Electro- and Photochemical Reduction of CO2 by Molecular Manganese Catalysts: Exploring the Positional Effect of Second-Sphere Hydrogen-Bond Donors

Article abstract of DOI:10.1002/cssc.202001940

A series of molecular Mn catalysts featuring aniline groups in the second-coordination sphere has been developed for electrochemical and photochemical CO₂ reduction. The arylamine moieties were installed at the 6 position of 2,2’-bipyridine (bpy) to generate a family of isomers in which the primary amine is located at the ortho- (1-Mn), meta- (2-Mn), or para-site (3-Mn) of the aniline ring. The proximity of the second-sphere functionality to the active site is a critical factor in determining catalytic performance. Catalyst 1-Mn, possessing the shortest distance between the amine and the active site, significantly outperformed the rest of the series and exhibited a 9-fold improvement in turnover frequency relative to parent catalyst Mn(bpy)(CO)₃Br (901 vs. 102 s^?1, respectively) at 150 mV lower overpotential. The electrocatalysts operated with high faradaic efficiencies (≥70 %) for CO evolution using trifluoroethanol as a proton source. Notably, under photocatalytic conditions, a concentration-dependent shift in product selectivity from CO (at high [catalyst]) to HCO₂H (at low [catalyst]) was observed with turnover numbers up to 4760 for formic acid and high selectivities for reduced carbon products.

Full text of DOI:10.1002/cssc.202001940

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Electro- and Photochemical Reduction of CO2 by Molecular
Manganese Catalysts: Exploring the Positional Effect of Second
Sphere Hydrogen-Bond Donors
Authors: Jonah W. Jurss, Kallol Talukdar, and Sayontani Sinha Roy
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ChemSusChem
10.1002/cssc.202001940
FULL PAPER
Electro- and Photochemical Reduction of CO by Molecular
2
Manganese Catalysts: Exploring the Positional Effect of Second
Sphere Hydrogen-Bond Donors
†
†
Sayontani Sinha Roy, Kallol Talukdar, and Jonah W. Jurss*
†
S.S.R. and K.T. contributed equally
S. Sinha Roy, K. Talukdar, Prof. Dr. J. W. Jurss
Department of Chemistry and Biochemistry
University of Mississippi
University, Mississippi 38677, USA
E-mail: jwjurss@olemiss.edu
Supporting information for this article is given via a link at the end of the document.
Abstract: A series of molecular Mn catalysts featuring aniline groups
continue to garner attention. Researchers have also modified the
second-coordination sphere of these systems in order to achieve
lower overpotentials and enhanced activities for electrochemical
and light-driven catalysis.[8–18] In this strategy, functional groups
are appended to the ligand framework to introduce specific
noncovalent, secondary interactions near the active site of
catalysts that serve to stabilize intermediates or promote key
steps (i.e. proton transfer) in the catalytic cycle.[19,20]
in the second-coordination sphere has been developed for
2
electrochemical and photochemical CO reduction. The arylamine
moieties were installed at the 6 position of 2,2’-bipyridine (bpy) to
generate a family of isomers in which the primary amine is located at
the ortho- (1-Mn), meta- (2-Mn), or para- (3-Mn) site of the aniline ring.
The proximity of the second-sphere functionality to the active site is a
critical factor in determining catalytic performance. Catalyst 1-Mn,
possessing the shortest distance between the amine and the active
site, significantly outperforms the rest of the series and exhibits a 9-
fold improvement in turnover frequency relative to parent catalyst
2
Since the initial report of electrocatalytic CO reduction by
fac-Mn(bpy)(CO) Br (MnBpy), a number of catalysts have been
3
reported with modified second-coordination sphere environments
that have improved the efficiency and/or altered the product
selectivity relative to the parent system.[8–10,13] In this class of
-
1
3
Mn(bpy)(CO) Br (901 vs 102 s , respectively) at 150 mV lower
overpotential. The electrocatalysts operate with high Faradaic
efficiencies (≥70%) for CO evolution using trifluoroethanol as a proton
source. Notably, under photocatalytic conditions, a concentration-
dependent shift in product selectivity from CO (at high [catalyst]) to
catalysts, Mn(L)(CO)
3
Br (where L = bipyridyl ligand) serves as a
–
pre-catalyst, which forms the active species [Mn(L)(CO)
3
] after
–
accepting 2e . This five-coordinate active species can bind and
HCO
2
H (at low [catalyst]) was observed with turnover numbers up to
activate CO
2
. However, a proton source is required to initiate
adduct to generate
This metallocarboxylic acid species is a
key intermediate in the conversion of CO to CO. Depending on
4760 for formic acid and high selectivities for reduced carbon products. catalysis by protonation of the CO
2
3 2
Mn(L)(CO) (CO
H).[
6,21]
2
the reaction conditions (applied potential, strength of the acid, etc),
the intermediate can either proceed through the “protonation-first
pathway” (protonation of the intermediate followed by reduction)
or the “reduction-first pathway” (1e reduction of the intermediate
followed by protonation) as summarized in Figure 1.[10,21]
Reduction of the cationic tetracarbonyl species generated in the
protonation-first pathway can be achieved at a significantly lower
potential than the reduced metallocarboxylic acid intermediate.
