Constructing High-Generation Sierpiński Triangles by Molecular Puzzling

Article abstract of DOI:10.1002/anie.201705480

Three generations of metalated trigonal supramolecular architectures, so-called metallo-triangles, were assembled from terpyridine (tpy) complexes. The first generation (G1) metallo-triangles were directly obtained by reacting a bis(terpyridinyl) ligand with a 60° bite angle and Zn^II ions. The direct self-assembly of G2 and G3 triangles by mixing organic ligands and Zn^II, however, only generated a mixture of G1 and G2, as well as a trace amount of insoluble polymer-like precipitate. Therefore, a modular strategy based on the connectivity of ?tpy?Ru²⁺?tpy? was employed to construct two metallo-organic ligands for the assembly of G2 and G3 Sierpiński triangles. The metallo-organic ligands L^A and L^B with multiple free terpyridines were obtained through Suzuki cross-coupling of the Ru^II complexes, and then assembled with Zn^II or Cd^II to obtain high-generation metallo-triangular architectures in nearly quantitative yield. The G1–G3 architectures were characterized by NOESY and DOSY NMR spectroscopy, ESI-MS, TWIM-MS, and transmission electron microscopy.

Full text of DOI:10.1002/anie.201705480

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Accepted Article
Title: Constructing High Generation of Sierpinski Triangles with
Molecular Puzzling
Authors: Pingshan Wang, Zhilong Jiang, Yiming Li, Ming Wang, Die
Liu, Jie Yuan, Mingzhao Chen, George R Newkome, Wei
Sun, and Xiaopeng Li
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Constructing High Generation of Sierpiński Triangles with
Molecular Puzzling
Zhilong Jiang,a,§ Yiming Li,a,b,§ Ming Wang,c Die Liu,a Jie Yuan,a Mingzhao Chen,a Jun Wang,a George
R. Newkome,d Wei Sun,e Xiaopeng Li,b,* Pingshan Wanga,*
Abstract: As a fractal geometry, Sierpiński triangle has attracted
increasing attention in science, technology, engineering, math and
art. Herein, three generations of metallo-trigonal supramolecular
architectures, metallo-triangles, were assembled using terpyridine
(tpy) complexes. The first generation (G1) of mononuclear metallo-
instead of mixture still remains a challenge in surface self-
assembly.
triangle was directly obtained by reacting
a
60o-orientated
bisterpyridinyl ligand and Zn(II) ion. The direct self-assembly of G2
and G3 triangles by mixing organic ligands and Zn(II). However, only
generated a mixture of G1 and G2, as well a trace amount of
insoluble polymer-like precipitate. Therefore, a modular strategy
based on the connectivity of <tpy-Ru(II)-tpy> was employed to
construct two predesigned metallo-organic ligands for the
assemblies of the second and third generations (G2 and G3) of
Sierpiński triangles. Metallo-organic ligands, LA and LB, with multiple
uncomplexed free terpyridines, were obtained through Suzuki-cross
coupling on the Ru(II) complexes, and then assembled with Zn(II) or
Cd(II) to achieve high generations of metallo-trigonal architectures
with nearly quantitative yield. G1-G3 architectures were
characterized by NMR, ESI-MS, TWIM-MS, NOESY, DOSY and
TEM.
Figure 1. Illustration of the molecular assembly based on a puzzling strategy.
