Highly Active Manganese-Based CO2Reduction Catalysts with Bulky NHC Ligands: A Mechanistic Study

Article abstract of DOI:10.1021/acs.inorgchem.0c01364

Because of the strong σ-donor and weak π-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO2 to CO with high activity. However, some NHC-based manganese complexes showed low catalytic activity an

Full text of DOI:10.1021/acs.inorgchem.0c01364

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Highly Active Manganese-Based CO2 Reduction Catalysts with Bulky
NHC Ligands: A Mechanistic Study
Yong Yang, Zhenyu Zhang, Xiaoyong Chang, Ya-Qiong Zhang, Rong-Zhen Liao, and Lele Duan*
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ABSTRACT: Because of the strong σ-donor and weak π-acceptor
of the N-heterocyclic carbene (NHC), Mn-NHC complexes were
found to be active for the reduction of CO2 to CO with high
activity. However, some NHC-based manganese complexes
showed low catalytic activity and required very negative potentials.
We report herein that complex fac-[MnI(bis-MesNHC)(CO)3Br]
[1; bis-MesNHC = 3,3-bis(2,4,6-trimethylphenyl)-(1,1′-diimidazo-
lin-2,2′-diylidene)methane] could catalyze the electrochemical
reduction of CO2 to CO with high activity (TOFmax = 3180
6
s−1) at a less negative potential. Due to the introduction of the
bulky Mes groups, a one-electron-reduced intermediate
{[Mn0(bis-MesNHC)(CO)3]0 (2•)} was isolated as a packed
“dimer” and crystallographically characterized. Stopped-flow Fourier-transform infrared spectroscopy was used to prove the direct
reaction between doubly reduced intermediate fac-[Mn(bis-MesNHC)(CO)3]− and CO2; the tetracarbonyl Mn complex
[Mn+(bis-MesNHC)(CO)4]+ ([2-CO]+) was captured, and its further reduction proposed as the rate-limiting step.
ure 1, right)] exhibited unprecedented activity [maximum
turnover frequency (TOFmax) of 2100 s−1] without addition of
INTRODUCTION
■
Selective reduction of carbon dioxide to yield high-value-added
products like CO, HCHO, MeOH, and CH4 or higher-Cx
products has attracted considerable attention. These reduction
reactions require multiple electron/proton transfer and involve
several bond cleavage/formation steps, so they are difficult to
catalyze, leading to applied potentials that are higher than the
thermodynamic values.1 Thereby, the development of effective
carbon dioxide reduction catalysts is challenging and essential
for utilizing CO2 as a C1 feedstock to produce green fuels.
Molecular transition metal-based complexes have been often
utilized for electrochemical CO2 reduction and for systemati-
cally investigating the reaction mechanisms because of their
rich structural and electronic tunability.2−5 Metal complexes,
including cobalt,6−10 nickel,11−14 iron,15−18 ruthenium,19−23
rhenium,24−27 palladium,28,29 manganese,30−42 etc., have been
shown to be capable of catalyzing electrochemical CO2
reduction. Among these catalysts, the development of Mn-
based electrocatalysts is a step forward toward affordable
catalysts due to the abundance of manganese in the Earth. The
first example, fac-[MnI(bpy)(CO)3Br] (bpy = 2,2′-bipyridine),
was reported by Bourrez et al.,31 and later several research
groups studied the reaction mechanisms of the related
catalysts33,37,38,40,41 and isolated the reduced Mn-bpy inter-
mediates.32,34 By changing the pyridyl ligand to NHC (N-
heterocyclic carbene), the Agarwal group,35,36,39 the Royo
group, and the Lloret-Fillol group42 developed Mn-NHC
electrocatalysts for selective reduction of CO2 to CO. In
particular, complex fac-[MnI(bis-MeNHC)(CO)3Br] [A (Fig-
Figure 1. Chemical structures of complexes 1 and A.
any Lewis acid. On the basis of FTIR-SEC studies and density
functional theoretical (DFT) calculations, Royo, Lloret-Fillol,
and their co-workers proposed the electrochemical CO2
reduction mechanism by A. However, the first and second
reduction potentials of this Mn-NHC complex are too close to
separate from each other so that a two-electron reduction
product is formed upon reduction. As a result, the FTIR-SEC
spectra are very complex and could not be well resolved
Received: May 10, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01364
© XXXX American Chemical Society
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A

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70%): 1H NMR (400 MHz, CDCl3) δ 7.47 (s, 1H), 7.24 (s, 1H), 6.97
(s, 2H), 6.90 (s, 1H), 2.34 (s, 3H), 1.99 (s, 6H).
experimentally. In addition, no reduced Mn-NHC intermediate
has been characterized by X-ray crystallography. On the other
hand, Kubiak and co-workers reported that the dimerization of
singly reduced Mn-bpy species could contribute to an
overpotential and limit the activity of Mn catalysts. Thereby,
bulky groups like Mesbpy (Mesbpy = dimesityl-2,2′-bipyridine)
were developed to eliminate dimerization and enhance the
catalytic activity of Mn-based CO2 reduction catalysts. We
adopted this strategy and report herein an earth-abundant Mn-
NHC catalyst, fac-[MnI(bis-MesNHC)(CO)3Br] (1), bearing
two bulky mesityl groups (Figure 1, left). As expected, complex
Under an atmosphere of nitrogen, the acetonitrile solution (5 mL)
containing 1-(2,4,6-trimethylphenyl)-1H-imidazole (0.931 g, 5.0
mmol) and dibromomethane (0.3 mL, 4.0 mmol) was added in a
pressure tube. The solution was heated to 160 °C for 1 h under
microwave radiation. After the mixture had cooled to room
temperature, a gray solid precipitated from the solution. The
precipitate was filtered and washed with cold THF until it became
a colorless solid and dried under vacuum to obtain the desired
1
compound (0.89 g, 65%): H NMR (400 MHz, CDCl3) δ 11.33 (s,
2H), 9.82 (s, 2H), 7.95 (s, 2H), 7.28 (s, 2H), 7.05 (s, 4H), 2.38 (s,
6H), 2.08 (s, 12H).
