Optically active metallo-supramolecular polymers derived from chiral bis-terpyridines

Article abstract of DOI:10.1021/ol901293r

Image Persented The optically active metallo-supramolecular polymers were successfully synthesized via complexation of Fe(II) ions with new bis-terpyridines containing chiral tetra-ethylene glycol units at the ortho-position of the peripheral pyridine rin

Full text of DOI:10.1021/ol901293r

ORGANIC
LETTERS
2009
Vol. 11, No. 16
3562-3565
Optically Active Metallo-Supramolecular
Polymers Derived from Chiral
Bis-terpyridines
Ravindra R. Pal,† Masayoshi Higuchi,*,† and Dirk G. Kurth*,‡
International Center for Materials Nanoarchitectonics, National Institute for Materials
Science, 1-1 Namiki, Tsukuba 305-0044, Japan, and Technologie der Material
Synthese, UniVersita¨t Wu¨rzberg, Rontgenring 11, 97070, Wu¨rzberg, Germany
HIGUCHI.Masayoshi@nims.go.jp; Dirk.Kurth@matsyn.uni-wuerzburg.de
Received June 10, 2009
ABSTRACT
The optically active metallo-supramolecular polymers were successfully synthesized via complexation of Fe(II) ions with new bis-terpyridines
containing chiral tetra-ethylene glycol units at the ortho-position of the peripheral pyridine rings. In addition, we also revealed solvent and
temperature effects toward assembly and dis-assembly behavior of polymers.
The self-assembly of organic ligands with metal ions has
afforded many fascinating architectures1 that are of interest
in various fields including catalysis,2 functional materials,3
and chemical biology.4 Recently, several reports5 covered
the development of optically active metallo-supramolecular
assemblies through tailored ligand systems and transition
metal ions, some of their derivatives are of great interest in
the field of asymmetric catalysis.6 However, there are only
limited reports7 dealing with synthesis of ditopic chiral bis-
terpyridines (btpy) and their optically active coordination
polymers. Although there have been reports on chiral btpy
and chiral metallo-supramolecular polyelectrolytes (MEPEs),
the detailed studies of solvent and temperature effects on
the optical activities have not been reported so far. We expect
that the introduction of chirally substituted tetra-ethylene
glycol (TEG),8 chains at the pyridine periphery will not only
influence the solubility but will also influence the architecture
† National Institute for Materials Science.
‡ Universita¨t Wu¨rzberg.
(1) (a) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspec-
tiVes; VCH: Weinheim, 1995. (b) Constable, E. C. In ComprehensiVe
Supramolecular Chemistry; Lehn, J.-M., Ed.; Pergamon: Elmsford, NY,
1996; Vol. 9, p 213. (c) Jones, C. J. Chem. Soc. ReV. 1998, 27, 289. (d)
Elhabiri, M.; Elbrecht-Gary, A.-M. Coord. Chem. ReV. 2008, 252, 1079.
(2) (a) Seo, J. S.; Whang, D.; Lee, H.; Jun, S. I.; Oh, J.; Jeon, Y. J.;
Kim, K. Nature 2000, 404, 982. (b) Yoshizawa, M.; Tamura, M.; Fujita,
M. Science 2006, 312, 251.
˚
(3) (a) Yaghi, O. M.; OKeeffe, M.; Ockwig, N. W.; Chae, H. K.;
(5) (a) Mamula, O.; von Zelewsky, A. Coord. Chem. ReV. 2003, 242,
87. (b) Barboiu, M.; Vaughan, G.; Graff, R.; Lehn, J.-M. J. Am. Chem.
Soc. 2003, 125, 10257. (c) El-Ghayoury, A.; Hofmeier, H.; Schenning,
A. P. H. J.; Schubert, U. S. Tetrahedron Lett. 2004, 45, 261. (d) Heller,
M.; Schubert, U. S. Macromol. Rapid Commun. 2001, 22, 1358. (e) Kiehne,
U.; Lu¨tzen, A. Org. Lett. 2007, 9, 5333. (f) Oyler, K. D.; Coughlin, F. J.;
Bernhard, S. J. Am. Chem. Soc. 2007, 129, 210. (g) Piguet, C. J. Incl.
Phenom. Macrocycl. Chem. 1999, 34, 361.
