Syntheses of 3-arm and 4-arm star-branched polystyrene Ru(II) complexes by the click-to-chelate approach

Article abstract of DOI:10.1002/pola.24487

Metal template synthesis is a useful methodology to make sophisticated macromolecular architectures because of the variety of metal ion coordination. To use metal template methodology, chelating functionalities should be introduced to macromolecules befor

Full text of DOI:10.1002/pola.24487

Syntheses of 3-Arm and 4-Arm Star-Branched Polystyrene Ru(II)
Complexes by the Click-to-Chelate Approach
CHUNHONG ZHANG,1 XIANDE SHEN,1 RYOSUKE SAKAI,2 MICHAEL GOTTSCHALDT,3 ULRICH S. SCHUBERT,3
SHIHO HIROHARA,4 MASAO TANIHARA,4 SHIGENOBU YANO,4 MAKOTO OBATA,5 NAO XIAO,6
TOSHIFUMI SATOH,6 TOYOJI KAKUCHI6
1College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
2Department of Materials Chemistry, Asahikawa National College of Technology, Asahikawa 071-8142, Japan
3Laboratory for Organic and Macromolecular Chemistry, Friedrich-Schiller-University Jena, Jena 07743, Germany
4Graduate School of Materials Science, Nara Institute of Science and Technology, Nara 630-0101, Japan
5Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan
6Division of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering,
Hokkaido University, Sapporo 060-8628, Japan
Received 9 September 2010; accepted 29 October 2010
DOI: 10.1002/pola.24487
Published online 3 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Metal template synthesis is a useful methodology
to make sophisticated macromolecular architectures because
of the variety of metal ion coordination. To use metal template
methodology, chelating functionalities should be introduced to
macromolecules before complexation. In this article, we dem-
onstrate the click-to-chelate approach to install chelating func-
tionality to polystyrene (PS) and complexation with Ru(II) ions
to form 3-arm and 4-arm star-branched PS Ru(II) complexes.
Azide-terminated PS (PS-N3) was readily prepared by atom
transfer radical polymerization using 1-bromoethylbenzene as
an initiator followed by substitution of bromine by an azide
group. The Cu(I)-catalyzed 1,3-dipolar cycloaddition of PS-N3
with 2-ethynylpyridine or 2,6-diethynylpyridine affords 2-(1H-
1,2,3-triazol-4-yl)pyridine (PS-tapy) or 2,6-bis(1H-1,2,3-triazol-4-
yl)pyridine (PS-bitapy) ligands bearing one or two PS chains at
the first-position of the triazole rings. Ru(II) complexes of PS-
tapy and PS-bitapy were prepared by conventional procedure.
The number-averaged molecular weights (Mns) of these com-
plexes were determined to be 6740 and 10,400, respectively, by
size exclusion chromatography using PS standards. These Mn
values indicated the formation of 3-arm and 4-arm star-
branched PS Ru(II) complexes [Ru(PS-tapy)3](PF6)2 and [Ru(PS-
bitapy)2](PF6)2 on the basis of the Mn values of PS-tapy (2090)
and PS-bitapy (4970). The structures of these complexes were
also confirmed by UV–vis spectroscopy and X-ray crystallogra-
phy of the Ru(II) complexes [Ru(Bn-tapy)3](PF6)2 and [Ru(Bn-
bitapy)2](PF6)2, which bear a benzyl group instead of a PS
C
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chain. 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 49: 746–753, 2011
KEYWORDS: atom transfer radical polymerization (ATRP); click
chemistry; Ru(II) complex; star-branched polystyrene; star polymer
the use of an initiator bearing chelating moieties, and the
other is the postfunctionalization of macromolecules with
chelating molecules. Fraser and coworkers5–7 extensively
used the first approach to form (mikto-arm) star polymers.
They used an atom transfer radical polymerization (ATRP)
initiator (e.g., halomethyl group and 2-bromopropionate)
covalently bonded with bipyridine at 4,40-positions and then
polymerized styrene, methyl methacrylate, and so forth,
before or after complexation with Ru(II) and Fe(II). In con-
trast, Schubert and coworkers8–10 used the second approach
to prepare block copolymers. They used 40-chloro-2,20:60,2"-
terpyridine as a chelating moieties and installed them by
Williamson-type reaction with hydroxy groups at the chain
end of polyethylene glycol and end-functionalized polysty-
rene (PS) prepared by anion polymerization.
INTRODUCTION Metal template synthesis is
a powerful
methodology to prepare macromolecules with sophisticated
architectures due to the large coordination numbers, variety
of coordination geometries, and liability and redox activity of
metal ions.1–3 To use metal ions to form certain architec-
tures, chelating functionalities, for example, polypyridyl
groups, polyamines, and diketones, must be incorporated
into macromolecules at a suitable position. Macromolecular
ligands are then subjected to complexation with certain
metal ions to form the certain architecture spontaneously.
