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Article

A Unified Electro- and Photocatalytic CO2 to CO Reduction
Mechanism with Aminopyridine Cobalt Complexes
Sergio Fernández, Federico Franco, Carla Casadevall, Vlad
Martin-Diaconescu, Josep María Luis, and Julio Lloret-Fillol
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06633 • Publication Date (Web): 10 Dec 2019
Downloaded from pubs.acs.org on December 10, 2019

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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
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redox potential (Scheme S1). Indeed, the CV of the solution after
electrolysis of 1(II) under CO2 is consistent with the analogous CV
in the presence of one equivalent of TBACO3H (Figure 4 C). The
latter experiment also showed that at the CoI/0 redox potential there
is catalysis which implies that carbonate is not involved in the
catalytic CO2 reduction process.
Computational modeling of the mechanism. With the aim to
give additional insight into the reactivity of electrochemically
generated CoI species with CO2, we studied the reaction energy
profile by DFT. The calculations were done at B3LYP-D3(SMD) /
aug-cc-pVTZ(-dH,-fC,N,O,-gCo) // B3LYP-D3(SMD) / 6-31+G*
level, which reproduced well the catalytic activity of related
systems.19b Computed Gibbs energies were corrected for the
catalytic conditions, i.e. substrate (CO2) and product (CO)
concentrations of 0.28 M and 50 [ respectively.27 For a detailed
description of the computational methodology and for the
optimized structure coordinates see sections 4.1 and 6 of the SI.
In aprotic conditions, CO2 is known to act as an oxide acceptor
assisting the reductive disproportionation reaction to CO and CO32.7b Nevertheless, residual water contained in anhydrous CH3CN
may have an important role in the protonation of the cobalt-CO2
adducts. To account for available protons, we studied the pH
dependency of the mechanism. At the low proton concentration of
reaction conditions, a proton assisted mechanism could be operative
but competitive with an aprotic CO2 reductive disproportionation
mechanism. Therefore, in the first part of this section, we will
discuss possible mechanisms for the formation of the key 1(I)-CO
intermediate under both proton-assisted and aprotic conditions.
Later, we will comment on the cobalt-catalyzed CO2 reduction
mechanism at the CoI/0 redox potential focusing on the effect of the
pH and the redox potential on the thermodynamics and kinetics of
the catalytic reaction.
Formation of 1(I)-CO. According to the experimental data, the
reduction of CO2-to-CO occurs at the first CoII/I reduction wave (ca.
–1.7 V), yielding 1(I)-CO and CoII-carbonate species as the main
reaction products. We have shown that, although the C-O bond
cleavage can take place, the reaction does not proceed catalytically.
In order to reproduce our experimental conditions at the CoII/I wave,
the theoretical CoII/I reduction potential (-1.91 V) was chosen to
calculate the energy profiles (Figure 5). As depicted in Figure 5 A,
in the proton-assisted mechanism, the nucleophilic CoI species
([LN4CoI(S)]+) formed by 1e- reduction of [LN4CoII(S)]+2 binds CO2
to form a higher in energy carboxylate adduct ([LN4CoIII-CO2]+),
with a 8.8 kcal·mol-1 energy barrier. Then, the subsequent 1ereduction gives the slightly endergonic [LN4CoII-CO2] at the
defined redox potential. Further protonation of the highly basic
[LN4CoII-CO2] species yields the thermodynamically favored
[LN4CoII-CO2H]+ (pKa = 28.4).
The subsequent C-O bond cleavage step has been proposed as the
rate determining step (r.d.s.) in the light-driven CO2-to-CO
reduction mechanism catalyzed by other macrocyclic Co
complexes.8b,28 In our case, the calculated Gibbs energy barrier for
the heterolytic C-O bond cleavage from [LN4CoII-CO2H]+ to give
[LN4CoII-CO(OH)]+ is 16.0 kcal·mol-1 (Figure 6A). This result is
in agreement with the previously reported data for complex C6 and
its variants showed in Chart 1.5c
However, we found that even at the low proton concentration
given by 0.4 µM of water, the C-O bond cleavage triggered via a
second protonation of [LN4CoII-CO2H]+ (Figure 5 A) is kinetically
more favored ( G‡1st CO2= 12.2 kcal·mol-1). The subsequent release
of a water molecule to form [LN4CoII-CO]2+ is entropically driven
due to the low concentration of water in organic solution. Likewise,
the recovery of the starting [LN4CoII(S)]2+ could be formed by the
CO release from [LN4CoII-CO]2+ which would complete the first
turnover cycle. The rate determining step of this postulated catalytic

