Crystallization of cesium complex containing bis(2-propyloxy)calix[4]crown- 6 and bis[(trifluoromethyl)sulfonyl]imide

Article abstract of DOI:10.1016/j.ica.2012.04.005

A complex of Cs⁺ with the calixcrown bis(2-propyloxy)calix[4] crown-6 (BPC6) and bis[(trifluoromethyl)sulfonyl]imide (NTf₂ ^-), an anion of ionic liquid (IL), was crystallized in the interface region between n-octanol and water. The compound was characterized by single crystal X-ray diffraction, ESI-MS, FT-IR, TGA and PXRD. Cs⁺ is coordinated with BPC6 by both the six oxygen atoms of the crown and the cation-π interactions, and with the anion NTf₂^- via one oxygen atom of the sulfonyl group. This single crystal explains the good extraction ability of BPC6 to Cs⁺ and reflects a coordinating interaction of NTf₂^- with the deficient complex [Cs·BPC6]⁺ during the extraction with an IL as diluent.

Full text of DOI:10.1016/j.ica.2012.04.005

Inorganica Chimica Acta 390 (2012) 8–11
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Note
Crystallization of cesium complex containing bis(2-propyloxy)calix[4]crown-6
and bis[(trifluoromethyl)sulfonyl]imide
⇑
Taoxiang Sun, Zheming Wang, Xinghai Shen
Beijing National Laboratory for Molecular Sciences (BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
Radiochemistry and Radiation Chemistry Key Laboratory of Fundamental Science, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
a r t i c l e i n f o
a b s t r a c t
A complex of Cs+ with the calixcrown bis(2-propyloxy)calix[4]crown-6 (BPC6) and bis[(trifluoro-
methyl)sulfonyl]imide (NTf2À), an anion of ionic liquid (IL), was crystallized in the interface region
between n-octanol and water. The compound was characterized by single crystal X-ray diffraction, ESI-
MS, FT-IR, TGA and PXRD. Cs+ is coordinated with BPC6 by both the six oxygen atoms of the crown
Article history:
Received 29 June 2011
Received in revised form 16 January 2012
Accepted 2 April 2012
Available online 17 April 2012
and the cation–p
interactions, and with the anion NTf2À via one oxygen atom of the sulfonyl group. This
singlÀe crystal explains the good extraction ability of BPC6 to Cs+ and reflects a coordinating interaction of
NTf2 with the deficient complex [CsÁBPC6]+ during the extraction with an IL as diluent.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquids
Single crystal
X-ray crystallography
Extraction
Cs+
1. Introduction
lanthanides or alkaline earth metal ions through an oxygen atom of
each sulfonyl group without the additional coordination of other
Room-temperature ionic liquids (ILs) have been considered as
alternative solvents for synthetic, catalytic, electrochemical and
separation sciences because of their unique properties such as low
vapor pressure, solvating properties and thermal stability [1–5].
Especially, they have become potential replacements of volatile
organic compounds (VOCs) in the extraction of metal ions, in partic-
ular, in the reprocessing of spent nuclear waste [6–11]. 137Cs, as a
major radioactive fission product in spent nuclear waste, contrib-
utes a large part of the heat load and radiation in high level liquid
waste (HLLW). Removal of 137Cs from HLLW will reduce waste vol-
ume and subsurface storage time, and facilitate handling and trans-
portation. The calixcrown–ILs systems have been regarded as a
potential method to extract 137Cs from HLLW [10,12]. For instance,
using a calixcrown bis(2-propyloxy)calix[4]crown-6 (BPC6, Fig. 1)
as extractant in the ILs 1-alkyl-3-methylimidazolium bis[(trifluoro-
methyl)sulfonyl]imide (CnmimNTf2), we found that Cs+ was effi-
ciently extracted into the IL phase as a complex of [CsÁBPC6]+ [10].
Besides, we also found that CnmimNTf2 themselves could extract
Cs+ with a considerable distribution ratio in the absence of extract-
ants [10], meaning that the function of CnmimNTf2 is both as diluent
and as extractant, that is, NTf2À is able to coordinate with Cs+.
molecules [13–15] and also can stabilize deficient transition metal
or uranyl complexes via distinct modes [16,17]. Temperature eleva-
tion is usually necessary in these syntheses. Recently, the com-
plexes [Ba(18-crown-6)][NTf2]2 and [K(18-crown-6)][NTf2] were
crystallized by Yan et al. at the interface of C2mimNTf2 and water,
in which an imidazolium based acid was used as a precursor [18].
À
The coordinating ability of NTf2 leads to an interesting ques-
tion in the extraction of metal ions wÀith an IL as diluent, that is,
is there an interaction between NTf2 and the deficient metal-
extractant complex? Using extended X-ray fine structure (EXAFS),
Dietz and co-workers found that water molecules rather than
NTf2À interacted with [SrÁDCH18C6]2+ during the extraction, where
DCH18C6 is dicyclohexano-18-crown-6 [19]. This is due to the fact
that Sr2+ has very high hydration energy [20]. The hydration energy
of Cs+ is low [20] so that the anion NTf2À probably coordinates with
the deficient Cs-extractant complex. The crystallÀization method
can demonstrate the coordinating ability of NTf2 and provide a
way to answer the question mentioned above. Without rising tem-
perature or using other precursors, however, no crystallization was
found during the extraction of Cs+ bÀy BPC6-CnmimNTf2 systems
[10]. If the interaction between NTf2 and [CsÁBPC6]+ does exist,
non-ionic circumstance instead of ionic circumstance (IL phase)
will facilitate the crystallization. In this paper, a new complex,
À
The coordinating ability of NTf2 has been realized and several
complexes were reported in recent years. NTf2À can coordinate with
[CsÁBPC6]+NTf2 was synthesized in the interface region between
À
n-octanol and water, and characterized by single crystal X-ray dif-
⇑
Corresponding author. Tel.: +86 10 62765915.
E-mail address: xshen@pku.edu.cn (X. Shen).
fraction, ESI-MS, FT-IR, TGA and PXRD.
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.04.005