Oligopyridine functionalities including heteropolycyclic com-
pounds such as phenanthroline are the most widely used
chelating motif for this purpose.4 There are two methods to
install a chelating functionality on a macromolecule: one is
Correspondence to: M. Obata (E-mail: mobata@yamanashi.ac.jp) or T. Kakuchi (E-mail: kakuchi@poly-bm.eng.hokudai.ac.jp)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 746–753 (2011) 2010 Wiley Periodicals, Inc.
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ARTICLE
Recently, Schibli’s group11 proposed a new approach to
install chelating moieties on biologically relevant molecules.
They used a 1,2,3-triazole ring as a coordinating functional-
ity; this is easily synthesized by Cu(I)-catalyzed 1,3-dipolar
cycloaddition of azide to acetylene derivatives, so-called click
chemistry.12 They synthesized a metal chelating peptide by
click chemistry of azide-terminated peptide (bombesin) with
L-propargyl glycine, in which amino acid bearing 1,2,3-tria-
zole acted as a tridentate chelating functionality, and they
prepared Re(I) and 99mTc complexes for nuclear medicine
application. They called this approach click-to-chelate.11,13,14
Other research groups demonstrated the versatility of this
approach for synthesizing metal chelating fluorophores,15
Zn(II) ion probes,16 ligands of Cu(I) complexes for click reac-
tion,17 sugar-containing metal chelaters,18 and Ag(I) coordi-
nation polymers.19 We demonstrated the synthesis of 2-(1-
benzyl-1,2,3-triazol-4-yl)pyridine (Bn-tapy) by click chemis-
try of benzyl azide with 2-ethynylpyridine and preparation
of the Re(I) complex.18 X-ray-crystallography of the Re(I)
complex showed clear evidence that the tapy moiety acts as
a bipyridine-like chelator. This approach affords a promising
method to introduce bipyridine-like functionality to macro-
molecules because of the excellent feasibility of azide substi-
tution and click chemistry.
were performed on a Jasco GPC-900 system equipped with
two Shodex K-805L columns (linear; 8 mm/ ꢀ 300 mm;
exclusion limit, 4 ꢀ 106) with a flow rate of 0.8 mL minꢁ1 at
ꢂ
40 C. The number-average molecular weight (Mn) and poly-
dispersity (Mw/Mn) were calculated based on a PS calibra-
tion. The matrix-assisted laser desorption/ionization time-of-
flight mass spectrometry (MALDI-TOF MS) measurements
were performed using an Applied Biosystems Voyager-DE
STR-H mass spectrometer with a 25-kV acceleration voltage.
The positive ions were detected in the reflector mode
(25 kV). A nitrogen laser (337 nm, 3 ns pulse width, 106–
107 W cmꢁ2) operating at 3 Hz was used to produce the
laser desorption, and 100 shots were summed. The spectrum
was externally calibrated using PS standard with a linear
calibration. Samples for the MALDI-TOF MS were prepared
by mixing the polymer (10 mg mLꢁ1, 2 lL), the matrix
(dithranol, 20 mg mLꢁ1, 100 lL), and the cationizing agent
(sodium trifluoroacetate, 10 mg mLꢁ1, 4 lL) in THF.
2,6-Bis(1-benzyl-1,2,3-triazol-4-yl)pyridine
2,6-Diethynylpyridine (254.1 mg, 2.00 mmol) and benzyl
azide (591.2 mg, 4.44 mmol) were dissolved in a mixture of
THF and H2O (1/1, v/v; 80 mL). Aqueous CuSO4 solution
(7.5 wt %, 1.27 mL) and 1 M sodium ascorbate (aq.;
0.76 mL) were added to the solution. The reaction mixture
was stirred at 55 ꢂC for 4 h. THF was removed under
reduced pressure, and water (200 mL) was added to the res-
idue. The insoluble powder was collected and dried in vacuo.
The crude product was purified by column chromatography
(silica gel, acetone) followed by recrystallization from ace-
tone and hexane to give 2,6-bis(1-benzyl-1,2,3-triazol-4-
yl)pyridine (Bn-bitapy) as colorless needle-like crystals
(381.3 mg, 48%).
In this study, we synthesized star-branched PS Ru(II) com-
plexes by the click-to-chelate approach. The coordination
structures of macromolecular complexes were established on
the basis of model Ru(II) complexes bearing benzyl groups
instead of PS, whose structures were confirmed by conven-
tional analytical techniques and X-ray crystallography.
