On the Use of Polyelectrolytes and Polymediators in Organic Electrosynthesis

Article abstract of DOI:10.1002/anie.201710659

Although organic electrosynthesis is generally considered to be a green method, the necessity for excess amounts of supporting electrolyte constitutes a severe drawback. Furthermore, the employment of redox mediators results in an additional separation problem. In this context, we have explored the applicability of soluble polyelectrolytes and polymediators with the TEMPO-mediated transformation of alcohols into carbonyl compounds as a test reaction. Catalyst benchmarking based on cyclic voltammetry studies indicated that the redox-active polymer can compete with molecularly defined TEMPO species. Alcohol oxidation was also highly efficient on a preparative scale, and our polymer-based approach allowed for the separation of both mediator and supporting electrolyte in a single membrane filtration step. Moreover, we have shown that both components can be reused multiple times.

Full text of DOI:10.1002/anie.201710659

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Title: On the Use of Polyelectrolytes and Polymediators in Organic
Electrosynthesis
Authors: Benjamin Schille, Niels Ole Giltzau, and Robert Francke
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On the Use of Polyelectrolytes and Polymediators in Organic
Electrosynthesis
[
a]
[a]
[a]
Benjamin Schille, Niels Ole Giltzau, and Robert Francke*
Abstract: Although organic electrosynthesis is generally considered
to be a green method, the necessity for excess amounts of
supporting electrolyte constitutes a severe drawback. Furthermore,
the employment of redox mediators represents an additional
separation problem. In this context, we have explored the
applicability of soluble polyelectrolytes and polymediators using the
TEMPO-mediated transformation of alcohols to carbonyl compounds
as test reaction. Catalyst benchmarking based on CV studies
indicates that the redox-active polymer can compete with
molecularly defined TEMPO species. While a high efficiency of the
alcohol oxidation was achieved in preparative-scale electrolysis, our
polymer-based approach allows for the separation of both mediator
and supporting electrolyte in a single membrane filtration step.
Moreover, we have shown that both components can be reused for
multiple cycles.
successfully applied silica gel supported piperidinium salts as
dispersed-phase supporting electrolytes for number of
a
[
5]
electrochemical transformations. A more recent approach by
Little et al. comprises a polymeric ionic liquid/carbon black
composite, which is recovered after completed electrolysis by
centrifugation.[ Attempts for the immobilization of mediators on
microparticles, such as poly(styrene)-supported phenyl iodide[7]
and TEMPO on silica gel,[8,9] have also been reported. While all
these dispersed-phase approaches facilitate the separation and
reuse of the supporting electrolyte, they are associated with
drawbacks such as low ionic conductivity and poor kinetics of
the electron transfer between electrode and mediator unit. In fact,
halide salts are needed as co-mediators in order to allow for
activation of the solid-supported mediator units.[
6]
7–9]
Compared to the dispersed-phase strategy, the attachment of
mediator and/or supporting electrolyte to soluble polymer
backbones (polymediators and polyelectrolytes) to create a
homogeneous electrolyte represents a promising approach for
improving both ionic conductivity and electron transfer kinetics.
To date, this has only been attempted by Steckhan et al., who
tested a polymer-supported triarylamine for the oxidation of
Compared to classical methods for the oxidation and reduction
of organic compounds, organic electrosynthesis offers several
economic, ecologic and practical advantages. The use of electric
current instead of redox reagents provides the opportunity for
reducing the overall cost, waste generation and energy
[10]
anisyl alcohol. While in principle, activity of the polymediator
[1]
consumption of a process. However, despite the substantial
progress that has been made in recent years, the potential of the
method has yet to be fully exploited and expressed. This is in
part due to the fact that the following issue has been neglected
for a long time: Although organic electrosynthesis is generally
considered as a green method, the employment of excess
amounts of supporting electrolyte constitutes a severe drawback.
