Pre-organized molecular tweezer stabilized by intramolecular hydrogen bonds: Solvent-responsive host-guest complexation

Article abstract of DOI:10.1016/j.tetlet.2016.03.080

A novel molecular tweezer consisting of 2,2′-iminodibenzoyl backbone and alkynylplatinum(II) terpyridine pincers is designed and synthesized. It shows moderate binding strengths towards two neutral organometallic guests, accompanying with interesting optical behaviours due to the involvement of donor-acceptor and metal-metal interactions. Notably, addition of polar solvent, hexafluoroisopropanol, cleavages intramolecular NH?O hydrogen bonds and thereby triggers conformational change for the molecular tweezer receptor. Consequently, molecular tweezer/guest complexation could be significantly influenced, benefiting for further construction of intelligent molecular machines and devices.

Full text of DOI:10.1016/j.tetlet.2016.03.080

Accepted Manuscript
Pre-Organized Molecular Tweezer Stabilized by Intramolecular Hydrogen
Bonds: Solvent-Responsive Host–Guest Complexation
Xiaoqin Lv, Yifei Han, Zhishuai Yang, Zijian Li, Zhao Gao, Feng Wang
PII:
S0040-4039(16)30302-1
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.03.080
TETL 47468
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
28 January 2016
23 March 2016
24 March 2016
Please cite this article as: Lv, X., Han, Y., Yang, Z., Li, Z., Gao, Z., Wang, F., Pre-Organized Molecular Tweezer
Stabilized by Intramolecular Hydrogen Bonds: Solvent-Responsive Host–Guest Complexation, Tetrahedron
Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.03.080
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Tetrahedron Letters
Pre-Organized Molecular Tweezer Stabilized by Intramolecular Hydrogen Bonds:
Solvent-Responsive Host–Guest Complexation
Xiaoqin Lv, Yifei Han, Zhishuai Yang, Zijian Li, Zhao Gao, and Feng Wang*
CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer
Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Abstract: A novel molecular tweezer consisting of 2,2'-iminodibenzoyl backbone and
alkynylplatinum(II) terpyridine pincers is designed and synthesized. It shows moderate binding
strengths towards two neutral organometallic guests, accompanying with interesting optical
behaviors due to the involvement of donor–acceptor and metal–metal interactions. Notably,
addition of polar solvent, hexafluoroisopropanol, cleavages intramolecular NH···O hydrogen
bonds and thereby triggers conformational change for the molecular tweezer receptor.
Consequently, molecular tweezer/guest complexation could be significantly influenced,
benefiting for further construction of intelligent molecular machines and devices.
Available online
Keywords:
molecular tweezer
host–guest chemistry
molecular recognition
solvent-responsive
2009 Elsevier Ltd. All rights reserved.
Introduction
(Scheme 1), for which the tweezer-like conformation is stabilized
via intramolecular NH···O hydrogen bonds.13 It is anticipated to
Molecular tweezer, representing the pre-organization of two
π-aromatic pincers with designated orientation and distance, is
capable for guest encapsulation and thereby receives enormous
attention for separation, sensor and optoelectronics applications.1
In recent years, coordination complexes consisting of d8
transition metal (such as Pt2+ and Pd2+) have been widely utilized
as the pincer units, attributing to their square planar geometries
and fascinating functionalities.2 It is worthy of note that, with the
elaborate choice of the complementary organometallic guests,
metal–metal interactions could be potentially embedded, leading
to intriguing photo-physical properties for the resulting non-
covalent complex.3 For example, molecular tweezer 1 with the
decoration of two electron-deficient alkynylplatinum(II)
terpyridine units has been reported by Yam et al. and us (Scheme
1).4-5 Due to the pre-organization effect imparted by the shape-
form molecular tweezer/guest complexes between 2 and the
neutral guests [Pt(C^N^C)(C≡N-C6H4-OMe-p)] and
3
[Au(C^N^C)(C≡C-C6H4-OMe-p)] 4 (Scheme 1) through electron
donor–acceptor and metal–metal interactions. Moreover,
considering that intramolecular hydrogen bonds embedded into
2,2'-iminodibenzoyl backbone of 2 are highly sensitive to the
solvent polarity changes, conformation change for the tweezer
receptor, as well as the binding strength for the resulting
tweezer/guest complexes is elucidated towards such external
stimuli, which is advantageous for the fabrication of intelligent
molecular machines and devices.
