Cooperative Ligands in Dissolution of Gold

Article abstract of DOI:10.1002/chem.202101028

Development of new, environmentally benign dissolution methods for metallic gold is driven by needs in the circular economy. Gold is widely used in consumer electronics, but sustainable and selective dissolution methods for Au are scarce. Herein, we describe a quantitative dissolution of gold in organic solution under mild conditions by using hydrogen peroxide as an oxidant. In the dissolution reaction, two thiol ligands, pyridine-4-thiol and 2-mercaptobenzimidazole, work in a cooperative manner. The mechanistic investigations suggest that two pyridine-4-thiol molecules form a complex with Au⁰ that can be oxidized, whereas the role of inexpensive 2-mercaptobenzimidazole is to stabilize the formed Au^I species through a ligand exchange process. Under optimized conditions, the reaction proceeds vigorously and gold dissolves quantitatively in two hours. The demonstrated ligand-exchange mechanism with two thiols allows to drastically reduce the thiol consumption and may lead to even more effective gold dissolution methods in the future.

Full text of DOI:10.1002/chem.202101028

Accepted Article
Accepted Article
Title: Cooperative Ligands in Dissolution of Gold
Authors: Eeva Heliövaara, Henri Liljeqvist, Mikko Muuronen, Aleksi
Eronen, Karina Moslova, and Timo Juhani Repo
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202101028
Link to VoR: https://doi.org/10.1002/chem.202101028
01/2020

10.1002/chem.202101028
Chemistry - A European Journal
COMMUNICATION
Cooperative Ligands in Dissolution of Gold
Eeva Heliövaara, Henri Liljeqvist, Mikko Muuronen, Aleksi Eronen, Karina Moslova and Timo Repo*
[*]
E. Heliövaara, H. Liljeqvist, Dr. M. Muuronen, A. Eronen, K. Moslova, Prof. T. Repo
Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014, Finland
E-mail: timo.repo@helsinki.fi
Dr. M. Muuronen
Current address: BASF SE, Carl-Bosch-Str. 38, 67056 Ludwigshafen, Germany
Abstract: Development of new, environmentally benign dissolution
methods for metallic gold is driven by needs in the circular economy.
Gold is widely used in consumer electronics, but sustainable and
selective dissolution methods for Au are scarce. Herein we describe
a quantitative dissolution of gold in organic solution under mild
conditions using hydrogen peroxide as an oxidant. In the dissolution
ligands to reduce the oxidation potential significantly, while item
(ii) was found to be the most important parameter to rationalize
the ligand dependence between different ligands and gold. Even
though 4-PS coordinates well with the gold surface in the thione
form, for efficient dissolution high concentrations are needed to
shift the complexation equilibrium toward bisliganded Au0 and to
stabilize the dissolved bisliganded AuI species. While the
dissolution with 4-PS proceeds in mild reaction conditions, its
limitation is the high consumption of the expensive ligand under
the oxidative stress. Here we describe a novel ligand-assisted
dissolution mechanism and demonstrate that a cooperative two-
ligand method provides an alternative, more atom-economic
approach to quantitatively dissolve gold in DMF solution.
reaction,
two
thiol
ligands,
pyridine-4-thiol
and
2-
mercaptobenzimidazole, work in
a
cooperative manner. The
mechanistic investigations suggest that two pyridine-4-thiol molecules
form a complex with Au0 that can be oxidized, while the role of
inexpensive 2-mercaptobenzimidazole is to stabilize the formed AuI
species via a ligand exchange process. Under optimized conditions,
the reaction proceeds vigorously and gold dissolves quantitatively in
an hour. The demonstrated ligand exchange mechanism with two
thiols allows us to drastically reduce the thiol consumption and may
lead to even more effective gold dissolution methods in the future.
Recycling of precious metals is at the core of the circular economy.