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cycle is the proton-assisted C-O bond cleavage with a kinetic barrier
as low as G‡1st CO2 ~ 12.2 kcal·mol-1, which is kinetically feasible
at room temperature. However, at a higher proton concentration
(pH < 24.5), the kinetics will be independent of the protonation
events and governed by the CO2 binding step ( G‡binding= 8.8
kcal·mol-1).
At this point, our modeled 2e- reduction mechanism, that
catalyzed the CO2 + 2H+ reduction to CO + H2O by 1(II), is similar
to the recently proposed mechanisms for similar systems under both
photo- and electrochemical conditions.5c,29 However, none of the
previously reported mechanisms gives an explanation for the
general non-catalytic behavior of these systems at the CoII/I wave.
Indeed, according to the CoII/CoII-CO mechanism, 1(II) should
catalyze the CO2-to-CO reduction at the CoII/I reduction potential
with fast reaction rates due to its low kinetic barrier. Nonetheless,
we have shown that our cobalt complex is not catalytic within the
CV timescale (100 mV/s) at the CoII/I wave, and only
substoichiometric amounts of CO were accumulated during
corresponding electrolysis experiments. Furthermore, we identified
the formation of 1(I)-CO, which is yet to be included as an
intermediate in the CO2-to-CO reduction catalyzed by
aminopyridine cobalt complexes.7b
In order to account for a model that fits our experimental
observations, we considered the further reduction of the cobaltbased intermediates involved in the CO2 reduction mechanism. In
this regard, it is remarkable that the 1e- reduction of [LN4CoII-CO]2+
is highly favored at the CoII/I reduction potential (E1/2(CoII/I-CO) =
–0.94 V; G(CoII/I-CO) = –22.3 kcal·mol-1). Then, [LN4CoI-CO]+
becomes the most stable intermediate of the Gibbs energy profile.
Indeed, the strong Co-CO bond is responsible for this stability with
respect to CoI. The nature of the CO binding and its -backbonding
character can be illustrated by the frontier molecular orbital analysis
in the CoII, CoI and formal Co0 oxidation states (Figure S36). In the
case of CoII-CO, there is not a significant -backdonation from the
Co center to the CO ligand, as it is expected for an electron poor
metal center. However, regarding CoI-CO and Co0-CO, two of the
singly occupied d orbitals of CoI/0 contribute to the backbonding character of the Co-CO bond as it is shown by the
canonic orbitals depicted in Figure 6 B. Moreover, the enhanced
stability in [LN4CoI-CO]+, provided by the presence of a -acceptor
ligand, can be explained by means of the 18 e- counting rule. While
the CO release from CoII (17 e-) is exergonic, the release from CoI
(18 e-) is highly endergonic ( GCoI-CO > 20.2 kcal·mol-1) which
prevents catalysis at the CoII/I redox potential. Similarly, the CO
release from Co0-CO is endergonic by 24.3 kcal·mol-1. Indeed, the
electronic structure of the formal Co0-CO is better described as
[(LN4)•`CoI-CO] (18 e-) since the -HOMO orbital is mainly
delocalized in the pyridine ring with a small contribution of the
metal center.
According to the energetic span model, the overall kinetic barrier
of a catalytic process (aEspan) should be calculated as
=

{

!
!

+&

%

! "#$%
! '$" %$

(8)

where GTDI, GTDTS and b8r correspond to the Gibbs energies of the
TOF Determining Intermediate (TDI), the TOF Determining
Transition State (TDTS) and the reaction, respectively.30 In our
case, the TDI corresponds to the [LN4CoI-CO]+ intermediate, and
at a working potential of –1.91 V, the TDTS is [LN4CoIICO···OH2]2+. Then, the energy barrier of the catalytic process is
given by a9span = G([LN4CoII-CO2H]+) + G([LN4CoI-CO]) +
Gr = 30.3 kcal·mol-1. The kinetic barrier of the catalytic cycle
includes the CO release from the TDI ( Grelease) to recover the
active species and the energy barrier of the first CO2 activation

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model is in agreement with the accumulation of 1(I)-CO at ca. –1.7
V evidenced by thin layer SEC (vide supra).

