DFT calculation and experimental validation on the interactions of bis(trifluoromethylsulfonyl)imide and hexafluorophosphate with cesium

Article abstract of DOI:10.1016/j.molstruc.2017.07.048

Bis(trifluoromethylsulfonyl)imide (NTf₂^?) and hexafluorophosphate (PF₆^?) are the most frequently used anions for hydrophobic ionic liquids (ILs) which have been considered as promising solvents in the extraction of cesium ions. The interactions of NTf₂^? and PF₆^? with Cs⁺ were explored in this work. The results of DFT calculation indicated that both Cs⁺ and Cs(18C6)⁺ prefer to interact with two NTf₂^? or PF₆^? anions in gas phase, where 18C6 is 18-crown-6. The complex of Cs(NTf₂)₂^? was observed in electrospray ionization mass spectrometry (ESI-MS), and the complexes of [Cs(18C6)NTf₂]₂ and [Cs(18C6)PF₆]₂ were crystallized in which Cs(18C6)⁺ interacted with two anions. The interactions of NTf₂^? with cesium resulted in a synergistic effect between dicyclohexano-18-crown-6 (DCH18C6) and NTf₂^? in the extraction of Cs⁺ using n-octanol as diluent. However, DFT calculation revealed that the complex Cs(DCH18C6)⁺ interacted with one NTf₂^? anion was more thermodynamically stable than that with two anions in organic phase, different from that in gas phase.

Full text of DOI:10.1016/j.molstruc.2017.07.048

Journal of Molecular Structure 1148 (2017) 206e212
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
DFT calculation and experimental validation on the interactions of
bis(trifluoromethylsulfonyl)imide and hexafluorophosphate with
cesium
a
,
b
,
*
b
c
a
c
Taoxiang Sun
, Wuhua Duan , Yaxing Wang , Shaowen Hu , Shuao Wang ,
b
a, **
Jing Chen , Xinghai Shen
Beijing National Laboratory for Molecular Sciences (BNLMS), Fundamental Science on Radiochemistry and Radiation Chemistry Laboratory, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
a
b
Institute of Nuclear and New Energy Technology (INET), Collaborative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University,
Beijing 100084, PR China
c
School for Radiological and Interdisciplinary Sciences, Soochow University, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
ꢀ
ꢀ
6
Article history:
Received 21 February 2017
Received in revised form
Bis(trifluoromethylsulfonyl)imide (NTf ) and hexafluorophosphate (PF ) are the most frequently used
2
anions for hydrophobic ionic liquids (ILs) which have been considered as promising solvents in the
extraction of cesium ions. The interactions of NTf
results of DFT calculation indicated that both Cs and Cs(18C6) prefer to interact with two NTf
anions in gas phase, where 18C6 is 18-crown-6. The complex of Cs(NTf
ionization mass spectrometry (ESI-MS), and the complexes of [Cs(18C6)NTf
crystallized in which Cs(18C6) interacted with two anions. The interactions of NTf
resulted in a synergistic effect between dicyclohexano-18-crown-6 (DCH18C6) and NTf
of Cs using n-octanol as diluent. However, DFT calculation revealed that the complex Cs(DCH18C6)
ꢀ
ꢀ
þ
2
and PF
6
with Cs were explored in this work. The
12 June 2017
þ
þ
ꢀ
ꢀ
2
or PF
6
Accepted 19 July 2017
Available online 20 July 2017
ꢀ
2
)
2
was observed in electrospray
2
]
2
and [Cs(18C6)PF
6
]
2
were
þ
ꢀ
2
with cesium
Keywords:
Ionic liquids
Cesium
DFT calculation
Crystal structure
ꢀ
2
in the extraction
þ
þ
ꢀ
interacted with one NTf
2
anion was more thermodynamically stable than that with two anions in organic
phase, different from that in gas phase.
©
2017 Elsevier B.V. All rights reserved.
1
. Introduction
Hydrophobic ILs have been considered as promising solvents for
separation processes in the advanced nuclear fuel cycle [7]. The
137
Room-temperature ionic liquids (ILs) are composed entirely of
organic cations and organic or inorganic anions, and have been
applied in many fields including synthesis, catalysis, energy and
separation sciences [1e4]. There are a large amount of ILs with
different cations and anions [5], and the functionalization of the
cations and anions can lead to more task specific ILs [6]. Among the
anions of ILs, bis(trifluoromethylsulfonyl)imide (NTf
fluorophosphate (PF
radioactive isotope Cs is one of the most troublesome isotopes in
the treatment of high level liquid waste (HLLW) due to its heat-
137
emitting property. It is of great importance to remove
Cs prior
to the waste vitrification, for not only reducing the waste volume
but also minimizing the long-term hazards [8]. The extraction of
þ
Cs has attracted much attention using crown ethers or calix crown
ꢀ
ꢀ
ꢀ
6
2
) and hexa-
ethers as extractants in combination with ILs based on NTf
2
or PF
ꢀ
6
) are very common which can construct hy-
[9e16].