EXPERIMENTAL
Materials and Analytical Techniques
Anal. calcd. for C23H19N7: C, 70.21; H, 4.87; N, 24.92. Found:
C, 70.73; H, 4.99; N, 25.04. 1H NMR (600 MHz, CDCl3,
All chemicals were analytical grade. 2-Ethynylpyridine was
purchased from Sigma-Aldrich. 2,6-Diethynylpyridine and
Bn-tapy were prepared according to methods in the litera-
ture.18,20 1H and 13C{1H} NMR spectra were recorded using
JNM-A400II (400 MHz; JEOL, Tokyo, Japan), AVANCE 400
(400 MHz; Bruker Biospin K. K., Yokohama, Japan), and JNM-
EC600 (600 MHz; JEOL) instruments. Infrared (IR) spectra
were recorded on a PerkinElmer Paragon 1000 FTIR instru-
ment. UV–vis spectra were recorded on a Jasco V-550 spec-
trophotometer. Electron spray ionization (ESI) and fast atom
bombardment (FAB) mass spectrometry (MS) were per-
formed using a JEOL JMS-SX102A instrument. Elemental
analysis was carried out using a YANACO CHN corder MT-6
instrument. Preparative size exclusion chromatography (SEC)
was performed on a JAI LC-9201 HPLC system equipped
with a JAI RI-50s refractive index detector and a JAI JAIGEL-
3H column (20 mm/ ꢀ 600 mm; exclusion limit, 7 ꢀ 104)
using tetrahydrofuran (THF) or CHCl3 (flow rate, 3.5 mL
3
Si(CH3)4 ¼ 0 ppm): d (ppm) ¼ 8.08 (d, J ¼ 7.8 Hz, 2H, 3,5-
3
pyridineH), 8.03 (s, 2H, 5-triazoleH), 7.85 (t, J ¼ 7.8 Hz, 1H,
4-pyridineH), 7.39–7.35 (m, 6H, PhH), 7.30–7.28 (m, 4H,
PhH), 5.57 (s, 4H, ACH2A). 13C{1H} NMR (150 MHz, CDCl3,
CDCl3 ¼ 77 ppm): d (ppm) ¼ 149.9, 148.7, 137.7, 134.6,
129.1, 128.8, 128.1, 122.0, 119.4, 54.3.
[Ru(Bn-tapy)3](PF6)2
RuCl3ꢃnH2O (67.4 mg, 0.33 mmol for n ¼ 0), Bn-tapy (235.3
mg, 1.0 mmol), and N,N-dimethylformamide (DMF; 4 mL)
were placed in a 10-mL flask. The mixture was heated at
ꢂ
145 C for 3 h. The mixture was concentrated under reduced
pressure, and saturated aqueous solution of NaPF6 was
added to the residue. The resulting orange powder was
collected by filtration. The powder was dissolved in CH3CN,
and the insoluble part was removed by filtration. The filtrate
was concentrated under reduced pressure. The residue was
purified by Sephadex LH-20 using a mixture of CHCl3 and
CH3OH (1/1, v/v) as an eluent to give a yellow powder
(88.5 mg, 0.0805 mmol, 24%).
minꢁ1) at 23 C. SEC measurements in THF were performed
ꢂ
on a Jasco GPC-900 system equipped with a Waters Ultra-
styragel column (linear; 7.8 mm/ ꢀ 300 mm; exclusion
limit, 1 ꢀ 107) and two Shodex KF-804L columns (linear;
8 mm/ ꢀ 300 mm; exclusion limit, 4 ꢀ 105) with a flow
rate of 1.0 mL minꢁ1 at 40 ꢂC. SEC measurements in CHCl3
Anal. calcd. for C42H36N12F12P2RuꢃH2O: C, 45.13; H, 3.43; N,
15.04. Found: C, 45.14; H, 3.35; N, 15.17. ESI-MS: m/z for
C
42H36N12F6PRu ([M ꢁ PF6]þ) calcd. 954.8, found 955.1. 1H
STAR-BRANCHED POLYSTYRENE Ru(II) COMPLEXES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
NMR (600 MHz, CD3CN, CHD2CN ¼ 1.93 ppm): d (ppm) ¼
8.66–8.62 (3H), 8.09–7.70 (9H), 7.40–7.14 (18H), 5.57–5.54
(6H, ACH2A). 13C{1H} NMR (150 MHz, CD3CN, CD3CN ¼ 1.3
ppm): d (ppm) ¼ 153.6, 153.3, 153.2, 152.5, 152.4, 152.2,
149.5, 149.4, 149.2, 149.1, 139.2, 139.1, 139.02, 138.98,
135.1, 135.1, 134.7, 134.7, 130.17, 130.15, 130.12, 130.1,
129.9, 129.4, 129.3, 128.9, 126.9, 126.8, 126.7, 126.4,
126.15, 126.12, 126.10, 125.8, 123.7, 123.4, 123.0, 122.9,
56.62 (ACH2A), 56.59 (ACH2A), 56.38 (ACH2A), 56.34
(ACH2A). UV–vis (c ¼ 45.5 lM, DMF, path length ¼ 1 cm): k
nmꢁ1 (e Mꢁ1 cmꢁ1) ¼ 383 (15.9 ꢀ 103).
tion from THF and CH3OH to give white solids (4.4 g, 88%).