( G‡1st CO2). Conversely to G‡1st CO2, a9span exceeds the kinetic
limit for a catalytic process at room temperature. Furthermore, this

A. Proton-assisted mechanism
2+

O
N 2.5

2.5

N

161o

C

Co

N
N

O
N

NCMe

N

N

N

O
1.8

Co

N
N

8.8

0.0

CO2

0.0

+ e-

N

O
N

N
N

N

Co

H2O
H+

3.1

G1st CO2

H+

N
N

Co

[LN4CoII-CO]2+ (d)

C

N

CO
+ e-

-11.6

125o

N

O

N

[LN4CoII-CO2] (d)

[LN4CoIII-CO2(S)]+ (t)

S

O

1.9

N

C O

N

-9.5

Co

1.8

N

N
N

O
NCMe

N

NCMe
N

2+

10.1

+ e-

N

Co

NCMe
N

[LN4CoII-CO···H2O]2+ (q)

143o

2.1 C
2.4

Co

N

O

N

N

N

C

3.5

O

-2.1

O

[LN4CoII(S)]2+ (q) [LN4CoI(S)]+ (t)

2+

N

O

4.8

Gbinding

0.8

C

S

6.0

N

3.0

1.8

[LN4CoIII-CO2]+ (s)

NCMe

N

C

2.0

N

TS1 (t)

Co

N

N

135o

OH2

Co

Co

O H

1.9

C

N

[LN4CoI(S)]+···CO2 (t)

-11.6
[LN4CoI(S)]+

[LN4CoII(S)]2+

117o

O

CO

+ e-

[LN4CoII-CO2H]+ (d)

S

N
N

-31.8

Co

N

Grelease

1.8

C

O

N
[LN4CoI-CO]+ (t)

CoII/I REDUCTION
AND CO2 BINDING

[CoI-CO]+ FORMATION
AND CO RELEASE

C-O BOND
CLEAVAGE

B. CO2 reductive disproportionation mechanism
S = MeCN LN4 = pyMetacn
Non- 1e-- 2e-- 3e-- reduced species

N

N
N

Co

N

C

N
N

O

O

N
N

N
N

Co
NCMe
N

[LN4CoII(S)]2+ (q)

N

O
1.8

Co

N

2.0

C

N
135o

N

O

N

N

2.4

N 1.9

N

125o

N

O

-9.4

2.7

N

[LN4CoII-CO2···CO2] (d)

N

Co

N
N

Co

O

N

-20.6

C

O
Eo = -0.94 V

[LN4CoIII(O2CO)]+ (s)

CoII/I REDUCTION
AND CO2 BINDING

BINDING OF A 2nd CO2
MOLECULE

S

CO

-22.6

[LN4CoII-CO]2+
(d)

+ e-

Co

+ e-

CO
S

O
O

N

C

O

N

-22.6
[LN4CoI(S)]+

[LN4CoII(S)]2+

N
N

GCO3

(d)

[LN4CoII(O2CO)]
(d)

+ e-

N

3)]

S

C 1.4
O
O C
O

+

O

TMA+ + [TMA···S]+

[LN4CoII(S)]2+ (q)

O

[LN4CoII-(CO2)2] (d)

N

TMA2CO3

O
O

[LN4CoII-(CO)(CO

N O C O

2.5

O C

-6.1

[LN4CoIII-CO2]+ (s)

N

O

C
Co

N

C

Co

[LN4CoII(S)]2+

TS3 (d)

3.1
O

+ e-, + CO2

Co

N
G2nd CO2

15.4

O
C 2.3
O
O C
O

N
N

10.9

2+

N

C

8.0

CO2
+ e-

O
O

TS2 (d)

125o

[LN4CoII-CO2] (d)

