Thermo-responsive extraction of cadmium(II) ion with TPEN-NIPA gel. Effect of the number of polymerizable double bond toward gel formation and the extracting behavior

Article abstract of DOI:10.1016/j.tet.2009.12.047

N,N,N′,N′-(Tetrakis-2-pyridylmethyl)ethylenediamine (TPEN) derivatives bearing the different number (1-4) of a double bond moiety on the pyridine ring are synthesized and subjected to copolymerization with N-isopropylacrylamide in the presence of AIBN. The obtained poly(TPEN-NIPA) gels show thermo-responsive swelling/shrinking behaviors and are employed for the extraction of cadmium(II) ion from the aqueous solution to examine the relationship of the gel characteristics and the extraction performance. The polymer gels composed of the TPEN derivative bearing three or four double bonds exhibit temperature-dependent change of swelling and shrinking in water. These gels extract Cd^II ion efficiently from the aqueous solution in the swelling state at 5 °C, while little extraction was observed at 45 °C with shrinking.

Full text of DOI:10.1016/j.tet.2009.12.047

Tetrahedron 66 (2010) 1721–1727
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Thermo-responsive extraction of cadmium(II) ion with TPEN-NIPA gel. Effect
of the number of polymerizable double bond toward gel formation and the
extracting behavior
Sachio Fukuoka a, Tatsuya Kida a, Yasutaka Nakajima a, Takayuki Tsumagari a, Wataru Watanabe a,
,
b
c
c,
*
Yusuke Inaba a, Atsunori Mori a , Tatsuro Matsumura , Yoshio Nakano , Kenji Takeshita
*
a Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe, Hyogo 657-8501, Japan
b Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, 2-4 Shirakatashirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan
c Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan; Integrated Research Institute, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
a r t i c l e i n f o
a b s t r a c t
N,N,N0,N0-(Tetrakis-2-pyridylmethyl)ethylenediamine (TPEN) derivatives bearing the different number
(1–4) of a double bond moiety on the pyridine ring are synthesized and subjected to copolymerization
with N-isopropylacrylamide in the presence of AIBN. The obtained poly(TPEN-NIPA) gels show thermo-
responsive swelling/shrinking behaviors and are employed for the extraction of cadmium(II) ion from the
aqueous solution to examine the relationship of the gel characteristics and the extraction performance.
The polymer gels composed of the TPEN derivative bearing three or four double bonds exhibit tem-
perature-dependent change of swelling and shrinking in water. These gels extract CdII ion efficiently from
the aqueous solution in the swelling state at 5 ꢀC, while little extraction was observed at 45 ꢀC with
shrinking.
Article history:
Received 30 October 2009
Received in revised form 23 December 2009
Accepted 23 December 2009
Available online 4 January 2010
Keywords:
TPEN
NIPA gel
Temperature-dependent extraction
Cadmium(II) ion
Radical polymerization
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
structure into a side chain of polymers is a method to avoid
leaching into water phases during extraction.
Solvent extraction technique has been of considerable interest
and extensively applied for separation of metal ions with a chelat-
ing agent. Organic molecules composed of multiple nitrogen atoms
are particularly important for the extraction of soft metals,1 such as
Hg, Cd, Au and Pd, and development of separation for d- or f-block
metals has been an attractive issue. A wide range of chelating agent
has been developed so far for the purpose of separation of, for
example, minor actinides (MA) from high level radioactive
wastes (HLW).2 TPEN, which is N,N,N0,N0-(tetrakis-2-pyr-
idylmethyl)ethylenediamine 1 suggested to be a hexadentate li-
gand with six nitrogen atoms to chelate a metal ion,3 is a potential
candidate for the practical and selective extracting agent for MAs
from HLW.4 Much effort has been paid to the use of TPEN for the
extraction, however, difficulties on the practical use of TPEN is its
highly water soluble and ease of protonation characteristics under
acidic conditions.3b Accordingly, incorporation of the TPEN
On the other hand, the polymer of N-isopropylacrylamide (NIPA)
has attracted interesting characteristics, that is, water soluble at
low temperature and becomes hydrophobic by raising the tem-
perature to higher than ca. 35 ꢀC.5 Thereby, the corresponding poly-
NIPA gels show thermo-responsive swelling/shrinking behaviors
on that temperature and have applied for metal extraction using
temperature-dependent change of chelation ability.6 Accordingly,
change of extraction characteristics of TPEN derivatives induced by
the thermo-responsive behaviors of such gel is intriguing if poly-
NIPA gel is prepared with a cross-linker, in which TPEN moiety is
incorporated.
N
N
N
N
N
N
* Corresponding authors. Tel./fax: þ81 788036181.
E-mail address: amori@kobe-u.ac.jp (A. Mori).
TPEN (1)
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.12.047

1722
S. Fukuoka et al. / Tetrahedron 66 (2010) 1721–1727
In our previous works, we have shown that a TPEN-NIPA gel was
NO2
O
Ac2O
hydroquinone
K2CO3, DMSO
synthesized by radical polymerization of NIPA in the presence of
a TPEN derivative bearing four polymerizable double bonds on the
pyridine ring and showed temperature-dependent change of
extraction behavior of AmIII and CdII, which was recognized as
a model metallic species of MAs.7 The TPEN-NIPA gel effectively
extracted CdII ion at swelling state (5 ꢀC), while little extraction of
CdII ion was confirmed at the elevated temperature (45 ꢀC) where
shrinking of the gel was observed.7 Our further concern has focused
on the relationship of the formation of polymer gel with the
number of the terminal polymerizable double bond, which would
be highly important toward improved molecular design of the
polymer gel. We herein report syntheses of TPEN derivatives
bearing the different number of the terminal double bond, forma-
tion of polymer gels with such TPENs, and studies on temperature-
dependent extraction behaviors of a CdII ion8 with the obtained
TPEN-NIPA gel.
+
HO
microwavea
N
O
N
O
100 °C
Δ
2
O
O
O
2 N NaOH aq.
SOCl2, CH2Cl2
0 °C
·HCl
Cl
OAc
OH
MeOH
r.t.
N
N
N
5
4
3
O
N
O
N
NH
2 , NaOH, KI
H2N
N
N
N
C16H33Me3NCl
THF/H2O, r.t. 48 h
(88%)
N
O
2. Results and discussion
O
N,N,N',N'-4PPEN
Scheme 1.