Eddaoudi, M.; Kim, J. Nature 2003, 423, 705. (b) Oh, M.; Mirkin, C. A.
Nature 2005, 438, 651. (c) Kurth, D. G.; Higuchi, M. Soft Mater. 2006, 2,
915.
(4) (a) Novokmet, S.; Alam, M. S.; Dremov, V.; Heinemann, F. W.;
Mu¨ller, P.; Alsfasser, R. Angew. Chem., Int. Ed. 2004, 43, 2. (b) Myari,
A.; Hadjiliadis, N.; Garoufis, A. J. Inorg. Biochem. 2005, 99, 616. (c)
Tashiro, S.; Kobayashi, M.; Fujita, M. J. Am. Chem. Soc. 2006, 128, 9280.
10.1021/ol901293r CCC: $40.75
Published on Web 07/21/2009
 2009 American Chemical Society

of the resulting MEPE. Thus, the presence of chiral TEG
chain in btpy encourages us for studies of solvent and
temperature effects on chiral MEPEs.
Scheme 1. Synthesis of Chiral bis-Terpyridine Ligands
On the basis of these perspectives, we present an approach
for the formation of self-assembled helical metallo-supramo-
lecular polyelectrolytes (η-MEPE). In contrast to previous
reports,9 the ditopic btpy ligand itself is not chiral but we
employ a chiral TEG chain at the 6-position of the pyridine
ring to induce the chirality at the next level of the supramo-
lecular architecture. To allow the formation of helical
MEPEs, we employ a 1,3-substituted phenyl unit to connect
the two chiral tpy metal ion receptors. In order to prevent
steric crowding in the coordination sphere, only one periph-
eral pyridine ring is functionalized.
The synthetic scheme of enantiopure btpy containing a
chiral TEG chain at the ortho-position of the pyridine ring
is shown in Scheme 1. The 6-Bromo-4′-(4-bromo-phenyl)-
[2,2′;6′,2′′] terpyridine, 1, was synthesized according to a
previously reported procedure,10 which is a useful protocol
for synthesis of new chiral synthon L1. The chemoselective
synthesis of ligand L1 was achieved using compound 1 and
1.2 equivalent of chiral TEG in the presence of two
equivalents of NaH as base. Ligand L1 was obtained in good
1
chemical and optical yield and characterized by H, 13C
NMR, 2D-COSY NMR, and mass spectrometry (Supporting
Information). The 1H NMR of compound 1 shows peaks for
proton Ha at δ ) 7.71 ppm (dd, J ) 7.25 Hz) and Hb at δ
) 7.76 ppm (d, J ) 15.9 Hz), whereas chiral ligand L1
shows peaks for proton Ha at δ ) 6.85 ppm (d, J ) 8.25
Hz) and Hb at δ ) 7.70 ppm (Supporting Information, Figure
S1). The upfield shift of the Ha proton and the unchanged
peak position of Hb in NMR confirm chemoselective
synthesis of chiral ligand L1. This new synthon opens up
the door for the synthesis of new chiral btpy using different
synthetic methodologies.
The chiral btpy ligand L2 was synthesized by Suzuki bis-
coupling11 of 1,3-benzenediboronic acid bis(pinacol) ester
3 with two equivalents of ligand L1 in modest yield.
Additionally, the chiral ligand L3 was synthesized by
Sonogashira cross-coupling12 of 1,3-diethynyl benzene 4 with
two equivalent of ligand L1 in low yield. The ligands L2
and L3 were purified by alumina column chromatography
and preparative GPC. All products were characterized by
NMR, mass spectrometry, and optical polarimetry.
(6) (a) Kwong, H.-L.; Yeung, H.-L.; Yeung, C.-T.; Lee, W.-S.; Lee,
C.-S.; Wong, W.-L. Coord. Chem. ReV. 2007, 251, 2188, and references
therein. (b) Yeung, H.-L.; Sham, K.-C.; Tsang, C.-S.; Lau, T.-C.; Kwong,
H.-L. Chem. Commun. 2008, 3801. (c) Sala, X.; Rodrˇıgues, A. M.;
Rodrˇıgues, M.; Romero, I.; Parella, T.; von Zelewsky, A.; Llobet, A.; Benet-
Buchholz, J. J. Org. Chem. 2006, 71, 9283.