Introduction
Developing technologies to avert climate change associated with
anthropogenic greenhouse gas emissions is vital to achieving a
sustainable future for mankind.[1] To address this global problem,
fuels and commodity chemicals can be generated from carbon
dioxide as a crucial step toward a carbon-neutral economy. The
–
large kinetic barrier associated with transforming CO
rich products necessitates the use of efficient catalysts, which can
2
into energy-
2
Thus, CO conversion via these two pathways operate in two
different potential regimes and can be clearly distinguished during
[
2]
facilitate this conversion using renewable energy .Transition
metal complexes are attractive candidates for this endeavor as
they possess tunable and well-defined active sites that are
amenable to mechanistic studies and rational design.[3–5]
Group 7 transition metal α-diimine tricarbonyl complexes,
specifically those of manganese and rhenium, are an extensively
studied catalyst group.[3,6,7] Their simple ligand design and high
[9,10]
electrocatalysis.
The protonation-first pathway is generally the
desired route of catalysis due to the higher energy demand of its
counterpart. However, accessing this pathway has only been
possible by coordinated electronic control and hydrogen-bonding
interactions introduced by second-sphere functional groups in the
presence of added Brønsted acids.[9,10] Previously reported fac-
Mn(bpy)(CO)
sphere methoxyphenyl
3
X type catalysts with pendant second-coordination
selectivity for the two-electron reduction of CO
2
to CO or HCO
2
H
[10]
and imidazolium[9] functional groups
1
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ChemSusChem
10.1002/cssc.202001940
FULL PAPER
–
Figure 1. Proposed CO
2
reduction pathways for the reduction of CO
2
to CO and HCO
2
H in the presence of a proton source catalysed by [Mn(bpy)(CO)
3
] .
have achieved catalysis at significantly lower overpotentials by
following this protonation-first pathway. However, catalysts that
can effectively access this pathway remain rare.
Interestingly, catalytic performance across the series did not
follow the anticipated trend as the catalyst with the -NH group in
2
closest proximity to the Re active site showed similar activity to
the one with the amine oriented away from the active site. In
contrast, the meta-isomer gave the best activity in terms of
overpotential, turnover frequency (TOF), and Faradaic efficiency
(FE) for CO production. Spectroelectrochemical studies suggest
that the meta-isomer accesses the catalytically active species at
a lower overpotential than its counterparts. Moreover, the ortho-
isomer appears to suffer from deactivation as the active site is
2
Formic acid and H have also been reported as minor
products in some electrochemical and light-driven reactions with
Mn-tricarbonyl catalysts. Recently, Daasbjerg and co-workers
were able to tune the selectivity of the reduced carbon products
by modifying the second-coordination sphere in a series of
substituted MnBpy complexes.[8] Their mechanistic studies
suggest that formic acid production can also follow two different
catalytic pathways (Figure 1). A manganese-hydride intermediate, blocked by undesirable coordination of the primary amine to the
Mn(L)(CO)
process, which is generated by protonation of the 2e– reduced
Mn(L)(CO) insertion into the Mn-H
]– species. Subsequent CO
bond is dependent on the applied potential. At modest potentials,
CO insertion occurs in the neutral hydride species and the
resultant Mn(L)(CO) (OCHO) intermediate is then reduced.
3
H, is proposed to be the key intermediate in this
reduced metal center.[14] Here, we report the analogous series of
manganese catalysts, the Earth-abundant and economical
alternative to Re (Figure 2). We sought to examine how changing
the metal center impacts catalytic performance and the overall
catalytic cycle. Moreover, we have extended our investigation to
[
3
2
2
3
2
photocatalytic CO reduction with the aid of an external
Conversely, at more negative potentials, the hydride species can
photosensitizer (PS) and a sacrificial electron donor (SD).
–
be reduced by 1e before the CO
2
insertion step. In both cases, a
proton-assisted bond cleavage then liberates formic acid and
regenerates the active catalytic species.[8]
Results and Discussion
In this context, we have recently reported a series of aniline-
functionalized fac-Re(bpy)(CO)
3
Cl complexes for electrocatalytic
Synthesis and Structure of the Complexes. The catalyst series
CO reduction where the -NH group was systematically installed
2
2
was synthesized by refluxing Mn(CO)
amount of the appropriate ligand in anhydrous diethyl ether under
atmosphere. Detailed synthetic procedures and
characterization data are provided in the Supporting Information
Figures S1-S8). In room temperature infrared and 1H NMR
5
Br with an equimolar
at the ortho-, meta-, or para- position of the aminophenyl
substituent to provide a well-defined distance between the
catalytic center and the second-sphere H-bond donor.[14]
N
2
(
spectra, the presence of a minor species of 1-Mn is visible in
acetonitrile but absent in chloroform, a noncoordinating solvent
(
Figures S9 and S10). This minor species is the product of rapid
solvolysis of the pristine complex and is identified as the CH
bound cation [1-Mn] . The cation was also independently
3
CN-
+
6
synthesized by reacting 1-Mn with AgPF in dry acetonitrile to
verify this assignment.
X-ray quality crystals of 1-Mn and 2-Mn were grown by slow
diffusion of diethyl ether into concentrated solutions of the
Figure 2. Catalysts studied in this work.
2
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ChemSusChem
10.1002/cssc.202001940
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the amine nitrogen (N3) is 3.965(2) Å from the bromine (Br1) in 1-
Mn, suggesting a weak intramolecular interaction that favors the
syn conformation.
