Highly Stable Spherical Metallo-Capsule from a Branched Hexapodal Terpyridine and Its Self-Assembled Berry-type Nanostructure

Article abstract of DOI:10.1021/jacs.7b10707

Discrete spherical metallo-organic capsules at the nanometer scale, especially those constructed from unique building blocks, have received significant attention recently because of their fascinating molecular aesthetics and potential applications due to their compact cavities. Here, the synthesis and characterization of a hexapodal, branched terpyridine ligand are presented along with the nearly quantitative self-assembly of the resulting tetrameric metallo-nanosphere. This metallo-nanosphere exhibited four quasi-triangular and six rhombus-like facets, all of which were made by the same hook-like bis-terpyridine. Collision-induced dissociation experiments were done to investigate overall stability. The metallo-architecture and host-guest chemistry were investigated with coronene and fully characterized by 1D and 2D NMR, ESI-MS, and transmission electron microscopy. Furthermore, this metallo-nanosphere was observed to hierarchically self-assemble into berry-type structures in an acetonitrile/methanol mixture, by virtue of counterion-mediated attractions. The functional molecular metallo-nanosphere presented here expands the reach of terpyridine coordination systems into molecular containers and other model systems.

Full text of DOI:10.1021/jacs.7b10707

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Article
Highly Stable Spherical Metallo-capsule From a Branched Hexapodal
Terpyridine and Its Self-Assembled Berry-type Nanostructure
Mingzhao Chen, Jun Wang, Die Liu, Zhilong Jiang, Qianqian Liu,
Tun Wu, Haisheng Liu, Weidong Yu, Jun Yan, and Pingshan Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10707 • Publication Date (Web): 27 Jan 2018
Downloaded from http://pubs.acs.org on January 27, 2018
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Highly Stable Spherical Metallo-Capsule From a Branched
Hexapodal Terpyridine and Its Self-Assembled Berry-type
Nanostructure
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Mingzhao Chen,‡ Jun Wang,‡ Die Liu, Zhilong Jiang, Qianqian Liu, Tun Wu, Haisheng Liu, Weidong
Yu, Jun Yan, Pingshan Wang*
Department of Organic and Polymer Chemistry, College of Chemistry and Chemical Engineering, Central South University,
Changsha, Hunan-410083, China
Terpyridine, Spherical structure, Hierarchical self-assembly, Host–guest chemistry
ABSTRACT: Discrete spherical metallo-organic capsules at the nanometer scale, especially those constructed from unique build-
ing blocks, have received significant attention recently because of their fascinating molecular aesthetics and potential applications
due to their compact cavities. Here, the synthesis and characterization of a hexapodal, branched terpyridine ligand is presented
along with the nearly quantitative self-assembly of the resulting tetrameric metallo-nanosphere. This metallo-nanosphere exhibited
four quasi-triangular and six rhombus-like facets, all of which were made by the same hook-like bisterpyridine. Collision-induced
dissociation experiments were done to investigate overall stability. The metallo-architecture and host–guest chemistry was investi-
gated with coronene and fully characterized by 1D and 2D NMR, ESI−MS, and transmission electron microscopy. Furthermore,
this metallo-nanosphere was observed to hierarchically self-assemble into berry-type structures in an acetonitrile/methanol mixture,
in virtue of counterion-mediated attractions. The functional molecular metallo-nanosphere presented here expands the reach of ter-
pyridine coordination systems into molecular containers and other model systems.
has been used to construct simple and elaborate 3D su-
pramolecular architectures, and it is worth mentioning
INTRODUCTION
Inspired by the aesthetically pleasing structures of vari-
ous natural virus capsids and naturally beautiful polyhe-
drons both in the biological and mineral worlds,1-3 chem-
ists have designed and synthesized a number of elegant
three-dimensional (3D) molecular mimicks.4,5 These ob-
jects not only permeate the literature as a form of nano-art
but also let us view chemistry from a different perspective.
