Electrochemical Strategy for Hydrazine Synthesis: Development and Overpotential Analysis of Methods for Oxidative N-N Coupling of an Ammonia Surrogate

Article abstract of DOI:10.1021/jacs.0c04626

Hydrazine is an important industrial chemical and fuel that has attracted considerable attention for use in liquid fuel cells. Ideally, hydrazine could be prepared via direct oxidative coupling of ammonia, but thermodynamic and kinetic factors limit the v

Full text of DOI:10.1021/jacs.0c04626

Subscriber access provided by Stockholm University Library
Article
Electrochemical Strategy for Hydrazine Synthesis:
Development and Overpotential Analysis of Methods
for Oxidative N–N Coupling of an Ammonia Surrogate
Fei Wang, James B. Gerken, Desiree M. Bates, Yeon Jung Kim, and Shannon S. Stahl
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04626 • Publication Date (Web): 10 Jun 2020
Downloaded from pubs.acs.org on June 10, 2020
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 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Electrochemical Strategy for Hydrazine Synthesis:
Development and Overpotential Analysis of Methods for
Oxidative N–N Coupling of an Ammonia Surrogate
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
Fei Wang, James B. Gerken, Desiree M. Bates, Yeon Jung Kim, and Shannon S. Stahl*
Department of Chemistry, University of Wisconsin - Madison, 1101 University Ave,
Madison, Wisconsin, 53706, United States
Abstract
Hydrazine is an important industrial chemical and fuel that attracted considerable attention for
use in liquid fuel cells. Ideally, hydrazine could be prepared via direct oxidative coupling of
ammonia, but thermodynamic and kinetic factors limit the viability of this approach. The present
study evaluates three different electrochemical strategies for the oxidative homocoupling of
benzophenone imine, a readily accessible ammonia surrogate. Hydrolysis of the resulting
benzophenone azine affords hydrazine and benzophenone, with the latter amenable to recycling.
The three different electrochemical N–N coupling methods include (1) a proton-coupled electron-
transfer process promoted by a phosphate base, (2) an iodine-mediated reaction involving
intermediate N–I bond formation, and (3) a copper-catalyzed N–N coupling process. Analysis of
the thermodynamic efficiencies for these electrochemical imine-to-azine oxidation reactions
reveals low overpotentials (h) for the copper and iodine mediated processes (390 and 470 mV,
respectively), but a much higher value for the proton-coupled pathway (h ~ 1.6 V). A similar
approach is used to assess molecular electrocatalytic methods for electrochemical oxidation of
ammonia to dinitrogen.
1
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 26
1
2
3
4
Introduction
5
6
7
8
Nitrogen-based fuels represent compelling carbon-free alternatives to conventional fossil
fuels for energy production. Ammonia (NH3) is an especially appealing energy carrier owing to its
high energy density and established commercial sources of production and distribution networks.1–
4 These considerations have led to the growing interest in electrocatalytic strategies for ammonia
oxidation to dinitrogen (N2) in an effort to support improved performance of ammonia-based fuel
cells.5–7 Hydrazine is a complementary fuel that has even higher energy density than ammonia,8
and it exhibits excellent performance in chemical and electrochemical power generation.9,10 While
these merits are partially offset by its toxicity and safety hazards, hydrazine-based fuel cells
employing non-precious metal electrocatalysts have already been demonstrated in electric
vehicles.11, 12 The appeal of hydrazine would be further improved if it could be synthesized
efficiently on an as-needed basis. The (electro)catalytic oxidative coupling of ammonia represents
an ideal target, but suitable methods do not exist. Dehydrogenative N–N coupling of ammonia into
hydrazine is very unfavorable (DG° = +44 kcal/mol13,14,15) and would require the use of a strong
oxidant or a suitable electrode potential to promote the reaction. But, any catalyst capable of
promoting ammonia-to-hydrazine conversion will inevitably promote the highly favorable
dehydrogenation of hydrazine to dinitrogen (DG° = –36 kcal/mol13,15, 16 ). A complementary
thermodynamic perspective is provided by the standard potentials for these reactions in aqueous
solution (Figure 1A).17,18 The values show that dehydrogenation of hydrazine is 1.3–1.5 V more
favorable than N–N coupling of ammonia to hydrazine, with the specific value depending on the
protonation states of the different species at different pH. Consistent with these insights, molecular
catalysts that have been reported for electrochemical ammonia oxidation in recent years generate
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
2
ACS Paragon Plus Environment