Synthesis of TPEN derivativesbearing the different number of the
polymerizable functional group was carried out as outlined in
Schemes 1 and 2. The synthetic strategy was based on the controlled
introduction of functionalized and unsubstituted chloromethy-
lpyridine derivatives into ethylenediamine. Chloromethylpyridine
bearing an allyloxy group at the 4-position of the pyridine ring 5 was
prepared in a manner as described previously with a slight modifi-
cation from commercially available 2-methyl-4-nitro-pyridine 1-
oxide and allyl alcohol and used as hydrochloride, which was iso-
lated directly from the mixture of the reaction of hydroxy-
methylpyridine 4 with thionyl chloride (Scheme 1).
system in the presence of NaOH, KI, and a catalytic amount of
C16H33Me3NCl (2 mol %) at room temperature afforded symmetric
N,N0-bifunctionalized TPEN derivative N,N0-2PPEN in 57% overall
yield.
Other TPEN derivatives were synthesized with N-Boc-ethyl-
enediamine 7, which was prepared by the reaction of ethylenedi-
amine with (Boc)2O.10 The reaction of 7 with chloromethylpyridine
hydrochloride in the manner for the reaction of 6 and 5 and fol-
lowing removal of the Boc group afforded N,N-difunctionalized
compound 8 in an excellent yield. On the other hand, reductive
amination of 2-pyridinecarbaldehyde with 8 lead to 9, which was
further treated with 5 in a similar manner to the reaction of 5
Reductive amination of 2-pyridinecarbaldehyde with ethyl-
enediamine lead to (N,N0-pyridylmethyl)ethylenediamine 6, which
was employed for the following reaction without further purifica-
tion. The following reaction of 5 and 6 in a biphasic THF/water
O
N
O
·HCl
2
N
2
N
Cl , NaOH, KI
H
N
N
CHO
NH2
Py
N
H2N
Py
N
H
N
C16H33Me3NCl
THF/H2O, r.t. 48 h
NaBH4, MeOH
reflux 1.5 h, r.t. 14 h
N
N
(57%)
6
Boc2O, CHCl3
O
r.t. 24 h
(94%)
N,N'-2PPEN
·HCl
Cl
O
N
O
1)
2
N
·HCl
N
N
2
NaOH,KI, C16H33Me3NCl
N
Py
Py
, NaOH, KI
Cl
THF/H2O, RT (86%)
N
N
NH2
N
BocHN
H2N
N
O
C16H33Me3NCl
2) CF3CO2H, CH2Cl2
RT (93%)
7
THF/H2O, r.t. 48 h
8
(51%)
N,N-2PPEN
N
CHO
N
CHO
NaBH4, MeOH
reflux 1.5 h, r.t.15 h
O
NaBH4, MeOH
reflux 1.5 h, r.t.1
O
O
·HCl
3 h
Py
Py
N
N
, NaOH, KI
Cl
N
N
Py
(50%)
N
N
N
N
HN
Py
(79%)
NH
C16H33Me3NCl
THF/H2O, r.t. 48 h
(69%)
BocHN
N
9
10
N-1PPEN
O
N
·HCl
, NaOH, KI
C16H33Me3NCl
O
1)
Cl
Py
·HCl
N
N
2
THF/H2O, r.t. 48 h (85%)
N
N
Cl , NaOH, KI
N
N
H2N
N
N
O
C16H33Me3NCl
2) CF3CO2H, CH2Cl2
r.t. 1 h (99%)
THF/H2O, r.t. 48 h
(39%)
11
O
O
N,N,N'-3PPEN
Scheme 2.

S. Fukuoka et al. / Tetrahedron 66 (2010) 1721–1727
1723
We then carried out formation of TPEN-NIPA gel with the
obtained derivatives. The polymerization was performed by radical
polymerization with AIBN as an initiator in DMF in the presence of
1.25 mol % of TPEN derivatives N,N-2PPEN, N,N0-2PPEN, N,N,N0-
3PPEN, and N,N,N0,N0-4PPEN7 (Scheme 3). All of the polymeriza-
tion proceeded within 18 h at 60 ꢀC. Insoluble polymer gels were
obtained by the reaction of TPEN derivatives bearing three or four
polymerizable double bonds. As shown in Table 1, the obtained
polymer gel derived from N,N,N0-3PPEN showed swelling in water
at room temperature and heating the gel in water at 45 ꢀC resulted
to exhibit shrinking. These behavior was similar to the case of
N,N,N0,N0-4PPEN.7 On the other hand, the polymer synthesized
with N,N0-2PPEN bearing symmetric two terminal double bonds
was found to give partially water-soluble gel. Although increasing
the amount of TPEN to 2.5 mol % resulted to afford the gel of better
quality, slow leaching was observed by standing the polymer gel in
water at room temperature. By contrast, gellation with N,N-2PPEN
under similar conditions was found to be unsuccessful to observe
dissolving in water within a few days.
These results suggest that employment of the TPEN derivative
bearing more than three allyloxy group is necessary to obtain the
polymer gel and the N,N0-difunctionalized derivative is preferable
for gellation compared to the N,N-difunctional one. Although the
gellation might be possible when two polymerizable functional
group exists, more than two double bonds in the TPEN derivative is
preferable to form stable polymer gel. The findings would be due to
the inferior reactivity in the radical polymerization of unfunction-
alized C–C double bond to acrylamide.
O
R
N
CONH
R
R
N
N
N
AIBN (3 mol%)
H
N
N
N
N
N
N
+
N
DMF, 60 °C,
18 h
O
N
R
N
R
R
R
TPEN-NIPA gel
R = OCH2CH=CH2 or H
R = OCH2CH−CH2 or H
Scheme 3.
and 6 afforded mono-functionalized derivative N-1PPEN in 69%
yield.
Reductive amination of 2-pyridinecarbaldehyde with 7 afforded
10. The reaction of 10 with 5 and following removal of Boc fur-
nished 11. Treatment of the obtained 11 with 5 afforded the TPEN
derivative bearing three double bonds N,N,N0-3PPEN.
