Formation of charge-transfer complexes based on a tropylium cation and 2,6-helic[6]arenes: a visible redox stimulus-responsive process

Article abstract of DOI:10.1039/c7cc00600d

Charge-transfer (CT) complexes formed from 2,6-helic[6]arene hosts and a tropylium cation are described in detail. In particular, it was found that the binding and release processes of the guest in the complexes could be efficiently controlled by a redox stimulus, and the responsive process could be visually observed by the color change of the solution. To our knowledge, this represents the first visible redox stimulus-responsive host-guest system formed from the tropylium cation.

Full text of DOI:10.1039/c7cc00600d

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DOI: 10.1039/C7CC00600D
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Formation of charge-transfer complexes based on tropylium cation
and 2,6-helic[6]arenes: a visible redox stimulus-responsive process
Received 00th January 20xx,
Accepted 00th January 20xx
Geng-Wu Zhang,a,b Qiang Shia,b and Chuan-Feng Chena,b,
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Charge-transfer (CT) complexes formed by 2,6-helic[6]arene hosts electron-deficient guests.15 Herein, we report a new kind of
and tropylium cation were described in details. Especially, it was charge-transfer complexation between 2,6-helic[6]arene 1 and
found that the binding and release processes of the guest in the its hexamethyl derivative 2 with tropylium cation G (Fig. 1).
complexes could be efficiently controlled by redox stimulus, and Especially, it was found that the binding and release processes
the responsive process could be visually observed by the color of the guest in the complexes could be efficiently controlled by
change of the solution. To our knowledge, this represents the first redox stimulus, and visually observed by the color change. To
visible redox stimulus-responsive host-guest systems formed by our knowledge, this represents the first visible redox stimulus-
tropylium cation.
responsive host-guest system formed by tropylium cation.
Stimuli-responsive host-guest1 complexations have attracted much
attention for their wide applications in responsive supramolecular
polymers,2 molecular machines,3 self-healing materials4 and other
fields.5 The development of novel responsive host–guest systems
with specific complexation modes has always been a very important
and attractive topic in supramolecular chemistry.6 Tropylium cation
was a stable aromatic system with a positive center and planar rigid
structure.7 Due to its unique properties, it often acts as an effective
-acceptor and ideal guest for charge-transfer complexation with
various -electron-rich hosts. To date, the host-guest chemistry of
concerning tropylium cation has included many different classes of
macrocyclic hosts, such as resorcinarenes,8 acetylene macrocycles,9
crown ethers,10 cryptands,11 pillararenes12 and cycloaramide.13 But
there is still no report on the stimulus-responsive host-guest
system formed by tropylium cation and macrocyclic hosts.
Recently, we14 have reported a novel macrocyclic arene named
2,6-helix[6]arene, which is composed of three chiral triptycene
moieties, and has a hex nut-like structure with a helical chiral cavity
and highly fixed conformation. Moreover, the macrocycle has
a large electron-rich and chiral cavity, which makes it not only
exhibit highly enantioselectivity towards chiral trimethylamine
group-containing compounds,14 but also show high affinities
towards a wide scope of organic cationic and even neutral
Fig. 1 Structures and proton designations of 2,6-helic[6]arene 1 and its derivative
2, and the tropylium tetrafluoroborate G.
First, we tested the complexation between the hosts and
1
guest G in solution using the H NMR spectroscopic method. As
shown in Fig. 2, when host 1 and one equivalent of guest G
were mixed in 1:1 (v/v) CDCl3/CD3CN (2.00 mM), the 1H NMR
spectra showed a distinctive difference from that of free host 1
and free G, which demonstrated the formation of the complex.
Besides, only one set of peaks were found in the NMR spectra,
indicating that the complexation process was fast exchange on
the proton NMR timescale. Meanwhile, the proton signals H2
and H3 of host 1 all downfield shifted to some extent (δ = -
0.03 and -0.23 ppm, respectively), but the signal of proton Ha
of guest G was found to exhibit a significant upfield shift (δ =
1.58 ppm), which might be due to the strong shielding effect of
the aromatic rings in the host. These observations suggested
that in the complexation between host 1 and G, the guest
molecule was located inside the cavity of the macrocycle.
a.
