Stoichiometric self-assembly of isomeric, shape-persistent, supramacromolecular bowtie and butterfly structures

Article abstract of DOI:10.1021/ja303177v

Two novel macromolecular constitutional isomers have been self-assembled from previously unreported terpyridine ligands in a three-component system. The terpyridine ligands were synthesized in high yields via a key Suzuki coupling. Restrictions of the pos

Full text of DOI:10.1021/ja303177v

Communication
pubs.acs.org/JACS
Stoichiometric Self-Assembly of Isomeric, Shape-Persistent,
Supramacromolecular Bowtie and Butterfly Structures
Anthony Schultz,‡,§ Xiaopeng Li,‡,§ Balaka Barkakaty,† Charles N. Moorefield,† Chrys Wesdemiotis,*,†,‡
and George R. Newkome*,†,‡
‡
†Department of Polymer Science and Department of Chemistry, The University of Akron, Akron, Ohio 44325, United States
S
* Supporting Information
ABSTRACT: Two novel macromolecular constitutional
isomers have been self-assembled from previously
unreported terpyridine ligands in a three-component
system. The terpyridine ligands were synthesized in high
yields via a key Suzuki coupling. Restrictions of the
possible outcomes for self-assembly ultimately provided
optimum conditions for isolation of either a molecular
bowtie or its isomeric butterfly motif. These isomers have
1
been characterized by ESI-MS, TWIM-MS, H NMR, and
13C NMR. Notably, these structural isomers have
remarkably different drift times in ion mobility separation,
corresponding to different sizes and shapes at high charge
states.
he past 20 years have brought major advances in the field
Figure 1. Hexagonal spoked-wheel motif (A) and isomeric triangular
T
of supramolecular chemistry. Lehn,1,2 Stang,3 Fujita,4−7
polygons (B,C) along with their corresponding terpyridine-based
metallocyclic representations (counterions omitted for clarity).
Schmittel,8−12 and others have reported numerous examples of
novel macromolecules that rely on <ligand−M−ligand′>
connectivity as a driving force for assembly. While homoleptic
self-assembly is typically quite facile,13,14 heteroleptic self-
assembly is often more difficult.11,15−18 Typically, directed and
self-assembly techniques are combined19−22 in the synthesis of
large metallocycles,23 rectangles,24−26 and other poly-
gons.15,27−29 These recent developments enhanced the insights
into molecular self-assembly of higher ordered superstructures.
Of the numerous examples, several have utilized <tpy−M2+−
tpy> (tpy = terpyridine) connectivity as a means of
constructing multicomponent systems.29−31
In particular, we have recently reported the use of <tpy−
M2+−tpy> connectivity for the facile construction of a
hexagonal spoked wheel,30 represented by Figure 1A, which
is composed of six individual triangles with shared sides that
exhibit isomeric patterns, such as end-to-end triangles (Figure
1B,C), possessing differing orientations about a mutual point.
This is an example of discrete control over the supra-
macromolecular self-assembly of bicyclic, polygonal constitu-
tional isomers; notably, monocyclic stereoisomers, syn/anti-
isomers, linkage isomers, and supramolecular isomers along
with isomeric nanobaskets have been reported.32 We herein
report the synthesis of novel bis- and tetrakis-terpyridine
ligands and their use in the construction of isomeric, triad-
based, bicyclic architectures.
MS coupled with electrospray ionization (ESI)-MS has afforded
mass, charge, and size differentiation in the characterization of
these species by allowing the observation of the higher charge
states without fragmenting the labile metal−ligand connections
of the self-assembled product.13,30,33
Prior to this technique, it was often impossible to get
acceptable MS data,13,34,35 and in cases where there were
1
multiple components, the H NMR spectra would often be
extremely complex and inconclusive. Also, if there were
undesirable side products resulting from kinetic instability,
their structural characterization could be difficult.
