Bis[alkynylplatinum(II)] Terpyridine Molecular Tweezer/Guest Recognition Enhanced by Intermolecular Hydrogen Bonds: Phototriggered Complexation via the "caging" Strategy

Article abstract of DOI:10.1021/acs.organomet.6b00429

The binding affinity between alkynylplatinum(II) terpyridine molecular tweezer and naphthol-derived guests can be significantly enhanced by embedding an intermolecular O-H - -N hydrogen bond, facilitating the achievement of a responsive host-guest recognition system via the photolabile "caging" strategy.

Full text of DOI:10.1021/acs.organomet.6b00429

Article
pubs.acs.org/Organometallics
Bis[alkynylplatinum(II)] Terpyridine Molecular Tweezer/Guest
Recognition Enhanced by Intermolecular Hydrogen Bonds:
Phototriggered Complexation via the “Caging” Strategy
Tengfei Fu, Yifei Han, Lei Ao, 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, People’s
Republic of China
S
* Supporting Information
ABSTRACT: The binding affinity between alkynylplatinum(II) terpyridine molecular
tweezer and naphthol-derived guests can be significantly enhanced by embedding an
intermolecular O−H---N hydrogen bond, facilitating the achievement of a responsive
host−guest recognition system via the photolabile “caging” strategy.
or the bottom-up fabrication of self-assembled nano-
structured materials, the fundamental molecular recog-
Scheme 1. Schematic Representation of the Hydrogen-
Bonding-Enhanced Complexation between Molecular
Tweezer 1 and the Complementary Guests
F
nition behaviors exert significant impacts on their structural and
functional complexities.1 Over the past decade, a variety of
host−guest recognition systems have been exploited, which
primarily consist of crown ether-, cyclodextrin-, calixarene-, and
cucurbituril-type macrocyclic receptors.2−5 Beyond this scope,
acyclic molecular tweezers, representing the preorganization of
two π-aromatic pincers with designated orientation and
distance, are capable of encapsulating the complementary
guests into their cavities.6 It is highly desirable to develop a
molecular tweezer/guest recognition motif with high binding
affinity, which would benefit its potential applications in the
fields of separation, sensing, and optoelectronics. To attain this
objective, one possible approach is to expand the π-surfaces of
the pincer/guest units, which is advantageous for strengthening
electron donor−acceptor interactions.7 However, some severe
issues, such as tedious synthesis and limited solubility, are
commonly encountered for the resulting noncovalent recog-
nition system, which limit its experimental feasibility.
In this context, introduction of intermolecular hydrogen
bonds represents an alternative strategy to improve non-
covalent molecular tweezer/guest binding strength, due to its
highly directional and stimuli-responsive character.8 We have
previously demonstrated that, for the alkynylplatinum(II)
terpyridine molecular tweezer receptor 1, guest 2 displays a
remarkable 80 times enhancement of the binding affinity with
respect to that of unsubstituted pyrene (Ka = (1.80 0.11) ×
105 M−1 for 1/2 versus (2.27 0.05) × 103 M−1 for 1/pyrene)
(Scheme 1).9 This is primarily ascribed to the formation of an
intermolecular N−H---N hydrogen bond between the amide
unit on 2 and the pyridine moiety on 1. Herein we sought to
expand the scope of guest toolboxes by screening a variety of
naphthalene- and benzene-derived guests featuring hydrogen
Received: May 28, 2016
© XXXX American Chemical Society
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bond donating groups (such as 1-/2-naphthol and 1-/2-
naphthalenamine) (Scheme 1). It is anticipated that different
types of hydrogen bonds (such as O−H---N and N−H---N)
can be formed between 1 and the complementary guests,
facilitating the examination of the versatility to enhance
molecular tweezer/guest complexation via the assistance of
hydrogen bonds. More importantly, the strength of inter-
molecular hydrogen bonds can be elaborately modulated, by
“caging” the hydroxyl and amine units on the guests,10 which
could efficiently revert to their original forms in respond to
external stimuli. Hence, it is highly expected that stimuli
responsiveness can be assigned to the resulting molecular
tweezer/guest recognition system via the “caging” strategy.
