Stabilization and controlled release of reactive molecules by solid-state van der waals capsules

Article abstract of DOI:10.1002/chem.201203551

Control by encapsulation: A dimeric capsule assembled from a tripodal host by van der Waals forces can stabilize a series of reactive molecules, such as cyclopentadiene and trichlorosilane, in the crystalline state even under relatively harsh conditions. Moreover, these reactive molecules can be controllably released into liquid media for further chemical reaction by thermal programming, leaving the insoluble host molecules behind for easy separation and reuse (see scheme). Copyright

Full text of DOI:10.1002/chem.201203551

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DOI: 10.1002/chem.201203551
Stabilization and Controlled Release of Reactive Molecules by Solid-State
van der Waals Capsules
[
a]
[a]
[a]
[b]
[b]
Wei Wei,* Wanlong Li, Zhongfeng Li, Weiping Su, and Maochun Hong*
In nature, reactive molecules are physically isolated from
the environment by biological hosts until they are required
on para-hexanoylcalix[4]arene can undergo efficient guest
exchange in a single crystal at the proper temperature. In
addition, the vdW hosts usually have the advantage of low
[9f]
[1]
for a particular reaction. To mimic the functionality of nat-
ural hosts, numerous molecular containers have been con-
cost and simple preparation in large quantities compared
[2–6]
[3]
[4]
[3–7]
structed
lent bonding
tive and short-lived species within their internal cavities.
by means of hydrogen, metal–ligand, or cova-
with solution-stable molecular containers.
These charac-
[5,6]
and some have been used to stabilize reac-
teristics have encouraged us to explore the use of solid-state
vdW capsules as host matrices for taming reactive guest spe-
cies by molecular encapsulation and controlled release.
Another key issue concerning the storage function of
vdW capsules is that the vdW structure must have a suffi-
cient robustness to ensure effective isolation of the reactive
species from each other. Therefore, we selected a tripodal
host molecule, 1, because it adopts an all-syn conformation
[6,7]
However, these materials can only be practically applied
under conditions in which 1) the molecular container is rigid
enough for guest storage at ambient temperature, 2) the re-
lease process is sufficiently sensitive to external stimuli, such
as heat or chemical agents, 3) after releasing guests, the host
molecules are easily separated from the system and can be
recycled multiple times, and 4) the preparation of the host is
relatively simple and low-cost. Therefore, the design and
synthesis of new recyclable host matrices for both storage
and controlled release of the reactive species, like natural
hosts, remain a great challenge for supramolecular chemists.
Recently, solid or crystalline materials that exhibit the
properties of molecular encapsulation have received consid-
[8]
erable attention. Among these materials, molecular capsu-
les that are assembled by van der Waals (vdW) forces only
[9–12]
have received particular attention.
It is evident that such
vdW assemblies are not sufficiently stable in solution be-
cause solvation effects are usually stronger than the vdW
forces holding the molecular capsules together. However,
such solid-state capsules are known to bind neutral guest
molecules within their interior cavities and, more important-
ly, the weak nature of vdW interactions can give these sys-
tems the flexibility necessary to allow guest capture and re-
[
11a]
and forms a thermodynamically favored cavity.
In the
crystal structure, two identical tripodal molecules can be ar-
ranged face-to-face with their pendant groups mutually in-
terdigitated, and thus create a capsule-shaped dimer that ac-
commodates the guest molecule. To the best of our knowl-
edge, this construction mode, which is based on the comple-
mentary shape of two identical tripodal molecules, has
[9]
lease under controlled conditions. For instance, Ripmeest-
er and co-workers reported that the controlled release of
some gases by thermal programming can be accomplished in
[9e]
a para-tert-butylcalix[4]arene vdW capsule.
Ananchenko
[11]
and co-workers also showed that a vdW nanocapsule based
rarely been adopted for the formation of vdW capsules,
although some examples involving hydrogen bonding or
[12]
electrostatic interactions have been reported. Such an in-
terdigitated structure should certainly be capable of provid-
ing steric hindrance great enough to effectively prevent the
escape of the guest molecule and is thus expected to endow
the capsule with the necessary rigidity to stabilize reactive
molecules under ordinary conditions.
[
a] Dr. W. Wei, W. Li, Dr. Z. Li
Department of Chemistry, Capital Normal University
1
00048 Beijing (P.R. China)
E-mail: weiwei_cnu@163.com
[
b] Prof. W. Su, Prof. M. Hong
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences, Fuzhou, Fujian 350002 (P.R. China)
E-mail: hmc@fjirsm.ac.cn
Herein, we show that the dimeric capsule (1) , assembled
2
from a pair of tripodal host 1 molecules by vdW forces, can
be used to stabilize a series of reactive appropriately sized
molecules in the crystalline state (Scheme 1). Within the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203551.
Chem. Eur. J. 2013, 19, 469 – 473
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
469

