Pyridinethiol-Assisted Dissolution of Elemental Gold in Organic Solutions

Article abstract of DOI:10.1002/anie.201810447

Dissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine-4-thiol (4-PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4-PSH to pyridine-4-thione (4-PS), coordination with Au⁰, and then oxidation of the Au⁰ thione species to Au^I simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a Au^I complex bearing two 4-PS ligands and SO₄^2? as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Full text of DOI:10.1002/anie.201810447

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Pyridinethiol-Assisted Dissolution of Elemental Gold in Organic
Solutions
Authors: Minna Räisänen, Eeva Heliövaara, Feda’a Al-Qaisi, Mikko
Muuronen, Aleksi Eronen, Henri Liljeqvist, Martin Nieger,
Marianna Kemell, Karina Moslova, Jani Hämäläinen, Kalle
Lagerblom, and Timo Juhani Repo
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201810447
Angew. Chem. 10.1002/ange.201810447
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10.1002/anie.201810447
Angewandte Chemie International Edition
COMMUNICATION
Pyridinethiol-Assisted Dissolution of Elemental Gold in Organic
Solutions
Minna Räisänena+, Eeva Heliövaara+, Feda’a Al-Qaisib, Mikko Muuronenc, Aleksi Eronen, Henri
Liljeqvist, Martin Nieger, Marianna Kemell, Karina Moslova, Jani Hämäläinen, Kalle Lagerblom, and
Timo Repo*
Abstract: Dissolution of elemental Au in organic solutions is a
contemporary approach to lower environmental burden related to
gold recycling. Here we describe fundamental studies on a highly
efficient method for dissolution of elemental Au which is based on
DMF solutions containing pyridine-4-thiol (4-PSH) as a reactive
ligand and hydrogen peroxide as an oxidant. Dissolution of Au
proceeds through several elementary steps: isomerization of 4-PSH
to pyridine-4-thione (4-PS); coordination with Au0; then oxidation of
Au0 thione species to AuI simultaneously with the oxidation of free
pyridine thione to elementary S and further to sulfuric acid. The final
dissolution product is an AuI complex bearing two 4-PS ligands and
somewhat toxic I2 using dithioxamides or tetraalkylthiuram
disulfides (e.g. tetraethylthiuram disulfide Et4TDS) as donors to
oxidize Au0 to AuIII (Fig. 1b).13-17 The third concept is based on
pyridinethiols but characterized by slow dissolution of Au in
ethanol solutions (0.06×10-3 mol/(m2h)) (Fig. 1c).18 As thiols
are commonly used as etch resists for Au0 surfaces, these
results were intriguing and counterintuitive. Therefore, we took
an initiative to carry out further studies and reveal mechanistic
details related to the 4-PSH-assisted dissolution.
2-
SO4 as a counter ion. The role of the ligand is crucial as it assists
the oxidation process and stabilizes and solubilizes the formed Au
cations.
Noble metals, including Au, Ag and Pt, are scarce elements in
earth’s crust and greatly needed in high-tech applications.1,2
They are, therefore, precious and their demand is continuously
growing. The possibility to recycle the elements from multi-metal
materials such as printed circuit boards (PCBs) is crucial when
sustainability and the circular economy are considered.3-7 In
general, recovery of Au in the mining industry is mostly based on
the hydrometallurgical cyanidation process, which relies on the
use of stoichiometric amounts of cyanide salts and produces
large amounts of hazardous waste.8,9 Consequently, a new
and fairly unexploited concept known as dissolution of noble
metals in organic solvents is highly attractive – it offers the
possibility to develop benign and selective dissolution
methods.10-18 One method
is
based
on pyridine,
dimethylformamide or imidazole solutions of thionyl chloride
(SOCl2) called organic aqua regia (Fig. 1a) which, regardless of
the toxicity of SOCl2, benefits from high selectivity towards
Pt/Au/Pd and Au/Pd mixtures.10-12 Another method is based on
Figure 1. Organic solvent-based methods for dissolution of elemental Au. a,
SOCl2 with py/Im/DMF b, Et4TDS/I2 c, 4-PSH/EtOH/O2 and comparison with
hydrometallurgical cyanide leaching. DR stands for dissolution rate. Structures
illustrate proposed Au species formed during the dissolution.


