Bifunctional Carbenium Dications as Metal-Free Catalysts for the Reduction of Oxygen

Article abstract of DOI:10.1021/jacs.0c04841

The development of catalysts for the oxygen reduction reaction is a coveted objective of relevance to energy research. This study describes a metal-free approach to catalyzing the reduction of O2 into H2O2, based on the use of redox-active carbenium species. The most active catalysts uncovered by these studies are the bifunctional dications 1,8-bis(xanthylium)-biphenylene ([3]2+) and 4,5-bis(xanthylium)-9,9-dimethylxanthene ([4]2+) which promote the reaction when in the presence of decamethylferrocene and methanesulfonic acid. Electrochemical studies carried out with [4]2+ suggest the intermediacy of an organic peroxide that, upon protonation, converts back into the starting dication while also releasing H2O2. Kinetic studies point to the second protonation event as being rate-determining.

Full text of DOI:10.1021/jacs.0c04841

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Communication
Bifunctional carbenium dications as metal-
free catalysts for the reduction of oxygen
Mohammadjavad Karimi, Rosmita Borthakur, Christopher L. Dorsey,
Chang-Hong Chen, Sébastien Lajeune, and Francois P. Gabbai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04841 • Publication Date (Web): 24 Jul 2020
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Bifunctional carbenium dications as metal-free catalysts for the
reduction of oxygen
Mohammadjavad Karimi, Rosmita Borthakur, Christopher L. Dorsey, Chang-Hong Chen, Sébastien Lajeune and François P.
Gabbaï*
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ABSTRACT: The development of catalysts for the oxygen reduction reaction is a coveted objective of relevance to energy research.
This study describes a metal‐free approach to catalyzing the reduction of O into H O , based on the use of redox-active carbenium
2 2 2
2
+
species. The most active catalysts uncovered by these studies are the bifunctional dications 1,8-bis(xanthylium)-biphenylene ([3] )
2+
and 4,5-bis(xanthylium)-9,9-dimethylxanthene ([4] ) which promote the reaction when in the presence of decamethylferrocene and
2+
methanesulfonic acid. Electrochemical studies carried out with [4] suggest the intermediacy of an organic peroxide that, upon
2 2
protonation, converts back into the starting dications while also releasing H O . Kinetic studies point to the second protonation event
as being rate-determining.
The catalytic reduction of oxygen is of relevance to biological
1
2
respiration and fuel cell research. Because of their high
reactivity toward oxygen, transition metal complexes have
been considered as catalysts for the oxygen reduction reaction
ORR).3 Given the multielectron nature of this process, the
4
(
design of catalysts in which two metals can synergistically
1
b, 3d, 5
reduce O
2
has also drawn considerable attention.
These
advances have been paralleled by related efforts in p-block
6
chemistry, leading to the discovery of reduced main group
compounds that spontaneously react with O
2
to form
7
peroxide-derivatives. Examples also exist where a hydrogen-
8
bond donor host or a Lewis acidic main group compound
2
fixate O in the presence of a reducing equivalent. When the
main group compound is a borane, this reduction produces
7a
peroxydiborates of type A (Scheme 1). This reactivity is
2
analogous to that of trityl radicals which also react with O to
9
form bridged peroxides of type B. The formation of such
bridged peroxides has also been observed with bifunctional
1
0
11
derivatives such as C and D. Inspired by these advances,
we have now questioned whether systems based on
1
2
electroactive carbon-based Lewis acids could mimic the
properties of transition metal complexes and act as
Scheme 1. Key precedents and general approach explored in
this study
1
3
14
carbenium-based catalysts for the ORR. It occurred to us
that such an objective could be attained based on the sequence
of steps depicted in the catalytic cycle shown in Scheme 1,
with the formation of a peroxo-bridged dimer and its
To test this hypothesis, we targeted systems based on the
9
-arylxanthylium moiety because of its documented
10, 16
+
2+ 10,
stability.
In addition to the known cations [1] and [2] ,
15
protonolysis as the key steps of the reaction.
17
2+
2+
we also synthesized [3] and [4] , two new dicationic
systems based on the biphenylene-1,8-diyl and 9,9-
dimethylxanthene-4,5-diyl backbones, respectively (Scheme
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-
2+
2
BF
4
xanthylium units, as shown in Figure 1 for [4] . The near
O
C
O
C
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degeneracy of these two orbitals suggests that the two-
electron reduction of these compounds will produce
molecules with diradical character (vide infra) as proposed for
H
H
O
O
2
O
O
O O
Li
Li
C
C
HBF4
•• 10
2
.