Br1
N3
N2
N1
2
Electrochemistry and Electrocatalytic CO Reduction. Cyclic
voltammetry experiments were then conducted with the isomers
under inert atmosphere in acetonitrile solutions containing 0.1 M
Mn
Bu
4
NPF
6
to evaluate their redox properties. Cyclic
C17
C19
voltammograms (CVs) were obtained with a glassy carbon disk
working electrode and internally referenced versus the
+
/0
C18
ferrocenium/ferrocene (Fc ) couple. Experimental details can be
found in the Supporting Information. The CV of 1-Mn exhibits one
quasi-reversible redox event with two successive irreversible
reductions (Figure 4). This behavior is different from 2-Mn, 3-Mn,
and the unsubstituted parent catalyst MnBpy (Figures 4 and S11,
Table S3). This deviation stems from the in situ solvolysis of the
pristine complex in electrolyte solution. While reversibility of the
first reduction improves with increasing scan rate, no anodic
waves corresponding to the subsequent reductions are observed
at scan rates up to 1 V/s. CVs of 2-Mn and 3-Mn show only one
quasi-reversible broad reduction event at -1.78 and -1.79 V,
respectively (Table S3). This behavior is similar to previously
reported Mn-bipyridyl complexes functionalized with a benzylic
Br1
N2
N3
N1
Mn
C17
[8]
C18
C19
amine group at the 6 position of 2,2’-bipyridine. Plotting the
reductive peak current of the first reduction wave versus the
square root of the scan rate for each catalyst shows a linear
dependency indicative of diffusion-controlled, homogeneous
systems (Figures S12-S15).
Figure 3. Crystal structures of 1-Mn (top) and 2-Mn (bottom) with thermal
ellipsoids rendered at the 70% probability levels. Hydrogen atoms and outer-
sphere solvent molecules have been omitted for clarity.
Next, the catalytic activity for CO
-sparged acetonitrile solutions. No appreciable current
increase was observed under CO atmosphere in anhydrous
CH CN solution. However, addition of a weak Brønsted acid,
2
reduction was studied in
CO
2
2
3
respective complexes in N,N-dimethylformamide (DMF). Crystal
structure and data refinement parameters are provided in Table
S1. Solid-state structures of the complexes show distorted
trifluoroethanol (TFE), to the solution triggers catalysis as evident
by moderate and large catalytic waves with peak currents
observed at around -1.6 and -2.2 V, respectively (Figure 4). This
behavior is assigned to the protonation-first and reduction-first
pathways, respectively, that have been observed with other fac-
octahedral geometries with the expected fac-Mn(CO)
3
core
(
Figure 3). Only one of two possible rotamers was found in both
1-Mn and 2-Mn where the amine groups are syn to the bromo
type catalysts.[9,10] The limiting catalytic currents (icat) at
Mn(CO)
3
ligands. The aminophenyl group imposes several structural
the two different potential regimes increase linearly with
increasing concentrations of added TFE before reaching a
maximum and subsequently falling to lower values (Figure S16).
The maximum [TFE] for each complex is different and the best
condition in each case is used to report the catalytic activities. It
should be noted that no catalytic current enhancement was
changes
Mn(bpy)(CO)
relative
to
unsubstituted
parent
complex
Br (MnBpy) (Table S2).[ Steric repulsion between
22]
3
the aminophenyl ring and an equatorial carbonyl elongates the
Mn-N2 bond and disrupts the planarity of the two pyridine donors.
The Mn-Br1 bond distance is slightly longer in 1-Mn relative to 2-
Mn and MnBpy, which is consistent with the lability of the bromide
ligand in 1-Mn observed spectroscopically in solution. Notably,
observed in N
2
-saturated TFE solutions suggesting that the
catalysts are not active for proton reduction in the potential
Figure 4. CVs of 1-Mn, 2-Mn, and 3-Mn at 0.5 mM concentrations in CH
3 4 6 2 2 2 2
CN / 0.1 M Bu NPF under N (black), CO (red), N with TFE (green), and CO with TFE
(blue). The TFE concentration was selected on the basis of the optimized condition. Scan rate = 100 mV/s; glassy carbon working electrode.
3
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ChemSusChem
10.1002/cssc.202001940
FULL PAPER
window studied.[20] In fact, comparing CVs of N
so-called reduction-first pathway. A plot of the catalytic current
versus the concentration of added TFE reveals that the optimal
concentration for 1-Mn, 2-Mn, and 3-Mn is 15%, 6%, and 15%
TFE, respectively (Figure S16). Under these conditions, 1-Mn
again shows the highest activity among the four catalysts studied
here (Figure S20). The estimated maximum TOFs were
determined as 901, 245, 296, and 102 s for 1-Mn, 2-Mn, 3-Mn,
and MnBpy, respectively (Figure S21, Table 2). We note that CVs
2
obtained with 2-Mn and 15% TFE in CO -saturated solution are
2
3
-sparged CH CN
/
4 6
0.1 M Bu NPF solutions with added TFE in the presence (Figure
4) and absence (Figure S17) of catalyst shows that the Mn
complexes actually suppress proton reduction at the glassy
carbon electrode.