All of the TPEN derivatives bearing the different number of the
double bond were synthesized in reasonable yields. Worthy of note
is the modified protocol of the reaction of chloromethylpyridine
with an amino group. Conventional synthesis of TPEN has been
performed by the reaction of chloromethylpyridine 5 with ethyl-
enediamine in aqueous NaOH to result in giving considerably poor
yield (<30%). Although the method was effective for the unsub-
stituted chloromethylpyridine, the use of the allyloxy derivative
induced a variety of side reactions leading to the formation of in-
separable byproducts. Employment of biphasic reaction in the
presence of a phase transfer catalyst and addition of potassium
iodide, which would cause in situ transformation of chloride on the
pyridine ring into iodide was found to cause drastic improvement
of the yield of TPEN derivatives.13 Subsequently, the improved
synthetic method resulted to give well-defined TPEN derivatives
with the different number of allyloxy group in a reasonable yields.
(Scheme 2).
With the polymer gel bearing the TPEN moiety in hand,
extraction studies were performed using a cadmium(II) ion employ-
ing the dried TPEN gel by the removal of water under reduced
pressure. We first examined the temperature-dependent extraction
performance of TPEN-NIPA gels in the swelling state (5 ꢀC) and the
shrinking state (45 ꢀC) at the pH value of 2.1 and 5.3, respectively.
As summarized in Figure 1, the percent extraction value (%E) of CdII
ion at each temperature and pH with TPEN-NIPA gels, in which
1.25 mol % of the corresponding TPEN derivative is incorporated,
was estimated by ICP-AES analysis. The calculation of %E was based
on the assumption that all of the TPEN derivative was incorporated
Table 1
Radical polymerization of NIPA with TPEN derivatives with the different number of polymerizable double bond and thermo-responsive behaviors of the obtained TPEN-NIPA
gela
TPEN derivative
(mol %)
Polymer
Thermo-responsive changec
N,N,N0,N0-4PPEN
(1.25)b
Gel (completely insoluble in water)
N,N,N0-3PPEN
N,N0-2PPEN
(1.25)
Gel (completely insoluble in water)
(1.25)
(2.50)
Partially water soluble
Gel (slow leaching)
N,N-2PPEN
(2.5)
Water soluble
a
Polymerization was carried out with NIPA (1.5 mmol), AIBN (3 mol %), and TPEN in 0.13 mL of DMF at 60 ꢀC for 18 h.
Prepared in a manner shown in Ref 6.
Swollen (left); shrinked (right).
b
c

1724
S. Fukuoka et al. / Tetrahedron 66 (2010) 1721–1727
into the polymer gel in the copolymerization. Both N,N,N0-3PPEN
and N,N,N0,N0-4PPEN extracted the cadmium(II) ion at 5 ꢀC highly
efficiently, while the extraction performance at 45 ꢀC was much
inferior to those at 5 ꢀC. The results suggest that excellent extrac-
tion of cadmium(II) ion is observed in the swelling state, while the
gel hardly extracted CdII in the shrinking state at 45 ꢀC. A slightly
higher extraction at 45 ꢀC was observed with N,N,N0-3PPEN bear-
ing three double bonds than with N,N,N0,N0-4PPEN suggesting that
TPEN bearing the less number of the double bond still involved
freedom to chelate the metal ion even in the shrinked state to cause
inferior temperature-dependent difference of extraction. Such an
extraction behavior was also observed at pH 2.1 although the per-
cent extraction value was found to be lower. TPEN-NIPA gel
N,N,N0,N0-4PPEN extracted ca. 23.2% of CdII at 5 ꢀC, while no ex-
traction was observed at the shrinking state (45 ꢀC). On the other
hand, N,N,N0-3PPEN composed of the three polymerized moiety
exhibited inferior thermo-responsive behavior. Since actual sepa-
ration of minor actinides is performed in an acidic aqueous solu-
tion, the thermo-responsive extraction behavior at the pH value of
2.1 is particularly noteworthy.
4. Experimental section
4.1. General
NMR (500 MHz for 1H, 125 MHz for 13C) spectra were recorded
on a Bruker Avance 500 spectrometer at the Center for Supports to
Research and Education Activities, Kobe University. Chemical shifts
are expressed in ppm using tetramethylsilane as an internal
standard (0 ppm). Coupling constants (J) were shown in Hertz (Hz).
IR (ATR) spectra were measured with Brucker Optics Alpha with Ge.
TLC analyses were performed on analytical TLC plates coated with
60 F254 (E. Merck) silica gel or alumina on aluminum foil. Column
chromatography was performed using silica gel Wakogel C200
(Wako Chemicals Co. Ltd.) or basic alumina (Wako Chemicals Co.
Ltd or Merck). High-resolution mass spectra were measured at Nara
Institute of Science and Technology with JEOL JMS-700. ICP-AES
analysis was carried out with SII SPS3100 at the Center for Supports
to Research and Education Activities of Kobe University. Tetrahy-
drofuran (anhydrous grade) was purchased from Wako Chemicals
Co. Ltd. and stored in a Schlenk tube under a nitrogen atmosphere.
Other chemicals were purchased and used without further
purification.
5 ˚C
pH=5.3
4.1.1. 4-(Prop-2-en-1-yloxy)-2-picoline 1-oxide (2). To a 20 mL of
round-bottle flask was added 4-nitro-2-picoline 1-oxide (494.3 mg,
3.2 mmol) in allyl alcohol (5 mL). The mixture was heated until to
homogeneous was dissolved in allyl alcohol, and then potassium
carbonate (493.2 mg, 3.6 mmol) was added. The reaction mixture
was stirred at 100 ꢀC for 7 h. Allyl alcohol was removed under the
reduced pressure, then the residue was poured into water and
extracted with CHCl3. The extract was washed with water, dried
over anhydrous Na2SO4 and concentrated in vacuo to afford 2
(584.2 mg) (>99%). The product was used in the next step without
further purification.
