7Li NMR studies on complexation reactions of lithium ion with cryptand C211 in ionic liquids: Comparison with corresponding reactions in nonaqueous solvents

Article abstract of DOI:10.1021/ic101686q

⁷Li NMR spectra of DEME-TFSA [DEME = N,N-diethyl-N-methyl-N-(2- methoxyethyl)ammonium; TFSA = bis(trifluoromethanesulfonyl)amide], EMI-TFSA (EMI = 1-ethyl-3-methylimidazolium), MPP-TFSA (MPP = N-methyl-N-propylpyridinium), DEME-PFSA [PFSA = bis(pentafluoroethanesulfonyl)amide], and DEME-HFSA [HFSA = bis(heptafluoropropanesulfonyl)amide] ionic liquid (IL) solutions containing LiX (X = TFSA, PFSA, or HFSA) and C211 (4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5] eicosane) were measured at various temperatures. As a result, it was found that the uncomplexed Li(I) species existing as [Li(X)₂]^- in the present ILs exchange with the complexed Li(I) ([Li?C211]⁺) and that the exchange reactions proceed through the bimolecular mechanism, [Li?C211]⁺ + [*Li(X)₂]^- = [*Li?C211]⁺ + [Li(X)₂]^-. Kinetic parameters [k_s/(kg m^-1 s^-1) at 25 °C, ΔH^?/(kJ mol^-1), ΔS^?/ (J K^-1 mol^-1)] are as follows: 5.57 × 10 ^-2, 69.8 ± 0.4, and -34.9 ± 1.0 for the DEME-TFSA system; 5.77 × 10^-2, 70.6 ± 0.2, and -31.9 ± 0.6 for the EMI-TFSA system, 6.13 × 10^-2, 69.0 ± 0.3, and -36.7 ± 0.7 for the MPP-TFSA system; 1.35 × 10^-1, 65.2 ± 0.5, and -43.1 ± 1.4 for the DEME-PFSA system; 1.14 × 10^-1, 64.4 ± 0.3, and -47.1 ± 0.6 for the DEME-HFSA system. To compare these kinetic data with those in conventional nonaqueous solvents, the exchange reactions of Li(I) between [Li?C211]⁺ and solvated Li(I) in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were also examined. These Li(I) exchange reactions were found to be independent of the concentrations of the solvated Li(I) and hence proposed to proceed through the dissociative mechanism. Kinetic parameters [k_s/s ^-1 at 25 °C, ΔH^?/(kJ mol^-1), ΔS^?/(J K^-1 mol^-1)] are as follows: 1.10 × 10^-2, 68.9 ± 0.2, and -51.3 ± 0.4 for the DMF system; 1.13 × 10^-2, 76.3 ± 0.3, and -26.3 ± 0.8 for the DMSO system. The differences in reactivities between ILs and nonaqueous solvents were proposed to be attributed to those in the chemical forms of the uncomplexed Li(I) species, i.e., the negatively charged species ([Li(X)₂]^-) in ILs, and the positively charged ones ([Li(solvent)_n]⁺) in nonaqueous solvents.

Full text of DOI:10.1021/ic101686q

Inorg. Chem. 2011, 50, 1619–1627 1619
DOI: 10.1021/ic101686q
7Li NMR Studies on Complexation Reactions of Lithium Ion with Cryptand C211 in
Ionic Liquids: Comparison with Corresponding Reactions in Nonaqueous Solvents
Atsushi Shirai and Yasuhisa Ikeda*
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34 O-okayama,
Meguro-ku, Tokyo 152-8550, Japan
Received August 19, 2010
7Li NMR spectra of DEME-TFSA [DEME = N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium; TFSA = bis-
(trifluoromethanesulfonyl)amide], EMI-TFSA (EMI = 1-ethyl-3-methylimidazolium), MPP-TFSA (MPP = N-methyl-
N-propylpyridinium), DEME-PFSA [PFSA = bis(pentafluoroethanesulfonyl)amide], and DEME-HFSA [HFSA =
bis(heptafluoropropanesulfonyl)amide] ionic liquid (IL) solutions containing LiX (X = TFSA, PFSA, or HFSA) and
C211 (4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane) were measured at various temperatures. As a result, it
was found that the uncomplexed Li(I) species existing as [Li(X)2]- in the present ILs exchange with the complexed
Li(I) ([Li C211]þ) and that the exchange reactions proceed through the bimolecular mechanism, [Li C211]þ
þ
3
3
[*Li(X)2]- = [*Li C211]þ þ [Li(X)2]-. Kinetic parameters [ks/(kg m-1 s-1) at 25 °C, ΔHq/(kJ mol-1), ΔSq/(J K-1
3
mol-1)] are as follows: 5.57 ꢀ 10-2, 69.8 ( 0.4, and -34.9 ( 1.0 for the DEME-TFSA system; 5.77 ꢀ 10-2, 70.6 (
0.2, and -31.9 ( 0.6 for the EMI-TFSA system, 6.13 ꢀ 10-2, 69.0 ( 0.3, and -36.7 ( 0.7 for the MPP-TFSA
system; 1.35 ꢀ 10-1, 65.2 ( 0.5, and -43.1 ( 1.4 for the DEME-PFSA system; 1.14 ꢀ 10-1, 64.4 ( 0.3, and -47.1
( 0.6 for the DEME-HFSA system. To compare these kinetic data with those in conventional nonaqueous solvents,
the exchange reactions of Li(I) between [Li C211]þ and solvated Li(I) in N,N-dimethylformamide (DMF) and dimethyl
3
sulfoxide (DMSO) were also examined. These Li(I) exchange reactions were found to be independent of the
concentrations of the solvated Li(I) and hence proposed to proceed through the dissociative mechanism. Kinetic
parameters [ks/s-1 at 25 °C, ΔHq/(kJ mol-1), ΔSq/(J K-1 mol-1)] are as follows: 1.10 ꢀ 10-2, 68.9 ( 0.2,
and -51.3 ( 0.4 for the DMF system; 1.13 ꢀ 10-2, 76.3 ( 0.3, and -26.3 ( 0.8 for the DMSO system. The
differences in reactivities between ILs and nonaqueous solvents were proposed to be attributed to those in the
chemical forms of the uncomplexed Li(I) species, i.e., the negatively charged species ([Li(X)2]-) in ILs, and the
positively charged ones ([Li(solvent)n]þ) in nonaqueous solvents.
