Dithiocarboxylic Acids: An Old Theme Revisited and Augmented by New Preparative, Spectroscopic and Structural Facts

Article abstract of DOI:10.1002/chem.201704235

The unstable dithiocarboxylic acids dithioacetic acid, 2-methyl-dithiopropionic acid, 2,2-dimethyl-dithiopropionic acid and dithiobenzoic acid were synthesized and characterized by NMR spectroscopy and GC/MS. The stable dithiocarboxylic acids 2,4,6-trimethyl benzoic acid, 2,4,6-tri-iso-propylbenzoic acid and 2,6-dimesityl benzoic acid were synthesized, isolated and characterized by spectroscopic methods and in parts by mass spectrometry and X-ray crystallography. The new data were used to re-evaluate literature data on the synthesis, spectroscopy and structural data of dithiocarboxylic acids as a fundamental class of organic compounds in general.

Full text of DOI:10.1002/chem.201704235

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Accepted Article
Title: Dithiocarboxylic acids - an old theme revisited and augmented by
new preparative, spectroscopic and structural facts
Authors: Norbert Werner Mitzel, Johanna Grote, Felix Friedrich,
Katharina Berthold, Loreen Hericks, Beate Neumann, and
Hans-Georg Stammler
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Dithiocarboxylic acids – an old theme revisited and augmented
by new preparative, spectroscopic and structural facts
Johanna Grote née Glatthor, Felix Friedrich, Katarína Berthold, Loreen Hericks, Beate Neumann,
Hans-Georg Stammler, and Norbert W. Mitzel*
In memoriam Professor Dr. Bernd Wrackmeyer
Abstract: The unstable dithiocarboxylic acids dithioacetic acid, 2-
methyl-dithiopropionic acid, 2,2-dimethyl-dithiopropionic acid and di-
thiobenzoic acid were synthesized and characterized by NMR spec-
troscopy and GC/MS. The stable dithiocarboxylic acids 2,4,6-trime-
thyl benzoic acid, 2,4,6-tri-iso-propylbenzoic acid and 2,6-dimesityl
benzoic acid were synthesized, isolated and characterized by spec-
troscopic methods and in parts by mass spectrometry and X-ray cry-
stallography. The new data were used to reevaluate literature data
on the synthesis, spectroscopy and structural data of dithiocarboxylic
acids as a fundamental class or organic compounds in general.
characteristic is their strong and disgusting odor, which together
with difficulties in handling due to instability make dithiocarboxy-
lic acids a still grossly underexplored class of compounds.[
Hence, instead of the free acids, their salts and esters were
more frequently used.[5,14]
2–6]
Unlike carboxylic acids, the dithio-analogues do not form thioan-
hydrates, but have a tendency to form disulfides (thioacyldisul-
fides) by oxidation with atmospheric oxygen (Scheme 1, d).[9,10]
Thuillier published thiol acids (b) as decomposition products as
well as the formation of hexathiaadamantanes (a) or different
trithianes (c) as aggregation products.[12,13]
Introduction
The number of carboxylic acids in organic chemistry is virtually
unlimited. They show ample occurrence in nature; to name but a
few: formic acid, lactic and citric acid. Various strategies for their
syntheses are basic textbook knowledge in organic chemistry.
Due to their stability, carboxylic acids are relevant in biological
and biochemical processes, for chemical industry and generally
in organic syntheses.[1,2] Dithiocarboxylic acids, in contrast, are
much less present in literature. Their 873 entries in chemical lite-
rature (Scifinder, number of search results) are lower by a factor
of about 1300.
Scheme 1. Aggregation and decomposition of dithiocarboxylic acids (a: R =
Me,[13,26] i-Pr, [26] b: R = Me, Et, Pr, Bu, cyclo-pentyl,[12] c: R = H,[17] Me[13,14]
)
Whereas the dithiocarboxylate functional group (–C(=S)S–) is
known since as early as 1824, Fleischer reported the synthesis
of thiophenol only in 1866 and in this context obtained dithio-
benzoic acid as a byproduct and as the first dithiocarboxylic
acid.[7–9] More than 50 years later Houben and Pohl brought the
class of dithiocarboxylic acids into better focus.[9] They reported
the syntheses of a variety of dithiocarboxylic acids and their cha-
racterization, partially by elemental analyses and by derivatizati-
on in the form of heavy metal salts.[9–11] However, reliable analy-
tical data concerning isolated, pure dithiocarboxylic acids are still
extremely scarce.[2,3,6] Houben and Pohl found dithiocarboxylic
acids to be strong acids, to be soluble in organic solvents, to be
of limited stability and to be generally of yellow, red or violet
color, due to the C=S group chromophore. Another important
Dithiocarboxylic acids and their derivatives are a group of highly
reactive compounds, which generally react under much milder
conditions than their dioxy-analogues.[2] Hartke found, that di-
thiocarboxylic acids form oligomers (dimers or trimers) after a
short time even at −30 °C in the neat state or in solution.[14] Due
to the lower electronegativity of σ-monovalent sulfur and its less
efficient orbital overlap with the adjacent carbon atom, the π-
system of a thiocarbonyl group is less stabilized than its carbo-
nyl analogue. Based on the smaller orbital overlap, weak and
relatively unstable C=S bonds tend to rearrange to more stable
C–S single bonds. This is why thiocarbonyl compounds show a
series of characteristic reactions which are mostly unknown or at
least unusual for carbonyl compounds (Scheme 1).[5]
Dithiocarboxylic acids are accessible by thiolation of appropriate
organic compounds with hydrogen sulfide or its salts, with ele-
mental sulfur or other thiolation agents. The most widely used
route is the reaction of carbon disulfide with strong nucleophi-
les.[2,3,5,6]
[*]
J. Grote née Glatthor, F. Friedrich, K. Berthold, L. Hericks, B.
Neumann, Dr. H.-G. Stammler, Prof. Dr. N. W. Mitzel
Anorganische Chemie und Strukturchemie
Centrum für Molekulare Materialien CM
Fakultät für Chemie
2
Universität Bielefeld
Universitätsstraße 25, 33615 Bielefeld (Germany)
E-mail: mitzel@uni-bielefeld.de
Supporting information for this article is given via a link at the end of
the document.
