Phthalocyanines as a π-π Adsorption Strategy to Immobilize Catalyst on Carbon for Electrochemical Synthesis

Article abstract of DOI:10.1055/s-0037-1611792

In most electrochemical syntheses, reactions are happening at or near the electrode surface. For catalyzed reactions, ideally, the electrode surface would solely contain the catalyst, which then simplifies purification and lowers the amount of catalyst needed. Here, a new strategy involving phthalocyanines (Pc) to immobilize catalysts onto carbon electrode surfaces is presented. The large π structure of the Pc enables adsorption to the sp ² -structure of graphitic carbon. TEMPO-modified Pc were chosen as a proof of concept to test the new immobilization strategy. It was found that the TEMPO-Pc derivatives functioned similarly or better than the widely used pyrene adsorption method. Interestingly, the new TEMPO-Pc catalyst appears to facilitate a cascade reaction involving both the anode and the cathode. The first step is the generation of an aryl aldehyde (anode) followed by the reduction of the aryl aldehyde in a pinacol-type coupling reaction at the cathode. The last step is the oxidation of a hydrobenzoin to create benzil. This work demonstrates the unique ability of electrochemistry and bifunctional catalysts to enable multistep chemical transformations, performing both reductive and oxidative transformations in one pot.

Full text of DOI:10.1055/s-0037-1611792

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Phthalocyanines as a – Adsorption Strategy to Immobilize
Catalyst on Carbon for Electrochemical Synthesis
Kevin J. Klundera,b
Ashley C. Cassa,b
Scott L. Andersona,b
Shelley D. Minteer*a,b
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Department of Chemistry, University of Utah, 315 South 1400
East, Salt Lake City, Utah 84112, USA
minteer@chem.utah.edu
Center for Synthetic Organic Electrochemistry, University of
Utah, 315 South 1400 East, Salt Lake City, Utah 84112, USA
b
Published as part of the Cluster Electrochemical Synthesis and
Catalysis
Received: 04.02.2019
Accepted after revision: 24.03.2019
Published online: 18.04.2019
bound catalysts also have the potential to be reused many
times over or possess the ability to be regenerated easily.
As a platform for modifying electrodes with surface-
DOI: 10.1055/s-0037-1611792; Art ID: st-2019-b0068-c
2
bound catalysts, sp -carbon is arguably one of the most fa-
Abstract In most electrochemical syntheses, reactions are happening
at or near the electrode surface. For catalyzed reactions, ideally, the
electrode surface would solely contain the catalyst, which then simpli-
fies purification and lowers the amount of catalyst needed. Here, a new
strategy involving phthalocyanines (Pc) to immobilize catalysts onto
carbon electrode surfaces is presented. The large  structure of the Pc
vorable supporting electrodes. Carbon is ubiquitous and in-
herently low cost. Carbon also has excellent chemical sta-
bility, is highly conductive, and kinetically fast. For these
reasons, carbon has been a historically popular electrode
5
material in the field of electrochemistry. Methods to im-
2
enables adsorption to the sp -structure of graphitic carbon. TEMPO-
mobilize catalytic moieties to carbon are numerous, which
include covalent grafting, dropcasting, polymer coatings,
modified Pc were chosen as a proof of concept to test the new immobi-
lization strategy. It was found that the TEMPO-Pc derivatives functioned
similarly or better than the widely used pyrene adsorption method. In-
terestingly, the new TEMPO-Pc catalyst appears to facilitate a cascade
reaction involving both the anode and the cathode. The first step is the
generation of an aryl aldehyde (anode) followed by the reduction of the
aryl aldehyde in a pinacol-type coupling reaction at the cathode. The
last step is the oxidation of a hydrobenzoin to create benzil. This work
demonstrates the unique ability of electrochemistry and bifunctional
catalysts to enable multistep chemical transformations, performing
both reductive and oxidative transformations in one pot.