4.8

1e-- oxidized species

C

Co

N

O

1.9

125o

O

N

Grelease

-42.9
[LN4CoI-CO]+ (t)

[LN4CoII(O2CO)]

[LN4CoII(O2CO)]

CO32- FORMATION
AND RELEASE

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[CoI-CO]+ FORMATION
AND CO RELEASE

[LN4CoII(O2CO)]

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Figure 5. Computed Gibbs energy profile for the [LN4CoI-CO]+ formation through the CO2 reduction to CO mediated by 1(II) at a working
potential of –1.91 V vs. Fc/Fc+ and pH 25. Energies and other relevant thermodynamic and structural parameters are given in kcal·mol-1, V
vs. Fc/Fc+, Å and degrees. The spin multiplicity of each intermediate is shown in parenthesis: singlet (s), doublet (d), triplet (t), quartet (q).
TMA = tetramethylammonium.

O

Co

N

1.8

G(Co-CO) < 0 < G(Co-CO) < G(Co-CO)

2+

H

Reduction

N

1.9

2.4

OH2

1.8

N

C O

N

C O

Co

N

G(Co-CO)

N

13.9

N4

II

[LN4CoII-CO]2+

2+

[L Co -CO(OH2)] (d)

11.6

22.3

H+

6.2

20.2

G(Co-CO)
II

TS4 (d)

20.2

[LN4CoI(S)]+

I

G(Co-CO)
N
N

N
N
N

O

1.9

C

H

O 117o

[LN4CoII-CO2H]+ (d)

2e-- reduced species
HETEROLYTIC C-O
BOND CLEAVAGE

1.9

Co

N

Co

19.8
[LN4Co0-CO]

[LN4CoII(S)]2+

H2O

-2.1

N

44.1
[LN4Co0(S)]

0

N 2.2

N

II

B

I

A

0

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2
3
4
5
6
7
8
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17
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1.8

-9.5

OH
C O

0.0
[LN4CoI-CO]+

2+

[(L N4 )·-CoI-CO] (18 e -)

N
N
[LN4CoII-CO(OH)]+ (d)

N

Co

1.8

N
N

[L N4 CoII-CO]2+ (17 e -)

C O

[LN4CoII-CO]2+ (d)

