From trigonal bipyramidal to platonic solids: Self-assembly and self-sorting study of terpyridine-based 3d architectures

Article abstract of DOI:10.1021/ja505414x

Using a series of tritopic 2,2':6',2'-terpyridine (tpy) ligands constructed on adamantane, three discrete 3D metallo-supramolecular architectures were assembled, i.e., trigonal bipyramidal, tetrahedron, and cube. The self-assembly used tritopic ligands as

Full text of DOI:10.1021/ja505414x

Article
pubs.acs.org/JACS
From Trigonal Bipyramidal to Platonic Solids: Self-Assembly and Self-
Sorting Study of Terpyridine-Based 3D Architectures
Ming Wang,†,∥ Chao Wang,†,∥ Xin-Qi Hao,*,‡ Xiaohong Li,§ Tyler J Vaughn,† Yan-Yan Zhang,#
Yihua Yu,# Zhong-Yu Li,⊥ Mao-Ping Song,‡ Hai-Bo Yang,⊥ and Xiaopeng Li*,†
†Department of Chemistry and Biochemistry, Texas State University, San Marcos, Texas 78666, United States
‡College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450052, China
§College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
⊥
#Shanghai Key Laboratory of Magnetic Resonance, Department of Physics and Shanghai Key Laboratory of Green Chemistry and
Chemical Processes, Department of Chemistry, East China Normal University, Shanghai 200062, China
S
* Supporting Information
ABSTRACT: Using a series of tritopic 2,2′:6′,2″-terpyridine (tpy)
ligands constructed on adamantane, three discrete 3D metallo-supra-
molecular architectures were assembled, i.e., trigonal bipyramidal,
tetrahedron, and cube. The self-assembly used tritopic ligands as corner
directing units and metal ions as glue units at the edge. The angles of the
linkers between adamantane and tpy head play a critical role in guiding
the assembled structures, which have the general formula of M3nL2n,
where M denotes metal ion and L denotes ligand. All complexes were
1
fully characterized by H, 13C NMR, diffusion-ordered NMR spectros-
copy, ESI-MS, and traveling-wave ion mobility-mass spectrometry. The binary mixtures of LA and LC or LB and LC underwent
a self-sorting process that led to the self-assembly of discrete 3D structures. The self-sorting behavior is solely based on the angles
precoded within the arm of tritopic ligands. Moreover, kinetic study of preassembled cube and tetrahedron demonstrated a slow
ligand exchange process toward a statistical mixture of hetero tetrahedrons with LA and LB.
corner directing units and end-capped Pt(II) or Pd (II)
components as edges.
INTRODUCTION
■
Self-assembly of three-dimensional (3D) metallo-supramolecu-
lar cages have received considerable attention over the past two
decades,1−11 because of their applications in diverse fields such
as host−guest chemistry,12 molecular recognition,13 reactivity
modulation,14 catalysis,15 template synthesis,16 and biology.17
Such discrete 3D supramolecular architectures possess large
void cavities and provide an ideal environment for the
encapsulation of guest molecules. Therefore, control over the
shape and size of 3D supramolecules has been one of the major
driving forces for chemists working in the fields. Among these
3D architectures, there are two important species, platonic and
archimedean polyhedra,18 which could be constructed by
carefully choosing the specific stoichiometry, the shape
information instilled in the components and reaction
conditions. In theory, if metal components act as vertices or
corners of these polyhedra, organic ligands should become as
edges or bridging units and vice versa.1a,e Actually, numerous
polyhedral architectures were built with metals as vertices and
ligands as edges, while polyhedral structures constructed by
using organic ligands as directing units in the corners and metal
components as edges were seldom reported.1b−d It is worth
noting that the dodecahedron,2a adamantanoids,2e bipyramidal
cage,2c,19 and cavitand-based cage20 reported by Stang, Fujita,
and Dalcanale mainly used pyridine-based organic ligands as
Expanding self-assembly to 2,2′:6′,2″-terpyridine (tpy),21
however, the construction of 3D architectures becomes even
more challenging, mainly attributed to the following two
reasons: (1) the fixed geometry of tpy-M(II)-tpy connectivity
(i.e., 180°) and (2) the synthesis of appropriate organic ligands
as directing units in the corners. Therefore, the self-assembly
based on tpy was focused on supramolecular polymers,22
macrocycles,23 grid array,24 and 2D architectures.25 To date,
very few 3D supramolecular cages and heteroleptic prisms were
reported based on tpy-M(II)-tpy connectivity and the
combination of terpyridine and bipyridine, respectively.26
Considering that tpy-based supramolecular complexes have
demonstrated remarkable utility in a myriad of applications,
e.g., light harvesting devices, optical display devices, switches,
molecular batteries,27 the self-assembly of new 3D architectures
using tpy building blocks may facilitate the design of novel
synthetic materials with molecular level precision and high
physical performance.
