Self-assembly from metal-organic vesicles to globular networks: Metallogel-mediated phenylation of indole with phenyl boronic acid

Article abstract of DOI:10.1039/c0cc00112k

Self-assembly of the conformationally flexible bismethylimidazolyl ligands with Pd(OAc)₂ is described. Depending on whether the ligands provide the hydrogen bonding donor, a switching of metal-organic vesicles to globular networks gelating solvents is achieved. The metallogels exhibit catalytic activity for the cross-coupling of indole with phenyl boronic acid.

Full text of DOI:10.1039/c0cc00112k

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Self-assembly from metal–organic vesicles to globular networks:
metallogel-mediated phenylation of indole with phenyl boronic acidw
Li Yang,a Liang Luo,a Shuai Zhang,a Xiaoyu Su,a Jingbo Lan,a
Chi-Tien Chenb and Jingsong You*a
Received 25th February 2010, Accepted 12th April 2010
First published as an Advance Article on the web 27th April 2010
DOI: 10.1039/c0cc00112k
Self-assembly of the conformationally flexible bismethylimidazolyl
ligands with Pd(OAc)2 is described. Depending on whether the
ligands provide the hydrogen bonding donor, a switching of
metal–organic vesicles to globular networks gelating solvents is
achieved. The metallogels exhibit catalytic activity for the cross-
coupling of indole with phenyl boronic acid.
so far.6 Herein, we surprisingly found that the self-assembly of
Pd(OAc)2 with the flexible bismethylimidazolyl building units,
depending on whether the ligands bear the hydroxyl groups,
was capable of creating metal–organic vesicles or globular
networks (Fig. 1).
A dimethyl sulfoxide (DMSO) solution of 1 reacted with a
DMSO solution of Pd(OAc)2 at a 1 : 1 molar ratio at room
temperature to form a clear yellow solution. Transmission
electron microscopy (TEM) studies distinctly revealed that 1
and Pd(OAc)2 spontaneously assembled into spherical particles
with a diameter of ca. 43 nm (Fig. 2a). The unstained images
showed a contrast difference between the periphery and
the inner part of the spheres, which is expected from the 2D
projection of 3D vesicular structures typically observed by
TEM. The structure is clear from a magnified image of a
vesicle (Fig. 2b). The wall thickness of vesicles of {[Pd-1](OAc)2}n
was estimated to be in the range of ca. 9–12 nm. A similar
observation was obtained for {[Pd-2](OAc)2}n (Fig. 2d and e).
Subsequently, the AFM images clearly showed that both 1
and 2 assembled into spherical particles with uniform shape
and size in DMSO (Fig. 2c and 2f). The average diameters
of the aggregates of {[Pd-1](OAc)2}n and {[Pd-2](OAc)2}n
estimated from the fitted histograms of the size distribution
after subtracting the tip-broadening parameters were 46.5 and
70.0 nm, respectively (Fig. 2g and S1bw).7 Taking for example
{[Pd-1](OAc)2}n, the histogram obtained from the individual
diameters of 300 particles shows a Lorentzian distribution
(R2 = 0.9661) with an average size of 46.5 nm and fwhm
(full width at half-maximum) of 20.9 nm (Fig. 2g). The
ratios of the diameter and height of aggregates generated by
{[Pd-1](OAc)2}n and {[Pd-2](OAc)2}n were ca. 24 and 27,
respectively, which illustrated a considerable flattening of
the spheres presumably owing to either the high local force
exerted by the AFM tip or the removal of the solvent after the
transfer of the nanosized assemblies from solution to a mica
surface.8 Further dynamic light scattering (DLS) experiments
The use of coordination chemistry to direct assembly of
small component molecules into versatile nanostructures, such
as spheres, tubes, rods, and fibers, has opened up exciting
opportunities for fabricating materials with potential applica-
tions in catalysis, electronic and photophysical materials,
host–guest chemistry and biosensing.1 Morphological transfor-
mation of these assemblies has been a hot topic during the past
decade accordingly. Imidazole derivatives as building blocks
are currently attracting special interest in the creation of
metal–organic assemblies.2 Recently, the metal ion-mediated
self-assembly of conformationally rigid and structurally simple
imidazole derivatives such as 1,3-bis(1-imidazolyl)benzene
(BIB) with AgNO3 formed helical nonracemic tubular coordina-
tion polymer gelators.3 The reaction of the conformationally
rigid 4,40-bis(1-imidazolyl)biphenyl (BIBP) with Zn(OSO2CF3)2
generated sheet-like coordination polymer microparticles that
could be further transformed to nanofibers gelating solvents
with sonication.4 In this communication, we wish to further
explore the self-assembly of the structurally simple and con-
formationally flexible imidazole derivatives with Pd(OAc)2, and
applications of their assemblies in catalysis (Scheme 1).5
Vesicles have achieved prominence due to potential applica-
tions in biomimetic models, drug or gene carriers, and nano-
structured materials, etc. Traditionally, vesicles consist of
amphiphilic components, mainly small organic molecules and
polymeric materials. However, a design strategy based on
coordination chemistry toward metal–organic vesicles has
been a challenging field of research, and few examples are known
a Key Laboratory of Green Chemistry and Technology of
Ministry of Education, College of Chemistry, and State
Key Laboratory of Biotherapy, West China Hospital,
Sichuan University, Chengdu 610064, People’s Republic of China.
