Synthesis and Photochromism of Crowned Spirobenzothiapyran: Facilitated Photoisomerization by Cooperative Complexation of Crown Ether and Thiophenolate Moieties with Metal Ions

Article abstract of DOI:10.1021/jo000175r

Spirobenzothiapyrans bearing monoaza-12-crown-4, -15-crown-5, -18-crown-6, and oligooxyethylene moieties were synthesized, and their photochromism was examined in the presence of cations in acetonitrile. The cation complexation by their crown ether moieties cannot induce thermal isomerization to their corresponding colored merocyanine form, unlike the corresponding spirobenzopyran derivatives. The UV-light-induced isomerization was, however, facilitated by the cation complexation of the crown ether moieties and the affinity of the merocyanine thiophenolate anion to metal ions, especially in the presence of Li⁺ and Ag⁺. The presence of Ag⁺ brought about the most remarkable effect in the facilitation of photoisomerization of the spirobenzothiapyrans and the thermal stability of the colored merocyanine form mainly due to the powerful interaction of the thiophenolate anion with the soft metal ion.

Full text of DOI:10.1021/jo000175r

4342
J . Org. Chem. 2000, 65, 4342-4347
Syn th esis a n d P h otoch r om ism of Cr ow n ed Sp ir oben zoth ia p yr a n :
F a cilita ted P h otoisom er iza tion by Coop er a tive Com p lexa tion of
Cr ow n Eth er a n d Th iop h en ola te Moieties w ith Meta l Ion s
Mutsuo Tanaka,† Kenji Kamada,† Hisanori Ando,† Takashi Kitagaki,‡
Yasuhiko Shibutani,‡ and Keiichi Kimura*,§
Osaka National Research Institute, AIST, 1-8-13, Midorigaoka, Ikeda, Osaka 563-8577, J apan,
Department of Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1,
Ohmiya, Asahi-ku, Osaka 535-8585, J apan, and Department of Applied Chemistry, Faculty of Systems
Engineering, Wakayama University, 930, Sakae-dani, Wakayama 640-8510, J apan
kimura@sys.wakayama-u.ac.jp
Received February 8, 2000
Spirobenzothiapyrans bearing monoaza-12-crown-4, -15-crown-5, -18-crown-6, and oligooxyethylene
moieties were synthesized, and their photochromism was examined in the presence of cations in
acetonitrile. The cation complexation by their crown ether moieties cannot induce thermal
isomerization to their corresponding colored merocyanine form, unlike the corresponding spiroben-
zopyran derivatives. The UV-light-induced isomerization was, however, facilitated by the cation
complexation of the crown ether moieties and the affinity of the merocyanine thiophenolate anion
to metal ions, especially in the presence of Li+ and Ag+. The presence of Ag+ brought about the
most remarkable effect in the facilitation of photoisomerization of the spirobenzothiapyrans and
the thermal stability of the colored merocyanine form mainly due to the powerful interaction of
the thiophenolate anion with the soft metal ion.
It is of great interest to apply photochromic compounds
devices does not seem to be very easy due to the poor
thermal stability of their merocyanine forms as compared
with spirobenzopyrans.
for photoresponsive devices. Various photochromic com-
pounds have been synthesized and their photochromism
has been examined extensively.1 Spirobenzopyrans2 and
spirobenzothiapyrans3,4 are well-known photochromic
compounds, which isomerize from the spiropyran forms
to their corresponding merocyanine forms by UV light,
and vice versa by visible light or heat. However, the
application of spirobenzothiapyrans to photochromic
It has been recognized, on the other hand, that the
incorporation of a crown ether moiety to spirobenzopyr-
ans facilitates photoisomerization to merocyanine forms
by the metal-ion complexation of the crown ether moiety
in the presence of metal ions.5-7 This concept prompted
us to design spirobenzothiapyrans bearing a crown ether
moiety, what we call crowned spirobenzothiapyrans,
expecting facilitated photoisomerization similar to the
crowned spirobenzopyrans. In our preliminary results,
significant facilitated photoisomerization was observed
with a crowned spirobenzothiapyran in the presence of
alkali metal ions, especially Li+.8 Here, we report the
syntheses and photochromism of crowned spirobenzothi-
apyrans carrying different crown ether rings in details.
† Osaka National Research Institute.
‡ Osaka Institute of Technology.
§ Wakayama University.