Due to the fixed ditopic geometry of tpy-M(II)-tpy
connectivity, using multitopic tpy ligands as directing units
becomes the only avenue toward 3D architectures. If tritopic
Received: May 29, 2014
© XXXX American Chemical Society
A
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tpy ligands are designed with appropriate geometry, in theory,
the possible structures of polyhedra have a general formula of
M3nL2n (M: metal ion; L: ligand), which are assembled by metal
ions with these ligands at 2:3 ratio as proposed in Figure 1.
study to investigate the self-sorting29 behavior of this series of
tritopic ligands. Finally, different cages were subjected for
kinetic study to evaluate the stability of assembled cages.
RESULTS AND DISCUSSION
■
Synthesis and Self-Assembly of Complex A. We
initially used Sonogashira coupling reaction to synthesize
ligand LA with acetylene bond between adamantane and tpy
(Scheme 1). Rather than the phenyl group in the previous
study,28 the extra acetylene was incorporated to increase the
flexibility and reduce geometry constraints.30 An elongated and
flexible linker may become less capable of transmitting the
directing effect of 109.5° provided by adamantane and thus,
facilitates the self-assembly of small structures (i.e., dimer and
tetrahedron). The bulky t-butyl moiety was introduced to
increase the solubility of the assemblies and simplify the NMR
spectra. Compounds 1 was directly accessible by the reaction
between 4-((trimethylsilyl)ethynyl)benzaldehyde31 and 1-(4-
tert-butylpyridin-2-yl)ethanone32 in a basic environment from a
one-pot reaction (see Supporting Information). While forming
the triketone intermediate species, desilylation also took place
due to the existence of excess NaOH. The further ring-closure
reaction with the addition of ammonium hydroxide gave 1 in a
decent yield. Ligand LA was synthesized in a single step from 1
and 1,3,5,-tri(4-iodophenyl)-adamantane (4) in a reasonable
yield by using Sonogashira coupling reaction and purified by
column chromatography (Al2O3). Ligand LA was fully
characterized by NMR (Figure 2) and ESI-MS. A stoichio-
metric ratio (2:3) of LA and Zn(NO3)2·6H2O were mixed in
MeOH/CHCl3 at 50 °C for 8 h, followed by the addition of
Figure 1. Self-assembly of of M3nL2n family with metals (M) as edges
and tritopic ligands (L) as vertices, respectively.