(7) (a) Bernhard, S; Takada, K.; D´ıaz, D. J.; Abrun˜a, H. D.; Mu¨rner, H.
J. Am. Chem. Soc. 2001, 123, 10265. (b) Barron, J. A.; Glazier, S.; Bernhard,
S.; Takada, K; Houston, P. L.; Abrun˜a, H. D. Inorg. Chem. 2003, 42, 1448.
(c) Kimura, M.; Sano, M.; Muto, T.; Hanabusa, K.; Shirai, H. Macromol-
ecules 1999, 32, 7951. (d) Bark, T.; Du¨ggeli, M.; Stoeckli-Evans, H.; von
Zelewsky, A. J. Chem. Soc., Perkin Trans.1 2002, 1881.
The self-assembly process of btpy was investigated with
Fe2+ as a central metal ion, since it forms well-defined
(achiral) octahedral complexes with well-distinguishable
metal-to-ligand charge transfer (MLCT) band in UV-vis
(8) (a) Sanji, T.; Sato, Y.; Kato, N.; Tanaka, M. Macromolecules 2007,
40, 4747. (b) Yamazaki, K.; Yokoyama, A.; Yokozawa, T. Macromolecules
2006, 39, 2432, and references cited therein. (c) Matthews, J. R.; Goldoni,
F.; Schenning, A. P. H. J.; Meijer, E. W. Chem. Commun. 2005, 5503.
(9) (a) Schubert, U. S.; Hofmeier, H.; Newkome, G. R. Modern
Terpyridine Chemsitry; Wiley-VCH: Weinheim, 2006; p 69. (b) Andres,
P. R.; Schubert, U. S. AdV. Mater. 2004, 16, 1043. (c) Constable, E. C.
Chem. Soc. ReV. 2007, 36, 246. (d) Vaduvescu, S.; Potvin, P. G. Eur.
J. Inorg. Chem. 2004, 1763. (e) Sauvage, J. P.; Collin, J. P.; Chambron,
J. C.; Guillerberz, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Cola, L. D.;
Flamigni, L. Chem. ReV. 1994, 94, 993.
13
spectra.10
The coordination reaction of L1 (Supporting
a,c,
Information, Figure S6a)14 and L2 (Figure 1a) with Fe(II)
to give rise dimer and polymer were monitored through
UV-vis spectrophotometric titration, respectively. These
experiments clearly showed that the coordination reaches end
point at 1:0.5 (Supporting Information, Figure S6b) and 1:1
(10) Han, F. S.; Higuchi, M.; Kurth, D. G. J. Am. Chem. Soc. 2008,
130, 2073.
(12) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605.
(13) Hasenknopf, B.; Lehn, J.-M. HelV. Chim. Acta 1996, 79, 1643.
references cited therein.
(11) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Han,
F. S.; Higuchi, M.; Kurth, D. G. Org. Lett. 2007, 9, 559.
(14) See Supporting Information.
Org. Lett., Vol. 11, No. 16, 2009
3563

stiochiometric ratio of metal to ligand (L2), further self-
assembly of chiral ligands (L1, L2, L3) were carried out by
refluxing an equimolar amount of ligand and Fe(OAc)2 in
acetic acid (CH3COOH) under argon atmosphere. The color
of the solution turned purple during complexation, indicating
the formation of the corresponding MEPEs. The MEPEs
with CH3COO- as counteranions are soluble in acetic acid,
methanol (CH3OH) and water (H2O). We also synthesized
-
FeL2′-MEPE with PF6 as counteranions to study the
optically active structures in CH3CN solvent. The coordina-
1
tion of Fe(II) and ligands (L1, L2) were confirmed by H
NMR, UV-vis spectroscopy and the molecular weights
determination in methanol solvents (FeL2-MEPE ) 1.7 ×
104 and FeL3-MEPE ) 1.4 × 104) by SEC-Viscometry-
RALLS method.14
The conformation of optically active polymers or oligo-
mers largely depends on the solvent media,8a,b,15 metal to
ligand coordination16 and their counteranions.16b,c As there
are no reports for different solvent effects on chiral MEPEs,
hence we elaborated our attention to this respect. The
absorption spectra of FeL2-MEPE (CH3OH) with
CH3COO- counteranions and FeL2′-MEPE (CH3CN) with
PF6- counteranions shows their characteristics MLCT band
almost at the same position (λ573) (Supporting Information,
-
Figure S8a) as in case of FeL2-MEPE with ClO4 coun-
teranions (λ574) (Figure 1b). Whereas, the absorption spectra
of different polymers FeL2-MEPE and FeL3-MEPE show
change in the MLCT band at 573 and 575 nm due to increase
in conjugation. The CD spectra of these polymers show
Figure 1. (a) UV-vis and (b) CD spectral changes in the titration
of L2 solution (CHCl3, c ) 1.10 × 10-5 M, l ) 1 cm) with
Fe(ClO4)2 solution (CH3CN, c ) 4.42 × 10-5 M) at 20 °C. Arrows
indicate the direction of spectral changes. The inset (a) shows the
change in absorbance at 574 nm and inset (b) shows the increase
nearly the same type of signal at the MLCT band (∆ε570
)
in CD intensities at 357 and 574 nm as a function of added Fe2+
.
