Steric hindrance effects in tripodal ligands for extraction and back-extraction of Ag+

Article abstract of DOI:10.1039/c3ra45700a

A novel series of tripodal ligands with thiophenylether arms connected to an anchoring nitrogen has been investigated. Seven tripodal ligands were synthesized by combining methyl, isopropyl, and tert-butyl residue bearing thiophenylether sites as groups w

Full text of DOI:10.1039/c3ra45700a

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PAPER
Steric hindrance effects in tripodal ligands for
extraction and back-extraction of Ag+†
Cite this: RSC Adv., 2014, 4, 9791
Yuki Hiruta,‡§a Takafumi Watanabe,§a Etsuko Nakamura,a Naoko Iwasawa,a
b
b
a
a
*
Hiroyasu Sato, Kensaku Hamada, Daniel Citterio and Koji Suzuki
A novel series of tripodal ligands with thiophenylether arms connected to an anchoring nitrogen has been
investigated. Seven tripodal ligands were synthesized by combining methyl, isopropyl, and tert-butyl residue
bearing thiophenylether sites as groups with different steric hindrance effects. The tripodal ligands allowed
for the extraction of Ag+ ions from the aqueous phase into a chloroform phase by forming 1 : 1 complexes
with Ag+. Back-extraction was performed with 1 M HNO3 aqueous solution. Each ligand showed different
extraction and back-extraction efficiency for Ag+, affected by changes in steric hindrance caused by the
various combinations of sidearms. These results are further supported by X-ray single crystal structural
analysis.
Received 10th October 2013
Accepted 27th January 2014
DOI: 10.1039/c3ra45700a
www.rsc.org/advances
On the other hand, studies focusing on the relationship
between back-extraction efficiency and ligand structure are
Introduction
hardly found, so far. In the back-extraction process, the target
component is transferred back to the aqueous phase by
extraction of the organic phase with an aqueous solution under
conditions shiing the distribution equilibrium of the target
component away from the organic phase. That distribution
equilibrium is controlled by the reagent concentrations, and the
pH of the aqueous phase. In a metal recycling process, the back-
extraction represents a second extraction step, and allows for
further selectivity improvement by choosing a suitable aqueous
phase composition. For example, Narita et al. successfully
recovered Rh prior to Pd and Pt by effective control of back-
extraction conditions.19
According to the HSAB concept,20 ‘so’ acids such as tran-
sition metal ions, and sulfur atoms characterized as ‘so’ bases
generally have a strong affinity for each other. Therefore, there
are a variety of sulfur containing structures reported as ligands
for so metal ions, in the form of macrocycles,21 crown ethers,22
lariat ethers,23 cryptands,24 calixarenes,25 and tripodal ligands.26
However, with a strong coordination bond formed between the
so sulfur donor site and a so transition metal ion like Ag+, the
use of an aqueous mineral acid is no longer sufficient for
the efficient release of the metal cation for back-extraction. In
that case, compounds with stronger interaction with Ag+, such
as ammonia or thiourea, are required in the aqueous phase.27
In the present work, the inuence of steric hindrance in a
tripodal ligand on the back-extraction efficiency with an
aqueous mineral acid was investigated. For this purpose, a new
series of tripodal ligands (6a–g) with three thiophenylether
sidearms connected to an anchoring nitrogen has been
synthesized and characterized. The effect of steric hindrance
induced by combinations of methyl, isopropyl, and tert-butyl
Coordination chemistry is relevant for complex formation and
molecular recognition. In this context, tripodal ligands having
multidentate structure have attracted special interest. A tripodal
ligand is characterized by its molecular structure with three
coordinating arms. Typically, each of the coordinating arms
having one or more donor atoms (N, O, S, P) is connected to an
anchoring nitrogen or phosphorous atom.1 As coordinating
arms, pyridyl,2 quinolyl,3 alkoxyl,4 and thioether5 groups have
been presented, among others. In particular, tripodal ligands
have been used as extractants for metals.6–10
From an economic and environmental standpoint, it is of
great importance to recover expensive metals for reuse and
recycling from electronic waste, or to remove metal contaminants
from wastewater.11–15 Examples include heavy metals, radioactive
precious metals, rare metals, and rare earth metals. Solvent
extraction is a very useful method to separate and concentrate
metal cations in solution, and is widely applied for separations
on industrial scale, such as in hydrometallurgy. There are many
reports on the extraction of metal cations using various kinds of
extractants, including studies focusing on the relationship
between extraction efficiency and ligand structure.16–18
aDepartment of Applied Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. E-mail: suzuki@applc.keio.
ac.jp; Fax: +81 45 566 1566; Tel: +81 45 566 1566
bRigaku Corporation, 3-9-12, Matsubara-cho, Akishima-shi, Tokyo, 196-8666, Japan
† Electronic supplementary information (ESI) available: NMR-spectra, HRMS
spectra, ESI-MS spectra. CCDC 936706 and 936707. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3ra45700a
‡ Present address: Division of Physical Pharmaceutical Chemistry, Faculty of
Pharmacy, Keio University, 1-5-30 Shibakoen, Minato-ku, Tokyo 105-8512, Japan.
§ These authors have equally contributed to this work.
