Complexation of palladium(II) with unsaturated dithioethers - A systematic development of highly selective ligands for solvent extraction

Article abstract of DOI:10.1002/ejic.201101406

There is a demand for new and robust Pd^II extractants due to growing recycling rates. Chelating dithioethers are promising substances for solvent extraction as they form stable square-planar complexes with Pd ^II. We have modified unsaturated dithioethers, which are known to coordinate Pd^II, and adapted them to the requirements of industrial practice. The ligands are analogues of 1,2-dithioethene with varying electron-withdrawing backbones and polar end-groups. The crystal structures of several ligands and their palladium complexes were determined as well as their electro- and photochemical properties, complex stability and behaviour in solution. Solvent extraction experiments showed the superiority of some of our ligands over conventionally used extractants in terms of their very fast reaction rates. With highly selective 1,2-bis(2-methoxyethylthio)benzene (4) it is possible to extract Pd^II from a highly acidic medium in the presence of other base and palladium-group metals. Copyright

Full text of DOI:10.1002/ejic.201101406

FULL PAPER
DOI: 10.1002/ejic.201101406
Complexation of Palladium(II) with Unsaturated Dithioethers – A Systematic
Development of Highly Selective Ligands for Solvent Extraction[‡]
Juliane Traeger,[a] Tillmann Klamroth,[b] Alexandra Kelling,[a] Susanne Lubahn,[a]
Ernst Cleve,[c] Wulfhard Mickler,[a] Matthias Heydenreich,[d] Holger Müller,[a] and
Hans-Jürgen Holdt*[a]
Keywords: Renewable resources / Palladium / Chelates / Ligand design / S ligands
There is a demand for new and robust PdII extractants due
to growing recycling rates. Chelating dithioethers are prom-
ising substances for solvent extraction as they form stable
square-planar complexes with PdII. We have modified unsat-
urated dithioethers, which are known to coordinate PdII, and
adapted them to the requirements of industrial practice. The
ligands are analogues of 1,2-dithioethene with varying elec-
tron-withdrawing backbones and polar end-groups. The
crystal structures of several ligands and their palladium com-
plexes were determined as well as their electro- and photo-
chemical properties, complex stability and behaviour in solu-
tion. Solvent extraction experiments showed the superiority
of some of our ligands over conventionally used extractants
in terms of their very fast reaction rates. With highly selective
1,2-bis(2-methoxyethylthio)benzene (4) it is possible to ex-
tract PdII from a highly acidic medium in the presence of
other base and palladium-group metals.
stripping the palladium from the organic phase is found to
be difficult.[4] In the field of basic research, there are cur-
rently still great efforts being undertaken to find new ex-
tractants.[5] These cover a wide range of functional groups:
oxygen-containing crown ethers,[5a] nitrogen- and sulfur-
containing protic ionic liquids,[5b] quaternary phosphonium
salts,[5c] urea derivatives,[5d] pyridinecarboxamides,[5e] thio-
urea derivatives,[5f] compounds that contain sulfide, sulfinyl
and sulfonyl groups[5g] and imidazole[5h] and triazole[5i] sub-
stitution products. Yet for most, the separation of PdII from
PtIV is not very effective.
Thioethers are widely known as soft donors that form
complexes with a range of transition metals.[6] Dithioethers
based on cis-1,2-dithioethene possess a rigid chelating unit,
which perfectly meets the geometrical demand for the
square-planar complexation of PdII.[7] Recently, Ananikov
et al. have reported sulfur-containing alkenes and their PdII
complexes.[8] However a general risk when dealing with
thioethers is the possible oxidation of the sulfur atom, espe-
cially in PGM refining processes when brought into contact
with an oxidising aqueous phase. It is noted that dihexyl
sulfide can be oxidised to a sulfoxide and thereafter to a
sulfone, which does not only consume the extracting agent,
but also may impair the selectivity of the system.[9] On this
account it is advisable to design ligands with reduced elec-
tron density on the sulfur atoms. This can be achieved by
introducing electron-withdrawing groups, such as cyano
groups, at the double bond or making the double bond part
of an aromatic ring.
Introduction
The recovery of platinum-group metals (PGMs) from
secondary sources such as automotive catalysts has gained
increasing importance, especially in countries without PGM
mines.[1] As measured by annual demand, palladium is one
of the most important PGMs. In industrial process streams,
it occurs in the divalent oxidation state and forms chlorido
complexes, of which the tetrachloridopalladate anion
[PdCl4]2– is the most common species. In industry, it is
mainly extracted by solvent extraction with hydroxyoximes
or long-chain thioethers through the formation of inner-
sphere complexes.[2–4] Due to slow extraction rates, the
equilibration times of such systems are quite long.[3,4] The
equilibrium times can be reduced with the help of phase-
transfer catalysts, such as amines in the case of β-hy-
droxyoximes,[4] but they decrease the selectivity towards
other metals. In addition, when using long-chain thioethers,
[‡] Patent application filed on August 22, 2011, EP11178303.1
[a] University of Potsdam, Institute of Chemistry, Department of
Inorganic Chemistry,
Karl-Liebknecht-Straße 24–25, 14476 Potsdam, Germany
Fax: +49-331-977-5055
E-mail: holdt@uni-potsdam.de
[b] University of Potsdam, Institute of Chemistry, Department of
Theoretical Chemistry,
Karl-Liebknecht-Straße 24–25, 14476 Potsdam, Germany
[c] Hochschule Niederrhein, Institute for Coatings and Surface
Chemistry,
Frankenring 20, 47798 Krefeld, Germany
[d] University of Potsdam, Institute of Chemistry, Department of
Analytical Chemistry,
Karl-Liebknecht-Straße 24–25, 14476 Potsdam, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101406.
Studies of maleonitrile dithiocrown ethers have shown
that they form stable PdII complexes and have also demon-
Eur. J. Inorg. Chem. 2012, 2341–2352
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H.-J. Holdt et al.
FULL PAPER
strated their potential to extract PdII with a good selectivity
Furthermore, we report the variable formation of com-
towards other soft metal ions.[10,11] Another important at- plexes that depend on the concentration ratio (PdII/extract-
tribute is their high surface activity caused by the ether ant). NMR spectroscopy and the single-crystal X-ray struc-
groups. However, because of their low synthetic yields, these tures prove that an inversion of the sulfur and thus, oscilla-
compounds have never been used on a large scale for sol- tion of the arms occurred in solution. For a comprehensive
vent extraction. As PdII is coordinated exocyclically understanding of the behaviour of an extractant, it is neces-
through the two sulfur atoms, the steric design does not sary to investigate the solid complex itself in addition to the
stringently need to contain macrocycles.
processes that occur in solution. Theoretical calculations
Hence the open-chain equivalents, which are easier to helped us to understand the electro- and photochemical
synthesise, are promising ligands to be utilised in industrial characteristics of the extractants and their complexes.
processes. PdII complexes of 1,2-bis(methylthio)maleo-
nitrile,[12] 1,2-bis(methylthio)benzene[7,12,13] and 4-methyl-
1,2-bis(methylthio)benzene[13] have been investigated, but
the ligands themselves have not received any attention con-
Results and Discussion
Synthesis of the Ligands and their Complexes
cerning their possible applicability in solvent extraction. As
the complexation of the metal occurs at the interface or in
the aqueous phase, it is important that the extractant used
is not too nonpolar, otherwise it takes too long to establish
the equilibrium.
We investigated a set of unsaturated dithioethers 1–6
(Scheme 1). The insertion of an unsaturated unit into a 4-
methylbenzene (1, 2) or benzene moiety (3, 4) and the intro-
duction of cyano groups at the double bond (5, 6) increased
the electron-withdrawing effect on the sulfur atoms. The
benzene dithioethers 1–4 were synthesised by the addition
of sodium to 4-methyl-1,2-benzenedithiol or 1,2-benzenedi-
thiol[16] and the alkylation of the dithiolates formed with 2-
bromoethanol or 2-chloroethyl methyl ether. The maleon-
itrile dithioethers 5 and 6 were prepared by the alkylation
of disodium (Z)-1,2-dicyanoethene-1,2-dithiolate[17] with 2-
bromoethanol and 2-chloroethyl methyl ether, respectively,
in the presence of NaI following a procedure reported by
Lange and coworkers.[18] We obtained single-crystal X-ray
structures of 3, 5 and 6.[19] NMR spectroscopy and elemen-
tary analysis proved the purity of all our ligands. In con-
trast to other thioethers, these substances are not volatile
and do not have the characteristic odour, which is impor-
tant for practical applications. The complexes [PdCl2(L)] (L
= 1–4, 6) were synthesised by a method similar to that de-
scribed by Drexler et al.[10] The crystal structures of all of
the PdII complexes were analysed, except for that contain-
ing 5, which was an insoluble powder with an undefined
composition.
