts of water. Addition of water (up to 10% v/v) to the solution of
A (2.0 mg/ml) in DMSO caused a precipitation of A. Hence, the
observed induced room temperature gelation of DMSO by the ionic
liquid 1 is not caused by the presence of small amounts of water, but
rather is attributed to the specific interactions of 1, A and DMSO.
Intrigued by this novel property of [C4−mim]BF4 1 to induce the
room temperature gelation of DMSO in the presence of small amounts
of A, we screened other common ionic liquids, such as [C4−mim]X,
where X=NO3, PF6, Br, and NTf2. It appeared that none of these ionic
liquids was able to produce a gel upon addition to the DMSO solution
of A neither at room temperature nor upon exposing the mixture to
the heating/cooling gel-forming procedure. Hence, it appeared that
BF4-anion was crucial for the observed phenomenon. In order to probe
the scope of ionic liquid-assisted gelation, we screened other
molecular solvents. Interestingly, the ionic liquid-induced gelation
of organic solvents was specific to DMSO as no gel formation was
observed when 1 was added to the solutions of A in methanol,
ethanol, iso-propanol, acetonitrile, tetrahydrofuran, dimethylforma-
mide, and acetone. Collectively, these data indicated that a very
specific and fine balance exists within this system, i.e., 1-A-DMSO,
which is responsible for the gelation process.
Tgel gradually changed as the concentration of the ionic liquid increased
to 40% v/v. Subsequent increase of the ionic liquid content had only a
marginal impact on the Tgel values and at 70% of 1, the temperature
decreased to ca. 50 °C.
It is noteworthy that gelation of ionic liquids by low molecular
weight organogelators requires extensive heating to dissolve the
organogelator into ionic liquids [4,5]. In all the cases shown in Fig. 3,
the gelation occurred at room temperature upon the addition of the
ionic liquid.
Small amounts of water in some ionic liquids are known to alter
their physical properties [18]. In view of a hygroscopic nature of
tetrafluoroborate ionic liquids as well as that of DMSO, small amounts
of water might be expected to affect the gelation. Since the formation
of the gels is done on a bench-top, it is likely that various amounts of
water (at least at ppm levels) will always be present, even if the ionic
liquids are extensively dried. In order to evaluate the effect of water
Next, we tested the effect of the cation on the gelation of DMSO in a
series of tetrafluoroborate ionic liquids/solids (Figs. 1 and 4). It
appeared that only imidazolium- and pyridinium-containing ionic
liquids had the ability to assist in the gelation of DMSO (Table 1).
Several structural features appeared to be important for the gelation.
The substituent on the nitrogen had a profound effect on the gelation
of A/DMSO solution.
Specifically, short alkyl chain-containing ionic liquids are more
efficient in the gelation process. For example, butyl-containing ionic
liquids 1 and 5 are superior to those ionic liquids or solids with
dodecyl-containing ionic liquids 4 and 8. Although 4 did not induce an
immediate formation of the gel upon addition to A/DMSO solution, a
notable increase of the viscosity was noted after a week for solutions
containing N30% of 4. Ionic solid 8 (up to 50% content) failed to gel the
A/DMSO mixture. Ionic liquids containing a benzyl group, 2 and 6,
were proven efficient gel inducers.
Hydroxy-containing ionic liquids, 3 and 7, required a slightly
smaller amount of A which may be attributed to an additional
hydrogen-bonding moiety that might be involved in the interactions
with A. The aromatic cation, either an imidazolium or a pyridinium, is
essential for the gelation as neither [C4444N]BF4, 9, nor NaBF4, 10, (up
to 60% w/v) induced the gelation of a A/DMSO mixture even after
14 days.
Further, we examined the ionic conductivity of the ionic liquid-
DMSO gel (Table 1). In the gel state, i.e., ionic liquid/A/DMSO system,
the mobility of ions in the ionic liquids was not impaired by the gel
formation, since the conductivity of the solution states, i.e., ionic
liquid/DMSO, and the gel states were virtually identical. These data
are in accord with the NMR experiments described above, which also
Fig. 3. Solutions and gels of 1 various concentrations of 1 in the A/DMSO mixture (top)
and the corresponding Tgel (bottom) as a function of [1], % (v/v); Tgel values are the
average of two or three measurements SD. Line is just for guiding an eye.

