DIBpillar[n]arenes (n = 5, 6): Syntheses, X-ray crystal structures, and complexation with n-octyltriethyl ammonium hexafluorophosphate

Article abstract of DOI:10.1021/ol1018344

Figure Presented. DIBPillar[n]arenes (n = 5, 6) were synthesized. They showed different host-guest properties with n-octyltriethyl ammonium hexafluorophosphate G due to their different cavity sizes. DIBpillar[5]arene showed no complexation with G, while D

Full text of DOI:10.1021/ol1018344

ORGANIC
LETTERS
2010
Vol. 12, No. 19
4360-4363
DIBPillar[n]arenes (n ) 5, 6): Syntheses,
X-ray Crystal Structures, and
Complexation with n-Octyltriethyl
Ammonium Hexafluorophosphate
Chengyou Han,† Fengying Ma,† Zibin Zhang,† Binyuan Xia,† Yihua Yu,‡ and
Feihe Huang*,†
Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, P. R. China, and
Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China
Normal UniVersity, Shanghai 200062, P. R. China
fhuang@zju.edu.cn
Received August 5, 2010
ABSTRACT
DIBPillar[n]arenes (n ) 5, 6) were synthesized. They showed different host-guest properties with n-octyltriethyl ammonium hexafluorophosphate
G due to their different cavity sizes. DIBpillar[5]arene showed no complexation with G, while DIBpillar[6]arene formed a 1:1 complex with G
with an association constant of 334 ((24) M-1 in chloroform. In this letter, the first pillar[6]arene crystal structure and the first investigation
of the host-guest chemistry of pillar[6]arenes are reported.
The preparation of novel macrocyclic hosts is very important
since it is one of the major driving forces to accelerate the
development of supramolecular chemistry.1,2 Pillar[5]arenes,
a new kind of macrocyclic hosts, were first synthesized by
Ogoshi et al.2a by a Lewis acid catalyzed condensation
reaction of 1,4-dimethoxybenzene and paraformaldehyde in
22% yield. Then, Cao and co-workers3 improved the yields
of pillararenes to 75-95% using the condensation of 1,4-
dialkoxy-2,5-bis(alkoxymethyl)benzene catalyzed by p-tolu-
enesulfonic acid with a disadvantage of starting materials
† Zhejiang University.
‡ East China Normal University.
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Y. J. Am. Chem. Soc. 2008, 130, 5022–5023. (b) Ogoshi, T.; Umeda, K.;
Yamagishi, T.; Nakamoto, Y. Chem. Commun. 2009, 4874–4876. (c)
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Yamagishi, T.; Nakamoto, Y. J. Org. Chem. 2010, 75, 3268–3273. (e)
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10.1021/ol1018344  2010 American Chemical Society
Published on Web 09/10/2010

