A tetrameric cage with D2h symmetry through alkyne metathesis

Article abstract of DOI:10.1002/anie.201404880

Shape-persistent covalent organic polyhedrons (COPs) with ethynylene linkers are usually prepared through kinetically controlled cross-coupling reactions. The high-yielding synthesis of ethynylene-linked rigid tetrameric cages via one-step alkyne metathes

Full text of DOI:10.1002/anie.201404880

Angewandte
Chemie
DOI: 10.1002/anie.201404880
Alkyne Metathesis
A Tetrameric Cage with D2h Symmetry through Alkyne Metathesis**
Qi Wang, Chenxi Zhang, Bruce C. Noll, Hai Long, Yinghua Jin, and Wei Zhang*
Abstract: Shape-persistent covalent organic polyhedrons
(COPs) with ethynylene linkers are usually prepared through
kinetically controlled cross-coupling reactions. The high-
yielding synthesis of ethynylene-linked rigid tetrameric cages
via one-step alkyne metathesis from readily accessible triyne
precursors is presented. The tetrameric cage contains two
macrocyclic panels and exhibits D2h symmetry. The assembly
of such a COP is a thermodynamically controlled process,
which involves the initial formation of macrocycles as key
intermediates followed by the connection of two macrocycles
with ethynylene linkages. With a large internal cavity, the cage
exhibits a high binding selectivity toward C70 (K = 3.9 ꢀ
103 LmolÀ1) over C60 (no noticeable binding).
thermal stability, ethynylene linkages can enable electron
conjugation if needed, which would be a valuable feature for
electrical, optical, and sensing applications. To date, ethyny-
lene-linked COPs have generally been prepared through
Sonogashira or Glaser-type coupling.[19–23] As coupling reac-
tions are kinetically controlled, the target molecular cages are
oftentimes obtained in low yields along with a large amount of
oligomeric or polymeric side products. High-dilution (or
pseudo high-dilution) conditions with a large excess of
catalysts are usually applied to minimize the “over-shooting”
problem, however with limited success. Alternatively, tem-
plates have been used to preorganize the monomers and
direct the syntheses of nanorings, rotaxanes, or cante-
nanes.[24–26] Herein, we report the template-free, dynamic
covalent assembly of a purely hydrocarbon molecular cage
through one-step alkyne metathesis. The tetrameric cage
consists of two macrocyclic panels and exhibits an uncommon
D2h symmetry. With a large internal cavity, the cage molecule
serves as a fullerene receptor and shows a high binding
selectivity for C70 over C60.
I
n recent years, discrete purely organic cage molecules, that
is, covalent organic polyhedrons (COPs), have attracted great
attention owing to their unique properties and interesting
applications in gas adsorption/separation,[1–3] host–guest rec-
ognition,[4–6] and as molecular “flasks”.[7,8] Moreover, their
great potential in emerging applications such as catalysis and
drug delivery is highly attractive. Recent advances in dynamic
covalent chemistry (DCvC) have provided powerful thermo-
dynamically controlled approaches towards COPs.[9–13] Most
COPs reported to date are assembled through dynamic imine
chemistry or boronic acid condensation.[11,14–16] Although
these COPs have shown intriguing applications in chemistry
Alkyne metathesis[27–35] has emerged as an alternative
viable dynamic covalent reaction. It has been widely practiced
in the synthesis of natural products,[28,36] shape-persistent
macrocycles,[37] and polymers.[38–40] However, alkyne meta-
thesis has only recently been applied to more challenging
COP synthesis, which involves innumerable possible oligo-
meric and polymeric intermediates along the pathway to the
target COP. In 2011, we reported the first application of
dynamic alkyne metathesis in the synthesis of an ethynylene-
linked shape-persistent rectangular prism.[6] In our previous
study, a (benzoyldiphenyl)acetylene moiety had to be instal-
led in the monomer unit to drive the equilibrium to the cage
product by precipitation of bis(benzoylbiphenyl)acetylene
byproducts. However, installation of the precipitating groups
in the monomer requires additional synthetic steps, and their
poor solubility causes difficult monomer purification and
premature precipitation of oligomeric intermediates.