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Under these conditions the magnesium salt (R–C(=S)SMgX) is
prevented from further fast alkylation to the corresponding es-
ter.[5,14] Note also, that dithiocarboxylic acids with an α-hydrogen
atom are prone to double deprotonation and formation of a dian-
ion (Scheme 3), leading to even more byproducts.[2,5,14]
Scheme 2. Different synthetic pathways to free dithiocarboxylic acids. [2,3,5]
Scheme 3. Twofold deprotonation of dithiocarboxylic acid with an α-hydrogen
atom
For the aromatic series of dithiocarboxylic acids sulfhydrolysis of
geminal di- or trihalides by using hydrogen sulfides and elemen-
tary sulfur or alkali (hydrogen) sulfides is a common method.[3,5]
Aromatic dithiocarboxylic acids were also synthesized by reacti-
ons of aromatic aldehydes with hydrogen sulfide. To obtain di-
thiocarboxylic acids the corresponding nitriles can be reacted
with hydrogen sulfide.[5]
The most popular method to synthesize dithiocarboxylic acids is
to react various alkyl or aryl Grignard reagents with carbon disul-
fide at low temperature.[2–5,9–12] Since Houben first synthesized
them in this way in 1907, the procedure has continuously been
refined and optimized[14] – mostly by replacing diethyl ether by
tetrahydrofuran and by slow addition of the Grignard reagent.
Due to the described instability, dithiocarboxylic acids have been
commonly transformed into their alkali metal cation, ammonium,
phosphonium and arsonium salts or dithiocarboxylic esters for
isolation, storage and further reactions.[2]
Table 1 contains an overview of available analytical data of se-
lected alkyl or aryl dithiocarboxylic acids. We have chosen di-
thiocarboxylic acids with heteroatom-free, short saturated alkyl
substituents (H-, Me-, Et-, n-Pr-, i-Pr-, n-Bu-, t-Bu-, i-Pen-, cy-Pr-,
cy-Pen-, cy-Hex-) and heteroatom-free aryl substituents (Ph-,
Tip- (2,4,6-triisopropylphenyl)), Mes-, Mes*- (2,4,6-tritertbutyl-
phenyl), Dmp- (2,6-dimesitylphenyl)) without any further functio-
nal groups to provide an meaningful overview. It is interesting to
point out that all synthetic
Table 1. Analytical data concerning the most common isolated dithiocarboxylic acids reported in
literature
protocols refer to the work of
Houben or Thuillier – and the
R
isol.
bp / pa
mpb
yieldc
18
NMR
MS
IR
UV-vis
EA
lit.
latter
refined
Houben’s
H
●
55-60
●
●
●
●
17
instructions from 1907. Although
all reports in Table 1 claim that
almost every dithiocarboxylic acid
has been isolated, from a todays
view, for most cases there is no
adequate evidence for their
unequivocal identity as free,
isolated compounds and their
monomeric structure. Due to the
fact that free dithiocarboxylic
acids have the same mass pro-
portions as their aggregates,
elemental analyses are no
sufficient proof for their identity as
free acids.
Me
Me
●
●
37/15
●
10
14
35–37/15
Me
Me
Me
Me
●
●
●
●
55
48
13
12
15
16
31–32/15
38–40/17
●
●
●
56
Et
Et
Et
●
●
●
41–42/13
53
12
53
12
11
14
48/17
●
●
n-Pr
n-Pr
n-Pr
●
●
●
73
5
14
11
12
59/13
59–60/13
40
●
●
●
i-Pr
●
●
●
45–46/13
76–77/13
84/33
56
40
4
12
12
11
11
12
12
n-Bu
i-Bu
●
●
Despite the availability of NMR
data for different dithiocarboxylic
acids, crucial characteristics like
the 13C NMR chemical shift of the
dithiocarboxylic carbon atom were
not accessible for most cases.
i-Pen
cy-Pr
cy-Pen
84/10
4
●
●
71–72/17
55–56/0.3
67–69/0.3
42
38
●
●
●
●
cy-Hex
Ph
●
●
●
25
12
18
19
100
70
Ph
Solely
resonances of the alkyl or aryl
substituents linked to the
the
assignment
of
Ph
Ph
Ph
●
●
●
20
21
22
●
●
●
●
●
67
40
C(=S)SH group is insufficient
proof of identity of the complete
Tip
●
●
76
51
●
23
24
Dmp
●
●
1
molecule. The assignments of H
a
b
c
boiling point bp in °C / pressure p in Torr; melting point mp in °C; yield in %.
NMR chemical shifts to SH group
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resonances are often insufficient and generally hampered due to
their strong concentration-dependence.[12] Only 2,6-dimesityl-di-
thiobenzoic acid (Dmp-C(=S)SH) has been fully characterized by
NMR spectroscopy including the downfield shifted 13C NMR
signal of the dithiocarboxylic carbon at 229.2 ppm.[24]
hexathiaadamantane (f), thiodiacetyl disulfide (g) and diacetyl
disulfide (h).