drophobic ILs in combination with organic cations such as the
Choosing an appropriate solvent is extremely important for a
separation process, as the properties of the solvent significantly
affect the extraction performance. The extraction mechanism in the
extraction of cesium ions using ILs as solvents is usually cation
exchange [17], different from the neutral complex extraction
mechanism by using traditional organic solvents. In the cation
exchange mechanism, cationic moieties (cesium ions or complexes
of cesium ions) are transferred into the IL phase and the IL cations
are simultaneously exchanged into the aqueous phase. In this
þ
imidazolium cations 1-butyl-3-methylimidazolium (C
4
mim ).
*
Corresponding author. Beijing National Laboratory for Molecular Sciences
BNLMS), Fundamental Science on Radiochemistry and Radiation Chemistry Labo-
ratory, College of Chemistry and Molecular Engineering, Peking University, Beijing
00871, PR China.
(
1
** Corresponding author.
E-mail addresses: sunstx@tsinghua.edu.cn (T. Sun), xshen@pku.edu.cn (X. Shen).
http://dx.doi.org/10.1016/j.molstruc.2017.07.048
022-2860/© 2017 Elsevier B.V. All rights reserved.
0

T. Sun et al. / Journal of Molecular Structure 1148 (2017) 206e212
207
context, an interesting question is whether the IL anions can co-
ordinate with the cesium cations or deficient complexes of cesium
ions, facilitating the extraction process. Some phenomena indeed
found at the bottom of the beaker. The solid was removed, washed
with water, and then taken for further characterization. ESI-MS (m/
þ
z): positive mode: 397.06328 [Cs(18C6)] , negative mode:
ꢀ
ꢀ
ꢀ
ꢀ
2 2
; 692.74113 Cs(NTf ) .
reflected the interactions of NTf
2
and PF
6
with cesium. In the
279.91657 NTf
2
þ
extraction of Cs with ILs, the pure IL C
4
mimNTf or C mimPF can
2
4
6
þ
extract Cs with unneglectable distribution ratios in the absence of
other extractants [11,13], whilst most of other metal ions can hardly
2
.4. Synthesis of [Cs(18C6)PF ]
6 2
þ
þ
be extracted by pure ILs. Cs is exchanged with the C
4
mim cation
1
mL of dichloromethane solution containing 18C6 (13.2 mg)
ꢀ
ꢀ
into the IL phase [18]. It is very likely that NTf
2
and PF
6
can coor-
was added into a flask containing CsPF (30 mg). The mixture was
6
þ
dinate with Cs , but investigations on the coordinating behavior
stirred for over 48 h, the supernatant was transferred to a beaker,
and 0.2 mL of n-heptane was added. After evaporating very slowly
for 7 days, colorless bulk crystals were found at the bottom of the
beaker. The solid was removed, washed with water, and then taken
for further characterization. ESI-MS (m/z): positive mode:
þ
are very limited. We reported a ternary complex of Cs with bis(2-
propyloxy)calix [4]crown-6 (BPC6) and NTf , in which Cs is co-
ꢀ þ
2
ordinated with BPC6 by the six oxygen atoms of the crown and the
ꢀ
cationꢀ
p
interactions, and also with the anion NTf
2
via one oxygen
atom of the sulfonyl group [19]. A crystal structure containing ce-
þ
þ
6
PF , negative mode:
3
4
97.06293 [Cs(18C6)] ; 939.11347, [Cs(18C6)]
2
ꢀ
ꢀ
6
coor-
sium and PF
dinated with the deficient Cs complex [20].
6
was reported by Salorinne et al., in which PF
ꢀ
22.83164 Cs(PF
6
)
2
.