Page 3 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
dinitrogen as the N–N coupled product, including those proposed to proceed via hydrazine
intermediates. 19–26
7
8
A: Thermodynamic Potentials for N2/N2H4/NH3 Interconversion
9
pH = 0
E° (V vs SHE)
0.274
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
+
+
N2
+
8 H+
+
6 e-
2 NH4
(i)
+
+
+
+
+
2 e-
4 e-
3 H+
5 H+
2 NH4
1.25
(ii)
(iii)
N2H5
+
N2
pH = 10
N2
N2H5
-0.21
E’ (V vs SHE)
-0.501
+
6 H+
+
6 e-
2 NH3
2 NH3
N2H4
(iv)
(v)
+
+
+
+
2 e-
4 e-
2 H+
4 H+
0.347
N2H4
N2
-0.924
(vi)
B: The Hayashi Process
NH
Ph
O
cat. ZnCl2
cat. CuCl
NH3
+
H2O
(vii)
+
Ph
Ph
Ph
Ph
NH
Ph
N
Ph
Ph
(viii)
(ix)
H2O
+
1/2 O2
+
2
N
Ph
Ph
Ph
O
+
+ 2 H2O + H+
+
NH2-NH3
2
Ph
N
N
Ph
Ph
Ph
Ph
C: Electrocatalytic Formal Hydrazine Synthesis
H
N
Ar
2
N
Ar
H2
+
+
Cat.
N
Ar
Ar
Ar
Ar
-
CuII/I
Strategies:
or
or
PCET
with base
NH
Ar
I2/I3
with redox mediator
NH
2
2
Ar
Ar
Ar
2 B
2 B
cat.
+
+
2 BH+
Ar
2 BH+
Ar
Ar
Ar
N
N
N
Ar
N
Ar
Ar
Ar
Figure 1. (Electrochemical) Oxidative N–N bond formation. A: Thermodynamic potentials of
N2/N2H4/NH3 interconversion in aqueous media; B: The Hayashi process for hydrazine synthesis
from ammonia; C: Our electrochemical approaches for formal hydrazine synthesis from ammonia
via catalytic N–N bond formation.
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 26
1
2
3
4
5
6
7
8
9
The considerations outlined above demonstrate that alternative strategies are needed for
electrochemical synthesis of hydrazine, and relevant insights come from industrial chemistry.
Some of the most important industrial processes employ ketones as recyclable 'protecting
groups',27 wherein condensation of ammonia with a ketone affords a primary imine as an ammonia
surrogate with only one N–H bond. Oxidative coupling of the imine affords the N–N-coupled
ketazine, and subsequent hydrolysis affords hydrazine and regenerates the ketone. These
precedents suggested that an analogous electrochemical method could be developed, leading to a
selective N–N coupling method that avoids overoxidation of hydrazine. The Hayashi
process,28,29,30 which employs benzophenone as the ketone protecting group and has been used to
generate hydrazine via the three-step sequence shown in Figure 1B, provided the platform for the
present investigation of electrocatalytic N–N bond formation. The focal point of this process is the
copper-catalyzed aerobic oxidative N–N coupling of benzophenone imine to afford the
corresponding azine. The condensation and hydrolysis steps have also been thoroughly studied.31
Herein, we demonstrate three different approaches to achieve electrochemical oxidative N–N
coupling of benzophenone imine (Figure 1C): (1) a proton-coupled electron-transfer process
promoted by a phosphate base, (2) an iodine-mediated reaction involving intermediate N–I bond
formation, and (3) a copper-catalyzed N–N coupling process, resembling the key sequence
involved in the Hayashi process. To our knowledge, these results represent the first formal
electrocatalytic synthesis of hydrazine from ammonia. The electrochemical data for each of these
processes are then analyzed to determine thermodynamic efficiency for the different approaches,
and the effective overpotentials are compared to other molecular catalyst systems that have been
reported for electrochemical oxidation of ammonia.
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
4
ACS Paragon Plus Environment

Page 5 of 26
Journal of the American Chemical Society
1
2
3
4
Results and Discussion
5
6
Electrolysis Methods for Oxidative Coupling of Benzophenone Imine.
7
8
9
4,4'-Difluorobenzophenone imine (1) was selected as the substrate for the oxidative coupling
studies as it allows straightforward analysis of the reactions by 19F NMR spectroscopy. In a
preliminary test, the bulk electrolysis of 1 was evaluated in acetonitrile under constant current
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
n
conditions, with Bu4NPF6 as supporting electrolyte, graphite rod as working electrode (anode)
and platinum wire as counter electrode (cathode). Only trace amounts of the desired N–N-coupled
azine product 2 was observed, together with a mixture of unidentified side products. It was
expected that 1 could undergo proton-coupled oxidation at the anode to generate an N-centered
radical (Figure 2A), but the substrate imine is the only species capable of serving as the proton
acceptor under the initial conditions. The poor product yield and high anode potential observed
under these conditions (cf. Figure 2B) prompted us to test various Brønsted bases in an effort to
facilitate proton-coupled electron transfer (PCET) at the electrode. The bases included 2,4,6-
collidine, 2,6-lutidine, carboxylates and phosphate (Figure 2C). No product was detected with
carboxylates as the base, while moderate-to-low product yields were observed with 2,4,6-collidine
(35%) and 2,6-lutidine (10%). Significantly improved results were obtained with dibutyl
phosphate, [MeBu3N][OP(O)(OBu)2], as the base, and optimization of the reaction conditions led
to an 84% yield of the desired azine product (Figure 2C, entry 7; see Table S1 of the Supporting
Information for full optimization data). The beneficial effect of the phosphate base is evident in
the cyclic voltammograms (CVs), which show a significant decrease in the anode potential when
the imine and phosphate base are combined. The onset potential drops from 1.2 V (vs Fc+/Fc) with
the imine alone to ~ 0.8 V upon addition of dibutyl phosphate (Figure 2B).
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 26
1
2
3
4
5
6
7
8
A: Oxidative Imine Coupling by Direct ET at the Electrode
NH
2
Ar
Ar
2B
2BH+
Ar
+
Ar
N
dimerization
2
N
N
Ar
Ar
Ar
9
Ar
relatively high concentration
at the electrode
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
B: Cyclic Voltammetry of Imine w/wo Phosphate
100
5 mM Imine
5 mM phosphate
5 mM Imine and 5 mM phosphate
75
H
N
50
Ar
Ar
Ar
B
+
BH+
25
0
N
Ar
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
E (V vs Fc+/Fc)
C: Optimization of A Base-Promoted Electrochemical Imine Coupling
Ar
Base (0.5 equiv),
Ar
NH
Ar
nBu4NPF6 (0.1 M)
2
Ar
N
N
CH3CN/MeOH (0.1 M),
RT, 4 mA, graphite II Pt, 2 F/mol
undivided cell
Ar
+ H2
Ar
Ar = 4-F-Ph
1
2
Yield/%a
Entry
Base
Current/mA
1
2
3
4
5
4
4
4
4
4
4
2
< 3%
n.d.
-
NaOAc
n.d.
4-MeO-PhCO2Na
2,4,6-Collidine
35%
10%
71%
84%
2,6-Lutidine
N(nBu)3MeOP(O)(OBu)2
N(nBu)3MeOP(O)(OBu)2
6
7
Figure 2. Base-Promoted Electrochemical Dehydrogenative Homocoupling of 1. A: Oxidative
imine coupling by direct electrolysis; B: Optimization of a base-promoted electrochemical imine
a
19
coupling. Yields were determined by F NMR analysis with α,α,α-trifluorotoluene as internal
standard.
Historical precedent for N–N coupling of iminyl reagents promoted by stoichiometric
iodine 32 – 34 raised the possibility of an electrochemical process for azine synthesis from
benzophenone imine, using iodine as an electrocatalytic mediator.35 CV studies of iodide showed
two distinct redox features, corresponding to the I– ® I3 and I3 ® I2 redox processes.36 Addition
–
–
of 1 to this mixture led to a distinct increase in current at the second oxidation feature, suggesting
–
that 1 reacts with iodine (but not I3 ) on the CV time scale. For example, the reaction of 1 with I2
is expected to generate an N-iodo imine species,37 and the resulting iodide by-product can undergo
6
ACS Paragon Plus Environment