There are hardly any mass spectrometric data of dithiocarboxylic
acids in literature. However, it was mass spectrometry that re-
vealed the oligomeric nature of dithioformic acid.[17] Tri-iso-
propyl-dithiobenzoic acid (Tip-C(=S)SH) has been successfully
characterized by high resolution mass spectrometry (HRMS).[24]
In infrared spectroscopy the S–H stretching vibrations of thiols or
sulfides are in general weak and found in the range between
−
1
2
540 and 2600 cm . In the case of dithiocarboxylic acids the S–
H stretching vibration at about 2550 cm−1 gives strong bands,
while the C=S bands appear close to 1215 cm−1 (by comparison;
Scheme 5. Trithioacetic anhydride (e), hexamethyl hexathiaadamantane (f),
thiodiacetyl disulfide (g), diacetyl disulfide (h) as aggregation and oxidation
products of dithioacetic acid
080–1210 cm−1 for thioesters). On the basis of IR spectroscopy
1
it is possible to distinguish between intact C(=S)SH acid functi-
ons and aggregated forms. For example the IR spectrum of the
Similarly, for to 2-methyl dithiopropionic acid (i-Pr-C(=S)SH),
2,2-dimethyl
dithiopropionic
acid
(t-Bu-C(=S)SH)
and
oligomeric dithioformic acid ([HC(S)SH] [17]
) shows no strong
x
dithiobenzoic acid (Ph-C(=S)SH) the corresponding alkyl or aryl
esters were found as byproducts. The attempt to remove the
solvent completely led to aggregation and oxidative
decomposition products; they were verified by GC/MS analyses.
In the case of the dithiobenzoic acid biphenyl was found to be a
preferred decomposition product. We were not able to identify all
of the large number of decomposition products (up to 45 diffe-
rent GC/MS signals). After considerable time of doing GC/MS
measurements we also observed a blocking of the noble metal
syringe of the GC/MS equipment. Although diluted solutions
were used, the stamp and the inner of the syringe were found to
be coated with a dark, ill smelling residue.
However, we obtained dithioacetic acid as a THF solution by the
standard procedure via a Grignard reaction in 33% yield (calcu-
lated from NMR data). Also i-Pr-C(=S)SH was obtained in 38%
yield in solution. In the case of t-Bu-C(=S)SH and Ph-C(=S)SH
we were able to determine NMR chemical shifts. However, the
determination of the concentrations was not possible, due to the
high dilution.
band at about 2550 cm−1 and no C=S band near to 1215 cm ,
−1
−
1 [17]
but one below at 1163 cm . In contrast, the IR spectrum of
Dmp-C(=S)SH shows a characteristic S–H stretching band at
−
1 [24]
2
501 cm . Despite the fact, that dithiocarboxylic acids are
strongly colored substances, there are virtually no available UV-
vis spectroscopy reference data.
The reason, why we explored the chemistry of dithiocarboxylic
acids was our intention to synthesize volatile (binuclear) heavy
metal complexes with dithiocarboxylate ligands. One possible
pathway to such substance involves the use of free dithiocarb-
oxylic acids. In the course of this work we realized the distinct
paucity of available information on this class of substances. This
report is intended to serve as a short overview and to provide
some examples of fully characterized and otherwise heteroatom-
free examples of the fundamental class of dithiocarboxylic acids.
Results and Discussion
Only Mes-C(=S)SH and Tip-C(=S)SH could be isolated in pure
form in 80% and 33% yield, respectively. Both were purified by
conversion into their sodium salts and reconversion into their
free acid. Mes*-C(=S)SH and Dmp-C(=S)SH were synthesized
by reacting the corresponding organolithium compound with
carbon disulfide and subsequent hydrolysis. The purification was
carried out in the same was as described above. We obtained
Mes*- and Dmp-C(=S)SH with 27% and 45% yield, respectively.
Syntheses and analytical data
We investigated in particular the syntheses, isolation and cha-
racterization of different free dithiocarboxylic acids with the follo-
wing substituents at the C(=S)SH unit: Me-, i-Pr-, t-Bu-, Ph-,
Mes-, Tip-, Mes*-, Dmp.
According to Thuillier’s protocol we synthesized Me-C(=S)SH,
i-Pr-C(=S)SH, t-Bu-C(=S)SH, Ph-C(=S)SH, Mes-C(=S)SH and
Tip-C(=S)SH.[12] Any attempt to isolate pure Me-, i-Pr-, t-Bu- and
Ph-C(=S)SH by evaporation, (vacuum) distillation, condensation
or crystallization failed. In contrast to earlier literature reports, we
were not able to isolate these as free acids and observed
decomposition and aggregation instead. Dithioacetic acid was
found to be the main product (GC/MS; 95.6 %, dithioacetic
methyl ester 4.4 %) before attempts of purification, whereas
after distillation or condensation the free acid could no longer be
detected in the residue. Instead of dithioacetic acid the GC/MS
measurements indicate the formation of aggregates and/or
oxidation products like trithioacetic anhydride (e), tetramethyl
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The 1H and 13C NMR spectra of the isolated dithiocarboxylic
acids (Mes-, Tip-, Mes*-, Dmp-C(=S)SH) showed the expected
Table 2. 13C-NMR and 1H NMR chemical shifts δ of the C(=S)SH group of
the isolated dithiocarboxylic acids (75 MHz / 300 MHz, CDCl )
3
signals concerning the Mes, Tip, Mes* and Dmp groups. Fur-
thermore the 13C NMR spectra show the characteristic
δ(13C) / ppm
δ( H) / ppm
1
Substituent R
13C(=S)SH signal (Table 2). The hydrogen atom of the C(=S)SH
group could be observed in the 1H NMR spectra as a broad
singlet, explaining why this led to difficulties in the past.
Due to the variations in concentration and the reactivity to diver-
se types of cell materials, IR spectroscopy of the free, instable
dithiocarboxylic acids (Me-, i-Pr-, t-Bu-, Ph-C(=S)SH) in solution
was not practical. Their disgusting smell also made such analy-
ses not attractive. IR spectra of the stable dithiocarboxylic acids
Mes
Tip
236.0
236.6
241.5
229.0
6.65
6.75
6.91
6.18
Mes*
Dmp
(
Mes-, Tip-, Mes*-, Dmp-C(=S)SH) show C–H stretching
−
1
determination of their molecular structure by X-ray diffraction. All
sulfur bonded hydrogen atoms could be located in the difference
Fourier maps and their positional and displacement parameters
were refined, also if the uncertainty of these values are naturally
large. We use these positions in the following discussion, but the
fact, that X-ray diffraction is locating electron density rather than
nuclear position must be kept in mind, when interpreting
parameters involving hydrogen positions.