E-mail: jsyou@scu.edu.cn
b Department of Chemistry, National Chung Hsing University,
Taichung 402, Taiwan
w Electronic supplementary information (ESI) available: Synthetic
procedures and characterization of 1–7 and 8; catalytic procedure
for 2-phenylation of indole; gelation test; AFM; TEM; DLS; IR
spectra; ESI-MS spectra; crystal structure of {[Pd-3]Cl2}2 (8). CCDC
759098. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0cc00112k
Scheme 1 Chemical structures of ligands BIB, BIBP and 1–7.
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gel concentrations (CGCs) of the {[Pd-3](OAc)2}n, {[Pd-4](OAc)2}n,
{[Pd-5](OAc)2}n and {[Pd-6](OAc)2}n gelators in DMSO were
as low as 0.58 wt%, 0.70 wt%, 0.76 wt% and 0.95 wt%,
respectively. All the gelators may be considered as
‘‘supergelators’’ accordingly.9 It is notable that transition from
one to another could not occur when the gels and vesicles were
heated.
Fig. 1 A schematic representation of the self-assembly of ligands 1–7
with Pd(OAc)2.
The morphology evolution of gelation observed by TEM
images provided considerable insight into the formation
mechanism of a gel. Taking the aggregation behavior of
{[Pd-5](OAc)2}n as a representative example, the size of the
spherical assemblies appeared to be not strongly dependent on
the concentrations. At a lower concentration (0.06 wt%,
below CGC), {[Pd-5](OAc)2}n formed nanospherical particles
with a diameter of ca. 38–43 nm, which were dispersed in
the solution (Fig. 3c). However, at a higher concentration
(2.0 wt%, above CGC), the initially formed aggregates
(ca. 35–41 nm) unprecedentedly interconnected and further fused
to evolve into extended superstructures, which created a three-
dimensional globular network leading to gelation (Fig. 3a and b).
This class of gelation mechanism is unusual, as most previously
reported examples are based on self-aggregation of gelator mole-
cules into long, entangled fibrous networks. Similar TEM images
were observed for other gelators including {[Pd-3](OAc)2}n,
{[Pd-4](OAc)2}n and {[Pd-6](OAc)2}n (Fig. S2–S4w).
Fig.
2 TEM images (unstained) of (a) {[Pd-1](OAc)2}n, and
X-Ray diffraction experiments for {[Pd-L](OAc)2}n
proved to be difficult possibly due to their tendency to form
amorphous structures. However, the electrospray ionization
time-of-flight (ESI-TOF) mass spectrum studies gave us valuable
information. The ESI-TOF spectrum of the complex of 3 and
Pd(OAc)2 in a 1 : 1 molar ratio showed a peak at m/z 1061.7,
which corresponds to the species [Pd232(OAc)4ꢁ2H2O]
(Fig. S6w). This result suggested the formation of [2 + 2]
species of {[Pd-3](OAc)2}2. Instead, we obtained single crystals
suitable for X-ray diffraction analysis of {[Pd-3]Cl2}2 (8) by
slow diffusion of acetone into the mixture of 3 and PdCl2 in
DMSO. The crystallographic studies revealed the discrete
dinuclear metallocyclic rectangular unit and the lack of
coordination between the hydoxyl group and metal ion
(for the crystal structure, see Fig. S5w).2c,d,10 Therefore, it is
reasonable to assume that vesicles and metallogelators might
be formed by hierarchical self-assembly of discrete dinuclear
macrocyclic units. Although the specific interactions are not
(d) {[Pd-2](OAc)2}n in DMSO (1.3 wt%) on a carbon-coated copper
grid. (b) and (e) A zoomed-in image marked in (a) and (d), respectively.
Tapping-mode AFM height image of aggregates of (c) {[Pd-1](OAc)2}n,
and (f) {[Pd-2](OAc)2}n in DMSO (0.02 wt%) on a freshly cleaved
mica surface after the solvent was evaporated (scale bar: 250 nm).
