Self-assembly and (hydro)gelation triggered by cooperative π-π And unconventional C-H×××X hydrogen bonding interactions

Article abstract of DOI:10.1002/anie.201307806

Weak C-H×××X hydrogen bonds are important stabilizing forces in crystal engineering and anion recognition in solution. In contrast, their quantitative influence on the stabilization of supramolecular polymers or gels has thus far remained unexplored. Herein, we report an oligophenyleneethynylene (OPE)-based amphiphilic Pt^II complex that forms supramolecular polymeric structures in aqueous and polar media driven by π-π and different weak C-H×××X (X=Cl, O) interactions involving chlorine atoms attached to the Pt^II centers as well as oxygen atoms and polarized methylene groups belonging to the peripheral glycol chains. A collection of experimental techniques (UV/Vis, 1D and 2D NMR, DLS, AFM, SEM, and X-Ray diffraction) demonstrate that the interplay between different weak noncovalent interactions leads to the cooperative formation of self-assembled structures of high aspect ratio and gels in which the molecular arrangement is maintained in the crystalline state. Growing hand in hand! Multiple unconventional C-H×××X (X=O, Cl) hydrogen-bonding interactions, assisted by π-π interactions, are strong enough to drive the cooperative formation of supramolecular polymers and gels in polar and aqueous media. The aggregates consolidate themselves in the crystals, as shown by combined studies in solution and crystalline state. Copyright

Full text of DOI:10.1002/anie.201307806

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Communications
DOI: 10.1002/anie.201307806
Weak Interactions
Self-Assembly and (Hydro)gelation Triggered by
À
Cooperative p–p and Unconventional C H···X
Hydrogen Bonding Interactions**
Christina Rest, Marꢀa Josꢁ Mayoral, Katharina Fucke, Jennifer Schellheimer,
Vladimir Stepanenko, and Gustavo Fernꢂndez*
Angewandte
Chemie
700
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À
Abstract: Weak C H···X hydrogen bonds are important
a significant role in the stabilization of self-assembled
stabilizing forces in crystal engineering and anion recognition
in solution. In contrast, their quantitative influence on the
stabilization of supramolecular polymers or gels has thus far
remained unexplored. Herein, we report an oligophenylenee-
thynylene (OPE)-based amphiphilic PtII complex that forms
supramolecular polymeric structures in aqueous and polar
media driven by p–p and different weak C-H···X (X = Cl, O)
interactions involving chlorine atoms attached to the PtII
centers as well as oxygen atoms and polarized methylene
groups belonging to the peripheral glycol chains. A collection
of experimental techniques (UV/Vis, 1D and 2D NMR, DLS,
AFM, SEM, and X-Ray diffraction) demonstrate that the
interplay between different weak noncovalent interactions
leads to the cooperative formation of self-assembled structures
of high aspect ratio and gels in which the molecular arrange-
ment is maintained in the crystalline state.
structures.[8] With these precedents in mind, we conceived
an amphiphilic PtII complex 2 in which two hydrophobic
oligophenyleneethynylene (OPE)[9] scaffolds featuring three
hydrophilic triethylene glycol (TEG) chains each are coordi-
nated to
a
central PtII ion through pyridine rings
(Scheme 1).[10] Our system satisfies the above prerequisites,
Scheme 1. Structural formula of the OPE-based ligand (1) and PtII
complex (2).