Thus, following the completion of a reaction, the supporting
electrolyte must be separated from the reaction mixture. Unless
it is recovered and reused, it constitutes a source of waste. More
specifically, tetraalkylammonium salts are often employed as
supporting electrolytes, since they provide good solubility in
organic solvents and high electrochemical stability.[2] However,
the good solubility often leads to difficulties with separation of
the salts, and in many cases column chromatography is required
for this purpose. Since the supporting electrolyte represents a
source of waste and a key-expense factor, the reuse of the
separated salts for multiple cycles is highly desirable for
industrial applications.[3] A further separation issue is caused by
redox mediators, which are often used to facilitate electron
transfer and to avoid electrode passivation.[4] The simplification
of the separation of both supporting electrolyte and mediator
from the reaction medium would certainly make electroorganic
synthesis a more appealing synthetic tool.
was observed, the system still suffered from poor electron
transfer kinetics and complete degradation of the soluble
polymer in the course of electrolysis. This study prompted us to
investigate the requirements for a successful use of soluble
polymers in electrosynthesis, and to develop a more robust and
active system, which allows for practical current densities and
repeated use. Herein we report the first protocol for the selective
electrooxidation of alcohols to carbonyl compounds using
recyclable polyelectrolytes and polymediators in combination
with membrane separation processes for recovery of the
polymers (see Scheme 1).
The separation issue was addressed by Fuchigami et al., who
Scheme 1. Concept for the use of soluble polymediators and polyelectrolytes
in electrosynthesis.
[
a]
Benjamin Schille, Niels Ole Giltzau, Dr. Robert Francke
Institute of Chemistry, Rostock University
Albert-Einstein-Str. 3a, 18059 Rostock, Germany
E-mail: robert.francke@uni-rostock.de
With respect to the redox-active unit, we selected 2,2,6,6-
tetramethylpiperidin-1-oxyl (TEMPO), a robust, versatile and
well-characterized mediator for homogeneously catalyzed
electrochemical transformations.[11,12] To construct suitable
Supporting information for this article is given via a link at the end of
the document.
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COMMUNICATION
homo- and copolymers for our application, we have modified a
procedure from Schubert et al. for the synthesis of redox-active
polymers applied in redox-flow batteries.[13] The resulting
structures HP-1 and HP-2 (Figure 1) contain redox-active
TEMPO moieties and electrochemically stable alkylammonium
perchlorate units covalently attached to a poly(methacrylic acid)
backbone. Each polymer is accessible from commercially
available starting materials in only two straightforward and
scalable steps (see SI).
is with 70 mV relatively close to the 57 mV expected for a
reversible single electron transfer in a diffusion controlled
process (whereby both E
0 P
and ΔE are in good agreement with
the literature),[17] ΔE
is only 35 mV for HP-2. This is a significant
P
deviation from the behavior of other linear polymers in solution
having ferrocene or anthraquinone units attached to the chain,
where ΔE
P
values of 60 mV and higher were reported for the
same scan rate.[
16,18]
While further investigation of this unusual
behavior is ongoing, we can conclude from the results presented
in Figure that HP-2 renders diffusion-controlled current
2
densities which are i) of the same order as for TEMPO and
therefore promising for application in indirect electrosynthesis
and ii) adjustable by variation of the concentration of the redox-
active repeat units.
Figure 1. Structures of polyelectrolyte HP-1 and polymediator HP-2.
For optimized solubility and diffusion properties of HP-2, we
adjusted M
w
to about 2700 g mol-1 (corresponds to ~11
monomer units) by using a chain transfer additive in the radical
polymerization of the methacrylic acid derivatives. Initially, we
also studied copolymers with different ratios between redox-
active and supporting electrolyte units. Since these
polyelectrolyte mediators are significantly less redox-active
compared to HP-2 (for more details see SI), we focussed our
efforts on the homopolymer-based approach.