The main goal of this work was to synthesise a number
of unsaturated “open-chain” dithioethers 1–6 (Scheme 1)
and to examine their potential to extract [PdCl4]2– from
streams originating from automotive catalysts. Our focus
was to create new extracting agents for practical applica-
tions and not only laboratory examination. First and fore-
most, this requires a relatively simple one-pot synthesis, and
the compounds must be robust as they will be used in the
extraction cycle repeatedly. By varying the electron-with-
drawing effect on the sulfur atoms, the oxidation potential
of the extractant and the stability of the PdII complexes
formed were influenced. To ensure a good interaction with
the aqueous phase, 2-hydroxyethyl or 2-methoxyethyl
groups were inserted. A comparison of the extraction kinet-
ics of the new substances with two industrial ones, dihexyl
sulfide[14] and Aloxime® 840,[15] demonstrated the advan-
tage of these new extracting agents. Although the electron-
withdrawing effect of the benzene and maleonitrile units
make the sulfur atoms harder, it was shown that the selec-
tivity for PdII over other metals that occur in automotive
catalysts (some of which in much higher concentrations) is
excellent.
Single-Crystal X-ray Structures
The single-crystal X-ray structures of 3 and 6 (Figure 1)
indicate the preorganisation of the lone pairs of the sulfur
atoms for the complexation of PdII. It is typical for dithio-
ethers that contain a maleontrile[10] or o-benzene unit[20]
that the S···S distance is less than the sum of the van der
Waals radii, which is around 3.7 Å. The values for 3, 5 and
6 are in agreement with this [2.984(1), 3.084(2) and
3.121(1) Å, respectively]. The molecular structures of
[PdCl2(L)] (L = 1–4, 6) have planar five-membered chelate
rings (Figures 2 and 4 show the structures of [PdCl2(6)] and
[PdCl2(3)], respectively) with customary Pd–S bond
lengths.[10,21] In the complexes that contain 1–4 and in that
of 6, the S···S distances increase to values between 3.17 and
3.19 Å. A comparison of dithioether 3, which contains an
o-benzene backbone, with the corresponding complex
Scheme 1. Synthesis of the ligands: a) Na, Br(CH2)2OH, EtOH; b)
Na, Cl(CH2)2OCH3, EtOH; c) NaI, Br(CH2)2OH, acetone; d) NaI,
Cl(CH2)2OCH3, acetone.
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Complexation of Palladium(II) with Unsaturated Dithioethers
[PdCl2(3)] shows a slight extension of the C1–S1 and C2– CAM-B3LYP/LANL2DZ[24] level of theory. In addition,
S2 bonds by 0.02 Å and a shortening of the aromatic electrostatic potential (ESP)-derived charges[25] were calcu-
C1=C2 bond by 0.03 Å. In the complexation of 6, which is lated. In 3 and 6, the electron density of the HOMO is
the maleonitrile derivative, the changes in the C–S and mainly situated at the lone pair of the sulfur atoms and
double bond lengths are more pronounced. The C1–S1 and slightly at the double bond (6)[19] or the aromatic C=C
C2–S2 bond lengths are lengthened by 0.06 and 0.09 Å, bond adjacent to the sulfur atoms (3, Figure 3). Further-
respectively, and the C1=C2 distance is shortened by 0.1 Å. more, the calculation of ESP charges of the cations con-
This is in accordance with a noticeable shift of the C=C firms that oxidation takes place at the sulfur atom. Voltam-
vibrational band in the IR spectra. The band of the com- metric data (Table 1) show that the introduction of electron-
plex is shifted by 62 cm–1 compared to that of the free li- withdrawing groups significantly increases the oxidation
gand, which is similar to the trend seen in the crystal struc- potential of the unsaturated dithioethers in the following
tures and IR spectra of comparable maleonitrile dithio- order: unsubstituted double bond [(Z)-1,2-bis(methylthio)-
crown ether complexes.[10]
ethene] Ͻ 4-methylbenzene backbone (1, 2) Ͻ benzene
backbone (3, 4) ϽϽ double bond with cyano groups (5, 6).
The oxidation potential of 5 and 6 is nearly twice that of
the reference, (Z)-1,2-bis(methylthio)ethene[22b] (BMTE).
Looking at the values of the PdII complexes of 1–4, it is
apparent that there is a significant enhancement of the oxi-
dation potential caused by complexation. This is in line
with the calculated HOMO energy levels of 3 and
[PdCl2(3)]. Compared to the ligand, the energy level of the
complex is found at 0.17 eV lower values. The HOMO of
the optimised structure of [PdCl2(3)] is mainly situated at
the chlorine atoms (Figure 3). In addition, a comparison of
the ESP charges of the neutral complex with the corre-
sponding cation reveals a shift of the partial negative charge
at the PdCl2 unit from –0.57 to –0.03 e. Therefore the oxi-
dation of the PdCl2 complexes that contain benzene dithio-
ether ligands is believed to occur at the chlorine atoms and
not at the sulfur atoms, whose electron density is further
reduced by complexation. A noticeable difference is ob-
served between the oxidation potentials of [PdCl2(1)] and
[PdCl2(3)], which possess 2-hydroxyethyl end-groups at the
two side arms, and [PdCl2(2)] and [PdCl2(4)], which contain
2-methoxyethyl end-groups. The values of the ligands do
not differ in that way, which indicates the influence of the
end groups on the electrochemical character of the palla-
Figure 1. Molecular structures of 3 and 6 (thermal ellipsoids at
50%).
Figure 3. HOMOs of 3 and [PdCl2(3)].
Figure 2. Molecular structure of [PdCl2(6)] (thermal ellipsoids at
50%) showing the Pd···O2 interaction [2.918(4) Å].
Table 1. Differential pulse voltammetry data for BMTE, 1–6 and
[PdCl2(L)] (L = 1–4, 6) in MeCN.[a]
Ep [V][b] Ep
[V][b] Complex Ep [V][b] Ep
[V][b]
Ox
Red
Ox
Red
Ligand
Electrochemistry and DFT Calculations
BMTE
+0.93
+1.21
+1.22
+1.29
+1.29
+1.79
+1.84
–
–
–
–
–
1
2
3
4
5
6
[PdCl2(1)] +1.63
[PdCl2(2)] +1.72
[PdCl2(3)] +1.62
[PdCl2(4)] +1.74
–0.59
–0.67
–0.57
–0.62
The literature broadly describes that the oxidation of un-
saturated thioethers leads to the formation of sulfones.[22]
Oxidation generally occurs at the highest occupied molecu-
lar orbital (HOMO), hence DFT calculations were under-
taken. We performed geometry optimisations for 3, 6 and
–1.06
–1.09
[PdCl2(6)] +1.81
–0.49
[PdCl2(3)] using the Gaussian 09[23] program package at the [a] All potentials are given vs. SCE. [b] ErrorϮ0.02 V.
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H.-J. Holdt et al.
FULL PAPER
dium complexes. This could be related to the weak Pd···O respectively. Thus a weak interaction of the oxygen atom
interaction (vide infra).
with a d orbital of the palladium is possible. This is also
indicated by the HOMO of the optimised structure
[PdCl2(3)] (Figure 3). In addition, a dispersion-driven hy-
drogen bond, as recently reported between water and
PtII,[28] could in principle be formed by the orientation of
the OH proton towards the Pd centre. We see no indication
for such bonding in the calculations, which is to be expected
as dispersive interactions are not covered by the CAM-
B3LYP functional. Nevertheless, the calculated Pd–O dis-
tance of 2.93 Å is in good agreement with that from the X-
ray structure.
