Controlling Structure beyond the Initial Coordination Sphere: Complexation-Induced Reversed Micelle Formation in Calix[4]pyrrole-Containing Diblock Copolymers

Article abstract of DOI:10.1021/jacs.8b09620

A diblock copolymer containing a strapped calix[4]pyrrole-based ion pair recognition subunit has been synthesized via RAFT polymerization. As prepared, the polymer is hydrophobic and devoid of any particular morphological form. However, upon ion pair comp

Full text of DOI:10.1021/jacs.8b09620

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Communication
Controlling Structure Beyond the Initial Coordination
Sphere: Complexation-Induced Reversed Micelle Formation
in Calix[4]pyrrole-containing Diblock Copolymers
Xiaodong Chi, Gretchen Peters, Chandler Brockman, Vincent Lynch, and Jonathan L. Sessler
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09620 • Publication Date (Web): 08 Oct 2018
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Controlling Structure Beyond the Initial Coordination Sphere:
Complexation-Induced Reversed Micelle Formation in
Calix[4]pyrrole-containing Diblock Copolymers
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Xiaodong Chi,a Gretchen M. Peters,a Chandler Brockmana, Vincent M. Lynch,a and Jonathan L.
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Sessler*a,b
aDepartment of Chemistry, The University of Texas at Austin, 105 E. 24th Street-A5300, Austin, Texas 78712-1224, USA.
bCenter for Supramolecular Chemistry and Catalysis, Shanghai University, Shanghai 200444, China.
Supporting Information Placeholder
ABSTRACT: A diblock copolymer containing a strapped
calix[4]pyrrole-based ion pair recognition subunit has been
synthesized via RAFT polymerization. As prepared, the polymer
is hydrophobic and devoid of any particular morphological form.
However, upon ion pair complexation, the copolymer self-
assembles to generate reverse micelles in organic media. The
reverse micelle formed in this way may be used to extract alkali
cation and cesium halide anion salts from an aqueous source into
an organic receiving phase. The polymer proved more effective as
an extractant than the corresponding free ion pair receptor.
aryl-triazole-based anion receptors to extract chloride anion salts.9
It is our view that such focused recognition and extraction tasks
would benefit further from the use of ion pair receptors
incorporated into the polymer.
Relative to simple ion (anion or cation) receptors, ion pair
receptors generally display enhanced affinities for the ions in
question as the result of, e.g., allosteric effects and enhanced
electrostatic interactions between the co-bound ions.10,11 To date,
ion pair receptors have seen use in such diverse areas as salt
solublization,12 ion extraction,13 and through-membrane
transport.14 We were thus keen to test whether an appropriately
designed diblock copolymer containing an ion pair receptor would
prove effective as an ion pair extractant. Our thinking, shown in
Scheme 1, was that ion pair recognition would favor micelle
formation thus aiding the extraction process.
Controlling structure beyond the first coordination sphere
through complexation is a current challenge in supramolecular
chemistry.1 It is being tackled elegantly through use of
mechanically interlinked molecular systems,2 and is becoming
increasingly appreciated in the context of catalysis.3 Long-range,
complexation-based changes in order also provide the basis for
certain stimulus-responsive polymers and self-assembled
ensembles.4 Remote structural effects are particularly important
in the area of liquid-liquid extraction where recent work has
served to highlight the role of higher order organized structures,
such as micelles and reversed micelles, in determining the
efficacy of a given molecular extractant.5 However, actually
controlling these effects has proved challenging.
Such considerations led to the design of 1, whose synthesis is
shown in Scheme 2. First, the functionalized strapped-
calix[4]pyrrole monomer, SC4Py was prepared via precursor M1.
This latter species was extended to produce SC4Py. Since
monomer SC4Py contains a pyrrole ring, it was deemed
incompatible with most living polymerization strategies, such as
atom transfer radical polymerization (ATRP).15 Thus, it was
subject to RAFT polymerization. First, the hydrophobic block
PMMA was synthesized using S-1-dodecyl-s′-(α,α′-dimethyl-α′′-
acetic acid) trithio-carbonate (DDMAT) as a chain transfer agent
(CTA). RAFT polymerization with SC4Py gave the PMMA-b-
PSC4Py block copolymer 1 (Scheme 2).
Recently, we showed that a free-standing calix[4]pyrrole-based
ion pair receptor could be used to extract specific ion pairs from
an aqueous source phase by means of ion pair complexation-
induced micellization.5b Here, we report here a new diblock
Scheme 1. Diblock Copolymer 1 and Illustration of the Self-
Assembly Processes giving Multi-aggregated Reversed Micelles.
copolymer,
poly(methyl
methacrylate)-block-poly(SC4Py)
(PMMA-b-PSC4Py; 1) that contains both a crown ether-like
cation recognition motif and a calix[4]pyrrole anion binding
subunit. As prepared, diblock copolymer 1 is hydrophobic and
lacking any well-defined structure. However, upon complexation
with an appropriately chosen ion pair in dichloromethane solution
or at an organic-water interface it becomes amphiphilic and self-
assembles into multi-aggregated reversed micelles. Compared to
the free-standing ion pair receptor on which it is based, namely
the crown ether-functionalized calix[4]pyrrole M1, copolymer 1
is much more effective as an extractant for a range of test alkali
cation and cesium halide anion salts.
