Complexation of neutral 1,4-dihalobutanes with simple pillar[5]arenes that is dominated by dispersion forces

Article abstract of DOI:10.1039/c2ob25251a

The complexation of neutral 1,4-dihalobutanes with simple pillar[5]arenes was investigated. The results indicate the formation of interpenetrated complexes, where the dispersive interactions dominate the complex stability. Typically, 1,4-diiodobutane disp

Full text of DOI:10.1039/c2ob25251a

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PAPER
Complexation of neutral 1,4-dihalobutanes with simple pillar[5]arenes that is
dominated by dispersion forces†
a
b
b
b
b
a,b,c
a,b
Xiaoyan Shu, Jiazeng Fan, Jian Li, Xiaoyang Wang, Wei Chen, Xueshun Jia*
and Chunju Li*
Received 3rd February 2012, Accepted 29th February 2012
DOI: 10.1039/c2ob25251a
The complexation of neutral 1,4-dihalobutanes with simple pillar[5]arenes was investigated. The results
indicate the formation of interpenetrated complexes, where the dispersive interactions dominate the
complex stability. Typically, 1,4-diiodobutane displays the strongest binding strength with ethylpillar[5]-
4
−1
arene [K = (1.0 ± 0.1) × 10 M ], up to 120 fold as compared with 1,4-difluorobutane.
a
Introduction
Pillararenes (PAs) are new calixarene (CA) analogues, and are
1
–4
described as “fascinating cyclophanes with a bright future”.
They are made up of hydroquinone units linked by methylene
bridges, and possess symmetrical pillar architectures. This struc-
tural feature makes PAs superior to CAs in the construction of
threaded complexes. Furthermore, PAs are more rigid than tra-
ditional CAs, which affords the possibility of their highly effec-
4
tive binding of specially designed guests. Our recent works
have reported the surprising host–guest properties of simple
5
alkyl-substituted pillar[5]arenes (AlkP5As) towards the neutral
bis(imidazole) and dinitrile guests. It is well known that CAs
6
generally interact strongly with cationic guests, except for calix-
pyrroles, which have shown considerable promise in the area of
anion recognition and sensing. P5A’s neutral guest recognition
abilities are very unique. Searching new neutral axles and further
clarifying the binding mechanisms (especially the driving forces)
and the inclusion characteristics of this new class of supramole-
cular host are thus significantly interesting.
Scheme 1 Structure of AlkP5A hosts and neutral guests.
the dominant driving force for the complexation was van der
Waals dispersion forces (also known as London forces).
Although dispersion forces are relatively weak intermolecular
interactions, they often play a large role in some supramolecular
In this work, we choose a series of neutral dihaloalkane
derivatives (Scheme 1) as guest molecules and screen their inter-
actions with AlkP5As, which results in formation of some stable
interpenetrated complexes both in solution and in the solid
8
complexes. Furthermore, their importance in the stacking
7
between nucleobases and in the folding of proteins has also been
recognized.
state. The driving forces, binding geometries, and binding selec-
tivities are comprehensively discussed. The results suggest that
a
College of Environmental and Chemical Engineering, Shanghai
Results and discussion
University, Shanghai 200444, P.R. China. E-mail: xsjia@
mail.shu.edu.cn, cjli@shu.edu.cn
Department of Chemistry, Shanghai University, Shanghai 200444,
b
Complexation of dihalobutanes by EtP5A
P.R. China
c
As shown in Fig. 1, in the presence of about 1 equiv. of EtP5A,
State Key Laboratory of Applied Organic Chemistry, Lanzhou
University, Lanzhou, 730000, P.R. China
the signals of DIBu’s methylene protons (H and H ) exhibit a
a
b
1
†
Electronic supplementary information (ESI) available: H NMR and
C NMR spectra of hosts, Job plots, determination of the association
very substantial upfield shift (Δδ = −2.85 for H ), and broaden-
b
1
3
ing effects compared to the free axle. Typically, the broadening
constants; and crystal structures. CCDC 852484, 863689–863691. For
ESI and crystallographic data in CIF or other electronic format see DOI:
effects were so remarkable that the signals of H could not be
a
1
1
0.1039/c2ob25251a
observed in the H NMR spectrum. These results reveal that the
This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 3393–3397 | 3393

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pair in the C–F bond gets dragged very firmly towards the
fluorine atom. This makes the DFBu guest molecule much less
polarizable, and therefore leads to weaker dispersion forces upon
complexation with P5A host. In contrast, since iodine is highly
polarizable and the C–I bond is almost nonpolar (the electro-
negativities of carbon and iodine are equal), the dispersion
forces between DIBu and host are significantly strong. That is
why DIBu displays the largest association constant of (1.0 ± 0.1)
4
−1
×
10 M , up to 120 fold compared with DFBu. On the other
hand, the size-fit between host and guest may be another, but not
the crucial, reason for the large difference of K values. The van
a
10
3
der Waals volume (Vvdw = 89.9 Å ) of DFBu represents a
small fraction of the P5A cavity (i.e., 225 Å ), resulting in a
weak binding.