Electronic Spectra
The UV/Vis spectra of the unsaturated dithioethers 1–6
and the palladium complexes are in good agreement with
the electrochemical data reported above as they confirm the
electron-withdrawing character of the ligand backbones. In
the spectra of 5 and 6, the characteristic band of the dithio-
maleonitrile push–pull π-electron system[26] is observed at
341 and 340 nm, respectively (see Figure 7 for the spectrum
of 6). This is caused by internal charge transfer from the
sulfur atoms to the maleonitrile unit and is suppressed by
the complexation of the sulfur atoms. The UV/Vis spectra
of 1–4 are shaped by three bands that occur near 215, 247
and 300 nm (Figure 6, a). DFT calculations for 3[19] imply
that the band at 300 nm is induced by a HOMO–LUMO
charge transfer from a sulfur atom to the aromatic ring.
This is similar to the transfer known in the dithiomaleo-
nitrile unit but occurs at a lower wavelength, denoting that
the electron-withdrawing effect is smaller. The second tran-
sition is from HOMO-1, which is also dominated by the
lone pair of a sulfur atom and energetically close to the
HOMO, into the LUMO. The absorption near 215 nm is
caused by the excitation from the HOMO to the π* orbital
of the aromatic ring, which represents the LUMO+1. In the
corresponding complex, [PdCl2(3)], a small band at approx-
imately 380 nm displays a d–d transition. A shoulder near
300 nm is caused by an excitation into the LUMO, which
is situated at the metal.
Figure 4. Molecular structures of (R,R)-[PdCl2(3)] and (S,R)-
[PdCl2(3)] (thermal ellipsoids at 50%).
However, the two arms are very flexible in solution and
can easily switch between the diastereomic forms. Based on
1H NMR spectroscopic data, Abel et al. postulated a py-
ramidal inversion at sulfur in dithioether complexes of PdII
in solution.[12,29] At low temperatures, they observed signals
from two diastereomers – meso and dl – which, on warm-
ing, coalesced to time-averaged bands. Interestingly, al-
though we dissolved the isolated isomers, we found the
same (Table 2). For dichlorido[1,2-bis(2-methylthio)benz-
ene]palladium(II), Abel et al. determined a coalescence
Stereoisomerism of the Complexes in the Solid State and
Solution
In thioether complexes, sulfur atoms can be centres of temperature of –45 °C and computed an energy barrier of
chirality. The PdII complexes synthesised with 1 and 2 are ca. 50 kJ/mol. The coalescence of the signals of [PdCl2(2)]
present in the meso form and those with 4 and 6 give a and [PdCl2(4)] occurred between –60 and –30 °C, which
racemate. With 3, the meso form and the racemate were suggests that the energy barrier is in the same magnitude as
obtained. To the best of our knowledge, [PdCl2(3)] is the that of dichlorido[1,2-bis(2-methylthio)benzene]palladi-
first palladium complex of an unsaturated 1,2-dithioether um(II). The other complexes were only poorly soluble in
for which both diastereomeric forms could be isolated (Fig- dichloromethane and gave no spectra worthy of evaluation.
ure 4). The crystal structures of the racemic complexes The diastereomers found at low temperatures are present in
show that one of the oxygen atoms is situated above the unequal abundance with an approximate ratio of 1:4. Ac-
palladium at a distance that is slightly shorter than the sum cording to the study mentioned above, the racemate (anti
of the van der Waals radii (3.1 Å)[27] (see the Supporting conformation) predominates on account of its higher
Information and Figure 2). For example, the distances in thermodynamic stability. From the number of peaks in the
[PdCl2(3)] and [PdCl2(6)] are 3.058(3) and 2.918(4) Å, 13C NMR spectrum of [PdCl2(4)] (Table 2), it was seen that
Table 2. 13C NMR chemical shifts [ppm] in CD2Cl2 for 2, 4 (25 °C), [PdCl2(2)] and [PdCl2(4)] in two diastereomeric forms (–60 °C).
2
[PdCl2(2)][a]
[PdCl2(2)][b]
4
[PdCl2(4)][a]
[PdCl2(4)][b]
SC=CS
CH2CH2O
OCH3
SCH2CH2
CH3C
137.4, 132.7
71.2, 71.1
58.7, 58.6
33.4, 32.8
21.0
135.2, 132.5
69.5, 69.4
58.4, 58.4
42.7, 42.4
20.6
135.1, 133.0
68.2, 68.2
58.1, 58.0
44.1, 43.7
20.6
137.1
71.0
58.7
32.9
–
135.4
69.7
58.4
42.9
–
135.3
68.4
58.1
44.1
–
[a] Major species. [b] Minor species.
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Complexation of Palladium(II) with Unsaturated Dithioethers
both diastereomers retain σ and C2 symmetry, respectively. Table 3. Complex stability constants determined by UV/Vis ti-
tration for [PdX2(L)] and [Pd(L)2]2+ (L = 1–4 and 6) in water (X
The 1H NMR spectrum of [PdCl2(4)] exhibits a lot of over-
laps in the range of the SCH2CH2 and CH2CH2O protons.
Nonetheless, it can be seen from the heteronuclear single
quantum coherence (HSQC) NMR spectrum (Figure 5)
that the respective SCH2CH2 and CH2CH2O protons on
the two arms are no longer magnetically equivalent –
SCH2CH2 and CH2CH2O each give two coupling signals
that each relate to two protons. The spectra of asymmetric
[PdCl2(2)][19] support this conclusion. Compared to the car-
bon resonances of the free ligands, in particular those of
SCH2CH2 are extremely downfield shifted in [PdCl2(2)] and
[PdCl2(4)]. These changes are more pronounced than those
observed by Drexler at al. for palladium complexes with
maleonitrile dithiocrown ethers,[10] which indicates the
higher complex stability of the aromatic open-chain dithio-
ether complexes.
= Cl–, H2O).
2+
L
logK[PdX (L)]
logK[Pd(L) ]
2
2
1
2
3
4
6
7.98ϮϽ 0.01
6.01ϮϽ 0.01
7.99Ϯ0.03
5.97Ϯ0.12
4.74Ϯ0.09
5.81Ϯ0.01
5.65ϮϽ 0.01
5.01Ϯ0.02
4.67Ϯ0.02
–
Figure 5. Aliphatic region of the HSQC NMR spectrum of
[PdCl2(4)] in CD2Cl2 at –60 °C showing the two diastereomeric spe-
cies (meso and racemate).
Figure 6. a) UV/Vis absorption spectra of 3 [c0(3) = 5ϫ10–5 m] in
the presence of 0.0 (black), 0.2 (red), 0.5 (green), 0.7 (blue), 1.0
(cyan), 1.5 (magenta) and 2.0 equiv. (orange) of PdII in H2O. Inset:
titration curve at 375 nm. b) Calculated distribution of [Pd(3)2]2+
(red) and [PdCl2(3)] (black) at various palladium/ligand ratios.
Complex Stability Constants
With the help of UV/Vis titration experiments, the sta-
In compliance with the extraction results described be-
bility constants of the PdII complexes of 1–4 and 6 were low, the stability constant of [PdCl2(6)] is much smaller
determined (Table 3). The titrations were also adjuvant to than that of [PdX2(L)] (L = 1–4). This can be attributed to
observe the progress of complex formation. Because com- the higher electron density on the sulfur atoms in the benz-
plexation is assumed to take place in the aqueous phase or ene dithioethers 1–4 in comparison to the maleontrile di-
interface in solvent extraction, water was chosen as solvent thioether 6, which has already been shown by the lower
for all UV/Vis measurements. Figure 6 shows the titration oxidation potential of the aromatic compounds. The value
curve of 3 as an example of 1–4. With excess ligand, the is also smaller than the logK value of the comparable crown
reaction involves a species with a 1:2 (Pd:L) composition. ether complex,[11] which is believed to be caused by the
In the presence of a higher metal concentration, this species higher loss of entropy of the open-chain derivative in the
degrades and forms the more stable [PdX2(L)] complex. De- course of complexation. The type of end group has a con-
rivative 6 just gives the complex with a 1:1 (Pd:L) composi- siderable influence on the logK[PdX (L)] values of the aro-
2
tion (Figure 7), and the titration spectra show an isosbestic matic compounds. Complexes with 2-hydroxyethyl end-
point.
groups are more stable in water than those with 2-methoxy-
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H.-J. Holdt et al.
