An air-stable organometallic low-molecular-mass gelator: Synthesis, aggregation, and catalytic application of a palladium pincer complex

Article abstract of DOI:10.1002/anie.200701486

(Chemical Equation Presented) An efficient organometallic gelator can gel a variety of organic solvents even in concentrations as low as 0.2 wt %. π Stacking of the heteroarene moieties, van der Waals interactions between the alkyl chains, and metal-metal

Full text of DOI:10.1002/anie.200701486

Communications
DOI: 10.1002/anie.200701486
Gel Catalysis
An Air-Stable Organometallic Low-Molecular-Mass Gelator:
Synthesis, Aggregation, and Catalytic Application of a Palladium
Pincer Complex**
Tao Tu,* Wilfried Assenmacher, Herwig Peterlik, Ralf Weisbarth, Martin Nieger, and
Karl Heinz Dötz*
Dedicated to Professor Fritz Vögtle
Currently, gels derived from low-molecular-mass compounds
are attracting increasing interest owing to the simplicity of the
gelator molecules, their physical properties, and potential
applications. A variety of organo- and hydrogelators based on
hydrogen bonding, p stacking, and van der Waals interactions
has been reported to immobilize organic fluids or water to
form swellable materials, even at extremely low concentra-
Palladium CNC pincer bis(imidazolylidene) complexes 1
[
6]
and 2a were reported to catalyze cross-coupling reactions;
however, their poor solubility hampered a broader applica-
[
6a]
tion in catalysis. Substitution with longer alkyl chains is a
common means to enhance the solubility of compounds in
organic solvents and, moreover, is expected to favor aggre-
gation through intermolecular van der Waals interactions,
[
1–3]
[2,7]
tions.
Structural requirements for low-molecular-mass
which may assist gel formation.
Thus, we concentrated on
gelators (LMMG) are poorly understood, and a rational
correlation of molecular structure and gelation ability in a
given solvent still remains a challenge. Despite the major role
that organometallic compounds play in synthesis, catalysis,
and materials chemistry, they have yet to be investigated as
CNC pincer palladium carbene complexes of type 3 bearing
two C16 alkyl substituents as potential low-molecular-mass
organometallic gelators (Scheme 1).
[
4]
gelator-type soft materials; to date, only two examples, a
sugar chromium carbene amphiphile and a cholesterol
titanocene derivative, were reported as organometallic low-
[
5]
molecular-mass organogelators. With the aim to introduce
the unique properties of organopalladium compounds in
organic synthesis into organometallic gelators, we extended
our studies towards robust functional organometallic com-
pounds as low-molecular-mass gelators. Herein, we report the
first example of a palladium-based air-stable organometallic
LMMG, which is able to gelate a variety of organic solvents
and to catalyze CÀC bond formation even in the gel state.
Scheme 1. Palladium CNC pincer bis(carbene) complexes.
The pincer complexes, such as 3, are readily accessible
from commercial starting materials in a modified modular
[
8]
high-yield synthesis. Upon heating to reflux in a variety of
organic solvents and subsequently cooling to room temper-
ature, complex 3 forms thermoreversible swellable materials
within a few hours. Gelation experiments with 1wt% (wt/v;
À3
[*] Dr. T. Tu, Prof. Dr. K. H. Dötz
9.81” 10 m) gelator in a selection of protic and aprotic
KekulØ Institute of Organic Chemistry and Biochemistry
University of Bonn
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax: (+49)228-73-5813
E-mail: tao.tu@uni-bonn.de
doetz@uni-bonn.de
Homepage: http://www.chemie.uni-bonn.de/oc/ak_do/
solvents are summarized in Table 1. Turbid gels were formed
from methanol (MeOH), acetic acid (HOAc), and dichloro-
ethane (DCE), whereas dimethyl sulfoxide (DMSO), dime-
thylformamide (DMF), dimethylacetamid (DMA), and tet-
rahydrofuran (THF) produced translucent or transparent gels
independent of the cooling rate. Because CNC complex 3 is
stable towards air and moisture, long-lived gels are accessible
under ambient conditions. When the concentration is
increased to 2 wt%, stable gels are also formed from
dichloromethane (DCM), dioxane, benzene, toluene, and
acetonitrile (ACN) at room temperature. The gels darken in
color with increasing coordination ability of the solvent.