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substituted thiophenylether sidearms on extraction and back- 2-thio-substituted benzoic acid 2a–c was dissolved in THF
extraction efficiencies of Ag+ was evaluated. In addition, X-ray (200 mL) and then cooled in an ice-bath (0 ꢀC). 1 M BH3$THF (70
crystal structure analyses of the tripodal ligands with Ag+ have mL) was added to the ask, and the reaction was stirred at 0 ꢀC
been performed for studying the binding structure.
for 1 h and then at room temperature for 3 h. It was then cooled
in an ice bath and quenched by the careful addition of water.
Solid K2CO3 was added until the solution was saturated, and the
layers were separated. The organic layer was evaporated to
dryness, while the aqueous phase was extracted with dieth-
ylether 2 times. All of the organic phases were combined,
washed with 1 M NaOH, water, and brine, dried over Na2SO4,
evaporated, and dried under vacuum.
Experimental
Reagents and instruments
All reagents for the syntheses of tripodal ligands (6a–g) were
purchased from the following commercial suppliers and were
used without further purication: Wako Pure Chemical (Osaka,
Japan), Tokyo Kasei Industry (Tokyo, Japan) and Aldrich
Chemical (St. Louis, MO, U.S.A.). 1H-NMR and 13C-NMR spectra
were recorded at room temperature on a JEOL ECA-500 spec-
trometer at 500 MHz and 125 MHz, respectively. All chemical
shis are relative to an internal standard of tetramethylsilane
(d ¼ 0.0 ppm), and coupling constants are given in Hz. High-
resolution mass spectra were obtained on a Waters Xevo G2-S
QTof MS. ESI-mass spectra were obtained on a LCMS-2010EV
(SHIMADZU, Kyoto, Japan). Flash chromatography separation
was undertaken using a YFLC-Al-560 chromatograph (Yamazen
Co., Osaka, Japan). ICPS-8000 inductively coupled argon plasma
atomic emission spectrometry (SHIMADZU, Kyoto, Japan) was
used for the determination of the concentration of each metal
ion in aqueous solution. The deionized water used had a
resistivity of 18.2 MU at 25 ꢀC. The pH values were veried using
an IOL-50 ion-meter (DKK-TOA Corp., Tokyo, Japan) with a glass
pH electrode. All X-ray crystal structure measurements were
performed with a Rigaku Saturn70 diffractometer using multi-
(2-(Methylthio)phenyl)methanol (3a). Compound 2a (14.1 g,
83.7 mmol) was used as the starting material, and 3a was
1
obtained as clear oily liquid (12.2 g, 94.3%). H-NMR (CDCl3,
500 MHz) d (ppm): 2.49 (s, 3H), 4.75 (s, 2H), 7.18–7.20 (m, 1H),
7.26–7.29 (m, 2H), 7.37 (d, J ¼ 7.4 Hz, 1H). 13C-NMR (CDCl3, 125
MHz) d (ppm): 16.21, 63.59, 125.61, 126.68, 128.11, 128.49,
136.76, 138.98.
(2-(Isopropylthio)phenyl)methanol (3b). Compound 2b
(4.98 g, 25.4 mmol) was used as the starting material, and 3b
was obtained as light yellow oily liquid (4.54 g, 98.2%). 1H-NMR
(CDCl3, 500 MHz) d (ppm): 1.30 (d, J ¼ 6.9 Hz, 6H), 2.47 (br, 1H),
3.38 (sep, J ¼ 6.9 Hz, 1H), 4.79 (s, 2H), 7.24–7.25 (m, 2H), 7.39–
7.45 (m, 2H). 13C-NMR (CDCl3, 125 MHz) d (ppm): 23.30, 38.97,
64.11, 127.48, 128.21, 128.57, 133.04, 133.94, 142.40.
(2-(tert-Butylthio)phenyl)methanol (3c). Compound 2c was
used as the starting material, and 3c was obtained as light
1
yellow oily liquid (5.81 g, 93.0%). H-NMR (CDCl3, 500 MHz) d
(ppm): 1.31 (s, 9H), 2.52 (br, 1H), 4.87 (s, 2H), 7.25–7.28 (m, 1H),
7.35–7.38 (m, 1H), 7.47 (d, J ¼ 7.5 Hz, 1H), 7.54 (d, J ¼ 7.8 Hz,
1H). 13C-NMR (CDCl3, 125 MHz) d (ppm): 31.30, 47.76, 64.67,
127.70, 128.75, 129.45, 131.15, 138.96, 145.94.
Alkyl-substituted (2-(bromomethyl)phenyl)sulfates (4a and
4b) were prepared as previously reported.29
˚
layer mirror monochromated Mo-Ka (l ¼ 0.71075 A) radiation.
The structure was solved by direct methods, and expanded
using Fourier techniques. Non-hydrogen atoms were rened
anisotropically. Structural renements were obtained with full-
matrix least-squares based on F2 using the program
SHELXL-97.28
(2-(Bromomethyl)phenyl)(tert-butyl)sulfate (4c). Compound
3c (5.81 g, 29.6 mmol) was dissolved in diethylether (300 mL).