1. Introduction
(Mnþ),8-21 and electrolytes for capacitors or batteries,22-31
have been investigated.
In recent years, ionic liquids (ILs) have been given atten-
tion as environmentally benign alternatives to conventional
organic solvents because of their attractive properties such as
thermal stability, nonflammability, high ionic conductivity,
and wide electrochemical potential windows.1-3 In fact,
applications of ILs to various fields, e.g., media for syntheses
of organic compounds,4-7 extraction media for metal ions
In order to develop such applications, especially separa-
tions of Mnþ using ILs as extraction media, data on the
solvation and reactivity of metal complexes in ILs are
necessary. However, limited information is available on the
dynamic properties such as ligand exchange or substitution
reactions of metal complexes in ILs and the solvent effects on
their reactions. To our knowledge, only the following
reactions have been reported, that is, the substitution reac-
tions of [Pt(apa)Cl]þ [apa = 2,6-bis(aminomethyl)pyridine]
*To whom correspondence should be addressed. E-mail: yikeda@nr.
titech.ac.jp. Phone: þ81 3-5734-3061.
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2011 American Chemical Society
Published on Web 01/28/2011
pubs.acs.org/IC

1620 Inorganic Chemistry, Vol. 50, No. 5, 2011
Shirai and Ikeda
with thiourea and iodide in 1-butyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)amide (BMI-TFSA),32 the
substitution reactions of [Pt(ppp)Cl]þ (ppp = 2,20:60,200-
terpyridine) with thiourea and thiocyanate in 1-ethyl-3-
methylimidazolium (EMI)-based ILs,33,34 the reversible
binding reaction of NO to (TMPS)Fe(OH) (TMPS = meso-
[tetrakis(3-sulfonatomesityl)porphyrin]) in EMI-TFSA,35
the complexation reactions of Li(I) with cryptand C211 in
DEME-TFSA [DEME = N,N-diethyl-N-methyl-N-(2-met-
hoxyethyl)ammonium],36 and the OPPh3 (triphenylphos-
phine oxide) exchange reaction of [UO2(OPPh3)4]2þ in BMI-
NfO (NfO = nonafluorobutanesulfonate).37 Most of these
studies have examined whether the reactions of metal com-
plexes in ILs proceed in a manner similar to those in the
conventional solvents. Eldik et al. have found that the ratesof
substitution reactions in Pt(II) complexes in ILs are consid-
erably slower than those in the corresponding reactions in
methanol or water33,34 and suggested that a likely reason is
the interaction between the anionic component of ILs and the
positively charged metal complex. We also observed that the
kinetic parameters [7.2 ꢀ 103 M-1 s-1 (M = mol dm-3) at
Scheme 1. Schematic Structures of ILs Used in This Study
25 °C, ΔHq = 55.3 kJ mol-1, and ΔSq = 16.1 J K-1 mol-1
]
for the OPPh3 exchange reaction of [UO2(OPPh3)4]2þ in
BMI-NfO are largely different from those (1.4 ꢀ 105
M-1 s-1 at 25 °C, ΔHq=7.1 kJ mol-1, and ΔSq=-122 J
K
-1 mol-1) in CD2Cl237 and proposed that this difference is
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As mentioned above, data on the reactivities of metal
complexes in ILs have not been obtained sufficiently. Hence,
as part of the acquisition of basic data on the reactivities of
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of Li(I) between the Li(I) complexed with cryptand C211
(4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane) and the
uncomplexed Li(I) in DEME-TFSA, EMI-TFSA, MPP-
TFSA (MPP = N-methyl-N-propylpyridinium), DEME-
PFSA [PFSA = bis(pentafluoroethanesulfonyl)amide], and
DEME-HFSA [HFSA = bis(heptafluoropropanesulfonyl)-
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solvents have been reported,38-47 we have studied the exchange
reactions of Li(I) between the Li(I) complexed with cryptand
C211 and the solvated Li(I) in N,N-dimethylformamide
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Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1621
(DMF) and dimethyl sulfoxide (DMSO) dissolving LiTFSA to
compare with the reaction behavior in ILs.
Kinetic Analyses of Exchange Reactions. Sample solutions for
kinetic studies were prepared by dissolving LiTFSA, LiPFSA,
or LiHFSA and appropriate amounts of C211 into ILs in a dry
7
globebox. Kinetic analyses were carried out using a Li NMR
2. Experimental Section
line-broadening method.51 7Li NMR spectra of sample solu-
tions were measured using a JEOL-JNM-LA300WB (7Li,
116.65 MHz) NMR spectrometer at various temperatures.
The apparent first-order rate constants (kobs) for the Li(I)
Materials. DEME-TFSA (Kanto Chemical Co., Inc.), EMI-
TFSA (Japan Carlit Co., Ltd.), and MPP-TFSA (Kanto) were
purified by stirring with active carbon in water for 72 h, followed
by filtration of active carbon. A small amount of water in these
ILs was removed by evaporation in vacuo at 80 °C for more than
72 h. Lithium bis(trifluoromethanesulfonyl)amide (LiTFSA)
was prepared by neutralization of HTFAS (Kanto) with Li2CO3
(Kanto, 99.95%) in an aqueous solution, followed by evapora-
tion of water. Lithium bis(pentafluoroethanesulfonyl)amide
(LiPFSA; Kishida Chemical Co., Ltd., 99.5%) and LiHFSA
(Wako, 99%) were dried by evaporation in vacuo at 80 °C for
more than 72 h. The DEMECl compound was prepared by using
the same method as that reported previously.48 Cryptand C211
(Merck, 95%) was dried in vacuo at room temperature for 3
days and used without further purification. DMSO (Wako Pure
Chemical Ind., Ltd.) and DMF (Kanto) were dried by storage
over 4A molecular sieves (Wako) for 1 week. All other chemicals
were of reagent grade and were used as received.
