Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in acetonitrile and binary solvents with protic solvents

Article abstract of DOI:10.1016/j.molliq.2015.11.053

In acetonitrile (MeCN), the specific interactions of alkali metal (M⁺ = Li⁺ or Na⁺) and alkaline earth metal ions (M^{2 +} = Mg^{2 +}, Ca^{2 +}, or Ba^{2 +}) with various phosphorus anions,

Full text of DOI:10.1016/j.molliq.2015.11.053

MOLLIQ-05264; No of Pages 10
Journal of Molecular Liquids xxx (2015) xxx–xxx
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Journal of Molecular Liquids
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Complexing ability of alkali metal and alkaline earth metal ions with
organic phosphinate or phosphates in acetonitrile and binary solvents
with protic solvents
b
Xiaohui Chen a, Masashi Hojo a, , Zhidong Chen
⁎
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
b
a r t i c l e i n f o
a b s t r a c t
In acetonitrile (MeCN), the specific interactions of alkali metal (M+ = Li+ or Na+) and alkaline earth metal ions
Article history:
Received 12 May 2015
Received in revised form 28 October 2015
Accepted 28 November 2015
Available online xxxx
(M2+
= , , ) , i.e., diphenylphosphinate,
Mg2+ Ca2+ or Ba2+ with various phosphorus anions, L−
diphenylphosphate, and bis(4-nitrophenyl)phosphate, have been examined by means of UV–visible spectrosco-
py. The formation of “reverse-coordinated” or coordinated species, M2L+ or ML+, has been observed in the
presence of excess amounts of the metal ions to the anions. Between all the M+ or M2+ ions and
1.0 × 10−3 mol dm−3 diphenylphosphinate ion (n-Bu4N+Ph2PO−2 ), both the precipitation of the non-charged
species (ML or ML2) and the successive re-dissolution of the precipitates take place. The addition of the alkaline
earth metal ions of just the equi-molar to L− causes almost complete dissolution of the precipitates through the
soluble ML+ coordinated species. As for the diphenylphosphate ion [n-Bu4N+(PhO)2PO−2 ], no apparent interac-
tion can be insisted between the alkali metal ions or Mg2+ and 5.0 × 10−4 mol dm−3 diphenylphosphate ion,
based on just no precipitation occurrence. Only Na+ and Ba2+ can cause obvious precipitation with
5.0 × 10−4 mol dm−3 bis(4-nitrophenyl)phosphate. A good evidence, however, has been provided by the con-
ductometric titration of 5.0 × 10−4 mol dm−3 n-Bu4N+(PhO)2PO−2 with LiClO4 or Mg(ClO4)2 in MeCN that the
“strong” interaction still operate between L− and Li+ or Mg2+ regardless of no precipitation (nor the successive
re-dissolution). The addition of protic solvents, such as water, MeOH, or EtOH, influences significantly the chem-
ical interaction between the metal ions and the anions in MeCN. The solubility products (Ksp) and the “reverse
coordination” or coordination constants (K2 = [M2L+]/([M+]2[L−], K1 = [ML+]/([M2+] [L−]) have been evaluat-
ed for the systems.
Keywords:
“Reverse” coordination
Indifferent electrolyte
Solubility product
Solvation
UV–visible spectroscopy
Hydrolysis in biochemical system
© 2015 Published by Elsevier B.V.
1. Introduction
concentrated alkali metal or alkaline earth metal (M+ or M2+) perchlo-
rates have been explained successfully by the concept of the specific
As one of the sixth most abundant elements in the human body,
phosphorus plays an important role in life process. Metal phosphates
have been gathering great interest, widespread investigating in bio-
chemical [1] and pharmaceutical fields [2]. Alkali metal and alkaline
earth metal ions have very specific functions in biological systems,
although alkali metal- or alkaline earth metal-phosphates [3] are less
recognized than transition metal-phosphates [4–6] due to their weaker
coordination ability. Many studies [7–9] have authenticated that the
metal ions can play a structural role on catalyzing the hydrolysis of
phosphate diester.
chemical interaction between M+ or M2+ and simple anions. That is,
the direct chemical interaction between the metal cations and the
leaving-group anions (X−) can generate favorably the carbocation
(R+) as the reaction intermediate even in “aqueous” solution containing
organic solvents.
In low solvating media of relatively high permittivities (20 b εr b 65),
the coordination or “reverse” coordination phenomena of alkali metal
(M+) or alkaline earth metal (M2+) ions with various anions, such as
Cl− [17,18], SCN− [19], tropolonates [20,21], sulfonates [22,23,24], and
carboxylates [22,25,26], have been examined over past three decades
in our group by means of voltammetry, conductometry, UV–visible,
and NMR spectroscopy. Where the term of “reverse” coordination
represents a species (such as M2X+) driven by a single-charged anion
(X−) with two or more of alkali metal ion (M+) through the chemical
interaction above the Coulombic force [27]. We have reported that
higher ion-aggregation (over the ion pair formation between ions)
takes place not only in low permittivity media (εr b 10) [28] but also
In recent decades, the effects of added salts on the hydrolysis rates of
various compounds have been examined in our laboratory [10–16]. The
exponential increases in hydrolysis rates of SN1 substrates (R-X) in
binary mixtures between H2O and many organic solvents containing
⁎
Corresponding author.
E-mail address: mhojo@kochi-u.ac.jp (M. Hojo).
http://dx.doi.org/10.1016/j.molliq.2015.11.053
0167-7322/© 2015 Published by Elsevier B.V.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

2
X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Scheme 1. Successive formation of ML and M2L+ (M
diphenylphosphinate ion in MeCN.
