Specific coordination phenomena of alkaline earth metal ions with aromatic sulfonate ions in alcohols and binary solvents of acetonitrile-alcohols

Article abstract of DOI:10.1016/j.molliq.2014.08.005

The specific interactions between alkaline earth metal (Mg²⁺, Ca²⁺, or Ba²⁺) and p-toluenesulfonate (L^-), 1,5-naphthalenedisulfonate (L^2-), or 1,3,6-naphthalenetrisulfonate (L^3-) ions (from

Full text of DOI:10.1016/j.molliq.2014.08.005

Journal of Molecular Liquids 199 (2014) 445–453
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Specific coordination phenomena of alkaline earth metal ions
with aromatic sulfonate ions in alcohols and binary solvents
of acetonitrile–alcohols
b
c
Xiaohui Chen a, Keita Ayabe a, Masashi Hojo a, , Zhidong Chen , Masato Kobayashi
⁎
a
Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780-8520, Japan
Department of Applied Chemistry, Changzhou University, Changzhou, Jiangsu 213164, China
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
The specific interactions between alkaline earth metal (Mg2+, Ca2+, or Ba2+) and p-toluenesulfonate (L−), 1,5-
naphthalenedisulfonate (L2−), or 1,3,6-naphthalenetrisulfonate (L3−) ions (from the tetraethylammonium salt
of L−, L2−, or L3−) have been examined by means of UV–visible spectroscopy in primary alcohols (from methanol
to hexanol) as well as in the binary acetonitrile–alcohols (MeCN–MeOH, MeCN–EtOH), ethanol–methanol
(EtOH–MeOH) and methanol–water (MeOH–H2O) solvents. The precipitation of non-charged species
(e.g. ML0) and the successive re-dissolution of the precipitates, with increasing concentration of M(ClO4)2,
have revealed the formation of cationic charged species or “reverse coordinated” species, M2L2+, even in the
protic media as well as in the aprotic solvent MeCN. The solubility products (Ksp) and the “reverse coordination”
Article history:
Received 23 June 2014
Received in revised form 2 August 2014
Accepted 4 August 2014
Available online 14 August 2014
Keywords:
Reverse coordination in protic solvent
Solvent donicity
constants (2M2+ + L2− ⇆ M2L2+, K2(−2) = [M2L2+] / [M2+ 2 [L2−]) have been evaluated. In ethanol, both phe-
]
Solubility product
nomena of the precipitation of ML0 and the successive re-dissolution to produce M2L2+ are observed for Ca2+ or
Ba2+, but not for Mg2+. In butanol, the interaction between Mg2+ and the L2− causes the complete precipitation
of MgL0 (pKsp = 10.39) and also the successive re-dissolution of Mg2L2+ (log K2(−2) = 8.08). Even in methanol,
the interaction between Ba2+ and L2− results in precipitation (log Ksp = 8.28) and the “reverse coordinated”
species, Ba2L2+ (log K2(−2) = 5.58). The interaction of Ba2+ with L− or L3− causes no precipitation in methanol;
however, in all the other alcohols, it results both in precipitation (BaL2 or Ba3L2) and the “reverse coordinated”
Tripe ion formation through coordination
DFT calculation
species, BaL+ or Ba2L+. The formulation for the formation constants (K2(−3)) for M2+ and L3− is newly presented
2
and the constants (2Ba2+ + L3− ⇆ Ba2L+, K2(−3) = [Ba2L+] / [Ba2+
]
[L3−]) are evaluated in ethanol and
propanol as well as in the binary EtOH–MeOH solvents, up to 70% (v/v) MeOH. The donicities toward M2+ of
the media have been related to the pKsp and “reverse coordination” constants for L−, L2− and L3−
.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
manufacture of paints to detergents and deodorants [5] and enable
the variation of properties for studying the ion–ion and ion–solvent
interactions [6].
The association modes of metal ions and anions in various solvents
have acquired great attention due to their extensive use in many
areas of fundamental research, e.g. the evaluation of major physico-
chemical properties of solution [1], mechanistic studies in organic
chemistry and supramolecular chemistry [2], and the prediction of
drug solubility and chemical stability in pharmaceutical sciences [3]. Re-
cently, the coordination reactions of the alkali metal and alkaline earth
metal ions have been extensively investigated, due to their widespread
application in the industry, their importance in maintaining the ionic
equilibrium of the human body and especially their relevance in
pharmaceutics [4]. The mixtures of aqueous–organic or organic–organic
mixed solvents find a broad application in industries such as the
It is a common idea that the alkali metal or alkaline earth metal salts
in low concentrations as electrolytes totally disassociate in aqueous so-
lution. Through the series of studies, however, we have demonstrated
that the specific interaction between alkali metal or alkaline earth
metal ions and simple anions can operate in non-aqueous solvents
by means of conductometry [7], polarography [8], UV–visible [9], 1
H
and 13C NMR spectroscopic methods [10]. The formation of both the co-
ordinated and the “reverse-coordinated” species in aprotic solvents has
been proposed [11]: with half-equivalent amount of LiClO4 or NaClO4
added to carboxylate ions (L−) [12], the direct coordinated species
(ML−2 ) may form; however, precipitation (ML0) takes place in the
presence of an equivalence of the metal ion, which is followed by the
re-dissolution of the precipitates, due to the formation of the “reverse-
coordinated” species (M2L+), with increasing metal concentration.
⁎
Corresponding author. Tel.: +81 88 844 8306; fax: +81 88 844 8359.
E-mail address: mhojo@kochi-u.ac.jp (M. Hojo).
http://dx.doi.org/10.1016/j.molliq.2014.08.005
0167-7322/© 2014 Elsevier B.V. All rights reserved.

