Crystallization in ionic liquids: Synthesis, properties, and polymorphs of uranyl salts

Article abstract of DOI:10.1021/cg501277d

Crystallizations in uranyl-containing ionic liquids yielding crystals {X}_a{[UO₂]_b[Y]_c} with selected anions and cations (X = [Bmim]⁺ for Y = Cl^-, NO₃^-, and SCN^-; Y = Cl^- for X = [Emim]⁺, [Emmim]⁺, [Bmim]⁺, and [Bmmim]⁺; Emim = l-ethyl-3-methyl-imidazolium, Emmim = l-ethyl-2,3-dimethylimidazolium, Bmim = l-butyl-3-methylimidazolium, and Bmmim = l-butyl-2,3-dimethylimidazolium) were performed herein. Through standard crystallographic analyses, Hirshfeld surface analyses, and multiple characterization techniques, compounds with common cations/anions were investigated. For compounds [Bmim]₂[(UO₂)₂(μ-OH)₂(NO₃)₄] (1), [Bmim]₃[UO₂(NCS)₅] (2), and [Bmim]₂[UO₂Cl₄] (3TT and 3TG), common [Bmim]⁺ cations with different conformations were studied with respect to packing and interactions (for 1). The coordinated [(UO₂)₂(μ-OH)₂(NO₃)₄]^2-, [UO₂(NCS)₅]^3-, and [UO₂Cl₄]^2- anions that have historically been related to nuclear fuel cycles were demonstrated with respect to geometry and distortion. For compounds with common [UO₂Cl₄]^2- anions [Emim]₂[UO₂Cl₄] (4), [Emmim]₂[UO₂Cl₄] (5), [Bmmim]₂[UO₂Cl₄] (6), 3TT, and 3TG, observed interionic interactions that have been previously impeded by limited structural information were discussed fully in relation to different cations and temperatures. Moreover, multistep phase transformations of 2, which have been undefined in solution studies, have been identified through differential scanning calorimetry analyses and polarizing optical microscopy. The polymorph transformations between 3TT and 3TG in solution, as controlled by uranyl concentration, were studied using optical microscopy and powder X-ray diffraction. The thermal stability, IR/Raman, and UV-vis/luminescence spectra of these compounds were also investigated.

Full text of DOI:10.1021/cg501277d

Article
pubs.acs.org/crystal
Crystallization in Ionic Liquids: Synthesis, Properties, and
Polymorphs of Uranyl Salts
Feng Qu, Qian-Qian Zhu, and Chun-Li Liu*
Beijing National Laboratory for Molecular Sciences, Fundamental Science Laboratory on Radiochemistry & Radiation Chemistry,
College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, P.R. China
S
* Supporting Information
ABSTRACT: Crystallizations in uranyl-containing ionic
liquids yielding crystals {X}a{[UO2]b[Y]c} with selected anions
and cations (X = [Bmim]+ for Y = Cl−, NO3 , and SCN−; Y =
−
Cl− for X = [Emim]+, [Emmim]+, [Bmim]+, and [Bmmim]+;
Emim = l-ethyl-3-methyl-imidazolium, Emmim = l-ethyl-2,3-
dimethylimidazolium, Bmim = l-butyl-3-methylimidazolium,
and Bmmim = l-butyl-2,3-dimethylimidazolium) were per-
formed herein. Through standard crystallographic analyses,
Hirshfeld surface analyses, and multiple characterization
techniques, compounds with common cations/anions were investigated. For compounds [Bmim]2[(UO2)2(μ-OH)2(NO3)4]
(1), [Bmim]3[UO2(NCS)5] (2), and [Bmim]2[UO2Cl4] (3TT and 3TG), common [Bmim]+ cations with different
conformations were studied with respect to packing and interactions (for 1). The coordinated [(UO2)2(μ-OH)2(NO3)4]2−,
[UO2(NCS)5]3−, and [UO2Cl4]2− anions that have historically been related to nuclear fuel cycles were demonstrated with respect
to geometry and distortion. For compounds with common [UO2Cl4]2− anions [Emim]2[UO2Cl4] (4), [Emmim]2[UO2Cl4] (5),
[Bmmim]2[UO2Cl4] (6), 3TT, and 3TG, observed interionic interactions that have been previously impeded by limited
structural information were discussed fully in relation to different cations and temperatures. Moreover, multistep phase
transformations of 2, which have been undefined in solution studies, have been identified through differential scanning
calorimetry analyses and polarizing optical microscopy. The polymorph transformations between 3TT and 3TG in solution, as
controlled by uranyl concentration, were studied using optical microscopy and powder X-ray diffraction. The thermal stability,
IR/Raman, and UV−vis/luminescence spectra of these compounds were also investigated.
have all been reacted with ILs, IL precursors, or even neutral
imidazoles under certain conditions to produce interesting
results. In some cases, uranium oxides are used as starting
materials,26 and the desired ligands coordinated with uranyl can
also be introduced through additional salts27 and acids.28−30
However, when compared with investigations performed on
pure ILs, which have enabled the synthesis of and studies on
compound series (e.g., how they differ from anionic
components31−33 or vary due to imidazolium cations with
respect to chain length,7,34,35 methyl substituents,7 and
polymorphs36,37), systematic and easy-to-perform synthesis
methods are important to support future investigations on
uranyl salts,38 as have been reported for pure ILs. As clearly
indicated by Desiraju, “The creative and intellectual challenge
in designing and constructing a new crystal structure is similar
to that in organic synthesis”,8 such that novel structures may
also be obtained in IL solutions by proper synthesis strategies
using pure ILs.
INTRODUCTION
■
Ionic liquids (ILs) are attractive to investigators due to their
unique physical and chemical properties and have thus been
widely examined in many areas.1,2 Among their usages,
crystallization in ILs is a complex and fascinating field of
research.3−5 Both the unique coordination features that cannot
generally be observed using molecular solvents and the
interaction specifics that are difficult to obtain in the liquid
state of ILs can be investigated.6,7 However, the crystallography
data that have immensely promoted our understanding of
intermolecular interactions8 have not been fully explored
regarding the crystallographic potential in IL systems.
A special interest in f metal-containing ILs has recently arisen
in various fields, resulting in many high-quality investigations
involving crystallographic studies.9−12 Specifically, promising
applications of ILs in nuclear fuel cycles13−17 have encouraged
extensive studies on uranium-based IL systems, including the
relatively common crystallization of uranium salts in ILs and
novel explorations of crystallization methods in such complex
systems.3−6,13,18,19 Using the widely studied uranyl-imidazolium
structures as an example, the starting uranyl salts with
inorganic/organic ligands, such as UO2(NO3)2·6H2O,18,20,21
and UO2Cl2·3H2O,22,23 and unique synthesized compounds,
such as UO2(NCS)2·nH2O24 and UO2SO4(propylamine)2,25
The typical IL [Bmim][Tf2 N] (Tf2 N
= bis-
(trifluoromethylsulfonyl)imide) was obtained through a two-
Received: August 27, 2014
Revised: October 20, 2014
© XXXX American Chemical Society
A
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Article
step synthesis. The first combination reaction by 1-methyl-
imidazole and chlorobutane was performed without foreign
components, and the second double decomposition reaction by
[Bmim]Cl and Li[Tf2N] was supported by a phase separation.
These two reaction types have been widely used to synthesize
uranyl salts in ILs, and combination reactions are generally
preferred when compared to double decomposition reactions.