[*]
Dr. M. Räisänen+ , E. Heliövaara+ , Dr. F. Al-Qaisi, A. Eronen, H.
Liljeqvist, Dr. M. Nieger, Dr. M. Kemell, K. Moslova, Dr. J.
Hämäläinen, Dr. K. Lagerblom, and Prof. T. Repo*
Department of Chemistry, University of Helsinki, A.I. Virtasen aukio
1, FI-00014, Helsinki (Finland)
Here we report rapid and quantitative dissolution of Au in DMF
solution of 4-PSH and H2O2. The achieved rate, 0.10 mol/(m2h),
is 1600-fold higher than that measured for the earlier 4-
PSH/EtOH protocol.18 The reaction mechanism is supported by
identification of intermediate species and rationalized by
quantum-chemical calculations. In terms of the circular economy,
the study opens opportunities to develop technologically and
economically viable dissolution of elemental gold.
E-mail*: timo.repo@helsinki.fi

[a]
[b]
[c]
Dr. M. Räisänen+
Current address: Academy of Finland, Hakaniemenranta 6, 00530,
Finland
Dr. F. Al-Qaisi
Current address: Department of Chemistry, University of Petra,
961343, Amman, Jordan
Dr. M. Muuronen
The study was initiated by conducting Au dissolution
experiments using our previously published protocol18: a small
amount of fine Au powder was added to 16 mM EtOH solution of
4-PSH and the mixture was stirred for 23 h under 10 bar of O2 at
22 °C. This resulted in only 14% of dissolved Au (Table 1, entry
1). To increase Au dissolution, a series of reactions was carried
out varying the solvent, oxidant, ligand concentration and
Department of Chemistry, University of California, Irvine, 1102
Natural Sciences II, Irvine, California 92697-2025, United States
Current address: BASF SE, Carl-Bosch-Strasse 38, Ludwigshafen,
Germany
[+]
M. Räisänen and E. Heliövaara contributed equally
Supplementary information and chemical compound information are
available in the online version of the paper. Correspondence and
requests for material should be addressed to T.R. and those related
to computational studies to M.M.
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10.1002/anie.201810447
Angewandte Chemie International Edition
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Figure 2. Elementary reaction steps during the dissolution of elemental Au. Step I: isomerization of pyridine-4-thiol to pyridine-4-thione and coordination with the
Au0 surface, Step II/III: oxidation of Au0 to AuI and simultaneous oxidation of free pyridine-4-thione, Step IV: further oxidation of S8 to sulfuric acid.
temperature (see SI). As a result, we found that Au could be
quantitatively dissolved upon heating at 60 C in DMF solutions
of 4-PSH using H2O2 as an oxidant (Table 1, entry 2). Next, a
series of experiments was performed with Au powder in DMF
solutions with 4-PSH concentrations of 20, 102, 203 and 406
mM, resulting in 1%, 50%, 67% and 84% of dissolved Au in only
8 min (Table 1, entries 3-6) showing that the dissolution
efficiency correlates directly with the 4-PSH concentration.
Importantly, quantitative dissolution of Au sample was achieved
either with extended reaction time and low 4-PSH concentration
or within 25 min using 203 mM 4-PSH concentration (Table 1,
entries 2, 7).
the main Au cation in the solution. In addition to 1, AuI
complexes 5 and 6 bearing 2 and 4-PS ligands were identified
with HRMS (Fig. 2, step III, SI Figs. S34-S37).
We were able to isolate 1 in crystalline form with the SO42- as the
counter anion and use XRD for structural analysis (SI 3.3).
Accordingly, the AuI centers coordinate two neutral 4-PS ligands
2-
and SO4 balances the overall charge of two adjacent AuI
complexes (Fig. 2). In the solid state, the dimeric structure is
further stabilized by a weak aurophilic interaction (3.1314 Å).
Together with formation of 1 in the dissolution reaction, a
significant amount of yellow crystals also precipitate from the
DMF solution after prolonged storage. Upon XRD analysis, the
crystals were identified as elemental sulfur (S8). As strongly
oxidizing conditions are needed for efficient dissolution of
elementary gold, S8 might have an important role; it can react
with H2O2 and form SO2 and successively SO3, which then
generates sulfuric acid in contact with H2O (Fig. 2, step IV). The
combination of sulfuric acid and H2O2 is thought to form a strong
oxidant. However, further addition of H2SO4 does not improve
dissolution as the amount of available 4-PS is reduced by
1
According to H NMR studies conducted during the dissolution,
