A discrete amphiphilic organoplatinum(II) metallacycle with tunable lower critical solution temperature behavior

Article abstract of DOI:10.1021/ja5093503

Oligo(ethylene glycol) (OEG)-decorated supramolecular assemblies are distinguished by their neutral character and macroscopic temperature-sensitive phase transition behavior. OEG functionalization is an emerging strategy to obtain thermoresponsive macrocyclic amphiphiles, although known methods organize the hydrophilic and hydrophobic segments by covalent bonding. Coordination-driven self-assembly offers an alternative route for organizing OEG-functionalized precursors into nanoscopic architectures, resulting in well-defined metallacycle cores surrounded by hydrophilic scaffolds to impart overall amphiphilic character. Here a tri(ethylene glycol)-functionalized thermosensitive amphiphilic metallacycle was prepared with high efficiency by means of the directional-bonding approach. The ensembles thus formed showed good lower critical solution temperature behavior with a highly sensitive phase separation and excellent reversibility. Moreover, the clouding point decreased with increasing metallacycle concentration and addition of K⁺.

Full text of DOI:10.1021/ja5093503

Communication
pubs.acs.org/JACS
A Discrete Amphiphilic Organoplatinum(II) Metallacycle with
Tunable Lower Critical Solution Temperature Behavior
Peifa Wei,† Timothy R. Cook,‡,∥ Xuzhou Yan,† Feihe Huang,*,† and Peter J. Stang*,‡
†State Key Laboratory of Chemical Engineering, Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China
‡Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States
∥Department of Chemistry, University at Buffalo, 359 Natural Sciences Complex, Buffalo, New York 14260, United States
S
* Supporting Information
synthesize topologically controllable and diverse smart materials
with fine-tuned thermoresponsiveness to external stimuli.10
A
ABSTRACT: Oligo(ethylene glycol) (OEG)-decorated
supramolecular assemblies are distinguished by their
neutral character and macroscopic temperature-sensitive
phase transition behavior. OEG functionalization is an
emerging strategy to obtain thermoresponsive macrocyclic
amphiphiles, although known methods organize the
hydrophilic and hydrophobic segments by covalent
bonding. Coordination-driven self-assembly offers an
alternative route for organizing OEG-functionalized
precursors into nanoscopic architectures, resulting in
well-defined metallacycle cores surrounded by hydrophilic
scaffolds to impart overall amphiphilic character. Here a
tri(ethylene glycol)-functionalized thermosensitive amphi-
philic metallacycle was prepared with high efficiency by
means of the directional-bonding approach. The ensembles
thus formed showed good lower critical solution temper-
ature behavior with a highly sensitive phase separation and
excellent reversibility. Moreover, the clouding point
decreased with increasing metallacycle concentration and
addition of K+.
handful of thermoresponsive MCAs with covalently connected
OEGs have been reported.11 However, tedious multistep organic
synthesis and purification processes are hallmarks of this
approach. Supramolecular self-assembly provides an alternative
way to realize thermal sensitivity in discrete architectures. The
spontaneous formation of metal−ligand bonds is a valuable
methodology to construct supramolecular coordination com-
plexes.12 This design strategy is particularly relevant for
amphiphilic materials, as the core and periphery of the precursors
and final metallacycles can be carefully controlled. The
orientation and extent of functionalization can be tuned, along
with the shape and dimensions of the internal cavity, providing
facile routes toward amphiphilic metallacycles13 with thermo-
responsiveness. Here the thermoresponsiveness afforded by
OEG functionalities is preserved on a neutral hexagonal
metallacycle core, resulting in an amphiphilic metallacycle with
a LCST that is responsive to both metallacycle concentration and
addition of K+.
Our group reported a class of amphiphilic rhomboids
comprising neutral metallacycle cores decorated with hydrophilic
moieties, obtained via Pt−carboxylate bond formation.13b Those
systems demonstrated hierarchical self-assembly of dimension-
ally controllable nanostructures. Although that work pioneered a
new route to synthesize amphiphilic metallacycles, function-
alization of these amphiphilic metallacycles has not yet been
reported. Our current efforts utilize the charge-neutral tri-
(ethylene glycol) (Tg) as a pendant group to impart temperature
sensitivity into amphiphilic metallacycles.