The separation of unfavorable products has proven to be more
difficult in ILs for nonvolatile and relatively high viscosity ILs.3
As procedures involving ILs can be improved by utilizing each
unique circumstance and by following the rules working in
ILs,39 are there opportunities to avoid unfavorable products in
IL solutions? Regarding previous investigations performed in
Figure 1. Anionic components of uranyl investigated in this work: (a)
the square bipyramid [UO2Cl4]2−, (b) the pentagonal bipyramid
[UO2(NCS)5]3−, and (c) the hexagonal bipyramid [(UO2)2(μ-
OH)2(NO3)4]2−.
in pure ILs, such as [Bmim]Cl,36,37 and [Bmim]PF6,53−58 as
well as for metal-containing polymorphs, such as
[Bmim]2[Mo6Cl14].59 However, the conformation of [Bmim]+
cations have not been fully studied in union of interactions and
crystal packing58 and were thus selected for our study. To
perform crystallization on a series of cations, the [Emim]+,
[Emmim]+, [Bmim]+, and [Bmmim]+ cations (Emim = l-ethyl-
3-methyl-imidazolium, Emmim = l-ethyl-2,3-dimethylimidazo-
lium, Bmim = l-butyl-3-methylimidazolium, and Bmmim = l-
butyl-2,3-dimethylimidazolium) were used for their structural
properties, allowing systematic studies.60,61 The crystal
structures of cation-varied compounds [Emim]2[UO2Cl4] (4),
[Emmim]2[UO2Cl4] (5), [Bmmim]2[UO2Cl4] (6), 3TT, and
3TG, as observed in interionic interactions, are discussed in
relation to the different cations and temperatures. Additionally,
novel transformations were observed with multiple character-
izations of compounds 2, 3TT, 3TG, and 6. The multistep
phase transformations of compound 2, which are dependent on
temperature, were identified using differential scanning
calorimetry (DSC) analyses and polarizing optical microscopy
(POM) and strongly support and well explain previously
undefined phenomenon in solution studies.24 The polymorph
transformations between 3TT and 3TG in solutions controlled
with uranyl concentration at room temperature, as investigated
using optical microscopy and powder X-ray diffraction
(PXRD), have uncovered a unique procedure at high uranyl
concentrations in IL solutions. The structure variations of 3TT,
3TG, and 6 in solid form at temperatures between room
temperature and 180 K were confirmed with DSC and the
temperature-dependent PXRD spectra. The thermal stability,
IR/Raman, and UV−vis/luminescence spectra were also
investigated.
−
Cl−- and NO3 -based IL solutions strongly related to nuclear
fuel cycles,11,15 [NTf2]−-based ILs have been widely used for
good stability, low viscosity, high conductivity, and generally
weak coordination by [NTf2]− cations.12,33 Formation of uranyl
speciations (coordinated anions) in such IL systems appeared
to be controlled. The results from experiments26,40,41 and
calculations42,43 have proven that the main speciation of uranyl
in Cl− and [NTf2]− containing ILs should be [UO2Cl4]2−
anions. Similarly, crystallizations performed with UO2(NO3)2·
6H2O and [NTf2]−-based ILs have afforded [(UO2)2(μ-
OH)2(NO3)4]2− anions in products.18 Thus, [NTf2]−-based
ILs have been designated as ideal background solutions for
crystallization on uranyl salts without affecting the uranyl
speciations (coordinated anions) by generating uranyl-[NTf2]−
complexes. The metal concentration in such solutions can be
controlled by adding [NTf2]−-based salts,44 such as UO2-
(Tf2N)2·xH2O.28,40 Moreover, the crystallizations of uranyl-
imidazolium structures have provided a degree of difficulty with
respect to different imidazolium cations. Crystal structures
generally with [Emim]+ cations have been investigated in
depth,18,24,45 while crystal structures with [Bmim]+ cations,
which are of extensive interest in solution studies,11,14,28 are
difficult to synthesize. This difference in crystallization may
indicate the possibility of the selective formation of crystals
with certain cations (such as [Emim]+) for the solution mixture
(such as [Emim]+/[Bmim]+) that may be required in
applications39 as “designed” solvents. In this work, we
demonstrate that selective crystal formations of anions and
cations can be performed in solutions constituted by
UO2(Tf2N)2·xH2O salts, [NTf2]−-based ILs, and additional
ILs. The novel structures obtained not only promote systematic
studies on uranyl coordinations and interionic interactions that
have been difficult to conclude on in previous limited structural
studies but also support solution investigations on transition
features that have been difficult to obtain in IL solutions.
The crystallizations performed herein include anion and
cation variations. For the crystallizations on a series of anions,
EXPERIMENTAL SECTION
■
Synthesis. Caution! Whereas all of the uranium compounds used
in this investigation contained natural uranium salts, standard
precautions should be employed when handling radioactive materials.
All reagents were of analytical grade and used without further
purification. l-Ethyl-3-methylimidazolium chloride ([Emim]Cl, >99%),
l-ethyl-2,3-dimethylimidazolium chloride ([Emmim]Cl, >99%), l-
butyl-3-methylimidazolium chloride ([Bmim]Cl, >99%), l-butyl-2,3-
dimethylimidazolium chloride ([Bmmim]Cl, >99%), 1-butyl-3-meth-
ylimidazolium nitrate ([Bmim][NO3], >98%), 1-butyl-3-methylimida-
zolium thiocyanate ([Bmmim][SCN], >98.5%), and lithium bis-
(trifluoromethylsulfonyl)imide (Li[Tf2N], >99%) were purchased
from Lanzhou Greenchem ILS, LICP, CAS, China. The [Emim]Cl,
[Bmim]Cl, [Emmim]Cl, [Bmmim]Cl, and [Bmim][NO3] were dried
in a vacuum box at 70 °C for 24 h prior to use. The [Bmim][SCN]
was dried in vacuo at 50 °C for 24 h. The [Bmim][Tf2N] was
synthesized by reacting [Bmim]Cl with Li[Tf2N] according to
procedures from the literature12,62,63 and then dried in a vacuum
box at 110 °C for 12 h. The elemental analysis calculated for
C16H15F6N2O4S2 ([Bmim][Tf2N]) was C, 28.64; H, 3.61; and N
10.02%. Found: C 29.00%, H 3.76%, N 9.98%.
Cl−, NO3 , and SCN− anions were selected for their historical
−
relationship to nuclear fuel cycles in pyro-reprocessing
methods, wet reprocessing systems, and analytical measure-
ments, respectively.11,14,15,46−50 Moreover, square bipyramid
[UO2Cl4]2−, hexagonal bipyramid [(UO2)2(μ-OH)2(NO3)4]2−,
and pentagonal bipyramid [UO2(NCS)5]3− anions (Figure 1)
formed in the anion-varied compounds [Bmim]2[(UO2)2(μ-
OH)2(NO3)4] (1), [Bmim]3[UO2(NCS)5] (2), and [Bmim]2-
[UO2Cl4] (3TT and 3TG), which include the common
coordination geometries of uranyl cations,51,52 are extremely
suitable for coordination and distortion investigations. As the
counterions in these structures, the [Bmim]+ cation is one of
the most well studied IL cations for performing polymorphism
B
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Article
Table 1. Crystallographic Data for 1, 2, 3TT, and 3TG
compound
1
2
3TT
3TG
formula
C16H32N8O18U2
1100.55
C29H45N11O2S5U
978.09
C16H30Cl4N4O2U
690.27
C16H30Cl4N4O2U
690.27
formula weight
T (K)
290
290
290
290
crystal system
space group
a (Å)
triclinic
monoclinic
P21/c
monoclinic
P21/n
triclinic
P1
̅
P1
̅
5.52386(17)
15.9591(5)
19.0897(7)
104.586(3)
91.067(3)
90.718(3)
1,628.09(10)
2
16.7336 (7)
17.8778 (7)
14.9349 (8)
90
8.5341(5)
17.5319(10)
8.8715(6)
90
8.8731(4)
8.9844(4)
9.2813(5)
73.820(4)
66.506(5)
71.928(4)
634.82(6)
1
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
110.211 (5)
90
106.449(7)
90
4,192.8 (3)
4
1,273.02(14)
2
Z
ρcalcd (g cm−3
)
2.245
1.549
1.801
1.806
μ (mm−1
)
10.017
4.160
6.812
6.830
Rint
0.0283
0.0437
0.0385
0.0278
data/restraints/parameters
GOF
6379/4/373
1.079
8566/4/427
1.043
2505/0/126
1.033
2492/17/164
1.035
a
R1, wR2 [I ≥ 2σ(I)]
0.0386
0.0428
0.0280
0.0182
0.0767
0.1000
0.0598
0.0394
a
R1, wR2 (all data)
0.0632
0.0763
0.0462
0.0184
0.0868
0.1173
0.0663
0.0394
a
2
2
2
R1 = Σ(|F0| − |FC|)/Σ|F0|, wR2 = [Σw(F0 − FC )2/Σw(F0 )2]1/2
.