exchange reactions were obtained from τLi values (kobs
=
1/τLi), which are the mean lifetimes of Li(I) of the complexed
7
site. The τLi values were evaluated from the best fitting of Li
NMR line shapes or obtained from the line width (kobs = 1/τLi
=
πW1/2,obs - πW1/20), where W1/2,obs and W1/20 are the line
widths (Hz) at half-height of 7Li NMR signals with and without
exchange, respectively. We used a 1.0 M LiCl/D2O solution as
an external standard of the chemical shifts of 7Li NMR.
3. Results and Discussion
3.1. Complex Formations of Li(I) with C211 in DEME-
TFSA, EMI-TFSA, and MPP-TFSA. Figure 1 shows 7Li
NMR spectra of DEME-TFSA solutions containing
LiTFSA (0.24 m, m = mol kg-1) and C211 (RC211
=
Syntheses of DEME-PFSA and DEME-HFSA. These hydro-
phobic ILs were synthesized according to an anion metathesis
method described in previous papers.49,50 Each aqueous solu-
tion (ca. 100 mL) containing LiPFSA (29.7 g, 7.67 ꢀ 10-2 mol)
and LiHFSA (30.2 g, 6.19 ꢀ 10-2 mol) was added slowly to
aqueous solutions (ca. 100 mL) containing DEMECl (13.8 g,
7.60 ꢀ 10-2 mol; 11.1 g, 6.13 ꢀ 10-2 mol), followed by stirring
for 24 h at room temperature and separation of the DEME-
PFSA and DEME-HFSA phases. In order to remove residual
LiCl in the obtained DEME-PFSA and DEME-HFSA, the
ultrapure water (ca. 200 mL) was added to CH2Cl2 solutions
containing crude DEME-PFSA and DEME-HFSA, followed
by shaking and standing. Furthermore, ultrapure water (ca.
200 mL) was added to the separated CH2Cl2 phase, followed by
shaking and separation. These procedures were repeated seven
times. To confirm the removal of LiCl in IL phases, the content
of Liþ and Cl- ions in the aqueous phases obtained in every
treatment was measured by using a inductively coupled plasma
atomic emission spectrometer (PerkinElmer OPTIMA 3000)
and an ion chromatograph (Dionex ICS-1500). After evapora-
tion of water and CH2Cl2 in ILs, organic impurities were
removed by stirring active carbon as mentioned above. Finally,
the resulting DEME-PFSA and DEME-HFSA were dried by
evaporation in vacuo at room temperature for more than 72 h.
Measurement of the Water Content of ILs. The water content
of each dried IL was determined by a Karl Fischer titration
method (Metrohm 831 KF coulmeter), i.e., 6.5 ppm for DEME-
TFSA, 11.6 ppm for EMI-TFSA, 6.3 ppm for MPP-TFSA, 6.5
ppm for DEME-PFSA, and 11.7 ppm for DEME-HFSA.
Measurements of NMR spectra of ILs. 1H, 13C, and 19F NMR
spectra of purified ILs were measured using JEOL JNM-
LA300WB (1H, 300.40 MHz) and JEOL ECX-400 FT-NMR
(13C, 100.53 MHz; 19F, 376.17 MHz) spectrometers. Chemical
shifts of 1H and 13C NMR were measured using tetramethylsi-
lane as an internal standard, and those of 19F NMR were
determined using a D2O solution containing 0.05 M NaF
as an external reference. As a result, each IL was confirmed to
be a single component (see Figures S1-S5 in the Supporting
Information).
[C211]/[LiTFSA] = 0.0-1.0). One signal is found to be
observed at -1.83 ppm in the DEME-TFSA solution of
RC211 = 0.0. In our previous study on the structure of
Li(I) in DEME-TFSA by using Raman spectroscopy,48
the Li(I) species in DEME-TFSA was confirmed to exist
as [Li(TFSA)2]- under the present experimental condi-
tions, i.e., [LiTFSA]=0.24 m, RTFSA=([DEME-TFSA] þ
[LiTFSA])/[LiTFSA]=10.0. Therefore, the signal at -1.83
ppm can be assigned as that due to [Li(TFSA)2]-. Further-
more, it is found that a new signal is detected at -1.02 ppm
and its intensity increases with an increase in RC211 and that
in the DEME-TFSA solution of RC211 = 1.0 the signal
observed at -1.83 ppm disappears and only one signal is
observed at -1.02 ppm. These phenomena indicate that the
signal at -1.02 ppm can be assigned to that of 1:1 complex
([Li C211]þ) of Li(I) with C211 and suggest that the
3
[Li(TFSA)2]- species are complexed with C211 stoichiomet-
rically; i.e., the stability constant (K) for [Li(TFSA)2]-
þ
C211 =[Li C211]þ in DEME-TFSA is very large. This is
consistent with the facts that the C211 compound with a
3
˚
cavity diameter of 1.6 A forms a very stable complex with
39
Li(I) (diameter = 1.56 A), that the K values for the
˚
complex formation of [Li C211]þ in organic solvents are
3
very large, e.g., K(at25°C) inmethanol, ethanol, DMF, and
DMSO are 108.0, 108.4, 106.9, and 105.8, respectively,52 and
that the K values for the complex formation of Ag(I) with
C222 in ILs are larger than those in DMSO.53 In addition,
the Popov and Lincoln groups have studied the structure of
the Li(I) complex with C211 in nonaqueous solvents40,41 and
reported that the Li(I) complex with C211 has the same
inclusive structure as that in the solid state.54 On the basis of
these reports, it seems likely that the [Li C211]þ complex in
3
DEME-TFSA has the inclusive cryptate structure incorpor-
ating Li(I) into the cavity of C211.