Page 7 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
oxidation at the electrode and lead to the observed current increase (Figure 3B). Initial attempts to
perform bulk electrolysis with tetrabutylammonium iodide (TBAI) as the mediator in acetonitrile
resulted in only trace product formation (Figure 3C, entry 1; see Table S2 of the Supporting
Information for full optimization data). Inclusion of 8 equiv of MeOH under the conditions,
however, increased the yield to 38% (Figure 3C, entry 2). The use of trifluoroethanol (TFE) or
hexafluoroisopropanol (HFIP) instead of methanol led to inferior results (33% and 19% yield,
respectively (Figure 3C, entries 3-4). Methanol and other alcohols are postulated to serve as proton
donors that facilitate H2 formation at the cathode, and the resulting alkoxide co-products may then
diffuse into the bulk solution where they can serve as bases to promote formation of the N-iodo
imine and azine formation (cf. "B" in Figure 3A).38 Bromide and chloride-based mediators showed
no formation of the desired azine product, even while leading to significant conversion of starting
material (Figure 3C, entry 6-7). KI exhibited even better performance than TBAI, and use of 10
mol % KI with 12 equiv of MeOH led to an 86% yield of the desired azine (Figure 3C, entry 10).
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
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 26
1
2
3
A: A Hypothesis for Iodide-Mediated Imine Coupling
4
5
6
7
8
2B
+ H2
2I-
I2
NH
Ar
Ar
+
-
I-
I-
Ar
Ar
I
N
2BH+
N
N
NH
Ar
Ar
Ar
+
B
BH+
B
BH+
Ar
9
Ar
Ar
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
B: Cyclic Voltammetry of Iodide w/wo Imine
I2
I-
I
NH
Ar
N
50
30
Ar
B
BH+
Ar
Ar
3I-
I3
-
-
2I3
3I2
2.5 mM nBu4NI
10
With 6 equiv Imine
-10
-1
-0.5
0
0.5
1
E (V vs Fc+/Fc)
C: Optimization of A Iodide-Mediated Electrochemical Imine Coupling
Ar
KI (10 mol%), KPF6 (0.1 M)
Ar
NH
Ar
CH3CN/MeOH (0.1 M), RT, graphite II Pt
5 mA, 3.0 F/mol, undivided cell
2
Ar
N
N
Ar
+ H2
Ar
Ar = 4-F-Ph
1
2
Entry
Catalyst
Additive
(8 equiv)
Yield/%a
nBu4NI (5 mol%)
nBu4NI (5 mol%)
nBu4NI (5 mol%)
nBu4NI (5 mol%)
KI (5 mol%)
1
-
< 3%
38%
33%
19%
61%
n.d.
2
MeOH
TFE
3
4
HFIP
5
MeOH
MeOH
MeOH
MeOH
MeOH
6
Et4NBr (5 mol%)
nBu4NCl (5 mol%)
KI (7.5 mol%)
7
n.d.
8b
9b,c
80%
86%
KI (10 mol%)
Figure 3. Iodide-Mediated Electrochemical Dehydrogenative Homocoupling of 1. A: A
hypothesis for iodide-mediated imine coupling; B: Cyclic voltammetry of iodide w/wo imine,
conditions: 2.5 mM nBu4NI in CH3CN (10 mL) with KPF6 (0.1 M) as supporting electrolyte, with
glassy carbon as working electrode (~ 7.0 mm2) and a platinum wire (1.0 cm, spiral wire) as
counter electrode, scan rate = 20 mV/s; C: Optimization of an iodide-mediated electrochemical
a
imine coupling. Yields were determined by 19F NMR analysis with α,α,α-trifluorotoluene as
internal standard; bConsumed charge is 2.2 F/mol; cWith 12 equiv of MeOH as additive.
The Hayashi process28,29 and other recent reports39 , 40 demonstrate that copper catalysts
mediate the oxidative homocoupling of imines under aerobic conditions. Mechanistic studies have
shown that CuII promotes efficient N–N coupling even in the absence of O2 (stoichiometrically),
raising the possibility that Cu salts could serve as effective mediators for electrochemical N–N
8
ACS Paragon Plus Environment

Page 9 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
coupling (Figure 4A). CV analysis of [Cu(CH3CN)4]PF6 in the presence and absence of imine
substrate 1 revealed that the presence of 1 substantially shifted the CuII/I redox potential. Moreover,
the redox features became irreversible together with a current increase upon addition of 1,
implicating the possibility of electrocatalytic turnover (Figure 4B). Pyridine is an effective ligand
in aerobic Cu-catalyzed N–N coupling reactions,39,40 and also proved to be beneficial to bulk
electrolysis reactions by ensuring that the imine substrate is not the sole ligand available for the
Cu catalyst. Electrolysis of 1 in the presence of [Cu(CH3CN)4]PF6 and pyridine led to a low yield
of the azine product (15%, Figure 4C, entry 1). Evaluation of various anionic Brønsted bases (e.g.,
K3PO4, K2CO3, or NaHCO3), however, revealed that substantially improved results could be
obtained with bicarbonate (86%, Figure 4C, entry 4; see Table S3 of the Supporting Information
for details).
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
9
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 26
1
2
3
A: A Hypothesis for Copper-Mediated Imine Coupling
4
5
6
2B:
2BH+
[CuII]
H
N
2
Ar
Ar
7
Ar
Ar
8
9
N
[CuII]
[CuII]
[CuI]
N
H
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
N
2
Ar
Ar
Ar
Ar
NH
Ar
2
2
Ar
B: Cyclic Voltammetry of Cu(I) w/wo Imine
60
With 2 equiv imine
With 10 equiv imine
With 20 equiv imine
40
5 mM Cu(I)
2Imine
20
2B
2BH+
CuII/I
Azine
0
-20
-0.4
-0.2
0
0.2
0.4
E (V vs Fc+/Fc)
0.6
0.8
1
1.2
C: Optimization of A Copper-Mediated Electrochemical Imine Coupling
Ar
[Cu(CH3CN)4]PF6 (20 mol%)
Pyridine (3.0 equiv)
Base (4.0 equiv), KPF6 (0.2 M)
Ar
NH
Ar
2
Ar
N
N
@ 0.1 V vs Fc+/Fc
CH3CN (0.1 M), 45 oC, graphite II Pt
divided cell
Ar
+ H2
Ar
Ar = 4-F-Ph
1
2
Yield/%a
Entry
Base
Pyridine
1
-
3.0 equiv
3.0 equiv
3.0 equiv
3.0 equiv
4.0 equiv
2.0 equiv
3.0 equiv
15%
4%
2
K3PO4
K2CO3
KHCO3
KHCO3
KHCO3
KHCO3
3
34%
86%
86%
75%
77%
4
5
6
7b
Figure 4. Copper-Mediated Electrochemical Dehydrogenative Homocoupling of 1. A: A
hypothesis for copper-mediated imine coupling; B: Cyclic voltammetry of [Cu(CH3CN)4]PF6
w/wo imine, conditions: 5 mM [Cu(CH3CN)4]PF6 in CH3CN (10 mL) with nBu4NPF6 (0.1 M) as
supporting electrolyte, with glassy carbon as working electrode (~ 7.0 mm2) and a platinum wire
(1.0 cm, spiral wire) as counter electrode, scan rate = 20 mV/s; C: Optimization of a copper-
a
19
mediated electrochemical imine coupling. Yields were determined by F NMR analysis with
α,α,α-trifluorotoluene as internal standard; bUse of [Cu(CH3CN)4]PF6 (10 mol%) as mediator.
Analysis of Overpotentials for the Different Electrochemical N–N Coupling Processes
The above results highlight three independent strategies for electrochemical oxidative N–N
coupling of benzophenone imine, and good yields of the azine were obtained in each case.
10
ACS Paragon Plus Environment