1 displayed higher activity (TOFmax = 3180
6 s−1) and
Synthesis of the fac-[MnI(bis-MesNHC)(CO)3Br] Complex (1).
In the dark under a nitrogen atmosphere, Mn(CO)5Br (70 mg, 0.26
mmol) was dissolved in dry THF (15 mL), to which potassium tert-
butoxide (52 mg, 0.46 mmol) was added. The reaction mixture was
heated to 60 °C, and 3,3-bis(2,4,6-trimethylphenyl)-[(1,1′-
diimidazolium)methane] dibromide (108 mg, 0.2 mmol) was slowly
added to the suspended solution. The suspension was stirred at 60 °C
for 24 h. After the mixture had been cooled to room temperature, the
solvent of THF was removed under vacuum and the resulting residue
was suspended in CH2Cl2 (15 mL). The solution was filtered, and the
orange-yellow filtrate was collected and removed under vacuum to
yield the crude product. The product was purified by column
performed at a potential less negative than that of complex A
toward electrochemical reduction of CO2 to CO. Moreover,
we carefully studied the infrared (IR) spectral evolution of 1
via stoichiometric reduction by a chemical reducing agent KC8.
For the first time, one-electron-reduced intermediate
[Mn0(bis-MesNHC)(CO)3]0 (2•) related to Mn-NHC-based
CO2 reduction catalysts was isolated and crystallographically
characterized. Doubly reduced Mn-NHC complexes have long
been proposed as the active species for CO2 reduction, and
herein, we demonstrated the direct reaction between fac-
[Mn−(bis-MesNHC)(CO)3]− and CO2 via stopped-flow Four-
ier-transform infrared spectroscopy (FTIR) and a catalytic
intermediate, tetracarbonyl Mn complex [Mn+(bis-MesNHC)-
(CO)4]+ ([2-CO]+), was observed.
1
chromatography to yield yellow powder (0.114 g, 73%): H NMR
(400 MHz, DMSO-d6) δ 7.84 (s, 2H), 7.37 (s, 2H), 7.02 (s, 2H),
6.97 (s, 2H), 6.84−6.75 (d, 2H), 6.47−6.37 (d, 2H), 2.29 (s, 6H),
2.08 (s, 6H), 1.82 (s, 6H); FTIR νCO 2009 (s), 1929 (s), 1888 (s)
cm−1; ESI-MS (positive mode) [1 − Br]+ 523.16. Elemental analysis
calcd (%) for C28H28N4O3BrMn: C, 56.08; H, 6.75; N, 8.72. Found:
C, 56.13; H, 6.52; N, 8.49.
Chemical Reduction. Complex 1 (0.008 mmol, 5.0 mg) was
dissolved in THF (1 mL), followed by the addition of KC8 (0.009
mmol, 1.2 mg). The color of the solution turned yellow to yellow-
green gradually over 5 min. The FTIR spectrum of the solution shows
that νCO stretching bands of complex 1 disappeared and new νCO
stretching bands of neutral Mn0 complex 2• arose. The solution was
centrifuged to remove the black solid (product of KC8), and the
solution of complex 2• was obtained.
EXPERIMENTAL SECTION
■
General Considerations. Anhydrous acetonitrile (CH3CN),
tetrahydrofuran (THF), and n-hexane were distilled and dried over
molecular sieves, and THF and n-hexane were stored over a Na−K
alloy. Tetrabutylammonium hexafluorophosphate (TBAP, Sigma-
Aldrich, 98%) and tetrabutylammonium bromide (TBAB, Sigma-
Aldrich, 98%) were dried under a vacuum at 100 °C overnight and
stored in the glovebox. Potassium graphite (KC8) was synthesized
according to the literature and stored at −20 °C in a glovebox. Other
reagents were used as received: 2,4,6-trimethylphenylamine (Heowns,
98%), a glyoxal solution (Macklin, 40% in H2O), dibromomethane
(Energy Chemical, 98%), and manganese pentacarbonyl bromide
[Mn(CO)5Br, Aldrich, 98%].
To obtain the two-electron-reduced species 2−, KC8 (0.018 mmol,
2.4 mg) was added to the THF solution containing complex 1 (0.008
mmol, 5.0 mg). Due to the weak solubility in THF, the reduced
species 2− precipitated from the solution. The precipitate mixed
together with a black solid (product of KC8), and we cannot acquire
pure complex 2−. Therefore, we replaced KC8 with a portion of
sodium as the reductant. A portion of metal sodium (0.4 mmol, 10
mg) was added to the THF solution of complex 1 (0.008 mmol, 5
mg), and the solution was stirred until the FTIR spectrum proved that
the νCO stretching bands of complex 1 and its neutral complex 2•
disappeared. The suspension was centrifuged, and the pure yellow
solid of reduced species 2− was obtained.