The UO2(NO3)2·6H2O used in this study was purchased from
The elemental analysis calculated for C16H30Cl4N4O2U ([Bmim]2-
[UO2Cl4]) yielded C 27.84%; H 4.38%; N 8.12%. Found: C 27.81%,
H 4.34%, N 8.18%.
[Bmim]2[UO2Cl4]. 3TT was synthesized from UO2(Tf2N)2·xH2O
(0.10 mmol, 0.1010 g, x = 10), [Bmim][Tf2N] (3.33 mmol, 1.3965 g),
and [Bmim]Cl (1.67 mmol, 0.2917 g). Yellow plate crystals of 3TT
were obtained after one month. Yield: 0.004 g (6% based on uranyl
salts). The elemental analysis calculated for C16H30Cl4N4O2U
([Bmim]2[UO2Cl4]) yielded C 27.84%; H 4.38%; N 8.12%. Found:
C 27.98%, H 4.39%, N 8.21%.
[Bmmim]2[UO2Cl4]. 6 was synthesized from UO2(Tf2N)2·xH2O
(0.09 mmol, 0.0909 g, x = 10), [Bmim][Tf2N] (1.66 mmol, 0.6982 g),
and [Bmmim]Cl (0.83 mmol, 0.1566 g). Yellow crystals of 6 were
collected after 2 weeks. Yield: 0.037 g (57% based on uranyl salts).
The elemental analysis calculated for C18H34Cl4N4O2U ([Bmmim]2-
[UO2Cl4]) yielded C 30.10%; H 4.77%; N 7.80%. Found: C 29.68%,
H 4.72%, N 7.73%.
[Bmim]2[(UO2)2(μ-OH)2(NO3)4]. 1 was synthesized from
UO2(Tf2N)2·xH2O (0.35 mmol, 0.3347 g, x = 7), [Bmim][Tf2N]
(3.00 mmol, 1.2581 g), and [Bmim][NO3] (2.00 mmol, 0.4024 g).
Yellow crystals of 1 were obtained after one month. Yield: 0.019 g (5%
based on uranyl salts). The elemental analysis calculated for
C16H32N8O18U2 ([Bmim]2[(UO2)2(μ-OH)2(NO3)4]) yielded C
17.46%; H 2.93%; N 10.18%. Found: C 17.31%, H 2.86%, N 10.07%.
[Bmim]3[UO2(NCS)5]. 2 was synthesized by adding UO2(Tf2N)2·
xH2O (0.20 mmol, 0.2021 g, x = 10) and [Bmim][Tf2N] (0.50 mmol,
0.2097 g) to an NMR tube, followed by drying in a vacuum box at 70
°C for 12 h. Then, the [Bmim][SCN] (3.00 mmol, 0.5919 g) was
added to the NMR tube followed by drying in vacuo at 50 °C for 12 h.
The sealed tube was placed at room temperature and left undisturbed
for two months, and yellow crystals of 2 were collected via filtration,
washed with hexyl hydride, and dried in a vacuum box. Yield: 0.005 g
(2% based on uranyl salts).
Sinopharm Chemical Reagent Co., China. UO3 was prepared by
heating UO2(NO3)2·6H2O to 550 °C under ambient air conditions.64
UO2(Tf2N)2·xH2O was synthesized by reacting UO3 with HTf2N
according to the literature.28 The product was dried in a vacuum box at
110 °C for 12 h and then stored in a glovebox. The composition of the
UO2(Tf2N)2·xH2O was determined via elemental analysis prior to
weighing. For example, the elemental analysis calculated for
C4H20F12N2O12S4U (UO2(Tf2N)2·10H2O) was C, 4.75%; H, 1.99%;
and N 2.77%. Found: C 4.71%, H 2.04%, N 2.78%. All ILs and
synthesized products were weighed in a glovebox (H2O < 1 ppm). The
elemental analysis was performed on an Elementar Vario MICRO
CUBE analyzer.
All compounds were obtained through the general process
described for [Emim]2[UO2Cl4].
[Emim]2[UO2Cl4]. 4 was synthesized by adding UO2(Tf2N)2·xH2O
(0.07 mmol, 0.0707 g, x = 10), [Bmim][Tf2N] (1.66 mmol, 0.6982 g),
and [Emim]Cl (0.83 mmol, 0.1217 g) to a reaction tube, followed by
drying in a vacuum box at 70 °C for 12 h. Then, the sealed tube was
stored at room temperature and left undisturbed for 2 weeks, and
yellow crystals of 4 were collected via filtration, washed with
dichloromethane, and air-dried. Yield: 0.016 g (35% based on the
uranyl salts). The elemental analysis calculated for C12H22Cl4N4O2U
([Emim]2[UO2Cl4]) was C 22.73; H 3.50; N 8.83%. Found: C
22.98%, H 3.63%, N 8.38%.
Caution: In this work, because the uranyl salts are highly soluble in
IL mixtures, the yield is relatively low; however, a continuous process
can be carried out in the solution after the crystals have been
separated.
[Emmim]2[UO2Cl4]. 5 was synthesized from UO2(Tf2N)2·xH2O
(0.09 mmol, 0.0909 g, x = 10), [Bmim][Tf2N] (1.66 mmol, 0.6982 g)
and [Emmim]Cl (0.83 mmol, 0.1333 g). Yellow crystals of 5 were
obtained after 1 week. Yield: 0.026 g (43% based on uranyl salts). The
elemental analysis calculated for C14H26Cl4N4O2U ([Emmim]2-
[UO2Cl4]) yielded C 25.39%; H 3.96%; N 8.46%. Found: C
25.39%, H 4.05%, N 8.46%.
Crystallographic Data Collection and Refinement. The
crystallographic data of 1 (290 K), 2 (290 K), 3TG (290 K), 3TT
(290 K), 6 (290 K), and 6 (180 K) were collected on an Agilent
Technologies Gemini kappa-geometry diffractometer equipped with
an Atlas detector using Mo Kα radiation (0.71073 Å). The
crystallographic data of 3TT (180 K), 3TG (180 K), 4 (180 K),
and 5 (180 K) were collected with an Agilent Technologies
[Bmim]2[UO2Cl4]. 3TG was synthesized from UO2(Tf2N)2·xH2O
(0.14 mmol, 0.1415 g, x = 10), [Bmim][Tf2N] (3.33 mmol, 1.3965 g),
and [Bmim]Cl (1.67 mmol, 0.2917 g). Yellow crystals of 3TG were
collected after 3 weeks. Yield: 0.023 g (23% based on uranyl salts).