Most of recently reported 3D capsules (organic and inor-
ganic) with size- and shape-discrimination were formed
via hydrogen bonding6,7 dynamic covalent chemistry,8,9 or
coordination.10-12 These 3D molecular structures have
attracted significant attention as a result of the potential
applications arising from their hollow cavities such as
host-guest chemistry,13,14 chemical separations,15,16 cata-
lytic chemistry,17,18 reaction control inside the cavity and
more.19,20
that N-donor bridging accounted for most of chemistry in
this area. Notable work was demonstrated by Fujita,27,28
Stang,29-31 Nitschke,32,33 Clever34,35 and others36-40 since
the 1990s. Some of well-constructed 3D spherical metal
complexes successfully synthetized by Fujita et al. were
made from functionalized angled bispyridine ligands and
palladium(II) ions having the formula MnL2n, in which n
is geometrically constrained to be 6, 12, 24, 30, and even
48.41 The above reported discrete 3D architectures were
mainly synthetized upon reaction of ditopic or tritopic
ligands.
Terpyridine (tpy) units have been highlighted for their
use in the preparation of unusual coordination nanostruc-
tures, especially those which are unattainable through
pyridine-based and other ligands.42-44 Though limited new
3D supramolecular structures demonstrate the utility of
the coordinating self-assembly of the tpy ligand with met-
al ions,45-47 one-step assembly for a stable metallo-capsule
is quite challenging partly due to the limitation of the
proper geometry and orientation required for the tpy-
M(II)-tpy connectivity. More recently, Newkome et al.
reported a series of concentration-dependant 3D metallo-
spheres by utilizing rigid or flexible multitopic tpy lig-
ands.48-52 In another recent example, a Ru6L4 metallo-
The synthesis of various categories of discrete 3D su-
pramolecular structures is a challenge due to many ardu-
ous limitations; one major drawback is that the undefined
directionality of multiple coordinating ligands tend to
form polymer networks or mixtures.21 For pure organic
systems, Cooper22 and Mastalerz9 et al. presented the
formation of different 3D architectures including octahe-
dral, cubic and cuboctahedral structures by using imine-
connectivity and boronate ester chemistry. Atwood23,24
and Rebek25,26 et al. reported a series of functional
nanocapsules based on noncovalent interactions. Over the
past few decades, the coordination-driven self-assembly
nanosphere was completed in 35% yield, where
L was a
tripodal ligand. However, attempts to exchange the coor-
dinating metal ions from Ru2+ to Cd2+, Fe2+ or Zn2+ for the
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Figure 1. Cartoon representation of [Zn6L4]12+ and stable
[Zn12LH4]24+ metallo-nanosphere based on tripodal and hexapo-
dal ligands.
sake of quantitative self-assembly failed to gain the ex-
pected M6L4 (M = Cd, Fe or Zn). In addition, a 3D metal-
lo-capsule derived from the rigid pentaterpyridine or hex-
aterpyridine has not been reported so far. Herein, we pre-
sent a C6 symmetrical homologous hexapodal ligand LH
that could assemble with Zn2+ to generate a metallo-
nanosphere [Zn12LH4]24+ in nearly quantitative yield. The
successful construction of the metallo-nanosphere may be
due to the more compact characteristics achieved upon
complexation with the zinc ions, thus providing more
Following the initial results, a more complicated hex-
apodal organic ligand LH was designed and synthesized.
A benzene core was functionalized with six 120° angular
terpyridines, where all terpyridyl arms could rotate freely.
Treating LH with Zn2+ yielded metallo-nanosphere
benefits for encapsulating guest molecules (Figure
1).
Based on intermolecular interactions (e.g., π−π stacking,
counterion-mediated attraction), we utilized [Zn12LH4]24+
for encapsulating a coronene guest, and investigated the
self-assembling into berry-type nanostructures.
[Zn12LH4]24+ quantitatively via self-assembly (Scheme
1
).
The sphere-like structure was supported by 1D and 2D
NMR and ESI-MS. The nano capsule, made of 16 com-
-
ponents, has a high degree of symmetry. It possessed four
quasi-triangular and six rhombus-like faces; notably, all
faces/edges were of free-form curved surfaces. Compared
with the rhombus-like dimer [Zn2LB2]4+, the quasi-
triangular face was made from the same hook-like bis-
terpyridine LB; that is, the ligand with the same angles
and degree of rotational freedom gave different products.
