Nonfluorous, highly CO2-soluble chelating ligands for scCO 2 metal ion extraction

Article abstract of DOI:10.1246/cl.2006.1000

A series of new nonfluorous chelating ligands derived from glucose have been designed and synthesized. All compounds are highly soluble in liquid or supercritical carbon dioxide. Solubility trend of these chelating compounds would be rationalized by the i

Full text of DOI:10.1246/cl.2006.1000

1000
Chemistry Letters Vol.35, No.9 (2006)
Nonfluorous, Highly CO2-soluble Chelating Ligands for scCO2 Metal Ion Extraction
Hai-Jian Yang
Key Laboratory for Analytical Chemistry of the State Ethnic Affairs Commission,
College of Chemistry and Materials Science, South-Central University for Nationalities,
Wuhan 430074, P. R. China
(Received June 29, 2006; CL-060731)
A series of new nonfluorous chelating ligands derived from
glucose have been designed and synthesized. All compounds are
highly soluble in liquid or supercritical carbon dioxide. Solubil-
ity trend of these chelating compounds would be rationalized
by the inductive effect of substituent on the benzene ring and
the easy accessibility of benzylic ether oxygen and neighboring
hydrogens. Preliminary metal ion extraction efficiency test
showed that all our compounds are selective for Sr2þ and Pb2þ
extraction.
O
R
O
Br
NaH
THF
O
O
n-Bu4NI
O
HO
1
2a-2f
O
O
OH
O
O
HO
66% aq. AcOH
rt.
R
R
O
O
O
O
O
O
3a-3f
4a-4f
O
O
O
O
Supercritical fluids are increasingly used as green media for
extraction. By far the most commonly used fluid is CO2 because
it is inexpensive, nontoxic, environmentally benign, and readily
available in relatively pure form and has moderate critical con-
stants (Tc ¼ 304:2 K; Pc ¼ 72:8 bar, ꢀc ¼ 0:45 g cmꢀ3).1–3 Al-
though supercritical carbon dioxide (scCO2) has been widely
used for the supercritical fluid extraction of organic compounds,
especially nonpolar compounds, it has difficulty in direct
extraction of metal ions because of a charge neutralization re-
quirement and weak solvent–solute interactions.4,5 To overcome
this drawback, many researchers have examined the use of met-
O
2
O
DMAP
O
O
O
R
CH2Cl2, rt.
O
O
O
5a-5f
Compounds
R
5a
5b
5c
5d
5e
F
5f
CH3
OCF3
t-Bu
H
Br
Scheme 1. Synthesis of new chelating ligands.
al-chelating ligands, surfactants, or microemulsions.6 Some suc-
9,10
cess in extracting metal ions such as Kþ, Naþ, Csþ,7,8 Cu2þ
,
Ni2þ
,
11 Zn2þ, Cd2þ, Pb2þ, Hg2þ, Sr2þ, Co2þ, Cr3þ, Au3þ, Bi3þ
,
140
Fe3þ, Sb3þ, V5þ, Mo3þ
,
As3þ
,
Ga3þ, and lantha-
12–14
15,16
nides,17,18 has been found using ligands (ꢁ-diketones, dithiocar-
bamates, organic phosphorus acids, crown ether, etc.) via in situ
chelation-SFE (supercritical fluid extraction) method. Unfortu-
nately, in most cases, the extraction efficiency is not always
good. Therefore, searching new, highly efficient chelating
ligands is still a challenge.19–21
The mostly used metal-binding ligand is ꢁ-diketone re-
agents because it is readily available and somewhat CO2-phi-
lic.19 In the present work, we have designed and synthesized a
series of new CO2-philic glucose derivatives containing bis-ꢁ-
diketone-like functional group, that is bis acetoacetyl group,
via simple procedure with high yield. We also report their solu-
bilities and metal ion extraction efficiencies in liquid and super-
critical CO2 (99.99% purity).
130
120
110
100
90
5a
5b
5c
5d
5e
5f
3
4
5
6
7
8
9
10 11 12 13 14 15
Chelating Ligands Concentration (x 10-3 mol/L)
Figure 1. Solubilities of 5a–5f (40 ꢁC).
The new chelating ligands were synthesized according to the
reaction sequences shown in Scheme 1. All the compounds are
new except compounds 3c and 4c which were prepared by refer-
ence method,22 and their structures were well confirmed by IR,
1H NMR, 13C NMR, and elementary analysis.
For observation of dissolution of compounds in scCO2, a
high-pressure view cell consisted of a stainless-steel block with
two sapphire windows were used. It has an initial volume of 3.61
mL and with a screw tuning the volume to test the cloud point at
different volume and pressure. The compounds in the cell were
stirred by a magnetic stirring bar. The temperature was control-
led using a temperature controller jacket with circulator.
Dissolution conditions of chelating ligands were determined
by visual observations using the high-pressure view cell. The
solubility results were shown on Figure 1 and the solubility se-
quence was observed as 5c < 5a < 5b < 5f < 5d < 5e. Even