persistent diphenylpyridine spacer,
1
permits for the
accommodation of various electron-rich guest molecules into its
cavity. Based on these findings, we have further self-assembled
such recognition motifs to afford well-ordered supramolecular
polymeric assemblies.5
On this basis, it is of particular interest to trigger reversible
guest encapsulation/release from molecular tweezer receptor.6
One of the feasible protocols to attain the objective is to
implement dynamic elements to the backbone unit. In this
context, a variety of switchable molecular tweezers have been
reported, which undergoes conformational changes in response to
external stimuli such as pH,7 cations,8 anions,9 light,10 voltage11
and mechanical forces.12 We sought to develop an alternative and
convenient strategy to modulate on–off switching behavior for
the resulting tweezer/guest recognition system. Based on this
consideration, molecular tweezer 2 is designed in this manuscript
Scheme 1. Schematic representation for the non-covalent host–
guest complexes derived from the tweezer receptors 1–2 and the
complementary guests 3–4, as well as the synthetic route towards
2.

2
Tetrahedron Letters
Results and discussion
complexation for these two compounds. For the resulting
complex 2/3, an obvious bathochromic-shifted band (λmax = 552
nm) emerges in UV-Vis spectrum (Fig. 2a), which is
characteristic for the communication between electron-rich
alkynylplatinum(II) diphenylpyridine guest 3 and electron-poor
alkynylplatinum(II) terpyridine pincers on 2. Simultaneously, a
metal-metal-to-ligand charge-transfer (MMLCT) emission band
appears in the near infrared region (λmax ≈ 800 nm) (Fig. 2a),
together with a broad excitation peak ranging from 400 nm to
600 nm (Fig. S8, ESI†). The presence of MMLCT absorption and
emission bands provides direct evidence for the involvement of
Pt···Pt metal–metal interactions in complex 2/3.
The designated molecular tweezer 2 was synthesized in a
straightforward manner (Scheme 1). Briefly, the esterification
reaction between 4-hydroxyphenylacetylene and commercially
available 2,2'-dicarboxydiphenylamine 5 in the presence of N-(3-
dimethylaminopropyl)-N′-ethylcarbodiimide
hydrochloride
(EDC•HCl) and 4-dimethylamino pyridine (DMAP) provided the
intermediate 6, which was subsequently converted to the targeted
compound 2 via copper(Ι)-catalyzed coupling reaction. The
1
proposed structure for 2 was confirmed by H NMR, 13C NMR
and MS spectroscopy (Fig. S4–6, ESI†). For proton H11, a singlet
1H NMR resonance peak appears at the noticeably downfield
position (δ = 11.1 ppm) (Fig. S4, ESI†), supporting the existence
of NH···O intramolecular hydrogen bonds between the amine
and two adjacent carbonyl oxygen atoms on the 2,2'-
iminodibenzoyl skeleton.
3
2
1
0
3
2
1
0
a 1.2
b 1.2
0.8
0.8
2 2/3 3
2 2/4 4
Since it failed to grow single crystal of 2 suitable for X-ray
crystallography measurement, we tuned to use DFT (density
functional theory) calculation to understand its structural
information. For the optimized geometry of 2 (Fig. 1), both
amide hydrogens form intramolecular hydrogen bonds with the
oxygen atom (bond length = 1.96 Å and 1.98 Å, respectively),
which is highly consistent with the 1H NMR experimental results.