Similar phenomena were also observed in Li NMR
7
spectra of EMI- and MPP-TFSA solutions containing
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1622 Inorganic Chemistry, Vol. 50, No. 5, 2011
Shirai and Ikeda
Figure 1. 7Li NMR spectra of DEME-TFSA solutions containing
LiTFSA (0.24 m) and C211. RC211 = 0.0 (a), 0.2 (b), 0.4 (c), 0.5 (d), 0.6
(e), 0.8 (f), and 1.0 (g). Temperature = 25 °C.
Figure 3. Plot of the line width at half-height of the 7Li NMR signal due
to [Li C211]þ in the DEME-TFSA solution containing LiTFSA and
3
C211 (RC211 = 1.0) against η/T.
seen from this figure, both signals become sharp up to
80 °C and then broad with an increase in the temperature.
On the other hand, the signals of Li NMR spectra for
7
DEME-TFSA solutions of RC211 = 1.0 and 0.0 were
found to be observed around -1.0 and -1.9 ppm and
become sharp with an increase in the temperature from 25
to 140 °C, respectively (see Figure S8 in the Supporting
Information). Hence, it is likely that the phenomena
shown in Figure 2 are due to a decrease in the viscosity
and the exchange of Li(I) between [Li C211]þ and
3
[Li(TFSA)2]- sites with increasing temperature. In fact,
the viscosity coefficient of DEME-TFSA is 69 mPa s at
25 °C,56 which is much larger than those of conven-
tional nonaqueous solvents such as DMF (0.796 mPa s
at 25 °C) and DMSO (1.96 mPa s at 25 °C).57 In order to
confirm the effect of the viscosity, we examined the
temperature dependence of the line width at half-height
Figure 2. 7Li NMR spectra of a DEME-TFSA solution containing
LiTFSA (0.24 m) and C211 (0.12 m) measured at various temperatures.
RC211 = 0.5.
7
(W1/2, Hz) of the Li NMR signal (Figure S8A in the
Supporting Information) due to [Li C211]þ in the
3
DEME-TFSA solution of RC211 = 1.0, in which the
[Li(TFSA)2]- species is not present, and hence the effect
of the exchange of Li(I) on the line width should be
neglected. Under this condition, the line width (W1/2) is
given by the following equations58 because the relaxa-
tion time of 7Li (I = 3/2) should be mainly controlled by
the quadrupole relaxation mechanism
LiTFSA and C211 (see Figures S6 and S7 in the Support-
ing Information); that is, two signals were observed
at -1.82 and -1.05 ppm for the EMI-TFSA system
and -1.85 and -1.04 ppm for the MPP-TFSA system,
and the signals at -1.82 and -1.85 ppm disappeared in
both systems of RC211 = 1.0. Furthermore, from Raman
spectroscopic studies on the structures of Li(I) in EMI-
and MPP-TFAS,55 Li(I) species has been confirmed to
exist as [Li(TFSA)2]-.
W1=2 ¼ 1=πT2
2
¼ 3ð2I þ 3Þ=½40πI2ð2I - 1Þꢁðe2Qq=pÞ τc
ð1Þ
ð2Þ
From these results, it is proposed that, in the TFSA-
based ILs containing C211 and LiTFSA, the Li(I) species
exist as the complexed Li(I) ([Li C211]þ) and the un-
3
complexed Li(I) ([Li(TFSA)2]-), respectively, regardless
of the cationic components of ILs.
τc ¼ CVη=kT þ τ0
where e2Qq/p is the quadrupole coupling constant, η is
the viscosity of the solution, V is the molecular volume,
τc is the correlation time, τ0 is the correlation time at
viscosity = 0, and C is the experimentally determined
3.2. Complexation Reactions of Li(I) with C211 in
DEME-TFSA, EMI-TFSA, and MPP-TFSA. Figure 2
shows the temperature dependence of Li NMR spectra
7
of the DEME-TFSA solution containing LiTFSA (0.24
m, m = mol kg-1) and C211 (0.12 m, RC211 = 0.5), i.e.,
[Li C211þ] = 0.12 m and [Li(TFSA)2-] = 0.12 m. As
(56) Zhou, Z.-B.; Matsumoto, H.; Tatsumi, K. Chem.;Eur. J. 2005, 11,
752.
3
(57) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents,
Physical Properties and Method of Purification; Wiley: New York, 1986.
(58) Yaita, T.; Ito, D.; Tachimori, S. J. Phys. Chem. B 1998, 102, 3886–
3891.
(55) Umebayashi, Y.; Mitsugi, T.; Fukuda, S.; Fujimori, T.; Fujii, K.;
Kanzaki, R.; Takeuchi, M.; Ishiguro, S. J. Phys. Chem. B 2007, 111, 13028–
13032.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1623
Figure 4. 7Li NMR spectra of a EMI-TFSA solution containing LiTF-
SA (0.24 m) and C211 (0.12 m) measured at various temperatures.
RC211 = 0.5.