Mn,THF ¼ 1940, Mw/Mn ¼ 1.13.
2-(1H-1,2,3-Triazol-4-yl)pyridine
Azide-terminated PS (PS-N3; Mn,THF ¼ 1940, 3.1 g, 1.6 mmol)
and CuBr (22.9 mg, 1.6 mmol) were placed in a Schlenk
tube. The tube was evacuated and backfilled with argon three
times. The following was added to another flask: PMDETA
(0.333 mL, 1.6 mmol), 2-ethynylpyridine (0.323 mL,
3.2 mmol), and distilled DMF (15.32 mL). The mixture was
degassed by argon bubbling for 30 min and transferred to the
Schlenk tube. The reaction mixture was stirred at 90 ꢂC for
24 h. After removal of the solvent under reduced pressure,
the residue was diluted with CH2Cl2 (100 mL), and washed
with distilled water (100 mL ꢀ 2). After drying over Na2SO4,
the solvent was removed under reduced pressure. The crude
polymer was purified by reprecipitation from THF and CH3OH
four times and dried in vacuo to give a white solid (2.49 g,
80%). Mn,THF ¼ 2090 and Mw/Mn ¼ 1.11. Mn,CHCl3 ¼ 2910
and Mw/Mn ¼ 1.18.
[Ru(Bn-bitapy)2](PF6)2
RuCl3ꢃnH2O (42.4 mg, 0.20 mmol for n ¼ 0), Bn-bitapy
(160.0 mg, 0.41 mmol), and DMF (4 mL) were placed in a
ꢂ
10-mL flask. The mixture was heated at 145 C for 3 h. The
mixture was concentrated under reduced pressure, and satu-
rated aqueous solution of NaPF6 was added to the residue.
The resulting orange powder was collected by filtration. The
powder was dissolved in CH3CN, and the insoluble part was
removed by filtration. The solvent was removed under
reduced pressure. The residue was purified by Sephadex LH-
20 using a mixture of CHCl3 and CH3OH (1/1, v/v) as an
eluent and followed by recrystallization from CH3CN and
CH3OH to give orange needle-like crystals (32.6 mg, 14%).
2,6-Bis(1H-1,2,3-triazol-4-yl)pyridine
PS-N3 (Mn,THF ¼ 1940, 0.4 g, 0.154 mmol) and CuBr (2.21
mg, 0.154 mmol) were placed in a Schlenk tube. The tube
was evacuated and back-filled with argon three times. The
following was added to another flask: PMDETA (32.1 lL,
0.154 mmol), 2,6-diethynylpyridine (9.7 mg, 0.0762 mmol),
and distilled DMF (1.5 mL). The mixture was degassed by
argon bubbling for 30 min and transferred to the Schlenk
tube. The reaction mixture was stirred at 90 ꢂC for 24 h.
After removal of the solvent under reduced pressure, the
residue was diluted with CH2Cl2 (50 mL), and washed with
distilled water (50 mL ꢀ 2). After drying over Na2SO4, the
solvent was removed under reduced pressure. The residue
was purified by preparative SEC using THF as an eluent
followed by reprecipitation from THF and CH3OH and dried
in vacuo to give white solid (0.1855 g, 62%). Mn,THF ¼ 4970
and Mw/Mn ¼ 1.09. Mn,CHCl3 ¼ 5310 and Mw/Mn ¼ 1.06.
Anal. calcd. for C46H38N14F12P2RuꢃCH3OH: C, 46.66; H, 3.50;
N, 16.21. Found: C, 46.35; H, 3.25; N, 16.45. FAB-MS: m/z for
C
46H38N14F6PRu ([M ꢁ PF6]þ) calcd. 1032.9, found 1033.2.
1H NMR (600 MHz, CD3CN, CHD2CN ¼ 1.93 ppm): d (ppm)
3
¼ 8.59 (s, 4H, 5-triazoleH), 8.23 (t, J ¼ 7.8 Hz, 2H 4-pyridi-
neH), 8.18 (d, 3J ¼ 7.2 Hz, 4H, 3,5-pyridineH), 7.36 (t, 3J ¼
7.2 Hz, 4H, 4-PhH), 7.31 (t, 3J ¼ 7.5 Hz, 8H, 3,5-PhH), 7.12
(d, 3J ¼ 7.2 Hz, 8H, 2,6-PhH), 5.38 (s, 8H, ACH2A). 13C{1H}
NMR (150 MHz, CD3CN, CD3CN ¼ 1.3 ppm): d (ppm) ¼
151.5, 150.7, 138.8, 134.4, 130.11, 130.07, 129.3, 126.5,
121.2, 56.5. UV–vis (c ¼ 42.4 lM, DMF, path length ¼ 1 cm):
k nmꢁ1 (e Mꢁ1 cmꢁ1) ¼ 394 (16.7 ꢀ 103).