Gold is a valuable component in electronic waste, but one major
barrier toward its efficient recycling is the inability to selectively
dissolve Au under mild and benign conditions.[1-4] Among the
classical methods; aqua regia,[5] chlorine gas,[5] amalgam
formation,[5] and cyanide leaching,[6,7] which is particularly widely
used. Unfortunately, these methods require harsh reaction
conditions or hazardous reagents,[5-7] or both, in addition they are
also unselectively dissolving all metals simultaneously. Recently,
a new approach based on organic solvents and reactive ligand
systems has been introduced.[8-25] These methods are mainly
based on S-donor ligands[8-13,17,18] such as dithioxamides,[10]
tetraethylthiuram disulfides,[8-13] pyridinethiols,[17,18] and/or
Figure 1. The concept of 4-PS-assisted gold dissolution. The thione form of 4-
PS coordinates to the Au0 surface, and the formed bisliganded Au0 species
assist the one-electron oxidation and dissolution of AuI. The in situ formed
sulfate anion balances the charge.[18]
A series of different aromatic thiols capable of thiol–thione
tautomerism were selected for the background study, namely, 2-
mercapto-benzimidazole (1), 2-mercaptobenzothiazole (2),
thiobenzamide (3), 2-thio-2-thiazoline (4) and pyridine-2-thiol (5)
(Table 1). We initiated the study by immersing Au powder (with
particle size of 1.5–3 m) into DMF solutions of a thiol in 200-fold
excess. Hydrogen peroxide was used as an oxidant, and the
reaction time was set for 20 min. Under these reaction conditions
the Au powder was dissolved nearly quantitatively with 4-PS while
the other thiols could only dissolve trace amounts of gold (see
Supporting Information Section S2.1 for details). However, the
ability of 1 to dissolve 40% of the gold under these conditions
during the first 20 min is noteworthy. The key difference between
the two ligands able to dissolve gold is their affinity to form
bisliganded Au0 species that can be oxidized.[18] Accordingly, the
formation energy of Au0 bisliganded species from monoliganded
Au0 species is clearly exergonic with 4-PS when computed at DFT
level (ΔG= -11.2 kcal/mol) and slightly endergonic with 1 (∆G= 1.3
kcal/mol)[18], while the oxidation potentials of both Au0L2 species
are nearly same (L= thione ligand). Indeed, the ability of 4-PS to
form bisliganded complexes with gold drives the one-electron
oxidation and is superior among the thiols studied so far.
halides[19-24]. However, for efficient dissolution of gold, powerful
[8-13]
and aggressive oxidants, such as fairly toxic I2
or corrosive
SOCl2,[14-16] are needed. Understanding the dissolution
mechanism of gold is essential to learning how to design
sustainable dissolution methods for other precious metals.
Oxidation of gold is a thermodynamically uphill and challenging
task. However, it is envisioned that if the energy cost is small
enough, facile dissolution of gold can take place even with mild
oxidants. We reported recently a pyridine-4-thiol (4-PS) -assisted
dissolution of Au in N,N-dimethylformamide (DMF) solutions
using H2O2 as a benign oxidant (Figure 1).[18] We concluded that
the dissolution with thiols depends on three parameters: (i) ability
of thiol to undergo thiol–thione tautomerization, (ii) ability of thione
to coordinate with Au0 and form bisliganded Au0 species, and (iii)
the oxidation potential of this bisliganded species. The oxidation
potential (iii) was observed to be highly ligand dependent, which
is because gold’s 6s1 electron should be delocalized on the
1
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4
2
3
4
5
1
1
1
1
1
1
1
1
20
20
20
20
20
20
4
2
120
120
120
120
120
120
120
120
120
120
120
120
5[c]
6[c]
6[c]
AuI complexes are coordinatively labile and have an intriguing
propensity to undergo ligand-exchange reactions.[27,28] This
directed us to investigate the ligand-exchange reactions, that is to
design a novel, two-ligand method for dissolution of gold. On the
basis of the above, 4-PS was an obvious choice as one of the
ligands, while ideally an excess of inexpensive thiols 1-4
(collectively SL; see Table 1) could be used to stabilize the
generated AuI centers. In this respect, we focused on reactions
where 4-PS was used in 2-fold excess compared to Au0. As an
expected result, there was no dissolution of gold, even in
extended 120 min reactions when 4-PS was used alone (Table 1,
entry 3). Next, we repeated the experiment, but now added thiols
1–5 in a small, 20-fold excess to the reaction mixture (Table 1,
entries 4−8). While the addition of thiols 2–5 had no influence on
the dissolution, thiol 1 provided an impressive 60% of dissolved
gold. Further enhancement in the dissolution of Au was observed
by increasing the amount of 1. Gratifyingly, using 50-fold or higher
excess of 1 gave quantitative dissolution reactions (Table 1,
entries 11 and 12).