Notably, to avoid steric hindrance for the 2,2'-dicarboxydiphenyl
skeleton, two alkynylplatinum (II) terpyridine pincers on 2 tends
to distort from each other to form a V-shape structure (Pt–Pt
distance: 18.5 Å), which is more twisted than that of 1 (Pt–Pt
distance: 7.56 Å based on its single crystal structure).4c
0.4
0.0
0.4
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength / nm
Wavelength / nm
Figure 2. a) UV-Vis absorption and fluorescent spectra (CHCl3,
0.10 mM for each component) of 2 (red lines), and 1 : 1 mixture
of 2 and 3 (black lines). b) UV-Vis absorption and fluorescent
spectra of 2 (red lines), and 1 : 1 mixture of 2 and 4 (black lines).
When equivalent amount of alkynylgold(III) diphenylpyridine
guest 4 is added into the chloroform solution of 2, no significant
colour change occurs for the resulting complex 2/4 (Fig. 2b, inset,
middle). Meanwhile, only intensity changes occur for the MLCT
absorption (460 nm) and emission (600 nm) bands (Fig. 2b).
Such phenomena are quite different from those of the
aforementioned complex 2/3. It is evident that, although both
guests 3 and 4 possess the square planar d8 metal ions, different
metal ions exert distinct impact on their photo-physical
behaviours.
Figure 1. Optimized geometry of 2 via DFT calculation. Pt
atoms are described by Lanl2dz, whilst the residual atoms in 2
are described by PBEPBE/3-21G.
The optical properties for 2 in dilute CHCl3 solution (1 × 10−4
M) were investigated by means of UV-Vis and fluorescent
spectroscopy. In detail, two absorption bands with the maximum
peaks locating at 379 nm and 463 nm appear in the visible region
(Fig. 2a). Such phenomena are distinct from that of 1, for which
only one broad absorption peak dominates in the same region
(Fig. S7, ESI†). According to the previously reported literatures,4
the visible light absorption for 1 is predominately assigned to the
admixture of π(C≡CR)→π*(tBu3tpy) ligand-to-ligand charge
transfer (LLCT) and dπ(Pt)→π*(tBu3tpy) metal-to-ligand charge
transfer (MLCT) bands. Since 2 features the ancillary ligand with
more electron-rich character, LLCT bands would be significantly
influenced. As a consequence, the band centred at 463 nm is
primarily assigned to the MLCT band, whilst the relatively high-
energy absorption band (379 nm) should be designated to the
LLCT absorption band. Meanwhile, an emission band centred at
600 nm is visualized for 2 (Fig. 2a), which is similar to that of 1
and thereby originates from the 3MLCT alkynylplatinum(II)
terpyridine band.4
Figure 3. Partial 1H NMR spectra (300 MHz, CDCl3, 298 K, 1.00
mM for each component) of a) 3; b) 1 : 1 mixture of 2 and 3; c) 2;
d) 1 : 1 mixture of 2 and 4; and e) 4.
1H NMR measurements were performed to get more detailed
information on the non-covalent tweezer/guest complexation
behaviours. For complex 2/3, the terpyridine protons on 2 (-0.44,
-0.43, -0.46 and -0.54 ppm for H1-4, respectively), as well as the
diphenylpyridine protons on 3 (-0.71 and -0.27 ppm for He,h,
respectively) undergo obvious upfield shifts, whilst proton Hf on
3 moves downfield (0.29 ppm) (Fig. 3a–c). In the meantime, only
slight chemical shift changes occur for the 2,2'-iminodibenzoyl
protons such as H8-11. Such phenomena support the close spatial
proximity between guest 3 and the two pincers on 2. The
maximum point for Job plots,14 deriving from the chemical shift
We then turned to study molecular tweezer/guest
complexation, by mixing equivalent amounts of 2 (bright yellow
solution) and 3 (pale yellow solution) together. For the resulting
mixture in chloroform solution, a dark brown color promptly
appears (Fig. 2a, inset, middle), indicating non-covalent

changes for H4, locates at the mole fraction of 0.5 (Fig. S9–S11,
ESI†), revealing 1: 1 binding stoichiometry between 2 and 3. In
terms of 2/4, similar chemical shift tendencies are also observed,
with the exception that Hh' exhibits a slightly downfield shift (-
0.12 ppm) (Fig. 3c–e).