The catalytic current associated with the protonation-first
pathway, observed at around -1.6 V, coincides with the first (or
only) reduction of the complexes (Figure 4). The limiting catalytic
current at this low overpotential regime increases linearly with
-1
[
2
TFE] up to a concentration of 20%, 18%, and 20% (v/v) for 1-Mn,
-Mn, and 3-Mn, respectively. While each aniline-substituted
catalyst outperforms MnBpy, the catalytic half-wave potential
cat/2) and the ratio of icat/i (where icat is the limiting catalytic
current and i is the corresponding peak current in the absence of
provided in Figure S22 for comparison.
These results demonstrate that the second-sphere
functional groups enhance the catalytic activity of the reduction-
first pathway as well, where 1-Mn, possessing the shortest
distance between the amine and the active site, operates at a
similar or lower overpotential compared to the other catalysts and
at a considerably faster rate. Indeed, 1-Mn exhibits a remarkable
9-fold improvement in turnover frequency relative to parent
(
E
p
p
substrate) vary considerably across the series (Table 1),
demonstrating the importance of the primary amine’s position with
respect to the active site. At a scan rate of 100 mV/s, the most
-
1
prominent current enhancement (icat/i
p
= 5.7), which occurs at the
3
catalyst Mn(bpy)(CO) Br (901 vs 102 s , respectively) at 150 mV
most positive catalytic potential (Ecat/2 = -1.47 V) within the series,
is seen with 1-Mn. This current enhancement is three times that
of the unsubstituted benchmark catalyst MnBpy and at 230 mV
lower overpotential. Likewise, the remaining aniline-substituted
catalysts show higher catalytic activity at the first reduction
relative to MnBpy and at ~110 mV more positive potential (Table
lower overpotential, and has a TOF that is roughly three times that
of 2-Mn and 3-Mn (Table 2).
Table 2. Summary of the results of electrocatalysis at the high overpotential
regime[
a]
1
). The difference in overpotential clearly indicates that the
TOF (s-1)
Catalyst
1-Mn
E
cat/2 (V)
ɳ (V)
icat/i
p
enhanced catalytic performance does not originate from
differences in thermodynamic driving force, but rather from the
second-coordination sphere contributions of the pendant amine
groups. The turnover frequency (TOF) of each catalyst was
estimated by performing
experiments at the optimal catalytic condition (see Supporting
Information for details).[25,26] Ideal “S-shaped” catalytic waves
-1.95
-1.94
-2.02
-2.10
0.59
0.58
0.66
0.74
51.0
27.4
26.2
14.4
901.4
245.2
296.0
102.1
2
-Mn
-Mn
a series of cyclic voltammetry
3
MnBpy
were not observed for 2-Mn, 3-Mn, and MnBpy at scan rates up
to 3 V/s; however, scan rate independent TOF values were
obtained at fast scan rates consistent with steady-state behavior
[a]Potentials are versus Fc+/0, rounded to the nearest 0.01 V, and measured at 
100 mV/s. Glassy carbon disk working, platinum wire counter, and silver wire
quasi-reference electrodes. The catalytic parameters were measured in CO
saturated CH CN / 0.1 M Bu NPF solutions containing 15%, 6%, 15%, and
5% TFE for 1-Mn, 2-Mn, 3-Mn, and MnBpy, respectively. [b]Overpotential (ɳ)
was calculated as, ɳ = │Ecat/2 – E0CO│. Here, the standard potential used for CO
=
2
-
[
26]
(
Figures S18-S19). The calculated TOFs of 1-Mn, 2-Mn, and 3-
3
4
6
-1
Mn at this low overpotential regime are 33.5, 17.0, and 11.1 s ,
1
respectively.
2
0
[8,27]
reduction under these conditions is E CO = -1.36 V.
Table 1. Summary of the results of electrocatalysis at the low overpotential
regime
[
a]
A
series of controlled potential electrolysis (CPE)
experiments were conducted in an airtight electrochemical cell to
characterize the reaction products and to examine the selectivity
and stability of the complexes (Figures S23-24). The headspace
of the cell was analyzed by gas chromatography and the
electrolyzed solutions were analyzed by 1H NMR to detect and
quantify solvated products. Details of the experimental setup,
product quantification, and Faradaic efficiency (FE) calculations
can be found in the Supporting Information (SI).
Catalyst
E
cat/2 (V)
-1.47
-1.59
-1.58
-1.70
ɳ (V)[b]
0.11
i
cat/i
5.7
2.9
3.2
1.9
p
TOF (s-1)
33.5
1-Mn
2-Mn
3-Mn
0.23
17.0
0.22
11.1
MnBpy
0.34
11.4
All of the aniline-substituted catalysts were found to be
selective toward CO production with high Faradaic efficiencies.