RESULTS AND DISCUSSION
Ligand synthesis and metallo-nanosphere assembly.
The original design involved a nonplanar hook-like ditop-
ic bisterpyridyl ligand LB (Scheme 1). A twofold Suzuki-
coupling reaction of 4,4''-dibromo-1,1':2',1''-terphenyl
(with 60° angle) and 3-terpyridinyl-B(OH)2 was conduct-
ed by employing Pd(PPh3)4 as the catalyst (Supporting
Information, Scheme S1). After purification by column
chromatography (Al2O3) eluting with the mixed solvent of
petroleum ether and CH2Cl2, LB was obtained as pure
white solid (75%). When treating LB with
Zn(NO3)2·6H2O in a stoichiometric ratio of 1:1 in
The key hexapodal terpyridine LH was generated
through a six-fold Suzuki-coupling reaction of hexa(4-
bromophenyl)benzene and 3-terpyridinyl-B(OH)2, with
Pd(PPh3)4 as the catalyst (Scheme S2). LH was purified
through recrystallization from MeOH/CHCl3. The 1H
NMR spectrum of LH showed two singlets at 8.65 ppm
and 8.00 ppm having a 2:1 radio assigned to tpyH3',5' and
Ph-Ha, respectively. Other protons assignments were con-
firmed by the 2D COSY and 2D NOESY. Direct evidence
for the the ligand was confirmed by the high-resolution
ESI-MS with m/z peak at 2400.85 (M+Na+) (ESI, Figure
S25).
CHCl3/MeOH (1:1.5, v/v) at 75 °C for 6 h (Scheme 1),
the coordination dimer [Zn2LB2]4+ was obtained (95%) as
a pale yellow solid after counterion exchange with excess
NH4PF6 in MeOH. Chemical shifts (1H NMR) of
[Zn2LB2]4+ showed a single peak for tpyH3',5', confirming
that the product was a pure complex (Figure S1). Further
evidence was provided by ESI-MS (Figure S1); the in-
tense signals in charge states 4+ (m/z 455.09), 3+ (m/z
655.11) and 2+ (m/z 1055.16), with its distinguishable
isotope patterns for each charge state, verified the 3D
metallo-macrocyclic, rhombus-like dimer [Zn2LB2]4+
Mixing LH and Zn(NO3)2·6H2O in a 1:3 molar ratio in
CHCl3/MeOH (2:3, v/v) at 75 °C for 12 h, a clear solution
was achieved. After cooling at ambient temperature, ex-
cess NH4PF6 in MeOH was added to produce a pale-
yellow precipitate, which was filtered and washed thor-
-
with 4 PF6 counterions.
24+
-
oughly with MeOH. Complex [Zn12LH4
]
with 24 PF6
Scheme 1. (A) Synthetic route to ligand LB and LH. (B)
counterions was obtained (96%) as a faint yellow solid.
Initial analysis on the metallo-sphere was conducted by
Self-Assembly of rhombus-like dimer [Zn2LB2]4+ and
metallo-nanosphere [Zn12LH4]24+
.
1H NMR (Figure
2A), which showed two single peaks at
8.87 ppm and 8.27 ppm with a 2:1 ratio attributed to
tpyH3',5' and Ph-Ha, respectively. The results were similar
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of [Zn12LH4]24+ was shown through the peaks of tpyH6,6''
and tpyH5,5'' shifting upfield characteristically upon com-
plexation, owing to the electron shielding effects. Other
proton peak assignments were confirmed based on the 2D
COSY and 2D NOESY NMR experiments (Figures S15
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4
5
6
7
8
1
and S16). Remarkably, in comparison with the H NMR
spectrum of ligand LH, the original two signals of Ph-
He,f,g,h were split into four signals in spheric [Zn12LH4]24+
,
indicating Ph-He,f and Ph-Hg,h were no longer the same.