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Figure 7. UV/Vis absorption spectra of 6 [c0(6) = 5ϫ10–5 m] in the
presence of 0.0 (black), 0.2 (red), 0.5 (green), 0.7 (blue), 1.0 (cyan),
1.5 (magenta) and 2.0 equiv. (orange) of Pd2+ in H2O. Inset: titration
curve at 340 nm.
Figure 8. Solvent extraction kinetics of 2 (magenta), 4 (cyan), 5
(blue), 6 (green), dihexyl sulfide (red) and Aloxime® 840 (black) in
CHCl3/H2O = 1:1 [c(L)o = 10–2 m, c0(Pd)w = 10–4 m, c(Cl)w
=
1.5ϫ10–2 m]. The extraction yield, E, was determined by measuring
c(Pd)w with inductively coupled plasma optical emission spectrome-
try (ICP OES).
ethyl groups. The logK[Pd(L) ]2+ values decrease in the order:
2
Figure 9 shows the influence of the end groups on the
extraction kinetics within a series of 1,2-dithio-4-methyl-
benzene derivatives. Acyclic 2 was tested in comparison to
its crown ether analogue, 4-methylbenzodithio-12-crown-
4,[30] and nonpolar, acyclic 1,2-bis(methylthio)-4-methyl-
benzene.[13] All of them exhibit very good extraction yields,
but the time taken to reach the extraction equilibrium dif-
fered widely. The extraction took the longest with nonpolar
1,2-bis(methylthio)-4-methylbenzene. The reaction rate in-
creases according an enhancement of the surface activity in
the order of: methyl Ͻ 2-methoxyethyl group. 4-Methyl-
[Pd(1)2]2+ Ͼ [Pd(2)2]2+ Ͼ [Pd(3)2]2+ Ͼ [Pd(4)2]2+. Again,
compared to ether end-groups, a hydroxy functionality en-
hances the complex stability in water. Complexes that pos-
sess a 4-methylbenzene backbone are more stable than
those that contain a benzene backbone.
Solvent Extraction
We performed solvent extraction batch experiments with benzodithio-12-crown-4, which possesses a similar polarity
the unsaturated dithioethers 1–6 and various reference com- to 2, reacted the fastest. This difference in kinetics can be
pounds. All tests were carried out in a chloroform/water explained by the rigidity of the crown ether. Compared to
system, where the extractant was present in excess in the open-chain 2, it features better preorganisation for the com-
organic phase. The composition of the aqueous phase, plexation of PdII. Interestingly the extraction yield of 6 is
which contained PdII, was progressively modified from a less than that of 2 and 4. This correlates with the order of
model solution closer to industrial conditions. Figure 8 the stability constants of their 1:1 PdII complexes. UV/Vis
demonstrates the ability of 2, 4 and 6 to transfer PdII from spectroscopy has shown that chlorido aqua PdII complexes
the aqueous into the organic phase. Compared to the two exist in aqueous media with low chloride concentrations.[31]
industrially used extractants, dihexyl sulfide[14] and Alox- Water is a relatively weak electron donor and can be substi-
ime® 840,[15] all of our ligands that contain 2-methoxyethyl tuted more easily than chloride. Consequently, with an in-
end-groups establish the extraction equilibrium extremely crease of the Cl– concentration, [PdCl4]2– becomes the pre-
fast. In particular, 2 and 4, which show extraction yields vailing species. For media that contain a high concentration
near 100% within only 30 min, are very promising new ex- of hydrochloric acid, such as the streams in PGM refining
tractants. Ligand 5 formed an insoluble precipitate, which industries, the extractants used have to be strong enough to
accumulated between the phases and could not be analysed. compete with Cl– for the complexation of PdII. As seen
Thus, although the palladium content of the aqueous phase from Figure 10, there is a huge difference in the extraction
instantly dropped after contact with 5, it is unsuitable as an performance between 2, 4 and 6. The extraction yield of 6
extractant. Surprisingly, 1 and 3 did not extract any PdII. dramatically decreases even on the addition of very small
This was caused by their high water solubility, revealed by quantities of HCl. At a concentration of 0.26 m HCl, 6 ex-
analysing both phases with UV/Vis spectroscopy. Both li- tracts almost no PdII at all. The same trend is observed
gands form palladium complexes, but under the conditions with the corresponding crown ether maleonitriledithio-12-
chosen, their concentration in the aqueous phase was high crown-4,[32] although it is shifted to higher HCl concentra-
enough to keep the palladium there.
tions. As it is more preorganised, the crown ether is a
2346
www.eurjic.org
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 2341–2352

Complexation of Palladium(II) with Unsaturated Dithioethers
stronger chelating agent than open-chain 6. However, uti- The organic phase contained 133 mg/L palladium, whereas
lising HCl concentrations higher than 1 m also depressed the concentration of all the other metals was smaller than
the extraction yield to negligible values. In contrast, 2 and 0.2 mg/L. This experiment also demonstrates the redox sta-
4 constantly exhibit extraction yields near 100% even at bility of the system under strongly oxidising conditions.
extremely high HCl concentrations. This is in accord with
their higher complex stability constants.
Figure 11. Solvent extraction batch experiment with a solution ob-
tained by leaching an automotive catalyst with aqua regia (subse-
quent dilution with the same volume of water) and 4 in chloroform.
The composition of the feed solution (blue) and organic phase (pur-
ple) was determined with ICP OES [c(4)0 = 10–2 m, H2O/CHCl3 =
1:1].
Conclusions
Figure 9. Solvent extraction kinetics of 2 (magenta), 4-methylbenzo-
dithio-12-crown-4 (green) and 1,2-bis(methylthio)-4-methylbenzene
A new series of chelating ligands for the selective coordi-
(black) in CHCl3/H2O = 1:1 [c(L)0 = 10–2 m, c0(Pd)w = 10–4 m, c(Cl)
nation of PdII, based on dithiomaleonitrile, dithiobenzene
= 1.5ϫ10–2 m]. E was determined by measuring c(Pd)w with ICP
w
and dithiotoluene, has been designed and synthesised. They
have a simple one-pot synthesis and a high stability towards
oxidation, which makes them interesting for applications in
solvent-extraction industries. The varying electron-with-
drawing effect of the backbones does not only influence the
electrochemical properties, as shown by the voltammetric
measurements, UV/Vis spectra and DFT calculations, but
also the complex stability and the kind of complex formed
in solution. The dithiomaleonitrile-based compounds are
weaker ligands and form only 1:1 (Pd:L) complexes,
whereas aromatic 1–4 also build species of 1:2 composition.
In batch experiments, 2 and 4 particularly showed the
capability to be used as solvent extractants. They are based
on 1,2-dithio-4-methylbenzene and o-dithiobenzene, respec-
tively, and contain 2-methoxyethyl arms. It was shown that
the control of the polarity by the end groups had a big
impact on the equilibrium times. Compared to conven-
tionally used extractants such as dihexyl sulfide or hy-
droximes, our ligands had drastically increased reaction
rates. The influence of varying the backbones of our ligands
on the complex stability is reflected in a dramatic disparity
of the extraction performance in the presence of chloride.
For an application in industry, it is important that the
extractant is cheap to produce, very stable against oxidation
OES.
Figure 10. Dependency of the E value of maleonitriledithio-12-
crown-4 (black), 2 (red), 4 (cyan) and 6 (green) on the HCl concen-
tration added to the aqueous phase [CHCl3/H2O = 1:1, c(L)0
=
10–2 m, c0(Pd)w = 10–4 m, c0(Cl)w = 1.5ϫ10–2 m]. E was determined
by measuring c(Pd)w with ICP OES.