C
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Crystal Growth & Design
Article
Figure 2. Interionic interactions that are at least 0.1 Å shorter than the van der Waals radii in compound 1 and in the structures from ref 18: (a)
Anion−anion contacts in compound 1, (b) C−H···O interactions around the [Bmim]+ cation with a G′T conformation in compound 1, (c) C−H···
O interactions around the [Bmim]+ cation with a TE′ conformation in compound 1, (d) C−H···O interactions around the [Mmim]+ cation in
[Mmim]2[(UO2)2(μ-OH)2(NO3)4] from ref 18, (e) C−H···O interactions around the [Emim]+ cation in [Emim]2[(UO2)2(μ-OH)2(NO3)4] from
ref 18, and (f) C−H···O interactions around the [Mmmim]+ cation in [Mmmim]2[(UO2)2(μ-OH)2(NO3)4] from ref 18.
SuperNova A diffractometer fitted with an Atlas detector using Mo Kα
radiation (0.71073 Å). Using Olex2,65 all of the structures were solved
based on the Superflip66 structure solution program, which uses
charge-flipping, and were subsequently refined with the ShelXL67
refinement package using least-squares (CGLS for 4) minimization.
The hydrogen atoms were placed at the calculated positions and were
included in the structure without further parameter refinement, with
the exception of the hydroxo bridge hydrogen atoms in compound 1
that were located from the Fourier difference map and refined
isotropically.18 The geometrical features of structures (Tables S2−S6,
Supporting Information) were calculated with Mercury 3.1 software
(Cambridge Crystallographic Data Centre, Cambridge, UK).
Hirshfeld Surface Analysis. Hirshfeld surface analyses68,69 and
fingerprint plots70 were obtained using the program CrystalExplorer
3.171 to perform visual investigations and quantitative analyses on
crystal interactions. These analyses were not performed on compound
3TG (290 K) due to disorder.
Powder X-ray Diffraction. PXRD data were collected on a Rigaku
Dmax 2400 diffractometer using Cu Kα radiation (1.54178 Å) in the
range of 5° < 2θ < 60° at room temperature. The temperature
dependence of the PXRD data was acquired on a Bruker Advance 8
diffractometer using Cu Kα radiation (1.54178 Å) in the range of 5° <
2θ < 60° at 180 K and 5° < 2θ < 40° at 210, 240, 270, and 298 K.
Spectroscopy Studies. Solid-state UV−Vis reflectance spectra
were recorded on a Shimadzu UV-2450 spectrometer in the range of
340−510 nm at room temperature using a BaSO4 plate as the
background (100% reflectance). FTIR spectra were acquired on a
NICOLET iN10 MX spectrometer in the wavenumber range of
4000−600 cm−1. Raman spectra were recorded on a Raman 950 FT-
Raman spectrometer. Luminescent spectra (with an excitation
wavelength of 330 nm) were measured on a HITACHI F7000
fluorescence spectrophotometer equipped with a low-temperature
system.
Polarizing Optical Microscope. Polarizing optical microscopy
(POM) analyses were acquired on a Leica DLMP microscope with a
hot stage, from −80 °C to +80 °C, and at a heating and cooling rate of
0.2 °C/s (12 °C/min). The samples were placed between two glass
slips under nitrogen during the testing process. All photos and videos
were recorded with a digital camera.
RESULTS AND DISCUSSION
■
Crystal Structural Analysis for Anion-Varied Com-
pounds with [Bmim]+ Cations. The crystallographic data for
1, 2, 3TT, and 3TG are given in Table 1. The simulated results
from the single crystallographic diffraction measurements are
strongly supported by the PXRD data (Figures S1−S4,
Supporting Information). These compounds are formed by
[Bmim]+ cations and typical uranyl coordination anions
[(UO2)2(μ-OH)2(NO3)4]2−, [UO2(NCS)5]3−, and
[UO2Cl4]2−, respectively. Although [Bmim]+ cations have
been extensively studied with these typical anions in different
solutions, crystal structures have not yet been obtained. The
reported compounds are generally with the similar [Emim]+
cations.18,24,45 For compounds with imidazolium cations, alkyl
chain length variations may cause diverse physicochemical
property alterations, such as melting point, viscosity, ionic
conductivity, and self-diffusion coefficient.72 Thus, the crystal
structures obtained with [Emim]+ cations could not convinc-
ingly act as a “structural messenger” for uranyl coordination and
cation−anion interactions in [Bmim]+ cation-based ILs.
Moreover, the conformation of [Bmim]+ cations that have
been identified to possess an important role regarding the
structures and physicochemical properties of ILs by exper-
imental and theoretical methods,36,37,53−59,73 have not yet been
investigated for uranyl salts. As follows, the conformation of
[Bmim]+ cations in 1, 2, 3TT, and 3TG serve a part on
interionic interactions and packing. All of the conformations of
[Bmim]+ cations used in this work (Table S2, Supporting
Information), such as TT, TG, G′T, TE′, TE, and TG′ are
named based on torsion angles θ1 and θ2 (based on θ0 being
Thermo Analysis. The thermogravimetry analysis/differential
scanning calorimetry (TGA-DSC) analysis was performed on an
SDT Q600 simultaneous TGA-DSC instrument from room temper-
ature to 600 °C at a heating rate of 10 °C/min under flowing N2 (100
mL/min). The DSC analysis was performed using 9−10 mg exactly
weighed samples on an Q100 DSC instrument from −80 °C to +80
°C at a heating and cooling rate of 3 °C/min (10 °C/min for
compound 2) under flowing N2 (50 mL/min).
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positive) outlined by Tsuzuki et al.73 and further illustrated by
Saouane et al.58
the associated 2D plots. The Hirshfeld surface of [Bmim]+
cations with G′T and TE′ conformations (Figures S12 and S13,
Supporting Information) were generated, marking the directed
interactions with red areas that supported the result perform-
ance in Figure 2 and Table S3. From the Fingerprint plots
(Table 2 and Figures S12 and S13, Supporting Information),
Compound 1 crystallized in the triclinic space group P1
(Table 1) and consists of two [Bmim]+ cations and two half
[(UO2)2(μ-OH)2(NO3)4]2− anions in an asymmetric unit.
Compounds with [(UO2)2(μ-OH)2(NO3)4]2− anions have
been extensively studied with various countercations, such as
imidazolium,18,20 ammonium,74 and pyridinium.75 The com-
pound 1 [Bmim]+ cations with G′T and TE′ conformations,
which maintain specific cation−anion interactions (Table S3,
Supporting Information), could be compared with previous
well-illustrated structures18 that have demonstrated the
counterions [Mmim]+, [Emim]+, and [Emmim]+. Because
there are numerous weak interactions in the structures, in order
to obtain a significant and comparable result, only the interionic
interactions that were at least 0.1 Å shorter than the van der
Waals radii76 were investigated. As observed in Figure 2a, the
O−H···O interactions between the [(UO2)2(μ-OH)2-
(NO3)4]2− anions in compound 1 are formed between the
axial oxygens of uranyl and the bridging hydroxo groups, as
reported in the compound [Mmim]2[(UO2)2(μ-OH)2(NO3)4].