Bromine-Terminated PS
CuBr (0.55 g, 3.84 mmol) was placed in a Schlenk tube, and
the tube was evacuated and backfilled with argon three
times. A degassed mixture of styrene (13.25 mL, 115.2
mmol), 1-bromoethylbenzene (0.52 mL, 3.84 mmol), and
N,N,N0,N00,N00-pentamethyldiethylenetriamine (PMDETA; 0.80
mL, 3.84 mmol) was added to the tube. The tube was heated
[Ru(PS-tapy)3](PF6)2
2-(1H-1,2,3-Triazol-4-yl)pyridine (PS-tapy; Mn,THF ¼ 2090,
160 mg, 0.0765 mmol) and RuCl3 (5.2 mg, 0.025 mmol)
were placed in a Schlenk tube. The tube was evacuated and
back-filled with argon three times. Degassed dry DMF
(0.75 mL) was added to the tube under an argon atmos-
phere. The reaction mixture was stirred at 100 ꢂC for 48 h.
After cooling to room temperature, a 10-fold excess of
NH4PF6 (122.3 mg, 0.72 mmol) was added. The mixture was
stirred for 10 min at room temperature. The reaction mix-
ture was diluted with CHCl3, and the insoluble part was
removed by filtration. The filtrate was evaporated under
vacuum, and the residue was dissolved in CHCl3 (15 mL)
and washed with water (20 mL ꢀ 2). After drying over
Na2SO4, the solvent was removed under reduced pressure.
The crude product was purified by preparative SEC using
CHCl3 as an eluent to give [Ru(PSt-tapy)3](PF6)2 as yellow
solids (89.5 mg, 56%). Mn,CHCl3 ¼ 6740 and Mw/Mn ¼ 1.07.
ꢂ
at 90 C for 30 min. The resulting mixture was diluted with
CH2Cl2 and washed with distilled water three times. The or-
ganic layer was dried over Na2SO4 and evaporated to remove
the solvent. The crude product was purified by reprecipita-
tion from THF and CH3OH to give white solids (5.14 g,
43%). Mn,THF ¼ 2100, Mw/Mn ¼ 1.13.
Azide-Terminated PS
Bromine-terminated PS (PS-Br; Mn,THF ¼ 2100, 5.0 g, 2.4
mmol), NaN3 (0.78 g, 12 mmol), and distilled DMF (40 mL)
were added to a flask. The mixture was stirred at room tem-
perature for 8 h. The resulting mixture was evaporated to
remove DMF. The crude product was purified by reprecipita-
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ARTICLE
SCHEME 1 Synthesis of triazolylpyridine ligand-bearing PS.
[Ru(PS-bitapy)2](PF6)2
2,6-Bis(1H-1,2,3-triazol-4-yl)pyridine (PS-bitapy; Mn,THF
bearing one or two PS chain(s) at the first-position of the
triazole ring(s). After purification of PS-tapy by reprecipita-
tion from THF and methanol and of PS-bitapy by preparative
SEC, the stretching band due to the azide group completely
disappeared from the IR spectra. The Mn values of PS-tapy
and PS-bitapy were estimated to be 2090 and 4970, respec-
tively. The Mw/Mn values of PS-tapy and PS-bitapy remained
comparable with that of the starting PS-N3. The IR spectra
and SEC traces suggested the formation of (1,2,3-triazol-4-
yl)pyridine type ligand bearing PS chain(s) as shown in
Scheme 1. To provide further insight into the polymer struc-
ture, a MALDI-TOF MS measurement of PS-tapy was carried
out. As shown in Figure 2, only one series of peaks was
observed in the MS spectrum. The peak interval is 104.1,
which is identical with the molecular weight of styrene.