The Ph-He,f at δ = 7.40 ppm was split into two doublets at
δ = 7.48 and 7.43 ppm, while the Ph-Hg,h at δ = 7.15 ppm
was split into two doublets at δ = 7.73 and 7.65 ppm (Fig-
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ure
2A). The two different chemical environments was
compelling evidence for the endo–exo conformation of
the tpy arms, where all six arms were limited in what
might be an unfavourable conformation (all are in one
direction) after forming the favoured metallo-
nanosphere.53,54 In addition, the diffusion-ordered NMR
spectroscopy (DOSY) spectrum was done to roughly es-
timate radii for the metallo-nanosphere structure (Figure
2
B), with a single band near a diffusion coefficient D=
2.59 × 10−10 m2 s−1. According to the Stokes-Einstein
Equation, the diameter of the spherical complex was ca.
4.6 nm. The result was consistent with the molecular
modelling data.
1
Figure 2. (A) Comparison of the H NMR spectra (500 MHz) of
star-shaped ligand LH in CDCl3 and assembled metallo-
nanosphere [Zn12LH4]24+ in CD3CN; (B) DOSY of [Zn12LH4]24+
(500 MHz, 25 °C , CD3CN).
The HR ESI-MS spectrum of [Zn12LH4]24+ revealed a
series of peaks at m/z 839.25, 914.51, 1003.23, 1107.79,
1232.86, 1386.04, 1577.31 and 1823.34 corresponding to
to LH, indicating that a single and highly symmetrical
complex was generated. More evidence for the formation
Figure 3. (A) ESI-MS of [Zn12LH4]24+; (B) Three charge states of [Zn12LH4]24+, Theoretical (top, blue) and experimental (bottom, red).
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the charge states from 14+ to 7+, respectively (Figure A).
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The experimental m/z values and isotope patterns for all
charge states were consistent with the corresponding theo-
retically calculated values of [Zn12LH4]24+ (Figure
3B).
−
−
Upon changing the counterion from PF6 to BPh4 , the
structure of [Zn12LH4]24+ remained stable (Figure S21).
The clear-cut size of the metallo-nanosphere facilitated
further characterization by TEM (Figure
4).The sample
was dissolved in MeCN at the concentrations of ~10−7 M,
and then the solution was drop casted onto a carbon-
coated copper grid (Cu, 400 mesh); extra solution was
absorbed by filter paper to avoid aggregation. TEM imag-
es provided visualization of [Zn12LH4]24+, displaying both
the size and shape of dispersed molecules directly. The
outline of a single molecule located on the film where
dots with uniform size could be observed. The measured
mean diameter of 4.2 nm correlates well with the size
shown in optimized molecular models.
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Figure 5. ESI-MS of [Zn12LH4]24+ at m/z 1577.31 with different
collision energies and ESI-MS of [Zn12LH4]24+ (PF6)21(NO3)3 at
m/z 1546.17 as a representative dissociation peak and the frag-
mentation behavior.
Figure 4. TEM image of [Zn12LH4]24+. The high-magnification
TEM image showed the round-shaped dots and the size is con-
sistent with energy-minimized structures from molecular model-
S30-S32), which showed similar ligand-binding strengths of
bis-cyclometalated Ir(III) coordination complexes.58
ing of [Zn12LH4]24+
.
Host–guest chemistry with Coronene. The internal vol-
ume of [Zn12LH4]24+ is estimated to be ca. ~9800 Å3 (the av-
erage distance across the interior is ∼30 Å, Figure S36);48,51
thus, the large cavity should be an ideal place for conducting
host-guest chemistry and molecular recognition. Previous to
this, metal-linked capsules have been used in encapsulating
different guest molecules, the great work was reported by Fu-
jita,28,60 Nitschke61,62 and others63-65. Notably, the functional
molecular capsules use large aromatic panels rather than wire-
like frameworks to reinforce the structure of the capsule, en-
suring the guest molecules can be fully enclosed.66-67 However,
the tpy-based 3D supramolecular capsule has only been docu-
mented with the anion encapsulation.50,52 From a structural
standpoint, the closed molecular nanosphere [Zn12LH4]24+ as
well as the hexaphenylbenzene core from ligand LH may be
employed as an ideal candidate for host-guest interactions.
