Encapsulation of Halocarbons in a Tetrahedral Anion Cage

Article abstract of DOI:10.1002/anie.201502399

Caged supramolecular systems are promising hosts for guest inclusion, separation, and stabilization. Well-studied examples are mainly metal-coordination-based or covalent architectures. An anion-coordination-based cage that is capable of encapsulating halocarbon guests is reported for the first time. This A₄L₄-type (A=anion) tetrahedral cage, [(PO₄)₄L₄]^12-, assembled from a C₃-symmetric tris(bisurea) ligand (L) and phosphate ion (PO₄^3-), readily accommodates a series of quasi-tetrahedral halocarbons, such as the Freon components CFCl₃, CF₂Cl₂, CHFCl₂, and C(CH₃)F₃, and chlorocarbons CH₂Cl₂, CHCl₃, CCl₄, C(CH₃)Cl₃, C(CH₃)₂Cl₂, and C(CH₃)₃Cl. The guest encapsulation in the solid state is confirmed by crystal structures, while the host-guest interactions in solution were demonstrated by NMR techniques.

Full text of DOI:10.1002/anie.201502399

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201502399
German Edition: DOI: 10.1002/ange.201502399
Anion Coordination
Encapsulation of Halocarbons in a Tetrahedral Anion Cage**
Dong Yang, Jie Zhao, Yanxia Zhao, Yibo Lei, Liping Cao, Xiao-Juan Yang, Martin Davi,
Nader de Sousa Amadeu, Christoph Janiak, Zhibin Zhang, Yao-Yu Wang, and Biao Wu*
Abstract: Caged supramolecular systems are promising hosts
for guest inclusion, separation, and stabilization. Well-studied
examples are mainly metal-coordination-based or covalent
architectures. An anion-coordination-based cage that is capa-
ble of encapsulating halocarbon guests is reported for the first
time. This A4L4-type (A = anion) tetrahedral cage,
[(PO4)4L4]12À, assembled from a C3-symmetric tris(bisurea)
ligand (L) and phosphate ion (PO43À), readily accommodates
a series of quasi-tetrahedral halocarbons, such as the Freon
components CFCl3, CF2Cl2, CHFCl2, and C(CH3)F3, and
chlorocarbons CH2Cl2, CHCl3, CCl4, C(CH3)Cl3, C(CH3)2Cl2,
and C(CH3)3Cl. The guest encapsulation in the solid state is
confirmed by crystal structures, while the host–guest interac-
tions in solution were demonstrated by NMR techniques.
chlorinated hydrocarbons are suspected carcinogens,[4] while
Freon compounds, which are useful refrigerants, can cause
ozone depletion in the stratosphere and display strong and
long-term greenhouse effects.[5] Thus, the detection and
capture of these species is imperative. Various synthetic host
systems, including metal coordination compounds and
organic hosts, can bind some halocarbon guests, such as the
commonly used solvents, CH2Cl2, CHCl3, and CCl4, and
halogenated aromatics.[6–8] However, reports on the encapsu-
lation of Freon components, many of which are gases at room
temperature, are very rare. Atwood et al.[6] reported Freon
and methane storage in the lattice voids of calix[4]arene
[9]
´
molecules. Very recently, Miljanic et al. reported a porous
noncovalent organic framework for fluorocarbons and chlor-
ofluorocarbons (CFCs). It is noteworthy that the encapsula-
tion of Freon in a well-defined, discrete cage remains
unknown.
We have been interested in anion coordination chemis-
try.[10–13] Recently we reported the first anion-coordination
tetrahedral cage (1, A4L4-type, “A” denotes anion; Scheme 1)
C
hemistry in a confined space has attracted much interest in
many applications, including recognition and separation of
a guest, stabilization of reactive molecules or reaction
intermediates, drug delivery, biomolecular sensing, and cat-
alysis.[1] Chemists have devoted much effort to create
enforced cavities, such as the zeolite-like complexes, organic
(covalent) cages, metal coordination complexes, and non-
covalent organic frameworks.[2] In particular, discrete molec-
ular cages have been extensively studied in host–guest
chemistry.[3] Whereas the majority of caged structures are
based on metal coordination, with a small portion of covalent
systems, we demonstrate in the current work that guest
inclusion can also be achieved by anion coordination cages.