Recently, we have developed triphenolsilane-based alkyne
metathesis catalysts that are compatible with 5 ꢀ molecular
sieves, which act as scavengers of small alkyne byproducts, for
example 2-butyne.[41] In the presence of molecular sieves,
simple propynyl-substituted monomers can undergo alkyne
metathesis with high conversion in a closed system using
triphenolsilane-based catalysts. Therefore, in this study,
simple triynes 3a and 3b are designed as the monomers
(Scheme 1a). Tetrahedron-shaped tetrameric cage 4Td is
expected to be the product, as compounds 3a and 3b are
C3-symmetric with an edge-to-face angle of 608, which closely
matches the edge-to-face angle of a tetrahedron (54.78). The
syntheses of 3a and 3b are straightforward starting from
readily available acetyl benzene 1a and 1b. SiCl4-catalyzed
condensation reaction followed by Negishi coupling afforded
À
and materials science, imine or B O linkages are susceptible
to hydrolysis in the presence of acid, base, or even mois-
ture,[17,18] which leads to the decomposition of COPs and is
a potential drawback for certain applications. In this regard,
COPs with more robust ethynylene linkages have attracted
our attention. Besides the rigidity and high chemical and
[*] Q. Wang, C. Zhang, Dr. Y. Jin, Prof. Dr. W. Zhang
Department of Chemistry and Biochemistry
University of Colorado, Boulder, CO 80309 (USA)
E-mail: wei.zhang@colorado.edu
Homepage: http://chem.colorado.edu/zhanggroup
Dr. B. C. Noll
Bruker AXS Inc.
5465 East Cheryl Parkway, Madison, WI 53711 (USA)
Dr. H. Long
National Renewable Energy Laboratory
Golden, CO 80401 (USA)
[**] We thank the National Science Foundation (DMR-1055705) and
Alfred P. Sloan Foundation for the financial support of this research,
and Youlong Zhu for helpful discussions. This research used
capabilities of the National Renewable Energy Laboratory Compu-
tational Sciences Center, which is supported by the Office of Energy
Efficiency and Renewable Energy of the U.S. Department of Energy
under Contract No. DE-AC36-08GO28308.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201404880.
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Communications
macrocyclic panel with another monomer and a third arm
bridging two macrocycles to form a cage. This agrees with the
observed 2:1 ratio of two signals for each set of originally
chemically equivalent protons of the monomer unit in
1H NMR spectra of 4D2h-A4 and 4D2h-B4.
The structure 4D2h was unambiguously determined by
single-crystal X-ray diffraction (Figure 2). Needle-like single
Scheme 1. Synthesis of tetrameric cage 4.
3a and 3b in good overall yields. The alkyne metathesis of
monomer 3a/3b (27 mm) was then conducted at 558C for 16 h
using catalyst 5 (1 mol% per propyne moiety). GPC analysis
of the crude product mixture showed the predominant
formation of a single species. The metathesis products were
isolated in good yields and characterized by 1D 1H and
13C NMR spectroscopy, gCOSY, ROESY, GPC, and MALDI-
TOF MS.
As expected, MALDI-TOF MS of the metathesis product
(R = C3H7) showed a strong signal at m/z = 1863 (Supporting
Information, Figure S4), which corresponds to a tetramer.
However, surprisingly, 1H NMR spectrum of the tetramer
showed splitting of each set of originally chemically equiv-
alent protons of the monomer unit into two signals in 2:1 ratio
(total eight sets of aromatic proton signals). This is incon-
sistent with the expected highly symmetrical tetrahedral
structure 4Td (Figure 1a), which should show only four sets of
Figure 2. Crystal structure of 4D2h-A4: a) Side view 1; b) Side view 2;
c) top view; d) crystal packing views along (011) direction; e) simpli-
fied view of 4D2h-A4; f) simplified view of the crystal packing; the two
stacks of the cages oriented differently are color-coded in yellow and
magenta. Solvent molecules are omitted for clarity.
crystals of 4D2h-A4 were obtained through slow evaporation
from a solution of 4D2h-A4 in CH2Cl2 and acetonitrile co-
solvent. 4D2h-A4 was crystallized in the monoclinic space
group P121/c1. It has been a challenge to obtain crystal
structures of pure hydrocarbon cages owing to their sensitivity
to solvent loss and the easy collapse of the crystals.[42] After
multiple failed trials, we finally succeeded in determining the
structure and packing of the cage 4D2h-A4, albeit with weak
X-ray diffraction. The two macrocyclic panels (top and
bottom of the cage) are slightly puckered and oriented in
a slipped stack fashion relative to each other with the distance
of 9.1 ꢀ between them (Figure 2a,c). Two biphenyl acetylene
arms bridge the two panels with the edge-to-face angle of
49.48, resulting in an overall Z-shape geometry of the cage
(Figure 2b). The dimension of the cage interior at the widest
point is 19.4 ꢀ. The packing structure shows that there are
two stacks of parallel cages that are arranged at an angle of
141.78 to each other (Figure 2d,f). There is no connectivity
between the cavities of the cages. The interior cavity of each
cage is filled with one disordered acetonitrile molecule and
two propyl groups from the two neighboring cages. We did not
Figure 1. a) Expected structure of tetramer 4Td with Td symmetry;
b) Observed structure of tetramer 4D2h with D2h symmetry.