The morphology of xerogels obtained from different
solvents (after evaporation of the solvent) was investigated by
transmission electron microscopy (TEM). Selected typical 3D
networks of xerogels demonstrated that morphologies
obtained from different solvents are quite different
(Figure 1). Larger fibers are present in xerogels obtained
Dr. W. Assenmacher, Dr. R. Weisbarth, Dr. M. Nieger
Institute of Inorganic Chemistry
University of Bonn (Germany)
Prof. Dr. H. Peterlik
Faculty of Physics
University of Vienna
Boltzmanngasse 5, 1090 Vienna (Austria)
[
**] T.T. thanks the Alexander-von-Humboldt-Foundation for a research
fellowship. We thank Prof. Dr. Werner Mader and Prof. Dr. Johannes
Beck for technical support. Financial support from the DFG (SFB
624) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
6
368
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Chemie
[
a]
Table 1: Gelation ability of complex 3 in various solvents.
mm long and ca. 10 nm wide) are straighter and more parallel
[
b]
than the fibers obtained from DMSO (Figures 1b and c).
Benzene, ACN, and DCM form straight ribbon morphologies
with a length of several micrometers and a width of
Entry
Solvent
Phase
Color
Gel type
Tg [8C]
1
2
3
4
5
6
7
8
9
MeOH
HOAc
DCM
DCE
DMSO
DMF
DMA
THF
dioxane
benzene
toluene
ACN
G
G
P
yellow
red
yellow
light yellow
orange-red
orange-red
orange-red
orange
yellow
turbid
turbid
turbid
turbid
transparent
transparent
transparent
translucent
turbid
54
52
[
10]
[
c]
[d]
approximately 100 nm.
–
The gel-to-solution phase-transition temperatures (T ) of
G
G
67
59
58
50
55
g
gels in selected solvents were determined by differential
[
1,9]
G*
G*
G
G*
G*
G*
scanning calorimetry (DSC; Table 1).
At 1wt% concen-
tration, the T values obtained for MeOH, HOAc, and
g
DMSO were corroborated by the “dropping ball
[
[
[
c]
c]
c]
[c]
53
50
[
11]
[c]
method”.
The DSC thermograms recorded for gel 3/
1
1
1
0
1
2
yellow
yellow
yellow
turbid
turbid
turbid
[c]
DMSO (2 wt%) revealed well-defined thermoreversible sol-
51
[
c]
[d]
PG
–
gel transitions with unchanged T (638C) and enthalpy
g
À1 [10]
parameters (DH = 2 Jg ). In all tested gels, the transition
[
a] Gelator concentration: 1 wt%; G: gel formed at RT; G*: gel formed by
[
9]
temperature from gel to solution (Tg/H) was found to be at
cooling to 58C; PG: partial gel, P: precipitate. [b] Determined by DSC.
least 58C higher than that from solution to gel (Tg/C); this
[
c] Gel formed with 2 wt% at RT. [d] Tg could not be determined
[
12]
accurately by DSC.
hysteresis behavior is characteristic of LMMGs. To exam-
ine the effect of concentration of the gelator on the solution–
from MeOH, HOAc, and DCE, while very dense networks
result from DMF, DMSO, DMA, and THF.
gel transition temperature (T ), a series of 3/DMSO gels with
g
[
10]
concentrations varying from 0.2 to 3.0 wt% was studied by
DSC (Figure 2). Tg/H and Tg/C increase with increasing
concentration of gelator 3 in DMSO, which demonstrates
that the stability of the gel increases with gelator concen-
tration.
Gels from MeOH and DMA (Figures 1a and d) reveal a
helical (P and M) ribbon morphology at the microscopic level,
which is rarely observed in gel networks formed from achiral
compounds. The dimensions of the individual fibers vary with
the solvent used. Their lengths amount to several micro-
meters with pitches of about 1280 nm from DMA and
The efficiency with which carbene complex 3 gelates a
broad variety of solvents with different network morpholo-
gies may be based on the planarity of the metal-chelating
pincer ligand, which allows for aggregation by intermolecular
p stacking, possibly enhanced by van der Waals interactions
between the alkyl chains and by metal–metal interactions.