Figure 5. 7Li NMR spectra of a MPP-TFSA solution containing LiTF-
SA (0.24 m) and C211 (0.12 m) measured at various temperatures.
RC211 = 0.5.
containing LiTFSA (0.24 m) and C211 (0.12 m, RC211
=
dimensionless parameter. If our assumption is reason-
able, a plot of W1/2 against η/T should give a linear
relationship. Figure 3 shows such a plot, where the η/T
values for neat DEME-TFSA were used.22 The result-
ing linear relationship indicates that the sharpening of
the 7Li NMR signals in the range of 25-80 °C in
Figure 2 is due to a decrease in the viscosity of
DEME-TFSA. Hence, the broadening of the 7Li
NMR signals in the range of 90-140 °C should be
attributed to the exchange reaction of Li(I) between
0.5), as shown in Figures 4 and 5. Sharpening of the
signals in the low-temperature region of these figures is
also attributable to the effect of the viscosity of solutions,
because the viscosity coefficients of EMI- and MPP-
TFSA are also much larger than conventional nonaqu-
eous solvents, i.e., 33 and 151 mPa s.61
From these results, it is concluded that the exchange
reactions of Li(I) between [Li C211]þ and [Li(TFSA)2]-
3
sites, i.e., the complexation reactions of Li(I) with C211,
occur in DEME-, EMI-, and MPP-TFSA solutions con-
taining LiTFSA and C211.
[Li C211]þ and [Li(TFSA)2]- sites. Moreover, both
3
signals in Figure 2 are found to shift to lower field with
an increase in the temperature. Such a lower field shift
was also observed in the 7Li NMR spectra for DEME-
TFSA solutions of RC211 = 1.0 and 0.0 (see Figure S8 in
the Supporting Information). These phenomena are
considered to be due to changes in the bulk magnetic
susceptibility (χv) with an increase in the temperature
based on the relationship59,60
Measurements for the exchange reactions were carried
out using sample solutions (samp-1-samp-3) listed in
Table 1. The apparent first-order rate constants (kobs) for
the exchange reactions (rate = τLi-1[Li C211þ] = kobs
-
3
[Li C211þ]) were evaluated from 1/τLi values using a two-
3
site model of a NMR line-broadening method. The resulting
kobs values were plotted against the concentrations of un-
complexed Li(I) species, [Li(I)free] (=[Li(TFSA)2-] =
4
4
[LiTFSA] - [Li C211þ] = [LiTFSA] - [C211]). The plots
δobsðLiÞ ¼ - πχv ¼ - πχgF
ð3Þ
3
3
3
are found to give straight lines passing through the origin in
three systems, as shown in Figures 6-8. Hence, the rate
7
where δobs(Li), χg, and F are the chemical shift of Li for
equation is expressed as rate = kobs[Li C211þ] = ks[Li-
[Li C211]þ, the mass magnetic susceptibility, and the density
3
3
(I)free][Li C211þ] (ks = second-order rate constant). This
3
of the sample, respectively. If δobs(Li) is dependent on only the
change in χv of sample solutions, the plot of δobs(Li) against F
should give a linear relationship with a slope of 2.06, because
the χg value of neat DEME-TFSA was estimated to be
-0.491 ꢀ 10-6 cm3 g-1. In fact, the plot of δobs(Li) against
F gave a linear relationship with a slope of 2.07 (see Figure S9
in the Supporting Information). Furthermore, the differences
(ΔδC-F) in the chemical shifts of the signals due to
dependence of kobs on [Li(I)free] suggests that the complexa-
tion reactions of Li(I) with C211 in DEME-, EMI-, and
MPP-TFSA proceed through the following bimolecular
mechanism proposed by Shcori et al.62
[Li C211]þ and [Li(TFSA)2]- in Figure 2 were found to be
3
constant (see Figure S10 in the Supporting Information).
From these results, the lower field shift observed in Figure 2 is
concluded to be due to the changes in χv.
A similar temperature dependence was also observed in
the 7Li NMR spectra of EMI- and MPP-TFSA solutions
where the asterisk is used to denote the exchange species.
The ks values were obtained from the slopes in Figures 6-8
(59) Live, D. H.; Chan, S. I. Anal. Chem. 1970, 42, 791–792.
(60) Teweldemedhin, Z. S.; Fuller, R. L.; Greenblatt, M. J. Chem. Educ.
1996, 73, 906–909.
(61) Matsumoto, H.; Sakaebe, H.; Tatsumi, K. J. Power Sources 2005,
146, 45.

1624 Inorganic Chemistry, Vol. 50, No. 5, 2011
Shirai and Ikeda
Table 1. Components and Water Content of Sample Solutions for Complexation Reactions of Li(I) with C211 in ILs
sample no.
ILs
[Li(I)]/m (Li salt)
RC211 (water content/ppm)
samp-1
samp-2
samp-3
samp-4
samp-5
DEME-TFSA
EMI-TFSA
MPP-TFSA
DEME-PFSA
DEME-HFSA
0.24 (LiTFSA)
0.24 (LiTFSA)
0.24 (LiTFSA)
0.20 (LiPFSA)
0.16 (LiHFSA)
0.2 (10.3)
0.1 (10.1)
0.1 (10.3)
0.1 (7.4)
0.4 (11.6)
0.3 (11.4)
0.3 (11.6)
0.3 (8.5)
0.5 (12.2)
0.5 (12.7)
0.5 (12.9)
0.5 (9.4)
0.6 (12.9)
0.7 (14.0)
0.7 (14.0)
0.7 (10.4)
0.8 (14.0)
0.8 (14.6)
0.8 (14.6)
0.8 (10.9
0.9 (15.3)
0.9 (15.2)
0.9 (11.5)
0.3 (13.9)
0.5 (15.2)
0.9 (17.5)
Figure 6. Plots of kobs values against [Li(I)]free for the complexation
reaction of Li(I) with C211 in DEME-TFSA.