Page 11 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Nonetheless, each of the reactions was optimized independently to maximize the yield, and the
different conditions and experimental parameters make it difficult to compare their relative
performance. One important metric that arises from the potential utility of hydrazine as an energy
carrier is the overpotential associated with each reaction, which corresponds to the difference
between the anode potential needed to support electrochemical N–N coupling and the
thermodynamic potential for the reaction under the different reaction conditions.
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
Several recent reports have outlined an effective protocol for evaluating overpotentials for
electrochemical reactions in non-aqueous solutions.17,41,42 The foundation of this approach is
determination of the H+/H2 open-circuit potential (OCP) under catalytic reaction conditions that
employ buffered electrolytes. (Acid/base buffering is essential to establish a stable thermodynamic
reference.) The H+/H2 OCP may be used together with (i) the standard aqueous cell potentials for
the reactions of interest and (ii) the free energies associated with the transfer of molecules from
water to organic solvent to obtain thermodynamic potentials in the organic medium. Miller and
coworkers recently employed this approach to obtain the free energies and thermodynamic
potentials for the equilibria of N2 with NH3 and with N2H4 in acetonitrile (Scheme 1A and 1B).17
These equations may be used to derive the equilibrium potential for NH3 and N2H4 (Scheme 1C),
and
a
similar approach was used to obtain thermodynamic potentials for
N2/hydrazinium/ammonium equilibria, relevant to the analysis below (cf. Scheme S2).43 The
involvement of protons in each of these reactions lead to a Nernstian dependence of the
thermodynamic potential on the pKa of the acid.
Full analysis for the catalytic reactions presented above required determination of the
thermodynamic potentials for the 2e–/2H+ oxidative coupling of imine 1 to azine 2 and the free
energy for azine ammonolysis. The latter was estimated from DFT calculations: ΔG°ammonolysis
=
11
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 26
1
2
3
4
5
6
7
8
9
14.4 kcal/mol (Scheme 1D; B3LYP, 6-311+G(2d,p) basis set, see section 8 of the Supporting
Information for details). Then, the sum of the reactions in Scheme 1C and 1D yields the
azine/imine equilibrium free energy and thermodynamic potential (Scheme 1E).
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 1. Thermodynamics of Interconversion of N2/N2H4/NH3 and Azine/Imine in MeCN
ΔG° = -4.9 + (6*1.364*pKa) kcal/mol
A)
B)
C)
N2
+
6 BH+
+
6 e-
2 NH3
+
+
+
6 B
4 B
2 B
E° = 0.035 - (0.059*pKa) V
ΔG° = 36.7 + (4*1.364*pKa) kcal/mol
E° = -0.398 - (0.059*pKa) V
N2
+
4 BH+
+
4 e-
N2H4
N2H4
+
2 BH+
+
2 e-
2 NH3
ΔG° = -41.6 + (2*1.364*pKa) kcal/mol
E° = 0.902 - (0.059*pKa) V
D)
E)
Azine (2)
+
2 NH3
2 Imine (1) + N2H4
2 Imine (1) + 2 B
ΔG° = 14.4 kcal/mol
Azine (2) + 2 BH+ + 2 e-
ΔG° = -27.2 + (2*1.364*pKa) kcal/mol
E° = 0.59 - (0.059*pKa) V
The thermodynamic data for the azine/imine equilibrium and the previously reported data17
enabled creation of a Pourbaix-type diagram that correlates thermodynamic potentials of the
relevant redox reactions with the electrolyte pKa (Figure 5). Each of the lines exhibits a change in
slope where the protonation state of the substrate and/or product results in a change in the e–/H+
ratio for the overall reaction. For example, the azine/imine potential, which lies between the
N2H4/NH3 and N2/NH3 redox couples, exhibits a change in slope at the pKa of the iminium ion
(pKa = 13.3).
This diagram enables straightforward assessment of the thermodynamic efficiency of the
three electrochemical imine coupling methods presented above. CV studies were performed in
buffered electrolytes with different pKa values at room temperature (i.e., somewhat different
conditions from those shown in Figures 2-4), and E1/2 values were obtained from anodic waves
(see Tables S4-S6 of the Supporting Information). The redox potentials observed for PCET-
mediated N–N coupling exhibits an approximately Nernstian slope of 50 mV/pKa unit, consistent
12
ACS Paragon Plus Environment

