One-Step Multicomponent Self-Assembly of a First-Generation Sierpiński Triangle: From Fractal Design to Chemical Reality

Article abstract of DOI:10.1002/anie.201407285

A novel terpyridine-based architecture that mimics a first-generation Sierpiński triangle has been synthesized by multicomponent assembly and features tpy-Cd^II-tpy connectivity (tpy=terpyridine). The key terpyridine ligands were synthesized by

Full text of DOI:10.1002/anie.201407285

Angewandte
Chemie
DOI: 10.1002/anie.201407285
Sierpi n´ ski Triangles
One-Step Multicomponent Self-Assembly of a First-Generation
Sierpi n´ ski Triangle: From Fractal Design to Chemical Reality**
Rajarshi Sarkar, Kai Guo, Charles N. Moorefield, Mary Jane Saunders, Chrys Wesdemiotis,* and
George R. Newkome*
Abstract: A novel terpyridine-based architecture that mimics
a first-generation Sierpi n´ ski triangle has been synthesized by
II
multicomponent assembly and features tpyꢀCd ꢀtpy connec-
tivity (tpy = terpyridine). The key terpyridine ligands were
synthesized by the Suzuki cross-coupling reaction. Mixing two
II
different terpyridine-based ligands and Cd in a precise
stoichiometric ratio (1:1:3) produced the desired fractal
architecture in near-quantitative yield. Characterization was
accomplished by NMR spectroscopy, mass spectrometry, and
transmission electron microscopy.
Figure 1. Conceptual progression of a 1!3 dendritic branching pattern
and its geometric relationship to the classical Sierpi n´ ski triangle.
[10]
N
aturally occurring species often self-assemble into fractal
[
1,2]
shapes and forms.
In 1915, the Polish mathematician
Waclaw Sierpi n´ ski formulated an equilateral triangular frac-
tal, which was constructed by connecting smaller equilateral
the structural walls of the polygons possessing rigid organic
vertices. Therefore, programmed structural features are easily
incorporated into the poly(terpyridine) building blocks,
[3]
triangles (Figure 1). Later, Mandlebrot described these
triangular fractals as Sierpi n´ ski gaskets (triangles or
[
1,4]
[23]
sieves).
can be found by using the formula N = 3 , where N is the
The number of triangles in a Sierpi n´ ski triangle
generating avenues to novel nanomolecular constructs in
n
[24]
quantitative or near-quantitative yields. Both homoleptic
n
[
5]
number of triangles and n the number of iterations.
and heteroleptic connectivity has been used extensively in
these assemblies, but heteroleptic assembly with a need for
multicomponent building blocks still remains challenging
Simple fractal motifs have been successfully integrated
into synthetic organic entities over the past forty years in the
[
6–9]
[25–27]
form of dendrimers.
between the 1!3 branched dendrimers and the Sierpi n´ ski
triangle, which is presented in Figure 1. Recent advances
There is a unique relationship
owing to a tendency to produce competitive products.
[6]
Recently, we have reported various heteroleptic materials,
[
10,11]
[28]
such as a hexagonal spoked wheel, molecular bowties and
[12]
[29]
[30]
in supramolecular chemistry have paved the way to a better
understanding of how to quantitatively assemble various
simple architectures in one step,
triangular predecessor constructs.
Coordination-driven self-assembly
,2’:6’,2’’-terpyridine (tpy) building blocks has highlighted
their ability to form stable linear tpyꢀM ꢀtpy complexes as
butterflies,
and a molecular rhombus,
which were
obtained by either multicomponent or one-step assembly.
Herein, we report the design and synthesis of a first-gener-
ation molecular Sierpi n´ ski triangle using two unique ligands
[
13–22]
including the desired
[
13,14]
II
using
tailored
with tpyꢀCd ꢀtpy connectivity (Figure 2).
2
Retrosynthetic analysis of the first-generation Sierpi n´ ski
triangle (Figure 1) revealed that it would require two easily
accessible components for its assembly: a tetrakis(terpyridine)
II
“
K” unit for the walls and the core region and a ditopic 608-
[
*] R. Sarkar, K. Guo, Dr. C. N. Moorefield, Prof. C. Wesdemiotis,
Prof. G. R. Newkome
Departments of Polymer Science and Chemistry
The Maurice Morton Institute for Polymer Science
The University of Akron
directed “V” unit for the vertices. Cadmium was chosen
owing to a unique combination of thermodynamic stability
and kinetic lability for its complexes. A 1:1 ratio of these “K”
and “V” ligands was coupled with precisely three equivalents
II
302 Buchtel Common, Akron, OH 44325 (USA)
of Cd ; the overall stability of the highly symmetric poly-
E-mail: wesdemiotis@uakron.edu
newkome@uakron.edu
Homepage: http://www.dendrimers.com
triangular architecture was projected to be favored over any
oligomeric possibilities. In essence, there would be no loose
ends or uncoordinated ligands.