2
800 nm from MeOH. The fibers are approximately 320 nm
(
DMA) and 1600 nm (MeOH) wide. They are composed of
tightly intertwined thinner fibers (ca. 5 nm diameter from
MeOH); smaller helical ribbon morphologies, which are
approximately 100 nm wide and several micrometers long
with a pitch of approximately 500 nm, could be observed in
1
Temperature-dependent H NMR spectroscopy studies of gel
3/[D ]DMSO (1wt%) were applied to probe the role of
6
[
10]
the gel networks obtained from toluene and THF.
aromatic rings in the gelation process (Figure 3). Broad
proton signals for the gel state (258C) indicate extensive
aggregation. Upon warming to 658C by 1 08C steps, the broad
signals observed for the heteroaromatic and the NCH2
hydrogen atoms steadily sharpen and are shifted downfield;
for example, the signal of the g proton of the pyridine ring is
shifted from d = 7.73 to 7.93 ppm over the temperature range
of 408C. This effect is reversible and may reflect reduced
aromatic p–p interactions between gelator molecules upon
warming, thus suggesting that the gelator molecules coordi-
Long, finer, and denser 3D networks are formed in the
gels from DMF and DMSO. The fibers from DMF (several
Figure 1. Selected TEM images of gels formed by pincer complex 3
Figure 2. Sol-gel transition temperatures (Tg/H and Tg/C) for gels 3/
(
concentration 1 wt%). a) MeOH; b) DMSO; c) DMF; d) DMA.
DMSO as a function of gelator concentration.
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Communications
1
Figure 3. Temperature-dependent H NMR spectra (aromatic and
NCH alkyl region) of gel 3/[D ]DMSO (1 wt%).
2
6
nate the solvent to form solvates within a modified network
that is able to cage additional solvent molecules to form a gel.
The structure and dimensions of the gel network of 3/
[
1,13]
DMSO have been studied by X-ray diffraction (Figure 4).
The most prominent features are reflections at 2q = 68 and 128
L001 and L002), which are assigned to a lamellar structure with
(
a mean distance of 15.2 Š, which may reflect the distance
between stacked layers of the complex. The peak at 2q = 238
(
d001) indicates another unit with a typical distance of 3.9 Š,
which is assumed to correspond to p stacking and Pd···Pd
interactions.
Figure 5. a) Molecular structure of 2b·CHCl in the crystal. b) View
along the a axis showing part of a truncated 2ꢀ2ꢀ2 array of unit cells.
3
A possible aggregation model of gelator 3 may be based
on the molecular structure of the N-butyl analogue 2b·CHCl3,
which was established by X-ray structure analysis (Figure 5).
This complex forms parallel sheets organized in bilayers that
are held together by hydrogen bonds between alkyl hydrogen
assembled in aggregates resulting from p stacking of the
pincer ligands (plane separation 3.4 Š) and Pd···Pd interac-
À
tions (3.48 Š) and are connected by the I counterions; these
atoms and CHCl guest molecules (H···Cl separation 2.81Š)
dimensions correspond to the data obtained from the X-ray
diffraction study of gel 3/DMSO; however, the separation
between the stacked layers is slightly smaller in the crystal of
3
and by additional van der Waals interactions between alkyl
[
10]
chains with shortest distances of 2.45 Š. The bilayers are
2
b. This aggregation principle is reminiscent of that reported
[
6]
for the unsolvated bromide analogue 2a. We propose that
gelation of 3 involves intercalation of additional solvent
molecules supported by long alkyl chains into the bilayer,
which is consistent with the ribbon morphologies observed in
the gel network.
Above T , carbene complexes 2b and 3 are efficient
g
catalysts for cross-coupling reactions, for example, Suzuki,
[
10]
Heck, and Sonogashira reactions.
Complex 3 reveals
similar catalytic activity to that reported for complexes 1
[
6]
and 2a; complex 2b is even more reactive. These results
prompted us to test the catalytic activity of carbene complex 3
in the gel state. So far, only two examples of palladium(II)
coordination compounds (one of which is a polymer gel) have
been applied to catalytic reactions in the gel state; both of
them showed only low turnover numbers in the oxidation of
[
14]
benzylic alcohol to benzaldehyde.