In recent years, increasing effort has been devoted to the
preparation of polymeric extractants.6 For example, our group
created copolymers containing calix[4]pyrroles and showed they
could extract certain halide anion salts.7 Piatek and co-workers
used a polymer bearing heteroditopic side chain receptors to
extract NaNO3.8 Flood and co-workers used polymers contained
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Scheme 2. Synthetic route leading to a strapped calix[4]pyrrole-
bearing diblock copolymer, PMMA-b-PSC4Py (1).
Further addition of CsF beyond one stoichiometric equiv
induces gradual downfield shifts in the aromatic proton signals of
the benzene subunit, particularly H1 (Figure S17). These 1H NMR
spectral changes are ascribed to the complexation of a second
cesium cation by the crown ether subunit. The addition of ≥1.0
equiv of CsF to M1 also leads to an upfield shift in the NH proton
signals (Figure S17). Such upfield shifts are consistent with the
presence of interactions between a cesium cation complexed by
the crown ether moiety and the bound fluoride anion.
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The ion pair complexation seen in the case of the model system
M1 led us to consider whether diblock copolymer 1 could be used
to promote the formation of supramolecular aggregates via
presumed ion pair complexation. As prepared, copolymer 1
displays no amphiphile-like features (Figures S19 and S22).
However, when treated with CsF in CH2Cl2, the polymer becomes
amphiphlic, as evidenced by dynamic light scattering (DLS),
transmission electron microscopy (TEM), and energy dispersive
X-ray analyses (EDX) studies. This change in character is
ascribed to recognition of the CsF by the strapped calix[4]pyrrole
subunits, rendering those portions of the diblock copolymer polar
and the overall system amphiphilic (cf Scheme 1).
Prior to studying the effects of salts on the morphology of
polymer 1, its expected ion pair recognition features were studied
using precursor M1 as a model host system. Initial evidence that
compound M1 could act as an ion pair receptor for cesium
fluoride (CsF) came from a single-crystal X-ray diffraction
analysis of the CsF complex. Crystals were obtained via the slow
evaporation of a CHCl3 / CH3OH solution in the presence of CsF.
Figure 2. (a) TEM images of the aggregates formed by treating diblock
copolymer 1 in dichloromethane with CsF (b) DLS results corresponding to
(a). The concentration of the copolymer is 2 mg/mL and that of CsF is 1 mM.
Dynamic light scattering (DLS) analyses were then carried out
so as to probe the relationship between the presumed ion pair
complexation and the observed self-assembly. At low CsF-to-
calix[4]pyrrole ratios (below 0.4 based on the number of sites),
the average size of the ensembles were found to range from 3 nm
to 10 nm. This was taken as evidence that little aggregation is
occurring. As the relative ion pair concentration increases, larger
nanoparticles are formed (Figure 2 and S18). Presumably, this
reflects the fact that more ions are binding to the appended
calix[4]pyrrole subunits, rendering the polymer more amphiphilic.
Figure 1. Single crystal structure of the complex M1•(CsF). The CsF
is shown in space-filling representations. Other anions have been
omitted for clarity. Cs purple, F light green.
There are two CsF complexes in the asymmetric unit (Figure 1
and Figures S28 and S29, Supporting Information). In both
complexes, the fluoride anion is hydrogen bonded to the four
pyrrole N-H protons of the cone-shaped calix[4]pyrrole subunit.
In one complex (ca. 50% occupancy) the Cs+ ion is coordinated
directly to the F– ion and is found within the cavity of the
macrocyclic receptor. In the other complex, the Cs+ cation is
bound within the bowl-like pockets created by the pyrrole rings
of two adjacent calix[4]pyrroles.
To ascertain the morphology of the aggregates formed in the
presence of CsF, transmission electron microscopy (TEM)
analyses of the amphiphilic form of 1 were performed on air-dried
samples (Figure 2a). The TEM analyses proved consistent with
the formation of spherical aggregates with an average diameter of
about 90 nm, as would be expected for the formation of multi-
micelle aggregates (MMAs) (Figure 2a).
The ability of M1 to bind cesium fluoride in solution was
1
probed via H NMR spectroscopy using a mixture of CDCl3 and
CD3OD (9:1, v/v) as the solvent (Figure S17). In a first study,
receptor M1 was dissolved in CD3OD/CDCl3 (1:9, v/v) and was
subject to titration with between 0.25 and 1.00 equiv of CsF.
Under these conditions, two sets of resonances are seen for all the
proton signals (Figure S17). At the point where 1.0 equiv of CsF
has been added, the NH proton signals are downfield-shifted and
EDX analyses of the micelles formed from amphiphile 1 were
carried out by scanning a small area of the surface of the micelle
and the blank background. The spectrum recorded from the
micelle surface revealed peaks corresponding to Cs and F, as well
as those for the carbon support (Figure 3b). In contrast, no
discernible Cs or F signals were observed in the spectrum of the
background (Figure 3d). This result is taken as evidence that CsF
ion pairs are incorporated into the calix[4]pyrrole subunits within
the multi-micelles formed upon exposure of amphiphile 1 to CsF.