Previously reported cation−anion interactions18 were divided
into the following four types according to the oxygen atoms
involved (Figure 2): the oxygen atom from the bridging
̅
Table 2. Relative Percentage Contributions of Selected
Interactions for Imidazolium [(UO2)2(μ-OH)2(NO3)4]2−
Salts with Respect to Overall Hirshfeld Surface Area
cation
C−H···O
C−H···H
C−H···N
C−H···C
[Bmim]+ (G′T)
[Bmim]+ (TE′)
[Emim]+
[Mmmim]+
[Mmim]+
42.1
44.4
50.9
54.1
54.6
47.8
44.9
36.4
33.7
31.0
1.8
1.9
1.8
0.8
1.0
1.0
1.2
0.6
the polar C−H···O and nonpolar H···H were identified as the
major interactions in those structures. From Table 2, the
relative percentage contributions of C−H···O and the H···H
interactions changed with imidazolium cation variation. The
percentage of H···H interactions increased and the C−H···O
interactions decreased (Table 2) with increasing the alkyl
groups in the structure from [Mmim]+ to [Bmim]+.
hydroxo groups, denoted as C−H···Obri, with a H···O
interaction distance of 2.573(3) Å and a C−H···O contact
angle of 173.5(3)°; the axial oxygens of uranyl, denoted as C−
H···Oura, with a H···O interaction distance of 2.565(8)−
2.619(3) Å and a C−H···O contact angle of 122.7(3)−
155.1(3)°; the oxygen atom from the nitrate groups that
coordinated to the uranyl, denoted as C−H···Ocoo, with a H···O
interaction distance of 2.453(7)−2.602(3) Å and a C−H···O
contact angle of 145.4(3)−164.0(3)°; and the oxygen atom
from the nitrate groups that were not coordinated to the uranyl,
denoted as C−H···Ofre, with a H···O interaction distance of
2.533(3)−2.601(3) Å and a C−H···O contact angle of
129.9(3)−167.0(7)°. In compound 1, C−H···Obri, C−H···
Ocoo, and C−H···Ofre were formed in two manners by the
[Bmim]+ cations with G′T and TE′ conformations, with short
H···O distances and small C−H···O angles (Table S3,
Supporting Information). As shown in Figure 2b, the
[Bmim]+ cation with the G′T conformation formed one C−
H···Obri, one C−H···Ofre, and two C−H···Ocoo. The [Bmim]+
Compound 2 crystallized in the monoclinic P21/c space
group (Table 1), which consists of one pentagonal bipyramid
[UO2(NCS)5]3− anion and three charge-balanced [Bmim]+
cations with conformations of TT, TG′, and TE in the
asymmetric unit. As expected, the uranyl cation is coordinated
by thiocyanates using nitrogen linkages, as observed in the
series of counter cations, e.g., imidazolium,24 ammonium,27,81
and others.82 The UO bond distance of 1.766(8) and
1.765(9) Å, the U−N bond distance of 2.398(11)−2.513(10) Å
and the OUO angle of 171.2(9)° have been reported in
the compound [Emim]3[UO2(NCS)5].23 Herein, the [Bmim]3-
[UO2(NCS)5] have relatively short UO bond distances of
1.761(4) and 1.757(5) Å, a close U−N bond distance of
2.430(7)−2.468(6) Å, and a large OUO angle of
178.0(2)°, suggesting less distortion of the [UO2(NCS)5]3−
anion in compound 2. In comparison with our structure
investigations, the more interesting features of compound 2
may have arisen from the phase transformations that remained
undefined in previous solutions studies, which will be illustrated
later in this work.
cation with TE′ conformation has found five C−H···Ocoo
,
including one double-bonded C−H···Ocoo (Figure 2c). Among
those C−H···O interactions performed in Figure 2b−f, the C−
H···Ocoo may be recognized as major interactions for its
attendance in all structures (the only form in Figure 2c) by
suggesting the strongest contacts on holding the shortest H···O
distance in each structure (Table S3, Supporting Information).
Furthermore, this conclusion can be supported by the C−H···
Ocoo interactions formed on the single acidic ring hydrogen
atom (between two nitrogen atoms), as this hydrogen atom is
known for its polarity to form the strongest interactions with
anions in ILs.34,77,78
Compound 3TT crystallized in the monoclinic space group
P2 /n, whereas 3TG crystallized in the triclinic space group P1
̅
1
(Table 1). The asymmetric unit of the two crystals are
composed of a half [UO2Cl4]2− anion and one [Bmim]+ cation
with TT and TG conformations, respectively (for 3TG, the
disordered butyl chain is revised in two positions with
restraints, including a TG conformation with a percentage of
0.466 and a TT conformation with a percentage of 0.534). The
geometry of [UO2Cl4]2− is square bipyramidal,83 with uranyl
oxygen atoms as the apexes and four chlorides in equatorial
positions; various counterions, such as imidazolium,22,23,45
ammonium,84 and others85−87 have been reported to stabilize
such structures. Because no significant differences were
observed for those ions themselves, we will discuss the
interionic interactions and packing features lead by imidazolium
cations later, together with cation-varied compounds.
In addition to standard crystallographic analyses, Hirshfeld
surface analyses and full fingerprint plots have enabled a full
understanding of interionic interactions in imidazolium
salts.31,32,79,80 By treating the interactions in the molecule as
a whole, Hirshfeld surface analyses indicated all existing types of
interactions in structures vividly by color differentiation on the
“interaction surface”. The generated fingerprint plots by certain
interactions with their relative percentage share are provided on
Compounds 1, 2, 3TT, and 3TG are suitable for
investigating uranyl coordination geometries. The square
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Table 3. Crystallographic Data for 4, 5, and 6
compound
4
5
6
formula
formula weight
T (K)
C12H22Cl4N4O2U
634.17
C14H26Cl4N4O2U
662.22
C18H34Cl4N4O2U
718.32
180
180
290
crystal system
space group
a (Å)
monoclinic
P21/n
monoclinic
P21/n
monoclinic
P21/c
10.0580(4)
9.7278(4)
10.4943(4)
90
9.6181(5)
9.8110(5)
12.0087(6)
90
10.4266(10)
13.6211(7)
10.4331(9)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
94.009(4)
90
97.510(5)
90
116.508(11)
90
1,024.28(8)
2
1,123.46(10)
2
1,326.0(2)
2
Z
ρcalcd (g cm−3
)
2.056
1.958
1.799
μ (mm−1
)
8.456
7.714
6.544
Rint
0.0783
0.0214
0.0322
data/restraints/parameters
GOF
2012/0/108
1.094
2179/0/118
1.060
2586/0/136
1.013
a
R1, wR2 [I ≥ 2σ(I)]
0.0397
0.0207
0.0348
0.1012
0.0419
0.0748
a
R1, wR2 (all data)
0.0429
0.0303
0.0570
0.1060
0.0469
0.0894
a
2
2
2
R1 = Σ(|F0| − |FC|)/Σ|F0|, wR2 = [Σw(F0 − FC )2/Σw(F0 )2]1/2
.
bipyramid [UO2Cl4]2− anions did not undergo significant
distortions because of the UO bond length [1.755(3)−
1.759(3) Å], Cl−Cl interatomic distance [3.736(1)−3.794(1)
Å], and lack of Cl−Cl−Cl−Cl torsion in the 3TT/3TG
structures (Table S6, Supporting Information). The OUO
angle of 178.0(2)°, N−N interatomic distance of 2.771(8)−
2.928(8) Å, and N−N−N−N torsion angle of 0.7(4)−4.9(4)°
(Table S5, Supporting Information) indicate a slight distortion
of the pentagonal bipyramid [UO2(NCS)5]3− anion in
compound 2. As expected, in compound 1, the OUO
angle of 177.7(3)−178.1(2)°, O−O interatomic distance of
2.13(1)−2.72(1) Å, and O−O−O−O torsion angle of 0.3(6)−
15.7(5)° (Table S5, Supporting Information) demonstrate a
significant distortion of the hexagonal bipyramid [(UO2)2(μ-
OH)2(NO3)4]2− anion due to the different chemical environ-
ment of the oxygen atoms.
reference for the crystallization method used in this work.