Figure 11 demonstrates the capability of 4 under condi- and able to extract PdII from a medium that has a high
tions close to praxis. We extracted a solution, which was hydrochloric acid content. On the other hand, the complex
obtained by leaching an automotive catalyst with aqua re- formed should not be too stable to enable fast elution.
gia followed by 1:1 dilution with water. In addition to an These criteria are perfectly met by 4, which renders it the
HCl concentration of nearly 4.5 m, this solution did not extractant of choice. We demonstrated the high potential of
only contain a higher concentration of PdII but also PtIV 4 by selectively extracting PdII from a solution of a leached
and RhIII, as well as a number of base metals, partially in automotive catalyst, which also contained a high concentra-
10-fold excess. The selectivity of 4 was found to be excellent. tion of other PGMs, base metals and hydrochloric acid.
Eur. J. Inorg. Chem. 2012, 2341–2352
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2347

H.-J. Holdt et al.
FULL PAPER
(ε, mol–1 dcm–1) = 298 (1927), 248 (10224), 216 (18856) nm. MS
(70 eV): m/z (%) = 272 (43) [M]+, 214 (36) [C10H14OS2]+, 167 (100)
[C9H11OS]+, 91 (65) [C7H7]+. C13H20O2S2 (272.42): calcd. C 57.32,
H 7.40, S 23.54; found C 57.11, H 7.45, S 23.64.
Experimental Section
General: All reactions were carried out in dry solvents under an
argon atmosphere. Melting points (m.p.) were measured with a cap-
illary. NMR spectra of 1–6 were recorded with a Bruker Avance-
1,2-Bis(2-hydroxyethylthio)benzene (3): Yield: 2.19 g (94%); Rf =
0.11 (CHCl3); m.p. 84.5–86.0 °C. 1H NMR (300 MHz, CDCl3,
25 °C, Me4Si): δ = 7.38–7.43 (m, 2 H, CHCHCS), 7.18–7.24 (m, 2
H, CHCHCH), 3.72 [t, 3J(H,H) = 5.7 Hz, 4 H, CH2CH2O], 3.12
[t, 3J(H,H) = 5.7 Hz, 4 H, SCH2CH2], 2.58 (s, 2 H, OH) ppm.
13C NMR (75 MHz, CDCl3, 25 °C, Me4Si): δ = 136.8 (CHCS),
130.9 (CHCHCS), 127.5 (CHCHCH), 59.9 (CH2CH2O), 37.8
1
300 spectrometer. The H NMR chemical shifts are reported rela-
tive to the signal of Me4Si (δ = 0 ppm) and the 13C NMR chemical
shifts relative to the solvent signal of CDCl3 (δ = 77 ppm). The
assignments of the NMR signals were confirmed by 2D HMBC
and HMQC NMR experiments. IR spectra were recorded with a
Thermo Nicolet NEXUS FTIR instrument. UV/Vis spectroscopic
measurements were performed with a Perkin–Elmer Lambda 950
spectrophotometer using quartz cuvettes. The EI MS spectra were
recorded using a Thermo Quest SSQ 710 spectrometer. HRMS
spectra of the complexes were measured with an Agilent 1200 series
HPLC coupled with a 6210 ESI-TOF-MS using electrospray ionis-
ation. Elemental analyses (C, H, N, S) of 1–6 were performed with
an Elementar Vario EL elemental analyser and those of the com-
plexes with an Elementar Vario Micro Cube and Thermo Finnigan
EA 1112 series elemental analyser. The Pd content of [PdCl2(L)]
was determined by diluting the complex (1 mg) in 65% HNO3
(1 mL) and measuring with an Optima 5300 DV ICP OES instru-
ment from Perkin–Elmer (λ = 340.458 nm).
(SCH CH ) ppm. IR (KBr): ν = 3329 (O–H), 3250 (O–H), 2971
˜
2
2
(C–H), 2954 (C–H), 2923 (C–H), 2873 (C–H), 1449 (C–C), 1057
(C–OH), 1039 (C–OH), 1023 (C–OH), 1008 (C–OH) cm–1. UV/
Vis (H2O): λ(max) (ε, mol–1 dcm–1) = 297 (1535), 246 (10108), 214
(16574) nm. MS (70 eV): m/z (%) = 230 (32) [M]+, 186 (26)
[C8H10OS2]+, 167 (35) [C9H11OS]+, 153 (100) [C8H9OS]+.
C10H14O2S2 (230.34): calcd. C 52.14, H 6.13, S 27.84; found C
52.10, H 6.10, S 27.75.
1,2-Bis(2-methoxyethylthio)benzene (4): Yield: 2.34 g (95%); Rf =
0.93 (CHCl3); nD = 1.583. 1H NMR (300 MHz, CDCl3, 25 °C,
20
Me4Si): δ = 7.31–7.36 (m, 2 H, CHCHCS), 7.13–7.19 (m, 2 H,
3
CHCHCH), 3.59 [t, J(H,H) = 6.8 Hz, 4 H, CH2CH2O], 3.37 (s, 6
General Procedure for the Synthesis of 1–4: Benzene-1,2-dithiol or
toluene-3,4-dithiol (9.5 mmol) in ethanol (3 mL) was added to a
solution of sodium (20 mmol) in ethanol (21 mL). The resulting
solution was heated under reflux, and 2-bromoethanol or 2-chloro-
ethyl methyl ether (20 mmol) was added. The solution was stirred
under reflux for 15 h. The precipitate was removed, and the solvent
was evaporated. The yellow, oily residue was purified by column
chromatography on silica gel using chloroform as an eluent to af-
ford 1, 2 and 4 as colourless oils and 3 as white crystals.
H, OCH3), 3.11 [t, 3J(H,H) = 6.8 Hz, 4 H, SCH2CH2] ppm.
13C NMR (75 MHz, CDCl3, 25 °C, Me4Si): δ = 136.9 (CHCS),
129.5 (CHCHCS), 126.6 (CHCHCH), 70.9 (CH2CH2O), 58.7
(OCH ), 32.7 (SCH CH ) ppm. IR (KBr): ν = 2980 (C–H), 2925
˜
3
2
2
(C–H), 2877 (C–H), 2824 (C–H), 2808 (C–H), 1446 (C–C), 1114
(C–O), 1094 (C–O), 1042 (C–O) cm–1. UV/Vis (H2O): λ(max) (ε,
mol–1 dcm–1) = 300 (1310), 247 (9203), 214 (15695) nm. MS
(70 eV): m/z (%) = 258 (44) [M]+, 200 (54) [C9H12OS2]+, 167 (60)
[C9H11OS]+, 153 (100) [C8H9OS]+. C10H14O2S2 (230.34): calcd. C
55.78, H 7.02, S 24.81; found C 55.68, H 6.99, S 24.92.
1,2-Bis(2-hydroxyethylthio)-4-methylbenzene (1): Yield: 2.16 g
20
(93%); Rf = 0.12 (CHCl3); nD = 1.621. 1H NMR (300 MHz,
General Procedure for the Synthesis of 5 and 6: A suspension of
disodium (Z)-dicyanoethene-1,2-dithiolate (45 mmol) and a spatula
tip of sodium iodide in acetone (80 mL) was heated. 2-Bromo-
ethanol or 2-chloroethyl methyl ether (91 mmol) was added. The
suspension was heated under reflux and stirred for 20 h. The pre-
cipitate was removed, and the solvent was evaporated. The residue
was dissolved in chloroform and washed with water. The organic
layer was dried with anhydrous MgSO4, and the solvents evapo-
rated to dryness to yield a yellow solid, which was purified by
recrystallisation from diethyl ether to afford 5 as light brownish
and 6 as colourless crystals.