þ
ꢀ
ꢀ
The coordination of NTf
2
or PF
6
with cesium is very important
2.5. X-ray crystal structure determination
þ
for understanding the mechanism in the extraction of Cs , but the
characterization method is very limited. Theoretical calculation is a
useful approach to investigate the coordinating interactions. Ali
The crystallographic data were collected on an Image Plate X-
Ray Diffractometer. Using Olex2, the structure was solved with the
Superflip structure solution program using Charge Flipping and
refined with the XL refinement package using Least Squares mini-
mization [21e23]. The details of data collection, data reduction, and
crystallographic data are summarized in Table 1.
et al. optimized the structures of the complexes of CsNTf
Cs(BPC6)NTf by density functional theory (DFT) calculation [16],
and the calculated structure of Cs(BPC6)NTf was similar with our
crystal structure [19]. To obtain more information about the in-
2
and
2
2
þ
ꢀ
þ
ꢀ
teractions between Cs and NTf
2
, more complexes of Cs and NTf
2
should be calculated. Furthermore, to the best of our knowledge,
there has been no theoretical calculation dealing with the inter-
2
.6. Extraction experiments
þ
ꢀ
action between Cs and PF
6
. Crystallization, as mentioned above, is
þ
1 mL of n-octanol containing single or mixtures of DCH18C6 and
another powerful means to illustrate the interactions between Cs
ꢀ ꢀ
C4mimNTf2 with total concentration of 0.02 mol/L and 1 mL of
aqueous solution containing 0.002 mol/L of CsNO and trace
Cs were added into a plastic tube, followed by
vibrating for 2 h at 298.2 K. Then the mixture was centrifuged and
and NTf or PF . Particularly, using the same ligands in the syn-
2 6
3
thesis of ternary complexes will facilitate the comparison of these
137
þ
amount of
two anions in the interaction with deficient Cs complexes. In this
ꢀ ꢀ
þ
work, we calculated the interactions of NTf and PF with Cs as
2 6
137
þ
þ
phase separated, and
Cs both in the organic phase and in the
well as with the deficient complex Cs(18C6) (18C6 is 18-crown-6),
aqueous phase were measured by ultra-low level liquid scintillation
QUANTULUS 1220 with Ultima Gold™ AB as the scintillation so-
lution (0.1 mL sample and 10 mL scintillation solution). The dis-
tribution ratio of Cs (D) was defined as the ratio of the
radioactivity of
and synthesized the ternary complex of Cs with 18C6 containing
ꢀ
ꢀ
NTf
2
or PF
6
. We hope that our work will facilitate the under-
standing of the extraction using ILs as solvents.
þ
137
Cs in the organic phase to that in the aqueous
2
. Experimental
2.1. Materials and methods
Table 1
2 2 6 2
Crystallographic data for the complexes of [Cs(18C6)NTf ] and [Cs(18C6)PF ] .
HNTf
2 6 2 2
(95%), HPF (~55%wt in H O) and CsOH (50 wt.% in H O,
[
Cs(18C6)NTf
2
]
2
[Cs(18C6)PF
6 2
]
9
9.9% trace metals basis) were purchased from Aldrich. The crown
Chemical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C
28
H
48Cs
2
F
12
N
2
O
20
S
4
C
24
H
48Cs F O P
2 12 12 2
ether 18-crown-6 (18C6, ꢁ99.5%) was purchased from Fluka, and
1354.74
180.0(1)
monoclinic
1084.38
298(2)
monoclinic
dicyclohexano-18-crown-6 (DCH18C6) was synthesized in INET.
CsNTf
2
and CsPF
6
were prepared by the reaction of CsOH in aqueous
and HPF , respectively, followed
solution with equal mole of HNTf
by evaporation of water. All other reagents were of analytical grade
and used without further purification.
2
6
C2/c
C2/c
24.9730(10)
9.0296(3)
23.5008(9)
90.00
109.240(5)
90.00
5003.4(3)
4
1.798
2688
1.739
3.33, 26.02
9520
0.0329
25.2084(17)
8.5402(6)
21.4061(14)
90
117.576(2)
90
4084.9(5)
4
1.763
2144
1.968
2.553, 27.568
51751
0.0306
b (Å)
c (Å)
ꢂ
a
b
g
( )
2
.2. Electrospray ionization mass spectra
(
ꢂ
ꢂ
)
( )
3
V (Å )
Z
Electrospray ionization mass spectra (ESI-MS) were acquired in
both positive and negative modes with a Bruker Apex IV FTMS.
L of an aqueous solution of CsNTf (0.08 mol/L) was diluted 20
times by methanol and then injected into the equipment.
