Pillar[5]-bis-thiacrown: An Adaptive Tricyclic Host Selectively Recognizing an Organic Guest by Dimetalation

Article abstract of DOI:10.1002/chem.201805275

Some biological receptors change their shapes and rigidity by metalation to recognize substrates precisely via adaptive guest binding process. Herein we present a semi-flexible tricyclic host molecule whose conformation is rigidified by dimetalation to up

Full text of DOI:10.1002/chem.201805275

A Journal of
Accepted Article
Title: Pillar[5]-bis-thiacrown: An Adaptive Tricyclic Host Selectively
Recognizing Organic Guest via Dimetalation
Authors: Eunji Lee, Hyunsoo Ryu, Huiyeong Ju, Seulgi Kim, Ji-Eun
Lee, Jong Hwa Jung, Shunsuke Kuwahara, Mari Ikeda, Yoichi
Habata, and Shim Sung Lee
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To be cited as: Chem. Eur. J. 10.1002/chem.201805275
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COMMUNICATION
Pillar[5]-bis-thiacrown: An Adaptive Tricyclic Host Selectively
Recognizing Organic Guest via Dimetalation
[
a]
[a]
[a]
[a]
[b]
[a]
Eunji Lee, Hyunsoo Ryu, Huiyeong Ju, Seulgi Kim, Ji-Eun Lee, Jong Hwa Jung, Shunsuke
[c]
[d]
[c]
[a]
Kuwahara, Mari Ikeda, Yoichi Habata,* and Shim Sung Lee*
Abstract: Some biological receptors change their shapes rigid
by metalation to recognize substrates precisely via adaptive
guest binding process. Herein we present a semi-flexible
tricyclic host molecule whose conformation is rigidified by
dimetalation to uptake organic guests selectively. Considering
two metal binding sites and an empty space between them,
pillar[5]-bis-thiacrown (L) was rationally synthesized. The
extensively explored in the areas of functional supramolecular
systems.[
6]
Despite the big advance of pillar[n]arenes in the last one
decade, their metallosupramolecular chemistry have still been an
unexplored research area.[ Thus, the sophisticated modification
of pillar[n]arenes for utilizing in the metallosupramolecules
remains a challenge. In case of calix[n]arenes, fused with crown
ether, the coordination chemistry of calixcrowns have been
studied as a landmark multifunctional host system.[ We have
also reported dimetalated complexes and continuous
coordination products of calix[4]-bis-(thia)crowns some of which
exhibit a photoluminescence switching.[
7]
8,9]
2 3 2
tricyclic host L forms a disilver(I) complex 1, [Ag L(NO ) ], with
1
an Ag···Ag separation (9.976 Å). Binding studies based on H
NMR including 2D NOESY and DOSY experiments towards α,ω-
9]
dicyanoalkanes [CN(CH
demonstrated that the dimetalated L, Ag
2
)
n
CN, n = 2-6, shortly C2-C6]
L preferentially
In biological molecular recognition such as enzyme-substrate
and protein-RNA, large flexible hosts change their shapes rigid by
2
recognizes C2 over other guests than that of free L. Furthermore,
the dimetalated host 1 uptakes only C2 in the presence of other
guests. Crystal structures support that the space between two
silver(I) centers in 1 plays a decisive role on the selective guest
binding forming an Ag-C2-Ag@L arrangement via the length-
selective recognition. This work demonstrates the chemical
example of the adaptive guest binding and presents a new
perspective on the metallosupramolecules of pillararenes.
metalation to recognize guests precisely via adaptive guest
[10]
binding.
In chemical sense, some flexible macrocycles form
dinuclear complexes which uptake a polyatomic anion via
cascade complexation.[ Lehn et al have reported dinuclear
11]
cryptand complexes in which an organic guest incorporates
between two metal ions.[
12]
We recently have reported the
cascade complexation of a 40-membered macrocycle which
[
13]
recognizes an organic guest via dimercury(II) complexation.
