Anisotropic, Organic Ionic Plastic Crystal Mesophases from Persubstituted Imidazolium Pentacyanocyclopentadienide Salts

Article abstract of DOI:10.1021/acs.chemmater.9b02338

We describe the synthesis, supramolecular organization, and thermal characteristics of an unprecedented family of symmetric 1,2,3,4,5-pentaalkylimidazolium ([(C_n)₅im]⁺) salts equipped with halide, nitrate, or pentacyanocyclopentadienide ([Cp(CN)₅]^-) counterions. Salts containing relatively small anions were obtained as low-melting solids, whereas those with [Cp(CN)₅]^- anions were found to be ionic liquids even below room temperature. A permethylated derivative, [(C₁)₅im][Cp(CN)₅], proved to be exceptional. Upon heating, the salt self-organized into a new type of organic ionic plastic crystal (OIPC) mesophase, which was termed M_hex and whose anisotropic structure featured hexagonally ordered, rotating anionic stacks positioned within a continuum composed of disordered cations. The structure of the mesophase resembles that of classical columnar liquid-crystalline phases, despite the absence of long, flexible chains. In the M_hex phase, the cations surrounding the anionic columns effectively fulfill the role of "softening" structural constituents, much in the same way as flexible chains. The discovery of the novel mesophase, which displays a two-dimensional, and thus intrinsically anisotropic, lattice resulting from the rotation of entire ionic assemblies around a columnar axis, represents a new paradigm in the field of OIPCs. Relatively high ionic conductivities were measured in the M_hex phase, particularly after doping with the corresponding sodium salt, Na[Cp(CN)₅], demonstrating the materials' potential for use in electrochemical applications such as sodium-ion batteries.

Full text of DOI:10.1021/acs.chemmater.9b02338

Article
pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Anisotropic, Organic Ionic Plastic Crystal Mesophases from
Persubstituted Imidazolium Pentacyanocyclopentadienide Salts
Karel Goossens,*,† Lena Rakers,‡ Benoît Heinrich,§ Guillermo Ahumada,†,∥ Takahiro Ichikawa,⊥
Bertrand Donnio,§ Tae Joo Shin,# Christopher W. Bielawski,*,†,∥,¶ and Frank Glorius*,‡
†Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea
‡
̈
̈
̈
̈
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstraße 40, 48149 Munster, Germany
§
́
́
Institut de Physique et Chimie des Materiaux de Strasbourg (IPCMS), UMR 7504, CNRS−Universite de Strasbourg, 23 rue du
Loess, BP43, 67034 Strasbourg Cedex 2, France
∥Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
⊥Department of Biotechnology and Life Science, Tokyo University of Agriculture and Technology, Nakacho, Koganei, Tokyo
184-8588, Japan
#UNIST Central Research Facilities (UCRF) and School of Natural Science, UNIST, Ulsan 44919, Republic of Korea
¶Department of Energy Engineering, UNIST, Ulsan 44919, Republic of Korea
S
* Supporting Information
ABSTRACT: We describe the synthesis, supramolecular organization, and thermal
characteristics of an unprecedented family of symmetric 1,2,3,4,5-pentaalkylimidazo-
lium ([(Cn)5im]+) salts equipped with halide, nitrate, or pentacyanocyclopentadienide
([Cp(CN)5]−) counterions. Salts containing relatively small anions were obtained as
low-melting solids, whereas those with [Cp(CN)5]− anions were found to be ionic
liquids even below room temperature. A permethylated derivative, [(C1)5im][Cp-
(CN)5], proved to be exceptional. Upon heating, the salt self-organized into a new
type of organic ionic plastic crystal (OIPC) mesophase, which was termed Mhex and
whose anisotropic structure featured hexagonally ordered, rotating anionic stacks positioned within a continuum composed of
disordered cations. The structure of the mesophase resembles that of classical columnar liquid-crystalline phases, despite the
absence of long, flexible chains. In the Mhex phase, the cations surrounding the anionic columns effectively fulfill the role of
“softening” structural constituents, much in the same way as flexible chains. The discovery of the novel mesophase, which
displays a two-dimensional, and thus intrinsically anisotropic, lattice resulting from the rotation of entire ionic assemblies
around a columnar axis, represents a new paradigm in the field of OIPCs. Relatively high ionic conductivities were measured in
the Mhex phase, particularly after doping with the corresponding sodium salt, Na[Cp(CN)5], demonstrating the materials’
potential for use in electrochemical applications such as sodium-ion batteries.
Likewise, halogen-free, cyano-substituted anions, such as
[N(CN)2]− and [C(CN)3]−, have been reported to yield ILs
that feature low viscosities and high ionic conductivities.21,22
Moreover, some of us recently reported a series of [N(CN)2]−
salts that self-organized into liquid-crystalline (LC) meso-
phases.23
Although LC phases are often observed upon the attach-
ment of long alkyl or fluoroalkyl chains, or mesogenic groups,
to cationic species,4,5 the use of relatively short substituents
commonly results in nonmesomorphic ILs or, in some cases,
salts that adopt organic ionic plastic crystal (OIPC) phases.
OIPCs constitute an emerging field of materials science and
are intensively investigated as “solid” electrolytes for use in
applications that require high ionic conductivities.24−30 The
1. INTRODUCTION
A wide variety of organic cations have been used to obtain
ionic liquids (ILs) as “designer” solvents1−3 as well as to
synthesize ionic liquid crystals (ILCs) expected to show
anisotropic physical properties.4−7 Besides other applications,
both classes of materials have attracted widespread interest for
their potential use as nonvolatile electrolytes in various
electrochemical devices, such as lithium-ion or sodium-ion
batteries, fuel cells, and dye-sensitized solar cells.8−15
1,3-Disubstituted imidazoliums are emblematic cations
which are among the most frequently selected partners for
incorporation into ILs and ILCs.16−19 The nature of the
imidazolium substituents as well as the type of the anion
selected can strongly impact the physical properties of the
corresponding salts, including their melting and clearing
temperatures, decomposition temperature, viscosity, and
ionic conductivity. For example, fluorinated anions are
known to increase thermal stability and reduce viscosity.20
Received: June 14, 2019
Revised: November 2, 2019
© XXXX American Chemical Society
A
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plasticity of OIPC mesophases and their ability to flow under
mechanical stress stem from structural disorder due to
rotational or reorientational short-range motions of the
cationic and/or anionic species that are otherwise part of a
long-range correlated lattice.31 The short-range dynamic
disorder leads to enhanced translational motion for individual
ions as well as to lattice vacancies.32 Whereas N-alkyl-N-
methylpyrrolidinium cations have been extensively used to
obtain OIPC phases, examples of imidazolium-based plastic
crystals are scarce.33−39 MacFarlane and co-workers attributed
the lower tendency of imidazolium salts to adopt OIPC phases
to charge delocalization which ultimately weakens interionic
interactions.40 Consequently, short-chain-substituted imidazo-
lium salts often melt to a liquid state before a stable plastic
crystal phase is formed.
even at elevated temperatures. The following discussions and
implications of the findings will be focused accordingly.