ꢀ3
)
r
calcd (g$cm
F(000)
(mmꢀ1)
2
0
m
2
m
ꢂ
q
min
,
q
max ( )
Total reflections
Unique reflections (Rint
2
2 2
.3. Synthesis of [Cs(18C6)NTf ]
)
Data/Restraints/Parameters
R1, wR2 [I ꢁ 2 (I)]
R1, wR2 (all data)
4903/0/307
4684/0/235
1
mL of chloroform solution containing 18C6 (10.5 mg) was
s
0.0371, 0.0871
0.0474, 0.0934
0.63, ꢀ1.16
1.057
0.0510, 0.1548
0.0584, 0.1637
1.005, ꢀ1.260
1.120
added into a flask containing CsNTf (30 mg). The mixture was
stirred for over 24 h and the supernatant was transferred to a
beaker. After evaporating for 2 days, colorless bulk crystals were
2
ꢀ
3
Dr (eÅ
)
Goodness of fit (GOF)

208
T. Sun et al. / Journal of Molecular Structure 1148 (2017) 206e212
phase. Each experiment was performed at least twice and the re-
sults agreed within an error of 3%.
2.7. Computation methods
All theoretical calculations were performed using the Gaussian
9 software package [24]. The structures of the complexes were
0
fully optimized by DFT using the hybrid B3LYP exchange correlation
functional [25,26] in conjunction with the Stuttgart-Dresden rela-
tivistic effective core potential (RECP) basis set for cesium and 6-
31G(d,p) for other atoms. All the optimized structures are
confirmed to be global minima structures on the potential energy
hypersurface by vibrational frequency analysis at the same level of
theory. The binding energies (Eint) were corrected with the basis set
superposition error (BSSE) using the Boys-Bernardi counterpoise
technique [27]. The final expression for the Eint was as follows:
Eint ¼ ECP(AB) ꢀ Emin(A) ꢀ Emin(B) þ
DZPVE
(1)
where ECP is the counterpoise corrected electronic energy, Emin is
the electronic energy in the minimum-energy geometry, and ZPVE
is the zero-point vibrational energy. Calculations taking into ac-
count solvation effects were carried out by using integral equation
formalism version of the polarizable continuum model (IEFPCM)
[28e31], where the dielectric constant used for n-octanol was
10.34.
3
. Results and discussion
ꢀ
ꢀ
þ
with Cs
3.1. Interactions of NTf
2
and PF
6
þ
Fig. 1 shows the structures of the complexes of Cs with one,
ꢀ
ꢀ
ꢀ
ꢀ
þ
two and three NTf
2
or PF
6
as well as one NTf
2
or PF
6
with two Cs
þ
ꢀ
2
optimized by DFT calculation. In the complexes of Cs with NTf
only the oxygen atoms from the sulfonyl group of NTf
,
ꢀ
þ
ꢀ
ꢀ
6
and PF .
coordinate
Fig. 1. Optimized structure of the complexes of Cs with NTf
2
2
to the cesium ion, and the CseO distances are listed in Table 2. For
ꢀ
the conformer of CsNTf
2
, the two eCF
3
fragments of NTf
2
are cis to
Table 2
each other, identical to that in the literature [16], allowing three
oxygen atoms coordinating to Cs with CseO distances of 2.947,
þ
þ
ꢀ
2
Values of bond length in the calculated structures of Cs and Cs(18C6) with NTf
þ
ꢀ
and PF
6
.
ꢀ
ꢀ
3
.130 and 3.135 Å, respectively. The conformer of Cs(NTf
2
)
2
belongs
trans to
Bond length (Å)
to C
each other. Each NTf
the CseO distances are 3.024 and 3.047 Å, respectively. In the
2
symmetry with the two eCF
3
fragments of both NTf
2
ꢀ
þ
ꢀ
binds to Cs through two oxygen atoms, and
CseO (NTf
2
)
CseO (Crown)
2
CsNTf
2
2.947, 3.130, 3.135
3.024, 3.047
3.156, 3.160
3.088, 3.144
3.020, 3.057
2
ꢀ
ꢀ
conformer of Cs(NTf
2
)
3
, the structure belongs to C
3
symmetry, and
Cs(NTf
2
)
2
þ
ꢀ
Cs(NTf )2ꢀ
the distances between Cs and the two oxygen atoms of each NTf
are 3.156 and 3.160 Å, respectively. The calculated Eint of Cs with
2
2
3
þ
þ
Cs
2
NTf
2
Cs(18C6)NTf
Cs(18C6)(NTf )
2
3.115e3.283
3.240e3.513
ꢀ
one, two, and three NTf
2
are ꢀ317.8, ꢀ399.7 and ꢀ269.4 kJ/mol,
ꢀ
2 2
3.174, 3.192, 3.210, 3.494
þ
respectively, indicating that Cs is most likely stabilized by two
ꢀ
CseF (PF
6
)
CseO (Crown)
ꢀ
ꢀ
þ
NTf
2
anions. In the structure of NTf
2
coordinating with two Cs ,
each Cs interacts with the two oxygen atoms from one sulfonyl
þ
CsPF6
2.935
3.083
3.207e3.280
ꢀ
2ꢀ
Cs(PF
Cs(PF
6
)
)
2
group and the CseO distances are 3.088 and 3.144 Å, respectively.