5 ˚C
45 ˚C
45 ˚C
N,N,N’.N’-4PPEN
N,N,N'-3PPEN
pH=2.1
4.1.2. 2-Chloromethyl-4-(prop-2-en-1-yloxy)pyridine hydrochloride
(5). To a 300 mL of round-bottom flask was added compound 2
(5.8 g, 35 mmol) in acetic anhydride (60 mL) and the mixture wad
stirred at 100 ꢀC for 3 h. Acetic anhydride was evaporated under the
reduced pressure, then the residue was basified with satd Na2CO3
aq and extracted with CHCl3. The extract was dried over anhydrous
Na2SO4 and concentrated in vacuo. The residue was purified by
column chromatography on silica gel (30:1–20:1 n-hexane/EtOAc)
5 ˚C
5 ˚C
45 ˚C
45 ˚C
to
afford
4-(prop-2-en-1-yloxy)pyridin-2-ylmethyl acetate
N,N,N’.N’-4PPEN
N,N,N'-3PPEN
(3, 5.6 g, 27 mmol) in 77% yield. To a 200 mL of round-bottom flask
was added compound 3 (5.6 g, 27 mmol) in methanol (42 mL) and
1 M NaOH aq (28 mL, 28 mmol) at room temperature. The reaction
mixture was stirred at the same temperature for 1.5 h. The solvent
was evaporated and the residue was extracted with CHCl3. The
extract was dried over anhydrous Na2SO4 and concentrated in
vacuo to yield [4-(prop-2-en-1-yloxy)pyridin-2-yl]methanol
(4, 3.5 g, 21 mmol) (78%). To a solution of compound 4 in CH2Cl2
was added thionyl chloride in CH2Cl2 dropwise at 0 ꢀC, After
stirring for 2 h at 0 ꢀC, large amount of diethyl ether was added and
the solvent was evaporated to afford hydrochloride salt. 1H NMR
Figure 1. Temperature-dependent extraction of CdII ion with N,N,N0,N0-4PPEN-NIPA
gel and N,N,N0-3PPEN-NIPA gel at pH 5.3 (top) and pH 2.1 (down).
3. Conclusion
In summary, we have synthesized TPEN derivatives bearing
the different number of polymerizable allyloxy group and the
prepared derivative was subjected to the formation of TPEN gel.
When the derivative bearing three and N,N0-difunctionalized two
allyloxy group was found to form the polymer gel in addition to
the case of tetrakis-functionalized TPEN. Among these, polymer
gels composed of three or four polymerizable double bond on
TPEN showed temperature-dependent extraction behaviors.
The TPEN-NIPA gel composed of N,N,N0-3PPEN exhibited
superior extraction performance at low temperature although
the thermo-responsive change of extraction was inferior to
N,N,N0,N0-4PPEN, while N,N,N0,N0-4PPEN-NIPA gel showed the
excellent temperature-dependent change with slightly lower
extraction.
(methanol-d4)
d
4.93 (s, 2H), 4.96 (td, J¼1.4, 5.4 Hz, 2H), 5.43 (dd,
J¼1.3, 10.7 Hz), 5.53 (dd, J¼1.3, 17.4 Hz), 6.08–6.15 (m, 1H), 7.51 (dd,
J¼2.6, 6.9 Hz, 1H), 7.67 (d, J¼2.6 Hz, 1H), 8.65 (d, J¼6.9 Hz, 1H).
HRMS (EIþ) found: m/z 183.0452. Calcd for 183.0451 (as dehydro-
chlorinated product:C9H10ClNO).
4.1.3. N,N,N0,N0-Tetrakis[4-(prop-2-en-1-yloxy)pyridin-2-ylmethyl]-
ethylenediamine (N,N,N0,N0-4PPEN): General procedure for alkylation
of ethylenediamine with 2-(chloromethyl)pyridines hydrochloride in
THF/H2O. To a test tube equipped with a magnetic stirring bar were
added ethylenediamine (20.1 mL, 0.3 mmol), 2-(chloromethyl)

S. Fukuoka et al. / Tetrahedron 66 (2010) 1721–1727
1725
pyridine hydrochloride
5
(264.1 mg, 1.2 mmol), hexadecyltri-
H2O (4 mL). KI (664.0 mg, 4 mmol) was then added, and the
resulting mixture was stirred vigorously at room temperature for
48 h. The reaction mixture was extracted with CH2Cl2 and the
extract was dried over anhydrous Na2SO4 and concentrated in
vacuo. The residue was purified by column chromatography on
alumina (hexane/EtOAc¼1:1) to afford {2-[bis(pyridin-2-ylme-
thyl)amino]ethyl}carbamic acid tert-butyl ester (586.2 mg,
methylammonium chloride (1.9 mg, 0.006 mmol), NaOH (96.0 mg,
2.4 mmol), THF (0.6 mL) and H2O (0.6 mL). KI (199.2 mg, 1.2 mmol)
was then added, and the resulting mixture was stirred vigorously at
room temperature for 48 h. The reaction mixture was extracted
with CH2Cl2 and the extract was dried over anhydrous Na2SO4 and
concentrated in vacuo. The residue was purified by column chro-
matography on alumina using 30:1 CHCl3/MeOH as an eluent to
afford 0.17 g of N,N N0,N0-4PPEN as a pale yellow oil (88% yield). 1H
1.7 mmol) in 86% yield. 1H NMR (CDCl3, 500 MHz):
d 1.43 (s, 9H),
2.77 (br s, 2H), 3.23 (br s, 2H), 3.93 (s, 4H), 5.86 (br s, 1H), 7.17 (dd,
J¼5.0, 6.5 Hz, 2H), 7.45 (d, J¼7.5 Hz, 2H), 7.65 (t, J¼7.5, Hz, 2H),
8.55 (d, J¼5.0 Hz, 2H).
NMR (CDCl3)
d
2.86 (s, 4H), 3.80 (s, 8H), 4.50 (d, J¼5.4 Hz, 8H), 5.29
(dd, J¼1.2, 10.4 Hz, 4H), 5.37 (dd, J¼1.2, 17.4 Hz, 4H), 5.94–6.02
(m, 4H), 6.66 (dd, J¼2.2, 5.7 Hz, 4H), 7.05 (d, J¼2.2 Hz, 4H), 8.27
To a 100 mL of round-bottom flask was added {2-[bis(pyridin-
2-ylmethyl)amino]-ethyl}carbamic acid tert-butyl ester (1.2 g,
3.4 mmol) in CH2Cl2 (10.3 mL). Trifluoroacetic acid (17.2 mL) was
added dropwise over a period of 30 min at 0 ꢀC. After stirring at
room temperature for 1 h, solvent was evaporated, then the
residue was dissolved in aq NaOH and extracted with CH2Cl2.