3
3c
For the mono-bromide guest, 1-bromobutane (BrBu), the host-
induced NMR response in CDCl3 was also a fast exchange
1
Fig. 1 H NMR spectra (500 MHz) of (a) DIBu, (b) DIBu + EtP5A,
and (c) EtP5A in CDCl at 4.6–5.2 mM.
(Fig. S14†). The complexation induced upfield shifts and broad-
3
ening effects are obviously less remarkable than those for di-
bromoalkanes, indicating a weaker binding. As expected, the K
a
value of BrBu with EtP5A [(5.2 ± 0.4) × 10 M− ] is much
1
Table 1 Association constants
K
a
for the host–guest inclusion
smaller than that observed for dibromide guest DBrBu [(4.9 ±
complexation in CDCl
3
at 298 K
3
−1
0
.3) × 10 M ]. This is mainly due to a clear advantage of
(M−
1
)
Host
Guest
K
a
cooperative dispersion, hydrogen bonding and C–H⋯π inter-
actions in the DBrBu⊂EtP5A complex.
EtP5A
EtP5A
EtP5A
EtP5A
EtP5A
EtP5A
EtP5A
EtP5A
EtP5A
MeP5A
BuP5A
OctP5A
DFBu
DClBu
DBrBu
DIBu
(8.6 ± 0.5) × 10
(1.9 ± 0.2) × 10
(4.9 ± 0.3) × 10
(1.0 ± 0.1) × 10
(5.4 ± 0.3) × 10
(3.5 ± 0.4) × 10
(3.0 ± 0.2) × 10
(1.4 ± 0.1) × 10
(5.2 ± 0.4) × 10
(1.6 ± 0.1) × 10
(4.3 ± 0.4) × 10
(4.1 ± 0.3) × 10
3
3
4
2
2
2
3
DOHBu
DN Bu
X-ray crystallographic analysis
3
DBrPro
DBrPen
BrBu
DBrBu
DBrBu
DBrBu
Further support for formation of the interpenetrated geometries
came from X-ray crystallographic analysis of the single crystals
by slow evaporation of the host–guest solutions in dichloro-
methane. We obtained the crystal structures of four complexes:
3
3
3
DClBu⊂EtP5A,
DIBu⊂BuP5A,
DFBu⊂BuP5A,
and
DClBu⊂OctP5A (Fig. 2, S18–S20†). In the solid state the guest
is included in the center of the AlkP5A host, which is consistent
with the result in solution. THe typical crystal structure of
DClBu⊂EtP5A complex is shown in Fig. 2. There exit multiple
C–H⋯π interactions between axle’s methylenes and host’s di-
alkoxybenzene units (Fig. 2a), weak C–H⋯Cl hydrogen bonds
between the host’s ethyls and the guest’s chlorine atoms
(Fig. 2b), and weak C–H⋯O hydrogen bonds between the axle’s
methylenes and the host’s oxygen atoms (Fig. 2c).
9
host engulfs the guest, which thus leads to an efficient shield
toward guest protons. On the other hand, the host is deshielded
by the presence of a guest molecule, since proton signals of
EtP5A derived from aromatic and ethyl protons display
downfield displacement (Δδ = +0.15–+0.17 ppm). Other investi-
gated 1,4-dihalobutane guest molecules (DFBu, DClBu, and
DIBu) present a similar behavior (Fig. S9–S11†). Thus, these
dihaloalkane axles must have a similar binding mode with
EtP5A, i.e., the wheel is fully threaded by the axles. These
inclusion complexes can be considered to have [2]pseudorotax-
ane structures. In the present recognition motifs, multiple
C–H⋯π interactions, C–H⋯halogen hydrogen bonds, dipole–
dipole forces and dispersion forces may exist. Due to the high
electronegativity of fluorine, the carbon–fluorine bond is very
polar and the acidity of DFBu’s methylenes is relatively strong,
which should lead to the enhanced dipole–dipole, C–H⋯π, and
Complexation of DBrBu by different AlkP5As
The interactions of DBrBu with other simple AlkP5As, MeP5A,
BuP5A and OctP5A, have also been investigated (Fig. S15–
S17†). Similar NMR changes are observed, representing the
interpenetrated complex formation. Among these four hosts,
hydrogen bonding interactions. However, the K value of DFBu
a
−1
3
−1
with EtP5A [(8.6 ± 0.5) × 10 M ] is the smallest one among
the four 1,4-dihalobutanes (Table 1). And the constant increases
in the order of F < Cl < Br < I, i.e., with the increasing polariz-
ability of the guest. This undoubtedly indicates that the inter-
action is dominated by dispersion forces. It is well known that
the dispersion forces depend on the movement of electrons to
produce temporary (instantaneous) dipoles. The shared electron
MeP5A gives the smallest K value [(1.6 ± 0.1) × 10 M ], and
a
the other three hosts (EtP5A, BuP5A and OctP5A) exhibit
similar binding affinities. For example, the binding constant of
DBrBu with EtP5A is 3.1 times larger than that for MeP5A. One
reasonable explanation is that there may be more C–H⋯Br and
dispersive interactions between the ethyls of EtP5A with the axle
than the methyls of MeP5A.
