Gold(III) extraction by 1,3-Bis(2'-acetoxymethylthiobutyl-3'- thiobutylpropyl)-6-methyluracyl from hydrochloric acid solutions

Article abstract of DOI:10.1134/S0036023607110265

Gold(III) extraction by 1,3-bis(2'-acetoxymethylthiobutyl-3'- thiobutylpropyl)-6-methyluracyl from 0.5 M HCl solutions to chloroform is studied. The reagent has a high efficiency, separates gold(III) ions from the sum of nonprecious and ferrous metals, and can be repeatedly used after stripping. The extraction mechanism is solvation, with Au-S coordination bonds formed with all sulfur atoms of the reagent; the solvation number changes from 1 to 0.25 with increasing gold(III) aqueous concentration.

Full text of DOI:10.1134/S0036023607110265

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 11, pp. 1798–1802. © Pleiades Publishing, Inc., 2007.
Original Russian Text © P.A Khisamutdinov, Yu.I. Murinov, I.P. Baikova, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 11, pp. 1908–1912.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Gold(III) Extraction by 1,3-Bis(2'-acetoxymethylthiobutyl-3'-
thiobutylpropyl)-6-methyluracyl
from Hydrochloric Acid Solutions
R. A. Khisamutdinov, Yu. I. Murinov, and I. P. Baikova
Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences,
pr. Oktyabrya 71, Ufa, 450054 Bashkortostan, Russia
Received October 19, 2006
Abstract—Gold(III) extraction by 1,3-bis(2'-acetoxymethylthiobutyl-3'-thiobutylpropyl)-6-methyluracyl from
0.5 M HCl solutions to chloroform is studied. The reagent has a high efficiency, separates gold(III) ions from
the sum of nonprecious and ferrous metals, and can be repeatedly used after stripping. The extraction mecha-
nism is solvation, withAu–S coordination bonds formed with all sulfur atoms of the reagent; the solvation num-
ber changes from 1 to 0.25 with increasing gold(III) aqueous concentration.
DOI: 10.1134/S0036023607110265
During the last several decades, the extractive sepa- compounds will be retained in the extract and not form
intermediate phases.
ration of gold has become prominent among other
physicochemical methods used in hydrometallurgy and
analysis. A significant number of efficient sulfur
reagents have been proposed for use in the extractive
separation of gold(III); organic sulfides (thioethers) are
of paramount importance. The 1970s–1980s were the
period of most intensive search for new sulfured
reagents and the development of gold extraction chem-
istry. In studies of the extraction properties of organic
sulfides, it was found that along with unambiguous
strengths and advantages over other classes of com-
pounds in gold(III) extraction, thioethers have a weak-
ness because of their reducing properties. As a conse-
quence, gold(I) and even gold metal appear in extracts,
as well as sulfide-oxidation products (sulfoxides) [1],
appreciably degrading the selectivity of gold(III)
extraction.
To find new highly efficient and selective extractants
for gold(III), in this work we study the extraction prop-
erties of an alkylthio derivative of 6-methyluracyl.
EXPERIMENTAL
We used 1,3-bis(2'-acetoxymethylthiobutyl-3'-
thiobutylpropyl)-6-methyluracyl (S),
C4H9SCH2C O
O
O
C
C4H9SCH2CHCH2
H
N
C
C
C
Having analyzed the related literature, we found that
only few reagents with sulfide sulfur atoms being the
only heteroatoms in their molecules were recently stud-
ied as extractants for gold(III) (but not for palla-
dium(II)) [2–4]. Compounds containing in addition to
sulfur other heteroatoms, such as oxygen and nitrogen,
have received much more attention. Gold(III) extrac-
tion was studied with a thioamide derivative [5], thio-
urea derivatives [6–10], N-thiocarbamoylbenzamidine
[11], thiacalix[4]aniline [12], and other reagents.
O
N
CH3
CH2CHCH2SC4H9
O
O CCH2SC4H9
.
which was first synthesized by V.P. Krivonogov with
colleagues at the Institute of Organic Chemistry, Ufa
Scientific Center. Its synthesis and some physical–
chemical properties were described earlier [13]. The
compound is a light brown, clear, thick liquid insoluble
in water but well soluble in common organic solvents.
The identity and purity of the reagent were verified by
elemental and functional chemical analyses, thin-layer
and gas–liquid chromatography, and IR, 1ç NMR, and
In our opinion, the future of the chemistry of
gold(III) extraction lies in the search for reagents that
have not only selective properties but also a high extrac-
tion capacity. Increasing the number of sulfide sulfur
atoms in an organic molecule can probably help to
solve this problem with the provision that extracted 13C NMR spectra.
1798