6
3
ꢀ
2ꢀ
ꢀ
In the complexes of CsPF
6
, Cs(PF
6
)
2
and Cs(PF
6
)
3
(Fig. 1), all PF
6
þ
Cs
2
PF
6
3.087
þ
anions coordinate with Cs via three fluorine atoms, and the point
Cs(18C6)PF6
3.055, 3.067, 3.144
3.163, 3.163, 3.214, 3.214
3.136e3.296
3.319e3.597
ꢀ
6 2
Cs(18C6)(PF )
group of these three complexes are C3v, C
3
1
and C . The CseF dis-
tances in these complexes are listed in Table 2, and their average
values for the three complexes are 2.935, 3.083 and 3.254 Å,
ꢀ
2ꢀ
3
increasing gradually as more PF
calculated Eint values reveal that Cs with two PF
6
coordinates with cesium ions. The
The higher interaction energies in the complexes of Cs(NTf )
2
þ
ꢀ
2ꢀ
6
anions is the
and Cs(PF )
6
3
as compared with those in the complexes of
Cs(NTf2)2 and Cs(PF6)2 can be explained by electrostatic repulsion.
Fig. 2 shows the electrostatic potential (ESP) of each NTf in
Cs(NTf ) and PF6 in Cs(PF6)3 . The blue color over oxygen atoms
in NTf2 indicates the electronegative nature, and the red color over
the eCF group indicates the electropositive nature. It can be seen
that mutual penetration occurs at the sites of the eCF3 group
ꢀ
ꢀ
most stable structure with an Eint value of ꢀ475.0 kJ/mol, while the
þ
ꢀ
ꢀ
interaction energies for Cs with one and three PF
6
anions
2
2
ꢀ
ꢀ
2ꢀ
are ꢀ344.1 and ꢀ225.6 kJ/mol, respectively. As compared with
2
3
ꢀ
ꢀ
þ
þ
ꢀ
NTf
2
, PF
6
shows slightly lower binding energies when coordinating
þ
with Cs . The conformer of Cs
each Cs cation interacts with PF
2
PF
6
belongs D3d point group, and
3
ꢀ
6
via coordinating with three
fluorine atoms with a CseF distance of 3.087 Å.

T. Sun et al. / Journal of Molecular Structure 1148 (2017) 206e212
209
extractants are usually added in ILs to achieve a high distribution
þ
ratio and good extraction selectivity toward Cs . The crown ethers
þ
have been demonstrated to be qualified [11e13]. After Cs is
ꢀ
ꢀ
6
complexed by the macrocyclic compounds, whether NTf
2
or PF
coordinates with the deficient complexes is very interesting. We
reported previously the crystal structure of Cs(BPC6)NTf
2
[19],
ꢀ
þ
suggesting the coordination of NTf
2
with the deficient Cs(BPC6)
complexes. We herein employed the most frequently used macro-
cyclic compound 18C6 to theoretically calculate the interactions of
ꢀ ꢀ þ
2 6
and PF with the complex Cs(18C6) as well as to syn-
both NTf
thesize the complexes of Cs and 18C6 with NTf
The optimized structures of the complexes of Cs(18C6) with
þ
ꢀ
ꢀ
6
2
and PF .
þ
ꢀ
ꢀ
NTf and PF are illustrated in Fig. 4. One can see that the cesium
2
6
atom is located out of the plane of the six oxygen atoms of 18C6 due
to its larger ionic radii as compared with the ring size of 18C6,
þ
affording Cs the opportunity to interact with other ligands. In the
ꢀ
complex of Cs(18C6)NTf
2
, the two eCF
3
fragments of NTf
2
are trans
to each other, affording two oxygen atoms from different sulfonyl
þ
ꢀ
groups to coordinate with Cs . In the complexes of Cs(18C6)(NTf
2
)
2
,
ꢀ
one NTf
2
anion keeps the similar coordinating behavior as that in
þ
2
, while the other one coordinates with Cs via two
Cs(18C6)NTf
oxygen atoms from one sulfonyl group, which should be explained
by the steric effect. In the structure of Cs(18C6)PF
6
, there are three
þ
fluorine atoms interacting with Cs , consisting with the optimized
þ
ꢀ
ꢀ
structure of Cs with PF
6
. In the structure of Cs(18C6)(PF
6
)
2
,
ꢀ
differently, only two fluorine atoms from each PF
6
anions interact
þ
with Cs . The steric effect should be also responsible for this
difference.