446
X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
Although it is generally acknowledged that alkali metal or alkaline
naphthalenedisulfonate (L2−), and 1,3,6-naphthalenetrisulfonate
(L3−) ions (cf. Chart 1) in primary alcohols as well as MeCN is inves-
tigated by means of UV spectroscopy. The interactions between the
alkaline earth metal and 1,5-naphthalenedisulfonate ions are care-
fully examined in binary mixed solvents of MeCN–MeOH, MeCN–
EtOH, and even EtOH–MeOH and MeOH–H2O. The solubility prod-
ucts and “reverse coordination” constants have been successfully
evaluated for the systems.
earth metal ions exhibit no coordination ability in dilute aqueous solu-
tion, aside from the chelate formation of alkaline earth metal ions
with a powerful chelate reagent, such as EDTA (ethylenediamine-N,N,
N′,N′-tetraacetic acid), these metal ions may exhibit a more covalent
character similar to transition metal compounds when polar organic
solvents are used [13].
Acetonitrile (MeCN) is an archetype for dipolar aprotic solvents with
simple molecular structure, which has attracted considerable interest
in the study of the interplay between ion solvation and association of
electrolyte solutions [14]. Acetonitrile of a relatively high permittivity
(εr = ca. 36) [15] is not only an aprotic solvent but also a protophobic sol-
vent [16], having poor solvation ability (DN = 14.1, AN = 19.3 [17], cf.
Table 1) toward both metal cations and anions. In many aprotic solvents,
including MeCN, of higher permittivities, it has been reported [11] that
the higher ion-aggregates, triple ions and quadrupoles, can be produced
from trialkylammonium halides, R3NH+X−, lithium trifluoroacetate,
etc., basically through hydrogen bonding and coordination forces, respec-
tively. The higher ion-aggregation from lithium fluoroalkanoates has
been conclusively evidenced in another protophobic aprotic solvent,
propylene carbonate [19] with the high permittivity of ca. 65 [15].
Contrastingly, protic solvents, such as water and primary alcohols
with relatively higher donicity and acceptivity (Table 1), can strongly
solvate both cations and anions. Even very small amounts of water
(~0.5%) added to acetonitrile can significantly influence the coordina-
tion reaction between magnesium and p-toluenesulfonate ions [9b].
Although the permittivities of many primary alcohols (EtOH, 1-PrOH,
1-BuOH, 1-HexOH) are lower than that of MeCN, the donor (DN) and
acceptor (AN) numbers of the alcohols under bulk conditions are
much larger than those of MeCN (Table 1). Only ion pair formation
has been found between Li+ and the tropolonate ion (C7H5O−2 ) in meth-
anol [9a]. The actual donicity of bulk methanol (DN = 31.3 [18]) is
much higher than the original value (DN = 20) reported by Gutmann
[17].
2. Experimental section
2.1. Chemicals
Tetraethylammonium p-toluenesulfonate was purchased from Al-
drich. Tetraethylammonium 1,5-naphthalenedisulfonate was prepared
as the previous method [9b]: A 1.0 g of 1,5-naphthalenedisulfonic
acid tetrahydrate (Aldrich) was dissolved in methanol and was titrated
with Et4NOH (20 wt.% in H2O, Aldrich) in methanol up to the equiv-
alence point. The solution was evaporated to dryness at 50 °C, and
the salt was dried in vacuo at 150 °C. Tetraethylammonium 1,3,6-
naphthalenetrisulfonate was prepared from 1,3,6-naphthalenetrisulfonic
acid in a similar method. However, the 1,3,6-naphthalenetrisulfonate
was dried at 120 °C in vacuo, and the conductometric titration with
trifluoromethanesulfonic acid suggested that the hydrated waters in the
salts can be negligible.
1,3,6-Naphthalenetrisulfonic acid was prepared from the sodium
salt as follows: 35 g of sodium 1,3,6-naphthalenetrisulfonate (pur-
chased from TCI Shanghai, Chemical, China) was dissolved in pure
water of 2 L, and the sodium ions were exchanged to protons with
an ion-exchange column. The Na+ concentration was determined
by an atomic absorption spectrophotometer and was kept to be less
than 0.1 μg/mL. The elute solution was evaporated to dryness in a ro-
tary evaporator at b35 °C, and the acid crystals were dried in vacuo
at 35 °C.
Some sulfonic acids, such as methanesulfonic and p-toluenesulfonic
acids, have been observed to be weak acids in benzonitrile (DN = 11.9,
AN = 15.5) to form the 1:2-type homoconjugated species by means of
conductometry [20]. Meanwhile, conductometric titrations with Et3N
of di- and trisulfonic acids in MeCN have revealed the strong
homoconjugation for the di- and trisulfonic acids [21]. In the previous
study [9b], the precipitation and re-dissolution reactions of alkali
metal (Li+, Na+) or alkaline earth metal (Mg2+, Ca2+, Ba2+) ions
with p-toluensulfonate and 1,5-, 2,6-, and 2,7-naphthalenedisulfonate
ions in MeCN have been examined by means of UV spectroscopy. The
addition of small amounts of water has influenced the specific reactions
so strongly.
Metal perchlorates without water, Mg(ClO4)2 and Ba(ClO4)2 (all Al-
drich), were used as received. Calcium perchlorate tetrahydrate from
Aldrich was dried in vacuo at 150 °C to obtain anhydrous Ca(ClO4)2.
Commercially obtained acetonitrile (MeCN) solvents of GR and super
dehydrated grades (Wako), containing b0.1 and b0.001% (v/v) H2O,
respectively, were used as received. Alcohols (MeOH, EtOH, 1-PrOH,
1-BuOH, 1-HexOH) all from Wako were used. The water contents are
certificated b0.1% (v/v) in MeOH and b0.2% (v/v) in all the other alco-
hols. Water was purified by means of a MilliQ system (Millipore
Corp.). The percentage of a solvent in binary solvents is all expressed
by the volume fraction [% (v/v)] in the present paper.
In the present work, the chemical interaction of alkaline earth
2.2. Apparatus and procedure
metal ions (Mg2+, Ca2+, Ba2+) with p-toluensulfonate (L−), 1,5-
UV–visible absorption spectra were measured at room temperature
using a Shimadzu double-beam spectrophotometer (model UV-2550)
in a 0.1 cm path-length quartz cuvette. When precipitation occurred,
the solution was sonicated for a few minutes in a Branson ultrasonic
bath (model Yamato 2510, 42 kHz and 125 W) and the supernatant
solution was measured after centrifugation. Sometimes, a long aging
time was needed to complete a precipitation reaction.