We then chose to assess the activities of these compounds
for the catalytic ORR using decamethylferrocene (Fc*) as the
reducing agent,19 methanesulfonic acid (MeSO
H) as the
O
C
C
O
O
C
C
O
O
3
C
O
O
proton source, and MeCN as the solvent. These experiments
were monitored spectrophotometrically by measuring the
O
O
C
+
C
appearance of Fc* at 780 nm (Figure 2). The time course
0
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0
graphs were obtained under a uniform set of conditions, using
a catalyst loading of 1.0% with respect to Fc* (3.33 µM)
[1]+
[2]2+
[3]2+
[4]2+
Scheme 2. General synthetic approach adopted for the
synthesis of the [3][BF and [4][BF and structures of the
dissolved in an O
2
-saturated solution containing 1.66 mM
4
]
2
4 2
]
MeSO H (see SI for details). A comparison of the resulting
3
cations investigated in this study.
traces shows some distinctive features. Out of the four
cationic systems surveyed, dications [3]2 and [4] displayed
+
2+
a
b
the highest activity, affording turnover frequency (TOF)
O1
O2
O1
O2
-2
-1
-1
-1
values of 5.7 (± 0.2)×10
s
and 6.1 (± 0.1)×10 s ,
respectively. Dication [2]2 displayed diminished activity
+
-2 -1
with a TOF value of 2.6 (± 0.2)×10 s , and the monocation
1] gave very slow conversion with a turnover frequency of
+
[
-3 -1
C1
C2
C2
6
.7 (± 0.3)×10 s .
C1
O3
c
d
LUMO
-8.944 ev
LUMO+1
-8.980 ev
2
+
2+
Figure 1. Top: Structure of [3] (a) and [4] (b) in the crystal
with selected ellipsoids drawn at the 50% probability level.
Bottom: LUMO (c) and LUMO+1 (d) of [4] (isovalue 0.04)
2
+
Figure 2. Benchmarking of the catalysts in O
MeCN solutions containing MeSO H and Fc* as monitored
by UV-vis spectroscopy (see text for details). Red trace [4] ,
2
-saturated
computed at B3LYP/6-31+G(d) level of theory.
3
2+
). The 13C-NMR spectra of these new derivatives show a
2
2+
2+
+
blue [3] , dark green [2] , light green [1] , black background
resonance at 171.3 ppm for [3][BF
4][BF corresponding to the carbenium centers.
Crystallographic analysis of [3][BF and [4][BF indicates
that the two carbenium centers are separated by 4.068(3) Å
4 2
] and 173.5 ppm for
(uncatalyzed). The inset shows the iodometric titration of
[
4 2
] ,
2 2 2
H O on the final solution obtained after catalytic O reduction
4
]
2
4 2
]
2+
using [4] as the catalyst. The rise in absorbance at 361 nm is
-
2
+
2+
due to the formation of I .
and 4.29(1) Å in [3] and [4] , respectively (Figure 1). These
3
distances are compatible with the µ-1,2 complexation of
1
8
The selectivity of each reaction was assessed by an
second-period diatomic molecules.
Theory calculations show that these dications have gas-phase
structures approaching perfect C symmetry. In these
computed structures, the separation between the carbenium
Density Functional
anaerobic iodometric titration1
9-20
,
showing 93-97%
2+
2 2
selectivity for H O as exemplified for [4] in the inset of
2
21
Figure 2. These initial results underscore the role played by
the linker in the activity of these catalysts. They also show
that bifunctional systems are superior to the monofunctional
2
+
2+
centers (4.25 Å for [3] , 4.72 Å for [4] ) is notably larger
than those determined crystallographically, reflecting the
unscreened repulsion between the two cationic moieties. The
LUMO and LUMO+1 of [3] and [4] are almost
energetically degenerate (∆E ~ 4×10 eV) and span both
carbenium. To shed light on the mechanism by which O
reduction proceeds, we carried out kinetic studies using the
2
2+
2+
2+
-2
best catalyst, namely [4] . We found that the reaction rate
depends on neither the concentration of Fc* nor the
2
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2
. By contrast, initial slope measurements
concentration of O
1
2
3
4
5
6
7
8
9
show a first-order dependence both on the catalyst and on the
+
acid, yielding the second-order rate law rate = kcat[cat][H ]
-1 -1
with kcat = 367 (± 6) M s . This rate law could be interpreted
based on a mechanism in which the reduction of the catalyst
2
by two electrons precedes a fast reaction with O (Scheme 3,
path a). In this mechanism, the resulting peroxide complex
undergoes stepwise protonation. The stepwise nature of these
protonation events is consistent with a first, rather than a
second-order dependence observed with respect to the acid
concentration. We propose that the second protonation is the
rate-determining step because the charged nature of
1
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+
intermediate [4-OOH] would make its protonation less
favored than that of 4-O
2
. The higher catalytic activity of [4]2
+
should thus be directly correlated to the predisposition of the
corresponding peroxide, namely 4-O , toward protonolysis.