þ
ꢀ
2
The calculated Eint between Cs(18C6) with one and two NTf
anions are ꢀ225.6 and ꢀ233.5 kJ/mol, respectively, while those
þ
ꢀ
6
between Cs(18C6) with one and two PF
anions are ꢀ263.6
and ꢀ311.5 kJ/mol, respectively. The results of interaction energy
þ
ꢀ
ꢀ
or PF
6
suggest that Cs(18C6) prefer to interact with two NTf
anions. A summary of the bond length in the complexes of
2
þ
ꢀ
ꢀ
6
is presented in Table 2. One can see
Cs(18C6) with NTf
2
and PF
that the CseO (Crown) distances are lengthened by the interaction
ꢀ ꢀ þ
2 6
or PF with Cs .
of NTf
The calculated results of the interactions between Cs(18C6)
þ
ꢀ
ꢀ
and NTf
2
or PF
6
were validated by the corresponding crystal
þ
ꢀ
ꢀ
6
structure. The crystallization of Cs and 18C6 with NTf
2
and PF
gives the complexes of [Cs(18C6)NTf and [Cs(18C6)PF ] ,
2
]
2
6 2
respectively, which are both dimers with bridging the anions, as
shown in Fig. 5. Select bond lengths are listed in Table 3.
ꢀ
In the crystal structure of [Cs(18C6)NTf
2
]
2
, two NTf
2
anions
þ
connect two Cs(18C6) moieties via coordinating to cesium ions.
ꢀ
2ꢀ
ꢀ
2ꢀ
6 3
in Cs(PF ) .
Fig. 2. ESP of NTf
2
in Cs(NTf
2
)
3
and PF
6
ꢀ
The two eCF
Cs(18C6)NTf
that in the calculated structure of Cs(18C6)(NTf
dination sphere is formed by the six oxygen atoms of the crown
3
fragments of the NTf
2
anion are trans to each other in
[
2 2
] . Each cesium is nine-coordinate, different from
ꢀ
2 2
) , and the coor-
ꢀ
2ꢀ
2ꢀ
between the ESP of NTf
2
in Cs(NTf
2
)
3
. In Cs(PF
6
)
3
, no penetration
of ESP was not observed, meaning that the electrostatic repulsion
ꢀ
ꢀ
ꢀ
ether and three oxygen atoms from NTf . Each NTf coordinates to
2
2
between PF
6
is smaller.
one cesium via one oxygen atom in each sulfonyl group and to the
other cesium ion via one oxygen atom only from one sulfonyl
group. The CseO distances between the oxygen atoms in the crown
moiety and the cesium ion range from 3.028(3) to 3.235(3) Å,
comparable to those in other Cs-crown ether complexes [32]. Be-
Some of the calculated results were validated by the electro-
spray ionization mass spectrometry (ESI-MS), as shown in Fig. 3.
The peaks at 545.73360 in the positive mode belonged to Cs
and 692.73862 in the negative mode belonged to Cs(NTf
observed in the ESI-MS spectra. Interestingly, a peak at 958.56466
belonged to Cs
þ
2
NTf
2
ꢀ
2
)
2
were
þ
sides, these values are slightly longer than those of the CseO dis-
3
(NTf
2
)
2
was also detected. We also tried to verify
ꢀ
þ
ꢀ
6
by ESI-MS.
tances between the cesium ion and the oxygen atoms in NTf
2
,
the calculation of the interactions between Cs and PF
Unfortunately, due to the poor solubility of CsPF , we did not
in ESI-MS.
which are ranging from 3.049(3) to 3.174(3) Å. The CseO distances
6
ꢀ
ꢀ
þ
6
between the cesium ion and the oxygen atoms in NTf
NTf
19]. This should be due to the fact that the cesium ion is deeply
2
in [Cs(18C6)
observe the peaks belonged to Cs(PF
6
)
2
or Cs
2
PF
2 2 2
] are much shorter than that in the complex of Cs(BPC6)NTf
[
ꢀ
ꢀ
þ
3
.2. Interactions of NTf
2
and PF
6
with Cs(18C6)
located in the cavity of BPC6 in Cs(BPC6)NTf
2
, which weakens the
ꢀ
interaction of NTf
2
with cesium ion.