= Li and Na) for the
In the present work, the complexing ability of M+ or M2+ with
diphenylphosphinate, diphenylphosphate, and bis(4-
Fig. 1. UV spectra of 1.0 × 10−3 mol dm−3 diphenylphosphinate ion (L−) of n-Bu4N+L−
(0.1 cm path-length) in MeCN with increasing concentration of LiClO4.
nitrophenyl)phosphate is examined by means of UV spectroscopy in
MeCN, a protophobic and aprotic solvent. The influences of protic sol-
vents, such as water, methanol, or ethanol, are also investigated. The co-
ordination or “reverse” coordination formation constants of anions have
been evaluated using UV visible spectroscopic data. The aim of this
paper is to try convincing one that alkali metal and alkaline earth
metal ions in poor salvation media can exhibit unexpectedly strong co-
in the media of relatively high permittivity (20 b εr b 65) [29]. We have
attributed the higher ion-aggregation to the coordination (of M+ or
M2+) as well as hydrogen bonding forces (of R3NH+) in addition to
Coulombic forces [30]. Holmes [31] has examined the hydrogen-
bonding between imidazole and diphenylphosphate by 1H NMR, IR,
and X-ray technique.
ordination ability, not so well as 4d-shell metal ions (such as Ag+, Cd2+
)
The concept of triple ions in low permittivity media (εr b 12 or 23.2)
has been originated by Fuoss and Kraus [32]. The triple ion formation
between an ion pair and a free ion due to the Coulomb force [32,33]
should be discussed more. Nevertheless, the triple ion mechanism or
theory [34] has been applied in studying on lithium batteries [35], ion-
aggregation [36], the color developer of dyes [37,38], and supramolecular
construction [39].
in aqueous solution. We would like to recognize that the interaction be-
tween phosphates and Mg2+ or Ca2+ in biochemical systems should be
through the real chemical force and not just the electrostatic interaction.
2. Experimental
2.1. Chemicals
In DMF, the higher aggregations of dihydrogenphosphate have been
observed [40]. NMR relaxation studies have shown that the sodium ion
interacts with the dibutyl phosphate to form ion-aggregates in concen-
trated aqueous solution of sodium dibutyl phosphate [41]. In the solu-
bility study [23], the specific interaction has been observed between
Li+ and diphenylphosphate [(PhO)2PO−2 ], in acetone, but somehow
strangely not in acetonitrile.
Acetonitrile (MeCN) is a relatively high permittivity solvent
(εr = 35.94 [42]) of low donor and acceptor numbers (DN = 14.1,
AN = 19.3 [43]), attracting considerable interest in the study of the
interplay between ion solvation and association of electrolyte solu-
tion [44]. In MeCN, the formation of the precipitates and successive
formation of “reverse-coordinated” species has been reported for
M+ and M2+ with nitrophthalates or sulfonates (p-toluenesulfonate,
1,5-naphthalenedisulfonate, and 1,3,6-naphethalenetrisulfonate)
[22,24].
Diphenylphosphinic,
diphenylphosphate,
and
bis(4-
nitrophenyl)phosphate acids were purchased from Aldrich.
Tetrabutylammonium diphenylphosphinate (n-Bu4N+Ph2PO2−) was
prepared as follows: A 1.0 g of diphenylphosphinic acid was dissolved
in methanol, and was titrated by n-Bu4NOH (Wako) in methanol up to
an equivalence point. The solution was evaporated to dryness at 50 °C,
and the salt was dried in vacuo at 120 °C. Tetrabutylammonium
diphenylphosphate [n-Bu4N+(PhO)2PO2−
diphenylphosphate acid in a similar way.
]
was prepared from
Metal perchlorates without hydrate water, LiClO4, NaClO4, Mg(ClO4)2,
and Ba(ClO4)2 (all Aldrich), were used as received. Calcium perchlorate
tetrahydrates from Aldrich was dried in vacuo at 150 °C to obtain anhy-
drous Ca(ClO4)2. Triethylamine (Et3N), Et4NClO4, acetonitrile, acetone
(the spectroscopic grade), methanol, and ethanol all from Wako were
used as received. The water contents of acetonitrile, methanol, and etha-
nol are certificated to be less than 0.1, 0.1, and 0.2%, respectively. Water
Fig. 2. Changes in absorbance (λmax
diphenylphosphinate ion with increasing concentration of alkali metal ions in MeCN:
=
ca. 226 nm) of 1.0 × 10−3 mol dm−3
Fig. 3. UV spectra of 1.0 × 10−3 mol dm−3 diphenylphosphinate (0.1 cm path-length)
(○) LiClO4; (●) NaClO4.
with increasing concentration of Mg(ClO4)2 in MeCN.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
3
Table 1
Precipitation and re-dissolution reactions of the diphenylphosphinate ion (n-Bu4N+ L−
)
with alkali metal or alkaline earth metal perchlorates in MeCN.
Equilibrium
constantsa
n-Bu4N+ salt
Li+
Na+ Mg2+ Ca2+
Ba2+
Diphenylphosphinate
ion
▲
▲
▲
▲
▲
(pKsp
(pKsp
)
)
8.28 8.06 11.89
8.38 8.16 12.20
11.35 10.43
11.66 10.74
b
(1.0 × 10−3 mol dm−3
)
○
△
○
○
○
log K2, log K1 5.88 5.20 4.36
4.11
3.63
Explanatory notes: Solid circles and triangles represent the complete and partial precipita-
tion, respectively. The complete precipitation means here that the absorbance of a “ligand”
anion (L−) reaches b1/10 of the initial value at an equivalence 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
Solubility products (Ksp) and “reverse” coordination constants (K2, K1), cf. the Exper-
=
ca. 226 nm) of 1.0 × 10−3 mol dm−3
imental section in this paper.
Fig. 4. Changes in absorbance (λmax
b
Thermodynamic solubility products (Ksp) corrected with the activity coefficients of ions.
diphenylphosphinate ion with increasing concentration of alkaline earth metal ions in
MeCN: (○) Mg(ClO4)2; (●) Ca(ClO4)2; (△) Ba(ClO4)2; (■) Et4NClO4 in addition to
5.0 × 10−4 mol dm−3 Mg(ClO4)2.
The mean activity coefficients of ions are evaluated from the limiting Debye–Hückel equa-
tion, log γ = −A|Z+ Z−| μ1/2, cf. Ref. [46].
ꢂh
i
ꢃ
was purified by means of a MilliQ system (Millipore Corp.). The percent-
age of the solvent in binary solvents is all expressed by the volume frac-
tion [% (v/v)] in the present paper.