pyridine-4-thiol (4-PSH) is prone to isomerize to pyridine-4-
thione (4-PS) which is the major component in the studied d7-
DMF solutions (Fig. 2, Step I, SI Figs. S4, S5). We assume that
the isomerization of 4-PSH is highly beneficial for 4-PS
coordination to Au0 surfaces and, in this way, for initiating the
dissolution. Upon heating the solution to 60 C and introducing
H2O2, signals belonging to 4-PS disappear and signals
belonging to 4,4’-dipyridyl sulfide 2, 4,4’-dipyridyl disulfide 3 and
4,4’-dipyridyl trisulfide 4 appear in the 1H NMR spectrum (Fig. 2,
step II, SI Figs. S6, S7). This was also confirmed by HRMS
studies wherein transformation of 4-PSH to 2, 3 and 4 was
clearly established (SI Figs. S28-S33). At the same time, HRMS
analysis of the reaction mixture revealed that AuI(4-PS)2+ 1 is
2-
protonation, but added SO4 anion (e.g. (NH4)2SO4) slightly
assists the dissolution reaction (Fig. 2 and SI 3.4).
Evidently, due to the marked excess of 4-PSH needed for
2-
efficient dissolution and the observed SO4 anion of the AuI
complex  the relation between in situ formed S8 and the
dissolution rate of Au appeared to be important. To confirm this
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10.1002/anie.201810447
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COMMUNICATION
hypothesis, we attempted to use external S8 as an accelerator
by introducing one equivalent of S8 into the reaction. With this
approach, the dissolution of Au with high 4-PSH concentrations
(406 and 203 mM) and external S8 had a minor effect on the
dissolution efficiency (Table 1, entries 8, 9). In contrast, when
the concentration of 4-PSH was markedly reduced (20 mM), the
added S8 greatly enhanced the dissolution of Au (Table 1, entry
11 vs. 3). These results imply that the dissolution reaction
depends strongly on the presence of S8. With low 4-PSH
concentrations there is less free ligand and in situ formation of
S8 is the rate limiting factor. Notably, no rise in the dissolution
efficiency was observed with the above stoichiometric amounts
of S8. On the other hand, excess of 4-PSH is needed for
effective dissolution. As 4-PS is a monodentate, neutral and
weakly coordinating ligand, a slight excess drives the equilibrium
towards bisligand Au species, which are then prone to oxidize
and form 1 (see also below).
capability of 2-PSH, 3 and 2-MBIH; but the use of S8 as an
additive does not improve the Au dissolution rate. For example,
2-MBIH gives a similar 8% Au dissolution with and without S8 in
otherwise similar reaction conditions (SI 3.4). This result implies
that the dissolution of Au with 2-MBIH does not rely on rapid
oxidation of thiol ligand to S8. Instead, it seems likely that the
dissolution rate is dependent on the coordination ability of the
thione isomer of the ligands to the Au surface and on the
oxidation potential of the formed reactive intermediate.
The ligand influence on the one electron oxidation of Au0 was
computationally rationalized following the suggested mechanism
that the dissolution is initiated by ligand coordination (Table 2).
In principle, gold could be present in solution as mono- or
bisligand complexes, although only the latter is experimentally
observed. According to RPA19,20 and DFT calculations at
TPSS-D321,22/def2-TZVP23 level in solution (see SI), the
coordination of the second ligand to the monoligand Au complex
lowers the oxidation potential of Au0, 1.2 V with 4-PSH and 1.6 V
with 2-MBIH (Table 2, entries 1, 3). Thus it seems that the
formation of bisligand Au0 centers is crucial for the oxidation
reaction. Such Au0 species are most likely formed at the corners
and edges of the gold surface, thus rationalizing a similar
etching order observed in experiments with Au thin films (SI 4).
According to the calculations, the significant lowering of the
oxidation potential of bisligand Au0 species is caused by
delocalization of the 6s1 electron on the ligands, thus allowing
the one-electron oxidation of the ligand instead of the metal.
This theoretical insight further supports the experimental results
which show that Pd and Pt were not dissolved with pyridinethiol
ligands as their dissolution would require two-electron oxidation
(SI 5.2).
Table 1. Optimization of dissolution conditions for Au powder.
T
(C)
Entry
Solvent
Oxidant
Time
Dissolved
Au (%)*
c4-PS
(mM)