Page 13 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
with a one electron-one proton transfer pathway (Figure 5, green squares). The applied potential
is >1.5 V above the thermodynamic azine/imine potential, indicating a very large overpotential for
7
8
9
this pathway. The I2/I3 and CuII/I redox couples are unaffected by the electrolyte pKa because their
–
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
potential-determining steps do not involve protons. In addition, these processes occur at potentials
much lower than that of the PCET process (Figure 5, red triangles and purple diamonds,
respectively). For example, at a pKa of 13.3, corresponding to the pKa of the iminium species 1H+,44
–
the overpotentials for each of the three reactions are 1.6 V (PCET, Figure 2), 0.47 V (I2/I3 , Figure
3) and 0.39 V (CuII/I, Figure 3), respectively.
Ar
Ar
Ar + 2e-
+ 2e-
Ar
+ 2H+
NH
Ar
NH2
N
+ 4H+
N
N
N
2
Ar
2
Ar
Ar
Ar
Ar
Ar
Ar
Eiminium/azine
Eimine/azine
1.5
1
ENH
+
+
/N H
2
4
5
0.5
0
ENH /N H
2 4
3
ENH
+
/N
2
4
-0.5
-1
EN2H
+
/N
5
2
ENH /N
3
2
EN2H /N
4
2
-1.5
5
10
15
pKa
20
25
Figure 5. Pourbaix-type diagram for azine (2)/benzophenone imine (1), N2/NH3/N2H4 species,
and redox potentials for the three electrochemical processes for azine synthesis. Potentials for
processes (gray lines) involving N2 can be found in ref. 17. The blue line is the thermodynamics
of azine 2 and imine 1 equilibrium. Green squares correspond to the PCET potentials with different
buffer solutions: 5 mM 1 in CH3CN (10 mL) with nBu4NPF6 (0.1 M). Red triangles correspond to
–
I2/I3 potentials with different buffer solutions: 5 mM nBu4NI in CH3CN (10 mL) with 50 mM 1
and nBu4NPF6 (0.1 M). Purple diamonds correspond to the relevant CuII/I potentials with different
buffer solutions: 5 mM [Cu(CH3CN)4]PF6 in CH3CN (10 mL) with 100 mM 1 and nBu4NPF6 (0.1
M). All the CV studies were performed with glassy carbon as working electrode (~ 7.0 mm2) and
a platinum wire (1.0 cm, spiral wire) as counter electrode, scan rate = 20 mV/s.
13
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 26
1
2
3
4
5
6
7
8
The high overpotential associated with the PCET pathway is attributed to generation of a
high-energy iminyl radical prior to N–N coupling. Although the presence of a Brønsted base
supports a PCET pathway, allowing the process to proceed at a lower potential than in the absence
of added base, the applied potential is still much higher than the reactions catalyzed by I2 and CuII.
The mechanisms with I2 and CuII avoid high-energy intermediates by bonding/coordination of
iminyl intermediates to iodine and copper.32,36,40
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
The data in Figure 5 show that the potentials for the I2/I3 and CuII/I processes approach the
–
+
+
thermodynamic potential for oxidation of NH4 to N2H5 at pKa = 13.3. This observation suggests
+
the possibility of electrochemical oxidation of NH4 with these catalysts; however, I2 reacts with
ammonia to generate the explosive compound NI3,45 while CuII forms Cu/ammine complexes with
+ 46
much lower redox potentials that will be incapable of generating N2H4 or N2H5 . Moreover, the
direct formation of hydrazine as a product will likely result in rapid oxidation of hydrazine to N2,
which is favoured by >1.25 V (cf. Figure 1A). These considerations highlight the strategic benefit
of using benzophenone imine as an ammonia surrogate as it supports formation of a single N–N
bond and allows the process to proceed at a potential that preserves much of the energy stored in
hydrazine.
Comparison with Other Molecular Electrocatalysts for Oxidative N–N Coupling
As noted in the Introduction, a number of transition metal complexes have been identified as
catalysts for ammonia oxidation to N2.21–26 Meyer and coworkers demonstrated stoichiometric
oxidation of Ru-polypyridyl ammine complexes into metal-imido and/or metal nitride species that
lead to N–N coupling and release of N2.19,20 This work provided the basis for the first molecular
14
ACS Paragon Plus Environment