Coordination-driven supramolecular chemistry13-26 in solution
may provide an alternative approach to achieve different
generations of Sierpiński triangles in a precisely-controlled
manner. Indeed, Newkome and coworkers reported a terpyridine
(tpy) based molecular Sierpiński metallo-triangle G2, which was
assembled based on a 60º-oriented bisterpyridine (V) and
1,2,3,4-tetraterpyridine (K) and Cd(II).27 In the exploration of
next generation Sierpiński triangle, however, the generation of
undesired low generation of triangles or coordination polymers
As one of the most well-known and versatile fractal figures,
Sierpiński triangle was first formulated by
a
Polish
mathematician Waclaw Sierpiński.1 Up to date, Sierpiński
triangle has attracted increasing attention in science, technology, by self-sorting of individual ligands cannot be avoided through
engineering, math and art. In particularly, the recent
development of supramolecular chemistry2-4 and/or self-
assembly5, 6 allows chemists and physicists to explore this fractal
geometry at molecular level. Along with theoretical prediction,7
different generations of Sierpiński triangles have been
approached by halogen bonding,8 hydrogen bonding9 and
simple stoichiometry control in the multicomponent self-
assembly.
In tpy-based supramolecular chemistry, it is well-
documented that Ru(II) and Fe(II) possess stronger coordination
with tpy than Cd(II) and Zn(II).28-32 Moreover, <tpy-Ru2+-tpy>
connectivity with remarkable stability is able to tolerate the harsh
coordination10-12 in ultrahigh vacuum using surface self-assembly. chemical process, for example, high temperature, basic
However, constructing discrete generation of Sierpiński triangle
condition, column chromatography etc. With regard to
application, Ru(II)-tpy complexes have been extensively studied
dye-sensitized solar cell,33,34. Unfortunately, <tpy-Ru(II)-tpy>
connections were seldom used to assemble large and complex
supramolecular architectures because of absence of selectivity
and reversibility. We herein report the construction of different
generations (G1 to G3) of Sierpiński triangles using the
combination of Ru(II) and Zn(II) or Cd(II) in a step-wise
approach.28,30,35-39 In the first step, metallo-organic ligands based
on <tpy-Ru(II)-tpy> connectivity were designed and synthesized
as the building blocks with enhanced selectivity and specificity.
In the second step, these building blocks were assembled with
Zn(II) or Cd(II) through highly reversible coordination to form
large architectures as molecular puzzling (Figure 1 and Scheme
1). Such molecular puzzling is reminiscent to the specificity of an
enzyme based on the structural and conformational
complementarity between enzyme and substrate.
[a]
[b]
[c]
[d]
[e]
Dr. Z. Jiang, Y. Li, Dr. D. Liu, J. Yuan, M. Chen, Prof. Dr. P. Wang
College of Chemistry and Chemical Engineering, Central South
University, Changsha, Hunan-410083, China
E-mail: chemwps@csu.edu.cn
Y. Li, Prof. Dr. X. Li,
Department of Chemistry, University of South Florida, Tampa,
Florida 78666, United States
E-mail: xiaopengli1@usf.edu
Prof. Dr. M. Wang
State Key Laboratory of Supramolecular Structure and Materials,
College of Chemistry, Jilin University, Changchun, Jilin-130012,
China
Prof. Dr. G. R. Newkome
Departments of Polymer Science, Maurice Morton Institute of
Polymer Science, Department of Polymer Engineering and
Chemistry, The University of Akron, Akron OH-44325, United States
Prof. Dr. W. Sun
College of Mineral Processing and Bioengineering, Central South
University, Changsha, Hunan-410083, China
In our initial study, Zn(II) was used to replace Cd(II) in the
self-assembly of V and K with a precisely stoichiometrical ratio
(V: K: Zn = 3 : 3 : 9) according to previous study.27 However, a
mixture of the Sierpiński triangle G2 [Zn9V3K3]18+ and small
§Those authors contributed equally to this work.
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10.1002/anie.201705480
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metallo-triangle G1 [Zn3V3]6+ was generated due to the self-
sorting of V with Zn(II). All attempts to obtain G2 as the single
assembly proven unsuccessful even after numerous adjusting
the sequence of the addition, reaction temperature, time,
solvents and counterions (Scheme S3 and Figure 2, and also
see Electronic Supporting Information, ESI). We speculated both
triangle G2 [Zn9V3K3]18+ and small metallo-triangle G1 [Zn3V3]6+
might possess the similar thermodynamic stability and entropy
favored effect.