Figure 8. Plots of kobs values against [Li(I)]free for the complexation
reaction of Li(I) with C211 in MPP-TFSA.
electrostatic interaction is important in the complexation
reactions of Li(I) with C211 in the present ILs.
3.3. Complexation Reactions of Li(I) with C211 in
DEME-PFSA and DEME-HFSA. According to the
above mechanism, the complexation reactions of Li(I)
with C211 should be affected by the anionic components
of ILs because of the differences in the interactions
between Liþ and anionic species of ILs. Hence, the
complexation reactions of Li(I) with C211 in DEME-
PFSA and DEME-HFSA were examined. Figure 9 shows
7Li NMR spectra of DEME-PFSA solutions containing
LiPFSA (0.20 m) and C211 (RC211 = 0.0-1.0). These 7Li
NMR spectral changes are found to be very similar to
those in Figures 1 and S6 and S7 in the Supporting Infor-
mation. Hence, the signals at -1.03 and -1.70 ppm are
assigned to be those due to the [Li C211]þ complex and
3
the Li(I) species without coordination of C211 [the un-
complexed Li(I) species], respectively. Similar Li NMR
7
Figure 7. Plots of kobs values against [Li(I)]free for the complexation
reaction of Li(I) with C211 in EMI-TFSA.
spectra were observed in the DEME-HFSA solution
containing LiHFSA (0.16 m) and C211 (RC211 = 0.3-
1.0) (see Figure S11 in the Supporting Information).
Although the structures of the uncomplexed Li(I) species
in DEME-PFSA and DEME-HFSA have not been
known, it seems likely that the uncomplexed Li(I) species
exist as [Li(PFSA)2]- and [Li(HFSA)2]- because the
structures of PFSA and HFSA are similar to that of
TFSA except for the difference in the lengths of the
perfluoroalkyl chains.
and are listed in Table 2. The kinetic parameters for ks were
evaluated from the semilogarithmic plots of ks against
the reciprocal temperature (Eyring plot) and are shown in
Table 3. As seen from this table (TFSA-based IL systems),
the kinetic parameters are almost consistent with each other,
indicating that the present complexation reactions are
independent of the cationic components of ILs. This is
reasonable because the Li(I) species in DEME-, EMI-,
and MPP-TFSA exist as the same anionic species ([Li(TF-
SA)2]-) and should interact with the cationic species
7Li NMR spectra of the DEME-PFSA solution con-
taining LiPFSA (0.20 m) and C211 (0.10 m, RC211 = 0.5)
and the DEME-HFSA solution containing LiHFSA
(0.16m) andC211(0.08m, RC211 = 0.5) were also measured
by changes in the temperature. The results are shown in
Figures 10 and S12 in the Supporting Information and are
([Li C211]þ) in the same manner. This suggests that the
3
(62) Shchori, E.; Jagur-Gordzinski, J.; Luz, Z.; Shporer, M. J. Am. Chem.
Soc. 1971, 93, 7133–7138.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1625
Table 2. Second-Order Rate Constants (ks) at Various Temperatures for the Complexation Reactions of Li(I) with C211 in ILs
ks/(m-1 s-1
)
temp/°C
DEME-TFSA
EMI-TFSA
MPP-TFSA
DEME-PFSA
DEME-HFSA
110
115
120
125
130
135
140
36.4 ( 1.2
49.5 ( 1.5
66.0 ( 1.8
87.5 ( 2.2
115 ( 3
41.0 ( 1.6
55.2 ( 2.0
73.7 ( 2.4
37.9 ( 0.8
50.8 ( 1.1
60.7 ( 1.5
59.0 ( 1.2
77.6 ( 1.5
101 ( 2
131 ( 3
169 ( 3
216 ( 4
284 ( 8
46.7 ( 0.3
61.1 ( 0.4
79.5 ( 0.6
103 ( 1
128 ( 4
170 ( 6
221 ( 8
117 ( 3
151 ( 4
198 ( 6
133 ( 2
170 ( 2
219 ( 4
193 ( 4
Table 3. Kinetic Parameters for the Complexation Reactions of Li(I) with C211 in ILs and Nonaqueous Solvents
system
mechanism
kobs/s-1a
ks/(m-1 s-1 a
)
ΔHq/(kJ mol-1
)
ΔSq/(J K-1 mol-1
)
DEME-TFSA
EMI-TFSA
MPP-TFSA
DEME-PFSA
DEME-HFSA
DMF
Bb
B
B
B
5.57 ꢀ 10-2
5.77 ꢀ 10-2
6.13 ꢀ 10-2
1.35 ꢀ 10-1
1.14 ꢀ 10-1
69.8 ( 0.4
70.6 ( 0.2
69.0 ( 0.3
65.2 ( 0.5
64.4 ( 0.3
68.9 ( 0.2
76.3 ( 0.3
-34.9 ( 1.0
-31.9 ( 0.6
-36.7 ( 0.7
-43.1 ( 1.4
-47.1 ( 0.6
-51.3 ( 0.4
-26.3 ( 0.8
B
Dc
D
1.10 ꢀ 10-2
1.13 ꢀ 10-2
DMSO
a Values at 25 °C. b B = bimolecular mechanism. c D = dissociative mechanism.
Figure 9. 7Li NMR spectra of DEME-PFSA solutions containing
LiPFSA (0.20 m) and C211. RC211 = 0.0 (a), 0.1 (b), 0.3 (c), 0.5 (d), 0.7
(e), 0.9 (f), and 1.0 (g). Temperature = 25 °C. *: 7Li NMR signal of a D2O
solution containing LiCF3SO3 (0.1 m; external standard).
