Sulfur-bridged oligo(benzoic acid)s as a novel family of metal extractants

Article abstract of DOI:10.1246/cl.2008.1228

Sulfur-bridged phenol dimer and tetramer were converted into corresponding oligo(benzoic acid)s, which have high complexation ability toward lanthanide ions as revealed by solvent extraction study. Copyright

Full text of DOI:10.1246/cl.2008.1228

1228
Chemistry Letters Vol.37, No.12 (2008)
Sulfur-bridged Oligo(benzoic acid)s as a Novel Family of Metal Extractants
Naoya Morohashi,ꢀ1 Kazutoshi Nagata,1 Shinya Tanaka,1 Yoshihiro Ohba,2 and Tetsutaro Hattoriꢀ1
1Department of Environmental Studies, Graduate School of Environmental Studies,
Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579
2Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering,
Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510
(Received August 26, 2008; CL-080816; E-mail: morohashi@orgsynth.che.tohoku.ac.jp)
Sulfur-bridged phenol dimer and tetramer were converted
we have already reported the preparation of the P–S–P ligand 52
from the phenol dimer 42 (n ¼ 2) and its unique coordination
modes to the palladium(II) ion that were caused by the cooper-
ative coordination of phosphorous and sulfur atoms to the metal
center.6 Therefore, as new ligands, we designed a sulfur-bridged
benzoic acid tetramer 64, in which the hydroxy groups of 44 are
directly replaced with carboxy groups, and its oxidized deriva-
tive 74. Herein, we report the synthesis of sulfur-bridged oligo-
(benzoic acid)s 62, 64, and 74 and the investigation of their coor-
dination ability toward metal ions by a solvent extraction study.
The sulfur-bridged benzoic acid dimer 62 and tetramer 64
were prepared from the phenol dimer 42 and tetramer 44,7 re-
spectively, via the palladium-catalyzed carboxylation of the cor-
responding triflate followed by hydrolysis.8,9 Furthermore, the
oxidation of 64 by NaBO3 yielded the corresponding sulfonyl
compound 74.9
into corresponding oligo(benzoic acid)s, which have high com-
plexation ability toward lanthanide ions as revealed by solvent
extraction study.
Many studies have been conducted on chelating ligands es-
pecially in the field of solvent extraction of metal ions for the
purpose of separation, concentration, and recovery of the metals
because of increasing demand for use as functional materials.1
In a solvent extraction experiment,2 it has been observed
that thiacalix[4]arene 1 and its oxidized derivatives 2 and 3
(Chart 1) exhibit high extractability toward metal ions, which
is attributed to the coordination of the sulfur functions to a metal
center in cooperation with phenoxy oxygens, as revealed by X-
ray structural analysis.3 Furthermore, the extraction selectivity
of metal ions can be controlled by the oxidation state of the
bridging sulfur. From the view point of application of 1–3 as
extractants, however, the following disadvantages were noted:
(1) they can extract metal ions only in a relatively high pH region
and (2) their extraction rate is low.2a We assumed that the direct
replacement of the hydroxy groups of 1–3 with carboxy groups
would yield a new extractant that can rapidly extract various
metal ions even in a low pH region owing to the high acidity
and high affinity of carboxy groups toward metal ions. However,
the direct replacement of the hydroxy groups with other func-
tional groups by conventional C–O bond cleavage of aryl tri-
flates or other esters using transition-metal catalysts is rather
difficult, particularly in the case of calixarenes with small ring
size because of the steric hindrance caused by the sterically
crowded cyclic structure.4
Fortunately, a single crystal of 74 that was suitable for an X-
ray structural study was obtained by the vapor diffusion of hex-
ane to a CH2ClCH2Cl solution of 74 (Figure 1).9 It should be not-
ed that 74 exhibits a pseudocyclic structure, in which two car-
boxy groups of the terminal benzoic acid form intramolecular
hydrogen bondings, as indicated by the geometry and the
˚
interatomic distance between O1–O8 (2.630 A) and O2–O7
˚
(2.678 A), respectively. Furthermore, two carboxy groups of
the inner benzoic acids form intermolecular hydrogen bondings
with the sulfonyl oxygen of a neighboring molecule to form a
3-D network structure (Figure S5).10
A solvent extraction study was performed to evaluate the
coordination ability of oligo(benzoic acid)s toward metal ions.9
To a 30-cm3 vial tube were pipetted an aqueous solution
(10 cm3) containing metal ion (½Metalꢁaq,init ¼ 1:0 ꢂ 10ꢃ4 M),
Me4NCl (0.1 M) as well as a pH buffer (0.05 M) and a 10 cm3
of 4-methyl-2-pentanone or CHCl3 solution ([64 and 74] =
5:0 ꢂ 10ꢃ4 M, [62] = 1:0 ꢂ 10ꢃ3 M) and then shaken at 300
strokes/min for 1 h at ambient temperature (ca. 20 ꢄC). The per-
cent extraction, E%, values of various metal ions by 62, 64, and
74 were calculated by the following equation.
On the other hand, we recently reported that a linear tetramer
44 (n ¼ 4) exhibits high extractability toward soft metal ions that
