of 8
Pendant Hydrogen-Bond Donors in Cobalt Catalysts Independently
Enhance CO
2
Reduction
Alon Chapovetsky,
,
Matthew Welborn,
,
§
John M. Luna,
Ralf Haiges,
Thomas F. Miller III,
*
,
§
and Smaranda C. Marinescu
*
,
Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
§
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
*
S
Supporting Information
ABSTRACT:
The bioinspired incorporation of pendant proton
donors into transition metal catalysts is a promising strategy for
converting environmentally deleterious CO
2
to higher energy
products. However, the mechanism of proton transfer in these
systems is poorly understood. Herein, we present a series of
cobalt complexes with varying pendant secondary and tertiary
amines in the ligand framework with the aim of disentangling
the roles of the
fi
rst and second coordination spheres in CO
2
reduction catalysis. Electrochemical and kinetic studies indicate
that the rate of catalysis shows a
fi
rst-order dependence on acid,
CO
2
, and the number of pendant secondary amines, respectively. Density functional theory studies explain the experimentally
observed trends and indicate that pendant secondary amines do not directly transfer protons to CO
2
, but instead bind acid
molecules from solution. Taken together, these results suggest a mechanism in which noncooperative pendant amines facilitate a
hydrogen-bonding network that enables direct proton transfer from acid to the activated CO
2
substrate.
INTRODUCTION
The catalytic conversion of carbon dioxide (CO
2
) into chemical
fuels holds promise for mitigating the adverse e
ff
ects of fossil
fuels on the environment.
1
9
In nature, the selective and
reversible conversion of CO
2
to carbon monoxide (CO) is
catalyzed by the enzyme CO-dehydrogenase (CODH) through
the transfer of two electrons and two protons.
2
,
10
Binding and
reduction of CO
2
occur through bifunctional activation by the
two metal centers in the NiFe cluster and additional
stabilization through hydrogen bonding from appropriately
positioned residues, as revealed by structural studies of the
active site.
11
These studies suggest that a transition-metal
center surrounded by ligands with pendant proton donors is an
e
ff
ective design motif for arti
fi
cial CO
2
reduction catalysts.
Pendant proton donors also facilitate catalysis of the
hydrogen evolution reaction (HER). In nature, HER occurs
at the FeFe-hydrogenase (FFH) active site, which
like
CODH
exhibits two metal centers surrounded by pendant
proton donors.
2
,
12
In FFH, protons are shuttled to and from
the active metal center by the secondary amine of an
azadithiolene moiety.
13
The structure of this active site has
inspired the development of nickel phosphine complexes
bearing pendant tertiary amines; these complexes have proven
to be extremely active HER electrocatalysts, with a mechanism
that involves the cooperative interaction of H
2
with both the
metal center and the pendant amines.
4
,
14
,
15
Incorporation of pendant proton donors into molecular
catalysts has only recently been explored in the context of CO
2
reduction. The electrochemical CO
2
-to-CO activity of iron
porphyrin and metal bipyridine complexes was shown to
increase with the incorporation of pendant phenol or
trimethylanilinium moieties into the ligand sca
ff
old.
6
,
16
24
The prepositioned phenol groups were proposed to stabilize
the initial Fe
(0)
CO
2
adduct through H-bonding, as con
fi
rmed
by density functional theory (DFT) studies, and to facilitate the
intramolecular protonation.
17
,
18
Nickel and cobalt phosphine
complexes bearing pendant tertiary amines have been recently
shown to catalyze the electrochemical conversion of CO
2
to
formic acid, as well as the reverse reaction.
25
27
Additionally,
nickel and cobalt cyclam systems bearing four secondary
amines were shown to electrochemically convert CO
2
into
CO.
1
,
3
,
5
,
9
,
28
33
DFT studies proposed that CO
2
is activated
through cooperative hydrogen-bonding interactions between
the Ni-bound substrate and the secondary amines in the ligand
framework.
