Salen-Based Amphiphiles: Directing Self-Assembly in Water by Metal Complexation

Article abstract of DOI:10.1002/anie.201908010

Tuning morphologies of self-assembled structures in water is a major challenge. Herein we present a salen-based amphiphile which, using complexation with distinct transition metal ions, allows to control effectively the self-assembly morphology in water, as observed by Cryo-TEM and confirmed by DLS measurements. Applying this strategy with various metal ions gives a broad spectrum of self-assembled structures starting from the same amphiphilic ligand (from cubic structures to vesicles and micelles). Thermogravimetric analysis and electric conductivity measurements reveal a key role for water coordination apparently being responsible for the distinct assembly behavior.

Full text of DOI:10.1002/anie.201908010

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Salen-Based Amphiphiles: Directing Self-Assembly in Water by
Metal Complexation
Authors: Filippo Tosi, Marc C. A. Stuart, Sander J. Wezenberg, and
Ben Lucas Feringa
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201908010
Angew. Chem. 10.1002/ange.201908010
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10.1002/anie.201908010
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Salen-Based Amphiphiles: Directing Self-Assembly in Water by
Metal Complexation
Filippo Tosi,[a] Marc C. A. Stuart,[a][b] Sander J. Wezenberg*[a][c] and Ben L. Feringa*[a]
Abstract: Tuning morphologies of self-assembled structures in
water is a major challenge. Here we present a salen-based
amphiphile which, using complexation with distinct transition metal
ions, allows to control effectively the self-assembly morphology in
water, as observed by Cryo-TEM and confirmed by DLS
measurements. Applying this strategy with various metal ions gives
a broad spectrum of self-assembled structures starting from the
same amphiphilic ligand (from cubic structures to vesicles and
micelles). Thermogravimetric Analysis and Electric Conductivity
measurements reveal a key role for water coordination apparently
being responsible for the distinct assembly behavior.
rings[52] or for surface functionalization.[53] They are also easily
synthesized by an imine condensation and the metalation step is
usually straightforward and high yielding. Here we report the
synthesis and self-assembly in water of an amphiphilic salen
ligand and its metal complexes of the late first row transition
metals. In the design of our target molecule we took advantage
of the modular synthesis of salen ligands by separately
preparing the hydrophilic diamine and hydrophobic salicylic
aldehyde components. The amphiphilic salen ligand that we
envisioned (Figure 1), is then obtained in a final condensation
step. In the present study, it is shown that this salen framework
allows for remarkable diversification in self-assembly behavior
by making different complexes (i.e. Cu, Ni, Co, Fe, Mn).
In recent years, there has been a growing interest in the
study of novel amphiphiles due to their potential application in
various fields,[1] including drug and gene delivery,[2–7] responsive
materials[8–10] as well as catalysis.[11–21] Specific self-assembled
structures are formed, depending on the characteristics of the
amphiphile, ranging from micelles,[22] vesicles[23] and inverted
structures[24] to more complex architectures (for example
nanotubes,[25] sheets[26] or ribbons[27]). A major challenge is to
control the morphology of the self-assembled structure in water
in an effective and simple manner. In this regard, accessing
more than one morphology with only minor modification of the
parent amphiphile is a difficult task;[28] it generally requires
significant structural modification and extensive chemical
synthesis. We envisioned that transition metal complexation to a
readily accessible ligand, forming the core of the amphiphile,
would present a unique opportunity to access a broad range of
aggregates.
=
Figure 1. Design of amphiphilic metallo-salen complexes.
The synthesis of the salen ligand (Scheme 1) started from
the known MOM-protected phenol 1,[54] which was first
deprotected by acidic hydrolysis and then formylated in the
ortho-position using paraformaldehyde to afford salicylaldehyde
3. The chiral diamine precursor 6 with pendant tetraethylene
glycol chains was synthesized starting from the previously
reported N-Boc protected compound 4,[55] which was doubly
functionalized with glycol chains to obtain compound 5. After
Boc deprotection, the TFA salt 6 was treated with base and
subsequent condensation with aldehyde 3 in a 1:2 ratio gave the
amphiphilic salen ligand L1.