is almost equal to that of 1.5 Furthermore, it was expected that
the replacement of hydroxy groups of 44 should be possible ow-
ing to the reduced steric hindrance compared to that of 1. In fact,
OH
OH
OH
PPh2
PPh2
H
S
S
1: X = S
2: X = SO
3: X = SO2
X
4
n-1
But
But
But
But
R
R = tBu and tOct
4n
52
CO2H CO2H
CO2H
CO2H
CO2H
CO2H
S
X
X
X
But
But
But
But
6
7
But
4 : X = S
4 : X = SO2
But
62
Figure 1. X-ray structure of compound 74. Hydrogen atoms
and solvents are omitted for clarity.
Chart 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.12 (2008)
1229
a
Table 1. The E% values of metal ions by 62, 64, and 74
that 74 has a higher extractability toward lanthanide ions than
that of sulfonylcalix[4]arene 3, which has a high affinity toward
hard metal ions. Furthermore, the time profile of E% was studied
(Figure S2).10 The extraction of Ho3þ by 64 and 74 attains equi-
librium within 5 min, indicating that these benzoic acid tetramers
can achieve efficient extraction within a short time. On the other
hand, the extractabilities of 64 and 74 toward other lanthanide
ions were examined (Figures S3 and S4),10 and the pH depend-
ence of E% toward several lanthanide ions was observed to be
similar, indicating that it is difficult to separate lanthanide ions
according to their ion size by using 64 and 74 at this stage.
In conclusion, sulfur-bridged oligo(benzoic acid)s were
easily prepared from their corresponding phenol dimer and tetra-
mer. X-ray crystallography revealed that 74 exhibits an interest-
ing pseudocyclic structure stabilized by the intramoleculer hy-
dorogen bonding of carboxy groups. The solvent extraction
study revealed that the synthesized oligomers could act as effi-
cient extractants for lanthanide ions.
Ligands
Metal
pHb
62
64
74
Agþ
6.0
7.0
8.0
10.0
5.0
10
51
16
0
16
89
12
0
5
76
0
37
34
86
88
100
99
Cu2þ
Ni2þ
Mg2þ
Ho3þ
5
5
5.5
6.0
7.0
3.0c
5
17
80
99
34
99
d
d
—
—
a4-Methyl-2-pentanone was used as organic phase. bpH val-
ues before extraction. CHCl3 was used as organic phase.
c
dNot examined owing to low solubility of 62 and 64 in
CHCl3.
E% ¼ ð½Metalꢁaq,init ꢃ ½MetalꢁaqÞ=½Metalꢁaq,init ꢂ 100%
where [Metal]aq are the concentrations of the metal ions in an
aqueous phase after extraction.
The authors wish to thank Prof. M. Yamashita and Dr. T.
Kajiwara of Tohoku University, for their kind assistance in
X-ray analysis.
Table 1 lists the E% values of the metal ions at a represen-
tative pH. Contrary to expectations, oligo(benzoic acid)s 62, 64,
and 74 exhibited a moderate E% value for only Cu2þ among soft
to intermediate metal ions (Agþ, Cu2þ, and Ni2þ).11 These re-
sults indicate that the extractability of oligo(benzoic acid)s 62,
64, and 74 toward soft to intermediate metal ions is lower than
that of 1 and 44. It has been already revealed that the high com-
plexation ability of 1 and 44 toward soft to intermediate metal
ions results from the ligation of the bridging sulfur as a soft
doner with the cooperative binding of the adjacent phenolic oxy-
gens, as shown in Figure S1. On the contrary, it is assumed that
the cooperative coordination of the carbonyl oxygen and the
bridging sulfur of oligo(benzoic acid)s toward the metal center
may be difficult because of the long distance between the two
atoms, causing their low extractability toward soft to intermedi-
ate metal ions.
On the other hand, Mg2þ, categorized as a hard metal ion,
could be extracted only by 74. We noted an important feature
of oligo(benzoic acid)s in that they exhibited high extractability
toward lanthanide ions, e.g., Ho3þ, which were hardly extracted
by 1.2b Furthermore, pH values, above which 62, 64, and 74 ex-
hibited high extractabilies, at pH values above 7.0, 6.0, and 5.5,
respectively. The mechanism of this extraction was suggested to
be chelate extraction via the release of the protons of carboxy
groups owing to the dependence of E% on the aqueous pH. In
chelate extraction, extraction at a lower pH implies high extract-
ability. Therefore, the extractability of oligo(benzoic acid)s to-
ward lanthanide metal ions follows the order 74 > 64 > 62, indi-
cating that the number of benzoic acid molecules and the oxida-
tion state of the bridging sulfur play a critical role in the extrac-
tion. The improvement in the extractability toward lanthanide
ions due to oxidation of the bridging sulfur of 64 is probably at-
tributable to the increase in the acidity of benzoic acid due to the
electron-withdrawing effect of the sulfonyl function and/or the
coordination of the sulfonyl oxygen to the metal center as a hard
donor as shown in Figure S1.10 It should be noted that Ho3þ could
be quantitatively extracted by 74 even at pH 3.0, when CHCl3
was used as organic phase (Table 1 and Figure S4). This indicates
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9
Detailed synthetic methods, spectral data, conditions of sol-
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Published on the web (Advance View) November 1, 2008; doi:10.1246/cl.2008.1228