Page 15 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
electrocatalysts for NH3 oxidation to N2. In 2019, Hamann and Smith showed that the mononuclear
Ru complex [(tpy)(dmabpy)RuII(NH3)](PF6)2 (tpy = 2,2':6',2''-terpyridine; dmabpy = 4,4'-
bis(dimethylamino)-2,2'-bipyridine) is an effective electrocatalyst in THF.21 Independently,
Nishibayashi and Sakata demonstrated catalytic NH3 oxidation with a mononuclear Ru complex
(bpy-dicarboxylate)RuL2 (bpy-dicarboxylate = 2,2'-bipyridyl-6,6'-dicarboxylate, L = isoquinoline)
in acetonitrile.24 Although the majority of studies were conducted with a triarylaminium-based
oxidant, CV studies provided evidence for electrocatalysis. More recently, Warren26 and Peters23
demonstrated iron-based catalysts for electrochemical ammonia oxidation in acetonitrile. Peters
and co-workers used an FeII(TPA) complex, [FeII(TPA)(NH3)2](OTf)2, [TPA = tris(2-
pyridylmethyl)amine], as the catalyst, while Warren and coworkers showed that ferrocene (Fc) is
an effective electrocatalyst. Each of these catalytic reactions showed exquisite formation of N2 as
the N–N coupling product, consistent with the much higher reactivity of N2H4 (and N2H2), relative
to NH3, if they are formed as intermediates.
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
Analysis of the overpotential has not been rigorously addressed in these reactions, in part,
due to the lack of buffered reaction conditions24,26 or the absence of a suitable thermodynamic
references.21 Nonetheless, reasonable estimates may be obtained by employing an analysis similar
to that described above. In order to estimate the overpotential for the Ru catalyst system reported
by Hamann and Smith, the H+/H2 OCP was measured in THF under the reported conditions (0.20
M NH4PF6 , 0.34 M NH3 in THF; see section 9 in the Supporting Information for details).21 The
resulting value, –0.86 V vs. Fc+/Fc, was then used to determine the thermodynamic N2/NH3
potential (–0.81 V vs. Fc+/Fc), and the difference between this value and the applied electrolysis
potential (0.20 V vs. Fc+/Fc) reveals an effective overpotential of ~ 1.0 V. The other three catalyst
systems were used in acetonitrile, allowing literature data17 to be used to estimate the N2/NH3
15
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 16 of 26
1
2
3
4
potential; however, only the Fe(TPA)-catalyzed reaction was conducted in buffered electrolyte
5
6
+
(i.e., with both NH4 and NH3 present).23 In this case, the catalytic onset potential of 0.70 V vs.
7
8
9
Fc+/Fc may be compared to N2/NH3 potential of –0.94 V in acetonitrile at the NH4 /NH3 electrolyte
+
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
pKa of 16.5 (cf. Figure 5) to obtain an estimated overpotential of ~1.6 V. The analysis is more
complicated for the other two reactions due to the lack of buffered electrolyte.24,26 Nevertheless,
overpotentials of 0.94 and 1.1 V vs. Fc+/Fc are estimated for the ammonia oxidation reactions
catalyzed by ferrocene and (bpy-dicarboxylate)RuL2 by assuming an electrolyte pKa of 16.5.
Table 1. Overpotential Comparison of Molecular Electrocatalysts for N–N Coupling.
2 NH3
6 e-
+
6 H+
+
N2
E° (N2/NH3)
CV/Electrolysis
Potential (V) a
Electrocatalyst/Mediator
Solvent
Ref.
h (V)
(V)a @ pKa = 16.5
[(trpy)(dmabpy)RuII(NH3)](PF6)2
[(bpy-dicarboxylate)RuL2]
Ferrocene
THF
–0.81
–0.94
–0.94
–0.94
0.20
0.20
0
1.0
1.1
21
24
26
23
CH3CN
CH3CN
CH3CN
0.94
1.6
[FeII(TPA)(NH3)2](OTf)2
0.7
2 Imine
2 e-
+
2 H+
+
Azine
E° (Azine/Imine)
(V)a @ pKa = 13.3
CV/Electrolysis
Potential (V) a
Electrocatalyst/Mediator
Solvent
Ref.
h (V)
Base/PCETb
Iodide
CH3CN
CH3CN
CH3CN
–0.19
–0.19
–0.19
1.45
0.28
0.20
1.6
this
work
0.47
[Cu(CH3CN)]PF6
0.39
aPotentials are reported vs. Fc+/Fc. b At pKa = 13.3, the imine substrate is the Brønsted base that promotes the PCET
initiated N–N coupling reaction.
In spite of the approximate nature of the overpotential estimates for the ammonia oxidation
catalysts, the values represent relevant benchmarks for consideration in future studies,47 and they
also provide relevant points for comparison with the imine oxidative coupling methods described
here. The PCET-based N–N coupling method mediated by a Brønsted base exhibits an
overpotential of 1.6 V, which is as large or larger than that observed for the ammonia oxidation
16
ACS Paragon Plus Environment

Page 17 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
reactions. In contrast, the I2 and Cu-catalyzed methods feature a comparatively low overpotential,
presumably reflecting their ability to stabilize reactive intermediates (i.e., an iminyl radical).
Future progress toward lower overpotential ammonia oxidation will presumably arise from
application of similar concepts, for example, identifying catalysts that can stabilize relatively high-
energy intermediates such as hydrazine- and/or azine-derived species.
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
The I2 and Cu-catalyzed methods for oxidative N–N coupling of imines operate at potentials
1.03 V and 0.95 V, respectively, above the thermodynamic N2/NH3 potentials. The similarity
between these values and the overpotentials associated with the ammonia oxidation catalysts in
Table 1 is probably not a coincidence and may be rationalized by the need for each of these
processes to access a similarly reactive species capable of forming an N–N bond. The lower
overpotential in the present system reflects its ability to stop at a high energy azine product without
cascading further to the lower energy N2 product.
Conclusion
In this study, we have demonstrated three complementary methods to achieve electrochemical
N–N coupling of benzophenone imine. Use of the imine as an ammonia surrogate ensures that the
reaction stops at formation of a single N–N bond, avoiding the kinetically facile and
thermodynamically favorable oxidation of hydrazine to dinitrogen that would occur in reactions
that use ammonia as a feedstock. By leveraging known processes in the chemical industry, these
results establish a formal strategy for electrochemical synthesis of hydrazine from ammonia.
Analysis of the overpotentials for the three different processes shows that the Cu-catalyzed
reaction exhibits good thermodynamic efficiency, establishing an important foundation for
17
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 18 of 26
1
2
3
4
5
6
development of improved catalysts and potential consideration of methods for practical
implementation of the concepts described herein.
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
ASSOCIATED CONTENT
Supporting Information.
Complete experimental details, procedures for bulk electrolysis and CV data and analysis, and
DFT calculation (PDF).
AUTHOR INFORMATION
Corresponding Author
* stahl@chem.wisc.edu
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy
Sciences, under Award #DE-FG02-05ER15690. FW was partially supported by a postdoctoral
fellowship from SIOC. Spectroscopic instrumentation was partially supported by a generous gift
from Paul J. and Margaret M. Bender, the NIH (1S10 OD020022-1) and the NSF (CHE-1048642).
The computational cluster at UW-Madison is funded by the NSF (CHE-0840494).
References
1. Erisman, J. W.; Sutton, M. A.; Galloway, J.; Klimont, Z.; Winiwarter, W. How a century of
ammonia synthesis changed the world. Nat. Geosci. 2008, 1, 636-639.
2. Lan, R.; Tao, S. Ammonia as a suitable fuel for fuel cells. Front. Energy Res. 2014, 2, 35.
3. Afif, A.; Radenahmad, N.; Cheok, Q.; Shams, S.; Kim, J. H.; Azad, A. K. Ammonia-fed fuel
18
ACS Paragon Plus Environment