WATER RELEASE

[L N4 CoI-CO]+ (18 e -)
CO RELEASE FROM Co-CO

Figure 6. A) Heterolytic C-O bond cleavage step starting from [LN4CoII-CO2H]+ to form [LN4CoII(CO)(OH)]+ intermediate. B)
Thermodynamics of the CoII/I/0 one electron reductions and each corresponding CO release step. Selected singly occupied molecular orbital
of CoII/I/0-CO complexes (isovalue 0.07). Energy profiles computed at –1.91 V vs. Fc/Fc+ and pH 25.
shown, a9span strongly depends on the stability of 1(I)-CO but also
Alternatively, the reductive disproportionation mechanism has
on redox and protonation events which are controlled by the applied
been also computed to explain the formation of 1(I)-CO in the
redox potential and the pH of the medium, respectively. Indeed, the
absence of H+ (Figure 5 B). In this case, after the first CO2 binding,
variation of these two factors can switch the operative mechanism
another CO2 molecule binds to [LN4CoII-CO2]2+ to form the
for the formation of 1(I)-CO from a pH-independent reductive
thermodynamically downhill [LN4CoII-(CO2)2]2+ with a kinetic
-1. The subsequent C-O bond cleavage to
disproportionation mechanism to a proton assisted CO2 reduction
barrier of 10.9 kcal·mol
mechanism.
obtain a [LN4CoII-(CO)(CO3)]2+ is exergonic and proceeds with a
barrier of 8.5 kcal·mol-1. Then, a second CoII molecule can assist
Catalytic CO2 reduction. According to cyclic voltammetry,
the release of carbonate to form [LN4CoII(O2CO)] and [LN4CoIIfurther reduction to formal Co0 intermediates is needed in order to
2+, which reduction at working potential is strongly exergonic
CO]
activate the catalytic process. Moreover, the catalytic wave
(equation 9). Therefore, the 1(I)-CO formation through the
increases in current when H2O is added to the solution and it shifts
disproportionation mechanism has a lower Gibbs energy barrier
to more positive potentials. Therefore, we have evidence to support
than in the proton assisted mechanism at pH values higher than
that catalysis is assisted by the presence of H+. As above shown, the
25.3.
catalytic wave it is not affected by the presence of added carbonate,
and then it can be excluded from the mechanism. These
On the contrary, the energy span for the reductive
experimental evidence, together with the previous DFT study, led
disproportionation mechanism ( G‡2nd CO2 + Grelease + GCO3 =
us to hypothesize a reaction mechanism in which: i) first [LN4CoI69.2 kcal·mol-1) is by far higher than in the proton assisted
II-carbonate
CO]+ is reduced to the formal [LN4Co0-CO] (E1/2(Co0/I) = –2.77 V,
mechanism due to the additional stability of the Co
Figure 7 A); ii) and then a second CO2 binding occurs forming the
species.
corresponding carboxylate adduct [LN4CoII-CO2(CO)]. Thereafter,
N4
II
N4
II
2+
protonation and further 1e- reduction yields [LN4CoI-CO2H(CO)].
[L Co -CO(CO3)]
[L Co -CO]
- MeCN
At this point, a second protonation breaks the C-O bond forming
+
+
(9)
the [LN4CoI-(CO)2]+ intermediate by the extrusion of a water
N4
II
2+
[L Co -NCMe]
[LN4CoII(CO3)]
molecule. In contrast with the mechanism described in Figure 5, the
CO release from [LN4CoI-(CO)2]+ is thermodynamically favored,
These results clearly show that the formation of 1(I)-CO is both
and the 18 e- intermediate [LN4CoI-CO]+ is recovered closing a
thermodynamically and kinetically favored. The high stability of
catalytic cycle.
1(I)-CO and the partial sequestration of the starting CoII in the form
We have evaluated how the thermodynamics ( Gr) and kinetics
of cobalt carbonate kinetically prevents the catalytic CO2 reduction
(aEspan) of the catalytic process are modified in terms of both the
II/I redox potential, in agreement with the detection of
at the Co
redox potential and pH. Although this type of analysis has its
1(I)-CO and cobalt carbonate species in solution after electrolysis.
precedents in heterogeneous catalysis, it is uncommon in the study
Both theoretical and experimental results highlight the complexity
of molecular systems.31 The variation of the redox potential and pH
of the cobalt catalyzed CO2 reduction mechanism. As it has been

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As it can be inferred from the 2D plot, the increase in the proton
concentration will drive the reaction to region B. Once in region B,
the reaction rate is given by the CoI/0 electron transfer process. That
is why we have included the Marcus electron transfer barrier to
better describe the reaction kinetics of this step. Then, as aEspan
solely involves an electron transfer, the reaction rate only depends
on the reduction potential. The subsequent CO2 binding,
protonation and reduction steps are thermodynamically favored,
and the overall energy profile becomes downhill in Gibbs energy.
For instance, at –2.35 V and pH < 33 the aEspan is 18.0 kcal·mol-1.
Finally, in region C the kinetic barrier depends on the CoI/0
thermodynamics and also on the kinetics of the protonation of the
carboxylate adduct [LN4CoII-CO2(CO)] (Figure S43).
In summary, our model allows for the rationalization of the
experimental observations. First, it describes a regime where the
catalytic reaction is kinetically unfavorable at low overpotentials
and high pH values. This data is also in agreement with the lack of
catalytic current at the CoII/I redox potential, even upon addition of
water to the reaction media, and with the detection of CoI carbonyl
species. In addition, our model gives an explanation of the peak
shift and current increase measured by CV at the CoI/0 redox
potential in the presence of water (vide supra).
The mechanistic proposal for the CO2 reduction at the CoII/I redox
wave suggests that catalysis could be activated by avoiding the
1(II/I)-CO reduction. However, we noticed that this 1e- reduction is
much more favored than the CoII/I process. Therefore, under
electrochemical conditions the formation of 1(I)-CO is difficult to
avoid.
A beneficial strategy to facilitate the metal carbonyl labilization
is the use of photocatalysis since it can operate at very low
concentrations. For bimolecular catalysis/photosensitizer reactions,
at very low concentrations the electron transfer rate is under
diffusion control. Therefore, at low enough catalyst concentration,
the 1(II/I)-CO reduction rate could be lower than the CO release
allowing the CoII/CoII-CO mechanism. Another beneficial strategy
to promote catalysis could be based on the metal carbonyl
labilization. In this regard, photocatalysis can facilitate it. It is wellknown that light induces the M-CO bond cleavage in
organometallic carbonyl species.32