5
2
6
2
7
2
9[c]
8
2
60[c]
20d
8[e]
99f
>97f
76g
20h
40i
9
none
10
11
12
13
14
15
4
50
100
100
20
20
2
2
none
4
1
*Au concentration analyzed by flame atomic absorption spectroscopy
(FAAS). All measurements were carried out in air/acetylene flame by
using a Au hollow cathode lamp (see Supporting Information for
details). [a]Au powder (2 mg, 10 µmol, particle size 1.5–3.0 µm), 4-PS
(226 mg, when applicable), 1 (306 mg, when applicable), S8 (20 µmol,
5.2 mg), H2O2 (33 w% in water, 92.8 µl), DMF (5 ml), 60C, 20 min.
bAu powder (2 mg, 10 µmol), 4-PS (2.3 mg), H2O2 (92.8 µl), DMF (5
ml), 60C, 120 min. cAs in [a], but 1 (31 mg, when applicable), 2 (34
mg, when applicable), 3 (28 mg, when applicable), 4 (24 mg, when
applicable), 5 (22.6 mg, when applicable), 120 min. dAs in [c], but no
4-PS. eAs in [c], but 4-PS (4.5 mg) and 1 (6.2 mg). fAs in [c], but 1
(155 or 77.5 mg), gAs in [f], but no 4-PS. hAs in [c], but 4-PS (4.5 mg,
4 equiv), iAs in [c], but 4-PS (1.1 mg, 1 equiv).
We continued the study to identify the role of 4-PS in these
reactions. A clear inhibition in the dissolution efficiency was
observed when decreasing the 4-PS concentration to 1 equiv
(Table 1, entry 15). When repeating the reaction without 4-PS, low
yields were obtained (Table 1, entry 9). The acceleration effect is
nicely demonstrated in entries 8 and 9, which show 40% increase
in dissolution when 4-PS is used as an initiator. Based on that, 4-
PS can account for the reactive species needed to initiate the
dissolution in the two-ligand system. Experiments showed that
with longer reaction times AuI is reduced back to Au0 and that the
optimal reaction time is 2 h. All in all, the results emphasize the
cooperativity of these two ligands and that high concentrations of
1 shift the complexation equilibrium toward the bisliganded AuI
species in short time periods. The reactivity with the two-ligand
method is unprecedented, as 4-PS alone does not dissolve Au in
the low ligand concentrations used and 1 alone does so to only a
minor extent (Table 1, entries 8 versus 9). Full details of reaction
optimization can be found in Supporting Information Section S2.
To continue our experimental investigations, thiol 1 was studied
with H NMR under the dissolution conditions. It seems to occur
1
in its thione form, as a characteristic S–H signal is absent in DMF-
d7/H2O2 solution (see Supporting Information Figures S5-S12).
Also, the number of signals (equivalent carbons) in the 13C NMR
spectrum further implies that 1 is present in a C2–symmetric
thione form; if otherwise, the absence of a NH group would
destroy the symmetry of the molecule. A similar tendency was
observed earlier for 4-PS.18 Unavoidably, aromatic thiols are
easily oxidized in the DMF-d7/H2O2 solution, and accordingly, a
new singlet appears at 9.5 in the aromatic region of the spectrum,
indicating the formation 1H-benzimidazole (Figure 2, 7, 7%), an
oxidized derivative of 1. The formation of 7 and 8, as well as the
presence of 4’,4’-dipyridylsulfide (4-DPS) as a second major
oxidation product of 4-PS, were also confirmed by HRMS analysis
(Figure 2, 7 and 8; Supporting Information Figures S17 and S18).