ꢀ
ꢁ
M
2þ þ L− ⇆ MLþ; K1 ¼ MLþ
=
M2þ ½L−ꢀ :
ð4Þ
2.2. Apparatus and procedure
3. Results and discussion
3.1. The diphenylphosphinate ion (L−)
UV–visible absorption spectra were measured at room temperature
using a Shimadzu double-beam spectrophotometer (model UV-2550)
in 0.01, 0.05, and 0.1 cm path-length quartz cuvettes. When precipita-
tion occurred, the solution was sonicated for a few minutes in a Branson
ultrasonic bath (model Yamato 2510, 42 kHz and 125 W) and the super-
natant solution was measured after centrifugation with a Hitachi centri-
fuge (model CT4D). Sometimes, a long aging time was needed to
complete a precipitation reaction. Electrical conductance was measured
at 25 0.02 °C with an Agilent LCR meter model HP4284 at 1 kHz in a
3.1.1. Specific interaction between the diphenylphosphinate ion (L−) and
M+ or M2+
The diphenylphosphinate ion (Ph2PO−2 : L−) of n-Bu4N+ L−
(1.0 × 10−3 mol dm−3) in acetonitrile (MeCN) gives a broad peak
around 226 nm (ε/cm−1 mol−1 dm3 = ca. 1.35 × 104) but the aromatic
band around 265 nm is very weak (Fig. 1). When LiClO4 is added to the
solution, the L− absorbance gradually decreases with increasing
concentration of LiClO4, and reaches its minimum value of 0.099 at an
equivalence of Li+ (1.0 × 10−3 mol dm−3), accompanying white pre-
cipitates. However, the further addition of LiClO4 causes the precipitates
to re-dissolve partially at 0.10 mol dm−3 Li+ and completely at
0.50 mol dm−3 Li+ or more. The peak absorbance of the solution re-
conductivity cell with a cell constant of 0.4959 S cm−1
.
The evaluation method of solubility products (pKsp) and “reverse” co-
ordination formation constants (K2 and K1 for Eqs. (2) and (4), respec-
tively) between metal cations (M+, M2+) and “ligand” ainons (L−
)
have been proposed in the previous paper [22].
(a) Alkali Metal ions
covers fully the initial value, showing a slight blue shift (λmax
=
223.5 nm).
The addition of an equivalence of NaClO4 to the L− ion also causes
precipitation similarly, but the re-dissolution of the precipitates of NaL
is not completed even in the presence of an excess amount of Na+ (cf.
Fig. 2). Scheme 1 indicates the manner of the ML (M = Li and Na) pre-
cipitation and the successive re-dissolution of ML through M2L+.
ꢀ
ꢁ
ML ⇆ Mþ þ L−; Ksp ¼ Mþ ½L−ꢀ;
ð1Þ
ð2Þ
ꢂ
ꢃ
ꢀ
ꢁ
ꢀ
ꢁ
2
2 Mþ þ L− ⇆ M2Lþ; K2 ¼ M2Lþ
=
Mþ ½L−ꢀ :
(b) Alkaline earth metal ions
h
i
2
ML ⇆ Mþ þ 2 L−; Ksp
¼
M2þ ½L−ꢀ ;
ð3Þ
Fig. 5. Changes in absorbance (λmax = ca. 226 nm) of 1.0 × 10−3 mol dm−3
diphenylphosphinate ion with increasing concentration of LiClO4 in MeCN–H2O mixtures:
(○) 0; (●) 1.0; (△) 2.0; (▲) 5.0% (v/v) of H2O.
Scheme 2. Successive formation of ML2 and ML+ (M = Mg, Ca, and Ba) for the
diphenylphosphinate ion in MeCN.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

4
X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Et4NClO4. As is shown in Fig. 4, however, the absorbance value of L− re-
mains constant without the re-dissolution of precipitation.
Table 2
Precipitation and re-dissolution reactions of the diphenylphosphinate ion with alkali
metal ions in binary mixtures of MeCN–H2O or MeCN–MeOH.
The coordination behavior of Ca2+ with L− is very similar to that of
Mg2+ (cf. Fig. 4). The precipitation takes place in the presence of
5.0 × 10−4 mol dm−3 Ca(ClO4)2, then the re-dissolution occurs
suddenly with 1.0 × 10−3 mol dm−3 Ca2+. Strange to say, the precipi-
tation of BaL2 occurs only to a smaller extent than that of MgL2 and
CaL2. Scheme 2 shows the precipitation of ML2 (M = Mg, Ca, and Ba)
and the successive re-dissolution of ML2 through ML+. The equilibrium
constants of the reactions between M+ or M2+ and the
diphenylphosphinate ion (L−) in MeCN are listed in Table 1. The “re-
verse coordination” or coordination constants indicate the interaction
increases in the order of Na+ b Li+ and Ba2+ b Ca2+ b Mg2+. In the pre-
vious paper [25], the sudden formation of ML+-type species has been
observed from the precipitates of ML2 for the benzoate ion in MeCN,
and furthermore the C6H5CO2Ca+ ion has been detected, indeed, by
the electrospray ionization mass spectroscopy [24].
Metal iona
Equilibrium constantsb
MeCN–H2O
[H2O% (v/v)]
1.0
2.0
5.0
7.0
▲
▲
No
−
−
No
−
▲
3.11
3.21
No
−
(pKsp
(pKsp
)
)
7.20
7.30
○
4.93
▲
6.64
6.74
○
6.68
6.78
○
4.98
▲
3.54
3.64
○
Li+
log K2
No
−
−
No
−
(pKsp
(pKsp
)
)
Na+
log K2
4.15
−
MeCN–MeOH
[MeOH% (v/v)]
2.0
5.0
10
▲
▲
No
–
–
No
−
No
−
−
No
−
(pKsp
(pKsp
)
)
7.30
7.40
○
5.02
▲
6.75
6.85
○
6.83
6.93
○
5.21
▲
4.09
4.19
○
3.1.2. The influences of added water and methanol on the interaction
between M+ or M2+ and the diphenylphosphinate ion (L−)
Li+
We have urged that the properties of “residual water” (the water re-
mains after drying) in non-aqueous solvents are much different from
those of the bulk water and that the presence of such “minor” water
can be often ignored [27]. However, additional water or protic solvents
in MeCN may cause strong influences on the reactions between alkali
metal or alkaline earth metal ions and the diphenylphosphinate ion
(L−) because of the stronger salvation toward both the metal and L−
ions by the additional water.
log K2
(pKsp
(pKsp
)
)
Na+
log K2
3.45
−
For the Explanatory notes, cf. Table 1.
a
MClO4.
b
Solubility products (Ksp) and “reverse” coordination constants (K2), cf. the Experimental
Fig. 5 shows the influences of added water on the precipitation and
the successive re-dissolution of lithium diphenylphosphinate (LiL) in
MeCN. The added water of a small content disturbs the precipitation
of LiL, and the absorbance minimum increases from 0.099 in 0.0% H2O
to 0.343 and 0.586 in 1.0 and 2.0% H2O, respectively. The solubility prod-
ucts (pKsp) are evaluated to be 8.28, 7.20, and 6.68 in 0.0, 1.0, and 2.0%
H2O, respectively (cf. Tables 1 and 2). The re-dissolution of precipitates
seems to be promoted apparently by the additional H2O. At
0.10 mol dm−3 LiClO4, the absorbance increases as 0.602, 0.733, and
1.36 for 0.0, 1.0, and 2.0% H2O, respectively. The precipitation and re-
dissolution reactions are completely inhibited in 5.0% H2O.
section.