Figure 10. 7Li NMR spectra of a DEME-PFSA solution containing
LiPFSA (0.20 m) and C211 (0.10 m) measured at various temperatures.
RC211 = 0.5.
found to be similar to those shown in Figures 2, 4, and 5.
This indicates that the Li(I) species exchange between
[Li C211]þ and [Li(PFSA)2]- or [Li(HFSA)2]- sites. Ki-
DEME-HFSA > DEME-TFSA. The detailed information
on the structures of Li(I) species in DEME-PFSA and
DEMA-HFSA must be necessary to discuss the effect of
the anionic components of the present ILs on the complexa-
tion reactions of Li(I) with C211.
3.4. Complexation Reactions of Li(I) with C211 in
Nonaqueous Solvents. The complexation reactions of Li-
(I) with C211 in DMF and DMSO were examined to
compare with the corresponding reactions in ILs men-
tioned above because the chemical forms of the Li(I)
species in the solutions prepared by dissolving LiTFSA
into DMF and DMSO should be different from those of
the Li(I) species in TFSA-based ILs, i.e., [Li(TFSA)2]-.
Salomon et al. have reported that the association con-
stants (Ka) for LiTFSA and LiClO4 in CH3CN are 4.73
and 16.5, respectively.63,64 Furthermore, the studies on
3
netic analyses were carried out using sample solutions
(samp-4 and samp-5) of Table 1 in a manner similar to that
mentioned above. Figure 11 shows the plots of kobs against
[Li(I)free] (=[Li(X)2-] (X = PFSA or HFSA) = [LiX] -
[C211]) at various temperatures. The ks values were ob-
tained from the slopes of Figure 11 and are listed in Table 2.
Their kinetic parameters were also obtained from the
semilogarithmic plots of ks against the reciprocal tempera-
ture and are summarized in Table 3. As seen from Table 3,
the kinetic parameters for the DEME-PFSA and DEMA-
HFSA systems are different from those for the DEME-,
EMI-, and MPP-TFSA systems, especially their ks values
are about 2 times larger than those in the DEME-, EMI-,
and MPP-TFSA systems. These results indicate that the
anionic components of ILs affect the complexation reaction
as expected. However, the effect cannot be explained by the
difference in the lengths of the perfluoroalkyl chains in
TFSA, PFSA, and HFSA because the ks values increase
in the order of the following IL systems: DEME-PFSA >
(63) Croce, F.; D’Aprano, A.; Baujundiash, C.; Koach, V. R.; Walker,
C. W.; Salomon, M. J. Electochem. Soc. 1996, 143, 154–159.
(64) Salomon, M. J. Solution Chem. 1993, 22, 715–725.

1626 Inorganic Chemistry, Vol. 50, No. 5, 2011
Shirai and Ikeda
Figure 12. 7Li NMR spectra of a DMF solution containing LiTFSA
(0.24 m) and C211 (0.12 m) measured at various temperatures. RC211 =
0.5. The chemical shifts of 7Li were determined using a D2O solution
containing 0.10 M LiCl as an external reference.
Table 4. Apparent First-Order Rate Constants (kobs) at Various Temperatures
for the Complexation Reactions of Li(I) with C211 in DMF or DMSO
kobs/s-1
RC211 (DMF system)
RC211 (DMSO system)
temp/°C
0.1
0.5
0.9
0.1
0.5
0.9
110
115
120
125
130
135
6.66
8.90
11.7
15.2
20.0
26.0
6.80
9.14
12.1
16.0
20.9
27.2
6.75
8.98
11.8
15.4
19.7
25.4
12.6
17.2
23.9
32.3
44.6
62.0
13.3
18.4
24.9
33.8
45.6
61.9
12.5
16.5
Figure 11. Plots of kobs values against [Li(I)free] for the complexation
reactions of Li(I) with C211 in DEME-PFSA (A) and DEME-HFSA (B).
considered to be attributed to the exchange reaction of
Li(I) between [Li C211]þ and [Li(DMF)n]þ sites. Similar
3
the association of LiClO4 in DMF using Raman and 7Li
and 13C NMR65-67 suggest that Li(I) species are not
associated with TFSA but solvated by DMF or DMSO
([Li(solvent)n]þ). The stability constants for the complex
formation of Li(I) with C211 in DMF and DMSO are
known to be 106.9 and 105.8 at 25 °C, respectively.52
Hence, the Li(I) species in DMF or DMSO containing
phenomena were also observed in a DMSO solu-
tion containing LiTFSA (0.24 m) and C211 (0.12 m,
RC211 = 0.5) (see Figure S14 in the Supporting In-
formation). The kobs values for DMF or DMSO solutions
with various RC211 were obtained and are listed in Table 4.
As seen from this table, the kobs values are constant
without depending on the components of sample solu-
tions. These results are consistent with those of Popov et
al.,40 indicating that the exchange reactions of Li(I)
LiTFSA and C211 are considered to exist as [Li C211]þ
3
and [Li(solvent)n]þ. Figure 12 shows the 7Li NMR spectra
of a DMF solution containing LiTFSA (0.24 m) and
C211 (0.12 m, RC211 = 0.5) measured at various tem-
between [Li C211]þ and [Li(solvent)n]þ (solvent =
3
DMF or DMSO) sites proceed through the following
dissociative mechanism.