Crystal Preparation. Crystals of 1 were grown by diffusion of
diethyl ether into the CH2Cl2 solution of complex 1. The CCDC
number of complex 1 is 1960153. Crystals of 2• were grown by slow
diffusion of hexane into a THF solution under −20 °C in an argon-
filled glovebox. The crystals were selected using a microscope in a
glovebox, and the selected crystal sample was immersed in
perfluoropolyether for manipulation. The crystal structure of complex
2• cannot be refined well for three reasons. (1) During the test, the
crystal easily cracked into several pieces, resulting in the bad quality of
the crystal. (2) The size of the crystal is small and thin, leading to a
high 2θ angle, and diffraction data were not available. (3) H2O and O2
reacted with complex 2• and destroyed the structure of complex
during the test, although crystals were capped with perfluoropo-
lyether. The CCDC number of complex 2• is 1960154.
The 1H nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker 400 MHz spectrometer at room temperature. Solvent
residual peaks are treated as internal references, and 1H chemical
shifts are published relative to Me4Si (δ = 0). Elemental analysis was
performed with a Thermoquest-Flash EA 1112 elemental analyzer for
C, H, and N. Mass spectrometry (ESI-MS) was performed on a Q-
Exactive instrument. Fourier-transform infrared (FTIR) spectra were
recorded on the Bruker Alfa or V80 spectrometer. Crystal structure
determinations were carried out using Bruker D8 VENTURE with
Mo Kα radiation under a N2 stream at 100 K. The collected data were
integrated by the software of Bruker SAINT, and the software of Olex
was used to induce the structure of the complexes.
Synthesis of 3,3-Bis(2,4,6-trimethylphenyl)-[(1,1′-
diimidazolium)methane] Dibromide. The compound was
synthesized according the literature procedures43,44 (Figure S1).
The mixture of acetic acid (5.0 mL), a formaldehyde solution (37%,
1.5 mL), and an oxalaldehyde solution (37%, 2.3 mL) was heated to
70 °C while being stirred. An aqueous solution (2 mL) containing
2,4,6-trimethylaniline (2.695 g, 20 mmol) and ammonium acetate
(1.54 g, 20 mmol) was added dropwise into the solution mentioned
above, followed by addition of acetic acid (5.0 mL). The reaction
mixture was stirred at 70 °C for 18 h. After being cooled to room
temperature, the resulting brown solution was added slowly to a
sodium bicarbonate solution (1.2 M, 150 mL) under stirring, after
which the generated brownish solid was filtered. The crude product
was dried under vacuum and purified by column chromatography to
yield the product of 1-(2,4,6-trimethylphenyl)-1H-imidazole (2.6 g,
Electrochemistry. All of the electrochemical experiments were
performed with an electrochemical workstation (CHI 760) under
rigorous air free conditions. Cyclic voltammetry (CV) measurements
were carried out in a CH3CN solution (0.1 M TBAP as the
B
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supporting electrolyte). The glass carbon (3 mm) electrode, Pt wire,
and Ag/AgNO3 electrode were used as the working electrode, counter
electrode, and reference electrode, respectively. Ferrocene (Fc) acted
as an internal standard. All potentials reported in this paper are
referenced versus the Fc+/0 redox couple and calculated to the Fc+/0
reference using E(Fc+/0) = E(Ag/AgNO3) − 0.08 V.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Complex 1 was
prepared by a reaction of the bis-MesNHC ligand (in situ
generated from the ligand precursor by addition of KOtBu)
and [Mn(CO)5Br] with a moderate yield (see the reaction
scheme in Figures S1 and S2).42 Complex 1 was explicitly
A homemade H-type electrochemical cell that was separated by a
glass frit into cathodic and anodic compartments was used for
controlled-potential electrolysis (CPE) experiment. The typical
working conditions of the cathodic compartment are as follows. A
glass carbon plate (3.14 cm2) was used as the working electrode, Ag/
AgNO3 as the reference electrode, and 0.1 M TBAP/CH3CN as the
electrolyte solution. For the anodic compartment, Pt mesh was
employed as the counter electrode and a CH3CN solution containing
0.1 M TBAP and 0.5 M tetraethylammonium acetate tetrahydrate
acting as a sacrificial agent (note that this reagent contains hydrated
water, and the glass frit that separates the anode and cathode of H-
type electrochemical cell will not completely prevent water from
crossing over during electrolysis; oxidation of acetate produced CO2
that does not contaminate the system) was used to avoid the
oxidation of electrolyte TBAP. The electrolyte was purged with
carbon dioxide for 15 min before electrolysis. The product of carbon
monoxide was analyzed by a FuLi Instruments GC9790 Plus gas
chromatograph with a thermal conductivity detector through an off-
line system (He as the carrier gas).
Fourier-Transform Infrared Reflectance Spectroelectro-
chemistry (FTIR-SEC). A homemade FTIR-SEC instrument was
used for this study. The schematic diagram and pictures of the cell are
shown in Figure S21. The cell consists of a glass carbon working
electrode (10 mm), a Pt counter (15 mm × 30 mm) electrode, and a
Ag/AgNO3 reference electrode and a CaF2 plate as the optical
window. The IR beam is directed to focus on the working electrode
through the optical window, where it is reflected and ultimately
directed to the Bruker Vertex 80 detector. The dry CH3CN solution
(0.2 M TBAP) prepared under an atmosphere of Ar or CO2 is used as
the electrolyte.