14a
1
2
3
4
5
6
7
EtOH
DMF
DMF
DMF
DMF
DMF
DMF
O2
16
16
20
102
203
406
203
23h
23h
22
60
60
60
60
60
60

99a
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2

1b
8 min
8 min
8 min
8 min
25 min

50b

67b

84b

99c
With added S8

86d
8
DMF
DMF
DMF
DMF
DMF
H2O2
H2O2
H2O2
H2O2
H2O2
406
203
102
20
8 min
8 min
8 min
8 min
8 min
60
60
60
60
60

73d
9

61d
10
11
12

33d

30e
20
*Au concentration analyzed by flame atomic absorption spectroscopy (FAAS).
All measurements were carried out in air/acetylene flame by using an Au
hollow cathode lamp (see SI for details). a Conditions: 1 mg of Au powder (5
µmol, particle size 1.5-3.0 µm), 27 mg of 4-PSH (243 µmol, c[4-PSH] = 16
mM), H2O2 (33% in water, 46,4 µl, when applicable), O2 (10 bar, when
applicable), DMF (15 ml), 23 h. b Conditions: 2 mg of Au powder (10 µmol,
particle size 1.5-3.0 µm), H2O2 (92.8 µl), DMF (5 ml), 8 min. c[4-PSH] = 20,
102, 203 or 406 mM. c Conditions: Au thin film (300 nm), 294 mg of 4-PSH
(c[4-PS] = 203 mM), H2O2 (33% in water, 61 µl), DMF (13 ml). Mass of Au ca.
1.3 mg. d Conditions: as in b, but Au : S8 = 1:1. e Conditions: as in b, but
Au : S8 = 1:4.

Table 2. Calculated oxidation potentialsb and Au complex formation

energiesc
.
Entry
Ligand
Monoligand
complex
Bisligand
complex
oxidation
potential
Formation
energy of
bisligand
complex[c]
(kcal/mol)
ΔG(L-
Dissolved
metal

(%)*a
To determine the ligand dependence of the Au dissolution,
oxidation
potential
experiments with 4-PSH, pyridine-2-thiol (2-PSH), 4,4’-dipyridyl