Limited by geometrical constraints, n is limited to be 1, 2, 4 or
10, corresponding to bipyramidal-like dimer (M3L2), tetrahe-
dron (M6L4), cube (M12L8), and dodecahedron (M30L20),
respectively. The latter three belong to the platonic solids
family. We initiated this study using a tritopic tpy ligand
constructed on adamantane care as vertices and Zn(II) as
gluing elements in the edges to assemble a supramolecular
cube.28 In contrast, we herein present the design and self-
assembly of three discrete cages, i.e., bipyramidal-like dimer,
tetrahedron, and cube by applying different linkers with
appropriate angles between adamantane core and tpy units,
such as m-phenyl, p-phenyl, and 2,5-thienyl (Scheme 1). Based
on these discrete self-assembled 3D structures, we extended our
Scheme 1. Synthetic Route of Ligands and Complexes Zn3nL2n
B
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determine the molecular composition and structural informa-
tion.23a,33 In ESI-MS (Figure 2B), one prominent set of peaks
with charge states from 10+ to 22+ were observed (due to the
−
loss of a different number of PF6 ). And the isotope pattern of
each peak closely matched the corresponding simulated isotope
pattern of [Zn12LA8] (Figure S1) for the desired cubic
structure with molecular weight up to 17797.7 Da. It suggests
that the arm of the ligand is flexible enough to accommodate
the strain of the three-armed ligand under entropy-driven force
by bending from 109.5° angle to 90°. In order to separate any
superimposed fragments (i.e., tetrahedron) and detect the
possible presence of overlapping isomers or conformers,
TWIM-MS was introduced as the advanced level of MS
analysis. As conventional ion mobility-mass spectrometry (IM-
MS),34 TWIM-MS is an effective approach to determine the
analyte mass, charge, and shape by analyzing the drift time of an
ion through the ion-mobility region (drift region).23a,c,33
Typically, the compact ions drift faster than extended ions at
the same charge state; while the high charge state ions drift
faster than the low charge state ions at the same m/z. As shown
in Figure 2C, each charge state of the formed cube [Zn12LA8]
was detected with narrow drift time distribution, indicating the
absence of other isomers or structural conformers. The size
information, i.e., experimental collision cross section (CCS),35
was also calculated and correlated to the theoretical CCS
obtained from molecular modeling (vide infra).
Synthesis and Self-Assembly of Complex B. Our initial
study demonstrated that increasing the length of the linear
linker between tpy and adamantane was not a feasible strategy
to assemble smaller 3D structures, i.e., M3L2 and M6L4. Besides
changing the length of linkers, we reasoned that incorporating
suitable angular linkers would lead to the formation of possible
smaller architectures. In light of this, we synthesized ligand LB
(Figure 3) by introducing 2,5-thienyl with the bend angle
149°36 to replace 1,4-phenyl (180°) linker used in LA. 2 was
synthesized from 5-((trimethylsilyl)ethynyl)thiophene-2-carbal-
dehyde,37 1-(4-tert-butylpyridin-2-yl)ethanone, and NH4OAc
using a similar procedure as compound 1. The use of NH4OAc
is to avoid the unexpected reduction of the acetylene bond into
the alkene with the presence of ammonium hydroxide. The
synthesis process of ligand LB and self-assembly of complex B
followed the same procedure as ligand LA and complex A. The
1
Figure 2. (A) H NMR spectra (400 MHz) of ligand LA in CDCl3
and complex A [Zn12LA8] in CD3CN. (B) ESI-MS and (C) 2D ESI-
TWIM-MS plot (m/z vs drift time) of complex A. The charge states of
intact assemblies are marked.
excess of NH4PF6 salt to give a white precipitate (complex A)
after a thorough washing with water.
The H NMR of complex A (Figure 2A) showed a simple
pattern of peaks for a tpy-metal complex. In the aromatic
region, there are eight sets of aromatic protons from tpy units
1
1
simple H NMR pattern of complex B with one set of signals
shows similar chemical shift to complex A (Figure 3A),
indicating the formation of highly symmetric architecture. The
3,3″-tpy and 6,6″-tpy protons were shifted to upfield
(particularly for 6,6″ protons of tpy: Δδ = 0.92 ppm) and all
other peaks (tpy and phenyl) were slightly shifted to downfield.
More structural evidences were provided by 2D-COSY and
HSQC (see Figures S13 and S15).