The stability study of [Zn12LH4]24+ According to the
concept of “density of coordination sites” (DOCS),55 more
coordination sites show more constraints, and the new pattern
of metallo-nanosphere [Zn12LH4]24+ may possess a high de-
gree of enhanced stability owing to its multiple coordination
sites. Currently, the collision-induced dissociation technique
has been employed to study the stability of small molecules56-
57
and terpyridine complexes.58-59 Therefore, we conducted the
ESI-MS collision-induced dissociation experiments to investi-
gate the stability of [Zn12LH4]24+, which revealed the excep-
tional stability of [Zn12LH4]24+ at collision energies ranging
from 5 to 40 eV with initial dissociation at 20 eV. 8+ ions at
m/z 1577.31 and 9+ ions at m/z 1380.10 were chosen as repre-
sentative peaks (Figure 5 and S31). Interestingly, at m/z
−
1567.06, an NO3 ion remained, likely the result of an incom-
−
−
The initial study of aromatic encapsulating interactions was
conducted utilizing coronene. Ligand LH and Zn(NO3)2•6H2O
were mixed in a molar ratio of 1:3 and then coronene was added
in excess to the reaction in a solvent of CHCl3/CH3OH. The pre-
pared host–guest complexes could be confirmed by 1H NMR
spectrum and ESI-MS. As shown in Figure 6A, the one-pot syn-
plete counterion exchange from NO3 to PF6 (ion exchange
was performed by adding excess amount of NH4PF6); as a
result, remaining NO3− anions may be held at the center of the
hollow cavity. As the collision energies increase, the signal of
incomplete exchange disappeared owing to the weak intercala-
tion. Nevertheless, a new signal at m/z 1546.17 (+8
1
thesis produced two H NMR signals for coronene, one was free
ions) was rising, probably from the fracture of one of the tpy-
Zn(II)-tpy moieties. The ESI-MS spectrum of [Zn12LH4]24+
(PF6)21(NO3)3 at m/z 1546.17 and 1358.28 was considered as
the fragmentation behavior at 8+ and 9+, respectively (Figures
coronene (exo), the other was encapsulated (endo), noted by the
significant chemical shift from 9.05 to 7.60 ppm. This suggests
the presence of coronene⊂[Zn12LH4]24+ complexation. Compared
1
1
to the H NMR spectrum of host complex [Zn12LH4]24+, the H
NMR of host–guest complex coronene⊂[Zn12LH4]24+ became a
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showed the upfield (endo) coronene signal was positioned within
the [Zn12LH4]24+ band, suggested that coronene was within the
cavity. Further evidence for the encapsulating was obtained by the
ESI-MS spectrum (Figures 6B and S23), the HR ESI-MS of host–
1
2
3
4
5
6
guest complex exhibited peaks for {coronene⊂[Zn12LH4]24+
-
-
-
10PF6 }10+ and {coronene⊂[Zn12LH4]24+-9PF6 }9+ at m/z 1263.01
and 1419.23, respectively. All ESI-MS signals agreed well with
their theoretical calculated isotopic values.
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S
elf-assembly berry-type nanostructure. More inter-
estingly, the metallo-nanosphere [Zn12LH4]24+ was found
to self-assemble into berry-type structures in a mixed po-
lar solvent. Liu et al. initially presented the self-assembly
of viral-capsid-type vesicle-like structures from cationic
metal–organic nanocages, and demonstrated that macro-
cations could act as counterion-mediated attraction simi-
larly to POM macroanions self-assembly behavior.68-70 So
far, those vesicle-like structures were obtained mostly
from pyridine-based metallo-supramolecular structures.
Nevertheless, such type of terpyridine-functionalized nan-
osphere has rarely been reported.
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Because the hydrophilic centers (charges at the Zn(II)
metal centers) and the hydrophobic parts (hexapodal aro-
matic ligands), electrostatic and hydrophobic interactions
could cause metallo-nanosphere [Zn12LH4]24+ (the coun-
terions were nitrates) to exist as macrocations in polar
solutions. Therefore, we chose a mixed solution of meth-
anol and acetonitrile to study the solution aggregation
behaviours. The sample was dissolved in CH3CN/MeOH
(1:1, v/v) at the concentrations of ~10−6 M, TEM experi-
ments provided evidence of the presence and morphology
of the berry-type structures. As shown in Figure
the berry-like balls had a relatively uniform size (
nm), and there were a few berry-type ball aggregations
(Figures A-B and S34), likely due to the multiple inter-
molecular interactions (e.g., π−π stacking). Moreover,
some of the berry-type structures were collapsed and
hence showed a biomembrane-like nature, which was
similar to the reported result.68 The sizes were also con-
sistent with the dynamic light scattering (DLS) results,
which gave an experimental average hydrodynamic size
1
Figure 6. (A) Comparison of the H NMR spectra (500 MHz, in
CD3CN) of the mixture containing coronene⊂[Zn12LH4]24+ and
empty [Zn12LH4]24+, coronene (middle) and [Zn12LH4]24+ (bot-
tom); (B) Two charge states of coronene⊂[Zn12LH4]24+, Theoret-
ical (top, blue) and experimental (bottom, red).