6
electropolymerization, and adsorption. Of these methods,
adsorption is popular and almost exclusively uses pyrene
– interactions with a carbon surface. Modified electrodes
using pyrene as an immobilization moiety have found utili-
zation in a wide range of applications, with hundreds of ar-
7
ticles now highlighting the advantage of the system. While
quite popular, pyrene has been reported as having stability
issues, even when eight pyrene units are attached to a sin-
8
gle redox center. To address these issues, we were motivat-
Keywords electrochemistry, phthalocyanines, alcohol oxidation, elec-
ed to find an alternative adsorptive functional group, since
there is a lack of adsorptive functional groups other than
pyrene in the literature. Ostensibly, a new pyrene alterna-
tive would simply consist of a large aromatic complex, that
is low cost, easy to derivatize, and has favorable chemical
stability.
trosynthesis, pyrene
Immobilizing catalysts to electrode surfaces is of great
importance for applied electrochemistry as a whole, includ-
1
2
3
ing: sensors, energy conversion, electrofuels, solar fuels,
4
and electrochemical synthesis. The attachment of catalysts
to electrode surfaces can turn an inert electrode surface
into one that is highly catalytic. In terms of electrochemical
synthesis, surface-bound catalysts have a number of advan-
tages, such as low catalyst loading, simplified product puri-
fication, lowered over-potentials which limit unfavorable
side reactions, and reduced energy consumption. Surface-
Herein, we report phthalocyanines (Pc) as a low-cost
pyrene substitute. Phthalocyanines are currently used in
many industrial applications and are mass-produced on the
9
ton scale. Pc also have a rich history in the field of electro-
chemical and chemical catalysis due to their stability and
high activity.10 Typically, phthalocyanines are used for their
catalytic metal center,11 for applications such as thiol oxida-
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K. J. Klunder et al.
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tion,12 CO reduction, or oxygen reduction.14 In this work,
13
methoxy-TEMPO in solution as well as with the already re-
ported pyrene-TEMPO catalyst.
2
it was hypothesized that the large aromatic Pc ring could be
used to anchor catalytic moieties to a carbon surface. In-
deed, phthalocyanines are well-known to adsorb/interact
with graphitic and metallic surfaces. Specifically, phthalo-
cyanines have a reported adsorption energy of a few 100 kJ
TEMPO was chosen as it has well-known activity for the
oxidation of alcohols. Historically, alcohol oxidation typi-
17
cally utilized stoichiometric oxidants, such as chromium
oxides, Dess–Martin periodinane,18 or activated DMSO
–1
–1 15
19
mol , while pyrene is reported as 42 kJ mol .
(Swern), and more recently aerobic oxidations. In the past
To test this hypothesis, TEMPO-derivatized phthalocya-
nines were chosen. Currently, only two reports exist for the
derivatization of a phthalocyanine with TEMPO, both are a
few years, TEMPO has gained significant attention for alco-
hol oxidation and a current review proposes its use for
commercial applications.20 TEMPO-modified electrodes are
well-known; however, only a handful of reports are related
to adsorption-based immobilization.21
Scheme 1 shows the twostep synthesis of the previously
unreported CoTPc catalyst used in this work. The reaction
starts with the synthesis of 4-TEMPO phthalonitrile. Here,
yields of 4-TEMPO phthalonitrile were ca. 80% with no opti-
mization. The starting material 4-nitrophthalonitrile is rel-
atively inexpensive ($1.9 US per gram) and available from
many chemical retailers. Nitro-displacement reactions are
16
demonstration for the detection of ascorbic acid. The Pc
described in both reports contained a zinc metal center and
was not used as an adsorbed catalyst on carbon. Our work
herein presents the synthesis and characterization of cobalt
phthalocyanine modified with TEMPO (CoTPc), which until
now is an unknown compound. The catalyst was integrated
into composite electrodes and tested for activity and cata-
lytic stability. Finally, the CoTPc catalyst is compared to 4-
Step one
N
N
O
O
N
N
N2
N
O
DMF
R.T.
HO
N
O
N
O
~5 gram scale
Step two
O
N
O
N
O
N
CoCl2
DBU
N
N
N
N
Co
N
O
O
O
~
130 °C
N
N
hexanol
N
N
O
O
N
N
O
N
O
~1 gram scale
Scheme 1 Synthesis of 4-TEMPO-phthalonitrile and cobalt 4-TEMPO-phthalocyanine (CoTPc)
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K. J. Klunder et al.
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popular within the field of derivatizing phthalocyanines.22
In general, alcohols, thiols, or amines can be used in these
displacement reactions, which makes the scope of possible
derivatives large.