Noncovalent complexation was first examined between 1 and
the complementary guests naphthalene and 2-naphthol (Figure
S1). For 1 itself, severe signal broadening behaviors are
UV−vis spectrum, gradually decrease in intensity upon
progressive addition of 2-naphthol (Figure 1b, inset). Nonlinear
curve-fitting of the collected absorbance data at 460 nm (eq S1)
provides a Ka value of (1.20
0.04) × 104 M−1 for 1/2-
naphthol (Figure 1b), which is highly consistent with the above
ITC experiments. In the meantime, a similar value (Ka = (1.50
0.10) × 104 M−1) can be achieved from the fluorescent
titration measurements (eq S2) by monitoring the progressively
decreasing MLCT/LLCT emission band of 1 centered at 600
nm (Figure 1c). In contrast, when unsubstituted naphthalene
serves as the guest, both MLCT/LLCT absorption (Figure 1d)
and emission (Figure S3) bands display very slight changes for
1. Such results denote very weak complexation strength
between 1 and naphthalene, which prevents us from obtaining
an accurate Ka value.
On the basis of the above experimental results, it is evident
that the presence of a hydroxyl group on the naphthalene guest
facilitates an enhancement in noncovalent binding affinity to a
large extent. We then resorted to using DFT theoretical
calculations to elucidate the binding mechanism. For the
optimized geometries of 1/2-naphthol, it is capable of forming
an intermolecular O−H---N hydrogen bond between the
hydroxyl group on 2-naphthol and the pyridine unit on 1, as
manifested by the short H---N distance of approximately 1.6 Å,
together with the O−H---N angle of 173° (Figure 2).
1
observed for the aromatic proton resonances in H NMR
spectrum, primarily ascribed to the formation of self-associated
structures in CHCl3 solution. This phenomenon is still
maintained upon adding an equimolar amount of naphthalene
to 1. In stark contrast, for a 1:1 mixture of 1 and 2-naphthol,
1
well-defined sharp signals emerge for the resulting H NMR
spectrum. In particular, H5 appears in the remarkably downfield
region (δ = 8.79 ppm), whereas the terpyridine protons H1−4
on 1 are located at 8.66, 7.31, 8.21, and 8.29 ppm, respectively.
Such phenomena indicate distinct complexation behaviors
between naphthalene and 2-naphthol guests toward the same
molecular tweezer receptor.
We then turned to understand the binding thermodynamics
for the resulting noncovalent recognition system. For complex
1/2-naphthol, the binding stoichiometry is determined to be
1:1, as manifested by the abrupt changes in the isothermal
titration calorimetry (ITC) curves (Figure 1a). Fitting the
exothermic isotherm data with the one-site model provides the
Ka value of (1.18
0.06) × 104 M−1 for 1/2-naphthol.
Additionally, the MLCT (metal-to-ligand charge transfer) and
LLCT (ligand-to-ligand charge transfer) absorption bands of 1,
predominately located in the region of 400−500 nm in the
Figure 2. Optimized structures of complexes 1/2-naphthol and 1/
naphthalene via DFT methods.
Moreover, the electron density predominately distributes over
the electron-rich 2-naphthol moiety in the HOMO orbital,
while it is fully occupied by electron-deficient alkynylplatinum-
(II) terpyridine motifs in LUMO and LUMO+1 orbitals. Such
phenomena definitely support the involvement of sufficient
electron donor−acceptor interactions between 1 and 2-
naphthol. In terms of 1/naphthalene (Figure 2), although the
electron distribution in LUMO and LUMO+1 orbitals is
analogous to that of 1/2-naphthol, in the HOMO orbital it
mainly distributes on the diphenylpyridine backbone, which
unambiguously indicates a rather weak charge transfer
interactions between naphthalene and the electron-deficient
pincers on 1. Hence, it is evident that the formation of an
intermolecular hydrogen bond, together with the strengthening
Figure 1. (a) ITC data for titrating 2-naphthol (8.00 mM in CHCl3)
into the CHCl3 solution of 1 (0.40 mM); (b) UV/vis absorption
spectral changes of 1 (5.00 × 10−5 M) upon addition of 2-naphthol;
(c) emission spectral changes of 1 (5.00 × 10−5 M) upon addition of
2-naphthol; (d) UV/vis absorption spectral changes of 1 (5.00 × 10−5
M) upon addition of naphthalene.
B
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of donor−acceptor interactions, contributes to the remarkably
enhanced binding affinity for 1/2-naphthol.
To obtain an in-depth understanding of hydrogen bond
enhanced complexation behaviors, the substitution groups on
the guest moiety were further varied (Table 1 and Figures S4−
reported that the hydroxyl group can be conveniently protected
by the o-nitrobenzyl dimethyl ether caging unit, which is
restored to its original form in response to UV irradiation.11
Hence, we have further designed compound 7 (Figure 4), with
Table 1. Binding Constants (Ka) of 1 with Various
Naphthalene and Benzene Derivatives in CHCl3 via UV−Vis
a
Titration
Figure 3. 1H NMR spectra (300 MHz, CDCl3, room temperature) of
(a) 1; (b) 7; (c) a 1:1 mixture of 1 and 7; (d) a 1:1 mixture of 1 and 7
after irradiation at 365 nm for 15 min; and (e) a 1:1 mixture of 1 and
6.
a
b
[1] = 5 × 10−5 M in CHCl3. No interaction can be detected on the
basis of UV−vis titration experiments.