Bulk electrolyses performed near the plateau current potential
confirm the consistent and selective production of CO with FEs ≥
70% and ≥ 76% at the low and high overpotential regimes,
respectively (Table 3). Trace amounts of formate (FE ≤ 5%) were
[a]Potentials are versus Fc+/0, rounded to the nearest 0.01 V, and measured at 
100 mV/s. Glassy carbon disk working, platinum wire counter, and silver wire
quasi-reference electrodes. The catalytic parameters are measured in CO
saturated CH CN / 0.1 M Bu NPF solutions containing 20%, 18%, 20%, and
0% TFE for 1-Mn, 2-Mn, 3-Mn, and MnBpy, respectively. [b]Overpotential (ɳ)
was calculated as, ɳ = │Ecat/2 – E0CO│. Here, the standard potential used for CO
=
2
-
3
4
6
2
2
1
also detected by NMR for each catalyst; a representative H NMR
0
[8,27]
reduction under these conditions is E CO = -1.36 V.
spectrum is shown in Figure S25. Importantly, no H
in any of the experiments, consistent with the cyclic voltammetry
conducted under N atmosphere with added TFE. In an effort to
2
was detected
In the high overpotential regime, a significant catalytic wave
appears at around -2.2 V for each catalyst in the series. The more
negative potential of this catalytic process is characteristic of the
2
optimize the applied potential, a series of CPEs were performed
with 1-Mn at different potentials and the catalyst was found to be
4
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FULL PAPER
Table 3. Summary of controlled potential electrolysis (CPE) experiments [a]
Low overpotential regime
High overpotential regime
Catalyst
E
appl (V) [b]
-1.53
H+ source
20% TFE
18% TFE
20% TFE
20% TFE
Charge [c] (C)
1.97 ± 0.07
0.84 ± 0.03
0.56 ± 0.06
0.51 ± 0.04
FECO (%) [c]
75 ± 2
E
appl (V) [b]
-2.05
H+ source
25% TFE
6% TFE
Charge [c] (C)
4.83 ± 0.17
4.44 ± 0.09
3.05 ± 0.12
2.42 ± 0.08
FECO (%) [c]
83 ± 3
1-Mn
2-Mn
3-Mn
-1.65
83 ± 4
-2.10
76 ± 2
-1.65
70 ± 3
-2.17
20% TFE
20% TFE
93 ± 3
MnBpy
-1.80
73 ± 3
-2.30
77 ± 3
[
a]
All CPEs were performed in an airtight two-chamber electrochemical cell equipped with a glassy carbon rod working electrode, silver wire pseudo-reference
electrode, and platinum mesh counter electrode. The electrode chambers were separated with a fine glass frit. The working electrode chamber contained 0.5 mM
catalyst in CO -saturated CH CN / 0.1 Bu NPF
solution. [b] The applied potentials were reported versus Fc+/0 and correspond the peak catalytic current observed
for each system. [c] Accumulated charges and FEs are reported as the average of three runs. Formic acid was detected in all experiments with FE ≤ 5%. No
quantifiable H was detected during 1 h electrolysis.
2
3
4
6
2
2
selective for CO reduction at all applied potentials studied (Table
S4) with no significant change in FECO
.
Table 4. Experimental (CO) frequencies of different intermediates detected by
IR-SEC experiments under N
[a]
2
Infrared
Spectroelectrochemical
Analysis.
Infrared
spectroelectrochemistry (IR-SEC) was conducted under inert
atmosphere to investigate the possible pathways to generate the
catalytically active 2e reduced species.[6,22] At resting potential,
all three complexes show three characteristic CO vibrational
modes (CO) at similar frequencies compared to MnBpy (Tables
Complex
1-Mn
2-Mn
3-Mn
–
2025, 1937,
917
2025, 1937,
1914
2024, 1937,
1912
3
Mn(L)(CO) Br
1
2
2
1
045, 1961,
[
[
Mn(L)(CO)
Mn(L)(CO)
3
3
3
3
3
(CH
(CH
]●
3
CN)]+
2044[b]
-
-
-
4
and S5). We note that ligand exchange is noticeably faster and
1945
more complete in the supporting electrolyte solution used for IR-
SEC compared to neat acetonitrile, and 1-Mn readily dissociates
the Br– ligand giving rise to peaks corresponding to [1-Mn]+
013, 1915,
1902
CN)]●
3
-
-
(
Figure S26). Complex 2-Mn also shows an equilibrium with [2-
974, 1890,
1872
+
[Mn(L)(CO)
[Mn(L)(CO)
Mn] under these conditions, but dissociation occurs to a much
smaller extent. With an applied potential at the onset of the first
+
1974, 1930,
872, 1847
1976, 1929,
1875, 1848
1974, 1929,
1876, 1845
reduction wave, peaks associated with [1-Mn] start to diminish
]
2
1
and a set of lower energy peaks emerge. In order to assign these
+
peaks we also performed IR-SEC experiments on [1-Mn] , from
1912,
811 (br)
1911,
1810 (br)
1910,
1810 (br)
[
Mn(L)(CO)
]–
1
which the same three lower energy peaks appeared at a similar
applied potential (Figure S27). Thus, we assign this species to
[a]The IR spectra are shown in Figures S26-S29. Experimental (CO)
frequencies of related complexes are summarized in Table S3. [b]Lower energy
peaks are overlapped by the prominent species (2-Mn) in the solution.
•
[
Mn(L1)(CO)
3
(CH
3
CN)] (L1 = ortho-substituted bipyridyl ligand).
When the applied potential was shifted more cathodically to match
that of the second reduction, a new intermediate began to appear
at the expense of the aforementioned species. The (CO)
0
0
stretches of this intermediate are consistent with a Mn -Mn
dimeric complex.[22] Further scanning toward the third reduction
wave produces a concomitant rise in CO stretches corresponding
Photocatalytic CO
complexes under electrocatalytic conditions encouraged us to
explore their activity in photochemical CO reduction.