Aer cooling to 0 ꢀC, PBr3 (4.00 g, 14.8 mmol) was added and
Synthesis
ꢀ
the reaction was stirred at 0 C for 0.5 h and room tempera-
2-Thio-substituted benzoic acid (2a and 2b) was prepared as ture for 1 h. Aer the reaction was completed, the reaction
previously reported.29
was quenched with methanol. Then, water was added, and
2-(tert-Butylthio)benzoic acid (2c). 1 (4.92 g, 31.9 mmol) was the organic layer was washed with saturated NaHCO3 three
dissolved in a mixture of acetic acid (46 mL), 60 % perchloric times and dried over Na2SO4, evaporated and dried under
acid (10 mL), acetic anhydride (8 mL) and THF (40 mL). Then, vacuum. 4c was obtained as colorless oily liquid (6.70 g,
tertiary butyl alcohol (4.68 g, 63.1 mmol) was added and the 87.4%).
solution was stirred for 18 h at room temperature, followed by
1H-NMR (CDCl3, 500 MHz) d (ppm): 1.32 (s, 9H), 4.88 (s, 2H),
2 h at 50 ꢀC. Aer the reaction was completed, the reaction 7.24–7.27 (m, 1H), 7.33–7.36 (m, 1H), 7.56 (d, J ¼ 7.7 Hz, 2H).
mixture was allowed to cool to room temperature, and excess 13C-NMR (CDCl3, 125 MHz) d (ppm): 31.38, 32.80, 47.47, 128.47,
saturated aqueous NaHCO3 solution was added to hydrolyze 129.60, 131.13, 132.62, 139.02, 142.96.
remaining acetic anhydride. Aer that, the pH of the solution
General procedure for the synthesis of 2-thio-substituted
was lowered to pH 1 by addition of 1 M HCl solution. The acidic phenylmethanamines (5a–c). The corresponding alkyl-
aqueous phase was extracted with ethyl acetate 3 times, and the substituted (2-(bromomethyl)phenyl)sulfate 4a–c was dissolved
combined organic layer was washed with water two times and in ethanol (160 mL). Then, 28–30 wt% aqueous ammonia
brine once, dried over Na2SO4, and the solvent evaporated. The solution (80 mL) and THF (80 mL) were added, and the reaction
resulting residue was used for the following reaction without was stirred at room temperature overnight. Aer the reaction
further purication.
was completed, the solvent was evaporated and the residue was
General procedure for the synthesis of 2-thio-substituted dried under vacuum, before being puried by ash chroma-
phenylmethanol compounds (3a–c). The corresponding tography (silica gel).
9792 | RSC Adv., 2014, 4, 9791–9798
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(2-(Methylthio)phenyl)methanamine (5a). Compound 4a
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MHz) d (ppm): 16.09, 23.24, 38.35, 55.81, 56.06, 125.00, 125.50,
126.85, 126.98, 127.28, 128.84, 129.33, 131.96, 135.08, 137.61,
137.69, 140.90. HRMS: calculated for C26H32NS3 454.1652 [M +
H]+; found 454.1701. Elemental analysis: calcd C 68.83, H 6.89,
N 3.09, S 21.20; found C 68.77, H 6.96, N 2.94, S 21.36.
(3.61 g, 16.6 mmol) was used as the starting material, and 5a
was obtained as a white powder (2.20 g, 86.4%). Eluent for
chromatography: chloroform–methanol 94 : 6 / 90 : 10 /
1
85 : 15. H-NMR (CD3OD, 500 MHz) d (ppm): 2.54 (s, 3H), 4.24
(s, 2H), 7.26–7.28 (m, 1H), 7.41–7.47 (m, 3H). 13C-NMR (CD3OD,
125 MHz) d (ppm): 16.53, 41.98, 127.17, 128.97, 130.83, 131.19,
132.74, 139.62.
N,N-Bis(2-(isopropylthio)benzyl)-1-(2-(methylthio)phenyl)-
methanamine (6c). Compound 4b (2240 mg, 9.13 mmol),
compound 5a (700 mg, 4.57 mmol) and K2CO3 (2.83 g, 20.5
mmol) in acetonitrile (330 mL) resulted in 6c as lightly yellow
oily liquid (1500 mg, 68.3%). Eluent for chromatography:
hexane–chloroform 75 : 25 / 20 : 80 / 0 : 100. 1H-NMR
(CDCl3, 500 MHz) d (ppm): 1.24 (d, J ¼ 6.9 Hz, 12H), 2.40 (s, 3H),
3.28 (sep, J ¼ 6.9 Hz, 2H), 3.72 (s, 2H), 3.83 (s, 4H), 7.12–7.21 (m,
7H), 7.36 (dd, J ¼ 7.7 Hz, 1.6 Hz 2H), 7.67 (d, J ¼ 7.2 Hz, 1H),
7.76 (dd, J ¼ 7.7 Hz, 1.2 Hz, 2H). 13C-NMR (CDCl3, 125 MHz) d
(ppm): 16.05, 23.23, 38.32, 55.81, 56.05, 124.95, 125.44, 126.83,
126.95, 127.25, 128.84, 129.35, 131.96, 135.10, 137.67, 137.72,
141.00. HRMS: calculated for C28H35NS3 482.1965 [M + H]+;
found 482.2012. Elemental analysis: calcd C 69.80, H 7.32, N
2.91, S 19.97; found C 69.71, H 7.47, N 2.79, S 20.41.