Stopped-Flow FTIR Spectroscopy. The stopped-flow FTIR
measurements were performed using a Bruker Vertex 80 infrared
spectrometer fitted with a stopped-flow apparatus (TgK Scientific SF-
73) and a transmission mixing cell. A Bruker Vertex 80 instrument
equipped with a liquid nitrogen-cooled RT-DLaTGS detector and a
4000 cm−1 low-pass filter was controlled via the Bruker OPUS
software. The stopped-flow apparatus is shown in Figure S18. The cell
is made from two CaF2 plates, with a cell volume of a few microliters
and a 100 μm path length. The cell is connected via PEEK tubing to a
stepper motor syringe pump; the pump is controlled via the apparatus
of a TgK Scientific SF-73 instrument that is placed in a glovebox.
Prior to a measurement, the stopped-flow apparatus and transmission
mixing cell were purged with Ar for 30 min, ensuring that no air was
in the system. Then dry THF was injected into the mixing cell. In a
typical measurement, one syringe was filled with a CO2-saturated
THF solution and another filled with the solution of complex 2−. The
flow volume and rate of the syringe pump were set to 300 μL and 1
mL s−1, respectively. The scanner velocity was fixed at 320 kHz, and
the wavenumber resolution set to 4 cm−1. Spectra were recorded
using a double-sided interferogram acquisition method. Overall, the
setting mode provided a maximum acquisition rate of 54 spectra per
second.
1
characterized by H NMR spectroscopy (Figures S3 and S4),
elemental analysis, FTIR spectroscopy (Figure S5), and X-ray
crystallography. The single-crystal X-ray diffraction structure of
1 is shown in Figure 2 (left), and its crystallographic data are
Figure 2. Single-crystal X-ray crystal structure of complexes 1 and 2•.
Hydrogen atoms have been omitted for the sake of clarity. Selected
bond lengths and bond angles for complex 1: C1−N1, 1.361; N1−C2,
1.443; C2−N2, 1.458; N2−C3, 1.359; C1−Mn1, 2.058; C3−Mn1,
2.055; C4−Mn1, 1.836; C5−Mn1, 1.771; C6−Mn1, 1.824; C4−O1,
1.145; C5−O2, 1.142; C6−O3, 1.152; C1−Mn1−C3, 83.87; N1−
C2−N2, 109.29. Selected bond lengths and bond angles for complex
2•: C1−N1, 1.392; N1−C2, 1.468; C2−N2, 1.436; N2−C3, 1.353;
C1−Mn2, 2.043; C3−Mn2, 2.104; C4−Mn1, 1.823; C5−Mn1, 1.762;
C6−Mn1, 1.640; C4−O1, 1.215; C5−O2, 1.215; C6−O3, 1.219;
C1−Mn2−C3, 81.51; N1−C2−N2, 111.37.
listed in Tables S1 and S2. The coordination sphere of Mn is in
the octahedral configuration, with Mn−CNHC bond lengths of
2.055−2.058 Å, slightly longer than those of the previously
reported complex A by 2.041−2.043 Å.42 These results reveal
that the electron-donating ability of bis-MesNHC is weaker than
that of bis-MeNHC. One-electron-reduced product
[Mn0(bis-MesNHC)(CO)3]0 (2•) was synthesized via reduc-
tion of 1 with KC8 (see details in the Experimental Section).
The X-ray crystals of one-electron-reduced products were
grown by vapor diffusion of pentane into a THF solution of the
complex under −20 °C in the glovebox. As shown in Figure 2
(right), the reduction of complex 1 results in the loss of its
bromo ligand, and the Mn coordination sphere of 2• is in the
square pyramid configuration. The bulky MesNHC ligand
inhibited Mn−Mn bond formation in the solid state, forming a
five-coordinate, 17-electron complex 2•; however, two
complexes 2• form a closely packed “dimer” in a solid, noted
as [2•−2•], with a shortest Mn−Mn distance of 3.490 Å
(Figure S6), which is close to the Mn−Mn distance (3.572 Å)
of the optimized structure of the “dimeric” Mn0 complex
(Figure S7). In comparison, the Mn−Mn distance of
[Mn(bpy)CO3]2 is 2.998 Å, much shorter than the Mn−Mn
distance of [2•−2•].30 On the other hand, the IR spectrum of
2• in THF is very different from that of [2•−2•] in the solid
state (Figure 5), indicating the relatively strong interaction of
two Mn0 complexes in [2•−2•]. The selected bond lengths and
angles as well as crystallographic data are listed in Tables S3
and S4. This is the first time that a crystal structure of a singly
reduced Mn intermediate related to Mn-NHC-based carbon
dioxide reduction catalysts is documented. Comparison of the
crystal structures of complexes 1 and [2•−2•] reveals
significant shortening of the Mn−C(CO) bonds in [2•−2•] as
Computational Details. The DFT calculations were performed
using the hybrid B3LYP-D345,46 functional as implemented in the
Gaussian 16 program package.47 Geometries were optimized in the
gas phase using the SDD48 pseudopotential for Mn and Br, while 6-
31G(d,p) basis sets were used for all other atoms. Analytic frequency
calculations were then carried out to obtain the vibrational
frequencies, and a scaling factor of 0.961 was used to simulate the
IR spectra.
C
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compared to 1 (1.640, 1.762, and 1.823 Å vs 1.771, 1.824, and
1.836 Å, respectively). The C−O bond length increased from
1.142, 1.145, and 1.152 Å in the crystal structure of 1 to 1.215,
1.215, and 1.219 Å in [2•−2•], respectively. These
observations are similar to those in the literature.32,34 The
metal center of complex [2•−2•] has an electron cloud density
that is higher than that of complex 1, and the π back-donation
from the filled d orbital to the empty π• orbital of CO
strengthens the Mn−C(CO) bonds and weakens the C−O
bonds. As a result, the Mn−C(CO) bonds are elongated while
the C−O bonds shortened.