When the peaks from the cyclic voltammograms are in-
–2
tegrated, a surface coverage of 4.0 ± 0.3 nmol cm for CoTPc
and 1.9 ± 0.4 nmol cm–2 for pyrene are observed. Error bars
are from three separate measurements where the electrode
was repeatedly sanded to expose a fresh surface. The inte-
gration results imply that there is ca. 2× the TEMPO on the
surface of the CoTPc electrode, which is reasonable consid-
ering each Pc contains four TEMPO molecules. This also im-
plies that there is ca. 2× as much pyrene-TEMPO on the sur-
face given that each pyrene only has one TEMPO moiety.
Related to this, if any of the CoTPc is lost from the surface, it
would have a significant detrimental impact on the overall
current. This may explain why the pyrene-TEMPO has
somewhat higher currents at higher concentrations. Over-
all, Figure 2 is compelling evidence that Pc could be used as
an alternative to pyrene as an adsorption moiety.
The CoTPc synthesis was straightforward; however,
yields were low initially. It was found to be advantageous to
decrease solvent and base amounts relative to the dinitrile.
Details on the synthesis can be found in the Supporting In-
formation. While a full optimization of CoTPc synthesis was
not undertaken, ca. 50% yields at gram-scale were realized.
The dinitrile was characterized by 1H NMR and GC–MS.
CoTPc was confirmed through UV-vis and ESI-MS, as shown
in Figures S5 and S6. The UV-vis spectra of CoTPc had the
typical Q and Soret features for Pc, and the molar absorptiv-
–1
–1
ity was calculated to be 133,100 M cm . Exceptionally
high molar absorptivity is typical of Pc.
Figure 1 Left: Cyclic voltammetry of 2 mM CoTPc and 1 mM 4-me-
–
1
thoxy-TEMPO at 50 mVs . Right: Square wave voltammetry of 2 mM
CoTPc. A 3 mm glassy carbon electrode was used with a 1:1 acetonitrile
and dichloroethane mixture containing 0.1 M TBABF4.
Figure 2 Cyclic voltammetry (25 mV s–1) of various concentrations of
benzyl alcohol in 0.3 M sodium carbonate at pH 10, using a carbon
composite electrode with (left) CoTPc and (right) pyrene-TEMPO
Phthalocyanines can also be characterized through elec-
trochemistry, given that they have characteristic redox
properties. Figure 1 shows typical redox behavior reported
(II/I)
for cobalt phthalocyanines with an apparent Co
couple
near –0.25 V vs. SCE and a ring reduction at –1.5 V vs. SCE
Cyclic voltammetry is a powerful tool to investigate sta-
bility and activity of a surface-bound catalyst. Figure 3 rep-
resents a stress test for the desorption of the catalyst from a
graphite surface. The black traces are initial voltammo-
grams in a solution containing the substrates of benzyl al-
cohol, phenethyl alcohol, and butanol. The black trace is
generated after the electrode has undergone cycling in a 0.3
M carbonate buffer solution. Even after cycling in the buffer,
large catalytic currents are still observed for all three sub-
strates. The red traces in Figure 3 indicated 30 cycles in the
buffer solution containing substrate (60 total cycles on the
electrode). Moving across the top row of Figure 3, the CoTPc-
modified electrodes see significant loss of catalysis as a
function of the substrate. The loss of activity is amplified
the most with the substrate phenethyl alcohol.
and Co( near –1.7 V vs. SCE. There is also a redox process
I/0)
23
which is presumably from TEMPO-related formation of the
24
oxoammonium cation species on TEMPO. The generation
of the oxoammonium species is critical for the oxidation of
alcohols.
Figure 2 has cyclic voltammograms of CoTPc and
pyrene-TEMPO composite electrodes for the oxidation of
benzyl alcohol. As expected, the current increases with in-
creasing substrate concentration. The new CoTPc catalyst
performs similar to the pyrene-TEMPO-modified electrode.
A somewhat unexpected result is that the pyrene-TEM-
PO catalyst is passing about 2 mA more current than the
CoTPc electrodes at higher substrate concentrations. Differ-
ences in the coverage of the catalyst may explain the higher
currents, where smaller pyrene molecules are better able to
coat the graphite surface. However, Figure S2 shows that
peaks relating to surface adsorbed catalyst are actually
higher for the CoTPc-modified electrode.
From Figure 3, it is also apparent that the cobalt metal
center is active in an oxidative process with benzyl alcohol.
