Unusual binding selectivity with non-selective homoditopic pillar[5]arene oxime: serendipitous discovery of a unique approach to heterobinuclear metalation in solution

Article abstract of DOI:10.1039/c7cc00237h

A heterobinuclear complexation strategy on homoditopic pillar[5]arene was developed by using a pillar[5]arene with two rims decorated with benzaldehyde oximes. The unique selective recognition process was found to result from vesicular formation based on

Full text of DOI:10.1039/c7cc00237h

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Unusual binding selectivity with non-selective homoditopic
pillar[5]arene oxime: serendipitous discovery of a unique
approach to heterobinuclear metalation in solution
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
Xiangyang Yuan,a Yiming Jia,a Yimin Cai,a Wen Feng,∗ Yiming Li,b Xiaopeng Li,∗ and Lihua Yuan∗
DOI: 10.1039/x0xx00000x
www.rsc.org/
14
heterobinuclear complexation strategy on homoditopic one rim by controlling host/guest molar ratios. In our
A
pillar[5]arene was developed by using a pillar[5]arene with two previous work, preorganizing ten diglycolamide moieties on
rims decorated with benzaldehyde oximes. The unique selective both sides of the pillararene platform was found to result in
recognition process was found to result from vesicular formation high efficiency towards partitioning of Eu(III) and Am(III) with
based on a controllable self-assembly, leading to binding of only one metal ion binding to pillararene ligands under
thorium (IV) only on one rim, with the other rim being unoccupied extraction conditions. 15 This is not unexpected given the
for subsequent complexation of a second different metal ion.
extremely low concentration of radioactive metal ion.
However, the presence of excessive metal ions still lead to
binuclear complexation on both rims of the pillar.14
Pillararenes have gained significant popularity in the past years
due to their ability to implement host-guest interactions with a
wide range of neutral, cationic or anionic species. 1 These
macrocyclic hosts are featured by its unique construction
involving a certain number (n=5~15) of hydroquinone units
linked by methylene (-CH2-) groups at the 2,5-positions of
aromatic rings. 2 Among them, functionalized homoditopic
pillar[5]arenes (P5A), i.e., those with two rims fully decorated
with same functional groups, have found widespread
applications in molecular recognition,3 catalysis,4 separation
technology,5 transmembrane channels,6 metallic or anionic
Selective binding of two different metal ions can be readily
achieved at two different sites using heteroditopic ligands,16
while it is difficult to do so with homoditopic ligands17 in high
specificity, particularly in the presence of excess metal ions.
Installation of two different metal ions may have implications
in biological enzymes where two metal ions display different
functions.18 So far, it is a demanding challenge to utilize
homoditopic P5A for selective one-rim complexation. Herein
we report on our discovery that this can be realized by using
oxime-based pillar[5]arene
3 (Fig. 1) via a route based on
sensor, 7 drug delivery, 8 and liquid-crystal materials, 9 and
bilayer vesicle formation. This work provides a useful strategy
of utilizing homoditopically functionalized pillararene for one-
rim host-guest interaction, which allows for subsequent
10
constructing mechanically interlocked structures
or
supramolecular polymers.11 Sometimes, use of one-rim on a
pillar framework is desired to serve a specific purpose.12 In this
regard, synthesis of non-symmetric pillararenes (NSP) with just
one rim being functionalized enables selective complexation of
guests or construction of amphiphilic molecule.13 However,
low yield and tedious workup preclude NSP or heteroditopic
pillararenes from being widely applied. This prompted us to
raise the question: Is it possible to selectively exploit only one
rim of homoditopic pillar[5]arenes in recognition process while
the other rim remains untouched? In effect, it is
a
commonplace for a pillararene to bind one cationic species on
Fig. 1. Schematic representation of an approach to heterobinuclear complex from
monothorium complex with homoditopic pillar[5]arene oxime 3 via vesicular formation
(vide post). Excess thorium nitrate and cupric nitrate salts are used in the recognition
process.
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introduction of otherwise different metal ion on the other rim. of 1 equiv. of Th(IV), also indicating a 1:1 binding stoichiometry
During our investigation on the recognition properties of of the complex between and Th(IV).
