Selective and effective binding of pillar[5,6]arenes toward secondary ammonium salts with a weakly coordinating counteranion

Article abstract of DOI:10.1021/ol301757q

The selective and effective binding of secondary ammoniums with a weakly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF) counteranion by per-ethylated pillar[5,6]arenes is reported. The construction of a first pillararene-based self-sorting system consisting of two wheels and two axles is also described.

Full text of DOI:10.1021/ol301757q

ORGANIC
LETTERS
2012
Vol. 14, No. 16
4126–4129
Selective and Effective Binding of
Pillar[5,6]arenes toward Secondary
Ammonium Salts with a Weakly
Coordinating Counteranion
Chunju Li,*,†,‡ Xiaoyan Shu,† Jian Li,† Jiazeng Fan,† Zhenxia Chen,*,§ Linhong Weng,§
and Xueshun Jia*,†
Department of Chemistry, Shanghai University, Shanghai 200444, P. R. China, and
State Key Laboratory of Molecular Engineering of Polymers and Department of
Chemistry, Fudan University, Shanghai 200433, P. R. China
cjli@shu.edu.cn; zhxchen@fudan.edu.cn; xsjia@mail.shu.edu.cn
Received June 28, 2012
ABSTRACT
The selective and effective binding of secondary ammoniums with a weakly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF)
counteranion by per-ethylated pillar[5,6]arenes is reported. The construction of a first pillararene-based self-sorting system consisting of two
wheels and two axles is also described.
Rotaxanes, and their pseudorotaxane precursors, have
been attracting considerable attention not only for their
topological importance but also due to their inventive
applications.1 They have usually been constructed by using
supramolecular macrocyclic hosts as the ‘wheels’ and linear
guest molecules as the ‘axles’. Pillararenes are a new class of
cyclophanes that exhibit intriguing and peculiar hostꢀ
guest properties. They are made up of hydroquinone units
linked by methylene bridges and possess the symmetrical
pillar architecture.2ꢀ5 Pillararenes’ structural characteris-
tics make the hosts suitable to develop pseudorotaxanes
with linear guest molecules. Our previous papers have
reported the formation of a series of threaded complexes
between pillar[5]arenes (P5As) with dicationic 1,4-bis-
(imidazolium)butanes, bis(pyridinium) derivatives and
(2) (a) Cragg, P. J.; Sharma, K. Chem. Soc. Rev. 2012, 41, 597–607.
(b) Xue, M.; Yang, Y.; Chi, X.; Zhang Z.;, Huang, F. Acc. Chem. Res.
2012, 45, DOI: 10.1021/ar2003418.
† Shanghai University.
‡ State Key Laboratory of Molecular Engineering of Polymers, Fudan
University.
(3) (a) Zhang, Z.; Luo, Y.; Chen, J.; Dong, S.; Yu, Y.; Ma, Z.; Huang,
F. Angew. Chem., Int. Ed. 2011, 50, 1397–1401. (b) Si, W.; Chen, L.; Hu,
X.-B.; Tang, G.; Chen, Z.; Hou, J.-L. Angew. Chem., Int. Ed. 2011, 50,
12564–12568. (c) Kou, Y.; Tao, H.; Cao, D.; Fu, Z.; Schollmeyer, D.;
Meier, H. Eur. J. Org. Chem. 2010, 6464–6470. (d) Strutt, N. L.; Forgan,
R. S.; Spruell, J. M.; Botros, Y. Y.; Stoddart, J. F. J. Am. Chem. Soc.
2011, 133, 5568–5671. (e) Hu, X.-B.; Chen, L.; Si, W.; Yu, Y.; Hou, J.-L.
Chem. Commun. 2011, 47, 4694–4696. (f) Han, C.; Yu, G.; Zheng, B.;
Huang, F. Org. Lett. 2012, 14, 1712–1715. (g) Ogoshi, T.; Hashizume,
M.; Yamagishi, T. Chem. Commun. 2010, 46, 3708–3710. (h) Ogoshi, T.;
Kanai, S.; Fujinami, S.; Yamagishi, T.; Nakamoto, Y. J. Am. Chem.
Soc. 2008, 130, 5022–5023. (h) Ogoshi, T.; Nishida, Y.; Yamagishi, T.;
Nakamoto, Y. Macromolecules 2010, 43, 3145–3147.
§ Department of Chemistry, Fudan University.
(1) (a)Chen, Y.;Liu,Y. Chem. Soc. Rev. 2010, 39, 495–505. (b) Harada,
A.; Hashidzume, A.; Yamaguchi, H.; Takashima, Y. Chem. Rev. 2009,
109, 5974–6023. (c) Chen, G.; Jiang, M. Chem. Soc. Rev. 2011, 40, 2254–
2266. (d) Tian, H.; Wang, Q. C. Chem. Soc. Rev. 2006, 35, 361–374.
(e) Nepogodiev, S. A.; Stoddart, J. F. Chem. Rev. 1998, 98, 1959–1976.
€
(f) Hanni, K. D.; Leigh, D. A. Chem. Soc. Rev. 2010, 39, 1240–1251.
(g) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H.-J.; Kim, K. Acc.
Chem. Res. 2003, 36, 621–630. (h) Chen, C.-F. Chem. Commun. 2011, 47,
1674–1688. (i) Mullen, K. M.; Beer, P. D. Chem. Soc. Rev. 2009, 38,
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r
10.1021/ol301757q
Published on Web 08/06/2012
2012 American Chemical Society