3394 | Org. Biomol. Chem., 2012, 10, 3393–3397
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Fig. 2 Crystal structure of the DClBu⊂EtP5A complex. EtP5A is green, DClBu is blue, oxygens are red, and chlorines are magenta. Dashes rep-
resent C–H⋯π interactions or hydrogen bonds. (A) C–H⋯π parameters: H⋯ring centre distances (Å), C–H⋯ring angles (°) A, 3.15, 148; B, 3.07,
1
51; C, 2.90, 169; D, 3.50, 147; E, 3.38, 118; F, 3.41, 120; G, 2.89, 146; H, 3.27, 138; I, 3.33, 137. (B) C–H⋯Cl hydrogen-bond parameters: H⋯Cl
distances (Å), C–H⋯Cl angles (°) A, 3.17, 136; B, 3.20, 140; C, 2.99, 167; D, 3.05, 167; E, 3.19, 148; F, 3.43, 154; G, 3.31, 137; H, 3.28, 151; I,
3.28, 139; J, 3.11, 141. (C) C–H⋯O hydrogen-bond parameters: H⋯O distances (Å), C–H⋯O angles (°) A, 3.14, 148; B, 2.80, 162; C, 3.33, 142;
3.10, 150.
Linker length effect
selective binding behaviors imply broad applications of the new
recognition motifs in the construction of new pillararene-based
functional supramolecular systems, such as mechanically inter-
locked structures and supramolecular polymers.
The linker length effect was then studied. From Table 1, the Ka
values for DBrPro and DBrPen with the host are reduced by
factors of 16 and 3.5, respectively, compared with that for
DBrBu. That is to say, DBrBu, possessing four methylenes in its
linker, is the most suitable axle for an EtP5A wheel. Both the
increase and the decrease of methylene linker length will reduce
the dispersion interactions, and thus lead to an obviously un-
favorable effect on the complex formation.
Experimental
Materials and methods
2
f
AlkP5A hosts (MeP5A, EtP5A, BuP5A and OctP5A) were pre-
pared by condensation of the corresponding 1,4-dialkoxyben-
zene with paraformaldehyde and BF ·O(C H ) as a catalyst. All
For comparison purposes, we also selected for our study
another two neutral 1,4-butylene derivatives, i.e., 1,4-butanediol
3
2 5 2
(DOHBu) and 1,4-diazidobutane (DN Bu). From Fig. S12 and
3
of the halogenated guest compounds were commercially avail-
S13,† the formation of [2]pseudorotaxane-type complexes was
observed since methylene protons H and H broadened and
shifted upfield upon complexation with EtP5A. The association
constants are both in the vicinity of 10 M in CDCl (Table 1).
1
13
able and used as received. H NMR and C NMR spectra were
recorded on a Bruker AV500 instrument.
a
b
2
−1
3
Since oxygen and nitrogen are less polarizable than chlorine,
bromine and iodine, the resulting weaker dispersive interactions
Determination of the association constants
between the host with DOHBu and DN Bu bring about the
weaker host–guest binding affinities.
For each of the host–guest complex, chemical exchange is fast
on the NMR time scale. To determine the association constant,
NMR titrations were done with solutions which had a constant
concentration of AlkP5A and varying concentrations of guest.