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. Fax: 8610-62554449; E-mail: cchen@iccas.ac.cn.
b. University of Chinese Academy of Sciences, Beijing 100049, China.
Electronic Supplementary Information (ESI) available: Additional 1H NMR
†
spectra, ESI-MS for the complexes, determination of the association constants and
theoretical study. See DOI: 10.1039/b000000x
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and guest G. As shown in Fig. 3, when equivalent 2 and G were
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DOI: 10.1039/C7CC00600D
mixed in the solution of CDCl3 and CD3CN, obvious cross-peaks were
detected between the proton of G and protons H1, H2 and H4 in the
benzene ring of host 2, which indicated that the guest threaded
through the central cavity of host 2 to form a 1:1 complex. Under
the same tested conditions, it was also found that host 1 and guest
G showed similar complexation behavior to that of 1·G (Fig. S3).
Fig. 2 Partial 1 H NMR spectra (400 MHz, 298 K, 1:1 v/v CDCl3 /CD3 CN) of (a) free
host 1, (b) host 1 with 1.0 equiv. guest G, (c) free guest G, (d) host 2 with 1.0
equiv. guest G and (e) free host 2 . [1]0 = [2]0 = 2.00 mM.
1
Moreover, molar ratio plot based on the H NMR titration data
showed the complexation between host 1 and guest G was in a 1:1
mode. The association constant Ka of complex 1·G was calculated to
be (2.20±0.20)×103 by using a nonlinear curve fitting method (Fig.
S4).16 In the same tested conditions, host 2 and G showed similar
complexation behavior to that in complex 1·G and formed a 1:1
complex 2·G with the association constant of (1.19±0.18)×102 M-1
(Fig. S6), which was smaller than that of complex 1·G. This result
might be ascribed to the multiple hydrogen bonding interactions
between the hydroxy groups of host 1 and the guest.
Fig. 4 Calculated structure of the complexes at the B3LYP/6-31G level. (a) Top
view and (b) side view of 1·G. (c) Top view and (b) side view of 2·G .
The electrospray ionization mass spectra (ESI-MS) also
confirmed the formation of 1:1 complexes between the hosts and G,
In order to gain insight into the structural characteristics of the
in which the strong m/z peaks at 985.35 and 1170.44 corresponding complexes formed by the hosts and the guest, DFT calculations of
to the positively charged species [1·G -BF4] and [2·G -BF4] were
observed, respectively (Fig. S8 and S9).
the free hosts 1 and 2, the free guest G, and their complexes were
also carried out at the B3LYP/6-31G level. Firstly, we simulated the
optimized structures of the free hosts 1, 2, and the free guest G
according to their X-ray crystal structures. From the ESPs of the
optimized structures, it was easily found that the benzene rings
surrounding the cavity of the free hosts were electron-rich, while
the free guest was electron-poor (Fig. S13). These findings
suggested that there might exist electrostatic interaction in the
formation of the host-guest complexes. The optimized structures of
the complexes 1·G and 2·G were also calculated. As shown in Fig. 4,
the guest molecules were located in the center of the cavity of the
hosts in a tilted manner. Meanwhile, multiple C-H··· interactions
and hydrogen-bonding interactions between the protons of G and
the aromatic rings of the hosts were found, and the distances
ranged from 2.45 to 3.07 Å. It was further found that in the
complexes, the Mulliken atomic charges of the host 1 and 2 were
0.2150 and 0.2079, respectively, indicating that partial positive
charges were transfered from the guest to the hosts in the
formation of the complexes.
When the colorless solutions of host 2 and of guest G were
mixed together, a significant color change occurred, indicating the
formation of charge-transfer complex (Fig. 5a). At the same
time, there also existed the charge transfer band in the UV-vis
spectrum (Fig. 5b). For complex 1·G, the similar changes in
color and UV-vis spectrum were observed, too (Fig. S10).
