Hierarchical self-assembly of triangular metallodendrimers into the ordered nanostructures

Article abstract of DOI:10.1016/j.cclet.2016.03.017

We designed and constructed a new family of 60° dendritic dipyridyl donors, from which two novel triangular metallodendrimers were successfully prepared via coordination-driven self-assembly. Inspired by the existence of multiple intermolecular interactions (e.g., π-π stacking and CH-π interactions) imposed by the DMIP-functionalized poly(benzyl ether) dendrons, their hierarchical self-assembly behaviors were studied in various mixed solvents by using scanning electron microscopy (SEM). Interestingly, it was found that the morphologies of the obtained metallodendrimers were highly depended on the dendron generation. For example, the first-generation metallodendrimer was able to hierarchically self-assemble into the spherical nanostructures in various mixed solvents. However, the nanofibers were observed for the second-generation metallodendrimer under the similar conditions. Furthermore, the driven force for the formation of such ordered nanostructures was investigated by using ¹H NMR and fluorescence spectroscopy.

Full text of DOI:10.1016/j.cclet.2016.03.017

Chinese Chemical Letters 27 (2016) 607–612
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
Hierarchical self-assembly of triangular metallodendrimers into the
ordered nanostructures
a,
Bo Jiang a, Li-Jun Chen a, Ying Zhang b, Hong-Wei Tan b, Lin Xu a, , Hai-Bo Yang
*
*
a Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University,
Shanghai 200062, China
b Department of Chemistry, Beijing Normal University, Beijing 100050, China
A R T I C L E I N F O
A B S T R A C T
Article history:
We designed and constructed a new family of 608 dendritic dipyridyl donors, from which two novel
triangular metallodendrimers were successfully prepared via coordination-driven self-assembly.
Received 1 February 2016
Received in revised form 4 March 2016
Accepted 10 March 2016
Available online 17 March 2016
Inspired by the existence of multiple intermolecular interactions (e.g.,
p–p stacking and CH–p
interactions) imposed by the DMIP-functionalized poly(benzyl ether) dendrons, their hierarchical self-
assembly behaviors were studied in various mixed solvents by using scanning electron microscopy
(SEM). Interestingly, it was found that the morphologies of the obtained metallodendrimers were highly
depended on the dendron generation. For example, the first-generation metallodendrimer was able to
hierarchically self-assemble into the spherical nanostructures in various mixed solvents. However, the
nanofibers were observed for the second-generation metallodendrimer under the similar conditions.
Furthermore, the driven force for the formation of such ordered nanostructures was investigated by
using 1H NMR and fluorescence spectroscopy.
Keywords:
Coordination-driven self-assembly
Hierarchical self-assembly
Metallodendrimers
Ordered nanostructures
Triangular metallacycles
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
presumably due to the difficulty in finding the appropriate
corner unit. Moreover, the triangular metallacycles sometimes
During the past few decades, dendrimers have received
increasing attention not only for their aesthetic attributes but
also because of their wide range of applications in nanomedicine,
molecular electronics, and catalysis [1–4]. Among them, metal-
lodendrimers represents an important class of dendrimers
because the incorporation of metals provides the resulting
metallodendrimers with the wider applications such as biological
mimetics, supramolecular redox sensors, and light-harvesting
antennae [5–8]. Coordination-driven self-assembly [9–25], which
is based on metal–ligand coordination interaction, has provided a
particularly powerful strategy for construction of supramolecular
metallodendrimers with well-defined shapes and sizes. Although
a variety of metallodendrimers have been prepared via coordina-
tion-driven self-assembly, most of them have been limited to
the rhomboidal or hexagonal ones. The successful examples
of triangular metallodendrimers have been relatively few,
suffer from the noticeable equilibrium with the molecular
squares when the flexible building blocks are employed [26–
28]. Thus, the construction of triangular metallodendrimers with
well-defined shape and size is still a challenge [29]. Moreover, the
hierarchical self-assembly behaviors of triangular metalloden-
drimers have been relatively unexplored.
Herein, we successfully synthesized two discrete peripherally
dimethyl isophthalate (DMIP) functionalized poly(benzyl ether)
metallodendrimers featuring a well-defined triangular cavity at
their cores via coordination-driven self-assembly (Scheme 1).
Interestingly, it was found that the dendron generation played an
important role in hierarchical self-assembly behaviors of the
obtained triangular metallodendrimers. For example, the first-
generation of metallodendrimer was able to hierarchically self-
assemble into the spherical nanostructures in various mixed
solvents. However, the nanofibers were observed for the second-
generation of metallodendrimer under the similar conditions.
Furthermore, the 1H NMR and fluorescence spectroscopy studies
revealed that the formation of ordered nanostructures was mainly
*
Corresponding authors.
driven by intermolecular
p–p stacking interaction imposed by the
E-mail addresses: lxu@chem.ecnu.edu.cn (L. Xu), hbyang@chem.ecnu.edu.cn
poly(benzyl ether) dendrons.
(H.-B. Yang).
http://dx.doi.org/10.1016/j.cclet.2016.03.017
1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