7
peratures. From comparison with Li NMR spectra for
DMF solutions of RC211 = 0.0 and 1.0 (see Figure S13 in
the Supporting Information), the signals at 0.47
and -0.56 ppm were assigned to be due to the solvated
½Li C211ꢁþ þ ½ꢂLiðsolventÞnꢁþ
3
slow
f C211 þ ½LiðsolventÞnꢁþ þ ½ꢂLiðsolventÞnꢁþ
Li(I) and the [Li C211]þ species, respectively. Two peaks
3
fast
f ½ꢂLi C211ꢁþ þ ½LiðsolventÞnꢁþ
ð5Þ
are found to become broad with an increase in the
temperature. This phenomenon is similar to those in 7Li
NMR spectra for Li(I) exchange reactions in a DMF
solution containing LiClO4 (0.25 m) and C211 (0.25 m).39
3
The kinetic parameters for kobs were evaluated from the
semilogarithmic plots of kobs against the reciprocal
temperature and are listed in Table 3. These kinetic
parameters are found to be in fair agreement with those
for DMF or DMSO solutions containing C211 and
7
Hence, the Li NMR spectral changes in Figure 12 are
LiClO4, that is, 1.30 ꢀ 10-2 s-1, 64.5 ( 2.5 kJ mol-1
,
(65) James, D. W.; Mayes, R. E.; Leong, W. H.; Jamiel, I. M.; Zhen, G.
Faraday Discuss. Chem. Soc. 1988, 85, 269–281.
(66) Cahen, Y. M.; Handy, P. R.; Roach, E. T.; Popov, A. I. J. Phys.
Chem. 1975, 79, 80–85.
(67) Berman, H. A.; Stengle, T. R. J. Phys. Chem. 1975, 79, 1001–1005.
and -64.8 ( 5.9 kJ K-1 mol-1 for DMF and 2.32 ꢀ
10-2 s-1, 64.5 ( 2.5 kJ mol-1, and -57.8 ( 5.9 kJ K-1
mol-1 for DMSO.39

Article
From these results, it is proposed that the complexation
Inorganic Chemistry, Vol. 50, No. 5, 2011 1627
those in the chemical forms of the uncomplexed Li(I)
species; i.e., the negatively charged species ([Li(X)2]-) in
reactions in ILs proceed in a manner different from those
in conventional nonaqueous solvents. The difference in
the reaction mechanism is explained by those in the
chemical forms of the uncomplexed Li(I) species in ILs
and nonaqueous solvents, that is, [Li(solvent)n]þ in non-
aqueous solvents and [Li(X)2]- in ILs. The approach of
ILs can more easily approach the [Li C211]þ complex than
3
the positively charged species ([Li(solvent)n]þ) in nonaqu-
eous solvents.
From these results, it is concluded that interactions be-
tween the anionic components of ILs and the positively
charged metal complexes or ions are the important factor
for controlling the complex formation reactions in ILs. This
is consistent with the results of Eldik et al. and our previous
study.
positively charged species ([Li(solvent)]þ) to [Li C211]þ
3
in DMF or DMSO should be more difficult than that of
the negatively charged species ([Li(TFSA)2]-, [Li(HF-
SA)2]-, and [Li(HFSA)2]-) to [Li C211]þ in ILs. This is
3
considered to cause the differences in the reaction me-
chanism in nonaqueous solvents and ILs.
Acknowledgment. The authors are thankful to Dr. H.
Miyashiro, Dr. Seki, and Y. Ohno of Central Research
Institute of Electric Power Industry for their kind
support.
4. Conclusion
7
We measured Li NMR spectra of DEME-TFSA, EMI-
TFSA, MPP-TFSA, DEME-PFSA, and DEME-HFSA so-
lutions containing LiX (X = TFSA, PFSA, or HFSA) and
C211 at various temperatures. As a result, it was found that
the Li(I) species in these IL solutions can form a 1:1 complex
Supporting Information Available: 1H, 13C, and 19F NMR
spectra of DEME-TFSA, EMI-TFSA, MPP-TFSA, DEME-
PFSA, and DEME-HFSA (Figures S1-S5), 7Li NMR spectra
of EMI-TFSA solutions containing LiTFSA (0.24 m) and C211
(Figure S6), 7Li NMR spectra of MPP-TFSA solutions contain-
ing LiTFSA (0.24 m) and C211 (Figure S7), 7Li NMR spectra of
DEME-TFSA solutions of RC211 = 1.0 (A) and 0.0 (B) mea-
sured at various temperatures (Figure S8), a plot of the 7Li
NMR chemical shift (δobs(Li)) against F (Figure S9), a plot of
([Li C211]þ) with C211 stoichiometrically, that the Li(I)
3
species exchange between [Li C211]þ and the uncomplexed
3
Li(I) ([Li(X)2]-) sites through the bimolecular mechanism
[Li C211]þ þ [*Li(X)2]- = [*Li C211]þ þ [Li(X)2]-, and
3
3
that the Li(I) exchange reactions are not affected by the
cationic components but anionic ones in ILs. Furthermore,
7Li NMR spectra of DMF and DMSO solutions containing
C211 and LiTFSA were measured at various temperatures. It
7
ΔδC-F against temperature (Figure S10), Li NMR spectra of
DEME-HFSA solutions containing LiHFSA (0.16 m) and C211
(Figure S11), 7Li NMR spectra of DEME-HFSA solutions
containing LiHFSA (0.16 m) and C211 (0.08 m) measured at
various temperatures (Figure S12), 7Li NMR spectra of DMF
solutions containing LiHFSA (0.24 m) and C211 (Figure S13),
and 7Li NMR spectra of DMSO solutions containing LiTFSA
(0.24 m) and C211 (0.12 m) measured at various temperatures
(Figure S14). This material is available free of charge via the
Internet at http://pubs.acs.org.
was found that the Li(I) species exist as [Li C211]þ and
3
[Li(solvent)n]þ (solvent = DMF or DMSO) in these solu-
tions and that the exchange reactions of Li(I) between
[Li C211]þ and [Li(solvent)n]þ sites occur via the dissociative
3
mechanism. The differences in the reaction mechanisms
between ILs and nonaqueous solvents were attributed to