Efficient air-stable organometallic low-molecular-mass gelators for ionic liquids: synthesis, aggregation and application of pyridine-bridged bis(benzimidazolylidene)-palladium complexes

Article abstract of DOI:10.1002/chem.200802116

Novel pincer-type, pyridine-bridged bis(benzimidazolylidene)-palladium complexes 5-7 were synthesised from cheap commercial precursors under microwave assistance. Although simple in structure, carbene complexes 5a,b are efficient low-molecular-mass metall

Full text of DOI:10.1002/chem.200802116

FULL PAPER
DOI: 10.1002/chem.200802116
Efficient Air-Stable Organometallic Low-Molecular-Mass Gelators for Ionic
Liquids: Synthesis, Aggregation and Application of Pyridine-Bridged
Bis(benzimidazolylidene)–Palladium Complexes
[
a]
[a]
[b]
[c]
Tao Tu,* Xiaoling Bao, Wilfried Assenmacher, Herwig Peterlik,
[
b]
[a]
Jçrg Daniels, and Karl Heinz Dçtz*
Dedicated to Professor Li-Xin Dai on the occasion of his 85th birthday
Abstract: Novel pincer-type, pyridine-
bridged bis(benzimidazolylidene)–pal-
ladium complexes 5–7 were synthesised
from cheap commercial precursors
under microwave assistance. Although
simple in structure, carbene complexes
light microscopy for a selection of or-
ganic and ionic liquids. The achiral ge-
lators are able to induce the formation
of helical fibres. The thermal stability
of the gel samples increases with the
gelator concentration as demonstrated
by thermoreversible DSC studies. Tem-
perature-dependent NMR and X-ray
diffraction studies, as well as compari-
sons with pincer complex analogues
bearing shorter alkyl chains, suggest
that the 3D networks responsible for
gelation are based on non-covalent in-
teractions, such as p-stacking, van der
Waals interactions, and hydrogen and
metal–metal bonding. Ionic liquids and
gels obtained from them and 5a,b dis-
play comparable high conductivities,
which characterises pyridine-bridged
bis(benzimidazolylidene)–palladium
pincer complexes as air-stable metallo
gelators that efficiently immobilise
ionic liquids in low gelator concentra-
tion indicating—beyond catalysis—
their potential applications in electro-
chemical devices.
5
a,b are efficient low-molecular-mass
metallogelators. They gelate not only a
broad variety of protic and aprotic or-
ganic solvents, but also different types
of customary ionic liquids (such as imi-
dazolium, pyridinium, pyrazolidinium,
piperidinium and ammonium salts) at
Keywords: aggregation · benzimi-
ꢀ
1
concentrations as low as 0.5 mgmL .
The morphologies of the resulting 3D
gel networks composed from long and
thin fibres were studied by TEM and
dazolylidene complexes
·
ionic
liquids · metallogels · palladium ·
sol–gel processes
Introduction
metallic complexes were synthesised and successfully ap-
plied as homogenous catalysts in CꢀC and CꢀN bond for-
[2]
Since the first stable N-heterocyclic carbene (NHC) com-
mation and olefin metathesis. The unique higher stability
of NHC pincer complexes towards oxygen, moisture and
heat makes them attractive not only as catalysts under aero-
bic conditions, but also as organometallics in material scien-
[1]
plex was reported in 1991, a large number of NHC organo-
[
a] Dr. T. Tu, X. Bao, Prof. Dr. K. H. Dçtz
Kekulꢀ-Institute of Organic Chemistry and Biochemistry
University of Bonn
Gerhard-Domagk-Strasse 1, 53121, Bonn (Germany)
Fax : (+49)-228-73-5813
[3]
ces. In this respect, however, examples demonstrating their
[4]
application—especially towards soft matter —are almost
neglected.
Currently, the development of simply structured mole-
cules to immobilise fluids to form nano-scaled soft materials
in a bottom-up approach is an exciting research frontier. In
spite of a rapidly increasing number of reports in this field,
a rational correlation of molecular structure and gelation
ability in a given solvent still remains a major challenge.
Due to their physical properties these low-molecular-mass-
E-mail: doetz@uni-bonn.de
tao.tu@uni-bonn.de
[
b] Dr. W. Assenmacher, Dr. J. Daniels
Institute of Inorganic Chemistry
University of Bonn (Germany)
[
c] Prof. Dr. H. Peterlik
Faculty of Physics
University of Vienna
Boltzmanngasse 5, 1090 Vienna (Austria)
[5,6]
gelators (LMMGs)
represent a particular type of soft
[7]
matter and as such are attractive targets for various types
of potential applications. For example, ionic liquids (ILs)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802116.
Chem. Eur. J. 2009, 15, 1853 – 1861
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1853

have received considerable attention due to their high ionic
conductivity and wide electrochemical window combined
with high thermal and chemical stability, inflammability, ex-
tremely low volatility and a wide liquid range, which makes
[8a]
them attractive for application in synthesis, material scien-
[
8b]
[8c]
ces
solar cells, Li-ion batteries,
and commercial processes
such as dye-sensitised
[9]
[10,11]
electrochemical actua-
[
12]
[13]
tors,
electrochromic windows and numerical displays.
Their quasi-solidification through gelation by LMMGs, espe-
cially in very low gelator concentrations, is a demanding and
still difficult task which is expected to eliminate leakage
problems.