Halocarbons are toxic chemicals that may cause severe
problems to human health and to the environment. Some
[*] D. Yang, J. Zhao, Dr. Y. Zhao, Prof. Y. Lei, Prof. L. Cao,
Prof. X.-J. Yang, Prof. Y.-Y. Wang, Prof. B. Wu
Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry of the Ministry of Education
Scheme 1. Assembly of the [A4L4]12À tetrahedral anion cage 2 from
phosphate anion and ligand L and the encapsulation of halocarbons.
College of Chemistry and Materials Science
Northwest University, Xi’an 710069 (China)
E-mail: wubiao@nwu.edu.cn
assembled by phosphate and a C3-symmetric tris(bisurea)
ligand.[10] However, this cage is too small to include a guest. To
facilitate the guest-inclusion chemistry of such anion-based
supramolecular systems, we designed the larger ligand L
(Scheme 1; see Supporting Information for the synthesis). A
DFT modulation of [(PO4)4L4]12À performed at the HF/STO-
3G level (Supporting Information) provided evidence for the
formation of a tetrahedral cage. The targeted guests were
(quasi)tetrahedral halocarbons (especially the highly chal-
lenging Freons) in consideration of the good match in shape
(both are tetrahedral) and size (the internal cavity of cage 2
was estimated to be about 229 Š3, while the volume of some
halocarbons ranges from 71 to 134 Š3; Supporting Informa-
Dr. M. Davi, Dr. N. d. S. Amadeu, Prof. Dr. C. Janiak
Institut für Anorganische Chemie und Strukturchemie
Universität Düsseldorf
Universitätsstrasse 1, 40225 Düsseldorf (Germany)
Dr. Z. Zhang
Division of Solid State Electronics, Department of Engineering
Sciences, Uppsala University
Uppsala 75237 (Sweden)
[**] This work was supported by the National Natural Science
Foundation of China (21271149 and 21325102). We thank Prof.
Sichun Zhang and Xingyu Si for their help in the ESI-MS measure-
ments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502399.
8658
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Angewandte
Chemie
tion) between the guest and host. Furthermore, the triphe-
First, treatment of L and [K([18]crown-6)]3PO4 in aceto-
À
nylbenzene spacer of ligand L may benefit C X···p inter-
actions with the halocarbon guest.
nitrile with an excess (ca. 40 equiv relative to cage 2) of CFCl3
(CFC-11) afforded complex 2ꢀCFCl3, in which a CFCl3
molecule is included (Figure 2). The CFCl3 resides on the
As expected, ligand L readily forms the tetrahedral cage 2,
which is capable of encapsulating a wide range of environ-
mentally problematic halocarbons, including the Freon com-
ponents CFCl3 (CFC-11), CF2Cl2 (CFC-12), CHF2Cl (HCFC-
22), and C(CH3)F3 (HFC-143a),[14] the widely used solvents
CH2Cl2, CHCl3, and CCl4, as well as chlorinated, “tetrahe-
dral” hydrocarbons C(CH3)Cl3, C(CH3)2Cl2, and C(CH3)3Cl
(Scheme 1). X-ray crystal structures clearly show the inclu-
sion and interaction of these guests with the tetrahedral cage,
and solution studies also confirmed their binding by the host.
Treatment of L with 1 equiv of (TMA)3PO4 (generated
from (TMA)OH and H3PO4; TMA = tetramethylammo-
nium) afforded the expected A4L4-type tetrahedral cage
[(PO4)4L4]12À (2). In contrast to the empty cage 1, the crystal
structure of complex (TMA)12[(PO4)4L4] (2ꢀTMA+) clearly
indicates the presence of a TMA+ guest inside the tetrahedral
cavity (Figure 1). The main backbone of 2 is similar to cage
Figure 2. a) Crystal structure of the CFCl3 inclusion complex,
[2ꢀCFCl3]12À; b) weak interactions around CFCl3.[17]
center of the tetrahedral cavity, forming extensive weak
À
contacts with the host, including a C H···Cl hydrogen bond
À
(C···Cl 3.399 Š, ]C-H···Cl 1058) and multiple C Cl···p
interactions[16] (Cl···C(ar) 3.420 to 3.604 Š) with the central
triphenylbenzene spacer. These interactions, though quite
weak independently, make a significant contribution to the
guest binding when organized together.