HFIP-triggered release of guest 3 from the cavity of molecular
tweezer 2.
Next, we shed light on the molecular tweezer/guest binding
thermodynamics by means of isothermal titration calorimetry
(ITC) measurements (Fig. 4a–b). For both complexes 2/3 and 2/4,
the appearance of exothermic signals suggests the enthalpy-
driven non-covalent complexation. Moreover, the abrupt
exothermic isotherm changes validate 1:1 binding stoichiometries,
1
which are highly consistent with the H NMR Job plots’ results.
Depending on the non-linear curve fitting of the titration data, the
binding constant (Ka) values for complexes 2/3 and 2/4 are
determined to be (1.25 ± 0.07) × 104 M-1 and (1.19 ± 0.09) × 104
M-1, respectively (Fig. 4a–b). Notably, for both guests 3–4,
molecular tweezer 2 exhibits relatively lower binding strengths
than those of 1 (Ka = (3.43 ± 0.09) × 104 M-1 and (2.81 ± 0.25) ×
104 M-1 for 1/3 and 1/4, respectively) (Fig. S12 and S18, ESI†),
highlighting the crucial role of backbone unit on the stability of
tweezer/guest complexation. Moreover, more enthalpies are
released for 1 than those of 2 (ΔH values: -5.65 kcal/mol for 1/3
versus -3.66 kcal/mol for 2/3, and -4.47 kcal/mol for 1/4 versus -
3.27 kcal/mol for 2/4). Considering that both tweezer receptors
possess the same π-pincers, the decreased binding strength for 2
is primarily ascribed to its twisted conformation (Fig. 1), which
leads to insufficient π-surface overlapping between the pincers
and the complementary guests. Another possible reason lies in
the presence of electron-rich ancillary ligand on 2, which
presumably influences the tweezer/guest binding affinity via
electronic effects.
Figure 5. Partial 1H NMR spectra (CDCl3, room temperature) of
a) 3; b) 2; and an equimolar mixture of 2 and 3 ([2] = [3] = 1.00
mM) with different percentage of HFIP (v/v): c) 0%; d) 5%; e)
10%; f) 15%; g) 20%.
UV-Vis absorption spectroscopy was further employed to
evaluate molecular tweezer/guest binding strength in response to
HFIP. When 0%, 4%, 6% (v/v) amounts of HFIP are titrated into
the chloroform solution of 2/3, Ka values are determined to be
(7.55 ± 0.33) × 103 M-1, (1.81 ± 0.19) × 103 M-1, and (0.63 ± 0.16)
× 103 M-1, respectively (Fig. 6 and Fig. S16, ESI†), denoting that
small amount of HFIP gives rise to several ten times decreases
for the binding affinity. Similar tendencies are also observed for
2/4 (Ka values are determined to be (6.18 ± 0.19) × 103 M-1 and
(1.32 ± 0.05) × 103 M-1 for 0%, 6% amounts of HFIP,
respectively, Fig. 6 and Fig. S17, ESI†). In contrast, for the
counterpart receptor 1, 6% (v/v) amounts of HFIP lead to only 2.7
and 1.8 times decrease for Ka,1/3 and Ka,1/4, respectively (Fig. S18).
Hence, it is evident that the main function of HFIP is disrupting
intra-molecular NH···O hydrogen bonds on 2, whilst solvent
polarity changes brought by HFIP play a minor role for the
binding strength variations. Interestingly, when trace amount of
HFIP (1%) is added, an abnormal binding strength reinforcement
is observed for both 2/3 (Ka = (1.17 ± 0.07) × 104 M-1) and 2/4
(Ka = (9.01 ± 0.34) × 103 M-1) (Fig. 6). Such phenomena are
probably ascribed to the strengthening of dipole-dipole van der
Walls interactions, which offsets the weakening effect of
hydrogen bonds to a certain extent.