[
9]
Inspired by the dimetalation of calix[4]-bis-(thia)crowns and
the inclusion of dinitriles[ into pillar[5]arene, we have interested
in the modification of the pillar[5]arene scaffold to develop a
candidate for the dimetalation-mediated guest recognition. In the
present study, we wished to explore on the pillar[5]-bis-crown type
host because of the following reasons as depicted in Figure 1. (i)
The pillar[5]-bis-crown is capable of inclusion of α,ω-
dicyanoalkanes, but not so selectively in its semi-flexible form
alone. (ii) Upon dimetalation, the semi-flexible crown units
become rigid and the empty space between two metal centers
(M···M) might recognize a specific organic guest via the length-
selective binding to form an M-guest-M arrangement.
3]
Pillar[n]arenes are relatively new class of macrocyclic host which
features a more rigid and symmetrical cavity compared to
calix[n]arenes due to the different connection of repeating units.[1]
Indeed, inclusion complexes of pillar[5]arene derivatives with a
variety of organic guests including diamines, dinitriles and ionic
liquid molecules adopting polyrotaxane and pseudo-rotaxane
studied.[
2,3]
structures
have
widely
been
Some
pseudo[1]catenane type pillar[5]arene bicycles showing chiral
inversion by external stimuli such as solvent, organic guest,
temperature and pH have been introduced.[4] We recently have
reported a bicyclic pillar[5]thiacrown whose planar chiral inversion
is triggered by mercury(II) ion and controlled by counter anion.[5]
Pillar[n]arene based nanostructured assemblies also have been
[
a]
Dr. E. Lee, H. Ryu, H. Ju, S. Kim, Prof. J. H. Jung, Prof. S. S. Lee
Department of Chemistry and Research Institute of Natural Science
Gyeongsang National University, Jinju 52828 (South Korea)
E-mail: sslee@gnu.ac.kr
[
b]
c]
Dr. J.-E. Lee
Figure 1. Schematic representation of the adaptive guest binding via
dimetalation and α,ω-dicyanoalkanes (C2-C6) as guests.
Center for Research Facilities, Gyeongsang National University
Jinju 52828 (South Korea)
Prof. S. Kuwahara, Prof. Y. Habata
[
To verify above hypothesis, we have proposed pillar[5]-bis-
dithiacrown (L, Scheme 1) incorporating two thiacrown units
which might induce the dimetalation of soft metals. Considering
the affinity of nitriles towards soft metals (M) and the M···M
Department of Chemistry, Toho University
2-2-1 Miyama, Funabashi, Chiba 274-8510 (Japan)
E-mail: habata@chem.sci.toho-u.ac.jp
Prof. M. Ikeda
Department of Chemistry, Education Center, Faculty of Engineering
Chiba Institute of Technology
[d]
2
n
distance (~1 nm), α,ω-dicyanoalkanes CN-(CH ) -CN (n = 2-6,
C2-C6) with different lengths have been employed as guests. In
the soft metal ions, silver(I) is a predominant candidate due to its
affinity towards sulfur donors (thiaphilicity) and dinitriles. The
tricyclic host L was synthesized by a bicyclization between
pillar[5]arene tetrabromide 5 (Figure S1) and 2-mercapto ethyl
ether in the basic condition (yield 35 %, Figure S2). The key
2-1-1 Shibazono, Narashino, Chiba 275-0023 (Japan)
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
precursor 5 was obtained via a mixture of dibromide 4, 1,4-
dimethoxybenzene, paraformaldehyde and BF ∙O(C H ) . The
3 2 5 2
crystal structures of 5 and L which includes one n-hexane were
characterized (Figures S3 and S4).
After confirmed the inclusive complexation between L and
dinitrile guests except C6, the preparation of corresponding solid
complexes was performed. It was possible to isolate all the
inclusion complexes with C2-C6 suitable for X-ray analysis and
their crystal structures were obtained (Figure 3 and Table S1). In
the crystal structures, all guests form a 1:1 product with a type
guest@L stabilized by C-H···π and C-H···O interactions (Figure
S27). Each of C2, C3 and C4 is fully encapsulated by L (Figure
3a-c). While, C5 and C6 are slightly bent in the cavity and one end
of each guest comes out through the open space of the thiacrown
loop due to the longer size than the loop size of L (Figure 3d,e).