Initially, a THF solution of 2,3,5,6-tetrabromocatechol
and iodomethane (Scheme 1) was heated at reflux in an inert
atmosphere (N , K CO , 12 h) to generate 3. Boronic acid 1
was prepared using commercially available 4-formylphenyl-
boronic acid. The desired “K” ligand 4 was easily synthesized
Prof. M. J. Saunders
Department of Biological Sciences, Florida Atlantic University
777 Glades Rd., Boca Raton, FL 33431 (USA)
[31]
2
2
3
[
**] We gratefully acknowledge support from the National Science
Foundation (CHE-1151991 to G.R.N., CHE-1308307 to C.W.) and
The Ohio Board of Regents.
(
72%) from 3 by treatment with 1 using standard Suzuki
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407285.
cross-coupling reaction conditions (K CO , [Pd(PPh ) Cl ])
2
3
3
2
2
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1
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peaks; a sharp singlet was observed at 4.02 ppm, which
1
3
corresponds to the OCH markers. The C NMR spectrum
3
displayed a peak at 56.40 ppm for the OCH group also.
3
The facile one-step assembly of Sierpi n´ ski triangle 5
(Scheme 1) utilized an exactly 1:1 solution of 2 and 4 in CHCl3
to which a methanolic solution of Cd(NO ) ·4H O (3 equiv)
3
2
2
was added. The solution was stirred for 30 minutes at 258C;
then, excess NH PF was added to effect counterion exchange
4
6
ꢀ
to PF6 . The desired PF6 complex precipitated and was
filtered and repeatedly washed with MeOH to remove excess
NH PF . Complex 5 was obtained without any further
4
6
purification as a light yellow solid in > 95% yield.
The Sierpi n´ ski triangle 5 was completely characterized by
H, COSY, and NOESY NMR spectroscopy and ESI and
1
traveling-wave ion mobility (TWIM) mass spectrometry,
along with transmission electron microscopy (TEM). The
1
H NMR spectrum of 5 showed the characteristic features of
II
a tpyꢀCd ꢀtpy complex, with a sharp and simple pattern
being indicative of a discrete structure with a high degree of
inherent structural symmetry. The structural simplicity and
Figure 2. Terpyridine-based Sierpi n´ ski triangle 5.
1
symmetry were reflected by the H NMR spectrum. The
imbedded methoxy markers within each ligand appeared in
the product as two distinct singlets at 3.98 ppm (from “V”)
and 3.87 ppm (from “K”) with a precise 1:1 ratio, initially
supporting the formation of a product that possesses D3h
symmetry. Notably, peaks that would be expected for
polymeric structures and impurities were not observed. The
aromatic region exhibited the peak ratio expected for three
different sets of 3’,5’-tpy protons for arms a, b, and c (8.92,
8
.91, and 8.74 ppm, respectively; Figure 2 and Figure 3); the
downfield shifts resulted from deshielding upon complex-
[28]
ation. The 6,6’’-tpy protons were noticeably shifted upfield
7.92, 7.68, and 7,80 ppm, respectively, for a, b, and c) when
(
[23]
compared with the free ligands, as expected. All of the
peaks in the H NMR spectrum were assigned and the
1
assignments verified with the aid of 2D-COSY and 2D-
NOESY experiments; they are in complete agreement with
the proposed structure.
1
3
The C NMR spectrum of complex 5 exhibits three
signals for the different and readily identifiable 3’,5’-tpy
Scheme 1. Synthesis of the key terpyridine building blocks 2 (“V”) and
4
(“K”).
[
29]
1
under an atmosphere of argon.
Its H NMR spectrum
showed the characteristic peak at 3.76 ppm for the installed
OCH marker and the presence of two completely different
3
1
3
arms (a and b, Scheme 1). The C NMR spectrum showed
a peak at 60.76 ppm, which corresponds to the OCH3
substituents.
All of the proton peaks were assigned using COSY and
1
3
NOESY NMR spectroscopy. The C NMR assignments and
the mass-spectrometric data are in full agreement with the
ligand structures. The colorless “V” ligand 2 was also
[29]
synthesized (80%) by a slightly modified procedure from
commercially available 4,5-dibromo-1,2-dimethoxybenzene
under Suzuki cross-coupling conditions. It was easily charac-
1
Figure 3. Stacked H NMR spectra (500 MHz) of ligands 2 (bottom)
1
terized by H NMR spectroscopy and mass spectrometry. The
ꢀ1
and 4 (top) in CDCl and of complex 5 in CD CN (0.25 mgmL
;
3
3
aromatic region of ligand 2 showed one set of characteristic
terpyridine peaks and one set of aromatic phenyl-spacer
center). Arrows indicate assigned resonance shifts that occur upon
complex formation.