For the electrochemical characterization of HP-2, cyclic
Figure 2. Left: Cyclic voltammetry of TEMPO and HP-2 ([TEMPO unit] = 1 mM,
voltammetry was carried out in a 0.1 M solution of NBu
4 4
ClO in
electrolyte: 0.1 M NBu
= Ag/0.01 M AgNO ), v = 50 mV s . The current density j has been normalized
with respect to the peak current density j . Right: Plot of the peak current
densities j
(with background correction) vs. v0.5 and c, respectively.
4 4 3 2
ClO in CH CN + 5 vol% H O, WE = glassy carbon, RE
-
1
CH CH/H O (95:5), whereby the polymer content was adjusted
3
2
3
to a 1 mM concentration of the redox-active repeat unit. The CV
of HP-2 (blue line in Figure 2, left) shows reversible redox
P
P
0
behavior at E = 0.44 with a 140 mV anodic shift compared to
free TEMPO (black line), which we used as a benchmark for the
electroanalytical studies. In each case, plots of the peak current
Table 1. Comparison between TEMPO and HP-2 with regard to redox
behavior and mass transport.
P
density j vs. the square root of the scan rate v show a linear
2 -1 [b]
Mediator
TEMPO
HP-2
E
0
[V][a]
ΔE
p
[mV][a]
70
D [cm s ]
dependency (see Figure 2, top right), indicating that both for
TEMPO and HP-2 the charge transfer is similar to a process
controlled by linear diffusion.[14] The diffusion coefficient DHP-2
0.30
0.44
3.2·10-5
vs. v0.5 according to the
8.8·10-6
was calculated from the slope of j
Randles-Sevcik treatment (for details see SI). With 8.8·10 cm2
P
35
−
6
-
1
−
1
[a] Determined at v = 50 mVs , [TEMPO unit] = 1 mM. [b] Determined via
s , this value is somewhat lower as compared to DTEMPO with
Randles-Sevcik treatment (see Figure 2, top right and SI).
3
.2 10−5 cm s , whereby the latter value is in good agreement
2
−1
[15]
with the literature.
The decrease in D follows the behavior
We continued our investigation with characterizing the
electrocatalytic behavior of HP-2 The voltammetric profiles
recorded at 50 mVs-1 in absence and presence of benzylalcohol
and N-methyl imidazole (NMI, employed as proton scavenger)
are depicted in Figure 3 (left). In presence of alcohol and NMI,
HP-2 shows a pronounced catalytic wave without cathodic peak
in the reverse scan. Interestingly, the ratio between the
maximum current densities with and without substrate (jcat/j ,
P
reflecting the rate of the homogeneous reaction), is significantly
higher compared to molecular TEMPO under the same
predicted by the Stokes-Einstein equation in the sense that
diffusion is curbed with increasing molecular weight. The ratio
between DHP-2 and DTEMPO suggests that the redox-centers are
non- or weakly interacting and that the transport mechanism
during the electrochemical conversion at the electrode is best
described as classic diffusion (for more details see SI).[16]
Upon increasing the concentration of redox active repeat units, a
P
linear increase of the peak current density j is observed (see
Figure 2, bottom right), which confirms the diffusion-type
limitation of the electron transfer. Noteworthy, the separation
conditions. A plot of jcat/j
bottom right) for both HP-2 and TEMPO at
concentration of mediator units, whereby a linear dependency is
P
vs. [PhCH
2
OH] is shown in Figure 3
between anodic and cathodic peak potential (ΔE
smaller for HP-2 as compared to TEMPO. While ΔE
P
) is significantly
for TEMPO
(
a 1
mM
P
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observed in each case. These plots show that i) the reaction is
first order with regard to the alcohol substrate,[19] and that ii) HP-
the cell voltage in a bulk electrolysis, a sufficiently high ionic
conductivity σ is crucial. We have therefore determined σ in
2
exhibits the superior homogeneous rates in the studied
CH
NBu
3
CN at different concentrations (Figure S22). A comparison to
ClO typical benchmark salt, shows significantly
concentration regime.