Figure 1 shows the molecular structure of the Tip-substituted
dithiocarboxylic acid. Its C–S distances and S–C–S angles have
a conformable magnitude for C(=S)SH groups (in comparison to
the data for other compounds containing C(=S)SH groups, I-V,
Scheme 4).[30–34] The atoms of its C(=S)SH unit are coplanar and
within the margin of error this plane is almost orthogonal to the
ring plane of the Tip-substituent, the angle of the mean planes is
vibrations in the rage between 2970 and 2850 cm . Further-
more all spectra show the characteristic S–H stretching vibra-
tions in the small interval between 2500 and 2530 cm−1 and C=S
stretching vibrations in the region between 1010 and 1150 cm .
On the basis of NMR- and IR-spectroscopy it is possible to
demonstrate that the C(=S)SH-group is intact and not
aggregated. Elemental analyses gave additional information on
the purity of the free dithiocarboxylic acid (see experimental
section).
−
1
Single crystal X-ray diffraction
Up to now there is no structure determination by X-ray diffraction
(
XRD) of single crystals for free and simply substituted dithio-
[
36]
carboxylic acids (CCSD data base, February 2017).
Com-
pounds with C(=S)SH groups that have been examined by XRD
are only in combination with other functional groups. In three of
these cases the solid state structures show intramolecular hy-
drogen bonds to these additional functional groups and may
therefore distort the structural characteristics compared to pure
dithiocarboxylic acid (Scheme 4, I, II, III).[30,31,33] Salt V is a dimer
8
8.7(1)°. The S(1)⋯H(2) measures 3.01(7) Å, which is the same
as the sum of the van der Waals radii of H and S (3.05 Å).[ It
can thus only be speculated if the orientation of the SH
hydrogen atom indicates a form of weak intramolecular hydro-
gen bonding.
39]
Besides this possible intramolecular hydrogen bond, there is a
more pronounced intermolecular hydrogen bond, S(1)⋯H(2’), of
2.87(7) Å in length. It involves an S⋯H–S angle of 120(3)°. Both
values are indicative of weak hydrogen bonds.[29] Though weak,
it leads to a chain pattern running along the a axis as shown in
Figure 1, in contrast the solid state structure and crystal packing
pattern of the dioxy analog acid Tip-C(=O)OH exhibit a dimer
structure via two intermolecular hydrogen bonds.[35]
(
with two linear hydrogen bonds) of the dithiocarboxylic acid
[
32]
unit. The inter- and intramolecular association patterns of the
C(=S)SH groups in IV are unclear.[34]
[
30]
Scheme 4. 2-Hydroxy-dithiobenzoic acid (I),
2-amino-1-cyclo-pentene-1-
[33]
[
31]
carbodithioic acid (II), 2-imino-cyclo-pentanedithioic acid (III), 2-oxo-cyclo-
[
34]
pentane dithiocarboxylic acid (IV),
dicyano(dithiocarboxy)-methanide salt
[
32]
(V).
All bulkily substituted dithiocarboxylic acids (Tip-, Mes*-, Dmp-
C(=S)SH) are deep red to pink colored, crystalline solids, from
which we have now obtained single crystals. These allowed the
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Figure 1. Molecular structure and crystal packing of the Tip-substituted
dithiocarboxylic acid.
Table 3. Characteristic intra- and intermolecular structural parameters of
Tip-C(=S)SH
C(1)–S(1)
C(1)–S(2)
S(2)–H(2)
S(1)⋯H(2)
1.6379(15) Å
1.7197(15) Å
1.42(6) Å
C(1)–S(2)–H(2)
S(1)⋯H(2’)
107(3)°
2.87(7) Å
120(3)°
S(1)⋯H(2’)–S(2’)
plane (C(=S)SH)
Figure 3. Crystal packing of Dmp-C(=S)SH by SH⋯π interaction
3.01(7) Å
±0.006 Å
(d(SH⋯centroid) = 2.46(2) Å). Carbon bonded hydrogens were omitted for
clarity.
Symmetry code for H(2’): –x, y, z
The intramolecular C–S distances and S–C–S angle adopt
values comparable to those of other C(=S)SH groups (in compa-
rison to I-V, Scheme 4).[30–34] The dihedral angle between the
C(=S)SH unit and the ring mean plane of the Dmp substituent is
78.7(1)°.
Figure 2. Molecular structure of the Dmp-substituted dithiocarboxylic acid
Table 4. Characteristic intramolecular structural parameters of Dmp-
C(=S)SH
Figure 4. Molecular structure of the Mes*-substituted dithiocarboxylic acid
C(1)–S(1)
C(1)–S(2)
S(2)–H(2)
S(1)⋯H(2)
1.6307(15) Å
1.7278(15) Å
1.22(2) Å
S(1)–C(1)–S(2)
C(1)–S(2)–H(2)
C(1)–S(1)⋯H(2)
plane (C(=S)SH)
124.81(9)°
97.3(10)°
52.6(4)°
Table 5. Characteristic intra- and intermolecular structural parameters of
Mes*-C(=S)SH.
2.82(2) Å
± 0.001 Å
C(1)–S(2)
1.7362(19) Å
1.633(2) Å
1.63(4) Å
S(2)–C(1)–S(1)
C(1)–S(2)–H(2)
H(2)⋯centroid
H(2)⋯Cring
122.98(11)°
104.3(13)°
Figure 2 shows the molecular structure of the Dmp substituted
dithiocarboxylic acid. Its C(=S)SH plane is again coplanar within
the limits of error. The hydrogen atom is bent inward at a much
smaller angle C(1)–S(2)–H(2) of 97(1)°. This and the absence of
an intermolecular hydrogen bond leads to a much smaller dis-
tance between H(2) and S(1) of 2.82(2) Å in this case; this can
be interpreted on one hand as an intramolecular H(2)⋯S(1)
hydrogen bond, on the other hand as a result of a SH⋯π
interaction, which reveals a distance of 2.46(2) Å from a centroid
of one aromatic ring to the H–S hydrogen atom forming a dimer
as shown in Figure 3. The centroid–S(2) distance is 3.58(1) Å.