Photochemical experiments were performed in
CH CN:triethanolamine (TEOA) solvent mixture using
Ru(bpy) ](PF as an external photosensitizer, 1,3-dimethyl-2-
2
Reduction. The high performance of the Mn
2
to the five-coordinate radical species [Mn(L1)(CO)
reduced anion [Mn(L1)(CO)
3
]• and the 2e–
3
] . The radical species could not
a
–
[26]
3
be spectroscopically isolated because the carbonyl absorption
bands of the latter species grow in intensity as the potential is held
for longer times.
[
3
6 2
)
phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial
electron donor, and a solar-simulated light source. Control
experiments were performed to confirm that the presence of each
component in the reaction mixture is essential for generating
IR-SEC of 2-Mn and 3-Mn is more straightforward and
similar to that of MnBpy. Clean formation of [Mn(L)(CO)
3
]
2
and
–
[
Mn(L)(CO)
3
] is observed at applied potentials corresponding to
CO/HCO
All of the Mn complexes were found to produce CO as the
primary CO reduction product along with HCO H as a minor
2 2
H from CO (Table S6).
the onset and peak of the first reduction wave, respectively
Figures S28-S29). No other intermediate species were detected
(
2
2
on the IR-SEC timescale for these two complexes. We note that
the dinuclear intermediates were not observed with the rhenium
analogues of 1-Mn and 2-Mn in IR- or UV-Vis-SEC experiments.
In addition, no solvolysis was observed with the pristine rhenium
complexes.[14]
product (Table 5). Under identical photochemical conditions, 1-
Mn was found to be the best catalyst in the series having TONs
of 121 and 23 for CO and HCO H production, respectively, with a
2
carbon selectivity (CS) of 96% (Figure 5A).
A Hg drop
homogeneity experiment[29] was performed with 1-Mn, which
showed nearly identical photocatalytic activity in the presence and
5
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ChemSusChem
10.1002/cssc.202001940
FULL PAPER
Table 5. Summary of the results of photocatalysis[a]
A
B
Catalyst
[Catalyst]
CO
2
HCO H
H
2
CS
(mM)
(TON)
(TON)
(TON)
(%)
1
2
3
-Mn
-Mn
-Mn
0.1
0.1
0.1
0.1
121
35
23
5
6
<1
<1
3
96
95
95
95
89
90
91
16
3
MnBpy
1-Mn
33
23
Figure 5. A) CO formation vs time by photocatalytic CO
2
reduction with 1-Mn,
2
-Mn, 3-Mn, and MnBpy. The data is average of three runs and contains ≤ 5%
0.01
0.001
0.0001
161
247
2340
76
30
105
680
deviation. Condition: 0.1 mM catalyst, 0.1 mM [Ru(bpy)
3 2
]2+, 50 mM BIH in CO
1
-Mn
-Mn
680
4760
saturated CH
concentration for CO
and BIH were kept constant for all studies.
3
CN:TEOA (5:1) solution. B) Product ratio vs. catalyst
]2+
2
photoreduction by 1-Mn. Concentrations of [Ru(bpy)
3
1
[a]All experiments were performed in CH
specified amount of catalyst, 0.1 mM [Ru(bpy)
CO atmosphere. Turnover numbers (TON) are calculated based on [catalyst].
CN:TEOA (5:1) in presence of the
](PF , and 50 mM BIH under
3
absence of elemental mercury in the reaction mixture (Figure
S30). Catalyst 2-Mn performed similarly to that of MnBpy in
regards to CO production. The least active catalyst in the series
was 3-Mn, which performed worse than the parent complex.
Additional studies with the best catalyst 1-Mn demonstrate that
lowering the catalyst concentration elevates the observed TONs
3
6 2
)
2
The TON values are reported as the averages of three runs and have ≤ 5%
deviations associated with them. Irradiation time = 12 h. Solar-simulated LED
light source.
2
In situ IR studies in a N -sparged solution of 1-Mn in the
2
for CO reduction, presumably because deleterious bimolecular
presence of TEOA, BIH, and the PS were performed in a thin-
layer cell to detect the intermediates leading to the active catalytic
catalyst-catalyst deactivation pathways are minimized as
previously proposed.[11,27] Very low catalyst concentrations may
species of photochemical CO reduction (Figure S31). The (CO)
2
also provide insight into how site-isolated surface-bound
photocatalysts will behave. Indeed, high stability has been
observed with rhenium-based photocatalysts immobilized in a
porous organic polymer.[28] Interestingly, the product selectivity
shifts significantly toward HCO H production as the catalyst
2
concentration is lowered (Figure 5B). Similar experiments were
frequencies associated with the intermediates observed in this
experiment are given in Table S8, along with those of related
intermediates detected by IR spectrophotochemical and
spectroelectrochemical reduction of reported Mn complexes.
During photocatalysis, [Ru(bpy)
3
]2+ is photoexcited and
reductively quenched by BIH before the electron is transferred to
the Mn complexes.[5,31,32] As discussed above, catalysis can occur
performed with MnBpy to compare the results to those with 1-Mn
(
Table S7). A shift in product selectivity was also observed with
by two different pathways depending on the driving force. Given
MnBpy, but to a smaller extent, suggesting that the aniline
substitution plays an important role in this phenomenon. To the
best of our knowledge, this concentration-dependent switch in
the reduction potential of [Ru(bpy)
3
]+ is -1.71 V vs Fc
+/0 [35,36]
,
our
photocatalytic system presumably follows the low-overpotential
protonation-first and CO insertion-first pathways in producing CO
and HCO H, respectively, as summarized in Figure 1. In the
product selectivity for photocatalytic CO
homogeneous Mn(CO)
described.