608
B. Jiang et al. / Chinese Chemical Letters 27 (2016) 607–612
Scheme 1. Synthetic routes of dendrimer ligands a and b.
2. Experimental
weighed accurately into a glass vial. To the vial was added 1.2 mL
of acetone and 0.2 mL of H2O, and the reaction solution was then
˚
2.1. Synthesis of the ligand a
stirred at 55 C for 15 h to yield a homogeneous orange solution.
Then acetone (3.0 mL) was added, followed by the addition of a
saturated aqueous solution of KPF6 into the bottle with continuous
stirring (5 min) to precipitate the product (Scheme 2). The reaction
mixture was centrifuged, washed three times with water, and
dried. The pare-yellow product A (22.5 mg, 98%) was collected and
redissolved in CD2Cl2 for NMR analysis. 1H NMR (CD2Cl2,
Under an atmosphere of nitrogen, 5.0 mL of toluene and 2.0 mL
of Et3N were added to a mixture of 4-ethynylpyridine hydrochlo-
ride (180.0 mg, 1.29 mmol), compound 7 (250.0 mg, 0.32 mmol),
CuI (15 mg, 0.08 mmol), and Pd(PPh3)4 (75 mg, 0.06 mmol). The
mixture was stirred at 95 8C for 27 h. Then the solvents were
removed in vacuo, and the residue was purified by column
chromatography on silica gel (dichloromethane/methanol = 20/1)
to give a (93 mg, 35.1%) as a yellow solid. Rf = 0.6 (dichloro-
methane/methanol = 20/1). M.p. 232 8C. 1H NMR (CDCl3,
400 MHz):
8.34 (d, 6H, J = 8.4 Hz), 8.29 (s, 12H), 7.88–7.92 (m, 18H), 7.09 (s,
12H), 5.38 (s, 12H), 3.93 (s, 36H), 1.38 (m, 72H), 1.20 (s, 108H). 31
NMR (CD2Cl2, 161.9 MHz):
12.95 (1JPt–P = 2716.7 Hz). CSI-TOF-
MS of A, [M–3PF6
À 3+: 1908.53, found: 1908.62; [M–4PF6
1395.16, found: 1395.20; [M–5PF6
À 5+: 1087.13, found: 1087.19.
d 9.15 (s, 6H), 8.70 (d, 12H, J = 5.6 Hz), 8.57 (s, 6H),
P
d
À 4+
] :
400 MHz):
d
8.93 (s, 2H), 8.64 (d, 4H, J = 6.0 Hz), 8.55 (s, 2H),
]
8.29 (d, 2H, J = 8.4 Hz), 8.27 (s, 4H), 7.83 (d, 2H, J = 8.8 Hz), 7.50 (d,
4H, J = 6.0 Hz), 5.35 (s, 4H), 3.91 (s, 12H). 13C NMR (CDCl3,
]
100 MHz):
d
166.1, 150.3, 144.6, 138.6, 133.0, 131.4, 131.3, 130.5,
2.4. Synthesis of the triangular metallodendrimer B
130.4, 129.8, 128.5, 127.3, 125.8, 123.2, 120.6, 94.2, 88.1, 75.0,
52.7. ESI-MS: 825.55(M + H+).
Following the procedure for the preparation of A. Compound b
(15.09 mg, 10.19
(1.8 mL), water (0.3 mL) yielded B (25.6 mg, 99%) as a pare-yellow
solid. 1H NMR (CD2Cl2, 400 MHz):
9.15 (s, 6H), 8.70 (d, 12H,
mmol), 10 (10.83 mg, 10.19 mmol), acetone
2.2. Synthesis of the ligand b
d
Following the procedure for the preparation of a. Compound 8
(220.0 mg, 0.15 mmol), 4-ethynylpyridine hydrochloride (110.0 mg,
0.79 mmol), CuI (15 mg, 0.08 mmol), Pd(PPh3)4 (40 mg,
J = 5.6 Hz), 8.40 (d, 6H, J = 8.4 Hz), 8.18 (s, 12H), 7.85–7.91 (m,
18H), 7.75 (s, 24H), 7.61 (s, 12H), 7.59 (s, 6H), 7.09 (s, 12H), 5.38 (s,
12H), 5.10 (s, 24H), 3.89 (s, 72H), 1.38 (m, 72H), 1.18 (s, 108H). 31
P
0.035 mmol), toluene (3.0 mL) and Et3N (2.0 mL) yielded
(100.0 mg, 43.6%) as a yellow solid after purification by column
chromatography (dichloromethane/methanol = 20/1). Rf = 0.4
(dichloromethane/methanol = 20/1). M.p. 180 8C. 1H NMR (CDCl3,
400 MHz): 8.92 (s, 2H), 8.64 (d, 4H, J = 4.0 Hz), 8.33 (d, 2H,
b
NMR (CD2Cl2, 161.9 MHz):
d
12.96 (1JPtÀP = 2723.2 Hz). CSI-TOF-
À 4+: 1887.5, found: 1887.5; [M–5PF6À 5+: 1481.0,
MS of B, [M–4PF6
]
]
found: 1481.2.
d
3. Results and discussion
J = 8.0 Hz), 8.167 (s, 4H), 7.82 (d, 2H, J = 8.4 Hz), 7.73 (s, 8H), 7.60 (s,
4H), 7.57 (s, 2H), 7.50 (d, 4H, J = 4.4 Hz), 5.35 (s, 4H) 5.08 (s, 8H), 3.88
The newly designed 608 dendritic dipyridyl donors a and b were
synthesized through a two-step sequence as shown in Schemes S3
in Supporting information. The DMIP-functionalized poly(benzyl
ether) dendrons were introduced by an etherification reaction of
compound 6 with the corresponding dendritic bromides. The
subsequent Sonogashira coupling of 7 or 8 with 4-ethynylpyridine
hydrochloride yielded 608 donor ligands a or b in the presence of
Pd(PPh3)4 and CuI as catalysts.
(s, 24H). 13C NMR (CDCl3, 100 MHz):
d 166.1, 159.0, 150.3, 144.8,
138.7, 137.7, 132.3, 131.5, 130.5, 130.1, 128.5, 127.3, 127.1, 126.6,
125.9, 123.3, 120.1, 94.33, 88.0, 75.6, 70.3, 52.7, 30.1. MALDI-TOF MS
of b: calcd. for C86H68N2O22 [M + 2H+]2+: 1482.43, found: 1482.36.
2.3. Synthesis of the triangular metallodendrimer A
The dipyridyl donor ligand a (10.04 mg, 12.2
organoplatinum 1808 acceptor 10 (12.88 mg, 12.2
m
mol) and the
mol) were
The metallacycles A and B were well characterized by multiple
nuclear (1H and 31P) NMR spectroscopy, which revealed the
m

