Studies on complexation of alkaline earth metal cations with acyclic polyether dicarboxylic acids

Article abstract of DOI:10.1021/je980058h

Acyclic polyether dicarboxylic acids having butyl and tetradecyl side alkyl chains at the α-position of the carboxylic acid sidearm unit were prepared by the reaction of bisphenol and 2-bromoalkanoic acid in the presence of sodium hydride with quantitativ

Full text of DOI:10.1021/je980058h

1072
J . Chem. Eng. Data 1998, 43, 1072-1075
Stu d ies on Com p lexa tion of Alk a lin e Ea r th Meta l Ca tion s w ith
Acyclic P olyeth er Dica r boxylic Acid s
J on g Seu n g Kim *
Department of Chemistry, Konyang University, Nonsan 320-711, Korea
Sa n g Ch u l Lee, Eu n Ta e Kim , J a e Hoon Ch o, a n d Moon Hw a n Ch o
Department of Chemistry, Kangwon National University, Chuncheon 200-701, Korea
J ou n g Ha e Lee
Korea Research Institute of Standards and Science, Taejon 305-606, Korea
Acyclic polyether dicarboxylic acids having butyl and tetradecyl side alkyl chains at the R-position of the
carboxylic acid sidearm unit were prepared by the reaction of bisphenol and 2-bromoalkanoic acid in the
presence of sodium hydride with quantitative yields. Complexation studies and solvent extraction on
complexes of these ethers with alkaline earth metal ions indicate that 1,2-bis((2-2′-carboxybutyloxy)-
phenoxy)ethane (2) shows an excellent selectivity for the calcium ion.
In tr od u ction
pellets and on a deposited KBr window in the case of
solid product and oil. 1H NMR spectra were recorded with
a 400 MHz (Bruker ARX-400) spectrometer, and the
chemical shifts (d) were reported downfield from the
internal standard, tetramethylsilane. Unless specified
otherwise, reagent grade reactants and solvents were
obtained from chemical suppliers and used as received.
Tetrahydrofuran was freshly distilled from sodium metal
ribbon or chunks. Dichloromethane was freshly distilled
from lithium aluminum hydride. A series of bisphenols as
starting materials were prepared by the method reported
by Heo (1983).
General preparation methods for acyclic polyether di-
carboxylic acids were as follows. After removal of the
protecting mineral oil from NaH (50% dispersion in mineral
oil, 6.00 g, 0.34 mol) by washing with pentane under
nitrogen, a solution of bisphenol (10.0 g, 34.4 mmol) in 150
mL of dry tetrahydrofuran (THF) was added. The mixture
was stirred for 2 h at room temperature. 2-Bromoalkanoic
acid (86.2 mmol) in 20 mL of dry THF was added dropwise
at room temperature during a period of 2 h. Upon the
complete addition, the reaction mixture was stirred for an
additional 10 h at room temperature. After careful addi-
tion of water to the reaction mixture in an ice bath to
destroy the unreacted excess NaH, which gave a homoge-
neous solution, THF was removed in vacuo leaving an
aqueous mixture. Into this basic solution was poured 100
mL of ethyl acetate to extract the unreacted bisphenol and
organic impurities. The aqueous layer was washed with
ethyl acetate (2 × 50 mL). Upon the acidification with
concentrated HCl to pH 1, crude product was extracted
with methylene chloride (3 × 50 mL) and dried over
MgSO4. Removal of methylene chloride in vacuo provided
a colorless oil, which was recrystallized from 100 mL of
diethyl ether to give white crystals in 90% yield.
A number of macrocyclic polyethers have been synthe-
sized and employed for the separation of alkali, alkaline
earth, and transition metal ions as a result of their
complexation abilities (Cram and Trueblood, 1985; Dietrich
et al., 1993; Inoue and Gokel, 1990). Complexation of a
metal ion with polyethers that bear side alkyl chains has
been studied (Vo¨gtle and Weber, 1979). Several of these
studies have focused on the determination of the selectivity
and efficiency of podand-mediated extraction of metal ions
from an aqueous phase into an organic medium or trans-
port of metal ions through the organic media into an
aqueous phase (Strzelbicki and Bartsch, 1981). The Com-
plexation ability was markedly enhanced by the introduc-
tion of proton-ionizable groups such as carboxylic, phos-
phonic, and sulfonic acids onto the crown ether backbone
(Bartsch et al., 1982, 1992). Metal ion extraction by such
chelating agents bearing proton-ionizable groups does not
involve concomitant transfer of one or more aqueous phase
anions into the organic medium. This factor was found to
be of immense importance to potential applications in
which hard anions such as chloride, nitrate, and sulfate
would be involved.