9
,
30
,
34
,
35
Electrochemical studies of nickel complexes
supported by mono-, di-, tri-, and tetramethylated cyclam
ligands revealed that both the catalytic activity and faradaic
e
ffi
ciency for CO
2
-to-CO conversion decrease upon methyl-
ation of the ligand framework.
30
,
35
Methylation of the cyclam
ligands impacts both the
fi
rst and second coordination spheres
of the nickel complexes. Hence, the e
ff
ects of the two cannot be
decoupled and a direct correlation between the number of
pendant proton donors and the activity of the catalyst could not
be established.
Received:
December 19, 2017
Published:
February 23, 2018
Research Article
Cite This:
ACS Cent. Sci.
2018, 4, 397
404
© 2018 American Chemical Society
397
DOI:
10.1021/acscentsci.7b00607
ACS Cent. Sci.
2018, 4, 397
404
This is an open access article published under an ACS AuthorChoice License, which permits
copying and redistribution of the article or any adaptations for non-commercial purposes.
We previously reported a cobalt complex bearing four
pendant secondary amine (NH) groups incorporated in the
ligand sca
ff
old that is an e
ffi
cient electrocatalyst for the selective
reduction of CO
2
to CO.
36
Methylation of all four secondary
amines produces a 300-fold reduction in activity, indicating that
the pendant amines are important for catalysis.
36
Unlike the
metal cyclam series, the pendant amines lie completely outside
of the primary coordination sphere of the metal center,
allowing for isolation of the role of pendant protons. In
addition, our ligand framework allows for discrete control over
the number and con
fi
guration of the proton donors present in
the outer sphere of the metal center without impacting its
primary coordination environment.
37
These structural features
enable us to decouple the roles of the
fi
rst and second
coordination spheres in a systematic and well-controlled
manner. In the current work, we investigate an expanded
series of cobalt aminopyridine compounds through the
synthesis of mono-, 1,2-di-, 1,3-di-, and trimethylated ligand
sca
ff
olds (
Figure 1
). We combine experimental and theoretical
approaches to elucidate the mechanism by which these catalysts
bind and reduce CO
2
in an e
ff
ort to isolate and quantify the
e
ff
ect of pendant amine protons on catalysis. This work
provides new design principles for tuning the e
ff
ect of the
second coordination sphere on CO
2
reduction.
RESULTS
Syntheses and Physical/Electrochemical Character-
ization.
Complexes
2
(II)
through
5
(II)
are synthesized using
procedures similar to those reported for complexes
1
(II)
and
6
(II)
(
Figure 1
).
36
Single crystal X-ray di
ff
raction studies reveal a
consistent motif for all six complexes, with a cobalt metal center
coordinated by four pyridines in a square planar fashion
(
Figures S1
S4, Tables S1
S4
). Complexes
1
(II)
6
(II)
adopt a
saddle conformation in which each set of opposing amines
points outward from the face of the complex. There is little
variance in the Co
N
Pyridine
and Co
N
PendantAmine
bond lengths
(1.95(2) Å and 3.06(4) Å on average, respectively) among the
complexes. The measured p
K
a
values of complexes
1
(II)
5
(II)
range between 2.48 and 3.10 (
Table 1
). Given their acidity,
complexes
1
(II)
5
(II)
are expected to be singly deprotonated in
solution.
38
Attempts to further titrate samples
1
(II)
5
(II)
in
order to obtain additional p
K
a
values have been unsuccessful
and resulted in decomposition. Electrochemical character-
ization of complexes
1
6
under N
2
reveals a reversible one-
electron reduction with
E
1/2
between
1.41 and
1.65 V vs
Fc
+/0
, attributed to the Co
(II)
/Co
(I)
couple (
Table 1
,
Figures
S5
S14
). An irreversible reduction feature is observed between
2.46 and
2.87 V vs Fc
+/0
and is attributed to the reduction
of Co
(I)
to Co
(0)
(
Figures S15
S19
). At fast scan rates,
complexes
1
and
2
show a positively shifted return oxidation,
indicating that the couple is slow or involves a chemical
rearrangement.
Mechanistic Studies of CO
2
Reduction Catalysis.