T. Sun et al. / Inorganica Chimica Acta 390 (2012) 8–11
9
Table 2
O
O
O
Selected bond lengths (Å) and angles (°).
O
O
O
S
O
S
À
[CsÁBPC6]+NTf2
O
F3C
N
CF3
N
N
Cs(1)–O(1)
Cs(1)–O(2)
Cs(1)–O(3)
Cs(1)–O(4)
3.130(3)
3.206(4)
3.279(4)
3.185(4)
3.152(4)
3.223(3)
3.507(8)
52.84(9)
52.56(12)
52.65(12)
53.74(11)
52.61(9)
105.56(7)
92.02(16)
71.42(14)
76.94(14)
70.08(17)
94.10(16)
135.05(12)
143.1(7)
90.2(5)
n-1
O NTf2
O
O
O
Cnmim+
−
BPC6
Cs(1)–O(5)
Cs(1)–O(6)
Cs(1)–O(9)
Fig. 1. Structures of BPC6 and the cation Cnmim+, the anion NTf2 of ILs.
À
O(1)–Cs(1)–O(2)
O(2)–Cs(1)–O(3)
O(4)–Cs(1)–O(3)
O(5)–Cs(1)–O(4)
O(5)–Cs(1)–O(6)
O(1)–Cs(1)–O(6)
O(1)–Cs(1)–O(9)
O(2)–Cs(1)–O(9)
O(3)–Cs(1)–O(9)
O(4)–Cs(1)–O(9)
O(5)–Cs(1)–O(9)
O(6)–Cs(1)–O(9)
O(9)–S(1)–N(1)
O(9)–S(1)–N(2)
O(11)–S(2)–O(12)
O(13)–S(2)–O(14)
S(1)–N(1)–S(2)
S(2)–N(2)–S(1)
2. Experimental
2.1. Materials and methods
The ILs 1-alkyl-3-methylimidazolium bis[(trifluoromethyl)sul-
fonyl]imide (CnmimNTf2) were synthesized via a metathetical reac-
tion as previously reported [10]. BPC6 (>95%) was obtained from
INET, Tsinghua University, PR China, and was used as received. n-
Octanol of analytical grade was purchased from Beijing Yili Fine
Chemical Co., Ltd. and used without further purification.
115.9(11)
119.8(17)
120.4(8)
131.6(8)
2.2. Crystallization of [CsÁBPC6]+NTf2
À
A 0.5 mL of n-octanol solution containing BPC6 (15 mmol dmÀ3
)
and CnmimNTf2 (15 mmol dmÀ3, n = 2, 4, 6, 8 and 10) was added
slowly to a 0.5 mL of the aqueous solution of CsCl (50 mmol dmÀ3).
Standing for 3 days gave thin plate like crystals in the interface
region between n-octanol and water. Anal. Calc. for C46H54NO12S2
F6Cs: C, 49.15; N, 1.25; H, 4.85. Found: C, 49.23; N, 1.33; H, 4.89%.
2.0 kW sealed anode source using graphite monochromated Mo
K
a radiation (k = 0.71073 Å) [21,22]. The structure was solved by
direct methods and refined by full-matrix least-squares on F2 using
the SHELX program [23]. The hydrogen atoms were added according
to the ideal geometry, and were not refined for good refinement
À
2.3. X-ray crystallography
convergence. NTf2 showed disorder thus treated in two orienta-
tions. The details of data collection, data reduction, and crystallo-
graphic data are summarized in Table 1. Selected bonds and
angles are given in Table 2.
The crystallographic data for the single crystal were collected at
room temperature on a Nonius Kappa CCD diffractometer with a
Table 1
2.4. Other characterizations
Crystallographic data for the complex.
[CsÁBPC6]+NTf2
À
Electrospray ionization mass spectra (ESI-MS) of the crystals
dissolved in acetonitrile were acquired in both positive-ion and
negative-ion mode with a Bruker Apex IV FTMS. FT-IR spectrum
was measured on a Thermo-Fischer Nicolet iN10 MX spectrometer
between 600 and 4000 cmÀ1. Thermogravimetric analysis (TGA)
was performed on a Q600 SDT thermoanalyzer under N2 atmo-
sphere with the temperature ranging from 25 to 1000 °C at a heat-
ing rate of 5 °C minÀ1. Powder X-ray diffraction (PXRD) data were
Chemical formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
C46H54CsF6NO12S2
1123.93
293(2)
triclinic
ꢀ
P1
10.6429(2)
13.5876(3)
17.5651(4)
86.2114(8)
87.9841(7)
89.4630(10)
2532.93(9)
2
b (Å)
c (Å)
a
(°)
measured on a DMAX-2400 diffractometer using Cu K
a radiation
b (°)
(k = 1.5406 Å).
c
(°)
V (Å3)
Z
3. Results and discussion
qcalcd (g cmÀ3
)
1.474
k (Mo K
a
) (mmÀ1
)
0.71073
To simulate the extraction procedure, n-octanol containing
CnmimNTf2 (n = 2, 4, 6, 8 and 10) and BPC6 was used to contact with
the aqueous solution of CsCl. All the samples with different ILs could
form colorless crystals at the interface and they are the same phase
(see later). The crystal synthesized by C8mimNTf2 was subjected in
the structure determination and other charactÀerizations.
Crystal size (mm)
Tmin, Tmax
hmin, hmax (°)
Total reflections
0.22 Â 0.15 Â 0.07
0.877, 0.942
3.395, 25.028
19201
0.0558
5142
Unique reflections (Rint
Observed reflections (I P 2
)
r
(I))
Parameters
642
b
R1a, wR2 (I P 2
r
(I))
0.0476, 0.1121
0.1034, 0.1263
0.960
The molecular structure of [CsÁBPC6]+NTf2 is shown in Fig. 2.
The host molecule BPC6 has its calix[4]arene part in the 1,3-alter-
nate conformation, with the crown ether and two phenyl groups
on the same side. The cesium ion locates at the center of the crown,
coordinated by the six ether oxygen atoms, with the CsÀO dis-
tances in range from 3.129 to 3.280 Å, comparable to those in other
R1a, wR2 (all data)
b
Goodness-of-fit (GOF)
D
qc (e ÅÀ3
)
0.497, À0.417
Maximum and meand
D/r
0.001, 0.000
a
b
c
R1
wR2 = [
=
R
||Fo| À |Fc||/
R|Fo|.
2
R
w(Fo À Fc2)2/
R .
w(Fo2)2]1/2
Cs-calixcrown complexes [24,25]. The cation–
p interaction is ob-
Maximum and minimum residual electron density.
Maximum and mean sigma/shift.
served between Cs+ and the two upper phenyls. The short CsÁ Á ÁC
d
contacts between Cs+ and the upper-rim C atoms of the two