(V)b
(V)b
disulfide
3
and 2-mercaptobenzimidazole (2-MBIH) were
M→L-M-L)
-11.2
-10.6
1.3
conducted under the same reaction conditions. At 60 C and
with 200-fold excess of the ligand, 4-PSH dissolves 84%, both 2-
PSH and 3 dissolve 11% and 2-MBIH dissolves 8% of Au.
According to 1H NMR and HRMS analysis of the reaction
mixtures, with 2-PSH the thione/thiol equilibrium resides on the
thione side. In the presence of the oxidant, 2-PSH is readily
oxidized to 2,2’-dipyridyl disulfide which is the major component
in the solution (SI Figs. S15-S20). Interestingly, in the presence
of H2O2, neither 3 nor 2-MBIH produce sulfides in amounts that
1
2
3
4-PSH
2-PSH
2-MBIH
4.1
4.3
4.3
2.9
3.0
2.7
84
11
8
*Experimental conditions: DMF (5 ml), 60 C, 8 min, M0/LH/H2O2 1:200:100.
Au concentrations analyzed by FAAS (see SI for details). a Au0 (2 mg, 0.010
mmol), 4-PSH (226 mg, 2.033 mmol) or 2-PSH (226 mg, 2.033 mmol) or 2-
MBIH (305 mg, 2.033 mmol), H2O2 (33% in water, 92.6 µl), DMF (5 ml). b
Computed absolute oxidation potentials (in V) at TPSS-D3/def2-
TZVP/COSMO levels. c Formation free energies of the bisligand metal
complexes from the monoligand complexes at TPSS-D3/def2-TZVP/COSMO
(left) levels. Energies are in kcal/mol.
1
are detectable by H NMR (SI 3.1.2). The slow formation of S8
from thiol ligands could partially explain the very low dissolution
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10.1002/anie.201810447
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The oxidation potentials of reactive intermediates increase in the
order (2-MBI)2Au0<(4-PS)2Au0<(2-PS)2Au0 and the significantly
lower reactivity of 2-PSH in the dissolution of Au can be
rationalized by considering the 0.1 V higher oxidation potential of
(2-PS)2Au0 compared with the (4-PS)2Au0 complex (Table 2,
entries 1-3). On the other hand, the (2-MBI)2Au0 complex
exhibits a 0.2 V lower oxidation potential than (4-PS)2Au0 but its
endothermic formation energies hamper the overall dissolution
reaction (Table 2, entry 3). Unambiguously, the rate of
dissolution is dependent on the formation energy of the bisligand
intermediate and its oxidation potential.
oxide layer on the metal surface. This leads unexpectedly to a
higher experimental dissolution efficiency with Au. From the
extended ligand series, (2-MBI)2Cu0 and (2-MBI)2Ag0 species
exhibit lower oxidation potentials than their 4-PS analogues, but
the dissolution reactivity towards Ag0 (as with Au0 above) is
reduced by endothermic complex formation energies (Table 3,
entry 2). (2-MBI)2Cu0 has an exothermic formation energy and
shows
a
high dissolution rate. This rationalized the
experimentally observed minor dissolution of Ag0 with 2-MBIH
assisted reactions while Cu0 is efficiently dissolved.
Table 3. Calculated oxidation potentialsc and Ag and Cu complex formation
4-PSH assisted dissolution can also be successfully applied to
other coinage metals, namely silver and copper, i.e. metals with

energiesd
.
the stable oxidation state of +1. Using
a high 4-PSH
Entry
Ligand
/metal
Monoligand
complex
oxidation
Bisligand
complex
oxidation
potential
Formation
energy of
bisligand
Dissolv.
metal
concentration (c4PSH = 742 mM) at 60 C, the reaction was
very fast and after 8 min 61% of Ag and 72% of Cu were
dissolved (Table 3, entries 1, 3). When 4-PSH loading was
reduced, the reaction slowed down and with 10-fold ligand
excess only minor amounts of the metals dissolved (4% Cu,
17% Ag). Yet a significant accelerating effect was observed for
Cu when the experiment was repeated with S8 additive in
otherwise similar dissolution conditions (20% Cu, SI 5.4). To
identify the structures of the resulting AgI and CuI complexes,
the products were isolated and analyzed with FTIR, TGA and
elemental analysis (EA). The structures of CuI complex 9 and
AgI complex 10 resemble that of 1. The counter anion in 9 and
10 can be clearly identified from FTIR spectra as the observed
broad peaks around wave numbers 1100 cm-1 (strong) and 600