C(1)–S(1)
S(2)–H(2)
2.06(4) Å
S(1)⋯H(2)
plane(C(=S)SH)
3.03(4) Å
2.27(4)–2.71(4) Å
3.5148(9) Å
± 0.011 Å
S(2)⋯centroid
Figure 4 shows the molecular structure of Mes*-C(=S)SH in the
solid state. The hydrogen atom of the planar C(=S)SH group is
bent inward comparable to Tip-C(=S)SH. Here the distance
S(1)⋯H(2) measures 3.03(4) Å, which is the same as the sum of
the van der Waals radii of H and S (3.05 Å).[39] Again the
distance and angles of the C(=S)SH group have a conformable
magnitude for C(=S)SH groups (in comparison to the data for
other compounds containing C(=S)SH groups, I-V, Scheme
4 [
).
30–34]
The dihedral angle between the C(=S)SH unit and the
ring mean plane of the Mes* substituent is 99.73(10)°.
Conspicuous is the long S(2)–H(2) distance of 1.63(4) Å and is
accompanied by a notably short distance of 2.06(4) Å of this
hydrogen atom to the centroid of the neighbouring π-system.
Also if the uncertainty of the hydrogen atom position is taken into
account, the centroid–S(2) distance of 3.51 Å reveals an SH⋯π
interaction, which forms a chain pattern as shown in Figure 5.
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in dependence of the size of the stabilizing organic group
carrying the C(=S)SH unit.
All structural data have been obtained from dithiocarboxylic
acids with aromatic substituents, and in all cases the dithiocarb-
oxylic acid groups are oriented about orthogonal to the aromatic
ring thus resulting in a minimum for possible conjugation. The
present data do not allow concluding, whether this is an inherent
structural feature for all dithiocarboxylic acids or due to the more
or less big groups in the two ortho-positions in all three cases
investigated in this work.
Dithiocarboxylic acids are precursors for dithiocarboxylates (R–
−
C(=S)S ), which are excellent ligands containing two equivalent
Figure 5. Crystal packing of Mes*-C(=S)SH. Carbon bonded hydrogens were
omitted for clarity.
soft sulfur atoms for coordination and a delocalized negative
charge. We have used them to prepare complexes of 1:1
stoichiometry with monovalent coinage metal(I) ions. They show
various binding motifs and shapes of aggregation. These results
will be communicated in forthcoming reports.
Although SH⋯π interactions are known to be relatively strong in
comparison to OH⋯π, NH⋯π, CH⋯π interactions from theore-
tical studies,[ there are no corresponding molecular structures
or crystal packing motifs in the CCSD data base, which show a
similarly short distance between an SH group and a π-system.
There are only few crystal structures, which show SH⋯π distan-
ces shorter than the sum of C and H van der Waal radii. The
crystal packing of 1,1’-biphenyl 2,2’-dithiole shows an intermo-
lecular SH⋯π interaction, which measures 2.50 Å (the shortest
46]
Experimental Section
NMR spectra were recorded on a BRUKER AVANCE II 300, a BRUKER DRX
500 and a BRUKER AVANCE III 500 instrument. The chemical shifts (δ)
were measured in ppm with respect to the solvent (CDCl
7.26 ppm, 13C NMR δ = 77.16 ppm; C : 1H NMR δ = 7.16 ppm, 13
C
3
: 1H NMR δ =
intermolecular SH-centroid distance, d(SH⋯centroid) = 3.605 Å;
CCSD database, August 2017).[47] Here we found a shorter
D
6 6
NMR δ = 128.06 ppm. Elemental analyses were performed with CHNS
elemental analyzer HEKATECH EURO EA. For all GC/MS measurements
a SHIMADZU GC-2010/GC/MS-QP 2010 S (column: RTX-200 CROSSBOND,
trifluoropropylmethyl polysiloxane) instrument was used. IR spectrosco-
pic measurements were performed with a BRUKER ALPHA spectrometer.
The vibrations (υ) were measured in cm−1 and the intensity is labelled
with strong (s), medium (m), weak (w) and broad (br).
SH⋯π distance of 2.46(2) for Dmp-C(=S)SH and 2.06(4) Å
length for Mes*-C(=S)SH (d(SH⋯π) = 3.58(1) resp. 3.51(1) Å)
demonstrating strong SH⋯π interactions.
Conclusions
Herein we reported the syntheses and characterization of un-
stable dithiocarboxylic acids with Me, i-Pr, t-Bu, and Ph groups
bonded to the C(=S)SH unit as solutions. They did not allow iso-
lation of the pure compounds and aggregate or decompose
upon attempts of separation from the solvent. Isolation is
possible for dithiocarboxylic acids with larger organic groups
including Mes, Tip, Mes* and Dmp. For the latter compounds it
Moisture- and air-sensitive reactions were carried out by using freshly
distilled and dry solvents from solvent stills. 2,4,6-Tri-iso-propylphenyl
bromide, 2,4,6-tri-tert-butylphenyl bromide, 2,6-Dimesityl-1-iodo-benzene
and the corresponding lithium salt were prepared according to literature
protocols.[37,38,40] 2,4,6-Trimethylphenyl bromide (FLUKA), iodine (VWR),
carbon disulfide (J. T. BAKER, FISHER SCIENTIFIC), n-butyl lithium and 1,3-
dichlorobenzene (ACROS), magnesium, methyl iodide and 2-bromo-
propane and phenyl bromide (MERCK) were purchased.