ꢀ
ꢀ
ꢀ
The above results demonstrate that NTf
with Cs in the absence of other ligands. In the extraction,
2
and PF
6
can coordinate
In the crystal structure of [Cs(18C6)PF
connect two Cs(18C6) moieties via coordinating to cesium ions,
6
]
2
, similarly, two PF
6
þ
þ

210
T. Sun et al. / Journal of Molecular Structure 1148 (2017) 206e212
2
Fig. 3. Positive and negative mode of the ESI-MS of CsNTf solution.
ꢀ
and each PF
6
coordinates to one cesium ion via two fluorine atoms
and to the other cesium ion via one fluorine atom, thus cesium is
also nine-coordinate. The CseF distances are ranged from 3.143 to
3.528 Å. The CseO (Crown) distances are from 3.064 to 3.263 Å, a
little longer than those in [Cs(18C6)NTf
2
]
2
. In the DFT calculations,
þ
ꢀ
the binding energies between Cs(18C6) and PF
6
are a little lower
þ
þ
ꢀ
than those between Cs(18C6) and NTf
2
, suggesting the stronger
ꢀ
6
interaction between Cs(18C6) and PF
tances in [Cs(18C6)PF are a little longer than those in [Cs(18C6)
NTf due to the stronger coordination of PF
. The CseO (Crown) dis-
6 2
]
ꢀ
þ
2
]
2
6
with Cs . The co-
ꢀ
6
with Cs weakens the complexation of 18C6 to-
þ
ordination of PF
ward Cs , thus the CseO (Crown) distances are lengthened.
þ
After comparing the crystal structures of [Cs(18C6)NTf
2 2
] and
[
Cs(18C6)PF with their corresponding calculating results, one
6 2
]
can find some differences such as the bond lengths and the coor-
ꢀ
dination number. Nevertheless, the coordinating behavior of NTf
2
ꢀ þ
or PF with Cs(18C6) is generally consistent in the crystal struc-
6
tures and the calculated structures, that is, it is definitely demon-
þ
strated that the deficient complex Cs(18C6) interact with two
ꢀ
ꢀ
6
anions of NTf
It is worthwhile mentioning that the structure of [Cs(18C6)
NTf is different from that of K(18C6)NTf as reported in the
2
or PF , forming 1:2 complexes.
2
]
2
2
þ
literature [33]. In K(18C6)NTf
nected by only one NTf
crown oxygen atoms, K is located in the plane of the six crown
oxygen atoms, thus NTf coordinates to K from two sides of the
2
, two K(18C6) moieties are con-
. Different from Cs residing out of the six
þ
ꢀ
ꢀ
6
and PF .
Fig. 4. Optimized structure of the complexes of Cs(18C6) with NTf
2
ꢀ
þ
þ
2
Hydrogen atoms are omitted for clarity.
ꢀ þ
2

T. Sun et al. / Journal of Molecular Structure 1148 (2017) 206e212
211
ꢀ
ꢀ
þ
þ
the coordination of NTf
2
and PF
6
with Cs and Cs(18C6) . The
ꢀ
ꢀ
þ
coordination of NTf
2
and PF
6
with Cs should be the crucial one of
the reasons why the pure ILs such as C
extract Cs from aqueous solution in the absence of extractants as
aforementioned. In the presence of extractants such as crown
4
mimNTf
2
and C
4
mimPF
6
can
þ
ꢀ
ꢀ
6
in ILs coordinates
ethers or calixcrown ethers, whether NTf
2
or PF
þ
the deficient Cs complexes and then contributes to the extraction
is hard to be revealed because related methods and techniques are
very limited. Nevertheless, we herein studied the coordination ef-
fect on the extraction using the traditional organic solvent n-
octanol, in which both the crown ethers DCH18C6 (18C6 was not
4 2
used because of its high aqueous solubility) and C mimNTf were
added into the organic phase, to investigate the synergistic effect
ꢀ
þ
between DCH18C6 and NTf
2
during the extraction of Cs .
þ
The variation of the distribution ratios of Cs is shown in Fig. 6.