Table 1
Properties of the solvents concerned to the present study.
DNa
ANa
εr
b
Solvents
Acetonitrile (MeCN)
Water (H2O)
Methanol (MeOH)
Ethanol (EtOH)
Propanol (1-PrOH)
Butanol (1-BuOH)
Hexanol (1-HexOH)
14.1
19.3
54.8
41.3
35.94
78.36
32.66
24.55
20.45
17.51
13.3
40.3c, 18.0d
31.3c, 19d
27.8c, 20d
(27)e
2.3. Evaluation of “reverse coordination” formation constants
37.1
_
The “reverse coordination” formation constants between the metal
cations (M2+) and the “ligand” anions (L−, L2−) are evaluated by the
UV–visible spectroscopic data as the method previously proposed
[9b]. The formation constants of “reverse coordination” between the al-
kaline earth metal cations (M2+) and the 1,3,6-naphthalenetrisulfonate
ion (L3−) are evaluated as follows.
26.2c
_
_
_
a
Gutmann's donor and accepter number, Ref. [17].
The permittivity values cited from Ref. [15].
For bulk water, methanol and ethanol, Ref. [18].
b
c
d
Isolated H2O and ROH molecules (not as the bulk solvents) in 1,2-dichloroethane,
The equilibrium constants for the precipitation reaction (the solubil-
Ref. [17].
ity products, Ksp) and “reverse coordination” (K2(−3)) at higher M2+
e
Estimated from the donor numbers of MeOH and EtOH.

X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
447
concentrations, compared to L3−, are expressed by Eqs. (1) and (2),
respectively.
h
i
h
i
3
2
M3L2⇄3M2þ þ 2L3−; Ksp
¼
M2þ
L3−
;
ð1Þ
ð2Þ
ꢀ
ꢁ
h
i
h
i h
i
2
2M2þ þ L3−⇄M2Lþ; K2ð−3Þ
¼
M2Lþ
=
M2þ L3−
:
The solubility s of M3L2 or the total “ligand” concentration, ct, in
solution (and not in precipitation) is expressed by Eq. (3).
ꢀ
ꢁ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
h
i
h
i
Â
Ã−3
2
s ¼ ct ¼ L3− þ M Lþ
¼
Ksp M2þ
1 þ K2ð−3Þ M2þ
:
ð3Þ
ꢀ ½
2
Fig. 1. The UV spectra of 1.0 × 10−4 mol dm−3 1,5-naphthalenedisulfonate (0.1 cm path-
length) in EtOH with increasing concentration of Ba(ClO4)2.
The observed absorbance Abs of L3− (and M2L+) can be rationalized
by Lambert–Beer's law as,
place with further increasing Ba2+ concentration, which should
be caused by the formation of the “reverse-coordinated” species of
Ba2L2+. Whereas, those of BaL have never been re-dissolved by a large
excess amount of Ba2+ in MeCN [9b], as mentioned above.
Abs ¼ ε cl ꢁ ε sl
ð4Þ
where ε, c, and l are the molar absorptivity (cm−1 mol−1 dm3) of
L3− (or M2L+), the concentration (mol dm−3), and the path-length
(cm), respectively. Eq. (5) is given by introducing Eq. (3) into Eq. (4).
Fig. 2 shows the changes in absorbance (at λmax) of L2− with increas-
ing concentration of alkaline earth metal ions (Mg2+, Ca2+, Ba2+) in
ethanol. Neither the precipitation (nor the re-dissolution) occurs be-
tween Mg2+ and L2− in EtOH, whereas Mg2+ can interact with L2− to
form precipitates of MgL and the “reverse coordinated” species of
Mg2L2+ in MeCN [9b]. The precipitation between Ca2+ and L2− takes
place at the equivalence point, however, the precipitates begin to re-
dissolve after 0.01 mol dm−3 Ca2+ and completely re-dissolve at
1.0 mol dm−3 Ca(ClO4)2. The precipitates of CaL are apt to re-dissolve
successively to form Ca2L2+ more easily than those of BaL, therefore,
the values of “reverse coordination” constant (Table 2), log K2, for
Ca2L2+ and Ba2L2+ are given to be 7.49 and 7.11, respectively. In
the present paper, for simplicity, we may describe log K2 and not
log K2(−2) for the “reverse coordination” constants between M2+ and
L2− unless some confusion is anxious.
ꢀ
ꢁ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi−3
h
i
Â
Ã
2
Abs ¼ ε l Ksp M2þ
1 þ K2ð−3Þ M2þ
ð5Þ
Actually, with higher M2+ concentrations and larger K2(−3) values,
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
h
i
Eq. (5) can be arranged to be Abs ¼ ε l Ksp M2þ K2ð−3Þ
.
3. Results and discussion
3.1. The specific interactions between alkaline earth metal and
1,5-naphthalenedisulfonate (L2−) ions in alcohols
We [9b] have already reported that Mg2+ interacts with the 1,5-
naphthalenedisulfonate ion (L2−) to form the 1:1 precipitation
The interaction of the alkaline earth metal ions with the 1,5-
naphthalenedisulfonate ion (L2−) in EtOH seems to be apparently
different from that in MeCN, cf. Ref. [9b]. For the first time, however,
we have discovered the specific interaction (M2L2+) between the
alkaline earth metal ions (M2+) and a divalent anion (L2−) in a protic
solvent. Scheme 1 shows the reaction scheme for the precipitation of
ML (M = Ca and Ba) and the successive re-dissolution of the precipi-
tates in EtOH.
(MgL), and the precipitation is successively re-dissolved (Mg2L2+
)
by the addition of a large excess of Mg(ClO4)2 in MeCN, whereas
Ca2+ and Ba2+ ions can scarcely produce the soluble species of
M2L2+ from the precipitates in the same solvent. In protophobic
aprotic solvents, such as MeCN (DN = 14.1, AN = 19.3[17]) [9b]
and acetone (DN = 17.0, AN = 12.5 [17]) [9a,22], due to the lower
donicity, the alkali metal cations (especially Li+) have a good oppor-
tunity to interact with simple anions (excluding ClO−4 , PF6−, etc.) to
form not only the coordination-type species (LiL−2 ) and the ion pair
(LiL0) but also the “reverse-coordinated” species (Li2L+). Conversely,
such specific interactions between Li+ and mono-charged anions have
not been detected in the solvents of relatively high donicity, such as
MeOH (DN = 31.3, AN = 41.3, cf. Table 1) [9a], DMF (DN = 26.6
[17]) and DMSO (DN = 29.8 [17]) [22]. Furthermore, the chemical in-
teraction between metal cations and anions could be largely inhibited
by addition of the protic solvents, such as H2O or MeOH, to MeCN [9].
