Organic Redox Systems Based on Pyridinium-Carbene Hybrids

Article abstract of DOI:10.1021/jacs.9b04249

New redox systems with three oxidation states are highly sought-after, for example, for redox-flow battery applications, selective reducing agents, or organic electronics. Herein, we describe a straightforward and modular synthesis of a new class of such

Full text of DOI:10.1021/jacs.9b04249

Subscriber access provided by BUFFALO STATE
Article
Organic Redox-Systems Based on Pyridinium-Carbene Hybrids
Patrick W Antoni, Tim Bruckhoff, and Max M Hansmann
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b04249 • Publication Date (Web): 24 May 2019
Downloaded from http://pubs.acs.org on May 24, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Organic Redox-Systems Based on Pyridinium-Carbene Hybrids
Patrick W. Antoni, Tim Bruckhoff and Max M. Hansmann*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Georg-August Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany.
ABSTRACT: New redox systems with three oxidation states are highly sought-after, e.g. for redox-flow battery applications,
selective reducing agents or organic electronics. Herein, we describe a straightforward and modular synthesis of a new class of such
a three-state redox system based on the intermolecular reaction of a large variety of pyridinium salts with carbenes. Those hybrids
represent organic (super) electron donors with tailored electro-chemical properties and feature three stable oxidation states, which
could be fully characterized including X-ray diffraction. We elaborate which electronic factors govern to obtain either stable
radicals through one electron transfer or instead favor 2e- processes. Indeed, based on X-ray data a verification for a potential
compression mechanism is given which originates through a large structural distortion in the first oxidation event. By geometrically
locking this hybridization change a potential expansion can be realized. The new class of stable organic radicals are highly
persistent and even moderately stable towards air. Additionally, we demonstrate that our modular synthesis approach is also
applicable to remarkably strong multi-electron (4e-) donors by utilizing bridged pyridinium salts. Based on the stability and
reversibility of the new redox system, we could demonstrate by charge–discharge experiments the use of the hybrid molecules as
novel anolyte materials for nonaqueous redox-flow batteries.
INTRODUCTION
design, based on the structure of the stable pyridinyl radical G,12
pyridinium salt H as an efficient electrolyte in the one electron
cycling (H/H’) for redox-flow batteries.13 Besides, pyridinyl
radicals are also of recent interest as transient highly reactive
intermediates in photo catalysis.14 Currently, a major trend
developed in the stabilization of transition metal or main group
open shell species by carbene entities.15 Here, we will establish a
simple synthetic route to a new class of organic redox system
which will combine the concepts of pyridinyl radicals and carbene
stabilization, analyze the electron transfer mechanism, generate
multi-electron donors and finally apply the redox system in
charge-discharge cycling experiments for redox-flow batteries.
Organic redox systems provide a basis for the understanding of
electron transfer processes and offer a variety of applications
ranging from organic electronics, data storage, photovoltaics to
batteries.1 Tetrathiafulvalene (A)2 and viologens such as paraquat
(B)3 represent two well-studied examples of organic “Weitz
type”4 redox systems existing in three stable oxidation states
(Figure 1). Tetraaza-substituted olefins also belong to this class
but predominantly received attention as organic reductants as a
result of their electron rich ground state.5 Tetrakis(dimethyl-
amino)ethylene (TDAE, C), cyclic tethered variants (D) or bis-
pyridinylidenes (E)6 have been utilized in organic synthesis as one
electron or super-electron donors respectively.7 While previous
work on super-electron donors as pioneered by Murphy et al.
focused on the strength of the reducing power, little is known
about the structural changes involved in the oxidation process or
the stability of the radicals involved. Indeed, in most cases the
electron rich compounds are prepared and reacted in situ and no
data of either the neutral or radical oxidation state is given.
Importantly, in two electron oxidation/reduction processes the
question arises if the two electron transfer occurs stepwise
(normally ordered) or “concerted” as a result of potential
compression/inversion.8 Importantly, Lainé et al. proposed based
on DFT modeling a potential compression mechanism for the
single step two electron reduction process of pyridinium salts such
as F.9 A key point of his theory for the potential compression in
such branch expanded pyridinium systems is a large structural
rearrangement process involving different electromers.9
Furthermore, stable organic radicals are particularly interesting as
a result of their applications such as organic magnetic materials,
spin-memory devices,10 or components in redox-flow batteries.11
During the last years organic redox-flow batteries have emerged
as a highly attractive option for energy storage, however a key
challenge remains in identifying novel highly stable anolytes.11b
In this context the groups of Minteer, Sigman and Sanford could
N
N
Me2N
Me2N
NMe2
NMe2
N
N
N
N
S
S
S
S
Me2N
NMe2
A
C
D
E
(E = +0.32/+0.71 V) (E = -0.78/-0.61 V)
(E = -0.76/-0.82 V)
(E = -1.24 V)
Ph
N
N
N
O
OMe
O
Ph
O
Ph
+ e-
- e-
+ e-
- e-
+ e-
- e-
Ph
N
Ph
N
N
N
N
N
N
N
Me
(E = -1.12 V) (E = -0.69 V)
B
B'
B''
F
G
H
H'
Figure 1. Selection of electron rich olefins (A-E) with their
corresponding redox potentials (vs. SCE), pyridinium salt F as well as
stable radicals G and H’.
RESULTS AND DISCUSSION
Utilizing our recently discovered intermolecular carbene
addition/elimination mechanism,16 we reacted two equivalents of
4,5-dimethyl substituted imidazolin-2-ylidene
disubstituted pyridinium salt 1II (Scheme 1). Addition in 4-
position of the pyridinium core affords intermediate 2 which leads
upon deprotonation with a second equivalent of carbene to the
neutral “carbene-pyridinium” hybrid 3aII as an orange solid in
a with 2,6-
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 11
-conjugation
-conjugation
Acc
55% yield. This synthetic approach is not limited to classical N-
Ph
Ph
Ph
N
1
2
3
4
5
6
7
Do
heterocyclic carbenes such as IMes (b) or saturated IMes (c) but
can also be performed with a series of stable carbenes such as
cyclic (alkyl) amino carbene (CAAC)17 (d) and diamido carbene
(DAC)18 (e) to afford hybrid molecules 3d and 3e respectively. In
order to vary the electronics on the pyridinium core, a small
library of electron donating (1I) to withdrawing (1IV) substituted
pyridinium salts were reacted with IMes carbene (b) to give 3bI –
3bIV.
N
Do/Acc
-conjugation
R
N
Ph
Ph
Ph
R
R
favored by
donor
favored by
acceptor
increasing distortion
Do
(
)
Acc
( )
Figure 3. Degree of structural distortion of the neutral hybrids based
on the substitution.
8
9
In case of the more π-accepting DAC moiety (3eII) the N–aryl
moiety is nearly in plane (0.08 Å) indicating π-delocalization,
while the aryl moiety is rotated perpendicular (Figure 2 and 3; for
a summary of structural parameters see Figure S1). Note, an
Scheme 1. Synthetic approach towards 2,6-diphenyl-N-aryl-
pyridinium-carbene hybrids.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R
Y
-
X
Y
-
X
imidazole+-CH2 bond polarization such as seen in N-heterocyclic
BF4
R
R
base or
1 eq. carbene
H
R
THF
+
-
X
Y
olefins,20 also favors a N–distortion/pyramidalization process in
order to avoid an anti-aromatic 8π-electron system (3’;
Scheme 2). The N–pyramidalization is also detected in solution by
15N-NMR of the N–dihydropyridine core, following the trend
δ [ppm] = –291 (3bII), –287 (3cII), –280 (3dII) to –258 (3eII).
Cyclic voltammetry measurements of 3 show two quasi-
reversible redox events for the stepwise oxidation to radical cation
4 as well as dication 5 (Scheme 2) ranging from E = –1.18 V to –
0.14 V vs. Fc/Fc+ (Figure 4). The position of the redox couple can
be tailored by changing the π-accepting properties of the carbene
moiety, while the variation in the N–aryl pyridinium-moiety
relates to the separation of the two redox potentials. Upon
reducing the electron density in the N–aryl moiety [R = p-NMe2
(3bI), p-tBu (3bII), p-CF3 (3bIII) to m,m-CF3 (3bIV)] the two
redox-events merge towards a one-step two electron reduction as
observed for electron deficient F (Figure 1).
R
R
Ph
N
R1-4
Ph
BF4
rt or
-78°C
Ph
N
R1-4
Ph
Ph
3a-eI-IV
N
R1-4
Ph
carbene
1I-IV
a-e
2
(
)
O
O
Et
N
N
N
N
N
N
N
N
N
Mes
Ph
Mes Mes
Mes Mes
Mes
Ph
Dipp
Mes
Mes
Et
N
R2
Ph
Ph
N
Ph
Ph
N
R2
Ph
N
R2
Ph
Ph
N
R2
Ph
R1-4
3bI-IV
3aII
3cII
3dII
3eII
CF3
R1
=
NMe2
R3
=
R2
=
CF3
R4
=
CF3
IV
I
II
III
In contrast to donor-acceptor substituted 1,4-dihydro-
pyridines, which have been exploited as fluorescent materials or
biological relevant scaffolds,19 compounds 3 feature an unusual
donor-donor substitution. This is reflected in the stability of 3aII–
3dII, which are prone towards oxidation and must be handled
under inert atmosphere, whereas 3eII is stable towards air.