AHCTUNGTRENNUNG
Up to now, three categories of gelators—polymeric gel-
ACHTUNGTRENNUNG
[14]
[15]
[16]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ators,
inorganic nanoparticles
and LMMGs —are
5a,b are obtained as firm DMSO gels after cyclopalladion
of bis(benzimidazolium) halides 2a,b and cooling to ambient
temperature after reaction.
known to quasi-solidify ionic liquids; however, their applica-
tion generally suffers from limitations such as high gelator
concentration required for gel formation. To the best of our
knowledge, no organometallic compound has been applied
in this context. In view of the major role that organometal-
lics play in synthesis, catalysis and supramolecular chemistry,
it is surprising that their application to soft matter and
Gelation tests with organic solvents: To test the gelation
ability of carbene complexes 5–7, a broad range of protic
and aprotic solvents with different polarity and coordination
ability were screened. Table 1 summarises the results of gel-
[17]
LMMG has been widely neglected.
Recently, we reported the
first example of a pincer bis-
NHC palladium complex 1 as
an efficient air-stable organo-
metallic LMMG, which not
only is able to gelate a broad
variety of polar and apolar,
protic and aprotic organic sol-
vents even in concentration as
Table 1. Gelation ability of CNC pincer complexes 5 and 6 in various or-
ganic solvents.
[a]
Entry
Solvent
5a
5b
6
1
2
3
4
5
6
7
8
9
10
11
1
1
DCM
TCM
DCE
DMSO
DMF
DMA
THF
ACN
AcOH
MeOH
benzene
toluene
acetone
G*
PG*
G
G
G
G*
PG*
G
G
G
G
G
G
G
I
PG*
S
G
S
S
S
[b]
[b]
G
G*
I
P
ꢀ
3
low as 0.2 wt% (2.0ꢂ10 m), but also could be applied as a
G
G
[18a]
[c,d]
supramolecular catalyst in the gel state.
Compared to
I
I
[c]
[b]
imidazolylidenes, benzimidazolylidenes behave quite differ-
S
[
b]
[
19]
PG
P
P
I
WG
G
ently; thus, we speculated that the extended planar metal
hybrid p-system and the s-donor properties increased by
benzannulation may further enhance the gelation ability and
[
d]
2
3
P
I
[d]
I
ꢀ
1
[
a] Gelator concentration: 10 mgmL ; G: Gel formed at room tempera-
[20b]
catalytic activity.
Following our recent interest in
ture; PG: Partial gel; G* or PG*: Gel or partial gel formed by cooling to
[18]
LMMGs derived from functional organometallics, we now
report on a convenient synthesis of pyridine-bridged bis-
58C; WG: Weak gel; S: Solution; P: Precipitate; I: Insoluble. [b] Gel
ꢀ
1
formed with 20 or 30 mgmL at room temperature. [c] Because of low
ꢀ1
solubility, a partial gel was observed at 5 mgmL . [d] A firm gel was
formed by adding a few drops of other solvents.
ACHTUNGTRENNUNG( benzimidazolylidene) pincer palladium complexes starting
from cheap commercially available precursors and on a
comparative study of their ability to gelate a representative
selection of ionic liquids.
AHCTUNGTRENNUNGa tion tests of 5 and 6 with selected solvents at a concentra-
ꢀ
1
tion of 10 mgmL . Transparent gels could be found with
gelators 5a,b in DMSO, DMF and DMA; the other gels are
AHCTUNGTRENNUNG
Results and Discussion
turbid. The colour of the gels darkens with the increasing
coordination ability of the solvents as observed for the gels
[18a]
Synthesis of pyridine-bridged bis(benzimidazolylidene)
pincer palladium complexes 5–7: The amination of 2,5-di-
fluoropyridine with two equivalents of benzimidazole to
give bis(benzimidazolyl)pyridine followed by N-alkylation at
658C for 30 h with different alkyl halides in a sealed tube
afforded the pyridine-bridged bis(benzimidazolium) salts 2–
in almost quantitative yields. Subsequent cyclometalation
with Pd(OAc) in DMSO under microwave conditions
40 W, 1608C, 25 min) resulted in the formation of bis(benz-
formed by carbene complex 1.
When the gelator concen-
ꢀ
1
tration was decreased to 0.5 mgmL , stable gels still could
be formed with carbene complexes 5a,b and DMSO, which
could be stored at room temperature without any visible
collapse. To test the effect of the alkyl chain length on the
aggregation process, carbene complexes 6 and 7 were also
involved in the gelation tests. With a complex bearing short-
er chain lengths (-C H ), turbid gels were formed with
1
4
ACHTUNGTRENNUNG
2
8
17
(
DCM, DCE, ACN and AcOH, and transparent gels ap-
1854
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Chem. Eur. J. 2009, 15, 1853 – 1861

Low-Molecular-Mass Gelators for Ionic Liquids
FULL PAPER
ꢀ
1
peared with toluene and benzene at 10 mgmL . Although a
Table 2. Gelation ability of CNC pincer complexes 5a,b in selected ionic
liquids.
[
a]
ꢀ
1
gel could not be formed with 5a or 5b at 10 mgmL in
MeOH, reflecting their poor solubility even at reflux condi-
tion, a yellow turbid gel was observed with gelator 6 in
MeOH at 20 mgmL (Table 1, entry 10). These results may
support our assumption that—in comparison with bis(imida-
zolylidene) gelator 1—extending the planar metal-hybrid p
system by introducing benzimidazolylidenes into the pyri-
5
T
a
5b
Tg/H [8C]
Entry Ionic liquid
[
b]
[b]
phase
WG
g/H [8C]
phase
ꢀ
1
[c]
[d]
1
2
3
4
5
6
7
8
IL-1a: [C
IL-1b: [C
IL-1c: [C
IL-2: [C Py]NTf
4
ImC
ImC
ImC
1
]BF
4
64
66
87
98
96
83
72
73
WG
–
4
1
]OTf WG
WG
WG
G
G
G
82
90
99
89
90
80
76
4
1
]NTf
2
G
G
G
G
G
G
8
2
IL-3: [C PpC ]NTf
4
4
4
1
2
dine-bridged palladium–bis
A
H
U
G
R
N
U
G
IL-4: [C
IL-5: [C
PrC
N3C
NC
1
]NTf
2
1
]NTf
]NTf
2
G
WG
gelation ability. However, no gels could be obtained with
carbene complex 7 and selected organic solvents, which may
be caused by less efficient van der Waals interactions be-
tween the shorter alkyl chains.