À
T
he occurrence of multiple weak C H···X interactions
involving electronegative atoms, anions, or p-systems is
a phenomenon of high relevance in numerous natural
processes, such as membrane transport, protein–ligand inter-
actions, and drug–receptor recognition.[1] These weak hydro-
gen-bonding interactions[2] are largely recognized as impor-
tant forces in the area of crystal engineering.[3] In recent years,
as it features, besides a relatively large aromatic surface, a Cl-
PtII-Cl fragment and a large number of methylene groups
polarized by electronegative oxygen heteroatoms that can
potentially participate in weak hydrogen bonding. We thus
expected 2 to self-assemble by a combination of cooperative
À
C H···X and p–p interactions, given that the presence of
À
C H···X bonds have also been exploited as additional binding
sterically demanding Cl atoms and up to six bulky TEG chains
would hinder the stacking of the monomeric units in a parallel
fashion,[11] and consequently, the occurrence of metallophilic
Pt···Pt interactions.[12]
sites[4] or as relevant weak noncovalent interactions[5] in anion
recognition by discrete preorganized receptors. In contrast,
the use of these forces to stabilize self-assembled structures in
solution or gel-phase materials has remained unexplored to
date.
The amphiphilic PtII complex 2 can be readily obtained in
69% yield by a complexation reaction between pyridine-
based ligand 1 and [PtCl2(PhCN)2] in benzene at 878C
(Scheme 1, for characterization details see the Supporting
Information).[13]
Herein, we show for the first time that cooperative p–p
and multiple unconventional C H···X hydrogen-bonding
À
interactions are strong enough to induce the supramolecular
polymerization and (hydro)gelation of an amphiphilic PtII
system.
The design of this complex was originally motivated by
the fact that halogens, when bound to metal ions, turn into
sufficiently strong hydrogen-bond acceptors[6] that their
The absorption spectrum of 1 exhibits a maximum at
approximately 332 nm in a wide range of solvents of different
polarity (Figure S1 in the Supporting Information). In water,
however, a small red-shifted shoulder at approximately
380 nm becomes visible (Figure 1a and Figure S1). This
observation, along with the appearance of a low-intensity
red-shifted maximum in emission studies (Figure S2), suggest
stacking of the aromatic OPE units.[14] Temperature-depen-
dent UV/Vis studies (3.5 ꢀ 10À5 m, 279–327 K, water) show
a simultaneous bathochromic shift of the absorption max-
imum from 326 to 335 nm and appearance of a shoulder at
380 nm upon decreasing temperature (Figure 1a). The cool-
ing curve obtained by monitoring the spectral changes
extracted from temperature-dependent experiments at
300 nm is sigmoidal and indicates that 1 self-assembles in
a non-cooperative fashion in water (Figure 1a, inset), afford-
ing an association constant of 4.8 ꢀ 104 mÀ1 (Figure S3 and
Table S1).[15] The absence of additional intermolecular inter-
actions other than p-stacking appears to be responsible for
this non-cooperative behavior that ultimately results in the
formation of aggregates of relatively low size (ca. 140 nm), as
extracted from dynamic light scattering (DLS) experiments
(Figure S4). Transmission electron microscopy (TEM) studies
reveal the formation of worm-like aggregates with a regular
width (1.8 Æ 0.2 nm) and lengths of several tens of nanometers
À
interaction with polarized C H hydrogen-bonding donors
becomes favorable.[7] Recent studies from our group highlight
that chlorine atoms coordinated to PdII ions can also play
[*] C. Rest, Dr. M. J. Mayoral, J. Schellheimer, Dr. V. Stepanenko,
Dr. G. Fernꢀndez
Institut fꢁr Organische Chemie and Center for Nanosystems
Chemistry, Universitꢂt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
E-mail: gustavo.fernandez@uni-wuerzburg.de
Homepage: http://www-organik.chemie.uni-wuerzburg.de/lehr-
stuehlearbeitskreise/fernandez
Dr. K. Fucke
Institut fꢁr Anorganische Chemie, Universitꢂt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
[**] We thank Prof. Frank Wꢁrthner for many helpful discussions and his
support, Ana Reviejo for graphic design, and the Alexander von
Humboldt Foundation for financial support (Sofja Kovalevskaja
Program).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201307806.
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a certain concentration, stable yellow gels are formed in
various alcohols and water (see Supporting Information),
most likely driven by strong solvophobic interactions and an
efficient solvation of the polar TEG chains of 2 in these
media.