2. RESULTS AND DISCUSSION
2.1. Synthesis of Pentaalkylimidazolium Salts. As
shown in Scheme 2, condensing commercially available
Scheme 2. Synthesis of the Pentaalkylimidazolium Salts
a
[(Cn)5im][X] (5-X-n)
In search of new ionic compounds and molecular
architectures that adopt LC or OIPC phases, or that behave
as ILs at reasonably low temperatures, we considered salts with
1,2,3,4,5-pentaalkyl-1H-imidazol-3-ium ([(Cn)5im]+) cations.
Although the influence of installing alkyl or aryl substituents in
the 2-positions of imidazoliums on the thermal characteristics
of the corresponding ILs and ILCs has been explored,41−47
peralkylated derivatives (i.e., [(Cn)5im][X] with n ≥ 2) have
not yet been reported in detail.48,49 Persubstitution was
envisioned to result in a protective hydrophobic shell around
the cationic heterocycle and to facilitate long-range molecular
ordering, particularly in combination with planar counterions.
Recently, some of us prepared 4,5-dialkylsubstituted imida-
zoles with relatively long alkyl substituents by utilizing
Radziszewski-type chemistry as a key step. The corresponding
1,3-dimethylimidazolium salts were obtained upon methylation
and applied as lipid analogues in model cell systems or used as
precursors for N-heterocyclic carbenes for the stabilization of
nanoparticles.50−54 We also reported 1,2,3-trimethyl-4,5-bis(n-
pentadecyl)imidazolium iodide to be the first example of a LC
pentasubstituted imidazolium salt.55
a
Reagents and conditions: (i) NEt3, 3-benzyl-5-(2-hydroxyethyl)-4-
methylthiazol-3-ium chloride, EtOH, reflux, and Ar (3 h); (ii) VOCl3,
O2, CH3CN, and r.t. (18 h); (iii) 1-n, NH4OAc, HOAc, EtOH, 110
°C, and Ar (18 h) (yields: 4-7: 60%, 4-11: 69%, and 4-15: 29%); (iv)
(1) CnH2n+1Br (15 equiv), K2CO3, THF, 80 °C, and Ar (3 days); (2)
an isolated mixture of 5-Br-n and the 1,2,4,5-tetraalkylimidazole
byproduct: CnH2n+1Br (15 equiv), K2CO3, THF, 80 °C, and Ar (3
days) (yields for (1) + (2): 5-Br-7: 28 + 49%, 5-Br-11: 27 + 32%, and
5-Br-15: 52%) [remark: 5-I-1 was prepared from 1,2,4,5-tetramethyl-
1H-imidazole in 95% yield]; (v) AgNO3 (1.0 equiv), H2O, and r.t.
(1.5 h) (for 5-NO3-1) or AgNO3 (1.1 equiv), EtOH, and r.t. (15 h)
(for 5-NO3-n (n = 7, 11, and 15)) (yields: 5-NO3-1: 95%, 5-NO3-7:
78%, 5-NO3-11: 80%, and 5-NO3-15: 73%); and (vi) Na[Cp-
(CN)5]57 (0.9 equiv), H2O, and r.t. (1 h) (for 5-Cp(CN)5-1) or
Na[Cp(CN)5]57 (1.0 equiv), CH3CN, and r.t. (1 h) (for 5-Cp(CN)5-
n (n = 7 and 11)) or 65−70 °C (1 h) (for 5-Cp(CN)5-15) (yields: 5-
Cp(CN)5-1: 77%, 5-Cp(CN)5-7: 99%, 5-Cp(CN)5-11: 98%, and 5-
Cp(CN)5-15: 99%).
We describe here how the previously reported synthetic
methodology may be extended to obtain [(Cn)5im][X] salts
that feature alkyl substituents larger than a methyl group (i.e.,
n-C7H15, n-C11H23 or n-C15H31). Derivatives with planar
nitrate ([NO3]−) or “π-electronic” pentacyanocyclopentadie-
nide ([Cp(CN)5]−) anions (Scheme 1) were subsequently
Scheme 1. Examples of 1,2,3,4,5-Pentaalkyl-1H-imidazol-3-
ium Salts with Halide (X− = Br− and I−), [NO3]−, or
[Cp(CN)5]− Anions
aldehydes CnH2n+1CHO 1-n with 1,2-diketones 3-n51 in the
presence of NH4OAc afforded the corresponding 2,4,5-trialkyl-
1H-imidazoles 4-n. Subsequent N-alkylation using an excess of
CnH2n+1Br (n = 7, 11, and 15) yielded the desired
[(Cn)5im][Br] salts (5-Br-n).56 In parallel, the [(C1)5im]+
salt 5-I-1 was also prepared from 1,2,4,5-tetramethyl-1H-
imidazole and CH3I under Menschutkin-type conditions. To
understand how the nature of the anion influenced the thermal
characteristics of the corresponding salts, anion exchange for
[NO3]− or [Cp(CN)5]− counterions was accomplished by
treating the halide salts with AgNO3 or Na[Cp(CN)5], which
was prepared from NaCN and CS2 (see Scheme S1).57 Based
on combustion elemental analysis data, all of the halide salts as
well as 5-Cp(CN)5-1 and 5-Cp(CN)5-15 were obtained as
quasi-anhydrous compounds, whereas the other [Cp(CN)5]−
salts and the [NO3]− salts were obtained as hemihydrates.58
prepared from the halide salts by anion metathesis. The
thermal characteristics of the aforementioned compounds as
well as those of permethylated homologues were also
examined. During the course of these studies, it was discovered
that [(C1)5im][Cp(CN)5] adopted an unprecedented, aniso-
tropic OIPC mesophase and was found to exhibit high ionic
conductivity values, especially after doping with a Na+ salt.