To
explain the 300-fold decrease in activity between complexes
1
and
6
, we previously proposed that CO
2
binding and catalysis
in complex
1
occurs through the formation of a Co
(0)
CO
2
adduct, which is stabilized through two intramolecular H-
bonding interactions from the pendant secondary amines.
36
This stabilization cannot occur for
6
, which lacks pendant
secondary amines. The current study provides additional kinetic
analysis of complexes
1
and
6
, as well as the additional context
of complexes
2
5
, to elucidate the mechanism by which these
catalysts bind and reduce CO
2
.
Electrochemical reduction of
1
under varying concentrations
of CO
2
(0 to 0.2 M) gives rise to current increases at potentials
near that of Co
(I)
/Co
(0)
reduction (
Figure S20
). The potential
corresponding to the maximum current displays a positive shift
with increased [CO
2
]. This behavior is Nernstian and suggests
a thermodynamically favorable interaction between
1
(0)
and
CO
2
, consistent with the formation of a CO
2
-bound
preassociation complex.
36
Controlled potential electrolysis
(CPE) studies of
1
under CO
2
saturation generate trace
amounts of CO, suggesting that even in the absence of any
added proton donors the CO
2
-to-CO conversion occurs. This
result is in agreement with the p
K
a
measurements, which
indicate that the NH moieties are acidic. Hence, useful
quantitative CO
2
binding constants cannot be extracted from
the positive shifts observed in the titration of complexes
1
5
with CO
2
.
32
,
39
Figure 1.
Syntheses of complexes
1
(II)
6
(II)
. The oxidation state of the cobalt center in each complex is indicated by the superscript.
Table 1. Parameters for Complexes 1
6
a
complex
E
1/2
(CoL
2+/+
)
vs Fc
+/0
E
(CoL
+/0
)
vs Fc
+/0
i
cat
/
i
p
k
obs
(s
1
)
FE
(%) p
K
a
1
1.65
2.46
208.8 16,900 98 2.74
2
1.66
2.73
189.9 14,000 98 2.66
3
1.53
2.41
130.0
6,200 98 2.53
4
1.52
2.41
113.7
5,200 98 3.10
5
1.44
2.87
11.4
50 90 2.48
6
1.41
2.58
7.7
20 36 na
a
Reduction potentials (
E
), normalized current densities (
i
cat
/
i
p
),
catalytic rate constants (
k
obs
), Faradaic e
ffi
ciencies (FE), and p
K
a
values for complexes
1
6
.
ACS Central Science
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ACS Cent. Sci.
2018, 4, 397
404
398
Titration of
1
with acid (2,2,2-tri
fl
uoroethanol, TFE) under
CO
2
saturation gives rise to a series of plateaus with a
maximum at
2.7 V vs Fc
+/
0
(
Figure S24
,
Table S6
). The
normalized peak catalytic current (
i
cat
/
i
p
) is related to the rate
of the catalytic reaction, as described in more detail in
Supporting Information eqs 1
5
.The
i
p
values used
correspond to the peak current obtained from either the
irreversible Co
I/0
reduction or the reversible Co
II/I
couple. A
plot of the catalytic rate constant,
k
obs
, vs [TFE] shows a linear
correlation, indicating a reaction that is
fi
rst order in acid
(
Figure S25
). A titration with CO
2
at 1.2 M TFE followed by
an analogous analysis produces a linear correlation between the
catalytic rate constant and [CO
2
], consistent with a reaction
that is
fi
rst order in CO
2
(
Figures S22 and S23, Table S5
).
Titrations with TFE-
d
3
are performed under CO
2
saturation
(
Figures S27
S29, Table S7
), giving rise to a H/D kinetic
isotope e
ff
ect (KIE) of 1.4(2). This result suggests that protons
are involved in the rate-limiting step (RLS).