IL-6: [3C
8
1
2
ꢀ
1
[
a] Gelator concentration: 5 mgmL ; G: Gel formed at room tempera-
ture; WG: Weak gel formed at room temperature. [b] Determined by
DSC. [c] Not be dissolved completely upon heating. [d] Not detected.
Although pyridine-bridged, bis(benzimidazolylidene)–pal-
ladium complexes are poorly soluble in acetone, no gel was
obtained after cooling to room temperature. However, addi-
tion of a few drops of THF increases the solubility of 5a
ture. Compared with 5a, carbene complex 5b is less soluble
in ionic liquids based on imidazolium salts. Especially, even
ꢀ
1
ꢀ1
and leads to gel formation (10 mgmL , Table 1, entry 13).
at a low gelator concentration of 5 mgmL , compound 5b
The same procedure was applied to the 5a/toluene system
cannot be dissolved completely in IL-1a upon heating; how-
ever, the resulting soluble part is able to form a weak gel
(Table 2, entry 1). Weak gels were also observed with most
imidazolium salts (IL-1a–c) for metallogelators 5a,b
(Table 2, entries 1–3). The weak gels obtained from IL-1a,b
collapsed after 4–6 days at room temperature; however, that
ꢀ
1
(10 mgmL ); upon addition of a few drops of DMSO, a
translucent gel was obtained (Table 1, entry 12). Compared
with 5b, carbene complex 5a is less soluble in AcOH even
ꢀ
1
at 10 mgmL ; however, the soluble part after refluxing
could form a weak gel, which makes it difficult to determine
the gelator concentration. After addition of a few drops of
DMSO, a homogeneous liquid phase was observed with gel
ꢀ
obtained from IL-1c (with NTf2 as counterion) is quite
stable for several months. Therefore, we focused on selected
ꢀ
1
ꢀ
5
a/AcOH (10 mgmL ) at reflux conditions in a sealed vial
ionic liquids containing NTf , with which firm gels were ob-
2
forming a firm gel after cooling to room temperature.
tained with carbene complexes 5a,b (Table 2, entries 4–8).
The solubility of carbene complexes 5a,b in most other
ionic liquids under heating conditions is quite good: at
Gelation tests with ionic liquids: As shown in Table 2, pyri-
dine-bridged carbene complexes 5a,b can gelate not only or-
ganic solvents, but also a variety of ionic liquids at
ꢀ1
50 mgmL metallogelators 5a,b easily dissolve in IL-6, but
still efficiently gelate a selection of ionic liquids even at very
low gelator concentrations. For example, the lowest gelator
concentration required to solidify IL-5 by 5a,b is
ꢀ1
5
mgmL gelator concentration. Typical types of ionic liq-
uids, such as imidazolium salts (IL-1a–c), pyridinium salt
ꢀ1
0
.5 mgmL after cooling to room temperature.
Transmission electron microscopy: The morphology of xero-
gels obtained from different organic solvents (after evapora-
tion of the solvent) was investigated by transmission elec-
tron microscopy (TEM). Compared with gels formed with
bis(imidazolylidene) complex 1, slender gel fibre networks,
which are composed of long (several mm) and thin (10-
1
00 nm wide) fibres, were observed in the gels formed with
bis(benzimidazolylidene) complexes 5a,b in most selected
solvents. In most cases, gels formed with 5a,b reveal a twist-
ed helical ribbon morphology at the microscopic level,
which is quite rarely observed in gel networks formed from
[18a]
achiral gelators.
The dimensions of the individual fibres
(
IL-2), piperidinium salt (IL-3), pyrazolidinium salt (IL-4)
vary with the gelator and solvents used. Selected TEM
images of gels with helical morphologies (P and M) formed
by carbene complexes 5a,b are presented in Figure 1. Heli-
cal fibres that are about 100 nm wide and 500–600 nm in
pitch were observed with gel 5a/DCE (Figure 1a); thinner
(ca. 70 nm wide) and longer (up to 100 mm long) fibres with
a pitch of about 500 nm were encountered in the gel 5b/
DCE (Figure 1c). More helical fibres with a pitch of approx-
imately 250 nm and 50 nm in width appeared in the TEM
and ammonium salts (IL-5 and IL-6) have been involved in
this test (Table 2).
Except IL-2 which was synthesised according to a litera-
[21]
ture protocol,
all other ionic liquids are commercially
available. At room temperature, carbene complexes 5a,b
are insoluble in all selected ionic liquids. Upon heating to
around 165–1858C in a sealed vial with shaking, the result-
ing clear liquids form gels after cooling to room tempera-
Chem. Eur. J. 2009, 15, 1853 – 1861
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www.chemeurj.org
1855

K. H. Dçtz, T. Tu et al.
Figure 2. TEM images of gels formed by CNC pincer complexes 5a in
ꢀ
1
solvents containing a few drops of cosolvent (10 mgmL ): a) Gel 5a/
AcOH+DMSO) and b) gel 5a/(acetone+THF).
(
fered from decomposition when irradiated with the electron
beam, which hampered the recording of satisfactory TEM
images. Therefore, dark-field optical microscopy was in-
volved to get a direct visual insight into the gel morpholo-
gies obtained from gelators 5a,b and various ionic liquids.
Selected images are shown in Figure 3. In most cases,
straight and long cotton-like fibre (10–25 mm long and ca.