Table 1. The effect of 4-PS and thiol ligands (SL) on Au dissolution.
When Au powder was introduced to DMF-d7 solution containing
4-PS and 1 in a 1:10 ratio (reaction molar ratio Au/4-PS/1/S8/H2O2
1:2:20:2:100), the 1H NMR spectrum is dominated by signals
arising from thione 1 (4:1 ratio of thione; see Supporting
Information Figures S13–S15). Because of the overlapping peaks
in 1H NMR, the assignment of the formed gold species is not
unambiguous, whereas high resolution ESI-TOF mass
spectrometry turned out be a highly useful analytical method for
their identification. With high 4-PS loadings, 200 equiv (Table 1,
entry 1), the main dissolved AuI species was [Au(4-PS)2]+ (Figure
2, 9), along with minor amounts of [Au(4-PS)(4-DPS)]+ (Figure 2,
10). After the completion of the dissolution reaction, we added an
equimolar amount of 1 (200 equiv) into the same reaction solution.
During 15 min stirring, the mass peak at m/z 419 attributed to
[Au(4-PS)2]+ decreased, and the peak assigned to [Au(1)2]+
simultaneously increased (Figure 2, 11, Supporting Information
Figures S22-S28). After a 15 min interval, the main AuI complex
Entry
SL
(No.)
SL
(equiv)
4-PS
(equiv)
Time
(min)
Dissolved
Au (%)*
1
2
3
none
1
none
200
200
none
2
20
85[a]18
40[a]
ND[b]
20
none
none
120
2
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detected was [Au(1)2]+ (Figure 2, 11). Interestingly, a new mass
peak at m/z 458 formed soon after adding 1 to the solution. It is
assigned to a novel mixed-ligand species Au(4-PS)(1)+ (Figure
2, 12, Supporting Information Figure S29), which is likely a
fingerprint of the hypothesized ligand-exchange reaction between
4-PS and 1 on the AuI cation.
to form the Au(1)2 complex from the corresponding Au(4-PS)2
complex in both redox states are shown in Figure 4: 4-PS clearly
has the greatest affinity to form bisliganded complex with Au0; the
Au0(4-PS)2 complex is 17.6 kcal/mol more stable than the
corresponding Au0(1)2 complex, thus rationalizing the higher
affinity of 4-PS to dissolve gold. This is also an important
parameter, as the Au0(1)2 complex is easier oxidized than the
Au0(4-PS)2 complex. The ligand preference is still the same for
the oxidized complexes, but the relative stability of the AuI(1)2
complex is increased by 4 kcal/mol, going from 17.6 to 13.2
kcal/mol, while the absolute computed values are susceptible to
errors rising especially from solvation. The observed trend
indicates that bisliganded 1 gold complexes can exist after one-
electron oxidation, especially when considering that AuI has an
overall higher tendency to form liganded complexes than Au0. The
mixed-ligand systems have the complexation free energies and
oxidation potentials between homocoordinating gold, implying
that these mixed complexes can also be active species in the
dissolution process.
We extended our dissolution studies to cover other group 11
metals. In our preliminary experiments, copper and silver were
dissolved under mild conditions with 1 as the only ligand (see
Supporting Information S2.2 and S3.2.3). The main AgI species
identified in HRMS analysis was bisliganded analogue of [Au(1)2]+,
[Ag(1)2]+, respectively (Figure 3, 13, Supporting Information
Figure S72). DMF/H2O2 solutions of 1 contain a mass peak at m/z
222 in HRMS, which is attributed to an in situ C–S coupled side-
product between 1 and the solvent DMF (C10H11N3OS+H+,
Figure 3, 8). This coupling product is also supported by HRMS of
CuI species containing two 8 molecules coordinated to CuI
centers (Cu(8)2+, m/z 505, Figure 3, 16, see Supporting
Information S3.2.3 for details).