Furthermore, ESI-MS (Figure 3B) and TWIM-MS (Figure
3C) spectra of complex B identified one discrete species. The
corresponding isotope patterns observed were in well agree-
ment with the calculated m/z ratios of tetrahedron (M6L4) with
molecular weight at 8970.3 Da (Figure S2). The TWIM-MS
spectrum (Figure 3C) clearly showed that no superimposed
fragments, overlapping isomers, or conformers of the assembly
existed in this complex. All these characterizations suggested
that we obtained a smaller 3D tetrahedron architecture, viz.,
[Zn6LB4], than cube by introducing the 2,5-thienyl groups
between the adamantane core and tpy units to change the
angles of the arms. It is worth noting that Newkome and co-
workers recently reported a truncated tetrahedron assembled
1
and phenyl groups, as expected. The broad H signals are
typical for a large complex due to its slow tumbling motion on
the NMR time scale.3d 3′,5′-tpy protons are significantly shifted
to downfield, due to the lower electron density upon
coordination with metal ions. Both the protons at 3,3″ and
6,6″ position of tpy shifted upfield (particularly for 6,6″ protons
of tpy: Δδ = 0.95 ppm), which indicated the formation of
complex with Zn(II), due to the electron shielding effect.25a,d
The other hydrogen signals of aromatic protons were slightly
shifted to downfield. The hydrogen signals of t-butyl group also
showed a very sharp peak, suggesting a high degree of structural
symmetry of the so-formed complex. The full assignments of
1H NMR shown in Figure 2A were based on 2D-correlation
spectroscopy (COSY) (see Figure S8). Heteronuclear single
quantum correlation (HSQC) NMR spectrum also supported
the generation of single structure with high symmetry and thus
excluding other structural isomers (see Figure S10).
This complex was further characterized by ESI-MS and
traveling-wave ion mobility-mass spectrometry (TWIM-MS) to
C
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1
Figure 4. (A) H NMR spectra (400 MHz) of ligand LC in CDCl3
and complex C [Zn3LA2] in CD3CN. (B) ESI-MS and (C) 2D ESI-
TWIM-MS plot complex C (m/z vs drift time). The charge states of
intact assemblies are marked.
1
Figure 3. (A) H NMR spectra (400 MHz) of ligand LB in CDCl3
and complex B [Zn6LA4] in CD3CN. (B) ESI-MS and (C) 2D ESI-
TWIM-MS plot (m/z vs drift time) of complex B. The charge states of
intact assemblies are marked.
Figure S3. The calculated molecular weight was the same as the
theoretical value of trigonal bipyramidal-like dimer at 4304.5
Da, from a series of prominent peaks by losing different
numbers of counterions. From TWIM-MS spectra (Figure 3C),
no superimposed fragments, overlapping isomers, or con-
formers of the assembly were found in complex C. All these
results demonstrate that the complex C indeed possesses the
trigonal bipyramidal-like architecture as expected.
Size Characterization by 2D DOSY NMR and CCS of
TWIM-MS. Although these three polyhedra have been well
documented by the NMR and MS results, all attempts to grow
X-ray-quality single crystals have been proven to be
unsuccessful to date. Alternatively, molecular simulation with
Materials Studio showed distinct size differences of these
complexes A, B, and C, which have diameters of 6.9, 4.4, and
2.9 nm, respectively (Figure 5). To validate these simulation
results, diffusion-ordered NMR spectroscopy (DOSY) was used
as an advanced technique to measure the trend of the size
change and the results are shown in Figure 5. The observation
of a single band at log D = −9.96, −9.66, and −9.48 for
complexes A, B, and C, respectively, demonstrated the
appreciable size decrease from complex A to complex C.
These results confirmed the formation of three discrete 3D
species after complexation of those different bridging ligands. In
addition, this cube (complex A) with 5.6 nm edge length is
by tritopic terpyridine ligand and Ru(II) with smaller size using
tris(4-bromophenyl)benzene as core structure and 1,3-phenyl
as linker.26f Compared to the lower yield of Ru(II) truncated
tetrahedron with column separation, our tetrahedron was
directly obtained after self-assembly through precipitation with
a decent yield (91%).