1
is possible to obtain a full set of analytical data including H and
1
3C NMR spectra, IR spectra, UV-vis spectra and elemental
analyses. The structures of Dmp-C(=S)SH and Tip-C(=S)SH
show a preference of dithiocarboxylic acids for weak S⋯H–S
hydrogen bonding. The Tip-C(=S)SH molecules aggregate in the
form of chains through links to neighbor molecules, whereas
Dmp-C(=S)SH shows a unique intramolecular S⋯H–S hydrogen
bonding motif within one C(=S)SH group, leading to a four-
membered HSCS ring and additionally SH⋯π interactions. This
makes dithiocarboxylic acids completely different from typical
carboxylic acids in their hydrogen-bonded dimers.[41–45]
Conversely, the Mes*-C(=S)SH molecules show no structure
determining intermolecular hydrogen bonds, but as well as Dmp-
C(=S)SH SH⋯π-type interactions in the solid state. With the
background of a critical reevaluation of the experimental and
analytical data concerning dithiocarboxylic acids collected in
literature over the last 150 years, our new data provide unequi-
vocal proof for the presence of simple examples of free dithio-
carboxylic acids, but also demonstrate the limitations of stability
Scheme 8. Atom numbering used for NMR assignment
Unstable dithiocarboxylic acids (Me-C(=S)SH, i-Pr-C(=S)SH, t-Bu-
C(=S)SH, Ph-C(=S)SH)
General procedure: A freshly prepared Grignard reagent in diethyl ether
(or commercially available solution for t-Bu, ACROS) was diluted with THF.
Then carbon disulfide was added dropwise at −5 °C. The reaction mixtu-
re was stirred for 20 h at room temperature and then hydrolyzed with
hydrochloric acid (half conc.) at 0 °C. The aqueous phase was extracted
with diethyl ether until it was colorless. The combined organic phases
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Chemistry - A European Journal
10.1002/chem.201704235
FULL PAPER
+
+
were concentrated and analyzed. Isolation was not possible due to the
instability of the compounds.
150 mL; n. d.; GC/MS m/z [fragment] 134 [M] , 101 [M−SH] , 57
[M−C(=S)SH]+.
Dithiobenzoic acid, Ph-C(=S)SH. Mg 2.40 g, 100 mmol; PhBr 15.8 g,
Dithioacetic acid, Me-C(=S)SH. Mg 3.89 g, 160 mmol; MeI 22.8 g, 160
1
2 2
00 mmol; Et O 40 mL; THF 60 mL; CS 8.5 g, 110 mmol; half conc. HCl
mmol; Et
2
O 60 mL; THF 320 mL; CS
2
13.2 g, 170 mmol; half conc. HCl
+
+
+
40 mL; yield n. d.; GC/MS m/z [fragment] 154.0 [M] , 121.0 [M−SH] ,
1
7
3
2
50 mL; yield 33% (calcd. from NMR); GC/MS m/z [fragment] 92.0 [M] ,
6.9 [M−CH
.20 (s, 3H, CH
.85 (s, 3H, CH
77.1 [M−C(=S)SH]+; 1H NMR [300 MHz, C
] , 59.1 [M−SH] , 45.0; 1H NMR [300 MHz, C
+
+
6 6
D , δ/ppm] 4.7 (s, 1H, SH),
3
6
D
6
, δ/ppm]
, δ/ppm]
, δ/ppm]
7.2 (t, 3JH,H = 7.8 Hz, 2H, meta-H), 7.4 (t, JH,H = 7.3 Hz, 1H, para-H), 7.9
3
1
3
), 5.60 (s, 1H, SH); H NMR [300 MHz, CDCl
3
(d, 3JH,H = 7.4 Hz, 2H, ortho-H); H NMR [300 MHz, CDCl
1
), 6.85 (s, 1H, SH); 13C NMR [75.5 MHz, C
3
, δ/ppm] 6.3 (s,
3
6
D
6
1
H, SH), 7.4 (t, 3JH,H = 7.8 Hz, 2H, meta-H), 7.6 (t, 3
J
H,H = 7.4 Hz, 1H,
3
Table 5. Crystal and refinement data for the structure determinations
Tip-C(=S)SHa Mes*-C(=S)SHb
para-H), 8.1 (d, JH,H = 7.3 Hz, 2H, ortho-H).
Dmp-C(=S)SH
Formula
C
16
H
24
S
2
C
19
H
30
S
2
25 26 2
C H S
Stable dithiocarboxylic acids (Mes-C(=S)SH, Tip-C(=S)SH, Mes*-
C(=S)SH, Dmp-C(=S)SH)
M
r
280.47
322.55
390.58
Cryst.size/mm
Cryst. system
Space group
F(000)
0.19×0.07×0.03
monoclinic
0.3×0.1×0.1
monoclinic
0.26×0.15×0.03
triclinic
2
,4,6-Trimethyldithiobenzoic acid, Mes-C(=S)SH. To
a
freshly
prepared Grignard reagent based on mesityl bromide (6.0 mL, 40 mmol)
in THF (20 mL) more THF (20 mL) and carbon disulfide (1.2 mL, 20
mmol) were added dropwise at –5 °C. The mixture was stirred for 3 h at
room temperature. The reaction mixture was hydrolyzed with
hydrochloric acid (75 mL, half conc. HCl). The aqueous phase was
extracted with diethyl ether (3 × 20 mL). The combined organic phases
were dried with magnesium sulfate and the solvent was removed in
vacuum. For purification the free acid was converted into its sodium salt
by reaction with sodium hydride (1 eq.) in pentane (ca. 50 mL). Water (ca.