More remarkably, the CF2Cl2 (CFC-12) molecule, which is
a gas at room temperature (b.p. À29.88C), can be entrapped
in the tetrahedral cage when bubbled (for about 0.5 h)
through a mixture of phosphate and L. The structure of
[K([18]crown-6)]12[(PO4)4L4]·CF2Cl2 (2ꢀCF2Cl2, Figure 3a)
Figure 1. a) Crystal structure of the tetrahedral cage [(PO4)4L4]12À (2)
3À
with an encapsulated TMA+ ion; b) hydrogen bonds around the PO4
ion; c) CH···p interactions around the TMA+ guest.[17]
1,[10] with four ligands lying on the triangular faces and
3À
phosphate ions on the vertices of the tetrahedron. Each PO4
À
is coordinated by three bisurea arms through 12 strong N
H···O hydrogen bonds to reach coordination saturation. The
PO4···PO4 separations (16.3–17.6 Š) are longer than that in
cage 1 (15 Š),[10] corresponding to a larger cavity. The
included TMA+ guest is located at the center of the cage,
with the four methyl groups pointing to the four vertices,
while the other eleven TMA+ ions are outside the cage. All of
the hydrogen atoms of the encapsulated TMA+ are involved
in CH···p interactions with the central aryl rings of the
ligands,[15] which, in combination with the size and geometry
complementarity and electrostatic attraction, may provide
sufficient stabilization of the inclusion complex.
Figure 3. Crystal structures of the Freon inclusion complexes,
2ꢀCF2Cl2 (a), 2ꢀCHF2Cl (b), and 2ꢀC(CH3)F3 (c).[17]
resembles 2ꢀCFCl3, with a similar host-guest binding mode
À
À
including C Cl···p and C F···p interactions (Supporting
Information, Figure S9). Another two gaseous Freon guests,
CHF2Cl (HCFC-22, b.p. À40.88C) and C(CH3)F3 (HFC-143a,
b.p. À478C), can also be encapsulated within cage 2 as
confirmed by the crystal structures of 2ꢀCHF2Cl and 2ꢀC-
(CH3)F3 (both with [K([18]crown-6)]+ countercations, Fig-
ure 3b,c; Supporting Information, Figures S10, S11). In these
cases, multiple weak interactions between host and guest were
also observed.
To examine whether an “empty” cage 2 can exist without
3À
the encapsulated TMA+ cation, the assembly of L and PO4
was performed in the presence of the large [K([18]crown-6)]+
countercation in acetone or acetonitrile. Encouragingly, it
appeared that cage 2 is quite stable and can be isolated as
2ꢀCH3COCH3 and 2ꢀCH3CN, respectively, in which a sol-
vent molecule is weakly bound (Supporting Information,
Figures S7, S8). This established the desired ability of cage 2
to include a neutral guest without electrostatic interaction,
which was then used in the capture of organics that are of
more environmental impact.
Next, we switched to the chlorinated hydrocarbon sol-
vents. The inclusion complexes 2ꢀCH2Cl2, 2ꢀCHCl3, and
2ꢀCCl4 ([K([18]crown-6)]+ countercation; Supporting Infor-
mation, Figures S12–S14) are very similar to each other, and
2ꢀCCl4 is discussed here (Figure S14). In the structure, the
four Cl atoms of CCl4 interact with the surrounding triphe-
À
nylbenzene moieties of the ligands through two C H···Cl and
À
fifteen C Cl···p interactions (Figure S14b). Since the inclu-
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Angewandte
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sion complexes were obtained from acetonitrile with a small
amount of the guest (ca. 0.3–3% mol relative to solvent), it
seems that the chloromethanes are bound more tightly than
CH3CN. This may result from both the favorable shape of
CCl4 and multiple interactions involving Cl atoms.