As the ligand, salen was our first choice since these
ligands and their metal complexes are known for their
remarkable self-assembly properties.[29–32] Because of their
modular structure, they have been successfully employed as
supramolecular building blocks,[33,34] for example, in the
formation
of
Langmuir
gels,[42–44]
films,[35]
boxes,[36–39]
metal-organic
helical
structures,[40,41]
fibers,[45,46]
frameworks,[47,48] covalent organic frameworks[49–51] and nano-
The structure of the amphiphile was confirmed by 1H-NMR,
showing both alkyl and tetraethylene glycol chains (Figure S7),
in addition to HRMS. The complexes L1-Cu and L1-Ni were
obtained in good yields by metalation of L1 with the
corresponding acetate salts (Scheme 1). The iron complex L1-
[a]
F. Tosi, Dr. M. C. A. Stuart, Dr. S. J. Wezenberg, Prof. Dr. B. L.
Feringa
Stratingh Institute for Chemistry
University of Groningen
.
Fe was obtained in a similar way using FeCl3 3H2O as the metal
Nijenborgh 4, 9747 AG, Groningen, The Netherlands
E-mail: b.l.feringa@rug.nl, s.j.wezenberg@lic.leidenuniv.nl
Dr. M. C. A. Stuart
Groningen Biomolecular Sciences and Biotechnology Institute
University of Groningen
source. As for Cu and Ni, the synthesis of the cobalt complex
L1-Co was performed using Co(OAc)2 4H2O, which in this case
was followed by oxidation with molecular oxygen in the presence
.
[b]
[c]
of AcOH. The manganese complex L1-Mn was synthesized
.
Nijenborgh 7, 9747 AG, Groningen, The Netherlands
Dr. S. J. Wezenberg
using a similar protocol, starting from L1 and Mn(OAc)2 4H2O,
followed by oxidation in the presence of molecular oxygen and
an excess of LiCl. The successful synthesis of all metal
complexes was confirmed by HRMS, showing the expected
isotopic patterns (Figure S8 - S13), as well as IR and UV-Vis
spectroscopy. The diamagnetic complex L1-Ni was additionally
characterized by NMR.
Leiden Institute of Chemistry
Leiden University
Einsteinweg 55, 2333 CC, Leiden, The Netherlands
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201908010
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Scheme 1. Synthesis of the amphiphilic salen ligand L1 and its metal complexes. (i) HCl, THF, 16 h; (ii) MgCl2, paraformaldehyde, TEA, THF, reflux, 16 h; (iii)
Cs2CO3, p-toluenesulfonate tetraethylene glycol monomethyl ether, THF, reflux, 16 h; (iv) TFA, TIPS, CH2Cl2, rt, 16 h; (v) NaOH (1 M), CH2Cl2, then MeOH, reflux,
.
.
.
.
.
1 h; (vi) Cu(OAc)2 H2O/Ni(OAc)2 4H2O/FeCl3 3H2O, MeOH, reflux, 2 h; (vii) Co(OAc)2 4H2O, CH2Cl2/MeOH 1:1, rt, 1 h then AcOH, rt, air, 3 h; (viii) Mn(OAc)2 4H2O,
MeOH, reflux, 1 h then LiCl (30 equiv), rt, air, 1 h.