Page 19 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
cells: a comprehensive review. Renewable Sustainable Energy Rev. 2016, 60, 822-835.
4 . Cheddie, D. Ammonia as a hydrogen source for fuel cells: A review. In Hydrogen
Energy−Challenges and Perspectives; Minić, D., Ed.; INTECH: Rijeka, Croatia, 2012; pp 333-
362.
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
5. Klerke, A.; Christensen, C. H.; Nørskov, J. K.; Vegge, T. Ammonia for hydrogen storage:
challenges and opportunities. J. Mater. Chem. 2008, 18, 2304-2310.
6. Adli, N. M.; Zhang, H.; Mukherjee, S.; Wu, G. Review-ammonia oxidation electrocatalysis
for hydrogen generation and fuel cells. J. Electrochem. Soc. 2018, 165, J3130-J3147.
7. Schüth, F.; Palkovits, R.; Schlögl, R.; Su, D. S. Ammonia as a possible element in an energy
infrastructure: catalysts for ammonia decomposition. Energy Environ. Sci. 2012, 5, 6278-6289.
8. Rothgery, E. F. Hydrazine and its derivatives, In Kirk-Othmer Encyclopedia of Chemical
Technology; John Wiley & Sons: New York, 2004; Vol. 13, pp 562-607.
9. Serov, A.; Kwak, C. Direct hydrazine fuel cells: A review. Appl. Catal. B 2010, 98, 1-9.
10. Sanabria-Chinchilla, J.; Asazawa, K.; Sakamoto, T.; Yamada, K.; Tanaka, H.; Strasser, P.
Noble metal-free hydrazine fuel cell catalysts: EPOC effect in competing chemical and
electrochemical reaction pathways. J. Am. Chem. Soc. 2011, 133, 5425-5431.
11. Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.; Kobayashi, T. A platinum-
free zero-carbon-emission easy fuelling direct hydrazine fuel cell for vehicles. Angew. Chem. Int.
Ed. 2007, 46, 8024-8027.
12. Serov, A.; Padilla, M.; Roy, A. J.; Atanassov, P.; Sakamoto, T.; Asazawa, K.; Tanaka, H.
Anode catalysts for direct hydrazine fuel cells: From laboratory test to an electric vehicle. Angew.
19
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 26
1
2
3
4
5
6
Chem. Int. Ed. 2014, 53, 10336-10339.
7
8
9
13. For the condensed phase thermochemistry data of hydrazine, see:
https://webbook.nist.gov/cgi/cbook.cgi?ID=C302012&Units=SI&Mask=2#Thermo-Condensed
(accessed April 19, 2020).
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
14. For the gas phase thermochemistry data of ammonia, see:
https://webbook.nist.gov/cgi/cbook.cgi?ID=C7664417&Units=SI&Mask=1#Thermo-Gas
(accessed April 19, 2020).
15. For the gas phase thermochemistry data of hydrogen, see:
https://webbook.nist.gov/cgi/cbook.cgi?Name=hydrogen&Units=SI (accessed April 19, 2020).
16. For the gas phase thermochemistry data of nitrogen, see:
https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379&Units=SI&Mask=1#Thermo-Gas
(accessed April 19, 2020).
17. Lindley, B. M.; Appel, A. M.; Krogh-Jespersen, K.; Mayer, J. M.; Miller, A. J. M.
Evaluating the thermodynamics of electrocatalytic N2 reduction in acetonitrile. ACS Energy Lett.
2016, 1, 698-704.
+
18. The redox potential for the N2/N2H5 couple in Figure 1A(iii) (–0.21 V) deviates somewhat
from the value reported in Table 1 of ref. 17 (–0.23 V); however, it matches that in the Supporting
Information of the same publication. The redox potential –0.21 V is the correct value on the basis
of standard heats of formation and entropies for the different reaction components.
19 . Ishitani, O.; White, P. S.; Meyer, T. J. Formation of dinitrogen by oxidation of
[(bpy)2(NH3)RuORu(NH3)(bpy)2]4+. Inorg. Chem. 1996, 35, 2167-2168.
20
ACS Paragon Plus Environment

Page 21 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
20. Ishitani, O.; Ando, E.; Meyer, T. J. Dinitrogen formation by oxidative intramolecular N–N
coupling in cis,cis-[(bpy)2(NH3)RuORu(NH3)(bpy)2]4+. Inorg. Chem. 2003, 42, 1707-1710.
21 . Habibzadeh, F.; Miller, S. L.; Hamann, T. W.; Smith III, M. R. Homogeneous
electrocatalytic oxidation of ammonia to N2 under mild conditions. Proc. Natl. Acad. Sci. U.S.A.
2019, 116, 2849-2853.
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
22. Bhattacharya, P.; Heiden, Z. M.; Chambers, G. M.; Johnson, S. I.; Bullock, R. M.; Mock,
M. T. Catalytic ammonia oxidation to dinitrogen by hydrogen atom abstraction. Angew. Chem. Int.
Ed. 2019, 131, 11744-11750.
23. Zott, M. D.; Garrido-Barros, P.; Peters, J. C. Electrocatalytic ammonia oxidation mediated
by a polypyridyl iron catalyst. ACS Catal. 2019, 9, 10101-10108.
24. Nakajima, K.; Toda, H.; Sakata, K.; Nishibayashi, Y. Ruthenium-catalysed oxidative
conversion of ammonia into dinitrogen. Nat. Chem. 2019, 11, 702-709.
25. Dunn, P. L.; Johnson, S. I.; Kaminsky, W.; Bullock, R. M. Diversion of catalytic C−N bond
formation to catalytic oxidation of NH3 through modification of the hydrogen atom abstractor. J.
Am. Chem. Soc. 2020, 142, 3361-3365.
26. Boroujeni, M. R.; Greene, C.; Bertke, J. A.; Warren, T. H. Chemical and electrocatalytic
ammonia oxidation by ferrocene. ChemRxiv 2019, doi.org/10.26434/chemrxiv.9729635.v1.
27 . Schirmann, J.-P.; Bourdauducq, P. Hydrazine. Ullmann's encyclopedia of industrial
chemistry; Wiley-VCH: Weinheim, Germany, 2002; pp 79-96.
28. Hayashi, H. Hydrazine synthesis by a catalytic oxidation process. Catal. Rev. 1990, 32,
229-277.
21
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 26
1
2
3
4
5
6
7
8
29. Hayashi, H.; Kainoh, A.; Katayama, M.; Kawasaki, K.; Okazaki, T. Hydrazine production
from ammonia via azine. Ind. Eng. Chem. Prod. Res. Dev. 1976, 15, 299-303.
30. A commercial ketazine process uses methyl ethyl ketone and proceeds in a "one-pot"
process involving oxidation of the corresponding imine with H2O2 to generate an oxaziridine,
followed by reaction with ammonia to generate a hydrazone. The Hayashi process involves Cu-
catalyzed N–N homocoupling of benzophenone imine. The latter process has not yet been
commercialized, primarily reflecting challenges in product separation and benzophenone recycling.
In principle, electrochemical N–N coupling could employ various primary imine precursors;
however, imines with a-C–H bonds, such as that derived from methyl ethyl ketone, can
tautomerize to enamines and undergo decomposition. This consideration prompted us to select
benzophenone imine as the ammonia surrogate for the present study. Further context for these
considerations is elaborated in ref. 28.
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
31. Hayashi, H.; Somei, J.; Okazaki, T. Ammonia-hydrazine conversion processes XVI.
Conversion of benzophenone azine into hydrazine catalyzed by sulphonic acids in a two-phase
system. Appl. Catal. 1988, 41, 213-224.
32. Morton, A. A.; Stevens, J. R. Condensations by sodium instead of by the Grignard reaction.
II. Reaction with benzonitrile. Preparation of diphenylketazine. J. Am. Chem. Soc. 1931, 53, 2769-
2772.
33. Song, L.; Tian, X.; Lv, Z.; Li, E.; Wu, J.; Liu, Y.; Yu, W.; Chang, J. I2/KI-Mediated oxidative
N−N bond formation for the synthesis of 1,5-fused 1,2,4-triazoles from N-aryl amidines. J. Org.
Chem. 2015, 80, 7219-7225.
22
ACS Paragon Plus Environment