1
P2 /c
1
P2 /c
P1̅
608
704
416
a /Å
5.84478(9)
13.3984(2)
20.4410(4)
90
17.0215(5)
9.6592(2)
11.4153(3)
90.0
8.3561(2)
9.10008(19)
14.7252(3)
101.8810(18)
98.2156(19)
101.1035(19)
1055.62(4)
2
b /Å
c /Å
α /°
β /°
93.4511(17)
90
95.801(2)
90.0
100 mL) was added and phases were separated. Finally the sodium salt
γ /°
in aqueous solution was acidified with hydrochloric acid and extracted
with diethyl ether for several times, until the aqueous solution was
colorless. 2,4,6-Trimethyl-dithiobenzoic acid (3.1 g, 16 mmol) was
V /Å3
1597.84(5)
4
1867.22(8)
4
Z
1
isolated as cherry-red oil with a yield of 80%. H NMR [300 MHz, CDCl
3
,
ρ
calc /mg·mm–3
1.166
1.147
1.229
δ/ppm] 2.30 (s, 3H, para-CH
3
) 2.39 (s, 6H, ortho-CH
3
), 6.65 (s, 1H,
, δ/ppm] 19.2
C6) 21.1 (C7), 128.7 (C4), 131.3 (C5), 138.2 (C3), 146.3 (C2), 236.0
C1). EA [for C10 / %] calcd. C 61.18, H 6.16, S 32.66; found C
1.75, H 6.29, S 32.19.
C(=S)SH), 6.88 (s, 2H, Ar-H); 13C NMR [75 MHz, CDCl
(
(
μ /mm–1
3
2.852
2.499
2.312
2θ-range /°
7.9–145.4
41231
5.218–151.32
9391
6.2–144.7
17976
12 2
H S
Reflns collected
Unique reflns
6
4122
4347
4190
2
,4,6-Tri-iso-propyl-dithiobenzoic acid, Tip-C(=S)SH. To a freshly
R
int
0.0166
3933
0.0249
3982
0.0340
prepared Grignard reagent based on 1-bromo-2,4,6-tri-iso-propyl
benzene (2.5 mL, 10 mmol) in THF (30 mL) carbon disulfide (0.7 mL, 11
mmol) was added dropwise at –5 °C. The mixture was stirred for 11 h at
room temperature. Afterwards the reaction mixture was hydrolyzed with
hydrochloric acid (40 mL, half conc. HCl). The precipitate was filtered off
and washed with water and ethanol (20 mL each). For purification the
free acid was converted into its sodium salt by reaction with sodium
hydride (analogous to Mes-C(=S)SH). 2,4,6-Tri-iso-propyl-dithiobenzoic
acid (920 mg, 3.3 mmol, 33%) was isolated in form of an grapefruit-pink
Refl I>2σ(I)
3795
Parameters
260
311
254
R
1
/wR
2
0.0330/0.0962
0.0422/0.1233
0.0358/0.0958
[I>2σ(I)]
R
1
/wR
2
0.0342/0.0972
0.0452/0.1265
0.0397/0.0995
[
all data]
GoF
1.054
1.081
1.031
solid. 1H NMR [500 MHz, CDCl
3JH,H = 6.9 Hz, 1H, para-CH(CH
3 2
)
), 7.00 (s, 2H, Ar-H); 13C NMR [75 MHz, CDCl
CH(CH , δ/ppm] 23.9
3
3
3
, δ/ppm] 1.25 (m, 18H, CH ) 2.87 (hept,
), 3.27 (hept, 3JH,H = 6.8 Hz 2H, ortho-
Δρmax/min /e·Å–3
0.36/−0.33
1572721
0.57/−0.54
1572722
0.34/−0.25
1572723
CCDC number
3
)
2
a
(C7, C9), 24.6 (C7, C9), 30.4 (C6), 34.3 (C8), 121.4 (C4), 142.2 (C5),
149.4 (C3),236.6 (C1); IR [υ/cm−1] 1602, 1456, 1436, 1423, 1381, 1362,
1126, 1096, 1061, 1049, 942, 915, 876, 804, 755, 643, 606; EA [for
Refined as a non-merohedral twin. The second domain is rotated by
−
179.96° around 001, ratio 67:33. bRefined as a non-merohedral twin. The
second domain is rotated by -180° around 001, ratio 74:26
16 24 2
C H S / %] calcd. C 68.51, H 8.62, S 22.86; found C 68.04, H 8.58, S
3
43.46 (CH ), 237.15 (C(=S)SH).
21.77.
2
-Methyl-dithiopropionic acid, i-Pr-C(=S)SH. Mg 0.50 g, 21 mmol; i-
O 15 mL; THF 30 mL; CS 1.89 g, 25 mmol;
half conc. HCl 100 mL; yield 38% (calcd. from NMR); GC/MS m/z
2
,4,6-Tri-tert-butyl-dithiobenzoic acid, Mes*-C(=S)SH. To a solution of
PrBr 2.63 g, 21 mmol; Et
2
2
supermesityl bromide (2.1 g, 6.4 mmol) in diethyl ether (12 mL) n-butyl
lithium (1.6 M in hexane, 3.9 mL, 6.2 mmol) was added at 0 °C. The
reaction mixture was allowed to achieve room temperature overnight.
Carbon disulfide (0.40 mL, 6.7 mmol) were added at −70 °C. The
reaction mixture was allowed to reach room temperature and was stirred
for 5 h. Then the reaction mixture was hydrolyzed with hydrochloric acid
fragment] 120.0 [M] , 87.0 [M−SH] , 76.9 [M−Pr] ; 1H NMR [300 MHz,
CDCl
, δ/ppm] 1.31 (d, 3JH,H = 6.7 Hz, 6H, CH(CH ), 3.36 (hept, 3JH,H
.7 Hz, 1H, CH(CH
), 5.70 (s, 1H, SH); 13C NMR [75.5 MHz, CDCl
δ/ppm] 24.0 (CH(CH ), 50.87 (CH(CH ), 246.0 (C(=S)SH).