organic molecules and metal cations with macrocyclic ligands,
As generally found with homoditopic macrocycles,22 the
homoditopic pillar[5]arenes were found to serve as an mononuclear complexes, once formed, show marked
excellent molecular platform for preorganizing various tendency to encapsulate a second metal ion in solution, to give
functional groups such as N,N,N',N'-tetraoctyl-3- stable binuclear complexes, particularly when metal to ligand
3
19
a
oxapentanediamide (TODGA) and carbamoyl methylphosphine molar ratio is greater than 2:1.14 Even at 1:1 molar ratio, the
oxide (CMPO) for the extraction and separation of presence of both mononuclear and binuclear complexes is still
actinide/lanthanide elements.14, 20 As a further step, oximes often inevitable. Therefore, the question that follows is: how
(Ri-CH=N-ORii), one of the widely used chelating agents for could it be possible for such a selectivity to be observed on a
transition metal ions such as Cu2+, Zn2+, Co2+, UO22+ and Th(IV), homoditopic pillar[5]arene even with addition of excess
21 are thus incorporated into a pillar[5]arene molecule in this thorium ion? The first clue to the answer comes from the
work, which leads to the design of pillar[5]ararene based observation that an off-white solid precipitated out if an
benzaldehyde oxime (P5ABO
synthesized by coupling bromo-substitued pillar[5]arene with to stand for over 48 h, suggesting the possibility of aggregation
4-hydroxybenzaldehyde, followed by reacting the resultant in solution. Indeed, dynamic light scattering (DLS) revealed the
with hydroxylamine hydrochloride. The yield is over 90% for presence of aggregates with the average size of 152 nm in
) 3 (Scheme 1a). Compound 3 was acetone solution of 3 and thorium(IV) nitrate (1:1) was allowed
2
each step.
acetone (Fig. S7, ESI
†), indicating self-assembling of the
Oxime-based ligands are known to have
a strong monothorium complex. The critical aggregation concentration
coordination ability and a variety of coordination modes. A (CAC) in acetone was determined to be 1.9 × 10-5 M (Fig. S8,
routine screening in acetonitrile of various lanthanide ions ESI†). Further experiments were performed at concentrations
(La3+, Nd3+, Gd3+, Pr3+, Er3+, Yb3+, Sm3+, Lu3+and Eu3+) and a above its CAC. Scanning electron microscopy (SEM)
typical actinide ion (Th4+) (Fig. S3, ESI
of complexing Th(IV) by the macrocyclic ligand
metal ions. However, to our great surprise, absorptions of the the observations from atomic force microscopy (AFM) (Fig. S10,
complex in acetonitrile during UV-vis titration experiments ESI ). Luckily, expanding the SEM image discloses a vesicle
†
) showed the preference experiments revealed spherical nanoscale particles at an
over other average size of 138 nm. (Fig. S9, ESI ), which is consistent with
3
†
†
were found to decrease at the outset of increasing the with a defect that a hole exists on the spherical particle,
concentration of thorium nitrate, but soon cease to change indicating its empty inside (Fig.2a & 2b). A more convincing
beyond ca. 1.0 equiv. of the salt (Fig. S4, ESI
1:1 binding mode between and Th(IV). Results from Job’s electron microscopy (TEM) images where a distinctly darker
plot experiments indicated a 1:1 stoichiometry for Th(IV) edge with a width of 5.6 nm is observed (Fig. 2c & 2d). Besides,
complex (Fig. S5, ESI ). The failure to observe the binding of encapsulating of rhodamine B also proved the existence of
two metal ions even in the presence of 3.0 equiv. of thorium vesicles (Fig. S16 ESI ).
nitrate is hardly rationalized given the two equivalent rims of To verify the crucial role played by the presence of thorium
†), implicating a evidence for forming vesicular assemblies is from transmission
3
3
⊃
†
,
†
the pillar[5]arene that are exposed to metal ions in solution. ion in forming vesicles, fluoride anion was then added to the
1HNMR titration experiments were then conducted in acetone- solution to disrupt the aggregation. Fluoride is known to react
d6/acetonitrile-d3 (4:1, v/v) to further explore the with Th(IV), forming an insoluble precipitate.23 As anticipated,
complexation process. The hydroxyl proton (Ha in
diagnostic of the complexation between P5ABO and thorium technique. Upon adding Th(IV), vesicles were observed again
ion (Fig. S6, ESI ). Binding saturation occurs after the addition (Fig. S11, ESI ). In contrast, macrocycle alone was unable to
form vesicles or aggregates in the chosen solvent system. This
3) is spherical assemblies disappeared in solution as shown by SEM
†
†
3
Scheme
1
a) Synthesis route of P5ABO
3
;
b) schematic illustration of the
Fig. 2 SEM images (a, b) and TEM images (c, d) of
3⊃Th(IV) vesicular aggregates
proposed mechanism for thorium(IV)-induced vesicle formation and disassembly
process in the presence of fluoride or a second metal ion.
from acetone-water (1:1, v/v) solution. b) and d) are the enlarged view.
2 | J. Name., 2012, 00, 1-3
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clearly indicates that the formation of the Th complex induces a continuous decrease in absorption until 1 equiv. of
constitutes an important step towards production of vesicles. Cu(II) ion is added, beyond which a plateau is achieved (up to 4
High resolution electrospray ionization (HR-ESI) MS equiv.) (Fig. 3a), suggesting addition of a
experiments uncovered a peak with the highest intensity at
m/z = 917.7, corresponding to the complex [M+Th4+-H+]3+. The
identity of the 1:1 complex is also corroborated by matching
the isotope distribution with the simulated one (right inset in
Fig. S12, ESI†). In addition, IR spectroscopy showed the strong
band at 1606.6 cm-1 of
ν
(C=N) in
3
, which shifts to 1599.9 cm-
).