Electrochemistry. The electrochemistry of 1 was studied
under Ar- and CO2-saturated conditions in acetonitrile
solutions containing 0.1 M tetrabutylammonium hexafluor-
ophosphate (TBAP) as an electrolyte. Under an Ar
atmosphere, CV of complex 1 (0.5 mM) displays a broad
reduction wave at −2.37 V (all potentials reported in this
paper are vs Fc+/0) (Figure 3a). Bromo ligand dissociation is
at −1.38 V was observed with a low current density. The cyclic
voltammogram of A also displayed such an oxidation wave that
is assigned to the oxidation process of the Mn0 dimer. In our
case, due to the introduction of the bulky Mes groups, complex
2• could not dimerize in solution. Thereby, without further
evidence we could not assign this oxidation wave. In
comparison with the literature-documented complex A,42 the
reduction potential of complex 1 is lower by 48 mV (Figure
S10a). This is due to the weaker electron-donating ability of
bis-MesNHC in comparison with that of bis-MeNHC, as
evidenced by the longer Mn−CNHC bond in complex 1.
Under CO2-saturated conditions, the cyclic voltammogram
of complex 1 reveals a strong enhancement of the catalytic
current at the first reduction wave with an onset potential at
approximately −2.13 V and the current density at −2.37 V is
11 times higher than those under an Ar atmosphere (Figure
3b). For comparison, the cyclic voltammograms of complexes
1 and A under CO2-saturated conditions are plotted in Figure
S10b. Overpotential (η) is defined by the difference between
equilibrium potential (ECO ) and catalytic potential of the CO2
2
reduction reaction. Under our conditions, the equilibrium
potential is difficult to estimate due to the unknown water
concentration but both 1 and A share the same equilibrium
potential as the reaction conditions are identical. The catalytic
potential, as recommended by Appel and Helm,49 is
determined from the potential at half of the catalytic current,
defined as Ecat/2 (Figure S10b). The overpotential difference
between A and 1 could be estimated as ηA − η1 = |ECO
−
Figure 3. Cyclic voltammograms of 1 (0.5 mM) under (a) an Ar
atmosphere and (b) a CO2-saturated atmosphere in anhydrous
CH3CN with 0.1 M TBAP as the electrolyte at a scan rate of 100 mV
s−1.
2
Ecat/2(A)| − |ECO − Ecat/2(1)| = |Ecat/2(A) − Ecat/2(1)| = 70 mV.
2
Therefore, the overpotential of complex 1 is smaller than that
of A by 70 mV due to the slightly weaker electron-donating
ability of the bis-MesNHC.
The addition of water to the electrolyte could increase the
catalytic current density, while the maximum current density
was observed at 0.55 M H2O. Further addition of water led to
a dramatic decrease in catalytic activity (Figure S11a).
However, the catalytic system becomes unstable in the
presence of water. For instance, at 0.55 M H2O, the cyclic
voltammogram response gradually decreased upon multiple
scans. The current density could be recovered after the glassy
carbon electrode had been polished (Figure S11b). In addition,
controlled-potential electrolysis experiments were performed
under CO2-saturated CH3CN with 0.1 M TBAP in the
presence of 0.55 M H2O at an applied potential of −2.3 V. The
current density decreased quickly in the first 10 min, and
deposits obviously appeared on the surface of the electrode
after 20 min (Figure S12). Those results revealed that complex
1 is unstable during the process of the electrocatalytic reaction
under CO2-saturated CH3CN in the presence of H2O.
Thereby, dry conditions were used in the following study.
To study the kinetics of the electrocatalytic CO2 reduction
by complex 1, the cyclic voltammograms under catalytic
conditions were studied by varying the catalyst concentration
and the scan rate. A linear increase in the catalytic current on
[1] was observed, indicating a first-order reaction in catalyst
(Figure S13). The catalytic current plateaus of 1 are relatively
independent of scan rate in the range of 0.6−1.0 V s−1 (Figure
4a). The plot of ic/ip versus the inverse square root of the scan
rate highlights that steady-state conditions and the pure kinetic
regime are accomplished at high scan rates of >0.6 V s−1
(Figure 4b). In this case, TOF of complex 1 was calculated
from the ic/ip ratio using eq 141,50
very common for diimine Mn−Br complexes [Mn(N^N)-
(CO)3Br] (N^N = diimine ligand), leading to the formation of
an equilibrium mixture of [Mn(N^N)(CO)3(sol)]+ (sol =
solvent molecule) and [Mn(N^N)(CO)3Br].5 Such a ligand
dissociation reaction will broaden the reduction wave of the
first electron transfer and shift the reduction wave toward the
anodic direction. In our case, the NHC ligand is a stronger
electron donor and will favor the ligand dissociation, as a result
of which the bromo ligand dissociation cannot be avoid. Figure
S8 shows that the cathodic peak current changes linearly with
the square root of the scan rate in the range of 0.05−6.4 V s−1,
which implies reductive diffusion-limited processes for 1. The
single reduction wave of A was previously assigned as a two-
electron process,42 and the same may apply to 1. To prove the
two-electron reduction process, we have recorded the cyclic
voltammograms of 1 and ferrocene (the oxidation of ferrocene
is a standard one-electron process) under an Ar atmosphere
(Figure S9). The consumed charge of the reduction wave at
−2.37 V (2.098 × 10−5 C) for 1 is ∼2 times as large as the
oxidation wave of ferrocene (1.066 × 10−5 C), indicating a
two-electron process for 1. On the basis of previous studies of
Mn-NHC catalysts, the two-electron process corresponds to
the consecutive ECE processes.35,36,39,42 Upon the first
electron reduction of 1 (the first “E” process), the Br− ligand
dissociates from fac-[Mn0(bis-MesNHC)(CO)3Br]− (1−; the
“C” process), leading to the formation of [Mn0(bis-MesNHC)-
(CO)3]0 (2•). Then, the Mn0 species 2• is reduced to give
[Mn−(bis-MesNHC)(CO)3]− (2−) upon the second electron
reduction (the second “E” process). During the reverse scan,
2• displays an oxidation wave at −2.02 V. The oxidation wave
D
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Figure 4. (a) Cyclic voltammograms of complex 1 (0.5 mM) in CO2-
saturated CH3CN with 0.1 M TBAP as the electrolyte at different
scan rates (0.1−1.0 V s−1). (b) Plot of TOF vs scan rate, with an inset
of ic/ip vs the inverse square root of the scan rate, highlighting that
steady-state conditions are accomplished at high scan rates (0.6−1.0
V s−1).