B. Jiang et al. / Chinese Chemical Letters 27 (2016) 607–612
609
Scheme 2. Self-assembly of dipyridyl donors a and b with the corresponding diplatinum(II) acceptor 10 into supramolecular metallacycles A and B.
Fig. 1. The partial 1H NMR spectra (400 MHz, in CD2Cl2, 298 K) of acceptor 10,
dendriticdipyridinedonorb, andtheircorrespondingself-assembledmetallacycleB.
Fig. 2. The 31P NMR spectra (161.9 MHz, in CD2Cl2, 298 K) of acceptor 10,
metallacycle A, and metallacycle B.
formation of discrete and highly symmetric species. For example,
the protons on the pyridine rings of donor b exhibited obvious
downfield shifts (ca. H , 0.06 ppm; H , 0.41 ppm), which was
a
b
caused by the loss of the electron density upon coordination of the
pyridine-N atoms with the Pt(II) metal center (Fig. 1). Moreover,
each 31P{1H} NMR spectrum of the metallacycles A and B displayed
a sharp singlet at 12.95 ppm for A and 12.96 ppm for B; these peaks
were shifted upfield from that of the starting platinum acceptor 10
by ꢀ5.16 ppm (Fig. 2). This change, as well as the decrease in
corresponding simulated result. Similarly, in the case of metalla-
cycle B, two peaks that agreed well with the corresponding
simulated isotope patterns were found in the CSI-TOF-MS
spectrum (Fig. S24 in Supporting information). All the aforemen-
tioned data support the highly efficient construction of dendri-
mers-functionalized triangular metallacycles with a well-defined
shape and size through coordination-driven self-assembly, thus
avoiding the time-consuming procedures and low yields often
encountered in covalent synthesis protocols.
All attempts to grow X-ray-quality single crystals of triangular
metallodendrimers A and B were unsuccessful. Thus, the PM6
semiempirical molecular orbital method was employed to opti-
mize the molecular geometry of A and B. It was found that each
metallodendrimer featured a roughly planar triangular ring at its
core surrounded by the dendron subunits at corners (Fig. 3 and Fig.
S5 in Supporting information). Moreover, the size of the triangular
metallodendrimers was also determined. For instance, in the case
coupling of flanking 195Pt satellites (ca.
DJ = -212.0 for A; DJ =
-218.5 Hz for B), is consistent with the electron back-donation
from the platinum atoms.
Furthermore, the structure of metallacycles A and B were
confirmed by CSI-TOF-MS [30], which provided strong support for
the formation of desired triangular metallodendrimers. For
example, the CSI-TOF-MS spectrum of A displayed three peaks
(Fig. S23 in Supporting information), corresponding to different
charge states that were resulted from the loss of PF6À counterions,
[M–3PF6À]3+, [M–4PF6À]4+, and [M–5PF6À]5+, where M represents
the intact assembly. Further investigation revealed that each
isotope pattern of these peaks was in good agreement with the