With this in mind, to provide greater insight into the
factors that control the selectivity and efficiency of alkaline
earth cation extraction by a lipophilic acyclic polyether
dicarboxylic acid, synthetic ligands (1-9) have been pre-
pared. On the basis of two-phase solvent extractions and
potentiometric titrations, we report the results of calcium
selective complexation.
Exp er im en ta l Section
Syn th eses. Melting points were measured with a Mel-
Temp feature of a Fisher-J ohns melting point apparatus
without any correction. IR spectra were obtained with a
Perkin-Elmer 1600 Series FT-IR using potassium bromide
P oten tiom etr ic Titr a tion . The potentiometer was an
ORION model 701A digital ion meter, which could be read
10.1021/je980058h CCC: $15.00 © 1998 American Chemical Society
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J ournal of Chemical and Engineering Data, Vol. 43, No. 6, 1998 1073
Sch em e 1. Syn th etic Rou te for P r ep a r a tion
Acyclic P olyeth er Dica r boxylic Acid s (1-9)
solid. Their melting points, 1H NMR spectra, and IR
spectra are listed in Table 1.
P oten tiom etr ic Titr a tion . Protonation constants and
stability constants of synthesized organic ligands in this
study were determined in 90 vol % methanol. Table 2
shows the first (log âH L) and second protonation constant
2
(log âH L), respectively, for each ligand. Upon the elonga-
2
tion of the ethylene glycol unit connected between the two
benzene rings, both protonation constants gradually in-
creased. Stability constants for the complexation of alka-
line earth metal ions were measured and are listed in Table
3. For most metal ions, the order of stability constant is
Ca2+ > Sr2+ Z Ba2+ . Mg2+. As the number of ethylene
glycoloxy units decreases, the stability constants toward
the calcium ion gradually decrease. It was reported that
when a ligating agent complexes with a specific metal ion,
major factors that influence the stability constant are
radius of metal ion, oxidation state of metal ion, species of
donor atoms, solvation effect, anion effect, and dipolar
interaction between the organic ligand and metal ion
(Vo¨gtle and Weber, 1985). So, the calcium selectivity
indicates that when the pseudo-cyclic conformation of the
organic ligand is carried out, the length of the monoeth-
ylene glycol unit is sufficient to fit with the calcium ion to
give a maximized ion-dipolar interaction and electrostatic
interaction. The stability constants based on both ef-
ficiency and selectivity toward an alkaline earth cation are
influenced by the introduction of a lipophilic group, C4H9.
The lipophilic ligand shows better conformation when a
metal ion is introduced. This is probably attributed to a
micelle effect with which the cyclic formation takes place
much more easily. From the distribution diagram, L2- was
observed to exist at over pH 6, indicating two hydrogens
are completely dissociated. The distribution species of
metal ion (M), complex (LM), and LMH-1 (MLOH), which
combines with the OH anion, give information on 1:1
maximum complexation in the pH range 4-6. At pH over
10, basic conditions, most of the metal complexes were
observed to exist as MLOH.
to 0.001 pH unit. The glass combination electrode was an
ORION Ross Electrode model 81-02. The filling solution
was made of 3 M TMACl in 90 vol % methanol (90 mL
MeOH/100 mL solution). The measurement was under-
taken at (25 ( 0.1) °C under a nitrogen atmosphere. The
pH meter was standardized against oxalate buffer solutions
before titration. Oxalate buffer solution (pH 3.73) were
made by ammonium oxalate monohydrate and oxalic acid
in 90 vol % methanol. Succinate buffer solutions (pH 6.73)
were made from succinic acid and sodium succinate salt
in 90 vol % methanol. Tetramethylammonium chloride (0.1
M) was used to adjust the ionic strength. A glass combina-
tion electrode, a microburet with 5.0 mL capacity, and 50
mL of 1 mM ligand in 90 vol % methanol were introduced
into a thermostated titration vessel equipped with a
magnetic stirring bar. The concentration of titrant, tet-
ramethylammonium hydroxide, was 20.3 mM. Titrations
were conducted in triplicate. Protonation and stability
constants were calculated by the program BEST (Martell,
1992).
Extr a ction P r oced u r es. Solvent extraction was fol-
lowed from the procedures reported by Bartsch et al. (1992).
Advantages of this method that it is known to speed the
experiment and it uses a small amount of organic ligand.
For competitive extractions of alkaline earth metal chloride
solution, a microextraction technique was used. An aque-
ous solution of alkaline earth metal chloride (2.00 mL,
0.125 M with 0.2 M cesium hydroxide for pH adjustment)
and 2.00 mL of 0.01 M organic ligand in chloroform in a
10 mL centrifuge tube were mixed by vortex mixer for 5
min. After centrifuging, the equilibrium pH of the upper
aqueous layer was measured. Then a 1.00 mL sample of
the chloroform layer was taken and stripped with 1.00 mL
of 0.1 M HCl solution. The concentration of metal ion in
the aqueous phase was determined by an atomic absorption
spectrometer.