In order to avoid the formation of small metallo-triangle G1, it
is necessary to prevent from the self-sorting of V and metal ion
alone. Based on geometrical analysis of the structure, a
molecular puzzling strategy was utilized to re-design the
synthetic pathway, or the step-wise approach with metallo-
organic ligand. This strategy can certainly circumvent the self-
sorting coordination when the assembly including multiple
components owing to the high specificity and selectivity of
metallo-organic ligand. In principle, the self-sorting of individual
ligand could be blocked if V- and K-shaped polyterpyridinyl
ligands were linked with Ru(II) ion to obtain a <tpy-Ru(II)-tpy>
complex moiety [V-Ru(II)-K] or LA (Scheme 2). In the presence
of Zn(II), the self-assembly of Sierpiński triangle G2 should be
formed as the sole structure. The directly reacting V, K and
RuCl3 in 1:1:1 stoichiometry ratio only produced a mixture of
Ru(II)-complexes which are hard to separate the target one
because of the closed polarity.
-
18PF6
-
6PF6
Thanks to the stable <tpy-Ru(II)-tpy> connectivity in Pd-
based catalyzed reaction and column chromatography, LA was
prepared with 7-step reactions starting from 2-methoxyphenol
(Scheme S1). The target metallo-organic ligand LA (42%) was
obtained by a final 3 folds of Suzuki coupling reaction with 4-
terpyridinyl-B(OH)2 using Pd0 as
a catalyst after column
separation (Al2O3, CHCl3/MeOH). The asymmetrical LA
-
36PF6
contained four uncomplexed free tpys, which could be used as
1
multiple junctions during the next step assembly. In the H NMR
spectrum of LA (Figure 3A), two singlets at ca. 9.1 and 9.3 ppm
were assigned to the tpyH3',5' of <tpy-Ru2+-tpy> moieties; four
singlets of uncomplexed tpyH3',5' exhibited at ca. 8.6-8.8 ppm.
Other peaks related to the complexed and uncomplexed
terpyridines, also the methyl and alkyl groups were fully
assigned according to the 1H/13C NMR and 2D-NOE NMR
spectra (Figure 3A and ESI). The HR-MS spectrum of LA (Figure
S47 in ESI) indicated two clear peaks of m/z = 1145.30 and
- 2+
- 3+
763.85 corresponding to [LA-2PF6 ]
and [LAH+-2PF6 ] ,
respectively.
The assembly of a dinuclear Sierpiński triangle G2 (Scheme
2) was carried out with Zn(II) and metallo-organic ligand LA in
MeOH/CHCl3. After counterion exchange with excess NH4PF6,
the precipitate was washed with D. I. water and MeOH to
remove the excess NH4PF6, the pale red powder of molecular
Sierpiński triangle G2 was obtained (>95%). As expected, LA
prevented from the self-sorting of V and K individually with
metals, as well the random connections between V, K and
metals.
G3
Scheme 1. Chemical illustration of three Sierpiński triangles G1, G2 and G3.
m/z
400
600
800
1000
1200
1400
Figure 2. ESI-MS of a mixture of G1 and G2 by direct self-assembly of V, K
and Zn(II).
Scheme 2. The synthetic route of supramolecular Sierpiński triangle G2.
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10.1002/anie.201705480
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the charge states of 13+ to 5+ from successive loss numbers of
PF6 . The
-
Figure 3. 1H NMR (500 MHz) of (A) metallo-ligand LA in CDCl3 and (B)
Sierpiński triangle G2 in CD3CN.
Scheme 4. Chemical illustration of synthetic route of supramolecular G3.
experimental m/z values and isotope patterns for each charge
state were consistent excellently with the respective calculated
values of molecular weight at 10159 Da. Additional evidence
was provided by ESI-TWIM-MS experiments (ESI-MS coupled
with traveling-wave ion mobility spectrometry),42-45 each charge
states of 14+ to 6+ (Figure 4B) showed a single band with a
narrow drift time distribution which excluded the existence of
other isomers or components.