Furthermore, the tetrahedral host 2 can also capture
a series of chlorinated and methylated methanes. The
structures of the complexes [K([18]crown-6)]12[(PO4)4L4ꢀG]
(G = C(CH3)Cl3, C(CH3)2Cl2, and C(CH3)3Cl) (Supporting
Information, Figures S15–S17) are close to 2ꢀCCl4. The
difference is that the number of weak interactions decreases
significantly with decreasing number of Cl atoms on the guest
À
(Figures S15, S16). As a result, only two C H···Cl bonds and
À
six C Cl···p interactions can be identified in 2ꢀC(CH3)3Cl
Figure 4. High-resolution ESI-MS spectrum of the inclusion complex
2ꢀC(CH3)Cl3; inset: measured and simulated peaks at m/
z=2293.6530.
(Figure S17). This trend is also reflected in the solution
binding studies discussed below.
The guest inclusion behavior of cage 2 in solution was
studied by NMR spectroscopy. All of the thirteen structurally
characterized inclusion complexes display very large down-
field shifts of the urea NH signals (Dd = 2.75–3.58 ppm;
Supporting Information, Figures S18–S31) in [D6]DMSO
compared to the free ligand, indicative of strong hydrogen
bonding to PO43À (Figure S32). Similar downfield shifts of the
urea NH protons were observed when 1.0 equiv of PO43À ions
(as both the [K([18]crown-6)]+ and TBA+ salt) were added to
L (5 ” 10À3 m; Figure S33). Moreover, titration experiments
proved the 1:1 (4:4) binding ratio of L and phosphate ion in
solution (Figure S34).
isotopic patterns are consistent with the simulated peaks.
Meanwhile, the sample 2ꢀC(CH3)Cl3 (Figure 4) displayed
peaks for the [A4L4] species with the guest, [(PO4)4L4(18-
crown-6)xKxHy(C2H3Cl3)(H2O)3(C4H10O)(C2H3N)]3À at m/z =
2293.6530 (x = 4, y = 5), 2394.3550 (x = 5, y = 4), 2495.0512
(x = 6, y = 3), 2596.0817 (x = 7, y = 2) and 2696.7741 (x = 8,
y = 1) (Figure S47). These NMR and MS results imply the
persistence of the tetrahedral cage in solution.
The guest inclusion was further confirmed by 19F NMR
spectra of cage 2 with added CFCl3 (ca. 40 equiv) or bubbled
CF2Cl2 gas. For CFCl3, a signal that is shifted substantially
upfield to À3.05 ppm relative to the free CFCl3 molecule
(0 ppm, Figure 5) was observed, indicative of guest inclusion
3À
When PO4 ions (as TMA+ salt) were titrated to
1
a solution of L (5 ” 10À3 m), the H NMR of TMA+ showed
a signal in the normal range (d = 2.99 ppm) and a peak in
upfield (d = 0.15 ppm) with an integral ratio of about 11:1,
demonstrating the encapsulation of a TMA+ ion by the cage.
An obvious upfield shift of the signal of H9 proton (Dd =
À0.53 ppm; see Scheme 1 for the numbering of protons)
occurred because of the shielding effects (Supporting Infor-
mation, Figure S34). Furthermore, H1 (Dd = 0.15 ppm) and
H2 (Dd = 0.24 ppm) of the central aromatic backbone shifted
downfield due to interaction with the trapped TMA+ (Fig-
ure S35). Titration of TMA+ into a solution of cage 2 (3.0 mm)
in acetonitrile was carried out and the changes of integral area
of trapped guest were plotted and fitted with a Hill function
(Figure S44), giving an apparent association constant Ka of
588mÀ1 for TMA+ ion.
Figure 5. 19F NMR spectrum of cage 2 with added CFCl3 (376 MHz,
[D6]DMSO, 208C).