However, even in ethanol of relatively high donicity, the
precipitation and successive re-dissolution phenomena between
Ba2+ and L2− are observed (Fig. 1). Tetraethylammonium 1,5-
naphthalenedisulfonate [(Et4N+)2L2−] has given a strong band at
228 nm (ε/cm−1 mol−1 dm3 = 6.1 × 104) and a smaller band at around
290 nm in MeCN [9b]. In EtOH, the wavelength of strong peak of L2−
(λmax = 227.5 nm) is almost the same or just slightly shorter
than that in MeCN. With increasing concentration of Ba(ClO4)2, the
absorbance at around 227 nm decreases gradually, and the band
The interactions between alkaline earth metal ions and L2− in other
primary alcohols (MeOH, 1-PrOH, 1-BuOH) were also examined. Fig. 3
suddenly disappears in the presence of an equivalence of Ba2+
,
Fig. 2. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of alkaline earth metal ions in
EtOH: (○) Mg(ClO4)2; (●) Ca(ClO4)2; (▲) Ba(ClO4)2.
1.0 × 10−4 mol dm−3, which is accompanied by the complete precip-
itation. However, the successive re-dissolution of precipitates takes

448
X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
Table 2
Precipitation and re-dissolution reactions of alkaline earth metal and 1,5-
naphthalenedisulfonate (L2−) ions in acetonitrile and alcohols.
Metal ionsa Equilibrium constantsb MeCN MeOH EtOH 1-PrOH 1-BuOH
Mg2+
Ca2+
Ba2+
●
No
–
No
–
●
(pKsp
(pKsp
)
)
10.92
11.11
○
7.15
●
N 9.82 8.26
N10.01 8.47
No
–
10.39
10.93
○
8.08
●
10.80
11.34
△
7.04
●
c
c
c
–
–
No
–
No
–
log K2
▲
●
(pKsp
(pKsp
)
)
10.74
11.07
○
7.49
●
11.00 10.62
11.33 11.06
○
○
5.60
▲
log K2
●
●
(pKsp
(pKsp
)
)
N 9.82 8.28
N10.01 8.49
No
10.89
11.43
No
Fig. 3. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Mg(ClO4)2 in different
solvents: (●) EtOH; (△) 1-BuOH; (○) MeCN.
○
△
log K2
–
5.58
7.11
–
–
Explanatory notes: Solid circles and triangles represent apparent complete and partial
precipitation, respectively. The complete precipitation means here that the absorbance
of the “ligand” anion (L2−) reaches b1/10 of the initial value at the equivalent or any
amount of a metal ion. Open circles and triangles represent complete and partial re-
dissolution of precipitation, respectively. The mark “No” indicates no precipitation or no
re-dissolution.
Ca2+, and the absorbance minimum of 0.126 is given in the presence
of 1.0 × 10−3 mol dm−3 Ca2+. Apparently, the excess amounts of
Ca2+ to L2− assist the precipitation reaction.
a
In EtOH, an equivalent amount of Ca2+ actually has caused the com-
plete precipitation. At any rate, the precipitates would re-dissolve
completely by an excess amount of Ca(ClO4)2 in both MeOH and
EtOH. The precipitation of CaL in 1-BuOH takes place to a larger extent
than in MeOH or EtOH. The pKsp values of CaL increased in the order
of MeOH b EtOH b 1-BuOH (cf. Table 2). Unfortunately, the solubility
of Ca(ClO4)2 in 1-BuOH (εr = ca. 17.5) is not enough to examine at
1.0 mol dm−3 Ca(ClO4)2.
M(ClO4)2.
b
Solubility products (Ksp) and “reverse coordination” constants (K2), cf. the Experimental
section in Ref. [9b]. The uncertainties (errors) in Ksp values and “reverse coordination”
constants may be less than 0.01 and 0.05, respectively, in this table and Tables 3–4.
c
Thermodynamic solubility products (Ksp) corrected with the activity coefficients of
ions. The mean activity coefficients of ions are evaluated from the limiting Debye–Hückel
equation, log γ = −A|Z+ Z−| μ1/2, cf. Ref. [28].
shows the differences in the interaction between Mg2+ and L2− in EtOH
and 1-BuOH, and in MeCN as the reference solvent. In MeCN, the precip-
itation completes at an equivalence of Mg2+, and then re-dissolves
completely in the presence of 1.0 mol dm−3 Mg(ClO4)2. In EtOH,
however, the absorbance (at λmax) of L2− is almost unchanged with
the increasing Mg2+ concentration (vide supra), indicating that no
apparent interaction takes place between Mg2+ and L2−. In 1-BuOH,
the complete precipitation occurs at an equivalence of Mg2+, and the
precipitates begin to re-dissolve at a 10-fold concentration of Mg2+
added to the 1.0 × 10−4 mol dm−3 L2− solution.
Fig. 5 shows that the interaction of Ba2+ with L2− is similar to that of
Ca2+ in MeOH, apart from that the precipitation of BaL takes place to a
larger extent than CaL. The absorbance minimum (at λmax) of 0.076 is
given by the addition of 2.0 × 10−4 mol dm−3 Ba2+. As the permittiv-
ities and donor numbers of primary alcohols decrease, the precipitation
proceeds more completely but the formation of the “reverse-coordinated”
species seems to become more difficult: the absorbance (at λmax
)
are 0.372, 0.069, 0.061 in EtOH, 1-PrOH and 1-BuOH, respectively, in the
presence of 0.5 mol dm−3 Ba(ClO4)2.