Page 23 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
34. Guo, Q.; Lu, Z. Recent advances in nitrogen–nitrogen bond formation. Synthesis 2017, 49,
7
8
9
3835-3847.
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
35. For a recent review on iodide-mediated electroorganic synthesis, see: Liu, K.; Song, C.;
Lei, A. Recent advances in iodine mediated electrochemical oxidative cross-coupling. Org. Biomol.
Chem. 2018, 16, 2375-2387.
36. Wang, F.; Stahl, S. S. Merging photochemistry with electrochemistry: functional-group
tolerant electrochemical amination of C(sp3)–H Bonds. Angew. Chem. Int. Ed. 2019, 58, 6385-
6390.
37. Bosnidou, A. E.; Duhamel, T.; Muñiz, K. Detection of the elusive nitrogen-centered
radicals from catalytic Hofmann–Löffler reactions. Eur. J. Org. Chem. 2019, DOI:
10.1002/ejoc.201900497.
38. Lennox, A. J. J.; Goes, S. L.; Webster, M. P.; Koolman, H. F.; Djuric, S. W.; Stahl, S. S.
Electrochemical aminoxyl-mediated α-cyanation of secondary piperidines for pharmaceutical
building block diversification. J. Am. Chem. Soc. 2018, 140, 11227-11231.
39. Laouiti, A.; Rammah, M. M.; Rammah, M. B.; Marrot, J.; Couty, F.; Evano, G. Copper-
catalyzed oxidative alkynylation of diaryl imines with terminal alkynes: A facile synthesis of
ynimines. Org. Lett. 2012, 14, 6-9.
40. Ryan, M. C.; Kim, Y.-J.; Gerken, J. B.; Wang, F.; Aristov, M. M.; Martinelli, J. R.; Stahl,
S. S. Mechanistic insights into copper-catalyzed aerobic oxidative coupling of N–N bonds. Chem.
Sci. 2020, 11, 1170-1175.
41. Roberts, J. A. S.; Bullock, R. M. Direct determination of equilibrium potentials for
23
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 24 of 26
1
2
3
4
5
6
7
8
hydrogen oxidation/production by open circuit potential measurements in acetonitrile. Inorg.
Chem. 2013, 52, 3823-3835.
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
42. Pegis, M. L.; Roberts, J. A. S.; Wasylenko, D. J.; Mader, E. A.; Appel, A. M.; Mayer, J. M.
Standard reduction potentials for oxygen and carbon dioxide couples in acetonitrile and N,N-
dimethylformamide. Inorg. Chem. 2015, 54, 11883-11888.
43. In principle, diprotonated hydrazine should also be included in the diagram in Figure 5;
2+
2+
however, the pKa of N2H6 in acetonitrile has not been reported. The aqueous pKa of N2H6 is
estimated to be in the range of -0.5 to 0.27, according to the following studies: a) Doherty, A. M.
M.; Dadcliffe, M. D.; Stedman, G. Kinetics of oxidation of nitrogen compounds by cerium (IV).
J. Chem. Soc., Dalton Trans. 1999, 3311-3316; b) Rich, L. R. Inorganic Reactions in Water;
Springer-Verlag: Berlin, Heidelberg, 2007: Section 1.
44. If there is no stronger base, the benzophenone imine will serve as the Brønsted base. The
iminium pKa value was obtained by 1H NMR spectroscopy using acids of known pKa close to that
of the iminium (see Table S6 of the Supporting Information). Reference acid pKa values are from:
Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.; Leito, I.; Koppel, I. A. Extension of
the self-consistent spectrophotometric basicity scale in acetonitrile to a full span of 28 pKa units:
unification of different basicity scales. J. Org. Chem. 2005, 70, 1019-1028.
45. Watt, G. W.; Foerster, D. R. Reactions of iodine in liquid ammonia. J. Inorg. Nucl. Chem.
1960, 13, 313-317.
46. Sun, Z.; Cao, H.; Venkatesan, P.; Jin, W.; Xiao, Y.; Sietsma, J.; Yang, Y. Electrochemistry
during efficient copper recovery from complex electronic waste using ammonia based solutions.
24
ACS Paragon Plus Environment

Page 25 of 26
Journal of the American Chemical Society
1
2
3
4
5
6
Front. Chem. Sci. Eng. 2017, 11, 308-316.
7
8
9
47. The method for estimating overpotentials employed here is considerably more reliable than
approximations based on adapting aqueous thermodynamic and/or reference potentials to reactions
conducted in organic solvent. For example, use of the latter method led to an overpotential estimate
approximately 400 mV lower than that in Table 1 for the (bpy-dicarboxylate)RuL2 system (cf. ref.
24).
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
25
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 26 of 26
1
2
3
4
TOC Graphic
5
6
7
8
direct
N2H4
H2
+
+
2 NH3
electrooxidation
X
H
Ar
N
2
N
Ar
H2
+
+
Cat.
9
N
Ar
Ar
Ar
Ar
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
Electrocatalysis
Strategy:
-
CuII/I
or
or
PCET
I2/I3
Overpotential (η): 1.6 V
0.47 V
0.39 V
1
ACS Paragon Plus Environment