Figure 5. FTIR spectra of fac-[MnI(bis-MesNHC)(CO)3Br] (black,
1), the singly reduced species (red, 2• and unknown species X), solid
[2•−2•] dissolved in THF (purple), and the doubly reduced species
[Mn−(bis-MesNHC)(CO)3]− (orange, 2−) in THF together with the
ATR-IR spectrum of solid [2•−2•] (blue).
2
3
i
j
j
j
j
j
j
y
z
i
y
n
p
j i z
Fv
RT
i
y
z
z
z
z
c
j
j
j
j
j
z
z
z
z
z
j
TOFmax = 0.1992j
j
z
z
2 z
z
ip
ncat
k
{
(1)
k
{
k
{
IR stretching bands of these two isomers are very close to each
other. Thereby, the bands labeled with stars and triangles are
not likely due to the isomerization between fac- and mer-2•. In
addition, the ATR-IR spectrum of the crystal solid of [2•−2•]
displays three νCO bands at 1878, 1853, and 1811 cm−1;
redissolving solid [2•−2•] in THF produced a yellow solution,
showing three νCO bands at 1975, 1886, and 1864 cm−1, and
these bands are almost identical to the bands denoted with
triangles for the singly reduced species. We thereby conclude
that the bands denoted with triangles are assigned to the
solvated, singly reduced species 2•. It should be mentioned
that the calculated IR spectra of monomeric 2• and dimeric
[2•−2•] are largely different from each other, further
supporting our assignment. Unfortunately, the bands denoted
with stars remain unresolved, and this unknown species is
denoted as X.
where F is the Faraday constant, R the gas constant, T the
temperature, v the scan rate, np the number of electrons
involved in the noncatalytic Faradaic process (two electrons
for 1), ncat the number of electrons required for a single
catalytic cycle (two electrons for CO2 to CO), ic the catalytic
current plateau, and ip the peak current of the noncatalytic
wave. As shown in Figure 4b, TOFmax (3180
6 s−1) was
observed at a scan rate in the range of 0.6−1.0 V s−1.
Controlled-potential electrolysis (CPE) experiments were
performed in a homemade H-type electrochemical cell to
evaluate the stability of complex 1. As shown in Figure S14a,
the current density gradually decreased from −1.9 to −1.5 mA
cm−2 over 110 min in a CO2-saturated CH3CN solution
containing 0.1 M TBAP at an Eappl of −2.3 V. Gas
chromatographic analysis confirmed that the product of
electrochemical reduction of CO2 is CO with an average
Faradaic efficiency of 95% (Figure S14b). A charge of 32 C
passed over 110 min, corresponding to a TONCO value of ∼25.
Under the same conditions but in the absence of complex 1, no
CO and H2 were detected in the CPE experiment. Isotope
labeling experiments were conducted in a 13CO2-saturated
CH3CN solution containing 1.0 mM complex 1 at an applied
potential of −2.3 V. After electrolysis for 1 h, the gas phase
product was analyzed by GC-MS and the results showed that
13CO was the main product with a small amount of 12CO
(Figure S15; 11.4:1 13CO:12CO). Thereby, we can conclude
that complex 1 catalyzes electrochemical reduction of CO2 to
CO in a CO2-saturated CH3CN solution.
Chemical Reduction and Infrared Spectroscopy. The
FTIR spectra of 1 and its singly and doubly reduced species in
THF together with the spectrum of 2• in solid (denoted as
[2•−2•]) are depicted in Figure 5, and their DFT-calculated
CO stretching bands listed in Table S5. Similar to complex
A,42 complex 1 displays three characteristic νCO bands at 2009,
1929, and 1888 cm−1. Upon addition of 1.1 equiv of KC8 to a
THF solution of 1, the resulting solution displayed two sets of
νCO stretching bands with the lower-intensity bands at 1975,
1886, and 1872 cm−1 (labeled as purple triangles) and higher-
intensity bands at 1941, 1848, and 1828 cm−1 (labeled as red
stars). As time elapsed, the set of lower wavenumbers was
gradually converted to the set of higher wavenumbers (Figure
S16). For the five-coordinate, singly reduced species 2•, there
are two possible configurations: fac- and mer-2• (Figure S17).
