Construction of double-stranded metallosupramolecular polymers with a controlled helicity by combination of salt bridges and metal coordination

Article abstract of DOI:10.1021/ja0619096

We describe the construction of the first double-stranded metallosupramolecular helical polymers. We designed and synthesized a supramolecular duplex comprised of complementary m-terphenyl-based strands bearing a chiral amidine or achiral carboxylic acid

Full text of DOI:10.1021/ja0619096

Published on Web 05/09/2006
Construction of Double-Stranded Metallosupramolecular Polymers with a
Controlled Helicity by Combination of Salt Bridges and Metal Coordination
Masato Ikeda,† Yoshie Tanaka,† Takashi Hasegawa,†,‡ Yoshio Furusho,*,† and Eiji Yashima*,†,‡
Yashima Super-structured Helix Project, ERATO, Japan Science and Technology Agency (JST), 101 Creation Core
Nagoya, 2266-22 Moriyama-ku, Nagoya 463-0003, Japan, and Department of Molecular Design and Engineering,
Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan
Received March 21, 2006; E-mail: furusho@yp-jst.jp; yashima@apchem.nagoya-u.ac.jp
Multiple-stranded helical structures are ubiquitous in Nature,
particularly in biomacromolecules, such as DNA and collagen.1
They are constructed through the cooperative action of several
noncovalent forces and are significantly linked to their elaborate
biological functions. Inspired by the biopolymers with multiple-
stranded helical structures, chemists have exerted much effort to
Figure 1. The synthesis of the double-stranded metallosupramolecular
helical polymers 3R and 3S. R1 ) (R)-1-phenylethyl for 1R‚2 and 3R; (S)-
1-phenylethyl for 1S‚2 and 3S.
intertwine artificially designed oligo- or macromolecular strands
to form multiple-stranded helical structures by utilizing noncovalent
interactions.2 We have recently reported the rational design and
synthesis of complementary double helices with optical activity,
of which the strands consist of crescent-shaped m-terphenyl groups
bearing chiral amidine and achiral carboxyl groups.3 The double
helix formation with a controlled helicity is driven by the chiral
amidinium-carboxylate salt bridges, which are tolerant to other
various functional groups, thereby making a wide range of designs
possible.4
Supramolecular polymerization has recently emerged as a new
protocol to synthesize macromolecules by holding together small
molecules through noncovalent bonds, and a number of such
supramolecular polymers have been reported to date.5 However,
to the best of our knowledge, double-stranded supramolecular
helical polymers stable in solution are hitherto unknown.6 With
the aim of constructing double-stranded metallosupramolecular
helical polymers with optical activity, we designed new ami-
dinium-carboxylate duplexes bearing pyridine groups at the four
ends, 1R‚2 and 1S‚2 (Figure 1). The pyridine groups are utilized
for the metal coordination site to form the metallosupramolecular
strands, which can be intertwined through the amidinium-carboxy-
late salt bridges to give rise to the double-stranded metallopolymers
3R and 3S, as illustrated in Figure 1. In this paper, we describe the
synthesis and characterization of the first double-stranded metal-
losupramolecular helical polymers with a controlled helicity along
with their atomic force microscopy (AFM) visualization.
Supramolecular polymerization of the duplexes 1R‚27 and 1S‚27
with cis-diphenylbis(dimethyl sulfoxide)platinum(II)8 (cis-PtPh2-
(DMSO)2) was first investigated by 1H NMR spectroscopy (Figure
2). Upon mixing 1R‚2 and 2 equiv of cis-PtPh2(DMSO)2 in 1,1,2,2-
tetrachloroethane-d2 (TCE-d2), the 1H NMR spectrum of the mixture
became significantly broadened and showed a signal due to free
DMSO at 2.51 ppm, indicating that the supramolecular polymer-
ization has occurred. The resonances of the NH protons remained
at a low magnetic field of ca. 13.5 ppm,3 showing that the salt
bridges were retained after the polymerization.