(2-(Isopropylthio)phenyl)methanamine (5b). Compound 4b
(4.51 g, 18.4 mmol) was used as the starting material, and 5b
was obtained as a white powder (2.68 g, 80.4%). Eluent for
chromatography: chloroform–methanol 100 : 0 / 98 : 2 /
50 : 50. 1H-NMR (CD3OD, 500 MHz) d (ppm): 1.29 (d, J ¼ 6.9 Hz,
6H), 3.41 (sep, J ¼ 6.9 Hz, 1H), 4.34 (s, 2H), 7.37–7.44 (m, 2H),
7.52 (d, J ¼ 7.5 Hz, 1H), 7.61 (dd, J ¼ 7.5 Hz, 1.2 Hz, 1H). 13C-
NMR (125 MHz, CD3OD) d (ppm): 23.36, 40.58, 42.56, 129.23,
130.91, 131.03, 135.25, 136.27, 136.68.
(2-(tert-Butylthio)phenyl)methanamine (5c). Compound 4c
(2.31 g, 8.92 mmol) was used as the starting material, and 5c
was obtained as a white powder (1.30 g, 69.3%). Eluent for
chromatography: chloroform–methanol 90 : 10. 1H-NMR
(CD3OD, 500 MHz) d (ppm): 1.30 (s, 9H), 4.47 (s, 2H), 7.44–7.47
(m, 1H), 7.50–7.53 (m, 1H), 7.62 (d, J ¼ 7.8 Hz, 1H), 7.69 (d, J ¼
7.8 Hz, 1H). 13C-NMR (CD3OD, 125 MHz) d (ppm): 30.29, 42.19,
129.54, 130.22, 130.25, 133.19, 138.58, 139.69.
Tris(2-(isopropylthio)benzyl)amine (6d). Compound 4b
(1180 mg, 5.43 mmol), compound 5b (504 mg, 2.78 mmol) and
K2CO3 (2.00 g, 14.5 mmol) in acetonitrile (250 mL) yielded 6d as
golden brown oily liquid (773 mg, 61.2%). Eluent for chroma-
1
tography: hexane–chloroform 80 : 20 / 20 : 80 / 0 : 100. H-
NMR (CDCl3, 500 MHz) d (ppm): 1.24 (d, J ¼ 6.6 Hz, 18H), 3.27
(sep, J ¼ 6.6 Hz, 3H), 3.81 (s, 6H), 7.12–7.15 (m, 3H), 7.19–7.21
(m, 3H), 7.36 (dd, J ¼ 7.6 Hz, 1.2 Hz, 1H), 7.75 (dd, J ¼ 7.5 Hz, 1.2
Hz, 1H). 13C-NMR (CDCl3, 125 MHz) d (ppm): 23.23, 38.30,
56.07, 126.82, 126.93, 129.38, 131.98, 135.13, 141.13. HRMS:
calculated for C30H39NS3 510.2278 [M + H]+; found 510.2327.
Elemental analysis: calcd C 70.67, H 7.71, N 2.75, S 18.87; found
C 70.40, H 8.03, N 2.56, S 19.07.
N-(2-(tert-Butylthio)benzyl)-N-(2-(methylthio)benzyl)-1-(2-
(methylthio)phenyl)methanamine (6e). Compound 4a (451 mg,
2.08 mmol), compound 5c (206 mg, 1.06 mmol) and K2CO3
(800 mg, 5.79 mmol) in acetonitrile (100 mL) gave 6e as white
solid (423 mg, 87.0%). Eluent for chromatography: hexane–
chloroform 50 : 50 / 10 : 90. 1H-NMR (CDCl3, 500 MHz) d
(ppm): 1.23 (s, 9H), 2.40 (s, 6H), 3.71 (s, 4H), 3.99 (s, 2H), 7.12–
7.17 (m, 7H), 7.31–7.35 (m, 1H), 7.49 (dd, J ¼ 7.8 Hz, 1.4 Hz, 1H),
7.67 (d, 7.2 Hz, 2H), 7.85 (dd, J ¼ 7.8 Hz, 1.1 Hz, 1H). 13C-NMR
(CDCl3, 125 MHz) d (ppm): 16.11, 31.19, 47.32, 55.87, 56.82,
124.98, 125.53, 126.45, 127.27, 128.90, 129.11, 129.64, 132.32,
137.72, 137.74, 138.86, 144.89. HRMS: calculated for C27H33NS3
468.1809 [M + H]+; found 468.1855. Elemental analysis: calcd C
69.33, H 7.11, N 2.99, S 20.57; found C 68.65, H 7.26, N 2.89, S
20.59.