Complexes
2
6
are studied in a similar manner (
Table 1
,
Figures S30
S41
). All complexes exhibit a scan-rate
independent, linear relationship between the rate and [TFE],
indicative of a reaction that is
fi
rst order in [TFE]. Controlled
potential electrolysis (CPE) studies in the presence of 1.2 M
TFE reveal that
1
5
produce CO with excellent faradaic
e
ffi
ciencies (
90%) and turnover numbers (
Figures S42
S46,
Table S8
); by contrast, complex
6
is a poor catalyst with low
faradic e
ffi
ciency (36%), producing negligible amounts of CO
or other common CO
2
reduction products, such as hydrogen or
formic acid. The titration results, coupled with results from
CPE, indicate that complexes
1
5
are competent catalysts, thus
allowing for direct comparison of their catalytic performance.
Interestingly, a linear correlation is observed between the
catalytic rate constant and the number of secondary amines on
the metal complex for
1
5
(
Figure 2
a), suggesting that the
pendant secondary amines play a critical role in catalysis.
Because the measured rates for
3
and
4
are similar, we
hypothesize that the pendant secondary amines act in a
noncooperative manner, since the di
ff
ering orientation of the
pendant amines in these complexes has no e
ff
ect on the
catalytic rate. The rates in
Figure 2
a vary linearly with the
number of pendant amines minus one, consistent with a model
in which one pendant amine is singly deprotonated and unable
to donate a hydrogen bond; complex
5
(which has only one
secondary amine) operates at the same rate as complex
6
and
100 times slower than complexes
3
and
4
. We also note that the
Co
(I)
/Co
(0)
reduction potential in complexes
1
5
does not
correlate with catalytic performance (
Table 1
), suggesting that
other factors govern the catalytic activity of these cobalt
aminopyridine complexes.
40
,
41
Taken together, the experimental results begin to paint a
picture of the CO
2
reduction mechanism. Linear rate
dependence on both [CO
2
] and [TFE] indicates that the
reaction is
fi
rst order in each, and the observation of a positive
shift in onset potential with the addition of CO
2
, but not TFE,
indicates favorable binding between the complex and CO
2
. The
hydrogen kinetic isotope e
ff
ect (
k
H
/
k
D
) of 1.4(2) suggests that
protons are involved in the RLS. Finally, the linear rate
dependence of the catalytic rate on the number of pendant
secondary amines indicates that they play a central role in the
reaction mechanism. However, key mechanistic details remain
unresolved, including the precise manner in which the pendant
proton donors facilitate the catalytic mechanism, the nature of
the CO
2
preassociation with the complex, and the competition
between inter- and intramolecular proton-transfer steps in the
reduction of CO
2
; DFT calculations are employed in the
following sections to address these points.
Formation of the CO
2
-Bound Preassociation Com-
plex.
DFT calculations are used to examine the structure and
energetics of the CO
2
-bound preassociation complex suggested
by the electrochemical experiments. Initially focusing on
1
, the
geometry of the CO
2
-bound complex is optimized in its various
accessible oxidation states (corresponding to
1
(II)
,
1
(I)
, and
1
(0)
for the unbound complex). Of these, only
1
(0)
has a stable
minimum when bound with CO
2
, consistent with the Nernstian
shifts in
Figure S20
. In this bound complex, CO
2
binds to the
metal center via the carbon atom with a bond length of 2.06 Å,
with the pendant amines pointed away from the CO
2
binding
site (inset of
Figure 2
b). Upon binding, charge transfers from
the Co center to the CO
2
, leading to oxidation of the metal
center to a +1 state and a bent CO
2
geometry like that of the
gas-phase anion. The anionic character of the bound CO
2
is
further supported by CHELPG charge analysis
42
(
Table S9
).
We now examine the role of the pendant amine protons in
CO
2
binding. Previous studies have shown that intramolecular
hydrogen bonds can stabilize the bound CO
2
.
17
,
18
,
43
In
principle, such interactions are available in complex
1
(I)
CO
2
if the pendant secondary amines undergo umbrella
fl
ipping to
orient their protons toward the bound CO
2
. To test this
possibility, we compute the energetics and barriers for the
conformational change associated with forming either one or
two hydrogen bonds between the pendant amine protons and
the CO
2
ligand. The structure without intramolecular hydrogen
Figure 2.