Compound 4 provides a fine distinction in unit cell dimensions
and interionic contacts. As described, the interionic interactions
that were at least 0.1 Å shorter than the van der Waals radii76
were investigated in this work. Thus, four interionic
interactions (Figure 3a), C4−H4A···Cl1, C5−H5A···Cl2,
Crystal Structural Analyses for Cation-Varied Com-
pounds with [UO2Cl4]2− Anions. The crystallographic data
for 4, 5, and 6 are given in Table 3. The PXRD obtained for
these structures are all in good agreement with the simulated
results from single crystallographic diffraction measurements
(Figures S5−S7, Supporting Information), especially for
compounds 3TT, 3TG, and 4 at 180 K (Figures S8−S10,
Supporting Information), which exhibited high reliability for
the refinement of these structures. Our interest in structure
analyses of those compounds has mainly arisen from electro-
chemical investigations that are related to advanced nuclear
cycles.11,14,15 As described in previous studies, a variety of
interionic interactions have been found in [Emim]2[UO2Cl4];
however, further investigations were hampered for lacking this
structure type.88
Figure 3. C−H···Cl (blue) and C−H···O (pink) interactions in
structures (a) 4 and (b) 5, which are at least 0.1 Å shorter than the van
der Waals radii.
C5−H5B···Cl2, and C3−H3···O1, with H···Cl/H···O contact
distances of 2.840(1), 2.832(1), 2.828(1), and 2.501(5) Å,
respectively, and contact angles of 151.4(3), 156.6(3),
146.0(3), and 157.7(3)°, respectively, were herein observed
in compound 4 (Table S4, Supporting Information). Compared
with the H···Cl or H···O contact distances of 2.787, 2.891, and
2.535 Å and angles of 164.20, 142.08, and 168.33° for C5−
H5A···Cl2, C5−H5B···Cl2, and C3−H3···O1, respectively,88
the results obtained here have relatively short H···Cl contact
distances and large C−H···Cl interaction angles and indicate a
new interaction in C4−H4A···Cl1. In compound 5 (Figure 3b),
three new C−H···Cl interactions and two C−H···O inter-
Compounds 4 and 5 crystallized in a monoclinic P21/n space
group (Table 3) and consisted of one [Emim]+/[Emmim]+
cation and a half [UO2Cl4]2− anion in the asymmetric unit.
Compound 4 represents a structure that has been typically
reported45 and re-evaluated88 elsewhere, and it serves as a
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Figure 4. C−H···Cl (blue) and C−H···O (pink) interactions in compounds 3TT, 3TG, and 6 that are at least 0.1 Å shorter than the van der Waals
radii: (a) 3TT at 290 K, (b) 3TG at 290 K, (c) 6 at 290 K, (d) 3TT at 180 K, (e) 3TG at 180 K, and (f) 6 at 180 K.
Figure 5. Packing diagram of compounds 3TT, 3TG, and 6, which are connected by C−H···Cl (blue) and C−H···O (pink) interactions that are at
least 0.1 Å shorter than the van der Waals radii: (a) 3TT at 290 K forming an octal ring (yellow), (b) 3TG at 290 K yielding chain rings (yellow), (c)
6 at 290 K forming tetra rings (yellow), (d) 3TT at 180 K, (e) 3TG at 180 K, and (f) 6 at 180 K yielding tetra rings (yellow).
actions are formed. Using the hydrogen atoms H2, H5B, and
H5C, respectively, shorter H···Cl contact distances of 2.727(1),
2.797(1), and 2.7277(9) Å were observed (Table S4); the C−
H···O interaction occurs through atoms H5A and H6A. By
comparing the interactions in 5 and 4, the C1−H1 group in
compound 4 (Figure 3a), which is generally considered to be
highly acidic, is not related to interionic interactions; the two
hydrogen atoms from the additional substituted methyl group
in 5 are involved in C−H···Cl interactions. Despite the
differences in interionic interactions, compounds 4 and 5 both
yield three-dimensional (3D) structures through close C−H···
Cl and C−H···O interactions (Figure S11, Supporting
Information).
In addition to compounds 4 and 5, compounds 3TT, 3TG,
and 6 with longer butyl chains also formed C−H···Cl and C−
H···O interactions. As shown in Table S4, two common C−H···
Cl interactions involving C2 and C4 are found in both 3TT
(Figure 4a) and 3TG (Figure 4b), with longer H···Cl contact
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Table 4. Relative Percentage Contributions of Selected Interactions for Imidazolium [UO2Cl4]2− Salts to Overall Hirshfeld
Surface Area
cation
C−H···Cl
C−H···O
C−H···H
C−H···N
C−H···C
[Bmmim]+ (6−290 K)
[Bmmim]+ (6−180 K)
[Bmim]+ (3TT-290 K)
[Bmim]+ (3TT-180 K)
[Bmim]+ (3TG-180 K)
[Emmim]+ (5−180 K)
[Emim]+ (4−180 K)
18.8
18.8
21.0
20.8
19.3
22.4
26.9
8.4
8.5
60.5
60.2
59.0
58.7
60.7
55.9
46.0
1.5
3.4
0.8
1.1
1.2
0.6
1.6
3.3
7.3
2.8
2.8
1.6
1.8
4.4
8.2
8.2
8.5
9.8
10.6
distances and a larger interaction angle for 3TT. [UO2Cl4]2−
clusters and [Bmim]+ cations are connected by these C−H···Cl
interionic interactions to form coordinated frameworks (Figure
5). 3TT is a 2D grid layer in which chain rings are connected
by bridging [UO2Cl4]2− clusters that partly participate in
individual octal rings (Figure 5a), with two positions for the
[UO2Cl4]2− anions. 3TG yields a one-dimensional (1D) chain
ring in which one [UO2Cl4]2− cluster and four [Bmim]+ cations
are connected by two pairs of bifurcated interactions (C2−
H2···Cl1 and C4−H4A···Cl1), forming tetra rings through
double-bonded [Bmim]+ cations along the c-axis (Figure 5b).
Compound 6 crystallized in the monoclinic space group P21/
c with a TT conformation for the butyl group that is also found
in compound 3TT. The interionic interactions of C2−H2···Cl1
and C4−H4C···Cl2 observed in 6 are weaker than those found
in 3TT, with longer H···Cl interaction distances and smaller
angles. The C2−H2···O1 interaction found here has a long H···
O interaction distance to the C6−H6A···O1 interaction
observed in 5 (Table S4, Supporting Information). Compound
6 is a 1D ring in which two [UO2Cl4]2− clusters and two
[Bmim]+ cations are connected by two pairs of C2−H2···Cl1;
C4−H4C···Cl2 and C2−H2···O1 interact, forming tetra rings
through triple-bonded [Bmim]+ cations along the c-axis (Figure
5c). Compared with 3TT, the direction of the butyl group in 6
was affected by methyl substitution, which lies outside of the
rings.
The crystal structures of 3TT, 3TG, and 6 at 180 K (Table
S1, Supporting Information) are slightly different from those at
290 K with respect to interionic interactions (Figure 4 and
Table S4, Supporting Information). The 2D layer for 3TT at
room temperature is connected by an additional C4−H4B···O1
interaction (Figure 4d), forming a pseudo 3D structure at 180
K due to this interaction (Figure 5d), and its H···Cl interaction
distance [2.723(1)−2.741(1) Å] is shorter than its distance at
room temperature [2.801(2)−2.806(1) Å]. In 3TG, a weak
C3−H3···Cl2 interaction (Figure 4e) with an H···Cl distance of
2.854(1) Å and a C−H···Cl angle of 145.6(4)° connects the 1D
rings, forming a 2D layer (Figure 5e). For compound 6, the
connection in the 1D ring (Figure 5f) becomes closer through
the formation of new C6−H6B···O1 contacts.
