Mediator-Enabled Electrocatalysis with Ligandless Copper for Anaerobic Chan-Lam Coupling Reactions

Article abstract of DOI:10.1021/jacs.1c02103

Simple copper salts serve as catalysts to effect C-X bond-forming reactions in some of the most utilized transformations in synthesis, including the oxidative coupling of aryl boronic acids and amines. However, these Chan-Lam coupling reactions have historically relied on chemical oxidants that limit their applicability beyond small-scale synthesis. Despite the success of replacing strong chemical oxidants with electrochemistry for a variety of metal-catalyzed processes, electrooxidative reactions with ligandless copper catalysts are plagued by slow electron-transfer kinetics, irreversible copper plating, and competitive substrate oxidation. Herein, we report the implementation of substoichiometric quantities of redox mediators to address limitations to Cu-catalyzed electrosynthesis. Mechanistic studies reveal that mediators serve multiple roles by (i) rapidly oxidizing low-valent Cu intermediates, (ii) stripping Cu metal from the cathode to regenerate the catalyst and reveal the active Pt surface for proton reduction, and (iii) providing anodic overcharge protection to prevent substrate oxidation. This strategy is applied to Chan-Lam coupling of aryl-, heteroaryl-, and alkylamines with arylboronic acids in the absence of chemical oxidants. Couplings under these electrochemical conditions occur with higher yields and shorter reaction times than conventional reactions in air and provide complementary substrate reactivity.

Full text of DOI:10.1021/jacs.1c02103

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Mediator-Enabled Electrocatalysis with Ligandless Copper for
Anaerobic Chan−Lam Coupling Reactions
Benjamin R. Walker,§ Shuhei Manabe,§ Andrew T. Brusoe, and Christo S. Sevov*
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ABSTRACT: Simple copper salts serve as catalysts to effect C−X
bond-forming reactions in some of the most utilized transformations
in synthesis, including the oxidative coupling of aryl boronic acids
and amines. However, these Chan−Lam coupling reactions have
historically relied on chemical oxidants that limit their applicability
beyond small-scale synthesis. Despite the success of replacing strong
chemical oxidants with electrochemistry for a variety of metal-
catalyzed processes, electrooxidative reactions with ligandless
copper catalysts are plagued by slow electron-transfer kinetics,
irreversible copper plating, and competitive substrate oxidation.
Herein, we report the implementation of substoichiometric
quantities of redox mediators to address limitations to Cu-catalyzed
electrosynthesis. Mechanistic studies reveal that mediators serve multiple roles by (i) rapidly oxidizing low-valent Cu intermediates,
(ii) stripping Cu metal from the cathode to regenerate the catalyst and reveal the active Pt surface for proton reduction, and (iii)
providing anodic overcharge protection to prevent substrate oxidation. This strategy is applied to Chan−Lam coupling of aryl-,
heteroaryl-, and alkylamines with arylboronic acids in the absence of chemical oxidants. Couplings under these electrochemical
conditions occur with higher yields and shorter reaction times than conventional reactions in air and provide complementary
substrate reactivity.
Rh,51 Ir,52 Cu,53−58 and Pd.59 However, catalysts for all of
these reactions are metal complexes with strongly chelating
ligands. While such ligands serve to modulate redox potentials,
improve ET kinetics, and inhibit metal aggregation, some of
INTRODUCTION
■
Reactions that form challenging C−X bonds, including C−N,
C−F, C−O, and C−CF3, are critically important for the
discovery and design of drug-related molecules.1,2 These
the most common oxidative transformations such as Pd-
catalyzed C−H oxygenation or Cu-catalyzed coupling require
“ligandless” metal halides or acetates as catalysts.60−72
Replacing prohibitive chemical oxidants in these trans-
formations through electrooxidation remains an unmet goal
because ligandless metal salts can undergo competing
reduction and plating at the cathode (Figure 1a).