(%)*a

complexc
potential


(V)b
(V)b
(kcal/mol)
ΔG(L-M
→L-M-L)
-8.5
3.0
-19.6
-5.6
4-PSH/Ag
2-MBIH/Ag
4-PSH/Cu
2-MBIH/Cu
3.5
3.6
3.4
3.6
2.8
2.7
2.8
2.5
61a
15a
72b
78b
1
2
3
4
*Experimental conditions: DMF (5 ml), 60 C, 8 min, M0/LH/H2O2 1:200:100.
Metal concentrations analyzed by FAAS (see SI for details). a Ag0 (2 mg,
0.019 mmol), 4-PSH (412 mg, 3.709 mmol, when applicable) or 2-MBIH (557
mg, 3.709 mmol, when applicable), H2O2 (33% in water, 172 µl). b Cu0 (2 mg,
0.032 mmol), 4-PSH (700 mg, 6.297 mmol, when applicable) or 2-MBIH (945
mg, 6.297 mmol, when applicable), H2O2 (33% in water, 292 µl). c Computed
absolute oxidation potentials (in V) at TPSS-D3/def2-TZVP/COSMO levels. d
Formation free energies of the bisligand metal complexes from the monoligand
complexes at TPSS-D3/def2-TZVP/COSMO (left) levels. Energies are in
kcal/mol.
cm-1 are characteristic of the ν3 and ν4 vibrational modes of SO4
,
2-
respectively24-27 (SI 5.5). According to EA (CHNS) and TGA, the
CuI center binds two 4-PS ligands and a sulfate anion stabilizes
the overall charge of the dimeric structure, while each AgI center
Recycling of noble metals from secondary sources including
electronic waste is becoming increasingly important when
modern societies are heading towards a circular economy. In
this respect, dissolution of Au films is essential and methods
other than cyanidation or Hg/Au amalgam extractions are also
needed to fulfill the criteria of green chemistry and
sustainability.28 We studied the dissolution of 300 nm thick Au
films (1 cm × 2.2 cm on mica substrate) and noted that etching
began at the edges and then steadily progressed towards the
center of the film (Fig. 3a). The studied Au film was fully
dissolved in a DMF solution of 4-PSH/H2O2 (cPSH = 203 mM)
in 25 min, which corresponds to an Au dissolution rate of 0.06
mol/(m2h). Remarkably, with added S8, an even more efficient
dissolution rate was achieved (0.10 mol/(m2h) corresponding to
a 1600-fold enhancement when compared to our previous 4-
2-
binds only one 4-PS ligand and SO4 serves as the counter
anion.
Next, dissolution experiments with 2-PSH and 2-MBIH were
performed for Cu0 and Ag0 similarly as before for Au0; Cu and Ag
powders were hardly affected by 2-PSH solutions while
reactions with high 2-MBIH concentration dissolved Ag 15% and
Cu 78% in 8 min reaction time. When the ratio of 2-MBIH to Cu0
was reduced from 200:1 to 10:1 and S8 was used as an additive,
Cu0 was dissolved 48% in 8 min reaction time and nearly
quantitatively (ca. 90%) in elongated 20 min reaction time (SI
5.4).
The reactivity differences between 2-MBIH with Cu0 and Au0
demonstrate
a preferred dissolution selectivity among the
coinage metals. This was further investigated with theoretical
studies on the dissolution of Cu0 and Ag0 with 4-PSH and 2-
MBIH ligands. As observed with the Au complexes, the
coordination of the second 4-PS ligand markedly lowers the
oxidation potential of Cu0 and Ag0 and delocalization of the
4s1/5s1 electron on the ligands favors the one-electron oxidation
of the metal (Table 3, entries 1, 3). Due to the 0.1 V lower
oxidation potential of (4-PS)2Cu0 and (4-PS)2Ag0 species
compared to (4-PS)2Au0, they are predicted to be oxidized
slightly faster. Although the theoretical results support faster
dissolution of metallic Ag and Cu compared to Au, the true
efficiency of the dissolution is affected by e.g. formation of an
PSH/EtOH method (0.0610-3 mol/(m2h))18
.