10
T. Sun et al. / Inorganica Chimica Acta 390 (2012) 8–11
Fig. 2. The structure of [CsÁBPC6]+NTf2 in 20% thermal ellipsoids. C atoms are not
labeled. X1A and X1B are the centeroids of the two phenyl rings, and the distances
(dashed bonds) between the Cs and the two centeroids are 3.572 (X1A) and 3.540
(X1B) Å, respectively.
À
phenyls are 3.445 to 3.677 Å, the two distances of Cs+ to the center-
oids of the two phenyl rings are 3.572 and 3.540 Å, and the Cs+ ion
is 3.404 and 3.406 Å above the phenyl planes. These are inÀdicative
of the existence of Cs+–
p interaction [26–28]. The NTf2 anion
compensates the positive charge of [CsÁBPC6]+ moiety, and one
oxygen atom of a SO2 group further coordinates with Cs+ ion, with
a long CsÀO distance of 3.507 Å. In the structure the anion shows
disorder, with the N and the non-coordinated SO2 group in two ori-
entations (N1/N2 40:60). In the lattice (Fig. 3), the column like
molecules are arranged head-to-end along the b direction, and fur-
ther arrange in a nearly hexagonal closest packing extending along
a and c directions.
Fig. 4. ESI-MS of the single crystal in positive mode (a) and in negative mode (b).
negative mode were observed in highly diluted conditions. A peak
of alkyltrifluoroborate anion with two [K(18-crown-6)]+ in ESI-MS
was observed in the literature [29]. However, no ion pairings were
observed in our experiments even at high concentrations, owing to
the steric hindrance. In the FT-IR spectrum of the single crystal
(Fig. 5), the absorption peak of 1056 cmÀ1 is assigned as the
stretching vibration of S@O. Comparing with the value of
1059 cmÀ1 in C8mimNTf2, it shifts to lower frequency because of
the coordination between the sulfonyl group and cesium ion. The
stretching vibration of C@C group of the phenyl ring in BPC6 shifts
from 1458 to 1452 cmÀ1 after it trapping Cs+.
Fig. 4 shows the ESI-MS of the crystal in acetonitrile. A peak in
positive mode is displayed at m/z = 843.3 that is attributed to
[CsÁBPC6]+. In negative mode, a peak at m/z = 280.1 corresponding
À
to NTf2 is observed. It was reported that ion parings were ob-
served at high concentrations and naked ions in the positive or
The thermal stability of the single crystal after grinding was
examined by TGA under N2 atmosphere from 25 to 1000 °C
(Fig. 6). The complex was decomposed in two continuing degrada-
tion steps within the temperature range from 315 to 465 °C and
then further slowly lost weight with increasing temperature. The
first step from 315 to 425 °C with a weight loss of 39.5% corre-
sponds to the decomposition of BPC6. From 425 to 465 °C the
weight loss of 18.3% is the decomposition of NTf2À anion. Compar-
ing the TGA curve of the single crystal with that of BPC6, one can
1059
C8mimNTf2
1458
BPC6
1452
1056
Crystal
4000
3000
2000
1000
Wavenumbers / cm−1
Fig. 3. Packing of [CsÁBPC6]+NTf2 molecules in the crystal structure, viewed along
À
b direction.
Fig. 5. FT-IR spectra of the single crystal, BPC6 and C8mimNTf2.