Strikingly, in the solid-state structure of 3bII and 3dII (see SI) the
N–aryl moiety is strongly pyramidalized out of the 1,4-dihydro-
pyridine plane (distance N–plane: 0.27 Å), avoiding overlap of the
N–lone pair with the electron rich plane (Figure 3 left).
Scheme 2. Redox-states of pyridinium-carbene hybrids.
X
Y
X
Y
X
Y
X
Y
X
Y
R
R
R
R
R
R
R
R
R
R
- e-
- e-
+ e-
+ e-
R2
N
R1
R2
R2
N
R1
R2
R2
N
R1
R2 R2
N
R1
R2
R2
N
R1
R2
3
3'
4
4'
5
Figure 2. Selected X-ray solid-state structures of 3bII (A) and 3eII (B)
with their corresponding views from the side (C/D). Hydrogen atoms
and solvent molecules omitted for clarity. Ellipsoids shown with 50%
probability. Selected bond parameters in [Å] and [°]: 3bII: N1–C1
1.383(2); N2–C1 1.377(3); C1–C2 1.408(3); C2–C3 1.441(3); C3–C4
1.352(3); C4–N3 1.447(3); N3–C5 1.418(3); N1–C1–C2–C3 –151.5;
3eII: N1–C1 1.432(1); N2–C1 1.433(1); C1–C2 1.375(1); C2–C3
1.456(1); C3–C4 1.353(1); C4–N3 1.411(1); N3–C5 1.454(1); N1–
C1–C2–C3 162.5.
Figure 4. Cyclic voltammograms of 3aII-3eII. [nBu4NPF6 (0.1 M),
Pt/glassy carbon 200 mV s−1, versus Fc/Fc+, in thf]. Redox potentials
0
0
E1 /E2 in [V]: 3aII: –1.13/–0.88; 3bI: –1.18/–0.85; 3bII: –1.10/–0.83;
3bIII: –0.93/–0.79; 3bIV: –0.85/–0.74; 3cI: –0.94/–0.66; 3dI: –0.77/–
0.38; 3eI: –0.41/–0.14.
ACS Paragon Plus Environment

Page 3 of 11
Journal of the American Chemical Society
Stoichiometric chemical oxidation of 3a–3e with one
(69% spin density), 4eII containing the significant more π-
accepting DAC is best described as 4’ exhibiting 55% spin-
density on the carbene moiety. Oxidation of 3 with two
equivalents of AgSbF6 affords the corresponding colorless
dications 5a – 5e in good yields (54-75%), which can be fully
characterized as stable compounds. The formation is detectable by
1H NMR in which the former 1,4-dihydro-pyridine moiety [δ(1H)
~ 5 – 6.5 ppm] is shifted into the pyridinium range [δ(1H) ~ 8
ppm]. The step wise oxidation process starting from the neutral
compounds can also be monitored by UV-vis spectroelectro
chemistry (UV-vis SEC; Figure 8) and matches with the UV-vis
spectra of the isolated compounds (see SI).
1
2
3
4
5
6
7
8
equivalent of AgSbF6 affords the corresponding stable radical
cations 4a–4e, which could be fully characterized including X-ray
diffraction (Figure 5; for 4bIII see SI). Note, the new radicals are
remarkably persistent even in air. Time dependent UV–vis
spectroscopy for 4bII shows a decomposition half-life of t1/2 ~ 5–
6 h in air at 35°C (Figure S93), while under inert atmosphere the
radical remains highly persistent. Interestingly, in the solid-state
structure of the monomeric radical cation the N–aryl fragment is
positioned in the heterocyclic plane, while the former carbene
moiety is slightly twisted (22°) (Figure 5). Note, based on the
structural data obtained a significant redox-induced structural
distortion with a large inner reorganization energy in the first
oxidation step can be detected, while this is not the case for the
second oxidation (vide infra). Electron withdrawing substituents
on the N–aryl pyridinium moiety stabilize the strongly distorted
“spring-loaded” geometry (Figure 3 left) thereby increasing the
inner reorganization energy for the first electron transfer step.
This finding is in line with a transient shift from normal ordered
redox potentials towards potential compression (3bIII/3bIV). For
the first time experimental X-ray structural data for this so far
only theoretically proposed9 potential compression mechanism for
pyridinium systems is given.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. X-ray solid-state structure for radical cation 4cII. Hydrogen
atoms omitted for clarity. Ellipsoids shown with 50% probability.
Selected bond parameters in [Å] and [°]: N1–C1 1.354(3); N2–C1
1.345(3); C1–C2 1.431(3); C2–C3 1.416(3); C3–C4 1.363(3); C4–N3
1.403(3); N3–C5 1.440(2); N1–C1–C2–C3 158.2.
According to its mesomeric structures, the radical could either
be localized on the carbene or pyridinyl moiety (4/4’; Scheme 2).
X-band EPR studies (Figure 6) indicate delocalization of the spin-
density with large N-hyperfine coupling constants to all N-atoms
[4bII: 2xN (carbene): 6.7 MHz; 1xN (pyridinyl): 14.7 MHz; for all
EPR spectra see SI], in line with theoretical calculated values
(Figure 7 and Figures S99-S102).
Figure 7. SOMOs (B3LYP/def2-TZVP), Mulliken spin-densities and
EPR-hfcs [MHz] calculated at the M06-2X/cc-pVDZ//B3LYP-
D3/def2-SVP level of theory for 4bII (A), 4cII (C), 4dII (C) and 4eII
(E).
Figure 6. Selected X-band EPR spectra with their simulated EPR
parameters [in MHz] for A: 4aII: 2xN: 6.8, 1xN 15.1, 6xH: 3.3, 2xH:
1.3; B: 4bII: 2xN: 6.7, 1xN: 14.7, 2xH: 2.5, 2xH: 0.6; C: 4cII: 2xN:
5.5, 1xN: 15.5, 4xH: 7.2, 2xH: 3.2 and D: 4dII 1xN: 14.1, 1xN: 13.3,
1xH: 5.1, 1xH: 5.2.
Indeed, calculated spin-densities indicate a significant carbene
dependency (Figure 7): While 4bII is best represented by
mesomeric structure 4 featuring mostly pyridinyl radical character
Figure 8. Spectroelectrochemistry of 3bII and 3eII in CH2Cl2.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 11
Typically the neutral species 3 absorb at λ ~ 400 – 500 nm,
reduced pyridinium starting material (for an X-ray structure of 6
1
2
3
4
5
6
7
8
while the radical cations 4 are bathochromically shifted and split
into two absorptions, whereas the dications 5 exclusively absorb
in the UV region. Interestingly, radical cation 4eII is intensively
blue colored showing a strong broad absorption at λ = 623 nm
(Figure 8) presumably due to charge transfer from the pyridinyl to
DAC moiety.
see Figure S14), presumably arising through reduction by formed
3aVII. This is quite remarkable as the reduction potential of 1VII is
high [E ~ –1.56 V (vs. Fc/Fc+, see SI)] outside of the triaryl hybrid
series 3bI – 3bIV (see above). Indeed, X-ray crystallography
supports the geometrical fixation of the N–aryl plane being
orthogonal to the N–1,4-dihydropyridine plane thereby inhibiting
a hybridization change at nitrogen (Figure 10 C and D; for the X-
ray of 3bVII see SI). In agreement, the first redox potential: E1 = –
1.58 V (E2 = –0.86 V; vs. Fc/Fc+) is significantly more negatively
shifted compared to the triaryl series 3bI – 3bIV (Figure 11). The
large separation of the redox events of ΔE = 720 mV agrees with a
normally ordered step wise electron transfer and can be classified,
as introduced by Lainé et al.,9b as potential expansion through a
geometrically locked distortion.
After analysis of the triarylpyridinium hybrid series we
investigated the pyridinium scope of our synthetic approach.
Interestingly, N–methyl lutidinium 1V reacts cleanly with two
equivalents IMes carbene b to afford 3bV in 56% yield (Scheme
3). Importantly, no deprotonation of the alkyl moieties occurs.
Intermediate 2bV could be isolated upon using only one
equivalent of carbene and transformed stepwise into 3bV by
addition of one equivalent of KHMDS. The connectivity could be
clearly established by X-ray diffraction (Figure 9A), which
revealed a bending of the alkyl moiety out of plane (sum of angles
at N: 347°). CV measurements indicate two quasi-reversible
oxidations at a negative potential (E1 = –1.63 V and E2 = –1.10 V
vs. Fc/Fc+; Figures S35-S36) in a typical range for super electron
donors and significantly more negatively shifted compared to the
triaryl-series 3aII – 3eII. Chemical oxidation with one or two
equivalents AgSbF6 leads to the stable radical cation 4bV (EPR
see Figure 9B) and dication 5bV, the latter also verified by X-ray
diffraction (see SI).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 4. Reaction pathways of N-alkyl/N-aryl pyridinium-salts
with carbenes.