aromatic proton signals. We excluded the possibility of two
dimeric cages being interlocked, since 1) the dimeric cage of
3a (or 3b) would be highly strained and disfavored; 2) we did
not observe any dimer species in the MALDI-MS; and 3) the
1H NMR spectra of the cage product at various temperatures
consistently show sharp and distinct signals rather than broad
and complicated ones. The tetrameric cage 4D2h (Figure 1b)
with D2h symmetry was then proposed, in which two macro-
cycle panels are connected by two diphenylacetylene side
arms. This structure gives rise to the splitting of the three arms
of a monomer into two types: two identical arms forming the
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observe any inter/intramolecular p–p interactions. Owing to
the absence of functional groups with directing capability, the
van der Waals interactions between neighboring molecules
controlled cross-coupling of two macrocyclic building blocks
8 and 9 (Scheme 2). Macrocycle 8 was isolated in excellent
yield (90%) through alkyne metathesis of compound 7. This
indicates the formation of 8 is a thermodynamically favored
À
and C H···p interactions between the propyl chain and
aromatic moieties appear to be the major forces to direct
the crystal packing of the cage. This is one of few purely
hydrocarbon cage crystal structures.
The formation of the cage 4D2h is somewhat surprising, as
C3-symmetric building blocks generally lead to tetrahedron-
shaped structures in the dynamic assembly process through
metal coordination or hydrogen bonding.[43–45] To gain better
understanding of the cage formation process, we monitored
the reaction progress. Aliquots of the reaction mixture were
withdrawn at different time intervals, and analyzed by GPC
and 1H NMR spectroscopy. The GPC traces showed the initial
formation of high molecular weight oligomers and their
gradual conversion to 4D2h-A4 (Supporting Information,
1
Scheme 2. Synthesis of 4D2h by kinetic control. I) cat. 5, CCl4, 558C,
90%; II) trimethylsilylacetylene, [Pd(PPh3)2Cl2], CuI, piperidine, THF,
808C; III) K2CO3, MeOH, PhMe, 4 h, RT; IV) Sonogashira cross-cou-
pling reactions under various conditions.
Figure S5). A closer look at the process through H NMR
data analysis revealed the initial conversion of monomer 3a
to a substantial amount of macrocycle 6 within 0.5 h
(Figure 3). The authentic sample of macrocycle 6 was
process and no significant angle strain is involved, further
supporting the possibility that macrocyclic species 6 forms
first and directs the assembly of 4D2h. Complementary
macrocyclic building block 9 was then obtained by Sonoga-
shira coupling of 8 with trimethylsilyl acetylene (TMSA)
followed by desilylation. Kinetically controlled cross-coupling
reactions have played an important role in the construction of
well-defined, 2D and 3D molecular architectures.[37,46,47]
However, the attempted cross-coupling of 8 and 9 failed to
yield cage 4D2h under our tested reaction conditions (for
details, see the Supporting Information). MALDI-TOF MS,
GPC, and 1H NMR spectroscopy analyses of the crude
product mixtures in multiple trials showed the formation of
oligomers and polymers without any noticeable amount of the
cage products. More exotic reaction conditions that might
lead to the desired cage formation were not further explored.
To confirm that the formation of cage 4D2h is a dynamic
and reversible process, we conducted the scrambling experi-
ment between 4D2h-A4 and 4D2h-B4. A 1:2 mixture of 4D2h-
A4 and 4D2h-B4 were subjected to alkyne metathesis (558C,
CCl4, 16 h). The GPC trace of the crude reaction mixture
showed a new peak with a broad shoulder (Figure 4).
MALDI-MS of the crude reaction mixture showed all
possible scrambled products, A3B, A2B2, AB3 together
with A4 and B4, indicating that the system is dynamic and
the cage 4D2h is not kinetically trapped.
The cage 4D2h-A4 has a large cavity, with a distance
between the top and bottom panels of about 9.0 ꢀ based on
the crystal structure. The shape-persistency and the rigid
backbones consisting of aromatic moieties make cage 4D2h-
A4 an attractive host for guest molecules, such as fullerenes.
To investigate the host–guest binding interactions between
4D2h-A4 and C60 or C70, we conducted 1H NMR titration
experiments at 298 K in [D8]toluene. Interestingly, 4D2h-A4
showed a very weak binding interaction with C60 according to
the 1H NMR data obtained from the titration experiment. We
did not observe any significant chemical shift changes of the
Figure 3. 1H NMR spectra of monomer 3a (a); Authentic sample of
macrocyclic intermediate 6 (b); crude mixture after 0.5–4 h (c–h); cage
product 4D2h-A4 (i). Spectra were recorded in CDCl3.
obtained by conducting alkyne metathesis of monomer 3a
in a closed system in the absence of molecular sieves that are
typically used to scavenge 2-butyne byproduct. After stirring
at 558C for 1 h, macrocycle 6 was isolated in 22% yield,
together with unreacted monomer 3a. This experiment
supports the notion that the macrocycle 6 is present in
a significant amount as a key intermediate during the
formation of cage 4D2h-A4. The formation of a tetramer
with D2h symmetry is therefore likely guided by the initial
predominant formation of macrocyclic panels (face-directed)
rather than by the geometrical angle of the monomer arms
(edge-directed). Intrigued by this observation, we attempted
to prepare cage 4D2h (R = n-C7H15) through kinetically
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distance about 11.7 ꢀ (Figure 5b). Based on the computer
modeling, the binding energy of 4D2h-A4 with C70 is
10 kcalmolÀ1 lower than that of binding with C60 (À48.7 vs.