Figure 1. Selected TEM images of gels formed by CNC pincer complexes
ꢀ
1
5
a,b (10 mgmL ): a) Gel 5a/DCE; b) gel 5a/DMA; c) gel 5b/DCE; and
5
00 nm wide) gel networks were observed under the micro-
scope. In the gel morphologies of 5a/IL-1b, 5b/IL-1c and
a,b/IL-5, shining rod-like particles were observed within
d) gel 5b/AcOH.
5
[22]
image of xerogel 5b/AcOH (Figure 1d). As shown in Fig-
ure 1b, a denser gel network with thinner (around 10 nm
wide) and long helical fibres was observed with gel 5a/
these straight and long fibre networks, which may reveal
that a lamellar assembly is also responsible for the gel for-
mation with gelator 5a,b and ionic liquids. Short and twisted
wool-like fibres were found with the gels 5a/IL-1c and 5a/
[22]
DMA; a similar morphology was found with gel 5a/DMF.
[22]
Very dense gel networks result from gels 5a,b/DMSO, 5b/
DMF and 5b/DMA. More straight and broad fibres were
IL-2. As observed in the xerogel networks obtained from
gelators 5a,b and some organic solvents, helical-like fibres
(approximately 100 mm long and 0.5–0.8 mm wide, with a
pitch of 20 mm) could be found in the gel 5a/IL-3 (Fig-
ure 3a). This type of helical fibres may also exist in the
other gel samples such as 5b/IL-2 (Figure 3b); the low reso-
lution of light microscopy in comparison to TEM hampers
the observation of more detailed information.
[22]
obtained from gels 5a,b with ACN, DCM, TCM and THF,
which may suggest gels formed by a parallel columnar self-
assembly.
TEM was also applied to investigate the morphologies of
gels obtained from mixtures of solvents. Ribbon-type fibres,
[22]
approximately 150 nm wide and several mm long, were ob-
served in the dense network of gel 5a/(toluene+DMSO). In
contrast, the morphologies of gels obtained from two other
mixtures of solvents are quite different. Bold fibres (0.5–
Thermal stability of the gels: The gel–sol phase-transition
temperatures (T ) of gels in selected solvents were deter-
g
[5,23]
1
mm wide) were present in the xerogel network of 5a/
mined by differential scanning calorimetry (DSC).
The
(AcOH+DMSO) (Figure 2a),
at the end of which small hair-
like twisted fibres (ca. 10 nm
wide) emerge. A Chinese chrys-
anthemum-like xerogel network
(
Figure 2b) appears in the mor-
phology of gel 5a/(ace-
tone+THF). The diameter of
ACHTUNGTRENNUNG
the flower is around 5–10 mm.
The fibres emerging from the
core of flower are approximate-
ly 100 nm wide.
Dark-field optical microscopy:
Figure 3. Selected dark-field optical microscopic images of gels formed by CNC pincer complexes 5a,b
ꢀ
1
(
Xero)gels of ionic liquids suf- (5 mgmL ) with ionic liquids: a) Gel 5a/IL-3; and b) gel 5b/IL-2.
1856
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Low-Molecular-Mass Gelators for Ionic Liquids
FULL PAPER
DSC thermograms revealed well-defined thermo-reversible
Figure 4. Similar results were observed with gel 5b/IL-6
(from 66 to 928C).
gel–sol transitions with accurate T and enthalpy parameters,
g
which indicate that the 3D networks responsible for gelation
are built up by non-covalent interactions, such as p-stacking,
hydrogen bonding and van der Waals interactions. In all
1
Temperature-dependent H NMR study: Similar to metallo-
gelator 1, the efficiency observed for carbene complexes
5a,b and even 6 gelating a broad variety of organic solvents
and ionic liquids may be caused by the extended planar
metal-chelating pincer ring, which allows for aggregation by
intermolecular p-stacking, possibly enhanced by van der
Waals interactions between the alkyl chains and by metal–
tested gels, the gel–sol phase-transition temperatures (Tg/H
)
were found to be approximately 158C higher than those ob-
served for the sol–gel conversion (Tg/C), a hysteresis behav-
[
18a,24]
iour which is characteristic of LMMGs.
(benzimidazolylidene)–palladium complexes 5a,b quite effi-
ciently gelate DMSO at concentrations even as low as
Although bis-
ACHTUNGTRENNUNG
metal interactions. Therefore, temperature-dependent
ꢀ1
1
0
.5 mgmL (0.05 wt%, w/v), these conditions are hard to
H NMR spectroscopic studies of gel 5a,b/[D ]DMSO (at
6
ꢀ1
ꢀ1
realise. Therefore, a series of 5a,b/DMSO gels with concen-
5 mgmL and 10 mgmL ) were applied to probe the role
of the metal-hybrid aromatic rings during the gelation pro-
cess. Almost no clear proton signals were detected at room
ꢀ
1
trations varying from 1 to 30 mgmL were studied by DSC
[24]
and the “dropping ball method” in order to examine the
effect of concentration of the gelator on the sol–gel transi-
tion temperatures. When the gel concentration is lower than
ꢀ1
temperature for gel sample 5b/[D ]DMSO (5 mgmL ) and
broad signals appeared even at 308 K, which indicates an ex-
tensive aggregation in the gel state.
6
ꢀ1
[22]
2
mgmL , it is difficult to determine T by DSC, because
Upon warming to
g/H
of the low melting point of DMSO (292 K) and the difficulty
to transfer the low concentration gel samples into the alumi-
nium crucibles. Thus, we applied the dropping ball method
in the Tg/H tests. Figure 4 shows that Tg/H and Tg/C increase
348 K by 10 K steps, the broad signals observed for the
hetero-aromatic and the N-CH2 hydrogen atoms steadily
sharpen and are shifted downfield: The signals of the g
proton of the pyridine ring and N-CH are shifted from d=
2
with increasing concentration of gel
A
H
U
G
R
N
U
G
7.82 and 4.32 to 8.02 and 4.52 ppm, respectively, (Dd is
which demonstrates that the thermal stability of the gels
formed with organic solvents increases with increasing gel-
0.2 ppm) over the temperature range of 40 K (308–348 K).