Synthesis and Self-Assembly of Complex C. Following
this result, the introducing of 1,3-phenyl (120°) is considered
as a feasible method to reach even smaller polyhedra dimer
(M3L2). 3-((trimethylsilyl)ethynyl)benzaldehyde38 was used as
starting material to synthesize 3. As shown in Scheme 1, the
synthesis route of ligand LC and complex C are the same as the
1
synthesis of ligand LA and complex A. The H NMR of
complex C showed an expected upfield shift of 3,3″-tpy and
6,6″-tpy protons compared to ligand LC, according to the
results of complex A and B (Figure 4A). The signals of the
other peaks (tpy and phenyl) were shifted to downfield as
expected.
ESI-MS (Figure 4B) and TWIM-MS (Figure 4C) spectra of
complex C also showed trigonal bipyramidal as the sole
product. The observed charge states of complex C (Z = 3+ to
6+) and the corresponding isotope patterns were shown in
D
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instrumentation, there is not yet a suitable set of calibrants
for metallo-supramolecular species to provide accurate
experimental CCS from drift time data. Therefore, we used
proteins as calibrants according to previous study by
Wesdemiotis and Cronin.23c,33b Theoretical CCSs for 50
candidate structures of the [Zn12LA8], [Zn6LB4], and
[Zn3LC2] were calculated using the trajectory (TJ), which
considers both long-range interactions and momentum transfer
between the ions and the gas in the ion mobility region, and
gives the most reliable CCS prediction.35 The slight
fluctuations of theoretical CCS indicated a highly rigid
structure. It is worth mentioning that several leading groups
in the metallo-supramolecular field, e.g., Lehn,41 Stang,42 and
Fujita,3c,30 have documented that the counterions are highly
disoriented in crystals, and that their exact locations are
unknown; for this reason, these groups have omitted
counterions in the molecular modeling studies.43 Furthermore,
Bowers and co-workers showed that the experimental CCSs for
the rectangle and triangle were nearly identical for the various
charge states of these respective species. Adding or subtracting
a counterion, had a negligible effect on the size of these
complexes.44 Given the giant size of our cube and tetrahedron
and the number of counterions in the examined charge states
(+14 to +23 for cube, +9 to +12 and tetrahedron), attempting
to place the anions arbitrarily until structures are found that
match the experimental CCS at each charge state is not
tractable. Therefore, we omitted the counterions to simplify the
structural modeling and theoretical CCS calculation.
Figure 5. Energy-minimized structures of complexes A (left), B
(middle), and C (right) from molecular modeling and 2D DOSY
NMR spectra of complexes A (left), B (middle), and C (right) (500
MHz, CD3CN, 300 K).
significantly larger than other metallo-supramolecular cubes
reported in the literature.39
As another evidence, the TWIM-MS data were used to derive
the collision cross sections (CCSs) of the ions being separated
by ion mobility. The experimental CCS deduced from TWIM-
IM was correlated to the theoretical CCS calculated from
molecular modeling for cube and tetrahedron at various charge
states (Table 1). Unfortunately, unlike linear drift tube IMS,
TWIM-MS instruments require calibration using peptides or
proteins ions of known CCSs to relate drift time.33d,40 Further,
this type of metallo-supramolecules may interact differently
with the collision gas (N2) as compared to metal-free proteins.
Despite the increasing use of commercial TWIM-MS
As shown in Table 1, the theoretical CCS by TJ method is in
an excellent agreement with the average experimental CCS
results supporting proposed structures. For instance, the
experimental and theoretical TJ CCSs of complex A are
3258.7
149.8 and 3212.5
43.3 Å2, respectively; the
experimental and theoretical TJ CCSs of complex B are 1510.9
79.3 and 1599.8 20.8 Å2, respectively. The size of complex
A is similar to myoglobin and the size of complex B is
comparable to the size of cytochrome c. Due to lack of
appropriate protein calibrants in the size range, the
experimental CCS of complex C was not calculated.
Table 1. Experimental and Theoretical Collision Cross
Sections (CCSs)
Photophysical Properties. Normalized optical absorption
and photoluminescence (PL) emission spectra of ligands and
complexes (10−5 M) were shown in Figure S5. The maximum
absorption peaks of these ligands and complexes are distinctly
different (318 nm, 353 and 282 nm for ligand LA, LB, and LC;
342, 383, and 289 nm for complex A, B, and C, respectively.)