though now we can not rationalize this solubility trend fully,
at least we can conclude that the compound with electron-with-
drawing substituent on benzene ring has better solubility than the
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.9 (2006)
1001
Table 1. Metal ions extraction efficiency without additive
PFOATa
In summary, we designed and synthesized a series of non-
fluorous novel chelating reagents for supercritical carbon diox-
ide metal extraction. All the newly synthesized ether-containing
glucoses with two acetoacetyl groups are highly soluble in liquid
or supercritical carbon dioxide. It appears that the ether oxygen
and the neighboring benzylic hydrogens will enhance solubility
of glucose-derived chelating compounds through Lewis acid–
base interaction with CO2.3 Solubility trend of our synthetic
chelating compounds would be rationalized by the electronic ef-
fect of substituent on the benzene ring and the easy accessibility
of benzylic ether oxygen and neighboring hydrogens. Our glu-
cose-derived chelating compounds exhibit relative CO2-philici-
ty; 5c (H) < 5a (t-Bu) < 5b (Me) < 5f (OCF3) < 5d (Br) < 5e
(F). Preliminary metal extraction test showed that all our chelat-
ing ligands are selective for Sr2þ and Pb2þ ions. Further studies
to explain a detailed relationship between structure and proper-
ties (solubility and extraction efficiency) are under going and
will be reported.
Metal ion extraction efficiency/%
Comp. [M]:[Comp.]
Co
Cu
Sr
Cd
Zn
Pb
5a
5b
5c
5d
5e
5f
1:50
1:50
1:50
1:50
1:50
1:50
7
2
2
1
0
0
9
0
5
3
4
0
33
4
0
0
0
6
20
9
17
0
18
11
1
9
31
21
17
29
0
0
20
17
6
0
a40 ꢁC; CO2 pressure, 150 bar; stirring for 30 min; 0.01 mL distilled water.
one with electron-donating substituent. It appears that the ether
oxygen and benzylic hydrogens next to the ether oxygen en-
hance the solubility of surfactant through specific Lewis acid–
base interactions with CO2, but only if the ether oxygen and
neighboring hydrogens are in a readily accessible position.
Especially, the higher solubility of chelating ligands with
electron-withdrawing substituent on the benzene ring can be
attributed to the higher acidity of benzylic hydrogen and the
subsequent stronger interaction with CO2. Further explanation
will be reported in a near future.
This work was supported by Key Natural Scientific Fund
of South-Central University for Nationalities (YZZ05001) and
‘‘Youth Chen-Guang Project’’ of Wuhan Bureau of Science
and Technology (20065004116-34).
All experiments of metal ion extraction from spiked filter
paper using the in situ chelation-SFE were performed with a
lab-built SFE apparatus described by Wai, et al.23 Cocktail-like
extraction procedure was used for extraction efficiency test. The
metal ion extraction efficiency of our chelating ligands was
tested with and without PFOAT (tetraethylammonium per-
fluoro-1-octanesulfonate) as additive. As shown on Table 1,
without additive, the extraction efficiencies were very low or
negligibly small, indicating that chelating ligands may form
complexes with metal ions but either their equilibrium constant
might be very small or their solubility in scCO2 is low.8 In order
to increase the extraction efficiency, PFOAT was added as addi-
tive. We suggested that the salt dissociates and gives a CO2-phi-
lic PFOAꢀ anion. The PFOAꢀ anion undergoes exchange with
the Clꢀ anion of the complex in the aqueous phase, and conse-
quently the affinity of the complex for scCO2 increases owing
to the scCO2-philic PFOAꢀ chain. As our expectation, the ex-
traction efficiency increased. Interestingly, our new chelating li-
gands showed selectivity for the extraction of Sr2þ and Pb2þ
(Table 2). A comparing experiment was conducted with PFOAT
as the single additive. The result showed that the extraction
efficiency was low (at most 30% for Sr2þ, Pb2þ, and other metal
ions) confirming that it is the metal-chelating reagent and not
PFOAꢀ that is doing the extraction.
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Table 2. Metal ion extraction efficiency with PFOAT as
additivea
Metal ion extraction efficiency/%
Comp. [M]:[Comp.]:[PFOAT]
Co
Cu
Sr
Cd
Zn
Pb
5a
5b
5c
5d
5e
5f
1:50:50
1:50:50
1:50:50
1:50:50
1:50:50
1:50:50
3
36
2
8
29
10
26
9
72
87
76
83
76
73
8
38
32
0
12
40
25
0
88
84
86
87
85
78
21
10
1
0
0
20
5
12
a40 ꢁC; CO2 pressure, 150 bar; stirring for 30 min; 0.01 mL-distilled water.
Published on the web (Advance View) July 29, 2006; doi:10.1246/cl.2006.1000