Because of their multi-metallic composition, we considered
PCBs as one of the most challenging substrates and identified
the so-called ‘Au finger’ part suitable for dissolution studies.
Accordingly, this type of sample contains 23.4 m-% of Cu and
only 0.25 m-% of Au (and 0.008 m-% of Ag) (SI 6). In this regard,
a direct 4-PSH leaching of Au is not rational due to the high Cu
content compared to that of Au. Therefore, we treated the
sample first with NH3/(NH4)2SO4 and H2O2 to extract Cu and Ag
(Fig. 3b, step I).29 The Au containing residue was then filtered
and immersed in a DMF solution of 4-PSH/H2O2 resulting in a
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10.1002/anie.201810447
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COMMUNICATION
significant 95% of dissolved Au in 30 min (Fig. 3b, step II). As
the 4-PSH-assisted dissolution method is selective for Group 11
metals, the recovery of these metals from Pt and Pd mixtures is
also possible.
his support of the computational studies by the NSF under CHE-
1464828.
Data availability
Supplementary information and chemical compound information
are available in the online version of the paper. CCDC 1852214
(1) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Keywords: gold • gold recycling • desulfurization • pyridinethiol
chemistry • reaction mechanism
a
b
1
2
3
4
S. Freakley, Q. He, C. Kiely, Catal. Lett. 2015, 145, 71.
C. W. Corti, R. J. Holliday, Gold. Bull. 2004, 37, 20.
U. Jadhav, H. Hocheng, Sci. Rep. 2015, 5, 14574.
C. Yue, H. Sun, W.-J. Liu, B. Guan, X. Deng, X. Zhang, P. Yang,
Angew. Chem. Int. Ed. 2017, 56, 9331.
Figure 3. Dissolution of Au from thin film substrates and PCB samples. a)
Progress of the etching process from edges towards center of the film, b)
Dissolution of Au in PCB samples. Step I: extraction of Cu and Ag with
NH3/(NH4)2SO4/H2O2 and Step II: quantitative dissolution of Au in 30 min with
4-PSH/H2O2 method.
5
6
E. D. Doidge, I. Carson, P. A. Tasker, R. J. Ellis, C. A. Morrison, J. B.
Love, Angew. Chem. Int. Ed. 2016, 55, 12436.
As shown here, 4-PSH as a reactive ligand and hydrogen
peroxide as an oxidant forms together a highly efficient
combination for dissolution of metallic Au. The reaction is driven
by the ability of a thione isomer of 4-PSH to coordinate to an Au
surface and to lower oxidation potentials of the Au atoms on the
surface. Simultaneously, a part of 4-PSH is oxidized to S8 and to
E. Lahtinen, L. Kivijärvi, R. Tatikonda, A. Väisänen, K. Rissanen, M.
Haukka, ACS Omega 2017, 2, 7299.
7
8
9
S. Foley, US patent WO2016168930A1, 2016.
G. Hilson, A. J. Monhemius, J. Cleaner Prod. 2006, 14, 1158.
A. Behnamfard, M. M. Salarirad, F. Veglio, Waste Manage. 2013, 33,
2354.
2-
the SO4 anions needed in the reaction to solubilize the
10 W. Lin, R.-W. Zhang, S.-S. Jang, C.-P. Wong, J.-I. Hong, Angew. Chem.
Int. Ed. 2010, 49, 7929.
generated AuI species. The excess of 4-PSH can be significantly
reduced and the reaction rate markedly accelerated by adding
external S8, which under optimized conditions leads to very fast
and quantitative dissolution of Au.
11 W. Lin, Rare Metals 2012, 31, 92.
12 W. Lin, Noble Metals 2012, 335.
13 A. Serpe, F. Artizzu, M. L. Mercuri, I. Pilia, P. Deplano, Coord. Chem.
Rev. 2008, 252, 1200.
The success with 4-PSH ligand is a balanced combination of its
good coordinative ability with Au0 and the low oxidation potential
of the generated bisligand intermediate. As a result of one-