4
4
, a
-1
decreased values for HP-1 (for instance 2.5 mS cm for a
solution containing 0.3 M repeat units as compared to 9.6 mS
cm-1 for a solution of NBu
4 4
ClO of the same concentration of
supporting electrolyte units). However, the conductivity of HP-1
is still in the appropriate range for bulk electrolysis.
With the analytical data in hand, we elaborated a protocol for
preparative scale electrolysis. The optimized conditions
comprise an electrolyte consisting of HP-1 (0.05 M supporting
electrolyte units), 1.5 - 5 mmol alcohol substrate, HP-2 (15 mol%
mediator units) and 2 mmol lutidine dissolved in CH
to facilitate H evolution as cathodic half-reaction, we added H
5 vol%) and used a Pt cathode. A comparison between different
3
CN. In order
2
2
O
(
anode materials revealed that inexpensive carbon roving gives
the best results. Bundled into a brush-type electrode (see Figure
S20), it provides a high surface area while passivation during
electrolysis is minimized compared to platinum, glassy carbon
and graphite. An inexpensive size-exclusive membrane was
used as a separator to prevent discharge of the polymediator at
the cathode. Using this set-up, the conversion of several
benzylic, allylic and aliphatic alcohols was carried out at 0.56 V
Figure 3. Left: Cyclic voltammetry of HP-2 ([TEMPO unit] = 1 mM) in absence
and presence of 0.1 M PhCH
2
OH and 0.45 M NMI (experimental conditions:
see Figure 2). Top right: Plot of the maximum catalytic current jcat vs. the
-
1
square root of the concentration of PhCH
2
OH (v = 50 mVs ). Bottom right: Plot
of jcat vs. V (0.1 M PhCH OH and 0.45 M NMI).
2
3
vs. Ag/AgNO (see Table 3).
For a quantitative treatment of the kinetics of the alcohol
oxidation, we determined the homogeneous rate constants kcat
Table 3. Scope of the electrocatalytic alcohol oxidation using HP-1 as
supporting electrolyte and HP-2 as mediator.
(
see Table 2) using CV under purely kinetic control and pseudo
[a]
[
19]
first-order conditions (see Figure 3, bottom right).
TEMPO and for HP-2 cat reaches a plateau value (jmax) at
sufficiently high v. This behavior allows for the extraction of kcat
which is about 60 times higher for HP-2 compared to TEMPO
for details see SI). The improved rate of the homogeneous
Both for
j
Controlled E[c]
Controlled i
i [mA]
,
Product[b]
,
Q [F]
FE [%][d]
Q [F]
FE [%][d]
(
1
2
.8
94
87 (84)
reaction in the case of HP-2 can be explained by the electron
withdrawing substituent in position 4 of the TEMPO unit (i. e. the
acyloxy linker) and the resulting anodic shift of Eox. Similar
behavior was recently reported by Stahl et al., who found that
the electrocatalytic activity of N-oxyl radicals rather depends on
the mediator potential than on steric influences.[11] It is important
to highlight that the results presented in Table 2 only reflect the
reaction rate under kinetic control, and that a scenario where
mediator transport is rate-limiting (for instance at low alcohol
concentrations) would turn the outcome in favor of molecular-
defined TEMPO due to its higher diffusion coefficient.
[e]
92 (49)[
87 (83)[
83
f]
1
.8
1.9
.0
4.5
4.5
4.5
e]
1.8
84
2
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
89
97
58
84
94
89
65
96
88
Table 2. Comparison of the electrocatalytic behavior of HP-2 and TEMPO.
-
2
[a]
-1 [b]
Mediator
TEMPO
HP-2
j
max [mA cm ]
k
cat [s ]
[f]
2
4.5
6.5
100 (3)
g]
1.9
79[
1.8
7.0
5
1.8
3.0
94 (8)[f]
301
[
a] Catalytic current for the oxidation of PhCH
first-order conditions (see Figure 3, bottom right). [b] Pseudo-first order rate
constant for the oxidation of PhCH OH, calculated from jcat (for details see SI).