GOLD(III) EXTRACTION
1799
For extraction studies, solutions of extractant S were
D
prepared from exact weights and chloroform was used
a diluent.
55
50
45
40
35
30
25
20
Gold(III) solutions in hydrochloric acid were pre-
pared from 999.9% metal gold as follows: an exact ali-
quot was dissolved in aqua regia, and then the solution
was converted to hydrochloric acid one. The gold(III)
concentration in stock and working solutions was deter-
mined spectrophotometrically by the intrinsic color of
the AuCl–4 ion (the absorption band peaking at 313 nm)
after plotting a calibration curve, which fully coincided
with that reported in the literature [14]. Trace organics
in raffinates interfered with the analysis, which did not
make it possible to determine gold(III) spectrophoto-
metrically. Therefore, atomic absorption spectrophom-
etry (Hitachi 508) was used to determine the gold(III)
concentration in raffinates. Aqueous gold(III) solutions
(stock solutions and raffinates) were burnt in acety-
lene–air flame; a lamp with a hollow cathode made of
the element to be determined served as the monochro-
mated radiation source.
0
0.5
1.0
1.5
2.0
2.5
cHCl, mol/L
Fig. 1. Gold(III) extraction vs. aqueous HCl concentration
–4
–3
(Ò
= 1.5 × 10 mol/L; Ò = 1.5 × 10 mol/L; 25°ë).
S
Au(III)
drances to the coordination of metal ions to sulfur
atoms.
The extraction was carried out in thermostated glass
separatory funnels 3 mL in capacity at 25 0.1°ë. Stir-
ring was performed with a Universal shaker 327. The
ratio between the aqueous and organic phases was
maintained at 1 : 1. Extraction equilibrium in the sys-
tem was acquired after 5–7 min of contact between the
aqueous and organic phase; in all experiments, the
phase-contact time was 10 min. Phase separation after
stirring stopped was clear and virtually instantaneous.
The gold(III) extraction by extractant S is plotted in
Fig. 1 as a function of aqueous hydrochloric acid con-
centration. The steady decrease in the extraction with
increasing aqueous acidity indicates that the extraction
process is induced by the substitution of a donor atom
of the extractant for a chloride ion in AuCl–4 . Because
gold(III) extraction from low-acidity solutions is quite
efficient, extraction was studied from 0.5 M HCl solu-
tions, in which AuCl–4 is virtually the only gold(III)
species [15].
Chloroform is an optimal diluent for gold(III)
extraction: no intermediate phases were formed over
the whole range of the gold(III) and extractant concen-
trations. The use of other common solvents (e.g., tolu-
ene) generated an intermediate phase during extraction
even at very low gold(III) concentrations.
Figure 2 shows gold(III) extraction isotherms for the
test extractant. The slope of the initial segments of the
YAu(III) × 10–4, mol/L
3.5
The electronic absorption spectra of the extractant,
aqueous gold(III) solutions, and extracts were recorded
on a Specord M40 spectrophotometer. The IR spectra
of the extractant and extracted complexes were
recorded as KCl and KRS5 salt plates on a Specord
M80 spectrophotometer at 2000–250 cm–1. NMR spec-
tra were recorded on a Bruker AM 300 spectrometer
(300 MHz) in D-chloroform.
3.0
2.5
2.0
1.5
1.0
0.5
4
3
2
RESULTS AND DISCUSSION
1
Extractant S contains not only sulfide sulfur atoms
but also nitrogen and oxygen atoms in various func-
tional groups. Therefore, it is pertinent to study this
polyfunctional reagent as an extractant for gold(III)
from both for theory and application. Previously [13],
we studied the ligand properties of the extractant with
respect to chloro complexes of palladium(II), plati-
num(II), and platinum(IV); we showed that only sulfur
atoms participate in complexing. Thioalkyl radicals
have a spatial freedom and do not generate steric hin-
0
1
2
3
4
XAu(III) × 10–4, mol/L
Fig. 2. Gold(III) extraction isotherms from 0.5 M HCl solu-
tions at 25°ë: (1) 0.00025, (2) 0.00035, (3) 0.00050, and
(4) 0.00075 mol/L.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 11 2007

