A highly efficient approach to the self-assembly of hexagonal cavity-cored tris[2]pseudorotaxanes from several components via multiple noncovalent interactions

Article abstract of DOI:10.1021/ja073744m

A method of incorporating both metal-ligand and crown ether-dialkylammonium self-assembly techniques in the efficient synthesis of metallacyclic tris[2]pseudorotaxanes is described. These multifunctional supramolecules can be prepared by the stepwise or o

Full text of DOI:10.1021/ja073744m

Published on Web 10/27/2007
A Highly Efficient Approach to the Self-Assembly of Hexagonal Cavity-Cored
Tris[2]pseudorotaxanes from Several Components via Multiple Noncovalent
Interactions
Hai-Bo Yang,*,† Koushik Ghosh,† Brian H. Northrop,† Yao-Rong Zheng,† Matthew M. Lyndon,§
David C. Muddiman,§ and Peter J. Stang*,†
Department of Chemistry, UniVersity of Utah, 315 South 1400 East, RM 2020, Salt Lake City, Utah 84112, and
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695
Received May 24, 2007; E-mail: hbyang@chem.utah.edu; stang@chem.utah.edu
Noncovalent synthetic strategies, in contrast to most covalent
strategies, allow for the spontaneous, selective formation of highly
stable and complex supramolecular structures with specific archi-
tectures and properties through the dynamic self-assembly of
reversible yet strong linkages.1 The majority of known supramo-
lecular structures to date, however, have been prepared using a
single recognition motif, with those examples that do utilize multiple
binding algorithms being restricted to multistep processes2 or limited
to functionalized polymers or crystal engineering.3 Biological
systems, by contrast, commonly exhibit multifunctionality wherein
they possess many recognition motifs in their structure and
function.4 Similar, purely synthetic multifunctional systems may
also be designed by employing orthogonal, that is, non-interacting
and self-consistent, supramolecular recognition motifs in which a
variety of noncovalent interactions allow for multiple complemen-
tary components to self-assemble into supramolecular species with
complex functional architectures. Yet emulating nature’s use of
multicomponent, orthogonal assembly poses considerable chal-
lenges.
block it will, when combined with a complementary 120° diplati-
num acceptor, provide access to tris-DB24C8 derivatives. Such
functionalized metallacyclic hexagons may then host dialkylam-
monium ions to provide, in a stepwise fashion, discrete supramo-
lecular architectures employing multiple noncovalent interactions.
This new type of cavity-cored tris[2]pseudorotaxane may also be
readily generated in one concerted step simply by combining the
donor, acceptor, and ammonium building blocks, thereby demon-
strating the power of orthogonal self-assembly.
Many nondirectional interactions, such as hydrogen bonding,
electrostatic forces, donor-acceptor, van der Waals forces, and
other weak interactions, contribute to biological self-assembly.
Synthetic chemists have also used such interactions in the construc-
tion of intricate supramolecular architectures such as rotaxanes and
pseudorotaxanes.5 Compared to these weak noncovalent interactions,
metal coordination allows for the formation of stronger, directional
metal-ligand dative bonds that play an essential role in the
construction of polygons and polyhedra with well-defined shapes
and considerable stability.6 By utilizing the non-interfering or-
thogonal nature of these two types of interactionssweak and
strongsnovel supramolecular species have been obtained from
specifically designed precursors via multiple noncovalent interac-
tions in one simple step.3 This important, multipronged method of
construction has been little explored in the self-assembly of discrete
nanoscopic supramolecular arrays, especially for those with pre-
determined shapes, geometries, and symmetries.7
We have demonstrated that planar hexagonal structures can be
self-assembled by combining three 120° pyridyl donor building
blocks and three 120° diplatinum acceptors.6a Furthermore, the
addition of functional groups at the vertex of individual building
units, even with high generation dendrons, does not hinder their
self-assembly.8 Encouraged by the fact that macrocyclic polyethers
are able to complex dialkylammonium ions (R2NH2+) to form
rotaxanes and pseudorotaxanes,9 which have been studied exten-
sively, we envisioned that by attaching a dibenzo-24-crown-8
(DB24C8) macrocycle on the periphery of a 120° donor building
Figure 1. Molecular structures of 1-4, as well as a graphical representation
of the self-assembly of hexagonal tris-DB24C8 derivatives 5 and 6 (Method
I, step I), pseudorotaxane 9 (Method II, step I), and tris[2]pseudorotaxanes
7 and 8 (Methods I and II, step II).