A
similar phenomenon was observed with gel 5a/
ꢀ
1
Aa tor concentration.
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[D ]DMSO (5 mgmL ). When the concentration was in-
6
creased to 10 mgmL , broad signals were observed even at
28 K, which also confirmed the results derived from the in-
ꢀ
1
3
crease of T with the gelator concentration. Upon warming
g
from 328 K to 348 K, the proton signals steadily sharpen
and are shifted downfield with a Dd of 0.16 ppm for the g
[22]
proton. This effect is thermo-reversible and reflects strong
aromatic p–p interactions between the gelator molecules.
X-ray structure of complex 7: Although carbene complex 7
is not able to gelate selected liquids, its crystal structure
may help to further understand the self-assembling process
of bis ACHUTNGRENNUG( carbene) complexes 5–7. Exposure of a solution of 7
in CHCl3 to vapor of MeOH induced the formation of
orange needle-shaped single crystals of the composition of
Figure 4. Sol–gel transition temperatures as a function of gelator concen-
tration: Tg/H and Tg/C for gels 5a,b/DMSO (* means that Tg/H was mea-
sured by the “dropping ball method”) and Tg/H for gels 5a,b/IL-6.
7·0.81CHCl . Its structure is depicted in Figure 5, and crys-
3
tallographic details are provided in the Supporting Informa-
tion. The terminal methyl groups of the butyl chains and the
guest molecules (CHCl ) are partially disordered. Unlike
3
The T of gels obtained with all ionic liquids studied at
the structures reported for CNC pincer palladium carbene
g/H
ꢀ1
[18a,26,27]
5
mgmL gelator concentration were determined by DSC
and corroborated by the “dropping ball method”, the results
are summarised in Table 2. Compared with the T (at
complexes,
the structure is very symmetric (Fig-
ꢀ
ure 5a): Two I ions are located vertically above and below
the plane of the metal-chelating pincer ring with equal dis-
tances to the metal (2.79 ꢃ). The extended metal-hybrid p
system slightly deviates from planarity, the benzene rings
are slightly out of the plane (ca. ꢁ3.28) that is defined by
palladium, the pyridine and imidazole rings. Strong p–p in-
teractions (ca. 3.3 ꢃ, Figure 5b) are observed between the
extended palladium-hybrid planes. Unlike the metal–metal
g/H
ꢀ1
1
0 mgmL ) for gels prepared with organic solvents, the T
g/H
ꢀ
1
of gels obtained from selected ionic liquids (at 5 mgmL )
are higher and generally above 658C (Table 2). With ionic
liquids, the Tg/H also increase with the gelator concentration
which confirms a similar thermal behaviour as previously
observed for organic solvents. For example, an increase of
the gelator concentration of gel 5a/IL-6 from
3
2
to
0 mgmL induced an increase of the gel–sol phase-transi-
tion temperatures (T ) from 63 to 888C as shown in
bonding found in the crystal of the butyl analogue of car-
ꢀ1
[18a]
bene complex 1,
there is no evident Pd···Pd interaction,
which may reflect the steric hindrance by the counter ion
g
Chem. Eur. J. 2009, 15, 1853 – 1861
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1857

K. H. Dçtz, T. Tu et al.
van der Waals interactions with the shortest distance of
1
.76 ꢃ between the hydrogen atoms of terminal butyl chains
belonging to two adjacent molecules (Figure 5c). Some
weak intermolecular interactions are observed within the
ꢀ
units; for example, the distance between I and the b hydro-
gen atom of the pyridine ring of the adjacent molecule is
3
.16 ꢃ. As shown in Figure 5a, the solvate guests have very
strong hydrogen-bonding interactions with the butyl chains,
which may reveal how the gelator cages the solvent during
the gelation. The shortest H···Cl distance is 2.18 ꢃ, the
others vary from 2.42 to 2.69 ꢃ. By this means, two elongat-
ing fibres may assemble through hydrogen-bonding interac-
tions with solvent guests to form a parallel columnar pack-
ing pattern in the crystal network (Figure 5d).
Although there are no evident metal–metal interactions
observed in the crystal structure (solid state) of carbene
complex 7 as a consequence of the steric hindrance by the
counterions, we believe that Pd···Pd interactions still play a
key role during the aggregation process to form a gel for the
following reasons. Bis(benzimidazolium) salts 2a,b and 3 ef-
ficiently gelate a variety of organic solvents (especially a
broad variety of alcohols) even at the gelator concentration
ꢀ
1 [20a]
of 5 mgmL ,
but, unlike gelators 5a,b, are unable to
gelate DMSO, DCE and other solvents. In addition, the
structure and dimensions of the gel network of 5a,b/DMSO
[5,28]
have been studied by X-ray diffraction (Figure 6).