According to our DFT calculations (Table S5), the calculated
The reduction of a Mn0 solution by KC8 leads to the
formation of complex 2−, which precipitated from the THF
solution as a yellow solid together with graphite (the reaction
product of KC8). As a result, we could not obtain pure
complex 2−. Thereby, we used metallic sodium as a reductant
and likely obtained the pure yellow precipitates of complex 2−
that showed low-energy νCO bands at 1863, 1710, and 1669
cm−1 (Figure 5), in good agreement with the formation of
Mn− species.42 The doubly reduced species of Mn-NHC CO2
reduction catalysts have not been previously isolated and
experimentally characterized, while such reduced species of
Mn-bpy-type CO2 reduction catalysts, for instance, fac-
[MnI(Mes bpy)(CO)3Br] (Mesbpy = 6,6′-dimesityl-2,2′-bipyr-
idine), were reported by the Kubiak group.34
Reaction of CO2 with Mn−. With the doubly reduced
species in hand, we tested the direct chemical reaction between
2− and CO2, monitored by the stopped-flow FTIR spectros-
copy. As shown in Figure S18, one syringe was filled with a
CO2-saturated THF solution and another syringe was filled
with a suspension solution of doubly reduced species 2−. Due
to the poor solubility of 2−, its suspension solution was used
and thereby kinetic values of the reaction were not calculated
due to the complex situation. Upon mixing, the FTIR spectral
changes versus time were monitored (Figure S19) and selected
spectra are plotted in Figure 6. No CO bands of 2− were
detected, indicating the poor solubility of 2− in THF and the
rapid reaction between 2− and CO2 via very likely the so-called
metallocarboxylate intermediate [2-CO2]−.51 As the reaction
proceeded, the CO bands at 1976, 1886, and 1868 cm−1
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intensity of the bands at 1686 and 1644 cm−1 (denoted with
circles) rapidly increased, and these bands are assigned to
HCO3 /CO3 stretching.55 The bands at 2089, 1997, and
1971 cm−1 (denoted with stars) matched well with the FTIR
spectrum of the tetracarbonyl Mn complex [2-CO]+.
Unfortunately, we did not observe the CO bands of the
metallocarboxylate intermediate, implying that its concen-
tration is extremely low. These spectral changes match well
with those observed in the stopped-flow FTIR experiments.
Apparently, the tetracarbonyl Mn complex [2-CO]+ is stable at
the Mn+ state. The buildup of [2-CO]+ and the undetectable
amount of other intermediates suggest that [2-CO]+ might be
the resting state.
Catalytic Cycle. The CO2 reduction mechanisms of
tricarbonyl Mn-bpy and Mn-NHC complexes have been
discussed by several research groups.35,39,42 On the basis of
our observations and previous mechanistic studies of other Mn
CO2 reduction catalysts, we could propose the following
reaction mechanism (Figure 7). The starting complex 1 is first
−
2−
Figure 6. Plot of FTIR spectral change vs time of 2− upon addition of
CO2 (0.28 M) in the stopped-flow experiment. Note that a band at
1670 cm−1, assigned to free CO32−, is observed but not shown in
Figure 6 for the sake of clarity.54,55
(denoted with triangles) assigned to 2• appeared as the most
intense bands and the intensities of the CO bands at 1941,
1848, and 1828 cm−1 assigned to X (denoted with circles)
gradually increased. Without knowing the structure of X, we
were not able to propose any chemical convention related to X.
A fourth intermediate was observed with CO bands at 2085,
1993, and 1969 cm−1 (denoted with stars) during the course of
the reaction between 2− and CO2. Royo, Lloret-Fillol, and co-
workers also observed similar CO stretching bands at 2090,
2002, and 1969 cm−1 for the A-catalyzed electrochemical CO2
reduction reaction, and this intermediate was assigned to the
tetracarbonyl intermediate [Mn(bis-MeNHC)(CO)4]+ on the
basis of DFT calculations.42 Rochford and co-workers reported
a tetracarbonyl Mn complex [Mn+([(MeO)2Ph]2bpy)(CO)4]+
{[(MeO)2Ph]2bpy = 6,6′-bis(dimethoxyphenyl)-2,2′-bipyri-
dine} with CO stretching bands at 2106, 2026, 2018, and
1982 cm−1, and this complex is stable but expels a CO
molecule after one-electron reduction to produce the
tricarbonyl Mn0 species.41 In addition, Cowan and co-workers
also observed a tetracarbonyl Mn complex [Mn(bpy)(CO)4]+
during CO2 reduction in situ at the electrode surface by
vibrational sum-frequency generation spectroscopy.52 Recently,
Andrew and co-workers directly synthesized a tetracarbonyl
Mn complex [Mn(bpy)(CO)4][SbF6] with CO bands at 2130,
2046, 2024, and 1984 cm−1, and this complex is an efficient
electrocatalyst for the reduction of CO2 in the presence of
water.53 In our case, when a THF solution of complex 1 was
stirred for 36 h under 10 atm of CO, we observed new CO
stretching bands at 2086, 1995, and 1969 cm−1 (denoted with
stars in Figure S20) and this species is assigned to the
tetracarbonyl manganese complex [Mn(bis-MesNHC)(CO)4]+
([2-CO]+). The CO bands at 2085, 1993, and 1969 cm−1 in
the stopped-flow experiment are consistent with the CO
stretching bands of this tetracarbonyl complex. Therefore, the
fourth intermediate is assigned as the tetracarbonyl inter-
mediate manganese complex.