+
+
i
+
[
3
3
)
2
=
6
3
)
2
3
,
3
)
2
3 2
)
(
×
half conc. HCl). The aqueous phase was extracted with diethyl ether (3
70 mL). The combined organic phases were dried with magnesium sul-
fate and the solvent was removed in vacuum. For purification 2,4,6-tri-
2
,2-Dimethyl-dithiopropionic acid, t-Bu-C(=S)SH. t-BuMgBr 1.7 M in
Et O, 30 mL, 51 mmol; THF 250 mL; CS 3.9 g, 52 mmol; half conc. HCl
2
2
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Chemistry - A European Journal
10.1002/chem.201704235
FULL PAPER
tert-butyl-dithiobenzoic acid was converted into its sodium salt by reacti-
on with sodium hydride (0.15 g, 7 mmol) in hexane (23 mL). Water was
added and the phases separated. Then hydrochloric acid was added to
the aqueous phase and afterwards extracted with diethyl ether (3 x 50
mL). The combined organic phases were dried with magnesium sulfate
Keywords: dithiocarboxylic acid • sulfur • crystal structure •
NMR spectroscopy • hydrogen bonding • SH⋯π interaction
[
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and the solvent was removed in vacuum. 2,4,6-Tri-tert-butyl-dithioben-
zoic acid (0.55 g, 1.7 mmol, 27%) was isolated as a red solid. 1H NMR
[2] Houben-Weyl, Methoden der Organischen Chemie; Bd. 5 Carbonsäuren
und Carbonsäurederivate, 1985, pp. 193ff; pp. 891ff.
[
300 MHz, CDCl
C(CH
CDCl
44.3 (C3), 150.1 (C5), 241.5 (C1); IR [υ/cm−1] 2961, 2907, 2866 (υ(C–
H)), 2518 (st, υ(S–H)), 1598, 1548, 1534, 1461, 1394, 1360, 1259, 1148,
093, 1057, 881, 797, 741; EA [for C19 / %] calcd. C 70.75, H 9.37,
S 19.88; found C 67.95, H 9.29, S 17.29.
3
, δ/ppm] 1.31 (s, 9H, para-C(CH
3
)
3
), 1.59 (s, 18H, ortho-
), 6.91 (br s, 1H, C(=S)SH), 7.45 (s, 2H, CH); 3C NMR [75 MHz,
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[3] E. Jansons, Russ. Chem. Rev. 1976, 45, 1035–1051.
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3
[
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, δ/ppm] 31.2 (C9), 33.7 (C7), 35.3 (C8), 39.2 (C6), 124.2 (C4),
1
[
5] S. Scheithauer, R. Mayer, Thio- and Dithiocarboxylic Acids and Their
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30 2
H S
[
[
[
[
[
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[
[
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dithiobenzoate (0.5 g, 1.2 mmol) was acidified with hydrochloric and
extracted with diethyl ether (3 × 50 mL). The combined organic phases
were dried with magnesium sulfide and the solvent was removed in
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was isolated in a pink, crystalline form with a yield of 61%. GC/MS m/z
+
]+; 1H NMR [500 MHz, CDCl
[
fragment] 388 [M] , 373 [M−CH
3
3
, δ/ppm]
3
848.
14] K. Hartke, N. Rettberg, D. Dutta, H.-D. Gerber, Liebigs Ann. Chem. 1993,
081–1089.
2
.09 (s, 12H, H10), 2.30 (s, 6H, H11), 6.17 (s, 1H, C(=S)SH), 6.87 (s, 4H,
[
H8), 7.12 (d, JH,H = 7.6 Hz, 2H, H4), 7.44 (t, 3JH,H = 7.6 Hz, 1H, H5); 13
NMR [125 MHz, CDCl , δ/ppm] 21.3 (C11) 21.3 (C10), 128.1 (C8), 129.2
C5), 129.5 (C4), 136.6 (C7, C9), 136.7 (C7, C9), 137.3 (C6), 137.6 (C2),
3
C
1
3
[
[
[
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(
1
2
7
45.2 (C3), 229.0 (C1); IR [υ / cm−1] 2962, 2909, 2851 (υ(C–H)),
500(υ(S–H)), 1611, 1447, 1373, 1259, 1088, 1054, 1014, 934, 850, 796,
51, 739, 687; EA [for C25H S / %] calcd. C 76.87, H 6.71, S 16.42;
26 2
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Scheme 9. Numbering used for NMR assignment.
[
X-Ray diffraction experiments
Suitable crystals of the compound Tip-, Mes*- and Dmp-C(=S)SH were
[
[
27] T. G. Levi, Gazz. Chim. Ital. 1924, 54, 395–397.
obtained by slow evaporation of a saturated solution of n-pentane (Tip,
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meter of the diffractometer into a nitrogen gas cold stream solidifying the
oil. Data collection was performed on a SuperNova diffractometer using
Cu-Kα radiation (λ = 1.54184 Å) at 100.0(1) K. Using Olex2,[48] the
structures were solved and refined with the ShelX program package.[48]
Crystal and refinement details, as well as CCDC numbers are provided in
Table 5. The supplementary crystallographic data for this paper can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
2
011, 47, 6939–6941.
We thank Klaus-Peter Mester and Dr. Andreas Mix for NMR
spectra, Brigitte Michel for elemental analyses and Philipp
Niermeier, Jan Horstmann and Jan-Hendrik Lamm for gas chro-
matography mass spectra. We acknowledge support by Deut-
sche Forschungsgemeinschaft through the Priority Program
SPP 1807 “Control of London dispersion interactions in mole-
cular chemistry” (MI477/28-1).
[
36] We have chosen dithiocarboxylic acids with short saturated alkyl
substituents (H-, Me-, Et-, n-Pr, i-Pr, n-Bu, t-Bu-, i-Pen, cy-Pr, cy-Pen-,
cy-Hex-) and sheer aromatic substituents (Ph-, Tip-, Mes-, Mes*-, Dmp-
) without any further functional group to give an expressive overview.
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1
[
This article is protected by copyright. All rights reserved.

Chemistry - A European Journal
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Chemistry - A European Journal
10.1002/chem.201704235
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Although described since more than
J. Grote, F. Friedrich, K. Berthold, L.
Hericks, B. Neumann, H.-G.
150 years, dithiocarboxylic acids
remained a scarcely characterized
class of compounds; here we
describe synthesis and isolation of
true free acids and critically review
earlier data.
Stammler, and N. W. Mitzel*
Page No. – Page No.
Dithiocarboxylic acids – an old
theme revisited and augmented by
new preparative, spectroscopic and
structural facts
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