Trapping White Phosphorus within a Purely Organic Molecular Container Produced by Imine Condensation

Article abstract of DOI:10.1002/anie.201708246

Three tetrahedral organic cages have been obtained by condensing a triamino linker with a set of three ostensibly analogous triformyl precursors. Despite the large number of imine bonds formed, the corresponding cages were obtained in exceptionally high yields. Both theory and experimental results demonstrate that intramolecular CH???π interactions within all of the cage frameworks play an important role in abetting the condensations and contributing to the near-quantitative synthetic yields. The three cages of this study exhibit high thermodynamic and kinetic stability. A variety of small neutral guest molecules with complementary sizes and geometries may be used as templates in the cage forming reactions. Among the guests that may be used in this way is white phosphorus (P₄), whose inherent reactivity towards oxygen is almost fully attenuated when bound within one of the cages.

Full text of DOI:10.1002/anie.201708246

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Accepted Article
Title: Trapping White Phosphorus within a Purely Organic Molecular
Container Produced via Imine Condensation
Authors: Hao Li, Tianyu Jiao, Liang Chen, Dong Yang, Xin Li,
Guangcheng Wu, Pingmei Zeng, Ankun Zhou, Qi Yin,
Yuanjiang Pan, Biao Wu, Xin Hong, Xueqian Kong, Vincent M
Lynch, and Jonathan L Sessler
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708246
Angew. Chem. 10.1002/ange.201708246
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Angewandte Chemie International Edition
10.1002/anie.201708246
COMMUNICATION
Trapping White Phosphorus within a Purely Organic Molecular
Container Produced via Imine Condensation
‡
‡
Tianyu Jiao, Liang Chen, Dong Yang, Xin Li, Guangcheng Wu, Pingmei Zeng, Ankun Zhou, Qi Yin, Yuanjiang
Pan, Biao Wu, Xin Hong, Xueqian Kong, Vincent M. Lynch, Jonathan L. Sessler and Hao Li*
Abstract: Three tetrahedral organic cages have been obtained
by condensing a triamino linker with a set of three ostensibly
analogous triformyl precursors. In spite of the large number of
imine bonds formed, the corresponding cages were obtained in
exceptionally high yields. Both theory and experimental results
demonstrate that intramolecular CH···π interactions within all of
the cage frameworks play an important role in abetting the
condensations and contributing to the near-quantitative synthetic
yields. The three cages of this study exhibit high stability, both
thermodynamically and kinetically. A variety of small neutral
guest molecules with complementary sizes and geometries may
be used as templates in the cage forming reactions. Among the
can also control linkage geometries, the high-yielding syntheses
of purely organic cage with defined shapes and sizes presents
different challenges. Even although a number of complex
organic structures, including elegant cage systems prepared via
dynamic covalent bond formation,[13] such as imine
condensations,[14] are now known, examples where purely
organic, imine-based cages are used as so-called “molecular
flasks” to recognize guest molecules or control their reactivity in
homogeneous solutions are rare. This may reflect the fact that
the cavities of imine-based cages are typically occupied by
organic solvent molecules, which interferes with host-guest
binding.
guests that may be used in this way is white phosphorus (P
whose inherent reactivity towards oxygen is almost fully
attenuated when bound within one of the cages of this study.
4
),
Here, we report the rational design and high-yield synthesis of a
set of purely organic tetrahedral cages obtained by condensing
three closely related triformyl precursors and tris(2-
aminoethyl)amine (TREN). The high-yielding nature of the
preparations is ascribed in part to the preorganized torsionally
twisted conformations that these three triformyl precursors can
adopt, which permit stabilizing CH···π interactions[15] to occur
within the cage frameworks. These imine-based cages exhibit
remarkably enhanced stability against imine exchange, as well
as decomposition when exposed to moisture or subject to
dilution. Two of these tetrahedrons have been demonstrated to
function as molecular containers in toluene. They can
Since Cram’s seminal report[1] involving the trapping of
cyclobutadiene, molecular containers and capsules have
enjoyed a privileged role for their ability to capture and stabilize
inherently unstable chemical entities. Examples of species
stabilized in this way include unusual Diels-Alder products,[2]
strained hydrocarbons,[3] labile reaction intermediates,[4] and
explosives.[5] In 2009, Nitschke reported[6] the stabilization of a
4
white phosphorus (P ) molecule using a mixed inorganic-organic
accommodate
phosphorus. Upon encapsulation, the bound P
a
variety of small guests, including white
is stabilized both
cage. Very recently, Wu and coworkers reported[7] the use of a
cage assembled by means of anionic (phosphate) nodes for the
same purpose. Porous coordinative networks,[8] supramolecular
lattices involving fullerenes,[9] as well as carbon nanotubes,[10]
4
in solution and solid state and can be stored for at least a few
months without evidence of appreciable decomposition or
oxidation.