[Cp(CN)5]− was identified as a stable and compact mesogenic
building block that maintains a “charge-segregated” stacking,
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a
Table 1. Phase Transition Temperatures and Thermal Data Recorded for the Pentaalkylimidazolium Salts 5-X-n
b
c
d
e
f
compound
5-I-1
5-Br-7
5-Br-11
5-Br-15
5-NO3-1
5-NO3-7
transition
T (°C)
ΔH (kJ mol−1
)
ΔS (J K−1 mol−1
)
T1% (°C)
g
Cr → Iso
214
46
22.7
1.8
∼47
∼231
∼188
∼183
∼193
∼267
∼175
a
a
a
g
S → Iso
S → Iso
S → Iso
∼6
a
a
h
40, 54, 59
42.0
a
a
h
78, 81
82.8
g
Cr → Iso
154
13.2
−12.3
11.4
1.1
33.1
77.6
16.4
11.9
-
∼31
∼−46
∼39
3
∼102
∼225
32
23
-
36
321
a
a
g
S1 + Iso → S2
S2 → S1 + Iso
S1 + Iso → Iso
S → Iso
−8
19
g
a
45
52
72
g
5-NO3-11
5-NO3-15
5-Cp(CN)5-1
∼185
∼204
∼369
g
S → Iso
Cr → Mhex
Mhex → Iso
g → Iso
S → Iso
S → Iso
234
240
14
−6
16
5-Cp(CN)5-7
5-Cp(CN)5-11
5-Cp(CN)5-15
∼192
∼175
∼221
9.7
92.9
a
b
See Table S2 for additional data and information. Abbreviations: Cr = crystalline phase; g = glass; S, S1, and S2 = solid phase(s) (not or only
c
partially crystalline); Mhex = plastic crystal mesophase with 2D hexagonal symmetry; and Iso = isotropic liquid phase. Onset temperatures obtained
by DSC during the second heating run at a rate of 10 °C min−1 and under an atmosphere of N2. A small hole was pierced into the lid of the DSC
d
e
f
sample pans. Enthalpy change. Entropy change. Temperature at which 1% weight loss was measured by TGA at a rate of 5 °C min−1 and under
g
h
an atmosphere of N2 (neglecting initial small weight losses attributed to the release of H2O). Peak temperature. An accurate value for the entropy
of fusion, ΔSfus, could not be obtained because of the overlap between transitions.
2.2. Thermal Characteristics and Structural Elucida-
tion of Crystal and Plastic Crystal Phases. The thermal
characteristics of the synthesized pentaalkylimidazolium salts
were examined using thermogravimetric analysis (TGA),
differential scanning calorimetry (DSC), and polarized-light
optical microscopy (POM). The key results are summarized in
Table 1 (see also Figures S8−S23). The decomposition
temperatures of [(C1)5im][X] salts were measured to be
significantly higher than those of the homologues with longer
alkyl chains and decreased in the following order: [Cp(CN)5]−
> [NO3]− > I−. None of the amphiphilic salts 5-X-n (X− = Br−,
[NO3]−, and [Cp(CN)5]−; n = 7, 11, and 15) were found to
adopt LC mesophases. The compounds melted directly to
isotropic liquids without passing through intermediate LC
phases and, in the case of [Cp(CN)5]− salts, were found to be
viscous liquids at ambient temperature. Likewise, variable
temperature, small- to wide-angle X-ray scattering (SWAXS)
measurements on powder samples of 5-X-n (X− = Br−,
[NO3]−; n = 11 and 15) revealed that the pristine samples
were solids of low crystallinity and that cooling from the melts
yielded even lower crystallinities because of slow crystallization
kinetics (Figures S36−S39). We refer to Section S8 in the
Supporting Information for a comparison with the thermal
characteristics of long-chain-substituted 1-alkyl-3-methylimida-
zolium ([Cnmim]+) and 1,3-dialkylimidazolium ([(Cn)2im]+)
salts and for an in-depth discussion of the effect of symmetric
pentasubstitution on the mesomorphic properties of imidazo-
lium salts. The salts 5-I-1 and 5-NO3-1 also did not exhibit
mesophases based on POM and DSC observations.
tendency toward homeotropic alignment (Figures 1 and S1−
S5).
Significantly different results were obtained upon analysis of
5-Cp(CN)5-1. DSC measurements revealed two pronounced
thermal events during heating and cooling of the salt (Figure
S16) apart from a small enthalpy change due to a crystal-to-
crystal transition (Cr1 → Cr2, see Table S2). POM studies
revealed that 5-Cp(CN)5-1 melts to a plastic, birefringent,
enantiotropic mesophase at ∼234 °C before clearing to an
isotropic liquid at ∼240 °C. The observed textures indicated
that a columnar type of mesophase formed with a pronounced
Figure 1. POM images of the Mhex phase of 5-Cp(CN)5-1 at 239 °C.
(Bottom) (a) Picture taken immediately after pressing the sample
with a needle. (b−d) Pictures showing the further evolution of the
texture over the course of 40 s during which time no pressure was
applied to the sample [the same sample area is shown in images (a−
d)]. All images were recorded using crossed polarizers.
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To gain additional insight into the structure adopted by 5-
Cp(CN)5-1 in the Cr1 phase, single-crystal X-ray diffraction
(XRD) data were collected at −150 °C. Although an
alternating packing59 of the cations and anions, which are
complementary in size and shape, may be expected, the solid-
state structure of 5-Cp(CN)5-1 features stacks of tilted
[Cp(CN)5]− anions in the direction of the a-axis that are
laterally separated by [(C1)5im]+ cations whose ring planes are
oriented parallel to the columnar axes (Figures 2 and S28−
S31). The packing is stabilized by intermolecular C−H···N
hydrogen bonds between the methyl groups of the cations and
the cyano nitrogen atoms of the anions (Table S5).
Figure 3. Synchrotron-based SWAXS pattern that was recorded for
the Mhex mesophase of 5-Cp(CN)5-1 at 240 °C (the X-ray wavelength
used was 1.00 Å). See also Table S8 in the Supporting Information.
≈ 131.2 Å2 and hmol to Vmol/Scol = 4.1( 0.2) Å (Vmol was
estimated to be 536( 20) Å3 from reference dilatometric
data).