Figure 2. Species found in high-resolution ESI-TOF mass spectra when
dissolving Au0 with 4-PS and 1.
We further studied the two-ligand method with low loadings of 4-
PS and 1, 2 and 20 equiv, respectively (Table 1, entry 8). The
formation of the mixed-ligand species Au(4-PS)(1)+ together
with [Au(1)2]+ was again identified by ESI-MS (see Supporting
Information Figure S52). Additional efforts were made to confirm
the formation of the mixed-ligand complex. We synthesized Au(4-
PS)2Cl as a model complex and studied the isolated complex
under the dissolution conditions. When a slight excess (2.5-fold)
of 1 was added, the mass peak m/z 419 attributed to the original
[Au(4-PS)2]+ slowly decreased, simultaneously as the peak m/z
458 assigned to a mixed-ligand complex [Au(4-PS)(1)]+ appeared
(SI Figs. S81-S84). When a 10-fold excess of 1 was used instead,
the equilibrium shifted from [Au(4-PS)2]+ toward the mixed-ligand
complex and further on toward [Au(1)2]+. An indicator of the
ligand-exchange on AuI is also the appearance of free 4-PS
sulfide in the mass spectrum (m/z 221, SI Figures S85-87). These
results give insight into
a spontaneous, step-wise ligand-
exchange reaction between 4-PS and 1 on the coordination
sphere of AuI. As importantly, when the dissolution reaction was
started in the reverse order, by addition of 1, the [Au(1)2]+ complex
was formed. Now, the addition of 4-PS to the solution also
generated the formation of the Au(4-PS)(1)+ complex. This result
underlines the reversibility of the ligand-exchange reaction on AuI
and the central role of the mixed-ligand complex in the process
(Figure 4). Accordingly, the bisliganded AuI complexes are
present in equilibria determined by the applied reaction conditions.
Figure 3. CuI and AgI species found in high-resolution ESI-TOF mass spectra
in dissolution experiments with 1.
To gain further insight into the ligand-exchange reaction and gold
complexes, we studied theoretically their structures and
energetics (TPSS-D3/def2-TZVP/COSMO in DMF). The
computed free energies for step-wise ligand exchange reactions
3
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Figure 4. (a) A schematic picture of the cooperative two-ligand dissolution of Au0; Step I: Generation of Au0 bisliganded species, Step II:
Oxidation of Au0(L1)2 to AuI(L1)2, Step III: Ligand-exchange reaction and formation of mixed-ligand species AuI(L1)(L2), Step IV: Formation of
bisliganded AuI(L2)2. (b) The complexation free energies for Au0 and AuI and the corresponding oxidation potentials of 9, 11 and 12.
In summary, we described a conceptually new gold dissolution
method. It benefits from the use of two cooperative thiol ligands:
2-mercaptobenzimidazole (1) and pyridine-4-thiol (4-PS), which
together form a reactive ligand mixture and give a quantitative
dissolution of Au under mild conditions. The reaction benefits from
the favorable formation energy for the bisliganded Au0 complex
with 4-PS. After initial complexation, the dissolution proceeds by
ligand-assisted oxidation of Au0(4-PS)2 to AuI(4-PS)2. The formed
AuI centers are prone toward ligand-exchange reactions (Figure
4a, steps III and IV), and by using an excess of ligand 1 the
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Acknowledgements
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Entry for the Table of Contents
Dissolution of elemental gold with cooperative thiol ligands: Gold is dissolved promptly with a combination of two different thiol
ligands via ligand-exchange reaction. Pyridine-4-thiol serves as the initiating ligand and 2-mercaptobenzimidazole ligands stabilize
the formed AuI cations in the dissolution.
Research group Twitter username: @HelsinkiCatLab
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