After using H NMR spectroscopy to test the in situ generation
of this peroxide starting from [4][BF and Zn (see SI), a
dilute solution of 4-O (20 µM) in MeCN was generated in a
cuvette and subjected to pronotolysis by addition of MeSO
2
1
4 2
]
2
3
H
(
10 equiv.). UV-vis monitoring showed regeneration of [4]2
+
2
Scheme 3. The catalytic cycle for O reduction mediated by
in just ~10 sec. The same experiment showed that 3-O
2
also
2+
[
4] .
0.31 V and -0.53 V versus Fc+/0 (Figure 3), when recorded
. This behavior, which is analogous
2+
undergoes relatively fast conversion into [3] (~20 sec.). The
+
2+
peroxides derived from [1] and [2] underwent sluggish
-
acidolysis, in agreement with their lower catalytic activity
under an atmosphere of N
to that of [2] and [3] , is consistent with the formation of
the radical cation [4] followed by that of 4 . The formation
of these two species can be viewed as resulting from the
successive one-electron population of the LUMO and
2
(see SI). We propose that the higher reactivity of 4-O
2
results
2+
2+
from the rigidity of the dimethylxanthene backbone and the
presence of an oxygen atom which destabilizes the C-O-O-C
motif, a conclusion supported by peroxide ion affinity
calculations (see SI).
Given that the above mechanism hinges on the double
reduction of the platform, we resorted to electrochemistry to
•+
••
2+
LUMO+1 orbitals of [4] . A similar stepwise reduction of
the two carbenium units was proposed by Suzuki in the case
2+ 10
of [2] . The reversibility of the formation of these two
probe the validity of this scenario. The cyclic voltammogram
2
species is lost when the CV is recorded under O (Figure 3).
2+
(
CV) of [4] shows two reversible reduction events at E1/2
n
Figure 3. Left: CVs on a 1.0 mM solution of [4][BF
.0 mM (black), 10.0 mM (violet), 25.0 mM (orange), 50.0 mM (green), 100 mM (red). Inset: CVs of [4][BF
in MeCN under an atmosphere of N (blue) and O (orange). Right: CV of [4][BF in an O
] at a scan rate of 500 mV.s . The feature at 1.45 V is assigned to the two-electron oxidation of [4-O
and [4] . Inset: Spectroelectrochemical data recorded with a 0.105 mM solution of [4][BF in MeCN in the presence of [ Bu
0.1 M). The spectra plotted with solid lines (1-4) were recorded during reduction at -0.5 V vs. Fc over the course of ~10 sec. Those
4
]
2
in 0.1 M [ Bu
4
][PF
6
] in MeCN, in the presence of various amounts of MeSO
3
H:
n
0
4
]
2
in 0.1 M [ Bu
4
6
][PF ]
2
2
4
]
2
2
-saturated MeCN solution containing
] liberating O
2
n
-1
0
.1 M [ Bu
4
][PF
6
2
2+
n
4
]
2
4 6
][PF ]
+/0
(
+/0
with dashed lines (5-8) were recorded during oxidation at 1.5 V vs. Fc over the course of ~10 sec.
3
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These changes indicate that both [4]•+ and 4 quickly react
••
shows that metal-free systems can mimic the catalytic
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with O
2
to form the peroxide species 4-O
2
. These species are
), which was
unambiguously characterized by Suzuki. While we failed in
the isolation of 4-O , its formation can be inferred from
electrochemical measurements. Indeed, when the scan rate is
reactivity of transition metal-based dinuclear oxygen
reduction catalysts. Given our interpretation that the second
protonation of the organic peroxide intermediate is rate-
limiting, we are currently exploring the incorporation of
proton shuttles in such organocatalysts.
well known as in the case of C (2-O
2
1
0
2
2
+
increased to 500 mV/s in the CV of [4] and when, at the
same time, the current is scanned cathodically, we obverse an
ASSOCIATED CONTENT
Supporting Information
+/0
irreversible oxidation event at 1.45 V vs Fc , close to the
anodic extreme of the solvent window (Figure 3). We assign
this feature at 1.45 V to the oxidation of the peroxide
2
, formed after the second reduction (Figure 3).