2
reduction with
3
type systems has not been previously
2
2
CH
3
CN/TEOA solution nearly all of 1-Mn is quickly converted into
+
[
1-Mn] . Previous studies of MnBpy type catalysts suggest that
We note that Reisner and coworkers have reported tunable
–
–
after 1e reduction and Br dissociation, formation of a dimeric
species is observed. Indeed, upon irradiation, the initial CO bands
of 1-Mn diminish and a new set of peaks appear which resemble
productive
selectivity
using
pyrene-functionalized
3
Mn(bpy)(CO) Br catalysts anchored to carbon nanotube
electrodes as a function of catalyst surface loading.[29] At higher
surface loadings, CO formation was favored via a dimeric Mn0-
Mn0 intermediate while formate production was significantly
the dimeric species [Mn(L1)(CO) ] observed in IR-SEC. Longer
3 2
light exposure results in the concurrent decay of the dimeric
species and growth of a species which is tentatively assigned to
enhanced at lower surface loadings, presumably via a monomeric
Mn-hydride species. This behavior is consistent with the
concentration-dependent product selectivities reported here. A
similar observation was reported by Kang and co-workers with a
•
[
1
Mn(L1)(CO)
895, and 1882 (sh) cm .
3
H] with C-O stretching frequencies CO at 1988,
Peaks consistent with
]– evolve when the solution is irradiated for a
-1 [8,13]
[
Mn(L1)(CO)
3
prolonged period of time (~3 min). Importantly, the hydride
species observed during this experiment is consistent with the
phosphonate-functionalized Mn(bpy)(CO)
3
Br system attached to
reduction,
a dye-sensitized TiO platform for photocatalytic CO
2
2
generation of HCO
2
H via CO
2
insertion into the metal-hydride
which exhibited a surface-loading dependence favoring formate
production at low catalyst loadings.[30] These results demonstrate
during the catalytic cycle.[ The hydride intermediate can also be
6]
2
protonated to produce H , which was detected as a minor product
that site isolation (or very low catalyst concentrations in the
in the photolysis experiments. The protons required for these
reactions can be supplied by BIH or TEOA during photolysis.
present homogeneous study) of molecular MnBpy-type catalysts
+
H (or HCO –
can determine product selectivity, where HCO
2
2
)
The production of formic acid generally proceeds via a
metal-hydride intermediate; however, the hydricity of the metal-
production is favored via a mononuclear catalytic cycle.
hydride species relative to that of formate is a critical factor that
dictates the reaction energetics.[
37–39]
The hydricity of formate has
been experimentally determined to be 44 kcal/mol in
6
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ChemSusChem
10.1002/cssc.202001940
FULL PAPER
acetonitrile.[40] Therefore, any metal-hydride that is capable of
hydride transfer to CO to generate formate should possess a
2
occurs at the lowest overpotential (110 mV) within the series. The
second-sphere amine groups also influence catalysis in the high
overpotential regime by significantly enhancing the TOF
compared to the unsubstituted counterpart (MnBpy). The
measured TOF values follow the expected trend with the values
hydricity that is less than 44 kcal/mol in order for the reaction to
be exergonic.[
37–39]
The calculated hydricity of
a
relevant
0
benzylamine-substituted Mn(bpy)(CO)
3
H species, [Mn-H] , was
recently reported as 48.4 kcal/mol, which accounts for a slow
transfer of hydride to CO
to produce formate.[8] We expect the
hydricity of the corresponding metal-hydride of 1-Mn to be similar
and capable of slow hydride transfer to CO , consistent with the
formation of HCO H as one of the reduced carbon products.
2
changing according to the distance between the -NH group to the
2
Mn center (1-Mn > 2-Mn > 3-Mn ≥ MnBpy). Indeed, the TONs in
the photocatalytic experiments also follow a similar trend within
the aniline-substituted series (1-Mn > 2-Mn > 3-Mn). However, 3-
Mn was found to be less active photocatalytically than the parent
MnBpy catalyst.
2
2
Role of the Primary-Amines in Catalysis. Previously it has been
Our spectroscopic investigations reveal that 1-Mn converts
into [1-Mn]+ in coordinating solvents. The intramolecular
interaction between the -NH group and the bromide ligand in 1-
2
Mn is likely responsible for labilizing the Mn-Br bond in the
absence of external stimuli. This occurrence expedites the
formation of the catalytically active species, affording this catalyst
demonstrated that amide, amine, and urea groups in the second-
coordination sphere of iron porphyrins,[22,23]
a
cobalt
can
macrocycle,[ and fac-M(bpy)(CO)
24]
X type systems
[8,9,25]
3
boost catalysis by either directly interacting with the catalytic
intermediate or by creating a hydrogen-bonding network with
neighboring Brønsted acid molecules to stabilize the intermediate. a competitive advantage.
In addition to nitrogen-based functional groups, other notable
examples include the addition of phenolic,[13,27] ether,[10,17] and
imidazolium[9] moieties on the ligand framework. We
hypothesized that the primary amine groups will engage in
hydrogen-bonding interactions during catalysis and accelerate C-
O bond cleavage / O-H bond formation to eliminate water. In line
with previous reports on the distance-dependent catalytic
In addition to being a hydrogen-bond donor, primary amine
groups have also been incorporated into the ligand framework of
CO
2
reducing electrocatalysts to provide stability to the complex
during catalysis[ and as a substrate (CO
47]
) capturing agent.