In the exploration of Sierpiński triangle G3 with
hexaterpyridine H as the center fitting, direct self-assembly of V,
K, H and metals in a precisely stoichiometrical ratio of 3:6:1:18
also generated a mixture of small metallo-triangle G1 [Zn3V3]6+
derived from self-sorting of V and Zn2+, Sierpiński triangle G2
[Zn9V3K3]6+, as well an insoluble polymer-like solid which should
be caused from a multicomplexition of polyterpyridines and
Zn(II) (Figures S4 and S43). According to the structural and
conformational complementarity, metallo-organic ligand LB
(Scheme 4) was designed as the key intermediate in the
molecular puzzling with hexaterpyridine ligand H. The synthesis
of LB was accomplished with a similar synthesis procedure of LA.
Figure 4. (A) ESI-MS and (B)TWIM-MS spectra of Sierpiński triangle G2.
The structural characterization was first performed with 1H
NMR and 2D-NOESY spectroscopies (Figure 3B). Compared
with LA, the protons of <tpy-Ru(II)-tpy> for assembled G2 clearly
showed the chemical shifts as the coordinated <tpy-Zn(II)-tpy>
derived from the uncomplexed tpys shifted downfield except the
protons of tpyH6,6” positon after coordination with Zn(II). Some
protonic signals of the pyridine rings were overlapped after
assembling; nevertheless, the two sets of terpyridinyl peaks, i.e.,
<tpy-Ru(II)-tpy> and <tpy-Zn(II)-tpy> moieties, could be
validated accurately through 2D-COSY and 2D–NOESY NMR
spectra. The singlets of CH3O- and triplet of –CH2O- implied the
formation of a single molecular Sierpiński triangle G2. The 2D-
DOSY NMR spectrum for G2 (Figure S29) displays a single
After
a
tribromoterpyridine-Ru(II) mono-capping adduct
Br3TpyRuCl3 (Scheme S2 in ESI) reacting with a 60o-angle
orientated bisterpyridine V in a stoichiometrical ratio of 2.1:1, the
obtained brominated metallo-complex was conducted with a final
six-folds Suzuki-coupling reactions with 4-terpyridinyl-B(OH)2
using Pdo as the catalyst under argon. A purified product LB
(62%) was acquired through the column chromatography (Al2O3,
200-300 mesh eluting with CHCl3/MeOH). The 1H NMR
spectrum of LB obviously showed two singlets at 9.03 and 9.45
ppm attributing to two kinds of complexed tpyH3’,5’, as well three
singlets of uncomplexed tpyH3’,5’. Another strong evidence was
raised from the two singlets for –OCH3 and one ternary peak for
–OCH2- at ca. 3.2-4.0 ppm with the exactly integration ratio of
3:2:3. The further validation of the LB was achieved by 2D-
NOESY NMR spectra and ESI-MS (Figure S35 and S36 in ESI),
all data were consisting with the theoretical and expected values.
The assembly of Sierpiński triangle G3 was carried on in
MeOH/CHCl3 (1:1) by adding Cd2+ solution (MeOH) drop-wise to
diffusion band indicating that
a discrete component was
assembled solution and no other by-product and oligomer co-
existed. The diffusion coefficient of 2.24x10-10 m2s−1 at 298 K
also showed that the experimental hydrodynamic radius (rH=2.66
nm) was consistent with the calculated molecular radius
(rm=2.65 nm).42
The ESI-MS and TWIM-MS data (Figure 4A) further
undoubtedly convinced the metallo-architecture of G2 with high
purity. The ESI-MS (Figure 4A) revealed a series of peaks with
-
LB slowly. After exchanging the counterion to PF6 with excess
NH4PF6, the precipitate was washed with H2O and MeOH, and
the final G3 was collected as pale red powder (>95%).