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Resu lts a n d Discu ssion
Syn t h esis. The syntheses of crowned spirobenzo-
thiapyrans 1-3 and its noncyclic analogue 4 were carried
out according to the synthetic route outlined by Scheme
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10.1021/jo000175r CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/09/2000

Synthesis of Crowned Spirobenzothiapyran
J . Org. Chem., Vol. 65, No. 14, 2000 4343
Sch em e 1
F igu r e 1. Absorption spectra for acetonitrile solution of
spirobenzothiapyran derivative incorporating monoaza-12-
crown-4 moiety 1 in the presence of metal ions and NH4+ under
dark conditions.
Sch em e 2
1. The conversion of the phenol groups of 1a -3a to the
corresponding thiophenol groups of 1d -3d was made
subsequently by the reaction with (CH3)2NCSCl in DMF
in the presence of NEt3, by the treatment in refluxing
toluene, and by the hydrolysis with KOH in H2O-EtOH.4
The crowned thiosalicyl aldehyde was then reacted with
2-methylene-1,3,3-trimethylindoline in refluxing ethanol
to yield the spirobenzothiapyran bearing monoaza-12-
crown-4 1, which was purified by recrystallization from
ethanol after preparative gel-permeation chromatography
(GPC). Some products were used for the subsequent
reaction without further purification because of their
lability. Especially in the case of the crowned nitrosalicyl
aldehyde synthesis, the product decomposed gradually
during the GPC purification. In a similar way to 1,
spirobenzothiapyrans bearing monoaza-15-crown-5 2,
monoaza-18-crown-6 3, and an oligooxyethylene moiety
4 were synthesized and purified by GPC.
is very different from that of corresponding crowned
spirobenzopyran, the pyran ring of which is opened
readily by metal-ion complexation of the crown ether
moiety without UV irradiation (Scheme 3).6 The same
tendencies concerning the cation effect under dark condi-
tions were found with 2, 3, and 4. Exceptionally, the
addition of Hg+, Zn2+, and Cu2+ under dark conditions
changed the absorption spectrum of the spirobenzothia-
pyrans remarkably. In the presence of Hg+, Zn2+, and
Cu2+, the present spirobenzothiapyran derivatives hardly
exhibited any significant photochromism, probably due
to the irreversible complex formation.
Upon irradiation of 365-nm UV light on the 1 solution
containing Li+, a drastic change in the absorption spec-
trum was seen as shown in Figure 2. On the other hand,
any significant spectral change was hardly observed
without any metal ion and with an equimolar amount of
K+, Rb+, and Cs+. The addition of Na+ and NH4+ allowed
some photoinduced spectral change of 1. The cation-
facilitated photoisomerization was promoted in the order
P h ot oisom er iza t ion of Cr ow n ed Sp ir ob en zo-
th ia p yr a n s. Absorption spectra of 1 were measured in
acetonitrile in the presence and absence of a metal ion
(alkali and alkaline-earth metal ions, NH4+, Ag+, Tl+,
and Pb2+) at room temperature (Figure 1). The spectra
were hardly changed by the equimolar amount addition
of any of the metal ions. This means that isomerization
to the merocyanine form do not proceed by metal-ion
complexation of the crown ether moiety under dark
conditions (Scheme 2). This isomerization behavior of 1
+
Li+ > NH4 > Na+ > K+, Rb+, Cs+, which is almost
consistent with the order in the cation-complexing ability
of 12-crown-4 ring. This means that the cation complex-
ation of the 12-crown-4 moiety of 1 clearly stabilizes the

4344 J . Org. Chem., Vol. 65, No. 14, 2000
Tanaka et al.
F igu r e 2. Absorption spectra for acetonitrile solution of
F igu r e 3. Absorption spectra for acetonitrile solution of
spirobenzothiapyran derivative incorporating monoaza-12-
spirobenzothiapyran derivative incorporating monoaza-15-
+
+
crown-4 moiety 1 in the presence of alkali metal ions and NH4
crown-5 moiety 2 in the presence of alkali metal ions and NH4
on UV-light irradiation.
on UV-light irradiation.
Sch em e 3
F igu r e 4. Absorption spectra for acetonitrile solution of
spirobenzothiapyran derivative incorporating monoaza-18-
+
crown-6 moiety 3 in the presence of alkali metal ions and NH4
on UV-light irradiation.
merocyanine form induced by the photoisomerization.