N,N-Bis(2-(tert-butylthio)benzyl)-1-(2-(methylthio)phenyl)-
methanamine (6f). Compound 4c (1060 mg, 4.09 mmol),
compound 5a (415 mg, 2.71 mmol) and K2CO3 (1.21 g, 8.76
mmol) in acetonitrile (150 mL) yielded 6f as golden brown oily
liquid (542 mg, 51.9%). Eluent for chromatography: hexane–
chloroform 50 : 50 / 0 : 100. 1H-NMR (CDCl3, 500 MHz) d
(ppm): 1.22 (s, 18H), 2.40 (s, 3H), 3.68 (s, 2H), 3.95 (s, 4H), 7.11–
7.16 (m, 5H), 7.32–7.35 (m, 2H), 7.49 (d, J ¼ 7.8 Hz, 2H), 7.64 (d, J
¼ 7.5 Hz, 1H), 7.84 (d, J ¼ 7.7 Hz, 2H). 13C-NMR (CDCl3, 125
General procedure for the synthesis of alkyl-substituted N,N-
bis(2-thiobenzyl)-1-(2-thiophenyl)-methanamines (6a–g). The
corresponding
alkyl-substituted
(2-(bromomethyl)phenyl)
sulfate (4a–c), 2-thio-substituted phenylmethanamine (5a–c)
and K2CO3 were dissolved in acetonitrile and then stirred at
room temperature for 1 day. Aer the reaction was completed,
solids were removed by ltration, and the solvent was removed
by evaporation. The resulting residue was puried by ash
chromatography (silica gel).
Tris(2-(methylthio)benzyl)amine (6a). Compound 4a
(1443 mg, 6.64 mmol), compound 5a (506 mg, 3.30 mmol) and
K2CO3 (2.00 g, 14.5 mmol) in acetonitrile (200 mL) were used. 6a
was obtained as a white powder (1024 mg, 72.4%). Eluent for
chromatography: chloroform–methanol 100 : 0 / 99 : 1 /
90 : 10. 1H-NMR (CDCl3, 500 MHz) d (ppm): 2.40 (s, 9H), 3.74 (s,
6H), 7.13–7.25 (m, 9H), 7.69 (d, J ¼ 7.5 Hz, 3H). 13C-NMR
(CDCl3, 125 MHz) d (ppm): 16.13, 55.79, 125.05, 125.54, 127.31,
128.84, 137.55, 137.67. HRMS: calculated for C24H27NS3
426.1384 [M + H]+; found 426.1387. Elemental analysis: calcd C
67.72, H 6.39, N 3.29, S 22.60; found C 67.73, H 6.55, N 3.22, S
22.61.
N-(2-(Isopropylthio)benzyl)-N-(2-(methylthio)benzyl)-1-(2-(methyl-
thio)phenyl)methanamine (6b). Compound 4a (1180 mg, 5.43
mmol), compound 5b (504 mg, 2.78 mmol) and K2CO3 (2.00 g,
14.5 mmol) in acetonitrile (250 mL) were used. 6b was obtained
as lightly yellow solid (773 mg, 61.2%). Eluent for chromatog-
raphy: hexane–ethyl acetate 88 : 12 / 80 : 20 / 50 : 50.
1H-NMR (CDCl3, 500 MHz) d (ppm): 1.25 (d, J ¼ 6.6 Hz, 6H), 2.41
(s, 6H), 3.29 (sep, J ¼ 6.6 Hz, 1H), 3.73 (s, 4H), 3.84 (s, 2H), 7.10–
7.22 (m, 8H), 7.36 (dd, J ¼ 7.8 Hz, 1.2 Hz, 1H), 7.68 (d, J ¼ 7.2 Hz,
2H), 7.76 (dd, J ¼ 7.5 Hz, 1.2 Hz, 1H). 13C-NMR (CDCl3, 125
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MHz) d (ppm): 16.12, 31.21, 47.31, 55.92, 56.84, 124.96, 125.55,
126.43, 127.26, 128.92, 129.12, 129.66, 132.37, 137.86, 137.92,
138.89, 145.16. HRMS: calculated for C30H39NS3 510.2278 [M +
H]+; found 510.233. Elemental analysis: calcd C 70.67, H 7.71, N
2.75, S 18.87; found C 70.09, H 8.01, N 2.65, S 19.12.
Tris(2-(tert-butylthio)benzyl)amine (6g). Compound 4c
(541 mg, 1.83 mmol), compound 5c (261 mg, 1.33 mmol) and
K2CO3 (810 mg, 5.86 mmol) in acetonitrile (100 mL) resulted in
6g as a white solid (386 mg, 76.5%). Eluent for chromatography:
hexane–chloroform 50 : 50
/
10 : 90. 1H-NMR (CDCl3,
500 MHz) d (ppm): 4.0 (s, 27H), 3.92(s, 6H), 7.13–7.16 (m, 3H),
7.33–7.35(s, 3H), 7.49 (dd, J ¼ 7.7 Hz, 1.2 Hz, 3H), 7.82 (dd, J ¼
7.8 Hz, 1.2 Hz, 3H). 13C-NMR (CDCl3, 125 MHz) d (ppm): 31.20,
42.27, 56.71, 126.38, 129.10, 129.59, 132.41, 138.89, 145.37.
HRMS: calculated for C33H45NS3 552.2748 [M + H]+; found ¼
552.2795. Elemental analysis: calcd C 71.81, H 8.22, N 2.54, S
17.43; found C 71.49, H 8.49, N 2.48, S 17.70.
Extraction experiments
Scheme 1 (a) CH3I, i-PrBr or t-BuOH (b) BH3$THF, THF (c) PBr3, Et2O
(d) NH3 aq., EtOH (e) K2CO3, CH3CN.