R2
N
R2
Trip
R2
N
R2
N
N
Mes
Mes
2
+
N
Mes
Mes
H
- N
- carbene-H+
H
BF4
R1
-
BF4
1VI
R1 = Me (
)
N
R
N
iPr
Trip
3bVI
3aVII
3bVII
(R1 = Me; R2 = H) 66 %
(R1 = Trip; R2 = Me)
(R1 = Trip; R2 = H) 58 %
6
R1 = Trip =
1VII
)
(
iPr
iPr
Scheme 3. Synthesis of trialkyl-hybrid 3bV with its corresponding
redox-states.
Mes
N
OTf-
N
N
N
N
N
N
Mes
N
N
Mes
Mes
Mes
KHMDS
Mes Mes
- e-
Mes Mes
- e-
N
H
Mes
+
b
+ e-
or
(56 % yield)
+ e-
N
N
N
N
N
E = -1.63 V
(vs. Fc/Fc+)
E = -1.10 V
5bV
(vs. Fc/Fc+)
OTf-
2bV
3bV
4bV
1V
Figure 10. X-ray solid-state structures of 3bVI (A), monomeric radical
cation 4bVI (B) and neutral 3aVII viewed from the top (C) and from
the side (D). Hydrogen atoms and solvent molecules omitted for
clarity. Ellipsoids shown with 50 % probability. Selected bond
parameters in [Å] and [°]: 3bVI: N1–C1 1.412(1), N2–C1 1.409(1),
C1–C2 1.372(1), C2–C3 1.463(1), C3–C4 1.343(1), C4–N3 1.396(1),
N3–C5 1.449(1), N1–C1–C2–C3 171.4; 4bVI: N1–C1 1.372(2), N2–
C1 1.374(2), C1–C2 1.424(2), C2–C3 1.435(2), C3–C4 1.358(2), C4–
N3 1.371(2), N3–C5 1.461(2), N1–C1–C2–C3 157.2; 3aVII: N1–C1
1.418(4); N2–C1 1.413(4); C1–C2 1.371(4); C2–C3 1.476(4); C3–C4
1.340(4); C4–N3 1.391(4); N3–C5 1.432(3); N1–C1–C2–C3 176.8;
C3–C4–N3–C5 177.8.
Figure 9. X-ray solid-state structure of 3bV (A) and X-band EPR
spectrum of radical-cation 4bV with its simulation (B). Hydrogen
atoms omitted for clarity. Selected bond parameters for 3bV in [Å]
and [°]: N1–C1 1.401(4); N2–C1 1.409(4); C1–C2 1.375(4); C2–C3
1.463(4); C3–C4 1.347(4); C4–N3 1.426(4); N3–C5 1.470(4).
Simulated EPR parameter [in MHz]: 2xN: 9.3, 1xN: 14.3, 2xH: 3.1,
6xH: 7.5, 3xH: 13.4, 2xH: 1.2.
Remarkably, while classical nucleophiles show low
regioselectivity for nucleophilic attack in 2- or 4-position of the
pyridinium core,21 two equivalents carbene b react cleanly with
structurally simple N–alkyl pyridinium salts such as N–methyl
pyridinium (1VI) to afford exclusively the 4-addition product 3bVI
in 66% yield (Scheme 4; X-ray see Figure 10A). Intrigued by the
relationship between redox potential and structural distortion we
investigated if strong reductants can be obtained by geometrically
constraining the N–aryl fragment. Therefore 2,4,6-triisopropylaryl
(Trip) substituted pyridinium salt 1VII was selected and reacted
with carbene a to afford 3aVII (Scheme 4). Besides the desired
product we could also detect the dimerization product 6 of the
Figure 11. Potential expansion through geometrical fixation.
ACS Paragon Plus Environment

Page 5 of 11
Journal of the American Chemical Society
Interestingly, we noticed a different reaction outcome when a
58% and 72% yield, respectively. Note, organic diradicals
stabilized by carbene entities have recently attracted large interest
as a result of their attractive magnetic and photochemical
properties.24 Interestingly, while there has been a debate about the
dimerization of bridged pyridinyl radicals,25 the X-ray solid-state
structure of 4bXI clearly demonstrates the two pyridinyl radical
moieties pointing away from each other with a N(3)–N(4) through
space distance of d ~ 5.0 Å (Figure 12B). In order to investigate
the degree of radical-radical interaction X-band EPR as well as
UV-vis studies were performed. EPR studies with solid 4bXI did
not show a half-field signal down to 240 K, while in solution the
EPR signal displayed only a broad single wave which could not
be further resolved to a triplet spectrum (Figure S75). We also
prepared the monomeric reference system 3bXII, which displayed
a well resolved EPR spectrum (Scheme 6, see also Figure S73).
UV-vis SEC of 3bXII and 3bXI indicate nearly identical absorption
properties (Figures S91 and S92) in agreement with the UV-vis
spectrum of isolated radical cation 4bXII and diradical 4bXI
(Figure S98). Also T-dependent UV-vis spectroscopy down to –
90 °C showed little change in absorption in agreement with a
monomeric species as observed with X-ray diffraction (Figures
S96 and S97). Those results indicate only a weak coupling
between the two pyridinyl radical sites and clearly no
dimerization. It should be highlighted that currently strong multi-
electron donors have found significant interest. For example,
Himmel et al. reported the strongest neutral organic four electron
donor featuring redox events at E1/2 = –0.96 V and E1/2 = –0.43 V
(vs. Fc/Fc+).26 Even though our system is clearly not as strongly
coupled as the reported hexakis(guanidine) redox system and
could be viewed in the extreme case as two individual hybrid
molecules, both 3bX and 3bXI demonstrate significantly stronger
4e- donor properties as the current record value.
1
2
3
4
5
6
7
8
simple sterically less demanding N–aryl pyridinium salt such as
1VIII was reacted with carbene b. Performing the reaction at –
78 °C gave rise to the so far not detected, highly distorted 2-
addition product 7bVIII, which could be structurally verified by X-
ray diffraction (Scheme 5i). In contrast performing the reaction at
room temperature leads to a 1:1.3 mixture of 4- and 2-addition
products 3bVIII and 7bVIII, which can both be separated by
crystallization. Calculations at the B3LYP/cc-pVDZ level of
theory indicate 3bVIII to be the thermodynamic and 7bVIII the
kinetic product (ΔG = –11.0 kcal/mol; see SI). In case the 4-
position is substituted such as in 1IX the exclusive formation of
the 2-adduct (7bIX) can be forced (Scheme 5ii).22
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 5. Formation of 2-addition products with N-aryl
pyridinium-salts and the corresponding X-ray solid-state structure.
Mes
IMes (2 eq.)
N
N
(i)
- carbene-H+
Ar
Mes
N
-78°C
rt
-
7bVIII
BF4
82% yield
G ~ 11 kcal/mol
N
-
1VIII
BF4
Ar
N
N
Mes
Mes
IMes (2 eq.)
- carbene-H+
Ar =
3bVIII
-
N
BF4
1:1.3
3bVIII:7bVIII
Ar
(ii)
IMes (2 eq.)
Mes
N
- carbene-H+
OTf-
N
N
OTf-
1IX
N
Mes
73% yield
7bIX
All hybrid molecules, except 2-addition product 7bVIII, feature
Scheme 6. Synthesis of multi-electron hybrid donors.
two reversible single electron oxidation steps (for a summary see
Figure S17). The radical cation of 7bVIII presumably dimerizes in
4-position, which can be efficiently inhibited by blocking this
position. Indeed, the CV of 7bIX featuring a 4-tert-butyl group
shows two reversible redox waves at E1 = –1.46 V and E2 =
–1.09 V (see SI). Importantly, even the sterically least hindered
N–methyl pyridinium hybrid 3bVI forms upon oxidation a
monomeric pyridinyl radical (4bVI) clearly shown by its X-ray
solid-state structure (Figure 10B). Note, we do not observe the
formation of radical-cation π-complexes as typically observed for
bipyridines,23 or proposed for pyridinyl radicals,12 presumably due
to the steric shielding given by the carbene moiety. Even though
the radicals derived from 2,6-non (H) substituted pyridinium salts
are significantly more reactive towards air [for example: 4bXII
(vide infra): t1/2 ~ 20 min at 25 °C, see Figure S95] they are stable
under inert atmosphere (t1/2 > 15 h at 50 °C for 4bXII, Figure S94).
Note, while recently developed photocatalytic cycles rely on the
N–C bond cleavage in pyridinyl radicals,14 we could not detect
such a potential decomposition pathway presumably due to the
stabilization given by the imidazolium (“carbene”) moiety.
In order to demonstrate the power of the here described
synthetic approach we investigated the synthesis of multi-electron
donors by utilizing alkyl bridged bispyridinium salts (Scheme 6).
Indeed the reactions of carbene b with bispyridinium salts 1X or
1XI, featuring a n-propyl spacer element, cleanly afford the neutral
hybrids 3bX and 3bXI in 56% and 61% yield respectively (for an
X-ray structure of 3bX see Figure 12A). CV measurements
indicate two quasi-reversible two electron oxidations at E1 = –
1.56 V (3bX)/–1.53 V (3bXI) and E2 = –1.01 V (3bX)/–0.91 V
(3bXI). In agreement, chemical oxidation with two equivalents
AgSbF6 affords the stable dicationic diradicals 4bX and 4bXI in
Mes
Mes
N
R
R
N
R
N
N
R
N
2 OTf-
R
N
N
N
Mes
4
+
Mes
- 2 carbene-H+
N
N
R
R
Mes
Mes
-
R
BF4
3bX
R = Me (
)
56% yield
61% yield
3bXI
1X
H (
)
R = Me (
H (
)
N
1XII
OTf-
1XI
)
+ 2e- - 2e-
Mes
Mes
N
R
N
R
Mes
N
N
N
N
N
N
N
R
R
Mes
Mes
4bX 58% yield
Mes
R = Me (
)
3bXII
69% yield
4bXI 72% yield
H (
)
Figure 12. X-ray solid-state structures of 3bX (A) and diradical 4bXI
(B). Hydrogen atoms and solvent molecules omitted for clarity.