Figure 2b,c show the atomic force microscopy (AFM)
images of diluted gel solutions of 2 in ethanol (4.9 ꢀ 10À3 m)
and water (2.2 ꢀ 10À3 m) onto mica. For both gels, the images
reveal a network consisting of long flexible nanofibers with
a consistent diameter of 5.8 Æ 0.4 nm and lengths of up to
500 nm (Figure 2b,c, Figures S7 and S8). Comparable net-
works of aggregates are also visualized by SEM studies
(Figures S9 and S10). The fact that in alcohols and water both
the morphology of the gel nanofibers and UV/Vis spectra of
the gel materials are very similar suggests an analogous
arrangement of the monomeric units of 2 in both media.
Figure 1. a) Temperature-dependent UV/Vis experiments of
1 (3.5ꢃ10À5 m, 279–327 K) in water. Arrows indicate the spectral
changes upon decreasing temperature. Inset: Fraction of aggregated
species (aagg) against T and fit of the spectral changes at 300 nm to
the isodesmic model. b) TEM micrograph of 1 onto a carbon-coated
copper grid.
that subsequently entangle into a porous network (Figure 1b
and Figure S5). These are most likely formed by the p-
stacking of the OPE units with a small rotation angle to avoid
steric repulsions between the bulky TEG chains.
Coordination of the pyridine-based ligand 1 to a Cl-PtII-Cl
fragment leads to notable changes in the optical and
supramolecular properties. The self-assembly of amphiphilic
PtII complex 2 can be readily monitored by the naked eye, as
the transition from monomeric to aggregated species is
accompanied by a color change from pale to deep yellow
(Figure 2a). UV/Vis studies show the presence of a transition
centered at approximately 350–354 nm in most organic
solvents even at millimolar concentration, characteristic of
molecularly dissolved species (Figure S6). However, the color
of the solutions of 2 in alcohols and water turn deep yellow
and a red-shift of the absorption maximum to approximately
370 nm takes place, which can be attributed to the formation
of self-assembled species (Figure 2a and Figure S6). Above
Figure 3. a) Temperature-dependent UV/Vis experiments of 2 (MeOH/
H2O (70:30), 1.28ꢃ10À4 m). Arrows indicate the spectral changes upon
decreasing temperature. b) Cooling curves obtained by monitoring the
absorption of 2 in MeOH/H2O (70:30) at 420 nm at four different
concentrations. Colored lines represent the fits to the ten Eikelder-
Markvoort-Meijer cooperative model.
Figure 2. a) Photograph illustrating the color change associated to the
monomer-aggregate/gel transition of 2 in different solvents at 9 mm.
AFM images obtained by drop-casting diluted gel solutions of 2 in
water (b) and ethanol (c) onto mica.
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Temperature-dependent UV/Vis experiments in water at
1.02 ꢀ 10À5 m show that the transition at 370 nm undergoes
negligible changes upon increasing temperature, indicating
that the molecules of 2 are strongly bound in this medium
(Figure S11a). The extension of the aromatic surface upon
metal coordination as well as the involvement of the Cl-PtII-Cl
fragment in intermolecular interactions are most likely
responsible for the enhanced degree of aggregation of 2 in
water compared to that of ligand 1. However, the monomer–
aggregate equilibrium can be fully tracked by addition of
a suitable amount of methanol to an aqueous solution of 2
(see Figure S11).[16] Figure 3a shows the temperature-depen-
dent UV/Vis experiments of 2 in methanol/water (70:30) at
1.28 ꢀ 10À4 m. Above 313 K, an absorption at 353 nm corre-
sponding to molecularly dissolved species can be observed.
Upon decreasing temperature, depletion of this transition is
accompanied by the emergence of a red-shifted band at
370 nm that is assigned to self-assembled species (Figure 3a).