The self-assembly behavior of the parent amphiphile and
its metal complexes was studied by Cryo-TEM using a sample
concentration of
2 mM (for detailed sample preparation
procedures, see Supporting Information). The observed
morphologies are presented in Figure 2.[56] The ligand L1 was
found to self-assemble in water in the form of a cubic structure
(Figure 2a). This structure is typically characterized by a bi-
continuous bilayer of inverted micelles, which shows a porous
system clearly visible in the convolutions of the soft material.[24]
Interestingly, under the same experimental conditions, the metal
complexes showed substantially different morphologies. The Cu
and Ni complexes both gave aggregates that were characterized
as sponges (Figure 2b-c).[57] Apparently, the geometrical
constraint of the salen core of the amphiphile in a square planar
geometry, as a result of Cu or Ni complexation, results in a very
distinct self-assembly behavior with respect to the free ligand.
The observed structures, which are smaller than the cubic
structure generated by the free ligand L1, still belong to the
same aggregation domain (namely inverted micelles) and
therefore show a similar type of porous and ordered bilayer
(Figure 2b-c).
Figure 2. (a) Self-assembly of L1 into cubic aggregates, QII; (b) Self-assembly
of L1-Cu into sponges, QII; (c) Self-assembly of L1-Ni into sponges, QII; (d)
Self-assembly of L1-Co into vesicles, Lα; (e) Self-assembly of L1-Fe into
vesicles, Lα; (f) Self-assembly of L1-Mn into spherical micelles (L1 phase) in
dotted circles.
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The presence of the metal in the soft material was
confirmed by EDX analysis (Figure 3). Elemental mapping
clearly showed the presence of Cu and Ni in the sponge
aggregates and not in the water solution, although apparently for
Cu some leaching occurred (Figure S2).
퐶푃푃 = 푉/(푙푐 ∗ 푎0)
Equation 1. Critical Packing Parameter (CPP) definition.
A change in substitution pattern, length or chain terminus
of an amphiphile is known to influence these three terms.[60] In
our case, the molecular scaffold of the amphiphile was left intact
and by merely changing the metal center the morphology
obtained upon self-assembly was altered. Considering the CPP
equation, which is an expression of the ratio between
hydrophobic and hydrophilic balance in the amphiphile, we
imagined that the differences observed could be explained in
terms of the geometrical and electronical characteristics of our
metal complexes. The largest deviations in aggregation from the
parent ligand L1 were observed with the complexes of Co, Fe
and Mn, which have a 3+ oxidation state, rather than the 2+
oxidation state of Cu and Ni. Furthermore, they possess an axial
ligand and have the possibility to coordinate an additional
electron donating ligand. Unlike Cu and Ni, the metal centers of
the Co, Fe and Mn salen amphiphiles may coordinate water as
an external ligand.[61–63] Water coordination should lead to a
higher hydrophilic character (a0) resulting in a decrease of the
CPP as is reflected in the structural change from cubic to
lamellar and eventually micellar aggregates for Mn complexes.
At the same time, the hydrophobic volume is reduced, as the
metal participates in hydrating the amphiphile. The generation of
an octahedral complex, also sterically different from the square
planar complexes of Cu and Ni, would cause a significant
change in CPP. Overall, the hydration of the amphiphile is
therefore expected to drive the self-assembly process from
inverted micelles (CPP>1 for L1-Cu and L1-Ni) to bilayers
(½<CPP<1 for L1-Co and L1-Fe) and even micelles (CPP<½ for
L1-Mn).
To demonstrate water coordination, we prepared the aqua-
complexes of Co, Fe and Mn starting from the model ligand L2,
which is similar to L1, but lacks the hydrophobic and hydrophilic
chains (Figure S3). Thermogravimetric Analysis (TGA) showed
water desorption upon heating of the samples of Co and Fe
(±160 °C and ±196 °C respectively, see Figure S3-S4).[64]
However, in the case of Mn we observed decomposition of the
complex and formation of HCl (Figure S5).[65] Since the self-
assembly behavior of the Mn amphiphile was surprisingly
different, we hypothesized that upon initial water coordination
the Cl ion partially dissociates leading to an ion pair, of which the
formation has been reported for the core salen structure.[66] The
charge formation upon chloride dissociation was successfully
proven by Electric Conductivity (EC) experiments using L2-Mn,
showing a 1:1 electrolyte dissociation (13.36 µS cm-1),[67] which
could not be observed for the model complexes L2-Co and L2-
Fe. Due to the much better solubility of the charged species in
water, the CPP is decreased. Hence, the formation of spherical
micelles in our case can be explained by charge formation. Our
proposed water-binding model is illustrated in Figure 4.