In the aprotic solvent, MeCN, the calcium ion has been found to
cause the complete precipitation at an equivalence to L2−; however,
CaL precipitates have never re-dissolved by a large excess amount of
Ca(ClO4)2, cf. Ref. [9]. Fig. 4 shows that, in a protic solvent, MeOH,
Ca2+ can react with L2− to cause partial precipitation of CaL. The absor-
bance (at λmax) of L2− begins to decrease at 1.0 × 10−4 mol dm−3 of
3.2. In the binary solvents of MeCN–MeOH and MeOH–H2O
Fig. 6 shows the precipitation and the successive re-dissolution of
MgL in the binary MeCN–MeOH solvents. In the 10% (v/v) MeOH
mixed solvent, the precipitation takes place incompletely, compared
to that in sole MeCN, and the successive re-dissolution of MgL is
much promoted: the re-dissolution of MgL precipitates begins as low
Fig. 4. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Ca(ClO4)2 in different
solvents: (●) MeOH; (△) EtOH; (▲) 1-BuOH; (○) MeCN.
Scheme 1. Successive formation of ML and M2L2+ (M = Ca and Ba) for the 1,5-
naphthalenedisulfonate ion in EtOH.

X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
449
Table 3
Precipitation and re-dissolution reactions between alkaline earth metal and 1,5-
naphthalenedisulfonate (L2−) ions in binary MeCN–MeOH and MeOH–H2O solvents.
Metal iona
Equilibrium constantsb
MeCN–MeOH [MeOH contents/% (v/v)] 10
50
No
–
70
No
–
75
No
–
80
No
–
100
No
–
Mg2+
Ca2+
Ba2+
●
(pKsp
(pKsp
)
)
10.18
10.38
○
8.15
●
c
c
c
–
–
–
–
–
No
–
No
–
No
–
No
–
No
–
log K2
●
▲
–
▲
▲
(pKsp
(pKsp
)
)
–
10.19 9.17
10.39 9.37
△
–
8.55 8.26
8.76 8.47
○
5.52 5.60
●
–
–
–
No
–
○
5.45
●
8.74 8.47
8.95 8.68
△
–
○
log K2
–
–
●
–
●
●
▲
Fig. 5. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Ba(ClO4)2 in different
solvents: (●) MeOH; (△) EtOH; (▲) 1-PrOH; (□) 1-BuOH; (○) MeCN.
(pKsp
(pKsp
)
)
9.20
9.40
No
–
8.28
8.49
○
–
–
No
–
○
5.20
○
–
log K2
–
5.58
MeOH–H2O [H2O contents/% (v/v)]
1.0
2.0
5.0
20
30
50
as 1.0 × 10−3 mol dm−3 Mg2+. The pKsp value and the “reverse coordi-
nation” constants are listed in Table 3. The interaction between the diva-
lent cation and the divalent anion is almost inhibited in the 20% MeOH.
In the previous paper [9b], we have reported that the addition of small
amounts of water (1.0–5.0%) to MeCN tend to inhibit the Mg2L2+ spe-
cies to form from the MgL0 precipitates, although the precipitation of
the non-charged species itself was not so much influenced by these
additional waters and that the addition of 10% water or more than
that causes an increase of solubility of the MgL salt without assisting
the formation of the Mg2L2+ species. We have also reported that the
effect of water upon the specific interaction between Li+ and the
tropolonate ion in MeCN is similar to but much more distinct than
that of MeOH [9a].
Fig. 7 shows the precipitation and the successive re-dissolution of
CaL in MeCN–MeOH and MeOH–H2O mixtures. In sole MeCN, the pre-
cipitates of CaL have formed completely at an equivalent amount of
Ca2+, and CaL precipitates have been found not to be re-dissolved
even by the large excess amount of Ca(ClO4)2 [9b]. When 20% MeOH
are added to MeCN, the precipitation and re-dissolution phenomena
are not influenced so much: the precipitates form completely at an
equivalence of Ca2+ and are just partly re-dissolved by the addition of
1.0 mol dm−3 Ca(ClO4)2. With increasing contents of MeOH, the precip-
itation reaction becomes incomplete and at the same time the re-
dissolution is promoted. Calcium perchlorate of 1.0 mol dm−3 causes
the complete recovery in absorbance as the MeOH content in the
solvent reaches 70% (cf. also Table 3).
Ba2+
●
●
●
●
▲
No
–
(pKsp
(pKsp
)
)
9.22
9.43
○
9.39
9.56
△
9.48 7.82 6.51
9.68 7.97 6.64
No
c
–
No
–
No
–
No
–
log K2
5.46
5.25
–
For the Explanatory notes, cf. Table 2.
a
M(ClO4)2.
b
Solubility products (Ksp) and “reverse coordination” constants (K2), cf. the Experimental
section in Ref. [9b].
c
Cf. Table 2, note c) for the thermodynamic solubility products (Ksp) corrected with the
activity coefficients of ions.
We have proposed that the properties of the residual amount water
in (aprotic) organic solvents should resemble to an ether [11,23], and
we have termed it “dihydrogen ether” [24–26]. Through hydrogen
bonding, however, the water molecules may interact with protic
solvents, such as MeOH. In order to evaluate the effects of water on
the specific interaction between Ca2+ and L2− in MeOH, the change in
absorbance (at λmax) of 1.0 × 10−4 mol dm−3 L2− is examined in
MeOH–H2O mixtures. With the addition of 1.0% water to MeOH, some
small changes occurred in both the precipitation and re-dissolution
reactions (cf. Fig. 7). The values of pKsp and log K2 are evaluated to be
6.57 and 4.01 in 1.0% H2O–MeOH, while those values are 8.26 and
5.60 in the sole MeOH. The addition of 2.0% H2O to MeOH, however, al-
ters the interaction suddenly: the absorbance of L2− decreases just
Fig. 7. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Ca(ClO4)2 in MeCN–MeOH
mixtures: (○) 0; (●) 20; (△) 50; (▲) 70; (□) 80; (■) 100% (v/v) of MeOH; and in
MeOH–H2O mixtures: (▽) 1.0; (▼) 2.0; (◇) 5.0% (v/v) of H2O.