The critical bis-terpyridine ligand 3 was successfully
synthesized (Scheme 1) by Suzuki cross-coupling of 4,5-
dibromo-1,2-bis-carbomethoxybenzene (2)36 with excess 4′-(4-
boronatophenyl)-2,2′:6′,2″-terpyridine (1) in a H2O:toluene:t-
BuOH mixture (3:3:1 v/v/v) for 48 h. Column chromatog-
raphy (Al2O3, CHCl3) followed by recrystallization afforded
(51%) bis-terpyridine 3. Characterization included pertinent 1H
NMR resonances, such as a singlet at 8.71 ppm and a doublet at
8.65 ppm assigned to the 3′,5′- and 6,6″-tpyH, respectively; all
other resonances were readily assignable. An ESI-MS peak at
m/z 809.2, consistent with [M+H]+, also supported the
structural assignment. Ligand 3 maintained the 1H NMR
Recent advances in mass spectrometry (MS) have been key
factors in better understanding the principles of supramolecular
construction. The use of traveling wave ion mobility (TWIM)-
Received: April 2, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja303177v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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a
characterized by the expected two sets of resonances (1H
NMR) with an integration ratio of 1:1 for the complexed and
metal-free terpyridine ligands (Figure S1). The MALDI-MS
was consistent with dimer formation, exhibiting a peak at m/z
1843.91.
Scheme 1. Synthesis of 3, 5, 6, and 7
Reaction of the bis-terpyridine dimer 6 with the alkyl-
modified tetrakis-terpyridine 5 and Zn(BF4)2 in a 2:1:4
stoichiometric ratio afforded the heterometal bowtie polygon
10 (Scheme 2), with no observable tetrameric starting material.
Scheme 2. Synthesis of the ZnII Analogues of the Bis-
triangles 10 and 12 with 2 equiv of 6 (25 °C, 1 h)
a
Ligands 3, 5, and 7 were synthesized from 2, 4, or 4a, respectively,
with 1 and isolated in good yields.
marker resonance (CO2CH3) at 4.0 ppm, allowing for
structural comparisons.
Alkoxy-modified, tetrakis-terpyridine 5 was prepared (84%,
Scheme 1) from 1 and 4 using a protocol similar to that used
for 3. The 1H NMR spectrum of 5 (Figure 3) exhibited a single
set of resonances for the terpyridine moieties that was nearly
identical to that of 3, as well as the expected symmetrical
absorptions for the aryl core; an ESI-MS peak for [M+H]+ at
m/z 1508.0 corroborated the assignment. Initial attempts to
obtain the desired bowtie 9 using 3, 5, and Cd2+ (or Zn2+), in a
4:1:6 stoichiometry, respectively, gave a triangle 8 as the major
component along with bowtie 9, as shown by both ESI-MS
(Figure 2) and NMR analyses. Notably, prior attempts using
1
The H NMR spectrum of 10 exhibited peaks in the expected
6:1 ratio corresponding to the −CO2CH3:−OCH2R moieties
(see Supporting Information). Although this creates the
potential for self-assembly of 6 and the meta-substituted
terpyridines of 5, the strain of this type of bonding would be
enthalpically disfavored over the structure depicted in Scheme
2.
COSY analysis simplified assignment of all resonances in the
1
1
aromatic region of the H NMR. Notable attributes of the H
NMR spectrum of 10 (Figure 3) include three discernible sets
of 3′,5′ resonances, two of which partially overlap but exhibit an
ideal 1:2 integration, and the 3,3″ and 6,6″ resonances, which
displayed the desired downfield and upfield trends, respectively.
Results from ESI-MS and TWIM-MS (Figures 4 and S2)
showed that bowtie 10 with either ZnII or CdII was the
predominant component. Based on the sharp drift time
Figure 2. One-dimensional ESI-MS of the mixture of 8 and 9, with
−
pertinent charge states marked. Counterions (BF4 ) are omitted from
the structures for clarity.
this procedure and the methoxy-modified analogue of bis-ligand
3 (3-OMe, SI-p16) led to more favorable yet similar
undesirable mixtures.