Interestingly, it seems that CoPc is only active for the oxida-
tion of benzylic alcohols and is completely inactive in the
solvent window for phenethyl alcohol and butanol oxida-
tion. The activity of the metal center is a potential major
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Figure 3 Stability tests of CoTPc-loaded composite electrodes. Columns represent various substrates, and the rows relate to the catalyst type. Cyclic
–
1
voltammetry was taken at 25 mV s for all trials in 0.3 M carbonate. Electrodes were cycled in buffer for 30 cycles by CV before substrate addition. The
red trace is after 30 cycles in buffer and 30 cycles in the substrate (60 cycles total).
advantage of Pc over pyrene, in that the adsorption moiety
can also have organometallic electrocatalytic properties. Bi-
functional catalysts can enable cascade reactions. Cascade
through the electrolysis. Possibly, it is for this reason that a
seminal report of alcohol oxidation using pyrene-TEMPO
had a small amount of the catalyst dissolved in solution
during electrolysis. If the catalyst is present in solution,
even in very low concentration, the electrode can be regen-
erated in situ over time.
25
24
reactions are often found in biological systems and have
26
27
been proposed for total synthesis and CO conversion.
2
Discussed later is a cascade reaction that appears to be pro-
moted by the CoTPc catalyst.
The previous report used a 5·10–5 M concentration of
pyrene-TEMPO catalyst in the electrolysis solution, and this
concentration was chosen here as the minimum concentra-
tion for CoTPc.24 In order to make the CoTPc disperse in
aqueous solutions, the surfactant sodium dodecylsulfate
(SDS) was used in a low concentration. The dispersions
were stable for weeks, and details on the preparation can be
found in the Supporting Information.
XPS analysis was performed on electrodes exposed to
the surfactant/CoTPc solution (Figures S3 and S4), and it
was found that there was a loss of ca. 70% of the Co signal
from that of a fresh electrode. However, there was still sig-
nificant Co present on the electrode surface. The surfactant
is likely facilitating the release of CoTPc from the surface
but given that the electrolysis can maintain currents for
hours, the CoTPc must be able to re-absorb to the surface.
The ability of the CoTPc catalyst to maintain activity for
prolonged alcohol oxidation (bulk electrolysis) is shown in
Figure 4. Instead of carbon composites, high surface area
vitreous carbon was used, which helped to maintain a large
active surface area and maintain higher currents. Figure 4
has a comparison of CoPc, CoTPc, and pyrene-TEMPO cata-
Given the lack of stability of CoTPc, it was hypothesized
that CoTPc was becoming detached from the surface. X-ray
photoelectron spectroscopy (XPS) was performed on car-
bon loaded with CoTPc to determine if the catalyst was be-
ing lost. Details on these measurements can be found in the
Supporting Information. As shown in Figures S3 and S4, it
was found that cobalt was still present on the surface even
after an hour of electrolysis in the presence of substrate.
However, there was a measurable loss of ca. 30% of cobalt
and nitrogen from the surface over time. Nitrogen levels
were lost at almost the exact same rate as the cobalt. Loss of
catalysis could be related to both loss of CoTPc as well as
TEMPO decomposition. Degradation of TEMPO at a similar
pH has been reported under prolonged electrolysis.28 At the
present, it is unclear what the culprit for diminished cataly-
sis is, but a solution to the degradation problem is present-
ed below.
After characterizing the stability of the catalyst as well
as the activity towards differing substrates, bulk electrolysis
was attempted. Initial attempts at bulk electrolysis of alco-
hols were not successful, as the reaction halted midway
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Table 1 Results from the Bulk Electrolysis of Various Alcohol-Containing Substratesa
Substrate-electrode
Scale (mmol)
Product
Conversion (%) Yield (%)
Faradaic efficiency (%)
Benzyl alcohol-CoTPc
benzyl alcohol–CoTPc
1
1
benzil
91
82
48b
70b
85
79
aldehyde
(
Divided)
benzyl alcohol-pyrene TEMPO
benzyl alcohol-4-methoxy-TEMPO
phenethyl alcohol-CoTPc
piperonyl alcohol-CoTPc
butanol-CoTPc
1
aldehyde
aldehyde
aldehyde
aldehyde
carboxylic
aldehyde
aldehyde
aldehyde
aldehyde
aldehyde
aldehyde
35
46
28b
37b
na
70
69
na
23
na
8
1
2
< 5
2
78
45b
na
2
< 5
4-trifluoromethyl-benzyl alcohol-CoTPc
1
100
100
100
100
100
100
>90
12a
54c
88c
90c
74c
92c
4-trifluoromethyl-benzyl alcohol-CoTPc (divided)
2
46
90
90
67
81
piperonyl alcohol-CoTPc (divided)
1
4-bromobenzyl alcohol-CoTPc (divided)
4-chlorobenzyl alcohol-CoTPc (divided)
3-methylbenzyl alcohol-CoTPc (Divided)
0.4
0.4
0.8
1
b
benzyl alcohol-4-methoxy-TEMPO (3 mM)
aldehyde
benzil
74
4
69
11
b
a
All conditions are for an undivided cell unless stated otherwise.