The cooling curves (l = 420 nm; cooling rate = 0.2 KminÀ1)
extracted from these experiments at four different concen-
trations are clearly non-sigmoidal and reminiscent of a phase
transition (Figure 3b), which is a distinctive feature of
cooperative supramolecular polymerization processes.[17]
Analysis of the curves through a recently developed nucle-
ation–elongation model[18] afforded excellent fits (Figure 3b
and Figure S12). The self-assembly of 2 can be thus divided
into a thermodynamically unfavorable dimerization process
and a subsequent highly favorable elongation step. The
enthalpy of elongation (DHe) was calculated to be approx-
imately À120 kJmolÀ1, the
(Figure S15), indicating that both aromatic and TEG units are
strongly involved in the formation of self-assembled struc-
tures. Rotating-frame Overhauser Effect Spectroscopy
(ROESY) NMR experiments show the absence of cross-
peaks in CD2Cl2, a solvent in which 2 exists in a monomeric
form (Figure S16). In contrast, multiple through-space cou-
pling signals between protons which are in spatial proximity
are in fact observed in CD3OD (Figure S17).[19] Upon closer
inspection, we noticed clear cross-peaks between protons
(Hd) belonging to the outer phenylene rings and protons Ha
and Hb of the pyridine units (Figure 4a). According to this
coupling pattern, the only possible molecular arrangement is
that in which one Cl-PtII-Cl fragment lies on top of the outer
phenylene ring of the molecule immediately below in the
stack, most likely stabilized by p–p and Cl···p interactions, as
illustrated in Figure 4c. This structural proposition is clearly
supported by all intramolecular (Hc···Hd and Hb···Hc) and
intermolecular (Ha···Hc and Hb···Hc) cross-peaks observed in
the ROESY spectrum (highlighted in red and green, respec-
tively, in Figure 4a). Additionally, the appearance of coupling
signals between most of the protons of the TEG chains (He–k
)
and protons Ha–d (Figure 4b) suggests that the TEG chains of
one monomer should be wrapped around the aromatic core of
a neighboring unit of 2.
By slow evaporation of an aggregate solution of 2 in
methanol, yellow single crystals suitable for X-ray diffraction
could be obtained. The resulting crystal structure could be
ꢀ
solved and refined in the triclinic space group P1 with half
a molecule in the asymmetric unit (Table S5 and Fig-
dimerization (K2) and elonga-
tion (K) constants are 63.8 and
approximately
8 ꢀ 103 mÀ1
respectively, and the coopera-
tivity factor (s), described as
s = K2/K,
is
7.9 ꢀ 10À3
(Table S3).
To shed some light on the
driving force of this coopera-
tive self-assembly process we
have performed 1D and 2D
NMR spectroscopic studies. As
the 1H NMR spectra of 2 in
D2O is severely broadened as
a result of a strong aggregation
(Figure S14), we have chosen
as a solvent CD3OD. In this
medium, an aggregation pro-
cess similar to that in water
takes place (see Figure S6c)
however, the NMR signals are
now sufficiently sharp. Con-
centration-dependent
1H NMR spectroscopy experi-
ments (CD3OD, 400 MHz, 0.4–
13 mm, 298 K) show a simulta-
neous shielding and broaden-
ing of all resonance signals
upon increasing concentration
Figure 4. a,b) Selected areas of the ROESY NMR spectrum (CD3OD, 600 MHz, 14.1 mm, 293 K) of 2. The
red and green circles highlight intra- and intermolecular through-space coupling signals, respectively.
c) Representation of the molecular organization of 2 to form self-assembled structures on the basis of
ROESY studies.