Modifications aimed at eliminating the formation of trimer 8
focused on restricting the degrees of freedom. Thus, a RuII
dimer was targeted to instill a nonreversible complex between
the two outer 60° ligands. Dimer 6 was prepared (40%) from
the stoichiometric reaction of bis-ligand 3 with
37
RuCl2(DMSO)4 in a refluxing CHCl3:MeOH mixture (1:1
v/v), followed by column chromatography (Al2O3, CHCl3 →
Figure 3. 1H NMR overlay of starting materials 5 (top) and 6
(bottom) with the Zn bowtie 10 in the center. Arrows depict assigned
resonance shifts that occur upon complex formation.
MeCN:H2O:sat. KNO3(aq) 25:1:1 v/v/v); small oligomers
1
were also isolated (22%), based on H NMR. Dimer 6 was
B
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Journal of the American Chemical Society
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Figure 5. Comparison of the estimated collision cross sections of
bowtie 10 and butterfly 12 at different charge states, i.e., 4+ to 7+.
bis-triangular supramolecular structures that were elucidated by
1H and COSY NMR, as well as ESI-TWIM-MS. While the one-
dimensional ESI-MS data were identical for the isomers, the
TWIM-MS data revealed notable differences regarding detailed
size and structure information. Comparison of the drift times of
the two isomers showed a significant difference in the low
charge states for the two isomers, whereas the drift times of the
higher charge states remained nearly identical. Further
utilitarian studies of these isomers and related materials are
ongoing.
Figure 4. (A) ESI-MS and (B) 2D ESI-TWIM-MS plot (mass-to-
charge ratio vs drift time) of 10. The charge states of intact assemblies
are marked.
ASSOCIATED CONTENT
* Supporting Information
■
S
All pertinent spectral data and complete experimental
procedures. This material is available free of charge via the
Internet at http://pubs.acs.org.
distribution of the different charge states, no other component
was detected in TWIM-MS, reflecting the highly rigid shape-
persistence of this bowtie-type architecture. Additionally, ZnII
and CdII give complexes of the same size, as the corresponding
drift times of each charge state are nearly identical (Table S3).
In view of this result, can one discern the difference between
two isomeric structures, for example, via differences in MS drift
times or collision cross sections? This was tested by preparing
the related butterfly motif 12 by the self-assembly of ligands 7
[accessed by Suzuki coupling of 1,2,3,4-tetrabromo-5,6-bis-
hexoxybenzene (4a, Scheme 1) to the terpyridine boronic acid
1] and 6 (Scheme 2). The tetrakis-terpyridyl product revealed
two distinct sets of resonances that are easily differentiable (see
spectra in the Supporting Information). Based on modeling of
isomers 10 and 12, there is a difference in size or theoretical
cross section value for the two species; thus, one would expect a
difference between these isomers in the TWIM-MS.
Butterfly 12 was readily assembled, resulting in a 1D ESI-MS
(Figure S5) nearly identical to that of 10 (Figure S4); however,
the TWIM-MS drift times were different at low charge states,
e.g., 4+ to 6+. Notably, the bowtie complex 10 has a
significantly larger collision cross section than that of its
butterfly-shaped isomer 12 (Figure 5). As the charge increases,
both isomers have very similar drift times or cross sections
(Table S3). Thus, the charge distribution affects the sizes of
these two complexes at low charge states, but once the 7+
charge state is reached, there are marginal differences between
the two.
AUTHOR INFORMATION
Corresponding Author
wesdemiotis@uakron.edu; newkome@uakron.edu
■
Author Contributions
§A.S. and X.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support is acknowledged from the National Science
Foundation (DMR-0705015 and CHE-1151991, G.R.N.;
DMR-0821313 and CHE-1012636, C.W.) and the Ohio
Board of Regents. Bowtie and butterfly images obtained from
zazzle.com and cyrussaintrid.com.
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