have also been used to trap P
of purely organic cages self-assembled by means of dynamic
covalent reactions including imine condensation to stabilize P
4
in solid states. However, the use
Three relatively rigid triformyl derivatives, namely La, Lb, and Lc,
were mixed with
aminoethyl)amine (TREN, A1), in a 1:1 stoichiometric ratio in
CDCl at room temperature (see the Supplementary Information
SI) for details). Under these imine-forming conditions, three
tetrahedral cages, La A1 , Lb A1 , and Lc A1 (Scheme 1),
a
triamino linker, namely tris(2-
4
and other highly reactive species is much less well-developed,
compared to their coordination chemistry-based counterparts.
This could be explained by the fact that, compared to cage
systems containing metal linkages[11] that have specific
coordination preferences or anion-based linking “nodes”[12] that
3
(
4
4
4
4
4
4
were obtained in yields of 90%, ≥99%, and ≥99%, respectively.
[*]
T. Jiao, L. Chen, X. Li, G. Wu, P. Zeng, A. Zhou, Q. Yin, Prof. Y.
Pan, Prof X. Hong, Prof X. Kong, Prof H. Li
Department of Chemistry, Zhejiang University
Hangzhou 310027, China
E-mail: lihao2015@zju.edu.cn
Dr. D. Yang, Prof B. Wu
Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry of the Ministry of Education, College of Chemistry and
Materials Science, Northwest University
Xi’an 710069, China
Dr. V.M. Lynch, Prof. J. L. Sessler
Department of Chemistry, The University of Texas at Austin
Austin,Texas,78712-1224,USA
Deparment of Chemistry, Shanghai University
Shanghai 200444,China
These authors contributed equally to this work.
[‡
]
Scheme 1. Structural formulas of the triformyl precursors La, Lb, Lc, and the
Supporting information for this article is given via a link at the end of
the document.
corresponding cages La
4 4 4 4 4 4
A1 , Lb A1 , Lc A1 formed by means of imine
condensation with a triamino linkage A1.
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Angewandte Chemie International Edition
10.1002/anie.201708246
COMMUNICATION
4 4 4 4 4 4
Figure 2. Single crystal structures of a) La A1 , b) Lb A1 , c) Lc A1 . Most
hydrogen atoms and disordered solvent molecules are omitted for the sake of
clarity. Carbon, gray; nitrogen, light blue; hydrogen, red. The close H-centroid
contacts are shown by using red arrows, indicating CH···π interactions. The
values of dihedral angles and distances are averaged ones obtained from the
corresponding crystal structures.
Scheme 2. Structural formulas of the triformyl precursors Ld and Le.
a phenyl ring and thus experiencing a shielded magnetic
environment.
We hypothesize that the intramolecular CH···π interactions[
15]
inferred in the case of cages Lb
provide a driving force that favors self-assembly. Consistent with
this suggestion was the finding that the yields of Lb A1 and
Lc A1 (≥99%) are reproducibly higher than that of La A1
90%). Further support came with the synthesis of two triformyl
precursors, namely, Ld and Le (Scheme 2). Combining A1 with
either Ld or Le in a 1:1 ratio in CDCl produced only an
uncharacterized mixture of oligomeric materials, rather than a
hypothetical cage product Ld A1 or Le A1 The planar
4 4 4 4 4 4
A1 , Lb A1 and Lc A1
4
4
4
4
4
4
(
3
4
4
4
4
.
trialdehyde Ld does not adopt the “preorganized” twisted
conformation needed to support the CH···π interactions inferred
in the case of cages Lb
cannot be incorporated into a tetrahedral cage (e.g., the
hypothetical Ld A1 ) without adopting a destabilizing twisted
conformation. In terms of Le, this precursor contains two fluorine
atoms whose presence precludes the formation of stabilizing
CH···π interactions, which also leads to a failure to form a cage
4 4 4 4
A1 and Lc A1 . As a consequence, it
Figure 1. Partial 1H NMR spectra (400 MHz, CDCl
Lb A1 , c) Lc A1 , d) a 1:1 mixture of La A1 and Lb
mixture of La, Lb and A1.