S12). In detail, 1/1-naphthol is also prone to forming
intermolecular O−H---N hydrogen bonds, resulting in a similar
binding strength to that of 1/2-naphthol. The presence of
ortho-substituted groups (such as naphthol-derived guests 3 and
4) hampers the formation of intermolecular hydrogen bonds,
primarily due to the steric hindrance effect. In comparison,
when the substitution group is located on the para-position,
both guests 5 and 6 still maintain sufficient binding strength
toward molecular tweezer 1. Notably, naphthalene guests
bearing dihydroxyl substituents show a relatively higher binding
affinity (Ka ∼105 M−1) than the monohydroxyl counterparts
toward the receptor 1.
Moreover, when the hydroxyl group is replaced by the amine
unit, the resulting guests 1- and 2-naphthalenamine display
considerably weaker binding affinity toward 1 (Table 1). Such
phenomena are highly plausible, since nitrogen shows a
relatively lower electronegativity (3.04) than that of oxygen
(3.44). Meanwhile, 1-naphthalenemethanol also shows a
relatively low binding affinity toward 1 (Table 1), ascribed to
its relatively weak hydrogen bond donating capability. These
results demonstrate the significant role of O−H---N hydrogen
bonds for the enhanced complexation between 1 and naphthol-
derived guests. On the other hand, when phenol-derived
compounds (such as phenol and methyl p-hydroxybenzoate)
serve as the complementary guests, they show remarkably lower
binding affinity than the corresponding naphthalene derivatives,
primarily attributed to the weakened donor−acceptor inter-
actions for the resulting noncovalent complexes.
Figure 4. (a) Schematic representation of the phototriggered
modulation of binding selectivity in a three-component supra-
molecular recognition system consisting of 1, 7, and 8. (b) Fluorescent
spectral changes of the mixtures of 1, 7, and 8 upon UV irradiation at
365 nm. (c) Dependence of emission intensity at 800 nm with
prolonged irradiation time.
the attachment of a photolabile group on guest 6. As an initial
step, the photostability for 7 was examined. Although it is quite
stable in the dark state, deprotection of the caging group on 7
occurs upon irradiation by UV light (365 nm). Briefly, for a
1
2.00 mM solution of 7 in CDCl3, the H NMR resonance for
the benzylic proton located at 5.64 ppm, as well as the aromatic
proton resonance located at 8.55 ppm, progressively decreases
in intensity and totally disappears after irradiation for 15 min
1
(Figure S16). In the meantime, the newly formed H NMR
resonances coincide very well with those of guest 6, indicating
the smooth removal of the photocaged group. The complete
conversion from 7 to 6 within 15 min is further monitored via
HPLC experiment (Figure S17), which is highly consistent with
1
After validating the crucial role of the O−H---N hydrogen
bond for enhanced molecular tweezer/guest complexation, we
endeavored to construct a stimuli-responsive supramolecular
recognition system via the “caging” strategy. It is widely
the above H NMR study, and thereby validates the efficiency
of the “decaging” process.
We then turned to exploit noncovalent complexation
behaviors between 1 and 7. Due to the absence of a hydrogen
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bond donating group, 7 does not tend to be encapsulated into
the cavity of molecular tweezer 1, as reflected by the persistence
of H NMR signals for a 1:1 mixture of 1/7 (Figure 3b,c).
ACKNOWLEDGMENTS
■
This work was supported by the Fundamental Research Funds
for the Central Universities (WK3450000001,
WK2060200012), CAS Youth Innovation Promotion Associa-
tion, and the National Natural Science Foundation of China
(21274139).
1
Upon UV irradiation, the resulting mixture shows remarkable
changes for their 1H NMR signals (Figure 3d), which correlate
well with those of complex 1/6 (Figure 3e). Simultaneously,
depending on the fluorescent measurements, the MLCT/
LLCT emission bands of 1/7 display a significant decrease in
their intensities (Figure S19). Hence, it can be concluded that
photocleavage of 7 leads to the favorable noncovalent
complexation between 1 and the decaged species.
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Synthetic route to compounds 1 and 7, characterizations,
1
detailed H NMR, UV−vis, and fluorescent measure-
ments between 1 and the complementary guests (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: drfwang@ustc.edu.cn.
Notes
The authors declare no competing financial interest.
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