Indeed, the observed crystal structures of the inclusion
complexes agrees well with the stability order obtained in solution.
Scheme 1. Synthesis of pillar[5]-bis-thiacrown (L).
In the NMR study for the inclusion complexation between L
and dinitrile guests (C2-C6), the signals for free and complexed
guests are found, indicating that the inclusion process is a slow
exchange system on the NMR time scale (Figure 2). For C2, a
singlet peak of the threaded guest, C2@L, appears at the upfield
(
-1.33 ppm) due to the strong shielding effect (Figure 2b). While,
a negligible peak for the free C2 at 2.83 ppm was observed,
indicating that a 1:1 inclusion complex is quite stable with the
5
association constant, K
a
≥ 10 . Similarly, C3 and C4 showed to
give C3@L and C4@L species in the upfield areas (Figure 2c,d),
with very small peaks for the free guests. For C5, two sets of
peaks corresponding free C5 and C5@L clearly appear,
indicating a less stable complexation (Figure 2e). Further, the
longest guest C6 remains as a free form (Figure 2f), indicating
almost no reactivity. The corresponding association constants
(
a
log K ) are described in the later part. Consequently, L forms
inclusion complexes with dinitriles regardless of their lengths
except C6. In general, longer guests show a weaker affinity
because the exceeded length destabilizes the inclusion product
in the limited internal space of L. Besides, NOE correlations
between L and alkyl chain protons of C2, C3 and C4 (Figures
S13-S15) were observed, indicating the threading of these guests
into L. However, negligible or no NOE correlations between L and
the longer guests C5 and C6 were observed (Figures S16 and
S17). These data are further confirmed by 2D diffusion-ordered
Figure 3. Crystal structures of the inclusion complexes of L with C2-C6.
Since L shows the inclusion complexation with a broad range
of the linear dinitrile guests employed, it has a limitation for the
specific recognition. In this situation, as depicted in Figure 1, the
dimetalation-mediated approach was carried out to improve the
recognition selectivity via the adaptive guest binding. To a mixture
of L and two equivalents of silver(I) nitrate, one equivalent of each
dinitrile guest (C2-C6) was added (Figure 4). As shown in Figure
1
spectroscopy (DOSY) H NMR experiments (Figures S21-S23).
4c, C2 was threaded into the cavity of L almost completely.
However, C3 was distributed inside (55%) and outside (45%) the
cavity (Figure 4d). Furthermore, the longer guests C4-C6 remain
completely outside the cavity in the same condition (Figure 4e-f).
2
These data illustrate that the disilver(I) metalloligand, Ag L,
recognizes C2 more selectively over other guests than L does
alone. Consequently, the observed results might be evidence for
the proposed hypothesis on the adaptive guest binding via the
dimetalation-mediated recognition shown in Figure 1.
Based on the NMR data, the association constants (log K
the 1:1 inclusion complexes of L and Ag L with C2-C6 were
obtained and compared. (Figure 5 and Table S5). In case of the
reaction of C2 with L, the log K value was obtained from the UV-
titration data to reduce the experimental error (Figures S28 and
a
) for
2
a
S29).[ Comparison of the log K
) with those for Ag L, (green bars) clearly show a decrease in
the respective cases. In both hosts, C2 shows the highest log K
value and, as mentioned, longer guests show lower values.
Notably, this trend is moderate for L but very sensitive for Ag L.
For instance, the stability differences (Δlog K ) approximating to
0.8 (for L) and 2.0 (for Ag L) were observed solely by variation of
14]
results for L (red bars in Figure
a
5
2
a
Figure 2. 1H NMR (500 MHz) spectra of L (6.0 mM) in the presence of C2-C6
3 6
(1 equiv) in CDCl /DMSO-d (5:1 v/v) (a) L only, (b) C2, (c) C3, (d) C4, (e) C5
2
and (f) C6. The peaks marked with asterisks indicate solvents (chloroform,
DMSO and water) and TMS.
a
2
2
one -CH - unit of the guests. In another word, the van der Waals
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Chemistry - A European Journal
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COMMUNICATION
in the cavity with regard to their lengths. In case of C3, such effect
is less.