In a 50 mL centrifuge tube, an aliquot of chloroform containing
1 ꢁ 10ꢂ5 to 2 ꢁ 10ꢂ3 M of each ligand (6a–g) and an equal
volume of aqueous buffer solution containing 10 mg Lꢂ1 of Ag+
as AgNO3, were mixed. Buffer solutions were prepared from
10 mM acetic acid and the pH values were adjusted by varying
the ratio of potassium nitrate and potassium hydroxide, with
the total potassium ion concentration adjusted to 100 mM. The
of bromo derivatives and 1 equivalent of amine derivatives
(Scheme 1). This synthesis method enables the access to ligands
having combinations of various substituents. Though there are
some literature reports about tripodal ligands with three iden-
tical coordinating arms for extraction, to the best of our
knowledge, the effect of combinations of sidearms in tripodal
ligands has not yet been evaluated in terms of extraction and
back-extraction efficiencies. In this work, the effect of the
combination of sterically demanding groups on the extraction
and back-extraction efficiency was examined. All ligands have
multiple sulfur atoms as “so” bases and efficiently extract Ag+
ions as “so” acid according to the HSAB concept.
ꢀ
centrifuge tubes were then shaken at 25 C over 3 h. Aer the
two phases were separated by centrifugation, the pH values
(pH electrode) and the metal concentrations (ICP-AES) in the
aqueous phase were measured. Moreover, the organic phase
was shaken at 25 ꢀC over 3 h with 1 M HNO3 as back extraction
solvent. Aer separation in the same way, Ag+ concentrations in
the aqueous phase (back extraction phase) were measured.
The extraction efficiencies, back extraction efficiencies, and
total extraction efficiencies were determined based on the Ag+
concentrations in each set of aqueous phases according to the
following equations,
Determination of the extraction constant and conrmation of
1:1 complex stoichiometry
Extraction behavior of Ag+ with ligands in chloroform: the
distribution ratio (D) of the silver ion is dened as
Extraction efficiency (%) ¼ (Cw ꢂ Ce)/Cw ꢁ 100
Back extraction efficiency (%) ¼ Ce0 /(Cw ꢂ Ce) ꢁ 100
Total extraction efficiency (%) ¼ Ce0 /Cw ꢁ 100
CAg;org
D ¼
(1)
CAg;aq:
where CAg, org and CAg, aq. are the total concentrations of Ag+ ions
in the organic and aqueous phases, respectively.
where Cw, Ce, and Ce0 correspond to the Ag+ ion concentrations
in the aqueous phase before extraction, aer extraction, and in
the back extraction aqueous phase, respectively.
Extraction efficiencies for other ions were evaluated accord-
ing to the same procedure as described for Ag+.
The extraction equilibrium can be formulated as
ꢂ
Ag+
+ NO3 (aq.) + nLorg # [LnAgNO3]org
(2)
(aq.)
The extraction constant, Kex, of Ag+ with the ligand is
expressed as
Results and discussion
½LnAgNO3ꢃ
org
Â
ꢂÃ
Kex
¼
(3)
Molecular design and synthesis
n
½Agþꢃ NO3
½Lꢃ
aq:
org
aq:
As sterically demanding groups in the sidearms, the Ag+
extracting ligands include combinations of methyl and iso-
As the total concentration of Ag+ ions in the organic phase
propyl, or methyl and tert-butyl substituents. The target corresponds to [LnAgNO3], the distribution ratio of Ag+ can be
compounds were obtained by the SN2 reactions of 2 equivalents rewritten as
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Table 1 Extraction constants (Kex) of each ligand for Ag+, and slope
and correlation coefficient (R2) of linear regression lines (data extrac-
ted from Fig. 1)
½LnAgNO3ꢃ
aq:
org
D ¼
(4)
½Agþꢃ
Substitution of eqn (3) into (4) results in the following
6a
6b
6c
6d
6e
6f
6g
ꢂ
n
D/[NO3
]
¼ Kex[L]org
(5) Slope
R2
1.00
0.999
5.89
1.02
0.999
5.97
1.01
0.996
6.04
1.03
0.996
6.12
1.00
0.996
5.10
1.01
0.987
4.57
1.00
0.999
4.40
aq.
log Kex
Taking the logarithm of eqn (5) yields
ꢂ
log D/[NO3
]
¼ log Kex + n log[L]org
(6)
aq.
spectra of 6a in the presence of AgNO3 showed changes of the
chemical shis compared to the spectra recorded in the absence
of AgNO3 (shown in the ESI†). These results further demonstrate
the interaction of the ligand with Ag+. Table 1 lists the extraction
constants (Kex) of each ligand for Ag+, estimated according to eqn
(6) from the intercept of the linear regression lines shown in
Fig. 1. With an increasing number of t-Bu substituents in
compounds 6a, 6e–g, Kex decreased from 5.89 to 5.10, 4.57 and
4.40, respectively. This might be the result of increasing binding
distances between Ag+ and the donor S atoms, as well as the
hindrance of binding between the counter anion and Ag+,
caused by the bulky t-Bu substituent directly connected to the S
atoms. On the other hand, with increasing number of i-Pr
substituents in compounds 6b–d, Kex slightly increased to 5.97,
6.04 and 6.12, respectively. The inductive effect of alkyl groups
increases in the order of methyl, i-Pr, and t-Bu. It has been
reported that the inductive effect sometimes outweighs the
steric effect on the complex formation constant of a ligand with a
metal ion.30 In analogy to that work, it is assumed that the steric
hindrance effects of the i-Pr substituents in compounds 6a–d are
outweighed by the inductive effects.