Similarly, the facilitated photoisomerization of 2 and 3
reflected the cation-complexing ability of their crown
ether moieties remarkably, being the most notable with
Li+ and Na+, respectively (Figures 3 and 4). In addition,
some interaction of thiophenolate anion of the merocya-
nine form with a metal ion, especially high-charge-den-
sity cations such as Li+, as well as the cation complex-
ation by the crown ether moieties probably contributes
to the stabilization of the merocyanine form. The ad-
ditional interaction may be suggested by the absorption
spectra of 3 in the presence of Li+, Na+, and K+ (Figure
4). A peak shift of the merocyanine absorption spectrum
appeared corresponding to the cation radii, the Li+ addi-
tion causing the most significant blue shifts.7 To the
contrary, the solution of 4 did not show any significant
photoinduced spectral change with Na+, K+, Rb+, or Cs+
(Figure 5). This indicates that the metal-ion complexation
of crown ether moieties of crowned spirobenzothiapyrans
is a crucial factor to facilitate photoisomerization. Figure
5 may suggest that the diethoxyamino group of 4
showed remarkable facilitated photoisomerization (Fig-
ure 6), but there was hardly any noteworthy ion selectiv-
ity among Ca2+, Sr2+, and Ba2+. The addition of Mg2+ did
not enhance the photoisomerization of crowned spiroben-
zothiapyran so much as the other alkaline-earth metal
ions. The facilitation for the photoisomerization of crowned
spirobenzothiapyran 1 in the presence of alkaline-earth
metal ions is also considered to be derived from the
cooperative interaction of the crown ether moiety and the
thiophenolate anion with metal ions. In the case of
alkaline-earth metal ions, the ionic interaction derived
from thiophenolate anion seems to contribute to the
facilitated photoisomerization more effectively than the
complexation only by the crown ether moiety, as com-
pared with in the presence of alkali metal ions. Similar
tendencies for the addition effect of alkaline-earth metal
ions on the facilitated photoisomerization were observed
with crowned spirobenzothiapyrans 2 and 3, and even
with the noncyclic analogue 4.
+
The facilitated photoisomerization of crowned spiroben-
zothiapyrans in the presence of heavy metal ions were
also followed, expecting a powerful interaction between
interacts with NH4 to some extent.
In the presence of alkaline-earth metal ions, the
absorption spectra of 1 on UV-light irradiation also

Synthesis of Crowned Spirobenzothiapyran
J . Org. Chem., Vol. 65, No. 14, 2000 4345
F igu r e 5. Absorption spectra for acetonitrile solution of
F igu r e 7. Absorption spectra for acetonitrile solution of
crowned spirobenzothiapyran 1 in the presence of heavy metal
ions on UV-light irradiation.
spirobenzothiapyran derivative incorporating noncyclic moiety
+
4 in the presence of alkali metal ions and NH4 on UV-light
irradiation.
F igu r e 8. Absorption spectra for acetonitrile solution of
spirobenzothiapyran derivative incorporating noncyclic moiety
4 in the presence of heavy metal ions on UV-light irradiation.
F igu r e 6. Absorption spectra for acetonitrile solution of
crowned spirobenzothiapyran 1 in the presence of alkaline-
earth metal ions on UV-light irradiation.
the merocyanie thiophenolate anion and a heavy metal
ion, which may enhance their photoisomerization ef-
ficiently. UV-light irradiation on a solution of 1 in the
presence of Ag+ and Tl+ caused drastic spectral change
as shown in Figure 7. The Pb2+ addition exhibited some
enhancement effect on the photoisomerization of 1.
Similar results were obtained with the other crowned
spirobenzothiapyrans 2 and 3, and even with the non-
cyclic analogue 4, as illustrated by the photoinduced
absorption-spectral changes for 4 in Figure 8. We previ-
ously reported that spirobenzopyran bearing monoaza-
12-crown-4, which corresponds to 1, shows remarkable
facilitated photoisomerization in the presence of Li+ in
THF solution.6 In control experiments, the presence of
Ag+ allowed only a slight spectral change although Li+
caused a dramatic absorption-spectral change in aceto-
nitrile solution of the crowned spirobenzopyran under
dark conditions. These results clearly show that the Ag+-
facilitated photoisomerization of the spirobenzothiapyran
derivatives is caused mainly by high affinity of the
thiophenolate anion to Ag+ but not by Ag+ complexation
of the crown ether moiety. This is also supported by the
fact that Ag+ still promotes the photoisomerization of
noncyclic analogue 4 strikingly.