The T of gels 3 allows us to test their catalytic reactions in
Figure 4. X-ray diffraction pattern of gel 3/DMSO (3 wt%; after sub-
g
traction of the solvent-based background).
the gel state up to approximately 508C. As a model reaction,
6
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Chemie
we studied the double Michael addition of a-cyanoacetate to
and the precipitate was purified three times by dissolving in CHCl
3
and precipitating with cold Et O, thus affording a yellowish solid
methyl vinyl ketone, which can be catalyzed by NCN and PCP
2
1
[
15,16]
(776 mg, 76%). H NMR ([D ]DMSO, 500 MHz, 338 K): d = 7.93 (t,
6
pincer palladium complexes.
Strong and easy-to-handle
1
(
4
H, J = 8 Hz, pyridine g-H), 7.79 (d, 2H, J = 2 Hz, imidazole-H), 7.37
d, 2H, J = 8 Hz, pyridine b-H), 7.10 (d, 2H, J = 2 Hz, imidazole-H),
.01(t, 4H, J = 7.5 Hz, NCH -C H ), 1.85 (quintet, 6H, J = 2 Hz,
4
wt% gels 3/DMF and 3/DMSO (prepared in situ) were
chosen as catalysts under very slow stirring to avoid destruc-
tion of the gel network (entries 2 and 3, Table 2). The gel 3/
DMSO revealed a catalytic activity higher than that observed
for DMF, probably owing to the different structures of gel
network. In order to assure that the gel state is responsible for
the observed catalytic activity, a saturated solution of complex
2
15 31
alkyl-chain H), 1.19 (quintet, 4H, J = 7.5 Hz, alkyl-chain H), 0.71(m,
8H, alkyl-chain H), 0.60 (br m, 38H, alkyl-chain H), 0.21ppm (t, 6H,
1
3
J = 7 Hz, NC15
66.90 (C-Pd), 149.58 (C-ortho), 146.52 (C-para), 123.82 (C-imida-
zole), 119.03 (C-imidazole), 110.25 (C-meta), 53.33 (NCH C H ),
H30CH ). C NMR (CDCl , 125 MHz, 298 K): d =
3
3
1
2
15 31
3
2
1.87 (NC H -CH C H ), 31.68 (NCH -CH -C H ), 29.65, 29.60,
13 26 2 2 5 2 2 15 31
3
in DCM was applied as catalyst (only ca. 0.08 mgPd per
9.53, 29.44, 29.29, 29.25, 26.19, 22.60 (alkyl-chain C), 13.98 ppm
mLDCM could be detected by ICP-MS); under these
conditions the double Michael addition was hardly faster
than in the blank test (entry 4, Table 2). DMSO and DMF as
additives in the blank test did not accelerate the reaction.
Similarly, the poor solubility of complex 2b also resulted in
low catalytic activity (entry 1, Table 2). Embedding the
catalyst into a 3D gel network allows for its easy recovery
after the reaction. Ongoing studies will focus on further
improving the catalytic activity of pincer-type complexes by
adjusting the gelating abilities with respect to the reaction
conditions required for efficient catalysis.
+
(NC H CH ). MS (MALDI): m/z 892.5 [MÀI] ; elemental analysis
1
5
30
3
(%) calcd for C H I N Pd·2H O (1019.299): C 48.89, H 7.35, N 6.63;
4
3
73
2
5
2
found: C 48.80, H 7.10, N 6.41.
Received: April 5, 2007
Published online: July 19, 2007
Keywords: carbene complexes · gel catalysis · gelators ·
.
palladium · self-organization
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k [10
s
]
t [h]
=
1
2
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1
2
3
4
5
2b
4.3
24.1
10.4
5.5
44
7.9
18.5
35
gel 3/DMSO (4 wt%)
gel 3/DMF (4 wt%)
3
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blank
3.8
51
2
[
a] Determined by comparison of the integration of the a-CH signal of
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2
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[
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interactions may be responsible for the aggregation. Complex
[
3
represents the first air-stable organometallic low-molecular-
[
[
[
10] See Supporting Information.
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[
2
10]
Synthesis of 3: A solution of [CHNCHC H ]I
(915 mg, 1.0 mmol)
1
6
33
and Pd(OAc) (224 mg, 1.0 mmol) was stirred in DMSO (8 mL) for
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2
3
h at room temperature and then for 12 h at 808C. Subsequently, the
reaction mixture was heated to 1658C for 2 h and then cooled to room
temperature, which led to gelation. Cold Et O was added (200 mL)
2
Angew. Chem. Int. Ed. 2007, 46, 6368 –6371ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6371