Fig. 3 shows the UV spectral changes of L− (1.0 × 10−3 mol dm−3
)
with increasing concentration of Mg(ClO4)2 in MeCN. The absorbance
suddenly decreases to reach its minimum in the presence of an equiva-
lence of Mg(ClO4)2 (5.0 × 10−4 mol dm−3). However, even 7.0 × 10−4
or 1.0 × 10−3 mol dm−3 Mg2+ causes the re-dissolution of the precip-
itates and also a drastic increase in the intensity of the absorption band
around 226 nm. With the further addition of Mg2+, the absorbance
band recovers completely the initial value, (showing a blue shift to
223 nm), which should indicate the formation of the coordinated spe-
cies, MgL+.
We would like to confirm concisely that the increase in the ionic
strength is not the main factor for the recovering of the absorbance
in the presence of higher Mg(ClO4)2 concentrations. The ionic strength
of the MgL2-precipitated-solution (the supernatant contains
1.0 × 10−3 mol dm−3 n-Bu4NClO4) has been increased up to 0.1 by
The influences of MeOH on the interaction between Li+ and L− in
MeCN are very similar but smaller than those of H2O (Fig. 6). In 2.0%
MeOH and 1.0% H2O, solubility products (pKsp) are observed to be
7.30 and 7.20, respectively, and “reverse” coordination constants (log
K2) are 5.02 and 4.93 (cf. Table 2). In 10% MeOH, no apparent precipita-
tion or re-dissolution reactions are observed between Li+ and L−. We
can easily notice that the useful solvation parameters for the added
protic solvents of N0.5% H2O and MeOH in MeCN should be those
(DNbulk = 40.3 and 31.3) given by Marcus [45] and not the originally
given by Gutmann (DN = 18.0 and 19) [43] for H2O and MeOH,
respectively.
The influences of added water and methanol have been also exam-
ined on the interaction between Na+ and L−. It is worth noticing that
the added water or methanol gives larger influences on the interaction
between Na+ and L− than that for Li+. In 5.0% MeOH, for instance, the
precipitation of NaL occurs only slightly (pKsp = 4.09), while that of
LiL still takes place to a relatively large extent (pKsp = 6.83, cf.
Table 2). In the reactions with sulfonate anions [22,24], however, the
metal ions of larger size or lower charge density have been less affected
by the added H2O and MeOH.
Fig. 7(a) shows the influences of added water (1.0–5.0%) on the pre-
cipitation and the successive re-dissolution of CaL2. Without H2O, as
shown Fig. 4, the distinct precipitation takes place at an equivalence of
Ca2+. In the presence of 1.0 and 2.0% H2O, no remarkable influences
are observed in the precipitation and re-dissolution reactions, that is,
the pKsp values for 0.0, 1.0, and 2.0% H2O are given to be 11.35, 11.31,
and 10.96 and log K1 = 4.11, 4.11, and 3.93, respectively (cf. Tables 1
Fig. 6. Changes in absorbance (λmax
diphenylphosphinate ion with increasing concentration of LiClO4 in MeCN–MeOH mix-
tures: (○) 0; (●) 2.0; (△) 5.0; (▲) 10% (v/v) of MeOH.
=
ca. 226 nm) of 1.0 × 10−3 mol dm−3
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
5
Fig. 7. (a). Changes in absorbance (λmax = ca. 226 nm) of 1.0 × 10−3 mol dm−3 diphenylphosphinate ion with increasing concentration of Ca(ClO4)2 in MeCN–H2O mixtures: (○) 0;
(●) 1.0; (△) 2.0; (▲) 4.0; (□) 5.0% (v/v) of H2O. (b). Changes in absorbance (λmax = ca. 226 nm) of 1.0 × 10−3 mol dm−3 diphenylphosphinate ion with increasing concentration of
Ca(ClO4)2 in MeCN–MeOH mixtures: (○) 0; (●) 5.0; (△) 10; (▲) 15% (v/v) of MeOH.
and 3). The absorbance minimum increases to 0.611 in 4.0% H2O, and fi-
nally it almost disappears in 5.0% H2O. The influences of MeOH on the
coordination reaction between Ca2+ and L− are smaller than H2O
[Fig. 7(b)]. With increasing contents of MeOH, the precipitation reaction
takes place to lesser extent. Precipitation may take place in 10% MeOH,
but it is almost inhibited in 15% MeOH. The higher solubility of CaL and
the sudden increase of absorbance with increasing Ca2+ [(0.5–
1.0) × 10−3 mol dm-3)] in 5.0% MeOH kept us from evaluating appropri-
ate K1 values [cf. Fig. 7(b)].
The added water (0–10%) has influenced in a different way for Mg2+
with L−. When 0.50% H2O is added to MeCN, the dissolution of MgL at
1.0 × 10−3 mol dm−3 Mg2+ (without added water, Fig. 4) is obstructed
remarkably and the absorbance of L− recovers its original value at as
high as 1.0 × 10−2 mol dm−3 Mg2+. The addition of 10% H2O causes
no precipitation between Mg2+ and L−. Compared with H2O, methanol
causes much smaller influences on the interaction between Mg2+ and
L−. The precipitation and re-dissolution reactions disappear in 30%
MeOH.
Table 3
Precipitation and re-dissolution reactions of the diphenylphosphinate ion with alkaline
earth metal ions in binary mixtures of MeCN–H2O or MeCN–MeOH.
Metal
Equilibrium
iona
constantsb
MeCN–H2O
[H2O% (v/v)]
The influences of added water (1.0–5.0%) have been examined also
for BaL2. With the addition of 1.0% H2O, very interestingly, the absor-
bance minimum value (at an equivalence) decreases from 0.451 to
0.346. In 5.0% H2O, the precipitation and re-dissolution reactions are
not observed. In general, the water influences on the interaction of
Ba2+ and L− are similar to those on Ca2+ and L−.
The influences of added MeOH on the behavior of precipitation and
the successive re-dissolution of BaL2 are rather complicated. When
MeCN contains 10% MeOH, the log Ksp is evaluated to be 10.73. This
value is still very close to that in 5.0% MeOH (pKsp = 10.96, cf.