1
in its Th complex, a change of 6.7 cm-1 (Fig. S13, ESI
†
Particularly, the pronounced band shift of 42.4 cm-1 of N-O of
the complexes with respect to (Fig. S13, ESI ) strongly
3
†
suggests that coordination occurs mainly through its oxygen
atoms of N-O bonds of oxime groups. All these results, taken in
concert, seem to present a scenario for the selective binding of
only one metal ion on a non-selective homoditopic P5A: the
macrocycle binds one thorium ion first to form amphililic
3⊃Th(IV) complex, followed immediately by the self-assembly
of this amphiphilic complex to form vesicles before proceeding
to add a second thorium ion on the other rim of P5A.
To provide further evidence for the rationalized pathway
through mononuclear Th-complex to vesicle formation,
computational simulation was carried out at B3LYP/6-31G level
(Fig. S17 & Table S1, ESI†). Since a P5ABO molecule is about
Fig.3 a) Plot of the UV-vis absorbance of 3 (2.5 ꢀM) at 264 nm versus equivalents of
metal nitrate when titrating first with Th(IV), and then with Cu(II) in acetonitrile); b)
high-resolution ESI MS spectrum and simulated spectra (insert red) of heterobinuclear
complex Th(IV)⊂3⊃Cu(II).
3.0 nm long as determined computationally, the wall of the
vesicle is reasonably assumed to consist of two layers with
each layer aligned with side-to-side complexes (Scheme 1). The
analysis from theoretical calculations and the experimental
results above only indicated the vesicles that are formed by
mononuclear complex. As a control, the possibility of forming
vesicles via binuclear complex was examined. Cupric(II) ion
was selected to replace Th(IV) ion for the experiment because
of its strong coordinating ability towards oximes.24 UV-vis
second metal ion to the other rim of pillararene platform.
These results clearly demonstrate a stepwise binding process
from mononuclear complex to heterobinuclear complex with
homoditopic ligand
complex was further corroborated by high resolution
spectrometry of the sample prepared from solution
containing (5.0 mmol) and 1 equiv. of thorium nitrate in
3. The formation of the heterobinuclear
titration experiments showed
absorbance with addition of cupric nitrate to a solution of
acetonitrile till up to ca. 2.0 equiv. (Fig. S14, ESI ), suggesting
the formation of dicopper complex. Job’s plot offered 1:2
(ligand to metal) stoichiometry (Fig. S5, ESI ), which is
consistent with HR-ESI data (Fig. S12C, ESI ). However, no sign
of assemblies was observed from DLS profiles (Fig. S15b, ESI ).
a
gradual decrease of
a
1
in
3
†
acetone/methanol, which shows identical peaks (m/z = 938.4
[M+Th4++Cu2+-3H+]3+) with isotopic distribution matching those
†
observed for Th(IV)
⊂3⊃Cu(II) complex (Fig. 3b & Fig. S12D, ESI
†
†
). The presence of aggregation prevents us from acquiring the
†
accurate binding constants.26 It should be noted that the
results from DLS experiments revealed absence of aggregation
when 2 equiv. of Cu(II) ions was added to a solution of
This again indicates that the selectivity for mononuclear
complexation on one rim and vesicle formation are dependent
on a specific metal ion, such as thorium(IV) used here. In
aggregated
3⊃Th(IV) complex (Fig. S15c, ESI†), indicating
addition, the fact that Cu(II) alone failed to form
a
disruption of the vesicles in the presence of a second metal ion.
The observed thorium(IV)-induced vesicular self-assembly and
disassembly process underlines a mechanistic pathway for the
formation of the heterobinuclear complex with a homoditopic
pillar[5]arene (Scheme 1b). The significant difference in metal-
ion-induced assembly between Th(IV) and Cu(II) ions may be
ascribed to the discrepancy in their coordination ability, size of
metal ions and the effect of solvents. 27
mononuclear complex explains the non-aggregation behaviour
of P5ABO in the presence of cupric(II) ion because the
amphiphilicity of the complex is lost when two cupric(II) ions
are introduced to both rims of the pillar.
With the other unoccupied rim on a Th-complex available
for further complexation, we wondered if it is possible to make
use of
a
homtoditopic pillar[5]arene for generating
heterobinuclear complexes. We started to explore the
possibility by a continuous UV-vis titration method. 25 The
In conclusion, we demonstrate a strategy that enables
exclusive formation in solution of mononuclear complex by
spontaneously assembling into vesicles with a homoditopic
pillar[5]arene, and more importantly, subsequent formation of
heterobinuclear complex. Our approach also circumvents
absorption of
3 at 264 nm decreases with increasing the
amount of Thorium(IV) nitrate; however, a marginal change is
observed beyond 1 equiv. of the salt (up to 2 equiv.), indicative
of binding one Th(IV) ion. Further addition of cupric salt
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We are grateful to the National Natural Science Foundation
of China (201471105) and Open Project of State Key
Laboratory of Supramolecular Structure and Materials
(SKLSSM201629) for funding this work. XL also gratefully
acknowledges the support from US NSF (CHE-1506722).
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