Ellipsoids shown with 50% probability. Selected bond parameters in
[Å] and [°]: 4bXI: N1–C1 1.365(4); N2–C1 1.367(4); C1–C2
1.423(4); C2–C3 1.433(4); C3–C4 1.358(4); C4–N3 1.370(4); N4–C5
1.374(4); C5–C6 1.358(4); C6–C7 1.428(4); C7–C8 1.426(4); C8–N5
1.369(4); C8–N6 1.369(4); N1–C2–C2–C3 155.4; C6–C7–C8–N6
152.1.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 11
Finally, in order to explore applications of our new redox
cyclization to the neutral/cationic (8’/8) oxidation states, thereby
1
2
3
4
5
6
7
8
systems we investigated the use of these hybrids as anolyte for
redox-flow batteries. Organic redox-flow batteries have recently
emerged as highly attractive options for energy storage and thus
there is a large interest in identifying novel high density anolytes
operating at relatively low potentials.11b,27 As a proof of principle
we subjected 3bVI, featuring the lowest molecular weight below
the molecules investigated, to 1e- charge-discharge cycling. A
symmetric static H-cell with 5 mM solutions of isolated neutral
3bVI and cationic 4bVI was loaded [DMF; 0.5 M
tetrabutylammonium hexafluorophosphate (TBAPF6)] and
galvanostatic charging (5 mA) with reticulated vitreous carbon
electrodes was performed (see SI). This setup adapted from
Sanford et al. allows to evaluate the anolyte’s electrochemical
cycling stability without applying the full redox flow cell.13
Remarkably, the redox system 3bVI/4bVI is stable over 100
charge–discharge cycles (cut off potentials at E = –1.2 V and –1.9
V vs. Ag/AgNO3) with perfect Coulombic efficiency and only ca.
5% capacity loss (Figure 13, see also Figure S61). Note, literature
known reference 8/8’ (Scheme 7) featuring the identical 1,4–
dihydro-pyridine core was reported to show ~ 20% capacity fade
after 75 charge-discharge cycles,13b indicating a significantly
higher stability given by the carbene entity.
restraining the redox potential to E ~ –1 V. While computational
evolution resulted in H (Figure 1) with remarkably high cycling
stability and a record potential of E ~ –1.2 V (vs. Fc/Fc+),13b the
here presented hybrid class allows to cycle at a much lower redox
potential E ~ –1.6 V as a result of the stable electron rich neutral
redox state. In preliminary work we also investigated the 2e-
cycling13c (3bVI ↔ 4bVI ↔ 5bVI) by placing equal amounts of
-
radical cation 4bVI as isolated BF4 salt into both sides of the
symmetrical H-cell and applying cut-off potentials at 0 and –1.9 V
with 5 mA galvanostatic charging. Interestingly, several charge-
discharge cycles can be performed, showing two plateaus at E ~ –
1.6 V and –1.0 V in cell voltage curves (Figure 13, bottom) in
agreement with the CV data. However, a relatively strong
capacity decay was detectable (~ 40% over 25 cycles; see Figures
S63 and S64) indicating the instability of the dicationic oxidation
state under the specific cycling conditions. While multi-electron
cycling remains rare for nonaqueous anolytes,13c it seems very
likely that an evolutionary design strategy should lead to robust
2e- cycling. Based on the outlined hybrid approach we are
currently investigating smaller carbene entities to access higher
energy densities for practical applications.
In summary, we report a rapid, simple and modular synthetic
approach to a new class of hybrid molecules which feature a fully
reversible redox system with three stable oxidation states. While
the approach is compatible with a variety of stable carbenes it is
also applicable to a broad range of alkyl and aryl pyridinium salts.
Importantly, we isolate and structurally compare the neutral and
mono-oxidized radical compounds and relate the reducing power
and single electron transfer mechanism with the structural
information given by X-ray crystallography. Furthermore, the
structurally new class of stable pyridinyl radicals can be tailored
electronically and shows remarkable persistency even towards air.
Considering the simplicity of this approach and that both
pyridinium salts and carbenes are largely available, the
methodology can be expected to lead to a variety of new redox
architectures. As a proof of concept we could show that alkyl
bridged pyridinium systems lead to exceptional strong multi-
electron (4e-) donors. In general, any pyridinium salt can be
converted in one step into a new redox system and therefore
broaden the interest in applications such as strong (multi e-)
reducing agents or redox-flow batteries. As a proof of principle,
we could show that low potential (E ~ –1.6 V) charge–discharge
experiments with the new hybrids demonstrate a high cycling
stability with no significant capacity decrease over 100 cycles,
thereby clearly highlighting the large potential of this hybrid class
as new structural motive in redox-flow chemistry. Future work is
focused on the evolution of the here described redox systems.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 7. Comparison of redox cycling of 3bVI/4bVI with literature
known 8/8’.
Neutral
Radical-cation
N
N
Mes
N
N
N
N
Mes
Mes
- e-
+ e-
Mes
Mes
Mes
- e-
+ e-
N
N
N
E = -1.58 V
(vs. Fc/Fc+)
3bVI
4bVI
5bVI
Neutral-radical
Ph
Cation
O Ph
O
Ph
O
- e-
- e-
+ e-
+ e-
E = -1.08 V
(vs. Fc/Fc+)
N
N
8'
N
8
8''
EXPERIMENTAL SECTION
For full characterization data for all compounds, see SI.
Representative procedures and selected characterization data for:
Neutral 3bII: To a suspension of 1II (282 mg, 0.63 mmol, 1.00 eq.)
in thf (10 mL) was added free IMes carbene (400 mg, 1.31 mmol,
2.10 eq.). The yellow/orange solution was stirred for 14 h and the
solvent evaporated under reduced pressure. The obtained solid
was extracted with Et2O (20 mL + 20 mL) and filtered. The
solvent of the orange solution was removed under reduced
pressure to give 3bII as a bright orange solid (282 mg,
0.422 mmol, 67 %). Single crystals suitable for X-ray diffraction
can be obtained by slow evaporation from a saturated Et2O
Figure 13. One electron cycling of 3bVI/4bVI (top) with the
corresponding capacity curve (middle) and two electron cycling
(bottom).
Importantly, it has been reported that the anionic redox state
(8’’; Scheme 7) is reactive and not stable.13d Thus this molecule is
not suitable for charge-discharge experiments, limiting the
1
solution. m.p. 250 °C; H-NMR (C6D6, 400 MHz, 298 K): 7.49
(d, J = 7.0 Hz, 4H), 7.18 (d, J = 7.9 Hz, 4H), 7.04 – 6.93 (m, 6H),
ACS Paragon Plus Environment

Page 7 of 11
Journal of the American Chemical Society
6.84 (s, 4H), 6.35 (s, 2H), 5.56 (s, 2H), 2.25 (s, 6H), 1.96 (s, 12H),
Radical cation 4bV: To a mixture of solids of 3bV (40 mg, 0.093
1.19 (s, 9H); 13C-NMR (C6D6, 101 MHz, 298 K): 150.3, 141.7,
140.5, 140.3, 139.2, 136.7, 135.6, 129.7, 129.3, 128.8, 125.0,
124.5, 123.9, 120.1, 117.0, 116.6, 85.1, 33.9, 31.8, 21.1, 17.9;
1H/15N HMBC (400/41 MHz, C6D6, 298K): -290.4, -247.1; IR
[cm-1]: ṽ = 3048, 3026, 2961, 2915, 2862, 1609, 1594, 1570,
1516, 1508, 1472, 1444, 1412, 1378, 1361, 1349, 1330, 1301,
1288, 1255, 1216, 1202, 1184, 1163, 1153, 1135, 1117, 1102,
1083, 1072, 1053, 1031, 1014, 994, 966, 933, 922, 903, 893, 851,
820, 757, 731, 689, 674, 633, 623, 575, 547, 525, 506; UV-vis
[nm]: (in CH2Cl2): 284 (ε = 12498 M-1cm-1), 462 (ε = 23752 M-
mmol, 1.00 eq.) and AgSbF6 (32 mg, 0.093 mmol, 1.00 eq.) was
added thf (10 mL) at room temperature and stirred for 30 min.
The red solution was filtered, the amount of solvent was reduced
to 1 mL under reduced pressure and the crude product was
precipitated by addition of Et2O (20 mL). The solid was washed
twice with Et2O (5 mL) until the supernatant became colorless.