The cation-dependent photoisomerization of spiroben-
zothiapyran derivatives 1 - 4 suggests that the dual-
mode facilitation of photoisomerization by the cation
complexation of their crown ether moiety and by the
cation affinity to their merocyanine thiophenolate anions.
Th er m a l Sta bility of Color ed F or m . To evaluate
thermal stability of colored merocyanine forms of the
crowned spirobenzothiapyrans, we studied their thermal
decoloration. For instance, time-course absorption-spec-
tral changes for acetonitrile solutions of 1 in the presence
and absence of Li+ were followed at room temperature
after turning off UV light. In the absence of Li+, the
thermal isomerization from the merocyanine form of 1
back to its spirothiapyran form was almost complete
within 5 s. On the contrary, the thermal back isomer-
ization in the presence of Li+ was very sluggish, the half-
time of the UV-induced merocyanine form being about 2
min. Visible-light irradiation did not promote the thermal
back isomerization at all. The slow back isomerization
in the presence of Li+ again proves the Li+-complexation-
induced high stability of the merocyanine form in crowned
spirobenzothiapyran 1.
As the thermal decoloration is a typical first-order
reaction for the isomerization to spiropyran form (S) from

4346 J . Org. Chem., Vol. 65, No. 14, 2000
Tanaka et al.
Ta ble 1. Th er m a l Decolor a tion Ra te Con sta n ts (10-2 s-1
)
Exp er im en ta l Section
+
no cation
Li+
Na+
K+
Rb+
Cs+
NH4
Ch em ica ls. Any chemicals for the synthesis were of avail-
able purity and used without further purification. For mea-
surements, spectroscopic-grade acetonitrile was used as a
solvent, while all salts were of the highest available purity
and used without further purification.
1
2
3
4
1.6
1.9
1.8
1.8
0.46
0.38
1.7
1.6
0.95
1.6
1.7
1.7
1.7
1.8
1.8
1.9
1.7
1.8
1.9
1.7
1.8
1.8
1.4
1.0
1.6
1.2
1.8
1.8
Syn th esis of Mon oa za -12-cr ow n -4 Der iva tive 1. 1a .
Under N2 atmosphere, monoaza-12-crown-4 (1.75 g, 10 mmol),
NEt3 (2.02 g, 20 mmol), and dry THF (20 mL) were placed in
a three-necked flask (100 mL), and the mixture was cooled at
0 °C. A dry THF (10 mL) solution of 3-chloromethyl-5-nitro-
salycylaldehyde (2.16 g, 10 mmol) was added dropwise to the
mixture. The reaction mixture was stirred for 3 h at 0 °C and
then stirred for additional 12 h at room temperature. The
reaction mixture was poured into water and extracted twice
with CHCl3. The crude product obtained by solvent evaporation
was used for the subsequent reaction without further purifica-
tion.
Mg2+
Ca2+
Sr2+
Ba2+
Ag+
Pb2+
TI+
1
2
3
4
1.0
1.7
1.7
1.4
0.29
0.48
0.47
1.1
0.43
0.47
0.49
1.5
0.46
0.50
0.51
1.4
0.21
0.18
0.29
0.43
1.5
1.7
1.7
1.6
0.38
0.34
0.45
1.0
merocyanine form (M), the thermal decoloration rate
constant (k) is defined with eq 1, using the time elapsed
1b. Under N2 atmosphere, crude 1a (2.83 g, 8 mmol), NEt3
(2.42 g, 24 mmol), and dry DMF (60 mL) were placed in a
three-necked flask (100 mL), and the mixture was cooled at
0 °C. A dry DMF (30 mL) solution of (CH3)2NCSCl (2.96 g, 24
mmol) was added dropwise to the mixture. The reaction
mixture was stirred for 3 h at 0 °C and then stirred for
additional 12 h at room temperature. The reaction mixture
was poured to water and extracted twice with CHCl3. The
crude product obtained by solvent evaporation was used for
the subsequent reaction without further purification.