Separation of the anodic reaction from the cathodic reaction
with a divided cell is one strategy for protecting the
homogeneous catalyst. While the use of divided cells poses
some limitations to the general synthetic community, the
approach has been successfully applied to electrooxidative
Wacker73,74 or C−H acetoxylation75−77 reactions catalyzed by
simple Pd(OAc)2 salts. Copper salts can similarly be confined
coupling reactions have long relied on the judicious design
of ligands that accelerate rates of reductive elimination from
organometallic intermediates.3−9 Recent approaches leverage
the reactivity of high-valent organometallic complexes bearing
inexpensive ligands to effect similar reactions.10−14 However,
accessing these high-valent organometallic complexes typically
requires oxidation with superstoichiometric quantities of
peroxides,15,16 N-fluoropyridiniums,12,17−19 xenon difluor-
ide,20,21 silver salts,22 hypervalent iodide,23 or pressurized
oxygen.24,25 Efforts to mitigate the hazards and wastes
associated with strong chemical oxidants have focused on
new strategies for electron transfer (ET) and oxidation,
including photoredox catalysis26−30 and electrosynthesis.31−40
Electrochemistry is particularly well-suited for net oxidative (or
reductive) organic transformations because reactions can be
designed for the anode while providing a benign reagent, like a
proton, for consumption at the opposite electrode. In doing so,
electrical energy and a proton can replace energetic chemical
oxidants.41
Received: February 23, 2021
Published: April 16, 2021
These advantages have motivated the recent development of
a wide range of catalytic electrooxidative methodologies that
employ complexes of Co,42,43 Fe,44 Ni,45,46 Ru,47 Mn,48−50
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RESULTS AND DISCUSSION
■
The importance of the Chan−Lam coupling reaction in
organic synthesis has motivated detailed mechanistic studies
that guided our reaction design.80,88−90 Previous reports
implicate a bimolecular disproportionation of CuII as a critical
step to forming a putative CuIII intermediate that undergoes
bond-forming C−N reductive elimination. As such, we sought
to establish an electrochemical system that could maintain high
concentrations of CuII without molecular oxygen or other
chemical oxidants. We envisioned three challenges that would
have to be addressed. First, CuII is easily reduced at an onset of
−0.8 V (vs Fc/Fc+) and outcompetes the desired cathodic
reaction of proton reduction in nonaqueous solvents (Figure
2a). The resulting CuI species can be further reduced to plate
Figure 1. (a) Challenges associated with electrooxidative, ligandless
transformations. (b) Redox mediator-enabled ligandless copper
Chan−Lam coupling (this work).
to an anodic chamber to protect from reductive plating but
have very poor kinetics for electrooxidation, allowing for
competitive oxidation of substrates. Because of these combined
challenges, there are no examples of electrochemical reactions
catalyzed by simple Cu salts in the absence of added chemical
oxidants.
This work details the development of electrooxidative C−N
coupling reactions in an undivided cell with simple Cu salts as
catalyst and a redox mediator that serves to (i) oxidize CuI to
the active CuII form, (ii) strip plated Cu0 to regenerate the
homogeneous Cu catalyst, and (iii) protect substrates from
oxidation. This general approach for protecting ligandless
metal catalysts with a mediator is successfully applied to the
Chan−Lam cross-coupling of (hetero)aryl or alkyl amines with
aryl boronic acids (Figure 1b). The electrochemical reactions
are scalable and typically occur with higher rates than the
conventional reactions under air. The battery-inspired
mediator is critically important, as electrochemical reactions
in its absence fail to generate product even in a divided cell.
Previous efforts to replace chemical oxidants with alternative
redox mechanisms have addressed some but not all of the
listed challenges. Chan−Lam coupling has been reported using
photoredox cocatalysts78 or electrochemical systems with Cu
electrodes as both the anode and cathode to sustain the
homogeneous catalyst by continuously stripping Cu metal.55,56
However, all reported systems rely on molecular oxygen and
are low-yielding in the absence of exogenous oxidants.
Application of a mediator-assisted strategy toward the
development of an electrochemical Chan−Lam coupling
reaction addresses the long-standing challenge of eliminating
oxygen or peroxide additives from this important trans-
formation.61,65−67,79,80 The requirement for hazardous oxidants
has precluded the use of Chan−Lam couplings on a large scale,
despite their ubiquitous application in drug discovery and
small-scale synthesis.24,81−84 The limited scale on which
Chan−Lam reactions are currently performed is particularly
frustrating because the reactions offer an easily accessible
approach to forming important C−N bonds with an
inexpensive copper salt as catalyst and abundant substrates.