The metallacycles were obtained via self-assembly reactions
following the formation of a Tg chain-functionalized 120°
dicarboxylate ligand, 2. When this donor was mixed with a 120°
organoplatinum(II) acceptor, 3, in a 1:1 ratio in D2O/acetone-d6
at 50 °C for 8 h, a [3+3] self-assembly occurred to give
amphiphilic metallacycle 1, containing hydrophilic Tg chains
around the hydrophobic core (Scheme 1a). The precursors 2 and
2a were used as model compounds to evaluate how a cyclic
structure affects the LCST of 1 relative to that of the free ligands.
1H and 31P NMR analyses of the product supported the
formation of hexagonal, three-fold-symmetric species. The
31P{1H} NMR spectrum of 1 possessed a sharp singlet at
he temperature-induced helix/coil transition of poly-
T
peptides is one of many examples of natural biomacro-
molecules undergoing dramatic conformational changes that
result in new physiochemical properties.1 These thermo-
responsive natural materials motivate the design and develop-
ment of artificial analogues, prompting both fundamental and
applied research efforts.2 These materials are soluble in solution
below a certain temperature, known as the lower critical solution
temperature (LCST). Above the LCST, increased aggregation
results in turbidity. Materials that exhibit LCSTs are useful for
molecular separation,3 smart surfaces,4 drug delivery,5 and
sensors.6 These applications prompted the design of polymeric
materials such as poly(N-isopropylacrylamide), which displays a
LCST of ∼32 °C in water.7 Non-polymeric LCST materials, such
as those based on simple discrete organic molecules or
macrocycles, are comparatively rare.8
Macrocyclic amphiphiles (MCAs), a class of fascinating
macrocyclic molecules containing both hydrophilic and hydro-
phobic segments, can spontaneously aggregate to form multi-
dimensional and hierarchical self-assemblies, driven by micro-
phase separation of the hydrophilic and hydrophobic groups into
ordered periodic structures, due to their mutual repulsion.9
Oligo(ethylene glycol)s (OEGs) have been widely used to
Received: September 10, 2014
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Scheme 1. (a) Self-Assembly of 2 and 3 To Give an
Amphiphilic Discrete Organoplatinum(II) Metallacycle and
(b) Cartoon Illustration of Its Thermosensitivity and
Potassium Cation Responsiveness
Figure 1. (a,b) 31P NMR and (c−e) partial 1H NMR spectra (CD2Cl2,
293 K) of free building blocks 2 (e) and 3 (a,c), and hexagonal
metallacycle 1 (b,d).
∼16.30 ppm with concomitant 195Pt satellites (JPt−P = 2310.5
Hz), consistent with a single phosphorus environment (Figure
1b). This peak shifted upfield relative to that of acceptor 3 by
∼2.94 ppm. Moreover, in the 1H NMR spectrum of 1, protons
H2, H3, and H4 on 1 showed downfield shifts compared with
those of 2 (Figure 1d,e), in accord with coordination of the O-
atom to the Pt centers. Electrospray ionization time-of-flight
mass spectrometry (ESI-TOF-MS) provided further evidence for
the formation of 1. Three peaks were found to support the
assignment of a [3+3] assembly (Figure 2), including those
corresponding to an intact hexagonal core with charge states
arising from the loss of counterions (m/z = 1458.40 for [M +
2Na + H + K + NH4]5+ (Figure 2a), m/z = 1462.00 for [M + 2Na
+ H + K + NH4 + H2O]5+ (Figure 2b), and m/z = 1817.45 for [M
+ 2Na + 2NH4]4+ (Figure 2c). The Na+ and K+ ions were
introduced as trace impurities either from the glass vessels used
or from the addition of NH4Cl to solutions of 1 to facilitate
ionization. All the peaks were isotopically resolved and agreed
very well with their calculated theoretical distributions.