1c. Crude 1b (2.65 g, 6 mmol) and dry toluene (100 mL)
were introduced to a three-necked flask (300 mL) under N2
atmosphere, and then the mixture was refluxed for 6 h. After
evaporation of toluene, the product was purified by GPC. The
isolated yield from 1a was 46%.
1d . 1c (1.76 g, 4 mmol) and EtOH (100 mL) were introduced
into a three-necked flask (300 mL) under N2 atmosphere. An
aqueous solution (100 mL) of KOH (2.24 g, 40 mmol) was
added, and then the reaction mixture was stirred for 2 h at
room temperature. After addition of CH3COOH (2.40 g, 40
mmol), the reaction mixture was poured into water and
extracted twice with CHCl3. The crude product obtained by
solvent evaporation was used for the subsequent reaction
without further purification.
1. Crude 1d (1.30 g, 3.5 mmol), 1,3,3-trimethyl-2-methyl-
eneindolin (606 mg, 3.5 mmol), and dry EtOH (100 mL) were
placed in a three-necked flask (300 mL) under N2 atmosphere,
and the reaction mixture was refluxed for 6 h. The EtOH
evaporation afforded a crude product, which was purified by
GPC. The isolated yield from 1d was 49%, and, therefore, the
overall isolated yield of 1 from monoaza-12-crown-4 was 23%.
Syn th esis of Mon oa za -15-cr ow n -5 Der iva tive 2. Com-
pound 2 was prepared according to the same procedures as
for 1, and the overall isolated yield was 21%.
(t), initial concentration of M at t ) 0 ([M]0), and
concentration of S at a given time ([S]t). [M]0 and [S]t are
expressed by the initial absorbance of a spirobenzothi-
apyran solution (A0), final absorbance at t ) ∞ (A∞),
absorbance at a given time (At), and the molar absorption
coefficient for M (ꢀ) at 550 nm, eq 1 can be then converted
to eq 2. According to eq 2, the thermal decoloration rate
constants of spirobenzothiapyran derivatives 1-4 in the
absence and presence of an equimolar amount of a cation
were determined spectrophotometrically, being sum-
marized in Table 1. Significant stabilization for mero-
cyanine forms was attained in the presence of Li+ with
1 and 2, showing the contribution of the metal-ion
complexation by the crown ether moiety to the merocya-
nine stabilization. Another remarkable thing is drastic
merocyanine stabilization observed in the presence of Ag+
with 1, 2, and 3. Even with the noncyclic analogue 4, the
presence of Ag+ allowed considerable effect on the mero-
cyanine stability, although the stability enhancement by
4 is not so dramatic as that by the crowned spiroben-
zothiapyrans. The results for the thermal decoloration
rate constants are clearly reflected in those for the
facilitated photoisomerization.
In conclusion, the present crowned spirobenzothiapy-
rans exhibit an intriguing type of photochromism differ-
ent from their corresponding spirobenzopyran derivatives
reported previously. Their isomerization to the corre-
sponding merocyanine form does not proceed even in the
presence of any crown-ether-complexing cation without
UV-light irradiation. The cation effect on their photoi-
somerization suggests that the dual-mode facilitation of
photoisomerization by the cation complexation of their
crown ether moiety and by the cation affinity to their
merocyanine thiophenolate anion. The thermal stability
of colored merocyanine form in the present spirobenzo-
thiapyran derivatives was enhanced markedly in the
presence of metal ions such as Li+ and Ag+.
Syn th esis of Mon oa za -18-cr ow n -6 Der iva tive 3. Com-
pound 3 was prepared according to the same procedures as
for 1, and the overall isolated yield was 16%.
Syn th esis of Non cyclic An a logu e 4. 2-Ch lor oeth yl
Eth yl Eth er . 2-Ethoxyethanol (90 g, 1 mol), pyridine (94.8 g,
1.2 mol), and benzene (500 mL) were placed into a three-
necked flask (1 L). Thionyl chloride (142.8 g, 1.2 mol) was
added dropwise to the mixture at room temperature, and then
the reaction mixture was refluxed for 16 h. After the reaction
mixture was allowed to cool at room temperature, aqueous HCl
(18 wt %, 100 mL) was added dropwise, and the organic layer
was rinsed twice with water. The benzene solution of 2-chlo-
roethyl ethyl ether (ca. 500 mL) was used for the subsequent
reaction after drying over P2O5.