The main absorption bands of ligands were associated with the
π−π* transition through the conjugated backbone.45 For
complexes, the high-energy absorptions in the region between
250 and 400 nm are ligand-centered transitions (π−π*).46 The
absorption spectra of ligand LB and complex B with thionyl
units as linkers were obviously red-shifted than ligand LA and
complex A, due to the donating-effect of the thienyl groups
which lower the energy gap between HOMO and LUMO
orbitals of the complex. In contrast, the spectra of LC and
complex C were observed with the blue-shifted curve, due to
the decrease of conjugation.47 The PL emission spectra of these
ligands and complexes also have similar differences (Figure
S5b). It suggested that the introduction of different units with
specific angles as the linker between adamantane core and tpy
heads could get various 3D cage with different optical
properties.
drift times
[ms]
CCS (calcd avg)
[Å2]
complex
CCS
CCS average
7.39(+14)
6.50(+15)
5.73 (+16)
5.18 (+17)
4.74(+18)
4.41 (+19)
4.19(+20)
3.75 (+21)
3.53(+22)
3.42(+23)
4.08 (+9)
3.75 (+10)
3.31 (+11)
2.98(+12)
6.73 (+3)
4.96 (+4)
3.86 (+5)
3.09 (+6)
3126.2
3117.4
3108.5
b
3212.5 43.3
3138.7 3258.7
a
(149.8)
3184.4
3251.5
3345.9
3350.8
3425.8
3537.8
A
[Zn12LA8]
a
b
1393.7 1510.9 (79.3)
1599.8 20.8
1463.8
1479.6
1502.2
−
−
−
−
B [Zn6LB4]
C [Zn3LC2]
b
809.8 12.4
−
Self-Sorting Behavior. With all three discrete 3D
structures, the next question raised by us is which structure
a
b
Average value of CCS. TJ value obtained using MOBCAL.
E
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forms in the case of a mixture of LA, LB, and/or LC. The
ligands could exclusively self-sort into distinct complex A, B,
and C, or else assemble into a statistical mixture. To answer this
question, LA and LC were first mixed in equimolar ratio
accompanied by stoichiometric amount of Zn(II) ions for the
self-assembly in MeOH/CHCl3 at 50 °C for 8 h, followed by
the addition of excess of NH4PF6 salt to give a white
precipitate. The ESI-MS and TWIM-MS results are depicted
in Figure 6, showing distinguishable complex A and C. The
Figure 7. (A) ESI-MS and (B) 2D ESI-TWIM-MS plot of the complex
assembled by LB and LC (m/z vs drift time). The charge states of
intact assemblies are marked.
However, when LA and LB were mixed with equimolar ratio,
a statistical mixture of tetrahedrons was observed in ESI-MS
containing both ligands with a binomial distribution (Figure 8).
It is interesting to note that the tetrahedron structure is more
versatile and preferable than cube due to the composition of
[Zn6LA3LB]. Nevertheless, we were unable to give a plausible
explanation for the formation of heterotetrahedron with LA
Figure 6. (A) ESI-MS and (B) 2D ESI-TWIM-MS plot of the complex
assembled by LA and LC (m/z vs drift time). The charge states of
intact assemblies are marked.
drift time of each ion at different charge states is consistent with
the drift time obtained for individual complex in Figures 2 and
4, confirming that no statistical mixture of two ligands were
assembled. Despite the possibility of forming myriad oligomeric
structures and statistical mixture, discrete cube and trigonal
bipyramidal were strongly preferred in the self-assembly,
representative a self-sorting process.