Figure 5. Association constants (log K
a
) for the 1:1 inclusion complexation of L
/DMSO-d (5:1, v/v) at 298 K.
or Ag L with dinitrile guests (C2-C6) in CDCl
2
3
6
While the reason for the higher association constants of L than
those of Ag L remains uncertain. The NOESY data (marked with
2
green circles in Figures S13-S15) suggested that the driving force
of the inclusion complexation between L and C2-C4 is mainly
intermolecular C-H···π and C-H···O interactions. However,
2
combination of Ag L and guests show almost no NOE correlations
(
Figures S18-20), probably the CN-Ag bond formation weakens
the van der Waals interaction between L and guests (mainly C2)
remarkably. As mentioned, the van der Waals interaction between
the free L and guests is quite strong but less selective in
recognizing the guests. On the contrary, almost no van der Waals
1
Figure 4. H NMR (500 MHz) spectra of (a) L (6.0 mM) and (b) L + AgNO
3
(
2 equiv), and (c-g) a mixture of L + AgNO
equiv of (c) C2, (d) C3, (e) C4, (f) C5 and (g) C6 in CDCl
v/v). The peaks marked with asterisks indicate solvents (chloroform, DMSO
and water) and TMS.
3
(2 equiv) in the presence of 1
/DMSO-d (5:1
3
6
interactions exist between Ag
2
L and guests including C2. These
results illustrate that the greater discrimination of C2 over other
interaction between L and guests is less selective, meanwhile the
2
guests via Ag L is mainly due to the length-selective recognition
Ag
assumed in Figure 1, this result might be explained that the empty
space between two silver(I) ions in Ag L can effectively blocks the
entry of longer guests C4-C6 into the cavity, but uptakes C2
2
L has a more enhanced selectivity for C2 over L alone. As
of the guests.
To confirm the length-selective recognition of C2 in the solid
state, first we tried to prepare a disilver(I) complex of L. When
silver(I) nitrate was reacted with L, a disilver(I) complex of type
2
+
+
selectively to form a favorable Ag -C2-Ag binding arrangement
[
2 3 3 2 3 3
Ag (L)(CH OH)(NO ) ]∙CH OH∙CHCl (1) was isolated (Figure
2 3 3 2 3 3 2 3 2 3 2 3 2 3 n
Figure 6. Formations of (a) [Ag (L)(CH OH)(NO ) ]∙CH OH∙CHCl (1), (b) {[Ag (μ-C2)(L)(NO ) ]·4CHCl } (2) and (c) {[Ag (C3)(L)(NO ) ]·5CHCl } (3), showing
the dimetalation-supported selective inclusion of C2. In L, one n-hexane in the cavity is omitted. Noncoordinated solvent molecules in 1-3 are omitted.
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COMMUNICATION
6
a). In 1, as expected, two Ag(I) atoms (Ag1 and Ag2) locate in
lengths, in keeping with the occurrence of the length-selective
recognition shown in solution.
Having confirmed the length-selective recognition via the
dimetalated L in solution and the solid state, a series of
competition experiments by employing mixtures of guests
including C2-C6 were carried out in the similar condition. As
the thiacrown rings and the Ag1···Ag2 separation is 9.98 Å. Each
thiacrown ring coordinates to each Ag(I) center in a tridentate
fashion via OS donors. The coordination environment of five-
2
coordinated Ag1 center is completed by one bidentate nitrate.
While the coordination environment of six-coordinated Ag2 center
is completed by one bidentate nitrate and one methanol. As can
be seen in Figure 6, the O1-S1-O2-S2-O3 segment in the
thiacrown ring of L shows a TTTT arrangement (T: trans, G:
gauche) between two adjacent donors, adopting a semi-flexible
square shape. Upon dimetalation (Figure 6a), the thiacrown ring
of 1 is rigidified because the O1-S1-O2-S2-O3 segment is
rearranged via the Ag-S and Ag-O bond formations, leading to a
TGGG arrangement between two adjacent donors.
shown in Figure 7, the NMR spectra of a mixture of L and AgNO
3
(2 equiv) obtained after stepwise additions of each guest from C3
to C6 showed no release of C2 and no uptake of C3-C6 occurred
in the whole process. This result indicates that the C2 uptake is
fast and of sufficient strength to preclude displacement by other
guests. So, this system might be utilized to the separation and
detection of small organic molecules, although further studies on
the additional modifications are required.