, 298K) of a) La
3
4 4
A1 , b)
4
4
4
4
4
4
4
4
4
4
A1 , and e) a 1:1:2
Initial characterization was made by means of 1H NMR
spectroscopic (Figure 1a-c) and mass spectrometric analyses
(
4 4
i.e., Le A1 ). Finally, density functional theory (DFT)
calculations were carried out (Figure 3) to quantify the
stabilization effect of the intramolecular CH···π interactions
within the tetrahedral cages. The calculation was performed at
the level of m06-2x functional[16] and 6-311+g(d,p) basis set,[17]
(
see the SI). Slow evaporation of chloroform solutions yielded
diffraction grade single crystals of La A1 , Lb A1 , and Lc A1 .
4
4
4
4
4
4
The resulting X-ray diffraction structures (Figure 2a-c) revealed
that in the case of all three cages, the triformyl residues derived
from La, Lb, and Lc adopt twisted conformations in the solid
state. In the cases of Lb A1 and Lc A1 , the dihedral angles
4 4 4 4
between the adjacent phenyl rings (average 70.8 and 66.9 ) are
based on the crystal structures of La
4
A1
4
, Lb
4
A1
4
and Lc
4
A1
4
(
see SI for details). The stabilization energies of the
intramolecular CH···π interactions (ΔECH···π) in La
and Lc A1
4
4 4
A1 , Lb
A1
4
o
o
4
4
were calculated to be ─22.1, ─29.3 and ─31.1
o
generally larger than those in La
4
A1
4
(average 51.5 ). The larger
kcal/mol, respectively. These calculation-based results further
support our argument that CH···π interactions act as one of the
stabilizing forces underlying the high-yields observed in the
syntheses of these three cages.
torsional twist seen in Lb A1 and Lc
4
4
4
A1
4
is ascribed to the ethyl
groups and methyl groups in Lb and Lc, respectively, that
introduce greater steric hindrance than the phenyl protons
present in La. In all three cages, one of peripheral phenyl
protons points towards an adjacent phenyl moiety. The
corresponding H-centroid distances are approximately 2.82,
4 4 4 4 4 4
The tetrahedral cages La A1 , Lb A1 , and Lc A1 are stable
for extended periods under normal laboratory conditions. In
CDCl
3
solution, little evidence of decomposition (≤5%) was seen
2
.71, and 2.70 Å in the case of La
4
A1
4
, Lb
4
A1
4
, and Lc
4
A1
4
,
1
when 1 mM solutions were monitored by H NMR spectroscopy
over the course of no less than seven days. These three cages
did not undergo polymerization or decomposition during
respectively. This leads us to suggest that the observed
structures are stabilized in part by the presence of energetically
favorable CH···π interactions. The presence of CH···π
interactions within these cages is supported (Figure 1b,c) by
crystallization from CH
diffraction analysis. When these solid materials were redissolved
and subject immediately to 1H NMR spectroscopic
in CDCl
2 2
Cl , as inferred from the powder X-ray
solution phase 1H NMR spectroscopic analysis. In CDCl
resonance ascribed to proton a' in both Lb A1 and Lc
, the
A1
3
3
4
4
4
4
analysis, the corresponding spectra of cages were restored,
indicating these cage compounds are stable in the solid state.
undergoes a significant upfield shift relative to the starting
material as would be expected for a proton that is located above
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Angewandte Chemie International Edition
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COMMUNICATION
precursors, whether aldehyde- or amine-derived, is connected
by three imine bonds once incorporated into the cage framework.
Full component exchange thus requires cleaving three imine
bonds. This imparts a relatively high energy barrier for exchange.
Consistent with this conclusion is that when the triangular prisms
La
aminoethyl)amine (A2); cf. Figure S28) were mixed under
conditions analogous to those used in the case of La A1 and
Lb A1 (vide supra), a hybrid prism, namely LaLbA2 , was
obtained. We explain this difference in reactivity (i.e., no
exchange in the case of La A1 + Lb A1 and exchange for
La A2 + Lb A2 ) to the fact that each A2 residue in the
triangular species, i.e., La A2 and Lb A2 , is connected to the
2 3 2 3
A2 and Lb A2 (prepared from La and Lb and bis(2-
4
4
4
4
3
4
4
4
4
2
3
2
3
2
3
2
3
framework by only two imine linkages. The enhanced kinetic
inertness that we ascribe to the multiple dynamic covalent
connections bears analogy to what was reported by Moore et
al.[13h] in the case of a tetrahedral cage system self-assembled
by means of alkyne metathesis.