Figure 2. 1H NMR spectra of (A) cis-PtPh2(DMSO)2 (1.0 mM), (B) 1R‚2
(1.0 mM), and (C) a polymerization mixture of 1R‚2 (1.0 mM) and cis-
PtPh2(DMSO)2 (2.0 mM) in TCE-d2 at 25 °C.
were measured by pulsed field gradient NMR experiments using
the BPPSTE pulse sequence.9 The diffusion constant for the supra-
molecular polymer 3R was determined to be D ) 6.0 × 10-11 m2
s-1, while a higher value of D ) 1.6 × 10-10 m2 s-1 was obtained
for the monomer 1R‚2. These values mean that the hydrodynamic
volume of 3R is ca. 20 times higher than that of 1R‚2.10 The average
hydrodynamic radius of 3R derived from the DLS data was RH
)
9.5 nm.7 The DLS and DOSY experiments were also carried out
at different ratios of cis-PtPh2(DMSO)2 to 1S‚2; the assembly size
reached its maximum at a ratio of 2.0, that is, the stoichiometry
used for the supramolecular polymerization (Figure S7).7 In contrast,
the light scattering signals for 1R‚2 were undetectable. Thus, these
results support the polymeric structure of 3R.
More information on the structures of the metallosupramolecular
polymers was obtained by absorption and CD spectroscopies (Figure
3). The absorption spectrum of 3R in TCE showed a typical metal-
to-ligand charge transfer (MLCT) band in the long wavelength
regions (370-450 nm), and a marked bathochromic shift (∆λ )
ca. 15 nm) was observed for the absorption of ca. 320 nm when
compared to the monomer 1R‚2, providing strong evidence for the
Pt-N coordination leading to the large π-conjugation system of
3R.11 In addition, 3R and 3S exhibited perfect mirror image Cotton
effects of each other, thus reflecting the absolute configurations of
the (R)- and (S)-phenylethyl groups at the amidine residues,
respectively. It should be mentioned that, in the CD spectra of 3R
and 3S, distinct Cotton effects were observed around the MLCT
band regions, suggesting that the chirality of the phenylethyl groups
To assess the hydrodynamic dimensions of the supramolecular
1
polymers, DOSY H NMR (diffusion-ordered spectroscopy) and
DLS (dynamic light scattering) experiments were carried out at 25
°C in TCE.7 The diffusion constants for 1R‚2 and 3R in TCE-d2
† ERATO.
‡ Nagoya University.
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J. AM. CHEM. SOC. 2006, 128, 6806-6807
10.1021/ja0619096 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank Dr. J. Kumaki and Dr. S.-i.
Sakurai (JST) for their technical guidance in the AFM observations.
We also acknowledge Dr. T. Kawauchi (JST) for his help in the
DOSY experiments.
Supporting Information Available: Experimental procedures for
the synthesis and characterization of the duplexes 1R‚2 and 1S‚2, and
double-stranded helical polymers 3R and 3S. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 3. (A) Absorption and (B) CD spectra of 1R‚2, 1S‚2, 3R, and 3S in
TCE (1.0 mM) at ca. 25 °C.
References
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& Co.: New York, 2002.
(2) For reviews on multiple-stranded artificial helical polymers and oligomers,
see: (a) Constable, E. C. Tetrahedron 1992, 48, 10013-10059. (b) Lehn,
J.-M. Supramolecular Chemistry: Concepts and PerspectiVes; VCH:
Weinheim, Germany, 1995. (c) Piguet, C.; Bernardinelli, G.; Hopfgartner,
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(3) Tanaka, Y.; Katagiri, H.; Furusho, Y.; Yashima, E. Angew. Chem., Int.
Ed. 2005, 44, 3867-3870.
(4) For recent examples of amidinium-carboxylate salt bridges, see: (a)
Corbellini, F.; Di Constanzo, L.; Cregs-Calama, M.; Geremia, S.;
Reinhoudt, D. N. J. Am. Chem. Soc. 2003, 125, 9946-9947. (b) Otsuki,
J.; Iwasaki, K.; Nakano, Y.; Itou, M.; Araki, Y.; Ito, O. Chem.sEur. J.
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Rosair, C.; Rotello, V. M. Tetrahedron Lett. 2004, 45, 557-560.
(5) For reviews on (metallo)supramolecular polymers and oligomers, see: (a)
Rehahn, M. Acta Polymer 1998, 49, 201-224. (b) Supramolecular
Polymers, 2nd ed.; Ciferri, A., Ed.; Marcel Dekker: New York, 2000.
(c) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem.
ReV. 2001, 101, 4071-4097. (d) Schubert, U. S.; Eschbaumer, C. Angew.
Chem., Int. Ed. 2002, 41, 2892-2926. (e) Dobrawa, R.; Wu¨rthner, F. J.
Polym. Sci., Part A: Polym. Chem. 2005, 43, 4981-4995. (f) Stone, M.