1800
KHISAMUTDINOV et al.
isotherms indicates the efficiency of metal-ion extrac- regation of metallic gold is not observed, likely because
tion. The run of the isotherms also indicates that in the
saturation regions the solvation number q is 0.25; i.e.,
gold(III) is extracted as (AuCl3)4 · S.
It is noteworthy that gold(III) extracts after separation
from raffinates do not undergo visible evolution for sev-
eral days. The decoloring of the organic phase, associ-
of the pyrimidine ring in the extractant molecule.
The sulfur atoms S1 of the extractant, which are far-
ther from the carboxy group, are more reactive in com-
plexing with palladium(II), platinum(II), and plati-
num(IV) than the atoms S2; they are the first to complex
ated with the reduction of gold(III) to gold(I), or the seg- because they have a higher negative charge [13]:
18
17
16
15
14 13
H3C H2C H2C H2C S2 H2C C O
–0.358
O
–0.005
O –0.275
–0.044
–0.027
H
H3C H2C H2C H2C S1 H2C CH H2C N
1
2
12
11
10
9
8
7
6
4
3
5
O
N
–0.072 CH3
–0.044
–0.403
CH2 CH CH2 S1 CH2 CH2 CH2 CH3
6'
7' 8'
9'
10'
11'
12'
O –0.275
–0.005
O C CH2 S2 CH2 CH2 CH2 CH3
–0.385
13' 14'
15'
16'
17'
18'
To determine the reactivity of the sulfur atoms of the The signals from the carbon atoms (9) and (9') experi-
extractant, we studied complex formation by the addi- ence additional downfield shift; in the spectrum of the
tions method under the conditions of gold(III) extrac- extractant, these were the highest field signals of all
tion. The experiments were carried out as follows. To a near-sulfur methylene carbon atoms (31.64 ppm). From
the aforesaid, we can assume that the sulfur atoms S1 of
the extractant are mainly the first to coordinate during
gold(III) extraction, and this occurs at low metal-ion
solution of the extractant in d-chloroform, a hydrochlo-
ric acid solution of gold(III) was added in portions,
gold(III) was extracted, a raffinate was separated, and
1
the H NMR spectrum of the extractant was recorded. concentrations. As the gold(III) concentration
The Au : S ratio (mol/mol) was increased gradually increases, the other sulfur atoms are involved in com-
until reaching 2 : 1. Although gold is a weak shifting plexing, but not oxygen or nitrogen atoms.
In the 1ç NMR spectrum of the extract obtained at
the set maximum Au : S ratio, equal to 4 : 1, near-sulfur
methylene protons experience an extra downfield
chemical shift of ~0.4 ppm and higher. The methylene
protons (8), (8'), (15), and (15'), which appear at
2.5−2.8 ppm in the spectrum of the extractant, in the
spectrum of the extract produce a broad signal at
2.8−3.3 ppm, and the signals from the methylene pro-
tons (14) and (14') experience an extra chemical shift to
3.5 ppm. Therefore, all sulfur atoms of the extractant
are in this case involved in complexing with gold(III).
In the IR spectrum of the extracted compound, iso-
lated by evaporating chloroform from the extract, there
is a strong band at 361 cm–1, associated with the anti-
symmetrical stretching vibrations ν(Au–Cl) in the
trans-Cl–Au–Cl group in an AuCl3 · L molecule [16,
agent in NMR spectra, the additions method made it
possible to trace the evolution of chemical shifts for
signals from several proton groups.
1
The H spectra change gradually so that the well
observable triplet signal from the methylene protons (9)
and (9') at 2.55 ppm are the first to experience an extra
chemical shift. As the gold(III) concentration increases
further, this signal appears in the region of proton sig-
nals from the near-sulfur methylene groups (8), (8'),
(15), and (15'), being shifted by ~0.19 ppm to low
fields. The signals from other methylene protons in
alkyl chains also experience insignificant chemical
shifts. The triplet signal from the protons of the termi-
nal methylene groups at 0.87 ppm in the spectrum of
the extractant is doubled after gold(III) addition is
stopped (0.92 and 0.97 ppm). However, the doublets
from diastereotopic methylene protons (14) and (14') at p. 196], and medium-intensity bands at 340–310 cm–1,
2.95 and 3.18 ppm remain virtually unchanged. In the which appear as shoulders and are also associated with
13C NMR spectrum of the extract (Au : S = 2 : 1), the
the stretching vibration modes ν(Au–Cl) and ν(Au–Cl)
(in the trans position to S). The position of the absorp-
signals from the carbon atoms of alkylthiomethyl chain
are doubled compared to the spectrum of extractant S. tion band at 361 cm–1 in the spectrum of the extracted
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 11 2007