Additionally, the peak at 1835.3 (m/z) is in a good agree-
ment with the theoretical isotopic molecular weight of
sodium-cationized PS (n ¼ 15) with the tapy chain end
¼
4970, 150 mg, 0.030 mmol) and RuCl3 (2.70 mg,
0.013mmol) were placed in a Schlenk tube. The tube was
evacuated and back-filled with argon three times. Degassed
dry DMF (0.3 mL) was added to the tube under an argon
atmosphere. The reaction mixture was stirred at 130 ꢂC for
48 h. After cooling to room temperature, a 10-fold excess of
NH4PF6 (122.3 mg, 0.72 mmol) was added. The mixture was
stirred for 10 min at room temperature. The reaction mix-
ture was diluted with CHCl3, and the insoluble part was
removed by filtration. The filtrate was evaporated under
vacuum, and the residue was dissolved in CHCl3 (15 mL) and
washed with water (20 mL ꢀ 2). After drying over Na2SO4,
the solvent was removed under reduced pressure. The crude
product was purified by preparative SEC using CHCl3 as
an eluent to give [Ru(PS-bitapy)2](PF6)2 as yellow solids
(40.3 mg, 27%). Mn,CHCl3 ¼ 10,400 and Mw/Mn ¼ 1.09.
X-Ray Structural Determination
Single crystal X-ray diffraction data were recorded on a
Rigaku RAXIS RAPID imaging plate area detector using
filtered Mo Ka radiation. All data sets were corrected for
Lorentzian polarization effects and for absorption. All struc-
tures were solved by the direct method (SIR9221). Hydrogen
atoms were placed into calculated positions and refined with
isotropic thermal parameters riding on those of the parent
atoms. Refinement of the nonhydrogen atoms was carried
out with the full-matrix least-squares technique.
RESULTS AND DISCUSSION
PS-Br was prepared by ATRP using 1-bromoethylbenzene as
an initiator and CuBr and PMDETA as a catalyst at 90 ꢂC
(Scheme 1). The Mn and Mw/Mn values of the resulting poly-
mer PS-Br were estimated to be 2100 and 1.13 by SEC using
PS standards. The resulting polymer PS-Br was treated with
sodium azide in dry DMF to give the PS-N3. The introduction
of the azide group was confirmed by its characteristic
stretching band at 2094 cmꢁ1 (Fig. 1). PS-N3 afforded a
unimodal SEC trace with the same Mw/Mn value (1.13) as
that of PS-Br. Cu(I)-catalyzed 1,3-dipolar cycloadditions
between PS-N3 and 2-ethynylpyridine or 2,6-diethynylpyri-
dine were conducted in degassed DMF using CuBr/PMDETA
as a catalyst at 90 ꢂC to give PS-tapy and PS-bitapy ligands
FIGURE 1 IR spectra of PS-Br, PS-N3, PS-tapy, and PS-bitapy.
STAR-BRANCHED POLYSTYRENE Ru(II) COMPLEXES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
unreacted PS-tapy and PS-bitapy by preparative SEC, Ru(II)
complexes of PS-tapy and PS-bitapy were obtained as yellow
solids. The Mn values of these solids were determined to be
6740 and 10,400, respectively.22 These Mn values suggest
the formation of a 3-arm complex [Ru(PS-tapy)3](PF6)2 and
a 4-arm complex [Ru(PS-bitapy)2](PF6)2, even though the Mn
values determined by SEC are on a relative scale calibrated
by linear PS standards (Figure 3).
To explore the detailed structure of the Ru(II) complexes of
PS-tapy and PS-bitapy, we synthesized Ru(II) complexes
bearing benzyl groups instead of PS chains (Chart 1). The
ligands Bn-tapy and Bn-bitapy were synthesized by the same
method as previously reported.18 The Ru(II) complexes
[Ru(Bn-tapy)3](PF6)2 and [Ru(Bn-bitapy)2](PF6)2 were pre-
pared by the reaction of RuCl3 with Bn-tapy or Bn-bitapy in
ꢂ
DMF for 3 h at 145 C and were obtained as yellow powders
in 24 and 14% yields, respectively. Elemental analysis and
mass spectrometry clearly indicated the formations of Ru(II)
complexes formulated as [Ru(Bn-tapy)3](PF6)2 and [Ru(Bn-
1
bitapy)2](PF6)2. [Ru(Bn-tapy)3](PF6)2 has a complex H NMR
spectrum plausibly because of a mixture of fac-isomers and
mer-isomers. Four peaks due to methylene carbon were
found at 56.62, 56.59, 56.38 and 56.34 ppm in 13C{1H} NMR
spectrum of [Ru(Bn-tapy)3](PF6)2 corresponding to one peak
from the C3 symmetric fac-isomer and three peaks from the
asymmetric mer-isomer. In contrast, [Ru(Bn-bitapy)2](PF6)2
has a very simple H NMR spectrum and 13C NMR spectrum,
1
FIGURE 2 MALDI-TOF MS spectra of PS-tapy in reflector mode.
showing only 10 peaks, clearly indicating an S4 symmetric
structure in acetonitrile solution. Recrystalization from a
mixture of acetonitrile and methanol affords yellow crystals
suitable for X-ray crystallography. Figure 4 shows an ORTEP
drawing of [Ru(Bn-bitapy)2]2þ, and Table 1 lists the selected
bond lengths and angles. Although the [Ru(Bn-bitapy)2]2þ
structure is distorted in the crystal packing, the space group
P-1 and the Z value of two means that two [Ru(Bn-
bitapy)2]2þ molecules, which distorted each other enantio-
merically, are packed in an asymmetric unit. Chart 2 shows
(C135H134N4Na: 1835.05). These results clearly indicated
that the tapy functionality was quantitatively introduced into
the PS chain. Therefore, the click reaction between PS-N3
and ethynylpyridine derivatives was demonstrated to be use-
ful and straightforward strategy to produce a polymer with a
desired chelating functionality.