86
N.W. Smith et al. / Journal of Molecular Liquids 157 (2010) 83–87
Fig. 4. Structures of ionic liquids and solids.
showed that the ionic liquid is not a part of the gel network. In the case
of the 1/DMSO/A gel, we also measured the conductivity as a function
of time and found that no significant changes during the sol to gel
transition were taking place.
We also found that the presence of small molecules in DMSO
solutions, including Zn-tetraphenylporphyrin, krypto-BODIPY [19]
and Congo red dyes (mM concentrations), had no impact on the
ability of the ionic liquids to induce the gel formation. This
observation should potentially create opportunities for designing
novel soft materials with tunable properties [20].
4. Conclusions
In summary, we have identified a new ability of ionic liquids to
assist a common organogelator in gelling the organic solvent. In the
presence of below the minimum gelation concentration of the
organogelator, tetrafluroborate ionic liquids induced a room temper-
ature gelation of DMSO.
Acknowledgement
In order to check whether the structure of the organogelator had any
effect on the ionic liquid-induced gelation, we have tested another
“super” organogelator, i.e., trans(1R,2R)-N,N′-1,4-cyclohexane diyl bis
dodecanamide [21]. No gelation was noted with any of the tetrafluor-
oborate-containing ionic liquids 1–8 (Figs. 1 and 4). This indicates that a
very fine balance exists between the structure of the organogelator and
the ionic liquid, which leads to the gelation of DMSO.
This work was supported by the donors of the American Chemical
Society Petroleum Research Fund (47965-G7). N.W.S. would like to
thank ACS Division of Medicinal Chemistry for the predoctoral
fellowship. J.M.K. acknowledges TCU-SERC. S.V.D. is a recipient of
the TCU-JFSR award. The authors are grateful to Professor Onofrio
Annunziata and Mr. Jamie Cuanzon for their helpful discussions and
for making the conductivity cell, respectively.
Table 1
Appendix A. Supplementary data
Properties of ionic liquid/A/DMSO gels and solutions.
Ionic liquid (% v/v)a
[A], mg/mlb
k, mS cm−1c
Supplementary data to this article can be found online at
doi:10.1016/j.molliq.2010.08.011.
Gel
Solutiond
1 (20)
2 (23)
3 (9)
1.7
1.7
1.3
–
14.6
12.9
10.2
–
15.2
12.9
10.4
–
References
e
4 (−)
[1] (a) P. Terech, R.G. Weiss, Chem. Rev. 97 (1997) 3133;
(b) J.H. van Esch, B. Feringa, Angew. Chem. Int. Ed. 39 (2000) 2263;
(c) M. George, R.G. Weiss, Acc. Chem. Res. 39 (2006) 489.
[2] (a) T. Welton, Chem. Rev. 99 (1999) 2071;
5 (23)
6 (20)
7 (13)
8 (−)e
1.7
1.4
1.3
–
15.3
13.2
12.4
–
15.6
13.3
12.3
–
(b) S.V. Dzyuba, R.A. Bartsch, Angew. Chem. Int. Ed. 42 (2003) 148;
(c) B. Wyyershausen, K. Lehmann, Green Chem. 7 (2005) 15;
(d) S. Lee, Chem. Commun. (2006) 1049;
See text for details.
a
Minimum ionic liquid concentration.
Minimum A concentration at the minimum ionic liquid content.
Electrolytic conductivity.
No gelator A is present.
No gelation occurred.
(e) X. Han, D.W. Armstrong, Acc. Chem. Res. 40 (2007) 1079;
(f) N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123.
[3] (a) M. Smiglak, A. Metlen, R.D. Rogers, Acc. Chem. Res. 40 (2007) 1182;
(b) H. Gao, Y.-H. Joo, B. Twamley, Z. Zhou, J.M. Shreeve, Angew. Chem. Int. Ed. 48
(2009) 2792.
b
c
d
e

N.W. Smith et al. / Journal of Molecular Liquids 157 (2010) 83–87
87
[4] (a) A. Ikeda, K. Sonda, M. Ayabe, S. Tamaru, T. Nakashima, N. Kimizuka, S. Shinkai,
(b) J.D. Holbrey, K.R. Seddon, J. Chem. Soc. Dalton Trans. (1999) 2133;
(c) S.V. Dzyuba, R.A. Bartsch, J. Heterocycl. Chem. 38 (2001) 265;
(d) S.V. Dzyuba, R.A. Bartsch, Chemphyschem 3 (2001) 161.