(a) Experimental catalytic rate constants,
k
obs
(s
1
), as a function of the number of pendant secondary amines for complexes
1
6
measured
in the presence of 1.5 M TFE and under CO
2
saturation at a scan rate of 100 mV/s. Rates are obtained from the plateau current. A linear
fi
t(
R
2
=
0.97) is shown in gray for complexes
1
5
. (b) Experimental catalytic rate constants (log scale) versus computed CO
2
binding energy for complexes
1
(I)
CO
2
through
6
(I)
CO
2
. The best-
fi
t line (
R
2
= 0.97) for complexes
1
(I)
CO
2
through
4
(I)
CO
2
is shown in gray. Inset: side and top views of
the geometry of complex
1
(I)
CO
2
. Atom colors: H = white, C = gray, N = blue, O = red, and Co = cyan.
ACS Central Science
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2018, 4, 397
404
399
bonds is the most stable, with formation of a single hydrogen
bond incurring a cost of 5.2 kcal/mol with a barrier of 10.7
kcal/mol, and with the second hydrogen bond incurring a
further 1.3 kcal/mol energy cost with a barrier of 7.2 kcal/mol
(
Figure S49
). Factors contributing to the unfavorability of
hydrogen-bond formation include ring strain in the ligand
sca
ff
old, the need to rotate CO
2
into a sterically unfavorable
con
fi
guration, and the relatively weak hydrogen-bond inter-
actions. Although this mode of binding di
ff
ers from that seen
previously in iron porphyrins,
18
it is con
fi
rmed by embedded
multireference con
fi
guration interaction singles and doubles
calculations (see
Supporting Information
).
41
,
44
To examine the e
ff
ect of methyl substitution on the binding
of CO
2
, we repeat the binding-energy calculations for
complexes
2
(I)
CO
2
through
6
(I)
CO
2
.Allsixbound
complexes are isostructural, with Co
C bond lengths ranging
between 2.06 and 2.17 Å (
Table S9
). The possibility of
intramolecular hydrogen bonding was also considered in all
fi
ve
NH-containing complexes and was found to be similarly
unfavorable in all cases.
In terms of CO
2
binding, the main di
ff
erence among
complexes
1
6
is the degree to which steric repulsions are
incurred between CO
2
and the methyl groups, which weakens
CO
2
binding. Complex
1
(I)
CO
2
has a CO
2
binding energy of
11.8 kcal/mol, whereas methylation of all four secondary
amines as in complex
6
(I)
CO
2
results in a binding strength of
0.4 kcal/mol. These weakened binding strengths lead to
reduced populations of
X
(I)
CO
2
preassociation complexes,
which manifests as a multiplicative factor in the rate under the
assumption that the mechanism proceeds via a CO
2
-bound
preassociation complex. Speci
fi
cally, we expect the catalytic rate
to be exponential in the CO
2
binding energy,
Δ
G
X
b
∝−
Δ
k
G
kT
exp
X
X
obs,
b
Figure 2
b tests the assumption that the catalytic mechanism
proceeds via a CO
2
-bound preassociation complex by plotting
the relationship between the catalytic rate constant and the
computed CO
2
binding energy for complexes
1
6
. For the
complexes that exhibit at least one available pendant proton
(i.e.,
1
4
), the rate is exponential with the CO
2
binding energy,
supporting a mechanism that involves formation of CO
2
-bound
preassociation complex. According to the trend from complexes
1
4
in
Figure 2
b, complexes
5
and
6
also bind CO
2
su
ffi
ciently
well to perform catalysis. However, as is discussed in the next
sections, complexes
5
and
6
are unable to employ the same
catalytic pathway as
1
4
due to the absence of an available
pendant proton. As mentioned previously, the measured p
K
a
values of complexes
1
(II)
5
(II)
range between 2.48 and 3.10
(
Table 1
), whereas the p
K
a
of TFE in DMSO is 23.5.