-0.13 M-1 cm-1), (∆ε570 ) -0.03 M-1 cm-1), followed by
a positive (∆ε340 ) 0.79 M-1 cm-1), (∆ε340 ) 0.52 M-1
cm-1) and a negative signal (∆ε293 ) -0.25 M-1 cm-1),
(∆ε293 ) -0.39 M-1 cm-1), respectively (Supporting Infor-
mation, Figure S7b). Surprisingly, we observed a very
interesting result in water as compared to organic solvents
of FeL2-MEPE with CH3COO- counteranions.
(inset, Figure 1a) ratio of L1:Fe(II) and L2:Fe(II) for tpy
and btpy, respectively. The presence of a very well-defined
isosbestic point at 321 nm also indicates that there is simple
equilibrium involved in case of L2. This result confirms the
formation of coordination polymer. Results from the spec-
trophotometric titration were corroborated by titrations
followed by Circular dichroism (CD) study. As depicted in
Figure 1b, clear spectral changes are observed upon addition
of Fe(II) metal ions. Interestingly, we are unable to observe
a CD signal for the neat ligands in chloroform. However,
upon addition of different amount of Fe2+ solution in
acetonitrile (CH3CN) to the solution of L2 in chloroform
(CHCl3), we observe a CD signal, which gradually increases
in intensity as Fe2+ is added to solution. The observed CD
signal is clearly an indication for the formation of optically
active structures upon metal ions coordination. As is evident
from the Job plots (inset, Figure 1b) the Cotton effect at the
MLCT band indicates the formation of an asymmetric
environment of the otherwise optically inactive Fe(tpy)2.
Furthermore, new peaks appeared at 357 nm (negative) and
320 nm (positive), which also show a function with respect
to added metal ions.
The UV-vis spectra showed a depression of overall
optical intensity with a shift of the MLCT band from λ573
to λ577, which is associated with the difference in the
dielectric constants (Supporting Information, Figure S8a).
In contrast, we observe strong changes in the CD spectra, a
negative Cotton effect with a very sharp increase in CD
intensity at 357 nm (∆ε357 ) -9.15 M-1 cm-1), followed
by a positive Cotton effect (∆ε316 ) 1.52 M-1 cm-1). This
solvent also shows an enhancement of the CD intensity of
the MLCT band (∆ε577 ) -1.03 M-1 cm-1). The CD spectra
of FeL2-MEPE in water show strong Cotton effects over
the absorption ranges (Supporting Information, Figure S8b),
as it was obtained in case of titration of L2 with Fe(ClO4)2
(Figure 1b), indicating the formation of a optically active
stable structures. Further, the CD sepctra of FeL2-MEPE
(15) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science
1997, 277, 1793. (b) Prince, R. B.; Brunsveld, L.; Meijer, E. W.; Moore,
It was well reported10 that, the solubility of a metallo-
supramolecular polymer largely depends on counteranions.
Hence, for study of solvent effects, we need MEPEs with
different counteranions. Therefore, after conformation of
J. S. Angew Chem., Int. Ed. 2000, 39, 228
.
(16) (a) Prince, R. B.; Okada, T.; Moore, J. S. Angew. Chem., Int. Ed.
1999, 38, 233. (b) Kim, H. J.; Zin, W. C.; Lee, M. J. Am. Chem. Soc.
2004, 126, 7009. (c) Piguet, C.; Bernarddinelli, G.; Hopfgartner, G. Chem.