14 F. Bigoli, M. A. Pellinghelli, P. Deplano, M. L. Mercuri, G. Pintus, A.
Serpe, E. F. Trogu, Chem. Commun. 1998, 2351.
15 F. Isaia, M. C. Aragoni, M. Arca, C. Caltagirone, F. Demartin, A. Garau,
V. Lippolis, Dalton Trans. 2013, 42, 492.
2-
electron oxidation, a soluble AuI complex is formed with SO4
16 L. Cau, P. Deplano, L. Marchio, M. L. Mercuri, L. Pilia, A. Serpe, E. F.
Trogu, Dalton Trans. 2003, 1969.
counter anion. Notably, the studies with 2-PSH and 2-MBIH
ligands illustrate how the dissolution of Au0 is markedly hindered
either by higher formation energies of the reactive bisligand
intermediates or by their higher oxidation potentials.
17 A. Serpe, L. Marchiò, F. Artizzu, M. L. Mercuri, P. Deplano, Chem. Eur.
J. 2013, 19, 10111.
18 M. T. Räisänen, M. Kemell, M. Leskelä, T. Repo, Inorg. Chem. 2007,
46, 3251.
The 4-PSH dissolution method was further applied to Cu0 and
Ag0 substrates resulting in high dissolution efficiencies. The
method possesses inherent selectivity towards metals which
undergo one-electron oxidation; Pt and Pd thin films remain
intact during the 4-PSH treatment. Furthermore, selectivity within
Group 11 was demonstrated by changing the ligand; 2-MBI as a
reactive ligand prefers only Cu0. This selectivity results from the
remarkably lower formation energy of the bisligand intermediate
Cu(2-MBI)2 in comparison to the Au and Ag species. The
dissolution was further implemented as an efficient method for
Au films and printed circuit board samples. Thus, the principles
learned from the studies reported herein could find future
applications in sustainable dissolution pathways for noble metals.
19 H. Eshuis, J. Yarkony, F. Furche, J. Chem. Phys. 2010, 132, 234114.
20 H. Eshuis, J. E. Bates, F. Furche, Theor. Chem. Acc. 2012, 131, 1084.
21 J. Tao, J. P. Perdew, V. N. Staroverov, G. E. Scuseria, Phys. Rev. Lett.
2003, 91, 146401.
22 S. Grimme, J. Antony, S. Ehrlich, H. A. Krieg, J. Chem. Phys. 2010,
132, 154104.
23 F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297.
24 H.-L. Jia, M.-J. Jia, G.-H. Li, Y.-N. Wang, J.-H. Yu, J.-Q. Xu, Dalton
Trans. 2013, 42, 6429.
25 J.-H. Yu, L. Ye, H. Ding, Y. Chen, Q. Hou, X. Zhang, J.-Q. Xu, Inorg.
Chem. Commun. 2007, 10, 159.
26 M. B. Salah, S. Vilminot, G. Andre, M. Richard-Plouet, F. Bouree-
Vigneron, T. Mhiri, M. Kurmoo, Chem. Eur. J. 2004, 10, 2048.
27 M. B. Salah, S. Vilminot, T. Mhiri, M. Kurmoo, Eur. J. Inorg. Chem.
2004, 2272.
Acknowledgements
The financial support of the Academy of Finland (#282555) is
28 L. Wade, Science 2013, 341, 1448.
29 A. Serpe, F. Artizzu, D. Espa, A. Rigoldi, M. L. Mercuri, P. Deplano,
Green Process Synth. 2014, 3, 141.
gratefully acknowledged. We thank Professor Filipp Furche for
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10.1002/anie.201810447
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION



M. Räisänena + , E. Heliövaara+ ,
Solution for Gold: The rapid
dissolution of gold is facilitated
by a reactive combination of
pyridine-4-thiol and H2O2 in
DMF solution. The ligand assists
the dissolution as it stabilizes
AuI centers and serves as the
F. Al-Qaisib , M. Muuronenc , A.
Eronen, H. Liljeqvist, M. Nieger, M.
Kemell, K. Moslova, J.
Hämäläinen, K. Lagerblom, and T.
Repo*

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Page No. – Page No.
source of
anion.
a sulfate counter
Pyridinethiol-Assisted
Dissolution of Elemental Gold in
Organic Solutions
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