45
Therefore, complexes
1
(II)
5
(II)
are expected to be singly
deprotonated in solution, reducing the number of pendant
protons available for catalysis by one. Additionally, computa-
tional studies on
1
(II)
estimate that its second deprotonation
would correspond to a p
K
a
of 5.2, and its third deprotonation
would correspond to a p
K
a
of 14.0. However, these additional
p
K
a
values cannot be measured experimentally, due to the
decomposition observed at pH above 3.5.
Nature of the Rate-Limiting Protonation Step.
Following binding, CO
2
must be twice protonated to complete
the catalytic cycle. Previous work on an analogous cobalt
tetraazamacrocyclic complex
indicates that this process
proceeds sequentially, with the
fi
rst protonation forming a
COOH ligand and the second protonation cleaving the C
OH
bond to form water and bound CO.
46
In this work, the
previously discussed experimental results suggest that a proton-
involving step is rate-limiting for the overall catalytic cycle. We
now use computation to investigate the various available
protonation pathways.
Reaction energies of
X
(I)
CO
2
(
X
=
1
6
) with a proton
from solution to yield
X
(II)
CO
2
Harecomputedwith
reference to the experimental free energy of solvation of the
proton in DMSO (details in
Supporting Information
). The
overall energy of this reaction ranges from
9.0 kcal/mol
(complex
1
(I)
CO
2
)to
18.2 kcal/mol (complex
3
(I)
CO
2
).
Having completed two reduction steps and a
fi
rst
protonation step, the catalytic cycle could involve either further
protonation (
EECC
mechanism,
Figure 3
,whereE=
electrochemical, C = chemical step) or reduction of
1
(II)
CO
2
H followed by protonation (
(E)ECEC
mechanism,
Figure S53
). Because the reduction of Co
(I)
to Co
(0)
is
irreversible, these mechanisms cannot be experimentally
distinguished and calculations are instead employed. The 1-
electron reduction potential of
1
(II)
CO
2
H is calculated to be
2.9 V vs Fc
+/0
, suggesting that reduction of
1
(II)
CO
2
H is not
kinetically competent at the potential of maximum catalytic
current (
2.7 V vs Fc
+/0
). This, along with precedent from the
aforementioned results of ,
46
suggests that the EECC
mechanism is dominant; the alternative (E)ECEC mechanism
is further detailed in the
Supporting Information
.
Proceeding along the EECC mechanism, the second
protonation may occur either on the protonated oxygen to
form a CO(OH
2
) adduct or on the unprotonated oxygen to
Figure 3.
Proposed EECC catalytic cycle illustrated with complex
1
,
where E = electrochemical, and C = chemical step.
ACS Central Science
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400
form a C(OH)
2
adduct. Calculations for
1
(II)
CO(OH
2
) and
1
(II)
C(OH)
2
show that the latter is less favorable by 26.4
kcal/mol, eliminating it from the catalytic pathway. After the
second protonation, the C
OH bond in
X
(II)
CO(OH
2
)
spontaneously breaks, forming
X
(II)
CO and water. The
energy of this reaction ranges from
8.4 kcal/mol (complex
1
(II)
CO
2
H) to
10.4 kcal/mol (complex
6
(II)
CO
2
H).
Figure 4
shows a comparison of the overall thermodynamic
driving force between the
fi
rst and second protonations for
complexes
1
6
. The clustering of complexes
1
and
2
versus
3
6
is believed to be due to solvation. This is further discussed in
the
Supporting Information
section titled
E
ff
ect of solubility
on the
fi
rst protonation energy.
In all six complexes, the
second protonation is less favorable, consistent with previous
studies on iron porphyrin complexes.
18
This result can be
understood as a di
ff
erence in nucleophilicity between the CO
2
and COOH intermediates, with the CO
2
ligand exhibiting
greater anionic character (
Tables S9 and S11
). Though
activation barriers were not calculated, the Bell
Evans
Polanyi
principle suggests that the energetically less favorable second
protonation will likewise have a slower reaction rate. In
summary, we argue that
X
(I)
CO
2
is twice protonated on the
same oxygen via the EECC mechanism to form water and
X
(II)
CO, with the second of these protonation steps
constituting the RLS.
Intra- versus Intermolecular Proton Transfer for the
Rate-Limiting Step.