As such, this work represents not only a rare example of
electrocatalysis with simple copper salts85−87 in an undivided
cell but alsoto the best of our knowledgethe first example
of Chan−Lam coupling to forge C−N bonds in the absence of
a stoichiometric oxidant.
Figure 2. (a) CV of Cu(OAc)2 (black, 20 mM) and aniline (red, 100
mM). (b) Targeted synergy of redox mediators and copper
electrocatalysts. CV conditions: 0.1 M KPF6 in MeCN, 100 mV/s
scan rate, room temperature, glassy carbon working electrode (WE)
and Pt counter electrode (CE).
Cu0 at the cathode or can undergo disproportionation with a
second CuI species to generate Cu0 metal and CuII, which can
be reduced further. Second, electrooxidation of ligandless CuI
to the desired CuII species is kinetically slow.55 The cyclic
voltammogram (CV) of Cu(OAc)2 reproduced in Figure 2a
reveals a reduction of CuII but no subsequent reoxidation on
the return scan (black trace). The only observed anodic peak is
the result of Cu0 stripping that had plated on the electrode
during the reductive sweep. Third, the slow and poorly defined
oxidation of CuI allows competitive oxidation and degradation
of the amine substrates (red trace).
These limitations directed our evaluation of redox-active
compounds that can assist electrochemically slow redox
processes of simple metal salts. Recently, redox mediators
have come to the forefront of electrocatalysis as a means to
mediate homogeneous ET, most commonly with an organic
substrate.91−95 Indirect redox of organic substrates does not
rely on ET directly at a heterogeneous surface, which often
leads to degradation. In analogy to these strategies, we
hypothesized that mediators could be employed to promote
the redox processes of a Cu catalyst, which similarly suffers
from problematic ET reactions at heterogeneous surfaces. With
properly tuned redox potentials and ET kinetics, the mediator
can undergo rapid oxidation at the anode and (i)
homogeneously oxidize CuI to CuII, (ii) oxidatively strip
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plated Cu0 to regenerate catalyst, and (iii) protect amine
substrates from oxidative degradation (Figure 2b).
Redox Mediator Design. We first evaluated a wide range
of conditions that are commonly employed for Cu-catalyzed
Chan−Lam coupling reactions but in an inert atmosphere and
without a redox mediator (Figure 3). As expected, electrolysis
Fc+) allow for competitive oxidation of the aniline (Eonset = 0.4
V).
Despite these promising yields, reactions with ferrocene
failed to reach full conversion regardless of the electron
equivalents added, which suggests that degenerate redox
shuttling occurs between the electrodes. Rather than relying
on electrochemistry to generate significant quantities of the
oxidized mediator, we instead employed the preoxidized
mediator as the starting material. Reactions with 20 mol %
ferrocenium hexafluorophosphate (Fc+) occurred with com-
plete conversion, and the coupled product was isolated in 81%
yield. Reactions with oxidized forms of mediators Fc-1 through
Fc-3 similarly formed coupled products in high yields, but the
unfunctionalized Fc+ was selected as the reaction mediator
because of its commercial availability, ease of handling, and
synthetic accessibility.
Reaction Development. With the standard conditions
summarized in Table 1, arylamine products could be prepared
a b
,
Table 1. Reaction Development
entry
deviation from standard
% conversion 1 % yield 3
1
2
3
4
5
6
7
8
none
92
77
66
7
16
14
29
88
82
8
92 (81)
2
60
7
0
14
25
77
73
8
34
2
73
no Fc+PF6
−
excluding Et3N
excluding NaOAc
excluding Cu(OAc)2
excluding e-chem
( ) Ni foam
(+)RVC/(−)Pt
under air
under air, no e-chem, no Fc+PF6
under air, no e-chem
Figure 3. Evaluation of ferrocenyl mediators for the electrocatalytic
reaction. Yields were determined by calibrated GC analysis with
dodecane as an internal standard.
9
−
10
11
12
13
of these solutions in both undivided and divided cells resulted
in trace yields (<5%). Our experience merging battery
chemistries with electrosynthesis for overcharge protection
and redox shuttling further guided the reaction design.41
Specifically, Chan−Lam reactions were conducted with redox
shuttles that undergo oxidation between −0.6 and +0.2 V (vs
Fc/Fc+). Mediators with potentials in this range can readily
oxidize CuI and Cu0 (E1/2 < −0.8 V) but are too mild to
oxidize amine substrates (E1/2 > +0.5 V).