Simi-
lar X-ray diffraction patterns could be observed with gels 1/
DMSO and 5b/DMSO with the only difference that the in-
tensities of the reflections are less prominent in the latter
ꢀ
1
(
Figure 6). The broad reflection at q=2.1 nm and the
ꢀ
1
sharp reflection at q=4.2 nm may suggest that the struc-
ture in the gel state resembles that of the crystal structure
shown in Figure 5d. The alkyl chains are lined up with a dis-
tance of approximately 1.5 nm (=2p/4.2) in real space
(
sharp reflection), whereas the Pd, the pyridine and imida-
zole rings are stacked with a corresponding distance of 3 nm
broad reflection), but in a less pronounced order, maybe
(
due to steric reasons. Differently, the gel 5a/DMSO exhibits
a clear lamellar arrangement (reflections at q=1.72, 3.53
ꢀ
1
and 5.2 nm ) with a corresponding distance of 3.6 nm in
real space. This reflects the significantly higher order of the
stacked layers of the complex with long alkyl chains and
bromide counterions instead of iodide. An additional fea-
ture, which is common to all three gels, is the peak at q=
ꢀ
1
1
6 nm , which corresponds to a distance of 3.8 ꢃ in real
space and is attributed to a p stacking and Pd···Pd interac-
tions. This distance is slightly larger in the gel state than in
the crystal. The intensity in the small-angle X-ray scattering
range towards low q values (Figure 5a) shows an increase
that may be caused by the arrangement of the basic units in
the formation of elongated fibres. The typical diameter of a
fibre is about 9 nm (which is similar to the fibre dimension,
ca. 10 nm wide, observed for TEM with gels 5a,b/DMSO),
which is derived from the shoulder in the intensities towards
low q-values.
3
Figure 5. a) Molecular structure of 7·0.81CHCl in the crystal; b) p inter-
actions between extended metal hybrid p rings of two adjacent molecules
to form units; c) van der Waals interactions between the alkyl chains re-
sulting in fibres; and d) side view showing part of a truncated 2ꢂ2ꢂ2
array of unit cells.
ꢀ
(
I ) in the solid sate. The p interactions between the mole-
cules are responsible for the formation of aggregation units,
which are assembled to fibres through strong intermolecular
1858
www.chemeurj.org
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1853 – 1861

Low-Molecular-Mass Gelators for Ionic Liquids
FULL PAPER
Figure 6. X-ray diffraction patterns of gels 1/DMSO and 5a,b/DMSO (3 wt%) after subtraction of the solvent-based background: left: logarithmic scale
and right: linear scale; the curves are vertically shifted for clarity.
Conductivity of the gels formed by ionic liquids: Ionic liq-
uids are a new type of electrolytes with a high conductivity
for ions. Currently, solid electrolytes, which are electrically
conductive solids with ionic carriers, attract much attention
because of their wide potential applications in the field of
electrochemical devices. Therefore, the immobilisation of
ionic liquids while maintaining high conductivity to produce
solid electrolytes is a very important issue. Since metallo-
very close to those of the pure ionic liquids. In some case, s5
is even higher than s (Table 3, IL-5 and IL-6), which may
0
fulfill the requirements of quasi-solidification of ionic liquids
ꢀ
1
to produce solid electrolytes. For 30 mgmL , the conductiv-
ity (s ) of gel 5a/IL-6 at room temperature is 6.39ꢂ
3
0
ꢀ
5
ꢀ1
10 Scm revealing that the conductivity further increases
with gelator concentration. This phenomenon is in contrast
with results reported in the literature stating a decrease of
the ionic conductivity with the organic gelator concentra-
ACHTUNGTRENNUNGg elators 5a,b quite efficiently gelate a variety of ionic liquids
[16a]
even at very low gelator concentrations, they provide a con-
venient method to obtain quasi-solid electrolytes. The con-
A
H
U
G
R
N
U
G
but may be rationalised in terms of the ionic proper-
ty of pyridine-bridged palladium–carbene complexes 5a,b.
The conductivities of iodide gels (5b) are slightly higher
than those of bromide gels (5a), which may reflect the
ꢀ
1
ductivity (s ) of different gels (5 mgmL ) formed by gel-
5
ACHTUNGTRENNUNGa tors 5a,b and eight ionic liquids are listed in Table 3. Gen-
ꢀ
ꢀ
better leaving group property of I compared to Br in the
complexes (Table 3).
Table 3. Ionic conductivities of selected pure ionic liquids and corre-
[
a]
The temperature dependence of the ionic conductivity
sponding gels formed from CNC pincer complexes 5a,b.
ꢀ
1
was studied with a series of 5 mgmL gels obtained from
5a,b and selected various ionic liquids. Figure 7 shows the
plots of ionic conductivities of the quasi-solidified ionic liq-
uids against temperature (25–1008C). As expected for the
pure ionic liquids, the ionic conductivities of the metallogels
also steadily increased upon heating. For some pairs of gels,
a conductivity cross-over is observed. For example, the con-
gel 5a/IL
gel 5b/IL
ꢀ
1
IL
0
s [Scm ]
ꢀ
1
0
ꢀ1
0
s
5
[Scm
]
s
5
/s
0
s [Scm
]
s /s
0
5
5
ꢀ
3
ꢀ3
ꢀ3[b]
ꢀ3
ꢀ3
ꢀ3
ꢀ
[b]
IL-1a
IL-1b
IL-1c
IL-2
IL-3
IL-4
3.98ꢂ10
2.90ꢂ10
4.40ꢂ10
1.18ꢂ10
1.58ꢂ10
2.88ꢂ10
2.20ꢂ10
6.07ꢂ10
3.78ꢂ10
2.89ꢂ10
4.29ꢂ10
1.12ꢂ10
1.48ꢂ10
2.86ꢂ10
2.20ꢂ10
6.15ꢂ10
0.95
1.00
0.98
0.95
0.94
0.99
1.00
1.01
3.82ꢂ10
2.91ꢂ10
4.28ꢂ10
1.13ꢂ10
1.55ꢂ10
2.78ꢂ10
2.29ꢂ10
6.22ꢂ10
0.96
1.00
0.98
0.95
0.98
0.97
1.04
1.02
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3
3
3
3
3
3
5
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ3
ꢀ5
ꢀ3
ꢀ3
ꢀ5
IL-5
IL-6
[
a] Conductivities of pure ionic liquids and gels 5a,b/IL were measured
at 258C. [b] 5b cannot be dissolved completely upon heating.