In addition to the observed influences on overall
architectures within these compounds, are there opportunities
to identify C−H···Cl interactions by strength or importance? A
proper consideration on the entire species C−H···Cl−U may
aid in this discussion,89 and the classic investigation on R−X···
X−R halogen interactions can be taken as an good example.90
According to the value of two R−X···X angles, two types of
interactions can be distinguished. In type I interactions, the two
R−X···X angles are equal. In type II interactions, one R−X···X
angle is approximately 90°, while the other is approximately
180°. These two types of interactions have also been well
illustrated for pyridinium salts with [UO2X4]2− anions (X = Cl,
Br), considering the relative sizes and polarizabilities of the
halogens.86 In this work, the C−H···Cl−U interactions with
low H···Cl−U interaction angles of 81.13(5) −98.25(4)°
generally maintain longer H···Cl interaction distances of
2.793(1)−2.854(1) Å (Table S4, Supporting Information)
and may be recognized as weak interactions.
Moreover, Hirshfeld surface analyses and full fingerprint
plots were respectively performed for [Emim]+, [Emmim]+,
[Bmmim]+, and [Bmim]+ cations with TT and TG
conformations (Figures S14−S20, Supporting Information) to
quantify the interactions in those structures. The identified C−
H···Cl and C−H···O interactions (Figures 3 and 4 and Table
S4, Supporting Information) are well supported by the
highlighted contacts in the corresponding Hirshfeld surface
with red areas. Associated full fingerprint plots (Figures S14−
S20, Table 4) indicate that the nonpolar H···H and polar C−
H···Cl and C−H···O are major interactions. The percentage of
H···H interactions increases with increasing alkyl group from
[Emim]+ to [Bmmim]+, while the C−H···Cl interactions
decrease. In Table 4, the relative percentage contributions of
C−H···Cl interactions are larger than the C−H···O inter-
actions. As observed for 3TT and 6, the percentage
contributions of H···H, C−H···Cl, and C−H···O interactions
change slightly with temperature.
Transformations and Structure Variations. Multistep
Phase Transformations of Compound 2. Investigations on
crystallized ILs have successfully provided detailed information
that has helped us to understand the physical properties of ILs
in liquid state.57 In solution studies on uranyl complexes, in
addition to powerful optical spectroscopy, crystal structure
analyses can supply partial evidence.28,91 However, failure of
obtaining crystals from IL solutions has frequently limited the
usage of these analyses. For example, the solution properties of
[Bmim]x[UO2(NCS)y] and the crystal structure of the
homology [Emim]3[UO2(NCS)5] have been reported, but
the [Bmim]x[UO2(NCS)y] crystal was difficult to obtain, even
at low temperatures.24 Hence, the successful crystallization of
compound 2 herein through our novel synthetic method has
In the 1D to 3D structures studied above, the number and
positions of the interacting [UO2Cl4]2− anion around the
alkylimidazolium cations could determine the structure
acquired here. The alkylimidazolium cations with short alkyl
chains (such as the ethyl chain in compounds 4 and 5) and low
temperatures (such as those for 3TT and 3TG at 180 K) may
be suitable to interact with more [UO2Cl4]2− clusters and could
at least be likely to form more contacts. The methyl
substituents in 6 and polymorphs in 3 appear to impact the
packing by avoiding steric hindrance interactions (Figure 5).
H
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provided an opportunity to gain features inside solid−liquid
phase transformations.7
crystals, indicating that reverse transformations in the solutions
were controlled by the uranyl concentration (Figure 8). The
As observed from the DSC spectra (Figure 6) and polarizing
optical microscope (POM) images (Figure S21), compound 2
Figure 6. DSC curves of compound 2: first (black), second (red), and
third (blue) cycles.
performed multistep phase transformations during heating and
cooling processes. In the first heating process from −80 to 80
°C, the crystal melted at 53 °C (DSC spectra) to form a liquid,
as shown in the POM photos (Figure S21) and videos (Video
S1, Supporting Information). By cooling the sample to −80 °C,
no obvious changes were observed in the POM imagines, and a
weak exothermic peak at approximately −50 °C was observed
in the DSC curves. During the second heating process, an
endothermic peak at approximately −53 °C and a broad
exothermic peak from −18 to 22 °C appeared before the
melting peak at 52 °C. As summarized in Figure 7, the observed
Figure 8. Photographs of crystal-to-crystal transformations between
3TT and 3TG confirm a dissolution/recrystallization mechanism that
is controlled by the uranyl concentration of the solutions. (a) 3TT
crystals after adding high concentrations of uranyl to the solution. (b)
The transformation from 3TT to 3TG. (c) 3TG crystals after
transformation. (d) 3TG crystals after adding ILs. (g) Transformation
from 3TG to 3TT. Panels (e) and (h) display a slight dissolution of
the 3TG crystals during the process. Panels (f) and (i) illustrate the
formation of 3TT crystals. The diameter of the round bottle is 15 mm,
and the scale bar in the figures represents 500 μm.
mixture of 3TT and 3TG during the transformation process
was further confirmed by PXRD data (Figure S22, Supporting
Information). The conversions observed here were not
completed after a span of three months, which is significantly
slower than the results97 reported by our group in a water
system, where conversions were completed in less than 10 days.
This completion rate indicates that long time scales may be
introduced using the unique and complex features in IL
mixtures.98
The structural conversions in the IL solutions above are
controlled by the uranyl concentration or, more broadly, the
ratio of the reactants, which is an important factor for
controlling transformations99,100 in molecular solvents and for
obtaining crystals59 in ILs. During the previous solution studies
on uranyl-containing ILs, the ratio of the reactants may not
have affected the results at relatively low uranyl concentrations
(below saturation).28,41 However, the solution-mediated trans-
formations between polymorphs 3TT and 3TG herein indicate
that these factors should be considered at high uranyl
concentrations.101
Structure Variations of Compounds 3TT, 3TG, and 6 in
Solid form. For pure ILs with [Bmim]+ cations, such as
[Bmim]PF6,58 the polymorphism may have occurred at low
temperatures due to [Bmim]+ conformation variation. In this
work, crystal-to-crystal transformations were not performed at
180 K for compounds 3TT, 3TG, and 6, as indicated by our
crystallographic data (Table S1, Supporting Information). The
observed slight alkyl chain variations are common in organic
compounds for contraction at low temperatures. Remarkably,
these minute differences in butyl chain are clearly distinguished
Figure 7. Possible phase transformations of 2.
pair of weak exo- (endothermic) peaks at approximately −50
(−53) °C for Tg− (Tg+) and broad exothermic peak from −18
to 22 °C for Ts may have been due to glass transitions and
solidifications, respectively.92 Solidification (crystallization)
during the second heating process was further confirmed by
associated POM images (Figure S21) and videos (Video S2,
Supporting Information), and an increasing bright solidified
region was clearly observed before melting. Thus, the multistep
phase transformations of 2 were identified by DSC and POM.
The low-temperature glass phase exhibited was in accordance
with the results that have been reported in solution studies.24
The relatively narrow temperature regions for the presence of
pure crystals during the second heating have demonstrated the
difficulty of crystallization in solutions by thermal shifting.
Transformations of 3TT ↔ 3TG in Solution at Room
Temperature. Transformations of uranium-organic frameworks
have been performed in molecular solvents93−97 due to their
fundamental scientific importance and significant application
potentials. However, to our knowledge, no such results have
been reported in IL solutions. Hence, we investigated the
transformations and structural properties of uranyl polymorphs.
An optical microscope was used to investigate the conversions
between thin plate 3TT crystals and block or needle-like 3TG
I
dx.doi.org/10.1021/cg501277d | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
by DSC and the temperature dependent on the PXRD spectra.