Drying under reduced pressure affords 4bV as olive green solid
(37 mg, 0.056 mmol, 60%). X-band EPR g = 2.0031 (2xN: 9.3
MHz; 1xN: 14.3 MHz; 2xH: 3.1 MHz; 6xH: 7.5 MHz; 3xH: 13.4
MHz; 2xH: 1.2 MHz); IR [cm-1]: ṽ = 3103, 2947, 1561, 1525,
1487, 1455, 1395, 1267, 1232, 1204, 1035, 862, 819, 735, 650,
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
+
1cm-1); HR-MS-ESI(+) calc. C48H50N3O2 [M+O2+H]+ 700.3898;
+
found 700.3883.
594, 576; HR-MS-ESI(+) calc. C29H34N3 [M-H]+ 424.2747;
Radical cation 4bII: To a mixture of solids of 3bII (50 mg,
0.07 mmol, 1.00 eq.) and AgSbF6 (27 mg, 0.08 mmol, 1.05 eq.)
was added CH2Cl2 (10 mL) at room temperature and stirred for
90 min. The red solution was filtered, the solvent evaporated
under reduced pressure and the resulting solid washed with
Et2O/CH2Cl2 (3/1; 10 mL). Drying under reduced pressure affords
4bII as dark red solid (72 mg, 0.04 mmol, 57 %). X-band EPR g =
2.0029 (2xN: 6.72 MHz; 1xN: 14.7 MHz; 2xH: 2.48 MHz; 2xH:
0.59 MHz); IR [cm-1]: ṽ = 3174, 3142, 2959, 2919, 2867, 1621,
1577, 1557, 1504, 1478, 1445, 1399, 1380, 1364, 1336, 1316,
1286, 1269, 1223, 1189, 1168, 1155, 1111, 999, 991, 925, 885,
860, 850, 821, 807, 784, 764, 744, 728, 699, 654, 612, 571, 559,
537; UV-vis [nm]: (in CH2Cl2): 389 (ε = 7660 M-1cm-1), 525 (ε =
found 424.2734.
Dication 5bV: To a mixture of solids of 3bV (20 mg, 0.047 mmol,
1.00 eq.) and AgSbF6 (32 mg, 0.093 mmol, 2.00 eq.) was added
thf (5 mL) at room temperature and stirred for 15 min. The gray
suspension was filtered and the amount of solvent was reduced to
1 mL under reduced pressure. The product was precipitated by
addition of Et2O (15 mL) and washed twice with Et2O (5 mL) to
afford 5bV as a colorless solid (32 mg, 0.036 mmol, 75 %). H-
1
NMR (CD3CN, 400 MHz, 298 K): 8.04 (s, 2H), 7.15 (s, 4H), 7.04
(s, 2H), 3.85 (s, 3H), 2.51 (s, 6H), 2.35 (s, 6H), 2.07 (s, 12H); 13C-
NMR (CD3CN, 101 MHz, 298 K): 159.5, 143.5, 138.9, 135.3,
135.2, 131.2, 130.2, 128.4, 127.0, 42.2, 22.6, 21.2, 17.9; IR
[cm-1]: ṽ = 3179, 3149, 3121, 3088, 3042, 3002, 2969, 2926,
2863, 1641, 1604, 1567, 1495, 1447, 1408, 1390, 1377, 1345,
1284, 1228, 1206, 1174, 1120, 1102, 1089, 1075, 1034, 1015,
993, 966, 931, 896, 885, 873, 860, 797, 780, 766, 743, 725, 716,
654, 642, 604, 590, 577, 552, 533, 508; UV-vis [nm]: (in thf): 285
+·
3662 M-1cm-1); HR-MS-ESI(+) calc. C49H52N3 [M+OMe]+·
698.4105; found 698.4090.
Dication 5bII: To a mixture of solids of 3bII (50 mg, 0.07 mmol,
1.00 eq.) and AgSbF6 (51 mg, 0.15 mmol, 2.05 eq.) was added
CH2Cl2 (10 mL) at room temperature and stirred for 90 min. The
grey suspension was filtered, the solvent evaporated under
reduced pressure and the resulting solid washed with Et2O/CH2Cl2
(5/1; 10 mL) and Et2O (5 mL). Drying under reduced pressure
(ε = 9289 M-1cm-1); HR-MS-ESI(+) calc. C29H35N2 [M]2+
212.6410; found 212.6414.
2+
Electrochemical measurements: The setup of the Siewert
group (Göttingen University) was used for CV/UV/vis-SEC, see
for example ref. 28. CVs were measured with a Gamry
Instruments Reference 600 and iR compensation using the
positive feedback method which is implemented in the PH200
software of Gamry. Samples were measured under inert
atmosphere in a nitrogen glove box at room temperature in dry thf
solution containing tetrabutylammonium hexafluorophosphate
(0.1 M). The setup consist of a three-neck flask with a three
electrode setup containing a glassy carbon working electrode
(GC: CH Instruments, ALS Japan; A = 7.1 mm²), a platinum wire
as a counter electrode, and an Ag/AgNO3 reference electrode
(0.01 M AgNO3 in 0.1 M nBu4NPF6 in CH3CN). The reference
electrode was freshly prepared by using a fritted sample holder
(Vycor glass), which was activated by storing in a CH3CN
solution for one night, followed by diluted HCl (1M) for one
night, dried and stored in thf for at least one additional night. To
the fritted sample holder was added a freshly prepared 0.01 M
AgNO3/0.1 M nBu4NPF6 solution in CH3CN and a silver wire.
The working electrode was cleaned before measuring a new
compound by standard methods: washed with water, polished
with an alox-slurry (0.05 μm), washed with millipore water,
sonicated in millipore water for 3 minutes, rinsed with millipore
water, and dried. Initially a blank sample only containing
electrolyte in thf (0.1M) was measured and the potential cycled
for 3-5 scans until a stable and clean potential was reached. Then
the 3-neck cell was emptied and a specific amount of compound
dissolved in 3 mL thf added and the CV measured. The system
was furthermore referenced internally by addition of diacetyl
ferrocene or ferrocene. The values obtained were corrected
against ferrocene (diacetyl ferrocene to ferrocene ΔE = 430 mV in
thf).16
1
affords 5bII as a colorless solid (55 mg, 0.04 mmol, 65 %). H-
NMR (CD2Cl2, 500 MHz, 298 K): 7.91 (s, 2H), 7.35 (t, J = 7.6
Hz, 2H), 7.28 (s, 4H), 7.25 (s, 2H), 7.20-7.17 (m, 4H), 7.14 (d, J =
8.8 Hz, 2H), 6.97 (d, J = 8.8 Hz, 2H), 6.80 – 6.73 (m, 4H), 2.45
(s, 6H), 2.18 (s, 12H), 1.13 (s, 9H); 13C-NMR (CD2Cl2, 126 MHz,
298K): 159.4, 155.4, 144.5, 136.9, 135.7, 135.3, 134.9, 131.9,
131.9, 131.8, 131.7, 131.0, 130.3, 128.8, 127.9, 126.8, 126.4,
35.3, 31.2, 21.5, 17.8; IR [cm-1]: ṽ =3166, 3142, 2965, 2873,
1620, 1578, 1550, 1503, 1478, 1442, 1412, 1397, 1365, 1325,
1284, 1269, 1251, 1223, 1188, 1175, 1152, 1113, 1091, 1080,
1031, 1001, 984, 931, 905, 884, 872, 861, 846, 829, 807, 783,
767, 743, 726, 697, 655, 610, 595, 585, 575, 564, 552, 540, 522,
509; UV-vis [nm]: (in CH2Cl2): 297 (ε = 14186 M-1cm-1); HR-
MS-ESI(+) calc. C48H49N32+ [M]2+ 333.6958; found 333.6963.
Neutral 3bV: To a suspension of 1V (169 mg, 0.63 mmol, 1.0 eq.)
in thf (10 mL) was added free IMes carbene (400 mg, 1.31 mmol,
2.10 eq.). The orange solution was stirred for 12 h and the solvent
evaporated under reduced pressure. The obtained solid was
extracted with pentane (15 mL + 5 mL) and filtered. The solvent
of the orange solution was removed under reduced pressure to
give 3bV as a bright orange solid (150 mg, 0.35 mmol, 56 %).
1
m.p. 158 °C; H-NMR (C6D6, 400 MHz, 298 K): 6.80 (s, 4H),
5.59 (s, 2H), 4.57 (s, 2H), 2.42 (s, 12H), 2.24 (s, 3H), 2.15 (s, 6H),
1.28 (s, 6H); 13C-NMR (C6D6, 100 MHz, 298 K): 137.9, 137.2,
136.4, 131.4, 129.2, 116.6, 104.8, 85.0, 33.0, 21.1, 20.9, 18.7; IR
[cm-1]: ṽ = 2908, 2872, 1599, 1568, 1483, 1447, 1391, 1373,
1347, 1327, 1236, 1179, 1145, 1112, 1090, 1077, 1031, 1010,
925, 913, 852, 810, 712, 671, 648, 606, 589, 577, 567, 509; UV-
vis [nm]: (in thf): 333 (ε = 29758 M-1cm-1); HR-MS-ESI(+) calc.
C29H34N3+ [M-H]+ 424.2747; found 424.2733.
