Unexpected self-assembly of chiral triangles from 90 chiral Di-Pt(II) acceptors

Article abstract of DOI:10.1021/ol4032649

Two unexpected chiral organometallic triangles rather than squares from newly designed 90 chiral di-Pt(II) acceptors were obtained through coordination-driven self-assembly. Their structures were well characterized by multinuclear NMR (¹H and ³¹P) and variable-temperature NMR experiments, ESI-TOF-MS, and elemental analysis. The PM6 semiempirical molecular simulation was employed for the interpretation of the formation and stability of such chiral triangles.

Full text of DOI:10.1021/ol4032649

Letter
pubs.acs.org/OrgLett
Unexpected Self-Assembly of Chiral Triangles from 90° Chiral
Di‑Pt(II) Acceptors
Jiang-Kun Ou-Yang,† Yan-Yan Zhang,‡ Meng-Lan He,† Jiang-Tao Li,∥ Xiaopeng Li,*,§ Xiao-Li Zhao,†
Cui-Hong Wang,† Yihua Yu,‡ De-Xian Wang,∥ Lin Xu,† and Hai-Bo Yang*,†
†Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
Shanghai 200062, P. R. China
‡Shanghai Key Laboratory of Magnetic Resonance, Department of Physics, East China Normal University, Shanghai 200062, P. R.
China
§Department of Chemistry and Biochemistry Texas State UniversitySan Marcos 601 University Drive, San Marcos, Texas 78666,
United States
∥Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: Two unexpected chiral organometallic triangles
rather than squares from newly designed 90° chiral di-Pt(II)
acceptors were obtained through coordination-driven self-
assembly. Their structures were well characterized by multi-
nuclear NMR (1H and 31P) and variable-temperature NMR
experiments, ESI-TOF-MS, and elemental analysis. The PM6
semiempirical molecular simulation was employed for the
interpretation of the formation and stability of such chiral triangles.
corner unit and the equilibrium between the triangles and the
hiral macromolecules widely existing in nature have
Cproven to play an important role during many biological corresponding squares.8 Herein, we report the unexpected self-
processes and be essential to some biosomes.1 Over the years,
assembly of chiral triangles rather than squares formed from
4,4′-bipyridine and 90° chiral di-Pt(II) acceptors derived from
1,1′-bi-2-naphthol (BINOL). The species of chiral triangles
were well confirmed by multinuclear NMR (1H and 31P) and
variable-temperature NMR experiments, ESI-TOF-MS, and
elemental analysis. The binding energy of chiral triangles was
explored by PM6 semiempirical molecular simulation for
interpretation of the formation and stability of such chiral
triangles.
The chiral acceptors 5a and 5b can be easily synthesized in a
few steps (see the Supporting Information). Single crystals of
5a and 5b, suitable for X-ray diffraction studies, were grown by
slow vapor evaporation of a mixed solvent of CH2Cl2 and
hexane (v/v 1/2) at ambient temperature for 7 days. According
to the crystal structure investigation, two binding sites of di-
Pt(II) nitrates 5a and 5b were found to be 89.0° and 87.3°,
respectively (Figure 1). On the basis of the directional-binding
approach,4a the molecular squares can be logically predicted
from the self-assembly of 90° di-Pt(II) nitrates and linear
ligands. Nevertheless, the mixtures of triangles and squares
were sometimes obtained when the flexible building blocks
were utilized. This phenomenon might be attributed to the
possible existence of the exchange between triangles and
construction of artificial chiral architectures that could mimic
biological systems to realize their functionality has aroused the
extensive interest of chemists.2 Thus, the design and
preparation of chiral macromolecules has now represented
one of the major frontiers of modern supramolecular chemistry
because of their wide applications in the areas of asymmetric
catalysis, chiral separation, molecular recognition, etc.3
During the past few decades, employing the strategy of
coordination-driven self-assembly to build discrete supra-
molecular two-dimensional (2-D) polygons and three-dimen-
sional (3-D) polyhedra has evolved to be one of the most
attractive fields within supramolecular chemistry and materials
science.4 A great number of well-defined discrete 2-D polygons,
such as squares, rhomboids, rectangles, and hexagons, etc., have
been successfully prepared. With the aim of improving their
functionality, recent efforts have focused on incorporating
functional groups into the final discrete metallacycles.5 For
example, functional subunits like ferrocene, diarylethene, and
́
Frechet-type dendrons have been successfully introduced to
prepare 2-D functionalized cavity-cored assemblies.6
Although a number of functional metallacycles via coordina-
tion-driven self-assembly have been reported, only a relatively
small number of chiral macrocycles exist in the literature.7 In
particular, the construction of chiral triangles is still a big
challenge due to the difficulty in finding the appropriate chiral
Received: November 12, 2013
Published: January 16, 2014
© 2014 American Chemical Society
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Figure 1. X-ray crystal structures of 5a (A) and 5b (B).