2
OH determined under pseudo
2
While the results presented above clearly indicate that dissolved
polymediators allow for efficient electrocatalysis, it is also
important to consider the properties of the polyelectrolyte HP-1.
Since the electrolyte resistance renders a major contribution to
1.8
3.0
87 (n. d.)[f]
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COMMUNICATION
therefore currently expanding our concept to other
electrochemical transformations and mediator units.
3.6
64[b]
[
a] Conditions: r. t., divided cell (separator: regenerated cellulose with 1 kDa
MW cut-off), anode: carbon roving, cathode: Pt sheet, solvent: CH CN/H
95:5 vol/vol). Anolyte: 20 mL solvent containing 54 mg HP-2 (15 mol%
3
2
O
(
Acknowledgements
mediator units), 313 mg HP-1 (corresponds to 0.05 M supporting electrolyte
units), 250 µL 2,6-lutidine, 1.5 mmol substrate. Catholyte: 20 mL solvent
containing 313 mg HP-1. [b] The starting material is the corresponding alcohol
We acknowledge financial support through a Liebig Fellowship
(Fonds der Chemischen Industrie) and by the German Federal
Ministry of Education and Research (BMBF, project number
3
(pentane-1,5-diol for δ-lactone). [c] E = 0.56 V vs. Ag/AgNO . [d] Faradaic
efficiencies are based on the yield determined with GC (internal standard). [e]
Isolated yield. [f] Electrolysis in absence of HP-2. [g] 5 mmol batch size.
031A123). We are also grateful for the contributions of Dr. Dirk
The Faradaic efficiencies (FE) were calculated on the basis of
the yields determined via GC (internal standard) after passing
Hollmann (Leibniz Institute for Catalysis, Rostock, Germany) to
the characterization of the polymers with EPR spectroscopy-
1
6
.8 - 2 F charge per mole of substrate, and are located between
4% and 97%. In addition, product isolations were carried out in
Keywords: Electrosynthesis • TEMPO • Polyelectrolyte • Redox
Mediator • Ultrafiltration
the case of p-methoxy benzaldehyde and p-nitro benzaldehyde,
whereby comparable FEs were obtained. In order to
demonstrate the scalability, we have converted 1.5 g 1-octanol
using the same cell and electrolyte volume, whereby 1-octanone
was obtained in 75% FE (for experimental details see SI). We
have also tested some of the reactions under controlled current
conditions (see Table 3, right), and found that the FEs are
similarly useful (72% - 100%). To confirm the active role of the
mediator under these conditions, control reactions in absence of
HP-2 were carried out, which rendered significantly lower FEs (0
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Figure 4. Recycling study for the use of HP-1 and HP-2 in the synthesis of
octane-1-one (galvanostatic electrolysis, conditions: see Table 3).
[
[
[
[
In conclusion, we have demonstrated the feasibility of using
soluble polyelectrolytes and polymediators in electrosynthesis
for an efficient recovery of both components by membrane
separation processes. In the particular case of polymer-bound
TEMPO (HP-2), we found that compared to free TEMPO the
rate of the alcohol oxidation is significantly improved. The
superior homogeneous kinetics can in part compensate the
unfavorable diffusion coefficient of HP-2, ultimately leading to
competitive current densities in the electrolysis. We expect that
our findings will have an important impact on future
developments of polyelectrolyte-polymediator systems, and are
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Entry for the Table of Contents (Please choose one layout)
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Combining electrosynthesis with
membrane filtration. The attachment
of TEMPO mediator and supporting
electrolyte to soluble polymers
enables a selective anodic conversion
of alcohols to carbonyls and allows for
efficient recovery/recycling of the
mediator and the salt by membrane
filtration.
Benjamin Schille, Niels Ole Giltzau
Robert Francke*
Page No. – Page No.
On the Use of Polyelectrolytes and
Polymediators in Organic
Electrosynthesis
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