Having identi
fi
ed the second protonation
of the CO
2
ligand as the likely RLS, we now investigate whether
protonation is more favorable via an intra- vs intermolecular
mechanism. Our experimental observations constrain this
mechanism in three ways, with (i) titrations indicating that
the RLS involves undissociated TFE, (ii) the overall catalytic
rates for complexes
1
6
suggesting that the RLS involves the
pendant secondary amines in a noncooperative manner, and
(iii) the H/D KIE indicating that the RLS involves protons.
Two mechanisms consistent with these observations include
intramolecular acid-assisted proton transfer from the pendant
amine to COOH and intermolecular pendant amine assisted
proton transfer from the acid to the COOH. For the
intramolecular mechanism, proton transfer follows rotation of
the COOH ligand and umbrella-
fl
ipping of the pendant amine
to form a hydrogen-bonding geometry (
Figure 5
); this
conformational rearrangement is energetically uphill by 10.3
kcal/mol with a barrier of 15.2 kcal/mol, and the subsequent
proton transfer step has a barrier of 3.9 kcal/mol. The high
energy barrier for reaching the intermediate disfavors this
mechanism, as does the fact that inclusion of a bound acid
molecule in the calculations further destabilizes the inter-
mediate by 1.1 kcal/mol, contradicting the experimental trend
of increasing rate with increasing acid concentration (
Figures
S51 and S52
).
Figure 4.
Protonation energies of complexes
1
(I)
CO
2
through
6
(I)
CO
2
(blue) and
1
(II)
CO
2
H through
6
(II)
CO
2
H (red).
Figure 5.
Two pathways for the rate-limiting protonation of complex
1
(II)
CO
2
H. Geometries of critical points are shown and labeled by their
energies in kcal/mol. Below: side view of a key intermediate from each pathway. Hydrogen bonds are labeled with dashed lines, and their lengths are
given in ångstroms. Fluorine atoms are shown in green.
ACS Central Science
Research Article
DOI:
10.1021/acscentsci.7b00607
ACS Cent. Sci.
2018, 4, 397
404
401
To investigate the intermolecular proton-transfer mecha-
nism, we consider the structure and energetics of
1
(II)
CO
2
H
complex, which contains a TFE molecule bridging the pendant
amine and COOH (
Figure 5
, blue box). Geometry
optimization reveals a stable binding energy of 4.3 kcal/mol,
and a short (1.81 Å) hydrogen bond, which suggests the
favorability of intermolecular proton transfer to the COOH
ligand. Note that the amine
acid hydrogen-bond distance is
too long to support a shuttle mechanism in which a proton is
transferred from the amine to the acid simultaneous with
proton transfer from the acid to the COOH ligand. We thus
conclude that the second protonation proceeds via direct
proton transfer from TFE to the COOH ligand, assisted by the
pendant amine proton.
Table S12
con
fi
rms that this analysis is
consistent with the previous section, indicating that all of the
corresponding barriers and intermediates shown in
Figure 5
are
lower in energy when considering the
fi
rst protonation step.
DISCUSSION
Figure 3
summarizes the proposed catalytic mechanism that
emerges from the combined analysis. Complex
X
(II)
(
X
=
1
6
)
is reduced by two electrons and binds CO
2
to yield
X
(I)
CO
2
.
The CO
2
adduct is twice protonated, with the latter being rate-
limiting and occurring via an intermolecular mechanism that is
noncooperatively facilitated by the pendant amines. Finally, the
C
OH
2
bond spontaneously cleaves to release water, and CO
dissociates from
X
(II)
CO to regenerate the catalyst.
Given this mechanism, the overall catalytic rate for complex
X
(
k
obs,
X
)is
=−
×
−Δ
kn k
X
((
1)[TFE] ) ([ ][CO ] e
)
GkT
XX
obs,
RLS
2
/
X
b
where
k
RLS
is the rate constant of the RLS per pendant amine
proton irrespective of
X
,(
n
X
1) is the number of available
amine protons in
X
,[
X
] is the concentration of
X
, and
Δ
G
X
b
is
the free energy of CO
2
binding. The
fi
rst grouping of terms
summarizes the kinetics of the rate-limiting step, and the
second summarizes the binding of CO
2
to form the
preassociation complex. A derivation of this rate equation can
be found in the
Supporting Information
section titled
Kinetics
of the EECC mechanism.