1
Figure 2. Partial H NMR spectra of the 6 (A), 5a (B), and 7a (C).
squares or not.8,9 It is significant, therefore, to explore the
specific self-assembly of such newly designed 90° chiral
acceptors.
Heating the mixtures of 4,4-bipyridine 6 and the
corresponding chiral di-Pt(II) nitrates 5a or 5b in a
stoichiometric ratio in aqueous acetone for 12 h resulted in
self-assemblies 7a and 7b, respectively (Scheme 1). The self-
1
assemblies 7a and 7b were dissolved in acetone-d6 for H and
1
31P NMR studies. In the H NMR spectrum of each assembly,
the hydrogen atoms of the pyridine rings exhibited downfield
shifts (α-H, 0.50−0.58 ppm; β-H, 0.57−0.64 ppm) resulting
from the transition of electron from the pyridine-N atom to the
Pt(II) metal center (Figure 2 and Figure S8, Supporting
Information). Moreover, the pyridine protons displayed two
Figure 3. 31P {1H} NMR spectra of 5a (A) and 7a (B).
1
NMR signals in both H and 31P NMR spectra along with the
solubility of these species ruled out the formation of oligomers.
The obtained result of one set of signals observed in the H
1
1
sets of doublets in the H NMR spectrum, which might be
and 31P {1H} NMR spectra could be traced back to the single
species or the fast exchange process between both squares and
triangles. If such an exchange process between chiral triangles
and squares existed, upon reducing the temperature, the
exchange process would become slower, and the separation of
1H NMR signals would be observed.12 Therefore, variable-
temperature NMR spectra of 7a and 7b for a given
concentration (0.09 mM) in acetone-d6 were recorded over
the temperature range of 223−323 K (Figures S11 and S12,
Supporting Information), respectively. Variable-temperature 1H
NMR spectra showed that the signals of 7a and 7b stayed
almost the same upon reducing the temperature from 323 to
223 K, and nearly no separation of signals was observed, which
suggested the formation of sole, discrete, and stable chiral
macrocycles.
caused by the hindered rotation of Pt−N bonds in a small-sized
triangle, thus leading to the different chemical environments
between the inner and outer triangles.10 The 31P{1H} NMR
spectrum of 7a displayed two doublets (ca. 11.59 ppm and
11.13 ppm) shifted upfield from the starting platinum acceptor
5a by approximately 4.45 and 4.52 ppm (Figure 3). This
change, as well as the decrease in coupling of flanking 195Pt
satellites (ca. Δ1JPPt = −48.71 Hz), was consistent with back-
donation from the platinum atoms. Moreover, the presence of
the two-spin AB-type system resulted from the existence of an
A doublet and a B doublet with a common coupling constant
JAB in 7a. Likewise, the signals in the 31P{1H} NMR spectrum
of 7b shifted upfield from the starting platinum acceptors by
approximately 4.06 ppm (Figure S9, Supporting Information).
However, only the sharp singlet with a set of 195Pt satellites on
the either side was observed in the 31P{1H} NMR spectrum of
7b, which indicated that the two phosphorus atoms were
equivalent in 7b. The abnormal phenomenon of 7a might be
attributed to the ortho-effect of methoxyl groups.11 The sharp
In order to confirm which species was achieved, an ESI-TOF-
MS technique was further utilized to explore mass spectra of
the chiral self-assemblies 7a and 7b. In the ESI-TOF-MS
spectrum of 7a, a peak at m/z = 1070.85, corresponding to the
Scheme 1. Self-Assembly of Chiral Macromolecules 7a and 7b via Coordination-Driven Self-Assembly
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charge state [M − 4 PF6]4+ of the chiral triangles, was found.