not easily being available. They also obtained a new type of
hexameric macrocyclic compounds called pillar[6]arenes.
The conformational mobility of pillar[5]arenes with different
linear alkyl chains had been fully studied by Ogoshi and
co-workers.2c-e Later, Li et al. studied the binding of
pillar[5]arenes to paraquats and bis(pyridinium) derivatives
in detail.4 Recently, we successfully synthesized a series of
copillar[5]arenes from co-oligomerization of different mono-
mers.5 Pillararenes have some advantages compared with
traditional hosts. First, they are highly symmetrical and rigid
compared to crown ethers, calixarenes, and cyclodextrins,
and this affords their selective binding to guests.2f,g,4 Second,
they are easier to be functionalized by different substituents
on the benzene rings than cucurbiturils, which enables tuning
of their host-guest binding properties easily. For the
investigation and prediction of guest-binding properties of
a new host, it is necessary to know its cavity size. However,
until now, the exact cavity sizes of pillar[6]arenes are still
unknown since their crystal structures have not been reported
yet. Furthermore, the host-guest chemistry of pillar[6]arenes
has not been explored. Herein, we report the first pillar[6]arene
crystal structure and the first exploration of the host-guest
chemistry of pillar[6]arenes.
P5 and P6 are well soluble in common organic solvents
such as dichloromethane, chloroform, n-hexane, and acetone.
The melting point of P6 (142 °C) is higher than P5 (139
°C) because of the better crystallinity of P6 resulting from
its higher symmetry. The UV-vis spectra of P5 and P6 in
chloroform are almost the same with a common peak at 295
nm due to the same aromatic rings.
According to the 1H NMR spectra of P5 and P6 in CDCl3
at 22 °C (Figure 1), we found that phenylene protons H1
DIBpillar[5]arene (P5) and DIBpillar[6]arene (P6) were
successfully synthesized from the condensation of 1,4-
diisobutoxy-2,5-bis(methoxymethyl)benzene using p-tolu-
enesulfonic acid as the catalyst (Scheme 1).3 First, hydro-
1
Figure 1
.
Partial H NMR spectra (400 MHz, CDCl3, 22 °C) of
(a) P5 and (b) P6.
Scheme 1
.
Syntheses of DIBpillar[5]arene (P5) and
DIBpillar[6]arene (P6)
upshifted from 6.85 ppm for P5 to 6.72 ppm for P6 because
of the shielding of more aromatic rings of P6. Methylene
protons H3 on P5 were observed to split into two overlapped
doublets, indicating that the mobility of H3 was suppressed
and slow on the NMR time scale at 22 °C caused by the
bulky isobutyl substituents on the benzene rings. Protons H3
located in the inner and outer spaces were shielded and
deshielded, respectively, by the electron-rich cyclic structure,
leading to the 1:1 split.2d However, P6 had a more flexible
structure and larger cavity. The mobility of H3 was relatively
faster so no splitting was observed at 22 °C in CDCl3. To
further investigate the conformational characteristics of P5
1
and P6, variable-temperature H NMR measurements were
done in toluene-d8 (Figure 2). Methylene protons H3 on P5
were not coalescent even at 50 °C. Methyl protons H5 of
the methyl groups at the end of the isobutyl groups had
synchronized changes with H3, which was not observed for
previously reported linear-alkyl-substituted pillar[5]arenes.2d
Upon heating to 75 °C, H3 and H5 were all coalescent into
one peak, indicating that the thermal motions of H3 and H5
were fast enough to overcome the suppression at this
temperature. The mobility of H3 on P6 was so fast that no
splitting was observed even at -70 °C. These demonstrated
that the protons on P6 had more mobility than those of P5
at the same temperature.
quinone (1), 1-bromo-2-methylpropane, and K2CO3 in
CH3CN were refluxed to obtain 1,4-diisobutoxybenzene (2)
in a yield of 70%. After bromomethylation of 2, 1,4-
bis(bromomethyl)-2,5-diisobutoxybenzene (3) was obtained
(94%). Methoxylation in dry THF using CH3ONa yielded
1,4-diisobutoxy-2,5-bis(methoxymethyl)benzene (4) quan-
titively. Condensation of 4 catalyzed by p-toluenesulfonic
acid in CH2Cl2 yielded P5 (73%) and P6 (4.6%).
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Single crystals of P5 and P6 were obtained by slow
evaporation of their respective solutions in a mixture of
dichloromethane and acetonitrile (1:2, V/V). Though P5 and
1568–1576.
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Org. Lett., Vol. 12, No. 19, 2010
4361

Figure 4. Capped stick top view of the crystal structure of P5.
Hydrogens except the ones on the methyl groups at the top side
were omitted for clarity. The methyl groups in the red squares are
inside the cyclic space, while the methyl groups in the red triangles
are outside the cyclic space.
Figure 2.
Partial variable-temperature 1H NMR spectra (500 MHz,
toluene-d8) of P5 (left) and P6 (right).
is consistent with the above-mentioned variable-temperature
1H NMR measurement results.
By the treatment of P5 and P6 as a regular pentagonal
pillar and a regular hexagonal pillar, respectively, and
ignorance of the substituents on the oxygen atoms of the
repeating units, we calculated the structural parameters of
P5 and P6 (Table 1). In terms of the cavity size, the diameter
P6 have the same repeating unit and are cyclic molecules,
their X-ray crystal structures (Figure 3) have an obvious
Table 1. Calculated Structural Parameters for P5 and P6a
A (Å)
B (Å)
H (Å)d
V (Å3)e
P5
P6
5.6b
7.7c
13.0b
14.5c
7.8
7.8
225
400
a Based on X-ray crystal structures reported here. b Based on the diameter
of the inscribed circle or the circumcircle of the regular pentagon. c Based
on the diameter of the inscribed circle or the circumcircle of the regular
hexagon. d Based on the distance between the two oxygen atoms on the
same benzene ring. e Based on the volume of the regular pentagonal pillar
or hexagonal pillar.
Figure 3
.
Ball-stick views of the crystal structures of P5⊃(CH2Cl2)
(CH3CN) (a and c) and P6⊃(CH3CN)4 (b and d). Hydrogens except
the ones on the solvent molecules included in P5 were omitted for
clarity. Carbon atoms are black; oxygen atoms are green; chlorine
atoms are yellow; and nitrogen atoms are deep blue.
of the internal cavity of P5 is ∼5.6 Å, analogous to
cucurbit[6]uril (∼5.8 Å)6 and R-cyclodextrin (∼4.7 to ∼5.3
Å).7 The diameter of the internal cavity of P6 is ∼7.7 Å,
analogous to cucurbit[7]uril (∼7.3 Å)6 and ꢀ-cyclodextrin
(∼6.0 to ∼6.6 Å).7 Cucurbiturils and cyclodextrins are
usually insoluble in organic solvents. The organic solvent
soluble pillararenes are good and necessary supplements to
the corresponding cucurbiturils and cyclodextrins with the
similar cavity sizes.
difference. P5 has a pentagon-like cyclic structure with one
dichloromethane molecule and one acetonitrile molecule
included in its cavity, while P6 has a hexagon-like cyclic
structure with four acetonitrile molecules included in its
cavity. From a capped stick view of the crystal structure of
P5 (Figure 4), it can be observed that the methyl groups on
the isobutyl groups are divided into two sets in the solid
state, one shielded set inside the electron-rich cyclic space
and the other deshielded set outside the cyclic space.2c This
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4362
Org. Lett., Vol. 12, No. 19, 2010