(g) The corresponding histogram (Lorentzian fit) of {[Pd-1](OAc)2}n.
(h) The intensity-weighted distribution of the aggregates obtained from
the DLS measurement of the sample of {[Pd-1](OAc)2}n (1.3 wt%) in
DMSO at 25 1C.
with {[Pd-1](OAc)2}n and {[Pd-2](OAc)2}n confirmed the
formation of supermolecular aggregates with the average
diameters of 46.4 and 66.2 nm, respectively, which were in
accordance with the AFM and TEM studies. Moreover, the
assemblies showed a narrow size distribution indicative of the
formation of well-equilibrated structures (Fig. 2h and S1cw).
Interestingly, as 1 and 2 with Pd(OAc)2 did not gelate
any fluids even upon heating and with sonication, their
analogues 3–6 bearing the appended phenolic hydroxyl groups
with Pd(OAc)2 induced the formation of remarkably stable
metallogels to entrap a variety of organic solvents. When the
two hydroxyl groups of 6 were blocked by the methoxy-
methoxy group to form the ether derivative 7, it resulted in a
complete loss of the gelating ability. This observation hinted
that the hydroxyl group of 3–6 would be absolutely critical for
the formation of gels.
A DMSO solution of 3 was added to a DMSO solution of
Pd(OAc)2 at a 1 : 1 molar ratio at room temperature to
spontaneously yield a complete and homogeneous liquid gelation
after aging for 3–4 h. Further investigation demonstrated that
the gelation also proceeded well in dimethylformamide
(DMF), dimethylacetamide (DMA), N-methylpyrrolidone
(NMP), and a mixture of DMSO and organic solvent
(v/v 2 : 1) (e.g., tetrahydrofuran, 1,4-dioxane, toluene,
m-xylene, and benzene, etc.) (Tables S1 and S2w). The critical
Fig. 3 (a) TEM image (unstained) obtained from the metallogel of
{[Pd-5](OAc)2}n in DMSO (2.0 wt%). (b) A zoomed-in image marked
in (a) (scale bar: 50 nm). (c) TEM image (unstained) obtained from the
solution of {[Pd-5](OAc)2}n in DMSO (0.06 wt%).
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completely clear at this stage, we speculated that spherical
particles might be stabilized by numerous subtle noncovalent
interactions of neighboring metallocycles such as van der Waals
and p–p stacking interactions. In the case of 3–6, the extra
hydrogen-bonding interactions among the phenolic hydroxyl
groups, the OAcꢂ anions and the solvent DMSO molecules
might serve as the bridge to connect metallogelators, which leads
to fusion of spherical particles to form well-developed three-
dimensional networks. The hydrogen-bonding interactions of
the hydroxyl groups could be demonstrated by IR studies of free
ligands and their corresponding metallogels. For example, in a
dry gel of {[Pd-3](OAc)2}n, the OH-stretching band of 3 was
shifted from 3441 cmꢂ1 to 3430 cmꢂ1 (Fig. S7w).
and to apply this strategy to other metallogel systems. The
work may open a way for the design of a new generation of
vesicles from nontypical amphiphilic architectures.
This work was supported by grants from the National NSF
of China (Nos. 20702035, 20602027 and 20972102) and
PCSIRT0846. We thank the Centre of Testing & Analysis,
Sichuan University for AFM, TEM, NMR, and X-ray
measurements.
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One advantage of metallogels is the possibility to impart
collective chemical and physical properties of metal ions that
are usually observed in the crystalline and solution state to highly
processable gel-phase materials.11 The opaque metallogels
formed by 3–6 and Pd(OAc)2 were stable enough to be no
longer soluble in other solvents including acetone, dichloro-
methane, THF, hexane, toluene, methanol, and itself without
any visible collapse even over a long period of time (several
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phenyl boronic acid (eqn (1)). The gel of {[Pd-3](OAc)2}n
afforded the corresponding cross-coupling product in ca.
50% yield. In sharp contrast, both a soluble complex of
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In summary, we have described the self-assembly of the
conformationally flexible bismethylimidazolyl ligands with
Pd(OAc)2. The ligands 1–2 that do not contain the hydroxyl
group and Pd(OAc)2 self-assemble into a new type of metal–
organic vesicles. The ligands 3–6 with the hydroxyl group and
Pd(OAc)2 evolve into a three-dimensional globular network
thereby resulting in the immobilization of organic fluids after
aging. Thus, depending on whether the ligands provide the
hydrogen bonding donor, a switching of vesicles to globular
networks is achieved. The metallogel materials further show
catalytic activity for the cross-coupling of indole with phenyl
boronic acid. Efforts are now in progress to investigate the full
scope of the transformation to obtain a detailed mechanism
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