2-Iod oeth yl Eth yl Eth er . The benzene solution of 2-chlo-
roethyl ethyl ether (ca. 500 mL), NaI (180 g, 1.2 mol), and
acetone (500 mL) was placed in a three-necked flask (2 L), and
the reaction mixture was refluxed for 4 days. After the reaction
mixture was cooled, the precipitated solid (NaCl) was filtrated
off. The filtrate was poured into water, and the benzene layer
was separated and washed twice with an aqueous solution of
Na2S2O3 (5 wt %, 50 mL). The benzene solution of 2-iodoethyl

Synthesis of Crowned Spirobenzothiapyran
J . Org. Chem., Vol. 65, No. 14, 2000 4347
ethyl ether (ca. 500 mL) was dried over P2O5 and used for the
following preparation.
and 2 × 10-4 M, respectively. The absorbance values of the
Pb2+ and Tl+ systems were normalized to compare with other
metal ion systems in Figures 7 and 8. The absorption spectra
under dark conditions were taken after allowing a measuring
solution to stand under dark conditions overnight. The absorp-
tion-spectral measurements under photoirradiated conditions
were carried out after UV-light irradiation for 30s. The light
was irradiated on a measurement cell in perpendicular direc-
tion to a measuring incident light. The UV light (675
mW/cm2), obtained by passing light of a 250-W Hg lamp
through a light filter (λo; 365 nm, λ/2; 9.5 nm, transmittance
0.53), was introduced to the cell compartment for a spectro-
photometer by using a glass fiber guide and was irradiated
on the quartz cell containing a solution.
Ben zylbis(eth oxyeth yl)a m in e. Benzylamine (21.4 g, 0.2
mol), Na2CO3 (84.8 g, 0.8 mol), and acetonitrile (500 mL) were
introduced into a three-necked flask (2 L), and the mixture
was then refluxed under N2 atmosphere. The benzene solution
of 2-iodoethyl ethyl ether (ca. 500 mL) was added dropwise to
the mixture and then stirred for 48 h under refluxing condi-
tions. After the solvent evaporation, CHCl3 and a KOH (5 wt
%, 400 mL) aqueous solution were added, and the organic layer
was separated. The product was isolated by vacuum distilla-
tion (bp 87 °C/0.2 mmHg) with an overall yield of 17% from
2-ethoxyethanol.
Bis(eth oxyeth yl)am in e. Benzylbis(ethoxyethyl)amine (43.3
g, 0.17 mol), 5 wt % Pd/C (1.5 g), p-toluenesulfonic acid (0.5
Deter m in a tion of Th er m a l Decolor a tion Ra te Con -
sta n ts. The thermal decoloration of spirobenzothiapyran
derivatives was followed in acetonitrile in the absence and
presence of an equimolar amount of a cation, by measuring
the absorbance at 550 nm after UV irradiation for 1.5 min.
The UV irradiation conditions were the same as for the
absorption-spectral measurements.
g), and EtOH (150 mL) were placed in
a stainles-steel
autoclave (300 mL). The mixture was stirred at 120 °C for 6 h
under 7.5 atm H2 pressure. The Pd/C was filtrated off from
the reaction mixture. Vacuum distillation (bp 78 °C/12 mmHg)
of the crude product afforded pure bis(ethoxyethyl)amine with
an isolated yield of 71%.
4. Compound 4 was prepared according to the same proce-
dures as for 1, and the overall isolated yield from bis-
(ethoxyethyl)amine was 25%.
Absor p tion -Sp ectr a l Mea su r em en ts. The absorption
spectra were measured at room temperature using acetonitrile
as the solvent. The concentrations of metal ions and spiroben-
zothiapyrans were 2 × 10-4 M. Perchlorate salts of alkali and
alkaline-earth metal ions and NH4+ and nitrate salts of heavy
metal ions were used as the metal salts. As Pb2+ and Tl+
nitrates were poorly soluble in acetonitrile, the concentration
of spirobenzothiapyrans and metal ions was 5 × 10-5 M. In
Figure 5, the concentrations of metal ions and 3 were 2 × 10-3
Ackn owledgm en t. We thank Yamada Science Foun-
dation for partial support of this work.
Su p p or tin g In for m a tion Ava ila ble: Data about proper-
ties and identification (1H NMR, IR, and mass spectra and
elemental analysis) for newly synthesized compounds 1-4.
This material is available free of charge via the Internet at
http://pubs.acs.org.
J O000175R