In the given extraction condition, DCH18C6 shows no extraction
þ
þ
toward Cs , while C
example, 0.02 mol/L C
4
mimNTf
mimNTf
2
can somewhat extract Cs . For
in n-octanol achieves a distribu-
4
2
þ
tion ratio of 0.015. The extraction of Cs by C
should be attributed to the coordination of NTf
4
mimNTf
2
in n-octanol
ꢀ
þ
2
with Cs and the
þ
exchange of C
mix and (D
4
mim into aqueous phase. The difference between
D
A
þ D
B
) representing the increase of D after mixing
extractants can intuitively reflect the synergistic effect in the
extraction system. One can see that the values of Dmix are much
larger than those of (D
between DCH18C6 and NTf
A
þ D
B
) in Fig. 6, thus the synergistic effect
ꢀ
2
is very obvious. Synergistic effect
usually suggests a formation of ternary complex during extraction,
ꢀ
so one can infer that NTf
2
coordinate with the complex of
þ
Cs(DCH18C6) during the extraction with n-octanol. As to the
extraction of Cs using ILs as solvents, it is hard to perform the
þ
same extraction experiment because the ILs act both solvents and
extractants.
þ
þ
The above calculated results indicated that Cs or Cs(18C6)
ꢀ
prefer to interact with two NTf
2
anions in gas phase. However,
structures in organic phase cannot be definitely identical to those in
gas phase due to the solvation effect. Thus we optimized the
þ
ꢀ
2
anions in n-
structures of Cs(DCH18C6) with one and two NTf
Fig. 5. Crystal structure of Cs(18C6)NTf
Hydrogen atoms are not showed and C atoms are not labeled for clarity.
2
and Cs(18C6)PF
6
in 20% thermal ellipsoids.
þ
octanol. To maintain charge neutrality, a C
included in the optimization of Cs(DCH18C6) with two NTf
4
mim cation was
þ
ꢀ
2
an-
ions. This is consistent with our extraction experiment as
mimNTf and DCH18C6 were both added into n-octanol as
C
4
2
Table 3
Selected bond length (Å) for the crystal structures of [Cs(18C6)NTf
PF
2
]
2
and [Cs(18C6)
extractants. The optimized structure of Cs(DCH18C6)NTf
2
and
6 2
] .
[Cs(18C6)NTf
2
]
2
6 2
[Cs(18C6)PF ]
CseO1
CseO2
CseO3
CseO5
CseO6
CseO7
CseO8
CseO9
3.049(3)
3.105(3)
3.174(3)
3.087(3)
3.193(3)
3.061(3)
3.184(3)
3.028(3)
3.235(3)
CseF1
CseF2
CseF3
CseO1
CseO2
CseO3
CseO4
CseO5
CseO6
3.213(11)
3.528(16)
3.144(6)
3.064(4)
3.264(4)
3.095(4)
3.224(5)
3.104(4)
3.226(4)
0
0
CseO10
þ
K(18C6) moiety and the complex forms as a coordination polymer.
In the lattice of [Cs(18C6)NTf (Fig. S1), the column like dimers are
2 2
]
arranged head-to-end, and are further arranged in a nearly hex-
agonal closest packing. The match of the cation radius and the ring
size of 18C6 should be one of the critical factors to crystallize the
ꢀ
complexes of the alkali metal ions with 18C6 and NTf
2
.
3
.3. Coordination effect on extraction
þ
Fig. 6. Extraction of Cs with single and mixtures of DCH18C6 and C
4
mimNTf
2
in n-
þ
octanol.
Aqueous
phase:
mimNTf
C(Cs )
¼
0.002
mol/L;
Organic phase:
The DFTcalculations and the experimental results demonstrated
C(DCH18C6) þ C(C
4
2
) ¼ 0.02 mol/L.

212
T. Sun et al. / Journal of Molecular Structure 1148 (2017) 206e212
ꢀ
þ
ꢀ
þ
than that between NTf
2
and Cs . The coordination of NTf
2
with Cs
þ
induced the synergistic effect in the extraction of Cs by DCH18C6
and C
4
mimNTf
2
in n-octanol. However, DFT calculation indicated
þ
that Cs(DCH18C6) in n-octanol prefer to interact with only one
ꢀ
NTf
2
anion. This study provides some useful information for the
þ
extraction of Cs by using ILs as solvents.
Acknowledgments
The study was supported by National Natural Science Founda-
tion of China under Project 21401115 and 51425403, and the Pro-
gram for Changjiang Scholars and Innovative Research Team in
University (IRT13026).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2017.07.048.
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the reaction (2) as shown below in n-octanol were calculated to be
þ
7
8.4 kJ/mol, indicating that Cs(DCH18C6) in n-octanol prefer to
ꢀ
interact with only one NTf
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2
anions, which is different from that in
[
[
CsðDCH18C6ÞNTf2
þ C mimNTf 4CsðDCH18C6ÞðNTf Þ $ðC mimÞ
(2)
4
2
2 2
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[
[
[
[
[
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both NTf
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2 6
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6
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