2
Recently, it was experimentally demonstrated that appended
aniline, benzylic amine, and thiourea groups can interact with
dissolved CO
2
,[8,44,48] which ultimately boosts the catalytic activity
performance
of
second-coordination
1-Mn should have the greatest advantage due
to the close proximity of the -NH group to the catalytic center
sphere
modified
by increasing the local concentration of substrate near the
catalytic center. However, FT-IR and 1H NMR experiments
eliminate this possible function of the aniline groups in our series
(Figures S33-S34).
catalysts,[
13,14,41,45]
2
following bromide dissociation. Catalysts 2-Mn and 3-Mn also
have an advantage relative to unsubstituted MnBpy, but to a
lesser extent given the greater distances between the amine and
the active site. The electrochemical and light-driven catalytic
reactions reported herein clearly support our hypothesis and
3
Neumann and coworkers have reported a Re(bpy)(CO) Cl-
based catalyst featuring a pendant thiourea group that can act as
an internal proton source. To investigate this possibility for the
aniline-substituents in 1-Mn, 2-Mn, and 3-Mn, varying
reveal a beneficial role of -NH
2
groups in the ligand structure.
concentrations of free aniline were added to CO
2
-saturated
Given the short distance between the amine and the active
site of 1-Mn, we anticipate that the amine can hydrogen bond
directly with the manganese-carboxylate and -carboxylic acid
intermediates. DFT calculations were performed to assess the
feasibility of these interactions in 1-Mn. As shown in the optimized
geometries of Figure S32, the amine can engage in H-bonding
anhydrous CH CN solutions containing 0.5 mM of each catalyst
3
including MnBpy (Figure S35). Under these conditions, no
catalytic current increase was observed. In fact, the catalytic
current decreases with increasing concentrations of aniline in
solution. This precludes the function of aniline as an internal
proton source in these systems. We also explored whether aniline
could independently serve as a co-catalyst in the presence of a
proton source. However, a noticeable decrease in catalytic
with the CO
2
-adduct with an O---H distance of 1.79 Å, well within
the range of typical O---H hydrogen bonding interactions.[
Likewise, a hydrogen bond between the amine and the carboxylic
acid adduct is observed with an O---H distance of 1.94 Å. These
calculations do not preclude the possibility that the amines
indirectly enhance catalysis by organizing the Brønsted acid
molecules around the active site for more efficient proton transfer,
but they demonstrate that the second-sphere functionality of 1-
46]
current was observed with the addition of aniline to CO
catalyst solutions containing 15% TFE in CH CN (Figure S36).
This observation highlights the importance of properly
2
-saturated
3
a
positioned intramolecular hydrogen-bond donor during catalysis.
It is worth mentioning that the previously reported rhenium
analogue of 1-Mn showed signs of catalyst deactivation under
Mn is capable of direct interaction with CO
during catalysis.
In addition to enhancing the activity of the catalysts, the
aniline substituents also anodically shift the catalytic potentials.
This affect is most apparent in the low overpotential regime.
Catalysis at two different potential regimes has been observed
2
-derived intermediates
anhydrous CH
3
CN conditions and had a low Faradaic efficiency
of 38% for CO production.[ In the presence of a proton source
(4% TFE), 1-Re exhibited good stability and a FE of 89% for CO
evolution. We postulated that the primary amine of 1-Re was able
to bind to the reduced metal center and inhibit catalysis. In this
work, no obvious deactivation was observed with 1-Mn during
14]
before with fac-Mn(bpy)(CO)
3
X type catalysts with second-
2
CO reduction with added TFE. A comparison to 1-Re could not
be made under anhydrous conditions as 1-Mn requires a proton
source to initiate catalysis.
coordination sphere methoxyphenyl,[10] benzylic amine, and
imidazolium[9] functional groups appended to the ligand
framework. It is evident from the electrochemical data that the
close proximity of the primary amine group to the active site in 1-
Mn results in the most effective second-sphere catalytic
enhancement of the protonation-first pathway, which notably
[8]
7
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ChemSusChem
10.1002/cssc.202001940
FULL PAPER
Conclusion
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5
2
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manganese catalysts while reducing their overpotential and
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We thank the National Science Foundation for generous funding
through a CAREER award (CHE-1848478).
[
[
[
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[
Conflict of Interest
The authors declare no conflict of interest.
26] E. S. Rountree, B. D. McCarthy, T. T. Eisenhart, J. L.
Dempsey, Inorg. Chem. 2014, 53, 9983–10002.
Keywords: manganese • homogeneous catalysis • CO
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FULL PAPER
Entry for the Table of Contents
A series of isomeric Mn catalysts featuring pendant aniline groups was developed for electrochemical and light-driven CO
The ortho-substituted catalyst significantly outperforms the rest of the series and exhibits a 9-fold improvement in TOF relative to
Mn(bpy)(CO) Br at 150 mV lower overpotential. Photocatalytically, a concentration-dependent shift in product selectivity from CO (at
high [cat]) to HCO H (at low [cat]) is observed.
2
reduction.
3
2
1
0
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