4 4 4 4 4 4
The high stability seen for cages La A1 , Lb A1 , and Lc A1
Figure 3. The side view of the tris-phenyl fragments within the cage
led us to explore their ability to act as possible hosts. As
prepared, none of these systems demonstrated a propensity for
either gas absorption in the solid state or small molecule guest
frameworks of a) La
measured to be 2.82 and 2.71 Å, in the case of La
respectively. Reduced density gradient (RDG) figures of CH···π interactions
top view from one of the four vertices of the cages) in the case of c) La A1
and d) Lb A14. DFT calculations at the m06-2x/6-311+g(d,p) demonstrate that
the stabilization energies of intramolecular CH···π interactions between the
three phenyl rings are 5.5 and 7.3 kcal/mol, in the case of La A1 and
Lb A1 , respectively.
1
A1
4
and b) Lb
1
A1
4
.
The H-centroid distances are
1
A1 and Lb A1
4
1
4
,
recognition in CDCl
in terms of CDCl
3
solution. This latter result was rationalized
, whose concentration as solvent greatly
(
1
4
1
3
exceeds that of any putative guest, occupying the cage cavity as
a competitive “inhibitor”. When either Lb or Lc was combined
with A1 in a 1:1 ratio in toluene-d , as opposed to CDCl , only an
8 3
̶
̶
1
4
1
4
uncharacterized mixture of oligomeric products was obtained.
Analogous studies could not be carried out with La due to its
4 4 4 4 4 4
The stability of cages La A1 , Lb A1 , and Lc A1 appears to be
poor solubility in toluene-d
Lb A1 and Lc A1 in toluene-d
toluene-d is too large to act as either a guest or template for
cages Lb A1 and Lc A1 . Toluene-d was thus considered to
be an ideal solvent with which to explore whether solvents other
than CDCl could template the production of cages Lb A1 and
Lc A1 . In fact, when species with complementary sizes and
geometries, including THF, CCl , CHCl , cyclohexane, benzene,
8
. The failure of the formation of
both thermodynamic and kinetic in origin. Support for this
suggestion came from the finding that, relative to control open-
chain systems, the imine linkages within the tetrahedral cage are
stabilized against dilution or exposure to water. For instance,
when a 4 mM solution of La A1 in CDCl is diluted to 1 mM, no
4 4 3
appreciable decomposition of the cage was seen as evidenced
4
4
4
4
8
results from the fact that,
8
4
4
4
4
8
3
4
4
by the fact that no detectable quantities of free La were seen
4
4
1
under conditions of H NMR spectroscopic analysis (Figure S22).
4
3
norbornene, were added into mixtures of Lb (1 mM) or Lc (1
mM) with A1 (1 mM) in toluene-d (the concentrations of
potential guests are around 0.25─0.5 mM), Lb A1 or Lc A1
4
were produced as indicated by the sharp and clearly resolved
resonances seen in their corresponding 1H NMR spectra (see
the SI). The 1:1 host-guest binding was further confirmed by
mass spectrometry for cage Lc A1 in the case of all guest
4 4
templates. See the SI for the details and the calculation of the
corresponding association constants.
In contrast, a 3:1 mixture of benzaldehyde (18.5 mM) and A1
(
3
6.1 mM) in CDCl yielded (Figure S21) an equilibrium mixture in
8
which both the aldehyde and imine species were present in a
4
4
4
1
roughly 1:7 ratio. Moreover, the H NMR spectra of cages
La
several weeks after CDCl
was also found that La could be condensed with A1 in water-
saturated CDCl to produce La A1 (Figure S48c). On this basis,
4
A1
4
, Lb
4
A1
4
, and Lc
4
A1
4
remain essentially unchanged for
3
solutions were saturated with D
2
O. It
3
4
4
we conclude that the cage frameworks are responsible for the
enhanced hydrolytic stability of the imine linkages in cages
4
We then investigated whether white phosphorous (P ), an
inherently reactive species with a tetrahedral geometry, could
serve as a template for cage formation. Toward this end, 2 molar
equivalents of P were added into a mixture of Lb and A1 (1 mM
4
La
4 4 4 4 4 4
A1 , Lb A1 , and Lc A1 .