ꢀ
1
erally, a conductivity exceeding 1 mScm , which is signifi-
cantly higher than that of Li-conducting solid polymer elec-
[10]
trolytes applied in various devices, is appealing for a solid
electrolyte. This property is realised for all gel samples stud-
ied with the exception of those formed with IL-6 (Table 3,
entries 1–7). For comparison, the ionic conductivities (s ) of
0
pure ionic liquids were also measured. Unlike the evident
difference in conductivity between the pure ionic liquids
and the ionic liquid gels prepared from reported organic
Figure 7. Temperature-dependent ionic conductivities of quasi-solidified
ionic liquids formed by CNC pincer complexes 5a,b (gelator concentra-
[16]
ꢀ1
LMMGs, the ionic conductivities of the gel samples are
tion: 5 mgmL ).
Chem. Eur. J. 2009, 15, 1853 – 1861
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1859

K. H. Dçtz, T. Tu et al.
ꢀ
3
ꢀ1
ꢀ1
ductivities (s ) of gels 5a/IL-1c and 6a/IL-4 are 4.29ꢂ10
1413 mScm and 12.88 mScm ). A CEM Discover microwave instru-
5
ꢀ
3
ꢀ1
ment was applied for the palladation.
and 2.86ꢂ10 Scm at 258C, (Table 3, IL-1c and 6), and
[
20]
ꢀ
3
ꢀ3
ꢀ1
The synthesis of bis(benzimidazolium) salts (2–4) , crystallographic
data of carbene complex 7, details of gel preparation and characterisa-
tion, as well as additional TEM and dark-field microscopic images are
provided in the Supporting Information.
they increase to 13.13ꢂ10
008C. Similar phenomena were also observed with gels 5a/
IL-3 and 5a/IL-5 (Figure 7). As shown in Table 3, the con-
ductivity (s ) of gels 5b/IL-6 is slightly higher than that of
and 13.43ꢂ10 Scm
at
1
5
Synthesis of bis(benzimidazol-2-ylidene)–palladium complexes (5–7): A
5
a/IL-6, and this tendency is kept upon heating, which indi-
solution of pyridine-bridged benzimidazolium salt (1.0 mmol) and Pd-
AHCTUNGTNERNUG( OAc) (0.224 g, 1.0 mmol) was stirred in DMSO (8 mL) for 1 h at room
2
temperature under vacuum. After refilling the flask with argon, the mix-
ture was exposed to microwave radiation with stirring in the open vessel
model for 25 min at 1608C (at 40 W with a CEM Discover instrument).
cates that the counterion affects the gelator property and
the conductivity of the quasi-solids. The gel–sol transition
temperature (Tg/H) of 5 mgmL samples prepared from
ꢀ
1
5
a,b and all selected ionic liquids are in the range of 65–
Cold Et O was added (200 mL) and the precipitate was purified three
2
1008C (Table 2), the s versus T plots are quite linear indi-
times by dissolving in hot CHCl
ing a yellowish solid.
3
2
and precipitating with cold Et O, afford-
5
cating that the activation energies for the phase transitions
are similar and that gels prepared from gelators 5a,b and
ionic liquids maintain the isotropic properties on a micro-
scopic scale, although ionic liquids are immobilised to form
a 3D network on a macroscopic scale. This property may
offer potential applications of pyridine-bridged bis-
1
Carbene complex 5a: Yield: 76%; H NMR ([D
6
]DMSO, 500 MHz,
3
8
7
48 K): d=7.95 (t, J=8.5 Hz, 1H), 7.81 (d, J=8.5 Hz, 2H), 7.76 (d, J=
.5 Hz, 2H), 7.39 (d, J=7.5 Hz, 2H), 7.02–7.10 (m, 4H), 4.37 (t, J=
.8 Hz, 4H), 1.30 (quintet, J=7.5 Hz, 4H), 0.88 (quintet, J=7.5 Hz, 4H),
1
3
0.74 (m, 4H), 0.61–0.70 (m, 44H) 0.24 ppm (t, J=7.0 Hz, 6H); C NMR
[D ]DMSO, 125 MHz, 338 K): d=175.85, 151.52, 147.68, 134.41, 130.31,
27.79, 127.10, 114.39, 114.00, 110.59, 48.55, 32.07, 30.66, 29.81, 29.67,
9.63, 29.56, 29.44, 29.40, 26.59, 22.82, 14.59 ppm; HR-MS (MALDI,
(
1
2
6
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(benzimidazolylidene) pincer carbene complexes in electro-
chemical devices.
+
DCTB): m/z calcd: 946.4401; found: 946.4620 [MꢀBr] ; elemental analy-
sis calcd (%) for C51
H
77Br
2
N
5
Pd·2H
2
O (1059.3792): C 57.65, H 7.68, N
6
.59; found: C 57.78, H 7.22, N 6.54.