Organic layer was dried over anhydrous Na2SO4 and concentrated
in vacuo to afford compound 8 (0.77 g, 3.2 mmol) (93%). 1H NMR
(CDCl3): 2.41 (br s, 2H), 2.60 (t, J¼6.0 Hz, 2H), 2.73 (t, J¼6.0 Hz,
2H), 3.75 (s, 4H), 7.03 (ddd, J¼1.0, 4.8, 7.6 Hz, 2H), 7.36 (d,
J¼7.6 Hz, 2H), 7.53 (td, J¼1.9, 7.6 Hz, 2H), 8.42 (ddd, J¼1.0, 1.9,
4.8 Hz, 2H).
(d, J¼5.7 Hz, 4H); 13C NMR (CDCl3)
d 52.3, 60.5, 68.4, 108.9, 109.2,
118.4, 132.1, 150.0, 161.2, 165.3. HRMS (EIþ) found: m/z 648.3428.
Calcd for 648.3424.
4.1.4. N,N0-Bis[4-(prop-2-en-1-yloxy)pyridin-2-ylmethyl]-N,N0-bis
(pyridin-2-ylmethyl)ethylenediamine (N,N0-2PPEN). To a 50 mL of
two-neck flask were added compound ethylenediamine (0.34 mL,
5 mmol), molecular sieves 3A (0.50 g) in MeOH (10 mL) under
a nitrogen atmosphere. A solution of pyridine-2-carboxyaldehyde
(0.95 mL, 10 mmol) in MeOH (10 mL) was added slowly, and the
mixture was refluxed for 1.5 h. After cooling to room tempera-
ture, NaBH4 (567.5 mg, 15 mmol) was added to the mixture,
which was stirred overnight at room temperature. After removal
of molecular sieves, the solvent was evaporated. The residue was
dissolved in satd Na2CO3 aq and extracted with CH2Cl2. The
organic layer was dried over anhydrous Na2SO4 and concentrated
in vacuo to afford crude N,N0-bis(pyridin-2-ylmethyl)ethylenedi-
amine (6)9 (1.23 g). The product was used in the next step
without further purification.
To a test tube were added compound 6 (72.7 mg, 0.3 mmol),
5 (132.1 mg, 0.6 mmol), hexadecyltrimethylammonium chloride
(1.9 mg, 0.0006 mmol) and NaOH (48.0 mg, 1.2 mmol) in THF
(0.6 mL) and H2O (0.6 mL). KI (99.6 mg, 0.6 mmol) was then
added, and the resulting mixture was stirred vigorously at room
temperature for 48 h. The reaction mixture was extracted with
CH2Cl2 and the extract was dried over anhydrous Na2SO4 and
concentrated in vacuo. The residue was purified by column
chromatography on alumina (CHCl3/MeOH¼30:1) to afford
N,N0-2PPEN (92.4 mg, 0.17 mmol) in 57% yield. 1H NMR (CDCl3)
4.1.7. N,N-Bis[4-(prop-2-en-1-yloxy)pyridin-2-ylmethyl]-N0,N0-bis
(pyridin-2-ylmethyl)ethylenediamine (N,N-2PPEN). To a test tube
equipped with a magnetic stirring bar were added compound 8
(72.7 mg, 0.3 mmol),
5
(132.1 mg, 0.6 mmol), hexadecyl-
trimethylammonium chloride (1.9 mg, 0.0006 mmol) and NaOH
(48.0 mg, 1.2 mmol), THF (0.6 mL), and H2O (0.6 mL). KI (99.6 mg,
0.6 mmol) was then added, and the resulting mixture was stirred
vigorously at room temperature for 48 h. The reaction mixture was
extracted with CH2Cl2 and the extract was dried over anhydrous
Na2SO4 and concentrated in vacuo. The residue was purified by
column chromatography on alumina (EtOAc/MeOH¼20:1) to af-
ford N,N-2PPEN (82.8 mg, 0.15 mmol) in 51% yield. 1H NMR (CDCl3,
500 MHz):
d 2.75–2.77 (m, 4H), 3.72 (s, 4H), 3.77 (s, 4H), 4.49 (td,
J¼1.4, 5.3 Hz, 4H), 5.28 (dd, J¼1.3, 10.6 Hz, 2H), 5.37 (dd, J¼1.3,
17.3 Hz, 2H), 5.94–6.02 (m, 2H), 6.65 (dd, J¼2.5, 5.7 Hz, 2H), 7.04 (d,
J¼2.5 Hz, 2H), 7.08 (ddd, J¼1.0, 4.9, 7.5 Hz, 2H), 7.43 (d, J¼7.5 Hz,
2H), 7.55 (td, J¼1.7, 7.5, 2H), 8.28 (d, J¼5.7 Hz, 2H), 8.46 (d,
d
2.96 (s, 4H), 3.91 (s, 8H), 4.53 (d, J¼5.2 Hz, 4H), 5.31
J¼4.9 Hz, 2H); 13C NMR (CDCl3, 125 MHz):
d 52.2, 52.3, 60.55,
(dd, J¼1.3, 10.4 Hz, 2H), 5.39 (dd, J¼1.3, 17.2 Hz, 2H), 5.96–6.03
(m, 2H), 6.69 (dd, J¼2.2, 5.8 Hz, 2H), 7.11–7.13 (m, 4H), 7.44
(d, J¼7.7 Hz, 2H), 7.57 (td, J¼1.7, 7.7 Hz, 2H), 8.28 (d, J¼5.8 Hz,
60.62, 68.3, 108.6, 108.9, 118.2, 121.7, 122.6, 132.0, 136.2, 148.8,
149.9, 159.5, 161.5, 165.1. HRMS (EIþ) found: m/z 536.2900. Calcd
for 536.2901.
2H), 8.46 (d, J¼4.8 Hz, 2H); 13C NMR (CDCl3)
d 52.2, 60.5,
60.6, 68.3, 108.6, 109.0, 118.2, 121.8, 122.6, 132.0, 136.2, 148.8,
149.9, 159.4, 161.4, 165.1. HRMS (EIþ) found: m/z 536.2900.
Calcd for 536.2903.