Time (min)
Time (min)
a
b
0
0
20
40
60
80
0
20
40
60
0
-3
-6
-9
-3
-6
-9
0
-12
0
-1
-2
-3
-1
-2
-3
Model: One Sites
Chi^2/DoF = 1627
Model: One Sites
Chi^2/DoF = 3151
N
K
0.903
1.25E4
±
±
±
0.0133
739
72.07
N
K
0.978 ±0.0190
1.06E4 ±876
DH -3662
DS 6.47
DH -3273 ±86.63
DS 7.44
0
1
2
3
4
0
1
2
3
Molar Ratio
Molar Ratio
Figure 4. ITC measurements (CHCl3, 298 K) for the titration of a)
3 and b) 4 into the chloroform solution of 2.
12000
8000
4000
0
Subsequently, we envisaged binding reversibility for
molecular tweezer/guest complexation. Hexafluoroisopropanol
(HFIP), which is a well-known fluorinated solvent capable of
interrupting hydrogen bonding interactions,15 is firstly titrated
into the chloroform solution of 2 (2.00 mM). Upon adding 8.33%
(v/v) amount of HFIP, significant upfield shift (Δδ = -0.53 ppm)
is observed for the amine proton H11, accompanying with the
obvious downfield shifts (Δδ = 0.20 and 0.15 ppm) for the
neighbouring protons H5 and H6, respectively (Fig. S14, ESI†).
Such phenomena indicate that intramolecular NH···O hydrogen
bonds on the 2,2'-iminodibenzoyl backbone, which are crucial to
preserve the tweezer-like conformation for 2, are disrupted
towards the HFIP solvent. On this basis, the impact of HFIP on
the binding strength of 2/3 was evaluated with the gradual
addition of HFIP (Fig. 5). Progressive downfield shifts are
visualized for the terpyridine protons H1-2 on 2 and the
diphenylpyridine proton Hh on 3, which is contrary to the trend of
0
1% 2% 4% 6%
0
1% 2% 4% 6%
Fraction of HFIP in CHCl
3
Figure 6. Ka values of 2/3 (■) and 2/4 (■) upon varying the
amounts of HFIP (v/v) in chloroform.
Conclusion
In summary, a novel molecular tweezer 2 has been designed
and synthesized, for which the tweezer-like conformation is pre-
organized by virtue of intramolecular NH···O hydrogen bonds on
2,2'-iminodibenzoyl skeleton. Molecular tweezer 2 exhibits
moderate binding affinities towards the neutral guests, which are
1
2/3 complexation process. Hence, H NMR results suggest the

4
Tetrahedron Letters
primarily driven by electron donor–acceptor and metal–metal
develop switchable π-aromatic receptor, which is promising for
the further development of stimuli-responsive supramolecular
materials and devices.
interactions. Notably, molecular tweezer/guest complexation is
liable to solvent polarity changes, mainly attributing to the
cleavage of intramolecular hydrogen bonds on the tweezer
receptor. The current study demonstrates the efficiency to
Khoury, N. Kyritsakas and J.-M. Lehn, J. Am. Chem. Soc., 2004,
126, 6637.
† Electronic Supplementary Information (ESI) available.
Synthesis, characterization, UV-Vis titration data and other
materials. See DOI: 10.1039/x0xx00000x.
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Graphical Abstract

Highlights
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The designed molecular tweezer is pre-organized by intramolecular hydrogen bonds;
It shows moderate binding affinity towards the organometallic guests；
The molecular tweezer/guest complexes display interesting optical behaviours;
Addition of HFIP leads to the release of guest from the molecular tweezer receptor.