Using the nonlinear curve-fitting method, the association con-
stant was obtained for each host–guest combination from the fol-
3
Conclusions
11
In conclusion, we have demonstrated the inclusion complexation
behaviors of neutral 1,4-dihalobutanes with simple alkyl-substi-
tuted pillar[5]arenes. The results obtained indicate undoubtedly
the formation of interpenetrated complexes, where the dispersion
forces are the dominant contributions. The halogen-substituent
pattern on 1,4-butylene unit drastically affects the binding abil-
ities and selectivities. The association constant increases in the
order of F < Cl < Br < I. The present study will further clarify
the recognition mechanisms and the inclusion characteristics of
the new supramolecular building block, pillararene, and can
provide new opportunities for supramolecular chemists in this
field. The formation of interpenetrated geometries, the easy
lowing equation :
2
A ¼ ðA1=½P5Aꢀ Þ ð0:5½Gꢀ þ 0:5ð½P5Aꢀ þ 1=KaÞ ꢁ ð0:5 ð½Gꢀ
0
0
0
0
2
0:5
þ ð2½Gꢀ ð1=Ka ꢁ ½P5Aꢀ ÞÞ þ ð1=Ka þ ½P5Aꢀ Þ Þ ÞÞ
0
0
0
where A is the chemical shift change of aromatic proton (H ) on
1
AlkP5A host at [G] , A∞ is the chemical shift change of H1
0
when the host is completely complexed, [P5A]0 is the fixed
initial concentration of the AlkP5A host, and [G] is the initial
0
concentration of guest. (Fig. 3 and 4) Assuming 1 : 1 inclusion
complexation stoichiometry between AlkP5As and these neutral
guests, the plot of Δδ (the chemical shift change of AlkP5A’s
aromatic proton H ) as a function of [guest] for each examined
5
availability of both the wheels and axles, and the highly
1
0
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Org. Biomol. Chem., 2012, 10, 3393–3397 | 3395

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reflections, 26 098 independent reflections, 1639 parameters, 25
restraint, F(000) = 3112, R = 0.2447, wR = 0.5428 (all data),
1
2
R = 0.1970, wR = 0.4940 [I > 2σ(I)], max. residual density
1
2
−3
2
1
0.302 e Å , and goodness-of-fit (F ) = 2.082. CCDC 863691.
X-ray crystal data for DFBu⊂BuP5A. Crystallographic data:
colorless, C19.75H29.50FO2.50, FW 325.93, triclinic, space group
P1ˉ, a = 12.2219 (11), b = 14.4860 (13), c = 21.9366 (19), α =
3
8
9.672 (2), β = 89.514 (2), γ = 76.1320 (10), V = 3770.4 (6) Å ,
−3
−1
Z = 8, D = 1.148 g cm , T = 296 (2) K, μ = 0.080 mm , 24
c
0
29 measured reflections, 13 910 independent reflections, 820
parameters, 0 restraint, F(000) = 1416, R1 = 0.1521, wR =
2
0
.2913 (all data), R = 0.0911, wR = 0.2284 [I > 2σ(I)], max.
1 2
−3
2
residual density 1.396 e Å , and goodness-of-fit (F ) = 1.031.
CCDC 863690.
X-ray crystal data for DClBu⊂OctP5A. Crystallographic
data: colorless, C59.50H ClO , FW 929.84, monoclinic, space
1
99
5
3
Fig. 3 Partial H NMR spectra (500 MHz, CDCl , 298 K) of MeP5A
group P2(1)/c, a = 21.897 (4), b = 21.540 (4), c = 25.200 (5), α
at a concentration of 1.0 mM upon addition of DBrBu. From bottom to
top, the concentration of DBrBu was 0, 0.3, 0.8, 2.0, 4.3, 8.1, 13.4,
3
=
1
γ = 90.00, β = 98.280 (4), V = 11 761 (4) Å , Z = 8, D =
c
.050 g cm− , T = 173 (2) K, μ = 0.108 mm , 55 903 measured
3
−1
2
0.3 mM.
reflections, 20 013 independent reflections, 1171 parameters, 18
restraint, F(000) = 4104, R = 0.2188, wR = 0.3894 (all data),
1
2
R = 0.1394, wR = 0.3347 [I > 2σ(I)], max. residual density
1
2
−3
2
1
.130 e·Å , and goodness-of-fit (F ) = 1.508. CCDC 863689.
Acknowledgements
We thank the National Natural Science Foundation of China
(Nos: 20902057, 21002061 and 21142012), and the State Key
Laboratory of Applied Organic Chemistry, Lanzhou University,
for financial support. We also thank Dr Hongmei Deng (Labora-
tory for Microstructures, Shanghai University) for NMR
measurements.
Fig. 4 The non-linear curve-fitting (NMR titrations) for the complexa-
tion of MeP5A host (1.0 mM) with DBrBu in CDCl at 298 K. The con-
centration of DBrBu was 0, 0.3, 0.5, 0.8, 1.0, 2.0, 3.0, 4.3, 5.8, 8.1,
3.4, 20.3 mM.
3
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