4.1.8. N,N,N0-Tris(pyridin-2-ylmethyl)ethylenediamine (9)11. To a 25 mL
of round-bottom flask were added compound 8 (242.3 mg, 1 mmol),
pyridine-2-carboxyaldehyde (94.8 mL, 1 mmol) and 3 Å molecular
sieves in MeOH (4 mL). The reaction mixture was refluxed for 1.5 h,
and cooled to room temperature. NaBH4 (113.5 mg, 3 mmol) was
added to the mixture, which was stirred overnight at room temper-
ature. After removal of molecular sieves, solvent was evaporated to
leave a crude oil, which was purified by column chromatography
4.1.5. (2-Aminoethyl)carbamic acid tert-butyl ester (7)10. To
a 300 mL of round-bottom flask was added ethylenediamine
(13.4 mL, 200 mmol) in CHCl3 (100 mL). A solution of di-tert-butyl
dicarbonate (4.4 g, 20 mmol) in CHCl3 (50 mL) was added dropwise
over a period of 2 h at 0 ꢀC. After stirring at room temperature for
24 h, the mixture was washed with brine. The organic layer was
washed with water, dried over anhydrous Na2SO4 and concentrated
in vacuo to afford compound 7 (3.0 g, 18.8 mmol) as a colorless oil
in 94% yield. 1H NMR (CDCl3): 1.36 (s, 9H), 1.80 (s, 2H), 2.74
(t, J¼5.8 Hz, 2H), 3.11 (q, J¼5.4 Hz, 2H), 4.89 (br s, 1H).
on alumina using MeOH as an eluent to afford 9 (165.2 mg,
1
0.50 mmol) in 50% yield. H NMR (CDCl3, 500 MHz):
d 2.87 (s, 4H),
3.84 (s, 4H), 3.94 (s, 2H), 7.12 (ddd, J¼1.1, 4.9, 7.5 Hz, 2H), 7.16 (ddd,
J¼1.1, 4.9, 7.5 Hz, 1H), 7.36 (d, J¼7.8 Hz, 1H), 7.46 (d, J¼7.9 Hz, 2H), 7.60
(td, J¼1.7, 7.6 Hz, 2H), 7.64 (td, J¼1.7, 7.6 Hz, 1H), 8.48 (d, J¼4.9 Hz, 2H),
8.50 (d, J¼4.9 Hz, 1H).
4.1.6. N,N-Bis(pyridin-2-ylmethyl)ethylenediamine (8)11. To a test
tube equipped with a magnetic stirring bar were added 7
(320.4 mg, 2 mmol), 2-chloromethylpyridine hydrochloride
(656.1 mg, 4 mmol), hexadecyltrimethylammonium chloride
(12.8 mg, 0.04 mmol), NaOH (320 mg, 8 mmol), THF (4 mL), and
4.1.9. N-[4-(Prop-2-en-1-yloxy)pyridin-2-ylmethyl]-N,N0,N0-tris(pyridin-
2-ylmethyl)ethylenediamine (N-1PPEN). To a test tube equipped
with a magnetic stirring bar were added compound 9 (100.0 mg,
0.3 mmol), 5 (66.0 mg, 0.3 mmol), hexadecyltrimethylammonium
chloride (1.9 mg, 0.0006 mmol), NaOH (24.0 mg, 0.6 mmol), THF

1726
S. Fukuoka et al. / Tetrahedron 66 (2010) 1721–1727
(0.6 mL), and H2O (0.6 mL). KI (49.8 mg, 0.3 mmol) was then added,
andtheresultingmixturewasstirredvigorouslyatroomtemperature
for 48 h. The reaction mixture was extracted with CH2Cl2 and the
extract was dried over anhydrous Na2SO4 and concentrated in vacuo.
The residue was purified by column chromatography on alumina
(EtOAc/MeOH¼20:1) to afford N-1PPEN (99.8 mg, 0.21 mmol) in 69%
Na2SO4 and concentrated in vacuo. The residue was purified by
column chromatography on alumina (20:1 EtOAc/MeOH) to afford
N,N,N0-3PPEN (69.9 mg, 0.12 mmol) in 39% yield. 1H NMR (CDCl3)
d
2.77 (s, 4H), 3.73 (s, 4H), 3.74 (s, 2H), 3.77 (s, 2H), 4.47–4.49 (m,
6H), 5.28 (td, J¼1.3, 10.6 Hz, 3H), 5.37 (td, J¼1.3, 17.3 Hz, 3H),
5.94–6.02 (m, 3H), 6.63–6.65 (m, 3H), 7.04 (d, J¼2.4 Hz, 3H), 7.09
(ddd, J¼1.2, 4.8, 7.6 Hz, 1H), 7.43 (d, J¼7.6 Hz, 1H), 7.56 (td, J¼1.8,
7.6 Hz, 1H), 8.26–8.28 (m, 3H), 8.47 (d, J¼4.8 Hz, 1H); 13C NMR
yield. 1H NMR (CDCl3, 500 MHz):
d 2.76 (s, 4H), 3.73 (s, 2H), 3.76
(s, 2H), 3.77(s, 4H), 4.50(td, J¼1.5, 5.4 Hz,2H), 5.30(dd, J¼1.3,10.6 Hz,
1H), 5.39 (dd, J¼1.3, 17.2 Hz, 1H), 5.96–6.03 (m, 1H), 6.66 (dd, J¼2.4,
5.8 Hz,1H), 7.06 (d, J¼2.4 Hz,1H), 7.09–7.12 (m, 3H), 7.43 (d, J¼7.6 Hz,
1H), 7.44 (d, J¼7.6 Hz, 2H), 7.56 (td, J¼1.7, 7.6 Hz, 3H), 8.29
(d, J¼5.8 Hz, 1H), 8.47–8.48 (m, 3H); 13C NMR (CDCl3, 125 MHz):
(CDCl3)
d 52.3, 52.4, 60.60, 60.62, 60.65, 68.3, 108.5, 108.57, 108.61,
108.9, 109.0, 109.2, 118.20, 118.23, 121.8, 122.6, 131.99, 132.05, 136.2,
148.8, 149.9, 145.0, 159.5, 161.5, 165.07, 165.08. HRMS found: m/z
592.3162. Calcd for 592.3168.
d
52.1, 52.2, 60.4, 60.6, 68.2, 108.6, 108.9, 118.2, 121.7, 122.58, 122.61,
132.0,136.15,136.19,148.70,148.75,149.8,159.41,159.42,161.4,165.1.
HRMS found: m/z 480.2638. Calcd for 480.2635.