The geometric parameters described above indicated that
the local molecular arrangements in the crystal phases and the
mesophase are similar, with the differences attributable to
thermal expansion (see also Section S6.2 in the Supporting
Information). In the mesophase, the [Cp(CN)5]− anions are
π-stacked into columns, albeit in an irregular manner as
evidenced by the broad hπ scattering signal centered at ∼3.6 Å
(Figure 3). Heating above the melting point also reduces the
correlation between the respective, tilt-induced (c.f., hmol > hπ)
orientations of the [Cp(CN)5]− stacks and the relative
positions of the surrounding [(C1)5im]+ cations. This process
is accompanied by the onset of rotation of the [Cp(CN)5]−
stacks. The disorder causes the formation of columns with an
average cylindrical shape and facilitates merging of the
peripheral cations to form a continuum (Figure 4). The
broad scattering signal hion, centered at ∼5.9 Å (Figure 3),
arises from the average, close-packed distances between the
[(C1)5im]+ cations that have lost long-range positional order.
The resulting time- and space-averaged hexagonal symmetry
and organization, as observed by SWAXS, is similar to that of
hexagonal columnar LC mesophases (Colhex) adopted by
Figure 2. (Top) Packing in the crystal structure of 5-Cp(CN)5-1,
showing the columnar stacking of the [Cp(CN)5]− anions and the
linkage of the molecules through nonclassical C−H···N hydrogen
bonds (blue dotted lines). (Bottom) View along the b-axis, showing
the ring-to-ring interactions between [Cp(CN)5]− anions (red dotted
lines). See also Section S6.2 in the Supporting Information.
The single-crystal XRD results for the Cr1 phase of 5-
Cp(CN)5-1 were used to derive the orthorhombic unit cell
parameters of its Cr2 phase from the SWAXS powder pattern
that was recorded at 200 °C (Figure S33 and Table S7). The
cross-sectional area Scol of one column of stacked [Cp(CN)5]−
anions along the a-axis surrounded by [(C1)5im]+ cations in
the Cr2 phase was calculated as (b × c)/4 = 126.6 Å2. The
thickness of one columnar slice was estimated as hmol = Vmol
/
Scol = a/4 = 3.94 Å (where Vmol is the molecular volume). The
results were then compared to the geometric parameters of the
thermotropic mesophase that were derived from synchrotron-
based SWAXS measurements at 240 °C. In addition to several
broad scattering signals, the SWAXS pattern contained four
sharp reflections characterized by an inverse d-spacing ratio of
1:√3:2:√7 (Figure 3 and Table S8). The signals were indexed
as the (10), (11), (20), and (21) reflections of a two-
dimensional (2D) hexagonal lattice with a lattice parameter
ahex ≈ 12.31 Å; hence, the columnar mesophase was designated
Figure 4. Schematic view of the supramolecular packing in the Mhex
mesophase of 5-Cp(CN)5-1. The 2D unit cell parameter is indicated
by ahex. The phase features rotating columnar stacks of [Cp(CN)5]−
anions that are arranged in a hexagonal lattice and positioned within a
continuum formed by molten [(C1)5im]+ cations that play the role of
disorderly components. In classical LC Colhex mesophases, such a
continuum is composed of molten chains.
2
as Mhex. The 2D lattice area Scol corresponded to (√3/2)ahex
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classical discotic mesogens that consist of planar aromatic
cores with peripheral alkyl chains, in which case the molten
alkyl chains form a continuum.60
Despite the structural similarity to Colhex phases, the
mesophase adopted by 5-Cp(CN)5-1 is not a liquid crystal
phase that involves molten chains but rather an ionic plastic
crystal (OIPC) phase for which the mesomorphism originates
from dynamic disorder. Indeed, 5-Cp(CN)5-1 solely consists
of cations and anions that lack flexible chains and have a
defined conformation because of their rigid structure (except
for the ability of the methyl groups in the [(C1)5im]+ cations
to rotate about their C−C or C−N bond axes61). In OIPC
phases, the individual cations and/or anions typically rotate
around the lattice points of a three-dimensional (3D) lattice,
leading to an isotropic or weakly anisotropic arrangement of
quasi-globular objects. In contrast, the OIPC phase of 5-
Cp(CN)5-1 features a 2D, and thus intrinsically anisotropic,
lattice resulting from the rotation of entire ionic assemblies
around a columnar axis. The molecular arrangements in the
crystalline solid phases, featuring approximately cylindrical
assemblies of stacked anions “wrapped” by cations, prefigure
the aforementioned motions in the OIPC phase. We note that
5-Cp(CN)5-1 is among the few examples of halogen-free
OIPC materials that are known to date, complementing, for
Figure 5. Packing in the crystal structure of 5-I-1, showing the
absence of a columnar stacking motif. The molecules are linked
through nonclassical C−H···N (blue dotted lines) and C−H···I
(purple dotted lines) hydrogen bonds as well as hydrogen-to-ring
(“C−H···π”) interactions (red dotted lines). See also Section S6.1 in
the Supporting Information.
tris[3,4-bis(alkyloxy)phenyl]pyrylium [BF4]− salts bearing
short alkyl chains.75
Finally, the thermodynamic parameters of the aforemen-
tioned phase transitions were evaluated. The entropy of fusion,
ΔSfus, that was measured for 5-Cp(CN)5-1 (23 J K−1 mol−1,
see Table 1) is significantly lower than the upper limit of 40 J
K−1 mol−1 for OIPCs that was proposed by MacFarlane and
co-workers.31,40 The measured ΔSfus value even approaches
Timmermans’ original criterion of ΔSfus < 20 J K−1 mol−1 for
plastic crystal behavior.76 Based on this criterion, the entropy
data support the hypothesis that the Mhex phase of 5-
Cp(CN)5-1 involves motions of the [(C1)5im]+ cations as
well as the [Cp(CN)5]− anions.31,40 The relatively small ΔSfus
values of compounds 5-Br-7 and 5-Cp(CN)5-11 can be
ascribed to incomplete crystallization during the DSC
measurements.
instance, more archetypical [N(CN)2]−,21,62−64 [SCN]−,65,66
36,68
[CH3SO3]−,37,67 and [H2PO4]−
salts.