CDCl3, 25 °C, Me4Si): δ = 7.32 [d, 3J(H,H) = 8 Hz, 1 H,
3
CHCHCS], 7.21 (s, 1 H, CCHCS), 7.01 [d, J(H,H) = 8.0 Hz, 1 H,
CCHCH], 3.73 [t, 3J(H,H) = 5.7 Hz, 2 H, CH2ChH2O], 3.67 [t,
3J(H,H) = 5.7 Hz, 2 H, CH2CjH2O], 3.12 [t, J(H,H) = 5.7 Hz, 2
3
3
H, SCgH2CH2], 3.07 [t, J(H,H) = 5.7 Hz, 2 H, SCiH2CH2], 2.65
(br. s, 2 H, OH), 2.32 (s, 3 H, CH3C) ppm. 13C NMR (75 MHz,
CDCl3, 25 °C, Me4Si): δ = 138.1 (CH3CCH), 137.5 (CHCaS), 132.5
(CHCfS), 132.1 (CHCHCS), 131.1 (CCHCS), 128.4 (CCHCH),
60.0 (CH2ChH2O), 59.8 (CH2CjH2O), 38.5 (SCgH2CH2), 37.7
(SCiH CH ), 21.0 (CH C) ppm. IR (KBr): ν = 3383 (O–H), 2921
˜
2
2
3
(C–H), 2873 (C–H), 1459 (C–C), 1063 (C–OH), 1039 (C–OH), 1012
(C–OH) cm–1. UV/Vis (H2O): λ(max) (ε, mol–1 dcm–1) = 296 (1720),
247 (11704), 216 (22061) nm. MS (70 eV): m/z (%) = 244 (45)
[M]+, 200 (75) [C9H12OS2]+, 167 (100) [C9H11OS]+, 91 (65)
[C7H7]+. C11H16O2S2 (244.37): calcd. C 53.97, H 6.60, S 26.24;
found C 54.01, H 6.57, S 26.08.
(Z)-1,2-Bis(2-hydroxyethylthio)ethene-1,2-dicarbonitrile (5): Yield:
0.94 g (43%); Rf = 0.03 (CHCl3); m.p. 67.5–68.5 °C. 1H NMR
3
(300 MHz, CDCl3, 25 °C, Me4Si): δ = 3.92 [t, J(H,H) = 5.7 Hz, 4
H, CH2CH2O], 3.32 [t, 3J(H,H) = 5.7 Hz, 4 H, SCH2CH2], 2.04 (s,
2 H, OH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C, Me4Si): δ =
121.8 (C=C), 112.2 (CN), 61.3 (CH2CH2O), 37.8 (SCH2CH2) ppm.
1,2-Bis(2-methoxyethylthio)-4-methylbenzene (2): Yield: 2.38 g
IR (KBr): ν = 3325 (O–H), 2210 (CϵN), 2001 (CϵN), 1492 (C=C),
˜
(92%); Rf = 0.80 (CHCl3); nD = 1.575. 1H NMR (300 MHz,
20
1061 (C–OH), 1014 (C–OH), 999 (C–OH) cm–1. UV/Vis (H2O):
λ(max) (ε, mol–1 dcm–1) = 341 (13902), 275 (4450), 216 (5896) nm.
MS (70 eV): m/z (%) = 230 (20) [M]+, 169 (13) [C6H5N2S2]+, 159
(19) [C5H5NOS2]+, 45 (100) [C2H5O]+. C8H10N2O2S2 (230.30):
calcd. C 41.72, H 4.38, N 12.16, S 27.84; found C 41.81, H 4.32,
N 12.21, S 27.79.
CDCl3, 25 °C, Me4Si): δ = 7.26 [d, 3J(H,H) = 7.9 Hz, 1 H,
3
CHCHCS], 7.13 (s, 1 H, CCHCS), 6.96 [d, J(H,H) = 7.9 Hz, 1 H,
CCHCH], 3.53–3.63 (m, 4 H, CH2Cj,hH2O), 3.38 (s, 3 H, OClH3),
3.35 (s, 3 H, OCmH3), 3.04–3.13 (m, 4 H, SCi, gH2CH2), 2.31 (s, 3
H, CH3C) ppm. 13C NMR (75 MHz, CDCl3, 25 °C, Me4Si): δ =
137.7 (CH3CCH), 137.2 (CHCaS), 132.5 (CHCfS), 130.9
(CHCHCS), 129.6 (CCHCS), 127.3 (CCHCH), 71.0 (CH2ChH2O),
70.9 (CH2CjH2O), 58.7 (OClH3), 58.6 (OCmH3), 33.2 (SCgH2CH2),
(Z)-1,2-Bis(2-methoxyethylthio)ethene-1,2-dicarbonitrile (6): Yield:
0.76 g (31%); Rf = 0.660 (CHCl3); m.p. 36.3–37.2 °C. 1H NMR
3
32.5 (SCiH CH ), 21.0 (CH C) ppm. IR (KBr): ν = 2980 (C–H),
(300 MHz, CDCl3, 25 °C, Me4Si): δ = 3.66 [t, J(H,H) = 5.9 Hz, 4
˜
2
2
3
3
2924 (C–H), 2876 (C–H), 2823 (C–H), 2808 (C–H), 1459 (C–C),
H, CH2CH2O], 3.39 (s, 6 H, OCH3), 3.32 [t, J(H,H) = 5.9 Hz, 4
1114 (C–O), 1095 (C–O), 1040 (C–O) cm–1. UV/Vis (H2O): λ(max)
H, SCH2CH2] ppm. 13C NMR (75 MHz, CDCl3, 25 °C, Me4Si): δ
2348
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 2341–2352

Complexation of Palladium(II) with Unsaturated Dithioethers
= 121.1 (C=C), 112.1 (CN), 70.6 (CH2CH2O), 59.0 (OCH3), 34.8 calcd. C 34.72, H 4.48, Pd 23.66, S 14.26; found C 34.87, H 4.41,
(SCH CH ) ppm. IR (KBr): ν = 2212 (CϵN), 2002 (CϵN), 1504 Pd 23.96, S 14.22.
˜
2
2
(C=C), 1109 (C–O), 1040 (C–O) cm–1. UV/Vis (H2O): λ(max) (ε,
mol–1 dcm–1) = 340 (13902), 276 (4255), 215 (5803) nm. MS
(70 eV): m/z (%) = 258 (21) [M]+, 199 (4) [C7H7N2OS2]+, 59 (54)
[C3H7O]+, 45 (100) [C2H5O]+. C10H14N2O2S2 (258.35): calcd. C
46.49, H 5.46, N 10.84, S 24.82; found C 46.55, H 5.49, N 10.93,
S 24.85.
Dichlorido[1,2-bis(2-hydroxyethylthio)benzene]palladium(II)
[PdCl2(3)]:Yield: 0.30 g (73 %); m.p. 199.5 (needles), 203.5 °C
(prisms). IR (KBr): (needles) ν = 3387 (O–H), 3347 (O–H), 1077
˜
(C–OH), 1052 (C–OH), 1012 (C–OH), 1005 (C–OH) cm–1. IR
(KBr): (prisms) ν = 3452 (O–H), 1073 (C–OH), 1066 (C–OH), 1020
˜
(C–OH), 1006(C–OH). FIR (ATR): (needles) ν = 321 cm–1 (Pd–
˜
Cl); FIR (ATR): (prisms) ν = 326 cm–1 (Pd–Cl). HRMS (ESI):
General Procedure for the Synthesis of [PdCl2(L)] (L = 1–6): The
ligands (0.11 mmol) were dissolved in MeOH (5 mL) and a solution
of Na2PdCl4 (0.1 mmol) in MeOH (15 mL) was added. This solu-
tion was allowed to stand at 5 °C for 4 d. The orange precipitate
formed was collected by filtration and washed with MeOH. The
single crystals of the complexes that occurred in the (R,S)-form
were prism-shaped, whereas those that occurred as a racemate were
needle-shaped.
˜
calcd. for C10H14ClO2PdS2 [M – Cl]+ 370.9153, 372.9147; found
370.9155, 372.9141. C10H14Cl2O2PdS2 (407.65): calcd. C 29.46, H
3.46, Pd 26.10, S 15.73; found C 29.39, H 3.43, Pd 26.11, S 15.63.
Dichlorido[1,2-bis(2-methoxyethylthio)benzene]palladium(II)
[PdCl (4)]: Yield: 0.26 g (60%); m.p. 174.2–176.5 °C. IR (KBr): ν =
˜
˜
2
1116 (C–O), 1096 (C–O), 1042 (C–O) cm–1. FIR (ATR): ν =
320 cm–1 (Pd–Cl). HRMS (ESI): calcd. for C12H18ClO2PdS2 [M –
C l ] + 3 98 . 9 46 7 , 40 0. 94 60 ; fou nd 39 8. 9 4 78 , 4 00. 94 72.
C12H18Cl2O2PdS2 (435.70): calcd. C 33.08, H 4.16, Pd 24.42, S 14.72;
found C 33.22, H 4.11, Pd 24.27, S 14.65.