This value is comparable to that reported for the oxidation of
Additional
experimental
and
computational
details.
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Crystallographic data in cif format. Optimized structures in xyz
format. These materials are available free of charge via the
Internet at http://pubs.acs.org.
derivative 4-O
1
0, 22
C (2.08 V vs. SCE).
greatly enhanced when two equivalents of cobaltocene is
added to the electrochemical cell under an atmosphere of N
The current of this oxidation wave is
Accession Codes
CCDC 2000553-2000554 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
followed by introduction of oxygen (see SI). This current
enhancement is assigned to the bulk chemical reduction of
2
, leading to an increased
concentration of 4-O Spectroelectrochemistry was
employed to support this interpretation. The low energy
absorption bands of the xanthylium chromophores in
2+
[
4] followed by exposure to O
2
.
10
23
AUTHOR INFORMATION
Corresponding Author
+/0
[4][BF
4
]
2
are quenched when a potential of -0.5 V vs. Fc is
(Figure
applied in air, in agreement with the formation of 4-O
2
+/0
3). When the potential is switched to 1.5 V vs. Fc , the
absorption bands of [4][BF are restored, showing clean
conversion back to the starting dication with no notable loss
of material (Figure 3). Having gathered evidence for the
formation of a diradical and its rapid conversion into a
François P. Gabbaï – Texas A&M University, Department of
Chemistry, College Station, Texas 77843, United States;
orcid.org/0000-0003-4788-2998; Email: francois@ tamu.edu
francois@tamu.edu
4 2
]
Author
presumably bridged peroxide species, we decided to test if
4] could behave as an electrocatalyst for this chemistry. To
2+
[
Mohammadjavad Karimi – Texas A&M University,
Department of Chemistry, College Station, Texas 77843,
United States
this end, CVs were recorded under O
different concentrations of MeSO H. As shown in Figure 3,
addition of MeSO H leads to an increase in the current,
2
, in the presence of
3
Rosmita Borthakur– Texas A&M University, Department of
Chemistry, College Station, Texas 77843, United States
Christopher L. Dorsey– Texas A&M University, Department
of Chemistry, College Station, Texas 77843, United States
Chang-Hong Chen– Texas A&M University, Department of
Chemistry, College Station, Texas 77843, United States
Sébastien Lajeune– Texas A&M University, Department of
Chemistry, College Station, Texas 77843, United States
3
pointing to reaction turn-over or electrocatalysis. The current
increase is especially acute at potentials corresponding to the
second reduction, indicating that the generation of the putative
peroxide-bridged species is an essential element of the
catalytic cycle (Scheme 3). However, this catalyst is not fast
enough to be kinetically characterized by CV as a well-
defined plateau region could not be reached, even at high acid
19
concentrations.
Since the first reduction wave loses
, we propose
which resists fast
protonolysis as indicated by the minute current increase
observed at the first wave upon addition of MeSO H. Instead,
this intermediate is poised for a second reduction, leading to
4-O as depicted in Scheme 3 (path b). Owing to its neutral
nature, 4-O is more sensitive to acidolysis, leading to the
observed electrocatalytic current and production of H . To
Author Contributions
reversibility when the CV is recorded under O
2
•+
C.L.D. and C.-H.C. contributed equally.
2
the intermediacy of a superoxide [4-O ]
Notes
3
The authors declare no competing financial interest.
2
ACKNOWLEDGMENT
This work was supported by the Welch Foundation (grant A-
2
2
O
2
1
423) and Texas A&M University (Arthur E. Martell Chair of
complete the characterization of this electrocatalyst, we have
also evaluated the overpotential of this catalyst when in the
presence of 1.66 mM of MeSO H, a concentration used in the
3
Chemistry). R. B. thanks the Science and Engineering Research
Board (SERB) and the Indo-US Science and Technology Forum
(
(
IUSSTF) for the SERB INDO-US postdoctoral fellowship
2016/36). We thank Dr. Michael Nippe for helpful discussions
19
chemical reduction experiment. Application of a literature
19, 24
method
that involves measurement of the open circuit
potential and the E1/2 of the pure catalyst affords ηeff of 490
mV.
In conclusion, we describe a new conceptual approach to
2 2 2
the reduction of O into H O , based on bifunctional triaryl
and instrument access.
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