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À
À
Figure 5. a) Packing of 2 driven by C H···Cl (green) and C H···O (red) hydrogen-bonding interactions. b) Overall packing of the crystal structure
seen along the OPE units. OPEs (blue) and TEG chains (red). C gray, O red, N blue, Pt dark blue, Cl green, H white. Displacement ellipsoids are
set at 50% probability, non-hydrogen-bonding hydrogen atoms in (a) and all hydrogen atoms in (b) are omitted for clarity. c) Hirshfeld surface
fingerprint plot of 2. Colors describe the frequency of a specific distance with blue being less frequent and light green being most common.
ure S20).[20] The presence of bulky Cl-PtII-Cl and TEG
fragments forces the OPE units to pack in a slipped fashion
through translationally stacked CH···p interactions (Fig-
Slightly hidden is Region 2, which represents C H···C and
À
À
C H···p short contacts. These interactions are considerably
À
longer than the C H···O hydrogen bonds and will thus not be
as strong, but with 22.3% they present a large portion of the
surface as well. With only 4.5% of the surface, C H···Cl
interactions (Region 3) are slightly less important for the
overall stability of the crystal packing, even though they are of
À
ure 5a,b). This arrangement is supported by C H···Cl and
À
À
C H···O hydrogen bonding interactions (Table S6). Each
À
chlorine atom is involved in up to four C H···Cl interactions
with O-CH2 groups belonging to the TEG chains of four
different molecules, facilitating the growth of the structure in
one dimension (see Figure 5a, green dotted lines).
À
comparable length than the C H···C interactions. However,
these forces appear to be more relevant in solution and the gel
state, as PtII complex 2 forms gels cooperatively whereas
pyridine-based OPE precursor 1, which lacks the Cl-PtII-Cl
fragment, only forms small aggregates in a non-cooperative
fashion. Finally, Region 4 shows that the crystal packing has
relatively large voids that can be attributed to the dimensions
and high degree of flexibility of the monomeric units.
To summarize, we have shown for the first time that
cooperative p–p and multiple unconventional CH···X (X = O,
Cl) hydrogen-bonding interactions are the driving force for
supramolecular polymerization and (hydro)gelation process-
es.[23] Complementary UV/Vis, NMR spectroscopy, and X-ray
studies demonstrate that the molecular arrangement of
amphiphilic PtII complex 2 in solution bears close resem-
blance to that in the crystalline state. Understanding the
relationship between molecular packing in crystals and
aggregates is one of the biggest challenges that chemistry
faces today.[24] Our results make clear that CH···X hydrogen
bonding can also play an important role, not only in crystal
À
A further growth is driven by multiple C H···O inter-
actions between three of the TEG chains of one molecule and
another three TEG chains of a neighboring unit in an exotic
À
“handshake-like” fashion, in part involving aromatic C H
groups (Figure 5a, red dotted lines). This crystalline packing
perfectly matches the slipped molecular arrangement of 2 in
solution and also explains all cross-peaks between most of the
protons of the TEG chains (He–k) and protons Ha–d (see
Tables S7–10) observed in ROESY studies. To visualize the
interactions of this molecule, the Hirshfeld surface (Fig-
ure S21)[21] has been calculated and the corresponding finger-
print plot is shown in Figure 5c (see also Figure S22). Four
regions show interesting features: Region 1 corresponds to
À
C H···O hydrogen bonds, which make a total of 16.6% of the
surface. This percentage is relatively high for a single type of
interaction and clarifies its importance in the self-assembly of
2. These interactions are also the shortest contacts and can be
assumed as the strongest interactions (see Table S6).[22]
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Received: September 5, 2013
Revised: October 14, 2013
Published online: December 18, 2013
Keywords: amphiphiles · cooperativity ·
.
supramolecular polymerization · X–H hydrogen bonds ·
p-conjugated systems
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a sharp 1H NMR spectrum is observed. However, during the
time required for ROESY studies (30–180 min) the solution
becomes viscous because of the strong tendency of 2 to form gels
and ultimately broad NMR signals are obtained (Figure S14a).
To overcome this, 2 (18 mg) was dissolved in a minimum amount
(0.05 mL) of CD2Cl2, followed by removal of the solvent until
a dry film is formed and subsequent redissolution of the solid in
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