In a separate experiment, a solution of La
heated at 50 C for 2 days. Under these conditions, no mixed or
so-called hybrid cages were observed as inferred from H NMR
4
4 4 4
A1 and Lb A1 was
o
1
8
in each) in toluene-d and the resulting solution was heated at
≤
323 K for 8 h. This process yielded a P
4
31
⊂ Lb
4
A1
4
complex, as
spectroscopic and ESI-MS analyses (see Figure 1d and Figure
S24a in the SI). On the other hand, combining La, Lb, and A1 in
1
inferred from both H (Figure S40) and P NMR spectroscopic
measurements (Figure 4c), as well as mass spectrometric
analyses (Figure S41). Under these reaction conditions, the
yield of P
evidence of guest exchange was seen when 20 molar equiv. of
CHCl were added to the toluene-d solution of P ⊂ Lb A1
produced via the above P -templated condensation, indicating
CDCl
different tetrahedral cages (i.e., La
La Lb A1 , and Lb A1 ) (see Figure 1e and Figure S24b in the
SI). We ascribe the lack of exchange seen when La A1 and
Lb A1 were mixed to the fact that each of the constituent
3
in a 1:1:2 ratio produces a statistical mixture of five
4
A1 , La Lb A1 , La Lb A1
4
3
1
4
2
2
4
,
4 4 4
⊂ Lb A1 was calculated to be around 75%. No
1
3
4
4
4
4
4
4
4
3
8
4
4
4
4
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Angewandte Chemie International Edition
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COMMUNICATION
the Lb
4
A1
4
cage in the solid state. Slow evaporation of the
solvent (i.e., toluene-d
white crystalline powder (Figure 4a) in about 50% yield. The P
molecules within the Lb A1 cage exhibits remarkable inertness
8 4 4 4
) of P ⊂ Lb A1 complex produced a
4
4
4
even when the white powder was exposed to air for three
months, as indicated by solid state 31P NMR spectral studies
(
Figure 4d, top: red trace). Powder X-ray diffraction spectral
analysis also provided support for the conclusion that the cage
framework of P ⊂ Lb A1 complex remains essentially intact
under these conditions (cf. Figure S46). However, when the
powder was dissolved in CDCl and the solvent removed, P
4
4
4
3
4
was released from the cage (cf. Figure S47) and underwent
rapid oxidation or decomposition, as indicated by the solid state
31P
NMR spectrum (Figure 4d, bottom: blue trace). This
observation is consistent with the conclusion that host-guest
effects are responsible for the stability of P when trapped within
the Lb A1 cage.
4
4
4
In conclusion, we have obtained three tetrahedral cages by
means of imine condensation and in exceptionally high yields.
The “preorganized” torsionally twisted conformations of the
triformyl ligands are thought to allow for stabilizing
intramolecular CH···π interactions within the resulting cages and
thus may account for the high synthetic yields seen during the
multi-imine formation reactions leading to cage formation.
Compared to control imine-containing compounds, the
tetrahedral cages of this study exhibit remarkably enhanced
stability. This enhanced stability opens up opportunities to use
these cage compounds as hosts. When the substrates were
used as templates in the noncompetitive solvent toluene, cages
Lb A14 and Lc A14 proved capable of encapsulating several
guest molecules with complementary sizes and geometries. One
guest that may be captured in this way is white phosphorous.
The resulting complex proved remarkably stable both in solution
and in the solid state. To our knowledge, the tetrahedral cage
4
Lb A14 represents the first example wherein a system
containing only covalent organic bonds and neutral building
Figure 4. Information for the complex P
of the P ⊂ Lb A1 complex in solution and in the solid state, both of which
remain unchanged for months, b) single crystal structures of P ⊂ Lb A1 , c)
31P NMR spectrum (162 MHz, toluene–d , 298K), and d) solid state 31P MAS
NMR spectra (243 MHz) of the P ⊂ Lb A1 , before (red trace, top) and after
blue trace, bottom) the powder sample was dissolved in CHCl followed by
immediate solvent evaporation.