paraquat derivatives as well as neutral bis(imidazole)
derivatives, alkanedinitriles, and 1,4-dihalobutanes.4
It is well-known that secondary ammonium salts have
been widely used in fabricating interpenetrated geometries
as linear axle components. Crown ethers,1h,6 cucurbiturils,1g,7
calixarenes,8 and other cyclic macrocycles9 have been success-
fully threaded onto suitable secondary ammonium salts to
fabricate pseudorotaxane topologies. Especially, the thread-
ing of secondary ammonium derivatives through the annulus
of large crown ethers was the genesis of the interpenetrated
structures such as rotaxanes and catenanes. Very recently,
when we prepared this paper, Huang et. al3f reported the
complexation between per-methylated P5A and n-octylethyl
ammonium hexafluorophosphate (PF6ꢀ), giving a moderate
Previous works have well demonstrated the sizeꢀfit
relationship between the alkyl chain and P5A’s cavity.4
Thus the secondary ammonium 1þ guest containing an
n-butyl unit was expected to strongly bind with P5A.
Considering that the diameter of the internal cavity of
2b
˚
P6A (ca. 6.7 A) is similar to those for β-cyclodextrin
(β-CD, ca. 6.0 A)10 and cucurbit[7]uril (CB7, with a portal
˚
˚
diameter of ca. 5.4 A and an internal diameter of ca.
7.3 A),1g,10,11 we want to explore whether adamantane de-
˚
rivatives, which fit the cavity size of β-CD and CB7, would
be suitable guests for P6As. Therefore, we designed am-
monium 2þ which had an adamantane group. Both guests
contain a 9-anthracene unit in the other end. The 9-an-
thracene unit is too bulky to be bound by P5A and P6A,
which will simplify our discussion. Potentially, 1þ and 2þ
would be excellent matches, in size and shape, with P5A
and P6A, respectively.
We first studied the complexation of the two secondary
ammoniums with the PF6ꢀ counteranion by per-ethylated
P5A/P6A (EtP5A/EtP6A). However, no obvious interac-
tion was observed between 2•PF6 and EtP6A (Figure 1b),
and the binding between 1•PF6 and EtP5A was very
weak (Ka = 61 ( 8 Mꢀ1).12 It is well documented that ion-
pairing effects hamper the complexation of charged species
by neutral receptors,13 and the type of counteranions
often affects the association strength during the course of
hostꢀguest complexation dramatically.4b,8,14 We therefore
explored whether and to what extent the use of a very
weakly coordinating counteranion, BArFꢀ (Scheme 1),
would improve the hostꢀguest complexation.
association constant (Ka) value of 1.09 ((0.31) ꢁ 103 Mꢀ1
.
To date, the binding abilities of secondary ammonium salts
by pillar[6]arenes (P6As) have not been investigated as yet.
Herein, we report the highly effective self-assembly of
[2]pseudorotaxanes between P5A/P6A withtwo secondary
ammonium cations with the tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate (BArFꢀ) counteranion (1•BArF and 2•BArF)
induced by very loose ammonium ion pairs, and the con-
struction of a pillararene-based self-sorting system consisting
of four components.
Scheme 1. Structure and Proton Designations of EtP5A/EtP6A
Hosts and Secondary Ammonium Guests
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for n-octylethyl ammonium hexafluorophosphate and per-methylated
P5A [Ka = (1.09 ( 0.31) ꢁ 103 Mꢀ1, ref 3f], which may be caused by the
increase of the guest size.
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Org. Lett., Vol. 14, No. 16, 2012
4127