electrolysis experiments under blue light irradiation. Previous
studies by T. C. Lau, M. Robert and co. suggested that light
irradiation could indeed facilitate the CO release in the case of the
[FeI(qpy)CO]+ adduct over the reduction to Fe0 carbonyl species.8a
For these set of experiments, we carefully controlled the reaction
temperature (25 ºC) with a jacketed electrochemical cell connected
to a cryostat. CV of 1(II) under blue LED light (447±20 nm) in CO2saturated solution showed the disappearance of the reoxidation peak
at –0.8 V (Figure S46).10 This feature is reproducible upon
successive switch on/off cycles.

PSIr1
1 µA

1(II)
PSIr10/-

Co(II/I)
Co(I/0)

PSIr2

PSIr2+/0
-3.0

-2.6

-2.2

-1.8

E(V) vs.

-1.4

A

15
13

CO

10

µmol

Catalysis and the effect of light irradiation. With the aim of
testing our hypothesis, we designed the following experiments to
promote catalysis at the 1(II/I) redox couple via the CoII/CoII-CO
mechanism.
We studied 1(II) as a homogeneous catalyst for the light-driven
CO2 reduction in combination with two different cyclometalated Ir
photosensitizers. The typically used [IrIII(ppy)3] (PSIr1) with an
E1/2(PSIr10/-) redox potential of –2.67 V, low enough to promote the
reduction of 1(I/0)-CO and [IrIII(ppy)2(bpy)](PF6) (PSIr2), with a
E1/2(PSIr2+/0) of –1.78 V at which the formation of 1(0)-CO is not
accessible (Figure 8). Experiments were performed with 1(II) (50
µM) and the photosensitizer (200 µM) in CO2 saturated
CH3CN:Et3N mixed (4:1 v/v) irradiated at 447±20 nm for 24 h at
25 °C. Gases evolved were quantified by GC, with CO and H2 as
the only detected products (Figures 9, S44). Remarkably, although
PSIr2 provides a redox potential 820 mV less negative than PSIr1,
both photosensitizers result in a similar reaction rate and catalytic
activity (TON of CO 69±2 and 68±3 for PSIr1 and PSIr2,
respectively). These data confirmed that the in-situ generated CoI
species is able to promote a selective conversion of CO2-to-CO as
anticipated from the electrochemical and computational studies.
DLS analysis indicates that nanoparticles are not responsible for the
main catalytic activity observed (Figure S45).
On the other hand, in an attempt to avoid the CO-poisoning
process under electrochemical conditions, we also performed

-1.0

Fc+/Fc0

Figure 8. CVs of 1(II) (black), PSIr1 (red) and PSIr2 (green) at 0.5
mM concentration in anhydrous TBAPF6/CH3CN (0.1 M) solution.
v = 0.1 V·s–1, Ø = 0.1 cm.

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3

H2

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B

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8

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16
Time (h)

20

24

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10

TONCO

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Figure 9. A) CO (circles) and H2 (triangles) evolution under
photocatalytic conditions ([1(II)] = 50 µM, [PS] = 0.2 mM, 4:1 v/v
CH3CN:Et3N, WLED = 447 nm). PSIr1 (red) and PSIr2 (green) were
used as photosensitizers. B) TON of CO over time under bulk

H+ + e-

N
N

CoII

N
N
N

CoIII

C
O

N

v1 > v2
N
N

CoI
N

UV-vis-SEC
H-NMR, XAS

I/0

N

e-

O

CoI
N

CO2, H+

C

O

H2 O
Catalytic under CPE
at ca. E(CoI/0)
Electrocatalysis in the
dark

2
N
N
N

CoII
N

e- v2
C

O

N
N
N

CoI
N

C

O

v1 < v2

N
N

CoII

N

N

CO2H
C

O

1(I)-CO

1(I)

v1

NCCH3

eE(CoII/I)