Table 3). The precipitation takes place in 20% MeOH, but finally, neither
precipitation nor re-dissolution appears in 30% MeOH.
0.5
1
2
3
4
5
10
▲
▲
▲
▲
−
−
○
−
No
−
No
−
−
No
−
(pKsp
(pKsp
)
)
12.40 12.14 11.89
12.71 12.45 12.19
○
Mg2+
Ca2+
Ba2+
○
4.08
▲
11.31 10.96
11.62 11.26
○
○
3.87
▲
log K1
4.43
−
▲
10.05
10.32
○
(pKsp
(pKsp
)
)
○
3.93
▲
No
−
No
−
log K1
4.11
▲
3.49
▲
(pKsp
(pKsp
)
)
10.79 10.89 9.96
11.10 11.19 10.25
○
○
○
No
log K1
3.89
3.96
−
−
MeCN–MeOH
[MeOH% (v/v)]
2.0
5.0
10
15
20
25
30
▲
▲
▲
▲
No
−
−
(pKsp
(pKsp
)
)
12.47 11.35 10.72 9.77
12.79 11.67 11.04 10.09
Mg2+
Ca2+
Ba2+
○
4.20
▲
○
○
3.84
○
−
No
−
log K1
4.00
No
−
▲
9.72
(pKsp
(pKsp
)
)
9.42
10.03 9.74
−
○
−
▲
○
−
▲
No
−
▲
log K1
▲
▲
No
−
−
(pKsp
(pKsp
)
)
10.94 10.96 10.73 10.41 10.00
11.25 11.27 11.05 10.73 10.32
○
3.75
○
3.08
○
3.28
○
3.40
○
3.34
No
−
log K1
For the Explanatory notes, cf. Table 1.
Fig. 8. Changes in absorbance (λ = ca. 206 nm) of 5.0 ×
10−3 mol dm−3
diphenylphosphate ion (0.05 cm path-length) with increasing LiClO4 and NaClO4 concen-
trations in MeCN (triangles) and acetone (circles). The open and solid symbols represent
LiClO4 and NaClO4, respectively.
a
M(ClO4)2.
b
Solubility products (Ksp) and “reverse” coordination constants (K1), cf. the Ex-
perimental section.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

6
X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Table 4
Precipitation and re-dissolution reactions on the diphenylphosphate ion (L−) or Et3N-HL
with alkali metal or alkaline earth metal perchlorates in MeCN.
Equilibrium
Mixed solution
Li+ Na+ Mg2+ Ca2+ Ba2+
constantsa
Diphenylphosphate
▲
–
−
▲
5.56
−
○
3.73
No
−
−
No
−
▲
●
b
(5.0 × 10−3 mol dm−3 L− for
alkali metal ions)
(pKsp
(pKsp
)
)
13.07 13.57
13.29 13.79
○
(5.0 × 10−4 mol dm−3 L− for
alkaline earth metal ions)
○
○
3.61
b
log K2, log
–
4.72
K1
▲
●
(pKsp
(pKsp
)
)
12.71 13.78
12.93 14.00
Et3N-HL
(5.0 × 10−4 mol dm−3
)
○
○
log K1
4.50
3.85
For the Explanatory notes, cf. Table 1.
Fig. 10. UV spectra of 5.0 × 10−4 mol dm−3 diphenylphosphate ion (0.1 cm path-length)
with increasing concentration of Ba(ClO4)2 in MeCN.
a
Solubility products (Ksp) and “reverse” coordination constants (K2, K1), cf. the Exper-
imental section.
b
The pKsp and log K2 values are not evaluated because the precipitation of LiL in MeCN
is not enough, cf. Fig. 8.
Fig. 9(a) shows the absorbance (λ
= ca. 206 nm) of
5.0 × 10−4 mol dm−3 diphenylphosphate ion (L−) with increasing con-
centration of alkaline earth metal ions in MeCN. Almost no absorbance
changes appear by the addition of Mg2+, i.e., no apparent interaction be-
tween L− and Mg2+. With increasing concentration of Ca(ClO4)2, how-
ever, the absorbance firstly decreases and reaches its minimum at an
equivalence of Ca(ClO4)2 (2.5 × 10−4 mol dm−3), then the equi-molar
Ca(ClO4)2 (5.0 × 10−4 mol dm−3) causes suddenly the re-dissolution
of precipitates and also the drastic increase in absorbance, the pKsp
and log K1 are listed as 13.07 and 4.72 in Table 4. Almost complete pre-
cipitation is caused by an equivalence of Ba(ClO4)2, and the precipita-
tion continues up to 1.0 × 10−2 mol dm−3 Ba2+. The precipitates of
BaL2 partially re-dissolve at 0.10 mol dm−3 Ba2+, and finally the absor-
bance recovers the original value at 0.20–1.0 mol dm−3 Ba2+. The UV
spectral changes of n-Bu4N+ L− in the presence of increasing
Ba(ClO4)2 concentrations are shown in Fig. 10. Scheme 3 shows the (re-
verse) coordinated species of ML+ (M = Ca, Ba), i.e., the 1:1 complex
formation from the ML2 precipitate.
3.2. Diphenylphosphate (L− and HL)
3.2.1. Specific interaction between M+ or M2+ and the diphenylphosphate
ion (L−) or the mixture of diphenylphosphate (HL) with Et3N
Diphenylphosphate [(PhO)2PO2H, phosphoric acid diphenyl ester]
in the group of week acids is fairly strong in water (pKa = ca. 1.5)
[23]. The UV absorption spectrum of tetrabutylammonium
diphenylphosphate [n-Bu4N+ (PhO)2PO−2 ] exhibits a band at around
206 nm in MeCN.
In MeCN, no apparent interaction could be detected between Li+ or
Na+ and 5.0 × 10−4 mol dm−3 diphenylphosphate (L−). However, in a
higher L− concentration solution, 5.0 × 10−3 mol dm−3, the precipita-
tion of LiL or NaL takes place (Fig. 8). The pKsp and log K2 values can
be evaluated for Na+ but not for Li+ and L− (cf. Table 4). Further details
will be discussed for the interaction between Li+ and L− along with be-
tween Mg2+ and L− (vide infra).