From the above results, we can unambiguously deduce
that the guest’s adamantane unit is included in the cavity of
the host, which thus leads to efficient shielding of the guest
protons (Hgꢀj).15 An ESI mass experiment also confirms
the hostꢀguest binding. In the ESI spectrum of an equi-
molar mixture of 2•BArF and EtP6A (Figure S20), only
two intense peaks were observed, one for the 1:1 complex
[2⊂EtP6A]þ (m/z 1424.7) and one for uncomplexed guest
2þ (m/z 356.2). The 1:1 binding stoichiometry was also
demonstrated by a Job plot based on proton NMR data
(Figure S21).
Table 1. Association Constant (Ka/Mꢀ1) for Complexation of
Neutral Guests with P5A Dimers in CDCl3 at 298 K
1•PF6
1•BArF
2•PF6
2•BArF
a
a
EtP5A
EtP6A
61 ( 8
3.4 ((0.4) ꢁ 104b
38 ( 4
a
a
3.4 ((0.2) ꢁ 103
a NMR changes are too small to allow the calculation of Ka. b 0.05%
(v%) DMSO-d6 in CDCl3.
The association constant of 2•BArF⊂EtP6A was mea-
sured by 1H NMR titration methods to be 3.4 ((0.2) ꢁ 103
Figure 1. 1H NMR spectra (500 MHz, 298 K) of (a) 2•PF6;
(b) 2•PF6 þ EtP6A; (c) EtP6A; (d) 2•BArF þ EtP6A, and
(e) 2•BArF in CDCl3 at 4.0ꢀ4.5 mM. “9” indicates CHCl3/H2O.
“*” indicates solvent impurities.
M
ꢀ1 in CDCl3 (Table 1 and Figure S23). To the best of our
knowledge, the resulting hostꢀguest complex represents
the most efficient recognition motif based on P6As.17
Thermodynamic parameters (Figures S28 and S29) indi-
cate that the complexation between EtP6A and 2•BArF is
synergistically contributed to by both enthalpy and entropy
changes (ΔH° = ꢀ11.1 kJ molꢀ1, TΔS° = 9.0 kJ molꢀ1).
The complexation of 2•BArF by the smaller EtP5A host
was then examined. As expected, no signal changes were
observed for the guest upon addition of EtP5A. Since the
adamantane unit was too large in comparison with P5A’s
cavity, the 2•BArF⊂EtP5A inclusion complex did not form.
Figure S15 shows the sequential addition of 0.3, 0.7, and
1.2 equiv of EtP5A toa solution of 1•BArFin CDCl3. Slow
exchange on the NMR time scale was observed for this
complex. From integrations of all peaks, the stoichiometry
of the complex was determined to be 1:1, which was also
been proved by ESI mass experiment (Figure S19). The
resonances of the new species are consistent with the
formation of an interpenetrated geometry. The peaks for
n-butyl protons of 1•BArF exhibit substantial upfield
shifts and broadening effects compared to the free axle
(Δδ = ꢀ1.34 to ꢀ3.10 ppm for Hgꢀj) as a consequence of
inclusion-induced shielding effects,16 indicating that the
wheel is fully threaded by the axle’s butyl unit. The forces
stabilizing the complex are very significant, since the free
guest is never observed in the presence of 1 equiv of the
EtP5A wheel or more, suggesting very strong binding
affinities in CDCl3. The association constant for this
complex in CDCl3 can not be determined using the
Figure 1e and 1d show the 1H NMR spectra of 2•BArF
in CDCl3 recorded in the absence and presence of ∼1 equiv
of EtP6A. Upon addition of the host, the NMR response
of 2•BArF was fast. The peaks for the methylene protons
and adamantane protons of 2•BArF exhibited substantial
upfield shifts and broadening effects compared to the
free axle. The Δδ value for methylene Hf was ꢀ0.21 ppm.
And the broadening effects were so remarkable that the
proton signals of methylene Hg and adamantane group
(Hh,
j) could not be observed in the 1H NMR
i, i0, and
1
spectrum. Variable-temperature and -concentration H
NMR experiments showed that the complexation induced
broadening effects of guest’s adamantane signals in higher
temperatures/concentrations were less remarkable than
those in lower temperatures/concentrations (Figures S26
and S27). For example, at 308 K, the adamantane signals
of 2•BArF could be observed in the NMR spectrum of a
1:1 hostꢀguest mixture at 13.0 mM, with obvious upfield
shifts compared to the free axle as a consequence of
inclusion-induced shielding effects (Figure S27d).15 At
the same time, the signals corresponding to the “outer”
protons (Ha, Hb and Hd) of the anthracene group remain
almost unaffected with respect to the free axle. Yet, the
“inner” Hc and He experience broadening and small down-
field shifts, which is characteristic of the protons being
located at just outside the host’s cavity portal.16
(17) Although the binding behavior of P5As has been well investi-
gated, the hostꢀguest properties of P6As have been rarely explored.
Only two types of guests, tetraalkyl ammonium hexafluorophosphate
and paraquat bis(hexafluorophosphate), were reported to be suitable
guests for P6A. See ref 5bꢀ5d.
(15) (a) Arena, G.; Gentile, S.; Gulino, F. G.; Sciotto, D.; Sgarlata, C.
Tetrahedron Lett. 2004, 45, 7091–7094. (b) Liu, Y.; Guo, D.-S.; Zhang,
H.-Y.; Ma, Y.-H.; Yang, E.-C. J. Phys. Chem. B 2006, 110, 3428–3434.
(16) Mock, W. L. Top. Curr. Chem. 1995, 175, 1–24.
4128
Org. Lett., Vol. 14, No. 16, 2012