1
Conclusions
Carbene complex 5b: Yield: 79%; H NMR ([D
6
]DMSO, 500 MHz,
48 K): d=8.02 (t, J=8.2 Hz, 1H), 7.87 (d, J=7.8 Hz, 2H), 7.83 (d, J=
3
8
8
0
.3 Hz, 2H), 7.44 (d, J=8.0 Hz, 2H), 7.05–7.15 (m, 4H), 4.52 (t, J=
.0 Hz, 4H), 1.33 (quintet, J=7.5 Hz, 4H), 0.92 (quintet, J=7.5 Hz, 4H),
In conclusion, a novel pincer-type of pyridine-bridged bis-
(benzimidazolylidene)–palladium complexes was synthesised
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
13
.77 (m, 4H), 0.43–0.54 (m, 44H) 0.24 ppm (t, J=7.0 Hz, 6H); C NMR
from cheap commercially available precursors under micro-
wave assistance and successfully applied as efficient organo-
metallic gelators for a variety of protic and aprotic organic
solvents as well as ionic liquids effective in concentrations
6
([D ]DMSO, 125 MHz, 338 K): d=176.34, 151.34, 147.50, 134.20, 130.43,
1
2
27.84, 127.16, 114.57, 114.08, 110.61, 61.28, 50.15, 32.08, 30.85, 29.81,
9.62, 29.56, 29.43, 26.33, 22.83, 14.63 ppm; HR-MS (MALDI, DCTB):
+
m/z calcd: 992.4258; found: 992.4613 [MꢀI] ; elemental analysis calcd
(
%) for C51
H
77
I
2
N
5
Pd·H
2
O (1137.3409): C 53.81, H 6.99, N 6.15; found: C
ꢀ
1
as low as 0.5 mgmL . p-Stacking of the extended planar
metal-hybrid (hetero)arene moieties, van der Waals interac-
tions between the alkyl chains, and metal–metal interactions
may be responsible for the aggregation as supported by tem-
perature-dependent NMR spectroscopy and X-ray diffrac-
tion studies. Since pure ionic liquids and corresponding gels
prepared from them and 5a,b show comparable high con-
ductivities, pyridine-bridged bis(benzimidazolylidene)–palla-
dium complexes represent the first examples of air-stable
metallogelators which efficiently immobilise ionic liquids in
very low gelator concentration, indicating potential applica-
tions—beyond catalysis—also in electrochemical devices.
53.85, H 6.93, N 6.15.
1
Carbene complex 6: Yield: 71%; H NMR ([D ]DMSO, 500 MHz,
6
338 K): d=7.81 (t, J=8.0 Hz, 1H), 7.65 (d, J=7.5 Hz, 2H), 7.61 (d, J=
8
.3 Hz, 2H), 7.23 (d, J=7.5 Hz, 2H), 6.85 (quintet, J=7.5 Hz, 4H), 4.25
(
t, J=8.0 Hz, 4H), 1.10 (quintet, J=7.5 Hz, 4H), 0.72 (quintet, J=
7
6
.5 Hz, 4H), 0.56–0.60 (m, 4H), 0.43–0.54 (m, 10H) 0.10 ppm (t, J=
.8 Hz, 6H); C NMR ([D ]DMSO, 125 MHz, 338 K): d=176.2, 151.2,
6
13
147.6, 134.1, 130.3, 127.9, 127.2, 114.5, 114.1, 110.6, 50.2, 31.9, 30.9, 29.4,
2
7
C
9.3, 26.4, 22.8, 14.6 ppm; HR-MS (MALDI, DCTB): m/z calcd:
+
68.1754; found: 768.2006 [MꢀI] ; elemental analysis calcd (%) for
35
H
45
I
2
N
5
Pd·0.5H
2
O (904.0852): C 46.45, H 5.12, N 7.74; found: C 46.46,
H 5.49, N 7.48.
1
Carbene complex 7: Yield: 68%; H NMR ([D
6
]DMSO 500 MHz,
298 K): d=7.74 (t, J=8.3 Hz, 1H), 7.64 (d, J=8.0 Hz, 2H), 7.60 (d, J=
8
7
.3 Hz, 2H), 7.22 (d, J=7.8 Hz, 2H), 6.81–6.90 (m, 4H), 4.24 (t, J=
.5 Hz, 4H), 0.95–1.11 (m, 4H), 0.66 (quintet, J=7.5 Hz, 4H), 0.10 ppm
1
3
(
1
t, J=7.3 Hz, 6H); C NMR ([D
51.32, 147.46, 134.12, 130.35, 127.83, 127.14, 114.64, 114.13, 110.61, 61.11,
33.09, 19.94, 14.65 ppm; HR-MS (ESI): m/z calcd: 782.9547, 656.0502;
6
]DMSO, 125 MHz, 298 K): d=176.08,
Experimental Section
+
+
found: 782.9588 [M] , 656.0503 [MꢀI] ; elemental analysis calcd (%) for
Pd·0.5DMSO (904.0852): C 40.48, H 3.88, N 8.13; found: C
0.63, H 3.92, N 8.14.
27 29 2 5
C H I N
All commercial reagents and solvents were used directly as purchased
4
without further purification. IL-2 was synthesised by
method. The other ionic liquids were bought from Fluka and Roth. All
a
literature
[
21]
1
reactions were carried out in air unless otherwise noted.
H
and
1
3
C NMR spectra were recorded on Bruker 500 DRX/400 DPX/300 DPX
spectrometers. MALDI-TOF mass spectra were recorded on a Bruker
Daltonics Announces Autoflex TOF/TOF mass spectrometer using
DCTB as matrix; ESI-MS spectra were recorded on a micrOTOF-Q
Bruker Daltonik instrument. Conductivities of gel samples and pure
ionic liquids were measured by a Mettler Toledo FiveGo (FG3) conduc-
tometer within a temperature range of 0–1008C. Before every testing, the
Acknowledgements
T.T. thanks the Alexander-von-Humboldt-Foundation for a research fel-
lowship. We thank Prof. Dr. S. Hçger, Prof. Dr. W. Mader and Prof. Dr.
J. Beck for technical support. Financial support from the DFG (SFB 624
“Templates”) is gratefully acknowledged.
ꢀ
1
conductometer was calibrated with standard solutions (84 mScm
,
1860
www.chemeurj.org
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1853 – 1861

Low-Molecular-Mass Gelators for Ionic Liquids
FULL PAPER
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Received: October 31, 2008
Published online: January 2, 2009
Chem. Eur. J. 2009, 15, 1853 – 1861
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1861