ReV. 1997, 97, 2005.
3564
Org. Lett., Vol. 11, No. 16, 2009

in aqueous solution show different sign around 340 nm and
same sign for the MCLT band from that in organic solvents.
This result suggests that FeL2-MEPE adopts a different
conformation in water as compared to organic solvents. The
possible explanation for strong changes in CD intensity of
MEPE is due to hydrogen bonding17 between water mol-
ecules and ethylene oxide chains of chiral TEG units of btpy,
but the possibility of aggregation of aromatic parts in water
is not ruled out as reported by Lee et al.18 for coordination
polymers.
It is well-known that the changes in temperature affect
chiral conformations.8b,c,15,19a,b Recently, Schubert et
al.5d,19c,d have reported the temperature effect on metallo-
supramolecular polymers. Inspired from these reports, the
temperature dependent CD spectra will allow us to study
the self-assembly of MEPE. The temperature dependent
studies of optically active FeL2-MEPE was carried out in
a temperature range from 10-50 °C in water. The UV-vis
spectra (Supporting Information, Figure S9) show a gradually
decrease in the absorption intensity of the MLCT band with
increasing temperature, and at 50 °C we note the almost
complete absence of this band indicating that MEPE has
dis-assembled. A similar effect is observed in the CD spectra,
which are shown in Figure 2. The CD intensity decreases
without change in shape of spectra with increasing temper-
ature. The changes of CD spectra by increasing temperature
may be due to the decomposition of metallosupramolecular
polymers by the cleavage of metal complexes. The temper-
ature dependent peak intensities of the MLCT band of FeL2-
MEPE in the UV-vis (Supporting Information, Figure S9)
and CD spectra (Figure 2) are shown in the insets. These
results show that the MEPEs are equilibrium structures. At
elevated temperatures the MEPE dis-assembled, which is
indicated by the loss of the characteristic MLCT band. While
we did not observe a reversible effect in water, we do note
that in methanol the MLCT band is recovered upon cooling.
As we noted above, equilibrium in water is slow at room
temperature and, therefore, recovery of the MLCT band is
not observed under the experimental conditions (Supporting
Information, Figure S10).
Figure 2
.
CD spectra of FeL2-MEPE in water (c ) 2.00 × 10-5
M, l ) 1 cm) at different temperatures. Arrows indicate the direction
of spectral changes. The inset shows the relationship of CD intensity
of the MLCT band (λ ) 577 nm) with increasing temperature.
In conclusion, we synthesized a new chiral synthon L1,
which is the basis for the development of chiral btpys. The
chiral btpy ligands L2 and L3 with a 1,3-connectivity and
chiral TEG chains at the ortho-position of periphery were
synthesized and applied for generation of metal ion induced
new optically active metallo-supramolecular polymers. The
change in CD intensities as a function of added Fe2+ confirm
the formation of optically active coordination polymers. The
strong CD intensities of MEPEs indicate that they adopt
chiral conformations and the temperature dependence of
UV-vis and CD spectra indicates the dis-assembling of
optically active metallo-architectures at higher temperature.
Studies to determine the conformational change of FeL2-
MEPE in water to organic solvent are in progress.
Acknowledgment. We thank Dr. Katsuhiro Isozaki for
images and the Ministry of Education, Culture, Sports,
Sciences and Technology, Japan, for financial support.
Supporting Information Available: Experimental pro-
cedures, full spectroscopic data for all new compounds,
UV-vis, CD spectra and proton NMR data of ligands and
MEPEs. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173.
(18) Kim, H.-J.; Lee, E.; Park, H.-S.; Lee, M. J. Am. Chem. Soc. 2007,
129, 10994, and references cited therein.
(19) (a) Hasegawa, T; Morino, K.; Tanaka, Y.; Katagiri, H.; Furosho,
Y.; Yashima, E Macromolecules 2006, 39, 482. (b) Yashima, E.; Maeda,
K Macromolecules 2008, 41, 3, and references cited therein. (c) Hofmeier,
H.; Hoogenboom, R.; Wouters, M. E. L.; Schubert, U. S. J. Am. Chem.
Soc. 2005, 127, 2913. (d) Chiper, M.; Fournier, D.; Hoogenboom, R.;
Schubert, U. S. Macromol. Rapid Commun. 2008, 29, 1640
.
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