For 3TT, 3TG, and 6, as shown in Table S2, the 3TG structure
changed significantly at torsion angles θ1 and θ2 (mainly
contributed by TT conformation disappearance at 180 K),
whereas that for compounds 3TT and 6 changed only at
torsion angle θ2. The relatively significant change of compound
3TG to compounds 3TT and 6 was observed in the DSC and
PXRD spectra. During heating from 180 K to room
temperature, as described in Figure 9, a small peak was
respectively, is consistent with its long UO bond distance of
1.773(5) Å (Table S6, Supporting Information). The Raman
shift between 600 and 700 cm−1 for the TT and TG
conformers of the [Bmim]+ cations given by the theoretical
calculations102 were determined to be approximately 623 and
658 cm−1 for polymorphs 3TT and 3TG, respectively. The
OUO symmetric frequency could be IR-active due to the
asymmetric molecular environment;103 the absence of this
frequency in compounds 4 and 6 (Table S7, Supporting
Information) may be due to the weakened asymmetric
environment brought on by the methyl substitution. In Table
S8, the signals at 3479 and 1034 cm−1 in the IR spectra of
compound 1 can be labeled as an OH-bridge bond,20 and the
Raman shift at 847 and 810 cm−1 for compound 2 could be due
to the symmetric stretch of the uranyl and the bonded C−S
stretch, respectively.27
The UV−vis and fluorescence spectra are given in the
Supporting Information. Compounds 4, 5, 3TT, 3TG, and 6
have nearly the same absorption on the UV−vis spectra due to
the common [UO2Cl4]2− cations (Figure S27, Supporting
Information); the small variation in position and the sharp peak
could be attributed to the slight difference introduced by the
imidazolium cations. The significantly different UV−vis spectra
of compounds 1 and 2 are due to their hexagonal bipyramid
[(UO2)2(μ-OH)2(NO3)4]2− and pentagonal bipyramid
[UO2(NCS)5]3− anions. The fluorescence spectra of 2, 3TT,
and 3TG are weak at room temperature, but all compounds
demonstrated high intensity after cooling with liquid nitrogen
(Figure S28, Supporting Information).
Figure 9. DSC curves of 3TG showing the structural transition at
approximately −30 °C during the multistep heating process: (A) first,
(B) second, and (C) third.
demonstrated in the DSC curves (multistep heating process) of
3TG, indicating a structure transition at approximately −30 °C
(243 K). This result is supported by the temperature
dependence of the PXRD data (Figure S23, Supporting
Information); the changes in the spectra were primarily
introduced in the temperature range between 210 and 270 K.
No such result was found in the DSC or the PXRD spectra for
compounds 3TT or 6 (Figures S24 and S25, Supporting
Information).
CONCLUSIONS
■
In summary, crystallization of a series of crystals with selected
anions (Cl−, NO3 , and SCN−) and cations ([Emim]+,
−
[Emmim]+, [Bmim]+, and [Bmmim]+) in IL mixtures was
demonstrated. The formation of uranyl speciations [UO2Cl4]2−,
[UO2(NCS)5]3−, and [(UO2)2(μ-OH)2(NO3)4]2− as well as
the preference of [Emim]+, [Emmim]+, and [Bmmim]+ cations
over [Bmim]+ in the crystal structures indicate the possibility of
selective crystallization in complex IL solutions. During this
work, similarities between crystal construction and organic
synthesis have been exhibited for IL systems. Interionic
interactions were studied in our work; previously, conclusions
regarding these interactions have been difficult to make because
of limited structural information. A systematic investigations on
homologies with [UO2Cl4]2− and [(UO2)2(μ-OH)2(NO3)4]2−
anions was conducted using standard crystallographic and
Hirshfeld surface analyses. Moreover, the novel phase trans-
formations observed for [Bmim]3[UO2(NCS)5] crystals not
only support but also indicate transition features that have been
difficult to obtain in IL solutions by thermal shifting. The
polymorph transformations between 3TT and 3TG in solutions
at room temperature and structure variations of compounds
3TT, 3TG, and 6 in solid form at temperatures between room
temperature and 180 K may also help to explain the specific
changes in solutions.
Properties. Thermal Properties. The thermal properties of
compounds 1, 3TT, 3TG, 4, 5, and 6 were measured on a
TGA-DSC instrument under flowing N2 (Figure S26,
Supporting Information). The melting and decomposition
processes were observed for homology 4, 5, 3TT, 3TG, and 6.
As identified via the DSC curves, the melting points for 4, 5,
3TT, 3TG, and 6 are 132, 195, 124, 125, and 125 °C,
respectively. The higher melting points for 4 and 5, and the
lower and similar melting temperatures for 3TT, 3TG, and 6
can be attributed to their short/long alkyl chains.34 The weight
loss procedures at approximately 300 °C for these five
compounds in the TGA curves are for the removal of organic
components, and the observed weight losses were 57.4% (4),
55.0% (5), 52.2% (3TT), 56.2% (3TG), and 59.4% (6),
corresponding to the calculated data (based on U3O8) of 55.8%
(4), 57.6% (5), 59.3% (3TT), 59.3% (3TG), and 60.9% (6),
respectively. The TGA cure of compound 1 exhibits an initial
weight loss of 1.0% (calculated, 1.6%), approximately 100 °C
for a dehydration process, and then a decomposing process
from 220 to 800 °C with a weight loss of 55.7% (calculated,
51.1%). The imperfect TGA data reported herein are mainly
due to limiting the compound sample to less than 1 mg to
avoid instrument contamination. The possible overflowing of
these compounds from the crucible during heating under N2/
air flow may have also contributed to the defective TGA data.
Spectroscopic Properties. The selected IR and Raman data
for 3TT, 3TG, 4, 5, and 6 are given in Table S7. The low
frequency of the OUO asymmetric/symmetric absorption
at 908/827 cm−1 for compound 4 on IR/Raman spectra,
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures S1−S28, Tables S1−S8, X-ray crystallographic data
(CIF), and Videos S1 and S2. This material is available free of
charge via the Internet at http://pubs.acs.org.
J
dx.doi.org/10.1021/cg501277d | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
(30) Parker, T. G.; Cross, J. N.; Polinski, M. J.; Lin, J.; Albrecht-
Schmitt, T. E. Cryst. Growth Des. 2014, 14, 228−235.
(31) Wang, X.; Vogel, C. S.; Heinemann, F. W.; Wasserscheid, P.;
Meyer, K. Cryst. Growth Des. 2011, 11, 1974−1988.
(32) Laus, G.; Bentivoglio, G.; Kahlenberg, V.; Wurst, K.; Nauer, G.;
Schottenberger, H.; Tanaka, M.; Siehl, H.-U. Cryst. Growth Des. 2012,
12, 1838−1846.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: liucl@pku.edu.cn.
Notes
The authors declare no competing financial interest.
(33) Lauw, Y.; Ruther, T.; Horne, M. D.; Wallwork, K. S.; Skelton, B.
̈
ACKNOWLEDGMENTS
W.; Madsen, I. C.; Rodopoulos, T. Cryst. Growth Des. 2012, 12, 2803−
2813.
■
This work was supported by the NSFC (Grant 91026010) and
the GSF (Grant [2007]840, [2012]851) of China.
(34) Endo, T.; Masu, H.; Fujii, K.; Morita, T.; Seki, H.; Sen, S.;
Nishikawa, K. Cryst. Growth Des. 2013, 13, 5383−5390.
(35) Yang, M.; Mallick, B.; Mudring, A.-V. Cryst. Growth Des. 2014,
14, 1561−1571.
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