N

II/I

E(Co -CO) < E(Co )

H+

C

Traces of CO under CPE
at ca. E(CoII/I)
Photoredox catalytic
cycle

CO2
N

N

O H

pKa = 15.6

KCO2 >> 104

1

N

O

N

electrolysis conditions (1 mM of 1(II) in 0.1 M TBAPF6/CH3CN
under CO2 at Eappl = –2.46 V in the dark (black trace) and under
irradiation (blue trace).

1(II)

CO

CV, IR-SEC
CO

CO

N
N

N
CoI
N

C

O

C O

H+, eH2 O

Scheme 4. Proposed unified mechanism for photo- and electrochemical CO2 reduction catalyzed by 1(II) with relevant catalytic
intermediates based on experimental evidence (dotted boxes) and DFT calculations.
When a constant Eappl potential of –2.46 V is held for 6 h under
irradiation a substantial improvement of the catalytic activity of 1(II)
is observed (TONCO = 13, FYCO = 38%) with respect to the
performance in dark (TONCO = 5.5, FYCO = 26%), in terms of both
catalytic turnovers and faradaic yield for CO production (Figure
S47). Prolonged electrolysis highlights a sustained electrocatalytic
current, leading to almost 20 turnovers of CO after more than 10 h
and maintaining the same average efficiency. This is consistent with
a beneficial effect of blue-light photoirradiation on catalysis,
consisting of a light-induced cleavage of the accumulated stable
Co-CO species in solution, thus favoring a partial regeneration of
the catalyst. On the other hand, the effect of irradiation is barely
observed during light-assisted electrolysis at –1.70 V under CO2
atmosphere, suggesting a smaller effect of light absorption on the
1(I)-CO species (Figures S49 and Table S18).
A unified photo- and electrochemical CO2 reduction
mechanism. Gathering together all studies, in Scheme 4 we present
in a simplified manner our proposal for the most likely pathways
for the 2e- photocatalytic and electrocatalytic CO2 reduction to CO
and the connections between them. In this study, we show that the
formation of a very stable metal carbonyl under electrocatalytic
conditions is detrimental for the catalyst turnover. At the end of the
first catalytic cycle, the catalysis is interrupted by the formation of
1(I)-CO. However, this is not the case under photocatalytic
conditions, which is able to reduce CO2 to CO at a redox value as
low as -1.78 V. As it is shown in Scheme 4, the main difference
between the electrocatalytic and photocatalytic conditions is the
competition between the formation of 1(I)-CO (v2) and CO release
(v1) both from 1(II)-CO. Under electrochemical conditions, the fast
1(II/I)-CO reduction by the electrode surpasses the CO release,
producing 1(I)-CO. Then, in electrocatalytic conditions, the
catalysis is only achieved when system is forced to evolve towards
low valent carbonyl species (CoI/0 blue cycle, Scheme 4). Instead,
under photocatalytic conditions, the CO release is faster than the
bimolecular 1(II/I)-CO reduction from the reduced PS. Since the
latter process depends on the catalyst and PS concentrations, under

diluted conditions, it is expected that the reduction rate can be slow
down, facilitating the CO release and the following intermediates
of the photocatalytic cycle (green cycle, Scheme 4). An interesting
connection between both catalytic cycles is the promotion of 1(I)
from 1(I)-CO by light labialization of the M-CO bond in
organometallic species. Indeed, light can be taken as an advantage
to allow the electrocatalytic performance at CoII/I reduction
potential. Another catalytic cycles interconnection is the potential
formation of Co(II)(CO)(CO2H) (blue cycle, Scheme 4) from
Co(II) (CO2H) + CO (green cycle, Scheme 4), which is slightly
exergonic (-2.4 kcal·mol-1). However, further progress in this
catalytic cycle is not viable under photocatalytic conditions due to
the energetic uphill Co(II/I)(CO)(CO2H) (-1.90 V) reduction,
together with the less favorable C-O cleavage in
Co(II)(CO)(CO2H) than in Co(II)(CO2H). Therefore, at
photochemical redox conditions, Co(II)(CO)(CO2H) can be
assigned as an off-cycle resting state.
Finally, we would like to remark that a large number of potential
interconnections between both catalytic cycles highlights the
challenge and the need for an in-depth analysis, even in CO2
reduction prototype reactions. To further progress into the
understanding, more elaborated approaches should be taken, such
as using graph theory to unravel all potential pathways and their
weight into the global mechanism for given reaction conditions.