With the amphiphilic metallacycle in hand, we investigated its
thermoresponsive behavior. The long Tg chains of 2, 2a, and 1
rendered them fully soluble in aqueous solution at 25.0 °C (1.00
mM). Clear aqueous solutions of 1 turned turbid upon mild
heating and returned to clear upon cooling. This reversible
transformation was the first evidence that 1 exhibited LCST
behavior in an aqueous solution. For model compound 2a, an
aqueous solution at 1.00 mM became opaque until the
temperature exceeded 80.0 °C, indicative of a LCST that is too
high for practical applications. Model compound 2 showed no
LCST behavior, attributed to its high solubility in water
precluding aggregation. These control experiments reveal the
importance of the metallacycle core in achieving reversible LCST
behavior. The combination of the hydrophilic OEGs and
hydrophobic hexagonal scaffolds is necessary to impart the
amphiphilic behavior that is the foundation for the thermal
responsiveness.
Figure 2. Experimental (red) and calculated (blue) ESI-TOF-MS
spectra of 1: (a) [M + 2Na + H + K + NH4]5+, (b) [M + 2Na + H + K +
NH4 + H2O]5+, and (c) [M + 2Na + 2NH4]4+.
It is well known that the thermal sensitivity of OEGs results
from reversible disruption of H-bonds between OEG chains and
surrounding water molecules.10 Below the clouding point
(Tcloud), H-bonding interactions between the Tg chains of 1
and exogenous water molecules dominate, whereas the Tg chains
become non-polar and intra-/intermolecular H-bonding inter-
actions increase above Tcloud. Thus, at high temperatures, the
surface of 1 is hydrophobic, promoting aggregation of the Tg
chains ultimately into micellar structures that induce light
scattering. This scattering provides a means to evaluate the Tcloud
of 1, namely by investigating the change in transmittance at 700
nm with a temperature-controlled UV/vis spectrometer. The
optical transmittance at 700 nm was used for Tcloud measure-
ments because the transmittance change at this wavelength was
clearly noticeable. Figure 3a shows the temperature dependence
of the light transmittance of aqueous solutions of 1. In all cases
the transmittance decreased drastically in response to a minor
change in temperature around Tcloud, indicating a highly sensitive
phase separation. We also investigated the reversibility of this
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Figure 3. (a) Transmittance as a function of temperature for aqueous
solutions of 1 with different concentrations. Heating rate = 0.200 K
min−1. (b) Concentration dependence of the clouding point of 1 in
aqueous solution. c = 3.00 mM.
Figure 5. (a) Transmittance as a function of temperature for different
molar ratios of 1 and K+ at a heating rate of 0.200 K min−1. (b)
Dependence of the clouding point on the molar ratio of 1 and K+ in
aqueous solution. c = 2.00 mM.
LCST system. Upon cooling to 25.0 °C, the cloudy 3.00 mM
solution became transparent, exhibiting a dehydration/hydration
transition. The solution of 1 showed no signs of fatigue under
modest heating/cooling cycling, demonstrating that this process
was reversible (Figure 4a). Figure 3b shows that a stepwise
extended length of 5.62 nm (Figure S10). Transmission electron
microscopy (TEM) was used to reveal changes in the
microstructures induced by the LCST behavior (Figure S11).
Spherical micellar structures of 1 were observed at 0.500 mM.
The diameters were ∼7.00 nm below 29.0 °C. When the solution
was heated to 30.0 °C, the nanoparticles increased to ∼100 nm
on average. This aggregation process is consistent with Tcloud
=
29.0 °C, matching the value determined by absorption
techniques (Figure 3b). The TEM results indicate that solutions
were populated with monomers below the LCST, as the mean
diameter of the aggregates was on par with the size of a discrete
hexagon. This monodispersion of 1 is reasonable because its
hydrophilicity is too high compared with its hydro-
phobicity.11a,12e The aggregation process of 1 in aqueous
solution was further evidenced by dynamic light scattering. At
23.0−29.2 °C, the average hydrodynamic diameter (DH) was
∼9.00 nm (Figure 4b). When the temperature was raised above
T
cloud (29.5 °C), DH continually increased, with a dramatic rise
between 30.0 and 34.0 °C. The temperature dependence of DH
was in good agreement with the results of the transmittance
measurement and the TEM investigation.
As indicated above, the LCST behavior of 1 derived from
competition between two kinds of interactions: H-bonding
between the surrounding water molecules and 1, and intra- and
intermolecular H-bonding interactions of 1. Since both effects
rely on interactions of the OEG chains, introduction of K+ was
expected to disrupt these phenomena by binding to the Tg
groups.14 Preliminary investigations involved addition of an
Figure 4. (a) Change in transmittance of 2.00 mM 1 when cycling
between 25.0 and 27.0 °C. (b) Dependence of hydrodynamic diameter
of aggregates in 0.500 mM solution of 1 on temperature. Insets:
Photographs of the solutions below and above Tcloud
.