Spectro-electrochemical measurements were measured with
the setup of the Siewert group (Göttingen University) and
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 11
recorded with a Gamry Instruments Reference 600. The samples
Crystallographic data for 3bX
Crystallographic data for 3bVII
Crystallographic data for 5bV
Crystallographic data for 4bIII
Crystallographic data for 3bVI
Crystallographic data for 4bVI
Crystallographic data for 7bVIII
Crystallographic data for 4bXI
Crystallographic data for 4cII
Crystallographic data for 3bV
1
2
3
4
5
6
7
8
were measured in CH2Cl2 starting from the neutral compounds,
containing a three electrode setup (platinum wire, platinum net
and Ag/AgNO3 reference electrode) in a UV-vis cell (1mm
diameter from ALS Co., Ltd; SEC-C spectroelectrochemical cell)
under nitrogen. For a picture of the setup see Figure S76. In order
to guarantee a clean oxygen free setup the measurement was
performed in a nitrogen glove box. The reference electrode was
freshly prepared by using a fritted sample holder, which was
activated by storing in a CH3CN solution for one night, followed
by diluted HCl (1M) for one night, dried and stored in thf for at
least one additional night. To the fritted sample holder was added
a freshly prepared 0.01 M AgNO3/0.1 M nBu4NPF6 solution in
9
AUTHOR INFORMATION
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Corresponding Author
CH3CN and
a
silver wire. The light source was
a
deuterium/tungsten light source and detected with a BWTEK
ExemplarLS. A blank spectrum and a reference spectrum with
just solvent was taken in advance and was subtracted from the
measured data. In general, a UV-vis spectrum was taken every 10
seconds, while the electric current was scanned from negative
potentials to positive potentials (usually around 1 V) by a step size
of 2-2.5 mV/s.
* mhansma@uni-goettingen.de
ORCHID
Max M. Hansmann: 0000-0003-3107-1445
Notes
Electrochemical charge-discharge experiments: Charge-
discharge experiments were performed in analogy to reports by
Sanford et al.13 in a custom made H-cell in a nitrogen filled
glovebox. Both cell chambers were separated by a P5-frit (15 mm
diameter, porosity 5 from Robu®). Reticulated vitreous carbon
(RVC) electrodes (100 ppi Duocel®) were cut into rods of the
dimensions 0.5 cm x 0.5cm x 4cm and positioned ca. 2 cm deep in
solution (see Figure S60). To rule out contamination processes the
electrodes were single used. As reference an Ag/AgNO3 reference
electrode (0.01 M AgNO3 in 0.1 M nBu4NPF6 in CH3CN) was
used. The reference electrode was freshly prepared by using a
fritted sample holder, which was activated by storing in a CH3CN
solution for one night, followed by diluted HCl (1M) for one
night, dried and stored in DMF for at least one additional night.
To the fritted sample holder was added a freshly prepared 0.01 M
AgNO3/0.1 M nBu4NPF6 solution in CH3CN and a silver wire.
Tetrabutylammonium hexafluorophosphate (electrochemical
grade from Sigma Aldrich) was molten under high vacuum prior
to use. A 0.5 M tetrabutylammonium hexafluorophosphate
solution in DMF (anhydrous grade dried additionally over 4Å
molecular sieves) was freshly prepared. For 1e- cycling the
following procedure was used: Solutions (0.5 M TBAP) of neutral
(5 mM) and radical cation (5 mM) were separately prepared and
filled into the chambers of the cell (each 6 mL). Then a 5 mA
current was applied with cut-off potentials at -1.9 V and -1.2 V
(against the internal Ag/AgNO3 reference). For 2e- electron
cycling: A 5 mM solution (0.5 M TBAP) of radical cation was
equally filled into the left and right side of the chambers. Then a 5
mA current was applied with cut-off potentials at -1.9 V and 0 V
(against the internal Ag/AgNO3 reference). During charge-
discharge experiments both cells were stirred constantly at 500
rpm.
The author declare no competing financial interests.
ACKNOWLEDGMENT
This work was supported by the Fonds der chemischen
Industrie (scholarships to M. M. H. and P. W. A.) and DFG (INST
186/1237-1). Isabelle Rüter is thanked for her synthetic help with
some of the compounds. The high performance computing
facilities of the University of Göttingen/MPI are thanked. Dr.
Christopher Golz is acknowledged for X-ray analysis, Prof. Inke
Siewert for generously granting access to her electrochemical
infrastructure, Prof. Franc Meyer for providing his UV-vis
spectrometer as well as Prof. Manuel Alcarazo for his continuous
support.
ABBREVIATIONS
NHC, N-heterocyclic carbene; CAAC, cyclic (alkyl)amino
carbene; DAC, diamido carbene; SEC, spectro electrochemistry;
TBAP tetrabutylammonium hexafluorophosphate; hfc, hyperfine
couplings
REFERENCES
(1) (a) “Organic Redox Systems: Synthesis, Properties, and
Applications” Eds. Nishinaga, T. Wiley 2016. (b) Deuchert, K.;
Hünig, S. Multistage Organic Redox Systems — A General Structural
Principle. Angew. Chem. Int. Ed. 1978, 17, 875–886.
(2) Bendikov, M.; Wudl, F.; Perepichka, D. F. Tetrathiafulvalenes,
Oligoacenenes, and Their Buckminsterfullerene Derivatives:ꢀ The
Brick and Mortar of Organic Electronics. Chem. Rev. 2004, 104,
4891–4946.
(3) Striepe, L.; Baumgartner, T. Viologens and Their Application as
Functional Materials. Chem. Eur. J. 2017, 23, 16924–16940.
(4) (a) Weitz, E. Radikale, Quasi‐Radikale, merichinoide
Verbindungen und Chinhydrone. Ein Beitrag zur Farbentheorie.
Angew. Chem. 1954, 66, 658–677. (b) Hünig, S.; Garner, B. J.;
Ruider, G.; Schenk, W. Über zweistufige Redoxsysteme, XIII
Bipyridylium‐, Bipyrylium‐ und Bithiopyrylium‐Salze. Liebigs Ann.
Chem. 1973, 1036–1060.
(5) Broggi, J.; Terme, T.; Vanelle, P. Organic Electron Donors as
Powerful Single‐Electron Reducing Agents in Organic Synthesis.
Angew. Chem. Int. Ed. 2014, 53, 384–413.
(6) Murphy, J. A.; Garnier, J.; Park, S. R.; Schoenebeck, F. Zhou, S.-
z.; Turner, A. T. Super-Electron Donors:ꢀ Bis-pyridinylidene
Formation by Base Treatment of Pyridinium Salts. Org. Lett. 2008,
10, 1227–1230.
ASSOCIATED CONTENT
Supporting
Information.
Experimental
procedures,
characterization data, spectra, crystallographic data as well as
computational details included in the supporting information. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Crystallographic data for 3dII
Crystallographic data for 3bII
Crystallographic data for 3aVII
Crystallographic data for 6
Crystallographic data for 3eII
(7) For an overview see: (a) Murphy, J. A. Discovery and
Development of Organic Super-Electron-Donors. J. Org. Chem. 2014,
ACS Paragon Plus Environment

Page 9 of 11
Journal of the American Chemical Society
79, 3731–3746. (b) Doni, E.; Murphy, J. A. Evolution of neutral
organic super-electron-donors and their applications. Chem. Commun.
2014, 50, 6073–6087.
(8) Evans, D. H. One-Electron and Two-Electron Transfers in
Electrochemistry and Homogeneous Solution Reactions. Chem. Rev.
2008, 108, 2113–2144.
Chem. Soc. 2013, 135, 2486–2488. (d) Kim, Y.; Kim, K.; Lee, E.
Oxime Ether Radical Cations Stabilized by N‐Heterocyclic Carbenes.
Angew. Chem. Int. Ed. 2018, 57, 262–265. (e) Back, J.; Park, J.; Kim,
Y.; Kang, H.; Kim, Y.; Park, M. J.; Kim, K.; Lee, E. Triazenyl
Radicals Stabilized by N-Heterocyclic Carbenes. J. Am. Chem. Soc.
2017, 139, 15300–15303. (f) Ruamps, M.; Bastin, S.; Rechignat, L.;
Sournia-Saquet, A.; Valyaev, D. A.; Mouesca, J.-M.; Lugan, N.;
Maurel, V.; Cesar, V. Unveiling the redox-active character of
imidazolin-2-thiones derived from amino-substituted N-heterocyclic
carbenes. Chem. Commun. 2018, 54, 7653–7656. with CAACs, see:
(g) Kinjo, R.; Donnadieu, B.; Celik, M. A.; Frenking, G.; Bertrand, G.
1
2
3
4
5
6
7
8
(9) (a) Fortage, J.; Peltier, C.; Peruchot, C.; Takemoto, Y.; Teki, Y.;
Bedioui, F.; Marvaud, V.; Dupeyre, G.; Pospisil, L.; Adamo, C.;
Hromadová, M.; Ciofini, I.; Lainé, P. P. Single-Step versus Stepwise
Two-Electron Reduction of Polyarylpyridiniums: Insights from the
Steric Switching of Redox Potential Compression. J. Am. Chem. Soc.
2012, 134, 2691–2705. (b) Lachmanova, S.; Dupeyre, G.; Tarábek, J.;
Ochsenbein, P.; Perruchot, C.; Ciofini, I.; Hromadová, M.; Pospisil,
L.; Lainé, P. P. Kinetics of Multielectron Transfers and Redox-
Induced Structural Changes in N-Aryl-Expanded Pyridiniums:
Establishing Their Unusual, Versatile Electrophoric Activity. J. Am.