Fig. 6. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Mg(ClO4)2 in MeCN–MeOH
mixtures: (○) 0; (●) 10; (△) 20; (▲) 50% (v/v) of MeOH.

450
X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
Fig. 8. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Ba(ClO4)2 in MeCN–MeOH
mixtures: (○) 0; (●) 50; (△) 70; (▲) 75; (□) 100% (v/v) of MeOH.
Fig. 10. Changes in absorbance (λmax = ca. 227 nm) of 1.0 × 10−4 mol dm−3 1,5-
naphthalenedisulfonate ion with increasing concentration of Ba(ClO4)2 in MeOH–H2O
mixtures: (□) 0; (■) 1.0; (○) 2.0; (●) 5.0; (△) 10; (▲) 20; (▽) 30; (▼) 50% of H2O.
slightly at 1.0 × 10−3 mol dm−3 Ca2+. Finally, no decrease of the ab-
sorbance is observed in 5.0% H2O. As a short conclusion, the influ-
ence of water of a small amount (1.0–5.0%) in MeOH is large
enough to alter the precipitation and successive re-dissolution reac-
tions of CaL.
decrease, however, the log K2 values of CaL2+ and BaL2+ remain con-
stant or slightly increased with increasing contents of N50% MeOH (cf.
also Table 3).
Fig. 10 shows the influences of additional water (1.0–50%) on the
successive reactions of precipitation and re-dissolution (the BaL0 and
Ba2L2+ formation) for the 1,5-naphthalenedisulfonate ion (L2−) in
MeOH–H2O. It is curious to report that the solubility of BaL decreases
with increasing H2O contents, i.e., the pKsp value of BaL increases from
8.28 for no additional water to 9.22, 9.39 and 9.48 for 1.0, 2.0 and 5.0%
H2O, respectively. On the other hand, the re-dissolution of the precipi-
tates is inhibited with increasing contents of H2O. Firstly, the re-
dissolution is gradually reduced with increasing water contents (1.0–
10%). With the water content of 20%, the absorbance (λmax) minimum
of L2− is still observed at 0.10 mol dm−3 Ba2+. With 30% H2O, the absor-
bance of L2− suddenly decreases at 1.0 × 10−2 mol dm−3 Ba2+ and
continues to decrease monotonously up to 1.0 mol dm−3 Ba(ClO4)2.
The larger amounts of water cause an increase of solubility of the BaL
salt. Neither precipitation nor re-dissolution is observed in the presence
of 50% water.
Fig. 8 shows the absorbance changes of 1,5-naphthalenedisulfonate
ion with increasing concentration of Ba(ClO4)2 in the MeCN–MeOH
mixtures. In sole MeCN, an equivalence Ba2+ has caused the complete
precipitation of L2−, and those precipitates have never been re-
dissolved [9b]. Even the MeOH content reaches 50%, both the precipita-
tion and the successive re-dissolution of non-charged species (BaL)
are not influenced so much, and the precipitation would not be re-
dissolved even in the presence of 1.0 mol dm−3 Ba(ClO4)2. A great
difference is given at 75% MeOH and the absorbance (at λmax) of
1.0 × 10−4 mol dm−3 L2− is completely recovered at 0.2 mol dm−3
Ba2+ (cf. also Table 3). Among the three alkaline earth metal ions, we
may notice that Ba2+ is the least influenced by the additional MeOH for
the reactions between M2+ and L2− in the binary MeCN–MeOH media.
The interaction of Ba2+ with L2− is rather similar to that of Ca2+
with L2− in MeCN–MeOH. The similarity can be seen in the solubility
products (pKsp) and “reverse coordination” constants (log K2) displayed
in Fig. 9. The “reverse coordination” constants of Ca2L2+ and Ba2L2+ are
close to each other, however, the precipitates of CaL are totally re-
dissolved by an excess amount of Ca2+ in 70% MeOH, while that of
BaL are totally re-dissolved not in 70% but in 75% MeOH. In the binary
MeCN–MeOH solvent system, the pKsp values of both CaL and BaL
3.3. In the binary MeCN–EtOH mixed solvents
The coordination reactions between alkaline earth metal ions and L2−
were investigated also in MeCN–EtOH mixed solvents. The precipitation
of MgL take place completely when an equivalence of Mg2+ is added to
1.0 × 10−4 mol dm−3 1,5-naphthalenedisulfonate (L2−) in the binary
MeCN–EtOH solvent of 10 or 20% EtOH. When the EtOH content
reaches 30%, however, the precipitation becomes incomplete. The
re-dissolution of the precipitates is promoted with increasing
EtOH contents. The “reverse coordination” constant of Mg2L2+
,
log K2, in the 10 and 30% EtOH solvents is evaluated to be 7.15
and 8.57, respectively. However, the interaction between Mg2+
and L2− was totally inhibited in 50% EtOH.
Although the complete precipitation of CaL has taken place at an
equivalence of Ca2+ not only in MeCN but also in the sole EtOH, the
re-dissolution of the precipitates happens differently in both sole
solvents: the precipitates would never be re-dissolved by an excess
amount of Ca(ClO4)2 in MeCN (vide supra), while the absorbance of L2−
(or Ca2L2+) can be totally recovered by the addition of 0.02 mol dm−3
Ca2+ in EtOH. The re-dissolution is gradually promoted as the
EtOH contents increase in the binary solvent: the absorbance
increased as 0.082, 0.219, and 0.558 for 20, 30, and 50% EtOH at
1.0 mol dm−3 Ca(ClO4)2. However, the precipitates of BaL are
not re-dissolved by 1.0 mol dm−3 Ba(ClO4)2 even in 50% EtOH–
MeCN.
Fig. 9. The changes of solubility products (pKsp) and “reverse coordination” constants (log
K2) vs. MeOH contents in MeOH–MeCN for the interaction between Ca2+ (●,○) or Ba2+
(▼,▽) and 1,5-naphthalenedisulfonate ion. The solid and open symbols represent pKsp
and log K2, respectively.

X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
451
Table 4
Precipitation and re-dissolution reactions of Ba2+ with the mono-, di- and trivalent aromatic sulfonate ions in alcohols.