In acetone, the precipitation of LiL occurs to a larger extent than that
of NaL. The Li+ ion may have a stronger tendency than Na+ to associate
with L− in acetone (εr = 20.56) [42]. The pKsp and log K2 values have
been evaluated from 5.0 × 10−3 diphenylphosphate ion (Fig. 8) to be
6.28 and 4.07, respectively, for Li+ and also 5.20 and 4.29 for Na+. By
means of polarography on DME, coincidently, we [23] have reported
(rather larger) overall formation constants of LiX−2 and Li2X+ to be
106.5 and 107.1, respectively, in acetone, where X = (PhO)2PO2.
Fig. 9(b) shows the changes in absorbance (λ = ca. 206 nm) of
5.0 × 10−4 mol dm−3 diphenylphosphate (HL) containing an equiva-
lent amount of Et3N with increasing of alkaline earth metal ions in
MeCN. Comparing Fig. 9(a) with 9(b), it is obvious that the alkaline
earth metal ions react with both (n-Bu4N+) L− and Et3N–HL (or
Et3NH+–L−) in a very similar way. Strictly speaking, the proper interac-
tions (without any obstruction) should be observed only with (n-
Bu4N+) L− and not with Et3N-HL (or Et3NH+—L−). However, in the
Fig. 9. (a). Changes in absorbance (λ = ca. 206 nm) of 5.0 × 10−4 mol dm−3 diphenylphosphate ion with increasing of alkaline earth metal ions in MeCN: (○) Mg(ClO4)2; (●) Ca(ClO4)2;
(△) Ba(ClO4)2. (b). Changes in absorbance (λ = ca. 206 nm) of 5.0 × 10−4 mol dm−3 diphenylphosphate (HL) containing 5.0 × 10−4 mol dm−3 Et3N with increasing of alkaline earth metal
ions in MeCN: (○) Mg(ClO4)2; (●) Ca(ClO4)2; (△) Ba(ClO4)2.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
7
Scheme 3. Successive formation of ML2 (M
=
Ca and Ba) and ML+ for the
diphenylphosphate ion (L−) in MeCN.
following sections, we examine the metal ion reaction with Et3N-HL for
experimental simplicity. It may be worth mentioning that the hydrogen
bonding interaction (in addition to the Coulombic interaction) between
R3NH+ and (PhO)2PO−2 is much stronger than the mere Coulombic in-
teraction between R4N+ and (PhO)2PO−2 in several protophobic aprotic
solvents, such as MeCN, benzonitrile, nitrobenzene, and propylene car-
bonate, according to the conductometric data [23]. Nevertheless, n-
Bu4N+ L− and Et3N-HL give similar pKsp and log K1 values for Ca2+
and Ba2+ in MeCN (cf. Table 4).
Fig. 12. Absorbance (λ = ca. 206 nm) of 5.0 × 10−4 mol dm−3 diphenylphosphate con-
taining 5.0 × 10−4 mol dm−3 Et3N in the presence of Ba(ClO4)2 in MeCN–H2O mixtures:
(○) 0; (●) 1.0; (△) 2.0; (▲) 3.0; (□) 5.0; (■) 10% (v/v) of H2O.
At an equivalence point (5.0 × 10−4 mol dm−3) in the titration
curve with LiClO4, the observed specific conductance (k/S cm−1) of
0.1425 × 10−3 equals 285 of the molar conductivity (Λ/S cm2 mol−1).
In the solution, we can expect the presence of two pairs of
5.0 × 10−4 mol dm−3 Li+(PhO)2PO2− and n-Bu4N+ClO−4 .
By means of the present method utilizing the successive reactions of
precipitation and re-dissolution, no interaction between a metal ion and
L− can be indicated unless the precipitation reaction occurs properly.
Nevertheless, we may predict the log K2 of ~4.2 for Li+ and log K1 of
~5.0 for Mg2+ with the L− ion in MeCN, judging from the log K2 and
log K1 values for the interaction between M+ or M2+ and the analogous
compound (diphenylphosphinate, Ph2PO−2 ), given in Table 1.
The observed Λ value (ca. 118) of Li+ (PhO)2PO2− at the equiva-
lence point has been found to be smaller than the calculated Λ one
(130.6) for the independent Li+ and (PhO)2PO2−. The Λclcd[Li+(-
PhO)2PO−2 ] = Λ0[n-Bu4N+(PhO)2PO2−](123.0) + Λ0(LiClO4) (174.8)
[29] − Λ0(n-Bu4N+ ClO4−)(167.2) [29] = 130.6. By the way, the Λ0
value of n-Bu4N+(PhO)2PO2− has not directly been measured yet,
therefore, the values is estimated by the calculation from Λ0[Et4N+(-
PhO)2PO−2 ] (149.1) [23] and the difference (26.1) between Et4N+ and
n-Bu4N+ [17]. We should note that any tetraalkylammonium and per-
chlorate ions have no chemical interaction except for the Coulombic
force, that is, R4N+X− or Li+ClO4− is a strong electrolyte in higher per-
mittivity media such as MeCN. For instance, the association constants
of n-Bu4NClO4, LiClO4, and Et4N+(PhO)2PO−2 have been reported to be
Ka = 0, 13.6 [29], and ca. 4.0 [23], respectively.
The specific interaction between Li+ or Mg2 + and 5.0 ×
10− 4 mol dm− 3 L− in MeCN cannot be obviously detected from
the UV spectral changes, as described above. Without observing distinct
phenomena of precipitation and the successive re-dissolution, we can-
not insist that Li+ or Mg2+ keeps still the strong interaction with L−
.
However, conductometric titrations of 5.0
×
10−4 mol dm−3
tetrabutylammonium diphenylphosphate [n-Bu4N+ (PhO)2PO2−] with
LiClO4 and Mg(ClO4)2 have supplied us with the good evidence for the
interaction. The deviation point has been given at an equivalence
point for LiClO4 (5.0
× ) or for Mg(ClO4)2
10−4 mol dm−3
(2.5 × 10−4 mol dm−3), as shown in Fig. 11 (a) or Fig. 11 (b), respec-
tively, indicating that the chemical interaction operates actually be-
tween Li+ or Mg2+ and 5.0 × 10−4 mol dm−3 L−. It may be worth
mentioning again that the both systems give no precipitation indeed.
The solubility of non-charged species for Li+ and Mg2+ with L− must
be too high to form precipitates under those conditions.