Yields were calculated by H NMR spectroscopy using 4-nitrophthalonitrile as internal standard.
Yields were calculated by mass. Phenethyl alcohol and butanol products were identified by GC–MS, and the exact yield is unknown. The yield and faradaic
b
c
1
efficiency are related to the identifiable products, the remaining percentage was either lost during workup or are from side reactions which could not be quanti-
fied. Conversion percentage is the amount of substrate recovered after electrolysis compared with the initial substrate concentration.
lysts. Surprisingly, even at low concentrations of 0.0002 M,
the dissolved catalyst of 4-methoxy-TEMPO was still mea-
surably active. As an attempt to directly compare catalysts,
the concentration of 4-methoxy-TEMPO was 4X the con-
centration of CoTPc, since CoTPc has four TEMPO molecules
per Pc molecule. However, the currents with 4-methoxy-
TEMPO were much lower than the adsorption-based cata-
lyst, and electrolysis involving 4-methoxy TEMPO was
stopped after ca. 8 hours, given that the time needed for full
electrolysis would be impractical. Similar to 4-methoxy-
TEMPO, the pyrene catalyst also deactivated before the full
electrolysis was complete. Both of these catalyst deactivat-
ed near the midway point of the reaction, and the results
are shown in Table 1. As expected, the major product for
these reactions was benzaldehyde.
duction of benzaldehyde (BA) would need to be favored
over hydrogen evolution in order to create hydrobenzoin. At
the bottom of Scheme 2 is evidence of catalyzed BA reduc-
tion when in the presence of CoTPc over that of a non-mod-
ified electrode. Drastic current increases are seen when the
electrode is in the presence of CoTPc and CoPc. Logically,
the main reaction at the cathode will involve the one occur-
ring at the lowest overpotential which in the case of a CoTPc-
modified electrode would be benzaldehyde reduction.
CoTPc had large sustained currents for multiple hours of
electrolysis. Perhaps the increased and prolonged current
over the pyrene catalyst was due to CoTPc having four times
the TEMPO moieties. In fact the charge passed well exceed-
ed what was expected for the reaction. Interestingly, when
1
the product was isolated and characterized by H NMR
spectroscopy and GC–MS, the nearly single product species
was benzil. To the best of our knowledge, benzil is not a
known oxidation product of benzyl alcohol, but it is likely
formed from the oxidation of hydrobenzoin, which in theo-
ry could be formed through coupling at the cathode.
Scheme 2 represents the hypothesized route for the forma-
tion of benzoin. For the coupling reaction to occur, the re-
Figure 4 Bulk electrolysis of 1 mmol of benzyl alcohol with various
TEMPO- and Pc-based catalyst. Applied potential was 0.8 V vs. SCE us-
ing a vitreous carbon working electrode in 15 mL of solution.
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K. J. Klunder et al.
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HO
OH
The coupling products were not limited to benzyl alco-
hol. GC–MS and H NMR data for the oxidation of trifluoro-
O
+
O
1
+
2 e-
methyl benzyl alcohol showed a significant amount of cou-
pling products, some of which were not fully oxidized to
the diketone. The coupling is a potential major reason for
the low yield of the aldehyde with this substrate. During the
workup, it was the aim to isolate aldehydes, and coupling
products were most likely lost to the plug of silica used in
purification.