34
12
73
DMA or DMF instead of MeCN
2 V CVE instead of 6 mA CCE
a
b
Reactions conducted on 0.3 mmol scale. Yields were determined by
GC analysis using dodecane as an internal standard. Isolated yield in
parentheses. DMA = dimethylacetamide, DMF = dimethylformamide,
CVE = constant-voltage electrolysis, CCE = constant-current
electrolysis, RVC = reticulated vitreous carbon, and e-chem =
electrochemistry.
Mediator-assisted couplings were conducted with 20 mol %
of the mediator. Of the tested additives (see Figure S1 in the
Supporting Information for a complete list of evaluated
mediators), only ferrocene-based mediators were compatible
with the reaction conditions. Reactions with ferrocene (Fc) as
a mediator dramatically improved product yields (47%)
compared to any nonmediated electrolysis reaction. Similar
or better yields were obtained from reactions conducted with
less-oxidizing mediators (Fc-1 through Fc-3) with potentials
between 0 and −0.15 V (vs Fc/Fc+). However, the benefit of
the mediator was lost when the redox potential of the
ferrocenyl derivative was lowered to more negative potentials
(Fc-4). Low yields observed from reactions with the weak
oxidant Fc-4 likely stem from the mediator’s inability to readily
oxidize CuI. Reactions with mediators that are more oxidizing
than Fc (Fc-5 through Fc-8) occurred with complete
consumption of the aniline but low yields. These results
suggest that mediators with potentials above +0.1 V (vs Fc/
in high yields with an inexpensive mediator and catalyst in just
5 h under air-free conditions and constant current electrolysis
in an undivided cell (entry 1). Control experiments reveal that
Fc+, Et3N, NaOAc, Cu(OAc)2, and electrochemistry are all
required for products to be formed under nitrogen (entries 2−
6). While reactions can be performed with a variety of anodic
materials (entries 7 and 8), reactions with Pt cathodes were the
highest yielding. The observed improvement likely stems from
decreased overpotentials for proton reduction at Pt compared
to reduction at alternative cathodic materials.96−100
While the transformation is reported under inert conditions
to highlight the use of electrochemistry as the means for
oxidative turnover, reaction setup and electrolysis can be
performed in air (entry 9). The slightly lower yield likely stems
from degenerate redox events in the presence of air. An added
benefit to the electrochemical reaction is the decreased
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h
Chart 1. Substrate Scope
a
b
c
d
Under air, no electrochemistry. Under air, without Fc+, no electrochemistry. 40 °C. Reaction was performed at double the concentration.
e
f
g
Reaction was performed at triple the concentration. Without NEt3. FcMe2 was used instead of Fc+. Minor variations to the applied current or
h
electron equivalents for specific substrates are detailed in the Supporting Information. The reported yields are isolated yields from reactions of
0.30 mmol of amine and 0.45 mmol of boronic acid.
reaction time compared to conventional reactions in air.
Reactions in air without both electrochemistry and mediator
formed products in just 8% yield (entry 10). Surprisingly, the
mediator proved beneficial to even the nonelectrochemical
reactions where yields were improved from 8% to 34% in the
same timeframe (entry 11). Collectively, these results highlight
the importance of both electrochemistry and redox mediators
for Cu-catalyzed Chan−Lam reactions.
Substrate Scope. The developed methodology was
applied to couplings of aryl-, heteroaryl-, and alkylamines
with arylboronic acids (Chart 1). Coupled products were
isolated in good yields following electrooxidative reactions
under inert conditions for arylamines with both electron-
donating and electron-withdrawing substituents. Steric hin-
drance ortho to nitrogen had little effect on product yields (5−
7). Electron-rich alkylamines that were susceptible to oxidative
degradation underwent coupling but were more sensitive to
the steric environment around nitrogen. While products of
reactions with primary amines were isolated in good yields,
yields from reactions with secondary amines were lower (14
and 15 vs 18 and 19). A similar effect was observed with
hindered primary amines such as the valine derivative (17) that
underwent coupling in reduced yields. No racemization was
observed for this amino acid derivative as well as for the
enantiopure benzylamine 16 under the mild reaction
conditions. Coupling reactions of heteroarylamines occurred
with lower conversion than with arylamines, but acceptable
yields of products could be isolated (20−22). The lower
reactivity likely stemmed from competing coordination to the
Cu catalyst, but yields could be improved with longer
electrolysis to pass additional Faradaic equivalents. Finally,
the standard reaction could be easily scaled 100-fold using a
media bottle as the undivided electrochemical cell. Under air-
free conditions, compound 3 was formed as a single product in
over 85% GC yield and was isolated in 72% yield (3.7 g).