Chem. Lett. (2001) 1154;
(b) W. Kubo, S. Kambe, S. Nakade, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida,
J. Phys. Chem. B 107 (2003) 4374;
[10] (a) G. Rangits, L. Kollár, J. Mol. Catal. A: Chem. 246 (2006) 59;
(c) K. Hanabusa, H. Fukui, M. Suzuki, H. Shirai, Langmuir 21 (2005) 10383.
[5] (a) N. Mohmeyer, D. Kuang, P. Wang, H.W. Schmidt, S.M. Zakeeruddin, M.J.
Gratzel, J. Mater. Chem. 16 (2006) 2978;
(b) B.A. Voss, J.E. Bara, D.L. Gin, R.D. Noble, Chem. Mater. 21 (2009) 3027.
[6] (a) For some examples see:J. Wang, S. Zhang, H. Wang, Y. Pei, J. Chem. Eng. Data
54 (2009) 3252;
(b) T. Erdmenger, J. Vitz, F. Wiesbrock, U.S. Schibert, J. Mater. Chem. 18 (2008)
5267.
[11] L.C. Branco, J.N. Rosa, J.J.M. Ramos, C.A.M. Afonso, Chem. Eur. J. 8 (2002) 3671.
[12] S. Petit, R. Azzouza, C. Fruitm, L. Bischoff, Tetrahedron Lett. 49 (2008) 3663.
[13] J.-C. Legeay, J.J.V. Eynde, J.P. Bazureau, Tetrahedron 61 (2005) 12386.
[14] Y.-F. Hu, T.-M. Guo, Langmuir 21 (2005) 8168.
(b) M.A. Iglesias-Otero, J. Troncoso, E. Carballo, L. Romaniui, J. Chem. Thermodyn.
40 (2008) 949;
(c) R.F. de Sousa, J.C. Padilha, R.S. Goncalves, M.O. de Souza, J. Rault-Berthelot, J.
Power Sources 164 (2007) 792;
[15] E. Juhász, K.N. Marsh, Pure Appl. Chem. 53 (1981) 1841.
[16] T. Kishida, N. Fujita, K. Sada, S. Shinkai, Langmuir 21 (2005) 9432.
[17] (a) For some examples, see:P.S. Schulz, K. Schneiders, P. Wassecheid, Tetrahedron
Asymmetr. 20 (2009) 2479;
(d) J. Wang, H. Wang, S. Zhang, H. Zhang, Y. Zhao, J. Phys. Chem. B 111 (2007) 6181;
(e) J. Rodrigez, J.F. Brennecke, J. Chem. Eng. Data 51 (2006) 2145;
(f) J. Wang, Y. Tian, Y. Zhao, K.A. Zhui, Green Chem. 5 (2003) 618.
[7] K. Hanabusa, K. Hiratsuka, M. Kimura, H. Shirai, Chem. Mater. 11 (1999) 649.
[8] Recently, the gel formation was achieved by simply mixing two distinct ionic liquids
gelation:. J. Kagimoto, N. Nakamura, T. Kato, H. Ohno, Chem. Commun. (2009) 2405.
[9] (a) J.G. Huddleston, H.D. Willauer, R.P. Swatloski, A.E. Visser, R.D. Rogers, Chem.
Commun. (2001) 1765;
(b) Y. Zhang, S. Gao, J. Wang, J. Tng, J. Phys. Chem. B 112 (2008) 2031;
(c) A.D. Headley, S.R.S. Kotti, J. Nam, K. Li, J. Phys. Org. Chem. 18 (2005) 1018.
[18] K.R. Seddon, A. Stack, M.-J. Torres, Pure Appl. Chem. 72 (2000) 2275.
[19] (a) A. Loudet, K. Burgess, Chem. Rev. 107 (2007) 4891;
(b) G. Ulrich, R. Ziessel, A. Harriman, Angew. Chem. Int. Ed. 47 (2008) 1184.
[20] M. Tamada, T. Watanabe, K. Horie, H. Ohno, Chem. Commun. (2007) 4050.
[21] K. Hanabusa, M. Yamada, M. Kimura, H. Shirai, Angew. Chem. Int. Ed. Engl. 35
(1996) 1949.