2 H+
+
Hydrobenzoin
O
HO
OH
O
-
4 e-
4 H+
-
Table 1 contains a brief summary for selected benzyl al-
cohol oxidations as well as a few samples of benzylic and
non-benzylic alcohol oxidations. When in a divided cell, the
coupling products seen with CoTPc were completely absent,
which is reasonable, since it is expected that benzoin cre-
ation is enabled by the cathode. The substrate piperonyl
had no coupling products as well; however, this reaction
was stopped at the theoretical charge needed for full elec-
trolysis. The yield was also somewhat low with this reac-
tion, and it is possible that the coupling products were re-
moved during the workup.
Benzil
Foreshadowing from Figure 3, the substrates of
phenethyl ether and butanol were not successfully fully ox-
idized. In less than 30 minutes, these reactions had signifi-
cant losses in current. Both of the reactions were made
acidic and extracted with ethyl acetate and analyzed with
GC–MS. The major products appeared to be the aldehyde
for phenethyl ether and the carboxylic acid for butanol. At
the present time, it is unclear why these substrates deacti-
vate the electrode so quickly. Related, the previous study
Scheme 2 Top: Scheme for the coupling reaction involving benzalde-
–
1
hyde to form benzil. Bottom: Cyclic voltammograms at 50 mV s in 0.3
M sodium carbonate (pH = 10) in the presence and absence of benzal-
dehyde (BA).
24
It is not entirely surprising that these coupling products
are being formed. Electrochemical reduction of aldehydes
to form hydrobenzoins has been reported at lead and mer-
with pyrene-TEMPO focused solely on benzylic alcohols.
In summary, phthalocyanines were tested as an adsorp-
tion moiety to immobilize molecular electrochemical cata-
lyst. As a proof of concept, a new TEMPO-modified cobalt
phthalocyanine was synthesized and characterized exten-
sively with electrochemistry. The catalyst showed excellent
near-term stability for the oxidation of alcohols in aqueous
systems and had initial small-scale voltammetry similar to
that of the pyrene-TEMPO catalyst. A strategy was devel-
oped to obtain longer-term catalysis utilizing low concen-
trations of dissolved CoTPc catalyst. The CoTPc catalyst ap-
peared to enable a cascade pinacol-type coupling of alde-
hydes which demonstrates both the utility of
electrochemical synthesis as well as the bifunctionality of
the phthalocyanine catalyst. Overall, this initial study is
motivating to explore the concept of phthalocyanine-based
adsorption for catalyst immobilization in more detail given
that phthalocyanines are low-cost, easily derivatized, readi-
ly adsorb onto carbon, and shown here can enable unique
chemistry.
29
cury electrodes. In fact, benzoin compounds are found as
products in partial electrolysis experiments, as shown in
the GC–MS in the Supporting Information. Typically, a sup-
pression of side reactions (hydrogen evolution) is necessary
to reduce benzaldehyde in alkaline solutions, related,
phthalocyanines have been shown to be able to suppress
side reactions in the selective reduction of CO , which oc-
2
curs at potentials similar to what BA is reduced at in our
30
system. In Scheme 2 it is also shown that CoPc-modified
electrodes are also catalytic for BA reduction, giving some ev-
idence that the metal center may be involved in the reaction.
To try and determine if CoTPc was fully responsible for
the coupling reaction, experiments were performed with a
higher concentration of 4-methoxy-TEMPO. The increased
concentration (3 mM) allowed the reaction to maintain ca-
talysis for a longer period of time. It was found that benzil
could be created under this set of conditions, however, it
was not the main product in the reaction (see Table 1). Ben-
zil byproduct from a 4-acetamido-TEMPO-based oxidation
of benzyl alcohol has also been recently reported in similar
Funding Information
31
amounts. It then appears that BA reduction is in competi-
tion with hydrogen evolution for a variety of systems, and
in the case of CoTPc, BA reduction is seemingly favored.
The authors would like to thank the Division of Chemistry (1740656)
and the National Science Foundation Center for Synthetic Organic
Electrochemistry for funding (CHE-1740656).
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Acknowledgment
(11) Sorokin, A. B. Chem. Rev. 2013, 113, 8152.
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The XPS instrument is part of the Utah Nanofab, sponsored by the
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ident for Research, and the Utah Science Technology and Research
(
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(
(
(
(USTAR) initiative of the State of Utah. We thank Tonya N. Jensen and
J. Paolo Perez for assistance with the XPS instrument.
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Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G