Et4N+ salts
pKa
Equilibrium constantsb
MeOH
EtOH
1-PrOH
1-BuOH
1-HexOH HH
a
p-Toluenesulfonate
−0.43
No
–
▲
▲
▲
▲
–
(1.0 × 10−3 mol dm−3
)
8.01c
(pKsp
)
)
10.48
10.85
○
7.58
●
(11.00)
(11.33)
○
7.11
●
11.59
12.07
○
8.9
●
(10.6)
(11.06)
△
13.10
13.71
○
10.32
●
(10.89)
(11.43)
△
d
(pKsp
log K1 (−1)
(pKsp
(pKsp
log K2 (−2)
(pKsp
No
–
○
–
1,5-Naphthalenedisulfonate
(1.0 × 10−4 mol dm−3
−0.60
−0.76
▲
●
–
)
)
)
(8.28)
(8.49)
○
5.58
No
d
No
–
–
–
1,3,6-Naphthalenetrisulfonate
(1.0 × 10−4 mol dm−3
●
●
–
●
–
)
)
)
–
27.60
28.21
○
27.66
28.46
○
d
(pKsp
No
△
△
log K2 (−3)
–
9.76
9.86
–
–
Explanatory notes: Solid circles and triangles represent apparent complete and partial precipitation, respectively. The complete precipitation means here that the absorbance of a “ligand”
anion (L−, L2−, and L3−) reaches b1/10 of the initial value at the equivalent or any amount of a metal ion. Open circles and triangles represent complete and partial re-dissolution of
precipitation, respectively. The mark “No” indicates no precipitation or no re-dissolution.
a
The first pKa value of HA, H2A, or H3A in water, cf. SciFinder: Calculated using Advanced Chemistry Development (ACD/Labs) Software V11.02 (© 1994–2014 ACD/Labs).
Solubility products (Ksp) and “reverse coordination” constants (K1, K2), cf. the Experimental section in the present paper and in Ref. [9b].
The pKa values in acetonitrile, cf. Ref. [27].
Cf. Table 2, note c) for the thermodynamic solubility products (Ksp) corrected with the activity coefficients of ions.
b
c
d
3.4. The specific interactions between Ba2+ and p-toluenesulfonate (L−) or
is gradually recovered after the minimum, and totally recovered at
1 × 10−2 mol dm−3 Ba2+, accompanied with the formation of “re-
verse coordinated” species of ML+, cf. Eq. (6).
1,3,6-naphthalenetrisulfonate (L3−) in alcohols
The specific interactions of monosulfonate and trisulfonate ions
with Ba2+ were also examined in the alcohols. The pKa values of
the sulfonic acids are shown in Table 4. The permittivity of MeOH is
close to that of MeCN; however, the donor number and acceptor
number of MeOH are much larger than those of MeCN. Neither
the normal coordination species nor the “reverse-coordinated”
species is observed between Ba2+ and p-toluenesulfonate ([Et4N+
CH3C6H4SO−3 ], L−) in MeOH: the absorbance (at λmax) of L− remains
almost unchanged with increasing concentration of Ba(ClO4)2
(Fig. 11), while the precipitation of BaL has taken place to a large
extent between Ba2+ and 1,5-naphthalenedisulfonate (L2−) ions
in MeOH (vide supra). In EtOH, the UV absorption spectrum of the
p-toluenesulfonate ion [Et4N+ CH3C6H4SO3−] gives a strong band at
222 nm (ε/cm−1 mol−1 dm3 = 1.1 × 104), which shows a slight
blue shift, compared with the band in MeCN (λmax = 223 nm). The
interaction between Ba2+ and L− causes some incomplete precipitation
at the equivalence point (5.0 × 10−4 mol dm−3 Ba2+): the absorbance
(at λmax) minimum of 0.345 in EtOH, cf. 0.031 in MeCN. The absorbance
ML20 þ M2þ→2MLþ
ð6Þ
The precipitation of BaL2 takes place to a larger extent in 1-PrOH
and 1-BuOH than in EtOH. Neither precipitation nor successive re-
dissolution occurs between Ca2+ and L− in all the alcohols even
1-HexOH.
In MeCN, tetraethylammonium 1,3,6-naphthalenetrisulfonate (L3−
)
gives a strong UV band at λmax = 238 nm (ε/cm−1 mol−1 dm3 = ca.
7.7 × 104), apart from the broad band around 280 nm of the naphtha-
lene body. The alkaline metal ions cause the complete precipitation at
1.5 × 10−4 mol dm−3 for 1.0 × 10−4 mol dm−3 L3−, but almost no
re-dissolution of the precipitates at 1.0 mol dm−3 M(ClO4)2, except
for Mg(ClO4)2. In MeOH, neither precipitation nor re-dissolution is
caused for L3− even by Ba2+ (Fig. 12). Regardless in MeOH, both precip-
itation and re-dissolution (between or among Ba2+ and L3−) can take
place in the other primary alcohols (from EtOH to 1-HexOH). The com-
plete precipitation at the equivalence point ([Ba2+]:[L3−] = 1.5:1.0)
Fig. 12. Changes in absorbance (λmax = ca. 238 nm) of 1.0 × 10−4 mol dm−3 1,3,6-
naphthalenetrisulfonate ion with increasing concentration of Ba(ClO4)2 in MeCN and
alcohols: (●) MeOH; (△) EtOH; (▲) 1-PrOH; (○) MeCN; and in EtOH–MeOH mixtures:
(▼) 20; (□) 50; (■) 70% (v/v) of MeOH.
Fig. 11. Changes in absorbance (λmax = ca. 222 nm) of 1.0 × 10−3 mol dm−3 p-
toluenesulfonate ion with increasing concentration of Ba(ClO4)2 in MeCN and alcohols:
(○) MeCN; (●) MeOH; (△) EtOH; (▲) 1-PrOH; (□) 1-BuOH.