We next evaluated the scope in arylboronic acid under the
anaerobic electrochemical conditions. Unlike reactions con-
ducted with oxygen, no oxygenation or dimerization of the
boronic acid was observed, and reactions generally form the
target compound as a single product.66,101 Classically, Chan−
Lam reactions are the highest yielding with electron-rich
arylboronic acids and are more challenging with electron-
deficient substrates.67,102 The opposite trend in reactivity was
observed with this electrochemical methodology. Substrates
with electron-withdrawing groups were coupled in the highest
yields (31−33), while couplings of substrates with strongly
donating groups para to the boronic acid were more
challenging (26). Reactions reverted to the expected reactivity
patterns when Chan−Lam coupling was performed under
conventional conditions without electrochemistry. As an
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example, the low-yielding electrochemical reaction of electron-
rich 26 (31%) improved to 72% when in air with Fc+. In
contrast, electrochemistry was essential for coupling of the
electron-deficient substrate to form 33 (88% yield vs 15% in air
and Fc+).
The investigation of the reaction scope reveals that the
developed protocol is not simply an air-free alternative to
Chan−Lam coupling reactions. Rather, this methodology
provides complementary reactivity for reactions of challenging
substrates that have previously been low yielding.67,102
Additionally, electrochemical reactions are higher yielding
than those performed in air alone. Finally, these studies reveal
that the mediator has a beneficial impact on yields of
nonelectrochemical reactions performed in air (see examples
3, 14, 20, 26, and 28).
indicates that the added Fc+ mediator was reduced to Fc and
was undergoing oxidation at high potentials. For comparison,
no oxidation was observed at high potentials in CVs of pure
Fc+ (blue trace). Together, these data revealed that Fc+ rapidly
oxidizes CuI to CuII under the reaction conditions.
We next analyzed deposits on the cathodes from reactions
with and without added mediator. Visually, Pt cathodes from
high-yielding reactions containing Fc+ appeared unchanged
from before electrolysis (Figure 5a). In contrast, an even
Role of the Mediator. The dramatic impact of the redox
mediator on the success of the electrochemical Chan−Lam
reaction inspired mechanistic studies that could provide
insights for the design and implementation of mediators in
other challenging electrocatalytic methodologies. We first
evaluated the role of Fc+ in homogeneous oxidation of low-
valent Cu salts to regenerate the active CuII species. CVs of
CuI(OAc)the Cu species generated after C−N bond
formationscanned from 0 to −1.7 V revealed a single
reduction event with a peak current at −1.5 V (Figure 4, red
Figure 5. (a) Photo and EDX-SEM image of Pt electrode from a
reaction containing Fc+ showing elemental platinum in red. (b) Photo
and EDX-SEM image of a Pt electrode from a reaction without Fc+
showing elemental copper in yellow.
coating of metallic copper was visible on cathodes from
reactions performed without Fc+ (Figure 5b). Elemental
analysis by scanning electron microscopy with energy
dispersive X-rays (EDX-SEM) of these cathodic surfaces
confirmed the visual observations and underscored the
dramatic impact of the mediator on maintaining a homoge-
neous catalyst. Images reproduced in Figure 5 reveal either a
pristine Pt surface from reactions with added Fc+ or complete
surface coverage by Cu0 in the absence of the mediator.
Coating of the Pt surface only further diminishes the selectivity
for the desired proton-reduction reaction over Cu plating.
Finally, we demonstrated that plated cathodes were stripped of
Cu deposits within seconds of submerging the electrodes in a
solution of Fc+. CVs of the resulting solution were consistent
with the conversion of Fc+ to Fc and the concomitant
formation of CuII (Figure S2 in the Supporting Information).