GOLD(III) EXTRACTION
1801
complex is virtually the same as in the spectrum of the
logD
AuCl4 ion. This is because the stretching frequencies of
trans-Cl–Au–Cl in AuCl3 · L complexes are almost
unaffected by the nature of extractant S, which signifies
the low cis-effect of extractant S on Au–Cl bonds [16,
p. 199]. Oxygen and nitrogen atoms are not involved in
complexing, as proven by the fact that the strong
absorption bands at 1700 and 1656 cm–1, which belong
to the stretching modes of ν(ë=é) and ν(ë=é) +
ν(ëN) in the spectrum of the extractant, virtually do not
change their positions in the spectrum of the extracted
complex.
1.55
1.50
1.45
1.40
1.35
1.30
1.25
1.20
The electronic spectrum of reagent S in chloroform
displays a single, very strong resolved absorption band
at 37720 cm–1, which is due to the absorption of the
uracyl ring. In the electronic spectrum of the gold(III)
extract, there are a weak resolved band at 31240 cm–1
and weak absorption bands in the visible, which are
resolved only as shoulders. The set of absorption bands
and their positions indicate that an AuCl3 · L coordina-
tively solvated complex exists in the organic phase. The
strong absorption band at 31950 cm–1, corresponding to
3.20 3.25 3.30 3.35 3.40 3.45 3.50
103/T, ä–1
Fig. 3. Temperature effect on gold(III) extraction from
–4
0.5 M HCl solutions (Ò
= 1.6 × 10 mol/L; Ò = 5 ×
Au(III)
S
–4
10 mol/L).
ant withstood at least five cycles of gold(III) extrac-
tion/stripping, holding its efficiency and capacity
unchanged.
the intrinsic color of the AuCl–4 ion, does not appear.
To study the selectivity of the extractant, we
extracted complex hydrochloric acid solutions that con-
tained platinum(IV), iron(III), nickel(II), copper(II),
and zinc(II) in addition to gold(III). The analysis of raf-
finates showed that gold(III) was almost totally trans-
ferred to the organic phase, while the nonprecious and
The isolated extracted gold(III) complex, after dry-
ing, was a yellow-brown glassy mass, which could be
powdered. The decomposition onset temperature was
145°ë; the compound was stable during several days in
light under the ambient standard conditions.
Figure 3 displays a gold(III) extraction versus tem- ferrous metals remained in the aqueous phase.
perature curve, which shows that the gold(III) recovery
decreases with increasing temperature.
From the above-described studies, we can infer that
1,3-bis(2'-acetoxymethylthiobutyl-3'-thiobutylpropyl)-
From the whole data set, we can infer that the
extraction of gold(III) by extractant S follows the sol-
vation mechanism. The gold(III) extraction reaction at
cAu > cS is
6-methyluracyl, as an extractant for gold(III), combines
a high efficiency, capacity, selectivity, and redox stabil-
ity, and as distinct from the other tioethers, it can be
reused.
4AuCl–4 + S = (AuCl3)4 · S + 4Cl–.
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