452
X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
Scheme 2. Successive formation of M2L3 and M2L+ (M = Ba) for the 1,3,6-naphthalenetrisulfonate ion (L3−) in EtOH and 1-PrOH.
should be based on the formation of Ba3L2. The successive re-dissolution
of the precipitates is attributed to the formation of the Ba2L+ species in
EtOH and 1-PrOH, cf. Eq. (2) and Scheme 2. The most probable occupa-
tions by two barium ions in the 1-, 3-, and 6-positions are discussed in
the final section of the present paper.
we have to report an irregular fact that the solubility of BaL (barium
1,5-naphthalenedisulfonate) decreases with the addition of the smaller
amount of water (1.0–10%), which is also displayed in Fig. 13. The donor
numbers of the binary MeOH–H2O system are estimated just assuming
the linearity between 31.4 of MeOH and 40.3 of H2O, up to 30% H2O.
We are so interested in the large different influences with solvent
alcohols, especially, the difference between MeOH and EtOH that we
examine the specific phenomena in the binary MeOH–EtOH mixed sys-
tem. In the 20% MeOH solvent, still occurs the almost complete precip-
itation at 2.0 × 10−4 mol dm−3 Ba2+, and the successive re-dissolution
from Ba3L2 is promoted. With the 50% MeOH content, the precipitation
becomes incomplete, and the successive re-dissolution is promoted.
Even at 70% MeOH–EtOH, precipitation formation and re-dissolution
phenomena can be observed (Fig. 12).
Fig. 13 shows the solubility products (pKsp) and the “reverse coordi-
nation” constants (log K1(−1), log K2(−2), or log K2(−3)) for the interac-
tion between Ba2+ and the sulfonate ions (L−, L2−, and L3−) in
different alcohols. In the primary alcohols of the lower donor numbers,
such as 1-BuOH (DN = 26.2, cf. Table 1), the precipitation reaction is apt
to take place to a larger extent than in methanol (DN = 31.3). In other
wards, with increasing donicity of the alcohols, the pKsp values decrease.
At the same time, the log Kn values would be depressed. For the interac-
tion between Ba2+ and L3− in the binary EtOH–MeOH solvent system,
the pKsp values decrease as 27.6, 26.3, 23.0, and 20.3 in a good accor-
dance with the log K2(−3) of 9.8, 9.3, 8.4, and 7.5 in 0, 20, 50, and 70%
MeOH (the DN of 27.8, 28.5, 29.55, and 30.25), respectively. However,
Fig. 13. The solubility products (pKsp) and “reverse coordination” constants (log Kn,
n = 1 or 2) vs. donor numbers (DN) of alcohols for the interaction between Ba2+
and TsO−, 1,5-NDS2−, and 1,3,6-NTS3−: (●,○) p-toluenesulfonate (TsO−); (▼,▽)
1,5-naphthalenedisulfonate (1,5-NDS2−); (▲,△) 1,3,6-naphthalenetrisulfonate
(1,3,6-NTS3−). The solid and open symbols represent pKsp and log Kn, respectively. The
binary mixed solvents of EtOH–MeOH and MeOH–H2O are used for 28 b DN b 31 and
31.5 b DN ≤ 34, respectively.
Fig. 14. The optimized structures of 1,3,6-naphthalenetrisulfonates to which two Ba2+
ions are coordinated at (a) 1,3-, (b) 1,6-, and (c) 3,6-positions.

X. Chen et al. / Journal of Molecular Liquids 199 (2014) 445–453
453
Table 5
Calculated relative energies for Ba2L+ (L: 1,3,6-naphthalenetrisulfonate) in EtOH.
Positions of Ba2+
Relative energy/kcal mol−1
1,3−
1,6−
3,6−
+1.19
+0.71
0.00
3.5. Computational prediction of the structures of Ba2L+ in EtOH
For predicting the coordinating structures of Ba2L+ shown in
Scheme 2, we performed geometry optimization using GAMESS
program package [29]. All geometries were optimized with the density
functional theory (DFT) employing the long-range corrected BOP (LC-
BOP) exchange-correlation functional [30]. Except for hydrogen
atoms, all core electrons were treated by the model core potential
(MCP) [31], and the valence electrons were described via MCP-dzp
basis set [32], while the diffuse functions were augmented to oxygen
atoms. For hydrogen atoms, the cc-pVDZ set [33] was adopted. In this
paper, we only report the results for the lowest energy conformers. In
the present calculations, the ethanol solvent was taken into consider-
ation by the conductor-like polarizable continuum model (C-PCM)
[34] with the solvation model density (SMD) [35].
Fig. 14(a)–(c) show the projected views of optimized structures of
1,3,6-naphthalenetrisulfonates to which two Ba2+ ions are coordinated
at (a) 1,3-, (b) 1,6-, and (c) 3,6-positions. For all three structures, Ba2+
ion coordinates to two O atoms of sulfonate. Accordingly, the S\O
lengths coordinated by Ba2+ (1.49–1.50 Å) were slightly longer than
those at free sulfonate (1.47–1.48 Å). Table 5 summarizes the relative
energies for these Ba2L+ structures obtained by the DFT calculations
in ethanol. The 3,6-coordinated structure is the most stable among
these three species, although the difference from the most unstable
1,3-coordinated structure is less than 1.2 kcal/mol.
Chart 1. Aromatic sulfonic acids examined in the present study.
Japan, and at Research Institute for Information Technology, Kyushu
University, Japan.
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,
and L3−) have been observed in sole alcohols as well as the binary
solvent mixtures. We have demonstrated that the precipitation of the
non-charged species, i.e. ML02, ML0, and M3L20 and the successive forma-
tion of the “reverse coordinated” species (ML+, M2L2+, and M2L+) take
place not only in an aprotic solvent, acetonitrile, but also in protic media,
i.e. alcohols and their binary mixtures (including H2O). Even in metha-
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the “reverse coordinated” species. However, no apparent coordination
phenomena have been observed between all the alkaline earth metal
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successive re-dissolution reactions occur between Ba2+ and L−, L2− or
L3− at different degrees. The reactivities of alkaline earth metal ions
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Acknowledgment
The present work was partially supported by a Grant-in-Aid for the
commission project of “Research Program on Climate Change Adapta-
tion” from Ministry of Education, Culture, Sports, Science, and Technol-
ogy, Japan. The present calculations were performed using the
computers at Research Center for Computational Science, Okazaki,
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