To conrm the complex stoichiometry with Ag+, as well as to
estimate the extraction constant (Kex) of each ligand for Ag+, the
distribution ratio D was determined as a function of the ligand
ꢂ
concentration. Plots of log(D/[NO3
]aq.) versus the concentration
of each ligand with different steric hindrance, log[L]org, at xed
pH of 4.0 are shown in Fig. 1. In all cases, straight lines with a
slope of one are observed, conrming a 1 : 1 complex formation
according to eqn (6). A schematic representation of a complex is
shown in Scheme 2. This indicates that the stoichiometry of
complexes formed with Ag+ is identical for all ligands, inde-
pendent of the sidearms having different steric hindrance
characteristics. ESI-MS mass spectra of mixtures of each ligand
and AgClO4 also revealed the formation of a 1 : 1 complex
through the metal coordination interaction, with two peaks
(derived from Ag+ isotopes) assigned to the species [6a–g + Ag]+,
1
respectively (shown in the ESI†). In addition, H and 13C-NMR
Crystal structure studies
X-ray single crystal structural analysis was performed to deter-
mine the molecular structures of complexes formed between
Fig. 1 Plots of log(D/NO3ꢂ) versus log[L]org for the extraction of 10 mg
L
ꢂ1 Ag+ as AgNO3 with 6a–g into chloroform; aqueous phase: pH 4.0.
Scheme 2 General schematic representation of the structure of Fig. 2 X-ray single crystal structures of (a) 6a and (b) 6dA (c) 6dB
complexes formed between the tripodal ligands and Ag+.
complexing Ag+.
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the ligands and Ag+. Since single crystals were more readily
obtained as perchlorate salts than nitrate salts, AgClO4 was
applied in crystallization experiments. Nevertheless, crystals
sufficiently large for X-ray analysis were only achieved with
ligands 6a and 6d. X-ray crystal structures of the two complexes
Ag(6a)ClO4 and Ag(6d)ClO4 are shown in Fig. 2, together with
selected structural parameters listed in Table 2. With respect to
Ag(6d)ClO4, two crystallographically independent complexes
(6dA and 6dB) were observed (Fig. 2b and c). The binding
characteristics of ligands 6a and 6dA and 6dB with Ag+ are
basically identical, with both compounds forming the same
5-fold coordinated complexes. Ag+ is coordinated by one N atom
Extraction and back-extraction behavior
The extraction efficiencies (%) of Ag+ with each ligand as a
function of equilibrium pH in the aqueous phase are shown in
Fig. 3. Ag+ was efficiently extracted with ligands 6a–d (increasing
number of sterically demanding i-Pr groups) independent of the
pH of the aqueous phase. The extraction efficiencies of Ag+ with
6e–g (increasing number of t-Bu groups) decreased with
decreasing pH. This behavior was particularly pronounced for
6f and 6g. The results in Fig. 4 show the extraction efficiencies
(%) of 6a for various other ions as a function of equilibrium pH
in the aqueous phase. Independent of the pH of the aqueous
phase, no signicant extraction was observed for most ions,
with the exception of Hg2+, which was extracted at an efficiency
of about 8%. According to the HSAB concept, this extraction can
be attributed to the relatively strong affinity of the sulfur atoms,
characterized as ‘so’ bases, for the ‘so’ acid Hg2+. However, in
accordance with the only limited extraction efficiency for Hg2+,
these results demonstrate that ligand 6a has a suitably high
selectivity for extraction of Ag+. The extraction efficiencies (%) of
Ag+ with each ligand under identical pH condition of the
aqueous phase (pH 4.0) are shown in Fig. 5a. The values of the
extraction constants (Table 1) are directly reected in
the extraction efficiencies. In the case of ligands 6b–d (with i-Pr
groups), the extraction efficiency was constantly high. In the
case of ligands 6e–g (with t-Bu groups), the extraction efficiency
decreased with increasing number of t-Bu groups. The back-
extraction efficiencies (%) of Ag+ with each ligand for a 1 M
HNO3 aqueous phase are shown in Fig. 5b. In the case of
compound 6a, the back-extraction efficiency was only about
35% and not sufficient. This might be attributed to the strong
interaction of S and Ag+.32 In the case of ligands 6b–d, the back-
extraction efficiency increased with increasing number of i-Pr
groups.