The 120° donor precursor 1, with both crown ether and pyridine
binding sites, can be easily prepared via an etherification reaction
of chloromethyl-DB24C8 and 3,5-bis(4-ethynlpyridinyl)phenol
(Scheme S1). Stirring the mixture of 1 (3.08 mM) and 120° di-Pt-
(II) acceptor 2 or 3 in a 1:1 ratio in CD2Cl2 at 298 K for 30 min
resulted in the formation of [3 + 3] hexagons with pendant
DB24C8s at alternate vertexes (step I, Figure 1). Multinuclear NMR
(1H and 31P) analysis of the reaction mixtures revealed the formation
of discrete, highly symmetric species. The 31P {1H} NMR spectrum
of hexagon 5, for example, displayed a sharp singlet (ca. 13.7 ppm)
shifted upfield from the starting platinum acceptor 2 by ap-
proximately 3.6 ppm. Additionally, the protons of the pyridine rings
exhibited downfield shifts (R-HPy, 0.12 ppm; â-HPy, 0.45 ppm)
resulting from loss of electron density upon coordination of the
pyridine N atom with the Pt(II) metal center (Figure 2C).
ESI-TOF mass spectrometry provides further evidence for the
formation of new hexagonal assemblies. In the mass spectrum of
5, for example, peaks at m/z ) 2995.89, 1423.47, and 1108.99,
† University of Utah.
§ North Carolina State University.
corresponding to [M - 2OTf]2+, [M - 4OTf]4+, and [M - 5OTf]5+
,
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J. AM. CHEM. SOC. 2007, 129, 14187-14189
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Figure 2. Partial 1H NMR spectra recorded at 500 MHz (CD2Cl2, 298 K)
of (A) dibenzylammonium 4, (B) precursor 1, (C) hexagonal tris-DB24C8
derivative 5, and (D) tris[2]pseudorotaxane 7 (>95%), along with some
signals for partially complexed and uncomplexed species at equilibrium.
Figure 4. Calculated (top) and experimental (bottom) ESI-MS spectra of
tris[2]pseudorotaxanes 7 (A) and 8 (B).
Figure 3. ESI-TOF-MS spectra of hexagonal tris-DB24C8 derivative 5
(vertical lines are the theoretical abundances).
Figure 5. The 1H NMR titration isotherm of tris[2]pseudorotaxane 8
recorded at 500 MHz in CD2Cl2 at 298 K (9 indicates the change in
chemical shift of the proton signal corresponding to the γ-H of the crown
ether).
respectively, were observed and their isotopic resolutions are in
excellent agreement with the theoretical distributions (Figure 3).
The collective analytical results indicate that the tris-DB24C8
derivative can be easily prepared (>95% yields) through coordina-
tion-driven self-assembly, which avoids the time-consuming pro-
cedures and lower yields often encountered in covalent synthetic
protocols.
signals associated with the pyridyl functionalities of 1 shift upon
metal coordination, while only those signals associated with
DB24C8 shift upon complexation of 4.
Subsequent investigations into self-assembling hexagonal cavity-
cored tris[2]pesudorotaxanes 7 and 8 were undertaken by adding 3
equiv of dibenzylammonium triflate salt 4 to the CD2Cl2 solution
of hexagonal tris-DB24C8 derivatives 5 and 6 (1.44 mM for 5,
1.15 mM for 6), respectively (Method I, step II, Figure 1). Within
10 min, a pale yellow solution was obtained. Compared to the
hexagonal assemblies 5 and 6, the 31P {1H} NMR spectra of new
species 7 and 8 do not show any significant change, indicating that
the incorporation of dibenzylammonium ions does not substantially
change the chemical environment of the phosphorus atoms. The
1H NMR spectra of 7 and 8, however, exhibited characteristic shifts9
associated with the complexation of 4 by DB24C8. For example,
The host:guest stoichiometry for supramolecular assemblies 7
and 8 was established by using the nonlinear least-squares fit
1
method10 based on the H NMR titration experiments (Figure 5,
see also Supporting Information). Results for both hexagonal tris-
DB24C8 derivatives (5 and 6) indicate a 1:3 stoichiometry with
relation to the dibenzylammonium guests, providing further support
for the formation of tris[2]pseudorotaxanes 7 and 8. Furthermore,
thermodynamic binding constants were obtained through the
titration experiments. Fitting the data to a 1:3 binding mode gave