Two-P h a se Extr a ction . To further prove the influence
of the lipophilicity and the polyether length of the organic
ligand on metal ion complexation, solvent extraction ex-
periments with all ligands except 1, 4, and 7 (see Table 4)
for alkaline earth metal ions were carried out. Solvent
extraction using 1, 4, and 7, which do not contain lipophilic
side chains, could not be carried out with this experiment
because of their low solubility in chloroform. A profile for
stripped metal ion concentrations by HCl aqueous solution
after extracted with the organic ligand 2 which performs
the best complexation behavior among nine organic ligands
in chloroform, is shown in Figure 1. The extracted metal
ion concentration was observed to increase with pH in-
crease. Calcium ion was selectively extracted at about 40-
50% from the source phase. Table 4 shows the selectivity
order of extracted metal ions as Ca2+ > Sr2+ Z Ba2+
.
Mg2+, indicating the same propensity of calcium ion
selectivity over other alkaline earth metal ions as observed
from potentiometric titration experiment. Hence the ex-
traction ability of an organic ligand is strongly relate to
that of the stability constant for complexation. The total
loaded amount of metal ions is over 90% in all cases.
Especially, ligand 2 shows the best extracting behavior to
give 99.4 of loading percentage. In Table 4, the selectivity
of calcium ion for 2, 5, and 8 in which butyl groups are
branched was observed to be somewhat better than that
for 3, 6, and 9 with tetradecyl group substituted. Walkow-
iak et al. (1992) have reported that when the pendent side
Resu lts a n d Discu ssion
Syn th eses. The synthetic route for the preparation of
acyclic polyether dicarboxylic acids is shown in Scheme 1.
Bisphenol was used as a starting material, which can be
prepared from the reaction of catechol with bis(2-chloro-
ethyl) ether in NaOH solution. Reaction of bisphenol with
2.5 equiv of 2-bromoalkanoic acid in the presence of 5 mol
of sodium hydride as a base provided the desired products
in over 90% yield. Attempted reactions with potassium
hydride instead of sodium hydride were found to be less
effective. Recrystallization from diethyl ether gave a white

1074 J ournal of Chemical and Engineering Data, Vol. 43, No. 6, 1998
Ta ble 1. Yield s a n d Sp ectr a l Da ta for Acyclic P olyeth er Dica r boxylic Acid s 1-9 Dr a w n in Sch em e 1
compd yield, %
Tm, °C
1H NMR spectra (CDCl3), ppm
IR spectra, cm-1
1
2
91
92
165-168 4.33 (s, 4 H), 4.65 (s, 4 H), 6.71-7.10 (m, 8 H)
127-130 0.86 (t, 6 H), 1.10-1.60 (m, 8 H), 2.80-2.11 (M, 4 H),
4.36 (s, 4 H), 4.57 (t, 2 H), 6.91 (s, 8 H)
3450 (O-H), 1710 (CdO), 1107 (C-O)
3455 (O-H), 1711 (CdO)
3
4
5
6
95
92
90
93
108-110 0.77-1.00 (t, 6H), 1.10-2.00 (br s, 48H), 2.12-2.40 (m, 4 H),
3445 (O-H), 1713 (CdO)
4.36 (s, 4H), 4.57 (t, 2H), 6.87-7.07 (m, 8H)
140-141 3.92-3.98 (m, 4 H), 4.21-4.24 (m, 4 H), 4.62 (s, 4 H),
6.93-6.99 (m, 8 H)
158-162 0.90 (t, 6 H), 1.20-1.80 (m, 8 H), 1.80-2.10 (m. 4 H),
3.80-4.20 (m, 8 H), 4.55 (t, 2 H), 6.94 (s, 8 H)
3455 (O-H), 1713 (CdO)
3389 (O-H), 1703 (CdO)
92-93
0.77 (t, 6 H), 1.10-1.77 (m, 48 H), 2.10-2.40 (m, 4 H),
3.96 (m, 4 H), 4.15-4.23 (m, 4 H), 4.47 (t, 2 H),
6.87-7.07 (m, 8 H)
3450 (O-H), 1710 (CdO), 1110 (C-O)
7
8
9
90
91
90
69-71
97-99
72-73
3.75 (s, 4 H), 3.85 (s, 4 H), 4.14 (s, 4 H), 4.59 (s, 4 H),