Ru(II) complexes of PS-tapy and PS-bitapy were prepared by
heating with RuCl3 as a metal source in DMF followed by the
addition of NaPF6 (Scheme 2). After the removal of
FIGURE 3 SEC traces for synthesis of Ru(II) complexes of PS-
tapy (a) and PS-bitapy (b). Top: crude Ru(II) complexes; middle:
PS ligands; and bottom: purified Ru(II) complexes.
SCHEME 2 Synthesis of 3-arm and 4-arm star-branched PS
Ru(II) complexes.
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FIGURE 4 ORTEP drawing of [Ru(Bn-bitapy)2]2þ with thermal
ellipsoids shown at the 50% probability level.
py)2](PF6)2, suggesting a similar coordination on Ru(II) ions.
Assuming the same molar absorption coefficient in the band
for Ru(II) complexes of PS-tapy and PS-bitapy, the UV–vis
spectra confirm the absolute molecular weights of Ru(II)
complexes of PS-tapy and PS-bitapy at 9300 and 12,000,
respectively. These values are close to the calculated values
of 9120 for [Ru(PS-tapy)3](PF6)2 and 11,000 for [Ru(PS-
˚
TABLE 1 Selected Bond Lengths (in A) and Angles (in deg) for
CHART 1 Ru(II) Complexes [Ru(Bn-tapy)3](PF6)2 and [Ru(Bn-
[Ru(Bn-bitapy)2](PF6)2
bitapy)2](PF6)2.
RuꢁN(1)
2.017(3)
2.050(3)
RuꢁN(2)
schematic representation of coordination geometry for
[Ru(Bn-bitapy)2]2þ and [Ru(terpy)2]2þ, in which the equiva-
lent bond lengths and angles are averaged.23 The coordina-
tion geometry of [Ru(Bn-bitapy)2]2þ is quite similar to that
of [Ru(terpy)2]2þ. An interesting difference was found in the
bond length between Ru and N(central pyridine): [Ru(Bn-
RuꢁN(5)
2.071(3)
RuꢁN(8)
2.019(3)
RuꢁN(9)
2.051(2)
RuꢁN(12)
2.069(2)
N(1)ꢁRuꢁN(2)
N(1)ꢁRuꢁN(5)
N(1)ꢁRuꢁN(9)
N(1)ꢁRuꢁN(12)
N(2)ꢁRuꢁN(12)
N(5)ꢁRuꢁN(9)
N(1)ꢁRuꢁN(8)
N(2)ꢁRuꢁN(5)
N(8)ꢁRuꢁN(9)
N(8)ꢁRuꢁN(12)
N(8)ꢁRuꢁN(2)
N(8)ꢁRuꢁN(5)
N(12)ꢁRuꢁN(5)
N(9)ꢁRuꢁN(2)
N(9)ꢁRuꢁN(12)
78.45(14)
78.22(14)
102.14(10)
101.60(12)
95.11(12)
93.26(11)
175.82(14)
156.56(13)
78.19(10)
78.12(11)
97.41(14)
105.94(14)
91.76(12)
89.42(11)
156.26(13)
bitapy)2]2þ (2.02 Å) is longer than that of [Ru(terpy)2]2þ
,
plausibly because of the five-membered triazole ring. The
angle between the two benzyl groups of [Ru(Bn-bitapy)2]2þ
was calculated to be ꢄ145ꢂ. The Ru(II) complexes [Ru(Bn-
tapy)3](PF6)2 and [Ru(Bn-bitapy)2](PF6)2 demonstrate that
tapy and bitapy, readily prepared by click reaction, act as
bipyridine- and terpyridine-like chelators for Ru(II) ions.