It is worth mentioning once more that the balance (ca. 118) between
285 and 167.2, attributed to the molar conductivity of Li+(PhO)2PO2−
,
seems to be enough smaller than the sum value (130.6) of independent
Λ0 values of Li+ and (PhO)2PO−2 . Therefore, we can predict the “strong”
Fig. 11. (a). Conductometric titration curve of 5.0 × 10−4 mol dm−3 diphenylphosphate ion [n-Bu4N+(PhO)2PO2−] with increasing concentration of LiClO4 in MeCN. (b). Conductometric
titration curve of 5.0 × 10−4 mol dm−3 diphenylphosphate ion [n-Bu4N+(PhO)2PO−2 ] with increasing concentration of Mg(ClO4)2 in MeCN.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

8
X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Fig. 13. (a). Absorbance (λ = ca. 206 nm) of 5.0 × 10−4 mol dm−3 diphenylphosphate containing 5.0 × 10−4 mol dm−3 Et3N in the presence of Ba(ClO4)2 in MeCN–MeOH mixtures: (○) 0;
(●) 10; (△) 15; (▲) 20% (v/v) of MeOH. (b). Changes in absorbance (λ = ca. 206 nm) of 5.0 × 10−4 mol dm−3 diphenylphosphate containing 5.0 × 10−4 mol dm−3 Et3N in the presence of
Ba(ClO4)2 in MeCN–EtOH mixtures: (○) 0; (●) 5.0; (△) 10; (▲) 20; (□) 30; (■) 50% (v/v) of EtOH.
interaction between (PhO)2PO2− and Li+ in MeCN, even without observ-
ing precipitates. The observed specific conductance of 0.129 × 10−3
from the titration with Mg(ClO4)2 may also support the “strong” inter-
action between Mg2+ and L− under no precipitation conditions.
is obviously restrained but the re-dissolution is promoted apparently
by the added H2O. The (reverse) coordination constants (log K1) are
given as “pseudo” values to be 4.25, 3.95, and 3.08 in 1.0, 2.0, and 3.0%
H2O, respectively. These values may be valid just relatively. In 5.0%
H2O, the absorbance decrease (i.e., precipitation) occurs slightly at
2.5 × 10−4 and 5.0 × 10−4 mol dm−3 Ba2+. The 10% H2O addition
causes no apparent precipitation or re-dissolution reactions between
Ba2+ and L−.
3.2.2. The influences of added H2O, MeOH, and EtOH on the interaction be-
tween M+ or M2+ and the mixture of HL with Et3N
Fig. 12 shows the influences of added H2O on precipitation and the
successive re-dissolution reactions of BaL2 in MeCN. The precipitation
Table 5
Influences of the concentration on the 1:1 mixture of Et3N and diphenylphosphate (Et3N-HL)
on the precipitation and re-dissolution reactions for Ba(ClO4)2 in MeCN–MeOH.
MeCN–MeOH [MeOH% (v/v)]
Equilibrium
Mixed solution
constantsa
10
15
20
30
50
Et3N-HL
(5.0 × 10−4 mol dm−3
▲
▲
No
–
–
No
–
▲
8.97
9.68
○
)
(pKsp
(pKsp
)
)
12.74
12.96
○
(10.94)
(11.16)
○
(3.78)
▲
11.26
11.97
○
log K1
3.90
Et3N-HL
(5.0 × 10−3 mol dm−3
▲
No
–
–
No
–
)
(pKsp
(pKsp
)
)
8.52
9.24
○
Fig. 15. Changes in absorbance (λmax = ca. 290 nm) of 5.0 × 10−4 mol dm−3 bis(4-
nitrophenyl)phosphate containing 5.0 × 10−4 mol dm−3 Et3N with increasing of alkali
metal ions: (○) LiClO4; (●) NaClO4.
log K1
For the Explanatory notes, cf. Table 1.
Solubility products (Ksp) and “reverse” coordination constants (K1), cf. the Experimen-
tal section.
3.35
3.06
2.68
a
Fig. 14. UV spectra of 5.0 × 10−4 mol dm−3 bis(4-nitrophenyl)phosphate (0.1 cm path-
length) containing 5.0 × 10−4 mol dm−3 Et3N with increasing concentration of NaClO4
in MeCN.
Fig. 16. UV spectra of 5.0 × 10−4 mol dm−3 bis(4-nitrophenyl)phosphate (0.1 cm path-
length) containing 5.0 × 10−4 mol dm−3 Et3N with increasing concentration of Ba(ClO4)2
in MeCN.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
9
Scheme 4. The partial ionization of bis(4-nitrophenyl)phosphate by the addition of Et3N in MeCN.
the band at around 295 nm, it decreases gradually and reaches its min-
imum of 0.067 at 1.0 × 10−2 mol dm−3 NaClO4, accompanying white
precipitates (cf. Eq. (5)). Then, the band absorbance begins to increase
with the addition of more than 1.0 × 10−2 mol dm−3 Na+, and recovers
almost the original value at 1.0 mol dm−3 Na+ (cf. Eq. (6)). Meanwhile,
the band shows a remarkable blue shift to 287 nm, suggesting the for-
mation of a “reverse-coordinated” species of Na2L+. The evaluated
values of pKsp and log K2 between Na+ and L− in MeCN are 6.70 and
3.75, respectively. However, no apparent interaction is detected be-
tween Li+ and L− based on the almost constant absorbance values of
the Et3N-HL mixture (Fig. 15), except for the blue shifts with increasing
Li+ concentrations.
Scheme 5. Successive formation of BaL2 and BaL+ for the bis(4-nitrophenyl)phosphate ion
(L−) in MeCN.
Both MeOH (DNbulk = 31.3 [45]) and EtOH (DNbulk = 27.8 [45]) are
protic solvents of higher donicities. Not only MeOH but also EtOH may
inhibit the precipitation between Ba2+ and L−, and the influences
may increase in the order of EtOH b MeOH b H2O [cf. Fig. 13 (a) and
(b)]. With increasing contents of MeOH and EtOH, the precipitation re-
actions become incomplete. The slight precipitation occurs in 15%
MeOH or 30% EtOH, and the interaction is totally hidden by 20%
MeOH and 50% EtOH.
Et3N–HL þ Naþ ⇆ NaþL− þ Et3NHþ;
NaþL− þ Naþ ⇆ Na2Lþ:
ð5Þ
ð6Þ
Table 5 lists the equilibrium constants of the reactions between Ba2+
and the (1:1) Et3N-HL (5.0 × 10−3 and 5.0 × 10−3 mol dm−3) mixture
in MeCN–MeOH. Even in 15–30% MeOH, the precipitation and “reverse”
coordination can take place between 5.0 × 10−3 mol dm−3 Et3N-HL and
Ba2+ to a relatively large extent. The (reverse) coordination constants of
5.0 × 10−3 mol dm−3 Et3N-HL have been given as “pseudo” values to be
log K1 = 3.35, 3.06, and 2.68 in 15, 20, and 30% MeOH, respectively. The
above results indicate clearly that, by performing experiments with
higher concentrations (e.g., a ten-fold) of L−, we have still the chance
to obtain equilibrium constants even though no precipitation takes
place for a metal ion with a lower L− concentration. Table 5 may con-
vince us that the equilibrium values evaluated from different L−
(Et3N-HL) concentrations are well consistent.