Scheme 1. Synthesis of capsule compound (1)
tive guests.
2
ꢀguest with various reac-
solid-state capsule, the homodimerization of cyclopenta-
diene and the hydrolysis of trichlorosilane were suppressed,
even under relatively harsh conditions. More importantly,
these guest molecules can be controllably released for fur-
ther chemical reaction into liquid media by thermal pro-
gramming and the insoluble host molecules are easily sepa-
rated and reused.
Figure 1. View of the dimeric capsule (1)
a space-filling model and its hydrogen atoms are not represented for
clarity.
2
ꢀ2. Encapsulated 2 is shown as
Although 2–5 can be handled in air at ambient temperature,
they are all potentially reactive, forming polymeric perox-
ides, in particular on exposure to light. Interestingly, the
compounds obtained show exceptional stability toward heat,
atmospheric oxidation, and light when crystallized with host
molecule 1 to form the inclusion compounds (1) ꢀG (in
[15]
The host molecule, 1, was readily synthesized according to
[
11a]
the literature in high yield.
The dimeric capsule, (1) ꢀ2,
2
was prepared by adding 2,3-dimethyl-1,3-butadiene (2) to a
solution of 1 in dichloromethane and was isolated as color-
less block crystals by slow evaporation of the solvent at
room temperature. Single-crystal X-ray analysis shows a di-
2
which G=2, 3, 4, or 5). At room temperature, these com-
pounds were stable in air without release of the guest mole-
cules for a period of several weeks. For compounds with vol-
atile guests, 2–5, a slight weight loss occurred at tempera-
tures was above 408C, but this process was quite slow,
unless the temperature exceeded 1008C. For instance, com-
pounds (1) ꢀ2 and (1) ꢀ4 retain approximately 90% of the
[13]
meric capsule structure in the unit cell.
As shown in
Figure 1, guest 2 is disordered between methyl and vinyl,
and is sealed in the cavity of the capsule by its hydrophobic
walls without any obvious interaction between the host and
1
guest. H NMR spectroscopic measurement of the single-
2
2
crystal samples dissolved in CDCl shows that the ratio of
guest molecules even after 12 h at 608C. When exposed to
sunlight in air for several days, the corresponding inclusion
compounds did not show any changes in terms of their color
3
the host to the guest is 2:1. Alternatively, this inclusion com-
pound can also be readily prepared by directly mixing con-
centrated solutions of 1 with excess 2 in dichloromethane.
The resultant white precipitate is a nonstoichiometric com-
pound in which approximately 64% of the capsules contain
1
or H NMR spectra (see the Supporting Information).
The effects of molecular encapsulation were unambigu-
ously demonstrated by the stabilization of the more reactive
cyclopentadiene (6), which rapidly homodimerize under or-
dinary conditions
[14]
the guest molecule.
Despite the different host–guest
ratios, both the single-crystal and the nonstoichiometric
[16]
compounds are denoted identically by (1) ꢀ2 to emphasize
.
After adding a concentrated solution of 1 in dichlorome-
2
the encapsulation behavior of the molecular capsule.
thane to freshly prepared 6, the crystalline precipitate
Similarly to the procedure for the synthesis of (1) ꢀ2,
formed was filtered off and dried immediately to obtain the
2
nearly identical capsule compounds were synthesized in
corresponding compound, (1) ꢀ6, the crystal form of which
2
1
high yields when isoprene (3), furan (4), and 2-methylfuran
was confirmed by its XRPD pattern (Figure 2a). H NMR
(
5) were employed as guests instead of 2. These molecules
spectroscopic measurements demonstrated that the guest oc-
cupancy in the capsule was 72% (Figure 2b). However, at-
were selected because of their moderate size, which is well
matched to that of the cavity in the capsule.
tal X-ray analysis and X-ray powder diffraction (XRPD)
confirmed that the compounds obtained have similar struc-
tures, all of which consist of dimeric capsules as was ob-
[11a]
Single-crys-
tempts to grow single crystals of (1) ꢀ6 failed because dicy-
2
[13]
clopentadiene (9), produced by the dimerization of 6, led to
[
11a]
a different type of crystal packing.
Similarly to (1) ꢀ2,
2
the molecules of 6 in the capsule possessed exceptional sta-
served for (1) ꢀ2 (see the Supporting Information).
bility at elevated temperature, with approximately 92, 77,
2
470
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 469 – 473