Fig. 3 2D NOESY spectrum (400MHz, CDCl3/CD3 CN, 1:1, v/v) of 2 and G (4.00 mM
for each).
The 2D NOESY spectroscopic experiments were further carried
out to confirm the formation of the complexes between the hosts
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7CC00600D
0.5
2
2 + G
G
0.4
0.3
0.2
0.1
0.0
500
550
600
650
700
750
800
Wavelength/nm
Fig. 5 Color changes (a) and UV-vis spectra (b) of guest G (4.00 mM) upon
complexation with 1.0 eq. of host 2 in 1:1 (v/v) CHCl3 -CD3 CN solution at 298 K.
Fig. 6 Partial 1 H NMR spectra (400MHz,CDCl3 /CD3 CN, 1/1, v/v): (a) 4.00 mM free
guest G (b) after addition of 1.0 equiv of 2 to (a); (c) after addition of 1.0 equiv of
NaBH4 to (b); (d) after addition of 2.0 equiv of NOBF4 to (c); (e) after addition of
1.0 equiv of NaBH4 to (d); (f) after addition of 2.0 equiv of NOBF4 to (e); (g) after
addition of 1.0 equiv of NaBH4 to (f).
The redox stimulus-responsive complexation behaviors
between the hosts and guest G in solution were then tested by
the 1H NMR spectroscopic method. We chose the sodium
borohydride as reductant and nitrosonium tetrafluoroborate
as the oxidant. As shown in Fig. 6c, when one equivalent of
NaBH4 was added into the solution of complex 2·G , the
complexed guest was reduced to cycloheptatriene and
dissociated from the cavity of the host. Upon the addition of
oxidant NOBF4, guest molecule was oxidized to carbenium ion,
and complexed with the host again. Meanwhile, by alternately
adding the oxidant and reductant, the 1H NMR spectra
changed accordingly, and the redox-responsive cycles could be
repeated at least 10 times (Fig. 7).
More importantly, this stimulus-responsive behavior was
accompanied by a color change. As shown in Fig. 7a, when the
host and the guest were mixed to form the complex, the color
of the solution was changed from colorless to brown. Upon the
addition of NaBH4 to the solution, the color returned from
brown to colorless due to the dissociation of the complex. And
adding the oxidant could make the color of the solution turn
brown again. This color change process could be repeated for
many times along with the redox response of the complex.
These results could provide a visual method to observe
stimulus-responsive process of the complex. The same changes
also occurred on complex 1·G (Fig. S11 and S12). When the
reductant was added into the solution of 1·G to make the
guest be reduced to cycloheptatriene, the complex dissociated
while the color of the solution changed from reddish brown to
yellow. Upon the addition of NOBF4 to the system, the guest
molecule was oxidized to the carbenium ion, the complex was
re-formed, and the solution of the color turned back to reddish
brown. The redox-responsive cycles could be repeated at least
5 times for the complex 1·G .
9.0
8.5
8.0
7.5
7.0
6.5
0
2
4
6
8
10
Number of Cycles
Fig. 7 (a) Color changes of 2 (4.00 mM, 298 K, 1:1 v/v CDCl3 /CD3 CN) in the
process of redox stimulus-responsive. (b) Redox switching of complex 2·G
monitored by 1 H NMR spectroscopy. The chemical shifts of H2 were recorded.
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In conclusion, we have demonstrated that the 2,6-helic[6]arene
and its hexamethyl derivative could form stable complexes with
tropylium tetrafluoroborate, which were supported by the H NMR
1
spectra, 2D NOESY spectra, ESI-MS and theoretical calculations.
Moreover, the color change of the solution and formation of the CT
band in UV-vis spectra indicated the charge-transfer interactions in
the complexes. Especially, it was found that the binding and release
process of the guest in the complexes could be efficiently controlled
by redox stimulus, and the responsive process could also be visually
observed by the color change of the solution. To our knowledge,
this represents the first visible redox stimulus-responsive host-guest
system formed by tropylium cation. The results presented herein
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new supramolecular assemblies with specific structures and
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