4 4 4
⊂ Lb A1 , including a) a photograph
4
4
4
4
4
4
8
4
4
4
(
3
that P
higher binding affinity within the cavity of Lb
CHCl . However, when the amount of CHCl
more than 460 molar equiv, P
cavity of Lb A1
4
might represent a better template that undergoes a
A1 than that of
was increased to
4
4
3
3
4
was gradually released from the
, as inferred from H NMR spectroscopic studies
4
4
1
4
4
(
Figure S45). Although further analyses of the exchange
chemistry are warranted, on the basis of these findings, we
propose that P ⊂ Lb A1 is more stable than CDCl ⊂ Lb A1 in
toluene-d solution.
Single crystals of P
4
4
4
3
4
4
8
4
⊂ Lb
4
A1
4
complex suitable for X-ray
diffraction analysis (Figure 4b) were obtained by slow vapor
diffusion of acetonitrile into the above reaction mixture. The
blocks is used to encapsulate and stabilize P . The present
4
system provides a stable complex that appears devoid of the
lability or exchange kinetic drawbacks that can potentially plague
systems based on coordination-, anion complexation-, or lattice
capture-based approaches. We thus propose that tetrahedral
cages, such as those described here, may have a role to play as
hosts for various reactive species, including reaction
intermediates and various high energy molecules. Tests of this
possibility are ongoing in our laboratories.
resulting structure provided unambiguous evidence for
complexation. In the solid state, the P-P bond lengths in P
P
4
4
⊂
Lb
slightly shorter than those in free P
Interestingly, each P atom in P ⊂ Lb
towards an adjacent phenyl moiety in a Lb residue in Lb
4
A1
4
complex are in the range of 2.171─2.179 Å, which is
(about 2.18 to 2.22 Å).
A1 complex points
A1
4
4
4
4
4
4
.
This orientation is different from that reported recently by the
group led by Wu, in which each P atom point towards one of the
4 4 4
vertices of the tetrahedron. Within P ⊂ Lb A1 , the P-centroid
Acknowledgements
distances are in the range of 3.266-3.444 Å, which are
significantly shorter than those reported by Wu’s group (i.e.,
3
H.L thanks the financial support from Zhejiang University, the
Chinese “Thousand Youth Talents Plan”, and The National
Natural Science Foundation of China (No. 21772173). J.L.S.
expresses appreciation to the National Science Foundation
.53-3.64 Å).[ The close contact seen in the present system is
7]
rationalized in terms of orbital overlap between the P atoms in P
and the corresponding phenyl rings in the cage, which might be
4
responsible for the higher binding affinity of the cage towards P
compared to CHCl
The P ⊂ Lb A1 complex is extremely stable in toluene-d
4
(
CHE – 1402004) and the Robert A. Welch Foundation (F-0018)
3
.
for partial support of this work.
4
4
4
8
, even
in the presence of air. The 1H and 31P NMR spectra of P
⊂
4
Keywords: Host-guest systems • Molecular recognition • Self-
Assembly • Organic cages • Labile compound stabilization
Lb
starts to crystallize from the solution. Given the solubility issues,
we focused further efforts on determining the stability of P within
4 4
A1 remains unchanged for several days until the complex
4
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COMMUNICATION
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Angewandte Chemie International Edition
10.1002/anie.201708246
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 2:
COMMUNICATION
‡
‡
Tianyu Jiao, Liang Chen, Dong
Yang, Xin Li, Guangcheng Wu,
Pingmei Zeng, Ankun Zhou, Qi Yin,
Yuanjiang Pan, Biao Wu, Xin
Hong, Xueqian Kong, Vincent M.
Lynch, Jonathan L. Sessler and
Hao Li*
Title
Trapping White Phosphorus
within a Purely Organic
Molecular Container Produced
via Imine Condensation
Three tetrahedral organic cages have been synthesized by means of imine condensation in close to
quantitative yields. The high yields are ascribed to the preorganized torsionally twisted conformations
of the trialdehyde precursors, which allows for stabilizing intramolecular CH···π interactions within the
cages. These cages exhibit enhanced stability, which appears to be both thermodynamic and kinetic in
origin, especially compared to analogous imine derivatives lacking a cage framework. Two of the three
cage molecules proved capable of encapusulating a variety of guests with complementary sizes and
geometires in toluene, including white phosphorus in the case of one of the cages. The latter complex
and the guest it contained proved remarkably stable, both solution and the solid state, including upon
exposure to oxygen.
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