Controlled Release of Reactive Molecules
COMMUNICATION
[
17]
Figure 3. IR spectra of a) (1)
2
ꢀCHCl
3
,
b) (1)
2
ꢀ8, and c) the solid
(
1)
2
ꢀ8 in water at 708C for 48 h.
Figure 2. a) XRPD patterns for (1)
from single-crystal X-ray diffraction data of (1)
2
ꢀ6. Simulated data are generated
1
2
ꢀ5. b) H NMR spectrum
the Supporting Information). Consequently, the CH OH/
3
of (1)
2
ꢀ6 dissolved in CDCl
tial H NMR spectra of (1)
tures for 12 h under vacuum.
3
. The signals of 6 are marked with *. c) Par-
ꢀ6, which was treated at different tempera-
H O system is an ideal liquid medium, in which the capsule
2
1
2
compound is expected to release the guest molecules in a
heterogeneous manner at a given temperature by adjusting
the composition of the CH OH/H O system. As shown in
3
2
and 73% of the guest molecule retained after treatment
under vacuum for 12 h at 40, 60, and 808C, respectively (Fig-
ure 2c). Taking the high reactivity and low boiling point of 6
Figure 4, the percentage of guest 6 released from (1) ꢀ6 dif-
2
fered greatly with temperature and composition of the
CH OH/H O medium. For example, (1) ꢀ6 remained intact
3
2
2
(
approximately 408C) into account, it is amazing that ap-
at room temperature but completely released its guest mole-
proximately 30% of guest 6 was still contained within the
crystalline capsule compound after treatment at 1208C for
cules at 608C within 5 h in CH OH/H O (v/v=7:3). These
results suggest a convenient way to control the release of
guest molecules for further reaction.
3
2
12 h (Figure S7 in the Supporting Information). Moreover,
the observation that the amount of 9 present failed to in-
crease even with the elevation of the temperature implies
the effective isolation of 6 by the capsule (Figure 2c). As an
analogue of 6, the freshly prepared methylcyclopentadiene
To investigate the temperature-controlled initiation of the
reaction of 6 in the liquid medium, excess dimethyl fuma-
rate was added to a 6:4 mixture of CD OD and D O. At
3
2
room temperature, no perceptible NMR signals attributable
(
7), an equimolar mixture of 1-methyl and 2-methylcyclo-
pentadiene, was also successfully included in the dimeric
capsule, thus suggesting the versatility of this encapsulation
process (Figure S13 in the Supporting Information).
More importantly, the reactive molecules encapsulated
can be controllably released into the proper liquid medium
by thermal programming. Firstly, because of the highly hy-
drophobic nature of 1, the entrapped guest can be efficiently
protected from surrounding water molecules by the capsule.
Compound (1) ꢀtrichlorosilane (8) represents an example
2
of this effect. The hydrolysis of 8 is completely inhibited
even when (1) ꢀ8 was exposed to hot water for days, as con-
2
firmed by the characteristic IR signals of 8 at n˜ =2248, 1060,
À1
and 833 cm (Figure 3). By contrast, when immersed in
methanol, the capsule compounds can release the guest mol-
ecules through the transformation of its crystal form into an-
other undefined form (Figure S6 in the Supporting Informa-
tion). Although slightly soluble in pure methanol, host 1 was
insoluble in a methanol/water mixed system, even when the
volumetric proportion of methanol was 80% (Figure S14 in
Figure 4. Percentage of 6 released from solid (1)
2
ꢀ6 at different tempera-
tures after 5 h. The lines correspond to CH
volume ratios varying from 0.4 to 0.8.
3
OH/H
2
O media with different
Chem. Eur. J. 2013, 19, 469 – 473
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
471

W. Wei, M. Hong et al.
to guest 6 were found, whereas the formation of the Diels–
Alder adduct of 6 and dimethyl fumarate, dimethyl bicyclo-
Experimental Section
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[2.2.1]hept-5-ene-2,3-dicarboxylate (10), was observed upon
Single-crystal X-ray diffraction data were collected on a Bruker Smart
APEX II CCD-based diffractometer by using graphite-monochromated
MoKa radiation (l=0.71073 ꢁ). X-ray power diffraction (XRPD) pat-
terns were collected in a sealed glass capillary on a Rigaku DMAX 2500
heating the system to 508C (Figure 5). The appearance of
1
powder diffractometer with ultra 18 kW Cu radiation. H NMR data
were collected by using Bruker AVANCE III 600 spectrometers. See the
Supporting Information for more experimental details.
Acknowledgements
This study was supported by the National Natural Science Foundation of
China (Project No. 21001076 and No. 21271025), the Funding Project for
Academic Human Resources Development in Institutions of Higher
Learning under the Jurisdiction of Beijing Municipality (PHR20100718)
and the Scientific Research Common Program of Beijing Municipal Edu-
cation Commission (KM201110028007).
Keywords: host–guest systems · inclusion compounds ·
solid-state structures · supramolecular chemistry · X-ray
diffraction
Figure 5. a) Reaction between the crystalline capsule compound (1)
and dimethyl fumarate in CH OH/H O mixed medium at elevated tem-
perature. b) H NMR spectrum of the reaction mixture in the presence of
excess dimethyl fumarate in CD OD/D O (v/v=5:5) at 508C after 12 h.
2
ꢀ6
3
2
1
3
2
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Received: October 5, 2012
Published online: December 18, 2012
Chem. Eur. J. 2013, 19, 469 – 473
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