Likewise, the ESI-TOF-MS spectrum of 7b exhibited the signal
at m/z = 1070.85, related to [M − 4 PF6]4+ of the chiral
triangles. Their isotopic distributions were in excellent
agreement with the theoretical distributions of the triangles
(Figure 4). Close examination of the ESI-TOF mass spectra of
in this study, the chiral triangles were exclusively obtained,
which might be mainly caused by the flexibility of chiral di-
Pt(II) acceptors 5a and 5b based on BINOL.14 Usually less
rigid building blocks are believed to induce relative less strain in
the triangular structure compared to the corresponding square
structures, which makes the molecular triangles more
favorable.15 Furthermore, the entropic preference for the
small ring might be another controlling thermodynamic factor
that led to the selective formation of chiral triangles in this
case.9
The investigation of inherent chirality of 5a, 5b, 7a, and 7b
was performed by using circular dichroism (CD) spectra. The
CD spectrum of 7b exhibited one major single band at ∼340
nm corresponding to the electronic spectrum of 7b (Figure S7,
Supporting Information) due to the ligand (naphthyl)-to-metal
Pt(II)) charge transfer (LMCT), which was similar to 5b, but
with higher intensity. This observation suggested that the
enhancement of CD signals in 7b was consistent with the
presence of multiple ligands in the metallocycle (Figure 6).16
Figure 4. Calculated (top) and experimental (bottom) ESI-TOF-MS
of 7a [M − 4 PF6]4+ (A) and 7b [M − 4 PF6]4+ (B).
7a and 7b revealed that only the signals derived from triangles
agreed well with their theoretical distributions (Figure S10,
Supporting Information). The multinuclear NMR (1H and 31P)
and variable-temperature NMR experiments and mass spectra
ensured that chiral triangles rather than chiral squares were
obtained exclusively.
The geometrical structures of 7a and 7b were optimized by
PM6 semiempirical molecular orbital methods (Figure 5),
Figure 6. CD spectra of 5b and 7b in acetone.
But the CD signals of 5a and 7a were too low to be clearly
observed, which might be related to the relatively low optical
rotation of di-Pt(II) nitrate 5a ([α]25°C(CH2Cl2) = +4.9) and
triangle 7a ([α]25 °C(acetone) = +24.2), compared to that of 5b
and 7b (see the Supporting Information).
In summary, a novel family of chiral triangles rather than
squares has been obtained unexpectedly from the newly
designed 90° chiral di-Pt(II) acceptors via coordination-driven
self-assembly. It was one of few examples where chiral triangles
were exclusively obtained from 90° building blocks. The chiral
triangles were determined by multinuclear NMR, ESI-TOF-
MS, variable-temperature NMR experiments, and elemental
analysis. PM6 semiempirical molecular simulation was
employed for the interpretation of the formation and stability
of chiral triangles. This phenomenon might be attributed to a
combination of the flexibility of di-Pt(II) acceptors in solution
and entropic advantage of forming triangles. This work thus not
only enriched the library of chiral macromolecules but also
provided an enhanced understanding of the formation of chiral
triangles. Further investigation on the breadth and scope of
these newly designed chiral triangles is underway.
Figure 5. Optimized structures of 7a (left) and 7b (right).
which indicated that the internal radii of chiral triangles 7a and
7b were 1.1 and 1.4 nm, respectively. Moreover, this simulation
method was further employed to explore the formation of
triangles and squares in term of energy. The simulations of the
self-assemblies unambiguously revealed that the formation of
triangles was much easier than the squares in aspect of energy.
The binding energy of 7a was −6.547 eV under solution
conditions, which was lower than that of the squares by 5.84
eV. In the case of 7b and the corresponding squares, the energy
difference was of 85.55 eV favorable to the triangles. The
calculation results strongly supported that the formation of
chiral triangles was more favorable, which agreed well with the
results observed in the NMR spectra, variable-temperature
NMR experiments, and ESI-TOF-MS spectra.
ASSOCIATED CONTENT
■
As mentioned above, according to the directional-bonding
approach,4 the self-assembly of rigid linear linkers and 90°
building blocks is expected to yield a molecular square.
Moreover, the equilibrium between triangles and squares
sometimes exists when 90° precursors are applied.13 However,
S
* Supporting Information
Experimental details, characterization, and spectral data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Letter
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: x_l8@txstate.edu.
*E-mail: hbyang@chem.ecnu.edu.cn.
Notes
The authors declare no competing financial interest.
(10) (a) Kryschenko, Y. K.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J.
Am. Chem. Soc. 2003, 125, 5193−5198. (b) Han, Q.; Wang, L.-L.; Li,
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ACKNOWLEDGMENTS
■
The work was financially supported by NSFC (No. 21322206
and 21132005), the Key Basic Research Project of SSTC (No.
13JC1402200), and the Program for Changjiang Scholars and
Innovative Research Team in University. X.L. gratefully
acknowledges support from the Research Enhancement
Program of Texas State UniversitySan Marcos.
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