1
equimolar amount of KPF6 to a D2O solution of 1. In the H
NMR spectrum, the proton signals of the benzyl (H4) and glycol
chains on 1 all showed slightly downfield shifts, indicating an
interaction between K+ and Tg chains (Figure S12). To get
detailed information about the dependence of Tcloud on the molar
ratio of 1 and K+, we investigated the aqueous solution properties
by the cloud-point method. When the molar ratio of K+ to 1 was
changed from 0 to 1.1, Tcloud decreased from ∼26.5 to ∼24.5 °C
(Figure 5a). Tcloud further decreased by another 1.00 °C when we
adjusted the molar ratio to 1.2. Figure 5b shows that, with
increasing molar ratio of K+ to 1, Tcloud decreased correspond-
ingly. A possible explanation for this phenomenon is that, with
decrease in [1] from 3.00 to 0.500 mM leads to a sharp increase
of Tcloud, indicating that the LCST behavior depends on the
concentration of the metallacycle. This trend is consistent with
that observed for other molecules that show LCST behavior.10a
The first step in determining the sizes of the supramolecular
aggregates as the temperature was changed was to establish the
monomeric metallacycle dimensions. A simulated molecular
model of 1, optimized with the Molecular Mechanics Universal
Force Field, indicated a planar hexagonal framework with
exohedral functionalization of the pendent Tg units and an
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Journal of the American Chemical Society
Communication
the introduction of additional K+, the complexation percentages
of K+ and Tg chains within a given metallacycle might increase
accordingly. This increase could influence H-bonding between
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In conclusion, a tri(ethylene glycol)-functionalized thermo-
sensitive amphiphilic metallacycle 1 was prepared with high
efficiency by means of the directional-bonding approach. It shows
good LCST behavior, including both highly sensitive phase
separation and excellent thermal reversibility. A stepwise
decrease in [1] led to a sharp increase in Tcloud. Moreover, the
turbidity temperature decreased with the introduction of K+. As
such, the Tcloud of 1 was tunable by two methods, changing either
the concentration of 1 or the molar ratio of K+ to the amphiphilic
metallacycle. Given the improved optical, magnetic, and
electronic properties of supramolecular coordination com-
plexes,12,13 diverse aggregates produced by macrocyclic amphi-
philes,9c,d and the favorable properties of emerging thermo-
responsive materials,2−6 we expect that the fundamental results
presented here will pave the way to construct novel multi-
functional stimuli-responsive materials.
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AUTHOR INFORMATION
■
Corresponding Authors
fhuang@zju.edu.cn
stang@chem.utah.edu
Schluter, A. D. Chem. Commun. 2008, 5948−5950.
̈
Notes
(11) (a) Jun, Y. J.; Toti, U. S.; Kim, H. Y.; Yu, J. Y.; Jeong, B.; Jun, M. J.;
Sohn, Y. S. Angew. Chem., Int. Ed. 2006, 45, 6173−6176. (b) Zhang, X.;
Wang, C. Chem. Soc. Rev. 2011, 40, 94−101. (c) Li, L.; Che, Y.; Gross, D.
E.; Huang, H.; Moore, J. S.; Zang, L. ACS Macro Lett. 2012, 1, 1335−
1338. (d) Ogoshi, T.; Shiga, R.; Yamagishi, T.-a. J. Am. Chem. Soc. 2012,
134, 4577−4580. (e) Ogoshi, T.; Kida, K.; Yamagishi, T. -a. J. Am. Chem.
Soc. 2012, 134, 20146−20150.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
F.H. thanks the National Basic Research Program
(2013CB834502), the NSFC/China (21125417, 21434005),
the State Key Laboratory of Chemical Engineering, and the
Fundamental Research Funds for the Central Universities for
financial support. P.J.S. thanks the NSF (1212799) for financial
support. Dr. Xiaoji Cao from Research Center of Analysis and
Measurement of Zhejiang University of Technology is thanked
for her assistance with ESI-TOF-MS measurements.
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