1H NMR single point method.4f,18 The addition of a very
small amount of polar DMSO-d6 (0.05%, v%) to a CDCl3
solution of 1•BArF and EtP5A resulted in a rapid NMR
response. The Ka value for 1•BArF⊂EtP5A is determined
to be 3.4 ((0.4) ꢁ 104 Mꢀ1 (Figures S24 and S25) in
1
the CDCl3/DMSO-d6 mixed solvent by H NMR titra-
tion methods, which is 560 times larger than that for
1•PF6⊂EtP5A (61 ( 8 Mꢀ1) in pure CDCl3. The drastic
counteranion effects unambiguously indicate the signifi-
cant superiority of BArFꢀ to PF6ꢀ, due to the inducing
effect of the weakly coordinating BArFꢀ that gives free
“naked” secondary ammoniums.
The larger EtP6A host can also bind with 1•BArF,
showing a very small association constant (38 ( 4 Mꢀ1),
which is reasonable since the n-butyl group of 1þ is very
small in comparison with P6A’s cavity.
Figure 2. ESI mass spectrum of an equimolar mixture of
1•BArF, 2•BArF, EtP5A, and EtP6A in methanol solution.
The concentration of host/guest is about 0.5 μmol/L.
Based on the results above, the construction of a four-
component self-sorting system was then considered. Self-
sorting phenomena are familiar in biological systems, such
as DNA replication, translation, and transcription. The
present self-sorting system consisting of two pillararenes
(EtP5A and EtP6A) and two ammonium ions (1•BArF
and 2•BArF) should have high fidelity since 2•BArF is not
able to be bound by EtP5A and the Ka value of 1•BArF
with EtP5Ais muchlarger(890 times) thanthat for EtP6A.
Thermodynamic properties thus control the preference of
EtP5A for 1•BArF and EtP6A for 2•BArF. As can be seen
from Figure S18, the pseudorotaxanes 1•BArF⊂EtP5A and
2•BArF⊂EtP6A are significantly dominant in the equimo-
lar mixture of 1•BArF, 2•BArF, EtP5A, and EtP6A, since
the response signals of the four-component mixture (Figure
S18b) are almost the superposition of those for 1•BArF/
EtP5A (Figure S18a) and 2•BArF/EtP6A (Figure S18c).
An ESI mass spectrum of the equimolar mixture of
1•BArF, 2•BArF, EtP5A, and EtP6A in CH3OH also
confirmed perfect self-sorting. As can be seen from
Figure 2, only two intense peaks for the hostꢀguest com-
plexes are observed, one for [1⊂EtP5A]þ (m/z 1154.7) and
one for [2⊂EtP6A]þ (m/z 1424.7). Meanwhile, the signal
for [1⊂EtP6A]þ is very weak at m/z 1332.6, while no com-
plex ion of [2⊂EtP5A]þ is detected at all (m/z 1246.5). To
the best of our knowledge, this is the first pillararene-based
self-sorting system.
coordinating BArFꢀ that gives free “naked” secondary
ammonium cations. Guest 2•BArF having an adamantane
unit fits the cavity of EtP6A very well but cannot be bound
by EtP5A. The binding affinity of 1•BArF, possessing an
n-butyl group, by EtP5A is much stronger than that for
EtP6A. Based on the strong binding and high selectivity, a
high-fidelity self-sorting system consisting of two pillarar-
enes and two secondary ammonium salts is constructed.
We believe that these excellent motifs will be applicable in
the fabrication of large and complex supramolecular sys-
tems, and the self-sorting behavior observed for the two
pillar[n]arenes could be used as the basis for separating
pillar[5]arene and pillar[6]arene.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (Nos.
20902057, 21002061, 21142012, and 21101031), Leading
Academic Discipline Project of Shanghai Municipal Edu-
cation Commission (No. J50101), and the State Key
Laboratory of Molecular Engineering of Polymers (Fudan
University). We thank Dr. Hongmei Deng (Laboratory
for Microstructures, Shanghai University) for NMR
measurements.
Supporting Information Available. Additional1H NMR
spectra, Job plots, ESI mass spectra, and determination of
the association constants. This material is available free of
charge via the Internet at http://pubs.acs.org.
Inconclusion, the highly effective self-assembly ofpillar-
[5,6]arene/secondary ammonium salt [2]pseudorotaxanes
can be fabricated through the inducing effect of the weakly
(18) Hoffart, D. J.; Tiburcio, J.; Torre, A.; de la; Knight, L. K.; Loeb,
S. J. Angew. Chem., Int. Ed. 2008, 47, 97–101.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 16, 2012
4129