4.2. General procedure for radical polymerization of NIPA
with TPEN derivatives
4.1.10. {2-[(Pyridin-2-ylmethyl)amino]ethyl}carbamic acid tert-butyl
ester (10)12. To a 50 mL of two-neck flask were added compound 7
(801.1 mg, 5 mmol), 3 Å molecular sieves in MeOH (10 mL) under
a nitrogen atmosphere. A solution of pyridine-2-carboxyaldehyde
(535.6 mg, 5 mmol) in MeOH (10 mL) was added slowly, and the
mixture was refluxed for 1.5 h. After cooling to room temperature,
NaBH4 (567.5 mg, 15 mmol) was added to the mixture, which was
stirred overnight at the same temperature. After removal of mo-
lecular sieves, the solvent was evaporated. The residue was dis-
solved in satd Na2CO3 aq and extracted with CH2Cl2. The organic
layer was dried over anhydrous Na2SO4 and concentrated in vacuo.
The crude product was purified by column chromatography on
alumina using EtOAc as an eluent to afford 10 (987.9 mg, 3.9 mmol)
To a 25 mL of sealed tube equipped with a magnetic stirring bar
were added N-isopropylacrylamide (170 mg, 1.5 mmol) and TPEN
derivative (0.019 mmol). The mixture was dissolved in 0.13 mL of
DMF and AIBN (0.023 mol % or 0.045 mol %) was added in one
portion. The resulting mixture was heated at 60 ꢀC for 18 h. Then,
the mixture was cooled to room temperature and washed with
water repeatedly. Water was removed under reduced pressure at
50 ꢀC to leave a colorless solid. TPEN-NIPA gels obtained were
found to swell at room temperature in water and shrinking was
observed when the mixture was heated to 45 ꢀC. The ratio of
swelling/shrinking was 4.5–7.0 for 4PPEN and 3PPEN, which was
estimated by the observation of the volume change of both states.
in 79% yield. 1H NMR (CDCl3, 500 MHz):
d 1.43 (s, 9H), 2.91
4.3. Extraction of cadmium(II) ion with TPEN-NIPA gel
(t, J¼5.5 Hz, 2H), 3.36 (q, J¼5.5 Hz, 2H), 4.03 (s, 2H), 5.35 (br s, 1H),
7.21 (dd, J¼5.2, 7.7 Hz,1H), 7.33 (d, 7.7 Hz,1H), 7.67 (td, J¼1.7, 7.7 Hz,
1H), 8.56 (d, J¼5.2 Hz, 1H).
A 1 mM of aqueous Cd(NO3)2 solution was prepared. The pH
value of the solution was controlled to 2.1 and 5.3 by the addition of
1 M of aq NH4NO3 and 1 M of HNO3. TPEN-NIPA gel (14.5 mg;
1.25 mol % of TPEN contents), whose concentration of the TPEN
4.1.11. N,N,N0-Tris[4-(prop-2-en-1-yloxy)pyridin-2-ylmethyl]-N0-
(pyridin-2-ylmethyl)ethylenediamine (N,N,N0-3PPEN). To
a test
moiety was controlled to be 1.5 mmol, was added to 0.75 mL of the
tube with ground glass were added compound 10 (251.3 mg,
1 mmol), 5 (220.1 mg, 1 mmol), hexadecyltrimethylammonium
chloride (6.4 mg, 0.02 mmol), and NaOH (80.0 mg, 2 mmol) in THF
(2 mL) and H2O (2 mL). KI (166.0 mg, 1 mmol) was then added,
and the resulting mixture was stirred vigorously at room tem-
perature for 48 h. The reaction mixture was extracted with CH2Cl2
and the extract was dried over anhydrous Na2SO4 and concen-
trated in vacuo. The residue was purified by column chromatog-
raphy on aluminum oxide (Merck, 1:1 n-hexane/EtOAc) to afford
0.34 g of (2-{N-[4-(prop-2-en-1-yloxy)pyridin-2-ylmethyl]-N-
(pyridin-2-ylmethyl)amino}ethyl)-carbamic acid tert-butyl ester
(85%), which was employed for the following reaction without
further purification.
To a 50 mL of round-bottom flask was added (2-{N-[4-(prop-2-
en-1-yloxy)pyridin- 2-ylmethyl]-N-(pyridin-2-ylmethyl)amino}e-
thyl)carbamic acid tert-butyl ester (0.34 g, 0.86 mmol) in CH2Cl2
(2.6 mL). Trifluoroacetic acid (4.3 mL) was added dropwise over
a period of 15 min at 0 ꢀC. After stirring at room temperature for 1 h,
solvent was evaporated, then the residue was dissolved in NaOH aq
and extracted with CH2Cl2. Organic layer was dried over anhydrous
Na2SO4 and concentrated in vacuo to afford compound 254 mg of
N-[4-(Prop-2-en-1-yloxy)pyridin-2-ylmethyl]-N-(pyridin-2-ylmethyl)
ethylenediamine (11, 99%).
aqueous solution. Vigorous stirring of the mixture was continued
for 60 min at 5 ꢀC or 45 ꢀC. An aliquot of the solution (0.2 mL) was
taken, passed through a membrane filter (0.2 mm), and diluted with
distilled water to 4 mL, which was subjected to ICP-AES analysis.
The percent extraction value was calculated as.
n
ꢀh
i
h
iꢁ.h
i
o
%E ¼ 100$D=ðD þ 1Þ; D ¼
[CdII]ini: concentration of CdII in water before extraction; [CdII]:
concentration of CdII in water after extraction.
CdII ꢁ CdII
CdII
ini
ini
The percent extraction values (%E) of N,N,N0-3PPEN were 11.1
(45 ꢀC) and 61.2 (5 ꢀC) at pH¼5.3; 10.3 (45 ꢀC) and 36.6 (5 ꢀC) at
pH¼2.1, respectively, while %E of N,N,N0.N0-4PPEN: at 1.4 (45 ꢀC)
and 53.7 (5 ꢀC) at pH¼5.3; 0 (45 ꢀC) and 23.2 (5 ꢀC) at pH¼2.1.
Acknowledgements
This work was partially supported by Innovative Nuclear
Research and Development Program by Ministry of Education,
Sports, Culture, Science, and Technology (MEXT), Japan. The
authors thank the Support Network for Nanotechnology Research
of Nara Institute of Science and Technology supported by MEXT for
the measurement of high-resolution mass spectra. The authors
thank Dr. Hiroshi Oaki for fruitful discussion on the thermo-
responsive behaviors of TPEN-NIPA gels.