8
A-C,
∼
33
8
of the berry-type structure of d = 30
±1.0 nm (Figure 8D).
Scanning transmission electron microscope (STEM) and
STEM mapping (zinc and nitrogen) analyses supplied the
further evidences of elemental compositions for berry-like
nanospheres (Figure S35).
Scheme 2. Schematic illustration of self-assembly of nano-
sphere [Zn12LH4]24+ into spherical berry structures.
Figure 7. (A) 2D DOSY spectrum (500 MHz) for the reaction
mixture containing Coronene
⊂
[Zn12LH4]24+ and empty
[Zn12LH4]24+ in CD3CN (peaks of coronene were labeled with
blue cycles).
little broad, likely due to free [Zn12LH4]24+ and coro-
nene⊂[Zn12LH4]24+ coexisting. By controlling the proportion of
coronene, the encapsulated coronene 1H NMR signal showed
different intensity (Figure S19); the larger proportion of coronene
in the reaction mixture resulted in a more intense signal of the
encapsulated coronene. The 2D DOSY spectrum (Figure 7)
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ACKNOWLEDGMENT
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This research was supported by the National Natural Science
Foundation of China (21274165 for PW, 51634009 and 51374247
for Wei Sun); the Distinguished Professor Research Fund from
Central South University of China; Authors thank the collabora-
tion with College of Mineral Processing and Bioengineering, Cen-
tral South University of China; The authors grateful acknowledge
Prof. Dr. Carol D. Shreiner for her professional consultation. Au-
thors acknowledge the TEM and NMR measurements from the
Modern Analysis and Testing Center of Central South University
of China.
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Figure 8. TEM images of berry-type structures generated from
−
[Zn12LH4]24+ (24 NO3 as counterions, in CH3CN/MeOH). (A)
Aggregated berry-typed ball network; (B) Two connected ball
aggregations; (C) Single ball dispersed. (D) Dynamic light scatter-
ing (DLS) of berry-type structures.
CONCLUSIONS
In summary, a well-defined spherical molecular architecture
was constructed in nearly quantitative yield by using a hexap-
odal, branched terpyridine ligand via one-step assembly. The
strategy of increasing coordination numbers stabilized the
large, hollow 3D metallo-organic complex. High resolution
ESI-MS, NMR, 2D-NOESY, and DOSY spectroscopies as
well as TEM measurements unambiguously supported the
formation of spherical metallo-architecture. More importantly,
the fascinating metallo-nanosphere retained a relatively big
and compact internal cavity, and thus became a reasonable
encapsulating model for conducting research on host-guest
chemistry and molecular recognition. An aromatic interaction
was achieved by utilizing coronene to generate host-guest
complex coronene⊂[Zn12LH4]24+. Further investigations on
molecular reactor applications is currently ongoing.63,64,71 In
addition, the nanosphere [Zn12LH4]24+ was shown to sequen-
tially aggregate into large berry-like ball.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at http://pubs.acs.org at DOI. Experi-
1
mental procedures and characterization data, including H, 13C,
COSY, NOESY, and DOSY spectra of the new compounds and
ESI-MS spectra of related compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
* P. Wang: chemwps@csu.edu.cn
ORCID
Pingshan Wang: 0000-0002-1988-7604
Author Contributions
‡ M.C. and J.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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Highly Stable Spherical Metallo-Capsule From a Branched Hexapodal Terpyridine and Its Self-Assembled Berry-
type Nanostructure
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