Figure 7. Proposed electrochemical CO2 reduction mechanism of
complex 1.
reduced to generate the Mn0 species, which is prone to
bromide ligand dissociation, leading to the formation of 2•,
and it can take one more electron to generate the doubly
reduced 2−, which reacts fast with carbon dioxide and yields a
metallocarboxylate intermediate [2-CO2]−. Usually, Mn-bpy
catalysts require Brønsted or Lewis acids to facilitate the
formation of the metallocarboxylic acid or metallocarboxylate.5
For instance, [Mn(Mesbpy)(CO)3]− reacts with CO2 in the
presence of MeOH to afford the Mn-COOH species where the
C−OH bond is cleaved upon reduction and protonation,
leading to the formation of CO.34 When MeOH is replaced by
a Lewis acid Mg2+, a metallocarboxylate species [Mn(I)−
CO2Mg]+ is then proposed instead of the Mn-COOH species;
Spectroelectrochemistry. A homemade IR spectroelec-
trochemistry cell as shown in Figure S21 that consists of a glass
carbon working electrode, a Pt counter electrode, and a Ag/
AgNO3 reference electrode was used for this study. As
depicted in Figure S22, the Fourier-transform infrared
spectroelectrochemistry (FTIR-SEC) spectral changes of 1
under a CO2 atmosphere at an applied potential of −2.2 V in a
CH3CN solution showed that the intensity of the CO
stretching bands of 1 slightly decreased (denoted with
triangles) and the differential spectra revealed the growth of
new bands at 2089, 1997, 1971, 1686, and 1644 cm−1. The
F
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the solvated CO2 molecule as an electrophile attacks the
oxygen atom of the metallocarboxylate, leading to the
formation of CO and carbonate.38 In comparison with the
Mn-bpy catalysts, Mn-NHC catalysts are more electron rich in
general, and thereby, their metallocarboxylate species are more
electrophilic and can react with solvated CO2 in the absence of
additional acids. In our case, we proposed that the solvated
CO2 molecule acts as an oxygen atom acceptor, reacts with this
metallocarboxylate, and thereafter cleaves one C−O bond of
the carboxylate, leading to the formation of the tetracarbonyl
complex [2-CO]+. This complex is stable against CO liberation
and requires further reduction to enable CO dissociation.
Then, the tricarbonyl 2• is regenerated and the catalytic cycle
closed. The buildup of the tetracarbonyl Mn+ intermediate
suggests that the reduction of this intermediate to the Mn0
state might be the rate-limiting step when the doubly reduced
catalyst is involved. Notably, Cowan and co-workers also
observed the accumulation of the tetracarbonyl Mn-bpy
species on the Au−Hg electrode using vibrational sum-
frequency generation spectroscopy.51 This common phenom-
enon shared by both tricarbonyl Mn-bpy and Mn-NHC
complexes suggested that more attention should be paid to the
tetracarbonyl Mn species in order to understand further the
limiting step of CO2 reduction reaction by Mn catalysts.
AUTHOR INFORMATION
Corresponding Author
■
Lele Duan − Department of Chemistry and Shenzhen Grubbs
Institute and Guangdong Provincial Key Laboratory of Energy
Materials for Electric Power, Southern University of Science and
Technology (SUSTech), Shenzhen 518055, P. R. China;
orcid.org/0000-0003-1662-5817; Email: duanll@
sustech.edu.cn
Authors
Yong Yang − Department of Chemistry, Southern University of
Science and Technology (SUSTech), Shenzhen 518055, P. R.
China
Zhenyu Zhang − Department of Chemistry, Southern University
of Science and Technology (SUSTech), Shenzhen 518055, P. R.
China
Xiaoyong Chang − Department of Chemistry, Southern
University of Science and Technology (SUSTech), Shenzhen
518055, P. R. China
Ya-Qiong Zhang − Key Laboratory of Material Chemistry for
Energy Conversion and Storage, Ministry of Education, School
of Chemistry and Chemical Engineering, Huazhong University
of Science and Technology, Wuhan 430074, P. R. China
Rong-Zhen Liao − Key Laboratory of Material Chemistry for
Energy Conversion and Storage, Ministry of Education, School
of Chemistry and Chemical Engineering, Huazhong University
of Science and Technology, Wuhan 430074, P. R. China;
orcid.org/0000-0002-8989-6928
CONCLUSIONS
■
We have described an earth-abundant metal complex fac-
[MnI(bis-MesNHC)(CO)3Br] (1) that is capable of catalyzing
the electrochemical reduction of CO2 to CO with high activity
with a TOFmax of 3180 6 s−1. The introduction of two bulky
Mes groups on the NHC ligand significantly prevents the
dimerization of the singly reduced Mn species, weakens the
electron-donating ability of NHC due to steric hindrance, and
thereby decreases its onset potential. Furthermore, low-valence
intermediates were characterized by FTIR spectroscopy, and
the single-crystal X-ray diffraction structure of the singly
reduced species 2• was successfully determined. The rate-
limiting step for 1-catalyzed electrochemical CO2 reduction is
proposed to be the reduction of the tetracarbonyl Mn+ species
to its Mn0 state on the basis of time-course FTIR spectroscopy
studies. These findings enriched the CO2 reduction chemistry
of Mn-NHC catalysts and paved the way for the design of
more efficient earth-abundant Mn-based CO2 reduction
catalysts.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01364
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was supported by the Shenzhen Nobel Prize
Scientists Laboratory Project (C17783101), the National
Natural Science Foundation of China (21771098), the Science
and Technology Innovation Commission of Shenzhen
Municipality (JCYJ20170817111548026), the Shenzhen
Clean Energy Research Institute (CERI-KY-2019-003), and
the Guangdong Provincial Key Laboratory of Energy Materials
for Electric Power (2018B030322001).
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