Figure 5 shows the UV–vis spectra of [Ru(Bn-tapy)3](PF6)2
and [Ru(Bn-bitapy)2](PF6)2 in DMF. Characteristic absorption
bands were found at 383 nm for [Ru(Bn-tapy)3](PF6)2 and
at 394 nm for [Ru(Bn-bitapy)2](PF6)2. These absorption
bands are shifted to shorter wavelength than that of
2þ
Ru(bpy)3 complex (450 nm in acetonitrile) because of the
poor electro-accepting property of triazole ring.18 The Ru(II)
complexes of PS-tapy and PS-bitapy afford similar UV–vis
spectra to those of [Ru(Bn-tapy)3](PF6)2 and [Ru(Bn-bita-
STAR-BRANCHED POLYSTYRENE Ru(II) COMPLEXES, ZHANG ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CHART 3 Schematic representation of 3-arm and 4-arm star-
branched PS geometries.
direction of the PS chain deviated by ꢄ17.5ꢂ from the best
plane, which is a marginal value between 35.26ꢂ for tetra-
gonal and 0ꢂ for planar configurations.
CHART 2 Schematic representation of coordination geometry
for [Ru(Bn-bitapy)2]2þ and [Ru(terpy)2]2þ 23
Terpy stands for
.
CONCLUSIONS
terpyridine.
We successfully prepared 3-arm and 4-arm star-branched
PSs with a Ru(II) complex core by the click-to-chelate
approach. PS-Br was readily synthesized by ATRP and easily
converted to azide groups by mixing with NaN3 in DMF at
an ambient temperature. The introduction of chelating func-
tionalities, namely, triazolylpyridine, was carried out using
click chemistry to afford triazolylpyridine (tapy) ligand bear-
ing one PS chain (PS-tapy) and bistriazolylpyridine (bitapy)
ligand bearing two polystylene chains (PS-bitapy). Ru(II)
complexes of PS-tapy and PS-bitapy were prepared by a con-
ventional procedure for the preparation of Ru(II) oligopyri-
dine complexes. The coordination structures of the Ru(II)
complexes were established to be of octahedral coordination
on the basis of SEC analyses and UV–vis spectra using Ru(II)
complexes with benzyl groups instead of PS. In particular,
bitapy)2](PF6)2 using the Mn values of PS-tapy and PS-bitapy
determined by SEC in CHCl3, and are a little greater than
those calculated on the basis of the Mn values determined by
SEC in THF. Therefore, the SEC analyses and UV–vis spectros-
copy clearly indicate that PS-tapy and PS-bitapy can be coor-
dinated on a Ru(II) ion to form star-branched 3-arm and 4-
arm PS architectures. The 3-arm PS complex is likely to be a
mixture of the fac- (tripodally-branched 3-arm PS) and mer-
configuration (planar-branched 3-arm PS; Chart 3). In con-
trast, the crystal structure of [Ru(Bn-bitapy)2](PF6)2 affords
more detailed information for 4-arm PS architecture. The 4-
arm PS complex is not actually a planar structure. On the
basis of the crystal structure of [Ru(Bn-bitapy)2](PF6)2, the
FIGURE 5 UV–vis spectra of
[Ru(Bn-tapy)3](PF6)2 (broken line)
and Ru(II) complex of PS-tapy
(solid line) (a) and [Ru(Bn-bita-
py)2](PF6)2 (broken line) and Ru(II)
complex of PS-bitapy (solid line)
(b) in DMF.
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10 Lohmeijer, B. G. G.; Schubert, U. S. J Polym Sci Part A:
X-ray crystallography clearly indicates the coordinating fash-
ion of the bitapy ligand that is quite similar to that of terpyr-
idine. Because of ATRP technology, the easy conversion
from bromine to azide, and click chemistry, the click-to-
chelate approach was found to be a powerful tool for metal
template synthesis of macromolecular with a sophisticated
architecture.
Polym Chem 2003, 41, 1413–1427.
11 Mindt, T. L.; Struthers, H.; Brans, L.; Anguelov, T.; Schweins-
berg, C.; Maes, V.; Tourwe´, D.; Schibli, R. J Am Chem Soc 2006,
128, 15096–15097.
12 Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B.
Angew Chem Int Ed 2002, 41, 2596–2599.
The authors thank Shohei Katao at NAIST for performing the
X-ray crystallography analysis. This work was supported by
Grants-in-Aid for Scientific Research (no. 21550156), the
Kyoto-Nanotechnology Network of the Ministry of Education,
Culture, Sport, Science and Technology (MEXT), and a grant
from the Japan–German exchange program supported by the
Japan Society for the Promotion of Science (JSPS) of the
Japanese Government. This study was also supported by Natu-
ral Science Foundation of Heilongjiang Province, China (Grant
No. B201013).
13 Mindt, T. L.; Muller, C.; Melis, M.; de Jong, M.; Schibli, R.
¨
Bioconjugate Chem 2008, 19, 1689–1695.
14 Struthers, H.; Spingler, B.; Mindt, T. L.; Schibli, R. Chem Eur
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15 Schweinfurth, D.; Hardcastle, K. I.; Bunz, U. H. F. Chem
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16 Huang, S.; Clark, R. J.; Zhu, L. Org Lett 2007, 9, 4999–5002.
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