In the following stage, the disilver(I) complex 1 was used as a
reactant for reactions with C2-C6. These reactions enabled us to
isolate two complexes (2: guest = C2 and 3: guest = C3) (Figure
In summary, we synthesized a tricyclic pillarcrown for the first
time to explore an adaptive guest binding system which uptakes
an organic guest selectively via the metal binding. For this, we
have prepared a pillar[5]-bis-thiacrown (L) which reacts with a
broad range of linear dinitrile guests (C2-C6) regardless of their
lengths. A series of NMR study has confirmed that its prototype
disilver(I) complex selectively recognizes C2, in combination with
the crystallographic studies. The superior guest recognition can
be drawn that two silver(I) ions bound to L not only block the entry
of C3-C6 into the cavity but also promote the uptake of C2 to form
a favorable Ag -C2-Ag binding arrangement in the cavity with
regard to their lengths. When two silver(I) ions bind to the
thiacrown units, the local conformational changes are induced
from semi-flexible to rigid which are similar to the adaptive guest
binding found in nature. From the mixtures of C2-C6, the selective
uptake of C2 was also performed indicating that the proposed
system in this work or further combinations of dimetalation and
crown rings may have wider applications in the recognition and
separation of guest substrates.
6b,c). The X-ray analysis revealed that 2 is a typical cascade
complex of type [Ag (μ-C2)(L)(NO ]·4CHCl in which C2 links
2
3
)
2
3
two silver(I) centers to give an Ag-C2-Ag arrangement via the Ag-
CN bond formation. The Ag···Ag separation (10.36 Å) in 2 is not
significantly different from that in 1 (9.98 Å), indicating the ligand
framework is well adapted via metalation for the length-selective
recognition.
+
+
In case of C3, as mentioned, a 2-D coordination polymer type
product {[Ag (C3)(L)(NO ) ]·5CHCl } (3) was obtained and
2 3 2 3
n
structurally characterized (Figure 6c). In 3, the role of two silver(I)
ion (Ag1 and Ag2) is quite different. For example, the Ag1 atom
locates in the thiacrown and binds to one end of C3 via Ag1-CN
bond. Meanwhile, Ag2 atom locates outside the cavity and shows
no interaction with C3. Instead, the Ag2 atom links the host
molecules via S-Ag-S bond linkage to give the 2-D coordination
network (Figure S30). For C4-C6, however, no solid products
3
were isolated. One-pot reactions of L, AgNO and guests (C2 or
C3) gave same products. Consequently, the above solid
structures illustrate that C2 is bound inside the cavity of the
disilver(I) complex more efficiently than other guests with different
Acknowledgements
This work was supported by NRF (2016R1A2A205918799 and
2
017R1A4A1014595), South Korea. The Support Program for
Strategic Research Foundation at Private Universities
S1201034) and KAKENHI-PROJECT-17K05844 from the
(
Ministry of Education, Sports, Science and Technology of Japan
are acknowledged.
Keywords: pillararenes • host-guest systems • macrocycle •
silver • supramolecular chemistry
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Chemistry - A European Journal
10.1002/chem.201805275
COMMUNICATION
Table of Contents
COMMUNICATION
We present a semi-flexible tricyclic
pillar[5]-bis-thiacrown whose
conformation is rigidified by
dimetalation to uptake organic
guests selectively.
Eunji Lee, Hyunsoo Ryu, Huiyeong Ju,
Seulgi Kim, Ji-Eun Lee, Jong Hwa
Jung, Shunsuke Kuwahara, Mari
Ikeda, Yoichi Habata,* and Shim Sung
Lee*
Page No. – Page No.
Pillar[5]-bis-thiacrown: An Adaptive
Tricyclic Host Selectively
Recognizing Organic Guest via
Dimetalation
This article is protected by copyright. All rights reserved.