Catalyst recovery by oxidative stripping represents degenerate
redox events: electrochemical Cu plating paired with Fc
oxidation followed by back-electron transfer during the
oxidative stripping of Cu0 with Fc+. It is for this reason that
reactions require 4 electron equiv instead of the theoretical 2
F/mol for high yields. However, electrolysis beyond the
theoretical capacity does not irreversibly waste electrons
because the electrons are simply shuttled through the
degenerate pathway for catalyst recovery.41 Together, these
data implicate the mediator’s role in stripping Cu metal
deposits from the cathode to maintain an active Pt surface for
proton reduction and to regenerate catalytic CuII salts.
Figure 4. CVs probing Cu(I) oxidation by Fc+ with Cu(OAc) (red),
Cu(OAc) with Fc+ (black), and Fc+ (blue). CV conditions: 20 mM in
MeCN with 0.1 M KPF6 as electrolyte and 100 mV/s scan rate at
room temperature using glassy carbon WE and Pt CE. Potentials were
calibrated with Fc as an internal reference.
trace). This current response is consistent with CuI/0
reduction. The only oxidation event on the return scan to 0
V was the stripping of plated Cu0 indicated by the non-
Nernstian peak centered at −0.45 V. Notably, there is no
second redox event that can be attributed to oxidation of CuI.
Additional evidence that CuI is not electrochemically oxidized
was provided by analysis of the baseline current on the initial
reductive sweep of the CuI-containing solution. Specifically,
CuI should be unstable at the starting potential of the CV
sweep (+0 V), and a positive baseline current would be
observed for electrochemical oxidation of the bulk CuI species
in solution. Additionally, any CuII that was generated at these
high potentials would be reduced at the CuII/I couple with an
onset of −0.8 V. However, no current was observed at either of
these potential ranges, supporting a lack of electrochemical
activity for the CuI salt under the reaction conditions.
Our final investigation into the function of the mediator
targeted its role on preventing oxidation of amine substrates.
Reactions without Fc+ form low yields of coupled product
despite complete conversion of the amine substrate. In
contrast, yields from reactions with the mediator closely
mirror substrate conversion. To gain insight into the oxidative
half-cell reactions, we performed mediated and unmediated
Chan−Lam couplings with a Ag/Ag+ quasi-reference electrode
CV scans after the addition of 1 equiv of Fc+ to the same
solution of CuI(OAc) (Figure 4, black) matched those of
CuII(OAc)2 and revealed a CuII/I reduction that was not
observed from CVs of CuI alone. Additionally, the baseline
current above the potential of the Fc/Fc+ was positive, which
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Authors
to monitor the absolute operating potential of the anode
(Figure 6a). Anodic reactions in solutions containing Fc+
Benjamin R. Walker − Department of Chemistry and
Biochemistry, The Ohio State University, Columbus, Ohio
43210, United States
Shuhei Manabe − Department of Chemistry and Biochemistry,
The Ohio State University, Columbus, Ohio 43210, United
States
Andrew T. Brusoe − Chemical Development, Boehringer
Ingelheim Pharmaceuticals, Inc., Ridgefield, Connecticut
06877-0368, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c02103
Figure 6. (a) Anodic voltage profile of a standard reaction with Fc+
(black) and a reaction without Fc+ (red). (b) CVs of anodic redox
events of a standard reaction mixture (red) and of pure Fc+ (black).
Conditions: 100 mM in MeCN with 0.1 M KPF6 as electrolyte and
100 mV/s scan rate at 60 °C using glassy carbon WE and Pt CE.
Potentials calibrated to Fc as an internal standard.
Author Contributions
§B.R.W. and S.M. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
occurred at +0.2 V (black trace), which is a potential
consistent with oxidation of Fc (Figure 6b, black). In contrast,
the anodic operating potential during unmediated reactions
was +0.8 V (red trace). This high voltage is indicative of amine
oxidation (Figure 6b, red) and provides a rational for the
observed high conversions with low product yields under
unmediated conditions.
We thank the National Institutes of Health (NIH R35
GM138373) for financial support.
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CONCLUSION
■
In summary, this work details a strategy for performing
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c02103.
■
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Experimental procedures, cell design, characterization of
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AUTHOR INFORMATION
Corresponding Author
Christo S. Sevov − Department of Chemistry and
Biochemistry, The Ohio State University, Columbus, Ohio
43210, United States; orcid.org/0000-0002-5384-873X;
Email: sevov.1@osu.edu
■
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