Similarly, self-sorting behavior was also observed for a
mixture of LB and LC at equimolar ratio. Both ESI-MS and
TWIM-MS clearly showed distinguishable complexes B and C
after self-assembly (Figure 7). The geometrical mismatch
between LA and LC or LB and LC ligands disfavors hetero
cage formation during the self-sorting process. Although
numerous studies on self-sorting were reported, the self-sorting
of 3D metallo-supramolecular structures was seldom addressed
because spontaneous self-sorting of discrete structures out of a
collection of multiple possibilities is always challenging. Among
these 3D self-sorting, Stang1c,48 and Dalcanale20c reported the
size selective self-sorting of triangular prisms and cavitand-
based coordination cages depending on the length of the
pyridyl anchoring units. In contrast, the self-sorting behavior of
our system is solely based on the angles of the tpy arms. Fujita
and co-workers reported the self-assembly using a mixture of
two angular ligands; however, a statistical mixture of cages was
observed instead of discrete structures by self-sorting.3d
Moreover, this is the first reported self-sorting study focusing
on tpy-based 3D self-assembly in one pot.
Figure 8. (A) ESI-MS spectrum of the complex assembled by LA and
LB. (B) Enlarged spectrum of the region from m/z 746 to 754.
F
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and LB, because of the angular difference of LA and LB is 31°,
which is slightly large than that (i.e., 29°) of LB and LC.
Kinetic Study of Preassembled 3D Structures. Though
the self-sorting behavior was not observed by mixing LA and
LB, they were an ideal system to conduct kinetic study of
preassembled complex A and B. The solution of preassembled
complex A and B (CH3CN, 1 mg/mL) were mixed at room
temperature (1:1 by volume). The time dependent on ligands
exchange and resulting structural change were monitored by
the ESI-MS (Figure 9). After 5 h of mixing, the one-ligand-
the self-assembly behavior. Discrete 3D metallo-supramolecules
with a general formula of M3nL2n including cube, tetrahedron,
and trigonal bipyramidal-like dimer were obtained by employ-
ing these ligands as corner directing units and Zn2+ metal ions
as edges, based on <tpy-Zn(II)-tpy> connectivity. NMR, ESI-
MS, TWIM-MS, and DOSY unambiguously supported for the
size, shape, and molecular composition. These results clearly
demonstrate that the microscopic variations of directing
geometry will result in the macroscopic differences for final
3D architectures. Furthermore, the binary mixtures of LA and
LC or LB and LC underwent a self-sorting process that led to
the self-assembly of discrete 3D structures. The self-sorting in
our system is solely based on the angles precoded within the
arm of tritopic ligands. Additionally, kinetic study of
preassembled cube and tetrahedron demonstrated a slow
ligand exchange process toward a statistical mixture of hetero
tetrahedrons with LA and LB. Ongoing study of dodecahedron
(M30L20) is focused on the ligand design with shorter and more
rigid linkers. More importantly, these cages may become an
excellent system to study host−guest interaction due to their
distinct cavity and face window sizes.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
X_L8@txstate.edu
xqhao@zzu.edu.cn
Author Contributions
∥These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Xiaopeng Li gratefully acknowledges the support from the
Research Enhancement Program of Texas State University and
the Welch Foundation (AI-0045). Xin-Qi Hao thanks the
NSFC/China (No. 21102135) for financial support. Hai-Bo
Yang thanks the NSFC/China (nos. 21132005 and 91027005)
and the SMSTC (no. 13JC1402200) for financial support.
Xiaohong Li thanks the NSFC/China (no. 21305098) and
SRFDP (no. 20123201120014).
Figure 9. (A) The full ESI spectrum of the mixture of preassembled
complex A and B at 0 min. Expanded regions show the 20+ signal for
(B) pure A and 10+ signal for (C) pure B as control. Time-dependent
spectra of mixing at preassembled complex A and B (D) 10 min, (E)
30 min, (F) 5 h, (G) 1 d, (H) 2 d, and (I) 7 d.
exchanged tetrahedron [Zn6LALB3] was observed, indicating a
slow ligand exchange process, which is consistent with the
previous kinetic study of ligand exchange between two M12L24
cages by Fujita and co-workers.49 After 7 days, all cube signals
disappeared, however, heterocube with LA and LB was not
detected, confirming the versatility of composition of
tetrahedron. Overall, our kinetic study demonstrated the
exceptional stability of preassembled 3D structures based on
weak tpy-Zn(II)-tpy connectivity in the reassembly process.
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