Although the structure of the Mhex OIPC phase is, to the
best of our knowledge, unprecedented, comparison to
nonionic systems may be valuable. For instance, the so-called
“rotator phases” that are adopted by long-chain n-alkanes in
certain temperature ranges in which they are still mainly in
their “stiff”, all-trans conformations also consist of units that
rotate about parallel axes, which, in those cases, correspond to
the chain backbones.69 In the columnar mesophases that were
reported by Ros and co-workers, the rotating units are columns
of π-stacked indene- and pseudoazulene-based aromatic cores
that are arranged in a 2D hexagonal sublattice.70,71 The
molecules did not contain peripheral chains but featured
polarizable atoms (halogens, S) and/or polar groups (−CN,
>CO) that facilitated stacking in the solid state, which was
proposed to be a prerequisite for the formation of soft
columnar mesophases, and assisted in decorrelating the
neighboring columnar stacks at higher temperatures to form
the plastic phases.70,71
Compound 5-Cp(CN)5-1 also exhibits solid-state molecular
stacking, albeit only with regard to the planar anions. In
contrast, the nonmesomorphic iodide salt 5-I-1 lacks a
columnar stacking motif in the crystalline solid state (Figures
5 and S25−S27).72 Collectively, these results identify [Cp-
(CN)5]− as a stable and compact mesogenic building block as
it appears to maintain a so-called “charge-segregated” stacking,
even at elevated temperatures.73 Although the corresponding
[(C1)5im]+ cations are not mesogenic, they effectively function
as “softening” structural constituents whose distribution
around the stacks of [Cp(CN)5]− anions becomes uniform
above a certain temperature, thereby losing their correlated in-
plane orientations.74 Although it is not yet clear whether the
fast rotations of the methyl substituents are important to
facilitate the overall motions of the cations, the rotational
motions of the peripheral [BF4]− anions were proposed to play
an active role in the decorrelation of neighboring cationic
columns found in the LC mesophases adopted by 2,4,6-
2.3. Ionic Conductivity Displayed by Compound 5-
Cp(CN)5-1 and Its Mixtures with Na[Cp(CN)5]. Because the
M
hex mesophase described above featured structural character-
istics that were potentially conducive for anisotropic ion
conduction, a series of ionic conductivity measurements were
performed. The analyses were done upon cooling over a range
of temperatures using the alternating current impedance
method in conjunction with a sample cell equipped with
comb-shaped gold electrodes.77,78 The pure salt 5-Cp(CN)5-1
was measured first and, as shown in Figure 6, an Arrhenius plot
was constructed using the recorded data. The ionic
conductivity in the Mhex phase (σ = 2.6 to 3.0 × 10−5
S
cm−1) was determined to be comparable to that in the
isotropic liquid phase. This indicates that the formation of a
long-range, anisotropic 2D lattice in the former enables a
mechanism of effective ion conduction that compensates for
the higher viscosity of the mesophase. Evaluation of the data
yielded an activation energy, Ea, of 19 kJ mol−1 for ion
conduction in the Mhex phase, which was approximately two
times lower than the Ea value that was calculated for the
isotropic liquid phase (∼42 kJ mol−1). The results indicate a
relatively low energy barrier to ion mobility in the Mhex phase,
in accordance with the finding that the constituent ions are not
engaged in a long-range 3D lattice.
To evaluate the effect of metal salt additives,79−83 mixtures
of 5-Cp(CN)5-1 and Na[Cp(CN)5] were also measured. Na+
salts with weakly coordinating heterocyclic anions have
recently attracted interest for use in sodium-ion batteries,84,85
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plastic crystals and liquid crystals to the self-assembly of
matter.
Future efforts may explore a systematic structure−property
relationship of the [Cp(CN)5]− salts in order to identify the
molecular requirements for the emergence of the OIPC
mesophase. Various structural variations of the imidazolium
motif, including changes leading to symmetry reduction,
should be synthetically accessible, such as modifications of
(i) the number and positions of methyl groups on the
imidazolium rings (i.e., [(H)x(C1)5−xim]+, where x = 2−5) or
(ii) the nature and disposition of substituents on the
imidazolium cores (i.e., [(R)x(C1)5−xim]+, where x = 2−3,
and R = −CN, −Cl, ..., or [(R1)x(R2)5−xim]+, where x = 2−
3). The former materials may find utility in OIPC applications
that require anisotropic proton conductivity. The latter can be
expected to exhibit tunable thermal phase characteristics and
surface activities in a manner that depends on the packing
complementarities between the [Cp(CN)5]− anions and the
modified imidazolium cations.
Figure 6. Dynamic ionic conductivity data (Arrhenius plots) recorded
for 5-Cp(CN)5-1 and for mixtures of 5-Cp(CN)5-1 and Na[Cp-
(CN)5] (molar ratios: 0.975:0.025 and 0.95:0.05). The thermal phase
transitions are indicated by dashed lines.
but Na[Cp(CN)5] has, to the best of our knowledge, not yet
been studied for this purpose.86 5-Cp(CN)5-1/Na[Cp(CN)5]
mixtures with molar ratios of 0.975:0.025 and 0.95:0.05 were
prepared by slowly evaporating acetonitrile solutions of the
two components followed by heating the samples to their
isotropic liquid states. Upon cooling, plastic mesophases
formed without phase separation.87 The textures that were
observed by POM were similar to those displayed by pure 5-
Cp(CN)5-1 (Figure S7). Introducing the Na+ salt caused slight
decreases in the phase-transition onset temperatures (Figures 6
and S20). Relatively high ionic conductivities in the range of
1.5 to 2.0 × 10−4 and 1.8 to 2.1 × 10−4 S cm−1 were measured
for the nonaligned mesophases adopted by 5-Cp(CN)5-1/
Na[Cp(CN)5] (0.975:0.025) and 5-Cp(CN)5-1/Na[Cp-
(CN)5] (0.95:0.05), respectively. The substantial increase in
ionic conductivity upon addition of the Na+ salt, by a factor of
8, indicates that conductivity values in the mS cm−1 range may
be realized along with potential utility as anisotropic
electrolytes in sodium-ion batteries.
4. METHODS
4.1. General Procedures. Nuclear magnetic resonance (NMR)
spectra were recorded at ambient temperature (22.0( 1.0) °C, unless
stated otherwise) on a Bruker ARX-300 spectrometer (operating at
1
300 MHz for H), a Bruker AV-300 spectrometer (operating at 300
MHz for 1H), a Bruker AV-400 spectrometer (operating at 400 MHz
for 1H), or a Bruker AVANCE III HD spectrometer (operating at 400
MHz for 1H). Fourier transform infrared (FT-IR) spectra were
recorded in attenuated total reflectance (ATR) mode on an Agilent
Cary 630 FT-IR spectrometer using a germanium crystal (Ge-ATR).