6.88-6.95 (m, 8 H)
3427 (O-H), 1718 (CdO), 1107 (C-O)
0.93 (t, 6 H), 1.20-1.71 (m, 8 H), 1.77-1.88 (m, 4 H), 3.57-4.50 3430(O-H), 1715 (CdO), 1106 (C-O)
(m, 14 H), 6.85-7.12 (m, 8 H)
0.79 (t, 6 H), 1.11-1.90 (m, 48 H), 2.10-2.40 (m, 4 H),
3.96 (m, 4 H), 4.15-4.23 (m, 4 H), 4.30-4.41 (m, 4 H),
4.47 (t, 2 H), 6.87-7.07 (m, 8 H)
3429 (O-H), 1710 (CdO), 1107 (C-O)
Ta ble 2. P r oton a tion Con sta n ts for Acyclic P olyeth er
Dica r boxylic Acid s in 90 vol % Meth a n ol a t 25 °C a n d µ )
0.1 M
Ta ble 4. Selectivity a n d Efficien cy of Alk a lin e Ea r th
Meta l Ca tion s fr om Aqu eou s Solu tion in to Ch lor ofor m by
Acyclic P olyeth er Dica r boxylic Acid s
a
b
ligand
log âHL
log âH
selectivity order and
2L
ligands
selectivity coefficientsa
total loading %b
99.4
1
2
4
5
7
8
4.51 ( 0.01
4.92 ( 0.03
4.63 ( 0.01
5.63 ( 0.02
5.68 ( 0.02
5.98 ( 0.01
8.19 ( 0.02
8.48 ( 0.02
8.13 ( 0.02
9.34 ( 0.01
9.23 ( 0.04
9.60 ( 0.04
Ca2+ > Sr2+ > Ba2+ > Mg2+
2
1.5
2.1
14.2
Ca2+ > Sr2+ > Ba2+ > Mg2+
3
5
91.9
94.0
1.3 2.3 10.4
Ca2+ > Ba2+ > Sr2+ > Mg2+
1.5 3.9 7.6
log âHL is for the equilibrium L2- + H+ T HL-. log âH is
a
b
2L
for the equilibrium HL- + H+ T H2L.
Ca2+ > Ba2+ > Sr2+ > Mg2+
1.8 2.9 5.1
6
90.8
Ta ble 3. Sta bility Con sta n ts for Acyclic P olyeth er
Dica r boxylic Acid s w ith Alk a lin e Ea r th Meta l Ca tion s in
90 vol % Meth a n ol a t 25 °C a n d µ ) 0.1 M
Ca2+ > Ba2+ > Sr2+ > Mg2+
1.2 3.1 7.7
Ca2+ > Ba2+ > Sr2+ > Mg2+
1.4 2.3 5.9
8
90.0
87.2
b
b
log KML
9
ligand
Mg2+
Ca2+
Sr2+
Ba2+
1
2
4
5
7
8
1.27 ( 0.10 6.63 ( 0.03 5.27 ( 0.01 4.95 ( 0.02
1.08 ( 0.12 7.86 ( 0.08 6.28 ( 0.05 5.35 ( 0.04
a
Mole ratio of Ca2+/other alkaline earth metal cations. (total
Ra
R
6.07 ( 0.03 4.45 ( 0.02 5.52 ( 0.02
6.80 ( 0.05 4.35 ( 0.03 5.54 ( 0.03
6.04 ( 0.02 4.72 ( 0.01 5.94 ( 0.02
6.00 ( 0.04 5.19 ( 0.02 5.85 ( 0.04
metal ion concentration loaded)/(ligand concentration) × 100 (%).
R
R
a
b
R: too small for calculation. log KML is for the equilibrium
M2+ + L2- T ML.
alkyl chain is too long like the C14H29 tail, the ligand
reveals large lipophilicity to give a poor complexation
ability due to entropic disfavor. Therefore, with this point,
the lipophilicity with two butyl group in this study could
be sufficiently enough for metal ion complexation in organic
solution.
In conclusion, acyclic polyethers having a dicarboxylic
end group able to carry out efficient transport of alkaline
earth metal ion were successfully synthesized. Complex-
ation studies and solvent extraction on complexes of these
ethers with alkaline earth metal ions indicate that the
ligand 2 show an excellent selectivity for the calcium ion.
The high calcium ion selectivity and efficiency appear to
arise from a unique combination of lipophilic group,
number of polyether oxygens, and carboxylic end groups.
To further investigate the influence of donor atom species
and number of donor atoms on complexation, syntheses of
acyclic polyethers containing nitrogen atom as a donor and
complexation studies are in progress, and the results will
be reported.
F igu r e 1. Profiles for molar concentrations of extracted metal
ions using ligand 2.
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Received for review March 3, 1998. Accepted September 2, 1998.
This study was supported by the academic research fund of
Ministry of Education, Republic of Korea (BSRI-96-3440).
J E980058H