1
split into a doublet (reflecting H–19F coupling). At this juncture
in the titration, the ß-pyrrolic protons are seen to resonate at higher
field (Figure S17), as do the benzene CH protons, especially H1.
These findings are interpreted in terms of the fluoride anion being
bound to the calix[4]pyrrole NH protons with the cesium cation
being held within the electron rich cup of the calix[4]pyrrole
(rather than the crown ether ring). Thus, under these conditions
and at this stoichiometry the solution phase structure recapitulates
only one of the complexes seen in the solid state (cf. Figure 1).
We next sought to address the question of whether the diblock
copolymer 1 could extract simple salts from an aqueous source
phase. Toward this end, 5 ml dichloromethane solutions of
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polymer 1 for which the effective calix[4]pyrrole concentration
was 2.0 mM, were contacted with 5 mM solutions of, respectively,
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CsF, CsCl, CsBr, KF, and NaF (5 mL each). It was expected that
extraction-induced changes in the ion concentrations of the
aqueous phases would be reflected in differences in the solution
conductivity (SI, Figure S15). The conductivity of all five
solutions decreased as a function of time. For example, the
conductivity of the CsF solution decreased from 580 to 360 µS/cm
after 30 h of liquid-liquid extraction, with a leveling off then being
observed (Figure 4a). The higher overall extraction values for
CsCl compared to CsF is consistent with the relative aqueous
solvation energies (∆Gh) of chloride and fluoride anions (∆Gh = –
465 kJ mol–1 for F–, –340 kJ mol–1 for Cl–,–315 kJ mol-1 for Br–).16
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Figure 4. Conductivity of aqueous inorganic salt solutions (5 mL
each at an initial concentration of 5 mM) after liquid-liquid
extraction with various extractants (effective calix[4]pyrrole
concentration: 2 mM) at different analysis times. (a) PMMA-b-
PSC4Py; (b) M1; (c) PMMA. (d) Extraction efficiency17 recorded
after 30 h using the conditions of studies (a-c).
In summary, a diblock copolymer, containing a calix[4]pyrrole-
derived ion pair receptor has been synthesized. Upon
complexation with simple ion pairs in dichloromethane solution
or at an organic-aqueous interface, the amphiphile polymer 1 self-
assembles into reversed micelles that remain within the organic
phase. The present diblock copolymer proved capable of
extracting ion pairs into an organic receiving phase more
effectively than the control calix[4]pyrrole-based ion pair receptor
M1. This result underscores the potential benefits of polymer-
based approaches to extraction, as well as the specific use of
systems that allow control over structure beyond the first
coordination sphere.
Figure 3. TEM-EDX studies of the aggregates formed upon
exposure of amphiphile 1 to CsF in dchloromethane. (a-b) Spectrum
recorded from the micelle surface; (c-d) spectrum recorded from
the background. The concentration of the copolymer is 2 mg/mL and
[CsF] = 1 mM.
ASSOCIATED CONTENT
A similar study looking at the cations was carried out. For this
purpose, CsF, KF, and NaF were studied. As shown in Figure 4a,
polymer 1 was found to extract cesium fluoride much more
effectively than other fluoride salts. This finding, which is in
accord with the hydration energies (∆Gh = -245 kJ mol–1, –295 kJ
mol–1, and -365 kJ mol–1 for Cs+, K+, and Na+, respectively),16
leads us to suggest that these materials may ultimately enable the
separation of cesium halide salts from aqueous mixtures.
Supporting
Information:
Experimental
details
and
characterization data. Cif files for M1 (CCDC number: 1845439);
and M1+ CsF (CCDC number: 1845440). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Control extraction studies involving M1 and PMMA were then
carried out using conditions analogous to those employed for
amphiphile 1. In contrast to what was seen for the block
copolymer, model compound M1 proved relatively ineffective as
an extractant or any of the salts considered in the present study
(Figure 4a,b) with no evidence of extraction being seen in the case
of PMMA alone (Figure 4c). On this basis, we suggest that the
combined use of an ion pair recognition and a polymer backbone
that is capable of undergoing ion pair-induced micelle formation
can lead to improvements in extraction relative to the
corresponding receptor system alone.
Corresponding Author
*sessler@cm.utexas.edu
ORCID numbers
Xiaodong Chi: 0000-0002-6726-8584
Gretchen M. Peters: 0000-0002-3634-3182
Chandler Brockman: 0000-0003-1773-4601
Vincent M. Lynch: 0000-0002-5260-9913
Jonathan L. Sessler: 0000-0002-9576-1325
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The work was supported by the U.S. Department of Energy Office of
Basic Energy Sciences (Grant DE-FG02-01ER15186) and the R. A.
Welch Foundation (Grant F-1018).
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