À38.4 kcalmolÀ1). Presumably C70 is bigger and more ellip-
soidal than C60, and resembles the shape of the cavity, thus
providing a better fit inside the cage and a stronger binding
interaction.
In conclusion, tetrameric cages 4D2h with an uncommon
D2h symmetry were obtained through one-step alkyne meta-
thesis from readily accessible C3-symmetrical propynyl-sub-
stituted monomers in good yields. The structure of the cage
4D2h-A4 was fully characterized by 1D 1H and 13C NMR
spectroscopy, gCOSY, ROESY, GPC, MALDI-TOF MS, and
single-crystal X-ray diffraction. The formation of the cage is
likely a face-directed dynamic assembly process, which
involves the formation of dimeric macrocycle panels as the
key intermediates. Our attempts to synthesize the cage 4D2h
through cross-coupling of two macrocyclic building blocks
under various reaction conditions failed, showing that
a dynamic covalent approach could be advantageous com-
pared to kinetically controlled approaches for constructing
complex molecular architectures. Finally, the cage 4D2h-A4
shows selective binding interactions for C70 (K = 3.9 ꢂ
103 LmolÀ1) over C60 (no noticeable binding).
Figure 4. The GPC traces (normalized) of 4D2h-A4, 4D2h-B, and the
crude mixture of the scrambling experiment.
cage protons when the solution of 4D2h-A4 in toluene was
titrated with increasing amount of C60 up to 6.2 equiv
(Supporting Information, Figure S9). In contrast, the binding
interaction between 4D2h-A4 and C70 was evident. The
addition of C70 (0–8.6 equiv) to the solution of 4D2h-A4 in
toluene induced significant chemical shift changes of the
aromatic and aliphatic protons that are in the close proximity Experimental Section
to the fullerene guest (Figure 5a). The protons Ha, Ha’, and Hd
X-ray intensity data were collected on a Bruker D8 VENTURE
diffractometer system equipped with a multilayer mirror monochro-
mator and a Cu Ka microfocus sealed tube (l = 1.54178 ꢀ). The
structure was solved and refined using the Bruker SHELXTL
Software Package. 4D2h-A4: C155.20H149.80Cl4N4.60
,
0.032 mm ꢂ
0.117 mm ꢂ 0.159 mm, space group P121/c1, Z = 2; 1 1.100 gcmÀ3
,
F(000) 2362 eÀ. The final anisotropic full-matrix least-squares refine-
ment on F2 (765 variables) converged at R1 = 11.39% (observed
data), wR2 = 38.15% (all data). Goodness-of-fit 1.785; largest peak/
hole in the final difference electron density synthesis 1.234 eÀ ꢀÀ3
/
À1.128 eÀ ꢀÀ3 with an RMS deviation of 0.121 eÀ ꢀÀ3. CCDC 999558
contains the supplementary crystallographic 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.
1
Figure 5. a) The chemical-shift changes in H NMR spectra obtained
during the titration of 4D2h-A4 (0.14 mm) with an increasing amount of
C70 (0–8.6 equiv). b) The computational model (side view) of
C70@4D2h-A4; methyl groups are used for simplification. The NMR
titrations were conducted in [D8]toluene.
Received: May 1, 2014
Revised: June 7, 2014
Published online: August 21, 2014
Keywords: alkyne metathesis · dynamic covalent assembly ·
fullerene receptors · organic molecular cages ·
thermodynamic control
point toward the cavity and show pronounced upfield shifts
upon fullerene binding. The Hd’ protons that are located at the
inner side of the macrocyclic rings are pushed away from the
cavity as the result of the puckered shape of the macrocycles,
leading to the small upfield shift of Hdꢁ signals upon fullerene
binding. Aromatic proton Hb and the methylene protons (He’)
are shielded in the presence binding of the fullerene guest.
Analysis of the Job Plot (Supporting Information, Fig-
ure S9) shows 1:1 stoichiometry between C70 and the cage
4D2h-A4 with the binding constant of 3.9 ꢂ 103 LmolÀ1. The
energy-minimized structure of C70@4D2h-A4 shows that upon
fullerene binding the bottom and top macrocyclic panels
become perfectly co-facial rather than the original “slipped”
conformation of the empty cage, with an enlarged interpanel
.
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