Figure 3. EDX mapping of L1-Cu (a) and L1-Ni (b) sponges on holey carbon
grid: (top) elemental mapping of C, red and Cu (a) Ni (b) green; (bottom) EDX
spectrum of the mapping.
A striking difference in assembly behavior was observed
with the Co, Fe and Mn complexes, all of which have a
pentacoordinated metal ion. The complexes L1-Co and L1-Fe
were found to form lamellar vesicles[23,58] (Figure 2d-e). These
vesicles feature a bilayer very distinct from the nanostructures
formed by the starting ligand L1. Furthermore, the L1-Mn
complex self-assembles into spherical micelles[22] (Figure 2f),
which is an aggregate very distinct from the one formed by the
starting ligand L1. Dynamic Light Scattering (DLS) showed
sharp peaks for L1-Cu, L1-Ni, L1-Co and L1-Fe with an
average Dh value above 70 nm, confirming the presence of large
aggregates as observed by Cryo-TEM (Figure S24). In contrast
to the above mentioned amphiphiles, L1-Mn showed an average
Dh value around 16 nm, confirming the presence of much
smaller aggregates as observed by Cryo-TEM in the formation
of spherical micelles (Figure 2f). It is important to note that not
only different aggregates are obtained for the metal complexes,
but by using the same ligand scaffold a wide range of self-
assembled amphiphilic structures can be obtained.
In order to explain the major differences in assembly
behavior, we qualitatively considered the Critical Packing
Parameter (CPP) of the amphiphile (Equation 1),[59] i.e. the ratio
between the volume of the lipophilic chain (V), its length (lc) and
the interfacial area occupied by the hydrophilic component (a0).
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Figure 4. Proposed water-binding model and resulting aggregates with different metal ions.
In conclusion, we have developed a powerful, modular
approach, based on an amphiphilic salen scaffold, to access a
diverse set of self-assembled structures in water. Cryo-TEM
measurements demonstrated that metalation of the salen ligand
gave access to a wide range of aggregates. These include:
cubic assemblies for the free ligand, sponges in the case of
Cu(II) and Ni(II) complexes, vesicles for Co(III) and Fe(III), and
micelles for Mn(III). TGA and EC studies support the hypothesis
that water coordination gives rise to the observed differences in
aggregation behavior, which can be related to the CPP. As far
as we know, our approach is unprecedented in terms of
effectively controlling self-assembly of a single amphiphilic
structure in water and the diverse structural morphologies
obtained by only changing its metal center, and controlling water
binding. These findings open the path for future developments in
the field of responsive self-assembly and catalysis in confined-
space.
and Science (Gravitation program 024.601035) is gratefully
acknowledged.
Keywords: self-assembly • transition metals • amphiphile •
water-binding • aggregation
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EC was measured at 22 °C in a sample concentration of 2,5*10-4 M.
A
comparison with
a NaCl solution and the concentration
dependence are reported in the Supporting Information (Page S12).
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10.1002/anie.201908010
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
Layout 2:
COMMUNICATION
Filippo Tosi, Marc C. A. Stuart, Sander
J. Wezenberg* and Ben L. Feringa*
Page No. – Page No.
Salen-Based Amphiphiles: Directing
Self-Assembly in Water by Metal
Complexation
Transition metal ion complexation allows the effective control of the self-assembly
morphology in water of new salen amphiphilic ligands, covering a broad range of
aggregates. Spectroscopic investigation sheds light on the role of water
coordination responsible for this process.
This article is protected by copyright. All rights reserved.