T. Sun et al. / Inorganica Chimica Acta 390 (2012) 8–11
11
combines the cesium ion with both the six oxygen atoms of the
100
80
60
40
20
0
crown and the cation–
p
interactions, explaining itsÀgood extraction
ability to Cs+. The coordinating interaction of NTf2 with the defi-
cient complex [CsÁBPC6]+ provides an insight into the extraction
mechanism of Cs+ with BPC6 in ILs.
Acknowledgments
Crystal
This work was supported by National Natural Science Founda-
tion of China (Grant No. 20871009). We thank Ms. Xiaoran He,
Ms. Zhixian Wang, Ms. Fuhui Liao and Mr. Shifu Weng for their
help with ESI-MS, EA, PXRD and FTIR measurements, respectively.
BPC6
C8mimNTf2
200
400
600
800
1000
Temperature / oC
Appendix A. Supplementary material
Fig. 6. TGA curves of the single crystal, BPC6 and C8mimNTf2.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.04.005.
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C4mimNTf2
C6mimNTf2
C8mimNTf2
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10mimNTf2
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o
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The crystals synthesized using different ILs CnmimNTf2 (n = 2, 4,
6, 8 and 10) were investigated by powder X-ray diffraction (Fig. 7).
The main peaks of 2h = 8.5°, 10.1°, 11.8°, 16.6°, 18.0°, 18.9°, 25.9°
and 30.4° in the PXRD patterns for ILs with different alkyl chains
agree well with the simulated one based on the structure of
[CsÁBPC6]+NTf2 and the alkyl chain of the imidazolium cation
À
did not influence the crystal structure.
We subsequently tried to crystallize BPC6 and NTf2 with Na+
À
and K+, according to the literature [30,31]. However, no crystalliza-
tion was found in the interface region between n-octanol and
water. This may be due to the large cavity of BPC6 compared with
the ionic radius of Na+, K+ and the weak coordinating ability of
NTf2À to Na+ and K+. Crystallization of metal ions with BPC6 and
À
NTf2 is influenced both by the match of the cavity of BPC6 with
the iÀonic radius of metal ions and by the coordinating ability of
NTf2 to metal ions.
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4. Conclusion
A new complex of Cs+ containing BPC6 and NTf2 was crystal-
À
lized in the interface region between n-octanol and water. BPC6