T.; Moore, J. S. J. Am. Chem. Soc. 2005, 127, 5928-5935.
(6) Several infinite double-stranded helical polymers that exist only in the
solid state have been reported to date. For examples, see: (a) Carlucci,
L.; Ciani, G.; W. v. Gudenberg, D.; Proserpio, D. M. Inorg. Chem. 1997,
36, 3812-3813. (b) Mamula, O.; von Zelewsky, A.; Bark, T.; Bernar-
dinelli, G. Angew. Chem., Int. Ed. 1999, 38, 2945-2948. (c) Erxleben,
A. Inorg. Chem. 2001, 40, 2928-2931.
Figure 4. AFM (A) height and (B) phase images of 3R prepared by casting
a dilute solution (4 µg mL-1) in TCE on HOPG. (C) The cross-section
profile along the yellow line in the image (A). (D) The magnified image
corresponding to the area indicated by the square in the image (B). (E) A
space-filling drawing of a possible right-handed double helical structure of
the trimer of 3R obtained by MM calculation.7
is transferred to the Pt(II) complex moieties, and accordingly, the
double-stranded metallopolymers 3R and 3S adopt an excess one-
handed helical structure in solution.12
(7) See Supporting Information for details of the synthesis and characterization
of the compounds 1R‚2, 1S‚2, 3R, and 3S.
AFM provided direct evidence for the polymeric structure of
3R. Panels A-D of Figure 4 show typical tapping-mode AFM
images of 3R cast from a dilute solution in TCE ([3R] ) 2 µM) on
highly oriented pyrolytic graphite (HOPG). The AFM images
showed that the polymers self-assembled into regular two-
dimensional bundles with a constant height of 1.4 nm.13 In the high-
resolution AFM image (Figure 4D), the bundle structures were
resolved into individual polymeric 3R chains packed parallel to each
other with a chain-chain spacing of ca. 2.6 nm. Although we could
not identify the helical structure of 3R, these AFM observations
are in good agreement with the molecular mechanics (MM)-
calculated structure of 3R (Figure 4E).14 The average length of the
polymers was approximately 100 nm, corresponding to ca. 40
repeating units.
In summary, we have successfully synthesized the first artificial
double-stranded metallosupramolecular helical polymers consisting
of two complementary metallopolymer strands that are intertwined
through chiral amidinium-carboxylate salt bridges. We believe that
the combination of salt bridges and metal coordination used in this
study can be applied to the construction of a wide variety of
multiple-stranded supramolecules, which is now under investigation
in our laboratory.
(8) Lanza, S.; Minniti, D.; Moore, P.; Sachinidis, J.; Romeo, R.; Tobe, M. L.
Inorg. Chem. 1984, 23, 4428-4433.
(9) For a recent review on DOSY, see: Cohen, Y.; Avram, L.; Frish, L.
Angew. Chem., Int. Ed. 2005, 44, 520-554.
(10) The hydrodynamic volumes were calculated using the Einstein-Stokes
equation. Delpuech, J.-J., Ed. Dynamics of Solutions and Fluid Mixtures
by NMR; John Wiley and Sons Ltd.: New York, 1995.
(11) The Cotton effect intensities of 3R and 3S showed almost no temperature
dependency between +25 and -10 °C.
(12) We measured the CD spectra of the supramolecular polymers prepared at
different ratios of cis-PtPh2(DMSO)2 to 1S‚2; the CD intensity at the MLCT
region also showed a maximum value at a ratio of 2.0 (Figure S7B). We
also performed a competition experiment using acetic acid as the
competitor, which can unravel the duplex, but is inert to the Pt-pyridine
coordination. Upon the addition of acetic acid to the solution of 3S, the
CD intensity decreased considerably (Figure S8A) because the duplex
appeared to unravel.7 In addition, the Cotton effects around the MLCT
region disappeared completely, while the average hydrodynamic radius
of the mixture hardly changed in the presence of excess acetic acid (Figure
S8B). These results also support the double helical structure of the
assembly assisted by salt bridges and metal coordination (Figure 1).
(13) In contrast, no fibrous structure could be found in the AFM images of
the samples prepared by casting a solution of 1R‚2, but rather only globular
objects were observed.7
(14) The helical 3R may have a right-handed double helix with an excess one-
handedness on the basis of the X-ray structure of an analogous double
helical oligomer of 1R‚23 and the MM calculation results together with
the temperature-independent changes in the Cotton effect intensity of 3R.
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