610
B. Jiang et al. / Chinese Chemical Letters 27 (2016) 607–612
Fig. 3. Geometric structure of B optimized by the PM6 semiempirical molecular
orbital method.
Fig. 5. Selected TEM images of the triangular metallodendrimer A in different
solvents. (a) Dichloromethane and toluene (v/v, 9/1); (b) acetone and ethanol (v/v,
10/1).
of B, the triangular metallacycle had an internal diameter of
2.6 nm.
Recently, DMIP-functionalized poly(benzyl ether) dendrons,
which showed multiple intermolecular interactions (e.g.,
p–p
stacking and CH– interactions), have been employed for the
p
construction of organogel and the ordered nanostructures [31–
33]. Inspired by these reports, the hierarchical self-assembly
behaviors of triangular metallodendrimers were studied in various
solvents by using scanning electron microscopy (SEM). Interest-
ingly, it was found that both the metallacycles A and B were able to
hierarchically self-assemble into the ordered nanostructures. As
shown in Fig. 4, the first-generation metallodendrimer A exhibited
spherical morphology with a relatively uniform distribution in
various mixed solvents, such as dichloromethane and toluene (v/v,
9/1), dichloromethane and anisole (v/v, 10/1), acetone and ethanol
(v/v, 10/1). In order to obtain the additional information about the
nature of spherical morphology, transmission electron microscopy
(TEM) measurements were performed. Interestingly, it was found
that the spherical structures prepared in dichloromethane and
toluene exhibited a clear contrast between the interior and
periphery, which is the typical characteristic of vesicular structures
(Fig. 5a). On the contrary, the spherical structures prepared in
acetone and ethanol did not show such clear contrast between the
interior and periphery, suggesting the existence of solid sphere
(Fig. 5b). Notably, the well-defined spherical nanostructures have
received increasing attention because of their various applications
in the fields of catalysis, sensors, and drug delivery, etc. [34–
36]. However, the second-generation metallodendrimer B, which
has the same metallacyclic scaffold at the core with A, displayed
the nanofibers morphology in the similar mixed solvents. These
results indicated that the dendron generation has significant
Fig. 6. The partial 1H NMR spectra of B ([B] = 7 Â 0À4 mol/L) in the CD2Cl2–CCl4
mixed solvents with different CCl4 volume ratios at room temperature.
influences on the hierarchical self-assembly behaviors of triangu-
lar metallodendrimers.
With the aim to investigate the driving forces for the formation
of such ordered nanostructures, the 1H NMR study was performed
with different solvent compositions. According to the previous
reports [33], the tetrachloromethane was selected as the poor
solvent because it could effectively induce the enhancement of
Fig. 4. Selected SEM images of the triangular metallodendrimers A (a–c) and B (d–e) in different solvents. (a) Dichloromethane and toluene (v/v, 9/1); (b) dichloromethane and
anisole (v/v, 10/1); (c) acetone and ethanol (v/v, 10/1); (d) dichloromethane and toluene (v/v, 20/1); (e) dichloromethane and anisole (v/v, 3/1); (f) dichloromethane and
tetrachloromethane (v/v, 3/1).

B. Jiang et al. / Chinese Chemical Letters 27 (2016) 607–612
611
400
300
200
100
0
Acknowledgments
CCl4 Content
L. Xu acknowledges the financial support of the National
Natural Science Foundation of China (No. 21302058). H.-B. Yang
acknowledges the financial support of the Key Basic Research
Project of Shanghai Science and Technology Commission (No.
13JC1402200).
(Vol%)
0%
30%
40%
50%
60%
70%
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2016.03.
017.
400
500
600
700
Wavelength / nm
Fig. 7. Fluorescence emission spectra of B ([B] = 5 Â 10À5 mol/L) in the CH2Cl2–CCl4
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