Dichlorido[1,2-bis(2-hydroxyethylthio)-4-methylbenzene]palla-
dium(II) [PdCl2(1)]: Yield: 0.20 g (47%); m.p. 163.0–164.7 °C. IR
(KBr): ν = 3474 (O–H), 3428 (O–H), 1070 (C–OH), 1020 (C–OH),
˜
1005 (C–OH) cm–1. FIR (ATR): ν = 325 (Pd–Cl) cm–1. HRMS
˜
(ESI): calcd. for C11H16ClO2PdS2 [M – Cl]+ 374.9310, 386.9304;
found 384.9304, 386.9293. C11H16Cl2O2PdS2 (421.67): calcd. C
31.33, H 3.82, Pd 25.24, S 15.21; found C 31.42, H 3.82, Pd 25.14,
S 15.34.
Dichlorido[(Z)-1,2-bis(2-methoxyethylthio)-1,2-dicyanoethene]palla-
dium(II) [PdCl2(6)]: Yield: 0.27 g (63%); m.p. 141.0–148.0 °C. IR
(KBr): ν = 2233 (CϵN), 1566 (C=C), 1114 (C–O), 1075 (C–O) cm–1.
˜
FIR (ATR): ν = 325 cm–1 (Pd–Cl). HRMS (ESI): calcd. for
˜
C10H13Cl3N2O2Pd2S2 [M – H++ Cl–+Pd2++e–]– 577.7546; found
577.7653. C10H14Cl2N2O2PdS2 (435.66): calcd. C 27.57, H 3.24, N
6.43, Pd 24.43, S 14.72; found C 27.80, H 3.30, N 6.38, Pd 24.23, S
14.60.
Dichlorido[1,2-bis(2-methoxyethylthio)-4-methylbenzene]palla-
dium(II) [PdCl2(2)]: Yield: 0.23 g (52%); m.p. 113.3–115.5 °C. IR
(KBr): ν = 1114 (C–O), 1097 (C–O), 1043 (C–O) cm–1. FIR (ATR):
˜
ν
= 3 2 6 ( P d – C l ) c m – 1 . H R M S ( E S I ) : c a l c d . f or
˜
C
13H20Cl2O2PdS2Na [M + Na]+ 470.9208, 472.9198; found X-ray Structure Determination and Refinement: Suitable crystals for
470.9211, 472.9201; calcd. for C13H20ClO2PdS2 [M – Cl]+ 412.9624, single-crystal X-ray diffraction were mounted on a glass fibre and
414.9617; found 412.9629, 414.9624. C13H20Cl2O2PdS2 (449.73): placed in the cryosystem of the diffractometer. Crystals of 5, 6,
Table 4. Crystal data and structure refinement for 3, 5 and 6.
3
5
6
Empirical formula
M [gmol–1]
Crystal description
Crystal colour
Crystal size [mm]
Crystal system
Space group
Unit cell dimensions
a [Å]
b [Å]
c [Å]
a [°]
b [°]
C10H14O2S2
230.33
needle
colourless
1.30. 0.29. 0.21
orthorhombic
P212121
C8H10N2O2S2
230.30
needle
colourless
1.00. 0.40. 0.10
monoclinic
C2/c
C10H14N2O2S2
258.35
block
colourless
0.500. 0.347. 0.240
triclinic
¯
P1
8.7515(10)
9.5547(16)
12.8493(17)
90
90
90
1074.4(3)
4
1.424
488
0.466
23.290(3)
4.8885(2)
21.742(2)
90
121.683(8)
90
2106.4(3)
8
1.452
7.9106(15)
8.5343(17)
10.803(2)
97.441(16)
100.640(15)
113.250(15)
641.8(2)
2
γ [°]
V [Å]
Z
ρcalcd. [gcm–3]
F (000)
1.337
272
0.403
960
0.481
μ [mm–1]
Tmax./Tmin.
2 θmax. (°)
0.8308/0.8298
50.26
0.7924/0.5261
50.00
0.8843/0.8852
50.00
Collected reflections
Independent reflections
Rint
Reflections with IϾ2σ(I)
Parameters
R1/wR2 [IϾ2σ(I)]
R1/wR2 [all data]
Min./max. Δρ [10–6 epm–3]
GooF
6884
1906
0.0682
1704
134
0.0307/0.0698
0.0348/0.0713
–0.270/0.166
0.972
9388
1836
0.0621
1682
157
0.0490/0.1214
0.0528/0.1240
–0.328/0.484
1.133
4207
2133
0.0580
1506
146
0.0370/0.0870
0.0545/0.0911
–0.336/0.207
0.895
Eur. J. Inorg. Chem. 2012, 2341–2352
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2349

H.-J. Holdt et al.
FULL PAPER
Table 5. Crystal data and structure refinement for the palladium complexes.
[PdCl2(1)][a]
[PdCl2(2)][a]
[PdCl2(3)][a]
[PdCl2(3)][b]
[PdCl2(4)][b]
[PdCl2(6)][b]
Empirical for-
mula
C11H16Cl2O2PdS2
C13H20Cl2O2PdS2
C10H14Cl2O2PdS2
C10H14Cl2O2PdS2 C12H18Cl2O2PdS2 C10H14Cl2N2O2PdS2
M [gmol–1
]
421.66
prism
449.71
block
407.63
prism
407.63
needle
435.68
needle
435.65
needle
Crystal descrip-
tion
Crystal colour
orange
orange
orange
orange
yellow
brown
Crystal size [mm] 0.500ϫ0.325ϫ0.175
0.340ϫ0.253ϫ0.190 0.400ϫ0.350ϫ0.260 0.900ϫ0.338ϫ0.040 1.100ϫ0.457ϫ0.110 1.64ϫ0.11ϫ0.05
Crystal system
Space group
Unit cell dimen-
sions
monoclinic
P21/n
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21/c
monoclinic
Cc
monoclinic
P21/c
a [Å]
b [Å]
c [Å]
a [°]
7.9338(7)
15.8975(11)
12.1618(12)
90
11.3469(5)
11.2729(3)
14.6773(7)
90
8.0445(6)
14.1803(9)
12.5617(10)
90
8.6545(12)
19.1338(19)
8.4479(11)
90
10.6170(12)
23.048(2)
8.5484(10)
90
12.0070(18)
7.5194(7)
17.959(3)
90
b [°]
101.248(7)
90
1504.5(2)
4
111.894(3)
90
1742.00(12)
4
103.541(6)
90
1393.12(18)
4
103.469(11)
90
1360.4(3)
4
128.125(8)
90
1645.5(3)
4
108.651(11)
90
1536.3(4)
4
γ [°]
V [Å3]
Z
ρ calcd. [g cm–3
]
1.862
1.715
1.944
1.990
1.759
1.884
F(000)
840
1.856
904
1.609
808
2.001
808
2.049
872
1.700
864
1.825
μ [mm–1
]
Tmax./Tmin.
2 θmax. [°]
Collected reflec-
tions
0.6090/0.6035
58.82
12184
0.8136/0.5766
50.00
21912
0.5429/0.5351
50.00
8893
0.7702/0.7684
50.00
8706
0.6621/0.6580
55.00
6827
0.7762/0.7745
58.90
9461
Indep. reflections 3447
3057
2454
2396
3537
2699
Rint
0.0589
3138
0.0568
2752
0.0472
2264
0.0635
1979
0.0328
3400
0.0628
2246
Reflections with
IϾ2σ(I)
Parameters
167
182
197
161
191
196
R1/wR2 [IϾ2σ(I)] 0.0425/0.1100
R1/wR2 [all data] 0.0461/0.1120
0.0523/0.1108
0.0576/0.1129
–0.883/0.879
0.0191/0.0472
0.0218/0.0481
–0.493/0.513
0.0241/0.0511
0.0333/0.0529
–0.753/0.794
0.0254/0.0650
0.0264/0.0653
–0.591/0.495
0.0352/0.0732
0.0466/0.0759
–0.668/1.663
Min./max. Δρ
[10–6 epm–3
GooF
–1.240/0.796
]
1.082
1.209
1.035
0.955
1.046
1.032
[a] (R,S)-configuration of the sulfur atoms. [b] (R,R) and (S,S)-configuration of the sulfur atoms.