High-resolution mass spectra (HRMS) were recorded in electrospray
ionization (ESI) mode on a Bruker Daltonics MicroTof mass
spectrometer, a Thermo Scientific LTQ Orbitrap XL mass
spectrometer, or a Waters Xevo G2-XS QTof mass spectrometer.
Elemental analyses (carbon, hydrogen, nitrogen, and sulfur) were
performed using a Thermo Scientific Flash 2000 Organic Elemental
Analyzer. Optical textures were observed using an Olympus BX53-P
polarized-light optical microscope that was equipped with a rotatable
graduated sample platform and an Instec HCS402 dual heater
temperature stage. The latter was equipped with a precision XY
positioner and coupled to an Instec LN2-SYS liquid nitrogen cooling
system and an Instec mK2000 programmable temperature controller.
Images were recorded with a QImaging Retiga 2000R CCD camera
that was coupled to the microscope via a C mount. DSC data were
recorded under nitrogen (50 mL min−1) on a TA Instruments DSC
Q2000 module equipped with an RCS90 cooling system at a heating
rate of 10 °C min−1 and a cooling rate of 5 °C min−1. The quantity of
sample analyzed was typically 4−5 mg. A small hole was pierced into
the lid of the aluminum sample pans. TGA data were recorded under
nitrogen (60 mL min−1) on a TA Instruments TGA Q500 module at
a heating rate of 5 °C min−1 and using a platinum sample pan. The
quantity of sample analyzed was typically 5−8 mg. Simultaneous
TGA−DSC measurements were performed using a TA Instruments
SDT Q600 module at a heating rate of 10 °C min−1 and under
nitrogen (100 mL min−1). An alumina sample pan was used. The
quantity of sample analyzed was approximately 6.5 mg. To determine
the crystal structures of compounds 5-I-1 and 5-Cp(CN)5-1, X-ray
intensity data were collected at 123 K on a Rigaku XtaLAB P200
diffractometer equipped with a Pilatus 200K detector, using ω scans
and Cu Kα radiation (wavelength λ = 1.54187 Å). See Section S6 in
the Supporting Information for further details. Most of the SWAXS
patterns were obtained with a transmission Guinier-like geometry. A
linear focalized monochromatic Cu Kα1 beam (λ = 1.5405 Å) was
obtained using a sealed tube generator (600 W) equipped with a bent
quartz monochromator. In all cases, the pristine powder was filled in
Lindemann capillaries of 1 mm diameter and 10 μm wall thickness or
in homemade sealed cells of 1 mm path length and equipped with 11
3. CONCLUSIONS
We report the synthesis of a series of pentaalkylimidazolium
salts as well as studies of their thermal characteristics and
supramolecular organization. None of the amphiphilic
compounds with relatively long alkyl chains and Br−,
[NO3]−, or [Cp(CN)5]− counterions appear to be LC,
although they are all low-melting salts. The amphiphilic
[Cp(CN)5]− salts are ILs below room temperature. In
contrast, [(C1)5im][Cp(CN)5] was found to self-organize
into an anisotropic, plastic crystal mesophase upon heating.
The phase features rotating columnar stacks of [Cp(CN)5]−
anions that are arranged in a 2D lattice and positioned within a
“continuum” formed by molten [(C1)5im]+ cations that play
the role of disorderly components. This binary system
represents an original type of OIPC phase, which holds
potential for use in electrochemical applications that require
anisotropic ion conduction. Indeed, preliminary data show that
high ionic conductivities can be achieved in the OIPC phase,
particularly after doping with a constituent Na+ salt. In a
broader perspective, the discovery represents a new paradigm
in the field of OIPCs and establishes new design parameters for
preparing highly ion-conductive, anisotropic electrolytes with
tunable thermal characteristics. The results pertain to various
domains of materials science, ranging from ILs through ionic
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μm-thick aluminum windows. The diffraction patterns were recorded
with a curved Inel CPS120 counter gas-filled detector linked to a data
acquisition computer (periodicities up to 90 Å) and on image plates
scanned by Amersham Typhoon IP with 25 μm resolution
(periodicities up to 120 Å). The sample temperature could be varied
between 20 and 200 °C with a precision of 0.01 °C, and exposure
times varied from 1 to 5 h. Synchrotron-based X-ray scattering
measurements of the mesophase adopted by compound 5-Cp(CN)5-1
were performed at the PLS-II 6D UNIST-PAL Beamline of the
Pohang Accelerator Laboratory (PAL), Pohang, Republic of Korea.
The X-rays coming from the bending magnet were monochromated
using Si(111) double crystals and focused at the detector position by
the combination of a second, sagittal-type monochromator crystal and
a toroidal mirror system. The diffraction patterns were recorded by a
Rayonix MX225-HS 2D CCD detector (225 × 225 mm2 square active
area, full resolution 5760 × 5760 pixels) with 2 × 2 binning. The peak
positions in the one-dimensional intensity profiles, which were
obtained from azimuthal averaging of the 2D patterns, were used for
phase-type assignments. SWAXS patterns were recorded using
12.3984 keV X-ray radiation (wavelength λ = 1.00 Å) and a
sample-to-detector distance of ca. 436 mm. Diffraction angles were
calibrated using a lanthanum hexaboride (LaB6) standard (NIST
SRM 660c). Samples were contained in a borosilicate glass (glass
#50) capillary with an outer diameter of 0.8 mm and a wall thickness
of 10 μm and were irradiated for 5−10 s per measurement, depending
on the saturation level of the detector. The capillaries were inserted
into a custom-made brass holder that was placed into a Linkam
TS1500V temperature stage to achieve temperature control. The
samples were allowed to equilibrate at each temperature before
starting a measurement. Dynamic ionic conductivity data were
recorded with a Solartron (Schlumberger) SI 1260 impedance/gain-
phase analyzer at a cooling rate of 2 °C min−1, using a frequency range
of 100 Hz to 20 MHz and a voltage of 0.6 V. The temperature was
controlled using a Mettler-Toledo HS82 dual heater temperature
stage that was coupled to a Mettler-Toledo HS1 programmable
temperature controller. A glass sample cell with comb-shaped gold
electrodes with thicknesses of 0.8 μm and spacings of 300 μm was
used for all the measurements. The cell constant was determined
using an aqueous KCl solution (0.01 M). Mixtures of 5-Cp(CN)5-1
and Na[Cp(CN)5] were prepared by slowly evaporating acetonitrile
solutions of the two components followed by heating the samples to
their isotropic liquid states.