2
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carbon atoms at 158.39, 155.32, and 154.93 ppm, respectively,
along with two very close, yet distinct peaks for the OCH3
substituents at 56.02 and 56.00 ppm, which further supports
the proposed structure (Supporting Information, Figure S12).
Sierpi n´ ski triangle 5 was further characterized by ESI–MS
Table 1: Drift times and collision cross sections for the Sierpi n´ ski
triangle 5.
2
2
Charge
Drift time [ms] CCSexp [ꢀ ] CCStheory [ꢀ ]
6
7
8
9+
1
+
+
+
8.93
6.49
5.01
4.11
3.40
2.76
1309.2
1295.5
1288.7
1297.2
1288.5
1242.3
1286.9
1388.3
[22–24,32–36]
coupled with TWIM mass spectrometry.
In ESI–MS
(
Figure 4), a series of dominant peaks were generated at
0+
1+
1
average of all charge states
single molecules upon deposition of a dilute solution (ca.
ꢀ5
ꢀ
1
0 m) of complex 5 in MeCN with PF6 counterions on
carbon-coated copper grids (300 mesh). The molecular
framework was observed as a uniform dispersion of individual
molecules with triangular shape, clear edges, and discernable
vertices. The average distance (5.6 nm) between the vertices
correlates well with the size obtained from the optimized
molecular model (Figure 5).
Figure 4. A) ESI–MS and B) 2D ESI–TWIM MS plot of 5. The charge
states of intact assemblies are marked.
Figure 5. Low-magnification TEM image of the Sierpi n´ ski triangle 5
showing a uniform field of particles. The high-magnification TEM
image clearly exhibits triangular motifs and a slightly larger and
rounded picture of a proposed aggregate (where the two triangles
illustrate a potential arrangement). Computer-generated models illus-
m/z values of 763.4, 854.2, 965.2, 1103.9, 1282.4, and 1520.3,
which correspond to charge states from 11 + to 6 + from the
trate the different OCH (singlet markers) environments that were
3
1
observed in H NMR dilution experiments to ascertain the presence of
ꢀ
loss of various numbers of PF6 anions. The isotope pattern of
individual versus stacked species.
each peak is in agreement with the corresponding simulated
isotope pattern. Additional evidence for formation of the
Sierpi n´ ski triangle 5 was provided by ESI–TWIM MS
The TEM image (Figure 5) also suggests that stacking or
aggregation occur at higher concentration, where two Sier-
pi n´ ski triangles lie atop each other to generate a “Star of
David”-like motif. This phenomenon was supported by
(
Figure 4); the corresponding spectrum showed a set of
single and narrow bands for charge states from 11 + to 6 + ,
which is in agreement with the presence of one single
structure 5.
1
H NMR experiments, where at higher concentrations,
The structure of complex 5 was further confirmed by
comparison of the experimental collision cross sections
a small extra peak was observed as a shoulder to the peak
assigned to the OCH markers of ligand “K”; thus, the NMR
3
(
CCSs) of the charge states 11 + to 6 + , which were deduced
spectrum is a combination of stacked (two or more) and
individual species. At lower concentrations, a single peak was
observed for these markers. The “V” ligand marker signals
are unaffected by the concentration (see the Supporting
Information for the details of the NMR dilution experiment,
Figure S17).
from their drift times measured by ESI–TWIM MS, with the
theoretical CCSs of the complex without any counterions (100
energy-minimized structures obtained by molecular model-
ing). Experimental and theoretical CCSs of triangle 5 are
listed in Table 1.
TEM afforded the visualization of the triangle 5, which
revealed a direct correlation of both the size and shape of
In conclusion, we have achieved the self-assembly of
a first-generation Sierpi n´ ski triangle with D3h symmetry using
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.
Angewandte
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II
tpyꢀCd ꢀtpy connectivities in near quantitative yield through
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3
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NMR spectroscopy, ESI and TWIM mass spectrometry, and
TEM provided great insight into the size, shape, symmetry,
and molecular structure of the assembled product. The
application of ligands with complementary directivity to the
assembly of higher-generation structures is currently under-
way.
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4
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Communications
Sierpi n´ ski Triangles
Mathematical mimicry leads to the syn-
thesis of a Sierpi n´ ski triangle, which was
obtained by multicomponent assembly
and features terpyridine–Cd –terpyridine
R. Sarkar, K. Guo, C. N. Moorefield,
M. J. Saunders, C. Wesdemiotis,*
II
G. R. Newkome*
&&&& — &&&&
connectivity. Complementary ligand
architecture and metal complex lability
act in synergy to achieve the desired
outcome.
One-Step Multicomponent Self-Assembly
of a First-Generation Sierpi n´ ski Triangle:
From Fractal Design to Chemical Reality
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!