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Figure 6. TEM images and energy-minimized structures of Sierpiński triangle
G2 (Upper) and G3 (Bottom).
In summary, different generations of metallo-architectures
with the geometry of Sierpiński triangle were designed and
assembly based on the structural and conformational
complementarity of building blocks, or molecular puzzling. The
stepwise strategies were employed to overcome the formation of
undesirable metallo-supramolecules and coordination polymers.
The key intermediates, metallo-organic ligands, were obtained
through the Pdo catalyzed multiple Suzuki cross-coupling on
brominated <tpy-Ru(II)tpy> complexes with high specificity and
selectivity. It is expected that this molecular puzzling strategy will
allow us to create more complicated fractal architectures with
desired shapes, sizes and functionality.
Figure 5. (A) The ESI-MS and (B) TWIM-MS spectra of G3.
The assembled architecture G3 was fully characterized by
1H NMR, 2D-NOESY, ESI-MS and TWIM-MS. After
complexation with LB, the 1H NMR peaks (Figure S39) at
aromatic region exhibited the broad and overlapped signals due
to the large size and multiple sets of aromatic rings.
Nevertheless, three aliphatic peaks indicated two singlets for –
OCH3 and one triplet for –OCH2- could interpret the assembly.
The diffusion-ordered NMR spectroscopy (DOSY) spectrum
(Figure S42) displayed a distinct single band with the diffusion
coefficients of 1.58x10-10 m2s−1 at 298 K; the experimental
hydrodynamic radius (rH=3.77 nm) was consistent with the
calculated molecular radius (rm=3.83 nm), recommending the
construction of a discrete single molecular structure. ESI-MS
and TWIM-MS measurements were used further to validate the
architecture G3. As shown in Figure 5A, ESI-MS spectrum of G3
revealed a series of peaks the charge states from 18+ to 10+
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (21274165 for PW and 51634009 for WS);
the Distinguished Professor Research Fund from Central South
University of China (for PW); NSF (CHE-1506722 and DMR-
1205670 for XL; CHE-1151991 for GRN); Research Corporation
for Science Advancement (23224 for XL); ACS Petroleum
Research Fund (55013-UNI3 for XL); Authors acknowledge the
NMR measurements from The Modern Analysis and Testing
Center of CSU.
-
derived from losing corresponded PF6 , a single and narrow
signals on the 2D TWIM-MS plot (Figure 5B) suggested a single
architecture. The experimental m/z values and isotope patterns
for each charge state are consistent well with the respective
theoretical molecular weight at 19980.99 Da.
Transmission electron microscopy (TEM) experiments were
facilitated visualization of morphological G2 and G3 (Figure 6),
revealing directly both the size and shape of individual
molecules upon deposition of a dilute (∼10−7 M) MeCN solution
of Sierpiński trianglar samples on carbon-coated grids (Cu, 400
mesh). The outline of triangular shaped patterns located on the
film with edges and corners can be observed. The average
lengths (5.9 and 8.6 nm, respectively) of the two edges of G2
and G3 perfectly fit the size that was obtained from the
optimized molecular modeling. The UV-vis absorptions and
fluorescence spectrum were also evaluated (Figures S50 and
S51).
Conflict of interest
§Those authors contributed equally to this work.
The authors declare no conflict of interest.
Keywords: terpyridine, self-assembly, modular strategy,
Sierpiński triangular, supramolecular chemistry
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10.1002/anie.201705480
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
Molecular Sierpiński triangle
assembled from the rigid, puzzle-like
metallo-organic ligands
Zhilong Jiang, Yiming Li, Ming Wang,
Die Liu, Jie Yuan, Mingzhao Chen, Jun
Wang, George R. Newkome, Wei Sun,
Xiaopeng Li,* Pingshan Wang*
Page No. – Page No.
Constructing High Generation of
Sierpiński Triangles with Molecular
Puzzling
This article is protected by copyright. All rights reserved.