To a test tube equipped with a magnetic stirring bar were added
compound 11 (89.5 mg, 0.3 mmol), 5 (132.1 mg, 0.6 mmol), hex-
adecyltrimethylammonium chloride (1.9 mg, 0.0006 mmol), NaOH
(48.0 mg, 1.2 mmol), THF (0.6 mL), and H2O (0.6 mL). KI (99.6 mg,
0.6 mmol) was then added, and the resulting mixture was stirred
vigorously at room temperature for 48 h. The reaction mixture was
extracted with CH2Cl2 and the extract was dried over anhydrous
References and notes
1. Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533–3539.
2. Kolarik, Z. Chem. Rev. 2008, 108, 4208–4252 and references therein.

S. Fukuoka et al. / Tetrahedron 66 (2010) 1721–1727
1727
3. (a) Blindauer, C. A.; Razi, M. T.; Parsons, S.; Sadler, P. J. Polyhedron 2006, 25,
513–520; (b) Ogata, T.; Takeshita, K.; Fugate, G. A.; Mori, A. Sep. Sci. Technol.
2008, 43, 2630–2640; (c) Ogata, T.; Takeshita, K.; Tsuda, K.; Mori, A. Sep. Purif.
Technol. 2009, 68, 288–290; (d) Shimojo, K.; Naganawa, H.; Noro, J.; Kubota, F.;
Goto, M. Anal. Sci. 2007, 23, 1427–1430; (e) Mikata, Y.; Yamanaka, A.; Yamashita,
A.; Yano, S. Inorg. Chem. 2008, 47, 7295–7301; (f) Heitzmann, M.; Bravard, F.;
Gateau, C.; Boubals, N.; Berthon, C.; Pecaut, J.; Charbonnel, M. C.; Delangle, P.
Inorg. Chem. 2009, 48, 246–256; (g) Ekberg, C.; Fermvik, A.; Retegan, T.; Skar-
nemark, G.; Foreman, M. R. S.; Hudson, M. J.; Englund, S.; Nilsson, M. Radio-
chimica Acta 2008, 96, 225–233; (h) Takeshita, K.; Watanabe, K.; Nakano, Y.;
Watanabe, M. Hydrometallurgy 2003, 70, 63–71; (i) Takeshita, K.; Watanabe, K.;
Nakano, Y.; Watanabe, M. Chem. Lett. 2003, 32, 96–97.
4. (a) Jensen, M. P.; Morss, L. R.; Beitz, J. V.; Ensor, D. D. J. Alloys Compd. 2000,
303–304, 137–141; (b) Cukrowski, I.; Cukrowska, E.; Hancock, R. D.; Anderegg,
G. Anal. Chim. Acta 1995, 312, 307–321; (c) Hirayama, N.; Iimuro, S.; Kubono, K.;
Kokusen, H.; Honjo, T. Talanta 1996, 43, 621–626; (d) Watanabe, M.; Mirvaliev,
R.; Tachimori, S.; Takeshita, K.; Nakano, Y.; Morikawa, K.; Mori, R. Chem. Lett.
2002, 31, 1230–1231; (e) Mirvaliev, R.; Watanabe, M.; Matsumura, T.; Tachimori,
S.; Takeshita, K. J. Nucl. Sci. Technol. 2004, 41, 1122–1124.
136–143; (d) Tokuyama, H.; Iwama, T. Langmuir 2007, 26, 13104–13108; (e)
Kanazawa, R.; Yoshida, T.; Gotoh, T.; Sakohara, S. J. Chem. Eng. Jpn. 2004, 37,
59–66.
7. (a) Takeshita, K.; Matsumura, T.; Nakano, Y. Prog. Nucl. Energy 2008, 50,
466–469; (b) Takeshita, K.; Ishida, K.; Nakano, Y.; Matsumura, T. Chem. Lett.
2007, 36, 1032–1033.
8. Recent studies on the separation of CdII ions (a) Cavus, S.; Gurdag, G.; Sozgen,
K.; Gurkaynak, M. A. Polym. Adv. Technol. 2009, 20, 165–172; (b) Cavus, S.;
Gurdag, G. Polym. Adv. Technol. 2008, 19, 1209–1217; (c) Rathore, N. S.; Leopold,
A.; Pabby, A. K.; Fortuny, A.; Coll, M. T.; Sastre, A. M. Hydrometallurgy 2009, 96,
81–87; (d) Huang, J.; Zeng, G.; Fang, Y.; Qu, Y.; Li, X. J. Membr. Sci. 2009, 326,
303–309; (e) Quintelas, C.; Rocha, Z.; Silva, B.; Fonseca, B.; Figueiredo, H.;
Tavares, T. Chem. Eng. J. 2009, 149, 319–324; (f) Perez-Quintanilla, D.; del Hierro,
I.; Fajardo, M.; Sierra, I. J. Mater. Chem. 2006, 16, 1757–1764.
9. Mialane, P.; Nivorojkine, A.; Pratviel, G.; Aze´ma, L.; Slany, M.; Godde, F.; Simaan,
A.; Banse, F.; Kargar-Grisel, T.; Bouchoux, G.; Sainton, J.; Horner, O.; Guilhem, J.;
Tchertanova, L.; Meunier, B.; Girerd, J. J. Inorg. Chem. 1999, 38, 1085–1092.
10. Eisenfu¨hr, A.; Arora, P. S.; Sengle, G.; Takaoka, L. R.; Nowick, J. S.; Famulok, M.
Bioorg. Med. Chem. 2003, 11, 235–249.
5. (a) Tanaka, T.; Nishio, I.; Sun, S.-T.; Ueno-Nishio, S. Science 1982, 218, 467–469.
6. (a) Fujinaga, K.; Yamato, Y.; Seike, Y.; Okumura, M. Anal. Sci. 1997, 13, 141–144;
(b) Takeshita, K.; Tanaka, M.; Nakano, Y.; Seida, Y. J. Chem. Eng. Jpn. 2003, 36,
1253–1258; (c) Tokuyama, H.; Kanehara, A. React. Funct. Polym. 2007, 67,
11. Kawabata, E.; Kikuchi, K.; Urano, Y.; Kojima, H.; Odani, A.; Nagano, T. J. Am.
Chem. Soc. 2005, 127, 818–819.
12. Kovbasyuk, L.; Kra¨mer, R. Inorg. Chem. Commun. 2006, 9, 586–590.
13. Sato, M.; Mori, Y.; Iida, T. Synthesis 1992, 539–540.