For the rate-limiting step, each pendant amine can
noncooperatively bind an acid molecule, activating and
enhancing the local concentration of proton donors around
the COOH adduct. The noncooperative nature of this
hydrogen bonding makes the degree of catalytic enhancement
dependent only on the number of available pendant proton
donors, such that
k
RLS
is independent of the number of pendant
amines (
Figure 2
a).
The second grouping of terms in the rate expression
represents the thermodynamics of CO
2
binding to form the
preassociation complex. As seen in
Figure 2
b, the experimental
catalytic rate is exponentially related to the computed CO
2
binding free energy for complexes
1
4
. This trend does not
extend to complexes
5
and
6
where the
fi
rst grouping of terms
sets the overall rate to zero, due to the absence of an available
pendant proton.
CONCLUSIONS
We introduce and characterize a series of cobalt aminopyridine
complexes that vary as a function of the number of pendant
proton donors and allow for the well-controlled analysis of
contributions from the
fi
rst and second coordination spheres in
CO
2
reduction catalysis. Electrochemical studies show that the
CO
2
reduction activity of these complexes depends strongly on
the number of secondary amines incorporated in the ligand
framework. The observed linear dependence of the rate of
catalysis on the number of pendant proton donors has not been
previously reported for CO
2
reduction. Computational studies
reveal the mechanism by which the pendant amines facilitate
rate-limiting intermolecular proton transfer via noncooperative
hydrogen bonds to acid in solution. By enabling systematic
control over the number of proton relays present in the second
coordination sphere, the reported complexes provide a relevant
model for biological systems and homogeneous catalysts for
small molecule activation. Furthermore, these complexes o
ff
er a
framework for tuning the e
ff
ect of the second coordination
sphere on CO
2
reduction, and more generally, on multi-
electron, multiproton reduction reactions.
ASSOCIATED CONTENT
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website
at DOI:
10.1021/acscentsci.7b00607
.
Experimental details and crystallographic data (
PDF
)
Crystallographic data for
2
(II)
(
CIF
)
Crystallographic data for
3
(II)
(
CIF
)
Crystallographic data for
4
(II)
(
CIF
)
Crystallographic data for
5
(II)
(
CIF
)
CCDC 1590218
1590221 contain the supplementary crystal-
lographic data for this manuscript. These data can be obtained
free of charge via
www.ccdc.cam.ac.uk/data_request/cif
,orby
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.
AUTHOR INFORMATION
Corresponding Authors
*
E-mail:
smarines@usc.edu
.
*
E-mail:
tfm@caltech.edu
.
ORCID
Thomas F. Miller III:
0000-0002-1882-5380
Smaranda C. Marinescu:
0000-0003-2106-8971
Author Contributions
A.C. and M.W.: equal contribution.
Notes
The authors declare no competing
fi
nancial interest.
ACKNOWLEDGMENTS
This work was supported by the University of Southern
California and the National Science Foundation (NSF) through
the CAREER award (CHE-1555387) and the Chemistry of Life
Processes Program (CHE-1611581). This material is based
upon work performed by the Joint Center for Arti
fi
cial
Photosynthesis, a DOE Energy Innovation Hub, supported
through the O
ffi
ce of Science of the U.S. Department of Energy
under Award Number DE-SC0004993. We are grateful to the
USC Wrigley Institute for a Norma and Jerol Sonosky summer
fellowship to A.C. M.W. thanks the Resnick Sustainability
Institute for a postdoctoral fellowship. We are grateful to NSF
(Grant CRIF 1048807) and USC for their sponsorship of
NMR spectrometers and an X-ray di
ff
ractometer.
ACS Central Science
Research Article
DOI:
10.1021/acscentsci.7b00607
ACS Cent. Sci.
2018, 4, 397
404
402
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