(R,S)-[PdCl2(2)] and (R,R;S,S)-[PdCl2(3)] were selected under per-
fluoroether oil (perfluoropolyalkyl ether, viscosity 1600 cSt). The X-
ray data were collected with an Imaging Plate Diffraction System
IPDS-2 (STOE) at 210 K with graphite-monochromated Mo-Kα ra-
diation (λ = 0.71073 Å). Spherical absorption corrections were ap-
plied for 3, 6, (R,S)-[PdCl2(1)], (R,S)-[PdCl2(3)] and (R,R;S,S)-
[PdCl2(3)], (R,R;S,S)-[PdCl2(4)] and (R,R;S,S)-[PdCl2(6)]. A numeri-
cal absorption correction was performed for 5 and (R,S)-[PdCl2(2)].
The structures were solved by direct methods using the program
SHELXS[33] and refined against F2 with full-matrix least-squares
techniques using the program SHELXL-97.[34] All non-hydrogen
atoms were refined anisotropically. In (R,R;S,S)-[PdCl2(4)], two
atoms (O2, C5) were disordered, exhibiting two split positions with
occupancies of 70 and 30%. The hydrogen atoms of the hydroxy
groups in all structures, and the carbon-bound hydrogen atoms in 5,
(R,S)-[PdCl2(3)] and (R,R;S,S)-[PdCl2(6)] were localised from differ-
ence Fourier maps, the Uiso(H) values were set to 1.2 Ueq(C,O) or
1.5 Ueq(Cmethyl). The carbon hydrogen atoms in 3, 6, (R,S)-
[PdCl2(1)], (R,S)-[PdCl2(2)], (R,R;S,S)-[PdCl2(3)] and (R,R;S,S)-
[PdCl2(4)], as well as the hydrogen atoms of the methyl groups [C(5)
and [PdCl2(3)]. To visualise the structures, the graphic program OR-
TEP[35] was used. Experimental details and the crystallographic data
are reported in Table 4 for the ligands, and Table 5 for [PdCl2(L)].
CCDC-834567 (for 3), -834562 (for 5), -834568 (for 6), -834566 {for
(R,S)-[PdCl2(1)]}, -834564 {for (R,S)-[PdCl2(2)]}, -834565 {for
(R,S)-[PdCl2(3)]}, -834569 {for (R,R;S,S)-[PdCl2(3)]}, -834563 {for
(R,R;S,S)-[PdCl2(4)]} and -834570 {for (R,R;S,S)-[PdCl2(6)]} con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Voltammetric Measurements: Electrochemical measurements were
performed with a PGSTAT302N electrochemical analyser (Met-
rohm) and cell stand C2 (Bioanalytical Systems) using a platinum
disc working electrode (2 mm2), a platinum wire auxiliary electrode
and an anhydrous Ag/Ag+ reference electrode. The experiments were
performed using degassed MeCN solutions with 0.1 m [nBu4N][PF6]
as the supporting electrolyte at a scan rate of 100 mV/s, and the
ferrocene/ferrocinium couple as an internal reference. All potentials
are given vs. Fc/Fc+ = 85 mV, peak separation 71 mV, ΔIp = 1.01.
H3 and C(8)H3] in (R,R;S,S)-[PdCl2(6)] were calculated in their ex- DFT Calculations: All calculations were carried out using the
pected positions and refined using a riding model with d(C–H) =
Gaussian09[23] programme package. We used the LANL2DZ[24] basis
0.93 Å (aromatic), 0.97 Å (methylene) or 0.96 Å (methyl) and Ui- set and the CAM-B3LYP[24] functional. For 3, 6 and [PdCl2(3)], ge-
so(H) = 1.2 Ueq(Car,Cmethylene) or Uiso(H) = 1.5 Ueq(Cmethyl). Intermo- ometry optimisation was carried out starting from the experimental
lecular hydrogen bonds appear in the crystal structures of [PdCl2(1)]
crystal structure. For the optimised geometries, ESP-derived charges
2350
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 2341–2352

Complexation of Palladium(II) with Unsaturated Dithioethers
were calculated according to the Merz–Singh–Kollman scheme,[36]
where an atomic radius of 1.63 Å was used for Pd. The same set of
calculations was carried out for the corresponding cations of all
three compounds. For 3, also excited state energies and the respective
oscillator strength were obtained by the time-dependent DFT[37] ap-
proach using the same functional and basis set.
Acknowledgments
We thank the Arbeitsgemeinschaft industrieller Forschungsvereini-
gungen “Otto von Guericke” e.V. (AIF) and ReMetall Drochow
GmbH for financial support and Cognis for the supply of Alox-
ime® 840. Special thanks are due to Dr. Christine Fischer (Leibniz
Institute for Catalysis, University of Rostock) for MS measurements,
Sigrid Imme (Technische Universität Berlin), and Sylvia Pirok (Max
Planck Institute of Colloids and Interfaces) for the elementary analy-
ses of the palladium complexes.
NMR Experiments on [PdCl2(2)] and [PdCl2(4)]: The measurements
were performed with a Bruker AVANCE 600 spectrometer operating
1
at H and 13C frequencies of 600.24 and 150.94 MHz, respectively.
All spectra were recorded and processed using BRUKER standard
1
software (TOPSPIN 2.1). Typical numbers of scans were 16 for H
and 1024 for 13C NMR spectra. For low-temperature measurements,
a BVT 3200 variable-temperature unit was used. The internal ther-
mocouple was calibrated using an external Pt100 thermocouple,
which was applied at the same position in the probe where the sam-
ple (NMR tube) is usually located. The chemical shifts are reported
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1
relative to the solvent signal of dichloromethane: H NMR δ(1H) =
5.31 ppm and 13C NMR δ(13C) = 53.7 ppm. The other complexes
were not soluble enough to give spectra worthy of evaluation.
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tra were recorded with a Perkin–Elmer Lambda 950 spectrophotom-
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with 1–6 (2 mL, c = 5ϫ10–5 m) and adding palladium solutions
(1000 ppm palladium absorption standard solution in 5% w/w HCl
from Sigma–Aldrich diluted to c = 5ϫ10–4 m) in 10 μL steps every
5 minutes with a Metrohm 805 Dosimat connected to a 846 Dosing
Interface. Deionised water (Elga, Purelab ultra) was used as the sol-
vent.
Solvent Extraction Experiments and ICP OES: All solvent extraction
batch experiments were carried out at room temperature. Equal vol-
umes of the aqueous and organic (10–2 m ligand in CHCl3 uvasol
from Merck) phases were mixed with a Heidolph Multi Reax vibrat-
ing shaker (ca. 1900 rpm) and separated by centrifugation. The metal
content of the aqueous phase was determined before and after the
extraction with an Optima 5300 DV ICP OES from Perkin–Elmer.
To prepare the model solutions, a 1000 ppm palladium absorption
standard solution in 5% (w/w) HCl from Sigma–Aldrich and 30%
(w/w) suprapur hydrochloric acid from Merck were diluted. The
solution of the automotive catalyst was obtained by leaching the
monolith (1 kg) with heating with aqua regia (2 L) until the evol-
ution of the gas was finished. Afterwards it was diluted with the
same volume of deionised water (Elga, Purelab ultra). The oxidation
potential of this solution was 1.05 V (vs. SHE). For the measurement
of the organic samples, it was necessary to mix chloroform with
water with the help acetic acid to obtain one phase. In this way it
was possible to use an aqueous stock solution to prepare the stan-
dards and carry out the calibration in the same matrix as the sam-
ples. On this account, the organic (1.5 mL, sample or pure CHCl3)
was added to the aqueous solution (1.5 mL, pure water or aqueous
standard solution respectively) and mixed with suprapur glacial ac-
etic acid (7 mL, Merck). This was measured using a GemCone nebu-
liser and a baffled cyclonic spray chamber. The measurement of the
aqueous samples was performed with a cross-flow nebuliser and a
Ryton Scott spray chamber.
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Eur. J. Inorg. Chem. 2012, 2341–2352
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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H.-J. Holdt et al.
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Received: December 16, 2011
Published Online: February 28, 2012
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Eur. J. Inorg. Chem. 2012, 2341–2352