4.2. Synthetic Procedures. The reactions described below were
performed in flame-dried or oven-dried glassware under an
atmosphere of argon, unless specified otherwise. The given reaction
temperatures correspond to the temperatures of the preheated oil
baths that surrounded the reaction vessels. Unless specified otherwise,
all reagents were purchased from commercial sources (Sigma-Aldrich,
Alfa Aesar, Acros Organics, TCI Europe, and TCI Japan) and used
without further purification. Anhydrous tetrahydrofuran (THF) was
obtained by distillation in a continuous still under an atmosphere of
argon over Na/benzophenone as the drying agent. The other
anhydrous solvents were purchased from Carl Roth or Sigma-Aldrich
and stored over molecular sieves under an atmosphere of argon.
Anhydrous triethylamine was purchased from Sigma-Aldrich, distilled,
and stored under an atmosphere of argon. The solvents that were
used for column chromatography were of technical grade and were
flash-distilled prior to use. During the syntheses of the precursors and
the halide salts, reaction progress was monitored by thin-layer
chromatography (TLC) on precoated, aluminum-backed plates
(Merck Kieselgel 60 F254). TLC spots were visualized by irradiation
with a UV lamp and by immersion into a KMnO4 stain. Purification
by flash column chromatography was conducted using silica gel (60 Å,
35−70 μm, Kieselgel, Acros) with a head pressure of argon (0.1−0.3
atm).
stirred for 18 h at 110 °C under an atmosphere of argon. After cooling
to room temperature (r.t.), the mixture was quenched with a saturated
aqueous solution of NaHCO3 and extracted with dichloromethane (3
× 30 mL per mmol). The combined organic layers were dried over
anhydrous MgSO4. After filtration, the solvent was removed under
reduced pressure. The residue was purified by flash column
chromatography (eluent: dichloromethane/methanol 100:0 → 93:7
v/v). See Section S1.2.1 in the Supporting Information for further
details and analytical data.
4.2.2. Synthesis of 5-I-1. Compound 5-I-1 was synthesized by
slowly adding iodomethane (80.32 mmol, 5.0 mL) to a stirred
solution of 1,2,4,5-tetramethyl-1H-imidazole (40.27 mmol, 5.00 g) in
15 mL of dry acetonitrile in an oven-dried Schlenk pressure tube. The
reaction mixture was covered from light and stirred for 18 h at 80 °C
under an atmosphere of nitrogen. The solvent and excess of
iodomethane were removed under reduced pressure. The crude
product was washed with diethyl ether (3 × 40 mL) and recrystallized
from acetone. Drying under reduced pressure at 50 °C afforded the
desired product as a white powder. Yield: 95% (10.16 g). δH (400
MHz, CD2Cl2): 2.24 (s, 6H, (imidazolium C-4/C-5)−CH3), 2.74 (s,
3H, (imidazolium C-2)−CH3), 3.70 (s, 6H, N−CH3). δC (101 MHz,
CDCl3): 9.1, 12.7, 33.2, 125.7, 142.8. IR (Ge-ATR): see Section
S1.2.7 in the Supporting Information. HRMS (ESI, CH2Cl2, m/z):
calcd for [C8H15N2]+ ([M − I−]+), 139.1230; found, 139.1233. Calcd
for C8H15IN2 (266.13): C, 36.11; H, 5.68; N, 10.53. Found: C, 36.00;
H, 5.70; N, 10.86.
4.2.3. General Method for the Synthesis of 5-Br-n (n = 7, 11, 15).
The appropriate 2,4,5-trialkyl-1H-imidazole 4-n (1 equiv), the
appropriate 1-bromoalkane CnH2n+1Br (15 equiv), and K2CO3 (2
equiv) were dissolved in dry THF ([2,4,5-trialkyl-1H-imidazole] = 0.3
M) in a flame-dried Schlenk pressure tube. The reaction mixture was
stirred for 3 days at 80 °C under an atmosphere of argon. After
cooling to r.t., the mixture was diluted with water. The resulting
biphasic system was extracted with dichloromethane (3 × 20 mL per
mmol). The combined organic layers were dried over anhydrous
MgSO4. After filtration, the solvent was removed under reduced
pressure. The residue was purified by flash column chromatography
(eluent: dichloromethane/methanol 100:0 → 93:7 v/v). To increase
the isolated yield of pure product, the column fractions that contained
a mixture of 1,2,4,5-tetraalkyl-1H-imidazole and 1,2,3,4,5-pentaalkyl-
1H-imidazol-3-ium bromide were concentrated and resubjected to the
aforementioned reaction conditions. See Section S1.2.2 in the
Supporting Information for further details and analytical data.
4.2.4. Synthesis of 5-NO3-1. The following steps were performed
in the dark. A solution of compound 5-I-1 (3.86 mmol, 1.0279 g) in
10 mL of water was slowly added to a solution of silver nitrate (3.86
mmol, 0.6561 g) in 15 mL of water under stirring. The reaction
mixture was stirred for 1.5 h at r.t. Insoluble products were filtered
using a paper filter. The filtrate was concentrated under reduced
pressure at 40 °C. The residue was redissolved in 20 mL of ethanol,
and the solution was filtered using a PTFE syringe filter (0.1 μm pore
size). After removing the solvent under reduced pressure at 40 °C, the
residue was redissolved in 10 mL of dichloromethane and the solution
was kept in a freezer overnight. Although visually there were no signs
of precipitation, the solution was filtered using a PTFE syringe filter
(0.1 μm pore size). Finally, the solution was concentrated and dried
under reduced pressure at 60 °C to afford the desired product as a
white powder. Small amounts of the product were dissolved in water
and tested with NaBr or AgNO3; the lack of a precipitate indicated
that AgNO3 and the iodide salt had been successfully removed. Yield:
95% (0.737 g). δH (400 MHz, DMSO-d6): 2.21 (s, 6H, (imidazolium
C-4/C-5)−CH3), 2.57 (s, 3H, (imidazolium C-2)−CH3), 3.61 (s, 6H,
N−CH3). δC (101 MHz, DMSO-d6): 7.9, 9.6, 31.5, 124.9, 142.6. IR
(Ge-ATR): see Section S1.2.7. HRMS (ESI, CH3OH, m/z): calcd for
[C8H15N2]+ ([M − [NO3]−]+), 139.1230; found, 139.1228. Calcd for
C8H15N3O3·0.25H2O (205.73): C, 47.75; H, 7.51; N, 20.88 (we note
that, upon standing open to air, the compound rapidly absorbed
additional water). Found: C, 47.42; H, 7.62; N, 20.45.