CONCLUSIONS
We have presented a detailed mechanistic investigation of
electrochemical CO2-to-CO reduction catalyzed by a new cobalt
catalyst (1(II)) based on a highly basic tetradentate aminopyridyl
ligand. To the best of our knowledge, FTIR-SEC provides the first
in-situ spectroscopic evidence for the formation of a CoI-CO (1(I)CO, CCO = 1910 cm-1) resulting from the electrochemical CO2-toCO reduction at the non-catalytic CoII/I redox wave. This
observation has relevant mechanistic implications since it shows
that: 1) the electrochemically generated CoI species (1(I)) is
nucleophilic enough to bind the CO2 molecule and 2) the C-O bond

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cleavage can occur at room temperature, at mild applied potentials
and with no added protons in acetonitrile. DFT modeling of the
reaction mechanism has corroborated that both the CO2 binding and
the C-O bond cleavage steps are kinetically feasible at the CoII/I
redox potential. However, the CO release from 1(I)-CO is a key
limiting step which prevents the recovery of the catalytically active
species 1(I). Computational modeling of the different catalytic
mechanisms in a broad potential and pH windows allowed for the
rationalization of our experimental observations. The catalytic
mechanism is triggered by the one-electron reduction of 1(I)-CO to
the corresponding formal Co0 which can only be afforded close to
the CoI/0 redox potential. Photocatalytic experiments under bluelight irradiation confirm the ability of 1(I) towards catalytic CO2
reduction, even when the E1/2 of the PSIr is not suitable for the 1(I/0)CO reduction. It is proposed that under photocatalytic conditions
the CO release from 1(II)-CO is kinetically favored over the 1(I)-CO
reduction due to the low concentration of catalyst and
photosensitizers.
Finally, light-assisted electrocatalysis was successfully employed
to improve the catalytic performance of 1(II) at -2.46 V reduction
potential. The irradiation, favors the activation of inactive carbonyl
species and reaching higher efficiency for CO production. In view
of these findings, light-induced metal carbonyl dissociation was
revealed as a promising strategy to mitigate CO catalyst poisoning.
Finally, we have proposed a unified mechanistic view of the
existing differences between photo- and electrochemical CO2-toCO reduction catalysis (Scheme 4). The results presented here will
help to rationalize the behavior of other reported cobalt-based
molecular electrocatalysts and to find out new approaches for the
optimization of earth-abundant molecular catalysts.

ASSOCIATED CONTENT
Supporting Information
Methods of synthesis, characterization of reaction intermediates,
catalytic studies and DFT studies. This material is available free of
charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION
Corresponding Author
E-mail: josepm.luis@udg.edu
E-mail: jlloret@iciq.es

Author Contributions
‡These authors contributed equally.

ACKNOWLEDGMENT
We would like to thank the European Commission for the ERCCG-2014-648304 (J.Ll.-F) project. The Spanish Ministry of
Science is acknowledged for a FPU fellowship to S.F. and C.C. We
also thank Catexel for a generous gift of tritosyl-1,4,7triazacyclononane. The financial support from ICIQ Foundation
and CELLEX Foundation through the CELLEX-ICIQ and the
Starting Career Program is gratefully acknowledged. We also thank
CERCA Programme and DIUE 2014SGR931 (Generalitat de
Catalunya) for financial support and MINECO project CTQ201680038-R and PGC2018-098212-B-C22. We acknowledge SOLEIL
and DIAMOND for provision of synchrotron radiation facilities
and we would like to thank Dr. Gautier Landrot for assistance in
using beamline SAMBA.

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