To investigate the differences in host-guest binding
properties between P5 and P6, we studied their binding to
n-octyltriethyl ammonium hexafluorophosphate (G). Com-
and He,f,g,h,i could not be found. These missing peaks could
not be found even in the higher field that is below 0 ppm. A
possible reason for this is that these methylene protons are
located in the cavity of P6 and shielded by its electron-rich
cyclic structure after the formation of a threaded structure
between P6 and G.4,8 Another possible reason is the
complexation-induced broadening effect. The variable-tem-
perature 1H NMR and EXSY spectra of a mixture of P6 and
G at -2 °C also confirmed the complexation between them.
A mole ratio plot indicated that the complexation stoichi-
ometry between P6 and G was 1:1. This was confirmed by
a low-resolution electrospray ionization mass spectroscopy
peak at m/z 1618.0 corresponding to [P6⊃G-PF6]+. The
association constant of the 1:1 complex P6⊃G in chloro-
form-d was determined to be 334 ((24) M-1 by an NMR
titration method. These experiments demonstrated that G fits
the cavity of P6, but it is too big for the cavity of P5.
In summary, we successfully synthesized two new hosts
DIBpillar[5]arene P5 and DIBpillar[6]arene P6. Despite their
same repeating unit, they have different melting points,
different proton NMR chemical shifts for the same protons,
and different conformational mobilities. We reported the first
pillar[6]arene crystal structure. It was found that the cavity
size of P5 is analogous to cucurbit[6]uril and R-cyclodextrin,
while that of P6 is analogous to cucurbit[7]uril and ꢀ-cy-
clodextrin. The host-guest binding property of P6 was
investigated for the first time with n-octyltriethyl ammonium
hexafluorophosphate G as a model guest. P6 shows com-
plexation with G, while no complexation was observed
between G and P5. With their unique symmetric pillar
structures, pillararenes are good and necessary supplements
to the existing widely used macrocycles and will be widely
used in molecular recognition and material chemistry to
construct novel fascinating supramolecular structures.
1
pared with the H NMR spectra (spectra a and c in Figure
5) of P5 and G, the 1H NMR spectrum (spectrum b in Figure
1
Figure 5
.
Partial H NMR spectra (400 MHz, CDCl3, 22 °C) of
(a) 7.5 mM P5; (b) 7.5 mM P5 and G; (c) 7.5 mM G; (d) 7.5 mM
P6 and G; and (e) 7.5 mM P6.
5) of an equimolar solution of P5 and G shows no chemical
shift changes and no signal doubling, indicating no com-
plexation between P5 and G in chloroform-d. The 1H NMR
spectrum (spectrum d in Figure 5) of an equimolar solution
of P6 and G in chloroform-d shows only one set of peaks,
showing fast-exchange complexation between P6 and G on
the 1H NMR time scale at 22 °C. After complexation, phenyl
protons H1, methylene protons H3, methine protons H4, and
methyl protons H5 on P6 shifted downfield (spectra d and e
in Figure 5). No chemical shift changes were observed for
bridging methylene protons H2. The peaks corresponding to
methyl protons Hd and Hj at the two ends of G could be
observed after complexation (spectrum d in Figure 5), while
the peaks corresponding to methylene protons Ha, Hb, Hc,
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (20774086,
20834004, J0830413) and the Fundamental Research Funds
for the Central Universities (2010QNA3008).
Supporting Information Available: Synthetic procedures,
characterizations, crystal data, and other materials. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL1018344
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.
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