rise to the association constants: Ks.1 ) (1.06 ( 0.15) × 104 M-1
,
Ks.2 ) (6.38 ( 0.10) × 103 M-1, and Ks.3 ) (4.16 ( 0.56) × 103
M-1 for 7, and Ks.1 ) (1.90 ( 0.28) × 104 M-1, Ks.2 ) (6.38 (
0.34) × 103 M-1, and Ks.3 ) (1.76 ( 0.16) × 103 M-1 for 8. These
values suggest that both hexacationic tris-DB24C8 hosts (5 and 6)
have a similar ability to bind dibenzylammonium guest(s) as does
DB24C8 in nonpolar solvent such as dichloromethane.9a
1
in the H NMR spectrum of 7, a 0.45 ppm downfield shift of the
+
signal for the benzylic methylene protons adjacent to the NH2
center was observed. Moreover, the protons HR, Hâ, and Hγ of
DB24C8 exhibited upfield shifts of 0.03, 0.08, and 0.28 ppm,
respectively (Figure 2D). In addition, the structures of hexagonal
cavity-cored tris[2]pesudorotaxanes have also been confirmed by
ESI mass spectrometry. In the ESI mass spectra of 7 and 8, peaks
attributable to [M - 4OTf]4+ for 7 (m/z )1684.6) and [M - 5
OTf]5+ for 8 (m/z )1284.3) were found. These peaks were
isotopically resolved, and they agree very well with the theoretical
distribution (Figure 4). Further characterization with two-dimen-
sional spectroscopic techniques (1H-1H COSY and NOESY) is in
agreement with the formation of the hexagonal cavity-cored tris-
[2]pseudorotaxanes (see Supporting Information). It is important
to note how 1H NMR spectroscopic results reveal the orthogonality
of self-assembly in this stepwise approach to 7 and 8: only those
Hexagonal cavity-cored tris[2]pseudorotaxanes 7 and 8 can also
be obtained via an alternative stepwise approach (Method II, Figure
1) wherein the order of orthogonal self-assembly steps is reversed.
Stirring a 1:1 mixture of donor precursor 1 and ammonium salt 4
in CD2Cl2 afforded pseudorotaxane 9 (Method II, step 1). The
addition of 1.0 equiv of 120° diplatinum acceptor 2 or 3 resulted
(Method II, step 2) in the formation of tris[2]pseudorotaxanes 7
and 8, respectively. Multinuclear NMR (1H and 31P) analysis of
the reaction mixtures revealed the same characteristics as those
observed in the spectra of 7 and 8 prepared stepwise by Method I,
confirming the formation of the same species by this alternative
stepwise procedure. The power of this orthogonal approach is
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C O M M U N I C A T I O N S
tions in one simple self-assembly step. We are currently extending
this strategy to combine coordination-driven self-assembly with
other noncovalent assembly methodologies to further explore its
application to the construction of multifunctional supramolecular
architectures.
Acknowledgment. P.J.S. thanks the NIH (GM-057052) and the
NSF (CHE-0306720) for financial support. B.H.N. thanks the NIH
(GM-080820) for financial support. We thank Prof. Mei-Xiang
Wang and Dr. Han-Yuan Gong for their help with the calculation
of thermodynamic binding constants. D.C.M. and M.M.L thank
NCBC and NCSU Chemistry for generous financial support.
Figure 6. Schematic representation of the one-pot self-assembly of tris-
[2]pseudorotaxanes 7 and 8 by using Method III.
Supporting Information Available: Synthesis and analytic data
of 1, hexagonal tris-DB24C8 derivatives 5 and 6, and tris[2]-
pseudorotaxanes 7 and 8, experimental data for 1H NMR titration
experiments, and computational procedures and structures for 5, 6, and
7 obtained from modeling. This material is available free of charge
via the Internet at http://pubs.acs.org.
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(11) Upon stirring at 298 K for approximately 48 h, the 1H NMR spectra of
assemblies 7 and 8 do not show any significant change, demonstrating
the stability of these novel supramolecular assemblies.
Herein, we have combined coordination-driven self-assembly
with the crown ether-dialkylammonium binding motif to generate
novel tris[2]pseudorotaxanes in which the molecular structures
incorporate a discrete hexagonal cavity as their main scaffold and
pendant pseudorotaxanes at alternate vertexes. During self-assembly,
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