˚
˚
(Ag–N ¼ 2.606 A in 6a and 2.456 A in 6d), three S atoms (Ag–S ¼
˚
˚
˚
2.558 ꢄ 0.006 A in 6a and 2.54 A in 6dA and 2.49 A in 6dB) and
ꢂ
˚
˚
one O atom from ClO4 (Ag–O ¼ 2.576 A in 6a and 3.29 A in 6dA
and 6dB). The distance between silver and each sulfur atom is
˚
slightly shorter than the mean (2.675 ꢄ 0.015 A) of 51 such
bonds taken from the X-ray literature.31 Regardless of the
substituent (Me or i-Pr), the Ag+–S bond lengths of 6a and 6d are
nearly the same. This result implies that the sterically more
demanding i-Pr moiety does not inuence the binding ability of
sulfur to Ag+, and it is adequate for the extraction constants (Kex
values) of ligands 6a–d to slightly increase with increasing
number of i-Pr groups according to the inductivꢂe effect. On the
other hand, the distance between Ag+ and ClO4 of Ag(6a)ClO4
and Ag(6d)ClO4 are clearly different. This indicates that the
presence of a sterically hindering substituent inuences the
distance between Ag+ and the counter anion.
Table 2 Selected bond lengths for 6a and 6d complexing Ag+
6a
6dA
6dB
Extraction and back-extraction efficiencies have been inves-
tigated using nitrate salts, due to their solubility in water over a
wide concentration range, while X-ray analysis was done on
complexes with perchlorate as the counter anion, because more
ready crystallization. For comparison purposes, some extraction
and back-extraction experiments were performed with AgClO4.
The extraction efficiency and back-extraction efficiency (%) of
Ag+ as AgNO3 or AgClO4 with 6d under identical pH condition of
Ag(1)–S(3)
Ag(1)–S(4)
Ag(1)–S(5)
Ag(1)–N(7)
2.5665(8) Ag(1)–S(1) 2.54(1)
2.5593(8) Ag(1)–S(1) 2.54(1)
Ag(1)–S(1) 2.49(1)
Ag(1)–S(1) 2.487(9)
2.548(1)
2.606(2)
Ag(1)–S(1) 2.542(7) Ag(1)–S(1) 2.49(2)
Ag(1)–N(1) 2.456(4) Ag(1)–N(1) 2.456(4)
Ag(1)–O(38) 2.576(2)
Ag(1)–O(1) 3.29(1)
Ag(1)–O(1) 3.29(1)
Fig. 3 Extraction of Ag+ with various ligands as a function of the Fig. 4 Extraction of various ions with 6a as a function of the equilib-
equilibrium pH of the aqueous phase; ligand concentration 1 mM.
rium pH of the aqueous phase; ligand concentration 1 mM.
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Fig. 6 Total extraction efficiencies for the extraction of 200 mg Lꢂ1
Ag+ as AgNO3 with 6a or 6d; aqueous phase: pH 4.0; organic phase:
ligand concentration 1 mM; back-extraction solvent: 1 M HNO3.
the aqueous phase (pH 4.0) are shown in Table 3. No signicant
differences in the extraction and back-extraction efficiencies of
AgNO3 and AgClO4 were observed.
From X-ray single crystal structural analysis of complexes
formed with 6a and 6d, the distance between the Ag+ ion and
the counter anion was larger in the complex of 6d than of 6a.
Therefore, the difference of the distance between Ag+ and the
counter anion might affect the back-extraction efficiencies.
Similar to ligands 6b–d with increasing number of i-Pr groups,
the back-extraction efficiency of compounds 6e–g increased
with increasing number of t-Bu groups. These results again
reect the decreasing values of the extraction constants
(Table 1), with increasing number of t-Bu groups.
The total extraction efficiencies (%) combining the extrac-
tion and back-extraction processes of Ag+ with each ligand are
shown in Fig. 5c. With increasing number of i-Pr substituents
(6a–d), the total extraction efficiencies increased up to 74%. The
total extraction efficiencies decreased down to 57% with
increasing number of t-Bu substituents. The total extraction
efficiency of 6d was highest among the synthesized ligands (6a–
g), because of the relatively high efficiency of both extraction
and back-extraction. These results indicate that the steric
hindrance of the alkyl-substituted thioether arms of the tripodal
ligands affects the total extraction efficiency.
The reusability of ligands (6a and 6d) was evaluated as
Fig. 5 (a) Extraction, (b) back-extraction, and (c) total-extraction shown in Fig. 6. The total extraction efficiency of both ligands is
efficiencies of Ag+ with various ligands; aqueous phase: pH 4.0;
organic phase: ligand concentration 1 mM; back-extraction solvent: 1
hardly altered aer three repeated cycles. These results
indicate that these ligands are applicable to the repeated
M HNO3 aqueous solution.
extraction of Ag+.
Conclusions
Table 3 Counter anion dependent extraction and back-extraction
A series of tripodal ligands for Ag+ with thiophenylether side-
efficiencies of 6a and 6d for Ag+
arms carrying different sterically demanding groups have been
successfully synthesized. The extraction and back-extraction
efficiencies of these ligands depend on the number and size of
sterically hindering groups. The extraction efficiencies decrease
with increasing number of t-Bu groups. The back-extraction
efficiencies increase with increasing number of i-Pr and t-Bu
groups. The total extraction efficiency of ligand 6d having three
Ligand
6a
Anion
Extraction
Back-extraction
Total-extraction
NO3
ClO4
NO3
ClO4
98.8%
99.0%
99.4%
98.4%
35.4%
35.7%
74.5%
72.1%
35.0%
35.3%
74.0%
71.0%
6d
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