4.2.1. General Method for the Synthesis of Imidazole Precursors
4-n (n = 7, 11, and 15). 1,2-Diketone 3-n (1 equiv), aldehyde 1-n
(1.2 equiv), and NH4OAc (2.4 equiv) were dissolved in dry ethanol
([1,2-diketone] = 0.5 M) in a flame-dried Schlenk pressure tube, after
which three drops of HOAc were added. The reaction mixture was
4.2.5. General Method for the Synthesis of 5-NO3-n (n = 7, 11,
15). The following steps were performed in the dark. A solution of the
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appropriate bromide salt 5-Br-n (1 equiv) in ethanol ([5-Br-n] = 0.04
M) was slowly added to a solution of silver nitrate (1.1 equiv) in
ethanol ([AgNO3] = 0.1 M) under stirring. The reaction mixture was
stirred for 15 h at r.t. Subsequently, 6 mL of dichloromethane was
added. Insoluble products were filtered using a paper filter. The
filtrate was concentrated under reduced pressure at 40 °C. The
residue was redissolved in 20 mL of dichloromethane and washed two
times with 5 mL of water. The organic layer was dried over anhydrous
MgSO4. After filtration, the solvent of the filtrate was removed under
reduced pressure. See Section S1.2.3 in the Supporting Information
for further details and analytical data.
4.2.6. Synthesis of 5-Cp(CN)5-1. Under stirring, a solution of
compound 5-I-1 (0.353 mmol, 0.0940 g) in 3 mL of water was slowly
added to a solution of sodium pentacyanocyclopentadienide57 (0.316
mmol, 0.0674 g) in 2 mL of water. The reaction mixture was stirred
for 1 h at r.t. The off-white precipitate formed was filtered, washed
with copious amounts of water, and then dried under reduced
pressure at 50 °C for 48 h. Yield: 77% (0.080 g). δH (400 MHz,
DMSO-d6): 2.20 (s, 6H, (imidazolium C-4/C-5)−CH3), 2.56 (s, 3H,
(imidazolium C-2)−CH3), 3.61 (s, 6H, N−CH3). δC (101 MHz,
DMSO-d6): 8.0, 9.7, 31.6, 101.7 (−CN in [Cp(CN)5]−), 113.0
(Cring in [Cp(CN)5]−), 125.0, 142.6. IR (Ge-ATR): see Section
S1.2.7. HRMS (ESI, DMSO, m/z): calcd for [C8H15N2]+ ([M −
[Cp(CN)5]−]+), 139.1230; found, 139.1226. Calcd for C18H15N7
(329.36): C, 65.64; H, 4.59; N, 29.77. Found: C, 65.26; H, 4.54;
N, 29.40.
4.2.7. General Method for the Synthesis of 5-Cp(CN)5-n (n = 7,
11, 15). Under stirring, a solution of the appropriate bromide salt 5-
Br-n (1 equiv) in dry acetonitrile ([5-Br-n] = 0.03 M) was slowly
added to a solution of sodium pentacyanocyclopentadienide57 (1
equiv) in dry acetonitrile ([Na[Cp(CN)5]] = 0.07 M). The reaction
mixture was stirred for 1 h at r.t. (in the case of 5-Cp(CN)5-7 and 5-
Cp(CN)5-11) or at 65−70 °C (in the case of 5-Cp(CN)5-15).
Subsequently, 30 mL of dichloromethane was added and the mixture
was washed with 10 mL of water. The organic layer was separated and
concentrated under reduced pressure at 40 °C. The residue was
further dried overnight under reduced pressure at 40 °C, after which it
was redissolved in 2.5 mL of dichloromethane. The solution was kept
in a freezer overnight. Although visually there were no signs of
precipitation, the solution was filtered using a PTFE syringe filter (0.1
μm pore size). Finally, the solution was concentrated and dried under
reduced pressure at 50 °C. See Section S1.2.4 in the Supporting
Information for further details and analytical data.
Author Contributions
K.G. and L.R. contributed equally to this work. All authors
have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
K.G., G.A., and C.W.B. were supported in part by the Institute
for Basic Science (IBS-R019-D1). C.W.B. also acknowledges
the BK21 Plus Program funded by the Ministry of Education
and the National Research Foundation of Korea. L.R. and F.G.
gratefully acknowledge the Deutsche Forschungsgemeinschaft
(SFB 858). B.H. and B.D. thank the CNRS and the University
of Strasbourg for support. K.G. and T.I. thank the Institute of
Global Innovation Research at Tokyo University of Agriculture
and Technology for financial support. The authors are grateful
to Geonhui Park for synthesizing 5-I-1 and for recording the
high-resolution mass spectrometric data. The authors thank
Seong-Hun Lee for his assistance with the synchrotron
SWAXS measurements. The experiments at the PLS-II 6D
UNIST-PAL Synchrotron Beamline were supported in part by
the Korean Ministry of Science and ICT (MSIT), Pohang
University of Science and Technology (POSTECH), and the
UNIST Central Research Facilities (UCRF).
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.chemmater.9b02338.
■
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: karel.cw.goossens@gmail.com (K.G.).
*E-mail: bielawski@unist.ac.kr (C.W.B.).
*E-mail: glorius@uni-muenster.de (F.G.).
■
ORCID
Karel Goossens: 0000-0003-0134-9565
Benoît Heinrich: 0000-0001-6795-2733
Guillermo Ahumada: 0000-0002-1507-0816
Bertrand Donnio: 0000-0001-5907-7705
Christopher W. Bielawski: 0000-0002-0520-1982
Frank Glorius: 0000-0002-0648-956X
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K
DOI: 10.1021/acs.chemmater.9b02338
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