Chem. Soc. 2015, 137, 11349–11364. (c) Fortage, J.; Peltier, C.;
Nastasi, F.; Puntoriero, F.; Tuyeras, F.; Griveau, S.; Bedioui, F.;
Adamo, C.; Ciofini, I.; Campagna, S.; Lainé, P. P. Designing
Multifunctional Expanded Pyridiniums: Properties of Branched and
Fused Head-to-Tail Bipyridiniums. J. Am. Chem. Soc. 2010, 132,
16700–16713.
9
Synthesis and Characterization of
a
Neutral Tricoordinate
Organoboron Isoelectronic with Amines. Science 2011, 333, 610–613.
(h) Bissinger, P.; Braunschweig, H.; Damme, A.; Krummenacher, I.;
Phukan, A. K.; Radacki, K.; Sugawara, S. Isolation of a Neutral
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Boron‐Containing
Radical
Stabilized
by
a
Cyclic
(Alkyl)(Amino)Carbene. Angew. Chem. Int. Ed. 2014, 53, 7360–
7363. (i) Mahoney, J. K.; Martin, D.; Thomas, F.; Moore, C. E.;
Rheingold, A. L.; Bertrand, G. Air-Persistent Monomeric
(Amino)(carboxy) Radicals Derived from Cyclic (Alkyl)(Amino)
Carbenes. J. Am. Chem. Soc. 2015, 137, 7519−7525. (j) Strya, S.;
Melaimi, M.; Moore, C. E.; Rheingold, A. L.; Augenstein, T.; Breher,
(10) (a) Sanvito, S. Molecular spintronics. Chem. Soc. Rev. 2011, 40,
3336–3355. (b) Kato, K.; Osuka, A. Platforms for Stable
Carbon‐Centered Radicals. Angew. Chem. Int. Ed. 2019, DOI:
10.1002/anie.201900307.
(11) (a) Magnetic Properties of Organic Materials; Lathi, P. M., Ed.;
Marcel Dekker: New York, 1999, 1–728. (b) Winsberg, J.;
Hagemann, T.; Janoschka, T.; Hager, M. D.; Schubert, U. S.
Redox‐Flow Batteries: From Metals to Organic Redox‐Active
Materials. Angew. Chem. Int. Ed. 2017, 56, 686–711.
(12) (a) Kosower, E. M.; Poziomek, E. J. Stable Free Radicals. I.
Isolation and Distillation of 1-Ethyl-4-carbomethoxypyridinyl. J. Am.
Chem. Soc. 1964, 86, 5515–5523. (b) Itoh, M.; Nagakura, S. The
Electron Spin Resonance and Electronic Spectra of 4-Substituted
Pyridinyl Radicals. Bull. Chem. Soc. Jpn. 1966, 39, 369–375. (c)
Kosover, E. M. Stable Pyridinyl Radicals. in Topics Current Chem.
1983, 112, 117–162.
(13) (a) Sevov, C. S.; Brooner, R. E. M.; Chénard, E.; Assary, R. S.;
Moore, J. S.; Rodriguez-Lopez, J.; Sanford, M. S. Evolutionary
Design of Low Molecular Weight Organic Anolyte Materials for
Applications in Nonaqueous Redox Flow Batteries. J. Am. Chem. Soc.
2015, 137, 14465–14472. (b) Sevov, C. S.; Heckey, D. P.; Cook, M.
E.; Robinson, S. G.; Barnett, S.; Minteer, S. D.; Sigman, M. S.;
Sanford, M. S. Physical Organic Approach to Persistent, Cyclable,
Low-Potential Electrolytes for Flow Battery Applications. J. Am.
Chem. Soc. 2017, 139, 2924–2927. (c) Hendriks, K. H.; Sevov, C. S.;
Cook, M. E.; Sanford, M. S. Multielectron Cycling of a Low-Potential
Anolyte in Alkali Metal Electrolytes for Nonaqueous Redox Flow
Batteries. ACS Energy Lett. 2017, 2, 2430–2435. (d) Sevov, C. S.;
Hendriks, K. H.; Sanford, M. S. Low-Potential Pyridinium Anolyte
for Aqueous Redox Flow Batteries. J. Phys. Chem. C. 2017, 121,
24376–24380.
(14) (a) Klauck, F. J. R.; James, M. J.; Glorius, F. Deaminative
Strategy for the Visible‐Light‐Mediated Generation of Alkyl
Radicals. Angew. Chem. Int. Ed. 2017, 56, 12336–12339. (b) Ociepa,
M.; Turkowska, J.; Gryko, D. Redox-Activated Amines in C(sp3)–
C(sp) and C(sp3)–C(sp2) Bond Formation Enabled by Metal-Free
Photoredox Catalysis. ACS Catal. 2018, 8, 11362–11367. (c) Wu, J.;
He, L.; Noble, A.; Aggarwal, V. K. Photoinduced Deaminative
Borylation of Alkylamines. J. Am. Chem. Soc. 2018, 140, 10700–
10704.
(15) For the stabilization of main-group radicals with NHCs, see: (a)
Tumanskii, B.; Sheberla, D.; Molev, G.; Apeloig, Y. Dual Character
of Arduengo Carbene–Radical Adducts: Addition versus
Coordination Product. Angew. Chem. Int. Ed. 2007, 46, 7408–7411.
(b) Rottschäfer, D.; Neumann, B.; Stammler, H. G.; Gastel, M. v.;
Andrada, D. M.; Ghadwal, R. S. Crystalline Radicals Derived from
Classical N‐Heterocyclic Carbenes. Angew. Chem. Int. Ed. 2018, 57,
4765–4768. (c) Abraham, M. Y.; Wang, Y.; Xie, Y.; Gilliard, R. J.,
Jr.; Wei, P.; Vaccaro, B. J.; Johnson, M. K., Schaefer, H. F.; Schleyer
III, P. v. R.; Robinson, G. H. Oxidation of Carbene-Stabilized
Diarsenic: Diarsene Dications and Diarsenic Radical Cations. J. Am.
F.;
Bertrand,
G.
Crystalline
Cyclic
(Alkyl)(amino)carbene‐tetrafluoropyridyl Radical. Chem. Eur. J.
2015, 21, 8441–8446. (k) Munz, D.; Chu, J.; Melaimi, M.; Bertrand,
G. NHC‐CAAC Heterodimers with Three Stable Oxidation States.
Angew. Chem. Int. Ed. 2016, 55, 12886–12890. (l) Li, Y.; Mondal, K.
C.; Samuel, P. P.; Zhu, H.; Orben, C. M.; Panneerselvam, S.; Dittrich,
B.; Schwederski, B.; Kaim, W.; Mondal, T.; Koley, D.; Roesky, H.
W. C4 Cumulene and the Corresponding Air‐Stable Radical Cation
and Dication. Angew. Chem. Int. Ed. 2014, 53, 4168–4172. (m)
Hansmann, M. M.; Melaimi, M.; Bertrand, G. Crystalline Monomeric
Allenyl/Propargyl Radical. J. Am. Chem. Soc. 2017, 139,
15620−15623; with DACs see: (n) Deardorff, C. L.; Sikma, R. E.;
Rhodes, C. P.; Hudnall, T. W. Carbene-derived α-acyl
formamidinium cations: organic molecules with readily tunable
multiple redox processes. Chem. Commun. 2016, 52, 9024−9027; for
a general overview, see: (o) Martin, C. D.; Soleilhavoup, M.;
Bertrand, G. Carbene-stabilized main group radicals and radical ions.
Chem. Sci. 2013, 4, 3020−3030. (p) Wang, Y.; Robinson G. H.
Carbene Stabilization of Highly Reactive Main-Group Molecules.
Inorg. Chem. 2011, 50, 12326−12337. (q) Nesterov, V.; Reiter, D.;
Bag, P.; Frisch, P.; Holzner, R.; Porzelt, A.; Inoue, S. NHCs in Main
Group Chemistry. Chem. Rev. 2018, 118, 9678-9842. (r) Kim, Y.;
Lee, E. Stable Organic Radicals Derived from N‐Heterocyclic
Carbenes. Chem. Eur. J. 2018, 24, 19110–19121. (s) Kundu, S.;
Sinhababu, S.; Chandrasekhar, V.; Roesky, H. W. Stable cyclic
(alkyl)(amino)carbene (cAAC) radicalswith main group substituents.
Chem. Sci. 2019, 10, 4727−4741.
(16) Antoni, P. W.; Hansmann, M. M. Pyrylenes: A New Class of
Tunable, Redox-Switchable, Photoexcitable Pyrylium–Carbene
Hybrids with Three Stable Redox-States. J. Am. Chem. Soc. 2018,
140, 14823–14835.
(17) (a) Lavallo, V.; Canac, Y.; Prasang, C.; Donnadieu, B.; Bertrand,
G. Stable Cyclic (Alkyl)(Amino)Carbenes as Rigid or Flexible,
Bulky, Electron‐Rich Ligands for Transition‐Metal Catalysts: A
Quaternary Carbon Atom Makes the Difference. Angew. Chem. Int.