The NOESY spectrum also revealed the formation of
cage 2 (Supporting Information, Figure S37). Cross-peaks are
formed between all adjacent NH protons as well as H7,H8/
H9, H5/H6,H8, and H5/H6,H9, indicating the spatial prox-
imity of the nitrophenyl and o-phenylene groups as was found
in the crystal structure (Figure S38). Meanwhile, the H atoms
of the trapped TMA+ guest show the expected through-space
interactions with H1, H2, and H3 protons on the central
triphenylbenzene fragment. The 31P NMR spectrum of com-
plex 2ꢀTMA+ in [D6]DMSO (Figure S6) showed only one
signal at 9.14 ppm (triphenylphosphine as external standard),
suggesting the formation of a single complex.
within the cage. The 19F NMR spectrum of CF2Cl2-including
cage also displayed an upfield signal (À10.68 ppm) compared
to À7.36 ppm of free CF2Cl2 (Figure S39). Moreover, varia-
ble-temperature 19F NMR measurements have been done
about free CFCl3 (CFC-11) and complex 2ꢀCFCl3 in CDCl3.
To probe the stability of the inclusion complex, the solid-state
sample of 2ꢀCFCl3 was stored under ambient conditions for
three months before the NMR measurements. In the
19F NMR spectrum of free CFC-11, the signal observed at
lower temperatures disappeared at the range 20–228C,
slightly below its boiling point (24.58C) (Figure S40a). How-
ever, when caged, the guest CFC-11 started to lose at above
258C and disappeared completely at 408C (Figure S40b).
These results demonstrate that, in the solid state, CFC-11 can
be caged and kept reasonably stable for a long time.
High-resolution ESI-MS of 2ꢀTMA+ exhibited intense
signals for [(PO4)4L4(TMA)2H4]6À at m/z = 918.9258 (Sup-
porting Information, Figure S46 and Table S1). The observed
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Raymond, Angew. Chem. Int. Ed. 1998, 37, 1840 – 1843; Angew.
Chem. 1998, 110, 1940 – 1943.
To further understand the inclusion properties of cage 2,
competition experiments were carried out. First, we examined
the inclusion/release process of guest by taking C(CH3)Cl3 as
an example. 1H NMR studies in [D6]DMSO revealed that the
signal of trapped C(CH3)Cl3 (À0.51 ppm) decreased and the
free C(CH3)Cl3 signal increased by time (Figure S41). Mean-
while, the downfield shifts of H1 and H2 moved back upon the
release of C(CH3)Cl3. Through the change of the integral
areas, the half-life period of the release of C(CH3)Cl3 was
calculated to be 16.4 min (Figure S42). Moreover, the asso-
ciation constant Ka was estimated to be 7.4mÀ1 for C(CH3)Cl3
(Figure S45). Furthermore, a mixture of equimolar CCl4,
C(CH3)Cl3, C(CH3)2Cl2, and C(CH3)3Cl were added to
a [D6]DMSO solution of cage 2. The relative integral areas
of signals corresponding to trapped C(CH3)Cl3 (À0.51 ppm),
C(CH3)2Cl2 (À0.93 ppm) and C(CH3)3Cl (À1.41 ppm)
revealed about 28% C(CH3)Cl3, 4.5% C(CH3)2Cl2, and
scarcely any C(CH3)3Cl (Figure S43), leaving 66% for CCl4.
The result implies an order of affinity of C(CH3)3Cl < C-
(CH3)2Cl2 < C(CH3)Cl3 < CCl4, which might be attributed to
the increasing host-guest interactions with more Cl atoms.
In summary, we report the first examples of the inclusion
chemistry of an anion-coordination-based cage structure. This
tetrahedral cage shows very good size and shape comple-
mentarity to a wide range of quasi-tetrahedral halocarbons.
The results represent a new concept for constructing confined
space for the inclusion of specific guests. Construction of new
caged structures and exploration of the inclusion of other
guests are currently underway.
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Keywords: anion coordination · anion-based tetrahedral cages ·
Freon · halocarbons · inclusion compounds
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[17] CCDC 1052762 – 1052774 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: March 14, 2015
Revised: April 20, 2015
Published online: June 5, 2015
Angew. Chem. Int. Ed. 2015, 54, 8658 –8661
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8661