The UV spectral changes of 5.0 × 10−4 mol dm−3 Et3N–HL, as the
function of Ba(ClO4)2 concentration, are shown in Fig. 16. The absorp-
tion band at around 290–295 nm almost disappears on the addition of
an equivalence of Ba2+ (2.5 × 10−4 mol dm−3), accompanying white
precipitates. The absorbance begins to increase with the further addi-
tion Ba2+, forming the (reverse) coordinated species of BaL+. The
“pseudo” values of pKsp and log K1 between Ba+ and L− in MeCN are ob-
tained to be 13.68 and 4.81. The partial ionization of HL and the coordi-
nation reaction are illustrated by Schemes 4 and 5. According to Fig. 17,
a
slight precipitation reaction may take place between bis(4-
nitrophenyl)phosphate and Mg2+ or Ca2+. Only Na+ and Ba2+ cause
observable interactions with L− through the remarkable precipitation
and re-dissolution reactions. However, we never believe that the
bis(4-nitrophenyl)phosphate ion cannot interact with Li+, Mg2+, or
3.3. Bis(4-nitrophenyl)phosphate (HL)
3.3.1. Specific interaction between M+ or M2+ and the mixture of HL with
Et3N
Ca2+ in MeCN. The solubilities of the non-charged species for Li+
,
Mg2+, and Ca2+ should be just too high to form precipitates in the sol-
vent. Needless to mention, the chemical interaction operates actually
between Li+, Mg2+, or Ca2+ and 5.0 × 10−4 mol dm−3 L−.
Fig. 14 shows the UV spectral changes of 5.0 × 10−4 mol dm−3
bis(4-nitrophenyl)phosphate (HL) containing an equivalent amount of
Et3N (5.0 × 10−4 mol dm−3), as the function of NaClO4 concentration.
The UV spectrum of Et3N-HL shows two distinct absorption bands at
around 218 nm and 295 nm (ε/cm−1 mol−1 dm3 = ca. 2.0 × 104).
With increasing concentration of NaClO4, if we may pay attention to
Fig. 18. Absorbance (λmax
= ca. 290 nm) of 5.0 ×
10−4 mol dm−3 bis(4-
Fig. 17. Changes in absorbance (λmax = ca. 290 nm) of 5.0 × 10−4 mol dm−3 bis(4-
nitrophenyl)phosphate containing 5.0 × 10−4 mol dm−3 Et3N with increasing of alkaline
earth metal ions in MeCN: (○) Mg(ClO4)2; (●) Ca(ClO4)2; (△) Ba(ClO4).
nitrophenyl)phosphate (0.1 cm path-length) containing 5.0 × 10−4 mol dm−3 Et3N in
the presence of NaClO4 in MeCN–H2O mixtures: (○) 0; (●) 1.0; (△) 2.0 (▲) 3.0% (v/v)
of H2O.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053

10
X. Chen et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
earth metal ions through the chemical (coordination) force above the
Coulombic force in poor solvating media. The results in the present
study may give an important clue to recognize the kinetic mechanism
in hydrolysis reactions under “non-aqueous solvent conditions” or in bi-
ological systems, based on the direct interaction between “indifferent”
metal ions and the anion species released from substrates.
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Ba(ClO4)2 in MeCN–H2O mixtures: (○) 0; (●) 0.50; (△) 1.0 (▲) 2.0% (v/v) of H2O.
3.3.2. The influences of added H2O and MeOH on the interaction between
Na+ or Ba2+ and the mixture of HL with Et3N
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Fig. 18 shows the influences of added H2O on the precipitation and
the successive re-dissolution of NaL. With increasing contents of H2O,
the solubility of NaL increases distinctly. The pKsp values are 6.70, 5.80,
and 4.63 in 0.0, 1.0, and 2.0% H2O, respectively. In 3.0% H2O, no precipi-
tation reaction is observed. Similarly, no apparent interaction can be de-
tected also in 10% MeOH.
Fig. 19 shows the influences of added H2O on the precipitation and
the successive re-dissolution of BaL2. The added 0.5% H2O affects obvi-
ously both precipitation and re-dissolution between Ba2+ and the
anion, despite the absorbance minimum appears at an equivalence of
Ba2+. Slight precipitation takes place in 1.0% H2O. The “pseudo” solubil-
ity products (pKsp) are 13.68, 12.36 and 10.72 in 0.0, 0.50, and 1.0% H2O,
respectively. The interaction is totally obscured by 2.0% H2O. Compared
with water, MeOH affects weakly the precipitation and re-dissolution.
The interaction is totally hidden by 5.0% MeOH. The remarkable influ-
ences on Ba2+ of low contents of protic solvents should be caused by
the stronger solvation toward the bis(4-nitrophenyl)phosphate ion.
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4. Conclusion
The coordination ability of alkali metal (M+) and alkaline earth
metal (M2+) ions with diphenylphosphinate, diphenylphosphate, and
bis(4-nitrophenyl)phosphate ions has been confirmed not only in sole
MeCN but also in the binary solvents with H2O, MeOH, and EtOH. The
precipitation and the successive re-dissolution reactions have been ob-
served between every M+ or M2+ and 1.0 × 10−3 mol dm−3
diphenylphosphinate. The “reverse coordination” or coordination con-
stants suggest that the chemical interaction should increase in the
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order of Na+
b
Li+ and Ba2+
b
Ca2+
b
Mg2+
.
With
5.0 × 10−4 mol dm−3 diphenylphosphate, however, only Ca2+ and
Ba2+ (the strength order of Ba2+ b Ca2+) can cause the obvious precip-
itation in MeCN. Nevertheless, we are confident that all the phosphinate
and phosphonate ions should interact with the alkali metal and alkaline
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Plenum, London, 1973.
Please cite this article as: X. Chen, et al., Complexing ability of alkali metal and alkaline earth metal ions with organic phosphinate or phosphates in
acetonitrile and binary..., J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.11.053