Ed. 2005, 44, 5705–5709; for reviews on CAACs, see: (b)
Soleilhavoup, M.; Bertrand, G. Cyclic (Alkyl)(Amino)Carbenes
(CAACs): Stable Carbenes on the Rise. Acc. Chem. Res. 2015, 48,
256–266. (c) Roy, S.; Mondal, K. C.; Roesky, H. W. Cyclic
Alkyl(amino) Carbene Stabilized Complexes with Low Coordinate
Metals of Enduring Nature. Acc. Chem. Res. 2016, 49, 357–369. (d)
Melaimi, M.; Jazzar, R.; Soleilhavoup, M.; Bertrand, G. Cyclic
(Alkyl)(amino)carbenes (CAACs): Recent Developments. Angew.
Chem. Int. Ed. 2017, 56, 10046–10068. (e) Paul, U. S. D.; Radius, U.
What
Wanzlick
Did
Not
Dare
To
Dream:
Cyclic
(Alkyl)(amino)carbenes (cAACs) as New Key Players in
Transition‐Metal Chemistry. Eur. J. Inorg. Chem. 2017, 3362–3375.
(18) (a) Hudnall, T. W.; Bielawski, C. W. An N,N′-Diamidocarbene:
Studies in C−H Insertion, Reversible Carbonylation, and Transition-
Metal Coordination Chemistry. J. Am. Chem. Soc. 2009, 131, 16039–
16041. For a review, see (b) Moerdyk, J. P.; Schilter, D.; Bielawski,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 11
C. W. N,N′-Diamidocarbenes: Isolable Divalent Carbons with Bona
quinone flow battery. Science 2015, 349, 1529–1532. (c) Soloveichik,
Fide Carbene Reactivity. Acc. Chem. Res. 2016, 49, 1458–1468.
(19) (a) Zhou, Y.; Qian, L.; Liu, M.; Wu, G.; Gao, W.; Ding, J.;
Huang, X.; Wu, H. The influence of different N-substituted groups on
the mechanochromic properties of 1,4-dihydropyridine derivatives
with simple structures. RSC Adv. 2017, 7, 51444–51451. (b) Lavilla,
R. Recent developments in the chemistry of dihydropyridines. J.
Chem. Soc., Perkin Trans. 1, 2002, 1141–1156.
(20) Roy, M. M. D.; Rivard, E. Pushing Chemical Boundaries with N-
Heterocyclic Olefins (NHOs): From Catalysis to Main Group Element
Chemistry. Acc. Chem. Res. 2017, 50, 2017–2025.
G. L. Flow Batteries: Current Status and Trends. Chem. Rev. 2015,
115, 11533–11558. (d) Wei, X.; Pan, W.; Duan, W.; Hollas, A.;
Yang, Z.; Li, B.; Nie, Z.; Liu, J.; Reed, D.; Wang, W.; Sprenkle, V.
Materials and Systems for Organic Redox Flow Batteries: Status and
Challenges. ACS Energy Lett. 2017, 2, 2187–2204.
(28) Du, J.-P.; Wilting, A.; Siewert, I. Are Two Metal Ions Better than
One? Mono‐ and Binuclear α‐Diimine‐Re(CO)3 Complexes with
Proton‐Responsive Ligands in CO2 Reduction Catalysis. Chem. Eur.
J. 2019, 25, 5555–5564.
1
2
3
4
5
6
7
8
9
(21) Sowmiah, S.; Esperanca, J. M. S. S.; Rebelo, L. P. N.; Afonso, C.
A. M. Pyridinium salts: from synthesis to reactivity and applications.
Org. Chem. Front. 2018, 5, 453–493.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(22) A structurally related intramolecular systems was described, see:
Garnier, J.; Thomson, D. W.; Zhou, S.; Jolly, P. I.; Berlouis, L. E. A.;
Murphy, J. A. Hybrid super electron donors – preparation and
reactivity. Beilstein J. Org. Chem. 2012, 8, 994–1002.
(23) For an overview see: Zhang, D.-W.; Tian, J.; Chen.; L. Zhang;
L.; Li. Z.-T. Dimerization of Conjugated Radical Cations: An
Emerging Non‐Covalent Interaction for Self‐Assembly. Chem. Asian
J. 2015, 10, 56–68.
(24) For carbene derived diradicals, see for example: (a) Mondal,
K.C.; Roesky, H. W.; Schwarzer, M.C.; Frenking, G.; Tkach, I.; Wolf,
H.; Kratzert, D.; Herbst-Irmer, R.; Niepötter, B.; Stalke, D.
Conversion of a Singlet Silylene to a stable Biradical. Angew. Chem.
Int. Ed. 2013, 52, 1801–1805. (b) Mondal, K. C.; Roesky, H. W.;
Schwarzer, M. C.; Frenking, G.; Niepötter, B.; Wolf, H.; Herbst-
Irmer, R.; Stalke, D. A Stable Singlet Biradicaloid Siladicarbene:
(L:)2Si. Angew. Chem. Int. Ed. 2013, 52, 2963−2967 (c) Hansmann,
M. M.; Melaimi, M.; Munz, D.; Bertrand, G. Modular Approach to
Kekulé Diradicaloids Derived from Cyclic (Alkyl)(amino)carbenes. J.
Am. Chem. Soc. 2018, 140, 2546−2554. (d) Ghadwal, R. S.;
Rottschafer, D.; Neumann, B.; Stammler, G.; Andrada, D. M. Kekulé
diradicaloids derived from a classical N-heterocyclic carbene. Chem.
Sci. 2018, 9, 4970–4976. (e) Messelberger, J.; Grünwald, A.; Pinter,
P.; Hansmann, M. M.; Munz, D. Carbene derived diradicaloids –
building blocks for singlet fission? Chem. Sci. 2018, 9, 6107–6117.
(25) For studies on bridged pyridinyl diradicals, see: (a) Hermolin, J.;
Kosower, E. M. Stable free radicals. 13. Reinvestigation of the
pyridinyl diradical, 1,1'-trimethylenebis(4-(carbomethoxy)pyridinyl).
J. Am. Chem. Soc. 1981, 103, 4813–4816. (b) Muramatsu, T.;
Ikegami, Y.; Hanaya, K.; Onodera, S. The Thermal and
Photochemical Behavior of the Cyclomers Derived from 1,1′-(1,3-
Propanediyl)-bis(pyridinyl) Diradicals. Bull. Chem. Soc. Jpn. 1990,
63, 1413–1421. (c) Itoh, M.; Kosover, E. M. Stable free radicals. IV.
Intramolecular association in pyridinyl diradicals. J. Am. Chem. Soc.
1968, 90, 1843–1849. (d) Tero-Kubota, S.; Sano, Y.; Ikegami, Y.
Monomer-dimer
equilibrium
of
the
1-methyl-2-
methoxycarbonylpyridinyl radical in 2-methyltetrahydrofuran
solution. Kinetic and thermodynamic studies by electron spin
resonance spectroscopy. J. Am. Chem. Soc. 1982, 104, 3711–3714. (e)
Itoh, M. Intramolecular association in pyridinyl cation radicals. J. Am.
Chem. Soc. 1971, 93, 4750-4754.
(26) (a) Eberle, B.; Kaifer, E.; Himmel, H.-J.
A Stable
Hexakis(guanidino)benzene: Realization of the Strongest Neutral
Organic Four‐Electron Donor. Angew. Chem. Int. Ed. 2017, 56, 3360-
3363; for an analysis see: (b) Eberle, B.; Hübner, O. Ziesak, A.;
Kiefer, E.; Himmel, H.-J. What Makes a Strong Organic Electron
Donor (or Acceptor)? Chem. Eur. J. 2015, 21, 8578−8590. For more
examples, see: (c) Porter, W. W. III; Vaid, T. P. Rheingold, A. L.
Synthesis and Characterization of a Highly Reducing Neutral
“Extended Viologen” and the Isostructural Hydrocarbon 4,4‘ꢀ‘‘ꢀ‘-Di-
n-octyl-p-quaterphenyl. J. Am. Chem. Soc. 2005, 127, 16559−16566.
(d) Han, Z.; Vaid, T. P.; Rheingold, A. L. Hexakis(4-(N-
butylpyridylium))benzene:ꢀ A Six-Electron Organic Redox System. J.
Org. Chem. 2008, 73, 445−450.
(27) (a) Janoschka, T.; Martin, N.; Martin, U.; Friebe, C.;
Morgenstern, S.; Hiller, H.; Hager, M. D.; Schubert, U. S. An
aqueous, polymer-based redox-flow battery using non-corrosive, safe,
and low-cost materials. Nature 2015, 527, 78–81. (b) Lin, K.; Chen,
Q.; Gerhardt, M. R.; Tong, L.; Kim, S. B.; Eisenach, L.; Valle, A. W.;
Hardee, D.; Gordon, R. G.; Aziz, M. J.; Marshak, M. P. Alkaline
ACS Paragon Plus Environment

Page 11 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Insert Table of Contents artwork here
Structural distortion
Potential compression
Multi-electron donors
R1
N
R1
R1
N
R
R
R
- e-
1 step
R1
Y
X
- e-
Y
X
Y
X
R2
R2
N
17
18
+ e-
+ e-
R1
N
R2
R2
R1
R1
R1
R
R
R
Stable 1e-
R1 = Ar, Me, H
R2 = Ar, alkyl
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
charge/discharge
cycles!
Stable radicals
cycle
2
cycle
50
cycle
100
ACS Paragon Plus Environment